Langmuir 1996,11, 3536-3541
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Molecular Aggregation State-Photopolymerization Behavior Relationship of Lithium 10,12-Heptacosadiynoate Monolayer Keisuke Kuriyama, Hirotsugu Kikuchi, Yushi Oishi,? and Tisato Kajiyama* Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan Received June 29, 1994. In Final Form: June 12, 1995@ This report is the first systematic study of the temperature effect on polymerization of lithium 10,12heptacosadiynoate monolayer with relation to its aggregation structure. A molecular aggregation state of the monolayer on the water surface was investigated on the basis of the subphase temperature (Tsp) dependencesof the elastic modulus and the electron diffraction (ED)pattern ofthe monolayer. The monolayer on the water surface was classified into a molten monolayer, a crystalline one and a glassy one, depending on Tspin comparison with the melting temperature (T,) of the monolayer on the water surface. The photopolymerization behaviors of the monolayers in various aggregation states were also investigated by the ultraviolet (UV) light irradiation time dependences of UV-visible absorption spectrum. "he photopolymerizationwas less reactive in the case of the monolayer in a molten state. On the other hand, the poly(diacety1ene)(PDA)blue form monolayer, which had the absorption peak at 640 nm, was formed upon photoirradiation to the crystalline monolayer. Moreover, in the case of the glassy monolayer, PDA red form monolayer, which had the absorption peak at 540 nm, was found to be formed by UV light irradiation. The delocalization length of a n-electron in the PDA red form would be shorter than that in the PDA blue form, as suggested by the wavelength of the main absorption peak corresponding to n-n* transition. The difference in the delocalization length of n-electron between the PDA blue form and the PDA red form could be explained by the lattice strain on conjugated PDA main chains caused during the polymerization reaction.
Introduction Nonlinear optical properties of conjugated polymers have been attracting considerable interest because of their potential use in optical devices, such as optical switching, nonlinear memory, and optical waveguide.'-4 In particular, poly(diacety1enes) (PDAs) have been extensively studied for their practical applications, because they possess a remarkably large value of the third-order nonlinear optical susceptibility, ~ ( ~ 1However, . the effective value of x(31for PDAs is not sufficient at the present time. The magnitude of x[3)for conjugated polymers such as PDAs strongly depends on the effective delocalization length of n-electron along a polymer main hai in.^-^ Therefore, in order to increase the magnitude of for PDA, n-electrons must be highly delocalized along the PDA backbone. PDA can be obtained by the solid state (topochemical) polymerization of diacetylene monomer single crystal.8 Then, a large single crystal of highly ordered monomer is required to obtain PDA with longer delocalization length of the n-electron along a polymer main chain. Moreover, for applications to integrated optical devices, PDA molecules must be assembled into
* Author to whom correspondence should be addressed. 7 Present address: Department of Applied Chemistry, Faculty
of Science a n d Engineering, Saga University, 1Honjo-machi, Saga 1 840, Japan. Abstract published inAdvance ACSAbstracts, August 15,1995. (1)Williams, D. J. Nonlinear Optical Properties of Organic and Polymeric Materials; American Chemical Society: Washington, DC, 1983. (2)Sautret, K.C.; Hermann, J. P.; Frey, R.; Pradere, F.; Ducing, J.; Baughman, R. H.; Chance, R. R. Phys. Rev. Lett. 1976,36,956. (3)Townsend, P. D.; Baker, G. L.; Schlotter, N. E.; Klausner, C. F.; Etemand, S. Appl. Phys. Lett. 1982,19, 53. (4)Kobayashi, T.Nonlinear Optics and Semiconductors; SpringerVerlag: Berlin, 1989. ( 5 ) Rustagi, K.C.; Ducing, J. Opt. Commun. 1974,10,258. (6)Hermann, J. P.; Ducing, J. J. Appl. Phys. 1974,45,5100. (7)Agrawal, G.P.; Cojan, C.; Flytzanis, C. Phys. Rev. 1978,B17, 776. (8) Wegner, G. Pure Appl. Chem. 1977,49,443.
