2336
Langmuir 1991, 7, 2336-2341
Analysis of Color Transitions and Changes on Langmuir-Blodgett Films of a Polydiacetylene Derivative Norihisa Mino,' Hideharu Tamura, and Kazufumi Ogawa Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., 3-15, Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan Received May 31,1990. In Final Form: November 13,1990 Color transitions and color changes of polymerized Langmuir-Blodgett (LB) films of a diacetylene derivative (10,12-pentacoeadiynoicacid, PDA) with annealinghas been investigated by measuring UV and visible spectra, differential scanningcalorimeter (DSC) curves, and IR spectra. Reversibility of the color transitions of the polymerized PDA LB films held during heating from 23 (room temperature, RT) to 50 "C,but not above 70 "C. On the other hand, on a DSC curve of the PDA LB film, an endothermal peak appeared at 67 "C,and on that of the polymerized PDA LB film, two peaks appeared at about 70 and 190 "C. The temperatures (about 60 "C) at which a drastic color change appeared were close to those of the endothermal peak (67 "C) on the PDA LB film and the first endothermal peak (62 "C)on the polymerized PDA LB film. This indicates that the color transitions (means reversible absorption peak shift between about 650 and about 540 nm caused by heating or cooling) of the PDA polymer at temperatures below 50 "C is caused by a fluctuation of side chain groups linked to the polymer backbone chain (polydiacetylenic bond). The color change (means irreversible absorption peak shift from about 650 to about 540 nm caused by heating or cooling) at temperatures between 70 and 170 "C is caused by a structural disorder of the side chain groups.
Introduction As diacetylene derivatives have two conjugated triple bonds in a molecule, the derivatives are noted as one of functionalorganicmaterials a t present. For example, since a good single crystal can be obtained easily by using an evaporation technique,' it can be polymerized topochemically to form a polydiacetylenic bond. As a A conjugated bond such as the polydiacetylenic bond shows better response for light than inorganic materials, the third-order nonlinear-optical effects of the polymerized diacetylene derivatives have been investigated in detail for optical application^.*-^ On the other hand, with diacetylene derivatives having a hydrophilic group and a hydrophobic group in a molecule, an ultrathin photosensitive film can be obtained easily by using LB technique^.^ Thus diacetyleneLB films have been investigated for applications as a new resist which could be used for the photolithography process in semiconductor production.6J The polymerized diacetylene derivatives undergo color transitions in solution, which are reversible changes from blue to red caused by changing temperature, pH, or UV light irradiation. The color transition was explained by Baughman8 who described the bond exchange of the backbone chain from a polydiacetylene structure to a polybutatriene structure. On the other hand, Lim et al.9 proposed that a conformation of the side chain group changes by rod-coil transition of the backbone chain. Nevertheless, the mechanism of the reversible color shift (1) Tokura, Y.; Iehikawa, K.; Kanetake, T.; Koda, T. Phys. Reu. B Condens. Matter 1987,36,2913. (2) Kanetake, T.; Bhikawa, K.; Hasegawa, T.; Koda, T.;Takeda, T.; Haeegawa, M.; Kubodera, K.; Kobayaehi,H. Appl. Phys. Lett. 1989,54,
2287. (3) Carter,G. M.; Hryniewia, J. K.; Thakur, M. K.; Chen, Y. J.; Meyler, S. E. Appl. Phys. Lett. 1986,49, 998. (4) Ogawa, K.; Mino, N.; Tamura, H.; Sonoda, N. Langmuir 1989,5, 1415. (5) Garito, A. F.; Singer, K. D. Laser Focrur 1982, 18,59. (6) Carter, G.M.; Chan,Y. J. ACS Symp. Ser. 1983,233, 213. (7) Barraud, A. Thin Solid F i l m 1983,99, 317. (8) Baughman, R. H. J. Appl. Phys. 1972,43, 4362. (9) Lim, K. C.; Heeger, A. J. J. Chem. Phys. 1986,82, 622.
and the irreversible color shift has not been explained clearly at present. In this paper, we wish to report an annealing effect for the color transitions and color changes of the 10,le-pentacosadiynoic acid (PDA) LB films. The thermal properties were investigated with a differential scanning calorimeter (DSC) and thermogravimetry (TG),and UVvisible and IR spectra were investigated by using a UVvisible spectrophotometer and a FT-IR spectrophotometer, respectively, The mechanism for color transitions and changes of polymerized PDA LB films have been discussed.
