pergamon emulsion polymerization

Pergamon Prog. Polym. Sci., Vol. 19,703- 753, 1994

© 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved.

0079 6700/94 $26.00 0079-6700(94)E0003-J

E M U L S I O N P O L Y M E R I Z A T I O N

QvN WANG, SHOUKUAN Fu* and TONGYIN YU Department of Macromolecular Science, Fudan University, Shanghai 200433, China

(*To whom correspondence should be addressed)

C O N T E N T S

1. Introduction 2. Recent advances in the kinetics of emulsion polymerization

2.1. Kinetics of particle growth 2.1.1. Theoretical treatments 2.1.2. Kinetic parameters and mechanisms

2.2. Kinetics of particle formation 3. Polymerization

3.1. Soapless emulsion polymerization 3.1.1. Polymerization process 3.1.2. Applications

3.2. Miniemulsion polymerization 3.2.1. The formation and stability of miniemulsions 3.2.2. The kinetics of miniemulsion polymerization 3.2.3. Copolymerization in miniemulsion systems

3.3. Microemulsion polymerization 3.4. Dispersion polymerization

3.4.1. The mechanism of steric stabilization 3.4.2. Design and synthesis of polymeric steric stabilizers 3.4.3. Mechanism of dispersion polymerization 3.4.4. Dispersion polymerization techniques 3.4.5. Applications of non-aqueous polymer dispersions

3.5. Preparation and application of monodisperse polymeric microspheres 3.6. Composite polymer latexes

3.6.1. The preparation and characterization of composite polymer latexes 3.6.2. The thermodynamic approach and model predictions 3.6.3. Structure and properties

3.7. Surface-functionalized polymeric dispersions 3.7.1. Preparation and characterization 3.7.2. Applications

References

703 704 706 706 708 718 720 720 721 723 723 724 725 725 726 729 730 732 734 735 736 736 740 741 741 743 744 745 746 747

1. I N T R O D U C T I O N

The m e t h o d o f emuls ion p o l y m e r i z a t i o n was first r e p o r t e d in 1932 by L u t h e r and Heuck . 1 It u n d e r w e n t initial d e v e l o p m e n t ove r the pe r iod 1930-1950 an d la ter became a ve ry successful indus t r ia l process . E m u l s i o n p o l y m e r i z a t i o n is a c o m p a r t m e n t a l i z e d p o l y m e r i z a t i o n r eac t ion tak ing place in a large n u m b e r o f r eac t ion loci d ispersed in a c o n t i n u o u s ex te rna l phase. This m e a n s tha t emul s ion p o l y m e r i z a t i o n is car r ied ou t in h e t e r o g e n e o u s systems, c o m m o n l y wi th an a q u e o u s phase an d a n o n - a q u e o u s phase. T h e m o n o m e r and p o l y m e r usual ly be long to the n o n - a q u e o u s phase . In inverse

703

704 Q. WANG et al.

systems, water-soluble monomers are dispersed in a non-aqueous medium. A typical emulsion polymerization system consists of water (or non-aqueous medium), monomer(s), emulsifier (surfactant) and initiator; typically, 30% monomer, 65% water and remainder surfactant, modifier and other additives. Usually, the monomer is sparingly soluble in water and generates a water-insoluble polymer which is swollen by monomer. Emulsification initially results in micelles (1019-1021 dm -3) swollen with solubilized monomer and surfactant-stabilized monomer droplets (1012-1014 dm-3). The micelles are usually 0.005-0.01 ~m in size, whereas the monomer droplets are of the order of 1-10~m. As initiator decomposition takes place, a new phase appears, the latex particles, which contain macromolecules of fairly high degree of polymerization, and are swollen with monomer and stabilized by surfactant. The diameter of these latex particles is around 0.1 I~m and the number density is usually in the range 1016-1018 dm -3.

Emulsion polymerization has some advantages over other polymerization processes. (1) Since the system remains perfectly fluid throughout the entire polymerization process, the heat generated by the exothermic free radical polymerization can be readily dissipated into the aqueous phase. (2) The rate of polymerization is usually much higher than that in an equivalent bulk process. (3) The formed polymer often has a higher molecular weight than that formed in bulk or solution polymerization. (4) The polymer is formed as a latex so that it can be more readily handled and it has an obvious advantage if the polymer is to be applied as a latex. (5) One can easily control the molecular weight by addition of chain transfer agents and thus control the properties of the final product. (6) The polymerization process and the resulting polymer latex is water based, so environmental hazards are minimized.

However, the emulsion polymerization process along with the resulting polymer also has certain inherent disadvantages. For example, since the formed latex contains surfactants and initiator decomposition products, which are often difficult to remove, emulsion polymers are usually excluded from applications requiring materials of high purity. Moreover, as an industrial process, the effective reactor volume available for polymerization is reduced by the volume of the aqueous dispersion medium, compared with bulk polymerization.

Emulsion polymerization is one of the most widely used polymerization techniques in industry, and emulsion polymers are utilized in various fields, including coatings, paints, inks, adhesives, and rubbers. More recently, emulsion polymers have been used as fine materials in chromatography, electron microscopy, drug delivery, and biochemical measurements. There seems to be an increasing interest in emulsion polymerization, from both an industrial and academic perspective.

2. RECENT ADVANCES IN THE K I N E T I C S OF E M U L S I O N P O L Y M E R I Z A T I O N

Although the emulsion polymerization technique has been widely used in industry since the 1930s, it was not until the late 1940s that a theoretical basis was successfully established for the process and results, both qualitative and quantitative, were reported.

In 1945, Harkins postulated that the main locus of emulsion polymerization is in the stabilized polymer particles rather than in the monomer droplets. He put together a

EMULSION POLYMERIZATION '705

qualitative mechanism reasonably consistent with experimental data. 2,3 In the late 1940s, Smith and Ewart 4-6 treated Harkins' physical picture in a quantitative fashion. The so-called 'Smith-Ewart Theory' has served as a framework for later research efforts. In many of the papers giving various modifications and extensions to the original work of Smith and Ewart, authors extended the treatment of the nucleation or particle formation mechanisms, 7-15 particle growth mechanisms,16-24 determinants of particle size distribution, 25-3° and particle morphology. 31-35

It is now widely accepted that an emulsion polymerization process can be classified into three separate stages.

Stage I. In this initial stage, the formation or nucleation of latex particles takes place. The system is characterized by the presence of surfactant micelles and monomer droplets, and an increase in latex particle number and particle size. This stage usually corresponds to a conversion range of 0-10%.

Stage II. This second stage starts when the nucleation of particles is complete. It is characterized by the absence of surfactant micelles, a constant number of monomer- swollen latex particles, a constant monomer concentration within the latex particles and an increase in particle size. This stage usually corresponds to a conversion range of 10 40%.

Stage III. This final stage begins when monomer droplets disappear, and almost all of the remaining monomer is now confined to the latex particles. It is characterized by a constant number of latex particles and a decreasing monomer concentration within the latex particles. This stage usually corresponds to a conversion range of 40-100%.

These three stages can be observed in a conversion time or rate- t ime plot, as shown in Fig. 1. First there is a period of increasing rate, stage I. This is because the primary free radicals generated from initiator decomposition initiate the polymerization and thus the particle number increases during the nucleation period. Then there is a period of constant polymerization rate, stage II. This is because the particle number, monomer concentration, and the average number of free radicals within each particle are approximately constant. Eventually, there is a period during which the polymerization rate starts to decrease, stage III. This is simply because the monomer droplets are no longer present, the latex particles can get no more monomer from 'the reservoir' and so the concentration of monomer within the particles decreases monotonically within this stage. There may also be an increase in the polymerization rate, which is often attributed to the gel effect.

It is also widely accepted that the main locus of polymerization in a conventional emulsion system is the interior of the latex particles, rather than the monomer droplets, micelles, or the aqueous phase, although under certain circumstances, polymerization may be carried out in monomer droplets (as in the case of miniemulsion polymerization, or microemulsion polymerization), or in the micelles (as in the case of micellar polymerization).

Since the early 1980s, Napper 's group (at the University of Sydney) has extensively studied homopolymerization in emulsion-based systems. They have employed a set of methods, including seeded emulsion polymerization, radiolysis initiation, and the use of particle size distribution data and high conversion kinetic data, to study particle growth and particle nucleation mechanisms. The observables they obtained and used in their mechanistic investigations are the rate of polymerization (measured dilato- metrically), the particle size and particle size distribution (measured by electron microscopy), and the molecular weight distribution. What can be considered as the unique

7 0 6 Q. W A N G et aL

I

X 0 . 5

150

t (min)

~ 0 30O

Fio. 1. Typical plot of fraction conversion (x) and rate (dx/dt) vs. time in an emulsion polymerization system.

feature of their work is that, instead of merely interpreting the data, they used these data to test the postulated mechanisms, i.e. they tried to confirm one of the postulated mechanisms and to refute conflicting ones, using sufficient independent data. By doing so, they have established a set of confirmed mechanisms responsible for the formation and growth of latex particles in an emulsion polymerization system. Their main results concerning the kinetics of particle formation and growth are summarized in the following two sections.

It has been established that, in a seeded emulsion polymerization system, conditions can be chosen to start the polymerization in either stage II or III, so that the complexities involved in the particle formation process can be obviated. It is for this reason that the kinetics of particle growth, i.e. the kinetics of stages II and III, are discussed first.

2.1. Kinetics of particle growth

2.1.1. Theoretical treatments - During stage II or III, i.e. in the process of particle growth, the particle number density N c remains constant. The rate of polymerization in the latex particles is given by eqn. (1), where x is the fractional conversion of monomer to polymer, CM is the monomer concentration within the latex particles

0 is the number of moles of monomer initially (i.e. at the locus of polymerization), n m present, and NA is Avogadro's constant. In stage II, CM and kp are both constant, at least within good approximation. In stage III, CM decreases, and one has (for a monomer sparingly soluble in water) x = 1 - (CM/C°), where C ° is the value of CM at the start of stage III and where, for convenience, x is set zero at the start of stage III. Thus, in stage III, eqn. (2) is operative, where fi is the average number of free radicals per particle and Vs is the swollen volume of the particles. Equations (1) and (2) are very similar except that x is replaced by - In(1 - x) and A by kp/NAVs, and can

EMULSION POLYMERIZATION 707

be analyzed in a similar way. Note that ~ and (in stage III) kp may vary with conversion.

x d - = kpNcCM/(n° NA)ft =_ Ah (stages I, II, III) dt

- d l n ( 1 - x ) / d t = (kp/NAVs)~ (stage III)

(1)

(2)

The average number of free radicals per particle, ~, is given by eqn. (3), where N n is the number of latex particles containing n free radicals; for convenience, one adopts the normalization ~ Nn = 1. The kinetic events dictating the time evolution of ~ are as follows: entry of free radicals into the latex particles (pseudo-first-order rate coefficient, p, i.e. the average number of free radicals entering a single latex particle per second); exit (desorption) of free radicals from the growing particles (first-order rate coefficient, k, i.e. the average fraction of free radicals exiting a latex particle per second); and bimolecular termination between two free radicals in a particle (pseudo- first-order rate coefficient, c = kt/NA Vs, where k t is the second-order termination rate coefficient). Thus, one has the Smith-Ewart equations for the time evolution of Nn (eqn. (4)).

4 = Z n N n

d N n / d t = p[Nn_ 1 - Nn] ~- k[(/~/-~- 1)Nn+ 1 - nNn]

+ c[(n + 2)(n 3- 1)Nn+2 - n(n + 1)N,]

(3)

(4)

The Smith-Ewart equations must take into account aqueous events, which have an important effect on the kinetics of emulsion polymerization and are extremely complicated. It has been shown 36-39 that these complexities can be condensed into a single quantity, termed 'fate parameter' , ~, resulting in great simplification. This dimensionless quantity, c~, which is confined to the range -1 _> c~ >_ + 1, is a complicated function of the various rate coefficients for the aqueous phase processes, 3v-39 but its physical implication is that it gives the relative importance of heterotermination (~ = - 1 ) and re-entry (o~ = + 1) in aqueous phase kinetics. Equation (5) encapsulates all the complexities of aqueous phase kinetics into a simple form.

/9 = PA -~- c~k~ (5)

where PA is the component of the entry rate coefficient arising entirely from free radicals generated from initiation, i.e. in the absence of exit.

Equations (1-5) admit of no time-dependent solution, inter alia because the various rate coefficients may vary with conversion. However, it is a straightforward matter to obtain numerical solutions, although, since the equations are coupled, an appropriate algorithm must be used (e.g. the Gear method). 4° Meanwhile, various approximate time-dependent solutions of eqns. (1-4) have been discussed by a number of authors.36-38, 44

In the high ~ limit, i.e. when ~ is comparatively large, the system obeys 'pseudo- bulk' kinetics, which means the effect of compartmentalization (the fact that the free radicals are in isolated latex particles) can, under certain circumstances, be neglected. As shown by Ballard e t a/., 42'43 when ~ is high or when both c~ = +1 and k >> p, c,

7 0 8 Q. W A N G et al.

eqn. (4) can be replaced by eqn. (6).

d~/dt = p - kn - 2c~ 2 = P A + ( o t - - 1)k~ - 2 c ~ 2 (6)

In the low ~ limit, c is so large that mutua l terminat ion occurs instantaneously when a free radical enters a particle which already contains another free radical. No particle contains more than one free radical in such a system, which is termed 'zero-one'; the condi t ions are c >> k, p. Then, eqn. (4) becomes eqns. (7) and (8), which can be formally derived f rom eqn. (4) by making the steady-state approximat ion for dNz /d t and ignoring higher order terms.

dNo/dt = --(PA + akN1)No + (PA + akN1 + k ) N 1 (7)

dN1/dt = ( P A + akNl)No - (PA + akN1 + k)N1 (8)

As shown by Whang et al.,37 eqns. (7) and (8) have analytical solutions, which can be used with eqn. (1) at long times to compute the values of PA and k f rom experimental x(t) curves, as indicated in eqns. (9-12), where s and i are the long time slope and intercept of the x(t) curve and ~0 is the initial value of ~.

lim x(t) = i + st (9) l---* ~X~

k = A ln ( r ) / (2a i ) , PA = Gk (10)

G = [2as 2 + A(1 - a)s] /A(a - 2s) (11)

F = l + { Z G + ( 1 - a ) + 4 a ~ o } / { Z [ 4 G 2 + ( a - 1 ) Z + 4 G ( a + l ) ] 1/2} (12)

Note that eqns. (10-12) are replaced by simpler expressions for the special case where a = 0. 37 It has also been pointed out that in the limit for which these results are applicable, the values of PA and k deduced f rom the data can be very sensitive to small errors in the steady-state ~, given by eqn. (13), which is the steady-state solution of eqns. (7) and (8).

~ s s = P / ( Z p + k ) (13)

The result is particularly useful since it shows that, in the limit as the entry rate coefficient becomes large, the steady-state ~ achieves the limiting value of 0.5; this limiting case was called 'Case 2' by Smith and Ewart. 5 Note that ~ only assumes this limiting value of 0.5 under special circumstances (c >> p >> k), and is found in relatively few experimental situations. One should never assume, wi thout firm evidence, that a part icular system will exhibit Case 2 kinetics. Smith and Ewart designated as 'Case 1' the si tuation when h < 0.5, and as 'Case 3' when ~ > 0.5.

