Langmuir 2001, 17, 7865-7872
7865
Dielectric Properties of Polyelectrolyte Multilayers Michael F. Durstock† and Michael F. Rubner* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 22, 2001. In Final Form: September 24, 2001 A relatively new technique based on the sequential adsorption of oppositely charged polymers affords the fabrication of multilayer thin films with controlled molecular architectures. In this work, we report the first study on the impedance and dielectric characteristics of multilayer films created by the sequential adsorption of layers of poly(allylamine hydrochloride) (PAH) with poly(acrylic acid) (PAA) and sulfonated polystyrene (SPS). We show large changes in the dielectric characteristics of these films with changes in temperature, moisture content, and deposition conditions. In particular, large increases in the conductivity and dielectric “constant” as the temperature and moisture content are increased were interpreted in terms of a change in the mobility and effective concentration of small ions in the film. A small but nonnegligible concentration of such ions was found to be incorporated into these films at a level that depends on the solution processing parameters such as the pH and salt content.
Introduction The ability to controllably deposit ultrathin multilayers of polymers onto a substrate has recently been advanced through a relatively new sequential adsorption technique.1 This new process involves alternately dipping a substrate into dilute aqueous solutions of a positively charged polyelectrolyte (polycation) followed by a negatively charged polyelectrolyte (polyanion) with a rinsing step in between. By repeating this process an arbitrary number of times, a thin film can be built up due to the electrostatic interaction between the two oppositely charged polyelectrolytes. Since its inception,2 this approach has received a growing amount of attention and has been used for a large number of systems including conducting,3-8 electroluminescent,9-15 and nonlinear optical polymers,16,17 as well as biomaterials18-21 and various inorganic materials.22-26 Thin films created using this process have been studied extensively in terms of their adsorption characteristics,2,27-37 but there still remain a few outstanding issues † Air Force Research Lab (AFRL/MLBP), 2941 P Street, WPAFB, OH 45433.
(1) Decher, G. Science 1997, 277, 1232-1237. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (3) Cheung, J. H.; Fou, A. C.; Rubner, M. F. Thin Solid Films 1994, 244, 1232. (4) Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806-809. (5) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (6) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (7) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712-2716. (8) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 27172725. (9) Durstock, M. F.; Rubner, M. F., in Organic Light-Emitting Materials and Devices, Proceedings of SPIE; Kafafi, Z., Ed.; SPIEs The International Society for Optical Engineering, Bellingham, WA, 1997; Vol. 3148, pp 1269131. (10) Durstock, M. F. Ph. D. Thesis, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,1999. (11) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (12) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067. (13) Hong, H.; Davidov, D.; Avany, Y.; Chayet, H.; Faraggi, E. Z.; Neumann, R. Adv. Mater. 1995, 7, 846. (14) Onoda, M.; Yoshino, K. Japanese J. Appl. Phys. 1995, 34, L260. (15) Tian, J.; Wu, C. C.; Thompson, M. E.; Sturm, J. C.; Register, R. A.; Marselia, M. J.; Swager, T. M. Adv. Mater. 1995, 7, 395.
that need to be addressed for a complete understanding. The incorporation of residual small ions into the film and their mobility, the ionic cross-link density, water content, and deposition conditions (solution pH and salt content) can all significantly affect the electrical properties of these films. Consequently, a firm understanding of these effects is required to be able to understand and design the electronic properties of devices made from these polyelectrolyte multilayers. (16) Wang, X.; Balasubramanian, S.; Li, L.; Jiang, X.; Sandman, D. L.; Rubner, M. F.; Kumar, J.; Tripathy, S. K. Macromol. Rapid Commun. 1997, 18, 451. (17) Lvov, Y.; Yamada, S.; Kunitake, T. Thin Solid Films 1997, 300, 107. (18) Decher, G.; Lehr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677. (19) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396-5399. (20) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. J. Am. Chem. Soc. 1995, 117, 6117. (21) Lvov, Y.; Haas, H.; Decher, G.; Mohwald, H.; Mikhailov, A.; Mtchedlishvily, B.; Mrogunova, E.; Vainshtein, B. Langmuir 1994, 10, 4232. (22) Sun, Y.; Hao, E.; Zhang, X.; Yang, B.; Shen, J.; Chi, L.; Fuchs, H. Langmuir 1997, 13, 5168. (23) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13069. (24) Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125. (25) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (26) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (27) Baur, J. W. Ph.D. Thesis, Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, 1997. (28) Yoo, D., Ph.D. Thesis, Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, 1997. (29) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309-4318. (30) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 42134219. (31) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160164. (32) Schmitt, J.; Grunewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Losche, M. Macromolecules 1993, 26, 7058-7063. (33) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (34) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 948-953. (35) Ramsden, J. J.; Lvov, Y. M.; Decher, G. Thin Solid Films 1995, 254, 246-251. (36) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (37) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 81538160.
