Giant reduction in dynamic modulus of kappa-carrageenan magnetic gels

Effects of magnetization on the complex modulus of kappa-carrageenan magnetic gels have been investigated. The magnetic gel was made of a natural polymer, kappa-carrageenan, and a ferrimagnetic particle, barium ferrite. The complex modulus was measured before and after magnetization of the gel by dynamic viscoelastic measurements with a compressional strain. The gels showed a giant reduction in the storage modulus of approximately 10(7) Pa and also in the loss modulus of approximately 10(6) Pa due to magnetization. The reduction increased with increasing volume fraction of ferrite, and it was nearly independent of the frequency. It was also found that the change in the modulus was nearly independent of the magnetization direction and irradiation time of the magnetic fields to the gel. The magnetic gels demonstrating the giant reduction in the dynamic modulus showed a large nonlinear viscoelastic response. It was observed that the magnetic gel was deformed slightly due to magnetization. The observed giant complex modulus reduction could be attributed to the nonlinear viscoelasticity and deformation caused by magnetization. Magnetism, nonlinear viscoelasticity, and effects of magnetization on the morphological and shape changes were discussed.     

Giant Complex Modulus Reduction of κ‐Carrageenan Magnetic Gels

Summary: Effects of magnetization on the complex modulus of κ‐carrageenan magnetic gels have been investigated. The magnetic gel was made of a natural polymer, κ‐carrageenan, and a ferromagnetic particle, barium ferrite. The complex modulus of the magnetic gel was investigated by dynamic viscoelastic measurements with a compressional strain. It was first observed that the magnetic gels showed giant storage modulus reduction ≈10⁷ Pa before and after magnetization. The reduction was nearly independent of the frequency, and it increased with increasing the volume fraction of the ferrite. The maximum reduction in the storage modulus reached 14.9 MPa which corresponds to 76.5% of the modulus before magnetization. It was also found that the change in the modulus was nearly independent of a magnetization direction. Magnetism and morphology of the magnetic gels were also presented.

Strain dependence of the storage modulus at 1 Hz for κ‐carrageenan gel (□) and its magnetic gel before (○) and after (•) magnetization (ϕ = 0.39). The geometry of magnetization and strain directions is perpendicular.     

Anisotropy in Longitudinal Modulus of Polymer Gels Containing Ferrite

The effects of magnetization direction on the longitudinal modulus of magnetic gels, which consist of magnetized barium ferrite and poly(vinyl alcohol), have been investigated using 10 MHz ultrasonic waves. The modulus change due to magnetization depends on both the crosslinking density and magnetization direction. The modulus change increased and decreased when the strain direction was perpendicular and parallel to magnetization, respectively.     

Nonlinear viscoelasticity, percolation and particles dispersibility of PVA/aluminum hydroxide composite gels

We investigated the rheological properties of a composite gel consisting of poly(vinyl alcohol) and aluminum hydroxide particles, and discussed the relation among nonlinear viscoelasticity, percolation and particles dispersibility. The dynamic viscoelastic measurements revealed that the storage modulus at volume fractions ? < 0.04 satisfied with the Krieger-Dougherty equation representing random dispersion of particles. The storage modulus did not show any nonlinear viscoelastic response at ? < 0.04. However, the storage modulus at ? > 0.06 took a value which is far larger than that expected by the equation, indicating heterogeneous distribution of particles. Additionally, the nonlinear viscoelastic response was recognized clearly at ? > 0.06, suggesting a partial contact between particles. The storage modulus at ? > 0.18 showed a further increase satisfied with the percolation theory, therefore, the volume fraction is considered to be the percolation threshold of 3-dimension. Microscopic observations of the gel showed a clear network with a mesh size of few μm that is considered to be a partial network of particles.     

