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Langmuir-Blodgett Deposition of Octadecyl Methacrylate Monolayers on Glass and Their E-Beam Polymerization G. Mu¨ller* and C. Riedel Institute of Surface Modification, Permoserstrasse 15, D-04303 Leipzig, Leipzig, Germany Received October 11, 1995. In Final Form: February 5, 1996X π-A isotherms of octadecyl methacrylate (ODMA) were recorded in the range 10-42 °C to investigate the experimental parameters for LB deposition of ODMA monolayers onto hydrophilic glass substrates. In the range 10-20 °C two monolayer phase transitions were detected in the π-A isotherm. Monolayer stability and the onset of monolayer collapse were investigated using relaxation phenomena. Isotherm recording was supported by Brewster angle microscopy (BAM), by which different tilt-oriented textures were detected. For monolayers of ODMA deposited onto glass at 10 °C and 16 mN/m, identical textures were found by BAM at the subphase and at the substrate. E-beam polymerization (10 MeV) of the deposited monolayer in a nitrogen atmosphere was indirectly proved by the change in the resistance of the layer with respect to evaporation in vacuum, where the Brewster angle reflectivity of the layers and their edges were used as an indication. It is suggested that the loss of tilt orientation detected by BAM in the deposited monolayer and at the layer edge results from polymerization.
1. Introduction LB technology is commonly used to prepare layers with a highly ordered molecular arrangement. From the beginning of research in this field remarkable efforts were directed to the polymerizability of unsaturated long chain amphiphiles. The main subjects of investigation were unsaturated aliphatic acids and their derivatives such as amides and esters.1-4 For molecules with double bonds in the middle, for example oleic or elaidic acid, the ordering of the chains is restricted because of the kink in the molecular chain. Thus, mainly amphiphiles with the double bond near the polar head group (acrylates, vinyl esters, and derivatives5,6) or at the unpolar end of the aliphatic chain have been investigated. Esters of acrylic and methacrylate acid, above all octadecyl acrylate20 and octadecyl methacrylate,7 have been extensively investigated by Shibazaki et al. These authors found that due to the polymorphic behavior of the crystalline bulk phase the polymerization by UV or e-beam excitation takes place only in the hexagonal R-form whereas polymerization starting from the more densely packed β-form is inhibited because it is connected with an expansion. The preparation and polymerization of LB multilayers including the kinetics of polymerization was extensively described only for octadecyl acrylate but not for octadecyl methacrylate.5,6 Only a few details are reported about the variations permitted for monolayer formation of octadecyl methacrylate, which are smaller than those for the acrylate. We thus intended to investigate the film stability as well as the conditions for the transfer of the monomeric ester onto solids. Since π-A isotherms only partly reflect the microscopic properties of the monofilm, our recording of the common surface pressure-area * To whom correspondence should be addressed. Telephone: 49 0341 235 2339. Fax: 49 0341 235 2313. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Beredjick, N.; Burlant, W. J. Polym. Sci., Polym. Chem. Ed. 1970, 8, 2807. (2) Ackermann, R.; Inacker, O.; Ringsdorf, H. Kolloid Z. Z. Polym. 1971, 249, 1118. (3) Dubault, A.; Casagrande, C.; Veyssie, M. J. Phys. Chem. 1975, 79, 2254. (4) Hatada, M.; Nishii, M. J. Polym. Sci. 1977, 15, 927. (5) Shibasaki, Y.; Fukuda, K. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1979, 20, 427. (6) Fukuda, K.; Shiozawa, T. Thin Solid Films 1980, 68, 55. (7) Shibasaki, Y.; Nakahara, H.; Fukuda, K. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2387.
