Study of molecular mobility of some aromatic polyamides

STUDY OF MOLECULAR MOBILITY OF SOME AROMATIC POLYAMIDES* V. S. DOMKIN, G A. KUZICETSOV, N. I NIKIFOROV, T. YE. SHISHENKOVA a n d L. N FOME~KO /kll-lJmon Seienttfie Research Institute of Synthetic Resins

(Recewed 25 March 1974) The molecular moblhty of polymctaphenylene-mophthalamlde and its momerm copolymers was examined using the dmlectrm and mechanical loss method The existence of two relaxation processes was estabhshed for all these polymers. The high temperature process is due to segmental moblhty. Low temperature relaxation depends on the motion of amtde groups; this process determines the low brittle point of polymetaphenylene-lsophthalamide. IN VIEW of t h e i r high t h e r m a l s t a b i l i t y a r o m a t i c p o l y a m i d e s are of c o n s i d e r a b l e i n t e r e s t in v a r i o u s b r a n c h e s of i n d u s t r y . H o w e v e r , t h e c o n n e c t i o n b e t w e e n t h e i r p r o p e r t i e s a n d s t r u c t u r e , processing a n d u t i l i z a t i o n h a s n o t b e e n c o m p l e t e l y elucidated, a l t h o u g h t h e few p a p e r s p u b l i s h e d so far i n d i c a t e t h a t t h e p r o p e r ties o f t h e s e p o l y m e r s are s o m e w h a t unusual. T h u s , one of t h e few t h e r m a l l y stable, rigid chain p o l y m e r s u s e d for m a k i n g plastic p r o d u c t s , e g. p o l y - m p h e n y l e n e i s o p h t h a l a m i d e ( P P I P ) , h a v i n g high i m p a c t s t r e n g t h , [1] r e m a i n s p l a s t i c o v e r a wide r a n g e o f t e m p e r a t u r e (the difference b e t w e e n t h e b r i t t l e p o i n t a n d t h e glass t e m p e r a t u r e is 340°). I t is n a t u r a l to a s s u m e t h a t this c o m b i n a t i o n o f p r o p e r t i e s is due to m o l e c u l a r s t r u c t u r a l f e a t u r e s o f a r o m a t i c p o l y a m i d e s . T h i s p a p e r deals w i t h t h e m o l e c u l a r m o b i l i t y o f P P I P a n d its isomeric c o p o l y mers. P P I P produced by polycondensatlon of m-phenylenedlammc (m-PDA) and lsophthalyl dmhlorlde (ID) and eopolymers thus obtained were investigated m which, compared with P P I P , some groups m the recta-position (5-80% of the overall amount of acid, or amlde units) are replaced by groups m the para-posttmn. Monomer umts m the chain are arranged at random tks with P P I P , copolymers were obtained by low temperature emnlslou eopolycon. densatlon [2] Samples for testing were prepared by compression at a pressure of 500 kg/em t at 340~350 °. Molecular moblhty of polymers was studmd by a dynamm mechanical method and by relaxatmn of dipole polarizatton. Dynamm mechanical tests were carrmd out with a torsion balance [3] using the method of free damped oscfllatmns at a frequency of 1 c/s at temperatures of -- 190 - - -~ 3 2 0 °. Samples were rectangular strips of 1 × 4 × 50 m m A T P - 1 0 C * Vysokomol. soyed. AI7: No. 6, 1341-1345, 1975. 1541

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broadband bridge (Ando Electrm Co., Japan) was used to measure dzelectrm charactel~stms over the frequency range off----30-3 × 106 c/s and at --60 -- ~-150 ° The samples were discs 64 m m m dmmeter and 1 m m think. Electrodes were formed on the samples by hob vacuum spraying of mlver. In addltmn, ultrasonm absorptmn was deterrmned m P P I P at temperatures of 20-350 ° at a frequency of 2 × 106 c/s by methods prewously described [4]. Glass temperature Tg was determined thermomechanlcally using apparatus prevzously descmbed [5]. fun o~ O'f#

