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Weak chemical bonds and chemical relaxation and rupture processes in polymers
Weak chemical bonds and chemical relaxation and rupture processes in polymers
1855
6. L . L . STOTSKAYA, G. A. ORESHKINA, S. T. BASHKATOVA, L. F. KIM and B. A. KRENTSEL, J. Polymer Sci. Polymer Syrup., 42, 95, 1973 "7. L.L. STOTSKAYA, T. A. SITNIKOVA and B. A. KRENTSEL, Europ. Polymer J. 7: 1661, 1971 8. V. Ye. SELEZNEVA, L. L. STOTSKAYA, T. A. SITNIKOVA and D. S. ZHUK, Pat. 316700 (U.S.S.R.) Published in B. I., No. 30, 1971 9. B. A. KRENTSEL', B. E. DAVYDOV, L. L. STOTSKAYA and V. S. SEREBRYANIKOV, Pat. 368801 (U.S.S.R.). Published in B. I., No. 10. 1973 10. V.S. SEREBRYANIKOV, B. E. DAVYDOV, G. P. KARPACHEVA and L. STOTSKAYA, In: XXIII Internat. Syrup. on Macromolec. Madrid, vol. 1, p. 396, 1974 I 1. V. S. SEREBRYANIKOV, L. L. STOTSKAYA, G. L. KARPECHEVA and B. E. DAVYDOV, In: Tez. II Vses. soveshch, po KPZ i ionradikal'nom solyam (Proceedings of the IInd All-Union Conference on Charge-transfer Complexes and Ion-radical Salts). p. 54, Zinatne, Riga, 1973; In: Tez. Vses. Konf. po khimii atsetilena (Proceedings of Vth All-Union Conference on the Chemistry of Acetylene). p. 3658, Metsniyereba, Tbilisi, 1975 12. L. L. STOSKAYA, G. A. ORESHKINA, V. S. SEREBRYANIKOV and G. P. KARPACHEVA, In: Europ. Symp. on Electric Phenomena in Polymer Sci. Pisa, p. 44, 1978 13. R. F. PARCiC.LL and C. B. POLLARD, J. Amer. Chem. Soc. 72: 2385, 1950 14. M. CAUDEMAIL Ann. Claim. 1 : 161, 1956 15. M. ITOH, J. Amer. Chem. Soc. 92: 886, 1970 16. R. E. MI'!J.~R and W. F. WYNNE-JONES, I. Chem. Soc., 7--8, 2375, 1959 17. A. K. BABKO, Fiziko-ldaimicheskii analiz kompleksnykh soyedinenii v rastvore (Physico-chemical Analysis of Complexes in Solution). Izd. AN USSK, Kiev, 1955 18. H. BENESI and J. 14"ILDEBRAND, J. Amer. Chem. Soc. 71 : 2703, 1949
Polymer Science U.S.S.R. Vol. 26,1'4o. 8, pp. 1855-1861, 1984
"Printedin Poland
0032--3950/84 $10.00+.00 © 1985 PerilamonPress Ltd.
WEAK CHEMICAL BONDS AND CHEMICAL RELAXATION AND RUPTURE PROCESSES IN POLYMERS* G. M. BARTENEV Institute of Physical Chemistry, U.S.S.R. Academy of Sciences
(Received 24 February 1983) Data concening the rupture, pyrolysis and the mass-spectrometry of polymers give evidence that there are weak chemical bonds that determine, in many cases, the rates of rupture and chemical relaxation. For carbon-chain polymers, there are characteristically three levels of thermal degradation with the following values of activation energy for the rupture of the C - C bond: 130-140, 220 and 310 kJ/mole. The process of chemical relaxation, ¢$c, in poly* Vysokomol. soyed. A26: No. 8, 1660-1664, 1984.
