Nanying Ning

New understanding of microstructure formation of the rubber phase in thermoplastic vulcanizates (TPV)

The breakup of the rubber phase in an ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) blend at the early stage of dynamic vulcanization is similar to that in an unvulcanized EPDM/PP blend because of the low crosslink density of the EPDM phase. In this work, the minimum size of the rubber phase in the unvulcanized EPDM/PP blend was first calculated by using the critical breakup law of viscoelastic droplets in a matrix. The calculated results showed that the minimum size of the rubber phase in the unvulcanized blend was in the nanometer scale (25-46 nm), not the micrometer scale as reported in many works. Meanwhile, the actual size of the rubber phase in the thermoplastic vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization was observed by using peak force tapping atomic force microscopy (PF-AFM). The results indicated that the EPDM phase indeed broke up into nanoparticles at the early stage of dynamic vulcanization, in good agreement with the calculated results. More interestingly, we first revealed that the micrometer-sized rubber particles commonly observed in TPV were actually the agglomerates of rubber nanoparticles with diameters between 40 and 60 nm. The mechanism for the formation of rubber nanoparticles and their agglomerates during dynamic vulcanization was then discussed. Our work provides guidance to control the microstructure of the rubber phase in TPV to prepare high performance TPV products for a wide range of applications in the automobile and electronic industries.

Dramatically improved dielectric properties of polymer composites by controlling the alignment of carbon nanotubes in matrix

Aligned multi-walled carbon nanotubes (MWCNTs)/polyvinyl alcohol composite films were prepared by using an easy and controllable electrospinning-in situ film-forming (EF) technique. A high dielectric constant (k), a low dielectric loss, a consistently high breakdown strength, and a high energy density were obtained by using this technique. The dramatically improved dielectric properties are ascribed to the good dispersion and alignment of MWCNTs in the matrix, facilitating the formation of a large number of separated nano-capacitors (high k and low direct current (DC) conductance). For comparison purposes, the same composite films were prepared by solution casting (SC). At the same MWCNT content, the SC method yielded a higher k, but a significantly higher dielectric loss and much lower breakdown strength and energy density because of the random dispersion of MWCNTs in the matrix and the formation of a MWCNT network, which result in a large increase in DC conductance. The formation mechanism of the different microstructures and the relationships between the microstructures and dielectric properties are clarified. Our results indicate that high-performance MWCNTs/polymer dielectric composites can be obtained by controlling the microstructure of the composites by using the EF technique, which widens the applications of dielectric materials.

Graphene encapsulated rubber latex composites with high dielectric constant, low dielectric loss and low percolation threshold

A dielectric composite with high dielectric constant, low dielectric loss and low percolation threshold was prepared by using the combined strategy of encapsulating of graphene oxide nanosheets (GONS) on carboxylated nitrile rubber (XNBR) latex particles and the in situ thermal reduction in GONS at a moderate temperature. The encapsulation of GONS on XNBR latex particles was mainly realized via the hydrogen bonding interactions between GONS and XNBR during latex mixing. A segregated graphene network was obtained at a low content of thermally reduced graphene (TRG), resulting in a low percolation threshold (0.25 vol.%). The dielectric constant at 100 Hz obviously increased from 23 for pure XNBR to 2211 and 5542 for the composite with 0.5 vol.% and 0.75 vol.% of TRG, respectively. The dielectric loss of the composites retained at a low value (less than 1.5). Meanwhile, the elastic modulus only slightly increased with the presence of 0.1-0.5 vol.% of TRG, keeping the good flexibility of the dielectric composites. This study provides a simple, low-cost and effective method to prepare high performance dielectric composites, facilitating the wide application of dielectric materials.

