Abel García-Bernabé

Relaxation behavior of acrylate and methacrylate polymers containing dioxacyclopentane rings in the side chains

The synthesis of poly[(2,2-dimethyl-1,3-dioxolan-4-yl) methyl acrylate)] (PACGA) and poly[(2,2-dimethyl-1,3-dioxolan-4-yl) methyl methacrylate] (PMCGA) is reported. Both polymers present dielectric and mechanical β subglass absorptions at −128 and −115 °C, respectively, at 1 Hz, followed by ostensible glass–rubber or α relaxations centered in the vicinity of 0 and 67 °C, respectively, at the same frequency. The values of the activation energy of both the mechanical and dielectric β absorptions lie in the vicinity of 10 kcal mol−1. The critical interpretation of the relaxation behavior of PMCGA suggests that dipolar intramolecular correlations play a dominant role in the response of the polymer to an electric field. The subglass relaxations of PACGA and PMCGA are further compared with the relaxation behavior of poly(1,3-dioxane acrylate), poly(1,3-dioxane methacrylate), and other polymers in the glassy state. The strong conductive processes observed in PMCGA at low frequencies and high temperatures were studied under the assumption that that these processes arise from Maxwell–Wagner–Sillars effects occurring in the bulk combined with Nernst–Planckian electrodynamic effects caused by interfacial polarization in the films.

Conductivity of composite membrane-based poly(ether-ether-ketone) sulfonated (SPEEK) nanofiber mats of varying thickness

Nanofiber mats of SPEEK70wt%–PVB30 wt% (polyvinyl butyral)-based composite membranes were prepared by varying the electrospinning time in order to obtain mats with different thicknesses. These mats were embedded in SPEEK65wt%–PVA35wt% (polyvinyl alcohol) polymer solution to fill the pores in the fibers. The obtained membranes with different mat thicknesses have been characterized by water uptake, ionic exchange capacity, scanning electron microscopy, mechanical properties and proton conductivity. Microtensile test studies reveal that the maximum tensile strength increases as the thickness of the SPEEK–PVB nanofiber mats increases, resulting in more flexible composite membranes compared to a pure SPEEK–PVA membrane obtained by casting. The proton conductivity occurs more easily through the nanofiber than through the matrix phase, and the best conductivity (0.038 S cm−1) was measured at 120 °C for the composite membrane of SPEEK–PVB nanofiber mats obtained after 12 hours of electrospinning time. This value suggests that our composite membranes have high potential to function in the temperature range between 100 and 140 °C without losing their strength and while maintaining their high proton conductivity, making them an excellent candidate for fuel cells that operate at intermediate temperatures.

Size-Tunable Micron-Bubbles Based on Fluorous–Fluorous Interactions of Perfluorinated Dendritic Polyglycerols

This paper describes the behavior of various generations of polyglycerol dendrimers that contain a perfluorinated shell. The aggregation in organic solvents is based on supramolecular fluorous-fluorous interactions, which can be described by means of (19)F NMR spectroscopy. In order to study the interaction and aggregation phenomena of dendrimers with perfluorinated shell and perfluoro-tagged guest molecules we investigated [G3.5]-dendrimer with a perfluorinated shell in the presence of perfluoro-tagged disperse red. Noteworthy, the interaction intensities varied in an unexpected manner depending on the equivalents of perfluoro-tagged guest molecules added to the dendrimers in solution which then formed supramolecular complexes based on fluorous-fluorous interactions. We found that these complexes aggregated around residual air in the solvent to form stable micron-sized bubbles. Their sizes correlated with the interaction intensities measured for certain dendrimer-guest molecule ratios. Degassing of the solutions led to a quasi phase separation between organic and fluorous phase, whereby the dendrimers formed the fluorous phases. Regassing the sample with air afforded bubbles of the initial size again.

Dielectric properties of poly(triethylene glycol p,p′-bibenzoate-co-(R)-(+)3-methyl adipate)

Abstract In this study, a copolyester of poly(triethylene glycol p,p′-bibenzoate-co-(R)-3-methyl adipate) was prepared. The mesophase texture, the thermal and dielectric properties (a.c.-dynamical as well as thermostimulated depolarization currents) were studied. X-ray diffractograms reveal a relatively stable mesophase smectic Sc, whereas DSC shows on heating only the glass transition and the endotherm due the isotropization of the mesophase. Dynamic dielectric measurements show a broad sub-Tg relaxation together with an α loss peak (associated with the glass transition) overlapped with a significant conductive contribution. After separating this contribution from the dipolar component of the dielectric loss, we have modeled the relaxation according to classical empirical models. Thermostimulated depolarization currents also show three relaxations. The fine structure of these relaxations is analyzed by the thermal sampling technique.

