G. Goulart Silva

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.

Solid state polymeric electrolytes based on poly(epichlorohydrin)

Elastomers constituted of the homopolymer of epichlorohydrin, its copolymer with ethylene oxide and its terpolymer with ethylene oxide and allyl-glycidyl-ether were studied as polymeric electrolytes in polymer/LiClO4 systems. The variation of the ionic conductivity with salt concentration indicated that the copolymer and the terpolymer show the highest conductivities, ca. 10−5 S cm−1 at 30 °C. Also, the fitting of the ionic conductivity versus inverse of temperature curves with the VTF equation indicated a conductivity mechanism similar to poly(ethylene oxide) for the copolymer and the terpolymer. The potential stability window determined by cyclic voltammetry is 3.5 V and the polymers are stable to heating up to 250 °C. These results confirm the feasibility of using the copolymer and the terpolymer systems with LiClO4 as polymeric solid state electrolytes.

Free volume‐size dependence on temperature and average molecular‐weight in poly(ethylene oxide) determined by positron annihilation lifetime spectroscopy

Positron lifetime measurements were carried out in a series of poly(ethyl- ene oxide)—PEO— of different average molecular weights (M # w): 1000, 1500, 6000, 10,000, 300,000, and 4 M. The mean radius (R # ) and the mean free volume size (Vf) values were determined using a semiempirical equation that correlates the ortho- positronium (o-Ps) lifetime (t3) and size of holes existing in the amorphous phases. The hole mean radius values determined at room temperature from lifetime spectra were found to be between 2.68 and 2.97 A, and the hole volumes between 80 and 110 A 3 . Free volume size evolution was studied with temperature variation until the melting tem- perature of the PEO samples. The degree of crystallinity and the melting temperatures were determined by Differential Scanning Calorimetry.

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.

Electrochemical behavior of polyurethane ether electrolytes:carbon black composites and application to double layer capacitor

Abstract The thermal and electrochemical properties of polyurethane electrolyte/carbon black composites have been studied. The material structure presented amorphous features and the electrical response was described in terms of equivalent circuits accounting for the different relaxation processes occurring in the medium. The conductivity behavior did not fit satisfactorily either the effective medium theory or conventional percolation models. Flexible electrochemical capacitors using the composite as active material in association with large area collectors have been built and tested. The performance of the capacitors were characterized by a variety of electrochemical techniques, including impedance spectroscopy, cyclic voltammetry, galvanostatic charge–discharge methods, and long term cycling tests. The device prepared with composite electrodes containing 20 wt% of carbon black exhibited a capacity of 0.7 F cm −3 under 2 V providing an energy density of 0.4 W h l −1 .

A comparison of ionic conductivity, thermal behaviour and morphology in two polyether–LiI–LiAl5O8 composite polymer electrolytes

Abstract Two polymer hosts have been used to prepare composite electrolytes, i.e. PEO ( M W =1×10 6 ) and a triblock copolymer [poly(propylene glycol–ethylene glycol–propylene glycol)] with M W =2000. The effect of the LiAl 5 O 8 addition on the properties of electrolytes prepared with polyether–LiI was studied. The aluminate ceramic powder was prepared by coprecipitation and calcined at 1150°C. The main goal of the present investigation is to compare the two classes of composite electrolytes with polyether–LiI with a composition fixed at n =molar ratio of [O]/[Li]=20. LiAl 5 O 8 powder was dispersed by ultrasound in the electrolyte matrix in a range of concentrations up to 20 wt%. Differential scanning calorimetry curves indicated that all materials are semi-crystalline, and the triblock composites show a smaller glass transition ( T g =−28°C at 20 wt% of ceramic) than that of PEO electrolytes (−16°C at 20 wt% of ceramic). Thermomechanical analyses were performed using penetration mode and the composite electrolytes exhibited a significant increase in mechanical stability. The ionic conductivity of both systems (PEO and triblock–LiI–LiAl 5 O 8 ) is 5×10 −4 S/cm at 80°C. The triblock composite shows a conductivity of one order of magnitude higher than that of the PEO material near room temperature as a consequence of the decrease in crystallinity after a first heating. This result raises the interest in the development of composites based on a triblock polyether.

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.

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.

Pyrolysis of organotin compounds: A preparative method for nanometric tin dioxide powders

Nanometric tin dioxide powders were obtained by pyrolysis of organotin oxide based compounds such as Sn3O3Bu6 (1) and Sn4O6Bu4 (2) (Bu = n-butyl). The thermal behaviour of 1 and 2 was studied by thermogravimetric analysis (TG) and with simultaneous differential thermal analysis (DTA). Pyrolysis experiments were carried out in three different atmospheres (N2, air and O2) and the residual powders were characterised by X-ray diffraction (XRD) and the morphology was investigated by scanning electron microscopy (SEM). The XRD data showed the formation of pure SnO2 when the decomposition process was accomplished in air or O2, however, in N2, and for Sn4O6Bu4, the X-ray study indicated the formation of a mixture of SnO/SnO2. The micrographs revealed grains of 30–40 nm, approximately.

Micro‐Raman study of polydioxolane/LiClO4 and NaClO4 electrolytes

A micro‐Raman study of polydioxolane (PDXL)/NaClO4 and PDXL/LiClO4 polymeric electrolytes has been carried out in a wide range of salt concentration, at room temperature. The ion–ion interactions and the precipitation of salt microcrystals in the host polymer have been evidenced by means of a line shape analysis of the band associated to the totally symmetric stretching mode (ν1) of the ClO4− anion. The Raman spectra results reveal changes of the crystallinity of the PDXL/MCLO4 systems which are directly related to the salt concentration.