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Langmuir 2008, 24, 11346-11349
Hydrogen-Bonded Polymer Multilayers Probed by Neutron Reflectivity Eugenia Kharlampieva,†,§ Veronika Kozlovskaya,†,§ John F. Ankner,‡ and Svetlana A. Sukhishvili*,† Department of Chemistry and Chemical Biology, SteVens Institute of Technology, Hoboken, New Jersey 07030, and Oak Ridge National Laboratory, Spallation Neutron Source, Oak Ridge, Tennessee 37831 ReceiVed August 2, 2008. ReVised Manuscript ReceiVed September 8, 2008 We present a neutron reflectivity study of the internal structure of multilayers made of a weak polyelectrolyte and a neutral component where interactions between adjacent layers are controlled by hydrogen-bonding. We found the degree of interpenetration of polymer layers expressed as the interlayer roughness to be strongly correlated with the strength of intermolecular interactions between the adjacent layers. In addition, polymer layers become more diffuse with a distance from the substrate. Our results demonstrate that hydrogen-bonded films exhibit a close correlation between their structure and properties, which is essential for various applications.
Introduction Recent years have seen increasing interest in layer-by-layer (LbL) deposition techniques due to their ability to create highly tailored polymer thin films with a nearly unlimited range of incorporated functional groups. The utility of neutron reflectivity (NR) to resolve the structure of polyelectrolyte multilayers (PEMs) is supported by earlier studies and has been applied to electrostatically bound films.1-3 Specifically, using selectively deuterated polymer layers, the internal structure of PEMs of strong polyelectrolytes, such as poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), was resolved. Polymer layers with individual layer thicknesses of several nanometers were shown to be intermixed within the film, but strong evidence of internal layering was also found.4 For example, for PSS/PAH films deposited at low ionic strength, an interlayer roughness between adjacent layers was found of 1.2-1.6 nm, comprising ∼0.3-0.4 dbl, where dbl is the PAH/PSS bilayer thickness.1,2 Schlenoff and Jomaa showed that interfacial roughness and polymer interpenetration increased when PAH/PSS multilayer films were exposed to high concentrations of salt, post-assembly.5 Similar effects of ionic strength on the interface roughness of PAH/PSS films during growth were reported by Lo¨sche et al.1 Recently, Gopinadhan et al. showed that heating solutions during deposition resulted in internal roughening of multilayer films similar to that caused by an increase in the ionic concentration of deposition solutions.6,7 Very recently, Jean et al. applied NR to multilayer films made of alternating sheets of rigid cellulose * To whom correspondence should be addressed. Telephone: 201-2165544. E-mail:
[email protected]. † Stevens Institute of Technology. ‡ Oak Ridge National Laboratory. § Present address: Department of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332. (1) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (2) (a) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (b) Kellogg, G. J.; Mayes, A. M.; Stockton, W. B.; Ferreira, M.; Rubner, M. F. Langmuir 1996, 12, 5109. (3) Kugler, R.; Schmitt, J.; Knoll, W. Macromol. Chem. Phys. 2002, 203, 413. (4) Decher, G.; Eckle, M.; Schmitt, J.; Struth, B. Curr. Opin. Colloid Interface Sci. 1998, 3, 32. (5) Jomaa, H. W.; Schlenoff, J. B. Macromolecules 2005, 38, 8473. (6) Gopinadhan, M.; Ivanova, O.; Ahrens, H.; Gu¨nther, J.-U.; Steitz, R.; Helm, C. A. Macromolecules 2005, 38, 5228. (7) Gopinadhan, M.; Ivanova, O.; Ahrens, H.; Gu¨nther, J.-U.; Steitz, R.; Helm, Ch. A. J. Phys. Chem. B 2007, 111, 8426.
