Synthesis and phase behaviour of chiral liquid crystalline monomers based on menthyl groups, smectic polymers and cholesteric elastomers

The synthesis of new chiral liquid crystal monomers (M1 −M4 ) derived from menthyl groups, the corresponding smectic polymers (P1 −P4 ), and cholesteric elastomers (E1 −E4 ) based on M2 and crosslinking agent (CA ) is presented. The chemical structures and purity were characterised by Fourier transform infrared, 1H nuclear magnetic and elemental analyses. The phase behaviour and mesomorphism were investigated by differential scanning calorimetry, polarising optical microscopy, thermal gravimetric analysis and X-ray diffraction. The selective reflection of light for M1 −M4 was characterised with ultraviolet/visible/near infrared. The structure–property relationships of M1 −M4 and P1 −P4 were studied. The effect of the content of mesogenic crosslinking units on phase behaviour and mesomorphism of E1 −E4 was discussed. By inserting a flexible spacer between the mesogenic core and the terminal menthyl groups, M1 −M4 all showed strong mesomorphism and exhibited an oily streak and focal conic textures of the cholesteric phase, or the broken fan-shaped texture of the smectic C* phase and the platelet texture of a cubic blue phase. P1 −P4 exhibited batonnet textures of the smectic A phase, indicating that the liquid crystal polymers with siloxane chains tended to form a lower order smectic phase. For M1 −M4 or P1 −P4 , the melting temperature Tm , the isotropic temperature Ti and the glass transition temperature Tg increased, and the mesophase temperature range widened with increasing rigidity of the mesogenic units or decreasing spacer length. E1 −E4 exhibited a cholesteric texture because of the introduction of the nematic crosslinking unit. By increasing the content of the crosslinking unit, the general tendency is toward increased Tg and decreased Ti .