Detectors viscometry

Synthetic, nonionic polymers generally elute with little or no adsorption on TSK-PW columns. Characterization of these polymers has been demonstrated successfully using four types of on-line detectors. These include differential refractive index (DRI), differential viscometry (DV), FALLS, and MALLS detection (4-8). Absolute molecular weight, root mean square (RMS) radius of gyration, conformational coefficients, and intrinsic viscosity distributions have... 

Hie hydrolytic depolymerization of nylon-6 was followed by gel permeation chromatography (GPC), viscometry, and gravimetry. GPC determinations were performed on a Waters 150C chromatography system using benzyl alcohol as die eluant, two Plgel 10-p.m crosslinked polystyrene columns, and a differential refractometer detector. The flow rate was 1 mL/min. The concentration of the polymer solutions was 0.5 wt% and dissolution was accomplished at 130°C. 

The experimental techniques for the study of conformational branched properties in solution are the same as used for linear chains. These are, in particular, static and dynamic light scattering, small angle X-ray (SAXS) and small angle neutron (SANS) scattering methods, and common capillary viscometry. These methods are supported by osmotic pressure measurements and, nowadays extensively applied, size exclusion chromatography (SEC) in on-line combination with several detectors. These measurements result in a list of molecular parameters which are given in Table 1. 

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. 

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... 

Absolute MWD can be measured using light scattering or viscometry combined with universal calibration. Compositional drift over the MWD of a polymer can be measured using a UV spectrophotometer and a differential refractive index detector. The increase in the available information also expands the complexity of data analysis. We discuss some of the concerns regarding data analysis that arise in multidetector SEC. 

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. 

Cotton fibers are single cells composed primarily ( 96%) of the polymer cellulose. In our laboratory (5), cotton fibers were dissolved directly in the solvent DMAC-LiCl. This procedure solubilizes fiber cell wall components directly without prior extraction or derivatization, processes that could lead to degradation of high MW components. MW determinations have been carried out by a size-exclusion chromatography (SEC) system using commercial columns and instrumentation with DMAC-LiCl as the mobile phase. Incorporation of viscometry and refractive index (RI) detectors (6) allowed application of the universal calibration concept (7) to obtain MW distributions (MWDs) based on well-characterized narrow-distribution polystyrene standards (5). The universal calibration concept used by incorporation of dual detectors bypasses the need for cellulose standards. There are no cellulose standards available. Polystyrene standards for a wide range of MWs dissolved readily in DMAC-0.5% LiCl with no activation necessary. 

Finally, SEC is merely a separation technique based on differences in hydrodiynamic volumes of molecules. No direct measurement of molecular weight is made. SEC itself does not render absolute information on molecular weights and their distribution or on the structure of the polymers studied without the use of more specialized detectors (e.g., viscometry and light scattering). With these detectors, a self-calibration may be achieved for each polymer sample while it is... 

This method uses the high-performance liquid chromatography (HPLC) equipment for sample handhng and requires molar mass sensitive detectors (such as light scattering and/or viscometry) to obtain a mean property values from each detector (Mw and/or IV, respectively). The FIA result from a concentration detector yields polymer content in a sample, which can also be determined with other well-established methods. The FIA approach requires expensive and well-maintained equipment, and will not save much time or solvent furthermore, no distribution information is available. 

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. 

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. 

J. W. Mays and N. Hadjichristidis, Polymer characterization nsrng dilnte solntion viscometry, in Modern Methods of Polymer Characterization (H. G. Barth and J. W. Mays, eds.), John Wiley Sons, New York, 1991, pp. 227-269. C.-Y. Kno, T. Provder, M. E. Koehler, and A. F. Kah, Use of a viscometric detector for size exclnsion chromatography, in Detection and Data Analysis in Size Exclusion Chromatography (T. Provder, ed.), American Chemical Society, Washington, DC, 1987, pp. 130-154. 

Copolymer Analysis. Even though the overall copolymer composition can be determined by residual monomer analysis, it still is necessary to have reliable quantitative techniques for copolymer composition measurements on the actual copolymer, mainly because concentration detectors for SEC or HPLC are sensitive to composition and because the conversion histories are not always available. Some of the techniques used to determine copolymer composition are melt viscometry (46), chemical analysis, elemental analysis, infrared spectroscopy (IR), Nuclear Magnetic Resonance (NMR), ultra-violet spectroscopy (UV), etc. Melt viscometry, chemical and elemental analysis are general techniques that can be applied to almost any polymer. The spectroscopic techniques can be applied depending on the ability of the functional groups present to absorb at specific wavelengths. 

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