The crystalline morphology semi-crystalline polymers is of paramount importance as the organization of the polymer chains on the molecular, nano-, and micro-scale dictate macroscopic properties such as mechanical integrity, optical clarity, degradability, thermal processability, and thin film stability. Small molecule additives used to suppress and control crystallization can pose undesirable environmental or health risks. Thus we are interested in using manipulations in polymer topology and the use of topological blends to provide the desired control over morphological and phsyical in order to circumvent the disadvantages associated with additives. Specifically, we are studying the crystalline morpoholgoy of poly(ε-caprolactone) (PCL) in thin film geometries where it can be used in applications such as anti-adhesion biomaterial films and substrate-mediated drug delivery. We began by comparing the thin film properties of cylic PCL (c-PCL) to its exact linear analogue (l-PCL). In our initial study, we discovered that for 6 kDa polymers cast into 100 nm thick films, c-PCL6k films resisted dewetting during melting and recrystallizaiton whereas l-PCL6k films always dewet, regardless of end group chemistry. This result was counterintuitive given that studies comparing cyclic polymers to their linear counterparts in the bulk have shown that cylclic polymers have lower viscosity, higher mobility, and fewer entanglements, properties that should have made the c-PCL6k film more susceptible to dewetting. Although both c-PCL6k and l-PCL6k films exhibited a flattened spherulite morphology prior to melting, subtle morphological differences were noted in optical and atomic force microscopy images of the films. Thus, ongoing work is aimed at understanding how differences in crystallization kinetics gives rise to morphological differences between cyclic and linear PCL in thin film geometries. These studies employ higher molecular weight polymers and thicker films so that neither the cyclic PCL nor the linear PCL films dewet. Preliminary results point to yet another deviation from bulk behavior, with cyclic PCL (lower viscosity) spherulites growing more slowly than linear PCL (higher viscosity) spherulites. We attribute these findings to differences in how topology is accommodated by polymer chains crystallizing under confined versus unconfined geometries.

Crystallization of poly(ε-caprolactone) under topological and thin film confinement

255th National Meeting (2018)

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