Modeling of particle‐dispersed melting mechanism and its application in corotating twin‐screw extrusion

Particle‐dispersed melting is a complex but important melting mechanism in the corotating twin‐screw extruder. In this study, the complex multi‐particle‐dispersed system was simplified into a single‐particle melting model. The finite‐difference method was introduced to solve this problem. The simulation results show that the melting of a particle may involve two steps: the heating stage and melting stage. The heating time and melting time depend on solid concentration, initial melt and solid temperature, and shear rate. Calculations indicate that high solid concentration and solid temperature, low melt temperature and shear rate will result in a more uniform temperature distribution after polymer melting. The model offers valuable information for designing the melting zone in a corotating twin‐screw extruder, especially at high screw speed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2461–2468, 2001     

Shear modulus determination in model hydrogels by means of elongated magnetic nanoprobes

The mechanical characterization of complex soft matter by quasi-static magnetometry using nanoscopic magnetic probes is demonstrated for model hydrogels doped with two types of elongated magnetic nanoparticles. Chemically crosslinked poly(acrylamide) (PAAm) hydrogels serve as the matrix in which nickel nanorods or weakly magnetized hematite (α-Fe₂O₃) ellipsoids are embedded as local probes. We investigated the swelling behavior of the ferrogels in order to verify that their equilibrium swelling degree in water is not influenced by the probes, shows a good correlation with the Frenkel–Flory–Rehner model. The proposed magnetomechanical method relies on a correlation between the shear modulus of the PAAm hydrogel matrix and the coercive fields of the corresponding isotropic ferrogels. By extending the Stoner–Wohlfarth model for single-domain blocked magnetic particles by a term for particle rotation in an elastic matrix, information on the shear modulus of the matrix can be obtained. Comparison of the results with the expected relation from rubber elasticity theory illustrates both the general potential as well as the limits of the approach. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Microscopic theory of gelation and elasticity in polymer-particle suspensions

A simplified mode-coupling theory (MCT) of ergodic-nonergodic transitions, in conjunction with an accurate two-component polymer reference interaction site model (PRISM) theory for equilibrium structural correlations, has been systematically applied to investigate gelation, localization, and elasticity of flexible polymer-hard particle suspensions. The particle volume fraction at the fluid-gel transition is predicted to depend exponentially on reduced polymer concentration and size asymmetry ratio at relatively high colloid concentrations. In contrast, at lower particle volume fractions, a power-law dependence on polymer concentration is found with effective exponents and prefactors that depend systematically on the polymer/particle size ratio. Remarkable power-law and near universal scaling behavior is found for the localization length and elastic shear modulus. Multiple experiments for gel boundaries and shear moduli are in good agreement with the no adjustable parameter theory. The one exception is the absolute magnitude of the shear modulus which is strongly overpredicted, apparently due to nonequilibrium dense cluster formation. The simplified MCT-PRISM theory also captures the qualitative aspects of the weak depletion-driven "glass melting" phenomenon at high particle volume fractions. Calculations based on an effective one-component model of structure within a low particle volume fraction framework yield qualitatively different features than the two-component approach and are apparently all in disagreement with experiments. This suggests that volume fraction and size asymmetry dependent many-body screening of polymer-mediated depletion attractions at finite particle concentrations are important.     

The scaling behavior of a highly aggregated colloidal suspension microstructure and its change in shear flow

The nature of the network structure and the evolution of structural change in shear flow were investigated for metal particle dispersions in terms of fractal aggregation of colloidal particles. Polymer-stabilized metal particle inks were prepared via a polyvinyl chloride coating dispersed in solvent. The fractal dimension of 1.74 was calculated with the scaling model based on the power law relationship between the elastic modulus and volume fraction. This scaling behavior can be explained by considering the deformable network structure of soft materials. While the elastic property of the floc was dominant, the limit of linearity was found at the inter-floc link, which is relatively weak and brittle. The steady shear results reveal two mechanisms that contribute to the breakdown of the microstructure in metal particle inks at increasing shear rate. Scaling of steady shear viscosity shows that these mechanisms are related to both inter-floc interactions and the elasticity of the floc itself. Further, these results suggest that individual flocs deform with weak inter-floc interactions and rupture into smaller flocs or aggregates at high shear stress, which is associated with the increased shear rate.     

