Langmuir 1988,4,903-906 Systems Analysis for the Control of Toxics. S. K. Friedlander wishes to express his appreciation for a senior Humboldt award from the German government during the course of this research.
Nomenclature preexponential factor, hg/(s.m2.Torr) diameter of elementary soot spheroids, nm d, initial diameter of soot spheroids, 20 nm activation energy, kcal/mol surface reaction rate constant, Tg/(s-m2) k kB Boltzmann constant, 1.38 X 10- erg/mol m mass of an oxygen molecule, 5.31 X g
A0
k
903
M
soot loading per cross sectional area of the filter, hg/cm2 catalyst loading per filter cross sectional area, Mcat mg/cm2 molecular weight of carbon, 12 g/mol MWC N number of soot spheroids in filter/cm2 Avogadro number, 6.023 X loz3mol-' partial pressure of oxygen, Torr time, s total reaction time, s temperature, K density of the elementary soot spheroid, 2 g/cm3 Regist;ry No. Na, 7440-23-5;02,7782-44-7; quartz, 14808-60-7.
Study of Photoreaction Processes of PDA Langmuir Films K. Ogawa," H. Tamura, M. Hatada,? and T. Ishihara Semiconductor Research Center Matsushita Electric Ind. Co., Ltd., 3-15, Yagumo-Nakamachi, Moriguchi, Osaka, 570 Japan, and Osaka Lab for Radiation Chemistry, Japan Atomic Energy Research Institute, 25-1, Mii-Minami Machi, Neyagawa, Osaka, 572 Japan Received July 10, 1987. I n Final Form: February 12, 1988 Pentacosadiynoic acid (PDA) monolayers on aqueous subphase containing Ca2+and on water subphase (Langmuir (L) film) were irradiated by UV light, and the reaction of the L films was monitored by surface area change under constanbpressure condition and by surface pressure change under constant-area condition. The highest rate of reaction was obtained at a surface area of ca. 28 A2/molecule, indicating that the PDA molecules have the most suitable molecular arrangement in the monolayer. The change of the optical absorption spectra of the L films also supports this finding. It was also found that the arrangement of the molecules in the Langmuir-Blodgett (LB) film is related to that of the monolayer from which the LB film was prepared.
Introduction It is known that diacetylene derivatives polymerize to give a polymer having a linear chain of conjugated double bonds when they are exposed to UV light in the solid state. Polymers of this type have attracted interest because they have special properties such as nonlinear optical properties or anisotropic electric conductivity.14 The polymerization of Langmuir-Blodgett films of diacetylene derivatives having a hydrophilic group on one end was also investigated by many researchers to elucidate the reactivity in relation to the arrangement of molecules in the Langmuir-Blodgett (LB) The photoreactivity of diacetylene derivatives whose substituents were replaced with different groups that will affect the molecular arrangement in the LB films has been further studied.*" Studies are also in progress on a chemical change of LB films of different diacetylene derivatives, because the chemical change induced by heat, pressure, ultraviolet (W)light, etc., is accompanied by a color change from blue to red which can be applied to optical recording media.12 It is of interest to study the reactivity of the LB films of these compounds prepared at different surface pressures to the results of the monolayer studies. It is of more interest to know the photochemical reactivity of the monolayers spread on an aqueous subphase as a function of molecular density, which is related to the conformation and arrangement of the molecules in the monolayer, because the molecular density can be measured directly with the surface pressure-area curve of monolayer films on an aqueous subphase (L film). Osaka Labs for Radiation Chemistry. 0743-7463/88/2404-0903$01.50/0
In this paper, we wish to report the change of the reactivity of pentacosadiynoic acid (CH3(CH2)11C=CC=C(CH,),COOH; PDA) L films under irradiation of UV light at different surface pressures and molecular areas and some preliminary results on PDA LB f i i s prepared at two different surface pressures. PDA was selected as film substance in this study, because the photopolymerization of this compound had already been investigated in detail on LB films.
Experimental Section Materials. PDA was obtained from Wako Pure Chemical Industries, Ltd., and used as received. Calcium chloride was also obtained from the same manufacturer and used after removal of (1)Wilson, E. G. J. Phys. C 1974,252, 655. (2) Bloor, D.;Ando, D. J.; Preston, F. H.; Stevens, G. C. Chem. Phys. Lett. 1974, 24, 407. (3) Bloor, D.; Chance, R. R. Polydiacetylenes; Martinus Nijihoff: Boston, MA, 1985. (4) Mehring, H.; Roth, S. Electronic Properties of Polymers and Related Compounds; Springer-Verlag: Berlin, Heidelberg, 1985; p 234. (5) Tieke, B.; Graf, H.-J.; Wegner, G.; Naegele, B.; Ringsdorf, H.; Banerjie, A.; Day, D.; Lando, 3. B. Colloid Polym. Sci. 1977,255(6), 521. ( 6 ) Bubeck.. G.: . Tieke.. B.:. Weaner, . G.Ber. Bunsen-Ges. Phys. Chem. 1982; 86, 495.
(7) Bubeck, C.; Tieke, B.; Wegner, G.Mol. Cryst. Liq. Cryst. 1983,96, 109.
