Tetra-needle-shaped zinc oxide whiskers (T-ZnOw) and barium titanate (BT) nanoparticles were embedded into polypropylene (PP) matrix to construct ternary nanocomposites with improved dielectric properties. The nanocomposites were fabricated via a melt blending and subsequent compression molding approach. The microstructure, crystallization behavior, and dielectric properties of as-fabricated nanocomposites were investigated in detail. Results reveal that the selecting processing approach could afford isotropic nanocomposites with T-ZnOw and BT nanoparticles homogeneously dispersed throughout the PP matrix. The T-ZnOw/PP binary composites exhibited typical characteristics of percolation system with a relatively small threshold of 13 vol%. When the content of T-ZnOw approaching the percolation threshold, the dielectric constant and loss tangent of the binary composites were dramatically enhanced. After introducing BT nanoparticles, the resultant ternary composites showed further improved dielectric performance. For the ternary T-ZnOw/BT/PP with the BT content of 15 vol% and T-ZnOw content of 9.76 vol%, dielectric constant and loss tangent at 1 kHz reached 11 and 0.04, respectively.

An efficient strategy is developed for the fabrication of graphene-filled polypropylene (PP) nanocomposites with graphene nanosheets orderly oriented in the in-plane direction. The nanocomposites with an anisotropic coefficient as high as 35 000 in electrical resistivity were fabricated by a sequential biaxial stretching process. Polymethylmethacrylate (PMMA) was employed to bridge graphene to the non-polar PP matrix, which facilitates the homogeneous dispersion and the orientation of the chemically converted graphene nanosheets. A PMMA/graphene masterbatch was firstly prepared and blended into the PP matrix. During the biaxial stretching, the PMMA/graphene phase was transformed from beads to sheets, which induced the in-plane orientation of the graphene nanosheets. As a consequence, the storage modulus and the conductivity of the nanocomposites were improved in the in-plane direction. The effects of graphene content and draw ratio on the anisotropy of the PP/PMMA/graphene nanocomposites were discussed in detail. This strategy of orientation-effectiveness and cost-effectiveness can be potentially integrated with commercialized biaxial stretching processes to produce high-quality anisotropic polyolefin/graphene composite films.

Herein we report the highly improved electromechanical actuation of thermoplastic polyurethane (TPU) by blending with polydimethylsiloxane (PDMS) to construct a bicontinuous structure. TPU/PDMS blend films with various PDMS loadings were fabricated through a simple solution-assisted casting method. Infrared spectroscopy measurements confirmed that TPU and PDMS are thermodynamically incompatible with each other. For TPU80 with 80 parts of PDMS, a bicontinuous phase structure was achieved. The TPU80 film showed greatly decreased elastic modulus and improved elongation at break compared to pristine TPU. It also showed the highest dielectric constant among the TPU/PDMS blend films with various contents of PDMS due to strong interfacial polarization. Most importantly, the TPU80 film exhibited a maximum areal strain of 2.3% under an electric field of 40 V μm−1, which is about 60 times higher than that of pristine TPU. The results described in this work demonstrate that the construction of a bicontinuous interface structure at the micrometer scale is very effective to develop elastomers with superior electromechanical actuation performance.

Dielectric elastomer generators (DEGs), which follow the physics of variable capacitors and harvest electric energy from mechanical work, have attracted intensive attention over the past decade. The lack of ideal dielectric elastomers, after nearly two decades of research, has become the bottleneck for DEGs’ practical applications. Here, we fabricated a series of polyurethane-based ternary composites and estimated their potential as DEGs to harvest electric energy for the first time. Thermoplastic polyurethane (PU) with high relative permittivity (∼8) was chosen as the elastic matrix. Barium titanate (BT) nanoparticles and dibutyl phthalate (DBP) plasticizers, which were selected to improve the permittivity and mechanical properties, respectively, were blended into the PU matrix. As compared to pristine PU, the resultant ternary composite films fabricated through a solution casting approach showed enhanced permittivity, remarkably reduced elastic modulus, and relatively good electrical breakdown strength, dielectric loss, and strain at break. Most importantly, the harvested energy density of PU was significantly enhanced when blended with BT and DBP. A composite film containing 25 phr of BT and 60 phr of DBP with the harvested energy density of 1.71 mJ/cm3 was achieved, which is about 4 times greater than that of pure PU and 8 times greater than that of VHB adhesives. Remarkably improved conversion efficiency of mechano-electric energy was also obtained via cofilling BT and DBP into PU. The results shown in this work strongly suggest compositing is a very promising way to provide better dielectric elastomer candidates for forthcoming practical DEGs.Keywords: composite; dielectric elastomer generator; energy conversion; energy density; polyurethane;

