- Email: [email protected]
The thermal degradation of aromatic polybenzoxazoles
2276
I. Y]g. KARDASH et al.
9. L. MANDELKERN, W. R. KRIGBAUM, H. A. SHERAGA and P. I. FLORY, J. Chem. Phys. 20: 1392, 1952 10. V. N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL', S t r u k t u r a makromolekul o v r a s t v o r a k h (The Structure of Macromoleeules in Solutions). Izd. " N a u k a " , 1964 11. V. Ye. ESKIN and K. Z. GUMARGALIEVA, Vysokomol. soyed. 2: 265, 1960 (Not translated in Polymer Sci. U.S.S.R.) 12. V. Ye. ESKIN and L. N. ANDREYEVA, Vysokomol. soyed. 3: 435, 1961 (Not translated in P o l y m e r Sci. U.S.S.R.) 13. G. MEYERHOFF, Z. phys. Chem. 4: 335, 1955
THE THERMAL DEGRADATION OF AROMATIC POLYBENZOXAZOLES* I. YE. KARDASH, A. YA. ARDASHNIKOV and A. N. PRAVEDNrKOV L. Ya. Karpov Instituteof Physical Chemistry (Received 16 September 1968)
THE rigid-chain polymers in which the chains consist of aromatic heterocycles and benzene rings (polyimides, polybenzimidazoles, polybenzoxazoles, polyoxadiazoles etc). are at present the most promising heat resistant polymers. The problems connected with the clarification of the cause of this large heat resistance, and its dependence on the polymer chain structure, are therefore of great interest. The large numbers of this type of synthesized polymers makes it often impossible to examine the effect of structure. With a few exceptions, little is known about the degradation products, but this is essential to get some idea about the mechanism of the processes taking place at high temperatures in the polymers. The paper reports our study of the degradation kinetics of polybenzoxazoles (PB) (based on 3,3'-dihydroxybenzidine and various aromatic dicarboxylic acid dichlorides [1]) having the following structure:
n
* V y ~ k o m o l . soyed. A l 1 : No. 9, 1996-2001, 1969.
Thermal degradation of aromatic polybenzoxazoles
-
-
C
--C
2277
-- (PB-DPO)
--0--
N n
PB-I was used as an example in the study of the product composition, and of the changes occurring within the polymer during degradation. EXPERIMENTAL
The degradation kinetics of PB were investigated as follows. A poly-o-hydroxamide sample [1] (about 150 rag) was placed on a vacuum balance, heated to 100-150°C to remove any residual solvent, and then subjected to eyclodehydration at 350°C for 1 hr. Tile produced PB was heated at 10-Sam vacuum at constant temperature, and the kinetics of the weight change were recorded. This method permits the tracking of the PB sample without contact with air up to desctruction, and the moisture absorption by the sample. The gaseous degradation products were analysed by mass- and infrared-spectrometry, and the solid ones by qualitative analysis and infrared spectrometry (using samples pressed into tablets with KBr). RESULTS
Preliminary degradation tests on PB-I [1] had shown the polymer weight to remain practically constant when heated up to 500°C for a fairly long time (about 3 hr). Quite a large amount of volatile products started to be liberated above 500°C and the polymer blackened, but retained its original shape, as in the case of aromatic polyimides [2]. The weight losses of the polybenzoxazoles PB-T, PB-I and PB-DPO at different temperatures are shown as a function of time in Fig. la-c. Sufficiently high temperatures will cause the degradation rate to decrease at 20% conversion. The comparison of these rat~s indicated the polymers to fall into the following sequence with respect to heat resistance: P B - D P O < P B - I < P B - T . The logarithm of the degradation rate as a function of 1/T was used to determine the apparent activation energies of the degradation; this was found to be the same for all 3 PB, i.e. 584-3 kcal/mole. The different chain structure present in the polymers will obviously affect the pre-exponential factor. The calculations show these factors to be in the following ratio to each other: k0PB.~po : kO~B.I, : k0PB.T----12"6 : 1"26 : 1. As the chain flexibility decreased in the same order [3], it can be concluded t h a t an introduction of various "armouring" units into the polymer chain would increase the degradation rate due to an increase of the lore-exponential factor of the reaction. This could really be true only if the strength of the introduced bonds were similar to t h a t of C--C bonds (e.g. the C - - 0 bond). I f this was not so, we should expect thav the thermal stability would be reduced as a result of reducing the activation energy. Yor example, the insertion of an S02-grou p into the PB chain substantially reduced the thermal stability (the bond in the CeHs--SO~--C~I-I a grouping is much weaker than in C6Hs--CeH5 or in C6Hs--O--
