photopolymerization of butyl acrylate microemulsions 1. post-polymerization

Polymer International 40 (1996) 41-49

Photopolymerization of Butyl Acrylate Microemulsions I Post-polymerization

lgnac Capek

Polymer Institute, Slovak Academy of Sciences, Dubravska cesta 9, 809 34 Bratislava, Slovakia

(Received 6 July 1995; revised version received 21 November 1995; accepted 7 December 1995)

Abstract: The oil-in-water-type microemulsion polymerization of butyl acrylate initiated by UV light was investigated. The polymerization showed two non- stationary rate intervals with a short rise to a maximum. The rate of polymerization was found to be proportional to the 0.7th power of the incident light intensity. The number of radicals per particle was found to be much below 0.5. The initiating radicals are supposed to be formed by the decomposition of the excited (SDS/BA)* intermediates (micelles). Desorbed radicals were found to increase the number of new particles after the cessation of illumination. The number of polymer particles increased during the whole polymerization. The micellar mechanism was proved to apply to the present system. The dependence of the molecular weight, or the relative viscosity of the microemulsion versus conversion is described by a curve with a maximum.

Key words: microemulsion, photopolymerization, rate of polymerization, nucleation of particles, post-polymerization, desorbed radicals.

INTROD UCTION

Microemulsion polymerizations offer a convenient access to well-defined microlatex particles being typi- cally one order of magnitude smaller than particles obtained by conventional emulsion polymerizations, polymers displaying high molecular weights and high rates. There was no apparent constant rate period and no gel effect in thermally and photoinitiated microemulsions which have been studied-see for example Refs 1-7.

The most significant difference between microemulsions (transparent or translucent) and macroemulsions lies in the fact that increasing the stirring rate or the emulsifier concentration usually improves the stability of macroemulsions. This is not the case with microemulsions, which appear to be dependent on specific interactions among the constituent molecules. If these interactions are not realized, neither increasing the stirring rate nor increasing the emulsifier concentration will produce a microemulsion. Once the conditions are right, spontaneous formation of microemulsion occurs and little mechanical work is required.

There are two possible nucleation mechanisms for butyl acrylate oil-in-water microemulsion polymerization using water soluble (1) nucleation in the microemulsion droplets (monomer-swollen micelles) by capture of radicals from the aqueous phase, and (2) homogeneous nucleation in the aqueous phase. The first nucleation mechanism (micellar) is found to be operative in these systems. In addition, coagulation (agglomeration) of initially formed (primary) polymer particles was found to be a significant mechanism con- tributing to the final number of particles.

In the case of oil-soluble initiators (or photoinitiators) several mechanisms for the production of radicals have been proposed :' (1) in the monomer-swollen polymer particles, with radicals formed desorbing to the continuous phase, (2) in the continuous phase, radicals are generated from the fraction of the oil-soluble initiator dissolved in water and (3) according to the suspension polymerization approach.

The model for the butyl acrylate oil-in-water microemulsion has been discussed in previous described as consisting of a monomer core phase, an interface (the droplet shell) and an aqueous phase. The

41 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain

42 Ignuc Cupek

interface is considered to be a monolayer of adsorbed butyl acrylate monomer (co-emulsifier) and emulsifier (SDS) molecules. Thus the BA monomer partitions between the core and shell of micelles, and the aqueous phase. It is accepted that the monomer concentration during the polymerization in different phases is determined by the thermodynamic equilibrium.

The partitioning of radicals (absorption and desorption events) plays an important role in determining the polymerization behaviour. In order to further clarify the role of desorbed radicals, the post-polymerization is investigated.

One particular advantage of a photopolymerization is that the initiating source can be removed from the system within a few seconds. Moreover, as for chemical initiation, the lifetime of the initiating free radicals produced by UV light in the reaction medium is usually of the order of microseconds. A second advantage of the method is that it permits experiments over a wider range of temperatures than is usually impossible with any chemical initiator.

