Apparent Dewetting of Ultrathin Multilayered Polyelectrolyte Films

Oct 6, 2007 - (SPS), and two hydrolytically degradable polyamines (polymers 1 and 2) are incubated in physiologically relevant environments...
0 downloads 0 Views 872KB Size
Download PDF
Langmuir 2007, 23, 11603-11610

11603

Apparent Dewetting of Ultrathin Multilayered Polyelectrolyte Films Incubated in Aqueous Environments Jingtao Zhang, Nathaniel J. Fredin, and David M. Lynn* Department of Chemical and Biological Engineering, UniVersity of WisconsinsMadison, 1415 Engineering DriVe, Madison, Wisconsin 53706 ReceiVed June 11, 2007. In Final Form: August 11, 2007 We have investigated and characterized changes in film morphology and surface structure that occur when ultrathin multilayered polyelectrolyte films fabricated from linear poly(ethylene imine) (LPEI), sodium poly(styrene sulfonate) (SPS), and two hydrolytically degradable polyamines (polymers 1 and 2) are incubated in physiologically relevant environments. Characterization of the physical erosion profiles of films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 (∼80 nm thick) by atomic force microscopy (AFM), reflective optical microscopy, and scanning electron microscopy (SEM) demonstrated that these materials undergo large-scale changes in surface structure and morphology upon incubation in phosphate-buffered saline (PBS) at 37 °C. The patterns and structures generated during this transformation (e.g., nucleation and growth of holes, coalescence of holes, formation of cell-type structures, and the subsequent breakup of these features into droplets) are similar in many ways to those observed for the dewetting of thin films of conventional polymers, such as polystyrene, on nonwetting surfaces. The processes reported here are sufficiently slow (they occur over ∼100 h) and occur under sufficiently mild conditions (e.g., incubation in PBS at 37 °C) to permit characterization and quantification of the structures and features that arise during the course of these transformations. The apparent dewetting of these ultrathin films upon exposure to aqueous environments creates future opportunities to investigate and characterize processes of mass transport in this class of ionically cross-linked assemblies.

Introduction Methods for the layer-by-layer deposition of oppositely charged polyelectrolytes on surfaces provide powerful approaches to the bottom-up assembly of ultrathin multilayered films.1-4 Among the many practical advantages and attractive features of these methods are the versatility of the aqueous-based assembly process, the exceptionally broad range of polyelectrolytes and other materials that can be incorporated, and the ability to deposit smooth, conformal films on a variety of objects and substrates having complex geometries. These and other practical considerations have led to widespread interest in the assembly of multilayered polyelectrolyte films for a wide range of potential applications.1-9 Multilayered polyelectrolyte films are generally considered to be ionically cross-linked, kinetically stabilized assemblies,1,10 and many potential applications of these materials benefit from the relative stability of these ionically cross-linked films in aqueous media or other environments. However, several recent studies demonstrate that the polyelectrolytes in these assemblies can diffuse or be transported over large distances under certain conditions and that these materials can undergo large-scale changes in internal structure or surface morphology upon exposure (1) Decher, G. Science 1997, 277, 1232-1237. (2) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348. (3) Hammond, P. T. AdV. Mater. 2004, 16, 1271-1293. (4) Peyratout, C. S.; Dahne, L. Angew. Chem., Int. Ed. 2004, 43, 3762-3783. (5) Ai, H.; Jones, S. A.; Lvov, Y. M. Cell Biochem. Biophys. 2003, 39, 23-43. (6) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203-3224. (7) Sukhishvili, S. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 37-44. (8) De Geest, B. G.; Sanders, N. N.; Sukhorukov, G. B.; Demeester, J.; De Smedt, S. C. Chem. Soc. ReV. 2007, 36, 636-649. (9) Sukhorukov, G. B.; Rogach, A. L.; Garstka, M.; Springer, S.; Parak, W. J.; Munoz-Javier, A.; Kreft, O.; Skirtach, A. G.; Susha, A. S.; Ramaye, Y.; Palankar, R.; Winterhalter, M. Small 2007, 3, 944-955. (10) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153-8160.

