Effect of distances between three-dimensional network units on crystallization kinetics of vulcanized polysiloxane elastomers

EFFECT OF DISTANCES BETWEEN THREE-DIMENSIONAL NETWORK UNITS ON CRYSTALLIZATION KINETICS OF VULCANIZED POLYSILOXANE ELASTOMERS* V. Y u . LEVIN, V. A. MOSKALENKO, A. A . ZHDANOV, G. L . SLONIMSKII, K . A. ANDRIANOV a n d D. YA. TSVANKIN I n s t i t u t e of Hetero-Organic Compounds, U.S.S.R. A c a d e m y of Sciences (Received

1969)

5 September

W E HAVE s t u d i e d c r y s t a l l i z a t i o n k i n e t i c s o f t h r e e - d i m e n s i o n a l p o l y o r g a n o s i l o x a n e c l a s t o m e r s [1] a n d h a v e p o i n t e d o u t t h a t w i t h d i s t a n c e s b e t w e e n n e t w o r k u n i t s s m a l l e r t h a n 100 S i 0 ~ u n i t s , i.e. w i t h n ~ 1 0 0 a n o t i c e a b l e r e d u c t i o n is o b s e r v e d i n t h e r a t e o f c r y s t a l l i z a t i o n , r e s u l t i n g i n c e s s a t i o n o f c r y s t a l l i z a t i o n w h e n n _~ 40. It was of interest to investigate crystallization kinetics of some cross-linked p o l y o r g a n o s i l o x a n e s w i t h a w i d e r a n g e o f n v a l u e s ( f r o m 100 t o 3000). A polydimethylsiloxane (PDMS) elastomer, methylvinyl siloxane (MVS) elastomer. containing very large numbers of vinyl groups and a methylphenyl siloxane (MPS) elastomer, containing a sisnificant number of methylphenyl groups were investigated. The number of network units was altered b y the amount of dicumyl peroxide and calculated by swelling in benzene according to a well known formula [2]. A list of the elasotmers studied is given in Table ]. Crystallization kinetics were studied calorimetrically b y methods previously described [3]. Results obtained were calculated using the Avrami equation [4]. a = 1 --

e-k~m

where ~ is the proportion of material subjected to phase transformation b y time v; k is the rate constant of crystallization; m is the p a r a m e t e r which characterizes the type of growing structures. T A B L E 1. V U L C A N I Z E D POLYORGAI~OSILOXANE ELASTOMERS SELECTED FOR I N V E S T I G A T I O N

(Unvulcanized molecular weight 500,000) PDMS

MVS 2tlc

Mc

Mc *

240,000 60,000 30,000 19,000

MPS

3200 800 400 250

85,000 2500 1500 8000

1100 340 200 110

210,000 75,000 46,000 30,000

2800 1100 620 400

* M is the molecular weight of a pnlysiloxanechain segment between the units. * Vysokomol. soyed. A I 2 : No. 11, 2588-2592, 1970.

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V. Yu. LEvlN et al.

Typical crystallization isotherms are shown in Fig. la. These isotherms arranged in Avrami coordinates are shown in Fig. lb. Similar isotherms were obtained for all the polymers listed in Table 1. Parameters calculated from these isotherms, which characterize crystallization of various PDMS samples, are shown in Table 2. This Table indicates t h a t elastomers cross-linked with PDMS starting from n = 2 5 0 , crystallize not!ceably faster than an unvulcanized elastomer. With high n values the crystallization rate constant of the vulcanized elastomer exceeds the rate constant of the unvulcanized elastomer by approximately two orders of magnitude. Thus, for vulcanized PDMS (n = 3200) at --58 ° k = 178 × 10-3 and for unvulcanized PDMS ]c=2.11 × 10 -3 at the same temperature. Figure 2 shows the dependence of log /c on the degree of super-cooling for various PDMS samples. It should be noted t h a t the melting points of crystallized vulcanized PDMS (as with MVS) do not differ from the melting points of vulcanized elastomers,* i.e. the difference observed in rates of crystallization of vulcanized and vulcanized PDMS is not determined by the varying degree of supercooling. 0

