Direct Force Measurements of Polymerization ... - ACS Publications

Elasticity and Molecular Rearrangement during Polymerization of the Mixed Monolayer of 10,12-Pentacosadiynoic Acid and Ru(dpphen)32+. Langmuir 2003, 1...
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5652

Langmuir 1997, 13, 5652-5662

Direct Force Measurements of Polymerization-Dependent Changes in the Properties of Diacetylene Films S. R. Sheth and D. E. Leckband* Department of Chemical Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received December 16, 1996. In Final Form: July 14, 1997X Two-dimensional polymers have numerous potential applications as surface coatings. In this work, we used the surface force apparatus to investigate the interfacial properties of monomeric and polymerized Langmuir-Blodgett films of the diacetylene 10,12-pentacosadiynoic acid (PCA). Using direct force and contact angle measurements, we investigated the impact of polymerization and film preparation conditions on the molecular surface properties of the layers. Humidity-dependent variations both in the water contact angles and in the directly measured interfacial energies of these layers were used to assess the film stability under different environmental conditions. Direct force measurements demonstrated directly that polymerization prevents molecular reorientations and consequent changes in the interfacial properties of the polymer films. Further, the polymerized diacetylene layers were stable to repeated subjection to large compressive loads over a course of several hours. A major limitation of the low-dimensional diacetylene polymers prepared from PCA is the difficulty of forming homogeneous, defect-free films over large areas. Our findings further demonstrate directly the impact of such heterogeneity on the interfacial properties of both monomeric and polymerized diacetylene films.

Introduction The modification of the surfaces of solid substrates with ultrathin coatings of organic layers has been an attractive approach for tailoring materials’ surface properties.1-4 Variations in the type and composition of the films can result in materials that possess controlled physical and chemical surface properties for specific applications.4 For this reason Langmuir-Blodgett (LB) deposition has been an attractive route for selective modification of surfaces. The composition of the films is readily controlled. Moreover, complicated architectures can be built up through the successive deposition of films with the same or different composition. The primary drawback of LB films is their limited stability under a variety of chemical or physical conditions. The monomers within the layers are held only by weak physical bonds. As a consequence these surface layers can undergo significant rearrangements when subjected to changes in the ambient conditions. For example, LB films swell in water or organic vapors.5,6 Surface groups can rearrange or completely invert their orientations in order to minimize the interfacial energy.6 Moreover, lateral diffusivity within the layer can lead to surface reorganizations in response to large compressive or tensile stresses. In multilayer assemblies, interlayer diffusion can further negate the advantages of controlled multilayer constructions. Some of these problems are circumvented by covalent, chemical self-assembly.3,4 The latter method * Author to whom correspondence should be addressed: phone, 217-244-0793; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993, 261, 585. (2) Spevak, W.; Nagy, J. O.; Charych, D. H. Adv. Mater. 1995, 7, 85. (3) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (4) Ulman, A. In An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: San Diego, CA, 1991. (5) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1992, 96, 7752. (6) Chen, Y. L. E.; Gee, M. L.; Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. J. Phys. Chem. 1989, 93, 7057.

S0743-7463(96)02107-5 CCC: $14.00

results in more robust, chemical coupling between the substrate and primary layer. Successive build-up of selfassembled films can be achieved,4 although the chemistry involved is often not as straightforward as that in standard Langmuir deposition methods. Two-dimensional polymerization has been explored, in part, to alleviate some of the limitations associated with LB films. In particular, diacetylenes are monomeric amphiphiles with two triple bonds in the hydrocarbon tails of the molecules.4 They can be photopolymerized by UV-irradiation to form extended polymers both on solid supports and at the air-water interface.7-11 Polymerization imparts increased mechanical strength to the monolayer and is expected to increase the strength of adhesion between the layer and substrate. As a consequence, these materials and other similarly polymerizable amphiphiles have potential applications as stabilizers of lipid membranes,12,13 as supporting matrices for biosensing molecules,1,2,14 and in liposomes for drug-delivery.13 Due to their optical properties, polydiacetylenes are also used as optical components in sensors and in nonlinear optical devices.4,15 Molecular recognition events have been detected by the induced spectral change of the polymer due to specific viral binding to the sialic acid-functionalized polymer.1,14 Consequently, the formation of stable, ultrathin polymer films is crucial for PDA’s envisaged application as a protective surface coating for biomedical or optoelectronic applications. Investigations of mono- and multilayers of the diacetylene 10,12-pentacosadiynoic acid (PCA) deposited on solid supports have focused largely on the morphological perturbations associated with film transfer from the (7) Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. (8) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631. (9) Hupfer, B. and Ringsdorf, H. Chem. Phys. Lipids 1983, 33, 263. (10) Wegner, G. Naturfosch., Teil B 1969, 24, 824. (11) Wegner, G. Pure Appl. Chem. 1977, 49, 443. (12) Chapman, D.; Charles, S. A. Chem. Br. 1992, March, 253. (13) Sells, T.; O’Brien, D. F. Macromolecules 1994, 27, 226. Lamparski, H.; O’Brien, D. F. Macromolecules 1995, 28, 1786. (14) Reichert, A.; Nagy, J. O.; Spevak, W.; Charych, D. H. J. Am. Chem. Soc. 1995, 117, 829. (15) Wenzel, M.; Atkinson, G. H. J. Am. Chem. Soc. 1989, 111, 6123.