10.1021/la010954i CCC: $20.00 © 2001 American Chemical Society Published on Web 11/09/2001
7866
Langmuir, Vol. 17, No. 25, 2001
In this study, we examine the electrical (dielectric and impedance) characteristics of sequentially adsorbed multilayers of poly(allylamine hydrochloride) (PAH) with both poly(acrylic acid) (PAA) and sulfonated polystyrene (SPS) to further develop an understanding of the incorporation of small residual ions into the multilayer film. Michaels et al.38-42 have performed similar dielectric studies on traditional polyelectrolyte complexes that are formed when a polycation and a polyanion are mixed together in the same solution and precipitate out due to the attractive electrostatic interaction between the two. These complexes form a polar matrix capable of dissociating ion pairs into free cations and anions and they have been studied for use as ion exchange membranes, antistatic coatings, chemical sensors, and to a lesser extent as solid polymer electrolytes when mixed with a salt.38,43 To our knowledge, these very early studies represent the only detailed dielectric measurements made on polyelectrolyte complexes. In essence, the sequential adsorption of oppositely charged polyelectrolytes can be considered as the controlled formation of a polyelectrolyte complex. This layerby-layer technique, however, affords much better control over the deposition process and consequently yields much more uniform films than was obtained previously by simply precipitating polyelectrolyte solutions. Since dielectric properties are highly sensitive to the presence of any small ions, such studies provide valuable information that is not easily realized by other techniques. Our results will be compared to those of Michaels et al. and to other recent studies by Decher et al.1,34,36 and Schlenoff et al.44,45 discussing the incorporation of residual small ions into sequentially adsorbed multilayer films. Experimental Section The poly(acrylic acid) (PAA) was purchased from Polysciences (Mw ∼ 90,000) and was received in the form of a 25% aqueous solution. It was then diluted with pure water to obtain the desired concentration for sequential adsorption. The poly(sodium 4-styrenesulfonate) (SPS) and the poly(allylamine hydrochloride) (PAH) were both obtained from Aldrich in solid form (Mw ∼ 70 000 for both). Solutions were made by dissolving the polymer in water to obtain the desired concentration and then filtering through 2.5 micron filter paper. The polymer concentration for all solutions used in this study was 10-2 M based on the repeat unit molecular weight. The solution pH was measured using an Orion model 230A pH meter and it was adjusted by the slow addition of either dilute NaOH or HCl. All water used in the process was filtered through a Milli-Q filtration system. Among other contaminants, this removed any residual ions present in the water to give a resistivity of at least 18 Mohm cm at the output. Samples were fabricated by sequentially adsorbing layers of PAH with PAA or SPS onto ITO coated glass. The conductive ITO was deposited (sputtered) by Donnelly Applied Films onto a glass substrate to give a thin film with a sheet resistance that was less than 15 Ω/square. It was then patterned, using standard photolithographic techniques, and cut by DCI, Inc., to give 2 in. × 1 in. glass substrates with several ITO stripes that were 3 mm wide and several thousand angstroms thick. The ITO coated glass substrates were then cleaned before use according to the (38) Michaels, A. S. Ind. Eng. Chem. 1965, 57, 32-40. (39) Michaels, A. S.; Falkenstein, G. L.; Schneider, N. S. J. Phys. Chem. 1965, 69, 1456-1465. (40) Michaels, A. S.; Mir, L.; Schneider, N. S. J. Phys. Chem. 1965, 69, 1447-1455. (41) Michaels, A. S.; Miekka, R. G. J. Phys. Chem. 1961, 65, 17651773. (42) Falkenstein, G. L., Ph.D. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, 1963. (43) Toyota, S.; Nogami, T.; Mikawa, H. Solid State Ionics 1984, 13, 243-247. (44) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626-7634. (45) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552-1557.
