Competing Effects of Amorphous and Crystalline Networks on the Mechanical Behavior of Poly(vinyl alcohol) Hydrogel

We investigate the effect of amorphous and crystalline networks on the mechanical behavior of poly(vinyl alcohol) physical hydrogels by comparing the rheological properties and flow-induced structural evolution for samples annealed at different temperatures with rheometers and in situ small-angle X-ray and neutron-scattering measurements. Four strain regions, namely, the linear (I), yielding (II), linear (III), and strain-hardening (IV) regions, are revealed in the tensile response. A critical annealing temperature 74 °C is observed, which corresponds to the structural transition from isolated crystal to crystalline network and the mechanical transition from quasi-linear to nonlinear response. For samples annealed below 74 °C, mechanical response depends on the deformation of the soft amorphous network, and the destruction in network connectivity plays a key role in the slight yielding. For samples annealed above 74 °C, the hard crystal network provides high strength and stiffness to the gels, resulting in loss modulus and Young’s modulus an order of magnitude higher than those annealed below 74 °C. In region II, the rupture of crystalline network leads to the more drastic yielding. For all samples, crystal disaggregation along the flow direction is found to be constrained by the secondary nucleus length. Moreover, the flow-induced recrystallization results in the strain-hardening behavior in region IV. These results reveal the role of soft and hard network structures in determining the mechanical performance of physical hydrogels.