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\chapter{\label{chapt:liquidcrystal}LIQUID CRYSTAL} |
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|
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\section{\label{liquidCrystalSection:introduction}Introduction} |
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|
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Long range orientational order is one of the most fundamental |
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properties of liquid crystal mesophases. This orientational |
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anisotropy of the macroscopic phases originates in the shape |
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anisotropy of the constituent molecules. Among these anisotropy |
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mesogens, rod-like (calamitic) and disk-like molecules have been |
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exploited in great detail in the last two decades\cite{Huh2004}. |
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Typically, these mesogens consist of a rigid aromatic core and one |
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or more attached aliphatic chains. For short chain molecules, only |
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nematic phases, in which positional order is limited or absent, can |
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be observed, because the entropy of mixing different parts of the |
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mesogens is paramount to the dispersion interaction. In contrast, |
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formation of the one dimension lamellar sematic phase in rod-like |
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molecules with sufficiently long aliphatic chains has been reported, |
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as well as the segregation phenomena in disk-like molecules. |
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|
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Recently, the banana-shaped or bent-core liquid crystal have became |
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one of the most active research areas in mesogenic materials and |
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supramolecular chemistry\cite{Niori1996, Link1997, Pelzl1999}. |
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Unlike rods and disks, the polarity and biaxiality of the |
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banana-shaped molecules allow the molecules organize into a variety |
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of novel liquid crystalline phases which show interesting material |
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properties. Of particular interest is the spontaneous formation of |
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macroscopic chiral layers from achiral banana-shaped molecules, |
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where polar molecule orientational ordering is shown within the |
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layer plane as well as the tilted arrangement of the molecules |
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relative to the polar axis. As a consequence of supramolecular |
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chirality, the spontaneous polarization arises in ferroelectric (FE) |
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and antiferroelectic (AF) switching of smectic liquid crystal |
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phases, demonstrating some promising applications in second-order |
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nonlinear optical devices. The most widely investigated mesophase |
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formed by banana-shaped moleculed is the $\text{B}_2$ phase, which |
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is also referred to as $\text{SmCP}$\cite{Link1997}. Of the most |
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important discover in this tilt lamellar phase is the four distinct |
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packing arrangements (two conglomerates and two macroscopic |
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racemates), which depend on the tilt direction and the polar |
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direction of the molecule in adjacent layer (see |
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Fig.~\ref{LCFig:SMCP}). |
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|
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\begin{figure} |
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\centering |
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\includegraphics[width=\linewidth]{smcp.eps} |
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\caption[] |
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{} |
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\label{LCFig:SMCP} |
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\end{figure} |
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|
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Many liquid crystal synthesis experiments suggest that the |
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occurrence of polarity and chirality strongly relies on the |
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molecular structure and intermolecular interaction\cite{Reddy2006}. |
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From a theoretical point of view, it is of fundamental interest to |
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study the structural properties of liquid crystal phases formed by |
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banana-shaped molecules and understand their connection to the |
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molecular structure, especially with respect to the spontaneous |
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achiral symmetry breaking. As a complementary tool to experiment, |
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computer simulation can provide unique insight into molecular |
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ordering and phase behavior, and hence improve the development of |
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new experiments and theories. In the last two decades, all-atom |
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models have been adopted to investigate the structural properties of |
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smectic arrangements\cite{Cook2000, Lansac2001}, as well as other |
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bulk properties, such as rotational viscosity and flexoelectric |
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coefficients\cite{Cheung2002, Cheung2004}. However, due to the |
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limitation of time scale required for phase transition and the |
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length scale required for representing bulk behavior, |
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models\cite{Perram1985, Gay1981}, which are based on the observation |
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that liquid crystal order is exhibited by a range of non-molecular |
