Mechanical and relaxation properties of polystyrene molded at high pressures



Atactic polystyrene glasses were molded at high pressures by cooling the polymer melt at pressures up to 5000 atm. Changes found in the properties of the samples included: a lower mechanical damping factor at temperatures from 50°K to 300°K; a higher compressive elastic modulus; a maximum in compressive yield stress at a molding pressure around 1000 atm; faster relaxation at yield; appearance of an exothermic dilational relaxation below Tg when the samples were heated; and faster relaxation in methyl ethyl ketone vapor. The observed behavior is interpreted in terms of a model based on pressure-induced heterogeneities in local order. The initial effect of increased molding pressure is to produce a more compact polymer which has less segmental mobility and more extensive interchain forces. Above molding pressures of about 1000 atm, a second effect develops due to the apparent formation of localized conformations which are stable under the high-pressure vitrification conditions, but not at room conditions. The spontaneous decay of these high-energy regions leads to subsequent formation of microvoid defects and more rapid structural relaxation effects. The existence of an optimum molding pressure somewhat below 1000 atm is indicated for polystyrene.