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Water Sorption of Acid-Doped Polyaniline Solid Fibers: Equilibrium and Kinetic Response Mayur M. Ostwal,† John Pellegrino,*,‡ Ian Norris,‡ Theodore T. Tsotsis,† Muhammad Sahimi,† and Benjamin R. Mattes‡ Department of Chemical Engineering, University of Southern California, Los Angeles, California 90089, and Santa Fe Science and Technology, Inc., 3216 Richards Lane, Santa Fe, New Mexico 87507
Measurements are reported for the sorption equilibrium and transport of water into commercially produced polyaniline solid fibers from a 50% relative humidity air stream at ambient pressure of ∼0.1 MPa and 300 K. The data have been collected using a single solid fiber morphology, PANION, but with different acid dopants used to change the polyaniline from its emeraldine base (insulating), PANI-EB, to the emeraldine salt form (conducting), PANI-ES. The sorption process was well described by unsteady Fickian diffusion into an infinite cylinder. The rates and equilibrium capacities depended on the acid dopant utilized, but did not vary monotonically with the anion size or the acid strength. The measured adsorption/desorption water capacities varied between 20 and 75 (mg of H2O)/(g of dry polymer), depending on the anion dopant. The BF4- and H2PO4- doped fibers had the highest capacities. When the mass loading was recalculated on a (H2O molecule/polymer repeat unit) basis, most doped fibers had a capacity of ∼0.5 H2O per PANI-ES repeat unit. The exceptions were the BF4- and H2PO4- doped fibers, for which the value was ∼1. The apparent diffusion coefficients varied between 0.05 and 0.6 × 10-12 m2/s and were usually larger during adsorption than desorption. The water capacity of the polyaniline solid fibers is at the upper end of what is usually observed for glassy polymers and provides the possibility for exploiting electronic conductivity, good mechanical strength properties, and desiccant qualities for advanced humidity control and sensing applications. Introduction Polyaniline (PANI) is a member of the family of electrically conducting polymers. A great number of articles, reviews, and books about conducting polymers and their applications are available in the literature,1-7 and therefore, only a brief description will be presented here. The intrinsically conductive nature of these polymers arises from a unique bonding structure along the polymer backbone, consisting of alternating double (π) and single (σ) bonds. If an electron is added to the conjugated polymer backbone (via reduction, n-type doping) or removed from it (via oxidation, p-type doping) during the chemical or electrochemical doping process, then the charge can freely travel down these conjugation paths when an electrical potential is applied, assuming the polymer’s conjugation length is sufficiently long. The electrical conductivity for the doped polymers can vary from semiconducting (10-5 to 10-1 Ω-1cm-1) to metallic (102 to 105 Ω-1cm-1), depending on the extent of doping. The conductivity achieved depends strongly on the type of dopant, the polymer characteristics (such as specific repeat unit, molecular mass, polydispersity, and chain defects such as branching and chemical heterogeneity), and how the polymer was processed. For example, stretching doped conducting polymer films and fibers can increase their conductivity by 2 orders of magnitude as a result of the anisotropic alignment of the polymer chains.8 * Corresponding author’s full current address and contact information: CEAE Department, ECOT-411, University of Colorado, Boulder, CO 80309-0428. Tel.: (303) 735-2631. Fax: (303) 492-7317. E-mail:
[email protected]. † University of Southern California. ‡ Santa Fe Science and Technology, Inc.
Figure 1. Polyaniline repeat structure. When it is half-oxidized and half-reduced, it is called emeraldine base (EB). The reduced units are referred to as “benzenoid” and the oxidized as “quinoid”. Reversible acid/base chemical doping of polyaniline occurs when PANI-EB is reacted (doped) with a strong protonic acid to form the emeraldine salt (PANI-ES).
Polyaniline has three oxidation states; however, only the emeraldine base (EB) form (shown in Figure 1) can be chemically doped by a protonic acid to become electrically conductive without a change in its oxidation state. Because there are two nitrogen atoms in the polymer repeat unit, a secondary amine and a tertiary imine, PANI-EB is weakly basic with a pKa ) 8.6. Unlike all other conjugated polymers, which require redox doping to “turn on” the electrical conductivity pathways, any protonic acid activates PANI-EB. This acid/base chemistry is shown in Figure 1 for the interaction (doping) of PANI-EB with a generic aqueous protonic acid (HX) and for the reverse reaction (dedoping) with a typical base, that is, ammonium hydroxide.
