Correlation between collective and molecular dynamics in pH-responsive cyclodextrin-based hydrogels

UV Raman and Brillouin light scattering (BLS) experiments have been used in this study to explore the
complex phase change behavior occurring in pH-responsive polysaccharide hydrogels as a function of
temperature. Due to the different physical quantities measured by the two techniques, the joint analysis
of Raman and BLS spectra has provided an unprecedented large-scale characterization of the molecular
rearrangements and of the different kinds of hydrophilic and hydrophobic interactions that cooperate to
determine the phase transformation observed in these hydrogels during the heating of the gel. As the
main result, the analysis of the Raman and BLS spectra showed the existence of a correlation between
the local (molecular) and collective properties of the gels during the phase transformation undergone by
the system, which is markedly triggered by pH. The joint set of experimental results suggests a model
according to which the mechanism of pH dependence in the hydrogels under investigation is dominated
by the interactions involving the hydrophobic parts of the polymer skeleton, whereas the solvation process
observed under heating of the gels is driven by the progressive distancing of the polymer domains among
them, as monitored by the Brillouin sound velocity.