Liquid crystalline polymers and elastomers

The results of experiments and theory correlate increasingly well in the investigation of liquid crystalline polymers. Current work focuses on rheology and the behavior of blends of thermotropic LC polymers, due to reemerging economic interest in these materials. Research on liquid crystalline elastomers concentrates on potential applications such as artificial muscles and lasing.     

Liquid crystalline elastomers: dynamics and relaxation of microstructure

The equilibrium mechanical response of nematic elastomers can be soft or hard depending on the relation between the imposed strains and the nematic director, in particular, if the local nematic director is able to respond by rotating. The dynamical response proves to be equally unusual. We examine the linear dynamic mechanical response of monodomain nematic elastomers under shear and the aspects of time-temperature superposition of the dynamical data across phase-transition regions. In the low-frequency region of the master curves, one finds a dramatic reduction of rubber plateau modulus and the rise in internal dissipation: in the shear geometries compatible with dynamic soft elasticity. Power-law variation of the storage modulus with frequency G' proportional, variant omega(a) agrees very well with the results of static stress relaxation, where each relaxation curve obeys the analogous power law G' proportional, variant t(-a) in the corresponding region of long times and temperatures.     

苯乙烯型环硅氧烷交联PP的研究

含氢环硅氧烷与不饱和的1-辛烯在铂催化剂下发生硅氢加成反应,然后继续与二乙烯基苯发生硅氢加成反应,得到相应的含乙烯基苯基的环硅氧烷.利用HAAKE流变仪研究了此产品在PP加工过程中的交联作用,以1‰(wt)的DCP作为引发剂,当合成的有机硅交联剂的添加量达到2.5%时,PP具有一定的交联效果.     

热致液晶高分子与聚酰胺原位复合的研究进展

综述了近年热致液晶聚合物(TLCP)与聚酰胺(PA)的原位复合材料的最新进展,对原位复合材料的形态、力学性能、增容研究以及熔体的流变行为做了系统的阐述.此外,介绍了原位混杂增强复合材料,并简单讨论了TICP/PA的增强机理.     

Artificial muscles based on liquid crystal elastomers

This paper presents our results on liquid crystal (LC) elastomers as artificial muscle, based on the ideas proposed by de Gennes. In the theoretical model, the material consists of a repeated series of main-chain nematic LC polymer blocks, N, and conventional rubber blocks, R, based on the lamellar phase of a triblock copolymer RNR. The motor for the contraction is the reversible macromolecular shape change of the chain, from stretched to spherical, that occurs at the nematic-to-isotropic phase transition in the main-chain nematic LC polymers.We first developed a new kind of muscle-like material based on a network of side-on nematic LC homopolymers. Side-on LC polymers were used instead of main-chain LC polymers for synthetic reasons. The first example of these materials was thermo-responsive, with a typical contraction of around 35-45% and a generated force of around 210 kPa. Subsequently, a photo-responsive material was developed, with a fast photochemically induced contraction of around 20%, triggered by UV light.We then succeeded in preparing a thermo-responsive artificial muscle, RNR, with lamellar structure, using a side-on nematic LC polymer as N block.Micrometre-sized artificial muscles were also prepared. This paper illustrates the bottom-up design of stimuli-responsive materials, in which the overall material response reflects the individual macromolecular response, using LC polymer as building block.     

液晶多肽及其应用研究

基于国内外最新研究文献,系统论述了近年来液晶多肽的大分子链结构、液晶性能及应用前景等方面的研究进展。指出液晶多肽在一定条件下将出现各向同性态到各向异性态的可逆转变,伴随着大分子链从线圈状到螺旋状的可逆转变。聚[γ－苄基－L－谷氨酸酯]有序度参数可达0.875。二次谐波的产生证明了有些液晶多肽呈现奇异的胆甾相结构,其胆甾相由极性向列型片晶组成,且分子偶极矩和长轴方向平行。多肽既可显示溶致液晶性又可显示热致液晶性,其液晶相多为胆甾型,有时呈向列型。外加场如取向场及电场对多肽的液晶性也会产生强烈影响。液晶多肽可加工成液晶态水凝胶、纤维和薄膜等,可望用作全息照相材料、光学元件、彩色滤色器以及溶致液晶电池等。     

