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Contributions of gel sol and fractions to the thermal stability of γ-irradiated poly(vinyl acetate)
L'ZT .r'--X'VW~
Polymer Degradation and Stability 39 (1993) 151-154
"?x:
.':
.'
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NI:i,,U
Contributions of gel sol and fractions to the thermal stability of y-irradiated poly(vinyl acetate) S. Basan Department of Chemistry, Cumhuriyet University, 58140 Sivas, Turkey (Received 25 September 1991; accepted 13 October 1991)
The thermal stability of the gel and sol fractions of poly(vinyl acetate) irradiated with y-rays to various doses in the post-gel region has been investigated by dynamic thermogravimetry. It has been found that the gel fractions are more stable than the sol fractions and the thermal stability of y-irradiated bulk poly(vinyl acetate) lies between those of the gel and sol fractions. However, the presence of increasing amounts of gel did not improve the thermal stability of poly(vinyl acetate) irradiated to high doses.
INTRODUCTION
EXPERIMENTAL
It was shown, by TG experiments, in the first part of this work ~ that y-irradiation affects the deacetylation stage of the thermal degradation of poly(vinyl acetate) (PVAc) and crosslinking predominates over chain scission. 2,3 The gelation dose of v-irradiated PVAc has been found to be 100 kGy. 2,4 In irradiations beyond that dose, i.e. in the post-gel region, since the structure and molecular size of the PVAc become very complex, it is not possible to define a relationship between the thermal behaviour of PVAc and the extent of v-irradiation. The contributions of the gelled and soluble parts of the irradiated PVAc on its thermal stability can be followed, however, for partially crosslinked samples. The purpose of the present study is therefore to investigate the thermal degradations of gel and sol fractions of v-irradiated PVAc separately and to try to understand their effects on the overall thermal stability of v-irradiated PVAc.
The sample used in this work and the details of irradiation conditions were given in the first part of this work.' Immediately after irradiation, accurately weighed amounts of PVAc samples were treated with ethyl acetate for the separation of gel and sol fractions. Sol fractions were extracted and insoluble gels were separated using sintered glass filters and dried in a vacuum oven at 40°C. The thermograms of gel and sol fractions and irradiated PVAc were recorded using a Shimadzu (Kyoto, Japan) DT-30 Model Thermal Analyzer under nitrogen at a heating rate of 10°C/min.
RESULTS A N D DISCUSSION
It is well known that the major effect of high-energy radiation on PVAc irradiated in v a c u o is crosslinking and the formation of an insoluble network (gel). 2.4.5 The thermal stability of PVAc has been shown to be directly dependent upon its molecular weight for a linear
Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 151
S. Basan
152
unbranched structure. 6 This type of dependence of thermal stability upon molecular weight implies an end effect. In other words, the relatively high concentration of end groups can be assumed to be responsible for the low thermal stability of low molecular weight PVAc. Thus, the thermal stability of PVAc would be expected to be improved with increasing molecular weight, and even more stable structures could be obtained by crosslinking. For this reason, the gel and sol fractions of the irradiated PVAc samples were separated and the thermal degradations of the two fractions were investigated separately. Dynamic thermograms are illustrated in Fig. 1 for the gel and sol fractions of irradiated PVAc. The TG curves show that gel fractions are more stable than the irradiated PVAc and its sol fraction. The increase in the amount of residue is especially worth mentioning in the case of samples subjected to 102 and 164 kGy. For gel fractions, the maximum rates of weight loss are observed to increase slightly with irradiation dose up to 644 kGy, but the maximum rate of weight loss of irradiated PVAc for the same dose is decreased with irradiation dose. 100
In order to make a quantitative comparison of the thermal stabilities of irradiated PVAc and its gel and sol fractions, the activation energies of the thermal degradation reactions were calculated from the dynamic thermograms according to the method of Freeman and Carrol v and graphs are presented in Figs. 2 and 3. The values of activation energy determined from the gradients of the lines in Figs 2 and 3 are listed in Table 1. The data show a gradual decrease in Ea values of gel and sol fractions similar to irradiated PVAc as shown previously) Values of activation energy are also plotted against dose in Fig. 4. From a comparison of the three curves, it can be seen that the gel fractions of irradiated PVAc are more stable than the sol fractions of irradiated PVAc. In addition, the thermal stabilities of both gel and sol fractions decrease with increasing dose. Table 2 shows the temperatures (Ts~, Th and Tm,=) at which 5% and 50% weight loss and maximum rate of weight loss are observed. Ts~ represents low conversion, while Th and Tm,=
50 40 45 35
~ <~,,
3~
-1(
-20
' - 30 50-
~
,
i
15
8 L
20 lO 5 5 E o : 45 40
1C
o 9o! 80 382
~y~
7C
! C
- -30
r, s -4o ~8
c
<1 -50
"7 '6
35
-60
60i 410 050203C ~
;L ~
/~1 ,~-1025 20 15 30 5
-7001 0-4 1 0-8 I I I 2.0 I 1.2 1.6 A(I/T)Atn(1-C) Fig. 2. Determination of activation energies of gel fraction of y-irradiated PVAc. (1) 102 kGy; (2) 164 kGy; (3) 218
%00
~oo
400
300
~oo
Temperature, *C
~oo
~o°
Fig. 1. Thermograms of gel and sol fractions of y-irradiated PVAc, heating rate 1if'C/rain, nitrogen atmosphere. - - , Irradiated PVAc; ...... , gel fraction; ...... , sol fraction.
