Water Interface. A

Sep 15, 1997 - This method provides direct information on the molecular dynamics based on the rotational micro-Brownian motion of a polymer main chain...
0 downloads 0 Views 171KB Size
Download PDF
Langmuir 1997, 13, 5685-5690

5685

Molecular Motion in Polymer Monolayers at the Air/Water Interface. A Time-Resolved Study of Fluorescence Depolarization Nobuhiro Sato, Kazutoshi Sugiura, Shinzaburo Ito, and Masahide Yamamoto* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received April 28, 1997. In Final Form: July 29, 1997X The microscopic motion of anthracene-labeled poly(ethyl acrylate), poly(ethyl methacrylate), poly(octadecyl acrylate), and poly(octadecyl methacrylate) in the air/water interfacial monolayers was examined by the time-resolved fluorescence depolarization method. This method provides direct information on the molecular dynamics based on the rotational micro-Brownian motion of a polymer main chain in a quasi two-dimensional plane. The degree of fluorescence polarization for the anthracene probe was analyzed using the sum of two exponential functions with time constants in a nanosecond range. These polymers formed stable monolayers, while the relaxation time of rotation was dependent on both the main-chain structure and the side-chain length as well as on the applied surface pressure. The steric hindrance of the R-methyl group and the close packing of the long alkyl side chains reduced the mobility of polymer segments in the monolayer and lengthened the rotational relaxation time from a few nanoseconds to a 100 ns. The rotational diffusion coefficients were also evaluated to be on the order of 106-107 rad2/s.

Introduction The dynamics, orientation, and higher order structure of molecules at a quasi two-dimensional plane are an attractive issue of research in materials science and interfacial science. The effect of space dimensionality upon the various properties of materials is an interesting subject. The monolayer formed at the air/water interface is a feasible system providing a quasi two-dimensional field.1-3 Furthermore, the Langmuir-Blodgett (LB) film, an organized molecular assembly extensively studied, is prepared by the successive deposition of the monolayer at the air/water interface onto a solid substrate. Properties of the monolayer play a crucial role in the quality of the fabricated LB films. Polymer monolayers, in particular, have many advantages4 in the quality of built-up films such as the thermal and mechanical stabilities,5 thinness of each layer, and the uniform dispersion of functional groups.6 Besides their practical importance, they are valuable in the field of research on “polymer science in the two-dimensional space”.7,8 The dynamic behavior of the polymer monolayer, which reflects the properties of the polymers on the water surface, has not been fully clarified. The dynamics of the monolayer at the air/water interface is expected to be more * To whom correspondence should be addressed. Tel: 81-75753-5622. Fax: 81-75-753-5632. E-mail: [email protected]. kyoto-u.ac.jp. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Urquhart, R. S.; Hall, R. A.; Thistlethwaite, P. J.; Griesen, F. J. Phys. Chem. 1990, 94, 4173. (2) Lucassen, J.; Akamatsu, S.; Rondelez, F. J. Colloid Interface Sci. 1991, 144, 434. (3) Barcegol, H.; Meunier, J. Nature 1992, 356, 256. (4) Tredgold, R. H. Thin Solid Films 1987, 152, 223. (5) (a) Schneider, J.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989, 22, 205. (b) Schneider, J.; Erdelen, C.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989, 22, 3475. (6) Ohmori, S.; Ito, S.; Yamamoto, M.; Yonezawa, Y.; Hada, H. J. Chem. Soc., Chem. Commun. 1989, 1293. (7) (a) Vilanove, R.; Rondelez, F. Phys. Rev. Lett. 1980, 45, 1502. (b) Vilanove, R.; Poupinet, D.; Rondelez, F. Macromolecules 1988, 21, 2880. (c) Poupinet, D.; Vilanove, R.; Rondelez, F. Macromolecules 1989, 22, 2491. (8) (a) Takahashi, A.; Yoshida, A.; Kawaguchi, M. Macromolecules 1982, 15, 1196. (b) Takahashi, A.; Yoshida, A.; Kawaguchi, M. Macromolecules 1983, 16, 956. (c) Kawaguchi, M.; Komatsu, S.; Matsuzaki, M.; Takahashi, A. J. Colloid Interface Sci. 1984, 102, 356.

