Electrochemical behaviour of copper/LDPE composites in the simulated uterine solution

Two metal/insulator composites, copper/low-density polyethylene (LDPE) microcomposites and copper/LDPE nanocomposites, were prepared in a uniform metal mass fraction. The corrosion characteristics of the two composites in the simulated uterine solution have been studied by cyclic voltammetry, potentiodynamic polarization measurement and electrochemical impedance spectroscopy. Results obtained during this study show that there are obvious differences on the corrosive rates and mechanisms between the two materials. The nanocomposites have stable corrosive reaction, hardly passivation, low corrosion rate and high transformation ratio of copper to soluble cupric ions compared with microcomposites, which indicate that the nanocomposites are perfect to be the novel material for intrauterine device. The corrosion behaviour of the microcomposites was controlled by both the processes of the pervasion of Cu²⁺ and the transfer of the charge, while that of the nanocomposites was only controlled by the transfer of the charge. This paper also compared the corrosion behaviours of the composites and bulk copper in the simulated uterine solution, there were more insoluble subproducts in the corrosion process of bulk copper than the composites. Further, the electrochemical method in this study is proved to be useful to characterize the corrosion behaviour of high electrical resistivity materials.     

无压烧结Al2O3/Si3N4纳米复合陶瓷的力学性能

本文对Al2O3／Si3N4体系进行无压烧结。获得试样相对密度大于98％，采用物相分析，烧结体中并没有Si3N4颗粒存在而是形成SIALON相。Si3N4和Al2O3反应生成的β-SIALON相颗粒不仅分布在Al2O3晶粒晶界处也存在于Al2O3晶粒内部，形成独特的“内晶型”结构。当受到外力时既能诱发穿晶断裂，又能引起裂纹偏转，从而起到增强的作用。由于产生晶界滑移，韧性有所下降。     

An improved technique for exfoliating and dispersing nanoclay particles into polymer matrices using supercritical carbon dioxide

An environmentally benign process, which uses supercritical carbon dioxide (sc-CO₂) as a processing aid, is developed in this work to help exfoliate and disperse nanoclay into the polymer matrices. The process relies on rapid expansion of the clay followed by direct injection into the extruder where the mixture is dispersed into the polymer melt. Results from the mechanical properties, rheological studies, and X-ray diffraction (XRD) show that this method represents a significant improvement relative to direct melt blending in single or twin-screw extruders or other methods using sc-CO₂. The greatest mechanical property response was a result of directly injecting pre-mixed sc-CO₂ and nanoclay into the polypropylene melt during extrusion. It was observed that for concentrations as high as 6.6 wt% (limited only by present process capabilities), XRD peaks were eliminated, suggesting a high degree of exfoliation. Mechanical properties such as modulus increased by as much as 54%. The terminal region of the dynamic mechanical spectrum was similar to that of the base polymer, contrary to what is frequently reported in the literature.     

Polyimide/montmorillonite nanocomposites based on thermally stable, rigid-rod aromatic amine modifiers

The preparation and processing of most of polymer/clay nanocomposites need high temperature. This limited the application of commonly used organic modifiers of long carbon-chain alkyl ammonium salts because of their low thermal stability. In this study, we synthesized two novel thermally stable, rigid-rod aromatic amines. Montmorillonite (MMT) treated by these amines exhibited larger layer-to-layer spacing, higher thermal stability than that treated by commonly used 1-hexadecylamine and also high ion-exchange ratio (>95%). They were applied to prepare nanocomposites with polyimide (PI) by in situ polymerization. XRD, TEM were used to obtain the information on morphological structure of PI/MMT nanocomposites. DMA, TGA, DSC, universal tester were applied to characterize the mechanical and thermal properties of the nanocomposites. When the MMT content was below 3 wt%, the PI/MMT nanocomposites were strengthened and toughened at the same time. The introduction of a small amount of MMT also led to improvement in thermal stability, slight increase in glass transition temperature, marked decrease in coefficient of thermal expansion and decrease in solvent uptake. MMT treated by these aromatic amines exhibited better dispersibility and (probably) interfacial interaction with PI matrix than that treated by 1-hexadecylamine. The nanocomposites based on these MMT resultantly exhibited better mechanical, thermal and solvent resistance properties than those based on 1-hexadecylamine treated MMT.     

