radiation curing of methacryloyloxyalkyl carbonates
TRANSCRIPT
Radiation Curing of Methacryloyloxyalkyl Carbonates
I. M. BARKALOV, D. P. KIRYUKHIN, and V. M. MUNIKHES, Institute of Chemical Physics, USSR Academy of Sciences, 142432
Chernogolovka, USSR
Synopsis
The results of studying the radiation curing of methacryloyloxyalkyl carbonates (MC), a new type of oligomer, are presented. These oligomers are notable for their high rates of radiation curing. The radiation yield G(-M) is 2 X 105 for MC and 2 X lo4 for triethylene glycol dimethyacrylate. The polymerization rate of MC appeared to be proportional, independent of the conversion degree, to the irradiation dose rate in the power of 0.9-1.0 (for dose rates ranging from 0.4 to 15 rad/s). In regard to the temperature dependence of the polymerization rate of MC for small conversion degrees, two temperature regions with different values of effective activation energy (18-20 kcal/mol and 2 kcal/mol, respectively) were observed. When an irradiated MC sample is being unfrozen, its poly- merization occurs in the region of devitrification (220-240°K). As distinct from mass polymerization, in the polymerization of MC solutions in acetone and benzol the mobility of growing chains increases so that the bimolecular termination becomes possible and the limiting conversion of double bonds is derived. Rather small irradiation doses necessary for curing MC and the proportionality of the radiation-induced polymerization rate to the dose rate make these oligomers valuable for various industrial applications.
INTRODUCTION
The use of radiation-cured polymerizing oligomers in the production of coatings and composite materials is a promising trend in applied radiochemistry. Radiation curing of unsaturated oligomers, as distinct from heat curing, makes it possible to increase the rate of curing considerably and, consequently, the ef- ficiency of the process equipment. Intensive investigations in the field of ra- diation curing having already resulted in a number of processes that use radiation curing.l A major factor that determines the economical efficiency of radiation curing is an irradiation dose necessary for obtaining the curing degree required. It is reasonable that various oligomers and oligomer-containing compositions that require a minimum irradiation dose for their curing are being sought.
In this report the results of a study of the kinetics of the radiation polymer- ization of a new type of oligomer (i.e., methacryloyloxyalkyl carbonates) are presented. A rather high curing rate is peculiar to this type of oligomer.
EXPERIMENTAL
The oligomers investigated were synthesized according to the method de- scribed in ref. 2. Their main characteristics are listed in Table I. Polymerization was initiated by the y-rays of 6oCo source. A calorimetric technique3 was used for investigating the kinetics of the radiation polymerization and the phase state of the oligomers.
A sealed calorimetric glass vessel that contained an oligomer sample was used
Journal of Polymer Science: Polymer Chemistry Edition, Vol. 21,1401-1416 (1983) 0 1983 John Wiley & Sons, Inc. CCC 0360-6376/83/051401-16$02.60
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CURING METHACRYLOYLOXYALKYL CARBONATES 1403
for experiments. All measurements were carried out in the absence of atmo- spheric oxygen.
The amount of unexpended methocrylate groups in irradiated oligomer samples was determined by the method of infrared (IR) spectroscopy; the ab- sorption band of the methyl groups (1348 cm-l) was used as an internal stan- dard.* In addition, the curing degree was estimated by gravimetric titra- t i ~ n . ~
The ESR spectra of y-irradiated samples were registered with a model JKhF-2 spectrometer a t a microwave field power of ca. lOW4W in a temperature range of 77-40O0K.
RESULTS AND DISCUSSION
Direct Radiation Curing.
To obtain kinetic information from calorimetric measurements, the poly- merization heat was determined for MC-2 and MC-8. With this end in view, the oligomers were cured to large conversion degrees (60-80%) and the integral heat evolution was measured. The amount of double bonds expended in the cured samples was then determined by IR spectroscopy for MC-8 and by gravi- metric titration for MC-2. A comparison of the results obtained for the two quantities in a number of identical experiments made it possible to determine the polymerization heat AH which turned out to be 35 f 2 kcal/mol for MC-2 and 28 f 2 kcal/mol for MC-8. It should be noted that the polymerization heat for methylmethacrylate is 13.9 kcal/mol.6
A typical curve of heat evolution which resulted from the polymerization of methacrylate groups in the y-irradiated oligomers is shown in Figure 1. The kinetic curves for MC-2 and MC-8, shown in Figures 2 and 3, respectively (for details refer to refs. 7 and 8), were calculated from calorimetric measurements; the value of AH was used and the calorimeter thermal lagging r = 100 s) was taken into account.
