Polydispersity applications

R. O. Buckius and D. C. Hwang, Radiation Properties of Polydispersions Application to Coal, ASME Journal of Heat Transfer, Vol. 102, p. 99,1980. 

The observation of molecular size or polydispersity and the subsequent determination of relative molecular mass, (MJ or molecular mass (weight) distribution (MWD), is the most common analytical application of SEC. The goal of these types of experiments is to either observe the solvated size of one or more molecular species or to observe the distribution of sizes present in a mixture... 

The use of hexafluoroisopropanol (HFIP) as an SEC eluent has become popular for the analysis of polyesters and polyamides. Conventional PS/DVB-based SEC columns have been widely used for HFIP applications, although the relatively high polarity of HFIP has led to some practical difficulties (1) the SEC calibration curve can exhibit excessive curvature, (2) polydisperse samples can exhibit dislocations or shoulders on the peaks, and (3) low molecular weight resolution can be lost, causing additive/system peaks to coelute with the low molecular weight tail of the polymer distribution... 

Since the SEC/LALLS technique always yields a weight-average molecular weight (l )y for the slightly polydisperse fraction at V, a small overestimation of the sample Rn is expected (, 1 ). As noted previously (Results) a 1% to decrease in the narrow MWD polystyrene Mp values (Table I) accompanied application of the band-spreading correction ... 

In the absence of long-chain branching, the DRI is expected to be zero and would have positive values for polymers with long-chain branching. It should be noted that the application of the DRI is limited to polymers with a narrow MWD, Mw/Mn< 2, since it cannot delineate the differences arising from polydispersity and long-chain branching. 

Application in the Field of Scattering. Let us consider two important distribution functions, hc (x) and lu. (x). These functions shall describe the thicknesses of crystalline layers and the distances (long periods) between them, respectively. In this case we take into account polydispersity of the crystalline layers, if (at least) the two parameters dc and ac/dc are determined which are defined as the average thickness of the crystalline layers,... 

A more elegant way to introduce polydispersity is founded on Tensor calculus. For an application in scattering theory cf. e.g. Burger and Ruland [20]. 

Classical Analysis. The classical analytical methods are even applicable for polydisperse samples and rest on the CLD (Sect. 8.5.3) and on Vonk s [189] distance distribution function (DDF) ([189-191] [101] p. 168)... 

Dextrans are also attractive as macromolecular carriers of paramagnetic chelates because of their hydrophilicity, the different available molecular weights with narrow polydispersity, and the versatility of activation methods applicable. Several DTPA- or DOTA-loaded carboxymethyl dextran (CMD) derivatives have been prepared and tested in blood pool MRI.136-139 The relaxivities reported for these compounds are, however, relatively moderate. 

Unlike the PEG molecules formed from anionic polymerization techniques, there now exist highly discrete forms of the polymer made by controlled addition of small PEG units to create chains of exacting molecular size. These discrete PEGs have a single molecular weight and do not display the polydispersity of the traditional PEG polymers. See Chapter 18 for a complete discussion of discrete PEG-based reagents and their applications. 

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