to aqueous media.11-25 Here, we demonstrate that polyelectrolyte multilayers ∼80 nm thick fabricated using sodium poly(styrene sulfonate) (SPS) and two hydrolytically degradable polyamines undergo large-scale transformations and changes in surface morphology when incubated in physiologically relevant environments. The patterns and structures that evolve during the transformation and erosion of these ionically cross-linked thin films are similar in several ways to those observed during the dewetting of thin films of conventional polymers (e.g., polystyrene)26-33 and, to our knowledge, have not been observed or described for this broad class of ionically cross-linked materials. Several past studies have demonstrated that multilayered assemblies fabricated from strong and/or weak polyelectrolytes can undergo changes in surface morphology or internal structure upon exposure to environmental conditions (e.g., changes in solution pH or ionic strength) that disrupt internal physical cross(11) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 1253112535. (12) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J. C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159-1162. (13) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458-4465. (14) Zacharia, N. S.; DeLongchamp, D. M.; Modestino, M.; Hammond, P. T. Macromolecules 2007, 40, 1598-1603. (15) Jomaa, H. W.; Schlenoff, J. B. Langmuir 2005, 21, 8081-8084. (16) Jomaa, H. W.; Schlenoff, J. B. Macromolecules 2005, 38, 8473-8480. (17) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725-7727. (18) Sui, Z.; Schlenoff, J. B. Langmuir 2004, 20, 6026-6031. (19) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017-5023. (20) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (21) Fery, A.; Scholer, B.; Cassagneau, T.; Caruso, F. Langmuir 2001, 17, 3779-3783. (22) Fredin, N. J.; Zhang, J.; Lynn, D. M. Langmuir 2005, 21, 5803-5811. (23) Fredin, N. J.; Zhang, J.; Lynn, D. M. Langmuir 2007, 23, 2273-2276. (24) Kujawa, P.; Sanchez, J.; Badia, A.; Winnik, F. M. J. Nanosci. Nanotechnol. 2006, 6, 1565-1574. (25) Qin, S.; Wei, D. S.; Liao, Q.; Jin, X. G. Macromol. Rapid Commun. 2006, 27, 11-14. (26) Reiter, G. Phys. ReV. Lett. 1992, 68, 75-78. (27) Reiter, G. Langmuir 1993, 9, 1344-1351.

10.1021/la701720k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007

11604 Langmuir, Vol. 23, No. 23, 2007

links in these materials.15-25 For example, polyelectrolyte multilayers have been reported to undergo swelling or surface smoothing in solutions having concentrations of salt sufficient to reversibly disrupt ionic interactions.16,17 In addition, Rubner and co-workers have reported films fabricated from weak polyelectrolytes that phase separate into microporous or nanoporous morphologies in response to changes in solution pH.19,20 We reported recently that multilayered films fabricated from plasmid DNA and polyamine 1 undergo large-scale changes in surface morphology (e.g., from uniform and smooth to a complex, nanoparticulate morphology) when incubated in aqueous environments.22,23 Finally, and of particular relevance to this current investigation, Qin et al. recently described the rapid formation and evolution of micrometer-scale holes upon the heating of films fabricated from two strong polyelectrolytes in water at 60 °C.25 The work reported here arose from our interest in the design of ultrathin films that erode and release anionic polymers when incubated in aqueous environments.34-41 For example, we have shown in several past studies that hydrolytically degradable polyamines such as polymer 1 can be used to fabricate films that erode and sustain the release of anionic polymers such as SPS or DNA from surfaces when incubated in phosphate-buffered saline (PBS).35,36,40,41 As an extension of this work, we reported recently that the fabrication of films using combinations of polymer 1 and a second, more hydrophobic degradable polyamine

Zhang et al.

Characterization of the physical erosion profiles of films having the structure (1/SPS)n(2/SPS)m using atomic force microscopy (AFM) revealed that the erosion of these multicomponent films occurs by a process that is significantly more complex than the erosion of films fabricated exclusively from either polymer 1 and SPS or polymer 2 and SPS. For example, whereas films having the structure (1/SPS)8 erode in a uniform manner without the generation of significant changes in film topography,22 films having the structure (1/SPS)4(2/SPS)4 undergo large-scale transformations and gross changes in surface structure upon incubation in PBS at 37 °C. The patterns and structures generated during this transformation (e.g., nucleation and growth of holes, coalescence of holes and formation of cell-type structures, and the subsequent breakup of these features into droplets) are similar in many ways to those observed for the dewetting of thin films of conventional polymers, such as polystyrene, heated above their glass transition temperatures.26-33 The early stages of this process are similar to the formation and evolution of holes in the assemblies of strong polyelectrolytes heated in water at 60 °C (as described above).25 However, in contrast to these past studies (for which the rapid formation of holes was followed by rapid film dissolution), the processes reported here are sufficiently slow (they occur over ∼100 h) and occur under sufficiently mild conditions (e.g., incubation in PBS at 37 °C) to permit characterization and quantification of the full range of transformations and surface features using optical microscopy and AFM. The apparent dewetting of these thin films upon exposure to aqueous environments creates future opportunities to investigate processes of mass transport in these ionically cross-linked assemblies. Materials and Methods