2278
I. Y~E. KARDASWr e~ al.
--Cells) , as was shown earlier [41. T h e i n t r o d u c t i o n o f two S02 groups into t h e p o l y m e r chain reduced it still more [4]. T h e kinetic s t u d y of the d e g r a d a t i o n o f P B containing d i p h e n y l o x i d e as well as diphenyl sulphone groups,
A,% 20
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E
b
A,!
zl
2#.
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/8
12 ~
8
I
I
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l
2
I
I
12
I
I
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f
I
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¢60
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160
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A,% 30 3
2#
2 18
!
tO
1
"/2
8 #0
80 ~20 Time , rain
t60
200
50
fO0 150 Time, m[n
200
Fzo. 1. Weight loss (A) as a function of heating time at different temperatures for : a--PB-T, b--PB-I, c--PB-DPO and d--CH-PB-DPO. Degradation temperatures, °C: a : 1 - - 5 7 7 , 2--590, 8--605, 4--617, 5--628; 5:1--565, 2--581, 8--600, 4--617, 5--636, 6--646; c:13--51, 2--553, 3--569, 4--591; d: 1--462, 2--481, 3--500, 4--531.
Thermal degradation of aromatic polybenzoxazoles
2279
showed (Fig. ld) the process to have an activation energy of 40 kcal/mole. This confirmed our assumption t h a t the introduction of bonds less strong t h a n C-- 0 or C--O will reduce the heat resistance because of reducing the activation energy of the thermal degradation process. The following gaseous products were identified during PB-1 degradation: CO, CO2, H 2, HCN, C6H G and CH 4. In addition there was a crystalline product
100 8O
6O 40 2O
80 60 ~0 20
80 80
20
#
88 8O ~O
5 ~8
I
I
I
f6
1/-/
12
I
I
10 8 }),,fO-Z cm -1
FIG. 2. Infrared spectral changes in the PB-I film during thermal degradation for 1 hr at: •--420, 2--500, 3--550, 4--600 and 5--650°C.
2280
I. YE. KARDASHet
al.
which easily dissolved in water and had a decomposition temperature of about 90°C. The qualitative analysis of this product for COss-, NHd+ and CN- ions showed it to be a mixture of NHdHCO s and NHdCN; the formation of these products is thought to be the result of a reaction between NHa, HsO, CO s and HCN produced by the degradation. The presence of NHdHCO a indicates that water is produced during PB-I degradation. The production of water, ammonia and carbon dioxide is evidence of hydrolyric processes taking place during degradation. Assuming these compounds to form as a result of reaction between the end groups of macromolecules, and that each will give rise to one HsO molecule, the degradation of 1 g of polymer of m.w. 30,000 will give rise to about 5 × 10-4 g water. In actual fact 4 × 10 -a g water/g polymer (about 0.015 g NH~HCOa) were produced. This quantity seems to be associated with incomplete cyelization of the poly-o-hydroxamides, and the presence of a low m.w. fraction in the polymer. The reaction of the end groups of this fraction results in a m.w. increase, as shown earlier (3) at lower temperatures (350°C), and in an increase in the water produced. The latter will cause the hydrolysis of the benzoxazole ring during degradation and the formation of amide bonds; hydrolysis of the latter will give rise to amine and carboxyl groups, and these will decompose to ammonia and COs. It should be noted that