CHEMICAL-REACTIONS OCCURRING IN THE THERMAL-TREATMENT OF PC PMMA BLENDS/

The chemical reactions occurring in the thermal treatment of bisphenol -A polycarbonate( PC) and poly(methyl methacrylate) (PMMA) blends have been investigated by nuclear magnetic resonance (NMR), mass spectrome try (MS), size exclusion chromatography (SEC), and thermogravimetry (T G). Our results suggest that in the melt-mixing of PC/PMMA blends, at 230 degrees C, no exchange reactions occur and that only the depolymer ization reaction of PMMA has been observed. In the presence of an este r-exchange catalyst (SnOBu2), an exchange reaction was found to occur at 230 degrees C, but no trace of PC/PMMA graft copolymer has been obs erved. Instead, an exchange reaction between the monomer methyl methac rylate (MMA), generated in the unzipping of PMMA chains, and the carbo nate groups of PC has been suggested. This is due to the diffusion of MMA.at the interface or even into the PC domains, where it can react w ith PC producing law molar mass PC oligomers bearing methacrylate and methyl carbonate chain ends and leaving the undecomposed PMMA chains u naffected. The TG curves of PC/PMMA blends prepared by mechanical mixi ng and by casting from THF show two separated degradation steps corres ponding to that of homopolymers. This behavior is different from that of a transparent film of PC/PMMA blend, obtained by solvent casting fr om DCB/CHCl3, which shows a single degradation step indicating that th e degradation rate of PC is increased by the presence of PMMA in the b lend. The thermal degradation products obtained by DPMS of this blend consist of methyl methacrylate (MMA), cyclic carbonates arising from t he degradation of PMMA and PC, respectively, and a series of open chai n bisphenol-A carbonate oligomers with methacrylate and methyl carbona te terminal groups. The presence of the latter compounds suggests a th ermally activated exchange reaction occurring above 300 degrees C betw een MMA and PC. The presence of bisphenol-A carbonate oligomers bearin g methyl ether end groups, generated by a thermally activated decarbox ylation of the methyl carbonate end groups of PC, has also been observ ed among the pyrolysis products. (C) 1998 John Wiley & Sons, Inc.