A_xBA_x-type block-graft polymers with soft methacrylate middle segments and hard styrene outer grafts: Synthesis, morphology, and mechanical properties

Novel copolymers with controlled architectures can function as new building blocks for well-defined nanostructures on the basis of micro-phase separation, unlike conventional ABA triblock copolymers. A series of well-defined A _xBA_x-type block-graft copolymers consisting of soft middle segments (dodecyl methacrylate (DMA)) and hard outer graft chains (styrene (St)) were synthesized by ruthenium-catalyzed living radical block and graft polymerization. NMR spectroscopy and size-exclusion chromatography combined with multiangle laser light scattering confirmed the well-defined structure of the A_xBA_x block-graft copolymers with backbones and graft chains of controlled lengths. Transmission electron microscopy and transmission electron microtomography revealed a series of morphologies for the copolymers. Morphological changes were observed from PSt "honeycomb" cylinders to lamellae and poly(DMA) cylinders with increasing PSt-graft content, whereby the phase diagram was shifted significantly to lower volume fractions of the larger-number component (St) relative to those of the corresponding ABA triblock copolymers. More specifically, poly(DMA) cylinders were observed even before the St content reached 50wt%. The A_xBA_x and ABA copolymers with 17-30 wt% of St exhibited characteristics of a thermoplastic elastomer with tensile strengths of 1-6 MPa and elongations at break of 70-300%. These mechanical properties can be related well to the microphase structures of the A_xBA_x and ABA copolymers.