Synthesis and Functionalization of Silver Nanoparticles for the Preparation of High Permittivity Nanocomposites

Over the last decade, a growing interest in nanochemistry has emerged due to the interesting features of nanomaterials that vary with size, shape and surface structure. In particular, metal nanoparticles have received much attention due to their properties that enable their use in various scientific disciplines. Although metal nanoparticles exhibit a number of properties that differ from bulk, some properties, such as their infinite permittivity, remain unchanged. As a result, metal nanoparticles have also been used to prepare nanocomposites with polymers in order to provide dielectric materials featuring high permittivities which can be used for applications such as energy storage (capacitors) or as materials for the conversion of electrical energy into mechanical motion (actuators). Despite the large number of publications on the preparation of nanocomposites exhibiting high permittivities which have emerged over the years, there is still room for further improvement in the current materials properties. For instance, dielectric losses are still quite high in some materials, and the use of certain types of filler lead to a large deterioration in the mechanical properties of the nanocomposites, especially with increasing filler content. In addition, a large number of the fillers used for the preparation of the nanocomposites feature poor size and shape control as well as poorly defined surface properties thus adding to the complexity of understanding the resulting material properties. This work tries to address some of the current issues concerning the preparation of dielectric materials. Therefore, silver nanoparticles (AgNPs) were used as filler, while polydimethylsiloxane (PDMS) was employed as the polymeric matrix. The advantages of using AgNPs as filler consist of their relative facile preparation, as well as the possibility of controlling their surface properties due to their resistance towards oxidation and corrosion. The possibility of preparing AgNPs in large amounts with control over the average size of the particles was realized by conducting the polyol synthesis of AgNPs in a Segmented Flow Tubular Reactor (SFTR). A SiO2 layer was grown around the AgNPs to prevent the loss of the insulating nature of the composite due to the formation of conductive pathways, and the thickness dependency of the dielectric properties of the core-shell particles was also investigated in this work. Furthermore, the SiO2 shell also provided the possibility of further surface functionalization, which was conducted in order to compatibilize the core-shell particles with the PDMS matrix. PDMS was chosen as the polymeric matrix due to its good electromechanical properties, which include high elasticity, low viscosity as well as low conductivity and low tangent losses (tan ÎŽ). The resulting nanocomposites featured enhanced permittivities compared to PDMS, while further optimization in the reaction conditions as well as in the processing procedure yielded nanocomposites with high flexibility that can undergo strains as high as 800 % at a silver content of 20 vol%. Other properties such as electric conductivity and the tan ÎŽ were kept low which emphasizes the potential of the nanocomposites to being used as flexible dielectric materials.

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

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

Poleable Dielectric Elastomer Composites

Yee Song Ko

As part of the recent efforts on renewable energies, energy harvesting by exploiting environmental motions and vibrations to generate electricity is being enthusiastically developed as a novel kind of green energy source. Piezoelectric materials are a logical choice for such applications, as they can readily generate electric charges when subjected to mechanical force without the need for additional equipment. While small deformations can be easily covered by traditional piezoelectrics, new materials have to be developed to access strains in excess of af few percent. This thesis lays out steps for the fabrication of a piezoelectric elastomer for use as active sensor and energy harvesting applications containing solely organic components. To achieve this a composite approach was chosen in which organic piezoelectric materials, so-called frozen-dipole electrets, were embedded as particulate fillers in a highly elastic polydimethyl siloxane (PDMS)matrix. Nanoparticles of a frozen-dipole electret material had to be prepared and characterized first, followed by their dispersion in the PDMS matrix, and finally the characterization of the composite elastomer. Poly-2-hydroxyethyl methacrylate (PHEMA) particles were produced byminiemulsion polymerization, and Disperse Red 1 (DR1) molecules were incorporated at the same time to turn the material into a frozen-dipole electret. The DR1 was introduced either as a blend or by copolymerization with amethacrylate-functionalized DR1 derivative. The study of PHEMA particles indicates that it would be very challenging to render them piezoelectric, due to their high conductivity, the environmental influence on their properties and unfavourable components introduced with the production method. In contrast, PMMA copolymer particles could easily be polarized. Polymethylmethacrylate (PMMA) copolymers with either methacrylate-functionalized DR1 or nitroaniline was used to produce nanoparticles via the nanoprecipitationmethod. Nanoparticles with up to 70 mol% DR1 could be obtained. These materials were chosen because of their known properties as electrets in order to serve as reference materials for the proof of concept of the organic piezoelectric elastomers. They were analysed by impedance spectroscopy and the thermally stimulated depolarization current technique to assess their relaxation and polarization behaviour. PMMA particles could be blended with PDMS without any additional additives to produce composite elastomers which can be turned into piezoelectric materials when poled in an electric field at high temperatures. Depending on the chain length of the PDMSmatrix, maximum strains of up to 600% could be reached. A custom built Berlincourt measuring setup was used to quantify the longitudinal piezoelectric coefficient d33 which was found to depend strongly on the force with which the composite is loaded. The transvere piezoelectric coefficient d31 was measured with a separate setup, based on a tensile testing machine. Piezoelectricity could be measured atmacroscopic strains as high as 200%. d31 initally decreases but stabilizes after a certain time. Most importantly, composites with PMMA-DR1 copolymer particles retain their piezoelectricity after exposure to elevated temperatures for several days.

EPFL2017