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Decomposition of benzoyl peroxide in triacetylcellulose in air
0032.-3950/79/0601,-1508507.5010
]Polymer Science U.S.S.R. VoL 21, pp, 1508-.1519. ~ ) Pergamon Press Ltd. 1980. Printed in Poland
DECOMPOSITION OF BENZOYL PEROXIDE IN TRIACETYLCELLULOSE IN AIR* L. S. ROGOVA, L. :N. GUSEVA, YU. A. MIXHXYEVand D. YA. TOPTYGIN I n s t i t u t e of Chemical Physics, U.S.S.R. A c a d e m y of Sciences
(Received 13 June 1978) Kinetics of con~mmption of benzoyl peroxide (temperature 92-115 °) in triacetylcellulose in the presence of oxygen a n d kinetics of macromolecular rupture are described b y first order equations. Effective rate constants of benzoyl peroxide cons u m p t i o n agree with corresponding constants of maeromolecular degradation o f triacetylaellulose a n d show a linear increase with the increase in the initial concentration of benzoyl peroxide a t temperatures higher t h a n 100% A reaction system is proposed which involves monomolecular a n d chain breakdown of benzoyl peroxide a n d t h e breakdown a n d isomerization of peroxide macroradicals.
DVRr~G the decomposition of benzoyl peroxide (BP) in triacetylcellulose (TAC) in vacuum kinetics of consumption of BP and kinetics of maeromoleeular rupture are described by first order equations up to high degrees of conversion of BP and rate constants of consumption of BP and of rupture show a linear increase with an increase in the initial concentration of BP in TAC [1]. The reaction system proposed [1] which describes these relations involves two methods of decomposition of BP -- a monomoleeular mechanism and a mechanism induced by eyclohexadienyl radicals. Based on ideas described in the literature kinetics of consumption of BP in polymers in air correspond to monomolecular decomposition of BP as a consequence of the inhibition by oxygen of induced decomposition of BP. These kinetics were observed in the presence of oxygen during the decomposition of BP in polystyrene and polyolefins [2-5]. A study was made of kinetic regularities of the consumption of BP inhibited by oxygen and macromoleeular rupture in TAC. TAC of a molecular viscosity weight of M , = 330,000 a n d a degree of acetylation of 62.5, produced b y the Okha combine was used for the study. A description was given o f m e t h o d s used for the purification of TAC, BP, casting of films used in the s t u d y and methods of determining M , , the n u m b e r of ruptures per initial maeromolecule n and determining t h e concentration of B P in TAC during decomposition [1]. Decomposition of B P in TAC was effected in test tubes of about 30 cm 3. The sample (film thickness 15-20 pro) was placed in the lower p a r t of the test tube immersed into an oil t h e r m o s t a t to 3 era. Temperature was maintained with an accuracy of 4-0.5 °. The samples were weighed before and after heating a n d gravimetric loss was calculated allowing for M , . The content of B P in volatile products (condensed on the walls of test tubes) was less t h a n 1 ~o of B P introduced using m a x i m u m temperature, relaxation time a n d B P c o n c e r t *
Yysokomol. soyed, h21: No. 6, 1373-1382, 1979. 1508
Decomposition of benzoyl peroxide
1509
tration. Special experiments have shown that rate constants of decomposition of BP for TAC films studied are independent of film thickness within the range of 10 to 40 gin. Consequently, the reactions of oxygen take place in the kinetic region. To determine macromolecular peroxides during the experiments, nondisintegrated BP and low molecular weight products of decomposition were completely washed out using isopropyl alcohol, the extent of extraction being controlled by U-V spectra. The amount of peroxides remaining in the film was then determined by conventional methods. Experimental results of the consumption of B P and rupture in TAC are tabulated. Figure l a shows linear transformations of curves of the consumption of B P in coordinates of a first order equation for various initial concentrations of B P at 115 °. I t can be seen t h a t in air effective rate constants of the decomposition of B P in TAC show a linear dependence on the initial concentration of BP, i.e. the effective rate constant of B P consumption m a y be written as ke=ko+kc[BP]o,
(1)
the value of k o corresponding to spontaneous decomposition and k¢[BP]o, to chain decomposition. We observed a similar mechanism during the decomposition of BP without oxygen [1]. Figure lb shows the dependence of effective rate constants of decomposition of B P on the initial concentration of BP, derived from linear transformations of kinetic curves of consumption of B P at temperatures of 92, 98, 107 and 115 °. I t can be seen t h a t with a reduction in the temperature of decomposition of BP the proportion of induced decomposition decreases considerably. At a temperature of 92 ° there is no induced decomposition in the concentration range of B P studied and kc92.= 0. I t is possible t h a t in other polymers the rate constant of consumption in air is independent of the initial concentration of BP as a consequence of comparitively low experimental temperatures. For comparison, Figure lb shows the dependence of ke on the concentration of B P in vacuum at a temperature of 98 ° (Fig. lb, curve 5) derived previously [1]. At this temperature in the presence of oxygen the effect of chain decomposition of B P is much lower t h a n in vacuum (Fig. lb, curve 3). Atmospheric oxygen considerably reduces the effect of induced decomposition of B P without affecting its spontaneous decomposition (k0=4.SX10-~min -1 in air and in vacuum). Regardless of the similarity of general mechanisms of decomposition of B P with and without oxygen, shown by the fact t h a t kinetically the consumption of B P is described in both cases by a first order equation, the value of kc (eqn. (1)) is considerably lower in the presence of oxygen t h a n in the absence of oxygen and decreases at a higher rate with a reduction in temperature. :Figure 2a shows the dependence of effective constants ke on temperature in coordinates of an Arrhenius equation for various initial concentrations of ]BP. The same :Figure (curve 4) shows in Arrhenius coordinates rate constants of epontaneous decomposition of BP, derived b y extrapolation of straigh$ lines in Fig. l b to zero concentration of BP. The effective activation energy of the consumption of B P in air increases with an increase in the initial concentration of BP. F o r example, when [BP]0=0.05mole/kg it is close to 28 kcal/mole and with con-
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Decomposition of benzoyl peroxide
1511
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1512
L . S . RoGovA et al.
