Surface induced order and dynamic heterogeneity in ultra thin polymer films: A 1H multiple-quantum NMR study

Proton multiple-quantum NMR techniques have been applied to study surface induced order and anisotropic dynamics in ultra thin films (upto about four monomer thick) of a flexible polymer, poly-dimethylsiloxane (PDMS). The 1H–1H residual dipolar coupling, and dynamic order parameter of the surface bound layer are estimated at different PDMS coverages. Results show that the thin PDMS films exhibit discrete dynamic layers with step-wise increase in their motional freedom with increasing film thickness. Dynamic heterogeneity is more pronounced at one monomer thick film, which is absent for sub-monomer coverage. These finding are attributed to the possible conformational changes of PDMS backbone.