Giuseppe Carmine Gallone

Standards for dielectric elastomer transducers

Dielectric elastomer transducers consist of thin electrically insulating elastomeric membranes coated on both sides with compliant electrodes. They are a promising electromechanically active polymer technology that may be used for actuators, strain sensors, and electrical generators that harvest mechanical energy. The rapid development of this field calls for the first standards, collecting guidelines on how to assess and compare the performance of materials and devices. This paper addresses this need, presenting standardized methods for material characterisation, device testing and performance measurement. These proposed standards are intended to have a general scope and a broad applicability to different material types and device configurations. Nevertheless, they also intentionally exclude some aspects where knowledge and/or consensus in the literature were deemed to be insufficient. This is a sign of a young and vital field, whose research development is expected to benefit from this effort towards standardisation.

Functionalized carbon nanotubes as a filler for dielectric elastomer composites with improved actuation performance

Among the broad class of electro-active polymers, dielectric elastomer actuators represent a rapidly growing technology for electromechanical transduction. In order to further develop this applied science, the high driving voltages currently needed must be reduced. For this purpose, one of the most widely considered approaches is based on making elastomeric composites with highly polarizable fillers in order to increase the dielectric constant while maintaining both low dielectric losses and high-mechanical compliance. In this work, multi-wall carbon nanotubes were first functionalized by grafting either acrylonitrile or diurethane monoacrylate oligomers, and then dispersed into a polyurethane matrix to make dielectric elastomer composites. The procedures for the chemical functionalization of carbon nanotubes and proper characterizations of the obtained products are provided in detail. The consequences of the use of chemically modified carbon nanotubes as a filler, in comparison to using unmodified ones, were studied in terms of dielectric, mechanical and electromechanical response. In particular, an increment of the dielectric constant was observed for all composites throughout the investigated frequency spectrum, but only in the cases of modified carbon nanotubes did the loss factor remain almost unchanged with respect to the simple matrix, indicating that conductive percolation paths did not arise in such systems. An effective improvement in the actuation strain was observed for samples loaded with functionalized carbon nanotubes. (Some figures may appear in colour only in the online journal)

Dielectric behaviour versus temperature of a monoepoxide

The conductivity and dielectric relaxation behaviour of a glass-forming monoepoxide, cresyl glycidyl ether (CGE), were studied from to by impedance spectroscopy, in the temperature interval 83 - 333 K. Data analysis indicated the existence of a transition temperature marking the separation between two different relaxation regimes, in accordance with previous results given by Stickel and co-workers. The temperature is well above the glass transition temperature , and coincides with the temperature at which the timescales of the main and secondary relaxation processes coincide, so at there is a unique overall relaxation process. The splitting layout of CGE differs from that of the very common diepoxide diglycidyl ether of bisphenol-A (DGEBA), studied previously, and also the relaxation at high temperature has a correspondingly different dynamical behaviour. The relaxation strength behaviour is also analysed and compared with that of DGEBA; the similarities are discussed.

Electrical properties of fluidified portland cement mixes in the early stage of hydration

Abstract The electrical conductivity and the dielectric constant of cement mixes have been studied in the initial stages of setting to correlate the time evolution of the electrical parameters with the chemical and microstructural modifications. The formulations, based on a mix of Portland cement and microsilica, were fluidified with various amounts of an acrylic polyelectrolite and had a w/c ratio of 0.24. Measurements were carried out at ambient temperature for times of up to 40 h, in the frequency interval 30 Hz–200 kHz. The conductivity showed a marked decrease correlated with the loss of connectivity of the macroscopic porosity (depercolation). The time to the drop of conductivity depended linearly on the concentration of superplasticizer whose retarding effect was clearly evidenced. A dielectric amplification phenomenon was observed which vanished after the porosity depercolated. The temperature profiles of mixtures with different concentrations of the superplasticizer were drawn and a simple model was developed to compute the specific thermal power as a function of time. A substantial coincidence was found, on the time axis, between depercolation and maximum rate of setting. Vicat tests were also carried out.

Viscoelastic characterisation of pig liver in unconfined compression

Understanding and modelling liver biomechanics represents a significant challenge due to the complex nature of this organ. Unfortunately, there is no consensus on liver viscoelastic properties, and results are strongly dependent on sample type and status, adopted testing method, and testing conditions. Standard force-triggered tests (e.g. step response or dynamic mechanical tests) necessitate an initial contact between sample and testing apparatus, which may result in significant pre-stress to very soft and highly hydrated samples. In a previous study we proposed the epsilon dot method (ε̇M): a testing and analysis framework to address the drawbacks of standard mechanical tests. Focusing on ex-vivo unconfined bulk compressive tests, here we use both the ε̇M and dynamic mechanical analysis (DMA) to derive liver viscoelastic parameters in the region of small strains or the linear viscoelastic region (LVR). As liver samples were visibly deteriorated at the end of frequency sweep tests, a modified approach was adopted to reduce DMA testing times. This approach, termed step-reconstructed DMA (SRDMA), is based on dynamic measurements around specific frequencies and then reconstruction of liver behaviour in the entire frequency range of interest. The instantaneous elastic modulus obtained from SRDMA tests (2.65 ± 0.30 kPa) was significantly higher than that obtained with the ε̇M (2.04 ± 0.01 kPa). We show that the overestimation of stiffness is due to data acquisition in a local rather than an absolute LVR, highlighting the importance of using a rapid and zero pre-stress approach to characterise very soft and highly hydrated biological tissues.

