Permeation of gases in polystyrene molded at elevated pressures

Atactic polystyrene was molded at pressures up to 3000 atmospheres, and the effects of this pressure history were studied by permeability measurements using helium, neon, and argon as probe molecules. The permeabilities were significantly reduced by molding at higher pressures. The diffusion coefficients decreased monotonically with molding pressure, reflecting more extensive interchain cohesion. In contrast, the solubility coefficients showed a broad minimum around a molding pressure of 1000 atm, indicating that changes in local order were induced by higher pressures. These changes were shown to affect permeation, mechanical characteristics, and other properties. An optimum molding pressure somewhat below 1000 atm is indicated for polystyrene.