2764
Langmuir 1992,8, 2764-2770
Disperse Red-Labeled Poly(methy1 methacrylate) Monolayer at Interfaces Studied by Second-Harmonic Generation and Surface Potential Kotaro Kajikawa,' Takashi Yamaguchi, Takeehi Anzai, Hideo Takezoe, and Atsuo Fukuda Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152, Japan
Shuji Okada, Hiro Matsuda, and Hachiro Nakanishit Research Institute for Polymers and Textiles, 1 - 1 -4 Higashi, Tsukuba, Ibaraki 305, Japan
Takashi Abe and Hiroshi It0 Central Research Institute, Mitsui Toatsu Chemical Inc., 1190 Kasama-cho, Sakae-ku, Yokohama 247, Japan Received March 27, 1992. In Final Form: July 27, 1992 A Disperse Red-labeled poly(methy1methacrylate) monolayer at the aidwater interface has been investigated by surface second-harmonicgeneration (SHG) and surface potential measurements. Upon spreading,the polymer forms a domain having noncentrosymmetricC," symmetry. The local field factor determined by SHG in the gas phase indicates no change in the distance between the dye moiety. On the basis of the local field factor and the optical absorption spectra, we found that domains gather each other and the dye moiety slightly moves from the aidwater interface to the water in the expanded phase. Consequently,the molecular tilt angle decreases in the expanded phase. These are observed not only by SHG but also by surface potential measurement. Thus,we can conclude that the polymer monolayer is ideally compressed at the aidwater interface. The influence of the depition of the monolayer onto a glass substrate has also been studied to clarify the cause of fii-forming ability. While the monolayer deposited by the horizontal depositionchangesthe structureinto a centrosymmetricfashion,the monolayer deposited by vertical moving-wall deposition maintains their structure at the air/water interface. Thia differenceoriginatesfrom hydrophilicityof the dye moiety contacting to either hydrophilicor hydrophobic substrates. In conclusion, the formation of the monolayer at the &/water interface is caused by dye moiety and the polar sites of PMMA. 1. Introduction A polymer monolayer is more attractive from the application viewpoint than a monolayerof small molecules because of its chemical and physical stability. While many surface probes have been applied for monolayers of small molecules, few probes were applied for polymer monolayers. Most of discussions on the polymer monolayers were baaed on the surface pressure-area isotherm ( F A isotherm). Recently, some noncontact surface probes, exemplified by ellips~metry,'-~ surface light scattering: and X-ray diffraction,' were adopted to clarify the structure of polymer monolayersat the air/water interface. Kawaguchi et al.lS and Sauer et aL4 demonstrated that ellipsometry is a powerful tool for the investigation of a polymer monolayer at the aidwater interface. Some ~
~~
'Present address: Institute for Chemical Reaction Science, Tohoku University, 2-1-1,Katahira, Aoba-ku, Sendai, Miyagi 980, Japan. (1)Kawaguchi,M.;Tohyama, M.; Mutoh, Y.; Takahashi,A. Langmuir 1988,4,4071 (2)Kawaguchi, M.; Tohyama, M.; Takahashi, A. Langumuir 1988,4, A1 1 _--.
(3)Nagata, N.; Kawaguchi, M. Macromolecules 1990,23,3967. (4)Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G.; Kim, M. W. Macromolecules 1989,22,2332. (5) Motachmann, H.;Reiter, R.; Lawall, R.; Duda, G.; Sta", M.; Wegner, G.;Knoll, W. Longmuir 1992,8,1784. (6)Kawaguchi, M.; Sano, M.; Chen, Y. L.; Zografi, G.; Yu, H. Macromolecules 1986,19,2606. (7)Schloesman,M.L.;Schwartz, D. K.; Kawamoto, E. H.; Kellogg, G . J.; Pershan, P. S.; Kim, M. W.; Chung, T. C. J. Phys. Chem.1991,96, 6628.
