J. Phys. Chem. 1991, 95, 7 109-7 1 14
7109
Molecular Arrangements of Pentacosadlynoic Acid Langmulr-Blodgett Films before and after Electron Beam Irradlatlons Kazufumi Ogawa Central Research Laboratories, Matsushita Electric Industrial Co.. Ltd., 3- 15, Yagumo-Nakamachi, Moriguchi, Osaka 570 Japan (Received: December 17, 1990)
Electron beam (EB) induced polymerization of IO,12-pentacosadiynoicacid (PDA) Langmuir-Blodgett (LB) films has been studied in relation to a molecular density or molecular arrangement of the films by using X-ray diffraction analysis and infrared (IR) spectroscopy. The molecular arrangement or density of the PDA LB films was controlled by subphase conditions when the films were built up, such as pH, temperature of the subphase, or salt concentration in the subphase. It was revealed by the X-ray diffraction analysis that, in the low-density (A-type) PDA LB film, the PDA molecules were bent to a larger extent than those in the high-density (B-type) PDA LB film and that the polymerization proceeded topochemically; that is, the thickness decreased little after the EB irradiation in a helium atmosphere. On the other hand, in the B-type PDA film, the thickness decreased about 10% by the EB irradiation. By IR reflection-absorption (RA) measurements, it was confirmed that conjugated diacetylenic bonds disappeared and conjugated double bonds appeared after the EB irradiation. These results support my previous conclusions' that 1,4polymerization, i.e., polydiacetylene-typepolymerization, occurs easily in the A-type PDA LB film and 1,2- or 3,4-polymerization, Le., polyacetylene-type polymerization, occurs in the B-type PDA LB films.
Introduction Diacetylene derivative polymers polymerized by UV irradiation in a solid phase have the main chains with the planar structure and alternating conjugated double and triple bonds, Le., polydiacetylenic bonds.2-' The polydiacetylenic bonds may be considered as a fully conjugated one-dimensional 77-electron system extended over long distance. Since the polymer main chains having conjugated double and triple bonds exhibit a number of interesting features with regard to optical and electronic properties such as a nonlinear optical effect or an electric conductivity, these polymers are studied widely as functional materials.H In addition to these interesting features, the polymerized diacetylene derivatives exhibit color change from blue to red caused by temperature change or UV irradiation. The color change was explained by Baughman9 as the bond exchange of the backbone chain from a polydiacetylenic structure to a polybutatriene structure. On the other hand, Lim et aI.'O reported that a conformation of the side-group changes by rod-coil transition of the backbone chain. The mechanism, however, is not explained clearly at present. Using some materials that have a hydrophilic group and a hydrophobic group in a molecule, Blodgett and Langmuir first succeeded in preparing ultrathin films by their own technique.I'J2 A characteristic of the Langmuir-Blodgett (LB) technique is that it is possible to prepare thin films having a desired molecular arrangement or density by building up monolayers under properly selected conditions. Thus, in recent years, the LB technique is thought to be one of the most important means to construct molecular devices" having some functions at a molecular level, ( I ) Ogawa, K. J . Phys. Chem. 1989,93,5305. (2) Wegner, G . 2. Naturforsch. 1969,824, 824. ( 3 ) Wegner, G.Makromol. Chem. 1972. 154,35. (4) Wegner, G.; Schermann, W. Kolloid Z . 2.Polym. 1974,252, 655. (5) Wilson, E. G. J . Phys. (0,Solid Srare Phys. 1975,8, 727. (6)Bloor, D.; Ando, D. J.; Preston, F. H.; Stevens, G. C. Chem. Phys. Len. 1974.24.407. (7) Bloor, D.; Chance, R. R. Polydiacerylenes; Martinus Mijihoff Publishers: Boston, MA, 1985. (8) Mehring, H.; Roth, S.Electronic Properties of Polymers and Related Compounds; Springer-Verlag: Berlin, 1985. (9)Baughman, R. H.J . Appl. Phys. 1972,43,4362. (IO) Lim, K. C.; Heeger, A. J . J . Chem. Phys. 1985,82. 522. ( I 1 ) Blodgett, K. B. J . Am. Chem. Soc. 1935,57, 1007. (12) Blodgett, K. B.; Langmuir, I. Phys. Reo. 1937,51, 964. ( 1 3 ) Mcalear, J . H.; Wehrung, J. M. 'Digest of Tech. Paper 1981 Symposium on VLSl Tech.", 1981,82.