Atmospheric oxygen strongly inhibits radiation curing and causes the occur- rence of an induction period. For this reason further kinetic measurements were
Fig. 1. Calorimetric curve of radiation curing of MC-2. Radiation intensity 2 rad/s; temperature, 22°C.
1404 BARKALOV, KIRYUKHIN, AND MUNIKHES
Fig. 2. Kinetic curves of MC-2 polymerization. Radiation intensity: 10 rad/s (1-4), 45 rad/s (5, 61, 2 rad/s (7); 1, 2,5, and 7 are samples that contain inhibitor; 3 ,4 , without inhibitor; 2,4, 6, evacuation at room temperature; 3,5, nonevacuated; 1,7, NP bubbling; temperature 22-23°C.
made in the absence of this inhibitor. Rather high rates of radiation polymer- ization of MC at the onset of the process were noteworthy. The kinetics of the radiation polymerization of MC-2 and that of triethylene glycol dimethacrylate (TGD), widely used in the radiation-curing technique,l were compared under identical conditions. It can be seen in the initial portions of the kinetic curves (Fig. 4) that the radiation yield G(-M) of expended double bonds of methacrylate groups per 100 eV of absorbed energy is 2 X lo5 for MC-2 and 2 X lo4 for TGD. Thus, at the onset of the reaction the rate of expenditure of methacrylate-group double bonds in MC-2 is an order of magnitude greater than that in TGD.
In prolonged irradiation the polymerization rate of MC-2 decreases sharply and at doses of about 6 krad the polymerization rates of MC-2 and TGD become equal. As irradiation proceeds, the degree of double-bond expenditure in TGD becomes greater than in MC-2. Even at high doses (ca. 10 Mrad) 25% of the double bonds in the cured MC-8 sample remain unexpended. It should be noted that the cured sample of MC-2 with a 50% expenditure of double bonds repre- sents a material almost insoluble in organic solvents (the content of the insoluble fraction is about 99% after prolonged extraction).
@p m
9
0 40 80 120 mitt
Fig. 3. Kinetic curves of radiation polymerization of MC-8 at dose rates of y-radiation from a W o source (rad/s); 1-5; 2-5; 8,9-1,6; W.8; 7 4 4 . Temperature: 1,3,6,7-22O; 2-36O; P 8 O ; 5--6O; 8--16O; 9 - - 3 3 O ; r-conversion degree.
CURING METHACRYLOYLOXYALKYL CARBONATES 1405
50 m i n
Fig. 4. Kinetic curves of radiation polymerization of MC-2 (1) and TGD (2) at a dose rate of y-radiation from a WCo source-20 radls. Temperature 22OC.
The dependence of the radiation polymerization rates of MC-2 and MC-8 on dose rates (initiation rates) has been investigated for different conversion degrees (1-40%). The polymerization rate, independent of the conversion degree, ap- peared to be proportional to the dose rate in the power of 0.9-1.0 (for dose rates ranging from 0.4 to 15 rad/s). It is important to emphasize that the power is almost 1.0, even in the first stages of the process (conversion degree about 1%). This characteristic of the dependence is evidence of the predominance of a mo- nomolecular termination of the kinetic chains. Apparently, the steric isolation of the growing radicals occurs at the onset of polymer-chain development and a biomolecular recombination of radicals becomes impossible. It should be noted that it is a bimolecular termination of the kinetic chains that is characteristic of the early stages of chemically initiatedg and radiation-induced1° curing of polyesteracrylates.
The fact that the radiation curing rate of MC increases in proportion to the radiation intensity may be advantageous in practical applications because it is possible to intensify the process by using high-powered electron accelerators.