kGy; (4) 305 kGy; (5) 382 kGy; (6) 470 kGy; (7) 564 kGy; (8) 644 kGy. Axes are labelled according to the Freeman-Carroll equation, A ln(dC/dt)/A In(1 - C) = n (E/R) A(1/T)/A In(1 - C), where C is percent conversion, E is activation energy in kJ/mol and R is the universal gas constant.
153
Thermal stabili,'y of y-irradiated P V A c 300
280 5 E 260
-10
+
+
o~
240
+
++
-2O
0I
~ 220
5 -3o
~200
o
• •
+ ++ o
Oo
•
++
0
0 0
•
+ -
~ I
r,
1
100
200
I
I
I
300
400
500
u -4o ¢-
Dose,
+ I q
I
600
I
700
800
kGy
Fig. 4. Change of activation energy of thermal degradation of gel and sol fractions of PVAc with dose. +, Irradiated PVAc; O, gel fraction; O, sol fraction.
-50
-60 ¸
_7010
I 0-4
I 0.8 A (llT)Atn
I 1-2
I 1-6
I 20
Table 2. Temperatures of 5% and 50% conversion and the maximum rate of the thermal degradation reaction for gel and sol fractions T5% (°C)
(1-C)
Fig. 3. Determination of activation energies of sol fraction of ),-irradiated PVAc. (1) 102 kGy; (2) 261 kGy; (3) 382 kGy; (4) 470 kGy; (5) 564 kGy; (6) 644 kGy. Axes are labelled according to the Freeman-Carroll equation, A ln(dC/dt)/A In(1 - C) = n - (E/R) A ( 1 / T ) / A In(1 - C), where C is percent conversion, E is activation energy in kJ/mol and R is the universal gas constant.
Gel
Gel
Sol
Gel
Sol
Gel
Sol
102 132 164 218 261 305 339 382 470 564 644
306 303 300 299 298 296 295 293 291 290 298
316 -315 317 317 316 315 313 310 316 305
349 352 350 352 352 351 352 352 350 347 348
354 -354 357 355 356 354 354 352 354 352
346 346 347 349 348 349 349 349 344 345 346
344 -346 348 347 346 348 348 349 348 344
Dose (kGy) g a (kT/mol) 102 132 164 218 261 305 339 382 470 564 644
280-34 -267-70 262.88 257.48 250-20 236.68 240-32 226.80 208.46 196.99
3207+-I-I-~_+$
•
31o-
+ + ++
~
o
o
o
~n 3 0 0 ~- 2 9 0 280
o
• o
oo
o o
o I
I
I
I
350 -
•o o •o
~
8 g~8
340
[
L
I
8oe
~ 8 811 ~
o I
o L
I
I
I
I
380 370360-
Cor."
E. (kJ/mol)
Cor."
-0.996 --0.999 -0.999 -0.999 -0.996 -0.998 -0-997 -0-991 -0-996 -0-993
238.23 240.78 239.82 251.44 225.21 226.88 226-38 219.85 201.05 196-53 181.80
-0-998 -0.997 -0-998 -0-996 -0.998 -0.997 -0.996 -0-997 -0.996 -0.9996 -0.993
" Correlation coefficient of the lines in Figs 2 and 3.
