Separation factors polymers

FIG. 22-73 Plot of separation factor versus permeability for many polymers, Oci/Nci. Abscissa— Fast Gas Permeability, p(02) Barrers. Ordinate— Selectivity, (X (O2/N2). ... 

Enantioseparation is typically achieved as a result of the differences in interaction energies A(AG) between each enantiomer and a selector. This difference does not need to be very large, a modest A(AG) = 0.24 kcal/mol is sufficient to achieve a separation factor a of 1.5. Another mechanism of discrimination of enantiomers involves the preferential inclusion of one into a cavity or within the helical structure of a polymer. The selectivity of a selector is most often expressed in terms of retention of both enantiomers using the separation factor a that is defined as ... 

B.-B. Li et al. [64] have studied the separation of EtOH-H20 solutions by pervaporation (PV) using chitosan (CS), poly (vinyl alcohol)-poly(acrylonitrile) (PVA-PAN) and chitosan-poly(vinyl alcohol)/poly(acrylonitrile) (CS-PVA/PAN) composite membranes. It was found that the separation factor of the CS-PVA/PAN composite membrane increased with an increase of PVA concentration in the CS-PVA polymer from 0 to 40 wt%. With an increase in the membrane thickness from 12 to 18 pm, the separation factor of the CS-PVA/PAN composite membrane increased and the permeation flux decreased. With an increase of ethanol-water solution temperature, the separation factor of the CS membrane decreased and the permeation flux of the CS membrane increased while the separation factor and the permeation flux of PVA/PAN and CS-PVA/PAN composite membranes increased. 

It may be seen that a very high separation factor of organic liquid isomers through polymer membranes has been obtained for PrOH isomers [84],... 

Not only chiral separations have been achieved with Mi-stationary phases. It has also been demonstrated that the MIP could distinguish between ortho- and para-isomers of carbohydrate derivatives. For example, a polymer imprinted with o-aminophenyl tetraacetyl P-D-galactoside was used to analyze a mixture of p-and o-aminophenyl tetraacetyl P-D-galactoside. As expected, the imprinted ortho analyte eluted after the non-imprinted para component see Fig. 5. Although baseline separation was not obtained, a separation factor of a = 1.51 was observed [19]. 

In a different approach, Lin et al. have used particles derived from a ground MI-bulk polymer and mixed with a polyacryl amide gel for chiral separation. Using a polymer imprinted with L-phenylalanine, D-phenylalanine could be separated from the template with a separation factor of 1.45 [35]. Although the combination of MIP with capillary electrochromatography is still not widely used, the ability to separate enantiomers in nanoliter samples promises interesting developments for the future. 

Finally the synthesis of inorganic-polymer composite membranes should be mentioned. Several attempts have been made to combine the high permeability of inorganic membranes with the good selectivity of polymer membranes. Furneaux and Davidson (1987) coated a anodized alumina with polymer films. The permeability increased by a factor of 100, as compared to that in the polymer fiber, but the selectivities were low (H2/O2 = 4). Ansorge (1985) made a supported polymer film and coated this film with a thin silica layer. Surprisingly, the silica layer was found to be selective for the separation mixture He-CH4 with a separation factor of 5 towards CH4. The function of the polymer film is only to increase the permeability. No further data are given. 

The CSPs prepared by the molecular imprint technique have also been used for chiral resolution by CEC [98-100]. Lin et al. [91] synthesized L-aromatic amino acid-imprinted polymers using azobisnitriles with either photoinitiators or thermal initiators at temperatures ranging from 4°C to 60° C. Methacrylic acid (MAA) was used as the functional monomer and ethylene glycol dimethacrylate (EDMA) was used as the cross-linker. The resulting polymers were ground and sieved to a particle size less than 10 pm, filled into the capillary columns, and used for enantiomeric separations of some amino acids at different temperatures. The relationships of separation factor and column temperatures were demonstrated to be linear between the logarithm of the separation factors and the inverse of the absolute temperature (Fig. 24). The authors also compared the obtained chiral resolution with the chiral resolution achieved by HPLC and reported the best resolution on CEC. The chromatograms of the chiral resolution of dl-... 