The temperature dependence of the polymerization rate of MC-2 on different conversion degrees is shown in Figure 5. These curves were plotted on a series of radiation polymerization kinetic curves obtained for different temperatures. For small conversion degrees two temperature regions with different values of effective activation energy E,ff were observed. A comparatively large value of E,ff 18-20 kcal/mol is typical of the low-temperature region; Varying from 1 to 40%, this value is almost constant with the conversion degree.
A noticeably smaller value of E,ff that increases as the transformation degree increases is characteristic of the high-temperature region. Thus for the region E,ff = 2 kcal/mol and E,ff = 8 kcal/mol for a 1 and 50% conversion degree, re- spectively. The same behavior is observed for MC-8.8 This complicated character of the temperature dependence of the polymerization rate may be explained qualitatively. The termination of polymer chains seldom occurs at low temperatures because of the high viscosity of the systein near the oligomer devitrification region. The process develops according to the mechanism of “living” chains.l’ I t is natural that in a viscous medium the polymer chain growth is restricted by the diffusive supply of a reaction methacrylate group to a growing radical, and it is for this reason that a large value of E,ff is observed
1406 BARKALOV, KIRYUKHIN, AND MUNIKHES
I 1 8 8 I ’ ‘%
44 $6 38 4.0 44 Fig. 5 . Temperature dependence of radiation polymerization rate of MC-2. Conversion degree:
1-1%; 2-2,5%, 3-30%, 4-40% 5-50%.
in this temperature region. The temperature dependence of polymerization rates is completely determined by that of the constant of the polymer chain growth rate.
As the temperature approaches the high-temperature region, the viscosity of the medium drops, and as a result two important effects are noted. First, the polymer chain termination plays an important role. We believe that the linear termination of chains is due to the fact that the growing radical occurs in the microregion of a three-dimensional netlike structure in which access of the re- action methacrylate groups is geometrically impeded. Second, the chain growth rate is not limited by diffusion. It is these two factors that lead to a considerable decrease in E,ff. However, the mobility of oligomeric molecules in the medium decreases as the conversion degree increases, and further diffusion and chain growth are impeded. Accordingly, the value of E,ff increases as shown in Figure 5.
Extremely high rates of radiation curing of a new type of oligomer, metha- cryloyloxyalkyl carbonate, and the proportion of its curing rate to the irradiation dose rate have enabled us to recommend it as a base for coatings and composite material^.^.'^
Postradiation Curing.
Cooling of oligomer samples vitrifies them completely. When these samples are unfrozen in a calorimeter a peculiar change in the heat capacity (a “step”) is observed. This change corresponds to the transition of the system from a
CURING METHACRYLOYLOXYALKYL CARBONATES 1407
223 248 273 293 3f3 1
Fig. 6. Calorimetric curves of unfreezing oligomers irradiated by y-rays from a Co60 source a t -196p, dose 9 X lo3 rad. I-MC-3,2-MC-4,3-AC-2,4-MC--I, 5-MC-2,6-MC-7.
vitreous to a liquid state. Figure 6 shows that devitrification of all the monomers investigated proceeds in the range of 220-235°K. For MC-7, however, it occurs a t 240-245°K. It was impossible to obtain crystalline oligomer even with slow cooling.
The y-radiolysis of vitreous samples is not accompanied by polymerization; only an accumulation of stabilized radicals takes place. When oligomers irra- diated in the vitreous state are unfrozen in a calorimeter the heat evolution as- sociated with their postpolymerization is registered immediately after the system has passed from a vitreous to a liquid state (Fig. 6).
This phenomenon is typical of the postpolymerization of vitrescent systems; the diffusive supply of monomer to the growing active center is sufficient only when the system passes to the liquid state.ll
The increase in the oligomer postpolymerization rate with temperature (refer to the heat-evolution, curves in Fig. 6) is described by Arrhenius' law. The
' , "" J16 4 7 &? 40 4], /2 83 B3 7-
Fig. 7. Temperature dependencies of the initial rate of postradiation curing. Obtained from experimental data given in Fig. 6. Irradiation conditions and designations are the same as in Fig. 6.
