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ABSTRACT: This paper reports the synthesis, characterization, and thermal stability of cation-exchange membranes based on naphthalenic copolyimides. To improve the mechanical stability, the membranes were cross-linked through the reaction of sulfonic acid groups with hydrogen atoms of activated phenyl groups located in neighboring chains using appropriate catalysts. The ion-exchange capacity of the membranes is higher than 2 meq/(g of dry membrane), and the number of molecules of water per ion-exchange group lies in the vicinity of 20. The response of the membrane electrode assembly (MEA) to electrical perturbation fields in a wide range of frequencies was measured, and the equivalent electrical circuit governing the response is discussed. The conductivity of the MEAs at 75 °C is of the order of 10−2 S/cm. Hydrogen crossover in the membranes was determined by electrochemical measurements and the permeability coefficient of the gas in the membranes determined. Finally, the single cell exhibits a fair performance, the power of the cell at 75 °C being only 225 mW/cm2 or lower, at 0.5 V.
12/2010;
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ABSTRACT: This article describes the diffusion and permeability of oxygen, carbon dioxide, methane, ethane, ethylene, propane, and propylene in 1-octene based polyethylene of densities 0.94, 0.92, 0.904, and 0.87. The isotherms obtained in the time-lag experimental device display a diffusion coefficient and permeability behavior similar to that of glassy polymers. We apply the dual model to semicrystalline polymers assuming that Henry's sites are related to the amorphous phase, which decreases when the crystallinity percentage increases. Whereas the interphase of the polymeric matrix and the crystalline phase prevails and acts as Langmuir sites. Their effect is to increase both, the tortuosity of diffusion trajectories and the chain immobilization. We now explain this effect using thermodynamic considerations. In fact, the tortuosity is related to the change in activation entropy, and the chain immobilization to the cohesive energy of the polymeric matrix. In those terms, the diffusion coefficient does not follow the same crystalline percentage dependence than the solubility. According to the previous findings, the solubility changes in proportion to amorphous percentages. Instead, diffusion coefficient has exponential dependence. Furthermore, we show that the permeability changes as a consequence of the modification of diffusion and solubility, according to the product of both quantities. Comparison with previous publication has been included. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 634–642, 2010
Journal of Polymer Science Part B Polymer Physics 02/2010; 48(6):634 - 642. · 1.53 Impact Factor
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ABSTRACT: Diffusion of gases in polymers below the glass transition temperature, Tg, is strongly modulated by local chain dynamics. For this reason, an analysis of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements considering the viscoelastic behavior of polymers is proposed. Carbon-13 PFG NMR measurements of [13C]O2 diffusion in polymer films at 298 K are performed. Data obtained in polymers with Tg above (polycarbonate) and below (polyethylene) the temperature set for diffusion measurements are analyzed with a stretched exponential. The results show that the distribution of diffusion coefficients in amorphous phases below Tg is wider than that above it. Moreover, from a PFG NMR perspective, full randomization of the dynamic processes in polymers below Tg requires long diffusion times, which suggests fluctuations of local chain density on a macroscopic scale may occur. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 231–235, 2010
Journal of Polymer Science Part B Polymer Physics 12/2009; 48(2):231 - 235. · 1.53 Impact Factor
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ABSTRACT: This work describes the preparation of mixed matrix membranes by casting from poly(1,4-phenylene ether−ether−sulfone) chloroform solutions containing dispersed zeolitic imidazolate frameworks. Diffusive studies of CO2 in the pristine poly(1,4-phenylene ether−ether−sulfone) membrane and composite membranes were performed at 6 bar and 298 K, using pulsed field gradient NMR techniques. The evolution of the heterogeneity of the diffusion environments as seen by NMR was monitored in terms of the diffusion time and a stretching parameter. The values of the self-diffusion coefficient increase with filler content, from 2.1 × 10−8 cm2 s−1 for pristine membranes to 9.3 × 10−8 cm2 s−1 for membranes with 30 wt % of filler. Sorption and permeation experiments carried out at different pressures were used to determine the dual-mode model parameters that describe the transport processes. Apparent diffusion coefficients of CO2 in the membranes were obtained from the time lag method, from parameters of the dual-mode model, and directly from the derivatives of the steady flux, expressed in terms of concentration and pressure, with respect to pressure. In general, the values of the apparent diffusion coefficients obtained by the three methods for pristine membranes are in rather good agreement with the self-diffusion coefficient obtained by the NMR technique. In composite membranes, the values of the self-diffusion coefficients are nearly 2 times those obtained using permeation and sorption experiments. The discrepancies between the values of the self-diffusion coefficients and the results obtained for the diffusion coefficient by other techniques are discussed. The filler contributes greatly to gas permeation by increasing the gas solubility in the composite membranes.