Durstock and Rubner following procedure. They were sonicated for 15 min in a 3:1 solution of H2O:Lysol and then in pure H2O. They were then dried and again sonicated for 15 min in each of the following solvents: 1,1,1-trichloroethane, acetone, and methanol. Finally the substrates were rinsed with pure water before use. The sequential adsorption process was performed automatically in a Carl Zeiss HMS Series microscope slide stainer. The substrates were typically held in the polyelectrolyte solutions for 15 min followed by three separate water rinse baths (not pH adjusted to match the solutions) for 2 , 1, and 1 min. This was done sequentially for both polyelectrolytes and then the whole process was repeated to build up the desired number of bilayers. Film thickness measurements were performed on a Tencor surface profiler with a 5 mg stylus force. Aluminum lines that were 2 mm wide and several thousand angstroms thick were evaporated perpendicular to the ITO lines at a base pressure of at least 2 × 10-6 Torr. Impedance and dielectric measurements were carried out on a Solartron model 1260 Impedance/Gain-Phase Analyzer that has a frequency range of 10 µHz to 32 MHz. In most cases, the AC amplitude was 10mV with no DC bias applied. The data were obtained as real and imaginary impedance values and were then transformed into the dielectric domain.46,47 Data analysis and simulations were done using the ZView program made by Scribner Associates, Inc. The devices were tested in either a resistively heated sample chamber or in a quartz tube placed in a tube furnace. The temperature was monitored by a type-K thermocouple placed in close proximity to the sample itself. In most cases, pure Argon was purged through the chamber during testing. For the cases in which the samples were exposed to a humid environment, however, the argon was first bubbled through water to give an environment with approximately 85-90% relative humidity (%RH).
Results Temperature-Dependent Behavior of PAH/PAA Multilayer Films. Polyelectrolyte multilayer thin films for frequency dependent dielectric measurements were first fabricated from the weak polyelectrolytes PAA and PAH. Figure 1 shows the temperature-dependent dielectric properties (dielectric “constant” ′ and loss ′′) of a PAH/PAA multilayer thin film fabricated with the PAH and PAA dipping solutions both set at a pH of 3.5 and with a total film thickness of approximately 900 Å. Complex impedance plots (i.e., Cole-Cole) for this system are also presented in the figure. To interpret these data, we assume that the dielectric response of polyelectrolyte multilayers can be modeled as a simple equivalent circuit of the type shown in Figure 2a. This model is frequently used to explain the impedance behavior of ion-containing polymer films with ion-blocking electrodes.48,49 The film response is represented as a parallel RC combination that takes into account the bulk resistance RB of the film as well as the bulk capacitance CB. The interfacial (double layer) capacitance Ci arises from the accumulation of mobile ions at electrode and possibly internal interfaces under the influence of the external applied field, and will be discussed in more detail later. RS, on the other hand, represents the combined resistances of the leads and/or contacts, most notably the ITO resistance. The ideal response of this equivalent circuit (shown in Figure 2b,c for two different values of RB) is qualitatively similar to that obtained from the PAH/PAA (pH 3.5/3.5) multilayer film at elevated temperatures. Specifically, a single, (46) Macdonald, J. R. Impedance Spectroscopy; John Wiley & Sons: New York, 1987. (47) Blythe, A. R. Electrical Properties of Polymers; Cambridge University Press: Cambridge, 1979. (48) Linford, R. G. In Electrochemical Science and Technology of Polymers; Linford, R. G., Ed.; Elsevier Applied Science Publishers: New York, 1987. (49) Bruce, P. G. In Polymer Electrolyte Reviews-1; MacCallum, J. R.; Vincent, C. A., Eds.; Elsevier Applied Science: New York, 1987.