1408 BARKALOV, KIRYUKHIN, AND MUNIKHES
TABLE I1
Oligomer MC-1 MC-2 MC-3 MC-4 MC-7 MC-8 E,ff kcal/mol 24 24 20 22 19 20
transformation of the initial portions of the calorimetric curves (Fig. 6) in the Arrhenius coordinates is shown in Figure 7. This enabled us to determine the effective activation energies (E& of the postpolymerization of the oligomers investigated (see Table 11).
As in direct radiation curing, large values of E,ff in the posteffect are associated with the fact that from the very beginning polymer chain growth, is limited in the low-temperature region by the diffusive approach of a methacrylate group to the growing active center. It should be noted that in methylmethacrylate polymerization the activation energy of chain propagation is only Ep = 4.6 kcal/moP3 because chain growth is not limited by diffusion in the first stages.
A series of calorimetric curves, similar to those in Figure 6 for MC-2 and MC-8, was obtained for various preirradiation doses. With an increase in the dose, the postpolymerization proceeds at lower temperatures but the slope of the curves in the Arrhenius coordinates remains constant; that is, the activation energy does not change (Fig. 7).
It is quite natural to consider that the rise in the process initial rate caused by the increase in the preirradiation dose is associated with a rise in the active center concentration. As shown in Figure 8, the initial postpolymerization rate of MC increases linearly with irradiation dose and is valid for doses up to lo7 rad. This dependence proves the linear character of kinetic chain termination during MC polymerization.
At small preirradiation doses (0.001-0.005 Mrad) the availability of oxygen dissolved in the oligomer sharply inhibits the process and results in an induction period. However, a t doses of 0.05-0.1 Mrad, the inhibitory action of oxygen is no longer observed. This is due to a rise in the initiation rate, since a dose in- crease leads to an increase in the quantity of active centers.
ms K34 io o6 f O L d
Fig. 8. Dependence of initial rate of postradiation curing of MC-2 on preirradiation dose.
CURING METHACRYLOYLOXYALKYL CARBONATES 1409
Fig. 9. ESR spectra of MC-2 y-irradiated at 77'K, irradiation dose 0,lMrad; (1) after irradiation before heating; (2) after irradiation the sample is heated to 150'K.
On the Mechanism of Initiation and Polymer Chain Growth
The radical stage of the low-temperature radiolysis of most vinyl monomers consists in the detachment of an H-atom from the molecule and its addition to double bonds. In low-temperature radiolysis of MC-2 (at doses 3 0.05 Mrad) mainly the radicals formed on joining an H-atom to a methacrylate group are stabilized. The ESR spectrum of a MC-2 sample y-irradiated at 77°K is shown in Figure 9 (for details see ref. 14). From the initial portion of the radical ac- cumulation curve it was found that the radiation yield of radicals GR = 3 f 1 1/100 eV.
As the heating of the sample proceeds the radicals R, stabilized in low-tem- perature radiolysis (referred to below as monomers) initiate the polymer chain growth and chain to the growing polymer chain radicals RP with peculiar ESR spectra (Fig. 9, spectra 1,2) identified in the polymerization of various methacrylic der ivat i~es . '~J~
As Figure 10 (curve 1) shows, initiation of the polymer chain, that is, the R, - Rp change, occurs at 140"K, which is 100°K lower than the devitrification temperature. This fact testifies to a high mobility of an oligomeric molecule in relation to the -C-0-C- bond because the formation of RP takes place in
Fig. 10. Change of relative concentration of radicals (1) on unfreezing MC-2 y-irradiated at 77OK (dose 0.05 Mrad), calorimetric curves of unfreezing nonirradiated MC-2 (2), and y-irradiated sample (dose 0.05 Mrad) (3).
