Mechanical,dielectric, and electromechanical properties of silicone dielectric elastomer actuators

Silicone elastomer actuators were investigated to develop a simple and industrially scalable product with improved mechanical properties, such as a low modulus, high tearing strength, and good resilience, and enhanced electromechanical actuation properties. Silicone elastomers were fabricated via a hydrosilylation addition reaction with a vinyl‐end‐functionalized poly(dimethyl siloxane) (V), a multivinyl‐functionalized silicone resin, and a crosslinker in the presence of a platinum catalyst. For the larger electromechanical actuation response, the silicone dielectric elastomer actuator had to have a larger molecular weight of poly(dimethyl siloxane), a smaller hardener content, and a resin‐free composition. However, the silicone elastomer actuators needed to include a small amount of resin to improve the tearing strength. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40030.     

Glycerol as high‐permittivity liquid filler in dielectric silicone elastomers

A recently reported novel class of elastomers was tested with respect to its dielectric properties. The new elastomer material is based on a commercially available poly(dimethylsiloxane) composition, which has been modified by embedding glycerol droplets into its matrix. The approach has two major advantages that make the material useful in a dielectric actuator. First, the glycerol droplets efficiently enhance the dielectric constant, which can reach astonishingly high values in the composite. Second, the liquid filler also acts as a softener that effectively decreases the elastic modulus of the composite. In combination with very low cost and easy preparation, the two property enhancements lead to an extremely attractive dielectric elastomer material. Experimental permittivity data are compared to various theoretical models that predict relative permittivity changes as a function of filler loading, and the applicability of the models is discussed. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44153.     

Preparation and electrodeformation of silicone dielectric elastomers containing poly(propylene glycol diacetate) with different molecular weights

In this study, poly(propylene glycol diacetate)s (PPGDAs) with different molecular weights were obtained by the esterification reaction of poly(propylene glycol) and acetic anhydride. We effectively reduced the residual moisture and hydrophilicity of PPGDA. Then, poly(dimethyl siloxane) (PDMS) was modified by the addition of only 5 wt % PPGDA, which possessed a high dielectric constant (k) and a large actuated strain at a low electric field. PPGDA was used to enhance the molecular polarity because of the more polar oxygen atoms and the greater number of ester groups. The great increase in k and the low elastic modulus of the PPGDA–PDMS composites lead to a great increase in the electromechanical sensitivity. When the molecular weight of PPGDA was about 4000, the PPGDA–PDMS composites had the largest actuated strain. As a result, compared to the pure silicone elastomer (8.94%), it exhibited a greater strain of 17.31% at a low electric field of 10.5 V/μm (an increase of ca. 1.94 times). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45329.     

Vegetable fillers for electric stimuli responsive elastomers

Dielectric elastomer actuators (DEAs) have been studied widely in recent years for artificial muscle applications, but their implementation into production is limited due to high operating voltages required. The actuation behavior of dielectric elastomer under an applied electric field is predicted by Maxwell's pressure and thickness strain equations. According to these equations, the best electromechanical response is achieved when the relative permittivity is high and elastic modulus is low. The potential source for additives increasing the relative permittivity of rubbers can be vegetable powders that have much higher dielectric constant than common elastomers. In the present research, the dielectric and actuation properties of polyacrylate rubber (ACM) were studied after the addition of different vegetable‐based fillers such as potato starch, corn starch, garlic, and paprika. The results were compared to ACM filled with barium titanate. The compounds containing vegetable fillers showed higher relative dielectric permittivity at 1 Hz frequency than the compounds containing barium titanate due to higher interfacial polarization. The actuation studies showed that lower electric fields are required to generate certain actuation forces when the starches and garlic are used in the rubber instead of barium titanate. Therefore, the vegetable‐based fillers can be used to improve actuation performance of DEAs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45081.     

