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Ultrathin Films of Smectic Liquid Crystals on Solid Substrates M. Woolley, R. H. Tredgold," and P. Hodge Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom Received August 5, 1994. In Final Form: October 21, 1994@ Two cyanobiphenyl materials and a variety of alkyl 4'-alkenoxybiphenyl-4-carboxylates with different chain lengths have been deposited on a wide variety of substrates by evaporation in vacuo and, where possible, by the Langmuir-Blodgett technique to form multilayer films. These films have been characterized by optical microscopy, low-angle X-ray dieaction, and, where appropriate, FTIR. The materials have also been studied by DSC and under the polarizing microscope using thicker layers pressed between two glass plates. All the materials studied exhibited at least one smectic phase. It was impossible to wet any of the substrates used by the cyanobiphenyl materials and thus the films formed from these materials degenerated into droplets. The two materials having an isopropylester termination and one of the materials having an ethyl ester termination formed smectic phases at or near room temperature when deposited by evaporation in vacuo. Two materials could also be deposited by the Langmuir-Blodgett technique, in one case forming a bilayer structure and in the other case forming a monolayer structure.
Introduction The layer structure observed in Langmuir-Blodgett (LB) films is in many ways similar to the structure of smectic liquid crystals, but the relationship between the two forms of matter has never been thoroughly investigated. Several groups of workers (for example Decher et a1.l) have studied materials which can be assembled into multilayers by the LB technique at room temperature but which appear to be in the smectic phase at substantially higher temperatures. Daniel et aL2 attempted to form LB films from those cyanobiphenyl materials which have been extensively studied in the smectic phase but reported that this was impossible. Recently we have studied the formation of multilayers of amphiphilic materials by thermal evaporation (TE) onto solid substrates (Ai-Adibet aL3and Jones et aL4). In these papers we showed that this technique can, in some cases, make it possible to form regular multilayer structures of amphiphilic materials which cannot be handled by the LB technique. As a long term objective we hope to be able to form ordered layer structures of polymerizable materials by thermal evaporation and to use them in microlithography either with irradiation by UV light or by the e-beam technique. If we are successful, there are obvious important technical applications of such a process. The tendency of TE films to form 2-D crystallites has already been shown by us3and could be avoided if it were possible to work with material in either the smectic A or B phases. Accordingly, we have obtained or synthesized a number of materials capable of existing in a smectic phase and have attempted to form ordered layers of these materials, usually about 100 nm thick, by the TE technique. Where possible, we have also formed and studied films of these materials by the LB technique. As will be seen below, some of the results which we have obtained are rather surprising. Abstract published in Advance A C S Abstracts, January 15, 1995. (1)Decher, G.;Sohling, U. Ber. Bunsenges. Phys. Chem. 1991,95, @
1538. ( 2 ) Daniel, M.F.; Lettington, 0. C.; Small, S. M. Thin Solid Films
1983. ~ . _ , 99. -_ - ,61. (3)Ali-Adib, Z.;Hodge, P.; Tredgold, R. H.; Woolley, M.; Pidduck, A. J. Thin Solid Films 1994,242, 157. (4) Jones, R.; Tredgold, R. H.; Ali-Adib, Z.; Dawes, A. P. L.; Hodge, P. Thin Solid Films 1991,200,375. ~~
Figure 1. Synthetic route to compounds 3-7.
Experimental Methods The cyanobiphenylmaterials used both in their original form and also as starting materials for other substances containing the biphenyl moiety were purchased from Merck, and the other substances used in the synthesis of the materials studied were purchased from Aldrich. The synthetic work did not involve any new chemistry and will not be discussed in detail here. A schematic diagram of the synthetic route t o the materials used, 3-7, is shown in Figure 1. The structures of the materials synthesized were checked by NMR and FTIR, and purity was assessed by elemental analysis. The LB trough employed has been described elsewhere,5and the methods employed to evaporate materials have been discussed However, we have improved on our in our previous original technique by using a Peltier effect device to either heat or cool the substrate to temperatures ranging from 50 "C above or below ambient. A built in thermocouple incorporated in the Peltier device and immediately below the substrate registered the temperature at this point. The materials were deposited on a variety of different substrateswhich are mentioned individually in the Experimental Results section of this paper. The films formed were characterized by a polarizing optical microscope fitted with a hot stage, low-angle X-ray diffraction, and, where appropriate,FTIR. The X-ray studies were carried out with a R a y " RX3D diffractometer using Cu Ka radiation, ( 5 ) Tredgold, R. H. Rep. Prog. Phys. 1987,50,1609.
