Marian Smoluchowski Institute of Physics, Department of Statistical Physics and Mark Kac Center for Complex Systems Research, Jagiellonian University, ul. Łojasiewicza 11, 30-348 Kraków, Poland.
Faculty of Materials Engineering and Physics, Cracow University of Technology, ul. Podchorążych 1, 30-084, Kraków, Poland.
Phys Rev E. 2023 Mar;107(3-1):034707. doi: 10.1103/PhysRevE.107.034707.
We study the self-organization in a monolayer (a two-dimensional system) of flexible planar trimer particles. The molecules are made up of two mesogenic units linked by a spacer, all of which are modeled as hard needles of the same length. Each molecule can dynamically adopt two conformational states: an achiral bent-shaped (cis-) and a chiral zigzag (trans-) one. Using constant pressure Monte Carlo simulations and Onsager-type density functional theory (DFT), we show that the system consisting of these molecules exhibits a rich spectrum of liquid crystalline phases. The most interesting observation is the identification of stable smectic splay-bend (S_{SB}) and chiral smectic-A (S_{A}^{}) phases. The S_{SB} phase is also stable in the limit, where only cis- conformers are allowed. The second phase that occupies a considerable portion of the phase diagram is S_{A}^{} with chiral layers, where the chirality of the neighboring layers is of opposite sign. The study of the average fractions of the trans- and cis- conformers in various phases shows that while in the isotropic phase all fractions are equally populated, the S_{A}^{*} phase is dominated by chiral conformers (zigzag), but the achiral conformers win in the smectic splay-bend phase. To clarify the possibility of stabilization of the nematic splay-bend (N_{SB}) phase for trimers, the free energy of the N_{SB} and S_{SB} phases is calculated within DFT for the cis- conformers, for densities where simulations show stable S_{SB}. It turns out that the N_{SB} phase is unstable away from the phase transition to the nematic phase, and its free energy is always higher than that of S_{SB}, down to the transition to the nematic phase, although the difference in free energies becomes extremely small when approaching the transition.
我们研究了柔性平面三聚体粒子单层（二维系统）中的自组织。分子由两个介晶单元通过间隔基连接而成，所有这些单元都被建模为相同长度的硬针。每个分子可以动态地采用两种构象状态：无手性的弯曲形（顺式）和手性的锯齿形（反式）。使用恒压蒙特卡罗模拟和昂萨格型密度泛函理论（DFT），我们表明，由这些分子组成的系统表现出丰富的液晶相谱。最有趣的观察是稳定的向列型 S 型弯曲（S_{SB}）和手性向列-A（S_{A}^{}）相的识别。在仅允许顺式构象的极限情况下，S_{SB}相也是稳定的。占据相图相当大一部分的第二相是具有手性层的 S_{A}^{}相，其中相邻层的手性相反。研究各种相中反式和顺式构象的平均分数表明，在各向同性相中，所有分数的分布是均匀的，而 S_{A}^{*}相主要由手性构象（锯齿形）主导，但在向列型 S 型弯曲相中，无手性构象占据优势。为了澄清三聚物向列型 S 型弯曲（N_{SB}）相稳定的可能性，我们在 DFT 中计算了 N_{SB}和 S_{SB}相对于顺式构象的自由能，对于模拟显示稳定 S_{SB}的密度。结果表明，N_{SB}相在远离向列相转变的情况下是不稳定的，并且其自由能始终高于 S_{SB}，直到向列相转变，尽管在接近转变时自由能的差异变得极小。