1410 BARKALOV, KIRYUKHIN, AND MUNIKHES
Fig. 11. Kinetic inhibition of MC-2 polymerization under isothermal conditions a t 246 (a) and 275°K (b). Preirradiation dose 0.1 Mrad.
the vicinity of Tg.17 The ESR method registers only the first act of chain polymer growth. Further chain growth requires the translation of monomer molecules to the growing active center. This mobility can be realized only when the matrix is devitrifying, and it is in this devitrification region that the calorimeter registers effective polymerization (Fig. 10, curve 3). Almost all radicals change to RP and the use of the radical coefficient in postpolymerization [Rp]/[R,] = 1. It is also important that the concentration of Rp remain constant in the temperature range of 140-320°K (Fig. 10, curve 1). The growing radicals are caught by the three- dimensional, netlike structure and remain therein until the temperature ap- proaches that of vitrification of a polymer matrix under formation. The low translatory mobility a t temperatures below Tg permits the formation of short- chain macroradicals only, before the chain growth terminates. The inhibition of the reaction is observed not only in the early stages of chain growth but also in the temperature region in which the system is in a supercooled liquid state. Indeed, if a sample in the calorimeter is thermostated at some temperature (246”K), a sharp decrease in the polymerization rate is observed [Fig. ll(a)]. This inhibition cannot be explained by the monomer expenditure or by the “death” of macroradicals because Rp = constant in this temperature range. After postpolymerization at constant temperature has terminated it may be resumed by increasing the sample temperature [Fig. 1 l(b)]. This isothermal inhibition of the reaction and its subsequent resumption due to heating may be observed repeatedly with the same sample. For the first time this phenomenon was ob- served in the postpolymerization of crystalline monomers.18 In MC-2 postpo- lymerization, the chain growth termination occurs in the liquid state at small conversion degrees because of the formation of a rigid, three-dimensional, netlike structure.
As shown in ref. 19 and confirmed by model computations on a computer, it is possible in spite of the reaction inhibition to estimate the temperature behavior of the constant of the polymer chain growth rate from the initial portion of the calorimetric curve. Taking into account that [R,] = [ R M ] = 7.5 1OI6 g-l (at a dose of 0.05 Mrad) and [R,] = constant at 140-32OoK, we can estimate [from the data on the temperature dependence of the rate (Fig. 10, curve 3)] the constant rate of the chain rate:
K, = 0.1 exp ( 2200~T2000) cm3/s
The overrated values of the activation energy and preexponential factor are likely to be connected by the compensation effect, near Tg and other phase
CURING METHACRYLOYLOXYALKYL CARBONATES 1411
transitions.lg The estimation of the constant of the chain growth rate directly in the field of y-radiation a t 250°K resulted in almost the same value as in postpolymerization; that is, K, N
Thus one of the peculiarities of MC-2 postpolymerization is the origin of a polymer chain a t a temperature 100°K lower than the Tg of the matrix. As a matter of fact, all radicals “prepared” by radiolysis change into the radicals of the growing polymer chain. The growth of the polymer chain proceeds without termination in accordance with the “living chain” mechanism. The inhibition of the polymerization observed under isothermal conditions is associated not with the “death” of active centers and the monomer expenditure but with an increase in the activation barriers of translation of oligomeric molecules due to an increase in the conversion degree and rigidity of the three-dimensional netlike structure. It should be noted that extrapolation of our data and those derived from other works20 shows that the order of magnitude of the growth rate constant for MC-2 a t 260-265°K is close to that of a less viscous monomer-methyl- methacrylate.
cm3/s.
Radiation Solution Polymerization
The monomolecular termination of kinetic chains is typical of the polymer- ization of MC oligomers, even in its earliest stages. Evidently, the intercon- nection of 2-3 oligomeric molecules is accompanied by the steric isolation of the growing centers; consequently the bimolecular recombination of the growing centers is impeded. Thus the growing center steric isolation which develops in polymerization leads to the suppression of termination, hence to a greater polymerization rate. It was not clear why for oligomers of this type, as distin- guished from other polymerizing oligomers, steric isolation of the growing center took place a t the very start of chain growth. To explain this peculiarity aggre- gates with a more or less ordered arrangement of oligomeric molecules, or asso- ciates, were supposed to form in liquid MC, these associates causing an efficient polymerization of MC. Their role may consist in the formation of a rigid, three-dimensional, netlike structure a t the earliest stages of polymerization.
It is reasonable to suppose that the solution polymerization of MC will con- siderably influence the mechanism of the growing center steric isolation in the early stages of the process.
