Precipitation stepwise, fractionation

In CRYSTAF, the analytical process is followed by monitoring the polymer solution concentration during crystallization by temperature reduction. Aliquots of the solution are filtered (through an internal filter inside the vessel) and analyzed by a concentration detector at different temperatures, as shown in Fig. 21. The whole process is similar to a classical stepwise fractionation by precipitation with the exception that, in this new approach, no attention is paid to the polymer being precipitated but to the one that remains in solution. 

Fractionation. Kett-McGee developed the ROSE process for separating the heavy components of cmde oil, eg, asphaltenes, resins, and oils, in the 1950s. This process was commercialized in the late 1970s, when cmde oil and utility costs were no longer inexpensive. In the ROSE process (Fig. 11), residuum and pentane ate mixed and the soluble resins and oils recovered in the supetctitical phase. By stepwise isobatic temperature increases, which decrease solvent density, the resin and oil fractions ate precipitated sequentially. 

Purification of photoprotein. The dialyzed photoprotein solution was centrifuged to remove precipitates, and then subjected to fractional precipitation by ammonium sulfate, taking a fraction precipitated between 30% and 50% saturation. The protein precipitate was dissolved in 50 ml of 10 mM sodium phosphate, pH 6.0, containing 0.1 mM oxine ( pH 6.0 buffer ), dialyzed against the same buffer, and the dialyzed solution was adsorbed on a column of DEAE-cellulose (2.5 x 13 cm) prepared with the pH 6.0 buffer. The elution was done by a stepwise increase of NaCl concentration. The photoprotein was eluted at 0.2-0.25 M NaCl and a cloudy substance (cofactor 1) was eluted at about 0.5 M NaCl. The photoprotein fraction was further purified on a column of Sephadex G-200 or Ultrogel AcA 34 (1.6 x 80 cm) using the pH 6.0 buffer that contained 0.5 M NaCl. 

Fractionation by Stepwise Elution. Information obtained from the analytical separation was applied for a preparative purification. Lignin peroxidase concentrate was bound to a Q-Sepharose colunm (0= 5 cm, V = 1000 ml) after ultrafiltration and eluted stepwise with 0.08 M, 0.18 M and 0.28 M sodium acetate, pH 6.0. The fraction which was eluted with 0.28 M buffer (V= 3.91, 4400 U/1) was purified further. It was bound to Q-Sepharose and eluted with 0.18 M and 0.3 M sodium acetate. En rnie in the latter fraction was precipitated and dissolved in glycerol as previously described. The volume was 15 ml. 

Fractional extraction is free from the disadvantages encountered in fractional precipitation. Here, the technique consists in extracting the polymer with a series of solvent/precipitant mixtures, the proportion of solvent being increased stepwise. Since one begins with the poorest solvent mixture - in contrast to fractional precipitation - the first fraction contains the low-molecular-weight com-... 

Long ago it has been established that nitrocellulose is not a homogeneous substance. The earliest experiments in this field were made by Stepanov [100] who used the method of fractional precipitation from an acetone solution by the addition of water. Stepanov added different amounts of water stepwise to a solution of nitrocellulose in acetone to obtain fractions varying slightly, corresponding with the nitrogen content. The first and least soluble fraction contained 13.15% of nitrogen, while the N-content of the last, fifth portion, was 12.90%. 

Samples of branched polyethylene were investigated by the combination of crystal-lizability fractionation and fractionation by molar mass. The first step was the precipitation of the polymer sample onto the glass-beads in a column (150 x 8 mm) used for the fractionation by crystallinity. The precipitation was performed by cooling a solution of 10 g/1 polyethylene in o-dichlorobenzene from 140 to 40 °C within 60 min. Subsequently, the polymer was extracted from the column by the same solvent at stepwise increased temperature. In an example given, the first fraction was eluted at 40 °C and additional 17 fractions at temperatures each raised by 2-10 K. The finer steps were employed in the middle of the fractionation procedure the last one reached even from 110 to 140 °C. About 10 min equilibration time proved to be adequate. The fractions were analyzed subsequently by SEC on a polystyrene-gel column. The whole process was automated 128). 

Fractionation of an asphaltene by stepwise precipitation with hydrocarbon solvents (heptane to decane) allows separation of the asphaltene by molecular weight. The structural parameters determined using the x-ray method (Table II) show a relationship to the molecular weight (16). For the particular asphaltene in question (Athabasca), the layer diameters (La) increase with molecular weight to a limiting value similar relationships also appear to exist for the interlamellar distance (c/2), micelle height (Lc), and even the number of lamellae (Nc) in the micelle. 

The functionality distribution of HTPB 134> can be obtained using the double detection procedure after derivatization of the OH groups with phenyl isocyanate. However, a true fractionation can been obtained by stepwise elution from silica gel n8,120) and, coupled with GPC, it enables establishment of the relationship between functionality and molecular weight distributions. Precipitation fractionation can also be used... 

Highly degraded cellulosic materials, such as viscose rayon, D.P. 250 to 350, may be dispersed in 8-12 % caustic at low temperatures. Upon increasing the temperature in a stepwise manner, the dispersed material may be fractionally precipitated. " This method of fractionation is of particular interest since viscosity data indicate that the state of dispersion of degraded cellulosic materials in caustic solution and in cuprammonium solution is similar. "... 

It will be noted that this type of complex-formation is entirely different from that in which complexes are formed between amylose and certain polar, organic compounds. In contrast to the precipitates of the latter complexes (which are of a distinct, crystalline appearance), the starch-alkaline-earth hydroxide complexes are amorphous, curdlike flocculates. These complexes di,s.sociate on diluting them with water, and the starch redissolves. According to the patent description, the amylose complexes dissolve much more easily than the amylopectin complexes hence, fractionation must occur if water is added stepwise. Likewise, fractionation will take place if the starch complexes are partially neutralized, by the gradual addition of an acid. For obvious reasons, such acids as carbonic acid and sulfuric acid (which give insoluble calcium salts) are preferred. Furthermore, it is claimed that gradual addition of caustic alkali to a starch solu-... 

The mechanism of this reaction is still discussed controversially [31]. A problem is the polymodal molecular mass distribution. Usually three fractions are obtained cyclic oligomers with n = 5, 6, a fraction with intermediate chain lengths (n = 30-50), and a high molecular fraction with = 10 -10 g mol [32]. A fractionation is achieved by a stepwise precipitation of the polysilanes from a solution in toluene with an alcohol like methanol or isopropanol. 

Stepwise retrogradation of amylose mixtures and GPC separation of the fractions should reveal whether a preferential precipitation exists, possibly as a result of preformed double helices in solution. 

While SEC aims to separate a polymeric mixture only with respect to size (hydro-dynamic volume), investigations of substituent distribution requires a separation with respect to the chemistry of the constituents. Spurlin, very early on, recognized this problem and fiactionated cellulose nitrate by repeated dissolution and precipita-tion. The fractions obtained showed increasing viscosities and, related to this, increasing flexibihty of solution-cast thin films. Saake et al. also made use of this common principle in polysaccharide isolation, in particular the (stepwise in this instance) precipitation of CMC by ethanol from aqueous solution. ... 

The simplest procedure for polymer fractionation is to dissolve the polymer at low concentration in a poor solvent and then to bring about stepwise phase separation (i.e. precipitation ) of polymer fractions by either changing the temperature or adding a non-solvent. The highest molar mass species phase separate first and so the fractions are obtained... 

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