Fracture toughness of highly ordered liquid crystalline epoxy thermosets achieved by optimized composition of curing agents

Abstract

Liquid crystalline (LC) epoxy resin was prepared by using different compositions of aromatic amine as curing agents, in order to control curing rates and chemical compositions. The progress of the curing reaction was investigated based on the gel fraction and epoxy groups of conversion determined by Fourier-transform infrared spectroscopy. The ordered networked polymer structure was analyzed by polarized optical microscopy and X-ray diffraction. Highly oriented network chains in the obtained epoxy thermosets were promoted by the incorporation of flexible chains in the network and the provision of sufficient time for vitrification. Furthermore, it was clarified that a curing temperature higher than T g is required to promote the transition to the smectic LC phase in order to prepare highly ordered epoxy thermosets. The increase in the formed smectic LC phase in the network chains resulted in significant higher fracture toughness and achieved up to 2.7 times higher value.

Thermal Conductivity and Orientation Structure of Liquid Crystalline Epoxy Thermosets Prepared by Latent Curing Catalyst

“…The higher relative intensity for the m-PDA system indicates that more of the S m phase was formed, whereas the CEUI system formed more of the nematic phase. In our previous paper, we reported on the LC epoxy thermosets, the halo strength of which is significantly lower than that of its smectic peak [36]. Therefore, we think that the m-PDA and CEUI systems consist of a mixture of smectic and nematic phases.…”

“…LC epoxy resins prepared with less-reactive curing agents have longer gelation times and can provide a range of possible mesogenic groups. We have also reported on the improvement of the alignment and toughness of cured materials using a mixture of LC epoxy resin and a rigid, highly reactive curing agent, where the curing agent was present in small amounts, contained flexible chains, and exhibited low reactivity [36]. The use of latent curing catalysts increased the concentration of mesogen groups in the LC epoxy thermosets and extended the gelation time, giving the mesogen groups more time to align; this alignment is expected to improve the orientation of networked polymer chains.…”

Advancements in The Cross-Linking and Morphology of Liquid Crystals

“…hardening and then gradually decreases [33]. Studies indicate that m-PDA, in comparison to other compounds, leads to a superior alignment of liquid crystal mesogens [34].…”

“…Phenylenediamine (PDA) isomers are frequently encountered during the LCER curing procedure. The use of m-phenylenediamine (m-PDA) as a hardener causes changes in the process when the conversion of the epoxy group increases significantly in the early stage of hardening and then gradually decreases [33]. Studies indicate that m-PDA, in comparison to other compounds, leads to a superior alignment of liquid crystal mesogens [34].…”

Advances in Liquid Crystal Epoxy: Molecular Structures, Thermal Conductivity, and Promising Applications in Thermal Management

“…[9,11,[21][22][23] An example is shown in Figure 2, illustrating the internal ordered structure and thermal conduction diagram of an epoxy network. In the cured E-51 epoxy (2,2-bis(4-glycidyloxyphenyl)propane (DGEBA)) system, there is no discernible ordered micro-region, resulting in low k. [24] However, when LCE is introduced, the cured epoxy undergoes a transformation from a disordered to an ordered microscale structure, facilitated by the self-assembly of the mesogenic units. Furthermore, the LCE domains can be further oriented through the application of external fields such as magnetic or electric fields.…”