Photo-crosslinking and surface-attachment of polyvinyl alcohol nanocoatings by C,H insertion to customize their swelling behavior and stability in polar media

The hydrophilic polymer polyvinyl alcohol (PVA) is a well-known coating material with interesting properties. However, PVA coatings swell in polar solvents or dissolve completely, most likely leading to problems with layer stability. We report a successful strategy to limit swelling and covalently attach PVA coatings to a substrate simultaneously, based on C,H-insertion crosslinking. For this purpose, a series of benzophenone-modified PVA (PVA-BP) with different degrees of benzophenone-modification chi(BP) are synthesized. PVA-BP is spin-coated on silanized silicon wafers as a nanoscopic film and its photoreactivity, stability and swelling behavior are investigated. The crosslinking progress is monitored by the gel content Y and the equilibrium degree of swelling EDS. The results show that PVA-BP is capable of crosslinking and surface adhesion when irradiated with UV light. UV/Vis spectroscopy reveals an isosbestic point at 240 nm and a decrease in the absorption maximum of the carbonyl pi-pi* transition at 298 nm during photoreaction in the film, indicating the reactive conversion of BP. The crosslinking kinetics are studied by applying the percolation theory to Y. The results show that increasing chi(BP) accelerates the crosslinking reaction until a threshold is reached at which further modification leads to predominantly unproductive photoreactions. Swelling of PVA-BP can be significantly reduced by increasing chi(BP) and crosslinking time. PVA-BP with chi(BP) = 5% exhibits an EDS value of 8.4 +/- 2.0 after 1 min and 2.4 +/- 0.1 after 30 min of irradiation, indicating that swelling can be successfully adjusted by crosslinking. In conclusion, modification of PVA with benzophenone is an effective method for simple photochemical crosslinking to limit swelling and improve surface adhesion of PVA coatings.