The kinetics of the solution polymerization of MC-8 was investigated a t 295°K. Figures 12(a), (b) present a series of kinetic curves for the solution polymerization of MC-8; acetone and benzol were the solvents. These curves result from ca- lorimetric measurements a t various dose rates and oligomer concentrations. A t first the solution polymerization of MC-8 proceeds less actively, compared with the polymerization of pure MC-8, but in mass polymerization of MC-8 the number of double bonds expended quickly reaches a maximum (ca. 50% at a dose rate of 1.3 rad/s); larger degrees of conversion (ca. 90%) are possible in the polymerization of a 55% solution of MC-8 acetone [cf. curves 4 and 7 in Fig. l2(a)]. This means that the three-dimensional, netlike structure formed in the 55% solution is looser than that in mass polymerization and allows for oligomeric molecules to reach the growing centers more effectively.
The three-dimensional, netlike structure formed in benzol solutions is as rigid as that in mass polymerization because the limiting conversions of double bonds
1412 BARKALOV, KIRYUKHIN, AND MUNIKHES
?5 -
100 min
I ///
(b) Fig. 12. (a) Kinetic curves of radiation polymerization of MC-8 in acetone. Dose rate 1.3 rad/s,
room temperature. Content of MC-8 in solution: (1) lo%, (2) 19%, (3) 27%, (4) 55%, (5) 83%, (6) 8W0, (7) 1ooo/0. (b) Kinetics of MC-8 polymerization in benzol. Dose rate 14 rad/s, room temperature. Content of MC-8 in solution: (1) 16%, (2) 24%, (3) 16% (in acetone), (4) 43%, (5) 65%, (6) 83%, (7) 100%.
are nearly the same as those of a pure MC-8 sample. The difference between the polymerization rates of MC-8 solutions in benzol and acetone of the same concentration [cf. curves 1 and 3, Fig. 12(b)] is due mainly to the different ra- diation yields of the active centers that initiate polymerization.
The concentration dependence of the MC-8 polymerization rate appeared to be unusual. Figure 13 presents the dependence of the reduced rate W, = W/(1 - r) ( W is the polymerization rate, r is the conversion of monomer) on MC-8 solution concentrations at various conversion degrees. For small degrees of conversion a decrease in the polymerization rate is observed with an increase in the solvent percentage of 20-30%. This sharp drop in the polymerization rate with the small amounts of the solvent available can be explained neither by a decrease in the initiation rate nor by a decrease in the monomer concentration. We may assume that the observed polymerization rate drop is the result of the formation of a loose, three-dimensional, netlike structure during solution poly-
CURING METHACRYLOYLOXYALKYL CARBONATES 1413
P Y
Fig. 13. Dependence of radiation polymerization reduced rate of MC-8 on its concentration in acetonic solution a t different degrees of conversion: (0) 1%, (0) 5%, (a) lo%, (0 ) 20%, (A) 30%. Dose rate 1.3 rad/s, room temperature.
merization. It is due to this fact that the probability of bimolecular termination of the growing radicals on the first stages of the process increases; hence the re- duced polymerization rate decreases.
A second peculiarity is the invariability of the reduced rate as the concentration of acetone and benzol varies from 20 to 80%. This is likely to be connected with the fact that the reduced rate is determined within this concentration range not by the average concentration of MC-8, but by its local concentration in the vi- cinity of the growing active center. This conclusion allows us to assume that the reduced rate is restricted by the diffusive supply of the monomer to the growing active center even in the earliest process stages (ca. 1% conversion).
The supposition that the looseness of the three-dimensional, netlike structure leads to an increase in the chain termination rate and, as a result, to a decrease in the reduced polymerization rate is confirmed by a change in the dependence of the process rate on the y-radiation intensity in passing from mass to solution polymerization. The mass polymerization of MC-8 and other oligomers of a similar type is characterized by the monomolecular termination of kinetic chains (the polymerization rate is proportional to the dose rate in the power n = 0.9-1.0). In the polymerization of MC-8 in acetonic and benzol solutions the value of n decreases to 0.4-0.5 for 14% solutions. This value of n remains invariable for conversion degrees to about 20%.
