Octadecylsiloxane Monolayers - ACS Publications - American

Chapter 22

Prepolymerized Langmuir—Blodgett Films of /j-Octadecylsiloxane Monolayers 1

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Downloaded by UNIV MASSACHUSETTS AMHERST on October 11, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0615.ch022

Atul N. Parikh , Jonathan Wood , Ravi Sharma , and David L. Allara

departments of Materials Science and Chemistry, Pennsylvania State University, University Park, PA 16802 Materials Science and Engineering Division, Eastman Kodak Company, Rochester, NY 14650-2158

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Monolayers of w-octadecylsiloxane (ODS) have been prepared by Langmuir-Blodgett film transfer of pre-polymerized films onto oxidized silicon substrates at a surface pressure of 20 mN.m' Characterization by infrared spectroscopy, single-wavelength ellipsometry, and contact angle measurements show that highly organized structures are formed which are nearly identical to those reported for ODS films made by solution self-assembly. Atomic force microscopy images reveal that the macroscopic structure of the LB film consists of large, dense domains on the size scale of -10 um. 2

Recently it has been shown(l) that densely packed, polymerized ^-alkylsiloxane [RSiO (OH)y] monolayer films, usually prepared by self-assembly from hydrocarbon solution,(2, 3) can be obtained by Langmuir-Blodgett film transfer from the air-water interface. In the above study,(l) H-octadecyltriethoxysilane [OTE; H3C(CH2)i7 Si(OC2H5)3] was spread at the air-water interface and LB films subsequently transferred onto mica substrates to yield polymerized w-octadecylsiloxane(ODS) monolayers. Characterization by wetting, ellipsometry, and surface force measurements showed thefilmsto be quite hydrophobic and formed at high coverages. While these observations suggest a highly organized molecular packing, no direct structural evidence of the molecular structure was obtained. In this paper, we present preliminary results in which a combination of ellipsometry, wetting, atomic force microscopy (AFM), and infrared spectroscopic (IRS) measurements are applied to characterize details of the structures of ODS LB monolayers. In order to simplify the optical characterizations, oxidized silicon surfaces were used instead of mica, while mica was used in the parallel AFM measurements. This combination of characterization tools allows a direct comparison of the LB film structure with that reported previously for ODS monolayers prepared by self-assembly techniques. (4) x

0097-6156/95/0615-0355$12.00/0 © 1995 American Chemical Society In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Experimental Section Materials. Highly polished silicon wafers were used as film substrates for IRS, contact angle and ellipsometry measurements whilefreshly-cleavedmica sheets were used as substrates for AFM measurements. For IRS measurements the wafers (Harrick Scientific; Ossining, NY) had wedged faces to minimize interference fringes. The materials H-octadecyltriethoxysilane (OTE; Petrarch-Huls America, Bristol, PA),ethanol (Quantum, Tuscola, IL), chloroform (Kodak, Rochester, NY) and methanol ( Baker Analyzed, G. T. Baker) were used as recieved. For all cleaning and deposition procedures, deionized water (18.2 MQ) was used. All glassware was cleaned by the sequence: 20-25 min. immersion in an ultrasonic bath contining 2% RBS solution (Phosphate-free RBS, Pierce, Rockford, IL) at 50 °C, extensiverinsingin deionized water, at least 1 hr soaking in fuming nitric acid (Reagent grade, Kodak, Rochester, NY), thoroughrinsingin deionized water followed by ethanol, and finally air drying. For the Langmuir film preparation, the 0.01 M HNO3 sub-phase was prepared by diluting 0.1 N H N 0 (pre-packaged ampoules; J. T. Baker, Philipsburg, NJ) in the required proportions of deionized water. 3

