Abstract
Much interest has been shown recently in using electron-beam (e-beam) processing to cure polymers and composites. Attempts to cure phenylethynyl end-groups with e-beams are reported. The synthesis of a novel phenylethynyl-terminated polydimethylsiloxane (PET-PDMS) by reaction of 4-phenylethynylphthalic anhydride with an amine-terminated siloxane is described and the resin shown to be susceptible to e-beam cure, in contrast to the commercial phenylethynyl-terminated resin, PETI-5, which is not. Crosslinking via the ethynyl bond and scission of the siloxane chain is observed. PET-PDMS cures at 325°C and Raman spectroscopy indicates that the thermal and e-beam cure mechanisms differ. PET-PDMS demonstrates some success as a reactive diluent in the e-beam cure of PETI-5. To understand the e-beam cure mechanism of the phenylethynyl end-group, blends of PETI-5 with an amine-terminated siloxane, and with a siloxane with an unreactive end-group are studied. Whilst the latter does not initiate curing reactions in PETI-5, the former results in total ethynyl loss after a two-hour exposure. The amine is thought to accelerate radical reactions during irradiation. Irradiations involving the unreactive siloxane, simple amines and PETI-5 do not result in ethynyl loss, possibly due to poor siloxane solubility. Attempts to repeat these experiments on an industrial-scale are unsuccessful. The products of irradiation differ from those obtained on the laboratory-scale apparatus, revealing the importance of irradiation conditions on the e-beam cure mechanism of phenylethynyl-terminated resins. A maleimide-terminated polydimethylsiloxane (MT-PDMS) is synthesised from the reaction of maleic anhydride and an amine-terminated siloxane and appears to undergo structural changes on e-beam irradiation. Although Raman spectroscopy of the cured material is inconclusive, multivariate analysis of the data indicates crosslinking and siloxane cleavage are occurring. The thermal and e-beam cure mechanisms of MT-PDMS appear to differ. MT-PDMS causes changes in the e-beam behaviour of a commercial bismaleimide, though the nature of this effect is uncertain.