Study of the kinetic features of emulsion copolymerization of vinylidene fluoride with hexafluoropropylene

Polymer Science U.S.S.R. Vol. 25, No. 11, pp. 2714-2720, 1983 Printed in Poland

0032-3950[83 $10.00+.00 © 1984 Pergamon Press Ltd.

STUDY OF THE KINETIC FEATURES OF EMULSION COPOLYMERIZATION OF VINYLIDENE FLUORIDE WITH HEXAFLUOROPROPYLENE * S. S. I v A ~ c l ~ v , V. 13. BI)3)TOV, A. I. A~DREYEVA, G. A. OTI~DI~A a n d Y ~ . A. Z A ~ o m ~ K o Okhtinksky Production Department of "Plastpolimer" {Received 1 June 1982)

The kinetic features of the emulsion copolymerization of vinylidene fluoride with hexafluoropropylene have been studied both in the absence of and in the presence of a n emulsifier. I t has been shown that without emulsifier, at a small content of copolymer in the latex, the process occurs by a radical mechanism, in quasi-homogeneous conditions. The introduction of an emulsifier into the reaction medium, whilst not essentially changing the mechanism, permits principally emulsion polymerization to take place. The kinetic features of the system are mainly due to the heterophase-type process, the dispersivity of the system and the size of the interphase surface. The use of an emulsifier permits the process to be carried out at a high rate and the production of eopolymer of high MW whose polydispersivity is not changed. An increase in the amount of copolymer introduced does not cause an increase in the MW of the product, but only a broadening of its MWD, at the expense of high molecular weight fractions.

COPOLY~ERS of vinylidene fluoride (VDP) a n d h e x a f l u o r o p r o p y l e n e (HF13) w i t h v a r i o u s ratios of m o n o m e r units (from r u b b e r - l i k e p r o d u c t s to elastic plastics) p r e s e n t m u c h i n t e r e s t in v a r i o u s technical a r e a s a n d h a v e b e e n p r o d u c e d ind u s t r i a l l y for a c o m p a r a t i v e l y long t i m e [1, 2]. T h e y are p r e f e r a b l y m a d e b y emulsion p o l y m e r i z a t i o n [3]. Nevertheless, t h e r e is no i n f o r m a t i o n in t h e l i t e r a t u r e on t h e kinetic f e a t u r e s of the e o p o l y m e r i z a t i o n of V D F a n d H F P in emulsion, from the viewpoint of the dispersity of the latex product and the molecular p r o p e r t i e s of t h e e o p o l y m e r . I n t h e p r e s e n t work, t h e c o p o l y m e r i z a t i o n of V D F a n d H F P in emulsion, b o t h in t h e presence a n d t h e absence of a n emulsifier h a s b e e n studied. T h e d i s p e r s i v i t y of the l a t e x f o r m e d a n d t h e m o l e c u l a r characteristics of the c o p o l y mer, in relation to the p o l y m e r i z a t i o n conditions, h a v e b e e n d e t e r m i n e d . The eopolymerization of VDF with t t F P was carried out in an aqueous medium in presence of ammonium persulphate (AP) aud a perfluorinated emulsifier, ammonium porfluorononoate, by a method desoribed in the literature [3, 4], under 0.9 Pa pressure in a stainless * Vysokomol. soyed. A25: No. 11, 2335-2340, 1983. 2714

