Mechanism for thermal shrinkage of oriented polystyrene monofilaments

Thermal shrinkage behaviour of oriented atactic polystyrene monofilaments, which show a brittle-to-ductile transition in the vicinity of Δn = ?2 × 10^?3 (at a temperature of 20°C and a stretching rate of 100%/min), is investigated by thermomechanical analysis (t.m.a.). The maximum contraction ratio $\text{ΔL}{}_1\text{L}{}_0$ at a constant rate of heating decreases linearly with increasing external tensile stress, and the limiting contraction ratio (at zero external load) has a broad maximum near the transition point. The time dependence of the isothermal contraction ratio $\text{[ΔL(∞) ? ΔL(t)]}\text{ΔL(∞)}$ is represented by an exponential function and an Arrhenius type temperature dependence is realized with an activation energy of several tens of kcal/mol. The degree of birefringence after isothermal shrinkage (Δn(∞)) increases linearly with an increase in external tensile load. Degrees of birefringence before isothermal shrinkage (Δn(0)) and Δn(∞) are related to each other almost linearly. The two-state model is effective in understanding these thermal shrinkage behaviours.