- Email: [email protected]
Pyrolysis of polyacrylonitrile and related polymers—VI. Acrylonitrile copolymers containing carboxylic acid and amide structures
European Polymer Join'hal, 1972, Vol. $, pp. 257-269. Pergamon Press. Printed in England.
PYROLYSIS OF P O L Y A C R Y L O N I T R I L E A N D RELATED P O L Y M E R S - - V I ACRYLONITRILE
COPOLYMERS
ACID AND
AMIDE
CONTAINING
CARBOXYLIC
STRUCTURES
N. GRASSIE AND R. McGUCHAN* Chemistry Department, The University of Glasgow. Glasgow, W.2, Scotland
(Received 25 September 1971) Abstract--Acidic comonomers exert a strong initiating effect on the exothermic nitrile group oligomerization in polyacrylonitrile. In contrast to the sharp intense transitions for pure homopolymers, the exotherms for the copolymers are very broad with much reduced differential temperatures. This is explained by the free radical mechanism in the homopolymer changing to a concerted or ionic mechanism in the copolymers. The detailed mechanism of acid initiation is complex; the possibility of imide intermediates is discussed. From the point of view of carbon fibre productioa, the thermogravimetric behaviour of these copolymers is better than that of the homopolymer since the chain scission and volatilization which occur during intense exotherms are greatly reduced. Acrylamide also initiates the exotherm but in this case the copolymer exotherm can be more intense than that of the homopolymer so that the free radical nature of the reaction is retained in acrylamide initiation. In contrast, the behaviour of a closely related copolymer system containing amideoxime structures resembles that of the acid rather than the acrylamide copotymers.
INTRODUCTION PREVIOUS p a p e r s (1'2) in this series have been p r i m a r i l y c o n c e r n e d with the t h e r m a l b e h a v i o u r o f p o l y a c r y l o n i t r i l e ( P A N ) in p a r t i c u l a r the t h e r m a l o l i g o m e r i z a t i o n o f the nitrile g r o u p s which results in cyclized structures c o n t a i n i n g c o n j u g a t e d - - C = N - sequences. Since this r e a r r a n g e m e n t is exothermic, differential t h e r m a l analysis ( D T A ) is a very p o w e r f u l tool for studying the reaction. In earlier w o r k (3.~) t h e influence o f impurities a n d c o m o n o m e r s on the nitrile r e a c t i o n was studied by following the d e v e l o p m e n t o f c o l o r a t i o n a n d insolubility o r the decrease in the nitrile a b s o r p t i o n in the i.r. spectrum. D T A should give a m o r e r a p i d , direct a n d a c c u r a t e assessment o f these influences a n d the p r o f o u n d changes in the e x o t h e r m characteristics in the presence o f additives has a l r e a d y been d e m o n s t r a t e d . (5) I n this a n d subsequent papers, we examine the influence o f a wide r a n g e o f com o n o m e r s on the t h e r m a l b e h a v i o u r o f P A N a n d in p a r t i c u l a r the effect o f the n a t u r e a n d c o n c e n t r a t i o n o f c o m o n o m e r s on the e x o t h e r m . It is i m p o r t a n t also to c o r r e l a t e this study with the use o f acrylic fibres as c a r b o n fibre precursors. A t present, Courtelle ( C o u r t a u l d s Limited), a c o m m e r c i a l a c r y l o n i t r i l e fibre c o n t a i n i n g methyl acrylate as the p r i n c i p a l c o m o n o m e r , is the m o s t widely used p r e c u r s o r for c a r b o n fibre m a n u facture. It is n o t i m p r o b a b l e t h a t o t h e r c o p o l y m e r c o m p o s i t i o n s might have s u p e r i o r processing qualities. It is o f interest, therefore, to investigate the effect o f c o m o n o m e r s * Present address: E.R.D.E., Waltham Abbey, Essex. E.PJ. 8/2--o 257
258
N. GRASSIE and R. M c G U C H A N
o n t h e ease o f p r e o x i d a t i o n , (6) t h e e x o t h e r m i c i t y o f d i r e c t p y r o l y s i s a n d t h e c a r b o n yield at elevated temperatures. In this paper, copolymers containing various carboxylic a c i d s , a c r y l a m i d e a n d a c r y l a m i d e o x i m e a r e i n v e s t i g a t e d . It w o u l d b e p r e d i c t e d f r o m e a r l i e r s t u d i e s Ca.~) t h a t t h e s e c o m o n o m e r s s h o u l d a c c e l e r a t e t h e e x o t h e r m i c r e a c t i o n and thus move the DTA transition to lower temperatures.
EXPERIMENTAL
Polymers Copolymers were prepared at 40 ° in a slurry c7~ consisting of 300 ml water, a redox initiator (30 mg potassium persulphate and 15 mg sodium bisulphite) and the appropriate monomer mixture. Table I lists the monomer mixtures used and the approximate copolymer compositions derived from microanalysis data; comonomer contents in the range 5-20 per cent were considered most desirable for the purposes of this study, although some of the polymers lay outside this range. In addition to the polymers listed, two other systems were studied. Firstly, sodium acrylate copolymers were derived from the acrylic acid copolymers by treatment with dilute sodium hydroxide. SAI and SA2 are 21/I and 5/1 copolymers in which the conversion of the acid units to the sodium salt was estimated at approximately 90 per cent. The second system was a copolymer of acrylonitrile and acrylamideoxime obtained by treatment of polyacrylonitrile with hydroxylamine. ~7) This converts some of the nitrile groups to amideoxime structures. Two samples were prepared, AO1 and AO2, for which the compositions were estimated to be 10/1 and 4/1 respectively. TABLE 1. PREPARATION OF POLYMERS
Polymer code
Comonomer
Monomer mixture AN(ml)/comonomer
Polymer yield (~)
Polymer composition (mole ratio) AN/comonomer
AA1 AA2 AA3
Acrylic acid
20 20 20
0-5 ml I "0 ml 2' 0 ml
6 8 20
50/1 21/I 5/1
MA.A1 MAA2 MAA3 MA.A4
Methacrylic acid
20 20 20 20
0" 25 ml 1-0 ml 0" 5 ml 2" 0 ml
4 9 8 9
25/1 10/1 7/1 2/I
IA1 IA2 IA3 IA4
Itaconic acid
20 20 20 20
0" 5 2"0 3-0 4-0
g g g g
20 12 6 2
50/1 25/1 15/I 10/l
AM I AM2 AM3 AM4 AM5 AM6
Acrylamide
20 20 20 20 20 20
1" 0 g 5-0 g 7-5 g 5"0 g 10 g 20 g
6 20 16 8 10 13
50/1 20/1 10/1 8/I 5/1 2/1
Thermal analysis A Du Pont 900 Thermal Analyzer incorporating differential thermal analysis (DTA), thermogravimetry (TG) and differential scanning calorimetry (DSC), was used. In DTA and T G analyses, 10 mg powder samples were heated at 10°/min in a nitrogen flow. For selected copolymers, T G analyses in air and vacuum (10 -z tort) were also obtained. In the DSC determinations of heats of reaction smaller samples (3-5 mg) were heated at 10°/rain under nitrogen. Thermal Volatilization Analysis (TVA) ~s) was also used. 100 mg samples were heated at 10°/rain under vacuum (10 -5 torr) and the gaseous degradation products monitored by a system of Pirani
Pyrolysis of Polyacrylonitrile and related Polymers--VI
259
gauges. The gauges are preceded by traps at various temperatures between 0 ° and -- 196 ° ; curves are obtained which depend upon the condensability of the evolved gases, and give some information about the nature of the products. Infra-red spectra of these products were obtained by distilling the trapped products into a gas cell. In presenting TVA curves, indicated temperatures refer to the trap temperatures at the approriate gauges.
