Evidence of pre-crystalline-order in super-cooled polymer melts revealed from simultaneous dielectric spectroscopy and SAXS

The existence and the formation of pre-ordered structures in super-cooled polymer melts as the initial step during the complex process of polymer crystallization are discussed. Most findings and interpretations are based on scattering and microscopic experiments despite the small contrast between the phases. Such experiments test small differences in morphology between the assumed precursors of the crystals and the surrounding melt. In contrast, relaxation experiments are probing the motions and related dynamics in the system and are therefore independent of differences in density. During the process of crystallization, material is transformed from the liquid to the solid state. Consequently, motions typical for a liquid become impossible and do not longer contribute to the measured signal. For pre-ordered structures one also expects changes in mobility due to changes in chain conformation during pre-ordering, but without additional morphological information the interpretation of relaxation experiments is often speculative. Therefore, a combination of both types of experiments seems essential. We performed simultaneously dielectric relaxation and small-angle X-ray scattering (SAXS) experiments during isothermal crystallization of polycaprolactone. A newly developed sample cell is described. Pronounced effects in the dielectric signal are observed significantly before changes in SAXS intensity or crystallinity can be detected. The observations strongly support the idea of pre-crystalline-order in the polymer melt before the formation of crystals.