Thermal degradation of poly(tertbutyl aziridine)-poly(methyl methacrylate) graft copolymer

Polymer

Degradation

and Stability 0

Pll:

ELSEVIER

SOl41-3910(96)00114-O

55 (1997)

275-279

1997 Elsevier Science Limited

Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00

Thermal degradation of poly(tertbuty1 aziridine)-poly(methy1 methacrylate) graft copolymer Shagufta Zulfiqar,“* M. Zafar-uz-Zaman,” Arshad Munir”t & I. C. McNeill’ bPolymer Research,

“Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan Chemistry Department, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ,

(Received

UK

28 March 1996; accepted 10 April 1996)

The degradation behaviour of a high molecular weight PMMA sample in which approximately 2% of the MMA units have been modified and grafted with PTBA chains of molecular weight 5000, to give a product containing about 50% by weight of PTBA, has been investigated. The techniques used were the same as in recently reported investigations of PTBA, PTBA-PMMA block copolymer and poly(N-methyl-N-tertbutyl-aminoethyl methacrylate) (PMTBAEM), and included TG, DTA and thermal volatilisation analysis (TVA). Products from the TVA experiments were separated and characterised. The graft copolymer, is less stable than PMMA and comparable in stability to PTBA. It appears that the grafted side chains degrade primarily to PTBA-related oligomeric material, accompanied by a small proportion of volatile products also derived from the PTBA chains, whereas the main backbone gives predominantly MMA monomer. 0 1997 Elsevier Science Limited. All rights reserved.

1 INTRODUCTION

has a single tertiary amine structure in each side group. The present communication continues this series of studies by an examination of a PTBA-PMMA graft copolymer. Cationic ring-opening polymerisation of TBA has living character because the self-termination of the propagating species is very slow compared with propagation, so that after all the monomer present is used up, there is a short period of time during which the concentration of active species is still more than 95% of that initially formed. In the present investigation, a graft copolymer of PTBA and PMMA was synthesised by a reported procedure4 which involved initial generation of alkoxide sites on the PMMA and then utilisation of these sites to terminate ‘living’ PTBA segments and in so doing form a graft. Because the preparative route involves the intermediate formation of some methacrylate salt groups in the PMMA, and subsequent reaction of these is not complete, the graft copolymer also has a small proportion of these salt groups. Its

The thermal degradation mechanisms of block and graft copolymers have received much less attention than those of random copolymers. The ‘living polymer’ character of polymers made from N-tertbutyl aziridine (TBA) has opened up a new route to the preparation of block and graft copolymers. The thermal degradation behaviour of the homopolymer poly(tertbuty1 aziridine) (PTBA) has been investigated,’ and a study of the block copolymer of TBA with methyl methacrylate (MMA) has also recently been reported.* A structurally related material is poly(ZV-methyl-N-tertbutyl-aminoethyl methacrylate) (PMTBAEM) and the behaviour of this polymer has also been investigated.3 These materials are all of interest in having tertiary amine structures. PTBA and the block copolymer have nitrogen in the TBA chain structure, whereas PMTBAEM * To whom correspondence should be addressed. tPresent address: Pakistan Atomic Energy Commission, Islamabad, Pakistan. 275

276

S. Zuljiqar

et al.

Scheme 1.

structure may be represented as shown in Scheme 1, in which y and z are each about 2-3% ofx.

2 EXPERIMENTAL 2.1 Polymer preparation 2.1 .l Modified PMMA precursor Reagents and solvents were purified before use by standard procedures. High molecular weight PMMA used as a starting material was acquired from BDH Ltd and used as supplied. Modification of this polymer was achieved by partial hydrolysis of the ester groups. A heterogenous alkaline hydrolysis was carried out’ using 5 g of PMMA and 50 ml of a 0.5 M NaOH solution in acetone: the reaction mixture was stirred for 3 h at 25”C, after which the polymer was separated and the acetone-soluble portion was added to methanol. PMMA is insoluble in acetone, but as the percentage of modified ester groups increases, the solubility of the product in acetone increases, so enabling separation of the modified polymer from the PMMA. The product was dried and stored in a desiccator. 2.1.2 Graft copolymer formation A 10 g sample of PTBA of molecular weight 5000 was synthesised by a reported procedure6 using THF as solvent and methyl triflate as initiator. To 5 g of the modified PMMA, this solution, previously dissolved in THF, was added. The solution was stirred for 2 h after which it was added to methanol. The precipitate formed contained both unreacted PTBA (used in excess of the stoichiometric ratio) and the graft copolymer. Separation of these made use of the

