High diffusion barrier and piezoelectric nanocomposites based on polyvinylidene fluoride-trifluoroethylene copolymer and hydrophobized clay

Nanocomposites based on polyvinylidene fluoride– trifluoroethylene copolymer and up to 4 vol % of hydrophobized clay nanoparticles are investigated. The structure, piezoelectric properties, and oxygen permeability of solvent cast films are analyzed before and after annealing above the Curie temperature of the polymer. Exfoliation of the clay takes place at concentrations up to 1 vol %, beyond which it rapidly drops and is absent at a concentration of 4 vol %. The presence of clay does not change the crystallinity of the polymer, but leads to a threefold decrease of the oxygen permeability at a concentration of 0.5 vol %. Annealing at 130 8C increases the crystallinity, the proportion of b phase up to 94%, and the piezoelectric coefficient by 20–40% at clay fractions below 1 vol %. Annealing also leads to a remarkable 3- to 10-fold decrease of O2 permeability and to intriguing changes of the activation energy for O2 transport, which decreases from 56 kJ/mol for the as-cast polymer to below 10 kJ/mol for the polymer and exfoliated composite.

The Effect Of Processing Conditions On The Morphology, Mechanical And Electrical Properties Of Batio3-Polymer Composites

Sara Dalle Vacche, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

Ferroelectric ceramic – polymer composites have a unique mix of electrical and mechanical properties. Piezoelectric and pyroelectric activity, a wide range of dielectric constants and high breakdown strength are combined with mechanical flexibility, formability and low acoustic impedance. Furthermore their properties may be tailored by the judicious choice of the polymeric matrix and ceramic filler, of their volume fraction, and of the type of connectivity, making these materials very attractive for applications as embedded capacitors, sensors and actuators. Composites with 0-3 connectivity, i.e. a continuous polymer matrix filled with a ceramic phase discontinuous in all directions, are particularly interesting because of their relative ease of fabrication and processing. The functionality of these composite materials is mostly due to the dielectric, ferroelectric and piezoelectric properties of the ceramic phase, which therefore often needs to constitute more than 50% in volume. On the other hand the possibility of integrating these composites as embedded devices in host structures, which are often polymer based, or using them as efficient actuators depends greatly on their thermomechanical properties. Most of the studies on ferroelectric ceramic-polymer composites, however, focus on their electrical properties, and relatively few systematic studies on their thermal and mechanical behaviour have been published so far. Furthermore the ceramic volume fractions considered are often much lower than those used in most applications [1, 2], studies of highly filled composites mainly being limited to lead-based ferroelectric materials [3]. In the present study 0-3 BaTiO3 – polymer composites were prepared, containing up to 60% volume fraction of ceramic phase. BaTiO3 was chosen as the ceramic filler as it is a common piezoelectric material with high dielectric constant, having the additional benefit from an environmental point of view of being lead-free. For the matrix, a low viscosity epoxy resin and a polyvinylidene fluoride based polymer were chosen. The first is a thermoset resin commonly used for several applications, and the second is a thermoplastic semicrystalline polymer which displays piezoelectric activity. A common route for achieving very high loading of ceramic filler is the use of solvents to reduce the viscosity during processing. However, as the materials prepared via the solvent route were found to have poor mechanical properties, attributed to the presence of residual solvent, most of the effort focused on obtaining high ceramic volume fractions via a solvent free process. The thermomechanical, dielectric, ferroelectric and piezoelectric properties of the obtained composites were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. X-ray diffraction was carried out to determine the crystal structure of the BaTiO3 in the pure powder and in the composites, confirming that processing did not alter the crystallographic structure of the ceramic phase. Scanning electron microscope observation of fracture surfaces of the composites evidenced that a satisfactorily homogeneous dispersion of BaTiO3 in the matrix could be obtained, although aggregates with diameter of some micrometers and some porosity were detected. The thermal and mechanical behaviour of the composites was studied by differential scanning calorimetry and dynamic mechanical analysis. The glass transition temperature was found not to significantly depend on the amount of filler, while both the storage and loss moduli increased upon addition of BaTiO3. Furthermore, all the composites displayed a region of linear stress-strain behaviour smaller than the pure matrices. Finally, experimental results also evidenced the possibility to obtain low leakage current and high breakdown voltage.

2010

Low-Stress UV-Curable Hyperbranched Polymer Nanocomposites for High-Precision Devices