S0743-7463(97)00432-0 CCC: $14.00

sensitive to the surroundings than that of the LB film because the subjacent water acts like a solvent, giving rise to additional attractive and/or repulsive interactions between the molecules. The viscoelastic properties of the polymer monolayer at the air/water interface have been well studied9-13 because they offer fundamental information on the macroscopic characters of the monolayers. The fluorescence method has been widely employed to observe the microscopic dynamics. For example, Kim and Yu14 obtained a lateral diffusion coefficient of the poly(tert-butyl methacrylate) monolayer at the air/water interface using the fluorescence recovery after photobleaching (FRAP) method. Ito et al.15 also reported the intralayer diffusion coefficient of the segments of acetalized poly(vinyl alcohol) by means of the electronic excitation energy transfer between fluorescent probes. The time-resolved fluorescence depolarization method gives more direct and detailed information about the microscopic dynamics.16 This method has been utilized to investigate the local motion of the polymer chain in a solution,17,18 the reorientation of the molecules adsorbed on a surface-modified quartz substrate,19 and the mobility of the molecules in biological membranes20 and LB films.21 Here, we report the application of the time-resolved fluorescent depolarization method to polymer monolayers on the water surface. The surface pressure of the monolayer, the side-chain length, and the main-chain (9) Inokuchi, K. Bull. Chem. Soc. Jpn. 1955, 28, 453. (10) Jarvis, N. L. J. Phys. Chem. 1966, 70, 3027. (11) Sacchetti, M.; Yu, H.; Zografi, G. Langmuir 1993, 9, 2168. (12) Peng, J. B.; Barnes, G. T.; Abraham, B. M. Langmuir 1993, 9, 3574. (13) Sato, N.; Ito, S.; Yamamoto, M. Polym. J. 1996, 28, 784. (14) Kim, S.; Yu, H. J. Phys. Chem. 1992, 96, 4034. (15) Ito, S.; Oki, S.; Sato. N.; Yamamoto, M. Macromolecules 1996, 29, 724. (16) Sasaki, T.; Yamamoto, M.; Nishijima, Y. Macromolecules 1989, 22, 4009. (17) Ono, K.; Okada, Y.; Yokotsuka, S.; Sasaki, T.; Yamamoto, M. Macromolecules 1994, 27, 6482. (18) Ono, K.; Okada, Y.; Yokotsuka, S.; Sasaki, T.; Yamamoto, M. Polym. J. 1994, 26, 1345. (19) Wirth, M. J.; Burbage, J. D. Anal. Chem. 1991, 63, 1311. (20) Kinosita, K., Jr.; Kataoka, R.; Kimura, Y.; Gotoh, O.; Ikegami, A. Biochemistry 1981, 20, 4270. (21) Kimura, N.; Tsuneta, R.; Araiso, T.; Mukasa, K. Chem. Phys. Lipids 1991, 57, 39.

© 1997 American Chemical Society

5686 Langmuir, Vol. 13, No. 21, 1997

Sato et al.

Figure 1. Structure of the sample polymers. Subscript f means the fraction of the anthracene unit. Table 1. Molecular Weight and Anthracene Fraction of Sample Polymers sample

