Associative and Entanglement Contributions to the Solution Rheology of a Bacterial Polysaccharide

We report the viscosity of semidilute solutions of a bacterially synthesized polysaccharidea partially deacetylated poly-N-acetylglucosamineas measured by microrheology. This polymer, commonly called polysaccharide intercellular adhesin (PIA), is synthesized by Staphylococcal strains; it is a principal component of the biofilms of these bacteria. We show that the concentration-dependent viscosity of PIA at a pH in which it is associated can be predicted using the Heo–Larson equation for entangled polymers [J. Rheol. 2005, 49 (5), 1117−1128], if the molecular parameters of the equation are measured in its associated state. This agreement is consistent with PIA adopting a concentration-dependent scaling of the viscosity that is dominated by entanglements and intermolecular associations, as described in the theory of Rubinstein and Semenov [Macromolecules 2001, 34 (4), 1058−1068]. The zero-shear specific viscosity, η_sp, measured in the concentration range, c_PIA = 0.1–13 wt %, scales as η_sp ∼ c_PIA^1.27±0.15 up to an entanglement concentration, c_e = 3.2 wt %, after which η_sp ∼ c_PIA^4.25±0.30. In the presence of urea, a known disruptor of associations, these scaling shifts to η_sp ∼ c_PIA^1.02±0.2 and η_sp ∼ c_PIA^2.57±0.6, respectively; no shift in c_e is observed. The urea effect is consistent with an associative contribution to viscosity in the aqueous solution case. The invariance of c_e suggests that the rheology of this polymer–solvent system also includes an entanglement contribution. With independent estimates of the PIA weight-average molar mass, M_w, entanglement molecular weight, M_e, hydrodynamic radius, R_H, and excluded volume, ν, we use the Heo–Larson equation to predict η_sp as a function of c_PIA. With the use of parameters from the associated stateparticularly the hydrodynamic radiuswe find good agreement between the model and data for aqueous PIA solutions. This study offers a means to predict the rheology of associating polysaccharides using correlations for nonassociating polymers adjusted with minimal a priori data from their associated state.