Cnllr1itis rrrlll StWfiKES. 67 ( 1992) 95100 Elscvicr Science Publishers B.V.. Amsterdam

95

Flocculation of’ polymer latex dispersion induced by a polyelectrolyte

Takeshi Nashima”, Hideki Sudob and Kunio Furusawab ’ Nc~timtal Research Laboratory 0s Metrology, Untezorto, Tsukuba, Ibaraki 305, Japan b Tiw lJnioersit?l oj’ Tsukubu, Departrnerc of Chemistry, Teworcdai, Tsd<llba, lbaraki 305, Japan

(Received 2 January 1992; accepted 22 April i 992)

Abstract

The stability of high surface charge latex dispersions containing non-adsorbing polyelcctrolytc was investigated. Poly- (styrcnq’sodium p-vinylphcnylsulphonatc) copolymer Iaticcs and sodium polystyrcncsulpboortc systems wcrc used. Floe-

culation wx confirmed using three observational methods - transmission mcasurcmcnt. microscopy and dynamic laser lighi scattering particle anulysis. It was found that in flocculation the cluster size of the flocculated particles split into two peaks - a small pcuk consisting almost cntircly or single particles and ;1 larger peak which incrcascd with time. Morcovcr. the floe which formed was in an grdcrcd state and changed into :L crystalline phase after sedimcntalion.

h’e_~r~t/.s: Dispersion: flocculation; polyclcctrolytc: polymer latex

Introduction

The stability of dispersions is greatly affected by the addition of polymers. Po!ymers which adsorb onto particle surfaces stabi!ize or destabilize ?hc dispersion by the steric effect of adsorbed layers or the bridging effect between particles. Polymers which do not adsorb onto the surface may also destabilize the dispersion. This effect is interpreted as the volume restriction effect of particles and polymers which come up from the interparticle region in which polymer mo!ecu!es cannot exist without deformation. As a result of the difference in polymer concentrations inside and outside the region, osmotic pressure acts to draw particles together. This effect is called depletion floccu!ation.

DepleCon flocculation has been observed in many systems, both aqueous [l-4] and non-aque- ous [5,6]. Also, in the case where polymer adsorbs

Corrcspor&#lce to: T. Nashima, National Rcscarch Labora- tory of Metrology. Umezono, Tsukuba, lbaraki 305. Japan.

onto particles, the depletion effect operates after saturation adsorption by excess polymer [7].

The volume restriction effect may be expected to act quite vigorously whez the polymer is a polyelectrolyte, since polyelectrolytcs have a more expanded conformation because of their charge. Actually, in a dispersed system of high surface charge latex plus sodium polystyrenesulphonate, phase separations were observed which seem to be the result of this ef?ect. It is remarkable that in this system the crysta!!ine phase emerges on phase separation. It has been reported, however, that no flocculation was observed after the addition of sodium polystyrenesulphonate [8]. The reason for this contrary result seems to be due to the difference in dispersed particles; the dispersant in Ref. [S] is a sterically stabilized system with sma!I charge effects, while our systems consist of highly charged latices and polymer.

In this work, the system of po!y(styrene/sodium p-vinylphenylsulphonate) !&ices and sodium poly- styrenesulphonate (PSSNa) was investigated. As

0 I66-6622/92/SO5.00 0 1992 - Elscvier Science Publishers B.V. All rights reserved.

the latex and the polymer have the same charge.

the polymer does not adsorb onto the latex surface.

Flocculation of this system was traced by means

of a dynamic light scattering particle analyzer and

microscopic observation.

particles. By using the Einstein-Stokes equation

we can calculate the hydrodynamic radius (Rh) of

the particles:

Experimental

High surface charge styrenc/sodium p-vinyl-

phenylsulphonate copolymer latices were prepared

by the surfactant-free emulsion polymerization

method. Two latices with different diameters were

prepared. The polymerization conditions and char-

acterization data arc Iisted in Table I. The larger

Iatex was used for dynamic light scattering

experiments.

R, = iiT/67@ (1)

where k, T and D are the Boltzmann constant, the

absolute temperature, and the diffusion cocllicient

obtained by the light scattering mcasurcmcnts,

respectively, and 11 is the viscosity of the liquid

surrounding the particle. In this work, R,, values

were calculated using the viscosity value of water.

Three lots of commercial grade sodium poly-

styrcncsulphonate wete used, the molecular

weights of which were 100 * IO’ (PSSNa 100)

50 - IO’ (PSSNa 50) and 5 * 10” (PSSNa 5). As the

aquecus solutions of PSSNa showed high pH

values (pH > IO), the solutions were dialyzed

against distiIlsd water so as to remove sodium

hydroxide.

