The photoelastic effect in graft copolymers

1532

V. •. TSVETKOV and M. G. VITOVSKAYA REFERENCES

1. I. Ya. SLONIM, Uspekhi khimii 31: 609, 1962 2. C. W. WILSON and G. E. PAKE, J. Polymer Sci. 10: 503, 1953 3. I. Ya. SLONIM. A. N. LYUBIMOV and B. M. KOVARSKAYA, Chem. prumysl 13/38: 606, 1963 4. V. L. KARPOV, N. M. POMERANTSEV and N. M. SERGEYEV, Vysokomol. soyed. 5: 100, 1963 5. I. Ya. SLONIM, Ya. G. URMAN and A. G. KONOVALOV, Plast. massy, No. 5, 58, 1963 6. N. N. TIKHOMIROVA and V. V. VOYEVODSKII, Optika i spektroskopiya 7: 829, 1959 7. M. I. HERRING and J. A. S. SMITH, J. Chem. Soc., No. 1, 273, 1960 8. A. V. K E S S E N I K H , V. N. L U S H C H I K O V , A. A. MANENKOV and Yu. V. TARAN, Fiz. tverdogo tela 5: 443, 1963 9. T. M. S H A W and R. H. ELSKEN, J. Appl. Phys. 26: 313, 1955 10. A. N. LYUBIMOV, A. F. VARENIK and I. Ya. SLONIM, Zavod. lab. 28: 991, 1962 11. A. N. LYUBIMOV, A. S. NOVIKOV, F. A. GALIL-OGLY, A. V. GRIBACHEVA and A. F. VARENIK, Vysokomol. soyed. 3: 1511. 1961

THE PHOTOELASTIC EFFECT IN GRAFT COPOLYMERS* V. N . T S V E T K O V a n d M. G. V I T O V S K A Y A I n s t i t u t e of Maeromolecular Compounds, U.S.S.R. Academy of Sciences

(Received 20 July 1963) I N R E C E N T L Y p u b l i s h e d p a p e r s i t w a s s h o w n b y flow b i r e f r i n g e n c e m e a s u r e m e n t s t h a t t h e o p t i c a l p r o p e r t i e s o f g r a f t c o p o l y m e r s i n s o l u t i o n c a n differ m a r k e d l y f r o m t h e p r o p e r t i e s o f t h e i r c o m p o n e n t s [1, 2]. For the purpose of finding to what extent these molecular properties are preserved in the solid copolymer a study was made of certain graft copolymers. EXPERIMENTAL The materials studied wore three samples of polystyrene grafted on to polymethylmothacrylato (PS-PMMA) and two graft copolymors of polystyrene (side chain) and polyn-butylmothacrylato (main chain), t The samples differed in molecular weight and in the ratio of the components (see Table). * Vysokomol. soycd. 6: No. 8, 1387-1390, 1964. t The graft copolymers wore prepared in A. A. Korotkov's laboratory (Institute of Macromolecular Compounds) by S. P. Mitsengondlor and G. A. An(lroyeva by catalytic polymerization, and kindly presented to us for this work, for which we express our gratitude.

1533

Photoelastie effi~ct in graft eopolymers CHARACTERIST[(',S OF GRAFT C()POLYMI,]RS

Sample No.

M×103 main chain

Polymer

I'S-PMMA

1 2 3 4 5

31X10 a grafted chain

j

2 2 24 2 2

600 600 6OO 20O 60O

Ditto

PS-Pm-BMA Ditto

PSin i copolymer, i (.)('

ti

86 13 88 80 88

Fihns for the photoolasicity measurements woret prepared by evaporation of benzene solutions. They wore brittle and unsuffieiontly transparent., which greatly hindered the birofrin~eneo measurements. B y a m e t h o d described p r e v i o u s l y [3] the d e p e n d e n c e of the birefringence, An, on time r, was studied b y application a n d removal of a c o n s t a n t s t r e t c h i n g load. P, in the t e m p e r a t u r e regiou from mechanical softening to flow of the c o p o l ~ n e r s . I n the glassy s t a t e the optical i s o t r o p y of the films r e m a i n e d practically u n c h a n g e d on stretching. The interval corresponding to the high-elastic s t a t e is ve~- n a r r o w for these copolymers, the glass t e m p e r a t u r e (T,) a n d flow t e m p e r a t u r e (Tr) are v e r y close to one another. Figatres la and b show the f u n c t i o n An/P=~o(r), the time d e p e n d e n c e of the birefringence corresponding to unit s t r e t c h i n g stress for the PS-PMMA a n d P S - P - n - B M A copolymers respectively, a t certain t e m p e r a t u r e s .

