聚对氧化偶氮苯酚酯系列热致性液晶高分子的合成及液晶态研究

本文用界面缩聚法合成聚对氧化偶氧苯酚酯系列热致性液晶高分子。并用DSC,X-光衍射和偏光显微镜对其结构和液晶相进行了表征与研究。观察液晶态织态结构,计算液晶态时分子链间距,并观察聚合物取向液晶态的结晶结构。     

高分子相分离的分子动力学模拟

用粗粒化分子动力学(MD)模拟方法从分子层次研究两组分聚合物共混体系相分离过程中的动力学. 在相分离初期, 相区尺寸不随时间增加而变化; 在相分离中期, 相区尺寸与时间有很好的标度关系, 标度指数(α=1/3)符合Lifshiz-Slyozov提出的以扩散为主导的蒸发-凝聚机理的标度预测; 在相分离后期, 体系实现宏观相分离, 相区尺寸不再随时间改变而变化. 体积分数小的高分子链尺寸在相分离过程中先收缩再扩张, 在实现宏观相分离后, 高分子链尺寸又回到本体状态尺寸.     

新型液晶／高聚物复合膜的富氧性能及结构表征 Ⅱ．液晶／高聚物复合膜的结构表征

用ＩＲ、ＤＳＣ、ＷＡＸＤ、正交偏光显微镜、ＳＥＭ、ＴＥＭ等手段，表征了所合成的液晶／烃橡胶、硅橡胶三元共混复合膜的织态结构及液晶态转变，结果发现，液晶ＰＥＥＣＢ－４．０５由于与ＳＢＲ和ＰＤＭＳ共混，Ｔｋ－ｓ转变温度下降了３７．５℃，体系存在不完全的相分离，液晶ＰＥＥＣＢ－４．０５在体系中形成液晶富集区（ＬＣ－ｒｉｃｈ ｄｏｍａｉｎ），大小约０．２－２．２μｍ，在压力的作用下，连续的液晶富集区形成宽     

聚丙烯酸酯侧链液晶聚合物的合成与表征

以对羟基苯甲酸、氯乙醇和丙烯酸为主要原料,经醚化、酯化和酰氯化反应合成了中间体和含液晶基元的丙烯酸酯单体,后者经自由基聚合合成了聚丙烯酸酯侧链液晶聚合物。用偏光显微镜观察了单体和聚合物的织态结构,用DSC和IR对聚合物进行了表征。结果表明,单体和聚合物均呈现向列型液晶织态结构,聚合物在较宽的温度范围内有很好的液晶性。     

席夫碱型液晶冠醚的合成与性质

以4-(4'-烷氧基联苯基-4-羧基)苯甲醛和二氨基二苯并-14-冠-4为原料,通过溶液缩合反应,合成了一系列席夫碱型液晶冠醚.并用元素分析、旋光仪、核磁共振、红外光谱、快原子轰击质谱、基质辅助激光解析电离飞行时间质谱、示差扫描量热法和偏光显微镜对其进行了表征.发现化合物2,4,8I和8Ⅱ加热至各自的熔点以上都能形成液晶态,在液晶态可以观察到手性近晶C相、近晶相、胆甾相和向列相的典型织构.随分子末端烷氧基碳原子数增加,化合物(除2A和4A外)的熔点(Tm)和液晶态的清亮点(Ti)呈规律性变化,近晶相范围渐增,而近晶相-向列相转变温度和向列相温度范围递减.     

聚甲基丙烯酸甲酯/聚醋酸乙烯酯共混相分离过程的标度行为与逾渗结构

本文用激光光散射和光学显微镜方法研究了聚甲基丙烯酸甲酯/聚醋酸乙烯酯共混体系不稳相分离过程最大散射强度I_m(t,T)和相应波矢q_m(t,T)随时间变化规律及相区的逾渗结构.实验结果表明:I_m(T,t)和q_m(t,T)与时间t满足简单的标度关系I_m(t,T)～t~β,q_m(t,T)～t~(-α),且标度关系β=3α成立.揭示了相态结构的分维特征.给出了计算相态结构分维数的简便方法,其分维数D值约为1.64±0.03.与逾渗模型给出的D值接近.     

乙基醋酸纤维素溶致性液晶的研究

乙基醋酸纤维素溶于三氟乙酸、二氯乙酸、醋酸中.在浓度大于临界浓度时,形成胆甾型液晶态.溶液随浓度的增加,从各向同性态经两相共存转变成液晶态.在加热液晶溶液时,溶液转变成各向同性态.降低温度,液晶相再生成.在形成液晶过程中,存在过冷现象.在一定的条件下,液晶相以球状的形式存在.溶液的双折射△n在两相共存与完全液晶态间相互转变时会发生较大变化.乙基醋酸纤维素大分子链的刚性随有机酸溶剂体系的酸强度的增大而增大,使临界浓度C_1~*随溶剂酸强度的增大而减小.     

