Viscosity detectors refractometer

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. 

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)... 

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.

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.

Peak Shapes. In the case of the Wesslau MWD, the shapes of the peaks from the three detectors are always the same. For the Flory-Schulz distribution, the peak shapes are slightly different and the differences increase with increasing polydispersity. As the polydispersity increases, the LS and viscosity signals become narrower relative to the concentration detector signal and they also become less skewed. Figure 3 shows the peak variance of the viscosity and LS signals relative to the concentration detector peak variance as a function of polydispersity. The concentration detector peak variance increases from 0.25 mL when the polydispersity is 1.1 to 3.65 mL when the polydispersity is 3.3. The LS peak variance increases more slowly. The viscometer variance is in between the two but closer to the LS peak behavior. Figure 4 shows the relative skew of the peaks compared with the refractometer, where the skew is defined as... 

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. 

Because cyclohexane does not require an added stabilizer in the mobile phase, this means that ultraviolet (UV) detection at 220 nm can be used. This is important, as this type of detector is more sensitive than a refractometer. In fact, as natural rubber is a polymer with a very high molecular weight, it is recommended that low-concentration solutions at around 0.2 mg/mL are injected, so as to overcome viscosity effects and avoid excessive shearing of the macromolecules [7]. Of course, a light-scattering detector, or viscometer, can be added to the system to access the branching rate [8,9], which is an important parameter for natural rubber. 

Chromatograms of polybistrifluoroethoxyphosphazene using different detectors (a) differential refractometer, (b) differential viscometer, and (c) intrinsic viscosity. Column PLgel mixed bed. Mobile phase acetone, cyclohexanone. Temperature 30°C, 40°C. [From T. H. Mourey et al. (1989)... 

A series of random, block and graft copolymers of VC with styrene (S), butadiene (B), MMA, VAc, and VDC was characterized with the help of three consecutive detectors a differential refractometer (RI), an ultraviolet absorption detector (UV) and an automatic viscometer [33]. Figure 5.2 summarizes the relation between solution viscosity and molecular mass for these copolymers. 

Precision on molecular weight, molecular size, and intrinsic viscosity is typically less than 0.3% RSD, whieh permits excellent feedback for control of the polymerization process. The integrated detector array consists of three primary detectors a light scattering detector that measures molecular weight, a four-eapillary differential viscometer that determines molecular density and size, and a differential refractometer that measures concentration. 

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