Synthesis of Barium Ferrite Ultrafine Particles by Coprecipitation in the Presence of Polyacrylic Acid

The barium ferrite ultrafine particles were synthesized by coprecipitation in an aqueous solution with polyacrylic acid (PAA) as a protective agent. Thermal analysis by TGA/DTA showed that the precursor could yield barium ferrite after calcination above 700 degrees C for 2 h. By analyses of the XRD and electron diffraction pattern, the formation of pure barium ferrite was confirmed and the appropriate molar ratio of Ba/Fe in aqueous solution was determined to be 1/11. The TEM measurements indicated that the average diameter of the precursor was 4.5 nm, and the diameters of the particles calcined at 700 and 800 degrees C were 23-34 and 49-82 nm, respectively. The magnetic properties characterized by a SQUID magnetometer showed that the barium ferrite ultrafine particles calcined at 700-800 degrees C had a saturation magnetization of 36.9-60.8 emu/g, a remanent magnetization of 19.0-31.0 emu/g, a coercivity of 117.3-221.8 Oe, and a squareness ratio of 0.51. The magnetization was also observed to increase with a decrease of temperature at 5-400 K. These magnetic properties all reflected the nature of ultrafine particles and also were influenced by the morphology and microstructure of final products. Copyright 2001 Academic Press.     

Rapid determination of molar ratios in binary systems by 14 MeV neutron activation analysis

The magnetic characteristics of barium ferrite, a compound widely used for magnetic materials, depend on the molar ratio of iron(III) oxide to barium oxide. On account of this fact, activation analysis using 14 MeV neutrons was applied for the rapid and non-destructive determination of the molar ratio of iron(III) oxide to barium oxide in barium ferrite. Iron was detected as⁵⁶Mn produced from the⁵⁶Fe(n, p)⁵⁶Mn reaction, and barium as^137mBa originating from the¹³⁸Ba(n, 2n)^137mBa reaction. A linear relation was obtained between the ratio of counts of⁵⁶Mn and^137mBa and the molar ratio of iron(III) oxide to barium oxide; the corrected gradient of the experimental calibration curve, obtained with^137mBa internal standard, agreed well with the calculated value. The molar ratios of iron(III) oxide to barium oxide obtained by activation analysis and by chemical analysis were in good agreement.     

Enhanced magnetism in electrodeposited-based CoNi composites containing high percentage of micron hard-magnetic particles

CoNi–barium ferrite magnetic composites with a high percentage of micrometric particles have been prepared by electrodeposition over silicon-based substrates. A cationic surfactant synthesized in our laboratory (4-ethylazobenzene-4′-(oxyethyl)trimethylammonium iodide-AZTMAI) has been used in order to favour the inclusion of magnetic particles into the alloy deposit. This surfactant reduces during cobalt–nickel electrodeposition and it is not embedded into the deposit. Moderate surfactant concentrations (1–5 g l⁻¹), room temperature, the application of a magnetic field during the electrodeposition and a double-pulse technique favour the maximum incorporation of barium ferrite up to around 30–35 wt%. Magnetic properties of composites prepared tend to hard-magnetic ones, with different magnetization curves when silicon/seed-layer/composite samples are oriented parallel or perpendicularly to the applied magnetic field. Constricted-type magnetization curves were obtained.     

X型六角晶系钡铁氧体纳米晶的制备和表征

用硬脂酸凝胶法制备了Co₂-X型六角晶系钡铁氧体纳米晶,在750℃热处理得到的纳米晶形貌为球形,粒径范围为15～25nm.随着热处理温度的升高,粒子逐渐长大并呈块状.振荡样品磁强计测试结果表明,Co₂-X型六角晶系钡铁氧体纳米晶具有与常规体材料不同的磁性能,其比饱和磁化强度σ_s低于后者.产物的矫顽力、比饱和磁化强度随粒子的长大呈规律性的变化.     

Monodisperse cobalt ferrite nanomagnets with uniform silica coatings

Ferro- and ferrimagnetic nanoparticles are difficult to manipulate in solution as a consequence of the formation of magnetically induced nanoparticle aggregates, which hamper the utility of these particles for applications ranging from data storage to bionanotechnology. Nonmagnetic shells that encapsulate these magnetic particles can reduce the interparticle magnetic interactions and improve the dispersibility of the nanoparticles in solution. A route to create uniform silica shells around individual cobalt ferrite nanoparticles--which uses poly(acrylic acid) to bind to the nanoparticle surface and inhibit nanoparticle aggregation prior to the addition of a silica precursor--was developed. In the absence of the poly(acrylic acid) the cobalt ferrite nanoparticles irreversibly aggregated during the silica shell formation. The thickness of the silica shell around the core-shell nanoparticles could be controlled in order to tune the interparticle magnetic coupling as well as inhibit magnetically induced nanoparticle aggregation. These ferrimagnetic core-silica shell structures form stable dispersion in polar solvents such as EtOH and water, which is critical for enabling technologies that require the assembly or derivatization of ferrimagnetic particles in solution.     

