Twin-Chain cryogels: probing the nanostructure evolution at freezing through Small Angle Neutron Scattering

Twin-Chain (TC) networks are cryogels containing two polyvinyl alcohols (PVAs), whose pre-gel mixtures undergo a polymer-polymer phase-separation, forming micro-domains that act as porogens during the freezing process. As a result, TC gels show a sponge-like morphology, fundamental to improve adhesion and fluid transport at the gel-substrate interface for applications ranging from the cleaning of artworks to cosmetics and drug delivery. While the effects of freezing on single-polymer, monophasic PVA solutions have been widely investigated through scattering techniques (e.g. Small Angle X-ray, Neutron and Light scattering, SAXS, SANS and LS, respectively), the gelation kinetics of PVA mixtures characterized by a liquid-liquid phase separation has not yet been explored. In this work, the behavior of TC solutions at freezing was investigated through SANS. The scattering signal, acquired over time during the cryostructuration process and at the end of gelation, showed the evolution of the nanoscale domains as PVA crystallites (the gels "physical crosslinks") and mesh-sizes formed. Our results show that TC gelation significantly differs from that of homogeneous PVA solutions, being generally slower and forming domains of different size. The process depends on the polymers concentration, molecular weight and the chain-chain interactions, altogether defining the systems phase-separation behavior at room and sub-zero temperatures.