Experimental measurement of coil-rod-coil block copolymer tracer diffusion through entangled coil homopolymers

The diffusion of coil-rod-coil triblock copolymers in entangled coil homopolymers is experimentally measured and demonstrated to be significantly slower than rod or coil homopolymers of the same molecular weight. A model coil-rod-coil triblock was prepared by expressing rodlike alanine-rich α-helical polypeptides in E. coli and conjugating coil-like poly(ethylene oxide) (PEO) to both ends to form coil-rod-coil triblock copolymers. Tracer diffusion through entangled PEO homopolymer solutions was measured using forced Rayleigh scattering at various rod lengths and coil molecular weights for the tracer, and various concentrations for the coil homopolymer solutions. For rod lengths, L, that are close to the entanglementh length, a, the ratio between the diffusivity of a triblock and the diffusivity of a coil homopolymer of the same molecular weight decreases monotonically and is only a function of L/a, in quantitative agreement with previous simulation results. For large rod lengths, diffusion follows an arm retraction scaling, which is also consistent with previous theoretical predictions. These experimental results support the key predictions of theory and simulation, suggesting that the mismatch in curvature between rod and coil entanglement tubes leads to the observed diffusional slowing.