crystals and flexible PAH and found they showed a well-ordered film structure.8 Deposition of lipid bilayers on PAH/PSS films was also explored by NR.9 We have recently used reflectivity to resolve the structure of PEMs composed of weak polyelectrolytes and showed more diffuse polymer layering as compared to strong polyelectrolytes.10 Here, we present a NR study of hydrogen-bonded LbL films composed of poly(N-vinylpyrrolidone)/poly(methacrylic acid) (PVPON/PMAA), poly(N-vinylcaprolactam)/PMAA (PVCL/ PMAA), poly(N-isopropylacrylamide)/PMAA (PNIPAM/PMAA), and poly(ethylene oxide)/PMAA (PEO/PMAA). Although these systems were intensively studied earlier11 using attenuated total reflection Fourier transform infrared (ATR-FTIR) and ellipsometry techniques, there are no existing studies of the internal structure of such films. Since many potential applications of PNIPAM-containing coatings and hydrogen-bonded multilayers, including control of cellular12 or bacterial adhesion,13 controlled release of functional molecules,14 ink-jet printing,15 fuel-cell membranes,16,17 or programmed adsorption and release of proteins and cells from the surfaces,18 are now being explored, structural information about such films is highly relevant.
Experimental Section The weight average molecular weights (Mw) of hydrogen-bonding polymers (PHB ) PVPON, PVCL, PNIPAM, PEO; Sigma-Aldrich) and hydrogenated PMAA (hPMAA, Polymer Source) were 360, 40, 300, 200, and 110 kDa, respectively. Si wafers of 4′′Ø with one side polished were purchased from EL-Cat Inc. (8) Jean, B.; Dubreuil, F.; Heux, L.; Cousin, F. Langmuir 2008, 24, 3452. (9) Delajon, C.; Gutberlet, T.; Steitz, R.; Mo¨hwald, H.; Krastev, R. Langmuir 2005, 21, 8509. (10) Kharlampieva, E.; Ankner, J. F.; Rubinstein, M.; Sukhishvili, S. A. Phys. ReV. Lett. 2008, 100, 128303. (11) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (12) Yang, S. Y.; Lee, D.; Cohen, R. E.; Rubner, M. F. Langmuir 2004, 20, 5978. (13) Lee, D.; Cohen, R. E.; Rubner, M. F. Langmuir 2005, 21, 9651. (14) Kozlovskaya, V.; Kharlampieva, E.; Mansfield, M. L.; Sukhishvili, S. A. Chem. Mater. 2006, 18, 328. (15) Sung, Y. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (16) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. J. Am. Chem. Soc. 2005, 127, 17228. (17) DeLongchamp, D.; Hammond, P. Langmuir 2004, 20, 5403. (18) Akiyama, Y.; Kikuchi, A.; Yamato, M.; Okanto, T. Langmuir 2004, 20, 5506.
10.1021/la802502c CCC: $40.75 2008 American Chemical Society Published on Web 09/24/2008
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Figure 1. Model, scattering density b/V versus layer depth z, used to fit the data. (a) Deposition sequence assuming perfect layering. (b) Simplified model divided into six slabs: Sub(strate), slabs I-IV centered on deuterated dPMAA layers, and Surf(ace). (c) Actual physical model incorporating interlayer roughness and intermixing.
Prior to PHB/PMAA film deposition, the Si crystals were cleaned as described above19 and primed with PAH (Mw ) 70 000), whose adsorption at pH 5 from 0.01 M HEPES buffer solution for 15 min yielded a monolayer with 1-2 nm dry thickness. This was followed by deposition of 24-bilayer PHB/PMAA films. All polymers were deposited from 0.5 mg/mL aqueous polymer solutions at pH 2 after a 5 min exposure. Deuterated PMAA (dPMAA, Mw ) 180 kDa, Polymer Source) was deposited with every fifth bilayer to provide neutron contrast. Figure 1a depicts an idealized version of such LbL deposition, wherein we assume sharp interfaces between immiscible polymers. Such perfectly sharp LbL models generate reflectivities that look nothing like our data. A model to fit the NR data incorporating a large amount of layerto-layer interdiffusion has been developed instead. First, note the minimal scattering contrast between protonated PMAA and PHB [the scattering density Σ (defined as Σ ) b/V, where b is the sum of monomer constituent-atom neutron scattering lengths and V is the monomer volume) of PNIPAM is shown in Figure 1a; Σ values for the other PHBs are similar]. This contrast is insignificant, and so we used a thickness-weighted average protonated layer scattering density Σp0 for the protonated PHB/PMAA layers [constant baseline in Figure 1b]. It was found necessary to employ models in which the thicknesses of dPMAA marker layers exceeded the nominal total thickness of one or more PMAA/PHB bilayers. The mass deposited within the dPMAA layers was accounted for to preserve mass balance. The nominal amount of dPMAA deposited is proportional to the area of the rectangle formed by the dotted dPMAA peaks in Figure 1b and the average protonated layer density: Md0 ) (Σd0 - Σp0)dd0, where Σd0 is the dPMAA bulk scattering density and dd0 is the nominal deuterated marker layer thickness. Distributing the material over a greater thickness dd can only be done by mixing the original deuterated layer with the surrounding protonated matrix. Since Md0 is constant, the scattering density of such a diffused layer is Σd ) Md0/dd + Σp0. Figure 1b shows a sequence of four marker layers (I, II, III, and IV) in which dd increases with distance from the substrate to 2, 4, 6, and 8 nm, respectively, with corresponding reductions in Σd. We structure the model around the sequence of marker layers as six distinct slabs [alternating white and shaded areas in Figure 1b]: Sub(strate), slabs I-IV, and Surf(ace). (19) Kharlampieva, E.; Kozlovskaya, V.; Tyutina, J.; Sukhishvili, S. A. Macromolecules 2005, 38, 10523.
