TY - JOUR A1 - Vukicevic, Radovan A1 - Vukovic, Ivana A1 - Stoyanov, Hristiyan A1 - Korwitz, Andreas A1 - Pospiech, Doris A1 - Kofod, Guggi A1 - Loos, Katja A1 - ten Brinke, Gerrit A1 - Beuermann, Sabine T1 - Poly(vinylidene fluoride)-functionalized single-walled carbon nanotubes for the preparation of composites with improved conductivity JF - Polymer Chemistry N2 - The surface of single-walled carbon nanotubes (SWCNTs) was functionalized with azide-terminated poly(vinylidene fluoride) (PVDF). Functionalization was confirmed by dispersibility, Raman spectroscopy, and thermogravimetric analyses. Raman spectra show disordering of the SWCNTs, thus, strongly suggesting that PVDF was covalently attached to SWCNTs. Functionalized SWCNTs were mixed with commercially available PVDF in a twin-screw extruder and thin films were obtained by melt-pressing. Films containing 0.5 and 1 wt% PVDF-functionalized SWCNTs exhibited significantly improved electrical conductivity compared to PVDF films containing pristine SWCNTs. Y1 - 2012 U6 - https://doi.org/10.1039/c2py20166f SN - 1759-9954 VL - 3 IS - 8 SP - 2261 EP - 2265 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Zhu, Jian A1 - Kollosche, Matthias A1 - Lu, Tongqing A1 - Kofod, Guggi A1 - Suo, Zhigang T1 - Two types of transitions to wrinkles in dielectric elastomers JF - Soft matter N2 - A membrane of a dielectric elastomer coated with compliant electrodes may form wrinkles as the applied voltage is ramped up. We present a combination of experiment and theory to investigate the transition to wrinkles using a clamped membrane subject to a constant force and a voltage ramp. Two types of transitions are identified. In type-I transition, the voltage-stretch curve is N-shaped, and flat and wrinkled regions coexist in separate areas of the membrane. The type-I transition progresses by nucleation of small wrinkled regions, followed by the growth of the wrinkled regions at the expense of the flat regions, until the entire membrane is wrinkled. By contrast, in type-II transition, the voltage-stretch curve is monotonic, and the entire flat membrane becomes wrinkled with no nucleation barrier. The two types of transitions are analogous to the first and the second order phase transitions. While the type-I transition is accompanied by a jump in the vertical displacement, type-II transition is accompanied by a continuous change in the vertical displacement. Such transitions may enable applications in muscle-like actuation and energy harvesting, where large deformation and large energy of conversion are desired. Y1 - 2012 U6 - https://doi.org/10.1039/c2sm26034d SN - 1744-683X VL - 8 IS - 34 SP - 8840 EP - 8846 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Risse, Sebastian A1 - Kussmaul, Björn A1 - Krüger, Hartmut A1 - Kofod, Guggi T1 - A versatile method for enhancement of electromechanical sensitivity of silicone elastomers JF - RSC Advances N2 - Dielectric elastomer actuators (DEAs) draw their function from their dielectric and mechanical properties. The paper describes the fabrication and various properties of molecularly grafted silicone elastomer films. This was achieved by addition of high-dipole molecular co-substituents to off-the-shelf silicone elastomer kits, Elastosil RT 625 and Sylgard 184 by Wacker and Dow Corning, respectively. Strong push-pull dipoles were chemically grafted to both polymer networks during a one step film formation process. All manufactured films were characterized using (13) C-NMR and FT-IR spectroscopy, confirming a successful attachment of the dipoles to the silicone network. Differential scanning calorimetry (DSC) results showed that grafted dipoles were distributed homogeneously throughout the material avoiding the formation of nano-scale aggregates. The permittivity increased with the amount of dipole at all frequencies, while the Young's modulus and electrical breakdown strength were reduced. Actuation strain measurements in the pure shear configuration independently confirmed the increase in electromechanical sensitivity. The ability to enhance electromechanical properties of off-the-shelf materials could strongly expand the range of actuator properties available to researchers and end-users. Y1 - 2012 U6 - https://doi.org/10.1039/c2ra21541a SN - 2046-2069 VL - 2 IS - 24 SP - 9029 EP - 9035 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Laflamme, S. A1 - Kollosche, Matthias A1 - Connor, Jerome J. A1 - Kofod, Guggi T1 - Soft capacitive sensor for structural health monitoring of large-scale systems JF - Structural control & health monitorin N2 - Structural integrity of infrastructures can be preserved if damage is diagnosed, localized, and repaired in time. During the past decade, there has been a considerable effort to automate the process of structural health monitoring, which is complicated by the inherent large size of civil structures. Hence, a need has arisen to develop new approaches that enable more effective health monitoring. In this paper, a new sensing technique for damage localization on large civil structures is proposed. Specifically, changes in strain are detected using a capacitance sensor built with a soft, stretchable dielectric polymer with attached stretchable metal film electrodes. A change in strain causes a measurable change in the capacitance of the sensor, which can be directly monitored when the sensor is fixed to a structure. The proposed method is shown here to permit an accurate detection of cracks. The proposed system deploys a layer of dielectric polymer on the surface of a structural element, and regularly monitors any change in capacitance, giving in turn information about the structural state. The smart material is composed of inexpensive silicone elastomers, which make the monitoring system a promising application for large surfaces. Results from tests conducted on small- scale specimens showed that the technology is capable of detecting cracks, and tests conducted on large- size specimens demonstrated that several sensor patches organized on a sensor sheet are capable of localizing a crack. The sensor strain also exhibits a high correlation with the loss