Viscosity detectors weight determinations

The use of a continuous GPC viscosity detector in conjunction with a DRI detector permits the quantitative determination of absolute molecular weight distribution in polymers. Furthermore, from this combination one can obtain Mark-Houwink parameters and the bulk intrinsic viscosity of a given polymer with a GPC calibration curve based only on polystyrene standards. Coupling these two detectors with ultraviolet and infrared detectors then will permit the concurrent determination of polymer composition as a function of molecular weight and... 

When the total polymer response, is known as a function of retention volume, the molecular weight distributlon can be obtained in the usual manner with the appropriate molecular weight calibration curve. The molecular weight calibration curve can be obtained (a) by using the Runyon (65) copolymer molecular weight scale approach, or (b) by using a hydrodynamic volume approach if the Mark-Houwink constants for the polymer of interest are known or can be determined, or (c) by using a hydrodynamic volume approach in conjunction with an on-line viscosity detector. 

The sample fluid could be any neat liquid or a sample of polymer solution. Under favorable conditions, a single viscosity determination on a polymer solution at high dilution can provide a direct measure of the polymer intrinsic viscosity, without the need of polymer concentration extrapolation. With this viscometer used as a continuous viscosity detector for SEC, it is possible to achieve SEC molecluar weight calibration by way of the universal SEC calibration methodology without the need of molecular weight standards for the unknown polymers. 

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. 

An alternative is to determine the polymers molecular weight/mass in the SEC eluent in situ, by use of on-line molecular mass sensitive detectors. Two such detectors are commercially available, the light scattering detector and the viscosity detector. These detectors are usually used in series with a mass concentration detector and require specialised data handling/software to compute the outputs from the twin detectors and to produce molecular weight/masses and distributions. 

For branched polymers and for linear polymers that do not have an established Mark-Houwink equation, SEC is typically used in conjunction with light scattering and viscosity detectors. These detectors measure the weight-average molar mass and viscosity of each elution volume. In principle, this experiment directly determines both M and [rj] for any polymer that is soluble in the SEC solvent, but in practice the determination also requires that the polymer have an appreciable difference in refractive index from the solvent so that in Eq. (1.88), dn/dc >0.05 mL g . Often a new polymerization is not understood sufficiently to know for certain whether the polymer produced will have branched chains present or... 

It is possible to add a second, molecular weight-sensitive detector to an SEC system to provide a direct means of absolute molecular weight calibration without the need to resort to external standards. These detectors represent refinements in classic techniques, such as light-scattering photometry, capillary viscometry (for intrinsic viscosity), and membrane osmometry for on-line molecular weight determination. Yau recently published a review of this subject with comparisons of the properties and benefits of the principal detectors currently in use (22). The present discussion is restricted to lightscattering and viscometry detectors because Yau s osmometry detector is not yet commercially available. The reader is referred to Chapter 4 for a comprehensive discussion of molecular weight-sensitive detectors. 

Molecular-mass detectors. Size-exclusion molar-mass calibration is a complicated matter. Calibration curves differ for different polymer types, and for most commercial polymers, direct molar-mass calibration is not possible because of the lack of suitable known molecular-weight standards of sufficiently narrow mass distribution, and of the same chemical structure. The alternative is to determine the polymers molar mass in the SEC eluent, in situ by the use of an on-line molar-mass-sensitive detector. Two such detectors are now commercially available, the light-scattering detector and the recently introduced viscosity detector[5-8]. 

Condition Ar atmosphere [21]/[BF3-OEt2] =40 solvent, CH2CI2 [21] =0.5mol " The weight average molecular weight Mw,sec-malls, polydispersity Mw,sEC-MALLs/ fn,SEC-MALLS, intrinsic viscosity [rj], and Mark-Houwink-Sakurada constant a ([ ] = KMw,sec-malls ) were determined by SEC in THF equipped with MALLS and viscosity detectors. 

Recent advances have greatly decreased the difficulties raised by these cautionary points. In particular, on-line viscosimetry and multi-angle lightscattering make it possible to determine the molecular weight and viscosity of samples as a function of elution volume. With such detectors, effects of solute-column packing interactions become unimportant, since the properties of narrow fractions can be measured. These detectors will be discussed in greater detail below. 

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