Investigation on magnetorheological elastomers based on natural rubber

Magnetorheological Elastomers (MR Elastomers or MREs) are a kind of novel smart material, whose mechanical, electrical, magnetic properties are controllable under applied magnetic fields. They have attracted increasing attentions and broad application prospects. But conventional MREs are limited to wide applications because their MR effects and mechanical performances are not high enough. This paper aims to optimize the fabrication method and to fabricate good natural rubber based MREs with high modulus by investigating the influences of a variety of fabrication conditions on the MREs performances, such as matrix type, external magnetic flux density, and temperature, plasticizer and iron particles. Among these factors, the content of iron particles plays a most important contribution in shear modulus. When the iron particle weight fraction is 80% and the external magnetic flux density is 1 T, the field-induced increment of shear modulus reaches 3.6 MPa, and the relative MR effect is 133%. If the iron weight fraction increases to 90%, the field-induced increment of shear modulus is 4.5 MPa. This result has exceeded the best report in the literatures researching the MREs on the same kind of matrix. The dynamic performances of MREs were also experimentally characterized by using a modified Dynamic Mechanical Analyzer (DMA) system. The effects of strain amplitude and driving frequency on viscoelastic properties of MREs were analyzed.     

Tear strength of elastomers over a range of rates,temperatures and crosslinking: tearing energy spectra

Measurement of tear strength of polyphosphazene (PNF), polychloroprene (CR) and styrenebutadiene rubber (SBR) has been carried out over a wide range of rates, temperatures and crosslinking. Data on strength of PNF have been found to be superimposable on a single master curve with the help of the WLF shift factor, indicating that enhancement of strength at high rates or low temperatures arises from simple viscoelastic loss mechanisms. The master curve may be termed as tearing energy spectra for a particular crosslinking density. Master curves at different elastic modulii are not superimposable. Strength of PNF is also much lower than that of SBR at a similar degree of crosslinking. This shows that mass per chain atom, which was important for predicting threshold strength is equally important under non-equilibrium conditions. Tearing energy spectra of filled SBR has also been obtained with the help of restricted tearing tests. Strength values obtained at various temperatures and rates for CR are, however, not superimposable on a master curve even with the help of special tests such as: (a) by partial swelling, (b) by doubling the curing agents, or (c) by peeling two layers apart. This anomaly is attributed to strain-induced crystallization of neoprene rubber on a molecular level.     

Mullins effect characterization of elastomers by multi-axial cyclic tests and optical experimental methods

Besides the typical hyperelastic behaviour, large elastic deformations with non-linear stress–strain behaviour, rubber-like materials may also exhibit some inelastic effects, like hysteresis and permanent set. One of them is a particular damage phenomenon called Mullins effect. This is visible when cyclic tension tests are performed with increasing values of deformation. Material is deformed up to a fixed strain value and then unloaded. When a second load is applied it is possible to observe a stress softening effect. In the present work uniaxial and equibiaxial tension tests have been carried out by a standard tensile machine and by an hydraulic bulge test experimental rig, respectively. In both tests optical methods have been used for strain measurement. Experimental data have been successively introduced in a numerical procedure that permitted to extract the best material parameters for two of the most known pseudo-elastic models [Ogden, R.W., Roxburgh, D.G., 1999. A pseudo-elastic model for the Mullins effect in filled rubber. Proceedings of the Royal Society London A 455, 2861–2877; Dorfmann, A., Ogden, R.W., 2004. A constitutive model for the Mullins effect with permanent set in particle-reinforced rubber. International Journal of Solids and Structures 41, 1855–1878] accounting for both stress-softening behaviour and residual strain.     

Thermal protection based on crystalline hydrates

Consideration is given to the possibility of using crystalline hydrates as a source of water, on whose evaporation most of the heat passing to the protected surface is expended. A version of the thermal protection based on silica fabric, whose pores are partly filled with a crystalline hydrate, is studied. Test results are presented. Experimental data are analyzed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 1, pp. 75–79, January–February, 2000.     

