Instruments intrinsic viscosity

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. 

A continuous capillary viscosity detector has been developed for use in High Performance Gel Permeation Chromatography (HPGPC). This detector has been used in conjunction with a concentration detector (DRI) to provide information on the absolute molecular weight, Mark-Houwink parameters and bulk intrinsic viscosity of polymers down to a molecular weight of about 4000. The detector was tested and used with a Waters Associates Model 150 C ALC/GPC. The combined GPC/Viscometer instrumentation was automated by means of a micro/mini-computer system which permits data acquisition/reduction for each analysis. 

In some cases, the selection of SEC for the particular application seems to be illogical. A typical example is the determination of the intrinsic viscosities of polymer samples by SEC. Here a simple and cheap glass viscometer is substituted by a rather expensive sophisticated SEC instrument. However, the advantage of SEC is evident when value of manpower, as well as speed and precision of determinations is taken into account. 

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. 

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. 

Evaluating the Polymer. The intrinsic viscosity (IV) of the polymer was determined at 25° C. with solutions in benzene or toluene containing 0.1 gram/liter polymer and calculated with the equation IV = (In rjTei)/C. In many cases some gel formation made I V determination for the whole polymer impracticable. Hoekstra viscosities of a number of polymers were measured at 100° C. with the Hoekstra (Wallace) plastometer (42). This instrument provides bulk viscosity data for rubbers on a scale 0 (low viscosity) to 100 (high viscosity). 

If a continuous viscosity detector is coupled to an FFF channel, viscosity distributions and intrinsic viscosities can be measured without calibrating the channel [76]. The coupling of one FFF instrument to another opens the possibility of obtaining two-dimensional property distributions of complex materials the combination of sedimentation- and flow-FFF provides the size-density distribution of complex colloids, whereas a combination of thermal- and flow-FFF yields the composition-molecular weight distribution of copolymers. 

SEC data for the unknown polydisperse sample can then be obtained on the same instrument using the same solvent and temperature in other words we measure V. and determine Jr Then, as long as we know the values of the intrinsic viscosity parameters K and a for the unknown sample, the molecular weight distribution (the s) and the number... 

It is possible in principle to derive K and a from a single whole polymer sample for which [ ] in the GPC solvent and M are known [21], This method is less reliable than the preceding procedure which involved intrinsic viscosities of two samples because the computations of M can be adversely affected by skewing and instrumental broadening of the GPC chromatogram. 

Unlike biopolymer dispersions where the intrinsic viscosity is known and the polymer concentration can be chosen a priori, often for fluid foods the concentration of soluble (e.g., pectins in fruit juices) and insoluble solids can be determined only posteriori, and the determination of their zero-shear viscosities is also difficult due to instrument limitation and due to the existence of yield stress. However, in many foods, it may be possible to identify the components, called key components, that play an important role in the rheological properties. 

As stated in Chapter 1, for the determination of intrinsic viscosity, [ ], of a polymer, viscosity values of several dilute solutions, when the relative viscosities ( / s) of the dispersions are from about 1.2 to 2.0, are determined. To facilitate such measurements, the so called Ubbelohde glass capillary viscometer is used that has a large reservoir to permit several successive dilutions of a polymer solution (Figure 3-19). Because intrinsic viscosity measurement is important, the test procedure for using the Ubbelohde viscometer is outlined here in brief (Cannon Instrument Co., 1982). 

The polymers were synthesized by the standard NMP/K2CO3 procedure according to literature (10,11). Glass transition temperatures and thermal stability measurements (TGA) were performed on a Du Pont DSC 1090 instrument at heating rates of 10 and 5°C/min, respectively. Intrinsic viscosity measurements were determined using a Cannon- Ubbelhodhe dilation viscometer in NMP (25°C). 

The measurement of intrinsic viscosity is simple and inexpensive when compared with other measurements related to the polymer MW. However, it can be time consuming, even if modern semiautomatic instruments are used for that purpose. As mentioned in Chapter 1, measurements of intrinsic viscosity were historically important in establishing the concept of macromolecules [29]. 

Continuous viscometer detectors are not subject to the same limitation as LALLS instruments. The reasons for this are as follows. If one represents a polymer molecule in solution as an equivalent hydrodynamic sphere (4), then the intrinsic viscosity of the solution [r ] is defined according to the... 

For the present fluid-mechanical tests a homologous series of polyacrylamide (PAAm) samples was used whose mean weights, of the molar masses were determined by means of a low-angle laser-light-scattering photometer. The PAAm samples exhibit virtually the same molecular weight distribution Myy/Mn = 2.5 the intrinsic viscosity [ 7 1 was deter-minded in a Zimm-Crothers rotational viscometer, since the polymer solutions are subjected to a very low shear rate in this instrument. The porous media flow tests were carried out with the aid of an instrument such as described in Refs. [1, 2]. Reference is also made to these studies as regards the test procedure and evaluation of the measured data. 

Many modern SEC instruments are provided with on-line viscometers to measure the intrinsic viscosity of the polymer sample during the analysis. In this case, the Mark-Houwink constants are estimated during the SEC analysis and do not need to be known a priori. 

Capillary Viscometers. These instruments are used for intrinsic viscosities and also for more viscous melts, solutions, and dispersions. The viscosity is given by the Hagen-Poiseuille expression. 

To overcome the problems associated with classical GPC of complex polymers, molar mass-sensitive detectors have been introduced into GPC instruments. Since the response of such detectors depends on both concentration and molar mass, they have to be combined with a concentration-sensitive detector. This set-up allows the direct measurement of molar mass in each analytical fraction and so there is no longer reliance on a calibration curve generated from reference polymer standards. Molar mass-sensitive detectors based on Rayleigh light scattering or intrinsic viscosity measurements can be used for this purpose [9]. 

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