Isotherm Measurements and Langmuir-Blodgett Film Deposition. All isotherm measurements and LB depositions were conducted on a commercial KSV 5000 (Helsinki, Finland) Langmuir trough. The procedures followed those described elsewhere(l). An acidic sub-phase was chosen since this condition leads to rapid hydrolysis of the triethoxy head-group to -Si(OH) groups but relatively slow formation of Si-O-Si crosslinks(5). A 45 uL aliquot of 2 mg.ml" OTE in chloroform/methanol (95/5 v/v) was spread drop-wise onto a freshly aspirated surface of the sub-phase. The solution was allowed to stand for -40 min. in order to allow complete solvent evaporation and subsequent hydrolysis and polymerization of the triethoxy head-group. A 30 min. standing time allows complete evaporation of solvent and headgroup crosslinking at - 0 mN nr* barrier pressure. Considerably longer standing times do not change the isotherm characteristics in any noticeable way. The sub-phase temperature during the isotherm run was held constant between 21 and 22 °C to an accuracy of ±0.1 °C. Multiple runs were made in order to ensure isotherm reproducibility and accuracy and the actual LB monolayer depositions were performed under identical conditions. Prior to monolayer deposition, silicon substrates were cleaned and oxidized using a combination of chemical (immersion in peroxysulfuric acid at -110 °C for 10-15 min.) and photochemical (UV/ozone treatment; cleaning unit purchased from Boekle Industries, Philadelphia, PA) methods. (4) Mica substrates were freshly cleaved and then rendered highly hydrophilic by plasma treatment as described earlier.(l) The depositions were carried out by the upward drawing method at a constant substrate withdrawal rate of 5 mm min.~l with surface pressures (IT) of 20 mN nr . The final LBfilmswere obtained by curing at -100 °C for -2 hrs in a vacuum oven. Allfilmswere stored in a dry atmosphere between use. 3

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Contact angle measurements. Advancing and receding contact angle measurements were conducted by the sessile drop and captive drop methods using a Rame-Hart Model 100 contact angle goniometer in which the chamber was maintained at 21.0 ±

In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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0.5°C and saturated with the test-liquid vapor. Details can be found elsewhere.(4) Advancing angles of decane, dodecane, tetradecane, and hexadecane were used to assess the critical surface tension (y ) values using the standard extrapolation method c

of Zisman.(6)

Single-Wavelength Ellipsometric Measurements. Ellipsometry measurements nm; 70° angle of incidence) were performed using a null-ellipsometer (Rudolph AutoEL-II, Fairfield, NJ). The experimental and analyses procedures are only summarized below since full details can be found elsewhere.(4) The measurement protocol for each sample involved the sequential measurement of the polarization parameters(7), A and *F, at three arbitrarily chosen spots on each sample immediately following substrate pre-cleaning and at variable times after film transfer. The overall, film thickness errors, in terms of sample-to-sample variations, are within ±1 A. Thickness were calculated from A and *F using a model of parallel, homogeneous layers with sharp, planar interfaces. The optical functions (refractive index or dielectric constant) of the initial SiCtySi substrates were derived from the A and *F values of bare substrates using a pseudo-two-medium [Si(>2-Si]/air model and subsequently applied to film thickness calculations using a three-medium air/RSiO / Si(>2-Si model in which the ODS film was treated as optically anisotropic in order to accountrigorouslyfor the effects of alkyl chain orientation.


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Transmission Infrared Spectroscopy. Infrared spectra were collected in the transmission mode using a Fourier transform spectrometer (Bomem Model MB-100, Quebec , Canada) operating at 2 cm" resolution with an unpolarized beam striking the sample at normal incidence. Details can be found elsewhere.(4) All spectra are reported as -log(T/T ), where T and T are the emission power spectra of each sample and its corresponding clean substrate obtained prior to deposition, respectively. 1

