Langmuir 2000, 16, 2797-2801
2797
Polymerization of 10,12-Pentacosadiynoic Acid Monolayer at Varying Surface Pressure and Temperature Zhou Huilin, Lu Weixing, Yu Shufang, and He Pingsheng* Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, China Received May 14, 1999. In Final Form: November 13, 1999 Isotherms of a PDA monolayer were measured on a subphase of 4 × 10-4 M CdCl2 at different temperatures. UV polymerization of the PDA monolayer was carried out at different temperatures and surface pressures, and expansions or contractions were observed to depend on the applied surface pressure. The polymerization rate constant was found to decrease with increased applied surface pressure, but it increased with temperature. From the temperature dependence of the polymerization of the PDA monolayer, the apparent activation energy of the 1,4-addition polymerization was estimated to be 63.6-89.7 kJ/mol depending on the surface pressure at which UV polymerization was carried out. The apparent activation energy had a minimum of 63.6 kJ/mol at the surface pressure of 20 mN/m, and there would be an obvious increase of the activation energy when the surface pressure deviated from this value. The surface pressure dependence of the apparent activation energy was interpreted in terms of molecular packing in the monolayer.
1. Introduction The study of polymerization by the LB technique has been measured quantitatively.1,2 The LB technique offers the possibility to illustrate the already recognized advantages of polymeric LB films, but also to study the polymerization process of reactive amphiphilic monomers in the two-dimensional state.3-6 Actually the LB technique offers several advantages: first, the average distance between reacting molecules can be easily controlled by varying the applied surface pressure, the temperature, etc.; second, polymerization kinetics can be followed in real time by recording the change of the surface area occupied by a monomer or repeating unit of the polymer backbone. The diacetylenes, R1-CtC-CtC-R2, exhibit many interesting features in their electronic7,8 and optical8,9 properties. Although extensive attention had been paid to the polymerization of PDA monolayers10,11,13 and multilayers12,13 due to their reactivity in the solid state and their ability to substitute for a length of alkyl chain without interfering in the molecular packing,14 few studies have been carried out on apparent polymerization kinetics, * To whom correspondence should be addressed. (1) Duran, R. S.; Zhou, H. C. Polymer 1992, 33, 4019. (2) Peltonen J. P. K.; He, P.; Rosenholm, J. B. Thin Solid Films 1992, 210/211, 372. (3) Bardosova, M.; Hodge, P.; Matsuda, H.; Nakanishi, F.; Tredgold, R. H. Langmuir 1999, 15, 631. (4) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. Langmuir 1993, 9, 2363. (5) Bodalia, R. R.; Duran, R. S. J. Am. Chem. Soc. 1993, 115, 11467. (6) Peltonen, J. P. K.; He, P.; Linden, M.; Rosenholm, J. B. J. Phys. Chem. 1994, 98, 12403. (7) Hunt, I. G.; Bloor, D.; Movaghar, B. J. Phys. C: Solid State Phys. 1983, 16, L 623. (8) He, P. In Advances in Organic and Polymeric Materials for High Technology; Huang, W., Wen, J., Eds.; Chemical Industry Press: Beijing, 1994; p 429 (Chinese). (9) Bloor, D.; Chance, R. R. Polydiacetylenes; Martinus Nijhoff Publishers: Boston, 1985. (10) Day, D. R.; Ringsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. (11) Tomioka, Y.; Tanaka, N.; Imazeki, S. J. Chem. Phys. 1989, 91, 5694. (12) Tieke, B.; Bloor, D. Makromol. Chem. 1979, 180, 2275. (13) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631.
