Langmuir 2008, 24, 13127-13131
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Controlling Layer Thickness and Photostability of Water-Soluble Cationic Poly(p-phenylenevinylene) in Multilayer Thin Films by Surfactant Complexation Jeremy S. Treger,† Vincent Y. Ma,† Yuan Gao,‡ Chun-Chih Wang,‡ Seaho Jeon,‡ Jeanne M. Robinson,‡ Hsing-Lin Wang,‡ and Malkiat S. Johal*,† Department of Chemistry, Pomona College, 645 N. College AVenue, Claremont, California 91711, and Chemistry DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed July 7, 2008. ReVised Manuscript ReceiVed August 13, 2008 In this work we build on prior studies of the novel water-soluble cationic conjugated polymer known as “P2” (poly{2,5-bis[3-(N,N,N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene}) with a focus on its incorporation into thin films for such applications as photovoltaics or electroluminescent devices. Multilayer assemblies were constructed using P2, the anionic surfactant sodium dodecyl sulfate (SDS), and the polyanion poly(sodium 4-styrene-sulfonate) (PSS) using the technique of layer-by-layer electrostatic self-assembly (LBL-ESA). SDS was observed to affect the layer thicknesses and absorbance characteristics of the films. We show that the optical properties and photo-oxidative resistance can be improved by varying the SDS content in the assemblies. Specifically, the surfactant-complexed poly(p-phenylenevinylene) (PPV) shows an enhanced absorption at longer wavelengths as well as improved photostability. Therefore, our work may have broad implications on the development of stable PPV-based materials in general and their efficient integration into thin films technologies.
Introduction Ever since poly(p-phenylenevinylene) (PPV) was first demonstrated to have electroluminescent properties in 1990,1 derivatives of PPV have been widely studied as potential components in devices ranging from photovoltaic cells2-4 and biosensors5,6 to electroluminescent devices.7 The conjugated system of alternating phenyl and vinyl groups common to all PPVs allows them to absorb visible light as well as conduct electricity along this characteristic backbone structure. We previously reported the synthesis of the water-soluble cationic polyelectrolyte poly{2,5-bis[3-(N,N,N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene},8 hereafter referred to as “P2” (Figure 1). This PPV derivative is distinguished by the two side chains joined to the benzene ring, each containing a quaternary ammonium ion. These impart a net positive charge on the polymer and have a steric effect on the conjugated backbone. For instance, the polymer twists along its backbone in order to give the cationic ammonium groups the greatest spatial separation.8,9 As in other PPV derivatives, the side groups greatly * Author to whom correspondence should be addressed. E-mail:
[email protected]; fax: (909) 607-7726; URL: http://pages. pomona.edu/∼msj04747/. † Pomona College. ‡ Los Alamos National Laboratory. (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature. 1990, 347, 539. (2) Jenekhe, S. A.; Yi, S. Appl. Phys. Lett. 2000, 77, 2635. (3) Kietzke, T.; Hoerhold, H.-H.; Neher, D. Chem. Mater. 2005, 17, 6532. (4) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15. (5) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. 1999, 96(22), 12287–12292. (6) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896. (7) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067. (8) Gao, Y.; Wang, C.-C.; Wang, L.; Wang, H.-L. Langmuir 2007, 23, 7760. (9) Wellman, D. Thesis, Department of Chemistry, Pomona College, 2007. (10) Treger, J. S.; Ma, V. Y.; Gao, Y.; Wang, C.-C.; Wang, H.-L.; Johal, M. S. J. Phys. Chem. B 2008, 112(3), 760–763. (11) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107.
Figure 1. Molecular structure of P2 (poly{2,5-bis[3-(N,N,N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene}).
