Odd–Even Effect on Nematic SmAd Phase Boundary and SmAd Structure in Homologous Binary Systems of Cyanobiphenyl Mesogens: 4-Alkyl-4′-cyanobiphenyl (nCB) and 4-Alkoxy-4′-cyanobiphenyl (nOCB)

Molecular aggregation in liquid-crystalline layers crucially affects their physics: smectic A (SmA)-nematic (N) phase transition

Recently, two molecular packing modes of the alkyl chain in smectic A (SmA) liquid crystal phases were revealed: normal and tilted types, which are named by the orientation relative to the layer normal. This study reveals the relationship between the packing mode and thermodynamic order of the SmA-nematic (N) phase transition. Two normal type and three tilted type mesogens were subject to thermodynamic and structural experiments. The DSC results showed that the SmA-N phase transitions of the normal and tilted types are of the second and first order, respectively. The analysis of the intensities of reflections in wide-angle X-ray diffraction related to the periodicity of the SmA layer yielded the distribution of the mass centers of molecules along the normal to the SmA layers. The resultant distribution offered a rationale for the correlation of the thermodynamic order of the SmA-N phase transition and molecular packing modes in the SmA phases based on the Meyer-Lubensky theory.

Aggregation structure of chiral cubic liquid crystals revealed by X-ray diffraction utilizing a new algorithm

Chiral aggregation structure spontaneously formed by achiral rodlike molecules, a long-time unsolved problem in liquid crystal science, has been clarified by applying a new crystallographic algorithm recently developed while utilizing aggregation characteristics of this type. Bicontinuously interwoven networks characterize it similarly to the neighboring Gyroid phase in a phase diagram against the alkyl chain length and temperature. However, the network connectivity is significantly different from the bicontinuous networks that have been either known for related compounds or assumed for this phase. The network is compatible with the homochiral arrangement of rodlike molecules with successive twists by a proper angle between adjacent junctions.

Effects of alkylthio groups on phase transitions of organic molecules and liquid crystals: a comparative study with alkyl and alkoxy groups

The effects of the alkylthio groups on the phase transition behavior of organic liquid crystal molecules were examined by comparing them with the effects of alkyl and alkoxy groups.

Crystal polymorphism of 8OCB liquid crystal consisting of strongly polar rod-like molecules

We report detailed experimental studies on the different crystal polymorphs of 8OCB (4′-octyloxy-4-cyanobiphenyl) obtained by cooling the sample from its melt. The 8OCB liquid crystal has a stable crystal phase which is known as commercial powder (CP) phase. In addition, it also exhibits various metastable crystal phases such as square-plate, needle phase and long parallelepiped (PP) phase. We identify the metastable crystal state often found on cooling the sample from its melt as PP phase which transforms to stable CP phase on aging at room temperature or by heating it above 312 K. Based on our experimental results, we find that the stable CP crystal phase of 8OCB is not a single homogeneous phase but is the coexistence of fibrillar nano-crystallites embedded in an amorphous phase. The heterogeneous nature of its stable crystal state is an intriguing feature of this pure compound consisting of relatively small rod-like molecules with a strong dipole moment along their long axis.

The studies on crystal polymorphism of 8OCB liquid crystal showing a metastable PP phase and a heterogeneous CP phase as the stable ground state.

Designing the disorder: the kinetics of nonisothermal crystallization of the orientationally disordered crystalline phase in a nematic mesogen

This article presents the molecular dynamics and solidification behavior of a 2,3-difluoro-4-propylphenyl 2,3-difluoro-4-(4-pentylcyclohexyl)benzoate nematic liquid crystal (5C4FPB3) observed by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). Polarized optical microscopy (POM) is also performed to confirm the phase transition temperatures. Our investigation reveals rare crystallization of the orientationally disordered crystal (ODIC) phase from the nematic phase and a glass transition of the crystal at cooling rates higher than 1 K min-1. The deconvolution of the dielectric spectra with derivative techniques is necessary because of the complex molecular dynamics in the crystalline phase. The BDS method enables us to capture the relaxation processes reflecting pre-crystallization molecular movements. The kinetics of nonisothermal crystallization is studied using the Ozawa, Mo, and isoconversional methods. The present studies suggest that the dominant factor of the crystal growth mechanism depends on the cooling rate. Two types of crystallization mechanisms are identified at cooling rates lower and higher than 5 K min-1. We design a diagram with crystallization and glass transition borders against the cooling rates. Estimations show that crystallization of the present compound can be bypassed at cooling rates higher than 78 kK min-1, at which a glass transition of the nematic phase occurs. We show various scenarios of the molecular order and the crystallization mechanism designed based on the process rate.