Infra-red spectra A Perkin-Elmer model 257 spectrophotometer was used. Polymers and degradation residues were examined in KBr discs. RESULTS T h e t h e r m a l a n a l y s e s a r e s u m m a r i z e d in T a b l e 2. T h e D T A e x o t h e r m s are c h a r a c t e r i z e d b y t h e A T v a l u e a n d t h e t e m p e r a t u r e a t w h i c h t h e p e a k is o b s e r v e d (Ts). T h e a p p r o x i m a t e s t a r t i n g t e m p e r a t u r e s o f t h e e x o t h e r m a r e a l s o i n c l u d e d (T~). I n t h e T G s t u d y , p y r o l y s i s t o 500 ° w a s c o n s i d e r e d sufficient t o a s s e s s t h e g e n e r a l w e i g h t loss c h a r a c t e r i s t i c s b u t s e l e c t e d p o l y m e r s w e r e h e a t e d t o 1000 ° t o e v a l u a t e t h e effect o f t h e c o m o n o m e r on the c a r b o n yield at elevated t e m p e r a t u r e s . The a m o u n t o f h e a t l i b e r a t e d in t h e e x o t h e r m w a s c a l c u l a t e d f r o m t h e a r e a u n d e r t h e D S C c u r v e ; t h e r e s u l t s a r e e x p r e s s e d in b o t h c a l o r i e s p e r g r a m o f c o p o l y m e r a n d k i l o c a l o r i e s p e r m o l e o f nitrile u n i t s , t h e l a t t e r v a l u e t h u s c o r r e c t i n g f o r t h e a m o u n t o f c o m o n o m e r w h i c h is a s s u m e d n o t to c o n t r i b u t e to the exothermicity. TABLE 2. THERMAL ANALYSIS OF COPOLYMERS (PYROLYSIS IN N2)
DTA*
~ Weight loss 500 ° i000 °
-- ~ h r (DSC) kcal/mole CN
Polymer
Composition
AT
Ts
7"i
$3 AAI AA2 AA3
homopolymer 50/I 21/I 5/1
44 13 6" 0 3"6
325 286 276 267
275 225 220 215
37 34 40 40
70 -54 56
114 -150 162
6" 05 -10" 1 9"2
SAI SA2
21/i 5/1
19 6" 5
300 286
230 225
3t 26
---
---
---
MA.A.1 MAA2 MAA3 MAA4
25/1 10/1 7/1 2/I
2- 0 1.5 1-5 1 "0
285 254 250 260
200 175 175 215
38 39 38 57
-55 ---
165 156 153 167
9- 3 9-6 10"0 16-0
IA 1 IA.2 IA3 IA4
50/1 25/1 15/1 10/1
5" 0 1" 8 1-0 1 "0
275 270 240 242
200 190 185 190
22 34 40 38
60 52 ---
-132 157 154
-7- 8 9.7 10-4
AM1 AM2 AM3 AM4 AM5 AM6
50/I 20/1 10/i 8/1 5/1 2/1
30 47 45 40 15 3
300 292 295 295 258 235
240 220 235 240 235 215
34 36 37 35 43 46
-------
--158 151 144 --
--9"5 9-4 9-7 --
AOI AO2
10/1 4/1
4"4 1" 5
268 266
175 160
19 26
67 --
144 --
8"9 --
*
AT--maximum temperature differential (°C). Tr--sample temperature at AT. Tr--approximate temperature of initiation of transition.
cal/g
260
N. GRASSIE and R. M c G U C H A N
The thermal analysis data for an acrylonitrile homopolymer, $3, is included for comparison, and the DTA and T G curves are shown in Fig. 1. A feature of particular interest is the increased weight loss under vacuum in the region of the exothermic reaction. This is attributed to the easier volatilization in vacuum of chain fragments formed by chain scission reactions concurrent with the rapid and intense exotherm. The corresponding curves for selected copolymers are illustrated in Figs. 2-6.
°o'_ i I o,~ °01-
t,oo
.9 40-
Temperature( ° C )
Fic. 1. DTA and TG curves for polyacrylonitrile, $3; 10 mg samples heated at 10°/rain in N2 ( ), air (- • ") and vacuum (- --). Effect o f comonomer on DTA exotherm
The results for the acrylic acid polymers demonstrate the initiating effect of the acid unit. As the acid content increases, the exotherm becomes less intense and broader with lower initiation and peak temperatures. The shape of the exotherm for the 5/1 copolymer is shown in Fig. 2. The change in the shape of the exotherm from that o f pure polyacrylonitrile suggests that the function of the acid unit is more complex than merely initiating the reaction, since this would have been expected simply to shift the sharp exotherms to lower temperatures. The broad exotherms and small AT values suggest a basically different initiation mechanism and relatively much slower propagation. The methacrylic and itaconic acid copolymers show more complex exotherms than the acrylic acid copolymers and, as illustrated by Figs. 3 and 4, the quoting of single AT values as in Table 2 does not properly define these exotherms. At very low acid contents, the low temperature initiation produces a shoulder on the major peak; as the acid content increases two or more distinct maxima are observed. At the higher acid contents, the low temperature component of the curve becomes the dominant peak. The net effect of this splitting of the exotherm is that the AT values are even smaller than for equivalent acrylic acid compositions, in which acid initiation is clearly less efficient. The results obtained for the sodium acrylate copolymers demonstrate that the salt structure is much less reactive than the free acid in initiating the reaction and the exotherms become more intense.
P?'rolysis of Polyacrylonitrile and related Potymers--VI
261
80,\\
" 1
O•O G0'402 ,/
\
*,
\, 20 -
r
2°
I
~"
/
JJ
i
l
i
l' 200
300 Temperature
400
(°C)
5/1 acrylonitrile-acrylic acid copolymer, A A 3 ; 10 mg samples heated at 10"/min in N2 ( ), air (. • -) and vacuum ( - - -).
FIG. 2. D T A and T G curves for
The exotherms for the acrylamide copolymers tend to be much more similar to those of pure PAN than to those of the acid copolymers; some compositions suggest a small increase in the intensity of the exotherms. At the 5/1 level, AT does show a reduction but the exotherm is still sharp and intense compared with acid polymers. Acrylamide apparently has some initiating ability, however, since the Ts and Tt values are lower than for pure PAN. The acrylamide oxime copolymers contain more reactive initiating structures than the acrylamide copolymers and the D T A behaviour is clearly more akin to the acid copolymers than to the amide copolymers.
lOO-
--.
-... "-.\
!
... \
•
!
I 60--
\
.5
\
\
\
!2o 4,
;0
. . . .
Temperature
,~0 (°C)
FIG. 3. D T A and T G curves for 7/I acrylonitfile-methacrylic acid copolymer, M A A 3 ; I0
mg samples heated at 10°/rain in N2 (
), air (- • ") and vacuum ( - - -).
262
N. GRASSlE and R. M c G U C H A N
"-~:',~ ...................... 8or-
I
201-
I
I
t
200
3(]0
400
Temperature ( ° C )
FIG. 4. D T A and TG curves for 15/1 ac~lonitfile-itaconic acid copolymer, L~3; 10 mg
samples heated at 10°/min in N, (
), air (. • ') and vacuum (- - -).
TG behaviour The three acid copolymer systems show similar thermogravimetric behaviours (Figs. 2-4). In marked contrast with pure PAN, the weight losses are very gradual initially and there is very little weight loss associated with the exotherm. However, this does not generally result in better weight retention at 500 ° since most of the polymers and PAN itself give weight losses in the range 35-40 per cent for pyrolysis in N2. In air and vacuum the weight losses at 500 ° are reduced in the copolymers because of the improved weight retention at the exotherm. Pyrolysis in air results in less weight loss than in nitrogen. This is probably due to oxidation reactions, the oxygen uptake compensating in part. The increased weight loss in vacuum suggests that fragmentation still occurs in the copolymers but to a lesser degree than in pure PAN. In the copolymers, scission does not appear to be concurrent with the exotherm but takes place gradually at higher temperatures. The most favourable weight loss characteristics are shown by the itaconic acid copolymers in which the difference between the nitrogen and vacuum curves is small. This is consistent with the difunctional initiating potential of the IA unit which could form cross-links if both acid groups are involved in initiating reactions. Cross-linking would oppose the formation of fragments by chain scission. The residue yields at 1000 ° do not vary much among the copolymers and show substantial improvement over the homopolymer. It is clear that the carbon yield at 1000 ° bears little relationship to the amount of residue at 500:. The sodium acrylate copolymers exhibit greater weight loss during the exotherm than the acrylic acid copolymers, which is consistent with the more intense exotherms. This weight loss is still quite small however and amounts to 10 per cent in vacuum for the 5/1 copolymer. Subsequent weight loss is gradual and the weight loss at 500° is less for the SA copolymers than the acid copolymers. It seems probable that the ionic structures in the residues reduce the volatility of the chain fragments. The acrylamide copolymers also show weight losses in N2 during the exotherm as illustrated in Fig. 5. Under vacuum, the weight loss is much less severe than in pure
Pyrolysis of Pol.vac~'lonitrile and related Polymers--VI
263
loo,.* i
°01_
o'° ,F \
\
\
//
/I' ;o
i z0k
L E
t zoo
t 300
I 400
(°C)
Temperature
FIG. 5. DTA and TG curves for 5/1 acrylonitrile-acrylamide copolymer, AM5; 10 mg samples heated at 10°/min in Nz ( ), air (. • .) and vacuum ( - - - ) .
