J.C. Machado

Preparation and thermal study of polymers derived from acrylamide

Abstract Polyacrylamide (PAA) and some of its N -alkyl substituted derivatives have been synthesised and characterised using thermal analysis. Thermal stability was studied by thermogravimetry (TG) and differential scanning calorimetry (DSC) was used to determine the glass transition temperatures ( T g ). The results showed an increase in degradation temperature ( T d ) as the hydrogens of the amide nitrogen were replaced by alkyl groups. An opposite tendency was verified for T g values. The observed increase in T d values is discussed in terms of the formation of an intermediate ion in the degradation, while the decrease in T g values is discussed in terms of polymer–polymer interactions such as hydrogen bonds.

Mechanical and thermal characterization of native Brazilian Coir fiber

Coir fiber native to the Brazilian northeast coast has been characterized by mechanical, thermal, and microscopy techniques. The tensile strength, initial modulus, and elongation at break were evaluated for untreated and alkaline-treated fibers. The results showed an enhancement of mechanical properties after 48-h soaking in 5 wt % NaOH. The thermal stability slightly decreased after this alkaline treatment. A thermal event was observed between 28 and 38°C. The heat capacity, Cp, as a function of temperature curves between −70 and 150°C, were obtained for the untreated and alkaline-treated coir fibers. The morphologies of the coir-fiber surfaces and cross sections were observed by scanning electron microscopy. The properties and the morphologies were discussed, comparing the native Brazilian coir fiber with the more extensively studied native Indian coir fiber.

Positron annihilation and differential scanning calorimetry investigations in poly(methylmethacrylate)/low molecular weight poly(ethylene oxide) polymer blends

Positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC) measurements were performed in atactic poly(methylmethacrylate) and low molecular weight poly(ethylene oxide) (PEO) polymer blends, prepared by codissolution in acetonitrile, covering the full range of composition. Results from the two techniques indicate that a window of miscibility is attained at around 20-30 wt % of the semicrystalline PEO.

Nanoheterogeneities in PEO/PMMA blends: A modulated differential scanning calorimetry approach

Modulated differential scanning calorimetry has been carried out on melt-mixed blends of poly(ethylene oxide)/atactic-poly(methyl methacrylate) (PEO/PMMA). Two PEO molecular weights have been used to prepare blends in the concentration range 10 to 80 wt % of PEO. Two glass transitions temperatures were observed for the fully amorphous blends, in the 10 to 30 wt % PEO range, using the differential of heat capacity with respect to temperature [dC p /dT] signal. The semicrystalline blends, 40, 60, and 80 wt % PEO, exhibited melting of PEO crystallites and the PEO-rich phase glass transition at -30 to -50°C. A second glass transition around 30°C was detected for the 40 wt % PEO blend when a cooling run was carried out, because PEO crystallization was avoided under these conditions. Therefore, heterogeneous amorphous phases were observed not only for fully amorphous blends, but also for semicrystalline ones. Further analysis of the dC p /dT signal, obtained from the MTDSC experiments by fitting with Gaussian curves, showed that there is an interphase that varies in amount between 10 to 50 wt %. Correlation of the MTDSC observations with NMR spectroscopy and SAXS/SANS literature results are discussed.

Positronium formation in lanthanide (III) trifluoroacetate with 3-picoline-N-oxide compounds

Positron annihilation lifetime and Doppler broadening annihilation radiation line shape on the salts Ln(CF3COO)3·xH2O and the adducts Ln(CF3COO)3(3-picNO)·xH2O (Ln=La–Pr, Sm–Lu and Y, and 3-picNO=3 picoline-N-oxide, x in the range 0.75–1.33) were performed. Positronium formation in the adducts, except for the europium complex, was observed. These results are discussed in terms of the electronic properties of the central metal ions.

