Letter pubs.acs.org/Langmuir
Minimizing Defects in Polymer-Based Langmuir−Blodgett Monolayers and Bilayers via Gluing Minghui Wang, Vaclav Janout, and Steven L. Regen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Polymeric surfactants were prepared by quaternization of poly(4-chloromethylstyrene) with N,N-dimethylN-n-dodecylamine and N,N-dimethyl-N-n-octylamine to give 1 and 2, respectively. Each of these polymers formed stable monolayers at the air/water interface. Injection of poly(acrylic acid) (PAA) beneath the surface of these films led to a substantial increase in their cohesiveness (i.e., “gluing”), as evidenced by a dramatic increase in their surface viscosity. Examination of monolayers of 1 by atomic force microscopy, after being transferred to silicon wafers that were surface-modified with n-octadecyltrichlorosilane, showed that the presence of PAA leads to intact film. In contrast, transfer of unglued monolayers resulted in poor coverage. Comparison of the barrier properties of single glued and unglued LB bilayers formed in the presence and in the absence of PAA have shown that PAA minimizes defect formation within these ultrathin assemblies.
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afforded 1 and 2, respectively. Poly(acrylic acid) was chosen as a gluing agent for this investigation based on past success with its use in gluing calixarene-based assemblies.6 Polymers 1 and 2 produced stable monolayers at the air/ water interface having collapse pressures of 32 and 21 mN/m, respectively (Figure 1). After being compressed to 18 mN/m, and then exposed to a 6 mm-slit opening of a canal viscometer, both monolayers showed a precipitous drop is surface pressure due to a relatively low surface viscosity (Figure 2). In sharp contrast, when each monolayer was first glued by injecting PAA into the subphase, a substantial increase in surface viscosity was noted, reflecting a greater cohesiveness within each assembly (Figure 2). Control experiments that were carried out in which the compressing bar was swept across an aqueous subphase containing PAA in the absence of 1 and 2 gave surface pressures that were less than 2 mN/m. It should be noted that monolayers of 1 and 2, when spread over an aqueous subphase containing PAA, were unstable; that is, considerable loss of both cationic polymers to the subphase was evident during three compression−expansion cycles. For this reason, no precise isotherms over PAA could be recorded. In contrast, when monolayers of 1 and 2 were first spread over a pure water subphase and compressed prior to injection of PAA, relatively stable monolayers were formed; that is, under these conditions, loss of 1 and 2 to the subphase containing PAA was greatly reduced. Because monolayers of 1 could be compressed to a higher degree, they were chosen for more detailed investigation. To judge the influence that gluing has on the transferability of monolayers of 1, we compared films that were deposited
angmuir−Blodgett films continue to be of broad interest because of their potential as ultrathin electronic and optical materials and as ultrathin coatings and membranes.1−6 Polymeric analogues (i.e., p-LB films) are of special interest in this regard because of their greater robustness.7−14 Here, we show how gluing (i.e., noncovalent cross-linking of LB assemblies through ionic and hydrophobic association with a water-soluble polymer) can minimize the formation of defects within p-LB monolayers and bilayers to give higher quality thin films.6,15 In previous studies, we have shown that water-soluble polymers can be used to enhance the cohesiveness of calixarene-based LB films.6,15 Here, we demonstrate the feasibility of extending this gluing approach to p-LB assemblies. For proof of principle, we have used two readily accessible polymeric surfactants (1 and 2) as LB-forming materials and poly(acrylic acid) (PAA) as the gluing agent (Chart 1). Thus, quaternization of poly(4-chloromethylstyrene) with N,Ndimethyl-N-n-dodecylamine and N,N-dimethyl-N-n-octylamine Chart 1
Received: December 17, 2011 Revised: February 20, 2012 Published: February 24, 2012 © 2012 American Chemical Society
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Figure 3. Height image (AFM, tapping mode, 2 × 2 μm) of an unglued monolayer of 1 that was deposited onto an OTS-modified silicon wafer. The deposition was made using a speed of 2 mm/min and a surface pressure of 25 mN/m.
smooth surface in which coverage appeared to be complete (Figure 4). Analysis of the latter by ellipsometry gave a polymer
Figure 1. Plot of surface pressure−area versus molecular area per repeat unit for (a) 1 and (b) 2 over an aqueous subphase at 25 °C.
