Twin chain polymer networks: Recent developments in hydrogels for Cultural Heritage

Abstract

Gels are transversal materials with key applications in multiple scientific and technological sectors, including the preservation of Cultural Heritage that is a fundamental drive for socioeconomic resilience. Recently, the new class of “twin-chain” (TC) polymer gel networks was developed, using freeze-thaw (FT) cycles on solutions of polyvinyl alcohol (PVA) with two different hydrolysis degree and molar mass. Taking advantage of polymer-polymer phase separation in the pre-gel solutions, a sponge-like, interconnected porosity is templated in the hydrogels during FT, which concurs to boost the cleaning capability of the gels versus soil and aged coatings that jeopardize paintings and other iconic artworks. This review covers the latest developments in this new class of gels, and their use in the conservation of works of art. The TC gels allowed time-effective restoration of masterpieces (paintings by Picasso, Pollock, Lichtenstein), which would have been risky and time-consuming with conventional restoration materials in wet cleaning. The review discusses gelation mechanisms, the partial replacement or decoration of PVA with non-toxic synthetic or bio-based polymers, the counterintuitive role of gels’ tortuosity in the cleaning process, and the upload of these gels with nanostructured cleaning fluids (microemulsions, micelles). Overall, the TC PVA hydrogels constitute an advanced tool to preserve Cultural Heritage and transfer it to future generations; moreover, they represent a class of sustainable soft matter materials with potential impact in several fields, spanning from detergency to the cosmetic, pharmaceutical and food industries, tissue engineering, and others.