10-4Mw

Mw/Mn

f/%

PEA PEMA PODA PODMA

4.94 9.57 1.72 11.2

2.08 1.34 1.34 3.15

0.61 0.93 0.32 0.98

structure of the polymers markedly affected on the rotational diffusion of the fluorescent probes attached to the polymer chain. Experimental Section Materials. The samples used were poly(ethyl acrylate) (PEA), poly(ethyl methacrylate) (PEMA), poly(octadecyl acrylate) (PODA), and poly(octadecyl methacrylate) (PODMA) labeled with an anthracene moiety in their side chains as a fluorescent probe (Figure 1). Anthracene, which has a sufficient molar extinction coefficient at the excitation wavelength 398 nm, is a probe suitable for the fluorescence depolarization study because of the large anisotropy of the transition moment and the relatively small molecular size. All samples were synthesized in the same way as the following procedure for PEA. An anthracene-containing monomer, 9-(10-methylanthryl)methyl acrylate was synthesized by the reaction of acryloyl chloride with 9-(10-methylanthracene)methanol, which was obtained through the reduction of 10methylanthracene-9-carboxaldehyde (Aldrich). This anthracenecontaining monomer and ethyl acrylate were copolymerized by the radical polymerization in benzene with azobis(isobutyronitrile) as an initiator, yielding anthracene-labeled PEA. The obtained polymer was purified by reprecipitation from benzene to methanol three times. Table 1 shows the molecular weights of the polymers and the fractions of the probe unit. A large fraction of the probe may give rise to the highly efficient excitation energy migration among the anthracene probes, and consequently interpretation of depolarization data becomes complicated. To suppress the energy migration as much as possible, the fractions were regulated to less than 1%, according to the result of the preliminary experiment described in the next section. Monolayer Preparation. Monolayers were prepared on a Teflon-coated aluminum trough by dropping the benzene solution (ca. 0.1 g/L, Dojin Spectrograde) of the polymer onto a clean surface of pure water that was ion-exchanged, distilled, and then treated with a water purification system (Barnstead NANO Pure II). The surface pressure change with the compression of monolayers was measured with a Wilhelmy plate of sand-blasted glass equipped with an electronic balance (A&D HR-60). The temperature was kept constant at 20.0 ( 0.1 °C by circulating water under the trough, whereas ambient temperature was under rough control. To avoid oxygen quenching, nitrogen gas was gently flowed over the water surface. The equipment was enclosed in a chamber. The macroscopic morphology of the monolayers was observed by Brewster angle microscopy (BAM). The experimental setup was as reported elsewhere.13

Figure 2. Schematic illustration of the measurement setup. In addition to the above illustration, nitrogen gas was flowed over the water surface and the equipment was enclosed in a chamber. Incident light was horizontally polarized (s-polarized). Fluorescence Measurement. Figure 2 is a schematic illustration of the measurement setup. An s-polarized pulsed beam (FWHM 20 ps, wavelength 398 nm) from a Ti:sapphire laser system (Spectra Physics model 3950) impinged upon the water surface at an angle of 4° by the surface plane. Fluorescence emission was collected along the surface normal. Each of the parallel and perpendicular components with respect to the polarization of the excitation beam was alternately extracted by rotating an analyzer 90° every 40 s, and then guided to a photomultiplier tube (Hamamatsu R3234) through a quartz optical fiber (3 mm diameter). To reduce the contamination by scattering from the surface, a short-cut filter (SC-42) and a monochromator (425 nm, slit width 3 mm) were placed before the detector; however, a slight influence of the Raman scattering of water remained. The time course of the fluorescence intensity was finally measured with a time-correlated single photon counting system. The FWHM of the response function for the total system including the detector was found to be about 600 ps.

Test for Depolarization by Energy Migration Depolarization of fluorescence occurs not only by the rotational diffusion of the transition moment but also by the electronic energy migration among the fluorescent probes. Interpretation of experimental results is complicated by the occurrence of energy migration. It is desirable to introduce as many probes as possible because the higher fluorescence intensity yields a better signal to noise ratio and reduces experimental error, while the increase in the plane density of the probes accelerates the rate of the migration. To determine the adequate probe fraction at which the contribution of the energy migration is negligible, a preliminary experiment on the fluorescence depolarization was carried out by using the monolayers deposited on a solid substrate. Since the molecular motion in the deposited monolayers is much slower than the fluorescence lifetime of anthracene, the depolarization is caused only by the energy migration. For this experiment, we employed a one-layer film of poly(isobutyl methacrylate) (PiBMA) labeled with various fractions of anthracene, which was deposited onto a hydrophilic quartz plate. The experimental method was similar to that described in the previous section. Figure 3 shows the mean relaxation time of the polarization, 〈T〉, as a function of the anthracene content. In the region of more than 2%, the value of 〈T〉 markedly decreased, indicating that the energy migration effectively occurs from this content and that the permissible fraction is up to 1%. The precise value has to be determined from the plane density of the probe; nevertheless the fraction obtained here is a fairly good ap-