Other experimental methods used were optic4

microscopic observation and measurement of the

transmission coefcient of light (500 nm). The latter

measurements were carried out after standing the

mixtures for one day (24 !I) following the addition

of PSSNa to latex dispersions. The concentration

of each species was adjusted to give the final value

of weight percent (based on total weight of the

dispersion) by mixing the concentrated stock

so!utions.

Results ilid discussion

A dynamic laser light scattering apparatus

(Otsuka elc. ELS-800) was used to observe the

growth of flocculated clusters. This apparatus has

been developed to measure the electrophorctic

mobility by means of dynamic light scattering,

however, we can make separate use of the light

scattering measurcmcnt. The dynamic light scatter-

ing method measures the diffusion coeficients of

On addition of PSSNa to iilt: iatex dispersions

the appearance of the sample generally varied as

illustrated in Fig. 1. In the first step the sample

changed from a milky white state to the appearance

of iridescence on the wall of the vessel. The next

step involved the settling of iridescent precipitates

with the surface iridescence remaining. Then, the

particles on the vcsscl walls also scttlcd out, and

I-ABLE I

Preparation conditions :rnd charnctcrization of polystyrcnc Iaticcs

Polymerization conditions

Hz0 Styrcnc K,S,O, N;1SS”

(cm’) (cm3) (J) (6)

I700 300 I.5 1.5 I600 400 1.2 3.0

“Sodium p-vinylphcnylsulphonatc. ‘Determined by transmission clcctron microscopy. ‘Measured in I * IO-’ M KCI at 25’C.

Characterization

MgS0,.7H20 Dinmctcrh Surklcc chzrrgc <-potential”

(6) (nm) (PC cm-‘) (mv)

0.0 170 7.13 -51.53 7.6 560 29.5 - 66.4

T. h’nslkm cl nl./Co/loids Srt&irccs 67 (1992) 9% 100 97

IX10 -4 M KC1 .,

Fig. 1, Illustration of the appcarancc of latex dispersions with varying PSSNa concentration; oblique lines express the appear- ancc of iridescence. Latex concentration, I.0 wt.“G: IO-’ M KCI.

at the highest PSSNa concentration the iridescence of the precipitate disappeared.

Figure 2 shows the variation of dispersion sta- bility with added polyelectrolyte concentration by measuring the transmission coefficient for the three PSSNa solutions of different molecular weights. It is shown that stability decreases with increasing

100 I

1

I

-2. -3. 0.

PSSNa concentration. log C (wtX)

Fig. 2. Variation ol dispersion stability due to addition of PSSNa solutions of various molecular weights: 0, 100. tOa; A, 50. 10J; U, .5- i0’. Latex concentration. I.0 wt.%; 10-’ M

KCI.

PSSNa concentration and that higher molecular weight PSSNa causes flocculation at lower concen- trations. When the salt concentration is varied, the flocculation concentration of PSSNa lowers for lower salt concentration (see Fig. 3). If this floccu- lation is related to the volume restriction effect, we can readily interpret these results. The extension of polyelectrolyte decreases with increasing salt concentration. For the samples including the Iow- est molecular weight PSSNa, however, the influ- ence of the salt concentration is very slight (Fig. 4). This may be because the high sodium ion concen- tration at such a high flocculation concentration of PSSNa overcomes the added sa!t concent:;:tion.

The flocculation process was traced with a dynamic laser light scattering particle analyzer. As the suitable particle concentration for light scatter- ing is much lower, the critical flocculation concen- tration (c.f.c.) of PSSNa became considerably higher than that of the light transmission expcri- ment. Since flocculation could not be detected for the small (170 nm) particle systems, the large (56C nm) particle systems were used in the light scat!ering experiments. At a iatex concentration of 0.05 wt.%, which was the concentration for the light scattering measurements, a PSSNa concen.. tration of 0.1 wt.% did not bring about variation in the measured particle size even after 2 days of storage. Fiocculation was observed from 0.2 wt.% PSSNa 50. Figure 5 shows the size distribution variation with time using the light scattering meas- urements. It was found that (i) the particle size distribution peak (mean size) shifts towards larger sizes with time; (ii) the particle size distribution eventually splits into two peaks; (iii) on comparing a 0.2 wt.% PSSNa sample with a 0.3 wt.% sampie, the one that has the lower PSSNa concentiation has a much slower flocculation speed. The initial particle sizes are too large for single latex particle values. The reason for this is not the light scattering measurement errors, nor fast flocculation, but the result of under-estimation of the medium viscosity. If the medium can be assumed to be homogeneous around the particles, we might estimate the viscos- ity of the medium surrounding the particle to be

E a0 -

Y c w

Fig. 4. As for Fig. 3 with PSSNit 5.