Polyelectrolyte Multilayers
Langmuir, Vol. 17, No. 25, 2001 7867
Figure 1. Temperature dependence of the (a) dielectric and (b) impedance characteristics of a PAH/PAA (pH 3.5/3.5) multilayer film (note that the arrow indicates increasing frequency).
relatively broad loss peak in the low-frequency range is seen to dominate the dielectric response. The weaker loss peak at higher frequencies (>105 Hz) that is apparent in the dielectric plot of the ideal circuit (Figure 2b) is due to the resistance of the electrode RS. This peak was also observed in the PAH/PAA (pH 3.5/3.5) films and was found to shift to higher frequencies when the length (and hence resistance) of the ITO electrode was decreased. Since this peak provides no useful information about the multilayer film or the electrode interfaces, we will present only experimental dielectric data generated up to 105 Hz. Returning to the experimental dielectric data presented in Figure 1a, it can be seen that the relatively large dielectric loss peak of the PAH/PAA (pH 3.5/3.5) film shifts to lower frequencies as the temperature is decreased from 108 °C. This shift corresponds to an increasing bulk resistance RB and consequently a lower conductivity. An illustration of this is shown in Figure 2b for the model circuit. In these experiments, the sample was first heated to 108 °C, with an argon purge, to ensure that any residual water was removed from the film. This is supported by the work of Farhat et al.50 in which they show, by TGA and FTIR, that all of the water is removed from similar polyelectrolyte multilayer films by heating under dry conditions to 100 °C. The measurements were then taken (50) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623.
Figure 2. (a) Proposed equivalent circuit and ideal (b) dielectric and (c) impedance characteristics for the following parameters: RB ) 80 kΩ and 20 kΩ, RS ) 50 Ω, CB ) 4.5 nF, Ci ) 50 nF, t ) 1000 Å, and A ) 6 mm2.
as the film was cooled back down to room temperature. At 108 °C, the dielectric “constant” of the PAH/PAA (pH 3.5/3.5) film approaches a value of about 100 at low frequencies. It is important to note that, up to about 110 °C, this behavior is completely reversible (i.e., the film shows the same characteristics upon being cooled and reheated). The relatively large dielectric “constant” of the PAH/ PAA (pH 3.5/3.5) film that is observed at elevated temperatures is characteristic of an ion-containing polymer with mobile ionic species that are able to move under the influence of the applied field. As the temperature decreases, the mobility of the ions becomes systematically “frozen-out” at the higher frequencies due to the fact that the ions are no longer able to respond to the alternating electric field. Thus, at lower temperatures, the large dielectric loss and “constant” associated with ionic polarization are only observed at very low frequencies. Note, for example, that, at room temperature, the PAH/PAA (pH 3.5/
7868
Langmuir, Vol. 17, No. 25, 2001
3.5) film exhibits the dielectric behavior of a good insulator with a dielectric “constant” of about 5-6 and low dielectric losses over the entire indicated frequency range. The Cole-Cole plots shown in Figure 1b can be used to estimate the ionic conductivity of the multilayer film. These plots are comprised of a semicircular arc at high frequencies that, at high temperatures, overlaps with a low-frequency “spike” similar to that found in the impedance plot of the model circuit shown in Figure 2c. The entire curve is displaced along the real axis by an amount equal to Rs, which is about 60 Ω here. In general, this behavior is characteristic of an ion-containing polymer between blocking electrodes.48,49 In the ideal impedance response, the diameter of the semicircular arc is equal to the value of RB, and this is used to experimentally determine the conductivity of the film. The vertical spike at low frequencies is due to the interfacial capacitance in series with the bulk resistance. As mentioned previously, this interfacial capacitance is usually attributed to the accumulation of mobile ions at the electrode interfaces within the film under the influence of the applied field. In heterogeneous systems, mobile ions can also accumulate at the internal interfaces of microdomains. For the homogeneous case48,49,51 the ions become polarized and buildup at the electrode interfaces to form electrode double layers. The double layers can be conceptualized as parallelplate capacitors with a very small separation between the plates.48,49 In the heterogeneous case, a mechanism similar to a Maxwell-Wagner type of polarization is evoked in which the ions build up at internal interfaces of microdomains due to the heterogeneity present in the film.39 Both models can qualitatively explain the observed behavior and be represented as the equivalent circuit shown in Figure 2a. Given the nature of the polyelectrolyte multilayer assembly process, it is expected that, under the usual conditions of assembly, the polymer layers are well interpenetrated forming essentially a homogeneous thin film. Many different studies in fact have confirmed that these are highly interpenetrated molecular organizations.1 It is not