S) can be determined. The imprinted polymers show differences in their sensitivities towards the enantiomers of the analytes (Fig. 12, left), whereas the non-chiral reference polymer shows no difference between the two enantiomers (Fig. 12, right). Therefore, a chiral discrimination by the imprinted polymers is proved this can be described by a separation factor a (Table 1). The separation factor is defined here as the ratio of the sensitivity of the template to the sensitivity of the antipode. 

For the (S,S)-imprinted polymer a separation factor of 1.19 was achieved, while for the (R,R)-imprinlcd polymer an a value of 1.23 was found. The reference polymer shows nearly no chiral discrimination (a = 0.98) [30]. 

Polymer Temperature (°C) Mass % of acetic acid in the feed Permeation rate (kg/m2 h) Separation factor (a) Ref. no. 

The details regarding preparation of clenbuterol imprinted polymers, HPLC columns and detection have been described previously (Crescenzi et al., 1998). A typical chromatogram showing the resolution of clenbuterol and timolol from a mixture at pH values 2.0 and 3.4 is shown in Fig. 4.2. In terms of the selectivity of the stationary phase, expressed as separation factor a, the values at pH 2.0 and pH 3.4 were 3.1 and 14.4, respectively. For control particles, the a value was 1. 

Figure 9.6 shows the separation factors measured by Nijhuis et al. [28] for various membranes with dilute toluene and trichloroethylene solutions. The separation factor of silicone rubber is in the 4000-5000 range, but other materials have separation factors as high as 40000. However, in practice, an increase in membrane separation factor beyond about 1000 provides very little additional benefit. Once a separation factor of this magnitude is obtained, other factors, such as ease of manufacture, mechanical strength, chemical stability, and control of concentration polarization become more important. This is why silicone rubber remains prevalent, even though polymers with higher selectivities are known. 

The values of permeability coefficients for He, O2, N2, CO2, and CH4 in a variety of dense (isotropic) polymer membranes and the overall selectivities (ideal separation factors) of these membranes to the gas pairs He/N2,02/N2, and CO2/CH4 at 35°C have been tabulated in numerous reviews (Koros and Heliums, 1989 Koros, Fleming, and Jordan et al., 1988 Koros, Coleman, and Walker, 1992). Moreover, several useful predictive methods exist to allow estimation of gas permeation through polymers, based on their structural repeat units. The values of the permeability coefficients for a given gas in different polymers can vary by several orders of magnitude, depending on the nature of the gas. Thevalues oftheoverall selectivities vary by much less. Particularly noteworthy is the fact that the selectivity decreases with increasing permeability. This is the well-known inverse selectivity/permeability relationship of polymer membranes, which complicates the development of effective membranes for gas separations. 

The resorcarene 23a, on the other hand, was obtained by O-alkylation of the corresponding octol. Its incorporation in a dimethylpolysiloxane backbone led to a stationary phase by which proteinogenic amino acids could be separated into their enantiomers by GC of their 7V(0,S)-trifluoroacetylmethylesters with separation factors aLD = 1.025-1.102.50 The question remains in this case (and in similar cases), whether the chiral amide functions have to be attached to the resorcarene skeleton, or if a direct attachment to the polymer backbone via suitable spacers would lead to similar results. The chiral resorcarene octaamides 23b prepared by... 

Fig. 9. Separation factor (a o1) and specific permeation rate (R) of substituted polyacetylenes and other polymers in pervaporation (30 °C)...

The generic permselectivity of a membrane can be described by the retention coefficient for liquid phase or the separation factor for gas phase. Separation factor will be defined and discussed in Chapter 7. In the case of liquid-phase membrane separation, the retention coefficient of the membrane can be characterized by some commonly used model molecules such as polyethylene glycol (PEG) polymers which have linear chains and arc more flexible or dextians which arc slightly branched. The choice of these model molecules is due to their relatively low cost. They are quite deviated from the generally... 

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