Development and characterization of thermoplastic biopolyester compounds and their processing by 3D bioplotting
The objective of this thesis is to improve the properties of thermoplastic biopolyesters by reactive modification and blending, and to develop thermoplastic biopolyester compounds with suitable rheological, thermal and mechanical properties for processing by 3D bioplotting. As a first step the effects of the structure of medium-chain-length polyhydroxyalkanoates (MCL-PHAs) on their thermal properties and crystallization kinetics were investigated. The predominantly homopolymeric poly(3-hydroxydecanoate), P(3HD)-98 (98 mol% HD), as well as poly(3-hydroxydodecanoate), P(3HDD), exhibited sharp crystallization peaks upon cooling, with the latter exhibiting faster crystallization rates. A chemical modification strategy involving reaction with dicumyl peroxide and triallyl trimesate coagent was implemented to introduce branching and improve the crystallization kinetics of P(3HD-98). Increases in the exothermic crystallization temperature by 8°C and in the overall crystallinity, and lower crystallization half-times were achieved upon chemical modification. Furthermore, blends containing various amounts of polycaprolactone (PCL) and poly(3-hydroxyoctanoate) P(3HO) were prepared by melt compounding. Even though addition of the amorphous P(3HO) resulted in a decrease in the crystallinity of PCL, the crystallization temperature of the blends increased, and the blends exhibited faster crystallization rates up to blend compositions of 30wt% P(3HO). 3D printing of these compounds requires detailed understanding of the viscosity, residence time and the forces encountered in a 3D bioplotter device. A model study was undertaken to investigate the effects of needle diameter and dispensing pressure on the shear rates, shear stresses, pressure drops and die swell of extruded miscible polycaprolactone (PCL) blends having a range of viscosities. Assuming simple capillary flow, flow curves were constructed. Relevant dimensionless numbers that reflected the material rheological properties and processing conditions including the capillary number (Ca), Bond number (Bo), Weissenberg number (Wi) and Elasticity number (El) were estimated. Based on the findings above, it was determined that the viscosity of the PCL/P(3HO) blend system was suitable for processing using the 3D bioplotter. Compositions ranging from 10-30 wt.% P(3HO) were ideal for processing, because of their faster crystallization kinetics, reduced stickiness and good flow properties. Owing to its favourable thermal and mechanical properties this blend system has promising potential in biomedical applications.