solubility difference in benzene: PTBA is insoluble, but the graft copolymer is soluble. The PTBA was separated and the graft copolymer was precipitated from the benzene solution using methanol, filtered and dried. The PTBA was also dried and then weighed in order to calculate the amount used in the process. It was concluded that about 2% of the original ester groups had been grafted by the procedure used. This conclusion was supported by GPC and atomic absorption spectroscopy measurements. If two segments of PTBA graft (each with on average 50 TBA units) are present for every 100 MMA units in the PMMA backbone, then the percentage by weight of PTBA in the graft copolymer will be about 51%. 2.2 Degradation studies The same techniques were used as employed in the previous studies on PTB,’ PTBA-PMMA block copolymer’ and PMTBAEM.3 Essentially these involved TG and DTA in dynamic nitrogen and thermal yolatilisation analysis (TVA) under continuous evacuation. The products of degradation by TVA were separated first into noncondensable gases, condensable gases and volatile liquids, cold ring fraction (tar/wax fraction materials volatile under vacuum at degradation temperature but not at ambient temperature) and involatile residue. Non-condensable gases were identified using an on-line Leda-Mass Multiquad quadrupole mass spectrometer. The condensable gas/liquid fraction was first separated into several fractions on the basis of volatility by subambient TVA (SATVA): the more volatile components were characterised by mass spectrometry and IR spectroscopy and the less volatile liquid components were further examined by GC-MS, The cold ring fraction products were examined by IR spectroscopy.

Thermal degradation of PTBA-PMMA

copolymer

277

3 RESULTS AND DISCUSSION 3.1 Thermal analysis experiments The TG curve for PTBA-g-PMMA is shown in Fig. 1. The copolymer is thermally stable up to about 225°C and then exhibits a two-stage degradation. In the first stage, up to 370°C 40% weight loss is observed; after the subsequent higher temperature stage, a residue of 7-8% remains at 550°C. The DTA curve for the copolymer is also shown in Fig. 1. There is an endotherm at approximately 150°C which is probably associated with softening of the polymer. Two endotherms at higher temperatures correlate with the two stages of decomposition on the TG curve. The TVA curve for the graft copolymer is reproduced in Fig. 2. The small initial amount of volatilisation from about 150°C can be attributed to some trapped volatile material released as the copolymer softens. Onset of degradation under continuous evacuation occurs at about 21O”C, with maxima in the TVA curve appearing at 250, 330 and 450°C. It is clear from the TVA traces that a range of products of different volatilities is present, including a small amount of noncondensable gases at the final degradation stage. Using on-line mass spectrometry during the TVA experiment, it was established that a small amount of carbon monoxide and traces of methane were formed. The amount of cold ring fraction produced from the copolymer was about half of that from PTBA homopolymer. Since PMMA does not normally give rise to cold ring fraction products,

L

I

I

100

200

I

I

300

400

500

Temperature, ‘C

nitrogen.

Heating rate: lO’C/min.

200

300

in dynamic

400

500

Temperature, “C

Fig. 2. TVA curve for degradation of PMMA-g-PTBA, under continuous evacuation at lO”C/min. Key to TVA” 00, . . . -45”,___-75”,----loo”C,_.traces: - 196°C.

this tends to suggest that on degradation the major part of the grafted PTBA chains must appear as cold ring fraction. There was also a large liquid fraction, which would therefore be expected to contain predominantly MMA from the PMMA chains. The residue of degradation amounted to about 7% of the original sample weight. 3.2 Characterisation of degradation products The SATVA curve for separation by controlled warm up of the condensable volatile product from degradation of the copolymer under TVA conditions is shown in Fig. 3. It indicates three main product fractions. The more volatile components were characterised by IR spectroscopy and mass spectrometry. At the first SATVA peak, carbon dioxide was present, whilst the second peak was due mainly to isobutene. Methyl methacrylate began to appear from about 16min and was the main product at the third peak, which comprised the liquid fraction of volatile products. A small amount of methanol was also present. The latter

16 24 Time, min

I

Fng. 1. TG and DTA curves for PMMA-g-PTBA

1M)

32

40

Fig. 3. Subambient TVA12 curve for separation by controlled warm up from -196°C of the condensable volatile products of degradation to 600°C of PMMA-gPTBA.