氨基改性纤维素纳米晶体的自组装行为研究

采用硫酸水解法制备纤维素纳米晶体(CNC),然后用环氧氯丙烷、氨水依次对其表面改性得到氨基改性的CNC (CNC-NH2),进一步采用蒸发诱导自组装(EISA)法制备CNC-NH2薄膜.通过偏光显微镜(POM)、原子力显微镜(AFM)、扫描电子显微镜(SEM)和流变测试对CNC-NH2悬浮液和薄膜的自组装行为进行研究.结果表明,CNC-NH2为棒状粒子,氨基的引入提高了其热稳定性.随着CNC-NH2悬浮液浓度增大,手性向列型液晶相逐渐形成,且干燥成膜后仍然能保持织态结构.流变测试表明,CNCNH2悬浮液显示独特的流变行为,可以观察到明显的相转变浓度和相转变温度.此外,考察了超声时间对CNC-NH2悬浮液成膜的影响.随着超声时间的延长,CNC-NH2膜由无色透明向虹彩色转变,且具有虹彩现象的膜对光的特殊波段产生吸收.     

乙基醋酸纤维素溶致性液晶的研究

非全取代的乙基纤维素在浓硫酸催化下与醋酸反应,生成乙基醋酸纤维素。乙基醋酸纤维素在适当的条件下可以形成溶致性液晶。随浓度的升高,溶液从各向同性态经两相共存,转变成完全的液晶态。在加热溶液时,可以看到两相共存状态的形成和液晶相的完全消失,降低温度,液晶相又可以再生成。在液晶相生成的过程中,存在过冷现象,液晶聚集的区域由许多微小的取向区域组成,它们的取向方向不相同。溶液的n—c曲线在各相同性、两相共存和液晶态区域内是直线,但在这些相态之间相互转变时出现转折点。溶液的双折射△n=ne—no在两相共存与液晶态相互转变时也会发生较大变化。乙基醋酸纤维素大分子链的刚性随有机酸溶剂体系的酸的强度增大而增大,使临界浓度C_1~*随溶剂酸的强度的增大而减小。     

氨基改性纤维素纳米晶体的自组装行为研究

采用硫酸水解法制备纤维素纳米晶体(CNC)，然后用环氧氯丙烷、氨水依次对其表面改性得到氨基改性的CNC (CNC-NH_2)，进一步采用蒸发诱导自组装(EISA)法制备CNC-NH_2薄膜.通过偏光显微镜(POM)、原子力显微镜(AFM)、扫描电子显微镜(SEM)和流变测试对CNC-NH_2悬浮液和薄膜的自组装行为进行研究.结果表明，CNC-NH_2为棒状粒子，氨基的引入提高了其热稳定性.随着CNC-NH_2悬浮液浓度增大，手性向列型液晶相逐渐形成，且干燥成膜后仍然能保持织态结构.流变测试表明，CNCNH_2悬浮液显示独特的流变行为，可以观察到明显的相转变浓度和相转变温度.此外，考察了超声时间对CNC-NH_2悬浮液成膜的影响.随着超声时间的延长，CNC-NH_2膜由无色透明向虹彩色转变，且具有虹彩现象的膜对光的特殊波段产生吸收.     

Slow dynamics of thin nematic films in the presence of adsorbed nanoparticles

Recent experiments indicate that liquid crystals can be used to optically report the presence of biomolecules adsorbed at solid surfaces. In this work, numerical simulations are used to investigate the effects of biological molecules, modeled as spherical particles, on the structure and dynamics of nematic ordering. In the absence of adsorbed particles, a nematic in contact with a substrate adopts a uniform orientational order, imposed by the boundary conditions at this surface. It is found that the relaxation to this uniform state is slowed down by the presence of a small number of adsorbed particles. However, beyond a critical concentration of adsorbed particles, the liquid crystal ceases to exhibit uniform orientational order at long times. At this concentration, the domain growth is characterized by a first regime where the average nematic domain size LD obeys the scaling law LDt approximately t1/2; at long times, a slow dynamics regime is attained for which LD tends to a finite value corresponding to a metastable state with a disordered texture. The results of simulations are consistent with experimental observations.     

Dichroism in dye-doped colloidal liquid crystals

Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.     

Density functional theory of liquid crystals and surface anchoring: hard Gaussian overlap-sphere and hard Gaussian overlap-surface potentials

This article applies the density functional theory to confined liquid crystals, comprised of ellipsoidal shaped particles interacting through the hard Gaussian overlap (HGO) potential. The extended restricted orientation model proposed by Moradi and co-workers [J. Phys.: Condens. Matter 17, 5625 (2005)] is used to study the surface anchoring. The excess free energy is calculated as a functional expansion of density around a reference homogeneous fluid. The pair direct correlation function (DCF) of a homogeneous HGO fluid is approximated, based on the optimized sum of Percus-Yevick and Roth DCF for hard spheres; the anisotropy introduced by means of the closest approach parameter, the expression proposed by Marko [Physica B 392, 242 (2007)] for DCF of HGO, and hard ellipsoids were used. In this study we extend an our previous work [Phys. Rev. E 72, 061706 (2005)] on the anchoring behavior of hard particle liquid crystal model, by studying the effect of changing the particle-substrate contact function instead of hard needle-wall potentials. We use the two particle-surface potentials: the HGO-sphere and the HGO-surface potentials. The average number density and order parameter profiles of a confined HGO fluid are obtained using the two particle-wall potentials. For bulk isotropic liquid, the results are in agreement with the Monte Carlo simulation of Barmes and Cleaver [Phys. Rev. E 71, 021705 (2005)]. Also, for the bulk nematic phase, the theory gives the correct density profile and order parameter between the walls.     

