Infrared and Resonance Raman Spectroscopic Study on the

Olivier J. Dautel, Mike Robitzer, Jean-Pierre Lère-Porte, Françoise Serein-Spirau, and Joël J. E. Moreau. Journal of the American Chemical Society ...
0 downloads 0 Views 345KB Size
Download PDF
3938

Langmuir 1996, 12, 3938-3944

Infrared and Resonance Raman Spectroscopic Study on the Photopolymerization Process of the Langmuir-Blodgett Films of a Diacetylene Monocarboxylic Acid, 10,12-Pentacosadiynoic Acid Atsushi Saito, Yoshie Urai, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169, Japan Received December 4, 1995X Absorption, infrared reflection absorption (IRA), and resonance Raman scattering (RRS) spectroscopies were applied to obtain direct information about the conformation change of a polydiacetylene (abbreviated to PDA) produced by UV irradiation of the Langmuir-Blodgett (LB) films of the cadmium salt of a diacetylene derivative, 10,12-pentacosadiynoic acid (DA), with the number of monolayers from 1 to 31. The rate and extent of the polymerization of DA monitored by the relative intensity change of the IRA bands due to CtC stretching vibrations were correlated with orientation changes of the alkyl groups of DA and changes in the conjugation length of the PDA backbone elucidated by RRS spectra in the CtC stretching vibration region. The results proved the following mechanism of the conversion from a blue (λmax ≈ 635 nm) to a red phase (λmax ≈ 540 nm) during the polymerization of DA. The conversion proceeds through two steps. The polymerization reaction itself is almost completed in the first step, although a residual reaction may take place in the second step. During the first step the plane formed by the all-trans zigzag backbone of the alkyl groups changes its arrangement in the following way: the orientation of the long axis of the alkyl backbone changes from a tilted state to a less tilted one, keeping the axis perpendicular to the long axis within the plane parallel to the substrate surface. This orientation change conforms to an increase in the interlayer spacing of the LB films of long-chain diacetylene monocarboxylic acids during their photopolymerization process observed through a small angle X-ray diffraction technique (Lieser, G.; et al. Thin Solid Films (1980, 68, 77). During the first step the PDA backbone keeps a fully extended conformation without any interruption of its conjugation, and the formation of the blue phase proceeds. When the polymerization proceeds to the second step, the regular plane of the all-trans alkyl group is converted to an irregular one containing gauche conformations. This conversion causes an interruption of the fully extended backbone structure and a reduction of the average conjugation length, resulting in the formation of the red phase.

Introduction The production of macroscopic polymer crystals by solidstate polymerization of diacetylene monomers, originally identified by Wegner,1 is now a well-known fact. It has also been established that the ordered packing state of cadmium salts of long-chain diacetylene monocarboxylic acids, CH3(CH2)mCtCsCtC(CH2)nCOOH (abbreviated to m-n diacid), in the form of Langmuir-Blodgett (LB) multilayers can be polymerized without destruction of the LB assembly by irradiation either with UV light or 60Co-γrays or by a thermal treatment2-8 (see Figure 1). The exposure of the freshly prepared LB films of the diacetylene monocarboxylic acids to UV or γ-ray irradiation results in the formation of a deep blue color, which changes into an intense purple-red after passing through a violet phase under continuous irradiation.2,3,6 A similar color change has also been observed on annealing the sample above 50 °C4,7 or on treating a partially polymerized sample (blue phase) with solvents for the diacetylene monocarboxylic acids such as ethanol and chloroform.4 The color change can be ascribed to a change in the electronic state associated with a structural change of the polymer backbone, and the origin for this process has been studied X

Abstract published in Advance ACS Abstracts, July 15, 1996.

(1) Wegner, G. Z. Naturforsch. 1969, 24B, 824. (2) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631. (3) Tieke, B.; Bloor D. Makromol. Chem. 1979, 180, 2275. (4) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. (5) Tieke, B.; Bloor, D.; Young, R. J. J. Mater. Sci. 1982, 17, 1156. (6) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. (7) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991, 7, 2336. (8) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1992, 8, 594.

