Block Copolymer Self-Diffusion in the Gyroid and Cylinder Morphologies

Abstract

Forced Rayleigh scattering (FRS) and pulsed-field-gradient NMR have been used to measure the self-diffusion coefficient, D, of a poly(ethylene oxide)−poly(ethylethylene) diblock copolymer in the molten state. The copolymer contains 42% PEO by volume and has a total molecular weight of 4100 g/mol. Upon heating from room temperature the sample transforms from crystalline lamellae to hexagonal cylinders, and then to a bicontinuous cubic “gyroid” phase (with Ia3d space group symmetry), before finally disordering at 175 °C. FRS measurements were performed in the gyroid and cylinder phases, and NMR measurements in the gyroid and disordered states. Cylinder samples both with and without shear alignment were employed. A hysteresis loop permitted measurements of D in both cylinder and gyroid phases at the same temperature (60 °C). FRS decays from cylindrical samples were described by a sum of two exponentials. For the aligned samples, values of the diffusivity along (Dpar) and across (Dperp) the cylinders were ex...