A law controlling polymer recrystallization showing up in experiments on s-polypropylene
Polymer, submitted (2007)
Barbara Heck1, Silvia Siegenführ1, Gert Strobl1 and Ralf Thomann2
1Physikalisches Institut, Albert-Ludwigs-Universität, 79104 Freiburg, Germany
2Institut für Makromolekulare Chemie, Albert-Ludwigs-Universität, 79104 Freiburg, Germany
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 dc depend on the crystallization temperature Tc 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 dc-1 versus Tc 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 dc-1 changes linearly with temperature, guided by a unique, i.e. sample-invariant, 'recrystallization line'. Crystallization and recrystallization line extrapolate for dc-1 → 0 to the same limiting temperature, Tc∞, and differ only in slope. The findings indicate that both the initial crystallization at Tc 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.