Nonlinear rheology and nanostructural features of twin-chain polymer networks with controlled porosity

Hypothesis: Twin-Chain Networks (TCNs) are polyvinyl alcohol (PVA)-based cryogels with enhanced porosity. They include two PVAs undergoing a polymer–polymer phase separation in pre-gel solution, granting the formation of sponge-like networks after gelation. Gel structural and transport properties, affecting networks’ tortuosity, can be optimized for specific applications, such as the cleaning of Modern and Contemporary Art, by selecting polymer pairs with specific micro-segregation behavior in the pre-gel solution. Experiments: In this work, TCNs were obtained by using PVAs of increasing molecular weight as porogens. Pre-gel solutions and gels morphology were observed through Confocal Laser Scanning Microscopy (CLSM), while properties like elasticity, crystallinity and characteristic dimensions at the nanoscale were explored through rheology, Differential Scanning Calorimetry (DSC) and Small Angle X-ray Scattering (SAXS). The gels’ yielding behavior in the non-linear viscoelastic region was related to the crosslinks size/local concentration and the tortuosity at the nanoscale (obtained through Fluorescence Correlation Spectroscopy, FCS, measurements). Findings: TCNs pores size increases with the porogen molecular weight. Despite a clear difference in the gels porosity at the micron-scale, only minor structural differences emerged through SAXS, DSC and linear rheology analysis. Gels deformation in the non-linear regime, analyzed through the Sequence of Physical Processes (SPP) approach, unraveled, for the first time, the hidden nanoscale features determining gels behavior at yielding, clarifying the role of the different porogens during cryostructuration. The nature of gels’ physical junctions was found to be intertwined with the gels’ tortuosity at the nanoscale, eventually affecting the cleaning ability of this new class of materials.