The dielectric relaxation processes of polymethyl methacrylates that have been functionalized with Disperse Red 1 (DR1) in the side chain (DR1-<i>co</i>-MMA) were studied with temperature dependent impedance spectroscopy and thermally stimulated depolarization current (TSDC) techniques. Copolymers with dipole contents which varied between 10 mol% and 70 mol% were prepared. All samples showed dipole relaxations above the structural-glass transition temperature (T<sub>g</sub>). The β-relaxation of the methyl methacrylate (MMA) repeating unit was most visible in DR1(10%)-<i>co</i>-MMA and rapidly vanishes with higher dipole contents. DSC data reveal an increase of the T<sub>g</sub> by 20 °C to 125°C with the inclusion of the dipole into the polymethyl methacrylate (PMMA) as side chain. The impedance data of samples with several DR1 concentrations, taken at several temperatures above T<sub>g</sub>, have been fitted with the Havriliak-Negami (HN) function. In all cases, the fits reveal a dielectric response that corresponds to power-law dipolar relaxations. TSDC measurements show that the copolymer can be poled, and that the induced polarization can be frozen by lowering the temperature well below the glass transition. Relaxation strengths ΔƐ estimated by integrating the depolarization current are similar to those obtained from the impedance data, confirming the efficient freezing of the dipoles in the structural glass state.
The research efforts for silicone based elastomers with high dielectric permittivity (Ɛ’) intensified significantly in the last years since such materials would allow the construction of dielectric elastomer actuators (DEA) with low operation voltages. Polar groups can be introduced to elastomers to adjust their permittivity. The results obtained regarding the functionalization of silicones with polar nitrile (CN) and trifluoropropyl (CF<sub>3</sub>) groups are presented. Those with CN groups were synthesized via anionic polymerization of nitrile containing cyclosiloxanes or via a post-polymerization modification of functional polysiloxanes. Polysiloxanes containing CF<sub>3</sub> groups were prepared by anionic copolymerization of 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclosiloxane with octamethylcyclotetrasiloxane. Importantly, we have found that all polysiloxanes have glass transition temperatures (Tg) well below room temperature (<-50°C). This ensures that the materials turn into true elastomers after cross-linking. In addition to this, a linear increase in Ɛ’ with increasing content of polar groups was observed with maximum values of Ɛ’ = 18 and Ɛ’ = 8.8 for polysiloxanes modified at every repeating unit with either CN or CF<sub>3</sub> groups, respectively.
It has been the dream of many scientists to create polymeric materials with simultaneously high dielectric permittivity, low glass transition temperature, and excellent elastomeric properties. Such material would be a highly attractive dielectricum in electromechanical actuators. Within this topic we are focusing on silicones because of their excellent elastomeric properties over a wide temperature and frequency range combined with low glass transition temperatures. To increase their low permittivity, we followed different approaches which include: blending the matrix with highly polarizable conductive and polar nanofillers and chemical modification of the silicones with polar side groups. This AC340presentation will show the advantages and disadvantages of the two strategies we have been following and will provide an assessment of their future potentials.
A new type of poleable dielectric elastomer is introduced herein. The elastomer contains polymer nanoparticles with frozen molecular dipoles, which can be oriented at elevated temperatures in an electric field <i>via</i> poling. The aim is to provide a soft material with high, tunable optical properties suitable for actuator and flexible electronics applications. To that end poleable polymeric nanoparticles with high dipole concentrations and glass transition temperatures well above room temperature will be needed to be used as filler in an elastomer matrix. The synthesis and characterization of such particles is presented in this manuscript. Polyhydroxyethyl methacrylate (PHEMA) nanoparticles were synthesized using miniemulsion polymerization. The particles were loaded with 4-[ethyl (2-hydroxyethyl) amino]-4-nitrobenzene, usually called Disperse Red 1 (DR1), which has a large dipole moment (μ = 7.5 – 9.5 D). The maximum dipole loadings is limited by the solubility of the dipole in the monomer solutions prior to polymerization. All samples show a glass transition temperature around 95 °C. Secondary electron microscopy (SEM) revealed spherical particles, the size of which was confirmed by dynamic light scattering (DLS) measurements. A composite was prepared by dispersing the particles in polydimethyl siloxane (PDMS).