affect the chemistry of the polymer and account for its unique optical properties.10 One of the advantages of working with P2 is its remarkably high photoluminescent quantum efficiency (PLQE). At 14% in aqueous solution, it is near the theoretical upper-limit for watersoluble PPVs.8 Furthermore, P2’s water solubility and net positive charge enable it to be incorporated into multilayer assemblies using techniques such as electrostatic self-assembly (ESA).11 The ESA process involves the interlaced deposition of a polycation and a polyanion to yield charge-alternating multilayers. The efficacy of this alternating deposition technique is based upon the electrostatic adsorption of a polyion from aqueous solution onto a substrate primed with a polyion of opposite charge. Charge overcompensation and entropic gain due to counterion expulsion are generally cited as the driving mechanisms behind layer formation.12
10.1021/la802080t CCC: $40.75 2008 American Chemical Society Published on Web 10/23/2008
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One major problem that has plagued PPVs, including P2, is their susceptibility to photo-oxidation.13 This is especially of concern to water-soluble PPVs. Light catalyzes an oxidation reaction at the vinyl linkages in the polymer backbone, decreasing conjugation length and reducing the lifetime and quality of the polymer’s optical properties.14-16 Previous studies have demonstrated the benefits that surfactants can have on the wettability and optical properties of polyelectrolyte multilayer assemblies.17-19 In this work, preformed complexes of P2 and the anionic surfactant sodium dodecyl sulfate (SDS) are adsorbed from aqueous solution directly onto a polyanionic layer of poly(sodium 4-styrene-sulfonate) (PSS) using the layer-by-layer ESA (LBL-ESA) method.20,21 We demonstrate that SDS is effective in significantly inhibiting the photo-oxidation of the films, as well as increasing the optical density in the visible wavelength region. Finally, we propose a model to explain the mechanism by which these effects occur. This work may have implications in the design and fabrication of thin films composed of photosensitive PPV derivatives, which may lead to stable materials for a variety of optical and electroluminescent applications.
Experimental Section Poly(ethylenimine) (PEI, mixture of linear and branched chains, CAS 9002-98-6), PSS (MW ∼ 1 000 000 g mol-1, CAS 2570418-1), and SDS (ACS reagent, > 99%, CAS 151-21-3) were used as received from Aldrich. The multilayer thin films were assembled on one-inch square glass substrates that had been cleaned by a 1-h immersion in a 70/30 by volume mixture of concentrated sulfuric acid (98%) and hydrogen peroxide (30%). Further details on the substrate preparation can be found in previous work.22 PEI was prepared at 1 mM (based on the molecular weight of the monomer repeat unit) at pH 7. P2 and PSS were both prepared at 0.4 mM concentration. Each polymer layer was constructed by exposing the substrate to the appropriate polymer solution for 5 min, followed by extensive rinsing with ultrapure water (resistivity > 18 MΩ cm). After rinsing, the film was dried under a stream of nitrogen gas. All multilayer assemblies were built on a base layer of PEI adsorbed to the glass substrate because of the low affinity of P2 for the glass surface. After the initial formation of the PEI layer, fabrication continued with a layer of PSS to prime the surface for the first layer of P2. Subsequent layers were built with alternating immersions in P2 and PSS. The procedure was repeated to yield the desired number of P2/PSS bilayers, and the multilayer assemblies are henceforth denoted as [P2/PSS]n, where n denotes the number of bilayers after the initial PEI/PSS bilayer. After each application of PSS, absorbance spectra of the films were recorded using a UV-visible absorption spectrometer (Varian Cary 300). We note that the multilayer films assembled on glass are uniformly clear and optically smooth. Visually we see no evidence of aggregation or inhomogeneity. Hence we eliminate the possibility that changes in the absorbance are due to scattering effects. (12) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (13) Cumpston, B. H.; Jensen, K. F. Syn. Metals 1995, 73, 195. (14) Bianchi, R. F.; Balogh, D. T.; Tinani, M.; Faria, R. M.; Irene, E. A. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 1033. (15) Scott, J. C.; Kaufman, J. H.; Brock, P. J.; DiPietro, R.; Salem, J.; Goitia, J. A. J. Appl. Phys. 1996, 79, 2745. (16) Manca, J.; Bijnens, W.; Kiebooms, R.; D’Haen, J.; D’Olieslaeger, M.; Wu, T. D.; De Ceuninck, W.; De Schepper, L.; Vanderzande, D.; Gelan, J.; Stals, L. Opt. Mater. 1998, 9, 134. (17) Johal, M. S.; Chiarelli, P. A. Soft Matter 2007, 1, 34–46. (18) Johal, M. S.; Ozer, B. H.; St. John, A.; Casson, J. L.; Wang, H.-L.; Robinson, J. M. Langmuir 2004, 20(7), 2792–2796. (19) El-Khouri, R. J.; Johal, M. S. Langmuir 2003, 19, 4880. (20) Decher, G. Science 1997, 277, 1232. (21) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 831, 210– 211. (22) Johal, M. S.; Casson, J. L.; Chiarelli, P.; Liu, D.-G.; Shaw, J. A.; Robinson, J. M.; Wang, H.-L. Langmuir 2003, 19, 8876.