isotherms and evaluation of the transferred layers was supported by Brewster angle microscopy (BAM), which is now a common technique for direct visualization of phase transitions and polymorphism in monolayers.17,18 LB multilayers of acrylates can be prepared only on hydrophobe substrates. But the methods of hydrophobizing, for example by coating with molten ferric stearate, result in optical reference surfaces which are not sufficiently homogeneous for BAM. Thus, we used hydrophilic glass substrates treated with chromosulfuric acid and restricted our investigations to monolayers. 2. Experimental Section 2.1. Substances and Substrates. Octadecyl methacrylate (ODMA) (Merck-Schuchardt) 95%, stabilized with hydroquinone monomethyl ether, was used without further purification. Cyclic differential scanning calorimetry gave a melting point at 26.4 °C and a solidifying point at 14.9 °C. Chloroform, 99.9% HPLC grade (Sigma-Aldrich), was used as solvent. High-purity water, 18.2 MΩcm, was obtained from a Milli-Q-Plus water purification system (Millipore). Commercial float glass was cleaned by successive treatment with chromosulfuric acid and with highpurity water in an ultrasonic bath. 2.2. Apparatus. The Langmuir-Blodgett trough type KSV 3000 (LOT) was Teflon coated, thermostated, and equipped with two symmetrically moving hydrophilic barriers and a Wilhelmy balance. Filter paper was used as Wilhelmy plate. The Brewster angle microscope type BAM 1 plus (Nanotechnology, Go¨ttingen) was employed both to observe the air-water interface approximately 20 mm away from the dipped substrate and to evaluate the monolayer deposited on the substrate. A linear electron accelerator type ELEKTRONIKA with 10 MeV electron energy, 4 µs pulse duration, 50 Hz pulse frequency, and 250 mA mean beam current was used to irradiate the sample.
3. Results 3.1. Isotherms and Film Stability at the Subphase. To find a useful range of experimental conditions for the preparation of ordered monolayers at the aqueous subphase and for their subsequent transfer onto solid substrates, the π-A isotherms of ODMA on water were investigated in the range between 10 and 42 °C (Figure 1). The films were compressed at a constant rate of trough area decrease of approximately 15 cm2 per min, which corresponds to a reciprocal strain rate or to a time of observation of 10-30 min.8 The surface concentration of
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Figure 1. π-A isotherms of ODMA.
the spread amphiphile was uniformly 2 × 10-10 mol/cm2, corresponding to a mean molecular are of approximately 75 Å2. The mean molecular area is the mean subphase area accessible per molecule. Isotherm recording was simultaneously assisted by Brewster angle microscopy, with which different textures could be observed at the air-water interface depending on the experimental conditions. In general, the shapes of the π-A isotherms are typical of the coexistence of two-dimensional gas and liquid-expanded phases in the surface pressure range between 0 and 0.5 mN/m and of liquid-expanded and liquid-condensed states (according to Harkins’s classification9,10) up to a pressure of approximately 21 mN/m. For practical reasons and with respect to the stability behavior of the monolayer there are two temperature ranges, one above and one below 17 °C, approximately. Above this temperature the films are too instable for subsequent deposition. Below this temperature there is a small surface pressure range above 21 mN/m where a stable highly ordered solid state exists, but the occurrence of the undesirable monolayer collapse begins already slightly above 25 mN/m, apparently depending on vibrations. To keep this small pressure gap useful for preparing stable films of high order, the upper limit of a practical working temperature seems to be about 17 °C. In the range below 17 °C, two kinks, at approximately 10 and 21 mN/m, were found in the π-A isotherms. According to the generalized phase diagram for long chain acids and esters and to the particular phase diagram of ethyl eicosanoate presented and discussed by Bibo et al.,15 these kinks might be attributed to a first-order and a second-order transition, respectively. Although not being discussed there in detail, similarly shaped isotherms including these kinks can be found in the figures of a paper on octadecyl acrylate.11 Although scarcely visible in our normal π-A plots, these kinks could be more clearly detected by plotting the compressibility versus the surface pressure, where the compressibility is defined as c ) -(1/ A)(∂A/∂π). The kink near 10 mN/m occurred as a peaklike increase in the compressibility-surface pressure plot with (8) Kato, T. Langmuir 1990, 6, 870. (9) Harkins, W. D.; Young, T. F.; Boyd, E. J. Chem. Phys. 1940, 8, 954. (10) Hann, R. A. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990; Chapter 2. (11) Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid Films 1988, 160, 43.