The original contribution of this paper is the kinetics of the microemulsion polymerization of butyl acrylate initiated by UV light under steady illumination or illumination for varying initial periods only. The addition of photoinitiator was used to estimate the initiation efficiency of the (SDS/BA)* associate (micelle). Special emphasis is given to the post-polymerization interval. We follow the effects of reaction conditions on the kinetic, colloidal and molecular weight parameters of the microemulsion polymerization.

EX PER I M ENTAL

Materials

Commercially available butyl acrylate (BA) monomer was purified by the usual methods. Extra-pure grade 2,2'-azobisisobutyronitrile (AIBN) was used as supplied. The emulsifier used was the reagent-grade sodium dodecylsulfate (SDS, from Fluka). Twice-distilled water was used as the polymerization medium.

Polymerization procedure

Batch polymerizations were run at 23°C using UV light of wavelength 365nm. In all runs (with or without AIBN) the recipe comprised lOOg water, 20g SDS, l o g BA and 0.025 g NaHCO, .

Polymerization technique

Polymerization experiments were performed on an optical bench using monochromatic light of wavelength A = 365nm at 23°C. Equipment, measurement of radi- ation intensity and photopolymerization technique have been described elsewhere.12

Polymer and latex characterization

The polymerization technique and the measurements of particle size and number (by static and dynamic light scattering) were the same as described earlier.' 3 + 1 4 The particle size measured is that of monomer-swollen particles. The radius of monomer-unswollen particles (r,) was estimated from the relation between swollen (r,) and unswollen radii:

rs/yu = {dM/(dM - CP/MO)}

where dM is the density of monomer, C, is the monomer concentration in the particles (moldm-,) and M , is the molecular weight of monomer. The monomer concentration in the particles was estimated to decrease lin- early with conversion beyond c. 10%. Conversion of monomer was determined by dilatometric measurements (checked by gravimetry). Limiting viscosity numbers were measured in acetone at 25°C and used to estimate the viscosity-average molecular weights ([q] = 6.85 x I@:'75).15*16 The conductivity measurements and dialysis of latexes have been described previously.17,'

RESULTS AND DISCUSSION

Microem ulsion

Microemulsions of BA, SDS and water were found to be formed spontaneously. The optical transparency of the oil-in-water microemulsion consisting of SDS (034 mol dm-3), BA (0.6 mol dm-3) and water is much lower (c. 50%) than that (295%) of the individual com- ponents at A = 365 nm. The polymer microemulsions were stable and bluish, but less transparent than the original monomer microemulsions, due to the larger particle size and the higher refractive index of poly(buty1 acrylate). The intensity of UV light passing through the reaction mixture is c. one order of magnitude larger for the monomeric microemulsion than for the polymer microemulsion. The number of micelles in the SDS/water system with [SDS] = 0.54m0ldm-~ (average micelle diameter D = 4 nm, emulsifier aggregation number N , = 66)," is c. 8 x 10z1dm-3.

Rate of polymerization

Figure 1 shows the conversion-time data of the microemulsion polymerization of butyl acrylate (BA) initiated by UV light (A = 365nm), summarizing the results of microemulsion polymerizations proceeding under steady and intermittent irradiation. The conversion curves take on a shape similar to that for 'dead-end' polymerization.20 In all runs a limiting conversion appears and no gel effect is observed.

POLYMER INTERNATIONAL VOL. 40, NO. 1, 1996

Photopolymerization of butyl acrylate microemulsions 43

s r

100

a0 -

60 -

0 12 24 36 4 8 60

Time / min

Fig. 1. Variation of monomer conversion in the microemulsion polymerization of BA photoinitiated by UV light with polymerization mode and reaction time. Recipe: lOOg water, 20g SDS, l og BA, 0.025 g NaHCO, . I , = 8.92 x einstein dm-3 s-'. (A) Steady irradiation; initial period of

irradiation: (A) 10min; (0) 5min.

