Features of emulsion polymerization of styrene USAing slightly soluble initiators

82~

P. ~E. |L~MENE~¢et at.

f r o m the values o f r l a n d r2 for a n unfilled system: t h e r e is a c e r t a i n t e n d e n c y t o w a r d s a d e c r e a s e in the relative activity o f styrene a n d a n i n c r e a s e in the relative activity o f A N . T h e results thus indicate t h a t the s t r u c t u r a l features o f the filler have a significant effect on the kinetics o f c o p o l y m e r i z a t i o n o f styrene a n d A N , a n d also in the relative r e a c t i v i t y o f the c o m o n o m e r s , as a result o f r e d i s t r i b u t i o n o f their local c o n c e n t r a t i o n s in the surface zones o f the solid particles. Translated by N. STANDEN REFERENCES 1. S. S. IVANCHEV, Radikalnaya polimerizatsiya (Radical Polymerization). Leningrad, 1985 2. S. S. IVANTSCHEV and A. W. DMITRENKO, Plaste und Kautschuk B32: 31, 1985 3. V. P. PRYANISHNIKOV, V. F. GUSYNIN, A. F. SOROKIN and K. L CHEPIZHNY, Russian Pat. 579246, ByulL Izobret. No. 41, 80, 1977 4. S. S. IVANCHEV, N. S. YENIKOLOPYAN, B. V. POLOZOV, A. A. SYROV, O. N. PRIMACHENKO and Z. N. POLYAKOV, Russian Pat. 787411, Byull. Izobret. No. 46, 97, 1980; Current West German Pat. No. 2852778; U.S. Pat. No. 421914; French Pat. No. 7834134; and Japanese Pat. No. 1029500. 5. S. S. IVANCHEV, A. V. DMITRENKO, V. A. DEMIDOVA and N. Ye. SHADRINA, Vysokotool. soyed. A28: 2095, 1986 (Translated in Polymer Sci. U.S.S.R. 28: 10, 2327, 1986) 6. Kh. S. BAGDASARYAN, Teoriya radikalnoi polimerizatsii (Theory of Radical Polymerization), Moscow, 1966

PolymerScienceU.S.S.R.Vol. 30, No. 4, pp. 826-834, 1988 Printed in Poland

0032-3950/88 $10.00+.00 © 1989PergamonPressplc

FEATURES OF EMULSION POLYMERIZATION OF STYRENE USING SLIGHTLY SOLUBLE INITIATORS * P. YE. [L'MENEV, G . I. LITVINENKO, V. A. KAMINSKII a n d 1. A. GRITSKOVA L. Ya. Karpov Physicochemical Research Institute M. V. Lomonosov Institute of Fine Chemical Technology, Moscow (Received 2 November 1986)

The emulsion polymerization of styrene in the presence of slightly soluble initiators is studied. A relation is observed between the polymerization rate and the initiator solubility in the aqueous phase. A mathematical model of the process is constructed for determining the concentration of the latex particles formed in the system. It is shown that the main contribution to particle formation is provided by the radicals arising on decomposition of the initiator dissolved in water. * Vysokomol. soyed. A30: No. 4, 814-820, 1988.

Emulsion polyfneri~tion of styrene

~2~

F_,MULSIOtqpolymerization (El') is a heterogeneous process differing from bulk polymerization by its high rate and the high MM of the polymer formed. The theory of EP was developed only for systems containing water soluble initiators. Polymerization takes place in the polymer-monomer particles (PMP), and is initiated when the radicals leave the aqueous phase. The polymerization rate w is given by the equation

dM w=" dt ---k~,[M]N~, -

(1)

where kp is the chain growth rate constant, [M] is the monomer concentration in the PMP, N is the number of particles in the system, n is the mean number of radicals in a single PMP. According to the Smith and Ewart theory [1, 2] the relation between N and the initial concentrations of initiators [1] and emulsifying agent IS] has the form

N®[II°"[S] °'6

(2)

