Melting behavior and morphology of freeze-dried ultra-high molar mass polyethylene

摘要

Freeze-dried ultra-high molar mass polyethylene (UHPE) was prepared from dilute solutions, and the resulting samples are a mixture of single-, few-, and multichain particles, each containing several or a number of small crystals. Thermal analysis of the freeze-dried samples by DSC indicated that the melting point decreased as the solution concentration decreased. The depression in melting point is most likely due to the decreasing crystal size. The effect of annealing the freeze-dried samples and the samples obtained after melting and recrystallization of the nascent UHPE at 122degreesC are discussed. Wide-angle x-ray diffraction (WAXD) measurements of average crystal dimensions normal to (110) and (210) and crystallinity were calculated for the freezedried samples. The crystallinity measured by WAXD is in agreement with that by DSC. By means of polarized optical microscopy (POM), -morphology of the freeze-dried samples was studied. The melt of the freeze-dried samples became optically isotropic after melting at a temperature beyond the equilibrium melting point (T-m(o)) of polyethylene, while the nascent UHPE retained birefringence even if the melting temperature exceeded 180degreesC. It is assumed that the freeze-dried samples have a structure of folded-chain lamellae, which melt completely at the temperature above T-m(o). However, the nascent UHPE was proposed to be extended-chain crystals, and the parallel chain conformation remains in the UHPE samples even if the melting temperature is beyond T-m(o), therefore, bireftingence can still be observed under POM at higher temperature. [References: 26]