Langmuir 2000, 16, 3337-3344
3337
Polymerization of Modified Diacetylenes in Langmuir Films A. S. Alekseev,† T. Viitala,*,‡,§ I. N. Domnin,| I. M. Koshkina,| A. A. Nikitenko,† and J. Peltonen*,‡ General Physics Institute, Russian Academy of Sciences, Vavilov Street 38, 117942 Moscow, Russian Federation, Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland, and Institute of Chemistry, St. Petersburg University, University pr. 2, 198904 St. Petersburg, Peterhof, Russian Federation Received March 24, 1999. In Final Form: September 27, 1999 The polymerization of a series of diacetylene Langmuir monolayers modified in the hydrophobic as well as in the hydrophilic part of the molecule was studied by measuring the kinetic changes of the mean molecular area and UV-vis spectra. The polymerization properties of the monolayers were dependent on the position of the diacetylenic unit and whether an ester or sulfonyl group was introduced in the hydrocarbon chain. The substances containing a sulfonyl group showed the highest polymerization activity. The blueto-red transition usually observed for diacetylenes in Langmuir and Langmuir-Blodgett films due to UV irradiation was not observed, instead the studied substances formed directly a red form polydiacetylene. A first-order consecutive reaction kinetic model with a steady-state approximation was successfully adapted to the experimental results of three of the studied compounds.
Introduction 1 on the solid-state
Since the pioneering work of Wegner polymerization of diacetylenes in the late 1960s and early 1970s, polydiacetylene (PDA) chemistry and physics have become an integral part of modern polymer science.2-15 The polymerization reaction of diacetylenes (DAs) has apart from solid-state reactions in single crystals also been applied to organized molecular systems such as model biomembranes,16-19 tubules,20 and liquid crystals.21,22 In * Corresponding author. E-mail:
[email protected]. † Russian Academy of Sciences. ‡ Åbo Akademi University. § Graduate School of Materials Research, Turku, Finland. | St. Petersburg University. (1) See, for example: Wegner, G. Z. Naturforsch. 1969, 24B, 824. Wegner, G. Macromol. Chem. 1971, 145, 85. (2) Bloor, D.; Chance, R. R. Polydiacetylenes; NATO ASI Series E; Applied Science; Martinus Nijhoff Publishers: Dordrecht, The Netherlands, 1985. (3) Eichele, H.; Schwoerer, M.; Huber, R.; Bloor, D. Chem. Phys. Lett. 1976, 42, 342. (4) Patel, G. N.; Witt, J. D.; Khanna, Y. P. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 1383. (5) Patel, G. N.; Bhattacharjee, H. R.; Preziosi, A. F. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 511. (6) Patel, G. N.; Lee, L. T. C. J. Macromol. Sci. Phys. 1983, B22, 259. (7) Kanetake, T.; Tokura, Y.; Koda, T. Solid State Commun. 1985, 56, 803. (8) Orchard, B. J.; Tripathy, S. K. Macromolecules 1986, 19, 1844. (9) Eckhardt, H.; Boudreaux, D. S.; Chance, R. R. J. Chem. Phys. 1986, 85, 4116. (10) Rubner, M. F.; Sandman, D. J.; Velazquez, C. Macromolecules 1987, 20, 1296. (11) Tanaka, H.; Gomez, M. A.; Tonelli, A. E.; Thakur, M. Macromolecules 1989, 22, 1208. (12) Sinclair, M.; McBranch, D.; Heeger, A. J. Synth. Met. 1989, 28, D645. (13) (a) Wenzel, M.; Atkinson, G. H. J. Am. Chem. Soc. 1989, 111, 6123. (b) Greene, B. I.; Orenstein, J.; Schmitt-Rink, S. Science 1990, 247, 679. (14) Hasegawa, T.; Ishikawa, K.; Kanetake, T.; Koda, T.; Takeda, K.; Kobayashi, H.; Kubodera, K. Chem. Phys. Lett. 1990, 171, 239. (15) Beckham, H. W.; Rubner, M. F. Macromolecules 1993, 26, 5198. (16) O’Brien, D. F.; Whitesides, T. H.; Klingbiel, R. T. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 95. (17) Hub, H. H.; Hupfer, B.; Koch, H.; Ringsdorf, H. J. Macromol. Sci. Chem. 1981, A15, 701.
