Ailton S. Gomes

Principais métodos de caracterização da porosidade de resinas à base de divinilbenzeno

This paper reviews the most important methods used to characterize the porosity of styrene-divinylbenzene resins. Methods such as adsorption of nitrogen for determination of surface area and mercury intrusion porosimetry for characterization of pore size distribution are related.

Variation of the mesh size of PEO-based networks filled with TFSILi: from an Arrhenius to WLF type conductivity behavior

The properties of ionically conducting membranes based on polyethylene oxide (PEO) networks crosslinked with urethane functions, containing lithium bis(trifluoromethane sulfone imide) (TFSILi) were studied. The temperature and salt concentration influence on the ionic conductivity of these materials was analyzed. The best conductivity value measured at 30°C was 7×10−5 S cm−1 at O/Li=43 for a PEO molecular weight between crosslinks of 2000 g mol−1. The curves obtained in the Arrhenius coordinates were typical of a William Landel Ferry (WLF) like behavior when the molecular weight of the PEO between crosslinks was 1000 or 2000 g mol−1. However, the network based on PEO600 followed an Arrhenius behavior, typical of a network where segmental movements are prevented. A strong interaction between TFSILi, ethylene oxide units and urethane crosslinks functions at high urethane concentration explains these results. This finding demonstrates that the two types of conductivity behavior (Arrhenius or WLF) can be found for the same type of polymer electrolyte depending on the predominant elementary conductivity process.

The use of EVA-containing mercapto groups in natural rubber-EVA blends. II. The effect of curing system on mechanical and thermal properties of the blends

Natural rubber (NR)-poly(ethylene-co-vinyl acetate) (EVA) blends have been studied by incorporation of mercapto-modified EVA (EVASH) combined with dicumyl peroxide (DCP) as a curing agent. The mechanical, thermal, and morphological properties of NR-EVA blends as functions of blend composition and compatibilizer and/or curative additions were investigated. An EVASH-DCP combined system leads to the greatest improvement in tensile strength when EVA is dispersed within the NR matrix. Higher performance on Shore A hardness was also achieved with this combined system. For blends characterized by NR domains dispersed in the EVA matrix, the efficiency of pure DCP in improving the tensile strength is higher. Morphological observations and selective extraction experiments indicate the cross-linking of NR phase in both systems. A cross-linking of the EVA phase is also suggested by DCP or an EVASH-DCP combined system, based on the decreasing degree of crystallinity of this phase.

Crosslinked Sulfonated Polysulfone-Based Polymer Electrolyte Membranes Induced by Gamma Ray Irradiation

Sulfonated aromatic polymers generally show high swelling at high proton conductivity. This disadvantage makes many of them unfit for proton exchange membrane applications. Crosslinking of the polymer is one way to overcome this problem. In this study, radiation-induced crosslinking was performed on a sulfonated polysulfone membrane, with doses ranging from 2.5 to 25.0 kGy (dose rate: 45 Gy/min) using gamma rays from a 60Co source. The pristine sulfonated polysulfones was obtained by mild sulfonation of bisphenol-A-polysulfone with trimethylsilyl chlorosulfonate as sulfonating agent. The proton conductivity of the membranes was characterized by means of electrical impedance spectroscopy techniques. Ion-exchange capacity, degree of sulfonation, water content and chemical stability membrane properties were characterized before and after irradiation. The results show that the mechanical, chemical and thermal stability of the membrane improve after irradiation. The degree of sulfonation and the proton conductivity exhibit a tendency to decrease with increasing irradiation total dose.

Degradation of polymeric restorative materials subjected to a high caries challenge.

OBJECTIVES The aim of this study was to evaluate the degradation of different resin filling materials after a caries challenge, by high performance liquid chromatography (HPLC) and contact angle (θ) measurement. METHODS Four different polymeric restorative materials (a resin composite, a polyacid-modified resin composite, an ormocer and a resin-modified glass ionomer cement) were tested. Five samples (30 mm × 6 mm × 2 mm) of each material were formed in a Teflon mold, following the manufacturers instructions. After pH cycles, the solutions were injected in an HPLC. The θ was obtained, before and after pH cycle, by a goniometer at 60% air humidity and 25°C. A distilled water drop (0.006 ml) was put on the material surface, and after 6 min, 10 measures were obtained at 20s intervals. Each sample received 4 drops, one at a time, on different areas. RESULTS HPLC results showed elution of byproducts in all materials. This was greater in the acid medium. Bis-GMA and TEGDMA were detected in TPH Spectrum and Definite residues. Analyses of the contact angle by ANOVA and Student-Neuman-Keulss test showed that the surfaces of TPH Spectrum, Dyract AP and Definite were altered, except Vitremer (p<0.05). SIGNIFICANCE All materials tested degraded on a caries simulated medium, suggesting that a great effort should be made to disseminate oral health information, since a high caries challenge environment (low pH) can lead to dental composite degradation, with potential toxic risks to patients.

