Preparation and characterization of biopolymer compounds containing poly-3-hydroxyalkanoates and polylactic acid
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This thesis is focused on developing cost effective and environmentally friendly techniques to improve the properties and processability of biopolyesters through compounding and reactive modification. Specifically, elastomeric medium-chain-length poly(3-hydroxyalkanoates) (MCL PHA) have been evaluated as potential impact modifiers for poly(lactic acid) (PLA) and poly-3-hydroxybutyrate (PHB), using conventional melt compounding. The Mark-Houwink constants, absolute molecular weight distributions and the absolute molecular weight (MW) averages of MCL PHAs with predominantly 3-hydroxyoctanoate (PHO), 3-hydroxynonanoate (PHN) or 3-hydroxydodecanoate (PHDD) content were determined and ranged between 18,200 for PHN to 172,000 Da for PHO. Detailed thermal and rheological characterization revealed that PHO had the highest viscosity, and was thus the best candidate as impact modifier for PHB and PLA. Melt compounded PHB/PHO and PLA/PHO blends showed improved tensile strain at break and unnotched impact strength upon addition of up to 30 wt.% PHO in PHB and 15 wt.% PHO in PLA. This was counteracted by decreased Young’s modulus due to lower blend crystallinity. The droplet-matrix morphology coarsened as PHO content increased beyond 5 wt.%, due to PHO coalescence attributed to viscosity mismatch between blend components. PHO was reacted using lauroyl peroxide to increase its viscosity through partial cross-linking, thus improving the morphology but the mechanical properties showed only moderate improvements, presumably due to high PHO gel content which compromised its elastomeric nature. Reactive compounding by radical mediated solvent-free grafting of triallyl trimesate (TAM) coagent was employed to improve blend properties. Reactively modified PLA had higher molar mass, melt viscosity and enhanced strain hardening. Additionally it showed a distinct crystallization peak upon cooling with disappearance of the cold crystallization peak, indicative of a nucleation effect. PLA modified using a multi-functional epoxide oligomeric chain extender yielded similar improvements in rheological properties, but no considerable change in crystallization. This coagent modification approach also increased the viscosity of PHO, and improved both extrudate appearance and handling. Coagent modified PLA/PHO blends demonstrated significant improvement in crystallization and rheological properties, similar to those seen in the coagent modified PLA alone, while the mechanical properties remained unaffected.