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Langmuir-Blodgett films of polymers containing nonlinear optical molecules in the side chains
202
Thin Solid Films, 210/211 (1992) 202 204
Langmuir-Blodgett films of polymers containing nonlinear optical molecules in the side chains T. Takahashi*, Y. M. Chen a, A. K. Rahaman a, J. K u m a r a a n d S. K. T r i p a t h y b Departments of ~Physics and bChemistry, University of Lowell, MA 01854 (U.S.A.)
Abstract Langmuir Blodgett (LB) film formation has been investigated for two different types of polymers in which nonlinear optical (NLO) moieties are attached in the side chain of polymethacrylate(PMA) and polyvinylalcohol(PVA) backbones. The LB films, where the polymer backbone is polymethacrylate were characterized by pressure area isotherms, polarized UV-visible spectra and second harmonic generation measurements. The NLO moieties in the side chain are inclined to the plane of the substrate at a shallow angle. The second order nonlinear coefficient and molecular hyperpolarizability were also calculated.
1. Introduction We have synthesized a number of polymers in which the N L O moieties separated by a flexible methylene spacer are covalently attached in the side chain of polymethacrylate (PMA) backbone [1]. We have also reported on the nonlinear optical properties of these polymers in spin coated thin film form that has been poled [2]. Recently, we have also synthesized polymers in which the N L O moieties are attached in the side chain of the polyvinylalcohol (PVA) backbone. These polymers possess an amphiphilic molecular structure and are expected to form suitable LB films. There have been only a few research reports on nonlinear optical polymer LB films [3-5]. In the present work the formation of LB films of candidate polymers and characterization of N L O moieties' orientation in the LB film are reported.
2. Experimental details The polymers used in this work are shown in Fig. 1. The synthesis procedure and polymer properties of the Type 1 polymers have been reported in an earlier communication [1]. Type 2 polymers were synthesized by ester formation between PVA (M w = 50 000, fully saponified) and carboxylic acid derivatives of the N L O dye. The detailed synthesis procedure will be published elsewhere. The measurement of surface pressure-area isotherms was carried out with the moving wall method LB film deposition apparatus (Nippon Laser & Electronics
*Visiting research scientist, Sekisui Chemical Co. Ltd, Tsukuba, Japan.
Type 1 Cll I ---(- C H 2- C - ' ~n
CH 3 PM5C8MA
( NO 2 )
(PM2C8MA)
Type 2 --(" CH2- ICH1 " ~ .x . . . . . . . . . --..(- C[I~- CH-)--~-x OH
O O - C ~ - C112-)~70
Az35 PVA ( x = 0.35)
N=N
NO 2
Az25 PVA ( × : 0.25)
Fig. 1. Polymer structure. (NL-LB-240-MWC)) equipped with a Whilhelmy balance for Type 1 polymers and a conventional Lauda M G W film balance for Type 2 polymers. The polarized UV-visible absorption spectra of LB films on the glass substrate were measured with a P e r k i n - E l m e r Lambda-9 UV-visible spectrophotometer with electric vectors oscillating parallel and perpendicular to the substrate. Second order nonlinear optical coefficient (d33), molecular hyperpolarizability and the orientation of N L O moieties (tilt angle) in the LB films were measured by S H G measurement from 1064 nm Q-switched N d - Y A G laser (Quantel 660A).
3. Results and discussion 3.1. Type 1 polymers It was extremely difficult to achieve complete vertical transfer of these fragile polymer films onto glass substrates
Elsevier Sequoia
T. Takahashi et al. / Polymer LB films with N L O molecules Pressure, mN/m 50.00 37.50 25.00
(b) 0.o0 -. 0.00
~"--'~~~.-T-"-~-----, 50.00 7~00
2~00
Surface Area, A2 / monomer unit Fig. 2. Pressure-area isotherm o f P M 5 C 8 M A on pure water (a); mixture o f P M 5 C 8 M A and stearic acid (4:1) on 2 x 10 -4 M C d C I 2 (b) at 20 °C.
0.060 0.050
~'f"-~,,
(b)
0.040 o E d:}
0.030
g <
\
k
0.020 \
".