Figure 2. UV-visible spectra of P2 multilayer assemblies (n ) 1-7) built with (a) 0 mM SDS and (b) 10 mM SDS. λmax is denoted by a dotted vertical line in both graphs, emphasizing the prominent red shift (∆λ) caused by the surfactant. All absorbance values were baseline-corrected by subtracting the spectrum of the blank slide.
To incorporate SDS into the films, the surfactant was directly added to the P2 solution to yield precomplexed P2 and SDS in aqueous solution. This approach was found to be more effective than other methods, such as complexing SDS with PSS and incorporating SDS as a separate layer altogether. The PEI/PSS primed substrate was exposed sequentially to the P2+SDS and PSS solutions to yield multilayers containing both polyelectrolytes and the surfactant. Each slide was built to [(P2+SDS)/PSS]7. The qualitative photo-oxidation behavior of the completed multilayer assemblies was investigated by periodic measurements of the absorbance spectra over several hours using UV-visible absorption spectroscopy. Spectra were measured from 350 to 700 nm at a scan rate of 150 nm/min. Multilayer assemblies were allowed to degrade in empty, capped, clear polypropylene centrifuge tubes under ambient laboratory conditions. Ellipsometric thickness measurements were performed on a Gaertner L2W dual-wavelength ellipsometer and taken at two wavelengths (543.5 and 632.8 nm) with an incident angle of 70°. A refractive index of 1.5 ( 0i was used to manually calculate ellipsometric film thickness from ∆ and Ψ parameters. The thickness of the native oxide layer on Si was determined for every substrate used. Multilayers were constructed on silicon wafers (Virginia Semiconductor) that had been precleaned using the method described above. The same native oxide surface layers for all substrate types provided reproducible surface conditions for film deposition.
Results and Discussion Multilayer thin films constructed without SDS exhibit a λmax ∼ 432.5 nm in the terminal layer (Figure 2a). Upon complexation with 10 mM SDS, the polyelectrolyte-surfactant assembly exhibits a prominent red shift in the absorbance spectrum (Figure
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Figure 3. Model of polymer-surfactant aggregation in thin film assemblies. (a) Thin films of P2 deposit onto underlying polyanion layers in a contorted conformation. (b) The complexation of P2 with SDS in solution stabilizes the polymer in an extended conformation.
2b). λmax increases to 470.5 nm in P2 films with 10 mM SDS. We interpret this red shift as an increase in the conjugation length induced by SDS molecules. This is consistent with our recent absorbance and photoluminescence measurements of P2/SDS in the bulk aqueous phase, where a significant red shift was observed up to a surfactant concentration close to the critical micelle concentration.10 These changes were interpreted as being due to ion-pair interactions between SDS and P2 leading to excimers with extended conjugations lengths. For the multilayer assemblies described in this work, the ion-pair complexation is illustrated in Figure 3, which shows how the backbone conformation of P2 may change from twisted (a), to predominantly extended (b) with the addition of SDS above the critical micelle concentration. We propose that surfactant molecules interfere with the electrostatic interactions of the side chains, inducing relaxation of the twisted backbone. The overall result is an increase in the conjugation length, and hence an increase in the absorbance wavelength. This is in contrast to typical polymer-surfactant complexes, where the presence of the amphiphile increases the coiling of the polymer.17 The initially twisted conformation of P2 is a result of the electrostatic repulsion between the cationic side chains leading to a reduction in the PPV conjugation length. The red shift observed upon SDS complexation is due to the untwisting of the P2 backbone, and not the coiling of the polymer. The binding SDS to P2 counteracts the electrostatic repulsion between the side chains, resulting in flattening of the benzene rings and a corresponding increase in the conjugation length. Figure 2 also shows an increase in absorbance across the entire spectrum in the presence of SDS. Thus, the presence of the surfactant leads to films with a greater overall film thickness compared to the surfactant-free films. Furthermore, 10 mM SDS was found to be the optimum concentration with respect to film thickness among several concentrations tested (Figure 4(a)). The increase in hydrophobicity of the complexes in solution may be contributing to the enhanced adsorption with increasing SDS concentration. Lower concentrations of the surfactant affected the absorbance to a lesser magnitude, but the effects of SDS did not increase linearly throughout the range of tested concentrations. Above an SDS concentration of 10 mM, the surfactant inhibited film growth (Figure 4b). Neither the films constructed using 20 mM SDS nor those constructed using 40 mM SDS built to the level of the surfactant-free film. It is likely that sufficiently high concentrations of SDS neutralize the charge of the polymer, preventing adsorption of both polyelectrolytes during ESA.
