Influence of film thickness on the crystalline morphology of a copolyesterurethane comprising crystallizable poly(ɛ-caprolactone) soft segments

In this work, a model approach to investigate changes in crystalline morphology during heating/cooling procedures in the context of programming and induction of the shape-memory effect is presented.

Atomic-force microscopy (AFM) was performed to investigate the variations in poly(ɛ-caprolactone) (PCL) crystalline morphology in nm thin films on a silicon substrate and a film with 20 μm thickness, prepared from a copolyesterurethane (named PDLCL) consisting of crystallizable poly(ω-pentadecalactone) (PPDL) hard segments and crystallizable PCL segments forming switching domains.

PCL crystals in switching domains melted/recrystallized repeatedly during heating/cooling cycles between 20 and 70 °C, while no evident variation in PPDL crystals forming hard domains was observed. When film thickness was 20 nm, PCL edge-on lamellae were observed, confined in the phase-separated morphology. Flat-on PCL lamellae, which broke out from the previous phase-separated morphology, were obtained at a film thickness of 170 nm. In contrast, large PCL spherulites were observed in the 20 μm thick film.

PCL crystalline morphology in PDLCL as well as the competition between crystallization and phase separation can be tailored by the film thickness and the substrate. Such AFM investigations on thin films can be a helpful approach for predicting the crystal morphology in micro-/nanoscaled objects.