In situ deuteron NMR investigations of sheared liquid crystalline polymers

The flow behavior of nematic liquid crystalline polysiloxanes of the side-chain type is studied by in situ 2H NMR spectroscopy on samples under shear in a cone-and-plate cell. The director orientation as a function of applied shear rate is determined from the quadrupole splitting of the spectra. The data analysis yields the two Leslie viscosity coefficients alpha2 and alpha3 and the flow-alignment parameter lambda = -(alpha3 + alpha2)/(alpha3 - alpha2). The values of lambda were determined for several homopolymers with only one type of side chain and random copolymers containing two different side chains. The results show that the flow behavior is related to the phase structure of the polymers, which varies with their composition. Only polymers with large amounts of smectic clusters in the nematic state show the tumbling instability (absolute value(lambda) < 1); other polymers are flow aligning (absolute value(lambda) > or = 1). For some polymers, a transition from tumbling at low temperature to flow aligning at high temperatures was observed.     

Prediction and observation of sustained oscillations in a sheared liquid crystalline polymer

Experimental observations of sustained oscillations of both shear stress and first normal stress differences are reported in flowing liquid crystalline polymers in a limited range of shear rates. The results can be described by considering the response of a rigid-rod model. Depending on the initial conditions, the frequency spectrum of the stress signal contains either one or two characteristic frequencies. This can be explained by the occurrence of either pure "wagging" or the coexistence of wagging and "log-rolling" behavior of the director.     

基于分子动力学模拟的主链型液晶聚合物的新模型

在传统的Gay-Berne (GB)/Lennard-Jones (LJ)模型的基础上，发展了一种用于模拟半刚性主链型液晶聚合物(LCP)的分子级模型，命名为Solo-LJ-SP-GB 模型.单一的LJ联合体和非线性弹簧被用于描述LCP分子中的间隔体.用分子动力学模拟半刚性主链型LCP系统(该系统由169条分子链组成，每两个刚性体之间的间隔体个数为6)时，该模型所需的计算时间不到传统的GB/LJ 模型所需时间的十分之一，大大地提高了计算效率.通过采用该模型模拟半刚性主链型LCP的相变问题，观察到了与半刚性主链型LCP分子中间隔体个数相关的热力学的奇偶效应以及从等方相到向列相的相转变过程.这些模拟结果与当前的试验结果相当符合，从而表明了该模型可以较为准确地描述出半刚性主链型LCP的结构特性. 关键词： Solo-LJ-SP-GB模型 液晶聚合物 分子动力学模拟     

Structural and stereochemical isomerism effects in thermotropic liquid crystalline polymers

Summary A critical survey is presented of the major contributions so far available on the influence of structural and stereochemical isomerism on the physical properties of thermotropic liquid crystalline polymers. The structure-property correlation in liquid crystalline polymeric materials is still far beyond to be satisfactorily rationalizable in terms of chemical and physical responses, as predictable at molecular level and modulable, from a practical point of view, at the stage of polymer formulation. In this connection it will be pointed out how isomerism effects in the polymer-repeating units can play sometimes a key role. For convenience, structural and stereochemical isomerism features shall be treated separately and the systems divided in ?side-chain? and ?main-chain? type, in accordance with the relative position of the mesogens within the repeating units. Work presented at the Second USSR-Italy Bilateral Meeting on Liquid Crystals held in Moscow, September 15–21, 1988.     

Effect of an external electric field on liquid crystalline polymers

The effect of an external electric field on the order parameter and on the isotropic-anisotropic phase transition temperature for semi-flexible liquid crystalline polymers is studied by a mean-field approximation. For the polymers whose electric dipole moments are parallel to the chain backbone, the critical transition temperature T_c is extensively changed by gDT ∼ ∥E∥², where E is the external electric field.     

Particle dynamics in sheared granular matter

The particle dynamics and shear forces of granular matter in a Couette geometry are determined experimentally. The normalized tangential velocity V(y) declines strongly with distance y from the moving wall, independent of the shear rate and of the shear dynamics. Local rms velocity fluctuations deltaV(y) scale with the local velocity gradient to the power 0.4+/-0.05. These results agree with a locally Newtonian, continuum model, where the granular medium is assumed to behave as a liquid with a local temperature [deltaV(y)](2) and density dependent viscosity.     

Early dynamics of guest-host interaction in dye-doped liquid crystalline materials

We have studied in detail the early dynamics of laser-induced molecular reorientation in a dye-doped liquid crystalline (LC) medium that exhibits a significant enhancement of the optical Kerr nonlinearity due to guest-host interaction. Experimental results agree quantitatively with theory based on a model in which the anisotropic dye excitation helps reorient the LC molecules through a mean-field intermolecular interaction.     

