Influence of stabilizers of thermooxidative degradation of poly-4-methyl-1-pentene on its viscoelastic properties

Stabil~ers of t,hermooxidatiw~ degradation of poly-4-methyl.l-pcntone

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11. Ye..A. BEKTUROV, Troinye polimernye sistemy v rastvorakh (Ternary Polymeric Systems in Solutions). p. 51, Nauka, Alma-Ata, ] 975 12. N. G. ILLARIONOVA, L. S. SEMENOVA, I. S. LISHANSgII, I. A. BARANOVSKAYA, A. I. GRIGORYEV and V. N. NIKITLN, Vysokomol. soyod. BI6: 192, 1974 (Ntd translated in Polymor Sei. U.S.S.R.) 13. V. I. KOLEGOV, Dis. na soiskaniye uch. st. kand. fiz.-mat, nauk. (Discussion at MottinT~ for Caud. Degree in Physics and Maths.), High Polymor Inst. AN SSSR. Loningr~l. 1976 14. V. Ye. ESKIN, I. A. BARANOVSKAYA, M. M. KOTON, V. V. KUDRYAVTSEV and V. P. SKLIZKOVA, Vysokomo1. soyed. A18: 2362, 1976 (Translatcd i~l Polymer Sci. U.S.S.R. 18: 10, 2699, 1976) 15. N. M. EMANUEL and D. G. KNORRE, Kurs khimh'.heskoi kinetiki (Chemical Kinetics Course), p. 355, Vysshaya shkola, Moscow, 1962 16. S. Ya. FRENKEL, Vvedeniye v statisticheskuyu teoriyu polimerizatsii (Introduction to Statistical The~)ry of Polymorizationi. p. 33, Nauka, Moscow-Leningrad, 1965 17. M. L. BENDER, Y. L. CHOW and F. CHI, OUPEK, J. Amer. Chem. Soc. 80: 20, 5380, 1958 18. L. FORST and D. KESSE, K h i m i y a i tekhnologiya polimerov, I, 77, 1965 19. V. I. KOLEGOV and S. Ya. FREN]KEL, Vysokomol. soyed. A18: 1680, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 8, 1919, 1976) 20. V.I. KOLEGOV, Vysokomol. soyed. AI8: 1689, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 8, 1929, 1976) 21. M. M. KOTON, Yu. N. SAZONOV and L. A. SHIBAYEV, Dokl. Akad. Nauk SSSR 224: 597, 1975

PolymerScienceU.S.S.R.Vol. 23. ~o. 6, pp. 1413-1420,1981 Printed in Poland

0032-3950/81/061413--08507.50/0 1982 Pergamon Pre~ Ltd~

INFLUENCE OF STABH,IZERS OF THERMOOXIDATIVE DEGRADATION OF POLY-4-METHYL-1-PENTENE ON ITS VISCOELASTIC PROPERTIES* I. I.

PEREPECHKO,

B. YA. 3~tRYASIN all(] A. I. K m v o x o s o v

All-Union Research Institute fi)r M(.(hcinal Polym(.rs (Received 11 March 1980)

The influence of stabilizers of thermooxidative degradation ,,f poly-4-methyl-lpentene on its viscoelastic properties has been investigated b y the tectmique of forced resonance vibrath)ns of a specimen held as a cantilever. The addition of stabilizers to the polymer influences molecular mobility and relaxation processes in the latter, thus changing the value of the dynamic elastic moduhm E ' in the low temperature, range. This change is related to the influence of tim stabilizers on the supermolecular * Vysokomol. soyed. A23: No. 6, 1275-1281. 1981.

