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Langmuir 2002, 18, 6602-6605
Elasticity of 10,12-Pentacosadiynoic Acid Monolayer and the Polymerized Monolayer at Varying pH and Temperatures Zou Gang, Fang Kun, Sheng Xia, and He Pingsheng* Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, China Received March 15, 2002. In Final Form: June 12, 2002 The π-A isotherms of a 10,12-pentacosadiynoic acid (PDA) monolayer were measured on a subphase of 2 × 10-4 M CdCl2 at varying pH and temperatures. UV polymerization of PDA monolayers was carried out at different pH values, surface pressures, and temperatures. The dynamic elasticity of PDA monolayer has been measured both before and after polymerization by means of a dynamic oscillation method, i.e., measuring the changes of surface pressure caused by an oscillating barrier at given frequency and amplitude. The dynamic elasticity of PDA and its polymer PPDA monolayer was found to decrease with increasing of temperature but to increase with increasing of pH. The dynamic elasticity of PDA monolayer increased with increasing of surface pressure, but it had a sudden decrease at 25 mN/m for PPDA monolayer. The dynamic elasticity of PPDA monolayer was even smaller than that of its monomer PDA monolayer and decreased with increasing of polymerization time. The explanation based on conformation change has been presented. It was interpreted in terms of molecular packing in the monolayer.
1. Introduction The good mechanical and thermal stability of materials is of importance for application purposes. It is also true for monolayer and Langmuir-Blodgett (LB) films.1 There are many reports concerning mechanical properties of monolayer and LB films.2,3 But to our knowledge, there are comparatively few studies4 concerning dynamic elasticity of the monomer monolayer and its resulting polymerized monolayer. The static elasticity of a monolayer, E ) -dπ/(dA/A)5 (where π is the surface pressure and A the surface area covered by monolayer) is generally determined from the slop of a π-A isotherm. The dynamic elasticity of a monolayer can be measured directly by the dynamic oscillation method. In the method the monolayer is subjected to small sinusoidal compressions and expansions with an oscillating barrier at a given frequency and amplitude, and the response of the surface pressure is monitored.6-9 On comparison to the static elasticity, the dynamic elasticity is more helpful for application and can provide more detailed information on intermolecular interaction.10 Additionally, relaxation behavior of monolayer can also be obtained from the difference in phase angle between stress and strain. If the film is purely elastic, the phase angle should be zero.
The diacetylenes (R1sCtCsCtCsR2) exhibit many interesting features in their electronic11 and optical12 properties. Although extensive attention had been paid to the polymerization of diynoic acid monolayer13-18 and multilayer19,20 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, few studies have been carried out on the dynamic elasticity of diynoic acid monolayer and its polymerized monolayer. In this paper, UV polymerization of 10,12-pentacosadiynoic acid (PDA) monolayer was carried out at different pH values, surface pressures, and temperatures. The dynamic elasticity of PDA monolayer and its resulting poly(10,12-pentacosadiynoic acid) (PPDA) monolayer were measured by a dynamic oscillation method. The surface pressure, pH, and temperature dependence of the dynamic elasticity of PDA and PPDA monolayer are presented and discussed.
* To whom correspondence may be addressed. E-mail: hpsm@ ustc.edu.cn.
(11) Hunt, I. G.; Bloor, D.; Movaghar, B. J. Phys. C: Solid State Phys. 1983, 16, L623. (12) Bloor, D.; Chance, R. R. Polydiacetylenes; Martinus Nijhoff Publishers: Boston, 1985. (13) Day, D. R.; Rindsdorf, H. J. Polym. Sci., Polym. Lett. Ed. 1978, 16, 205. (14) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631. (15) Tomioka, Y.; Tanaka, N.; Imazeki, S. J Chem. Phys. 1989, 91, 5694. (16) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991, 1, 2336. (17) Ohe, C.; Ando, H.; Sato, N.; Urai, Y.; Yamamoto, M.; Itoh, K. J. Phys. Chem. B 1999, 103, 435. (18) Zhou, H. L.; Lu, W. X.; Yu, S. F.; He, P. S. Langmuir 2000, 16, 2797. (19) Tieke, B.; Bloor, D. Makromol. Chem. 1979, 180, 2275. (20) Carpick, R. W.; Mayer, T. M.; Sasaki, D. Y.; Burns, A. R. Langmuir 2000, 16, 4639.
