Nonsolvent fractionation

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... 

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. 

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. 

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... 

Based on these ideas, the intrinsic viscosity (in 0 concentration units) has been evaluated for ellipsoids of revolution. Figure 9.3 shows [77] versus a/b for oblate and prolate ellipsoids according to the Simha theory. Note that the intrinsic viscosity of serum albumin from Example 9.1-3.7(1.34) = 4.96 in volume fraction units-is also consistent with, say, a nonsolvated oblate ellipsoid of axial ratio about 5. 

The first term reflects the fact that, in practice, volume fraction is not the concentration unit ordinarily used. Even for nonsolvated spheres, some factors will modify the Einstein 2.5 term merely as a result of reconciling practical concentration units with

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They showed further that the limiting slope (RTA2) of the plot of the osmotic pressure-concentration ratio tz/c against the polymer concentration in a binary solvent mixture should be proportional to the value of the quantity on the left side of Eq. (17),f with V2 representing the volume fraction of solvent in the nonsolvent-solvent mixture which is in osmotic equilibrium with the solution. The composition of the liquid medium outside the polymer molecules in a dilute solution must likewise be given by V2. The composition of the solvent mixture within the domains of the polymer molecules may differ slightly from that outside owing to selective absorption of solvent in preference to the nonsolvent. This internal composition is not directly of concern here. If the solution is made sufficiently dilute, the external nonsolvent-solvent composition v2 = l—Vi) will be practically equal to the over-all solvent composition for the solution as a whole. Hence... 

Principles and Characteristics Fractional solution procedures usually consist of consecutive extractions with solvents of increasing solvent power. These labour intensive methods benefit from a larger surface area to mass ratio. Other methods for fractionation by solubility rely on fractional precipitation through addition of a nonsolvent, lowering the temperature or solvent volatilisation (Section 3.7). 

Knockout or drown-out is generally not preferred as it involves adding a further extraneous material to the process. If it is to be successful, it requires a steep solubility curve versus the fraction of nonsolvent added. 

The products obtained in IBVE-aMeSt block copolymerization were fractionated with 2-propanol, a good solvent for poly(IBVE) and a nonsolvent for poly(aMeSt). Table II shows molecular weights and compositions of typical blocking products. Figure 7 illustrates examples of 1H-NMR spectra of the 2-propanol-soluble and -insoluble fractions. 

For both kinds of polymer-grafted particles, flocculation was induced either by changing the temperature or by adding a nonsolvent for the stabilising polymer. In this way critical flocculation temperature (c.f.T) and critical flocculation vol. fractions of non-solvent (c.f.v.) values were obtained, in general as a function of . 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

The data plotted in Figure 3.6 were obtained by the fractionation of a polydisperse polymer sample. Polydisperse polymers can be fractionated by a number of techniques. The most widely used technique is chromatography. Other methods include addition of a nonsolvent to a polymer solution, cooling a polymer solution, solvent evaporation, extraction, diffusion, and centrifugation. The molecular weight of the fractions may be determined using any of the classic techniques given in Table 3.3. 

Fractional precipitation is dependent on the slight change in the solubility with molecular weight. When a small amount of miscible nonsolvent is added to a polymer solution, the product with the highest molecular weight precipitates first. The procedure is repeated after the precipitate is removed. Molecular weights are run for each fraction and a curve developed that is similar to Figure 3.6. 

Figures 6 and 7 illustrate the preposed mechanism in OC. Using the specific example of a separation of a styrene n-butyl methacrylate copolymer, the first SEC separates the copolymer according to molecular size in solution. At any desired retention time, the flow in the first instrument is stopped and an injection made into the second instrument of a single molecular size "slice" of the chrcoiatogram. The solvent running in the second instrument is a mixture of tetrahydrofuran (THF) and n-heptane. THF is a solvent for both styrene cuid n-butyl methacrylate portions of the polymer molecules. However, n-heptane is a nonsolvent for the styrene-rich portions. As a result, vrfien the injection is made into the second instrument, the styrene-rich molecules will shrink relative to the n-butyl methacrylate-rich molecules. An immediate size distribution will be present vrfiich will reflect the composition differences. The smaller styrene-rich molecules will enter more pores of the column packing than their n-butyl methacrylate-rich counterparts and so be fractionated. Furthermore, since the styrene-rich molecules "hate" the mobile phase, they should find the surface area of the packing more "sticky" than the n-butyl methacrylate-rich molecules. Thus, again the styrene-rich molecules should be retarded relative to the others. According to this picture, the mechanisms of size exclusion, adsorption and partition are thus able to act synergistic ally to accomplish a composition separation.

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. 

When the symmetry factor was introduced by Volmer and Erdey-Gruz in 1930, it was thought to be a simple matter of the fraction of the potential that helps or hinders the transfer of an ion to or from the electrode (Section 7.2). A more molecularly oriented version of the effect of P upon reaction rate was introduced by Butler, who was the first to apply Morse-curve-type thinking to the dependence of theenergy-dis -tance relation in respect to nonsolvent and metal—hydrogen bonds. 

The fractionation column is operated at conditions effective to form a precipitate on the support. The temperature of the column is lowered to a temperature less than about 40°C at a rate of l-0.5Khr 1. The PE fractions may then be recovered from the fractionation column by displacing a recovery solvent/non-solvent mixture into the column. The relative concentrations of the solvent and the nonsolvent are based on a solvent gradient profile of the PE parent polymer. The temperature of the column is raised to a temperature ranging from about the melting point temperature of the UHMWPE... 

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