Microrheological study of PVA–borax physical gel: effects of charge screening

The charge screening effects on the viscoelasticity and the gelation properties of the mixture poly(vinyl alcohol) (PVA) and borax transient physical hydrogel was studied by diffusing-wave spectroscopy microrheology (DWS) and classical rheometry. The critical gel point of PVA–borax hydrogel was demonstrated at the high frequency range according to the uniqueness criterion of Winter and Chambon (WC) and was found to decrease with increasing sodium chloride (NaCl) concentrations. Time-cure superposition strategy has been adopted to elucidate the scaling laws that govern the sol–gel transition of PVA–borax hydrogel. Then, the Rubinstein and Semenov (R–S) regime was established allowing the estimation of the size of the largest relaxed cluster R z near the gel point. The increase of the ionic strength within the PVA–borax network was found to induce a decrease of R z . The combination of these findings suggested that NaCl triggered a contraction of PVA–borax physical network as a result of the charge screening effect, leading to the enhancement of the associativity as well as the density of the hydrogel. This work is expected to provide an insight into the behavior of physical gels within biological systems with high ionic strengths. Graphical abstract