Melt blending of poly(lactic acid) and poly(caprolactone): phase structure and property relationships

This thesis investigates the improving the mechanical properties of two brittle biodegradable polymers, poly (lactic acid) (PLA) and poly (vinyl acetate) (PVAc), by melt blending with a ductile polymer, poly (caprolactone) (PCL). An important aspect of polymer blends is determining the phase structure and where phase inversion occurs. This was achieved by applying an empirical model based on melt viscosity ratios to predict the phase inversion point in these systems. Ways of further improving properties in the PLA/PCL system were examined by adding compatibilisers to the formulation. Surface energy measurements were carried out to interpret the results obtained.

In the first part of the project (Chapter 4) blends of PLA/PCL were investigated over a range of compositions (PLA/PCL: 100/0, 80/20, 60/40, 50/50, 45/55, 40/60, 35/65, 30/70, 20/80 and 0/100). The co-continuous phase structure was predicted using an empirical model due to Paul and Barlow and it was expected to be found at 54 wt% of PCL. The microstructure of the blends was examined using both optical microscopy and scanning electron microscopy and the co-continuous phase was found between 50 and 55 wt% PCL. Mechanical properties showed a very good increase in ductility with balanced strength and modulus at the co-continuous phase morphology. Applying parallel and series models to predict how modulus varied at different blend compositions showed the occurrence of the co-continuous phase between 50-55% of PCL, which corresponded to the values calculated from the melt viscosities and seen in the optical micrographs and SEM.

Biodegradation behaviour was also investigated. For both biodegradability in compost at 50°C and degradation in phosphate buffer at 37°C, it was found that neat PCL and blends with a higher PCL content degraded faster than the blends in which PLA was the continuous phase.

In the second part of the project (Chapter 5) the empirical model was used to predict the interpenetrating network structure of polyvinyl acetate (PVAc)/polycaprolactone (PCL) blends. The ratio of melt viscosities of both components was used to predict the phase inversion point at 72 wt% PCL. PVAc/PCL blends at different ratios (PVAc/PCL: 100/0, 80/20, 60/40, 50/50, 40/60, 35/65, 30/70, 25/75, 15/85 and 0/100) were examined in terms of thermal properties, mechanical properties and phase structure. It was found from the optical microscopy and SEM images of PVAc/PCL blends that the co-continuous phase structure was formed between 65 and 75 wt% of PCL. This is consistent with the model prediction. The elongation at break of blends within the range of the interpenetrating network structure increased greatly and there was balanced between tensile strength, modulus and elongation at break. Thermal properties of PVAc/PCL blends were studied. It was found that the crystallinity of PCL decreased as a function of PVAc content. From the Fox equation, there was no shift in the glass transition temperature of the PVAc, indicating an immiscible blend.

In the final part of the project (Chapter 6), the effect of compatibilisers on PLA/PCL melt blends was studied at the composition found to give a co-continuous phase morphology (PLA50/PCL50). Four compatibilisers; Poly(L-lactide-co-Ɛ-caprolactone) di-block copolymer (Lboro copolymer), Poly(L-lactide-co-Ɛ-caprolactone) block copolymer (Resomer), Poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate) (EMA-GMA) and Poly(ethylene-co-methacrylic acid) (EMAA) were examined. Their thermal properties, mechanical properties, phase morphology, FTIR and surface energies were investigated.

Of the block copolymers (Lboro copolymer and Resomer) investigated, it was found that the Resomer gave significant improvements in both ductility and tensile strength of the blend at an addition level of 2 wt%. It also gave a significantly finer microstructure. No further improvements were found at addition levels above 2 wt%. The Lboro di-block copolymer had a relatively low molecular weight and acted as a plasticiser. It increased the crystallinity of PCL in the blend and caused an increase in modulus but had a deleterious effect on ductility, especially at a high addition level of 10 wt%. Of the reactive compatibilisers investigated (EMA-GMA and EMAA), it was found that EMA-GMA gave rise to a finer microstructure but decreased mechanical properties, whereas the EMAA gave an improvement in ductility and strength but no significant change in the microstructure. Surface energy measurements were carried out on PLA, PCL and the compatibilisers. From these results the effects of the compatibiliser could be explained: the surface energies of PLA, PCL, Resomer and EMAA were similar, whereas that of EMA-GMA was different, which agrees with the mechanical properties.