Water Sorption of Acid-Doped Polyaniline Powders ... - ACS Publications

Santa Fe Science and Technology, Inc., 3216 Richards Lane, Santa Fe, New Mexico 87507. Measurements are reported for the sorption equilibrium and ...
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Ind. Eng. Chem. Res. 2006, 45, 6021-6031

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Water Sorption of Acid-Doped Polyaniline Powders and Hollow Fibers: Equilibrium and Kinetic Response Mayur M. Ostwal,† Baohua Qi,‡,§ John Pellegrino,*,‡ Andrei G. Fadeev,‡ Ian D. 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 vapor into polyaniline (PANI) powders and asymmetric, microporous hollow fibers. Equilibrium isotherms at 298 K were measured on powders doped with 13 different organic and inorganic acids to change the polyaniline from its emeraldine base, PANI-EB (insulating) form, to the emeraldine salt, PANI-ES (conducting) form. The powders were exposed to air streams of varying relative humidity (RH) from 15 to 80%. The four acid dopants, H3PO4, HPF6, HBF4, and CF3SO3H, provided the highest water sorption figures-of-merit and also displayed nonlinear isotherms. At 50% RH, these four doped powders adsorbed 0.9-1.6 molecules of water per PANI-ES repeat unit. Measurements of the sorption kinetics on the hollow fibers were done with one dopant acid, H3PO4. The data were obtained under exposure to air streams at an ambient pressure of ∼0.1 MPa with relative humidity ∼50% and temperatures of 300-309 K. The sorption process was well-described by unsteady Fickian diffusion into an infinite hollow cylinder. The asymmetric and porous nature of the hollow fiber’s wall was represented by using an effective medium approach. The quantitative adsorption/desorption rates and equilibrium capacities depended on the experimental conditions with the measured water capacities being between 33 and 75 (mg of H2O)/(g of dry polymer) under most conditions, though at the highest relative humidity (∼80% at 300 K) the fiber adsorbed almost 250 (mg of H2O)/g. For relative humidities e 50%, when the mass loading was recalculated on a per (H2O molecule/polymer repeat unit) basis, the fiber had a capacity of ∼0.5-1 H2O per PANI-ES repeat unit. In this same range of humidities, the apparent diffusion coefficients varied between 0.21 and 0.67 × 10-12 m2/s, except for the case of desorption at 80% RH, wherein a value of 2.38 × 10-12 m2/s was obtained. In general, the apparent diffusion coefficients were always larger during desorption than during adsorption. The water capacity of the polyaniline hollow fibers is at the upper end of what is usually observed for glassy polymers and provides the possibility for exploiting their electronic conductivity, good mechanical strength properties, and desiccant qualities for advanced humidity control and sensing applications. Introduction The water vapor sorption and transport in the conducting polymer polyaniline (PANI) is of interest for possible applications in air dehumidification and water recovery, as well as the role of moisture content in a number of its other possible integrated-end uses, such as sensors and actuators.1 Polyaniline is one of the most promising conductive polymers due to its straightforward preparation procedure and excellent chemical stability, combined with relatively high levels of electronic conductivity.2 The emeraldine base (EB) form of PANI consists of phenylenediamine and quinoid diimine units (Figure 1). EB is insulating, but its iminic nitrogen sites can be protonated by strong acids to form an acid-base complex.2 This electrically conductive form is called the emeraldine salt (ES). The conductivity achieved depends strongly on the type of dopant utilized, the other polymer characteristics (such as the specific repeat unit, molecular mass, polydispersity, and chain defects, * Corresponding author. Current address: University of Colorado, CEAE Department, ECOT-441, Boulder, CO 80309-0428. E-mail: [email protected]. Tel.: (303) 735-2631. Fax: (303) 4927317. † University of Southern California. ‡ Santa Fe Science and Technology, Inc. § Current address: Fleetguard Emission Solutions, Cummins Engine Co., Inc., Columbus, IN 47201.

Figure 1. Polyaniline repeat structure. When it is half-oxidized and halfreduced, it is called emeraldine base (EB). The reduced units are referred to as “benzenoid”, and the oxidized units are referred to as “quinoid”. Reversible acid/base chemical doping of polyaniline occurs when PANIEB is reacted (doped) with a strong protonic acid to form the emeraldine salt (PANI-ES).

like branching and chemical heterogeneity), and the type of processing the polymer has undergone.3,4 The electrical conductivity for doped polyaniline can vary from 100-103 Ω-1 cm-1 depending on the type of dopant and the degree of doping. It is well-known that the electrical conductivity5-7 and chemical properties8-10 of PANI are modified by the presence

10.1021/ie060163h CCC: $33.50 © 2006 American Chemical Society Published on Web 07/13/2006

