Langmuir 1999, 15, 631-633
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Ultrathin Films of Polymerized Smectic Liquid Crystals. A Study of the Polymerization Process M. Bardosova,†,‡,§ P. Hodge,† H. Matsuda,§ F. Nakanishi,§ and R. H. Tredgold*,† Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K., Slovak Academy of Sciences, Bratislava, Slovak Republic, and National Institute of Materials and Chemical Research, Tsukuba, Japan Received February 5, 1998. In Final Form: October 6, 1998 We have studied the photopolymerization of an amphiphilic compound containing a biphenyl group which has, at one end, a fumarate propyl ester and, at the other, a vinyl group. We have photopolymerized the compound when it was spread as a monolayer at the air/water interface and have then assembled it into multilayer structures by the Langmuir-Blodgett technique. We have also formed multilayers of the monomers and then photopolymerized them. Both kinds of films have been characterized by X-ray diffraction, by polarizing microscopy, and by UV absorption spectroscopy. When monomers are assembled into a layer structure and are then irradiated with UV light, they will only polymerize when in the smectic phase, which can be produced by heating the film to a temperature above 54 °C, but when the multilayer consists of two-dimensional crystals, it will not polymerize. Because polymerization can take place at the air/water interface, we think it likely that the polymerization process takes place within layers rather than between layers.
Introduction 1
In 1972 Cemel et al. showed that it was possible to polymerize Langmuir-Blodgett (LB) multilayer films of vinyl stearate by irradiating the films with γ radiation from 60Co. Since then, a variety of amphiphilic monomers have been assembled by the LB technique and have then been polymerized by irradiation by UV light or by the e-beam method. Many of the papers describing this work have been reviewed in a book by Tredgold.2 However, one particularly interesting paper on this topic was published by Petty et al.3 after the appearance of this book. They polymerized films of ω-tricosenoic acid alternated with 1-docosylamine and also Y layers of ω-tricosenoic acid using UV light. They then dissolved the films and examined the resultant solution using gel permeation chromatography. They were thus able to show that true polymerization had taken place in both cases. A few cases of photopolymerization being carried out at the air/water interface and the resultant monolayers subsequently being assembled by the LB technique have also been reported. See, for example, Caruso et al.4 However, we are not aware of any reported case of a single compound being both first polymerized and then assembled by the LB technique and also being both first assembled and then polymerized. We thus report the use of both processes using the same compound for the first time. One of the principal difficulties which appears if LB films are polymerized arises from the fact that most of the monomers employed exist in a phase wherein the long axes of the molecules tilt with respect to the normal to the plane of the film. The direction of the tilt varies over the †
University of Manchester. ‡ Slovak Academy of Sciences. § National Institute of Materials and Chemical Research. (1) Cemel, A.; Fort, T.; Lando, J. B. J. Polym. Sci., Polym. Chem. Ed. 1972, 10, 2061. (2) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: Cambridge, U.K., 1994; Chapter 5. (3) Petty, M.; Tsibouklis, M.; Song, Y. P.; Yarwood, J.; Petty, M. C.; Feast, W. J. J. Mater. Chem. 1992, 2, 87. (4) Caruso, F.; Grieser, F.; Thistlethwait, P. J.; Furlong, D. N. Macromolecules 1994, 27, 77.
film so that a large number of small two-dimensional crystallites are produced. This structure is usually emphasized by the process of polymerization. In an effort to avoid this effect and produce a homeotropic film, we have studied the polymerization of multilayers when they are in the smectic A phase.5,6 We have been able to form multilayers both by the LB method and by thermal evaporation in vacuo using compound 1, shown in Figure 1, and it is the properties of films of this compound which we wish to discuss in this paper. It will be noted that the terminal double bond at one end of the molecule and the fumarate group at the other are both potentially capable of polymerization. Our earlier studies5,6 have shown that, at least to some extent, both groups must take part in this process. We have heated multilayers (formed either by the LB method or by thermal evaporation in vacuo) to a temperature above 54 °C, the temperature above which the smectic phase exists, and have exposed them to UV light. Such films become largely insoluble in THF, and we thus conclude that both polymerization and cross-linking have taken place. However, it was not possible to ascertain whether the polymerization and cross-linking take place within monolayers or between them. To clarify this point, we have also polymerized the compound at the air/water interface and then assembled multilayers by the LB method. We then compared the properties of multilayers which were first assembled and then polymerized in the smectic phase with those which were polymerized at the air/water interface and were afterward assembled. Experimental Section The details of the methods employed to form multilayers of monomers and to polymerize them in a layer structure are given in refs 5 and 6. A commercial LAUDA trough with one movable and one immovable barrier was employed to form the films which were (5) Bardosova, M.; Clarke, I.; Hodge, P.; Tredgold, R. H.; Woolley, M. Chem. Commun. 1996, 587. (6) Bardosova, M.; Clarke, I.; Hodge, P.; Tredgold, R. H.; Woolley, M. Thin Solid Films 1997, 300, 234.
10.1021/la9801460 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/23/1998
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Bardosova et al.
