Viscosity determination from cone-plate geometry

The cone-plate rheometer. The cone-plate rheometer is often used when measuring the viscosity and the primary and secondary normal stress coefficient functions as a function of shear rate and temperature. The geometry of a cone-plate rheometer is shown in Fig. 2.47. Since the angle Oo is very small, typically < 5°, the shear rate can be considered constant throughout the material confined within the cone and plate. Although it is also possible to determine the secondary stress coefficient function from the normal stress distribution across the plate, it is very difficult to get accurate data. 

Rheology is a powerful method for the characterization of HA properties. In particular, rotational rheometers are particularly suitable in studying the rheological properties of HA. In such rheometers, different geometries (cone/plate, plate/plate, and concentric cylinders) are applied to concentrated, semi-diluted, and diluted solutions. A typical rheometric test performed on a HA solution is the so-called "flow curve". In such a test, the dynamic viscosity (q) is measured as a function of the shear rate (7) at constant strain (shear rate or stress sweep). From the flow curve, the Newtonian dynamic viscosity (qo), first plateau, and the critical shear rate ( 7 c), onset of non-Newtonian flow, could be determined. 

Conversion versus time was determined gravlmetrlcally using one vial from each pair. The polymer was precipitated In methanol, vacuum dried, and weighed. The second vial was used to obtain viscosity. A Deer Rheometer (16) cone and plate geometry with a 65 mm diameter 0.0698 rad cone was used. A Rheometrlcs System Four (17) In the fluids model (50 mm, O.OA rad) was used In later work. 

Sakai, et al report the low-shear viscosity of poly-a-methylstyrene in a-chloronaphthalene(31). Polymer molecular weights extended from 60.2 kDa to 3.3 MDa. The study was limited to relatively large concentrations most systems had )7/f7o > 100 as determined with a rheogoniometer in cone-and-plate geometry. Figure 12.7 shows their measurements. The stretched-exponential form is seen to fit accurately, including the c = 0 solvent viscosity (not shown), which was included in the fits as a data point. 

---