Thermo-and photo-driven soft actuators based on crosslinked liquid crystalline polymers

Crosslinked liquid crystalline polymers(CLCPs) are a type of promising material that possess both the order of liquid crystals and the properties of polymer networks.The anisotropic deformation of the CLCPs takes place when the mesogens experience order to disorder change in response to external stimuli; therefore,they can be utilized to fabricate smart actuators,which have potential applications in artificial muscles,micro-optomechanical systems,optics,and energyharvesting fields.In this review the recent development of thermo-and photo-driven soft actuators based on the CLCPs are summarized.     

Anisotropic microstructure-induced reduction of the Rayleigh instability for liquid crystalline polymers

We analyze a macroscopic 3D model for flows of liquid crystalline polymers (LCPs), deduced from Doi-type [3,4] kinetic equations. The Doi model accounts for rigid-rod microstructure, which introduces elastic relaxation and polymer-induced viscosity in addition to a Newtonian solvent viscosity, thus capturing all effects contained in standard isotropic viscoelastic models for Maxwell and Oldroyd B fluids. The rod-like microstructure further introduces anisotropic effects in the form of drag on the rods, together with a short-range, Maier-Saupe intermolecular potential, whose critical points vary with LCP concentration and yield stable isotropic (at low density) and nematic (at high density) equilibrium phases. From this single model, we compare various physical mechanisms for reducing the capillary instability of inviscid cylindrical jets: solvent viscosity as studied by Rayleigh and Chandrasekhar; isotropic viscoelasticity, both with and without Newtonian solvent viscosity; anisotropic polymer friction; and finally, the nematic, highly aligned prolate phase at high LCP density. Realistic parameter values for LCPs correspond to a regime in which the LCP capillary number (polymer bulk free energy relative to surface tension) is above an identified critical value; in such regimes, the unstable growth rates of the isotropic and nematic phases are lowered arbitrarily close to zero if the molecular drag is sufficiently anisotropic even in the absence of solvent viscosity. In low capillary number regimes, where surface tension dominates LCP bulk free energy, the LCP growth rates are sandwiched below the inviscid Rayleigh curve and above an explicit positive lower bound.     

GPU-accelerated molecular dynamics simulation for study of liquid crystalline flows

We have developed a GPU-based molecular dynamics simulation for the study of flows of fluids with anisotropic molecules such as liquid crystals. An application of the simulation to the study of macroscopic flow (backflow) generation by molecular reorientation in a nematic liquid crystal under the application of an electric field is presented. The computations of intermolecular force and torque are parallelized on the GPU using the cell-list method, and an efficient algorithm to update the cell lists was proposed. Some important issues in the implementation of computations that involve a large number of arithmetic operations and data on the GPU that has limited high-speed memory resources are addressed extensively. Despite the relatively low GPU occupancy in the calculation of intermolecular force and torque, the computation on a recent GPU is about 50 times faster than that on a single core of a recent CPU, thus simulations involving a large number of molecules using a personal computer are possible. The GPU-based simulation should allow an extensive investigation of the molecular-level mechanisms underlying various macroscopic flow phenomena in fluids with anisotropic molecules.     

Microscopic dynamics of recovery in sheared depletion gels

We report x-ray photon correlation spectroscopy and diffusing wave spectroscopy studies of depletion gels formed from nanoscale silica colloids in solutions of nonabsorbing polymer following the cessation of shear. The two techniques provide a quantitatively coherent picture of the dynamics as ballistic or convective motion of colloidal clusters whose internal motion is arrested. While the dynamics possesses features characteristic of nonergodic soft solids, including a relaxation time that grows linearly with the time since shear, comparison with behavior of quenched supercooled liquids indicates that this evolution is not directly related to traditional aging phenomena in glasses.     

Stochastic dynamics of a sheared granular medium

We experimentally investigate the response of a sheared granular medium in a Couette geometry. The apparatus exhibits the expected stick-slip motion and we probe it in the very intermittent regime resulting from low driving. Statistical analysis of the dynamic fluctuations reveals notable regularities. We observe a possible stability property for the torque distribution, reminiscent of the stability of Gaussian independent variables. In this case, however, the variables are correlated and the distribution is skewed. Moreover, the whole dynamical intermittent regime can be described with a simple stochastic model, finding good quantitative agreement with the experimental data. Interestingly, a similar model has been previously introduced in the study of magnetic domain wall motion, a source of Barkhausen noise. Our study suggests interesting connections between different complex phenomena and reveals some unexpected features that remain to be explained.     