Molecular dynamics simulations of a hard sphere crystal and reaction-like mechanism for homogeneous melting

Molecular dynamics simulations of a hard sphere crystal are performed for volume fractions ranging from solidification point to melting point. A local bond order parameter is chosen to assign a nature, liquid or solid, to a particle. The probability for a liquid or solid particle to change state presents a typical sigmoid shape as the nature of its neighbors changes. Using this property, I propose a reaction-like mechanism and introduce a small number of rate constants. A mean-field approach to melting and a kinetic Monte Carlo algorithm on a lattice are derived from these chemical processes. The results of these models successfully compare with molecular dynamics simulations, proving that the main properties of melting can be captured by a small number of dynamical parameters.     

Computer simulation of chiral liquid crystal phases VIII. Blue phases of the chiral Gay-Berne fluid

The phase diagram of chiral calamitic liquid crystals was studied in the temperature-chirality parameter plane by means of computer simulation. Bulk systems composed of N = 2048 uniaxial chiral calamitic Gay-Berne molecules, i.e. with interactions described by the Gay-Berne potential and an additive term for the energy of the chiral interaction, were investigated using Monte Carlo (MC) simulations in the canonical ensemble (NVT). A rich polymorphism of chiral liquid crystal phases was observed along an isotherm with increasing chirality parameter describing the strength of the chiral interaction. In addition to the cholesteric phase (N*), for the first time a blue phase I (BP I) could be proven by computer simulation of a many-particle system based on model intermolecular interactions. Additionally, at high values of the chirality parameter, a phase with randomly oriented squirming double twist tubes was found as characteristic for the so-called spaghetti model for blue phase III (BP III). The structures of all phases were characterized by order parameters, a set of scalar and pseudoscalar orientational correlation functions, and especially by visual representations of selected configurations.     

Molecular simulation of liquid crystals: progress towards a better understanding of bulk structure and the prediction of material properties

This tutorial review covers recent progress in the field of computer simulation of liquid crystals. The development of the main "molecular-based" models for liquid crystals is described. These include lattice models, coarse-grained single site models based on hard and soft interaction potentials, atomistic models and multi-site coarse-grained models. A brief historical review is followed by an assessment of some of the new areas in this field, with an emphasis on understanding of molecular structure in liquid crystal phases and the prediction of bulk material properties. The article also looks to link the field of liquid crystal simulation with important developments in areas such as polymer simulation, lyotropic liquid crystals and model membranes.     

Orientational order in binary mixtures of hard Gaussian overlap molecules

Based on a standard constant-pressure Monte Carlo molecular simulation, we have studied liquid crystal phases of binary mixtures of nonspherical molecules. The components of the mixtures are two types of hard Gaussian overlap (HGO) molecules. The first type of molecule has a small molecularelongation parameter (short HGO molecules) and cannot form stable liquid crystal phase in the bulk by themselves. The second type of molecule has a large elongation parameter (long HGO molecules) and can form a liquid crystal phase easily. In the mixtures, the short HGO molecules can form an orientationally ordered phase because the long HGO molecules form confining surfaces to induce the alignment of the short molecules. We also study the isotropic-nematic phase transition in different mixtures composed of short and long HGO molecules with different elongations and concentrations. The obtained result implies that small anisotropic molecules can show liquid crystal behavior.     

Phase separation and gelation of polymer-dispersed liquid crystals

Polymerization-induced phase separation in polymer-dispersed liquid crystal is studied by computer simulations in two dimensions. The domain morphology resulting from phase separation is investigated by solving the coupled set of equations for the local volume fraction and the nematic order parameter, taking into account the viscoelastic effects and gelation due to polymerization. Comparing the morphology of phase separation by temperature quench, it is shown that the viscoelastic effects and gelation enable the polymer-rich phase to form a stable interconnected domain even when the polymer component is minority. The experimental evidence consistent with this characteristic feature is also given.     

Computer simulation of liquid crystals

Summary We review recent progress in the computer simulation of liquid crystals, with special emphasis on hard particle models. Surprisingly, the simplest molecular models, taking account only of molecular size and shape, are sufficient to generate a wide variety of liquid crystalline phases, closely analogous to those observed in real life. Thermodynamic stability of different phases is very sensitive to shape, and presumably will also be sensitive to further details of intermolecular interactions as they are incorporated into the model. Realistic atom-atom potential models of liquid crystals are available, but the associated simulations are quite expensive. Thus, while idealized models may be used to study quite general, fundamental properties of mesophases, the modelling of specific liquid crystal systems in a realistic way remains a great challenge. Progress continues to be made on both these fronts.