0.010
0.000
I
340.0
i
i
=
i
I
380.0
420.0
460.0
500.0
wavelength (nm)
Fig. 3. Polarized UV-visible spectra of P M 5 C 8 M A 11 layers: (a) p-polarization, (b) s-polarization at 45 ° incidence.
using a conventional film balance. Therefore monolayer characterization and transfer were pursued on the moving wall trough which is preferable for deposition of rigid monolayers due to a minimization of shear flow. Figure 2 shows the pressure-area isotherm of PM5C8MA. The isotherm has a long plateau region at a relatively low surface pressure and then becomes steeper. Monolayer films could be easily transferred onto hydrophilic glass substrates using the vertical deposition method with a transfer ratio of 1.0. The resulting multilayers were Y type. Figure 3 shows the polarized UV-visible spectra at 45 ° incidence from a multilayer of PM5C8MA. The absorbance in the p-polarized spectrum is weak in comparison with the s-polarized spectrum. The ratio for the absorbance for s- and p-polarization (1) enables us to calculate the molecular tilt angle 0 from the normal to the substrate by techniques described elsewhere [6].
(As)/(Ap) = sin20/(cos2~b sin20 + 2 sin2~b cos20) where ~b is the incident angle.
(1)
203
For 11 and 5 layers of PM5C8MA and 5 layers o f PM2C8MA, the value of 0 was estimated to be 60 °. This indicates that the NLO moieties (polar groups) in the side chain of all the polymers are lying at an angle of about 30 ° to the substrate surface. This probably permits opposite charges on two polar side groups to find the optimal interaction. Mixed monolayers of stearic acid and PM5C8MA were prepared in an attempt to optimize the orientation of the NLO moieties in the polymer monolayer. Figure 2 shows the pressure-area isotherm OF PM5C8MA and stearic acid by using a mole ratio of 4:1. The isotherm is steeper when compared to that of PM5C8MA. However, the 0 value did not change significantly and was found to be 65 ° . Films with other component ratios yield approximately the same 0 value. This is possibly due to the difference in rigidity between the polymer and the stearic acid molecules. For the transmitted SH measurements, the samples consisted of one monolayer deposited on only one side of the glass substrate. Both p-polarized and s-polarized incident fundamental beams generated a p-polarized SH beam (referred to as p-p SH and s-p SH, respectively). Although both SH intensities were of the same order of magnitude, the p-p SH beam was stronger than the s-p SH beam. No s-polarized SH light was detected. The SH intensity did not change significantly when the sample was rotated around its normal. Since there is in-plane isotropy, the SH expression for uniaxial crystals [2] can be used. The SH intensities of the samples were compared to the SH intensities of Y cut quartz, (dl 1 ( q u a r t z ) = 0.364 pm/V) and yield a d33 value of 1.8 pm/V for PM5C8MA. The second harmonic intensities for the incident sand p-polarized fundamental beams can be expressed in terms of the molecular hyperpolarizability (/~), the molecular tilt angle (0) and the NLO moieties' surface density (Ns). These detailed relationships are given in reference [7]. The molecular tilt angle 0 is first calculated from the ratio between s-p SH and p-p SH intensities. The NLO moieties' tilt angle was found to be around 55 ° in the PM5C8MA monolayer. This value is in good agreement with the value obtained from polarized UV-visible spectral data. The molecular hyperpolarizability/~ was computed according to the procedure outlined in ref. 6. The molecular NLO unit surface density Ns is determined from the pressure-area isotherm ( N s = 3 x 10+~5cm-2 for PM5C8MA). For the PM5C8MA monolayer, the/~ value was found to be 1.6 x 10 -38 (SI units).
3.2. Type 2 polymers Figure 4 shows the pressure-area isotherm of Az35PVA. As the area was compressed, the pressure increases steeply and a condensed surface film was
T. Takahashi et al. / Polymer LB films with NLO molecules
204
Type 2 polymer LB films using polarized UV-visible spectroscopy, SHG measurements and F T - I R spectroscopy is in progress.
