Stephan Busato

Controlling Phase Distributions in Macroporous Composite Materials through Particle-Stabilized Foams

Aqueous foams stabilized by ceramic and thermoplastic polymeric particles provide a general method for producing novel porous materials because their extraordinary stability against disproportionation and drainage allows them to be dried and sintered into solid materials. Here, we report the different microstructures that can be obtained from liquid foams stabilized by binary mixtures of particles when the interfacial energies between the particles and the air-liquid interfaces are manipulated to promote either preferential or competitive self-assembly of the particles at the foam interface. Modification of the interfacial energies was accomplished through surface modification of the particles or by decreasing the surface tension of the aqueous phase. Materials derived from liquid foams stabilized by poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles are investigated. However, as is shown, the method can be extended to other polymeric and ceramic particles and provides the possibility to manufacture a wide range of porous composite materials.

Designing macroporous polymers from particle-stabilized foams

Particle-stabilized liquid foams provide a general route for producing low-density macroporous materials from melt-processable and intractable thermoplastic polymers. In this paper, we demonstrate how these liquid foams can be used to design macroporous polymers with tailored microstructures and properties by adjusting the various processing parameters. By varying the size, concentration, and wettability of the particles in the colloidal suspensions and controlling the frothing, drying, and sintering conditions, macroporous materials with porosities between 33 and 95% and median pore sizes (D50) between 13 and 634 μm were obtained. This foaming process is applicable to a wide range of hydrophobic materials and is demonstrated here on commercially available polymeric powders of poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF), poly(ether imide) (PEI), and poly(ether ether ketone) (PEEK).

The use of Trametes versicolor laccase for the polymerization of aniline in the presence of vesicles as templates

The enzymatic polymerization of aniline to polyaniline (PANI) with Trametes versicolor laccase (TvL) as catalyst and dioxygen (O₂) as oxidant was investigated in an aqueous medium containing unilamellar vesicles with an average diameter of about 80 nm formed from AOT (=sodium bis(2-ethylhexyl) sulfosuccinate). Compared to the same reaction carried out with horseradish peroxidase isoenzyme C (HRPC) as catalyst and hydrogen peroxide (H₂O₂) as oxidant, notable differences were found in the kinetics of the reaction, as well as in the characteristics of the PANI obtained. Under comparable optimal conditions, which are pH 3.5 for TvL/O₂ and pH 4.3 for HRPC/H₂O₂, the reaction with TvL/O₂ was much slower than with HRPC/H₂O₂, i.e. ≈27 days vs. 1 day reaction time to reach equilibrium with >90% yield at 25 °C. Although in both cases, aniline monomer coupling occurred mainly via the carbon atom in para position of aniline, UV-vis-NIR absorption and EPR measurements indicate that the reaction with TvL/O₂ yielded mainly overoxidized products (with λ(max)=730 nm). These products had a lower amount of unpaired electrons if compared with the products obtained with HRPC/H₂O₂ (with λ(max)≈1000 nm, which is characteristic for the polaron state of PANI-ES, the emeraldine salt form of PANI). Similarly to previous findings with HRPC/H₂O₂, enzyme inactivation occurred during the polymerization also in the case of TvL/O₂. Since the aqueous PANI-vesicle suspensions obtained are of high colloidal stability, they can be used directly as ink in a conventional thermal inkjet printer for printing on paper or on surface treated polyimide films. Printed PANI-ES patterns on paper changed colour from green (emeraldine salt) to blue (emeraldine base) upon exposure to ammonia gas, demonstrating the expected ammonia sensing properties.

Quasi-static electric properties of insulating polymers at a high voltage for electro-bonded laminates

This paper reports on a high voltage measurement set-up for determining the relative permittivity and the volume resistivity of dielectric polymers. These properties were evaluated on thin films at electric fields up to 80?V? ?m?1 and in the quasi-static regime at frequencies lower than 1?Hz. It is found that the high field properties of FEP, PFA, polyimide and Mylar are comparable to the respective low field values, while for ferroelectric PVDF poling behavior becomes evident at high fields. High field properties of dielectric polymers are of particular importance in the design of devices relying on electrostatic attraction, such as electro-bonded laminates applied in tunable bending stiffness structures.

Perspectives for Reactive Molding of PPA as Matrix for High-performance Composite Materials

While liquid composite molding (LCM) technologies for thermoset composites are increasingly used for manufacturing high quality structural components, LCM technologies based on thermoplastic matrices are still in development. Reactive thermoplastic LCM processes are at present applicable only to a few engineering polymers. This work presents perspectives for reactive molding of a high-temperature resistant polyphthalamide (PPA). Thermoanalytical, rheological, and optical tests were performed to assess the behavior of low-melting PA 6T/6I oligomers with increasing temperature and to monitor the polymerization and crystallization reactions. The raw oligomeric material first undergoes a solid-liquid phase transition at around 135 C. At this stage, it shows a shear-thinning behavior and its viscosity is around 102 Pa s. At higher temperatures, the viscosity of the oligomeric melt decreases further until a high temperature crystallization process occurs at about 180 C. Polymerization starts at about 220 C and ends at 290 C. During the reaction the polymer crystallizes as soon as it is formed with an achievable degree of crystallinity considerably higher than that of melt-crystallized samples. In accordance with the results, a viable processing technology based on powder impregnated intermediate materials is proposed.

Electromechanical actuation of macroscopic carbon nanotube structures

The electromechanical actuation of macroscopic carbon nanotube (CNT) structures, including single and multi walled CNT mats and aligned ribbons, is analyzed and compared. From experimental evidence, actuation due to quantum chemical and electrostatic effects can be distinguished. Their respective contribution to the total actuation depends on two key parameters, namely Youngs modulus and charge density. While mechanical energy densities of actuated structures are appreciable, electromechanical conversion efficiencies are found to be impractically low.

Engineering macroporous composite materials using competitive adsorption in particle-stabilized foams.

The high absorption energies of partially wetted particles at fluid interfaces allow the production of macroporous composite materials from particle-stabilized foams. Competition between the different particle types determines how they are distributed in the foam lamella and allow the phase distribution to be controlled; a technique that is useful in the design and engineering of porous composites. Here, we report details on the effects of preferential and competitive adsorption of poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles at the foam interfaces on the consolidated macroporous composite materials. By varying the relative composition and surface energies of the stabilizing particles, macroporous composite materials with a broad range of phase distributions are possible.