Liquid crystalline twin epoxy monomers based on azomethine mesogen: synthesis and curing with aromatic diamines

Synthesis of multifunctional liquid crystalline epoxy resin embedded cyclic-siloxane chain with Tg-less behavior and enhanced toughness

A novel tetrafunctional mesogenic epoxy monomer with a cyclic-siloxane chain as a central part was successfully synthesized and characterized by proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, differential scanning calorimetry, and polarized optical microscopy. The transition temperature and liquid crystallinity of cyclic-siloxane type mesogenic epoxy were compared with those of linear-siloxane type mesogenic epoxy. Moreover, epoxy thermosets were prepared based on the cyclic- and linear-siloxane type mesogenic epoxy monomers and 4,4′-diaminodiphenylethane. The effects of epoxy backbone moiety on thermal and mechanical properties were investigated in detail. As a result, the cyclic-siloxane type mesogenic epoxy thermoset shows glass transition temperature-less behavior and low coefficient of linear thermal expansion without a decrease in toughness.

Epoxide and oxetane based liquid crystals for advanced functional materials

Liquid crystalline elastomers (LCEs) and liquid crystalline networks (LCNs) are classes of polymers very suitable for fabricating advanced functional materials. Two main pathways to obtain LCEs and LCNs have gained the most attention in the literature, namely the two-step crosslinking of LC side-chain polymers and the photoinitiated free-radical polymerisation of acrylate LC monomers. These liquid crystal polymers have demonstrated remarkable properties resulting from their anisotropic shapes, being used in soft robotics, responsive surfaces and as photonic materials. In this review, we will show that LCs with cyclic ethers as polymerisable groups can be an attractive alternative to the aforementioned reactive acrylate mesogens. These epoxide and oxetane based reactive mesogens could offer a number of advantages over their acrylate-based counterparts, including oxygen insensitivity, reduced polymerisation shrinkage, improved alignment, lower processing viscosity and potentially extended resistivity. In this review, we summarise the research on these materials from the past 30 years and offer a glimpse into the potential of these cyclic ether mesogens.

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.

Liquid Crystalline Epoxy Azomethine Thermoset

An investigation was carried out on the curing reaction of a liquid crystalline epoxy monomer, N, N'-bis[4-(2,3-epoxypropoxy)benzylydene]-1,4-phenylenediamine, with an aromatic diamine, sulfanilamide (SAA). The complexity of the curing involves the competition of the reactivities of a primary amine and a secondary amine and also a difference in reactivity of the amine and the amide functional groups from SAA. These phenomena have been studied by differential scanning calorimetry (DSC) under isothermal and non-isothermal conditions. The stability of the mesophase during cross-linking reaction was investigated by polarized optical microscopy (POM) and X-ray diffraction (WAXS).