Tip-splitting crystal growth observed in crystallization from thin films of poly(ethylene terephthalate)

A novel crystal morphology called terrace appearing in the crystallization of poly(ethylene terephthalate) (PET) is studied. This morphology is confirmed to be inherent to the polymer itself and is formed when the polymer crystallizes from a very thin melt film. The growing processes of the terrace are examined in detail using confocal scanning laser microscopy (CSLM) and atomic force microscopy (AFM). The terraces appear on the periphery of a two-dimensional spherulite at the final stage of the spherulite growth. They have a single crystal-like structure though their growth front or outer shape is not smooth but fluctuating; it shows a periodic pattern characteristic of a diffusion-controlled process. AFM reveals that the terrace is about 13 nm thick and its growth front is in contact with a further thinner melt layer about 2-4 nm thick, suggesting that amorphous polymer molecules are supplied to the growing front only from the thinner melt layer. As crystallization proceeds, long needle-like crystals protrudes from the terrace front, resulting in the so-called finger pattern. Such needle growth is then suppressed by accompanying the tip-splitting of some needle crystals to produce crabs pincers-like crystals. Monte Carlo simulations for the two-dimensional diffusion model reproduce the morphologies, where the surface tension and the crystal anisotropy are shown to be crucially important.