The Underestimated Effect of Existence or Absence of Intra-Crystalline Chain Dynamics (α_c-relaxation) on Morphology and Stability of Semicrystalline Polymers

Some polymers show translational motion of the chains in crystallites – the α_c-relaxation. Although it was recognized early by Boyd that α_c-mobile polymers have a higher crystallinity than crystal-fixed polymers, the relaxation process has been ignored in most crystallization models. We show that the α_c-relaxation has a strong influence on the crystallization process, changing morphology and stability of the sc structure. Using SAXS, we compare structural characteristics for PEO (α_c -mobile) and PCL (crystal-fixed) after isothermal crystallization and during heating. With NMR we can estimate the timescale of the α_c-relaxation depending on T. A direct comparison points out fundamental differences: PCL shows marginally stable lamellae with a narrow thickness distribution, starting to reorganize for T> T_c. In contrast, PEO shows a well-defined, narrowly distributed amorphous thickness d_a and a broad thickness distribution of the lamellae. For T>T_c, the lamellae are stable over a wide range. We hypothesize that due to the α_c-relaxation, the lamellae thicken directly behind the growth front up to a minimal d_a. This is supported by NMR-results: At high T_c the α_c-relaxation is fast enough to enable crystal reorganization to take place in a narrow zone directly behind the growth front.