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

Abstract

The effects of melt structure and shear flow on the polymorphic nature of β-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 Tm “the nominal melting temperatures” and Tm0 “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 Tm. 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 °C, the long period (L) increases from 29.2 to 48.9 nm (Strobl method) and the lamellar thickness (dc) increases from 9.2 to 12.9 nm with increasing Tm∗. 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 (σw = 0.020 MPa) have been applied to ordered melt, the shear-induced β-cylindrites can be observed. Length and number of the β-cylindrites decreases with increasing temperature of ordered melt (Tm∗ = 177–190 °C), and the content of β-iPP remained almost invariant when Tm∗ was set from 177 to 180 °C, however, as Tm∗ was above 180 °C, the β-iPP content obviously decreased. According to the results of shear-induced β-cylindrites from ordered melt, here we proposed the formation process of β-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.