(polymer 2) provides an approach to extending release and tuning the rates at which these assemblies erode.37 For example, films having the general structure (1/SPS)n(2/SPS)m erode and release SPS more slowly than films fabricated entirely from polymer 1 and SPS, and rates of film erosion were observed to correlate to the relative numbers of layers of polymer 1 and polymer 2 deposited during fabrication. (28) Jacobs, K.; Herminghaus, S.; Mecke, K. R. Langmuir 1998, 14, 965969. (29) Seemann, R.; Herminghaus, S.; Jacobs, K. Phys. ReV. Lett. 2001, 86, 5534-5537. (30) Seemann, R.; Herminghaus, S.; Neto, C.; Schlagowski, S.; Podzimek, D.; Konrad, R.; Mantz, H.; Jacobs, K. J. Phys. Condens. Matter 2005, 17, S267S290. (31) Stange, T. G.; Evans, D. F.; Hendrickson, W. A. Langmuir 1997, 13, 4459-4465. (32) Meredith, J. C.; Smith, A. P.; Karim, A.; Amis, E. J. Macromolecules 2000, 33, 9747-9756. (33) Becker, J.; Grun, G.; Seemann, R.; Mantz, H.; Jacobs, K.; Mecke, K. R.; Blossey, R. Nat. Mater. 2003, 2, 59-63. (34) Lynn, D. M. Soft Matter 2006, 2, 269-273. (35) Jewell, C. M.; Zhang, J.; Fredin, N. J.; Lynn, D. M. J. Controlled Release 2005, 106, 214-223. (36) Jewell, C. M.; Zhang, J.; Fredin, N. J.; Wolff, M. R.; Hacker, T. A.; Lynn, D. M. Biomacromolecules 2006, 7, 2483-2491. (37) Zhang, J.; Lynn, D. M. Macromolecules 2006, 39, 8928-8935. (38) Zhang, J.; Fredin, N. J.; Lynn, D. M. J. Polym. Sci. Polym. Chem. 2006, 44, 5161-5173. (39) Zhang, J.; Fredin, N. J.; Janz, J. F.; Sun, B.; Lynn, D. M. Langmuir 2006, 22, 239-245. (40) Zhang, J.; Chua, L. S.; Lynn, D. M. Langmuir 2004, 20, 8015-8021. (41) Vazquez, E.; Dewitt, D. M.; Hammond, P. T.; Lynn, D. M. J. Am. Chem. Soc. 2002, 124, 13992-13993.

General Considerations. Silicon substrates (e.g., 0.5 × 2.0 cm) used for the fabrication of multilayered films were cleaned with acetone, ethanol, methanol, and deionized water and dried under a stream of filtered compressed air. Surfaces were then activated by etching with an oxygen plasma for 5 min (Plasma Etch, Carson City, NV) prior to film deposition. The optical thicknesses of films deposited on silicon substrates were determined using a Gaertner LSE ellipsometer (632.8 nm, incident angle ) 70°). Data were processed using the Gaertner ellipsometer measurement program. Relative thicknesses were calculated by assuming an average refractive index of 1.58 for the multilayered films. Thicknesses were determined in at least four different standardized locations on each substrate and are presented as an average (with standard deviation) for each film. UV-visible absorbance values for PBS solutions used to determine film release kinetics were recorded on a Beckman Coulter DU520 UV/vis spectrophotometer (Fullerton, CA). For the characterization of surface morphology by scanning electron microscopy (SEM), an accelerating voltage of 5 kV was used to obtain images on a LEO DSM 1530 scanning electron microscope. Samples were coated with a thin layer of gold using a sputterer (30 s at 45 mA, 50 mTorr) prior to analysis. Optical microscopy of thin films on silicon substrates was performed on an Olympus BX-60 light microscope (Tokyo, Japan) operated in reflective mode. Film topography and surface roughness were obtained from height data imaged in tapping mode on a Digital Instruments MultiMode atomic force microscope (Digital Instruments, Santa Barbara, CA). Silicon cantilevers with a spring constant of 40 N/m and a radius of curvature of less than 10 nm were used (model NSC15/Al BS, MikroMasch USA, Inc., Portland, OR). For each sample, at least two different scans were obtained at randomly chosen points near the center of the film at each time point. Height data were flattened using a secondorder fit. Root-mean squared surface roughness (Rrms) was calculated over the scan area using the NanoScope software package (Digital Instruments, Santa Barbara, CA). Sizes and dimensions of the features on a surface were obtained from cross-sectional analysis of the AFM data.

