Effects of melt structure on shear-induced β-cylindrites of isotactic polypropylene

The effects of melt structure and shear flow on the polymorphic nature of beta-cylindrites were investigated by means of wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), polarized light microscopy (PLM), differential scanning calorimeter (DSC), and Scanning Electron Microscope (SEM). The temperature-resolved SAXS/WAXD experiments indicate that the NMP (near melting point) melting process (the annealing temperature between T-m the nominal melting temperatures and T-m(0) the equilibrium melting temperature) erases the polymer crystalline structure of unit lattice, however, the lamellar structures of iPP can survive in this melt for long time, even annealing (partially melting) temperature above the T-m. We call this incomplete relaxed melt (include survived lamellar structures) as ordered melt. The observations from SAXS experiments of ordered melt suggest that between 167 and 180 degrees C, the long period (L) increases from 29.2 to 48.9 nm (Strobl method) and the lamellar thickness (d(c)) increases from 9.2 to 12.9 nm with increasing T-m*. The long period and lamellar thickness changed slightly with increasing melting time. In order to study the effect of ordered structures in NMP melt on crystals structure after shear flow, the shear-induced specimens were prepared by extruding the near melting point (NMP) ordered melt of iPP through capillary die. It was found that, even only very low shear stress (sigma(w) = 0.020 MPa) have been applied to ordered melt, the shear-induced beta-cylindrites can be observed. Length and number of the beta-cylindrites decreases with increasing temperature of ordered melt (T-m* = 177-190 degrees C), and the content of beta-iPP remained almost invariant when T-m* set from 177 to 180 degrees C, however, as T-m* above 180 degrees C, the beta-iPP content obviously decreased. According to the results of shear-induced beta-cylindrites from ordered melt, here we proposed the formation process of beta-cylindrites, it is suggested that the survived lamellar structures may remain in the NMP ordered melt which can act as shear-precursor during the course of recooling and initiate the shear-induced crystallization. The structures (the size and density of survived lamellar structures) of ordered melt strongly influence by partially melting temperature. (C) 2012 Elsevier Ltd. All rights reserved.