High-frequency viscoelasticity of crosslinked actin filament networks measured by diffusing wave spectroscopy

We study the short-time relaxation dynamics of crosslinked and uncrosslinked networks of semi-flexible polymers using diffusing wave spectroscopy (DWS). The networks consist of concentrated solutions of actin filaments, crosslinked with increasing amounts of α-actinin. Actin filaments (F-actin) are long semi-flexible polymers with a contour length 1–100μm and a persistence length of 5–15μm; α-actinin is a small 200kDa homodimer with two actin-binding sites. Using the large bandwidth of DWS, we measure the mean-square-displacement of 0.96μm diameter microspheres imbedded in the polymer network, from which we extract the frequency-dependent viscoelastic moduli via a generalized Langevin equation. DWS measurements yield, in a single measurement, viscoelastic moduli at frequencies up to 10⁵Hz, almost three decades higher in frequency than probed by conventional mechanical rheology. Our measurements show that the magnitude of the small-frequency plateau modulus of F-actin is greatly enhanced in the presence of α-actinin, and that the frequency dependence of the viscoelastic moduli is much stronger at intermediate frequencies. However, the frequency-dependence of loss and storage moduli become similar for both crosslinked and uncrosslinked networks at large frequencies, G′(ω)∝G′′(ω)∝ω^0.75±0.08. This high-frequency behavior is due to the small-amplitude, large-frequency lateral fluctuations of actin filaments between entanglements. Received: 20 January 1998 Accepted: 12 February 1998     

Structure and linear viscoelasticity of flexible polymer solutions: comparison of polyelectrolyte and neutral polymer solutions

The current state of understanding for solution conformations of flexible polymers and their linear viscoelastic response is reviewed. Correlation length, tube diameter, and chain size of neutral polymers in good solvent, neutral polymers in θ-solvent, and polyelectrolyte solutions with no added salt are compared as these are the three universality classes for flexible polymers in solution. The 1956 Zimm model is used to describe the linear viscoelasticity of dilute solutions and of semidilute solutions inside their correlation volumes. The 1953 Rouse model is used for linear viscoelasticity of semidilute unentangled solutions and for entangled solutions on the scale of the entanglement strand. The 1971 de Gennes reptation model is used to describe linear viscoelastic response of entangled solutions. In each type of solution, the terminal dynamics, reflected in the terminal modulus, chain relaxation time, specific viscosity, and diffusion coefficient are reviewed with experiment and theory compared. Overall, the agreement between theory and experiment is remarkable, with a few unsettled issues remaining.     

Constitutive modeling of shape memory polymer based self-healing syntactic foam

In a previous study, it was found that the shape memory functionality of a shape memory polymer based syntactic foam can be utilized to self-seal impact damage repeatedly, efficiently, and almost autonomously [Li G., John M., 2008. A self-healing smart syntactic foam under multiple impacts. Comp. Sci. Technol. 68(15–16), 3337–3343]. The purpose of this study is to develop a thermodynamics based constitutive model to predict the thermomechanical behavior of the smart foam. First, based on DMA tests and FTIR tests, the foam is perceived as a three-phase composite with interfacial transition zone (interphase) coated microballoons dispersed in the shape memory polymer (SMP) matrix; for simplicity, it is assumed to be an equivalent two-phase composite by dispersing elastic microballoons into an equivalent SMP matrix. Second, the equivalent SMP matrix is phenomenologically assumed to consist of an active (rubbery) phase and a frozen (glassy) phase following Liu et al. [Liu, Y., Gall, K., Dunn, M.L., Greenberg, A.R., Diani J., 2006. Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modeling. Int. J. Plasticity 22, 279–313]. The phase transition between these two phases is through the change of the volume fraction of each phase and it captures the thermomechanical behavior of the foam. The time rate effect is also considered by using rheological models. With some parameters determined by additional experimental testing, the prediction by this model is in good agreement with the 1D test result found in the literature. Parametric studies are also conducted using the constitutive model, which provide guidance for future design of this novel self-healing syntactic foam and a class of light-weight composite sandwich structures.     

Modeling size-dependent thermo-mechanical behaviors of shape memory polymer Bernoulli-Euler microbeam

The objective of this paper is to model the size-dependent thermo-mechanical behaviors of a shape memory polymer(SMP) microbeam. Size-dependent constitutive equations, which can capture the size effect of the SMP, are proposed based on the modified couple stress theory(MCST). The deformation energy expression of the SMP microbeam is obtained by employing the proposed size-dependent constitutive equation and Bernoulli-Euler beam theory. An SMP microbeam model, which includes the formulations of deflection, strain, curvature, stress and couple stress, is developed by using the principle of minimum potential energy and the separation of variables together. The sizedependent thermo-mechanical and shape memory behaviors of the SMP microbeam and the influence of the Poisson ratio are numerically investigated according to the developed SMP microbeam model. Results show that the size effects of the SMP microbeam are significant when the dimensionless height is small enough. However, they are too slight to be necessarily considered when the dimensionless height is large enough. The bending flexibility and stress level of the SMP microbeam rise with the increasing dimensionless height, while the couple stress level declines with the increasing dimensionless height.The larger the dimensionless height is, the more obvious the viscous property and shape memory effect of the SMP microbeam are. The Poisson ratio has obvious influence on the size-dependent behaviors of the SMP microbeam. The paper provides a theoretical basis and a quantitatively analyzing tool for the design and analysis of SMP micro-structures in the field of biological medicine, microelectronic devices and micro-electro-mechanical system(MEMS) self-assembling.     

