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Effect of Molecular Weight on the Construction of Polyelectrolyte Multilayers: Stripping versus Sticking Zhijie Sui, David Salloum, and Joseph B. Schlenoff* Department of Chemistry and Biochemistry & Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306 Received September 10, 2002. In Final Form: December 19, 2002 Combinations of a polyanion and a polycation, with different molar masses but narrow molar mass distributions, are employed to construct thin films by the polyelectrolyte multilayering technique. All polyelectrolytes are assembled in the presence of added salt. Even highly charged polymers in the 104 Da range of molar mass exhibit atypical multilayering characteristics. If the molar mass, MM, of either of the polymers is in this range, the thickness increment realized on addition of the shorter polymer is partially lost on exposure to the solution of longer polymer, as surface polyelectrolyte is stripped off by its oppositely charged partner. Complete loss of low MM polyanion on exposure to low MM polycation inhibits multilayer growth altogether. The role of salt is elucidated in the balance between multilayer growth, which relies on kinetically irreversible adsorption, and polyelectrolyte stripping, which produces a stable solution dispersion of polyelectrolyte complex. Subtleties in the irreversibility of commonly employed combinations of high molar mass polymers are revealed by in situ waveguide measurements.
Introduction In the construction of ultrathin polyelectrolyte complex and multicomposite films by the alternate layering method,1-3 polymer molar mass is believed to be a variable of lesser importance compared, for instance, with polymer type or salt concentration. While this assumption may hold true for high molar mass polyelectrolyte, there are indications in the literature that the layer-by-layer addition of multilayer components does not always proceed in a smooth fashion, particularly for species of low molar mass4-6 or for nanoparticulates.7 It was recognized early2 that making thin films by the multilayer process is inherently a kinetically irreversible process, as is generally found for polyelectrolyte adsorption8seach component should add irreversibly to the surface, forming an insoluble complex that augments the thickness of the film. The process competing with surface addition is surface removal, to form a stable, solution dispersed complex. In the case of purely polymeric constituents, quasisoluble polyelectrolyte complexes, QPECs, are well-known stable solution-phase particulate morphologies9,10 of the same composition found in polyelectrolyte multilayers (PEMUs).11 Q-PECs are formed (1) Decher, G., Schlenoff, J. B., Eds. Multilayer Thin FilmsSequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, 2002. (2) Decher, G. Science 1997, 277, 1232. (3) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (4) Kolarik, L.; Furlong, D. N.; Joy, H.; Struijk, C.; Rowe, R. Langmuir 1999, 15, 8265. (5) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (6) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178. (7) Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chem. Lett. 1997, 125. (8) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (9) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Application; Hanser: Munich, 1994; Chapter 6. (10) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343. (11) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621.
simply by mixing dilute solutions of polycations and polyanions in nonstoichiometric proportions.9,10 If the same polymers are mixed at higher concentration, a gelatinous mass of insoluble polyelectrolyte complex results.12 Kinetic irreversibility was demonstrated with radiolabeled polymer in the 105 Da range,13 which was neither removed from the surface by oppositely charged polymer nor exchanged by like-charged polymer (nor displaced by salt) on the time scale of the multilayering. At long times, however, some loss of polymer was observed,13 an indication of slight reversibility. The work described herein was initiated to find out why multilayering is compromised by low molar mass components and to explore the lower limits for molar masss and the fate of polyelectrolytes that do not stick to the surface. Salt moderates the strength of the polymer-polymer contacts in complexed polyelectrolytes.14,15 Further, salt controls the kinetics of polyelectrolyte rearrangement and detachment, as shown by radiochemical studies,16,17 atomic force microscopy,14 and Q-PEC exchange kinetics.18 For a thorough understanding of the relative propensities of sticking vs stripping, the salt concentration should be defined. Experimental Section The characteristics of the polymers used are summarized in Table 1. Broad and narrow molar mass distribution (BMMD and NMMD, respectively) poly(styrenesulfonate) (sodium salt, PSS) and BMMD poly(diallyldimethylammonium chloride) (PDADMA) were obtained from Aldrich. Poly(4-vinylpyridine) (P4VP, Polymer Sources) was methylated according to the following procedure: P4VP (200 mg, 1.9 mmol) was dried at 110 °C and dissolved in 15 mL of a 50/50 mixture of DMF and nitroethane, which had (12) Dautzenberg, H. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; M. Dekker: New York, 2001; Chapter 20. (13) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (14) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (15) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184. (16) Pefferkorn, E.; Jean-Chronberg, A. C.; Varoqui, R. Macromolecules 1990, 23, 1735. (17) Schlenoff, J. B.; Li, M. PMSE Prepr. ACS Proc. 1995, 72, 269. (18) Izumrudov, V. A.; Bronich, T. K.; Zezin, A. B.; Kabanov, V. A. J. Polym. Sci. Polym. Lett. Ed. 1985, 23, 439.