The mean number of radicals in a particle [3] is Q

= ~-/o(a)H~(,~),

(3)

where a 2 = 16vp/koN; v = 4nRa/3 is the volume of PMP, and p = 2fkd [1] is the radical formation rate in water, ko is the chain breakage rate constant, I, (a) is a modified Bessel function of order v. In the case most characteristic of EP a <<1, it follows (3) that n = 0"5. In this limiting case the EP rate will not change during the process, until there is a free monomer in the system in the form of a droplet. Numerous workers have developed the theory of EP [4-6], and starting with various assumptions on the mechanism of PMP formation, have calculated w and the polymer MMD for systems initiated by a water soluble initiator. Oil soluble initiators are also used for EP initiation, for example azobisisobutyronitrile (AIBN), which is widely used in bulk polymerization. The EP initiation mechanism in such systems remains unexplained [5]. In spite of the fact that the main bulk of the initiator in this case is concentrated in monomer droplets, the EP rate, as a rule, is significantly higher than the bulk polymerization rate. The polymerization rate the dimensions of the latex particles, and the mean MM of the polymer formed are comparable with those attained with a water soluble initiator. If it is assumed that the radicals in PMP are formed in pairs on degradation of the initiator dissolved in the monomer, the mean number of radicals ~ is [7]

n=v

Pm ,Om P~tanh(v /~m~

(4)

where Pm= 2fkd [lira is the initiation rate in the monomer phase. As follows from (2) and (4), on increasing the PMP volume during polymerization the polymerization rate should increase. However, the kinetic conversion-time curve generally has a linear section corresponding to a constant polymerization rate. If the characteristic values p=, ko, and v are inserted in (4) it is found that ~ 0 . 5 , and consequently at specific initiation rates the EP rate in using an oil soluble initiator should be significantly lower than with a water soluble one. Furthermore, if it is assumed that the difference in EP in the presence of an oil soluble initiator from bulk polymerization consists only in the separation of the monomer phase into small droplets, a decrease of the mean degree of polymerization as well as the polymerization rate is to be expected. To remove these contradictions AI.Shabib and Dunn [8] proposed that all the radicals formed in initiator degradation i n the monomer pass into the water, after which initiation takes place just as in using a water soluble initiator. Almog and Levy [9] investigated the micro-suspension polymerization of styrene initiated with oil soluble initiators, and assuming the formation of polymolecular monomer to be a special feature of EP, proposed that the initiator solubility in water is fairly high,

828

p. yt~. IL'MI~NEVet al.

so that part of the monomer was polymerized by an emulsion mechanism because of the water solubility of the initiator. Since the initiation mechanism and the role 'of monomer droplets in EP using oil soluble initiators remains unexplained, it seems advisable to study the special features of EP initiated by such initiators. In this work the EP of styrene initiated by three oil soluble initiators differing significantly in their water solubility was studied. Pure styrene was treated with aqueous slkali to remove hydroquinone (stabilizer), washed with water to a neutral reaction, dried with calcined calcium chloride, and vacuum distilled twice. The grade E30 emulsifying agent (sodium alkyl sulphonate C15ZatSO3Na), a commercial product containing 92 ~ of the main substance was used without additional purification. The initiator [commercial (AJBN), was twice recrystaUized from methanol, and the benzoyl peroxide (BP) and lauryl peroxide (LP)] were twice reprecipitated by methanol from solution in chloroform. The inhibitor was pure sodium nitrite. Polymerization was carried out at 50°C. The solution of emulsifying agent and styrene was carefully degasscd under vacuum before loading into the dilatometer. The emulsion was obtained by means of a magnetic stirrer in the wide part of the dilatometer. The polymerization rate was determined by a dilatometric method. The MMD of polystyrene was determined by gel permeation chromatography, using "Knauer . . . . Waters" chromatographs, and "Chrompack" and "Waters" columns packed with/z-Styrogel of particle size 103, 104, 10 s and l0 s A. The chromatograms were treated on a "Spectra-Physics SP-4100" programmed integrator, using a calibration curve consisting of a three-degree polynomial. The water solubility of the initiator was determined by UV spectrometry on a "Specord UV-VIS" spectrophotometer. A s follows f r o m Fig. 1, with a l l the initiators t h e r e is a l i n e a r section, i.e. a c o n s t a n t r a t e section, o n the kinetic curves, w h e r e a s in a c c o r d a n c e with eqn. (4) t h e r a t e s h o u l d i n c r e a s e c o n s t a n t l y with increase in m o n o m e r conversion. T h e d a t a o f T a b l e 1 i n d i c a t e

