Molecular fractionation

Separation of small molecules into diflferent phases is a well-known phenomenon. Polymers that have a diflferent molar mass have sufficient free energy difference to have a tendency to separate and differences in their melting points will allow fractionation into different crystalline species. Lower molar mass materials will crystallize at low temperatures into separate lamellae often located between the dominant lamellae or at the spherulite boundaries. 

At each crystallization temperature there will exist a critical molar mass (Merit) such that the molecules of molar mass greater than Merit are able to crystallize at this temperature whereas molecules of molar mass less than Merit are unable 

The thermodynamic criterion for the existence of a triple point may be expressed as follows  

The most general statement that can be deduced from the scheme is that phase transformation starts 

Crystallization of most polymers is accompanied by the separation of different molecular species, a process referred to as molecular fractionation. In linear polyethylene, fractionation occurs due to differences in molar mass. The low molar mass material crystallizes at low temperatures in subsidiary lamellae located between the dominant lamellae and in the spherulite boundaries (see Chapter 7). 

The first direct evidence of molecular fractionation in polyethylene was provided by Bank and Krimm (1970). The first extensive studies were reported by Mehta and Wunderlich (1975). The data obtained by Mehta and Wunderlich indicated that, at each crystallization temperature, there exists a critical molar mass (M it) such that the molecules of molar mass 

The lower limit of segregation is set by the hypothetical equilibrium of crystallization. It is assumed that dynamic equilibrium is achieved between fully extended-chain crystals and the surrounding melt. At equilibrium, the molecular length of the crystallizable species corresponds closely to the lamellar thickness, and molecules too short or too long introduce defects and increase the free energy and are thus rejected from the crystal. The equilibrium melting point of a given molecular species is dependent not only on its molar mass but also on the molar masses of the other difFerent species present in the blended melt  

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Various ionization methods were used to bombard phenol-formaldehyde oligomers in mass spectroscopic analysis. The molecular weights of resole resins were calculated using field desorption mass spectroscopy of acetyl-derivatized samples.74 Phenol acetylation was used to enable quantitative characterization of all molecular fractions by increasing the molecular weights in increments of 42. 

Calf thymus (CT) DNA was first fragmented by sonication, then purified by a standard procedure. Agarose gel electrophoresis for the sonicated CT DNA revealed a distribution in molecular weight ranging from 30 kDa to 260 kDa which is equivalent in base pairs (bp) from 40 to 400 (1 bp = 660 Da). The DNA fragments were reacted with HEDS in the presence of l-cyclohexyl-3-(2-morpholinoethyl) carbodi-imide metho-p-toluenesulfonate (CMC/jTs). Finally, the reaction mixture was gel-filtered and the macro-molecular fractions, when displayed the characteristic absorption of 260 nm of the nucleic bases, were collected. 

Fractionation of the Sediment Extract. Ultrafiltration methods (17) have produced some useful data about the nominal molecular size of the active component. Crude sediment extract was filtered (Amicon, 50 psi) and separated into three fractions having the following nominal molecular fractions <10,000, <2,000, <500. The fractions were tested for hydrilla-inhibition, and the two lower M.W. fractions proved to be inactive against the plant. The fraction having a M.W. greater than 2,000 but less than 10,000 showed inhibitory action somewhat enhanced over that of the crude extract (12). 

Some negligible amounts of linear oligomers, obtained as a non-volatile, solid brown residue, were also formed on reaction of Illb with o-phenylenediamine these were almost completely soluble in toluene. No high molecular fraction was detected among the reaction products. 

Now, since each respective fraction shows a structural change along with the reaction course, it may be considered at the same time that it contains products from a higher molecular fraction. In other words, for instance i r A2 is produced from coal 2 which changes by hydrogenolysis and at the same time, the residue resultinq from the reaction from Fr Ai to Fr also appears in this fraction. 

Molecular fractions of polymeric lithium cyclopentadlenyl derivatives... 

FIGURE 16. Molecular fractions of polymeric lithium cyclopentadienyl derivatives in the solid-state. The cations of the lithocene anions 47 and 48 (Ph4P+ and [(12-crown-4)2Li], respectively) have been omitted for clarity. With 55 the cation is [(THF)4Li]+. AU three are solvent-separated ion pairs. 

