ISSN 0965545X, Polymer Science, Ser. A, 2014, Vol. 56, No. 2, pp. 205–210. © Pleiades Publishing, Ltd., 2014. Original Russian Text © S.A. Dubrovskii, A.N. Zelenetskii, S.A. Uspenskii, V.N. Khabarov, 2014, published in Vysokomolekulyarnye Soedineniya. Ser. A, 2014, Vol. 56, No. 2, pp. 206–212. 205 INTRODUCTION Sodium hyaluronate is the sodium salt of hyalu ronic acid, a highmolecularmass biopolysaccharide of the glycosaminoglycan family. A molecule of hyalu ronic acid is built of regularly alternating disaccharide units composed of residues of Dglucuronic acid and NacetylDglucosamine. In disaccharide units, ami nosaccharide is linked to Dglucuronic acid via β1,4 glycosidic bonds, while the disaccharide units are con nected via β1,3glycosidic bonds [1]. The carboxyl groups of hyaluronic acid impart it with polyelectro lyte properties, so that, in aqueous solutions, the acid is charged negatively. Note that, if a macromolecule occurs in the form of the sodium salt, almost all disac charide units are charged. The rheological properties of sodium hyaluronate solutions are of interest for the development of ideas about many biological processes. For example, it has been proposed that they determine the mechanical properties of synovial fluid and the study of these properties is important for gaining insight into the mechanisms of joint lubrication and diseases of joints. The rheological properties of sodium hyaluronate solutions have been the subject of many studies [2– 10]. As is shown in [3, 5–10], the viscosity values of aqueous solutions of this polymer strongly depend on shear rate . In a wide range of values, viscosity decreases with an increase in shear rate. However, at very low shear rates, which depend on the molecular mass and concentration of the polymer, Newtonian behavior is observed. The rheology of hyaluronic acid solutions is extremely sensitive to the presence of pro tein admixtures in solution, which lead to the aggrega tion of polymer chains (formation of transient physi cal crosslinks, that is, gels) [3, 6]. Aggregation is prac tically absent in the case of hyaluronic acid of the bacterial origin and may be distinct in the case of ani mal hyaluronic acid. Proteinfree solutions of sodium hyaluronate in a buffer salt solution simulating physi ological conditions are viscoelastic liquids featuring behavior typical for solutions of linear polyelectrolytes at a high concentration of the salt [6]. The addition of borax to the solution of sodium hyaluronate makes it possible to obtain 10 Bcontaining pharmaceutical preparations that show promise for the neutroncapture therapy of tumor diseases [11]. These preparations are complex compounds formed via the wellknown reaction of sugar binding with borate, a reaction that is in wide use in the analytical chemistry of boron and sugars [12]. The existence of hyaluronic acid complexes with borax has been con firmed in IR studies [11]. These complexes are stable γ · γ · Effect of Borax Additives on the Rheological Properties of Sodium Hyaluronate Aqueous Solutions S. A. Dubrovskii a, *, A. N. Zelenetskii b , S. A. Uspenskii c , and V. N. Khabarov c a Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia b Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, Profsoyuznaya ul. 70, Moscow, 117393 Russia c Martinex Research Centre, Vtoraya Vladimirskaya ul. 12, korp. 3, Moscow, 111123 Russia *email: sd@chph.ras.ru Received July 5, 2013; Revised Manuscript Received October 11, 2013 Abstract—The rheological properties of sodium hyaluronate aqueous solutions are studied, and the effect of borax additives on them is investigated. It is shown that, at low concentrations, sodium hyaluronate behaves as a typical linear polyelectrolyte in the limit of a high concentration of the salt in both a 0.1 M NaCl aqueous solution and a saltfree solvent. The addition of 1 mole of borax per basemole of the polymer to the solution of sodium hyaluronate significantly decreases the specific viscosity of the solution if no salt is added and has practically no effect on the viscosity of the solution in 0.1 M NaCl. The viscosity of a semidilute solution of sodium hyaluronate without the added salt decreases as the shear rate is increased in the range 1.5–656 s –1 . With an increase in temperature, viscosity decreases and its dependence on shear rate becomes less pro nounced. The same effect is exerted by small amounts of borax. The properties of saltfree solutions are explained by the presence of admixtures of lowmolecularmass ions in them that screen the Coulomb repul sion of charges linked to sodium hyaluronate chains, and the effect of borax may be rationalized by the screening effect of ions resulting from the hydrolysis of borax. DOI: 10.1134/S0965545X14020047 RHEOLOGY