A new class of hydrogels: PVA-based Twin-Chain Networks for the cleaning of Modern and Contemporary Art

Untypical artistic techniques, fleeting materials and tridimensional surfaces make the artworks of the last century impermanent and delicate, if compared to traditional, flat easel paintings. As a result, the cleaning of Modern and Contemporary paintings is a very delicate process: the removal of dirt or grime deposited on their surface can be challenging. Traditional cleaning techniques were found to be unsuitable for this purpose: free solvents spread on the surface and swell the paint layers, water-based fluids can leach the paint components, while confining systems like traditional gels are too rigid, and/or leave residues on the surface. Poly(vinyl alcohol) (PVA)-based cryogels can meet the demand for new, science-driven, materials for restoration. This class of hydrogels, obtained through a freeze-thawing (FT) process, exhibits unique properties, such as high water retentiveness, free water content and adaptability to rough surfaces, interconnected porosity, structural cohesion and ease of handling/removal. Cryogels can be loaded with cleaning fluids, or just be swollen in water, and then placed in contact with the surface to clean: the free water inside the network allows the detaching of dirt, that is eventually trapped in the polymer matrix. The characteristics of PVA cryogels can be tailored by adding a second polymer that acts as a semi-interpenetrating agent, and/or varying the number of FT cycles. The chemical nature of polymer chains, and their molecular weight, also affect the final gel structure. As a result, the gel porosity, the rheological behavior and the dynamics of polymer chains embedded in the network are expected to change. Twin-Chain Polymer Networks (TC-PNs) were obtained by mixing two PVAs with higher (H-PVA) and lower (L-PVA) molecular weight and hydrolysis degree. Despite the very similar structure of the two polymers, a liquid-liquid phase separation occurred in the pre-gel solution: L-PVA chains are expelled from the H-PVA continuous phase and form spherical blobs, which act as porogens during cryostructuration. Confocal and Scanning Electron Microscopies revealed the sponge-like structure of TC-PNs, with micron-sized and interconnected pores. Small Angle X-ray Scattering and Differential Scanning Calorimetry clarified details about the structure at the nanoscale, while Fluorescence Correlation Spectroscopy (FCS) revealed the polymers dynamics. On the other hand, rheology data suggested that the semi-interpenetrating polymers can alter the gelation process, acting also as structuring agents. The diffusion of cleaning nano-structured fluids (NSFs) through the gels matrix was investigated through FCS: polymer-surfactant interactions and gels porosity were proven to influence the diffusive components. Overall, TC-PNs exhibited unprecedented cleaning performances, unconceivable with traditional methods. TC-PNs were successfully used to treat two Jackson Pollock’s and one Pablo Picasso’s masterpieces (Peggy Guggenheim collection, Venice), bringing back the paints original hue and brightness.