Lactic acid based poly(ester-urethane) : modification via copolymerization, chain linking and blending

Abstract

The properties of biodegradable lactic acid based poly(ester-urethanes), PEU, were chemically and physically modified and the structure-property relationships investigated. The heat resistance of PEU was improved by copolymerization of lactic acid with DL-mandelic acid. The glass transition temperature of poly(L-lactic acid-co-DL-mandelic acid-urethanes) showed a marked increase with increased mandelic acid composition. Molecular weight depression was attributed to the steric hindrance of the bulky phenyl group of mandelic acid. Novel biodegradable and thermoplastic poly(ester-urethane) elastomers were synthesized by the copolymerization of lactic acid and ε-caprolactone. Properties, such as glass transition temperature and mechanical properties were strongly dependent on the composition of the copolymer. Small amounts of ε-caprolactone increased the strain of PEU, and at higher caprolactone content the poly(L-lactic acid-co-ε-caprolactone-urethane), P(LA/CL)U, exhibited elastomeric properties, having lower strength but significant elongation.

The rheological properties of PEU were enhanced by modification of the structure of the polymer chains. An increase in the amount of 1,6-hexamethylene diisocyanate (HMDI) as a chain extender caused branching, which was revealed by the broadened MWD and increased shear thinning at low frequencies. The chain linking technology for lactic acid prepolymers was further developed with the use of highly effective carboxyl and hydroxyl reactive chain extenders. Reaction between 2,2'-bis(2-oxazoline) (BOX) and the carboxyl groups of the lactic acid oligomer led to a hydroxyl terminated prepolymer with low acid value, which provided a significant increase in molecular weight in the HMDI linking reaction. The introduction of oxamide groups into the polymer structure increased the chain stiffness, which was detected in enhanced mechanical properties and an increase in the glass transition temperature.

The impact strength of poly(ester-urethane) was significantly improved by blending. The toughening was achieved with a finely dispersed P(LA/CL)U or copoly(L-lactide/ε-caprolactone) elastomer phase in the matrix PEU. Tensile modulus and strength showed a downward trend as a function of rubber content but remained at an acceptable level. Good compatibility and interactions at the rubber-matrix interface were observed. The relationship between phase separation and mechanical properties of rubber-toughened blends was investigated. Composition of the elastomer, i.e. ε-caprolactone content, was found to determine the formation of the heterophase structure. The degree of crosslinking in the P(LA/CL)U rubber was another important factor in the impact modification. Furthermore, the balance between impact strength and stiffness of the poly(ester-urethane) composites was considerably improved by the addition of particulate or fibrous fillers as a third component.