Thus in the earliest stages (ca. 1%) of the mass polymerization of MC-8 the growing active centers are strongly retained in the three-dimensional, netlike structure, thus permitting the monomolecular termination of kinetic chains. In solution polymerization, the mobility of the growing centers increases to an extent that makes bimolecular termination possible.
Postradiation Polymerization of MC Solutions
When cooled pure MC-8, like all the oligomers of this type, vitrifies completely. If a vitrified sample is being unfrozen its softening is observed at Tg = 210K [Fig. 1WI.
1414 BARKALOV, KIRYUKHIN, AND MUNIKHES
__- - _ _ _ - - - d \ * - - ‘.--- *
L UV j38 Ti3 20i 2‘23 A0 >55 ;70 280 290 295 3dox
Fig. 14. Calorimetric curves of unfreezing nonirradiated samples (solid lines) of pure MC-8 (a); 66% solutions of MC-8 in acetone (b); 61% solutions of MC-8 in acetone (c); 83% solutions of MC-8 in benzol (d), and irradiated (dotted lines). Dose 0.1 Mrad.
The addition of small amounts of acetone or benzol to MC-8 results in a re- duction of the devitrification temperature [Fig. 14(b)]. When the initial con- centration of acetone is increased the devitrification temperature continues to decrease and, starting from a 65% content of MC-8 in an acetonic solution after devitrification, an exothermal peak of crystallization and an endothermal peak of melting are observed on the calorimetric curve of “unfreezing” [Fig. 14(c)].
The postpolymerization of vitrified solutions of MC-8 in acetone [Fig. 14(a), (b), (c)] and benzol [Fig. 14(d)] irradiated at 77°K is observed immediately after the system has passed from a vitreous to a supercooled liquid state. The initial portions of the calorimetric postpolymerization curves are satisfactorily rectified in the Arrhenius coordinates (lgW, l/T). By decreasing the oligomer content in the solution the postpolymerization is shifted to lower temperatures and the effective activation energy decreases from 20 f 2 kcal/mol for pure MC-8 to 6 f 1 kcal/mol for a 66% solution of MC-8 in acetone. Thus the introduction of a solvent leads to a greater mobility in the system. It is this rise in mobility that causes a reduction of the devitrification temperature and effective activation energy.
Hence the radiation polymerization of MC-8 solutions has the following pe- culiar features:
(1) A sufficiently loose, three-dimensional, netlike structure, formed in the early stages of the process, leads to an increase in the rate of termination and a decrease in the polymerization rate. In later stages of the process the limiting value of double-bond conversion becomes larger than in mass polymerization.
(2) After the three-dimensional, netlike structure (gel-fraction) has been formed, the kinetics of the process is determined by the local concentration of monomer in the globule and not by its average concentration in the solution.
CURING METHACRYLOYLOXYALKYL CARBONATES 1415
CONCLUSIONS
A new type of polymerizing oligomer-methacryloyloxyalkyl carbonates-is characterized by rather high initial rates of radiation polymerization. Even in the earliest stages of radiation curing the polymerization rate is proportional to the irradiation intensity in the power of 1, which proves the predominance of monomolecular termination.
In postradiation curing of frozen monomers the polymerization occurs in the region of devitrification of samples.
Even with small doses of preliminary irradiation (0.050.1 Mrad) the initiation rate is so high that the inhibitory action of the dissolved oxygen is no longer ob- served.
In low-temperature radiolysis the radiation-chemical yield of radicals GR = 3 f 1 1/100 eV, and when unfreezing the system almost all stabilized radicals initiate polymer chains. Under these conditions the chains grow nearly without termination.
The inhibition of the reaction observed under isothermal conditions is asso- ciated neither with the “death” of active centers nor with the expenditure of the monomer but with an increase in activation barriers of translation of oligomeric molecules due to an increase in the conversion degree.
As distinct from mass polymerization, in solution polymerization of MC the mobility of growing chains increases so that biomolecular termination is possible and the limiting conversion of double bonds rises.
The rather small irradiation doses necessary for curing MC and the propor- tionality of the radiation polymerization rate to the dose rate make this type of oligomer valuable for various industrial applications.
References
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Received January 23,1981 Accepted September 13,1982