Shear-Induced Nucleation and Growth of Long Helices in Supercooled Isotactic Polypropylene

Flow-induced conformational ordering in the supercooled isotactic polypropylene (iPP) is studied with in situ Fourier transform infrared spectroscopy (FTIR) coupled to an extrusion slit die. At temperature around the normal melting point of iPP, helices with monomer numbers up to 12 and 14 can be induced by shear. A window of shear strength exists to induce helices with different lengths, which increases with temperature. After the cessation of,shear, the intensity of the 841 cm(-1) hand in FTIR spectra, corresponding to helices with 12 monomers, increases sharply in the First stage, which is followed by a slow growth process, whereas a reduction of shorter helices is observed after the cessation of shear. The different trends of intensity evolutions of the long and short helical bands suggest that a coupling between coil-helix transition and intermolecular ordering occurs with helices with a length of 12 monomers. which eventually leads to an isotropic-liquid-crystal transition. Therefore, the flow-induced precursor may be a liquid crystal phase, which will transform into crystal and initiate crystal growth later.