Kinetics of the radiation—induced polymerization of styrene in emulsion

Kinetics of the radiation—induced polymerization of styrene in emulsion

Kinetics of the Radiation-Induced Polymerization of Styrene in Emulsion V. STANNETT, H. SHIOTA, H. GARREAU, AND J. L. WILLIAMS Department of Chemical ...

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Kinetics of the Radiation-Induced Polymerization of Styrene in Emulsion V. STANNETT, H. SHIOTA, H. GARREAU, AND J. L. WILLIAMS Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27650 Received January 2, 1979; accepted February 2, 1979 The kinetics of the radiation-induced polymerization of styrene in emulsion have been reexamined in a system which obeys the simple Case II Smith-Ewart kinetics with potass!um Persulfate initiation. The discrepancies found with other radiation emulsion systems were found to disappear and close agreement with the idealized simple theory was found. The rate constants for propagation and the corresponding activation energy were found to be close to those reported in the literature. The termination rate constants were much smaller than the literature values for liquid systems but close to those reported for both chemicaland radiation-initiated emulsion systems. The lower values have been attributed to the higher viscosities encountered in the monomer-polymer particles. The efficiences of the radicals both in initiating polymerization and in creating the number of monomer-polymer particles were found to be in the range of 0.3-0.5. The reasons for the low efficiences are not clear but are consistent with results obtained by other investigators. INTRODUCTION

Research into the radiation-initiated emulsion polymerization of vinyl monomers has been underway since the early 1950s (1). The work has been motivated by both industrial and fundamental considerations. The high yield of radicals from the radiolysis of water combine with the high kinetic chain lengths to give excellent yields. There are other advantages. These have also been discussed elsewhere (2, 3). From the fundamental point of view radiation offers the possibility of an almost infinite range of radical flux rates which are independent of temperature. The majority of the radicals are neutral hydroxyl radicals or hydrogen atoms. In general, the kinetics of radiation emulsion polymerization appear to be different from those found with potassium persulfate-initiated systems. This is often true even in the case of styrene, a normally well-behaved monomer. However, the simple Case II Smith-Ewart kinetics (4)

are obeyed fully, even by styrene, only in a few systems notably with potassium persulfate as the initiator and with sodium lauryl sulfate as the emulsifier (5). It was thought of interest to study such an ideal standard system with radiation initiation. In this way any differences between the two methods of initiation would be clearly demonstrated. The present paper presents the results of such a study. MATERIALS AND METHODS

Styrene was obtained from Eastman Kodak Company and was 99.6% pure. It was distilled carefully under reduced pressure. After distillation, it was stored at -10°C. Immediately before use, it was repurified again by distillation under vacuum. The water used for the emulsion polymerization was once distilled in a Barnstead distillation unit and once more distilled in a glass distillation flask. Sodium lauryl sulfate was used, as sup130

0021-9797/79/100130-11 $02.00/0 Copyright © 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and lnterJaee Science, Vol. 71, No. 1, August 1979

KINETICS OF EMULSION POLYMERIZATION plied by the Fisher Scientific Company, without further purification.

Polymerization Procedure Pyrex glass ampoules of 20 mm o.d. and 140 mm length were used as the reaction vessels. The ampoules were filled with the required volume of freshly prepared sodium lauryl sulfate aqueous solution and purified styrene and then very carefully degassed by the f r e e z e - t h a w technique to a residual air pressure of approximately 10-5 Torr. After a final pumping cycle the ampoules were sealed and removed from the vacuum line. The ampoules were shaken vigorously before irradiation in order to ensure emulsification and were then irradiated in a " s w i m ming pool" source consisting of an array of cobalt-60 rods (doubly encapsulated in aluminum and stainless-steel sheaths) submerged in 10 ft of shielding water. The samples were placed in slots in a vaned Ferris wheel which was then enclosed in a water-tight aluminum canister. Details of this apparatus have been presented in an earlier publication (6). Agitation and temperature control were achieved by pumping water in a closed circuit from a thermostated bath through a nozzle on the inside of the canister lid. The jet of water issuing from the nozzle impinged upon the vanes of the wheel and so gave an endover-end agitation to the samples at a rate of approximately 6 rpm. Dosimetry was carried out using the Fricke method. At the end of a run the ampoules were taken from the source, opened, and the polymer isolated by coagulation in methanol followed by filtration, washing and drying in vacuum to constant weight. Conversion was calculated from the constant weight and the initial amount of styrene used. Errors due to posteffects were studied. Only negligible postpolymerization was found to occur within the time scales used. The polymerization tempera-

