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Conductivity Spectra of Polyphosphazene-Based Polyelectrolyte Multilayers†

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Conductivity Spectra of Polyphosphazene-Based Polyelectrolyte Multilayers† Conductivity Spectra of Polyphosphazene-Based Polyelectrolyte Multilayers† Yahya Akgo1l,‡ Christian Hofmann,‡ Yunus Karatas,§,| Cornelia Cramer,‡ Hans-Dieter Wiemho1fer,§ and Monika Scho1nhoff*,‡ Institut fu¨r Physikalische Chemie und Sonderforschungsbereich 458, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Corrensstrasse 30/36, D-48149 Mu¨nster, Germany, Institut fu¨r Anorganische und Analytische Chemie und Sonderforschungsbereich 458, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Corrensstrasse 30/36, D-48149 Mu¨nster, Germany, and International NRW Graduate School of Chemistry (GSC-MS), 48149 Mu¨nster, Germany ReceiVed: December 22, 2006; In Final Form: March 19, 2007 Polyelectrolyte multilayers are built up from ionically modified polyphosphazenes by layer-by-layer assembly of a cationic (poly[bis(3-amino-N,N,N-trimethyl-1-propanaminium iodide)phosphazene] (PAZ+) and an anionic poly[bis(lithium carboxylatophenoxy)phosphazene] (PAZ-). In comparison, multilayers of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are investigated. Frequency-dependent conductivity spectra are taken in sandwich geometry at controlled relative humidity. Conductivity spectra of ion-conducting materials generally display a dc plateau at low frequencies and a dispersive regime at higher frequencies. In the present case, the dispersive regime shows a frequency dependence, which is deviating from the typical behavior found in most ion-conducting materials. Dc conductivity values, which can be -10 -7 -1 attributed to long-range ionic transport, are on the order of σdc ) 10 -10 S‚cm and strongly depend on + - relative humidity. For PAZ /PAZ multilayers σdc is consistently larger by one decade as compared to PSS/ PAH layers, while the humidity dependence is similar, pointing at general mechanisms. A general law of a linear dependence of log(σdc) on relative humidity is found over a wide range of humidity and holds for both multilayer systems. This very strong dependence was attributed to variations of the ion mobility with water content, since the water content itself is not drastically dependent on humidity. Introduction multilayers and their properties,5-11 and of capsules with respect to release applications.12,13 For applications as separation membranes in Li batteries, Durstock and Rubner were the first to study the conductivity polymer electrolytes are a promising class of materials. The of PEM. They investigated films of PSS/PAH or PAA/PAH, challenge of current research in this field is to merge high ion respectively, where PAA denotes poly(acrylic acid), PSS is poly- conductivities with sufficient mechanical properties. For ex- (sodium 4-styrenesulfonate), and PAH is poly(allylamine hy- ample, this can be achieved by employing flexible backbones drochloride). They found dc conductivities, σdc, in the range of with short, mobile oligoether side chains providing a large 10-12-10-7 S‚cm-1,14 where the maximum of 10-7 S‚cm-1 was segmental mobility, which is necessary for ion motion, and achieved only at strong hydration. Recently, further studies of 1-3 cross-linking the backbones to enhance mechanical stability. conductivities in PEM suggested them as potential ion-conduc- Typically, such bulk polymers are then prepared as a thin tive materials for battery applications.15,16 When employing membrane. poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) As an alternative approach, polyelectrolyte multilayers (PEM)4 as a polyanion, at high humidity the largest value of the dc -5 -1 are promising, since their multiple electrostatic interactions conductivity (σdc ≈ 10 S‚cm ) was achieved, which is provides an excellent mechanical stability for the material, while already in a realistic range for applications.15 Furthermore, at the same time ultrathin membranes of nanometric dimensions following swelling in lithium triflate, conductivities of dry films can easily be fabricated. Polyelectrolyte multilayers can be made were enhanced, implying that σdc is limited by the low by layer-by-layer assembly, i.e., the alternating adsorption of concentration of mobile ions. In water-saturated films, however, polycations and polyanions. Research in the field of PEM has an increase of the salt concentration in solution did not have a vastly expanded in the past decade, as there are numerous significant influence on σdc, and it was concluded that the potential applications such as containers, sensors, and drug conductivity is limited by limited mobility of the charge delivery. Past review articles summarize the state of the art of carriers.15 Different polyelectrolyte pairs tested for their conductivity † Part of the special issue “International Symposium on Polyelectrolytes already involved polymers known as good ion conductors, such (2006)”. 