Viscosity detectors scattering

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. 

A universal calibration is therefore possible for SEC by plotting log ([q] M) vs. Vg when a viscosity detector is used. Absolute molar masses can be obtained using a light-scattering detector. 

Detector sensitivity light scattering and viscosity detectors are very sensitive towards higher molar masses while RI detectors are most sensitive at lower molar masses,... 

Figure 12. Mark-Houwink plot for dextran. Intrinsic viscosities were obtained from the viscosity detector and (M ysfrom the light scattering one.

Molecular weightjmass detectors include light scattering detectors and viscosity detectors. When SEC is used in the characterisation of polymer systems, its main aim will be the production of a molecular mass/weight distribution and where possible absolute molecular weights. Mass calibration is a complicated matter (section 9.3.5.1) in that calibration curves differ for different polymer types, and for many commercial polymers, direct molar mass calibration is not possible because of the lack of suitable, known molecular weight standards. 

An alternative is to determine the polymers molecular weight/mass in the SEC eluent in situ, by use of on-line molecular mass sensitive detectors. Two such detectors are commercially available, the light scattering detector and the viscosity detector. These detectors are usually used in series with a mass concentration detector and require specialised data handling/software to compute the outputs from the twin detectors and to produce molecular weight/masses and distributions. 

Gel permeation chromatography (GPC) is the established method for the determination of molar mass averages and the molar mass distributions of polymers. GPC retention is based on the separation of macromolecules in solution by molecular sizes and, therefore, requires a molar mass calibration to transform elution time or elution volume into molar mass information. This kind of calibration is typically performed with narrow molecular mass distribution polymer standards, universal, or broad calibration methods or molar-mass-sensitive detectors like light-scattering or viscosity detectors. 

For branched polymers and for linear polymers that do not have an established Mark-Houwink equation, SEC is typically used in conjunction with light scattering and viscosity detectors. These detectors measure the weight-average molar mass and viscosity of each elution volume. In principle, this experiment directly determines both M and [rj] for any polymer that is soluble in the SEC solvent, but in practice the determination also requires that the polymer have an appreciable difference in refractive index from the solvent so that in Eq. (1.88), dn/dc >0.05 mL g . Often a new polymerization is not understood sufficiently to know for certain whether the polymer produced will have branched chains present or... 

Size-exclusion chromatography (SEC) has become the technique of choice in measuring the molar-mass distributions of polymers that are soluble in easily handled solvents (Dawkins, 1989). The technique as widely practised is not an absolute method and a typical SEC system must be calibrated using chemically identical polymers of known molar mass with a narrow distribution unless a combined detector system (viscosity, light scattering and refractive index) is employed. 

The parameter [Equation (18)] is used to take into account deviations from 0 conditions (23). This approach has been evaluated with good success using polystyrene samples (21,24). If a viscosity detector is used in series with a right-angle light-scattering detector. Equation (16) can be used in an iterative procedure to correct for angular asymmetry (see p. 116). 

Furthermore, an on-line viscosity detector can be connected in tandem to the concentration detector (and the light-scattering detector). As we will learn in Section 3.3, the solution viscosity gives an important piece of information on the state of polymer molecules in solution. 

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