Summat ion ofeqn . (4) over n, after mult ipl icat ion by n, yields eqn. (14), 42 where (n 2) is the second m o m e n t of N n (~ being the first moment) . I f the system starts out initially with ~ = 0, PA can be est imated by using eqn. (15).

d~/dt = PA - - ( 1 - - a)k~ - 2c((n 2) - ~) (14)

dR PA = lim-=- (if~(/=0) = 0) (15)

t-~0 at

2.1.2. Kinetic parameters and mechanisms

2.1.2.1. Monomer concentration - In stage III, for a m o n o m e r that is very insoluble in water, CM in the monomer-s ta rved particles is trivially calculated f rom the amoun t


initially added, since essentially all monomer is located inside the latex particles (for more soluble monomers such as MMA, it is necessary to allow for the partitioning of monomer between the particles and the aqueous phase). 43 In stage I and II, the monomer concentration within the latex particles achieves an equilibrium saturation value, with the latex particles minimizing the free energy, which has components arising from both interfacial tension and mixing. These competing effects are considered in eqn. (16), where F is the interfacial tension between the latex particles and water, 0 is the volume fraction of polymer, V m is the partial molar volume of monomer (to an excellent approximation, Vm = Mo/dM, where M0 is the molecular weight of" the monomer), R is the gas constant, r is the unswollen radius of the particle, and X is the Flory-Huggins interaction parameter. The values of 0 and Wp (the weight fraction of polymer) are related by mass conservation, as given in eqn. (17). The value of CM is then found from eqn. (18). However, the values of F and X cannot be predicted for a given system at present and indeed, the physical basis of eqn. (17) is open to question. It is therefore necessary to determine the equilibrium monomer concentration in stage II directly from experiment, for example, by direct analysisf or by determination of the x value at which the transition from stage II to stage III takes place and CM is then found by assuming ideal mixing, using eqn. (19), 45'46 o r by the 'static swelling' method proposed by Ballard et al. 43

ln(1 - O) + q5 + XO 2 + (2FVm/rRT)O U3 -- 0 (16)

= [(dp/dM)Wp 1 4- (1 - { d p / d M } ) ] - ' (17)

C M = (l - q~) /V m (18)

C M = (1 - x ) g O / ( N c V s ) (19)

Given the value of CM, the swollen particle volume Vs can be calculated at any fractional conversion from eqn. (20), where V is the unswollen particle volume.

V s / V -= 0 -1 = d M / ( d M - C M M o ) (20)

2.1.2.2. Mode&for entry - The mechanism whereby primary free radicals generated in the aqueous phase of an emulsion polymerization system are able to enter surfactant micelles and/or monomer-swollen latex particles and initiate polymerization is of central importance in heterogeneous polymerization, because such entry events are relevant to both the nucleation of latex particles and their subsequent growth. Entry of free radicals into latex particles is more readily studied by the use of seed particles; if present at sufficiently high concentrations, preformed particles obviate the occurrence of secondary nucleation. 4s'47 This allows the average entry rate coefficient per particle p to be measured, 45 together with its dependence upon such factors as the free radical production rate, the latex particle size and number, the surfactant surface coverage, the electrostatic charges associated with both the entering free radicals and the particles into which they enter, ionic strength, the degree of swelling of the latex particles by the monomer, etc. 37'47-5° It is also possible to measure the activation energy for entry by using temperature-independent ,,/-ray initiation 51 or UV-initiation. 52

At least four different theories for entry have been proposed. (1) Collision Theory. Gardon 7 attempted to predict the rate of entry of free radicals into latex particles by using simple collisional considerations embodied in the kinetic

710 Q. WANG e t al.

theory of gases. This primitive theory predicts that entry rate should vary with the collision cross-section of the particles, i.e. with the square of the particle radius. (2) Surfactant Displacement Theory. It has been postulated 15 that the displacement of surfactant molecules from the surface of latex particles by incoming surface-active free radical species is the rate determining step for entry. This mechanism would imply that p should depend on the extent of surface coverage of the latex particles. (3) Oligomeric Entry. Implicit in the development of several of the theories of free radical entry 1°'12 and particle nucleation was the assumption that the rate-determining step is the diffusion of soluble oligomeric species from the bulk aqueous phase, where they are generated, to the latex particle surfaces. Entry should thus be influenced by any electrostatic repulsion between the entering free radical species and the electro- statically stabilized latex particles. It would also be expected that the entry rate would be proportional to the radius of the particles, rather than the square of the radius. (4) Colloidal Entry. This theory differs from the proceeding one in that the diffusing free radical species is not a soluble surface active oligomer but an insoluble colloidal entity that heterocoagulates with the latex particles. 39

The most extensive data on PA are for styrene, for a wide range of [I], with various initiator types and particle number. 39'45'47'48 Data on PA for styrene, BA, and BMA are given in Table 1.

The first important result is that the radical capture efficiency f is never 100% (it could be as low as 1-10%) for styrene for the persulfate concentrations (10 -3-10 -2 M) used in practice, which contradicts the unfortunately common assumption made in interpreting and predicting emulsion polymerization data. Moreover, the d a t a 38'39'45'47,48'53'54 show that f is not independent of [I]. These imply that thermodynamic considerations may be important for persulfate and that primary free radicals are not the entering species. Rather, the primary free radicals must add one or more monomer molecules before entry can occur. During addition, bimolecular termination can also occur, leading to a reduced capture efficiency. At low initiator concentrations (ca. 10 -5 M), the efficiency of free radical capture approaches the 100% predicted by Flory, 55 presumably because of reduced termination in the aqueous phase.

On the other hand, experimental results show no evidence of notable effects of particle surface coverage and ionic strength on the entry rate. 48 It is also shown in Table 1 that the entry rate does not change for positively charged primary free radicals compared with negatively charged species. 47,48

The k e values (105-106 dm 3 mol-1 s-l) in Table 1 eliminate the simple collision entry mechanism, because this is some 10 orders of magnitude smaller than that predicted by the collisional rate of an oligomer with latex particles. The fact that ke is unaffected by the surface coverage of particles by surfactant helps to rule out the surfactant displa-

TABLE 1. Entry rate parameters for various monomers at 50 °C

7 ke rs f Monomer Initiator (mo ldm -3) (dm3mo1-1 s -1) (dm3mo1-1 s -1) /~ (nm) (%) Ref.

Styrene K2S208 2.8 x 10 -1° 3 x 103 3 x 105-4 x 106 0.4 69 2 39 8.0 x 10 -11 5 x 103 5 x 105-3 x 104 0.5 113 2 39

V50 6.4 × 10 -12 1 x 104 1.2 x 106 0.4 69 1 39, 47 B M A K25208 3 × 10 -10 1.4 x 104 2 × 106 0.7 51 30 I0, 50 BA K2S208 1.2 x 10 -1° 6 x 104 4 x 106 7 83 10 50, 53


cement theory. The observation that k e is not significantly affected by ionic strength and surface charge refutes the applicability of the colloidal entry mechanism and the oligomeric entry mechanism.

The only entry model that is in accord with the observed dependence of PA on [I] and Nc has been given by Penboss et a l . , 38'39'47 who extended the earlier work of Hansen and Ugelstad. 36 This model takes into account the various events leading to entry and

2 gives eqn. (21), where 3' = ke/2kt,aq, fl ~-kt,aq "kI/k2, N = Nc/NA, where k e is the second-order rate coefficient of entry of a free radical arising directly from initiator, ki is the first-order rate coefficient for the production of such a free radical from initiator decomposition (kt is an effective rate coefficient which takes account of the aqueous phase propagation of a free radical produced directly from initiator decomposition to a sufficient degree of polymerization to be able to enter the particle). The rate parameters/3 and 7 can be found by non-linear least square fitting eqn. (21) to data on the dependence of PA o n [I] and Arc; it is found that eqn. (21) always provides a good fit to experiment. Table 1 displays the values of 3 and 7 so obtained for styrene, BA, and BMA. Given an assumed value of kt,aq, o n e may then obtain estimates of values of ke and ki, also provided in Table 1. A sensitivity analysis of the fitting procedure using extensive data on styrene 39 yields the uncertainties for ke for styrene listed in Table 1.

PI = 7[( u2 + 4/3[I]) 1/2 - N] (21)

It is assumed in this model that aqueous propagation to the critical size z is the rate- determining step, and a reasonably accurate steady-state analysis yields an equation similar to that developed by the HUFT nucleation theory.

p = (2kd[I]NA/Nc)[(2kt[T. ]/kp[maq])÷ 1] ' -z (22)

where [T. ] = (kd[I]/kt)l/2, is the total radical concentration in the aqueous phase. A value of the critical degree of polymerization z, 2-3, brings the prediction of the aqueous phase propagation model into reasonable quantitative agreement with experimental results. The model also implies that p should be proportional to the swollen surface area of the latex particles, provided [I] is the same. This agrees well with the p/r s relationship found by Fu et al. 5°

2.1.2.3. The fate parameter' It may be recalled that the fate parameter a gives the relative importance, for exited free radicals, of competing re-entry (c~ = + 1) versus heterotermination (a = -1) . Table 2 shows the values of a currently available from experiments. For chemical initiations, o~ is in the range 0 > a _>-1 for styrene, whereas for MMA, BA, and BMA, a appears closer to +1. For relaxation studies, c~ appears to be +1 for all monomers and initiator types. Reliable values for c~ with

TABLE 2. Values of a for emulsion polymerization systems at 50 °C

Monomer a(K2S207 initiator) a(7-radiolysis relaxation mode) Ref.

Styrene 1 > c~ > - 1 +1 37 M M A +1 > a > +0.5 +1 43 BA +1 > a _> +0.5 +1 54 B M A 1 > a > 0 +1 53

712 Q. WANG et al.

"),-initiation (i.e. not in the relaxation mode) are unavailable, but all data can be fitted by assigning the radiolytic a the same value as for the monomer using chemical initiation.

The value of a = +1 in the relaxation mode (in the absence of chemical or initiator) is readily explained. After removal from the radiolysis system there will be only a low concentration of (thermally generated) free radicals in the aqueous phase, and thus the most likely fate for an exited free radical is re-entry, rather than heterotermination.

For chemical initiators the dependence of the fate parameters on monomer can be qualitatively understood as follows. The kp for styrene is significantly lower than other monomers examined. This suggests that the value of a is determined by the length of time spent growing to a sufficient degree of polymerization in the aqueous phase. That is, if the exited free radical propagates quickly, it is much more likely to re-enter than to heteroterminate. The solubility of the monomer in the aqueous phase is also important in determining both the propagation rate and the number of monomers that must be added to allow entry to occur, both of which might influence a.

A general theory for determining a given appropriate rate coefficients has been developed, 39 but it requires values for various rate parameters such as the rate coefficient for heterotermination between exited free radicals and free radicals derived directly from initiator, and that for re-entry. A quantitative comparison between theory and experiment has yet to be carried out. Moreover, one sees in Table 2 that the values of a deduced from experiment are subject to quite a large uncertainty. However, it is found that it is unnecessary to know the precise value of a, because, in m o s t cases, 37'43'50 computed x ( t ) curves and deduced PA values calculated with a values covering the range of uncertainties given in Table 2 show no significant difference.

One can therefore conclude that the value of a can be obtained to within the precision needed to interpret and predict experimental data using an estimate of kp for the monomer and/or consideration of initiator type.

2.1.2.4. E x i t m o d e l s - The importance of exit (desorption) of free radicals from latex particles in emulsion polymerizations has only become evident in recent times. Even for relatively water-insoluble monomers such as styrene, exit of free radicals can be rate determining if the particle size is sufficiently small.

A large amount of data on exit, for a range of monomer systems, is now available. These data suggest that exit occurs through transfer to monomer followed by diffusion of the resulting small free radicals out of the particle before it has time to propagate. Some results have been obtained by assumption-flee methods, 37,45,5°,52,54,56-58 while others have been obtained by indirect (global modeling) techniques. 8'36'59-61 Obviously, the former are probably more reliable.

Exit data are found to be in excellent accord with the transfer/diffusion theory, 36'59 which involves the following steps: (1) transfer of free radical activity to monomer or to a chain transfer agent; (2) propagation of this new free radical; (3) diffusion of the free radical to the boundary of the particle before propagation; and (4) diffusion of the small free radical away from the particle through the aqueous phase. It has been shown that if these four events are all rate determining, the exit rate coefficient k is given by eqn. (23). Here, z is the degree of polymerization of the exiting free radical, and Dp and Dw are the diffusion coefficients for the exiting species in the particle and in the


aqueous phase, respectively. The parameter q is the partition coefficient of the exiting species between the organic latex particle and the aqueous phase. To a good approximation, q = CM/[Maq ]. The transfer/diffusion theory can be simplified in certain limits. For a sparingly soluble monomer, such as styrene, q >> 6Dw/Dp, so that eqn. (23) becomes eqn. (24). In the limit where diffusion within the particles is rate determining (i.e. 6Dw/Dp >> q), eqn. (23) reduces to eqn. (25).

k = (3zDw/r~)(k tr /kp) / ( q + 6Dw/Dp) (23)

k -~ (3zDw/r~)(ktr/kp)([Maq]/CM) (if q >> 6Dw/Dp) (24)

k ~- z (k t r /kp)Dp/2r ~ (if 6Dw/D p >> q) (25)

Another limit of interest is the 'transfer controlled limit', where eqn. (23) can be replaced by eqn. (26). In actuality, k can be given by eqn. (27), where ktd is calculated from transfer and diffusion (eqn. (23)) and kt0 is calculated from transfer only (eqn. (26)).

k - k t r C M (26)

k -1 = (ktd) -1 q- (kt0) -1 (27)

Extensive tests of eqns. (23-27) have been carried out on styrene systems. The results are in quantitative conformity with the prediction of eqn. (23), which can be shown to be the appropriate limit for this styrene system, namely, k is linear 45'5° in rs 2 and in the concentration of chain transfer agents. 57 Adams et al. 56 examined the effect of a decreased value of q by adding methanol, and excellent accord with theory was again found, including the transition from the transfer/diffusion limit (eqn. (23)) to transfer alone (eqn. (26)) as given in eqn. (27). The effect of inert diluents on k can be understood in terms of transfer/diffusion theory and the slow propagation of the transferred free radicals s9. Thus, for styrene, the transfer/diffusion theory gives good agreement with the data.

Data for other monomers are much more sparse. The only direct measurement is for BMA, 54 where results are again in good accord with eqn. (23). Some other results were deduced from modeling studies. 8,36,59,61

In summary, all reliable data on exit can be qualitatively and quantitatively explained in terms of the transfer/diffusion theory which would therefore seem to be quite reliable for predictions and for modeling studies.

2.1.2.5. Termination models It may be expected that the termination processes in emulsion polymerization can be different from those operative in low conversion bulk systems, because the presence of formed polymer in the latex particles ensures that entanglement effects are almost always present for chains of significant molecular weight.

The value of c and hence k t may be obtained using the 'high ~/constant k and c" method, in which eqn. (4) can be replaced by e q n . (28). 42'43

d~ /d t = p - k~ - 2c~ 2 = P A q- (ol - - 1)kt7 - 2 c t I 2 (28)

The requirement that k t be constant over the range of conversion used for data reduction (apart from trivial change due to variation in swollen volume) will usually confine the method to stage II, and s o k t can only be obtained at the weight fraction

714 Q. WANG et al.

TABLE 3. Experimental results and theoretical upper and lower bounds for k t (dm 3 mol - I s - i ) for M M A , BMA, and BA

M M A value

Quanti ty no C T A 2% CBr 4 BA value B M A value

Ref. 43, 63 63 50, 53 50, 54 Wp 0.33 0.33 0.57 0.43

k t (experimental) 3.3 × 10 4 2.1 × 10 5 8 × 10 2 7 x 10 2 k t (lower bound) 3 × 104 3 × 104 9 × 102 1.5 × 102 k t (upper bound) 1.7 × 10 5 1.7 x 10 5 3 × 10 4 5 × 10 4

corresponding to the equilibrium swelling of the particles by the monomer (typically Wp ~ 30-40%). To date, this method has been applied to BMA and BA. 53,54,62 Results are listed in Table 3.