and many studies have been carried out on the polymerization process of LB films of diacetylene derivatives.Ict6 The photoreactivity of the diacetylene derivatives whose substituents were replaced with some chemical groups that cause a change in the molecular arrangement in the LB films has also been studied.".'* The present study has been carried out to obtain more precise information on the reaction mechanism of pentacosadiynoic acid [PDA: CH3(CH2)IIC=CC=C(CH2)8COOH] LB films in relation to the molecular density or molecular arrangement that was reported in my previous report.' For this purpose, X-ray diffraction and infrared (IR) spectroscopic studies were also made before and after polymerization of the PDA LB films obtained under different buildingup conditions. Two typical PDA LB films were used: one is the low-density (A-type) film, and the other is the high-density (B-type) film, which were prepared under different subphase conditions and surface pressures. Experiments were also carried out for reference purposes on n-heneicosanoic acid [HEA: CH3(CH2)&OOH) and n-pentacosanoic acid (PCA: CH3(CH2),,COOH) which have no unsaturated bonds.
Experimental Section Materials. PDA, HEA, and PCA of reagent grade and chloroform of UV spectrum grade were purchased from Wako Pure Chemical Industries, Ltd., and used without purification. Calcium chloride was also supplied by the same manufacturer and used after Soxhlet extraction to remove any surface-active contaminations. Water for subphase was purified by ultrafiltration of deionized water to satisfy the ultrapurified grade required in the semiconductor device process (resistivity, 16 MQcm or higher; the number of particles exceeding 0.5-pm diameter, less than 30 particles/cm3; concentration of total carbon, lower than 100 ppb). The substrates for the deposition were quartz plates for X-ray diffraction analysis and AI/Si and Si plates for IR reflection-absorption (RA) and transmission (T) spectral measurements, respectively. (14)Bubeck, C.; Tieke, B.; Wegner, G. Ber. Bunsen-Ges. Phys. Chem.
__
1982. .. , 86. - -, 495. ..- .
(15) Bubeck, C.; Tieke, B.; Wegner, G. Mol. Crysr. Liq. Crysr. 1983,96,
109. (16) Tieke, B.; Lieser, 0.; Weiss, K . Thin Solid Films 1983, 99, 95. (17)Wegner, G. J . Polym. Sci., Polym. Lcrr. Ed. 1971,9,133. (18) Nakanishi, H.; Matsuda, H.; Kato, K.; Thocharis, C.; Jones, W. Polym. Prepr. Jpn. 1984,33, 2491.
0022-3654/91/2095-7 109$02.50/0 0 1991 American Chemical Society
7110 The Journal of Physical Chemistry, Vol. 95, No. 18, 199'I
751
-Et
50-
a
t
a'
8
'
0 10 Area (~2/mc4ecule)
Figure 1. Surface pressure-area curves for building up two PDA LB
films of different densities, HEA LB film, and PCA LB film: (a) A-type PDA film, (b) B-type PDA film; (c) HEA film; (d) PCA film.
