We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer ...exhibiting a uniaxial nematic (N) to twist-bend nematic (N
) phase transition. The elastic constants for director splay (K
), twist (K
) and bend (K
) exhibit the relations K
> K
> K
and K
/K
> 2 over the bulk of the N phase. Their behavior near the N-N
transition shows dependency on the parity of the number (n) of the rigid mesomorphic units in the flexible n-mers. Namely, the bend constant K
in the dimer and tetramer turns upward and starts increasing close to the transition, following a monotonic decrease through most of the N phases. In contrast, K
for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K
increases pretransitionally in both even and odd n-mers, but more weakly so in the trimer, while K
increases steadily on cooling without evidence of pretransitional behavior in any n-mer. The viscosities associated with pure splay, twist-dominated twist-bend, and pure bend fluctuations in the N phase are comparable in magnitude to those of rod-like monomers. All three viscosities increase with decreasing temperature, but the bend viscosity in particular grows sharply near the N-N
transition. The N-N
pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the N
phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A* phase.
We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, ...1,7-bis-4-(4'-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, N_{TB}, and the oblique helicoidal cholesteric, Ch_{OH}, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the N-N_{TB} and N-to-isotropic (I) transitions. The splay constant K_{11} is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K_{33} and twist K_{22} constants are measured by DLS. K_{33}, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and N-N_{TB} pretransitional regions. The measured ratio K_{11}/K_{22} is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the N-N_{TB} transition.
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Odd-even effects, oscillations in properties of materials comprised of an odd or even number of connected repeating units, are well-known phenomena in materials science. In organic materials, they ...are usually associated with the number of methylene groups in aliphatic chains. In this work, we unveil multiple signatures of a new odd-even effect in liquid crystals that occurs at the larger scale of molecular moieties that by themselves express liquid crystalline behavior. We demonstrate that oligomeric liquid crystals, with
n
= 1-4 number of rigid mesogenic segments connected by flexible aliphatic chains with an odd number of methylene groups, produce an odd-even effect in optical anisotropy and the bend elastic constant of the liquid crystal oligomer. This effect is different from the usual odd-even effects with respect to the parity of carbon atoms in an aliphatic chain and can be understood in term of the average molecular shape and the associations between
n
-mers based on the packing of these shapes. We also show that, in spite of the fact that there is no long-range electron density modulation, careful analysis of synchrotron SAXS results can provide important information about the molecular associations in the N and N
TB
phases that other techniques cannot access. This novel odd-even effect opens up a new mode to optimize phase and optical behavior.
We demonstrate a novel oligomeric odd-even effect of mesogenic segments connected by flexible chains with an odd number of methylene groups.
We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer ...exhibiting a uniaxial nematic (N) to twist-bend nematic (N
TB
) phase transition. The elastic constants for director splay (
K
11
), twist (
K
22
) and bend (
K
33
) exhibit the relations
K
11
>
K
22
>
K
33
and
K
11
/
K
22
> 2 over the bulk of the N phase. Their behavior near the N-N
TB
transition shows dependency on the parity of the number (
n
) of the rigid mesomorphic units in the flexible
n
-mers. Namely, the bend constant
K
33
in the dimer and tetramer turns upward and starts increasing close to the transition, following a monotonic decrease through most of the N phases. In contrast,
K
33
for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant
K
22
increases pretransitionally in both even and odd
n
-mers, but more weakly so in the trimer, while
K
11
increases steadily on cooling without evidence of pretransitional behavior in any
n
-mer. The viscosities associated with pure splay, twist-dominated twist-bend, and pure bend fluctuations in the N phase are comparable in magnitude to those of rod-like monomers. All three viscosities increase with decreasing temperature, but the bend viscosity in particular grows sharply near the N-N
TB
transition. The N-N
TB
pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the N
TB
phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A* phase.
We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer exhibiting a uniaxial nematic (N) to twist-bend nematic (N
TB
) phase transition.
We use Dynamic Light Scattering to study fluctuation modes in the nematic and twist-bend nematic (NTB) phases of flexible dimers. Experimental results on fluctuation modes in the NTB phase of a ...dimer/monomer mixture show a nearly continuous N-NTB transition. We use our results to test various theoretical models of this phase transition, which are based on different "local" mechanisms for the twist-bend modulation and are "coarse-grained" to describe collective fluctuation modes at optical length scales. The wavevector and temperature dependences of these modes are described.We also discuss an experimental light scattering geometry for measuring the elastic constant for compression of the "pseudolayered" structure of the NTB phase, and we report results for the temperature dependence of this parameter. These results allow us to make a comparison to predictions from two different theoretical models that describe the development of the NTB phase.Finally, we extend our studies to higher n-mers - specifically, trimer and tetramer compounds, whichare homologous of the dimer investigated in our first project - that exhibit the textures and other tell-tale indications of a pseudo-layered NTB phase. We report measurements of the orientational elasticities and viscosities of the n = 2 - 4 homologous n-mers, focusing on their behavior near the N-NTB transition. We show that this pretransitional behavior agrees qualitatively with a "coarse-grained" analysis of local models for the NTB ordering process. This result confirms that on length scales large compared to the helicoidal pitch, the N - NTB transition has the same symmetry as the nematic to smectic-A transition.
We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, ...1,7-bis-4-(4-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rod-like cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist bend nematic, NTB, and the oblique helicoidal cholesteric, ChOH, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pre-transitional regions near the N-NTB and N-to-isotropic (I) transitions. The splay constant K11 is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K33 and twist K22 constants are measured by DLS. K33 being the smallest of the three constants, shows a strong non-monotonous temperature dependence with a negative slope in both N-I and N-NTB pretransitional regions. The measured ratio K11/K22 is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist and bend fluctuations in the N phase are comparable to those of nematics formed by rod-like molecules. All three show strong temperature dependence, increasing sharply near the N-NTB transition.
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