S0743-7463(95)01503-4 CCC: $12.00

Figure 1. Structure of 10,12-pentacosadiynoic acid (abbreviated to DA) and its photopolymerization scheme.

by a variety of methods including X-ray diffraction,2 UVvis absorption spectroscopy,2-8 resonance Raman scattering (RRS) spectroscopy,3,5 and electron diffraction and electron microscopy.4-6 For example, Lieser et al.4 analyzed small angle X-ray diffraction patterns observed for the UV-induced polymerization process of the LB films of the 10-8 and 12-8 diacids and proposed a model which explains the color change as a crystallographic phase transition. Mino et al.7 studied the color transition of polymerized LB films of the 10-12 diacid by using UV-vis and infrared (IR) spectroscopies and differential scanning calorimetry and explained the color change as due to an onset of a fluctuation and/or a structural disorder of the side chain groups linked to the polymer backbone. Structures of mono- and multilayered polydiacetylenes © 1996 American Chemical Society

LB Films of 10,12-Pentacosadiynoic Acid

Langmuir, Vol. 12, No. 16, 1996 3939

prepared by self-assembling techniques were also studied by Batchelder et al.9 and Kim et al.,10 who indicated that the techniques produced stable “blue” polymers. In spite of these studies there have been no experimental data which provide direct information about structural changes of the alkyl groups as well as the polymer backbones during the polymerization. In the present paper we applied absorption, infrared reflection absorption (IRA), and RRS spectroscopies to study conformational changes associated with the polymerization induced by UV irradiation of the LB films of 10,12-pentacosadiynoic acid or the 11-8 diacid (CH3(CH2)11CtCsCtC(CH2)8COOH). (For simplicity, the monomeric diacetylene is abbreviated to DA and its polymerized form is abbreviated to PDA hereafter.) The high sensitivity of Fourier-transform IRA spectroscopy allowed us to observe the weak CtC stretching bands (ν(CtC)) of DA (with intensities of the order of 10-5 in the absorbance scale). By measuring the intensities of the ν(CtC) bands, we could determine the rate and extent of the polymerization of DA as a function of irradiation time. The rate and extent of the polymerization were correlated with the blue to red phase conversion clarified by UV-vis spectroscopy and with the orientation change of the alkyl chains elucidated by IRA spectroscopy. The change in the conjugation length of the PDA backbone during the polymerization was monitored by RRS spectroscopy. On the basis of these results we tried to present a reasonable model to a structural aspect of the color transition during the polymerization process in the LB films of DA. Experimental Section Materials. DA was purchased from Wako Pure Chemicals Co. Ltd. and used as received. Preparation of LB Films. The LB films of DA were prepared by using a Kyowa Kaimen Kagaku, HBM-AP2, Langmuir trough with a Whilhelmy balance. Water used for the preparation was purified with a Millipore water purification system (Milli-Q, 4-bowl). A silver film (of a thickness of 100 nm) deposited on a glass slide by a vacuum evaporation method was used as a substrate for the preparation. A monolayer was spread on the trough water by dropwise addition of a sample solution in chloroform (2 × 10-3 mol/L). A CdCl2 solution was added to the trough water to the level of 2 × 10-4 mol/L at pH 6.8 (adjusted by NaHCO3) for the preparation of the LB films of the cadmium salt of DA. The monolayer was transferred to the substrate by a vertical dipping method at a surface pressure of 20 mN/m and at 20 °C. The dipping speed was fixed at 5 mm/min. All the LB films were of a Y-type structure. Photopolymerization. UV light from a 200 W high-pressure mercury lamp (Oriel Lamp model no. 6283) was used to polymerize the LB films of DA. By using a quartz lens (f ) 10 cm), the light was collected onto each LB film to get an intensity of ca. 11 mW/cm2 at the surface of the film. Absorption Spectral Measurement. Absorption spectra of the LB films prepared at the silver substrate were measured by using a Unisoku Co. Ltd. rapid scan spectrometer, model RSP 601. An reflection absorption mode was employed for the measurements with an angle of incidence of about 60°. IRA Spectral Measurements. IRA spectra were recorded by using a Bio-RAD FTS-45A Fourier transform infrared spectrophotometer equipped with a liquid nitrogen-cooled MCT detector. A JEOL IR-RSC110 reflection attachment was used at the angle of incidence of 80°. Each spectrum was run for 1024 scans at a resolution of 4 cm-1. RRS Spectral Measurements. RRS spectra were measured by using a SPEX 1877 spectrophotometer equipped with a multichannel detector (Princeton Instruments Inc., ICCD-576 G/B) at a resolution of 4 cm-1. The 514.5 nm line from an Ar+ (9) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (10) Kim, T.; Crooks, R. M.; Tsen, M.; Sun. L. J. Am. Chem. Soc. 1995, 117, 3963.

