The influence of short-chain branching on the morphology and structure of polyethylene single crystals

Abstract

The influence of short-chain branching on the formation of single crystals at constant supercooling is systematically studied in a series of metallocene catalyzed high-molecular-weight polyethylene samples. A strong effect of short-chain branching on the morphology and structure of single crystals is reported. An increase of the axial ratio with short-chain branching content, together with a characteristic curvature of the (110) crystal faces are observed. To the best of our knowledge, this is the first time that this observation is reported in high-molecular-weight polyethylene. The curvature can be explained by a continuous increase in the step initiationstep propagation rates ratio with short-chain branching, that is, nucleation events are favored against stem propagation by the presence of chain defects. Micro-diffraction and WAXS results clearly indicate that all samples crystallize in the orthorhombic form. An increase of the unit cell parameter a(0) is detected, an effect that is more pronounced than in the case of single crystals with ethyl and propyl branches. The changes observed are compatible with an expanded lattice due to the presence of branches at the surface folding. A decrease in crystal thickness with branching content is observed as determined from shadow measurements by TEM. The results are in agreement with additional SAXS results performed in single crystal mats and with indirect calorimetry measurements. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1751-1762