Structure and gas transport properties

We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

We report here on the structure and gas transport properties of asymmetric membranes produced by the LB deposition of a polymeric lipid on porous supports. The effects of temperature on the structure and gas transport is described. The selectivity of CO2 over N2 permeation through the LB polymer films is determined. The polymerized lipid used in this study contains tertiary amines which may influence the CO2 selectivity over N2. The long term objective of our work is to understand how structure and chemistry of ultrathin films influence the gas permeation. 

The journal and patent literature was searched to identify the 87 polymers used in this work. They include members from the polymers, their structure, and gas transport properties are provided in the Appendix. Carbon dioxide is reported to permeate many polymers by a dual mode transport mechanism (13). It has also been noted that COj has a plasticizing effect on some polymers leading to decreased permselectivity at high pressure (14). For these reasons we have preferentially used pure gas permeability data obtained at 10 atm CO2 and 35°C. 

It should be however kept in mind that the membrane preparation procedure could influence its structure and gas transport properties. Thus, casting of the integrally skinned asymmetric membranes from p-DMePO solution using different nonsolvent additives produced the nodule structures in the surface skin layer of the membranes, which affected the permeance ratios for O2/N2 and CO2/CH4 [72]. In the homogeneous films of polyphenylene oxides considered in this chapter such structures apparently do not exist. 

Chern R. T., Repeat Unit Structure and Gas Transport Properties of Aromatic Polymers, Sep. Sci. andTechnol., 1990, 25, 1325-1338. 

The free volume approach has been an increasingly popular method to relate polymer structure to gas transport properties. The basic premise of this technique is that a polymer with an open, poorly packed structure will have a large unoccupied free volume through which a gas can diffuse with ease. In a typical model, set forth by Lee (d2.) a specific free volume, SFV, is derived from the difference between the molar volume, Vm, (determined from the experimental density of a polymer) and the occupied volume, Vo, (calculated using a group additive method, in this case, that of Bondi) (4(1). ... 

Chng, M.L., Xiao, Y., Chung, T.S., Toriida, M. and Tamai, S. 2007. The effects of chemical structure on gas transport properties of polyfaryl ether ketone) random copolymers. Polymer 4S 311-317. 

Those results motivated us, in the present work, to stndy the structure, physical characteristics and gas transport properties of membranes made of different grades of Pebax block copolymers and their blends with PEG (Table 13.1). 

The separation of gas mixtures by polymeric membranes has become a commercially important methodology for a number of different systems (1). Several recent review articles have discussed the interaction between polymer structure and gas permeability properties (2,3). The quantification of the effect of polymer structure on gas transport properties recently has been reported (4,5) and it is now possible to optimize gas transport properties for well defined polymeric materials. For those materials which do not have a well defined data base it is necessary to prepare and measure the gas transport properties. The polyamide-imides (PAI) are a class of polymeric materials which do not have an extensive data base for gas transport properties (6,7). Work by Yamazaki and coworkers (8) demonstrated that PAI materials could be prepared easily and in a manner whereby the amide bond could be prepared from a phosphite activated carboxylic acid and an aromatic amine. Yang and CO workers (9-11) have shown that novel dicarboxyl ic acids could be prepared from trimellitic acid anhydride (TMA) and aromatic diamines and that these dicarboxylic acids could be coupled with a second diamine to form regiospecific PAI materials. Our focus was to examine the effects of a phenylene diamine and its alkylated analogs on the gas transport properties of regiospecific PAI materials and to identify those structures which maximized both permeability and selectivity. 

It is well known that the performance of the air gas-diffusion electrode is influenced not only by the activity of the catalyst, but also by all transport processes taking place in its porous structure. In addition, the transport hindrances in the electrode are function not only of its overall structure, but also of the porous structure and the surface properties of the catalyst. Methods for diagnostic of the activity and the transport properties of air gas-diffusion electrodes were proposed [9]. 

Figure 2.24 Correlation of the oxygen permeability coefficient for a family of related polysulfones with inverse fractional free volume (calculated using the Bondi method) [33]. Reprinted with permission from C.L. Aitken, W.J. Koros and D.R. Paul, Effect of Structural Symmetry on Gas Transport Properties of Polysulfones, Macromolecules 25, 3424. Copyright 1992, American Chemical Society...

The chemical composition, structure, and, hence the properties of products with modified surface are determined both by observing the required sequence of operations, and chosen chemico-technological parameters of process the chemical nature of reagents (volatile and solid), temperature (in stages of preparation of surface, chemisorption and desorption), concentration of reagents (in gas phase and functional groups on surfaces of substrate), hydrodynamics of the process (rate of transport and removal of reagents, mobility or stationary condition of disperse solid phase). 

The polyimide is prepared from 1,5 naphthlene diamine (1,5 ND) and 3,3, 4,4 diphenylhexafluoroisopropylidene tetracarboxylic acid dianhydride (6F). A polyamide acid is obtained initially and this is cyclized to give the polyimide. Because polyimides are derivatives of polyamides, polyimides are often included in reviews of aromatic polyamides. Another reason polyimides are included in this paper is that they illustrate the role of Structure Level II on gas transport properties in a straightforward manner. 

To date, reports of investigations on the gas transport properties of main chain liquid crystalline polymers appear to have been limited to the work conducted in our laboratory. Chiou and Paul (4.) have briefly described the transport parameters of an extruded film of an LCP having a similar structure to the commercial product Vectra. This copolyester belongs to the family of napthylene thermotropic polymers (NTP s) commercialized by Hoechst-Celanese Corp. whose synthesis and properties have been described previously (iLS.). Transient permeation experiments were conducted with a series of gases. The effective... 

Recent evidence indicates that the influence of molecular structure on gas permeation through polymers is complex. For example, reports investigating series of structurally varied polyimides (5-7), polyacetylenes (2), polystyrenes (2) and silicone polymers (12) show that gas transport rates within a particular polymer class can vary dramatically depending upon the structure of the monomer present. These observations on materials where the monomer changes while the functional "link" remains constant suggest that structural factors other than the polymer class are significant in determing gas transport properties. 

Shao et al. (2005a) also used ethylenediamine (EDA) to cross-link Pis based on its unique linear structure, small molecular volume, and the potential of its functional groups to react with Pis. Thermal annealing processes at 100°C and 200°C were also performed on the membranes in order to study their effects on the separation performance of EDA-cross-linked Pis. The permeation results demonstrated that the EDA cross-linking can effectively decrease the permeability and increase the selectivity of Pis for He/N2 and H2/N2 separations. However, there were no such increases for O2/N2 selectivity, and CO2/CH4 selectivity was found to decrease with cross-linking. The gas transport properties of the EDA-cross-linked and then thermally treated Pis also show high selectivity for He/N2 and H2/N2 separations. 

This chapter covers multi-step work of tailoring C02-selective membrane material, from new copolymers designs to tailor-made copolymer/PEG blends, moreover thin film composite membrane performance are also discussed. The relationships between gas transport properties, structure, morphology and physical properties are analyzed. The performance at different operating conditions and with mixed gases is monitored as well in order to have a guideUne for scaling up the membranes. The benefits of these membranes are the simplicity of preparation, low cost and resistance toward acid gas treatment. 

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