Shrinkage and retractive forces during the annealing of drawn low‐density polyethylene

Abstract

Abstract Shrinkage of unconstrained low‐density polyethylene samples and the retractive stress of samples with ends fixed have been investigated as a function of the annealing time t A and temperature T A on material drawn at room temperature to draw ratios λ between 4 and 6. The shrinkage increases with t A and T A . The retractive stress on a sample annealed with ends fixed goes through a maximum as the sample is annealed and then drops to a limiting value which increases with T A as long as T A is at or below 80°C and rapidly decreases with higher T A . The drop from the maximum to the limiting retractive stress, slow at lower T A and rapid at higher T A , seems to be a consequence of rapid pulling of chain segments out of crystal block in which interfibrillar tie molecules are anchored. This process is facilitated by the high T A , which softens the crystal matrix. At constant end‐to‐end distance, the contour length of the tie molecules is irreversibly increased, and this causes a reduction in the contribution of the affected tie molecules to the overall retraction stress. Hence one finds a substantially higher retraction stress during first heating than during subsequent cooling and heating of the drawn sample.