Solvent-nonsolvent fractionation

The classical solvent precipitation fractionation technique provides reproducible fractionations for determining molecular weight distributions of CTPB and almost 100% recovery of the sample from the column. A solvent-nonsolvent combination which has been used effectively is the toluene—acetone-methanol system, where acetone and methanol are used as the nonsolvents. The precipitating fractions are required to stand approximately 24 hours to ensure complete separation. Each fraction is vacuum stripped of solvent at approximately 30 °C., and the molecular weight of each fraction is then determined by either VPO or intrinsic viscosity. 

The classical approach is based on the dependence of copolymer solubility on composition and chain length. A solvent/nonsolvent combination fractionating solely by molar mass would be appropriate for the evaluation of MMD, another one separating with respect to chemical composition would be suited for determining CCD or FTD. However, in reality, precipitation fractionation yields fractions which vary both in chemical composition and molar mass. Even high resolution fractionation would not improve the result. Narrower fractions can be obtained by cross-fractionation separating in two different directions. However, even in this case, it is almost impossible to obtain perfectly homogeneous fractions. 

By this method, one cuts the sample to be analyzed into many narrow fractions by using a column in which a solvent-nonsolvent gradient and a temperature gradient are working against one another. From the fractions are measured the mass and the DP. Therefore the elution curve is not related directly to a calibration curve, because all fractions are characterized separately. One can use a GPC column for this characterization thereby economizing the work considerably. 

Recognizing the difficulties which one could expect to have with a fractionation or extraction procedure for analyzing these polymers, we searched for suitable solvent-nonsolvents for the PVC and acrylic copolymers. Applica-... 

The physicochemical properties of alkylated polysaccharides have received some attention, and details of the structures and conformations of 0-methylcelluloses, and their interaction in micelle junctions, have been included in a thorough discussion of polysaccharide gels and networks. Information on the polydispersity of samples of partially methylated cellulose may be obtained from column fractionation and by fractional precipitation from a solvent-nonsolvent system, but, for a more complete characterization of polydispersity, fractionation with a series of solvent-nonsolvent systems is necessary. The solubility, in water, of polysaccharides that are mainly methylated may be considerably improved by introduction of a few suitable ionizing groups, for example, by reaction with monochloroacetic acid to introduce carboxymethyl ether groups. The general sorption and diflFusion features of hydrocarbons and other... 

As with the alkylated polysaccharides, the physicochemical properties of hydroxyalkylated polysaccharides have continued to be investigated, particularly those of commercial value. The general characteristics of 0-(2-hydroxyethyl)cellulose have been described, together with pro-cedmes for the viscosimetric measurement of solutions of the polymer, and the macromolecular properties of the polymer in solution, described earlier, have been reviewed. The effect of increasing the substitution of both water-soluble 0-(2-hydroxyethyl)- and 0-(2-hydroxypropyl)-cellulose is to decrease their afiBnities for water. Water-soluble 0-(2-hy-droxypropyl) cellulose can be fractionated in a way analogous to that for O-methylcellulose by utilizing solvent—nonsolvent mixtures. For polysac-... 

The classical method of solvent-nonsolvent fractionation according to MWD and compositional distribution relies on solubility differences among the various species. The method is empirical and tedious, involving characterization of phase-separated cuts. However, fractionations can be carried out with minimal equipment, and for some polymers are the only source of narrow compositional... 

Poly (S-leucinato-AT, 0) cobalt (III) -u- [3,3 -dithiobis (2,4-pentanedion-ato-0,0 )]).—Initially, 1.7460 g disulfur dichloride (12.93 mmol) in 6 mL dry dichloroethane was added dropwise to 5.0088 g Co(leu)(acac)2 (12.93 mmol) in 20 mL dimethylacetamide (DMAC) with 0.7095 g sodium carbonate (6.69 mmol) as a slurry in the dimethylacetamide solution under argon and with vigorous stirring. The mixture was stirred for 24 - 36 h and then precipitated with diethyl ether. Alternatively, the solvent was removed in vacuo at 45 C. The product was collected on a fritted funnel, washed with water, and dried in vacuo at lOO C yield, 5.5 g 95%. The polymer was fractionated (three times) by a DMAC/acetone or dimethyl sulfoxide/acetone solvent/nonsolvent precipitation to remove low molecular-weight material. 

Figure 19.1 shows a representation of the phase inversion process. The process starts with a polymeric solution thermodynamically stable, the composition of which is usually located at the axis polymer-solvent, being a binary mixture of these two components. The process begins when this solution gets in contact with the nonsolvent, and so, the fraction of nonsolvent in the solution increases (ternary mixture). 

FRACTIONATION Fractionation has been accomplished using the following solvent/ nonsolvent combinations dichloroethane/petroleum ether, dioxane/elhanol, methylene chloride/methanol. 

Solvent/ nonsolvent Benzene/ acetone, benzene/ n-butanol, Fractional (61)... 

Method of fractionation Solvent or solvent/nonsolvent mixture Remarks... 

FRACTIONATION METHODS Fractional precipitation in toluene/ methanol (solvent/nonsolvent) mixtures at40/20°C. ... 