131

ture was 30°C except with the experiments specifically concerned with temperature effects. Unless otherwise stated the dose rate was 0.093 Mrad/hr.

Determination of Polymer Particle Size One drop of latex was diluted to 20-30 ppm polymer with distilled water. The specimen for the electron microscope was made by drying one drop of 20-30 ppm polymer concentration latex on a c o p p e r grid which was previously coated with a thin film of carbon deposited by vacuum evaporation. Magnifications were generally 50,000×. Polymer particle numbers were calculated from the "m ean volume diameter." The mean volume diameter is determined as follows: Mean volume diameter = (En.~D~/~nO1/3 where ni is the number of particles of diameter Di. Mean volume diameters are more appropriate. The electron microscope used was a Jeol JEM 120.

Determination of Polymer Molecular Weight The number average molecular weights, lVln, were determined from intrinsic viscosity measurements in benzene at 30°C. The relationship used for the calculation was (7) 19In = 1.67 × 105[~'/] 1"37.

Determination of G(R .) Value for Soap Aqueous Solution and Styrene-Saturated Aqueous Solution The d i p h e n y l p i c r y l h y d r a z y l (DPPH) method was used to follow the radical generation rates, by spectrophotometric measurements. The extinction coefficient e of the aqueous solution was first measured on a 1.3%, by weight, solution, at 520 nm. The e value was found equal to 9.28 x 103. In order to solubilize DPPH in water, a small amount of methanol was added to all aqueous solutions. The slopes of abJournal of Colloid and lnterfiTce Science, Vol. 71, No. 1, August 1979

132

S T A N N E T T ET A t .

sorbance-irradiation time curves were checked to show that they were independent of the methanol concentration. This showed that the presence of methanol did not interfere with the determination of G values for soap aqueous solutions at the low concentrations used, It was also found that the soap concentration does not change the slopes of absorbency-irradiation time curves. The same effects were observed in the case of styrene-saturated soap aqueous solution, although the slope itself was lower.

ioo

8 5o

!

~L,,~ ~ .

- 1.3 wt

~

RESULTS 10

Polymerizations at Constant Temperature Typical conversion curves are shown in Figs. 1-3 which show also the effect of varying the monomer-to-water ratio at three different emulsifier concentrations. The dependence of the rates on the emulsifier concentration and on the radiation intensity (dose rate) is shown in Figs. 4 and 5. The dependence of the number of particles on the emulsifier concentration is shown in Fig. 6. Although there was considerable scatter in the rate versus emulsifier data the dependencies of the rate data indicated close

20

30 TIME

40

50

(min)

FIG. 2. The effect of the monomer-to-water ratio on the polymerization rate. O, W/M = 3.85; A, W/M = 2.71; [3, W/M = 1.43.

agreement with the classical Case II SmithEwart kinetics, i.e., independence of the monomer-water ratio, a 0.4 dependence on the dose rate and an approximate 0.7 dependence on the emulsifier concentration. Since the number of particles were proportional to the 0.6 ___ 0.1 power of the emulsifier concentration, the rates are clearly essentially dependent on the number of

100

lOO

z

j ~U 5o

O.

