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

Abstract

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.