such as apparent activation energy, etc., in the monolayer. As is well-known, the reactivity of the PDA crystal was packing parameter dependent and the packing parameters could be largely changed by varying the surface pressure and temperature of the monolayer. Studies on the surface pressure dependence of the apparent activation energy would contribute to an understanding of the relationship between the topochemical reaction and the packing parameters in the monolayer. In this paper the surface pressure and temperature dependence of UV polymerization of the PDA monolayer were presented; the apparent activation energy was estimated and the relationship between the activation energy and the applied surface pressure was discussed as well. 2. Experimental Section 2.1. Materials. The monomer 10,12-pentacosadiynoic acid (PDA) CH3-(CH2)11-CtC-CtC-(CH2)8-COOH was purchased from ABCR GmbH (Karlsruhe, Germany) and used without further purification. The PDA was dissolved in AR grade chloroform and filtered in order to take off as few polymerized solids as possible. Subphase solutions were prepared with CdCl2 (AR grade) and double distilled water (prepared by SYZ-A type quartz sub-boiling distiller) with the concentration of 4 × 10-4 M. 2.2. Film Balance and UV Polymerization. A homemade computer-controlled film balance was used to carry out the experiments. A program was designed to perform the kinetic experiments, i.e., recording the barrier speed as a function of time under UV irradiation. The film balance was placed in a dust-free box and protected against natural UV light with opaque plastic sheets. The box was temperature-controllable and could be filled with nitrogen if necessary. A 20 W low-pressure Hg lamp (λ ) 254 nm) hung 12 cm above the monolayer and was used for UV polymerization. The polymerization began after the temperature equilibrium (the temperature difference was less than 0.2 °C) was reached and the subphase temperature was kept constant and monitored during the experiment. 2.3. Spectroscopic Analysis of the Polymerized Monolayer. The polymerized monolayers were collected onto quartz substrates for spectroscopic analysis. The UV-visible absorption (14) Enkelmann, V. In Polydiacetylenes; Cantow, H.-J., Ed; SpringerVerlag: Berlin, 1984; p 91.
10.1021/la990587z CCC: $19.00 © 2000 American Chemical Society Published on Web 02/02/2000
2798
Langmuir, Vol. 16, No. 6, 2000
Zhou et al. Table 1. Experimental Temperatures and Applied Surface Pressuresa surface pressure (mN/m) temp (°C)
5
10
15
20
25
30
35
40
10 15 20 25 30
x x x x -
x x x x -
x x x
x x x x -
x -
x x x x -
x x x -
x x x x -
a Where x is marked for the condition of temperature and applied surface pressure at which the polymerization was carried out.
Figure 1. π-A isotherms of 10,12-pentacosadiynate at 10, 15, 20, 25, and 30 °C on a subphase of 4 × 10-4 M CdCl2, pH ) 7.2. spectrum was obtained using a Shimadzu UV-240 UV-visible spectroscope. 2.4. Experimental Procedure. The π-A isotherms were obtained on the subphase of 4 × 10-4 M CdCl2. A known amount of the PDA chloroform solution was spread on the subphase and chloroform was allowed to evaporate from the surface for 20 min. The resulting monolayer was then compressed at a constant rate of 9 mm/min. The polymerization of monolayer was performed in situ on the same Langmuir trough at a constant surface pressure. After 20 min of solvent evaporation, the PDA monolayer was compressed to a desired surface pressure that was kept constant throughout the polymerization by moving the barrier forward and backward. The UV-polymerization was started after several minutes of stabilization to regulate the surface pressure and both the mean molecular area and the average barrier speed were recorded as a function of time. The time of switching on the UV lamp was taken as the zero polymerization time.