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Fzo 1. Relatzon between dynamm mechamcal loss and temperature for P P I P copolymers containing 15 (1), 40 (2) and 80% p-PDA lmlts (3) at a frequency of 0.7c/s. T w o r e l a x a t i o n processes a r e o b s e r v e d in a r o m a t i c c o p o l y m e r s , as w i t h P P I P [3], using t h e d y n a m i c a n d m e c h a n i c a l loss m e t h o d o v e r t h e t e m p e r a t u r e r a n g e s t u d i e d (Fig. 1). T h e h i g h t e m p e r a t u r e process is o b s e r v e d in t h e glass t e m p e r a t u r e range. I t c a n be seen t h a t t h e p o s i t i o n o f m e c h a n i c a l loss m a x i m u m t a n ei on t h e t e m p e r a t u r e scale v a r i e s for c o p o l y m e r s w i t h different s t r u c t u r e s o f m o n o m e r units, w h i c h is c o r r e l a t e d w i t h t h e v a r i a t i o n o f Tg on c h a n g i n g t h e c o m p o s i t i o n o f c o p o l y a m i d e s (Fig. 2, c u r v e 1). As s h o w n b y Fig. 2, a n increase in t h e n u m b e r o f isomeric Tara-groups in c o p o l y a m i d e s p r o b a b l y r e s u l t s in a n increase o f Tg f r o m 270 to 310 ° as a r e s u l t o f higher m a c r o m o l e c u l a r rigid i t y . T h e a v e r a g e a c t i v a t i o n e n e r g y o f t h e h i g h t e m p e r a t u r e process, d e t e r -

Study of molecular mobd,ty of some aromatm polyamldes

1543

mined for P P I P from two frequencies (0.7 c/s) (dynamic mechanical method) and 2 × 106 c/s (ultrasonic method) appeared to be 155 kcal/mole. Such a high activation energy and the position of the high temperature maximum of mechanical loss in the glass transition range enables us to relate the high temperature relaxation process in P P I P and copolymers to the cooperative form of segmental motion It should be added that the size of segment m P P I P obtained b y viscosity measurement does not exceed 8-10 monomer units [6].

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FI~ 2 Relation between T s (1), relaxatmn time r (2) and act*ration energy AU (3) of the low temperature process and the content of p-PDA (pomts) or TD umts (trmngles) The second relaxation process was formed near --70 ° (at a frequency of 0.7 c/s). The position of mechanical loss maximum on the temperature scale is independent of copolymer composition, the maximum position being slightly affected b y the phase condition of copolymers, which is seen on comparing curves of copolyamides containing 15% p - P D A units (amorphous polymer) and 80% p - P D A units (crystalline polymer) (curves 1, 3 Fig. 1). When studying relaxation of dipole polarization in P P I P and copolymers a low temperature process was only observed (Fig. 3), since at temperatures higher than 200 ° loss due to conductivity prevails. I t appeared that within the range of variation of experimental data relaxation time r is practically independent of copolymer composition (Fig. 2, curve 2). The dependence of relaxation time on temperature is descrlbed b y the Arrhenius equation ~=T0 exp (AU/kT) for all copolyamides. The activation energy A U determined from the linear dependence of log f on inverse temperature is also independent of copoly-

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mer composition and is 14 kcal/mole (Fig. 2, curve 3). Figure 4 shows that the relationship between the frequency of maximum loss fmax and inverse temperature is common under the action of both electrical and mechanical fields. This indicates that in both cases the relaxation process is probably related to the same group of atoms. The low activation energy and the independence of temperature for the low temperature relaxation process proves that a small group of atoms undergoes local motion in this case. fan c~ OOq

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FzG. 3. Relation between dmlectrm loss and temperature for PPIP copolymers containing 15 (1), 50 (2) and 80% p-PDA umts (3) at a frequency of l0 s c/s. According to the specific effect of dielectric relaxation, it is essential t h a t the kinetic unit has a group of atoms with a constant dipole moment. The plane amide group between two phenyl nuclei fulfils this condition in P P I P and its copolymers. Consequently, the kinetic unit which is responsible for the low temperature relaxation process observed in this class of polymers incorporates an amide group. For the final explanation of the size of kinetic unit it was essential to establish whether it contained phenyl nuclei. In the samples studied amide groups are combined with phenyl nuclei either in the meta or para-position, the number of meta and para-substituted phenyl nuclei varying in different copolymers. In those types of molecular motion, where phenyl nuclei definitely take part relaxation parameters of the kinetic unit depend on the composition of the copolymer (e.g. Tg, or the maximum temperature tan J of the high temperature relaxation process). For the low temperature relaxation process values of ~ and A U, as shown, are independent of copolymer composition. According to Frosini's results [7], the value of T of the low temperature process is the same for P P I P , for which all phenyl nuclei are in the recta-position and for poly-lo-phenylene tetephthalamide (PPTA),

