Self-recovery and fatigue of double-network gels with permanent and reversible bonds

Double-network (DN) gels subjected to cyclic deformation (stretching up to a fixed strain followed by retraction down to the zero stress) demonstrate a monotonic decrease in strain with time (self-recovery). Observations show that the duration of total recovery varies in a wide interval (from a few minutes to several days depending on composition of the gel), and this time is strongly affected by deformation history. A model is developed for the kinetics of self-recovery. Its ability to describe stress-strain diagrams in cyclic tests with various periods of recovery is confirmed by comparison with observations on several DN gels. Numerical simulation reveals pronounced enhancement of fatigue resistance in multi-cycle tests with stress- and strain-controlled programs when subsequent cycles of deformation are interrupted by intervals of recovery. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 438-453