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Thermal degradation studies of alternating copolymers: I—maleic anhydride-vinyl acetate
Polymer Degradation and Stability 37 (1992) 223-232
Thermal degradation studies of alternating copolymers: I maleic anhydride-vinyl acetate I. C. McNeill Polymer Research Group, Chemistry Department, University of Glasgow, Glasgow G12 8QQ, Scotland, UK
A. Ya. Polishchuk & G. E. Zaikov Institute of Chemical Physics, Academy of Sciences of the USSR, Vorobyevskoye Chauss~e 2b, 117334 Moscow, Russia (Received 13 June 1991; accepted 24 June 1991)
The thermal degradation of the alternating copolymer of maleic anhydride and vinyl acetate has been studied by thermal volatilisation analysis (TVA). An investigation of the decomposition products and a comprehensive analysis of the residues have been made using subambient TVA, IR spectroscopy, tH and ~3CNMR spectroscopy, the GC-MS technique and UV spectroscopy. On the basis of this evidence, a mechanism of degradation has been suggested. Under programmed heating at 10°C min -1, two stages of breakdown, with rate maxima at 260 and 420°C, were observed. The products consisted of acetic acid, carbon dioxide, carbon monoxide, maleic anhydride and water. Acetic acid, carbon dioxide and traces of carbon monoxide were also observed in a detailed isothermal investigation at 205°C. It is suggested that, following the initial loss of acetic acid, structural rearrangements in the polymer chain can occur, with the formation of conjugated double bonds, so accounting for the colour of the partially degraded residues. Some hydroxyl groups are also formed. During degradation, insolubility develops, which has been attributed to reduction in the flexibility of the backbone and some intermolecular dehydration of OH groups. The interaction between the copolymer (or its residue) and water is discussed in relation to potential applications of this material.
INTRODUCTION
the thermal degradation of this copolymer has received little attention. Matsui & Aida6 studied the process in the temperature region 150-200°C in nitrogen and in air. They concluded that the main volatile product was acetic acid and that double bonds and crosslinks are formed in nitrogen, whereas epoxy groups are formed only in air. Caze & Loucheux7 briefly examined the possibility of different degradation reactions with the object of confirming their proposal of the influence of the charge-transfer complex (CTC) on the microstructure of the alternating copolymers. On the basis of the lowering of two carbonyl absorption bands, following the formation of conjugated double bonds, they suggested
Although the thermal degradation of a wide variety of polymers and copolymers has now been studied, there has been very little work done on the thermal stability and degradation behaviour of alternating copolymers, which provide a unique and interesting type of polymer structure, There have been many studies of the polymerisation of alternating copolymersI and in particular of the copolymerisation of maleic anhydride (MAn) and vinyl acetate (VAc), 1-5 but Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992ElsevierSciencePublishersLtd. 223
224
I.C. McNeill, A. Ya. Polishchuk, G. E. Zaikov
a mechanism of isothermal decomposition of the M A n - V A c copolymer. In common with Matsui & Aida, they described the primary decomposition in terms of first-order elimination of acetic acid. In the present investigation, the thermal degradation of the M A n - V A c copolymer has been studied by the thermal volatilisation analysis (TVA) technique, 8 with subsequent separation of the volatile products by subambient TVA (SATVA). 9 Measurements have been made of the amount of the major product, acetic acid. A comprehensive analysis of the products of degradation and residues has been carried out in order to evaluate probable steps in the degradation mechanism, A further objective of this study was to consider the interaction of water with the initial copolymer and explore how this is modified by different extents of thermal degradation,
EXPERIMENTAL
Copolymer preparation Vinyl acetate was purified according to a standard procedure. 1° Maleic anhydride was recrystallised from purified chloroform and dried under reduced pressure, te, a/-Azoisobutyronitrile (AIBN) was recrystaUised and dried. 7 Analar-grade acetone was dried for 24 h over CaSO4. Vinyl acetate and a solution of MAn in acetone were added to A I B N in a dilatometer in known amount. The dilatometer was attached to the vacuum line and the liquid was degassed four times, after which the dilatometer was sealed under vacuum. Polymerisation was carried out at 60°C to 20% conversion. The crude copolymer was twice precipitated in ethanol from acetone solution and finally dried under vacuum to constant weight. The experimental conditions are listed in Table 1.
Degradation techniques TVA data were collected under continuous evacuation in a differential condensation TVA system, 11 using programmed heating at 10°C/min and isothermal heating at 205°C for periods of 15rain up to 6h. Non-condensable gas was identified by using a quadrupole mass spectrometer (VG Micromass QX-200) on line with the TVA system. Condensable volatile degradation products were collected at -196°C and then separated in the TVA system by SATVA. 9 Products present in each SATVA fraction were analysed by IR spectroscopy and further separated and characterised using G C - M S . The cold ring fraction (CRF) of products volatile at degradation temperature but not at ambient temperature, which collected on the cooled upper part of the TVA tube, was investigated by IR spectroscopy. The involatile residue at various stages of degradation was examined by IR spectroscopy, including FTIR with A T R for undegraded and degraded polymer films, UV spectroscopy, 1H and 13C N M R spectroscopy. The main volatile degradation product, acetic acid, was collected as a liquid fraction and estimated by UV spectroscopy in acetone. The extinction coefficient has been calculated by least-squares regression analysis 12 of the dependence of the absorbance on concentration in acetone to equal 401m01 - l c m -1 at the absorption maximum.
RESULTS Characterisation of the ¢opolymer The copolymerisation of MAn with VAc is believed to involve the presence of a chargetransfer complex (CTC) which is assumed to exist in equilibrium with unassociated species I and to have the structure. -
Table 1. Experimentalconditions in the copolymerisation of maleicanhydride(MAn) with vinylacetateat 60"C Total concentration of monomers Total volume Initial mole fraction of MAn AIBN concentration Conversion
2-5 M 85 ml 0-5 10-2 M 20%
4
-
I
'CH2--CH ?
F I~1H---~ N 1
~C --
[ CH3
[ .C. C.. LO// \ O
[
-
We found rather unexpected evidence for the
Thermal degradation of alternating copolymers presence of the CTC during copolymerisation when the initially colourless solution developed a pink colour. The explanation of this colour may be similar to that suggested by Walling et al., 13 who observed coloured complexes of MAn with styrene or its derivatives, and by Weiss, TM who obtained coloured complexes between quinones and unsaturated hydrocarbons. According to Weiss, the colour follows from the structure: each ion has an odd number of electrons, like a free radical, with a corresponding unoccupied electronic level. This gives rise to a small excitation energy and hence to a light absorption in the visible region. Within the ions themselves, there is resonance on account of their conjugated system of bonds and this is accompanied by a number of polar valence structures which can contribute to the ground state a n d t h e excited state. This gives rise to a large transition moment and to strong absorption (charge resonance spectra). The pink colour was reduced in the precipitated copolymer and lost completely after heating at 100°C. This temperature was selected because the CTC is reported ~ to be unstable above 90°C, but the degradation process does not begin until 150°C The chemical structure of the M A n - V A c copolymer was verified by IR, ~H and ~3C N M R
225
a
1
3
4
2
[~Jl/\
r/~
/\ _ I . __~ I L e.0 5.0
41o
~L~_~--_/L_L 30I 20I ,i0
b 1
"
, ,80
~
, ,60
~
, ,40
,
, ,20
~
~ ,oo ppm
2 4 3 M~CH2--CH--CH--CH'~ I ] I o c c I o//\o / o-------c I1 CH3 (a) ~H NMR
~
~ s , 80
~ 3 , 80
, ~o
I
ao
5 2 4 3 ,,~CH2--CH--CH--CH,~, J 16 [_7 o .C t~ ~ ; 18o¢ \ o / o I/H3C (b) ~3C NMR
Fig. 2. NMR spectra of M A n - V A c copolymer. Key to
spectra (a) and (b):
fl
~ =o = z E
~
I
i
3soo
3000
J
I
~
I
2soo aooo
I
I
,soo
,ooo
spectroscopy and spectra for the undegraded copolymer are illustrated in Figs l(a), 2(a) and 2(b). These spectra are in agreement with those reported by Caze & Loucheuxfl whose interpretation of the main bonds and shifts we have followed.
