Optimization of the property profile of poly-L-lactide by synthesis of PLLA-polystyrene - block copolymers

Diblock and triblock copolymers of poly-L-lactide (PLLA) and polystyrene (PS) were synthesized and the mechanical properties of these copolymers studied. Free radical polymerization of styrene in the presence of 2-mercaptoethanol as functional chain transfer agent produced mono-functionalized PS-blocks which were used as macroinitiators in the subsequent ring opening polymerization (ROP) of L-lactide to produce the diblock copolymers. Furthermore a --bishydroxyl functionalized PS-block was synthesized by RAFT, which was then engaged as bifunctional initiator for the ROP of L-lactide to provide the triblock copolymers PLLA-PS-PLLA. Through the copolymerisation and high molar masses, it was possible to achieve an improved mechanical property profile, compared with pure PLLA, or the analogous blends of PLLA and PS. A weight fraction of PS of 10-30% was found to be the optimal range for improving the heat deflection temperature (HDT), as well as mechanical properties such a s ultimate tensile strength or elongation at break.