Viscoelastic behavior of carbon black and its relationship with the aggregate size

The viscoelastic behavior of binderless carbon black compacts under dynamic compression (0.2–10.0 Hz, static strain 1.0–2.5%, deformation amplitude 9.4–16 μm and packing density 0.18–0.45 g/cm³) is governed by the solid part of the compact, with the elastic character dominating the viscous character. The viscous character increases with increasing aggregate size, while the elastic character decreases. The interparticle movement in an aggregate contributes to the viscous deformation, while the connectivity among the aggregates contributes to the stiffness. The loss tangent of the solid part increases with increasing aggregate size up to 300 nm, which is optimum for viscoelasticity. A larger specific surface area weakly correlates with a lower storage/loss modulus of the solid part. The particle size does not correlate with the viscoelastic behavior. Both viscous and elastic characters increase with increasing static strain, due to the tightening of the solid part. The loss tangent of the solid part is up to 1.2, compared to 21 and 0.67 for exfoliated graphite and rubber respectively. The storage and loss moduli of the solid part (up to 21 and 180 kPa respectively) are below those of rubber, but are above or comparable to those of exfoliated graphite. Possible applications relate to mechanical isolation.     

Ageing under oscillatory stress: Role of energy barrier distribution in thixotropic materials

In this work the ageing dynamics of soft solids of aqueous suspension of laponite has been investigated under the oscillatory stress field. We observed that, at small stresses elastic and viscous moduli showed a steady rise with the elastic modulus increasing at a faster rate than the viscous modulus. However, at higher stresses both the moduli underwent a sudden rise by several orders of magnitude with the onset of rise getting shifted to a higher age for a larger shear stress. We believe that the observed behavior is due to interaction of barrier height distribution of the potential energy wells in which the particle is trapped and strain induced potential energy enhancement of the particles. Strain induced in the material causes yielding of the particles that are trapped in the shallower wells. Those trapped in the deeper wells continue to age enhancing the cage diffusion timescale and consequently the viscosity, which lowers the magnitude of strain allowing more particles to age. This coupled dependence of strain, viscosity and ageing causes forward feedback for a given magnitude of stress leading to sudden rise in both the moduli. Changing the microstructure of the laponite suspension by adding salt affected the barrier heights distribution that showed a profound influence on the ageing behavior. Interestingly, this study suggests a possibility that any apparently yielded material with negligible elastic modulus may get jammed at a very large waiting time.     

Thermal,mechanical, and rheological characterization of polypropylene/layered double hydroxide nanocomposites

Polypropylene (PP)/organomodified layered double hydroxide (LDH) nanocomposites were prepared in order to examine the influence of LDH content on thermal, mechanical, and rheological properties. The nanostructure examinations by X‐ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the exfoliated/intercalated dispersion of LDH. Incorporation of the LDH resulted in a noteworthy improvement in the thermal stability of PP. It was shown that the addition of LDH contributed to the reinforcement effect by increasing the elastic modulus. The mechanical performance, as evaluated by stress–strain curves, reveal that PP/LDH hybrid materials showed significant contribution toward increment in elastic modulus, tensile strength but at the expense of impact strength. The rheological response showed a strong influence of LDH particles on the flow behavior of the PP/LDH melt which resulted in enhancement of storage, loss of moduli, and complex viscosity of nanocomposites. Therefore, the nanocomposites have higher moduli but better processibility compared with pure PP. Overall, the results indicated that the LDH particles in nanometer size might act as potential reinforcing agent for polypropylene. POLYM. ENG. SCI., 52:2006–2014, 2012. © 2012 Society of Plastics Engineers     

Rheology of a low‐filled polyamide 6/montmorillonite nanocomposite

The rheological properties of a polyamide 6/clay nanocomposite with a low loading of clay (1 wt %) were studied. Linear viscoelastic measurements in oscillatory and steady shear with small strain amplitudes were carried out. The nanocomposite exhibited a higher elastic modulus, viscous modulus, and complex viscosity than neat polyamide 6 during dynamic and steady shear tests. Moreover, the addition of clay resulted in a reduction of the critical strain amplitude, an increase of the loss angle, and a reduction of the frequency at the intersection of the elastic and viscous moduli. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

