SEC/viscometry

TABLE 20.4 Mark-Houwink Constants for PEG, PEO, and PSC from SEC/Viscometry Using TSK-PW Columns... 

Polystyrene. Table 3 shows the results obtained for three broad MWD polystyrene samples. The agreement for M and M values obtained from SEC/Viscometry analysis and the nominaY values supplied by the vendors is excellent. In addition the Mark-Houwink parameters, K and a values also are in excellent agreement with each other as well as with literature values.(28,39) This shows the consistency of the analysis method and the technique for determining the dead volume between detectors. The lower M value for the NBS 706 sample is due to the low molecular weight tail associated with the sample. 

Polymethyl Methacrylate. Table 4 shows the results obtained for three commercially available polyraethyl methacrylate samples. Again the molecular weight averages obtained with SEC/Viscometry are in good agreement with the nominal values and the K and o values are self-consistent and in excellent agreement with the literature values.(40)... 

TABLE 4, SEC/VISCOMETRY RESULTS FOR THREE BROAD MWD PMMA SAMPLES... 

TABLE 5. SEC/VISCOMETRY RESULTS FOR A POLYVINYL CHLORIDE SAMPLE... 

SEC/Vlscometry methodology gave values of M = 151,000, M = 182,000 and [p] = 0.297 dl/g. Thus, a very accurate M value was obtained from SEC/Viscometry methodology. 

TABLE 7. SEC/VISCOMETRY RESULTS OF STAR PS-W (WHOLE POLYMER)... 

TABLE 8. SEC/VISCOMETRy RESULTS OF TWO POLYVINYL ACETATE SAMPLES... 

Figure 8. Molecular weight distribution of branched PVAc (B...) as determined by laser light scattering (LLS) and size exclusion chromatography (SEC)/viscometry. A comparison of results by the two methods is listed in Table 2.

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

One of the most difficult problems when characterizing copolymers and polymer blends by SEC-viscometry is the accurate determination of the polymer concentration across the SEC elution curve. The concentration detector signal is a function of the chemical drift of the sample under investigation. To overcome this problem, Goldwasser proposed a method where no concentration detector is required for obtaining Mn data [72]. In the usual SEC-viscometry experiment, the determination of the intrinsic viscosity at each slice of the elution curve requires a viscosity and a concentration signal ... 

Due to the problems encountered with SEC-LALLS and SEC-viscometry, a triple-detector SEC technology has been developed, where three on-line detectors are used together in a single SEC system. In addition to the concentration detector, an on-line viscometer and a LALLS instrument are coupled to the SEC... 

As has been discussed, the combination of SEC and molar-mass-sensitive detectors is a powerful tool for the analysis of complex polymers. However, it is important to distinguish between claimed versus actual capabilities and between potential expectations and demonstrated performances. Tables 1 and 2, taken from a critical review of different techniques, summarize the informational content and additional details of SEC-light scattering and SEC-viscometry coupling... 

In addition, the complex procedures related to SEC-light scattering and SEC-viscometry coupling are a potential source of error. According to Jackson and Barth [81] these include ... 

The computer models described provide a functional simulation of SEC-viscometry-LS analysis of linear polymers. The results for the Flory-Schulz MWD are in qualitative agreement with previous results for the Wesslau MWD. Both models emphasize the importance of determining the correct volume offset between the concentration detector and molecular weight-sensitive detectors. For the Flory-Schulz model, the peak shape, as well as the peak elution volume, can provide information about molecular weight polydispersity. Future work will extend the model to incorporate peak skew and polymer branching. 

For further information on the intrinsic viscosity determination in GPC, including the use of the light-scattering detector, see Ref. 8 and the entry GPC-SEC Viscometry from Multiangle Light Scattering. 

Fig.l SEC-viscometry characterization of a linear (a) and a branched (b) PA-12 synthesized by anionic polymerization, with different activators. Analysis conditions mixed eluant 5 95 v/v HFIP/CH2CI2 at 30°C Styragel-HR2/3/4/5 columns. 

Relative methods measure properties that depend clearly on molar mass, for example, the hydrodynamic volume of the polymer coils (SEC, viscometry) or their solubility as a function of chain length. However, these measurements can only be evaluated with respect to the molecular weight of the macromolecules if experimental calibration curves are available, which were generated by comparison with an absolute method of molar mass determination. 

SEC-viscometry combined with universal calibration can provide measurements of molecular weight distribution even when it is not possible to use a concen-... 

Another approach to the SEC-viscometry data is that of Kirkland et al. (20). The intrinsic viscosity is a fundament property of the polymer sample in solution, and thus polymers may be characterized in terms of their intrinsic viscosity distribution (IVD) without attempting to convert this into a molecular weight distribution. Moments of the IVD may be calculated similar to those for the MWD (21). The advantage is that the intrinsic viscosity distribution is di-... 

If universal calibration is used with SEC -viscometry, it is also possible to calculate the radius of gyration for linear polymers at each elution volume using the Flory-Fox equation (22),... 

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