Pressure, m N / m 60.0
50.0
Acknowledgments
40.0
Partial support for this work from Sekisui Chemical Co. Ltd., is gratefully acknowledged. We acknowledge NLE and MTI Corporation for the use of their demonstration trough. Technical assistance from Dr. P. Miller, Dr. L. Samuelson, D. Galotti and Dr. B. K. Mandal is gratefully acknowledged.
30.0
20.0 •
10.0 •
0.0
i 0
10
20
30
i
i
i
40
50
60
Surface Area, A 2 /
i
70
80
AZ side group unit
Fig. 4. Pressure-area isotherm of Az35-PVA on pure water at 20 cC.
formed. The limiting area is about 21/~2 per NLO unit. Since the vinyl alcohol unit is strongly hydrophilic, surface film seems to have a structure in which polymer main chain slips into water and side chains protrude away from the surface. Type 2 polymer monolayers were easily transferred onto hydrophilic glass substrates using the vertical deposition technique (a conventional film balance) with a transfer ratio of 1.0. The characterization of the orientation of the NLO moieties in
References 1 B. K. Mandal, T. Takahashi, M. Maeda, S. Kumar, A. Blumstein and S. K. Tripathy. Makromol. Chem., 192 (1991) 1009. 2 Y. M. Chen, A. K. Rahaman, T. Takahashi, B. K. Mandal, J. Y. Lee, J. Kumar and S. K. Tripathy, Jpn. J. Appl., 30 (4) (1991). 3 M. M. Carpenter, P. N. Prasad and A. C. Griffin, Thin Solid Films, 161, (1988) 315. 4 N. Carr, M. J. Goodwin, A. M. Mcroberts, G. W. Gray, R. Marson and R. M. Scrowston, Makromol. Chem., Rapid Commun., 8 (1987) 47. 5 K. Oguchi, Y. Yokoh, K. Sanui and N. Ogata. MRS Syrup. Proc., 175 (1989) 277. 6 P. Miller, Optical characterization of polymer thin films, PhD Thesis, University of Lowell, Physics Department, 1991. 7 Y. R. Shen, The Principles of Nonlinear Optics, Wiley, New York 1984.
Thin Solid Films, 210/211 (1992) 202 204
Langmuir-Blodgett films of polymers containing nonlinear optical molecules in the side chains T. Takahashi*, Y. M. Chen a, A. K. Rahaman a, J. K u m a r a a n d S. K. T r i p a t h y b Departments of ~Physics and bChemistry, University of Lowell, MA 01854 (U.S.A.)
Abstract Langmuir Blodgett (LB) film formation has been investigated for two different types of polymers in which nonlinear optical (NLO) moieties are attached in the side chain of polymethacrylate(PMA) and polyvinylalcohol(PVA) backbones. The LB films, where the polymer backbone is polymethacrylate were characterized by pressure area isotherms, polarized UV-visible spectra and second harmonic generation measurements. The NLO moieties in the side chain are inclined to the plane of the substrate at a shallow angle. The second order nonlinear coefficient and molecular hyperpolarizability were also calculated.
1. Introduction We have synthesized a number of polymers in which the N L O moieties separated by a flexible methylene spacer are covalently attached in the side chain of polymethacrylate (PMA) backbone [1]. We have also reported on the nonlinear optical properties of these polymers in spin coated thin film form that has been poled [2]. Recently, we have also synthesized polymers in which the N L O moieties are attached in the side chain of the polyvinylalcohol (PVA) backbone. These polymers possess an amphiphilic molecular structure and are expected to form suitable LB films. There have been only a few research reports on nonlinear optical polymer LB films [3-5]. In the present work the formation of LB films of candidate polymers and characterization of N L O moieties' orientation in the LB film are reported.
2. Experimental details The polymers used in this work are shown in Fig. 1. The synthesis procedure and polymer properties of the Type 1 polymers have been reported in an earlier communication [1]. Type 2 polymers were synthesized by ester formation between PVA (M w = 50 000, fully saponified) and carboxylic acid derivatives of the N L O dye. The detailed synthesis procedure will be published elsewhere. The measurement of surface pressure-area isotherms was carried out with the moving wall method LB film deposition apparatus (Nippon Laser & Electronics
*Visiting research scientist, Sekisui Chemical Co. Ltd, Tsukuba, Japan.
