Highly Durable Antifogging Materials Based on Polysilsesquioxane with Double Hydrophilic Groups: Effect of Bridged Tetraethylene Glycol Chains in Polysilsesquioxane Films

To investigate the effect of introducing a hydrophilic and flexible tetraethylene glycol chain into a polysilsesquioxane (PSQ) antifogging film, a poly[3-(2,3-dihydroxypropoxypropyl)-co-tetraethylene glycol]silsesquioxane (PSQ-Diol/EG) film was prepared via the sol-gel reaction of 3-glycidyloxypropyltrimethoxysilane (GPTMS) and tetraethylene glycol bis(triethoxysilylpropyl)ether (BTESP-EG). Their antifogging property, mechanical property, and durability were investigated. First, poly(glycidyloxypropyl-co-tetraethylene glycol)silsesquioxane (PSQ-Gly/EG) was synthesized via the sol-gel reaction of GPTMS and BTESP-EG. Thereafter, to prepare the PSQ-Diol/EG film, PSQ-Gly/EG was coated onto a glass substrate, and the ring-opening reaction of epoxy groups was conducted by immersing the film in a 0.5 mol/L H2SO4 solution. The PSQ-Diol/EG films exhibited a water uptake (WU) of 15-19%, similar to that of the poly[3-(2,3-dihydroxypropoxypropyl)]silsesquioxane (PSQ-OH) film (12-19%). The transparency of the PSQ-Diol/EG film was maintained even after exposure to water vapor because of the absorption of water by the PSQ-Diol/EG film. The PSQ-Diol/EG film exhibited a high scratch resistance of 2.4-3.5 determined by nanoindentation, comparable to that of the PSQ-OH film (2.7-3.6). In the durability test, the WU of the PSQ-OH film slightly decreased during repetitive changes in the relative humidity (RH) between 95 and 20% at 30 °C, and cracks were formed on the surface after leaving the film to stand at a high RH of 95% at 50 °C. Contrarily, the PSQ-Diol/EG film did not exhibit a decrease in WU and crack formation on the surface under the same conditions. The introduction of hydrophilic and flexible tetraethylene glycol chains into the PSQ antifogging film was effective in improving the durability while maintaining the antifogging property and mechanical strength. The polycondensation reaction of residual silanol groups observed for the PSQ-OH film was probably suppressed in the PSQ-Diol/EG film because of the space effect of the tetraethylene glycol units keeping the silanol groups away from each other.