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Interface and Confinement Induced Order and Orientation in Thin Films of Poly(ϵ-caprolactone)

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journal contribution
posted on 20.04.2016, 13:52 by Wilhelm Kossack, Anne Seidlitz, Thomas Thurn-Albrecht, Friedrich Kremer
Infrared transition moment orientational analysis (IR-TMOA), X-ray diffraction (XRD), and model calculations are combined to study interface and confinement induced order and orientation in thin (h ≈ 11 μm) films of poly­(ϵ-caprolactone) (PCL), as prepared by drop-casting on silicon wafers. Depending on the crystallization temperature, 303 K ≤ Tx ≤ 333 K, nonbanded spherulites with a diameter of 1 μm ≤ dS ≤ 500 μm form. Macroscopic order of the crystalline lamellae is imposed by interfacial layers and geometrical confinements of the spherulitc structures (dS > h): radial crystal growth is restricted to a disc of aspect ratio dS/h. Order is quantified by IR-TMOA and XRD pole figure measurements, which both rely on the relative orientation of the sample and the incident radiation and measure, in the case of PCL, the orientation distribution of complementary crystal directions. This enables one to (1) correlate the directions of the transition moments with the crystal axes and (2) estimate the volume fractions of flat-on or edge-on lamellae as induced by the substrate or the free interface as well as the fractions of surface induced or bulk nucleated spherulites in dependence on Tx. It turns out that the contribution of substrate induced spherulitic structures rises with Tx = 323 K up to a value of ∼12 vol %, whereas no indications of edge on lamellae at the free surface are found. For Tx ≤ 308 K nonconfined spherulites (dS < h) dominate the morphology, and furthermore no substrate induced layer is found.