Stress Relaxation and Creep Response of Glassy Hydrogels with Dense Physical Associations

Various glassy hydrogels are developed by forming dense physical associations within the matrices, which exhibit forced elastic deformation and possess high stiffness, strength, and toughness. Here, the viscoplastic behaviors of the glassy hydrogel of poly(methacrylamide-co-methacrylic acid) are investigated by stress relaxation and creep measurements. We found that the characteristic time of stress relaxation of the glassy gel is much smaller than that of amorphous polymers. The varying hydrogen bond strength leads to a broad distribution of structural activation energies, which in turn affects the range of characteristic time. In the presence of water, the weak hydrogen bond associations are easily disrupted under applied strain, enhancing segmental mobility and reducing relaxation time in the preyield regime, while in the postyield regime, the relaxation time increases slightly since the chain stretching increases the energy barrier. In creep tests, the creep strain rate accelerates at the initial stage due to stress-activated segments and then decelerates as chains are extensively stretched. The stress required for structural activation during creep is much lower than the Young's modulus of the gel, reflecting the poor structural stability. To further analyze the underlying mechanism of the glassy gel, a micromechanical model is established based on an extension on shear transformation zone theory. By incorporating a state variable for hydrogen bond density, this model can capture the intricate mechanical responses of glassy gels. Our findings reveal that glassy hydrogels are far from the thermodynamic equilibrium state, exhibiting rapid segment activation under external loading. This work provides insights to the dynamics and structural stability of glassy materials and can promote the design and applications of tough hydrogels.