=
~"
Degradation under p r o g r a m m e d heating conditions
, I I 3so0 3000 2soo 2000
, ,500
Wavenumber,
Fig. 1. IR
, ,ooo
crn-'
spectra of maleic anhydride-vinyl
acetate
copolyrner (a) before degradation, and (b) after heating for 60 min at 205°C under TVA conditions,
The TVA behaviour, shown in Fig. 3, indicates two stages of volatilisation, with rate maxima at 260 and 420°C, respectively. The separation of traces indicates the presence of materials covering a range of volatilities and including only at the second stage (above 250°C) some non-condensable gas, identified by using the on-line mass spectrometer as carbon monoxide.
I.C. McNeill, A. Ya. Polishchuk, G. E. Zaikov
226
CO n-
t--
100
, 150
I 200
J 250
l 300
Temperature,
~ 350
I 400
, 450
*C 3
Fig. 3. T V A behaviour of M A n - V A c copolymer under programmed heating at 10°C/min. - - , 0°C; . . . . . . . -45°C; . . . . . , -75°C; - - - , -100°C; - - - - . , -196°C.
Characterisation of the condensable oolatile products This fraction of degradation products w a s separated by SATVA prior to further investigation. The SATVA curve illustrated in Fig. 4 consists of three peaks. The first and second peaks were found by IR spectroscopy to be due to carbon dioxide and acetic acid, respectively, The third peak was due to liquid products, found by IR spectroscopy and G C - M S to consist of a mixture (chromatogram, Fig. 5) of which the main components were acetic acid, water and MAn. A small amount of the latter was also detected in the CRF products. The residue consisted of a black, insoluble material, contain-
~
l A
~
~ ~_
.~can
t00
200
300
:~ . T .
9.~9
19,04
28.38
Fig. 5. G C chromatogram in G C - M S investigation of the liquid fraction of volatile products of degradation of M A n - V A c copolymer to 500°C at 10°C/min under T V A conditions. MS assignments: peak 1, water; peak 2, acetic acid; peak 3, maleic anhydride.
ing negligible amounts of MAn. Its IR spectrum, however, was remarkable in respect of the presence of a 3400 cm -1 band, discussed further below. Degradation under isothermal conditions
The temperature 205°C was selected as corresponding to the onset of the first stage of degradation indicated by programmed heating.
-196 °
0°
/1
tr
l
o ~
IY"
0
10
acetic acid water
20 Time,
30
liquid
fraction (see text)
4.0
50
rain
Fig. 4. Subambient T V A curve for warm up from -196°C to 0°C of condensable volatile degradation products from degradation of M A n - V A c copolymer to 500°C at 10°C/min under T V A conditions.
Thermal degradation of alternating copolymers "0"
1.0
"~ .-
>- ® u
227
loo
~_.~--~ t---o-
~
c ~ L 0 01
~
u ~ ._
~
50
u < ~
de
0.8 0.6 0,4
< 0.2 15
i 30
I 45 Time,
L f~, i II 60 180
i 360
300
400
min
Fig. 6. Elimination of acetic acid from MAn-VAc copolymer during isothermal degradation at 205°C in the TVA apparatus, The collected condensable volatile degradation products were separated by S A T V A and analysed by IR spectroscopy and G C - M S . Product composition was similar to that for programmed heating except that the amount of MAn present was negligible. The acetic acid was estimated by UV spectroscopy on the basis of the absorption at 210 nm, giving the kinetic data of Fig. 6. Assuming first-order elimination and an Arrhenius temperature relationship, the calculuted rate constant at this temperature is consistent with those previously obtained 7 at 160 and 180°C. Characterisation o f the residues Two characteristics of the residues of degradation at 205°C for various periods of time which are interesting and significant are colour and solubility. Both were found to change with time, the colour changing from pink through white to yellow to brown, and the solubility decreasing. The colour change indicates some structural change which is developing progressively. This was followed by examining spectral changes in the visible region quantitatively. Visible spectra of solutions of the residues (or the soluble fraction), illustrated in Fig. 7, show growing absorption in the region associated with the development of conjugated double bond sequences, It must be noted, however, in relation to conclusions drawn from these data, that the residues are only completely soluble at degradation times of up to 30min. The change in solubility in acetone does not appear simply to reflect a change in the polarity of the chain structure, since attempts to find an alternative less polar solvent for the degraded copolymer were unsuccessful,
; 600
500 Wavelength,
i
S00
700 nm
Fig. 7. Visible spectra of MAn-VAc copolymer and its residuesafter heating for various periods at 205°C. Time of heating (min): a, 0; b, 15; c, 30; d, 45; e, 60.
:'~ :~ r, ,', ! ,,..,', : i
:, : • : ". - . . /r~ ,,.._
?, ..",
. ,
J J
6.0
, 5.0
, 4.0
, 3.0
j 2.0
ppm Fig. g. tHNMR spectra of (a) MAn-VAc copolymer and (b) its residue after 60 min degradation at 205°C. W N M R study In Fig. 8, the spectrum of the original copolymer is compared with those of residues from partial degradation. The undegraded copolymer exhibits the main resonances at 2.1, 2.6, 3-5-3.7 (doublet) and 5.6 ppm and some shifts were also observed around 1.2, 1.7, 6.4 and 7.7 ppm which are attributed to traces of charge-transfer complex or unpolymerised monomers. The most significant difference produced on degradation is the appearance of a new peak at 3-0 ppm which increases with time compared with the other resonances. A second important change is the transformation of the resonance at 2.6ppm from a singlet to a doublet, in which, after i h of degradation, both intensities are the same. The very limited solubility of the residual polymer after more than 3 h precluded further investigation by NMR, but even the data noted above
L C. McNeill, A. Ya. Polishchuk, G. E. Zaikov
228
I
180
I
160
/
140
I
I
I
I
I
120
100
80
60
4.0
20
ppm
Fig. 9. 13CNMRspectra of (a) MAn-VAc copolymer, and its residues after degradation at 205°C for (b) 30 and (c) 60 min.