TABLE 1. WATER

SOLUBILITY OF INITIATORS AT

20°C AND

POLYMERIZATION RATE

OF STYRENE IN BULK AND IN AN EMULSION IN USING THE THREE INITIATORS*

Initiator AIBN BP LP

Solubility, g/100 ml 0.036 0.00l 10 -4

Rate of bulk polymerization, ~o/min

EP rate t, ~o/min

0.03 0.02 0.03

0-75 0.12 0.05

* Initiator concentration 0.025 mole]l, of styrene. t Concentration of E-30 emulsifying agent 4 g/100 ml of aqueous physe.

the existence o f a n indirect c o n n e c t i o n b e t w e e n the i n i t i a t o r solubility in w a t e r a n d the E P rate. O n the o t h e r h a n d , the p o l y m e r i z a t i o n rates in b u l k a r e close for all free init i a t o r s (at the s a m e c o n c e n t r a t i o n s ) w h e r e a s the E P rates differ significantly. Thus, the E P r a t e in the p r e s e n c e o f LP, which h a s the lowest w a t e r solubility o f all three i n i t i a t o r s c o n s i d e r e d , exceeds the p o l y m e r i z a t i o n r a t e in b u l k o n l y b y a f a c t o r o f 1.5, w h e r e a s the p o l y m e r i z a t i o n r a t e in b u l k with D A A is a m e r e few p e r cent o f the E P rate. I n c o n t r a s t to E P i n i t i a t e d b y a w a t e r soluble initiator, when the M M D o f the p o l y s t y r e n e scarcely changes with conversion, in initiation o f E P with an oil soluble

l~mulslOnpolym~ization of styrene

8:~9

initiator the MMD is bimodal in character (Fig. 2) at low monomer conversions. The low molecular weight fraction with a maximum in the region of M -~ l0 s corresponds to the MMD obtained in bulk polymerization. The proportion of low molecular weight fraction is inversely related to the conversion, and on El) in the presence of AIBN, at a conversion close to 100 % the MMD of polystyrene hardly differs from the distribution

Y, %

~

I

Ctw• 105

8O 8

#0

#

__

"~"r

l

I

120

1

I

I

360 Time, rain

2

I

o.,

5 M,IO "6

Fio. 1 Fie. 2 Fro. 1. Styreneconversion as a function of time. Volumeratio of styrene/emulsifyingagent solution =1:2, emulsifyingagent E-30 [S]--4 g/100 ml, [I]=0.025 mole/i, of styrene. 1-AIBN, 2-BP, 3 - LP. F]o. . 2. MMD of polystyrene obtained by EP, initiated with AIBN [I]=0.025 mole/l, of styrene, emulsifyingaffcnt E - 3 0 , [SI = 4 g/100 ml, volumeratio of styr¢neto emulsifyingagent solution= 1 : 2. Polymer yield I (1) and 97 % (2).

obtained in using K2S20a [10]. In the case of EP initiated by BP and LP, the proportion of low molecular weight fraction remains significant until the end of polymerization. In order to explain the experimental results the possible variants in the initiation of EP in using oil soluble initiators will be considered. In a system with an oil soluble initiator the primary radicals are formed as a result of initiator decomposition in the monomer droplets, in the PMP, and in the emulsifying agent micelles, and also, to a slight extent in the aqueous phase. The part of the low molecular weight radicals containing several monomer units can leave the droplets, the PMP, and the micelles in the aqueous phase, after which these radicals can again be captured by other particles. The probability that a radical leaves a particle p is determined by the competition of three processes, i.e. growth, breakaway, and diffusion of radicals in a particle to its boundary:

p=~t(l--exp[--t,(tol+tl-1)]),,

where tr is the time during which

a primary radical joins m monomer units and becomes almost insoluble in water, t,=m/kp [M]; (1 < m < 10); to is the time for termination of two radicals, equal to V/ko; t~ is the lifetime of a radical in a particle, which is defined as tho characteristic time for decrease in concentration of the radicals in a particle of radius R. The quantity ti can

be determined by solving the differential expression C

t--DpAC1

0C 2 - dt - =DwAC 2

R~>r>0;

r> R

(5)

with the initial and boundary conditions

Cl(r , O ) = C o ,

C2(r , 0)--0,

aCt

Cx(R, t)=)~C2(R , t)