Linear viscoelasticity and structure of polypropylene-montmorillonite nanocomposites

Polypropylene (PP)/clay composites were prepared by melt mixing in a thermoplastic mixer using a polypropylene grafted with maleic anhydride (PPg) as the compatibilizer. Concentrations of an organophilic montmorillonite (MMT) between 2 and 15 wt% and concentration ratios of PPg/clay between 1:3 and 3:1 were employed to investigate the relationship between the structural characteristics of the hybrids and their rheological properties. The structure was analyzed with electron microscopy, X-ray diffraction and melt rheology. Thermogravimetric analysis and infrared spectroscopy were also used. The clay interlayer spacing increases after mixing with PP while the addition of PPg only facilitates the partial exfoliation of the clay platelets without changing that spacing. When clay loadings of 8 wt% or larger were used, an important fraction of the original clay particles was found to remain unmodified. The dynamic moduli show little effect of the presence of the inorganic material when no compatibilizer is added or the amount of PPg or clay is too small. As the extent of exfoliation increases, the linear viscoelastic behavior of the composites gradually changes with time while in the molten state, mainly at low frequencies. Evidence of solid-like behavior appears as the concentration of clay increases, for a given PPg/clay or PP/PPg concentration, or as the PPg concentration increases (for a given clay concentration). The concentrations of PPg and clay that induce percolation were observed to have an inverse relation. Evidence of regions with large concentration of MMT was obtained in the annealed samples of composites with solid-like rheological behavior. Additionally, infrared spectra of these materials suggest the simultaneous occurrence of chemical reactions between the PPg and the surfactant or products derived from its thermal decomposition during the annealing process.     

Morphology, structure and properties of a poly(1-butene)/montmorillonite nanocomposite

A specifically formulated nanocomposite based on isotactic poly(1-butene) (PB) and montmorillonite was studied, by wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and polarized light optical microscopy, investigating the polymorphism of the polymer, and examining the interaction between PB and the silicate. Montmorillonite was found to disrupt the ordered morphology of the polymer, determining a dramatic increase in the rate of the II→I phase transition. Interaction between polymer and clay was studied by TEM and SAXS also under a quantitative point of view. A significant enhancement of physical-mechanical properties was observed, even though exfoliation did not occur, but just a slight intercalation and a reduction in the size of tactoids.     

Nanoparticle induced network self-assembly in polymer-carbon black composites

A novel percolation phenomenon with inorganic nanoparticle loading in polyamide 6-carbon based nanoparticle hybrids was identified. Percolation threshold substantially shifts to lower carbon black (CB) volume fractions in the presence of optimum concentration of chemically modified montmorillonite (organoclay) while the effective organoclay concentration can be optimized to lower the slope of percolation curve maintaining electrical conductivity within static dissipative 10⁻⁶-10⁻⁹ S cm⁻¹ range. Organoclay/CB ‘nano-unit’ morphology was found in polyamide 6 ternary hybrids. It is composed of stacked organo-montmorillonite platelets that deform to wrap partially around one or two primary CB aggregates. This elementary nano-unit structure induces CB network self-assembly within polyamide 6 matrices. The structure was found to be prevalent throughout the polymer matrix. This morphology remains robust under wide range of thermal-deformation histories due to the strong preferred organoclay/polyamide 6/CB interactions that partially blocks the electron conduction and hopping mechanisms with clay ‘walls’ thereby reducing the slope of the percolation curve. Organoclay can be used as a dispersion control agent in these polymer-carbon systems to induce self-assembly of CB network at low CB content, simultaneously, partial blocking the electron hopping pathways to level the slope of percolation curves. High order exfoliation and nano-scale dispersion of organoclay is essential to induce this advanced percolation phenomenon.     

