Effect of Block Length and Stereocomplexation on the Thermally Processable Poly(ε-caprolactone) and Poly(Lactic acid) Block Copolymers for Biomedical Applications

The current research concentrates on the synthesis of linear block copolymers with their backbone consisting of hard and soft segments, that is, poly(L-lactic acid) (PLLA)/ poly(D-lactic acid) (PDLA) and poly(ε-caprolactone) (PCL), with the detailed investigation of the effect of block length on the thermal, mechanical, and crystallization behavior followed by their processing and biocompatibility evaluation. In the current work, sequential ring opening polymerization has been employed wherein PCL is used as a macroinitiator for the synthesis of diblock (PCL-PDLA and PCL-PLLA) and stereotriblock copolymers (PCL-PLLA-PDLA) of targeted molecular weight, which is confirmed by 1H NMR spectroscopy. The block length of PCL, in the case of diblock copolymer, is fixed; however, that of PLLA or PDLA is varied, and the enantiomeric blends thereof are made to achieve the merits of stereocomplexation. The length of the individual blocks is the same in the case of stereotriblock copolymer. The presence of two crystalline domains is manifested from differential scanning calorimetry and thermogravimertric analysis. An interesting improvement in the mechanical properties is observed upon increasing the block length of PDLA/PLLA in the block copolymer system which may be ascribed to the confined crystalliztion of PCL in the microdomain structure. Additionally, the synthesized materials are found to support the adhesion of MG-63 (human bone osteoscarcoma) cells as determined from in vitro studies indicating their potential in bone repair and regeneration. Further, the block copolymers possessing superior mechanical properties are thermally processed by injection molding to fabricate representative orthopedic fixation devices such as cancellous and cortical bone screws. Eventually, the thermo-mechanical stability of the cancellous bone screw made from the synthesized block copolymer is presented as compared to that made from the commercial PLA at the sterilization temperature of the biomedical devices.