Self-diffusion and constraint release in isotropic entangled rod-coil block copolymers

Understanding dynamic relaxation mechanisms in self-diffusion and constraint release processes of rod-coil block copolymers is important for many technological applications that employ neat melts or concentrated solutions. Using a model system composed of poly(alkoxyphenylenevinylene) rods and polyisoprene coils, reptation theories of entangled rod-coil block copolymers are investigated in the isotropic melt state. Self-diffusion was measured by forced Rayleigh scattering using a red laser line and a new blue photoswitchable dye that allow operation above the bandgap of most semiconducting polymers. In contrast to previous tracer studies where the diffusion of rod-coils through a coil homopolymer matrix is slowed relative to coil homopolymers because of a mismatch in the curvature of the rod and coil entanglement tubes, slowed diffusion is only present in self-diffusion measurements above a critical molecular weight. An activated reptation mechanism with constraint release is proposed as a modification to the description of entangled rod-coil block copolymer dynamics, where the slowing occurs when the time scale of rod block reptation is faster than the reorganization of the surrounding entanglement tube. This mechanism is supported by additional tracer diffusion experiments on polyalanine-b-poly(ethylene oxide) diblocks in aqueous entangled poly(ethylene oxide) matrix solutions and Kremer-Grest simulations where the matrix molecular weight is varied. The slowing of tracer diffusion in rod-coil block copolymers relative to coil homopolymers is significantly weaker for smaller matrix polymers, confirming the proposed constraint release effects.