Switchable Chaotropicity: Structure of polyoxometalate-cellulose ether solutions and gels under UV

Nanometer sized ions with low charge density, such as polyoxometalates (POMs), bind to non-ionic hydrated matter driven by the chaotropic effect in order to shed their weakly bound hydration water. In solutions of the non-ionic cellulose ether hydroxypropylcellulose, chaotropic POMs bind to the polymer at millimolar concentrations, whichs lead to an interplay of electric charging and physical crosslinking of the polymer chains and provides a simple gelation pathway for cellulose ethers. Here, we use the photocatalytic POMs PW12O40(3-) and SiW12O40(4-), in conjunction with HPC to produce highly viscous solutions with UV-responsive rheological properties. Upon UV-irradiation, the POM undergoes a reduction reaction with alcohol groups of HPC, which makes the charge of the POM increase leading to its stronger hydration, weaker binding with HPC and a decrease in crosslinking and viscosity. The POM is re-oxidized upon exposure to atmospheric O2, which leads to an increase in viscosity. We request SANS-beamtime, in order to investigate the effects of UV-irradiation on the solution structure of HPC in the presence of photocatalytic POMs. We will probe systematically the form factor of the HPC-polymer in dilute solution and the correlation length in entangled solution in the presence of photoreduced or re-oxidized POM in order to understand the molecular mechanism behind this concept of "switchable chaotropicity".