(Z

/'0

08~i //~0/ ,,1.5/lo.¢r

-0.5 -I.0

0-5

-1.5

10

20 Time, rain

30

-2.0 loSEk (1-all

FIG. 1. Isotherms of crystallization of PDMS (a); same in Avrami coordinates (b) (n =400) 1-- --50, 2-- --52, 3-- --54, 4-- --56, 5-- --58, 6-- --60 °. Figure 3a shows the dependence of the time of semi-completion of crystallization T~ on the value of M c. This Figure indicates t h a t the rate of crystallization has a maximum (time of semi-completion of crystallization is minimum at certain (very high) n values. Thus, optimum conditions of crystallization are observed for very infrequent cross-links of polyorganosiloxane chains. A similar relation was derived for vulcanized and unvulcanized MVS. * The authors are grateful to Yu. K. Godovskii, who determined melting points calorimetrically.

Effect of distances between three-dimensional network units

2933

Figure 3b shows the d e p e n d e n c e of the t i m e o f semi-completion of crystallization on M c for MVS. As w i t h P D M S i n f r e q u e n t units b e t w e e n polysiloxane chains considerably increase t h e r a t e of crystallization. A s t u d y of crystallization kinetics of vulcanized and u n v u l c a n i z e d MPS revealed t h a t for this elastomer, (q 'k -!

-2

-3 20

f5

fO

FIG. 2. Dependence of log k on the degree of super-cooling for different PDMS samples: unvulcanized elastomer (•); n = 250 (2); 400 (3); 800 (4); 3000 (5). e v e n with v e r y high n values, a r e d u c t i o n t a k e s place in the r a t e of crystallization, c o m p a r e d w i t h the u n v u l c a n i z e d m e t h y l p h e n y l e l a s t o m e r (Fig. 3c). (We note t h a t , in this p o l y m e r , the r e g u l a r i t y of chain s t r u c t u r e is considerably d i s r u p t e d because of the p h e n y l groups, which in itself m a r k e d l y slows down crystallization

[5]). W e n o t e d p r e v i o u s l y [6] t h a t crystallization of siloxane rubbers m a y be c o n v e n t i o n a l l y divided into p r i m a r y a n d s e c o n d a r y crystallization. The caloriz/O 4,o

(2

I

b ?

20

20

[ :,

3

20 10

3o ,

2.5

5-0

o

2"5

5"0

0

2"5

5"0 Mc "I0 -5

FIG. 3. Dependence of the time of half completion of crystallization on the Mc value for PDMS (a), MVS (b) and MPS (c): a, b: 1 - --50, 2-- --52, 3-- --54 and 4 - --58°; c: 1-- 70, 2-- --72,: 3-- --74, 4-- --76 ° . m e t r i c s t u d y of a v e r y slow process of s e c o n d a r y crystallization involves difficulties, X - r a y diffraction was t h e r e f o r e used [7]. The d e p e n d e n c e of sample c r y s t a l l i n i t y on t e m p e r a t u r e a n d r e t e n t i o n time a t each t e m p e r a t u r e was studied.

E f l b c t of d i s t a n c e s b e t w e e n t h r e e - d i m e n s i o n a l n e t w o r k u n i t s

C~

ii ~

oO

oo

©

s

r~

,~

r~

.e

Z'

. . .I. .I. .~~ . .~ ~ ~~" ~. ~& 6

b

~1~s~L

....

2935

2936

V. Yu. LEVIN et aL

for primary crystallization. This is very logical since several other factors, as well as macromolecular mobility, are of considerable importance for secondary crystallization. It was observed previously [9] that the dependence of the time of semi-completion of polyethylene crystallization on molecular weight has a minimum. .As in our case, an increase in the degree of super-cooling here too obscures the minimum. The range of reduction of the T½ value is explained b y a possible reduction in the free energy of formation of nuclei AF on increasing molecular weight and an increase in v½ on further increasing molecular weight is attributed to higher viscosity. Thus, the appearance of a minimum in the dependence of r~ on molecular weight is explained b y the existence of competing processes of reduction in the free energy of nucleus formation and increase in viscosity. I t has been pointed out [10] that nucleus formation is the limiting stage of crystallization of polyorganosiloxane polymers and from this point of view it m a y be assumed that even in the case of transition from unvuleanized elastomers to vulcanized elastomers a reduction takes place in A F , i.e. the rate of crystallization increases and a reduction in the rate of crystallization on subsequently reducing n is due to increased viscosity. However, even for linear polyorganosiloxane elastomers, there is no information concerning the dependence of free energy of nucleus formation on molecular weight. For crosslinked polymers there is no information available concerning the effect of the distance between network units on the free energy of nucleus formation and on the viscosity of the system. Our hypothesis therefore appears probable, b u t cannot yet be confirmed. Finally, we would like to emphasize that the maximum observed in the rate of crystallization during vulcanization of regular polyorganosiloxane elastomers, apparently, depends on specific properties of this class of polymers [3]. CONCLUSIONS