11/2009;
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ABSTRACT: This work describes proton transport in membranes cast from dimethyl sulfoxide solutions of polyelectrolytes obtained by polycondensation of 4,4′-diaminodiphenyl ether (ODA) and 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODADS) with 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), the moles of sulfonated diamine per mole of unsulfonated one being roughly 3/1. Pulsed field gradient (PFG) NMR studies reveal two kinds of water: water located in the pores of the membranes appearing in the range 5 to 1 ppm and a minor amount of water associated with the imide groups, appearing at 1 ppm. The diffusion coefficient of 1H in the first type of water is about 2 orders of magnitude higher than that measured in the second type and in both cases the values of this parameter severely decrease as the water content of the membranes decreases. The diffusion coefficients of bare protons, hydronium ions and water in the membranes were calculated using molecular dynamics techniques. For membranes with low water content, the diffusion coefficient of 1H is very close to the diffusion coefficients of water and hydronium ions obtained by simulation. At high concentrations the simulated values are higher than D(1H). The simulated values obtained for the diffusion coefficients of hydronium ion and water for membranes equilibrated with water are fairly close to those estimated, respectively, from proton conductivity and osmotic measurements. This work suggests that the study of cation-exchange membranes in the acidic form using NMR, conductivity, and molecular dynamics simulation techniques provides useful information on how structure and water content affect transport processes in membranes.
08/2009;
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ABSTRACT: This work describes the calculation of retardation time spectra by minimization of the square of the differences between experimental compliance results and those recalculated from the spectra. Spectra were computed taking analytical complex dielectric results as the basis of the minimization process. Comparison of the spectra computed from both the complex dielectric permittivity and the dielectric loss with those calculated analytically shows that minimization methods based on complex dielectric permittivity data are more accurate than those based only on loss dielectric results.
The Journal of chemical physics 10/2008; 129(10):104513. · 3.09 Impact Factor
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ABSTRACT: A comparative study of the dielectric relaxation behavior of two structurally close polymers containing aliphatic-aromatic side groups was carried out in order to get a better understanding on how slight differences in chemical structure affect the molecular responses to perturbation fields. Specifically, chain dynamics of poly(2-acryloyloxyethyl-2-naphthalene-2-ylacetate) and poly(2-methacryloyloxyethyl-2-naphthalene-2-ylacetate) were studied by broadband dielectric spectroscopy in the frequency range of 10(-2)-10(8) Hz and temperature window of 298-403 K. Also, the relaxation behavior of (2-acetyloxyethyl-2-naphthalene-2-ylacetate), model compound of the polymer side groups, was analyzed. The isotherms representing the dielectric loss in the frequency domain show important conductive contributions, especially at high temperature, which hide the low frequency side of the alpha relaxation. Conductivity also increases the real component of the complex permittivity in the low frequencies region. Retardation spectra were obtained by minimizing the sum of the squares of the difference between the experimental values of the complex permittivity for each frequency and the analytical ones, predicted by the linear phenomenological theory, using a Tikhonov regularization technique. The spectra present an apparent alpha peak with an excess wing at short time side resulting from the overlapping of the true alpha relaxation and a beta process. Three absorptions, named in increasing order of time gamma, beta, and alpha relaxations, are separated by deconvolution methods. The activation energies associated with the gamma process are 70.0+/-1.8, 68.0+/-1.4, and 74.8+/-0.8 kJ mol(-1) for (2-acetyloxyethyl-2-naphthalene-2-yl acetate), poly(2-acryloyloxyethyl-2-naphthalene-2-yl acetate) and poly(2-methacryloyloxyethyl-2-naphthalene-2-yl acetate), respectively. The respective activation energies associated with the beta relaxation are 121.7+/-2.4, 135.3+/-1.4, and 141.6+/-1.3 kJ mol(-1). Values of the shape parameters and the strengths of the relaxation processes were obtained as a function of temperature. The dynamic fragility of the polymers and the model compound was studied and compared with that reported for macromolecular and monomeric systems. Also, the evolution of the size of the correlated domains associated with the alpha relaxation was estimated. Finally, the gamma relaxation rather than the beta absorption obeys the criteria apparently held by the Johari-Goldstein beta processes.