60 -

40 - i 20 - n :

:

0 _______._ .._...._.........._..................... _____o .A” . . . . . .._ _

-2. -1. 0. PSSNa concentration. log C (HtX)

that of the polymer solution. In a depletion floccu- lation system, however, the polymer concentration lowers around the particle surface, so we have hesitated to adopt those vaiucs. Thcrcfore, the results are only valuable as a measure of relative size variations. In Fig. 6, the particle size variations with tin:5 ior the 0.1 wt.% and 0 ? wt.? PSSNa 100 are plotted. As explained above, the apparent particle size for a 0.2 wt.% sample increases with time and a double peak emerges after about 9 h. However, the particle size for a 0.1 wt.% sample changes only slightly with time. While the double peak emerges after 9 h, it does not instantly mean the onset of flocculation or phase separation because it is possible that the resolution of the

1 7 4 0 m i n . +:...y.‘. :.{::;:,: ~~

100 !OiKl ? 0000 Apparent particle size (nm)

: 0 min. ,,.,....,...,....,. ..

Apparent particle size (nm)

Fig. 5. Histogr;lms d pnrticlt: s;zc distribution vs time. NnCl

:onccntr;ltion = I - IO--’ ,II. PSSNa 50 conccnIr;llion: 0.1 wt.“i

(fdt); 0.3 wt.“; (right).

measurement was less than that for separating peaks. The smaller sized peaks of the 0.2 wt.% PSSNa sample arc not so dimerent from the initial size and are nearer to those of the 0.1 wt.% PSSNa sample. It can be seen that only the larger size cluster of the 0.2 wt.% PSSNa sampI,- grew with time.

It is remarkable that the size distribution of the flocculated particles splits into large and almost single particle clusters. This scheme is confirmed by microscopic observation. I‘hc flocculation can be observed 30 min alter lhe addition of PSSNa. Figure 7 shows a flocculated sample about 1 day after the addition of 0.2 wt.% PSSNa 100 (I - iO_” M KCI). It is clearly observed that there were many single particles among the flocculated clusters. Furthermore, it can be shown that the flocculated clusters have a crystalline structure. It was observed that these flocculated clusters

99

0 10 20 .. 4s

Time hours )

Fig. 6. Variation of apparent particle six with time. PSSNa 100 conccntr~~tion: c 0 .- 7 wt.‘G , 0: 0.1 wt.%, 11. Latex: diamctcr, 560 nm; concentration. 0.05 wt.%. NaCl concentration = 1 * 10 - .l IV.

Fig. 7. Micrograph showing ordcrcd clustering after addition of 0.’ \\~I.‘.‘~ PSSNa 100.

underwent frequent formation and dissolution with adherence and separation of single particles. The particles in the system frequently exchanged their positions between clusters and solitary particles, which suggests that the flocculated clusters are in equilibrium with single particles. This implies that flocculation is affected not only by the activity of particles but also by the solva?cd polymer in the volume restricted flocculation.

These results make it possible to interpret the fact that a low particle concentration sample for light scattering did not floccuiate at the same PSSNa concentration as in the light transmission experiment, and that the smaller particle latex did not flocculate in the light scattering measurement. The former is obvious because a strong depletion effect is necessary if particle activity is low. For the latter, if we knnw that the depletion effect on particles of the same polymer is less pronounced for a smaller particle than a larger one (small interparticle areas of small particles will bring small depletion potentials compared with the thermal energy), it is easy to understand that a higher polymer concentration is needed to flocculate the small particle dispersions.

Phase separation of the volume restricted system is predicted by theoretical studies ES-1 I] which indicate that the critical phase separation point, i.e. the flocculation concentration for free polymer, is lowered with increasing particle content in the low particle content regime. Although our system is not one of steric interaction (as in the theoretical studies) but of electrostatic repulsion, it seems that volume restriction flocculation occurs. Our system rather resembles that of the two repulsive charged particles intensively studied by Hachisu and co- workers [ 12,131. Hachisu demonstrated the appearance of crystalline superstructures in binary mixtures of latices. Yasrebi et al. [14] recognized that the flocculation of binary mixtures of hard spheres is comparable with the depletion floccu- lations induced by polymers_ In our systems, we may consider that one of two particles is substi- tuted by a “soft sphc.-c” of polyelectrolyte. The formation of ordered floes of latex particles may be similar to the binary hard sphere sys.zms, apart

100

from the discussion of whether or not superstruc- tures are formed.

We can conclude that for the electrostatic rcpul- sion system of latex dispersion plus polyelectrolytc, phase separation associated with flocculation also occurs, which seems to bc the resu!t of volume restriction efiects. It should be emphasized that the floes form crystalline structures and are in equilib- rium with single particles in the dlspcrsions used in this work.

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