0743-7463/95/2411-0683$09.00/00 1995 American Chemical Society
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684 Langmuir, Vol. 11, No. 2,1995
0-0-
CQH19
\ /
\ /
CN
Figure 2. Compounds 1 and 2. which corresponds to a wavelength of 0.1542 nm. "he infrared measurements were carried out using a Nicolet MX-1 spectrometer.
Experimental Results We started by investigating the two well-known cyanobiphenyl materials 1 and 2, shown in Figure 2,which change from the crystalline to the smectic A phase at 21.5 and 42.0 "Crespectively. Films of these materials were evaporated onto substrates maintained at a variety of temperatures in the range 20 to -10 "C. The substrates employed were clean hydrophilic glass, glass rendered hydrophobic by treatment with hexamethyldisilazane, fused quartz, silicon wafers bearing a thin oxide layer, freshly cleaved mica, and glass separately treated with the silanizing agents (3-cyanopropy1)triethoxysilaneand dimethyloctadecylchlorosilane. The rate of evaporation was about 20 n d m i n and the mean thickness, as measured by a commercialthickness monitor placed close to the substrate, was about 100 nm. In all these cases we showed by X-ray diffraction that a regular layer structure existed both above and below the transition temperature. However examination under the microscope showed that the films were not continuous but consisted of a large number of tiny droplets each having a typical diameter of 0.12 or 0.04 mm when silanized substrates were used. The appearance of such a film is shown in Figure 3. It is evident that these materials are unable to wet any of these substrates. This is a result which does not appear to have been previously reported. Olbrich et aL6studied cyanobiphenyl films depositedon various substrates using X-ray diffraction and, in a note added in proof, mentioned the appearance of droplets on their films. They appear, however, to believe that these droplets are superimposed on a uniform film rather than constitute the film, as we believe. Ocko' studied thin cyanobiphenyl films by X-ray diffraction but did not, apparently, look at his films through a microscope and thus would not have noticed the droplet phenomena. If we had simply relied on X-ray results we would not have noticed it. We put forward a tentative explanation of this result in the Discussion section of this paper. It seemed likely that a truly amphiphilic material would be more likely to wet a hydrophilic surface and we accordinglyembarked on a systematic study of amphiphilic materials the structures of which gave a reasonable hope of producing a smectic phase. Various alkyl 4'-(nalkenyloxy)biphenyl-4-carboxylateswith different chain lengths in the hydrophobic tails and with various ester head groups were synthesized, and those used are listed in Table 1. It was found that esters with larger head groups than isopropyl gave poor films with little structural order. The bulk liquid crystal phases were characterized by means of DSC and optical microscopy. Here comparison was made with the structures illustrated by Gray and Goodby.s The transition temperatures from both heating and cooling runs are shown in Table 2. The transition (6) Olbrich, E.; Marinov, 0.; Davidov, D. Phys. Rev. E. 1993, 48, 2713. (7)Ocko, B. M. Phys. Reu. Lett. 1990,64,2160. ( 8 ) Gray, G. W.; Goodby, J. W. G. Smectic Liquid Crystals; Leonard Hill: Glasgow and London, 1984; Plates 1-124.