A

a

A

,

b

"4 +2

°

"2 O,qemocalfiOA"-

tO0

tion . 2

-4

FI(;. 1. Time dependence of birefringence, An, corrc.~ponding to unit stretching stress, P: a--for PS-PMMA copolyrncr (sample l): 1--70°; 2--89::; 3--112~; /,--for P,q-P-,.BMA copolymor (sample 4): 1--83~; 2--95:~: 3--101 °. O r d i n a t e An/P × 101° (cm-O/dyno)(A). At t e m p e r a t u r e s close to the flow t e m p e r a t u r e (110-115 ~) a large, positive birefringence is observed in the fihns (Fig. 3) which slowly relaxes after removal of tlw h)ad, even over a period of sev(,ral hours.

1534

V. ~-N~'TSVETKOV . and M. G. VITOVSKAYA

In the region of the glass temperature the photoelastic effect is negative and close to the effect in pure polystyrene (curve 1, Fig. la). In the intermediate temperature region the relationship An/P=q~(T) has the typical form of curves 1 (Fig. la) and 1 and 2 (Fig. lb). On application of the load negative birefringence apperas almost instantaneously and this subsequently decreases as a result of increase in positive anisotropy (the rate of which is greater the higher the temperature) leading to change is sign of the cffcct. When the load is removcd the negative effect disappears instantaneously, whereas the positive effect relaxes slowly with time. The curves in Figures la and b are typical of all the specimens studicd, with thc cxception of curvcs 2. In these cases the time dcpendenees of An had the form characteristic of high-elastic, amorphous, linear polymers [4, 5]. The photoelastic cocfficient, E, was determined from the equilibrium values of An, and the temperature dependence of this is shown in Figure 2. For comparison this diagram also includes the corresponding curve for pure PMMA. DISCUSSION

Tile photoelastic properties of the graft eopolymers, described above, are associated with their particular molecular structure, and like the dynamooptical properties characteristic of such copolymcrs can serve as proof of the structure. In comparison with PMMA the polystyrene side chains have a large, negative anisotropy. This means that their maximum polarizability is in the direction normal to these chains, i.e. parallel to the main chain of the copolymer. Therefore as a whole the optical anisotropy of such macromolecules should be positive, as is observed in studies of the flow birefringcnce of solutions of these copolymers [1, 2]. Examination of the time dependence of birefringence in these graft copolymcrs shows that there are two mechanisms of molecular orientation during stretching, differing substantially in their relaxation times. 1. The negative birefringence is evidently associated predominantly with orientation along the direction of stretching of the units of the side chains of grafted polystyrcne (the inherent anisotropy of which is negative). This orientation has a short relaxation time. Figure 3 shows the temperature dependencc of the instantaneous, oricntational photoelastic effect (An/P)inst., for all the above graft copolymers. Despite considerable scatter the points lie on a curve that increases sharply irt absolute value at 90 °, which is close to T~ for polystyrene. This temperature practically coincides with the mechaifical softening temperature of PS-PMMA and PS-P-nBMA copolymers. 2. The second component of the birefringence of films of the graft copolymers in the high-elastic state and at temperatures close to the flow point is a positive

Photoela,stic effect in graft copolymer.~

1535

effect, t h a t increases with time and slowly disappears when the stretching load is removed. It is evident t h a t this effect is due to orientation of the main chain of the polymer as a whole, and has a long relaxation time. When t > T, free motion of segments of the main chain increases considerably. The orientation of these along the direction of stretching involves orientation of units of grafted polystyrene, situated close to the main chain, in the direction normal to the direction 60

20

700

t, o(-

0 +Ioo -7O

+50l

50

~-50

t, °C

0(2 ~b @c .g

-2"0 A

-100 FiG. 2.