this respect PDAs are one of the most systematically studied groups of materials. The large interest toward PDAs originates from their unique optical, mechanical, and chemical properties. During the past 3 decades DA derivatives have been the subject of a large number of studies concerning their production and structural characterization and applications in Langmuir-Blodgett (LB) technology.23-42 Concerning practical applications, thin solid films obtained (18) Hupfer, B.; Ringsdorf, H.; Schupp, H. Chem. Phys. Lipids 1983, 33, 355. (19) Reichert, A.; Nagy, J. O.; Spevak, W.; Charych, D. J. Am. Chem. Soc. 1995, 117, 829. (20) Lando, J. B.; Hansen, J. E.; Sudiwala, R. V.; Rickert, S. E. Polym. Adv. Technol. 1990, 1, 27. (21) Schen, M. A.; Kotowski, K.; Cline, J. Polymer 1990, 32, 1843. (22) Tsibouklis, J. Adv. Mater. 1995, 7, 407. (23) Roberts, G., Ed. Langmuir-Blodgett Films; Oxford University Press: Oxford, U.K., 1990. (24) Arslanov, V. V. Adv. Colloid Interface Sci. 1992, 40, 307. (25) Day, D.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. (26) Day, D.; Ringsdorf, H. Macromol. Chem. 1979, 180, 1059. (27) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1982, 99, 95. (28) Hupfer, B.; Ringsdorf, H. Chem. Phys. Lipids 1983, 33, 263. (29) Tieke, B.; Weiss, K. Colloid Polym. Sci. 1985, 263, 576. (30) Tokura, Y.; Nishhikawa, S.; Koda, T. Solid State Commun. 1986, 59, 393. (31) Go¨bel, H. D.; Gaub, H. E.; Mo¨hwald, H. Chem. Phys. Lett. 1987, 138, 441. (32) Bubeck, C. Thin Solid Films 1988, 160, 1. (33) Fischetti, R. F.; Filipowski, M.; Garito, A. F.; Blasie, J. K. Phys. Rev. B 1988, 37, 4714. (34) Halperin, K.; Peng, J. B.; Gadwood, R.; Dutta, P. J. Polym. Sci., Polym. Phys. 1989, 27, 1289. (35) Ogawa, K. J. Phys. Chem. 1989, 93, 5305. (36) Meller, P.; Peters, R.; Ringsdorf, H. Colloid Polym. Sci. 1989, 267, 97. (37) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991, 7, 2336. (38) Goettgens, B. M.; Tillmann, R. W.; Radmacher, M.; Gaub, H. E. Langmuir 1992, 8, 1768. (39) Yanusova, L.; Klechkovskaya, V.; Sveshnikova, L.; Stiopina, N.; Kruchinin, V. Liq. Cryst. 1993, 14, 1615. (40) Deckert, A. A.; Fallon, L.; Kiernan, L.; Cashin, C.; Perrone, A.; Encalarde, T. Langmuir 1994, 10, 1948. (41) Tillmann, R. W.; Hofmann, U. G.; Gaub, H. E. Chem. Phys. Lipids 1994, 73, 81. (42) Lio, A.; Reichert, A.; Ahn, D. J.; Nagy, J. O.; Salmeron, M.; Charych, D. Langmuir 1997, 13, 6524.
10.1021/la990351v CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000
3338
Langmuir, Vol. 16, No. 7, 2000
from amphiphilic DA molecules by the LB technique have many advantages in comparison with single-crystal PDAs.43 The thermal and mechanical stability of the polymerized DA thin films in conjunction with the specific characteristics of the film itself, or characteristics implemented by other functional groups, could in the future progressively be employed in microelectronics, integrated optics, biosensors, and the development of other molecular devices.44-47 The use of DA compounds for this purpose is largely justified by the ease of producing an ultrathin and highly ordered polymer thin film by simply irradiating the monomeric mono- or multilayer structure by UV light. The polymerization process can be easily followed by UVvis spectroscopy because of the fact that the PDA film exhibits an extremely high optical density in the visible region.48-52 This also enables the determination and control of the degree of polymerization and modeling of the reaction kinetics. However, only a couple of studies concerning the modeling of reaction kinetics of DAs in LB films have been published,53,54 and to our best knowledge, no paper has been published concerning the modeling of the polymerization kinetics of diacetylene Langmuir (L) films. This is quite surprising because successful modeling of the solid-state polymerization kinetics of DA crystals has been reported.1,2,55,56 The photochemical reaction of DA is known to be a topochemical polymerization (TP) reaction, which means that almost no change of the mean molecular area (Mma) occurs during the reaction.1 It is very well-known that the reaction does not occur unless the reactive groups have a certain relative distance and orientation with respect to each other.2,16,21,28,32,48,49 The packing of the molecules in the mono- or multilayer is influenced by various factors, such as the composition of the hydrophobic and hydrophilic parts of the molecule, the subphase composition and temperature, pressure, and the spreading solvent.17,27,28,35,39,48-52 A suitable combination of these factors or properties may lead to a floating monolayer that can be compressed to a solid state at the air-subphase interface. However, one should bear in mind that also a too tightly packed monolayer can inhibit the reaction by not leaving enough space for polymerization to occur;25,35,39,51,52 i.e., the optimal distance or orientation between the diyne units is not provided because of, for example, staggering. Most of the studied DAs in L and LB films have been straight-chain hydrocarbon compounds mainly consisting of carboxylic acid derivatives or two-chain lipids with various hydrophilic headgroups.17,25-41,48-54 All of these compounds have been shown to react readily at the air(43) Chemla, D. S., Ziss, J., Eds. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: Orlando, FL, 1987. (44) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (45) Pitt, C. W.; Walpita, L. M. Thin Solid Films 1980, 68, 101. (46) Grunfeld, F.; Pitt, C. W. Thin Solid Films 1983, 99, 249. (47) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993, 261, 585. (48) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631. (49) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. (50) Collins, M. A. J. Polym. Sci., Polym. Phys. 1988, 26, 367. (51) Tamura, H.; Mino, N.; Ogawa, K. Thin Solid Films 1989, 179, 33. (52) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1992, 8, 594. (53) Laxhuber, L. A.; Scheunemann, U.; Mo¨hwald, H. Chem. Phys. Lett. 1986, 124, 561. (54) Deckert, A. A.; Horne, J. C.; Valentine, B.; Kiernan, L.; Fallon, L. Langmuir 1995, 11, 643. (55) Chance, R. R.; Patel, G. N. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 859. (56) Baughman, R. H. J. Chem. Phys. 1978, 68, 3110.
