Chapter 26
Oxygen Transport Through Electronically Conductive Polyanilines 1
2
2
2
2
Yong Soo Kang , Hyuck Jai Lee , Jina Namgoong, Heung Cho Ko , Hoosung Lee , Bumsuk Jung , and Un Young Kim 1
1
Structure and Properties of Glassy Polymers Downloaded from pubs.acs.org by TUFTS UNIV on 08/05/18. For personal use only.
1
Division of Polymer Science and Engineering, Korea Institute of Science and Technology (KIST), P.O. Box 131 Cheongryang, Seoul, Korea Department of Chemistry, Sogang University, Mapo-ku, Seoul, Korea 2
Emeraldine base of polyaniline was synthesized by a chemical oxidation polymerization technique. The resulting emeraldine base film was treated with 4M HC1,1M NH OH, and subsequently with varying dopant HC1 concentrations. The oxygen and nitrogen permeabilities through the doped polyanilines decreased with their doping level, while their oxygen selectivity over nitrogen increased up to 12.2 when doped with 0.0150 M HC1 solution. When doped with 0.0175 M HC1, the membrane selectivity is expected to be higher but immeasurable because of extremely low permeability of nitrogen. The origin of such high selectivity is explored in terms of the facilitated transport and free volume change upon doping. Because the polarons generated upon doping react with oxygen molecules specifically and reversibly, these polarons can act as oxygen carriers and thereby facilitate oxygen transport. It is, therefore, expected that the oxygen permeability increases with the polaron (carrier) concentration. However, permeability for oxygen in these materials decreased with the increase in polaron concentration. Instead, the permeability correlated well with the d-spacing orfreevolume, measured via x-ray diffraction. It is found that although facilitated oxygen transport may occur, its contribution to oxygen permeability is insignificant. The free volume change upon doping seems to play a major role in determining gas permeation. 4
Electronically conductive polyaniline has been paid much attention as a potential membrane material for gas separation because of high selectivity, particularly very high selectivity of oxygen over nitrogen (1-4). In addition, it is thermally stable and soluble in NMP, not like common electronically conductive polymers such as
©1998 American Chemical Society
383
384 polyacetylene, polypyrrole, etc. (5,6). Therefore, it can be easily processed to form membranes. The reported transport properties strongly depended upon doping conditions and doping level (1-4). It has been known that the chemical and physical natures of polyaniline can be changed by doping. For example, paramagnetic polarons can be generated by doping with protonic acids such as HC1, HF, camphor sulfonic acid, etc. as illustrated in Figure 1 (7). Since oxygen is also paramagnetic, specific interaction H
Η Emeraldine Base
Η
Doping with Protonic acid , (e.g. HCI) Η
I!
Η Formation of Bipolaron
Η
j Internal Redox rxn
Η Η Formation of Two Polarons Polaron Separation
Η
Η
Formation of Polaron Lattice
Figure 1. Schematic doping mechanism of emeraldine base polyaniline.
385 between polarons and oxygen molecules is expected. This interaction has been investigated by EPR spectroscopy and electronic conductivity and found to be reversible (8). When the interaction between oxygen molecules and polarons is assumed to be 1 to 1, it can be expressed as a simple reversible chemical reaction: 0 +Po[0 -P] 2
(1)
2
where Ρ is the polaron, and [0 -P] is the oxygen-polaron complex. The polaron can possibly act as an oxygen carrier because it reacts with oxygen molecules specifically and reversibly (8). The oxygen transport could, then, be facilitated due to the presence of the oxygen carrier in addition to the normal Fickian permeation (9,10). The facilitated oxygen transport might result in a high oxygen permeability as well as a high oxygen selectivity over nitrogen. It has been reported that densification of polyaniline can occur when doped with protonic acids (11). The densities of as-cast, fully doped and redoped polyanilines were reported to be 1.3, 1.4 and 1.32 g/cm , respectively (11). The densification causes a reduction in the free volume, through which gas molecules can permeate. Therefore, it is also possible that the doping or dedoping process results in afreevolume change which dictates the gas permeability. In this study, it will be attempted to interpret the 0 permeability and 0 /N selectivity in terms of the facilitated transport due to the presence of the paramagnetic polarons and thefreevolume change upon doping with protonic acids. 2
3
2
2
2
EXPERIMENTAL Polymerization of aniline : Polyaniline was prepared with oxidative polymerization of aniline in aqueous acidic media (1M HQ) with ammonium persulfate as an oxidant by following the method used by Mattes et al. (1,2). The molar ratio of monomer/oxidant used was 4/1. The reaction was carried out at 0°C for 3 hours, and the precipitate was formed during the reaction. The precipitate was, subsequently,filteredand washed with deionized water until thefiltratewas colorless. The as-synthesized polyaniline in its protonated form was treated with 1M NH OH for 15 hours to yield emeraldine base powder, followed by drying under vacuum for over 48 hours at room temperature. Doping and dedoping of polyaniline membranes with aqueous HC1 solution : The emeraldine base powder was dissolved in NMP (8 wt %). The emeraldine base solution in NMP was cast onto a glass plate and the solvent was removed under 120°C for 3 hours. The as-cast membrane was, then, immersed into a 4M HC1 solution for 24 hours to give a fully doped membrane. The fully doped membrane was completely dedoped by immersion into 1M NH OH solution for 48 hours. The dedoped membrane was subsequently redoped with 0.0150, 0.0175, 0.0200 (± 0.0002), and 1 M HC1 solutions. Each membrane was dried under vacuum for 48 hours at room temperature. EPR experiment : Narrow strips of polyaniline membrane (1mm χ 5mm) were 4
4
386 put into an EPR cell. In order to study the interaction of oxygen molecules with polarons, the cell was connected to a large volume (1 L) of oxygen reservoir so that the applied pressure maintained constant during the experiment. The EPR line intensity was monitored with oxygen contact time from a Bruker EPR spectrometer (ER 200E-SRC). The spectrum obtained was doubly integrated to obtain the polaron concentration. X-ray experiment : X-ray diffractogram was obtained by an X-ray diffractometer (Rigaku Geigerflex D/Max-B System). The d-spacing was obtained using the Bragg's law
