Structure and mechanical properties of multiblock copolymers: toward the development of enhanced mechanical response materials

Abstract

Block polymers have attracted scientific interest for decades, and most studies have focused on the simplest molecular architectures: linear AB diblock and ABA triblock copolymers. Multiblock copolymers containing a large number of blocks are expected to have distinct microstructures and a mechanical response which is different from that of conventional diblock and triblock copolymers. This research addresses synthesis and characterization of poly(cyclohexylethylene)-polyethylene (CECECECEC) nonablock copolymers, poly(styrene-<italic>b</italic>-butadiene) (PS-PB) multiblock copolymers, and poly(lactide-<italic>b</italic>-butadiene) (PLA-PB) multiblock copolymers. CECECECEC nonablock copolymers having a large center C block were synthesized using sequential anionic polymerization followed by catalytic hydrogenation. The CECECECEC samples exhibited different morphologies with varying size of PE blocks. As the PE block size increased, the microstructure was transformed with the sequence of disordered homogeneous phase - lamellae with mixed phase of outer CECE blocks - layer-in-layer microstructure. Moreover, the secondary phase segregation of outer CECE blocks allowed tough mechanical behavior. PS-PB multiblock copolymers with alternating and random block sequences were synthesized using a combination of living anionic polymerization and polycondensation. Molecular characterization revealed the successful synthesis of the desired multiblock products through the proposed procedure. Structural analysis demonstrated a random bicontinuous-like morphology over a wide range of compositions, 0.69 ≤ <italic>f</italic>_PS ≤ 0.85. Tensile tests showed yielding followed by necking and an overall ductility that translates into much greater toughness than that typically found in glassy continuous SBS triblock copolymers. PLA-PB multiblock copolymers (0.5 ≤ <italic>f</italic>_PLA ≤ 0.9) were synthesized in a two-step procedure: PLA-PB-PLA triblock copolymers were prepared using ring-opening polymerization, followed by chain extension with the condensation reaction. Multiblock copolymer and homologous triblock materials exhibited nearly identical and well-ordered morphologies, in sharp contrast with the findings of PS-PB multiblock polymers. These results indicate a transition from classically ordered morphologies to a state of bicontinuous disorder for multiblocks containing <<italic>n</italic>> ≥ 10, where <<italic>n</italic>> is the average total number of blocks. In tensile tests, most PLA-PB multiblock copolymers exhibited dramatically enhanced mechanical properties compared to the corresponding LBL triblock copolymers. These results suggest that a multiblock copolymer strategy offers new possibilities to obtain unique microstructures and physical properties from many other combinations of polymers.