P A N . This was true also o f A M 4 for which AT is comparable with that o f $3. Thus the a m o u n t o f fragmentation appears to be sensitive to the temperature at which the exotherm occurs as well as the intensity o f the reaction. Pyrolysis in air exhibits no weight loss at the exotherm which demonstrates the unpredictability o f the influence o f oxygen on the exotherm. This is further emphasized by the results for the acrytamideoxime copolymers (Fig. 6) for which the inert T G curves reflect the small exotherms with very little volatilization in the exotherm region, but the pyrolysis in air is highly exothermic. This is therefore the opposite situation f r o m that in the acrylamide system.
F $0j
'",
~
\,.
i
._~ !
...
........]
'°i
z01 F 1
2 o'0
' 300 Temperature
' 4°0 (°C)
FIG. 6. DTA and TG curves for 4/1 acrylonitrile-acrylamideoxime copo/ymer, AO2; 10 mg samples heated at 10°/min in N2 ( ), air (" • .) and vacuum ( - - - ) .
264
N. GRASSIE and R. M c G U C H A N
D S C studies
The DSC data in Table 2 show that although the shape of the exotherm may vary enormously, the amount of heat liberated is not reduced in those copol.vmers. Indeed based on the nitrile content, the amount of heat may be substantially increased. The most probable explanation is that the observed exotherm is the net sum of both exothermic and endothermic reactions. The latter, which include chain scission and gaseous product formation, are more important in the homopolymer than in the copolymers thus depressing t h e / 5 H value for the homopolymer to the greatest extent. T V A curves
The TVA curves for representative acid copolymers are shown in Fig. 7. The acrylic acid copolymer curve is similar to that of polyacrylonitrile except that the initial peak is smaller (smaller exotherm) and there is some non-condensable product during the initial stages of pyrolysis; in pure PAN non-condensables are not observed until after the exothermic reaction. In the acid copolymer, it is likely that some decomposition of the acid structures to form carbon monoxide occurs but only to a minor extent. The condensable fraction of the products consists of HCN and NH3. The itaconic acid copolymer is similar with even more gradual initial reaction. The first small peak from this copolymer is water. The methacrylic acid copolymer shows a different pattern of noncondensable products with increased volatilization at about 400 °. Product analysis confirmed that this was methane being elin'Jnated from the methacrylic acid units. The main peak consists of hydrogen and carbon monoxide which is common to all the acid copolymers. The TVA curves for acrylamide copolymers are very similar to those of pure PAN. i
__
o~._.o,_7¢
. . . .
/ ~
1oo°
. . . . . . . . . . ~9~
/ ¢.
4,-li 1 0! E
~. ,L ~l Q
I
/f
.~
o.
i
~s " ....
.
ii
0
I
s/
,~
// ..
3 200
300
~0~
Oven Temp.erature{°C)
~$Q
FIG. 7. TVA curves for acid copolymers; (1) AA3, (2) MAA3 and (3) IA3.
Pyrolysis of Polyacrylonitrile and related Polymers--VI
265
The relative amount of ammonia was increased, which is consistent with the fact that the amide structure provides a second source for this product. The amideoxime copolymer, AO2, shows very broad low temperature volatilization yielding ammonia and water. The general pattern of evolution of the water suggests that, like the ammonia, it is formed by decomposition of the oxime structure rather than being an impurity in the copolymer. Coloration and i.r. spectra The acid and acrylamide copolymers show tan-brown coloration similar to pure P A N on pyrolysis to 300 °. There is a tendency to less intense coloration as the comonomer content is increased. The sodium acrylate copolymers, on the other hand, were black at 300 °. The i.r. spectrum of this black residue exhibits a broad nitrile absorption at 2180 cm -~ which is more intense than the residual 2240 cm - t absorption, the only nitrile peak in the unheated copolymer. The new absorption suggests ionic nitrile species in the residue and may also be attributed to ionic structures associated with the ~ N conjugation. The changes in the i.r. spectra of the free acid copolymers are illustrated by Fig. 8. On heating through the exotherm, the free nitrile absorption (2240 cm -~) is greatly reduced and intense absorptions, assigned to
FJ
I
2 3 00
2000
Crl~-1
FIG. 8. Infra-red spectra of (1) M . ~ 3
,&
,;00
,'0o
and (2) M A A 3 heated at 10°/rain to 300 °
conjugated - - C = N - - and imine structures, appear in the 1600 cm-~ region; other intense absorptions appear at 1370, 1240 and 1140 cm - I . These changes are consistent with these in P A N and have been discussed previously. (1'2) The interesting feature here is the disappearance of the carbonyl absorption due to the acid ~ o u p . In the 7/1 copolymer illustrated, this has shifted to lower frequencies and is observed as a shoulder at 1650-1680 cm - t on the main conjugation absorption. In 20/1 copolymers the carbonyl absorption disappears completely and it is concluded that the carbonyl group has become part of the conjugated structure. The itaconic acid copolymers exhibit a specific feature in that a transient anhydride species is formed during pyrolysis; this is consistent with the TVA evidence of water evolution in the
266
N, GRASSIE and R. M c G U C H A N
initial stages of pyrolysis. The intensities of the anhydride absorptions at 1850 and 1780 cm-1 suggest that a little intramolecular anhydride formation takes place:
C~=; H2"-"."~
--CH= ~'/C HZ"-'-'~
/ C\
"
HOOC CHzCOOH
C.//~ O=
+ HzO H2
I ] O--C II O The i.r. changes in the acrylamide copolymers (Fig. 9) show that the amide structure disappears during the exotherm so that it again appears that the carbonyl group is incorporated into the overall conjugation.
/ 2
30100
I 20OO
Cri'1-1
I
I
r
tOO0
12Q0
000
FiG. 9, Infra-red spectra of (1) AM5 and (2) AM5 heated at 10~/min to 260 °.
DISCUSSION The general structure of degraded polyacrylonitrile has been presented C1'2'9) as an imperfect ladder structure incorporating short conjugated sequences. C N - ~ ~
n-O-5
CN Small amounts of abnormalities, impurities or terminal structures initiate the cyclization which is continued by hydrogen transfer and reinitiation of the oligomerization by cyclization of the resulting radical;
I CN
Pyrolysis of Polyacr2,ionitrile and related Polymers--VI
267
to give the carbocyclic end unit in the structure shown. The reaction is free radical in nature and, since the oligomerization is exothermic, the reaction proceeds very rapidly under pyrolysis conditions. In this rapid degradation of PAN there is some weight loss associated with the exotherm, which has raised doubt ¢1°) as to whether the exotherm is adequately explained by the above cyclization process which does not involve weight loss. Volatilization is caused, in fact, by secondary reactions and it is interesting that the present study lends further support to this proposition. Thus, in the pyrolysis of the acid copolymers, the weight loss during the exotherm is negligible so that the assignment of the exotherm to the nitrile group oligomerization is perfectly justified. Acid comonomers produce profound changes in the DTA characteristics since even small amounts of these reactive initiating structures greatly increase the number of primary initiation centres in the polymer structure and, once initiated, the nitrile reaction can continue by the secondary reinitiation process. As the acid content is increased, most of the ladder sequences will be initiated through the acid structures. Since propagation of the nitrile reaction is relatively slow in the acid initiated pyrolyses, it is concluded that the free radical mechanism in pure PAN is replaced in these copolymers by a concerted or ionic mechanism similar to that previously proposed (3'+> for acid initiation
X
C
C
t-,,N
N
H
./
-~
C
["~N
I
H
H
Since each propagation step involves a hydrogen transfer, this mechanism would be expected to be less rapid than the free radical process in pure PAN. Looking at this acid initiation process in more detail, the generally accepted ester structure shown above is not supported by the present study. Since the structure is analogous to a vinyl ester, the carbonyl absorption should appear at a higher frequency than the original acid carbonyl absorption. In fact the carbonyl absorption moves to lower frequencies and it is concluded that it becomes part of the conjugation. It is likely therefore that initiation produces the more stable imide structure, the normal product from acid-nitrile interaction,C11) and that propagation proceeds via tautomeric forms of this as outlined below.
o......c..N..c... ° H
H
..