1414

I. I. PEREPECKKO e$ ~ .

organization of poly-4-methyl-l-pentene. In addition, it is shown that the value of the pre-exponential factor r0 in the Arrhenius equation is not constant, but depends on temperature and on the tyt)e of stabilizer being used. :PoLY-4-3WTHYL-I-I'Eh'TENE (PMP) is classed with the higher poly-a-olefins. Co0apared with other plyolcfins P M P has a n u m b e r of good properties, viz. a high melting point, translucency, low density, good dielcctiSc properties and good chemical stability. However, it is subject to m a r k e d t h e r m o o x i d a t i v e degradation, and various stal)ilizers arc actor(tingly introduced. Although several b~vestigations of P M P have been r e p o r t e d [1-6], the results o f analyses of molecular mobility in P M P are c o n t r a d i c t o r y , and t h e i r i n t e r p r e t a tion is open to qucst ion. Up to t he 1)resent no rcl~tionship has beck1 found betweert structural features a.nd the visc(;clastic propcrties of PMP, aml d a t a on the influence of stabilizers on thcsc properties are lacking. I t is generally t h o u g h t tha.t small a m o u n t s of stabilizers (cortcctltrations being normally some t e n t h s of a pcr(.cr~t.agc) do not affect the main physical p r o p e r t i e s o f polym(,rs. I t has, however, been d e m o n s t r a t e d that. stabilizers i n t r o d u c e d into p o l y c a r b o n a t e results in major changes in its viscoelastic b e h a v i o u r [7]. However, it. has yel to be decided w h e t h e r this p h e n o m e n o n bcars a gelleral c h a r a c t e r , or w h e t h e r the e x p e r i m e n t a l findings r e p o r t e d in [71] rcpresertt a" rate exception. It, is i m p o r t a n t that. one shottld know w h e t h e r stabilizers influence the d y n a m i c mechanical p r o p e r t i e s el" PMP, a~ld w h a t is thc m e c h a n i s m of the influencc o f stabilizers on viscoelastic properties. Our aim in the present iusta~lce was to investigate the molecular m o b i l i t y o f P M P over a wide range of t e m p e r a t u r e and to e x a m i n e the influence of small stabilizer concentrations on the viscoelastic b e h a v i o u r of PMP. • PMP specimens prepared by injection moulding were investigated by the technique of forced resonance vibrations of a specimen held as a cantilever [8] in the temperature interval from --170 to 230° at two fixed frequencies differing by a factor of ~ 6, which meant that activation energies of relaxational transitions co(fld be estimated. The resonance oscillatioIl frequency for a specimen in the specified temperature interval varied within the limits of 55-365 :I-rz. As a result of the investigati(m we obtained the value of Young's dynamic modulus E', the low frequency sonic velocity C, and the tangent of the meeha~ical loss angle, tan ~. The relative measuring error was ~ 4% for E', ~ 2~o f()r C and 5~o for tan ~. Point thermostattmg was carrie,d out over the (,ntire temperature interw~l, accurate to =]=0.5°. Several regions of mechanical r e l a x a t i o n associated with different t y p e s of molecular m o t i o u a p p e a r in the t e m p e r a t u r e dependences of the sonic v e l o c i t y C~=f(T) (Fig. l, curve 1) and of the t a n g e n t of the mechanical loss angle t a n 5 = f ( T ) (curve 2). In the p o l y - 4 - m e t h y l - l - p e n t e n e side chain t h e r e are two m e t h y l groups s i t u a t e d a t different angles relative to the C - - C bond, and h a v i n g different potential barriers to r o t a t i o n .

Stabilizers of t,hermooxidative degradation of poly.4-methyl-l-pentene

1415

The most low temperature maximum of tan ~, appearing at --145 °, relates to the relaxation process reflected on the C = f ( T ) plot below --170 °, and is associated with inhibited rotation of the CH 3 group which has a lower potential barrier to rotation. The other low temperature transition, at --83 °, is apparently due t,o "thawing" of inhibited rotation of the methyl group for which the p~t enti~d barrier is higher. The activation energy for this transition is 25.0 kJ]mole. The :text temperature transition, at --44 °, evidently appears (judging by the activation energy) (29.0 kJ/mole) as a result of illhibited rotation or twisting a,-ibrations of the side-chain as a whole.