Synthesis and characterization of monodisperse magnetic composite particles for magnetorheological fluid materials

Monodisperse magnetic composite particles (MCP) were prepared and characterized for a study of magnetic field-responsive fluids. Magnetic composite particles used are iron oxide-coated polymer composite particles, which were synthesized through in situ coating of iron oxide onto pre-existing polymer particles by the reduction of ferrous fluids. For a uniform and bulk coating of iron oxide, the porous structure was introduced into the substrate polymer particles through a two-step seeded polymerization method. Moreover, surface cyano-functionality was born from acrylonitrile unit of substrate polymer and it played an important role in obtaining successful uniform coating. The structure of the composite particle was analyzed by using a thermo gravimetric analysis (TGA) and a X-ray diffraction (XRD) analysis. The magnetization property of the particle was also observed. Then, the rheological properties of monodisperse magnetorheological (MR) suspensions of magnetic composite particles were examined under a magnetic field using a parallel-plate type commercial rheometer. From the rheological measurements, it was found that MR properties of the magnetic composite suspensions are dependent on the iron oxide content and the fluid composition.     

Effect of surface modification on magnetization of iron oxide nanoparticle colloids

Magnetic iron oxide nanoparticles have numerous applications in the biomedical field, some more mature, such as contrast agents in magnetic resonance imaging (MRI), and some emerging, such as heating agents in hyperthermia for cancer therapy. In all of these applications, the magnetic particles are coated with surfactants and polymers to enhance biocompatibility, prevent agglomeration, and add functionality. However, the coatings may interact with the surface atoms of the magnetic core and form a magnetically disordered layer, reducing the total amount of the magnetic phase, which is the key parameter in many applications. In the current study, amine and carboxyl functionalized and bare iron oxide nanoparticles, all suspended in water, were purchased and characterized. The presence of the coatings in commercial samples was verified with X-ray photoelectron spectroscopy (XPS). The class of iron oxide (magnetite) was verified via Raman spectroscopy and X-ray diffraction. In addition to these, in-house prepared iron oxide nanoparticles coated with oleic acid and suspended in heptane and hexane were also investigated. The saturation magnetization obtained from vibrating sample magnetometry (VSM) measurements was used to determine the effective concentration of magnetic phase in all samples. The Tiron chelation test was then utilized to check the real concentration of the iron oxide in the suspension. The difference between the concentration results from VSM and the Tiron test confirmed the reduction of magnetic phase of magnetic core in the presence of coatings and different suspension media. For the biocompatible coatings, the largest reduction was experienced by amine particles, where the ratio of the effective weight of magnetic phase reported to the real weight was 0.5. Carboxyl-coated samples experienced smaller reduction with a ratio of 0.64. Uncoated sample also exhibits a reduction with a ratio of 0.6. Oleic acid covered samples show a solvent-depended reduction with a ratio of 0.5 in heptane and 0.4 in hexane. The corresponding effective thickness of the nonmagnetic layer between magnetic core and surface coating was calculated by fitting experimentally measured magnetization to the modified Langevin equation.     

聚苯胺钡铁氧体纳米复合材料的制备、表征及性能

采用原位掺杂聚合法, 将聚苯胺(PANI)对粒径在60~80 nm的M型钡铁氧体颗粒(BaFe12O19)进行了包覆, 得到了具有棒状结构的复合材料. 通过X射线衍射(XRD)、傅里叶变换红外光谱(FTIR)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)等测试手段对材料的形貌和结构进行了表征. 结果表明, PANI链段与BaFe12O19颗粒之间存在作用力. 使用振动磁强计和四探针法测定了复合材料的磁性能与电性能后发现, 饱和磁化强度与矫顽力均随聚苯胺含量的增加呈规律性下降趋势, 而电导率呈上升趋势. 复合材料的吸收特性测试结果表明, 该材料反射率小于-20 dB时, 带宽可以达到15.07 GHz. 同时详细地讨论了纳米复合材料的聚合机理及相互作用.     

