Hydrogels Under Superchaotropic Control: Polyoxometalate Stabilization and pH-Responsive Crosslinking in Cellulose Ether Solutions

ChemRxiv

Abstract

Polymer-polyoxometalate (POM) systems represent an emergent class of functional hybrid materials. However, the pH-dependent stability of POMs limits their scope in water. We show here that the water-mediated, so-called superchaotropic, binding of α-Keggin POMs to a non-ionic biopolymer, hydroxypropylcellulose (HPC), i) selectively stabilizes superchaotropic POMs in water, and ii) enables pH-responsive HPC solutions and hydrogels. Raman and NMR spectroscopy revealed that binding to HPC sterically protects the superchaotropic PW12O403- and SiW12O404- against hydrolysis, extending their stability from acidic to near-neutral pH. In turn, POMs with higher charge, stronger hydration and thus without the ability to bind to HPC, such as H2W12O406-, PW11O397- and SiW11O398-, do not get stabilized. Cloud points, small angle neutron scattering and rotational rheology showed that pH-induced conversion from superchaotropic PW12O403- to non-superchaotropic PW11O397- switches HPC from a bound, crosslinked to an unbound, non-crosslinked state enabling pH-switchable viscosity and gel-sol transitions. Superchaotropic stabilization and pH-switching are proposed as general phenomena in superchaotropic POM/solute systems highlighting the potential of superchaotropicity in aqueous soft materials.

Authors 4

1. Vighnesh B. Lokare Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University

2. Amina Ledinic Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University

3. Nina Wehr Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University

4. Max Hohenschutz Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University