Experimental Section Materials. 10,12-Pentacosadiynoic acid (PDA, CH3(CH2)11C&C=C(CH&COOH) was of reagent grade, and chloroform as the solvent was of UV spectrum grade; all were obtained from Wako Pure Chemical Industries,Ltd.,and used without purification. Calciumchloride was also supplied by the same manufacturer and used after Soxhletextraction to remove any surface-active contaminants. The water used was of MOS grade for semiconductoruse and was purified by ultrafiltration of deionized water. Apparatus. A Langmuir trough (Joyce-Loebl Trough IV) was used for building up the PDA LB films. The preparations of the LB films were carried out in a class 100 clean room under yellow light cut below 500 nm. The temperature of the clean room was controlled at 23.0 & 1.0 "C, and the humidity was 40 5 % . Procedure. 1. Preparation of PDA LB Films. PDA samples (ca. 0.1 mg) were spread on an aqueoussolution of CaCl2 (3.0 X 10" mol/L) from chloroform solution (100 mg/L) with a graded pipet. The temperature was adjusted at 6 O C and the pH was adjusted at 5.4 by adding NaOH or HCl. For IR spectra measurement, PDA LB films were deposited on a Si wafer on which surface A1 was sputtered (Al/Si). For UV spectralmeasurement,they were deposited on a quartz plate. The substrate was moved across the PDA monolayer on the aqueous subphase to build up the PDA LB films, at constant velocity of 12 mm/min for upward and downward at constant surface pressure of 20 mN/m.
0743-7463/91/2407-2336$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No.10, 1991 2337
Color Changes of LB Films I
-E
i 50t
I
Subphose Conditions 3 . 0 ~ 1 O ~ ~ m o CaCLz L/L 6.0%
pH 5.4 70
I
0
51"
IO
20 Area
30
I
40
(A2/ molecule) IR
Figure 1. Preseure-area isotherms of the PDA molecule at pH 5.4 and 6.0 OC. The PDA LB f i i , i.e., the f i i built up by the above procedure, was of 25 layers, in which the first 12 layers were built up by Z-type deposition and then the deposition changed to that of Y-type. The transfer ratios depended a little on the type of deposition, but the ratio was almost 1for all built-up processes. After the building up, polymerization of the PDA LB films was carried out by irradiation of UV light (SHLlOOUV, Toshiba Corp.) in air at room temperature. The light intensity was 0.014 mW/cm2 at 365 nm. The amount of the irradiation dose was 50 mJ/cm2 at irradiation time of 60 min. The build-up condition is denoted by dots on the surface pressure-area curve, as shown in Figure 1. 2. Annealing PDA LB Films. The polymerized PDA LB films was heated ona digital hot plate (PMC-730,PMC Industries, Inc.) in air. The rate of heating was set at 0.5 OC/min. The temperature was kept constant within f0.3 OC at 0 OC and f0.8 "C at 100 "C. The films were cooled spontaneously after heating. 3. Spectroscopic Analysis of PDA LB Films. UV and visible absorption spectra of the PDA LB films were obtained by using a multichannel spectrophotometer (MCPD-llOA, Otsuka Electronics Co., Ltd.), which allowed real time measurement at annealing. IR spectra were taken before and after annealing on the PDA LB films of 25 layers built up on four-sided Si plates with deposited Al. A FT-IR spectrophotometer (FTIR-4000, Shimadzu Co., Ltd., 2 cm-' resolution) with a multiple external reflection (MER) attachment was used. The angle of incidence was 73O, and the number of external reflections was 7 times, as shown in Figure 2. One thousand interferograms were added. All spectra reported here were the result of subtraction of the curves measured with the clean Al/Si plates from those measured after depositing the respective films on the same plates, and assignments of the absorption bands were made in reference to the reporta by Hayashi et al.10 and Kimura et al.11 4. Thermal Analysis of PDA LB Films. DSC and TG curves of typical PDAs, i.e., PDA monomer, PDA polymer polymerized from polycrystal PDA (polycrystal PDA polymer) by UV light irradiation, and polymerized PDA LB film by UV light irradiation, were measured to clarify the mechanism of the color transition and change with annealing. The thermal properties were investigated by using differential scanning calorimetry (DSC-200,Deiko Electronics Co., Ltd.) and thermogravimetry (TG-DTA300, Seiko Electronics Co., Ltd.). The amount of the sample was about 15 mg.
Results and Discussion Color Transitions and Color Changes of Polymerized PDA LB Films. Visible spectra of the polymerized PDA LB films were measured during UV light irradiation at different temperatures from RT to 290 "C,as shown in Figure 3. In Figure 3, the spectra of the polymerized PDA ~~
(10) Hayashi, 5.; Umemura, J. J. Chem. Phya. 1976,63,1732. (11) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1988, 2,M.
