Influence of the a-Methyl Group on Elastic-To-Glassy Transition of Supramolecular Hydrogels with Hydrogen-Bond Associations

The molecular chemical structure has a decisive influence on physicochemical properties of polymer materials, which becomes more complex for hydrogels with dense associative interactions. Recently, we have developed a series of tough supramolecular hydrogels with robust hydrogen-bond associations, and the properties of gels severely depend on the composition of copolymers. Here, we examine the influence of the molecular structure on the mechanical and viscoelastic behaviors of these supramolecular hydrogels synthesized from copolymerization of acrylamide/methacrylamide and acrylic acid/methacrylic acid with a varied alpha-methyl group on the basic unit. The obtained four kinds of the as-prepared hydrogels exhibit distinct mechanical perform-ances and dynamics, evolving from elastic to glassy state with the increase in the number of alpha-methyl groups. Rheological dynamic spectra of these hydrogels follow the time-temperature superposition principle, which are used to elucidate the correlation between the chemical structure and the dynamics of gel materials. The incorporation of a hydrophobic alpha-methyl group effectively improves the chain rigidity and enhances the hydrogen-bond associations. Furthermore, the methyl group on the carboxylic acid unit strengthens the hydrogels more significantly than that on the acrylamide unit, which also results in distinct stability of the hydrogels in water. This comparative study discloses the effect of the alpha- methyl group on the dynamics of associative copolymers and the performances of supramolecular hydrogels, which should be informative to understand the structure-property relationship of other tough soft materials with associative interactions.