Cosolvent Gel-like Materials from Partially Hydrolyzed Poly(vinyl acetate)s and Borax

Abstract

A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. (11)B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.

Poly(vinylalcohol)-borate hydrogels with improved features for the cleaning of cultural heritage surfaces

“…The most evident difference is the drastic decrease of FWI in the formulations including the organic solvent, in strict agreement with results reported in the literature [14,18,19,25]. In the concentration range here investigated,…”

“…In Figure 4 the mechanical spectra of two formulations with different composition of liquid phase are shown and it is evident that dispersions containing acetone display much stronger elastic behavior than formulations with water only. This effect has been interpreted [14,20] by means of We may therefore conclude that the increased stiffness of the gels containing acetone, with constant PVA/borax ratio, is due to a larger amount of cross-links in comparison with the gels containing only water.…”

“…Suggestions have been advanced about the decrease of the FWI induced by the presence of an organic content in the liquid, as being the result of a different (increased) structuring of the water in the system [14,19]. Different types of dynamic polar interactions have been shown to develop between the molecules in mixtures of acetone and water [30] and it appears that as a result of these interactions in the gel environment more water becomes bound to the polymer strands.…”

“…PVA-borate hydrogels exhibit viscous behavior at a long time scale (low frequency), at which the network has sufficient time to rearrange (or disentangle) and elastic behavior at short time scale (high frequency), at which crosslinks cannot dissociate [4]. Several studies report that the increase of borate ions concentration contributes to the dispersion strength less than the polymer molecular weight and concentration [14,15], which therefore appear to be the dominant factors contributing to the elastic behavior and stability of the dispersions.…”

“…These preparations, based on poly(acrylic acid) as a gelator, minimize the negative effects related to the penetration of liquid, but some residues can remain on the surface after the cleaning process [17]. PVA/borax hydrogels, on the other hand, show mechanical features suitable for easy removal by peeling from surfaces, without need of strong mechanical or chemical actions, and have been therefore proposed for cleaning purposes [14,[18][19][20]. Water/ cosolvent mixtures can be embedded into these high viscosity polymeric dispersions, making them able to soften and swell water-insoluble materials, in a way similar to that proposed in a study concerning the solubilization of hydrophobic substances by hydrotropes in water [21].…”