Emulsion copolymerization of VDF with I-IFP

2715

steel autoclave of ll. capacity, with stirring. The quantity of initiator was varied from 5 × 10-3 to 40 × 10 -3 mole/L, that of emulsifier from 0 to 45 × 10-a mole/l. The purity of the VDF was not less than 99.9 volume%, and of H F P not less than 99.5 volume ~o; the molar composition of the reaction mixture was V]:)F : H F P = 1.2 : 1. The composition of the VDF/HFP copolymer was checked by elemental analysis and by I~rMR. The latex particle size was determined with a UEMB-100 electron microscope, with 20,000 × magnification. The MWD of the VDF-H_FP eopolymer was determined by" gel chromatography [5]. The MWD analysis was made on a Waters GPC-501 chromatograph on four 600 mm long, 6 man. diameter columns, filled with Korasil Type B rigid gel. TI-LF was used as eluent, at 1 ml/minute, 25 ° and the solution concentrations did not exceed 0-32 g/dl. The gel-chromatography analysis showed that the MWDs of V D F - H F P copolymers, prepared both with and without an emulsifier are uni-modal. For the calculation of the MW and MWD of the copolymers, a universal calibration was used, which was obtained from eight Waters polystyrene standars. T h e Mark-Kutm-Houwink ratio for V D F - H F P fractions in T H F at 25 °, determined in the presen~ work, is expressed by the equation: It/]= 1"17 × 10-3 M °'s3 The MWs of V D F - H F P copols~mer fractions were determined using a Fuoss-Mead type osmometer.

Emulsion copolymerizatoi~ of VlPF with H F P without an emulsifier. F i g u r e 1 r e p r e s e n t s t h e kinetic curves a t v a r i o u s t e m p e r a t u r e s . F o r i n f o r m a t i o n on t h e effect of c o p o l y m e r a c c u m u l a t i o n in t h e r e a c t i o n m e d i u m a t a g i v e n stage o f t h e process, t h e e o p o l y m e r i z a t i o n w a s carried o u t till t h e l a t e x c o n t a i n e d 4 - 5 wt. ~o o f e o p o l y m e r . I t b e c a m e e v i d e n t t h a t in t h e t e m p e r a t u r e r a n g e studied, t h e higher t h e t e m p e r a t u r e , t h e sooner t h e process r a t e b e c a m e s t e a d y . T h e o v e r a l l a c t i v a t i o n e n e r g y was 3 8 ± 8 k J / m o l e , as d e t e r m i n e d f r o m these kinetic curves. A n increase in a m m o n i u m p e r s u l p h a t e c o n c e n t r a t i o n f r o m 5 > ( 1 0 - 3 t o 40 X 10 -s mole/1, f a c i l i t a t e d m o r e r a p i d a t t a i n m e n t o f t h e s t e a d y process r a t e . I t w a s found, f r o m t h e d e p e n d e n c e o f this r a t e on [AP], t h a t t h e o r d e r o f t h e reaction, in r e s p e c t o f t h e i n i t i a t o r is 0 . 5 ± 0 . 0 5 . On t h e basis o f results on a m m o n i u m p e r s u l p h a t e d e c o m p o s i t i o n r a t e in a q u e o u s solutions [6], a s s u m i n g t h e efficiency of A P initiation equals u n i t y , t h e v a l u e o f t h e r a t i o kp/k°'6 w a s c a l c u l a t e d for t h e e o p o l y m e r i z a t i o n o f V D F w i t h H ~ P . I n t h e calculation o f this value, t h e c o n c e n t r a t i o n o f t h e m o n o m e r w a s t a k e n e q u a l t o t h e c o n c e n t r a t i o n o f t h e m i x e d c o m o n o m e r s in t h e gaseous phase. T h e m a g n i t u d e o f ]¢p/k~"5 a t 60 ° was

3 - 5 × 1 0 -~.

E l e c t r o n m i c r o s c o p e studies show t h a t in this e o p o l y m e r i z a t i o n w i t h o u t a n emulsifier, a t small m o n o m e r conversions, l a t e x particles c h a r a c t e r i z e d b y a low degree o f dispersion in r e s p e c t o f dimensions, were f o r m e d . T h e p a r t i c l e d i a m e t e r w a s 0 - 2 5 ± 0 . 0 3 /~m. T h e g e l - c h r o m a t o g r a p h i c a n a l y s e s o f copolymers, p r e p a r e d u n d e r these conditions, i n d i c a t e t h a t V D F - H F P e o p o l y m e r s are c h a r a c t e r i z e d b y a b r o a d n n i - m o d a l M W I ) w i t h a v a l u e of/~w//14~---7 a n d 2tlz//l~w---4-5. As t e m p e r a t u r e

S . S . IVANuu.~v e~ al.