degradation of the aromatic polyamides alone [5] will give similar products, which indicates similar processes to occur in the degradation of polyamides and PB. One cannot ignore, however, the possibility of radical reactions taking place in such a degradation of PB, because of the formation of CO, methane and benzene. Furthermore, an earlier degradation of a model compound, i.e. 2-phenylbenzoxazole [6], had shown it to be possibly an entirely homolytic process, leading to the same degradation products, i.e. CO, CO 2, Ha, HCN and C6H~. This similarity and also that of the temperatures giving rise to volatile products (2-phenylbenzoxazole starts to decompose at 530-550°C) indicates also homolytic decomposition of chemical bonds to occur in PB. The residue of the PB-I degradation was strongly carbonized and gave an ESR signal (narrow singlet). Its elemental analysis after decomposition at 615°C (24.50/o weight loss) was as follows (~/o): 82.52 C, 3.34 H, 6.56 N. Decomposition at 636°C (250/o weight loss) gave (~/o): 82.18 C, 4.75 H, 6.11 N, while the elemental analysis of the original polymer gave (%): 76.30 C, 3.11 H, 8-05 N. This shows the carbon and hydrogen contents to increase during degradation, while the nitrogen and oxygen contents decreased; there is thus preferential decomposition of the oxazole rings. Yet their content decreased only by 30~/o, although degradation was practically over under these conditions and the benzoxazole absorption lines had disappeared. Attention is also drawn to the retention of the 0 : N ratio in the samples after degradation (0 :N~-l-15: 1). The study of the infrared spectral changes, made on PB-I during degradation (Fig. 2) at about 500°C, where no volatile products are liberated, showed that a new absorption line appears in the 1690 cm -1 region; its intensity increased with increasing degrada-
Thermal degradation of aromatic polybenzoxazoles
2281
tion temperature. There was a simultaneous disappearance of the 935, 1555 and 1630 cm -1 lines, which are typical of the benzoxazole rings [7]. The infrared spectra of polymers after degradation were practically free from benzoxazole absorption lines of those polymers for which the elemental analysis was given above. These results show that the benzoxazole rings decompose (decrease of ~ - and O-content in the polymer, disappearance of benzoxazole rings from spectrum), and that a rearrangement of the oxazole rings takes place (preservation of the 0 : N ratio). The progress of the rearrangement was also confirmed by the appearance of the 1960 cm -1 line in the infrared spectra of the degradation products, i.e. the line attributed to the earhonyl group. As already stated, this line was found t~ appear at lower temperatures than those giving rise to volatile products from the decomposition of the oxazole ring. This is equivalent to a fairly large rate of rearrangement, compared with that of decomposition. The rearrangement seems to have a similar course to that of imino esters to tertiary amides [8]: 0--Cell 5
@_c /
o
~,\
--\__/ ~_/---- -,//--~, \=/
\ 0
.;--~, c / o - - K / ~ --,,__/-
//~
II \ = / "
[I \ = / ~--c--N< o ~J-\~,
--,/--\--c/°-/~-
\=/ / In addition there is possibly another route of rearrangement which also leads to the tertiary amide structure, as in the thermal rearrangement of oxazolines