Figure 2. Absorption spectral changes of the LB films of the cadmium salt of DA during polymerization induced by UV irradiation. The irradiation time is indicated at the right-hand side of each spectrum. (A) The number of monolayers in the LB film (L) ) 3; (B) L ) 5; (C) L ) 9; (D) L ) 15; (E) L ) 31. laser (SpectraPhysics model 165) and the 632.8 nm line from a He-Ne laser (NEC Co. Ltd., model GLG 2030) were used as excitation sources. The following precautions were taken to eliminate polymerization caused by irradiation with the excitation light of the LB films during the RRS measurements: (i) The laser power was kept as small as possible (ca. 8 mW for the 514.5 nm line and ca. 1 mW for the 632.8 nm line). (ii) A rotating sample method was employed. (iii) The samples were exposed to low-temperature nitrogen gas to keep the temperature at about 5 °C. The time required for accumulation of RRS scattering was from 10 to 30 min depending on the samples and the excitation wavelength.

Results and Discussion Absorption Spectral Changes during Photopolymerization. Figure 2 exhibits VIS absorption spectral changes during UV irradiation of the LB films of DA with the number of monolayers (L) from 3 to 31. The spectral changes are similar to those observed for the UV-lightinduced polymerization of a multilayer LB film of a longchain diynoic acid.6 Several minutes exposure of all the LB films to UV light results in the formation of a blue phase with λmax near 635 nm. After a prolonged exposure to the UV light, the LB films exhibit a shift in the absorption maximum toward shorter wavelength, which indicates a conversion of PDA to a red phase with λmax near 537 nm. The maximum intensity at 635 nm was observed after about 10 min of exposure for the LB films with L ) 3-9, while the maximum was observed after about 20-30 min of exposure for the films with L ) 1531; thus, the formation of the blue phase in the LB films with smaller L proceeds faster than that in the films with larger L.

3940 Langmuir, Vol. 12, No. 16, 1996

Saito et al.

Figure 3. IRA spectral changes in the CtC stretching vibration region of the LB films of the cadmium salt of DA during polymerization induced by UV irradiation. The irradiation time is indicated at the right-hand side of each spectrum. (A) The number of monolayers in the LB film (L) ) 9; (B) L ) 15; (C) L ) 31.