Dewetting of Ultrathin Polyelectrolyte Films Materials. Poly(sodium 4-styrenesulfonate) (SPS, MW ) 70 000) and sodium acetate buffer were purchased from Aldrich Chemical Co. (Milwaukee, WI). Test grade n-type silicon wafers were purchased from Si-Tech, Inc. (Topsfield, MA). Linear poly(ethylene imine) (LPEI, MW ) 25 000) was purchased from Polysciences, Inc. (Warrington, PA). Phosphate-buffered saline (PBS) was prepared by dilution of commercially available concentrate (EM science, Gibbstown, NJ). Poly(ester-amines) 1 (Mn ) 7700) and 2 (Mn ) 6200) were synthesized as described previously.39 All materials were used as received without further purification unless noted otherwise. Deionized water (18 MΩ) was used for washing steps and to prepare all buffer and polymer solutions. All buffers and polymer solutions were filtered through a 0.2 µm membrane syringe filter prior to use unless noted otherwise. Compressed air used to dry films and coated substrates was filtered through a 0.4 µm membrane syringe filter. Preparation of Polyelectrolyte Solutions. Solutions of LPEI and SPS used for the fabrication of LPEI/SPS precursor layers (20 mM with respect to the molecular weight of the polymer repeat unit) were prepared using a 50 mM NaCl solution in water. LPEI solutions contained 5 mM HCl to aid polymer solubility. SPS solutions used for the deposition of all other polyamine/SPS layers (20 mM with respect to the molecular weight of the polymer repeat unit) were prepared in water and the solution pH was adjusted to 4.9 using HCl. Solutions of polymers 1 and 2 used for dipping (5 mM with respect to the molecular weight of polymer repeat units) were prepared in sodium acetate buffer (100 mM, pH ) 5.1). Fabrication of Multilayered Films. Films were deposited on planar silicon substrates precoated with a multilayered film composed of 10 bilayers of LPEI and SPS (terminated with a topmost layer of SPS) to ensure a suitably charged surface for the adsorption of polymer 1, as previously described.39,40 These precursor layers were fabricated using an automated dipping robot (Riegler & Kirstein GmbH, Potsdam, Germany). Multilayered films fabricated from SPS and polymers 1 and 2 were fabricated on these precursor layers manually using an alternate dipping procedure according to the following general protocol: (1) Substrates were submerged in a solution of polyamine for 5 min, (2) substrates were removed and immersed in an initial water bath for 1 min followed by a second water bath for 1 min, (3) substrates were submerged in a solution of SPS for 5 min, and (4) substrates were rinsed in the manner described above. This cycle was repeated until the desired number of polyamine/SPS bilayers was reached. Films having the structure (1/SPS)4(2/SPS)4 were fabricated by first depositing four bilayers of polymer 1/SPS followed by deposition of four bilayers of polymer 2/SPS. Films were either used immediately or were dried under a stream of filtered, compressed air and stored in a vacuum desiccator until use. All films were fabricated at ambient room temperature. Characterization of Film Erosion and Changes in NanometerScale and Micrometer-Scale Topography. Experiments designed to investigate film erosion and changes in morphology were performed in the following general manner: Film-coated substrates were placed in a plastic UV-transparent cuvette, and 1.0 mL of phosphate buffered saline (PBS, pH ) 7.4, 137 mM NaCl) was added to cover the film-coated portion of the substrate. The samples were incubated at 37 °C and removed at predetermined intervals for characterization by optical microscopy, scanning electron microscopy (SEM), or atomic force microscopy (AFM). Films were rinsed under deionized water and dried under a stream of filtered compressed air prior to measurement. For experiments designed to monitor a decrease in film thickness, optical thickness values were determined in at least four different predetermined locations on the substrate by ellipsometry, and the sample was returned immediately to the buffer solution. In experiments for which reflective optical microscopy was used to observe film morphology in situ (that is, without removing the films from buffer), film-coated substrates were placed in the wells of a Lab-Tek II four-well chambered slide (Nalge Nunc International, Naperville, IL), covered with PBS, and incubated at 37 °C. At predetermined intervals, these films were characterized directly using a reflective optical microscope without removing the film-coated substrates from buffer.

