Shear-induced crystallization in novel long chain branched polypropylenes by in situ rheo-SAXS and -WAXD

The characteristic properties of a new class of long chain branched isotactic polypropylene (LCB-iPP) polymers and the effects of its molecular architecture on the nature of orientation-induced crystallization precursor structures and crystallization kinetics in flow were investigated. In these polymers, long chain branching (LCB) was introduced via in situ polymerization of propylene and a diene monomer using the metallocene catalyst technology. The LCB-iPP polymers exhibited improved melt strength and mechanical properties, such as flexural modulus and tensile yield strength. As determined by gel permeation chromatography coupled with multiangle laser light-scattering (GPC-MALLS) analysis, it was noted that the LCB-iPP polymers had a significant population of highly branched chains in the high molecular weight component of the molecular weight distribution. Dynamic viscoelastic properties obtained by small-amplitude oscillatory shear technique showed that the increase in the degree of LCB caused a broadening of the melt relaxation spectrum, especially in the longest relaxation time regime. The effects of the unique molecular architecture of the LCB-iPP polymers on shear-induced crystallization development was investigated by in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) techniques. The results showed that upon application of a step shear (rate = 60 s^-1, t_s = 0.25 s, T = 140 °C), the oriented crystals fraction was substantially higher in the LCB-iPP polymers than that in the linear iPP polymer. The shear-induced crystallization rate of the LCB-iPP polymers was analyzed by an Avrami model; the value of Avrami exponent, n, ranged from 1.8 to 2.8 indicating a rod or disklike crystal growth geometry. In addition, the crystallization kinetics was enhanced by more than an order of magnitude when compared to linear iPP polymer under shear. The details of the crystalline phases determined from the WAXD data showed that, in addition to monoclinic α-form crystals, they were triclinic γ-form crystals. The contribution of γ-form crystals to the total crystalline phase was independent of the LCB level and was attributed to the regio defects of the chain. Enhancements in various mechanical properties as well as their strong crystal orientation and improved crystallization kinetics in flow, are due to the unique molecular architecture of these LCB-iPP polymers, which in turn are strongly coupled to their broadened and complex relaxation behavior.