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of water in the polymer matrix. But only a few studies have been performed on the mechanism of water adsorption in PANI. Some studies6,8 indicate, for example, that there are two forms of water adsorbed by the material: a weakly bonded water molecule which possesses an activation energy for desorption of ∼-21 kJ/mol and a strongly bonded water molecule that can be desorbed only with the decomposition of the polymer itself. It has also been suggested that these water molecules form hydrogen bonds with the acid sites in the ES form of PANI.9 More recent work has also supported the view that the adsorbed water consists of two forms: a hydrogen-bonded water with a desorption energy of ∼-20.9 kJ/mol and another form with a desorption energy of ∼-63 to -75 kJ/mol.11 We have previously published12 isothermal, water sorption measurements made on dense PANI textile (PANION) fibers produced by an acidic solution processing route.13 These measurements were made at a single relative humidity (RH) (50%) on fibers that had been doped with a variety of acids. Their sorption (after 2 h), on a per (H2O molecule/polymer repeat unit) basis, was generally ∼0.5 H2O per PANI-ES repeat unit, except for 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 during desorption. A sampling12 of literature data14-16 for a variety of polymers and inorganic sorbants indicates the PANION fibers would compare favorably with most other polymeric materials that could be used as water adsorbents, but they have 3-5× less capacity than most inorganic desiccants. With respect to the other relatively robust, conducting polymers, polypyrrole (PPy) and polythiophene, some data exist for the former in the context of humidity sensing and electromechanical actuation.17,18 For example, Okuzaki and Funasaka17 reported that PPy films doped with BF4- adsorbed 0.32 H2O per PPy repeat unit (∼0.061 mass fraction) at 50% relative humidity and 298 K. The water sorption on both a per and mass fraction basis were less than what was measured for the PANION solid fibers.12 It was also reported that the activation energies of these PPy films in the sorption and desorption processes were -46 kJ/mol and -44.6 kJ/mol, respectively.18 On the other hand, the water sorption of an electropolymerized polythiophene has only been reported in the context of its use as a corrosion-protection coating.19 These authors stated that the film contained a water volume fraction of ∼1.2 when exposed to 3.5% (mass) NaCl solution, and that this value was indicative of the polymer being adherent, resisting water uptake, and being corrosion resistant. In light of these latter comments, we surmise that they meant to report the uptake as a water volume % and not a water volume fraction. Consequently, it must be noted that PANI has been the most prevalent conducting polymer studied for its water sorption characteristics, because of its ease of processing into thin films and ease with which the dopant anion in the polymer can be manipulated. In addition, the PANION fibers previously studied12 have mechanical properties approaching conventional textile fibers, and are also electronically conductive. For example, it has been found20 that the electronic conductivity of these fibers is proportional to relative humidity in a very reproducible fashion. Herein, we report further sorption studies on doped PANI powders that address how the water loading varies with thermodynamic driving force and time to reach equilibrium. In addition, we present additional kinetic water vapor adsorption results, under a variety of thermodynamic conditions, in noVel

Figure 2. SEM micrographs of the PANI-EB powders: (a) NESTE-EB and (b) SFST-EB. The scale bar is 50 µm.

microporous hollow PANI fibers21 that were produced under a basic solution processing route22-24 but were only doped with one of the optimal dopants (H2PO4-) from the prior studies. These new results were obtained at various temperatures and relative humidities. Our work on these specific hollow fibers was motivated by their potential applications in air dehydration, water recovery from atmospheric sources, and other potential uses where the particular structural format (that is, hollow fibers) may be of value. The interior surface (lumen) of these fibers was made relatively thick and dense to provide both structural integrity and a possible conduit for nonpermeating heat transfer fluids. Heat transfer could be an important aspect to improving the effectiveness of any sorption/desorption process, as coolants passing through the bore of the hollow fiber are expected to improve the adsorption kinetics and mass of water adsorbed, while heating fluids should improve the extent of water desorption and desorption kinetics. Integrated heat and mass transfer studies are not in this report. Materials and Methods Powders. Two types of polyaniline EB powders were used in these measurements. One was a commercially available EB from Neste Oy (Mw ≈ 93 700 g‚mol-1) and the other was internally produced (Mw ≈ 195 000 g‚mol-1). They are referred to as NESTE-EB and SFST-EB, respectively. The SFST-EB powders were produced by methods similar to those described in the prior literature.1,24-26 Figure 2 presents electron micro-

Ind. Eng. Chem. Res., Vol. 45, No. 17, 2006 6023 Table 1. Polyaniline Powder Samples powder ID

acid dopant

acid dopant pKa a

estimatedb van der Waals volume of dopant anion (Å3)

PANI_EB PANI_HCl PANI_HBr PANI_HI PANI_HF PANI_H3PO4 PANI_HBF4 PANI_HPF6 PANI_CF3SO3H PANI_acetic PANI_acrylic PANI_pyruvic PANI_oxalic PANI_TCA

none HCl (hydrochloric) HBr (hydrobromic) HI (hydroiodic) HF (hydrofluoric) H3PO4 (phosphoric) HBF4 (fluoroboric) HPF6 (hexafluorophosphoric acid) CF3SO3H (triflic) CH3COOH (acetic) CH2CHCOOH (acrylic) CH3COCOOH (pyruvic) HOOCCOOH (oxalic) CCl3COOH (trichloracetic)

