Molecular relaxation study of polystyrene: influence of temperature, draw rate and molecular weight

Molecular relaxation curves of five different polystyrene samples, four monodisperse of weight-average molecular weight ranging from 210 000 to 2 340 000, and one polydisperse, have been studied using birefringence and polarization modulation infrared linear dichroism, during and after a step-strain uniaxial deformation at temperatures between Tg and Tg+60°C. Relaxation measurements can be fitted with a set of exponential decay functions, thus defining three different relaxation times. The relaxation times thus obtained with birefringence and polarization modulation experiments are similar, and decrease with an increase in temperature. The first relaxation time (τ1), which is of the order of seconds, is independent of average molecular weight (Mw), while the second (τ2) and third (τ3) relaxation times increase with molecular weight. For example, at Tg+20°C, and for a PS molecular weight of 2 340 000, values of 0.7, 39 and 16 500 s were determined for τ1,τ2 and τ3, respectively. The power law dependence found for the third relaxation time scales as Mw1.6 at each temperature. This behavior is consistent with the prediction of the theoretical model of Doi–Edwards and allows the assignment of τ1 to the first, and τ3 to the second relaxation time of Doi–Edwards (τ2 is intermediate between those two). The decrease of the relaxation time ratio (τ3/τ1) with temperature, noted for all the PS molecular weights investigated, suggests a decrease in the number of entanglement points with an increase in temperature above Tg+20°C. Finally, for a given temperature, a linear dependence is observed between τ3 and τ1 in agreement with the τ3=2τ1N1.6 Doi–Edwards prediction, where N is the number of entanglements.