Figure 4. Height image (AFM, tapping mode, 2 × 2 μm) of a glued monolayer of 1 that was deposited onto an OTS-modified silicon wafer. The LB transfer was made using a speed of 2 mm/min and a surface pressure of 25 mN/m.
Figure 2. Surface pressure of polymeric monolayers as a function of time of exposure to a 6.0 mm slit opening of a canal viscometer at 25 °C for (a) 1 and (b) 2 over a 0.1 mM PAA solution (pH 2.5) and pure water (c and d, respectively). Prior to exposure to this opening, both monolayers were maintained for 180 min at 18 mN/m. During this equilibration period, the decrease in surface area was less than 10%.
thickness of 2.60 ± 0.10 nm. Profilometry by AFM (after scratching the surface with a razor blade to remove a segment of the film) gave a step height that corresponded to 2.77 ± 0.16 nm (Supporting Information). The advancing contact angle for water on this surface was 72 ± 3° and was unchanged after immersing in chloroform for 1 h; that is, the measured value was 71 ± 3°. To judge the influence that gluing has on p-LB bilayers, we compared the surface morphology, film thickness, and barrier properties of single bilayers of 1 that were formed in the absence and in the presence of PAA. Thus, examination of a glued bilayer of 1 by AFM revealed a surface that was smooth (Figure 5). In the case of unglued p-LB bilayer analogues, a very similar surface morphology was observed (Supporting Information). Analysis of the unglued bilayer by ellipsometry gave a film thickness of 3.56 ± 0.05 nm; prolifometry measurements gave a film thickness of of 3.47 ± 0.14 nm. Analysis of corresponding glued LB bilayers gave film thicknesses of 3.61 ± 0.04 nm (ellipsometry) and 3.64 ± 0.07 nm (profilometry). The similarity in the apparent film thickness of the glued and
onto silicon wafers that were surface-modified with noctadecyltrichlorosilane (OTS). Thus, LB deposition of a monolayer of 1 onto an OTS-modified silicon wafer by a single vertical down-trip, followed by removal of excess monolayer from the air/water interface, withdrawal into air and washing with water afforded a surface that was poorly covered, as shown by atomic force microcscopy (AFM) (Figure 3). That a negligible amout of polymer returned to the water surface upon removal of the wafer from the subphase was evident by the fact that recompression showed surface pressures that were less than 2 mN/m. It should be noted that this small residual surface pressure is not due to the presence of 1 but is due to the PAA in the subphase; that is, in the absence of 1, the same residual surface pressure can be detected. In sharp contrast, a similar deposition of a PAA-glued monolayer of 1 onto an OTS-modified silicon wafer yielded a 4615
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chloroform for 1 h at room temperature, that is, a good solvent for 1. Whereas such exposure resulted in a 77% reduction in film thickness for the unglued bilayer, similar exposure to chloroform led to a reduction in film thickness of only 18% for the glued bilayer. Measurement of the advancing contact angles for water on the surface of the glued bilayer water before and after washing with chloroform gave values of 86 ± 2° and 84 ± 3°, respectively. In contrast, washing of the unglued bilayer with chloroform resulted in a significant increase in the observed contact angle, going from 88 ± 2° to 102 ± 2°. Such a finding is consistent with a significant amount of the OTS underlayer becoming exposed during washing with chloroform, since the contact angle for water on the OTS-modified silicon wafer, itself, is 108° ± 2°.16 The gluing of p-LB monolayers and bilayers described herein represents a simple approach that can be used to enhance the cohesiveness of such films, and minimize the formation of defects. As such, it offers an opportunity for improving the structure and performance of such materials in ways that have not previously been possible. Although we believe this technique will prove to be broadly applicable, its full scope remains to be determined. Based on past findings with glued LB films derived from calix[n]arenes, it appears likely that selfhealing via hydrophobic interactions between the gluing agent and the LB monolayers plays an important role in minimizing defect formation.6 We think this is likely be the case for p-LB films as well. Thus, we believe that the future design of glued pLB films should take hydrophobic as well as ionic interactions into account.
