Indexes molecular weight determination

The GPC analysis of block copolymers is handicapped by the difficulty in obtaining a calibration curve. A method has recently been suggested to circumvent this difficulty by using the calibration curves of homopolymers. This method has been extended so that the calibration curves of block copolymers of various compositions can be constructed from the calibration curve of one-component homopolymers and Mark-Houwink parameters. The intrinsic viscosity data on styrene-butadiene and styrene-methyl methacrylate block polymers were used for verification. The average molecular weight determined by this method is in excellent agreement with osmometry data while the molecular weight distribution is considerably narrower than what is implied by the polydispersity index calculated from the GPC curve in the customary manner. 

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. 

Molecular-weight determinations are performed in good solvents for both blocks of the PEO copolymers. The most widely used analysis conditions are as follows eluents-tetrahydrofuran (THE) and chloroform (CHE) flow rate—1.0 ml/min detection—differential refractive index (dRI) detector. The temperature interval is between... 

Molecular weight determinations were obtained using a Brice-Phoenix Model BP-3000 Universal Light Scattering Photometer. Refractive Index Increments, dn/dc, were determined using a Bausch and Lomb Abbe Refractometer Model //3-L. 

Dong et al. [171] developed a scale using the polarizability of probe molecules. An advantage of this method is that polarizability is not a macroscopic property of the molecules in a condensed state. In this case, no probe molecule lies under the reference line. Nevertheless, the polarizability values are not easy to find in the literature, and their determination requires the use of Debye s equation, which in turn requires macroscopic data such as refractive index, molecular weight, and density of the probe [177]. 

Component Refractive Index Theoretical Area under Curve, % Molecular Weight, nominal, g/mol Molecular Weight, determined, g/mol... 

The refractive index (RI) of a mixture is a function of the composition of the mixture and the respective refractive indices of the constituents [8]. The mixture refractive index follows mixture laws such as the Lorentz-Lorenz law. Operational measuring instruments are usually differential refractometers or critical-angle re-fractometers [4]. A large disadvantage in the method is that it only provides meaningful results when a two-component system is considered. However, a differential refractometer is commonly used as a concentration detector in the effluent of a gel permeation chromatography (GPC) column for molecular weight determination. 

The most common detectors used in SEC are based on refractive index and UV absorption. Recently online LALLS detectors have been introduced which allow molecular weight determinations using SEC without calibration. The LALLS detector is placed in series with a concentration detector so that and C can be measured for each elution volume increment. Because the concentration of the polymer is very small, the scattering Eq. (7) reduces to... 

Fig. 3 Purification of the isolated polysaccharide CSP-1 from cultured Cordyceps. (a) Partial purification of polysaccharides from cultured Cordyceps mycelia by DEAE-cellulose chromatography. Extract was loaded onto the DEAE column (3.5x30 cm), and eluted with 0 to 0.5 M NaCI, as indicated by dotted line, in 10 mM Tris-HCI pH 7.4 having a flow rate of 30 mLTh. Rve milliliter fractions were collected, (b) The polysaccharide-enriched fractions were applied onto a Sephacryl S-300 column equilibrated with 0.2 M NaCI in 10 mM Tris-HCI buffer pH 8.0. The first-peak fractions (CSP-1) were collected, (c) HPLC profile of CSP-1 by using TSKgel G3000 SWXL (7.8 mm ID x 30 cm) column. Molecular markers are indicated by arrows in kOa. The polysaccharide profile was detected by refraction index detector. The insert shows the molecular weight determination of CSP-1 (reproduced from ref. [25] with permission from Elsevier)...

Molecular descriptors must then be computed. Any numerical value that describes the molecule could be used. Many descriptors are obtained from molecular mechanics or semiempirical calculations. Energies, population analysis, and vibrational frequency analysis with its associated thermodynamic quantities are often obtained this way. Ah initio results can be used reliably, but are often avoided due to the large amount of computation necessary. The largest percentage of descriptors are easily determined values, such as molecular weights, topological indexes, moments of inertia, and so on. Table 30.1 lists some of the descriptors that have been found to be useful in previous studies. These are discussed in more detail in the review articles listed in the bibliography. 

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