"Green" Poly(vinyl alcohol)/Starch based cryogels for the cleaning of works of art: Application, characterization, and investigation of the Amylose/Amylopectin structural role

Cultural Heritage assets are crucial to mankind, as they are drivers of welfare and economic improvement. If properly preserved, this patrimony can boost job creation, social inclusion, and cultural identity. Unfortunately, degradation processes inevitably threaten works of art. Colloids and material science have been providing effective solutions to preserve works of art in the last decades. In the specific case of cleaning of painted artworks, excellent results have been obtained using highly performing gels based on synthetic polymers like poly (vinyl pyrrolidone) (PVP) and poly (hydroxyethyl methacrylate) (pHEMA). However, there is still large room for the formulation of polymer networks that retain optimal cleaning ability but have higher eco-compatibility. In this perspective, we have developed and studied different biocomposite hydrogels based on poly(vinyl alcohol) (PVA) and rice starch (RS) obtained via freeze-thawing, with water as the only used solvent. The PVA/RS hydrogels have been extensively characterized from a morphological, rheological, and structural point of view and have been tested as cleaning tools on painted mock-ups with excellent outcomes, showing performances comparable with their state-of-the-art synthetic counterparts. Furthermore, the introduction of a biopolymer in the synthetic path improved the sustainability of the art cleaning formulations, while maintaining optimal and tunable mechanical behavior. Besides, the reduction of usage and disposal of materials based on synthetic polymers is an urgent and transversal need, and biocomposite PVA/starch-based systems could meet the requirements that different applications demand., being broadly tunable by simply varying the PVA:starch ratio in their formulation. Nevertheless, starch as a raw product comes with a high variety of features, especially regarding the composition of its polymeric portion (i.e., the amylose to amylopectin ratio), which is cardinal in determining the properties of the biocomposite systems. The investigation of the fundamental interactions between PVA, amylose, and amylopectin has therefore been deemed necessary to set a reliable base from which to start developing state-of- the-art materials, drastically reducing the usage of synthetic reagents without compromises in terms of performances. Said mutual interactions and their consequences have been investigated by means of direct laser imaging of fluorescently labeled systems, thermal analysis, and Small-Angle X-ray Scattering, coupling the results with rheological measurements and gel fraction trends to provide a preliminary theoretical framework, the aim of which is to support future developments of highly performing eco-sustainable materials.