Biphenyl-based liquid crystal precursors with alkanoate and hydroxyl group

<p>We synthesized a series of 4′-hydroxy-[1,1′-biphenyl]-4-yl alkanoate with a potentially reactive functional hydroxyl group as a LC precursor, which facilitates reaction with other chemical groups to tailor biphenyl-based liquid crystals (LCs) for specific applications. Several liquid crystalline materials were also synthesized based on these LC precursors to show high probability to generate various potential LCs. With increasing chain length, the melting point decreased and <i>R</i>_f (retardation factor: migration distance of substance ÷ migration distance of solvent front) of the synthesized LC precursor increased. This LC precursor series provides a useful first synthesis step to design tunable biphenyl/ester-based LCs.</p

Copper Sintering Pastes with Various Polar Solvents and Acidic Activators

Devices in the developing semiconductor market require high density, high integration, and detailed processing. Conventional wire bonding is inappropriate for fine-sized devices, and connected wires can be damaged by heat generation and external physical impact. Soldering is also used in advanced packaging technologies. However, disturbances and overhead joints can occur during bonding. Thus, sintering has been extensively utilized to overcome these drawbacks. Sintering pastes are pressurized and bonded, resulting in stable bonding during sintering. In this study, the composition of the Cu sintering material was examined using diverse additives and solvents. We manufactured sintering materials comprising Cu (1 μm), a solvent [methanol (MeOH), ethanol (EtOH), or ethylene glycol (EG)] and an acidic additive (benzoic acid, phthalic acid, or hexanoic acid). After the sintering process, the mechanical and electrical characteristics were compared to determine the optimal composition and bonding conditions. The optimum ratios between the acid and solvent were 4:6 (MeOH and EtOH) and 2:8 (EG) due to the high viscosity and effective long-term storage. All samples using EtOH as the solvent exhibited the highest sintering performances. The aromatic and carboxylic groups substantially improved the sintering performance and increased the electrical conductivity. Based on the O1s/Cu2p ratio (2.23%), the best sintering composition was EtOH/PA, which showed the highest electrical conductivity (ca. 104 S/m) and strength (34.0 MPa). The sintering process using various additives and solvents can be helpful to determine the sintering conditions while maintaining the electrical properties

Probing the Interplay of Ultraviolet Cross-Linking and Noncovalent Interactions in Supramolecular Elastomers

Ultraviolet (UV) irradiated supramolecular polybutadienes (PBs) containing 2-ureido-4-[1<i>H</i>]-pyrimidone (UPy) linkages were examined as a simple model for curable supramolecular elastomers. Via precise control of UV exposure, the cure and the degradation of the vinyl groups within the PB elastomeric core were investigated. The combination of UPy binding and covalent cross-linking by UV irradiation dramatically enhanced mechanical properties of these UPy-functionalized elastomers, yielding toughness enhancement up to ∼200× at the 5 min UV cure. UV-initiated cross-linking dominated the curing process up to ∼50 min exposure time. Beyond this cure time, dominant degradation of the vinyl linkages was observed. Control of this UV-initiated process yielded supramolecular elastomers with a covalently cross-linked phase induced by UV irradiation combined with a noncovalent UPy cross-linked phase induced by secondary hydrogen bonding interactions. Of particular note, it was determined that the presence of UPy hydrogen-bonded aggregates accelerated the UV cross-linking process during the initial stage of exposure. This observation was attributed to microphase-separated structure of UV-irradiated supramolecular elastomer, where UPy aggregation increased the probability of interaction between the pendant vinyls responsible for UV cross-linking. The systematic study of uniaxial tensile behavior of the UV-irradiated supramolecular elastomers offers new insight into the design and architecture of mechanically tunable supramolecular elastomers

Biphenyl-based liquid crystals for elevated temperature processing with polymers

<div><p>Due to the limited thermal stability of current commercially available liquid crystals (LCs), the incorporation into polymer composites through standard processing techniques, such as melt coextrusion, has been hindered. Motivated by this dilemma, a series of smectic B liquid crystalline structures based on the 4,4ʹ-alkyl substituted biphenyl moiety were synthesised through conventional methodologies and probed for their thermal stability and LC properties. Degradation temperatures were found to increase with increasing aliphatic chain length – up to 295 °C for C16 substituted structures, which is well above the processing temperatures of commercial polymers. Additionally, all compounds were found to be liquid crystalline in nature with crystal-to-smectic B transition temperatures ranging from 49.8 °C to 91.4 °C. Thermal stability, phase separation, and compatibility of LC/polystyrene composites were also examined. Less than 10% of 15A15 LC by weight in polystyrene exhibited good polymer miscibility, while phase separation occurred at loads higher than 15% by weight. We foresee the use of these LCs in applications that require elevated processing conditions to produce materials with enhanced mechanical or gas barrier properties.</p></div