© 1997 American Chemical Society

Polymerization-Dependent Changes in Films

liquid-vapor interface.16-18 However, for biomedical uses such as coatings for blood contacting materials or as transducers in biosensors, the interfacial properties as well as the robustness of the materials are of primary interest. In particular, when sialyl-functionalized polydiacetylene was used as a viral biosensing element, the recognition occurred entirely at the surface of the multilayer film of poly(diacetylene) (PDA).1,2,14 Moreover, uncontrolled surface reorganizations that occur upon contact with biological fluids can alter blood compatibility.19,20 Although a number of studies investigated the molecular surface properties of surfactant monolayers, it is not known how the interfacial properties change upon polymerization. Moreover, the mechanical robustness of the films under harsh conditions is also important criteria for the successful use of these materials. Previous contact angle measurements with supported diacetylene polymer suggested that polymerization stabilizes the film properties against environmental perturbations.21 The latter were bulk measurements, however, and the mechanisms underlying their observations were not verified at the submicroscopic level. In addition, the aging behavior of polymerized versus monomeric diacetylenes has not been examined. Diacetylene polymerization has been studied on solid supports and at the air-water interface.4,7-13,22-25 From the standpoint of their potential applications as surface coatings, however, the quality of the films which are first formed at the liquid-vapor interface and then transferred onto solid supports is critical. An important issue is the dependence of film properties on polymerization conditions such as the ionic composition of the subphase, the molecular packing density, the subphase temperature, the spreading solvent, the nature of solid substrates, and the irradiation time.16,17,23 A second important factor is the film damage that occurs during Langmuir-Blodgett (vertical) or LangmuirSchaeffer (horizontal) transfers to solid supports. Such damage is attributed to large stresses, which create cracks and defects in the layer.4,16-18,26 These defects could, in principle, be avoided by in situ polymerization of supported monomer monolayers or by the introduction of rubberlike properties into the polymer backbone, for example.27,28 Other reports demonstrated that two-dimensional polymers formed in situ on solid supports also contain defects, cracks, and grain boundaries.4,29,30 Such uncontrolled heterogeneity in the surface morphology limits the ability (16) Goettgens, B. M.; Tillman, R. W.; Radmacher, M.; Gaub, H. E. Langmuir 1992, 8, 1768. (17) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1478. (18) Johnson, S. J.; Tillmann, R. W.; Saul, T. A.; Liu, B. L.; Kenney, P. M.; Daulton, J. S.; Gaub, H. E.; Ribi, H. O. Langmuir 1995, 11, 1257. (19) Ratner, B. D. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, Ed.; Plenum: New York, 1985; Vol. 1, p 373. (20) Anderson, J. M.; Zhao, Q. H. MRS Bull. 1991, XVI, 75. (21) Uchida, M.; Tanizaki, T.; Kunitake, T.; Kajiyama, T. Macromolecules 1989, 22, 2381. (22) Tieke, B.; Lieser, G. J. Colloid Interface Sci. 1982, 88, 471. (23) Tieke, B.; Weiss, K. Colloid Polym. Sci. 1985, 263, 576. (24) Higashi, N.; Kajiyama, T.; Kunitake, T.; Prass, W.; Ringsdorf, H.; Takahara, A. Macromolecules 1987, 20, 29. (25) Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1976, 15, 764. (26) Gaines, G. L., Jr. In Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (27) Stupp, S. I.; Son, S.; Lin, H. C.; Li, L. S. Science 1993, 259, 59. Stupp, S. I.; Son, S.; Li, S.; Lin, H. C.; Keser, M. J. Am. Chem. Soc. 1995, 117, 5212. (28) Sisson, T. M.; Lamparski, H. G.; Ko¨lchens, S.; Esayadi, A.; O’Brien, D. F. Macromolecules 1996, 29, 8321-8329. (29) Mao, G.; Tsao, Y.; Tirrell, M.; David, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461. (30) Mao, G.; Tsao, Y.; Tirrell, M.; David, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1995, 11, 942.

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of these materials both to provide uniform surface layers and to effectively camouflage the underlying substrates.16-18 Although substrate heating or disruption of the PCA film during polymerization results in a more uniformly fluorescent surface coating, the submicroscopic heterogeneity was high nevertheless.18 While this limits the utility of such materials for the precisely tailoring of surface properties, few studies have examined directly the effects of such defects on the molecular surface properties of the resultant surface coatings. In this work, we have used a surface force apparatus (SFA) together with other surface analytical methods to investigate the molecular surface properties of monomer and polymer films of PCA deposited on mica and germanium crystals. The sample preparation was optimized for this study. To do this we investigated, with fluorescence and atomic force microscopy, the dependence of the film morphology on both the aqueous subphase composition and the transfer conditions. The thicknesses of the deposited LB films were measured by ellipsometry, atomic force microscopy (AFM), and by direct force measurements. These determined structural properties were then related directly to (i) the dependence of film surface energies on the ambient conditions and (ii) their chemical and mechanical stability. With direct force measurements, we quantified directly the effects of polymerization on the interfacial properties of the films and determined the stability of supported monomeric and polymerized films to environmental perturbations and mechanical stress. The following aspects of the surface force apparatus enabled us to investigate directly and quantitatively the polymerization-dependent changes in the properties of PCA monolayers. Namely, (i) the force measurements are more sensitive to the submicroscopic details of the surface than, say, contact angles, (ii) the size of the contact area can be measured directly, and (iii) changes in the film thickness in response to environmental or mechanical perturbations are measured directly and in situ. Our findings thus show directly at the submicrometer level the improved stability of both the interfacial and mechanical properties of these polymerized films relative to monolayers of the unpolymerized monomers. Materials and Methods Chemicals. 10,12-Pentacosadiynoic acid (PCA, a diacetylene, molecular weight 374) was purchased in a powder form from Farchan Laboratories (Gainesville, FL). Solutions of PCA were prepared in approximately 90% (v/v) HPLC grade chloroform supplied by Sigma-Aldrich and 10% (v/v) ACS certified methanol supplied by Fisher Scientific in a clean environment in a Class 100 laminar flow cabinet (Forma Scientific, Inc., OH). After the solvents were added in the above ratio to a precisely weighed quantity of PCA, the mixture was sonicated for up to one-half hour to enhance the dissolution. In some cases, the powder received was slightly blue, due to some polydiacetylene contamination. In all cases, the freshly prepared solutions were centrifuged at 16000g for 30 min to remove any polymer. The solution thus prepared was always stored at -20 °C and was never exposed to light and ambient atmosphere for more than a half hour when in use. Ruby muscovite mica (ASTM grade 3), the standard substrate for depositing monomeric and polymeric PCA in this work, was obtained from S & J Trading, Inc. Cadmium sulfate (CdSO4‚8/ 3H2O, formula weight 256.5) and lithium chloride (LiCl, formula weight 42.39) were acquired from Sigma and Fisher Scientific, respectively. The desired relative humidities were achieved by placing supersaturated aqueous solutions of salts in equilibrium with their vapor in the sealed SFA chamber or contact angle cell. At least 12 h was allowed for equilibration with the vapor. The

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Sheth and Leckband

Table 1. Salts and Their Equilibrium Relative Humidities

Table 2. Summary of Conditions Used for LB Film Preparation

no.

salt

equilibrium RH

1 2 3

anhydrous CaSO4 (Drierite) CaCl2‚6H2O CuSO4‚5H2O

0 31 98

method polymer

salts used in this work and their humidities created at the indicated temperature are given in Table 1.31 The fluorescent probe N-(Texas Red sulfonyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt (Texas Red DHPE, TR-PE), acquired from Molecular Probes, was used in some of the fluorescence studies of monomeric and polymeric PCA. TR-PE was added to monomeric PCA solutions at a concentration of 8 mol %. Methods. Johnson-Kendall-Roberts (JKR) Theory. In this work, the JKR theory32 was used to determine quantitatively the solid-vapor interfacial energy of the mica-supported diacetylene and polydiacetylene films. This theory relates the area of contact between deformable materials to the externally applied compressive load and the solid-vapor interfacial energy.32-34 In JKR theory, two crossed cylinders of radii R1 and R2, bulk elastic modulus K, and surface energy γ deform when brought into contact due to the influence of intersurface attractive forces. The contact area further increases under the compressive load, FL. At mechanical equilibrium the contact radius a is given by

a3 )

