Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
J Chem Phys. 2010 Oct 14;133(14):144911. doi: 10.1063/1.3503602.
We present a systematic dissipative particle dynamics (DPD) study on the phase behavior, structure, and dynamics of rodlike mesogens. In addition to a rigid fused-bead-chain model with RATTLE constraint method, we also construct a semirigid model in which the flexibility is controlled by the bending constant of k(φ). Using this notation, the rigid model has an infinite bending constant of k(φ)=∞. Within the parameter space studied, both two kinds of models exhibit the nematic and smectic-A phases in addition to the isotropic and solid phases. All of the phase transitions are accompanied by the discontinuities in the thermodynamical, structural, and dynamical quantities and the hysteresis around the transition points, and are therefore first order. Note that the obtained solid state exhibits an in-layer tetragonal packing due to the high density. For the rigid model, the simulations show that the liquid crystal phases can be observed for mesogens with at least five beads and the nematic phase is the first one to appear. More importantly, the phase diagram of seven-bead-chain models is obtained as a function of k(φ) and temperature. It is found that decreasing the value of k(φ) reduces the anisotropy of molecular shape and the orientational ordering, and thereby shifts the liquid crystal phases to the lower temperature end of the phase diagram. Due to the different k(φ) dependence of phase transition temperatures, the nematic phase range exhibits a more marked narrowing than the smectic-A phase as k(φ) is reduced, implying that the flexibility has a destabilizing effect on the nematic and smectic-A phases. We also have investigated the anisotropic translational diffusion in liquid crystal phases and its temperature and flexibility dependence. In our study, we find that the phases formed, their statical and dynamic properties, as well as the transition properties are in close accord with those observations in real thermotropic liquid crystals. It is clear that both the rigid and semirigid models we used are valuable models with which to study the behavior of thermotropic liquid crystals using DPD algorithm.
我们呈现了一个关于棒状介晶的相行为、结构和动力学的耗散粒子动力学（DPD）系统研究。除了具有 RATTLE 约束方法的刚性融合珠链模型外，我们还构建了一个半刚性模型，其中灵活性由弯曲常数 k(φ)控制。用这个符号表示，刚性模型的弯曲常数 k(φ)是无穷大的。在所研究的参数空间内，这两种模型都除了各向同性相和固相之外，还表现出向列相和近晶 A 相。所有的相变都伴随着热力学、结构和动力学量的不连续性以及相变点周围的滞后，因此都是一级相变。请注意，由于高密度，所得到的固相表现出层内四方堆积。对于刚性模型，模拟表明对于至少有五个珠子的介晶可以观察到液晶相，并且向列相是首先出现的相。更重要的是，得到了七珠链模型的相图作为 k(φ)和温度的函数。结果表明，降低 k(φ)的值会降低分子形状的各向异性和取向有序性，从而将液晶相推向相图的低温端。由于相变温度的 k(φ)依赖性不同，随着 k(φ)的降低，向列相范围的变窄更为明显，表明灵活性对向列相和近晶 A 相具有失稳作用。我们还研究了液晶相中的各向异性平移扩散及其对温度和灵活性的依赖性。在我们的研究中，我们发现形成的相、它们的静态和动态性质以及相变性质与真实热致液晶的观察结果非常一致。很明显，我们使用的刚性和半刚性模型都是使用 DPD 算法研究热致液晶行为的有价值的模型。
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