Viscosity detectors universal calibration plots

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 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 intrinsic viscosity and Mark-Houwink constants of standards can be determined from a static capillary viscometer or an on-line viscometer detector in an SEC system. If the intrinsic viscosity is to be used for constructing a universal calibration curve, it is important to use the identical conditions in performing the SEC analysis and the intrinsic viscosity measurement. A Mark-Houwink plot for five PAM standards and one PAA standard is shown in Figure 4. The intrinsic viscosity of PAM may decrease with time and becomes constant after about one week. It is recommended that the PAM solution be analyzed while still fresh. 

FIGURE 43.2. Plot of intrinsic viscosity vs. weight fraction of crown ether for polyurethane rotaxanes made by threading of a polymer Mn 16.6 kDa and Mh 58.7 kDa by universal calibration and viscosity detector in NMP at 60 °C) with 42-crown-14 in the melt at 80 °C (last four entries of Table 43.1) [33]. 

When an on-line viscometer is used together with the refractive index detector to generate the intrinsic viscosity [t]] in order to build the universal calibration curve. Sec. II.B, the intrinsic viscosity [t]] can also be used to determine the presence and degree of branching. This is done by plotting the log of [t]] versus log molecular-weight for each slice of the distribution. This plot is called the viscosity law plot, or the Mark-Houwink plot. It is described by the equation... 

In universal calibration, samples of a monodisperse polymer, often polystyrene, that is different from the polymer to be analyzed, are dissolved in the solvent of interest, and the intrinsic viscosities of the resulting solutions are measured. Then identical samples are injected into the column to be used, and the refractive index of the effluent is measured as a function of retention volume, V, which depends on V,. Then a calibration plot of [tj] M versus is prepared, and this plot is assumed to be valid also for the polymer to be analyzed. The retention time can also be used as the independent variable, since it is linear in at constant flow rate. It is convenient to fit an equation, for example a third-order polynomial, to the universal calibration curve. Carrying this concept a step further, if an on-line IV detector is used along with the DRI detector, the data from an analysis can be interpreted directly in terms of a molecular size distribution, and from this the MWD can be determined. 

---