G. M. BARTENEV
1856
mers is connected with the rupture of weak C - C bonds with an activation energy that is the same as the lowest level of activation energy for thermal degradation. In connection with this, the thermal-fluctuation process involved in the rupture of solid polymers may be considered to be a chemical relaxation process accelerated by the action of the stresses. ACCORDING to Zhurkov's kinetic concept of strength [1, 2] and the equation for the life-to-rupture of polymers, the activation energy for the rupture process, Uo (as the stress o'~0), is practically the same as the energy for the process of thermal degradation of the polymeric chains, Up [3, 4]. Table 1 shows data from the literature [2-6] for certain crystalline and glassy polymers, where the values of Uo should be understood to be the energy for rupture of the C - C bonds (or the C - N bond for the polyamide) in the polymeric chain. Except for PETF, Uo for all the polymers is substantially less than energy for rupture of an isolated C - C bond (in simple molecules) namely, Eo = 345 k J/mole, according to Cottrell [7] (for the C - N bond, Eo = 304 kJ/mole). For certain polymers (Table 1) different investigators have observed either a low level with Uo = 120-140 kJ/mole or a high level with Uo =220-230 kJ/mole. For example, for P M M A Peschanskii and Stepanov [5] found a value of Uo = 134 k J/mole (Table 1, specimen A) but Zhurkov et al. [3, 4] found a value Uo =225-230 k J/mole (specimen B). Other investigators have obtained either the first or the second value of Uo for PMMA. The reasons why Uo for polymers is less than the energy for the rupture of an isolated bond have been discussed [2]. The strength of C - C bonds in polymeric chains is reduced because of the effect of side branches, intermolecular interactions, the chemical action of oxygen (oxidised C - C bonds), the presence of hydroperoxide groups in the chains and other causes. Thus Gubanov and Chevychelov's calculations [8] of the energy for the rupture of bonds in polyTABLE 1. ACTIVATION ENERGII~ FOR THE RUPTURE PROCESS Uo (BELOW T,) AND FOR THERMAL DEORADATION IN VACUUM UI) FROM THE LITERATURE [2-6]
Polymer PE PP specimen A specimen B PS specimen A specimen B PMMA specimen A specim©n B Polycaproamidc PETP PTFE
[_
Uo
t Ua k J/mole 106 84-105
125 234
104 236
142 220
Absent 230
Stage of the process
I II I
II
\ 134 225-230 188 210-222 313
Absent 220 180 Absent ))
I
II II lI III
Weak chemical bonds and chemical relaxation and rupture processes in polymers
1857
meric chains, taking account of intermolecular interactions, led to,values of the energy Up close to 220-230 kJ/mole, that is, close to the second level for the strength of the C - C bond. For a polyamide (the C - N bond), they obtained a value of Uo o f 203 kJ/mole, which approximates to the experimental value of U, namely 188 kJ/mole. rh ~Ao
!
"'"B~~~I
II
-'L. . . .
r----~JE
2.5
50
75
/tm
FIG. 1. Typical dependence of a polymer's rate of degradation, h~, on the specimen's weight loss, Am, during pyrolysis: the section AAoB corresponds to the evolution of volatile products, BC to stage 1, CD to stage II and DE to stage HI in the degradation. Discrete levels for the energy of a chemical bond in a polymer chain have been observed by other independent methods. Pyrolysis is one such method [9, 10]. The rate of degradation, determined from the rate of loss of the specimen's mass m in vacuum at high temperature is different in the different stages of the process (Fig. 1). Analysis of the experimental data makes it possible to establish that there are three degradation processes for polymers in the general case; these occur simultaneously but differ markedly in their rates ~hz, ih 2, ~h3. After the rapid evolution of volatile impurities and the low-molecular weight fraction, stage I may be clearly distinguished (section BC), the first degradation process being practically terminated at the point C on BC. Stage II is then observed (the section CD), the second degradation process terminating at the point D on CD. Only a very slow third degradation process remains on the section DE. Generally 20-30 ~ of the substance is removed up to the point B because of the evolution of volatiles and the simultaneous occurrence of a~l three processes. If volatile impurities are not taken into account, the degradation proper of the polymer first fol.lows the section AB and most of the 20-30 ~ belongs to this part of the process. Then a certain further amount of the polymer is lost in section I and the losses amount to 40-45 ~ at the point C; in section II, the losses of the substance continue to 80--90 ~ . The polymer residue of 10-20 ~ is difficult to destroy (stage Ill). Each of the three processes is characterized by a different rate and activation energy (Table 2). The mechanism of the first process (stage I) is explained by the presence of weak chemical bonds ~n the polymer chain. According to Madorskii's data [9], the first process for PE is characterized by an activation energy of 105 kJ/mole and the second, by an energy of 263-267 kJ/mol¢; for
G . M . BARTENEV