FIG. 3.

FiG. 2. Temperature dependence of photoelastic coefficient: 1 - PS-PMMA graft, copolymer (sample 2); 2 -- atactic PMMA. FI(;. 3. Temperature dependence of instantaneous, negative photoela-stie effect, (An/P)inst.: a-- PS-PM'MA (Sample 1); b-- PS-PMMA (sample 3); c-- PS-P-n-BMA (sample 5); d--PS-P-n-BMA (sample 4). Ordinate-- (An/P)inst. × 101° (em2/dyne) (A). of stretching. Since the positive anisotropy of the graft copolymer molecule as a whole is m a n y times greater in absolute value than the negative anisotropy of the polystyrene (than its most anisotropic part), the positive photoelastic effect as it increases with time counterbalances the negative anisotropy t h a t arises initially and the film becomes positively anisotropic. As has already been stated above, the time dependence of the birefringeltce for films of sample 2 (which contains a comparatively small proportion of grafted polystyrene) is not of the same nature as t h a t described above. This is because the contribution of the polystyrene to the mechalfical properties of the copolymer is small in tiffs ease. However it is probable t h a t the two mechanisms of orientation of the macromolecules also apply to this copolymer. There existence is manifested in the considerable increase in the negative and positive values of the l)hotoelastic coefficient (Fig. 2) in comparison with the corresponding curve fbr pure l)olystyrene.

1536

V.N. TSVETKOVand M. G. VITOVSKAYA

Thus although the photoelastic properties of PS-PMMA and PS-P-n-BMA copolymers differ markedly from the photoelasticity of the components and does not correspond to a simple summation of the photoelasticity curves of the homopolymers neverthelcss some traces of the behaviour of the homopolymers is exhibited quite clearly in the orientation process. This fact has been reported several times in communications by Kargin and his collaborators [6-8] relating to the mechanical properties of and ordering processes in grafted structures. CONCLUSIONS

A study has been m ade of the photoelastic properties of copolymers of polystyrene grafted on to p o l y m e t h y l m e t h a c r y l a t e chains (PS-PMMA) and polystyrene grafted on to poly-n-butylmethacrylate (PS-P-n-BMA). I t was found t h a t the photoelastic properties of the graft copolymers differ markedly from those of the corresponding homopolymers. The birefringence observed on stretching films in the high-elastic state is the algebraic sum of negative and positive effects, which differ markedly in relaxation time. I t is suggested t h a t the first effect is the result of orientation of the units of the grafted polystyrene side chains and the second is the result of orientation of the copolymer molecule as a whole. Translated by E. O. I'HILLIPS REFERENCES 1. V. N. T S V E T K O V , S. Ya. M A G A R I K , S. I. KLENLN and V. Ye. ESKIN, Vysokomol.

soyed. 5: 3, 1963 2. V. N. TSVETKOV, S. I. KLENIN and S. Ya. MAGARIK, Vysokomol. soyed. 6: 400, 1964. 3. V. N. TSVETKOV and L. N. VERKHOTINA, Zh. tekh. fiz. 28: 97, 1958 4. V. N. TSVETKOV and M. G. VITOVSKAYA, Vysokomol. soyed. 3: 931, 1961 5. V. N. TSVETKOV and M. G. VITOVSKAYA, Vysokomol. soyed. 4: 577, 1962 6. V. A. KARGIN, P. V. KOZLOV, N. A. PLATI~and I. N. KONOREVA, Vysokomol. soyed. 1: 14, 1959 7. V. A. KARGIN, N. A. PLATI~ and Ye. P. REBLNDER, Vysokoraol. soyed. 1: 1547, 1959 8. N. A. PLATi~, V. P. SHIBAYEV, T. A. PATRIKEYEVA and V. A. KARGIN, Vysokomol. soycd. 3: 292, 1961