NA -7 -9 -10 3.2 2.1 0.5 -10 -15 4.8 4.3 2.4 1.2 0.7

NA 25.3 29.6 35.7 10.4 47.9 41.6 51.7 89 50.6 62.4 81.6 58.4 90.0

a CRC Handbook of Chemistry and Physics, 66th ed.; p D-163. The Merck Index, 10th ed.; p 7925. b Bondi, A. van der Waals Volumes and Radii. J. Phys. Chem. 1964, 68 (8), 441. Zhao, Y. H.; Abraham, M. H.; Zissimos, A. M. Fast calculations of van der Waals volume as a sum of atomic and bond contributions and its application to drug compounds. J. Org. Chem. 2003, 68, 7368. HyperChem, release 7.

graph images of the two types of powders. Besides the differences in overall size and Mw, the SFST-EB powder seems to have a greater surface rugosity. The NESTE-EB powders were doped at room temperature (∼298 K) with 13 aqueous acids. Five of these acids were organic acids, acetic acid, pyruvic acid, acrylic acid, oxalic acid, and trichloroacetic acid, while the remaining acids were inorganic acids, hydrofluoric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, tetrafluoroboric acid, phosphoric acid, hexafluorophosphoric acid, and triflic acid. The concentration of the acid in water was kept constant at 1 M, and the doping time for each acid was 4 h. Note that, since these acids have different pKa values, the PANI may not have been equally doped by all these acids (that is, the pH at 1 M is different for each acid), and thus, their electronic conductivities would be different. Since these studies with powders served as initial screening measurements, and very low pH’s could not be reached with all the organic acids, we decided to focus on evaluating the powders equilibrated with a constant concentration of acid and not pH. (N.B. Our studies with solid PANION fibers12 were done with doping at constant pH.) Table 1 lists the doped powder samples with the pKa and estimated anion volume of the dopant acids. Hollow Fibers. The PANI hollow fibers were produced using the procedures described in greater detail by Norris et al.21 In general, the fibers were produced using “spin-dope” solutions of between 15 and 20 mass % of high molecular mass polyaniline EB powder (Mw ≈ 280 000 g/mol) dissolved in N-methyl-2-pyrrolidinone containing the gel inhibitor 4-methyl piperidine (number of gel inhibitor molecules per EB tetramer repeat unit was between 1.0 and 1.2). The PANI hollow fibers were formed as integrally skinned asymmetric hollow fibers with the dense layer on the lumen side. The nascent fiber from the coagulation bath passed through two washing godet baths and finally collected on a bobbin by means of a Leesona fiber winder. The coagulation and godet baths were filled with a 10 mass % phosphoric acid aqueous solution so that the EB hollow fiber became fully doped with the desired dopant acid (phosphoric acid) by the time the hollow fiber was collected onto the bobbin. This step eliminated any subsequent postprocessing step to dope the fiber with the desired acid. In contrast, the bore fluid consisted of a 50 mass % phosphoric acid in order to obtain a dense, anisotropic skin on the lumen side. The PANI hollow fibers collected on the bobbin were then immersed in 10 mass % phosphoric acid/methanol extraction baths for at least 24 h to remove any residual solvent and the gel inhibitor,

while still maintaining the desired dopant acid in the hollow fiber. Methanol was chosen instead of water for this extraction process, because water could potentially collapse the pores upon drying due to capillary forces. The PANI hollow fibers were finally dried under ambient conditions for at least 24 h. A small segment of the PANI hollow fiber was removed from the bobbin and fractured in liquid nitrogen. A Philips XL30 scanning electron microscope was employed to record the cross section and surface morphology of the fractured hollow fibers. Figure 3 presents electron micrograph images of the typical morphology of the fibers tested in this study. This hollow fiber had an outer diameter of ∼390 µm and an inner diameter of ∼200 µm, and the thickness of the dense skin on the inner lumen was ∼30 µm. Atmospheric Pressure Equilibrium Adsorption Apparatus. The experimental apparatus for measuring the moisture adsorption characteristics of the polyaniline powders is depicted schematically in Figure 4. This apparatus allows measurements of the mass of any type of sample at a controlled temperature and humidity. In this apparatus, the Mettler AT Microbalance is placed inside a sealed, insulated incubator chamber that has high thermal inertia, so temperature control is (1 K. A small vent allowed the computer cables and electrical cords from the controllers outside the chamber to be connected to the instruments inside. The humidity inside the chamber is controlled by pumping wet and dry air into the chamber through plastic tubing. The source of moist air is provided by a ∼37.8 L (10 gal) water tank with an electrical heater that is located on the bottom of the sealed chamber. The humidity and temperature control system was internally developed to be able to control the temperature inside the chamber from room temperature to 90 K at (0.5 K and the humidity between 5% RH to 95% RH with a tolerance of (1% RH. In the humidity chamber, the Mettler AT Balance communicates bidirectionally with a PC host computer through a RS-232 serial connection. The humidity is measured continuously using a Honeywell HIH-3605A humidity sensor, while the temperature is monitored using a LM76 thermocouple from National Semiconductor Corp. An internally developed Labview software application enables both feedback control and continuous data logging of both the environmental conditions inside the chamber and the properties of the samples. Flow Adsorption/Desorption Apparatus. The flow adsorption/desorption apparatus was described in detail in our previous work,12 and hence, only a brief description will be given here. The apparatus has separate lines that are individually utilized

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Figure 3. SEM micrographs of the hollow PANI fiber: (a) cross section, where the scale bar is 100 µm; (b) close-up of cross section showing thick and dense lumen wall, where the scale bar is 20 µm; (c) close-up of cross section near the surface showing spongelike porous structure, where the scale bar is 5 µm; and (d) close-up of outer surface showing microporosity to facilitate mass transfer during ad/desorption, where the scale bar is 10 µm.

