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.