Multidimensional Spectroscopy of Polymers - American Chemical

Chapter 33

Fourier Transform IR and Fluorescence Spectroscopy of Highly Ordered Functional Polymer Langmuir—Blodgett Films

Downloaded by CORNELL UNIV on September 13, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch033

Tokuji Miyashita Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aoba Aramaki, Aoba-ku, Sendai 980, Japan

FT-IR spectroscopic measurements (polarized transmission and reflection absorption spectra (RAS) ofN-octadecylacrylamide(ODA) LB films indicate that ODA molecules are oriented nearly perpendicular to the surface of the plate, moreover, they are arranged in the plane along the dipping direction (in-plane order). ODA LB films have a biaxial orientation. The emission spectroscopic study for the aromatic chromophore incorporated into the highly ordered N-alkylacrylamide polymer LBfilmsis carried out. The emission spectra for the LBfilmswere measured as a function of the deposited layer and the alkyl chain length of N-alkylacrylamide polymers. The excimer emission intensity increased with increasing number of deposited layers when the effective interlayer energy transfer to the excimer forming site is taking place. The efficiency of vertical energy transfer in the Y-type polymer LBfilmsfor naphthalene, pyrene, carbazole and fluorene chromophores was safely judged from the increase in the excimer emission intensity as a function of the deposited layers in the emission spectra. The limiting energy transfer distances for each chromophores are correlated with critical energy transfer distance of Förster type. Langmuir-Blodgett (LB) method is one of the most useful techniques for fabrication of functional organic ultrathin films. The L B technique makes it possible to prepare thin films with controlled thickness at a molecular size and well-defined molecular orientation (1,2). Because of this superior feature, the L B method has been tried to apply, to polymer materials. We have continued to study the preparation of functional polymer L B films (3). Three approaches, schematically shown in Scheme 1 (A, B, and C) can be considered for preparation of polymer L B films. In the first approach (A), polymerizable amphiphilic monomer is spread onto a water surface and the condensed monomer monolayer is transferred onto a solid support (deposition), and then the polymer L B film is obtained through polymerization by U V light, electron beam, or γ-ray irradiation. In the second approach (B), amphiphilic monomer is polymerized separately by a usual polymerization method and the resulting amphiphilic preformed polymer is spread onto a water, forming the condensed polymer monolayer and then the deposition of the monolayer gives the 0097-6156/95/0598-0568S12.00/0 © 1995 American Chemical Society

Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.



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MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS


desired polymer L B film. Lastly, the approach ( Q , this is just an intermediate of the approach (A) and (B). We have succeeded in the preparation of fairly uniform polymer L B films using jV-alkylaciylamide series. For method A , #-octadecylaciylamide (ODA) monomer gave a stable condensed monolayer on a water surface and a uniform polymer L B film can be obtained by the polymerization of the monomer L B film (4). For method B , preformed poly(i^-dodecylacrylamide) (PDDA) formed a fairly uniform polymer L B film (5). Moreover, we have proposed a method to incorporate various functional groups as a comonomer of PDDA (6-5). In this paper, we report a study on the molecular orientation of alkyl substituents in O D A and PDDA L B films by FT-IR spectroscopy. Moreover, the photophysical behavior of the aromatic chromophores that are incorporated into PDDA L B films by the copolymerization method is discussed from fluorescence spectra. Experimental Section Materials. JV-alkylacrylamide monomers were synthesized by the reactions of the corresponding alky lamines and acryloyl chloride in the presence of trimethylamine at joom temperature. The products were purified by column chromatography and recrystallization. Poly(#-alkylacrylamides) and the copolymers were prepared by usual free radical polymerization. Chloroform used for spreading monolayers on the water was of spectroscopic grade. Distilled and deionized water (18 ΜΩ/cm) by Milli-QH was used for the subphase. Measurements. Measurement of surface pressure - area isotherms and the deposition of the monolayer were carried out with automatic working Langmuir trough (Kyowa Kaimen Kagaku H B M - A P using Wilhelmy - type film balance ). FT-IR transmission and reflection absorption spectra (RAS) were measured with a JOEL JIR-100 FT-IR spectrophotometer operated at a resolution of 4 cm" . CaF2 plate and Au-evaporated slide glass were used for transmission and R A S measurement, respectively. Fluorescence spectra and UV-visible absorption spectra were measured with a Hitachi 850 spectrofluorophotometer and a Hitachi U-3500 UV-visible spectrophotometer, respectively. 1