are sufficient to indicate certain rearrangements in the copolymer chain during isothermal degradation,
Table 2. Main absorptions (era-') in the IR spectrum of the MAn-VAc copolymer after various periods of heating at z0soc" Duration of Heating (rain)
13C N M R study
Further information regarding the rearrangement i s provided by 1 3 C N M R spectra. In Fig. 9, t h e spectra o f t h e undegraded copolymer a n d i t s residue after degradation for 30 and 6 0 m i n are compared. The initial sample exhibits resonances at 173, 171 (doublet), 70, 50, 40, 33 and 21 p p m ,
0
15
30
45
60
3 400 w
3 400 m
3 400 m
3 400 m
3 400 s
3 400 s
1860 s
1856 s
1853 m
1851 m
1 849 m
1 835 w
--
1795s 1740s
1785s 1740m
1780s 1740m
1775s 1740w
1770s 1740w
1755s --
1755s --
Infrared spectroscopicstudy The IR spectra of the undegraded copolymer and its residue after degradation for 60 min at 205°C under T V A conditions are shown in Figs l(a) and l(b), respectively. The spectrum of the copolymer before degradation exhibits the characteristic absorptions at 1860 and 1785cm -1 due to the anhydride ring and at 1740 cm -1 due to the ester function. Other significant bands are also listed in Table 2, in which the main changes occurring during degradation are summarised. In the initial
360 3 400 s
1620w 1620w 1620w 1620w 1620m 1630s 1630s 1 440 m
1 440 m
1 440 m
1 440 m
1 440 m
1 440 s
1 440 s
1375s 1375s 1375s 1375m 1375m 1375w 1375w .
and there are additional resonances at 167, 140, 135, 25 and 1 4 p p m attributed to the impurities noted above. These latter peaks disappear progressively during heating. The main changes are observed in the regions of 108 and 23 ppm, where new resonances appear: the first of these is lost again after 60 min, whereas the second becomes more intense as time of degradation increases. The appearance of resonances at 144 and 167 p p m in the spectrum of the residue after 60 min of degradation may also be significant, but due to the problem of decreasing solubility the data have to be interpreted with caution,
180
.
.
.
.
1290s
1290s
1220s 1220s 1220s 1220s 1220s 1220m 1220w 935m 935m 935m 935m 920m 910s 910s a Abbreviations: s, strong; m , m e d i u m ; w, w e a k .
period of degradation, the disappearance of the ester absorption is observed and the position of the two carbonyl bands shifts to lower wavenumbers, indicating conjugation with double bonds. After extensive degradation (6 h), the residue is clearly of different chemical structure from the starting material: the data for intermediate times show how the changes develop. A band grows at 3400cm -1 due to intramolecularly b o n d e d O H groups and other bands appear at 1755 and 1290 cm -1 (CO absorptions) and at 1630, 1440 and 910 cm -1 (CH absorptions).
DISCUSSION Thermal degradation mechanism Elimination of acetic acid from the copolymer can in principle lead to the formation of any of
Thermal degradation of alternating copolymers three chain structures: ---CH2----CH~-----CH---CH I I o//C\ /C%o O or ,,~,CH2--~H----~H---~H,,~, ? o ~ C . , , o / C %O
(I)
I I C--CH--
1
CH3 (!I)
.C C ~
0//"XO/~O or
----CH=CH----C. H--CH-I "~O
---CH=CH--CH---CH-I I o / / C \ o / C %O
(ma)
then the carbonyl band at 1755cm -1 in the residues suggests that the formation of a four-membered ring ketone is favoured over that of other ketones or aldehydes. Therefore we obtain, respectively --CH2--C----CH ---CH2--CH--CH
O=~C
CH3COOH + -~CH2-~CH2-~C-~--C-I I
229
(m)
Caze & Loucheux 7 proposed that the structure HI is not likely because it does not exhibit double bonds in conjugation with carbonyl groups. The evidence for such conjugation is the lowering of the carbonyl absorption bands of the anhydride
ring (1860, 1785 cm -1) in the partiallydegraded
II O (lb)
I C
II O (llb)
II C--
---CH~-----C--CH2 I C--CH-II o (I,lb) It is proposed that the chain structures containing O H groups formed either directly from l a - I I I a by rearrangement or from the ketones I b - I l l b by tautomerism would be the following: ___CH2 ~.___c,H --CH2---~--~H--
polymer residues which has also been observed in C~--~-C-C--C-this investigation. It seems possible, however, I OH OI H that structure III might be an intermediate in the (I¢) (llc) formation of longer conjugation, the existence of --CH=C--CH: which has been indicated by the U V spectroI I scopic data of Fig. 7. C-----C-Carbon dioxide is also a significant degradation [ OH product, the amount of which increases with (Ilk) time, but this process is slower than acetic acid elimination. The loss of carbon dioxide involves Of these structures, only life can form a polymer breaking of the anhydride ring, which is chain with long conjugation accompanied by confirmed by the disappearance of the band at hydrogen-bonded O H groups: 1860 cm -1 in the IR spectra of residues after a ,,~,CH=C--CH2 lengthy period of degradation. The most I I important bands in the spectrum after 6 h of c,t~____~._._CH~,~___~H2 ,-, ,-, ~ degradation are the single carbonyl absorption at H--O---H--O--~C----CH~--------C--CH2 1755 cm-' and the hydrogen-bonded O H absorpI I CH~------C--CH2 C----C tion at 3400 cm-L This indicates the presence of I I I carbonyl and O H in the structures resulting from H--O---H--O ~G,~ breakdown of the anhydride ring. A further possibility which cannot be excluded If we consider the intermediate state of the is rearrangement of le, !1¢ or I l k or all of them chain after elimination of CO2 from each of the to the structure IV¢, which also results in long above structures conjugated sequences in the polymer backbone: --CH2--CH = C--CH-I --cH=c CH "C = O (In) I II --CH2--CH2~C-CH---C-(lYe) I I "C-~-O (lla) OH
230
I . C . McNeill, A. Ya. Polishchuk, G. E. Zaikov
The presence of the structures mentioned above is consistent with the following pieces of experimental evidence, (a) The colour of the residues indicating conjugated sequences of various lengths (structures lllc, IVc).
resonance at 3-0 ppm in the 1H NMR spectrum even for very short degradation times. Furthermore, the evidence for the presence of O H groups leads us to propose that acetic acid elimination occurs in two ways. The first stage of degradation is therefore represented as follows:
(b) The appearance of doublets at 2.6 ppm in the mNMR spectra, corresponding to --CH----CH-- and at 108ppm in the 13C NMR spectra corresponding to ---C-----C-(structure II). (c) Resonances at 3.0 ppm in the ~H NMR and at 23ppm in the 13C NMR spectra, together with IR evidence, indicating the presence of ~wCH-- (all structures except
(--CH2---CH---CH--CH)n-I I ] ? 0//% /C%o O~C O I CH,
/
~~HJCOOH (--CH2---CH%H--CH)n.(---CH~CH--CH---CH),.--I I
u).
o%o c%
Before a mechanism is proposed, these data have also to be related to the time of degradation. The doublets referred to in (b) above appear only after 30 min of degradation. It may therefore be proposed that ll is formed after I, which is confirmed by the appearance of a
(---CH2
0) (m) The next step includes rearrangement of structures I and lll, with or without elimination of carbon dioxide, giving a complex chain structure which may be represented as:
C-----CH
I
(
I
CH~----C---CH2
I
CIm C H ) n 3 ( ~ C H 2 - ~ C H 2 - - - ~ C ) n , 0
oC%C%
I
C[ ~ m C ) n s ( - ~ C H - -[- C ~ C1[H
C C O / / N o / %O
H--O---
CH---C)~--I H--O---
The last step of isothermal degradation is total breakdown of the anhydride ring, leading to a chain structure such as (--CH2--C-----CH
I
(--CH---C---CH2
I
C---CH).7(--CH2~CH--CH
II
O.