c e n t r a t i o n s of 0.38 a n d 0.7 mole/kg, it increases to 36-37 kcal/mole. T h e a c t i v a t i o n e n e r g y E 0 = 28 k c a l / m o l e d e r i v e d for s p o n t a n e o u s d e c o m p o s i t i o n of B P agrees w i t h t h e effective a c t i v a t i o n e n e r g y of t h e c o n s u m p t i o n of B P in v a c u u m a n d is close to t h e v a l u e d e r i v e d in liquid solutions. Time, m i n tO
30
50
3* log K e ~6
2
O~
[BP] ! ÷ log [Bp] ° t( ., 02~rn i n -'
to I
5r-
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al
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2.55
[ BP.I r n o l e / k g
FIG. 1
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FIG. 2
Fro. 1. Linear transformations of kinetic curves showing the consumption of BP in TAC in air at 115° (a) and dependence on initial concentration of BP (b) of effective rate constants of the consumption of BP and macromolecular rupture of TAC (dark circles in curve 1) in air: a--[BP]0, mole/kg: 1--0.05; 2--0.2; 3--0.38; 4--0.7; b--temperature, °C: 1--115, 2--107, 3, 5--98, 4--92; 5--vacuum. FIG. 2. Dependence on inverse temperature of rate constants of the consumption of BP (a) and initial rates of rupture formation in TAC (b) in air; [BP]0, mole/kg: 1--0.7; 2--0,38; 3-- 0"05 and 4-- 0. A s t u d y w a s also maple of kinetic relations of m a c r o m o l e c u l a r r u p t u r e of T A G d u r i n g t h e c o n s u m p t i o n of B P in air. K i n e t i c curves of m a c r o m o l e c u l a r r u p t u r e of T A C for v a r i o u s initial c o n c e n t r a t i o n s of B P are s h o w n in Fig. 3a, t h e i r linear t r a n s f o r m a t i o n s in c o o r d i n a t e s of a first o r d e r e q u a t i o n - - i n Fig. 3b. W i t h a
Deeomposi$ion of benzoyl peroxide
1513
concentration of B F of 0.5 and 0.196 mole/kg experimental values of the limiting number of rupture n~ were used to plot the transformations. With concentrations of B P of 0.38 and 0.7 mole/kg the breakdown of TAC continues even after B P has been used up. On reaching high concentrations of B P reactions with oxygen apparently result i n a new process of breakdown of TAC. The effect of this process P
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Time, rain FIG. 3. Kinetic curves of macromolecular rupture (a) a n d linear transformations (b). Decomposition of B P in TAC in air a t 115 °, [BP]0, mole/kg: •--0.7, 2--0.38, 3--0.2, 4--0-05; 5--calculation curve for [BP]0=0.7 mole/kg.
increases with an increase in the initial concentration of BP; it was shown b y a special experiment that it does not depend on the formation of a product of decomposition of B P (benzoic acid) in the system and the formation of water and is not therefore the result of heterolytie breakdown of the initial polymer. I t could be assumed at first glance that further rupture is due to the formation of polymeric peroxide compounds in the system. However, further increase in the number of macromolecular rupture is not accompanied b y a change in kinetics of consumption peroxides subjected to titration. We have shown, furthermore, t h a t during the consumption of B P (concentration 0.7 and 0.38 mole/kg) at 115 °
1514
L . S . I~OGOVA et cd.
macromolecular peroxides and hydroperoxides are not formed in noticeable quantities. It may therefore be assumed that further rupture occurs not as a result of degenerate branching on polymeric peroxides, but as a consequence of oxidation of a polymer chemically modified by the formation of particles that are readily oxidized. For the highest BP concentration studied (0.7 mole/kg) the effect of further rupture as a result of chemical modification of TAC is significant already at the initial stages of decomposition of BP. Values of n~ and ke could not therefore be determined. For a concentration of [BP]0-----0.38 molc/kg it was established that curves of the breakdown of TAC and the decomposition of BP a r e identical up to a degree of conversion of 5 0 ~ and the value of n is proportional to t.he amount of peroxide used up (the same proportionality holds good for [BP]0~0.05 and 0.196 mole]kg). Bearing in mind this proportionality the value of n~ corresponding to breakdown uncomplicated by oxidation of the modified polymer, was calculated for [BP]0-----0.38 mole/kg. It appeared that for three concentrations of BP--0.05, 0.196 and 0.38 mole/kg--effective rate constants of degradation determined from results of Fig. 3b, coincide with corresponding values of ke of the decomposition of BP (Fig. lb, curve 1). A similar agreement of effective constants is also observed for different temperatures when [BP]0~0"38 mole/kg (Table). For both processes Ee----36kcal/mole. The activation energy obtained from the Arrhenius dependence of the initial rate of degradation w0is 30 kcal/mole. I t may be concluded from results ~that kinetic mechanisms of macromolecular rupture of a chemically unmodified polymer agree with kinetic regularities of the consumption of BP in this system. The absence of the formation of polymeric peroxide compounds in the concentration range of BP studied and the sole exponential relation of the conq sumption of BP to high degrees of conversion (up to 8 5 ~ in many experiments) is evidence of the fact that the effect of atmospheric oxygen observed is not due to the masking formation in the system of peroxide subjected to titration. The absence from the system of the chain reaction R02 ~-PH - ~ ROOH ~-PO~ is apparently due to more rapid reactions of loss with the participation of peroxide radicals. The similar effect of atmospheric oxygen in liquid solutions is related to inhibition of the chain decomposition of BP as a consequence of the oxidation of solvent radicals, which induce this reaction, to inactive peroxide radicals [6, 7]. However, in solid polymers radicals of the medium do not induce the decomposition of BP [1, 5] and their oxidation cannot therefore be the cause of the effect observed. I t should also be noted that with an increase in the temperature of decomposition of BP above 92 ° the effect of the chain process increases in spite of the free penetration of oxygen, without being limited by diffusion: This incomplete inhibition of the chain reaction can only be explained assuming a viariable nature of free radicals, which induce the decomposition of BP in vacuum and
Decomposition of benzoyl peroxide
1516
