Hydrogels and Organogels from Partially Hydrolyzed Poly(vinyl acetates) and Boron-based Crosslinkers: Development, Characterization, and Application to the Conservation of Art

Abstract

Soft, peelable hydrogels and organogels formed from partially hydrolyzed poly(vinyl acetate)s and different crosslinkers have been developed, characterized, and evaluated for cleaning treatments on works of art. Partially hydrolyzed poly(vinyl acetate)s were crosslinked with either borax to form hydrogels in aqueous glycol ether solutions or with aryl-diboronic acids to form organogels in organic liquids. According to 1H nuclear magnetic resonance spectroscopy, the aggregates of short-chain glycol ethers were not affected by incorporation into the gel network. The effect of short-chain glycol ether aggregation on the gel network was also investigated by 11B nuclear magnetic resonance spectroscopy, small angle neutron scattering, and rheology. The stability, crosslinking, polymer conformation, and bulk physical properties of the hydrogels were dependent on the structure of the glycol ether.

The organogel systems crosslinked with benzene-1,4-diboronic acid and 4,4’-biphenyldiboronic acid were found to gel a wide range of organic solvents. The crosslinking and bulk physical properties were studied as a function of solvent and concentration of polymer and crosslinker using 1H NMR, fluorescence, and rheology. These organogels could be peeled off a surface easily, leaving no residue detectable by UV–vis spectroscopy.

The potential of these hydrogels and organogels for use in cleaning objects of cultural heritage was assessed: while the addition of glycol ethers did not substantially enhance the cleaning efficacy of the hydrogel (as measured by the removal of artificial soil from a paint layer), the organogels were able to soften and remove deteriorated coatings from works of art. A 2-ethoxyethanol organogel was used to remove oxidized varnish from a 16th century reliquary decorated with painted gold leaf, and an ethanol gel was used to remove solvent-resistant coatings from 16th and 18th century oil paintings. In both cases, these organogels were able to soften and remove coatings that could not be removed by traditional moistened-swab methods. Atomic force microscopy and scanning electron microscopy images of other delicate surfaces (i.e., acrylic paint or brass sheet) before and after cleaning showed that the gel cleaning method is gentle and can avoid the formation of scratches and pinholes.