Surface and bulk morphology of cold-crystallized poly(ethylene terephthalate)

Surface Crystal Nucleation and Growth in Poly (ε-caprolactone): Atomic Force Microscopy Combined with Fast Scanning Chip Calorimetry

By using an atomic force microscope (AFM) coupled to a fast scanning chip calorimeter (FSC), AFM-tip induced crystal nucleation/crystallization in poly (ε-caprolactone) (PCL) has been studied at low melt-supercooling, that is, at a temperature typically not assessable for melt-crystallization studies. Nanogram-sized PCL was placed on the active/heatable area of the FSC chip, melted, and then rapidly cooled to 330 K, which is 13 K below the equilibrium melting temperature. Subsequent isothermal crystallization at this temperature was initiated by a soft-tapping AFM-tip nucleation event. Crystallization starting at such surface nucleus led to formation of a single spherulite within the FSC sample, as concluded from the radial symmetry of the observed morphology. The observed growth rate in the sub-micron thin FSC sample, nucleated at its surface, was found being much higher than in the case of bulk crystallization, emphasizing a different growth mechanism. Moreover, distinct banding/ring-like structures are observed, with the band period being less than 1 µm. After crystallization, the sample was melted for gaining information about the achieved crystallinity and the temperature range of melting, both being similar compared to much slower bulk crystallization at the same temperature but for a much longer time.

A Combinational Effect of "Bulk" and "Surface" Shape-Memory Transitions on the Regulation of Cell Alignment

A novel shape-memory cell culture platform has been designed that is capable of simultaneously tuning surface topography and dimensionality to manipulate cell alignment. By crosslinking poly(ε-caprolactone) (PCL) macromonomers of precisely designed nanoarchitectures, a shape-memory PCL with switching temperature near body temperature is successfully prepared. The temporary strain-fixed PCLs are prepared by processing through heating, stretching, and cooling about the switching temperature. Temporary nanowrinkles are also formed spontaneously during the strain-fixing process with magnitudes that are dependent on the applied strain. The surface features completely transform from wrinkled to smooth upon shape-memory activation over a narrow temperature range. Shape-memory activation also triggers dimensional deformation in an initial fixed strain-dependent manner. A dynamic cell-orienting study demonstrates that surface topographical changes play a dominant role in cell alignment for samples with lower fixed strain, while dimensional changes play a dominant role in cell alignment for samples with higher fixed strain. The proposed shape-memory cell culture platform will become a powerful tool to investigate the effects of spatiotemporally presented mechanostructural stimuli on cell fate.