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Atomic Force Microscopy (AFM) Measurements. AFM measurements were conducted in air in a clean, laboratory environment using a commercial Topometrix (Model TMX-2010 ) instrument with a nominal spring constant of -0.12 N nr . The integrated perimeter, Si3N4 probe tips were obtained from Digital Instruments. Measurements were performed on ODS films deposited at 20 mN n r onto mica substrates, chosen to provide large-scale flat terraces for optimum AFM characterization. Because of the highflatnessand comparable water wettability of the mica and Si02/Si substrates it is expected that highly comparable ODS films are formed.(4). Images were collected for low magnifications (50 um x 50 um) in a constant force mode. 1

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Results and Discussion Langmuir Isotherms. A representative compression isotherm obtained at -21 °C is shown in Figure 1. Three distinct regions in the isotherm are easily distinguished: I.) a flat portion with the average area per molecule, Am, ranging from -45 to -24 molecule" at n < 0.3 mN nr , II.) a rapidly increasing portion with 24>A,n>17 A^ 1

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molecule" and III.) a region of ultimate film collapse for II > 50mN nr*. These data are in excellent agreement with a previously reported isotherm collected under similar conditions(l) and are also in general qualitative agreement with isotherms reported(8,9) for Langmuir films prepared from the identical chain-length silanes, CH (CH2)i7NHCOCONHSi(OC H5)3 and CH (CH )i7SiCl (OTS), under conditions of sub-phase pH and temperature similar to the present experiments. Several salient features of the phase behavior of the present OTE Langmuir films can be derived from the above data. First, the observation of the steep rise in region I indicates that upon compression the monolayer structure undergoes only a single significant phase transition before collapse. Based on Am -24 A^molecule" at the transition, (to the left of the dashed line in Fig. 1), it appears that a liquidcondensed (LC) phase structure forms. Drawing on an analogy with typical Cig alkyl chain surfactant behavior,(10) the absence of an intermediate liquid-expanded (LE) phase indicates that the sub-phase temperature of -20 °C is well below the triple point temperature [the simultaneous LC, LE and gas (G) co-existence point] above which the LE phase can exist. This conclusion is strongly supported by the recent observation that for analogous ODS monolayers self-assembled on Si02/Si substrates(4,ll) a critical temperature (T ) of 28 (±5)°C exists below which no general LC phase character is observed in the ODS film. Further support is obtained from grazing incidence X-ray diffraction measurements of ODS Langmuir monolayers at the air/water interface(9) which show that the monolayer structure in region II of the isotherm consists of alkyl chains in a hexagonal arrangement, although with short correlation lengths. Second, the observation of no appreciable rise in surface pressure at Am -24 A molecule suggests that at Am >24 A molecule" , the monolayer is composed of structurally uncorrelated, polymerized islands randomly placed on the sub-phase surface. Assuming the existence of only the G phase between the domains, the average degree of polymerization is estimated to be -45-60, and consequently, an average size of a condensed domain would be -200-270 A. This value is lower than the size of-700 A estimated from the reported degree of polymerization equal to 155 for ODS Langmuir monolayers prepared using OTS precursor molecules(9). The above inference is consistent with the fact that the overall polymerization rate of the Si(OC H ) head-group of OTE is significantly slower than the -SiCl group of OTS molecules(12). The structure of these polymers must be quasi-two dimensional since the formation of a single linear oligomer of such a large degree of polymerization is difficult owing to the steric constrains on the Si-0-Si bond-angles and distances imposed by the pendant alkyl chain of 4.5-5.0 A diameter(13). It is quite plausible, however, that the G phase domains themselves are not single, polymerized oligomers but are composed of loosely correlated aggregations of cyclic trimers or tetramers. Extrapolation of the isotherm curve in region II to zero surface pressure yields a value of Amo ~22 A molecule" . This value falls in the range of 20-25 A molecule" which has been reported for single-chain w-alkanoic acid Langmuir monolayers(14) for which a film structure composed of closely spaced, highly organized islands has been observed. Finally, the high collapse pressure of -50 mN m* in region III of Fig. 1 indicates the appearance of a significantly enhanced stability in these ODS monolayers compared to typical single alkyl chain surfactant layers(15) where typical collapse