IRA Spectral Changes in the CtC Stretching Vibration Region of the LB Films during Photopolymerization. Figure 3 illustrates the IRA spectral changes in the CtC stretching vibration region observed during the photopolymerization of DA in the LB films with L ) 9, 15, and 31. According to Nikolova,11 diacetylene, 1,3-pentadiyne, and 2,4-hexadiyne give rise to the in-phase coupling modes of the stretching vibrations of the two CtC bonds (νs(CtC)) at 2219 (Σg+), 2239 (A1), and 2266 (A1g) cm-1, respectively, and the corresponding out-of-phase modes (νas(CtC)) at 2051 (Σu+), 2071 (A1), and 2190 (A2u) cm-1, respectively. (The symbol in parentheses following each frequency denotes the symmetry species to which the vibration mode belongs.) The frequencies of the IRA bands near 2180 and 2145 cm-1 in Figure 3 are close to those of the νas(CtC) bands observed for diacetylene and its derivatives, indicating that both of the IRA bands are associated to the νas(CtC) vibration of DA. The reason for the doublet feature has not been explained yet; presumably, it is ascribable either to Fermi resonance or to ‘hot bands’.12 Figure 3 indicates that the frequencies of the νas(CtC) bands slightly increase as the polymerization proceeds; for example, the 2141 cm-1 band of the LB films with L ) 31 shifts to 2144 cm-1 (Figure 3C). Presumably, a lateral packing interaction between the DA molecules in the LB films changes with the onset of polymerization, resulting in the frequency increase. Figure 4 plots the relative intensities of the band near 2145 cm-1 normalized to that observed at t ) 0 min against irradiation time for the LB films with L ) 3-31. The figure indicates that the relative intensities decrease following two steps, the rate of the decrease in the first step being much faster than that in the second step. The intensity decrease can be ascribed either to the polymerization of DA or to an orientation change of DA. It cannot be considered that the orientation change takes place from the beginning of the polymerization. Then, we concluded that the precipitous intensity decrease of the νas(CtC) band in the first step is due to the polymerization. The transition from the first to the second step takes place after about 10 min of UV light exposure for the LB films with L ) 3-9, while the transition takes place after about 20 min for the LB films with L ) 15-31. (A vertical line in each plot in Figure 4 indicates the irradiation time at which the transition from the first to the second step takes place.) The time of the transition for each LB film seems to correspond to that at which the intensity of the 635 nm band maximizes in Figure 2, indicating that the first polymerization step corresponds to the formation of the blue phase and the second step to the conversion from the (11) Nikolova, B. M. J. Mol. Struct. 1992, 273, 291. (12) Schrader, B. In Infrared and Raman Spectroscopy; Schrader, B., Ed.; VCH: New York, 1995; p 202.

Figure 4. Relative intensity changes of the CtC stretching band at 2144 cm-1 as a function of irradiation time. The ordinate indicates the intensities normalized to those observed at t ) 0 min. A vertical line inserted in each plot indicates the irradiation time at which the conversion from the first to the second step takes place (see text). (A) The number of monolayers in the LB film (L) ) 3; (B) L ) 5; (C) L ) 9; (D) L ) 15; (E) L ) 31.

blue phase to the red one. The DA molecules, which form a stacking state favorable for the polymerization in the original LB films, polymerize readily with the onset of UV irradiation, and the polymerization is virtually completed during the first step. Presumably, the gradual intensity decrease observed after 10-20 min of irradiation in Figure 4 is mainly due to an orientation change of the CtC bonds of DA induced by the transition from the blue to red phase, although a residual polymerization may proceed in the second step. From Figure 4 it is also clear that the extent of polymerization of the LB films with smaller L (the number of monolayers) is smaller than that of the films with larger L. An interaction between the substrate (the

LB Films of 10,12-Pentacosadiynoic Acid

Langmuir, Vol. 12, No. 16, 1996 3941

Figure 5. IRA spectral changes in the CH2 and CH3 stretching vibration region of the LB films of the cadmium salt of DA during polymerization induced by UV irradiation. The irradiation time is indicated at the right-hand side of each spectrum. (A) The number of monolayers in the LB film (L) ) 1; (B) L ) 3; (C) L ) 5; (D) L ) 9; (E) L ) 15; (F) L ) 31.