A more widely applied technique for obtaining kt from kinetic data is through eqn. (29), which is valid if ~ is sufficiently high for k not to be rate determining.

P + 1 - (29) t~(t) = nss P 1 ; P = exp[(8pAc)l/Zt] t~ss + no - - / ' / s s - - n O

where, ~0 is the initial value of ~ and Ass = (#A/C) 1/2. Equation (29) can be used to least-square fit 7-radiolysis relaxation data (given kp, including any dependence on Wp). This relation is derived assuming that k t is constant, which will usually be an excellent approximation over the small range of Wp for which relaxation takes place. Moreover, the dependence of k t o n Wp so deduced can then be iteratively improved through exact numerical simulation. The technique is therefore applicable to both stages II and III.

There has been an extensive application of this method to MMA by Ballard eta/. 43'63 The dependence of k t o n Wp was determined in a PMMA seed of high molecular weight, both in the absence of chain transfer agent (CTA) and with the addition of CTA to observe the effect of change of molecular weight of the growing chain on k t. Their results on kt data are the best available for any free radical system at high Wp since they were derived essentially free of any mechanistic assumptions. The requisite kp(wp) values were obtained directly from ESR measurements 64 (and have been verified by independent kinetic data65). No modeling assumptions were required, since the only free radical loss event is termination, whose rate is measured directly using 7-radiolysis in the relaxation mode.

It is generally accepted that termination at low Wp involves three steps66: (1) two radical coils gain proximity by translational diffusion; (2) the radical chain ends make contact through segmental diffusion; and (3) the barriers to chemical reaction are overcome. Since step (3) is rapid compared with the other two in polymerization systems, termination is expected to be entirely diffusion controlled. The values of k t

may be a function of the following: Wp, the molecular weight of the polymer matrix ( M m ) , and the degree of polymerization of the two terminating species. Since M m and Wp evolve in time, k t may also depend implicitly on time. The overall loss of free radical activity by termination is given by eqn. (30). Here n(p, t) denotes the concentration of chains of degree ofpolymerizationp at time t. Equation (30) can be rewritten in terms of an average kt, defined by eqn. (31), where [R.] is the overall free-radical


concentration. However,/~t will, in general, be a function of time and indeed of the full history of the system, e.g. varying with initiator concentration, because [I] may affect

Rlterminati°n = - Z Z kt(r's' WP ' M m ' On(r' On(s' t) r s

Rltermination = - ]~t[R.] 2

Mm"

(30)

(31)

The experimental variation of k t with Wp for MMA as reported by Ballard eta/. 43'63

shows some of these effects. First, note that the values of k t a r e several orders of magnitude less than those reported in the literature for Wp = 0. This illustrates that u s e o f k t values for Wp = 0 in global modeling (as is not uncommon) will lead to serious error. It can be seen that for Wp ~ 0.3, addition of a CTA (reducing the length of both terminating chains) results in an obvious increase in kt, which is in agreement with the intermediate Wp regime of the data.

The kinetics of termination can be represented in terms of a single rate coefficient (kt or c) only at low and high Wp; at low Wp, this kt will be almost constant, whereas at high wp, it will depend on Wp. In the intermediate regime, a value of the termination rate coefficient deduced from experiment using a single kt, rather than some form of eqn. (30), will be an apparent rate coefficient which may vary with system characteristics such as [I], etc. This may contribute to some of the wide variation of kt values reported in the literature.

In an emulsion polymerization system, residual termination may become applicable at quite low Wp values. For example, with monomers (such as MMA) which are moderately water soluble, the entering oligomeric free radical species is expected to be of moderately high degree of polymerization (say, 50-100), and will be likely to undergo rapid propagation on entering the particle to reach the critical degree of polymerization for entanglement. This would signal the onset of residual termination; if CTA is absent, this can occur at Wp as low as 0.35. Theories for residual termination have been given by Gardon 2° and by Soh and Sundberg. 67 However, the simplest theory is furnished by the 'rigid chain limit' and 'flexible chain limit' of Russell et al. 6s The rigid chain limit provides a lower bound for residual kt, which must be applicable at the highest Wp values. It supposes that the growing chain is completely rigid in the polymer matrix on the timescale of propagation. The termination rate coefficient can be found from the Smoluchowski equation, k t = 167rDd, where D is the 'diffusion coefficient' of the propagating chain end and d is the radius of interaction at which termination may be expected to occur instantaneously. One can identify d with the Lennard-Jones radius o- of the monomer and D can be found from the diffusion relation D = (r2)/6At. Here (r 2) is the mean-square displacement of the active chain end in time At. The time step At can be identified as that for propagation, At = (kpCM) -1, a n d (r2) 1/2 as the root mean-square end to end distance per monomer unit, denoted by a. One thus obtains eqn. (32). The upper bound for kt in the residual regime can be found by noting that the maximum value for residual termination will occur when the roving chain end is completely flexible. Such a chain has nodes of entanglement every Jc monomer units. This leads, 68 via an argument similar to that used to deduce eqn. (33), to determine the value of it; the relation 69 .Jc =jo/~ may be used, where jo is the entanglement spacing (in monomer units) of pure polymer and (h is the volume fraction of polymer, v° It can be seen that eqns. (32) and (33) contain no adjustable parameters, and the properties of the polymers required

716 Q. WANG et al.

for their evaluation (jo and a) can be obtained from physical measurements of the polymer. 7° The value of cr can be estimated either from the physical properties of the monomer, 71 or from measured o- values for analogous substances. 72

k t (residual, lower b o u n d ) = 47rkpCMa2~r/3 (32)

k t (residual, upper bound) = 87rkpCMa3jlc/2/3 (33)

A comparison between the predictions of eqns. (32) and (33) and experimental data for MMA has been made, 43'63 which suggests that the accord between the extremely simple models and experiment is excellent; the upper and lower bounds neatly encom- pass the observation. Moreover, the lower bound gives agreement with the observed k t over a wide range of conversion. It has also been found that eqn. (32) accurately predicts the high conversion emulsion polymerization of styrene providing further evidence for its general applicability. 73

Table 3 also compares the predictions of these equations with k t values for BA and BMA. Although in these cases the data are less extensive, they are in accord with the models. It is clear that these simple models are qualitatively and quantitatively applicable to emulsion (and other) polymerizations at high Wp. Currently lacking is a means of interpolating between these two limits, although it is apparent from the comparison made by Ballard e t al. 43'63 that empirical interpolation can suffice for modeling purposes.

Finally, one notes that the concept of residual termination, as quantified by eqns. (32) and (33) can be used to understand the sudden drop in k t observed at high conversions. This is due to the sudden drop in kp as propagation becomes diffusion controlled (see Section 2.1.2.6.). It has also been suggested 68 that a significant contribution to the gel effect in emulsion polymerization is due to the changeover from the upper (flexible chain) bound to the lower (rigid chain) bound of k t. Note in this last context that the gel effect in emulsion systems, if observed at all, occurs at much greater Wp than in equivalent bulk polymerization systems.

The current status of theories for termination at lower Wp, including the dependence of k t on chain length, is considerably less satisfactory. There are many models in the literature, but all with certain adjustable parameters, which have thus far only been evaluated by comparison with experimental conversion/time curves. One of the less empirical theories is that of Tulig and Tirrell, 74 based on reptation theory. 75 Friis and Hamielec 76 have given extensive empirical correlations of k t in terms of Wp; however, these were deduced from bulk conversion/time data under the assumption that bulk initiator efficiencyf was independent of conversion, an assumption that has subsequently required revision. 77'78 There are also the models of Soh and Sundberg, 67 Gardon, 2° Hamielec and co-workers 79 and of Cardenas and O'Driscoll, 8° but again, each suffers from similar deficiencies. The development of reliable models and sensitive experiments for kt at moderate Wp is clearly an important area for future development.

2.1.2.6. Propagation rate coefficients - The values of kp should be the same in an emulsion, bulk, or any other polymerization system, other conditions being the same. However, kp may depend on the value of Wp. That is, until very high Wp, kp should be essentially constant, because the rate-determining step in propagation is usually 'chemical', e.g. the crossing of the activation barrier for reaction between a


TABLE 4. Some k ° values of relevance to emulsion polymerization systems.

Temperature kp Monomer (°C) Method Ref. (dm 3 mo1-1 s l)

Styrene 15-30 SIP 81 10 7.0 exp(-29.5 kJ mol 1/RT) 45 65 EM 51 10 7"1 exp(-29 kJ mo1-1/Ry) 45-60 EM 52 107.29 exp(-30 kJ mo1-1/RT)

MMA 15-30 SIP 81 105.7 exp(- 18.2 kJ mol-l/RT) BMA 50 EM 54 6 x 102 BA 50 EM 53 4.5 x 102

macroradical and a monomer molecule. At very high conversions (often near the glass transition), kp decreases with Wp, as the rate-determining step involves diffusion of monomer to the chain end of the macroradical. This decrease will be obtained if the system has a high viscosity, while it is unlikely to be observed if the temperature is sufficiently high. It is useful to distinguish between the low conversion (chemical) value of kp, denoted k ° and that at high conversions.

There are many data available on kp values in the literature, but inspection of the data 82 shows that there is a wide range of reported values even under ostensibly the same conditions. The origin of the problem is that the data interpretation is usually model dependent. Recent work has shown that good agreement can be obtained between the kp values reported by different groups using different techniques, 51'7~'81 as long as the data interpretation involves minimal model-based assumptions. This suggests the correctness of the approaches used by these different groups. Some k ° values obtained from spatially intermittent polymerization (SIP, a bulk technique) 81 and the emulsion polymerization technique are tabulated in Table 4.

At high conversions, i.e. in the diffusion-controlled regime, the dependence of kp on Wp can be obtained entirely from kinetic data, without requiring ESR measurements. 65 In this regime, monomer diffusion is very slow, and thus eqns. (231) and (25) show that exit can be ignored. Moreover, ~ is likely to be high and thus the pseudo-bulk eqn. (6) may be reliably used. The discussion in Section 2.1.2.6. shows that at high Wp, k t can be accurately found from the rigid chain limit for residual termination (eqn. (32)). Equation (6) with k = 0 and eqn. (32) yield eqns. (34) and (35).

kp = 87rCMa2 rq2 / [ 3 NA Vs(p - an/at)] (34)

q =_ kp~ = -UAVsdln(1 - x ) /d t (35)

One sees that the quantity q may be obtained from experimental conversion/time data. Equation (34) then provides a means of deducing kp from conversion data, if p is known. A value of p can be obtained from an observed d~/dt as t ~ 0 in a seeded run which starts with ~ = 0, from eqn. (15). Since experiments indicate 73 that p depends weakly (if at all) on Wp, one can find p by starting a kinetic run at a wp below that at which kp becomes diffusion controlled. The data for these low Wp values yields d~,/dt from eqn. (1), given a k ° obtained by one of the above mentioned techniques; p can then be found from eqn. (15). One then takes values of q at Wp where kp becomes diffusion controlled and obtains for each Wp a first estimate of kp from eqn. (34) with d~/dt set to zero. Improved values of kp are obtained using an improved d~/dt found by solving eqns. (1-5) numerically with kp(wp) deduced from the first iteration. Iterations are continued unti l kp(wp) converges.

718 Q. WANG et aL

This method hhs been applied to seeded emulsion polymerizations of MMA 65 and styrene. 73's3 For the case of MMA, the agreement between the results from the kinetic method and the ESR results is excellent, both for the absolute value of kp and for the decrease in kp with increased Wp. This agreement between two totally different techniques strongly suggests the accuracy of both. Essentially the only assumptions made in the kinetic method are: (1) that the free radicals and monomer are uniformly distributed throughout the latex particles; and (2) the validity of the rigid chain limit for k t. A value of k°p is also required.

There is at present a dearth of reliable models for kp at high conversion. There are a number of expressions for kp in the literature that have been deduced from global modeling of bulk kinetics. 67,79,84 However, all have been based on the assumption that bulk initiation efficiency is independent of (or only weakly dependent on) kp, a hypothesis that is now realized to be in gross error. Indeed, because of this error, expressions for kp(wp) currently in the literature 67'79's4 give values that are many orders of magnitude less than experimental results at high conversion. 77's5 The diffusion controlled kp, denoted kp,d, can be given by an expression (eqn. (36)) which takes account of the contribution to kp arising from reaction/diffusion, the same dynamic effect which gives rise to eqns. (32) and (33). Here, Dp(wp) is the diffusion coefficient of monomer through the polymer matrix as a function of weight fraction of polymer. Thus, the value of kp at any conversion is found from combining the chemical contribution, k °, with the diffusion contribution, kp,d, as in eqn. (37).

kp,d = 47roNa(Dp + 1 kpCMa2) (36)

kp 1 = (ko,a) -1 + (k°p) -l (37)

k2,a + kp,d (k ° - 47roNaDp - 27roNak ° CM a2) - 47rcrNaDpk ° = 0

Dp = kp[k° (alr~rNA)-l (k ° - kp) -1 - ( CMa2 / 6)] (38)

Equations (36) and (37) can be combined to yield eqn. (38), which relates Dp, k 0, and kp,d, the value of kp,d being the positive root of eqn. (38). Equations (36-38) can be used to deduce Dp(wp) from kp(wp) data. Given Dp(wp), they can also be used to find kp; however, what is lacking at present is a theory which enables Op(wp) to be predicted.

2.2. Kinetics of particle formation

The mechanism of particle formation or nucleation is particularly difficult to investigate, because there is a plethora of physically reasonable contributions to this event, but only a limited number of experimental observables which can be used to test theoretical models.

The pioneering work of Harkins 2'3 and Smith and Ewart, 4-6 and most contem- porary textbooks, assume that particle nucleation occurs entirely through the entry of free radicals into micelles (micellar entryS). However, there is considerable evidence to support a rival theory, homogeneous nucleation, which was pioneered by Fitch, 1° and has become known as HUFT (Hansen-Ugelstad-Fitch-Tsai) theory. 36 Indeed, it is now generally accepted that homogeneous nucleation must be operative, at least at low (or zero) surfactant concentrations, where there are no micelles present.


At surfactant concentration near and above the CMC, the problem is that, while data are readily found consistent with either model, it has so far been proved impossible to find data to refute 5 one or the other model. For example, the widely observed relationship that the particle number Nc is approximately proportional to [I]2/5[S] 3/5, where [S] is the concentration of surfactant above the CMC, has long been quoted as evidence for micellar entry, but it was later shown by Roe 9 that the same two exponents are also predicted by the homogeneous nucleation mechanism, or indeed, by any model which inter alia assumes that cessation of complete surface coverage by surfactant is one of the governing events. Another example is the large change in particle number Arc as a function of [S], as [S] goes through CMC, which is taken as definitive evidence for micellar entry, but which can also be successfully predicted by coagulative nucleation theory, 86'87 an extension of HUFT theory which takes detailed account of coagulation of newly formed particles.

The above mentioned theories describe principal mechanisms which may be operative in a given system. These can be summarized as follows. (1) Micellar entry: 4,5 free radicals in the aqueous phase become absorbed into micelles containing solubilized monomer to form small particles directly. (2) Homogeneous nucleation: 12 oligomeric free radicals in the aqueous phase propagate until they attain a sufficiently high degree of polymerization to precipitate and thereupon yield particles which swell with monomer. (3) Coagulative nucleation: 1°'13-15 particles formed by either of both of the previous two mechanisms are colloidally unstable, and are termed 'precursor particles'. These form colloidally stable latex particles by coagulation and propagation growth. As mentioned above, experimental data, such as polymerization rate, particle formation rate, particle size, polydispersity, surface properties, etc., are not sensitive enough to elucidate or to refute postulated mechanisms and must be used in conjunction with other observables. Data which have proved to be sensitive to mechanistic assumptions include particle size distributions (PSDs) 14 and the entry rates of free radicals into pre-existing latex particles. 88

The most informative data are the full PSDs expressed as the volume distributions at the start of stage II and at various times in the early part of stage II. It has been observed 14'89 in a styrene/SDS system at 50 °C that the early time PSD (immediately after the nucleation period) is positively skewed and this skew decreases markedly at later times. This suggests that the nucleation rate, dNc/dt , must be initially an increasing function of time. The actual form of dN~/dt can be deduced from PSD data through solution of the time evolution equation of PSD, which is derived by mass balance in conjunction with the Smith-Ewart kinetic considerations, and given as eqns. (39-41), where ni(V, t) is the (relative) number of particles at time t with volume V and containing i free radicals, Kii(V) is the rate of volume growth of a particle containing i free radicals, given by eqn. (40), which is a function of V because of the dependence of CM on V; the term ci is the nucleation rate of a particle containing i tree radicals given by eqn. (41).