Melting of non-spherical ultrafine particles

The behaviour of lead particles, with size up to 100 nm, embedded in a SiO matrix have been investigated versus temperature by dark field electron microscopy. When the particle is not spherical, it is shown that the melting process initiates on the surface regions of the particle where curvature is maximum. The process is continuous and reversible as far as a solid core exists. When the temperature increases, the volume of the solid core decreases and its shape evolves towards a sphere. Final melting of the core occurs as a first order transition (with size effect). These direct observations are in good agreement with previous results deduced from a high-sensitive reflectance experiment which gives an information averaged on the size and the state of the particles. Emphasis is laid on the dimension of the particle perpendicular to the substrate which is not generally obtained through electron microscopy measurements and appears here as an important parameter.     

接枝聚乳酸/塑化淀粉共混物的制备与性能研究

为提高聚乳酸(PLA)/淀粉共混物界面作用和降低成本,引入甲基丙烯酸缩水甘油酯(GMA)接枝聚乳酸和塑化淀粉(TPS),通过挤出和注射成型制备接枝聚乳酸/塑化淀粉共混物(PLA-g-GMA/TPS)。红外光谱分析证实,GMA成功接枝到PLA分子链上。对共混物的力学性能、热机械性能、微观形貌、热性能及亲水性等进行了系统研究,结果表明,选择GMA用量为6%(接枝率为1.51%)和TPS用量为10%时的拉伸强度、断裂伸长率及弹性模量最佳,分别为42.6MPa、8.9%及260MPa。FE-SEM观察结果表明,低含量TPS中颗粒被基体包覆或嵌入,界面平整,界面结合力强。DMA和DSC结果显示,不同质量配比的PLA-g-6%GMA/TPS共混物的Tg、储存模量、结晶度、结晶温度及熔融温度仅在小范围内发生变化。吸水率和接触角结果表明,低含量TPS的共混物吸水率和接触角变化幅度均小于高含量TPS体系。     

Glassy dynamics and mechanical response in dense fluids of soft repulsive spheres. II. Shear modulus, relaxation-elasticity connections, and rheology

We apply the quiescent and mechanically driven versions of nonlinear Langevin equation theory to study how particle softness influences the shear modulus, the connection between shear elasticity and activated relaxation, and nonlinear rheology of the repulsive Hertzian contact model of dense soft sphere fluids. Below the soft jamming threshold, the shear modulus follows a power law dependence on volume fraction over a narrow interval with an apparent exponent that grows with particle stiffness. To a first approximation, the elastic modulus and transient localization length are controlled by a single coupling constant determined by local fluid structure. In contrast to the behavior of hard spheres, an approximately linear relation between the shear modulus and activation barrier is predicted. This connection has recently been observed for microgel suspensions and provides a microscopic realization of the elastic shoving model. Yielding, shear and stress thinning of the alpha relaxation time and viscosity, and flow curves are also studied. Yield strains are relatively weakly dependent on volume fraction and particle stiffness. Shear thinning commences at values of the effective Peclet number far less than unity, a signature of stress-assisted activated relaxation when barriers are high. Apparent power law reduction of the viscosity with shear rate is predicted with a thinning exponent less than unity. In the vicinity of the soft jamming threshold, a power law flow curve occurs over an intermediate reduced shear rate range with an apparent exponent that decreases as fluid volume fraction and/or repulsion strength increase.     

Effect of surface tension and surface elasticity of a fluid-fluid interface on the motion of a particle immersed near the interface

The motion of a particle immersed in a fluid near a fluid-fluid interface is studied on the basis of the linearized Navier-Stokes equations. The motion is influenced by surface tension, dilatational surface elasticity modulus, and surface shear modulus, as well as by gravity. The backflow at the location of the particle after a sudden impulse has some universal features that are the same as for a rigid wall with stick boundary conditions. At short times the flow depends only on the mass densities of the two fluids. The nature of the short-time flow is calculated from potential flow theory. At a somewhat later time the particle shows a pronounced rebound. The maximum value of the rebound and the time at which the maximum occurs depend on the elastic properties of the interface.     