Figure 5. Height image (AFM, tapping mode, 2 × 2 μm) of a glued LB bilayer of 1 on an OTS-modified silicon wafer.
unglued bilayers is surprising. A simple explanation that can account for this similarity is that the PAA that is picked up from the subphase penetrates both leaflets of the bilayer. In the case of the corresponding glued monolayer, exposure to air is likely to keep the PAA anchored on the films surface. That these polymeric LB bilayers are in a glued state is readily apparent by significant differences in their barrier properties and by their increased resistance toward disruption by chloroform relative to unglued bilayers. To assess their barrier properties, glued and unglued bilayers of 1 were deposited onto supports made from poly[1-(trimethylsilyl)-1propyne] (PTMSP), and their permeability assessed with respect to He and N2. As shown in Table 1, single permeance
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Table 1. Barrier Properties of LB Bilayers of 1a membrane
He
N2
He/N2
PTMSP unglued unglued glued glued
272 177 178 152 160
302 50.4 48.1 3.6 4.0
0.9 3.5 3.7 42 40
EXPERIMENTAL METHODS
General Methods. Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) (Gelest, Inc., Morrisville, PA), poly(acrylic acid) (Mw = 240 000) (Sigma-Alrdich, St. Louis, MO), and n-octadecyl trichlorosilane (OTS) (Sigma-Aldrich, St. Louis, MO) were used as obtained. Poly(4vinylbenzyl chloride) (Mw = 40 500, Mn = 20 300) was purchased from Polymer Source, Inc. (Montreal) and used as obtained. Housedeionized water was purified using a Millipore Milli-Q-filtering system containing one carbon and two ion-exchange stages. A Nima 612D film balance (Nima Technologies, Coventry England) was used for all monolayer experiments. 1H NMR spectra were recorded using a Bruker 500 MHz spectrometer and the solvent as a reference. Methods that were used for modifying the surface of silicon wafers with OTS, preparing cast films of PTMSP, measuring film thicknesses by ellipsometry and atomic force microscopy, and measuring gas permeabilities were similar to those previously reported.16−18 The refractive index values used for the silicon oxide, OTS, and LB layers were 1.46, 1.41, and 1.41, respectively. Methods used for fabricating LB films and measuring surface pressure−area isotherms and surface viscosities were similar to those previously described.18,19 In all cases, monolayers were spread on an aqueous subphase of 515 cm2 in area from chloroform/methanol (10/1, v/v) solutions having a concentration of 1 mg/mL. Synthesis of 1. To a solution of 0.248 g (1.63 mmol of repeat unit) of poly(4-vinylbenzyl chloride) in 1.50 mL of chloroform was added a solution made from 0.694 g (3.25 mmol) of N,N-dimethyl-Nn-dodecylamine and 4.00 mL of a mixture of chloroform and ethanol (3/1, v/v). After stirring, the resulting solution at room temperature under an argon atmosphere for 48 h, and 0.516 g of a cross-linked form of chloromethylated poly(styrene) was added to scavenge the excess amine (i.e., 4.23 mmol Cl/g, 20−50 mesh, 2.18 mmol of reactive CH2Cl groups). After gently shaking this heterogeneous mixture for 24 h at room temperature, the insoluble polymer beads were removed by filtration and washed three times with 20 mL of methanol/chloroform (1/4, v/v). The combined solution was concentrated under reduced pressure affording a colorless oil, which