Figure 4. Schematic of the atmospheric pressure equilibrium adsorption apparatus.

during the adsorption and desorption parts of the cycle. A humidity probe (Rotronic Instruments, HW3) is placed at the outlet of the sample chamber (aka, reactor) to measure the exit stream’s relative humidity. There is a buffer volume in the apparatus with a volume of ∼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 achieve a higher surface area for mixing and resistance to flow shortcircuits. Equilibrium Adsorption Measurements on Powders. After being doped, the polyaniline powders were under vacuum for 18 h. The mass of each doped powder used in the water absorption measurement was ∼1 g (when weighed at room temperature and 15 % RH). In this trial, we also included the undoped forms of SFST-EB and NESTE-EB to serve as controls. These measurements were made at ambient pressure

in Santa Fe, NM, which is nominally ∼0.081 MPa. At 298 K, the relative humidity was increased from 20 to 80% RH with steps of either a 5 or 10% RH change. The step size was based on preliminary measurements that determined a reasonable response signal in order to minimize uncertainty. In these experiments, the step time was 5000 s. The response kinetics of the powders were relatively rapid (a plateau value for mass change was obtained within ∼2500 s), and consequently, we are confident that the relative mass changes we tabulated represent a local equilibrium condition. Control measurements were performed to determine the change in mass registered without any polymer in the weighing dish (blank test.) Over the entire range of changing humidity from 20 to 80% RH, the absolute mass change for the blank was 1-1.5 mg. For the samples with the lowest and highest water sorption, this represents ∼7% and 0.3% uncertainty, respectively. We did not make any correction to the raw data based on the blank tests. Data Analysis on the Equilibrium Adsorption Measurements. The mass loading reference condition, mref, is the sample mass obtained by linear (or third-order polynomial, when warranted) regression of the measured masses between 15 and 50% RH back to 0% RH. The data are presented as (mg of H2O)/(g of PANI), with (g of PANI) ) mref. In addition, as a way to “quantify” the adsorption isotherms, we calculated the linear “slope” (pseudo-Henry’s law constant) in the range of 15-50% RH. Another informative way to consider the water sorption in doped polyaniline is by the number of water molecules per “ion exchange” repeat unit (each section in curly brackets of the “polaron state” in Figure 1 may be considered a “repeat unit”).12 Since the acid dopes the polymer backbone in a stoichiometric fashion, we can use the molecular mass of the anion, Mw,Xi, and the protonated PANI-ES repeat unit (Mw ≈ 182.22) to

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convert from (mg of H2O)/(g of “dry” polymer) (M∞Xi) to the number of H2O molecules/repeat unit, (nH2O)Xi, using the following algebraic relation:

(nH2O)Xi )

M∞Xi (182.22 + Mw,Xi) 18 015.2

(1)

Adsorption/Desorption Measurements on Hollow Fibers. In the kinetic measurements, since we are operating close to room temperature and require a measurement sensitivity down to sub-mg levels of water sorption, artifacts from wall adsorption in the apparatus are a concern. Therefore, at the beginning of each series of tests, measurements were first made in the absence of the fibers (also referred to as the “blank” tests). These control measurements of the adsorption/desorption of the system without fibers were then used to correct the measurements made with the fibers. The latter were made by placing a sample of fibers (∼1 g) inside the adsorption apparatus, and then evacuating them overnight. Following that, the fibers were exposed to humid air “fed” at a constant level of humidity (during adsorption) and to dry air (during desorption) consecutively for two cycles. Each cycle consisted of a period of 2 h of adsorption, followed by 2 h of desorption; the cycle was then repeated. Three different types of experiments were carried out, as follows: 1. The relative humidity (% RH) of the humid air and the total molar flow rate during adsorption were kept constant, and the temperature of the process (fibers and humid air) was varied. 2. The molar flow rate of water and the molar flow rate of air were kept constant, and the temperature of the process was varied. 3. The temperature of the process was kept constant, and the % RH in the feed was varied. To generate the feed with the preselected % RH at a specified temperature, water at a molar flow rate determined from the humidity charts was fed into the system using a syringe pump. The feed mixing chamber of the apparatus was heated at a temperature of 423 K, in order for the liquid water to be converted to vapor and become properly mixed with the dry air. A buffer volume was used for this latter purpose and to eliminate disturbances in the system. Initially, the humidified air mixture was vented to the atmosphere, until the desired % RH level was achieved; then it was passed through the adsorption apparatus. The humidity probe recorded the exit % RH continuously with an uncertainty of (1 and a resolution of 0.01 RH unit. The water feed during adsorption is kept at a level determined from the humidity charts; for all the experiments reported here, the dry air flow rate was set at a 500 cm3/min (STP). The volumetric flow rate was set high enough (the corresponding residence time in the apparatus was 50%. The notable exception to this latter trend was for the HI dopant. It is possible that side reactions may have occurred between the Neste-EB powder and HI; both hydrolysis and redox reactions are plausible. Redox reactions that would reduce the EB to the nonconductive (and more hydrophobic) leucoemeraldine (LEB) have not been reported by other researchers working with HI-doped PANI, and we did not note the tell-tale color change to pale yellow that accompanies the reduction of EB to LEB.1 Nonetheless, it is also important to note that the HI-doped solid PANION fibers12 adsorbed twice the amount

