Durable superhydrophilic coatings via surface-initiated photopolymerization

The covalent attachment of polymers has emerged as a powerful strategy for the preparation of multi-functional surfaces. For this work, we describe new chemistries to engineer durable superhydrophilic polymer brush films that provide anti-fogging properties. We use both tribology to study the mechanical properties of the resulting coatings and use vibrational sum frequency generation (SFG) spectroscopy to study how the conformation of hydrophilic polymer brushes changes in response to environmental conditions, i.e., changes in humidity (in air) and upon exposure to liquid water. Three hydrophilic brushes were prepared on silicon oxide surfaces by surface-initiated reversible deactivation radical polymerization of containing cationic (quaternary ammonium), anionic (sulfonate), and zwitterionic (containing both) monomers. The average tilt angle of methyl groups was analyzed and used to deduce the chain conformations of the polymer brushes. In air, the brush films absorb water and swell with increasing humidity. This is accompanied by the rotation of interfacial polymer chains. The degree of water uptake and chain conformation vary with the nature of the charged hydrophilic moieties. The hydrophilic polymer brush surfaces appear to remain relatively dry except in near-condensation conditions. In water, the quaternary ammonium groups of cationic and zwitterionic brushes are aligned nearly parallel to the surface. The anionic brush chains appear to assume nearly random conformations in water.