Electro-Optic (EO) polymers can be catagorized into three families in terms of the polymer structure: guest-host, side-chain thermal plastic, and thermally crosslinkable. 

1. Guest-host systems 

            Guest-host systems are physical mixtures of chromophores and the polymer host. The guest-host EO polymers have low T_stab because the chromophores are not attached to the polymer matrix and are free to rotate. Guest-host EO polymers are also very useful in the evaluation of new chromophores because these polymers are easy to prepare and easy to pole.

             Fig. 1(a) and 1(b) show the chemical structures of two guest-host polymers. Both of them have a poly(methyl methacrylate) (PMMA) host doped with two high µb chromophores, APII and FTC, respectively. The weight percent value are experimentally optimized for maximum EO effect. The 45 wt% APII/PMMA polymer has a refractive index of 1.65, a r₃₃ of 30 pm/V, and an optical loss of 4 dB/cm. The 22 wt% FTC/PMMA has index of 1.62 and an optical loss of 2 dB/cm. The r₃₃ of the poled PMMA/FTC polymer is 55 pm/V and it has been found unchanged over two months at room temperature. This unusually stable EO effect for a guest-host polymer is believed to be largely due to the long shape of the chromophore that reduces its freedom of rotational relaxation.

2.6.2. Thermoplastic linear polymer with chromophores as side groups

            The chemical structure of a commercial EO polymer PMMA-DR1 (from IBM Almaden Research Center, San Jose, CA) is given in. Fig. 1(c). The r₃₃ of this polymer is 10-15 pm/V with T_stab of approximately 80 ˚C. The optical loss is lower than 1 dB/cm. The refractive index of this polymer is 1.55. This polymer has excellent and consistent device processing properties, with high film quality and good compatibility with photoresist.

Fig. 1 Chemical structures of (a) Guest host APII/PMMA, (b) Guest host FTC/PMMA, and (c) Thermoplastic side-chain PMMA-DR1.

 2.6.3. Thermal cross-linkable polyurethane systems

            In this type of EO polymers, the chromophores are aligned and are covalently attached to polymer matrix through chemical bonds during poling at elevated temperatures. This attachment increases the thermal stability of chromophore alignment.

            Thermosetting PU-DR19 is a typical example of this family of EO polymers. Its chemical structure is shown in Fig. 2. The disperse red 19 chromophores, which lead to the linear electro-optic effect, are covalently attached to the polyurethane prepolymer at one end. The prepolymer and the cross-linker were mixed and dissolved in 1,4-dioxane and 1 µm thick films were made on substrates by spin casting.

Fig. 2. Chemical structure of thermosetting PU-DR19. (a) Prepolymer. (b) Triethanolamine (TEA) as cross-linker. (c) Cross-linked polymer matrix.

            In this thermosetting system both cross-linking and electric field poling take place above 110 degree C. Chromophore mobility is enhanced at that temperature and the chromophores can be aligned under the influence of an external electric field. Moreover, at the elevated temperature during poling, the NCO groups in the prepolymer react with the OH groups in the cross-linker to form a tightly linked three dimensional network. After cross-linking, the polymer film becomes resistant to common solvents and can be patterned using commercial photoresist. After the polymer is slowly cooled to room temperature and the electric field removed, the chromophores are locked in the aligned orientation. The poled polymer film shows strong and stable electro-optic properties. The important properties of this EO polymer are: r₃₃=10-12 pm/V, T_stab =90-120 ˚C, and n=1.65.

            A drawback of this polymer is the NCO groups in the prepolymer tend to reactive with water molecules in the moisture and lose their cross-linking ability. When this happens, there will be a surplus of liquid state cross-linker in the solid state thin film. Film has a hazy appearance and SEM examination reveals the existence of great number of circular pits in the film. This problem makes the synthesis, the thin film processing, and the EO and thermal stabilities less reproducible. A modified Trilink DR 19 polymer is developed to overcome this disadvantage, as shown in Fig. 3. The EO polymer consist of three components: tri-functionalized DR 19 chromophore (DRTO), tolylene diisocyanate (TDI), and TEA. Chromophore and TDI are first mixed in dioxane and heated in a 80 ˚C oil bath for 30-45 minutes, followed by another 5-15 minutes of heating after adding of TEA. Thin film with a thickness of 1 to 2 µm can be spin cast at 300 rpm.

Fig. 3. Chemical structure of thermosetting Trilink DR19. Upper half: Tri-functionalized DR 19 chromophore, TDI and TEA before cross-linking reaction. Lower half: Cross-linked polymer matrix.

            The advantage of this polymer system is that the moisture sensitive NCO groups are now a part of a commercial chemical TDI instead of a part of the prepolymer that is difficult to synthesize. TDI is a commercial chemical and a fresh supply of it with maximum chemical activity is easily available. There are three cross-linking sites on each chromophore that can lock the aligned chromophores at both ends to achieve a better thermal stability. TEA is used to adjust the chromophore number density while maintaining an equal number of the cross-linking reaction groups of OH and NCO. The concentration of the solution is 100 mg to 200 mg of the three components per milliliter of dioxane. The heating makes the three components to have enough degree of cross-linking to produce optical quality thin films. The film quality is inspected in a lamp box with a dark background for increased contrast. A hazy surface usually means that at least one of the components suffers phase separation and further heating may be needed if the cross-linking groups are not damaged by, for example, moisture and are still chemically active. The major properties of this polymer are: T_stab =100-120 ˚C, r₃₃=10-14 pm/V, and n=1.65. The trilink scheme not only slightly improves the EO coefficients and the thermal stability, but more importantly, makes these properties and the thin film quality more reproducible than PU-DR19.