A mathematical approach to assess research diversity: operationalization and applicability in communication sciences,political science,and beyond

Scientometrics - With today’s research production and global dissemination, there is growing pressure to assess how academic fields foster diversity. Based on a mathematical problem/solve...     

Physico‐Mechanical Properties of Biodegradable Starch Nanocomposites

Nanocomposites of cassava starch reinforced with waxy starch nanocrystals were prepared. They showed a 380% increase of the rubbery storage modulus (at 50 °C) and a 40% decrease in the water vapor permeability. X‐ray spectra show that the composite was more amorphous than the neat matrix, which was attributed to higher equilibrium water content in the composites. TGA confirmed this result and its thermal derivative suggested the formation of hydrogen bonding between glycerol and the nanocrystals. The reinforcing effect of starch nanocrystals was attributed to strong filler/matrix interactions due to the hydrogen bonding. The decrease of the permeability suggests that the nanocrystals were well dispersed, with few filler/filler interactions.

     

Specific induction of uncoiling in NORs of human acrocentric chromosomes by 5-azacytidine and 5-azadeoxicytidine

The organization of rDNA-containing chromatin was analysed by transmission electron microscopy after treatment of cultured human lymphocytes with 5-azacytidine (ACR) or 5-azadeoxicytidine (AdCR). The number of observed acrocentric chromosomes with satellites was significantly increased after treatment with low doses of ACR or AdCR during the last 24 h of culture, whereas with exposures during the last 7 h the number remained normal. The results suggest that the incorporation of ACR and AdCR in the early period of the S-phase may have reverted the non-satellized to satellized chromosomes. The cytidine analogues may have become more visible during secondary constriction thus changing the NOR structure leading to an increased number of satellized chromosomes.     

Dynamic mechanical analysis of particulate‐filled epoxy resin

The dynamic mechanical properties of epoxy composites filled with different amounts of quartz powder were investigated. The storage modulus and loss tangent were measured at frequencies between 7.8 and 323 Hz from room temperature up to 460 K. The influence of the filler content on the temperature and frequency behavior of the dynamic mechanical properties is discussed and explained in terms of models presented in the literature. In particular, the dependence of the composite damping with the quartz content is explained with regard to damping due to particle–particle and polymer–particle interaction. Also, the glass‐transition temperature as a function of filler content was obtained and was related to the results obtained for the apparent activation energies of the α relaxation, which were estimated with the Williams, Landel, and Ferry equation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 883–892, 2003     

Cure kinetics and shrinkage model for epoxy-amine systems

Manufacture of most of epoxy resins implies that cure needs to be carried out under pressure. Due to the significance of knowing the influence of the pressure factor in cure kinetics, cure shrinkage of a stoichiometric epoxy-amine system was measured using a pressure-volume-temperature (PVT) analyzer. Recording the specific volume change in the range of temperature from 100 to 180 °C and a pressure of 200 bar we could model the cure kinetics. The Runge-Kutta method was applied to obtain the kinetic constants of the cure reaction. In addition, using the differential scanning calorimeter (DSC) for measurements of 1 bar and the PVT analyzer for pressures of 200, 400, and 600 bar, we also model the kinetic constants as a function of pressure. The results obtained show that the effect of the temperature on the kinetic constants is higher than the effect of pressure. Therefore, both PVT and DSC are complementary techniques to describe the full range of cure kinetic process of epoxy mixtures.     

Comparative analysis of electric,magnetic, and mechanical properties of epoxy matrix composites with different contents of multiple walled carbon nanotubes

Epoxy composites based on aligned chemical vapor deposition–grown multiwall carbon nanotubes (MWCNTs), containing trapped iron nanoparticles, with weight fractions ranging from 0.03 to 1 wt%, were produced following a well‐known processing way. Electrical and mechanical properties as well as their densities were measured. The results are compared with previous studies investigating the magnetic percolation behavior in the same samples of composites. A percolation threshold at 0.4 wt% of MWCNTs was determined by the electrical property and it agrees with the threshold observed in the magnetic properties. A hop in the porosity and a depression of the bulk mechanical properties were also found at the threshold value. These results point out that there is an efficiency threshold related to the sonication stage of the fabrication process and the weight fraction of MWCNTs, and after this threshold, the initial nanotubes pans were not separated into individual MWCNTs or into small ropes. An optimized sonication stage is proposed and the improvement in the dispersion of the filler is shown. POLYM. COMPOS., 28:612–617, 2007. © 2007 Society of Plastics Engineers     

Accelerator adsorption onto carbon nanotubes surface affects the vulcanization process of styrene–butadiene rubber composites

The multiwalled carbon nanotubes (MWCNT) filled styrene–butadiene rubber (SBR) composites were prepared by incorporating MWCNT in a SBR/toluene solution and subsequently evaporating the solvent. These composites have shown a significant improvement in Young's modulus and tensile strength with respect to SBR gum without sacrificing high elongation at break. However, this improvement is less than expected at the higher filler content. Then, the influence of low concentrations of MWCNT on the vulcanization process of the SBR composites was studied by means of rheometer torque curves, swelling measurements, differential scanning calorimeter (DSC) analysis, and Fourier transform infrared (FTIR) spectroscopy. Also, their thermal degradation was studied by thermogravimetric analysis (TGA). It has been noticed that MWCNT affects the cure kinetics of SBR gum matrix reducing all parameters, i.e., the total heat rate and order of the reaction, scorch delay, maximum torque, and crosslink density. This effect increases as MWCNT content does, and it was attributed to the adsorption of the accelerator employed in the vulcanization (N‐tert‐butyl‐benzothiazole‐2‐sulfenamide) onto the MWCNT surface. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Yield and internal stresses in aluminum filled epoxy resin. A compression test and positron annihilation analysis

The influence of the filler content on the mechanical properties of an epoxy resin composite filled with aluminum powder was investigated. Compressive tests were performed at room temperature and at different strain rates. The response of the composites was also studied by positron annihilation lifetime spectroscopy. The dependence of the yield stress on the filler content is shown. The results are discussed in terms of a proposed model that takes into account the contribution of the filler powder. To this purpose information from positron spectroscopy is important since it allows to correctly evaluate the internal stresses introduced in the composite epoxy lattice by the metal filler.     

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.