possible, however, to completely rule out the possibility of ion accumulation at internal interfaces. In any case, these results clearly demonstrate that mobile small ions are present in these films. It can also be seen from Figure 1b that the spike is not completely vertical as in the ideal case, but rather makes an angle of approximately 77° with respect to the real axis. This deviation from ideality is frequently attributed to surface roughness at the electrode interfaces; however, it is known that these multilayer films have quite low surface roughness. Other possibilities have been discussed52-54 and include a large mobility ratio between positive and negative charge carriers, or some degree of incomplete blocking at the electrodes, both of which could potentially be occurring in these devices. The Cole-Cole plots in Figure 1b show that as the temperature is decreased from 108 °C, the diameter of the semicircle RB systematically increases due to a decreasing conductivity, and this behavior is again completely reversible. The conductivity of a PAH/PAA (pH 3.5/3.5) film is quite low: ranging from about 1 × 10-12 S/cm at 30 °C to about 4 × 10-9 S/cm at 108 °C. An Arrhenius plot of these data can be fit to a straight line producing an activation energy of about 1 eV with a correlation coefficient of 0.994. This implies that the (51) Day, D. R.; Lewis, T. J.; Lee, H. L.; Senturia, S. D. J. Adhes. 1985, 18, 73-90. (52) Macdonald, J. R. J. Chem. Phys. 1974, 61, 3977-3996. (53) Macdonald, J. R. J. Electroanal. Chem. 1975, 66, 143-147. (54) Macdonald, J. R. J. Electrochem. Soc. 1977, 124, 1022-1030.
Durstock and Rubner
conduction follows an Arrhenius-type activation process that is typical of ion conducting polymers below their glass transition temperatures Tg. At temperatures greater than 250 °C, an irreversible change in the dielectric behavior was observed (data not shown), characterized by a shift in the loss peak to lower frequencies with increasing temperature. This change in behavior is associated with a thermally driven reaction between the ionic groups of PAH and PAA.55 This wellknown thermal dehydration process produces a dense, cross-linked film in which ionic motion becomes more hindered. pH Dependent Behavior of PAH/PAA Multilayer Films. We have previously reported that the pH of the dipping solutions dramatically influences the resultant multilayer composition/organization of PAH/PAA films.28-30 The dielectric properties of these films were therefore expected to be sensitive to the processing conditions used to assemble the multilayers (i.e., solution pH). For the pH range examined in this study (pH 3.5-6.5), the degree of ionization of the adsorbed PAA chains increases with pH reaching close to 100% at a pH of 6.5 (determined by FTIR). The PAH chains on the other hand remain essentially fully charged over the pH range studied. The variation in the PAA charge density can significantly influence the resulting film and so PAH/PAA films, made under a variety of pH conditions, were studied in an attempt to more fully understand the nature of the sequential adsorption process. Multilayer films were made with the pH of both solutions set to three different values. A pH of 3.5 corresponds to a relatively low degree of ionization of the PAA and produces a PAH/PAA bilayer thickness of approximately 56 Å. Increasing the pH to 5.0 results in a larger degree of ionization of the PAA and an increase in the bilayer thickness to about 124 Å. Pushing the pH to 6.5 results in the PAA being almost completely ionized and thereby gives a very small bilayer thickness (ca. 5-8 Å).30 The total film thickness for each of these films was 1155, 1176, and 692 Å for the pH 3.5/3.5, 5.0/5.0, and 6.5/6.5 cases, respectively. As with the PAH/PAA (pH 3.5/3.5) film, all of these films exhibited a temperature dependent conductivity with an activation energy near 1 eV. The pH 5.0/5.0 and 6.5/6.5 films, however, had conductivities that were consistently a factor of 2 smaller than the 3.5/3.5 films. The dielectric response at 110 °C for each of these films is shown in Figure 3. At low frequencies, ′ is significantly greater in the pH 3.5/3.5 case than for either of the other two cases. The films made at the two higher pH values have similar low-frequency values for ′ with perhaps the pH 5.0/5.0 case tending to slightly higher values. In the plateau region at higher frequencies, the value of ′ is about 5-6 for the pH 3.5/3.5 and pH 5.0/5.0 cases, and about 4-5 for the pH 6.5/6.5 case. These results reflect the fact that as the solution pH is increased, the PAA chains exhibit a higher degree of ionization. This higher charge density results in the formation of a more highly ionically cross-linked film due to the presence of a larger number charged carboxylate groups that are available to form polymer-ion pairs. The fact that the low-frequency value of ′ decreases as the pH increases further suggests that the amount of small ions that gets incorporated into the film decreases as the PAA becomes more ionized. It should be noted, however, that even at a pH of 6.5/6.5, the film still exhibits appreciable (55) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978-1979.
