Steady shear and dynamic properties of biodegradable polyesters

The rheological properties of three biodegradable partially crystalline alipathic polyesters - poly ε caprolactone (PCL), polylactic acid (PLA), and polyhydroxybutyrate-co-hydroxyvalerate (PHBV)-were investigated. Low shear viscosity (η), first normal stress difference (N₁), and dynamic properties (G′ and G″), were measured for each polymer using rotational instruments. High shear viscosity data was obtained using a capillary rheometer. Gel permeation chromatography indicates that both PLA and PHBV undergo degradation during processing. Thermal degradation rate constants for PLA and PHBV were determined from the time-sweep data. Data indicates that material functions are proportional to the molecular weight for PCL and PLA samples, and directly related to the HV content in the PHBV samples. The relaxation spectra were calculated from the G′ and G″ data using the regularization method and the applicability of the nonlinear Wagner equation was checked on these materials. Results are discussed in terms of variations in the molecular characteristics on Theological properties and their possible influence on the processability.