Soon Man Hong

Electromagnetic interference shielding with 2D transition metal carbides (MXenes)

Materials with good flexibility and high conductivity that can provide electromagnetic interference (EMI) shielding with minimal thickness are highly desirable, especially if they can be easily processed into films. Two-dimensional metal carbides and nitrides, known as MXenes, combine metallic conductivity and hydrophilic surfaces. Here, we demonstrate the potential of several MXenes and their polymer composites for EMI shielding. A 45-micrometer-thick Ti3C2Tx film exhibited EMI shielding effectiveness of 92 decibels (>50 decibels for a 2.5-micrometer film), which is the highest among synthetic materials of comparable thickness produced to date. This performance originates from the excellent electrical conductivity of Ti3C2Tx films (4600 Siemens per centimeter) and multiple internal reflections from Ti3C2Tx flakes in free-standing films. The mechanical flexibility and easy coating capability offered by MXenes and their composites enable them to shield surfaces of any shape while providing high EMI shielding efficiency.

Characterization and processing of blends of poly(ether imide) with thermotropic liquid crystalline polymer

Abstract We investigated thermal, rheological, morphological and mechanical properties of an in situ composite of poly(ether imide) (PEI) and thermotropic liquid crystalline polymer (TLCP). Ultem 1000 was used as a matrix and Vectra B950 was used as the in situ reinforcing TLCP. Fibre-spinning of the blends was performed on a capillary rheometer. Differential scanning calorimetry thermograms of extruded fibres indicated that the thermal properties of PEI did not change noticeably with the amount of TLCP but thermogravimetric analysis showed that thermal stability of the blend was decreased with the amount of TLCP. Immiscibility was checked with thermal data. The rheological properties of the blends changed remarkably with temperature and composition. The tensile strength and modulus of blend fibre increased with TLCP content and spin draw ratio. The increase of tensile strength was more striking for the fibre of the blend containing more TLCP. Wide angle X-ray patterns suggested that the increase in tensile strength was due to the enhanced molecular orientation and resultant fibrillation of TLCP. A modified Tsai-Halpin equation was used to predict the aspect ratio of microfibrils for these blends. Morphology of the blend showed that PEI/TLCP fibres contained fine fibrils of almost infinite aspect ratio and nearly perfect orientation in the flow direction. The draw ratio effect on the mechanical properties was remarkable at low draw ratio, but levelled off soon. The amount of TLCP influenced the fibril formation. Morphological observation showed the effect of thermal history of the blend and its effect on mechanical performance. The blend showed maximum aspect ratio and aspect ratio change when TLCP content was 25 wt%. TLCP orientation and its steric effect seem to induce the optimum TLCP amount for fibril formation.

Synthesis and curing of liquid crystalline epoxy resins based on 4,4′-biphenol

Abstract Two liquid crystalline epoxy (LCE) resins based on biphenol mesogen, BP1 and BP2, were synthesized to produce a highly heat-resistant liquid crystalline network, and the curing behaviour of them was investigated using diaminodiphenylsulfone (DDS) and diaminodipheylester (DDE) as curing agents. The curing rate and heat of curing of LCE resins were measured with dynamic and isothermal DSC. The curing reaction of BP2 based on aromatic ester was faster than that of BP1, and the degree of cure was vice versa. A densely cross-linked BP2 network showed a much higher glass transition temperature (≈280°C) and elastic modulus than other epoxy resins reported up to now. BP2 formed a liquid crystalline network on curing with DDS and DDE, while BP1 showed a liquid crystalline network on curing with DDE. The liquid crystalline phase of the resulting network was maintained up to decomposition temperature (

Compatibilizing effect of a poly(ester imide) on the properties of the blends of poly(ether imide) and a thermotropic liquid crystalline polymer: 2. Morphology and mechanical properties of the in situ composite system

