In recent years, more and more evidence has been accumulated that calls into question the popular Lauritzen-Hoffman model explaining the formation of crystalline lamellae in semi-crystalline polymers. The still open question is why a specific lamellar thickness is chosen during crystallization of long entangled linear polymers. Here, we address this question by considering exlicitly entanglement constraints as well as the as yet underestimated potential influence of intra-crystalline chain mobility [1]. We compare the crystallization of poly(ethylene oxide) (PEO) and poly(e-caprolactone) (PCL) as representative cases of crystal-mobile [2] and crystal-fixed [3] polymers, resp. An intermediate case is represented by poly(oxymethylene) (POM). We study the morphology of the samples in terms of the thicknesses of the crystalline and amorphous domains as determined by small-angle X-ray scattering, including the newly developed option to also extract the width of the thickness distribution [4]. We find that, contrary to conventional wisdom, in PEO it is the amorphous rather than the crystalline thickness that is well-defined. NMR investigations of the timescale of intra-crystalline chain motion in PEO demonstrate that the lamellae re-organize already on the timescale of the attachments of stems. The morphology further depends on the crystallization conditions, where we distinguish slow and fast crystal growth as compared to the terminal relaxation time of the chains in the melt, again with qualitative differences between PEO and PCL. Finally, we use modern NMR techniques to assess the entangled structure [5] within the amorphous domains, and present correlations with the different morphologies.

Relations between amorphous and crystalline chain motions and the morphology of semicrystalline polymers

255th National Meeting (2018)

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