I
C
II
II
C)n,
O
The n values in these structures may vary and there is not sufficient evidence on which to base any comments about their relative values. The ring structures built into the backbone reduce flexibility and provide one reason for the reduction in solubility. A second contributing factor could be crosslinking, for which the simplest explanation would be dehydration between pairs of O H groups. This explanation is supported by the fact that some water is released during isothermal degradation. This type of
I
I
C--~C),9(--CH~-----C CH
I
H--O---
I
II
CH'~C)nlO'~ I
H---O--reaction would lead to crosslinks such as ,w,CH--C---CH21 L ~,CH=C----CH[ II C---~ HC C/~ ,-,I I or ~ y ~,C----C '~'(2---C.H II I I I H2C---C-----CH,w, HC~CHM~ Such structures are consistent with the spectroscopic data discussed above.
Thermal degradation of alternating copolymers Finally, it may be commented that under programmed heating some differences in mechanism are involved since, at high temperatures, elimination of MAn from the chain may occur in competition with scission of the ring to release carbon dioxide; the latter process under these conditions also involves some loss of carbon monoxide.
a A
/'-~
'
3500
~
b
The degradation mechanism of the MAn-VAc alternating copolymer, although basically consisting of a first step of acetic acid loss, followed by subsequent elimination of carbon dioxide from the MAn units remaining in the chains, is complicated by the possibility of more than one structure resulting in the initial step and by a variety of alternative structures formed as a result of CO 2 loss, which lead to the development of conjugation and consequently of colour in the residual copolymer. There is also a gradual loss of solubility, partly due to the reduction in flexibility in the backbone, but also as a result of crosslinking by intermolecular dehydration involving OH groups produced in some of the structures resulting from CO2 loss. Although acetic acid loss predominates at the lowest degradation temperatures, this reaction cannot be completely separated from the subsequent elimination of CO2.
~ z_ E
A
An understanding of the behaviour of the initial copolymer in presence of water and the way in which sorption of water is altered by controlled degradation was a secondary objective of this study. Sorption of water was considered in two situations, firstly from the atmosphere and secondly from the liquid phase, The first stage of interaction of the copolymer with atmospheric water vapour is characterised by reversible sorption. After drying under reduced pressure the sample recovered its original weight and the chemical structure of the copolymer did not change. Longer exposure to water vapour, however, over a period of three days or more, led to irreversible sorption of water, but not to further increase in water content in the sample or to any detectable change in structure, In contrast, water sorption from the liquid phase led through the stage of irreversible
r~
f~
/
General conclusions on degradation behaviour
Water sorption by the MAn-VAc copolymer
231
'
3000
2
s'00 2 0'0 0 "-~'-'~
V-4 I
I
'00
15
'
1000
c~
.
f~,
/
I
3500 3000 2soo 2d00
I
,500
1o'o0
c ~
~
I 3500
/
I 3000
-,'/
I
2500
~
I 2000
/ //~J
I 1500
f,~f/
I tO00
W,v.o,mb.r. era-' Fig. 10. IR spectra of (a) MAn-VAc copolymer, (b) productafter exposure to H20(liq.) for 5 min, and (c) product after exposure for 15min. sorption to hydrolysis of the copolymer, followed by its complete dissolution after two hours of exposure in water. The chemical structure of the copolymer at the stage of irreversible binding was the same for sorption from the gas and liquid phases. The changes in IR spectrum following exposure to H20(liq.) are shown in Fig. 10. Taking these results into account, we suggest that water sorption by the MAn-VAc copolymer includes reversible sorption up to the equilibrium value, then irreversible binding of the sorbed water, rupture of chemical bonds in the copolymer with change in its structure (hydrolysis) and, finally, dissolution. The critical concentration for hydrolysis is only reached in sorption from the liquid phase. The general pattern of water sorption by partially degraded copolymer is the same as for the original material and the differences are in the depth of the structural changes following the point of equilibrium sorption. Even the residue obtained after degradation for 15 min is insoluble
232
I . C . McNeill, A. Ya. Polishchuk, G. E. Zaikov
in water although some hydrolysis was observed. Degradation for 30 min gives a product in which the stage of irreversible sorption is restricted and, after one hour of degradation, sorption of water was found to be reversible. The change of the hydrophilic character of the copolymer due to degradation to different extents may be applied in systems w h e r e variation of the
hydrophobic/hydrophilic
balance is required. O n e such case is for controlled drug release. The regulation of the mechanism and velocity of drug release using the various modifications of the initial copolymer is a possible method of
providing programmed drug delivery and the possible use of the MAn-VAc alternating copolymer in this way is the subject of further
research.
ACKNOWLEDGEMENT Support for this project from the Royal Society and the U S S R A c a d e m y of Sciences is acknowledged with thanks.
REFERENCES 1. Cowie, J. M. G., Alternating Copolymers. Plenum Press, New York, 1985. 2. Georgiev, G. S., Golubev, B. V. & Zubov, V. P., Polym. Sci. USSR, 20A (1978) 1814. 3. Caze, C. & Loucheux, C., J. Macromol. Sci.--Chem., A9 (1979) 29. 4. Arnaud, R., Caze, C. & Fossey, J., J. Macromol. Sci.--Chem., A14 (1980) 1269. 5. Fujimori, N. & Brown, A. S., Polym. Bull., 15 (1986) 223. 6. Matsui, S. & Aida, H., High Polym. Jpn., 26 (1969) 10. 7. Caze, C. & Loucheux, C., J. Macromol. Sci.--Chem., A15 (1981)95. 8. McNeill, I. C., J. Polym. Sci., Part A, 4 (1966) 2479. 9. McNeill, I. C., Ackerman, L., Gupta, S. N., Zulfiqar, M. & Zulfiqar, S., J. Polym. Sci., Polym. Chem. Ed., 15 (1977) 2381. 10. Nozakura, S. I., Morishima, Y. & Murakashi, S., J. Polym. Sci., Polym. Chem. Ed., 10 (1972) 2853. 11. McNeill, I. C., Eur. Polym. J., 6 (1970) 373. 12. Norris, A. C., Computational Chemistry. Wiley, New York, 1981. 13. Walling, C., Briggs, E. R., Wolfstirn, K. B. & Mayo, F. R., J. Am. Chem. Soc., 70 (1948) 1537. 14. Weiss, J., J. Chem. Soc., 1942, 245.