Variation in the rate of microemulsion polymerization with conversion and polymerization mode is shown in Fig. 2. The polymerization rate at different conversions (from Fig. 1) was determined by nonlinear least-squares regression analysis. This dependence shows two distinct non-stationary regions (intervals 1 and 3 according to the micellar modelZ1). The results show that the rate of polymerization is a function of both conversion and illumination.

The effect of the polymerization mode on the microemulsion polymerization of BA initiated by UV light was tested. The limiting conversion for the polymerization proceeding under steady illumination

- u! m 4 1 A A A A A A

A

A

A

A

0 0 16 32 4 8 6 4 ao

Conversion In YO

Fig. 2. Variation of the rate of polymerization in microemulsion polymerization of BA photoinitiated by UV light with irradiation time and conversion. (A) Steady irradiation; initial period of irradiation: (A) 10min; (0 ) 5min. Other con-

ditions, see legend for Fig. 1.

appears at c. 80% (see Fig. 1). In the polymerizations with irradiation for varying initial periods, the final conversion was much below 80%. In the latter case the limiting conversion was found to increase with the time of irradiation. The cessation of irradiation leads to an abrupt decrease in both the rate of polymerization and the final conversion.

The limiting conversion of c. 90% found for the microemulsion polymerization of BA initiated by 2,2- azobisisobutyronitrile (AIBN) or benzoyl peroxide (DBP)* was attributed to the cage effect and/or immobilization of initiator molecules in the polymer matrix. The high amount of free monomer at 70-80% conversion (photopolymerization under steady illumination) and low T, of polyBA (c. -46"C)16 disfavour the glassy state approach. This behaviour may be discussed in terms of the decrease in monomer concentration and consequently the rate of initiation when the BA itself acts as a radical precursor."

To get more information about the fate of radicals formed in the BA microemulsion the following approaches were discussed :

using the following data:

CJ + log R, ( m ~ l d m - ~ s - ' ) : 0.86, 0.68, 0.51, 0.44

CJ + log I , (einstein dm-3 s-'): 1.57, 1.37, 1.1, 0.95

was found to be 0.7, which deviates from 0.5 found for the photopolymerization of acrylates in solution." The reaction order a > 0.5 indicates that a first-order radical loss process is operative. This behaviour results from the presence of large molecular weight macroradicals or emulsifier (derived) radicals (see later), which are immobilized in the polymer particle phase, and which continue to propagate until mutual termination occurs when two radical ends grow close enough to react together.

(2a) The radical decay process was evaluated from the dependences of In R,, against time (first-order kinetics) and l/Rpp against time (second-order kinetics) (values taken from Fig. 1-the steady illumination run). The results showed that the first-order plot was linear whereas the second-order plot was curved. This behaviour indicates that the free-radical loss process is influ- enced by first-order kinetics.

(2b) The radicals take part in the following sequence of termination reactions:'3

(1) The reaction order, a, from R, cc

ki

A' - Bit (1)

(2) k2

A' + A' __j dead polymer

in which the free radical B;, is assumed to be relatively stable (entangled), A' is the fast decaying radical, k, is the unimolecular rate constant and k, is the bimolecular rate constant.


44 Iynac Capek

Variations of the free-radical concentration in polymer particles during the post-polymerization are summarized in Table 1. The values of [R'] were estimated from R , (during post-polymerization), the liter- ature value of k , (1360dm3mol-' s - ' ) ~ ~ and the equilibrium monomer concentration (4.4 mol dm- 3).1 By simulation of [R'] values using the following equa-

TABLE l . Variations of free-radical concentration in the microemulsion post-polymerization of butyl acrylate with reaction time and irradiation interval

Time [R;!" 10' [R;,] x lo8 (min)

(mol dm-3)

