Sieving polymers

Figure 9.8 Ferguson plots for polyethylene oxide used as sieving polymer. The graphs represent plots of the polymer concentration and the log of electrophoretic mobility for six different samples. Graphs A, B, and C represent polymers with MW of 100, 300, and 900 kDa, respectively (From A. Guttman, Electrophoresis, 16 611 (1995). With permission.)...

Up to now, a variety of non-zeolite/polymer mixed-matrix membranes have been developed comprising either nonporous or porous non-zeolitic materials as the dispersed phase in the continuous polymer phase. For example, non-porous and porous silica nanoparticles, alumina, activated carbon, poly(ethylene glycol) impregnated activated carbon, carbon molecular sieves, Ti02 nanoparticles, layered materials, metal-organic frameworks and mesoporous molecular sieves have been studied as the dispersed non-zeolitic materials in the mixed-matrix membranes in the literature [23-35]. This chapter does not focus on these non-zeoUte/polymer mixed-matrix membranes. Instead we describe recent progress in molecular sieve/ polymer mixed-matrix membranes, as much of the research conducted to date on mixed-matrix membranes has focused on the combination of a dispersed zeolite phase with an easily processed continuous polymer matrix. The molecular sieve/ polymer mixed-matrix membranes covered in this chapter include zeolite/polymer and non-zeolitic molecular sieve/polymer mixed-matrix membranes, such as alu-minophosphate molecular sieve (AlPO)/polymer and silicoaluminophosphate molecular sieve (SAPO)/polymer mixed-matrix membranes. 

Particle Size and Shape. The polymerization process for producing macroporous synthetic polymers (539) leads to the formation of spherical particles whose size can be controlled within certain limits. The popular XAD polymers are usually sold with approximately 90 of the total weight encompassing smooth beads with 20-50-mesh sizes. Most users incorporate a suspension step to remove the fines in their purification of the polymer, but they do not remove the small number of particles larger than 20 mesh. The particle size and distribution vary with different polymer batches, and it is advisable to mechanically sieve polymer beads and choose only those within the 20-50-mesh size for preparation of the adsorption columns. 

Further optimization of DNA sequencing of M13mp 18 DNA (up to 550 bases) on a microfabricated glass chip was achieved on separation buffer composition (no borate), sieving polymer concentration (> 2% LPA), device temperature (<... 

Using geometrical considerations, the mobility ji of a solute in the polymer solution is related to that in free solution /i0 and the concentration c of the sieving polymer by ... 

When plotting the log of the reduced mobility vs. the log of the molecular mass of the analyte the typical sigmoidal curves known from DNA analysis are also obtained. The concentration of the sieving polymer has to be beyond its entanglement threshold concentration c. A typical plot is shown in Fig. 10, where different concentrations of dextran T 2000 have been added to the running buffer. Also included are the reduced mobilities of the PSS in the plain buffer. As can be seen, the sieving properties are improved with increasing dextran concentra-... 

In Fig. 11 the influence of the molecular mass of the sieving medium is demonstrated. There is no difference between dextran T 500 and T 2000 at identical concentration. Less efficient separations are only observed with dextran T 70. The mass concentration here is just around the overlap threshold. For practical reasons it is advisable to use the sieving polymer which gives the lowest viscosity in the buffer solution at the required concentration. 

The selection of the sieving polymer follows to a great extent the rules applied in DNA separations. Studies have revealed [27] that the efficiency of the sieving polymer depends on its chain stiffness and its hydrophobicity. The higher the flexibility of the polymer and the smaller its gyration radius due to hydrophobic interactions, the higher the concentration in the buffer has to be in order to achieve comparable separation efficiencies. 

When comparing different sieving polymers, it seems reasonable to keep the mesh size in the solution constant. As shown, beyond the threshold concentration the mesh size seems to be independent of the molecular mass of the polymer. Consequently, it is only important to compare the sieving properties as a... 

Using sieving polymer solutions as buffer additives can enhance resolution [33,34]. However, the mass range of the solutes is far below that necessary for the high resolving reptation regime. 

In a following section our results on optimization of the type of sieving polymer, its molecular mass, and its concentration will be described. 

The latter, however, is also influenced by other variables, e.g. the viscosity of the buffer solution. On the other hand, it has been shown that the viscosity has no direct relationship to the sieving potential of the polymeric additive. In characterizing the potential of a sieving medium one is more interested in mobility differences than in their absolute values. The addition of the sieving polymer already influences the migration velocity of small (mono-molecular) molecules. The migration time of a small molecule increases with increasing polymer concentration as shown in Fig. 7 for the standard (4-aminopyridine) with different polymeric additives. 

Whereas mobility differences stay constant differences between migration times increase with increasing separation time. This results in better resolution due to longer analysis times (corresponding to the use of a longer capillary), when increasing the concentration of the sieving polymer. Therefore,... 

Fig. 7 Dependence on sieving polymer concentration of the migration time of the internal standard 4-aminopyridine in the p(2-VPy) standard mixture...

Different sieving polymers of identical molecular masses show, however, significant differences in their selectivity. This becomes obvious from Fig. 13, where the influence on selectivity for all concentrations and molecular masses for all the studied sieving media is summarized. It can be seen... 

Fig. 12 Dependence of the size selectivity, m, on the concentration and molecular mass of the sieving polymer poly(ethylene glycol)...

Fig. 13 Superimposition of all investigated sieving media in one Dolnik plot. Corresponding sieving polymers carry identical symbols and line styles...

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