in air. In ~he absence of oxygen induced decomposition of BP, apparently, t a k e s place by the action of cyclohexadienyl radicals formed as a result of the addition of initiator radicals to benzene rings of BP and products of decomposition [1]. It is known that the interaction of cyclohexadienyl radicals with oxygen t a k e s place at a high rate and results in the formation of peroxycyclohexadienyl radicals [8-10] and HO~ radicals [8, 11, 12]. Comparing results of previous studies [8, 10] it may be concluded that the efficiency of HO~ radical formation increases with an increase in temperature. The inhibition of chain decomposition of BP observed by the authors may be explained by oxidation of active cyclohexadienyl radicals to peroxycyclohexadienyl radicals (r'0~), which cannot induce the decomposition of BP. The formation of HO~ radicals which, as observed for cyclohexadienyl radicals, also have labile hydrogen atoms, may be responsible for induced decomposition. Bearing in mind the foregoing and maintaining the theory concerning reactions of light radicals and macroradicals in solid polymers, put forward in separate studies [1, 5], we propose a probable system of reactions taking place in the presence of oxygen during the decomposition of BP to TAC
B P - ~ 2r" r'+~H r~H~02
k~ r ~ H
k, H O ~ product
r'O HO~+PB k ~ r ' + products r'+PH ~
[~...PH]c k ~ P ' + r H --~ l ~ ' + r H
Po -F
k,,, o~ RO~+ degradation
~-. . . . . . 1~0~ (isomerization) k¢', Os
RO~+r'O~ ~ 2RO~_k~ loss 2r'O~ k~
~1516
L.S. ROGOVAet aL
In this system PH, P" and R" are the macromolecule and macroradicals, re~spectively; symbol ¢ H denotes benzene rings with a concentration of 2[BP]0, c o n s t a n t during decomposition until the removal of products of decomposition ~vith benzene rings in the molecules may be ignored. According to this system, for the rate of decomposition of BP and its current -concentration we obtain the following formulae:
d[BP] dt
/
2k°kl~2[¢H] koq- k , k 3 [ ¢~~ k z q _ k 3
~ Y- I [BP]
" (2)
[BP]=[BP]oe -~'t, here k3:ko~
2koklk2[¢H] klks[¢H] + kc [PH](k~q-k3)
(3)
The experimental linear relation observed between ke and [BP]o=I/2[~H ], :shown in Fig. lb indicates that with the existing concentrations of BP the following ratio holds good kc[PH] > k l [ ¢ H ] (4) It is known that rate constants of free radical acceptance by aromatic compounds r ' ¢ ~ - H -* r.~H exceed rate constants of free radical loss [8] in a number ~*)fcases and exceed by several orders of magnitude the rate constants of element a r y reactions of hydrogen atom abstraction r'~-PH -~ rH-[-R" [13]. Consequently, t h e latter cannot compete with reactions of addition of radicals to benzene rings. However, in TAC the transfer of free valency to maeromolecules competes successfully with the reaction of addition and is recorded according to macromolecular -degradation. It may hence be assumed that transfer of valency to macromolecules :in solid polymers is not an elementary reaction of radical substitution. Transfer m a y be presented using a model of microheterogeneous distribution of low molecular weight additives in solid TAC, previously confirmed [14]. Using this model i t may be assumed that BP molecules are arranged between particles formed by TAC macromolecules creating a microphase in the polymer. This model makes it possible, first of all, to explain the absence of decomposition of BP induced by radicals of the medium in polymers, in contrast with liquid solutions with homogeneous distribution of additives and secondly, to present the process of free valency transfer to macromolecules "in the form of two sequential stages. The first stage of transfer is the sorption of light initiator radicals on the walls of polymer particles (this stage is introduced in the system with rate constant kc). I t is determined by the rate of displacement of the initiator radical to the particle in the microphase of the additive (at a distance of the order of several dozens of .~ngstrSms) and may compete with the reaction of addition of the radical to benzene rings, which is in agreement with ratio (4). The second stage of transfer is the reaction of radical substitution in radical complex (r...PH) to form macroradicals, including those responsible for macromolecular rupture of TAC.
Decomposition of benzoyl peroxide
1517
Results enable a comparison to be made between the rate of macromolecular rupture in air (Table) and in vacuum [1] during decomposition of BP at 98 °. I t appears that the rate of degradation decreases approximately 1.5 fold in the presence of atmospheric oxygen. To explain the inhibition of degradation, isomerization of peroxide radicals PO~ (with constant ]c~) was added to the reaction which competes with their decomposition. I f this reaction (with/c~) were absent
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FIG. 4. Dependence of the initial rate w0 of molecular rupture of TAC on the initial concentration of B P at 115 ° in air. FIG. 5. Correlation between inverse values o f the limiting n u m b e r of molecular r u p t u r e of" TAC and initial concentration of B P at 115 °.
the effect of degradation ought to increase during inhibition of chain decomposition of BP by the action of oxygen, since the elimination of the inactive chain direction of the consumption of BP should increase the effect of free valency transfer to the polymer. It follows from calculations that the effect of degradation ought to increase on reducing the rate of chain decomposition of BP even if this process took place as part of macroradical loss. Special experiments carried out at 115 ° in pure oxygen "show that on increasing oxygen concentration 6 times, compared with air, the rate of degradation decreased 1.2 fold in all on average. This enables the possibility of inhibition to be rejected by the competition of the degradation of macroradicals P" with oxidation to undecomposing PO~ peroxide radicals. It should therefore be assumed that competition of decomposition with isomerization of PO~ radicals alone is responsible for the reduction in the effect of degradation of TAC in the presence of oxygen, compared with vacuum. All reactions of the loss of free valencies included in the system agree with kinetic regularities of consumption of BP and degradation of TAC molecules. The loss of peroxide macroradicals occurs by interaction with light initiator radicals. Corresponding products of the addition of BP fragments TAC macromolecules were detected by the authors according to UV absorption typical of benzene derivatives (after careful removal from the films of BP residues and low
1518
L.S. RoGovA eta/.