0 0.5 1 2 2.5 3 4.5 6 6.5 8.5 9

14 17.5 22 32.5 45 74 87.5

104 128 134 162

5.6" 5.01

3.94

2.4

1.86 1.17

0.65

0.374

0.1 92

0.1 39

0.4

4.1

3.44 2.62

2.22

1.85

0.835 0.64

0.42

0.225 0.21 4

0.25

0.01 3

a The initial conversion (after the cessation of illumination) is 16% and the final conversion is c . 48%. ' The initial conversion is c . 6% and the final conversion is c . 35%.

t i ~ n : ~ ~

t/CRXlo - CR'I,) = l/[R.Ii, 2 k2 + t/[R'lo, 2 (3)

where [R'l0 = [Relo, + [R'lo, 2 , 0 denotes the initial stage, t is the reaction time, [R'l0 is the total radical concentration, [R'l0, is the concentration of stable (entangled) radicals and [R'],,, is the concentration of fast decaying radicals, the values of different parameters were estimated (see Table 2).

It is interesting to compare values of the parameters [R'lo, [R'l0, 1, [R'l0, and k2 for two different runs (see Table 2); in run 1 the light was turned off at 6% conversion and in run 2 the light was turned off at 16% conversion. The final conversion of run 1 was 24% and that of run 2 was 48%. Note that run 1 contains only fast decaying radicals, [R'],,, . It is expected that increasing conversion decreases the monomer concentration in the polymer particles and increases the fraction of immobilized macroradicals. Indeed, in run 2 a fraction of entangled radicals ([R'l0, 1) appears. At the same time, the rate constant k 2 decreased by one-third. Both the decrease of monomer concentration in the particles and the increase in the molecular weight hinder the move- ment of chain segments required for the second-order free-radical decay.

(3) It is well known that radicals desorbed from polymer particles have a major influence on the kinetic and molecular weight parameters of microemulsion and macroemulsion polymerizations. The exit of radicals was supposed to increase with decreasing latex particle size.25

The termination of (desorbed and mobile) radicals in the continuous phase proceeds by a second-order decay in both the macroemulsion polymerization of unsatu- rated monomers'' and the microemulsion polymerization of styrene4 and butyl acrylate." The low viscosity of the reaction medium favours the instantane- ous termination events.

The chain-transfer to monomer and desorption of radicals dominate the mechanism of thermally initiated microemulsion polymerization of styrene4 and butyl

TABLE 2. Constants" for the decay of free radicals in the microemulsion polyBA particles at 24°C

Run [Re lo x 10' [R.l0, x 1 O8 [R' lo. 2 x 10' k , x (dm3 mol-'s-')

(mol dm-3)

1 4.2 0 4.2 9.45 2 5.6 0.5 5.1 6.41

a [R' l0 is the initial total radical concentration, [Relo , , is the initial concentration of slowly decaying radicals, [R'l0, , is the initial concentration of fast decaying species and k , is the second-order constant for the fast and slow reactions, respectively.


Photopolymerization of butyl acrylate microemulsions

A

A

45

A

, A

n 5 6 - A A A

A A

-

A A A A 4 8 - A A -

acrylate.'O The large number of micelles favours the re- entry of desorbed radical^.^."

In the photorun there is no water-soluble initiator in the aqueous phase (without AIBN). This favours the importance of desorbed radicals in the initiation and nucleation events. Indeed, it was observed that the nucleation of new particles proceeded during the post- polymerization, i.e. the number of particles increased after the cessation of illumination. For example, the number of particles increased from c. 0-3 x 10'Sdm-3 (after the cessation of illumination at 16% conversion) to 0.6 x 10'' dm-3 (c. 50% final conversion) (see Fig. 2, run (A)). In the polymerization under steady illumination the number of particles at 50% conversion was observed to be 1.0 x 101Bdm-3 (Fig. 3). These results indicate that the nucleation of particles during post- polymerization is most important. The initiating radicals formed by the decomposition of excited intermediates disappear shortly after the cessation of

0.6

0.3

, A

n 5 6 - A A A

A A

-

A A A A 4 8 - A A -

0.6

0.3

A

m E D ,

m c

0 7

Z

illumination, which means that the nucleation events are performed by the desorbed radicals (re-entry).

