Molecular weight distribution measurements

Column dispersity (band spreading) causes the measured molecular weight distribution to be broader than the true molecular weight distribution (Fig. 3.5). Because Zorbax PSM columns exhibit very low band-spreading characteristics, these columns have historically provided better molecular weight distribution accuracy than many gel-type columns. 

FIGURE 3.5 Measured molecular weight distribution (by HPSEC) is broader chan the true molecular weight distribution. 

Mark returned to research after the modernization program was organized and operating smoothly. Limited to cellulose chemistry by the necessity of commercial feasibility, he studied cellulose acetate and the effect of the degree of acetylation on product solubility. At the same time he worked closely on the development of suitable analytical methods to measure molecular weight distribution and the degree of functionality. 

The relations presented so far have been in terms of summations for greater clarity. The equations given are valid for a distribution in which the variable (molecular weight) assumes only discrete values. However, differences between successive molecular weights are trivial compared to macromolecular sizes and the accuracy with which these values can be measured. Molecular weight distributions can therefore be regarded as continuous, and integral expressions are also valid. 

Assume that each fraction is monodisperse and calculate Mn, and a measure of the breadth of the number distribution for the recovered polymer. Note-. This is not a recommended procedure for measuring molecular weight distributions. The fractions obtained by the method described will not be monodisperse and the molecular weight distributions of successive fractions will overlap. The assignment of a single average molecular weight to each fraction is an approximation that may or may not be useful in particular cases.)... 

Fractionation depends on the differential solubility of macromolecules with different sizes. It has been displaced in many cases by size exclusion chromatography as a means for measuring molecular weight distributions, but it is till often the only practical way of obtaining narrow fractions in sufficient quantities for the study of physical properties of well-characterized specimens. It is also part of the original procedure for the calibration of solution viscosity measurements for estimation of molecular weights. 

Figure I Model of asphaltene particle and measured molecular weight distribution of asphaltenes in heavy Venezuelan crude. Asphaltene molecular size distribution computed by configurational diffusion model (from Spry and Sawyer [I40p.

Rashidzadeh, H. and Guo, B., Use of MALDI-TOF to Measure Molecular Weight Distributions of Polydisperse Poly(methyl methacrylate). Anal. Chem., 70, 131, 1998. 

The case of poly(isosorbide succinate) (PIS) shows that it is difficult to compare results from different groups. Differences in methods of preparation, measured molecular weight distribution, and methods to obtain molecular weight distributions lead to a broad range of Tg values, apparently not related to the value of Mn. 

The fragments are therefore large molecules that fall within the glu-tenin size range, and the separation by size-exclusion HPLC (SE-HPLC) is unaffected if time and intensity of sonication are optimum. Thus, sonication combined with SE-HPLC provides a sound method for determining the relative proportions of the main protein classes. If the solubilized protein is to be used for measuring molecular weight distribution or functional properties, however, the changes induced by sonication must be taken into account. 

SEC-LS is used to measure molecular weight distribution directly as a polymer elutes from the SEC without universal calibration. For each polymer-solvent system, the specific refractive index increment dn/dc is required, and for most instruments the solvent refractive index is also needed. Table 3 lists selected papers describing SEC-LS measurements of synthetic polymers, copolymers, polysaccharides, cellulosics, and related polymers. 

Mass spectrometry, for measuring molecular weight distribution [218,219]... 

Nowadays the most common technique for determining the molecular weight distribution of polymers is size-exclusion chromatography (SEC), also known as gel-permeation chromatography (GPC). Other methods for measuring molecular weight distributions exist, but the ease of use and imiversality of SEC makes it the technique of choice for this type of fractionation. 

Unfortunately, suitable standards are not available for all polymers. For a number of homopolymers suitable standards are commercially available. However, for copolymers it is notoriously difficult to determine absolute molecular weights by SEC. Even if copolymer standards were available, there are many different molecules (different combinations of molecular weight and chemical composition) that exhibit the same hydrodynamic volume. Also, the size of polymer molecules in solution is affected significantly by other properties, for example the degree of branching. For these reasons, SEC is often used to measure molecular weight distributions relative to different standards, such as polystyrene. Such relative measurements may be perfectly adequate for many purposes. For example, for monitoring polymerization reactions and product specifications the precision (repeatability) of the data tends to be much more important than their accuracy. 

The occurrence of such side reactions can partially account for the fact that the tuffy resin has a lower epoxide value than is implied by the measured molecular weight distribution [11]. The fact that the stoichiometric balance cannot be maintained due to incomplete dehydrohalogenation and the presence of monofunctional epoxides in the reacting mixture can appreciably limit the molecular weight attainable. 

ThFFF MALDI-ToF-MS is capable of directly measuring molecular weight distributions (MWD) for lower weight polymers. A 40% dioxane/60% heptane solvent mixture was used to fractionate a wide polydispersity PS with a nominal of 33 IcDa. One minute fractions, each containing several micrograms of polymer, were collected. The fractions that were selected for further analysis are shown as shaded areas in Figure 1.9a. 

Thus, perhaps one of the most important potential practical uses of rheology in the entire field of polymer science is as a method to detect and quantify the presence of long-chain branching. This potential use of rheology is more important than its use in measuring molecular weight distributions, because there are analytical methods for the latter, but for the measurement of LCB there is simply no alternative method that can detect minute levels of LCB (i.e, less than about 0.1 branch per 1,000 carbons) see Section 5.12.2. 

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