Polystyrene sulphonate Fractionation

Field flow fractionation has been used to fractionate and determine the molecular weight of sodium polystyrene sulphonate" and polystyrene sulphonate (molecular weight range 6.6 x 10 to 6.9 x 10 ). 

A C4 fraction and methyl alcohol (molar ratio methyl alcohol i-C4 = 1 1) is preheated to about 70°C before being sent to the reactor containing a fixed bed of sulphonated polystyrene resin. The reaction is exothermic (AH298=-37.7 kj/mol) and the heat produced is removed by means of cooling jackets so as to keep the temperature below 120°C. The reaction mixture from the top of the reactor is distilled and any unreacted butene is collected overhead with the azeotropic amount of methyl alcohol. The bottom contains pure MTBE. The distillate, together with additional methyl alcohol, is passed to a second reactor. The products from the second stage are extracted with water to remove residual methyl alcohol. The water-methyl alcohol solution is distilled to recover methyl alcohol, which is recycled. 

Many schemes for fractionating nucleotides, nucleosides and bases on sulphonated polystyrene resins have been published. The main difficulty with these methods is variation between resin batches (e.g. Anderson et al. 1963). Nucleotide separations can be achieved on DEAE-cellulose (Whatman Data Sheet 13, 1967) and DEAE-Sephadex (Piers et al. 1965b) but these media do not seem to be widely used. Gel filtration columns will separate some nucleotide components. Ligand exchange chromatography and partition chromatography of nucleosides are useful for minor components. 

Flow FFF is perhaps most promising in the area of water-soluble polymers. These polymers, which as a class are very difficult or impossible to separate by thermal FFF, can be fractionated according to diffusion coefficient or Stokes radius (which translate to molecular mass) in a flow FFF system using a water-compatible membrane such as cellulose acetate. Such a fractionation is shown in Figure 8.15, illustrating the programmed field separation of three sulphonated polystyrene components in a 510-//m-thick channel. The fact that the time of separation is somewhat longer than desired can be related to the excessive thickness of the channel, ten times thicker than the thinnest thermal FFF channel utilized. Recently we have been able to work successfully with a... 

The /i-Bondagel grades cover a wide fractionation range of 2 to 7000 kllodaltons. They are compatible with water and buffered eluents and offer a high temperature stability of up to 408 K. A large number of aqueous SEC separations on these packings has been reported In the literature, e.g. for sulphonated polystyrenes and other anionic polyelectrolytes, poly(vlnylalco-hols) , polysaccharides , proteins and enzymes . 

Field flow techniques have been reviewed in a number of articles [148-150]. Sedimentation field flow fractionation has found use in the separation of PVC [151, 152], polystyrene [151-153], poly(methyl methacrylate) [153, 154], poly (vinyl toluene) [155] and poly(glycidyl methacrylate) latexes [156] to produce particle-size distributions and particle densities. It has also been applied in polymer-aggregation studies [157], pigment [157] quality control and in the separation of silica particles [158] and its performance has been compared with that of ultracentrifugation [159]. Thermal field flow fractionation has been used successfully in the characterisation of ultra-high-molecular-weight polystyrenes [160, 161], poly(methyl methacrylate), polyisoprene, polysulphane, polycarbonate, nitrocellulose, polybutadiene and polyolefins [162]. In the difficult area of water-soluble polymers, poly(ethylene glycol), poly(ethylene oxide), poly(vinyl pyrrolidone) and poly(styrene sulphonate) have been analysed [163, 164]. In addition, compositional separations have been achieved for polystyrene-poly(methyl methacrylate) mixes [165] and comparisons between TFFF and SEC have been made [166]. 

Thielking and Kuliche" used a combination of flow field flow fractionation and multiple laser light scattering for the characterization of a sulphonated polystyrene solutions. These workers examined seven sulphonated polystyrene standards (1800 - 3 x 10 g mole ), which were taken as model substances for macromolecular polyelectrolytes. The technique was applied to O.IM sodium nitrate solutions of the polyelectrolytes. 

A pellicular chromatographic packing consists of a rigid core with a relatively thick outer shell. The outer shell is cross-linked by the inclusion of a difunctional monomer and for added stability the outer shell may be covalently attached to the core. An example of this type of packing is the material developed for carbohydrate analysis, CarboPac (Dionex), which is a pellicular anion exchange resin bead which has a 5 pm non-porous sulphonated polystyrene bead core with a 0.1 pm quaternary amine latex. These materials have gained widespread acceptance for analytical separations and small-scale fractionations. 

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