Anisotropic self-diffusion in a hexagonally ordered asymmetric PEP-PDMS diblock copolymer studied by pulsed field gradient NMR

The self-diffusion of an asymmetric poly(ethylene-co-propylene)-b-poly(dimethylsiloxane) diblock copolymer in the hexagonally ordered state is investigated using pulsed field gradient NMR. The order-to-disorder transition (ODT) occurs at T-ODT = 192 degrees C A marked nonexponentiality of the echo attenuations is found, and coming from high temperatures, an increase of this nonexponentiality is observed. This is caused by the onset of anisotropic diffusion along the cylinder axes within the hexagonal order whose orientations are randomly (isotropically) distributed in the sample. The echo attenuation curves are fitted by assuming isotropically oriented grains of parallel cylinders with preferentially longitudinal diffusion along the cylinders and slow molecular propagation from cylinder to cylinder, perpendicular to the cylinder axes. The ratio of diffusivities parallel and perpendicular to the cylinders is of the order of 50 at room temperature, decreasing notably with increasing temperatures. The magnitude and activation energy of the diffusivity along the cylinder axes are between those of the corresponding two homopolymers with the same molar masses as the block copolymer.