•Voltage ramp rate and diaphragm radius impact on electromechanical actuation.•Adding a small amount of plasticizers improves electromechanical strain.•Deflection strain as high as 1800% is achieved for polyurethane at 42 V/μm.Oligomer-plasticized polyurethanes were fabricated and their mechanical properties, dielectric properties, and electro-mechanical actuations in a diaphragm configuration were investigated. The effect of the ramp rate of electric field and the diameter of actuators on the actuation was tested in detail. We found that choosing appropriate parameters can suppress the leakage current and reduce the visco-elastic resistance, resulting in improved actuated strain. Under near optimal conditions, the polyurethane with 1 phr (per hundred resin) plasticizer displayed a deflection strain as high as 1800% (around 12% of diameter) at 42 V/μm, providing an effective actuated work of 1.8 J/cm3. The optimized conditions are material specific, however the present analysis can be extended to other dielectric elastomers to aid in the design and fabrication of better actuators.Download high-res image (109KB)Download full-size image

Herein we report a novel and efficient approach to fabricate dielectric elastomers with enhanced dielectric constant and high dielectric strength. Azobenzenes with strong permanent dipole moments were synthesized to co-crosslink with hydroxyl-terminated polydimethylsiloxane through a simple one-step process, which realized a type of robust, molecularly homogenous silicone rubber (SR). The chemical structure, dielectric and mechanical properties of the resultant azo-g-PDMS elastomers with azobenzne contents ranging from 0 to 13.2 wt% were carefully characterized. The dielectric constant of azo-g-PDMS films at 1 kHz increased from 2.72 to 4.88 with the increase of azobenzene contents. By grafting with 4.0 wt% of azobenzene, the breakdown strength of azo-g-PDMS reached 89.4 V μm−1, which is 36% higher than that of pristine SR. The electric field induced deformation of silicone rubber could be enhanced by grafting with azobenzenes. The azo-g-PDMS film with 7.1 wt% of azobenzenes displayed a maximum area strain of 17%. Meanwhile, the azo-g-PDMS films exhibited a short response time (about 0.5 s) to the change in the electric field. Some prototype electromechanical actuators based on this type of azo-g-PDMS films were fabricated, demonstrating that the azo-g-PDMS dielectric elastomer is a very promising candidate for artificial muscle applications.

Herein a diaphragm type dielectric elastomer actuator by blending azobenzene dyes into a polyurethane matrix (Azo/PU) is described. The effects of azobenzene content on the dielectric, mechanical, and electromechanical properties of the Azo/PU blends are discussed. The resultant blends with azobenzene content ranging from 1 to 8 wt% were characterized. The dielectric permittivity of PU increased from 8.0 to 11.1 (increasing of 38.5%) at 1 wt% azo content and reached a maximum of 35.6 (increasing of 343%) at 4 wt% azo content. By blending with a small content of azo the dielectric breakdown strength reached 57 kV mm−1, which is 86% higher than that of the neat PU. The 30 μm thick diaphragm of Azo/PU blend with 1 wt% azo content exhibited the highest actuated displacement at the center around 700 μm. Meanwhile the neat PU gave a displacement of 25 μm. That is a 26-fold increase in actuation is obtained by blending only 1 wt% azo dyes into the PU matrix. Infrared spectroscopy was used to characterize the structure of the Azo/PU blends. The particular chemical structure the Azo/PU blend containing 1 wt% of azo dyes is proposed to be responsible for the improvement in electromechanical actuation.

In this work, a series of diblock copolymers with a defined length of a soft poly(dimethylsiloxane) (PDMS) segment and different lengths of liquid crystalline (LC) poly[11-(4-cyano-4′-biphenoxy)undecyl methacrylate] (P11CBMA) segments were synthesized by using the atom transfer radical polymerization (ATRP) method. Their microphase separation behavior and dielectric properties were carefully investigated. The well-defined diblock copolymers PDMSm-b-P11CBMAn possess three different soft/rigid ratios (m = 62, n = 15, 22, and 40) and narrow molecular distributions (PDI ≤ 1.25). Due to the supramolecular cooperative motion effect, the copolymers can form lamellar morphology (WLC = 57.8%), predominant lamellar morphology with PDMS cylindrical defects (WLC = 66.8%), and PDMS cylinders embedded in the LC matrix (WLC = 78.3%) after annealing at 160 °C (above Ti) under N2 for 24 h, respectively. The dielectric properties of the block copolymers were strongly influenced by the microphase separated morphologies and orientational directions of mesogens. Compared to homopolymer P11CBMA, PDMSm-b-P11CBMAn showed higher dielectric constants due to the confinement effect of microdomain dimensions. The dielectric constants of diblock copolymers could be further improved by aligning mesogens parallel to the electric field through substrate treatments. These findings will inspire researchers to design and develop novel LC block copolymers for dielectric elastomer actuator applications.

