Interplay between Stereocomplexation and Microphase Separation in PS-b-PLLA-b-PDLA Triblock Copolymers

Polymers with different tacticities or chiralities can self assemble into stereocomplex structure (sc-structure) which may further self-assemble into sc-crystals. This stereocomplexation behavior has a great potential in enhancing the crystallization and mechanical properties of polymers. However, less is known about its interplay with microphase separation (MS) in controlling the microscopic structures of block copolymers. In this work, we have designed and synthesized two series of PS-b-PDLA-b-PLLA (PS-b-PLLA-b-PDLA) triblock copolymers. By introducing a PDLA block into a strong segregation PS-b-PLLA system, we were able to explore the interplay between stereocomplexation, crystallization, and MS. The first series of triblock polymers have a fixed total molecular weight and a PLA weight fraction about 0.3, corresponding to a cylinder MS structure. By increasing the relative PDLA:PLLA composition approaching to unity within the PLA block, we found that the long period, L-0, of the MS structure decreases due to the formation of sc-structure. However, crystallization was completely suppressed under the strong confinement as no characteristics of sc-crystal could be detected by DSC and WAXS. In the second series, triblock copolymers with the same PS-b-PLLA block but a varying PDLA block were synthesized. With increasing the PDLA length, the MS structure undergoes a transition from cylinder to lamellae. Interestingly, because of the sc-structure formation, the long period of the lamellar phase only increases slightly with increasing PDLA length. Despite the confinement of MS, exclusive sc-crystals could be formed within the lamellae. Our results demonstrate that the confinement of MS structure has a strong impact on sc-crystallization and sterocomplexation within the PLA domains, which provide a new strategy for further adjusting the microstructure as well as various properties of PLA-based materials.