Restricted diffusion in the regime of the order-to-disorder transition in diblock copolymers: a field gradient NMR study

The self-diffusion of diblock copolymers of polystyrene/cis-1,4-isoprene and polyisoprene homopolymers is studied using field gradient nuclear magnetic resonance. The block copolymers are shown to go through their order-to-disorder transition (ODT) at experimentally accessible temperatures. Their phase state is determined using small-angle X-ray scattering. Diffusion in the ordered, lamellar phase is found to be spatially restricted. Fast motion is possible along the lamellae and within a grain. The crossing over the enthalpic barriers between styrene and isoprene domains takes place on a much longer time scale. The ODT does not lead to an abrupt crossover to unrestricted diffusion. Local order remains even above the ODT, and the barriers disappear only gradually.