Thermal Properties and Segmental Dynamics of Polymer Melt Chains Adsorbed on Solid Surfaces.

The glass transition of supported polystyrene (PS) and poly(2-vinylpyridine) (P2VP) thin films in the vicinity of the substrate interface was studied by using a nanoplasmonic sensing (NPS) method. This "nanocalorimetric" approach utilizes localized surface plasmon resonance from two-dimensional arrangements of sensor nanoparticles deposited on SiO-coated glass substrates. The NPS results demonstrated the existence of a high glass transition temperature ( T) along with the bulk glass transition temperature ( T ≈ 100 °C for PS and P2VP) within the thin films: T ≈ 160 °C for PS and T ≈ 200 °C for P2VP. To understand the origin of the T, we also studied the thermal transitions of lone polymer chains strongly adsorbed onto the substrate surface using solvent rinsing. Interestingly, the NPS data indicated that the T is attributed to the adsorbed polymer chains. To provide a better understanding of the mechanism of the T, molecular dynamics simulations were performed on a PS film adsorbed on hydrophobic and hydrophilic substrates. The simulation results illuminated the presence of a higher density region closest to the substrate surface regardless of the magnitude of the polymer-solid interactions. We postulate that the highly packed chain conformation reduces the free volume at the substrate interface, resulting in the T. Moreover, the simulation results revealed that the deviation of the T from the bulk T becomes larger as the polymer-substrate interaction increases, which is in line with the experimental findings.