(4) If the rates of polymerization are divided by the number of particles and the appropriate constants, the average number of radicals per particle, Q, can be estimated. The values used were R,, max, N at R,, max, k , (BA) = 1360dm3mol-' s-1,24 and [BA],, = 4.4mol dm-3.'5 The estimated values for Q summarized in Table 3 are much lower than 0.5 and slightly increase in the presence of AIBN. The very low value of Q is attributed to the high desorption rate of monomeric radicals generated by the transfer reaction to monomer inside the particles and the low entry rate of radicals to particles.26 The re-entry rate constant into micelles is c. 2-3 orders of magnitude larger than the rate constant for bimolecular termination in the aqueous p h a ~ e . ~ , ~ - " Thus, the radicals are captured by micelles (Nmicelles %

The rate per particle, R,, ( x lo2' mol particle-' s - I), taken as a semi-quantitative equivalent of Q, varies with conversion as follows:

N p o l ymer-particles).

15.0(0*8%) > 11.2(3.3) > 3.1(12.5) > 1.7(20)

> 1-2(25 > lol(27.5) > 0-75)(33) > 0-37(42)

> 0.25(46) > 0*07(57) (4)

where the values in brackets denote conversion. The values used to estimate R,, were R, , N , k , . (1360dm3mol-' s - l ) and [BA] (found to decrease beyond 20-30% conversion). The decrease of Q with conversion can be attributed to the decrease in monomer concentration and/or the rate of initiation when the BA monomer itself acts as a radical precursor (see later).22

(5) The decrease in radical concentration is expected to proceed by both unimolecular (at very large conversions) and bimolecular termination. The entry rate of radicals to the particles is very low, which excludes the reaction between entered and 'original'

TABLE 3. Variations of kinetic and colloidal parameters of the microemulsion polymerization of BA initiated by AIBN"

[AIBN] x lo3 Rp, max x lo4 D (mol dm-3) (mol dm-3 s-' ) (nm)

b C

N x lo'* 0 (dm-3) (per particle)

b C

R Conversion ("/.I

b C

0 1 .o 2.0 4.0 5.0

4.1 61 48 4.7 60 48 6.0 58 47 8.0 56 45 8.9 54 44

0.29 1.4 0.1 4 0.28 1.46 0.1 7 0.34 1.6 0.1 8 0.35 1.8 0.23 0.4 1.9 0.22

1 .o 78 19 1.15 90 17 1.16 93 19 1.31 93 18 1.36 92 19

[SDS] = 0.54 mol dm-3, [BA] = 0.6 mol dm-3, I,, = 8.92 x 1 0-6 einstein dm-3 s-' A = 365 nm, 23°C.

At limiting conversion. At Rp, m a x .


46 Ignac Capek

radicals. Thus the radical activity of polymer particles is regulated by the chain-transfer events. The deactivation of radicals may also proceed during the agglomeration of nucleated micelles (primary particles).

According to the micellar theory," the relation between the rate of polymerization in interval 2 and the initiator concentration can be expressed by

R, K [initiator]" or R,, cc [ in i t ia t~r ]~ (5)

where x is 0.4 and y is very small or close to zero. The experimental values of the reaction order x = 0-4 and y = 0.1 (R, K [AIBN]0'4 and R,, cc [AIBNIO", evaluated from the results summarized in Table 3) obey eqn (5). The micellar mechanism, thus, is applied to butyl acrylate due to its low water solubility and the presence of large numbers of micelles. The initiated radicals (derived from AIBN) are located in both the aqueous phase and the monomer-swollen micelles during the whole polymerization. This favours the growth events even at high conversions (increased final (limiting) conversion) even though a certain AIBN fraction is immobilized in particles due to the cage effect.8

The specific interactions of (SDS/BA)*, which are responsible for the formation of radicals and initiation (without AIBN), are limited in the high conversion range. In this case the BA monomer concentrates (saturates the polymer) in polymer particles where it does not interact with SDS and therefore the formation of initiating radicals is depressed. The increase in final conversion by the addition of AIBN is due to the formation of initiating radicals at high conversion from the water-soluble fraction of the initiator.