Preparação e caracterização de organobentonita modificada com ionenos alifáticos

The preparation of bentonite-ionene complexes based on the adsorption of 2y-ionenes, where y = 4, 6, 10 or 12 and an ionene of the epichloridrin-dimethylamine EPI-DMA kind, was carried out in order to observe the behavior of the ionene spacer size in the expansion of the basal space (d-spacing, d001) of a Brazilian commercial bentonite. The complexes were characterized by adsorption spectromety in the infrared region, thermogravimetry, fluorescence and X-Ray diffraction. Ionenes were synthesized and characterized by infrared and nuclear magnetic resonance, the latter technique being useful for determining the average number molecular weight values. Based on the clay-ionenes complexes it could be observed that the ammonium polyquaternaries are adsorbed on the inner and outer clay surfaces, above the ion exchange capacity, CEC = 91 meq/100 g clay (calculated based on X-Ray fluorescence data). The clay basal expansion with the several polycations revealed that the spacer size is the most important factor, independent of ionene molecular weight values. Among the bentonite-ionene complexes, mainly between Bt-2,10 and Bt-2,12-ionene complexes which differ in molar mass by more than 30,000 g/mol, it was observed that the spacer size has no dependence on the ionene molecular weights.

Synthesis and electrochemical characterization of crosslinked poly(ethylene oxide) containing LiN(CF3SO2)2

Abstract The ionic conductivity of polymeric electrolyte systems comprising a network of poly(ethylene oxide) (PEO) crosslinked with urethane functions and containing LiN(CF3SO2)2 (TFSILi), was studied. Polyurethanes based on three different PEO molecular weights were prepared. Diffusion studies of TFSILi in swollen PEO-urethane membranes with acetonitrile were performed showing a diffusion coefficient for PEO2000 higher than that observed with LiClO4 in the same material, and were correlated with the variation of Tgs of the dried membranes which were determined as a function of salt concentration and crosslink density. The influence of a salt molecule on the Tg (for the TFSILi charged membranes) is equivalent to 25% of the influence of a covalent crosslink in the range of molecular weight between crosslinks used in this study. The relatively high slope for the plot Tg−1 vs salt concentration for the network in PEO600 allows to consider a high chain rigidification due to the introduction of salt. The conductivity of network based on PEO2000 with a molar concentration ratio O Li = 43 reached 10−5 S cm−1.

Functionalisation of polypropylene with fluorinated acrylic monomers in the molten state

Abstract The free radical grafting of flurorinated acrylic monomers (FA) onto polypropylene (PP) was investigated in the molten state. Such a modified PP would display low surface energies which are useful for numerous applications. Styrene (St) was added as a comonomer to improve FA grafting and reduce the chain degradation of PP. The underlying grafting mechanism in the presence of St was that St reacted first with PP macro-radicals and the resulting styryl radicals copolymerized with the grafting monomers. St’s contributions were most important when the concentration of the free radical generator was lowest. It was more difficult to graft FA onto a PP containing a stabilizing package than if it was free of any additives.

Rheological properties of blends of polycarbonate with poly(ethylene oxide)

Blends of polycarbonate (PC) and poly(ethylene oxide) (PEO), of composition ranging between 5 and 30% in wt of PEO, have been investigated in order to study the effect of the addition of PEO on the flow properties of PC. The blends were prepared in the mixing chamber of a HAAKE torque rheometer Rheomix 600. An attempt was made to convert data obtained by the torque rheometer to fundamental units, such as apparent viscosity, and compare these results to the Instron 3213 capillary rheometer data. Viscosity values of the blends have been found to decrease with the true shear rate, and the decrease is greater for compositions rich in PEO.

Determination of accessible chloromethyl groups in chloromethylated styrene-divinylbenzene copolymers

This paper describes an evaluation of the method based on the quaternization of pyridine for determining the chloromethyl groups content in styrene-divinylbenzene copolymers with different kinds of porosity. Some modifications of the method described by Feinberg and Merrifield are proposed depending on the porous characteristic of the copolymer to be analyzed. The quaternization reaction was limited by the access of pyridine to chloromethyl groups that is governed by the swelling equilibrium of the copolymer in the reactant.