Biodegradable magnetic gel: synthesis and characterization

Biodegradable polymer-based magnetic gels have been synthesized using hydroxypropyl cellulose and maghemite. These magnetic gels have a network of nanoparticles of hydroxypropyl cellulose (30–100 nm) and a homogeneous distribution of nanosized maghemite (~7 nm). This has been observed in a STEM micrograph. The surface structure of the gels has been observed by atomic force microscopy, while transmission electron microscopy has shown the distribution of iron oxide in HPC gel nanoparticles. These gels have magneto-elastic properties. The magnetic susceptibility and magnetization of these gels are measured by a superconducting quantum interference device magnetometer.     

Influence of nanoparticle surface chemistry on the thermomechanical and magnetic properties of ferromagnetic nanocomposites

The effect of nanoparticle surface chemistry on the thermal, mechanical, and magnetic properties of poly(methyl methacrylate) (PMMA) nanocomposites with cobalt ferrite nanofillers was studied by comparing nanofillers coated with oleic acid (OA; which does not covalently bond to the PMMA matrix) and 3‐methacryloxypropyltrimethoxysilane (MPS, which covalently bonds to the PMMA matrix). Thermogravimetric analysis revealed an increase in the thermal degradation temperature of the nanocomposites compared with the neat polymer. The effect of cobalt ferrite nanofiller on the glass transition temperature (T_g) of the nanocomposite was evaluated by differential scanning calorimetry. The T_g value of the material increased when the particles were introduced. Dynamic mechanical analysis indicated an increase in the storage modulus of the nanocomposite because of the presence of nanofiller and a shift in the peak of loss tangent toward higher temperature. Magnetic measurements indicated that both nanocomposites had a small hysteresis loop at 300 K and no hysteresis at 400 K. However, estimates of the nanofiller's rotational relaxation times and measurements of the zero field cooled temperature‐dependent magnetization indicate that the observed lack of hysteresis at 400 K is likely because of particle rotation in the polymer matrix. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

利用大幅振荡剪切研究Laponite 凝胶的流变性质

通过大幅振荡剪切(LAOS)流变学方法, 研究了NaCl浓度对Laponite悬浮体系的结构及非线性黏弹性的影响. 在线性黏弹性区, Laponite体系的储能模量G′随着NaCl浓度的增加而逐渐增大. 体系的非线性黏弹性用响应应力的Fourier变换三次谐波的相对振幅I_3/1与Lissajous曲线的定量参数G_M, G_L, η_M和η_L描述. 当NaCl浓度较低时, I_3/1随应变振幅γ₀的增加而缓慢增加; 当NaCl浓度较高时, I_3/1随γ₀的增加迅速增大, 达到平台值. 不同NaCl浓度试样的G_M和G_L随γ₀变化的曲线区别不大, 但η_M和η_L随γ₀变化曲线在非线性区域出现峰值且区别较大. NaCl浓度越高, η_M和η_L出现峰值的γ₀越小, 峰值越大. 结果表明, Laponite凝胶的非线性黏弹性与凝胶网络结构有关, 随着NaCl浓度的增加, 粒子间的静电相互作用距离缩短, 粒子间距减小, 形成了更紧密的网络结构. 但这种网络在较大的应变下很容易被破坏, 出现非线性黏弹性.     

A slender-body micromechanical model for viscoelasticity of magnetic colloids: comparison with preliminary experimental data

The storage modulus, G', together with the yield stress, is an essential quantity characterizing the rheological properties of magnetic field-responsive suspensions (magnetorheological fluids or MRF). In this work, we present both experimental and theoretical results on the viscoelastic properties of MRFs. Two MRFs are used: In one the solid phase consists of cobalt ferrite particles + silica gel, with silicone oil as liquid phase. The second system is formed by carbonyl iron + silica gel also dispersed in silicone oil. The cobalt ferrite particles are synthesized as monodisperse colloidal spheres with an average diameter of 850 nm. We describe a new model based on the slender-body approach for hydrodynamic interactions. The predictions of the model are compared to preliminary experimental G' data obtained in a controlled stress plate-plate rheometer. It is found that the model gives the correct order of magnitude for the highest fields in iron suspensions, but underestimates the experimental results obtained in ferrite ones. In the case of high permeability materials such as carbonyl iron, by the inclusion of high-order multipolar interactions and saturation effects we also predict the order of magnitude of the experimental results. When dealing with low permeability cobalt ferrite based MRFs, other effects, such as remanence (at low fields) and saturation (at high fields), must be considered.     