Apparatus and Circumstances. A Joyce-Loebl Langmuir Trough IV was used for building up the PDA, HEA, and PCA LB films. The LB films were prepared in a class 100 clean room under yellow light which was cut below 500 nm. The temperature of the clean rmm was controlled at 23.0 f 1 OC, and the humidity was at 40 f 5%. Procedures. 1. Preparetion of Film, PDA dissolved in CHCl:, (ca. 0.1 mg/L) was spread on aqueous subphases containing CaCI, of two different concentrations to build up films of different molecular density: one containing 3.0 X IO4 mol/L at pH 5.7 and 6.5 O C for preparation of the A-type PDA LB film (lowdensity film) and the other containing 1.3 X lo-' mol/L at pH 6.8 and 8.5 OC for the B-type PDA LB film (high-density film). The HEA and PCA films were also built up on the subphase of pH 6.5, 7.8 OC, and 1.3 X IO-' mol/L CaCI2 and of pH 6.8, 18 'C, and 1.3 X mol/L CaCI2, respectively. The pH of the subphase was adjusted by NaOH or HCI. The substrate was moved across the monolayer on the aqueous subphase at a constant velocity of 3 cm/min for downward and 1 cm/min for upward for building up the A- and B-type PDA and PCA LB films and 3 cm/min for downward and 3 cm/min for upward for building up the HEA films. During the building up of the PDA LB films, the surface pressure was kept at constant values denoted as points A and B in the surface pressure-area curves, as shown in Figure 1 . In Figure 1, similar plots obtained for the HEA and PCA LB films are also included for reference purposes and the surface pressure during the building up were also denoted as points C and D, respectively. The PDA LB films prepared were of 25 layers, in which the first some 12 layers were built up by Z-type deposition and then the deposition changed to that of Y type for the A-type PDA LB film, and the first some six layers were built up by 2-type deposition and then the deposition changed to that of Y type for the B-type PDA LB film. The HEA and PCA LB films were also of 25 layers, in which all the layers were built up by the Y type for the HEA LB films and by the Z type for the PCA LB films. The transfer ratios depended on the type of deposition. The ratio for PCA films on the upward stroke was changed from 0.6 to 1 .O with increasing number of layers, that on the downward stroke was almost 1 .O for every buildup process, and those for the Aand B-type PDA and HEA films at upward and downward strokes were almost I .O for all buildup processes. 2. Polymerization of PDA LB Films by EB Irradiations. The PDA LB films were polymerized by the irradiation of electron beam (EB)from a transformer-type accelerator (300 keV, 50 PA, Nissin High Voltage CO.).'~ The irradiation chamber was made of stainless steel (SUS 304) and was equipped with an irradiation window of aluminum foil (thickness 15 pm) at the top, through which the electrons penetrated. The bottom of the chamber was (19) Ogawa, K.;Tamura, H.; Hatada, M.; Ishihara, T. Colloid Polym. Sci. 1988. 266. 525.
Ogawa cooled by running water. For EB irradiation, a helium stream was introduced into the chamber during irradiation to keep oxygen away from the sample films (oxygen concentration less than 20 PPm). The EB irradiation dose was 4 Mrad for all LB films, and the dose rate of the irradiation was 0.02 Mrad/s at the beam current of 50 FA. The temperature of the LB films did not exceed 30 OC during irradiation. 3. Analysis of LB FilmP by X-ray Diffraction Technique. The X-ray diffraction patterns before and after the EB irradiation were measured to clarify the spacing length of the repeat unit of the monolayers by an X-ray diffractometer (RU-200 and RAD-C system, Rigaku International Co.; Target, Cu, 60 keV, 150 mA) at room temperature. 4. IR Measurememts. A Shimadzu Model 4OOO FTIR spectrophotometer (resolution, 2 cm-'; detector, MCT) was used to study the reaction process and molecular arrangement in the monolayers. The reflection-absorption (RA) spectra of the monolayers were measured on the AI/Si substrates with an RAS attachmentm and the polarizer which was set parallel to the plane of incidence. The angle of incidence was 73'. One thousand interferograms were accumulated to obtain an IR spectrum of the monolayers. All spectra reported here were results of subtraction of the curves measured with the clean AI/Si plates from those measured after reflection from the respective films on the same plates. The transmission (T) spectra of the collapsed PDA films on the aqueous subphases at different pH's and concentrations of calcium chloride were also measured on a Si substrate by the FTIR spectrophotometer. The assignments of the absorption bands were made in reference to the reports by Hayashi et al.?' Kimura et aL?* and Umemura et al.23
