Nonlinear Rheological Behavior of Dually Cross-Linked Elastomers on Start-Up and Cessation of Shear Deformation

Nonlinear shear behavior was examined for a representative dually cross-linked elastomer, poly(vinyl alcohol) (PVA)/Borax hydrogel having both permanent and transient cross-links, the former due to covalent bonds and the latter sustained by a complex of the hydroxy group and borate ion. The PVA chain backbone between the transient cross-links behaved as the transient network strand partially relaxing on thermal dissociation and/or mechanical breakage of the transient cross-links. For comparison, the nonlinear behavior was also examined for bulk SI(COOH)S block copolymer having glassy spherical domains of the S block as the permanent end-cross-links for the I blocks and the hydrogen bonds between carboxyl groups as the transient cross-links. The I block backbone between the carboxyl groups served as the transient network strands. Experiments revealed significant shear-softening of the PVA/Borax hydrogel even under a very slow and small shear deformation characterized with the Weissenberg number Wi = 0.2 and the total shear strain γ m = 0.03. Namely, the linear viscoelastic (LVE) responses of this gel vanished even under very mild shear conditions. The SI(COOH)S copolymer showed softening of a similar magnitude but under stronger shear conditions, Wi = 4.6 and γ m = 2.0. These nonlinear features can be related to the shear-induced breakage of the transient cross-links that results in anisotropy of the population of the network strands reformed in different directions as well as nonequilibrium motion of the strands connected to the broken transient cross-links. The difference between the PVA/Borax hydrogel and the SI(COOH)S copolymer, the nonlinearity much stronger for the former, could be related to the mobility of the transient cross-links (–OH/borate ion complex) along the PVA backbone and lack of the mobility of the carboxyl group along the I block backbone.