Thermal degradation studies of alternating copolymers: I maleic anhydride-vinyl acetate I. C. McNeill Polymer Research Group, Chemistry Department, University of Glasgow, Glasgow G12 8QQ, Scotland, UK
A. Ya. Polishchuk & G. E. Zaikov Institute of Chemical Physics, Academy of Sciences of the USSR, Vorobyevskoye Chauss~e 2b, 117334 Moscow, Russia (Received 13 June 1991; accepted 24 June 1991)
The thermal degradation of the alternating copolymer of maleic anhydride and vinyl acetate has been studied by thermal volatilisation analysis (TVA). An investigation of the decomposition products and a comprehensive analysis of the residues have been made using subambient TVA, IR spectroscopy, tH and ~3CNMR spectroscopy, the GC-MS technique and UV spectroscopy. On the basis of this evidence, a mechanism of degradation has been suggested. Under programmed heating at 10°C min -1, two stages of breakdown, with rate maxima at 260 and 420°C, were observed. The products consisted of acetic acid, carbon dioxide, carbon monoxide, maleic anhydride and water. Acetic acid, carbon dioxide and traces of carbon monoxide were also observed in a detailed isothermal investigation at 205°C. It is suggested that, following the initial loss of acetic acid, structural rearrangements in the polymer chain can occur, with the formation of conjugated double bonds, so accounting for the colour of the partially degraded residues. Some hydroxyl groups are also formed. During degradation, insolubility develops, which has been attributed to reduction in the flexibility of the backbone and some intermolecular dehydration of OH groups. The interaction between the copolymer (or its residue) and water is discussed in relation to potential applications of this material.
INTRODUCTION
the thermal degradation of this copolymer has received little attention. Matsui & Aida6 studied the process in the temperature region 150-200°C in nitrogen and in air. They concluded that the main volatile product was acetic acid and that double bonds and crosslinks are formed in nitrogen, whereas epoxy groups are formed only in air. Caze & Loucheux7 briefly examined the possibility of different degradation reactions with the object of confirming their proposal of the influence of the charge-transfer complex (CTC) on the microstructure of the alternating copolymers. On the basis of the lowering of two carbonyl absorption bands, following the formation of conjugated double bonds, they suggested
Although the thermal degradation of a wide variety of polymers and copolymers has now been studied, there has been very little work done on the thermal stability and degradation behaviour of alternating copolymers, which provide a unique and interesting type of polymer structure, There have been many studies of the polymerisation of alternating copolymersI and in particular of the copolymerisation of maleic anhydride (MAn) and vinyl acetate (VAc), 1-5 but Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992ElsevierSciencePublishersLtd. 223
224
I.C. McNeill, A. Ya. Polishchuk, G. E. Zaikov
a mechanism of isothermal decomposition of the M A n - V A c copolymer. In common with Matsui & Aida, they described the primary decomposition in terms of first-order elimination of acetic acid. In the present investigation, the thermal degradation of the M A n - V A c copolymer has been studied by the thermal volatilisation analysis (TVA) technique, 8 with subsequent separation of the volatile products by subambient TVA (SATVA). 9 Measurements have been made of the amount of the major product, acetic acid. A comprehensive analysis of the products of degradation and residues has been carried out in order to evaluate probable steps in the degradation mechanism, A further objective of this study was to consider the interaction of water with the initial copolymer and explore how this is modified by different extents of thermal degradation,
EXPERIMENTAL
Copolymer preparation Vinyl acetate was purified according to a standard procedure. 1° Maleic anhydride was recrystallised from purified chloroform and dried under reduced pressure, te, a/-Azoisobutyronitrile (AIBN) was recrystaUised and dried. 7 Analar-grade acetone was dried for 24 h over CaSO4. Vinyl acetate and a solution of MAn in acetone were added to A I B N in a dilatometer in known amount. The dilatometer was attached to the vacuum line and the liquid was degassed four times, after which the dilatometer was sealed under vacuum. Polymerisation was carried out at 60°C to 20% conversion. The crude copolymer was twice precipitated in ethanol from acetone solution and finally dried under vacuum to constant weight. The experimental conditions are listed in Table 1.
Degradation techniques TVA data were collected under continuous evacuation in a differential condensation TVA system, 11 using programmed heating at 10°C/min and isothermal heating at 205°C for periods of 15rain up to 6h. Non-condensable gas was identified by using a quadrupole mass spectrometer (VG Micromass QX-200) on line with the TVA system. Condensable volatile degradation products were collected at -196°C and then separated in the TVA system by SATVA. 9 Products present in each SATVA fraction were analysed by IR spectroscopy and further separated and characterised using G C - M S . The cold ring fraction (CRF) of products volatile at degradation temperature but not at ambient temperature, which collected on the cooled upper part of the TVA tube, was investigated by IR spectroscopy. The involatile residue at various stages of degradation was examined by IR spectroscopy, including FTIR with A T R for undegraded and degraded polymer films, UV spectroscopy, 1H and 13C N M R spectroscopy. The main volatile degradation product, acetic acid, was collected as a liquid fraction and estimated by UV spectroscopy in acetone. The extinction coefficient has been calculated by least-squares regression analysis 12 of the dependence of the absorbance on concentration in acetone to equal 401m01 - l c m -1 at the absorption maximum.
RESULTS Characterisation of the ¢opolymer The copolymerisation of MAn with VAc is believed to involve the presence of a chargetransfer complex (CTC) which is assumed to exist in equilibrium with unassociated species I and to have the structure. -
Table 1. Experimentalconditions in the copolymerisation of maleicanhydride(MAn) with vinylacetateat 60"C Total concentration of monomers Total volume Initial mole fraction of MAn AIBN concentration Conversion
2-5 M 85 ml 0-5 10-2 M 20%
4
-
I
'CH2--CH ?
F I~1H---~ N 1
~C --
[ CH3
[ .C. C.. LO// \ O
[
-
We found rather unexpected evidence for the
Thermal degradation of alternating copolymers presence of the CTC during copolymerisation when the initially colourless solution developed a pink colour. The explanation of this colour may be similar to that suggested by Walling et al., 13 who observed coloured complexes of MAn with styrene or its derivatives, and by Weiss, TM who obtained coloured complexes between quinones and unsaturated hydrocarbons. According to Weiss, the colour follows from the structure: each ion has an odd number of electrons, like a free radical, with a corresponding unoccupied electronic level. This gives rise to a small excitation energy and hence to a light absorption in the visible region. Within the ions themselves, there is resonance on account of their conjugated system of bonds and this is accompanied by a number of polar valence structures which can contribute to the ground state a n d t h e excited state. This gives rise to a large transition moment and to strong absorption (charge resonance spectra). The pink colour was reduced in the precipitated copolymer and lost completely after heating at 100°C. This temperature was selected because the CTC is reported ~ to be unstable above 90°C, but the degradation process does not begin until 150°C The chemical structure of the M A n - V A c copolymer was verified by IR, ~H and ~3C N M R
225
a
1
3
4
2
[~Jl/\
r/~
/\ _ I . __~ I L e.0 5.0
41o
~L~_~--_/L_L 30I 20I ,i0
b 1
"
, ,80
~
, ,60
~
, ,40
,
, ,20
~
~ ,oo ppm
2 4 3 M~CH2--CH--CH--CH'~ I ] I o c c I o//\o / o-------c I1 CH3 (a) ~H NMR
~
~ s , 80
~ 3 , 80
, ~o
I
ao
5 2 4 3 ,,~CH2--CH--CH--CH,~, J 16 [_7 o .C t~ ~ ; 18o¢ \ o / o I/H3C (b) ~3C NMR
Fig. 2. NMR spectra of M A n - V A c copolymer. Key to
spectra (a) and (b):
fl
~ =o = z E
~
I
i
3soo
3000
J
I
~
I
2soo aooo
I
I
,soo
,ooo
spectroscopy and spectra for the undegraded copolymer are illustrated in Figs l(a), 2(a) and 2(b). These spectra are in agreement with those reported by Caze & Loucheuxfl whose interpretation of the main bonds and shifts we have followed.