Plasticized thermoplastic polyurethanes for dielectric elastomers with improved electromechanical actuation

In this article, four different plasticizers are blended in thermoplastic polyurethane (PU) to improve its electromechanical actuation performance. The selected plasticizers include dibutyl phthalate, triphenyl phosphate, polyethylene glycol (PEG), and an unsaturated polyester PMG. The plasticization effect of various plasticizers on the mechanical properties, dielectric properties, and the electromechanical actuation of PU films is carefully characterized and compared. Results demonstrate that the actuated strain under low electric fields and the electromechanical coupling efficiency of PU can be substantially improved by blending with appropriate type and amount of plasticizers. The oligomer‐type plasticizers, PEG and PMG, act more efficiently in the improvement of actuation. An actuated strain in thickness of 1.54%, 140 times higher than that of pure PU, along with an electromechanical coupling efficiency of 0.60 under a low electric field of 5 V/μm was achieved for the PU plasticized with PMG suggesting an attractive approach toward advanced dielectric elastomer actuators. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45123.     

Crystallization,structure, and enhanced mechanical property of ethylene-octene elastomer crosslinked with dicumyl peroxide

Crosslinking of polyolefin elastomer (POE, ENGAGE™ 8480) with Dicumyl Peroxide (DCP) can have effects on its crystallization dynamics, crystal structure, and properties. The POE crosslinked uniformly has significantly lower crystalline ability than the one with only amorphous phase crosslinked, which, in turn, has weaker crystalline ability than neat POE. The crystallinity and melting point depend on how the POE is crosslinked. The neat POE and POE crosslinked in amorphous phase only, are investigated with DSC and in-situ tensile/synchrotron radiation (WAXD/SAXS). In situ tensile/synchrotron X-ray during a uniaxial stretching process indicates that severe crystal fragmentation is observed at a strain around 45%, and with further increase in strain. The stress in the crosslinked POE is significantly larger than neat POE. For both samples, crystal orientation increases sharply within the strain range up to 88% where orientation-induced new crystals aligned in stretching direction are observed. The long period increases more in stretching direction for the crosslinked POE, consistent with larger stress in this sample, and the stress difference is more pronounced at large strains (27.3 vs. 10.9 MPa at a strain 435%). Permanent set of the crosslinked POE is smaller, consistent with less oriented crystals observed after the test for permanent set.     

Body-temperature shape-shifting liquid crystal elastomers

Nematic monodomain liquid crystal elastomers (LCEs) undergo efficient temperature-induced reversible shape-shifting around the nematic-isotropic transition temperature (T_ni) due to the presence of the liquid-crystalline order of mesogens. Usually, the T_ni of nematic LCEs is much higher than the human body temperature, and therefore LCEs are not often considered for biomedical applications. This study describes an LCE system where the T_ni is tuned by substitution of the rigid mesogens RM257 with a flexible backbone PEGDA250. By systematically substituting the RM257 with PEGDA250, the T_ni of LCEs was observed to decrease from 66°C to 23°C. A rate-optimized LCE material was fabricated with 10 mol % rigid mesogens substituted with a flexible backbone that demonstrated T_ni at 32°C, in-between the room temperature of 20°C and the body temperature of 37°C. The T_ni allowed the programmed shape at room temperature, quick shape-shifting upon exposure to body temperature, and before-programmed shape when kept at body temperature. This LCE material displayed reversible length change of 23%, opacity change, and shape change between room temperature and body temperature.     

Enhancement of electromechanical properties of natural rubber by adding barium titanate filler: An electro-mechanical study

Natural rubber is one of the most potential electro-active polymers for sensors, actuators, and energy harvesting applications. Enhancing the characteristic properties of polymers by reinforcing with fillers that possess multifunctional attributes have attracted considerable attention. In the present study, barium titanate reinforced natural rubber composite is prepared by using two-roll mill mixing. Afterwards, mechanical, electrical, and electromechanical properties of the composites are extensively analyzed by reinforcing different amounts of barium titanate into the matrix of natural rubber. The fabricated dielectric composite shows excellent properties such as high dielectric constant, low dielectric losses, high dielectric breakdown strength, and extreme stretchability. It is observed that as the filler loading reaches the value of 11 parts per hundred rubber (phr), maximum agglomeration of the particles occurs. Maximum stretchability and highest ratio of dielectric constant to elastic modulus are obtained at 8 phr of barium titanate fillers and at the loading, a maximum actuation strain of 11.24% is achieved. This study provides a simple, economical, and effective method for preparing enhanced mechanical, electrical, and electromechanical properties of natural rubber composites, facilitating the wide applications of dielectric materials as actuators and generators.     