Viscoelasticity of Shear Thickening Fluid Based on Silica Nanoparticles Dispersing in 1-butyl-3-methylimidizolium Tetrafluoroborate

The viscoelasticity of shear thickening fluid (STF), a crucial property in the protective composite applications, with different silica nanoparticle concentrations in ionic liquid, 1-butyl-3-methylimidizolium tetrafluoroborate ([C₄min]BF₄), was studied at different temperatures and with shear frequencies through oscillatory shear, respectively. All STFs present strain thickening behavior. With increasing silica nanoparticle concentration, the critical shear strain for the onset of strain thickening decreased, while the complex viscosity, storage modulus, and loss modulus increased significantly. The critical shear strain increased with an increase of temperature, while the complex viscosity, storage modulus, and loss modulus decreased notably. The critical shear strain was constant with increasing the frequency of strain, while the complex viscosity decreases slightly. The storage modulus and loss modulus were independent with frequency in the strain thickening region. Nanoparticle clusters leading to strain thickening were demonstrated. The viscoelastic response of STFs to varying silica nanoparticle content, temperature, and frequency investigated here will help to design the specific application of STFs in soft protective composites and damping devices.     

Substrate effect on the melting temperature of gold nanoparticles

Previous experimental, molecular dynamics, and thermodynamic researches on the melting temperature of Au nanoparticles on tungsten substrate provide entirely different results. To account for the substrate effect upon the melting point of nanoparticles, three different substrates were tested by using a thermodynamic model: tungsten, amorphous carbon, and graphite. The results reveal that the melting point suppression of a substrate-supported Au nanoparticle is principally ruled by the free surface-to-volume ratio of the particle or the contact angle between the particle and the substrate. When the contact angle θ is less than 90°, a stronger size-dependent melting point depression compared with those for free nanoparticles is predicted; when the contact angle θ is greater than 90°, the melting temperature of the supported Au nanoparticles are somewhat higher than those for free nanoparticles.     

Microrheology of entangled polymer solutions

Microrheology of semidilute polymer solutions is investigated. In this paper we calculate a response function of a probe particle embedded in a semidilute polymer solution by analyzing the two-fluid model. We find that when the size of the probe particle is comparable to the viscoelastic length, the response from the longitudinal compression modes becomes more important than that of the transverse shear modes. As a result, depending on the circumstances, the obtained complex shear modulus cannot be well approximated by that measured in macroscopic rheology experiments. The present results are due to the dynamical asymmetry coupling and the existence of the cooperative dynamics, which are intrinsic to entangled polymer solutions.     

Using the bending beam model to estimate the elasticity of diphenylalanine nanotubes

The core recognition motif of the amyloidogenic beta-amyloid polypeptide, diphenylalanine peptide, has previously been shown to self-assemble into discrete, well-ordered, stiff nanotubes under mild conditions. The nanotubes keep the same morphology from room temperature up to 100 degrees C. In the presented study, we applied the bending beam model to atomic force microscopy images of diphenylalanine nanotubes suspended across cavities and obtained the Young's modulus 27 +/- 4 GPa and the shear modulus 0.21 +/- 0.03 GPa. We also showed that the elasticity of these nanotubes is stable within the same temperature range and at relative humidity from 0% to 70%. This study furthers our understanding of the structure and properties of these nanotubes, which are important for their potential applications in biotechnology.     

Theoretical study of heating of spherical nanoparticle in media by short laser pulses

Theoretical investigation is made of heating of a small solid (metal) spherical particle in a liquid or gaseous medium on exposure to intense laser radiation pulse. The solutions are obtained for quasi-stationary distributions of temperature inside and around of a particle and conductive heat transfer between the particle and the surrounding medium with allowance for the temperature dependence of the transfer coefficients. The heating of spherical nanoparticle in medium on exposure to laser radiation pulse and following cooling is considered on the base of analytical solutions. The time dependencies of particle temperature are obtained. Comparison of some predicted results of the heating of gold spherical nanoparticle in liquid media with experimental data is given and agreement of theoretical results with experimental data validates the model and theory developed.     

Molecular dynamics simulations of the melting of aluminum nanoparticles

Molecular dynamics simulations are performed to determine the melting points of aluminum nanoparticles of 55-1000 atoms with the Streitz-Mintmire [Phys. Rev. B 1994, 50, 11996] variable-charge electrostatic plus potential. The melting of the nanoparticles is characterized by studying the temperature dependence of the potential energy and Lindemann index. Nanoparticles with less than 850 atoms show bistability between the solid and liquid phases over temperature ranges below the point of complete melting. The potential energy of a nanoparticle in the bistable region alternates between values corresponding to the solid and liquid phases. This bistability is characteristic of dynamic coexistence melting. At higher temperatures, only the liquid state is stable. Nanoparticles with more than 850 atoms undergo a sharp solid-liquid-phase transition characteristic of the bulk solid phase. The variation of the melting point with the effective nanoparticle radius is also determined.     