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Figure 6. Equilibrium water sorption (mg/g) in acid-doped Neste-EB powders (as depicted in Figure 2a) against the water vapor partial pressure (Pvp, kPa) in the controlled environment at 298 K for (a) the organic acids and (b) the mineral acids. The data for the EB powder is reproduced on both plots. The uncertainty is ∼1.5 (mg of H2O)/(g of PANI) for all data points. The vertical line is approximately the 50% RH condition. Table 2. Equilibrium (at 298 K) Sorption Results for Doped Polyaniline Powders

powder IDa

(nH2O)Xi (@ ∼50% RH)

pseudo-Henry’s law coefficientb (mg‚g-1‚kPa-1)

acid dopant pKa

PANI_EB PANI_HI PANI_TCA PANI_acetic PANI_acrylic PANI_HF PANI_oxalic PANI_HBr PANI_pyruvic PANI_HCl PANI_H3PO4 PANI_HPF6 PANI_HBF4 PANI_CF3SO3H

0.22 0.24 0.44 0.47 0.57 0.69 0.70 0.70 0.75 0.80 0.92 0.92 1.20 1.60

12.79 8.54 16.07 22.01 25.33 39.07 29.12 29.59 31.50 40.98 37.80 42.30 69.44 59.74

NA -10 0.7 4.8 4.3 3.2 1.2 -9 2.4 -7 2.1 -10 0.5 -15

a Italics indicate samples with a nonlinear adsorption isotherm. b The slope of a linear fit applied to the data between 15 and 50% RH.

of water at 50% RH than the Neste-EB powder did, so the possibility of a chemical artifact cannot be ruled out in this case. Table 2 lists the number of water molecules per doped PANI repeat unit, (nH2O)Xi, at 50% RH and the pseudo-Henry’s law coefficient for the adsorption between 15 and 50% RH for the various acid-doped powders. An absolutely consistent correlation did not exist with respect to the water adsorption and the acid strength; nonetheless, the majority of the PANI powders doped with stronger acids had the highest water adsorption on a per repeat unit basis. The four acid dopants (other than trichloroacetic) with the most nonlinear adsorption isotherms also had the highest (nH2O)Xi and pseudo-Henry’s law coefficients. Since the nonlinear isotherm is indicative of a Langmuir or capillary condensation type of adsorption mechanism, we infer that these acids are opening up free volume or micropores in the powder. Kinetic Sorption Measurements on Hollow Fibers. On the basis of the results with powders, our previously reported measurements12 with PANION solid fibers, and considerations of dopant cost and robustness (under repeated cycling) in practical applications, we focused our further studies on the use of H3PO4 as the acid dopant.

Figure 7. Data for one adsorption (heavier lines) and desorption (lighter lines) cycle for hollow fiber and for the blank measurements (symbols: adsorption, 0, and desorption, O). The vertical axis is the flow rate of water (mg/min) leaving in the air stream. Note that only every second experimental data point has been plotted.

Figure 7 presents typical experimental results for the blank test, and for one adsorption and desorption cycle using the hollow fibers. The vertical axis is the exit flow rate of water (mg/min) based on the humidity sensor measurements. These data illustrate that the reactor wall equilibrates with the bulk water vapor quite quickly, and therefore, this influence on the mass balance (eq 9) should be negligible by the 90 min point. Figure 8 shows the experimental, dimensionless (normalized) exit concentration (Cb/C0) during one adsorption and desorption cycle for the hollow fiber (points) together with the simulated results (lines) based on the fitted parameters using eqs 2-10. The correspondence between the measured data points and the simulation illustrated in this figure are typical of all the measurements performed. It can be seen that the experimental results are well-fit by the model. Figure 9 presents a plot of the average measured amount that is adsorbed q120 (loading at 120 min) during adsorption for various temperatures. The data are presented for the cases of maintaining a constant 50% relative humidity and a constant

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Ind. Eng. Chem. Res., Vol. 45, No. 17, 2006 Table 3. Average Measured (at 120 min) Adsorbed and Desorbed H2O for PANI Hollow Fibers % RH

temp (K)

Pvp,H2O (kPa)

adsorption q120 (mg/g)

desorption q120 (mg/g)

(nH2O)Xia

50 50 30 50 80 42 30

303 309 300 300 300 303 309

2.125 2.976 1.071 1.785 2.856 1.785 1.785

63.97 ((0.15) 74.14 ((0.01) 33.20 ((0.21) 63.20 ((0.18) 248.34 ((1.17) 50.44 ((1.22) 35.95 ((0.57)

62.49 ((1.07) 76.43 ((0.26) 35.16 ((2.19) 63.30 ((0.04) 243.13 ((0.03) 51.23 ((0.14) 36.44 ((0.27)

0.99 1.15 0.51 0.98 3.85 0.78 0.56

a Number of water molecules per PANI-ES ion exchange repeat unit based on the average adsorption equilibrium results.

