Solid state molecular dynamic investigation of an inclusion ferroelectric: [(2,6-diisopropylanilinium)([18]crown-6)]BF4.

Many order-disorder-type phase transitions in molecule-based ferroelectrics are related to changes of molecular dynamics. If the molecular motions do not involve reorientations of dipole moments, their ordering fails to contribute directly to spontaneous electric polarization. For understanding ferroelectric mechanisms in these systems, it is important to clarify how such molecular dynamics changes induce structurally symmetry-breaking phase transitions and thus the appearance of spontaneous electric polarization. Systematic characterization of an [18]crown-6 based host-guest inclusion compound, [(DIPA)([18]crown-6)]BF4 (DIPA = 2,6-diisopropylanilinium), shows it is an excellent ferroelectric with a large dielectric anomaly, significant pyroelectricity, and SHG response, and rectangular polarizaiton-electric field hysterisis loops. By the combination of variable-temperature single-crystal structural determination and solid-state NMR observation, it is found that the slowing down of the rotation of the [18]crown-6 molecule and the tumbling of the BF4 anion causes the symmetry breaking, while the spontaneous polarization is induced by the relative displacement between the cationic and anionic sublattices. This investigation will contribute to a deeper understanding of the structure-property relationship in the emerging molecular ferroelectrics.

4-Methoxyanilinium perrhenate 18-crown-6: a new ferroelectric with order originating in swinglike motion slowing down.

A supramolecular adduct 4-methoxyanilinium perrhenate 18-crown-6 was synthesized, which undergoes a disorder-order structural phase transition at about 153 K (T(c)) due to slowing down of a pendulumlike motion of the 4-methoxyanilinium group upon cooling. Ferroelectric hysteresis loop measurements give a spontaneous polarization of 1.2 µC/cm2. Temperature-dependent solid-state nuclear magnetic resonance measurements reveal three kinds of molecular motions existing in the compound: pendulumlike swing of 4-methoxyanilinium cation, rotation of 18-crown-6 ring, and rotation of the methoxyl group. When the temperature decreases, the first two motions are frozen at about 153 K and the methoxyl group becomes rigid at around 126 K. The slowing down or freezing of pendulumlike motion of the cation triggered by temperature decreasing corresponds to the centrosymmetric-to-noncentrosymmetric arrangement of the compound, resulting in the formation of ferroelectricity.

Reticulated Polyamide Thin-Film Nanocomposite Membranes Incorporated with 2D Boron Nitride Nanosheets for High-Performance Nanofiltration.

Nanofiltration (NF) technology has been widely used in saline wastewater treatment due to its unique separation mechanism. However, the NF membrane, as the core of the nanofiltration technology, is restricted by the trade-off between permeability and selectivity, which greatly restricts the development of NF membranes. The interlamellar arrangement of 2D boron nitride nanosheets (BNNSs) can provide additional transport channels and selectivity, as well as strong adsorption capacity due to its high specific surface area, exhibiting significant potential for advanced membranes. In this work, BNNSs prepared by tannic acid (TA)-assisted exfoliation (TA@BNNSs) were successfully adopted to fabricate thin-film nanocomposite (TFN) membranes via interfacial polymerization (IP). The resultant TFN membranes' structure and properties were systematically characterized via various methods. The results demonstrated that the surface morphology of polyamide membranes evolved gradually from a nodular structure to a reticular topography, accompanied by the decrease of the thickness of the polyamide selective layer when incorporating TA@BNNSs into the membranes. This phenomenon can be mainly ascribed to that the uptake density and diffusion of piperazine (PIP) monomer were effectively regulated by BNNSs. This is validated by molecular dynamics and revealed by the adsorption of PIP in BN models, the diffusion coefficients, and interaction energies, respectively. In addition, the TFN membranes demonstrated improved permeance and stable solute rejection for the inorganic salts. Specifically, the water flux of PA-TA@BNNSs-10%/PMIA membrane could reach up to 109.1 ± 2.49 L·m-2·h-1 while keeping a high rejection of 97.5 ± 0.38% to Na2SO4, which was superior to most of the reported membranes in the literature. Besides, the PA-TA@BNNSs-10%/PMIA membrane exhibited an excellent stability in the long-term filtration process. The finding in this work provides a potential strategy for developing the next-generation 2D material-based membranes with high-performance for separation applications.

Ferroelectricity induced by ordering of twisting motion in a molecular rotor.

A novel mononuclear metal-organic compound, [Cu(Hdabco)(H(2)O)Cl(3)] (1, dabco = 1,4-diazabicyclo[2.2.2]octane) in which the Cu(II) cation adopts a slightly distorted bipyramidal geometry where the three Cl anions constitute the equatorial plane and the Hdabco cation and H(2)O molecule occupy the two axial positions, was synthesized. Its paraelectric-to-ferroelectric phase transition at 235 K (T(c)) and dynamic behaviors were characterized by single crystal X-ray diffraction analysis, thermal analysis, dielectric and ferroelectric measurements, second harmonic generation experiments, and solid-state nuclear magnetic resonance measurements. Compound 1 behaves as a molecular rotor above room temperature in which the (Hdabco) part rotates around the N···N axis as a rotator and the [Cu(H(2)O)Cl(3)] part acts as a stator. In the temperature range 235-301 K, a twisting motion of the rotator is confirmed. Below the T(c), the motions of the rotor are frozen and the molecules become ordered, corresponding to a ferroelectric phase. Origin of the ferroelectricity was ascribed to relative movements of the anions and cations from the equilibrium position, which is induced by the order-disorder transformation of the twisting motion of the molecule between the ferroelectric and paraelectric phases. Study of the deuterated analogue [Cu(Ddabco)(D(2)O)Cl(3)] (2) excludes the possibility of proton ordering as the origin of the ferroelectricity in 1.

Molecular simulations of the effects of substitutions on the dissolution properties of amorphous cellulose acetate.

The dissolution behavior of cellulose acetate (CA) is an extremely important property in its extensive applications and preparation of derivatives. In this paper, we proposed a molecular model building strategy to construct amorphous CA with various substituent distributions (different degrees of substitution and substitution positions). A protocol combing molecular dynamics simulation and density functional theory (DFT) was applied to systematically investigate the dissolution behavior of CAs, and the structural properties of CAs. The reduced cohesive energy and polarity of CAs caused by the increase in substituents would enhance its solubility. The interaction of solvent molecules and CAs and the diffusion of solvent molecules in CAs have a synergistic effect on the dissolution of CAs. The diffusion coefficient is the primary factor affecting the solubility. Moreover, substituents at different positions of the anhydroglucose units along the CAs chains would produce different steric hindrance effects, which in turn affect the dissolution behavior.

Highly flexible and superhydrophobic MOF nanosheet membrane for ultrafast alcohol-water separation.

High-performance pervaporation membranes have potential in industrial separation applications, but overcoming the permeability-selectivity trade-off is a challenge. We report a strategy to create highly flexible metal-organic framework nanosheet (MOF-NS) membranes with a faveolate structure on polymer substrates for alcohol-water separation. The controlled growth followed by a surface-coating method effectively produced flexible and defect-free superhydrophobic MOF-NS membranes. The reversible deformation of the flexible MOF-NS and the vertical interlamellar pathways were captured with electron microscopy. Molecular simulations confirmed the structure and revealed transport mechanism. The ultrafast transport channels in MOF-NS exhibited an ultrahigh flux and a separation factor of 8.9 in the pervaporation of 5 weight % ethanol-water at 40°C, which can be used for biofuel recovery. MOF-NS and polydimethylsiloxane synergistically contribute to the separation performance.