10.1021/ie050625b CCC: $30.25 © 2005 American Chemical Society Published on Web 08/27/2005
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This acid/base reaction does not change the polymer, as is the case with redox doping. It is assumed that the proton from an acid first reacts preferentially with the base imine nitrogen in the initial doping to first form a diamagnetic “bipolaron” form of protonated polyaniline until ∼50% of the available imine sites are coordinated with protons. Further acid doping above this level causes an internal redox reaction to yield the polysemiquinone stable radical cationic magnetic polaron form (shown in Figure 1), which exhibits Pauli (metallic) susceptibility. The fully doped PANI-ES (emeraldine salt form) powders have conductivities in the range 100 to 101 Ω-1cm-1, while doped stretched films are highly conductive and have conductivities in the direction of stretching up to ∼103 Ω-1cm-1 at room temperature. For separation applications using conducting polymers, it is important to bear in mind that the dopant counterion is closely bound to the doped polymer’s ionic repeat unit sites by electrostatic forces, which serve to maintain the overall charge neutrality of the system. Thus, the volume conductivity is truly electronic and not simply ionic in nature, although ionic conductivity may also be implicated in some transport mechanisms. Solution processing of PANI into fibers is extraordinarily difficult because of the following: (a) poor solubility in solvents compared with normal engineering plastics, (b) very rapid polymer gelation times at low total solids content, and (c) strong aggregation tendency due to interchain attractive forces. There have only been a few open literature reports9-17 regarding the subject of reproducible fiber spinning of PANI. Recently, Mattes et al.18 have reported the development of polyaniline fiber production techniques that have been used to create commercializable fibers (PANION); these techniques hold good promise for further significant advances in the field of solid-state electrochemical devices.19 While it is well-known that water present in the polyaniline matrix affects the conductivity of PANI in its emeraldine salt oxidation state,20,21 only a few studies have been performed on the mechanism of water sorption by PANI. The earliest work indicated that there existed two forms of water adsorbed by the materials weakly bonded water molecules that possess an activation energy of desorption of ∼5 kcal/mol and strongly bonded water molecules that are only desorbed during simultaneous decomposition of the polymer backbone.21,22 In a related study, it has been suggested that these water molecules form hydrogen bonds with the acid sites in the emeraldine salt form of polyaniline.23 More recent work has suggested that reversibly absorbed water consists of two forms, that is, the hydrogenbonded water (as found in previous studies with a desorption energy of 5 kcal/mol) and another form with a desorption energy of 15-18 kcal/mol.24 This energy for desorption of the water molecules exceeds that of a single hydrogen bond and could correspond to the formation of a chemical bond with the polymer backbone. In this paper, we have limited ourselves to working at a single temperature and mass transfer driving force in order to compare the water sorption kinetics and mass uptake of the PANI with respect to the dopant anion. We present initial results from investigations of the water vapor sorption in solid PANI fibers doped with eight protonic acids, including halide, mineral, and organic acids. Our work was motivated by potential applications of these fibers in air dehydration (for example, humidity control), water recovery from atmo-
Table 1. Polyaniline Solid Fiber Samples sample ID PS-1 PS-2 PS-3 PS-4 PS-5 PS-6 PS-7 PS-8 PS-9
acid dopant
estimated30,31 acid dopant van der Waals volume pKa of dopant anion (Å3)
none NA HCl (hydrochloric) -7 HBr (hydrobromic) -9 HI (hydroiodic) -10 H3PO4 (phosphoric) 2.13 HBF4 (fluoroboric) 0.5 HCOOH (formic) 3.75 CF3SO3H (triflic) -10 CH3COCOOH (pyruvic) 2.39
NA 25.3 29.6 35.7 47.9 41.6 40.8 89 81.6