Type 1 Cll I ---(- C H 2- C - ' ~n
CH 3 PM5C8MA
( NO 2 )
(PM2C8MA)
Type 2 --(" CH2- ICH1 " ~ .x . . . . . . . . . --..(- C[I~- CH-)--~-x OH
O O - C ~ - C112-)~70
Az35 PVA ( x = 0.35)
N=N
NO 2
Az25 PVA ( × : 0.25)
Fig. 1. Polymer structure. (NL-LB-240-MWC)) equipped with a Whilhelmy balance for Type 1 polymers and a conventional Lauda M G W film balance for Type 2 polymers. The polarized UV-visible absorption spectra of LB films on the glass substrate were measured with a P e r k i n - E l m e r Lambda-9 UV-visible spectrophotometer with electric vectors oscillating parallel and perpendicular to the substrate. Second order nonlinear optical coefficient (d33), molecular hyperpolarizability and the orientation of N L O moieties (tilt angle) in the LB films were measured by S H G measurement from 1064 nm Q-switched N d - Y A G laser (Quantel 660A).
3. Results and discussion 3.1. Type 1 polymers It was extremely difficult to achieve complete vertical transfer of these fragile polymer films onto glass substrates
Elsevier Sequoia
T. Takahashi et al. / Polymer LB films with N L O molecules Pressure, mN/m 50.00 37.50 25.00
(b) 0.o0 -. 0.00
~"--'~~~.-T-"-~-----, 50.00 7~00
2~00
Surface Area, A2 / monomer unit Fig. 2. Pressure-area isotherm o f P M 5 C 8 M A on pure water (a); mixture o f P M 5 C 8 M A and stearic acid (4:1) on 2 x 10 -4 M C d C I 2 (b) at 20 °C.
0.060 0.050
~'f"-~,,
(b)
0.040 o E d:}
0.030
g <
\
k
0.020 \
".
0.010
0.000
I
340.0
i
i
=
i
I
380.0
420.0
460.0
500.0
wavelength (nm)
Fig. 3. Polarized UV-visible spectra of P M 5 C 8 M A 11 layers: (a) p-polarization, (b) s-polarization at 45 ° incidence.
using a conventional film balance. Therefore monolayer characterization and transfer were pursued on the moving wall trough which is preferable for deposition of rigid monolayers due to a minimization of shear flow. Figure 2 shows the pressure-area isotherm of PM5C8MA. The isotherm has a long plateau region at a relatively low surface pressure and then becomes steeper. Monolayer films could be easily transferred onto hydrophilic glass substrates using the vertical deposition method with a transfer ratio of 1.0. The resulting multilayers were Y type. Figure 3 shows the polarized UV-visible spectra at 45 ° incidence from a multilayer of PM5C8MA. The absorbance in the p-polarized spectrum is weak in comparison with the s-polarized spectrum. The ratio for the absorbance for s- and p-polarization (1) enables us to calculate the molecular tilt angle 0 from the normal to the substrate by techniques described elsewhere [6].
(As)/(Ap) = sin20/(cos2~b sin20 + 2 sin2~b cos20) where ~b is the incident angle.