arbitrarineas, however, lies in the determination of the
film thickness. Actually, although their experimental results are quite similar, the determined thicknesses of the monolayers disagree with each other owing to the different analysis. For example, Kawaguchi et aL2 determined the thickness of a poly(viny1 acetate) (PVAc) monolayer as 130-320A, which is much thicker than that (around 6 A) determined by Sauer et ala4The difference is serious, since such thick and thin PVAc films imply a totally different chain conformation at the air/water interface, namely a three-dimensional piled structure and a two-dimensional flattened structure, respectively. In a monolayer of poly(methy1methacrylate) (PMMA),which is the backbone of the polymer used in the present study, nearly the same thickness of the order of 10 A was exceptionally concluded. Second-harmonic generation (SHG) provides UB with the orientational and symmetry information of a monolayer.g18 Most of the studies, however, have been re(8) Shen, Y. R. Liq. Cryst. 1989,5,636. R. Nature 1989,337,619. (10)Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30,1060. (11)Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30,L1626. (12)Chen, W.;Feller, M. B.; Shen, Y. R. Phys. Rev. Lett. 1989,63, 2666. (13)Mizrahi, V.; Stegeman, G. I.; Knoll, W.Phys. Rev. A 1989,39, 3666. (14) Shimizu, Y.; Kotani, M. Opt. Commun.1989,74, 190. (16)Maroweky, G.;Steinhoff, R. Opt. Lett. 1988,13, 707. (9) Shen, Y.
0743-7463/92/2408-2764$03.00/0 Q 1992 American Chemical Society
Dye-Labeled PMMA Monolayer at Interfaces by SHG stricted to a monolayer of smaller molecules such as hemicyanine dye,'OJ5 merocyanine dye," liquid crystal,8J2 and fattyacid.I6 A couple of works on polymer monolayers at the aidwater interface using SHG have been reported. Berkovic et al." reported nonlinear-optical properties in a polydiacetylene monolayer, although most of the part were concerned with third-harmonic generation (THG). Gaines showed in his review article'* that the vinylpyridine moiety in a poly(4-vinylpyridine-co-acrylonitrile) monolayer at the aidwater interface is oriented along the surface normal and shows no orientational change during the compression process according to SHG measurements; however, the results were preliminary and no detailed discussion was given. Recently, Hsiung et al.le measured SHG from a polymer monolayer and discussed the nonlinear optical properties from the view point of devices. The purpose of the paper is rather device-oriented. Here, we probe the compression process and deposition process of Disperse Red I (DR)-labeled PMMA monolayers a t the aidwater interface by the measurements of SHG and surface potential. The purpose of this paper is to clarify the behavior of flattened conformation of a polymer monolayer a t the aidwater interface and at the aidsolid interface. We demonstrate that SHG just fits for the research of interface science. The rapid report had been published.20 The DR moiety in the monolayer a t the aidwater interface is oriented in a noncentrosymmetricfashion, i.e., C,, symmetry with the rotation axis along the surface normal. Thus, fairly strong SH light arises from the dye moiety. We used the polymers of various dye fractions. In the polymer monolayer of low dye fraction, rather gradual decrease in the tilt angle and the dispersion of the dielectric constant arises. In the PMMA monolayer of the high dye fraction, abrupt changes in both values take place at expanded phase-condensed phase transition. In this transition region, the red-shift of the absorption band occurs, resulting in the change of the resonance condition to SH light. We also report the changes in molecular orientation due to various deposition processes. The monolayer deposited by vertical moving-wall deposition maintains their structure a t the air/water interface, whereas a t least the first monolayer suffers a change into a centrosymmetric fashion by the horizontal deposition.