3 3 0 --
Uo
Sol
Tmax (°C)
Dose (kGy)
represent high conversion. The change in Ts~, Th and Tm~ with irradiation dose shows a trend similar to that of the Ea values. Ts~, Th and Tma x are also plotted against dose in Fig. 5. It can be seen from Fig. 5, that the differences between the Ts~ values for gel and sol fractions is greater than for Th and Tmaxvalues. In addition, the tendency to a decrease in the Ts~ values with irradiation Table 1. Activation energy values, E,, for the thermal degradation of y-irradiated PVAc and its gel and sol fractions
Th (°C)
oo o I
o
•o I
o• I
370 U o
360-
~35o-
o,
8
8
~- 3 4 0 330
I 100
[ 200
I 300
I 400
Dose,
I 500
[ 600
I 700
I 800
kGy
Fig. 5. Change of temperatures of 5%, 50% conversion and
maximum rate of thermal degradation reaction for ),-irradiated PVAc, gel and sol fractions. +, irradiated PVAc; O, gel fraction; O, sol fraction.
154
S. Basan
dose is greater than for Th and /"max. This behaviour shows that the irradiation is more effective at the beginning of the thermal degradation reaction. The strong similarity between the curves in Figs 4 and 5 suggests that the increase in thermal stability of irradiated P V A c before the gelation dose, originates from the more stable gel fraction. Moreover, the decrease in the thermal stability of gel and sol fractions with dose is explained by the increasing n u m b e r of chain ends and double bonds due to chain scission, branching and the deacetylation reaction.
ACKNOWLEDGEMENT The author thanks Dr F. Yigit for the irradiation of PVAc samples in Hacettepe University.
REFERENCES 1. Basan, S., Polym. Deg. and Stab. In press. 2. Giiven, O., Yigit, F. & Basan, S., Paper presented at 1st National Maeromol. Colloquium, Ankara, Turkey, December 1984. 3. Charlesby, A. & Pinner, S. H., Proc. Roy. Soc., Lond. A, 2,49 (1959) 367. 4. Maxim, L. D., Marchessault, R. H., Stannet, V. & Kuist, C. H., Polymer, $ (1964) 403. 5. Basan, S., Yigit, F. & Giiven, O., Paper presented at Vth National Spectroscopy Symposium, Sivas, Turkey, September 1985. 6. Varma, I. K. & Sadhir, R. K., Angew. Makromol. Chem., 46 (1975) 1. 7. Freeman, E. S. & Carrol, B., J. Phys., Chem., 62 (1958) 394.
Polymer Degradation and Stability 39 (1993) 151-154
"?x:
.':
.'
."
NI:i,,U
Contributions of gel sol and fractions to the thermal stability of y-irradiated poly(vinyl acetate) S. Basan Department of Chemistry, Cumhuriyet University, 58140 Sivas, Turkey (Received 25 September 1991; accepted 13 October 1991)
The thermal stability of the gel and sol fractions of poly(vinyl acetate) irradiated with y-rays to various doses in the post-gel region has been investigated by dynamic thermogravimetry. It has been found that the gel fractions are more stable than the sol fractions and the thermal stability of y-irradiated bulk poly(vinyl acetate) lies between those of the gel and sol fractions. However, the presence of increasing amounts of gel did not improve the thermal stability of poly(vinyl acetate) irradiated to high doses.
INTRODUCTION
EXPERIMENTAL
It was shown, by TG experiments, in the first part of this work ~ that y-irradiation affects the deacetylation stage of the thermal degradation of poly(vinyl acetate) (PVAc) and crosslinking predominates over chain scission. 2,3 The gelation dose of v-irradiated PVAc has been found to be 100 kGy. 2,4 In irradiations beyond that dose, i.e. in the post-gel region, since the structure and molecular size of the PVAc become very complex, it is not possible to define a relationship between the thermal behaviour of PVAc and the extent of v-irradiation. The contributions of the gelled and soluble parts of the irradiated PVAc on its thermal stability can be followed, however, for partially crosslinked samples. The purpose of the present study is therefore to investigate the thermal degradations of gel and sol fractions of v-irradiated PVAc separately and to try to understand their effects on the overall thermal stability of v-irradiated PVAc.
The sample used in this work and the details of irradiation conditions were given in the first part of this work.' Immediately after irradiation, accurately weighed amounts of PVAc samples were treated with ethyl acetate for the separation of gel and sol fractions. Sol fractions were extracted and insoluble gels were separated using sintered glass filters and dried in a vacuum oven at 40°C. The thermograms of gel and sol fractions and irradiated PVAc were recorded using a Shimadzu (Kyoto, Japan) DT-30 Model Thermal Analyzer under nitrogen at a heating rate of 10°C/min.