Colloidal and molecular weight parameters

The average (monomer-swollen) particle size versus conversion plot (Fig. 3) shows that the average particle size decreases with conversion and the decrease is more pronounced in the range of low conversion.

The drastic increase in the size of the monomer- swollen polymer particles (c. 60 nm) at c. 5% conversion compared with the size of the original microemulsion droplets (5 nm) was experimentally detected by an increase in turbidity and decrease in the light intensity of the emerging light. Similar behaviour was observed in the thermally initiated microemulsion polymerization of BAS-" and ~ t y r e n e . ~ This drastic increase in size was attributed4 to the transport of monomer into polymer particles via diffusion from droplets and collision with microdroplets. The original microemulsion (colloidal stability, average particle size, . . .) changes with the conversion of monomer to polymer. The nature of the ther- modynamically stable microemulsion depends on specific interactions between emulsifier, co-emulsifier, water and m ~ n o m e r . ~ ' Slight changes in the structure of (co)emulsifier and the nature of the oil-phase

(conversion of monomer to polymer) turn the microemulsion into a mini- or microemulsion.

The average particle size variation (Fig. 3), however, is in good agreement with that simulated for the microemulsion polymerization of ~ t y r e n e . ~ Thus, the decrease in the average particle size with conversion is due to the redistribution of monomer to the newly formed polymer particles (continuous particle n~cleat ion) ,~ the conversion of monomer to polymer (volume shrinkage), and the 'immobilization' of emulsifier (bound to polymer) in the polymer particle phase.

In order to explain the effect of emulsifier immobilization on the particle size, the conductivity of microlatexes prepared with different initiation systems was followed.

30pS([AIBN] = 5 x 10-3moldm-3, 6OOC8-I 0 ) < 70ps([APS] = 5 x 10~3moldm-3 , 6OOC8-10 ) < 90 ps(initiated by UV light, this work) (6)

where APS is ammonium peroxodisulfate. As can be seen from these results and as expected, the conductivity increases in the direction from AIBN (carrying no charged units) to APS (with charged units). The microlatexes were prepared and cleaned under the same conditions (the same concentrations of BA and SDS, and the same dialysis time). In these runs the surface of (dialysed) polymer particles is covered with initiator (APS) or/and emulsifier (SDS) (bound to polymer) charged fragments. AIBN does not contribute to the surface charge density of polymer particles and therefore the conductivity (30pS) can be attributed to the SDS units bound to the polymer. Indeed, the SDS molecules were reported to take part in the chain-transfer events.28 The increased conductivity in the APS system results from the accumulation of negatively charged groups (derived from persulfate and bound to polymer) on the polymer particle surface. The highest conductivity, however, was observed in the microlatex prepared by photoinitiation. In this system the only charged molecules are SDS ones. Thus, the surface charge density of photo-produced particles should be attributed to SDS bound units. Here the initiating SDS radicals formed by the decomposition of excited SDS/BA intermediates ini- tiate the growth events. Similar results were obtained in Ref. 29 where the immobilization of emulsifier molecules decreased the size of particles and/or increased the stability of polymer particles.

The number of particles increases with conversion in the whole polymerization (Fig. 3). The increase of the particle number with conversion indicates that the monomer-swollen micelles are nucleated even at high conversion. This finding results from the very large number of micelles (Nmicelles dm-3) % Nparticles

A similar particle nucleation process was also found for the inverse microemulsion polymerization of acrylamide30 and the direct microemulsion poly-

(1018dm-3).


