@article{KofodPaajanenBauer2006, author = {Kofod, Guggi and Paajanen, Mika and Bauer, Siegfried}, title = {Self-organized minimum-energy structures for dielectric elastomer actuators}, issn = {0947-8396}, doi = {10.1007/s00339-006-3680-3}, year = {2006}, language = {en} } @article{KofodWirgesPaajanenetal.2007, author = {Kofod, Guggi and Wirges, Werner and Paajanen, Mika and Bauer, Siegfried}, title = {Energy minimization for self-organized structure formation and actuation}, issn = {0003-6951}, doi = {10.1063/1.2695785}, year = {2007}, abstract = {An approach for creating complex structures with embedded actuation in planar manufacturing steps is presented. Self-organization and energy minimization are central to this approach, illustrated with a model based on minimization of the hyperelastic free energy strain function of a stretched elastomer and the bending elastic energy of a plastic frame. A tulip-shaped gripper structure illustrates the technological potential of the approach. Advantages are simplicity of manufacture, complexity of final structures, and the ease with which any electroactive material can be exploited as means of actuation. (c) 2007 American Institute of Physics.}, language = {en} } @article{McCarthyRisseKatekomoletal.2009, author = {Mc Carthy, Denis N. and Risse, Stefan and Katekomol, Phisan and Kofod, Guggi}, title = {The effect of dispersion on the increased relative permittivity of TiO2/SEBS composites}, issn = {0022-3727}, doi = {10.1088/0022-3727/42/14/145406}, year = {2009}, abstract = {Polymer composites are currently suggested for use as improved dielectric materials in many applications. Here, the effect of particle size and dispersion on the electrical properties of composites of rutile TiO2 and poly(styrene- ethylene-butadiene-styrene) (SEBS) are investigated. Both 15 and 300 nm particles are mixed with SEBS, with amounts of sorbitan monopalmitate surfactant from 0 to 3.3 vol\%, and their dielectric and mechanical properties are measured. Composites with the 300 nm TiO2 particles result in increases of 170\% in relative permittivity over the pure polymer, far above those predicted by standard theories, such as Bruggeman (140\%) and Yamada (114\%), and improving dispersion with surfactant has little effect. The composites with 15 nm particles showed surprisingly large relative permittivity increases (350\%), but improving the dispersion by the addition of any surfactant causes the relative permittivity to decrease to 240\% of the pure polymer value. We suggest that the increase is due to the formation of a highly conductive layer in the polymer around the TiO2 particles.}, language = {en} } @article{StoyanovMcCarthyKolloscheetal.2009, author = {Stoyanov, Hristiyan and Mc Carthy, Denis N. and Kollosche, Matthias and Kofod, Guggi}, title = {Dielectric properties and electric breakdown strength of a subpercolative composite of carbon black in thermoplastic copolymer}, issn = {0003-6951}, doi = {10.1063/1.3154553}, year = {2009}, abstract = {We investigate the dielectric properties and electric breakdown strength of subpercolative composites of conductive carbon black particles in a rubber insulating matrix. A significant increase in the permittivity in the vicinity of the insulator to conductor transition was observed, with relatively low increases in dielectric loss; however, a rapid decrease in electric breakdown strength was inevitable. A steplike feature was ascribed to agglomeration effects. The low ultimate values of the electric field strength of such composites appear to prohibit practical use.}, language = {en} } @article{KofodMcCarthyKrissleretal.2009, author = {Kofod, Guggi and Mc Carthy, Denis N. and Krissler, Jan and Lang, G{\"u}nter and Jordan, Grace}, title = {Electroelastic optical fiber positioning with submicrometer accuracy : model and experiment}, issn = {0003-6951}, doi = {10.1063/1.3134002}, year = {2009}, abstract = {We present accurate electromechanical measurements on a balanced push-pull dielectric elastomer actuator, demonstrating submicrometer accurate position control. An analytical model based on a simplified pure-shear dielectric elastomer film with prestretch is found to capture the voltage-displacement behavior, with reduced output due to the boundary conditions. Two complementary experiments show that actuation coefficients of 0.5-1 nm/V-2 are obtainable with the demonstrated device, enabling motion control with submicrometer accuracy in a voltage range below 200 V.