RESULTS A N D DISCUSSION
It is well known that the major effect of high-energy radiation on PVAc irradiated in v a c u o is crosslinking and the formation of an insoluble network (gel). 2.4.5 The thermal stability of PVAc has been shown to be directly dependent upon its molecular weight for a linear
Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 151
S. Basan
152
unbranched structure. 6 This type of dependence of thermal stability upon molecular weight implies an end effect. In other words, the relatively high concentration of end groups can be assumed to be responsible for the low thermal stability of low molecular weight PVAc. Thus, the thermal stability of PVAc would be expected to be improved with increasing molecular weight, and even more stable structures could be obtained by crosslinking. For this reason, the gel and sol fractions of the irradiated PVAc samples were separated and the thermal degradations of the two fractions were investigated separately. Dynamic thermograms are illustrated in Fig. 1 for the gel and sol fractions of irradiated PVAc. The TG curves show that gel fractions are more stable than the irradiated PVAc and its sol fraction. The increase in the amount of residue is especially worth mentioning in the case of samples subjected to 102 and 164 kGy. For gel fractions, the maximum rates of weight loss are observed to increase slightly with irradiation dose up to 644 kGy, but the maximum rate of weight loss of irradiated PVAc for the same dose is decreased with irradiation dose. 100
In order to make a quantitative comparison of the thermal stabilities of irradiated PVAc and its gel and sol fractions, the activation energies of the thermal degradation reactions were calculated from the dynamic thermograms according to the method of Freeman and Carrol v and graphs are presented in Figs. 2 and 3. The values of activation energy determined from the gradients of the lines in Figs 2 and 3 are listed in Table 1. The data show a gradual decrease in Ea values of gel and sol fractions similar to irradiated PVAc as shown previously) Values of activation energy are also plotted against dose in Fig. 4. From a comparison of the three curves, it can be seen that the gel fractions of irradiated PVAc are more stable than the sol fractions of irradiated PVAc. In addition, the thermal stabilities of both gel and sol fractions decrease with increasing dose. Table 2 shows the temperatures (Ts~, Th and Tm,=) at which 5% and 50% weight loss and maximum rate of weight loss are observed. Ts~ represents low conversion, while Th and Tm,=
50 40 45 35
~ <~,,
3~
-1(
-20
' - 30 50-
~
,
i
15
8 L
20 lO 5 5 E o : 45 40
1C
o 9o! 80 382
~y~
7C
! C
- -30
r, s -4o ~8
c
<1 -50
"7 '6
35
-60
60i 410 050203C ~
;L ~
/~1 ,~-1025 20 15 30 5
-7001 0-4 1 0-8 I I I 2.0 I 1.2 1.6 A(I/T)Atn(1-C) Fig. 2. Determination of activation energies of gel fraction of y-irradiated PVAc. (1) 102 kGy; (2) 164 kGy; (3) 218
%00
~oo
400
300
~oo
Temperature, *C
~oo
~o°
Fig. 1. Thermograms of gel and sol fractions of y-irradiated PVAc, heating rate 1if'C/rain, nitrogen atmosphere. - - , Irradiated PVAc; ...... , gel fraction; ...... , sol fraction.
kGy; (4) 305 kGy; (5) 382 kGy; (6) 470 kGy; (7) 564 kGy; (8) 644 kGy. Axes are labelled according to the Freeman-Carroll equation, A ln(dC/dt)/A In(1 - C) = n (E/R) A(1/T)/A In(1 - C), where C is percent conversion, E is activation energy in kJ/mol and R is the universal gas constant.
153
Thermal stabili,'y of y-irradiated P V A c 300
280 5 E 260
-10
+
+
o~
240
+
++
-2O
0I
~ 220
5 -3o
~200
o
• •
+ ++ o
Oo
•
++
0
0 0
•
+ -
~ I
r,
1
100
200
I
I
I
300
400
500
u -4o ¢-
Dose,
+ I q
I
600
I
700
800
kGy
Fig. 4. Change of activation energy of thermal degradation of gel and sol fractions of PVAc with dose. +, Irradiated PVAc; O, gel fraction; O, sol fraction.