Synergetic toughening of poly(phenylene sulfide) by poly(phenylsulfone) and poly(ethylene-ran-methacrylate-ran-glycidyl methacrylate)

Poly(phenylene sulfide) (PPS) is a high-performance super-engineering plastic, but is brittle. In this study, super-tough PPS-based blends were successfully generated by melt blending PPS with poly(ethylene-ran-methacrylate-ran-glycidyl methacrylate) (EGMA) and poly(phenylsulfone) (PPSU) at (56/14/30) PPS/EGMA/PPSU composition, and their toughening mechanisms were investigated in detail. It was demonstrated the interfacial reaction between PPS and EGMA and partial miscibility between PPS and PPSU, both play important synergistic roles on the toughening. The interfacial reaction between PPS and EGMA contributes to the reduction of the PPSU domain size by the increased viscosity of the PPS matrix containing EGMA, and the increased mobility of EGMA chains by negative pressure effect. The partial miscibility between PPS and PPSU contributes to the increased interfacial adhesion between PPS and PPSU, resulting in effective propagation of the impact to the domains, and the increased mobility of not only PPSU chains but also PPS chains, causing a reduction in crystallization.     

Aspects of varying crosslinker and catalyst concentrations on tear strength of silicone polymer networks: Comparison of tear strengths using samples of different geometries

Crosslinker and catalyst concentrations have been varied to prepare different hydroxyfunctional poly(dimethylsiloxane) (HOPDMS) polymer network compositions. The tear strengths of these silicone polymer networks have been measured using different geometries, as trouser, crescent, and Graves (angled) specimens. It has been observed that the results of tear strength of Graves and crescent‐shaped specimens do not show a constant ratio with the concentration of crosslinker used for curing of HOPDMS networks. Instead, it has been observed and reported for the first time that the tear strengths of Graves and crescent‐shaped samples show a crossover at about 1.2% crosslinker. The observation of this crossover pattern for different compositions of silicone networks show that it is difficult to compare the results of the tear test of the same polymer performed on samples of different geometries with one another. The crossover pattern of the tear energy results for the test specimens of two different geometries has been explained in the light of essential work facture theory based on the geometry of the testing sample, crosslinking, and testing that alters the distribution of force over the width of the specimen. It was shown that the change in composition of the HOPDMS networks changes the order of ranking of Graves and crescent tear tests. With varying catalyst concentration in the silicone network composition, the tear property differences between the Graves and crescent‐shaped specimens are not significant. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43115.     

Property reinforcement of silicone dielectric elastomers filled with self‐prepared calcium copper titanate particles

In this article, submicron and micron calcium copper titanate (CCTO) crystallites with different morphologies were successfully designed and prepared by directly thermal treatment method and molten salt method, respectively. Then, the silicone elastomer filled with self‐prepared CCTO particles had high dieletric constant, low dielectric loss, and actuated strain which was greatly improved at low electric field. The dieletric constant at 50 Hz obviously increased from 2.15 for pure silicone elastomer to 4.37 and 4.18 for the submicron and micron CCTO/poly (dimethyl siloxane) (PDMS) composites. The dielectric loss of the composites retained at a low value (less than 0.06). Meanwhile, the elastic modulus of CCTO/PDMS composites was increased slightly only with a good flexibility. Compared to pure silicone elastomer (2.25%), the submicron and micron CCTO/PDMS composites with 2 wt % content exhibited a greater strain of 7.69% and 9.83% at a low electric field of 5 V/μm. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42613.     

