Effect of the molecular weight on the crystallinity of PCL-b-PLLA di-block copolymers

A systematic study on the synthesis and characterization of di-block copolymers based on aliphatic polyesters such as poly(l-lactic acid) (PLLA) and poly(ɛ-caprolactone) (PCL), performed varying both the length of the blocks as well as the relative content of each block in the copolymers, is reported. The block length and the molecular weight of each synthesized PCL-b-PLLA di-block copolymer were analysed by nuclear magnetic resonance spectroscopy. The influence of the block length and of the amount of each block on the thermal properties and the morphology, was evaluated by differential scanning calorimetry and by small angle X-Ray scattering experiments. In particular, the correlation between the molecular weight of each block and its amorphous and/or crystalline structure was obtained, evidencing that the crystallization of the PLLA block was not influenced by the presence of PCL and depends mainly on its molecular weight but the crystallization of PCL is strongly interfered by the crystallization of PLLA. In particular PLLA blocks are amorphous for short lengths (≤672 g/mol, that means ≤9.3 LA repeat units) and start to crystallize for molecular weight ≥964 g/mol, that means ≥13.4 LA repeat units.