Structural and molecular response in cyclodextrin-based pH-sensitive hydrogels by the joint use of Brillouin, UV Raman and Small Angle Neutron Scattering techniques

The response to pH variation of polymeric cyclodextrin-based hydrogels has been investigated by a multi-technique approach based on UV Raman and Brillouin light scattering (BLS) measurements together with Small Angle Neutron Scattering (SANS) experiments. The structural, viscoelastic and molecular modifications of the polymer brought about by the pH changes have been examined, over a spatial range going from mesoscopic to nanoscopic length-scale, by exploiting the complementary information of these three investigation methods. The data provide a picture where pH is the trigger promoting the change of the characteristic size of the hydrophilic pores in those hydrogels where the cross-linker has suitable structural and acid-base properties. The pH increase leads to the reinforcement of the polymer domains interconnections, thus providing an overall stiffer gel network on the length-scale probed by BLS. Structural changes of the polymer network upon pH rising are also witnessed by concomitant modifications of the Raman CH stretching bands. Raman data also indicate that the interactions of the solvent with polymer backbone affect the intermolecular ordering of water. This is evident especially at high pH where the destructuring of tetrahedral ice-like configurations is particularly significant. Such an effect might be ascribed to an increased exposition to the solvent of the ionic portions of the polymer surface.