1858
TABLE 2. ACTIVATION ENERGIES FOR THE THERMAL DEGRADATION OF POLYMERS E o A T VARIOUS STAGES OF DECOMPOSITION IN VACUUM FROM PYROLYSIS DATA [ 9 - 1 0 ]
Polymer PE with M= (10-20) x 10a Polymethylene PP with M= 1.75 x l0 s PS with M=2.3 x l0 s Poly-~t-methylstyrene PMMA specimen A with M= 1.5 x l0 s specimen B with M=5.1 x 106 Polyeaproamide PETP
Values of Eo, k J/mole
e-' 105-109 Absent i,
125 Absent 60 Absent
263-267 Absent 230-242 186-230 230
300 300 Absent
217 217 176 160
linear PE (polymethylene) only the third degradation process is observed with E~3) = 300 k J/mole, which is close to the energy for the dissociation of isolated C - C bonds (E9 = 345 kJ/mole). Let us now consider the physical significance of the three discrete values of activation energy for PE ( - C H 2 - C H 2 - C H 2 - C H 2 . . . ) . A model for the weak and strong C - C bonds in PE must be adopted to do this. It is principally the weak bonds that are ruptured in stage I and they should determine the kihetics of rupture o f PE. By making a comparison with the value o f Uo (Table 1), it is seen that Uo = 106 kJ/mole agrees with the energy for thermal degradation in stage I, EaIt) = 105 kJ/mole. This means that the life-to-rupture observed in the experiments of Zhurkov et aL is determined by the thermal-fluctuation rupture of weak bonds. In experiments on the life-to-rupture o f PE, the matter does not reach stages II and III since there is a sufficient number of weak bonds in the specimen, which lead to rupture. It may be assumed that, with tests on polymethylene, in which there are no weak bonds, Uo should be high (300 k J/mole). Madorskii's data for P M M A with M = 1"5 x l0 s (specimen A) are very typical [9]. Two processes were observed with Earl)= 125 and EatS)= 217 k J/mole. The third process, for which data were not brought forward, should correspond to Eut3) = 345 kJ]mole. With M=5-1 x 10s (specimen 13) the first process was not observed and the second degradation process occurred from the very start with an activation energy of 217 kJ/ /mole. Consequently, in the case of specimen B there are no weak bonds in the polymeric chains because of the specific nature of the synthesis of PMMA. These data explain the contradictory results for the life-to-rupture of P M M A (Table 1). Thus, according to some data, Uo= 134 kJ/mole, which is close to the lowest level, EL1)= 125 kJ/mole, for specimen A and according to other data Uo = 225-230 kJ/mole, which is close to the middle level E~2)=217 kJ/mole observed in both P M M A specimens. The various authors were, consequently, dealing with specimens of the same polymer that were different in their chemical structure. It may be seen from a comparison of the data in Tables 1 and 2 that the polymers have activation energies for the degradation process that approximate either to E~ I) or to Eo~2). Similar results were ob-
Weak chemical bonds and chemical relaxation and rupture processes in polymers
1859~
tained by pyrolytic mass-spectrometry [11, 12] in particular by mass-spectrometric thermal analysis. F o r example, the levels E a t ° = 125-134 and Eot2)= 209 kJ/mole are observed in the case of P M M A for a specimen of type A but only the level E~2) = 206 kJ/ /mole for a specimen of type B, as was found in the case of pyrolysis (Table 2). Figure 2 shows the mass-spectrogram of a butadiene-methylstyrene copolymer [13]; it follows from this that there are two levels of energy for the rupture of C - C bonds, which are shown in Table 3. The two maxima on the mass-spectrogram (at 352 and 426°C) correspond to two processes of chemical decomposition, ~c and 3c~, with activation ener-
E,i ,7/mole
I,arb.un.
"C-C
33 --
300
,S r',-' . . . .
°° o
J
800
100
\
I
oO0
~
AA
-~O#- - .e.---- A~----*----
t
I
J
,,,
de
200
7"o.5
~
°
.c-.
,?it
I 500 T °
I
//
///
/V
I
I
!
I
Vo
Eo
FIG. 2
Eo
vs0
FIG. 3
Fxo. 2. Mass-thermo$ram o f p o l y b u t a d i e n e m e t h y l s t y r e n e in v a c u u m at a h e a t i n g rate o f 0-17° C/see (1 is the intensity of evolution of the products). FIG. 3. Comparison of the activation energies for various processes involved in the decomposition of chemical bonds in polymers. TABLE 3. VALUES OF E o IN THE VARIOUS STAGES OF DECOMPOSITION IN VACUUM ACCORDING TO THIi DATA OF PYROLYTIC MASS-SPECTROSCOPY
[11, 12]
Values of Eo, kJ/mole Polymer PE PS PMMA specimen A specimen B with M=3-9 x 106 PETP Polybut adiene-methylstyrene (SKMS-30)* * Present author's data,
87-105 125-145
Absent 230-267
293 308
125-134 Absent
209 206 171 Absent
Absent
130-140
270--280
1860
G.M. BARTENEV
