Role of the Interfacial Interactions from an Adjacent Wall on Polymer Diffusion

The tracer diffusion of deuterated polystyrene (dPS; $M_{\mathrm{n}} = $23 - 1866 kg/mol) with a thickness (l) is measured diffusing away from hydroxyl-terminated, phenyl-terminate, and PS-grafted silicon substrates. For a hydroxyl-functionalized substrate, short polymer chains ($M_{\mathrm{n}} = $23 and 49 kg/mol; $l$ \textgreater \textgreater $R_{g})$ exhibit a diffusion coefficient that is comparable to bulk PS, whereas long polymer chains ($M_{\mathrm{n}} = $532 and 1866 kg/mol; $l$ \textless $R_{g})$ are significantly slower than the bulk case. This slowing down is consistent with the observation by Zheng et al. [1]. In particular, bimodal diffusion coefficient was observed for intermediate molecular weight ($M_{\mathrm{n}} = $168 kg/mol;$ l$ $\sim$ $R_{g})$. For phenyl-functionalized and PS-grafted substrates, no significant differences in the diffusion coefficients are observed although long polymer chains showed a moderate slowing down. These experiments demonstrate that the polymer diffusion of thin, confined films ($\sim$ $R_{\mathrm{g}})$ away from the substrate is determined by the friction due to surface-monomer contacts, and is sensitive to the chemical state of the substrate, providing a new insight into a role of the interfacial interactions on polymer dynamics. \\[4pt] [1] X. Zheng, et al., \textit{Phys. Rev. Lett}., 74, 407 (1995).