The characteristic morphological feature of semicrystalline polymers crystallized from the melt is a nanoscopic, non-equilibrium two-phase structure of thin lamellar crystals separated by disordered amorphous layers. The thickness of lamellar crystals varies largely between different polymer systems and depends to variable degree on the crystallization temperature T_c. It has a strong influence on the mechanical properties. Addressing the still highly debated mechanism for thickness selection we prove a general classification based on the interplay between the kinetics of crystal growth and of reorganisation processes enabled by intracrystalline chain dynamics (ICD). Although it was recognized already early on by Boyd that polymers with ICD have in general a higher crystallinity than polymers without ICD, the necessary classification with regard to the obvious impact on the crystallization process has been ignored in most crystallization models and a quantitative analysis is missing.
Using polarization microscopy and solid-state NMR we determine the timescale of both processes and compare three polymer systems, poly(caprolactone), poly(oxymethylene), and poly(ethylene oxide). Depending on the ratio of the timescales of crystal growth and ICD we observed a systematic increase of the crystal thickness as determined by small angle X-ray scattering. We conclude that the crystal thickness of semicrystalline polymers is mainly determined by the competition of these two processes, leading to different morphological and thermal characteristics of the polymers with and without ICD, including the commonly observed strong T_c-dependence for polymer systems with fast ICD.

Systematic increase of lamellar thickness d<sub>c </sub>with increasing effect of intracrystalline chain dynamic (ICD) on crystal growth.

Systematic increase of lamellar thickness d_cwith increasing effect of intracrystalline chain dynamic (ICD) on crystal growth.

Martha Schulz

Martin-Luther-University Halle-Wittenberg

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