Microscopically viewed structural change of PE during the isothermal crystallization from the melt: II. Conformational ordering and lamellar formation mechanism derived from the coupled interpretation of time-resolved SAXS and FTIR data

Abstract

Time-resolved small-angle X-ray scattering (SAXS) measurement was carried out to trace the structural change of polyethylene (PE) during the isothermal crystallization from the melt. All kinds of PE samples of high-density PE, linear low-density PE, and deuterated high-density PE were found to show essentially the same SAXS pattern changes, although the crystallization rates were different among them. The thus obtained SAXS data were combined with the previously reported FTIR data [Tashiro K., Sasaki S., Kobayashi M., Polym J, 1998;30:485] and could be interpreted quantitatively by dividing into the following three time regions: (1) immediately after the sample was cooled to the crystallization temperature, the density fluctuation in the molten state increased, resulting in the generation of the conformationally disordered short trans segments. (2) The disordered trans segments experienced the conformational ordering to the orthorhombic-type trans-zigzag form. These regular chain segments were aggregated to form a crystalline lamella of ca. 50Å thickness. This separation of the system into the high (lamella) and low density (amorphous) regions occurred with ca. 800Å period. (3) These isolated lamellae were stacked more densely by generating new lamellae in between the already existing lamellar layers, creating a structure of ca. 400Å period.