}, language = {en} } @article{StoyanovKolloscheMcCarthyetal.2010, author = {Stoyanov, Hristiyan and Kollosche, Matthias and McCarthy, Denis N. and Kofod, Guggi}, title = {Molecular composites with enhanced energy density for electroactive polymers}, issn = {0959-9428}, doi = {10.1039/C0jm00519c}, year = {2010}, abstract = {Actuators based on soft dielectric elastomers deform due to electric field induced Maxwell's stress, interacting with the mechanical properties of the material. The relatively high operating voltages of such actuators can be reduced by increasing the permittivity of the active material, while maintaining the mechanical properties and high electrical breakdown strength. Approaches relying on the use of highly polarizable molecules or conjugated polymers have so far provided the best results, however it has been difficult to maintain high breakdown strengths. In this work, a new approach for increasing the electrostatic energy density of a soft polymer based on molecular composites is presented, relying on chemically grafting soft gel-state pi-conjugated conducting macromolecules (polyaniline (PANI)) to a flexible elastomer backbone SEBS-g-MA (poly-styrene-co-ethylene-co-butylene-co-styrene-g-maleic anhydride). The approach was found to result in composites of increased permittivity (470\% over the elastomer matrix) with hardly any reduction in breakdown strength (from 140 to 120 V mu m(-1)), resulting in a large increase in stored electrostatic energy. This led to an improvement in the measured electromechanical response as well as in the maximum actuation strain. A transition was observed when amounts of PANI exceeded 2 vol\%, which was ascribed to the exhaustion of the MA- functionality of the SEBS-g-MA. The transition led to drastic increases in permittivity and conductivity, and a sharp drop in electrical breakdown strength. Although the transition caused further improvement of the electromechanical response, the reduction in electrical breakdown strength caused a limitation of the maximum achievable actuation strain.}, language = {en} } @article{ZhuStoyanovKofodetal.2010, author = {Zhu, Jian and Stoyanov, Hristiyan and Kofod, Guggi and Suo, Zhigang}, title = {Large deformation and electromechanical instability of a dielectric elastomer tube actuator}, issn = {0021-8979}, doi = {10.1063/1.3490186}, year = {2010}, abstract = {This paper theoretically analyzes a dielectric elastomer tube actuator (DETA). Subject to a voltage difference between the inner and outer surfaces, the actuator reduces in thickness and expands in length, so that the same voltage will induce an even higher electric field. This positive feedback may cause the actuator to thin down drastically, resulting in electrical breakdown. We obtain an analytical solution of the actuator undergoing finite deformation when the elastomer obeys the neo-Hookean model. The critical strain of actuation is calculated in terms of various parameters of design. We also discuss the effect of the strain-stiffening on electromechanical behavior of DETAs by using the model of freely joined links. (C) 2010 American Institute of Physics. [doi:10.1063/1.3490186]}, language = {en} } @article{KolloscheDoeringStumpeetal.2011, author = {Kollosche, Matthias and D{\"o}ring, Sebastian and Stumpe, Joachim and Kofod, Guggi}, title = {Voltage-controlled compression for period tuning of optical surface relief gratings}, series = {OPTICS LETTERS}, volume = {36}, journal = {OPTICS LETTERS}, number = {8}, publisher = {OPTICAL SOC AMER}, address = {WASHINGTON}, issn = {0146-9592}, pages = {1389 -- 1391}, year = {2011}, abstract = {This Letter reports on new methods and a consistent model for voltage tunable optical transmission gratings. Elastomeric gratings were molded from holographically written surface relief gratings in an azobenzene sol-gel material. These were placed on top of a transparent electroactive elastomeric substrate. Two different electro-active substrate elastomers were employed, with a large range of prestretches. A novel finite-deformation theory was found to match the device response