×~
c
I
H N
- propagation
o~c'-N-~C-.N.-'%-o I
H
o~C.N~c\~>C.o I
H
In this way the carbonyl groups can be incorporated into the conjugated sequences. The different initiation efficiencies observed in the present study are probably related to the stereochemistry of the acid structure relative to the adjacent nitrile
268
N. GRASSIE and R. McGUCHAN
groups. In the case of acrylic acid the most favourable steric arrangement would be as follows: H COOH H
X'YY CN
H
CN
Thus initiation would require rotation to a more favourable configuration. In the disubstituted acid copolymers, both possible configurations are sterically strained so that the probability of the acid structure being adjacent to the nitrile group is increased. In the acrylamide copolymers, a free radical mechanism is indicated; it is proposed that the initiation step is homolytic fission of the amide carbon-nitrogen bond.
"~.
,." ~
,,- ~
+'NH2
H obsfrocfion 1=
=.propagation
NH3+polymerrodiccl ey¢liz~tion
Since the nitrile oligomerization reaction is still apparent in copolymers with fairly high acrylamide contents, it is also feasible that the acrylamide unit can participate in the overall reaction in other ways; thus, = ~ ' ~ /C~N" I~0 NH=
+*NH2
~ NH3+ polymer radic~l
/~C~o
cycli!otlon
In the amideoxime copolymers, a concerted mechanism involving stepwise migration of the hydroxyl group is plausible.
" ~ ~
~~ ' ~ ~
H~N ,/ C~I~..~N H=N/%NJC~NoH O H
propagation
With respect to potential application to carbon fibre manufacture, the acid copolymers appear to be particularly attractive. Although the total amount of heat liberated is not reduced, pyrolysis in both inert and oxygenated environments is much less sharply exothermic than in pure PAN, so that the possibility of direct pyrolytic processing seems more reasonable for these copolymers. Even if pre-oxidation is still necessary, the lower temperatures at which the copolymer exotherrns occur indicate that more rapid preoxidation schedules should be applicable. The itaconic acid copolymers are particularly favourable since fragmentation is almost eliminated during pyrolysis. In all the acid copolymers, the carbon yields for direct pyrolysis are fairly good and much improved over pure polyacrylonitrile. Acknowledgement--The authors thank Rolls-Royce Limited, Derby, for sponsoring this research.
Pyrolysis of Polyacrytonitrile and related Polymers--VI
269
REFERENCES (i) (2) (3) (4) (5) (6) (7) (8) (9) (I0) (11)
N. Grassie and R. McGuchan, Eurap, Palym..]. 6, 1277 (1970). N. Grassie and R. McGuchan, Europ. Palym. J. "7, 1091 (1971). N. Grassie and I. C. McNeill, J. Polym. ScL 2'7, 207 (1958); 29, 211 (1959). N. Grassie and J. N. Hay, J. Palym. Sci. 56, 189 (1962). N. Grassie and R. McGuchan, Europ. Palym. J. 7, 1503 (1971). W. Watt, Proc. R. Sac. A319, 5 (1970). W. R. Sorenson and T. W. Campbell, Preparatire :V[ethods of Polymer Chemistry, pp. 237-238. Interscience, New York, 2nd Edition 1968. I. C. McNeill, Europ. Polym. d'. 6, 373 (1970). N. Grassie and R. McGuchan, Europ. Polym. J. 7, 1357 (1971). J. N. Hay, J. Pol.vm. Sci. A1, 6, 2127 (1968). J. M. Polya and I. M. Spotswood, Recl. Tray. chim. Pays-Bas Belg. 67, 927 (1948).
R~sum6----Les comonom~res acides exercent un fort effet amorceur sur l'oligom6risation exothermique des groupements nitriles du polyacrylonitrile. En contraste avec los transitions &roites et intenses obtenues pour les homopolym~res purs, les transitions exothermiques des copotym~res sont tres largos avec des diff6rences de temp&atures plus r6duites. Ceci est expliqu6 par le fait que la r6action proc~de par un m&anisme radicalaire darts l'homopolymere et par un m6canisme concert6 ou ionique clans les copolym~res Le m&anisme d6taill6 de l'amorqage par l'acide est complexe. On discute la possibilit6 d'existence d'interm6diaires imides. Du point de rue dela production de fibre de carbone le comportement thermogravim&rique de cos copolym~res est meilleur que celui de l'homopolym~re puisque la rupture de chalne et la volatilisation qui apparaissent pendant los transitions exothermiques, sont fortement r~duites. L'acrylamide amorce 6galement la transition exothermique mais clans cecas, la transition exothermique du copolym~re peut &re plus intense que celle de l'homopolym~re si bien, que la nature du radical libre de la r~action est conserv6e dans l'amorqage par l'acrylamide. Au contraire, le comportement d'un syst~me de copolym~re tr~s voisin contenant des structures amideoxime ressemble b. celui de l'acide plus que celui des copolymeres amorc6 par l'acrylamide. Sommario---Dei comonomeri acidi esercitano un forte effetto iniziatore sull'oligomerizzazione esotermica in poliacrilonitrile deI gruppo nitrile. In contrasto con la transizione brusca e intensa che si ha con omopolimeri purl, le esotermiche dei copolimeri sono molto larghe con temperature differenziali molto ridotte. Ci6 si spiega con il meccanismo dei radicali liberi negli omopolimeri che cambia in un meccanismo ionico oppure concertato nei copolimeri. II meccanismo dell'iniziazione acida e complesso; si discute la possibilitb, di prodotti intermedi imidici. Dal punto di vista della produzione di fibra di carbenio, il comportamento termogravimetrico di tall copoiimeri e migliore di quello degli omopolimeri, dato che sono grandemente ridotte la scissione di catene e ta volatizzazione che si verifica durante intense esotermiche. Pure l'acrilarrdde inizia l'esotermica, ma in questo caso l'esotermica dei copolimeri pu6 essere pifi intensa che quella degli omopolimeri cosicch~ la natura a radicali liberi della reazione viene mantenuta netl'iniziazione delt'acrilamide. In contrasto a ci6, il comportamento di un sistema copolimerico strettamento collegato contenente strutture ammidossimiche assomiglio pifi a quello di un acido che a quello di copolimeri di acrilammide. Zusammenfassung--Auf die exotherme Oligomerisierung der Nitritgruppen in Poiyacrylnitril haben saure Comonomere einen stark initiierenden Einflug. Im Gegensatz zu den scharfen intensiven UmwandIungen f~r reine Homopolymere sind die Exothermen fiir die Copolymeren sehr breit mit erheblich reduzierten differentiellen Temperaturen. Dies wird erkl/i.rt dutch einen freien Radikalmechanismus bei dem Homopolymeren, der bei den Copolymeren in einen konzertierten Meclaanismus tibergeht. Der genaue Mechanismus der sauren Initiierung ist komplex; es wird die M6glictxkeit yon Imid Zwischenstufen diskutiert. Vom Gesichtspunkt der Carbonfaser Herstellung aus betrachtet ist das thermogravimetrische Verhalten dieser Copolymeren besser als das des Homopolymeren, da die Kettenspaltung und die Verfltichtigung, die bei intensiven Exothermen stattfinden, stark reduzie~ sind. Aucla Acrylamid initiiert die Exotherme, in diesem Fall karm aber die Copolymer Exotherme intensiver sein als die des Homopolymeren, so dab b e i d e r Acrylamid Initiierung der freie Radikalmechanismus der Reaktion beibehalten bleibt. Das Verhalten eines nahe verwandten Copolymer Systems mit Amidoxim Strukturengleicht, im Gegensatz dazu, mehr dem der sauren als dem der Acrylamid Copolymeren.
