Continuous Viscometric Detection in Size Exclusion Chromatography

Abstract

Continuous viscometric detection is based on the measurement of pressure drop in an on-line small capillary tube in which chromatographic eluents flow at constant flow rate. This detector is always coupled with a concentration detector (usually refractometer) and located before it to avoid back pressure in the refractometer. In order to obtain reliable information for polymer samples, it is generally necessary to connect these two detectors to a computer which performs data acquisition and treatment.

First, we discuss the problem of shape, geometry and dimensions of the viscometer. The typical characteristics are the result of a compromise between contradictory targets, mainly small internal volume, low shear rate and low pressure drop. It is shown that Poiseuille's laminar flow is only obtained when coiling radius of the measurement tube is greater than 6 cm, which is not the case inside the refractometer. Accordingly, two pressure transducers are necessary to eliminate pressure drop data coming from refractometer.

In a second part, we show how to extract information from pressure variation data. By using concentration data, pure solvent pressure and sample pressure it is possible to calculate intrinsic viscosity extrapolated to zero concentration at each point of the chromatogram. By comparison with intrinsic viscosity of the polymer used for calibration, a correction of hydrodynamic volume according to Benoit's universal calibration leads to absolute molecular weights.

In addition, for a linear polymer, the knowledge of log [η] versus log M leads to the determination of Mark-Houwink relationship coefficients. For branched polymers, viscosity laws are curved and the comparison between the linear law corresponding to the linear equivalent polymer and the experimental law allows the determination of the g' branching parameter distribution.