Refractometer, absolute differential

Determination of the Refractive Index Increment. In order to be able to calculate results obtained from light-scattering measurements, the refractive index increment (dn/dc) of HEC must be known. For this reason, an absolute differential refractometer (Brice-Phoenix Instrument Company, Philadelphia 40, PA type 1120) was calibrated geometrically (18), using the next formula ... 

The most widely used molecular weight characterization method has been GPC, which separates compounds based on hydrodynamic volume. State-of-the-art GPC instruments are equipped with a concentration detector (e.g., differential refractometer, UV, and/or IR) in combination with viscosity or light scattering. A viscosity detector provides in-line solution viscosity data at each elution volume, which in combination with a concentration measurement can be converted to specific viscosity. Since the polymer concentration at each elution volume is quite dilute, the specific viscosity is considered a reasonable approximation for the dilute solution s intrinsic viscosity. The plot of log[r]]M versus elution volume (where [) ] is the intrinsic viscosity) provides a universal calibration curve from which absolute molecular weights of a variety of polymers can be obtained. Unfortunately, many reported analyses for phenolic oligomers and resins are simply based on polystyrene standards and only provide relative molecular weights instead of absolute numbers. 

An absolute value of M for each of these branched PVAcs was obtained from light-scattering measurements. In each case five polymer solutions were made up in tetrahydrofuran (THF) solvent and a Chromatix KMX-6 LALLSP instrument was employed to measure the intensity of light scattered from these solutions at 7° to the incident laser beam. A Chromatix KMX-16 laser differential refractometer was used to determine the refractive index increments, dn/dc, of the polymer solutions under ambient conditions. 

Problems caused by the determination of the unoccupied vapor space were avoided by Haynes et al., since they measure the pressure difference as well as the absolute vapor pressure. Also, the concentration is determined independently by using a differential refractometer and a normalized relation between eoneentration and refractive index. Degassing of the liquids remains a necessity. Time for establishing thermodynamic equilibrium could be somewhat shortened by intensive stirring (slight problems with increasing polymer concentration and solution viscosity were reported). 

Modern GPC systems incorporating viscometry detection together with the differential refractometer are able to generate universal calibration curves and from them determine absolute molecular-weight values. In 1967 Benoit and co-workers introduced the concept of universal calibration. Instead of plotting the log molecular-weight M versus retention time, the log of the product of intrinsic viscosity [t ] times molecular-weight M is plotted versus retention time. This product, [ri]M, is related to the hydrodynamic volume of the polymer in solution. 

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