Study of molecular mobdlty of some aromatm polyamldes

1545

(PPTA), for which phenyl nuclei are only in the para-position although T~ of P P I P and P P T A differ b y 250 ° (270 and 520 °, respectively [8]). I t should be noted t h a t our results for P P I P are in agreement with Frosini's results. Furthermore, the low temperature relaxation process observed for aliphatie [9] and partiaUy aromatm [17] polyamides (according to mechanical and dielectric studies) is linked precisely with the motion of amide groups. I t has the same kinetic parameters (r and A U) as those we obtained for the low temperature relaxation process of fully aromatic polyamides.

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FIG. 4. Logarithmic relation between the frequency of chelectrm (1) and meehamcal loss maxima of PPIP (2) and reverse temperature. Thus phenyl nuclei, their number and position had no effect on kinetic parameters of low temperature transition to polyamides of different structures. I t m a y be assumed that the low temperature relaxation process in P P I P and its isomer copolymers is due to torsional oscillations of a planar amide group between phenyl nuclei, which are not coplanar either with each other or with the plane of the amide group [10, 11]. In the polymers studied the amlde group is part of the main chain, therefore, satisfactory agreement is observed between results of mechanical and dielectric measurements. I t is assumed [12], that mobility of small atom groups incorporated in the main macromolecular chain at temperatures lower than Tg ensures low brittle

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V . S . DOMXl~ et al.

p o i n t s o f p o l y m e r s a n d t h e p o s s i b i l i t y o f r a p i d r e l a x a t i o n o f m e c h a n i c a l stress in t h e g l a s s y state. T h e l a t t e r c i r c u m s t a n c e d e t e r m i n e s s t r e n g t h p r o p e r t i e s a n d firstly r e s i s t a n c e t o i m p a c t stress. F r o m results o f this s t u d y it m a y b e a s s u m e d t h a t it is precisely t h e m o t i o n o f a m i d e g r o u p s a t t h e p o l y a m i d e s s t u d i e d a t t e m p e r a t u r e s higher t h a n - - 7 0 ° w h i c h e x p l a i n s t h e high s t r e n g t h (e.g. i m p a c t s t r e n g t h o f i n d i v i d u a l m e m b e r s o f this class o f p o l y m e r s reaches 200 k g c m / c m 2 ) .

Translated by E. SEM~RE REFERENCES

1. L. B. SOKOL0V, G. A. KUZNETSOV and L. N. FOMENKO, Spravochnik po plasticheskim massam (Handbook on Plastms). Izd. "Khimiya", 2, 1969 2 L . B . SOKOLOV and T. V. KUDIM, ]:)old. AN SSSR 154: 1139, 1964 3. G. A. KUZNETSOV, M. Ye. DLINNIKOV, N. I. NIKIFOR0V and V. A. VASIL'YEV, Plast. massy, No. 5, 70, 1970 4. G. A. KUZNETSOV, V. D. GERAS1MOV and L. B. SOKOLOV, Plast. massy, No. 4, 64, 1972 5. V. D. GERASIMOV, G. A. KUZNETSOV and L. N. FOMENKO, Zavodsk. lab. 29: 996, 1963 6. I. K. NEKRASOV, Vysokomo]. soyed. A13: 1707, 1971 (Translated m Polymer Sci. U.S.S.R. 13: 8, 1920, 1971) 7. V. FROSINI and E. BUTTA, J. Polymer Sei. BP: 253, 1971 8. G. A. KUZNETSOV, V. M. SAVINOV, L. B. SOKOLOV, V. K. BELYAKOV, A. I. MAKLAKOV and G. G. PIMENOV, Vysokomol. soyed. A l l : 1491, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 7, 1691, 1969) 9. V. A. BERNSHTEIN, N. A. KALININA and V. A. STEPANOV, Mekhamka polimerov, 919, 1972 10. M. G. NORTHOLT and J. J. VAN AARTSEN, J. Polymer Scl. B l l : 333, 1973 11. H. HERLINGER, P. P. H~RNER, F. DRUSCH]KE, H. KN~LL and H. FRIEDEMANN, Angew. Makromolek. Chem. 29-30, 229, 1973 12. R. F. BOYER, Polymer Engng. ScL 8: 161, 1968