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Figure 2. NR data for [(PHB/PMAA)4/PHB/dPMAA]4 films at pH 2.
To model interlayer penetration, Gaussian interdiffusion (error function density profiles) for all interfaces was assumed. For the marker layers in our deuterated slabs (I-IV), interfacial roughness full widths at half-maximum σd were assumed to be equal to the layer thickness dd (we found that smaller σd values consistently degraded goodness-of-fit). Assuming σd ) dd is consistent with classical treatments of diffusion from a slab.20 Most significantly, since substrates and surfaces for our different HB films are similar, by linking both scattering density and interfacial diffusion to dd, we effectively reduced the number of adjustable parameters in the model. When the deuterated marker layer spread over a thickness so large that it began to interact with adjacent marker layers, mixing was modeled by holding deuterated marker and protonated background thicknesses equal (dp ) dd) and uniformly decreasing layer contrast (Σd - Σp). Applying standard substrate and surface interfacial widths to the profile in Figure 1b and setting σd ) dd for slabs I-IV produces the scattering density profile in Figure 1c.
Results and Discussion As confirmed by ellipsometric measurements of dry film thicknesses as a function of layer number, strongly associated PVPON/PMAA, PVCL/PMAA, and PNIPAM/PMAA films showed linear growth, with individual bilayer thicknesses of 3, 3.9, and 4.0 nm, respectively (for density of 1 g/cm3), that agree with our previous findings21 (data not shown). At the same time, consistent with our previous reports,21,22 PEO/PMAA films showed an initially 3-4 times larger individual layer thickness for bilayer numbers less than 14, followed by a drastic decrease and partial layer fouling at larger layer numbers. The PEO/PMAA film used for NR measurements had a dry film thickness of 230 nm. Figure 2 (left panels) compares experimental NR data R as a function of momentum transfer Q for the four polymer systems. The PVPON/PMAA (a) and PVCL/PMAA (b) films exhibit broadened Bragg peaks, more complicated in structure than those (20) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, 1975. (21) Kharlampieva, E.; Sukhishvili, S. A. Polym. ReV. 2006, 46, 377. (22) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301.
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Table 1. Model Parameters for [(PVPON/PMAA)4/PVPON/ dPMAA]4/(PVPON/PMAA)4 Film section
slab
b/V [10-4 nm-2]
substrate
Si SiOx PAH PVPON/PMAA PVPON/PMAA dPMAA PVPON/PMAA dPMAA PVPON/PMAA dPMAA PVPON/PMAA dPMAA PVPON/PMAA
2.07 3.80 0.30 1.05 1.05 4.13 1.05 3.18 1.09 2.45a 1.53 1.65a 1.15
I II III IV surface a
d [nm] 1.5 1.9 1.3 5.7 3.6a 10.6 5.2a 8.7 7.5 7.5 7.5 11.3
σ [nm]
section
slab
b/V [10-4 nm-2]
d [nm]
1.0 1.5 1.3
substrate
Si SiOx PAH PNIPAM/PMAA PNIPAM/PMAA dPMAA PNIPAM/PMAA PNIPAM/PMAA dPMAA PNIPAM/PMAA PNIPAM/PMAA dPMAA PNIPAM/PMAA dPMAA PNIPAM/PMAA
2.07 3.80 0.50 1.05 1.05 2.36 1.05 1.05 2.20 1.05 1.05 1.84 1.05 1.35 0.65
1.5 1.0 3.1 7.3 3.4a 7.3 7.1 3.8a 7.1 6.4 5.2a 10.3 10.2a 12.4
3.6 3.6 5.2 5.2 7.5 7.5 7.5 7.5 11.3
Table 2. Model Parameters for [(PVCL/PMAA)4/PVCL/ dPMAA]4/(PVCL/PMAA)4 Film section
slab
b/V [10-4 nm-2]
d [nm]
substrate
Si SiOx PAH PVCL/PMAA PVCL/PMAA dPMAA PVCL/PMAA dPMAA PVCL/PMAA dPMAA PVCL/PMAA dPMAA PVCL/PMAA
2.07 3.80 0.50 1.30 1.30 2.51 1.30 2.14 1.46 1.84a 1.62 1.53a 1.05
1.5 1.4 1.7 7.6 6.0a 13.9 8.7a 11.6 10.6 10.6 10.6 14.8
II III IV surface a
I II III IV surface
Value is independently varied.