2. Experimental Section Figure 1 shows the chemical structure of the monolayer material, poly(2-(N-(4-((4-nitropheny1)azo)phenyl)ethylamino)ethyl acrylate-co-methylmethacrylate),whose averagemolecular weight is shown in Table I. It was measured by a gel permeation chromatography (GPC) with a column of Shodex GPC K-804 (Showa Denko K.K.). The stereotacticityof the polymer must be atacticdue to radical polymerization. We also confirmedthat the PMMA polymerized by the same procedure is attributed to be atacticby lH NMR. Hereafter,for convenience,we abbreviate the polymer to PMMA-DR(r), where x stands for the molar percent fraction of DR. The polymers of various dye fractions, x = 4.4,8.4,13.9%,wereused. They were dissolved in chloroform and spread onto the aidwater interface. The measurements of SHG in the monolayers at the air/water interface were accom(16) Rasing,Th.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. Reu. Lett. 1985.55. 2093. (17) Berkovic,G.;Superf~e,R.;Guyot-Sionnest,P.; Shen,Y.R.;Prasad, P.N.J. Ont. Soc. Am. B 1988.5., 668. --(18) G a k , G.L., Jr.%angmuir 1991, 7, 834. (19) Hsiung, H.; Rodriguez-Parada, J.; Bekerbauer, R. Chem. Phys. Lett. 1991,182, 88. (20) Kajikawa, K.; Anzai, T.; Takezoe, H.; Fukuda, A.; Okada, S.; Matsuda, H.; Nakanishi, H.; Abe, T.;Ito,H. Chem. Phys. Lett. 1992,192, 113. - 7
Langmuir, Vol. 8, No. 11,1992 2765
NO2
Figure 1. Chemical structure of the material used; PMMADR(x), where x stands for the molar fraction. Table I. Molecular Weight of the Polymers Used in This Measurement. M, Mw MwIMn PMMA 4.8 X 10' 9.4 X 1@ 2.0 2.3 X 1@ 4.8 X 104 2.1 PMMA-DR(4.4) 3.9 X lo' 8.6 X 10' 2.2 PMMA-DR(8.4) PMMA-DR(13.9) 1.9 X 10' 3.8 X 104 2.0 M,and M, mean number-averagemolecular weight and weightaverage molecular weight. The PMMA is a reference bemuse it waa produced by the same polymerization method, Le., radical polymerization as well as PMMA-DR(x). The calibration polymer used was PMMA. plished using the home-brewed Teflon trough. The molecular area per repeating unit was controlled by a movable barrier. We adopted three kinds of deposition methods to prepare LB films, i.e., the conventional vertical deposition using a Kuhntype Langmuir-trough (Takahashi Seiki Co. Ltd.), the vertical deposition using a moving-walltrough (NLE,NL-LB240),and the horizontal deposition with a Teflon guide to prevent bilayer deposition. In a surface SHG measurement, we used a Q-switched Nd YAG h e r (Quanta-Ray DCR-11) running at ita fundamental wavelength, 1064nm (12 Hz, 8 ns). The reflected SH signal waa obervedby a photomultiplier (Hamamatsu,R955) after removing the fundamental light by both a color filter and an interference filter. We used a polarizer and an analyzer, which were fully controlled by a microcomputer. In the determination of the absolute value of susceptibilities, we used a'hemicyanine monolayer at 210 A2/moleculeat the aidwaterinterface as a reference. The hemicyanine molecule has a second-order molecular polarizability along the molecular long axis, j3((( = 2.5 X 10-%esu, according to the measurement by Maroweky and Steinhoff.16 The absorption spectra of the monolayers were obtained by two kinds of spectrometers: MCPD-1000 (Oteuka Electronics Co., Ltd.)for the monolayerat the aidwater interfaceand U-3400 (HitachiLtd.)for the monolayer at the airholid (glass)interface. In MCPD-1000, incident light was guided to the water surface by an opticalfiber. The light transmitted through the monolayer was reflected by a metal mirror located below the water surface and was transmitted through the monolayer again and waa also guided by an optical fiber to the spectrometer. The surface potential measurements were carried out with a vibrating plate condenser by a null technique. An electrode of 10 mm x 10 mm was located at 1mm above the monolayer and was vibrated slightly by a speaker. Another electrode waa immersedin water. Thevoltagesignalconvertedfromthe current between the two electrodes was amplified by a lock-inamplifier (NFElectric Instruments, 6600A). The surface potential waa obtained by searching a null signal by applying a biaa voltage. The experimental details were described in a previous paper." 3. Analysis of SHG Profile Let us briefly describe the analysis of the SHG signal adopted in this study. Details in the theoretical expres(21) Yamaguchi,T.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J.
Appl. Phys. 1992,31,1160.
2766 Langmuir, Vol. 8, No.11, 1992
Kajikawa et al. 1.21
.
I
.
I
.
1.21
I
.
I
,
I
.
,
m 0.8
-s 0.4 C
I v)
0. 0.0 0
Polarization Angle (deg.)
30
60
90
Polarization Angle (deg.)
