SYNTHESIS AND PROPERTIES OF POLYSTYRENE／LAPONITE NANOCOMPOSITES

Exfoliated polystyrene (PS)/laponite nanocomposites were prepared successfully. The characteristic doo1 diffraction peak of organo-laponite disappeared in the XRD patterns of nanocomposites, indicating that the laponite layers were exfoliated and the ordered crystal structure of laponite was destroyed because of the styrene polymerization. TEM observations showed that the exfoliated laponite primary particles were dispersed randomly in the PS matrix with lateral dimensions from 1 nm to 10 rim. SEM results showed that the PS/laponite nanocomposite particles were almost monodispersed spheres with the size of about 120 rim. Because of the interaction between PS and laponite nanolayers, the nanocomposites exhibited higher thermal stability and glass transition temperature when compared to pure PS.     

Fabrication of core/shell hybrid organic–inorganic polymer microspheres via Pickering emulsion polymerization using laponite nanoparticles

Oil-in-water (o/w) emulsions of styrene, as monomer oil in water, were achieved successfully via Pickering emulsification with laponite nanoparticles as the sole inorganic stabilizers. The formed emulsions showed excellent stability not only against droplets coalescence (before polymerization) but also against microparticles coagulation (after polymerization). Generally, the number of composite polystyrene microparticles (PS) increased and their sizes decreased with the content of solid nanoparticles used in stabilizing the precursor o/w emulsions. This is consistent with the formation of rigid layer(s) of the inorganic nanoparticles around the PS microparticles thus a better stability was achieved. The composite microparticles were characterized using various techniques such as surface charge, stability, transmission electron microscope (TEM), scanning electron microscope (SEM) and Fourier transform infra-red (FT-IR). Coating films of the prepared latexes were applied to flat glass surfaces and showed reasonable adhesion compared to PS latex particles prepared with conventional surfactants. The effect of employed conditions on the features of the resulting emulsions in terms of stability and particle size has been discussed.     

Aggregation of laponite in the presence of magnesium ions and polyacrylic acid

The average mass and size characteristics of ageing laponite/MgCl₂ and laponite/MgCl₂/polyacrylic acid were determined in aqueous dispersions at pH 10 using a Coulter counter and laser diffractometry methods. Comparison between mass and size characteristics showed the two systems to temporarily acquire a structure where particles are more densely packed. The relatively higher packing observed for laponite/polymer was attributed to conformational changes of the polymer adsorbed on neighbouring surfaces. Analysis of the reduced mass and size distribution functions confirmed the self-similarity of the distributions. The reduced mass frequency continuously decreased, while the reduced size frequency showed a linear decrease combined with bell-shaped populations. The volume packing observed for the laponite/polymer was attributed to a typical class of aggregates of large size, which was present at the maximal concentration in the bell-shaped distribution. Received: 19 February 2001 Accepted: 6 April 2001     

硅酸镁锂的有机改性及对Cr(Ⅵ)的吸附特性

采用水热法合成了硅酸镁锂(Laponite), 然后利用十六烷基三甲基溴化铵(CTMAB)对其进行有机改性, 研究了改性后的有机硅酸镁锂(CTMABL)对Cr(Ⅵ)的吸附特性. 结果表明, 改性后样品的比表面积和孔容积变小而平均孔径增大. CTMABL样品的d₍₀₀₁₎值从改性前的1.23 nm增加到1.79 nm, 表明CTMAB进入Laponite层间. 随着溶液pH值的提高, CTMABL对Cr(Ⅵ)的吸附效率明显下降; pH<8.5时, CTMABL颗粒表面电势为正, 能够与Cr(Ⅵ)阴离子发生静电吸引从而提高吸附效率. 随着固液比增加, 对Cr(Ⅵ)的去除效率迅速上升, 当固液比达到4 g/L后去除效率趋于稳定. 离子强度对Cr(Ⅵ)吸附过程的影响不明显. CTMABL对Cr(Ⅵ)的吸附符合准二级动力学模型, 吸附传质速率受膜扩散和颗粒内扩散过程共同影响. 等温吸附过程符合Langmuir模型, 热力学分析结果表明吸附过程是一个自发的吸热反应. 综合分析认为表面配合作用是主要的吸附机制, 同时静电引力在吸附过程中起到了促进作用.     

Evidence for re-entrant behavior in laponite–PEO systems

We observe evidence of re-entrant behavior in dispersions of a discotic clay, laponite, with added polymer. Under basic conditions, neat laponite forms a disordered colloidal glass. Rheologically, this phase behaves as a viscoelastic solid, and dynamic light scattering shows evidence of non-ergodic behavior. Addition of low molecular weight poly(ethylene oxide) (PEO) melts the glass, resulting in a low-viscosity liquid with fast dynamics. We believe this is due to a depletion force caused by excess PEO chains in solution. A viscoelastic solid is re-formed with the addition of high molecular weight PEO, which we believe to be caused by polymer chains bridging between laponite particles. The physics in our system is quite different from the hard sphere/nonadsorbing polymer systems for which re-entrant glass transitions have been reported in the literature; however, we believe there may be similarities between these phenomena. To our knowledge, this is the first evidence of a type of re-entrant behavior in anisotropic colloids.     