evaporated silver film) and the LB films may cause a more or less unfavorable stacking state of the DA molecules, reducing the extent of polymerization of the LB films with the smaller L. IRA Spectral Changes in the CH Stretching Vibration Region during Polymerization. Figure 5 summarizes the IRA spectral changes in the CH stretching vibration regions during photopolymerization of the LB films of DA with L ) 1-31. The IRA bands near 2850, 2875, 2918, and 2956 cm-1 are due to CH2 symmetric (νs(CH2)), CH3 symmetric (νs(CH3)), CH2 asymmetric νas(CH2)), and CH3 asymmetric (νas(CH3)) stretching vibrations, respectively.13 The IRA band near 2933 cm-1 has been assigned to a band arising from Fermi resonance between the νas(CH3) band and an overtone of a CH3 asymmetric deformation vibration near 1450 cm-1.14 All the LB films before exposure to the UV light give the νs(CH2) and νas(CH2) bands at 2850 and 2918 cm-1, which means that the alkyl groups of DA in the LB films take on an all-trans conformation. Upon exposure to the UV light, the νs(CH2) and νas(CH2) bands shift to higher frequency, indicating that the alkyl groups are partly converted to those containing gauche conformations,15 and the latter band coalesces into a broad band with the Fermi component near 2933 cm-1. In order to obtain quantitative aspects of the IRA spectral changes, a curve resolution (13) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (14) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (15) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316.

Figure 6. Results of the curve resolution procedure performed on the IRA spectra shown in Figure 5D measured at irradiation time (t) ) 0 min (A), t ) 30 min (B), and t ) 100 min (C) (see text).

procedure was applied under the following assumptions: (i) All component bands are of a Lorenz type (The curve resolution procedure was performed also under the assumption that the components are of a Gaussian type; the results, however, gave a much poorer fit with the observed spectra than the procedure under the assumption of the Lorenz type.). (ii) The νs(CH2) bands for the alltrans and irregular conformations of the alkyl chains are situated at 2849 and 2854 cm-1, respectively.15 (iii) The νas(CH2) bands for the all-trans and irregular conformations of the alkyl chains are at 2918 and 2924 cm-1, respectively.15 (iv) The νs(CH3) and νas(CH3) bands are at 2873 and 2958 cm-1, respectively, irrespective of the conformations of the alkyl groups. (v) There exists a weak component band at 2895 cm-1, which may be ascribable to a Fermi resonance component arising from the νs(CH2) band and an overtone of a CH2 scissoring vibration.14 In reality, the frequencies of the νs(CH2) and νas(CH2) components associated with irregular conformations depend on the gauche contents. Then, assumptions ii and iii mean that the frequencies and the total intensities of the νs(CH2) and νas(CH2) components are represented by those of the 2854 and 2924 cm-1 components, respectively, as a whole. As illustrated in Figure 6, the curve resolution reproduces the observed spectra quite well, indicating the validity of the assumptions. In other words, the resolution

3942 Langmuir, Vol. 12, No. 16, 1996

Saito et al.

Figure 8. Schematic representations of the photopolymerization of DA in the LB films and the conversion from the blue phase to the red phase.

Figure 7. Intensity changes of the νs(CH2) (2849 cm-1), νas(CH2) (2918 cm-1), and νs(CH2) (2854 cm-1) bands of the LB films of the cadmium salts of DA during the photopolymerization as a function of irradiation time. A vertical line inserted in each plot indicates the irradiation time at which the conversion from the first to the second step takes place (see text). (A) The number of monolayers in the LB film (L) ) 1; (B) L ) 3; (C) L ) 5; (D) L ) 9; (E) L ) 15; (F) L ) 31.