A new kind of electro-active polymer composite composed of silicone elastomer and polyethylene glycol

In this work, a new kind of electro-active polymer composite composed of silicone and polyethylene glycol (PEG) was prepared by solution blending. Two types of PEG with average molecular weights of 600(PEG600) and 1500(PEG1500) were first blended with liquid silicone (DC3481) separately. Then, the dielectric, electrical, mechanical and electromechanical properties of pure silicone and the silicone/PEG composites were investigated. The silicone/PEG600 composite shows an increased dielectric constant and a decreased Youngs modulus, resulting in an improved figure of merit (FOM) and an actuation strain of 11.5% at 40 V µm−1, which is 64% higher than pure silicone. This indicates that the actuation properties of silicone are largely improved by the addition of PEG600. However, for the silicone/PEG1500 composite, a much higher dielectric constant and an increased modulus due to its semi-crystalline structure are obtained, which lead to a decreased FOM and a smaller actuation strain than that of pure silicone. In addition, a comparison between theoretical strain and the corresponding experiment value was made. And the results indicate that not only the FOM, but the dielectric loss and mechanical loss play an important role in the actuation properties.

Dielectric elastomer actuator with excellent electromechanical performance using slide-ring materials/barium titanate composites

Dielectric elastomers are referred to as artificial muscles because of their excellent properties. However, the need for high operating voltage limits their practical application. A reduction of the operating voltage can be achieved with novel elastomers offering intrinsically high electromechanical sensitivity. In this work, slide-ring materials with a necklace-like molecular structure are prepared as dielectric elastomer materials. These slide-ring materials are found to exhibit high dielectric constants, low elastic moduli, and high electromechanical sensitivity due to their special structural characteristics. Barium titanate particles modified by γ-methacryloxypropyl trimethoxy silane (KH570) are incorporated into the slide-ring materials to further improve the actuated performance of the slide-ring materials. A high actuated strain (26%) at a relatively low electric field (12 kV mm−1) is obtained on the circular membrane actuator without any pre-strains, much more excellent than those of other dielectric elastomers reported in the literature. In addition, an obviously larger displacement is achieved by the slide-ring composite than that of commercial dielectric elastomer VHB 4910 on a cone-type actuator at the same electric field. These results demonstrate that our research might help to establish a new synthetic route to high performance dielectric elastomers.

Largely improved actuation strain at low electric field of dielectric elastomer by combining disrupting hydrogen bonds with ionic conductivity

Dielectric elastomer actuators (DEAs) can lead to surprisingly large deformations by applying an electric field. The biggest challenge for DEAs is to get a large actuated strain at a low electric field. Herein, a novel approach was used to largely improve the actuated strain at a low electric field of a thermoplastic polyurethane (TPU) dielectric elastomer (DE) by introducing polyethylene glycol (PEG) oligomer into the matrix. The dielectric constant (er) of TPU was obviously increased by adding PEG due to the combined effect of the increase in the interfacial polarization ability of TPU/PEG blends by the ionic conductivity of PEG and the increase in dipole polarization ability of TPU chain segments by the disruption of hydrogen bonds of TPU chains. Meanwhile, the elastic modulus (Y) of TPU was obviously decreased due to the plasticizing effect of PEG on TPU. The simultaneous increase in er and decrease in Y resulted in a 7500% increase in actuated strain at a low electric field (3 V μm−1) by adding PEG. The actuated strain (5.22% at 3 V μm−1) is considerably higher than that of other DEs at the same electric field reported in the literatures. Our work provides a simple and effective method to largely improve the actuated strain at a low electric field of a DE, facilitating the application of DE in biological and medical fields.

High performance dielectric composites by latex compounding of graphene oxide-encapsulated carbon nanosphere hybrids with XNBR