Adsorptive Stripping Voltammetric Determination of 2,4-Dichlorophenol by Laponite Modified Carbon Paste Electrode

A laponite modified carbon paste electrode was prepared, characterized and applied for the 2,4-dichlorophenol (2,4-DCP) voltammetric determination. It takes advantage of the ability of laponite to adsorb phenols, as well as of its availability and very low cost. Kinetic and equilibrium data for 2,4-DCP adsorption by laponite in aqueous dispersions demonstrated that the adsorption process obeyed a pseudo first order kinetic model and was consistent with the formation of adsorbed multilayers on a surface with heterogeneous pore distribution. The composite paste electrode exhibited a heterogeneous surface with 65 % increased surface area and 27 % enhanced catalytic activity compared to the unmodified one. The adsorptive stripping voltammetric determination of 2,4-DCP at an electrode with an optimized graphite:laponite ratio of 55 : 15 w% using a 3 min accumulation time at pH 5.5 was found to be suitable for its quantification in the linear concentration range extended up to 50 μmol L⁻¹ with a sensitivity of 0.56 μA L μmol⁻¹ and a LOD of 0.2 μmol L⁻¹ (S/N=3).The 2,4-DCP electrochemical response was not affected by the presence of some structurally similar phenols, like catechol and p-nitrophenol, while resorcinol, 2-chlorophenol, and 4-chlorophenol presented interferences. The results were validated by 2,4-DCP determination in spiked tap water.     

Synthesis of LDHs and Rheological Properties of Laponite／LDHs Dispersions

The chemical composition, morphology of the particles of positively charged layered double hydroxides (LDHs) were studied by ICP, Mastersizer, TEM. The preliminary results indicated that with adding LDHs to 1% laponite dispersion the yield stress of laponite/LDHs mixed dispersions increased at first, then decreased dramatically to a minimum, and the yield stress rose again when LDHs increased further.     

Improvement of poly(amphiphilic pyrrole) enzyme electrodes via the incorporation of synthetic laponite-clay-nanoparticles

The electropolymerization of an enzyme-amphiphilic pyrrole ammonium-laponite nanoparticles mixture preadsorbed on the electrode surface provides the simultaneous immobilization of the enzyme and the hydrophilic laponite-clay-nanoparticles in a functionalized polypyrrole film. The presence of incorporated laponite particles within the electrogenerated polymer induces a strong improvement of the analytical performances (I_max and sensitivity) of amperometric biosensors based on polyphenol oxidase. These beneficial effects have been attributed to a marked enhancement of the apparent specific activity of the immobilized enzyme (from 0.21 to 0.85% of the specific activity of the free enzyme), the permeability of the host polymer being unchanged. This strategy of biosensor performance improvement was tested with cholesterol oxidase as an enzyme model. The presence of laponite additive in the poly(amphiphilic pyrrole) host matrix induces a similar enhancement of sensitivity and I_max for cholesterol biosensing as well as a large improvement of the storage stability of the polypyrrole-cholesterol oxidase electrode.     

An L‐Lactate Amperometric Enzyme Electrode Based on L‐Lactate Oxidase Immobilized in a Laponite Gel on a Glassy Carbon Electrode. Application to Dairy Products and Red Wine

A biosensor based on the immobilization of Lactate oxidase in laponite–organosilasesquioxane films on glassy carbon electrode for the quantification of L ‐lactate in wine and dairy products is presented. The bioelectrode showed a very high sensitivity (0.33±0.01) A cm^?2 M^?1 and a short time response (10 s) for less than 1 U of enzyme. No significant interferences, including ascorbic acid, were detected. For red wine, matrix effects assigned to polyphenols and anthocyanins were observed, which ware easily overcome by sample dilution. Our L ‐lactate determinations were in good agreement with those of two standard methods.     

Field induced radial crack patterns in drying laponite gel

We present a study of crack patterns in a layer of laponite gel allowed to dry in a static electric field. Crack patterns in natural and synthetic clays have been studied extensively with interesting results. Since clay platelets have a surface charge in aqueous solution, it is natural to expect the cracking patterns to be affected by an electric field. This is the first report of such an observation in a radial electric field. The nano-sized disc-like laponite particles carry a quadrupole moment due to their charge distribution. The interaction of the quadrupole moment with the field gradient in a non-uniform field of radial symmetry is probably responsible for the characteristic pattern observed. The cracks start radially from the positive electrode. The same geometry with no field does not produce the characteristic pattern, neither does a uniform field with rectangular geometry.