molecular weight products of decomposition). Similar products of addition were also observed [2] during the decomposition of BP in polystyrene in air. The system proposed gives the following formula for the rate of macromolecular degradation: dn 2ko/¢5 . . . . _~,~
w= ~
-
k~+k~' l~rJ0e
k-5-~i
(5)
According to this formula the initial rate of degradation is directly proportional to the initial concentration of BP: Wo----c[BP]0 (c is a constant). This relation is illustrated by Fig. 4. The effective activation energy characterizing the initial rate of degradation, determined from results shown in Fig. 2b is 30 kcal/mole, which is close to E 0 and is in agreement with equation (5). The kinetic equation of molecular rupture of TAC derived using the system proposed by integration of eqn. (5), takes the form n=
2/~0/~~
- -
k~
- -
[BP]0 _e_~.t 1
(61
The limiting concentration of rupture is related to the initial concentration o f [BP]0 b y the formula 1 k0 2he ÷ (71 n~ A[BP]0 A ' 2k0ks k~ where A = - - ks÷k~
• The kinetic equation of rupture (6) in logarithmic
k~÷/4'
coordinates takes the form nee
I n -
----]¢et
(8 t
noo--n
The applicability of ratios (7) and (8) is confirmed by results in Fig. 3b and Fig. 5. The ratio of the segment intersected on the ordinate axis by a straight line (Fig. 5) to the tangent of the gradient of this straight line which according to eqn. (7) is equal to the value of ke/ko, is 2.6 at 115 ° and close to the value o f he/k0----2.9 derived from the concentration dependence of rate constants of t h e consumption of B P at 115 ° (Fig. lb, curve 1). The agreement of rate constants of the consumption of BP and macromolecular rupture (Table and Fig. lb, curve 1) conforms to the proposed system and further confirms the validity of ratio (4). The effective activation energy of t h e consumption of B P (Ee=36 kcal]mole>E0), derived with initial concentrations of B P of 0.38 and 0.7 mole/kg is, apparently, due to the temperature dependence of the reaction of HO~ radical formation (reaction with constant k2) causing induced decomposition of BP. The role of this reaction decreases with a reduction in the concentration of BP and with a reduction of temperature; the value of Ee in these cases approximates to E 0. Translated by E. S E ~
Mechanical durability of heat resistant polymers
1519
REFERENCES 1. L. N. GUSEVA, Yu. A. MIKHEYEV and D. Ya. TOPTYGIN, Vysokomol. soyed. A20: 2006, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 9, 2253, 1978) 2. H. C. HAAS, J. Polymer Sci. 39: 493, 1959; 55: 33, 1961 3. A. V. TOBOLSKY, P. M. NORLING, N. H. FRICK and H. YU, J. Amor. Chem. See. 86: 3925, 1964 4. J. C. W. CHIEN and D. S. T. WANG, Macromoleeules 8: 920, 1975 5. R. RADO, Chem. listy 61: 785, 1967 6. K. NOZAKI and P. D. BARTLETT, J. Amer. Chem. See. 68: 1686, 1946 7. G. A. RUSSELL, J. Amer. Chem. Soc. 78: 1044, 1956 8. L. M. DORFMAN, J. A. TAUB and R. E. BUHLER, J. Chem. Phys. 36: 3051, 1962 9. K.-D. ASMUS, B. CERCEK, M. EBERT, A. HENGLEIN and A. WIGGER, Trans. F a r a d a y Soc. 63: 2435, 1967 10. O. Ye. YAKIMCHENKO, I. S. GAPONOVA, V. M. GOL'DBERG, G. B. PARIISKII, D. Ya. TOPTYGIN and Ya. S. LEBEDEV, Izv. AN SSSR, ser. khim., 354, 1974 11. M. B. LEDGER and G. PORTER, J. Chem. See. F a r a d a y Trans. I, 539, 1972 12. D. NONKHIBEL and G. UOLTEN, K h i m i y a svobodnykh radikalov (Chemistry of Free Radicals). Izd. "Mir", 1977 13. N. N. SEMENOV, O nekotorykh problemakh khimicheskoi kinetiki i reaktsionnoi sposobnosti (Some Problems of Chemical Kinetics and Reactivity). Izd. AN SSSR, 1958 14. A. G. ZATSEPIN, N. I. NAIMARK a n d A. I. DEMINA, Vysokomol. soyed. A18: 561, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 3, 641, 1976)
Polymer ScienceU.S.S.R. ¥ol. 21, pp. 1519-1529. (~) PergamonPress Ltd. 1980.Printed in Poland
0032-3950]79/0601-1519507.50]0
STUDY OF MECHANICAL DURABILITY OF HEAT RESISTANT POLYMERS UNDER CONDITIONS OF STRESS RELAXATION (USING POLYBENZOXAZOLE)* ~. B. BANYAVICHYUS,A. I. MARMAand A. A. ASKADSKII A n t a n a s Snechkus Polytechnic, K a u n a s I n s t i t u t e of Hetero-Organie Compounds, U.S.S.R. Academy of Sciences
(Received 16 June 1978) A study was made of relaxation properties of aromatic polybenzoxazole over a wide range of temperature a n d deformation. Detailed analysis of the relaxation behaviour of this polymer revealed substates distinguished b y different rates of relaxation processes in the range of glass-like states. I t was established t h a t linearity of mechanical behaviour of polybenzoxazole is observed over a fairly wide range oi temperature a n d deformation. I t was shown t h a t the principle of temperature-time analogy holds * Vysokomol. soyed. A21: No. 6, 1383-1392, 1979.
