Peroxide-initiated Modification of Polylactic acid (PLA) and Poly(3-hydroxyalkanoates) (PHAs) in the Presence of Allylic and Acrylic Coagents
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This thesis investigates the fundamentals of modification of polylactic acid (PLA) and poly(3-hydroxyalkanoates) (PHAs), focusing on improving the understanding of the reactivity of these polymers in the presence of peroxide and multifunctional coagents. The first objective was to examine the effects these modifications had on PLA and to compare them to a well understood polyolefin system, ethylene octene copolymer (EOC). The linear viscoelastic (LVE) properties and molecular weight distributions showed that in the presence of peroxide and coagent these systems were able to produce long-chain branched structures, with allylic coagents being more effective at altering the chain architecture. These reactions were found to proceed through a radical mechanism as oppose to other forms of ionic chemistry. Evaluation of the abstraction efficiencies (AE) and graft propagation of monofunctional coagents showed that PLA is a poor hydrogen donor and the effectiveness of the allylic coagents is likely a result of solubility between the polymer and coagent in the melt. The second objective was to investigate the chemical modification of poly(3-hydroxyalkanoates) (PHAs), with different lengths of side chains. Medium-chain-length PHAs (MCL-PHAs) showed an affinity for both allylic and acrylic coagents with increases in viscosity, the appearance of shear thinning, and bimodal molecular weight distributions. On the other hand, the short-chain-length PHAs (SCL-PHAs), poly(3-hydroxybutyrate) PHB, preformed very similar to what was observed with PLA, where allylic coagents out preformed the acrylate coagents. The AE of these materials gave significant insight into the reactivity. As the alkane side chain length was increased from SCL-PHAs to MCL-PHAs, the number of methylene group increased and as a result more hydrogen abstraction sites became available, thus resulting in higher AE. This implies there is a greater probability for coagents to graft onto the polymer backbone and therefore the promotion of branched structures.
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