3. Results and Discussion 3.1. Pressure (π)-Area (A) Isotherms of the PDA Monolayer at Different Temperatures. The compression isotherms of the PDA monolayer on the subphase of 4 × 10-4 M CdCl2 at temperatures of 10, 15, 20, 25, and 30 °C are shown in Figure 1. It is usually possible to obtain the mean molecular area at the surface pressure onset (Ao, the mean molecular area at which an appreciable rise in surface pressure is observed), the collapse molecular area (Ac) and the collapse pressure (πc) from the pressurearea isotherm. A large temperature dependence of Ao, Ac, and πc was observed. The mean molecular area increased with the subphase temperature. The Ao values were 0.25, 0.30, 0.35, 0.36, and 0.42 nm2/molecule at 10, 15, 20, 25, and 30 °C, respectively. The changing of Ac had the same temperature dependence as Ao and the Ac values were 0.18, 0.18, 0.20, 0.22, and 0.23 nm2/molecule at 10, 15, 20, 25, and 30 °C, respectively. The πc values decreased with increasing temperature and they were 53, 54, 51, 49, and 33 mN/m at 10, 15, 20, 25, and 30 °C, respectively. π-A isotherms of PDA on the 10-3 M CdCl2 subphase at temperatures of 5, 13, and 24 °C have been measured in a study on the polymerization of PDA multilayers and same temperature dependence has been obtained.13 The temperature dependence of the isotherms shows that the monolayers are more compressible at the higher temperature and that it is possible to change the intermolecular distance by varying both the temperature and the applied surface pressure. 3.2. Polymerization of the Monolayer at Different Temperatures. A comparison of UV polymerization with and without oxygen has been made and no discernible differences were detected between experiments carried
Figure 2. The average barrier speed vs time during polymerization of the PDA monolayer on the subphase of 4 × 10-4 M CdCl2 at 25 °C and at surface pressures of 15 (a) and 35 (b) mN/m.
out in air and in a nitrogen atmosphere. Two possibilities can account for this phenomenon: first, oxidation or ozonization do not affect the monolayer area during UV polymerization; second, oxygen can not permeate the closely packed alkyl chain (C12H25-) to react with the diacetylene moieties. In either circumstance, the monolayer polymerization can be monitored by recording the change in both the mean molecular area and the average barrier speed (the rate of barrier movement needed to maintain the isobaric condition) as a function of time. Table 1 shows the conditions of temperature and applied surface pressure at which the polymerization has been carried out. The UV polymerization kinetic curves of the PDA monolayer at various surface pressures were shown in Figure 2. In the polymerization at a surface pressure of 15 mN/m, the barrier rapidly moved backward for about 6 min, then slowly moved forward. During the first 6 min expansion, the mean molecular area increased from 0.26 nm2 to 0.34 nm2 (about 31%). This expansion can be ascribed to the shape of a polydiacetylene chain, about which two models have been proposed and are schematically presented in Figure 3. The first model (Kuhn chain) is built by planar segments of limited conjugation length. The second concept of a “wormlike” chain (Porod-Kratky chain14) presents a continuous curvature of the chain
10,12-Pentacosadiynoic Acid ML Polymerization
Langmuir, Vol. 16, No. 6, 2000 2799
Figure 4. π-A isotherms of 10,12-pentacosadiynate monomer and its polymer at 25 °C on the subphase of 4 × 10-4 M CdCl2, pH ) 7.2. Figure 3. Schematic representation of the shape of the polydiacetylene chains. Planar, fully conjugated chain (a); wormlike chain (b).
