Size-Scale Sensitivity in Multiphase Systems with a Liquid Crystalline Phase

具有液晶相的多相系统中的尺寸尺度敏感性

• 批准号: 0112358
• 负责人: Morton Denn
• 金额: --
• 依托单位: CUNY City College
• 依托单位国家: 美国
• 项目类别: Continuing grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-09-15 至 2005-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0112358&HistoricalAwards=false
• 关键词: Size; Scale; Sensitivity; Multiphase; Systems

Abstract/ CTS-0112358 / Denn, Morton M / CUNY City CollegeSize -Scale Sensitivity in Mutliphase Systems with a Liquid Crystalline PhaseLiquid crystalline systems contain an intrinsic length scale, the correlation length over which the liquidcrystalline order is preserved; this scale is of the order of a few micrometers in nematic liquid crystalline polymers (LCPs). The liquid crystalline order can affect the interphase structure in a multiphase system, which in turn can affect macroscopic properties. Thus, the system behavior depends on the interaction between the different length scales: the molecular nanoscale characterizing the interfacial region, the mesoscales characterizing the correlation length and the phase dimensions, and the macroscopic scale on which properties are determined. Blends containing small amounts of a LCP in a matrix of a flexible thermoplastic are of considerable technological interest. It is possible to develop "self-reinforced" composites that exploit the outstanding tensile properties of fibers made from LCPs by creating a fibrillar morphology in the LCP dispersed phase. In addition, LCPs can act as "flow modifiers" for conventional thermoplastics, effecting a reduction in extrusion pressure at low concentrations. Our prior research has shown that the linear viscoelasticity of LCP blends is insensitive to droplets smaller than the nematic correlation length, and that the dynamics of LCP droplets do not follow the same scaling as droplets of flexible polymers. Furthermore, we have shown in preliminary Monte Carlo calculations that the interfacial tension between a LCP and a flexible polymer depends on the far-field nematic orientation in the droplet.The proposed research comprises three complementary approaches to elucidate the effect of multiple length scales on the mechanics of blends containing LCPs: continuum theory, Monte Carlo calculations, and experiments.Continuum theory. The LCP orientation distribution in the droplet will be a major factor in the deformation mechanics, and transitions between radial and bipolar orientations, which will affect the interfacial tension, are expected with droplet deformation. The Principal Investigator plans initially to use a perturbation analysis of a Leslie-Ericksen (LE) material with equal Frank elastic constants to analyze the first-order effect of droplet deformation. The LE fluid is the most elementary nematic model, and it is the asymptotic limit of more complete models for LCPs that are extensions of the Doi theory. Then the Principal Investigator will undertake a full three-dimensional finite-element solution of the equilibrium distribution in a deformed sphere for a LE material with unequal Frank coefficients. The purpose of this computation is to understand the likely orientation distributions in droplets after shearing. The time scale for droplet response defines the frequency range in which interfacial effects can be observed in blend rheology. The Principal Investigator will initially determine the response of a LE fluid, using an expansion in spherical harmonics. This is an important calculation in light of the experimental observations for blend linear viscoelasticity and droplet dynamics. The principal investigator expects the transient response of the droplet in the linear regime to depend on the Frank elasticity when the elastic stresses in a "domain" become comparable to the Laplace pressure. Available data do not show this dependence, but they do show a dependence on initial strain that is absent for droplets of a flexible polymer. This behavior needs to be rationalized by an analysis of the type proposed here. The principal investigator will repeat the Palierne analysis of viscoelastic blends for a monodomain liquid crystalline droplet, first using the LE theory for simplicity. The computed rheological properties are likely to depend on the anchoring conditions imposed on the nematic phase at the boundary, which will define the nematic order in the domain interior. The principal investigator expects the response to be sensitive to the Frank elasticity terms in the LE equation in the neighborhood of singularities. The LE theory is structurally similar to multi-domain theories for LCPs, so these calculations should provide a framework for interpreting experimentally observed differences between multidomain and monodomain droplets. The principal investigator anticipates a dependence of the storage modulus G' on a new dimensionless group involving the correlation length. The principal investigator will then carry out the comparable calculation for the Doi theory and relevant extensions.Monte Carlo Calculations. The Bond Fluctuation Model (BFM) will be used to study the effect of nematic order on the interfacial tension between a LCP droplet and an amorphous matrix, exploring chain length, energy parameters, and polydispersity. Modification of the BFM by developing a rational method for placing interpolated points on the lattice near the interface is required to account for the large size differences between LCP and flexible chain monomers. The effect of shear flow on the chain orientation near the interface and on the interfacial tension will be explored using a pseudopotential model for self-avoiding lattice chains. Experiments. Linear viscoelastic measurements intended to isolate the effect of the interface and Frank orientational elasticity from macromolecular effects will be carried out on blends containing low molar mass liquid crystals (LMMLCs). The principal investigators preliminary measurements suggest that the linear viscoelastic behavior of the LMMLCs may be unusual (negative G') and shear history dependent. Thus, the first step will be to elucidate the linear viscoelastic behavior and to analyze the results in the context of the predictions of LE and Doi theories. The principal investigator will then study the rheology of the blends, employing LMMLCs that exhibit a phase transition to obtain nematic and isotropic dispersed phases with equal viscosities and identical chemical compositions.