R [F + 6πRγ + x12πRγFL + (6πRγ)2] K L

(1)

where R is the geometric average radius of the two crossed cylinders, (R1R2)1/2.32,34 In the force measurements, the external load (FL) was varied in small steps, and the contact diameter (2a) was measured directly from a video screen on which the interference fringes were displayed. The interfacial energy was then obtained by fitting the advancing and receding branches of the a3 vs FL graph to eq 1. Both K, the bending modulus of the mica and underlying glue, and γ were unknown. In each experiment K was determined independently in measurements with bare mica prior to the film deposition. The fitted values of K for consecutive measurements at a particular contact region on the mica-supported layer did not vary by more than 20%, consistent with previous reports.6 They did, however, differ by more than 20% when measured at different contact regions. The latter variation was attributed to the uneven thickness of the underlying glue layer.6,33 Despite the variation, these measurements provided a range for the fitted values of K. The pull-off force Fssthe tensile force required to separate the surfacessis related to the interfacial energy by 6,34

γ)

Fs 3πR

(2)

Consequently, the interfacial energy of the thin films can be determined either from a3 vs FL curves and fitting to eq 1 or from the pull-off force and eq 2. Sample Preparation: Langmuir-Blodgett (LB) Deposition. Samples of monomeric PCA and poly(diacetylene) (PDA) on mica were prepared by LB deposition. The unbuffered subphase contained either an aqueous 10-3 M CdSO4 solution or 10-3 M aqueous LiCl solution. All solutions were prepared with water with 18.2 MΩ cm resistivity (Milli-Q UV Plus system, Millipore Corp., USA). Different materials were used as solid supports for the films. In the SFA experiments, the substrates were very thin (∼3 µm thick) back-silvered mica sheets, which were glued silvered-side down to fused silica lenses. For contact angle measurements, (31) CRC Handbook of Chemistry and Physics, 58th ed.; Weast, R. C., Ed.; CRC Press: Boca Ratan, FL, 1978. (32) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Sect. A 1971, 324, 301. (33) Horn, R. G.; Israelachvili, J. N.; Pribac, F. J. Colloid Interface Sci. 1987, 115, 480. (34) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992.

1 2 3 4

no yes yes yes

subphase 1 mM CdSO4 1 mM CdSO4 1 mM LiCl 1 mM LiCl

polymerization monolayer/ pressure (mN/m) multilayer 15 15 0 < Π < 10 >Πcollapse

monolayer monolayer monolayer multilayer

fluorescence microscopy, and atomic force microscopy, the substrates were about 0.5 mm thick freshly cleaved mica sheets. Plasma-cleaned germanium crystals were used as substrates in the ellipsometry measurements. In all cases, the diacetylene deposition was accomplished by withdrawing the hydrophilic substrates vertically through the air-water interface at a constant film surface pressure. The retraction rate was 1-1.5 mm/min. The mica or germanium surface always emerged dry, which indicated that the surfaces were hydrophobic. Four different methods were used to prepare the LB films in this study. The conditions used for preparation and the resulting film structures are summarized in Table 2. In method 1, monolayers of unpolymerized PCA were deposited onto mica from a 10-3 M aqueous CdSO4 subphase at a constant surface pressure of 15 mN/m. In method 2, monolayers of polymerized PCA were prepared. PCA polymerization was accomplished by irradiation of the monomer monolayer at the vapor-liquid interface with UV light at a constant surface pressure of 15 mN/m. The mercury pen ray lamp (Hg pen source, Oriel Corp., CT) was held 15 cm from the surface until there was no further increase in the film area. This required 25 min, but the precise irradiation time depended on the amount of monomer present at the air-water interface. The resulting polymer monolayer was then deposited onto mica at a constant surface pressure of 15 mN/m. In the third procedure method 3, the monomers on a 10-3 M aqueous LiCl subphase were compressed to 5 ( 1 mN/m, which is below the film collapse pressure (monolayers). The PCA was irradiated at constant area for 2 min as described. During the irradiation, the film pressure increased to 8 ( 1 mN/m, and the resulting polymer film was transferred onto mica at a constant pressure of 8 mN/m. In contrast, in method 4, the monomers on a 10-3 M LiCl subphase were polymerized, following the monolayer collapse into multilayers, by irradiation of the films for 2 min. The polymerized multilayers were then deposited onto mica substrates. Method 4 gave supported multilayers of polymerized PCA. In all of the above methods, the subphase was at room temperature, and the measured transfer ratiosthe ratio of the area of the film removed from the water surface to the surface area coatedswas between 0.85 and 1. Surface Force Measurements. The SFA was used to evaluate the impact of polymerization on (i) the chemical and mechanical properties and (ii) the aging behavior of supported monomeric PCA and polymerized PDA films. The force measurements also corroborated the film thicknesses determined by ellipsometry and by AFM, and revealed any thickness changes that resulted from environmental perturbations. Measurements were conducted with a Mark II surface force apparatus interfaced with a video camera (Dage MTI), SONY S-VHS video recorder, and timer.35,36 The advancing and receding interfacial energies were determined directly with the SFA by two methods.6,33 First, in a pressure run, the surfaces were brought into contact under zero external load, and a, the radius of circular contact, was measured directly from the video monitor. The external compressive load FL was increased in small increments by pushing the layers against each other. This contact area increased as the samples deformed under the load.5,6 An advancing pressure run refers to the case in which the compressive load was increased. At an arbitrary point, the trend was reversed. During a receding pressure run, the load was reduced in small steps, and the radius of circular contact was measured. The receding pressure run culminated when the gradient of the tensile force between the (35) Tabor, D.; Winterton, R. H. S. Proc. R. Soc. London, Sect. A 1969, 312, 435. Israelachvili, J. N.; Tabor, D. Proc. R. Soc. London, Sect. A 1972, 331, 19. (36) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975.

