New dimeric LC-epoxyimine monomers with oxyethylene central spacers. Crosslinking study

Reprocessable and Chemically Recyclable Hard Vitrimers Based on Liquid-Crystalline Epoxides

The rapid increase in demand for recyclable and reusable thermosets has necessitated the development of materials with chemical structures that exhibit these features. Thus, functional mesogenic epoxide monomers bearing both ester and imine groups that can be vitrimerized and recycled are reported herein. The compounds show mesophase characteristics at 100-200 °C and can be converted into hard epoxides by a common curing reaction. The obtained hard epoxides have high isotropic thermal conductivity (≈0.64 W m^-1 K^-1 ), which is derived from their highly ordered microstructures. The cured products can be easily reprocessed through imine metathesis and transesterification, and decomposed products can be obtained through imine hydrolysis under acidic or basic conditions and subsequently be re-cured. Surprisingly, recycled materials can be repeatedly reprocessed or chemically decomposed. The reprocessed materials retain the properties of their pristine counterparts, and the recycled products preserve the advantages of the hard thermosets without alteration to any of their unique properties. A dehydration reaction occurs between the residual hydroxyl groups during the re-hardening, which dramatically increases the glass transition temperature by ≈60 °C. These reprocessable and recyclable vitrimers demonstrate the effectiveness and environmental friendliness of the molecular design strategy reported herein.

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.