The missing parameter in rheology: hidden solid‐like correlations in viscous liquids,polymer melts and glass formers

BACKGROUND: Determination of dynamic relaxation consists of measuring the viscous and the elastic components of a material by generally applying a small (oscillatory) deformation. The shear stress is transmitted to the material via contact with a substrate. Dating at least back to Stokes, the no‐slip boundary condition between the fluid and the substrate is supposed to be fulfilled during this measurement. We show that the viscoelastic parameters of fluids are usually not determined under no‐slip boundary conditions and do not originate from the first linear regime. Viscous and viscoelastic fluids (entangled and unentangled polymers, glass formers) measured under no‐slip conditions exhibit a fundamentally different response with a dominant terminal solid‐like response. RESULTS: We show that the terminal behaviour of fluids such as liquid polymers or glass formers measured at the sub‐millimetre scale and far above the glass transition is not viscous but solid‐like. Instead of a viscoelastic behaviour scaling as ω and ω² (ω is the frequency) for the viscous and the elastic moduli, respectively, the dynamic response is simplified; for low gap thickness, both viscous and elastic moduli are invariant with respect to the frequency (with the elastic modulus being larger than the viscous modulus) and enhanced by two to four orders of magnitude compared to the conventional viscoelastic response. Over a critical strain amplitude, the solid‐like response decreases and is progressively replaced by the conventional viscoelastic behaviour. We discuss the implications of this observation and reconsider the assumptions inherent to a rheology measurement. CONCLUSION: The identification of so far neglected macroscopic elasticity in the fluidic state far above the glass transition temperature in entangled and unentangled polymers and glass formers shows that the liquid state is dominated by long range intermolecular interactions. This information is fundamental to understand and to foresee dynamic behaviour; it sheds further light on nonlinear phenomena such as large time scale relaxations, rheo‐thinning, violation of the no‐slip boundary condition and spectacular shear‐induced instabilities (spurt effect, ‘shark‐skin’ instabilities, gross melt fracture, etc.) that are unpredictable in the frame of the conventional viscoelastic approach. It also implies that the viscoelastic times (reptation, Rouse) in polymers are not the longest relaxation times of these materials. Copyright © 2009 Society of Chemical Industry     

The effect of liquids on the dynamic properties of carbon black filled natural rubber as a function of pre-strain

A free oscillation technique has been adopted to measure the dynamic storage and loss moduli of carbon black filled natural rubber materials. These tests are conducted with small oscillations that are superimposed on a range of tensile pre-strains. In addition, the effect of temperature on the dynamic moduli is measured as well as the effect of swelling the materials to various extents by liquids with a range of viscosity. It is observed that the dynamic storage and loss moduli do not depend strongly on the pre-strain at small pre-strains. At higher pre-strains there is a marked increase in both the storage and the loss moduli. An increase in temperature causes a dramatic reduction in both the storage and loss moduli. The dynamic behaviour of the filled rubbers when swollen can be approximately ascribed to the combined effects of a reduction in the modulus of the rubber matrix (caused by the swelling action) and a reduction in the effective volume fraction of the filler. The liquids used had a range of viscosity of more than a factor of a thousand. Despite this, the loss moduli of the swollen rubbers varied by only about a factor of two. This insensitivity could be understood in terms of a previously developed theory, based on free volume considerations.     