Oni( V , t ) / Ot = p[n i_ 1 - ni] + k[ ( i + 1)ni+ l -- ini] + c[ ( i + 2)(i + 1)n/+ 2 - i ( i - 1)ni]

-- O ( g i i n i ) / O V q- ¢i (39)

K u = ikpMoCM/(NAdp) (40)

C i = ~ ( V - V m ) ~ i l ( d U c / d t ) ( t = oG) (41)

w h e r e , (~il = O, i ¢ 1; 511 = 1. It has been assumed that nucleation produces particles

720 Q. WANG e t al.

whose volume is Vm and that all new particles contain a single free radical. It is found that no form for dNc/dt, which is initially a decreasing function of time, can fit the data. 14

Detailed mechanistic treatments 13 for micellar entry and homogeneous nucleation mechanisms, without any account of coagulation, show that neither can give this observation; as is physically obvious, both must predict dNc/dt to be initially a non- increasing function of time. Therefore, the skewness of early time PSD refute single- step micellar entry and homogeneous nucleation mechanisms for particle formation. A further refutation of the miceUar entry mechanism is found with the observation of Liesegang rings, 9°'91 which appear to be inexplicable in terms of micellar entry, but is strongly suggestive of coagulation. In addition, it is well known that latex particles can form well below the CMC (and indeed, even in the complete absence of added surfactant, as described in Section 3.1), demonstrating that there must be an alternate mechanism to micellar entry in these systems. While modern data from a wide range of studies 10'14'15'89'92-94 suggest that micellar entry plays little (if any) role in latex particle formation, the Sydney group has shown through analysis of nucleation rate and particle number and accurate modeling for the rate of entry of free radicals into the pre-existing latex particles, that, at surfactant concentrations above CMC, micelles are directly involved in the nucleation process. 88 Another argument has been proposed by Dunn 95 that positive skewness of PSD at the end of stage I may not be conclusive proof of the participation of a coalescive step in the emulsion polymerization of styrene in the presence of micellar emulsifier, but may be explained by a transition from transfer termination to termination by the entry of a second initiator radical.

The question of the mechanism for particle formation in emulsion polymerization is still unsettled; many parameters in nucleation kinetics, especially those involving very small precursor particles, are poorly understood. Fortunately, means of investigating the thermodynamic properties of such small particles, for example, small-angle neutron scattering (SANS), have recently become available, 96 and so it can be fore- seen that a better understanding of the mechanism for particle formation will be obtained in the near future.

3. P O L Y M E R I Z A T I O N

3.1. Soapless emulsion polymerization

Soapless (or emulsifier-free) emulsion polymerization is a technique derived from conventional emulsion polymerization in which polymerization is carried out in the absence of emulsifiers. This technique has been extraordinarily useful for the preparation of model polymer colloids with narrow particle size distributions and well- characterized surface properties. Soapless emulsion polymerization eliminates the disadvantages of conventional emulsion polymerizations stemming from the use of emulsifiers, e.g. impurities in products caused by unremoved emulsifier and poor water-resistance of films formed by polymer latex.

Since 1965, when Matsumoto and Ochi 97 first described a soapless emulsion polymerization for the preparation of a monodisperse polystyrene latex, the technique has been widely described in the literature, especially for polystyrene. In order to explain the formation of stable latex particles in the absence of emulsifier, a number of mechanisms have been proposed, including a homogeneous nucleation


mechanism, 99'1°° an oligomer micellization mechanism, 1°1 and the coagulation mechanism. 14,86,89

3.1.1. Polymerization process - Although soapless emulsion polymerization is a kind of emulsion polymerization without added emulsifier, the system gains colloidal stability via the involvement of one of the following reactive components which acts as an emulsifier: (1) ionizable initiator, such as potassium persulfate 99'1°2 and azo- bis(isobutylamidine hydrochloride); 1°3 (2) hydrophilic comonomers, such as carboxylic monomers, 1°4,1°5 acrylamide and its derivatives; 1°6'1°7 and (3) ionic comonomers such as sodium styrene sulfonate 1°8 and the sodium salt of 2-sulfoethyl methacrylate (NaSEM), 1°s dimethyl vinyl pyridinium methyl sulfonate. 1°9 The soapless emulsion polymerization of systems with other additives has also been reported, including inorganic salts, such as calcium sulfite 11° and barium sulfate; 111 organic solvents, such as methanol 112 and acetone; 113 and phase-transfer catalysts, such as 18-crown-6.118 The kinetics and mechanisms of polymerization are different in these systems. The nucleation mechanism and colloidal stability are key features in an understanding of soapless emulsion polymerization mechanisms, and are therefore of significant interest.

Detailed studies on nucleation mechanisms of soapless emulsion polymerization have been carried out in the styrene/potassium persulfate/water system by Goodall et al.l°l It was found that there are a large amount of styrene oligomers ( M W ca 1000) present in the nucleation period as observed from GPC analysis of the polymer particles, and a micellar nucleation mechanism was suggested. The oligomers with ionic chain ends first aggregate to form micelles in the aqueous phase, the primary radicals generated from initiator decomposition diffuse into these micelles to initiate polymerization. Later, it was found that, as the polymerization proceeds, a reduction in surface charge density occurs as a result of increased surface area, and then the primary particles undergo coagulation to regain colloidal stability. 1°1,1°3 Once the stable latex particles are formed, polymerization takes place mainly within the latex particles swollen by the monomer, and particle growth resembles that of conventional emulsion polymerization. Since the latex particles in such systems are stabilized by the ionic fragments of initiator, the surface charge density is usually low and therefore the solid content of such systems is confined to a range below 10%. A typical value for particle number density in such systems is 1012 cm -3, compared with 1015 cm -3 for conventional systems. Thus, the polymerization rate of soapless emulsion polymerization is relatively low. It was also found that agitation can affect the acceleration in polymerization rate via an effect on monomer mass transfer from monomer droplets to the polymer particles. 111,114

Soapless emulsion polymerizations of styrene with non-ionic hydrophilic comonomers initiated by potassium persulfate have been studied by a number of authors, for glycidyt methacrylate 115 and 2-hydroxyethyl methacrylate. 116 It has been suggested that the particles nucleate via a homogeneous nucleation mechanism in such systems. The primary free radicals generated from initiator decomposition induce the aqueous phase copolymerization of styrene and the hydrophilic comonomer, forming copolymeric radicals, which, upon reaching a certain degree of polymerization, become insoluble in water and precipitate to form primary particles. Then, coagulation can occur due to insufficient surface charge density and mutual chain entanglements of the more water-soluble segments extending into the aqueous

722 Q. WANG et al.

phase. The nucleation stage ceases before 1% conversion, after which the particle number remains constant and the polymerization takes place within the latex

116 particles. A shell-growth mechanism has been proposed based on the observation that there is a linear relation of (conversion) 2/3 versus [KPS]I/2N 1/2. In such systems potassium persulfate (KPS) plays a dominant role in the particle nucleation process while the hydrophilic comonomer contributes extra stability to the resulting latex. The amount of hydrophilic comonomer also affects the shell thickness and the physical properties of the shell, and thus, the monomer swelling capacity, monomer diffusion rate, and the polymerization rate. The solid content can be increased up to about 45%, which is the highest for soapless systems so far reported.

When an ionic comonomer is present in a soapless emulsion polymerization system, the nucleation mechanism differs drastically with the hydrophilicity of the comonomer and its relative reactivity. For a comonomer which is much more reactive than the main monomer, such as sodium methallyl sulfonate (NaMS) with styrene as the main monomer, most of the comonomer polymerizes to form poly(NaMS) or a copolymer with high NaMS content during the initial period. These are highly water-soluble and may either dissolve in the aqueous phase to reduce the stability of the latex particles or be absorbed onto the particle surface to stabilize the particles, depending on the solubility and concentration of such species. The particles are generated mainly through a micellar nucleation mechanism as proposed by Goodall et al. based on molecular weight distributions at low conversions, l°l On the other hand, for comonomers with comparable reactivity to the main monomer, for example, in the case where 1,2-dimethyl 5-vinyl-pyridinium methylsulfate or 1-methyl 2-ethyl 5-vinyl- pyridinium bromide is used as comonomer and styrene as main monomer, primary particles are formed through homogeneous nucleation mechanism which then become stable particles via coagulation or growth. Ire'l°3 Another type of soapless emulsion polymerization is carried out with the presence of a surface active ionic comonomer, such as undecylenic isethionate sodium (NaUI) 117 or 2-sulfoethyl methacrylate sodium (NaSEM)] °8 The particle formation mechanisms are quite similar to those in a conventional emulsion polymerization system, i.e. when the comonomer concentration is below its CMC, the particles nucleate homogeneously. When the comonomer concentration is above its CMC, micellar nucleation mechanism is predominant.

It was reported 112'113 that the addition of an organic solvent, which is a solvent for the monomer but a non-solvent for the resulting polymer, can largely increase the polymerization rate and the solid content. Experimental results have shown that the effect of such solvents, e.g. methanol and acetone, largely depends on their ability to enhance the swelling capability of the polymer particles by the monomer. It has also been found that the addition of such solvents results in a much narrower distribution of the final latex particle size.

Chang e t al. 118 reported the soapless emulsion polymerization of styrene in the presence of a phase-transfer catalyst such as 18-crown-6. The phase-transfer catalyst formed a complex with a K + cation resulting from the decomposition of the initiator, potassium persulfate, and increased the solubility of the primary free radicals, and consequently, increased the polymerization rate and the solid content of the final latex.

Another extensively studied technique of soapless emulsion polymerization involves the addition of an inorganic powder, which offers strong potential advantages for the manufacture of a homogeneously dispersed composite material of organic and inorganic substances. 111,119 Yamagushi e t al. 120-122 studied the free radical


polymerization of vinyl monomers such as MMA in an aqueous medium with the addition of various kinds of inorganic powders. They found that in most cases, the surface of the powder is partly or totally covered with the polymer formed during the polymerization. It was also shown that the rate of polymerization varies remarkably with the type of inorganic powders used. The inorganic powders used in soapless emulsion polymerization can be classified into two types: one that participates in an initiation reaction and the other that is chemically inert to the polymerization reaction. The two types of inorganic powders affect polymerization kinetics in different ways. The latter might greatly affect the polymerization rate via the change of the physical environment of the polymerization loci, while the former may affect the polymerization via a combination of a change in initiation mechanism as well as initiation site and a change in the physical environment. The change in physical environment is complicated, but the result has been found to be an increase in monomer concentration within the polymerization loci and thus a higher rate of polymerization. The addition of an inorganic powder, such as calcium sulfite, which participates in the initiation reaction, results in an transition from persulfate decomposition initiation to the redox initiation which increases the rate of free radical generation and thus the rate of polymerization.

3.1.2. Applicat ions - Latexes resulting from soapless emulsion polymerization have a narrower particle size distribution compared with those resulting from conventional emulsion polymerization because the nucleation stage in soapless system is relatively short, and the number density of latex particles is lower than in a conventional system. Both tend to result in a narrower particle size distribution. The particle size of the latex resulting from soapless emulsion polymerization is governed by a number of factors, namely, the ionic strength of the system, the monomer concentration, the initiator concentration, the comonomer type and concentration, and the reaction temperature. Monodisperse latex particles with well-defined surface properties can be produced by soapless emulsion polymerization. The surfaces of the resulting latex particles can be controlled through the choice of initiator and comonomer, and moreover, the latexes do not contain low molecular weight emulsifiers. This is extremely advantageous for the production of adhesives and surface coatings, because this improves the water- resistance and weather-durability of the products.

Another application of soapless emulsion polymerization is the production of com- posites of inorganic filler and polymer. The incorporation of inorganic fillers into polymers was conventionally made by mechanical blending, but the non-uniformity of the resulting composite was usually problematic. There was no bonding at the interface between polymer and the filler, so failure would often result from interfacial defects upon impact. However, in the case of soapless emulsion polymerization with the presence of an inorganic filler, good uniformity can be achieved under most conditions, and under certain circumstances, chemical bonding may form at the interface between the polymer and the filler. In such cases, the strength of such a composite can be expected to be higher than that of an ordinary blend.

3.2. Miniemuls ion polymerizat ion

Initiation in conventional emulsion polymerization has been proved to take place either in monomer-swollen micelles 2'3 or in the aqueous phase, 1° and the monomer

724 Q. WANG et al.

droplets are usually ruled out, because it is obvious that the total surface area o f the monomer droplets is considerably smaller than that of monomer-swollen micelles. However, under certain circumstances, the monomer droplets can be the main loci of initiation, as shown by Ugelstad et al., 123 in a system which is generally known as 'miniemulsion'. Miniemulsions are different from conventional emulsion in that the former often need some sort of co-surfactant, usually fatty alcohols or long-chain alkanes, to form a stable dispersion with droplets ranging from 100 to 400 nm in diameter, and so the total surface area of the droplets is comparable with that of monomer-swollen micelles. Therefore, the kinetics of miniemulsion polymerization are unique, because the initiation and particle formation mechanisms are different from those operative in conventional emulsion polymerization. The published work in the miniemulsion field has been referred to stability 124'125 and polymerization kinetics,126'127 and more recently to the copolymerization processes.128-131

The process of miniemulsion polymerization bears in itself certain advantages over conventional emulsion polymerization, including: (1) higher system stability which increases the efficiency for industrial applications; (2) larger particle size of the resulting latex, which is readily controlled by the amount of co-surfactant; and (3) moderate polymerization rate which leads to better control in industrial processes. Besides, the process also provides a potential to produce composite latexes with better microphase separation and latexes with interpenetrating polymer network (IPN) structures. There- fore, miniemulsion polymerization has been extensively studied and is receiving increasing interest from research groups in both industrial and academic backgrounds.

3.2.1. The formation and stability o f miniemulsions - Miniemulsions are usually prepared via a three-step process: (1)pre-emulsification, the surfactant (e.g. sodium dodecyl sulfate, SDS or sodium hexadecyl sulfate, SHS) and the co-surfactant (e.g. cetyl alcohol, CA or hexadecane, HD) are dissolved in water; (2) emulsification, the oil (monomer or monomer mixture) is added into the aqueous solution of surfactant and co-surfactant and mixed by agitation; and (3) miniemulsification, the mixture resulting from steps 1 and 2 is then further homogenized by an ultrasonic disrupter.

The size of the monomer droplets of miniemulsions has been measured by transmis- sion electron microscopy (TEM). The results have shown that the droplet size of miniemulsions ranges from 100 to 400 nm in diameter, which is larger than ordinary monomer-swollen micelles (10-50 nm), but smaller than monomer droplets in conventional emulsion (103-104 nm). The morphology of monomer droplets of miniemulsion has been studied by TEM. 132 It has been found that, when the oil phase is absent, the surfactant (SDS) and co-surfactant (CA) (molar ratio 1 : 3) form rodlike particles with lengths of 100-200nm in dilute aqueous solution, and the length of the rodlike particles decrease with decreased CA concentration, and finally, globular particles are formed; when the oil (styrene) is added into the solution, the rodlike particles aggregate to form starlike particles, and when more styrene is added, the starlike aggregates round out and finally become droplets.