]Polymer Science U.S.S.R. VoL 21, pp, 1508-.1519. ~ ) Pergamon Press Ltd. 1980. Printed in Poland
DECOMPOSITION OF BENZOYL PEROXIDE IN TRIACETYLCELLULOSE IN AIR* L. S. ROGOVA, L. :N. GUSEVA, YU. A. MIXHXYEVand D. YA. TOPTYGIN I n s t i t u t e of Chemical Physics, U.S.S.R. A c a d e m y of Sciences
(Received 13 June 1978) Kinetics of con~mmption of benzoyl peroxide (temperature 92-115 °) in triacetylcellulose in the presence of oxygen a n d kinetics of macromolecular rupture are described b y first order equations. Effective rate constants of benzoyl peroxide cons u m p t i o n agree with corresponding constants of maeromolecular degradation o f triacetylaellulose a n d show a linear increase with the increase in the initial concentration of benzoyl peroxide a t temperatures higher t h a n 100% A reaction system is proposed which involves monomolecular a n d chain breakdown of benzoyl peroxide a n d t h e breakdown a n d isomerization of peroxide macroradicals.
DVRr~G the decomposition of benzoyl peroxide (BP) in triacetylcellulose (TAC) in vacuum kinetics of consumption of BP and kinetics of maeromoleeular rupture are described by first order equations up to high degrees of conversion of BP and rate constants of consumption of BP and of rupture show a linear increase with an increase in the initial concentration of BP in TAC [1]. The reaction system proposed [1] which describes these relations involves two methods of decomposition of BP -- a monomoleeular mechanism and a mechanism induced by eyclohexadienyl radicals. Based on ideas described in the literature kinetics of consumption of BP in polymers in air correspond to monomolecular decomposition of BP as a consequence of the inhibition by oxygen of induced decomposition of BP. These kinetics were observed in the presence of oxygen during the decomposition of BP in polystyrene and polyolefins [2-5]. A study was made of kinetic regularities of the consumption of BP inhibited by oxygen and macromoleeular rupture in TAC. TAC of a molecular viscosity weight of M , = 330,000 a n d a degree of acetylation of 62.5, produced b y the Okha combine was used for the study. A description was given o f m e t h o d s used for the purification of TAC, BP, casting of films used in the s t u d y and methods of determining M , , the n u m b e r of ruptures per initial maeromolecule n and determining t h e concentration of B P in TAC during decomposition [1]. Decomposition of B P in TAC was effected in test tubes of about 30 cm 3. The sample (film thickness 15-20 pro) was placed in the lower p a r t of the test tube immersed into an oil t h e r m o s t a t to 3 era. Temperature was maintained with an accuracy of 4-0.5 °. The samples were weighed before and after heating a n d gravimetric loss was calculated allowing for M , . The content of B P in volatile products (condensed on the walls of test tubes) was less t h a n 1 ~o of B P introduced using m a x i m u m temperature, relaxation time a n d B P c o n c e r t *
Yysokomol. soyed, h21: No. 6, 1373-1382, 1979. 1508
Decomposition of benzoyl peroxide
1509
tration. Special experiments have shown that rate constants of decomposition of BP for TAC films studied are independent of film thickness within the range of 10 to 40 gin. Consequently, the reactions of oxygen take place in the kinetic region. To determine macromolecular peroxides during the experiments, nondisintegrated BP and low molecular weight products of decomposition were completely washed out using isopropyl alcohol, the extent of extraction being controlled by U-V spectra. The amount of peroxides remaining in the film was then determined by conventional methods. Experimental results of the consumption of B P and rupture in TAC are tabulated. Figure l a shows linear transformations of curves of the consumption of B P in coordinates of a first order equation for various initial concentrations of B P at 115 °. I t can be seen t h a t in air effective rate constants of the decomposition of B P in TAC show a linear dependence on the initial concentration of BP, i.e. the effective rate constant of B P consumption m a y be written as ke=ko+kc[BP]o,
(1)
the value of k o corresponding to spontaneous decomposition and k¢[BP]o, to chain decomposition. We observed a similar mechanism during the decomposition of BP without oxygen [1]. Figure lb shows the dependence of effective rate constants of decomposition of B P on the initial concentration of BP, derived from linear transformations of kinetic curves of consumption of B P at temperatures of 92, 98, 107 and 115 °. I t can be seen t h a t with a reduction in the temperature of decomposition of BP the proportion of induced decomposition decreases considerably. At a temperature of 92 ° there is no induced decomposition in the concentration range of B P studied and kc92.= 0. I t is possible t h a t in other polymers the rate constant of consumption in air is independent of the initial concentration of BP as a consequence of comparitively low experimental temperatures. For comparison, Figure lb shows the dependence of ke on the concentration of B P in vacuum at a temperature of 98 ° (Fig. lb, curve 5) derived previously [1]. At this temperature in the presence of oxygen the effect of chain decomposition of B P is much lower t h a n in vacuum (Fig. lb, curve 3). Atmospheric oxygen considerably reduces the effect of induced decomposition of B P without affecting its spontaneous decomposition (k0=4.SX10-~min -1 in air and in vacuum). Regardless of the similarity of general mechanisms of decomposition of B P with and without oxygen, shown by the fact t h a t kinetically the consumption of B P is described in both cases by a first order equation, the value of kc (eqn. (1)) is considerably lower in the presence of oxygen t h a n in the absence of oxygen and decreases at a higher rate with a reduction in temperature. :Figure 2a shows the dependence of effective constants ke on temperature in coordinates of an Arrhenius equation for various initial concentrations of ]BP. The same :Figure (curve 4) shows in Arrhenius coordinates rate constants of epontaneous decomposition of BP, derived b y extrapolation of straigh$ lines in Fig. l b to zero concentration of BP. The effective activation energy of the consumption of B P in air increases with an increase in the initial concentration of BP. F o r example, when [BP]0=0.05mole/kg it is close to 28 kcal/mole and with con-
1810
c ? '
L. S. R o o o v i
~ oZ.
