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Herein we report the finding that spectral reflectance (SR) can be used to identify thermal transitions in semi-crystalline polymer thin films. By studying film thickness as a function of temperature, we found that semi-crystalline polymers exhibit characteristic expansion and contraction profiles during the melting (Tm) and crystallization (Tc) transitions, respectively. Prior to this discovery, studies on the crystalline Tm in thin films have involved more expensive and complex techniques such as atomic force microscopy (AFM), ellipsometry, and grazing-incidence wide-angle X-ray scattering (GIWAXS). We correlate Tm and Tc as measured by SR with differential scanning calorimetry (DSC) in the bulk and temperature-controlled GIWAXS or AFM in thin films. We show that SR is accurate for measuring changes in films with thicknesses of 500 nm down to 21 nm and for detecting melting point depression due to thin film confinement. Thus, we demonstrate that SR is a powerful tool for measuring thermal transitions in semi-crystalline polymer films with single-degree resolution.

We report the discovery of a new nanostructured morphology in ultrathin blend films of semi-crystalline poly(e-caprolactone) (PCL) and various styrene-isoprene block copolymers. The nano-rose morphology is visualized by atomic force microscopy as a monolayer of approximately 500 nm diameter crystalline spirals. Our finding that the nano-rose morphology occurs in a narrow film thickness window commensurate with the crystalline lamellar thickness suggests that this distinctive morphology results from redirection of growing PCL crystalline lamellae due to confinement effects during film casting. The nano-rose morphology was located in blends with several block copolymers of various architecture but was absent in blends with the individual homopolymer components. PCL molecular weight did not affect nano-rose size or film thickness window. However, solvent choice was an important factor, with nano-rose formation favored in non-polar and weakly polar solvents. This work supports theories that unbalanced surface stresses and impurity exclusion cause polymer crystals to grow unconventionally.