Free Radical-Mediated Reactive Extrusion of Commodity Polymers
The objective of this thesis is to improve the melt and solid-state properties of poly (propylene) (PP) and poly(lactide) (PLA) by reactive extrusion. Peroxide-mediated melt-state reactive extrusion in the presence of mesate and acrylate-based coagents was implemented to introduce branching and enhance the strain hardening, crystallization kinetics and solid state properties of these commodity polymers, to make them conducive to more high-value applications. The coagent modified PP possessed bimodal molecular weight and branching distributions, whose population varied with coagent structure. Besides changing polymer chain architecture, chemical modification generated a small amount of well-dispersed, coagent-derived nanoparticles. When compared with the parent material and PP samples treated with peroxide alone, coagent-modified materials demonstrated significantly higher crystallization temperatures and crystallization rates, as well as a finer spherulitic structure. Crystallization studies showed that whereas branching had a moderate effect on crystallization kinetics, heterogeneous nucleation effects dominated the crystallization of coagent-modified PP materials. Coagent modified PP specimens prepared by injection molding retained the modulus and tensile strength of the parent PP, in spite of their lower molar mass and viscosities, whereas their elongation at break and the impact strength were improved. This was attributed to the finer spherulitic structure of these materials, and to the disappearance of the skin-core layer that typically forms during the injection molding process. In the second part of this thesis, structure-property relations of PLA, branched by peroxide-mediated reactive extrusion in the presence various coagents was carried out. The coagent modified PLA, consisted of mixtures of linear, and long chain branched (LCB) PLA chains. A trifunctional coagent (triallyl trimesate, TAM) was extremely effective in producing LCB structures, which promoted substantial increases in viscosity, elasticity, as well as strain hardening characteristics. The reactively-modified injection molded branched PLA formulations had improved Izod impact strength, while maintaining their capacity to degrade hydrolytically were maintained. Furthermore, these materials developed high amounts of crystallinity, when cooled under controlled conditions, revealing a nucleating effect. The improvements in strain hardening resulted in microcellular foams with very high cell densities and sub-micron sized cell size, when these formulations were foamed at temperatures close to the crystallization temperature of the polymer.
URI for this recordhttp://hdl.handle.net/1974/23844
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