Langmuir, Vol. 23, No. 23, 2007 11605

Results and Discussion We fabricated multilayered polyelectrolyte films having the structure (1/SPS)4(2/SPS)4 using an alternate dipping process similar to that used in our past studies.37 All films were fabricated on planar silicon substrates to facilitate the characterization of film erosion using ellipsometry, AFM, and reflective optical microscopy. In all cases, these silicon substrates were precoated with a multilayered film ∼20 nm thick composed of 10 alternating layers of linear poly(ethylene imine) (LPEI) and sodium poly(styrene sulfonate) (SPS) terminated with a layer of SPS to provide a surface suitable for the adsorption of polymers 1 and 2, as described previously.39,40 Film thickness increased in a linear manner with respect to the number of polyamine/SPS layers deposited during fabrication; these results are consistent with the results of our past studies and are not repeated here.37 The thicknesses of films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 used in all erosion studies described below ranged from ∼70 to ∼100 nm, as determined by ellipsometry. To investigate the erosion profiles of these materials and characterize the surfaces of partially eroded films, film-coated silicon substrates were incubated in PBS (pH ) 7.4) at 37 °C and removed from the buffer at predetermined intervals for characterization by AFM. Figure 1A shows an AFM image (10 µm × 10 µm) of a film prior to incubation. Inspection of this image reveals that the surfaces of these films are smooth (Rrms ∼ 3.3 nm), uniform, and devoid of significant micrometer-scale topographic features prior to incubation. Figure 1B shows an image (100 µm × 100 µm) of the same film after incubation in PBS for 7 h. Inspection of this image reveals large changes in surface morphology from a surface that is smooth and continuous to a morphology having a cell-like structure composed of interconnected ridges. These results differ significantly from the results of our past studies using AFM to characterize the surfaces of films having the structure (LPEI/SPS)10(1/SPS)8, which demonstrated that the surfaces of these films remained smooth and uniform during film erosion.22 The average height of the ridges shown in the image in Figure 1B is 254 nm. This value is significantly greater than the thickness of the film prior to incubation (∼106 nm), suggesting that polymer flows to or is accumulated in these regions of the film during incubation and erosion. The dimensions of the features shown in Figure 1B were sufficiently large to permit straightforward characterization by reflective optical microscopy. Figure 1C shows a reflective optical micrograph of the same film in Figure 1B and confirms the presence of a complex, cell-like morphology on the surface of the silicon substrate. Figure 1D,E show SEM images of a film that was incubated in PBS for 4 h and coated with gold prior to imaging. Inspection of Figure 1E, which shows a region of the film that was scratched prior to imaging (see white arrows), reveals the presence of a continuous, ultrathin film covering the areas enclosed within the cell-like structures generated during incubation. We note here that characterization of films having the structure (LPEI/SPS)10(2/SPS)4(1/SPS)4 (i.e., fabricated by depositing layers of polymer 2 and SPS prior to the deposition of layers of polymer 1 and SPS, the reverse of the order of the film discussed above) by reflective optical microscopy demonstrated that the surfaces of these films remained relatively smooth and did not undergo large-scale changes in morphology when incubated under identical conditions (Figure 1F). We return to these observations again in the discussion below. The ability to characterize the surface features described above using reflective optical microscopy permitted convenient characterization of the evolution of these features as a function of

11606 Langmuir, Vol. 23, No. 23, 2007

Zhang et al.

Figure 1. Representative tapping mode atomic force microscopy images of a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film ∼100 nm thick deposited on a silicon substrate prior to incubation (A, 10 µm × 10 µm) and after incubation in PBS at 37 °C for 7 h (B, 100 µm × 100 µm). The scale in the z-direction is 600 nm for each image. (C) Reflective optical micrograph of the film in part B (100 µm × 100 µm). (D, E) Scanning electron micrographs of a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film after incubation in PBS at 37 °C for 4 h; (E) high magnification view of an area of the film shown in part D scratched prior to preparation of the film for imaging; white arrows indicate the edge of the scratch. (F) Reflective optical micrograph of a (LPEI/SPS)10(2/SPS)4(1/SPS)4 film deposited on a silicon substrate after incubation in PBS at 37 °C for 9 h (100 µm × 100 µm).