Molecular Motion in Polymer Monolayers

Langmuir, Vol. 13, No. 21, 1997 5687

cone axis coincides with the z-axis normal to the water surface (the xy-plane) and its half-angle, β, is fixed. The azimuth, φ, is assumed to have no preferential direction. The S0-S1 transition moment of the anthracene is directed along its shorter molecular axis. The transition moments of absorption and emission coincide with each other. With the excitation light incident along the y-axis and polarized in the direction of the x-axis, the excitation probability of M is proportional to cos2 φ. When the fluorescence from the moment with an azimuth φ is observed from the z-axis, each component of the intensities, ix and iy, parallel to the x-axis and the y-axis, respectively, is given as a function of φ. Figure 3. Fluorescence depolarization by the excitation energy migration between anthracene moieties measured with onelayer films of PiBMA deposited on a quartz substrate. When the fraction of anthracene unit is larger than 1%, marked energy migration occurs.

ix(φ) ) k sin2 β cos2 φ iy(φ) ) k sin2 β sin2 φ

(3)

where k is a constant. Then the fluorescence intensity, s(φ), observed from the z-axis as a function of φ is given as

s(φ) ) ix(φ) + iy(φ) ) k sin2 β (constant)

(4)

Thus S(t), the ensemble average of s(φ), is independent of the azimuth and proportional to the total emission intensity. On the other hand, the degree of polarization, p(φ), as a function of φ is given as

p(φ) )

ix(φ) - iy(φ) ix(φ) + iy(φ)

) 2 cos2 φ - 1

(5)

The observed degree of polarization from the whole system, Figure 4. Coordinate system of the measurement and analysis. The transition moment vector, M, rotates along the cone surface. β is a tilt angle and φ is an azimuthal angle. The xy-plane corresponds to the water surface in the actual measurement. Excitation light propagates along the y-axis and is polarized in the direction of the x-axis. Fluorescence emission is observed along the z-axis.

proximation because the area of the PiBMA unit at the surface was similar to that of the other polymer monolayers studied.

Fluorescence intensity observed from the surface normal, S(t), is given as

(1)

where I|(t) and I⊥(t) are the parallel and perpendicular components with respect to the polarization of excitation light, respectively. The degree of polarization as a function of time, P(t), is defined as

P(t) )

I|(t) - I⊥(t) S(t)

P ) 2 cos2 φ - 1 cos2 φ )

(2)

The fluorescent probe in the monolayer at the air/water interface is considered to keep a particular orientation due to some interaction between the hydrophilic units and water. In the current analysis, we adopt the model, as depicted in Figure 4, that the transition moment of the fluorescent probe, M, moves on the surface of a cone. The

(6)

∫cos2 φ J(φ) dφ

In the present model, J(φ) right after the excitation is given as

J(φ) )

Data Analysis

S(t) ) I|(t) + I⊥(t)

P, is given by cos2φ, the average of cos2 φ involving the probability density function of the excited moment J(φ).

1 cos2 φ π

(7)

Thus cos2φ ) 0.75 and P ) 0.5 are obtained as the initial degrees of polarization at time 0. According to the theory of rotational Brownian motion,22 the following relation holds as a result of the rotation around one axis

P ) P0(2 cos2 ω) - 1

(8)

where P and P0 are the degrees of polarization at time t and 0, respectively, and ω is the angle of rotation from time 0 to t. If P0 ) 0.5 and ω follows a one-dimensional diffusion equation with a rotational diffusion coefficient Dr,

P(t) ) 0.5 exp(-4Drt)

(9)

Thus the change of the degree of polarization with time (22) Weber, G. Adv. Protein. Chem. 1953, 8, 415.

5688 Langmuir, Vol. 13, No. 21, 1997

Sato et al.