Radiation
A 1 / SI
Substrate
Figure 2. Schematic diagram of the coordinate system for MER IR measurement. l.5-
co m
a
400
500
600
7 0
Wavelength (nm)
Figure 3. Visible spectra of the polymerized PDA LB f i iduring UV irradiation at different temperatures: A, RT; B, 50; C, 64; D, 80; E, 100; F, 160; G, 190; F, 210; I, 290 OC.
LB films can be divided into four groups. The first group consists of spectra having two absorption bands at about 650 and 600 nm (A and B),and this film is called "blue phase film"; the second group is the spectra having two absorption bands at about 540 and 500 nm (C-E), and this film is called "red phase film";the third group is the spectra having a broad absorption band at the region between about 450 and 540 nm (F-H), and the fourth group is spectra having little absorption in visible wavelength region (I). The spectra change drastically between 50 and 64"C (B and C). The reversibility of the spectral change on the polymerized PDA LB films was measured further in detail with annealing between RT and 50,60, and 70 "C,as shown in parts a, b, and c of Figure 4, respectively. With an increase in temperature from RT to 50 "C,the absorption peak at 650 nm shifted to 630 nm slowly, as shown in Figure 4a,A. With a decrease in the temperature from 50 "C to RT, the absorption peak at 630 nm shifted to 645 nm reversibly, as shown in Figure 4a,B. The peak moved 5 nm toward red after annealing. When the temperature was at 60 "C,the absorption peaks at 650 and 600 nm changed, and a new strong absorption peak and two new weak absorption peaks
Mino et al.
2338 Langmuir, Vol. 7,No. 10,1991 Io1
I-
(bl I
I
HeaIing
23OC 40
I
I ,
I 400
1
I
500
I
I
600
Wovelength ( n m )
1
I
I
700
400
1
I
500
1
I
600
I
I 400
700
600
500
Wavelength l n m l
700
Wovelength lnm)
Figure 4. Visible spectra of the polymerized PDA LB fiis during annealing at different temperatures: (a) 50; (b) 60; (c) 70 "C.Part A contains the spectra at heating process from RT to the different temperatures,and part B contains the spectra at cooling process from the different temperatures to RT.
appeared at 540,625,and 500 nm, respectively, as shown in Figure 4b,A. With cooling from 60 "C to RT, the absorption peaks at 500 and 540 nm did not move, and the other peak at 630nm shifted near to the original position of 650 nm, as shown in Figure 4b,B. The reverse of the absorption peak at 630 nm in Figure 4b,B was similar to that in Figure 4a,B. When the temperature became 70 OC, the absorption peaks observed at RT disappeared entirely and two new absorption peaks appeared at 500 and 540 nm, as shown in Figure 4c,A, and with cooling down to RT, the new peaks stayed at the same position, as shown in Figure 4c,B. The two new absorption peak positions were similar to those observed at 60 OC, as shown in Figure 4b,B. Accordingly, it was concluded that the reversibility of the color transition was maintained at the region between RT and about 50 O C , but not above 70 OC. Thermal Properties of PDA Polymers. The DSC curves obtained for three typical PDAs are shown in Figure 5. An endothermal peak, which indicates melting of the PDA monomer, was observed at 67.1 OC and the melting point of the PDA monomer was assigned as 62.3 "C, as shown in Figure 5a. For the polycrystal PDA polymer, three endothermal peaks were observed at 655,185,and 206 O C , as shown in Figure 5b. The first peak at 65.5 O C was similar to the melting point of the PDA monomer. The peak also had a shoulder at about 70 O C . For the polymerized PDA LB film, four endothermal peaks were observed at 62,72.8,183,and 196 "C,as shown in Figure 5c. As the first peak at 62 "C can be identified with the peaks at 67.1 and 65.5 "C in park a and b of Figures 5 , respectively, the peak should arise from the PDA monomer remaining in the polymer. The relative intensity of the peak at 62 "Cwas also weaker than those on the monomer and the polycrystal polymer. The second peak at 72.8 O C , which can be assigned to the shoulder at about 70 O C in Figure 5b, was stronger than that on the polycrystalline polymer. This may indicate that the polymerization degree of the PDA LB film was higher than that of the polycpystal PDA polymer. The third and fourth peaks at 183 and 196 O C should be the same peaks on the
174.6
206
65.5 t
I
I
1
(C)
174.7
62
I96
72.8 I
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1
100
I
I
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Temperature
200
2
io
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Figure 5. DSC curves of the typical PDAs: (a) PDA monomer; (b) polycrystal PDA polymer; (c) polymerized PDA LB films.