Alekseev et al. Chart 1. Molecular Structure of the Diacetylenes Used in This Study
water interface or as multilayers. Using a set of longchain diynoic acids, the influence of the structural features on the polymerization reaction and quality of the formed DA monolayers have been studied.27,49,50 The acids with the DA fragment close to the carboxylic headgroup formed the most condensed and stable monolayers. On the contrary, positioning the DA group in the middle of the alkyl chain facilitated TP because of higher flexibility needed for the change of the crystal structure during the polymerization. The molecular design of an amphiphilic DA must guarantee a reasonable compromise between these two conflicting tendencies. A much less studied DA group is the alcohol derivatives, which are mainly concerned in this study (Chart 1). The alcohol derivatives are interesting because they have a smaller polar headgroup than the carboxylic acid derivatives and they are less sensitive to pH changes of or small impurities in the subphase.57-59 Therefore, the optimal conditions for TP are reached simply by changing the surface pressure or by changing the composition of the hydrocarbon chain. The alcohol derivatives have also been found to form ordered domain type arrays, while the acid derivatives did not show this kind of arrangement.39 It has been shown that a greater volume change in diacetylenic acid than in the corresponding alcohol multilayers during polymerization leads to crack formation in the acid polymer.58 In this paper we have investigated the photopolymerization of a set of DA derivatives modified in the hydrophobic as well as in the hydrophilic part of the molecule (Chart 1). One of the main points was to study how a sulfonyl or ester group in the hydrocarbon chain affects the monolayer formation and polymerization kinetics. The photopolymerization kinetics was investigated using in situ reflection absorbance spectroscopy of the monolayers at the air-water interface. The changes in the Mma during UV irradiation of the monolayers were also measured. The color changes of the monolayers because of UV irradiation were used to adapt a simple first-order kinetic model for the photopolymerization reaction. Experimental Details Materials. Following the systematic investigation of amphiphilic DAs with different extensions and polarities of both the hydrophobic and hydrophilic parts of the molecules,60-62 we (57) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966. (58) Sarkar, M.; Lando, J. B. Thin Solid Films 1983, 99, 119. (59) Okada, S.; Matsuda, H.; Nakanishi, H.; Kato, M.; Otsuka, M. Thin Solid Films 1989, 179, 423. (60) Grudinin, A. L.; Koshkina, I. M.; Domnin, I. N.; Moritz, V.; Lemmetyinen, H. Russ. J. Gen. Chem. Engl. Transl. 1996, 66, 967. (61) Grudinin, A. L.; Koshkina, I. M.; Domnin, I. N. Russ. J. Org. Chem. Engl. Transl. 1993, 29, 1620.
