Crosslinked Borate HPG Equilibria and Rheological Characterization

Abstract

Abstract Viscoelastic properties of hydroxypropyl guar (HPG) crosslinked by borate ions were studied using steady shear and dynamic oscillatory shear experiments. Chemical equilibria involving boric acid, borate ions, and borate ions associated with cis-diol sites on the polysaccharide chains determine the number of crosslinks. These equilibria are functions of temperature and pH, and thus the rheology of the borate gels depend on temperature and pH. Dynamic oscillatory measurements on polymer solutions crosslinked by borax were performed over the temperature range from 10C to 65C, and the pH range from 6.35 to 9.5. The storage and loss moduli obey time-temperature superposition so that master curves can be constructed over 6 decades in frequency. Leibler, et al. have recently proposed a theory for the rheology of associating polymer systems. Following this theory, we have subtracted the solution moduli from the gel moduli and the resulting reduced moduli data follow a single time constant Maxwell model. Steady shear experiments were performed in capillary tubes at two different diameters to determine wall slip. The shear rates studied were such that the chains do not have sufficient time to relax completely. By analyzing the data from the two tubes, the gel was found to slip at the wall. Three different regions were seen in the stress - shear rate behavior of these gels. At low rates, the stress increases rapidly with shear rate. At intermediate shear rates, the stress remains constant and at high shear rates, the stress increases slowly with rate and the gel is found to slip in this region. The actual stress - shear rate behavior after accounting for slip and slip velocities were determined. Introduction Water soluble polymers can be crosslinked to form water-swellable gels using a number of crosslinking chemistries. Examples include covalently crosslinked gels used in controlled drug release or as soft contact lenses, gels formed by hydrogen bonding such as photographic gelatins, gelatin foods or agarose gels for electrophoresis, gels formed by hydrophobic interactions, or gels formed using inorganic metal ions as the crosslinking species in hydraulic fracturing operations. In this paper we present a study of the gelation of hydroxypropyl guar by borate ions. The various equilibrium reactions between borate ions and sugar residues are presented, followed by the dynamic oscillatory behavior and steady shear behavior of HPG crosslinked by borax gels. The surprising result is that the borate "gels" obey the laws of time-temperature superposition just like linear polymer melts or solutions. The superposition shift factors show an Arrhenius dependence on temperature which is in accord with the borate ion equilibria that governs crosslinking. P. 515^