Figure 5. Surface pressure-area isotherms. The octadecyl side-chain monolayers (bold lines) show larger areas and steeper slopes than the ethyl side-chain monolayers (thin lines).

exhibits a single-exponential decay. However, when there are several relaxation processes, the decay of P(t) is given as a sum of exponential functions with various relaxation times, τi.

P(t) ) P0

( ) t

∑i fi exp - τ

i

∑i fi ) 1

(10)

Although P0 should ideally be 0.5, this is not the case on account of the imperfect anisotropy of the probe molecule and of some fast relaxation processes with time constants shorter than the time resolution of the measurement. In the present analysis P0 was fixed to 0.39 so that the best fitting results could be obtained. The calculated decay curves of S(t) and P(t) were convoluted with the response function of our system and then fitted to experimental data by a nonlinear least squares method; thereby, we obtained the fluorescence lifetimes and the relaxation times of the degree of polarization. When S(t) was fitted to three-component exponential functions, one of them had the shortest lifetime of less than 0.1 ns. Fluorescence spectra of the monolayers at the air/water interface showed a large peak around 460 nm due to the Raman scattering of water when excited at 398 nm. Even by using a filter and a monochromator, we could not eliminate completely the scattered light. Therefore, ascribing the shortest component to the Raman scattering, we took the longer two components into account. For the P(t) decay, the data points within the time range of 500-600 ps after the maximum intensity of the excitation pulse were omitted in the fitting procedure to eliminate the contamination by the Raman scattering of water. Results and Discussion Macroscopic Properties. Figure 5 shows the pressure-area isotherms of the monolayers studied. The profile of each isotherm is in agreement with that reported.23,24 The monolayers having octadecyl side chains showed curves with steeper slopes on compression, while those with ethyl side chains had gentler slopes. This indicates that the monolayers of PODA and PODMA are less compressible, namely, more rigid. Direct visualization of the monolayer morphology supports the above results. Figure 6 is a BAM micrograph of PODA observed at the (23) (a) Crisp, D. J. J. Colloid Sci. 1945, 1, 49. (b) Crisp, D. J. J. Colloid Sci. 1945, 1, 161. (24) Mumby, S. J.; Swalen, J. D.; Rabolt, J. F. Macromolecules 1986, 19, 1054.

Figure 6. BAM image of the PODA monolayer. The compression direction is horizontal. The bright region corresponds to the monolayer; the dark region to the bare water surface. Angular domains flowing on the surface showed no deformation during the compression.

lift-off point of the isotherm. It obviously shows the formation of condensed and angular domains free from deformation. The domain of PODMA had a feature similar to PODA, as we reported previously.13 PEA and PEMA, in contrast, exhibited no distinct structure because they expanded homogeneously and sparsely. Thus the long alkyl side chains make the monolayers rigid by the large cohesive force among them. The difference in the main-chain structure also altered the isotherm. Under the same surface pressure, PODA occupied a smaller area than PODMA, indicating the closer packing of the alkyl side chains of PODA because of the small steric hindrance in the absence of the R-methyl group. The difference was more prominent between PEA and PEMA on account of the reduction of the cohesive force among side chains. However, the methacrylate polymer occupied a smaller area. Since the absence of the R-methyl group in PEA made the main chain flexible and hydrophilic compared with PEMA, the PEA chain readily expanded on the water surface, resulting in appreciable surface pressure even in a large area. It is also likely that the larger interaction between the main chains of PEMA caused by the R-methyl group have reduced area. Fluorescence Decay. As mentioned in the previous section, the measured fluorescence decays were fitted by three-component exponential functions. Omitting the shortest component arising from the Raman scattering of water, we applied the following expression to the actual fluorescence decay.

( )

S(t) ) x exp -

( )

t t + (1 - x) exp τ1 τ2

(11)

where x is the fraction of the τ1 component. Figure 7 shows an example of a fluorescence decay curve for PEMA at 1 mN/m. All the decay curves consisted of a longer lifetime component beyond 10 ns and a shorter one below 6 ns; the former fraction was around 60-90%. The oxygen quenching occurred to some extent in our surface monolayer system in spite of the nitrogen gas flow since the lifetime of the anthracene moiety in a degassed solution is about 13 ns. This was ascertained by the marked reduction of the fluorescence intensity in the absence of the nitrogen flow.