Polymerization-Dependent Changes in Films surfaces exceeded the spring constant, and the surfaces jumped apart. The entire cycle takes 30 min. The advancing and receding monolayer surface energies and the bulk elastic modulus of the mica-monolayer-glue system were obtained by fits of the advancing and receding branches to eq 1.6,32,34 The advancing runs gave the advancing interfacial energy, γa, and the receding runs gave the receding interfacial energy, γr. Typically, the monolayers were subjected to extremely high pressures (approximately 104 kPa) during pressure runs. The second approach is referred to as an adhesion run. The samples were brought to contact under zero external load (FL ) 0) and then separated. The interfacial energy γs was determined from the pull-off force normalized by the radius of curvature Fs/R and eq 2. The subscript s indicates that the surface energy was determined from the measured work of adhesion. The latter is twice the interfacial energy, or Wadh ) 2γs.34 We identified surface group rearrangements, aging behavior, and the degradation of the coatings in response to large compressive loads from the hysteresis in the measured surface energies at different relative humidities. Hysteresis in both contact angles and the measured adhesion has been attributed to molecular rearrangements or to the occurrence of other dissipative processes during the measurements.5,6,33 Adhesion hysteresis is manifested as a difference between γa and γr and between γs and γr. Significant perturbations were confirmed visually from the shapes of the interference fringes, which correspond to the microscopic surface contours.35,36 Water Contact Angle Measurements. The relative hydrophobicity and stability of the different diacetylene films were determined from contact angle measurements.37 The advancing and receding water contact angles were measured on micasupported diacetylene films (methods 1 and 2) at room temperature, and at the following relative humidities: 0%, 31%, and 98% RH. The samples were prepared as described for the SFA experiments. Measurements were conducted in a small chamber, which allowed manipulation of the droplet from outside. To control the humidity within the chamber, we placed inside the chamber a small Petri dish containing saturated salt solutions. Film stability was assessed on the basis of the time dependence of the advancing and receding water contact angles.5,6 To gauge the facility of rearrangements in the film, we measured changes that occurred in the advancing contact angle over a period of 60 s.5,6 Mica sheets coated with monomer or polymer were prepared as described and equilibrated at a particular relative humidity inside a custom-made sealed chamber for at least 12 h prior to taking data. Contact angles were then measured with a contact angle goniometer (Rame´-Hart, Inc., NJ, USA). Several different spots on the sample were accessed by moving the clear plastic dome holding the syringe above the aluminum base. The contact angles on both the monomer and the polymer layers on mica were measured at 0%, 31%, and 98% relative humidity. Because hysteresis may be due to (i) surface roughness, (ii) surface heterogeneity, (iii) surface reorganizations facilitated by bulk water, or (iv) the simultaneous effects of any combination of these,6,37 we used additional techniques to further characterize the films. Fluorescence Microscopy (FM). Since the SFA measures the average force between two surfaces over contact areas of up to 10 µm2,35,36 we determined the film heterogeneity over comparable areas to facilitate the proper interpretation of the force data. The surface morphology of the polymer was first assessed by epifluorescence microscopy. First, we followed the size of polymerized domains as a function of irradiation time. For this purpose we used the fluorescent TR-PE. The dye was miscible in the unpolymerized PCA monolayer but is squeezed out of the polymerized film.16 The polymerization could be followed from the changes in the TR-PE domains. To visualize the dye distribution in the samples, mica samples were coated with a monolayer of a 92/8 mol % mixture of PCA/TR-PE, and examined under a Leitz fluorescence microscope. Second, we monitored the autofluorescence of PDA. In addition to confirming the presence of fluorescent polymer, we placed a polarizer analyzer between the sample and the viewing eyepiece. This enabled us to identify anisotropic, crystalline polymer domains (37) Van Oss, C. J. Interfacial Forces in Aqueous Media; Marcel Dekker, Inc.: New York, 1994.

Langmuir, Vol. 13, No. 21, 1997 5655 within the film. We used this approach to (i) determine the effect of the counterion identity in the subphase on polymerization and (ii) to characterize the morphology of both supported and floating polymer films.16 Thickness Measurements. The steric thicknesses of the films were determined with the FECO technique of the SFA and by AFM. With the SFA, we determined the shift in the wavelength of the interference fringes, which correspond to the distance of closest approach, after burning off the layers by UV irradiation. The thicknesses were thus determined directly from the resulting changes in the wavelengths.5 From the AFM images, we determined the heights of the polymer structures. The steric thicknesses were compared with film thicknesses on germanium crystals determined ellipsometrically with a Type 43702-200E ellipsometer (Rudolf Research, NJ). Highly reflective plasma-cleaned germanium crystals were used as substrates. A refractive index of 1.45 was assumed in order to estimate the thickness of monomeric and polymeric PCA. The refractive index of bulk n-pentacosane is 1.5 and decreases with increasing chain length and with the degree of unsaturation.31 Thicknesses were measured on several 1 µm2 regions of the 4 cm2 sample surface. These values were compared with the theoretical monolayer thicknesses, which were calculated on the basis of the molecular dimensions and of the orientations of the molecular directors relative to the surface normal. The theoretical length of a PCA molecule in an all-trans configuration is 31 Å. In the case of the unpolymerized monolayers, a tilt angle of 30° with the surface normal was assumed, whereas a 33° tilt was assumed for the diacetylenes in the polymer monolayer.4,38 Atomic Force Microscopy (AFM). AFM images indicated the submicroscopic structure, roughness, and lateral heterogeneity of the materials investigated in this study. These images were then used to relate the film architecture to results obtained from SFA experiments and from contact angle measurements. Polymer films were examined in the constant force mode in air. Additionally, some films were solvent-etched by immersion in pure chloroform for 2 h. Solvent-etching was used to detect unpolymerized regions (monomeric PCA is soluble in chloroform) and to test the chemical robustness of PDA. The samples were between 2 days and 4 weeks old when imaged. Prior to imaging, they were stored in closed vials in a clean environment.

Results Pressure-Area Isotherms. Pressure-area isotherms of PCA monolayers were measured on the LangmuirBlodgett trough with different dissolved ions in the subphase. We first investigated the effects on the isotherms and resulting polymer morphology of Cd2+, Li+, and H+ (the latter by varying the pH) ions in the subphase. Diacetylene polymerization is a topochemical reaction.4 Consequently, counterions affect the monomer packing at the air-water interface and thereby influence polymerization and the polymer morphology.6,7,9,23 The isotherms of PCA monomers on unbuffered water subphases containing either 10-3 M CdSO4 or 10-3 M LiCl (pH 6) at room temperature are shown in Figure 1. Monolayers were compressed at 5 cm2/min. Both isotherms exhibited a “rise regime” in which the pressure increased with decreasing molecular area and a “plateau regime” in which the nonzero pressure did not change with decreasing area. On the CdSO4 subphase at 15 mN/ msthe surface pressure at which SFA samples were preparedsthe PCA formed a tightly packed monolayer at 22 Å2/molecule.9,16 The monolayer collapsed at 27 mN/m, and further compression resulted in multilayer formation.16 The subsequent “plateau regime” consisted of domains of multilayers interspersed with the monolayer phase.16 The isotherm measured on an unbuffered 10-3 M LiCl subphase exhibited similar behavior, but the collapse to multilayers occurred at 10 mN/m. As discussed below, the presence of monolayer or multilayer structures (38) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1483.