=
~"
Degradation under p r o g r a m m e d heating conditions
, I I 3so0 3000 2soo 2000
, ,500
Wavenumber,
Fig. 1. IR
, ,ooo
crn-'
spectra of maleic anhydride-vinyl
acetate
copolyrner (a) before degradation, and (b) after heating for 60 min at 205°C under TVA conditions,
The TVA behaviour, shown in Fig. 3, indicates two stages of volatilisation, with rate maxima at 260 and 420°C, respectively. The separation of traces indicates the presence of materials covering a range of volatilities and including only at the second stage (above 250°C) some non-condensable gas, identified by using the on-line mass spectrometer as carbon monoxide.
I.C. McNeill, A. Ya. Polishchuk, G. E. Zaikov
226
CO n-
t--
100
, 150
I 200
J 250
l 300
Temperature,
~ 350
I 400
, 450
*C 3
Fig. 3. T V A behaviour of M A n - V A c copolymer under programmed heating at 10°C/min. - - , 0°C; . . . . . . . -45°C; . . . . . , -75°C; - - - , -100°C; - - - - . , -196°C.
Characterisation of the condensable oolatile products This fraction of degradation products w a s separated by SATVA prior to further investigation. The SATVA curve illustrated in Fig. 4 consists of three peaks. The first and second peaks were found by IR spectroscopy to be due to carbon dioxide and acetic acid, respectively, The third peak was due to liquid products, found by IR spectroscopy and G C - M S to consist of a mixture (chromatogram, Fig. 5) of which the main components were acetic acid, water and MAn. A small amount of the latter was also detected in the CRF products. The residue consisted of a black, insoluble material, contain-
~
l A
~
~ ~_
.~can
t00
200
300
:~ . T .
9.~9
19,04
28.38
Fig. 5. G C chromatogram in G C - M S investigation of the liquid fraction of volatile products of degradation of M A n - V A c copolymer to 500°C at 10°C/min under T V A conditions. MS assignments: peak 1, water; peak 2, acetic acid; peak 3, maleic anhydride.
ing negligible amounts of MAn. Its IR spectrum, however, was remarkable in respect of the presence of a 3400 cm -1 band, discussed further below. Degradation under isothermal conditions
The temperature 205°C was selected as corresponding to the onset of the first stage of degradation indicated by programmed heating.
-196 °
0°
/1
tr
l
o ~
IY"
0
10
acetic acid water
20 Time,
30
liquid
fraction (see text)
4.0
50
rain
Fig. 4. Subambient T V A curve for warm up from -196°C to 0°C of condensable volatile degradation products from degradation of M A n - V A c copolymer to 500°C at 10°C/min under T V A conditions.
Thermal degradation of alternating copolymers "0"
1.0
"~ .-
>- ® u
227
loo
~_.~--~ t---o-
~
c ~ L 0 01
~
u ~ ._
~
50
u < ~
de
0.8 0.6 0,4
< 0.2 15
i 30
I 45 Time,
L f~, i II 60 180
i 360
300
400
min
Fig. 6. Elimination of acetic acid from MAn-VAc copolymer during isothermal degradation at 205°C in the TVA apparatus, The collected condensable volatile degradation products were separated by S A T V A and analysed by IR spectroscopy and G C - M S . Product composition was similar to that for programmed heating except that the amount of MAn present was negligible. The acetic acid was estimated by UV spectroscopy on the basis of the absorption at 210 nm, giving the kinetic data of Fig. 6. Assuming first-order elimination and an Arrhenius temperature relationship, the calculuted rate constant at this temperature is consistent with those previously obtained 7 at 160 and 180°C. Characterisation o f the residues Two characteristics of the residues of degradation at 205°C for various periods of time which are interesting and significant are colour and solubility. Both were found to change with time, the colour changing from pink through white to yellow to brown, and the solubility decreasing. The colour change indicates some structural change which is developing progressively. This was followed by examining spectral changes in the visible region quantitatively. Visible spectra of solutions of the residues (or the soluble fraction), illustrated in Fig. 7, show growing absorption in the region associated with the development of conjugated double bond sequences, It must be noted, however, in relation to conclusions drawn from these data, that the residues are only completely soluble at degradation times of up to 30min. The change in solubility in acetone does not appear simply to reflect a change in the polarity of the chain structure, since attempts to find an alternative less polar solvent for the degraded copolymer were unsuccessful,
; 600
500 Wavelength,
i
S00
700 nm
Fig. 7. Visible spectra of MAn-VAc copolymer and its residuesafter heating for various periods at 205°C. Time of heating (min): a, 0; b, 15; c, 30; d, 45; e, 60.
:'~ :~ r, ,', ! ,,..,', : i
:, : • : ". - . . /r~ ,,.._
?, ..",
. ,
J J
6.0
, 5.0
, 4.0
, 3.0
j 2.0
ppm Fig. g. tHNMR spectra of (a) MAn-VAc copolymer and (b) its residue after 60 min degradation at 205°C. W N M R study In Fig. 8, the spectrum of the original copolymer is compared with those of residues from partial degradation. The undegraded copolymer exhibits the main resonances at 2.1, 2.6, 3-5-3.7 (doublet) and 5.6 ppm and some shifts were also observed around 1.2, 1.7, 6.4 and 7.7 ppm which are attributed to traces of charge-transfer complex or unpolymerised monomers. The most significant difference produced on degradation is the appearance of a new peak at 3-0 ppm which increases with time compared with the other resonances. A second important change is the transformation of the resonance at 2.6ppm from a singlet to a doublet, in which, after i h of degradation, both intensities are the same. The very limited solubility of the residual polymer after more than 3 h precluded further investigation by NMR, but even the data noted above
L C. McNeill, A. Ya. Polishchuk, G. E. Zaikov
228
I
180
I
160
/
140
I
I
I
I
I
120
100
80
60
4.0
20
ppm
Fig. 9. 13CNMRspectra of (a) MAn-VAc copolymer, and its residues after degradation at 205°C for (b) 30 and (c) 60 min.
