Chapter 25
Polyimides for Electrooptic Applications 1,3
1,4
1
1
2
2
P. Kaatz , P. Prêtre , U. Meier , P. Günter , B. Zysset , M. Anlheim , M. Stahelin , and F. Lehr Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch025
2
2
1
Nonlinear Optics Laboratory, Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH Hönggerberg, CH-8093 Zürich, Switzerland SANDOZ Optoelectronics Research, SANDOZ Huningue SA, F-68330 Huningue, France
2
New modified polyimide polymers with pendant side group nonlinear optical (NLO) azo chromophores and moderate to high glass transition temperatures (140 °C < T < 190 °C) have been prepared. Corona poled films of these polymers possess large nonlinear optical susceptibilities of d = 23 pm / V and electro-optic (EO) coefficients of r = 6.5 pm / V at a wavelength of λ=1.3 μm. The structural pro perties (glass transition, molecular weight, chromophore density) and optical properties (refractive index, optical nonlinearity) of these poly imides can easily be varied to fulfill the requirements of potential electro-optic devices. Due to the relatively high glass transition temperatures of these polymers, long-term stability of the optical nonlinearity of typically one to hundreds of years at operating temperatures of 80-100 °C is predicted from accelerated time -temperature measurements. g
31
13
Novel amorphous nonlinear optical (NLO) polymers have been synthesized for applications as electro-optic materials (1). High glass transition temperatures, good temporal stability, good processability and the ease of modification of the NLO side chain chromophores are the most significant aspects of these new polymers. The dispersion of the linear and nonlinear optical properties of these polymers has been determined. Relaxation processes in nonlinear optical polymers are of considerable interest for obtaining a better understanding of the long-term stability of potential devices fabricated from these materials. Extensive relaxation measurements as a function of temperature have been performed above and below the glass transition, which plays the key role for the understanding of the orientational stability of poled amorphous polymers. It is found that the time-temperature relaxation behaviour of the NLO chromophores in a variety of polymer systems can be understood in terms of a phenomenological description of the glass transition with the aid of a scaling relation. 3
Current address: Department of Physics, University of Nevada, Las Vegas, NV 89154 Corresponding author
4
0097-6156/95/0601-0346$12.00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
25.
KAATZ ET AL.
Polyimides for Electrooptic Applications
347
Synthesis of NLO Polymers
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch025
The polymers described in this article were prepared by polymer analogous reaction of styrene-maleic-anhydride copolymers with aminoalkyl functionalized azo chromophores (2).
In such NLO polymers, a precursor polymer containing reactive functional groups for the reaction with the NLO active unit is first prepared. For the synthesis of the precursor polymers, maleic anhydride was chosen as comonomer because of its high reactivity towards various vinyl monomers leading to preferably alternating
In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
348
POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS
copolymers. This offers the possibility of the synthesis of tailored NLO polymers. These precursor polymers were treated with aminoalkyl functionalized azo chromophores leading to polyamic acids, which were cyclized in a one-pot procedure to the corresponding polyimides with N,N-dimethylformamide (DMF) or N-methyl2-pyrrolidone (NMP) as solvents . Three polymers denoted by A-095.11, A-097.07, and A-148.02 with the chromophores and glass transition temperatures Tg indicated in Table I were chosen for the detailed measurements described in this work.
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch025
Table I. Azo Dye Substitution Patterns and Glass Transition Temperatures Polymer
n
A-095.11 A-097.07 A-148.02
3 3 2
Ri CH CH H
R2 3
3
H CI H
T [°C] 137 149 172 g
Glass transition temperatures for the C2 and C3 spacer polymers as a function of dye concentration are drawn in Figure 1 together with the abbreviations for the three polymers used in this work. 220 200 r-.
U
180
L__l
h° 160 140
0
20 40 60 80 100 Chromophore concentration [ mole % ]
Figure 1. Glass transition temperatures as a function of chromophore content. Thin Film Preparation Thin films for optical and nonlinear optical measurements were spin cast from solutions of cyclopentanone/NMP (5:1) of varying polymer concentrations (typically 20 wt %). Films were backed on a hot plate at 200 °C for 1 h reducing the temperature to 170 °C for the rest of 1 day to prevent possible chromophore degra dation.
In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
25.
KAATZETAL.
Polyimides for Electrooptic Applications
349
Optical and Nonlinear Optical Properties
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch025
Refractive Indices. There exist several methods for the determination of refractive indices of spin cast polymer films. Quick, simple, and capable of reasonable accuracy is transmission spectroscopy (3). The difference between the refractive indices of the polymer and the substrate ( preferably fused silica ) produces a modulation of the transmission properties due to optical interference in the thin ( « 1 \im ) polymeric films. With the knowledge of the dispersion of the refractive indices of the substrate, the real part of the polymer's index can be fitted to a Sellmeier-type dispersion formula by the following equation 2
n AX)-\
=
f
^-2
+A
T
1/A^-l/A
(1)
2
where Ao, the absorption wavelength of the dominant oscillator, was obtained directly from measured absorption curves. The parameter A was kept constant as it describes the contribution from the polymer backbone to the refractive index, assuming that the dispersion from the chromophores is approximately described by a single oscillator term. Figure 2 is an example of a transmission spectrum showing a characteristic modulation of the transmissivity due to interferences in the thin film (~ 1 |jm). (Table II) 0.95
,p,0
g
i j k
=
-4
" (tfnff
f?fffk '
ft*
(3fi)g - (O ) • (mg -fi)' ) • (fflg - 4fi)' ) .-