有机硅纳米杂化改性环氧胶的研制和应用

通过溶胶-凝胶法制备了纳米有机硅溶胶。用透射电镜和红外光谱表征和分析了纳米粒子的形态和粒径,粒径为30－70nm。制备了用活性稀释剂糠醛和丙酮及有机硅纳米杂化改性的环氧胶。研究了胶液的初始黏度、渗透性、力学强度、弹性模量、粘接性、表面疏水性和防腐蚀性能。实验结果表明,环氧树脂含量、有机硅溶胶、硅烷偶联剂等对改性环氧胶的这些性能有显著影响,改性环氧胶双组分混合液的初始黏度可低至1.1mPa·s,渗透性强,固结体抗压强度高达40．8－81．5MPa,弹性模量为1．3－2．1×10^3MPa,剪切粘接强度5．1－11．9MPa,防水抗渗性及防腐蚀耐候性良好,可应用于灌浆防水加固补强、工业地坪涂料和防腐蚀涂料等多种用途。     

Dynamic mechanical behavior of flexible graphite made from exfoliated graphite

The low-frequency dynamic mechanical properties of low density flexible graphite (0.027–0.089 g/cm³, obtained by compressing exfoliated graphite at 0.3–1.7 MPa without a binder) are different in flexure and compression. The storage and loss moduli and the loss tangent are higher under flexure than compression. Under flexure, the storage modulus is essentially unaffected by increasing the static strain while the loss tangent and loss modulus decrease. Under compression, both storage and loss moduli increase with increasing static strain, while the loss tangent slightly decreases. The storage and loss moduli are higher and the loss tangent is lower for out-of-plane compression than for in-plane compression. The storage and loss moduli increase with increasing fabrication pressure, while the loss tangent decreases. Energy dissipation is much more effective under flexure than compression. Flexure appears to provide more sliding of the graphite layers than compression. For the highest energy dissipation under flexure, a low static strain is recommended; for high energy dissipation under compression, a high static strain is recommended.     

Elastic constants of lamellar and interlamellar regions in α and mesomorphic isotactic polypropylene by AFM indentation

A study of the nanoscale mechanical properties of isotactic mesomorphic and semi‐crystalline polypropylene (iPP) is presented. Two iPPs produced with metallocene and Ziegler‐Natta catalyst polymerization are used. The resulting fibers are characterized by wide angle X‐ray scattering, small‐angle X‐ray scattering (SAXS), and Raman spectroscopy. The spatial variability of the percentage crystallinity is evaluated based on the SAXS data. AFM indentation is performed to measure the elastic modulus of the fibers in the direction perpendicular and parallel to the fiber axis. Since the AFM probing is performed on a scale larger than the lamellar thickness, a statistical analysis of the AFM and SAXS data is necessary to infer the elastic moduli of the α crystals and of the inter‐lamellar regions. The elastic modulus of the crystalline lamellae in the direction perpendicular to the c‐axis of the α crystal probed in compression is estimated at approximately 3.3 GPa, while the effective modulus of the interlamellar regions ranges from 1.5 to 2.2 GPa. The method proposed can be applied to other material systems with similar layered structure to measure elastic moduli or hardness on length scales smaller than the resolution of the indentation test. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43649.     

二氧化硅溶胶体系的电导行为

二氧化硅溶胶的导电性与其储存稳定性及纳米材料的制备和应用密切相关。本文通过Stöber法制备了分散性较好的非晶型二氧化硅纳米微球,用激光粒度分析仪、高分辨透射电子显微镜和X射线衍射仪对微粒进行了表征。通过电导率的变化监测了不同条件下的溶胶-凝胶动力学过程,并详细地研究了影响二氧化硅溶胶体系电导行为的各个因素。实验结果表明,氨水用量对溶胶-凝胶过程的平衡时间有很大影响,氨水用量少,体系平衡时间短,反之亦然。二氧化硅的浓度、粒径、分散介质的温度、pH值和电解质浓度对溶胶体系的电导率都有显著的影响,并得出了二氧化硅的浓度和体系温度与电导率之间呈线性关系,同时发现,硅溶胶体系的电导率与胶体粒子总的比表面积成正比,与颗粒表面的ζ电位也密切相关。     

Effect of the particle size on the viscoelastic properties of filled polyethylene

