Evaluation of free volume in styrene-butadiene block copolymers probed by positron annihilation lifetime technique

The effects of ratio of polystyrene molecular weight in styrene-butadiene block copolymers on average free volume size, its numerical concentration, and size distribution were studied by positron annihilation lifetime measurements. At room temperature, bigger free volume size was observed in the polybutadiene (PB) unit, whose glass transition temperature is below room temperature (similar to 200 K), while that of polystyrene (PS) is similar to 363 K. Free volume size of copolymers decreased linearly with the increase of molecular weight ratio of PS, showing a good additivity. The numerical concentration of free volume decreased with increase of PS to PB in SE copolymer ratio, showing a crook at 12% of PS ratio of molecular weight in SE. Good correlations are recognized between specific volume, viscosity and refractive index and apparent free volume fraction obtained by V(f)xI(3). The temperature dependences of size, size distribution, and numerical concentration of free volume reveal that there are several phase transitions at about 120 to 150 K other than glass transitions which are found at 180 to 200 K.