-50
-60 ¸
_7010
I 0-4
I 0.8 A (llT)Atn
I 1-2
I 1-6
I 20
Table 2. Temperatures of 5% and 50% conversion and the maximum rate of the thermal degradation reaction for gel and sol fractions T5% (°C)
(1-C)
Fig. 3. Determination of activation energies of sol fraction of ),-irradiated PVAc. (1) 102 kGy; (2) 261 kGy; (3) 382 kGy; (4) 470 kGy; (5) 564 kGy; (6) 644 kGy. Axes are labelled according to the Freeman-Carroll equation, A ln(dC/dt)/A In(1 - C) = n - (E/R) A ( 1 / T ) / A In(1 - C), where C is percent conversion, E is activation energy in kJ/mol and R is the universal gas constant.
Gel
Gel
Sol
Gel
Sol
Gel
Sol
102 132 164 218 261 305 339 382 470 564 644
306 303 300 299 298 296 295 293 291 290 298
316 -315 317 317 316 315 313 310 316 305
349 352 350 352 352 351 352 352 350 347 348
354 -354 357 355 356 354 354 352 354 352
346 346 347 349 348 349 349 349 344 345 346
344 -346 348 347 346 348 348 349 348 344
Dose (kGy) g a (kT/mol) 102 132 164 218 261 305 339 382 470 564 644
280-34 -267-70 262.88 257.48 250-20 236.68 240-32 226.80 208.46 196.99
3207+-I-I-~_+$
•
31o-
+ + ++
~
o
o
o
~n 3 0 0 ~- 2 9 0 280
o
• o
oo
o o
o I
I
I
I
350 -
•o o •o
~
8 g~8
340
[
L
I
8oe
~ 8 811 ~
o I
o L
I
I
I
I
380 370360-
Cor."
E. (kJ/mol)
Cor."
-0.996 --0.999 -0.999 -0.999 -0.996 -0.998 -0-997 -0-991 -0-996 -0-993
238.23 240.78 239.82 251.44 225.21 226.88 226-38 219.85 201.05 196-53 181.80
-0-998 -0.997 -0-998 -0-996 -0.998 -0.997 -0.996 -0-997 -0.996 -0.9996 -0.993
" Correlation coefficient of the lines in Figs 2 and 3.
3 3 0 --
Uo
Sol
Tmax (°C)
Dose (kGy)
represent high conversion. The change in Ts~, Th and Tm~ with irradiation dose shows a trend similar to that of the Ea values. Ts~, Th and Tma x are also plotted against dose in Fig. 5. It can be seen from Fig. 5, that the differences between the Ts~ values for gel and sol fractions is greater than for Th and Tmaxvalues. In addition, the tendency to a decrease in the Ts~ values with irradiation Table 1. Activation energy values, E,, for the thermal degradation of y-irradiated PVAc and its gel and sol fractions
Th (°C)
oo o I
o
•o I
o• I
370 U o
360-
~35o-
o,
8
8
~- 3 4 0 330
I 100
[ 200
I 300
I 400
Dose,
I 500
[ 600
I 700
I 800
kGy
Fig. 5. Change of temperatures of 5%, 50% conversion and
maximum rate of thermal degradation reaction for ),-irradiated PVAc, gel and sol fractions. +, irradiated PVAc; O, gel fraction; O, sol fraction.
154
S. Basan
dose is greater than for Th and /"max. This behaviour shows that the irradiation is more effective at the beginning of the thermal degradation reaction. The strong similarity between the curves in Figs 4 and 5 suggests that the increase in thermal stability of irradiated P V A c before the gelation dose, originates from the more stable gel fraction. Moreover, the decrease in the thermal stability of gel and sol fractions with dose is explained by the increasing n u m b e r of chain ends and double bonds due to chain scission, branching and the deacetylation reaction.
ACKNOWLEDGEMENT The author thanks Dr F. Yigit for the irradiation of PVAc samples in Hacettepe University.
REFERENCES 1. Basan, S., Polym. Deg. and Stab. In press. 2. Giiven, O., Yigit, F. & Basan, S., Paper presented at 1st National Maeromol. Colloquium, Ankara, Turkey, December 1984. 3. Charlesby, A. & Pinner, S. H., Proc. Roy. Soc., Lond. A, 2,49 (1959) 367. 4. Maxim, L. D., Marchessault, R. H., Stannet, V. & Kuist, C. H., Polymer, $ (1964) 403. 5. Basan, S., Yigit, F. & Giiven, O., Paper presented at Vth National Spectroscopy Symposium, Sivas, Turkey, September 1985. 6. Varma, I. K. & Sadhir, R. K., Angew. Makromol. Chem., 46 (1975) 1. 7. Freeman, E. S. & Carrol, B., J. Phys., Chem., 62 (1958) 394.