Rheological measurements concentrated polymer solutions

The zero-shear viscoelastic properties of concentrated polymer solutions or polymer melts are typically defined by two parameters the zero-shear viscosity (f]o) and the zero-shear recovery compliance (/ ). The former is a measure of the dissipation of energy, while the latter is a measure of energy storage. For model polymers, the infiuence of branching is best established for the zero-shear viscosity. When the branch length is short or the concentration of polymer is low (i.e., for solution rheology), it is found that the zero-shear viscosity of the branched polymer is lower than that of the linear. This has been attributed to the smaller mean-square radius of the branched chains and has led to the following relation... 

The measurement of yield stress at low shear rates may be necessary for highly filled resins. Doraiswamy et al. (1991) developed the modified Cox-Merz rule and a viscosity model for concentrated suspensions and other materials that exhibit yield stresses. Barnes and Camali (1990) measured yield stress in a Carboxymethylcellulose (CMC) solution and a clay suspension via the use of a vane rheometer, which is treated as a cylindrical bob to monitor steady-shear stress as a function of shear rate. The effects of yield stresses on the rheology of filled polymer systems have been discussed in detail by Metzner (1985) and Malkin and Kulichikin (1991). The appearance of yield stresses in filled thermosets has not been studied extensively. A summary of yield-stress measurements is included in Table 4.6. 

The complete expression for is given by Darby and Pivsa-Art (1991) in terms of the viscoelastic properties of the fluid. These properties are complex, and cannot be predicted accurately from first principles or other basic properties, and must be measured for each polymer solution. However, a simplified expression has been given by Darby and Pivsa-Art (1991), in which these rheological parameters are contained within two constants, ki and k2, which depend only on the specific polymer solution and its concentration ... 

Solution Rheology. Polymers were hydrated in distilled, filtered water and were agitated gently until dissolution was complete. To prepare polymer solutions containing salt, concentrated sodium chloride solutions were added to polymer previously dissolved in distilled water. An alternative procedure was used to evaluate the effect of sahnity on solution rheology. Solid sodium chloride was slowly added to various concentrations of polymer in solution. To ensure complete dissolution, the solutions were allowed to equilibrate for approximately 24 h before viscometric measurements were obtained. Turbidity measurements were made with a turbidimeter (Hach) on 1500-ppm solutions in 3% NaCl and 0.3% CaCU brine, which we called 3.3% brine. 

After a preliminary analysis of the influence of the production mode on xanthan characteristics, this study was focussed on the effects of the origin and concentration of proteins on the state of aggregation of xanthan in solution estimated by rheological measurements. In a first while, purification treatments were applied to dilute polymer solutions in order to try to separate proteins from xanthan and thus to evaluate the amount and effect of proteins linked to the polysaccharide backbone. In the second phase of this study, the direct addition of proteins to dilute or concentrated xanthan solutions was analyzed, and the effects of salt, pH and origin of proteins were investigated. 

Rheological measurements were performed on xanthan solutions prepared by dilution of broths, 48(X) CX, A, C or powder Rhodopol 23, adjusted at similar polymer concentrations (between 5 and 7 g.l ). The effects of the addition of a protein-rich solution and of other parameters such as salinity and pH were further investigated. 

Rheological Measurements Polymer solutions with various concentrations were prepared in a small sample bottle by moderately heating for several days, and then used for rheological measurements. The study was carried out using a cone-and-plate rheometer (Haake CV-20N) with a shear rate range of 0 300 s at 30 C. 

Concentrated dispersions may be shear thickening, as opposed to the shear thinning of dilute polymer solutions. Some materials, such as latex paints, tend to form a structure. As the structure breaks down with shearing action, the viscosity decreases. Such materials are thixotropic. Some fluids have a yield stress. A thorough characterization of the rheology may require a number of different measurements. 

To fully assess the properties of suspension concentrates, three main types of measurements are required. Firstly some information is needed on the structure of the solid/solution interface at a molecular level. This requires investigation of the double layer properties (for systems stabilised by ionic surfactants and polyelectrolytes), adsorption of the surfactant or polymer as well as the extension of the layer from the interface (adsorbed layer thickness). Secondly, one needs to obtain information on the state of dispersion on standing, such as its flocculation and crystal growth. This requires measurement of the particle size distribution as a function of time and microscopic investigation of flocculation. The spontaneity of dispersion on dilution, i.e. reversibility of flocculation needs also to be assessed. Finally, information on the bulk properties of the suspension on standing is required, which can be obtained using rheological measurements. The methods that may be applied for suspension concentrates are described briefly below. 

An important means of assessing the rheological behavior of polymer melts and concentrated solutions is the use of dynamic mechanical analysis. This consists in testing the mechanical response, or strain, of a polymeric material to a time-dependent stress, or, conversely, measuring the stresses generated by time-dependent strains. For simplicity, a... 

---