skeleton. The chain repeat distance of the polydiacetylene polymer (0.49 nm) is almost equal to the intermolecular distance of the monomers (0.45-0.55 nm or so) before reaction. If the polydiacetylene chain is a Kuhn chain as shown in Figure 3(a), no obvious expansion can be expected. However, if the chain is a Porod-Kratky chain as shown in Figure 3(b), in which some segments seriously lean or even lie down and occupy much more area than the close packed, standing monomers, a large expansion can be expected. The observed expansion during the UVpolymerization indicated that the polydiacetylene chain should be a Porod-Kratky chain. The polymerization at a surface pressure of 35 mN/m contrasted sharply with that at the lower surface pressure. After a slight expansion the monolayer underwent a pronounced contraction for 15 min, while the mean molecular area decreased from 0.24 nm2 to 0.12 nm2 at 18 min. This contraction suggests two possibilities: first, another type of polymerization with short chain repeat distance, such as the reported 1,2- and 3,4-addition polymerization15 has occurred, whose chain repeat distance is only 0.24 nm; second, the surface pressure, at which the polymerization is carried out, is higher than the collapse pressure of the resultant 1,4-addition polymer, and the domains will collapse if they have polymerized, so the polymerization is accompanied by a collapse process which causes the contraction. To distinguish the possible 1,2- and 3,4-addition polymerization, the synchrotron radiation X-rays in-plane diffraction technique16 was used to investigate the chain repeat distance of two samples prepared at 15 and 35 mN/m surface pressure. However, no expected short chain repeat distance of a 1,2- and 3,4-addition product was detected. So the polymerization is the 1,4-addition, independent of surface pressure. The π-A isotherms of polymerized PDA were measured and are shown together with that of the PDA monomer in Figure 4. The isotherms were identical for the polymerized polymer monolayers at surface pressures of 35 mN/m and 15 mN/m. The collapse pressure of the PDA polymer monolayer is 31 mN/m, below which the polymer occupies a larger area than the monomer does, so the expansion is observed if polymerizing at surface pressures (15) Ogawa, K. Polym. Int. 1992, 28, 25. (16) Duta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulas, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228.
Figure 5. UV-visible spectra of the polydiacetylene polymerized at the surface pressure of 15 mN/m (a) and 35 mN/m (b).
below 30 mN/m. During the collapse of the polymer monolayer, the mean molecular area (area occupied by the chain repeat unit) decreased from 0.28 nm2 at 30 mN/m to 0.15 nm2 at 35 mN/m and the film thickness was measured to be doubled. So it is the collapse of the polymerized monolayer rather than another polymerization that causes the contraction at surface pressures above 30 mN/m. It may be concluded that the UV-polymerized products at the higher and the lower surface pressure were indeed the same 1,4-addition polymers and that the PDA polymer monolayer is stable at surface pressures lower than 30 mN/m above which it will collapse. The UV-visible absorption spectra of the polymers were measured as shown in Figure 5. For the polymer obtained at the surface pressure of 15 mN/m, there were two absorption peaks at 500 and 550 nm (Figure 5a), which is similar to the absorption found in the corresponding polymerized monolayer at 503 and 540 nm.10 For the polymer obtained at the surface pressure of 35 mN/m, there were two absorption peaks at 480 and 528 nm (Figure 5b). The blue shift of the absorption spectrum can be attributed to the different surface pressures at which the polymer monolayer was collected.11 The UV-visible absorption spectra indicated that the 1,4-addition polymerization occurred at both low and high surface pressures. Figure 6 shows the average barrier speed as a function of time during UV polymerization at a constant surface pressure of 5 mN/m for the PDA monolayer at temper-
2800
Langmuir, Vol. 16, No. 6, 2000
Zhou et al. Table 2. Rate Constant k, Apparent Activation Energy Ea and Preexponential Factor for Polymerization of the PDA Monolayer at Different Temperatures and Surface Pressuresa rate constant k temp (°C)
5 mN/m
10 mN/m
20 mN/m
30 mN/m
10 5.82 5.35 2.61 1.61 15 9.59 8.08 4.2 3.15 20 22.1 13.4 6.38 5.9 25 34.6 21.5 9.75 8.1 Ea (KJ/mol) 89.7 68 63.6 80.2 preexp 2.3 × 1017 1.9 × 1013 1.4 × 1012 1.1 × 1015 a The polymerization reactions were carried out on the subphase of 4 × 10-4 M CdCl2. The rate constant, k, at the surface pressures of 10, 20, and 30 mN/m were obtained by the same method as that at 5 mN/m discussed in the text.
Figure 6. Average barrier speed vs time during the polymerization of the PDA monolayer at a surface pressure of 5 mN/m at 10, 15, 20, and 25 °C.
