High Glass Transition Temperature Electro-Optic Polymers
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ABSTRACT Disperse Red-I type chromophores were attached to the backbones of PMMA, polyvinylphenol as well as the styrene-maleimide alternative copolymer and glass transition temperatures of these three polymer derivatives were in the range of 125, 165, and 245'C, respectively. We have also, for the first time, attached EO chromophores to some thermostable cardo-polymers. Fluorene derivatives in which the 2- and 7-positions were substituted with an electron accepting and an electron donating substitutent, respectively, were made polymerizable through their reactive functional groups at the 9,9'-positions. Various condensation polymers were prepared. These polymers are amorphous, film-forming, thermostable, and having Tg ranging from 150 to 300'C. These polymers exhibit strong electro-optic properties after electrode poling. Thermal stabilities of these polymers were compared by following the spectral decay of the chromophore and the decay of the electro-optic coefficient. The loss of EO activity at elevated temperature
can be realized as a combination of chromophore reorientation and chemical degradation. INTRODUCTION
The macroscopic electro-optic effect in polymers results from the collective response of the individual polar chromophores. From the theoretical expression for EO coefficient in Eq. 1 rijk
3 = -(8n/n,2nj2) (N/V) O zzz f,2f 2
(1)
three factors are important for maximizing the EO response, namely the number density (N/V) of the polar chromophores, the hyperpolarizability I3zzz of each chromophore, and the orientation factor which in turn is controlled by the poling conditions. Besides a large EO response, however, other properties are also critical for practical application to EO devices. One issue is insufficient poling stability which can occur if the Tg of the polymer is too close to the operating temperature of device. Other obstacles include thermal degradation of the electro-optic chromophores, and excess absorption loss at working wavelengths. In this paper, we will review some of our work on the EO polymers containing covalently-bound azo chromophores. We will then introduce a new class of EO cardo polymers which can have Tg in excess of 200 0C. AZO-BASED ELECTRO-OPTIC POLYACRYLATES
To illustrate the effect of chromophore hyperpolarizability, Figure 1 shows several methacrylate copolymers containing various covalently-bound azo chromophores. Relative to 477 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
Relative r133 of Different MA Copolymers 0.2 XOc_
MAI
_AO
1
MA9
1.5
MAll
2.25
(Tg 120)
(Tg 135)
(Tg 122)
X
Poled 0.5MV/cm at Tg For MA1, r33 = 4pm/V at 810nm
0 2N
MA8 (Tg 135)
1.45
MA15
0.38
(Tg 114)
MA16
0.63
(Tg I 15)
Figure 1. Several Azo-dye Polymers and their Relative Electro-Optic Coefficients. MAI, the polymer bound Disperse Red-i, addition of a double bond (MA8) as well as restriction of the rotation of one single bond by either ring fusion (MA9) or by hydrogen bonding (MAI 1) roughly increases the electro-optic coefficient by 1.5 times,
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