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Figure 2. Surface pressure relaxation of ODMA at 10 °C: ascending sections of curve, compression phases; descending sections, surface pressure relaxation with barriers stopped.
a starting point at π ) 10 mN/m and an end point at π ) 15 mN/m. The peak height above the average level (approximately 0.01 m/mN) in that surface pressure region ranged from 0.005 at 10 °C to 0.01 m/mN at 16 °C, respectively. A nearly identical compressibility increase was found for compressing as well as for expanding (decompressing) the film, with a delay of ∆π ≈ 1 mN/m depending on the direction of that operation (hysteresis). The second kink near 21 mN/m, and 18 Å2/molecule occurred as a sharp decrease of compressibility. At this maximum packing density only a very small increase of surface pressure is allowed before the monolayer collapse begins. At temperatures above 17 °C and up to 25 °C only the lower transition is detectable as a compressibility peak and the detectability of the upper transition gradually vanishes. To further improve information, the monolayer stability was directly investigated between 10 and 20 °C using two procedures. After successive monolayer compression steps of 0.5-1 mN/m every 2 min, either the barrier movement was stopped, allowing the surface pressure to relax, (∂π/ ∂t)A,T, or the molecular area reduction rate was determined with the surface pressure kept constant, (∂A/∂t)π,A. Figure 2 presents the dependence of (∂π/∂t)A,T on surface pressure for 10 °C. From these (∂π/∂t)A,T functions surface pressure relaxation times of approximately 30 s were estimated for surface pressures below 23 mN/m, approximately. Above this surface pressure, monolayer collapse starts and thus changes the relaxation characteristics. The (∂A/∂t)π,T data were collected 5 min after the onset of the automatic pressure control, with the barrier movement stopped. At 10-15 °C the onset of monolayer collapse is indicated by a sharp increase of the trough area reduction rate. At 17.5 °C this rate increases more continuously, beginning already at a lower surface pressure (Figure 3). The development of the monolayer structure observable by Brewster angle microscopy is illustrated in Figure 4. Circular domains surrounded by uncovered areas appeared due to self-organization even if the amphiphile was spread to mean molecular areas as high as 75 Å2 and the surface pressure did not exceed 0.05 mN/m. This range of the surface pressure-area isotherm is commonly referred to as the coexistence of the two-dimensional gaseous state and the liquid-expanded state, representing the coexistence of matter with very low and with already
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Figure 3. Molecular area reduction rate determined 5 min after compression stop and onset of surface pressure control.
moderately high density (texture type 1, Figure 4a).12 Also at higher temperature, for example at 25-30 °C, similar domains appear which are more homogeneous in size. Upon increasing the surface pressure the residual space on the surface is fully covered by domains (0.01-0.4 mm in diameter) with nearly hexagonal shaping and internal cakelike texture (texture type 2, Figure 4b), similar to those described by Overbeck for methyl arachidate.13 Molecules having equal azimuthal tilt orientation or equal bond orientation give rise to subareas of different brightness. With a second polarizer (analyzer) in the reflected beam, domain sectors show brightness inversion for different analyzer angles. Upon further increase of film compression beyond 10 mN/m, these hexagonal domains change to a pattern commonly called mosaic texture14 with kinked boundaries (Figure 4c). In concordance with the basic width of the compressibility peak mentioned above, i.e. between 10 and 15 mN/m, this change of pattern due to phase transfer could be observed by BAM, too, and is stepwise shown in Figure 5a-e. In order to keep the monolayer spot to be observed in the limited area of imaging of the microscope, the compression rate was, of course, necessarily too high to ensure equilibrium conditions for the phase transition. Immediately before the total loss of contrast upon further monolayer compression, i.e. before the transition near 21 mN/m, the mosaic texture changes to distinctive elongated domains with their longer axes oriented perpendicular to the direction of compression (texture type 4, Figure 4d). These textures resemble those described for fatty acids.16-18 Above a pressure of 21 mN/m the solid state is approached where the molecules are standing almost upright or are only slightly tilted. The BAM image of the monolayer appears optically homogeneous within the resolution power of the microscope (approximately 10 µm), see Figure 4e. Due to the limited laser power intensity, variations caused by regions of equal bond orientation could not be observed. Depending on the film (12) Katholy, S. Workshop, University of Potsdam, May 1995. (13) Overbeck, G. A. Thesis, University of Go¨ttingen, 1993. (14) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Langmuir 1993, 9, 555. (15) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (16) Rivie`re, S.; He´non, S.; Meunier, J.; Schwartz, D. K.; Tsao, M.W.; Knobler, C. M. J. Chem. Phys. 1994, 101, 10045. (17) Overbeck, G. A.; Mo¨bius, D. J. Phys. Chem. 1993, 97, 7999. (18) Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1993, 97, 8849.