" - ' ~ ~
~
@
.~ ~'
•
~
',tt ~
~
I
~1
@ 0
@ 0
~ l l
I
I
I,
t
0
0
,
I
i
Decomposition of benzoyl peroxide
1511
~6~66
~ i ~
I~ ~
oo
~
I
oo r ~ r.. , ~
~
I
1
~
~ "~ ~" "~ ¢¢
7
v
iq
6
,$
~666666 6
- -
d~
~.~
8
1512
L . S . RoGovA et al.
c e n t r a t i o n s of 0.38 a n d 0.7 mole/kg, it increases to 36-37 kcal/mole. T h e a c t i v a t i o n e n e r g y E 0 = 28 k c a l / m o l e d e r i v e d for s p o n t a n e o u s d e c o m p o s i t i o n of B P agrees w i t h t h e effective a c t i v a t i o n e n e r g y of t h e c o n s u m p t i o n of B P in v a c u u m a n d is close to t h e v a l u e d e r i v e d in liquid solutions. Time, m i n tO
30
50
3* log K e ~6
2
O~
[BP] ! ÷ log [Bp] ° t( ., 02~rn i n -'
to I
5r-
I
I
I
3+log~ o
2"0 3 2 t.O _
'$"
_
I
al
I
0"4
4jL
I 0"8
I
2.55
[ BP.I r n o l e / k g
FIG. 1
I
I
2.75 1 0 3 / T , "K-I
FIG. 2
Fro. 1. Linear transformations of kinetic curves showing the consumption of BP in TAC in air at 115° (a) and dependence on initial concentration of BP (b) of effective rate constants of the consumption of BP and macromolecular rupture of TAC (dark circles in curve 1) in air: a--[BP]0, mole/kg: 1--0.05; 2--0.2; 3--0.38; 4--0.7; b--temperature, °C: 1--115, 2--107, 3, 5--98, 4--92; 5--vacuum. FIG. 2. Dependence on inverse temperature of rate constants of the consumption of BP (a) and initial rates of rupture formation in TAC (b) in air; [BP]0, mole/kg: 1--0.7; 2--0,38; 3-- 0"05 and 4-- 0. A s t u d y w a s also maple of kinetic relations of m a c r o m o l e c u l a r r u p t u r e of T A G d u r i n g t h e c o n s u m p t i o n of B P in air. K i n e t i c curves of m a c r o m o l e c u l a r r u p t u r e of T A C for v a r i o u s initial c o n c e n t r a t i o n s of B P are s h o w n in Fig. 3a, t h e i r linear t r a n s f o r m a t i o n s in c o o r d i n a t e s of a first o r d e r e q u a t i o n - - i n Fig. 3b. W i t h a
Deeomposi$ion of benzoyl peroxide
1513
concentration of B F of 0.5 and 0.196 mole/kg experimental values of the limiting number of rupture n~ were used to plot the transformations. With concentrations of B P of 0.38 and 0.7 mole/kg the breakdown of TAC continues even after B P has been used up. On reaching high concentrations of B P reactions with oxygen apparently result i n a new process of breakdown of TAC. The effect of this process P
Q
7I 5
o
5
..~.~ 3
'
.~X..---X--
~'~
':~"1 "x
,
X
I
'X
,
I
I
_/7/
///
.
0"2//x ,
I
20
, ,
I
,
40
I
60
Time, rain FIG. 3. Kinetic curves of macromolecular rupture (a) a n d linear transformations (b). Decomposition of B P in TAC in air a t 115 °, [BP]0, mole/kg: •--0.7, 2--0.38, 3--0.2, 4--0-05; 5--calculation curve for [BP]0=0.7 mole/kg.
increases with an increase in the initial concentration of BP; it was shown b y a special experiment that it does not depend on the formation of a product of decomposition of B P (benzoic acid) in the system and the formation of water and is not therefore the result of heterolytie breakdown of the initial polymer. I t could be assumed at first glance that further rupture is due to the formation of polymeric peroxide compounds in the system. However, further increase in the number of macromolecular rupture is not accompanied b y a change in kinetics of consumption peroxides subjected to titration. We have shown, furthermore, t h a t during the consumption of B P (concentration 0.7 and 0.38 mole/kg) at 115 °
1514
L . S . I~OGOVA et cd.
macromolecular peroxides and hydroperoxides are not formed in noticeable quantities. It may therefore be assumed that further rupture occurs not as a result of degenerate branching on polymeric peroxides, but as a consequence of oxidation of a polymer chemically modified by the formation of particles that are readily oxidized. For the highest BP concentration studied (0.7 mole/kg) the effect of further rupture as a result of chemical modification of TAC is significant already at the initial stages of decomposition of BP. Values of n~ and ke could not therefore be determined. For a concentration of [BP]0-----0.38 molc/kg it was established that curves of the breakdown of TAC and the decomposition of BP a r e identical up to a degree of conversion of 5 0 ~ and the value of n is proportional to t.he amount of peroxide used up (the same proportionality holds good for [BP]0~0.05 and 0.196 mole]kg). Bearing in mind this proportionality the value of n~ corresponding to breakdown uncomplicated by oxidation of the modified polymer, was calculated for [BP]0-----0.38 mole/kg. It appeared that for three concentrations of BP--0.05, 0.196 and 0.38 mole/kg--effective rate constants of degradation determined from results of Fig. 3b, coincide with corresponding values of ke of the decomposition of BP (Fig. lb, curve 1). A similar agreement of effective constants is also observed for different temperatures when [BP]0~0"38 mole/kg (Table). For both processes Ee----36kcal/mole. The activation energy obtained from the Arrhenius dependence of the initial rate of degradation w0is 30 kcal/mole. I t may be concluded from results ~that kinetic mechanisms of macromolecular rupture of a chemically unmodified polymer agree with kinetic regularities of the consumption of BP in this system. The absence of the formation of polymeric peroxide compounds in the concentration range of BP studied and the sole exponential relation of the conq sumption of BP to high degrees of conversion (up to 8 5 ~ in many experiments) is evidence of the fact that the effect of atmospheric oxygen observed is not due to the masking formation in the system of peroxide subjected to titration. The absence from the system of the chain reaction R02 ~-PH - ~ ROOH ~-PO~ is apparently due to more rapid reactions of loss with the participation of peroxide radicals. The similar effect of atmospheric oxygen in liquid solutions is related to inhibition of the chain decomposition of BP as a consequence of the oxidation of solvent radicals, which induce this reaction, to inactive peroxide radicals [6, 7]. However, in solid polymers radicals of the medium do not induce the decomposition of BP [1, 5] and their oxidation cannot therefore be the cause of the effect observed. I t should also be noted that with an increase in the temperature of decomposition of BP above 92 ° the effect of the chain process increases in spite of the free penetration of oxygen, without being limited by diffusion: This incomplete inhibition of the chain reaction can only be explained assuming a viariable nature of free radicals, which induce the decomposition of BP in vacuum and
Decomposition of benzoyl peroxide
1516