*
thin films. In these points, the monolayer of diynoic acid on the water surface is suitable because of its ability to form a large two-dimensional single crystal with crystallographically superior q~ality.~JO In recent years, many studies have been carried out on both the polymerization process and photoreactivity of Langmuir-Blodgett (LB) films of various diacetylene derivative~.l'-~~ It was reported that the polymerization process and photoreactivity of diacetylene derivatives were strongly affected by the preparing conditions of the monolayer, such as pH of the subphase, species and concentration of the metal ion in the subphase. Also, the temperature during polymerization exerted much influence on the polymerization process and the photoreactivity of diacetylene derivatives. These preparing conditions of the monolayer and the temperature during polymerization were thought to govern the packing of the monomeric molecules,that is, the relative position of adjacent reactive 173-butadiyne-1,4-diyl groups. Thus, the aggregation structure of the monomeric monolayer of diacetylene derivatives might be an important factor of its polymerization, similarly to the case that diacetylene derivatives are in the bulk solid state. The relationship between the monomeric arrangement and the polymerizability of bulk diacetylene derivatives is well-established by Enkelman et d.16However, more investigations of the aggregation (9)KEyiyama,T.; Umemura, K.;Uchida, M.; Oishi,Y.; Takei, R. Chem. Lett. 1989,1515. (10)Kuriyama, K; Kajiyama, T. Bull. Chem. SOC.Jpn. 1993,66,2522. (11)Tomioka, Y.; Tanaka, N.; Imazeki, S. J. Chem. Phys. 1989,91, 5694. (12)Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978,16, 211. (13)Tamura, H.; Mino, N.; Ogawa, K. Thin Solid Films 1989,33, 179. (14)Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991,7, 2336. (15)Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983,99,95. Wegner, G.In Molecular Metals; Hatfield, W. E., Ed.; Plenum Press: New York, 1978. (16)Enkelman, V. In Polydiacetylenes; Cantow, H.-J., Ed.; SpringerVerlag, 1984.
0743-7463/95/2411-3536$09.00/00 1995 American Chemical Society
Aggregation State-Polymerization Relationship structures of the monolayers should be made to clarify the relationship between the molecular aggregation state of diacetylene monomer and its polymerization behavior. One of the authors have developed a n effective technique to investigate the aggregation structure of the fatty acid monolayer on the water surface on the basis of the electron diffraction (ED) study.17 Furthermore, thermal molecular motions in the fatty acid monolayer on the water surface have been studied from the thermomechanical measurement, that is, the subphase temperature (Tap) dependence of the two-dimensional modulus which is evaluated by the surface pressure-area h - A ) isotherm.18 The combination of thermomechanical and structural analyses has revealed that the molecular aggregation state in the monolayer on the water surface was strongly related to Tspin comparison with the melting temperature (T,) and the crystalline relaxation temperature (T,) of the fatty acid monolayers on the water surface. In this paper, a lithium 10,12-heptacosadiynoate monolayer was used as a monomer of PDA.lg The molecular aggregation state of the lithium 10,12-heptacosadiynoate monolayer on the water surface was investigated on the basis of the Tapdependences of both the elastic modulus and the ED pattern of the monolayer. Furthermore, photopolymerization behaviors of the monolayers in various aggregation states were investigated by the UV light irradiation time dependences of UV-visible absorption spectrum.