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bodies with high shape anisotropies, became the dominant models in |
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the field of liquid crystal phase behavior. Previous simulation |
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studies using hard spherocylinder dimer model\cite{Camp1999} produce |
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nematic phases, while hard rod simulation studies identified a |
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Landau point\cite{Bates2005}, at which the isotropic phase undergoes |
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a direct transition to the biaxial nematic, as well as some possible |
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liquid crystal phases\cite{Lansac2003}. Other anisotropic models |
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using Gay-Berne(GB) potential, which produce interactions that favor |
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local alignment, give the evidence of the novel packing arrangements |
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of bent-core molecules\cite{Memmer2002,Orlandi2006}. |
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|
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Experimental studies by Levelut {\it et al.}~\cite{Levelut1981} |
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revealed that terminal cyano or nitro groups usually induce |
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permanent longitudinal dipole moments, which affect the phase |
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behavior considerably. A series of theoretical studies also drawn |
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equivalent conclusions. Monte Carlo studies of the GB potential with |
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fixed longitudinal dipoles (i.e. pointed along the principal axis of |
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rotation) were shown to enhance smectic phase |
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stability~\cite{Berardi1996,Satoh1996}. Molecular simulation of GB |
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ellipsoids with transverse dipoles at the terminus of the molecule |
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also demonstrated that partial striped bilayer structures were |
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developed from the smectic phase ~\cite{Berardi1996}. More |
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significant effects have been shown by including multiple |
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electrostatic moments. Adding longitudinal point quadrupole moments |
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to rod-shaped GB mesogens, Withers \textit{et al} induced tilted |
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smectic behaviour in the molecular system~\cite{Withers2003}. Thus, |
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it is clear that many liquid-crystal forming molecules, specially, |
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bent-core molecules, could be modeled more accurately by |
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incorporating electrostatic interaction. |
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|
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In this chapter, we consider system consisting of banana-shaped |
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molecule represented by three rigid GB particles with one or two |
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point dipoles at different location. Performing a series of |
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molecular dynamics simulations, we explore the structural properties |
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of tilted smectic phases as well as the effect of electrostatic |
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interactions. |
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|
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\section{\label{liquidCrystalSection:model}Model} |
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|
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A typical banana-shaped molecule consists of a rigid aromatic |
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central bent unit with several rod-like wings which are held |
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together by some linking units and terminal chains (see |
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Fig.~\ref{LCFig:BananaMolecule}). In this work, each banana-shaped |
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mesogen has been modeled as a rigid body consisting of three |
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equivalent prolate ellipsoidal GB particles. The GB interaction |
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potential used to mimic the apolar characteristics of liquid crystal |
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molecules takes the familiar form of Lennard-Jones function with |
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orientation and position dependent range ($\sigma$) and well depth |
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($\epsilon$) parameters. The potential between a pair of three-site |
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banana-shaped molecules $a$ and $b$ is given by |
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\begin{equation} |
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V_{ab}^{GB} = \sum\limits_{i \in a,j \in b} {V_{ij}^{GB} }. |
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\end{equation} |
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Every site-site interaction can can be expressed as, |
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\begin{equation} |
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V_{ij}^{GB} = 4\epsilon (\hat u_i ,\hat u_j ,\hat r_{ij} )\left[ |
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{\left( {\frac{{\sigma _0 }}{{r_{ij} - \sigma (\hat u_i ,\hat u_j |
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,\hat r_{ij} )}}} \right)^{12} - \left( {\frac{{\sigma _0 |
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}}{{r_{ij} - \sigma (\hat u_i ,\hat u_j ,\hat r_{ij} )}}} \right)^6 |
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} \right] \label{LCEquation:gb} |
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\end{equation} |
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where $\hat u_i,\hat u_j$ are unit vectors specifying the |
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orientation of two molecules $i$ and $j$ separated by intermolecular |
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vector $r_{ij}$. $\hat r_{ij}$ is the unit vector along the |
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intermolecular vector. A schematic diagram of the orientation |
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vectors is shown in Fig.\ref{LCFigure:GBScheme}. The functional form |
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for $\sigma$ is given by |
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\begin{equation} |
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\sigma (\hat u_i ,\hat u_i ,\hat r_{ij} ) = \sigma _0 \left[ {1 - |
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\frac{\chi }{2}\left( {\frac{{(\hat r_{ij} \cdot \hat u_i + \hat |
