The low-angle bent-shaped molecules with 1,7-naphthalene central core and alkylthio tails can form a novel hexagonal columnar phase and a dark B4 phase. The columnar phase has a large two-dimensional hexagonal lattice with edges of 65–75 Å and exhibits polar switching with spontaneous polarization along the column axis. Calculated from the density (∼1 g·cm-3) and unit volume, the number of molecules that are necessary to fill a 4.6-Å stratum of each column were found to be ∼11. Such a large number of molecules can be accommodated only in the tube-like assembly, which may be the first example as formed by the usual bent-shaped molecule with a single alkylthio tail.

A series of syndiotatic P5CPn polymethylenes was prepared with 4-(trans-4-pentylcyclohexyl)phenoxy moieties linked to each backbone carbon atom via an alkyloxycarbonyl spacer and with even numbers of alkyl carbons n ranging from 2 to 14, and their thermotropic behaviors were investigated. The P5CPn, except P5CP2, formed smectic phases in which the rod-like polymethylene backbones were arranged in rectangular lattices, and the sidechain mesogens were aggregated into layers parallel to the shorter sides of the rectangular lattices. The packing of the mesogens changed with decreasing temperature from smectic C-like to smectic I-like (SmI-like) for n = 4−8 and from smectic Alike to SmI-like for n = 10−14. In the SmI-like phases, each mesogen along the main-chain axis was connected to every sixth backbone carbon atom, revealing a correlation between the packing of the mesogens and the main-chain conformation of a 3/2 helix. Conversely, P5CP2 formed a smectic phase with the main chains arranged in rows and with the mesogens barely aggregated into layers due to the short spacers.

Solid-state 13C NMR measurements were made for the B2 and B4 phases of the achiral banana-shaped molecule, P-14-O-PIMB, which exhibits a direct transformation from the B2 phase to the B4 phase. In both phases, an NMR resonance signal assigned to carbonyl carbons of the ester linkages appears as doublet peaks, showing that the two carbonyl carbons are circumstanced in different electronic environments on the NMR time scale. The chemical shifts of the two peaks are 165.6 and 163.9 ppm in the B2 phase, and these values are not changed in the transformation to the low-temperature B4 phase. To explain this distinct splitting of the carbonyl carbon signal, we take three assumptions into account:  (1) the molecules are accommodated in the unique phase, but the conformational exchange between two states takes place slowly; (2) the two side wings of each molecule experience fast interconformational jumps, but their “average conformer” is different, thus giving two different peaks in the spectra; and (3) the individual molecule claims the twisted conformation, where the two carbonyl carbons of the ester moieties are twisted away from each other by rotating out of the molecular core plane with different dihedral angles. From the present 13C NMR results, the first and second possibilities are ruled out, and it is concluded that the banana-shaped molecules assume the twisted conformation, which is attributable to the origin of the chirality of the B2 and B4 phases in the achiral banana-shaped molecular system.

Bent-shaped molecules based on the oxadiazole central core with various side wings and terminal chain groups have been synthesized, and their liquid-crystalline behavior was investigated by optical microscopic, X-ray, and electrooptic measurements. These molecules exhibit liquid-crystal polymorphism including both the calamitic and banana phases. Such a characteristic polymorphism is attributable to the larger bend angle of the oxadiazole core compared to that of the resorcinol core used in conventional banana molecules. Only one type of banana phase, designated as the Bx phase, is formed. It appears upon cooling from the nematic and smectic liquid crystals and exhibits chiral domains with a very weak birefringence (apparently optically isotropic). By applying an electric field, the Bx phase is altered to a high-birefringence B2 phase with a homochiral SmC(A)P(A) structure that exhibits an antiferroelectric response. From detailed analyses of the optical texture and X-ray patterns through the transformation from well-oriented calamitic phases, the Bx phase was found to exhibit a helical structure, which arises as a frustration from the ground-state B2 phase in such a manner that the blocks of B2 layers are twisted with respect to each other in a direction parallel to the layer plane similarly to the twisted grain boundary (TGB) phase.

Recent works on polymer crystallization have given considerable attention to the possibility that the crystal phase in the final semicrystalline state could be preceded by transient liquid crystal phases. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] At this aspect, it is interesting to investigate the smectic liquid crystallization process of polymer in which the developments of positional order as well as the orientational one are required. However, the liquid crystallization from the isotropic melt is hardly followed with the conventional scattering techniques which have been applied for the polymer crystallization because it generally takes place close to equilibrium; i.e., it proceeds completely and rapidly even at small supercooling. Recently, we have found that the following main-chain smectic liquid crystalline BB-3(1-Me) polyester shows strong cooling rate dependence of the liquid crystallization temperature. 19

We have prepared a series of dimeric compounds, mOAM5AMOm, which comprise two Schiff's base mesogens. These compounds formed three types of fluid smectic liquid crystals. When the alkoxy tail length is short (m=4 and 6), the alkoxy tail groups mix randomly with the alkyl spacer groups, resulting in the SmCAs phase. The compounds with m longer than 14 form the SmCAb phase with a bilayer character, in which the spacer and tail groups segregate from each other and consequently two mesogenic layers (so-called bilayer) are included within a repeat unit. In such a bilayer association of twin dimers, spontaneous polarization appears in each bilayer. In this SmCAb phase, furthermore, the molecules are tilted to the layer so that chirality is spontaneously formed in addition to ferroelectricity. The structure was identified as SmCAPA type. The compounds with intermediate m of 8–12 formed the frustrated SmCAf phase with a density modulation (frustration) along the layer. On cooling from the smectic phases, crystallization takes place for mOAM5AMOm with m=4–8, while the chiral Crblue phase with the TGB-like helical structure was clearly identified with m=14–18. The structure and properties of these exotic phases in twin dimers are described in detail.

A series of an ABA triblock copolymer with amorphous polymethacrylate A blocks and a main-chain liquidcrystal (LC) polyester center block were prepared with the molecular weight of the amorphous blocks (M n am ) ranging from 2300 to 10 000 and that of the LC block kept constant at 10 000. Irrespective of asymmetric compositions, all block copolymers invariably formed lamellar microstructures. The LC segments were more extended perpendicular to the interface to form smectic layers parallel to the lamellae and folded to be accommodated in lamellae, whereas the amorphous segments had dimensions similar to those of segments in amorphous block copolymer microdomains. Increases in M n,am enlarge the interfacial area between the amorphous and LC segments and increase the number of folds in LC segments. Thus, LC lamellar thickness decreases and counterbalances the increase in amorphous lamellar thickness so as to suppress the lamellar spacing increase. When the LC segment was in the isotropic phase, the lamellae in the copolymers at asymmetric fractions were disordered and developed on cooling simultaneously with LC formation. The lamellar formation is attributed to the smectic layers that prefer a flat microdomain interface from an energetic perspective.

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