Viscometry detection

Another very useful approach for obtaining molar mass information of complex polymers is the coupling of GPC to a viscosity detector. The viscosity of a polymer solution is closely related to the molar mass (and architecture) of the polymer molecules. The product of polymer intrinsic viscosity [p] and molar mass M is proportional to the size of the polymer molecule (the hydrodynamic volume). Based on the Einstein-Stokes theory, the molar mass, M, of an unknown sample can be determined directly in a GPC experiment from the known molar mass, of a polymer standard and its intrinsic viscosity, irrespective of sample 

This behavior is generally referred to as universal calibration (10). In conventional calibrations, the logarithm of molar mass is plotted versus elution volume different samples yield different calibration curves. In a universal calibration, the logarithm of the product of molar mass and intrinsic viscosity is plotted versus elution volume. In this plot, the calibration curves of very different samples all fall on a single line (the so-called universal calibration curve, see Fig. 14). 

Viscosity measurements in GPC can be performed by measuring the pressure drop across a capillary, which is proportional to the viscosity p of the flowing liquid (the viscosity of the ptue mobile phase is denoted as Po). The relevant parameter, intrinsic viscosity [p], is defined as the Hmiting value of the ratio of specific viscosity (p p = (p - Pol/ Ho) vanishing concentration c  

the concept of universal calibration also provides an appropriate calibration for polymers for which no calibration standards exist 

To obtain absolute molar mass calibration (unknown polymer, -x-x-) 

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Synchropak columns are very useful for characterizing hydrophilic, anionic, and nonionic, water-soluble polymers, CATSEC columns work best for characterizing cationic polymers utilizing both light scattering and/or differential viscometry detection over a wide range of molecular weights. 

Modern GPC systems incorporating viscometry detection together with the differential refractometer are able to generate universal calibration curves and from them determine absolute molecular-weight values. In 1967 Benoit and co-workers introduced the concept of universal calibration. Instead of plotting the log molecular-weight M versus retention time, the log of the product of intrinsic viscosity [t ] times molecular-weight M is plotted versus retention time. This product, [ri]M, is related to the hydrodynamic volume of the polymer in solution. 

Viscometry Detection Although single and dual capillary viscometers are stiU in use, most commercially available detectors offered today are based on the four capillary bridge design originally developed by Haney [17]. 

FIGURE 10.14 Analysis of polyvinylpyridines on SynChropak CATSEC 100,300, and 1000 columns in series (250 X 4.6 mm i.d.). Flow rate 0.37 ml/min. Mobile phase 0.1 % trifluoroacetic acid in 0.2 N sodium nitrate. Detection by differential viscometry. (Reprinted from Ref. 9 with permission.)... 

Many proteins and polymers have been analyzed on SynChropak GPC and CATSEC columns. Table 10.6 lists some of the published applications. The use of a surfactant to analyze the caseins in milk is illustrated in Eig. 10.12. Viruses have also been analyzed on SynChropak GPC columns, as seen in the chromatogram from Dr. Jerson Silva of the University of Illinois (Pig. 10.13). Dr. Nagy and Mr. Terwilliger analyzed cationic polymers on a series of CATSEC columns using differential viscometry as detection (Pig. 10.14) (9). 

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... 

A recent paper by Lairdinvestigated the efficacy of HPAM flocculation of kaolinite, illite and quartz by carrying out visible absorption experiments. He concluded that HPAM more effectively flocculates kaolinite than quartz or illite. This was also the conclusion of previous work by Allen et al. who studied the adsorption of HPAM onto kaolinite, quartz and feldspar at various HPAM concentrations and solution pH by X-ray photoelectron spectroscopy (XPS). Much of the previous work on polyacrylamide adsorption onto aluminosilicates monitored the adsorbed amount by viscometry, carbon analysis and radiotracer techniques. These methods rely on following adsorption by subtraction from that detected in solution. 

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy 36,134 143) which has shown that the carbon-lithium bond of allylic-lithium species can possess considerable covalent character. Worsfold has also previously published 43 > concentrated solution viscosity results where the ratio of flow times, before and after termination, of a poly(isoprenyl)lithium solution was about 15. This finding is clearly incompatible with the conclusion that viscometry cannot detect the presence of aggregates greater than dimeric. 

It should also be noted that the viscometric technique can detect the presence of star-shaped aggregates, having the ionic active centers. The addition of ethylene oxide to hydrocarbon solutions of poly(isoprenyl)lithium leads to a nearly two-fold increase in viscosity144). Conversely, this results in an approximately twenty-fold decrease in solution viscosity, after termination by the addition of trimethylchloro-silane. This change in solution viscosity is reflected in the gelation which occurs when difunctional chains are converted to the ionic alkoxy active centers 140,145,146). Branched structures have also been detected 147> by viscometry for the thiolate-lithium active center of polypropylene sulfide) in tetrahydrofuran. 

Measurements of hydrodynamic radius (Rh) and intrinsic viscosity [in] of PVP in Na2SC>4 (0.55 M) have been performed by using Dynamic Light Scattering (DLS) and viscometry. From these results, a coil - to - globule phase transition was detected. 

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]. 

The characterization of star-branched polymers has been performed using triple detection because it was not obvious, in the beginning of this study, that universal calibration could be applied to star-branched polymers. In fact, the GPC software used handles experimental data as a double dual detection, GPC-viscometry and GPC-LALLS. Experimentally, it has been found that excellent agreement between results from these two sets of data can be obtained. GPC-viscometry uses a universal calibration curve and GPC-LALLS is free of any molecular weight calibration curve. Therefore, the universal calibration works well with very long chain branched polymers, even with a very particular... 

SEC with On-Line Triple Detection Light Scattering, Viscometry, and R( Susan V. Greene... 

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