Figure 8. Measured (symbols) and fitted (lines) values for the dimensionless (normalized) exit flowrate of water during adsorption (squares; heavier line) and desorption (circles; lighter line) for one experiment (300 K, RH ) 50%). Note that only every second experimental data point has been plotted.

Figure 9. Average measured water mass sorption (q120) versus temperature at constant RH ) 50%, adsorbed (0)/desorbed (O), and at constant water feed (vapor partial pressure Pvp ) 1.785 kPa), adsorbed (4)/desorbed (]).

water feed rate of 6.889 mg/min (corresponding to an inlet concentration of 0.765 mol/L or a water vapor partial pressure (Pvp) of ∼1.785 kPa)sthis feed concentration also corresponds to RHs of 50%, 42%, and 31% at 300, 303, and 309 K, respectively. The amount of water adsorbed increases with increasing temperature at the constant % RH and decreases with temperature at constant water vapor partial pressure. These results are consistent with the facts that, to maintain a constant relative humidity with increasing temperature, the partial pressure (and fugacity) of the water vapor must increase, whereas when the water vapor partial pressure stays constant and the temperature increases, there is a decrease in the water vapor’s fugacity. Table 3 summarizes the results of the amounts of H2O adsorbed and desorbed during a 120 min cycle for different levels of the % RH and temperaturessthe q120, expressed in terms of (mg of H2O)/(g of fiber), is presented along with one standard deviation for that average calculated from the values

measured during repeated adsorption/desorption cycles. There are insignificant differences between either the adsorbed and desorbed amounts of water at any given temperature and % RH or between the cycles (e2 (mg of H2O)/(g of dry fiber)). At a constant temperature, the amount of water adsorbed increases with increasing relative humidity. Table 4 presents the average values for the parameters D and K calculated by fitting eqs 2-10 to the experimental data for adsorption and desorption. The estimated values for K between the adsorption and desorption parts of the cycle are fairly close to each other, but the same is not true for the D values. The diffusivities measured during adsorption are, for the most part, significantly lower than those measured during desorption. The calculated values are consistent with values recently reported32 for the diffusion coefficients of water vapor through semicrystalline polypropylene, which were ∼0.19 and 0.36 × 10-12 m2/s at 298 and 328 K, respectively. The more significant comparison of the transport parameters is with our previous measurements12 with solid polyaniline fibers. The fitting parameters and q120 for the similar cases with H3PO4 as the dopant and measurements at 300 K and 50% RH are in Table 5. There are only insignificant differences between the two types of fibers with respect to both the total mass ad/desorbed and in the fitting parameters for the adsorption cycle, but on the contrary, a more significant difference exists in the desorption cycle. Since these fibers are morphologically different on both macroscopic and microscopic levels, it will require future studies to elucidate the dominant transport influences. Figure 10a is a plot of average fitted adsorption/desorption diffusivities versus temperature for constant RH ) 50% and for experiments where the water feed concentration stays constant. One observes that, generally, the diffusivity increases as temperature increases. Figure 10b is a plot of the average fitted adsorption and desorption partition coefficients (K) versus temperature for constant RH ) 50% and for experiments where the water feed concentration stays constant. The K decreases as the temperature increases. The highly nonlinear water sorption equilibrium isotherms exhibited by many of the doped powder measurements (see Figure 6) are also shown by these hollow fibers. Figure 11 presents the (mg of H2O)/(g of PANI) (q120) versus water vapor partial pressure (or % RH) at constant temperature for the H3PO4-doped hollow fiber (300 K) and for the H3PO4-doped powder (298 K). The hollow fiber adsorbs significantly more H2O at the highest humidity than the powder would seem to if we simply extrapolated its measured isotherm. It is possible that the 2 K difference in temperature influences this, but it is more likely that structural aspects are more important. It is also noted that the degree of curvature shown by this hollow fiber’s isotherm, as humidity increases, is not inconsistent with that

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Figure 10. Average fitted (a) diffusivity, D, and (b) partition coefficient, K, versus temperature at constant RH ) 50%, adsorbed (0)/desorbed (O), and at constant water feed (vapor partial pressure Pvp ) 1.785 kPa), adsorbed (4)/desorbed (]). Table 4. Average Values of Fitting Parameters for Adsorption and Desorption % RH 50 50 30 50 80 42 31 pooled 1σa

temp (K)

water feed (ads) (mg/min)

303 309 300 300 300 303 309

D (× 1012) (ads) (m2/s)

K (ads) (L/(g of sample))

D (× 1012) (des) (m2/s)

K (des) (L/(g of sample))

0.208 0.338 0.182 0.182 0.166 0.319 0.412 0.026

3.81 2.99 3.87 4.35 10.77 3.16 2.20 0.149

0.421 0.564 0.368 0.426 1.536 0.444 0.220 0.085

3.10 3.75 4.04 4.26 9.53 3.36 2.20 0.115

6.889 6.889 6.889

a The pooled 1σ is for the standard deviations of the average values calculated from repeated adsorption and desorption cycles; this represents reproducibility, not uncertainty.

