Effect of mesogenic crosslinking units on the mesomorphic phases of cholesteric liquid crystalline polymers

Cholesteric Liquid-Crystalline Thermosets Derived from Side-Chain Liquid-Crystalline Epoxy Oligomers

A series of side-chain liquid crystalline (LC) oligomers bearing glycidyl ether groups were synthesized with cholest-5-en-3-ol(3 β)-4-(2-propenyloxy) benzoate, octyl 4-(4-allyloxy-benzoyloxy)-benzoate and epoxy monomer allyl glycidyl ether. The chemical structures and LC properties of the monomers and oligomers were characterized by use of various experimental techniques. All the chiral LC oligomers showed cholesteric mesophase with very wide mesophase temperature ranges. All the synthesized LC oligomers were cured with 4,4'-diaminodiphenylmethane at the same curing conditions. The thermal behaviors, mesomorphic phase and optical properties of the thermosets were characterized. The synthesized thermosets have high thermal stability. The cholesteric phases of LC oligomers were frozen in the polymer networks. Compared with those of corresponding oligomers, the reflection located at wide angles of thermosets became more diffuse, and the peak intensity decreased in X-ray diffraction analysis. The selective light reflection maximum λmax of the thermosets at room temperature shifted slightly to short wavelengths as the LC monomer bearing cholesteryl component was increased, indicating that the helical pitch P becomes shorter due to more chiral groups in the polymer network systems. The temperature dependences of both reflection wavelength and optical rotation under the influence of the crosslinking reaction were also investigated.