Dielectric relaxation studies on thermotropic side-chain liquid crystalline polymers

The dielectric activity of a thermotropic side-chain liquid crystalline polymer is analysed. The sub-glass relaxations are deconvoluted by two different empirically based equations (Fuoss–Kirkwood and Havriliak–Negami). The conductivity contributions are studied following a methodology proposed by Macdonald and Coelho. The effects of the orientation of the mesogenic group under electrical fields are also analysed. We propose a general method to estimate the curve of loss permittivity with a determined value of Sd based on the knowledge of two experimental curves, each corresponding to one well-known orientation. © 2001 Society of Chemical Industry

Memory function for dielectric relaxation

The second-order memory function (SOMF) for the dicyclohexylmetyl-2metyl succinate is obtained by using simple numerical manipulation of the experimental dielectric data. According to the prescription given in a previous paper [J. Chem. Phys. 109, 9057 (1998)], the frequency behavior of the real and imaginary parts of the SOMF is discussed in terms of the Havriliak-Negami equation of the dielectric function, and together with the three-variable model describing the evolution of the torque-autocorrelation function. Furthermore, in this paper we present the temperature dependence of the parameters, which characterize the SOMF behavior for two ester substances.

Proton Conducting Electrospun Sulfonated Polyether Ether Ketone-Graphene Oxide Composite Membranes

A series of novel composite membranes, based on sulfonated poly(ether ketone) (SPEEK) with a graphene oxide (GO) layer, were prepared. One contained a GO layer sandwiched between the SPEEK–polyvinyl alcohol (PVA) matrix (SPEEK/PVA@GO), and another deposited thin layers of GO on the nanofibers of SPEEK–polyvinyl butyral (PVB), with both sandwiched in the phase matrix of SPEEK–PVA (SPEEK/PVA@GO-NF). Various nanofiber thicknesses were studied by varying the electrospinning time. The prepared composite membranes with different nanofiber thicknesses were characterized by scanning electron microscopy, water uptake, ionic exchange capacity, thermogravimetric analysis, mechanical properties and proton conductivity. Our results showed that the proton conductivity of SPEEK/PVA@GO membranes increased with temperature, from 1 × 10−3 S cm−1 at 30 °C to 8.3 × 10−3 S cm−1 at 130 °C. These conductivity values are higher than those observed for the membrane with SPEEK/PVA@GO-NF nanofibers. However, a conductivity comparison of the different thicknesses of SPEEK/PVA@GO-NF nanofibers allowed us to conclude that conductivities increase with nanofiber thickness at all temperatures. Finally, the calculated activation energy of the SPEEK/PVA@GO membrane (1.4 kJ mol−1) was found to be one order of magnitude lower than that for pure SPEEK/PVA (17.3 kJ mol−1). This reduction indicated that the influence of temperature on the conductivity decreases when GO is inserted into SPEEK/PVA membranes. In the case of the SPEEK/PVA@GO-NF membrane, the activation energy decreased as a function of the nanofiber networks thickness.

Fractional Fokker–Planck equation approach for the interconversion between dielectric and mechanical measurements

This work describes a model of interconversion between mechanical and dielectric measurement. A previous version of this model has been proposed in the hereafter called “previous paper” [Diaz-Calleja, et al.Phys. Rev. E 72, 051505 (2005)], starting from a scaling relation between the translational and rotational viscosities present in a glass forming liquid near the glass transition temperature. Now, in order to improve the previous procedure, the following modifications have been made: (1) a definition of the rotational viscosity obtained from a fractional Fokker–Planck equation has been used, (2) the complex translational viscosity is taken as a non-Newtonian one, (3) a careful splitting of α and β relaxations is necessary, because the interconversion algorithm depends on the underlying molecular mechanism of each relaxation, and (4) the modulus and phase angle of complex viscosities were analyzed instead of real and imaginary parts of the complex viscosities. The proposed interconversion model, in the ...

Phosphoric Acid Doped Polybenzimidazole (PBI)/Zeolitic Imidazolate Framework Composite Membranes with Significantly Enhanced Proton Conductivity under Low Humidity Conditions

The preparation and characterization of composite polybenzimidazole (PBI) membranes containing zeolitic imidazolate framework 8 (ZIF-8) and zeolitic imidazolate framework 67 (ZIF-67) is reported. The phosphoric acid doped composite membranes display proton conductivity values that increase with increasing temperatures, maintaining their conductivity under anhydrous conditions. The addition of ZIF to the polymeric matrix enhances proton transport relative to the values observed for PBI and ZIFs alone. For example, the proton conductivity of PBI@ZIF-8 reaches 3.1 × 10−3 S·cm−1 at 200 °C and higher values were obtained for PBI@ZIF-67 membranes, with proton conductivities up to 4.1 × 10−2 S·cm−1. Interestingly, a composite membrane containing a 5 wt.% binary mixture of ZIF-8 and ZIF-67 yielded a proton conductivity of 9.2 × 10−2 S·cm−1, showing a synergistic effect on the proton conductivity.

INTERCONVERSION MODEL FOR MECHANICAL AND DIELECTRIC α‐RELAXATIONS MEASUREMENTS

This work describes a new algorithm of interconversion between mechanical and dielectric measurement. Another version of this algorithm has been proposed in a previous paper [1], starting from a scaling relation between the translational and rotational viscositieś, which is assumed in a glass forming liquid near the glass transition temperature.In order to improve the previous procedure, the following modifications have been made: 1) a new definition of the complex rotational viscosity in terms of a fractional power, 2) the complex translational viscosity is taken as a non‐Newtonian one, 3) the modulus and phase angle of viscosities were analyzed instead of real and imaginary viscosities and 4) the characterization of the α dielectric relaxation has been carried out by means of the dielectric retardation time spectrum.It is usual that the α and secondary relaxation processes are overlapped in the dielectric and mechanical spectra. Both types of processes have distinct physical basis and different molecula...