A. Maus, E. Hempel, T. Thurn-Albrecht, K. Saalwächter.
Memory effect in isothermal crystallization of syndiotactic
polypropylene — Role of melt structure and dynamics?
Eur. Phys. J. E 23, 91-101 (2007).
The crystalline-memory effect on the crystallization of syndiotactic
polypropylene is investigated by differential scanning calorimetry and
solid-state NMR spectroscopy. The influence of several parameters in the thermal
(pre-)treatment and the crystallization conditions is studied in detail. In
agreement with previous reports, the power law behavior of the overall crystal
growth rate is found to be remarkably different for melts with and without
memory. This has previously been interpreted in terms of changes in the
structure and/or the dynamics of the melt (disentangled state, local order), and
a variety of NMR experiments is used to detect such potential changes. All our
NMR results are identical for melts with and without memory, therefore excluding
any large effect of the “memory” on melt structure or dynamics exceeding the
percent level of the whole sample volume, and thus supporting more conventional
interpretations in terms of persisting nuclei. Samples that were
pre-crystallized at lower temperatures exhibit a larger memory effect, and the
potential nuclei fraction is a non-equilibrium structure and is restricted to
the 0.1% level if it is crystalline or highly ordered.