A law controlling polymer recrystallization showing up in experiments on s-polypropylene

Abstract

When polymers are crystallized at large supercoolings a subsequent heating is accompanied by recrystallization processes, which proceed to a fixed final melting point. We studied these processes with small angle X-ray scattering and DSC measurements for three s-polypropylenes with different stereoregularity and co-unit content. As is known from previous experiments, crystal thicknesses d_c depend on the crystallization temperature T_c only, being not affected by stereo defects or co-units. The new experiments, all carried out for low crystallization temperatures, again confirm this property; in plots of d_c^-1 versus T_c all points are located on a unique straight crystallization line. A similarly simple law controls the crystal thickness during the continuous structure reorganization on heating. Over an extended temperature range d_c^-1 changes linearly with temperature, guided by a unique, i.e. sample-invariant, 'recrystallization line'. Crystallization and recrystallization line extrapolate for d_c^-1  → 0 to the same limiting temperature, T_c^∞, and differ only in slope. The findings indicate that both the initial crystallization at T_c and the process of recrystallization use a pathway via a transient mesomorphic phase. The DSC thermograms of the samples show a multiple peak structure which varies with the heating rate. The SAXS results enable the peaks to be assigned to different melting processes.