gies of 130-140 and 270-280 kJ/mole. Only the Jc process cat be observed by relaxation spectrometry. This is explained by the fact that, after the decomposition of the weak C - C bonds, the initial polymer ceases to exist as such because of the chemical decomposition and the vaporization of a large part of the specimen. Let us now compare the activation energy for the Jc process as given by the data from mass-spectrometric thermal analysis (MTA) with that from relaxation spectrometry. In addition to the MTA results, there are data for the butadiene-methylstyrene copolymer obtained from high-temperature curves for stress relaxation [14, 15]. According to these data, Use= 143 kJ/mole and for SKEP, 150 kJ/mole. A summary of all the data for the values of activation energy for the rupture and pyrolysis of the polymers is shown in Fig. 3. It may be clearly seen that there are three levels of energy of the chemical bonds in the polymeric chains. Data from the following have been chosen in this figure: the "zero" activation energy for rupture Uo, (I); thermal degradation of the polymers by pyrolysis (II); thermal degradation of the polymers by mass-spectrometry (III) and relaxation spectrometry for the Jc chemical relaxation process in polybutadiene methylstyrene, (IV). The activation energy for the rupture of a polymer depends on the special features of the structure of its chain, which vary with the technological methods of their synthesis and heat treatment. For a given sample with a definite structure of the polymeric chains, Uo is the constant in Zhurkov's equation for the life-to-rupture and does not depend on the defect nature of the specimen (microcracks), the degree of molecular orientation, plasticization or other factors. Of the three possible degradation processes, the very lowest, the E~t~-process, has been observed by the methods of relaxation spectrometry. In this case, the process of rupture of the polymer below T~ is characterized by the activation energy Uo ~ Eo¢1~. In those cases when there are no weak bonds in the polymer or their concentration is very low, Uo E(D2) or E~3). The process of rupture of solid polymers may be considered, as a whole, to be one involving processes of chemical relaxation accelerated by the action of large stresses. =
Translated by G. F. MODLEN
REFERENCES
1. S. N. ZHURKOV, Izv. AN SSSR. Neorganickeskiyematerialy (Inorganic Materials). 3:1767, 1967 2. V. R. REGEL', A. L SLUTSKERand E. Ye. TOMASHEVSKII,Kineticheskayapriroda prochnosti tverdykh tel (Kinetic Nature of the Strength of Solids). Nauka, Moscow, 1974 3. S. N. ZHURKOV and S. A. ABASOV, Vysokomol. soyed. 3: 441, 1961 (Not translated in Polymer Sci. U.S.S.R.) 4. S. N. ZHURKOV, V. R. RF~EL' and T. D. SANFIROVA, Vysokomol. soyed. 6: 1092, 1964 (Translated in Polymer Sci. U.S.S.R. 6 : 6, 1201, 1964) 5. N. N. PESCHANSKAYAand V. A. STEPANOV, Fizika tvedoga tela 7: 2962, 1965 6. B.N. NARSULLAJEW,G. M. BARTENEW, S. N. KARIMOV and G. D. KORODENKO, Plaste und Kautschuk 26: 383, 1979 7. T. L. COTTRgLL, Strength of ChemicalBonds, 2d ed., Butterworths, London, 1958
Free radicals formed during the mechanical degradation of polypeptides
1861
8. A. I. GUBANOV and A. D. CHEVYCHELOV, Fizika tverdogo tela 5: 91, 1963 9. S. L. MADORSKY, Termicheskoye razlozheniye organicheskikh polimerov (Thermal Decomposition of Organic Polymers). Translated from the English under the editorship of S. R. Raftkey, Mir, Moscow, 1967 10. V. D. MOISEYEV, M. B. NEIMAN and A. I. KRYUKOVA, Vysokomol soyed. 1: 1552, 1959 (Translated in Polymer Sci. U.S.S.R. 2 : 1/2, 55, 1961) I 1. R. A. KHMEL'NITSKII, I. M. LUKASHENKO and Ye. S. BRODSKII, Piroliticheskaya massspektrometriya vysokomolekulyarnykhsoyedinenii (Pyrolityc Mass-spectrometry of High-molecular Compounds). Khimiya, Moscow, 1980 12. V. R. REGEL', O. F. POZDNYAKOV, A. V. AMELIN and Yu. A. GLAGOLEVA, In: Mate~ialy I Vses. Konf. po mass-spektrometrii (Proceedings of Ist All-Union Conference on Massspectrometry), p. 198, Izd. AN SSSK, Leningrad, 1972 13. G. M. BARTENEV, K. BOTUROV andL. T. ZHURAVLEV, Vysokomol. soyed. B26: 69, 1984 (Not translated in Polymer Sci. U.S.S.R.) 14. G. M. BARTENEV, K. BOTUROV, N. M. LYALINA and B. I. REVYAKIN, Vysokomol. soyed. A25: 309, 1983 (Translated in Polymer Sci. U.S.S.R. 25 : 2, 358, 1983) 15. G. M. BARTENEV, N. M, L Y A L ~ A and B. I. REVYAKIN, Vysokomol. soyed. B24: 567, 1982 (Not translated in Polymer Sci. U.S.S.R.)
Polymer Science U.S.S.R. Vol. 26, 'No. 8, pp. 1861-1871, 1984 Printed in Poland
0032-3950/84 $10.00+ .00 ~:) 1985 Pergamon Press Ltd
FREE RADICALS FORMED DURING THE MECHANICAL DEGRADATION OF POLYPEPTIDES* A. M. DUBINSKAYA Chemical Physics Institute, U.S.S.R. Academy of Sciences
(Received 24 February 1983) The rates of mechanical degradation of synthetic polymers with various structures, proteins, polyamino acids and cyclopeptidcs have been compared. EPR has been used to study the structure of the free-radical products of the mechanical degradation of proteins (trypsin, serum albumin, insulin, subtilisin and collagen), polyamino acids (polyleucine) and polypeptides (gramicidine C and bacitracine). It has-been shown that the rigidity of the polypeptide structure causes an increase in the rate of mechanical degradation and a reduction in its limiting value. A s a rule, h o m o l y t i c r u p t u r e of covalent b o n d s in the m a i n polymeric chain, with the f o r m a t i o n o f free radicals, occurs d u r i n g m e c h a n i c a l action o n macromolecules. I n proteins, macroradicals have been observed by E P R d u r i n g the mechanical c o m m i n u * Vysokomol. soyed. A26: No. 8, 1665-1673, 1984.