procedure gave reasonable estimates of the intensities of the νs(CH2) and νas(CH2) components associated with the all-trans conformations, which are the main concerns in the following discussion. Figure 7 plots the intensities of the νs(CH2) and νas(CH2) bands due to the all-trans alkyl chains (at 2849 and 2918 cm-1) against irradiation time for the LB films with the number of monolayers (L) from 1 to 31; the intensities are normalized to those measured at t ) 0 min. In the figure the relative intensity of the 2854 cm-1 component is also plotted against irradiation time; the result indicates that the polymerization in the LB films accompanies a partial conversion of the all-trans structure of the alkyl groups to an irregular structure containing gauche conformations. In addition, the plots in Figure 7 clarify the following points with regard to the intensity changes of the νs(CH2) and νas(CH2) bands associated with the all-trans alkyl groups (at 2849 and 2918 cm-1): (i) The νs(CH2) and νas(CH2) bands observed for the LB film with L ) 1 reduce their intensity in a similar manner from the beginning of irradiation. (ii) In contrast to the case of the LB film with L ) 1, the intensities of the νs(CH2) and νas(CH2) bands observed for the LB film with L g 3 decrease their intensity in quite a different manner; i.e., the intensity of the νas(CH2) band decreases precipitously with the onset of irradiation, while that of the νs(CH2) band remains constant in the initial irradiation period from 0 to 20-40 min. (iii) After 20-40 min of irradiation, both the νs(CH2) and νas(CH2) bands reduce their intensity in a similar manner, as in the case of the LB film with L ) 1. Comparing the plots for the relative intensity of the ν(CtC) band in Figure 4 with those for the νas(CH2) band due to the all-trans alkyl group in Figure

7, we can recognize that the intensity decreases of both bands are similar to each other; i.e., as in the case of the ν(CtC) band, the intensty of the νas(CH2) band decreases following two steps, the rate of the first step being much faster than that of the second step. (The transition from the first to the second step takes place after about 10 min of irradiation for the LB films with L ) 3-9, while the transition occurs after about 20 min for the LB films with L ) 15 and 31. A vertical line inserted in each plot in Figure 7 indicates the irradiation time at which the transition from the first step to the second one takes place. Compare the vertical lines in Figure 4 with those in Figure 7.) Then, the precipitous intensity decrease of the νas(CH2) band takes place during the formation of the blue phase. According to the surface selection rule,16 the intensity of an IRA band is proportional to the square of the component perpendicular to the surface of the transition moment of the IRA band. The transition moment of the νas(CH2) band is perpendicular to the plane formed by the all-trans zigzag chain of the alkyl groups of DA, while the transition moment of the νs(CH2) band is in the plane and perpendicular to the moment of the νas(CH2) band. Then, the above-mentioned intensity changes of the νs(CH2) and νas(CH2) bands during the blue phase formation can be explained by considering that the long axis of the alltrans alkyl groups of DA changes its orientation from a tilted to a less tilted one, while the axis perpendicular to the long axis in the plane remains parallel to the substrate surface. This orientation change to the perpendicular direction of the alkyl moieties of DA should cause the increase in the layer thickness of the LB films (see Figure 8) which has been actually observed during the formation of the blue phase of the cadmium salts of long-chain diynoic acids.4 Kim et al.10 studies the photopolymerization process of self-assembled mono- and multilayers of ω-functionalized diacetylene thiols and indicated that the polymerization causes a substantial decrease in the tilt angle between the hydrocarbon backbone and the surface normal of the substrate. This result also conforms to our observation. The fact that both the νs(CH2) and νas(CH2) bands show an intensity decrease in a similar manner after completion of the blue phase formation can be explained by considering that the conformation of the alkyl groups is converted to an irregular one containing gauche conformations, as illustrated in Figure 8. The conversion should accompany interruption of the fully extended backbone of PDA, reducing its average conjugation length. (16) Greenler, R. G. J. Chem. Soc. 1966, 44, 310.

LB Films of 10,12-Pentacosadiynoic Acid

Langmuir, Vol. 12, No. 16, 1996 3943

Figure 9. RRS spectral changes during the photopolymerization of the LB film of the cadmium salt of DA with the number of monolayers ) 15. (A) Excitation wavelength (λex) ) 632.8 nm; (B) λex ) 514.5 nm.