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Sheth and Leckband

Figure 1. Pressure-area isotherms of monomeric PCA on a CdSO4 subphase and on a LiCl subphase. PCA monomer was spread on an aqueous subphase containing the specified salt at 1 mM (pH 6) and 25 °C. Both isotherms exhibit a “rise” regime that corresponds to a tightly packed monolayer, and a “plateau” regime that corresponds to film collapse into domains of multilayers interspersed with the monolayer phase.

was confirmed by ellipsometry, AFM, and direct force measurement. Fluorescence Microscopy (FM). (a) Illumination Time and Extent of Polymerization. The mixed monolayer of PCA monomer and TR-PE at the air-water interface was irradiated for different times to follow the time-dependent increase in size of polymerized regions in the monolayer. The resulting (partially) polymerized monolayer was then deposited onto mica by LB transfer and examined with a fluorescence microscope. Upon polymerization, the PCA forms a tightly packed structure.4 The extent of polymerization was monitored qualitatively from the sizes and numbers of remaining TR-DHPE containing domains. The latter decreased with increasing irradiation time. Control experiments verified that the dye was not bleached by UV irradiation for the timeperiods used. From these data and the time-dependence of the film pressure change during irradiation, the polymerization according to method 2 appeared to be complete within 30 min. (b) Effects of Counterions. As shown in Figure 2, the counterions had a detectable influence on the polymer morphology. Poly(diacetylenes) are autofluorescent, and the fluorescence micrographs in Figure 2 (10× magnification) illustrate the effect of counterions on the morphology of the resulting floating polymer multilayers. When 10-3 M H+ (pH 3 subphase), Li+, or Cd2+ ions were present in the subphase, anisotropic, crystalline domains were visualized when the film was viewed with a polarizer analyzer placed between the sample and eyepiece. By contrast, the polymer exhibited fewer anisotropic domains when the subphase contained only pure water at pH 6 (10-6 M H+). No polymerization was detected following UV exposure of PCA monomers on a 1 mM NaOH subphase at pH 11 (10-11 M H+), but polymerization did occur in the presence of 10-3 M Li+ at that pH. Equal amounts of monomer were spread at the air-water interface, and all monolayers were irradiated from a distance of 3-4 cm above the surface at constant pressure for 2 min. Consequently, the detectable differences in the film morphologies were attributed to the particular counterionsurfactant interactions and not to differences in the monomer surface coverage or to the initial packing densities. The morphologies of the floating polymerized films also determined their transfer behavior. The effects of LB deposition on the polymer multilayers prepared by method 4 are shown in parts A and B of Figure 3A. These films were extensively damaged by LB transfer onto mica

Figure 2. Fluorescence microscopy images (10×) of multilayers of PCA Polymer at the air-water interface. The subphase contained (A) 10-3 M HCl, (B) 10-3 M LiCl, and (C) 10-3 M CdSO4. In each case, equal amounts of monomeric PCA were spread over equal areas of the air-water interface at 25 °C. Polymerization was carried out by irradiation of the monolayers for 2 min.

(Figure 3B). The film break-up was attributed to the large stresses experienced by the brittle polymer network during transfer.4,16-18,26,38 The multilayered structure and high crystalline content rendered the films more brittle and amenable to fracture during deposition.16 In contrast, the damage was reduced, if polymer monolayers prepared by method 2 were transferred instead (Figure 3C). The regions coated by the latter monolayers were consistently more uniform than those covered by shattered multilayers, although minute areas of either uncovered mica or regions

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Figure 4. Applied pressure versus a3 between two polymer monolayers. The radius a of the contact area between two PDA monolayers (method 2) in the surface force apparatus was recorded as a function of the applied load FL. In these measurements, the external load on the monolayers was increased (advancing run, open circles) and then decreased (receding run, filled circles). The theoretical fit (solid lines) to the advancing branch gave an advancing surface energy (γa) of 19 mJ/m2, and the fit to the receding branch gave the receding surface energy (γr) of 24 mJ/m2. The pull-off force and eq 2 gave γs ) 25 mJ/m2.

Figure 3. (A) Fluorescence images (10×) of polymer multilayers at the air-water interface (10-3 M CdSO4) and (B) following LB deposition onto mica. (C) Fluorescence image (10×) of a monolayer of PDA prepared on a 10-3 M CdSO4 subphase and transferred onto mica by LB deposition.

of unpolymerized monomers (monomers are fluorescently silent) were still evident. Nevertheless, because the polymer films prepared by method 2 yielded the most uniform layers, as determined by fluorescence microscopy, they were used in the direct force measurements. It was thought that the films exhibiting fewer crystalline domains in the polarized fluorescence images, such as polymer monolayers prepared on a pure water subphase (pH 6), might be less brittle, and incur less damage during the deposition. They were, however, extensively damaged during transfer. They appeared to be much more susceptible to mechanical shear and tore easily. We at-

tributed this to reduced ordering in the layer and to a likely reduction in the average molecular weight of the polymers. Direct Determination of Surface Energies from SFA Measurements. (a) Pressure RunssJKR Theory. The advancing and receding interfacial energies of the various films were determined from the measured changes in the area of contact between two opposed layers as the externally applied load was increased or decreased, respectively. The solid-vapor interfacial energy and the bulk elasticity modulus were then obtained from nonlinear least-squares fits of the data to eq 1. Figure 4 shows the fit to the data determined from consecutive advancing and receding pressure runs measured at 0% RH (relative humidity) on a polymer monolayer prepared according to method 2. The fits gave 19 and 24 mJ/m2 for the advancing and receding interfacial energies, respectively, and 4 × 1010 dyn/cm2 for the elasticity constant K of the mica and underlying glue layer. In this case, there was little hysteresis in the measured values. However, in other cases, the advancing and receding energies differed significantly. Values for the advancing and receding interfacial energies γa and γr, respectively, as well as the hysteresis (γr - γa) determined with other samples and under different conditions are summarized in Table 3. Three trends are immediately obvious from the data. First, the hysteresis was clearly more pronounced with the monomeric films at high relative humidities than for the polymer films. Second, the interfacial energies of both monomer and polymer films determined from both adhesion and pressure runs increased with the relative ambient humidity. Third, the fitted values of surface energies for the monomer film changed significantly with time at high RH. The temporal variation in the measured interfacial energies of the monomer films provided the most striking example of the improved stability afforded by polymerization. During pressure and adhesion runs at 98% RH, films prepared by method 1 were subjected to large compressive stresses under repeated contacts, often for >50 h. In particular, the responses of the monomeric (method 1) and polymeric films (method 2) to repeated