20

40

60

TIME

80

100

(min)

Fio. 1. The effect of the monomer-to-water ratio on the polymerization rate. O, W/M --- 3.85; /X, W/M = 2.71. Journal of Colloid and Interface Science, Vol. 71, No. 1, August 1979

I

I

I

I

I

20

40

60

80

100

TIME (rain)

FIG. 3. The effects of the monomer-to-water ratio on the polymerization rate. Symbols as in Fig. 2.

133

KINETICS OF EMULSION POLYMERIZATION

o )¢ 20

-

I

o~ "lw

:/

e 0

E Z o

10

--

9

--

8

--

7

--

6

--

5

--

4

--

3

--

N

/

0

~

SLOPE -- O. 7

/~

L

I

I

I

I I I I

I

I

I

I

t

t t I

.4

.5

.6

.7 .8 .9 I

2

3

4

5

6

7

SOAP

CONCENTRATION

8

9

(wt,~)

FIC~. 4. The dependence of the rate on emulsifier content.

particles to the first power. The molecular weights of the polymers, calculated as number average degrees of polymerization, are shown as a function of conversion in Fig. 7 and of the emulsifier concentration in Fig. 8. The molecular weights were also found to be quite constant with conversion up to about 75% conversion and then began to decrease somewhat and were independent of the monomer-to-water ratios. The variation with the emulsifier concentration was to the power of 0.7. The degrees of polymerization are plotted versus the rates of polymerization in Fig. 9, a firstorder dependence was found.

Effect of Temperature Rate and molecular weight measurements were conducted at 14, 30, and 40°C. The results are presented as an Arrhenius plot in Fig. 10. The activation energies are calculated to be 7.9 __+0.6 kcal/mole for both quantities.

Free Radical Yield in the Aqueous Phase The G (radical) values for the radiolysis of water with 1.0% methanol added to solubilize the DPPH and extrapolated to zero methanol concentration together with the styrene-saturated aqueous emulsifier solutions are presented in Fig. 11 as a function of the added emulsifier concentration. The methanol had only a small effect on the G values but the presence of styrene reduced the values from about 6.2 to 5.2 radicals per 100 MeV. Presumably the lower yield is due to energy transfer to the dissolved styrene. DISCUSSION

Molecular Weights and the Efficiency of Initiation The kinetic orders observed with both the rates and degrees of polymerization are consistent within the experimental error with the S m i t h - E w a r t Case II mechanism Journal of Colloid and Interface Science, Vol. 71, No. 1, August 1979

,-.I

a,

\

Z

--4 0

N

~

O

m

10

20

3O

4O

50

60

7O

80

90

I .4

I .5

I .6

I

III RATE

.8 .9 1

DOSE

.7

(mrads/hr)

I x10 2

2

I 3

I 4

FIG. 5. The d e p e n d e n c e of t he rate on the dos e rate, 2.0% e m u l s i f i e r c o n c e n t r a t i o n .

I .3

I

.2

I 5

I 6

I 7

9

I I 8

,q

,..1

Z Z

4~

KINETICS OF EMULSIONPOLYMERIZATION

135

ul O I

E 10 w

c l

5.0

Z 3.0 U m 2.0 I-

~

.

6

L 1.0 |

.4

i

|

|

.6 SOAP

|

|

|

.8 1.0

|

2.0

CONCENTRATION

|

|

3.0

|

5.0

{wt. "/~ )

FIG. 6. T h e d e p e n d e n c e of the n u m b e r o f particles on the emulsifier concentrations. ©, W/M = 3.85; A, W/M = 2.71; IN, W/M = 1.43.

(4). Furthermore, the linear relationship between the rates and degrees of polymerization are in accord since little chain transfer to monomer is expected. In spite of a large number of careful and tedious experiments there still remain other experiments which should be conducted, in particular, seeded experiments. However, recent work in these laboratories with grafting of styrene to poly(vinylchloride) seeds indicates that simple Smith-Ewart kinetics do indeed hold with the sodium lauryl sulfate-emulsified systems (8). The independence of the molecular weights with conversion up to about 75% conversion is gratifying. This is consistent with recent work by James and Piirma (9) who have discussed the reasons for the conflicting data in the literature where increasing molecular weights with conversion were reported. Since Smith-Ewart Case II kinetics appear to be obeyed, and the radical yields i

have been determined, a theoretical degree of polymerization X n c a n be calculated kpN.