in air. In ~he absence of oxygen induced decomposition of BP, apparently, t a k e s place by the action of cyclohexadienyl radicals formed as a result of the addition of initiator radicals to benzene rings of BP and products of decomposition [1]. It is known that the interaction of cyclohexadienyl radicals with oxygen t a k e s place at a high rate and results in the formation of peroxycyclohexadienyl radicals [8-10] and HO~ radicals [8, 11, 12]. Comparing results of previous studies [8, 10] it may be concluded that the efficiency of HO~ radical formation increases with an increase in temperature. The inhibition of chain decomposition of BP observed by the authors may be explained by oxidation of active cyclohexadienyl radicals to peroxycyclohexadienyl radicals (r'0~), which cannot induce the decomposition of BP. The formation of HO~ radicals which, as observed for cyclohexadienyl radicals, also have labile hydrogen atoms, may be responsible for induced decomposition. Bearing in mind the foregoing and maintaining the theory concerning reactions of light radicals and macroradicals in solid polymers, put forward in separate studies [1, 5], we propose a probable system of reactions taking place in the presence of oxygen during the decomposition of BP to TAC
B P - ~ 2r" r'+~H r~H~02
k~ r ~ H
k, H O ~ product
r'O HO~+PB k ~ r ' + products r'+PH ~
[~...PH]c k ~ P ' + r H --~ l ~ ' + r H
Po -F
k,,, o~ RO~+ degradation
~-. . . . . . 1~0~ (isomerization) k¢', Os
RO~+r'O~ ~ 2RO~_k~ loss 2r'O~ k~
~1516
L.S. ROGOVAet aL
In this system PH, P" and R" are the macromolecule and macroradicals, re~spectively; symbol ¢ H denotes benzene rings with a concentration of 2[BP]0, c o n s t a n t during decomposition until the removal of products of decomposition ~vith benzene rings in the molecules may be ignored. According to this system, for the rate of decomposition of BP and its current -concentration we obtain the following formulae:
d[BP] dt
/
2k°kl~2[¢H] koq- k , k 3 [ ¢~~ k z q _ k 3
~ Y- I [BP]
" (2)
[BP]=[BP]oe -~'t, here k3:ko~
2koklk2[¢H] klks[¢H] + kc [PH](k~q-k3)
(3)
The experimental linear relation observed between ke and [BP]o=I/2[~H ], :shown in Fig. lb indicates that with the existing concentrations of BP the following ratio holds good kc[PH] > k l [ ¢ H ] (4) It is known that rate constants of free radical acceptance by aromatic compounds r ' ¢ ~ - H -* r.~H exceed rate constants of free radical loss [8] in a number ~*)fcases and exceed by several orders of magnitude the rate constants of element a r y reactions of hydrogen atom abstraction r'~-PH -~ rH-[-R" [13]. Consequently, t h e latter cannot compete with reactions of addition of radicals to benzene rings. However, in TAC the transfer of free valency to maeromolecules competes successfully with the reaction of addition and is recorded according to macromolecular -degradation. It may hence be assumed that transfer of valency to macromolecules :in solid polymers is not an elementary reaction of radical substitution. Transfer m a y be presented using a model of microheterogeneous distribution of low molecular weight additives in solid TAC, previously confirmed [14]. Using this model i t may be assumed that BP molecules are arranged between particles formed by TAC macromolecules creating a microphase in the polymer. This model makes it possible, first of all, to explain the absence of decomposition of BP induced by radicals of the medium in polymers, in contrast with liquid solutions with homogeneous distribution of additives and secondly, to present the process of free valency transfer to macromolecules "in the form of two sequential stages. The first stage of transfer is the sorption of light initiator radicals on the walls of polymer particles (this stage is introduced in the system with rate constant kc). I t is determined by the rate of displacement of the initiator radical to the particle in the microphase of the additive (at a distance of the order of several dozens of .~ngstrSms) and may compete with the reaction of addition of the radical to benzene rings, which is in agreement with ratio (4). The second stage of transfer is the reaction of radical substitution in radical complex (r...PH) to form macroradicals, including those responsible for macromolecular rupture of TAC.
Decomposition of benzoyl peroxide
1517
Results enable a comparison to be made between the rate of macromolecular rupture in air (Table) and in vacuum [1] during decomposition of BP at 98 °. I t appears that the rate of degradation decreases approximately 1.5 fold in the presence of atmospheric oxygen. To explain the inhibition of degradation, isomerization of peroxide radicals PO~ (with constant ]c~) was added to the reaction which competes with their decomposition. I f this reaction (with/c~) were absent
I/n,,~ \ Og
1"5 ~
"
t~
~ 04
0.5
f ,
0
I
i
04 [S P]o,mole / kg FIG. 4
)
06
I
5
I
I
I
15 IIBPIo,kg/rnole FIG. 5
FIG. 4. Dependence of the initial rate w0 of molecular rupture of TAC on the initial concentration of B P at 115 ° in air. FIG. 5. Correlation between inverse values o f the limiting n u m b e r of molecular r u p t u r e of" TAC and initial concentration of B P at 115 °.
the effect of degradation ought to increase during inhibition of chain decomposition of BP by the action of oxygen, since the elimination of the inactive chain direction of the consumption of BP should increase the effect of free valency transfer to the polymer. It follows from calculations that the effect of degradation ought to increase on reducing the rate of chain decomposition of BP even if this process took place as part of macroradical loss. Special experiments carried out at 115 ° in pure oxygen "show that on increasing oxygen concentration 6 times, compared with air, the rate of degradation decreased 1.2 fold in all on average. This enables the possibility of inhibition to be rejected by the competition of the degradation of macroradicals P" with oxidation to undecomposing PO~ peroxide radicals. It should therefore be assumed that competition of decomposition with isomerization of PO~ radicals alone is responsible for the reduction in the effect of degradation of TAC in the presence of oxygen, compared with vacuum. All reactions of the loss of free valencies included in the system agree with kinetic regularities of consumption of BP and degradation of TAC molecules. The loss of peroxide macroradicals occurs by interaction with light initiator radicals. Corresponding products of the addition of BP fragments TAC macromolecules were detected by the authors according to UV absorption typical of benzene derivatives (after careful removal from the films of BP residues and low
1518
L.S. RoGovA eta/.