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Table 3. Interfacial Energies of PCA Monolayers Determined from JKR Theory film monomer (method 1) polymer (method 2)

relative humidity

γa ( σ (dyn/cm)

γr ( σ (dyn/cm)

γs ( σ (dyn/cm)

γ r - γa ( σ (dyn/cm)

γ r - γs ( σ (dyn/cm)

0% 31% 98% 31% 98%

5(1 7.3 ( 0.3 11 ( 2 9(2 18 ( 3

8(1 9.6 ( 0.3 18 ( 3 12 ( 2 22 ( 1

6(1 5(2 14 ( 2 9.6 ( 0.5 21 ( 3

2.3 ( 0.4 2.3 ( 0.5 7(1 3(1 4(3

1(1 4(2 4(1 2(1 2(3

Figure 5. Time evolution of the interfacial energy of monomeric PCA monolayers at different relative humidities. The monolayers were prepared according to method 1. Interfacial energies were determined by direct force measurements and fits of the data to eq 1. The relative humidities were 0%, 31%, and 98% at 25 °C.

compression-decompression cycles at 98% RH differed markedly from each other. Figure 5 shows the aging behavior of the unpolymerized monolayer films (method 1) at 0%, 31%, and 98% RH. The measured adhesion energy decreased dramatically over time at 98% RH and eventually stabilized. However, at 98% RH, repeated contact, compression, and separation of the monomer films caused significant monolayer damage, as evidenced by the appearance of severe deformations in the interference fringe contours. By contrast, with polymer films prepared by method 2, the surface energy γ remained relatively constant for up to 50 h at 31% RH. The polymer also exhibited neither aging (Figure 6) nor visible damage, even at 98% RH. Additionally, with the monomeric films, but not with the polymer monolayers, there was evidence for crosssurface polymerization at 98% RH. Instead of the characteristic jump-apart at the end of the pressure run, we observed a very slow, viscous detachment of the surfaces which persisted to separations of nearly 400 Å. The surfaces were highly deformed during separation. The latter behavior was similar to that expected for the separation of two highly deformable, adhesive surfaces and was attributed to in-situ cross-surface polymerization during the pressure runs. At the end of a pressure run, the opposed monolayers had been in contact for ∼1 h. Aided by the high moisture and large compressive stresses (up to 104 kPa), the monomer units on the opposed surfaces in the contact area may have adopted configurations appropriate for cross-surface polymerization. The latter reaction was likely catalyzed by the white light source that constantly illuminated the contact area. This was never observed with polymers prepared by methods 2 or 3.

Figure 6. Time evolution of the interfacial energy of a polymer monolayer. Monolayers of poly(diacetylene) (method 2) were deposited on mica, and the interfacial energy was obtained from pressure runs with the surface force apparatus and fits of the data to eq 1. The relative ambient humidity was controlled at 31% and 98% at 25 °C.

(b) Adhesion Measurements. The receding solidvapor interfacial energy was also obtained from the tensile force required to separate the surfacessthe pull-off forcesand eq 2. While the values of γr and γs should be similar in the absence of energy dissipation, data in Table 3 show that this was not the case. Such variation is partly attributed to the differences in the fitted value of K which can vary by as much as 20% during a pressure run. Under such high loads, the underlying glue could deform inelastically during the measurement.33 However, the value of the bending modulus K affects the value of γr obtained from the fit to eq 1, but γs, which is obtained from the pull-off force and eq 2, is independent of K and any substrate deformation. Nevertheless, while not as pronounced as in the pressure runs, there are still differences between γr and γs, which are ascribed to dissipative processes that occur during the pressure runs.5,6,30 The adhesion between the polymer multilayers (method 4) was significantly lower than with the other films since contact occurred primarily at asperities on the two surfaces. Atomic Force Microscopy (AFM). To characterize the submicroscopic features of the polymer films that might influence the surface energies measured with the SFA, we obtained AFM images of the unpolymerized PCA monolayers (method 1). The images showed that the monomers were not homogeneously distributed. Below film collapse, the film exhibited clusters (height ∼20 Å), which were dispersed homogeneously in the film (Figure 7). Since the cluster density increased with the surface pressure of the deposition, the aggregates were attributed to nucleating solid domains within the monolayer prior to its collapse.

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Figure 7. AFM image (constant force) of a PCA (monomer) monolayer on mica. The film was prepared according to method 1 (details in text). The image was obtained with a Digital Instruments Nanoscope IIIE AFM.

The low-resolution AFM images of polymer monolayers (method 2) suggested that the polymer coated a large fraction of the mica, consistent with the fluorescence micrographs (Figure 3C). However, higher resolution AFM images showed that the film was in fact very heterogeneous at both the microscopic and submicroscopic levels (Figures 8 and 9). In some regions, the polymer prepared by method 2 exhibited loose strands, similar to supported poly(diacetylene) structures reported elsewhere (Figure 8).38 The heights of the latter structures were 19 ( 2 Å. The chains were interspersed with more compact, ordered polymer domains (Figure 9). While our films do contain crystalline polymer domains (Figure 9), these images show that on a larger scale than reported by Goettgens et al.16 the films are not well-ordered. The heterogeneity reported in this work is consistent with the fluorescence images, which show anisotropic domains interspersed in an isotropic fluorescent field. The low-resolution AFM images of the multilayer polymer films (method 4) confirmed that the films were laterally heterogeneous and rough. The root mean square roughness was ca. 100 Å, in agreement with the SFA data. Due to the sample roughness, any effects of solvent etching of the films were not observable. Water Contact Angles. As shown in Table 4, the contact angles of the monomeric films were unstable, in agreement with the conclusions based on the SFA measurements. The advancing contact angle decreased by 30° and 25° within 60 s. This contrasted sharply with the stability of the angles measured on the polymer monolayers (Table 4). The latter contact angles changed by less than 5° in 60 s. Furthermore, the variability in the water contact angles measured at different spots on the sample surface was also much greater on the monomeric films. Although local clusters were evident in the AFM, the monolayer was laterally homogeneoussthat is, the clusters were distributed uniformly over the sample. Thus, the rapid decrease in the contact angle on these layers relative to that on the polymer is attributed to molecular surface reorganizations and not to heterogeneity. Since the monomers could easily rearrange in the