2716

increases, the MW of the copolymer is decreased. Thus at 55 °, ~J~-----150X10• and 75 ° it is 54 × 108. A kinetic study at high comonomer conversions (25 wt. % copolymer in the latex) showed t h a t the copolymerization rate remains constant only up to a definite conversion (ca. 6 w~. ~ ) , after which an increase in rate is observed (Fig. 2, curve 1). Moreover, the size of the latex particles is increased from 0.25#m (at a concentration of 5 ~ . ~/o) to 0.42 #m (at 25 wt. ~/o concentration).

Cc,~g

F 2

2#

x

_lo---T

18 8

0

I00

200 800 qO0 500 Time, min

FxG. 1

=

I

I

@

I0

lq

I 18

I 22

Cc,Wt%

FIG. 2

Fio. 1. Kinetic curves for eopolymerization of VI)F with HFP at 55 (1), 60 (2) medium, and 75° (3). [AP]-- 5.25 × 10-s mole/L, co--copolymer conoentration. Fio. 2. Relation of process rate on VI)F-HFP copolymer accumulation in reaction medium. Emulsifier concentration 0 (1) and 12.5 × 10-a molefl. (2). I n analysing the above regularities, the constancy of copolymer concentrations with polymerization time should be considered, also the poor water solubility of the comonomers [7] and the formation of primary radicals from the persulphate in the aqueous phase. By using the known ideas on particle nucleation mechanism [8] in polymerization without an emulsifier, one m a y assume the process goes b y the following route. Primary radicals formed in the water, b y adding molecules of monomer, form ollgomeric propagating radicals which on attaining critical sizes become agglomerated b y virtue of their insolubility and drop out of solution, forming polymer particles. Ultimately as these particles accumulate, together with the formation of new particles, oligomerie radicals are trapped b y the surface of the particles, where polymerization is thereby concentrated. The non-steady rate on the initial section of the kinetic copolymer accumulation curve (copolymer concentration in latex not exceeding 2 wt. %) must be connected with the appearance of new particles and with an appreciable increase in the surface when copolymerization occurs.

Emulsion copolymerizationof VI)F with HFP

2717

The steady part of the process indicates the retardation or complete cessation of new particle formation, when all the oligomerie radicals formed in the aqueous phase are trapped by the particles aIready formed. An increase in reaction rate with further growth of the copolymer concentration in the latex ( ~ 6 ~ ) is due to an increase in particle surface.

/~xld 15

10

w. !Of mdelL.

5, = 6

I 10

~1 ~ lq.

I 18

c¢, FI~. 3

I 22

t I0

I 20

30 "

#0

ce "10"a,rno/e/'/.. Fza. 4

~ o . 3. Dependence of MW of copolymer of VDF with HFP on extent of its accumulation in the reaction medium: ]--M~, 2--M~. I~G. 4. Dependence o£ process rate on emulsifierconcentration. l~igure 3 presents the relation of the MW values for V D P - H ~ P copolymer to degree of eopelymer accumulation in copolymerization without an emulsifier. I t is seen that as reaction proceeds, no increase in the mean MW is observed, indicating the constant probability of fracture of the t:inetic chain. At the same time, the ~w value grows more than 8 times. The growth in polydispersivity o f the polymer, linked with the increase in content of high molecular weight fraction is e~idently due to the fact that with an increase in degree of accumulation of copolymer in the propagating polymer-monomer particle, the probability o f chain transfer in the polymer grows, which may lead to the appearance and growth of branching of the macromolecular chains. There is information in the literature on the presence of branching in the chains of polyrinylfluoride [9] and VDFtetrafluoroethylene copolymer [10]. Data on the ability to vulcanize of a fluororubber [11] and also on grafting acrylates or vinyl derivatives in presence of peroxides [12] on to this copolymers are evidence for the possible appearance of branching in VDF-HI~P copolymer chains. However, to establish the real reasons for the growth of polydispersivity of the copolymer of VDF with HF1~ during the increase in conversion it is necessary to introduce additional methods for studying its properties in dilute solutions [5]. The copotygneriz~io~ of VDF with H F P i~ the l~resence of c6fl~oro-emulsifier. Figure 4 presents the dependence of the steady process rate on emulsifier con-

