Interdiffusion at the bilayer polymer interface: Evidence for reptation

Neutron reflection is used to study the interdiffusion in equal molecular weight polystyrene bilayer melts with a spatial resolution of 1nm. Interfacial widths and concentration profiles at the bilayer interface are obtained for annealing times up to and beyond the time (reptation time, τd) in which the molecule has moved by its own length. For annealing times t < τd, the reptation model predicts a mean square displacement of monomers whose time evolution is a power law. The detailed profile is expected to exhibit a discontinuity (sharp gradient) of density at the interface between the two polymers. For relatively light weight polymers of M ∼ 233,000, the mean square displacements of monomers is in general agreement with reptation predictions; the discontinuity at the interface is only observed when the molecular weights are large, M ∼ 1,000,000. The discontinuity is present even for low concentrations of the deuterated polymer where isotopic slowing down effects are negligible, thus confirming the sharp gradient as being due to reptation. 19 refs., 5 figs