presence of water, the measurements depended strongly on the rapidity with which data were taken after advancing the droplet. By contrast, with the cross-linked polymer monolayer, the individual monomers were fixed by rigid chemical bonds, and the advancing angle on polymer films was time-invariant. Consequently, monomeric films can rearrange easily in response to changes in ambient conditions and thereby alter their interfacial properties in an uncontrolled manner. The polymers exhibited lower advancing and receding contact angles than those of the monomers (Table 4). This was attributed to polymer film heterogeneity and to the influence of uncoated mica. Since mica is hydrophilic (water contact angles on bare mica are ∼1-2°), and since contact angles reflect the area-averaged surface energy,37,39 exposed mica would decrease the average interfacial energy. The nonzero water contact angle (Table 5) together with atomic force and fluorescence microscopy results, nevertheless, attested to extensive polymer coverage. Film Thickness Determinations. The film thicknesses were determined by (i) the fringes of equal chromatic order (FECO) technique of the SFA, (ii) by AFM, (iii) by ellipsometry, and (iv) by calculation. These results are summarized in Table 5. The thicknesses of both the monomer and polymer monolayer films prepared by methods 1 and 2 were 20 ( 2 and 18 ( 2 Å, respectively, as determined with the FECO technique of the SFA (Table 5). Polymer monolayers prepared by method 3 were 20 ( 2 Å thick. The AFM images confirmed the 19 ( 2 Å thickness of the polymer strands. The compact polymer domains were somewhat thicker at 22 ( 2 Å. Similarly, the 18 Å optical thicknesses of the PCA monolayers, determined with ellipsometry and an index of refraction of 1.45, agreed with both the SFA and AFM results (Table 5). These were less than the theoretically calculated values of 26 and 29 Å for, respectively, monomer and polymer films. The measured values corresponded to tilt (39) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; Wiley-Interscience: New York.

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Figure 8. High-resolution image of a PDA chain on mica. The image was obtained with a TopoMetrix Discoverer AFM. The film was prepared by method 2. The heights of the features in the image are 18-21 Å.

angles of 50° and 54° for, respectively, the monomer and polymer layers. The ca. 30° angles used in the calculations were, however, based on those determined for crystalline polymer bilayers that had been transferred onto carboncoated electron microscope grids from the air-water interface by the Langmuir-Schaeffer technique.17,38 The differences in the apparent tilt may be due to strong substrate effects and to the reduced packing densities of the mica-supported polymers. The experimentally determined monolayer thicknesses were, however, in good agreement with each other (Table 5). The >30 Å thicknesses measured with polymer prepared by method 4 verified that the films consisted of laterally heterogeneous multilayers. Not only did the latter thicknesses exceed those expected for a single monolayer, but they also varied between regions probed on the sample surfaces. The average value of 90 Å measured with the SFA was greater than the 30-60 Å measured by ellipsometry. This discrepancy is due to the differences in the two techniques. The direct force measurements report the actual steric thickness of the layers, while ellipsometry reports the area-averaged optical thickness. Despite this difference, the measurements confirmed that the films consisted of multilayers.16 Discussion Poly(diacetylenes) have been studied extensively as robust coatings for optical transducers or for surface passivation in biomedical applications, for example.1-4,12-16,18 Although the polymer films are damaged during transfers from liquid surfaces to solid supports,6,24 our findings demonstrate directly the substantially im-

proved chemical and mechanical robustness of supported polydiacetylene monolayers relative to those of monomers. The dependence of the film morphology on the counterions in the subphase was as expected, given the acidic property of the carboxylic acid headgroup and the packing requirements of polymerization. PCA is charged at pH > 6.5. The negative charge thus inhibited close packing in the monolayer and the formation of large, ordered polymer domains. Surface charge neutralization and electrostatic screening improved the packing and polymerization.4 Thus, large anisotropic polymer domains formed only in the presence of millimolar concentrations of positive counterions. Counterion influences on monolayer structures were reported previously.40,41 More quantitative statements, however, regarding counteriondependent order in the polymer films require more detailed structural analyses.40,41 The correlation of the polymer brittleness with monolayer damage during LB transfer is not unexpected. In these studies, the brittleness of the multilayers was largely attributed to the presence of extensive, rigid crystalline domains within the polymer layers that shattered easily during deposition. Polarized fluorescence microscopy showed that polymer multilayers prepared on a pure water subphase contained fewer crystalline domains. They also tore readily rather than fracturing into small, angular shards. By contrast, polymer monolayers prepared in the presence of millimolar levels of counterions exhibited the (40) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Science 1993, 261, 449. (41) Zadsadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726.

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Figure 9. AFM image (constant force) of a PDA monolayer on mica. The polymer film was prepared according to method 2. The image was recorded in air in constant force mode with a Digital Instruments Nanoscope IIIE AFM. Compact 22 ( 2 Å polymer domains are interspersed with linear chains of PDA (19 ( 2 Å). Table 4. Water Contact Angles Measured on PCA Films at Different Relative Humidities advancing angle

receding angle

film

% RH

0s

30 s

60 s

0s

30 s

60 s

monomer monolayers (method 1)

0 31 98 0 98 0 31 98

97 ( 3 102 ( 5 101 ( 4 49 ( 2 51 (1 64 ( 2 45 ( 2 43 ( 1

73 ( 5 80 ( 8 84 ( 4 44 ( 1 45 ( 1 59 ( 1 40 ( 1 39 ( 2

67 ( 4 75 ( 8 77 ( 2 42 ( 2 43 ( 1 58 ( 1 38 ( 1 39 ( 2

90 ( 5 95 ( 4 94 ( 3 48 ( 2 48 ( 1 59 ( 4 39 ( 2 40 ( 3

71 ( 4 83 ( 8 82 ( 4 46 ( 1 45 ( 2 56 ( 4 38 ( 1 39 ( 3

68 ( 5 77 ( 7 80 ( 3 44 ( 1 44 ( 2 55 ( 4 36 ( 1 37 ( 3

polymer monolayers (method 2) polymer multilayers (method 4)

Table 5. Measured Film Thicknesses sample monomer monolayer (method 1) polymer monolayer (method 2) polymer monolayer (method 3) polymer multilayer (method 4)

theoretical (Å)

SFA (Å)

AFM (Å)

ellipsometry (Å)

26

20 ( 2 22 ( 1

16.7 ( 0.8

29

18 ( 2 19 ( 2

n.d.

29

20 ( 2 21 ( 2

18 ( 1

>29

∼90

n.d.

