A_XBA_X-Type Block-Graft Polymers with Soft Methacrylate Middle Segments and Hard Styrene Outer Grafts:Synthesis,Morphology,and Mechanical Properties

摘要

Novel copolymers with con trolled 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 chro matography 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 con tent,whereby the phase diagram was shifted significantly to lower volume fractions of the larger-number compo nent(St)relative to those of the corre sponding ABA triblock copolymers.More specifically,poly(DMA)cylin ders 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 ther moplastic elastomer with tensile strengths of 1-6MPa and elongations at break of 70-300%.These mechani cal properties can be related well to the microphase structures of the A-xBA_x and ABA copolymers.