[9]: N--Ctt2 R--C
O
CH~
CH~-* R-0
CH~
--'/~--C--N 0
~/
2282
I. YE. KARDASH et al.
The study of the kinetics of the thermal degradation of a series of aromatic P B has thus shown t h a t the degradation rate depends on the polymer chain flexibility. The insertion of " ar m our i ng" groups increased this rate. Where the strength of the inserted bonds is similar to t h a t of C--C-bonds, the increase of the degradation rate is due to increase in the pre-exponential factor of the reaction, although the activation energy of the degradation does not significantly change. Where groups with weaker bonds are introduced, the rate of the degradation will also increase as a result of decreasing activation energy of the process. The s t u d y of the P B degradation products showed there to be 3 competing routes, i.e. rearrangement of the oxazole rings, homolytic decomposition of the chemical bonds, and hydrolysis by water formed during the condensation of the functional groups contained in the low m.w. fraction of the polymer. The authors wish to t h a n k N.N. Voznesenskaya and V. S. Yakubovich for supplying the polymer samples, A. Ya. Yakubovich and G. I. Braz for useful criticism of the results. CONCLUSIONS
(1) The kinetics of thermal degradation of aromatic polybenzoxazolcs, having different elemental unit structures, were studied. The effect of polymer chain flexibility was clarified, and also t h a t of bond strength on the rate of thermal degradation. (2) The composition of the degradation products and the changes taking place in the polymer during degradation were studied; the reactions leading to the degradation of polybenzoxazoles were examined. Translated by K. A. ALLEN
REFERENCES
1. G. I. BRAZ, I. Ye. KARDASH, V. S. YAKUB0VICH, G. V. MYASNIKOVA, A. Ya. ARDASHNIKOV, A. F. OLEINIK, A. N. PRAVEDNIKOV and A. Ya. YAKUBOVICH, Vysokomol. soyed. 8: 272, 1966 (Translated in Polymer Sei. U.S.S.R. 8: 2, 295, 1966) 2. S. D. BRUCK, Polymer 5: 435, 1964; 6: 49, 1965 3. I. Ye. KARDASH, A. Ya. ARDASHNIKOV, V. S. YAKUBOVICH, G. I. BRAZ, A. Ya.
4. 5.
6.
7. 8. 9.
YAKUBOVICH and A. N. PRAVEDNIKOV, Vysokomol. soyed. A9: 1914, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 9, 2160, 1967) A. Ya. YAKUBOVICH, A. F. OLEINIK, G. I. BRAZ, N. N. VOZNESENSKAYA, V. S. YAKUBOVICH, I. Ye. KARDASH and A. Ya. ARDASHNIKOV, Vysokomol. soyed. A9: 1782, 1967 (Translated in Polymer Sei. U.S.S.R. 9: 8, 2013, 1967) Ye. P. KRASNOV, V. M. SAVINOV, L. B. SOKOLOV,V. I. LOGUNOVA,V. K. BELYAKOV and T. A. POLYAKOVA, Vysokomol. soyed. 8: 380, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 3, 413, 1966) G. I. BRAZ, G. V. MYASNIKOVA, A. Ya. YAKUBOVICH, V. P. BAZOV, I. Ye. KARDASH and A. N. PRAVEDNIKOV, Khim. geterotsikl, soyed., No. 2, 215, 1967 P. BASSIGNANA, C. COGROSSI and M. GANDINO, Speetrochim. Aeta 19: 1855, 1963 K. B. VIBERG and B. I. ROWLAND, J. Am. Chem. Soe. 77: 2205, 1955 H. L. WEHRMEISTER, J. Organ. Chemic 30: 664, 1965
I. Y]g. KARDASH et al.
9. L. MANDELKERN, W. R. KRIGBAUM, H. A. SHERAGA and P. I. FLORY, J. Chem. Phys. 20: 1392, 1952 10. V. N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL', S t r u k t u r a makromolekul o v r a s t v o r a k h (The Structure of Macromoleeules in Solutions). Izd. " N a u k a " , 1964 11. V. Ye. ESKIN and K. Z. GUMARGALIEVA, Vysokomol. soyed. 2: 265, 1960 (Not translated in Polymer Sci. U.S.S.R.) 12. V. Ye. ESKIN and L. N. ANDREYEVA, Vysokomol. soyed. 3: 435, 1961 (Not translated in P o l y m e r Sci. U.S.S.R.) 13. G. MEYERHOFF, Z. phys. Chem. 4: 335, 1955
THE THERMAL DEGRADATION OF AROMATIC POLYBENZOXAZOLES* I. YE. KARDASH, A. YA. ARDASHNIKOV and A. N. PRAVEDNrKOV L. Ya. Karpov Instituteof Physical Chemistry (Received 16 September 1968)
THE rigid-chain polymers in which the chains consist of aromatic heterocycles and benzene rings (polyimides, polybenzimidazoles, polybenzoxazoles, polyoxadiazoles etc). are at present the most promising heat resistant polymers. The problems connected with the clarification of the cause of this large heat resistance, and its dependence on the polymer chain structure, are therefore of great interest. The large numbers of this type of synthesized polymers makes it often impossible to examine the effect of structure. With a few exceptions, little is known about the degradation products, but this is essential to get some idea about the mechanism of the processes taking place at high temperatures in the polymers. The paper reports our study of the degradation kinetics of polybenzoxazoles (PB) (based on 3,3'-dihydroxybenzidine and various aromatic dicarboxylic acid dichlorides [1]) having the following structure:
n
* V y ~ k o m o l . soyed. A l 1 : No. 9, 1996-2001, 1969.