35-60

more optimal balances of structural integrity and film compliance. As a result, they suffered less damage during transfer from the air-water interface. Nevertheless, the properties that gave rise to apparently better coating at the macroscopic level resulted in a greater number of submicroscopic film defects. Despite the

relatively homogeneous fluorescent field over large (>500 µm2) domains, higher resolution AFM images indicated that these apparently uniformly coated regions were actually composed of small crystalline polymer domains interspersed with a loose network of linear polymer chains. The latter polymer morphology was similar to supported poly(diacetylene) bilayers reported elsewhere.23 The absence of large regions of ordered, crystalline polymer as well as the presence of linear structures presumably resulted in a more compliant material, which was less prone to damage upon transfer to the solid mica supports. However, submicroscopic regions of exposed mica remained. Humidity-dependent changes in the surface energies determined directly by SFA measurements and indirectly from water contact angles, however, demonstrated clearly

5662 Langmuir, Vol. 13, No. 21, 1997

the improved chemical and mechanical stability of 2D polymer coatings over those of the monomeric films. Polymerization stabilized the interfacial properties of the PCA films and increased their mechanical stability. Monomer reorientations occurred readily in monolayers, and resulted in facile changes in the interfacial properties. These rearrangements were linked to observed film damage when the monolayers were subjected to large compressive loads. In contrast, PCA polymerization substantially reduced both the contact angle and adhesion hysteresis and stabilized the surfactant layer. In addition, the polymer monolayers were much more robust and not perceptibly damaged by large compressive loads. The humidity-dependent behavior of the monomer films can be understood in terms of moisture-dependent increases in the molecular mobility.5,6 The water contact angles increased slightly with increasing humidity. This suggests that the increased humidity facilitated the annealing out of clusters and defects in the film. More uniform packing, aided by water vapor adsorption and increased surfactant mobility, would lower the solidvapor interfacial energy, as observed. On the other hand, the apparent surface energies, as determined by direct force measurement increased with increasing humidity. This is also attributed to monolayer annealing. The initial low surface energy of the monomeric PCA at 5 ( 1 dyn/cm (0% RH) is attributed to the monolayer defects (clusters) that were observed in the AFM images (Figure 7). Such local roughness would decrease the effective intersurface contact and, hence, the adhesion and apparent surface energy. Increased molecular mobility would anneal the layer, reduce local roughness, and increase the area of contact between the surfaces, hence, the adhesion. Thus, moisture appeared to aid monolayer annealing, as evidenced by the increase in the directly measured surface energy to 7.3 ( 0.3 and 11 ( 2 dyn/cm at 31 and 98% RH, respectively. The latter values are comparable to those reported for monolayers of single- and double-chain surfactants.5,6 The greater apparent surface energy could also result from increased chain interdigitation between opposed monolayers. Both mechanisms would contribute to the apparent increase in surface energy γ, and both require some moisture-aided molecular rearrangements. Such molecular surface rearrangements were not evident with the poly(diacetylene) monolayers. Neither the water contact angles nor the directly measured surface energies at 98% RH exhibited large hystereses. This demonstrated directly the improved chemical and physical stability of the former films. Neither the chemical properties of the polymer interface nor the film structure changed in response to environmental perturbations. These findings are consistent with the reported contact angle stability of two-dimensional polymers.21 However, contact angles are relatively insensitive to submicroscopic features.42 By contrast, the SFA measurements provided direct evidence that in-plane polymerization prevents molecular surface reorganizations and stabilizes the monolayer surface properties. The direct force measurements also demonstrated the improved temporal stability of the polymer, even when subjected repeatedly to pressures as high as 104 kPa for long periods. This was in stark contrast to the timedependent adhesion energies of the monomer films measured with the SFA. The increased adhesion energy determined with the latter films at 98% RH and the slow (42) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. Xiao, X. D.; Liu, G.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (43) Marmur, A. J. Colloid Interface Sci. 1994, 168, 40.

Sheth and Leckband

decay to a limiting value were not evident with the polymeric layers. The increased intersurface adhesion of the former was attributed to (i) greater interdigitation between opposed PCA chains,5,6 (ii) defect annealing, and (iii) trans-surface polymerization at high relative humidities. With the monomer films, the subsequent timedependent decay in the adhesive force was consistent with film damage. The latter was corroborated by the rough appearance of the interference fringes observed with the force apparatus. In contrast, such aging and load-induced damage were entirely absent in similar measurements with the two-dimensional PCA polymer. These results thus demonstrate the improved mechanical stability of the latter. The polymer monolayers did exhibit small humiditydependent increases in their interfacial energies as determined by direct force measurement, while their advancing water contact angles were humidity independent. This is attributed to surface heterogeneity and to the different sensitivities of contact angle versus direct force measurements to the submicroscopic details of the surface. Again, in contrast to contact angles, the SFA measurements are more sensitive to the submicroscopic and molecular details of the materials. During the adhesion measurements at high RH, capillary condensation could occur in the small pores between the contacting surfaces. Small droplets condense much more readily in small pores than large droplets such as those used in contact angle measurements.43 The apparent surface energy determined with the SFA would therefore increase due to both increased surface hydration and to the Laplace pressure arising from water condensation between contacting asperities. The simultaneous absence of contact angle hysteresis and presence of adhesion hysteresis is thus attributed to the greater sensitivity of direct force measurements to the submicroscopic and molecular properties of the films. Our findings demonstrate that the polymerization of 10,12-pentacosadiyonic acid monolayers substantially improves both their chemical and mechanical stability. The aging behavior of the film was improved by PCA polymerization, even when the film was subjected to large compressive forces. The main drawback of the twodimensional polymer prepared from PCA and similar diacetylenes is, however, the difficulty of preparing highquality, homogeneous coatings. The quality of lowdimensional polymer films can be improved by manipulating fabrication conditions, as demonstrated in this work. In addition, the materials could be improved by the incorporation of rubber-like properties into the material which reduce the intralayer stresses that cause film fracture during LB transfer.28 In situ polymerization is also an attractive alternative for obtaining high-quality, low-dimensional polymer coatings.27-30 Recent reports demonstrated reasonable success with the in situ polymerization of self-assembled bolaform amphiphiles.29,30 The polymer films similarly exhibited improved stability over the monomeric ones, but they nevertheless exhibited unpolymerized domains and defects. Clearly, these fabrication limitations must be surmounted, in order to fully exploit the unique properties of these materials. Acknowledgment. This research was supported by the Whitaker Foundation. We thank Dr. Deborah Charych for her constructive comments and for the AFM images of several of the poly(diacetylene) films. We also thank Todd Purves and Joe Carlson for their assistance in obtaining several AFM images. LA962107Z