Effect of fluorinated nematic mesogens on phase behaviors and optical properties of chiral liquid crystalline polysiloxanes

A series of new chiral side-chain liquid crystalline polymers (P₁–P₇) have been synthesized with poly(methylhydrogeno)siloxane, two chiral liquid crystalline monomers, cholesteryl-4-allyloxybenzoate (M₁) and cholesteryl 4-(10-undecylen-1-yloxy) benzoate (M₂), and a nematic liquid crystalline monomer, 4-(trifluoromethyl)phenyl 4-(undec-10-enoyloxy)benzoate (M₃). The chemical structures and liquid crystalline properties of the synthesized polymers have been investigated by FTIR, ¹H-NMR, differential scanning calorimetry (DSC), polarizing optical microscopy (POM), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). All chiral polymers show wide mesophase temperature ranges and a high thermal stability with decomposition temperatures (T _d) at 5 % weight loss greater than 300 °C. P₁–P₄ display a single cholesteric phase, but P₅–P₇ containing more fluorinated units show a smectic A (SA) phase besides a cholesteric phase. The optical properties of the polymers have been characterized by circular polarization spectra and optical rotation analysis. The cholesteric polymers P₃ and P₄ exhibit different colors at room temperature, and the color can remain over 24 months. The maximum reflection bands of polymers P₁–P₄ shift to long wavelength with increasing the content of M₃ in the polymer systems. For P₅–P₇, the reflection wavelengths change sharply around the temperature of the SA–Ch phase transition. The specific rotation value of P₂ smoothly decreases from −8.2° to −0.29° when it is heated, but the specific rotation value of polymer P₇ changes from negative value to positive value on heating cycle. The optical properties of the polymers offer tremendous potential for various optical applications.