Annealing characteristics of ultraoriented high-density polyethylene

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Abstract
The annealing characteristics of ultraoriented high‐density polyethylene fibers have been studied using differential scanning calorimetry, thermomechanical analysis, and tensile testing. The melting point of unannealed fibers was estimated to be 130 °C using a 10 °C min −1 heating rate. Fiber annealing between 126 and 132 °C split the melting endotherm into two peak melting at 132 and 139 °C. The weight fraction of fiber which had reorganized to the lower melting point increased with annealing time, finally attaining a constant value. However, annealing of laterally constrained fibers below 136 °C did not result in structural changes as detected by calorimetry. Melting‐point variations with heating rates≳0.625 °C min−1 indicated the equilibrium melting point of the unconstrained fiber at zero heating rate as 138 °C, while a similarly extrapolated value using heating rates <0.625 °C was ∼132 °C. The linear expansion coefficients rapidly increased near 132 °C and were attributed to the structural reorganization detected by calorimetry. Fiber annealing markedly decreased the tensile modulus from 50 GPa towards the 1 GPa observed for the unoriented polyethylene. The annealing behavior was consistent with the thermal instability associated with the anisotropy of the surface free energies of the ultraoriented crystalline fibers as well as sources of melting‐point reduction such as defects.