Figure 3. Typical examplea of the p-polarized SH intensity profiles ae a function of the polarization angle of fundamental light, yp. (a) and (b) were observed in the PMMA-DR(l3.9) monolayer at 27.3 and 11.7 AZ,respectively. Figured. Opticalgeometryaround the monolayer. The medium 1,m, and 2correspondto the air,the monolayer,and the subphase or the Substrate, respectively. See the text for the details.
sions and simulations were already described in our previous paper.1° (In eq 12 of ref 10, D, B, and C should be read for B, C, and D, respectively.) They are, in principle, based on the theoretical expression by Bloembergen and Pershan.22 The expression was also applied for many individual system^.^^^ The optical geometry around the monolayer is shown in Figure 2. We define the z axis along the surface normal and the 2 and y axes in the surface plane. Medium 1 corresponds to the air of the dielectric constant el(Q) at frequency Q (=oor 2w) and medium 2 corresponds to subphase or a substrate of the dielectric constant cz(Q). The fundamental light at frequency w with an incident angle &(o)generates the SH light in the medium m, whose dielectric constant is em(Q). The SH light is reflected with the reflection angle 81(20). The polarization angle yp is defined as an angle of the polarization direction of the incident light with respect to the incident plane of the fundamental light, and the polarization angle r a is that of the reflected light. Thus,p-polarization and s-polarization ' and yi = 90' (i = p or a), respectively. correspondto = 0 Most rodlike molecules, such as azobenzene and hemicyanine, tend to form a Cmumonolayer unless they aggregate. Since the monolayers used in the present study also show Cmusymmetry as will be described in section 4, we present here the theoretical expression for a Cmu monolayer with the z axis along the surface normal. The second-order susceptibility tensor in a Cmumonolayer is described with three independent nonzero tensor componenta as
(1)
x,o
0
0
where x+xx = fiyyand fizz= xyzyby the symmetry operation. In the present paper, we used the surface susceptibility (two-dimensional susceptibility), since we treat the monolayer as a two-dimensional system. Suppose that the nonlinearity originates only from the dominant microscopic polarizability, @E[[, along the long axis, t, of polar moiety; the tensor components can be described as (22) Bloembergen, N.; Pershan, P. S.Phys. Reo. 1962,138,606. (23)Aktaipetrov, 0. A.; Akhmediev, N. N.; Baranova, I. M.;Miehina, E. D.; Novak, V. R. Zh. Eksp. Teor. Fiz. 1985,89,911. (24) Dick, B.; Gierulski, A.; Marowsky, G.; hider, G. A. Appl. Phys E 1986,38,107. (25) Zhang, T. G.; Zhang, C. H.; Wong, G. K.J. Opt. SOC.Am. E 1990,
7, 902.
where 0 is the molecular tilt angle with respect to the surface normal, Li(Q) is the local field factor for i direction at frequency $2, and N,is the surface number density. For convenience, we used xett as
Xzxx,eff
=
(;g)xzxx
(3)
The use of Xetrinstaadof x effectivelyincludes the multiple reflection within the monolayer.1° Kajikawa et al.l0 proposed a fitting procedure to determine both Xrrz,effl~xx,eff and f i z x , e d ~ x the x ,SH e~; intensity was measured as a function of yp and by keeping the polarizer8 parallel or perpendicular to each other, and the obtained profile was fitted to the respective theoretical expression I(yp,ya= yp)or I(yp,yn = yp+900). Thisfitting procedure determines x E z z ~ d x z x x ~ xlrx,d and ~ ~ ~used ~ as , two ~ f parameters. f In the present study, we adopted a one-parameterfitting for the sake of simplicity. To reduce the number of the parameters, x l z x , e ~ / ~ x r , d was first determined by the SH intensity ratio I(45O,Wo)/ I(90°,00) ( = f i z x , e ~ /as ~x proposed x , e ~ )by wlang et al.m Provided that the dispersion of e2 is negligible and that the dispersion and/or anisotropy of the local field factor is negligible, xxrx,e~/xzxx,~ directly gives the dispersion of the dielectric constant of the monolayer, em(&)/cm(w). We measured the p-polarized SH light as rotating the polarization angle of the fundamental light yp from Oo to 9oo at intervals of loo. The SH intensity profile against yp was fitted by the theoretical curve with ~ ~ ~ ~ , ~ nas/ a~ l x ~ , a r parameter. The examples of the profiles and the fitted curves are shown in parta a and b of Figure 3, which are for the L films (Langmuir film: a monolayer at the air/ water interface) of PMMA-DR(13.9) at the molecular areas 27.3 AVrepeating unit (r.u.) and 11.7 A2/r.u., respectively. For the determination of the absolute value of fizzand xzxx,from xzzz,etflXzrr,efr, we used = 2.31 for a spincoated DR-doped e2(o) = e2(2w) = 1.769 for water subphase and t 2 ( 4 = 2.131 and 42w) = 2.160 for a glass (26) Singer, K.D.; Kuzyk, M. G.; Soh, J. E.J. Opt. SOC.Am. B 1987, 4, 968.
