Design and characterization of biodegradable thermoplastic elastomers

In this thesis, biodegradable thermoplastic elastomers were developed through the structural design of copolymers based on caprolactone and lactide. The copolymers comprised of 2 segments, namely an amorphous middle block, made up of caprolactone and L-lactide, and crystallizable end blocks, made up of mainly poly(L-lactide). Triblock copolymers with fixed compositions but varying middle and end block lengths were studied and it was found that copolymers with long middle block length could achieve high elongation-to-break while end block length should be sufficiently high for defined PLLA crystals to form. The ratio of end block length to middle block length influenced the recovery properties of the copolymers where copolymers with longer middle block length showed better recovery. Copolymers with high caprolactone content in the middle block resulted in formation of new crystalline domains during deformation which inversely reduced the recovery properties of the copolymers. At fixed middle and end block lengths, the effect of end block disruption with DLLA was also studied. By varying the amount of DLLA in the end blocks, the properties of the copolymers could be tuned. High elongation-to-break and good cyclic recovery were displayed by copolymers with disrupted end blocks but excessive disruption resulted in reduced creep resistance of the copolymers. The influence of molecular architecture was studied with star-block copolymers. Their complex 3-dimensional structure and steric hindrance of the arms were found to influence the properties of the copolymers. Longer end block length was needed to form PLLA crystals which were crucial for elastomeric character of the copolymers. Hydrolytic degradation of the copolymers were performed and concluded that they degraded through bulk degradation mechanism. Copolymers with longer middle block and higher end block disruption showed faster degradation rate while those with high initial crystallinity showed slower rate. With degradation, their mechanical properties dropped drastically. Hydrolysis dominantly occurred in the middle block and was verified by accelerated degradation where CL-based oligomers, mainly originating from the middle block, were formed before LA-based oligomers were observed. It was concluded that biodegradable thermoplastic elastomers should be designed based on a triblock copolymer due to their simplicity in structure. However, the elastomers based on this structure should only be considered for applications that require elastomeric character initially and for about a month after deployment.Doctor of Philosophy (MSE