Internal orientations in externally deformed nematic polymer liquid crystals

The system of linear polymer liquid crystal (PLC) macromolecules, each modeled by semiflexible chain of alternate connected flexible and stiff rodlike sequences, is externally deformed. As a result, two orientation phases for hard rods are generated. One of them is nematic N+ with orientation parameter s>0; this has the place in monomer liquid crystal (MLC) systems and in PLCs. The nematic N- phase with s>0 appears in deformed PLCs only. This causes the fact that orientation of PLC chains, as a whole, is generated also by the system deformation. A discussion of that effect is the main goal of this article. The change of orientation is also discussed dependent on changes of the system temperature and parameters of the chain structure, such as the liquid crystal component concentration, contour lengths of stiff and flexible parts, and internal interactions parameters. Average shape of PLC chain and the shape anisotropy are calculated and discussed.     

The elasticity of nematic networks

Nematic rubbers are composed of crosslinked polymer chains with stiff rods either incorporated into their backbones or pendant as side chains. When nematic effects axe strong, such rubbers exhibit discontinuous stress-strain relationships and spontaneous shape changes. We model such a rubber using Gaussian elasticity theory, including the nematic interaction via a mean field. Results are presented for the cases of uniaxial extension and compression. Under uniaxial extension the rubber can undergo a first order phase transition to a uniaxial nematic phase. Under uniaxial compression first or second order transitions are possible to genuinely biaxial nematic states with biaxial strains. When nematic effects are very small (i.e. T >> T_c where T_c is the nematic-isotropic phase transition temperature of the rubber) we postulate that the model is a good approximation to a conventional, non-nematic elastomer, and fit our model to data from an isoprene rubber.     

Polyelectrolyte networks: elasticity,swelling, and the violation of the Flory-Rehner Hypothesis

This paper discusses the elastic behaviour of polyelectrolyte networks. The deformation behaviour of single polyelectrolyte chains is discussed. It is shown that a strong coupling between interactions and chain elasticity exists. The theory of the complete crosslinked networks shows that the Flory-Rehner Hypothesis (FRH) does not hold. The modulus contains contributions from the classical rubber elasticity and from the electrostatic interactions. The equilibrium degree of swelling is estimated by the assumption of a c1-network.     

Prediction of the internal energy contribution of polydimethylsiloxane during elongation

The aminolysis can effectively introduce primary amine (NH2) groups onto polyester materials, enabling a variety of subsequent surface biofunctionalization reactions. However, less attention has been paid to the basic knowledge of aminolysis reaction in terms of reaction kinetics and its influences on materials properties. In this study, taking the widely used poly(ε-caprolactone) (PCL) as a typical example, the influences of diamines and solvent property on the surface -NH 2 density are firstly assessed by using X-ray photoelectron spectroscopy (XPS) and colorimetric analysis. Results show that smaller diamine molecules and nonpolar alcohols could accelerate the reaction. The reaction kinetics with 1,6-hexanediamine is further investigated as a function of temperature, reaction time, and diamine concentration. During the initial stage, the reaction shows a 1 st order kinetics with the diamine concentration and has an activation energy of 54.5 kJ/mol. Ionization state of the NH 2 groups on the PCL surface is determined, revealing that the pK a of NH 3 + (<5) is much lower than that of the corresponding diamine molecules in solution. After aminolysis, surface hydrophilicity of PCL membrane is significantly enhanced, while surface elastic modulus and average molecular weight are decreased to some extent, and others such as weight, surface morphology and bulk mechanical strength are not apparently changed. The introduced NH 2 groups are found to be largely lost at 37 o C, but can be mostly maintained at low temperature.     