Polyelectrolyte Multilayers
Figure 3. Dielectric behavior at 110 °C of PAH/PAA films sequentially adsorbed with both of the dipping solutions set at the indicated pH.
values for ′ and ′′ at low frequencies. The conductivity of this film is also still relatively high (3 × 10-9 S/cm, ca. 110 °C). This implies that even when the molecules pair up to form very thin bilayers, there still exists some residual level of small ions present in the film. In a later section, these results will be compared to those for layers of PAH and SPS, two fully charged macromolecules, and similar conclusions will be drawn. Finally, it should be noted that it is not completely clear why the peak of the dielectric dispersion occurs at higher frequencies for films fabricated from the higher pH solutions. This may suggest a change in the mechanism of ionic mobility possibly due to different morphologies and/or chain organizations for films made at different solution pH’s.28-30 It may also be the reflection of a larger number of ions in the films made at lower pH values. As discussed in the next section, this can influence the mobility of ions in the film and thereby change the dielectric behavior. Postsequential Adsorption Treatment of a PAH/ PAA Multilayer Film. It is interesting to consider the fact that if a multilayer film is fabricated at relatively low pH values only a small fraction of the carboxylic acid groups of the PAA will be ionized and complex with the PAH. The others remain as free acid groups in the final PAH/PAA film. Since these groups are not associated with the PAH, there exists the potential to make use of them after the film has been created. For example, it has recently been shown that this approach can be used to grow nanoparticles in these films.56 If a film containing some of these free acid groups is immersed into an aqueous salt solution (NaCl used here) at a high pH, then in addition to the solution simply swelling the film,34 the acid groups also become ionized to the sodium carboxylate form (-COO-Na+). Figure 4 shows the dielectric behavior obtained at 110 °C for PAH/PAA (pH 3.5/3.5) films (each nominally 900 Å) that were dipped into various salt solutions after they were made. They were immersed for 1.5 h into 10-5 M NaOH aqueous solutions, having a pH of about 8-9, with different NaCl concentrations. (It should be noted that while there was some variation in the behavior of films made at different times, films that were made at the same time showed good reproducibility.)
Langmuir, Vol. 17, No. 25, 2001 7869
Figure 4. Dielectric behavior at 110 °C for PAH/PAA (pH 3.5/ 3.5) multilayer films dipped into aqueous salt solutions for 1.5 h after making the film.
As shown in the figure, exposing the film to a high pH has the effect of increasing the magnitude of the dielectric dispersion (samples A and B). The free acid groups that are present in the film become ionized at the higher pH and this results in an increase in the carrier concentration. As additional salt is added to the solution, the loss peak shifts to lower frequencies and the conductivity decreases (samples B, C, and D). This is attributed to a decrease in the carrier mobility with an increasing salt concentration. Such an effect is frequently observed in polymer electrolytes based on PEO:salt mixtures57 in which the decrease in conductivity with increasing salt content is usually attributed to an increase in the glass transition temperature. This causes a stiffening of the matrix that significantly decreases the mobility of the ions and this more than compensates for the increase in their concentration. A similar effect is believed to be responsible for the results shown here. Effects of Moisture on the Behavior of PAH/PAA Multilayer Films. In addition to changing the number of ions that are present, the ionic conductivity can be increased through the use of plasticizers. In solid polyelectrolyte films, the strong ion pairing that occurs between the polymer ions (i.e., the ionized polymer) and the small ions gives rise to a glassy and brittle nature and hence low values of the conductivity. Plasticizing the matrix with a compatible, low molecular weight solvent can have the effect of breaking up this association.58 For polyelectrolytes, water is usually the solvent of choice as evidenced in ion exchange and fuel cell membranes.59 Here, the effect that a humid environment has on the electrical properties of a PAH/PAA (pH 3.5/3.5) multilayer film, with a total film thickness of approximately 900 Å, is examined. Figure 5 shows that upon exposing the film (56) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354. (57) Gray, F. M. Solid Polymer Electrolytes, Fundamentals and Technological Applications; VCH Publishers: New York, 1991. (58) Barker, R. E., Jr. PureAppl. Chem. 1976, 46, 157-170. (59) Gray, F. M. Polymer Electrolytes; The Royal Society of Chemistry: London, 1997.