Composites of surface treated and non-treated colloidal calcium carbonate and high-density polyethylene with different filler loading were prepared. Their viscoelastic properties were studied by dynamic strain sweep and small-amplitude oscillatory shear, and compared to those of the corresponding composites of micron-sized calcite. The specific surface area of the filler enormously increases as the average particle diameter becomes smaller than 600 nm, leading to a strong tendency to agglomeration (soft flocks) and aggregation (hard clusters that need attrition to be disintegrated). In nanocomposites, more and stronger filler clusters are formed than in microcomposites due to the large contact area between the particles. The clusters have different shapes and maximum packing than the nearly spherical primary particles, thus enhance the moduli and viscosity of the composites. The obtained results indicate that the higher moduli and viscosity of the nanocomposites is not a direct consequence of the particle size but is due to the presence of more agglomerates and aggregates. Clusters that are local structures and do not represent a space-filling filler network enhance the moduli in the low frequency region more than at high frequencies and increase the storage more than the loss modulus. The presence of strong local structures in the nanocomposites leads to weak log moduli-log frequency dependence in the low frequency (terminal) region. Polymer adsorption on the particles' surface results in a transient filler-polymer network and slow dynamics of the bound polymer, which contribute to the moduli of the complex fluid. The sum of all these factors leads to gradual increase in moduli and to a shift of the crossover frequency to lower values. Above a certain filler volume fraction, the composite responds as a viscoelastic solid (storage modulus>loss modulus over the whole frequency range and both moduli are frequency independent in the terminal zone of the log-log plot).     

Molecular modeling approach to the prediction of mechanical properties of silica‐reinforced rubbers

Recently, we have suggested a nanomechanical model for dissipative loss in filled elastomer networks in the context of the Payne effect. The mechanism is based on a total interfiller particle force exhibiting an intermittent loop, due to the combination of short‐range repulsion and dispersion forces with a long‐range elastic attraction. The sum of these forces leads, under external strain, to a spontaneous instability of “bonds” between the aggregates in a filler network and attendant energy dissipation. Here, we use molecular dynamics simulations to obtain chemically realistic forces between surface modified silica particles. The latter are combined with the above model to estimate the loss modulus and the low strain storage modulus in elastomers containing the aforementioned filler‐compatibilizer systems. The model is compared to experimental dynamic moduli of silica filled rubbers. We find good agreement between the model predictions and the experiments as function of the compatibilizer's molecular structure and its bulk concentration. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40806.     

Rheological characterization of chitosan matrices: Influence of biopolymer concentration

Viscoelastic properties of chitosan (CH), chitosan‐poly(ethylene glycol) 400 (CH‐PEG), and chitosan‐poly(ethylene glycol) 400 with glyoxal as crosslinking agent (CH‐PEG‐Gly) systems were studied to analyze the effect of chitosan concentration (from 0.83 to 1.67%). Dynamic moduli increase as chitosan concentration increases for all systems. For CH and CH‐PEG systems the loss modulus (G″) is greater than the storage modulus (G′) with predominance of the viscous over the elastic behavior. This corresponds to the characteristic behavior of solutions (nonstructured systems). The presence of PEG 400 induces a complementary reinforcement of the mechanical properties of the system. Except for the lowest chitosan concentration, when glyoxal was added to the CH‐PEG systems, a gelled matrix was obtained. In this case, G′ is greater than G″, and practically independent of frequency. This behavior is typical of three‐dimensional networks and indicates true gel formation, showing clear elastic behavior (tan δ < 1). In creep and recovery analysis, CH‐PEG‐Gly systems exhibited distinct regions that were mathematically modeled using Burger's model. This analysis shows that the CH‐PEG‐Gly matrices (from 1.25 to 1.67%) recover almost totally (100%). Therefore, these matrices could be useful as systems for the development of films for topical hydrophilic drug delivery, and the levels of the residual viscosity (η₀) or the complex viscosity (η*) could be used to control drug release. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Investigations on rheological properties and gelation of tasar regenerated silk fibroin solution