PYROLYSIS OF P O L Y A C R Y L O N I T R I L E A N D RELATED P O L Y M E R S - - V I ACRYLONITRILE
COPOLYMERS
ACID AND
AMIDE
CONTAINING
CARBOXYLIC
STRUCTURES
N. GRASSIE AND R. McGUCHAN* Chemistry Department, The University of Glasgow. Glasgow, W.2, Scotland
(Received 25 September 1971) Abstract--Acidic comonomers exert a strong initiating effect on the exothermic nitrile group oligomerization in polyacrylonitrile. In contrast to the sharp intense transitions for pure homopolymers, the exotherms for the copolymers are very broad with much reduced differential temperatures. This is explained by the free radical mechanism in the homopolymer changing to a concerted or ionic mechanism in the copolymers. The detailed mechanism of acid initiation is complex; the possibility of imide intermediates is discussed. From the point of view of carbon fibre productioa, the thermogravimetric behaviour of these copolymers is better than that of the homopolymer since the chain scission and volatilization which occur during intense exotherms are greatly reduced. Acrylamide also initiates the exotherm but in this case the copolymer exotherm can be more intense than that of the homopolymer so that the free radical nature of the reaction is retained in acrylamide initiation. In contrast, the behaviour of a closely related copolymer system containing amideoxime structures resembles that of the acid rather than the acrylamide copotymers.
INTRODUCTION PREVIOUS p a p e r s (1'2) in this series have been p r i m a r i l y c o n c e r n e d with the t h e r m a l b e h a v i o u r o f p o l y a c r y l o n i t r i l e ( P A N ) in p a r t i c u l a r the t h e r m a l o l i g o m e r i z a t i o n o f the nitrile g r o u p s which results in cyclized structures c o n t a i n i n g c o n j u g a t e d - - C = N - sequences. Since this r e a r r a n g e m e n t is exothermic, differential t h e r m a l analysis ( D T A ) is a very p o w e r f u l tool for studying the reaction. In earlier w o r k (3.~) t h e influence o f impurities a n d c o m o n o m e r s on the nitrile r e a c t i o n was studied by following the d e v e l o p m e n t o f c o l o r a t i o n a n d insolubility o r the decrease in the nitrile a b s o r p t i o n in the i.r. spectrum. D T A should give a m o r e r a p i d , direct a n d a c c u r a t e assessment o f these influences a n d the p r o f o u n d changes in the e x o t h e r m characteristics in the presence o f additives has a l r e a d y been d e m o n s t r a t e d . (5) I n this a n d subsequent papers, we examine the influence o f a wide r a n g e o f com o n o m e r s on the t h e r m a l b e h a v i o u r o f P A N a n d in p a r t i c u l a r the effect o f the n a t u r e a n d c o n c e n t r a t i o n o f c o m o n o m e r s on the e x o t h e r m . It is i m p o r t a n t also to c o r r e l a t e this study with the use o f acrylic fibres as c a r b o n fibre precursors. A t present, Courtelle ( C o u r t a u l d s Limited), a c o m m e r c i a l a c r y l o n i t r i l e fibre c o n t a i n i n g methyl acrylate as the p r i n c i p a l c o m o n o m e r , is the m o s t widely used p r e c u r s o r for c a r b o n fibre m a n u facture. It is n o t i m p r o b a b l e t h a t o t h e r c o p o l y m e r c o m p o s i t i o n s might have s u p e r i o r processing qualities. It is o f interest, therefore, to investigate the effect o f c o m o n o m e r s * Present address: E.R.D.E., Waltham Abbey, Essex. E.PJ. 8/2--o 257
258
N. GRASSIE and R. M c G U C H A N
o n t h e ease o f p r e o x i d a t i o n , (6) t h e e x o t h e r m i c i t y o f d i r e c t p y r o l y s i s a n d t h e c a r b o n yield at elevated temperatures. In this paper, copolymers containing various carboxylic a c i d s , a c r y l a m i d e a n d a c r y l a m i d e o x i m e a r e i n v e s t i g a t e d . It w o u l d b e p r e d i c t e d f r o m e a r l i e r s t u d i e s Ca.~) t h a t t h e s e c o m o n o m e r s s h o u l d a c c e l e r a t e t h e e x o t h e r m i c r e a c t i o n and thus move the DTA transition to lower temperatures.
EXPERIMENTAL
Polymers Copolymers were prepared at 40 ° in a slurry c7~ consisting of 300 ml water, a redox initiator (30 mg potassium persulphate and 15 mg sodium bisulphite) and the appropriate monomer mixture. Table I lists the monomer mixtures used and the approximate copolymer compositions derived from microanalysis data; comonomer contents in the range 5-20 per cent were considered most desirable for the purposes of this study, although some of the polymers lay outside this range. In addition to the polymers listed, two other systems were studied. Firstly, sodium acrylate copolymers were derived from the acrylic acid copolymers by treatment with dilute sodium hydroxide. SAI and SA2 are 21/I and 5/1 copolymers in which the conversion of the acid units to the sodium salt was estimated at approximately 90 per cent. The second system was a copolymer of acrylonitrile and acrylamideoxime obtained by treatment of polyacrylonitrile with hydroxylamine. ~7) This converts some of the nitrile groups to amideoxime structures. Two samples were prepared, AO1 and AO2, for which the compositions were estimated to be 10/1 and 4/1 respectively. TABLE 1. PREPARATION OF POLYMERS
Polymer code
Comonomer
Monomer mixture AN(ml)/comonomer
Polymer yield (~)
Polymer composition (mole ratio) AN/comonomer
AA1 AA2 AA3
Acrylic acid
20 20 20
0-5 ml I "0 ml 2' 0 ml
6 8 20
50/1 21/I 5/1
MA.A1 MAA2 MAA3 MA.A4
Methacrylic acid
20 20 20 20
0" 25 ml 1-0 ml 0" 5 ml 2" 0 ml
4 9 8 9
25/1 10/1 7/1 2/I
IA1 IA2 IA3 IA4
Itaconic acid
20 20 20 20
0" 5 2"0 3-0 4-0
g g g g
20 12 6 2
50/1 25/1 15/I 10/l
AM I AM2 AM3 AM4 AM5 AM6
Acrylamide
20 20 20 20 20 20
1" 0 g 5-0 g 7-5 g 5"0 g 10 g 20 g
6 20 16 8 10 13
50/1 20/1 10/1 8/I 5/1 2/1
Thermal analysis A Du Pont 900 Thermal Analyzer incorporating differential thermal analysis (DTA), thermogravimetry (TG) and differential scanning calorimetry (DSC), was used. In DTA and T G analyses, 10 mg powder samples were heated at 10°/min in a nitrogen flow. For selected copolymers, T G analyses in air and vacuum (10 -z tort) were also obtained. In the DSC determinations of heats of reaction smaller samples (3-5 mg) were heated at 10°/rain under nitrogen. Thermal Volatilization Analysis (TVA) ~s) was also used. 100 mg samples were heated at 10°/rain under vacuum (10 -5 torr) and the gaseous degradation products monitored by a system of Pirani
Pyrolysis of Polyacrylonitrile and related Polymers--VI
259
gauges. The gauges are preceded by traps at various temperatures between 0 ° and -- 196 ° ; curves are obtained which depend upon the condensability of the evolved gases, and give some information about the nature of the products. Infra-red spectra of these products were obtained by distilling the trapped products into a gas cell. In presenting TVA curves, indicated temperatures refer to the trap temperatures at the approriate gauges.