The amount of surfactant adsorbed on the monomer droplets of miniemulsion has been measured by Delgado et al. using a two-phase-titration method with chloroform and mixed indicator (disulphine blue/dimidium bromide). 128 It has been found that the addition of a co-surfactant would significantly increase the surfactant adsorption. This is because the addition of the co-surfactant tends to reduce the size of the monomer droplets, thus causing an increase in the amount of surfactant adsorbed.


The centrifugational stability of miniemulsion has been investigated in a VAc-BA/ HD/SDS/water system by measuring the amount of monomer mixture separated from the miniemulsion after centrifugation. 12s The results have shown that the stability increases with the concentration of surfactant and the molar ratio of the co-surfactant/surfactant.

The effect of electrolyte on the electrokinetic properties of miniemulsion has been studied in a toluene/CA/SDS/water system, 133 finding that the electrokinetic sonic amplitude (ESA) increases initially with the electrolyte concentration within a certain range (ca. 10 -2 M), until a maximum occurs. This is due to either the displacement of adsorbed CA by SDS from the continuous phase or to co-ion adsorption, or to both. Beyond the maximum, the ESA then decreases with the collapse of the electrical double layer as the concentration of electrolyte is further increased.

3.2.2. The kinetics o f miniemulsion polymerization The kinetics of miniemulsion polymerization are very different from those of conventional emulsion polymerization. The conversion-t ime curve usually shows two intervals in the rate of polymerization: stage I with an increasing rate and stage II with a decreasing rate, and the constant rate stage, which is usually observed in conventional emulsion polymerization, is not observed in miniemulsion polymerization. 134 Stage I, the nucleation stage, is usually longer than that of a conventional process, this is because the interfacial layer between the monomer droplets and the aqueous phase consists of the surfactant-co-surfactant complex which leads to a lower free-radical capture efficiency of the monomer droplets and thus results in slow particle nucleation. 127,134 Beyond a maximum value, which usually occurs at conversions higher than 30%, the rate of polymerization decreases with conversion due to consumption of the monomer and the lack of supply from a larger monomer reservoir as for the case of conventional emulsion polymerization.

3.2.3. Copolymerization in miniemulsion systems - The copolymerization of vinyl acetate (VAc)-butyl acrylate (BA) in the presence of SHS and HD has been studied. 128-131 It was found that the addition of a small amount of the co-surfactant (HD) and the use of miniemulsification result in significant differences in the polymerization kinetics compared with those for conventional emulsion polymerization, with respect to the surfactant adsorption behavior, rate of polymerization, particle size and distribution, evolution of copolymer composition, and properties of the copolymer. In detail, the polymerization rate is slowed down, the final latex particle size is larger and the amount of coagulum formed during the polymerization is diminished. The magnitude of the gel effect is increased as the initial concentration of the surfactant is reduced and the co-surfactant is used. During polymerization, the copolymer formed in the miniemulsion polymerization process shows a lower content in VAc monomer units up to 70% conversion and the product copolymer shows greater microphase separation than that produced in a conventional process.12s These are explained by the unique monomer transport behavior and polymerization mechanisms operative in miniemulsion copolymerization.

A model for monomer transport in miniemulsion copolymerization has been proposed to explain the role of the cosurfactant in the polymerization process as well as the effect of the different components and process variables on the rate, of copolymerization, monomer distribution between phases, and composition of the copolymer. 129 It has been found that a long-chain co-surfactant increases the

726 Q. WANG et aL

stability of the miniemulsion and the capability of the monomer droplets to retain monomer. The latter effect tends to promote the swelling of the initiated droplets in the initial stage, but later, it results in a reduction in the equilibrium concentration of the monomers in the polymer particles. These results have been confirmed by experimental evidence, and it has been shown that the co-surfactant plays at least four roles in miniemulsion polymerization. (1) Its presence during the miniemulsification process allows the formation and stabilization of submicrometer monomer droplets whose surface adsorbs most of the surfactant. (2) Upon initiation of polymerization, the monomer droplets become the main loci of particle nucleation. At this stage, the co- surfactant helps to retain the monomer originally contained in the initiated droplets. (3) Later on during polymerization, its presence in uninitiated monomer droplets reduces the equilibrium concentration of monomer in the polymer particles. (4) As a result of the particle nucleation in the co-surfactant containing monomer droplets, its presence in the polymer particles increases their swelling capacity in post-polymerization swelling processes. 13°

The kinetics of miniemulsion copolymerization has been studied 131 in the VAc-BA/ SH/HD/water system and the rate of polymerization followed four regions. In region I, the particle nucleation stage, the rate of polymerization increased with conversion, reaching a maximum at the end of this region. In region II the rate of polymerization decreased with conversion, due to the preferential consumption of BA and the continuous decrease in the average number of free radicals per particle, as a consequence of the increase of the average free radical termination rate coefficient and the increased free radical desorption. The rate of polymerization reached its minimum value at the conversion at which BA was exhausted. Region III corresponds to the homopolymerization of the remaining VAc. The increase in the rate of polymerization in this region is the result of both a higher propagation rate coefficient for VAc, when compared with that for BA or for cross-propagation, and an increase in the average number of free radicals per particle due to a diffusion controlled free radical termination (gel effect). Finally, in region IV, the rate of polymerization decreased because of depletion of the monomer and a diffusion-controlled propagation rate. The number of particles produced in the miniemulsion polymerization is smaller than that produced in conventional emulsion polymerization and for the miniemulsion polymerization of VAc- BA mixture, the number of particles is proportional to the 0.25 power of the surfactant concentration, compared with the 0.6 power found in conventional emulsion polymerization for the same monomer mixture. On the other hand, the number of particles in the miniemulsion polymerization showed a 0.8 power dependence on the initiator concentration, whereas it is independent of the initiator concentration in conventional processes.

The evolution of the copolymer composition showed that the copolymers formed in the miniemulsion polymerization contained fewer VAc monomeric units than the copolymer formed at the same conversion in the conventional emulsion process. This fact was reflected in the value of the glass transition temperatures of the copolymer.

3.3. Microemulsion polymerization

Microemulsions may be defined as thermodynamically stable dispersions of either water or oil, in an oil or water continuous medium, respectively, stabilized by an


interfacial layer of surfactants. They have been shown to consist of droplets 5 80 nm in diameter, and thus are optically transparent. 135 These systems are significantly different from ordinary emulsions and miniemulsions, which are kinetically stable. The reason for this difference is that the van der Waals attractive force between droplets of the dispersed materials increases with droplet size, 136 and only becomes significant beyond some minimum size. The droplets of a microemulsion are below this size and are therefore stable. Their optical transparency is advantageous for photo- chemical experiments. Photoinitiation of polymerization in ordinary emulsion systems is possible, but the light is usually absorbed within a very shallow range from the wall of the reactor and cannot be effective enough unless a certain type of reactor is used, e.g. the small-diameter rotary reactor with an inner magnetic agitator described by Fu et al. 52 In the case of microemulsion systems, such problems do not exist, and photoinitiation can be carried out in any reactor.

Oil-in-water microemulsions can only be formed over a small range of surfactant concentration at high sur fae tant -monomer ratios and generally require the use of a co-surfactant, while water-in-oil microemulsions are formed much more easily, because the monomer acts as a co-surfactant and is partly located at the water oil interface, rather than in the micelles, so that the extent of the microemulsion domains in the phase diagram often broadens in the presence of the monomer. Unlike emulsions, which are essentially of water-in-oil or oil-in-water types, microemulsions can adopt a large variety of labile structural organizations. The latter are characterized by low size domains, the stabilization of which requires a fairly high amount of surfactant (ca. 10-15%). In water- or oil-rich regions, microemulsion consists of small globules of uniform size swollen with oil and dispersed in water (direct microemulsions), or swollen with water and dispersed in oil (inverse microemulsions). In the phase inversion region, where the mixture contains comparable amounts of oil and water, the generally accepted description is that of a bicontinuous structure with randomly connected oil and water domains. ~37'13s One remarkable feature is that microemulsions can shift from water-in-oil to oil-in-water dispersions without any apparent discontinuity. Lamellar structures have also been observed in some systems containing small amounts of surfactant. 13s This variety of structures offers great versatility for choosing the locus of polymerization.

Microemulsion polymerization was first studied around 1980 as a consequence of extensive studies on microemulsions developed after the oil crisis. This was because of the need of high molecular weight water-soluble polymers which are widely used in enhanced oil recovery, 139 as well as in water treatment, paper and mining industries. 14° Moreover, there are difficulties encountered in producing such polymers by conventional inverse emulsion polymerization systems, e.g. inverse latexes usually exhibit a broad particle size distribution and flocculate rapidly. 141'142 Recent studies have shown that the polymerization of a water-soluble monomer in a microemulsion medium, 143J44 rather than in an emulsion, could overcome some of these problems. In addition, microemulsion systems can offer a polymerization environment that is better for control of molecular weight and molecular weight distribution.

Studies of microemulsion polymerization focused on the partitioning of monomer and the co-surfactant, the kinetics of the polymerization, the particle nucleation mechanism, photoinitiated polymerization in microemulsions, and the particle size distribution and molecular weight distribution of the resulting microlatexes.

728 Q. WANG et al.

The monomer and co-surfactant partitioning in microemulsion polymerization has been studied by a 13C-NMR chemical shielding technique in a styrene oil-in-water microemulsion stabilized by sodium dodecyl sulfate and 1-pentanol. 145 The results have shown that more than 60% of the pentanol partitions into the interface and the concentration of pentanol in the aqueous phase and at the interface decreases with an increased amount of styrene in the system, attributing to the need for more pentanol in the oil core to enhance the solubilization of styrene. Only a small fraction (ca. 5%) of the styrene is solubilized in the interface, the majority residing in the oil core phase. A thermodynamic model has been developed to simulate the partitioning behavior in microemulsions. It predicts the phase compositions quite well when compared with experimental results obtained by the 13C-NMR method.

The kinetics of microemulsion polymerization have been studied by a number of groups. Candau et al. 146 and Carver et a1.147 studied the inverse microemulsion polymerization of acrylamide and acrylamide-sodium acrylate mixture in the presence of Aerosol OT (sodium 1,4-bis(2-ethylhexyl sulfosuccinate). Atik and Thomas 148 and Tang 149 reported the microemulsion polymerization of styrene using different types of initiations (water- or oil-soluble initiator and 7-irradiation). Results have shown that the kinetics of microemulsion polymerization follows neither classical solution nor emulsion rate laws and that it is indeed unique in that a rapid polymerization is always observed, regardless of the type of initiation used, and the total conversion to polymer is usually obtained within 100 minutes. It has been reported 15° that polymerization rate versus conversion curves show only two stages and neither a constant rate stage nor a gel effect, both of which are typical in a conventional emulsion polymerization, is observed. It is also suggested that the gel effect does exist in the course of microemulsion polymerization, but appears differently. 151 It is generally observed 15°'151 that the polymerization rate reaches a maximum within the first few minutes and then starts to decrease, usually at 20-30% conversion. In the second stage, the decrease in polymerization rate slows down at a certain value of conversion which varies with the reaction temperature. 151 This observation implies that as the polymerization proceeds, the concentration of polymer within the growing particles increases, giving rise to an increase in the viscosity of the reaction mixture within the particles and consequently, a slow down in the loss of free radicals occurs. 151 Though a similar phenomenon has been reported by Feng et al. 152 in a similar system investigated by in situ raman spectroscopy with AIBN as initiator, the author offered a different explanation.

Particle nucleation has been found to take place throughout the polymerization and it has been proposed that particle nucleation occurs in the microemulsion droplets or mixed micelles. 153'154 A mathematical model, which takes into account the nucleation in microemulsion droplets and the propagation radical entry mechanism, has been developed to simulate the polymerization kinetics of styrene oil-in-water microemulsions. 154 The predictions of the kinetic model agreed well with the experimental conversion-time data except in the high conversion region.

Photoinitiation of polymerization in a styrene/SDS/pentanol/water microemulsion has been studied using bibenzyl ketone as a photosensitizer. 153 It has been found essential to dilute the styrene with an equal volume of toluene to prevent turbidity during polymerization. The particle sizes of the microlatex produced are in the range 35-56 nm, considerably larger than the sizes for initial microemulsion droplets (10-30nm). This is attributed to a combination of coagulation and monomer


diffusion. The polymerization rate was found to be proportional to the 0.2 power of the concentration of the photosensitizer and the light intensity. In another kinetic study, 155 high quantum yields and biradical termination reactions, as well as high rates of polymerization were observed in an Aerosol OT/(water + AM)/decane microemulsion photoinitiated by benzophenone. It has been shown that initiation occurs by competing routes, i.e. either by decane radicals formed by quenching of benzophenone, or through radicals formed via electron transfer from benzophenone.

Polymers and copolymers formed by microemulsion polymerization exhibit high molecular weights which depend slightly on initiator concentration. 15°'~51 In addition, the molecular weight distributions are much narrower than those found in conventional emulsion polymerization. These observations are consistent with the postulation that most of the polymer chains growing inside the particles terminate by undergoing chain transfer to the monomer or to the co-surfactant, t5°'t53 It should be noted that the microstructures of copolymers produced by microemulsion polymerization are significantly different from those produced by other polymerization processes. Analysis of the microstructure of poly(acrylamide-co-sodium acrylate)s determined by 13C-NMR 156 showed that the reactivity ratios of both monomers are close to unity, significantly different from the reported literature values obtained for copolymers prepared in solution polymerization ("~A "~ 0.3, 7M ~ 0.95).15v This difference was attributed to a unique microenvironment which exerts an influence on the free-radical copolymerization of acrylamide with ionogenic monomers, as shown by numerous studies.158~159

The particle size distributions of microlatexes resulting from microemulsion polymerizations have been studied by a number of investigators. In some of the systems, the resulting microlatexes are monodisperse, the size of which are found to be larger than the initial sizes of the microemulsion droplets, ~5°'~5~ while in some other systems the final particle size distributions are found to be bimodal. 149'16° The reason for the bimodal distributions is not yet understood, but it is believed to be related to multistage kinetic behavior in such systems. 15e

An important and perhaps outstanding feature of microemulsion polymerization is that the resulting microlatex particles contain only several macromolecules. The average number of macromolecules per particle, N, can be estimated from the volume of the particle and the molecular weight of the polymer. It had been shown 146 that N is directly related to the acrylamide : surfactant weight ratio in acrylamide polymerization in water-in-oil microemulsions, and that the limiting case corresponding to a single polymer molecule per particle can be reached by using aerosol OT systems. Although it can be deduced that a high molecular weight macromolecule confined in such a small particle should be strongly collapsed or tightly coiled, the only experimental proof available is the glass transition behavior of a polystyrene microlatex showing a Tg 10 °C higher than for ordinary polystyrene samples. 151

Microemulsion polymerization is an interesting polymerization process with a great potential for the preparation of latexes of small size as well as high molecular weight polymers. More extensive investigations will be required to further understand the mechanism of the polymerization process.

3.4. Dispersion polymerization

Dispersion polymerization may be defined as the polymerization of a monomer dissolved in an organic liquid to produce an insoluble polymer in the form of a stable

730 Q. WANG et al.

colloidal dispersion. The colloidal stability of the resulting particle is provided by the adsorption of an amphiphilic polymeric stabilizer or a dispersant which is present in the organic medium on the surface of the polymer particles. Therefore, the process may also be viewed as a type of precipitation polymerization in which flocculation is prevented and particle size controlled. 161

In the 1970s, the interest in dispersion polymerization arose from the desire in the surface coatings industry to apply thin films of vinyl and acrylic polymers without resorting to multiple applications of dilute polymer solutioils, 162,163 because of the fact that, under appropriate conditions, dispersion polymerizations result in stable colloidal dispersions with very high disperse phase volumes, and the film-forming rate can be adjusted over a wide range via the choice of diluent volatility. The technique has been applied to virtually all monomers which undergo free-radical polymerization to produce polymers insoluble in an organic medium and to a more limited range of ionically initiated polymerizations.