Modified Diacetylenes in Langmuir Films carried out the synthesis and investigations of the monolayer polymerization properties of the DAs of Chart 1. The choice of the diynes (DA1-DA5) was based on the following reasons. 2,4-Hexadiyne-1,6-diol bis(p-toluenesulfonate) is one of the most extensively studied DA,1-3,55,56 possessing high TP activity as a bulk sample. We thus expected that an introduction of a sulfonyl group in the diacetylenic amphiphilic alcohol structure would lead to a high TP rate of the made-up monolayers (compounds DA3 and DA4). It has been shown by reflection and transmission electron diffraction techniques that monolayers of diacetylenic alcohol with an ester group in the hydrophobic part such as DA2 of this work contain small (50 Å) ordered regions.39 DA5 with an amide group in the hydrophobic part and an isonicotinic acid ester in the hydrophilic part of the molecule was prepared by taking into account the following considerations. First, amide groups forming intermolecular hydrogen bonds increase the monolayer cohesion, which is expected to enhance the TP process. Second, salts of isonicotinic acid esters polymerize rapidly under UV or γ irradiation, forming PDA soluble in chloroform.29 This enables their investigation by conventional spectroscopic methods. DA1,60 DA2,61 DA3,62 and DA560 were prepared by published procedures while DA4 was synthesized by the following procedure. 14-Hydroxy-10,12-tetradecadiynilic ester of hexadecanesulfonic acid was prepared by treatment of tetradeca-2,4-diyne1,14-diol with hexadecanesulfonyl chloride in a CH2Cl2 solution in the presence of K2CO3. The reaction mixture was stirred at 45 °C under sonication for 4 h. The crude product was purified by silica gel column chromatography (yield 21%, mp 63-64 °C). Langmuir Monolayer Formation. A computer-controlled KSV 3000 Langmuir trough (KSV Instruments Ltd., Finland) with a symmetric compression and a Pt-Wilhelmy balance was used for the monolayer studies. The trough was placed in a laminar flow cabin on a heavy stone table, and it was protected against natural UV light with an absorbing yellow semitransparent plastic sheet. The amphiphilic diynes DA1-DA4 were dissolved in CHCl3 (concentration ca. 0.5-1.0 mg/mL) while a mixture of CHCl3 (95 vol %) and CF3COOH (5 vol %) was used in the case of DA5. Monolayers were obtained by spreading the solutions on a water subphase having a pH of 5.8. The solvent was allowed to evaporate for a sufficiently long time before the compression of the monolayer was started. The monolayer compression speed was 10 mm/min. The water used as the subphase was purified with a Millipore Milli-Q filtering system (Millipore Corp., Bedford, MA) yielding a resistance of 18 MΩ cm. The temperature of the subphase was maintained at 19 ( 0.5 °C for all measurements. Monolayer Photopolymerization and Reaction Kinetics Study. The irradiation of the monolayers was carried out in ambient air using a 30 W low-pressure mercury lamp (maximum emission at 254 nm) placed 0.25 m above the monolayer. The reaction kinetics of the monolayer was monitored as the change in barrier speed and Mma while simultaneously keeping the surface pressure at a constant value. The UV light was not turned on before the monolayer was stabilized for a sufficiently long time; i.e., the barrier speed did not exceed 0.5 mm/min. In Situ Spectroscopy. The in situ spectroscopic studies of the photopolymerization of the diyne monolayers were carried out with a Photal MCPD-100 UV-visible spectrophotometer (Otsuka, Japan). The visible light of the source was guided to the air-water interface with an optical fiber with an angle of about 45° from the surface normal. The signal reflected from the mirror on the bottom of the trough was detected by another fiber probe, which was connected to the detector. The change in intensity of the reflected light as a function of UV irradiation time was detected simultaneously with the changes in barrier speed and Mma.
Results and Discussion Surface Pressure-Area Isotherms. The surface pressure-area isotherms of the studied diynes are presented in Figure 1. As compared with the isotherm of (62) Alekseev, A. A.; Domnin, I. N.; Grudinin, A. L.; Lemmetyinen, H. Unpublished results.
Langmuir, Vol. 16, No. 7, 2000 3339
Figure 1. Surface pressure-area isotherms: (a) (- -) DA1, (‚‚‚) DA2, (s) DA3, (-‚‚-) DA4 and (b) DA5.
DA1, the introduction of the ester group in the hydrocarbon chain (DA2) had only a minor effect on the compression isotherm. The most pronounced change in the isotherms as compared with that of DA1 (regarded as a reference compound) was observed when a sulfonyl group (DA3 and DA4) was introduced in the chain (Figure 1a). The DA1 has the simplest chemical structure among the studied substances with the lowest extrapolated Mma of 21.8 Å2/molecule. The introduction of an ester group (DA2) slightly increased the Mma to 22.2 Å2/molecule but simultaneously decreased the compressibility and increased the collapse pressure of the monolayer. This indicates that the ester group did not introduce any flexibility to the hydrocarbon chain but rather affected the characteristic packing of the molecules resulting in a monolayer of slightly increased rigidity. The more expanded nature, i.e., higher compressibility and a slightly larger Mma of 23.2 Å2/molecule, of the DA3 monolayer clearly showed that the flexibility of the hydrocarbon chain due to the sulfonyl group was higher than that of the DA1 and DA2 monolayers. The effect of the longer hydrocarbon chain between the -OH group and the diacetylenic moiety in DA4 was only seen as an increase in the extrapolated Mma (26.2 Å2/molecule) as compared with DA3. The compressibility of these two substances appeared identical. The extrapolated Mma of the DA5 monolayer was 51.2 Å2/molecule which is much higher than the Mma of any other compound of this study (Figure 1b). The DA5 monolayer exhibited a much higher compressibility than the monolayers shown in Figure 1a. This kind of behavior was expected because of the fact that a pendant linker with a relatively large pyridyl group was included in the polar part of the molecule. Polymerization of the Monolayers. The monomeric DAs of this study were optically inactive in the visible wavelength region, as was obvious from the spectral baseline that was measured before the compression and UV irradiation of the monolayer. The observation of the monolayer polymerization by UV-vis absorption spec-
3340
Langmuir, Vol. 16, No. 7, 2000
Alekseev et al.