Piezoresistive nanocomposites using alkyl-functionalized graphene (G-ODA) as a conducting filler and polydimethylsilicone (PDMS) as the polymer matrix were prepared and their piezoresistivity behavior was investigated. One-pot synthesis of G-ODA from graphite oxide and octadecylamine improved its dispersion in nonpolar xylene and PDMS with low surface free energy. Results show that the graphene nanosheets were homogeneously dispersed in the PDMS matrix and an ultra-low percolation threshold (0.63 vol%) of the composites was obtained. The G-ODA/PDMS composites with 1.19 vol% content of G-ODA show a remarkable positive piezoresistivity of high sensitivity (R/R0 > 400 under the pressure of 1.2 MPa), excellent repeatability, small hysteresis, and long-term durability. Under uniaxial compression, the resistance of the composites exponentially increased with the pressure. The resistance–pressure curves remain nearly unchanged after 1000 loading–unloading cycles. The results suggest that the G-ODA/PDMS nanocomposites provide a new route toward fabrication of soft piezoresistive sensors with high performance.

In this paper, we report the preparation and dielectric properties of reduced graphene oxide/polypropylene (rGO/PP) composites with an ultralow percolation threshold as low as 0.033 vol%. This value is the lowest among those that have been reported in graphene-filled composites. The rGO/PP composites were prepared through a latex technique, which consists of an in-situ chemical reduction of graphene oxide in PP latex and a subsequent filtration. Scanning electron microscopy and X-ray diffraction measurements demonstrate that the homogeneous dispersion of rGO nanosheets in the PP matrix was realized. A blue shift in Raman G band of the rGO nanosheets was observed in the rGO/PP composites, indicating the strong interaction between the rGO filler and the PP matrix. In the frequency range from 102 Hz to 107 Hz, the rGO/PP composites showed an insulator-to-conductor percolation transition as the increase of the rGO loading. Near the percolation threshold, the dielectric permittivity of the rGO/PP composites underwent a significant change of three orders of magnitude. Moreover, the permittivity was found to be temperature dependent.

Herein we report a novel and efficient approach to fabricate dielectric elastomers with enhanced dielectric constant and high dielectric strength. Azobenzenes with strong permanent dipole moments were synthesized to co-crosslink with hydroxyl-terminated polydimethylsiloxane through a simple one-step process, which realized a type of robust, molecularly homogenous silicone rubber (SR). The chemical structure, dielectric and mechanical properties of the resultant azo-g-PDMS elastomers with azobenzne contents ranging from 0 to 13.2 wt% were carefully characterized. The dielectric constant of azo-g-PDMS films at 1 kHz increased from 2.72 to 4.88 with the increase of azobenzene contents. By grafting with 4.0 wt% of azobenzene, the breakdown strength of azo-g-PDMS reached 89.4 V μm−1, which is 36% higher than that of pristine SR. The electric field induced deformation of silicone rubber could be enhanced by grafting with azobenzenes. The azo-g-PDMS film with 7.1 wt% of azobenzenes displayed a maximum area strain of 17%. Meanwhile, the azo-g-PDMS films exhibited a short response time (about 0.5 s) to the change in the electric field. Some prototype electromechanical actuators based on this type of azo-g-PDMS films were fabricated, demonstrating that the azo-g-PDMS dielectric elastomer is a very promising candidate for artificial muscle applications.

Piezoresistive nanocomposites using alkyl-functionalized graphene (G-ODA) as a conducting filler and polydimethylsilicone (PDMS) as the polymer matrix were prepared and their piezoresistivity behavior was investigated. One-pot synthesis of G-ODA from graphite oxide and octadecylamine improved its dispersion in nonpolar xylene and PDMS with low surface free energy. Results show that the graphene nanosheets were homogeneously dispersed in the PDMS matrix and an ultra-low percolation threshold (0.63 vol%) of the composites was obtained. The G-ODA/PDMS composites with 1.19 vol% content of G-ODA show a remarkable positive piezoresistivity of high sensitivity (R/R0 > 400 under the pressure of 1.2 MPa), excellent repeatability, small hysteresis, and long-term durability. Under uniaxial compression, the resistance of the composites exponentially increased with the pressure. The resistance–pressure curves remain nearly unchanged after 1000 loading–unloading cycles. The results suggest that the G-ODA/PDMS nanocomposites provide a new route toward fabrication of soft piezoresistive sensors with high performance.