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pressures of -30-40 mN.m-1 are observed. This stability is consistent with the presence of intermolecular cross-linking or polymerization in the present ODS monolayers. Transferred LB films on Si0 /Si Substrates. Monolayer films were prepared at n = 20 mN.nr and a transfer ratio of-1 was obtained in all depositions. The films always emerged completely dry (autophobic)fromthe sub-phase during the pull-up process. Wetting measurements revealed a water contact angle of 113(±3)°, in quantitative agreement with that reported previously for mica(l) and for a selfassembled ODS monolayer on Si02/Si.(4) Hydrocarbon wetting measurements led to y = 20.6 (±0.5) mN nr , in good agreement with the value of 20.5 mN n r reported for self-assembled ODS films(4,l 1). Ellipsometry measurements led to afilmthickness value of 24.2(±1) A. A value of-22 A is estimatedfromthe isotherm intercept, Amo - 22 A^ molecule" , by using a theoretical maximum film coverage of 5.42 chains nm"2 together with a 26.2 A maximum chain extension.(16) On the latter basis, the ellipsometric thickness of 24.2 A corresponds to an equivalent coverage of 92%. The latter value is in excellent agreement with that of 87-95% obtained for self-assembled ODS monolayers. (4) Figure 2 shows the IRS spectrum in the C-H stretching modefrequencyregion. The spectrum is characterized by the presence of three distinct peaks at -2848, -2916.1, and -2957 cm" which are assigned(17) to the methylene C-H symmetric (d ) and antisymmetric (d") stretching modes and the methyl C-H antisymmetric stretching modes, respectively. Based on previous correlations,(18, 19) the values of the d" and d frequencieslead to the conclusion that an average alkyl chain in the ODS film is highly conformationally ordered, close to an ail-trans state, and exists in a crystalline like environment. 2


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Atomic Force Microscopy Data. Figure 3 shows an AFM image (50 um x 50 um) of an OTE LB film transferred onto a companion, plasma-treated mica substrate at 11=20 mN nr . The image clearly reveals a large number of interconnected domains or islands of distorted circular shapes with smooth edges. The average domain sizes, ignoring the bridge regions, occur in a remarkably narrow distribution of-12.5 ±1.2 um and the height difference between the inter-domain boundary region and the domain surfaces is ~12±3 A. (20) The observation of distorted shapes of closely spaced, often touching domains is entirely consistent with the crystalline-like organization of chains shown by the IRS data. We note that the estimated 12 A domain height is significantly lower than the ellipsometrically determined film thickness of -24 A. This difference is consistent with a film structure in which islands of conformationally ordered all-trans extended chains are separated by boundary regions of disordered chains at reduced densities (G phase). This mixed morphology is reminiscent of the LC-G phase co-existence which obtains for equilibrium Langmuir films at arbitrary surface-pressures below the LC-LE-G triple point temperature(21). 1

Thermally Induced Structural Changes. The previous sections show that the LB films transferred at n=20 mN.m" are densely-packed with their alkyl chains in nearly ail-trans conformational sequences. It would be expected that such a dense structure 2


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Figure 1. Surface pressure vs average area per molecule for an octadecylsiloxane film at the air-water interface with an acidic sub-phase at 21 °C.

Figure 2. A representative transmission infrared spectrum in the C-H stretching region of an octadecylsiloxane Langmuir-Blodgett film withdrawn at a surface pressure 20 mN.m" onto an oxidized silicon substrate. 2



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Figure 3. An atomic force microscopy image (50 um scale) of a LangmuirBlodgett film of an octadecylsiloxane Langmuir-Blodgett film withdrawn at a surface pressure of 20 mN.m" onto mica substrates. 2


