Strain Hardening Behavior of Poly(vinyl alcohol)/Borate Hydrogels

Abstract

The large-amplitude oscillatory shear (LAOS) behavior of poly­(vinyl alcohol) (PVA)/borate hydrogels was investigated with the change of scanning frequency (ω) as well as concentrations of borate and PVA. The different types (Types I–IV) of LAOS behavior are successfully classified by the mean number of elastically active subchains per PVA chain (<i>f</i>_eas) and Deborah number (<i>D</i>_e = ωτ, τ is the relaxation time of sample). For the samples with Type I behavior (both storage modulus <i>G</i>′ and loss modulus <i>G</i>″ increase with strain amplitude γ, i.e., intercycle strain hardening), the critical value of strain amplitude (γ_crit) at the onset of intercycle strain hardening is almost the same when <i>D</i>_e > ∼2 (Region 3), while the value of Weissenberg number (<i>Wi</i> = γ<i>D</i>_e) at γ_crit is similar when <i>D</i>_e < ∼0.2 (Region 1). For intracycle behavior in the Lissajous curve, intracycle strain hardening is only observed in viscous Lissajous curve of Region 1 or in the elastic Lissajous curve of Region 3. In Region 1, both intercycle and intracycle strain hardening are mainly caused by the strain rate-induced increase in the number of elastically active chains, while non-Gaussian stretching of polymer chains starts to contribute as <i>Wi</i> > 1. In Region 3, strain-induced non-Gaussian stretching of polymer chains results in both intercycle and intracycle strain hardening. In Region 2 (∼0.2 < <i>D</i>_e < ∼2), two involved mechanisms both contribute to intercycle strain hardening. Furthermore, by analyzing the influence of characteristic value of <i>D</i>_e as 1 on the rheological behavior of PVA/borate hydrogels, it is concluded that intercycle strain hardening is dominated by strain-rate-induced increase in the number of elastically active chains when <i>D</i>_e < 1, while strain-induced non-Gaussian stretching dominates when <i>D</i>_e > 1