Figure 4. BAM of ODMA monofilms at the subphase at 10 °C; gas phase domains, type 1, 0.1 mN/m: b, cakelike texture, type 2, 8 mN/m; c, mosaic texture, type 3, 16 mN/m; d, compression texture, type 4, 20 mN/m; e, uniform film with spots of collapse, 24.5 mN/m.
history, in particular on the spreading process and mechanical vibrations, small spots begin to occur within the monolayer area due to locally restricted monolayer collapse.19
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Figure 5. BAM images of phase transfer in ODMA at 10 °C.
In particular at the lower temperatures (10-12 °C) optical uniformity of the film over the total trough area could be reached only after relatively long times of about (19) Lu, Z.; Nakahara, H. Chem. Lett. 1994, 2005. (20) Shibazaki, Y.; Nakahara, H.; Fukuda, K. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2947.
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2-3 h. Even within the laser spot, 600 by 800 µm2, different texture types could be observed at the same time. This is in agreement with reports of other authors who observed that phase transitions may take hours.16 3.2. Monolayer Deposition on Glass and E-Beam Polymerization. We used float glass substrates hydrophilized by chromosulfuric acid and not precoated in order to get appropriate reference surfaces for the Brewster angle microscopy to follow. Onto these surfaces only monolayer transfer could be realized, with the target crossing the air-water interface in the upward direction. On the basis of our stability investigations, the monolayers were transferred at 10 °C and using dipping speeds from 0.5 to 5 mm/min. With the transfer surface pressure ranging from 10 to 20 mN/m the transfer rates obtained from trough area measurements and considering the residual instability were uniformly 1.02 ( 0.01. Above 22 mN/m the transfer rate apparently increased significantly to 1.10 due to the superposition of the beginning monolayer collapse. Since the transfer rates are nearly the same and equal unity at 10 and 16 mN/m and since the deposition temperature is low, only small changes of the monolayer structure should be expected during deposition. Actually, the investigation of the deposited monolayer by BAM showed the same typical textures as previously recognized at the water subphase (texture types 2 and 3, Figure 6a and b). Although for the deposited monolayer the contrast in the images is less than that for the monolayer at the subphase because of the substrate inhomogeneity, light scattering, and a smaller difference of refractive index, the conclusion could be drawn that in this surface pressure range the monolayer only slightly changed its structure during deposition. For deposition pressures less than 15 mN/m both the texture and the contrast in the BAM image obtained with polarized light remained unchanged, suggesting that the tilt orientational structure in the domains is at least qualitatively the same at the aqueous subphase and at the substrate.13 For deposition at surface pressures above 21 mN/m, Brewster angle microscopy indicated monolayer homogeneity at the subphase as well as at the substrate. The border between the uncoated and coated area is clearly visible as a sharp edge due to the different refraction indexes of the substrate and the monolayer. By use of the analyzer, tilt orientational textures of type 2 are visible not only in the inner part of the covered area but also immediately at its edge toward the uncovered surface. For polymerization experiments, hydrophilic float glass targets were simultaneously coated under equal conditions. Some of them were irradiated with 10-MeV electrons of a linear accelerator in a nitrogen atmosphere at 25 °C. The dose applied was 20 kGy. As indirect proof of the polymerization of octadecyl acrylate multilayers, the decrease of IR absorption (CdC vibration band at 815 cm-1) has been successfully used by several authors.7,8 Since for monolayers this is difficult because of the very small absorption, we have tried to investigate the potential of BAM and other indirect methods. For comparison, the nonirradiated and irradiated substrates were inserted into a vacuum chamber at 10-4 mbar and 25 °C. On nonirradiated substrates the above-mentioned significant reflectivity difference caused by deposition could no longer be recognized after 20 min of vacuum treatment. The typical edge at the borderline of deposition totally vanished due to sublimation of the monolayer of monomeric ODMA. In contrast to this, the monolayer edges on irradiated substrates were clearly visible even after more than 3 h of the same vacuum treatment and after subsequent storage in air at room temperature for more than a week. Although in some cases the contrast was somewhat