Figure 3. In situ UV-vis absorption spectra of DA5 as a function of UV irradiation time at a surface pressure of 29 mN/m.
Figure 2. Change in Mma during UV irradiation of the monolayer at a constant surface pressure: (- -) DA1, 20 mN/ m; (‚‚‚) DA2, 35 mN/m; (s) DA3, 30 mN/m; (-‚‚-) DA4, 30 mN/m; (- - -) DA5, 29 mN/m.
troscopy was expected to be possible because of the high extinction coefficient of the polydiacetylene end product.48-52 Among the substances of this study, the DA4 monolayer showed the largest absorbance values and welldistinguished absorption bands as a result of the UVinduced polymerization. The monolayers of the rest of the substances in this study, on the contrary, showed only weak (DA2, DA3, and DA5) if any (DA1) absorbance after UV irradiation probably because of the formation of oligomers rather than a polymer backbone, which can be assumed to be the case for the DA4 monolayers. The Mma of the most condensed monolayer DA1 increased because of the UV irradiation and then reached a maximum after about 10 min, after which it began to decrease slowly (Figure 2). The expansion of the monolayer at the beginning of the UV irradiation could be due to an initially too tightly packed monolayer or due to a reaction of the diacetylenic moieties with the subphase or a combined effect of these two. The interaction of the reaction intermediate (possibly a carbene3) with the subphase was possible because of the fact that the DA moiety of DA1 is located quite close to the polar headgroup near the airwater interface. The conclusion of a very low polymerization activity of DA1 was supported by the absence of any absorbance band of the UV-irradiated L monolayer. The monolayers of DA2 and DA5 turned out to be slightly more reactive than DA1, showing measurable absorbances after UV irradiation. The absorbance bands were, however, very broad, ranging from ca. 400 to 600 nm with no welldefined form (see Figure 3 for DA5). The absorbance values were almost an order of magnitude less than the maximum absorbance of the DA4 monolayer, which showed the highest activity. An interesting feature of the DA5 compound was that it showed a polymer form which had an absorbance band located at clearly longer wavelengths (570 nm) than the bands of the other polymers studied here. The broad and low-intensity band of the DA2 and DA5 monolayers is believed to originate from the formation of oligomers of different structure. The absorption behavior of DA5 was probably due to the orientation of the isonicotinic acid ester in the hydrophilic part of the molecule, which presumably favored the formation of
Figure 4. In situ UV-vis absorption spectra of DA3 as a function of UV irradiation time at a surface pressure of 30 mN/m: (a) when the polymer is forming and (b) when the polymer is degrading.
oligomers with planar acetylenic bonds. The UV-induced changes of the Mma of DA2 and DA5 monolayers had the same trend as was observed for DA1 (Figure 2). Thus, the same kind of reasoning as was done for DA1 concerning the reactivity could also be done for DA2 and DA5. The low polymerizability of DA5 is also probably a consequence of the monolayer being in the liquid-expanded state (see Figure 1b) and thus too loosely packed for a successful topochemical polymerization. The DA3 and DA4 compounds seemed to be the most suitable substances for the production of a polymeric monolayer. Therefore, a thorough investigation was carried out for these monolayers. The changes in absorption spectra as a function of UV irradiation time at a surface pressure of 30 mN/m are shown in Figures 4 and 5 for DA3 and DA4, respectively. Similar measurements were also carried out at surface pressures of 10 and 20 mN/m with results not differing much from those shown in Figures 4 and 5. The respective changes in Mma at different surface pressures are shown in Figure 6.
Modified Diacetylenes in Langmuir Films
Figure 5. In situ UV-vis absorption spectra of DA4 as a function of UV irradiation time at a surface pressure of 30 mN/m: (a) when the polymer is forming and (b) when the polymer is degrading.
Figure 6. Change in Mma during UV irradiation of the monolayer at different surface pressures: (a) DA3 and (b) DA4. Surface pressure (nN/m): (____) 10, (- -) 20, and (‚‚‚) 30.
