Viscosity detectors universal calibration

To use universal calibration, intrinsic viscosity must be measured. An online, DV detector can measure specific viscosity, 7j,p, which is related to intrinsic viscosity by the expression... 

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. 

Figure 12 were superimposable on those for detector 2. Therefore, when the plot shown in Figure 14 is linear over the range of compositions involved in the sample, then (according to equations (1-4) ) the composition of the sample is the same at each retention volume. If the variation with retention volume is negligible the copolymer can then possibly be treated as is a homopolymer in GPC interpretation. In particular, intrinsic viscosity measurements could then lead to estimates of molecular weight via the universal calibration curve. 

A viscosity online detector in a size exclusion chromatography (SEC) instrument allows for a universal calibration for polymers with known K- and a-values. For polymers that are only soluble at high temperature, e.g., polyolefines, high-temperature detectors are available, which can be operated up to 200°C. In addition to molar mass measurements, viscosity detectors have also been employed successfully to obtain structural information of branched polymers [28]. 

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. 

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. 

As illustrated in Figure 15, the search for the MW calibration of an unknown polymer from the universal calibration curve requires an on-line SEC viscosity detector. Since narrow MW standards are not available for most commercial and experimental polymer samples, the determination of the polymer [b] calibration would not be possible without an on-line SEC viscosity detector. Once the [q] calibration for the unknown polymer is established, the polymer MW calibration can then be deduced from the universal calibration curve as indicated by the approaches shown in Figure 15. 

The size exclusion chromatography for this study was done in the routine manner execept for the inclusion of an online viscosity detector called a Differential Viscometer <3> (Viscotek Corp., Porter, Texas, USAl. This instrument together with an RI concentration detector permits the calculation of intrinsic viscosities across the chromatogram. An IBM PC data system with software is also provided (5). The software acquires data from both detectors, and performs calculations of intrinsic viscosity and molecular weight distributions using the Universal Calibration Method. 

Fig. 2. Effect of the uv detector noise on the estimation of the molecular weight as function of elution volume for a narrow polystyrene standard. The symbols represent molecular weights obtained using the universal calibration and viscosity measurements on collected fractions...

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. 

If no primary standards are available two approaches can be taken, either polystyrene standards are used to provide an apparent molecular weight or alternatively for homopolymers and copolymers the universal calibration can be employed. This method developed by Benoit utilises the fact that the elution volume of different polymer types in the same solvent is proportional to their molecular weight and their intrinsic viscosity. The intrinsic viscosity can be obtained either by a viscometer detector or be calculated from the Mark-Houwink equation... 

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. 

If a viscometer is used instead of a MALLS detector in SEC (again in conjunction with a concentration-sensitive detector), either by applying universal calibration or the newly emerging technique of SEC, the ratio (g ) of the intrinsic viscosities of the branched molecule and the linear standard at the same molecu-... 

Several procedures were proposed to assess the long-chain branching of macromolecules with help of SEC. They are based either on the universal calibration concept or on the employment of absolute detectors that continuously monitor light scattering and viscosity of the column effluent (see sections 11.6.1.4 and 11.7.3.2). 

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