Infra-red spectra A Perkin-Elmer model 257 spectrophotometer was used. Polymers and degradation residues were examined in KBr discs. RESULTS T h e t h e r m a l a n a l y s e s a r e s u m m a r i z e d in T a b l e 2. T h e D T A e x o t h e r m s are c h a r a c t e r i z e d b y t h e A T v a l u e a n d t h e t e m p e r a t u r e a t w h i c h t h e p e a k is o b s e r v e d (Ts). T h e a p p r o x i m a t e s t a r t i n g t e m p e r a t u r e s o f t h e e x o t h e r m a r e a l s o i n c l u d e d (T~). I n t h e T G s t u d y , p y r o l y s i s t o 500 ° w a s c o n s i d e r e d sufficient t o a s s e s s t h e g e n e r a l w e i g h t loss c h a r a c t e r i s t i c s b u t s e l e c t e d p o l y m e r s w e r e h e a t e d t o 1000 ° t o e v a l u a t e t h e effect o f t h e c o m o n o m e r on the c a r b o n yield at elevated t e m p e r a t u r e s . The a m o u n t o f h e a t l i b e r a t e d in t h e e x o t h e r m w a s c a l c u l a t e d f r o m t h e a r e a u n d e r t h e D S C c u r v e ; t h e r e s u l t s a r e e x p r e s s e d in b o t h c a l o r i e s p e r g r a m o f c o p o l y m e r a n d k i l o c a l o r i e s p e r m o l e o f nitrile u n i t s , t h e l a t t e r v a l u e t h u s c o r r e c t i n g f o r t h e a m o u n t o f c o m o n o m e r w h i c h is a s s u m e d n o t to c o n t r i b u t e to the exothermicity. TABLE 2. THERMAL ANALYSIS OF COPOLYMERS (PYROLYSIS IN N2)
DTA*
~ Weight loss 500 ° i000 °
-- ~ h r (DSC) kcal/mole CN
Polymer
Composition
AT
Ts
7"i
$3 AAI AA2 AA3
homopolymer 50/I 21/I 5/1
44 13 6" 0 3"6
325 286 276 267
275 225 220 215
37 34 40 40
70 -54 56
114 -150 162
6" 05 -10" 1 9"2
SAI SA2
21/i 5/1
19 6" 5
300 286
230 225
3t 26
---
---
---
MA.A.1 MAA2 MAA3 MAA4
25/1 10/1 7/1 2/I
2- 0 1.5 1-5 1 "0
285 254 250 260
200 175 175 215
38 39 38 57
-55 ---
165 156 153 167
9- 3 9-6 10"0 16-0
IA 1 IA.2 IA3 IA4
50/1 25/1 15/1 10/1
5" 0 1" 8 1-0 1 "0
275 270 240 242
200 190 185 190
22 34 40 38
60 52 ---
-132 157 154
-7- 8 9.7 10-4
AM1 AM2 AM3 AM4 AM5 AM6
50/I 20/1 10/i 8/1 5/1 2/1
30 47 45 40 15 3
300 292 295 295 258 235
240 220 235 240 235 215
34 36 37 35 43 46
-------
--158 151 144 --
--9"5 9-4 9-7 --
AOI AO2
10/1 4/1
4"4 1" 5
268 266
175 160
19 26
67 --
144 --
8"9 --
*
AT--maximum temperature differential (°C). Tr--sample temperature at AT. Tr--approximate temperature of initiation of transition.
cal/g
260
N. GRASSIE and R. M c G U C H A N
The thermal analysis data for an acrylonitrile homopolymer, $3, is included for comparison, and the DTA and T G curves are shown in Fig. 1. A feature of particular interest is the increased weight loss under vacuum in the region of the exothermic reaction. This is attributed to the easier volatilization in vacuum of chain fragments formed by chain scission reactions concurrent with the rapid and intense exotherm. The corresponding curves for selected copolymers are illustrated in Figs. 2-6.
°o'_ i I o,~ °01-
t,oo
.9 40-
Temperature( ° C )
Fic. 1. DTA and TG curves for polyacrylonitrile, $3; 10 mg samples heated at 10°/rain in N2 ( ), air (- • ") and vacuum (- --). Effect o f comonomer on DTA exotherm
The results for the acrylic acid polymers demonstrate the initiating effect of the acid unit. As the acid content increases, the exotherm becomes less intense and broader with lower initiation and peak temperatures. The shape of the exotherm for the 5/1 copolymer is shown in Fig. 2. The change in the shape of the exotherm from that o f pure polyacrylonitrile suggests that the function of the acid unit is more complex than merely initiating the reaction, since this would have been expected simply to shift the sharp exotherms to lower temperatures. The broad exotherms and small AT values suggest a basically different initiation mechanism and relatively much slower propagation. The methacrylic and itaconic acid copolymers show more complex exotherms than the acrylic acid copolymers and, as illustrated by Figs. 3 and 4, the quoting of single AT values as in Table 2 does not properly define these exotherms. At very low acid contents, the low temperature initiation produces a shoulder on the major peak; as the acid content increases two or more distinct maxima are observed. At the higher acid contents, the low temperature component of the curve becomes the dominant peak. The net effect of this splitting of the exotherm is that the AT values are even smaller than for equivalent acrylic acid compositions, in which acid initiation is clearly less efficient. The results obtained for the sodium acrylate copolymers demonstrate that the salt structure is much less reactive than the free acid in initiating the reaction and the exotherms become more intense.
P?'rolysis of Polyacrylonitrile and related Potymers--VI
261
80,\\
" 1
O•O G0'402 ,/
\
*,
\, 20 -
r
2°
I
~"
/
JJ
i
l
i
l' 200
300 Temperature
400
(°C)
5/1 acrylonitrile-acrylic acid copolymer, A A 3 ; 10 mg samples heated at 10"/min in N2 ( ), air (. • -) and vacuum ( - - -).
FIG. 2. D T A and T G curves for
The exotherms for the acrylamide copolymers tend to be much more similar to those of pure PAN than to those of the acid copolymers; some compositions suggest a small increase in the intensity of the exotherms. At the 5/1 level, AT does show a reduction but the exotherm is still sharp and intense compared with acid polymers. Acrylamide apparently has some initiating ability, however, since the Ts and Tt values are lower than for pure PAN. The acrylamide oxime copolymers contain more reactive initiating structures than the acrylamide copolymers and the D T A behaviour is clearly more akin to the acid copolymers than to the amide copolymers.
lOO-
--.
-... "-.\
!
... \
•
!
I 60--
\
.5
\
\
\
!2o 4,
;0
. . . .
Temperature
,~0 (°C)
FIG. 3. D T A and T G curves for 7/I acrylonitfile-methacrylic acid copolymer, M A A 3 ; I0
mg samples heated at 10°/rain in N2 (
), air (- • ") and vacuum ( - - -).
262
N. GRASSlE and R. M c G U C H A N
"-~:',~ ...................... 8or-
I
201-
I
I
t
200
3(]0
400
Temperature ( ° C )
FIG. 4. D T A and TG curves for 15/1 ac~lonitfile-itaconic acid copolymer, L~3; 10 mg
samples heated at 10°/min in N, (
), air (. • ') and vacuum (- - -).
TG behaviour The three acid copolymer systems show similar thermogravimetric behaviours (Figs. 2-4). In marked contrast with pure PAN, the weight losses are very gradual initially and there is very little weight loss associated with the exotherm. However, this does not generally result in better weight retention at 500 ° since most of the polymers and PAN itself give weight losses in the range 35-40 per cent for pyrolysis in N2. In air and vacuum the weight losses at 500 ° are reduced in the copolymers because of the improved weight retention at the exotherm. Pyrolysis in air results in less weight loss than in nitrogen. This is probably due to oxidation reactions, the oxygen uptake compensating in part. The increased weight loss in vacuum suggests that fragmentation still occurs in the copolymers but to a lesser degree than in pure PAN. In the copolymers, scission does not appear to be concurrent with the exotherm but takes place gradually at higher temperatures. The most favourable weight loss characteristics are shown by the itaconic acid copolymers in which the difference between the nitrogen and vacuum curves is small. This is consistent with the difunctional initiating potential of the IA unit which could form cross-links if both acid groups are involved in initiating reactions. Cross-linking would oppose the formation of fragments by chain scission. The residue yields at 1000 ° do not vary much among the copolymers and show substantial improvement over the homopolymer. It is clear that the carbon yield at 1000 ° bears little relationship to the amount of residue at 500:. The sodium acrylate copolymers exhibit greater weight loss during the exotherm than the acrylic acid copolymers, which is consistent with the more intense exotherms. This weight loss is still quite small however and amounts to 10 per cent in vacuum for the 5/1 copolymer. Subsequent weight loss is gradual and the weight loss at 500° is less for the SA copolymers than the acid copolymers. It seems probable that the ionic structures in the residues reduce the volatility of the chain fragments. The acrylamide copolymers also show weight losses in N2 during the exotherm as illustrated in Fig. 5. Under vacuum, the weight loss is much less severe than in pure
Pyrolysis of Pol.vac~'lonitrile and related Polymers--VI
263
loo,.* i
°01_
o'° ,F \
\
\
//
/I' ;o
i z0k
L E
t zoo
t 300
I 400
(°C)
Temperature
FIG. 5. DTA and TG curves for 5/1 acrylonitrile-acrylamide copolymer, AM5; 10 mg samples heated at 10°/min in Nz ( ), air (. • .) and vacuum ( - - - ) .
