Molecular weight distribution polymer fractionation

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.)... 

Matrix assisted laser desorption ionization (MALDI) is a frequently used ionization technique, but it is rarely used as an online detector. The sample stream is appUed to a target plate, and it is allowed to cocrystallize with the matrix, which is subsequently desorbed, ionized with a lasCT, and analyzed in the MS. MALDI/TOF has been successfully used to determine molecular weight distributions of fractions collected after thermal FFF separation of polydis-pCTse polymers. MALDI is a good ion source, due to the soft ionization with high effrciency and simple mass spectra, even for heavier molecules, since the m ority carry only a single charge. 

After recovery from the polymerization reaction mixture, PPS can be further treated in ways that modify the suitability of the polymer for specific applications. These treatments include an oxidative heat treatment (commonly referred to as curing), techniques to modify the molecular weight distribution of PPS (typically narrowing the molecular weight distribution by fractionation), and performing cation exchange reactions on PPS to alter the ion-exchangeable species present in the polymer. 

The phenomena we discuss, phase separation and osmotic pressure, are developed with particular attention to their applications in polymer characterization. Phase separation can be used to fractionate poly disperse polymer specimens into samples in which the molecular weight distribution is more narrow. Osmostic pressure experiments can be used to provide absolute values for the number average molecular weight of a polymer. Alternative methods for both fractionation and molecular weight determination exist, but the methods discussed in this chapter occupy a place of prominence among the alternatives, both historically and in contemporary practice. 

The primary polymerization product ia these processes has a relatively wide molecular weight distribution, and a separate step is often used to narrow the polydispersity. Such a narrowkig step may consist of high vacuum stripping to remove volatile polymer chains, often followed by a solvent fractionation step (35,36), sometimes a solvent fractionation step alone (37,38), or a fractional precipitation from organic solvent (32). The molecular weight distribution can also be narrowed by depolymerization at elevated temperatures ia the presence of a depolymerization catalyst (217—220). 

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

Among the techniques employed to estimate the average molecular weight distribution of polymers are end-group analysis, dilute solution viscosity, reduction in vapor pressure, ebuUiometry, cryoscopy, vapor pressure osmometry, fractionation, hplc, phase distribution chromatography, field flow fractionation, and gel-permeation chromatography (gpc). For routine analysis of SBR polymers, gpc is widely accepted. Table 1 lists a number of physical properties of SBR (random) compared to natural mbber, solution polybutadiene, and SB block copolymer. 

In industry and academia the need often arises to isolate portions of a polymer sample, whether it be to separate low molecular weight material from a sample or to actually fractionate the polymer across its molecular weight distribution. If gram quantities of isolated polymer are needed, true preparative chromatography equipment and techniques are usually necessary. 

A prehminary study of the use of larch AGs in aqueous two-phase systems [394] revealed that this polysaccharide provides a low-cost alternative to fractionated dextrans for use in aqueous two-phase, two-polymer systems with polyethylene glycol (PEG). The narrow molecular-weight distribution (Mw/Mn of 1-2) and low viscosity at high concentration of AG can be exploited for reproducible separations of proteins under a variety of conditions. The AG/PEG systems were used with success for batch extractive bioconversions of cornstarch to cyclodextrin and glucose. 

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