1855
6. L . L . STOTSKAYA, G. A. ORESHKINA, S. T. BASHKATOVA, L. F. KIM and B. A. KRENTSEL, J. Polymer Sci. Polymer Syrup., 42, 95, 1973 "7. L.L. STOTSKAYA, T. A. SITNIKOVA and B. A. KRENTSEL, Europ. Polymer J. 7: 1661, 1971 8. V. Ye. SELEZNEVA, L. L. STOTSKAYA, T. A. SITNIKOVA and D. S. ZHUK, Pat. 316700 (U.S.S.R.) Published in B. I., No. 30, 1971 9. B. A. KRENTSEL', B. E. DAVYDOV, L. L. STOTSKAYA and V. S. SEREBRYANIKOV, Pat. 368801 (U.S.S.R.). Published in B. I., No. 10. 1973 10. V.S. SEREBRYANIKOV, B. E. DAVYDOV, G. P. KARPACHEVA and L. STOTSKAYA, In: XXIII Internat. Syrup. on Macromolec. Madrid, vol. 1, p. 396, 1974 I 1. V. S. SEREBRYANIKOV, L. L. STOTSKAYA, G. L. KARPECHEVA and B. E. DAVYDOV, In: Tez. II Vses. soveshch, po KPZ i ionradikal'nom solyam (Proceedings of the IInd All-Union Conference on Charge-transfer Complexes and Ion-radical Salts). p. 54, Zinatne, Riga, 1973; In: Tez. Vses. Konf. po khimii atsetilena (Proceedings of Vth All-Union Conference on the Chemistry of Acetylene). p. 3658, Metsniyereba, Tbilisi, 1975 12. L. L. STOSKAYA, G. A. ORESHKINA, V. S. SEREBRYANIKOV and G. P. KARPACHEVA, In: Europ. Symp. on Electric Phenomena in Polymer Sci. Pisa, p. 44, 1978 13. R. F. PARCiC.LL and C. B. POLLARD, J. Amer. Chem. Soc. 72: 2385, 1950 14. M. CAUDEMAIL Ann. Claim. 1 : 161, 1956 15. M. ITOH, J. Amer. Chem. Soc. 92: 886, 1970 16. R. E. MI'!J.~R and W. F. WYNNE-JONES, I. Chem. Soc., 7--8, 2375, 1959 17. A. K. BABKO, Fiziko-ldaimicheskii analiz kompleksnykh soyedinenii v rastvore (Physico-chemical Analysis of Complexes in Solution). Izd. AN USSK, Kiev, 1955 18. H. BENESI and J. 14"ILDEBRAND, J. Amer. Chem. Soc. 71 : 2703, 1949
Polymer Science U.S.S.R. Vol. 26,1'4o. 8, pp. 1855-1861, 1984
"Printedin Poland
0032--3950/84 $10.00+.00 © 1985 PerilamonPress Ltd.
WEAK CHEMICAL BONDS AND CHEMICAL RELAXATION AND RUPTURE PROCESSES IN POLYMERS* G. M. BARTENEV Institute of Physical Chemistry, U.S.S.R. Academy of Sciences
(Received 24 February 1983) Data concening the rupture, pyrolysis and the mass-spectrometry of polymers give evidence that there are weak chemical bonds that determine, in many cases, the rates of rupture and chemical relaxation. For carbon-chain polymers, there are characteristically three levels of thermal degradation with the following values of activation energy for the rupture of the C - C bond: 130-140, 220 and 310 kJ/mole. The process of chemical relaxation, ¢$c, in poly* Vysokomol. soyed. A26: No. 8, 1660-1664, 1984.