Modulus-switching viscoelasticity of electrorheological networks

To form an electrorheological network (ERN), semiconducting nanoparticles were embedded in a polymer that was cross-linked to restrict particle motion. The microstructure ranged from random to aligned, depending on the degree of field-induced particle alignment during chemical network formation. We investigated in detail the softness effects of the matrix, having a relatively low storage modulus, on the dynamic rheological behavior of the ERN and analyzed its anisotropy. The anisotropy of the microstructure was probed rheologically with the modes of small-amplitude oscillatory shear (loading perpendicular to the field direction) and compression (loading in the field direction). The storage shear modulus was found to be a function of the applied electric field, particle volume fraction, and the pre-alignment electric field strength during the cross-linking reaction of the matrix, which governs the thickness of particle columns and intercolumn distance. Nonlinear behavior at small strain (below 0.1%) was conspicuous; this nonlinear viscoelasticity was accompanied by only a limited deformation of ordered connectivity. Throughout this study, we fabricated the ERN with the highly controllable modulus-switching effect acting in a shear-mode operation. Managing this anisotropy of an ERN by the electrical and chemical process is important in the design of smart materials that will provide improved stability and mechanical strength compared with fluid-type electrorheological materials and faster response time compared with that of conventional charged polymer gel.     

面向火星探测的环氧形状记忆聚合物复合材料的性能研究

国家火星探测任务是建设航天强国进程中的重大标志性工程,是中国航天走向更远深空的里程碑工程.智能材料这种集材料、结构和功能于一体的先进材料将会对火星探测任务有所助力.形状记忆聚合物及其复合材料作为一种典型的智能材料,可在有效减轻载荷的同时实现自主变形,已经在地球同步轨道航天器的应用中崭露头角.因此有必要研究这种新型环氧基形状记忆聚合物复合材料应用于火星探测工程的可能性.首先,针对“天问一号”火星探测器的任务需求,设计了一个具有自释放功能的着陆平台国旗装置.其中的锁紧释放装置由碳纤维增强的形状记忆聚合物复合材料制成,分别从静态拉伸力学性能、动态热机械性能和形状记忆性能三个角度评估了空间辐照和长期存储对形状记忆聚合物复合材料的影响.其中,空间辐照包括γ射线和紫外射线,辐照剂量分别为5×10⁵ rad和23.6 kCal.长期存储分为低温-196℃、室温25℃和高温85℃存储30天,和低温-196℃存储457天两组实验.最后,从“祝融号”火星车所携带相机拍摄的照片可以看到五星红旗被成功释放,旗面平整、图案清晰.这说明所研究的环氧基形状记忆聚合物复合材料可成功应用于火星探测...     

Predicting the nonlinear hereditary viscoelasticity of polymers

A mathematical model for the nonlinear hereditary viscoelasticity of polymer materials is proposed to predict deformation processes of various complexity — from simple relaxation and simple creep to complex deformation-relaxation and reverse relaxation processes with alternative loading and unloading. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 6, pp. 147–157, November–December, 2007.     

Non-symmetric viscoelasticity of anisotropic polymer liquids

The liquid crystalline (LC) polymers are considered as anisotropic viscoelastic liquids with nonsymmetric stresses. A simple constitutive equation for nematic polymers describing the coupled relaxation of symmetric and antisymmetric parts of the stress tensor is formulated. For illustration of non-symmetric anisotropic viscoelasticity, the simplest viscometric flows of polymeric nematics in the magnetic field are considered. The frequency and shear rate dependencies of extended set of Miesowicz viscosities are predicted. Received: 23 March 1999/Accepted: 13 December 1999     

显式方法精确模拟形状记忆聚合物热力学行为

通过构建一个热耦合的多轴可压缩应变能函数,得到应力-应变、应力-温度和应变-温度之间的函数关系,建立形状记忆聚合物的本构方程.本文引入三个基于对数应变的不变量使得模型（i）可以模拟可压缩情况;（ii）适用于单轴拉伸和等双轴拉伸至少两个基准实验;（iii）多轴有效.通过显式方法（i）给出自由能和熵的具体表达,证明模型热力学定律;（ii）给出应变-应力,温度-应力以及,温度-应变的形函数具体表达.多轴模型在特定的情况下可以自动退化到各自的单轴情况. 通过调节形函数的参数,最终得到的模型结果和实验结果能够精确匹配.新方法建立的本构模型得到的结果能更加准确地指导形状记忆聚合物的工程设计。     

