Langmuir 1994, lo, 3387-3389
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A Highly Ordered Structure in N-Octadecylacrylamide Langmuir-Blodgett Multilayers Tokuji Miyashita* and Takehiko Suwa Department of Molecular Chemistry and Engineering, Tohoku University, Aoba Aramaki, Aoba-ku, Sendai 980, Japan Received July 20, 1994. In Final Form: August 23, 1994@ FT-IR spectroscopic measurements (polarized transmission and reflection absorption spectra (RAS)) of N-octadecylacrylamide (ODA) LB films indicate that ODA molecules are oriented nearly perpendicular to the surface of the plate; moreover they are ordered in the plane along the dipping direction (in-plane order). That is, ODA LB films have a high biaxial orientation. A drastic dichroism in polarized FT-IR caused by the large anisotropic arrangement of ODAmoleculeswas observed,whereas that was not observed for the corresponding acrylate derivative.
Introduction Ordered Langmuir-Blodgett (LB) films have growing application in various fields such as microelectronics and biomimetic membranes, because it is possible to form the ultrathin films with highly ordered molecular array and with controlled thickness by LB technique.' We have attempted to fabricate uniform ultrathin polymer films having well-defined molecular orientation by the LB technique. In the previous work, we have showed that N-octadecylacrylamide and N-icosylacrylamide (ICA)form stable condensed monolayers and the LB multilayers; moreover, the ODA and ICA LB multilayers are polymerized by W irradiation yielding the uniform polymer LB multilayers, especially ODA LB multilayers can be polymerized completely.2 The polymerized ODA LB multilayers were insoluble in usual organic solvents and the possibility of application to a new type of high resolution resist was also i n d i ~ a t e d . ~An , ~ ordered molecular orientation is expected to be formed in ODA LB multilayers in relation to the quantitative polymerization. Many works on molecular orientation to the normal surface in LB films have been already appeared in the literature, where it is indicated that the alkyl tails of amphiphiles are oriented nearly perpendicular to the LB film plane and rotated around the axes, that is, uniaxial orientation to the z a x i ~ . ~As - ~far as we know, there are few reports on a biaxial orientation (in-plane order) in LB films.' In this paper, a highly ordered structure in ODA LB multilayers can be shown by FT-IR measurements (transmission and reflection absorption spectra (RAS)), especially, the polarized transmission spectra showed a drastic dichroism due to anisotropic orientation of ODA molecule to the dipping direction. Abstract published in Advance ACS Abstracts, September 15, 1994. (l)Kuhn, H.;Mobius, D.;Bucher, H. In Physical Methods of Chemutry; Weissberger, A., Rossiter, B. W., Eds.; Wiley-Interscience: New York, 1972; Vol. I, Part III-B, Chapter VII. (2) Miyaahita,T.; Sakaguchi, K.; Matauda, M. Polym. Commun. 1990, 31,461. (3) Miyashita, T.; Matauda, M. Thin Solid Films 1989,168,L47. (4) Miyaahita, T.;Yoshida, H.; Matsuda, M. Thin Solid Films 1987, 155,L11. (5) Takenaka,T.;Nogami,K.;Gutoh,H.;Gotoh,R.J. Colloidlnterface sci. 1971,35,395. (6) Kimura, F.;Umemura, J.; Takenaka, T. Langmuir 19862,96. ( 7 )Rahlt, J.F.;Burns, F. C.;Schlotter,N. E.; Swalen, J. D. J.Chem. @
Phys. 1983, 78,946. (8)Naselli, C.; Rabolt, J. F.; Swalen, J. D. J.Chem. Phys. 1986,82, 2136.
