Synthesis and properties of new chiral mesogenic monomers and side-chain smectic homopolymers containing (−)-menthyl groups

The synthesis of new chiral monomers (M1−M5) and the corresponding smectic homopolymers (P1−P5) containing menthyl groups is described. The chemical structures and purity were characterized by FT-IR, 1H NMR and elemental analyses. The specific optical rotations were evaluated with a polarimeter. The phase behavior and mesomorphism were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The selective reflection property of light was studied with UV/visible/NIR. The monomers M2−M5 formed a chiral smectic C (SC∗), and cholesteric or blue phase when a flexible linkage chain was inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect. M1 showed no mesomorphism, while M2−M5 showed enantiotropic SC∗ and cholesteric phases. Moreover, M5 also exhibited a cubic blue phase on cooling. With increasing temperature, the selective reflection of light shifted to the long wavelength region at the SC∗ phase range, and to the short wavelength region at the cholesteric phase range, respectively. The homopolymers P1−P5 all exhibited the batonnet textures of a smectic A phase. The melting, clearing, and glass transition temperatures increased, and the mesophase temperature ranges widened with increasing the aryl number in the mesogenic core.

Graphical abstractThe new chiral monomers M2−M5 can form SC∗, and cholesteric or blue phases when a flexible spacer is inserted between the mesogenic core and the terminal menthyl groups by reducing the steric effect, while their corresponding homopolymers P1−P5 only exhibit SA phases. Tm, Ti and Tg increased with increasing the rigidity of mesogenic units or decreasing the spacer length. With increasing temperature, the selective reflection of light shifted to the long wavelength region at the SC∗ phase range, and to the short wavelength region at the cholesteric phase range, respectively.Figure optionsDownload full-size imageDownload as PowerPoint slide