Table 5. Comparison of Solid PANION and Hollow Fiber Sorption Parameters at 50% RH, 300 K, and with H3PO4 as the Dopant fiber

D (× 1012) (ads) (m2/s)

K (ads) (L/(g of sample))

q120 (ads) (mg/g)

D (× 1012) (des) (m2/s)

K (des) (L/(g of sample))

q120 (des) (mg/g)

solid hollow

0.297 0.182

4.56 4.35

66.7 63.2

0.169 0.426

5.45 4.26

65.6 63.3

shown by several of the other doped powders (see Figure 6b). Thus, some common element is suggested. Figure 12 depicts a very dramatic change in the fitted diffusion coefficient during desorption and in the fitted K

(partition coefficient) values during both adsorption and desorption at the highest humidity. These results are not definitive with respect to any particular mechanism but are consistent with concentration-dependent structural rearrangements within the PANI. Discussion

Figure 11. Average measured water mass sorption versus water vapor partial pressure (% RH) at constant temperature for the PANI H3PO4 powder at 298 K adsorbed (0) and the PANI H3PO4 hollow fiber at 300 K adsorbed (b)/desorbed (O).

With respect to the kinetic measurements made on the hollow fibers, we observe that all the trends are quite consistent with fundamental thermodynamic effects. That is, as the water vapor’s partial pressure increases and the air stream’s saturation level stays constant, the equilibrium loading (q120) in the fibers increases because the fugacity driving force is increasing. When the water vapor’s partial pressure stays constant but the degree of saturation decreases (by raising the temperature), the fugacity is decreasing and the equilibrium sorption decreases. On the other hand, in all cases, as temperature increases the fitted diffusion coefficient increases, and the partition coefficient (K) decreases. Intuitively, we would expect the K to correlate with the q120. It sometimes does, but that it does not always is likely bound up in the fact that K “lumps” a variety of mass transfer phenomena in with an equilibrium coefficient and is arrived at by fitting. Since we did not perform sorption measurements at constant fugacity driving force and changing temperatures, it is not completely reasonable to apply an Arrhenius-type analysis

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Figure 12. Average fitted (a) diffusivity, D, and (b) partition coefficient, K, versus water vapor partial pressure (% RH) at constant temperature 300 K, adsorbed (0)/desorbed (O), for the PANI H3PO4 hollow fiber.

to the fitted values of K (or q120). Nonetheless, doing so with the K data obtained at constant water partial pressure provides an Arrhenius activation energy of ∼-57 kJ/mol. This value is significantly larger than the expected value of ∼-21 kJ/mol based on the results of Matveeva et al.11 and is inconsistent with the comparative ease of water sorption/desorption that we observed. Thus, we are satisfied to simply view K as a semiempirical parameter. The last column in Table 3 presents the number of H2O molecules/repeat unit based on eq 1 and by setting M∞Xi ) q120, the average equilibrium adsorption after 120 min. This quantity is an estimate of the mobile water under the adsorption-desorption conditions we applied. In general, the hollow fiber adsorbs ∼0.5 to 1 mobile H2O molecules per ion exchange repeat unit, depending on the experimental conditions. This range is quite consistent with what was measured12 for the solid PANION fibers at 300 K and 50% RH, that is, the H3PO4doped fiber had (nH2O)Xi ) 1.03 versus 0.98 for the hollow fiber and 0.92 for the powder (at 298 K, though) in this current study. A quite interesting point is the inference that the asymmetric, microporous hollow fiber structure is increasing the kinetics of the desorption process. We have collected only a limited amount of data thus far, but the following working hypothesis is emerging as consistent with the body of results. The equilibrium water sorption in the powders generally followed the same trend as we observed in our studies12 with dense fibers. That is, doping with inorganic acids resulted in greater water sorption than occurred with organic acids. We speculate that the water uptake in PANI-ES is a result of the combination of, at least, four factors: (i) strength of the acid in maintaining charge separation between the positive PANI backbone and the coanion; (ii) the size of the anion with respect to disrupting the packing of adjacent PANI chains; (iii) the actual equilibrum solubility of the acid in the polymer; and (iv) the hydration volume of the anion after it has been equilibrated. For many dopants, the equilibrium loading in the PANI-ES powders showed a nonlinear increase as the water vapor fugacity increased. This follows from the notion that the dopant is initially opening up accessible free volume and the sorption of water proceeds by a Henry’s law behavior but also participates in further structural rearrangement. We have previously observed29 that some gas sorption in PANI continues to cause