Results and Discussion X-ray Diffraction Analysis. X-ray diffraction patterns of Aand B-type PDA, HEA, and PCA LB films before and after irradiation with the EB dose of 4 Mrad are shown in Figure 2 a 4 , respectively. On the A-type PDA film, the diffraction peaks (2 e) appeared at 1.82,4.88, 5.68, and 9.60' before the irradiation and a t 1.86, 5.76,7.74, and 9.68O after the irradiation, as shown on patterns A and B in Figure 2a, respectively. The peaks a t 5.68 and 9.60' on pattern A and a t 5.76, 7.74, and 9.68O on pattern B were assigned to the peaks of (003), ( 0 0 5 ) and (003), (004), (005) of the (001) plane of d = 46.3 A. The intensities of the peaks (004) and (005) increased by the irradiation, while that of the peak at 1.82' decreased and that at 4 . 8 8 O disappeared, as shown in Figure 2a. This indicates that the peaks at 1.82 and 4.88' are due to the PDA crystals contained as an impurity (represented by "i" in Figure 2a), which have a different crystal structure from that of the main part of the A-type PDA film, and changed to the main structure by the irradiation. Consequently, the crystallinity of the film increased after irradiation. The diffraction in the 0 values between the (003) and (005) peaks did not change before and after irradiation, indicating that "topochemical reactions" occurred in the A-type PDA film. If the PDA molecule has all-trans hydrocarbon chains, and two hydrocarbon chains are aligned parallel to each other in the molecule, the calculated molecular length is about 33 A. The above-obtained d value of 46.3 A is, however, longer than this length and is unreasonable for the long spacing of the unit monolayer. Thus, although the first some 12 layers of the A-type PDA films were built up by 2-type deposition and then the de(20) Greenler, R. G. J . Chcm. Phys. 1966,44, 310. (21) Hayashi, S.;Umemura, J. J . Chcm. fhys. 1975, 63, 1732. (22) Kimura, F.; Umemura, J.; Takenaka, T. fungmuir 1986. 2. 96. (23) Umemura. J.; Kamata. T.; Kawai. T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (24) Takeuchi, H.;Arakawa, T.; Furukawa, Y.;Harada, 1. J. Mol. Strucr. 1987, 1.58, 179.
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 7111
Pentacosadiynoic Acid LB Films IO
A
A T y p PDA (001) plane d.46.3
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Figure 2. Change of the X-ray diffraction patterns of the LB films before (A) and after EB irradiations (B)at a dose of 4 Mrad: (a) A-type PDA film; (b) B-type PDA film; (c) HEA film; (d) PCA film. In (a), "in represents an impurity.
position changed to that of Y type, if all the built-up layers are assumed to have Y-type structure, the long spacing of the A-type PDA LB films is about 66 A when the molecules are oriented perpendicular to the film surface. Taking account of the molecular tilting, the d value of 46.3 A may reasonably correspond to the long spacing of the unit bilayers. Therefore, the tilt angles (angles relative to the normal to the surface of the substrate) of the PDA molecules are ca. 45' before and after irradiations. On the other hand, on the B-type PDA film, the diffraction peaks (2 0) appeared at 1.66,2.92,3.22,4.32,4.98,5.78, and 7-18' before irradiation and at 1.64, 4.86, 6.50, and 8.08O after irradiation as shown on pattern A and pattern B in Figure 2b, respectively. The peaks at 2.92.4.32, 5.78, and 7.18O on the pattern A are assigned to the peaks of (002), (003), (004). and (005) of the (001) plane of d = 61.1 A. This may also correspond to the long spacing of the unit bilayer consisting of the inclined molecules by the same reasons mentioned above. The peaks at 1.66, 3.22, and 4.98' on pattern A and at 1.64, 4.86, 6.50, and 8.08O on pattern B are assigned to the peaks of (001)', (OOZY, (003)', and (OOI)', (003)', (004)', (005)' in the (001)' plane of d = 54.3 A. The peaks of the (001) plane disappeared and the peaks of the (001)' plane became stronger after the irradiation, as shown in
Figure 2b. This indicates that the crystal structure with the (001) plane reacted with the EB irradiation and changed to the new crystal structure with the (001)' plane. The results that the long spacing of d = 61.1 A changed to d = 54.3 A after irradiation indicate that molecular alignment declined more or that the molecules were bent. If the hydrocarbon chains in the PDA molecule are on a straight line, the long spacing of 61.1 A means that the tilt angles of the PDA molecules are ca. 20' before irradiation, and the long spacing of 54.3 A suggests that the tilt angles of the rearranged PDA molecules are ca. 33' after irradiation. It may also be reasonable to suppose for the B-type PDA films that both of the declining and bending occurred. The weak peaks of the (001)' plane observed before the EB irradiation (pattern A in Figure 2b) may indicate that a little bit of the PDA monomers in the B-type film before the irradiation was reacted (or polymerized) by X-ray irradiation at the X-ray diffraction measurement, because the peaks of the (001)' plane on the pattern A agreed with those on pattern 8. The reason why the peaks (002)' disappeared on pattern B in Figure 2b cannot be explained at present. On the HEA films, the diffraction peaks (2 0 ) appeared at 1.67, 3.24, 4.78, 6.32, and 7.84' before irradiation and at 1.72, 3.26,