Amine/epoxy stoichiometric ratio dependence of crosslinked structure and ductility in amine-cured epoxy thermosetting resins

Epoxy-amine thermosetting resins undergo different reactions depending on the amine/epoxy stoichiometric ratio (r). Although many desirable properties can be achieved by varying the stoichiometric ratio, the effects of the variation on the crosslinked structure and mechanical properties and the contribution of these factors to the ductility of materials have not been fully elucidated. This study investigates the brittle-ductile behavior of epoxies with various stoichiometric ratios and performs curing simulations using molecular dynamics (MD) to evaluate the crosslinked structures. The molecular structure is predominantly branched in low-stoichiometric ratio samples, whereas the chain extension type structure dominates the high-stoichiometric ratio samples. As a result, the higher-stoichiometric ratio samples enhances the ductility of materials and the elongation at break increases form 1.4% (r = 0.6) to 11.4% (r = 1.4). Additionally, the tensile strength (105.4 MPa) and strain energy (7.96 J/cm³) are maximum at r = 0.8 and 1.2, respectively. On the other hand, the Young's modulus is negatively impacted and it decreased from 4.2 to 2.7 GPa with increasing stoichiometric ratio.     

A highly elastic and sensitive sensor based on GSP/HPAM composited hydrogel

Development of conductive hydrogels to mimic the structures and properties of human skin has attracted enormous scientific interests. However, applications of such materials are often restricted by their poor mechanics and moderate sensitivities. Herein, a highly-stretchable, self-healing, and conductive hydrogel, comprised of grape seed extracted polymer and hydrophobically associated polyacrylamide (GSP-HPAM), was fabricated by simple mixing and in-situ polymerization. Compared with the single HPAM gels, the rigid GSP polymer could form sufficient hydrogen bonds and ionic interactions, which endowed the gel with effective energy dissipation mechanism and greatly improved the uniformity of network. As a result, the GSP-HPAM gel exhibited enhanced mechanics, that is, the tensile strength, strain, and compression stress of GSP-HPAM hydrogel were 0.7 MPa, 3000% and 28.3 MPa, respectively. Furthermore, the gel demonstrated linear strain-dependant conductivity between 0% and 1000% with gauge factor around 3.43, superior to most hydrogel-based strain sensors. In addition, the gel was able to monitor human body motion, such as, finger bending and pulse rate. This multiple functional gel might find potential applications in artificial skin, soft robotics, and wearable devices.     

Economical conductive graphite-filled polymer composites via adjustable segregated structures: Construction,low percolation threshold,and positive temperature coefficient effect

The economical graphite-filled thermoplastic urethane/ultra-high molecular weight polyethylene (TPU/UHMWPE) composites with the segregated structure were constructed by the combination of mechanical crushing and melt blending method. The low percolation threshold of 1.89 wt% graphite in the adjustable segregated composites was obtained and high electrical conductivity was about 10⁻¹ S m⁻¹ at 10 wt% graphite loadings owing to the formation of three-dimensional conductive networks. Moreover, when the graphite loadings were over the percolation threshold, the remarkable positive temperature coefficient (PTC) effect of electrical resistivity for TPU/UHMWPE-Graphite composites were achieved, originating from the combined thermal motion of TPU and UHMWPE. Meanwhile, the outstanding repeatability of PTC effects was obtained after 5-time cycles. Therefore, economical conductive polymer composites were still the promising field in the practical application of PTC materials.     