(M)

fen--

P

where k o is the rate constant for propagation, N is the number of polymer particles, (M) is the concentration of monomer in the particles, and p is the rate of radical generation in the aqueou s phase. The values of 2n calculated in this way are presented in Table I together with the experimental values. The ratio can be considered as a measure of the efficiency of the initiation step. These are included in Table I and vary between 0.33 and 0.55 in excellent agreement with the results of Van der Hoff found over a wide range Of conditions (10). The reasons for such low efficiencies are not clear and presumably reflect the efficiency of capture of the primary radicals by DPPH compared with the monomer-polymer parJournal of Colloid and lnterJace Science, Vol. 71, No. 1, August 1979

136

STANNETT ET AL.

'~'i

N

4.0 o

3.0

|

a.

o

An ~

O

(C)

~o

(B)

2.0 dX

(A)

(9 Q

l.O

I

20

I 40

PERCENT

I

I

60 CONVERSION

80

FIG. 7. Variation of the degree of polymerization with conversion. Water-to-monomer ratios, same symbols as Fig. 6. Emulsifier concentrations: (A) 0.67%; (B) 1.3%; (C) 2.0%.

ticles, or some termination in the aqueous phase.

Effect o f Emulsifier Concentrations on N and the Rate and Degree of Polymerization The Smith-Ewart theory of emulsion polymerization (4) gives a theoretical estimate of the number of particles N.

i p \0.4 N = 0.4(S.as)°.6~--~) where s is the emulsifier concentration in grams per cm z, as is the area occupied bY 1 g of soap (1.04 × 107 cm2/g for sodium lauryl sulfate), p is the rate of radical genera~ tion (8.2 × 101~ per ml of water, calculated from the experimentally determined G Value), and /x is the rate of increase in the volume of the particles with conversion. kp "(M) Vu / z - 2NA -, 1 ---~Vm Journal of Colloid and Interface Science, Vol. 71, No. 1, August 1979

where Vu is the volume of one monomer unit (1.15 x 10z cm3/mole) and V m is the volume fraction of monomer in the particles (0.634). The theoretical number of particles can be calculated and compared with the experimental values. These are presented in Table II, the ratio can be regarded as a measure of the efficiency of the radicals, formed by radiolysis of the aqueous phase, in forming particles. These are included in Table II and are again in the 0.32-0.55 range, consistent with the molecular weight data and with Van der Hoff's results (10).

Determination of the Termination Rate Constant kt Van der Hoff has shown (10) how, based on Stockmayer's equations (11), the termination rate constant kt can be calculated. He applied this approach to potassium

KINETICS OF EMULSION POLYMERIZATION

I

137

o w

z EO

4.0

_N

3.0

.=

2.0

O

G.

1.0 a

I

I

i

.6 SOAP

i

,

.8

,i

I

1.0

I

2.0

CONCENTRATION

I

3.0

(wt.~)

FIG. 8. Variation of degree of polymerization with the emulsifier concentration.

persulfate-initiated emulsion polymerizations of styrene. Later Vanderhoff et al. (12) carried out similar calculations with a similar but y-initiated system (11). In this work calculations were also carried out using the equation

kt = ( Zkp(_M) ) 2 1 \ 2. f'p' where Z is the subdivision value (1.11 was the average value found in Ref. 10), p' is the rate of radical generation in ap-

propriate units, and f is the efficiency of initiation. The equation is valid where, as is reasonable in this system, there is negligible transfer of radical activity from the particle. The values are presented in Table III. They vary from 3.8 to 8.8 x 103M -1 sec -1 with an average value of 5.9 x 103. These are in good agreement with Vanderhoffet al. (12) who obtained an average value 5.25 x 103. Van tier Hoff (10) with a persulfate-initiated system at 50°C found values ranging from 3.1 to 28.7 × 104. Hummel et al. (14) and