The Journal of chemical physics 09/2008; 129(5):054903. · 3.09 Impact Factor
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ABSTRACT: Gas transport of carbon dioxide in poly[bisphenol A carbonate-co-4,4'-(3,3,5-trimethylcyclohexylidene)diphenol carbonate] films over a wide range of pressure is described. The interpretation of the experimental results in terms of the dual mode model allowed the evaluation of the parameters of the model that govern the gas permeation process. The value of the diffusion coefficient obtained for carbon dioxide at zero concentration was 2.4 x 10(-8) cm(2) s(-1), at 303 K. This parameter was also measured by using pulsed field gradient NMR finding that its value reaches a nearly constant value of (2.7 +/- 0.9) x 10(-8) cm(2) s(-1), at 298 K, for diffusion times greater than 20 ms. Both the diffusion and solubility coefficients were also computed by using simulation methods based on the transition states theory and the Widom method, respectively. The value obtained for the diffusion coefficient was 1.8 x 10(-8) cm(2) s(-1), at 303 K, which compares very favorably with the experimental measurements. The drop of the simulated solubility coefficient with increasing pressure is sharper than that of the experimental one, at low pressures, and similar, at high pressures.
The Journal of Physical Chemistry B 05/2008; 112(14):4253-60. · 3.70 Impact Factor
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ABSTRACT: A series of sulfonated diamines were synthesized which were further used to obtain relevant sulfonated naphthalenic copolyimides. Tough and ductile membranes were cast from solutions of the copolyimides in dimethylsulfoxide, which exhibit high ion-exchange capacity and high water uptake. The protonic conductivity of the membranes equilibrated with water lies in the range 1.0-8.6 S/m, at 25 degrees C, being of the same order of magnitude as that reported for perfluorinated acidic membranes. The values of the transport number of protons and sodium ions are close to the unit for very dilute electrolyte solutions, but they lie in the range 0.80-0.90 for moderate concentrations. The membranes exhibit rather high electroosmotic permeability. The similarity of the diffusion coefficients of protons and water in the membranes suggests that the Grottus mechanism governs the protonic conductive process in the acidic membranes equilibrated with water.
The Journal of Physical Chemistry B 01/2008; 111(49):13694-702. · 3.70 Impact Factor
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ABSTRACT: The non-Debye relaxation behavior of hyperbranched polyglycerol was investigated by broadband dielectric spectroscopy. A thorough study of the relaxations was carried out paying special attention to truncation effects on deconvolutions of overlapping processes. Hyperbranched polyglycerol exhibits two relaxations in the glassy state named in increasing order of frequency beta and gamma processes. The study of the evolution of these two fast processes with temperature in the time retardation spectra shows that the beta absorption is swallowed by the alpha in the glass-liquid transition, the gamma absorption being the only relaxation that remains operative in the liquid state. In heating, a temperature is reached at which the alpha absorption vanishes appearing the alphagamma relaxation. Two characteristics of alpha absorptions, decrease of the dielectric strength with increasing temperature and rather high activation energy, are displayed by the alphagamma process. Williams' ansatz seems to hold for these topologically complex macromolecules.
The Journal of Chemical Physics 10/2007; 127(12):124904. · 3.33 Impact Factor
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ABSTRACT: This article describes the solubility of carbon dioxide, ethylene and propane in 1-octene based polyethylene of 0.94, 0.92, 0.904, and 0.87 densities. The isotherms obtained in the gas sorption experimental device display a sorption behavior similar to that of glassy polymers. We apply the dual model to semicrystalline polymers assuming that Henry's sites are related to the amorphous phase, which decreases when the crystallinity percentage increases, whereas the surface of the crystalline phase acts as a Langmuir site with higher gas-polymer affinity than glassy polymers. The good concordance of the calculated kD values, using the Flory-Huggins theory of polymer diluent mixtures, with the experimental results suggest that Henry's gas sorption fulfills this theory and, therefore, it may be a suitable way to estimate polymer-gas enthalpic interactions. Particularly, the variation of kD with the crystallinity fraction is exponential and the proportionality of the total sorption with the amorphous content seems only apparent. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1798–1807, 2007
Journal of Polymer Science Part B Polymer Physics 06/2007; 45(14):1798 - 1807. · 1.53 Impact Factor
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ABSTRACT: This work reports the solubility of carbon dioxide, ethylene and propylene in low density polyethylenes with densities lying in the range 0.870–0.940 g/cm3. The sorption behavior is studied as a function of the crystallinity in terms of the Flory-Huggins theory of polymer solutions. For each gas, both the Henry's solubility constant referred to the amorphous phase of the films and its temperature dependence do not change significantly with the crystallinity of the LDPE films. The enthalpic polymer–gas interaction changes with the nature of the gases, but not with the crystallinity of the films. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007