7112 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
Ogawa
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A Type PDA
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Figure 3. Change of the RA spectra of the LB films before (A) and after EB irradiations (B) at a dose of 4 Mrad: (a) A-type PDA film; (b) B-type PDA film; (c) HEA film; (d) PCA film. 4.80,6.22, and 7.90' after irradiation as shown on pattern A and pattern B in Figure 2c, respectively. The peaks at 1.67, 3.24,4.78, 6.32,7.84O and at 1.72, 3.26,4.80, 6.22, 7.90° on patterns A and B in Figure 2c could be assigned to the peaks of (OOl), (002), (003), (004), and (005) of the (001) plane of d = 54.5 A, respectively. As the molecular length of the straight HEA molecule is about 28 A and the film was of the Y type, this d value may be the long spacing of the unit bilayer. The peaks (001) through (005) became weaker and shifted a little after the irradiation, as shown on pattern B in Figure 2c. This indicates that although the long spacing did not change, the crystallinity of the film decreased a little after the irradiation; Le., cross-linking reactions between hydrocarbon chains might occur in the HEA LB films. The long spacing of 54.5 A indicates that the HEA molecules aligned almost perpendicular to the substrate before and after irradiation. On the PCA films, the diffraction peaks (2 0) appeared at 4.46 and 7.34O before irradiation and at 1.54, 4.48, and 7.40° after irradiation as shown on pattern A and pattern B in Figure 2d, respectively. The peaks at 4.46 and 7.34O on pattern A in Figure 2d were assigned to the peaks of (003) and (005) of the (001) plane of d = 60.4 A, and the peaks at 1 .54, 4.48, and 7.40° on pattern B in Figure 2d were assigned to the peaks of (OOI), (003) and (005) of the (001) lane of d = 60.6 A. The molecular length of PDA is about 33 The fact that the X-ray diffraction patterns are similar to those of the HEA films may indicate that although the deposition of the PCA films is of 2 type, the prepared film is Y type. Therefore, the observed d value may also correspond to the long spacing of the unit bilayer. In contrast to HEA, the peaks of the (001) plane became stronger after irradiation, as shown by pattern B in Figure 2d. This indicates that the crystallinity of (001) was increased by the irradiation. If the hydrocarbon chain in the PCA molecule was on a straight line, the long spacing of 60.4 and 60.6 A suggests that the tilt angles of the PCA molecules are ca. 24' before and after irradiation.
1.
Accordingly, when A- or B-type PDA film was polymerized by the EB irradiation, polymer films of different crystal structures were obtained. The polymerization in the A-type PDA film proceeded topochemically, but in the B-type PDA film, the polymerization proceeded with changing layer structures. This indicates that the polymerization mechanisms on both types of PDA LB film are different from each other, as predicted by my earlier report.' Infrared RA Spectra of LB Films before and after the EB Irradiation. Infrared RA spectra of the LB films of A- and B-type PDA, HEA, and PCA are shown in Figure 3a-d. The absorption bands due to the CH3stretching vibrations (v,(CH3), 2960 cm-I; v,(CH3), 2880 cm-I), the CH, stretching vibrations (u,,(CH2), 2920 cm-I; u,(CH,), 2850 cm-I), the CO stretching vibration (u(C=O), 1720 cm-I), the CH, scissoring vibration (6(CH,), 1470 cm-I), the symmetric COO- stretching vibration (v,(COO-), 1430 cm-l), and the band progression due to the CH2wagging vibrations (w(CH,), 1350-1 200 cm-I) were observed in all the spectra. The weak absorption bands due to the stretching vibration of the conjugated C 4 (diacetylenic) group (u(c0nj C 4 ) , 2160 and 21 IO cm-I) were observed only in spectra A in Figure 3a,b. The presence of both v ( C 4 ) and v,(COO-) bands in Figure 3a,b indicates that the A- and B-type PDA films were composed of a mixture of acid and calcium salt of the PDA molecules. The fact that the relative intensities of the two bands are different between parts a and b of Figure 3 suggests that the acid and calcium salt compositions in the A- and B-type films were different. This may be due to the difference in the subphase conditions such as calcium concentration or pH at the preparation of the LB films. In order to explore the effect of the subphase conditions on the acid and calcium salt compositions in the films, the composition of calcium salt in the PDA LB films was plotted as a function of pH of the aqueous subphase containing two different calcium concentrations of 3 X IO4 and 1.3 X IO-' mol/L. The peak intensities of the u,(COO-)band in the spectra of the collapsed PDA films were used to evaluate the composition of calcium salt in the films. The results are shown in Figure 4.