A typical recipe for dispersion polymerization consists of 40-60% solvent or mixed solvents, 30-50% monomer or monomer mixture, 3-10% stabilizer, ca. 1% initiator based on monomer and other additives, such as co-stabilizers and chain transfer agents. At the start of the polymerization, the system is a homogeneous solution. During the course of polymerization, insoluble polymer is formed and precipitates. The precipitated polymer is stabilized against uncontrolled aggregation by a block or graft copolymeric stabilizer which consists of a soluble component and an 'anchor' component. The former component is soluble in the continuous phase and stretches into the continuous phase; the latter should be insoluble in the continuous phase and is adsorbed on or absorbed into, i.e. anchors on, the dispersion phase. The graft stabilizer may be generated in the course of the latex process by a grafting reaction between the dissolved stabilizing component and the polymerization monomer, or it may be preformed and then used in the required amount in the latex process. The primary particles are formed via homogeneous nucleation and, in some cases, may undergo aggregation to form colloidally stable particles. Monomer is absorbed from solution by the disperse phase once it forms, and subsequently, nearly all the polymerization takes place within the particles.

The property and structure of the stabilizer are found to have great influence on the kinetics of dispersion polymerization, and on the control of the final product. There- fore, an understanding of the mechanism of steric stabilizers has been the key feature in the development of reliable dispersion polymerization processes. Dispersion polymerization is exploited for a wide range of applications, including auto paint, reprographics, printing inks, adhesives, foams, polymer reinforcements, polyalkenes, and monodisperse polymeric microspheres.

3.4.1. The m e c h a n i s m o f s t e r i c s tabi l i za t ion - The effect of steric stabilization may be considered as arising from the increase in polymer chain segment density resulting from the interpenetration or compression of the polymer sheath when two particles approach one another. The repulsive force, which causes the particles to separate and thus leads to the effect of colloidal stabilization, may either result from the loss of entropy which derives from the limitation of configurations of the soluble polymer molecules; or result from an increase of enthalpy which derives from the exothermic nature of the solubilization of the stabilizing polymer. In the former case, the effect may also be referred to as 'entropic stabilization' which is characterized by a lower

E M U L S I O N P O L Y M E R I Z A T I O N 731

! ~'Y - ' : : ~. , - - - 7 - : 7 -" :.. ::. ,.:":..,, ,-'. ~B '.. """ ..'.~,

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I _

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critical flocculation temperature (LCFT). 164 In the latter case, in which enthalpic interaction between the solvent and the stabilizing polymer predominates, as in aqueous dispersions or highly polar media, flocculation occurs on heating to an upper critical flocculation temperature (UCFT). 165,166 Theoretical treatments of steric stabilization have involved various theories of polymer solution by taking account of the steric interaction Vs between two neutral spheres separated by a distance h with an adsorbed polymer layer of depth (5 surrounding each particle (Fig. 2). The total interaction potential (VT) may be written as

VT = VA + Vs (42)

where VA is the van der Waals attraction. For polymer colloids stabilized by firmly attached solvated polymer layers, the

thickness of the layer is such that, at the point of closest approach, the van der Waals attraction, V A, between the particles is reduced to a level at which the particles separate spontaneously by thermal motion. Figure 3 schematically illustrates the potential energy vs distance of separation diagram for two sterically stabilized particles and for different barrier thicknesses. It can be seen that there is no long-range repulsion and the particles are subject only to attractive forces until the outer fringes of the adsorbed molecules are in physical contact. For strongly attracting particles or a shallow steric barrier, there may be a secondary minimum, i.e. a small net attraction. The colloidal stability is therefore dependent on the dimensions and the solution properties of the adsorbed polymer, under the local conditions of temperature, concentration, and on the size and nature of the particle core.

As for the quantitative treatment of the steric interaction Vs, eqn. (43) is given, in which the elastic or volume restriction (compression) effect and the mixing or osmotic effect were treated separately.

Vs = 27rak TVzF2 (½ - X)Smix -~-27vakTV2F2Sel (43)

where a is the particle radius, V2 is the polymer molecular volume, £2 is the density of adsorbed polymer (number of chains per unit area), and X the Flory polymer-solvent interaction parameter, Smix and Sel a re geometric functions dependent on the segment concentration profile p(z) in the adsorbed layer normal to the interface. The segment concentration profiles for individual particles have been determined experimentally by small-angle neutron scattering (SANS). For high surface coverage, the hydrodynamic

732 Q. W A N G et al.

0

O.

cr

c

n

t - .o_

2 4--

I Thin steric J Deep stenc barrier

barrier

t',v \_

s s

$

I van der Waals a t t rac t ion ( ~ 1 i I I I I

FIG. 3. Potential energy vs distance of separation h for sterically stabilized particles with varying depth of steric barrier.

thickness of the stabilizing sheath is determined largely by the longer extended tails of adsorbed polymer, shorter chains and loops playing a relatively minor role. 167 This approach has proved to be of great practical value in understanding and predicting the behavior of sterically stabilized colloids. A novel approach based on an excluded volume effect has been recently proposed, in which separate treatments of the mixing and elastic effects have been avoided through the development of self-consistent field theories 164'168'169 because the mixing term in eqn. (43) has been proved to be much more important, and it can be easily estimated on the basis of the theta temperature of the stabilizing polymer in the dispersion medium. 164 When the temperature of the system approaches 0, and thus X approaches ½, flocculation is usually observed. There- fore, under the practical conditions of a dispersion polymerization, it is prudent to select a stabilizing molecule for which the medium is considerably better than a theta solvent.

Another determinant in eqn. (43) is the surface coverage (expressed by r2), which should approximate the close packing of solvated polymer chains on the particle surface. 170 In addition, the stabilizing polymer chains should be firmly attached to the particle surface to resist lateral displacement which would allow the particle cores to touch or closely approach each other. The most effective method to fulfill such requirements has been through the use of the above mentioned amphiphilic block or graft copolymers.

3.4.2. Design and synthesis o f polymeric sterie stabilizers - The theoretical discussion in Section 3.4.1 has demonstrated that the dispersion medium should be better than a theta solvent for the soluble component of the stabilizer under the reaction conditions of the colloid process. Table 5 is a list of combinations of soluble polymers and dispersion media which have been successfully employed.

Although there does not exist a critical value for the molecular weight of the soluble component, a minimum value in the range 1000-1500 has been considered necessary

EMULSION POLYMERIZATION

TABLE 5. Examples of polymer dispersions prepared by addition polymerization.

733

Stabilizing polymer Dispersion polymer Medium

Natural rubber PMMA, PVAC Alkane Butyl rubber PMMA Polyisobutylene PM MA Poly(lauryl methacrylate) PVC, PMMA, polyacetal Poly(ethyl hexyl acrylate) PVAC Poly(12-hydroxystearic acid) PMMA, PAN, PVDC, PVAC, PVP Poly(dimethylsiloxane) PMMA, PS, PAN, poly(c~-methylstyrene) Poly(t-butylstyrene) PS Polystyrene PAN Poly(ethylene phthalate) Poly(acrylic acid) Poly(vinylpyrrolidone) PS Hydroxypropylcellulose PS

Toluene Chloroform

Ethanol Methoxyethanol ethanol

for practical reasons. 161 It has been found that stability can be provided by molecular weights as low as 300-400,171 but an unsolved problem lies in constructing an effective stabilizer for polymer dispersions incorporating such low molecular weight groups. An effective anchor component would need molecular weights above 500-1500, depending on the degree of its insolubility in the medium and the strength of its association with the particles. On the other hand, when the anchor component is dissimilar to the dispersed polymer, a higher limit would arise from incompatibility of the dissimilar polymers. For example, when polystyrene was used as the anchor component of a block copolymer for stabilizing polystyrene dispersion, SANS measurements revealed that it was uniformly distributed on the particle surface; while the same stabilizer on poly(methyl methacrylate) clustered into discrete domains on the particle surface. 172

A consequence of the larger anchor group of a non-aqueous stabilizer molecule is the reduction in the rate of dissociation of micellar aggregates into single molecules compared with aqueous surfactants.173'174 A careful balance must therefore be struck between the anchor and soluble components of the stabilizer (anchor/soluble balance, ASB). The micellar equilibrium may also be adjusted by varying the solvency of the dispersion medium, which can be achieved with convenience by changing the concentration of polar monomer in solution prior to the commencement of dispersion polymerization.

Typical structures of non-aqueous stabilizers include random graft, comb graft, ABA, AB, and BAB blocks. The random graft copolymer can either form by means of hydrogen abstraction which occurs in the course of free-radical polymerization, 175 or by introducing active sites into the soluble chains. 176'177 It should be noted that, in the former case, the reactivity of the soluble polymer has a strong effect on the course of polymerization through the yield of graft.

The comb stabilizer may be easily synthesized from inexpensive starting materials, and leads itself to considerable variation in the composition of both the soluble and the anchor components. Such a stabilizer has been synthesized through the reaction of the terminal carboxyl group of poly(12-hydroxystearic acid) (PHSA) of molecular weight 1500-2000 with glycidyl methacrylate to form the soluble component. This is followed by the copolymerization of this component with an equal weight of (meth)acrylic monomers, e.g. MMA, and a small proport ion of glycidyl methacrylate or methacrylic acid (to which polymerizable or other functional groups may be

734 Q. WANG et al.

attached) to an overall molecular weight of 10,000-20,000.178 To date, a variety of comb stabilizers based on PHSA have been synthesized 17°'179-181 and applied in industrial dispersion polymerizations as well as in fundamental studies. 182,183

Block copolymers have also been widely employed in fundamental studies of dispersion polymerization in non-aqueous media. One of the favored compositions has been a soluble group of poly(dimethylsiloxane) (PDMS, block A) combined with an anchor group of polystyrene (PS, block B) to form AB 174 or ABA 184-186 block copolymers which have been synthesized via anionic polymerization of styrene initiated by n-butyl-lithium or dilithium naphthalene and then the reaction of the resulting polystyrene anion with a siloxane cyclic trimer, hexamethylcyclotri- siloxane. Block copolymers thus prepared have well-defined block lengths and narrow chain length distributions (Mw/Mn < 1.25). AB block copolymers of polystyrene- block-poly(ethylene-co-propylene) have also been used as stabilizers for PMMA and PVAc dispersions in alkanes.187

3.4.3. Mechanism o f dispersion polymerization - Like any heterogeneous polymerization in colloidal systems, the process of dispersion polymerization can be divided into two stages: a particle nucleation stage and a particle growth stage. Early investigations ass of dispersion polymerization kinetics revealed that the nucleation mechanism is initially homogeneous. When a homogeneous solution consisting of monomer, dispersion stabilizer, initiator, and the reaction medium is heated to the reaction temperature, polymerization commenced in solution to form oligomeric free radicals. These precipitate to form nuclei in the process associating with the stabilizer and thus become sterically stabilized. These primary particles may undergo aggregation or particle growth to form colloidally stable particles. Monomer is absorbed from the solution by the dispersed polymer phase once it forms and subsequently, almost all of the polymerization takes place within the monomer swollen particles. As further polymerization proceeded within the particles, the kinetics resembled that of bulk polymerization and an acceleration in the rate of polymerization was observed, the extent of which varies with the type of monomer used. 188,190

The kinetics of dispersion polymerization differ from those of emulsion polymerization in several aspects: the dependence rate on particle size or number is not observed because the nature of particle growth in a dispersion polymerization system is essentially microbulk. In addition, the fact that monomer is present in solution in the dispersion medium at the start of polymerization, not in a separate monomer phase, greatly influences the nucleation stage. 189

A general expression for the overall rate of polymerization Rp is given by Barret et al.) ss assuming polymerization takes place predominantly within the particles.

Rp = o~C d v1/Zkp( Ri/kt)1/2 (44)

where a is the monomer partition coefficient, polymer:diluent, Co is the monomer concentration in diluent, V is the particle volume fraction, kp is the propagation rate coefficient, k t is the termination coefficient, and Ri is the initiation rate. The rate of polymerization is proportional to the total volume of particles. The termination rate does not remain constant throughout the polymerization, but falls as the internal viscosity of the particles increases. At high conversions, the polymerization rate also falls as it becomes diffusion controlled. The polymerization rate versus conversion curve consequently rises to a maximum analogous to that observed in bulk polymerizations.


For a low value of c~, typical of M M A (o~ = 1) and vinyl acetate (~ = 2), polymer dispersions in aliphatic hydrocarbons, the increase in the rate of conversion over the initial part of the polymerization is directly proportional to time~ 191 For very polar monomers, such as acrylic acid, (acrylonitrile-acrylic acid), the effective value of c~ is large because the greater part of the monomer partitions at an early stage into the particles, even though the true equilibrium value o f ~ between diluent and unswollen polymer may be no higher than that of MMA. The kinetics then approximate to those of suspension polymerization: the rate of conversion rising rapidly to a high level, and thereafter being subject to the competing effects of acceleration, due to the gel effect, counterbalanced by falling monomer concentration.188

The acceleration rate in a system which produces polymer having a low Tg value, e.g. in the case of VAc or EA, is less pronounced than that in systems producing polymers with high Tg values, e.g. in the case of M M A or AN, since the viscosity within the particles is lower in the former case. 190

The appearance of the particles is smooth and spherical in MMA, EA, and VAc systems. The electron microscope reveals only a fine surface texture, and the stabilizer consumption corresponds to a monolayer covering the surface. In contrast, the particles are granular in surface texture and irregular in shape in VC or VDC systems, and the stabilizer consumption is excessive, consistent with continuous formation and aggregation of partly stabilized particles since there is relatively little absorption of monomer into the particles. 188

Particle size and number are controlled by the solvency of the medium for the precipitating polymer and for the stabilizer, by the concentration of the stabilizer and its A S B . 174'188'192 It has been found that particle size increases with decreased stabilizer concentrations as predicted by quantitative analyses of the nucleation kinetics. Furthermore, particle size has been shown to increase with the solvency of the dispersion medium for the stabilizer in the polymerization of styrene for a series of alcohols with different chain lengths stabilized by polyvinylpyrrolidone. This is explained by an increased stretching of stabilizer chains which reduces the particle surface area covered by each stabilizer molecule.

3.4.4. Dispersion polymerization techniques - In a typical dispersion polymerization, the reactants are dissolved in the reaction medium to form a homogeneous solution, and the polymerization is carried out in a single step. However, in order to produce concentrated polymer dispersions, another technique is employed in which the nucleation and growth stages are separated. A seed stage is formed by polymerizing a portion of monomer and stabilizer, after which the remaining monomer and stabilizers are fed over 1-3 hours to give a controlled reaction rate. A feed process is essential for the preparation of a concentrated latex, because above a monomer concentration of ca 35% in a single-step process, the exotherm can be difficult to control, and often, only coarse particle latexes are obtained.