Physical and Chemical Compatibilization Treatment with Modified Aminosilanes for Aluminum/Polyamide Adhesion

Metal/polymer bilayer composites feature high strength-to-weight ratios and low manufacturing costs despite the weak interfacial adhesion between their components. In this study, aluminum surfaces were modified to generate microporous architectures and hydroxyl moieties by various physical and chemical treatments, including thermal, plasma, anodizing, and hexafluorozirconic acid treatments to overcome the weak interfacial adhesion. The maximum shear strength of the obtained metal/polymer bilayer composites was achieved by anodizing treatment, whereas all treatment methods substantially improved the material toughness. In addition, modified compatibilizing agents with tailorable hydroxyl moieties were applied to enhance the interfacial adhesion using aminoethylaminopropyl trimethoxysilane (AEAPS) and modified AEAPS as a coupling agent. AEAPS modified by monoepoxide (glycidol) produced the strongest positive effect on the composite mechanical properties. These findings can be useful in a myriad of metal/polymer multilayer composites

Electromagnetic Interference Shielding Performance of Poly(styrene-<i>co</i>-butyl acrylate)/Carbon Nanotube Nanocomposites Fabricated by Latex Technology

With the increase in electronic devices emitting radio and electromagnetic waves, it has become increasingly crucial to improve electromagnetic interference (EMI) shielding properties. In this study, to provide efficient shielding performance, copolymer/carbon nanotube (CNT) nanocomposites were fabricated using latex technology with colloidal monodisperse copolymer particles and surface-modified nanofillers. The copolymer was synthesized using styrene and butyl acrylate to enhance impact resistance, and thus, colloidal resin having a low glass transition temperature was obtained. The nanofiller, i.e., CNT, was surface-modified by wrapping it with two types of hydrophilic polymers, poly(styrenesulfonate) or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), to improve the dispersibility of CNT in aqueous colloidal suspensions. After investigating the morphological, thermal, and rheological properties of the matrix resin, the EMI shielding effectiveness (SE) of nanocomposite films was analyzed by varying the nanofiller type and content and the number of film layers. The degree of reflection and absorption shielding effects of the nanocomposites were compared at the frequency range of 50 MHz–1.5 GHz using the measured S-parameter. Wrapping hydrophilic and electrically conductive polymers on CNTs improved their dispersity in the aqueous suspension, thereby enhancing the SE. With the same thickness, the performance was improved as the number of stacked layers increased. In particular, the absorption shielding of the nanocomposites was more dominant than the reflection shielding. These copolymer/surface-modified CNT nanocomposites can be employed in various applications that require EMI shielding performance

Electrically Conductive Silicone-Based Nanocomposites Incorporated with Carbon Nanotubes and Silver Nanowires for Stretchable Electrodes

Stretchable electrode materials have attracted great attention as next-generation electronic materials because of their ability to maintain intrinsic properties with rare damage when undergoing repetitive deformations, such as folding, twisting, and stretching. In this study, an electrically conductive PDMS nanocomposite was manufactured by combining the hybrid nanofillers of carbon nanotubes (CNTs) and silver nanowires (AgNWs). The amphiphilic isopropyl alcohol molecules temporarily adhered simultaneously to the hydrophobic CNT and hydrophilic AgNW surfaces, thereby improving the dispersity. As the CNT/AgNW ratio (wt %/wt %) decreased under the constant nanofiller content, the tensile modulus decreased and the elongation at break increased owing to the poor interaction between the AgNWs and matrix. The shear storage moduli of all nanocomposites were higher than the loss moduli, indicating the elastic behavior with a cross-linked network. The electrical conductivities of the nanocomposite containing the hybrid nanofillers were superior to those of the nanocomposite containing either CNT or AgNW at the same filler content (4 wt %). The hybrid nanofillers were rearranged and deformed by 5000 cyclic strain tests, relaxing the PDMS matrix chain and weakening the interfacial bonding. However, the elastic behavior was maintained. The dynamic electrical conductivities gradually increased under the cyclic strain tests due to the rearrangement and tunneling effect of the nanofillers. The highest dynamic electrical conductivity (10 S/m) was obtained for the nanocomposite consisting of 2 wt % of CNTs and 2 wt % of AgNWs