The success of dielectric elastomer materials in actuator technology as well as in energy harvesting is much influenced
by the material parameters, e.g. breakdown field, dielectric constant, and elastic modulus which have a direct impact on
the driving voltage. By increasing the dielectric constant of a material the activation voltage can be decreased, however
this increase is very often associated with a decrease in the breakdown field. In this proceeding, dielectric elastomer
materials based on polydimethylsiloxanes with increased strain at break and high breakdown fields are presented.
Dielectric elastomer actuators (DEAs) have attracted increasing attention over the last few years owing to their
outstanding properties, e.g. their large actuation strains, high energy density, and pliability, which have opened up a wide
spectrum of potential applications in fields ranging from microengineering to medical prosthetics. There is consequently
a huge demand for new elastomer materials with improved properties to enhance the performance of DEAs and to
overcome the limitations associated with currently available materials, such as the need for high activation voltages and
the poor long-term stability. The electrostatic pressure that activates dielectric elastomers can be increased by higher
permittivity of the elastomer and thus may lead to lower activation voltages. This has led us to consider composite
elastomeric dielectrics based on thermoplastic elastomers or PDMS, and conductive polyaniline or ceramic (soft doped
PZT) powder fillers. The potential of such materials and strategies to counter the adverse effects of increased
conductivity and elastic modulus are discussed.
In principle EAP technology could potentially replace common motion-generating mechanisms in positioning, valve
control, pump and sensor applications, where designers are seeking quieter, power efficient devices to replace
conventional electrical motors and drive trains. Their use as artificial muscles is of special interest due to their similar
properties in terms of stress and strain, energy and power densities or efficiency. A broad application of dielectric
elastomer actuators (DEA) is limited by the high voltage necessary to drive such devices.
The development of novel elastomers offering better intrinsic electromechanical properties is one way to solve the
problem. We prepared composites from cross-linked silicone elastomers or thermoplastic elastomers (TPE) by blending
them with organic fillers exhibiting a high dielectric constant. Well characterized monomeric phthalocyanines and
modified doped polyaniline (PANI) were used as filler materials. In addition, blends of TPE and an inorganic filler
material PZT were characterized as well. We studied the influence of the filler materials onto the mechanical and
electromechanical properties of the resulting mixtures. A hundredfold increase of the dielectric constant was already
observed for blends of an olefin based thermoplastic elastomer and PANI.
The dielectric constant (ε) of a polymer can significantly be increased by blending it with conducting fillers. Given our
interest in developing highly efficient and long-lasting actuators for muscle replacement, we set out to explore all key
issues which could help to reduce the required voltage and at the same time ensure long term stability. The presentation
describes experiments which prove that the water content in carboxylic acid-decorated phthalocyanines (Pcs), commonly
falsely referred to oligo-Pcs, is a critical factor determining the absolute value of ε. Several publications on ε values of these oligo-Pcs led to contradicting conclusions because the effect of water was not sufficiently considered. The water
content is relevant because o-Pcs are often used as fillers to increase ε of polymer matrices. This presentation also
describes an experimental evaluation on whether or not as-prepared polyaniline (PANI) and poly(divinyl benzene)-
encapsulated (PDVB) PANI can be reasonably used as high ε fillers in matrix materials. For this purpose several blends
with polystyrene-polybutadiene block copolymer gels (PS-<i>b</i>-PB) and polydimethyl siloxane (PDMS) were prepared and
their dielectric properties investigated. The former part of this presentation has in part already been published (D. M.
Opris et al. Chem. Mater. 20(21), 6889-6896, 2008), the latter is completely new.