| Abstract: | Atactic polystyrene was subjected to an elevated pressure–temperature cycle with the resulting densification, mechanical properties, and thermal scanning behavior observed. Most densifications were carried out with the PST as a viscous liquid. In this manner, ambient residual compactions greater than 2% were produced. Pressures up to 90,000 psi and temperatures to 320°C were employed. The technique used for vitrification from the high pressure–temperature region was found to drastically affect the mechanical behavior. If the polystyrene was vitrified from the treatment region by lowering the temperature, the material exhibited enhanced yield strength, by up to 40%. If the polystyrene was quenched by raising the pressure, the samples exhibited much lower mechanical strength. While the mechanical behavior of temperature-vitrified samples is enhanced compared to the pressure-vitrified materials, their densities are comparable. The compaction achieved is primarily determined by the pressure applied as the polymer vitrifies. Thermal scanning behavior of the pressure-vitrified materials show endothermic and exothermic responses below Tg, while the temperature-vitrified materials do not. Annealing the compacted polystyrene at room temperature caused little change in density. However, at temperatures above 60°C, the density relaxed rapidly. Samples which had been temperature vitrified and annealed such that the compaction completely relaxed, still maintained the enhanced mechanical properties of the densified materials. |