Figure 4. (a) Absorbance as a function of layer number for the assembly of P2 with varying concentrations of SDS. The baseline-corrected absorbance is plotted as a function of layer number for each of six concentrations of the surfactant: 0 mM (0), 0.05 mM ()), 0.2 mM ((), 10 mM (O), 20 mM (9), and 40 mM (b). In all cases, absorbance increases linearly before peaking at higher layers, but is strongest in the slide with 10 mM SDS. (b) SDS enhances P2 absorbance most at an optimum concentration of 10 mM. The absorbance of terminal layers of other concentrations are plotted on a logarithmic scale, illustrating the inhibitory effect of high concentrations of SDS on layer growth.
Figure 5. Ellipsometric thickness values for the P2 multilayer assembly as a function of layer number, with (open squares) and without (filled circles) SDS. The film with SDS was built at a surfactant concentration of 10 mM. The first point (at n ) 0) indicates the thickness of the native oxide layer on silicon.
Figure 5 shows the ellipsometric thickness of the multilayer assembly as a function of bilayer number, with and without 10 mM SDS. The assembly containing the surfactant is significantly thicker than the film without the surfactant. The ellipsometric thickness measurements across the film surface indicate a relatively smooth surface with variations within ∼2 Å. The observed patterns in the ellipsometric thickness values of these
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Figure 6. Absorbance difference as a function of wavelength of a 10 bilayer assembly when constructing the film with 10 mM SDS and 0 mM SDS. The absorbance differences were calculated by subtracting the spectrum of the 0 mM film from that of the 10 mM film. This graph shows that the addition of the surfactant greatly increases absorbance at visible wavelengths (∼500 nm). The solid line is a Gaussian fit to the data points (R2 of 0.975).
films independently support the aforementioned conclusions drawn from the absorbance measurements. In particular, as ellipsometry is insensitive to minor scattering effects, the increase in thickness observed is consistent with the increase in optical absorbance. To further investigate the effect of SDS on the optical properties of P2, we calculated the wavelength-dependent difference in the absorbance between the films made with 0 mM and 10 mM SDS concentrations (Figure 6). The resulting spectrum represents a subtraction of the top-layer spectrum of the 0 mM substrate from that of the 10 mM substrate. The difference spectrum shows an enhanced absorbance facilitated by the surfactant, particularly at longer visible wavelengths. Interestingly, the absorbance difference exhibits a Gaussian distribution centered at a wavelength of 500 nm and may represent a characteristic absorption band arising from an almost fully extended conformation of P2. Thus, the effect of SDS is an enhanced absorption of light at ∼500 nm, a region that is desirable for the efficient function of photovoltaic materials. In addition to improving layer-assembly and shifting absorbance to longer wavelengths in the visible region, SDS also reduces the rate of photo-oxidation in multilayer films. In films built to seven bilayers using 10 mM SDS, maximum absorbance levels were higher and also stayed closer to the maximum level after several hours of light and air exposure (Figure 7). The film with no SDS degraded at a rate of 1.38 × 10-5 absorbance units min-1 over a period of 3 h. In contrast, the film with 10 mM SDS degraded at 5.98 × 10-6 absorbance units min-1, which is less than half the rate of the surfactant-free film. Decay rates were also calculated at wavelengths lower and higher than λmax using the slopes of absorbance versus time graphs (Figure 8a). Both experienced the greatest rates of photo-oxidation near wavelengths of maximum absorbance, but throughout the spectra, rates were consistently lower in films constructed from 10 mM SDS. Further calculations of relative decay rates (min-1) for the films (Figure 8b) were performed by dividing degradation rates in the region of 375-500 nm (abs/min) by the maximum absorbance. The findings confirmed that the slower degradation was not merely due to differing absolute film thicknesses, but resulted from the
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Figure 7. Comparisons of films built from P2 with 0 mM (open squares) and 10 mM (filled squares) concentrations of SDS. The absorbance at λmax as a function of time indicates that the surfactant inhibits the degradation of the films. The film with no SDS degrades at a rate of 1.38 × 10-5 abs/min while the film with 10 mM SDS degrades at a rate of only 5.98 × 10-6 abs/min. T ) 0 corresponds to the completion of the layer assembly.