P A N . This was true also o f A M 4 for which AT is comparable with that o f $3. Thus the a m o u n t o f fragmentation appears to be sensitive to the temperature at which the exotherm occurs as well as the intensity o f the reaction. Pyrolysis in air exhibits no weight loss at the exotherm which demonstrates the unpredictability o f the influence o f oxygen on the exotherm. This is further emphasized by the results for the acrytamideoxime copolymers (Fig. 6) for which the inert T G curves reflect the small exotherms with very little volatilization in the exotherm region, but the pyrolysis in air is highly exothermic. This is therefore the opposite situation f r o m that in the acrylamide system.
F $0j
'",
~
\,.
i
._~ !
...
........]
'°i
z01 F 1
2 o'0
' 300 Temperature
' 4°0 (°C)
FIG. 6. DTA and TG curves for 4/1 acrylonitrile-acrylamideoxime copo/ymer, AO2; 10 mg samples heated at 10°/min in N2 ( ), air (" • .) and vacuum ( - - - ) .
264
N. GRASSIE and R. M c G U C H A N
D S C studies
The DSC data in Table 2 show that although the shape of the exotherm may vary enormously, the amount of heat liberated is not reduced in those copol.vmers. Indeed based on the nitrile content, the amount of heat may be substantially increased. The most probable explanation is that the observed exotherm is the net sum of both exothermic and endothermic reactions. The latter, which include chain scission and gaseous product formation, are more important in the homopolymer than in the copolymers thus depressing t h e / 5 H value for the homopolymer to the greatest extent. T V A curves
The TVA curves for representative acid copolymers are shown in Fig. 7. The acrylic acid copolymer curve is similar to that of polyacrylonitrile except that the initial peak is smaller (smaller exotherm) and there is some non-condensable product during the initial stages of pyrolysis; in pure PAN non-condensables are not observed until after the exothermic reaction. In the acid copolymer, it is likely that some decomposition of the acid structures to form carbon monoxide occurs but only to a minor extent. The condensable fraction of the products consists of HCN and NH3. The itaconic acid copolymer is similar with even more gradual initial reaction. The first small peak from this copolymer is water. The methacrylic acid copolymer shows a different pattern of noncondensable products with increased volatilization at about 400 °. Product analysis confirmed that this was methane being elin'Jnated from the methacrylic acid units. The main peak consists of hydrogen and carbon monoxide which is common to all the acid copolymers. The TVA curves for acrylamide copolymers are very similar to those of pure PAN. i
__
o~._.o,_7¢
. . . .
/ ~
1oo°
. . . . . . . . . . ~9~
/ ¢.
4,-li 1 0! E
~. ,L ~l Q
I
/f
.~
o.
i
~s " ....
.
ii
0
I
s/
,~
// ..
3 200
300
~0~
Oven Temp.erature{°C)
~$Q
FIG. 7. TVA curves for acid copolymers; (1) AA3, (2) MAA3 and (3) IA3.
Pyrolysis of Polyacrylonitrile and related Polymers--VI
265
The relative amount of ammonia was increased, which is consistent with the fact that the amide structure provides a second source for this product. The amideoxime copolymer, AO2, shows very broad low temperature volatilization yielding ammonia and water. The general pattern of evolution of the water suggests that, like the ammonia, it is formed by decomposition of the oxime structure rather than being an impurity in the copolymer. Coloration and i.r. spectra The acid and acrylamide copolymers show tan-brown coloration similar to pure P A N on pyrolysis to 300 °. There is a tendency to less intense coloration as the comonomer content is increased. The sodium acrylate copolymers, on the other hand, were black at 300 °. The i.r. spectrum of this black residue exhibits a broad nitrile absorption at 2180 cm -~ which is more intense than the residual 2240 cm - t absorption, the only nitrile peak in the unheated copolymer. The new absorption suggests ionic nitrile species in the residue and may also be attributed to ionic structures associated with the ~ N conjugation. The changes in the i.r. spectra of the free acid copolymers are illustrated by Fig. 8. On heating through the exotherm, the free nitrile absorption (2240 cm -~) is greatly reduced and intense absorptions, assigned to
FJ
I
2 3 00
2000
Crl~-1
FIG. 8. Infra-red spectra of (1) M . ~ 3
,&
,;00
,'0o
and (2) M A A 3 heated at 10°/rain to 300 °
conjugated - - C = N - - and imine structures, appear in the 1600 cm-~ region; other intense absorptions appear at 1370, 1240 and 1140 cm - I . These changes are consistent with these in P A N and have been discussed previously. (1'2) The interesting feature here is the disappearance of the carbonyl absorption due to the acid ~ o u p . In the 7/1 copolymer illustrated, this has shifted to lower frequencies and is observed as a shoulder at 1650-1680 cm - t on the main conjugation absorption. In 20/1 copolymers the carbonyl absorption disappears completely and it is concluded that the carbonyl group has become part of the conjugated structure. The itaconic acid copolymers exhibit a specific feature in that a transient anhydride species is formed during pyrolysis; this is consistent with the TVA evidence of water evolution in the
266
N, GRASSIE and R. M c G U C H A N
initial stages of pyrolysis. The intensities of the anhydride absorptions at 1850 and 1780 cm-1 suggest that a little intramolecular anhydride formation takes place:
C~=; H2"-"."~
--CH= ~'/C HZ"-'-'~
/ C\
"
HOOC CHzCOOH
C.//~ O=
+ HzO H2
I ] O--C II O The i.r. changes in the acrylamide copolymers (Fig. 9) show that the amide structure disappears during the exotherm so that it again appears that the carbonyl group is incorporated into the overall conjugation.
/ 2
30100
I 20OO
Cri'1-1
I
I
r
tOO0
12Q0
000
FiG. 9, Infra-red spectra of (1) AM5 and (2) AM5 heated at 10~/min to 260 °.
DISCUSSION The general structure of degraded polyacrylonitrile has been presented C1'2'9) as an imperfect ladder structure incorporating short conjugated sequences. C N - ~ ~
n-O-5
CN Small amounts of abnormalities, impurities or terminal structures initiate the cyclization which is continued by hydrogen transfer and reinitiation of the oligomerization by cyclization of the resulting radical;
I CN
Pyrolysis of Polyacr2,ionitrile and related Polymers--VI
267
to give the carbocyclic end unit in the structure shown. The reaction is free radical in nature and, since the oligomerization is exothermic, the reaction proceeds very rapidly under pyrolysis conditions. In this rapid degradation of PAN there is some weight loss associated with the exotherm, which has raised doubt ¢1°) as to whether the exotherm is adequately explained by the above cyclization process which does not involve weight loss. Volatilization is caused, in fact, by secondary reactions and it is interesting that the present study lends further support to this proposition. Thus, in the pyrolysis of the acid copolymers, the weight loss during the exotherm is negligible so that the assignment of the exotherm to the nitrile group oligomerization is perfectly justified. Acid comonomers produce profound changes in the DTA characteristics since even small amounts of these reactive initiating structures greatly increase the number of primary initiation centres in the polymer structure and, once initiated, the nitrile reaction can continue by the secondary reinitiation process. As the acid content is increased, most of the ladder sequences will be initiated through the acid structures. Since propagation of the nitrile reaction is relatively slow in the acid initiated pyrolyses, it is concluded that the free radical mechanism in pure PAN is replaced in these copolymers by a concerted or ionic mechanism similar to that previously proposed (3'+> for acid initiation
X
C
C
t-,,N
N
H
./
-~
C
["~N
I
H
H
Since each propagation step involves a hydrogen transfer, this mechanism would be expected to be less rapid than the free radical process in pure PAN. Looking at this acid initiation process in more detail, the generally accepted ester structure shown above is not supported by the present study. Since the structure is analogous to a vinyl ester, the carbonyl absorption should appear at a higher frequency than the original acid carbonyl absorption. In fact the carbonyl absorption moves to lower frequencies and it is concluded that it becomes part of the conjugation. It is likely therefore that initiation produces the more stable imide structure, the normal product from acid-nitrile interaction,C11) and that propagation proceeds via tautomeric forms of this as outlined below.
o......c..N..c... ° H
H
..
