Length Effects of Short Alkyl Side Chains on Phase-Separated Structure and Dynamics of Hydrophobic Association Hydrogels

Phase separation plays a crucial role in toughening hydrogels. Thus, regulating the phase-separation structure is vital to understanding the toughening mechanism in phase-separated hydrogels. Current synthesis strategies often provide limited control on phase-separated structures. In this work, a library of short-alkyl-side-chain-modified hydrogels is fabricated as model phase-separated hydrogels to investigate the length effects of short alkyl side chains on the phase-separated structure, apparent mechanics, and dynamics. Short-alkyl-chain-modified polymers undergo vapor-induced phase separation from highly concentrated solutions and coalesce into a well-connected polymer-rich phase. With increasing length of the side chains, the polymer-rich domain thickens due to enhanced hydrophobic interaction. Rheology suggests that longer alkyl side chains result in higher "glass"-transition temperatures and slower dynamics. However, by correlating the stretch rate and temperature dependence of both the small deformation properties (linear rheology) and the large deformation properties (tensile behavior), we find that regardless of the length of the side chain, hydrogels become tough and strong when the tensile test temperature approaches the glass-transition temperature or the stretch rate matches the relaxation time in the intermediate frequency regime. The strength and toughness of the gels obtained in this work are the combined effect of phase separation and glass transition. This work sheds light on the design principles for the mechanical elements in phase-separated hydrogels.