Figure 7. Logarithm of the reaction rate constant, k, as a function of inverse temperature. A linear regression method was used to draw straight lines from the experimental data.
atures of 10, 15, 20, and 25 °C. Upon switching on the UV lamp, a quick expansion immediately occurred and the expansion continued for 5 to 13 min depending on the temperature. The average barrier speed changed steadily from a small negative value to its negative maximum in a relatively short time, then gradually slowed to zero, when the expansion turned into a slow contraction and the average barrier speed increased smoothly and finally was maintained at 3-4 mm/min 30 min. It could be seen that the time needed to reach the negative maximum got shorter and the slope of the average barrier speed got steeper with increasing temperature, i.e., the polymerization rate increased with increasing temperature. As a rough estimation, it is possible to obtain the apparent activation energy of the polymerization in the twodimensional state by means of the temperature dependence of the barrier speed. In general, the reaction rate constant k and activation energy Ea has the Arrhenius relationship of
ln k ≈ Ea/RT
(1)
Taking the tangent of each kinetic curve shown in Figure 6 during the period from the reaction time of zero to the time the negative maximum was reached as the polymerization rate constant, k, the apparent activation energy, Ea, for polymerization of PDA monolayer can be calculated from the Arrhenius plot of lnk versus 1/T (Figure 7) and is found to be 89.7 kJ/mol (Table 2) for the PDA monolayer at the surface pressure of 5 mN/m. The polymerization
rate constant k at other surface pressures of 10, 15, and 20 mN/m is calculated by the same method as that at the surface pressure of 5 mN/m and these values are shown in Table 2. A plot of lnk versus 1/T (K) for polymerization in the PDA monolayer at different surface pressures is shown in Figure 7 and a good agreement with the Arrhenius relationship was observed. The activation energies, calculated from the slope of the linear fitted line in Figure 7 according to eq 1, are also listed in Table 2. Because there is no report on the apparent activation energy of the monolayer polymerization in the literature, the calculated values here have to be compared with the apparent activation energy obtained from the bulk polymerization of some diacetylenes. The apparent activation energies are 96 kJ/mol17 and 105.3 kJ/mol18 for solid-state polymerization of bis-(p-toluenesulfonate) of 2,4-hexadiyne-1,6diol (TS). The reason that the apparent activation energies of the monolayer polymerization (63.6-89.7 kJ/mol depending on surface pressure) are slightly smaller than in the bulk polymerization is that the monomer molecules in the monolayer state have more degrees of freedom than in solid state to reorient themselves to optimal positions to react with each other. The result is that the apparent activation energy in the bulk crystal is somewhat different from that in its spread film; this has been observed for TS.19 It has been indicated3 that the temperature sharply changed the π-A isotherms of the smectic liquid crystal Langmuir film and largely affected the whole polymerization process. And for the PDA monolayer in our experiment, temperature can also affect its π-A isotherms and its polymerization rate constant. Large changes in reaction rate might occur because of differences in both the preexponential factor and the apparent activation energy. The preexponential factor decreased steadily from the maximum of 2.3 × 1017 to the minimum of 1.4 × 1012 with increasing surface pressure from 5 to 20 mN/m, then increased to 1.1 × 1015 at the surface pressure of 30 mN/m. The apparent activation energy, which was 89.7 kJ/mol at the surface pressure of 5 mN/m, decreased steadily with increasing surface pressure and went down to 63.6 kJ/mol at the surface pressure of 20 mN/m, then increased steeply with increasing surface pressure and went up to 80.2 kJ/mol at the surface pressure of 30 mN/m. The apparent activation (17) Ba¨ssler, H. In Polydiacetylenes Cantow, H.-J., Ed.; SpringerVerlag: Berlin, 1984; p 9. (18) He, P.; Jiang, Z.; Zhu, Z. China University Sci. Technol. 1984, 14, 371 (Chinese). (19) He, P.; Peltonen J. P. K.; Rosenholm, J. B. J. Mater. Sci. 1993, 28, 5702.
