SYNOPSIS Mixtures of maleimidomethylated polystyrene with difurfuryl adipate or with furfuryl alcohol-esterified poly [ styrene-co-(maleic anhydride)] underwent thermally reversible Diels-Alder crosslinking. Reversal of crosslinking occurred rapidly at 150°C. The instability of the furfuryl group, however, limits the practicability of the reaction. 0 1992 John Wiley & Sons, Inc.
ABSTRACT:The kinetic mechanism of the thermal cure of a phenylethynyl-terminated imide model compound, 3,4-bis[(4-phenylethynyl)phthalimido]diphenyl ether (PEPA-3,4-ODA) and a phenylethynyl-terminated imide oligomer PETI-5 (MW 5000 g/mol) was studied. FTIR was used to follow the cure of the model compound, while thermal analyses (DSC) was used to follow the cure of the PETI-5 oligomer. The changes in IR absorbance of phenylethynyl triple bonds at 2214 cm 01 of PEPA-3,4-ODA as a function of cure time were detected at 318, 336, 355, and 373ЊC, respectively. The changes in the glass transition temperature, T g , of PETI-5 as a function of time were measured at 350, 360, 370, 380, and 390ЊC, respectively. The DiBenedetto equation was applied to define the relative extent of cure, x, of the PETI-5 oligomer by T g . For the model compound, the reaction followed first order kinetics, yielding an activation energy of 40.7 kcal/mol as determined by infrared spectroscopy. For PETI-5, the reaction followed 1.5th order, yielding an activation energy of 33.8 kcal/mol for the whole cure reaction, as determined by T g using the DiBenedetto method. However, the cure process of PETI-5 just below 90% by this method followed firstorder kinetics yielding an activation energy of 37.2 kcal/mol. ᭧
The cure reactions of phenylethynyl end-capped polyimides were investigated using solid-state 13C magic-angle spinning (MAS) nuclear magnetic resonance (NMR). A 13C-labeled model compound
(13C-PEPA-3,4‘-ODA) and an imide oligomer (13C-PETI-5) were synthesized and characterized. The thermal
cure process for 13C-PEPA-3,4‘-ODA was followed over the temperature range 318−380 °C and for13C-PETI-5 over the temperature range from 350 to 400 °C. Our NMR results showed that, for the model
compound, as curing proceeded, the percentage of polymeric structures containing double-bonded and
single-bonded carbon increased while the percentage of triple-bonded carbon gradually decreased and
finally disappeared at the elevated temperatures. The PETI-5 cure process was very similar to the PEPA-3,4‘-ODA cure process, and the percentage of double-bonded carbon structure of PETI-5 increased during
the cure process as the percentage of triple-bonded carbon decreased. Moreover, for the PETI-5 resin
system, a weak broad 13C signal due to a single-bonded structure was observed after cure. The carbonyl
groups remained relatively constant during the curing reactions for both the model compound and PETI-5
resin. The appearance of single-bonded structures in the cure of the model compound and PETI-5 can be
derived from polyene structures by a further intra- or intermolecular Diels−Alder reaction to form
cycloolefinic ring or branched structures. On the basis of the chemical shift data of several low molecular
weight compounds with aromatic ring structures and polyene structures, we cannot exclude the formation
of substituted aromatic ring structures from PEPA-3,4‘-ODA or from PETI-5.
Maleimide groups were incorporated into polystyrene by a mild Friedel–Crafts procedure using N‐chloromethylmaleimide. Films and solutions of the modified polymer underwent photocrosslinking at a rate that was not appreciably affected by addition of photosensitizer. Free radical, thermal, and Diels–Alder crosslinking were also investigated.
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