Tasar silk is a variety of non‐mulberry silk indigenous to the Indian subcontinent. We present the measured frequency‐dependent viscoelastic moduli of Tasar regenerated silk fibroin (RSF) solution using optical tweezers at two concentrations (0.16% and 0.25% w/v) and extend these measurements to the low frequency regime using a video microscopy technique. We extend the investigation on the rheological behavior of Tasar RSF for four more RSF concentrations, viz., 0.50%, 1.00%, 2.50% and 5.00% using video microscopy. In all the RSF samples, both storage and loss moduli are found to increase with frequency. At lower frequencies the loss modulus is more than the storage modulus and exhibit similar behavior until a crossover frequency beyond which the storage modulus exceeds the loss modulus at all frequencies. The relaxation time which is inversely related to the crossover frequency is found to rise sharply at 5% w/v, indicating the onset of gelation in the sample. These results are examined in relation to the viscoelastic parameters of mulberry silk, wherein the larger crossover frequencies at the same higher concentrations indicate relaxation times that are an order of magnitude smaller than those measured for Tasar RSF. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40104.     

Effect of solvent exchange on the structure and rheological properties of cellulose in LiCl/DMAc

Effect of solvent exchange on the structure of cellulose was investigated by Fourier transform infrared spectroscopy, wide angle X‐ray diffraction, and scanning electron microscopy analysis. The solvent exchange facilitated the dissolution of cellulose in LiCl/DMAc with no change of the crystalline structure of cellulose. In contrast, solvent exchange led to the fibrillation on the treated fiber surface and the trimmed rod‐like particles, further confirming the occurrence of particle disintegration. The rheological properties of three cellulose samples with different degrees of polymerization (DP) and different concentrations were investigated. Results indicated that the cellulose LiCl/DMAc solutions were non‐Newtonian fluids. At low deformation rates the cellulose solution behaved like a viscous liquid (loss modulus G″ being larger than storage modulus G′), but elastic properties developed at high angular frequency. The two domains of viscoelastic behavior were separated by the so‐called crossover point for G′ and G″, which was slightly shifted to lower frequencies as the testing temperature increased from 50 to 80°C. As the concentration and the average molecular weight (or DP) increased, the angular frequency at the crossover point increased also under the experimental conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

The elastic moduli of particulate-filled polymers

The static and dynamic elastic moduli of particulate composites, consisting of two phases, one of which has isotropic–elastic and the other linear viscoelastic properties, were studied. For this purpose a model defining the approximate equations for determining the elastic modulus of a composite from the properties of the constituent materials was used. Classical theory of elasticity was applied to this simplified model of a composite-unit cell. The following assumptions are made: (i) filler particles are spherical; (ii) fillers are completely dispersed; and (iii) the volume fraction of fillers is sufficiently small, so that any interaction among fillers may be neglected. A class of iron-filled epoxy composites was subjected to tests in order to compare the theoretical values with the experimental results. The elastic modulus calculated by the expression derived in this study seems to corroborate with the experimental results fairly well. Finally, by applying the correspondence principle to this expression, theoretical relationships for the dynamic storage and loss moduli were also derived.     

Association and physical gelation of ABA triblock copolymer in selective solvent

Association behavior and physical gelation mechanism of ABA triblock copolymer dissolved in B-selective solvent have been studied systematically from dilute to moderately concentrated solutions. Static and dynamic light scattering and nuclear magnetic resonance measurements for dilute solutions of poly(methyl methacrylate)-block-poly(tert-butyl acrylate)-block-poly(methyl methacrylate) (PMMA-PtBuA-PMMA) in 1-butanol (PtBuA selective solvent) indicated that PMMA-PtBuA-PMMA chains are molecularly dissolved above 50 °C. With decreasing temperature, the triblock copolymers form associated micelles consisting PMMA associated core and PtBuA shell. Linear dynamic viscoelastic measurements for solutions with moderate concentration (3.9-12.0 wt%) revealed that the system was viscous sol state at 60 °C. Drastic increase of shear storage modulus (G′) occurred with decreasing temperature, and at 25 °C, G′ showed rubbery plateau with weak frequency dependency, means the formation of elastic physical gel. The consistency between the temperature for micelle formation and that at the increase in G′ indicates that the physical gelation is owing to the network formation as the result of the association of PMMA chains and the bridging PtBuA chains connecting the PMMA cores. Master curves for the dynamic moduli were derived by time-temperature superposition along the frequency axis. Just above sol-gel transition concentration (C_gel), the master curves suggest the existence of fairy amount of aggregate that is not incorporated in the macroscopic network. With the increase in polymer concentration, the master curves become to reveal Maxwell-type viscoelasticity with narrow relaxation time distribution, suggesting the formation of transient network with easily generation and destruction of crosslinks. Concentration dependency of the plateau modulus is stronger than the theoretically expected, means the macroscopic transient network grows with polymer concentration by increasing the fraction of elastically effective bridging PtBuA chain above C_gel.     