Molecular Motion in Polymer Monolayers

Langmuir, Vol. 13, No. 21, 1997 5689

because subtraction was necessary to calculate the degree of polarization. However, the observed P(t) could be successfully fitted with the sum of two exponential functions as follows:

[ ( )

P(t) ) P0 x exp -

Figure 7. Fluorescence decay of PEMA under the surface pressure of 1 mN/m. The broken line represents the system’s response function.

Figure 8 shows the plots of mean fluorescence lifetimes derived from the equation below, as a function of the surface pressure.

〈τ〉 ) xτ1 + (1 - x)τ2

(12)

For the samples having ethyl side chains, PEMA had a longer 〈τ〉 than PEA, and both samples had longer lifetimes with the increase of surface pressure. By contrast, octadecyl samples had a 〈τ〉 value around 13 ns, which hardly changed with the surface pressure regardless of the presence of the R-methyl group. These results are accountable in terms of the shielding effect from the oxygen attack. Compared with PEA, PEMA coheres tightly and then prevents oxygen molecules from encountering the anthracene probe. Both polymers are so compressible that the tightness of monolayer increases on compression. The higher the surface pressure, the harder the oxygen molecules penetrate into the monolayer. The octadecyl samples clearly show the shielding effect. The long alkyl side chains are packed together so closely even under a low surface pressure that they cover up the probes and keep out oxygen. Since the effect of the side chain is predominant, the main chain and the surface pressure have no appreciable effects on the lifetime 〈τ〉. Fluorescence Depolarization. In this experiment, data points of P(t) considerably scattered due to low emission intensity particularly in the long time range,

(13)

where P0 is the initial degree of polarization, which was set to 0.39, as mentioned earlier. Figure 9 shows an example of P(t) decays observed under the surface pressure of 1 mN/m for all the samples studied, where the solid lines are the ones calculated by eq 13 with the best fit parameters listed in Table 2. The time constants greatly varied from 7.8 to over 200 ns for T1 and 0.34-4.8 ns for T2. The value of x was nearly constant for a given polymer even when the surface pressure changed but increased in the order PEA, PEMA, PODA, and PODMA. Practically, it is difficult to determine how T1 and T2 correspond to the real motion of the polymer chain. These two time constants may be assigned to the rotational relaxation times of two different modes, or they may be just two representatives for the multicomponent decay functions. In the following discussion, accordingly, we use the mean relaxation time 〈T〉 given below, instead of arguing each parameter.

〈T〉 ) xT1 + (1 - x)T2

Figure 8. Mean fluorescence lifetime as a function of surface pressure: (b) PEA; ([) PEMA; (O) PODA; (]) PODMA. For the ethyl side-chain monolayers (filled symbols), the lifetime became longer as the surface pressure increased. For the octadecyl sidechain monolayers (open symbols), the lifetime was almost constant irrespective of the surface pressure.

( )]

t t + (1 - x) exp T1 T2

(14)

The values of 〈T〉 are also listed in Table 2. 〈T〉 is the smallest for PEA, but increases markedly with surface pressure. This indicates that the PEA chain in the monolayer is microscopically very mobile on the water surface although the mobility is reduced under high surface pressure. The mobility for PEMA is smaller than that for PEA and not so sensitive to the surface pressure. The data obtained for PODA and PODMA showed little decay of the polarization in the time range of our measurement, and then 〈T〉 was much larger than the fluorescence lifetime of anthracene. Since the decay was too slow, the fitting parameters were somewhat arbitrary. Consequently, we concluded that all 〈T〉 values for PODA and PODMA were in a similar order under any surface pressure. The findings above are in good agreement with the macroscopic features and the fluorescence lifetimes previously mentioned and provide the following insight into the polymer dynamics in the monolayers at the air/water interface. The presence of an R-methyl group in the main chain sterically hinders the rotation around bonds and increases the cohesive force among the polymer chains, thus markedly restricting the main chain motion. On the other hand, the long alkyl side chains pack together so closely that the whole monolayer becomes vary hard; consequently, molecular motion is hardly allowed in the time range considered here. Evaluation of Rotational Diffusion Coefficients. Equation 9 shows that the relaxation time is directly related to a rotational diffusion coefficient Dr when a single exponential function represents the decay of the degree of polarization. In a multiexponential case, more than one diffusion coefficient corresponding to several relaxation processes could be obtained, but in fact, it is not easy to determine them distinctly because an assumption of the appropriate model is necessary. Here, we approximately evaluated Dr from the mean relaxation time 〈T〉 with eq 9, in spite of the double exponential decay. This offers rough but convenient estimation of the mobility of the probe molecule in the monolayer. Since the large values of 〈T〉 for PODA and PODMA involve some

