Atomistic Mechanism of Anisotropic Heat Conduction in the Liquid Crystal 4-Heptyl-4'-cyanobiphenyl: All-Atom Molecular Dynamics

Structural and Thermal Diffusivity Analysis of an Organoferroelastic Crystal Showing Scissor-Like Two-Directional Deformation Induced by Uniaxial Compression

A two-directional ferroelastic deformation in organic crystals is unprecedented owing to its anisotropic crystal packing, in contrast to isotropic symmetrical packing in inorganic compounds and polymers. Thereby, finding and constructing multidirectional ferroelastic deformations in organic compounds is undoubtedly complex and at once calls for deep comprehension. Herein, we demonstrate the first example of a two-directional ferroelastic deformation with a unique scissor-like movement in single crystals of trans-3-hexenedioic acid by the application of uniaxial compression stress. A detailed structural investigation of the mechanical deformation at the macroscopic and microscopic levels by three distinct force measurement techniques (including shear and three-point bending test), single crystal X-ray diffraction techniques, and polarized synchrotron-FTIR microspectroscopy highlighted that mechanical twinning promoted the deformation. The presence of two crystallographically equivalent faces and the herringbone arrangement promoted the two-directional ferroelastic deformation. In addition, anisotropic heat transfer properties in the parent and the deformed domains were investigated by thermal diffusivity measurement on all three axes using microscale temperature-wave analysis (μ-TWA). A correlation between the anisotropic structural arrangement and the difference in thermal diffusivity and mechanical behavior in the two-directional organoferroelastic deformation could be established. The structural and molecular level information from this two-directional ferroelastic deformation would lead to a more profound understanding of the structure-property relationship in multidirectional deformation in organic crystals.

Effects and mechanism of filler content on thermal conductivity of composites: a case study on plasticized polyvinyl chloride/graphite composites

Thermally conductive polymer composites that retain mechanics and processing properties have attracted significant attention because of promising high thermal conductivity. Herein, plasticized polyvinyl chloride (P-PVC)/graphite composites were successfully prepared via melt blending. Following the addition of graphite rising from 0 to 300 phr, the thermal conductivity of P-PVC/graphite composites increases from 0.18 to 3.01 W m−1 K−1. The thermal conductivity of P-PVC/graphite composites with 300 phr graphite is 17 times that of the P-PVC matrix. P-PVC/graphite composites with high thermal conductivity have excellent performance in thermal management for LEDs. Therefore, the high thermal conductivity allows for the LED’s temperature to drop 44%, compared with the P-PVC matrix, at 1.5 V. The notably cooling effect provides the ideas for the future application of the P-PVC/graphite composites in the thermal management for electronic components.

Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

We study by scanning thermal microscopy the nanoscale thermal conductance of films (40-400 nm thick) of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8). We demonstrate that the out-of-plane thermal conductivity is significant along the interlayer direction, larger for BTBT (0.63 ± 0.12 W m^-1 K^-1) compared to C8-BTBT-C8 (0.25 ± 0.13 W m^-1 K^-1). These results are supported by molecular dynamics calculations (approach to equilibrium molecular dynamics method) performed on the corresponding molecular crystals. The calculations point to significant thermal conductivity (3D-like) values along the 3 crystalline directions, with anisotropy factors between the crystalline directions below 1.8 for BTBT and below 2.8 for C8-BTBT-C8, in deep contrast with the charge transport properties featuring a two-dimensional character for these materials. In agreement with the experiments, the calculations yield larger values in BTBT compared to C8-BTBT-C8 (0.6-1.3 W m^-1 K^-1versus 0.3-0.7 W m^-1 K^-1, respectively). The weak thickness dependence of the nanoscale thermal resistance is in agreement with a simple analytical model.

Acceleration of Liquid-Crystalline Phase Transition Simulations Using Selectively Scaled and Returned Molecular Dynamics

The molecular dynamics (MD) technique to accelerate simulation of phase transition to liquid-crystalline (LC) phases is demonstrated on the model LC system 4-octyl-4'-cyanobiphenyl (8CB) smectic A phase. Simulation of a phase transition to a smectic phase is challenging because an intrinsically long simulation time and large system size are required owing to the high order and low onset temperature. Acceleration of the simulated transition of 8CB to the smectic A phase was ultimately achieved by selectively weakening the intermolecular Lennard-Jones interaction of alkyl chains and then returning the scaled interaction to the unscaled one. The total time needed to form the smectic A phase using selectively scaled and returned molecular dynamics (ssrMD) was five times shorter than that when using unscaled MD. Formation of the smectic A phase occurred only when induced polarization from the antiparallel dipole dimer point charge was included in the simulation. The use of ssrMD presented herein is anticipated to accelerate the theoretical development of self-assembled organic materials containing both rigid and flexible moieties, including LC materials.