Resonances in chaotic dynamics

We present a discussion and some numerical results on the actual possibility of making accessible, by numerical techniques, the complex singularities of the power spectrum (resonances) for a chaotic signal. Hénon's transformation is investigated in detail, showing that the position of the leading resonance in the complex frequency plane determines the kind of mixing rate in the time evolution.     

Structure and dynamics of liquid crystalline pattern formation in drying droplets of DNA

We investigate the formation of ringlike deposits in drying drops of DNA. In analogy with the colloidal "coffee rings," DNA is transported to the perimeter by the capillary flow. At the droplet edge, however, DNA forms a lyotropic liquid crystal (LC) with concentric chain orientations to minimize the LC elastic energy. During the final stages of drying, the contact line retracts, and the radial stress causes undulations at the rim that propagate inward through the LC and form a periodic zigzag structure. We examine the phenomenon in terms of a simple model based on LC elasticity.     

Tensile failure of crystalline polymers

In the creep experiment the brittle fracture of the unoriented semicrystalline polymers at very small and very high tensile load with the intermediate ductile region may be explained by the competition between crazing and shear band formation during the microcrack growth phase. The former type of microcrack growth leads to brittle fracture while the latter type yields necking which transforms the original lamellar structure into the final fibrous structure. The actual fate of the strained sample depends on the growth time of the craze, t_g, and of the shear band formation time, t_s. If t_g<t_s, the material will break in a brittle manner, and if t_g > t_s, the material will deform plastically. The failure of the fibrous material seems to occur when the ratio between the average distance and diameter of the microcracks reaches a value about 3. The microcracks seem to form primarily at defects of the microfibrillar structure, i.e., at the ends of microfibrils where the axial connection of subsequent crystal blocks through the amorphous layers by a great many taut tie molecules is either completely interrupted or at least drastically reduced. The stress concentration resulting from the opening of these defects into microcracks may rupture also some of the adjacent microfibrils. Such nucleation and subsequent lateral growth of the microcrack ruptures the taut tie molecules in its path. The ruptured molecules yield free radicals which can be monitored by electron spin resonance.     

Stochastic resonance in chaotic dynamics

It is suggested that chaotic dynamical systems characterized by intermittent jumps between two preferred regions of phase space display an enhanced sensitivity to weak periodic forcings through a stochastic resonance-like mechanism. This possibility is illustrated by the study of the residence time distribution in two examples of bimodal chaos: the periodically forced Duffing oscillator and a 1-dimensional map showing intermittent behavior.     

Attractor selection in chaotic dynamics

For different settings of a control parameter, a chaotic system can go from a region with two separate stable attractors (generalized bistability) to a crisis where a chaotic attractor expands, colliding with an unstable orbit. In the bistable regime jumps between independent attractors are mediated by external perturbations; above the crisis, the dynamics includes visits to regions formerly belonging to the unstable orbits and this appears as random bursts of amplitude jumps. We introduce a control method which suppresses the jumps in both cases by filtering the specific frequency content of one of the two dynamical objects. The method is tested both in a model and in a real experiment with a CO2 laser.     

Structural factors in deformation of crystalline polymers

The multiple α absorption of bulk-crystallized polyethylene (PE) was separated into the α₁ and the α₂ absorptions on the assumption that this α₂ absorption is associated with shear deformation of lamellar crystals, i.e., has the same characteristics as in single crystal mats. The separated α₁ mechanism is related to the molecular motions in the intermosaic block region. The α₁ process is very sensitive to static and dynamic deformation, whereas the α₂ process is not affected. Plastic deformation of bulk crystallized PE was analyzed in terms of true stress and true strain. The temperature dependence of the critical yield stress below 60°C showed the same magnitude of activation energy (26 kcal/mole) as that of α₁. The leading mechanism of deformation at lower temperatures is the breakdown of lamellar crystals into mosaic blocks. Compressive deformation of solid-state extrudates along the molecular axis, giving rise to kink bands, was analyzed with X-ray goniometry and in terms of the strain-rate dependence of the yield stress. The deformation of the crystals in the kink bands occurred by superposition of intercrystallite slip (α₁) and uniform shear deformation (α₂). It was concluded that consideration of intermosaic slip mechanisms (α₁), in addition to the shear deformation (α₂) and the interlamellar deformation (β), is effective and helpful to understand the deformation process of crystalline polymers.