The monomer to polymer conversion of DA3 as followed by in situ absorption spectroscopy was almost completed (the absorption band reached maximum intensity) during the first 6-7 min of UV irradiation at 30 mN/m (see Figure 4a). The absorption spectra showed an absorption maximum at 535 nm and a shoulder at about 500 nm. The Mma of the DA3 monolayer decreased by less than 5% from the initial value during this time which is a clear indication of a TP reaction (Figure 6a). With further UV irradiation (7-11 min) the Mma started to decrease and
Langmuir, Vol. 16, No. 7, 2000 3341
the absorption maximum centered at 535 nm shifted to about 515 nm (Figures 4a and 6a). Part of these effects could be a consequence of a reorganization process, but most likely these changes refer to the initiation of a degradation of the polymer backbone. After about 11 min of UV irradiation, the absorption bands centered at 515 and 500 nm started to decrease, which indicated that degradation was taking place17,28,35 (Figure 4b). When the monolayers were UV irradiated at 10 or 20 mN/m, the Mma started to decrease slightly later. This is suggested to result from a slower collapse of the degraded material at lower surface pressures. The polymer formed within the first 10 min had almost the same absorption behavior irrespective of the surface pressure, which indicates a degree of orientation high enough to permit TP at all pressures. This is also in accordance with an earlier study where it was found that the polymerization of DA acids could be carried out at any surface pressure.25 The highest photopolymerization reactivity was observed for DA4 (Figure 5). The absorption spectra for a monolayer UV irradiated at 30 mN/m had two distinct maxima at 511 and 474 nm. These maxima did not change position during the UV irradiation. The maximum absorbance value due to the monomer to polymer conversion was achieved after ca. 11 min of irradiation for the peak at 511 nm and after ca. 20 min for the peak at 474 nm. The Mma decreased by less than 5% during the first 11 min of UV irradiation, after which the decrease was slightly accelerated (Figure 6b). With further UV irradiation, the absorbance started to decrease because of polymer degradation and the Mma continued to decrease more rapidly. The two absorption maxima are suggested to arise from two different polymer forms with characteristic molecular weight. When the degradation starts, it first appears as a decrease in intensity of the high molecular weight component (the 511 nm band). The resulting polymer fractions with smaller molecular weight contribute to the intensity of the respective absorption band centered at 474 nm, thus increasing its intensity. The 474 nm band is hence a superposition of the initially forming short-chain polymer and the degradation product of the longer-chain polymer. With proceeding degradation also the 474 nm band vanishes. The same trends in the absorption spectra were also observed for the monolayers UV irradiated at 10 and 20 mN/m, thus clearly indicating that the organization of the molecules in the monolayer was identical at the studied surface pressures, with the same spatial constraints for the polymerization process. However, the Mma behaved differently for the monolayer irradiated at 10 mN/m as compared with the monolayers irradiated at 20 and 30 mN/m (Figure 6b). In the former case an expansion of the Mma was observed after 20 min of irradiation, which could be an indication of a slightly looser packing of the formed polymer backbone. This would consequently enable an ozonolysis of the conjugated backbone, which manifests itself as an increase in Mma.25 Previous studies on diacetylene L and LB films have mainly concerned molecules with one or two plain hydrocarbon chain(s). The blue form of a PDA polymer has been reported to have absorbance within the wavelength range 550-660 nm and that of the red form within 450-540 nm.4,7,17-19,26-30,33,38-42,48,49,52 Taking this as the basis, only the red form was observed for the UV-irradiated DA2, DA3, and DA4 monolayers. The absence of the blue form is quite unusual for PDA L or LB films, although similar behavior has been reported for phosphatidylcholine diacetylene vesicles.16,18 The chemical and space structures of the DA2, DA3, and DA4 monomer molecules with the ester or sulfonyl group in the acyl chain lead to a packing
3342
Langmuir, Vol. 16, No. 7, 2000
Alekseev et al.
and orientation which favored direct formation of the red polymer form as seen in Figures 4 and 5. This conclusion is supported by many studies which have shown that the formation of the blue form PDA polymer and subsequent transition to the red form polymer largely depends on the orientation of the side groups bound to the DA moiety.4,6-11,13,15,30,37,40,42,49,54 The side groups consequently affect the effective conjugation length of the formed polymer backbone so that the backbone in the blue polymer form is more planar than that in the red polymer form. A recent AFM study on PDA LB films nicely showed how the packing of the hydrocarbon chains at a molecular level changed from a cubic to a hexagonal arrangement when going from the blue to the red form.42 In that study it was also concluded that the hydrocarbon side chains in the red polymer form have more trans character than those in the blue polymer form, which also has been concluded in other studies.11,15 In our case, the absence of the blue form polymer may also refer to a case where the transition from the blue to the red form polymer was so fast that it was beyond the time resolution of the spectrophotometer. In summary, the better polymerization properties of DA3 and DA4 monolayers are suggested to be due to the following reasons. First, the sulfonyl group in the hydrophobic part of the molecule decreased the rigidity of the monolayer as compared with the other substances of this study (except for DA5 which also otherwise was an exception), increasing the TP reactivity. The same trend has been observed for the same substances polymerized on a solid substrate.62 Second, the flexibility of the DA4 molecules was much higher as compared with all others because of a (CH2)9 group between the hydrophilic and the diacetylenic unit. This gave more freedom for the molecules to pack in a favorable manner for the reaction and to reorganize during the polymerization process. Third, the distance between the DA unit and the water subphase as increased by the introduction of a (CH2)9 group probably contributed to the enhanced polymerization yield through a decreased interaction of the reaction sites with the subphase. Consistently, earlier papers have shown that an incorporation of a relatively long hydrocarbon chain between the diacetylene fragment and the hydrophilic headgroup improves the polymerization properties of a multilayer.27,49,50 Reaction Kinetics. Most of the DA derivatives found in the literature have shown a tendency to first produce a blue form of the polymer and upon further reaction or change of the chemical environment transform into the red form. Such a consecutive reaction can be described by the following reaction scheme:
M9 8B9 8R k k M
B
(1)
where M is the monomer, B is the blue form of the polymer, R is the red form of the polymer, and ki’s are the reaction rate constants. This kind of behavior was shown by Hofmann et al.63 to apply to an EMPDA monolayer when it was polymerized at the air-water interface and by Laxhuber et al.53 when a DA acid was polymerized as a multilayer. For a constant initiation rate, e.g., radiation polymerization, DAs are expected to follow first-order kinetics.2,67 Additionally, when a TP reaction in an L or (63) Hofmann, U.; et al. Unpublished results. (64) Rolandi, R.; Dante, S.; Gussoni, A.; Leporatti, S.; Maga, L.; Tundo, P. Langmuir 1995, 11, 3119. (65) Viitala, T. J. S.; Peltonen, J.; Linde´n, M.; Rosenholm, J. B. J. Chem. Soc., Faraday Trans. 1997, 93, 3185. (66) Viitala, T.; Peltonen, J. Biophys. J. 1999, 76, 2803. (67) Sixl, H.; Neumann, W. Mol. Cryst. Liq. Cryst. 1984, 105, 41.