excellently, without fitting parameters. The results clearly show that the grating underwent pure-shear deformation, and more surprisingly, that the mechanical properties of the electro-active substrate did not affect device actuation. (C) 2011 Optical Society of America}, language = {en} } @article{KofodRisseStoyanovetal.2011, author = {Kofod, Guggi and Risse, Sebastian and Stoyanov, Hristiyan and McCarthy, Denis N. and Sokolov, Sergey and Kr{\"a}hnert, Ralph}, title = {Broad-spectrum enhancement of polymer composite dielectric constant at ultra low volume fractions of silica-supported copper nanoparticles}, series = {ACS nano}, volume = {5}, journal = {ACS nano}, number = {3}, publisher = {American Chemical Society}, address = {Washington}, issn = {1936-0851}, doi = {10.1021/nn103097q}, pages = {1623 -- 1629}, year = {2011}, abstract = {A new strategy for the synthesis of high permittivity polymer composites is demonstrated based on well-defined spatial distribution of ultralow amounts of conductive nanoparticles. The spatial distribution Was realized by immobilizing Cu nanoparticles within the pore system of Alia microspheres, preventing direct contact between individual Cu particles. Both Cu-loaded and unloaded silica microspheres were-then used as fillers in polymer composites prepared with thermoplastic SEBS rubber is the matrix. With a metallic Cu content of about 0.26 vol \% In the compoilte, a relative increase of 94\% In real permittivity was obtained. No Cu-induced relaxations were observed in the dielectric spectrum within the studied frequency range of 0.1 Hz to 1 MHz. When related to the amount of conductive nanoparticles, the obtained composites achieve the highest broad spectrum enhancement of permittivity ever reported for a polymer based composite.}, language = {en} } @article{DoeringKolloscheRabeetal.2011, author = {D{\"o}ring, Sebastian and Kollosche, Matthias and Rabe, Torsten and Stumpe, Joachim and Kofod, Guggi}, title = {Electrically tunable polymer DFB laser}, series = {Advanced materials}, volume = {23}, journal = {Advanced materials}, number = {37}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {0935-9648}, doi = {10.1002/adma.201102465}, pages = {4265 -- 4269}, year = {2011}, language = {en} } @article{StoyanovKolloscheRisseetal.2011, author = {Stoyanov, Hristiyan and Kollosche, Matthias and Risse, Sebastian and McCarthy, Denis N. and Kofod, Guggi}, title = {Elastic block copolymer nanocomposites with controlled interfacial interactions for artificial muscles with direct voltage control}, series = {Soft matter}, volume = {7}, journal = {Soft matter}, number = {1}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c0sm00715c}, pages = {194 -- 202}, year = {2011}, abstract = {Soft, physically crosslinking, block copolymer elastomers were filled with surface-treated nanoparticles, in order to evaluate the possibility for improvement of their properties when used as soft dielectric actuators. The nanoparticles led to improvements in dielectric properties, however they also reinforced the elastomer matrix. Comparing dielectric spectra of composites with untreated and surface-treated particles showed a measurable influence of the surface on the dielectric loss behaviour for high filler amounts, strongly indicating an improved host-guest interaction for the surface-treated particles. Breakdown strength was measured using a test bench and was found to be in good agreement with the results from the actuation measurements. Actuation responses predicted by a model for prestrained actuators agreed well with measurements up to a filler amount of 20\%(vol). Strong improvements in actuation behaviour were observed, with an optimum near 15\%(vol) nanoparticles, corresponding to a reduction in electrical field of 27\% for identical actuation strains. The use of physically crosslinking elastomer ensured the mechanical properties of the matrix elastomer were unchanged by nanoparticles effecting the crosslinking reaction, contrary to similar experiments performed with chemically crosslinking elastomers. This allows for a firm conclusion about the positive effects of surface-treated nanoparticles on actuation behavior.