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

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.