structural rearrangements that feedback into the sorption process for quite some time. These physical rearrangements would seem to be reversible (hence, the relative reproducibility between successive adsorption measurements) but likely have a time scale associated with them that can lead to a hysteresis between adsorption and desorption. This hysteresis would be difficult to detect if the length scales of the diffusion measurements give rise to diffusion time scales that are longer than the time scales for the structural rearrangementssas is likely to be the case with very dense PANI structures. Our measurements with these asymmetric microporous structures suggests that the length scales for diffusion have been reduced significantly enough to observe the hysteresis between adsorption and desorption. The overall water sorption in the PANI asymmetric, microporous hollow fibers is on the same order as what we observed for the solid PANION fibers (at equivalent conditions), but the diffusion coefficient is approximately an order of magnitude larger than that reported by Passiniemi.33 In the latter report, powders were hot sintered to make a film, and the diffusion coefficient was derived from covolution of the unsteady diffusion relationship with conductivity measurements. The assumption that conductivity measurements (by AC impedance) would immediately respond to the local water content has not been critically examined and may not be valid. Therefore, we attribute the difference in the reported H2O diffusion coefficient to important differences between both materials and method and, thus, this is not troublesome. Conclusions Polyaniline powders doped with a variety of organic and inorganic acids indicate both linear and nonlinear adsorption isotherms at 298 K. The general trends with respect to the number of water molecules adsorbed per doped-repeat unit observed with the powders follow what had been previously noted for solid PANION fibers. That is, dopants such as HBF4 and H3PO4 were superior. Some notable difference were also seen, for example, CF3SO3H and HCl were considerably more efficient at increasing water sorption in the powders than they were in the solid fibers. Continuing our elucidation of polyaniline fibers as reversible water sorbents, asymmetric, microporous hollow fibers doped with H3PO4 dopant were shown to reversibly adsorb 0.5-1

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molecules of water per doped repeat unit at various temperatures (300-309 K) and relative humidities in the range 30-50%. At higher water vapor partial pressure (closer to saturation), this figure-of-merit increases 3-4-fold. Both the adsorption and desorption processes were well-fit by unsteady Fickian diffusion. The apparent diffusion coefficient for the desorption was higher than what was calculated for the adsorption. We have forwarded a testable hypothesis as to why this may be the case, but further measurement will be required to prove it. In their present form, the PANI-ES hollow fibers adsorbed as much water as many hydrophilic polymers (on a mass basis) but do not compare favorably with most inorganic desiccants. Nonetheless, it must be noted that these fibers have mechanical properties approaching conventional textile fibers and are electronically conductive. These attributes can support new applications in sensing and control of humidity that may offset the modest total water sorption capacity. Acknowledgment This work was performed under DARPA Contract No. NBCHC020069, and under the direction of contract managers Drs. Michael Gardos and Len Buckley, to whom the authors are grateful for the support. Literature Cited (1) Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds. Handbook of Conducting Polymers; Marcel Dekker: New York, 1998. (2) Chiang, J.-C.; MacDiarmid, A. G. ‘Polyaniline’: Protonic acid doping of the emeraldine form to the metallic regime. Synth. Met. 1986, 13, 193. (3) Hundley, M. F.; Adams, P. N.; Mattes, B. R. The influence of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) additive concentration and stretch orientation on electronic transport in AMPSA-modified polyaniline films prepared from an acid solvent mixture. Synth. Met. 2002, 129, 291. (4) Eaiprasersak, K.; Gregory, R. V. Effect of process conditions on the mechanical and electrical characteristics of wet spun polyaniline fibers and films. ANTEC ‘98 1998, 2, 1263. (5) Nechtschein, M.; Santier, C.; Travers, J. P.; Chroboczek, J.; Alix, A.; Ripert, M. Water Effects in Polyaniline: NMR and Transport Properties. Synth. Met. 1987, 18, 311. (6) Lubentsov, B. Z.; Timofeeva, O. N.; Khidekel, M. L. Conducting polymer interaction with gaseous substances. II. PANI-H2O, PANI-NH3. Synth. Met. 1991, 45, 235. (7) Taka, T. Humidity Dependency of Electrical Conductivity of Doped Polyaniline. Synth. Met. 1993, 57, 5014. (8) Lubentsov, B.; Timofeeva, O.; Saratovskikh, S.; Krinichnyi, V.; Pelekh, A.; Dmitrenko, V.; Khidekel, M. The study of conducting polymer interaction with gaseous substances. IV. The water content influence on polyaniline crystal structure and conductivity. Synth. Met. 1992, 47, 187. (9) Alix, A.; Lemoine, V.; Nechtschein, M.; Travers, J. P.; Menardo, C. Water Absorption Study in Polyaniline. Synth. Met. 1989, 29, 457. (10) Travers, J. P. Polyaniline: Hydration effects on spin dynamics. Synth. Met. 1993, 55, 457. (11) Matveeva, E. S.; Diaz Calleja, R.; Parkhutik, V. P. Thermogravimetric and calorimetric studies of water absorbed in polyaniline. Synth. Met. 1995, 72, 105. (12) Ostwal, M. M.; Pellegrino, J.; Norris, I.; Tsotsis, T. T.; Sahimi, M.; Mattes, B. R. Water sorption of acid-doped polyaniline solid fibers: equilibrium and kinetic response. Ind. Eng. Res. Chem. 2005, 44, 7860.

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ReceiVed for reView February 8, 2006 ReVised manuscript receiVed June 5, 2006 Accepted June 19, 2006 IE060163H