Differential viscosity detector

Figure 2.15 Schematic diagram of a differential viscosity detector system. A number of different detector arrangements have been reported.

Figure 2.16 Some typical experimental results for a differential viscosity detector. The difference in the response of a concentration detector (such as a differential refractometer) and a viscosity detector to the same solution of three polymers undergoing analysis should be noted.

Figure 2.12 Schematic of Viscotek differential viscosity detector. By permission of Roth...

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. 

Haney, M. A., The differential viscometer. II. On-line viscosity detector for size-exclusion chromatography, /. Appl. Polym. Sci., 30, 3037, 1985. 

DP (1) Differential pressure (viscosity detector) (2) Density profiling... 

The automation of the HPGPC/Viscometer system is achieved by interfacing the differential refractometer (DRI) and viscosity detector to a microcomputer for data acquisition. The raw data subsequently, are transferred to a minicomputer (DEC PDP-ll/HiI) for storage and data analysis. Details of the instrument automation are given elsewhere.(6)... 

DIFFERENTIAL PRESSURE CAPILLARY VISCOHETER AS AN IN-t INE VISCOSITY DETECTOR... 

The size exclusion chromatography for this study was done in the routine manner execept for the inclusion of an online viscosity detector called a Differential Viscometer <3> (Viscotek Corp., Porter, Texas, USAl. This instrument together with an RI concentration detector permits the calculation of intrinsic viscosities across the chromatogram. An IBM PC data system with software is also provided (5). The software acquires data from both detectors, and performs calculations of intrinsic viscosity and molecular weight distributions using the Universal Calibration Method. 

The most commonly-used detectors are those based on spectrophotometry in the region 184-400nm, visible ultraviolet spectroscopy in the region 185-900nm, post-column derivativisation with fluorescence detection (see below), conductivity and those based on the relatively new technique of multiple wavelength ultraviolet detectors using a diode array system detector (described below). Other types of detectors available are those based on electrochemical principles, refractive index, differential viscosity and mass detection. 

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 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. 

The only universal detector sensitive enough to detect asphalt (because of its relatively low molecular weight) is the Viscotek differential viscometer (108,109). It utilizes a Wheatstone bridge flow resistance scheme that measures intrinsic viscosity differences between the column eluant and the carrier solvent. Other viscosity detectors measure absolute intrinsic viscosity of the eluant and are not as precise. In Figure 19, several supercritically refined asphalt fractions having a variety of molecular weights (MJ are seen to have similar RI and IV... 

Viscosity detection in one form or another has been used with SEC for a considerable number of years [20]. This detection has ranged from manual measurement of fractions, to the use of pressure transducers to monitor the pressure changes as the polymer solution passes through the detector. More recently, a viscosity detector has become available which uses a pressure transducer to accurately measure a differential pressure, and is hence considerably more sensitive and applicable to modern SEC systems [21,22] (see also section 2.2.5.6). 

Fig.12A,B. LCCC chromatograms of blends of PMMA and PnBMA under the critical condition of PMMA. Column LiChrospher 300 A +1000 A, mobEe phase methyl ethyl ketone/cy-clohexane (72/28, v/v), detector A differential refractometer B capElary viscometer. The large injection solvent peak is suppressed in the chromatograms recorded by the viscosity detector.

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