Results and Discussion Molecular orientation in O D A L B film, hi the previous work, we have showed that O D A and iV-icosylacrylamide (ICA) form stable condensed monolayers giving L B films and the O D A and I C A L B films are polymerized by U V irradiation, especially O D A L B film can be polymerized completely. The polymerized O D A L B film was insoluble in usual organic solvents and the possibility of application to a new type of high resolution resist was also indicated (9). A n ordered molecular orientation is expected to be formed in ODA L B film. In this part, a highly ordered structure in O D A (Scheme 2) L B 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. A chloroform solution of O D A was spread onto a water surface. The surface pressure (π) - surface area (A) isotherms, which were already studied clearly in the previous paper (4), 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 15 mN/m with a transfer ratio of unity giving Y-type L B multilayers (20 layers). The transmission



33. MIYASHITA

Functional Polymer Langmuir-Blodgett Films

Scheme 2.


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spectrum and RAS of the ODA L B multilayers are shown in Figure 1. They are quite different each other. The N H stretching (3300 cm" ), C-H symmetric stretching (2850 cm* ), C=0 stretching (1653 an" ), O C stretching (1622 cm' ), and CH2 scissoring (1471 cm* ) absorption bands were not observed in R A S 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 N H , C=0 and C=C substituents are highly oriented in parallel and the long alkyl chain is standing up from the surface in the L B multilayers. If the long alkyl chain is completely perpendicular to the surface, the absorption assigned to the C - H asymmetric stretching (2953 cm' ) should not be observed in R A S . However, the CH2 and ÔNH (1543 cm' ) absorption bands can be observed in RAS, which indicates that the long alkyl chain is tilted regularly. Although the information on normal plane (uniaxial orientation) can be given by comparison of transmission spectra and RAS, this method is not useful to examine anisotropic orientation in the plane (biaxial orientation). In order to examine the two-dimensional ordered structure in L B multilayers, we measured polarized FT-IR spectra of O D A L B multilayers. The monolayer on the water-surface is transferred onto a CaF2 P ^ conventional vertical dipping method. The transmission spectra are taken with the radiation parallel to the dipping direction of L B multilayers (p-polarization) and perpendicular to the direction (^-polarization) (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 RAS. H i e large dichroism in p- and 5-polarization indicates that a highly ordered structure in plane (biaxial orientation) is also formed along the dipping direction in ODA L B multilayers. Hie vNH, vC=0, vC=C, ÔCH2 and v CH2 absorption bands appear strongly in ^-polarization, indicating that the N H , C=0, and C=C groups are perpendicular to the dipping direction. From the results shown in Figure 1 and the present dichroism (Figure 2), it is concluded that the NH, C=0, 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 5-polarization, moreover, the only asymmetric mode appears in RAS. These results mean that all the alkyl chains in ODA L B 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 N H . It can be said that the hydrogen bonding between the amide groups is a driving force for the molecular assembly. This statement is supported by the following results. Octadecylacrylate (OAc), being the corresponding acrylate derivative to ODA, gives a similar π - A isotherms with O D A monolayer. The condensed O A c monolayer can be transferred onto CaF2 plate with a transfer ratio of unity. The polarized transmission spectra of OAc L B 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 L B film was not observed. Moreover, no dichroism was also observed for the polarized FT-IR spectra of Noctadecylacetamide L B film (Figure 5). The highly ordered structure shown in Figure 3 apparently arises from only the acrylamide structure. 1

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33. MIYASHITA


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Figure 1. FT-IR spectra of ODA L B film with 20 layers (upper: transmission, lowenreflection absorption spectra (RAS)).


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Figure 2. Polarized FT-IR spectra of O D A L B film (uppens-polarization, lowenp-polarization).



33. MIYASHITA


Film dipping direction Figure 3. A highly two-dimensional ordered structure in O D A L B film


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Multidimensional Spectroscopy of Polymers - American Chemical

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