Mixed Cross-Link Systems in Elastomers

Rubbers are long-chain molecules which are plastic in nature in the raw or unvulcanized state. Vulcanization is an irreversible process by which the predominantly plastic rubber is converted to predominantly elastic and a three-dimensional network structure through the anchoring between two polymer chains. Chemically, this is done by an intermolecular cross-linking reaction. Cross-linking increases the retractive force and reduces the amount of permanent deformation remaining after removal of the deforming force. These links between polymer chains may be chains of sulfur atom or atoms, carbon-carbon bonds, polyvalent metal ions, etc., depending on the nature of the vulcanizing system. Properties of elastomers depend on how efficiently this cross-linking has been achieved and also which types of cross-linking agents are used. For example, the retractive force resisting a deformation is proportional to the number of network supporting polymer chains per unit volume of elastomers [1]. Again the strength of elastomers depends on the stress relaxation mechanism in the cross-linked material [2]. The modulus of a vulcanizate is proportional to the number of cross-links formed, while the tensile strength normally passes through a maximum with an increase in the number of cross-links. The resilience, heat build-up, and fatigue properties depend on the chemical nature of the cross-links and on the chemical structure of the base polymer [3]. Different types of cross-links have both advantages and disadvantages with respect to technical properties. To cite one example, a sulfur cross-linking system produces good tensile strength but poor aging properties. On the other hand, carbon-carbon cross-links produce good aging properties but poor tensile properties. Thus there are reasons for using a mixed cross-link system in order to obtain the right compromise.     

A simple model of stiff and flexible polymer chain adsorption: the influence of the internal chain architecture

In this study, we investigated the process of random sequential adsorption of stiff and flexible polymer chains on a two-dimensional square lattice. The polymer chains were represented by sequence of lattice points forming needles, T shapes, and crosses as well as flexible linear chains and star-branched chains consisted of three and four arms. The Monte Carlo method was employed to generate the model systems. The percolation threshold and the jamming threshold were determined for all systems under consideration. The influence of the chain length and the chain architecture on both thresholds was calculated and discussed. The changes in the ordering of the system were also studied.     

Ferroelectric LC-elastomers

Ferroelectric liquid crystalline elastomers combine the rubber elasticity of polymer networks with liquid crystalline phases and ferroelectric ordering. Ferroelectric switching leads therefore to a deformation of the polymer network and an elastic stress. The coupling between both effects can be varied by changing the topology of the netpoints.     

Mechanical properties of reversibly cross-linked ultrathin polyelectrolyte complexes

Tensile properties of microcoupons of polyelectrolyte complex, formed by the multilayering method, were determined using a micromechanical analysis system. The degree of internal ion-pair ("electrostatic") cross-linking was reversibly controlled by exposure to salt solution of varying concentration, which "doped" counterions into the films, breaking polymer/polymer ion pairs in the process. Linear stress-strain behavior was observed for a poly(styrene sulfonate)/poly(diallyldimethylammonium) multilayer up to 2% deformation. The dependence of modulus on cross-link density could be rationalized well by classical theories of rubber elasticity, including some insight on the topology of polyelectrolyte complexes.     

Elasticity of model poly(oxypropylene) networks

The equilibrium stress-strain properties and the swelling behavior of moderately cross-linked model networks of poly(oxypropylene) were studied. Results were in general agreement with the theory of rubber elasticity due to Flory. However, data on the highly cross-linked networks (M?_c ≈ 725) could not be satisfactorily described by the recent theories of elasticity or swelling. This is believed to be primarily due to the marked non-Gaussian character of the very short network chains and the substantial chemical modification of the polymer by the cross-linking moiety which inevitably occurs at high cross-link densities.     

An alternative approach to rubber elasticity theory based on a lattice model

An alternative rubber elasticity theory for bulk elastomers has been developed. It is based on the model of a cubic lattice which is filled completely with long flexible chains. The dependence of the configurational entropy on the orientation factor of Hermans has been derived for a uniaxial homogeneous orientation of the system. With the assumption of a constant stress-optical coefficient, the dependence of the true stress Γ on the deformation λ = L/L₀, with L the length of the sample, can be described approximately as τ ∝︁ in λ both for uniaxial extension and compression, which agrees quite well with experimental results on bulk elastomers.     