15 + + as Nafion as a known proton conductor or poly(ethylene oxide) * Corresponding author. Phone: 49-2518323419. Fax: 49-2518329138. 16 E-mail: [email protected]. (PEO) as a classical polymer electrolyte. ‡ Institut fu¨r Physikalische Chemie und Sonderforschungsbereich 458, Although organic polymers have been widely investigated Westfa¨lische Wilhelms-Universita¨t Mu¨nster. as polyelectrolytes, inorganic polymers, such as polyphosp- § Institut fu¨r Anorganische und Analytische Chemie und Sonderfor- schungsbereich 458, Westfa¨lische Wilhelms-Universita¨t Mu¨nster. hazenes, are still untouched in this field. Although polyphos- | International NRW Graduate School of Chemistry. phazenes comprise the largest class of inorganic polymers due 10.1021/jp068872w CCC: $37.00 © xxxx American Chemical Society Published on Web 05/03/2007 PAGE EST: 7.1 B J. Phys. Chem. B Akgo¨l et al. SCHEME 1: Structures of the Cationic (PAZ+) (2) and spectroscopic characterization of both polyelectrolytes as well Anionic (PAZ-) (3) Polyphosphazenes as the other intermediate polymer products (not shown in Scheme 1) can be found as well in the Supporting Information. Other Materials. Poly(sodium 4-styrenesulfonate) (PSS; Mw ) 70 000 g/mol) was purchased from Acros and dialyzed (Mw cutoff ) 10 000 g/mol) against ultrapure water (Millipore, F g 18 MΩ‚cm) and freeze-dried. Poly(ethylene imine) (PEI; Mw ) 50 000-60 000 g/mol, 50 wt % solution in water) and poly- (allylamine hydrochloride) (PAH; Mw ) 70 000 g/mol) were obtained from Sigma Aldrich and used without further purifica- tion. The utilized salts were sodium chloride (NaCl; Merck, Darmstadt), potassium acetate (KAC; Fluka), magnesium nitrate hexahydrate (Mg(NO3)2‚6H2O; Fluka), and lithium chloride (LiCl; Acros). All these chemicals were of analytical grade. LiCl was dried at T ) 110 °C for at least 90 min in an oven in order to remove water prior to preparing solutions from it. Throughout the preparation procedure, ultrapure water (H2O, three stage purification system (Millipore), resistivity >18 MΩ‚cm) was used. Indium tin oxide coated glass substrates (ITO thickness 180 nm; 40 mm × 15 mm, thickness 1.1 mm, area resisitivity g10 17 to their ease of substitution by a wide variety of nucleophiles, Ω/square) were provided by PGO, Pra¨zisions Glas und Optik only a few studies on different polyphosphazene-based polyca- GmbH, Iserlohn. Oxidized Si wafers were received from 18-22 23,24 tions and polyanions have been reported. These poly- SilChem GmbH, Freiberg. The nominal thickness of the SiO2 mers are potentially useful for a range of applications, such as layer was either 500 or 300 nm. 24 21 20 composite materials, lubricants, and gene delivery. Solutions. The concentrations of the polyelectrolyte solutions In this study, we employ a model polyphosphazene-based were 0.01 mol/L with respect to the monomer repeat unit. For polycation and polyanion for the first time as a building block the deposition cycles of PEI/PSS(PAH/PSS)49 0.1 M NaCl was for PEM. In contrast to the bulk material in many studies, the added to the PSS and PAH solutions, while salt-free PEI solution side chains are ionically modified in order to enable layer-by- was used. For deposition of PSS in combination with PAZ+,a layer assembly. The results of such multilayers are compared PSS solution with 0.5 M NaCl and a salt-free PAZ+ solution to those of the “classical” polyelectrolytes commonly employed were prepared. For multilayers of PAZ+/PAZ-, solutions of both for multilayer formation. polymers contained 0.1 M LiCl. In some cases PAZ+ solutions To study the ion dynamics in PEM on different time scales, were adjusted to pH 5.1 by a buffer consisting of 0.064 M we employed conductivity spectroscopy, which allows us to potassium hydrogen phthalate and 0.037 M sodium hydroxide. determine not only the dc conductivity but also the frequency Multilayer Preparation. Before starting the deposition, ITO- dependence of the conductivity. coated glass substrates or oxidized silicon wafers, respectively, were cleaned in RCA solution (H2O2/ammonia/water, 1:1:5 by Materials and Methods volume), which was heated to 70 °C for 15 min. Extensive Polymer Synthesis. The synthesis of the monomer trichloro- rinsing of the substrates with ultrapure water followed. Cleaned (trimethylsilyl)phosphoranimine was carried out according to hydrophilic substrates were stored in ultrapure water and used Wang et al.,25 with minor modifications.26 Poly(dichlorophos- within hours. phazene) (1) was synthesized from this monomer via living Polyelectrolyte adsorption and intermediate washing steps cationic polymerization using the route described by Allcock were performed automatically using a dipping robot (Riegler et al.27
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