G. M. BARTENEV
1856
mers is connected with the rupture of weak C - C bonds with an activation energy that is the same as the lowest level of activation energy for thermal degradation. In connection with this, the thermal-fluctuation process involved in the rupture of solid polymers may be considered to be a chemical relaxation process accelerated by the action of the stresses. ACCORDING to Zhurkov's kinetic concept of strength [1, 2] and the equation for the life-to-rupture of polymers, the activation energy for the rupture process, Uo (as the stress o'~0), is practically the same as the energy for the process of thermal degradation of the polymeric chains, Up [3, 4]. Table 1 shows data from the literature [2-6] for certain crystalline and glassy polymers, where the values of Uo should be understood to be the energy for rupture of the C - C bonds (or the C - N bond for the polyamide) in the polymeric chain. Except for PETF, Uo for all the polymers is substantially less than energy for rupture of an isolated C - C bond (in simple molecules) namely, Eo = 345 k J/mole, according to Cottrell [7] (for the C - N bond, Eo = 304 kJ/mole). For certain polymers (Table 1) different investigators have observed either a low level with Uo = 120-140 kJ/mole or a high level with Uo =220-230 kJ/mole. For example, for P M M A Peschanskii and Stepanov [5] found a value of Uo = 134 k J/mole (Table 1, specimen A) but Zhurkov et al. [3, 4] found a value Uo =225-230 k J/mole (specimen B). Other investigators have obtained either the first or the second value of Uo for PMMA. The reasons why Uo for polymers is less than the energy for the rupture of an isolated bond have been discussed [2]. The strength of C - C bonds in polymeric chains is reduced because of the effect of side branches, intermolecular interactions, the chemical action of oxygen (oxidised C - C bonds), the presence of hydroperoxide groups in the chains and other causes. Thus Gubanov and Chevychelov's calculations [8] of the energy for the rupture of bonds in polyTABLE 1. ACTIVATION ENERGII~ FOR THE RUPTURE PROCESS Uo (BELOW T,) AND FOR THERMAL DEORADATION IN VACUUM UI) FROM THE LITERATURE [2-6]
Polymer PE PP specimen A specimen B PS specimen A specimen B PMMA specimen A specim©n B Polycaproamidc PETP PTFE
[_
Uo
t Ua k J/mole 106 84-105
125 234
104 236
142 220
Absent 230
Stage of the process
I II I
II
\ 134 225-230 188 210-222 313
Absent 220 180 Absent ))
I
II II lI III
Weak chemical bonds and chemical relaxation and rupture processes in polymers
1857
meric chains, taking account of intermolecular interactions, led to,values of the energy Up close to 220-230 kJ/mole, that is, close to the second level for the strength of the C - C bond. For a polyamide (the C - N bond), they obtained a value of Uo o f 203 kJ/mole, which approximates to the experimental value of U, namely 188 kJ/mole. rh ~Ao
!
"'"B~~~I
II
-'L. . . .
r----~JE
2.5
50
75
/tm
FIG. 1. Typical dependence of a polymer's rate of degradation, h~, on the specimen's weight loss, Am, during pyrolysis: the section AAoB corresponds to the evolution of volatile products, BC to stage 1, CD to stage II and DE to stage HI in the degradation. Discrete levels for the energy of a chemical bond in a polymer chain have been observed by other independent methods. Pyrolysis is one such method [9, 10]. The rate of degradation, determined from the rate of loss of the specimen's mass m in vacuum at high temperature is different in the different stages of the process (Fig. 1). Analysis of the experimental data makes it possible to establish that there are three degradation processes for polymers in the general case; these occur simultaneously but differ markedly in their rates ~hz, ih 2, ~h3. After the rapid evolution of volatile impurities and the low-molecular weight fraction, stage I may be clearly distinguished (section BC), the first degradation process being practically terminated at the point C on BC. Stage II is then observed (the section CD), the second degradation process terminating at the point D on CD. Only a very slow third degradation process remains on the section DE. Generally 20-30 ~ of the substance is removed up to the point B because of the evolution of volatiles and the simultaneous occurrence of a~l three processes. If volatile impurities are not taken into account, the degradation proper of the polymer first fol.lows the section AB and most of the 20-30 ~ belongs to this part of the process. Then a certain further amount of the polymer is lost in section I and the losses amount to 40-45 ~ at the point C; in section II, the losses of the substance continue to 80--90 ~ . The polymer residue of 10-20 ~ is difficult to destroy (stage Ill). Each of the three processes is characterized by a different rate and activation energy (Table 2). The mechanism of the first process (stage I) is explained by the presence of weak chemical bonds ~n the polymer chain. According to Madorskii's data [9], the first process for PE is characterized by an activation energy of 105 kJ/mole and the second, by an energy of 263-267 kJ/mol¢; for
G . M . BARTENEV