聚合物单分子力谱的研究进展

在单分子水平研究聚合物体系的分子内及分子间相互作用, 对于揭示其结构-性能的关系, 进而实现对相应功能的调控极为重要. 基于原子力显微镜技术(AFM)的单分子力谱, 由于其操作简单且适用面广, 在单分子研究领域得到了广泛的应用. 本文概括了该技术在生物高分子及合成高分子体系的研究进展. 对于生物高分子体系, 主要介绍了核酸(DNA/RNA)、 蛋白质和多糖(淀粉)的单分子力谱研究及利用各自力学指纹谱对其它分子间的相互作用的研究. 对于合成高分子体系, 主要介绍了聚合物的一级结构与单链弹性的关系及溶剂和聚集态结构等对高分子单链力学性质的影响规律.     

橡胶高弹性大形变的唯象理论

通过简单的假定,由纯粹的数学论证推导出橡胶高弹性的储能函数公式,在橡胶大形变时,公式比统计理论的更能反映客观实际。     

Quantitative molecular characterization of cold-drawn semi-crystalline polymers. Case of polyethylene terephthalate

The influence of pre-orientation on the natural draw ratio achieved in necking has been studied for poly(ethylene terephthalate). The results are analysed with the aid of the non-Gaussian theory of rubber elasticity assuming that the amorphous phase behaves as a network formed by entanglements of chains. A model for separating the influence of amorphous and strain-induced crystalline regions on deformation is proposed and compared with the pseudo-affine model.     

Elastic properties and network structure in silicone gum vulcanizates

A model of a rubber network formed from finite, linear polymer chains is treated, using expressions from accepted elastic theory as well as relationships that have been less generally used or only implied. The resultant empirical mathematical expressions conform very closely to the predictions of the simple kinetic theory of rubber elasticity and are mathematically consistent, in that the major properties and measurements are interrelated, and apparent inconsistencies in the literature are reconciled. As a consequence, an empirical equation is developed for the relationship between elastic properties and network structure in silicone gum vulcanizates. This equation is then applied to the estimation of peroxide crosslinking efficiencies.     

Bond Scrambling and Network Elasticity

Dynamic networks (DNs) recently reported in the literature are based on cross‐linked supramolecular chains or on covalent chains with reversible bonds. As originally pointed out by Lehn, these networks should be regarded as dynamic materials exhibiting adaptive features due to continuous scrambling of their bonds and sequences. Results in the recent literature reveal that these networks undergo reversible long‐range deformation resembling that of rubber networks. The present analysis of this process in terms of the theory of composite networks is based on the expectation that the scrambling process should allow rupture of bonds in the undeformed state and their reformation in the stretched state. Accordingly, a permanent set of the resting length of DNs should generally be expected and set materials should retain long‐range elasticity relative to the set state. However, only a limited set is shown by DNs, implying that a strong memory of the initial network topology assists elastic recovery of the original dimensions. The analysis of reported experimental data further reveals that the stress‐strain dependence of dynamic networks accurately follows the classical rubber elasticity theory. In this respect, DNs show better rubber behavior than typical covalent networks. Consistently with theoretical predictions, this surprising finding suggests that bond scrambling relieves local strain constraints on the fluctuations of networks junctions and favors the recovery of the initial network topology. Scrambling therefore allows compliance under stress and enhanced recovery when stress is released. Unprecedented applications of these advanced materials thus become foreseeable.     

Elastic Polymer Coated Nanoparticles with Fast Clearance for 19F MR Imaging

¹⁹F magnetic resonance imaging (MRI) is a powerful molecular imaging technique that enables high-resolution imaging of deep tissues without background signal interference. However, the use of nanoparticles (NPs) as ¹⁹F MRI probes has been limited by the immediate trapping and accumulation of stiff NPs, typically of around 100 nm in size, in the mononuclear phagocyte system, particularly in the liver. To address this issue, elastic nanomaterials have emerged as promising candidates for improving delivery efficacy in vivo. Nevertheless, the impact of elasticity on NP elimination has remained unclear due to the lack of suitable probes for real-time and long-term monitoring. In this study, we present the development of perfluorocarbon-encapsulated polymer NPs as a novel ¹⁹F MRI contrast agent, with the aim of suppressing long-term accumulation. The polymer NPs have high elasticity and exhibit robust sensitivity in ¹⁹F MRI imaging. Importantly, our ¹⁹F MRI data demonstrate a gradual decline in the signal intensity of the polymer NPs after administration, which contrasts starkly with the behavior observed for stiff silica NPs. This innovative polymer-coated NP system represents a groundbreaking nanomaterial that successfully overcomes the challenges associated with long-term accumulation, while enabling tracking of biodistribution over extended periods.     