are sufficient to indicate certain rearrangements in the copolymer chain during isothermal degradation,
Table 2. Main absorptions (era-') in the IR spectrum of the MAn-VAc copolymer after various periods of heating at z0soc" Duration of Heating (rain)
13C N M R study
Further information regarding the rearrangement i s provided by 1 3 C N M R spectra. In Fig. 9, t h e spectra o f t h e undegraded copolymer a n d i t s residue after degradation for 30 and 6 0 m i n are compared. The initial sample exhibits resonances at 173, 171 (doublet), 70, 50, 40, 33 and 21 p p m ,
0
15
30
45
60
3 400 w
3 400 m
3 400 m
3 400 m
3 400 s
3 400 s
1860 s
1856 s
1853 m
1851 m
1 849 m
1 835 w
--
1795s 1740s
1785s 1740m
1780s 1740m
1775s 1740w
1770s 1740w
1755s --
1755s --
Infrared spectroscopicstudy The IR spectra of the undegraded copolymer and its residue after degradation for 60 min at 205°C under T V A conditions are shown in Figs l(a) and l(b), respectively. The spectrum of the copolymer before degradation exhibits the characteristic absorptions at 1860 and 1785cm -1 due to the anhydride ring and at 1740 cm -1 due to the ester function. Other significant bands are also listed in Table 2, in which the main changes occurring during degradation are summarised. In the initial
360 3 400 s
1620w 1620w 1620w 1620w 1620m 1630s 1630s 1 440 m
1 440 m
1 440 m
1 440 m
1 440 m
1 440 s
1 440 s
1375s 1375s 1375s 1375m 1375m 1375w 1375w .
and there are additional resonances at 167, 140, 135, 25 and 1 4 p p m attributed to the impurities noted above. These latter peaks disappear progressively during heating. The main changes are observed in the regions of 108 and 23 ppm, where new resonances appear: the first of these is lost again after 60 min, whereas the second becomes more intense as time of degradation increases. The appearance of resonances at 144 and 167 p p m in the spectrum of the residue after 60 min of degradation may also be significant, but due to the problem of decreasing solubility the data have to be interpreted with caution,
180
.
.
.
.
1290s
1290s
1220s 1220s 1220s 1220s 1220s 1220m 1220w 935m 935m 935m 935m 920m 910s 910s a Abbreviations: s, strong; m , m e d i u m ; w, w e a k .
period of degradation, the disappearance of the ester absorption is observed and the position of the two carbonyl bands shifts to lower wavenumbers, indicating conjugation with double bonds. After extensive degradation (6 h), the residue is clearly of different chemical structure from the starting material: the data for intermediate times show how the changes develop. A band grows at 3400cm -1 due to intramolecularly b o n d e d O H groups and other bands appear at 1755 and 1290 cm -1 (CO absorptions) and at 1630, 1440 and 910 cm -1 (CH absorptions).
DISCUSSION Thermal degradation mechanism Elimination of acetic acid from the copolymer can in principle lead to the formation of any of
Thermal degradation of alternating copolymers three chain structures: ---CH2----CH~-----CH---CH I I o//C\ /C%o O or ,,~,CH2--~H----~H---~H,,~, ? o ~ C . , , o / C %O
(I)
I I C--CH--
1
CH3 (!I)
.C C ~
0//"XO/~O or
----CH=CH----C. H--CH-I "~O
---CH=CH--CH---CH-I I o / / C \ o / C %O
(ma)
then the carbonyl band at 1755cm -1 in the residues suggests that the formation of a four-membered ring ketone is favoured over that of other ketones or aldehydes. Therefore we obtain, respectively --CH2--C----CH ---CH2--CH--CH
O=~C
CH3COOH + -~CH2-~CH2-~C-~--C-I I
229
(m)
Caze & Loucheux 7 proposed that the structure HI is not likely because it does not exhibit double bonds in conjugation with carbonyl groups. The evidence for such conjugation is the lowering of the carbonyl absorption bands of the anhydride
ring (1860, 1785 cm -1) in the partiallydegraded
II O (lb)
I C
II O (llb)
II C--
---CH~-----C--CH2 I C--CH-II o (I,lb) It is proposed that the chain structures containing O H groups formed either directly from l a - I I I a by rearrangement or from the ketones I b - I l l b by tautomerism would be the following: ___CH2 ~.___c,H --CH2---~--~H--
polymer residues which has also been observed in C~--~-C-C--C-this investigation. It seems possible, however, I OH OI H that structure III might be an intermediate in the (I¢) (llc) formation of longer conjugation, the existence of --CH=C--CH: which has been indicated by the U V spectroI I scopic data of Fig. 7. C-----C-Carbon dioxide is also a significant degradation [ OH product, the amount of which increases with (Ilk) time, but this process is slower than acetic acid elimination. The loss of carbon dioxide involves Of these structures, only life can form a polymer breaking of the anhydride ring, which is chain with long conjugation accompanied by confirmed by the disappearance of the band at hydrogen-bonded O H groups: 1860 cm -1 in the IR spectra of residues after a ,,~,CH=C--CH2 lengthy period of degradation. The most I I important bands in the spectrum after 6 h of c,t~____~._._CH~,~___~H2 ,-, ,-, ~ degradation are the single carbonyl absorption at H--O---H--O--~C----CH~--------C--CH2 1755 cm-' and the hydrogen-bonded O H absorpI I CH~------C--CH2 C----C tion at 3400 cm-L This indicates the presence of I I I carbonyl and O H in the structures resulting from H--O---H--O ~G,~ breakdown of the anhydride ring. A further possibility which cannot be excluded If we consider the intermediate state of the is rearrangement of le, !1¢ or I l k or all of them chain after elimination of CO2 from each of the to the structure IV¢, which also results in long above structures conjugated sequences in the polymer backbone: --CH2--CH = C--CH-I --cH=c CH "C = O (In) I II --CH2--CH2~C-CH---C-(lYe) I I "C-~-O (lla) OH
230
I . C . McNeill, A. Ya. Polishchuk, G. E. Zaikov
The presence of the structures mentioned above is consistent with the following pieces of experimental evidence, (a) The colour of the residues indicating conjugated sequences of various lengths (structures lllc, IVc).
resonance at 3-0 ppm in the 1H NMR spectrum even for very short degradation times. Furthermore, the evidence for the presence of O H groups leads us to propose that acetic acid elimination occurs in two ways. The first stage of degradation is therefore represented as follows:
(b) The appearance of doublets at 2.6 ppm in the mNMR spectra, corresponding to --CH----CH-- and at 108ppm in the 13C NMR spectra corresponding to ---C-----C-(structure II). (c) Resonances at 3.0 ppm in the ~H NMR and at 23ppm in the 13C NMR spectra, together with IR evidence, indicating the presence of ~wCH-- (all structures except
(--CH2---CH---CH--CH)n-I I ] ? 0//% /C%o O~C O I CH,
/
~~HJCOOH (--CH2---CH%H--CH)n.(---CH~CH--CH---CH),.--I I
u).
o%o c%
Before a mechanism is proposed, these data have also to be related to the time of degradation. The doublets referred to in (b) above appear only after 30 min of degradation. It may therefore be proposed that ll is formed after I, which is confirmed by the appearance of a
(---CH2
0) (m) The next step includes rearrangement of structures I and lll, with or without elimination of carbon dioxide, giving a complex chain structure which may be represented as:
C-----CH
I
(
I
CH~----C---CH2
I
CIm C H ) n 3 ( ~ C H 2 - ~ C H 2 - - - ~ C ) n , 0
oC%C%
I
C[ ~ m C ) n s ( - ~ C H - -[- C ~ C1[H
C C O / / N o / %O
H--O---
CH---C)~--I H--O---
The last step of isothermal degradation is total breakdown of the anhydride ring, leading to a chain structure such as (--CH2--C-----CH
I
(--CH---C---CH2
I
C---CH).7(--CH2~CH--CH
II
O.