Photopolymerization of butyl acrylate microemulsions 47

merization of ~ t y r e n e . ~ However, there were some differ- ences in variations of the number of polymer chains per particle with conversion. (1) Only one polyacrylamide chain was found inside each particle at the end of the polymerization, which suggests that, once nucleated, particles do not capture more radical^.^' (2) The number of polystyrene chains inside each particle was found to increase with conversion, one polymer chain per particle at 2% conversion and c. 4 at the end of p~lymerization.~

In the present system (under steady illumination), the number of poly(buty1 acrylate) chains inside each particle was estimated to be c. 12 throughout the polymerization. This finding indicates that the capture of additional radicals by the polymer particles is negligi- ble. Thus, the agglomeration of primary particles (nucleated micelles) and the chain-transfer events are the governing processes in the regulation of particle size (number) and polymer molecular weight.

The monomer-swollen micelles, thus, compete with the polymer particles in capturing radicals. However, owing to the much larger surface area (2 or 3 orders of magnitude) provided by micelles, the radical flux to polymer particles is very small. Thus, the time required for each polymer particle to capture a second radical is greater than that of chain-transfer to m ~ n o m e r . ~ . ~ ~

Variations of the viscosity-average molecular weight and the specific viscosity with conversion and polymerization mode are summarized in Fig. 4. This plot shows that M , increases up to c. 18% conversion, then decreases and reaches a plateau at 70-80% conversion. It was reported" that the molecular weight of polyBA in the macroemulsion polymerization parallels the rate of polymerization, i.e. the dependence is described by a curve with a maximum at 45% conversion.

7 .O

6.0

Lp 9 5.0

15 4.0

3.0

2 .o

A-

-A ' A

A

A

A A A

A

A

L A A L

A A b

7.0

6.0

5.0

4.0

3.0

2.0

The

s a E -. x v)

cn

w ._

8 5

0 10 36 54 72 90

Conversion in YO

Fig. 4. Variation of the viscosity-average molecular weight (A) and the relative viscosity (A) of the microemulsion latex in the microemulsion polymerization of BA photoinitiated by UV light with conversion. Other conditions, see legend for

Fig. 1 (steady irradiation, run A)

decrease of M , beyond 50% conversion was attributed to the decrease of monomer concentration in the particles. A similar trend is observed in the microemulsion polymerization of BA where the monomer concentration in the particles starts to decrease already at 25% conversion. Here the maximum is shifted to the low conversion range (c. 25%). This behaviour supports the earlier results concerning the monomer distribution between the particles, micelles and water with conversion.4.8,24.28

The poiyBA molecular weights are c. one order of magnitude larger in the photoruns than in the emulsion polymerization' and several times those in the thermally initiated microemulsion polymerization (under the same conditions).8

This difference may be discussed in terms of the different contributions of the water-phase termination, the entry rate of radicals to particles and compartmental- ization. The largest polymer molecules, however, were formed in the post-polymerization runs. For example, M , increased from 4 x lo6 (cessation of irradiation at 4% conversion) to 7.7 x lo6 (24% final conversion). In the run under steady illumination, A, at 24% conversion was somewhat lower-5 x lo6. From this behaviour it appears that bimolecular termination (especially the water-phase) is more depressed in the photopolymerization. The addition of AIBN (5 x mol dm-3) decreased M , at 24% conversion to 1 x lo6. Thus, the radicals (mobile) in water as well in micelles derived from AIBN seem to decrease the growth events.

The variation of the relative viscosity of the polymer latex is a complex function of the particle/particle and/or particle/micelle interaction, the particle size distribution and the number of particles. The appearance of a strong maximum at 25% conversion cannot be ascribed to slight variations in particle size and number. It is known that interactions between micelles and particles vary with the chemical structure or internal organization of polymer, monomer and emulsifier in micelles or particle^.^' The increase of the fraction of polymer chains in the nucleated micelles (or primary particles) favours interparticle interactions (the clus- tering process). Under monomer-starved conditions, however, the viscosity decreases because of the poorly attractive interactions and highly rigid interface films. Thus, the polymer molecules may influence the organization of the globules in solution and their transport. The interfacial transport of monomer in the monomer- saturated globules is assumed to lead to aggregation by bridging with polymer molecules (the flexible surface emulsifier/monomer film). In contrast, the highly rigid interfacial emulsifier/polymer surface film does not favour interparticle interaction and/or the transport events.