Journal of Applied Polymer Science 04/2007; 105(2):903 - 907. · 1.29 Impact Factor
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ABSTRACT: Full molecular dynamics was used to simulate separately the diffusion of naked protons and H3O+ hydrated protons across sulfonated poly(phenyl sulfone)s. Simulations were carried out for wet membranes with the following characteristics: ion-exchange capacity, 1.8 mequiv/g of dry membrane; water uptake, 10−30%; temperature range, 300−360 K. The diffusion coefficient of naked protons is nearly 1 order of magnitude higher than that of the hydrated protons for the membranes with the lower water uptake (10%). For the membranes with higher water uptake the ratio between the diffusion coefficients of the two particles reduces to about half an order of magnitude. The conductivity of the naked protons increases from 21.4 × 10-3 to 52.5 × 10-3 S/cm when the water uptake increases from 10% to 30%. For hydrated protons the conductivity increases from 1.54 × 10-3 to 7.57 × 10-3 S/cm. The conductivities obtained through simulations carried out at 300 K for the hydrated proton across membranes with water uptake 18% and 30% are roughly similar to those experimentally measured for a membrane with ion exchange capacity = 1.8 mequiv/g and water uptake = 24.3%. Simulated conductivities of both naked protons and hydrated protons follow Arrhenius behavior.
11/2006;
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ABSTRACT: This work reports the relaxation behavior of amorphous polymers the spectra of which do not explicitly present the secondary β relaxation. Poly(2,4-difluorobenzyl methacrylate), a polymer with complex motions in the side groups, was chosen to carry out this study. The apparent distribution of retardation times of the polymer calculated from dielectric loss isotherms in a wide range of temperatures (T > Tg) presents two peaks which do not merge into a single absorption at moderate temperatures. The β absorption appearing as a shoulder of the α relaxation in global thermal stimulated depolarization current experiments is not at first sight detected in the retardation spectra. Separation of the hidden β absorption from the longest time peak in the retardation spectra was carried out by fitting the inverse of the Havriliak−Negami equation to the peak using b = 1 and b < 1 for the skewness shape parameter of the β and α relaxations, respectively. Moreover, the shortest time peak in the retardation spectra was found to be the result of two overlapping peaks, named in increasing order of time γ and γ‘. Arrhenius plots for the α and β absorptions present a new scenario characterized for displaying both relaxations the same temperature dependence at temperatures not far above Tg. To investigate how small differences in the chemical structure may influence the dielectric response of polymers to perturbation fields, the results obtained for poly(2,4-difluorobenzyl methacrylate) are compared with those previously reported for poly(3-fluorobenzyl methacrylate). The differences in the responses of both polymers are interpreted in terms of molecular motions of the side groups.
06/2006;
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ABSTRACT: The relaxation behavior of poly(3-methylbenzyl methacrylate), poly(3-fluorobenzyl methacrylate), and poly(3-chlorobenzyl methacrylate) was thoroughly studied by broadband dielectric spectroscopy with the aim of investigating the influence of slight differences in chemical structure on the response of polymers to electric perturbation fields. Retardation spectra calculated from dielectric isotherms utilizing linear programming regularization parameter techniques were used to facilitate the deconvolution of strongly overlapped absorptions. Above the glass transition temperature, the spectra of the two halogenated polymers present a secondary γ process well separated from a prominent peak resulting from the overlapping of the α and β relaxations. The spectra of poly(3-methylbenzyl methacrylate) exhibit at long times a well-developed α absorption followed in decreasing order of time by two weak absorptions, named β and γ, whose intensities increase with temperature. The temperature dependence of the distance of the α peak from the β and γ peaks, expressed in terms of log(fmax,β/fmax,α) and log(fmax,γ/fmax,α), respectively, is studied. The Williams ansatz and the extended ansatz give a fairly good account of the relaxation behavior of the polymers. The stretch exponent associated with the α relaxation increases with temperature from ca. 0.2 at low temperatures to the vicinity of 0.5 at high temperatures. At low temperatures, the α relaxation is described by a Vogel-type equation, but at high temperature the β and α processes are roughly described by the same Arrhenius equation. In the whole temperature range, the activation energy o the γ relaxation is significantly lower than that of the β absorption. The mechanisms involved in the development of the secondary relaxations are qualitatively discussed.