Permeances at ambient temperature, 106 P/l (cm3/cm2·s·cm Hg), were calculated by dividing the observed flow rate by the area of the membrane (9.36 cm2) and the pressure gradient (10 psi) employed, using ca. 30 μm thick PTMSP supports All measurements were made at ambient temperatures. Values were obtained from 5−10 independent measurements; the error in each case was ±5%. a
measurements showed that unglued bilayers of 1 provided only a modest increase in the barrier properties of the PTMSP support. The fact that the He/N2 permeation selectivity lies close to that which is predicted by Graham’s law (i.e., 2.6) implies that permeation is dominated by Knudsen diffusion; that is, permeation occurs mainly through defects.5 In contrast, glued LB bilayers of 1 provide a much greater barrier toward N2, resulting in an order of magnitude increase in He/N2 selectivitiy. These findings clearly show that gluing helps to minimize the formation of defects within these p-LB bilayers. Measurements made for the corresponding glued monolayers indicate weaker barrier properties and reduced permeation selectivities relative to the glued bilayers (Supporting Information). Finally, to test whether gluing improves the stability of p-LB bilayers, unglued and PAA-glued bilayers of 1 that were deposited onto OTS-modified silicon wafers were immersed in 4616
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was then dried at room temperature for 48 h (0.5 Torr) to give 0.592 g (99%) of 1 as a glassy solid having 1H NMR (CD3OD, ppm): 7.50 (bm, 2H), 6.55 (bm, 2H), 4.60 (bs, 2H), 3.45 (bt, 2H), 2.95 (bs, 6H), 1.93 (bm, 3H), 1.45 (m, 20H), 0.89 (t, 3H). Calcd. for C23H40NCl·0.73H2O: C, 72.88; H, 10.56; N, 3.68. Found: C, 72.87; H, 10.40; N, 3.65. Polymer 2 was prepared using similar procedures and gave 1H NMR (CD3OD, ppm): 7.51 (bm, 2H), 6.57 (bm, 2H), 4.61 (bs, 2H), 3.46 (bt, 2H), 2.97 (bs, 6H), 1.95 (bm, 3H), 1.46 (m, 12H), 0.90 (t, 3H). Calcd. for C19H32NCl·0.59H2O: C, 71.22; H, 10.02; N, 4.37. Found: C, 71.22; H, 9.92; N, 4.32. Gluing of LB Monolayers. In a typical gluing procedure, a monolayer of 1 (32 μg) was spread onto a pure water surface of a film balance, and the solvent allowed to evaporate for 25 min prior to compression. The film was then compressed to 25 mN/m using a barrier speed of 25 cm2/min. A solution of PAA (5.5 mL, 10 mM PAA, pH 1.0) was then injected directly beneath the monolayer using 11 injections of 0.5 mL, which were evenly spaced along the air/water interface. The final concenration of PAA in the subphase was 0.1 mM, and the pH was 2.5. Prior to the dipping of two OTS-modified silicon wafers (1.4 × 2.4 cm, held back-to-back) at a rate of 2 mm/min, the subphase was allowed to equilibrate for 40 min. After the down-trip, the wafers remained immersed in the subphase for 1.5 h. For the formation of glued monolayers, the residual surfactant at the air/water interface was removed by aspiration, and the wafers then removed by one vertical up-trip at a speed of 60 mm/min. For the formation of glued bilayers, an up-trip was carried out using a speed of 2 mm/min while the surface pressure was maintained at 25 mN/m. Glued monolayers were washed prior to ellipsometry and AFM measurements by immersing in pure water for 5 min; glued bilayers were measured, directly, without immersion in water.