Surface and colloid chemical studies of gamma iron oxides for magnetic memory media

The specific interaction of dispersants and of matrix polymers with the surface of gamma iron oxide particles determines the dispersibility of these powders in coating formulations for magnetic tapes and disks, and also determines the adhesion and other mechanical properties of the cast tapes. These specific interactions have been found to be Lewis acid—base interactions of the surface sites of the gamma iron oxides with the active sites of the polymers or dispersants. The FeOH surface sites of the gamma iron oxides can react with either acidic or basic molecules, and the energy of such bonds can be assessed and predicted by measurement of the molar heats of adsorption of test acids and bases with the FeOH sites. We have measured the molar heats of adsorption of test acids and bases with two different methods; primarily with a flow microcalorimeter having down-stream UV concentration-detection, but also by FTIR transmission measurements of the infrared spectral shifts of complexation of test acids or bases with the surface FeOH sites.Polymer adsorption onto gamma iron oxides from multicomponent systems (three solvents and two polymer species) was studied by another FTIR technique, photo-acoustic spectroscopy, and the results of such adsorption on the dispersibility and dispersion stability of the gamma iron oxides were assessed by photon correlation spectroscopy. Photon correlation spectra of solutions of the basic polymer polyvinylmethylether in basic organic solvents showed that some of the polymer was in the form of one micron aggregates which caused rapid flocculation of the oxide, but upon addition of the acidic epoxy resin the polymer was less aggregated and the oxide became well dispersed.     

Negative Electrorheological Effect of Silicone Gels Containing Barium Titanate

Summary: We observed the negative electrorheological phenomenon on swollen silicone gels containing barium titanate. Contrary, no negative electrorheological effect was observed in unswollen silicone gels containing barium titanate and silicon gels without barium titanate. The change in storage modulus increased with increasing DC electric field strength. The effects of frequency, degree of swelling, and the field strength have been investigated.

Electric‐field dependence of the change in storage modulus at f = 0.1 Hz for swollen silicone/BaTiO₃ gels with varying CAT content; (○): CAT 0.1, (□): CAT 0.2, (⋄): CAT 0.3, (×): SG.     

Photochemical control of network structure in gels and photo-induced changes in their viscoelastic properties

Poly(amide acid) gels containing photosensitive azobenzene groups in the main chain have been synthesized and their viscoelastic properties before and after light irradiation have been investigated by dynamic viscoelasticity measurements. It was found that 405 nm light induced a local volume change and a two-fold increase in the storage modulus of the gels. We discuss the change in storage modulus upon light irradiation quite simply in terms of classical rubber elasticity theory, which cannot explain this large increase in storage modulus. The photo-induced increase in storage modulus may result from an increase in entanglement interactions of topological constraints fixed in the network structure, caused by photoisomerization of the azobenzene moieties. We suggest that topological constraints in the network structure of the gels were realized by light irradiation and calculate the resulting slip link ratio (index of the topological constraints) in the gel network.     

钡铁氧体包覆碳纤维的制备及毫米波衰减性能

通过溶胶凝胶法,在碳纤维表面均匀包覆了一层厚度约为1 μm的钡铁氧体(BaFe₁₂O₁₉)。采用SEM、FTIR、XRD、XPS等技术对碳纤维/钡铁氧体复合材料的组成、结构、性能进行了表征和分析。利用8 mm波雷达装置测试了碳纤维、碳纤维/钡铁氧体复合材料的毫米波衰减性能。实验结果表明：由于碳纤维/钡铁氧体复合材料兼具电损耗和磁损耗吸收,其8 mm波衰减性能明显优于单纯的碳纤维。