7870
Langmuir, Vol. 17, No. 25, 2001
Durstock and Rubner
Figure 5. Effect of a humid environment (argon at 85-90% relative humidity) on the room-temperature dielectric behavior of a PAH/PAA (pH 3.5/3.5) multilayer film.
Figure 6. Temperature dependence of the dielectric behavior of a PAH/SPS (pH 3.5/3.5) multilayer film made with no added salt in the dipping solutions.
to wet argon (85-90% relative humidity (%RH)), a broad dispersion in the dielectric response is observed. The lowfrequency value of ′ increases to about 325 and the dispersion extends over about 4 orders of magnitude (fwhm∼2.5 orders). Furthermore, the conductivity shows a dramatic 5 order of magnitude increase upon hydration, going from 3 × 10-12 S/cm in the dry state to 2 × 10-7 S/cm when hydrated. The reason for these large increases upon hydration is mainly attributed to a solvation effect. The water solvates the ions in the film and can cause an increase in both the effective concentration of carriers as well as their mobility. It should be noted that all of these values were based on a measured film thickness of about 900 Å in the dry state. It has been shown recently, however, that the thickness of sequentially adsorbed layers of PAH and SPS can increase by as much as 18% when they are hydrated.34,36 Any increase in the film thickness brought about by hydration would cause an increase in the calculated value for the conductivity and for ′ by the same factor. Consequently all of the values reported here for the hydrated films should be considered to be lower estimates. If the characteristics of a hydrated PAH/PAA film at room temperature (′ = 325, σ = 2 × 10-7 S/cm) are compared to those of a dry film at 108 °C (′ = 100, σ = 4 × 10-9 S/cm), then the effects of both water and temperature can be distinguished. Humidity has a significantly larger effect than temperature on the behavior of the PAH/PAA film. As discussed above, increasing the temperature resulted in a simple shifting of the dielectric dispersion to higher frequencies but did not affect its magnitude. This is primarily due to the temperature dependence of the ionic mobility. On the other hand, exposing the film to humidity (room temperature) resulted in both a shifting of the dielectric dispersion to higher frequencies, as well as in an increase in its magnitude to a value greater than that seen by increasing the temperature (′ = 325 vs 100). This suggests that in these PAH/PAA films, moisture not only acts to increase the mobility of the ions by plasticizing the matrix, but it also increases their effective concentration. An additional possibility is that the water may be acting to ionize some of the free acid groups present in the film and thereby increase the carrier concentration. To see if this effect was important, the PAA was replaced with SPS and the characteristics of the resulting PAH/SPS multilayer films were examined. As
discussed below, small ion conductivity appears to dominate the conduction process in all of these films. Temperature-Dependent Behavior of PAH/SPS Multilayer Films. The previous sections dealt with the behavior of sequentially adsorbed layers of a fully charged polyelectrolyte (PAH) together with one whose charge density can vary with the pH (PAA). In this section, however, the behavior of two fully charged polyelectrolytes will be examined. The polycation is the same that was used in the previous section, namely PAH. The polyanion is sulfonated polystyrene (SPS) which, being a much stronger acid than PAA, is also fully charged under the pH conditions that are typically used. Both the SPS and the PAH tend to form more extended chain conformations in solution due to the electrostatic repulsion of like charges along the chain. When they are fabricated into a multilayer film, they adsorb as very thin layers presumably with a high density of polymerpolymer contact ion pairs. Frequently, a salt such as NaCl is added to the solution to effectively shield the charges along the polymer chain from each other. This shielding reduces the amount of electrostatic repulsion and thereby causes the chains to adsorb in a more coiled conformation which, in turn, results in thicker bilayers. Multilayer films of PAH/SPS (pH 3.5/3.5) were made both with and without added NaCl in the dipping solutions used for sequential adsorption. For the films made without adding any salt into the solution, the bilayer thickness was very small (