I
Table 3. Model Parameters for [(PNIPAM/PMAA)4/PNIPAM/ dPMAA]4/(PNIPAM/PMAA)4 Film
σ [nm] 1.0 1.4 1.4 6.0 6.0 8.7 8.7 10.6 10.6 10.6 10.6 14.8
Value is independently varied.
found earlier in electrostatically assembled LbL films.1,2,4,10 The PNIPAM/PMAA film (c) reveals a better-defined Bragg peak indicative of a more organized structure within the multilayer. The NR profile for the PEO/PMAA film is dramatically different and does not show any features. These data show that the film is highly disordered with a surface significantly rougher than the others. The data were simulated using the model described in the Experimental Section; initial individual layer thicknesses used during modeling were taken from ellipsometry measurements and then adjusted to correspond to NR superlattice peaks. For all films, no satisfactory simulation of the NR results could be obtained when it was assumed that dPMAA layers were not diluted with hydrogenated material. The calculated reflectivities, overlaid on the data in Figure 2, were optimized for goodnessof-fit. In the scattering density profiles (on the right), one can see the sharply contrasting and well-defined natural silicon oxide layer at the surface of the Si crystal, followed by the polymer layers. These features were similar for all four films. Likewise, the surfaces were very rough (>10 nm) for all four films. Tables 1-4 show the model parameters used for all PHB/PMAA systems. Importantly, in these models, the relative thickness of PMAA versus PHB was fixed for any specific polymer system but varied in the different films. This ratio is determined by specific polymer conformations and the density of hydrogen-bonding sites in the polymer films. The average bilayer thicknesses of PVPON/ PMAA, PVCL/PMAA, and PNIPAM/PMAA partially deuterated films determined from NR were approximately 3, 4.3, and 3.6 nm, respectively, agreeing within e12% with the ellipsometry data given above. All PHB/dPMAA systems showed large internal roughness. The values of σ for the dPMAA layers closest to the substrate
a
σ [nm] 1.0 1.0 0.5 3.4 3.4 3.8 3.8 5.2 5.2 10.2 10.2 12.4
Value is independently varied.