Figure 2. Reflective optical micrographs of a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film deposited on a silicon substrate before (A) or after (B-I) incubation in PBS at 37 °C. Images in B-I are of films removed from buffer and characterized after (B) 3 h, (C) 5 h, (D) 9 h, (E) 18 h, (F) 27 h, (G) 38 h, (H) 65 h, and (I) 133 h. All images are 177 µm × 132 µm.

time. Figure 2 shows images of a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film prior to incubation and after incubation in PBS for up to 133 h. These images demonstrate clearly a complex series of morphological transformations that are similar in many respects to the patterns and features that evolve during the dewetting of thin films of polymers such as polystyrene on nonwetting

surfaces.26-33 For example, Figure 2B shows the formation of small holes in the film after 3 h of incubation in PBS. These holes are then observed to grow in size (Figure 2C) and coalesce to form cell-type structures similar to those shown in Figure 1 (Figure 2D) after ∼10 h. Finally, upon further incubation, the ridges of these cell-type structures decompose into droplets

Dewetting of Ultrathin Polyelectrolyte Films

Langmuir, Vol. 23, No. 23, 2007 11607

Figure 3. Reflective optical micrographs of (LPEI/SPS)10(1/SPS)4(2/SPS)4 films deposited on a silicon substrate after incubation in PBS at 37 °C for 3 h (A and D), 13 h (B and E), and 46 h (C and F). For the images in parts A-C, the film was removed from buffer and dried prior to imaging. For the images in parts D-F, images were recorded without removing the film from buffer (see the text). All images are 177 µm × 132 µm.

(Figure 2F,G) to yield a final film morphology characterized by small particulate structures (Figure 2I) after 133 h. The images shown in Figure 2 correspond to images of a film that was incubated in PBS buffer and then removed at predetermined intervals and dried under air prior to imaging. We conducted an additional series of experiments to determine whether the progression of changes in film morphology observed in Figure 2 occurred during the incubation of these materials (i.e., in situ) or whether they were the result of forces experienced during film drying. Figure 3 shows two sets of images corresponding to two identical films incubated in PBS at 37 °C that were either (i) dried prior to imaging (Figure 3A-C) or (ii) imaged without ever removing the film from buffer (Figure 3DF) at 3, 13, or 46 h. The features in Figure 3D-F acquired in situ are similar to those shown in the dried images shown in Figure 3A-C. These results demonstrate that the evolution of the surfaces features and patterns shown in Figure 2 occurs during the incubation of these films, and that these features do not simply result from forces that arise during the drying of these assemblies. Additional imaging of these materials by AFM permitted quantitative characterization of the features and processes that arise during the decomposition of these films. Figure 4A-D shows a series of AFM images (100 µm × 100 µm) acquired for a film ∼85 nm thick prior to incubation (Figure 4A) and after 1, 2, or 3 h of incubation in PBS (Figure 4B-D). These images show clearly that small holes (∼1.8 µm in diameter) form in these materials as early as 1 h after incubation in PBS and that these holes grow in both size and number over a period of 3 h. Figure 5A shows plots of average hole diameter and average hole depth versus time for data extracted from cross-sectional analysis of the images in Figure 4B-E. Inspection of these data reveals that hole diameter increases monotonically over time and that the depth of these holes remains constant (∼63 nm) over this time period. Parts G and H of Figure 4 show a threedimensional representation and corresponding cross-sectional analysis of two holes for a film incubated for 2 h. These images reveal the presence of raised rims (∼26 nm high) surrounding each hole. Figure 5B shows a plot of the average rim height versus time using data extracted from the images in Figure 4BD. These data demonstrate that the height of the rims surrounding these holes also increases monotonically over this 3-h time period. Finally, Figure 6 shows an AFM image of the array of punctate