5690 Langmuir, Vol. 13, No. 21, 1997

Sato et al.

Figure 9. Decay of the fluorescence polarization under the surface pressure of 1 mN/m: (a) PEA; (b) PEMA; (c) PODA; (d) PODMA. The solid lines are the calculated curves by using eq 13 and the parameters listed in Table 2; the broken lines represent the system’s response function. To remove the effect of light scattering, several points just after the maximum of the pump pulse were omitted in the fitting calculation. For PEA, the fitting calculation was not applied to the data points after 18 ns because they contain many errors owing to the low P(t) values. Table 2. Fitting Parameters for Decay of Fluorescence Polarization sample

surface pressure (mN/m)

x

T1 (ns)

T2 (ns)

〈T〉 (ns)

1 5 10 1 5 10 1 5 10 1 5 10

0.43 0.41 0.47 0.63 0.64 0.66 0.85 0.83 0.87 0.83 0.97 0.86

7.8 14 17 54 45 48 170 180 220 200 250 210

0.34 0.51 1.3 1.0 1.1 1.3 1.7 1.2 1.6 4.1 1.5 4.8

3.6 6.2 8.8 34 29 32 140 150 190 160 240 190

PEA PEMA PODA PODMA

Table 3. Rotational Diffusion Coefficients Calculated from the Mean Relaxation Time of Fluorescence Polarization 10-7Dr (rad2/s) PEA PEMA

surface pressure (mN/m) 1 5 10

6.9 4.0 2.8

0.73 0.86 0.78

uncertainty, Dr is calculated only for PEA and PEMA by the following relation.

Dr )

1 4〈T〉

(15)

Table 3 shows Dr under the various surface pressures studied. Dr for PEA is on the order of 107 rad2/s, while that for PEMA is on the order of 106 rad2/s. These values are much smaller than those of many organic molecules in solution. Finally, it is worth considering what actual molecular motions cause the rotational relaxation of the probe

molecule. In our samples, a methylene group and an ester group come between the polymer main chain and the anthracene moiety with the covalent bonds. The most probable process that changes the direction of the transition moment is the rotation around the C-O bond between the methylene and the ester carbonyl groups in the side chain. Consequently, a relatively rapid rotational motion of the side chain is expected, provided there is enough free space for the rotation. However, the relaxation is also affected by the micro-Brownian motion of the polymer main chain because Dr is small even in a large area. The two components of the decay function may be related to the different relaxation times between the side chains and the main chains of the polymers. Conclusion Through the time-resolved analysis of fluorescence depolarization, we found that both the main-chain structure and the side-chain length of the polymers have a great influence on their microscopic motion in the monolayer at the air/water interface. The steric hindrance of the R-methyl group in the main chain impedes the facile rotation around bonds so that the segmental motions of polymers are restrained. Close packing of long alkyl side chains caused by the strong cohesive force among them makes monolayers entirely rigid and markedly reduces both the mobility of polymer chains and the free space for rotation. Finally, it is worth noting that the fluorescence method has a sufficiently high sensitivity even for the investigation of the microscopic mobility of the very thin monolayer at the air/water interface. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas, Photochemical Reactions (No. 06239107) from the Ministry of Education, Science, Sports and Culture of Japan. S.I. thanks the Sumitomo Foundation for financial support. LA970432W