1858
TABLE 2. ACTIVATION ENERGIES FOR THE THERMAL DEGRADATION OF POLYMERS E o A T VARIOUS STAGES OF DECOMPOSITION IN VACUUM FROM PYROLYSIS DATA [ 9 - 1 0 ]
Polymer PE with M= (10-20) x 10a Polymethylene PP with M= 1.75 x l0 s PS with M=2.3 x l0 s Poly-~t-methylstyrene PMMA specimen A with M= 1.5 x l0 s specimen B with M=5.1 x 106 Polyeaproamide PETP
Values of Eo, k J/mole
e-' 105-109 Absent i,
125 Absent 60 Absent
263-267 Absent 230-242 186-230 230
300 300 Absent
217 217 176 160
linear PE (polymethylene) only the third degradation process is observed with E~3) = 300 k J/mole, which is close to the energy for the dissociation of isolated C - C bonds (E9 = 345 kJ/mole). Let us now consider the physical significance of the three discrete values of activation energy for PE ( - C H 2 - C H 2 - C H 2 - C H 2 . . . ) . A model for the weak and strong C - C bonds in PE must be adopted to do this. It is principally the weak bonds that are ruptured in stage I and they should determine the kihetics of rupture o f PE. By making a comparison with the value o f Uo (Table 1), it is seen that Uo = 106 kJ/mole agrees with the energy for thermal degradation in stage I, EaIt) = 105 kJ/mole. This means that the life-to-rupture observed in the experiments of Zhurkov et aL is determined by the thermal-fluctuation rupture of weak bonds. In experiments on the life-to-rupture o f PE, the matter does not reach stages II and III since there is a sufficient number of weak bonds in the specimen, which lead to rupture. It may be assumed that, with tests on polymethylene, in which there are no weak bonds, Uo should be high (300 k J/mole). Madorskii's data for P M M A with M = 1"5 x l0 s (specimen A) are very typical [9]. Two processes were observed with Earl)= 125 and EatS)= 217 k J/mole. The third process, for which data were not brought forward, should correspond to Eut3) = 345 kJ]mole. With M=5-1 x 10s (specimen 13) the first process was not observed and the second degradation process occurred from the very start with an activation energy of 217 kJ/ /mole. Consequently, in the case of specimen B there are no weak bonds in the polymeric chains because of the specific nature of the synthesis of PMMA. These data explain the contradictory results for the life-to-rupture of P M M A (Table 1). Thus, according to some data, Uo= 134 kJ/mole, which is close to the lowest level, EL1)= 125 kJ/mole, for specimen A and according to other data Uo = 225-230 kJ/mole, which is close to the middle level E~2)=217 kJ/mole observed in both P M M A specimens. The various authors were, consequently, dealing with specimens of the same polymer that were different in their chemical structure. It may be seen from a comparison of the data in Tables 1 and 2 that the polymers have activation energies for the degradation process that approximate either to E~ I) or to Eo~2). Similar results were ob-
Weak chemical bonds and chemical relaxation and rupture processes in polymers
1859~
tained by pyrolytic mass-spectrometry [11, 12] in particular by mass-spectrometric thermal analysis. F o r example, the levels E a t ° = 125-134 and Eot2)= 209 kJ/mole are observed in the case of P M M A for a specimen of type A but only the level E~2) = 206 kJ/ /mole for a specimen of type B, as was found in the case of pyrolysis (Table 2). Figure 2 shows the mass-spectrogram of a butadiene-methylstyrene copolymer [13]; it follows from this that there are two levels of energy for the rupture of C - C bonds, which are shown in Table 3. The two maxima on the mass-spectrogram (at 352 and 426°C) correspond to two processes of chemical decomposition, ~c and 3c~, with activation ener-
E,i ,7/mole
I,arb.un.
"C-C
33 --
300
,S r',-' . . . .
°° o
J
800
100
\
I
oO0
~
AA
-~O#- - .e.---- A~----*----
t
I
J
,,,
de
200
7"o.5
~
°
.c-.
,?it
I 500 T °
I
//
///
/V
I
I
!
I
Vo
Eo
FIG. 2
Eo
vs0
FIG. 3
Fxo. 2. Mass-thermo$ram o f p o l y b u t a d i e n e m e t h y l s t y r e n e in v a c u u m at a h e a t i n g rate o f 0-17° C/see (1 is the intensity of evolution of the products). FIG. 3. Comparison of the activation energies for various processes involved in the decomposition of chemical bonds in polymers. TABLE 3. VALUES OF E o IN THE VARIOUS STAGES OF DECOMPOSITION IN VACUUM ACCORDING TO THIi DATA OF PYROLYTIC MASS-SPECTROSCOPY
[11, 12]
Values of Eo, kJ/mole Polymer PE PS PMMA specimen A specimen B with M=3-9 x 106 PETP Polybut adiene-methylstyrene (SKMS-30)* * Present author's data,
87-105 125-145
Absent 230-267
293 308
125-134 Absent
209 206 171 Absent
Absent
130-140
270--280
1860
G.M. BARTENEV