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copy showed that the collapse patterns in the deposited layer were very similar to those at the subphase. In particular, we observed that the initial appearance of collapse was frequently concentrated along the borderlines between subareas of domains of the same or of different textures types. From this the question arises whether these zones of collapsed and thus mechanically more flexible material hinder the propagation of surface pressure to the enclosed or neighboring areas and therefore delay the phase transitions. We therefore assume that in deposited matter the disorder at least partly prevents polymerization. In the region of disorder, the locally increased vapor pressure seems to become the origin of lines of monolayer defects. Subsequent local evaporation as a natural process or accelerated by vacuum treatment seems to form narrow trenches. Those trenches investigated by AFM had a depth of nearly 3 nm, which is the approximate length of the stretched molecular chain of ODMA.
Figure 6. BAM of ODMA monolayers on float glass: a, structure type 2, deposited at 8 mN/m; b, structure type 3, deposited at 16 mN/m; c, edge of layer, deposited at 16 mN/m and irradiated.
smoothed at the edges, the BAM reflectivity difference remained the same. In all cases where the transfer was performed at lower surface pressures, the texture of the monolayer underwent remarkable changes. After irradiation only the borderlines of the transferred domains remained whereas the internal tilt orientational textures, which were clearly visible in the nonirradiated layer always vanished (Figure 6c). The disappearance of these textures might be attributed to tilt orientational changes in the layer due to polymerization. To verify this explanation further, specific atomic force microscopy (AFM) investigations of the monolayer on the substrate are necessary. Some interesting observations were made in partitions with partially collapsed monolayers. Brewster angle micros-
4. Conclusions We showed that for ODMA there is only a very small temperature and surface pressure range in which highly ordered and sufficiently stable monolayers can be produced at an aqueous subphase. At the necesarily low temperatures (less than about 17 °C) which can be applied, the phase transitions commonly occuring in the monolayer at the subphase proceed very slowly. Thus, in the range of lower surface pressures different phases commonly coexist in macroscopic areas comparable with that of the trough and become homogeneous only after a long relaxation time. When, after sufficient compression, the monolayer appears homogeneous under BAM, not too much time should lapse before starting film deposition to prevent monolayer collapse, the latter being more accelerated when the higher surface pressure of the upper transition (21 mN/m) is exceeded. As monolayer collapse is hardly detectable by surface pressure measurement alone, the Brewster angle microscope may serve as a useful and often necessary tool for monitoring. In the temperature range up to 17 °C the monolayers are sufficiently stable to be transferred apparently without drastic structural changes. This might be deduced from the comparison of textures found by Brewster angle microscopic visualization of monolayers before and after deposition. Of course, because of the limited number of experiments, structural changes cannot be totally excluded. Because of this apparent conservation of structure, several e-beam polymerization experiments were made with monomeric monolayers transferred not only at the highest possible surface pressure but also at lower ones. Thereby from conservation of contrast (obtained by BAM) on both sides of irradiated monolayer edges even after vacuum treatment, it was deduced that polymerization appeared. The vanishing of texturing within the greater domains after irradiation encourages us to suppose that polymerization is connected to changing the azimuthal tilt orientation. Polymerization of monolayers deposited at pressures above about 21 mN/m could only be proved by the conservation of edge contrast. Because of the intrinsic homogeneity of highly compressed monolayers, the usefulness of BAM to characterize them seems to be restricted only to the detection of defects amplified by increased evaporation due to local collapse. LA950858F
