Variation of free volume size, size distribution, and numerical concentration in shape memory polymer - Polystyrene polybutadiene copolymer - with temperature, studied by positron annihilation lifetime technique

Positron annihilation lifetime measurement was applied to the study of temperature dependencies of free volume parameters in polystyrene-polybutadiene copolymer, specially fabricated as a shape memory polymer. The size variation of free volumes in amorphous region was recognized at similar to 200 K (glass transition temperature, T-g,, of amorphous polybutadiene) and similar to 360 K (T-g, of amorphous polystyrene), whereas the transitions were observed at similar to 200 K, and 323 K (melting point of crystalline polybutadiene) for shape memory polymer. The shape memory mechanism of polystyrene-polybutadiene copolymer is discussed from a nanoscopic viewpoint through the behavior of size and numerical concentration of free volume holes. The anomalous thermal behavior of numerical concentration of free volume is concluded to be an indication of the shape memory effect. The configurational recovery of the nanoscopic regulation around crystalline polybutadiene bundles functioning as a hard segment proceeds so as to restore the minglement of soft polystyrene segment as the initial state.