Experimental Section Monolayer Preparation. 10,12-Heptacosadiynoicacid (chromatographic reference quality) was used without further purification. Benzene with spectroscopic quality was used as a spreading solvent. A benzene solution of 10,12-heptacosadiynoic acid was prepared with a concentration of 2.0 x 10-3 mo1.L-1.20 The subphase water was purified by a Milli-&I1system (Millipore Co., Ltd.). The dimensions ofthe trough were 404 mm in length, 150mm in width, and 5 mm in depth. The subphase temperature, Tap,was varied in a temperature range of 283-308 K by circulating constant-temperature water around the aluminum support of the trough. The control accuracy of Tspwas &1K, which was evaluated by using a thermocouple positioned ca. 1 mm below the water surface. The monolayer was prepared by spreading the benzene solution of 10,12-heptacosadiynoic acid on the water subphase containing 2.0 x mo1.L-l of LiOH.21-23The monolayer was compressed to a given surface pressure at an area change rate of 2 x nm2molecule-%l. Modulus Measurement of Monolayer. n-A isotherms were obtained at various Tapvalues with a microprocessorcontrolled film balance system (FSD-20, US1 system Co., Ltd.). The static areal elasticity, K, of the monolayer on the water surface was evaluated from the n-A isotherm by using the following e q ~ a t i o n l ~ , ~ ~
K, shows the maximum at the relatively higher surface pressure close to the collapsing pressure. The maximum value of K , is denoted by Ks(max). At such a high surface pressure, the collapsed monolayer fragments were often observed as an appearance of patchy pattern on the base monolayer (the substrate monolayer on the water surface) in the electron micrographs. However, even though the collapsed monolayer (17)Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992,8, 1563. (18)KaJiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y.Bull. Chem. Soc. Jpn.1992, 65, 864. (19)Tieke, B.;Lieser, G. J. Colloid Interface Sci. 1982, 88, 471. (20)Tieke, B.;Weiss, K. J . Colloid Interface Sci. 1984, 101, 129. (21)Day, D.;Lando, J. B. Macromolecules 1980,13, 1478,1483. (22)Miyano, K.; Mori, A. Thin Solid Films 1989, 168, 141. (23)Miyano, K.; Mori, A. Jpn. J. Appl. Phys. 1989,28, 252. (24)Chen, Y. L.; Sano, M.; Kawaguchi, M.; Yu, H.; Zografi. G. Langmuir 1986,349, 2.
Langmuir, Vol. 11,No. 9, 1995 3537 fragments were observed, molecules in the base monolayer were packed most densely and homogeneously. Therefore, it is reasonable to consider that homogeneouscompression force was transmitted throughout the monolayer. Then, in this paper, the temperature dependence ofKs(m,) was adopted for investigation of the aggregation state of the lithium 10,12-heptacosadiynoate monolayer on the water surface. Substrate Preparation. The hydrophilic S i 0 substrate (static water contact angle = 30") was prepared by vapordeposition of S i 0 onto a Formvar-covered electron microscope grid (200 mesh) for electron diffraction study. The hydrophilic Si0 substrate is suitable for the electron microscopic structural investigation of the lithium 10,12-heptacosadiynoate monolayer on the water surface, since the crystal system of the monolayer on the water surface is stably maintained on the hydrophilic S i 0 substrate upon transferring the monolayer by the upward drawing m e t h ~ d . ' ~ JThis ~ is because the hydrophilic group of acid molecule contacts with the hydrophilic substrate. In other words, this situation of monolayer-substrate interface is similar to that of monolayer-water interface with respect to the magnitude of interfacial free energy between the hydrophilic group and the substrate. Also, the surface of the hydrophilic Si0 substrate was confirmedto be smooth and amorphous, based on morphological and ED studies, respectively. Then in this study, the monolayer was transferred onto the hydrophilic Si0 substrate by the upward drawing method at a transfer rate of 100 mmmin-1 at various TSpvalues. Electron Diffraction of Monolayer. ED patterns were taken with a Hitachi H-7000 electron microscope, which was operated at an acceleration voltage of 75 kV and a beam current ofO.5pA. The electron beamwas l0pmin diameter. ED patterns were taken at the same temperature as TSpat which the monomeric monolayer was prepared on the water surface. The temperature of the sample holder was controlled using a cryotransfer system (Gatan Co., Ltd.). Also, the sample was scanned by the electron beam widely over a millimeter range in order to confirm the structural homogeneity of the sample. PhotopolymerizationProcedure of Monolayer. Polymerization of the monolayer on the S i 0 substrate was carried out by UV light irradiation (UL1-5EB-6A,50 W, Ushio Co., Ltd.) at a distance of 28 cm in a N2 gas atmosphere. The temperature during W light irradiation corresponded to TBP.The irradiation time was 5, 10,20,40, and 60 min. The UV-visible absorption spectra ofthe monolayers were obtained using a Shimadzu MPS2000.