of stiffness. KW - large-scale system KW - structural health monitoring KW - strain monitoring KW - capacitive sensor KW - dielectric polymer KW - stretchable sensor Y1 - 2012 U6 - https://doi.org/10.1002/stc.426 SN - 1545-2263 VL - 19 IS - 1 SP - 70 EP - 81 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - McCarthy, Denis N. A1 - Stoyanov, Hristiyan A1 - Rychkov, Dmitry A1 - Ragusch, Huelya A1 - Melzer, Michael A1 - Kofod, Guggi T1 - Increased permittivity nanocomposite dielectrics by controlled interfacial interactions JF - Composites science and technology N2 - The use of nanoparticles in polymer composite dielectrics has promised great improvements, but useful results have been elusive. Here, the importance of the interfacial interactions between the nanoparticles and the polymer matrix are investigated in TiO2 nanocomposites for dielectric materials using surface functionalisation. The interface is observed to dominate the nanocomposite properties and leads to a threefold increase in permittivity at volume fractions as low as 10%. Surface functionalisation of the filler nanoparticles with silanes allows control of this interface, avoiding significant degradation of the other important material properties, particularly electrical breakdown strength, and resulting in a material that is demonstrated successfully as an active material in a dielectric elastomer actuator application with increased work output compared to the pure polymer. Although further permittivity increases are observed when the interface regions have formed a percolation network, the other material properties deteriorate. The observation of percolation behaviour allows the interface thickness to be estimated. KW - Ceramics KW - Nanocomposites KW - Actuator KW - Interface KW - Electrical properties Y1 - 2012 U6 - https://doi.org/10.1016/j.compscitech.2012.01.026 SN - 0266-3538 VL - 72 IS - 6 SP - 731 EP - 736 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Kollosche, Matthias A1 - Zhu, Jian A1 - Suo, Zhigang A1 - Kofod, Guggi T1 - Complex interplay of nonlinear processes in dielectric elastomers JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - A combination of experiment and theory shows that dielectric elastomers exhibit complex interplay of nonlinear processes. Membranes of a dielectric elastomer are prepared in various states of prestretches by using rigid clamps and mechanical forces. Upon actuation by voltage, some membranes form wrinkles followed by snap-through instability, others form wrinkles without the snap-through instability, and still others fail by local instability without forming wrinkles. Membranes surviving these nonlinear processes are found to attain a constant dielectric strength, independent of the state of prestretches. Giant voltage-induced stretch of 3.6 is attained. Y1 - 2012 U6 - https://doi.org/10.1103/PhysRevE.85.051801 SN - 1539-3755 VL - 85 IS - 5 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Kussmaul, Björn A1 - Risse, Sebastian A1 - Wegener, Michael A1 - Kofod, Guggi A1 - Krüger, Hartmut T1 - Matrix stiffness dependent electro-mechanical response of dipole grafted silicones JF - Smart materials and structures N2 - The properties of dielectric elastomer actuators can be optimized by modifying the dielectric or mechanical properties of the dielectric elastomer. This paper presents the simultaneous control of both dielectric and mechanical properties, in a silicone elastomer network comprising cross-linker, chains and grafted molecular dipoles. Chains with two different molecular weights were each combined with varying amounts of grafted dipole. Chemical and physical characterization showed that networks with stoichiometric control of cross-linking density and permittivity were obtained, and that longer chain lengths resulted in higher electrical field response due to the reduction in cross-linking density and correspondingly in mechanical stiffness. Both actuation sensitivities were enhanced by 6.3 and 4.6 times for the short and long chain matrix material, respectively. Y1 - 2012 U6 - https://doi.org/10.1088/0964-1726/21/6/064005 SN - 0964-1726 VL - 21 IS - 6 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Risse, Sebastian A1 - Kussmaul, Bjoern A1 - Krüger, Hartmut A1 - Kofod, Guggi T1 - Synergistic improvement of actuation properties with compatibilized high permittivity filler JF - Advanced functional materials N2 - Electroactive polymers can be used for actuators with many desirable features, including high electromechanical energy density, low weight, compactness, direct voltage control, and complete silence during actuation. These features may enable personalized robotics with much higher ability to delicately manipulate their surroundings than can be achieved with currently available actuators; however, much work is still necessary to enhance the electroactive materials. Electric field-driven actuator materials are improved by an increase in permittivity and by a reduction in stiffness. Here, a synergistic enhancement method based on a macromolecular plasticizing filler molecule with a combination of both high dipole moment and compatibilizer moieties, synthesized to simultaneously ensure improvement of electromechanical properties and compatibility with the host matrix is presented. Measurements show an 85% increase in permittivity combined with 290% reduction in mechanical stiffness. NMR measurements confirm the structure of the filler while DSC measurements confirm that it is compatible with the host matrix at all the mixture ratios investigated. Actuation strain measurements in the pure shear configuration display an increase in sensitivity to the electrical field of more than 450%, confirming that the filler molecule does not only improve dielectric and mechanical properties, it also leads to a synergistic enhancement of actuation properties by simple means. KW - allycyanide KW - silicone-based dielectric elastomer actuators KW - permittivity enhancement KW - compatibilized filler molecules Y1 - 2012 U6 - https://doi.org/10.1002/adfm.201200320 SN - 1616-301X VL - 22 IS - 18 SP - 3958 EP - 3962 PB - Wiley-VCH CY - Weinheim ER -