How to automate the determination of Viscosity in dilute solutions?

Problem: Many bulk polymers, such as polyethylene terephthalate (PET), polyvinyl chloride (PVC), and nylon, are specified in terms of intrinsic viscosity (IV). This is because there is a strong and well-understood relationship between IV and molecular weight, a key parameter affecting the thermal and mechanical properties of polymers. Intrinsic viscosity is determined using dilute solution viscometry (DSV), a technique that is also increasingly used to study biological macromolecules such as hyaluronic acid, an ingredient in many pharmaceuticals and personal care products.

DSV, as the name suggests, involves the measurement of dilute polymer solutions. Comparing the viscosity of a solution of known concentration to that of a pure solvent yields the relative viscosity data needed for IV calculations. Data are traditionally obtained using glass instruments and by measuring the time it takes a solution or solvent to flow through a capillary under gravity.

This has many limitations. Glass capillary viscometers are not sensitive enough to work at very low concentrations, so multiple measurements are required to provide extrapolated data. Analysis times (including lengthy equipment cleaning) are very long and manual input is relatively high. Accurate temperature control is both important and a challenge for accurate viscosity measurement. Finally, solvent consumption and exposure can be problematic, as dissolution often requires the use of expensive solvents with unfavorable environmental and/or toxicological profiles.

Viscotek DSV system

Solution: Automation increases productivity, reduces manual entry and eliminates safety health and environment (SHE) concerns related to solvent exposure. However, with traditional glassware, the range is limited.

With this system, the solvent is first pumped through two identical capillaries. This baseline condition confirms the correct operation of the pressure sensor, which should register the same pressure drop. Switching the injection valve loads the sample loop with polymer solution, which is then pumped through the second capillary. During the measurement, the pump pushes the solvent through the first capillary and the polymer solution through the second capillary and the polymer solution at the same rate. Therefore, relative viscosity is simply the ratio of the two recorded pressure drops.

Relative viscometers are sensitive enough to measure very dilute solutions, so only one measurement is needed to generate IV data, speeding up testing. Simultaneous measurement of solvents and solutions further simplifies analysis. After measuring the polymer solution, clean the device by continuing to pump the solvent through the system. Solvent consumption and exposure are reduced by reducing the number of tests performed and simplifying cleaning.

Malvern company Viscotek has used this approach to develop a fully automatic temperature control system for DSV. It is an excellent choice for QC and development applications, offering very good precision and accuracy as well as high safety. The Viscotek DSV system further simplifies operation by combining a dual capillary viscometer with an automated sample preparation and injection system. Measurements are performed according to ASTM-approved method D 5225-92 at temperatures up to 150℃, making the instrument suitable for more demanding applications such as polyolefin analysis.