Effect of molecular weight on the rheological properties of atactic poly(vinyl alcohol)/dimethylsulfoxide/water solution

This article describes the molecular weight effect of atactic poly(vinyl alcohol) (a‐PVA) on the rheological properties of 7.5, 10.0, and 12.5 g/dL solutions of a‐PVA with number‐average degrees of polymerization (P_n) of 4000 and 1700 in dimethylsulfoxide/water mixture. a‐PVA with a P_n of 1700 solutions exhibited almost Newtonian flow behavior, whereas high molecular weight a‐PVA, with a P_n of 4000 solutions, exhibited shear‐thinning behavior. On the plot of storage and loss moduli of a‐PVA with a P_n of 1700 solutions, the dynamic storage modulus of a‐PVA, with a P_n of 1700 solutions, was smaller than the dynamic loss modulus over the frequency range of 10^?1 to 10² rad/s. However, the dynamic storage modulus of a‐PVA, with a P_n of 4000 solutions, was smaller than the dynamic loss modulus in the sol state and, in the postgel state, the dynamic storage modulus became larger than the dynamic loss modulus, indicating the evolution of viscoelastic solid properties. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 41–46, 2004     

Effect of synthesis variables on viscoelastic properties of elastomers filled with carbonyl iron powder

This work studies the influence of synthesis variables on the lineal viscoelastic properties of elastomers filled with soft magnetic particles. Three matrices [natural rubber (NR), high-temperature vulcanising silicone rubber (HTV-SR), and room-temperature vulcanising (RTV-SR)] and three volumetric particle contents (0%, 15%, and 30%) were studied. Anisotropic samples were synthesised with a softer matrix to obtain a larger magnetorheological (MR) effect, and the variation of their properties under an external magnetic field was examined. All samples were characterised within the lineal viscoelastic (LVE) region using a rheometer, because the MR effect is larger within this region. The influence of the matrix, particle content, and pre-structure on the viscoelastic properties of the synthesised samples was studied. The storage and loss modulus increased with the frequency owing to the viscoelastic behaviour of an elastomer in the rubbery phase. Both moduli also increased with the filler content. The influence of the filler is dependent on the matrix, and the maximum variation was seen in the NR-based samples. However, the maximum MR effect was seen in the samples with a softer matrix, and the effect was enhanced in the anisotropic samples. In this work, the MR effect on the loss modulus was studied, and the tendencies were found to be similar to those of the storage modulus. The main contribution of this work is that all dynamic behaviour results were comparable because all synthesis variables and characterisation conditions were identical. Therefore, how the particle content, frequency, and magnetic field affects each matrix can be studied.     

Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes

The paper investigates the possibility of using a shear wave reflection technique to monitor the viscoelastic behavior (represented by storage shear modulus and viscosity) of Portland cement paste at very early age. Three cement pastes with water/cement ratios equal to 0.4, 0.5 and 0.6 cured under water at a constant temperature of 25 °C were studied. By measuring the wave reflection coefficients and the phase angles of reflected ultrasonic waves, the dynamic storage shear moduli and the viscosity of the cement paste can be calculated. The calculated results of the storage modulus were compared with the results obtained directly from the oscillatory rheometric measurement. In addition, the viscosity calculated from the wave reflection measurements was compared with results obtained directly from the step rheometric method and a qualitative agreement was found. The results show that as a non-destructive method, the ultrasonic wave reflection method provides useful information about both the elastic and viscous behavior of cement pastes at very early age.