DwOC2

where Dp and D,, are the coefficients of diffusion of radicals in water and the particle respectively; 7 is the distribution factor for the distribution of the radicals between the monomeric and aqueous phases; C1 and 6'2 are the concentrations of radicals in the particle and in waterrespectively. When ?,/> 1, on solving the system (5) it is found that R2 t~--- --:- ~ :tD w

Table 2 shows the values of the characteristic times for droplets, micelles, and PMP. In these calculations it is assumed that for styrene kp= 100-200 1./mole'sec; k0= 107 l./mole.sec; [M]=10 mole/l, for the micelles and droplets, and [M]=5 mole/l, for TABLB 2. CHARACTERISTIC TIME tr, 1o, tl FOR MICELLES OF P M P , AND EMULSION DROPLETS* Particles MiceUe PMP Emulsion droplet

t,, sec

to, see

ti, sec

5 x (10-'~-10 -3) 10 - 3

10 - 5 10 - 3

10-5-10 -4 2.5 x ( 1 0 - 4 - 1 0 -3)

5 x ( 1 0 - ' ~ - 1 0 - 3)

x/pinko-

i

0.1-1

Diameters of micelles PMP, and droplets 0.01, 0.05 and 10/tm respectively.

PMP; Dz,=10 -7 cm2/sec; Dw= 10 -s cm2/sec; the diamaters of the micelle, PMP, and droplets are 100 A, 500 A, and 10/~m respectively. In the case of the radical distribution factor it is assumed, using the solubility of the initiators in water as a starting point, that ~-- 102-103. As follows from Table 2, the radical yield from monomer droplets and PMP can be ignored. The probability of a radical leaving a micelle in water is also small, but because of the high micelle concentration in the system the overall number of radicals leaving the micelles can be comparable with the number of radicals formed on initiator degradation in water. The proportion of radicals leaving the micelles is directly related to the initiator water solubility (decrease of ~). It also follows from Table 2 that because of the rapid chain termination, radicals formed on degradation of initiator in PMP can neither pass into the water nor form a polymer molecule of significant MM and consequently their contribution to polymerization initiation is almost zero. Initiation by the radicals decomposed in ~t particle will play a part only if the PMP diameter is Iess

Emulsion polymerization of styryne

831

than (ko/p,,,)ll6E 1000 A. Accordingly, the rate of EP initiated by an oil soluble initiator will be made up of the polymerization rate in droplets, which takes place in accordance with a homogeneous mechanism, and the rate of EP itself, taking place in the PIMP and initiated by radicals formed on decomposition of initiator soluble in water, and also, possibly, on decomposition of initiator in the micelles under conditions where one of the radicals passess into the water. The concentration N of PMP, formed in the system is using an oil soluble initiator will be calculated. It is assumed that the particles are formed only from emulsifying agent micelles (this is true in using an emulsifying agent such as E-30, which forms hardly any micro-emulsion when introduced into the aqueous phase). At the particle formation stage, when the particle dimensions are small, the rapid loss approximation applies; moreover, the particles will contain no more than one radical. Suppose f~(v, t) is the PMP concentration in a volume v containing i radicals (i= 0.1). The system of equations describing the formation and growth of particles has the form ~ = j ( v ) ( f t -fo)

aA

= j (v) ( f o - A ) -

o

(6)

+ [j(vo)(1 - p) + 2p (1 - p) p . Vo]

( v - Vo)