Dye-Labeled PMMA Monolayer at Interfaces by SHG
Langmuir, Vol. 8, No. 11,1992 2767
E~
2 E
40
-
Y
30 95-0.
20
8
5 a
20
10
Area
10
n ., 0
0
10
30
20
30 0
10
20
30 0
10
20
30
(A2/I.u.) Area (A2/1.u.) Figure 6. hzr,dfizza values of the monolayers against the area per repeating unit. (a), (b), and (c) correspond to those in PMMA-DR(4.41, PMMA-DR(8.41, and PMMA-DR(13.9), respectively. Note that ~ z z , ~ / ~ z z r =C cf i~ ( ~ w ) / c ~ignoring (o) the effect of local field factor.
a,
Area per repeating unit (A2)
(A2/I.u.)
Area
Figure 4. Typical example of n-A isotherm of the PMMADR(13.9) monolayer. N
2
n 0
10
Area
20
(A2/ r u )
30
0
10
Area
20
(A2/ I u )
30
0
10 20 30 Area(A*/ru)
Figure 7. fizzand xIrz of the monolayers against the area per repeating unit. (a), (b),and (c) correspond to those in PMMADR(4.4), PMMA-DR(8.41, and PMMA-DR(13.91, respectively. 400
I
I
I
500
600
700
800
at 495 nm. The spectra agree with those previously reported by Okada et ale2' Before describing the analysis of the SHG measurements, let us first discuss the symmetryof the monolayer. Most of the monolayers consisting of rodlike small moleculeshave Cmusymmetry with the rotation axisnormal substrate,10and &€ = 2.5 X esu of a hemicyanine to the surface. However,Aktsipetrovet al.23and Kajikawa moleculels used as a standard. The ratio of ~z~~ and xPxx et al.l0 reported the examples of symmetryother than CmU gives the average tilt angle B as when the rodlike molecules form aggregates. Therefore, we must first determine the symmetry of the monolayer. In the theoretical prediction in Cmusymmetry,s-polarized (4) SH light should not be generated by p-polarized or s-polarized fundamental light, i.e., 1(Oo,90) = 1(90°,900) assumingthat the distribution of B is given by a &function. = 0. We c o n f i i e d that this is actually the case. Moreover, By use of the obtained values xzzrand B as a function of the fittings were successfully made by assuming CmU N,,Lr(2w)LZ(w)&~ is obtained. Then, we can determine symmetry, as already shown in parts a and b of Figure 3. the effective local field factor L,(2w)Lz(w),which tends to These results safely conclude the monolayer symmetryto unity in the limit of N,= 0, and consequently estimate f l ~ ~be ~of Cmu. of the DR dye. According to the procedure described in section 3, we determined Xxzx,eff/Xzxx,eff against the area per repeating 4. Results unit, as shown in Figure 6. In the monolayers of the low dye fraction polymers, PMMA-DR(4.4) (Figure 6a) and 4.1. Compression Process. Figure 4 shows a typical PMMA-DR(8.4) (Figure 6b), Gzx,eff/&xx,eff = 1.0 in the surface pressure/area isotherm ( P A isotherm) of the gas phase. The Xxrx,ett/Xlxx,eftValues increase to 1.07 in the PMMA-DR(13.9) monolayer. In the region larger than expanded phase upon compression. In a PMMA-DR(13.9) 30 A2/r.u., the SH signal was unstable. The SH light monolayer (Figure 6c), Xxzx,eff/Xzxx,eff 1.07 in the gas and gradually became stable in the gas phase close to the the expanded phases. The Xxrx,etfl%xx,ett rather abruptly expanded phase (from 20 to 30 A2/r.u.). (The present increases to 1.12 at around 15A2/r.u.where the expandedpolymer monolayer forms a domain structure at the air/ condensed phase transition takes place. water interface. Therefore, strictly speaking, there is no Figure 7 shows x~~~ and xzXxin the compression process gas phase and it is impossible to distinguish the expanded of the monolayers: (a) PMMA-DR(4.4); (b) PMMAphase from the condensed phase. For the sake of DR(8.6); (c) PMMA-DR(13.9). The compression was convenience, however, we