Abstract Blends of a thermotropic liquid crystalline polymer (TLCP), [poly(ester amide), PEA, Vectra B950 from Hoechst Celanese] and poly(ether imide) (PEI, Ultem 1000 from G.E.) with the compatibilizer [poly(ester imide), PEsI] were extruded in a twin-screw extruder. The extruded strands were evaluated in terms of morphology and mechanical properties. The morphology of the compatibilized in situ composites was found to be significantly dependent on the concentration of the compatibilizer in the blend. For a TLCP phase content of 25 phr, a maximum reduction in phase size was observed when 1.5 phr by weight of compatibilizer was added to the blend. At high concentrations of the compatibilizer, flocculation of the TLCP phase was observed. Measurement of the tensile properties shows increased elongation as well as enhanced modulus and strength when properly compatibilized. This improvement is ascribed to better adhesion between the TLCP fibrils and the PEI matrix and better dispersion of the TLCP fibrils. Synthesized PEsI significantly improved the adhesion between the matrix phase (PEI) and fibril phase (PEA). However, maxima in tensile modulus, tensile strength and elongation were observed when excess compatibilizer was added. An emulsifying effect of the compatibilizer to coalesce the fibrils is believed to be the cause of the maxima in the tensile properties. Impact strength was seriously increased with the compatibilizer. A maximum in impact strength was also observed, but all compatibilized samples exhibited a higher impact strength than the non-compatibilized one. The reason is believed to be the failure mode difference between the tensile properties and the impact strength.

High Through-Plane Thermal Conduction of Graphene Nanoflake Filled Polymer Composites Melt-Processed in an L-Shape Kinked Tube

Design of materials to be heat-conductive in a preferred direction is a crucial issue for efficient heat dissipation in systems using stacked devices. Here, we demonstrate a facile route to fabricate polymer composites with directional thermal conduction. Our method is based on control of the orientation of fillers with anisotropic heat conduction. Melt-compression of solution-cast poly(vinylidene fluoride) (PVDF) and graphene nanoflake (GNF) films in an L-shape kinked tube yielded a lightweight polymer composite with the surface normal of GNF preferentially aligned perpendicular to the melt-flow direction, giving rise to a directional thermal conductivity of approximately 10 W/mK at 25 vol % with an anisotropic thermal conduction ratio greater than six. The high directional thermal conduction was attributed to the two-dimensional planar shape of GNFs readily adaptable to the molten polymer flow, compared with highly entangled carbon nanotubes and three-dimensional graphite fillers. Furthermore, our composite with its density of approximately 1.5 g/cm(3) was mechanically stable, and its thermal performance was successfully preserved above 100 °C even after multiple heating and cooling cycles. The results indicate that the methodology using an L-shape kinked tube is a new way to achieve polymer composites with highly anisotropic thermal conduction.

Copper Shell Networks in Polymer Composites for Efficient Thermal Conduction

Thermal management of polymeric composites is a crucial issue to determine the performance and reliability of the devices. Here, we report a straightforward route to prepare polymeric composites with Cu thin film networks. Taking advantage of the fluidity of polymer melt and the ductile properties of Cu films, the polymeric composites were created by the Cu metallization of PS bead and the hot press molding of Cu-plated PS beads. The unique three-dimensional Cu shell-networks in the PS matrix demonstrated isotropic and ideal conductive performance at even extremely low Cu contents. In contrast to the conventional simple melt-mixed Cu beads/PS composites at the same concentration of 23.0 vol %, the PS composites with Cu shell networks indeed revealed 60 times larger thermal conductivity and 8 orders of magnitude larger electrical conductivity. Our strategy offers a straightforward and high-throughput route for the isotropic thermal and electrical conductive composites.