Figure 3. Photomicrograph of a 500 nm thick layer of compound 1 evaporated a t room temperature on glass (magnification x 125, reproduced a t 80% of the original size). Table 1. Chemical Structures of Compounds 3-7
compd 3 4 5
n
R
3 6
Et
compd 6
Et
7
n 9 9
R Et iPr
'Pr temperatures on cooling are of most interest as the materials have then been through an annealing process. Ordered multilayers were also studied under the microscope using the heated stage, and the temperatures at which phase changes took place are also shown in Table 2. As will be seen, these transitions were near in temperature to those observed on cooling the bulk material. Compounds 5-7 offered the best opportunities for the direct deposition of films in the smectic phase by thermal evaporation since 5 has a smectic A phase down to 35 "C (and a smectic E phase almost to room temperature) and 6 has a smectic E phase almost to room temperature. In the case of 7, the E phase may extend downward to 10 "C, though it is.difficult to establish this fact with certainty. However, under these conditionsthese materials tend to re-evaporate and do not adhere to a solid substrate. Nevertheless it is possible to form good multilayers of these materials by TE at slightly lower temperatures and then to heat them to a temperature at which they are in a smectic phase. To illustrate this behavior, a freshly evaporated multilayer of 6 was illuminated by polarized light and a micrograph of this film is shown in Figure 5. An apparently polycrystalline structure is observed which probably corresponds to the 6
Langmuir, Vol. 11, No. 2, 1995 685
Ultrathin Films of Smectic Liquid Crystals
Table 2. Phase Transitions of Bulk Materials Obtained from DSC and Optical Microscopy and of Evaporated Films Obtained from Optical Microscopy phase transitions on cooling ("C) phase transitions on heating ("C) compd
K-I
method
K-SE
SE-SB
SE-SA
SB-SA
SA-I
I-SA
SA-SB
SA-SE
SB-SE
SE-K
I-K
DSC 102 121 123 122 120 105 optmic 122 123 121 120 film 120 119 DSC 85 91 105 100 87 82 4 optmic 85 103 103 83 film 87 86 DSC 66 58 35 23 5 optmic 62 59 37 film 58 50 DSC 68 73 83 99 91 80 70 30 6 optmic 73 85 93 94 85 73 30 film 75 73 DSC 59 49 41 34 10 7 optmic 58 47 40 35 film 48 46 a K denotes a crystalline phase and I denotes an isotropic phase. It should be noted that, in most cases, it is not possible to distinguish between the SEand K phase when studying the evaporated films by optical microscopy.
3
',! !
! ! ! !
0
-..*
.
e
I
2
4
6
8
10
Area per molecule (m2) Figure 4. Isotherms of compounds 4-7 taken at 25 "C over pure water.
smectic E phase. Heating to 76 "C produces a homogeneous structure (almost certainly smectic A) and cooling slowly to 71 "C reestablishes the smectic E structure. In order to study LB films of the materials 4-7, they were spread at the aidwater interface using ethyl acetate as a solvent. The isotherms obtained are shown in Figure 4. The two materials having an isopropyl termination (5 and 7) collapse at a pressure of about 15 mN m-l. The shoulder in the isotherm for compound 4 could possibly correspond to a phase change from a liquid-expanded to a liquid-condensed phase. We were unable to form Langmuir-Blodgett films of this material. Compound 5 exhibited a narrow "window" around a surface pressure of 13 mN m-l, whereY deposition was possible. Compound 6 could be deposited a t a pressure of 30 mN m-l with a deposition ratio of 1 on the upstroke and 0.3 on the down stroke. ComDound 7 denosited rather noorlv at aroiind 13 mN m-l and formed LB films in th6 Y mode. The layer structure of the evaporated and LB films was investigated by X-ray diffraction. A typical diffractogram, that of compound 5 stored for a week at room temperature, is shown in Figure 6. The layer spacings were determined from the positions of the 001 Bragg peaks and are listed in Table 3. The ~
Figure 5. Photomicrograph of a 115 nm thick layer of compound 6 evaporated onto a glass substrate a t -10 "C. Heating to 76 "C produces a homogenous structure and cooling slowly to 71 "C re-establishes a n apparently polycrystalline structure which we believe corresponds to the smectic E phase. The scratch in the layer was deliberately made in order to prove that there was still a film on the substrate in the homogeneous state (magnification x 125,reproduced at 80% of the original size).
table shows that the evaporated films of materials 4-6 exhibit a monolayer structure but evaporated films of 7 produce a bilayer structure. It is difficult to form good LB films of 4 and 5, and 6 produces a monolayer notwithstanding the fact that it appears to dip on both up and down strokes. Only 7 produces true Y layers with relative ease. It is interesting to note that Decher et al.' found
Woolley et al.
