Chain Dynamics and Segmental Orientation in Polymer Melts Confined to Nanochannels

We study changes in the dynamics of polymer chains confined to cylindrical nanochannels within aluminum oxide membranes. Specifically, a proton time-domain NMR technique is used to assess the effect of transient wall contacts on the time-averaged orientational order of poly(butadiene) segments in melts with different molecular weights (MW). Previous work has evidenced that the weakly interacting polymer, residing in,similar to 100 mu m long, 20 and 60 nm wide channels, shows no significant confinement-related changes in the segmental (alpha) relaxation time and only weak (less than a factor of 2) changes in the micrometer-scale diffusivity. In the relevant temperature range above 340 K, we here use samples with pores oriented at different angles with respect to the main magnetic field to study the macroscopic anisotropy of segmental rotations and the effect of slower motions in regimes III and IV of the tube model up to the milliseconds time scale. We show that the pore walls exert a significant orientation effect on the chains, measured in terms of a time-averaged order parameter with a related length scale of one to a few nanometers, coexisting for high molecular weight (MW) inhomogeneously with bulk-like behavior in the pore center. Low MW with fewer than about 10 entanglements as well as low MW liquids exhibit a homogeneous response, with an overall residual orientation that represents a diffusively averaged quantity reflecting the pore geometry. We support our findings by a simulation model based upon one-dimensional curvilinear chain diffusion along the primitive path. The study is complemented by deuterium NMR experiments on a labeled poly(dimethylsiloxane) sample, in which strong surface contacts prevent full diffusive averaging.