Surface melting and crystallization of polyethylene

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Abstract
Extended chain polyethylene crystals yield a striated surface on fracturing. By statistical analysis it is shown that the striations are random and that there is no present compelling reason for assumption of interior grain boundaries. The rough surface starts melting at 100[ddot]C or lower, 40[ddot] below the equilibrium melting point. The interior is unaffected by this annealing process. Measurements on micron-thin slices of extended chain crystals were carried out and showed less superheating before melting than in large crystals. Also, a decreased heat of fusion in the high temperature region was found. The surface reorganization process is shown to be a two-step process: melting followed by recrystallization into folded chain lamellae. The epitaxial lamellae thus grown are analyzed with respect to frequency of occurrence and fold length. Their growth features are discussed. From the instability of the rough surface, it is concluded that under normal crystallization conditions extended chain nuclei would be unstable. Time-dependent calorimetry and electron microscopy were used as tools in this research.

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