Viscosity detector, continuous

Development of a Continuous Gel Permeation Chromatography Viscosity Detector... 

A continuous capillary viscosity detector has been developed for use in High Performance Gel Permeation Chromatography (HPGPC). This detector has been used in conjunction with a concentration detector (DRI) to provide information on the absolute molecular weight, Mark-Houwink parameters and bulk intrinsic viscosity of polymers down to a molecular weight of about 4000. The detector was tested and used with a Waters Associates Model 150 C ALC/GPC. The combined GPC/Viscometer instrumentation was automated by means of a micro/mini-computer system which permits data acquisition/reduction for each analysis. 

This work describes the design, operation and application of the continuous GPC viscosity detector for the characterization of the molecular weight distribution of polymers. Details of the design and factors affecting the precision and accuracy of results are discussed along with selected examples of polymers with narrow and broad molecular weight distribution. 

Details of the data analysis for the GPC/Viscometer system have been reviewed by Ouano.(T ) The data reduction scheme is summarized in Figure 2 and briefly will be discussed here. The intrinsic viscosity of the effluent at a given retention volume [n](v) is determined from the DRI and continuous viscosity detector responses according to the following equation... 

The use of a continuous GPC viscosity detector in conjunction with a DRI detector permits the quantitative determination of absolute molecular weight distribution in polymers. Furthermore, from this combination one can obtain Mark-Houwink parameters and the bulk intrinsic viscosity of a given polymer with a GPC calibration curve based only on polystyrene standards. Coupling these two detectors with ultraviolet and infrared detectors then will permit the concurrent determination of polymer composition as a function of molecular weight and... 

The sample fluid could be any neat liquid or a sample of polymer solution. Under favorable conditions, a single viscosity determination on a polymer solution at high dilution can provide a direct measure of the polymer intrinsic viscosity, without the need of polymer concentration extrapolation. With this viscometer used as a continuous viscosity detector for SEC, it is possible to achieve SEC molecluar weight calibration by way of the universal SEC calibration methodology without the need of molecular weight standards for the unknown polymers. 

If a continuous viscosity detector is coupled to an FFF channel, viscosity distributions and intrinsic viscosities can be measured without calibrating the channel [76]. The coupling of one FFF instrument to another opens the possibility of obtaining two-dimensional property distributions of complex materials the combination of sedimentation- and flow-FFF provides the size-density distribution of complex colloids, whereas a combination of thermal- and flow-FFF yields the composition-molecular weight distribution of copolymers. 

A continuous-viscosity detector has been shown to be a good detection tool for thermal FFF analysis of polymer solutions [8]. Due to the high sample dilution in FFF, the viscosity detector response above the solvent baseline, AS, is only dependent on the intrinsic viscosity of every sample point, [17], multiplied by the concentration, c, at the corresponding points ... 

The opportunity to measure the dilute polymer solution viscosity in GPC came with the continuous capillary-type viscometers (single capillary or differential multicapillary detectors) coupled to the traditional chromatographic system before or after a concentration detector in series (see the entry Viscometric Detection in GPC-SEC). Because liquid continuously flows through the capillary tube, the detected pressure drop across the capillary provides the measure for the fluid viscosity according to the Poiseuille s equation for laminar flow of incompressible liquids [1], Most commercial on-line viscometers provide either relative or specific viscosities measured continuously across the entire polymer peak. These measurements produce a viscometry elution profile (chromatogram). Combined with a concentration-detector chromatogram (the concentration versus retention volume elution curve), this profile allows one to calculate the instantaneous intrinsic viscosity [17] of a polymer solution at each data point i (time slice) of a polymer distribution. Thus, if the differential refractometer is used as a concentration detector, then for each sample slice i. 

Dynatrol Viscosity Systems are designed for continuous measurement of viscosity in process streams. Dynatrol Viscosity Detectors are installed directly in process vessels without the need for sampling or analysis. Response is immediate and continuous. Utilizing this system, we carried out a series of experiments to study the change in viscosity of polymer solutions when we introduced a change in polymer chain ends. 

A form of the viscosity detector has been used for exclusion chromatography, and prior experiments employing a single capillary have demonstrated the feasibility and usefulness of a continuous on-line viscosity detector in conjunction with a concentration detector (RI or UV). The sensitivity of the device, however, is rather poor, the minimum detectable concentration being about lO" g/ml. To date, the instrument has only been used in a recent application in polymer chromatograjdiy. 

As shown in Figure 1 data from the viscometer detector and DRI are combined to yield the Intrinsic viscosity as a function of retention volume (la). This curve then was fit to a polynomial and a smoothed curved calculated. At this stage of data reduction the analyst can choose to continue to use the polynomial smoothed values of log [n](V) throughout, or continue to use the unsmoothed values for further data reduction. 

The combination of the differential refractive index (RI) detector and on-line viscometer allows the direct use of the universal calibration and thus true molecular weight determination. The RI detector is concentration-sensitive, and the viscometer records specific viscosity. The ratio of the specihc viscosity to the concentration is equivalent to intrinsic viscosity (as discussed in Section 6.1), and the continuous dependence of this ratio versus the retention volume could be related to the universal calibration curve, thus allowing the correlation of each point on the chromatogram with the true molecular weight. 

From that point, the necessity of continuously measuring viscosity, in addition to polymer concentration, became obvious. Several attempts were made to adapt existing viscometers as GPC detectors, but the problem of internal volume was critical. Ouano [2] published the first design of a single-capillary viscometer which was based on pressure measurement. Several similar designs [3-6] were pubfished and a commercially available instrument, the Waters Model 150CV (Waters Associates, Milford, MA, U.S.A.), based on a design described in Ref. 4, became commercially available. 

A special group of detectors are those which continuously measure the molar mass of sample components in the eluate. Their application allows to omit the calibration of the gel chromatographic system. So far, vapor phase osmometry for solutes with lower molar masses, and viscosity and light scattering measurements for polymers have been applied. 

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