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 7113
Pentacosadiynoic Acid LB Films
IO!
4 T y p PDA
3
4
5
8
7
6
9
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Figure 5. Possible molecular arrangements in the LB films of (a) A-type PDA, (b) B-type PDA, (c) HEA, and (d) PCA.
pn
Figure 4. Acid-salt composition curves on the subphase for building up A-type PDA LB films (a) and for B-type PDA LB films (b) as a function of pH. The concentrations were calculated by the intensities of the absorption band due to the u,(COO-). TABLE I: Iatwity Ratio of tbe u.(CH,) Band to tbe v.(CH3) Band
before EB irradiation after EB irradiation
1.6 1.6
1.4 1.5
1.3 1.4
2.0 1.8
From this figure, it was found that the A-type PDA film is composed of ca. 95% PDA acid and ca. 5% calcium salt, and the Btype PDA film is a mixture of ca. 30% acid and ca. 70% calcium salt. These results support that the molecular density of the B-type LB films is higher than that of the A-type LB film as shown in Figure 1, because metal salts have stronger intermolecular interaction than that of acid. In a typical LB film composed of an amphiphilic compound with the all-trans hydrocarbon chain, the intensity ratio of the u,(CH2) band to the u,(CH,) band in the RA spectra may be a sensitive measure of the orientation of the chain axis. If the hydrocarbon chain is aligned perpendicular to the substrate, the direction of the transition moment of the u,(CH,) band is parallel to the substrate and that of the u,(CH,) band is perpendicular to the substrate. Therefore, the intensity ratio should be zero. However, if the hydrocarbon chain is tilted, the ratio increases with increasing the tilt angle. From Figure 3a-d, the intensity ratio is found to decrease in the order of PCA film > A-type PDA film > B-type PDA film > HEA film, as shown in Table 1. This means that the tilt angle of the hydrocarbon chains in the films also decreases in the same order. The reason for the disagreement of the ordering between the X-ray diffraction analysis and 1R spectroscopy may be that, in the X-ray diffraction analysis, the calculation data of 45 and 3 3 O for the A- and B-type PDA films were obtained, respectively, under the supposition that the two hydrocarbon chains in the PDA molecule were on straight line. Nevertheless, the PDA molecule is bent at a diacetylenii: bond. If the bent angles are considered in the supposition, the ordering in the tilt angles obtained from the X-ray diffraction may become similar to that from IR spectroscopy. The u,,(CHJ bands of the A- and B-type PDA films were apparently stronger than those of the HEA and PCA films. This may depend on the difference in the orientation of the plane of the all-trans hydrocarbon chain. It is to be noted that after the EB irradiation, the u(conj m) bands in the PDA spectra (Figure 3a,b) disappeared, and instead the vcry wcak and broad band due to the conjugated C=C stretching vibrations (u(conj C-C)) appeared around 1600 cm-I. This indicatcs that the conjugated polymer having the polydiacetylenic or polyacclylenic bonds were produced by the irradiation. Although in thc HEA spectra (Figure 3c) a similar band was observed, it cannot be assigned to the conjugated double bonds,
Figure 6. Suggested reaction schemes for the two types of PDA LB films under EB irradiations: (a) A-type PDA film; (b) R-type PDA film.