Thermomechanics of structures made of elastomers in cyclic loading on the basis of a finite-element model

Finite-element discretization of equations of thermoviscoelasticity in solving problems of dissipative heating of elastomers carried out. The FEM equations produced on the basis of the moment system of finite elements can be used to calculate elastomer structures with an arbitrary degree of compressibility. Examples of solving test problems are used to analyze the convergence of numerical solutions for linear, quadratic, and cubic loss of approximation of displacements. Calculations are carried out of the self-heating temperature in a trapezoidal shock absorber with a hole under complicated loading and also a shock absorber of AKS type. The effect of relaxation processes and geometrical dimensions on heating a solid cylindrical shock absorber is examined.Voroshilovgrad Agricultural Institute. Translated from Problemy Prochnosti, No. 10, pp. 74–82, October, 1989.     

环状分子交联剂的合成及其改性硅橡胶膜的富氧性能

通过控制2,4,6,8-四甲基环四硅氧烷(D4H)和乙烯基三甲氧基硅烷(VTMO)的摩尔比合成了环状分子上含有硅氢原子和三甲氧基硅基的交联剂,并用IR进行了表征.以乙烯基摩尔分数为10%的硅橡胶和上述交联剂为基质材料,以氯铂酸为催化剂,经溶液浇铸和通过双重交联反应──乙烯基与硅氢的加成反应和三甲氧基硅基的水解、缩合反应制备了改性硅橡胶气体分离膜.气体渗透的测试结果表明,该膜具有良好的富氧性能,例如,在20℃和0.05 MPa压差下,PO2为595 Barrer,αO2/N2高达2.89.改性硅橡胶膜的成膜过程和交联反应可在室温下同时进行,与其它改性硅橡胶膜相比,不仅改性方法简单,且分离性能明显提高.     

基于动态共价交联网络自修复涂层的研究进展

涂层是材料抵御外界应力损伤的重要屏障,且随着科技的发展,智能涂层可在原先的基础上赋予涂层荧光、抗菌、检测、传感等先进功能。然而,此类涂层在使用过程中不可避免会受到机械破坏(如擦痕、刮伤等)及与内部各组分应力不匹配引发的宏观或者微观损伤,导致裂纹甚至开裂,结构损伤会引起功能的减弱甚至消失。因此,对涂层结构稳定性和功能持续性提出了更高要求。基于动态共价交联网络的自修复涂层是以动态共价键可逆反应为基础,在一定的外界刺激下建立原料分子与产物分子之间的热力学平衡,通过动态网络“重组”实现对涂层自修复。动态网络活化能的大小不仅能直接反映修复难易程度(反应速率),也能间接反映材料的力学表现。因此,本文将从化学热/动力学角度出发,分析自修复网络构筑与反应活化能之间的关系,并进一步概述基于动态共价交联网络的自修复涂层在传统涂料(层)、智能传感、光学变色、生物医药领域的应用。最后对动态共价自修复涂层目前存在的局限和未来的发展进行展望。     

New chiral liquid crystalline monomers, polymers, and elastomers derived from menthol derivatives: synthesis and mesomorphism

To investigate liquid crystalline properties and structure relationships of chiral compounds based on menthol, a series of new chiral monomers derived from menthol derivatives, a mesogenic crosslinking agent, the corresponding side chain homopolymers with siloxane backbone, and cholesteric elastomers were synthesized. The structures and purity of these chiral compounds obtained in this study were characterized using FTIR, ¹H NMR, and elemental analyses. The mesomorphism and thermal stabilities were investigated using differential scanning calorimetry, polarizing optical microscopy, thermogravimetric analysis, and X-ray diffraction. The selective reflection of light for the chiral monomers was studied with UV/visible/near IR. The effect of the crosslinking agent content on the phase transition temperatures of the elastomers was discussed. It was found that these chiral monomers seemed beneficial for the formation of the mesophases when a flexible spacer was inserted between the mesogenic core and the terminal menthyl groups. All obtained chiral monomers showed a cholesteric phase, chiral smectic C phase, and cubic blue phase. The crosslinking agent exhibited a smectic A (S_A) phase and nematic phase. However, their corresponding homopolymers with siloxane chains tended to form a lower order S_A phase. With an increase of the mesogenic rigidity, the melting temperatures, glass transition temperatures (T _g), and isotropic temperatures (T _i) of chiral monomers or homopolymers all increased. For the elastomers, general tendency was toward increased T _g and T _i with increasing the crosslinking agent content.     