}, language = {en} } @article{JordanMcCarthySchleppleetal.2011, author = {Jordan, Grace and McCarthy, Denis N. and Schlepple, N. and Krissler, Jan and Schroeder, H. and Kofod, Guggi}, title = {Actuated micro-optical submount using a dielectric elastomer actuator}, series = {IEEE ASME transactions on mechatronics}, volume = {16}, journal = {IEEE ASME transactions on mechatronics}, number = {1}, publisher = {Inst. of Electr. and Electronics Engineers}, address = {Piscataway}, issn = {1083-4435}, doi = {10.1109/TMECH.2010.2089991}, pages = {98 -- 102}, year = {2011}, abstract = {Analysis of the operating characteristics of a dielectric elastomer actuator (DEA) submount for the high-precision positioning of optical components in one dimension is presented. Precise alignment of a single-mode fiber is demonstrated and variation of the sensitivity of the submount motion by changing the bias voltage is confirmed. A comparison of the performance of the DEA submount with a piezoelectric alignment stage is made, which demonstrates that DEAs could present a very attractive, low-cost alternative to currently used manual technologies in overcoming the hurdle of expensive packaging of single-mode optical components.}, language = {en} } @article{AhnertAbelKolloscheetal.2011, author = {Ahnert, Karsten and Abel, Markus and Kollosche, Matthias and Jorgensen, Per Jorgen and Kofod, Guggi}, title = {Soft capacitors for wave energy harvesting}, series = {Journal of materials chemistry}, volume = {21}, journal = {Journal of materials chemistry}, number = {38}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {0959-9428}, doi = {10.1039/c1jm12454d}, pages = {14492 -- 14497}, year = {2011}, abstract = {Wave energy harvesting could be a substantial renewable energy source without impact on the global climate and ecology, yet practical attempts have struggled with the problems of wear and catastrophic failure. An innovative technology for ocean wave energy harvesting was recently proposed, based on the use of soft capacitors. This study presents a realistic theoretical and numerical model for the quantitative characterization of this harvesting method. Parameter regions with optimal behavior are found, and novel material descriptors are determined, which dramatically simplify analysis. The characteristics of currently available materials are evaluated, and found to merit a very conservative estimate of 10 years for raw material cost recovery.}, language = {en} } @article{KolloscheStoyanovLaflammeetal.2011, author = {Kollosche, Matthias and Stoyanov, Hristiyan and Laflamme, Simon and Kofod, Guggi}, title = {Strongly enhanced sensitivity in elastic capacitive strain sensors}, series = {Journal of materials chemistry}, volume = {21}, journal = {Journal of materials chemistry}, number = {23}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {0959-9428}, doi = {10.1039/c0jm03786a}, pages = {8292 -- 8294}, year = {2011}, abstract = {Strain sensors based on dielectric elastomer capacitors function by the direct coupling of mechanical deformations with the capacitance. The coupling can be improved by enhancing the relative permittivity of the dielectric elastomer. Here, this is carried out through the grafting of conducting polymer (poly-aniline) to the elastomer backbone, leading to molecular composites. An enhancement in capacitance response of 46 times is observed. This could help to extend the possible range of miniaturization towards even smaller device features.}, language = {en} } @article{KussmaulRisseKofodetal.2011, author = {Kussmaul, Bjoern and Risse, Sebastian and Kofod, Guggi and Wache, Remi and Wegener, Michael and McCarthy, Denis N. and Kr{\"u}ger, Hartmut and Gerhard, Reimund}, title = {Enhancement of dielectric permittivity and electromechanical response in silicone elastomers molecular grafting of organic dipoles to the macromolecular Network}, series = {Advanced functional materials}, volume = {21}, journal = {Advanced functional materials}, number = {23}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1616-301X}, doi = {10.1002/adfm.201100884}, pages = {4589 -- 4594}, year = {2011}, abstract = {A novel method is established for permittivity enhancement of a silicone matrix for dielectric elastomer actuators (DEAs) by molecular level modifications of the elastomer matrix. A push-pull dipole is synthesized to be compatible with the silicone crosslinking chemistry, allowing for direct grafting to the crosslinker molecules in a one-step film formation process. This method prevents agglomeration and yields elastomer films that are homogeneous down to the molecular level. The dipole-to-silicone network grafting reaction is studied by FTIR. The chemical, thermal, mechanical and electrical properties of films with dipole contents ranging from 0 wt\% to 13.4 wt\% were thoroughly characterized. The grafting of dipoles modifies the relative permittivity and the stiffness, resulting in the actuation strain at a given electrical field being improved by a factor of six.