This process causes the formation of the red phase, as confirmed by RRS spectral measurements in the next section. RRS Spectral Changes of PDA during Photopolymerization. Figure 9 illustrates the RR spectral changes in the CtC stretching region (the ν1 band17) excited by 632.8 and 514.5 nm laser lights, respectively, observed during the polymerization induced by UV irradiation of the LB films of the cadmium salt of DA with L ) 15. The RRS band was observed at an almost constant frequency near 2078 cm-1 in the spectra excited by the 632.8 nm line. On the other hand, the RRS spectra observed by the 514.5 nm excitation indicate that the ν1 band was observed at 2078 cm-1 at the beginning of polymerization and that, after about 10 min of irradiation, there appears shoulder bands at the higher frequency side of the 2078 cm-1 band; the intensity of the shoulder components increases upon further irradiation. It has been known that the frequency of the ν1 band depends linearly on the strain of the polydiacetylenes applied along the chain axis; e.g., according to Bloor et al.,18 the ν1 band of the fully polymerized crystal of 1,6-bis((p-tolylsulfonyl)oxy)-2,4-hexadiyne (TSDA) was observed at 2083 cm-1 at zero strain, while the band was observed at 2040 cm-1 upon application of an elastic strain of about 2%. The frequency of the 2078 cm-1 band in Figure 9 is close to that of the fully polymerized single crystal with no strain. In addition, according to Batchelder et al.9 and Kim et al.,10 polydiacetylene mono- and multilayers in the blue phase prepared by self-assembling techniques give the RRS band near 2075 cm-1. On the basis of these results, we can assign the 2078 cm-1 band in Figure 9A, and that observed at an initial stage of polymerization in Figure 9B can be ascribed to PDA with a fully extended backbone (17) Batchelder, D. N.; Bloor, D. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden-Wiley: Chichester, U.K., 1984; Vol. 11, p 133. (18) Bloor, D.; Kennedy, R. J.; Batchelder, D. N. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 1355.

Figure 10. Results of the curve resolution procedure performed on the RRS spectra shown in Figure 8B measured at irradiation time (t) ) 2 min (A), 10 min (B), 30 min (C), 50 min (D), and 80 min (E) (see text).

or PDA in the blue phase. The excitation wavelength of 632.8 nm is close to the absorption maximum at 635 nm of the blue phase, and it selectively enhances the ν1 band associated with this phase in Figure 9A. A curve resolution procedure was applied to the RRS spectra in Figure 9B in order to correlate the intensity increase of the shoulder component at the higher frequency side of the 2078 cm-1 peak with the conversion from the blue to red phase. The curve resolution was performed under the assumption that the RRS band due to the ν1 mode is composed of two components of a Lorenz type at 2080 and 2101 cm-1. As can be seen from Figure 10, the calculated spectra reproduce quite well the observed ones in Figure 9B. Although the existence of the two components of Lorenz type is an arbitrary assumption, the good fit indicates the validity of semiquantitative discussion on the basis of the results of the curve resolution. Parts A and B of Figure 11 plot the relative intensities of the component bands as a function of the irradiation time for the LB films with L ) 15 and 31, respectively. (The intensities in parts A and B of Figure 11 are normalized to those of the 2101 cm-1 component measured at 150 and 140 min, respectively.) From the figure it is clear that the relative intensity of the 2080 cm-1 component ascribable to the blue phase shows a rapid increase with the onset of irradiation, maximizes after about 30-60 min of irradiation, indicating the completion of the blue phase formation, and then gradually decreases. The intensity change of the 2080 cm-1 component in Figure 11A corresponds well to that observed for the RRS band at 2078 cm-1 in Figure 9A, because the latter band also maximizes its intensity at 30-60 min. (The correspondence may give more evidence for the validity of the results of the curve resolution.) Thus, the formation of the blue phase was completed appreciably

3944 Langmuir, Vol. 12, No. 16, 1996

Figure 11. Relative intensity changes of the component bands as a function of irradiation time observed for the RRS spectra during the photopolymerization of the LB films of the cadmium salt of DA (see text). (A) The number of monolayers (L) ) 15; (9) 2080 cm-1; (b) 2101 cm-1; (B) L ) 31; (9) 2080 cm-1; (b) 2101 cm-1.

