Formation mechanism, chain folding, and growth behavior of the intriguing fiber-like crystal of poly (ethylene oxide-b-ε-caprolactone) block copolymer in ultrathin films

The morphologies, crystallization behavior and chain orientation of a highly asymmetric poly (ethylene oxide-b-ε-caprolactone) (PEO-b-PCL) block copolymer ultrathin films were investigated by using wide-angle X-ray diffraction (WAXD), atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD) techniques. It is shown that the intriguing fiber-like crystal which seems to be an individual flat-on lamella was first observed in PEO-b-PCL ultrathin film according to our knowledge. The possible mechanism of forming the fiber-like crystals is chiefly ascribed to the rather low molecular weight of PEO-b-PCL and the mutual interaction between crystallization and microphase separation. At the same time, solution concentration and substrate surface energy play a crucial role. Subsequently, chain folding of the fiber-like crystal of PEO-b-PCL in ultrathin films was deduced based on the cell parameters and the lamellar thickness. Isothermal crystallization tests revealed that in the whole growth process, the crystal growths can be divided into two stages, both of which followed the diffusion-limited (DL) mechanism.