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Hsu, J. T.; Truong, Q.; Wilusz, E.; Regen, S. L. Exceptional Gas Permeation Selectivity of a Glued Langmuir-Blodgett Bilayer By pH Control. Langmuir 2004, 20, 8214−8219. (7) Kumaki, J.; Kajitani, T.; Nagai, K.; Okoshi, K.; Yashima, E. Visualization of Polymer Chain Conformations in Amorphous Polyisocyanide Langmuir-Blodgett Films by Atomic Force Microscopy. J. Am. Chem. Soc. 2010, 132, 5604−5606. (8) Erdogan, M.; Oabek, Z.; Capan, R.; Yagci, Y. Characterization of Polymeric LB Thin Films for Sensor Applications. J. Appl. Polym. Sci. 2012, 123, 2414−2422. (9) Ito, Y.; Miyake, T.; Suginome, M.; Katakura, R.; Mitsuishi, M.; Miyashita, T. Langmuir-Blodgett Films of Helical Rigid-Rod Poly(quinoxaline-2,3-diyl)s. Polym. J. 2010, 42, 406−410. (10) Aoki, H.; Takahasi, T.; Ito, S. Segmental Orientation of a Single Polymer Chain in Langmuir-Blodgett Films Studied by Defocus Fluorescence Imaging. Polym. J. 2011, 43, 218−221. (11) Wieland, D. C. F.; Degen, P.; Paulus, M.; Schroer, M. A.; Rehage, H.; Tolan, M. Manipulating Thin Polymer Films by Changing the pH Value. J. Appl. Phys. 2011, 110, 102221−10224. (12) Levitsky, I. A.; Kim, J.; Swager, T. M. Mass and Energy Transport in Conjugated Polymer Langmuir-Blodgett Films: Conductivity, Fluorescence, and UV−Vis Studies. Macromolecules 2001, 34, 2315−2319. (13) Matsui, J.; Yoshida, S.; Mikayama, T.; Aoki, A.; Miyashita, T. Fabrication of Polymer Langmuir-Blodgett Films Containing Regioregular Poly(3-hexylthiophene) for Application to Field-Effect Transistor. Langmuir 2005, 21, 5343−5348. (14) Somanathan, N.; Pandiyarajan, C. K.; Goedel, W. A.; Chen, W. C. Physicomechanical Studies on the Langmuir-Blodgett Films of Polythiophene Containing Mesogenic Side Chains. J. Polym Sci., Part B: Polym. Phys. 2009, 47, 173−182. (15) Wang, M.; Janout, V.; Regen, S. L. Glued Langmuir-Blodgett Bilayers From Calix[n]arenes: Influence of Calix[n]arene Size on Ionic Crosslinking, Film Thickness and Permeation Selectivity. Langmuir 2010, 26, 12988−12993. (16) Li, J.; Janout, V.; Regen, S. L. J. Gluing Langmuir-Blodgett Monolayers Onto Hydrocarbon Surfaces. J. Am. Chem. Soc. 2006, 128, 682−683. (17) Wang, Y.; Janout, V.; Regen, S. L. Creating Poly(ethylene oxide)-Based Polyelectrolytes for Thin Film Construction Using an Ionic Linker Strategy. Chem. Mater. 2010, 22, 1285−1287. (18) Hendel, R. A.; Nomura, E.; Janout, V.; Regen, S. L. Assembly and Disassembly of Langmuir-Blodgett Films on Poly[1-(trimethylsilyl)-1-propyne]: The Uniqueness of Calix[6]arene Multilayers As Permeation Selective Membranes. J. Am. Chem. Soc. 1997, 119, 6909− 6918. (19) McCullough, D. H. III; Grygorash, R.; Hsu, J. T.; Regen, S. L. Insight Into the Permeation Barrier of Glued Langmuir-Blodgett Bilayers. J. Am. Chem. Soc. 2007, 129, 8663−8667.
ASSOCIATED CONTENT
S Supporting Information *
Atomic force microscopic images showing step heights in a glued monolayer of 1 and glued and unglued bilayers of 1. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Grant No. DE-FG0205ER15720) is gratefully acknowledged.
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REFERENCES
(1) Blodgett, K. B. Monomolecular Films of Fatty Acids on Glass. J. Am. Chem. Soc. 1934, 56, 495−495. (2) Blodgett, K. B. Films Build by Depositing Successive Monomolecular Layers on a Solid Surface. J. Am. Chem. Soc. 1935, 57, 1007−1022. (3) Roberta, G. G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (4) Ulman, A. An Introduction to Ultrathin Films: From LangmuirBlodgett to Self Assembly; Academic Press: New York, 1991. (5) McCullough, D. H. III; Regen, S. L. Don’t Forget LangmuirBlodgett Films. Chem. Commun. 2004, 2787−2791. (6) (a) Wang, M.; Janout, V.; Regen, S. L. Hyper-thin Organic Membranes with Exceptional H2/CO2 Permeation Selectivity: Importance of Ionic Crosslinking and Self-Healing. Chem. Commun. 2011, 47, 2387−2389. (b) Li, J.; Janout, V.; McCullough, D. H. III; 4617
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