}, language = {en} } @article{KofodStoyanovGerhard2011, author = {Kofod, Guggi and Stoyanov, Hristiyan and Gerhard, Reimund}, title = {Multilayer coaxial fiber dielectric elastomers for actuation and sensing}, series = {Applied physics : A, Materials science \& processing}, volume = {102}, journal = {Applied physics : A, Materials science \& processing}, number = {3}, publisher = {Springer}, address = {New York}, issn = {0947-8396}, doi = {10.1007/s00339-010-6066-5}, pages = {577 -- 581}, year = {2011}, abstract = {A simple dip-coating technique was employed to manufacture coaxial actuators with multiple layers of alternating dielectric and conducting layers. A thin rubber string was coated with an electrode-insulator-electrode structure, giving rise to a thin, fiber-like actuator with coaxial geometry. The process was repeated to achieve a compact multilayer actuator with up to three coaxial dielectric layers. Mechanical and electromechanical characterization of the actuators is presented, showing actuation strains up to 8\% and proper voltage-thickness scaling behavior. Also presented is a capacitance vs. extension plot, demonstrating that these structures can be used for compact and accurate capacitive strain sensing.}, language = {en} } @article{McCarthyStoyanovRychkovetal.2012, author = {McCarthy, Denis N. and Stoyanov, Hristiyan and Rychkov, Dmitry and Ragusch, Huelya and Melzer, Michael and Kofod, Guggi}, title = {Increased permittivity nanocomposite dielectrics by controlled interfacial interactions}, series = {Composites science and technology}, volume = {72}, journal = {Composites science and technology}, number = {6}, publisher = {Elsevier}, address = {Oxford}, issn = {0266-3538}, doi = {10.1016/j.compscitech.2012.01.026}, pages = {731 -- 736}, year = {2012}, abstract = {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.}, language = {en} } @article{KolloscheZhuSuoetal.2012, author = {Kollosche, Matthias and Zhu, Jian and Suo, Zhigang and Kofod, Guggi}, title = {Complex interplay of nonlinear processes in dielectric elastomers}, series = {Physical review : E, Statistical, nonlinear and soft matter physics}, volume = {85}, journal = {Physical review : E, Statistical, nonlinear and soft matter physics}, number = {5}, publisher = {American Physical Society}, address = {College Park}, issn = {1539-3755}, doi = {10.1103/PhysRevE.85.051801}, pages = {4}, year = {2012}, abstract = {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.}, language = {en} } @article{ZhuKolloscheLuetal.2012, author = {Zhu, Jian and Kollosche, Matthias and Lu, Tongqing and Kofod, Guggi and Suo, Zhigang}, title = {Two types of transitions to wrinkles in dielectric elastomers}, series = {Soft matter}, volume = {8}, journal = {Soft matter}, number = {34}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1744-683X}, doi = {10.1039/c2sm26034d}, pages = {8840 -- 8846}, year = {2012}, abstract = {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.}, language = {en} } @article{VukicevicVukovicStoyanovetal.2012, author = {Vukicevic, Radovan and Vukovic, Ivana and Stoyanov, Hristiyan and Korwitz, Andreas and Pospiech, Doris and Kofod, Guggi and Loos, Katja and ten Brinke, Gerrit and Beuermann, Sabine}, title = {Poly(vinylidene fluoride)-functionalized single-walled carbon nanotubes for the preparation of composites with improved conductivity}, series = {Polymer Chemistry}, volume = {3}, journal = {Polymer Chemistry}, number = {8}, publisher = {Royal Society of Chemistry}, address = {Cambridge}, issn = {1759-9954}, doi = {10.1039/c2py20166f}, pages = {2261 -- 2265}, year = {2012}, abstract = {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.}, language = {en} }