Rheological Behavior of High Melt Strength Polypropylene-based Materials

Abstract

Two types of high melt strength polypropylenes (PP), including long-chain branching (LCB) PP and gel particle-filled PP, have been discussed. This dissertation aims to establish the structure-rheology-morphology-property relationship based on two sets of model PP systems. The linear viscoelasticity of LCB PPs has been systematically studied. The model PPs with a proportional amount of controlled branching structure were prepared with a coupling agent containing bifunctional reactive groups in a twin-screw extruder. The complex viscosity, elasticity, and storage modulus increase with LCB molar fraction. Moreover, a methodology was developed to determine the LCB molar fraction in PPs using gel permeation chromatography and rheological methods. The methodology has been demonstrated to be applied to a commercial high melt strength PP successfully.

This dissertation also focuses on the effect of gel content on PP's linear and nonlinear rheological properties. The PPs filled with the gel particles exhibit pronounced shear-thinning and enhanced zero-shear viscosity. The strain-hardening degree increases significantly with a low loading of gelled particles. Moreover, our SEM and rheological results indicate that the gel particle prepared via grafting-crosslinking modification exhibits a core-shell structure, consisting of a crosslinked PP core and a shell made of grafting chains and constrained matrix chains. Finally, the third part of this dissertation investigates how LCB influences PP's scratch and tensile performance. Scratch resistance of LCB PP was evaluated using a standardized scratch test, followed by visibility analyses, scratch profile characterization, and materials properties determination. The better elastic recovery and tensile strength of LCB PP contribute to the enhanced scratch resistance.