(8) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. (9) Wegner, G.J. Polym. Sci., Polym. Lett. Ed. 1971, 9, 133. (IO) Wegner, G. Pure Appl. Chem. 1977,49,443. (11) Nakanishi, H.; Matsuda, H.; Kato, K.; Thocharis, C.; Jones, W. Polym. Prepr. Jpn. 1984, 33, 2491. (12) Sandaman,D.J.; Tripathy, S. K.; Elman, B. S.; Samuelson, L. A. Synth. Metals 1986, 15, 229.
0 1988 American Chemical Society
Ogawa et al.
I
PDA: CH3(CHz)'i -C-C-C-C-(CHz)sCOCH
CaCk 6 . 5 ~ 1 0 - ~ ( r n o P / PpH=6.6T=6'C ) CaClz 2.6xlCT4(moP/P) pH=5,6T=6"C
Area I 82/rnolecule)
Figure 1. Surface pressurearea curve of PDA L f i for different subphase conditions. possible contamination of surface-active material by extraction with n-hexane, as mentioned previously. Deionized water of semiconductor grade was used as the aqueous subphase. Measurements of Surface Properties. In order to measure the photoreactivityof L films, a new direct photomeasuring system was developed after Gruninger et The details of experimental procedure are given el~ewhere,'~ and only a brief description is provided in this paper. The surface area isotherms of L films were obtained by using a Joyce-Leobel Trough IV in a class 100 clean room under yellow safety lighting. The room temperature was controlled at 23 f 1 "C, and humidity was controlled at 40 f 5 % . PDA L films were spread from chloroform solution (100 mg/L) and allowed to stand 10 min before the compression was started. Irradiation with UV Light. Two types of UV irradiation were made. (I) The film was compressed to a desired surface pressure, and the irradiation was carried out at constant surface pressure. In this case, surface pressure was recorded as a horizontal line on the surface pressure-area isotherm. After the irradiation, the L films were further compressed to record the rest of the surface area isotherm, which indicated any change in monolayer properties caused by the irradiation. (2) The L films were irradiated at constant area, with the surface area being kept constant at a desired value and the photochemical change of the L films recorded as change in surface pressure with time. The experiments were carried out at five surface areas, giving surface pressures of 35 and 20 mN/m (because a phase change was observed above and below about 30 mN/m). The UV light source was a 200-W deuterium lamp, and the intensity of the light on the surface was 0.05 mW/cm2. Optical Absorption Spectra. The optical absorption spectra of the L films were obtained by using a multichannel spectrophotometer (MCPD-11OA; Otsuka Electronics Co., Ltd.), which allows spectrophotometry during surface pressure-area measurements. Preparation of LB Films. The LB films of PDA were built up on a 3-in. Si wafer on which an oxide layer had been deposited before use. The PDA LB films were of 25-50 layers, the first 10 layers being built up by Z-type deposition and the rest by Y-type deposition.
Results and Discussion Figure 1 shows surface pressure-area isotherms of P D A L films spread o n different subphases. T h e surface pressure-area plot becomes more "condensed" in type as t h e concentration of Ca2+ increases, indicating that t h e number of calcium bridges increases with increasing Ca2+ in t h e subphase, thus making t h e L film more solid. T h e letters in Figure 1 indicate t h e surface areas a n d surface (13) Gruninger, H.; Mobius, D.; Meyer, H. J . Chem. Phys. 1983, 79,
3701. (14) Ogawa, K.; Tamura, H.; Hatada, M.; Ishihara, T. Thin Solid Films, in press.
dl
Subphose ; A , 8 : CaCk 6.5 x 10-4(mO!/ 1 ) pHs6.6 T=6"C C, D : CaCk 2.6 x 104(rnoP/f) oH.5.6 T = 6 T
T=6'C
Photoreactions of PDA Langmuir Films
-"+ Subphase : CoCPz 6 5T=:~ 1 6 ~ ( m o i1/ i
a
Low Denslty L
Film of PDA
,-
Nan Exposure Exposure
------ - - _ _ _ _ _ Exposure pH15.6 T = 6 T
I
I
-0.161
----- 380sec 1
E
z
E
~~
25oXx)m
400
450
500
Wovelength (nm)
b
1'
0.00 (')
Subphask
:
Pure Water pH.68 T = 6 T
'
I/
-
I
ob
IO0
------380 sec.
I
m
Osec.
__. ._. ... I 9 0 sec.
a Y
-0.15
240
250 2w Wwslenglh Inml
270
I
---- 660 sec.
Irradiation Time (second )
Figure 3. Change of surface pressure of PDA L film during UV
light irradiationon different subphase at different molecular areas: mol/L CaC12. Mo(a) Aqueous subphase containing 6.5 X lecular area: A, 21; B, 23 A2/molecule. (b) Aqueous subphase mol/L CaC12. Molecular area: C, 22; D, containing 2.6 X 28 A2/molecule. (c) Pure water subphase. Molecular area: E, 30 A2/molecule.