5 4

3

Q

@

=1) 0

1

N 3

4

5 6

10

RATE OF POLYMERIZATION

, 20 ( m o l e s / c c H20 - sec) x 10 - 7

FIG. 9. Variation of the degree of polymerization on the rates. Journal of Colloid and Interface Science, Vol. 71, No. 1, August 1979

138

STANNETT ET A t .

--

3O

0

'O

X A w

_20 Z

I

o. .I-

N 10

9

--

v

8

--

7

--

Z 0

6

--

5

--

4

--

3

--

2

--

0

E

-9

-

O

6

m

I,,I ,v

0 L

~ a

I

3.1

--

I

I

i

I

3.2

3.3

3.4

3.5

1/T

(1/°K)xI0

4

3.6

3

FIG. 10. Temperature dependence of the rates and degrees of polymerization.

Mezhirova et al. (15) also reported comparable values. The termination rate constants in liquid polymerizations are of the order of 107 M -1 sec -1. All the authors quoted, however, point out that kt should be much lower in the viscous latex particles. The kp values were calculated from the Case II SmithEwart rate equation: Rp = (N/2)kp(M) and are included in Table III. They varied from 32 to 68 M -1 sec -1 with an average value of 52 M -1 sec -1. This is in good agreement with the literature values (13). The efficiencies of initiation are all within the 0.2-0.5 range. It is interesting that similar values are found with potassium persulfate at 50°C and with ~/-initiated radicals at lower temperatures. It is possible Journal of Colloid and Interface Science, Vol, 71, No. 1, August 1979

that some recombination of radicals takes place before they enter the particles although other mechanisms may, of course, be operative.

Influence of Temperature The Arrhenius plots of both the rates and degrees of polymerization are presented in Fig. 10 and lead to activation energies of 7.9 ___ 0.6 kcal/mole. Since the activation energy for initiation is essentially zero for radiation initiation, the observed activation energy should, indeed, be close the Ep value for styrene. These are fully in accord with numerous literature values (13) for Ep for styrene.

139

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

o o

6 FI

O

5

i 0.5 SOAP

i

i

1.0

1.5

CONCENTRATION

(wt.

I 2.0 °/o)

FIG. 11. G (radical) values (DPPH method) as a function of the emulsifier concentration. O, Pure water (extrapolated from various methanol concentrations); A, 1% methanol in water; [2, styrene-saturated aqueous emulsifier solutions.

The number of particles decreases somewhat with increasing temperature. In a radiation initiated system (R .) is constant and kp increases with temperature. This would lead to a greater demand for emulsifier from the soap micelles resulting in a lower number of particles.

In conclusion it can be stated that the results of this investigation show that normal Smith-Ewart Case II kinetics appear to be followed in the radiation-initiated emulsion polymerization of styrene under condi-

TABLEI

T A B L E II

Efficiency of Initiation of Radicals Calculated from the Ratio of Experimental to Theoretical Degrees of Polymerization

Efficiency of Initiation of Radicals Calculated from the Ratio of the Experimental N u m b e r of Particles to the Calculated N u m b e r of Particles

Soap concentration ratio (wt%) 0.67 0.67 1.30 1.30 1.30 1.30 2.0 2.0 2.0

Experimental degree of polymerization (x 104) 1.57 1.65 2.49 2.56 2.50 2.54 2.90 3.10 2.98

Calculated degree of polymerization 5.3 5.6 1.36 1.36 1.04 1.04 1.43 1.28 1.29

x x x x x x x × x

10a 103 104 104 104 104 104 104 104

SUMMARY

Efficiency of initiation of radicals

Soap concentration (wt%)

