线性和非线性聚合物的相分离

分子共混物从20世纪60年代开始,引起很多科学家的兴趣.加工过程中,聚合物相分离过程中的热力学和动力学起了重要的作用,赋予了共混物一定的形貌,提供了很多人们需要的功能.关于聚合物的相分离问题,近年来无论在实验还是在理论上都得到了很大的发展.聚合物相分离的热力学是以Flory-Huggins的平均场理论为基础,动力学以CHC(Cahn-Hilliard-Cook)的线性理论为基础.在本文中,我们对亚稳相分离SD(Spinodal Decomposition)的热力学和动力学都进行了考察.描述了测量二元共混物的浓度涨落的静态散射实验,给出了在二元共混物体系的SD的近期理论计算.首先,从普适的Langevin方程推导CHC理论,从而给出CHC的唯象的平均场理论的微观图象.然后介绍模式耦合计算,从而将SD理论扩展到非线性区域.通过将短期SD分解与高分子共混物的对比揭示了定量计算的本质.最后,讨论了一些近年来在剪切场下相行为的研究以及该领域可能的发展方向.     

全息聚合物分散液晶的结构调控与性能

全息聚合物分散液晶(HPDLCs)是由富聚合物相与富液晶相周期性排列而成的结构有序高分子复合材料.HPDLCs通过单体/液晶复合体系的光聚合诱导相分离而形成,如何调控并定量化描述复合体系的光聚合反应动力学、凝胶化行为和相分离程度,进而获得结构规整、电光性能优异的HPDLCs是关键难题.专论概述了光引发体系、单体结构、纳米无机材料掺杂对HPDLCs结构及性能的影响.光引发阻聚剂通过引发和阻聚的竞争与协同,降低了光聚合反应速率、延迟了凝胶时间,促进形成衍射效率达90%的HPDLCs.超支化单体降低了复合体系黏度和光聚合反应速率,延迟了凝胶时间,促使形成衍射效率达94%、具有一维光子晶体结构的HPDLCs.丙烯酰胺单体优化了相分离结构,将HPDLCs的衍射效率提升至98%.纳米硫化锌掺杂在保持规整结构和高衍射效率的同时,大幅降低了HPDLCs的驱动电压.研究还确定了HPDLCs的相分离程度与凝胶时间的函数关系.构建兼具高衍射效率与低驱动电压的HPDLCs,推进其在彩色3D图像存储等领域的应用仍是重要课题.     

基于黏弹模型的聚合物溶液相分离的三维数值模拟

利用GPU加速技术实现黏弹模型的三维数值模拟,系统研究了中等浓度聚合物溶液的分相形貌和相分离动力学.模拟结果表明本体应力的不对称是体系出现相反转的重要原因.与实验观察的聚合物溶液相反转的演化过程一致,首先聚合物富集相形成网络结构,然后聚合物网络收缩断网,最后变为离散的液滴相.本体松弛模量的增加,一方面会抑制相分离初期的浓度涨落,导致相反转发生时间延迟;另一方面在相分离后期有助于形成大的聚合物富集液滴相,加速相区增长.剪切松弛模量的增加只有助于聚合物富集相保持网络结构,并在剪切松弛模量较大的情况下,不规则相结构的缓慢松弛导致了剪切应力在相分离后期长期存在.     

光致取向调控偶氮苯侧链液晶聚合物的阻隔特性研究

研究了液晶分子的排列方式对聚合物膜阻隔特性的影响,采用473 nm线偏振光照无定形偶氮液晶聚合物,使其介晶基元发生从无序到有序的取向排列.用膜透射率变化和锥光干涉图表征了分子的取向,其锥光干涉图为粗黑十字,说明在线偏振光下作用下液晶分子取向形成了单相畴沿面内排列的有序态.用金属表面氧化法进一步研究了取向态聚合物膜的阻隔...     

