Rheology Enhancement by Molecular Association Complexes

Associations between polyethylene oxide polymer (PEO) chains and species containing sulfate groups can be used to increase the viscosity and elasticity of polymer solutions. The condensation of sodium dodecyl polymer solutions. The condensation of sodium dodecyl sulfate (SDS) micelles onto the PEO chain is shown to lead first to chain coil collapse and then to network or gel structure formation as SDS concentration is increased. Stronger interactions are produced between PEO and either Congo-red dye (a large organic molecule with two widely spaced sulfate groups) or polystyrene sulfonate (PSS). In these cases the association complexes lead to minor changes in the solution viscosity but large changes in the primary normal stress and storage modulus G' at low frequencies. The structures formed by association are disrupted by flow but rapidly re-form. Association complexes are an attractive way of obtaining high-viscosity fluids that would be resistant to permanent shear degradation. Introduction Association complexes between polymers and solutes can be used to increase substantially the viscosity and elasticity of polymer solutions. The advantage of using association complexes rather than ultrahigh molecular weight polymers to achieve high viscosities is that the association complexes are resistant to permanent mechanical shear degradation (chain backbone scission). In this paper we present rheological data on interactions between PEO polymers and various solutes containing sulfate groups, specifically, SDS, Congo-red dye, and PSS. PSS. The interaction between PEO polymers and SDS surfactant has been investigated extensively. The techniques used include equilibrium dialysis, viscometry, NMR spectroscopy, and neutron scattering, as well as measurements of surface tension, conductivity, and intrinsic viscosity. Above the critical micelle concentration (CMC) of SDS (8 × 10(-3) mol/L [8 × 10(-3) mol/dm3]) the PEO/SDS association complex has been shown to consist of spherical SDS micelles attached along the polymer backbone at approximately 80- [8-nm] polymer backbone at approximately 80- [8-nm] intervals. This surfactant micelle adsorption leads to significant changes in the rheology of the polymer solution. Other solutes containing sulfate groups also associate with PEO in solution. Congo-red, a large organic molecule dye with two widely separated sulfate groups, was found to produce strong association complexes with polyvinyl alcohol and PEO. PSS polymers also showed strong associations with PEO. Experimental Procedures Dynamic Measurements. If a viscoelastic material is sheared sinusoidally according to ..........................(1) where (t) is the strain, max is the maximum strain, t is time, and is the angular frequency; the shear stress, is given by ....................(2) with a phase angle between 0 (for ideal elastic solids) and /2 (for ideal viscous liquids), where max is the maximum shear stress. Expansion of the sine function in Eq. 2 yields ..........(3) where. Eq. 3 defines the two dynamic moduli: G' is called the "storage modulus" since it is the elastic (or stored) part of the stress, and G" is the "loss modulus" since it part of the stress, and G" is the "loss modulus" since it is the viscous (or dissipative) part of the stress. The storage and loss moduli can be combined to define a quantity called the "dynamic or complex viscosity," given by ................(4) For simple fluids it is observed empirically that the complex viscosity * is the same function of frequency w as the steady shear viscosity is of shear rate that is, .............................(5) SPEJ p. 431