Junction-point-functionalized block copolymers were synthesized according to Scheme 3 [34]. Styrene was polymerized first using s-BuIi as initiator. After completion of polymerization a small excess of DMADPE was introduced to the reaction mixture (DMADPE/Ii = 1.2/1). The reaction was left for completion for 3 d at room temperature. Isoprene was then added, polymerized, and terminated by addition of degassed methanol In the case of the triblock copolymer synthesis, after completion of isoprene polymerization, a predetermined amount of a Me2SiCl2 solution in benzene was introduced to the reaction mixture (Cl/Ii = 1/2.2). The coupHng reaction was essentially complete in 3 d. Solvent/nonsolvent fractionation was employed in order to separate the triblock from excess diblock. 

Ogawa et al. (23) used a HFIP/toluene (20 80) mixture as the eluant for nylon 12. Column fractionation of the same polymer was also performed using benzyl alcohol/decalin as the solvent/nonsolvent pair. They were able to demonstrate that in the HFIP/toluene mixture, polystyrene narrow standards and nylon 12 narrow fractions were in compliance with the universal calibration... 

Fractionations of nylon 12 by column fractionation and by preparative-scale SEC in HFIP/CFjCOONa and in HFIP/toluene (20 80) mixtures were carried out by Ogawa and Sakai (24). They found that SEC fractionation in HFIP/toluene (20 80) is more effective than in HFIP/CFjCOONa. Column fractionation using benzyl alcohol and decalin as the solvent/nonsolvent pair is more practical in producing large quantities of narrow fractions, however. Molecular weight characterization of the fractions was accomplished by analytical SEC in HFIP/toluene (20 80) and by static LALLS work in straight HFIP. 

Uglea et al. (32) fractionated polyethylene terephthalate-ethylene isophthalate copolymer with the coacervate extraction technique, that is, successive extractions of the precipitated polymer using a solvent/nonsolvent pair, with an increasing solvent amount in the next round of the extraction process. The fractioned samples were dissolved in phenol/TCE (3 2, wt/wt) at ambient, and the SEC was later run in TCE-nitrobenzene (95 5, vol/vol) at 100°C. Elevated temperature (lOOX) was used, probably to increase the solubility of the copolymers in the eluant. 

Semicrystalline polymers can be fractionated according to their molecular weights by the solvent/nonsolvent approach at temperatures above their melting point, or by crystallizability by the temperature variation approach. Amorphous polymers are fractionated only according to their molecular weights. We will describe the fractionation of amorphous polymers first, followed by a discussion on the fractionation of semicrystalline polymers later. 

The precipitation fractionation method starts with a dilute polymer solution. Liquid-liquid separation, with the formation of a polymer-rich and a polymer-lean phase, is induced by either decreasing the temperature or by adding a specifled amount of a nonsolvent. A third, less common technique, involves the evaporation of solvent from a solvent/nonsolvent mixture. All these procedures increase the value of X, until it eventually exceeds its critical value, thus leading to phase separation. 

Rigorously speaking, for the case of solvent/nonsolvent precipitation fractionation the general mathematical treatment described above must be modified to treat the ternary solvent/nonsolvent/polymer system, but the trends are the same. It has been proposed that solvent/nonsolvent fractionation can be more effective for chain-length fractionation than temperature reduction, and in fact solvent/nonsolvent fractionation is generally a more commonly used technique to fractionate polymer chains according to molecular weight. 

A fully automated batch fractionation instrument called mc2-PREP is now available. Figure 10 shows a schematic of mc2-PREP. The two stirred fractionation vessels placed inside a temperature-programmable oven can fractionate polymers into eight fractions by either solvent/nonsolvent or temperatin-e variation techniques. Operator intervention is only required to precipitate and filter the polymer fractions after they are isolated from the parent polymer solution. 

Oxygen plasma ablation combined with scanning electron microscopy has been used to define the structure of hollow fiber membranes prepared from polysulfone in N-formylpiperidine/formamide solvent/nonsolvent mixtures (23). These asymmetric hollow fiber membranes possess a microscopically observable skin supported by a porous open cellular matrix. The oxygen plasma ablation studies indicated that the effective separating layer of the asymmetric hollow fiber membrane is only a small fraction of the thickness of this microscopically observable skin. Below the effective separating layer, this skin contains pores and channels with dimensions below the limits of resolution of the scanning electron microscope. 

Fig. 33 Effect of number-average molecular weight on Crystaf profiles. Each sample is a fraction of ethylene/1-hexene copolymers. Fractionation was carried out using solvent/nonsolvent extraction therefore, these samples have distinct molecular weights and contain similar amounts of 1-hexene [57]...

In the following discussions density, p (kg/m ), is defined as the weight of a nonsolvent-solvent mixture, free from polymer, in a unit volume of the film. Polymer is included in the latter unit volume. A quantity, u , is defined as a mass fraction in the solvent-nonsolvent mixture, also on a polymer-free basis. In order to emphasize the pseudobinary approach, subscripts I and 2 are used instead of n(nonsolvent) and s(solvent). Then the mass flux of the nonsolvent becomes... 

Subsequent studies by Ail baud. Gal lot and Skoulios dealt with block copolymers of MMA with hexyl methacrylate (HMA), lauryl methacrylate (LMA) and octadecyl methacrylate initiated with diphenylmethyl sodium at -70°C in THF. By solvent-nonsolvent fractionation of polymer pairs such as PMMA-b-PHMA and PHMA-b-PMMA they were able to distinguish differences in composition depending on which block was formed first. Thus when... 

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