10.1021/la026531d CCC: $25.00 © 2003 American Chemical Society Published on Web 02/06/2003
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Table 1. Characteristics of Polyelectrolytes Employed identifier
polymer
Mn
Mw/Mn
comments
PSS-103N “L-” PSS-801N PSS-7N “S-” PSS-70Ba PDADMAa,b P4VP-55N “L+” P4VP-6N “S+”
poly(styrene sulfonate) sodium salt poly(styrene sulfonate) sodium salt poly(styrene sulfonate) sodium salt poly(styrenesulfonate) sodium salt poly(diallyldimethylammonium chloride) poly(4-vinylpyridine) poly(4-vinylpyridine)
103 200 801 100 7 200 70 000 300 000-400 000 46 700c 5 060c
1.23 1.16 1.11 2.5 2.5 1.14 1.06
NMMD polyanion, high MM NMMD polyanion, high MM NMMD polyanion, low MM WMMD polyanion, high MM WMMD polycation, high MM NMMD polycation, high MM NMMD polycation, low MM
a
MM and MMD are nominal values supplied by the manufacturer. b This material is branched. c The MM for P4VP is before methylation.
been dried with a molecular sieve. Methyl iodide (0.18 mL, 2.9 mmol) was added and the mixture was maintained at 45 °C for 4 h. The product, PM4VP, was precipitated with ethyl acetate, filtered, and washed with ethyl acetate. The complete disappearance of the IR stretching frequency at 1414 cm-1 was taken to indicate >95% methylation.19 Silicon wafers (Si, 0.5 mm thick, 1-in. diameter, undoped, polished from Topsil Inc.) were cleaned in “piranha” solution (70% H2SO4 (conc)/30% H2O2(aq). Caution: highly corrosive; prepare fresh and do not store in closed containers) and then in H2O2/ammonia/water 1:1:7 vol/vol, rinsed in distilled water, and dried with a stream of N2. Multilayers were prepared on these Si wafers manually or with the aid of a robot.20 For layer-by-layer buildup using the manual method, the wafer was alternated between 1 mM PM4VP in 0.5 M NaClaq and 1 mM PSS in 0.5 M NaCl, with a triple rinse in distilled water between each layer. The wafer contacted the polymer solutions for 5 min each and the water rinses for 1 min each. Polymer solutions were constantly stirred with the aid of a stir bar. After each layer, the wafer was dried with N2 and the thickness was measured. Polyelectrolyte concentrations refer to the polymer repeat unit. For the robotic deposition, the Si wafer was affixed to a stainless steel shaft with Teflon tape. The shaft was rotated at 300 rpm by a small dc motor. A robotic platform (StratoSequence V, nanoStrata Inc.), accommodating eight 100-mL beakers, was programmed to expose the wafer alternately to the 1 mM polymer solutions, with water rinses between, for a total of 20 layers.21 The salt concentration in these solutions varied from 0 to 5 M. The thickness of the dried multilayers was measured with a Gaertner Scientific L116B Autogain ellipsometer, using 632.8nm light at 70°. A refractive index of 1.55 was employed for multilayers. A native oxide layer of 8-10 Å was subtracted from total measured thicknesses. The multilayer buildup was also followed in situ by using an optical waveguide technique described by Ramsden.22,23 A flowthrough cell had one surface defined by a planar optical waveguide made from Si1-yTiyO2 (y ∼ 0.4) and supported on a glass slide. Polyelectrolytes were deposited on the surface of the waveguide by pumping solutions through the cell. Integral diffraction gratings coupled light into and out of the waveguide chip, which was mounted on a goniometer (IOS-1, ASI AG, Zu¨rich). The angle between a linearly polarized external beam (λ ) 632.8 nm) and the grating was varied, while the power coupled into the waveguide was measured with a photodiode. At discrete angles, R, a possible guided mode is matched, incoupling takes place, and a sharp maximum is seen in a plot of incoupled power vs angle. Angle R is given by N ) n sin R + lλ/Λ, where N is the effective index for the mode, n is the refractive index of air, l is the mode order, λ is the wavelength of the light, and Λ is the diffraction grating period (here Λ ) 2400 mm-1). From R and N an apparent (or “optical”) thickness tf and refractive index nc of a uniform film contacting the waveguide may be estimated. The (19) Gao, Z.; Varshney, S. K.; Wong, S.; Eisenberg, A. Macromolecules 1994, 27, 7923. (20) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (21) These multilayers are amorphous, e.g., see: Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. In this work “layer” refers to the increment in thickness after exposure to one of the polyelectrolyte solutions and “layer pair” is used to describe a negative/positive “bilayer”. (22) Ramsden, J. J.; Prenosil, J. E. J. Phys. Chem. 1994, 98, 5376. (23) Ramsden, J. J.; Lvov, Y. M.; Decher, G. Thin Solid Films 1995, 254, 246.
