UBC Theses and Dissertations
Rheology and processing of poly(lactides) and their enantiomeric copolymers and blends Othman, Norhayani
Poly(lactide) PLA, a biodegradable thermoplastic produced from corn and other renewable agricultural resources, has received a large share of the interest in biodegradable materials due to environmental concerns and desire to reduce dependence on finite petroleum reserves. In this study, nearly monodisperse controlled microstructure PLA samples synthesized using a novel chiral dinuclear indium catalyst; and studied thermorheologically. Specifically, the effects of molecular structural parameters (i.e. weight-average molecular weight (Mw) and different ratios of lactides) on solution and melt rheological properties under shear and extension were studied. The solution properties and linear viscoelasticity (LVE) of melts indicated linear structure behavior. The zero-shear viscosity and relaxation time of PLAs showed a power law scaling of 3.4 with Mw. The K-BKZ constitutive equation was used and proved that strain hardening occurs at low temperatures, which is due to the dynamics of molecular relaxation, when the longest relaxation time exceeded the characteristic time for deformation. In an attempt to reduce PLA brittleness, copolymers of L-lactide with its enantiomer D-lactide or racemic mixture DL-lactide were synthesized. The effects of Mw and block length ratio on the thermal, rheological and mechanical behavior of the diblock copolymers were investigated. For comparison, blends of PDLLA and PLLA homopolymers of equivalent Mw to the diblock copolymers were prepared. Despite different thermal behavior, the linear viscoelasticity of block copolymers and blends in disordered state are relatively similar. Improvement in elongation at break and tensile strength were observed as compared to their counterpart homopolymer blends. Furthermore, the wall slip and melt fracture behaviors of four commercial PLAs with Mw in the range of 10⁴ to 10⁵ g/mol were investigated. PLAs with Mw greater than a certain value slipped. The slip velocity increased with decrease of Mw. The onset of melt fracture for the high Mw PLAs occurred at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt% of a poly(ε-caprolactone) (PCL) into the PLA that exhibits melt fracture was effective in eliminating and delaying its onset to higher shear rates.
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