Properties of Liquid Crystal Epoxy Thermosets Cured in a Magnetic Field

Effect of carbon nanotube (CNT) functionalization in Epoxy-CNT composites

The effect of carbon nanotube (CNT) functionalization in altering the properties of Epoxy-CNT composites is presented. The presence of functional groups effectively influenced the colloidal behavior of CNTs in the precursor epoxy resin and the hardener triethylenetetramine (TETA), which affected the synthesis process and eventually the interfacial interactions between the polymer matrix and the CNTs. The physical, thermal and electrical properties of the composites exhibited strong dependence on the nature of functionalization. At a 0.5 wt% CNTs loading, the enhancement in tensile strength was found to be 7.2, 11.2, 11.4 and 14.2 percent for raw CNTs, carboxylated CNTs, octadecyl amide functionalized CNTs and hydroxylated CNTs, respectively. Glass transition temperatures (T_g) also varied with the functionalization and composite prepared using hydroxylated CNTs showed the maximum enhancement of 34%.

Enhanced Thermal Conductivity of Liquid Crystalline Epoxy Resin using Controlled Linear Polymerization

A powerful strategy to enhance the thermal conductivity of liquid crystalline epoxy resin (LCER) by simply replacing the conventional amine cross-linker with a cationic initiator was developed. The cationic initiator linearly wove the epoxy groups tethered on the microscopically aligned liquid crystal mesogens, resulting in freezing of the ordered LC microstructures even after curing. Owing to the reduced phonon scattering during heat transport through the ordered LC structure, a dramatic improvement in the thermal conductivity of neat cation-cured LCER was achieved to give a value ∼141% (i.e., 0.48 W/mK) higher than that of the amorphous amine-cured LCER. In addition, at the same composite volume fraction in the presence of a 2-D boron nitride filler, an approximately 130% higher thermal conductivity (maximum ∼23 W/mK at 60 vol %) was observed. The nanoarchitecture effect of the ordered LCER on the thermal conductivity was then examined by a systematic investigation using differential scanning calorimetry, polarized optical microscopy, X-ray diffraction, and thermal conductivity measurements. The linear polymerization of LCER can therefore be considered a practical strategy to enable the cost-efficient mass production of heat-dissipating materials, due to its high efficiency and simple process without the requirement for complex equipment.

Thermomagnetic processing of liquid-crystalline epoxy resins and their mechanical characterization using nanoindentation

A thermomagnetic processing method was used to produce a biphenyl-based liquid-crystalline epoxy resin (LCER) with oriented liquid-crystalline (LC) domains. The orientation of the LCER was confirmed and quantified using two-dimensional X-ray diffraction. The effect of molecular alignment on the mechanical and thermomechanical properties of the LCER was investigated using nanoindentation and thermomechanical analysis, respectively. The effect of the orientation on the fracture behavior was also examined. The results showed that macroscopic orientation of the LC domains was achieved, resulting in an epoxy network with an anisotropic modulus, hardness, creep behavior, and thermal expansion.

Synthesis and properties of aligned all-aromatic liquid crystal networks

A series all-aromatic main-chain liquid crystalline (LC) polymers based on 6-hydroxy-2-naphtoic acid (HNA), biphenyl-4,4′-dicarboxylic acid (BA), resorcinol (RS), and a cross-linkable ethynyl functionality, 4,4′-(1,2-ethynediyl)bisbenzoic acid (EBA) were synthesized using a one-pot melt condensation method. In contrast with reactive LC oligomers where reactive functionalities were placed at the chain terminus, the reactive ethynyl group is now used as a cross-linking functionality placed in the polymer main-chain. Differential scanning calorimetric analysis reveals that these polymers can be cured at temperatures above 350°C in the nematic melt and exhibit excellent thermal properties, that is, temperature at 5% weight loss > 465°C, and reaches a maximum after cure glass transition temperature ( Tg) of 167°C. When only 5 mol% of BA was replaced with EBA, a slightly cross-linked nematic network, HNA/BA/RS/EBA-5, was obtained and thin films thereof could be stretched by 400% strain above Tg in four consecutive steps. A maximum order parameter [Formula: see text] of 0.54 could be achieved, and this moderately aligned LC network displays a room temperature storage modulus ( E′) of 29 GPa, a tensile strength of 330 MPa, a tensile modulus of 7 GPa, and an elongation at break of 7%. Cross-linking also improved the transverse properties of the nematic films. E′ of HNA/BA/RS/EBA-5 is 1.0 GPa at 150°C after being stretched by 400% versus 0.16 GPa for the non-cross-linked reference polymer. An affine deformation model was used to calculate E′ using [Formula: see text], and the predicted values are in close agreement with the experimental results.

Synthesis and Properties of a Liquid Crystalline Thermoset Epoxy Resin Containing 1,3,4-Oxadiazole Groups

A new liquid crystalline thermoset (LCT) based on 1,3,4-oxadiazole was prepared from liquid crystalline epoxy resin containing oxadiazole and diaminodiphenyl methane. The LCT was characterized and compared with the conventional thermoset. The Time—Temperature Transformation diagram was constructed to determine an appropriate set of cure cycles. The goal of this paper was centered on the evaluation of the mechanical performances of this new class of thermoset. In the case of LC thermoset, a higher Tg and good fracture resistance were noticed than that of the conventional thermoset. The LC thermoset exhibited higher thermal stability when compared with the conventional non-LC thermosets. The results tempted to consider the LCT materials as potential candidates for advanced composites.

Formation of fibril structures in polymerizable, rod-coil-oligomer-modified epoxy networks

This paper describes the in situ preparation of fibrils in epoxy networks in which the fibril-like structures are cured polymerizable rod-coil oligomers. The epoxy-terminated alpha,omega-modified PEO oligomers, which are ABA rod-coil-rod oligomers with a poly(ethylene oxide) coil unit and two aromatic azomethine liquid-crystalline rod units, were synthesized and then further blended with an epoxy precursor. Uniform nanoscale columnar structures were observed in the neat rod-coil oligomers as well as in the crosslinked liquid-crystalline state. During the curing of the blends, the supramolecular nanoscale columnar structures of the rod-coil oligomers are transformed into polymeric fibrils where the epoxy functional end groups have co-reacted with epoxy precursors to form a crosslinked network.