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:FIo. 1. Temperature dependenco of the low frequoney sonic velocity C (1) and of the tangent of the mecha.nical loss angle tan $ {2) for PMP. Of major interest are the results obtained in the region of transition from the glassy to the high elastic state. The most intense mechanical loss maximum appearing at 63 ° (frequency 200 Hz) is asymmetrical. In view of this we surmise that several relaxation processes take place in this temperature region. P M P is ~ partially crystalline polymer in which the amorphous phase apparently has several forms of order. Molecular mobility of a micro Brownian type may develop irdtially in less ordered parts of the amorphous interlayer. A further rise in temperature creates conditions conducive to a release of segmental mobility in more ordered parts of the P M P amorphom~ interlayer. •Certainly two relaxation processes appear in the glass transition region on the •~,urve of T vs. sonic velocity: at 33 ° (apparent activation energy Uapp~---265 kJ]mole) and at 51 ° (Uapp=399 kJ/mole). Calorimetric measurements give values of T , = 30 or 17-37 ° [8, 9]. Judging b y the activation energies the glass transition temperature for PMP will be T ~ 5 1 ° {frequency ~200 Hz). These results are substantiated by measurements made with the aid of a DSK-2 differential Perkin-Elmer type 'scanning calorimeter (Tg--~50°). There are two temperature transitions above T,: at 104 and 150 °. Of these l,he most interesting and controversial (from the point of view of interpretation)

I. I. l>~,reEo~n~o e~ a/.

1416

is that at 150 °. Wunderlich and Bauer [9] think t h a t the latter is a first order transition due to the presence of diverse polymorphic forms of PMP. I n our view the transition at 150 ° is a relaxation transition since, when the frequency is changed, it is displaced on the temperature scale (Uavp=307 kJ/mole).

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F1a. 2. Temperature dependeneo of E' (a) and of tan J (b) for the nonstabiliz~t I~]~-P specimen (1) and for PMP sl~e~eIls stabilized by an Irg~inox-T3vitex mixture (2), Trg~ox-Stafor mi~uro (3), Ionox (4) and Trganox (5). The following were used as thermooxidative degradation stabilizers: tetra(methylene-4-hydroxy-3,5-di-tert-butylphenyl propionate)methane, or Irganox 1010 (0.3~/o); 1,3,5-trimethyl-2,4,6-tri-3,5-di-tert-butyl-4-hydroxybenz~-lbenzene, or Ionox 330 (0-35°//o); Stafor 10 (0.3~); a synergetic mixture of Irganox 1010 and Stafor 10, and Irganox mixed with the optical bleaching agent Uvitex (0.05%). Figure 2a shows the temperature dependences of the elastic modulus E ' for the pure P M P (curve 1) and for the P M P containing different amounts of stabilizers (curves 2-5). The specimens under study will henceforward be denoted b y the appropriate numbers. I t is seen from Fig. 2a that the E ' = f ( T ) plot for P M P specimens containing stabilizers m a y be subdivided into two regions. In the first region (above T ~ = 3 3 °) the introduction of various types of stabilizers has little or no effect on the elastic modulus of PMP. We therefore conelude in the light of results of mechanical tets carried out at room temperature, that the introduction of stabilizers in 0.300//0 concentrations has no significant effect on the main mechanical properties of PMP. However, it is clear from t h e acoustic measurements that the influence of stabilizers cannot be ignored. An entirely different situation is observed below T~. It was found that on introducing stabilizers E' is reduced b y up to 250/o (for P M P containing Irganox E ' = 1.7 × 103 MPa at -- 100 °) compared with the unstabilized PMP (E'--~ 2.3 × 10* MPa). The reduction in E ' is most marked at temperatures below T2 in cases where antioxidants Irganox or Ionox are introduced (see Fig. 2a, curves 4 and 5).