Interplay between key variables of peroxide cured EPDM and evaluation of electromechanical efficiency for MV cables

Extensive work has been done on ethylene‐propylene‐diene monomer (EPDM) vulcanizates and the main factors influencing their cure efficiency. However, very little attention has been given to the evolution of material properties and the interplay between key variables formulation. The effect of ethylene/propylene content and concentration of peroxide and type on viscoelastic and dielectric properties of EPDM elastomers was investigated using dynamic mechanical analysis and electric property measurements at room temperature. In order to compare results, these measurements were obtained by FTIR spectroscopy. Differences between real and imaginary (loss) part of the permittivity of EPDM/ dicumyl peroxide (DCP) composites were more significant than those in EPDM/di(tert‐butylperoxyisopropyl)benzene (DTBPIB) composites. For peroxide DTBPIB, the dielectric percolation limit moves further with the increase of ethylene content of EPDM. The dielectric percolation limit of the peroxide DTBPIB is found to be approximately 60 g at a 75% of ethylene in EPDM. The FTIR analysis showed that a termoxidative degradation was promoted at mixes with peroxide DCP at 45 and 75% of ethylene in EPDM. Therefore, we could interestingly show the decrease in electrical properties particularly associated with termoxidative degradation of peroxide DCP. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46139.     

Evaluation of mechanical properties and hydrophobicity of room-temperature,moisture-curable polysilazane coatings

Polysilazane coatings have a broad need in real-life applications, which require low processing or working temperature. In this work, five commercially available polysilazanes have been spin-coated on polycarbonate substrates and cured in ambient environment and temperature to obtain transparent, crack-free, and dense films. The degree of crosslinking is found to have a significant impact on the hardness and Young's modulus of the polysilazane films but has a minor influence on the film thickness and hydrophobicity. Among all five polysilazane coatings, the inorganic perhydropolysilazane-based coating exhibits the largest hardness (2.05 ± 0.01 GPa) and Young's modulus (10.76 ± 0.03 GPa) after 7 days of curing, while the polyorganosilazane-derived films exhibit higher hydrophobicity. The molecular structure of polysilazanes plays a key role in mechanical properties and hydrophobicity of the associated films, as well as the adhesion of coatings to substrates, providing an intuitive and reliable way for selecting a suitable polysilazane coating material for a specific application.     

Polyvinylidene fluoride nanofibers obtained by electrospinning and blowspinning: Electrospinning enhances the piezoelectric β-phase – myth or reality?

Considerable effort has been devoted to improving the properties of PVDF (polyvinylidene fluoride), arguably the most technologically important piezoelectric polymer. Electrospinning has been found to be a particularly effective method of producing PVDF nanofibers with superior piezoelectric properties due to the resulting exceptionally high fraction of the piezoelectrically active crystalline β-phase. It is typically assumed that the high external electric fields applied during electrospinning enhance the formation of this β-phase, with the confused literature offering various unsatisfactory mechanistic explanations. However, by comparing PVDF nanofibers produced by two different processes (electrospinning and blowspinning), we show that the electric field is entirely unnecessary; indeed, the crystallization dynamics are principally driven by the applied mechanical stress, as evidenced by structurally identical 200 nm diameter PVDF fibers produced with and without external electric fields.     

Comparison of cure characteristics and mechanical properties of nano and micro silica‐filled CSM elastomers

In this study, the effect of micro and nano silica and their combination on mechanical and thermal properties of Chlorosulfonated Polyethylene compounds were investigated. Cure characteristics were studied using a Monsanto Moving Die Rheometer at 155°C. Incorporation of nano silica accelerated the vulcanization whereas the micro silica particles decelerated the curing process. Both micro and nano silica increased the crosslink density as evidenced by swelling test. However, this value has been more improved in CSM/nano silica composites. The physico‐mechanical properties of CSM/nano silica are superior compared to CSM/micro silica. Nano silica provided reinforcing efficiency which is not only because of higher specific surface area but also because of various interactions and especially physical interactions which are discussed in the text. Nano silica particles also improved the thermal properties more efficiently. Incorporation of 15 phr (part per hundred) nano and 5 phr micro silica to polymer improved the initial decomposition temperature for about 51°C and 16°C, respectively, using a TGA. The combination of micro and nano silica, showed that by coupling nano and micro fillers, the loading of fillers can be minimized. In other words, the hybrid samples with a lower filler loading behave as efficient as their separate counterpart with higher loading. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42668.