686 Langmuir, Vol. 11, No. 2, 1995
I
, 2.
6
4
8 10 B r a s Angle. 28. degreeo
12
14
Figure 6. X-ray diffraction pattern obtained from a thermally evaporated film of compound 5 stored for 1 week at room temperature. The first Bragg peak is not shown to scale. Table 3. Layer Spacings of Thermally Evaporated and LB Films method of layer s acing molecular length compd
4 5
6 6 7 7
deposition TE TE TE
LB TE LB
(I5
(A,
23.8 21.3 24.7 25.5 50.2 48.9
24.4 24.6 28.1 28.1 28.3 28.3
that LB films of the ethyl esters with C, and CSsaturated alkyl chains in the tail exhibit both monolayer and bilayer structures.2 Diffractograms from all films exhibited more than 5 orders of 001 Bragg peaks, which showed that the films were well-ordered.
Discussion Cyanobiphenyl liquid crystals are clearly unable to wet any of the many different surfaces that we studied. It is well-known that, in the smectic phase, these materials tend to exist in a structure which, in Langmuir-Blodgett terminology, would be described as a Y structure with very strong interpenetration. In other words, individual layers consist of equal numbers of upward- and downwardpointing molecules mixed in a more or less ordered structure. This effect is probably brought about by the interaction of the dipoles associated with the CN groups and also by the interaction of the induced dipoles associated with the biphenyl groups. Successivelayers are thus likely to be arranged in such a way that an upwardpointing dipole is more likely to be superimposed on another upward-pointing one rather than on a downwardpointing one. The interactions of the dipoles in successive layers will thus make an important contribution to the cohesive energy ofthe system. We assume that this energy
will always be larger than the interaction of the dipoles in the first layer with the induced dipoles in the substrate. If this is the case, then considering dipole interactions alone, it will always be more favorable energetically to build up piles of layers rather than to spread the material evenly over the substrate as is indeed observed. This postulated interaction of dipoles is an addition to the contribution to the cohesive energy arising from London forces and various other smaller effects. If these other contributions to the binding of the first layer to the substrate are greater than their contribution to interlayer binding, then the size of the dipole interactions will be critical in determining the nature of the stable configuration. A large dipole contribution will bring about the state of affairs which we have observed in the case of the cyanobiphenyl materials. However, as the dipole effect is reduced, the material should eventually form uniform layers on the substrate and wet it. The esters which we studied carry a smaller dipole than do the cyanobiphenyl materials. Furthermore by placing an ethyl or a propyl group in the material it should be possible to separate the dipoles in successive layers to some extent. Our experimental results confirm the predictions of this simple theory. All the ester materials that we have studied wet clean glass, and furthermore, the larger the group separating the dipoles in successive layers, the lower the transition temperatures between the various phases. We have attributed the phase changes observed in the thin films ofthe isopropyl materials, 5 and 7, to transitions from SAto SE,as they correspond to a change from a clear field of view to one similar to that shown in Figure 5. However, the temperatures a t which these changes take place are close to those at which these materials, in the bulk, change from I to SA. It is thus possible that, when these materials exist as thin films, their phase behavior changes and that these phase transitions represent a change from I to SC. This is a matter which still has to be resolved. The problem does not arise in the case of the ethyl-terminated material, 6. Unfortunately, if groups larger than propyl are employed, it is no longer possible to form LB films or even to form well-ordered evaporated films. It will be seen from the results reported above that even the materials terminated by the isopropyl group form poor LB films. We thus conclude that, at least for the materials studied, the isopropyl termination represents a transition. Materials having butyl terminations will not form LB films while materials having ethyl terminations only become liquid crystals well above room temperature. We are now studying evaporated layer structures formed from nonamphiphilic materials chosen to form smectic liquid crystals. Here the problems associated with dipoles do not arise. However using these materials it is not possible to form Langmuir-Blodgett films. Acknowledgment. We wish to thank the Engineering and Physical Sciences Research Council for financial support. LA9406214