particulate features (∼3.5 µm in diameter) that evolve upon the decomposition of the cell-type structures shown in Figure 4E,F and a representative cross-sectional analysis of two of these features. The heights of these particulate structures (∼470 nm) is approximately 5.5 times the height of the original film prior to incubation (∼85 nm), suggesting that large amounts of polymer accumulate in these regions during film decomposition and the breakup of the ridges that form during this process. The dewetting of thin films of polymers such as polystyrene on nonwetting surfaces has been investigated intensely from both experimental and theoretical standpoints26-33 and is generally considered to occur either by a spinodal dewetting mechanism or by a mechanism that involves the nucleation and growth of holes. In the latter case, film rupture and the nucleation of holes can occur either by (i) heterogeneous nucleation, which results from defects in the film or on the substrate surface and generally yields holes with uniform size distributions, or (ii) thermal nucleation, which generally leads to arrays of holes with a distribution of sizes.29,30 The patterns and structures that evolve during the transformations shown in Figures 1-4 (nucleation and growth of holes, coalescence of holes, and the formation and breakdown of cell-type morphologies), and the general trends shown in Figure 5 are similar to those observed during the dewetting of thin films of liquids and conventional polymers that occur by nucleation and growth. Inspection of the data in Figure 4C,D reveal that the majority of holes formed after 2 or 3 h of incubation are similar in size but that smaller holes do continue to form as time increases. This distribution of hole sizes appears similar in many respects to distributions of hole sizes observed during thermal nucleation in thin films of bulk polymer (as compared to uniform size distributions observed to result from heterogeneous processes) and could result from thermal processes, polymer degradation, or processes that result in the time-dependent physical loss of polymer from these materials upon incubation (as discussed below). The results above demonstrate that the incubation of ultrathin polyelectrolyte multilayers having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 in PBS creates conditions that permit the decomposition of these materials and the reorganization of the components of these films over large length scales. Although many different aspects of the transformations of these materials are similar to those observed during the dewetting of thin films

11608 Langmuir, Vol. 23, No. 23, 2007

Zhang et al.

Figure 4. (A-F) Representative tapping mode atomic force microscopy images (100 µm × 100 µm) of a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film ∼85 nm thick deposited on a silicon substrate prior to incubation (A) and after incubation in PBS at 37 °C for (B) 1 h, (C) 2 h, (D), 3 h, (E) 5 h, and (F) 12 h. The scale in the z-direction is 300 nm for each image. Three-dimensional representation (G) and corresponding cross-sectional analysis (H) of two holes for a film incubated for 2 h.

of conventional polymers, there are also many significant differences between the ionically cross-linked materials investigated here and thin bulk films on nonwetting surfaces that could serve to complicate direct comparisons of these behaviors. First, whereas the behavior of thin films of polystyrene, for example, is governed to a large extent by changes in van der Waals and other weak interactions, interactions within multilayered polyelectrolyte assemblies are generally dominated by long-range electrostatic interactions. Second, whereas the dewetting of thin films of polystyrene or other conventional polymers can be described as arising in the context of a single-component system, multilayered polyelectrolyte assemblies are, by definition, multicomponent systems composed of at least two different, oppositely charged polyelectrolytes. The assemblies investigated here, in fact, are composed of four different polyelectrolyte species: three different weak polycations and one strong polyanion. Additional consideration of the data presented above reveals that the presence of each of these four species is required for this transformation to occur [e.g., films having the structure (LPEI/SPS)10,22 (LPEI/SPS)10(1/SPS)8,22 and (LPEI/SPS)10(2/ SPS)8 do not exhibit this behavior (films fabricated from SPS and polymers 1 and 2 cannot be fabricated reliably on silicon in the absence of LPEI/SPS precursor layers)]. These data also demonstrate that the relative order in which these film components are incorporated into a film is also important [e.g., films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 undergo these transformations, but films having the structure (LPEI/SPS)10(2/SPS)4-

(1/SPS)4 do not; see Figure 1]. The reasons for these large differences in film behavior are not yet completely understood but could result from differences in the relative hydrophobicities of polymers 1 and 2 and their initial locations relative to the film/water interface or LPEI/SPS foundation layers. In addition, differences in the relative rates of hydrolysis of backbone ester bonds have been observed for polymers 1 and 2 in solution and could play important roles in promoting or preventing this behavior. Regardless, consideration of the depth of the holes observed in films ∼85 nm thick (e.g., ∼57 nm; as shown in Figure 4H) demonstrates that these large-scale changes in film morphology involve the diffusion, flow, transport, or rearrangement of substantial portions of more than simply the top four (2/SPS) layers of these materials [that is, although the order in which layers of (1/SPS) and (2/SPS) are deposited appears to be important, all layers appear to be involved in the transformation; the topmost layers of (2/SPS) do not appear to simply dewet the underlying layers of (1/SPS)]. Other differences also exist between the materials and transformations reported here and past studies describing the dewetting of thin films of conventional polymers. For example, whereas the dewetting of thin bulk films of conventional polymers can often be investigated simply by heating a thin film under air, the transformations described above appear to require the presence of a high dielectric solvent; no significant changes in the morphologies of these polyelectrolyte assemblies were observed when these films were heated dry at 37 °C under air and then