spheric sources, and ambient composition sensors, as well as a variety of other potential uses where water vapor transport in the fiber may be of importance. Materials and Methods Fibers. Solid fibers of polyaniline were prepared following the procedures patented by Mattes et al.18 All the fibers were from a single production run. They were autoclaved for 1 h with 15 psig steam as an initial conditioning step. All the fibers were further de-doped in 1 M NH4OH for 0.5 h and then rinsed completely. The fibers were then allowed to dry under ambient conditions for about 24 h. After autoclaving, but prior to the complete de-doping step, the fibers had the following nominal properties: diameter ∼65 µm ((2 µm), electronic conductivity 170 S/cm ((10 S/cm), elastic modulus ∼7.8 GPa ((0.6 GPa), and extension at break ∼3.1% ((0.4%). The conductivity and mechanical properties of these fibers were not optimized for any particular application and, prior to the autoclaving step, were within the ranges cited by Mattes et al.18 Sets of these fibers were then re-doped with one of the acids listed in Table 1 (except the EB samples). The fibers were doped by immersion in an aqueous solution of the acid at ambient temperature for 24 h and were then allowed to air dry for another 24 h. The solution pH was kept at 2 to ensure the same proton concentration in the solution during doping (this means that the acid concentration varied depending upon its pKa). This approach was taken to ensure that all the fibers contained the same stoichiometry of emeraldine salt, that is, they were all “fully doped” independent of the particular acid’s strength. The final mass of all sets of doped fibers was within (0.5 g (depending on the acid used for re-doping) of their starting value (∼3 g) prior to the autoclaving. Adsorption/Desorption Apparatus. Figure 2 presents a schematic of the apparatus. The adsorption volume, referred to as the “reactor” (component 9 in Figure 2), is a small stainless steel tube (length ) 7.62 cm and internal diameter ) 1.02 cm) with a volume of 6.24 cm3. The apparatus has separate lines that are individually utilized during the adsorption and desorption parts of the cycle. A humidity probe (Rotronic Instruments, HW3) is at the outlet of the reactor to measure the exit stream’s relative humidity, from which, in turn, one can calculate the exit mass flow rate of water and its concentration. The probe collects readings every 1 min and stores them in the computer. Component 4 in the apparatus is a buffer volume (∼300 cm3), which ensures proper mixing of the dry air and the water vapor so as to achieve the desired RH. It is filled with glass beads to provide more surface area and resistance to flow.
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Figure 2. Schematic of the adsorption/desorption apparatus.
Adsorption/Desorption Measurements. At the beginning of each series of tests, measurements are first made in the absence of the fibers (referred to as the “blank” tests). Since we are operating at close to temperature and require sensitivity to small levels of water sorption, artifacts from wall adsorption in the apparatus is a concern. Therefore, the control measurements of the adsorption/desorption of the system without fibers were used to correct the later measurements with fibers. The measurements were made by placing a sample of fibers (∼1 g) into the reactor and then evacuating overnight. Following that, the fibers were exposed to humid air (adsorption) and dry air (desorption) consecutively for two cycles. Each cycle consisted of 2 h of adsorption, followed by 2 h of desorption; the cycle was then repeated. All the experiments were done for relative humidity (RH) ) 50% at a temperature of 300 K during the adsorption. To generate the required RH at a specified temperature, water, at a rate determined from the humidity charts, was pumped into the system (at point 3 in Figure 2) using a syringe pump. Component 3 was heated to ∼423 K, so that the liquid water was converted to vapor and properly mixed with the dry air. During the adsorption part of the cycle, the dry air and water vapor
mixture passed through the buffer volume (component 4) to facilitate further proper mixing. Initially, the humidified air mixture was vented to the atmosphere in order to minimize the composition variations due to the buffer chamber’s dead volume, and also to ensure that the humid air was at the desired RH level before contacting the samples. Once the desired RH was achieved, valves 7 and 8 were closed and humid air was passed through the reactor. The humidity probe records the exit RH continuously with an uncertainty of (1 RH unit and a resolution of 0.01 RH units. The water feed during adsorption was kept at ∼6.9 mg/min, and the dry air flow rate was 0.5 L/min (standard temperature and pressure, STP). Thus, the water content of the air was ∼1.85 mol %. The volumetric flow rate (STP) was such that the residence time in the reactor was