(7)

where j(v) is the diffusional stream of radicals from water into the particle; 0 is the rate of increase of PMP volume, which is determined from the conditions of equilibrium swelling of the particle in the monomer; 0=_am-- kpO= [2]; d~ and d, are the monomer dp 1 -Om and polymer densities respectively; ¢~= is the volume proportion of monomer in the PMP;/z is the micelle concentration; Vo is the micelle volume; and g(v-vo) is a Dirac function. The system (6)--(7) differs from the system of equations for EP with a water soluble initiator [11 ] in that the formation of PMP from micelles is determined not only by falling of the radicals into the micelles from the aqueous phase [first term in the square brackets of eqn. (7)], but also by degradation of the initiator directly in the micelle with consequent exit of one of the radicals into the aqueous phase (second term). The stream of radicals to the particle j depends on the radical concentration in water C2 and for different boundary conditions on the PMP surface can change from j =21Cz v I/a to 22 C2 v213 [12] (where 21 and 22 are constants). The radical concentration C2 is determined from the quasi-steady-state equation

dt - P w + [ p 2 + 2 p ( 1 - p ) ] p m V o l ~ - j ( v o ) # ( 1 - p ) -

f

j(v)(fo+fl)dv=O

(8)

In this equation Pw is the rate of initiator degradation in water, equal to Pm/Y;the second term describes the exit of radicals from the micelles, and the two last terms cover the absorption of radicals by the micelles and PMP. To determine the concentration of mioelles in the system/z, the traditional assumption should be made that the total area

832

P. Yg. IL'MENIgVet al.

occupied by the emulsifying agent on the micelle and PMP surfaces is constant, until micelles are present in the system, i.e. Vo = Vo +

(Yo+A) v / dv,

(9)

where Po is the initial micelle concentration. To solve the system of equations (6)--(9) the following dimensionless parameters and variables are introduced

~---IZ/go--l-K2/3

o~=YlZoVo

F~=f~ Vo/Po

Kn -- ~ (Fo + FI) y~dy

oD

1

0

y=v/Vo

>>1

z-~ 2pm Vot Assuming thatj,~v t/3 (as shown by Smith and Ewart [2] and Pismen and Kuchanov [11] the type of relation between j and the size of the PMP has only a weak effect on N in systems containing water soluble initiator). In the new notation of the system, (6), and (7) take the form

~Fo=yt/a ~ + p ( 2 - p ) ~ (Fx-Fo) ~ ~(1-p)+ KI/3 ~Fl+rOFl=yl/3 ~ + p ( 2 - p ) ~ ( F o _ F 1 ) + ~ ( 1

(10)

[" ~ + p ( 2 - p ) ~ +2 -]t~~

(11) The solutions of equations (10) and (11) for arbitrary values of ~ and ~t can only be obtained numerically. The limiting cases will be considered. If ~t-lp (2-p)<
S ~([qm/V) °'` Es] In the other limiting case, when the radical leaving the micelles (~t-lp(2-p)>>l) comprise the main part of the radicals in the water, the system of equations (10) and (11) will contain a single dimensionless parameter •, which is independent of the concentration of emulsifying agent. Consequently, in this case Fo and F~ are independent oo

Of Po, and,the total PMP concentration N=po J"(Fo +F1)dy will be directly proportional I

to the concentration of the emulsifying agent. The system of equations (10) and (11)

Emulsion polymerization of styrene

833

is simplified when the values of the parameters x / p ( 2 - p ) > > l are large. Analysis of the system as in [11], shows that in this case up to the end of the first stage the particles are formed at a constant rate, the length of the first stage rl is ~ . . . .

and the

concentration of PMP is N ~ [lq°'4. The experimental curves for w-[I] (Fig. 3) give

I/J */,/~i.

y,*,

0"8

80

(r0

90

t~OO 0"08 [I J , mole /l.of st~lrene

1 2

lz0

290 Time, rnin

FIo. 3 FIG. 4 Fie. 3. EP as a function of AIBN concentration (1), BP, (2), an LP (3) emulsifying agent E-30, [S]ffi4 g/100 ml, volume ratio of styrene to emulsifying agent= 1:2. Fie. 4. Effect of water soluble inhibition NaNO2 on the. kinetics of EP for styrene initiated with AIBN [1]. lI]=0.025 mole/l, of styrene, / - w i t h o u t inhibitor, 2-lNaNO~l=0.25 mole/I, of styrene.