(1) Tieke, B. Adv. Mater. 1991, 2, 222. (2) Joly, M. In Surface and Colloid Science; Matijevic, E., Ed.; WileyInterscience: New York, 1972; Vol. 5, p 79. (3) Petty, M. C. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990; p 193. (4) He, P. S.; Peltonen, J. P. K.; Rosenholm, J. B. Chin. J. Polym. Sci. 1998, 16 (2), 147. (5) Behroozi, F. Langmuir 1996, 12, 2289. (6) Blank, M.; Lucassen, J.; Temple, M. V. D. J. Colloid Interface Sci. 1977, 33, 94. (7) He, P. S.; Peltonen, J. P. K.; Rosenholm, J. B. J. Mater Sci. 1993, 28 (21), 5702. (8) Yang, H. Y.; Zhu, P. P.; Cong, S. X.; He, P. S. Chin. Bull. Chem. 1996, 9, 32. (9) He, P. S.; Zou, G.; Fang, K. Chin. J. Chem. Phys. 2001, 14 (3), 371. (10) He, P. S.; Fang, K.; Zou, G. Colloids Surf., A 2002, 201, 265.
2. Experimental Section 2.1. Materials. 10,12-Pentacosadiynoic acid (PDA), CH3s (CH)11sCtCsCtCs(CH)8sCOOH, was purchased from ABCR GmbH (Karlsruhe, Germany) and used without further purification. 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
10.1021/la020260p CCC: $22.00 © 2002 American Chemical Society Published on Web 07/23/2002
Elasticity of a Monomer Monolayer
Figure 1. π-A isotherms of PDA monolayer at 17, 20, 23, 26, and 29 °C on a subphase of 2 × 10-4 M CdCl2, pH ) 5.6. distilled water (pH ) 5.8, prepared by a SYZ-A type quartz subboiling distiller) with a concentration of 2 × 10-4 M. The pH was adjusted by addition of NaOH (10-5 M). 2.2. Film Balance. All the experiments were carried out on a homemade computer-controlled Langmuir film balance. The surface pressure was measured with a Wilhelmy Pt plate. The Langmuir 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 was used for UV polymerization. 2.3. Measurement of the π-A Isotherms of PDA and PPDA Monolayer. A known amount of PDA chloroform solution was spread on the subphase, and chloroform was allowed to evaporate from the surface for 20 min, the resulting monolayer was compressed at a constant rate of 8 mm/min. The polymerization of monolayer was performed in situ on the same Langmuir trough at a constant surface pressure. After polymerization, the barrier moved back to the original position and the monolayer was compressed again after 10 min. The π-A isotherms of PDA and PPDA monolayer were recorded by the computer during the monolayer compression. 2.4. Measurement of the Dynamic Elasticity of PDA Monolayer. The dynamic elasticity measurements were performed in a VISCOS mode that could vary frequency and amplitude of the barrier movement in sinusoidal pattern by a specially designed program. When the PDA monolayer was compressed to a designed surface pressure, the computer turned the barrier movement into VISCOS mode automatically, and the periodic barrier oscillation movement started. The changes of surface pressure caused by the periodic barrier movement were recorded by the computer. Then the dynamic elasticity of PDA monolayer can be calculated with a formula of E ) |dπ(dA/A)|, where dπ and dA are the change amplitudes of the surface pressure and molecular area, respectively. All dynamic elasticity experiments were performed at a frequency of 40 mHz and relative amplitude (dA/A) of 1%. 2.5. Measurement of the Dynamic Elasticity of PPDA Monolayer. After 20 min of solvent evaporation, the PDA monolayer was compressed to a designed surface pressure, which kept constant throughout the polymerization by moving the barrier forward or backward. Then UV polymerization was started after several minutes of stabilization to regulate the surface pressure. The time of switching on the UV lamp was taken as the zero polymerization time. Until the designed polymerization time was attained, the lamp was turned off and the computer quickly turned the barrier movement into VISCOS mode. Then the dynamic elasticity of PPDA monolayer can be obtained by the similar method mentioned in section 2.4. All experiments were also performed at frequency of 40 mHz and relative amplitude of 1%.
3. Results and Discussion 3.1. PDA Monolayer Behavior at the Air/Water Interface. The π-A isotherms of PDA monolayer on a subphase of 2 × 10-4 M CdCl2 at different pH and temperatures are shown in Figure 1 and Figure 2. In both
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Figure 2. π-A isotherms of PDA monolayer at a pH of 5.6, 6.0, 6.2, 6.4, 6.6, 7.2, and 7.4 on a subphase of 2 × 10-4 M CdCl2 at 26 °C.