S. Zulfiqar et al.

278

part of this fraction was further examined by GC/MS. It was found that MMA was the predominant ( > 90%) component, but small amounts of several other products were present, all of which could be related in structure to the PTBA grafts. The most important of these was ltertbutyl-4-isopropenyl-l,2,3,4-tetrahydro-l,4-diazine (I). 1-Tertbutyl aziridine (II) was also present. Characterisation of the remaining products was difficult, but it appeared likely that the amine products (III) and (IV) were significant, together with a further, structurallyrelated material which could not be clearly identified (see Scheme 2). The cold ring fraction was examined by IR spectroscopy. The spectrum suggested that it consisted primarily of TBA-related oligomers, but the presence of some anhydride and ester absorptions indicated the presence of a small contribution of modified MMA oligomeric material. The residue of degradation was found to be insoluble in common solvents.

4 DISCUSSION Pure samples of PMMA degrade to give almost gives 100% monomer;7~s PTBA homopolymer more than 60% of cold ring fraction products, plus several cyclic (perhydrodiazine) materials together with acyclic aliphatic amines. The more volatile products from PTBA consist of isobutene together with some isobutane and aziridine.’ In the study of a PMMA-PTBA block copolymer the products were much as expected if each block were degrading as in the corresponding homopolymer, but some carbonyl-containing groups in

the cold ring fraction pointed to backbone scissions to give oligomers in which a few MMA or MMA-related structures were present. The latter result probably arises from the fact that the PTBA chains are less stable and are undergoing scissions before the PMMA sequences begin to depolymerise to monomer. In view of the above findings, it is not surprising that the copolymer of PMMA grafted with PTBA shows some similarities in behaviour to the block system. TBA-related oligomers predominate in the cold ring fraction and MMA monomer in the volatile products. Since the preparative route leaves about 2% of methacrylate salt units in the chain, however, an additional process is possible involving reaction between MMA units and the occasional salt groups: such ester/salt copolymers have been found to give inter-unit reaction leading to methanol and anhydride rings,’ both of which are confirmed as minor products in the graft system. The anhydride rings interfere with PMMA chain unzipping,‘0 and thus some ester and anhydride groups are found in the cold ring fraction, although it remains true that most of the original MMA units degrade to monomer. The anhydride rings are also the main source of the small amounts of CO, and CO formed during degradation. The nitrogen-containing volatile products can all be envisaged as resulting from scissions and in some cases rearrangements in the PTBA grafts. The most important of these is a perhydrodiazine molecule (also a significant, minor product from degradation of PTBA homopolymer), which is of approximately TBA dimer size. Although PMTBAEM has a structural relationship with PTBA and the copolymers, the CH3

CH3

CHr C-CH3

CH2-

I

\

/N\ mH2 CH2

CH2

CH3-

N I

CH2

CH3

/ CH2 I

:: -cH3 CH3

‘N’

CH3-

I

F\\

CH3

CH2

CH3

\N’

7 -cH3 CH3

(III)

w

Scheme 2.

CH3- C -CH3 I NH c/H2 AH2 ‘NH I

F\\

CH3

CH2

Thermal degradation of PTBA-PMMA

influence of the amine groups is necessarily reduced because these are not in the backbone structure and also degradation to monomer does not occur to any significant extent with this complex methacrylate polymer. The different degradation pathways involved have been discussed.3

REFERENCES 1. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeil], I. C., Polym. Degrad. Stab., 47 (1995) 59. 2. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeill, I. C., Polym. Degrad. Stab., 50 (1995) 33. 3. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeill,

copolymer

279

I.C., Polym. Degrad. Stab., in press.

4.Munir, A. & Goethals, E. J. .I., Polym. Sci. Polym. Chem. Ed., 19 (1981) 1985. 5.Talu, M. & Ozgun, H. B., Eur. Polym. J., 26 (1990) 5. 6.Zulfiqar, M., Zulfiqar, S., Zafar-uz-Zaman, M. & Munir, presented at 5th National Chemistry Islamabad, Pakistan, October 1993. 7.Gassie, N. & Melville, H. W., Proc. R. Sot. (Lond.), A199 (1949) 1. 8.McNeil& I. C., Eur. Polym. J., 4 (1968) 21. 9.McNeill, I. C., in Developments in Polymer Degradation, Vol. 7, ed. N. Grassie. Elsevier Applied Science, London, 1987, p.1. 10. McNeill, I. C., in Comprehensive Polymer Science, Vol. 6, ed. G. Eastmond, T. Ledwith, S. Russo and P. Sigwalt. Pergamon Press, London, 1989, p. 451. 11. McNeill, I. C., Eur. Polym. J., 6 (1970) 373. 12. McNeill, I.C., Ackerman, L., Gupta, S.N., Zulfiqar, M. & Zulfiqar, S., J. Polym. Sci., 15 (1977) 2381. A.,

Paper

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