Stabilizers of thermooxidative degradation of poly-4-methyl-l-pentene

1417

A synergetic system of Irganox and Stafor reduces E' slightly in this temperature region (E' approximates in this case to the value for the unstabilized PMP). Thus the stabilizing system Irganox-Stafor as it were "stabilizes" the mechanical properties of P M P at T < T s , and results in a higher level of E' compared with specimens 4 and 5. I t can be seen from the E'=f(T) plots for the stabilized P M P specimens that the introduction of stabilizers into the polymer does markedly influence the molecular mobility as well as relaxation processes. Typical in this respect are the relaxation processes relating to thawing out of rotation of methyl groups in P M P side-chains. This leads to a low temperature maximum appearing on the temperature dependence of tan J----f(T), which, in the nonstabilized polymer, is observed at 145 °. The activation energy of this relaxation process calculated b y the formula T~TOe U/RT

(r being the relaxation time, U the activation energy, R the universal gas constant, T temperature, °i and vo the pre-exponentia] factor) proved to he a function of the concentration of one or other of the stabilizers. For instance, in the nonstabilized P M P (specimen I) U----20.0kJ/mole, while in the specimens containing Ionox (specimen of type 4) and Irganox (specimen of type 5) U = 2 5 kJ/mole° In the polymer stabilized with the system Irganox-Stafor, U=21.0 kJ/mole. Thus a curious relation has come to light: the introduction of any of the stabilizers under study reduces the activation energy of the relaxation process relating to the motion of methyl groups, and the lowest activation energy is observed for the polymer t o which mixed stabilizers have been added. It was found that the activation energy characterizing the energy of thermal motion of Ctt groups required for rotation of the latter about the C--C bond is a sort indicator that m a y be used to estimate the efficiency of a stabilizer in PM:P. The fact that the activation energy is lowest for the relaxation process relating to rotation of methyl groups in a specimen containing mixed stabilizers is apparently attributable to a certain amorphization of specimens of this type (t~Te 3). As a result of this the height of the potential barrier limiting the rotation o f methyl groups is reduced. Interesting results were obtained on analyzing tan J - f (T) plots ibr all the specimens under study. We noted that tan Jmax relating to the rotation of CH~ groups becomes more diffuse in specimens containing mixed stabilizers. One naturally assumes that an increase in the half-width of the mechanical loss peak stems from a broadening of the distribution of one of the parameters characterizing relaxation processes. I t is known [10] that one of the factors leading to a bro~d distribution of relaxation times m a y be the distribution of r0~ values in Arrhenius equatiorLs. It is probable that the pre-cxponcntial factor will differ" for the various methyl groups of PMP, and so one may speak of a distribution function for this para-

1418

I.I. P~mP~.Cm~O ~ a/.

meter. It was demostrated in [10] that the distributiotl function for r0 is bellshaped in character, while the quantity T0 which is determined b y dynamic mechanical and dielectric methods, represents art average or "effective" value. It is generally assumed that the value of r0 is identical (10-1~-10 -la see) for all polymers, and is not temperature-dependent. However, it was found b y measurements at various frequencies t h a t ' To is temperature-dependent, and decreases, in the specimen containing a synergetic mixture of stabilizers (a specimen of type 3) from r 0 = 6 - 3 × 1 0 -1~ t~) 3.7×10 -t~ sec when the temperature rises from 142 to 158°K.

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val

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%

FIe. 3. 8chematio representation of the distribution function for methyl groups in PM-P relative to r, for the normtabilized (1) and stabilized specimens (2). Moreover, the addition of one or other type of stabilizer leads to a marked change in r0. Thus in specimens of types 4 and 5 the r0 values are respectively 1.8× 10 -14 and 1.1 × 10 -14 sec. In a specimen of type 3, To----6.3× 10 -1~ sec, and for the nonstabilized polymer T 0 = 2 . 9 × I 0 -is sec, i.e. the stabilizer mixture increases the value of T0 b y four orders. One would expect that this would be accompanied by a change in the width of the distribution function for the pre-exponential factor To and that the highest values of To will correspond to the broadest distribution function (Fig. 3). One would suppose that a broad distribution of To is due to regions with differing levels of supermolecular organization forming in the amorphous interlayer. More diffuse low-temperature peaks appearing on tan ~-----f (T) plots and attributable to the rotation of CI-Ia groups m a y point to the emergence or marked differentiation of various structural inhomogeneities in the polymer. Thus in our view the mechanism whereby the stabilizers influence the viscoelastic behaviour of PMP is apparently due to their influence on the supermolecular organization of the polymer. This assumption is favoured by the results of an analysis of the relaxation properties of PMP in the transition region of the P M P amorphous regions from the glasslike to the high-elastic state. Whereas the glass transition temperature Tg associated with thawing of segmental mobility in the more ordered regions of the polymer is scarcely influenced b y the content and type of ~tabilizers, and is 50 °, a marked change is observed in the apparent activation energy characterizing the efficiency of intermolecular interaction. It was found t h a t