Results and Discussion A chloroform solution of ODA was spread onto a water surface. The surface pressure (n)-surface area (A) isomers, which were already studied clearly in the previous paper: show the formation of the stable condensed monolayer. The ODA condensed monolayer was transferred onto solid supports (CaF2 plate for transmission spectra and Au-evaporated glass slide for RAS) at 15mN/m with a transfer ratio of unity giving Y-type LB multilayers (20layers). The transmission spectrum and RAS of the ODA LB multilayers are shown in Figure 1. They are quite different from each other. The NH stretching (3300 cm-l), C-H symmetric stretching (2850 cm-l), C=O stretching (1653cm-l), C-C stretching (1622cm-'1, and CHZ scissoring (1471 cm-l) absorption bands were not observed in RAS in which the electric field vector is perpendicular to the metal surface, whereas those absorption bands appear strongly in a standard transmission spectrum. This indicates that the transition moments of those groups are parallel to the surface of solid supports. The NH,C-0 and C=C substituents are highly oriented parallel to each other and the long alkyl chain is standing up from the surface in the LB multilayers. If the long alkyl chain is completely perpendicular to the surface, the absorption assigned to the C-H asymmetric stretching (2953cm-') should not be observed in RAS. However, the vasCHz and 6NH (1543 cm-l) absorption bands can be observed in RAS, which indicates that the long alkyl chain is tilted regularly. Although the information on molecular orientation to the normal plane (uniaxial orientation) can be given by comparison between transmission spectra and RAS,the method is not useful to examine anisotropic orientation in plane (biaxial orientation). In order to examine the two-dimensional ordered structure in LB multilayers, we measured polarized FT-IR spectra of ODA LB multilayers. The monolayer on the water surface is transferred onto a CaFz plate by the conventional vertical dipping method. The transmission spectra are taken with the radiation parallel to the dipping direction of LB multilayers (p-
0743-746319412410-3387$04.50/00 1994 American Chemical Society
Letters
3380 Langmuir, Vol. 10, No. 10, 1994 Transmission
,
vNH
vc=o h vc=c
Fibn dipping dirsUon
Figure 3. A highly two-dimensionalordered structure in ODA LB multilayers proposed from FT-IR measurement.
Reflection Absorption
octadecylanylate
I
6NH
O=C I
CH =CH2 4000
3000 2000 Wavenumber (cm")
ID00
Figure 1. FT-IR spectra of ODA LB multilayers with 20 layers (upper, transmission; lower, reflection absorption spectra
(MI). Transmission
Y
C=Q Y
c=c
4ooo
1
3000
m
1000
Wavenumber ("1)
Figure 4. Polarized FT-IR spectra of OAc LB multilayers (upper, s-polarization;lower, p-polarization). 6 NH
Figure 2. Polarized FT-IR spectra of ODA LB multilayers (upper, S-polarization; lower, p-polarization).
polarization) and perpendicular to the direction (spolarization) (Figure 2). It is very interesting that the two spectra are drastically different from each other and the difference is similar to that between transmission spectra and M S . The large dichroism in p- and spolarization indicates that a highly ordered structure in plane (biaxial orientation) is also formed along the dipping direction in ODA LB multilayers. The YNH,YC-0, vC=C,
dCHz, and v,CH2 absorption bands appear strongly in s-polarization, indicating that the NH, C-0, and C=C groups are perpendicular to the dipping direction. From the results shown in Figure 1and the present dichroism (Figure 21, it is concluded that the NH, C=O,and C=C groups are organized parallel to the surface of the substrate and moreover oriented perpendicularly to the dipping direction (Figure 3). On the CH2 stretching vibration bands, the absorption intensities in symmetric and asymmetric modes are reversed in p- and s-polarization; moreover, the only asymmetric mode appears in RAS. These results mean that all the alkyl chains in ODA LB multilayers are packed with the same orientation (tilting with a constant angle). The biaxial molecular orientation shown in Figure 3 is proposed, where the C-C and C-0 groups are placed in the same plane and the alkyl chain bends at the bond of CH2 and NH. It can be said that the hydrogen bonding between the amide groups is a driving
Letters force for the molecular assembly. This statement is supported by the following results. Octadecylacrylate (OAc), being the corresponding acrylate derivative to ODA, gives a similarR-A isotherm with the ODA monolayer. The condensed OAc monolayer can be transferred onto a CaFz plate with a transfer ratio of unity. The polarized transmission spectra of OAc LB film are shown in Figure 4. The spectra recorded with p- and s-polarized radiation gave nearly the same spectra. The drastic large dichroism shown in Figure 2 for ODA LB film was not observed. The highly ordered structure shown in Figure 3 apparently arises from only the amide structure. It is understandable that the hydrogen bonding between the amide groups promotes the organization of molecular assemblies; however, it is difficult to make clear why the order of the assemblies is formed along the dipping direction. One factor can be considered; on the transfer-
Langmuir, Vol. 10, No. 10, 1994 3389
ring process of monolayer onto a solid support, the flow on the water surface is produced by the movement of monolayer transferred on the solid s ~ p p o r t .The ~ flow appears to influence the molecular order. However, these factors are not clear and should be further studied. In the present work, it can be concluded that ODA molecules in the LB films are oriented nearly perpendicular to the surface of the plate; moreover they are ordered in the plane along the dipping direction (in-plane order). That is, ODA LB films have a high biaxial orientation.
Acknowledgment. The present work was supported by a Grant-in-Aid for Developmental Scientific Research (No. 06555282) from the Ministry of Education, Science and Culture, Japan. (9) Roberts, G. In Langmuir-Elodgett Films; Plenum Press: New
York, 1990.
