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(dime thylsiloxane) diblock copolymer in the hexagonally ordered state is investi gated using pulsed field gradient NMR. The order-to-disorder transition (OD T) occurs at T-ODT = 192 degrees C A marked nonexponentiality of the echo a ttenuations is found, and coming from high temperatures, an increase of thi s nonexponentiality is observed. This is caused by the onset of anisotropic diffusion along the cylinder axes within the hexagonal order whose orienta tions are randomly (isotropically) distributed in the sample. The echo atte nuation curves are fitted by assuming isotropically oriented grains of para llel cylinders with preferentially longitudinal diffusion along the cylinde rs 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 not ably with increasing temperatures. The magnitude and activation energy of t he diffusivity along the cylinder axes are between those of the correspondi ng two homopolymers with the same molar masses as the block copolymer.