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 SiO2-coated glass substrates. The NPS results demonstrated the existence of a high glass transition temperature (T-g,(high)) along with the bulk glass transition temperature (T-g,T-bulk approximate to 100 degrees C for PS and P2VP) within the thin films: T-g,T-high approximate to 160 degrees C for PS and T-g,T-high approximate to 200 degrees C for P2VP. To understand the origin of the T-g,T-high, 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-g,T-high is attributed to the adsorbed polymer chains. To provide a better understanding of the mechanism of the T-g,T-high, 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-g,T-high. Moreover, the simulation results revealed that the deviation of the T-g,T-high from the bulk T-g,T-bulk becomes larger as the polymer- substrate interaction increases, which is in line with the experimental findings.