基于定向电纺纤维膜的可调制偏振片的制备

基于定向静电纺丝技术制备了高取向性有序纤维薄膜, 利用有序纤维对液晶分子取向的诱导, 构建了可调制散射型偏振片. 填充混合液晶的有序聚甲基丙烯酸甲酯纤维薄膜在可见光范围内, 表现出明显的偏光特性. 混合液晶中光敏性偶氮液晶4-正丁基-4'-甲氧基偶氮苯在360 nm紫外光照射下进行顺反异构转变, 诱导混合液晶发生从各向异性到各向同性的相变. 利用混合液晶光致相变与有序纤维的耦合, 实现了薄膜偏光特性的光控切换.     

氰乙基纤维素/二甲基乙酰胺液晶溶液的研究

本文利用Abbe折射仪,热台偏光显微镜,小角光散射等方法研究了氰乙基纤维素/二甲基乙酰胺液晶溶液的形成,形态结构和某些性质。溶液随浓度的增加,从各向同性态经过两相共存状态,转变成为单一的液晶态。该液晶是胆甾型的。溶液的双折射Δn随浓度的增加或温度的降低而增大。在无外力作用时,液晶相由许多无规分布的取向微区组成。液晶溶液受切应力后形成“条纹结构”,大分子链沿切应力方向取向,并在各条纹中排列有序。     

一个判别液晶聚合物状态的新方法

伴随液晶相转变而产生的转变焓、转变熵可提供有关各相的结构、状态及有序度等信息.本文从动力学、热力学角度分析了液晶聚合物从各向同性熔体冷却时系统的热熵变化,建立了液晶聚合物升降温过程中的热熵图,提出了定量判断液晶聚合物从各向同性熔体冷却后所处状态的新方法.     

二元聚合物刷体系相分离的计算机模拟

利用耗散粒子动力学(DPD)方法研究了二元聚合物刷体系的相分离行为. 发现组分间相容性的差别对膜厚的影响很小, 对纵向相分离结构中相区尺寸的影响则较大. 溶剂质量对控制聚合物刷膜的厚度和密度具有决定作用, 同时对层状膜结构也具有调控作用. 结果表明, 通过控制环境条件, 二元聚合物刷膜材料在快速且可逆地调节表面润湿性方面具有潜在的应用价值.     

向列型液晶与硬粒子两相体系的织态演化动力学研究

用二维XY模型和CDS元胞动力学方法研究了向列型液晶与少量硬粒子组成的两相体系的织态演化动力学. 通过模拟给出了织态演化过程的偏光图案, 其中纯液晶体系的偏光图案与实验观察到的结果一致; 并计算得到了空间相关函数和结构因子以及相区域的特征长度随时间t增长的标度关系. 同时统计了体系中缺陷点的数目, 得到了缺陷数目随时间t减少的标度关系. 发现粒子与液晶的相互作用将导致序参量有序化过程、 相区域增长速度和缺陷湮灭速度减慢.     

苯并菲盘状侧链液晶聚合物的大分子工程

盘状液晶聚合物兼具盘状液晶的光电性能和聚合物的柔韧性以及优异的成膜加工性能,有望发展成为新一代先进有机聚合物柔性光电功能材料.本文介绍苯并菲盘状侧链液晶聚合物的研究进展,主要结合我们研究组的工作,重点评述采用受控/活性自由基聚合方法可控合成这类侧链液晶聚合物以及对分子量效应和间隔基长度影响等基本问题的阐明.我们采用可逆加成-断裂链转移(RAFT)聚合第一次成功实现了分子量窄分布的苯并菲聚丙烯酸酯侧链盘状液晶聚合物的可控制备.首先提出盘单元局部簇集的分立短柱堆积(DCS)模型,合理解释了聚合度20左右出现的显著分子量效应,尤其揭示并提出了盘状侧链液晶聚合物的正耦合效应(PCE)理论,即短间隔基的较强耦合作用更有利于其有序柱状相的形成,与棒状侧链液晶聚合物经典的柔性长间隔基去耦合理论形成鲜明对照,补充了缺失的理论短板.基于这些原则,我们设计合成的丁氧基苯并菲聚丙烯酸酯侧链盘状液晶聚合物,经飞行时间谱(TOF)测试,表现出比文献报道值高1~2个数量级的载流子迁移率.进一步在手性客体分子掺杂诱导组装形成单手性螺旋结构聚合物复合物,以及拓扑受限环状聚合物和嵌段共聚物的受控制备等方面开展了比较系统的大分子工程实践.盘状侧链液晶聚合物的可控制备及其显著不同于棒状液晶聚合物体系的一些基本特征的阐明,为这类重要有机聚合物半导体材料的理性设计与可控合成提供理论指导,也为加速推进其光电器件化应用奠定基础.     