2718

S . S . IVA.~CH~V e~ al.

centration, which is varied from 0 to 45 × 10:2 mole/1. (copolymer concentration in latex, 5 wt. %). I n the emulsifier concentration range close to 6.7 × 10 -3 mole]l, being the critical concentration for micelle formation, an increase (within the limits of error) of the process rate is observed which usually is evidence for a micelle-forming mechanism and for particle growth. Such an insignificant change in the case of the copolymerization of VDF and H F P monomers which are polar and partly water-soluble, is probably determined by the fact t h a t the role of an emulsifier micelle is t h a t of a stabilizer for the embryonic, growing particles. I n fact, during the copolymerization, monomer-insoluble polymer is formed in an emulsifier micelle and the reaction proceeds in heterophase conditions, inside the micelle. Such a particle is identical with a latex particle, in the case of copolymerization in absence of an emulsifier, the difference consisting only in the monomer concentration at the particle surface. The course of the graph, depicting the relation of reaction rate to extent of copolymer build-up in the reaction medium (Fig. 2, curve 2) in the presence of an emulsifier, confirms this. We see t h a t t h e n a t u r e of the relation of ~ to copolymer concentration both without emulsifier (curve 1) and in its presence in the system (curve 2) is practically the same with the exception of the lesser extent of the initial section (at a monomer concentration in the latex of 2-3 wt. %). As already pointed out, the growth of the rate in this section is linked with the production and stabilization of the growing particles. The rate of this initial process is appreciably higher in the presence of a fluoro-emulsifier than in its absence. The fact that the emulsifier accelerates the process of forming growing particles, and increase their stability is confirmed b y electronographic studies of the dependence of latex particle size on emulsifier concentration (Fig. 5). As can be seen, the emulsifier molecules actively influence the stabilization of latex particles as the emulsifier concentration increases from 0 to 40 × 10 -3 molefl., the particle size is decreased from 0.25 to 0.05 ]~m. I t is interesting to note that a comparison of the steady reaction rate and latex particle size, in relation to change in emulsifier concentration, solves the problem of the site of the eopolymerization. Figure 6 gives the interlink between process rate co and the value of lfR, representing the surface/volume ratio of a latex particle. The existence of a linearly proportional relation between these values indicates t h a t reaction rate is proportional to particle surface. All the above facts (proportional relation between process rate ~o and particle surface, similar relations of process rate to extent of eomonomer conversion in eopolymerization without an emulsifier and with ce-~12.5×10 -s mole]l.) indicate that the kinetic rules of reaction are based on dispersivity of the system, the size of the interphase surface and the heterophase process. Figure 7 represents the dependence of MW on emulsifier concentration at a constant value of copolymer accumulation in the reaction medium, of 5 wt. %. As would be expected, at a small ce value, growth of MW is not generally seen; its later increase causes an inappreciable rise in the ~ and ~ w of the samples,

Emulsion eopolymerization of VDF with HFP

2719

not thereby changing the breadth of their MWD, in spite of the increase in process rate. With an emulsifier concentration increase of from 10× 10 -8 to 40×10 -3 mole/]., /~n and A~w are increased not more t h a n 1.5 times. O#CITt"I ZO 15

R ~Izm

0.25 ~

70

/ 0.051--

i

I0

t ~ ~ 20 30

#0

, to ,

0

ole/z.