Thermal degradation of aromatic polybenzoxazoles
-
-
C
--C
2277
-- (PB-DPO)
--0--
N n
PB-I was used as an example in the study of the product composition, and of the changes occurring within the polymer during degradation. EXPERIMENTAL
The degradation kinetics of PB were investigated as follows. A poly-o-hydroxamide sample [1] (about 150 rag) was placed on a vacuum balance, heated to 100-150°C to remove any residual solvent, and then subjected to eyclodehydration at 350°C for 1 hr. Tile produced PB was heated at 10-Sam vacuum at constant temperature, and the kinetics of the weight change were recorded. This method permits the tracking of the PB sample without contact with air up to desctruction, and the moisture absorption by the sample. The gaseous degradation products were analysed by mass- and infrared-spectrometry, and the solid ones by qualitative analysis and infrared spectrometry (using samples pressed into tablets with KBr). RESULTS
Preliminary degradation tests on PB-I [1] had shown the polymer weight to remain practically constant when heated up to 500°C for a fairly long time (about 3 hr). Quite a large amount of volatile products started to be liberated above 500°C and the polymer blackened, but retained its original shape, as in the case of aromatic polyimides [2]. The weight losses of the polybenzoxazoles PB-T, PB-I and PB-DPO at different temperatures are shown as a function of time in Fig. la-c. Sufficiently high temperatures will cause the degradation rate to decrease at 20% conversion. The comparison of these rat~s indicated the polymers to fall into the following sequence with respect to heat resistance: P B - D P O < P B - I < P B - T . The logarithm of the degradation rate as a function of 1/T was used to determine the apparent activation energies of the degradation; this was found to be the same for all 3 PB, i.e. 584-3 kcal/mole. The different chain structure present in the polymers will obviously affect the pre-exponential factor. The calculations show these factors to be in the following ratio to each other: k0PB.~po : kO~B.I, : k0PB.T----12"6 : 1"26 : 1. As the chain flexibility decreased in the same order [3], it can be concluded t h a t an introduction of various "armouring" units into the polymer chain would increase the degradation rate due to an increase of the lore-exponential factor of the reaction. This could really be true only if the strength of the introduced bonds were similar to t h a t of C--C bonds (e.g. the C - - 0 bond). I f this was not so, we should expect thav the thermal stability would be reduced as a result of reducing the activation energy. Yor example, the insertion of an S02-grou p into the PB chain substantially reduced the thermal stability (the bond in the CeHs--SO~--C~I-I a grouping is much weaker than in C6Hs--CeH5 or in C6Hs--O--
2278
I. Y~E. KARDASWr e~ al.
--Cells) , as was shown earlier [41. T h e i n t r o d u c t i o n o f two S02 groups into t h e p o l y m e r chain reduced it still more [4]. T h e kinetic s t u d y of the d e g r a d a t i o n o f P B containing d i p h e n y l o x i d e as well as diphenyl sulphone groups,
A,% 20
a
E
b
A,!
zl
2#.