The behaviour observed for the specific viscosity, the molecular weight and the polymerization rate (the

POLYMER INTERNATIONAL VOL. 40. NO. 1, 1996

48 Ignac Capek

dependence of R, , the specific viscosity and the molecular weight versus conversion is described by a curve with a maximum at certain conversion) results from variations of monomer in the different phases. Under monomer-saturated conditions (low conversion range) polymerization is very fast and the polymer is very large. On the other hand, under monomer-starved conditions polymerization is slow and the polymer formed is small.

Photoph ysics of radical formation

It was found that direct photolysis of BA monomer or SDS/BA in bulk or solution did not lead to the formation of polymer. Polymer was observed, however, during the photolysis of the BA/SDS microemulsion. Thus, the organized association of SDS and BA molecules (microemulsion droplets) initiates the formation of radicals and polymer under irradiation.

It was observed that SDS molecules do not absorb at 365nm. The absorptivity of BA at 365nm (in solution) was estimated to be c. 0-1dm3mol-’s-’. The formation of translucent BA microemulsions did not allow the study of spectral characteristics of ground or excited states of BA or BA/SDS. It is known that the slight interactions of SDS/BA are responsible for the formation and high stability of monomeric and polymeric microemulsions. Such ‘donor-acceptor’ interactions under illumination are supposed to release initiating radicals.”

In order to evaluate the initiating efficiency of (SDS/ BA)* associates (micelles), the effect of photoinitiator (AIBN) on the kinetic and colloidal parameters of the microemulsion polymerization of BA was investigated. In this case, the variations of R , , D, and N with initiator type and concentration were estimated and are shown in Table 3. These results show that the rate of polymerization, the rate per particle and the number of particles increase with increasing AIBN concentration. The increase in the rate per particle or the number of particles by the addition of AIBN (even with the largest concentration-5 x mol dm-3) is relatively slight. The ratio (R) of the R , with and without AIBN increases slightly above 1.0. R is 1.36 for the largest concentration of AIBN (5 x 10-3moldm-3). These results show that the (SDS/BA)* associate (micelle) is an efficient photoinitiator.

CONCLUSIONS

Polymerizations of translucent oil-in-water microemulsions of SDS/water/BA initiated by UV light produce stable and bluish-translucent microlatexes. A common feature of the BA microemulsion polymerization is the short rise to a maximum rate and the

two-rate intervals process. The initiating radicals are formed by the decay of excited SDS/BA micelles.

The formation of new particles during post- polymerization is direct proof of the desorbed (transferred) radicals formed. The decrease of the radical concentration with conversion (or during post- polymerization) is attributed to several deactivation events, such as collisions of growing (nucleated micelles) primary particles and premature polymer particles, the water-phase termination, . . . .

The dependences of the rate of polymerization, the molecular weight and the relative viscosity of microemulsion versus conversion are described by a curve with a maximum. The number of polymer particles increases in the whole polymerization as a result of continuous nucleation. This is due to the much larger number of monomer-swollen micelles compared with that of polymer particles. The decrease of average polymer particle size with conversion is discussed in terms of the redistribution of monomer to the newly formed polymer particles (continuous particle nucleation), the conversion of monomer to polymer (volume shrinkage) and the ‘immobilization’ of emulsifier (bound to polymer) in the polymer particle phase.

The very large molecular weights of poly BA result from the decreased bimolecular termination in both the aqueous phase and the polymer particles. The growth events in micropolymer particles are determined by chain-transfer events.

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POLYMER INTERNATIONAL VOL. 40. NO. 1, 1996

photopolymerization of butyl acrylate microemulsions 1. post-polymerization

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