specify the region as expanded started at 28 A2/r.u. In PMMA-DR(4.4) and PMMAand condensed phases as in the text.) With further DR(8.4), %zz monotonically increases with decreasing area compreseion, the expanded phase (15-20 A2/r.u.) and the until the film collapses, while f i X xshows only a slight condensed phase (9-15 A2/r.u.) appear, and the monolayer increase. In contrast, x~~~and xurx of the PMMAcollapses at A = 9 A2/r.u. The discontinuityof the isotherm DR(13.9) monolayer have maxima at around 13 and 15 at 12A2/r.u.originatesfrom the relaxation during the SHG A2/r.u., respectively. measurement. Using x values and eq 4, we determined the average tilt Spectra I and I1 shown in Figure 5 were taken at the angle B of the dye moietywith respect to the surface normal molecular areas indicated by arrows in Figure 4, before as shown in Figure 8. In PMMA-DR(4.4) (Figure 8a) and and after the surface pressure starts to increase. While spectrum I of the monolayer in the gas phase has a peak (27) Okada, S.; Matauda, H.; M a d i , A.; Nakanishi, H.; A b ,T.;Ito, H.Jpn. J. Appl. Phys. 1992, 31, 365. at 480 nm, spectrum I1 in the condensed phase has a peak Wavelength (nm) Figure 5. Absorptionspectraof the PMMA-DR(13.9) monolayer observed at the areas per repeating unit indicated by arrows in Figure 4.
Kajikawa et al.
2768 Langmuir, Vol. 8, No. 11, 1992 BO,
35
.
.
.
.
.
.
.
-
0
0
20 30 Area (A2/ r.u.) 10
.
.
.
.
.
.
I
. . . . .
1
u 20 30 0 Area (AZ/r u )
10
10
Area
20
30
(A2/ r.u.)
Figure 8. Average moleculartilt angleswithrespect to the surface normal of the monolayers against the area per repeating unit. (a),(b), and (c) correspond to thoseinPMMA-DR(4.4),PMMADR(8.4))and PMMA-DR(13.9), respectively.
the covering over of the electrode area by the monolayer. After the increase,the surface potential shows only a slight change in the expanded phase and the condensed phases. Consequently, the apparent dipole moment indicates a rather monotonic decrease in this range. However, in the PMMA-ADR(13.9) monolayer, we can clearly observe an inflection point at 15 A2 corresponding to the expandedcondensed phase transition point. In contrast, there is no inflection point over the expanded and the condensed phases in a PMMA monolayer as shown in Figure l l b . 4.2. Deposition Process. It was impossibleto transfer the monolayer to a substrate by conventional vertical deposition. Therefore, we adopted the moving-wall type vertical deposition method. Although we had some difficulties in finding the deposition condition such as surface pressure, concentration of solute, and compression speed, it was possible to deposit the monolayer onto a hydrophilic glass substrate. The condition adopted for the deposition of the PMMA-DR(13.9) monolayer was as follows: the surface pressure of 10 mN m-l, the concentration of the solute of 0.8 X mol/L. As shown in Table 11, the SHG measurement provides us with the structural information: the monolayer deposited by moving-wall vertical deposition maintains Cmusymmetry during deposition and the estimated susceptibility components agree well with those of the monolayer at the aidwater interface. We also tried horizontal deposition onto a hydrophobic glass substrate with a Kuhn-Type trough at 15and 40mN m-l. Although we confirmed the existence of the polymer on a substrate by the absorption spectrum, we observed no or negligibly weak SHG from both samples deposited at 15 and 40 mN m-l. Surprisingly, however, the accumulation up to 30 layers gave rise to a fairly high x value. The details will be reported in a separated paper.
