pH-Induced Mechanical Transition in Polyion Complex Hydrogels: Shifting from Ionic Association to Hydrophobic Association

The introduction of monomers containing a hydrophobic group endows the polyion complex (PIC) hydrogels with enhanced mechanical strength and salt resistance without sacrificing their original functions. However, the synergistic mechanism between ionic interactions and hydrophobic associations as well as the relationship between network structure and performance remain unclear. In this study, we employed a model system: a PIC hydrogel composed of positively charged poly-(2-(diethylamino)-ethyl methacrylate) and negatively charged poly-(4-styrenesulfonic acid) to investigate the influence of ionic interactions and hydrophobic associations on their structure and mechanical behavior. Notably, we observed that pH can induce a mechanical transition in these PIC hydrogels from a stiff and tough state to a soft but still tough one. When the hydrogel is immersed in NaOH for a short time, the breakage of ionic bonds is expedited, resulting in a weakening of the ionic interactions between the polymer chains. However, increasing the immersion time further leads to an increase in the modulus and work of extension of the hydrogel. Through a systematic investigation of the network structure evolution during immersion in NaOH and the utilization of various hydrophobic monomers, it was discovered that the pH-induced mechanical transition can be attributed to a shift from ionic association to hydrophobic association in the gel network. Furthermore, it was found that the mechanical properties and the deformation capacity of the PIC hydrogels can be adjusted facilely through controlling the hydrophobicity of the side groups, which can further affect the pH-induced shape-changing behavior. This novel strategy of combining ionic association and hydrophobic association can be used to develop smart PIC hydrogels with multiple functions and outstanding performance.