Compatibilizing effect of a poly(ester imide) on the properties of the blends of poly(ether imide) and a thermotropic liquid crystalline polymer: 1. Compatibilizer synthesis and thermal and rheological properties of the in situ composite system

Abstract This study investigates the compatibilizing effect of a poly(ester imide) (PEsI) on the blends of poly(ether imide)(PEI, Ultem 1000 from G.E.) and thermotropic liquid crystalline polymer (TLCP)(poly(ester amide), PEA, Vectra B950 from Hoechst Celanese). The compatibilizer, PEsI, was synthesized. Composite fibres were prepared by extrusion. Compatibility, thermal and theological properties of the compatibilized in situ composite have been analysed. PEI and PEA blends are known from previous studies to be immiscible. Differential scanning calorimetry (d.s.c.) and dynamic mechanical thermal analysis (d.m.t.a.) results, however, show that PEsI is miscible with both PEI and PEA. This means that synthesized PEsI can be used as a compatibilizer for the PEI/PEA in situ composite system. The viscosity of the compatibilized in situ composite was increased by the compatibilizer owing to the strong interaction. Significant changes in the dispersion of the TLCP were observed when the compatibilizing agent was added. The size of the dispersed phase appears to be controlled by interfacial phenomena rather than rheological effects. Explanations for the interaction of PEsI with PEI and TLCP related to the interfacial phenomena are presented.

Relationship between the structure of the bridging group and curing of liquid crystalline epoxy resins

The influence of the bridging group between the mesogenic group and the oxirane ring on the curing behavior and the liquid crystalline phase of liquid crystalline epoxy (LCE) resins was investigated. Two LCE resins containing ether and ester bridging groups were synthesized for this purpose. The ether linkage stabilized the liquid crystalline phase of the LCE and the LCE network more than the ester linkage. The retardation effect of curing was observed in LCE with the ester linkage. The LCE with the ether bridge showed higher mechanical and thermal properties than that with the ester bridge. The liquid crystalline phase of the LCE monomer remained after the crosslinking reaction and it was stable up to 300°C.

Hybrid ionogel electrolytes for high temperature lithium batteries

Hybrid ionogels fabricated using 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) crosslinked with ladder-like structured poly(methacryloxypropyl)silsesquioxane (LPMASQ) were investigated as high temperature ionogel electrolytes for lithium ion batteries. In addition to the exceedingly low crosslinker concentration (∼2 wt%) required to completely solidify the ionic liquids, which provided high ionic conductivities comparable to the liquid state ionic liquid, these hybrid ionogels exhibited superior thermal stabilities (>400 °C). Rigorous lithium ion battery cells fabricated using these hybrid ionogels revealed excellent cell performance at various C-rates at a variety of temperatures, comparable with those of neat liquid electrolytes. Moreover, these hybrid ionogels exhibited excellent cycling performance during 50 cycles at 90 °C, sustaining over 98% coulombic efficiency. Highly advantageous properties of these hybrid ionogels, such as high ionic conductivity in the gel state, thermal stability, excellent C-rate performance, cyclability and non-flammability, offer opportunities for applications as high temperature electrolytes.

Novel polysilsesquioxane hybrid polymer electrolytes for lithium ion batteries

A novel inorganic–organic hybrid crosslinker was prepared through synthesis of a fully condensed, high molecular weight ladder-like poly(methacryloxypropyl)silsesquioxane (LPMASQ) in one pot with a facile, base-catalysed system. The fully condensed LPMASQ revealed good thermal (∼380 °C) and electrochemical stability (∼5.0 V) due to the absence of uncondensed silanol groups. LPMASQ also revealed good solubility in various organic solvents and fully gelated 1 M LiPF6 in ethyl carbonate–diethyl carbonate (EC–DEC, 3/7, v/v) electrolyte solution through fast thermal and photocuring even at a very low concentration of 2 wt%. These observations were attributed to the polymeric nature of LPMASQ containing over one hundred methacryl moieties on the rigid double-stranded siloxane backbone. To the best of our knowledge, formation of a gel polymer electrolyte with 2 wt% gelator is the smallest gelation concentration that has ever been reported. This leads to high ionic conductivity (∼6.0 mS cm−1), excellent Coulombic efficiency and battery cell performance, comparable with those of the neat liquid electrolyte. The small crosslinker content, thermal and electrochemical stability, fast thermal and photocuring and facile processing of the LPMASQ based GPEs, as well as excellent Li battery cell performances strongly hold great promise for future industrial battery applications.