42.1 A.-n
0
IO 20 30
40
50
( A 2 / r.u.) Figure 9. Reversibility of the average molecular tilt angles in the PMMA-DR(13.9) monolayer. Open and fiied circles indicate those in compressionprocess and expansionprocess,respectively. Area
u
0.0 o.2 0
10
20
30
Area (A*/r.u,)
Figure 10. Effective local field factor in the PMMA-DR(13.9) monolayer as a function of surface number density.
PMMA-DR(8.6) (Figure8b), 8 gradually decreases during compression. On the contrary, the PMMA-DR(13.9) monolayer gives constant B = 47.5O in the gas phase, and B rather abruptly decreases to 41° and stays constant in the expanded phase, as shown in Figure 8c. In order to investigatethe reversibility, we measured B of the PMMADR(13.9) monolayer in the compression-expansioncycle as shown in Figure 9. We compressed the monolayer to the condensed phase, stopped at 12 A2,and then started to expand. Open and closed circles indicate B values in the compression and expansion processes, respectively. The compression-expansioncycle indicates good reversibility of the monolayer, although slight hysteresis exists in the expanded phase. As described in section 3, we can estimate the effective local field factor, L = L,(~w)L,(o)~, as a function of the area per repeating unit. The result for the PMMADR(13.9) monolayer is shown in Figure 10. It is clear that the effective local field factor approaches an asymptotic limit, which is taken as unity. The surface potential for the PMMA-DR(13.9) monolayer and the apparent dipole moment estimated by the surface potential were plotted against the molecular area as shown in Figure l l a together with the T-A isotherm. For comparison,we also show those of a PMMA monolayer in Figure llb. We s read the polymer solution to form a monolayer of 40 2/r.u. In this molecular area, the potential is low at least at the beginning, since the polymer is not sufficiently expanded to cover the area between electrodes. The abrupt increase of the surface potential in the region close to the expanded phase is attributed to
AT
5. Discussion 5.1. Behavior in Less Condensed Area. In the P A isotherm shown in Figure 4, there is no or little surface pressure in a large molecular area, i.e., gas phase. By this observation together with a consideration of polymer backbone, the polymer used is attributed to a condensed type monolayer; upon spreading, the molecules associate and form a domain. Since there exists little interaction between the domains, no surface pressure appears. In this region, the SH signal is unstable, because of inhomogeneity in the observed area due to the so-called "sea and island structure". In the ellipsometry measurement of a PMMA monolayer in this region by Sauer et al.? the signal was ais0 unstable. The domainsgather and surface pressure appears by compression. The sudden increase of the surface potential well below the onset in the surface pressure shown in Figure 11is due to this domain gathering and hence the electrode area covering. The SH signal also becomes stable in the region close to the expanded phase. The domain formation and the inhomogeneity of ita surfacecoveringhave also been reported in the monolayers of small molecules. In the spiropyran monolayer at the &/water interface,the surface potential tends to be stable, if we wait for about half an hour to allow the domains to be uniformly distributed.21 In the polymer monolayer, nonetheless, no surface potential appeared even several hours after dropping the solution because of little spreading ability of the polymer monolayer. In a monolayer of hemicyanine mixed with fatty acid at the aidwater interface, unstable SH light generates and little surface potential is observed in a large molecular
Dye-Labeled PMMA Monolayer at Interfaces by SHG
Langmuir, Vol. 8, No.11, 1992 2769
(a>
g 0.0 o.o+
600
z
f
20
8
10
a
6
0
10
20
30
$ 01 0° L $ 0 0 10
50
40
-200
x
100
20
30
40
0 50
8 3
5
Area (A2/r.u.)
Figure 11. Apparent dipole moment p, surface potential AV, and surface pressure ?r against the molecular area per repeating unit for (a) the PMMA-DR(13.9) monolayer and (b) the PMMA monolayer. Table 11. Susceptibilities and Orientation of the Monolayer on a Substrate by Various Deposition Processesa vertical deposition conventional moving wall ~~~
transfer
SHG tilt angle
horizontal deDosition
~
~~
no
Yes xzZr= 2.22 x 10-20 m2/V xzxz = 1.30 X m2/V x 47.3O
Yes