Results and Discussion Molecular Aggregation State of the Monomeric Monolayer. Figure 1 shows the Tspdependence of log Kscmru, for the lithium 10,12-heptacosadiynoatemonolayer on the water surface and, also, the ED patterns of the monolayers transferred onto the hydrophilic substrates at a surface pressure of 20 "am-'. The ED pattern of the lithium 10,12-heptacosadiynoate monolayer was independent of the magnitude of surface pressure. It was confirmed on the basis of the bright field electron microscopic observation that every monolayer was morphologically homogeneous at a surface pressure of 20 mNm-l. The plot of the magnitude of log K,cmax) vs Tap shows the maximum at around 293 K as shown in Figure 1. The ED patterns at 293, 298, and 303 K showed crystalline triclinic spots, a crystalline Debye ring, and an amorphous halo, respectively. Since the amorphous monolayer on the water surface above 303 K showed low it is apparently in a molten state. Therefore, it is reasonable that the melting temperature, T , ofthe lithium heptacosadiynoate monolayer on the water surface is around 300 K.l0 On the other hand, the amorphous monolayer observed below 288 K is thought to be in a glassy state, because the conformational misalignment along a lithium 10,12-heptacosadiynoate chain is frozen owing to strikingly fast evaporation of solvent. This quench effect in the monolayer on the water surface was confirmed by the ED pattern of the monolayer prepared
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by cooling the amorphous monolayer down to Tspof 283 K from Tspof 303 K at a cooling rate of 15 K0h-l and also at a surface pressure of 12 mN-m-l.l0 The ED pattern of the slowly cooled monolayer exhibited crystalline triclinic spots, which was similar to the ED pattern of the monolayer at Tspof 293 K. On the basis of structural studies mentioned above, the monolayer on the water surface was classified into a molten monolayer, a crystalline one, and a glassy one, depending on Tsp. The photopolymerization behaviors of these monolayers in various aggregation states were investigated. PhotopolymerizationBehaviors of Monolayers in Various Aggregation States. Figure 2 shows the UV irradiation time dependences of UV-visible absorption spectra of the monolayers in various aggregation states as a function of Tsp. The appearance of the absorption peak in each UV-visible absorption spectrum is derived from the extension of conjugated chain. Figure 2 indicates that the photopolymerization of monolayer took place upon irradiation of UV light, since there is no absorption peak of the monomeric monolayer in an observed wavelength range of 350-850 nm before irradiation of UV light. The characteristic absorbances for the polymerized monolayer a t Tspof 308 and 303 K were relatively low even after the UV irradiation for 60 min (Figure 2a,b) in comparison with other absorption spectra. Therefore, it seems apparent from parts a and b of Figure 2 that the monolayer in a molten state is nonliable to the polymerization reaction. Furthermore, the absorption spectra for Tspof 308 and 303 K have the broad absorption bands instead of the definite absorption peak (structure), and no clear tendency between intensity of absorbance and the W irradiation time was observed. It may be reasonable to consider that the polymers with a wide distribution of delocalization length of n-electron were formed by the UV irradiation, and also, both the polymerization reaction induced by the UV irradiation and the decomposition process due to the excess energy of the W irradiation occurred in the local portion of the monolayer simultaneously. The diacetylene molecules in the monolayer are movable in a random fashion with respect to the rotational and/or translational movement and the distance between adjacent 1,3-butadiyne-1,4-diylgroups has the maximum limitation for the polymerization reaction (topochemical polymeri~ation);~J~9~~ the photoreactivity of diacetylene derivatives is strongly related to the relative position of adjacent 1,3-butadiyne-1,4-diylgroups. In the case of the monolayer in a molten state, the polymerization reaction (25) Baughman, R. H. J.Polym. Sci., Polym. Phys. Ed. 1973,11,603.