Figure 8. Comparisons of films built from P2 with 0 mM (open squares) and 10 mM (filled squares) concentrations of SDS. (a) The decay rate for the film with no SDS is much greater throughout the spectrum, especially in the visible region of the P2 spectrum. (b) Because it is important to know how long a film will take to photo-oxidize, we calculated relative decay rates for the films. Decay rates in the region of 375-500 nm (abs/min) were divided by the maximum absorbance to obtain relative rates (min-1). The results confirmed that slides built with 10 mM SDS photo-oxidize through a much smaller fraction of their film thickness in a given amount of time.
addition of SDS. Assuming that the photo-oxidation proceeded linearly as a function of time, approximate half-lives for the
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films can be calculated from the inverses of the relative decay rates. This is done by determining how long it would take for a film’s absorbance to degrade to one-half of its original value under the linear decay approximation. The surfactant-free film would be expected to last 3.83 h before its absorbance at its λmax (432 nm) was half of its original value. The multilayer assembly made using 10 mM SDS, on the other hand, would have an approximate half-life of 13.2 h at its λmax (470.5 nm), a greater than 3-fold improvement. At longer wavelengths, the effect is even more dramatic. At 500 nm, the surfactant-free multilayer film has an expected half-life of 2.31 h, while the film constructed from 10 mM SDS is expected to last 10.75 h before reaching half-degradation, well over 4 times as long. We suggest that the increased resistance to photo-oxidation resulting from surfactant complexation is due to SDS inhibiting diffusion of oxygen and moisture into the film assemblies due to its enhanced hydrophobicity. Photo-oxidation of PPV assemblies on glass is primarily a surface effect.23 This implies that oxidation occurs as oxygen/water molecules diffuse into the film and come into direct contact with the polymer. SDS, however, is a small molecule and can fill much of the open space in the film’s structure, making it much more difficult for oxygen to penetrate the film’s interior. Furthermore, it is worth mentioning that the ellipsometric thickness values reported in Figure 5 did not change over the course of 24 h. Thus, we conclude that the reduction in the absorbance under ambient conditions is not due (23) Norrman, K.; Alstrup, J.; Joergensen, M.; Krebs, F. C. Surf. Interface Anal. 2006, 38, 1302.
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to the loss of material or any other process such as aggregation that may change the thickness. The lack of thickness change is consistent with the slow breakage at the vinyl linkages in the polymer backbone, leading to no significant changes in mass, density, or thickness of the film. Thus, the absorbance measurements provide unequivocal evidence of photodegradation.
Summary Surfactant-complexed conjugated polyelectrolyte films constructed by ESA exhibit enhanced absorption of visible light (500 nm) and increased photostability versus films constructed from the pure polyelectrolyte. These results are consistent with a model in which the intercalated surfactant counteracts the electrostatic repulsion between PPV side chains, resulting in flattening of the benzene rings and increased conjugation length. These findings demonstrate that the longevity and optical properties of multilayer assemblies of conjugated polyelectrolytes can be significantly improved using simple small molecules. Acknowledgment. We thank Lewis Johnson (Pomona College) for useful discussions. We are grateful for funding from the Pomona College SURP program. H.-L.W. acknowledges support from the Basic Energy Science (BES) Biomaterials Program and the LANL Laboratory Directed Research and Development (LDRD) Fund. J.S.T. acknowledges support from the Arnold and Mabel Beckman Foundation. LA802080T
