Chain dynamics of a weakly adsorbing polymer in thin films

Thin films of weakly adsorbing poly(dimethyl siloxane) (PDMS) on porous alumina are examined with NMR fast field cycling (FFC) relaxometry and NMR transverse relaxometry. The longitudinal relaxation dispersion of polymer amounts corresponding to approximate monolayer coverage shows substantial deviation from the bulk and is characterized by a particularly weak temperature dependence. Thicker films, however, show relaxation behavior and temperature dependence more similar to the bulk polymer. Transverse relaxation times were found to cover a range of several orders of magnitudes for any sample investigated; their dependence on temperature is a function of the total amount of adsorbed polymer. While thick films see an overall increase of molecular mobility at higher temperatures, monolayer films are best characterized by the decreasing fraction of a short, i.e. relatively rigid, component. These effects are consistent with the concept of two regions, one in which chain dynamics deviate from bulk and another where chain dynamics are reduced but bulk-like, although chains inside each region may also experience motional heterogeneity.