Valérie Geiser

The aim of this thesis was to investigate the process-structure-property relationships of UV-curable hyperbranched polymer (HBP)/silica nanocomposites. Special attention was paid to the interplay between photo-conversion, rheological behavior, shrinkage, and stress dynamics. This knowledge was used to maximize the shape fidelity and dimensional stability of imprinted nano-patterns made with these nanocomposites. Two different processing routes, resulting in different nanocomposite morphologies, were compared. The first and more conventional approach was ultrasonic, solvent-assisted mixing with solid silica nanoparticles followed by UV curing, which led to a discrete dispersion of silica particles in the matrix. The second and more novel approach was a dual-cure process combining UV and sol-gel processing with liquid tetraethyl orthosilicate. The sol-gel process, using a low-viscosity organometallic precursor, overcame the potential processing issue of the highly viscous particulate nanocomposites and led to a hybrid material with a homogeneous silica network at nanometer scale. Rheological analysis of silica nanoparticle suspensions up to the concentrated regime in the HBP showed an exponential increase of the viscosity with the particle fraction for well-dispersed discrete particle systems. A liquid-to-solid transition occurred in the 5 to 10 vol% range, which was correlated with an immobilized layer of polymer hydrogen-bonded to the silanol groups on the surface of the particles, as was confirmed by calorimetric analysis. Polymerization kinetics of HBP nanocomposites and hybrids were analyzed by means of photo differential scanning calorimetry using an autocatalytic model. A time-intensity-superposition principle with power-law dependence was established, which invalidated the classic radiation dose equivalence principle. Gelation and modulus build-up were monitored using photo-rheology. The calorimetric and rheological data were combined in the form of time-intensity-transformation diagrams. It was found that gelation was delayed with respect to conversion up to 36% when lower UV intensities were used, which favored reduce polymerization stresses. The dynamics of polymerization shrinkage and internal stress build-up were investigated using photo-hyphenated interferometry and beam-bending methods. The linear shrinkage of the HBP was as low as 4.5%, which was further reduced to 3.3% with the addition of 20 vol% nanoparticles. The residual stress of HBP/SiO2 nanocomposites was below 5.5 MPa. That is well below the level of standard non-reinforced resins, in spite of an increased stiffness. For the sol-gel hybrids with 20 vol% SiO2, stress reduction by 50% with simultaneous stiffness increase by 20% with respect to corresponding particulate composites was demonstrated. The coefficient of thermal expansion was also lowered by 30%. Finally, nanogratings with a period of 360 nm and a step height of 12 nm were fabricated using UV-nanoimprint lithography with a glass or nickel master in a low-pressure (max. 6 bar) and rapid (≈1 min) process. Stable gratings were imprinted in the composite material containing up to 25 vol% of silica nanoparticles, despite the high viscosity of such compositions. Thanks to the exudation of a HBP-rich surface layer, the shape fidelity in terms of period of the replicated patterns was within 98% of the master, with shape distortion increasing with internal stress. The obtained nanogratings were used as a substrate for grated high refractive index TiO2 waveguides and then applied as wavelength-interrogated optical sensors (WIOS). An immunoassay with the polymer WIOS showed the same ultrahigh detection sensitivity as the standard glass-based devices. Therefore, the novel polymer-based sensor should be useful to probe contaminants in liquids with concentrations as low as a few ppb.

EPFL2010

Functionally graded polymer nanocomposites using magnetic fields

Tommaso Nardi

Functionally Graded Materials (FGM) are a class of materials in which the composition and the properties gradually change over the volume. The present PhD project aims to generate a full understanding of a novel synthetic strategy for polymeric FGMs based on the magnetophoretic motion of magnetic nanofillers in a polymer matrix under the application of an external magnetic field gradient. Due to its high rate, costâeffectiveness and lowâtoxicity, UV polymerization was considered as the first curing option for the magnetic nanocomposites. The curability problem deriving from the presence of opaque particles was tackled by studying two different systems based on the same hyperbranched acrylated polymer matrix (HBP): one containing bare Fe3O4 nanoparticles whereas the other loaded with Fe3O4@SiO2 coreâshell nanoparticles. The employment of Fe3O4@SiO2 NPs for the synthesis of magnetic polymeric films showed to be effective towards the inclusion of 10âtimes higher particle volume fractions compared to formulations including bare Fe3O4. By adding Fe3O4@SiO2 nanoparticles to the UVâcurable matrix, the motion of the magnetic responsive filler was induced upon the application of magnetic field gradients generated by permanent magnets. The process was speeded up through the employment of a simple setâup composed of two block magnets in repulsion configuration and through the functionalization of the particle surfaces with an acrylated silane. From the experimental analysis of the photocuring process, the residual stresses arising during the photopolymerization of functionally graded composite coatings based on the UVâcurable HBP matrix and Fe3O4@SiO2 nanoparticles were evaluated with a Finite Element Method approach. These coatings were able to consistently decrease the residual stresses at the coatingâsubstrate interface compared to those encountered either in composites with homogeneous compositions or in the pure polymer, provided that reinforcing fillers were concentrated at the coatingâsubstrate interface. If instead nanoparticles were concentrated at the free surface, graded nanocomposite coatings could reduce by as much as 40% the interfacial stress arising from thermal variations, as demonstrated through numerical simulations. Nanoindentation tests were carried out on the surface of polymer nanocomposites exhibiting either graded or homogeneous distributions of Fe3O4@SiO2 nanoparticles in the UVâcurable matrix. It was experimentally shown how only at relatively small indentation depths, large increases in modulus and hardnesswere obtained for graded composites with respect to their homogeneous counterparts. Through a Material Point Method approach, experimental nanoindentation tests were successfully simulated. Finally, nanocomposite systems based on epoxy matrices were considered. In particular, composites with gradients in permittivity were synthesized through the application of a magnetic field to suspensions of highâpermittivity Fe3O4@TiO2 particles in an epoxy resin. The application of the magnetic force not only induced the movement of the particles, but also caused their alignment in high aspect ratio structures, resulting in rather steep permittivity gradients. Numerical simulations clarified how the asâsynthesized materials, employed as insulators, have the potential to efficiently reduce the electric field stress at the electrodeâinsulator interface and have intrinsic durability advantages over their homogeneous counterparts.

EPFL2015