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UBC Theses and Dissertations

Synthesis and rheology of poly(lactide)s and their lignin composites Chile, Love-Ese


Synthetic plastics were first introduced 180 years ago, but the materials we have produced are likely to persist on our earth for thousands of years. Global shifts in thinking have urged researchers to focus their attention on bio-derived and biodegradable polymers. One such polymer is poly(lactic acid) (PLA). Despite its environmental benefits, PLA has several material weaknesses which hinder it’s use as a replacement for commodity plastics. Highly active and selective indium catalysts for the ring-opening polymerization of lactide isomers have recently been developed by the Mehrkhodavandi group. By utilizing these catalysts, modification of tacticity and end-group functionality of PLAs are possible, permitting exploration into the effect of these modifications on chain interactions in PLA. The thermal and rheological behaviours of PLAs with different microstructures were compared. The molecular weight between entanglements was greatest for the syndiotactically enriched PLAs, giving rise to the lowest zero-shear viscosity. In addition, hetero- and isotactically enriched PLA had higher flow activation energies than syndiotactic variants, implying the inclusion of transient aggregate regions within these polymers due to enhanced L- and D-interactions. A series of aryl-capped PLAs were synthesized by living ring-opening polymerization with a chain transfer agent using a previously reported dinuclear indium catalyst, [(NNO)InCl]₂(μ-Cl)(μ-OEt) (A). Thermal, rheological and mechanical techniques were employed to understand the extent and strength of association caused by arylated chain ends. It is shown that the end-group has a greater effect on the properties of low molecular weight PLAs due to the larger number density of aryl end groups; significant interactions can be induced under oscillatory shear conditions in the low frequency flow regime (terminal zone). The lignocellulosic biorefinery industry has been expanding in recent years and now provides researchers access to a range of bio-based composite materials through blending and copolymerization. Lignin-graft-PLA copolymers were synthesized via different routes and the PLA products were analyzed. Polymers were found to have cyclic structures at low lignin loading and star-like structures at higher lignin loading. Rheological studies were undertaken to derive useful structure-property relationships and optimize material properties.

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