On-line viscometer

The latest trend is to smaller beads in smaller columns, as this saves eluent and shortens the time for a chromatographic analysis. This argument can be correct if only one suitable detector is used. However, these modern small columns are not optimal for a combination of detectors. So-called multiple detection is a combination of some detectors with different measurement principles (differential refractometer, spectral photometer, light-scattering detector, on-line viscometer) behind the last column, mostly in series, seldom in a branched ( parallel ) order. In this way, the tedious preparative fractionation of a polymer sample can often be avoided. 

The most direct method for calculating My(V) across the chromatogram would be to use an on-line viscometer to measure [ri](V) and then to calculate My(V) from... 

Unfortunately, an on-line viscometer which can provide instantaneous [n] values at low and high temperatures is not available. Although batch viscometers have been used in the past (1, 13) the use of high speed SEC makes them useless due to the smaTT elution volumes. 

There are commercially available in-line or on-line viscometer devices. In-line devices are installed directly in the process while on-line devices are used to analyze a side stream of the process. Most devices are based on measuring the pressure drop and flow rate through a capillary. The viscosity is either determined at a single shear rate or, at most, a few shear rates. Complex fluids, on the other hand, exhibit a viscosity that cannot be so easily characterized. In order to capture enough information that allows, for example, a molecular weight distribution to be inferred, it is necessary to determine the shear viscosity over reasonably wide ranges of shear rates. 

Lesec, J. and Volet, G., Data treatment in aqueous GPC with on-line viscometer and light-scattering detectors, /. Liq. Chromatogr.,13, 831, 1990. 

Examples are Laser Differential Microanemometry (LMA) and Total Reflection Microscopy (TMA) (8). Both LMA and TMA measure the velocity profile of the fluid in tube flow. However, such optical techniques are generally not suitable for opaque and/or heterogeneous substances such as foods. Acoustic velocimetry seems to be more promising for determining the velocity profiles of opaque substances. Such an acoustic technique has been applied by Brunn et al (19) as an on-line viscometer for flow of mayonnaises in pipes. 

GPC with an on-line viscometer can be used instead of a LALLS detector to analyze branched polymers. In this case the intrinsic viscosity is measured so that the Mark-Houwink parameters are not needed. It is complementary to the LALLS instrument in intrinsic viscosities. 

In the production of a condensation polymer the melt viscosity of the final polymer (Y) was measured every 15 minutes by an on-line viscometer. This viscosity could be kept near its target value by manipulating a variable (X). By performing a very simple plant test in which the input (manipulated) variable was perturbed in a pseudo-random binary manner and the resulting viscosity recorded every 15 minutes by the on-line viscometer, the following dynamic-stochastic model was identified for the process ... 

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... 

From the Th-FFF retention data, it is possible to obtain a molar mass distribution after a suitable calibration for the determination of the Mark-Houwink constants (straight-line plot of log(D/DT) vs. log M [15]). Another possibility is to couple an absolute molar mass detector like MALLS (see Sect. 4.3.2) or a suitable detector combination such as an on-line viscometer coupled with a refractive index detector. This possibility does not require prior knowledge of DT... 

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. 

A polymer sample may consist of a mixture of species whose compositions differ enough to affect the responses of both the concentration-dependent detector and the molecular-weight-sensitive detector in a multidetector system. Examples are mixtures of different polymers or copolymers (Chapter 7) whose composition is not independent of molecular size. Conventional GPC cannot be used reliably to characterize such mixtures, but an on-line viscometer can be employed to measure molecular weight averages independent of any compositional variations [25]. Remember, of course, that such data characterize the mixture as a whole, and not just the major component. 

An in-line measurement is performed in a process line an on-line measurement is performed in a bypass loop from the main process line and the food may be returned to the main process line after measurement is performed. A near-line measurement is performed on a sample taken from a process line which is often discarded after measurement. Because foods are complex materials (e.g., suspensions, emulsions, gels), stmctural changes may take place during sampling (e.g., flow through a valve) for on-line and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, and rotational concentric cylinder, plate-cone, and mixer viscometers may be used for in-line, on-line, and near-line measurements. In this respect, Tamura et al. (1989) proposed a helical screw rheometer as an on-line viscometer. The empirical measurement methods described previously are used primarily in near-line measurements. 

With the use of an on-line viscometer and universal calibration, absolute MWDs can be obtained. Because both M and intrinsic viscosity are known, the Mark-Houwink coefficients a and K can be calculated for the injected polymer ... 

The necessary conditions for application of the universal calibration method and for calculation of molecular weights through Eq. (6) is the knowledge of the [r]]i values, which are obtained from the Mark-Houwink equations when the pertinent values of K and a constants are known. An alternative way is to make a continuous measurement of [t]]i at the different elution volumes with an on-line viscometer detector coupled to the usual concentration detector system. 

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. 

There is big theoretical and practical interest for the scaling relationship [q] = /(M) for HA. Theoretically, the Mark-Houwink-Sakurada (MHS) plot for HA could be obtained by using an on-line viscometer combined with an SEC system applied to an appropriate number of broad MMD HA samples. Unfortunately, the 7 range of on-line DV detectors is too high for HA even at very low flow rates. To obtain a reliable MHS plot for HA for an extended range of molar masses, Mendichi et. al. [275] used a modified on-line SCV... 

An alternative molecular weight-sensitive detector is the on-line viscometer. All current instrument designs depend upon the relationship between pressure drop across a capillary through which the polymer sample solution must flow and the viscosity of the solution. This relationship is based upon Poiseuille s law for laminar flow of incompressible fluids through capillaries ... 

If the intrinsic viscosity of the eluting unknown polymer is measured at each elution volume using an on-line viscometer, universal calibration can be used to calculate the molecular weight at each volume, and thus the molecular weight distribution, without knowledge of the Mark-Houwink coefficients. 

SEC-viscometry and universal calibration has been widely used to determine the MWD of synthetic polymers, and selected applications are listed in Table 1. On-line viscometers have been successfully used at high temperatures Pang and Rudin (50) measured the MWD of polyolefins dissolved in 1,2,4-trichlorobenzene at 145°C, and Stacy (17) measured the MWD of polyphenyl sulfide in 1-chloronaphthalene at 220 °C. 

The difficulty with copolymer analysis is in the measurement of the concentration of each elution volume. On-line viscometers measure the correct specific viscosity for copolymers. If universal... 

An important application of SEC -viscometiy in conjunction with universal calibration is to determine the Mark-Houwink coefficients for a given polymer system. The coefficients can provide information about solvent quality and molecular conformation. In addition, once the coefficients for a polymer-solvent system are known, that polymer can then be characterized using conventional universal calibration without an on-line viscometer. All references listed in Table 1 report the Mark-Houwink coefficients for the systems studied. 

We turn now to a recent method for SEC analysis of nonhomogeneous polymers using an on-line viscometer, without a concentration detector (41). This evidently applies also to copolymers with nonhomogeneous chemical compositions. The method is a generalization of the Goldwasser technique... 

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. 

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