18 3
/8
12 ~
8
I
I
#0
l
2
I
I
12
I
I
80 120 Time, rain
f
I
#O
¢60
80 fro Time, rain
160
A,%
A,% 30 3
2#
2 18
!
tO
1
"/2
8 #0
80 ~20 Time , rain
t60
200
50
fO0 150 Time, m[n
200
Fzo. 1. Weight loss (A) as a function of heating time at different temperatures for : a--PB-T, b--PB-I, c--PB-DPO and d--CH-PB-DPO. Degradation temperatures, °C: a : 1 - - 5 7 7 , 2--590, 8--605, 4--617, 5--628; 5:1--565, 2--581, 8--600, 4--617, 5--636, 6--646; c:13--51, 2--553, 3--569, 4--591; d: 1--462, 2--481, 3--500, 4--531.
Thermal degradation of aromatic polybenzoxazoles
2279
showed (Fig. ld) the process to have an activation energy of 40 kcal/mole. This confirmed our assumption t h a t the introduction of bonds less strong t h a n C-- 0 or C--O will reduce the heat resistance because of reducing the activation energy of the thermal degradation process. The following gaseous products were identified during PB-1 degradation: CO, CO2, H 2, HCN, C6H G and CH 4. In addition there was a crystalline product
100 8O
6O 40 2O
80 60 ~0 20
80 80
20
#
88 8O ~O
5 ~8
I
I
I
f6
1/-/
12
I
I
10 8 }),,fO-Z cm -1
FIG. 2. Infrared spectral changes in the PB-I film during thermal degradation for 1 hr at: •--420, 2--500, 3--550, 4--600 and 5--650°C.
2280
I. YE. KARDASHet
al.
which easily dissolved in water and had a decomposition temperature of about 90°C. The qualitative analysis of this product for COss-, NHd+ and CN- ions showed it to be a mixture of NHdHCO s and NHdCN; the formation of these products is thought to be the result of a reaction between NHa, HsO, CO s and HCN produced by the degradation. The presence of NHdHCO a indicates that water is produced during PB-I degradation. The production of water, ammonia and carbon dioxide is evidence of hydrolyric processes taking place during degradation. Assuming these compounds to form as a result of reaction between the end groups of macromolecules, and that each will give rise to one HsO molecule, the degradation of 1 g of polymer of m.w. 30,000 will give rise to about 5 × 10-4 g water. In actual fact 4 × 10 -a g water/g polymer (about 0.015 g NH~HCOa) were produced. This quantity seems to be associated with incomplete cyelization of the poly-o-hydroxamides, and the presence of a low m.w. fraction in the polymer. The reaction of the end groups of this fraction results in a m.w. increase, as shown earlier (3) at lower temperatures (350°C), and in an increase in the water produced. The latter will cause the hydrolysis of the benzoxazole ring during degradation and the formation of amide bonds; hydrolysis of the latter will give rise to amine and carboxyl groups, and these will decompose to ammonia and COs. It should be noted that degradation of the aromatic polyamides alone [5] will give similar products, which indicates similar processes to occur in the degradation of polyamides and PB. One cannot ignore, however, the possibility of radical reactions taking place in such a degradation of PB, because of the formation of CO, methane and benzene. Furthermore, an earlier degradation of a model compound, i.e. 2-phenylbenzoxazole [6], had shown it to be possibly an entirely homolytic process, leading to the same degradation products, i.e. CO, CO 2, Ha, HCN and C6H~. This similarity and also that of the temperatures giving rise to volatile products (2-phenylbenzoxazole starts to decompose at 530-550°C) indicates also homolytic decomposition of chemical bonds to occur in PB. The residue of the PB-I degradation was strongly carbonized and gave an ESR signal (narrow singlet). Its elemental analysis after decomposition at 615°C (24.50/o weight loss) was as follows (~/o): 82.52 C, 3.34 H, 6.56 N. Decomposition at 636°C (250/o weight loss) gave (~/o): 82.18 C, 4.75 H, 6.11 N, while the elemental analysis of the original polymer gave (%): 76.30 C, 3.11 H, 8-05 N. This shows the carbon and hydrogen contents to increase during degradation, while the nitrogen and oxygen contents decreased; there is thus preferential decomposition of the oxazole rings. Yet their content decreased only by 30~/o, although degradation was practically over under these conditions and the benzoxazole absorption lines had disappeared. Attention is also drawn to the retention of the 0 : N ratio in the samples after degradation (0 :N~-l-15: 1). The study of the infrared spectral changes, made on PB-I during degradation (Fig. 2) at about 500°C, where no volatile products are liberated, showed that a new absorption line appears in the 1690 cm -1 region; its intensity increased with increasing degrada-
