Suzana P. Nunes

Inorganic modification of proton conductive polymer membranes for direct methanol fuel cells

Abstract New organic–inorganic composite membranes based on sulfonated polyetherketone (SPEK) and sulfonated poly(ether ether ketone) (SPEEK) for application in the direct methanol fuel cell (DMFC) were synthesized. The membranes’ water and methanol permeabilities were evaluated in pervaporation experiments and the conductivity determined by impedance spectroscopy. The permeabilities were decreased by inorganic modification. The first approach was the in situ generation of SiO 2 , TiO 2 or ZrO 2 . The modification with ZrO 2 led to a 60-fold reduction of the methanol flux. However, a 13-fold reduction of conductivity was also observed. Modification with organically modified silane led to a 40-fold decrease of the water permeability without a large decrease of proton conductivity. A good balance of high conductivity and low water and methanol permeability was possible with a mixture of ZrO 2 and zirconium phosphate. In this case, a 28-fold reduction of water flux was observed with only 10–30% reduction of proton conductivity.

Switchable pH-Responsive Polymeric Membranes Prepared via Block Copolymer Micelle Assembly

A process is described to manufacture monodisperse asymmetric pH-responsive nanochannels with very high densities (pore density >2 × 10(14) pores per m(2)), reproducible in m(2) scale. Cylindric pores with diameters in the sub-10 nm range and lengths in the 400 nm range were formed by self-assembly of metal-block copolymer complexes and nonsolvent-induced phase separation. The film morphology was tailored by taking into account the stability constants for a series of metal-polymer complexes and confirmed by AFM. The distribution of metal-copolymer micelles was imaged by transmission electron microscopy tomography. The pH response of the polymer nanochannels is the strongest reported with synthetic pores in the nm range (reversible flux increase of more than 2 orders of magnitude when switching the pH from 2 to 8) and could be demonstrated by cryo-field emission scanning electron microscopy, SAXS, and ultra/nanofiltration experiments.

Dense hydrophilic composite membranes for ultrafiltration

Dense hydrophilic composite membranes for ultrafiltration were prepared from an asymmetric porous PVDF support coated with a thin (< 1 μ) layer of polyether-block-polyamide copolymer. Membranes with a molecular weight cut-off between 800 and 4500 g/mol and water permeability between 2.3 and 9.41 h− m−2 bar−1 were obtained. The dry membrane surface was characterized as a dense non-porous layer when observed by scanning electron microscopy. When compared to other commercial membranes and to the non-coated porous PVDF support in the ultrafiltration of oil-water waste, the performance of the composite membranes was comparable to the Amicon YM30 cellulose membrane with lower susceptibility to fouling.

Ultrafiltration membranes from PVDF/PMMA blends

Abstract Asymmetric membranes for ultrafiltration were obtained from PVDF/PMMA blends. Addition of 1% PMMA to the casting solution increases the water permeability 14-fold without loss of retention. Pore size distribution, evaluated by a modified bubble point test, showed a large increase in number of pores with size between 10 and 30 nm. The addition of PMMA also increases the size of finger-like cavities as shown by scanning electron microscopy.

Selective separation of similarly sized proteins with tunable nanoporous block copolymer membranes.

An integral asymmetric membrane was fabricated in a fast and one-step process by combining the self-assembly of an amphiphilic block copolymer (PS-b-P4VP) with nonsolvent-induced phase separation. The structure was found to be composed of a thin layer of densely packed highly ordered cylindrical channels with uniform pore sizes perpendicular to the surface on top of a nonordered sponge-like layer. The as-assembled membrane obtained a water flux of more than 3200 L m(-2) h(-1) bar(-1), which was at least an order of magnitude higher than the water fluxes of commercially available membranes with comparable pore sizes, making this membrane particularly well suited to size-selective and charge-based separation of biomolecules. To test the performance of the membrane, we conducted diffusion experiments at the physiological pH of 7.4 using bovine serum albumin (BSA) and globulin-γ, two proteins with different diameters but too close in size (2-fold difference in molecular mass) to be efficiently separated via conventional dialysis membrane processes. The diffusion rate differed by a factor of 87, the highest value reported to date. We also analyzed charge-based diffusive transport and separation of two proteins of similar molecular weight (BSA and bovine hemoglobin (BHb)) through the membrane as a function of external pH. The membrane achieved a selectivity of about 10 at pH 4.7, the isoelectric point (pI) of BSA. We then positively charged the membrane to improve the separation selectivity. With the modified membrane BSA was completely blocked when the pH was 7.0, the pI of BHb, while BHb was completely blocked at pH 4.7. Our results demonstrate the potential of our asymmetric membrane to efficiently separate biological substances/pharmaceuticals in bioscience, biotechnology, and biomedicine applications.