Fiber-reinforced composites based on epoxy resins modified with elastomers and surface-modified silica nanoparticles

The properties of fiber-reinforced composites made using epoxy resin formulations can be improved using modified epoxy resins. As epoxies are inherently brittle, they are toughened with reactive liquid rubbers or core–shell elastomers. Surface-modified silica nanoparticles, 20 nm in diameter and with a very narrow particle size distribution, are available as concentrates in epoxy resins in industrial quantities for the past 10 years. Some of the drawbacks of toughening like lower modulus or a loss in strength can be compensated when using nanosilica together with these tougheners. Apparently, there exists a synergy as toughness and fatigue performance are increased significantly. Some of these improvements in bulk resin properties can be found for fiber-reinforced composites as well. In this article, the literature published in the last decade is studied with a focus on mechanical properties. Results are compared, and the mechanisms responsible for the property improvements are discussed. A relationship between the improvements of the fracture energy of the cured bulk epoxy resins and the fracture energy of the fiber-reinforced composites could be established.     

蒙脱土对硅橡胶基磁流变弹性体性能的影响

考察蒙脱土对硅橡胶基磁流变弹性的动态力学特性和磁流变性能的影响，通过调节体系中蒙脱土的含量优化硅橡胶磁流变弹性体的综合性能。研究结果表明，当弹性体中蒙脱土的含量在4．6％时，硅橡胶基磁流变弹性体／蒙脱土复合材料的模量比纯弹性体的模量提高了近345％，硅橡胶磁流变弹性体的磁致模量达到1.76MPa，磁流变效应为63．7％。     

磁流变弹性体主动式自调谐吸振器控制系统的研究

为了实现对一种基于磁流变弹性体的主动式自调谐吸振器的减振控制,提出了变步长寻优与反馈控制相结合的控制算法,设计了以TMS320F2812DSP为核心的控制器。利用以上控制系统,在多模态实验平台上进行了吸振器的减振实验研究,结果表明该控制系统能够独立控制磁流变弹性体主动式自调谐吸振器的减振,寻优时间小于15s,并且减振效果良好     

Investigations on activated alumina based domestic defluoridation units

Investigations were carried out on the defluoridation of fluoride-spiked ground water in domestic defluoridation units (DDU) with activated alumina (AA). Specific safe water yield (SSY) was determined as a function of AA amount and adsorbent depth. Reuse potential of exhausted AA was assessed by regenerating and reusing AA in multiple defluoridation cycles. High fluoride uptake capacity (FUC) from ground water matrix as well as retaining approximately 95% FUC up to five cycles showed the suitability of AA for defluoridation in DDU. SSY, liters of safe water/kg AA, was dependent on the AA amount and its depth. There was a significant decrease in SSY with the decrease in AA depth in different DDUs, even though the amount was maintained constant. The derived data from four DDUs, with 3-5 kg AA and depth ranging from 5 to 13 cm, showed that DDU design is one of the most important parameter to be considered for optimizing SSY.     

Molecular manipulation based on allosteric crown-appended units and related systems

Synthesis of many potentially useful molecular units by chemists promotes the development of methods for constructing supramolecular architectures. Indeed, programmed supramolecular interactions play crucial roles in the preparation of well-defined nanoscale assemblies. This review highlights successful approaches in designing for specific functions systematically. The incorporation and synergistic communication of conformation-switchable segments into molecules bears great promise. Allostery, which is often observed in naturally occurring proteins, provides a hint for molecular manipulation. In supramolecular chemistry crown ethers can serve not only as substrate-binding sites but also as function-tuning sites. With the combination of synthetic versatility and well-tailored design, diverse capabilities (molecular recognition, catalytic properties, chirality, actuation, and so on) become possible at the molecular or mesoscopic level. The synthetic preparation of advanced molecular units allows the systems to become smarter, and promise nanosystems with programmable functions. This review considers mainly the use of crown ethers which act as anchoring as well as allosteric sites, from the standpoint of design for molecular manipulation. It also gives a brief discussion of recent progress in molecular manipulation, based on related systems. The "synthetic systems" described in the present review should contribute to not only to more elaborate "chemical systems", but also to the evolving field of "molecular machinery", that can utilize these systems.