Figure 1. Thickness as a function of layer number for four combinations of PM4VP and PSS. The combinations were as follows: diamonds, L+L-; squares, S+L-; triangles, S-L+; and circles, S+S-, as defined in Table 1. Polyelectrolytes were deposited at room temperature from 1 mM solutions in 0.5 M NaClaq onto silicon wafers. The relative error for each data point was (5%. technique and instrument have previously been employed for multilayer buildup studies, where the experimental setup and mathematical treatment of raw data are presented in detail.23,24 The optical waveguide was cleaned with hot sulfuric acid, rinsed with water, and then stored in buffer. All solutions were made up in the same buffer, which was phosphate buffered saline (PBS) of pH 6.8 and ionic strength 0.45 M. Polymers were PDADMA and PSS (1 mM each). Pure buffer, instead of water, was used as rinse. The flow rate of the solutions through the cell was 2.4 µL s-1. Measurements of R were taken every 13.4 s. Polyelectrolyte solutions were alternately pumped through the cell for 4 min each. The temperature was maintained at 24 °C.
Results and Discussion Combinations of low and high MM polyelectrolytes were employed for multilayering. The polydispersity for these samples was minimal and the charge on the polymers was high and pH independent. The terms “high” and “low” molar mass are relativeswithin the context of multilayer buildup, the “high” molar mass limit is reached when MM no longer has a significant impact on multilayer thickness. While an infinite combination of molar masses is possible, the use of ca. 104 and 105 Da polymers is representative of the two molar mass regimes in the present case. It should be pointed out that the “low” molar mass polymers still comprise a substantial number of repeat units (>35). The layer-by-layer buildup of different combinations of “short” and “long” (S and L, respectively) PM4VP and PSS at constant ionic strength is summarized in Figure 1. The L+L- combination exhibited uncomplicated buildup, with a slight upward curvature for the first few layers (as (24) Chluba, J.; Voegel, J.-C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800.
Construction of Polyelectrolyte Multilayers
Figure 2. Model of the possible fate of surface polyanion on exposure to a solution of polycation. The polyanion is either stripped off the surface (upper scenario) to yield a nonstoichiometric quasisoluble polyelectrolyte complex or (lower scenario) it adds to the surface. It is also possible to lose complex from the surface (k3), especially in the presence of salt. For multilayer growth to propagate in a linear fashion, k2 > k1 and k3 should be negligible on the time scale of the experiment. As shown herein, both of these assumptions are not necessarily met if “low” MM polyelectrolyte is employed.
commonly seen with PEMU construction25). Other experiments with high MM PM4VP and PSS (BMMD, Mw ∼ 300 000 and 70 000, respectively) gave similar results. In contrast, after a thin initial film is deposited, the S+Ssystem approaches a constant-thickness oscillatory behavior: a slight increase in thickness provided by a PSS (S-) exposure is removed by subsequent PDADMA (S+). Both S+L- and S-L+ exhibited intermediate behavior but provided a net increase in film thickness with layer number. In rationalizing the behavior of the polyelectrolytes shown in Figure 1 we refer to Figure 2, which represents two possible scenarios following the exposure of a growing multilayer to polyelectrolyte solution. In Figure 2, which, for clarity, does not show the polymers to be interpenetrating,2,21 the starting multilayer is capped with low MM PSS. On exposure to a solution of PM4VP the positive polymer can either compensate the existing charge on the surface and remain, as is normal for multilayer growth, or it can form a quasisoluble complex with the negative polymer, stripping it off the surface and decreasing film thickness. This decrease in film thickness, especially evident when both polymers are of low molar mass, is clear evidence for loss of polymer from the surface. Q-PECs are nonstoichiometric.9,12 The excess polymer charge in Q-PECs is thought to reside at the surface,12 as it does in PEMUs, stabilizing the nanoparticles formed. In the approaches of Dautzenberg9 and Kabanov10 to making stable aqueous dispersions of Q-PECs a great excess of one polyelectrolyte and polyelectrolytes of strongly different molar masses are commonly employed. Clearly, the former condition exists at all times when a growing multilayer is exposed to a polyelectrolyte solution. In assessing the fate of a surface polymer molecule one can identify both thermodynamic and kinetic factors. Thermodynamically, although the composition of PEMUs and Q-PECs are almost the same, there are many gains in entropy favoring Q-PEC formation, including increases in translational and configurational (including the overcompensated surface region of the Q-PEC) entropy. Enthalpic components due to the enhanced hydration of (25) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592.