Dynamics of a glass-forming triepoxide studied by dielectric spectroscopy

Dielectric measurements of an epoxy resin, N, N-diglycidyl-4-glycidyloxyaniline, have been carried out in the supercooled and glassy phase over a broad frequency range (10 2-6 109 Hz). The measurements reveal electrical transport due to ionic impurities as well as three different dipolar relaxations—in addition to the - and -relaxation, a slower 0 -relaxation is recognized, whose loss peak is disclosed after subtraction of the dc conductivity contribution. The glass transition is found to affect markedly the secondary relaxation, whose strength and shape parameters change across Tg. The major inference from the results concerns the existence of a transition in the dynamics, occurring some tens of degrees above Tg, in the vicinity of the temperature TS where the peaks of the - and -relaxations merge. Evidence in favour of such a transition is given by: (i) the change in the temperature dependence of the -relaxation time; (ii) independently, the change in the temperature dependence of the dc conductivity; (iii) the breakdown of the Debye-Stokes-Einstein model, replaced at lower temperatures by a fractional regime. Concerning the 0 -process, it shows a Vogel-Fulcher behaviour with the same temperature T0 as the -relaxation but, unlike this last, it is not involved in a splitting phenomenon with the -relaxation. Several hypotheses concerning the nature of the 0 -process are explored.

Temperature and pressure dependences of the relaxation dynamics of supercooled systems explored by dielectric spectroscopy

Abstract A wide-band (102 − 2 × 1010Hz) dielectric study of epoxy compounds was carried out under isobaric conditions (atmospheric pressure) by changing the temperature down to the supercooled and glassy phases. One of these systems (diglycidyl ether of bisphenol A (DGEBA)) was also measured under isothermal conditions at 293 K by changing the pressure from 0.1 up to 235 MPa. The analysis of variable-temperature measurements gave evidence of a connection between the α β-splitting phenomenon, the breakdown of the Debye-Stokes-Einstein (DSE) relation which turns into a fractional DSE law, and the transition of the α-relaxation dynamics between two different temperature regimes. The variable-pressure measurements revealed that the pressure dependence of the α-relaxation time in DGEBA is better described by a second order polynomial function rather than a Vogel-Fulcher-like function. The perfect scaling observed between couples of isobaric and isothermal spectra with the same value of the α-relaxation time su...

Enhancing the Electro-Mechanical Response of Maxwell Stress Actuators

The need for high electric fields to drive dielectric elastomers is still retaining their diffusion as actuators in some areas of potential application, as in the case of biomedical disciplines. The development of new materials offering superior electromechanical properties is thus an essential requirement in order to effectively reduce the driving fields. In this light, the present work is aimed to enhance the electromechanical properties of two silicone and polyurethane based dielectric elastomers, both by making particulate composites with high-permittivity ceramic fillers, and by blending with a highly polarisable polymeric phase. Due to a consequent worsening of the mechanical properties, pure composite architectures yielded only limited results on the overall electromechanical response. With the blend approach, instead, both an increase of the dielectric permittivity and an unexpected reduction of the tensile elastic modulus were observed, leading to an overall increase of the electromechanical response. In any case, a key role appears to be played by the nature and intensity of polarisation phenomena arising at interfaces between different phases.

COMPARISON OF SINE VERSUS PULSE WAVEFORM EFFECTS ON FATIGUE CRACK GROWTH BEHAVIOR OF NR, SBR, AND BR COMPOUNDS

Abstract Fatigue crack growth experiments on carbon black-filled rubber compounds have been carried out to evaluate the influence of testing conditions over different compound formulations. Investigations on the influence of waveform, data acquisition, and compound formulation have been performed on strip-tensile specimens reproducing the mode I of crack opening. The response of three different compound formulations (based on either natural rubber, butadiene rubber, or styrene-butadiene rubber) to the application of two different waveforms, pulse and sine, has been analyzed, showing significant differences in fatigue behavior and ranking of the various compounds. Compared to the sinusoidal waveform, the use of a pulse waveform provided an improved correlation of the tearing energy with the crack propagation speed. This difference was particularly evident in the case of natural rubber and butadiene rubber, while it resulted negligible in the case of styrene-butadiene rubber. Such a different behavior could...

Enhancement of the electromechanical transduction properties of a silicone elastomer by blending with a conjugated polymer

The need for high driving electric fields currently limits the diffusion of dielectric elastomer actuation in some areas of potential application, especially in the case of biomedical disciplines. A reduction of the driving fields may be achieved with new elastomers offering intrinsically superior electromechanical properties. So far, most of attempts in this direction have been focused on composites between elastomer matrixes and high-permittivity ceramic fillers, yielding to limited results. In this work, the electromechanical response of a silicone rubber (poly-dimethyl-siloxane) was improved by blending, rather than loading, the elastomer with a highly polarizable conjugated polymer (undoped poly-hexyl-thiophene). Very low percentages (1-6 wt%) of poly-hexyl-thiophene yielded both an increase of the dielectric permittivity and an unexpected reduction of the tensile elastic modulus. Both these factors contributed to a remarkable increase of the electromechanical response, which reached a maximum at 1 wt% content of conjugated polymer. This approach may lead to the development of new types of improved dielectric elastomers for actuation.