Relation between Energy Contribution to Elastic Force and Strain Dependence of Modulus for Polybutadiene Vulcanizates

It is well known that the modulus G = r/(λ - λ^?2) varies with deformation, thus deviating from the predictions of statistical theories of rubber elasticity which require it to be constant. It has also been found that there is a nonnegligible energy contribution to the elastic force. It is postulated that these two phenomena are related because both arise from energetic interaction between chains.

Based on the lateral order of chains indicated by x-ray fiber diagrams of elongated noncrystallizable elastomers, it is suggested that energetic interaction of chains is induced by strain orientation. Proportionality between these two is assumed. The orientation distribution functions of end-to-end vectors and of statistical chain segments are considered. The proportionality constants are determined from the energy contribution to the strain dependence of the coefficient of thermal expansion. With the aid of these constants the modulus, corrected for energy contribution, is calculated. The observed and calculated elongation dependence of G agree reasonably well.

It is concluded that an energy interaction between aligned chains can account for the deviation of the observed stress elongation relation from the predictions of entropy elasticity theories.     

Phase stability of colloidal spheres mixed with supramolecular rod-like polymers

Mixtures of colloids and supramolecular polymers exhibit stimuli-responsive phase behavior. In previous work (Peters and Tuinier, Physica A 510, 233 [2018]) the polymers were treated as fully flexible chains, while in experimental systems supramolecular polymers may have a certain degree of stiffness. Here we predict that for stiff rod-like supramolecular polymers phase separation can occur at much lower polymer concentrations than for flexible supramolecular polymers. Additionally, it is demonstrated that colloid–polymer interactions can significantly influence the equilibrium polymer size distribution, however this does not strongly affect the phase behavior of the mixture. At the low polymer concentrations at which the system already gets unstable, the effect of excluded volume interactions between polymers chains themselves is small. Finally, for an experimental system it is predicted that a variety of re-entrant phase transitions may be observed within a realistic temperature range as illustrated by a specific example.     

液晶弹性体刺激形变研究

开发可以通过外部刺激产生机械形变的人工致动材料是一个近年来的研究热点。其中,液晶弹性体因结合了聚合物网络的橡胶弹性和液晶的有序性而具有独特的性质,在热、光、电等的外界刺激下可以产生可逆的形状记忆效应。本文综述了液晶弹性体响应多种外界刺激产生各种形变的行为,主要介绍了有关热致形变液晶弹性体、电致形变液晶弹性体、化学刺激导致形变的液晶弹性体及光致形变液晶弹性体的研究进展,阐述了各类液晶弹性体产生形变的机理包括热致、电致和光致相转变,讨论了影响其响应性能的主要因素,并展望了这一领域的发展前景。     

基于弹性统计理论的硅橡胶纳米复合材料本构关系的建立

基于考虑了悬垂链的橡胶弹性统计模型,通过引入应变放大因子,建立了硅橡胶纳米复合材料的基于微观机制的本构关系,其中利用硅橡胶分子信息(分子量M、乙烯基含量wt_(Vi)%)、乙烯基反应程度(q)估算获得本构方程中的交联点间链段分子量(Mc),网络链(network strands)体积分数(Φ)等参数,通过拟合确定了与纳米粒子相关的部分参数(初始应变放大因子X_0,极限应变放大因子X_∞,衰减因子z),对掺杂白炭黑的单组分及长短链配合硅橡胶拉伸应力-应变数据进行拟合,在采用相同X_∞,z值情形下,拟合曲线仍能与实测值符合较好(拟合的Adj.R-Square值分别为0.99576、0.99596)。基于微观物理机制的本构关系能够成为联系微观分子结构参数与宏观应力的桥梁,本文工作有望为更有针对性地改进和优化硅橡胶的性能提供依据。