The number of particles in a batch polymerization remains virtually constant after the cessation of particle nucleation unless reaction conditions are altered, e.g. by a sudden addition of stabilizer to bring about re-nucleation. A re-nucleation process may cause a broadening of particle size distribution, which is commonly unfavored. This technique can be employed to produce very concentrated dispersions in which the particle diameter and volume fraction are controlled to achieve maximum packing. 193

736 Q. WANG e t al.

With a bimodal particle size distribution, a concentration of 85% has been obtained on an industrial scale. 194

Dispersion polymerization has also been applied to the preparation ofmonodisperse microspheres. The relatively short period of nucleation and the subsequent reduction in the solution polymerization rate as a result of diffusion of monomer into the particles are two factors which facilitate the formation of a monodisperse latex in a non-aqueous dispersion polymerization. The polymerization has been carried out both in stepwise processes 182 and in seed-and-feed processes. 195 The particle size is controlled by adjusting monomer concentration, stabilizer concentration, and the solvency of the dispersion medium.192 It has been found that the stability of sterically stabilized particles is insensitive to shear, and so stirring the reaction mixture has little effect on particle size. 196

3.4.5. Applications of non-aqueous polymer dispersions - The special properties of non- aqueous polymer dispersions, such as high polymer content, low viscosity, good shear stability, low heat of evaporation of the diluent, together with a wide choice of diluent volatility and stabilizer type are the mainspring of their development by the coating and electroreprographic industries.

The most important and perhaps the first industrial application of dispersion polymerization was for the preparation of automotive paints because the highly concentrated dispersions with favored rheological properties are of interest. In an early patent, 197 multifunctional groups were attached to the anchor group of a PHSA-graft-PMMA copolymer comb stabilizer to provide cross-linkable reactions with the dispersion polymer, so that a microgel fraction was generated. When the resulting dispersion was swollen by a stronger solvent, it conferred pseudoplastic or thixotropic properties on the coating composite, giving better control of atomization and flow of the coating during spray applications. Subsequently, processes have been developed for the controlled preparation of cross-linked microgel particles of acrylic polymers, which may then be added to conventionally prepared solution polymers to produce a desired rheology. 181'198-2°1 The cross-linking reactions involved in such processes have included the reaction of copolymerized hydroxyl monomers with isocyanates, 2°° or aminoplast resins, 198'2°2 and reactions between epoxides and carboxyl groups. 2°1 Automotive coatings incorporating microgels have found extensive use worldwide, and are generating increasing interest, especially because the use of microgels provides better control of the rheology of high-solid low molecular weight coatings during application. This is necessary because of environmental pressures to reduce the solvent content of coatings.

Dispersion polymerization products are also used for electroreprographics as developers and toners, 2°3'2°4 low-temperature-setting printing inks, and photoconduc- tive lithographic inks. 2°5'2°6 Polymers prepared as non-aqueous dispersions have also been employed to modify the mechanical properties of both thermoplastic and thermosetting plastics. In addition, stabilizers located on the surface can also act as functional groups in certain cases (Section 3.7).

3.5. Preparation and application of monodisperse polymeric microspheres

There has been increasing interest in polymeric microspheres since the first papers appeared in the 1950s, which describe a method used by Vanderhoff et al. to obtain


monosized latex particles. The first products of monodispersed polystyrene latexes supplied by Dow Chemistry Co. were used as calibration standards for electron microscopy. Indeed, emulsion polymerization was the first method used to prepare monodisperse polymeric microspheres, and is still the most important and widely used method at present. Various improvements and extensions of the conventional emulsion polymerization method have been developed during the past four decades. The size of monodisperse polymeric microspheres so far prepared covers the range 0.02-10001am, and applications of such microspheres are found in the fields of microscopy, chromatography, cytometry, cell separation, cancer treatment, DNA technology, etc. It is for this reason that an 'International Symposium on Polymeric Microspheres' was held in Fukui, Japan, in October, 1991, at which more than sixty papers were presented on the fundamental studies, preparation techniques, and applications of polymeric microspheres.

In the 1950s, monosized particles were obtained directly by using emulsion polymerization of vinyl monomers with little or no surfactant, 2°7,2°8 or by making use of the levelling out effect of particle size under Smith-Ewart Case II conditions during emulsion polymerization. 2°9'21° The determinants of particle size and PSD were studied and it was shown that the particle size of the final latex varies with the concentration of surfactant and the amount of monomer. 211 The PSD is determined mainly by the entry rate coefficient of free radicals into the latex particles, the length of the 'steady-state' interval, the average number of free radicals per particle, and the volume growth rate of the latex particles. 86'211 It has also been found that the particle size of the microspheres obtained directly by emulsion polymerization is confined to a range of 0.03-1 lam. Microspheres having sizes below this range can be obtained via microemulsion polymerization (Section 3.3), whereas microspheres with sizes above this range can be obtained via other methods, namely, seeded emulsion polymerization (with certain swelling techniques), 212-215 dispersion polymerization (Section 3.4), and soapless emulsion polymerization (Section 3.1). In this section, the swelling techniques and the techniques used during the preparation of microspheres for some special uses will be discussed.

In stepwise emulsion polymerizations, particles up to about 1 I.tm in diameter have been successfully prepared. Attempts to prepare larger particles by seeded polymerizations are hindered by the agglomeration of particles during polymerization. By using a process of repeated swelling and polymerization, the system can endure a relatively long time at conditions where collision between two particles is most likely to lead to coagulation. This will be the case when the particles contain about 60-90% of polymer. Agglomeration may be prevented by the addition of a sufficient amount of surfactant during polymerization. This in turn leads to the formation of new particles (termed 're-nucleation') in the aqueous phase. It has been reported that monodisperse particles of 5 Jam have been successfully prepared in outer space where the lack of gravity is a decisive advantage. It is generally considered impossible to prepare monodispersed particles larger than 3 tam by a simple seeded polymerization on earth. However, newer methods have been developed which allow for the production of monodisperse particles up to 100 ~tm in diameter with a standard deviation of less than 2 % . 216

The two-step swelling method has been utilized by Ugelstad eta/. 212 215 The first step swells monodisperse polymer, polymer-oligomer, or oligomer particles with a water-insoluble compound (Y) under conditions which allow compound Y to diffuse

738 Q. WANG e t al.

into the particles while there is no transport of material out of the particles. After swelling with Y, more water and emulsifier are added. The added organic solvent may be removed if necessary. The monomer or monomer mixture is then added. The conditions are now such that only monomer may diffuse through the aqueous phase to be absorbed into the particles containing polymer and compound Y, while the transport of Y out of the particles is effectively hindered. Obviously, the process requires the solubility of Y in water to be much lower than that of the monomer or monomer mixture added in the second step. The highly swollen particles thus formed are much more resistant to coagulation than particles containing 60-90% polymer, because in the two-step method, the total time in which the composition of particles corresponds to this vulnerable interval during polymerization is much less than in the ordinary, stepwise swelling procedure. In addition, the high swelling capacity may be utilized to avoid the critical conditions for coagulation by having a substantial amount of an inert additive present in the particles which is removed after polymerization. The two-step method has been applied for the preparation of monodisperse particles in the range of 1-100 ~tm in diameter.

Another interesting method has been suggested by Okubo et al., 217'218 which is one step and is termed a 'dynamic swelling method' (the DSM). The DSM does not need a swelling agent and is capable of producing monodisperse polymer particles above 5 ~tm in diameter. The swelling method is carried out by dispersing monodisperse polystyrene seed particles in ethanol-water solution with dissolved styrene monomer, benzoyl peroxide as initiator, and poly(vinyl alcohol) as stabilizer at 65 °C, followed by a cooling process at a rate o f - 1 °C min -1 , until - 5 °C is reached. Water is added to prevent redissolution of styrene from the particles into the continuous phase, and sodium nitrite is added to prevent polymerization in the continuous phase which may lead to renucleation. Very high monodispersity of the resulting polystyrene particles is observed.

A number of studies on the preparation of porous polymeric particles have appeared in recent years, and porous particles of styrene-(divinyl benzene) copolymers have been used as precursors for ion-exchangers, absorbants, and GPC column material. 219 Cross-linked polymer particles with permanent macroporous structures (pore size > 50 nm in diameter), useful as macropores in these particles allow biomolecules near 500,000 daltons to be separated. 22° In earlier preparations, such polymer particles were produced by adding an inert diluent to a suspension polymerization system which often gives relatively large particles with a broad particle size distribution. It is expected that improved separation efficiency, optimal packing, and a lower back- pressure may be achieved with monodisperse macroporous polymer particles compared with polydisperse particles. 221

A new approach to the use of monodisperse polystyrene seed particles as inert diluents for the preparation of macroporous polymer particles has been reported. 222 The synthesis process is generally divided into three stages: (1) swelling; (2) copolymerization; and (3) removal of diluent. The diluents are usually a linear polymer (e.g. polystyrene seed) or a mixture of linear polymer and solvent or nonsolvent. In the swelling stage, the linear, monodisperse polystyrene seed latex particles are swollen by monomers and a solvent type diluent (solvent or nonsolvent), in which the swollen particles behave as an individual bulk polymerization site. The mechanism of pore structure formation was investigated by Cheng e ta / . 223 They observed that the porosity was a consequence of phase separation in the presence of diluents. The


development of pore structure during the copolymerization process was divided into two stages, the first stage being the production and agglomeration of highly cross- linked microspheres to make a polymer particle; the second stage involving the binding together and subsequent fixation of the microspheres and agglomerates. 224 These results are based on changes in copolymerization kinetics, gel content, cross-linking density, particle morphology, surface area, pore volume, and pore size distribution data.

The fundamental properties of a dispersion are dependent on the shape and orientation of the dispersed particles. The preparation and characterization of monodisperse ellipsoidal polystyrene latex particles have been reported by a number of authors. 225-227 The preparation of such particles involves the embedding of monodisperse polystyrene microspheres into a matrix of poly(vinyl alcohol) which is then stretched above the Tg of polystyrene to a pre-set draw ratio. Although the process sounds quite simple, problems often arise from the fact that stretching of the poly(vinyl alcohol) film is not uniform.

Most of the methods for the preparation of monodisperse polymeric microspheres with different size, structure, and surface properties are aimed at special applications in various fields. Monodisperse macroporous particles with great variation in pore size and pore size distribution have been used in size exclusive chromatography and exhibit significant advantages over commercial polydisperse particles with regard to both separation efficiency and flow properties. A rapid protein liquid chromatographic technique developed by Pharmacia is also based on monodisperse porous particles, which gives high resolution of complex biological extracts such as proteins, peptides and nucleotides. Strongly hydrophilic particles prepared by surface modification have been applied to new ionic exchange systems resulting in high efficiency in protein separation. These can be operated at low pressures. 228

Particles with various groups on their surfaces have been prepared by seeded emulsion copolymerization, dispersion polymerization, and soapless emulsion polymerization (Sections 3.1, 3.4 and 3.7). These particles have turned out to be very useful for a number of immunoassay applications. In particle-counting immunoassays, the high degree of monodispersity and the fact that size and density can be varied at will allows the use of simple methods for the assay. Particles of 2 ~tm with hydroxy surface groups also turned out to give excellent results in different radioimmunoassays (RIA) and immunoradiometric assays (IRMA). Nustad et al . 229'230 applied these particles in RIA for carcinoembryonic antigen and triiodothyronime. The particles, which exhibited very low physical adsorption of labelled ligands (low nonspecific binding), were more effective and gave more reliable results than commercial DASP based on cellulose derivatives. Similar particles were applied in an IRMA for the determination of glandular kallikrein. 231 Determination of kallikrein in blood by an enzymatic assay and by RIA has been impaired by the presence of inhibitors which appeared to have no influence on IRMA with the hydrophilic particles. Hydrophilic particles have been used as an immunoadsorbent for the determination of IgM from human cells. It was concluded that they give a much more selective binding of IgM than the previously used commercial solid phase based on staphylococcus aurens.

Particles in the same size range with different functional groups were found suitable for calibration of flow cytometers. A flow cytometric assay for carcinoembryonic antigen (CEA) based on a mixture of the two particle types of sizes 7 ~tm and 10 ~tm, coated with antibody of the same specificity but different affinity, has been found to be

740 Q. WANG et aL

effective for eliminating the hook effect at high concentrations. A mixture of different types of particles is used in assays for the simultaneous detection of different antigens. Each type of particle is then coated with an antibody having unique specificity for the antigens to be determined. Macroporous polyacrylate particles with covalent attachment of antibodies have been especially designed for particle-based flow cytometric immunoassays and investigated in assays for CEA and alpha-fetal protein. 232

Ugelstad et al. 233 prepared monodisperse particles containing magnetite which have been applied for cell separations.234,235Rembaum et al. 234 grafted polyacrolein onto magnetic particles, rendering them suitable for the covalent attachment of antibodies. Kemshead et al. 235 used monoclonal antibodies together with 3 ~tm porous magnetic particles to remove neuroblastoma cells from bone marrow. Recently, the application of monodisperse magnetic particles has been extended to include isolation of bacteria, viruses and various subcellular components.

Magnetic particles for cell separation should fulfill the following criteria. (1) They should not aggregate in the media used in cell separation. (2) They should give very little magnetic resonance after being subject to the magnetic field. (3) They should not bind to cells non-specifically. (4) They should allow fast and complete magnetic separation of the cells labeled with particles. (5) They should be of a size which minimizes phagocytosis. Such particles have been prepared by Ugelstad et al. 215'236 and marketed as 'Dynabeads'. More than 300 papers have appeared dealing with applications of Dynabeads, including an immunomagnetic separation procedure for the removal of tumor cells from bone marrow for autologous bone marrow transplan- tation, depletion of normal T-cells in allografts, isolation of stem cells, quantification of lymphocytes in peripheral blood, immunomagnetic separation of bacteria, 238 etc. Recently, Dynabeads have found a steadily increasing application in DNA technology, including probes, affinity purification of DNA binding proteins, separation and isolation of RNA or single stranded DNA, and sequencing reactions.

It was recently reported that the use of dyed polymeric microspheres for hole- burning memories can eliminate the need for cryogenic operating temperatures. The new technique is based on the well-known morphologically dependent resonance (MDR), but differs from ordinary condensed matter hole-burning in that, because of the involvement of the narrowly distributed microspheres and the dye, memory can be written or read at room temperature. 239

3.6. Composite polymer latexes

Latex systems with well-designed morphologies are necessary for advanced engineering plastics with high impact strengths, improved toughening, for optimum peel strength of adhesives, and for many other high value-added products in fields such as membrane separation and biotechnology. Among the various composite polymer latexes, the first product was developed by Rohm & Hass Co. in 1957 and marketed as K-120, a plastic modifier. Currently, composite polymer latexes with different components and hence different morphological features, have been prepared via seeded emulsion polymerization processes and multistage emulsion polymerization. The monomers used in the preparation of composite polymer latexes have included almost all of the monomers which undergo free-radical polymerization. Considerable interest has been generated for a composition of acrylate with methacrylate or styrene.


The most advantageous aspect of composite polymer latexes is that they allow an optimal combination of special properties of both components. This largely depends on the morphological features of the latex particles, hence, the investigation of particle morphology and factors controlling it have been the goals of many papers published in recent years, notably those of Sundberg e t al. , 240 Dimonie e t al. , 242 Okubo e t al., 234"244 and Cheng e t al. 245

3.6.1. The preparation and characterization of composite polymer latexes Seeded emulsion polymerization or multistage emulsion polymerization techniques are the main methods for the syntheses of composite polymer latexes. In the first stage, a seed latex is usually prepared by conventional batch emulsion polymerization or copolymerization. In the second stage, another monomer or monomer mixture is polymerized in the presence of seed latex particles. 246 The second-stage polymerization can be carried out by different techniques, namely, batch or semi-continuous, with or without swelling. The morphologies of the resulting latexes have been found to differ greatly. 247

Characterizations of composite polymer latexes have involved a number of techniques, including TEM and scanning electron microscopy (SEM), as well as thin-layer chromatography (TLC), neutron magnetic resonance (NMR), SANS, and measurements of minimum filming temperature (MFT), along with mechanical properties of the films formed with composite polymer latexes.