Poly(l-lactide) comb polymer brushes on the surface of clay layers

Poly(l-lactide) (PLLA) comb polymers on poly(hydroxyethyl methacrylate) (PHEMA) backbone were prepared on the surface of clay layers by a combination of in situ atom transfer radical polymerization (ATRP) and ring-opening polymerization. An ATRP initiator with a quaternary ammonium salt end group was intercalated into the interlayer spacing of clay. PHEMA polymer brushes on the surface of clay layers were prepared by in situ ATRP. PLLA comb polymers on PHEMA backbone were prepared by ring-opening polymerization. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that with the increase of comb chain length more and more exfoliated structure was created. Aggregation of wormlike comb polymer brushes on the surface of clay layers was observed by TEM. Differential scanning calorimeter (DSC) results indicated that both the melting points and glass transition temperatures of the comb polymer brushes increase with the increase of comb chain length. The equilibrium melting temperature of the comb polymer brush on the surface of clay layers is lower than cleaved polymer. An atomic force microscopy (AFM) image proves the formation of wormlike structure by cleaved comb polymers.     

Comparison of nanocomposites based on nylon 6 and nylon 66

Nylon 6 and nylon 6,6 organoclay nanocomposites were prepared by melt processing using a twin screw extruder. The effects of polyamide type and processing temperature on the mechanical properties and the morphology of the nanocomposites were examined. Mechanical properties, transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), percentage crystallinity and isothermal thermo-gravimetric analysis (TGA) data are reported. A particle analysis was performed to quantitatively characterize the morphology; these results are later employed in modeling the modulus of these materials using composite theory. No significant difference was observed in the mechanical properties and morphology of PA-6 nanocomposites processed at two different temperatures. PA-6 nanocomposites had superior mechanical properties than those made from PA-66. The tensile strength of PA-66 nanocomposites deviated from linearity at high levels of MMT. WAXD and TEM results show that the PA-6 nanocomposites are better exfoliated than the PA-66 nanocomposites, which exhibit a mixture of intercalated and exfoliated structures. Mechanical properties were consistent with the morphology. DSC reveals a higher percentage of crystallinity in the PA-66 samples. Isothermal TGA shows only a 5% difference in the degradation of the organic modifier on the organoclay processed at 240 °C versus 270 °C. Particle analysis shows a higher average particle length and thickness, and a lower average particle density and aspect ratio in nanocomposites based on PA-66 versus PA-6. The Halpin-Tsai and Mori-Tanaka composite theories predict satisfactorily the behavior of the PA-6 nanocomposites, while the PA-66 nanocomposites were predicted acceptably up to a certain volume fraction where the non-linear behavior takes effect. All the results indicate that there is a lower degree of exfoliation in the nanocomposites produced with a PA-66 matrix apparently stemming from the chemical differences between PA-6 and PA-66.     

Thermo-mechanical properties of LLDPE/SiO2 nanocomposites

Two series of linear low density polyethylene (LLDPE)/SiO₂ nanocomposites were prepared. They were based on two types of commercial LLDPE, one prepared by metallocene (mLLDPE) and the other by traditional Ziegler-Natta (zLLDPE) catalysts, and silica nanoparticles surface treated with dimethyldichlorosilane. The silica nanonparticles used have an average diameter of 16 nm, and their weight fraction varied from 2 up to 10%. The structure and thermal-mechanical features of the nanocomposites were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMA) as well as tensile tests. The effect of nanoparticles on crystallinity, and hence to the morphology of the materials was studied. The secondary transitions were also affected by the filler presence, while the tensile properties were reinforced with varying the nanoparticle weight fraction. The addition of the nanofillers brought up an increase in the elastic modulus and the tensile strength of mLLDPE accompanied by an unusual dramatic increase in the elongation at break. The same trend, although to a lesser extent, was observed for the zLLDPE/SiO₂ composites. The increment of the elastic modulus of the composites with increasing filler content was simulated with three micromechanical models developed in previous works. The model which assumes an effective interface between the matrix and the nanoparticles provided the best fitting with the experimental data of mLLDPE/SiO₂.