Conductivities, thermal properties and Raman studies of poly(tetramethylene glycol) based polymer electrolytes

Abstract Polymer electrolytes prepared with lithium perchlorate and low molecular weight poly(tetramethylene glycol) (PTMG) homopolymer and PTMG/poly(ethylene glycol) (PEG) copolymer were investigated. Complex impedance measurements, thermogravimetric analysis, differential scanning calorimetry and Raman spectroscopy were used to characterize the systems. Ionic conductivity results at room temperature for the homopolymer are lower than those reported for the well-studied PEG, while those for the copolymer are similar. The most concentrated samples showed the presence of crystalline polymer–salt complexes for both the polymer hosts. An increase of the ionic association with increase of salt concentration is observed.

Kinetic study of the thermal decomposition on bis(dialkyldithiocarbamate) Cd(II) complexes by isothermal and non-isothermal thermogravimetry

Abstract The thermal decomposition kinetics of solid Cd(S 2 CNR 2 ) 2 complexes, where R=C 2 H 5 , n -C 3 H 7 , n -C 4 H 9 and iso-C 4 H 9 , has been studied using isothermal and non-isothermal thermogravimetry. Superimposed TG/DTG/DSC curves show that thermal decomposition reactions occur in the liquid phase. The kinetic model that best adjusted the experimental isothermal TG data was the one-dimensional phase-boundary reaction controlling process R 1 . The thermal analysis data suggest the thermal stability sequence Cd(S 2 CNBu n 2 ) 2 >Cd(S 2 CNPr n 2 ) 2 >Cd(S 2 CNBu i 2 ) 2 >Cd(S 2 CNEt 2 ) 2 , according to the order of stability of apparent activation energy.

Solid Eu(III) complexes studied by positron annihilation, optical and Mössbauer spectroscopies: insights on the positronium formation mechanism.

In this work, positron annihilation lifetime (PALS), Doppler broadening annihilation radiation lineshape (DBARL), Mössbauer and optical spectroscopies measurements were performed in Eu(III) dipivaloylmetanate complex, Eu(dpm)(3), at 295 and 80 K. The Eu(dpm)(3) complex is not luminescent at 298 K and does not form positronium. On the other hand, it is highly luminescent at 80K, but still does not form positronium. The absence of positronium formation at 80K cannot be explained by a ligand/metal charge transfer process. We found strong evidences that the electronic delocalization does not occur at both temperatures. Despite the Mössbauer results being inconclusive regarding the Eu(III)/Eu(II) reduction hypothesis, previous results showing positronium formation in other Eu(III) complexes suggest that this process is not occurring. Thus, more studies are needed to explain the absence of positronium in Eu(III) complexes.

Inhibition of positronium formation and its quenching reactions in binary solid solutions and mixtures of metal tris-2,2,6,6-tetramethyl-3,5-heptanedionate (dpm) complexes

Abstract The inhibition of positronium formation and its quenching reactions have been studied in binary solid solutions formed by Al(dpm) 3 as a matrix, and Co(dpm) 3 , Fe(dpm) 3 and Mn(dpm) 3 as guest molecules, by lifetime and Doppler broadening spectroscopies. In all the solid solutions studied the quenching of positronium lifetime by redox reactions was observed. The total inhibition and quenching rate constants determined for each system are similar. The full-width at half-maximum of the annihilation line increases with increasing guest molecule mole fraction, indicating the presence of redox reactions combining with inhibition processes. The corresponding mixtures were studied as blanks.

NANOCOMPÓSITOS DE POLIURETANA TERMOPLÁSTICA E NANOTUBOS DE CARBONO DE PAREDES MÚLTIPLAS PARA DISSIPAÇÃO ELETROSTÁTICA

Polyurethane/multi-walled carbon nanotube (MWCNT) nanocomposites have been prepared with nanotube concentrations between 0.01 wt% and 1 wt%. MWCNT as-synthesized samples with ~74 nm diameter and ~7 μm length were introduced by solution processing in the polyurethane matrix. Scanning electron microscopy (SEM) images demonstrated good dispersion and adhesion of the CNTs to the polymeric matrix. The C=O stretching band showed evidence of perturbation of the hydrogen interaction between urethanic moieties in the nanocomposites as compared to pure TPU. Differential scanning calorimetry and positron anihilation lifetime spectroscopy measurements allowed the detection of glass transition displacement with carbon nanotube addition. Furthermore, the electrical conductivity of the nanocomposites was significantly increased with the addition of CNT.