3.6.2. The thermodynamic approach and model predictions The results so far reported concerning the determinants of morphological features of composite latex particles 24°-245 fall into two categories: thermodynamic and kinetic. The thermodynamic factor determines the equilibrium morphology of the final composite latex particles; while the kinetic factor determines the ease with which such thermodynamically favored morphologies can be achieved. Cheng et al. 248 have proposed a theoretical model to predict the final morphology.

The thermodynamic analysis given by Cheng e t al. 248 is based on the approach presented by Torza and Mason 249 who dealt with the interfacial behavior of systems containing three mutually immiscible liquids. They examined the conditions necessary for a coacervate droplet (liquid 3) to engulf an initial droplet (liquid 1) when both are immersed in a continuous phase (liquid 2) by the spreading coefficient, Si, which is defined as:

Si = 7jk -- (Tij + 7ik) (45)

By assuming that the interfacial tension of liquid 1 against liquid 2 (712) is greater than that of liquid 3 against liquid 2 (723), only three possible sets of values for S exist. These correspond to the three different equilibrium configurations: complete engulfing (normal core-shell), partial engulfing (hemisphere) and non-engulfing (individual particles). Complete engulfing occurs only if $3 > 0, $2 < 0, and Sl < 0. On the other hand, when S 2 < 0, $3 < 0, and SI < 0, partial engulfing is preferred. Tonza and Mason demonstrated the general validity of their approach by making a number of interfacial tension measurements, calculating values of S and then observing the actual configuration of the three-phase systems. In most cases, when the liquid phases are highly mobile and thus the thermodynamics of the systems are influential in determining the final morphology, the predictions of engulfing based on S are

742 Q. WANG et aL

satisfactory. Recently, Sundberg et al. 24° utilized the approach of Torza and Mason to present a thermodynamic analysis of the morphology of a system comprising a polymer encapsulating a relatively large-sized oil droplet in the micrometer range. Their analysis revealed that the interfacial tension of each phase is the key factor governing the type of microcapsules formed. Dimonie et al. 241 experimentally supported the hypothesis that, in addition to the viscosity of the polymerization locus (related to the chain mobility), the interfacial tension of the polymer phases is one of the main parameters controlling the particle morphology in a composite polymer latex system.

The thermodynamic analysis of Cheng et al. 248 is similar to that presented by Sundberg et al. 24° and a mathematical model was derived to describe the free-energy changes corresponding to various morphologies of composite latex particles shown in Fig. 4. The model was applied and verified for predicting the final morphology for particles in the submicrometer size range. The methodology involves consideration of the free-energy changes for the following hypothetical pathways. The initial state was considered to consist of a polymer phase 1 (seed particles of polymer 1 swollen by the second stage monomer, i.e. monomer 2) dispersed in aqueous continuous phase

A (core-shell) A'(inverted core-shell)

C (hemisphere 1) C" (hemisphere 2)

D (individual particles)

Fie. 4. Various morphological structures of particles dispersed in water (hatched area, polymer 1; open area, polymer 2).


containing surfactant. The final state is one of the morphologies shown in Fig. 4. The only contribution to the free-energy change for this pathway is the creation of new interfaces and changes in the corresponding interfacial tensions. For latex particles dispersed in a continuous aqueous phase, those interfaces include polymer phase 1 - water, polymer phase 2 - water and polymer phase 1 - polymer phase 2. Polymer phase 2 is the polymer 2 (formed as a result of the polymerization of monomer 2 in the presence of polymer 1) swollen by its own monomer. The total concentration of monomer 2 depends on the conversion. The monomer distribution between polymer 1 and polymer 2 is assumed to be proportional to the volume fraction of each polymer.

The total free-energy change for all types of configurations shown in Fig. 4 can be expressed as:

AG = Z ")'~/A~y- 70A 0 (46)

where 3'ij is the interfacial tension between phase i and j, and A/y is the corresponding interfacial area, 3'0 is the interfacial tension of original polymer phase 1 (i.e. seed particles of polymer 1 swollen by monomer 2) dispersed in aqueous phase, and A0 is the interfacial area. Equations describing the free-energy change for each case shown in Fig. 4 were derived. 245 Calculations of the free-energy change for each case involved a trial and error solution method for which a computer program was prepared. The thermodynamically preferred morphology will be the one with the minimum interfacial free-energy change. This approach is feasible if all of the interfacial tensions of various phases 3qw (polymer phase 1 and water), 72w (polymer phase 2 and water), and "~12 (between two polymer phases), can be measured separately. The calculations have indicated that when the interfacial tension 712 is smaller, there is a greater chance of obtaining the desired core-shell morphology (Case A in Fig. 4). If the interfacial tension between the seed polymer and water is higher than that between the second- stage polymer and water (i.e., 3'lw >> "Y2w), a greater chance of having inverted core- shell morphology (Case A" in Fig. 4) occurs, when "Y2w ) ) "Ylw. In addition, when the interfacial tension between polymer phase 1 and polymer phase 2 increases, namely, when ")'12 > I'~lw - 72w[, the equilibrium particle morphology changes from core-shell to hemisphere (Case C or C" in Fig. 4) and finally, when ~q2 > (')qw + 72w), it changes to individual particles (Case D in Fig. 4). The above calculations are carried out under the condition that the volume ratio of polymer phase 2 polymer phase 1, Vr, equals 1. The value of Vr can also be varied to obtain the desired particle morphology, e.g. when 71w is greater than ")'2w, the tendency of decreasing phase separation can be enhanced by increasing the volume ratio.

An experimental examination of the model predictions was also carried out. 248 Interfacial tensions between polymer phases and water were measured by the drop- volume method, 25° tensions between the two polymer phases swollen with the second- stage monomer were estimated based on the work of Broseta et a/ . 251 Particle morphologies were examined by TEM where PS was stained with RuO4 and PMMA was stained with phosphotungstic acid. The effects of the volume ratio of the two polymer components, different types of surfactant and initiation on the final particle morphology were all examined. 248 The results strongly suggested the validity of the model predictions.

3.6.3. S t r u c t u r e s a n d p r o p e r t i e s - Besides the three types of particle morphologies discussed in the above mentioned model, there are other types of morphologies

744 Q. WANG et al.

formed in particular systems. These include the polymeric-oil-in-oil (POO), 252 raspberrylike morphologies, 253 latex particles with voids, 254 etc. Different properties have been found to arise from these specific structures.

The physical properties of composite polymer particles were first investigated by Hughes and Brown. 255 It was shown that the microphase separation of the latex particles is the determinant of the physical properties of the latex and a formed film.

The mechanical properties of the film formed by the composite polymer latex are determined by the viscoelastic relaxation under the surface tension and the interaction between the macromolecules at the particle interface. These are related to the glass transition temperature (Tg) of the polymer at the particle interface and are therefore related to the morphology of the particle. Thus, the filming ability of latexes can provide some information about the particle morphology. 256,257 It has been found that the minimum filming temperatures (MFT) of P M M A - P E A composite latexes with core-shell structures are lower than those for M M A - E A copolymers with the same chemical composition. The reason lies in the fact that, in the core-shell composite, the latex particle has a soft (PEA) shell and a hard (PMMA) core, and the filming property is governed solely by the shell polymer.

Measurements of mechanical properties of the film formed by P M M A - P E A core- shell latexes have revealed that the tensile strength and modulus are much higher than those of films formed from corresponding random copolymers. 258'26° This indicates the existence of an interfacial layer between the hard core and the soft shell, which enhances the binding of the two phases and therefore serves as a buffer zone and helps to scatter or transmit stress. Similar results were reported in the case of PVAc-PBA core-shell latexes. 259 The interfacial layer has been shown to contain a copolymer of the core monomer and the shell monomer, with a composition gradient ranging from shell-monomer-rich to core-monomer-rich. 258 This was also suggested by the Tg values of the latexes. 256-258

Other physical properties of composite polymer latexes were also investigated, including latex stability, 26° adhesion, 244 water resistance, 244 and optical properties 244,257 of films formed from composite latexes.

3.7. Surf ace-functionalized polymeric dispersions

Surface-functionalized latexes have received increasing attention during the past decade, due to their application in a broad range of fields, e.g. in biochemical and biomedical fields, in curable coatings, polymer modification, catalysis, information industry, microelectronics, dry and liquid toners, chromatographic packings, etc. A great deal of fundamental studies have been performed using various functional monomers differing in their physicochemical properties and their reactivity in (co)polymerization, in order to investigate and to control polymerization kinetics in heterogeneous systems as well as their location in the resulting latex particles. It has been clearly shown that the ultimate properties of the final latexes are strongly affected by whether the functional groups are preferentially incorporated at the water-polymer interface or within the particle. To date, polymeric dispersions with a variety of pendant functional groups have been prepared, including carboxyl groups, hydroxyl groups, vinyl groups, amino groups, pyrrolidine groups, etc. The preparation of such


dispersions has involved emulsion copolymerization, precipitation polymerization, and non-aqueous dispersion polymerization as well as surface modification of preformed latexes.

3.7.1. P r e p a r a t i o n a n d c h a r a c t e r i z a t i o n - The most widely used technique for preparing surface functionalized polymeric dispersion is the copolymerization of a functional monomer with a matrix monomer in heterogeneous systems. Most of these copolymerizations are carried out in the form of soapless emulsion polymerizations and dispersion polymerizations.

Methods used to prepare carboxylated polymer emulsion particles with carboxyl groups predominantly localized at the surface layer have been reported by Matsu- moto e t a / . 261-264 A simple stepwise copolymerization process is ruled out since it has been found that even for a very hydrophilic co-monomer such as acrylic acid, most of the carboxyl groups reside inside the particles rather than on the surface. 104'265-267 In order to localize the carboxyl groups at the particle surface, a number of techniques were explored including post-addition, neutralization of the unsaturated monomers in the process of emulsion copolymerization. Unfortunately, desired results were not obtained by these techniques. Some effective post-treatments have been reported, in which the localization of carboxyl groups at the particle surface is achieved either by direct alkali hydrolysis of ester groups at the surface of poly(ethyl acrylate) emulsion particles, 268 or by a redistribution of carboxyl groups in poly(styrene-co-methacrylic acid) emulsion particles from the interior of the particles. 269 An alternate technique involves the soapless seeded emulsion copolymerization of styrene (S) and butyl acrylate (BA) at high pH in the presence of a styrene (butyl acrylate)-(methacrylic acid) terpolymer seed which has been formed at low pH in the absence of surfactant. 27° The location of the carboxyl groups has been established with conductometric titration data. The results indicate that, with an appropriate monomer addition method, e.g. the adsorption method, 56% of the carboxyl groups are localized at the surface.

Micrometer-size monodisperse polymer microspheres with vinyl groups on the surface have been prepared by seeded dispersion copolymerization. 271 The seeded copolymerization of styrene and divinylbenzene was carried out in the presence of polystyrene seeds, which were prepared by dispersion polymerization with poly(acrylic acid) as the stabilizer. The amount of vinyl groups on the particle surface was estimated from bromine titration, and the results were satisfactory. The same technique can be employed to prepare micrometer-size monodisperse polymeric microspheres with chloromethyl groups at the surface, where the co-monomer is chloromethyl styrene. 272

Polymeric dispersions with surface-vinyl groups can also be prepared by soapless emulsion copolymerization with styrene or MMA as the main monomer, and ethylene glycol dimethacrylate (EDMA) as the multifunctional co-monomer. Indeed, soapless emulsion copolymerization processes have been employed to prepare a variety of polymeric dispersions with different surface functional groups, e.g. sulfonate, 273 amino, 274 etc. Some recent studies 275"276 have disclosed the use of macromonomers for the preparation of model particles exhibiting steric stabilization. The macromonomer often has a polymerizable group at one end and a functional (and usually hydrophyilic) group or functional chain at the other end, for example, a (hexyl methacrylate) terminated oligosaccharide termed 6-(2-methylpropenoylloxy)hexyl

746 Q. WANG e t al.

fl-D cellobioside: 277

H Oo -~OH~- O'~HO~ o/(CH2)~'O"~CH2 O OH

Such a monomer is copolymerized with styrene or MMA in the presence of a seed latex prepared via soapless emulsion polymerization of the main monomer.

Dispersion polymerization is perhaps the most convenient process for introducing functional groups or chains onto the surface of microspheres. By using an appropriate stabilizer, which not only provides colloidal stability, but also the desired functional group, the particle surface can be easily functionalized. For example, when ABA or AB type stabilizers are used, the lyophobic components anchor on the surface of the particles, while the lyophilic components stretch out into the continuous phase and serve as pendant functional groups. Such stabilizers are widely used in the production of automobile metallic finishes, in which the pendant chains may react with solubilized macromonomer to form a cross-linked polymeric film.181,197-2°1,287

3.7.2. A p p l i c a t i o n s - Polymeric microspheres with hydroxyl groups on their surfaces were prepared by precipitation copolymerization of glycidyl methacrylate (GMA), 2-hydroxyethyl methacrylate (HEMA), and ethylene glycol dimethacrylate followed by hydrolysis with dilute acidic or alkaline solutions. A similar process was used to prepare microspheres with amino groups on the surface, in which the precipitated polymer microspheres were ammoniated with ammonia water solution. Both kinds of microspheres can be used for cell labeling, 278 immunological agglutination tests, 279 the measurements of phagosomal reactive oxygen, 28°-282 and for measurements of phagosomal enzyme activity. 283'284

Polymeric microspheres with both epoxy groups and carboxyl groups on the surface were prepared by precipitation copolymerization of GMA and MMA in an isopropanol medium. The resulting microspheres were used as a diagnostic material with IgG introduced by the carbodimido method. These proved to have the advantage of non-specific aggregation because of their hydrophilic surface. 285

Polymeric microspheres with carboxyl and hydroxyl groups were prepared by emulsion copolymerization of MMA, HEMA, and dodecyl benzene sulfonate. Such microspheres can be easily labeled with various kinds of antigens or antibodies as well as with drugs or isotopes. They were used for the detection of characteristic markers on cell surfaces by TEM, for the isolation and purification of the corresponding counter- parts (immunoadsorbent), for the removal of toxic substances from circulating blood (artificial organs), and for targeting malignant tumors with specific antibodies, toxins, anticancer drugs, or isotopes (missile therapy).

Another application involves some interesting amphiphilic polymers, such as poly(N-isopropylacrylamide) which has a lower critical solution temperature (LCST) at 31 °C in water. Therefore, these solutions show a revisable phase separation above the LCST. Presumably, the LCST is a result of the balance between the hydrogen bond formation with water and the intermolecular hydrophobic force. This reversible phase separation behavior has been utilized for the separation and purification of proteins. 286

EMULSION POLYMERIZATION

TABLE 6. Some pairs of functional groups and the resulting cross-link.

747

Functional group A Functional group B Cross-linking formed

--COOH --C~---~C --COO--C--C--I OH

#-i ° --COOH -- II --O00-C-C--N-C--

O--C

_ooo -<i \N--C-- ~NH --C~--~C / II

O

~N--C-- ,~NH --COOH / II

O

,,/N-C \N--C--C--NH--C-- ,~J"4 H - - C I / II

O--C O

Non-aqueous dispersion and water-based latexes are widely used in surface coatings, automotive metallic finishes, and adhesives. The mechanical properties and durability of the resulting films are of great interest and are therefore studied extensively. One of the governing features of the above mentioned properties involves interparticle cross-linking. In order to achieve satisfactory cross-linking, polymer particles have been designed and prepared with two kinds of functional groups, one of which (referred to as functional group A) is localized on the surface of the particles and is hydrophilic, while the other (referred to as functional group B) is localized within the particles and is hydrophobic. The two kinds of functional groups have no contact when dispersed in the system, but in the film-forming process on evaporat ion of the dispersion medium (solvent or water), the particles get closer and functional group A may penetrate the surface of another particle to react with functional group B. Table 6 gives pairs of functional groups suitable for use in room-temperature cross-linkable coatings and adhesives.

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pergamon emulsion polymerization

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