may proceed only in the case that the distance between diacetylene molecules happened to be close enough to polymerize by its vigorous thermal molecular motions. For photopolymerization, the activation barrier for the addition process of a monomer is overcome via the photon energy.26 Thus, under sufficient UV irradiation, almost all molecules are ready to react, and whether the addition reaction will occur depends only on the relative position of adjacent 1,3-butadiyne-1,4-diylgroups. Furthermore, since the energy of the UV irradiation is rarely consumed in the polymerization reaction in this case, a part of the excess energy may cause a decomposition of conjugated chains, as suggested by the observed decrease in the absorption band intensity. The intensity of the absorption peak observed at Tspof 298 K increased with the UV irradiation time up to 10 min but started to decrease upon UV irradiation more than 20 min (Figure 2c). This indicates that the polymerization reaction proceeded by UV irradiation up to 10 min, while prolonged exposure of UV light caused a decomposition of conjugated polymer chains. It can be apparently concluded from the azimuthally extended arcs as shown in an inserted ED pattern at 298 K in Figure 1that the positional and/or conformational arrangements of diacetylene molecules in the monolayer at 298 K were relatively disordered even in a crystalline state owing to an active thermal molecular motion. Then, a fraction of polymerizablediacetylene molecules in the monolayer at 298 K should be rather small compared with that at lower temperature. This means that the polymerization reaction can occur locally in a limited portion of the monolayer in which the distance between diacetylene molecules is close enough. The remarkable absorption peak at 640 nm and the weak and broad shoulder at ca. 580 nm, which are assigned to the n-n* transition (excitonic absorption) and the phonon sideband of PDA, re~pectively,~~ were observed in the absorption spectrum for the monolayer at Tspof 293 K (Figure 2d). A highly ordered crystalline phase was recognized for Tspof 293 K as mentioned by the ED pattern in Figure 1. Hence, it can be expected that the PDA monolayer composed of fairly long conjugated chains is obtained by the photopolymerization of the highly ordered crystalline monolayer. PDA, which has the absorption peak a t 640 nm, is designated as the blue form.28 The absorption spectrum of the monolayer in the case of Tsp of 293 K shows a similar profile at each UV irradiation time. No degradation of the absorption peak (structure) due to the decomposition process was observed. Moreover, Figure 3 shows the UV irradiation time dependence of the absorption peak maximum at wavelength of 640 nm being shown in Figure 2d. The absorption intensity at 640 nm increases remarkably in an early stage of UV irradiation and saturates on further UV irradiation. This indicates that the polymerization reaction almost completes upon UV irradiation of around 60 min. If diacetylene molecular packing in the monolayer is highly ordered, they are sufficiently close together to polymerize, as shown in the case of Tspof 293 K. And the amount of diacetylene molecules which possess an opportunity to polymerize may be very large in the case of the highly ordered crystalline monolayer. Moreover, in the cases of the absorption spectra for the monolayer prepared a t 288 and 283 K, the absorption peak at 640 nm appeared in the early stage of irradiation (26) Prock, A.; Shand, M. L.; Chance, R. R. Macromolecules 1982,15, 238. (27) Tokura, Y.; Oowaki, Y.; Kaneko, Y.; Koda, T.; Mitani, T. J.Phys. SOC.Jpn. 1984, 53, 4054. (28) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980,68,77.
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