polycrystal PDA polymer, which may be assigned to the melting point of the polymerized PDA. As the polymerization degree of the PDA polymer can be estimated with the residue of the PDA monomer, optical densities of the two polymers (polycrystalPDAs polymer and polymerized PDA LB films) were measured before and after photopolymerization. The spectra of the polycrystal PDA polymer and the polymerized PDA LB films
Langmuir, Vol. 7, No. 10,1991 2339
Color Changes of LB Films
-Before
UV Light Irradiation ----After UVLight Irradiation
I
I
I
I
la)
-Before
UV Lqht Irradiation ----After UV Light Irradiation
250
200
300
350 ,
Wavelength Inm )
Figure 6. UV spectra of PDAs before and after UV light irradiation: (a) polycrystal PDAs; (b) PDA LB films. IO0
-
50
-
25
-
a c
7 2 5 0 " ~
75
I
0
100
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I
200 300 Temperature (OC 1
.
I
l
.
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400
Figure 7. A TG curve of the polymerized PDA LB film. are shown in parts a and b of Figure 6, respectively. After the UV irradiation, the absorption band at 254 nm due to the absorption of diacetylenic group disappeared on the PDA LB film, as shown in Figure 6b, but the same band remained as a weak peak on the polycrystal PDA polymer, as shown in Figure 6a. Thus the polymerization degree of the polymerized LB film was higher than that of the polycrystal PDA polymer. This result supports the DSC data. It was confirmed further by TG analysis that the decomposition temperature of the polymerized PDA LB film was 250 "C,as shown in Figure 7. This indicates that the endothermal peaks at about 180 and 200 "Cwere not due to the decomposition. Accordingly the polymerized PDA has two melting points at about 60 "C and 175 "C, as shown in parh b and c of Figure 5, where the first melting point at about 60 "C can be assigned to the melting point of the residual monomer (62.3 "C). The other two endothermal peaks and shoulder appeared at 185,206, and about 70 "C in Figure 5b, and the three peaks at 72.8,183, and 196 "C in Figure 5c might be caused by the polymer.
.
.
.
l
.
.
.
.
1500
l
.
lo00
.
.
.
500
Wavenumber ( cm-'
Figure 8. IR spectra of the monomer PDA LB films (a) and polymerized PDA LB films (b). The spectra in part b were measured at different temperatures: A, RT; B,50; C, 64; D, 82; E, 100, F,160; G, 190; H, 210; I, 290 "C.
Spectroscopic Analysis of Polymerized PDA LB Films. IR spectra of the PDA LB film and polymerized PDA LB films of 25 layers at different temperatures were measured, as shown in Figure 8. The absorption bands at 2965 and 2870 cm-l can be assigned to the CH2 antisymmetric vibration (CH2 vu) and the CH2 symmetric vibration (CH2 v,), respectively. The absorption bands at 1720 and 1700 cm-l can be assigned to the C=O stretching vibration and the COOH stretching vibration, respectively. The absorption bands at 1465cm-l can be assignedto the CH2 scissoringvibration (CH2 6s). The absorption band at 1420 cm-l can be assigned to the C-O stretching vibration (C-0 u). IR spectra changes of the polymerized PDA LB films were investigated further with increasing the temperature, as shown in Figure 8b from (A) through (I). Although the COOH band (at 1720 cm-l) measured at RT (A) and 50 "C (B)was very weak, it became stronger withincreasing the temperature from 64 to 100 "C (C-I) and disappeared above 160 "C (F). The reason for the appearance of the strong peak between 64 and 100 "C can be explained by the change from C-0 bond to the COOH bond by heating. On the spectrum at 160 "C (F),the new weak absorption band due to the C=O stretching vibration of acid anhydride also appeared at about 1800 cm-l, indicating that the dehydration of the COOH groups occurred at 100 "C (E). The temperature (64 "C)when the stronger C 4 stretching vibration band appeared was similar to that of the first melting point, as shown in Figure 5c. On the other hand, although the C-O band (at 1420 cm-9 was sharp at RT (A) and 50 "C (B),the band became broad and weak with increasing the temperature from 64 to 100 "C (C-E) and reversed to the original state at 160 "C (E). This result may support that the dehydration of the COOH group occurred beyond 100 "C.
Hcl[qq&
Mino et al.