不同分子量可德胶水悬浮液的粘弹性研究

采用动态粘弹性测量研究了不同分子量的生物大分子可德胶 (Curdlan)水悬浮液 (ASC)的流变学特性 .室温下观察到ASC具有弹性 (Solid like)行为 ,储能模量G′在测量范围内轻微依赖频率 ,而损耗模量G″和损耗角正切tanδ存在最小值 .ASC粘弹性随可德胶分子量和浓度的增加而增强 .ASC的流动特性符合Herschel Bulkley模型 ;其弹性行为可以通过渗流理论的标度弹性模型来描述 .网络结构是由于可德胶颗粒聚集或絮凝而形成的 ,当可德胶含量超过临界浓度cs=0 3 %时 ,弹性模量G′与可德胶浓度存在标度关系G′=Goεt,其中标度指数t=2 5 4.     

Mechano-chemical stability of gold nanoparticles coated with alkanethiolate SAMs

Molecular dynamics simulations are used to probe the structure and stability of alkanethiolate self-assembled monolayers (SAMs) on gold nanoparticles. We observed that the surface of gold nanoparticles becomes highly corrugated by the adsorption of the SAMs. Furthermore, as the temperature is increased, the SAMs dissolve into the gold nanoparticle, creating a liquid mixture at temperatures much lower than the melting temperature of the gold nanoparticle. By analyzing the mechanical and chemical properties of gold nanoparticles at temperatures below the melting point of gold, with different SAM chain lengths and surface coverage properties, we determined that the system is metastable. The model and computational results that provide support for this hypothesis are presented.     

Analysis of pulsed laser plasmon-assisted photothermal heating and bubble generation at the nanoscale

A study is presented of photothermal effects associated with nanosecond-pulsed laser-illuminated subwavelength metallic nanoparticles in aqueous solutions. Computational electromagnetic and fluid analysis are used to model fundamental aspects of the photothermal process taking into account energy conversion within the nanoparticle at plasmon resonance, heat transfer to the fluid, homogeneous bubble nucleation, and the dynamic behaviour of the bubble and surrounding fluid. Various nanoparticle geometries are modelled including spheres, nanorods and tori. The analysis demonstrates that the laser intensity and pulse duration can be tuned to achieve controllable bubble generation without exceeding the melting temperature of the particle. The analysis also shows that the particle geometry can be tuned to optimize photothermal energy conversion for bubble generation at wavelengths that span the UV to NIR spectrum. Multiparticle systems are studied and a cooperative heating effect is demonstrated for particles that are within a few radii of each other. This provides more robust bubble generation using substantially reduced laser energy as compared to single-particle systems. The modelling approach is discussed in detail and should be of considerable use in the development of new photothermal applications.     

Reversible Melting During Crystallization of Polymers Studied by Temperature Modulated Techniques (TMDSC, TMDMA)

Quasi-isothermal temperature modulated DSC and DMA measurements (TMDSC and TMDMA, respectively) were performed to determine heat capacity and shear modulus as a function of time during crystallization. Non-reversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in complex heat capacity and in the amplitude of shear modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for PCL PET and PEEK. This shows that also after long crystallization times the surfaces of the individual polymer crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are ‘living crystals’. This revised version was published online in August 2006 with corrections to the Cover Date.     

The rheology and microstructure of charged colloidal suspensions

The effects of electric charge interation and particle correlations on suspension rheology are examined. A one-component fluid analysis using a Smoluchowski equation for the equilibrium structure is applied to charged suspensions of spherical colloids under shear. The frequency dependent modulus and viscosity, predicted as functions of particle and added salt concentrations, are compared with published rheological measurements on model suspensions. Recent improvements in the statistical mechanical theories for the equilibrium microstructure, its nonequilibrium deformation, and the bulk shear stresses are included. The direct electrostatic interaction is found to drive the divergence in the shear viscosity near the liquid-solid phase transition. Extensions of the theory predict the elastic modulus of binary mixtures of charged colloids. Estimates of the primary electroviscous effect, hydrodynamic interactions, and errors in the Yukawa limiting form for the potential and applications of asymptotic theories are presented. Predictions for the rheology based on effective hard-sphere models are found to be reasonable when using a parameter fit from the equilibrium phase behavior. Mean-field mode coupling theories predict larger relaxation times than calculated from the Smoluchowski equation (=SE). A study of binary mixing effects on elasticity shows non-ideal behavior. It is noted that equilibrium structural information can be used to resolve discrepancies between the theoretical predictions and the measured rheology.