gies of 130-140 and 270-280 kJ/mole. Only the Jc process cat be observed by relaxation spectrometry. This is explained by the fact that, after the decomposition of the weak C - C bonds, the initial polymer ceases to exist as such because of the chemical decomposition and the vaporization of a large part of the specimen. Let us now compare the activation energy for the Jc process as given by the data from mass-spectrometric thermal analysis (MTA) with that from relaxation spectrometry. In addition to the MTA results, there are data for the butadiene-methylstyrene copolymer obtained from high-temperature curves for stress relaxation [14, 15]. According to these data, Use= 143 kJ/mole and for SKEP, 150 kJ/mole. A summary of all the data for the values of activation energy for the rupture and pyrolysis of the polymers is shown in Fig. 3. It may be clearly seen that there are three levels of energy of the chemical bonds in the polymeric chains. Data from the following have been chosen in this figure: the "zero" activation energy for rupture Uo, (I); thermal degradation of the polymers by pyrolysis (II); thermal degradation of the polymers by mass-spectrometry (III) and relaxation spectrometry for the Jc chemical relaxation process in polybutadiene methylstyrene, (IV). The activation energy for the rupture of a polymer depends on the special features of the structure of its chain, which vary with the technological methods of their synthesis and heat treatment. For a given sample with a definite structure of the polymeric chains, Uo is the constant in Zhurkov's equation for the life-to-rupture and does not depend on the defect nature of the specimen (microcracks), the degree of molecular orientation, plasticization or other factors. Of the three possible degradation processes, the very lowest, the E~t~-process, has been observed by the methods of relaxation spectrometry. In this case, the process of rupture of the polymer below T~ is characterized by the activation energy Uo ~ Eo¢1~. In those cases when there are no weak bonds in the polymer or their concentration is very low, Uo E(D2) or E~3). The process of rupture of solid polymers may be considered, as a whole, to be one involving processes of chemical relaxation accelerated by the action of large stresses. =
Translated by G. F. MODLEN
REFERENCES
1. S. N. ZHURKOV, Izv. AN SSSR. Neorganickeskiyematerialy (Inorganic Materials). 3:1767, 1967 2. V. R. REGEL', A. L SLUTSKERand E. Ye. TOMASHEVSKII,Kineticheskayapriroda prochnosti tverdykh tel (Kinetic Nature of the Strength of Solids). Nauka, Moscow, 1974 3. S. N. ZHURKOV and S. A. ABASOV, Vysokomol. soyed. 3: 441, 1961 (Not translated in Polymer Sci. U.S.S.R.) 4. S. N. ZHURKOV, V. R. RF~EL' and T. D. SANFIROVA, Vysokomol. soyed. 6: 1092, 1964 (Translated in Polymer Sci. U.S.S.R. 6 : 6, 1201, 1964) 5. N. N. PESCHANSKAYAand V. A. STEPANOV, Fizika tvedoga tela 7: 2962, 1965 6. B.N. NARSULLAJEW,G. M. BARTENEW, S. N. KARIMOV and G. D. KORODENKO, Plaste und Kautschuk 26: 383, 1979 7. T. L. COTTRgLL, Strength of ChemicalBonds, 2d ed., Butterworths, London, 1958
Free radicals formed during the mechanical degradation of polypeptides
1861
8. A. I. GUBANOV and A. D. CHEVYCHELOV, Fizika tverdogo tela 5: 91, 1963 9. S. L. MADORSKY, Termicheskoye razlozheniye organicheskikh polimerov (Thermal Decomposition of Organic Polymers). Translated from the English under the editorship of S. R. Raftkey, Mir, Moscow, 1967 10. V. D. MOISEYEV, M. B. NEIMAN and A. I. KRYUKOVA, Vysokomol soyed. 1: 1552, 1959 (Translated in Polymer Sci. U.S.S.R. 2 : 1/2, 55, 1961) I 1. R. A. KHMEL'NITSKII, I. M. LUKASHENKO and Ye. S. BRODSKII, Piroliticheskaya massspektrometriya vysokomolekulyarnykhsoyedinenii (Pyrolityc Mass-spectrometry of High-molecular Compounds). Khimiya, Moscow, 1980 12. V. R. REGEL', O. F. POZDNYAKOV, A. V. AMELIN and Yu. A. GLAGOLEVA, In: Mate~ialy I Vses. Konf. po mass-spektrometrii (Proceedings of Ist All-Union Conference on Massspectrometry), p. 198, Izd. AN SSSK, Leningrad, 1972 13. G. M. BARTENEV, K. BOTUROV andL. T. ZHURAVLEV, Vysokomol. soyed. B26: 69, 1984 (Not translated in Polymer Sci. U.S.S.R.) 14. G. M. BARTENEV, K. BOTUROV, N. M. LYALINA and B. I. REVYAKIN, Vysokomol. soyed. A25: 309, 1983 (Translated in Polymer Sci. U.S.S.R. 25 : 2, 358, 1983) 15. G. M. BARTENEV, N. M, L Y A L ~ A and B. I. REVYAKIN, Vysokomol. soyed. B24: 567, 1982 (Not translated in Polymer Sci. U.S.S.R.)
Polymer Science U.S.S.R. Vol. 26, 'No. 8, pp. 1861-1871, 1984 Printed in Poland
0032-3950/84 $10.00+ .00 ~:) 1985 Pergamon Press Ltd
FREE RADICALS FORMED DURING THE MECHANICAL DEGRADATION OF POLYPEPTIDES* A. M. DUBINSKAYA Chemical Physics Institute, U.S.S.R. Academy of Sciences
(Received 24 February 1983) The rates of mechanical degradation of synthetic polymers with various structures, proteins, polyamino acids and cyclopeptidcs have been compared. EPR has been used to study the structure of the free-radical products of the mechanical degradation of proteins (trypsin, serum albumin, insulin, subtilisin and collagen), polyamino acids (polyleucine) and polypeptides (gramicidine C and bacitracine). It has-been shown that the rigidity of the polypeptide structure causes an increase in the rate of mechanical degradation and a reduction in its limiting value. A s a rule, h o m o l y t i c r u p t u r e of covalent b o n d s in the m a i n polymeric chain, with the f o r m a t i o n o f free radicals, occurs d u r i n g m e c h a n i c a l action o n macromolecules. I n proteins, macroradicals have been observed by E P R d u r i n g the mechanical c o m m i n u * Vysokomol. soyed. A26: No. 8, 1665-1673, 1984.