03/2006;
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ABSTRACT: The relaxation behavior of poly(2,3-dichlorobenzyl methacrylate) is studied by broadband dielectric spectroscopy in the frequency range of 10(-1)-10(9) Hz and temperature interval of 303-423 K. The isotherms representing the dielectric loss of the glassy polymer in the frequency domain present a single absorption, called beta process. At temperatures close to Tg, the dynamical alpha relaxation already overlaps with the beta process, the degree of overlapping increasing with temperature. The deconvolution of the alpha and beta relaxations is facilitated using the retardation spectra calculated from the isotherms utilizing linear programming regularization parameter techniques. The temperature dependence of the beta relaxation presents a crossover associated with a change in activation energy of the local processes. The distance between the alpha and beta peaks, expressed as log(fmax;beta/fmax;alpha) where fmax is the frequency at the peak maximum, follows Arrhenius behavior in the temperature range of 310-384 K. Above 384 K, the distance between the peaks remains nearly constant and, as a result, the a onset temperature exhibited for many polymers is not reached in this system. The fraction of relaxation carried out through the alpha process, without beta assistance, is larger than 60% in the temperature range of 310-384 K where the so-called Williams ansatz holds.
The Journal of Chemical Physics 10/2005; 123(11):114904. · 3.33 Impact Factor
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ABSTRACT: The oxygen permeability and diffusion coefficients of hydrogel membranes prepared with copolymers of 2-ethoxyethyl methacrylate (EEMA)/2,3-dihydroxypropylmethacrylate (MAG) with mole fraction of the second monomer in the range between 0 and 0.75 are described. Values of the permeability and diffusion coefficients of oxygen are determined by using electrochemical procedures involving the measurement of the steady-state current in membranes prepared by radical polymerization of the monomers. The results obtained for the transport properties were analyzed taking into account the fractional free volumes, the cohesive energy densities and the glass transition temperatures of the hydrogels.
Biomaterials 07/2005; 26(18):3783-91. · 7.40 Impact Factor
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ABSTRACT: This work reports sorption processes of oxygen, carbon dioxide, methane, ethylene, and propylene in films of both vulcanized natural rubber and vulcanized rubber–regenerated cellulose composites. The curves representing the pressure dependence of the concentration of carbon dioxide in the composites clearly exhibit a slight concavity with respect to the abscissa axis as a result of adsorption processes taking place in Langmuir sites located in the glassy cellulose component. Adsorption processes are also detected in the sorption curves of ethylene at low pressures. The concavity with respect to the ordinate axis of the curve concentration of propylene versus pressure at high pressure is pretty well described by the Flory-Huggins formalism. The solubilities of the other gases mainly obey Henry's behavior, adsorption processes in the glassy component being in most cases negligible. Values of the interaction χ parameter for gas–natural rubber and gas–natural rubber composites are obtained from the comparison of the experimental solubility coefficients with those predicted by the Flory-Huggins theory. The theory suggests that Henry's constant is a linear function of the boiling temperature of the gases. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2131–2140, 2005
Journal of Polymer Science Part B Polymer Physics 06/2005; 43(16):2131 - 2140. · 1.53 Impact Factor
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ABSTRACT: The relaxation behavior of poly(5-acryloxymethyl-5-methyl-1,3-dioxacyclohexane), a polymer containing highly flexible side groups, is studied by broadband dielectric spectroscopy in the frequency and temperature ranges 10(-1)-10(9) Hz and 123-473 K, respectively. Above the glass transition temperature T(g) the dielectric loss in the frequency domain exhibits a prominent alpha absorption, followed in increasing order of frequencies by two secondary absorptions called beta and gamma. At temperatures slightly higher than T(g), the a relaxation is well separated from the beta, but as temperature increases overlapping between both relaxations augments forming an alphabeta absorption in the vicinity of 420 K. This latter absorption displays a shoulder on its high-frequency side corresponding to the y relaxation. The strength of the a relaxation decreases with increasing temperature, eventually vanishing at the temperature at which the alphabeta absorption is formed. The time retardation spectra of the isotherms are calculated and further used to facilitate the deconvolution of the overlapping relaxations. The fact that the temperature dependence of the beta relaxation also describes that of the alphabeta absorption suggests that both relaxations have the same nature. It seems that as temperature increases, the a relaxation feeds on the beta absorption until its complete disappearance. The gamma relaxation, in turn, seems to increase at the expense of the alphabeta process at high temperature.
The Journal of Chemical Physics 06/2005; 122(19):194905. · 3.33 Impact Factor
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10/2003;