Stabilizers of thermooxidative degradation of poly-4-methyl-l-p~'ntene

141,9

the apparent activation energy for the unstabilized polymer Uapp~---403kJ/mole. In specimen 5 Uavp----328 kJ/mole, and in the polymer containing mixed stabilizers (specimens of type 3) Uapp~806 kJ/mole. No less marked is the change in the activation energy for the relaxation process due to thawing of segmental motion ixt disordcred amorphous regions of PMP, the respective values for the above specimens being: 252, 126 and 475 kJ/mole. In view of these findings we conclude that the w~rious stabilizers introduced into PMP have a nonuniform influence on the character of supermolecular organization. We surmise ~hat a reduction in E ' for PMP in the low temperature r a n g e accompanying the addition of a stabilizer such as Ionox to the polymer is associated with crystallization of the specimens (of type 4). I t could be that antioxidants m a y serve as centres of crystallization. This must lead to reduced efficiency of intermolecular interaction within amorphous regions and to a corresponding reductior~ in the dynamic clastic modulus when T<
1420

X . I . PSm~TECKKO e~ a/.

the value of tan Jmax which is responsible for glass transition of the amorphous interlayer is increased and is displaced 7° on the t em perat ure scale (Fig. 2b) compared with ta n Jm~x for the specimens containing individual stabilizers. Amorphization of PMP upon adding a mixed stabilizer is bound to increase the relative a m o u n t of monomer units t h a t are in amorphous parts and this will consequeny increase the efficiency of intermolecular interaction in these amorphous regions. This results in higher activation energies relating to release of segmental motion in disordered and ordered regions of the amorphous interlayer, as well as in a higher value of E ' below Tg (Fig. 2a), which, for the polymer containing mixed stabilizers approximates to E ' for the nonstabilized polymer. This means t h a t a mixture of stabilizers not only effectively safeguards the polymer against thermooxidative degradation [10], but also "stabilizes" its viscoelastic properties, i.e. in the case under consideration there is a sort of structural-chemical stabilization of the polymer. Translated by R. J. A. HE-'~DRY REFERENCES I. B. G. RANBY, K. S. CHAN a n d H . BRUMBERGER, J. Polymer Sci. 58: 166, 545, 1962 2. J. H. GRIFFITH and B. G. ltANBY, J. Polymer Sci. 44: 143, 369, 1960

3. N.M. GRISSMAN,J. A. SAUER and A. E. WOODWARD, J. Polymer Sci. A2: 6, 5075, 1964 4. W. A. HEWETT and F. E. WEIR, J. Polymer Sci. AI: 1239, 1963 5. F. E. KARASZ, H. E. BAIR and J. M. O'REII.LY, Polymer 8: 10, 547, 1967 6. A. E. WOODWARD, Polymer 5: 6, 293, 1964 7. I. I. PFAtEPECHKO, L. A. KVACHEVA and I. I. LEVANTOVSKAYA, Vysokomol. soyed. A13: 702, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 3, 796, 1971) 8. I. I. PEREPECH]KO, Akusticheskiye metody issledovaniya polimerov (Acoustic Methods for Polymer Investigations), p. 60, Moscow, Khimiya, 1973 9. B. WUNDERLICH and G. BAUER, Teploemkost' lineinykh polimerov (Heat Capacity of Linear Polymers), p. 179, Mir, Moscow, 1972 10. R. M. KIMMEL and D. R. UHLMAN, J. Appl. Phys. 49: 4254, 1969 11. V. M. DEMIDOVA, I. I.VASILOVA, A. L. GOL'DENBERG and Ye. N. MATVEYEVA, Plast. massy, No. 7, 46, 1974