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polymer surface excess charge may also contribute to a small degree. Q-PECs are aggregates of molecules.9,12 There is little thermodynamic difference in aggregates of high and low molar mass polymer. While thermodynamics provides a driving force for Q-PEC formation during multilayering, the balance between sticking and stripping is likely controlled by kinetic factors. It is known that the adsorption of polyelectrolytes of high MM is kinetically irreversible on the time scale of multilayering. While it has been shown that the adsorption of polymers generally is apparently irreversible due to a very strong adsorption isotherm,8 the true test of irreversibility is a lack of exchange or selfexchange with other polymers in solution.13 Polyelectrolyte adsorption is a multistep process, beginning with the attachment of a few segments, flattening out of the molecule of the surface, and formation of more polymer/ polymer contacts, followed by penetration into the surface to give an amorphous blend of polymers.1,2,21 Stripping must occur before adsorbing polymer has had a chance to interpenetrate. Similarly, the existing surface polymer should be minimally interpenetratedsthis is aided by low molar mass. Surface polyelectrolyte is stripped off to form a Q-PEC of the same charge as the surface and is thus repelled by it. With a S/L combination, Figure 1, it is seen that L (be it L+ or L-) removes some of the S on the surface but does not erode the rest of the multilayer. A solution of S (+ or -) does not remove L on the surface, because the latter has already interpenetrated into the surface. For the S/S system, removal of the S-increment by S+ is quantitative after a few layers, so no growth is observed. The difference in behavior between S+L- and S-L+ was an unexpected subtlety: PM4VP is quite hydrophilic and so is probably more efficient at forming Q-PECs with Sand stripping it off the surface than is PSS at stripping S+. All polyelectrolyte solutions contained a defined amount of salt. Salt moderates electrostatic interactions between oppositely charged polyelectrolyte segments, Pol+ and Pol-, according to the following:14,15
Pol+Pol-m + Na+aq + Cl-aq h Pol+Cl-m + Pol-Na+m where the subscript “m” refers to the multilayer phase. Direct measurements of the swelling of PEMUs by salt ions14 showed it to be reversible, provided swelling was maintained below the point of PEMU decomposition.26 In addition to weakening polymer interactions, salt enhances the thickness of the surface charge overcompensation region25 and therefore the thickness of each “layer”. Salt is also known to “lubricate” the motions of polyelectrolyte molecules interacting with oppositely charged surfaces or molecules.16,27,28 In situ measurements, using atomic force microscopy, of the surface morphology of PEMUs showed that salt accelerates the smoothing of the surface.14 Extremely small polyelectrolyte interdiffusion coefficients were estimated. A combination of weaker interaction and faster molecular motions conspire to facilitate detachment of polyelectrolytes from the surface of a growing PEMU. In Figure 3 the thickness of a 20layer PEMU is shown as a function of salt concentration in the deposition solution. The S+S- system exhibited no significant film growth at any salt concentration in the (26) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (27) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 146. (28) Karibyants, N.; Dautzenberg, H. Langmuir 1998, 14, 4427.
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Figure 3. Thickness of 20-layer PM4VP/PSS multilayers as a function of the concentration of salt in the polyelectrolyte solution. Identity of multilayers are as in Figure 1. Relative errors for data points is (5%.
Figure 4. In situ optical thickness vs time of PSS-801N/ PDADMA multilayers on a waveguide as the solution contacting the waveguide alternates between polymer and rinse. The multilayer starts with PDADMA and halfway through each step a rinse in buffer is applied. Though the precision of the measurement is high, the accuracy of thickness values measured with the waveguide setup is estimated to be (30%.
range 0-1 M.29 L+L- and S+L- showed the typical pseudolinear growth of PEMU thickness with salt concentration. S-L+, by contrast, showed intermediate behavior, with enhanced thickness at low salt concentration, followed by a decrease at high salt. The intermediate behavior (increase followed by a decrease) of S-L+ in Figure 3 reflects a competition between increased thickness as a function of salt concentration due to greater overcompensation25 and saltfacilitated Q-PEC formation (of the type seen in Figure 1). The kinetic balance turns toward stripping at high salt concentration. It appears that, for S+S- and S-L+, some loss to solution as Q-PEC occurs even when no salt is added, since the 20-layer films at [NaCl] ) 0 are very thin. Most PEMUs are constructed from readily available high MM polyelectrolytes of wide MMD. For example, in prior studies, we have used, as have others, the PSS70B/PDADMA combination for numerous multilayers. These common BMMD polymers would be considered (29) Separate experiments with preformed L+L- films exposed to salt revealed that the critical concentration for multilayer decomposition was approximately 4.5 M NaCl.