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r_{ij} \cdot \hat u_j )^2 }}{{1 + \chi \hat u_i \cdot \hat u_j }} |
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+ \frac{{(\hat r_{ij} \cdot \hat u_i - \hat r_{ij} \cdot \hat u_j |
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)^2 }}{{1 - \chi \hat u_i \cdot \hat u_j }}} \right)} \right]^{ - |
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\frac{1}{2}}, |
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\end{equation} |
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where the aspect ratio of the particles is governed by shape |
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anisotropy parameter |
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\begin{equation} |
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\chi = \frac{{(\sigma _e /\sigma _s )^2 - 1}}{{(\sigma _e /\sigma |
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_s )^2 + 1}}. |
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\label{LCEquation:chi} |
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\end{equation} |
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Here, $\sigma_ s$ and $\sigma_{e}$ refer to the side-by-side breadth |
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and the end-to-end length of the ellipsoid, respectively. The well |
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depth parameters takes the form |
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\begin{equation} |
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\epsilon (\hat u_i ,\hat u_j ,\hat r_{ij} ) = \epsilon _0 \epsilon |
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^v (\hat u_i ,\hat u_j )\epsilon '^\mu (\hat u_i ,\hat u_j ,\hat |
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r_{ij} ) |
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\end{equation} |
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where $\epsilon_{0}$ is a constant term and |
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\begin{equation} |
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\epsilon (\hat u_i ,\hat u_j ) = \frac{1}{{\sqrt {1 - \chi ^2 (\hat |
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u_i \cdot \hat u_j )^2 } }} |
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\end{equation} |
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and |
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\begin{equation} |
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\epsilon '(\hat u_i ,\hat u_j ,\hat r_{ij} ) = 1 - \frac{{\chi |
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'}}{2}\left[ {\frac{{(\hat r_{ij} \cdot \hat u_i + \hat r_{ij} |
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\cdot \hat u_j )^2 }}{{1 + \chi '\hat u_i \cdot \hat u_j }} + |
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\frac{{(\hat r_{ij} \cdot \hat u_i - \hat r_{ij} \cdot \hat u_j |
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)^2 }}{{1 - \chi '\hat u_i \cdot \hat u_j }}} \right] |
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\end{equation} |
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where the well depth anisotropy parameter $\chi '$ depends on the |
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ratio between \textit{end-to-end} well depth $\epsilon _e$ and |
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\textit{side-by-side} well depth $\epsilon_s$, |
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\begin{equation} |
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\chi ' = \frac{{1 - (\epsilon _e /\epsilon _s )^{1/\mu} }}{{1 + |
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(\epsilon _e /\epsilon _s )^{1/\mu} }}. |
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\end{equation} |
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|
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\begin{figure} |
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\centering |
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\includegraphics[width=\linewidth]{banana.eps} |
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\caption[]{} \label{LCFig:BananaMolecule} |
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\end{figure} |
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|
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%\begin{figure} |
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%\centering |
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%\includegraphics[width=\linewidth]{bananGB.eps} |
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%\caption[]{} \label{LCFigure:BananaGB} |
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%\end{figure} |
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|
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\begin{figure} |
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\centering |
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\includegraphics[width=\linewidth]{gb_scheme.eps} |
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\caption[]{Schematic diagram showing definitions of the orientation |
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vectors for a pair of Gay-Berne molecules} |
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\label{LCFigure:GBScheme} |
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\end{figure} |
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|
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To account for the permanent dipolar interactions, there should be |
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an electrostatic interaction term of the form |
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\begin{equation} |
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V_{ab}^{dp} = \sum\limits_{i \in a,j \in b} {\frac{1}{{4\pi |
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\epsilon _{fs} }}\left[ {\frac{{\mu _i \cdot \mu _j }}{{r_{ij}^3 }} |
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- \frac{{3\left( {\mu _i \cdot r_{ij} } \right)\left( {\mu _i \cdot |
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r_{ij} } \right)}}{{r_{ij}^5 }}} \right]} |
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\end{equation} |
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where $\epsilon _{fs}$ is the permittivity of free space. |
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|
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\section{Computational Methodology} |
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|
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A series of molecular dynamics simulations were perform to study the |
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phase behavior of banana shaped liquid crystals. In each simulation, |
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every banana shaped molecule has been represented three GB particles |
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which is characterized by $\mu = 1,~ \nu = 2, |
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~\epsilon_{e}/\epsilon_{s} = 1/5$ and $\sigma_{e}/\sigma_{s} = 3$. |
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All of the simulations begin with same equilibrated isotropic |
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configuration where 1024 molecules without dipoles were confined in |
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a $160\times 160 \times 120$ box. After the dipolar interactions are |