Figure 3. The oscillation of surface pressure of PDA monolayer caused by periodic compression-expansion cycles of barrier around a surface pressure of 15 mN/m at 20 °C, pH ) 6.0.
cases the monolayers exhibit condensed phases that can be characterized by a steep rise of the isotherms upon compression. The pH and temperature dependence of the π-A isotherms were observed. With increase of temperature or decrease of pH, the mean area (at which an appreciable rise in surface pressure is observed) and the collapse molecular area increase, but the collapse pressure decreases. This indicates that the monolayer is more compressible at higher temperature but lower pH, which is in accord with our previous work18 and Tieke’s work.19 Thus it is possible to change the intermolecular distance and interaction by varying the pH, temperature, and surface pressure. 3.2. Dynamic Elasticity of PDA Monolayer. Actually the elasticity-surface pressure curve is the differential form of the change of surface pressure during compression and would be better to expose the surface state of a monolayer than its integral form of the π-A isotherm. It is determined by the condensability of the arranged molecules. The typical experimental curve of surface pressure oscillation caused by periodic compressionexpansion cycles at π )15 mN/m with the frequency of 40 mHz and relative amplitude of 1% at 20 °C is shown in Figure 3. The average value of dπ can be obtained directly from the Figure 3. Because all the experiments were carried out at a designed relative amplitude (dA/A) of 1%, the dynamic elasticity of the PDA monolayer can be calculated with the formula of E ) |dπ(dA/A)| ) 100 dπ and is plotted as the function of π, pH, and temperature in Figures 4-6. It is obvious from Figures 4-6 that the elasticity of PDA monolayer increases with increasing of π and pH but decreases with increasing of temperature. Because the intermolecular distance decreases and intermolecular interaction increases, the dynamic elasticity of PDA monolayer increases with increasing of pH or
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Figure 4. The dependence of elasticity of PDA and PPDA monolayer on surface pressure at 29 °C, pH ) 5.6.
Zou et al.
Figure 7. π-A isotherms of PDA monolayer and PPDA monolayer at 23 °C on the subphase of 2 × 10-4M CdCl2, pH ) 7.2.
Figure 5. The dependence of elasticity of PDA and PPDA monolayer on subphase pH around surface pressure of 15 mN/m at 26 °C. Figure 8. Schematic representation of the shape of the polydiacetylene chains: (a) planar, fully conjugated chain; (b) wormlike chain.
Figure 6. The dependence of elasticity of PDA and PPDA monolayer on subphase temperature around surface pressure of 15 mN/m, pH ) 5.6.
decreasing of temperature. The relationship between the elasticity and surface pressure implies that the intermolecular interaction in PDA monolayer increases greatly during compression. Therefore, there is phase transition in the PDA monolayer during the compression process in the measured range of surface pressure, not like linoleic acid monolayer, which shows no phase transitions.4 The phase angle between the surface pressure oscillation and the barrier oscillation movement is not observed for PDA monolayer, indicating that PDA monolayer is purely elastic. 3.3. UV Polymerization of the PDA Monolayer. UV polymerization experiments of the PDA monolayer have been performed at varying π, pH, and temperatures. After 25 min of UV irradiation, the polymerization was almost completed. The π-A isotherm of PPDA monolayer was measured and is shown together with that of a PDA monolayer in Figure 7. The collapse pressure of PPDA
monolayer is 25 mN/m, much lower than that of PDA monolayer. But PPDA monolayer occupies a larger area than the PDA monolayer does. This expansion can be ascribed to the form of polydiacetylene chains, about which two models have been proposed and are schematically presented in Figure 8. The first model (Kuhn chain) is built by planar segments of limited conjugation length. The chain repeating distance of the polydiacetylene polymer (0.49 nm) is almost equal to the intermolecular distance of the monomers (0.45-0.55 nm) before polymerization, and no obvious expansion can be expected. The second concept of a “wormlike” chain (Porod-Kratky chain21) presents a continuous curvature of the chain skeleton. The observed expansion during the UV polymerization indicates that the polydiacetylene chain should be a Porod-Kratky chain in which some segments seriously lean or even lie down and occupy much more area than the closely packed standing monomers. After polymerization, a trace amount of red product can be obtained by carefully sweeping the surface of subphase. The UV-visible absorption spectra of the polymer can also be seen in our previous work.18 3.4. Dynamic Elasticity of PPDA Monolayer. A UV polymerized PDA monolayer at a constant surface pressure for designed time, dynamic elasticity measurement followed (Figure 9). It is surprising that the dynamic elasticity of the resulting PPDA monolayer is much lower than that of its monomer PDA monolayer (in Figures 4-6). The fact can also be seen in Figure 7 that a UV-irradiated monolayer rises slightly slower than that of unirradiated (21) Enkelmann, V. In Polydiacetylenes; Cantow, H. J., Ed; Springerverlag: Berlin, 1984; p 91.