later than that observed for the LB films through absorption and IR spectroscopies (see Figures 2, 4, and 7). This can be explained by two factors. Firstly, the error in the curve resolution procedure is so large that the results do not always correlate with the real situations. Secondly, the RRS measurements were performed at 5 °C, while the absorption and IR measurements were done at 24 °C; the lower temperature may cause the time delay of the blue phase formation. As can be seen from Figure 11, the intensity of the higher frequency component at 2101 cm-1 increases gradually during the formation of the blue phase and, after completion of the blue phase formation, the intensity shows a rapid increase. Shand et al.19 measured the RRS spectra of a polydiacetylene having the structure (d(R)CsCtCsC(R)d)n with R ) (CH2)3OCONHCOOC4H9 (abbreviated to poly3BCMU) in a mixed chloroformhexane solvent and in chloroform. The former solution gives a broad absorption maximum (λmax) centered around 625 nm (referred to as a “blue solution”), and the latter gives λmax near 480 nm (referred to as a “yellow solution”). According to Shand et al.,19 the absorption maximum is closely related to the conjugation length (i.e., the length over which backbone planarity is maintained without interruption), and the longer the conjugation length, the longer the wavelength of the absorption maximum. Then, the blue solution consists of poly3BCMU with the polydiacetylene backbones of a longer average conjugation length, while the yellow solution contains the backbones with a smaller average conjugation length. By observing the RRS spectra of the blue and yellow solutions by changing the excitation wavelength from 625 to 455 nm, Shand et al.19 could selectively enhance the RRS bands due to the CtC stretching band (the ν1 band) from (19) Shand, M. L.; Chance, R. R.; LePostollec, M.; Schott, M. Phys. Rev. 1982, B25, 4431.

Saito et al.

poly3BCMU with specific conjugation lengths and concluded that the ν1 frequency can be approximated by ν1 ) 2083 cm-1 + A/N, where N is the number of repeat units in a conjugated chain and A = 250-330 cm-1. If it is assumed that this equation can be applied to the case of PDA, the frequencies of the component associated with the blue phase (2080 cm-1) in Figure 10, which are close to the first term of the right-hand side of the equation, are ascribed to PDA having fully conjugated backbones with a virtually infinite number of N. The frequency of the component at 2101 cm-1 corresponds to N ) 14-18. Although it is difficult to consider that this number reflects a physical reality, the intensity increase of the 2101 cm-1 component can be explained as due to reduction of the average conjugation length of PDA, which takes place during the conversion from the blue to the red phase. Conclusion As mentioned above, the combined use of the absorption, IRA, and RRS spectroscopies provides us with detailed information about the structural changes taking place during the polymerization of the LB films of DA. By monitoring the intensity decrease of the IRA bands due to the νas(CtC) mode of DA, we proved that the polymerization proceeds through two steps; the rate of intensity decrease in the first step was appreciably larger than that of the second step. During the first step the polymerization reaction is virtually completed and the blue phase is formed. At the same time the trans zigzag plane of the hydrocarbon backbone of DA changes its direction, so that the long axis of the plane changes its orientation from a tilted to a less tilted one, while the short axis of the plane remains parallel to the surface. This kind of orientation change of the all-trans alkyl chains is a characteristic feature associated with the blue phase formation and confirms the increase in the interlayer spacing observed for the polymerization process of the LB films of longchain diynoic acids through a small angle X-ray diffraction technique.4 During the blue phase formation the PDA backbone keeps a fully extended structure without any interruption of its conjugation. When the polymerization (or the formation of the blue phase) proceeds virtually to completion, there begins a corruption of the regular array of the all-trans alkyl chains. The corruption causes the interruption of the fully extended backbone structures of PDA and the reduction of the conjugation length, resulting in the formation of the red phase. The intensity decrease of the νas(CtC) band in the second step corresponds to the conversion process from the blue to the red phase. Then, the decrease is mainly due to the orientation change of DA induced by the conversion. LA951503Z