Evidence for spinodal decomposition and nucleation and growth mechanisms during membrane formation

Abstract The mechanisms of phase separation during membrane preparation were investigated by light scattering during immersion of cellulose acetate casting solutions in water / acetone coagulation baths. Data obtained for casting solutions with a concentration range (11–17 wt% cellulose acetate in acetone) normally chosen for membrane preparation, coagulated in baths containing up to 65 wt% acetone, could be well fitted with the linear Cahn theory for spinodal decomposition at least in the early stages of phase separation. Nucleation and growth was observed for casting solutions with polymer concentration lower than 6 or higher than 25 wt%. The morphology of membranes obtained by different mechanisms was compared.

Reduction of methanol permeability in polyetherketone–heteropolyacid membranes

Organic–inorganic membranes for direct methanol fuel cell (DMFC) application were prepared, using an organic matrix of sulfonated polyetherketone (s-PEK), different heteropolyacids and an inorganic network of ZrO2 or RSiO3/2. The membranes were characterized concerning their methanol and water permeability by pervaporation and ionic conductivity by impedance spectrometry. The bleeding out of the heteropolyacid from the membrane was quantified. The presence of the ZrO2 decreases the methanol and water permeability and reduces the bleeding out of the heteropolyacid. Molybdophosphoric acid (MoPA) was also investigated, showing a more pronounced bleeding out effect, which could be reduced by modifying the anion structure with the inclusion of Ni or Si. High conductivity values were obtained with membranes containing tungstophosphoric acid (TPA).

Hybrid films of poly(ethylene oxide-b-amide-6) containing sol–gel silicon or titanium oxide as inorganic fillers: effect of morphology and mechanical properties on gas permeability

Abstract In this work we describe the preparation of hybrid organic–inorganic films using the sol–gel process. Poly(ethylene oxide-b-amide-6), PEBAX, was used as an organic matrix. Silicon oxide or titanium oxide were prepared by hydrolysis and polycondensation of tetraethoxysilane, TEOS, or titanium tetraisopropoxide, TiOP, respectively. Hybrid films were characterized by electron microscopy, stress–strain tests and their gas permeability and selectivity properties were evaluated. Small angle X-ray scattering was used to investigate structural heterogeneity caused by the presence of SiO2 or TiO2 particles in the organic matrix. For films containing silicon oxide as an inorganic filler, gas permeability decreased as a function of the inorganic component content. When titanium oxide was used as a filler, gas permeability was considerably lower than that measured for films containing silicon oxide (80/20 PEBAX/inorganic precursor compositions). In these cases, the morphologies were very similar, but films containing titanium oxide were much more rigid. For CO2/N2 and CO2/H2 separation factors up to 52.9 and 7.1, respectively, were obtained for 80/20 PEBAX/TiOP films. For higher titanium oxide contents, gas permeability increased. In these cases, a TiO2 agglomeration was observed and the barrier performance decreased. Permeability and selectivity properties are discussed as a function of morphological and mechanical properties of these films.

Hybrids of perfluorosulfonic acid ionomer and silicon oxide by sol-gel reaction from solution: Morphology and thermal analysis

NAFION®/silicon oxide hybrids were prepared from solution by hydrolysis/polycondensation of alkoxy silanes. Transparent, but brittle films were obtained when TEOS was used as the inorganic polymer precursor, showing a lamellar structure by transmission electron microscopy. Part of the TEOS was substituted by TMDES to increase the film flexibility. For substitution higher than 10% phase separation was clearly observed by scanning and transmission electron microscopy. The thermal analysis of NAFION® and hybrids with different inorganic contents showed two main endothermic transitions, one assigned to the NAFION® ionic clusters and the other to the melting of the perfluorinated matrix. The cluster transition temperature decreases as the TEOS content increases up to 50 wt%, but than increases as the TEOS content reaches 80 wt%.

From Micelle Supramolecular Assemblies in Selective Solvents to Isoporous Membranes

The supramolecular assembly of PS-b-P4VP copolymer micelles induced by selective solvent mixtures was used to manufacture isoporous membranes. Micelle order in solution was confirmed by cryo-scanning electron microscopy in casting solutions, leading to ordered pore morphology. When dioxane, a solvent that interacts poorly with the micelle corona, was added to the solution, polymer-polymer segment contact was preferential, increasing the intermicelle contact. Immersion in water gave rise to asymmetric porous membranes with exceptional pore uniformity and high porosity. The introduction of a small number of carbon nanotubes to the casting solution improved the membrane stability and the reversibility of the gate response in the presence of different pH values.