D in Figure 2 and Figure 3b). It was also found that the surface pressure increased slightly a t the very early stage of the reaction and then decreased with increasing irradiation time (B in Figure 3a and D in Figure 3b) suggesting that some intermediate state precedes the reaction. The third indication of photochemical polymerization and of reactivity related to molecular orientation in an L film was obtained by observing the change in the optical absorption spectra of typical monolayersat high-density L film and low-density L film (A and D in Figure 1, respectively). These are shown in parts a and b of Figure 4, respectively, where the shorter wavelength region (230-280 nm) is shown in the inset. The optical density of the absorption a t 450-520 nm, which is assigned as the absorption by the polymerized PDA L films, increased with irradiation time when the irradiation was carried out at D (Figure 4a), while it remained unchanged at A (Figure 4b). The difference in the spectral changes was also observed in the region 230-280 nm; the peaks a t 243 and 255 nm decreased rapidly a t D but not a t A. These two absorption peaks were reported for polydiacetylene LB films by Tieke et a1.16 This supports the conclusion from the constant-area and constant-pressure experiments that the reaction took place a t D and only slightly a t A upon UV irradiation. It was further found that the optical density in the 230-280-nm region of LB films prepared from the mono(15) Tieke, B.;Lieser, G.;W e g n e r , G. J.Polym. Sci., Polym. Chem. Ed. 1979, 17,1631.
250
300
350 Wavelength
400
450
500
(nrn)
Figure 4. Spectral changes of PDA L films on UV light irradiation: (a) spectra of low-density film measured at point D; (b) spectra of high-density film measured at point A.
layer at condition D decreased while that of LB films prepared a t condition A decreased only slowly with increasing irradiation time, similar to the results obtained for the monolayer study. The low-density LB film prepared a t condition D became insoluble in ethanol (a solvent for PDA) after irradiation, but the high-density LB film prepared at condition A remained soluble after irradiation. These results indicate that the PDA LB films keep the orientation or arrangement of the monolayer from which the LB film was prepared. The PDA LB films were also irradiated with UV light by using different exposure times, and the LB film was then dipped in ethanol to remove unpolymerized PDA molecules. In the case of the low-density films, the normalized thickness T, which is the ratio of thickness after dipping to the original value To,was plotted as a function of dose, as shown in Figure 5. In this example, the plots were obtained for 50 layers of PDA LB film. The normalized thickness of the irradiated low-densityfilm shows a maximum at 40-50 mJ/cm2 and decreases with prolonged UV irradiation, indicating that polymerized PDA LB film decomposes with excessive exposure. For the high-density PDA LB film, which was built up at point A in Figure 1, no film substance was found after the same procedures, indicating that no photopolymerization occurred on this LB film. One possible set of conformations of PDA molecules in monolayers of various molecular areas, which explains the above photochemical reactivity at different molecular areas, is the following. At molecular areas of about 21, 22, and 23 A2/molecule in Figure 1 (A, C, and B), PDA L films
Ogawa et al.
906 Langmuir, Vol. 4, No. 4, 1988
may have the form indicated in eq 1,which is too tight to react by illumination.
I .o
Low Density L B Film of PDA
0'
I
0
20
d 40
Irrodidiation
At about 28 A2/molecule (D), the PDA molecule has enough mobility to react with adjacent molecules, and the molecular orientation of PDA molecules is such that the unsaturated bonds of one molecule are a t a suitable distance from those of an adjacent one, as indicated by eq 2:
c - e\c - c* \ - u\c
I/\
60
80
100
Dose (mJ/crn2)
Figure 5. Change of thickness of low-density PDA LB film,built up at point D in Figure 1 after being dipped in ethanol, as a function of irradiation dose.
proceeds as illustrated by reaction 2, producing polydiacetylene bonds, and that these bonds are also decomposed by further irradiation according to reaction 5:
-
0 0 0 0 0 000
A t molecular areas larger than 30 A2/molecule (E), the conformation of the PDA molecule has a more extended form as in eq 3 or 4 and reacts with adjacent molecules only with difficulty.
hu
+a
(3)
*
(4)
hu
Conclusions The photosensitivity of PDA L films depends on the arrangement or density of PDA molecules in the film: a high-density PDA L fiim does not exhibit photoreactivity, while the suitably low-density film exhibits very high photoreactivity, indicating that the PDA molecules need a suitable molecular arrangement for photoreaction. The results further seem to indicate that photopolymerization of diacetylene bonds is produced between PDA molecules by UV light irradiation when the molecules are arranged at a proper distance from one another. Similar phenomena were also recognized for LB films of high and low density, indicating that the arrangement of the molecules in the PDA LB films remained that of the monolayer from which the LB film was prepared. The polydiacetylene bond produced by photopolymerization is unstable and is decomposed by further irradiation.
Acknowledgment. We acknowledge the assistance of K. Ueda of Semiconductor Basic Research Laboratory, Matsushita Electric Industrial Co., Ltd., for preparation of LB films and thank Managing Director H. Mizuno of Matsushita Electric Industrial Co., Ltd., for his helpful comments.
The above experimental results may further indicate that the photopolymerization process of PDA L film
Registry No. PDA, 66990-32-7;PDA(homopolymer),6699033-8.
