Cybotactic nematic phases of photoisomerisable hockey-stick liquid crystals

<p>New five-ring hockey-stick liquid crystalline materials with 4-bromoresorcinol as the central core unit and an azobenzene-based side arm were synthesised and their mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and under a triangular wave electric field. Additional structural modification was done by introducing a lateral fluorine atom in the terminal ring of one of the side arms. It is found that regardless of the alkyl chain length or the lateral fluorine substitution, all of the prepared materials are liquid crystalline exhibiting nematic phases composed of cybotactic clusters of the SmC-type (N_CybC) in addition to a monotropic SmC phase for the longest homologue.</p

Solid State Phase Transitions in Poly(ethylene oxide) Crystals Induced by Designed Chain Defects

We have used Cu­(I)-catalyzed azide–alkyne cycloaddition to synthesize a new series of poly­(ethylene oxide)­s having in the center of their chains two 1,2,3-triazole (TR) rings separated by (CH₂)_<i>n</i> spacers with 2 ≤ <i>n</i> ≤ 4 (PEO₁₁-TR-(CH₂)_<i>n</i>-TR-PEO₁₁). The degree of crystallinity obtained by temperature-dependent WAXS measurements indicates that only one out of the two PEO₁₁ chains of the three polymers forms a 7₂ helix upon cooling to −10 °C and crystallizes into a monoclinic unit cell known from PEO homopolymer. A solid-state phase transition occurs for all samples during heating below their melting temperature. Solid-state ¹³C MAS cross-polarization and single-pulse NMR spectroscopy indicate the complete incorporation of the chain defects into the PEO crystals (PEO-TR phase) during this transition. The 2D WAXS pattern of an oriented PEO₁₁-TR-(CH₂)₂-TR-PEO₁₁ sample generates a structural model where the crystal lattice of the initial PEO phase becomes highly distorted during the solid-state phase transition due to C–H···π interactions of the aromatic TR rings. Furthermore, an additional phase transition occurs for PEO₁₁-TR-(CH₂)₄-TR-PEO₁₁ after melting of the PEO-TR phase. This phase has complex characteristics; i.e., the typical 7₂ helix of PEO forms, but the two TR rings and the methylene groups of the alkyl spacer are in different chemical environments