A real-time study of homogeneous nucleation, growth, and phase transformations in nanodroplets of low molecular weight isotactic polypropylene using AFM

摘要

Nanodroplets of isotactic polypropylene (iPP) were observed using temperature-controlled AFM in order to study the polymer's crystallization behavior. The nucleation, growth, and transformation of iPP crystals on heating have been directly imaged. The strong confinement of the polymer into nanoscale droplets has allowed the controlled observation of polymer nucleation as well as access to crystal growth at exceptionally high supercooling in iPP. Different modes of crystal growth were observed depending on the film thickness, including the formation of multiple independent homogeneous nuclei within single droplets. The temperature at which the onset of nucleation was observed in individual droplets was found to be dependent on the thickness as well as the volume of the droplets. For droplets with smaller thicknesses (<5 nm), the thickness of the droplets was found to be the dominating factor influencing the nucleation temperature. This is the first real-space, real-time observation of homogeneous nucleation in iPP, almost 125 degrees C below its melting point, which could signify crystal growth in the smectic form, and of the subsequent reorganization into the a-form.