As organic ionic crystals, C2‐, C3‐, and C4‐alkylene α,ω‐bis[N,N′‐(4‐alkylpyridinium)] PF6− salts are synthesized to investigate their thermal properties and phase behaviors along with chemical structure changes. The X‐ray crystal structure of 1,2‐bis[N,N′‐(4‐methylpyridinium)]ethane 2PF6− (2–1) showed that two pyridinium rings in one cation molecule and two pyridinium ring planes of neighboring molecules were exactly parallel. Multiple C−H⋅⋅⋅F hydrogen bonds were observed for all protons of the bis‐pyridinium with PF6− anions. Interestingly, most of the prepared C2‐bridged dicationic bis‐pyridinium PF6− salts showed solid–solid phase transitions, which are typical features of soft crystalline materials, such as plastic and liquid crystals. Colorfully aligned crystalline textures of the polarized optical microscope images are formed with strong birefringence for the soft crystalline phase of 1,2‐bis[N,N′‐(4‐n‐propylpyridinium)]ethane PF6−, showing a low entropy of fusion (ΔSf=25.0 J mol−1 K−1). The C2‐bridged n‐undecyl (2–11) and n‐dodecyl (2–12) salts exhibit a very soft solid phase, showing large heat absorptions and entropy changes during their multiple solid‐solid phase transitions. Additionally, the C3‐ and C4‐bridged dicationic (bis)pyridinium PF6− salts are prepared to determine the property changes with different bridge lengths. The longer alkylene connected salts are thermally more stable than the C2‐bridged compounds and show only one melting phase transition.
A free-standing ion-conductive gel is formed by spontaneous self-assembly of the amphiphilic ionic liquid 1-tetradecyl-3-methylimidazolium chloride (C14MIm·Cl) and the cross-linkable monomer 6-hexanediol diacrylate (HDODA) in a mixed solvent of 1-octene, 1-butanol, and water. The ionic conductivity of this ion gel is 24 mS cm–1 at 33 °C. To enhance the mechanical strength of the ion gels, the acrylate ionic liquid 1-(2-acryloyloxyundecyl)-3-methylimidazolium bromide (A-C11MIm·Br) was added, leading to significant morphological changes of the HDODA phase from spherical, ellipsoid, angular platelets to interconnected with increasing addition of the acrylate ionic liquid and consequent enhancement in the mechanical strength of the resulting ion gels. Small angle X-ray scattering data reveal that the ion gels are composed of bicontinuous phase. The formation of the anisotropic HDODA structures upon introduction of the acrylate ionic liquid was accompanied by a change of the bicontinuous phase to be undulated, which increased the ionic path through the formed film, resulting in reduced ionic conductivity. Such coaxial structured gels may be a promising route for developing highly ion-conductive as well as mechanically stable solid electrolyte systems.
A series of dicationic imidazolium bis(tetrafluoroborate) salts were newly synthesized, and their phase transition behaviors were correlated with thermal, scattering, optical and conductivity results. The bisimidazolium salts having side-chain lengths of C 6 -C 10 showed plastic crystal mesophases, while a liquid crystal mesophase was formed in the bis-imidazolium salts with long side-chains (C 11 and C 12 ). Soft plastic and liquid crystalline phases were also confirmed by wide-angle X-ray diffraction. For the bisimidazolium salts exhibiting a plastic crystal mesophase, the ionic conductivity suddenly increased at the melting temperature. However, the bis-imidazolium salts with long side-chains showed a slope increase during the liquid crystal-liquid crystal transition. Results and discussionThe synthetic scheme of the dicationic imidazolium BF 4 À salts is shown in Fig. 1. The bromide salt of 1,2-bis(N-alkyl-a P DS ss is calculated from the summation of DS ss values at all T ss transitions. 16 b DS f ¼ DH f (J mol À1 )/T (K). 17 c C4 showed T g at À31 C on the second heating scan. d Two heat absorption peaks only on the rst heating scan but no heat absorption peak on the second heating scan.This journal is
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