Simulation of phase‐separated structures of liquid‐crystalline polymer/flexible polymer blends

The phase‐separation kinetics of liquid‐crystalline polymer/flexible polymer blends was studied by the coupled time‐dependent Ginzberg–Landau equations for compositional order parameter ? and orientational order parameter S_ij. The computer simulations of phase‐separated structures of the blends were performed by means of the cell dynamical system in two dimensions. The compositional ordering processes of phase separation are demonstrated by the time evolution of ?. The influence of orientational ordering on compositional ordering is discussed. The small‐angle light scattering patterns are numerically reproduced by means of the optical Fourier transformation of spatial variation of the polarizability tensor α_ij, and the azimuthal dependence of the scattering intensity distribution is interpreted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2915–2921, 2001     

Phase and orientational ordering of low molecular weight rod molecules in a quenched liquid crystalline polymer matrix with mobile side chains

We study the phase diagram and orientational ordering of guest liquid crystalline (LC) rods immersed in a quenched host made of a liquid crystalline polymer (LCP) matrix with mobile side chains. The LCP matrix lies below the glass transition of the polymer backbone. The side chains are mobile and can align to the guest rod molecules in a plane normal to the local LCP chain contour. A field theoretic formulation for this system is proposed and the effects of the LCP matrix on LC ordering are determined numerically. We obtain simple analytical equations for the nematic/isotropic phase diagram boundaries. Our calculation show a nematic-nematic (N/N) first order transition from a guest stabilized to a guest-host stabilized region and the possibility of a reentrant transition from a guest stabilized nematic region to a host only stabilized regime separated by an isotropic phase. A detailed study of thermodynamic variables and interactions on orientational ordering and phases is carried out and the relevance of our predictions to experiments and computer simulations is presented.     

Orientational ordering of nanorods in diblock copolymers

ABSTRACT

Orientational ordering of rod-like nanoparticles in the lamellae phase of diblock copolymers has been considered theoretically using the model of a nanoparticle with two interaction centres. It has been shown that strongly anisotropic nanoparticles order spontaneously in the boundary region between the blocks where the orientational order is induced by the interface and by the interaction with monomer units in different blocks. The nematic order parameter possesses opposite signs in adjacent blocks which means that the nanorods are aligned parallel or perpendicular to the boundary between the blocks on different sides of their interface. Concentration and nematic order parameter profiles have been calculated numerically for different values of the nanoparticle length and compared with the results of recent computer simulations and with the results of the previous molecular theory based on nanoparticles of spherical shape.     

高分子液晶条带织构的计算机模拟

The relationship between the vector version and tensor version of Frank distortion energy for the liquid crystal was established, through which the relationship between the Frank elastic constants and the expansion coefficients of spatial derivatives for the orientational order parameter Sij of liquid crystalline polymer was obtained. Ginzberg-Landau equation for the orientational order parameter Sij was numerically solved by using the cell dynamical system, which involves a free energy functional containing Landau-de Gennes orientational free energy, Maier-Saupe anisotropic interaction free energy and Frank distortion free energy. Incase of the splay elastic constant being much lager than the bend elastic constant, the evolution process of band textures in liquid crystalline polymer during the shear relaxation and its small angle light scattering patterns were simulated. The influence of preshear rate on the band forming and band structure was investigated. It was found that the formation of longitudinal band textures is due to the much rapider longitudinal splay relaxation than the latitudinal bend relaxation, and the induced times of band forming and the characteristic length of band structure decrease with the increasing of preshear rate, which is consistent with the experimental results.     

