Characterisation of IPNs of thermoplastic elastomers

Tough but strong composites can form using thermoplastic elastomers as starting materials by controlling nanometer-scale texture. Block copolymers based on butadiene or isoprene and styrene are widely used industrially. They can, when blended, toughen polystyrene but the formation of interpenetrating polymer networks (IPNs) is another potentially better way to enable compatibilization. IPNs combine two or more polymers in network form with one network polymerised and/or crosslinked in the immediate presence of the other. Materials can be formed with controlled morphologies, by for example growing the dispersed phase. Jones et al produced polystyrene/poly(styrene-butadiene-styrene) IPNs using thermal methods. Phase separation is confined to domains ∼ 20-40 nm and materials can have good mechanical and impact properties. Some samples however embrittled due to thermal degradation of the butadiene regions in the SBS material. It is possible to form similar IPNs of PS/SBS using gamma irradiation, which provides a room-temperature alternative to traditional thermal methods. However the previous work was confined to an oil-extended grade of SBS where the influence of the oil extension was not known. Here two main types of IPNs of block copolymers (thermoplastic elastomers, TPEs) and PS were prepared using sequential polymerisation. In one type, the TPE was thermally crosslinked, and styrene was polymerised and crosslinked using gamma irradiation to produce tough transparent IPNs which show no evidence of thermal degradation. A second group was prepared thermally, but with varying ratios of PS and TPE. These are of interest as the ratio affects strength and toughness. In this presentation we emphasise morphological aspects at the nm level using transmission electron microscopy and small angle neutron scattering