I
C
II
II
C)n,
O
The n values in these structures may vary and there is not sufficient evidence on which to base any comments about their relative values. The ring structures built into the backbone reduce flexibility and provide one reason for the reduction in solubility. A second contributing factor could be crosslinking, for which the simplest explanation would be dehydration between pairs of O H groups. This explanation is supported by the fact that some water is released during isothermal degradation. This type of
I
I
C--~C),9(--CH~-----C CH
I
H--O---
I
II
CH'~C)nlO'~ I
H---O--reaction would lead to crosslinks such as ,w,CH--C---CH21 L ~,CH=C----CH[ II C---~ HC C/~ ,-,I I or ~ y ~,C----C '~'(2---C.H II I I I H2C---C-----CH,w, HC~CHM~ Such structures are consistent with the spectroscopic data discussed above.
Thermal degradation of alternating copolymers Finally, it may be commented that under programmed heating some differences in mechanism are involved since, at high temperatures, elimination of MAn from the chain may occur in competition with scission of the ring to release carbon dioxide; the latter process under these conditions also involves some loss of carbon monoxide.
a A
/'-~
'
3500
~
b
The degradation mechanism of the MAn-VAc alternating copolymer, although basically consisting of a first step of acetic acid loss, followed by subsequent elimination of carbon dioxide from the MAn units remaining in the chains, is complicated by the possibility of more than one structure resulting in the initial step and by a variety of alternative structures formed as a result of CO 2 loss, which lead to the development of conjugation and consequently of colour in the residual copolymer. There is also a gradual loss of solubility, partly due to the reduction in flexibility in the backbone, but also as a result of crosslinking by intermolecular dehydration involving OH groups produced in some of the structures resulting from CO2 loss. Although acetic acid loss predominates at the lowest degradation temperatures, this reaction cannot be completely separated from the subsequent elimination of CO2.
~ z_ E
A
An understanding of the behaviour of the initial copolymer in presence of water and the way in which sorption of water is altered by controlled degradation was a secondary objective of this study. Sorption of water was considered in two situations, firstly from the atmosphere and secondly from the liquid phase, The first stage of interaction of the copolymer with atmospheric water vapour is characterised by reversible sorption. After drying under reduced pressure the sample recovered its original weight and the chemical structure of the copolymer did not change. Longer exposure to water vapour, however, over a period of three days or more, led to irreversible sorption of water, but not to further increase in water content in the sample or to any detectable change in structure, In contrast, water sorption from the liquid phase led through the stage of irreversible
r~
f~
/
General conclusions on degradation behaviour
Water sorption by the MAn-VAc copolymer
231
'
3000
2
s'00 2 0'0 0 "-~'-'~
V-4 I
I
'00
15
'
1000
c~
.
f~,
/
I
3500 3000 2soo 2d00
I
,500
1o'o0
c ~
~
I 3500
/
I 3000
-,'/
I
2500
~
I 2000
/ //~J
I 1500
f,~f/
I tO00
W,v.o,mb.r. era-' Fig. 10. IR spectra of (a) MAn-VAc copolymer, (b) productafter exposure to H20(liq.) for 5 min, and (c) product after exposure for 15min. sorption to hydrolysis of the copolymer, followed by its complete dissolution after two hours of exposure in water. The chemical structure of the copolymer at the stage of irreversible binding was the same for sorption from the gas and liquid phases. The changes in IR spectrum following exposure to H20(liq.) are shown in Fig. 10. Taking these results into account, we suggest that water sorption by the MAn-VAc copolymer includes reversible sorption up to the equilibrium value, then irreversible binding of the sorbed water, rupture of chemical bonds in the copolymer with change in its structure (hydrolysis) and, finally, dissolution. The critical concentration for hydrolysis is only reached in sorption from the liquid phase. The general pattern of water sorption by partially degraded copolymer is the same as for the original material and the differences are in the depth of the structural changes following the point of equilibrium sorption. Even the residue obtained after degradation for 15 min is insoluble
232
I . C . McNeill, A. Ya. Polishchuk, G. E. Zaikov
in water although some hydrolysis was observed. Degradation for 30 min gives a product in which the stage of irreversible sorption is restricted and, after one hour of degradation, sorption of water was found to be reversible. The change of the hydrophilic character of the copolymer due to degradation to different extents may be applied in systems w h e r e variation of the
hydrophobic/hydrophilic
balance is required. O n e such case is for controlled drug release. The regulation of the mechanism and velocity of drug release using the various modifications of the initial copolymer is a possible method of
providing programmed drug delivery and the possible use of the MAn-VAc alternating copolymer in this way is the subject of further
research.
ACKNOWLEDGEMENT Support for this project from the Royal Society and the U S S R A c a d e m y of Sciences is acknowledged with thanks.
REFERENCES 1. Cowie, J. M. G., Alternating Copolymers. Plenum Press, New York, 1985. 2. Georgiev, G. S., Golubev, B. V. & Zubov, V. P., Polym. Sci. USSR, 20A (1978) 1814. 3. Caze, C. & Loucheux, C., J. Macromol. Sci.--Chem., A9 (1979) 29. 4. Arnaud, R., Caze, C. & Fossey, J., J. Macromol. Sci.--Chem., A14 (1980) 1269. 5. Fujimori, N. & Brown, A. S., Polym. Bull., 15 (1986) 223. 6. Matsui, S. & Aida, H., High Polym. Jpn., 26 (1969) 10. 7. Caze, C. & Loucheux, C., J. Macromol. Sci.--Chem., A15 (1981)95. 8. McNeill, I. C., J. Polym. Sci., Part A, 4 (1966) 2479. 9. McNeill, I. C., Ackerman, L., Gupta, S. N., Zulfiqar, M. & Zulfiqar, S., J. Polym. Sci., Polym. Chem. Ed., 15 (1977) 2381. 10. Nozakura, S. I., Morishima, Y. & Murakashi, S., J. Polym. Sci., Polym. Chem. Ed., 10 (1972) 2853. 11. McNeill, I. C., Eur. Polym. J., 6 (1970) 373. 12. Norris, A. C., Computational Chemistry. Wiley, New York, 1981. 13. Walling, C., Briggs, E. R., Wolfstirn, K. B. & Mayo, F. R., J. Am. Chem. Soc., 70 (1948) 1537. 14. Weiss, J., J. Chem. Soc., 1942, 245.