(549f) Relaxation of Polymer Melts In Nanofilms At the Polymer-Solid Interface

It is well known that the relaxation of polymer chains in bulk melt is fast, e.g., on the order of milliseconds to seconds. However, the relaxation could be orders of magnitude slower for some polymer melts in nanometer-thick films on solid substrates. To date, the governing mechanism behind this phenomenon remains unclear. Such a slow relaxation at molecular level could cause certain properties of the polymer nanofilms to change with time after the polymer is applied on a solid surface. This is a serious concern for the long-term performance and the lifetime of the polymer thin films in various applications. Without understanding the thermodynamics and the kinetics governing the relaxation, it is impossible to rationally design more robust thin-film materials.

In this presentation, we report the experimental results indicating that the polymer-solid interfacial interaction, which does not have a bulk counterpart, plays a key role in both thermodynamic driving force and kinetics of the relaxation. Two perfluoropolyethers (PFPEs) with the same backbone and different endgroups, one polar and the other non-polar, have been studied. The relaxation of the nanometer-thick PFPEs on a hydrophilic solid substrate was characterized by the surface energy analysis via the contact angle measurement. For the PFPE with polar endgroups, the surface energy “relaxes” with time and the relaxation time constant, obtained from Kohlrausch-Williams-Watts  (KWW) model, is ten orders of magnitude higher than that of bulk polymer. However, for the PFPE with non-polar endgroups, the surface energy relaxation was not observed at all. Based on the experimental results, a molecular-level model has been proposed. According to this model, the observed relaxation is thermodynamically driven by the attractive interaction between the polar endgroups of the polymers and the polar sites on the solid substrate. The very slow kinetics of the relaxation has been attributed to the heterogeneity of the polymer-solid interaction at the interface and the cooperative nature of the molecular motions in the relaxation.