显式方法精确模拟形状记忆聚合物热力学行为

通过构建一个热耦合的多轴可压缩应变能函数,得到应力-应变、应力-温度和应变-温度之间的函数关系,建立形状记忆聚合物的本构方程.本文引入三个基于对数应变的不变量使得模型（i）可以模拟可压缩情况;（ii）适用于单轴拉伸和等双轴拉伸至少两个基准实验;（iii）多轴有效.通过显式方法（i）给出自由能和熵的具体表达,证明模型热力学定律;（ii）给出应变-应力,温度-应力以及,温度-应变的形函数具体表达.多轴模型在特定的情况下可以自动退化到各自的单轴情况. 通过调节形函数的参数,最终得到的模型结果和实验结果能够精确匹配.新方法建立的本构模型得到的结果能更加准确地指导形状记忆聚合物的工程设计。     

Nonlinear viscoelasticity of fat crystal networks

The rheology of fat crystal networks under linear shear deformations has been extensively studied due to its role in material functionality and sensory perceptions. In contrast, there has been limited focus on their viscoelastic response under large shear deformations imposed during processing and product use. We probed the nonlinear viscoelastic behavior of fats displaying mechanics akin to ductile and brittle solids using large amplitude oscillatory shear (LAOS). Using the FT-Chebyshev stress decomposition method, and local measures of nonlinear viscoelasticity, we obtained rheological properties relevant to bulk behavior. We found that ductile fats dissipate more viscous energy than brittle fats and show increased plastic deformation. Structural characterization revealed the presence of three hierarchy levels and layered microstructures in ductile fats in contrast to only two hierarchies and random microstructures in brittle fats. We suggest that these structural features account for increased hypothesize dissipation, which contributes to their ductile-like macroscopic behavior.     

Some thermal and rheological properties of homogeneous interpenetrating polymer networks

Summary Homogeneous interpenetrating polymer networks, (IPNs), consisting of methacrylic and epoxy networks, were obtained at various compositions from a simultaneous polymerization of DGEBAMA and DGEBA. For each composition, the glass transition occurs at a well-defined temperature which is lower than the weighted average of the glass transition temperatures of each component. Tensile experiments showed a change of mechanical behaviour above some critical strain value. This phenomenon was corroborated by stress relaxation tests which allowed the determination of a complete relaxation below a critical strain. This strain is increasing with the temperature and decreasing with the crosslink density. Such a property disappeared after the addition of grafting molecules which prevented both networks from any relative sliding. In this way this behaviour appears to be a specific property of interpenetrating networks.With 10 figures     

Thermomechanical behavior of shape memory polymer beams reinforced by corrugated polymeric sections

With the ever-increasing applications of smart actuators, great attention is drawn to the SMP-based composites. In this paper the flexural behavior of corrugated SMP composite beams is studied. Employing a widely-accepted one-dimensional SMP model, based on the assumptions of Euler–Bernoulli beam theory, the governing equations are obtained. Further using a finite difference scheme the equations are solved. In this regard, different types of corrugated sections (rectangular, sinusoidal, …) with equal SMP content are studied and the mechanical properties of interest (load capacity, shape fixity, …) are compared. It is observed that reinforced single-cell patterns have completely different mechanical behaviors. For the sake of generality, the single-cell reinforced composite sections are studied in detail. Numerical results show an increase in load capacity of the structure. However, like any other reinforcing method, an inevitable small decrease in shape fixity is observed. In addition to the properties studied here, other desirable characteristics can be achieved by introducing a reinforcing cover. The results can be utilized in designing SMP-based actuators since the present work proposes a simple and efficient method for enhancing the load capacity of the SMP-based composites.     

The role of viscoelasticity in polymer sintering

An experimental study for polymer sintering has been carried out using pairs of powder particles. Although in many cases Newtonian sintering models successfully describe polymer sintering, they predict a faster coalescence rate than that observed with the polypropylene copolymer resins used in this study, indicating that factors other than the surface tension and the viscosity play a role in polymer sintering. Observations of coalescence under the microscope and rotational molding experiments suggest that melt elasticity slows down the process. Based on these findings, a mathematical model describing the complete polymer sintering process for viscoelastic fluids has been developed. The approach was similar to that of Frenkel (1945) and the convected Maxwell constitutive equations were used together with the quasi-steady state approximation. The proposed viscoelastic sintering model is capable of predicting the sintering rate slowdown observed in this study. Received: 18 August 1997 Accepted: 30 March 1998     

Confined thin film wrinkling on shape memory polymer with hybrid surface morphologies

Acta Mechanica Sinica - With appropriate stimuli, such as heat, humidity, or magnetic field, shape memory polymers (SMPs) can recover to their original shapes from temporary, programmed states....