×~
c
I
H N
- propagation
o~c'-N-~C-.N.-'%-o I
H
o~C.N~c\~>C.o I
H
In this way the carbonyl groups can be incorporated into the conjugated sequences. The different initiation efficiencies observed in the present study are probably related to the stereochemistry of the acid structure relative to the adjacent nitrile
268
N. GRASSIE and R. McGUCHAN
groups. In the case of acrylic acid the most favourable steric arrangement would be as follows: H COOH H
X'YY CN
H
CN
Thus initiation would require rotation to a more favourable configuration. In the disubstituted acid copolymers, both possible configurations are sterically strained so that the probability of the acid structure being adjacent to the nitrile group is increased. In the acrylamide copolymers, a free radical mechanism is indicated; it is proposed that the initiation step is homolytic fission of the amide carbon-nitrogen bond.
"~.
,." ~
,,- ~
+'NH2
H obsfrocfion 1=
=.propagation
NH3+polymerrodiccl ey¢liz~tion
Since the nitrile oligomerization reaction is still apparent in copolymers with fairly high acrylamide contents, it is also feasible that the acrylamide unit can participate in the overall reaction in other ways; thus, = ~ ' ~ /C~N" I~0 NH=
+*NH2
~ NH3+ polymer radic~l
/~C~o
cycli!otlon
In the amideoxime copolymers, a concerted mechanism involving stepwise migration of the hydroxyl group is plausible.
" ~ ~
~~ ' ~ ~
H~N ,/ C~I~..~N H=N/%NJC~NoH O H
propagation
With respect to potential application to carbon fibre manufacture, the acid copolymers appear to be particularly attractive. Although the total amount of heat liberated is not reduced, pyrolysis in both inert and oxygenated environments is much less sharply exothermic than in pure PAN, so that the possibility of direct pyrolytic processing seems more reasonable for these copolymers. Even if pre-oxidation is still necessary, the lower temperatures at which the copolymer exotherrns occur indicate that more rapid preoxidation schedules should be applicable. The itaconic acid copolymers are particularly favourable since fragmentation is almost eliminated during pyrolysis. In all the acid copolymers, the carbon yields for direct pyrolysis are fairly good and much improved over pure polyacrylonitrile. Acknowledgement--The authors thank Rolls-Royce Limited, Derby, for sponsoring this research.
Pyrolysis of Polyacrytonitrile and related Polymers--VI
269
REFERENCES (i) (2) (3) (4) (5) (6) (7) (8) (9) (I0) (11)
N. Grassie and R. McGuchan, Eurap, Palym..]. 6, 1277 (1970). N. Grassie and R. McGuchan, Europ. Palym. J. "7, 1091 (1971). N. Grassie and I. C. McNeill, J. Polym. ScL 2'7, 207 (1958); 29, 211 (1959). N. Grassie and J. N. Hay, J. Palym. Sci. 56, 189 (1962). N. Grassie and R. McGuchan, Europ. Palym. J. 7, 1503 (1971). W. Watt, Proc. R. Sac. A319, 5 (1970). W. R. Sorenson and T. W. Campbell, Preparatire :V[ethods of Polymer Chemistry, pp. 237-238. Interscience, New York, 2nd Edition 1968. I. C. McNeill, Europ. Polym. d'. 6, 373 (1970). N. Grassie and R. McGuchan, Europ. Polym. J. 7, 1357 (1971). J. N. Hay, J. Pol.vm. Sci. A1, 6, 2127 (1968). J. M. Polya and I. M. Spotswood, Recl. Tray. chim. Pays-Bas Belg. 67, 927 (1948).
R~sum6----Les comonom~res acides exercent un fort effet amorceur sur l'oligom6risation exothermique des groupements nitriles du polyacrylonitrile. En contraste avec los transitions &roites et intenses obtenues pour les homopolym~res purs, les transitions exothermiques des copotym~res sont tres largos avec des diff6rences de temp&atures plus r6duites. Ceci est expliqu6 par le fait que la r6action proc~de par un m&anisme radicalaire darts l'homopolymere et par un m6canisme concert6 ou ionique clans les copolym~res Le m&anisme d6taill6 de l'amorqage par l'acide est complexe. On discute la possibilit6 d'existence d'interm6diaires imides. Du point de rue dela production de fibre de carbone le comportement thermogravim&rique de cos copolym~res est meilleur que celui de l'homopolym~re puisque la rupture de chalne et la volatilisation qui apparaissent pendant los transitions exothermiques, sont fortement r~duites. L'acrylamide amorce 6galement la transition exothermique mais clans cecas, la transition exothermique du copolym~re peut &re plus intense que celle de l'homopolym~re si bien, que la nature du radical libre de la r~action est conserv6e dans l'amorqage par l'acrylamide. Au contraire, le comportement d'un syst~me de copolym~re tr~s voisin contenant des structures amideoxime ressemble b. celui de l'acide plus que celui des copolymeres amorc6 par l'acrylamide. Sommario---Dei comonomeri acidi esercitano un forte effetto iniziatore sull'oligomerizzazione esotermica in poliacrilonitrile deI gruppo nitrile. In contrasto con la transizione brusca e intensa che si ha con omopolimeri purl, le esotermiche dei copolimeri sono molto larghe con temperature differenziali molto ridotte. Ci6 si spiega con il meccanismo dei radicali liberi negli omopolimeri che cambia in un meccanismo ionico oppure concertato nei copolimeri. II meccanismo dell'iniziazione acida e complesso; si discute la possibilitb, di prodotti intermedi imidici. Dal punto di vista della produzione di fibra di carbenio, il comportamento termogravimetrico di tall copoiimeri e migliore di quello degli omopolimeri, dato che sono grandemente ridotte la scissione di catene e ta volatizzazione che si verifica durante intense esotermiche. Pure l'acrilarrdde inizia l'esotermica, ma in questo caso l'esotermica dei copolimeri pu6 essere pifi intensa che quella degli omopolimeri cosicch~ la natura a radicali liberi della reazione viene mantenuta netl'iniziazione delt'acrilamide. In contrasto a ci6, il comportamento di un sistema copolimerico strettamento collegato contenente strutture ammidossimiche assomiglio pifi a quello di un acido che a quello di copolimeri di acrilammide. Zusammenfassung--Auf die exotherme Oligomerisierung der Nitritgruppen in Poiyacrylnitril haben saure Comonomere einen stark initiierenden Einflug. Im Gegensatz zu den scharfen intensiven UmwandIungen f~r reine Homopolymere sind die Exothermen fiir die Copolymeren sehr breit mit erheblich reduzierten differentiellen Temperaturen. Dies wird erkl/i.rt dutch einen freien Radikalmechanismus bei dem Homopolymeren, der bei den Copolymeren in einen konzertierten Meclaanismus tibergeht. Der genaue Mechanismus der sauren Initiierung ist komplex; es wird die M6glictxkeit yon Imid Zwischenstufen diskutiert. Vom Gesichtspunkt der Carbonfaser Herstellung aus betrachtet ist das thermogravimetrische Verhalten dieser Copolymeren besser als das des Homopolymeren, da die Kettenspaltung und die Verfltichtigung, die bei intensiven Exothermen stattfinden, stark reduzie~ sind. Aucla Acrylamid initiiert die Exotherme, in diesem Fall karm aber die Copolymer Exotherme intensiver sein als die des Homopolymeren, so dab b e i d e r Acrylamid Initiierung der freie Radikalmechanismus der Reaktion beibehalten bleibt. Das Verhalten eines nahe verwandten Copolymer Systems mit Amidoxim Strukturengleicht, im Gegensatz dazu, mehr dem der sauren als dem der Acrylamid Copolymeren.