LB film is concerned, one does not have to take into account the very small time-dependent change of the area, being significantly different as compared with monolayers for which the area decreases significantly during the reaction process.64-66 Thus, by applying a consecutive first-order mechanism to eq 1, we get for the rate of the decay of monomer concentration68
d[M]/dt ) -kM[M] or [M] ) [M]0e-kMt
(2)
The intermediate blue form polymer is then formed from the monomer M at a rate kM[M] but decays to the red form polymer at a rate kB[B]. Thus, its net rate of formation is
d[B]/dt ) kM[M] - kB[B]
(3)
Consequently, the red form polymer is formed through the unimolecular decay of B:
d[R]/dt ) kB[B]
(4)
In this study the DAs did not form the blue form polymer but rather reacted directly into the red form. This indicates that the second step in eq 1 is much faster than the first step. Consequently, this leads to a steady-state approximation based on the assumption that during the reaction the concentration of the blue form polymer is negligibly small and below the detection limit of our measuring system. Equation 3 then becomes equal to zero. Then by solving [B] and inserting it into eq 4 and integrating with the boundary conditions for R, 0 f Rt, and for t, 0 f t, one gets
[R] ) (1 - e-kMt)[M]0 or
ln(1 - [R]/[M]0) ) -kMt
(5)
The measured spectra yield information about the end product but tell nothing about the kinetics of monomer concentration. Therefore, the mole fraction of so-called dead monomers cannot be evaluated based on this data set. Equation 5 then only describes the first-order reaction kinetics of the active monomers. Even though eq 5 is derived for a three-dimensional system, it can be applied for DAs in L and LB films that undergo a TP reaction with hardly any volume or area contraction. The theory (eq 5) can be fitted to the experimental data by assuming that the monomer concentration is equal to the maximum end product concentration and further replacing [R] with At - A0 and [M]0 with Ainf. A0, At, and Ainf are the absorbance values from the in situ reflection absorbance spectra at times 0, t, and the time when the absorbance has reached a maximum intensity (or just before the monolayer starts to degrade as revealed by a decreasing absorbance), respectively. This consequently gives us (because A0 ) 0)
ln(1 - At/Ainf) ) -kt
(6)
The replacement is justified because, for every monomer that reacts, there will be a change in the absorbance spectrum as a consequence of the produced polymer backbone. In conjunction with this the direct use of the (68) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, U.K., 1994.
Modified Diacetylenes in Langmuir Films
Langmuir, Vol. 16, No. 7, 2000 3343
Table 1. Reaction Kinetic Constants and Correlation Coefficients for Linear Regression Lines When Equation 6 Was Adapted to Experimental Dataa DA4 [0.32 (R ) 0.993)]
DA3 [0.24 (R ) 0.991)]
DA5 [0.33 (R ) 0.997)]
koverall [min-1] (Figure 8)
10 mN/m
20 mN/m
30 mN/m
10 mN/m
20 mN/m
30 mN/n
kI [min-1] k474 [min-1] k511 [min-1]
0.29 (R ) 0.995) 0.19 (R ) 0.997) 0.40 (R ) 0.999)
0.32 (R ) 0.996) 0.20 (R ) 0.998) 0.45 (R ) 0.995)
0.33 (R ) 0.994) 0.25 (R ) 0.999) 0.45 (R ) 0.997)
0.27 (R ) 0.996)
0.29 (R ) 0.992)
0.24 (R ) 0.988)
a
S.D. for the k values at most 0.05 min-1.