10,12-Pentacosadiynoic Acid ML Polymerization
Langmuir, Vol. 16, No. 6, 2000 2801
from originally tilting toward the nearest neighbors to tilting toward the next nearest neighbors.23 More energy is required for the monomers to polymerize with nearest neighbors. So the activation energy increased with increasing surface pressure in the range of 20-30 mN/m. The strain induced electronic state change of heptacosa10,12-diynoic acid polymer chains has been observed at an intermediate surface pressure between 14 and 20 mN/ m.11 This is consistent with our result that the minimum of activation energy occurs at a surface pressure of around 20 mN/m.
Figure 8. The apparent activation energy of the PDA monolayer polymerization vs applied surface pressure.
energy in relation to the applied surface pressure was shown in Figure 8. Qualitatively, the apparent activation energy is related to monomer packing (bond angle and length in the monomer lattice) and the C1-C4 reaction distance of adjacent diacetylene moieties. The decrease in activation energy with increasing surface pressure in the range of 5-20 mN/m may be attributed to the decrease of reaction distance among adjacent monomers. With the compression of the monolayer, the area available to molecules was steadily reduced and the distance between the C1 and C4 of adjacent diacetylene moieties was gradually shortened. And less energy was required to overcome the repulsive force and establish the critical distance necessary for bond formation between the adjacent molecules. As the monolayer was further compressed, the molecules’ interactions in the horizontal plane were steeply increased, and the orthorhombic lattice with the c-axis dimension corresponding to the planar zigzag repeat distance of side group alkyl chains20,21 underwent a large distortion along the a-axis direction accompanied with rotations and translations of methylene chains. That is, the distortion implies a transition to a pseudomonoclinic packing cell.11 The larger compressive strain along the a-axis perpendicular to the polymer conjugated backbone is thought to reduce the planarity of the polymer conjugated backbone. Consequently, less π electron delocalization along the conjugated backbone22 and an upward shift of polymerization energy barrier are expected. On the other hand, in the phase transition, the monomers rearranged themselves (20) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1483. (21) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. (22) Eckhardt, H.; Boudreaux, D. S.; Chance, R. R. J. Chem. Phys. 1986, 85, 4116.
4. Conclusions The mean molecular area at the surface pressure onset as well as at any given surface pressure increased with increasing subphase temperature. The monolayers were more compressible at higher temperatures and it was possible to change the intermolecular distance and molecular packing by varying both the temperature and the applied surface pressure. The UV-polymerization was determined to be by 1,4-addition at any surface pressure and this was accompanied by a two-dimensional expansion at surface pressures lower than 30 mN/m or a twodimensional contraction at surface pressures higher than 30 mN/m. The contraction was caused by the collapse of the polymerized PDA domains. The polymerization rate constant, k, of the PDA monolayer was found to increase with decreasing surface pressure and to increase with increasing temperature. The apparent activation energy for the PDA monolayer polymerization could be estimated from the Arrhenius plot. The apparent activation energy for the polymerization decreased steadily with increased surface pressure from 5 to 20 mN/m, then increased steeply with increased the surface pressure from 20 to 30 mN/m with a minimum at 20 mN/m. The surface pressure dependence of the apparent activation energy was related to the changes in molecular orientation, molecular distance, and molecular packing at different applied surface pressures. Acknowledgment. Support of this work under the National Science Foundation of China (29674027) is gratefully acknowledged. The authors are grateful to Prof. R. H. Tredgold (Department of Chem., University of Manchester, UK) for helpful discussions. Prof. J.B. Peng, (Department of Chemistry, The University of Queensland, Australia) is acknowledged for the synchrotron radiation X-ray in-plane diffraction measurement and analysis. LA990587Z (23) Tredgold, R. H. In Order in Thin Organic Films; Cambridge University Press: Cambridge, 1994; p 50.