a satisfactory description of the relation w,,, [I] °4, but this relation cannot be used to separate the contributions from the initiation mechanisms under consideration. To further test the role of the initiator dissolved in water in a polymer system containing AIBN the water soluble initiator sodium nitrite was introduced. The polymerization rate was at first close to that for polymerization in bulk, and the MM of the polymer produced was low just as after polymerization in bulk. After exhaustion of the initiator the rate increased to the values of the EP obtained without addition of inhibitors (Fig. 4). Accordingly, the experimental data and the calculations given confirm that in using oil soluble initiators of water solubility lower than that of AIBN the radicals arising on decomposition of the initiator dissolved in the aqueous phase play a main part. With decrease in solubility of the initiator in water, the contribution from polymerization in droplets to the overall polymerization rate increases. The results show that the use of oil soluble initiators or a mixture of these with water soluble initiators enables the M M D of the polymer obtained to be controlled over a wide range. Translated by N. STAND~N

834

G . M . BAStTENEVet aL

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

W. V. SMITH and R. H. EWART, J. Chem. Phys. 16: 592, 1948 W. V. SMITH and R. H, EWART, J. Amer. Soe. 70: 3695, 1948 W. H. STOCKMAYER, J. Polymer Sci. 24: 314, 1957 J. VANDERHOFF, J. Polymer Sci. 33: 487, 1958 S. S. IVANCHEV, Radikalnaya polimerizatsiya (Radical Polymerization). Leningrad, 1985 J. UGELSTAD and F. K. HANSEN, Rubber Chem. and Technol. 49: 536, 1976 N. HAWARD, J. Polymer Sci. 4: 273, 1949 W. A. G. AL-SHABIB and A. S. DUNN, Polymer 21: 49, 1980 Y. ALMOG and M. LEVY, J. Polymer Sci., Polymer Chem. Ed. 18: 1, 1980 S. V. ZHACHENKOV, G. I. LITVINENKO, V. A. KAMINSKII, P. Ye. IL'MENEV, A. V. PAVLOV, V. V. GURYANOVA, I. A. GRITSKOVA and A. N. PRAVEDNIKOV, Vysokomol. soyed. A27: 1249, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 6, 1400, 1985) 11. L. M. PISMEN and S. L KUCHANOV, Vysokomol. soyed. AI3: 1055, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1187, 1971) 12. V. I. LUKHOVITSKII, Vysokomol. soyed. A21: 319, 1979 (Translated in Polymer Sci. U,S.S.R. 21: 2, 349, 1979)

Polymer Science U.S.S.R. VoI. 30, No. 4, pp. 834-843, 1988 Printed in Poland

0032-3950/88 $10.00 +.00 © 1989 Pergan~on Press pie

RELAXATIONAL TRANSITIONS OF POLY(BUTADIENEACRYLONITRHJE) ABOVE THE GLASS TRANSITION TEMPERATURE* G. M . BARTENEV, S. V. BAGLYUK a n d V. V. TULINOVA Institute of Physical Chemistry, U.S.S.R. Academy of Sciences A. M. Gorky State Teaching Institute, Kicv (Received 3 November 1986)

Relaxation conditions in polybutadiene and its copolymers with acrylonitrile are studied by mechanical and structural relaxation methods over the temperature range - 170 to 450°C. Above Ts thirteen relaxional transitions of various nature are observed, most of which are associated with the butadiene component in the copolymers. The acrylonitrile component produces a relaxational transition nN, associated with the mobility of dipole-dipole crosslinks and affects the Twand the chemical degradation temperature. COPOLYMERS a r e n o t g e n e r a l l y c h a r a c t e r i z e d b y t h e p u r e l y statistical d i s t r i b u t i o n o f units in the p o l y m e r chain [1]. A c c o r d i n g l y , a g r e a t e r i n d i v i d u a l i t y o f the A a n d B units s h o u l d be m a n i f e s t e d in c o p o l y m e r s , especially in r e l a x a t i o n a l p h e n o m e n a , T h e

* Vysokomol. soyed.A~0: NO. 4, 821-828, 1988.