Chain Conformation And Dynamics Of Poly (Vinyl Alcohol) And Borax Complexing System

Complexation between poly(vinyl alcohol)(PVA)and borax in aqueous solution yields a dynamic poly electrolyte.Study of chain conformation of such a poly electrolyte is challenging due to the difficulty in separating different macromolecular species.In this study,we investigated conformation of dilute PVA chains complexed with borax in a wide concentration range between 0.02 and 78.6 mM using asymmetrical flow field-flow fractionation(AF4).We find that elution of the complexes is strongly influenced by the borax concentration owing to(1)the charged complex is repelled electrostatically by the charged bottom membrane of the AF4 channel,and(2)the Debye screening length reduces with increasing a charge density.The chain conformation of PVA-borax complexes depends on both polymer molar mass and borax concentration.At low borax concentrations of 0.02-0.5 mM,a transition from random coil to rod-like conformation is observed with increasing the chain size.At borax concentrations of 1 mM and higher,all chains become random coil owing to a reduced screening length of the electrostatic repulsion.Then we examined linear viscoelasticity(LVE)of semidilute non-entangled PVA and borax aqueous solutions:one borax molecule dissociates into two B(OH)3 molecules and two B(OH)4' ions,the latter can crosslink the PVA chains.Gelation was observed with increasing concentration of the borax in the solutions.Analysis of LVE with the reversible gelation model reveals that although the number density of crosslink:ing ions increases with increasing the concentration of borax,the fraction of the crosslinking ions decreases,which is attributed to intensified electrostatic repulsion when more sites of the PVA chains are occupied by the ionic crosslinkers.Finally,thermo-rheological behavior was examined for PVA and borax aqueous solutions.The PVA content was fixed to be?four times the entanglement concentration,and the borax concentration Cborax was varied in a wide range of 0 to 10.4mM.In this system,borate ions yielded from the hydrolysis of borax can crosslink the PVA chains to form a reversible network.At low Cborax=2.5mM,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 high Cborx? 4.0mM,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 thermo-rheological behavior provides rich information on the temperature-dependent structural evolution.Particularly,to analyze the high frequency data attributed to the ion dissociation process indicates that the failure of tTs is owing to a reduction of degree of gelation with increasing T,which leads to an acceleration of the chain relaxation with respect to the ion dissociation.