Table 4. Model Parameters for [(PEO/PMAA)4/PEO/dPMAA]4/ (PEO/PMAA)4 Film section
slab
b/V [10-4 nm-2]
d [nm]
σ [nm]
substrate
Si SiOx PEO/PMAA
2.07 3.80 1.10
1.5 235.0
0.5 0.5 25.0
surface
were 3.6, 6.0, and 3.4 nm for PVPON/PMAA, PVCL/PMAA, and PNIPAM/PMAA systems, respectively. These values of internal roughness are significantly larger than those reported for electrostatically deposited PAH/PSS films (usually σ < 2 nm).1-7 The absence of electrostatic regulation of the film growth could be responsible for the stronger interpenetration of polymer chains in the hydrogen-bonded polymer systems. A second observation is that for all except PEO/PMAA systems, the multilayer ordering of PVPON/PMAA, PVCL/PMAA, and PNIPAM/PMAA films decayed with increasing distance from the substrate, in spite of the fact that chains in PVPON/PMAA and PVCL/PMAA films are strongly associated and show linear film growth.21 Stronger chain coiling at larger distances from the substrate is probably driven by an increase in the chain entropy. In the case of PNIPAM/PMAA films, the film layering is more persistent, featuring a more moderate increase in σ (from 3.4 to 5.2 nm for dPMAA layers 1-3). We suggest that the better ordering of the PNIPAM/PMAA film as compared to PVPON/ PMAA and PVCL/PMAA films might be explained by the closeness of the deposition temperature (27 °C) for the PNIPAM solution to its lower critical solution temperature (LCST). Indeed, LCSTs of 30-32 °C23,24 for PNIPAM in solution, 27-32 °C for grafted PNIPAM chains,18 and ∼29 °C for PNIPAM adsorbed at a surface25 have been reported. Caruso and Quinn26 reported that hydrogen-bonded PNIPAM/poly(acrylic acid) (PAA) films fabricated at 30 °C showed lower surface roughness compared to films prepared at lower temperatures. Interestingly, in our experiments, a film containing another temperature-responsive polymer PVCL (LCST ∼ 35 °C27) showed stronger broadening of polymer layers. Because both PNIPAM and PVCL polymers are of high molecular weight, the possible influence of the (23) Furyk, S.; Zhang, Y.; Ortiz-Acosta, D.; Cremer, P. S.; Bergbreiter, D. E. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1492. (24) Yim, H.; Kent, M. S.; Mendez, S.; Lopez, G. P.; Satija, S.; Seo, Y. Macromolecules 2006, 39, 3420. (25) Gao, J.; Wu, C. Macromolecules 1997, 30, 6873. (26) Quinn, J.; Caruso, F. Langmuir 2004, 20, 20. (27) Yanul, N.; Kirsh, Yu.; Anufrieva, E. J. Therm. Anal. Calorim. 2000, 62, 7.
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molecular weight of polymers on LCST can be excluded.28 We believe that the interlayer broadening between these two systems is caused by the higher LCST of PVCL solutions. One of the intriguing directions for future work will be exploring the internal structure of PNIPAM/PMAA or PVCL/PMAA films as a function of the temperature difference from the LCST. Finally, the absence of any features in the reflectivity profile revealed complete intermixing of the PEO and PMAA layers (Figure 1d). There are no significant reflectivity features for the PEO-based multilayer, which we have fit to a thick, completely diffused layer. Such strong intermixing is consistent with ellipsometry results and ATR-FTIR data22 showing exponential growth of PEO/PMAA films and eventual film fouling due to weak hydrogen-bonding between PMAA and PEO chains.21 Earlier, it was shown that the weaker binding between the hydrogen-bonded layers correlated with a lower critical pH for dissolution in such films.21 Here, we see higher interdiffusion of PMAA and PEO layers revealed in the internal film structure.
Conclusion We have presented a study of the internal film structure of hydrogen-bonded films and shown that the degree of molecular intermixing within such films correlates with the strength of (28) Xia, Y.; Yin, X. C.; Burke, N. A. D.; Sto¨ver, H. D. H. Macromolecules 2005, 38, 5937.
intermolecular interactions and the mode of film growth (linear versus exponential). Specifically, we constructed several types of PMAA/neutral polymer multilayers, in which dPMAA was deposited within every fifth bilayer within the film, and probed these films using NR. The degree of interpenetration of polymer layers expressed as the interlayer roughness varied for the dPMAA layers closest to the substrate, ranging from 3.5 to 6 nm for PVPON/PMAA, PVCL/PMAA, and PNIPAM/PMAA. For these three systems, we found that polymer layers became more diffuse with distance from the substrate. For PEO/PMAA, films were completely interdiffused and no layers formed. This observation strongly correlates with weak intermolecular interactions and exponential growth of PEO/PMAA films. Recently, the PEO/ PAA system has demonstrated excellent performance as a protonexchange membrane.17,29 Our finding of the strong intermixing of polymers in such membranes suggests that weak intermolecular binding and molecular disorder might be one of significant factors determining such a performance. Structural information on the interpenetration of hydrogen-bonded multilayers is also important in the context of the growing interest in using hydrogen-bonded polymer films as release layers which provide an advantage of working in mild and/or physiological conditions.30 LA802502C (29) Lutkenhaus, J. L.; Hammond, P. T. Soft Matter 2007, 3, 804. (30) Ono, Sh. S.; Decher, G. Nano Lett. 2006, 6, 592.