Dewetting of Ultrathin Polyelectrolyte Films

Figure 5. (A) Plot of the change in hole diameter and hole depth versus time for a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film ∼85 nm thick deposited on a silicon substrate incubated in PBS at 37 °C. (B) Plot of rim height versus time for the same film characterized in part A. Data were obtained from a cross-sectional analysis of the AFM data shown in Figure 4 (see the text).

characterized by reflective optical microscopy. Finally, whereas mass is generally conserved when thin films of polystyrene are heated on a surface under air, our past studies of films fabricated from SPS and polymers 1 and 2 demonstrate that these materials erode and physically release material into solution when incubated in PBS.37 For example, our past studies demonstrate that films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 erode and release SPS (and, by inference, polymers 1 and 2) into solution over a period of ∼100 h. We cannot rule out on the basis of these current experiments the possibility that the early stages of the behavior described here (e.g., nucleation of holes) could be initiated by (or result from) the physical loss of certain components of these materials. However, inspection of the release profiles of these materials demonstrates that only ∼6% of the SPS that is released from these materials is released over the first 3 h of incubation in PBS.37 These data suggest that the nucleation and initial growth of holes in these materials is not the result of large-scale loss of SPS from these assemblies. Additional analytical work will clearly be required to determine the extent to which time-dependent changes in the chemical compositions of these multicomponent assemblies correlate to the changes in morphology described in this current study. We demonstrated recently that ultrathin multilayered films fabricated from polymer 1 and plasmid DNA undergo transformations and changes in surface morphology on the nanometer scale that are similar to those observed during the spinodal dewetting of thin films of conventional polymers.22,23 This past study suggested that the ability of the components of these films to reorganize over large distances was the result of a reduction in the ionic cross-linking of these assemblies upon incubation

Langmuir, Vol. 23, No. 23, 2007 11609

Figure 6. (A) Representative tapping mode atomic force microscopy image (100 µm × 100 µm) of a (LPEI/SPS)10(1/SPS)4(2/SPS)4 film ∼85 nm thick deposited on a silicon substrate after incubation in PBS at 37 °C for 39 h. The scale in the z-direction is 900 nm. (B) cross-sectional view of two particles on the surface showing particle dimensions.

in PBS at pH 7.4, not the result of the degradation of polymer 1.20 This proposition was partially supported by the observation that films incubated at pH 5.0 (which should result in a higher density of ionic cross-links relative to films incubated at pH 7.4) remained smooth and uniform and did not decompose. Similar control experiments conducted by incubating (LPEI/SPS)10(1/ SPS)4(2/SPS)4 films in acetate buffer at pH 5.0 demonstrated that these films did not exhibit large-scale changes in surface morphology. In combination with the results presented above, the results of this experiment suggest that changes in pH (and resulting changes in the ionic cross-linking of these materials) play an important role in creating conditions that allow the reorganization of the components of these films over scales of micrometers.

Summary and Conclusions We have investigated and characterized changes in film morphology and surface structure that occur when ultrathin multilayered polyelectrolyte films fabricated using SPS and two hydrolytically degradable polyamines (polymers 1 and 2) are incubated in physiologically relevant environments. Characterization of the physical erosion profiles of films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 by AFM, reflective optical microscopy, and SEM demonstrated that these materials undergo large-scale changes in surface structure and morphology upon incubation in PBS at 37 °C. The patterns and structures generated during this transformation (e.g., nucleation and growth of holes, coalescence of holes, formation of cell-type structures, and the subsequent break-up of these features into droplets) are similar in many ways to those observed for the dewetting of thin films of conventional polymers, such as polystyrene, on non-wetting surfaces. The processes reported here occur slowly enough (e.g.,

11610 Langmuir, Vol. 23, No. 23, 2007

over ∼100 h) and under sufficiently mild conditions to permit characterization and quantification of the structures and features that arise during the course of these transformations. The apparent dewetting of these films upon exposure to aqueous environments creates future opportunities to investigate and characterize processes of mass transport in these ionically cross-linked assemblies.

Zhang et al.

Acknowledgment. Financial support was provided by the National Institutes of Health (R21 EB02746) and the Arnold and Mabel Beckman Foundation. N.J.F. thanks the NSF for a Graduate Research Fellowship. D.M.L. is an Alfred P. Sloan Research Fellow. LA701720K