Thermal degradation of aromatic polybenzoxazoles
2281
tion temperature. There was a simultaneous disappearance of the 935, 1555 and 1630 cm -1 lines, which are typical of the benzoxazole rings [7]. The infrared spectra of polymers after degradation were practically free from benzoxazole absorption lines of those polymers for which the elemental analysis was given above. These results show that the benzoxazole rings decompose (decrease of ~ - and O-content in the polymer, disappearance of benzoxazole rings from spectrum), and that a rearrangement of the oxazole rings takes place (preservation of the 0 : N ratio). The progress of the rearrangement was also confirmed by the appearance of the 1960 cm -1 line in the infrared spectra of the degradation products, i.e. the line attributed to the earhonyl group. As already stated, this line was found t~ appear at lower temperatures than those giving rise to volatile products from the decomposition of the oxazole ring. This is equivalent to a fairly large rate of rearrangement, compared with that of decomposition. The rearrangement seems to have a similar course to that of imino esters to tertiary amides [8]: 0--Cell 5
@_c /
o
~,\
--\__/ ~_/---- -,//--~, \=/
\ 0
.;--~, c / o - - K / ~ --,,__/-
//~
II \ = / "
[I \ = / ~--c--N< o ~J-\~,
--,/--\--c/°-/~-
\=/ / In addition there is possibly another route of rearrangement which also leads to the tertiary amide structure, as in the thermal rearrangement of oxazolines
[9]: N--Ctt2 R--C
O
CH~
CH~-* R-0
CH~
--'/~--C--N 0
~/
2282
I. YE. KARDASH et al.
The study of the kinetics of the thermal degradation of a series of aromatic P B has thus shown t h a t the degradation rate depends on the polymer chain flexibility. The insertion of " ar m our i ng" groups increased this rate. Where the strength of the inserted bonds is similar to t h a t of C--C-bonds, the increase of the degradation rate is due to increase in the pre-exponential factor of the reaction, although the activation energy of the degradation does not significantly change. Where groups with weaker bonds are introduced, the rate of the degradation will also increase as a result of decreasing activation energy of the process. The s t u d y of the P B degradation products showed there to be 3 competing routes, i.e. rearrangement of the oxazole rings, homolytic decomposition of the chemical bonds, and hydrolysis by water formed during the condensation of the functional groups contained in the low m.w. fraction of the polymer. The authors wish to t h a n k N.N. Voznesenskaya and V. S. Yakubovich for supplying the polymer samples, A. Ya. Yakubovich and G. I. Braz for useful criticism of the results. CONCLUSIONS
(1) The kinetics of thermal degradation of aromatic polybenzoxazolcs, having different elemental unit structures, were studied. The effect of polymer chain flexibility was clarified, and also t h a t of bond strength on the rate of thermal degradation. (2) The composition of the degradation products and the changes taking place in the polymer during degradation were studied; the reactions leading to the degradation of polybenzoxazoles were examined. Translated by K. A. ALLEN
REFERENCES
1. G. I. BRAZ, I. Ye. KARDASH, V. S. YAKUB0VICH, G. V. MYASNIKOVA, A. Ya. ARDASHNIKOV, A. F. OLEINIK, A. N. PRAVEDNIKOV and A. Ya. YAKUBOVICH, Vysokomol. soyed. 8: 272, 1966 (Translated in Polymer Sei. U.S.S.R. 8: 2, 295, 1966) 2. S. D. BRUCK, Polymer 5: 435, 1964; 6: 49, 1965 3. I. Ye. KARDASH, A. Ya. ARDASHNIKOV, V. S. YAKUBOVICH, G. I. BRAZ, A. Ya.
4. 5.
6.
7. 8. 9.
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