Journal of Rheology, Vol.64, No.4, 991-1002, 2020
Thermorheological complexity of poly(vinyl alcohol)/borax aqueous solutions
Thermorheological behavior was examined for poly(vinyl alcohol) (PVA) and borax aqueous solutions. The PVA content was fixed to be approximately four times the entanglement concentration, and the borax concentrationC(borax)was varied in a wide range of 0-10.4 mM. In this system, borate ions yielded from the hydrolysis of borax can crosslink the PVA chains to form a reversible network. At lowC(borax) = 2.5 mM, the network strands are larger than the precursor chains. The stress relaxation is governed by the effective breakup, and the time temperature superposition (tTs) fails only slightly. In contrast, at highC(borx )>= 4.0 mM, the network strands are dense and each chain is divided averagely into several network strands. For this case, the stress relaxation is governed by chain relaxation activated by continuous ion dissociations, where tTs fails significantly. Analysis of the thermorheological behavior provides rich information on the temperature-dependent structural evolution. Particularly, analyzing the high frequency data attributed to the ion dissociation process indicates that the failure of tTs is owing to a reduction of the degree of gelation with increasingT, which leads to an acceleration of the chain relaxation with respect to the ion dissociation.