A specific heat anomaly in multiwall carbon nanotubes as a possible sign of orientational order-disorder transition

We present micro-calorimetric specific heat measurements on different multiwall carbon nanotubes of large outer diameter, as a function of temperature in the range 10-120 K. A clear anomaly at 60 K with the shape of a peak is present in two of the samples, and both the height and the characteristic temperature of the peak are independent of magnetic field and do not exhibit thermal hysteresis, discarding magnetic degrees of freedom related to Fe seeds or contributions from adsorbed gases. These features suggest that the anomaly may be caused by a structural change. As the anomaly is also unaffected by induced intertube disorder, it may be related with a melting of orientational dislocations of individual tubes within multiwall nanotubes, an effect that was theoretically predicted to occur in carbon nanotubes and represents a distinctive feature of asymmetric molecular systems.     

A comparison between the physico-chemical properties of tuber and cereal starches

Biofilms based on waxy maize and cassava starches (cereal and tuber starch, respectively), plasticized with glycerol were characterized through their crystallinity, dynamic-mechanical behavior (DMA), thermal degradation (TGA), moisture content and water vapor permeability (WVP). X-ray diffraction experiments show that both materials were mainly amorphous, with the waxy starch presenting a discreetly A-type X-ray pattern. Microscopic investigation of the cryo-fractured surfaces supported this observation. The glass transition of the glycerol-rich phase (measured by DMA) occurs at higher temperatures for cassava than for waxy maize starch, suggesting that the dispersion level of glycerol is higher in the former. TGA showed that maize starch has a slightly higher thermal stability than cassava starch, while glycerol interacts more strongly with the last one. The WVP was 18% higher in the case of the cassava starch film.     

Magnetic properties of multi-walled carbon nanotube-epoxy composites

Epoxy matrix composites have been prepared using as filler different weight fractions (0.03-1 wt%) of CVD grown multi-walled carbon nanotubes (MWCNTs) containing trapped iron nanoparticles. Magnetic characterization performed at room temperature with magnetic field between −19 and 19 kG shows that the nanocomposites are weakly ferromagnetic at room temperature. An abrupt variation in the dependence of both the coercivity field, B_C, and remanent magnetization, M_R, with the weight fraction of MWCNTs, ?, was found for ?=0.4%. Results suggest the formation of MWCNTs agglomerates for increasing weight concentrations, in accordance with SEM micrographs.     

Dynamical properties of epoxy composites filled with quartz powder

The dynamical mechanical properties of epoxy composites filled with different amounts of quartz powder were investigated. The loss tangent and the storage Young’s modulus were measured at frequencies between 5 and 323 Hz with a tension–compression device, from room temperature up to 460 K. The glass transition temperature and the apparent activation energy increase with filler content. The dynamic modulus also increases with quartz powder percentages. Test frequency and temperature dependence of these results were analyzed within the scope of models presented in the literature for composite materials.     

Direct relationships between volume variations at macro and nanoscale in epoxy systems. PALS/PVT measurements

The free volume in epoxy systems fully cured with aminic hardeners having different chemical structures was studied by means of positron annihilation lifetimes spectroscopy. The results are compared with those obtained from the analysis of the macroscopic specific volume changes by using pressure-volume-temperature experimental technique. An excellent correlation between the volumes measured at macro and nanoscales was found. From this fact, it can be asserted that the variation of the specific volume is mainly controlled by the variation in the average size of sub-nanometer size local free volumes, which are fixed by the chemical structure of hardener in fully crosslinked epoxy networks. These results give the first evidence of the direct relation existing between macro and nanoscales in thermosetting networks.     

A comparison of chitosan-silica and sodium starch glycolate as disintegrants for spheronized extruded microcrystalline cellulose pellets

Chitosan-silica coprecipitate (C-S) has recently been proposed as a tablet disintegrant. In this study we compared it with a 1:1 physical mixture of chitosan and silica (C/S) at the same composition as the coprecipitate, and with the widely used commercial disintegrant sodium starch glycolate (SSG), as regards to its behavior in spheronized extruded pellets of microcrystalline cellulose (MCC) containing hydrochlorothiazide as a typical poorly water-soluble drug. In all three cases, possible synergism between the disintegrant (0-5%) and sorbitol (0-50%) was also evaluated. All the formulations examined exhibited appropriate morphology and had satisfactory mechanical and flow properties. Drug release depended mainly on sorbitol content, however C-S accelerated drug release at all sorbitol levels (the fastest release was from 50% sorbitol pellets with C-S, which disintegrated), whereas C/S did not vary drug release from pellets, and SSG depressed drug release, especially from 50% sorbitol pellets.     

Variation in physical and mechanical properties with coating thickness in epoxy–diamine–aluminum system

The mechanical properties and glass‐transition temperature within different thickness organic coatings made of diglycidyl ether of bisphenol A epoxy resin and 3‐aminomethyl‐3,5,5‐trimethylcyclohexylamine hardener are determined. The coatings are deposited on aluminum alloy (1050) substrates after degreasing. Dynamic mechanical thermal analysis and differential scanning calorimetry experiments are carried out on debonded coatings before and after the material from the opposed surface to the polymer/metal interface is removed by polishing. The results clearly show that the values of the physical and mechanical properties in those coatings depend on their thickness, but there is no gradient of properties within such coatings. Therefore, at a given thickness, those properties are homogeneous within the coating. To gain a better fundamental understanding of this behavior, a qualitative model involving the chemical reactions that take place at the epoxy/metal interface and the related diffusion phenomena is given. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 891–895, 2005