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

DSV, as the name suggests, involves the measurement of dilute polymeric solutions. Comparing the viscosity of a solution of known concentration with that of the pure solvent gives the relative viscosity data required for IV calculation. Traditionally, glass apparatus is used and data are derived by measuring the time taken for the solution or solvent to flow through a capillary under gravity.

This has a number of limitations. Glass capillary viscometers are insufficiently sensitive to work at very low concentrations, so multiple measurements are necessary to provide data for extrapolation. Analysis times (including lengthy apparatus cleaning) are quite long and manual input relatively high. Accurate temperature control, critical for precise viscosity measurement, is also a challenge. Finally, solvent consumption and exposure can present problems since dissolution often demands the use of expensive solvents with unfavourable environmental and/or toxicological characteristics.

The Viscotek DSV system

The Solution: Automation offers improved productivity, reduced manual input and the elimination of Safety Health and Environment (SHE) issues associated with solvent exposure. However, with conventional glassware the scope is limited.

With this system, solvent is initially pumped through the two identical capillaries. This baseline condition confirms correct operation of the pressure transducers, which should both record the same pressure drop. Switching the injection valve loads the sample loop with polymer solution, which is subsequently pumped through the second capillary. During measurement, the pump is pushing solvent through the first capillary and polymer solution through the second at the same rate. Relative viscosity is therefore simply the ratio of the two recorded pressure drops.

The relative viscometer is sensitive enough to measure very dilute solutions so IV data can be generated from just a single measurement, accelerating testing. Solvent and solution are measured simultaneously, further streamlining analysis. The equipment is cleaned by continuing to pump solvent through the system after the polymer solution has been measured. By cutting the number of tests performed and simplifying cleaning, solvent consumption and exposure are reduced.

Viscotek, a Malvern company, has developed a fully automated, temperature-controlled system for DSV using this approach. Ideal for both QC and development applications, it delivers excellent precision and accuracy coupled with high levels of safety. The Viscotek DSV system combines a dual capillary viscometer with an automated sample preparation and injection system to further simplify operation. Measurements are made according to ASTM approved Method D 5225-92 at temperatures as high as 150ºC, making the instrument suitable for more demanding applications such as polyolefin analysis.

For more information, go to www.viscotek.com