Abstract
The melt diffusion of polymers confined to nanoscale cylinders was investigated by molecular dynamics simulations and depth profiling experiments. In the simulations, entangled polymers are confined within long cylindrical pores having effective diameters (deff) from 0.4Ree to 1.7Ree, where Ree is the square root of the mean-squared end-to-end distance of the polymer in the absence of confinement. The local dynamics of polymers confined to cylinders exhibit anisotropic relaxations. Perpendicular to the cylindrical axis, monomer motion is suppressed by the adjacent wall, while motion along the cylindrical axis is faster relative to the bulk dynamics. These anisotropic relaxations are discussed in light of our prior studies showing that chain conformations parallel to the cylinder axis are elongated relative to the bulk conformation, whereas in the perpendicular direction the chain conformations are compressed. Furthermore, our previous simulations found that the number of entanglements per chain decreases as deff decreases. Here, the effects of confinement on local dynamics, chain size, and entanglement density are combined to calculate polymer diffusion (Drep,z) along the cylindrical pore according to the reptation model. The center of mass diffusion coefficients (DMSD,z) along the cylindrical pore were also determined using long simulation times. Finally, using elastic recoil detection, polymer tracer diffusion coefficients (Dexp) along the cylindrical nanopores were measured for deuterated polystyrene diffusing into membranes preinfiltrated with polystyrene. Relative to the bulk diffusion coefficients, the diffusion coefficients along the cylinder (Drep,z, DMSD,z, Dexp) systematically increase as the extent of cylindrical confinement increases (smaller diameter). Moreover, normalized Drep,z and normalize DMSD,z from simulations are in good agreement when deff/Ree > 0.5, while normalized Dexp is substantially smaller at all degrees of confinement investigated. These are the first side-by-side comparisons of simulations and experiments of polymer diffusion in cylindrical nanopores, and the implications of faster polymer diffusion along the cylinder and parallel to the confining wall are discussed.