2340 Langmuir, Vol. 7, No. 10,1991
Scheme I1
Scheme I
Hc2[
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c * \
c
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0 0-0 0-0 0-0 0-
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The bands between 1350 and 1240 cm-l due to the twisting and wagging vibration of the (CH2)" group did not change from RT (A) to 50 "C (B), became weaker between 64 and 190 "C (C-G), with increasing the temperature, and disappeared above 210 "C (H and I). The band (at 720 cm-l) due to the rocking vibration of ( C H Z ) ~ did not change from RT to 100 "C (A-F), became weaker over 100 OC with increasing the temperature (F-H), and diseappeared at 290 "C (I). The reason for these might be similar to that observed for the endothermal broad band at 70 and 190 "C shown in Figure 5c. Although the CH2 scissoring vibration band (1465cm-l) was also strong at the region between RT and 160 "C, the band became weak above 190 "C (G). At 290 "C (I), all the bands were very weak, indicating that the film was decomposed at this temperature. Possible Mechanism of Color Transitions and Changes. The possible conformationchanges in the polymerized PDA LB film corresponding to the color transitions and changes are shown in Schemes I and 11. The part of the PDA molecule other than the diacetylenic group (HC1 and HC2) constitute large parts of its molecular weight, and in the PDA polymer, the side chain groups (S1 and S2) form the great part of the total molecular weight, as shown in Scheme I. Thus, as the melting energy of the PDA monomer sould be similar to the first melting (caused by the conformationchanges of the side chain group) point of the PDA polymer, it can be thought that the endothermal peak at 72.8 "C should correspond to the structural fluctuation of the side chain groups, as shown in Scheme IIb. This is supported by the fact that the endothermal peak (72.8 "C) of the polymerized PDA LB film was similar to the endothermal peak (67.1 "C) of the monomer PDA. This may be supported further by the results that the color transition occurred below 50 "C and the color change was caused above 70 "C. The color transition is caused by the change of the effective conjugated length of the polydiacetylene bond which depends on the order in the arrangement of the side chain groups. The color change occurs by the change of
c
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the effective conjugated length which is caused by the disorder of the polymer backbone. Below the first melting point, the side chain groups fluctuate and entangled but the backbone dose not fluctuate. Then the color transitions are observed, as shown in Figure 4a. This indicates that the side chain groups fluctuate reversibly, as shown in Scheme IIa,b. The reason for the appearance of the color change above the first melting point at 72.8 "C can be explained that as the side chain groups (S1 and 52)wobble violently and entangle, the side chain groups are locked each other after cooling, as shown in Scheme IIc. That is, the conformation of the side chain groups could not reverse to the previous state shown in Figure 2a. This is supported by the fact that the IR absorption bands associated with the side chain groups i.e., the bands at 1465 cm-l (CH2 6s) and between 1350 and 1240 cm-l (CH2 T and w ) became weaker above 64 "C. The endothermal peak at about 190 "C might be due to the change of polymer backbone structure, Le., the disorder of the polydiacetylenic bond (B) of the PDA polymer, as shown in Scheme IId, because the endothermal peak at about 190"Cwas observed only on the polymer PDA. Above 250 "C, the film decomposed, which is also supported by the TG curve and visible spectra changes between 190 and 290 "C. Accordingly, the color transition, which accompanies the absorption band shift on the spectra, with annelaing below 50 "C from 650 to 630nm is caused by the fluctuation of the side chain groups, because the temperature (50 "C) is lower than that where the color change occurs and the PDA monomer melts. At an annealing temperature of 60 "C, both the color transition and change were observed, because the polymerized PDA LB film has two states at about 60 "C; one is the fluctuated state of the side chain group, and the other is the state that the side chain group of polydiacetylenic bond was disordered structurally, as shown in parts b and c of Scheme 11,respectively. The color transition is caused to a certain extent by the former. With annealingabove70 "C,the color transition became irreversible and the color changed from blue to red, because
Color Changes of LB Film when the annealing temperature was above the melting point of the PDA monomer, the side chain groups were disordered and got tangled, as shown in Scheme IIc. Conclusions By measurement of the UV spectra, IR spectra, and DSC and TG curves, it was concluded that the color transition of polymerized PDA was caused by the fluctuation of the side chain groups and the color change was caused by the structural disorder of the side chain groups. The fluctuation is reversible and the structural disordering is irreversible. The color change occurs above about 70 OC. This indicates that the side chain groups of the PO-
Langmuir, Vol. 7, No. 10, 1991 2341 lymerized PDA are changed structurally at about the melting point of the monomer PDA and disordered over the melting point of the monomer, and beyond the melting point of the PDA polymer, the polymer backbones (polydiacetylenic bond) are further disordered.
Acknowledgment. The authors gratefully acknowledge Ms. Ueda for her assistance in preparing the LB films and thank Director Dr. Nitta of Matsushita Electric Industrial Co., Ltd., Central Research Laboratories for his helpful comments. Registry No. PDA (homopolymer),66990-33-8.