I

t

I0

20

w, W s, rnole/l., sec

FxG. 5

FIG. 6

FxG. 5. Relation of mean latex particle radius on emulsifier concentration. FIG. 6. Dependence of process rate on value of I/R.

t~,lO'a I

IO00F

0

"

o

t"-"-r

I0

20

2

, .T

30 qO Ce, I0"J, mole/l.

FIG. 7. Relation of MW value of VD F-HFP eopolymer to emulsifier concentration. 1--M~, 2--Mw. In this way, the kinetic features of the emulsified copolymerization of VDF and H F P have been studied, both in the presence and absence of an emulsifier. I t was shown t h a t in the latter case, with a small copolymer content in the latex, the reaction proceeds by a radical mechanism, in quasi-homogeneous conditions. Introduction of a fluoro-emulsifier into the reaction medium which does not change the essential nature of the reaction mechanism, at the same time permits a preferred emulsion polymerization to be achieved. By using an emulsifier, the copolymerization can be carried out at a higher rate and a V D F - H F P copolymer of greater MW can be prepared, not depending on its polydispersivity, which is important in some cases to solve technical problems.

2720

S . S . I v ) . N o ~ v e~ a/.

The kinetic features of the emulsified polymerization of VDF with H F P are mainly explained b y the heterophase nature of the process, the dispersivity of the system and the size of the interphase surface..4, micellar mechanism does n o t play an essential par~ though it m a y be the reason for some increases in the MW. As the eopolymer accumulates in the reaction medium, an aubo-accelerating effect is observed, linked with the growth of the surface of the particles being formed. An increase in the amount of comonomer introduced leads not to an increase in the MW of the product b u t to a widening of its MWD, at the expanse of high molecular weight, possibly even branched fractions. B y carrying out a kinetic analysis of the VI)F-HI~P copolymerization in combination with GPC analyses of the copolymer obtained one can direct changes in their MW and MW]). Translated by C. W. CAPP REFERENCES

l. Ftorpolimory, (Fluoropolymers). p. 22, (ed. by L. Voll), Mir, Moscow, 1975 2. Yu. A. PANSHIN, S. G. MALKEVI•H and Ts. S. DUNAYEVSKAYA, Ftoroplasty (Fluoroplasts), p. 182, Khimiya, Leningrad, 1978 3. U.S. Pat. 3,051,677, 1962 4. U.S. Pat. 3,178,399, 1965 5. S. R. RAFIKOV, V. P. BUDTOV and Yu. B. MONAKOV, Vvedeniye v fiziko-khimiyu polimerov (Introduction to the Physical Chemistry of Polymers). p. 328, Nauka, Moscow, 1978 6. E. F. NOSOV, Zh. fiz. khimii 40: 2921, 1966 7. L Yo. VOLOKHONOVICH, E. F. NOSOV and L. B. ZORINA, Zh. fiz. khimii 40: 268, 1968 8. V. I. YELISEYEVA, S. S. IVANCHEV and S. I. KU6~AI~OV, Emul'sionnaya polimerizatsiya i yee primeneniye v promyshlenosti (Emulsion Polymerization and Its Application in Industry). Khimiya, Moscow, 1976 9. M. L. W,~LLACH and M. A. KABAWAMA, J. Polymer SoL 4: 2667, 1966 10. G. A. 0TRADINA, V. P. BUDTOV, N. A. DOMNI~HEVA, L. N. VESELOVSKAYA, S. G. M~L~EVICH, T. G. MAKEYENKO, Vysokomol. soyed. Bl1: 572, 1979 (No~ translated in Polymer Sci. U.S.S.R.) 11. F. A. GALIL-OGLY,A. S. NOVIKOV and Z. N. NUDEL'MAN, Ftorkauchuki i rezini na ikh osnovye (Fluoro.rubbers and Resins Based on Them). p. 114, Khimiya, Moscow, 1966 12. U.S. Pa~. 1,137,555, Chem. Abstr. 58: 811, 1963