Dynamics and conformation of polymer chains in a porous medium

Conformational and transport properties of polymer chains in a porous medium are studied using computer simulations. Two types of links between the consecutive nodes of the self-avoiding walk (SAW) chains are considered: (i) constant bond and (ii) flexible (fluctuating) bond. In addition to normal diffusive motion for the global transport of chains, we observe unusual nondiffusive transport phenomena such as subdiffusive power-law dependence of the rms displacement (${\mathit{R}}_{\mathrm{rms}}$) of the chain with time (t), i.e., ${\mathit{R}}_{\mathrm{rms}}$=${\mathit{At}}^{\mathit{k}}$ with an exponent k less than 1/2 at low porosity. In two dimensions, we find that the power-law exponent k is nonuniversal as it depends on the barrier concentration (${\mathit{p}}_{\mathit{b}}$) and the chain length (${\mathit{L}}_{\mathit{c}}$) with the magnitude of k≃0.0-0.5. The chains with flexible bonds exhibit better relaxation with the value of k slightly higher than that for the chains with constant bonds at low porosities. The subdiffusion constant A shows a nonuniversal power-law dependence on the chain length, A∼${\mathit{L}}_{\mathit{c}}^{\mathrm{-}\mathrm{\alpha }}$, with α≃0.28-1.85 for chains with constant bond, and 0.33-1.27 for chains with flexible bonds. A similar but less pronounced effect of impurity barriers is also observed in three dimensions. The radius of gyration of the chains is affected by the presence of barriers; characteristics of the crossover from a SAW to collapse conformations are reported, along with the anomalous conformation as a function of barrier concentration. © 1996 The American Physical Society.