Molecular packing in two bicontinuous Ia3[combining macron]d gyroid phases of calamitic cubic mesogens BABH(n): roles in structural stability and reentrant behavior

1,2-Bis(4'-n-alkoxybenzoyl)hydrazine [BABH(n), n is the number of carbon atoms in the alkyl chain] exhibits two different Ia3[combining macron]d cubic phases depending on the chain length (5 ≤n≤ 13 and 15 ≤n≤ 22). The molecular packing modes, not only of molecular cores but also of alkyl chains, are investigated through a maximum entropy method (MEM) and an analysis of the chain-length dependence of the body diagonal of the unit cell. The analyses revealed the difference in molecular packing. The short-chain Ia3[combining macron]d structure of BABH(n) (5 ≤n≤ 13) is constructed by single-layered core motifs and bilayers of alkyl chains, in which the orientation is tilted from the normal to the layer. The long-chain Ia3[combining macron]d structure (15 ≤n≤ 22) is formed by double-layered core motifs and monolayers of alkyl chains, in which the orientation is normal to the layer. Based on the molecular packing modes, the reentrant behavior between the two Ia3[combining macron]d phases was clarified. It was revealed that the alkyl chains of BABH(n) molecules play an essential role in the formation and stability of the two Ia3[combining macron]d structures.

Shape segregation in molecular organisation: a combined X-ray scattering and molecular dynamics study of smectic liquid crystals

Temperature-dependent X-ray scattering studies have been carried out on 4-undecyloxy-4'-cyanobiphenyl (11OCB) and 4-(12,12-dimethyltridecyloxy)-4'-cyanobiphenyl (t-Bu-11OCB) in the smectic A phase, from which their layer spacings and translational order parameters were obtained. Marked differences between the layer structures of the two compounds were demonstrated, showing that the addition of the t-Bu group results in a higher translational order parameter and wider layer spacing for t-Bu-11OCB than 11OCB. Fully atomistic MD simulations of both compounds run for >1000 ns demonstrated the spontaneous formation of smectic mesophases from isotropic starting geometries, and experimental trends in order parameters and absolute layer spacings were shown to be replicated well. Further analysis showed that both the aromatic head-groups and the alkyl tail-groups exhibit interdigitation in the simulated smectic phases of both compounds, and the difference in the layer structures between 11OCB and t-Bu-11OCB could be attributed mainly to a shape segregation effect arising from the addition of the bulky t-Bu end-group to the alkyl chain.

Structure and molecular packing in smectic BCr and Ad phases of Schiff base liquid crystal compounds through the analyses of layer spacing, entropy and crystal structure

Single-crystal structural analyses and heat capacity measurements were performed on two Schiff base liquid crystal compounds, 5CBAA (4-chlorobenzylidene-4'-pentyloxyaniline) and 5ABCA (4-pentyloxybenzylidene-4'-chloroaniline). The alkyloxy-chain of a 5CBAA molecule was conformationally ordered in the crystal at room temperature. While that of 5ABCA was partially disordered in the room temperature phase but ordered in a low-temperature phase at 100 K. The structural phase transition involving the disordering of the conformation was observed at 107 K in the heat capacity of 5ABCA. Both compounds showed two liquid crystalline phases, SmB_Cr and SmA_d. The net entropy change associated with the chain disordering was essentially the same in them despite the difference in the orientation of their central -CH[double bond, length as m-dash]N- moiety. The layer-spacings of SmB_Cr and SmA_d phases were analyzed for their chain-length dependence in both series of mesogens (nCBAA and nABCA), as well as in the case of nBBAA (4-bromobenzylidene-4'-alkyloxyaniline). The results reveal that these smectic structures are composed of alternately stacked core- and chain-layers with an antiparallel arrangement of cores and a bent-form of molecules.

Examination of molecular packing in orthogonal smectic liquid crystal phases: a guide for molecular design of functional smectic phases

The reported layer spacings (d_smectic) of six homologues of mesogens exhibiting orthogonal smectic phases (SmE, SmB, and SmA phases) are reexamined. The slopes of the linear dependences on chain length (n, the number of carbon atoms in the hydrocarbon chain) are clearly categorized into two groups: 1.9 Å (CH₂)^-1 and 1.4 Å (CH₂)^-1. It is clarified that in the former the molecules take a rod-like form (rod-form; category-I), whereas in the latter the molecules are bent around the connection between the core and chain moieties (bent-form; category-II). The average relative positions of adjacent molecules within the smectic structures are deduced from the intercept of the linear functions of d_smectic against n. The relation between and the features of molecules belonging to the two categories are discussed for molecular design of functional smectic liquid crystals.