Shape memory and stress relaxation behaviour of oriented mono-dispersed polystyrene.

The shape memory and stress relaxation behaviour of oriented samples of mono-dispersed polystyrene have been studied. It was found that the transient stress dip of Fotheringham and Cherry could successfully predict the recovery providing that the initial deformation which produces the molecular orientation is undertaken at a sufficiently fast strain rate. This means that if recovery is to be predicted, relaxation of the molecular orientation must not occur during the drawing process. This is attributed to the low fraction of the viscosity term of the total stress for polystyrene. This makes it advantageous to maximise the total stored stress, allowing the smaller component to be more accurately determined. It is proposed that the critical strain rate for maximising the stored stress is related to the smallest relaxation time in the melt, the entanglement time, τ. The stress relaxation results showed two relaxation processes when drawn at a high strain rate (faster than the inverse of τ), but only one when relaxation could occur during drawing. No differences were observed due to molecular weight, indicating that the molecular reorganisation processes affecting shape memory recovery and stress relaxation are local processes and so independent of chain length.