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Figure 5. In situ optical thickness vs time of one PSS step of a multilayer buildup on a waveguide. This is the 16th layer. The upper curve is from PSS-801N and the lower curve from PSS-70B, injected at “A” and rinsed at “B”.
Figure 6. Part of a PSS/PDADMA buildup on an optical waveguide, starting at layer number 12 (PSS). The first three bilayers shown here were made with PSS-801N, the last three were with PSS-70B. The PSS-70B loses some thickness on rinse. No loss is observed on the PDADMA adsorption step, indicating the sample to be, for this system, in the “high” MM range, on average, despite having a broad MMD.
“high” molar mass. Significant differences in multilayer buildup between the 70 kDa and much higher MM PSS (up to 106 Da) have not been reported. Additionally, we have not found any difference (at the (10% level of precision) in the behavior of commercially available PDADMA, which is branched (due to high conversion of monomer feed), and linear PDADMA.25 However, in broad MMD polymers there is a significant amount of low MM material. We have always been concerned that the fraction of low MM polymer appearing in the multilayer may be enriched compared to solution due to the faster diffusion of low MM material (although we have no direct evidence that this is the case). The subtleties of PSS-70B/PDADMA multilayer buildup were examined by using the in situ waveguide method described by Ramsden et al.22-24 Figure 4 depicts the optical thickness vs time (layer number) for PSS-70B/ PDADMA with the first thickness increment corresponding to adsorption of the first PDADMA layer. Each step starts with a 4-min deposition and is followed by a 4-min
Construction of Polyelectrolyte Multilayers
rinse. The PSS-801N/PDADMA system, employing higher molar mass PSS, gave somewhat thicker films (about 25% thicker). Figure 5 shows an explanation for this. At the point “A” indicated in the figure, the PSS is injected. At point “B” the flow is switched to rinse buffer. Apparently, some complex is lost from the surface with the system employing lower MM PSS. The loss of material on this step is about 25%. Similar losses were not observed during the PDADMA adsorption/rinse step. The contrast between the two molar mass samples of PSS is further illustrated in Figure 6. Three PSS-801N/ PDADMA bilayers (the PSS is the first layer) in the middle of a multilayer buildup are followed by three PSS-70B/ PDADMA bilayers. The growth then loss of material (on rinsing) for the latter is evident. The conditions used for Figures 4-6 are not identical to those for the typical multilayer procedure, where a rinse in pure water, instead of buffer, is carried out after the polymer layer is added. We have not noticed such large differences ascribable to MM in the final PEMU thickness when using pure water to rinse. Pure water would tend to “freeze” the polymer conformations, maximize the polyelectrolyte interactions, and reduce loss.30 In summary, the concept of kinetic irreversibility in polyelectrolyte complexation, required for multilayer growth, is again supported and shown to be influenced by salt concentration. All conceivable variables (including (30) Unfortunately due to wide swings in refractive index between pure water and buffer we could not mimic exactly the typical procedure used for multilayering.
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polymer type, molar mass and concentration, salt type and concentration, temperature, solvent, deposition time, rinse time, whether the sample is dried between layers, and, if the molecule is a weak acid, pH) have some impact on layer thickness. The critical question is whether they are deemed significant. Since most depositions are performed at room temperature with aqueous NaCl solutions of “high” MM polymer, the most important factors remain polyelectrolyte type and salt concentration. However, molar mass is a concern in PEMU formation. Even molecules that would be considered “polymeric”, as opposed to “monomeric” or “oligomeric”, may have a propensity to form Q-PECs at the expense of multilayer growth. The “low” MM range used here was 5-10 000 Da, but this range may depend on interaction energy. For weakly interacting polymers, such as polycarboxylates with PDADMA,14 significantly longer molecules may be required to reach the “high” MM limit, especially in the presence of salt. The use of very hydrophilic polyelectrolytes as one of the partners should be avoided if there is some concern about whether the molar mass is sufficient to be immaterial. A corollary is that more hydrophobic combinations will have a lower likelihood for Q-PEC formation during multilayering. Acknowledgment. This work was supported by a grant from the National Science Foundation (DMR 9727717). We are grateful to K. H. Dahmen for help with waveguide experiments. LA026531D