A study of crystallization kinetics of some polyorganosiloxane elastomers shows that the dependence of crystallization rate constant of regular elastomers on the number of units has clearly defined maximum, situated in the region where there are very few units. For irregular polyorganosiloxane elastomers the existence of any number of units between the macromolecules invariably reduces only the rate of crystallization. Translated by E. SEMERE REFERENCES

1. V. Yu. LEVIN, Dissertation, 1967 2. P. J. FLORY and J. REHNEV, J. Chem. Phys. 2: 521, 1943; 18: 108, 1950 3. Yu. K. GODOVSKII, V. Yu. LEVIN, G. L. SLONLMSKII, A. A. ZHDANOV and K. A. ANDRIANOV, Vysokomol. soyed. All: 2444, 1969 (Translated in Polymer Sci. U.S.S.R. ll: 11, 2776, 1967) 4. MANDEL'KERN, Kristallizatsiya polimerov (Crystallization of Polymers). Izd. "Khimiya", 1966

Synthesis and study of polycarbamides

2937

5. K. A. ANDRIANOV, G. L. SLONIMSKII and V. Yu. LEVIN et al., Vysokomool. syed. A12: 1268, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 6, 1970) 6. K. A. ANDRIANOV, A. A. ZHDANOV, Yu. K. GODOVSKII, D. Ya. TSVANKIN and V. A. MOSKALENKO, Vysokomol. soyed. B12: 272, 1970 (Not translated in Polymer Sci. U.S.S.R.) 7. V. A. MOSKALENKO and D. Ya. TSVANKIN, Vysokomol. soyed. A l l : 383, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 2, 429, 1969) 8. V. A. MOSKALENKO, D. Ya. TSVANKIN and T. A. GALIL-OGLY, Vysokomol. soyed. A12: 548, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 3, 617, 1970) 9. L. MANDELKERN, J. G. FATOU and K. OHNO, J. Polymer Sci. B6: 615, 1968 10. G. L. SLONIMSKII and V. Yu. LEVIN, Vysokomol. soyed. 8: 11, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 11, 2139, 1966)

SYNTHESIS AND STUDY OF POLYCARBAMIDES CONTAINING A PHENANTHRIDINE RING* O. YA. FEDOTOVA, H . S. KOLESNIKOV (dec.), R . I. GRAMATIKATI and N. Yu.

SHAKUOVA

D. I. Mendeleyev Chemico-technological Institute, Moscow (Received 8 September1969 ) IT IS well known t h a t polyheterocyclic compounds, in which the macromolecules consist of alternating aromatic and heteroaromatie cycles, are of considerable theoretical importance and practical interest. A combination in these aromatic polymers of high heat resistance and other valuable physieo-chemical and mechanical properties makes t h e m of considerable value as materials used in m a n y branches of industry. Numerous papers have been published in recent years, both in experimental and review literature [1-5], dealing with polymers such as polyimides, polybenzoxazoles, polyoxadiazoles, polytriazoles and other polymers, containing five-membered hetero-cycles. Among polymers with six-membered hetero-cycles in the chain [6-8] polyquinazolones, polyquinazolindiones, polyphenanthridinylamides, polydiazapyrenilene alkyls (arlys). etc. have particularly high heat resistances. Hetero-cycles in these polymers are normally obtained as a result of polyheterocyelization, which does not facilitate complete cyclization. The purpose of our investigation was to synthesize and study different types of polymers with six-membered nitrogen-containing phenanthridine hetero-cycle in the chain 3

\N 115~,

/

10//~/ 8

* Vysokomol. soyed. A12: No. 11, 2593-2598, 1970.