by David Trowbridge, Ph.D., Polymer Chemist, CANNON Instrument Company
The distinguishing aspect of polymeric materials is the relatively large size of their molecules—so large that they have been termed macromolecules. Consequently, measuring the size of these macromolecules is of particular interest in the polymer industry; many physical and dynamic properties are related to a macromolecule's size and shape. The viscosity of dilute polymer solutions is sensitive to these structural features of macromolecules, making viscosity a useful measure to correlate with processing and physical properties.
The procedures involved in the measurement of polymer solution viscosity are described in the polymer science literature. Authors typically outline the use of glass capillary viscometers held to a tight temperature tolerance by immersion in a constant temperature bath. The flow times of solvent, t0, and solutions, ti, are recorded to 0.01 seconds by an analyst observing the fall of the fluid under a constant gravitational field.
The flow times for several polymer concentrations are recorded and the relative viscosity, ?r,i = ti /t0, is calculated for each concentration. The reduced viscosity, ?red,i, and the inherent viscosity, inh,i, for each solution are determined from the relative viscosities and from the concentrations, Ci, of the respective solutions: ?red = (?r,i-1)/Ci; ?inh = ln(?r,i)/Ci.
The reduced viscosity and the inherent viscosity are predicted to converge on the same value, the intrinsic viscosity, in the limit of zero concentration. Consequently, plots of ?red and ?inh versus concentration are extrapolated by linear equations to obtain y-intercept values that are combined to give the measured intrinsic viscosity. This is the general approach upon which intrinsic viscosity has grown to its current prominence.
However, the practice can be labor intensive. Some laboratory protocols require that the measured flow times be made in multiple repetitions in order to obtain an average value to be applied in the subsequent viscosity calculations. These repeated measurements are made for several polymer solutions covering a range of concentrations. The result is a sizable data matrix that requires significant time and effort to process.
The volume of data, the complex cascading calculations and the graphical analysis present an opportunity for automation that is keenly addressed by the CANNON Instrument Company. PolyVISC automated polymer solution viscometer performs each timing measurement using a flow sensing technology and an internal AirBath designed to regulate the temperature of the glass capillary with a precision of ± 0.02 C. The instrument includes an 11-vial sample carousel positioned in the AirBath to facilitate rapid and consistent temperature equilibration. The instrument closely duplicates manual viscosity procedures by using glass capillary designs similar to the widely specified Ubbelohde and Atlantic manual viscometers (see Figure 1).
The software controls the viscometer and manages the sample test parameters, configuration settings and measurement data in a secure database. For the intrinsic viscosity analysis, the software functions as the interface to the sheltered database. The analyst selects records for inclusion in the intrinsic viscosity determination. VISCPRO responds by providing the analyst with a summary of all the viscosity calculations, as well as a plot of the regressed ?red and ?inh data (see Figure 2). The plotting feature provides a clear depiction of the y-intercept for each of the extrapolated lines, and for their point of intersection. The features of the sample data can be quickly determined from the graphical analysis and the reported slopes of the linear least-squares lines can be used to make judgments regarding the polymer-solvent interaction. The software also allows the user to specify Mark-Houwink-Sakurada parameters for converting the intrinsic viscosity into molecular weight values. If the molecular weight has been determined by an independent method, then the branching factor, G, can be obtained.
Complete washing and drying of the viscometer is also automated. The intrinsic viscosity analysis can be performed using up to eight solution concentrations that can be measured without operator intervention. The analyst can set up the viscometer to function unattended over the course of the night and in the morning simply use the software to view the intrinsic viscosity results.
Conclusion
Figure 2. Table of records showing viscosity results for a series of concentrations. Also listed are the regression statistics, calculated molecular weight and plot of viscosity concentration dependence. Least-squares fit lines are extrapolated to the y-intercept, and their
intersection at the intrinsic viscosity value is indicated by a solid red circle.
This capability has opened up new opportunities for the polymer scientist. No longer is it necessary to limit intrinsic viscosity determinations in order to manage time and human resources in the laboratory. It is now feasible to routinely perform rigorous intrinsic viscosity measurements for screening research polymers, evaluating processing effects, quality checking production polymers, and studying a series of model polymers. The temperature of the Air Bath environment chamber is easily controlled and can be quickly changed to provide viscosity data over a range of temperatures. Also, the viscometer capillary is designed for simple changing and is provided in several sizes to cover a wide viscosity range. This flexibility allows the analyst to observe viscosity trends in the concentration range above the dilute solution regime. With VISCPRO driving the PolyVISC automated viscometer, research and industrial scientists have available a full complement of test conditions at which to make very accurate measurements of polymer solution viscosity.
For more information, contact David Trowbridge, Ph.D., Polymer Chemist, CANNON Instrument Company, at DTrowbridge@cannoninstrument.com or by phone at 814-353-8000.
AT A GLANCE
• Measuring the size of these macromolecules is of particular interest in the polymer industry
• Some laboratory protocols require that the measured flow times be made in multiple repetitions in order to obtain an average value to be applied in the subsequent viscosity calculations
• The software controls the viscometer and manages the sample test parameters, configuration settings and measurement data in a secure database
• No longer is it necessary to limit intrinsic viscosity determinations in order to manage time and human resources in the laboratory