摘要/ CTS-0112358 / Denn, Morton M / 纽约市立大学具有液晶相的多相系统中的尺寸尺度敏感性液晶系统包含固有的长度尺度，即保持液晶顺序的相关长度；在向列液晶聚合物 (LCP) 中，该尺度约为几微米。液晶顺序可以影响多相系统中的相间结构，进而影响宏观性质。因此，系统行为取决于不同长度尺度之间的相互作用：表征界面区域的分子纳米尺度、表征相关长度和相尺寸的介观尺度以及确定性质的宏观尺度。在柔性热塑性塑料基体中含有少量 LCP 的共混物具有相当大的技术意义。通过在 LCP 分散相中形成原纤维形态，可以开发出“自增强”复合材料，利用 LCP 制成的纤维的出色拉伸性能。此外，LCP 还可充当传统热塑性塑料的“流动改性剂”，在低浓度下降低挤出压力。我们之前的研究表明，LCP 共混物的线性粘弹性对小于向列相关长度的液滴不敏感，并且 LCP 液滴的动力学不遵循与柔性聚合物液滴相同的缩放比例。此外，我们在初步蒙特卡罗计算中表明，LCP 和柔性聚合物之间的界面张力取决于液滴中的远场向列取向。所提出的研究包括三种互补方法，以阐明多个长度尺度对含有 LCP 的共混物力学的影响：连续介质理论、蒙特卡罗计算和实验。 理论。液滴中的LCP取向分布将是变形力学的主要因素，并且预计随着液滴变形，径向和双极取向之间的转变将影响界面张力。首席研究员最初计划使用具有相等 Frank 弹性常数的 Leslie-Ericksen (LE) 材料的扰动分析来分析液滴变形的一阶效应。 LE 流体是最基本的向列模型，它是更完整的 LCP 模型的渐近极限，是 Doi 理论的扩展。然后，首席研究员将对具有不等弗兰克系数的 LE 材料的变形球体中的平衡分布进行完整的三维有限元求解。此计算的目的是了解剪切后液滴中可能的方向分布。液滴响应的时间尺度定义了在共混流变学中可以观察到界面效应的频率范围。首席研究员将首先使用球谐函数展开来确定 LE 流体的响应。根据混合线性粘弹性和液滴动力学的实验观察，这是一个重要的计算。主要研究人员预计，当“域”中的弹性应力与拉普拉斯压力相当时，线性状态下液滴的瞬态响应取决于弗兰克弹性。现有数据并未显示出这种依赖性，但它们确实显示出对初始应变的依赖性，而柔性聚合物液滴则不存在这种依赖性。这种行为需要通过对此处提出的类型进行分析来合理化。主要研究人员将重复对单域液晶液滴的粘弹性混合物进行 Palierne 分析，为简单起见，首先使用 LE 理论。计算的流变特性可能取决于边界处向列相施加的锚定条件，这将定义域内部的向列序。主要研究者预计响应对奇点附近 LE 方程中的 Frank 弹性项敏感。 LE 理论在结构上类似于 LCP 的多域理论，因此这些计算应该提供一个框架来解释实验观察到的多域和单域液滴之间的差异。主要研究人员预计储能模量 G' 依赖于涉及相关长度的新无量纲群。然后首席研究员将对Doi理论和相关扩展进行可比计算。蒙特卡洛计算。键涨落模型 (BFM) 将用于研究向列序对 LCP 液滴和非晶基体之间界面张力的影响，探索链长、能量参数和多分散性。需要通过开发一种在界面附近的晶格上放置插值点的合理方法来对 BFM 进行修改，以解决 LCP 和柔性链单体之间的巨大尺寸差异。将使用自回避晶格链的赝势模型探索剪切流对界面附近的链取向和界面张力的影响。实验。线性粘弹性测量旨在将界面和弗兰克取向弹性的影响与大分子效应分开，将对含有低摩尔质量液晶 (LMMLC) 的共混物进行。主要研究人员的初步测量表明，LMMLC 的线性粘弹性行为可能不寻常（负 G'）并且依赖于剪切历史。因此，第一步将是阐明线性粘弹性行为并在 LE 和 Doi 理论预测的背景下分析结果。然后，主要研究人员将研究共混物的流变学，利用具有相变的 LMMLC 来获得具有相同粘度和相同化学成分的向列相和各向同性分散相。