molecular weight products of decomposition). Similar products of addition were also observed [2] during the decomposition of BP in polystyrene in air. The system proposed gives the following formula for the rate of macromolecular degradation: dn 2ko/¢5 . . . . _~,~
w= ~
-
k~+k~' l~rJ0e
k-5-~i
(5)
According to this formula the initial rate of degradation is directly proportional to the initial concentration of BP: Wo----c[BP]0 (c is a constant). This relation is illustrated by Fig. 4. The effective activation energy characterizing the initial rate of degradation, determined from results shown in Fig. 2b is 30 kcal/mole, which is close to E 0 and is in agreement with equation (5). The kinetic equation of molecular rupture of TAC derived using the system proposed by integration of eqn. (5), takes the form n=
2/~0/~~
- -
k~
- -
[BP]0 _e_~.t 1
(61
The limiting concentration of rupture is related to the initial concentration o f [BP]0 b y the formula 1 k0 2he ÷ (71 n~ A[BP]0 A ' 2k0ks k~ where A = - - ks÷k~
• The kinetic equation of rupture (6) in logarithmic
k~÷/4'
coordinates takes the form nee
I n -
----]¢et
(8 t
noo--n
The applicability of ratios (7) and (8) is confirmed by results in Fig. 3b and Fig. 5. The ratio of the segment intersected on the ordinate axis by a straight line (Fig. 5) to the tangent of the gradient of this straight line which according to eqn. (7) is equal to the value of ke/ko, is 2.6 at 115 ° and close to the value o f he/k0----2.9 derived from the concentration dependence of rate constants of t h e consumption of B P at 115 ° (Fig. lb, curve 1). The agreement of rate constants of the consumption of BP and macromolecular rupture (Table and Fig. lb, curve 1) conforms to the proposed system and further confirms the validity of ratio (4). The effective activation energy of t h e consumption of B P (Ee=36 kcal]mole>E0), derived with initial concentrations of B P of 0.38 and 0.7 mole/kg is, apparently, due to the temperature dependence of the reaction of HO~ radical formation (reaction with constant k2) causing induced decomposition of BP. The role of this reaction decreases with a reduction in the concentration of BP and with a reduction of temperature; the value of Ee in these cases approximates to E 0. Translated by E. S E ~
Mechanical durability of heat resistant polymers
1519
REFERENCES 1. L. N. GUSEVA, Yu. A. MIKHEYEV and D. Ya. TOPTYGIN, Vysokomol. soyed. A20: 2006, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 9, 2253, 1978) 2. H. C. HAAS, J. Polymer Sci. 39: 493, 1959; 55: 33, 1961 3. A. V. TOBOLSKY, P. M. NORLING, N. H. FRICK and H. YU, J. Amor. Chem. See. 86: 3925, 1964 4. J. C. W. CHIEN and D. S. T. WANG, Macromoleeules 8: 920, 1975 5. R. RADO, Chem. listy 61: 785, 1967 6. K. NOZAKI and P. D. BARTLETT, J. Amer. Chem. See. 68: 1686, 1946 7. G. A. RUSSELL, J. Amer. Chem. Soc. 78: 1044, 1956 8. L. M. DORFMAN, J. A. TAUB and R. E. BUHLER, J. Chem. Phys. 36: 3051, 1962 9. K.-D. ASMUS, B. CERCEK, M. EBERT, A. HENGLEIN and A. WIGGER, Trans. F a r a d a y Soc. 63: 2435, 1967 10. O. Ye. YAKIMCHENKO, I. S. GAPONOVA, V. M. GOL'DBERG, G. B. PARIISKII, D. Ya. TOPTYGIN and Ya. S. LEBEDEV, Izv. AN SSSR, ser. khim., 354, 1974 11. M. B. LEDGER and G. PORTER, J. Chem. See. F a r a d a y Trans. I, 539, 1972 12. D. NONKHIBEL and G. UOLTEN, K h i m i y a svobodnykh radikalov (Chemistry of Free Radicals). Izd. "Mir", 1977 13. N. N. SEMENOV, O nekotorykh problemakh khimicheskoi kinetiki i reaktsionnoi sposobnosti (Some Problems of Chemical Kinetics and Reactivity). Izd. AN SSSR, 1958 14. A. G. ZATSEPIN, N. I. NAIMARK a n d A. I. DEMINA, Vysokomol. soyed. A18: 561, 1976 (Translated in Polymer Sci. U.S.S.R. 18: 3, 641, 1976)
Polymer ScienceU.S.S.R. ¥ol. 21, pp. 1519-1529. (~) PergamonPress Ltd. 1980.Printed in Poland
0032-3950]79/0601-1519507.50]0
STUDY OF MECHANICAL DURABILITY OF HEAT RESISTANT POLYMERS UNDER CONDITIONS OF STRESS RELAXATION (USING POLYBENZOXAZOLE)* ~. B. BANYAVICHYUS,A. I. MARMAand A. A. ASKADSKII A n t a n a s Snechkus Polytechnic, K a u n a s I n s t i t u t e of Hetero-Organie Compounds, U.S.S.R. Academy of Sciences
(Received 16 June 1978) A study was made of relaxation properties of aromatic polybenzoxazole over a wide range of temperature a n d deformation. Detailed analysis of the relaxation behaviour of this polymer revealed substates distinguished b y different rates of relaxation processes in the range of glass-like states. I t was established t h a t linearity of mechanical behaviour of polybenzoxazole is observed over a fairly wide range oi temperature a n d deformation. I t was shown t h a t the principle of temperature-time analogy holds * Vysokomol. soyed. A21: No. 6, 1383-1392, 1979.