Crystallite Growth Enhances Segmental Dynamics in Star-Shaped Semi-Crystalline Polymer

The impact of nano-metric fillers on segmental dynamics of polymers is of great interest for both fundamental and applied research. On the one hand their effects are expected to shed light on the still poorly understood principles of the glass transition, on the other hand it is a promising route to develop hybrid or composite materials exhibiting novel properties and enhance performance. Similarly to nano-particles also the intrinsic semi-crystallinity found in many polymers can modify the macroscopic properties. For isotactic polystyrene (iPS) it is known that the crystallites severely impact the segmental dynamics of adjacent amorphous domains. In case of linear molecules, those chains which contribute to both crystallites and amorphous regions, the dynamics of connecting segments is slowed down forming a so-called rigid amorphous fraction [1].

In contrast to this established effect, star-shaped iPS with a octafunctional polyhedral oligomeric silsesquioxanes molecule as center [2] exhibits strongly enhanced dynamics; the mean relaxation time in the semi-crystalline state is about one decade faster than in the purely amorphous state. A comparison of the relaxation time distribution function of the purely amorphous and semi-crystalline state reveals a considerable fraction of additional faster and extinction of slower relaxation modes in the latter. We assume that the particular architecture of the polymers stabilizes the overall sample volume upon crystallization thus leading to a reduced density in the amorphous domains which yields faster dynamics. This mechanism of thermally induced switching of segmental dynamics is an example for novel effects created by the modification of the molecular architecture.

[1] Natesan, B.; Xu, H.; Seyhan Ince, B. & Cebe, P. J Polymer Sci Part B: Polymer Phys, 2004, 42, 777

[2] Vielhauer, M.; Lutz, P. J.; Reiter, G. & Mülhaupt, R. J Polymer Sci Part A: Polymer Chem, 2013, 51, 947