Elasticity of a Monomer Monolayer
Figure 9. The oscillation of surface pressure of PPDA monolayer caused by periodic compression-expansion cycles of barrier around surface pressure of 15mN/m at 20 °C, pH ) 6.0.
monolayer, which indicates that there is a lower dense molecular arrangement in PPDA monolayer and the condensability and the elasticity of PPDA monolayer will be smaller. Under UV irradiation PDA monomer molecules are excited and react with each other. The point is that some PDA molecules have polymerized into macromolecules, while the unreacted molecules are still closely packed. Nevertheless, when the molecular weight of macromolecules is large enough, the polydiacetylene chain will play a more important role in the mechanical properties. The dynamic elasticity of the resulting polymerized PPDA monolayer is much lower than that of its monomer monolayer, indicating that the polydiacetylene chain would be a Porod-Kratky chain instead of a Kuhn chain, because the Kuhn chain is built by planar segments of limited conjugation length and will lead to the increase of the elasticity of PPDA monolayer due to the high rigidity of the acetylene bond. The molecules form Porod-Kratky chains in which some segments seriously lean or even lie down. The macromolecules of the polymerized PPDA monolayer can vary their conformation instead of changing bond length or bond angle to suit the change of applied force in oscillating movements of the barrier. The arrangement of the polymerized monolayer will not be as tight as that of its monomer in the monolayer and the dynamic elasticity should be smaller than that of the monomer monolayer. To prove the above idea, experiments with different UV-irradiation times were performed. The elasticity of polymerized PPDA monolayer as a function of polymerization time is shown in Figure 10. The elasticity decreases with increasing of polymerization time. The longer the UV-irradiation time, the more Porod-Kratky chains formed, which results in the less tight molecular arrangement and the smaller elasticity of the polymerized PDA monolayer. It is obvious from Figures 5 and 6 that the dynamic elasticity of PPDA monolayer increases with increasing of pH but decreases with increasing of temperature. With increasing of surface pressure, the elasticity of PPDA monolayer increases slightly but decreases suddenly at a surface pressure of 25 mN/m (Figure 4). The fact indicates that there is phase transition in the monolayer during the polymerization process at π ) 25 mN/m. Similar results can also be observed in the π-A isotherm of PPDA monolayer. The collapse surface pressure is below 26 mN/ m. During the collapse area of PPDA monolayer, the mean molecular area decreased from 0.23 to 0.13 nm2 and the
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Figure 10. The elasticity of PPDA monolayer as a function of UV-irradiation time around surface pressure of 15 mN/m at 23 °C, pH ) 5.6.
film thickness was measured to be doubled. It is in the collapse region of PPDA monolayer that small area change cannot cause the surface pressure change remarkably. So the dynamic elasticity of PPDA monolayer at 25 mN/m decreased a lot. The phase angle between the surface pressure oscillation and the sinusoidal movements of the barrier was not observed for the polymerized PDA monolayer either, which may indicate that PPDA monolayer is also purely elastic. 4. Conclusions With increase of temperature or decrease of pH, the mean area and the collapse molecular area of PDA monolayer increase, but the collapse pressure decreases. The results indicate that the monolayer is more compressible at higher temperature or lower pH. The dynamic elasticity of PDA and PPDA monolayers are found to decrease with increasing of temperature but increase with increasing of pH. The phase angle between the surface pressure oscillation and the sinusoidal movements of the barrier is not observed for PDA and PPDA monolayers, which indicates that both PDA and PPDA monolayers are purely elastic. Compared with the monomer PDA monolayer, the PPDA monolayer has much lower collapse pressure and occupies larger molecular area. The observed molecular area expansion of the monolayer during UV irradiation indicates that the polydiacetylene chain should be a Porod-Kratky chain, in which some segments seriously lean or even lie down and occupy larger area than the closely packed monomer molecules. The dynamic elasticity of PPDA monolayer is even smaller than that of its monomer monolayer and is found to decrease with increasing of polymerization time. This is interpreted in terms of molecular packing in the monolayer. The macromolecules of the polymerized PPDA monolayer can vary their conformation instead of changing bond length or bond angle to suit the change of applied force. The dynamic elasticities of PDA and PPDA monolayers are both found to increase with the increasing of surface pressure, except that the elasticity of PPDA monolayer decreased suddenly at π ) 25 mN/m. The reason is that the PPDA monolayer is in the collapse region and it is easy to form a multilayer, so a small area change cannot cause the surface pressure change remarkably. Acknowledgment. Support from the National Science Foundation of China (29974028) is gratefully acknowledged. LA020260P