Theoretical simulation of thermally induced phase separation in a main‐chain liquid‐crystalline polymer solution

Phase diagrams of main‐chain liquid‐crystalline polymer (MCLCP) solutions have been calculated self‐consistently on the basis of a simple addition of the Flory–Huggins free energy for isotropic mixing, the Maier–Saupe free energy for nematic ordering, and the Flory free energy for chain rigidity of the MCLCP backbone. The calculated phase diagram is an upper critical solution type overlapping with the nematic–isotropic transition. The phase diagram consists of liquid–liquid, liquid–nematic, and pure nematic regions. Subsequently, the dynamics of thermally induced phase separation and morphology development have been investigated by the incorporation of the combined free energy density into the coupled time‐dependent Ginzburg–Landau (model C) equations, which involve conserved compositional and nonconserved orientational order parameters. The numerical calculations reveal a variety of the morphological patterns arising from the competition between liquid–liquid phase separation and nematic ordering of the liquid‐crystalline polymer. Of particular interest is the observation of an inflection in the growth dynamic curve, which may be attributed to the nematic ordering of the MCLCP component, which leads to the breakdown of the interconnected domains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 913–926, 2003     

Self-organization of polymer dispersed nematic droplets

Self-organization of polymer dispersed nematic droplets has been observed experimentally. Self-organization is manifested in the spatial periodic distribution of drop clusters as well as in the orientational ordering of the symmetry axes of the droplet structures. The possible elastic and electrostatic mechanisms for the phenomena are discussed.     

Analysis of orientation fluctuation in fluids by small angle X-ray scattering

The small angle X-ray scattering of molecular fluids contains information on particular aspects of their orientational order. Examples are given for the case of the isotropic, nematic and cholesteric phases of mesogenic molecules. It is shown that the distribution of the molecular centers relative to the direction defined by the molecular long axes can be analysed by means of small angle X-ray scattering. An approximate expression for the circulation correlation function is given.     

Orientational ordering, conformation, and polarizability of molecules in the reentrant phase of discoid nematics

This paper presents data on orientational ordering in a reentrant discoid nematic and on changes in the polarizability of discogenic molecules in a homologous series. The phase transition of a nematic to the discotic column phase is accompanied by a jump of the orientational ordering parameter and is characterized by close relationship between the orientational and translational ordering of molecules. The lengthening of the flexible peripheral molecular chains in the reentrant nematic phase considerably changes their conformational state, decreasing the anisotropy of their polarizability. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 1, pp. 89–97, 1997.     

Stacking fault structure in shear-induced colloidal crystallization

We report measurements of the spatial distribution of stacking faults in colloidal crystals formed by means of an oscillatory shear field at a particle volume fraction of 52% in a system where the pair potential interactions are mildly repulsive. Stacking faults are directly visualized via confocal laser scanning microscopy. Consistent with previous scattering studies, shear orders the initially amorphous colloids into close-packed planes parallel to the shearing surface. Upon increasing the strain amplitude, the close-packed direction of the (111) crystal plane shifts from an orientation parallel to the vorticity direction to parallel the flow direction. The quality of the layer ordering, as characterized by the mean stacking parameter, decreases with strain amplitude. In addition, we directly observe the three-dimensional structure of stacking faults in sheared crystals. We observe and quantify spatial heterogeneity in the stacking fault arrangement in both the flow-vorticity plane and the gradient direction, particularly at high strain amplitudes (gamma> or =3). At these conditions, layer ordering persists in the flow-vorticity plane only over scales of approximately 5-10 particle diameters. This heterogeneity is one component of the random layer ordering deduced from previous scattering studies. In addition, in the gradient direction, the stacking registry shows that crystals with intermediate global mean stacking probability are comprised of short sequences of face-centered cubic and hexagonal close-packed layers with a stacking that includes a component that is nonrandom and alternating in character.     

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.