Figure 8. Data points calculated from the integrated absorbances between 400 and 650 nm obtained from measurements performed at different surface pressures, all included in the same graph. Monolayer of DA4 at different surface pressures: (/) 10 mN/m; (O) 20 mN/m; (9) 30 mN/m (solid line). Monolayer of DA3 at different surface pressures: (+) 10 mN/ m; (]) 20 mN/M; (2) 30 mN/m (dotted line).
Figure 7. Equation 6 adapted to the reaction kinetic data obtained from the in situ UV-vis spectra for a DA4 monolayer irradiated at different surface pressures: (a) 10 mN/m; (b) 20 mN/m; (c) 30 mN/m. (9) Calculated from the height of the absorbance peak at 511 nm. (/) Calculated from the height of the absorbance peak at 474 nm. (O) Calculated from the integrated absorbances between 400 and 650 nm. The straight lines are linear regression fits to the corresponding data points.
absorbance values can be done, because according to Lambert-Beers’ law (A ) Cl), the absorbance is directly proportional to the concentration of the formed product. The plot of the left-hand side of eq 6 versus time gives the reaction kinetic constant as the slope of the fitted straight line. Figure 7 shows the result of the fit for DA4 at different surface pressures. The spectra for DA4 (Figure 4b) show two distinct absorbance peaks, which are attributed to polymers of different chain lengths or alternatively polymer backbones with different conformations. Hence, in the case of DA4 we tested the model to three different series of absorbance values, first to the values obtained from the peak at 474 nm, second, to the values obtained from the peak at 511 nm, and finally, to the absorbance value integrated over the 400-650 nm region. The reaction kinetic constants are shown in Table 1. The rate constant was hardly affected by the surface pressure, supporting the earlier conclusion that the conditions for TP were fulfilled at every studied surface pressure. The k values were considerably lower when using the absorbance values obtained at 474 nm than at 511 nm. The k value obtained by integrating over the whole absorption regime (400-
600 nm) gave an k value which corresponded to the average of k474 and k511. If the processes would be parallel firstorder processes, then kI should equal the sum of k474 and k511. The kI value can thus be regarded as the apparent reaction kinetic constant. The use of the integrated absorbance values makes it easier to obtain comparable k values in cases where the exact position and fine structure of the absorbance peaks are difficult to determine. This was in fact the case in this study for DA3 (see the spectra for DA3 in Figure 4a). Because the surface pressure did not significantly affect the k values, we included all of the data points from measurements performed at different surface pressures in the same graph, yielding an average value for the reaction kinetic constants. Such a procedure has been done for DA3 and DA4 in Figure 8, and the respective k values are given in Table 1. One can notice that the kI value of DA3 is slightly lower than that of DA4. This is probably due to the fact that the triple bonds close to the polar headgroup in the DA3 molecule counteract the reorganization of the molecules during the reaction and thus slow the reaction. Furthermore, as discussed earlier, the possibility of the DA3 polymerization to be quenched by the subphase exists. In the case of DA4 the monolayer is less dense (Figure 1) and therefore the reorganization is not inhibited. The kinetic model was also adapted to the kinetic data obtained for DA5, but because of the very broad absorbance for the polymerized DA5 monolayer, the final results from the calculation in Table 1 should only be considered as indicative. The kinetic model could unfortunately not be adapted to the DA1 and DA2 molecules because they did not show any significant absorbance as discussed in the previous section.
3344
Langmuir, Vol. 16, No. 7, 2000
Conclusions Long-chain modified DA alcohol compounds were shown to be able to form monolayers at the air-water interface. The surface pressure-area isotherms were very dependent on the type of group, which was inserted in the hydrocarbon chain. The insertion of a sulfonyl group had the largest influence, which could be seen as a slightly more expanded monolayer as compared with the alcohol derivatives without a sulfonyl group. The course of the reaction in a DA monolayer was followed by in situ UV-vis spectroscopy. The polymerization properties of the modified DA derivatives were found to be dependent, apart from the position of the DA unit, on which kind of group that was inserted in the hydrocarbon chain. The derivatives with the DA moiety in or near the hydrophilic part of the molecule showed much lower polymerization capabilities than the compound where the DA unit was located in the middle of the hydrophobic hydrocarbon chain. This was attributed to unfavorable packing of the former molecules and possible
Alekseev et al.
reactions of the triple bonds with water, which quenches the polymerization reaction of the DA molecule. The introduction of the sulfonyl group in the chain markedly improved the polymerization properties. The reaction kinetic data obtained from the in situ UVvis spectra for three of the studied compounds were used to adapt a simple first-order reaction kinetic model. The model fitted quite well to the experimental data measured for a monolayer, even though it was derived based on three-dimensional reaction kinetics. The adaption of the model was possible because the reaction in the monolayers was topochemical to its nature; i.e., only very slight area or volume contraction occurred during the reaction. Acknowledgment. I.N.D. acknowledges Prof. B. Tieke for useful discussions. The Academy of Finland (Project Nos. 30591 and 42441) is acknowledged for financial support of this work. LA990351V