Experimental: number of polymer particles(cmZ)-~ (× 101b

Calculated: number of polymer particles(cm3)-~ (x 10~)

Efficiency of initiation of radicals

0.37 0.33 0.55 0.53 0.42 0.41 0.49 0.41 0.43

0.67 0,67 1,30 1,30 1.30 1.30 2.0 2.0 2.0

2.72 2.12 3.27 2.97 3.38 2.59 4.22 3.06 3.42

5.02 5.02 7.25 7.25 7.25 7.25 9.55 9.55 9.55

0.55 0.42 0.45 0.41 0.47 0.36 0,44 0.32 0.36

Journal of Colloid and Interface Science, Vol. 71, No. 1, August 1979

140

Soap concentration (wt%) 0.67 0.69 1.30 1.30 1.30 1.30 2.0 2.0 2.0

STANNETT ET AL. TABLE III

REFERENCES

Calculated Values of the Propagation and Termination Constants

1. BaUantine, D. S., Brookhaven National Laboratory Report 294(T-50) (1954). 2. Stannett, V., Gervasi, J. A., Kearney, J. J., and Araki, K., J. Appl. Poly. Sci. 13, 1175 (1969). 3. Barriac, J., Knorr, R., Stahel, E. P., and Stannett, V., Amer. Chem. Soc. Symp. Set. 24, 142 (1976). 4. Smith, W. V,, and Ewart, R. H., J. Chem. Phys. 16, 592 (1948). 5. Smith, W. V., J. Amer. Chem. Soc. 70, 3695 (1948). 6. lshigure, K., O'Neill, T., Stahel, E. P., and Stannett, V., J. Macromol. Sci.-Chem. A8(2), 353 (1974). 7. Mayo, F. R., Gregg, R. A., and Matheson, M. S., J. Amer. Chem. Soc. 73, 1691 (1951). 8. Mitri, K., Stahel, E. P., Memetea, T., and Stannett, V., J. Macromol. Sci.-Chem. All(2), 337 (1977). 9. James, H. L., and Piirma, I., Amer. Chem. Soc. Symp. Ser. 24, 197 (1976). 10. Van der Hoff, B. M, E., J. Poly. Sci. 33, 487 (1958). 11. Stockmayer, W. H., J. Poly. Sci. 24, 314 (1957). 12. Vanderhoff, J. W., Bradford, E. B., Tarkowski, H. L., and Wilkinson, B. W., J. Poly. Sci. 50, 265 (1964). 13. Brandrup, J., and Immergut, E. H., "Polymer Handbook," 2nd. ed., I1-50, p. 452. John Wiley, New York, 1975. 14. Hummel, D., Ley, G., and Schneider, C., Amer. Chem. Soc. Adv. Chem. Ser. 34, 60 (1962). 15. Mezhirova, L. P., Yakovleva, M. K., Matveeva, A. V., Abekin, A. D., Khomikoskii, P. M., and Medvedev, S. S., Vysokomolek. Soedin 1, 68 (1959).

Water styrene volume ratio 3.85 2.71 2.71 2.71 1.43 1.43 2.71 1.43 1.43

kp (liter.mole-1. sec-~)

DP x 10-4

kt × 10-3 (liter.mole -t. sec-~)

32 40 62 68 40 60 51 60 56

1.57 1.65 2.49 2.56 2.50 2.54 2.92 3.10 2.98

5.2 7.3 7.7 8.8 4.2 7.0 3.8 5.0 6.4

tions where the potassium persulfate initiated also leads to such compliance. The efficiency of the radicals produced by the radiolysis of the aqueous phase, however, is only in the 0.3-0.5 range. The reasons for this are not clearly understood at this time but are consistent with results obtained by other workers both with radiation and with chemical initiation. ACKNOWLEDGMENT Thanks are expressed to the Division of Isotopes, U. S. Atomic Energy Commission for their partial support of this work.

Journal ofColloid and lnterfi~ceScience, Vol. 71. No. 1, August 1979