Table IV. Enzymatic Hydrolysis of the High-Molecular Fraction of Steamed Birchwood Xylan. The substrate was fractionated by ultrafiltration prior to hydrolysis to remove impurities and the 1-5 DP oligosaccharides. Substrate concentration 10 gl— 1, initial pH 5, temperature 45°C, hydrolysis time 24 h...

Various criteria can be considered in the classification of the SEC applications. The most important are the analytical SEC procednres. The preparative applications, which encompass the purification of complex samples before their further treatment, draw rather wide attention. In this latter case, analytes are preseparated by SEC according to the size of their components and either macromolecular or low molecular fractions are subject to further analyses by other methods. The production oriented SEC did not find wide application in the area of synthetic polymers due to both the high price of organic solvents and the ecological considerations. 

Takeda, Y, Hizukuri, S., Takeda, C., Suzuki, A. (1987). Structures of branched molecules of amyloses of various origins, and molecular fractions of branched and unbranched molecules. Carbohydr. Res., 165, 139-145. 

The mixed lipid-cholesterol monolayers are unstable. Recent studies of these systems show that for molecular fractions of cholesterol larger than 30% the cholesterol separates from the film, as if there were a limited misability in two dimensions. 

Schreiber and co-workers have found some indication of molecular fractionation during capillary flow, with smaller molecules migrating preferentially towards the wall and larger molecules toward the center line (74). This conclusion has been disputed recently by Porter and co-workers however (75). 

It is of great value to know which types of sulphur compounds are present in a crude oil, because the various types may behave quite differently in respect to the properties of the crude and of the products that can be obtained from it by physical or chemical methods. According to recent investigations, in lower molecular oil fractions sulphur appears mainly in the form of alkane-thiols, cyclohexane-thiols and thio-alkanes in higher molecular fractions cyclic sulphur compounds of the thiophen type predominate. 

Sie67 allowed methyl linoleate to react with gaseous styrene at 28o°C and isolated an addition compound by molecular fractionation of the reaction products in the cascade fractionating still (Fig. 78). From the analytical data of the addition compound he concluded that a Diels-Alder mechanism was involved in the copolymerization reaction of styrene with the primarily conjugated linoleic ester (c/. Table XVI) ... 

The experimental data on fractionation by molecular weight are given in 52,53). Fig. 2 b shows changes in the MWD of PVPD (oligomer) after binding part of it in a polycomplex with PAA (polymer)S2) it is clear that the experimental pattern (Fig. 2b) corresponds to the theoretical one (Fig. 2a), i.e., the high-molecular fraction binds in a polycomplex while the low molecular one remains in the solution. 

Figure 3. Electron spin resonance spectra of the high molecular fraction of creosol oxidation product...

Most of the early studies concerned with molecular fractionation dealt with samples having a broad molar mass distribution. The crystallisation of binary mixtures of sharp fractions was studied to a lesser degree. The crystallisation of binary mixtures of linear polyethylene sharp fractions in the molar mass range from 1000 to 20,000 g mol1 depended upon the cooling rate, and two types of crystallisation were observed [155] ... 

The present studies were carried out using commercially available humic acid, obtained as the corresponding sodium salt (EGA HI,675-2) and dissolved in a phosphate buffer (pH - 6.99), and an acetate buffer (pH = 4.47), respectively. The humic acid solutions were dialyzed against pure buffer solution prior to their use in the complex formation experiments in order to remove any low molecular fractions that could pass through the dialysis membrane. 

The gas-liquid chromatographic evaluation of n can obviously be performed on a single distribution C E but not on mixtures of distributions. Commercial surfactants always consist of mixed distribution. Only when the procedure is applicable, rt=Lxnrt is calculated from the observed distribution of the molecular fraction (x ) for various values of n. It should be checked that x =l. 

In copolymerisations carried out with heterogeneous catalysts, the higher a-olefin is preferentially incorporated at sites producing shorter chains higher a-olefins are thus enriched in the low-molecular fractions of the copolymers obtained. This increases the content of extractables and tends to make these copolymers sticky, two generally undesirable properties [30]. 

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