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switched on, 2~ns NPTi cooling run with themostat of 2~ps and |
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barostat of 50~ps were used to equilibrate the system to desired |
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temperature and pressure. |
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|
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\subsection{Order Parameters} |
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|
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To investigate the phase structure of the model liquid crystal, we |
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calculated various order parameters and correlation functions. |
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Particulary, the $P_2$ order parameter allows us to estimate average |
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alignment along the director axis $Z$ which can be identified from |
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the largest eigen value obtained by diagonalizing the order |
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parameter tensor |
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\begin{equation} |
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\overleftrightarrow{\mathsf{Q}} = \frac{1}{N}\sum_i^N % |
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\begin{pmatrix} % |
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u_{ix}u_{ix}-\frac{1}{3} & u_{ix}u_{iy} & u_{ix}u_{iz} \\ |
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u_{iy}u_{ix} & u_{iy}u_{iy}-\frac{1}{3} & u_{iy}u_{iz} \\ |
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u_{iz}u_{ix} & u_{iz}u_{iy} & u_{iz}u_{iz}-\frac{1}{3} % |
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\end{pmatrix}, |
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\label{lipidEq:po1} |
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\end{equation} |
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where the $u_{i\alpha}$ is the $\alpha$ element of the unit vector |
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$\mathbf{\hat{u}}_i$, and the sum over $i$ averages over the whole |
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collection of unit vectors. The $P_2$ order parameter for uniaxial |
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phase is then simply given by |
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\begin{equation} |
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\langle P_2 \rangle = \frac{3}{2}\lambda_{\text{max}}. |
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\label{lipidEq:po3} |
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\end{equation} |
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In addition to the $P_2$ order parameter, $ R_{2,2}^2$ order |
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parameter for biaxial phase is introduced to describe the ordering |
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in the plane orthogonal to the director by |
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\begin{equation} |
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R_{2,2}^2 = \frac{1}{4}\left\langle {(x_i \cdot X)^2 - (x_i \cdot |
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Y)^2 - (y_i \cdot X)^2 + (y_i \cdot Y)^2 } \right\rangle |
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\end{equation} |
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where $X$, $Y$ and $Z$ are axis of the director frame. |
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|
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\subsection{Structure Properties} |
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|
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It is more important to show the density correlation along the |
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director |
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\begin{equation} |
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g(z) =< \delta (z-z_{ij})>_{ij} / \pi R^{2} \rho |
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\end{equation}, |
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where $z_{ij} = r_{ij} \dot Z$ was measured in the director frame |
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and $R$ is the radius of the cylindrical sampling region. |
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|
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\subsection{Rotational Invariants} |
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|
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As a useful set of correlation functions to describe |
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position-orientation correlation, rotation invariants were first |
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applied in a spherical symmetric system to study x-ray and light |
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scatting\cite{Blum1971}. Latterly, expansion of the orientation pair |
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correlation in terms of rotation invariant for molecules of |
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arbitrary shape was introduce by Stone\cite{Stone1978} and adopted |
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by other researchers in liquid crystal studies\cite{Berardi2000}. |
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|
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\begin{eqnarray} |
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S_{22}^{220} (r) & = & \frac{1}{{4\sqrt 5 }}\left\langle {\delta (r |
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- r_{ij} )((\hat x_i \cdot \hat x_j )^2 - (\hat x_i \cdot \hat |
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y_j )^2 - (\hat y_i \cdot \hat x_j )^2 + (\hat y_i \cdot \hat |
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y_j )^2 ) \\ |
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& & - 2(\hat x_i \cdot \hat y_j )(\hat y_i \cdot \hat x_j ) - |
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2(\hat x_i \cdot \hat x_j )(\hat y_i \cdot \hat y_j ))} |
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\right\rangle |
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\end{eqnarray} |
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|
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\begin{equation} |
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S_{00}^{221} (r) = - \frac{{\sqrt 3 }}{{\sqrt {10} }}\left\langle |
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{\delta (r - r_{ij} )((\hat z_i \cdot \hat z_j )(\hat z_i \cdot |
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\hat z_j \times \hat r_{ij} ))} \right\rangle |
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\end{equation} |
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|
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\section{Results and Conclusion} |
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\label{sec:results and conclusion} |
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|
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To investigate the molecular organization behavior due to different |
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dipolar orientation and position with respect to the center of the |
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molecule, |
