The Decisive Role of Intra-Crystalline Chain Dynamics for the Morphology of Semicrystalline Polymers

M. Schulz, A. Seidlitz, R. Kurz, R. Bärenwald, K. Saalwächter and T. Thurn-Albrecht

Institute of Physics, Martin Luther University Halle-Wittenberg

A special feature of some semicrystalline polymers is the existence of the so-called αc-relaxation, caused by a translational motion of the chains in the crystal. Although it was recognized already early on that these crystal-mobile polymers generally have a higher crystallinity than crystal-fixed polymers [1], differences in the semicrystalline morphology have not been analyzed in detail and the relaxation process has not been taken into account in most crystallization models. Using an extended method for quantitative analysis of small angle x-ray scattering data [2] we here compare the structural characteristics for two model polymers, namely PEO (crystal-mobile) and PCL (crystal-fixed). PCL follows the expectations of classical crystallization theories. The crystal thickness is well defined and determined by the crystallization temperature Tc. In contrast, for PEO the amorphous thickness is better defined. We hypothesize that due to the αc-relaxation, the crystalline lamellae thicken directly behind the growth front until the amorphous regions reach a minimal thickness. This assumption is consistent with NMR experiments, which give the time scale of intra-crystalline dynamics [3] and which show that crystal reorganization takes place on the same time scale as crystal growth itself. In keeping with this model, crystal-mobile and crystal-fixed polymers exhibit a different melting behavior during heating. The crystallization of PCL leads to the formation of marginally stable crystallites which constantly reorganize during heating, while in PEO due to the presence of the αc-relaxation, much more stable (thickened) lamellar crystals form, which melt only at much higher temperatures.

[1] Boyd, R.H.; Polymer 26, 323 (1985) (link)
[2] Seidlitz, A. et al. in Polymer Morphology, Wiley (2016) (link)
[3] Kurz, R. et al.; Macromolecules 50 (10), 3890 (2017) (link)