(1)
203
For 11 and 5 layers of PM5C8MA and 5 layers o f PM2C8MA, the value of 0 was estimated to be 60 °. This indicates that the NLO moieties (polar groups) in the side chain of all the polymers are lying at an angle of about 30 ° to the substrate surface. This probably permits opposite charges on two polar side groups to find the optimal interaction. Mixed monolayers of stearic acid and PM5C8MA were prepared in an attempt to optimize the orientation of the NLO moieties in the polymer monolayer. Figure 2 shows the pressure-area isotherm OF PM5C8MA and stearic acid by using a mole ratio of 4:1. The isotherm is steeper when compared to that of PM5C8MA. However, the 0 value did not change significantly and was found to be 65 ° . Films with other component ratios yield approximately the same 0 value. This is possibly due to the difference in rigidity between the polymer and the stearic acid molecules. For the transmitted SH measurements, the samples consisted of one monolayer deposited on only one side of the glass substrate. Both p-polarized and s-polarized incident fundamental beams generated a p-polarized SH beam (referred to as p-p SH and s-p SH, respectively). Although both SH intensities were of the same order of magnitude, the p-p SH beam was stronger than the s-p SH beam. No s-polarized SH light was detected. The SH intensity did not change significantly when the sample was rotated around its normal. Since there is in-plane isotropy, the SH expression for uniaxial crystals [2] can be used. The SH intensities of the samples were compared to the SH intensities of Y cut quartz, (dl 1 ( q u a r t z ) = 0.364 pm/V) and yield a d33 value of 1.8 pm/V for PM5C8MA. The second harmonic intensities for the incident sand p-polarized fundamental beams can be expressed in terms of the molecular hyperpolarizability (/~), the molecular tilt angle (0) and the NLO moieties' surface density (Ns). These detailed relationships are given in reference [7]. The molecular tilt angle 0 is first calculated from the ratio between s-p SH and p-p SH intensities. The NLO moieties' tilt angle was found to be around 55 ° in the PM5C8MA monolayer. This value is in good agreement with the value obtained from polarized UV-visible spectral data. The molecular hyperpolarizability/~ was computed according to the procedure outlined in ref. 6. The molecular NLO unit surface density Ns is determined from the pressure-area isotherm ( N s = 3 x 10+~5cm-2 for PM5C8MA). For the PM5C8MA monolayer, the/~ value was found to be 1.6 x 10 -38 (SI units).
3.2. Type 2 polymers Figure 4 shows the pressure-area isotherm of Az35PVA. As the area was compressed, the pressure increases steeply and a condensed surface film was
T. Takahashi et al. / Polymer LB films with NLO molecules
204
Type 2 polymer LB films using polarized UV-visible spectroscopy, SHG measurements and F T - I R spectroscopy is in progress.
Pressure, m N / m 60.0
50.0
Acknowledgments
40.0
Partial support for this work from Sekisui Chemical Co. Ltd., is gratefully acknowledged. We acknowledge NLE and MTI Corporation for the use of their demonstration trough. Technical assistance from Dr. P. Miller, Dr. L. Samuelson, D. Galotti and Dr. B. K. Mandal is gratefully acknowledged.
30.0
20.0 •
10.0 •
0.0
i 0
10
20
30
i
i
i
40
50
60
Surface Area, A 2 /
i
70
80
AZ side group unit
Fig. 4. Pressure-area isotherm of Az35-PVA on pure water at 20 cC.
formed. The limiting area is about 21/~2 per NLO unit. Since the vinyl alcohol unit is strongly hydrophilic, surface film seems to have a structure in which polymer main chain slips into water and side chains protrude away from the surface. Type 2 polymer monolayers were easily transferred onto hydrophilic glass substrates using the vertical deposition technique (a conventional film balance) with a transfer ratio of 1.0. The characterization of the orientation of the NLO moieties in
References 1 B. K. Mandal, T. Takahashi, M. Maeda, S. Kumar, A. Blumstein and S. K. Tripathy. Makromol. Chem., 192 (1991) 1009. 2 Y. M. Chen, A. K. Rahaman, T. Takahashi, B. K. Mandal, J. Y. Lee, J. Kumar and S. K. Tripathy, Jpn. J. Appl., 30 (4) (1991). 3 M. M. Carpenter, P. N. Prasad and A. C. Griffin, Thin Solid Films, 161, (1988) 315. 4 N. Carr, M. J. Goodwin, A. M. Mcroberts, G. W. Gray, R. Marson and R. M. Scrowston, Makromol. Chem., Rapid Commun., 8 (1987) 47. 5 K. Oguchi, Y. Yokoh, K. Sanui and N. Ogata. MRS Syrup. Proc., 175 (1989) 277. 6 P. Miller, Optical characterization of polymer thin films, PhD Thesis, University of Lowell, Physics Department, 1991. 7 Y. R. Shen, The Principles of Nonlinear Optics, Wiley, New York 1984.