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would lead to a lack of response to thermal stress. Analysis of IRS spectra taken at room temperature and 160 °C reveal that the d and d~ peak positions shift upward by -2.6 and -4.9 cm~l, respectively, upon heating. This shift is accompanied by a slight but progressive increase in the peak-widths. While these changes are consistent with increasing gauche content in the film, the small magnitude of the changes are quite noteworthy when compared with reported changes observed in bulk A?-alkane crystals(22) and other monolayer alkyl chain assemblies of comparably high organizations. For example, for w-alkanethiolate monolayers on Au(lll) surfaces heated over the same temperature range,(23) the d and d" peak positions shift upward by -4 and 8 cm" , respectively. The relatively smaller shifts of the ODS films can be taken as evidence for a smaller available space within the LB chain assemblies for accommodation of increasing chain disorder as compared to the alkanethiolate/Au SAMs. +


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Conclusion The characterization of ODS Langmuirfilmstransferred to hydrophilic substrates at a high surface pressure of 20 mN.m" shows that a highly organized film structure is obtained which closely approaches the limit of LC Langmuir phases of w-alkyl surfactant films and is quite similar to that obtained from solution self-assembly methods. Atomic force microscopy images reveal an islanded structure of large domains which indicates that intermolecular polymerization at the air-water interface is a relatively slow process which does not significantly hinder formation of highly organized, dense structures. 2

Acknowledgment This work was supported in part by the National Science Foundation (Grant No. DMR-900-1270; for DLA and ANP).

References 1 2 3 4 5 6 7 8 9

Wood, J.; Sharma, R. Langmuir 1994, 10, 2307-2310. For example, see: A. Ulman, Introduction to Ultra-thin Organic Films: From Langmuir Blodgett to Self-Assembly; Academic, San Diego, CA 1991. Maoz, R. and Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-496. Parikh, A. N.; Allara, D. L.; BenAzouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577-7590. Devreux, F.; Boilot, J. P.; Chaput, F.; Lecomte, A. Phys. Rev. A 1990, 41, 69016909. Zisman, W. A. Adv. Chem. Ser. 1964, No. 43, 1-51. See, for example: Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland : Amsterdam, The Netherlands 1977. Ariga, K.; Okahata, Y. J. Am. Chem. Soc. 1989, 111, 5618-5622. Barton, S. W.; Goudot, A.; Rondelez, F. Langmuir 1991, 7, 1029.


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Kellner, B. M. J.; Muller-Landau, F.; Cadenhead, D. A. J. Colloid Interface Sc. 1978, 66, 3. Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992, 360, 719-721. Pluedemann, E. P. Silane Coupling Agents; Plenum: New York, 1990. Ulman, A.; Adv. Mater. 1990, 2, 573-582 See, for example: Deamer, D. W.; Meek, D. W.; Cornell, D. G. J. Lipid Res. 1967, 8, 255 Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. Dubois, L. H.; Nuzzo, R.G.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558569. (a) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85-116 (b) MacPhail, R. A.; Strauss, H. L.;Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1982, 88, 334-341 (c) Snyder, R. G.; Hsu, S.L.; Krimm, S. Spectrochim. Acta Part A 1978, 34, 395-406. (d) Hill, I. R.; Lewin, I. W. J. Chem. Phys. 1979, 70, 842-581. Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 51455150. The above images are in strong contrast to those obtained in independent AFM studies of self-assembled ODS films on SiO /Si substrates where polygonal islands are observed with several tens of nanometer lateral spans [Allara, D.L.; Parikh, A.N.; Coulman, D.; Rondelez, F. manuscript submitted for publication]. These contrasting observations suggest that pre-polymerization at the air-water interface leads to much larger LC domains than do self-assembly conditions. This difference most likely arises because of more extensive relaxation of domain structures at the air-water interface than directly on solid substrates. See, for example: Knobler, C. M. Science 1990, 249, 870-876. Snyder, R.G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M . J. Phys. Chem. 1986, 90, 5623-5630. (a) Nuzzo, R.G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767773 (b) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Electron Spec. Rel. Phenom. 1990, 54/55, 1143-1152. 2

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Octadecylsiloxane Monolayers - ACS Publications - American

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