because the band did not change before and after the EB irradiation. Possible Model of Molecular Arrangement in PDA LB Films. From the results of X-ray diffraction analyses and IR RAS measurements, possible molecular arrangements for the A- and B-type PDA films can be illustrated in comparison with those of the HEA and PCA LB films, as shown in Figure 5 . The molecular configuration in the A-type film can be shown by Figure 5a, in which the PDA molecular chain makes sharp bends at C-8 and C-13 of the carbon chain. This configuration may be reasonable from the experimental results that the long spacing of the unit bilayer was found to be 46.3 A by the X-ray diffraction analysis and that the infrared intensi'y ratio of the u,(CH2) band to the u,(CH,) band in the RA spxtra ( 1 .C w. between those of the HEA film (1.3) and the PCA film (2.1)) and larger than that of the B-type PDA film (1.4). This is also consistent with the fact that the A-type PDA film is largely expanded (the molecular area is 26 A*/molecule), as shown in the surface pressurearea curve (a) in Figure I . This configuration also seems to be favorable for the polymerization by 1 ,Caddition upon the EB irradiation. In the B-type films, the vertically oriented diynoic groups may be more closely packed than that in A-type films and stacked parallel with each other in the most largely condensed region, as shown in Figure 5b. In this configuration, 1,2- and 3.4-addition rather than successive 1 ,Caddition would occur favorably, and this configuration seems to be reasonable to maintain Ihe long spacing of the unit bilayer of 61 . I A. This is also consistent with the fact that the intensity ratio of the u,(CH2) band to the u,(CH,) I
7114 The Journal of Physical Chemistry, Vol. 95, No. 18, 1991
band of the B-type PDA film (1.4) is larger than that of the HEA film (1.3) and smaller than that of the A-type PDA film (1.6) and that the molecular area (20 A*/molecule) is between those of the HEA and A-type PDA films, as shown in the surface pressure-area curve (b) in Figure 1. Suggested Reaction Schemes in the PDA LB Films under the EB Irradiation. Taking accounts of the above results and Raman study published previously,’ we can conclude that the polymerization mechanisms of the A- and B-type PDA films are different from each other perhaps due to the arrangements of the unsaturated groups in the PDA LB films prepared under different subphase conditions. Thus, the suggested reaction schemes for the PDA films can be shown in Figure 6. The polydiacetylene-type polymer film could be obtained for the A-type PDA, and the polyacetylene-type polymer film could be formed for the B-type film, as shown by the scheme (a) and (b) in Figure 6, respectively. Th,is reaction process may also mean that the polyacene-type polymerization occurs in the B-type films by high-energy beam irradiation such as an accelerated EB.
Conclusions The typical PDA LB films with two different molecular densities were found to be polymerized to form different polymer LB films by the EB irradiation. Molecular arrangement or density of the PDA LB films was controlled by subphase conditions, such as pH, temperature of the subphase, and salt concentration and by surface pressure at which the film was prepared. It was suggested by the X-ray diffraction analysis and infrared RAS measurements that in the
Ogawa low-density PDA films the molecule has a ’bent” structure, while in the high-density PDA films the molecule has a ‘straight” structure in the LB film. In the low-density PDA LB films, polymerization proceeds topochemically; Le., the thickness decreased little by the EB induced polymerization. On the other hand, in the high-density PDA LB films, the thickness decreases about 10%by the EB irradiation in a helium atmosphere. From IR spectroscopy, it was also found that the conjugated diacetylenic bonds disappeared and the conjugated double bonds appeared after the EB irradiation. On the basis of the previously reported results’ and of those obtained in the present study, it can be concluded that 1 ,4-polymerization, i.e., polydiacetylene-type polymerization, occurs favorably in the low-density PDA LB film, whereas 1,2or 3,4-polymerization, i.e., polyacetylene-type polymerization, occurs in the high-density PDA LB films.
Acknowledgment. I thank Prof. Dr. T. Takenaka, Institute for Chemical Research of Kyoto University, Dr. M. Hatada, Director of Osaka Laboratories for Radiation Chemistry of Japan Atomic Energy Institute, and Dr. T. Nitta, Director of Central Research Laboratories of Matsushita Electric Industrial Co., Ltd., for their helpful comments. I also acknowledge the assistance of Mr. Matsuda of Osaka Laboratories for Radiation Chemistry, Japan Atomic Energy Institute for the EB irradiations, and Mr. H. Tamura, Mrs. K. Aono, and Mr. Y.Yamagata, and Mr. N. Mino of Central Research Laboratories of Matsushita Electric Industrial Co., Ltd. Registry No. PDA, 66990-32-7; PDA (homopolymer), 66990-33-8.