Figure 1. Structure of the compound discussed in this paper.
Figure 2. Isotherms of the compound taken over pure water. polymerized at the air/water interface. The subphase was deionized in doubly distilled water. Deposition was performed at 22 mN m-1 with speed of 4 mm min-1 and a waiting time between strokes of 5 min. To polymerize these films, the monolayer was held at 22 mN m-1 while being irradiated using a UV lamp. During irradiation, the film was subjected to heat from the UV source which could cause substantial changes in monolayer behavior. To investigate this, we studied isotherms at different subphase temperatures (Figure 2) An increase of the subphase temperature leads to film collapse at lower pressures. To avoid overheating, the total time of UV irradiation (2 min) was divided into three steps (30 s + 30 s + 1 min), during which time the initial area decreased by about 30%. Films deposited after this procedure were clear and of good quality when observed under a polarizing microscope. Films change color during stage rotation in a uniform manner, indicating that there is some orientation due to the dipping process but that there is no two-dimensional crystalline structure. LB deposition was Y-type with deposition ratios which were between 0.95 and 1.05 in both down and upward directions. The films formed by both methods were characterized by polarizing optical microscopy, small-angle X-ray diffraction, and UV/vis spectroscopy. The X-ray measurements were obtained from a Raymax RX3D diffractometer using Cu KR radiation which corresponds to a wavelength of 0.1542 nm. The UV/vis spectral data were recorded on a Varian Cary1E spectrophotometer. The UV irradiation experiments were carried out using a Spectroline R51-F light source.
Figure 3. (a) X-ray diffractogram of newly deposited film formed from films irradiated at the air/water interface. (b) X-ray diffractogram of the same film stored for 1 month. (c) X-ray diffractogram of the sample after washing in dichloromethane.
Results
Figure 4. UV absorption spectra of a multilayer of the compound. The solid line corresponds to an unpolymerized film and the dotted line to a film first polymerized and then dipped.
We have already5,6 reported the properties of films deposited as layers of monomers and subsequently polymerized. Films heated to above 54 °C and polymerized exhibited a thickness for each monolayer of 3.1 nm. Films held for 1 week at room temperature and irradiated by UV light did not appear to polymerize and exhibited a thickness for each monolayer of 2.9 nm, suggesting that the molecules had a tilt angle of about 30°. We examined films polymerized at the air/water interface by small-angle X-diffraction and found that they have a relatively high degree of order (Figure 3a). In the case of freshly deposited films, a peak corresponding to third Bragg order of diffraction from a structure that repeats every four layers was present. Storage for 1 month at room temperature caused a reorganization of the multilayer (Figure 3b). However, even a sample which had been washed in dichloromethane still showed traces of layered structure when examined by the X-ray diffraction (Figure 3c). The main difference between films polymerized in the homeotropic state after LB deposition or deposition in vacuo and films polymerized at the air/ water interface and subsequently deposited onto a solid
substrate is that for the former a layer thickness of 3.1 nm is observed while for the latter a thickness 2.9 nm is obtained, which is the same as that for the nonpolymerized multilayered structure. In our earlier studies6 (Figure 2 of that reference), we showed that there was significant absorption at the place in the UV spectrum corresponding to the biphenyl group only in the case of films having a repeat distance of 2.9 nm but not when the repeat distance was 3.1 nm. We deduced from this result that the axis of the biphenyl group is normal to the plane of the film in the latter case but not in the former one. To make a further study of the difference between these two kinds of polymerized films, their UV/vis spectra were compared. For the UV studies films were deposited onto quartz substrates and their spectra recorded. Both samples were of approximately equal thickness (200 nm), as was shown by ellipsometry. Multilayers of samples polymerized in a trough were yellowish in color when observed in a direction nearly tangential to the plane of the film. The UV maxima positions (254, 265, and 284 nm; Figure 4) could be clearly distinguished in the case of an unpoly-
A Study of the Polymerization Process
merized multilayer. For films first polymerized and then dipped, the maxima coalesced into a broad peak whose absorbance values are 1.5-2 times bigger than corresponding values for nonpolymerized material. All of these maxima include one arising from the presence of a phenyl ring, and their increase indicates that the biphenyl group is tilted to the normal of the film. This model is quite different from the one we presented for films polymerized in the homeotropic phase in ref 6.
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of the material. However, it is clear that both the processes which we have studied (LB and then irradiation with UV light or irradiation followed by assembly by the LB method) produce a largely insoluble film, and thus a polymerized and cross-linked structure occurs. These results give support to the hypothesis that the polymerization process is within the individual monolayers rather than between monolayers. However, the structures produced differ in that one has a repeat distance of 3.1 nm and the other has a repeat distance of 2.9 nm.
Conclusions The results presented above indicate that polymerization involving the fumarate group and also polymerization involving the vinyl group require a layer structure to be effective but cannot take place in the solid crystalline form
Acknowledgment. The authors thank the Royal Society for financial support and the Canon Foundation for a fellowship (M.B.). LA9801460