A novel dielectric composite with high dielectric constant (k), low dielectric loss, low elastic modulus and large actuated strain at a low electric field was prepared by a simple, low-cost and efficient method. The graphene oxide nanosheet (GO)-encapsulated carbon nanosphere (GO@CNS) hybrids were fabricated for the first time via π–π interaction and hydrogen bonding interaction by simply mixing the CNS and GO suspension. The assembly of GO@CNS hybrids around rubber latex particles was realized by hydrogen bonding interaction between carboxylated nitrile rubber (XNBR) and GO@CNS hybrids during latex compounding. The thermally reduced GO (RGO)@CNS/XNBR composites were then obtained from GO@CNS/XNBR by vulcanization and in situ thermal reduction, resulting in the formation of a segregated filler network. The results showed that k at 103 Hz obviously increased from 28 for pure XNBR to 400 for the composite with 0.75 vol% of the hybrids because of the formation of a segregated filler network and the increased interfacial polarization ability of the hybrids after in situ partial thermal reduction. Meanwhile, the composite with 0.75 vol% of the hybrids retained low conductivity (10−7 S m−1), resulting in low dielectric loss (<0.65 at 103 Hz). In addition, the elastic modulus only mildly increased with the addition of 0.75 vol% of the hybrids, retaining the good flexibility of the composites. More interestingly, the actuated strain at 7 kV mm−1 obviously increased from 2.69% for pure XNBR to 5.68% for the composite with 0.5 vol% of RGO@CNS, and the actuated strain at a lower electric field (2 kV mm−1) largely increased from 0.23% for pure XNBR to 3.06% for the composite with 0.75 vol% of RGO@CNS, which is much higher than that of other dielectric elastomers reported in previous studies, facilitating the application of the dielectric elastomer in biological and medical fields, where a low electric field is required.

Tailoring Dielectric and Actuated Properties of Elastomer Composites by Bioinspired Poly(dopamine) Encapsulated Graphene Oxide

In this study, we obtained dielectric elastomer composites with controllable dielectric and actuated properties by using a biomimetic method. We used dopamine (DA) to simultaneously coat the graphene oxide (GO) and partially reduce GO by self-polymerization of DA on GO. The poly(dopamine) (PDA) coated GO (GO-PDA) was assembled around rubber latex particles by hydrogen bonding interaction between carboxyl groups of carboxylated nitrile rubber (XNBR) and imino groups or phenolic hydroxyl groups of GO-PDA during latex compounding, forming a segregated GO-PDA network at a low percolation threshold. The results showed that the introduction of PDA on GO prevented the restack of GO in the matrix. The dielectric and actuated properties of the composites depend on the thickness of PDA shell. The dielectric loss and the elastic modulus decrease, and the breakdown strength increases with increasing the thickness of PDA shell. The maximum actuated strain increases from 1.7% for GO/XNBR composite to 4.4% for GO-PDA/XNBR composites with the PDA thickness of about 5.4 nm. The actuated strain at a low electric field (2 kV/mm) obviously increases from 0.2% for pure XNBR to 2.3% for GO-PDA/XNBR composite with the PDA thickness of 1.1 nm, much higher than that of other DEs reported in previous studies. Thus, we successfully obtained dielectric composites with low dielectric loss and improved breakdown strength and actuated strain at a low electric field, facilitating the wide application of dielectric elastomers.

Large increase in actuated strain of HNBR dielectric elastomer by controlling molecular interaction and dielectric filler network

Titanium dioxide (TiO2) as the high-dielectric-constant filler and a nontoxic, renewable, and biodegradable epoxidized soybean oil (ESO) as the plasticizer were introduced into a hydrogenated nitrile-butadiene rubber (HNBR) matrix to form a dielectric elastomer (DE) with high actuated strain at a low electric field. The results showed that the actuated strain increased from 5.8% for pure HNBR to 13.6% for the 30 wt% ESO/(10 wt% TiO2/HNBR) composite. The increase in actuated strain was due to the increased dielectric constant by the addition of TiO2 and the weakened HNBR molecular interaction with the presence of ESO. In addition, the actuated strain of the 30 wt% ESO/(40 wt% TiO2/HNBR) composite increased by almost 170% over that of the 40 wt% TiO2/HNBR composite at 30 kV mm−1, mainly because of the breaking up of the TiO2 filler network by the addition of ESO, but the greatly weakened HNBR molecular interaction also played a role. The corresponding mechanism was proposed based on the interaction between HNBR and ESO. Furthermore, a relatively high tensile strength (about 6 MPa) was successfully achieved for the 30 wt% ESO/(40 wt% TiO2/HNBR) composite, a big advantage for the practical application of a DE. Our work demonstrated that a filler/polymer DE with a relatively high actuated strain and tensile strength could be successfully achieved by controlling the molecular interaction and dielectric filler network, which provides a new idea for the preparation of high performance DEs.

Largely improved electromechanical properties of thermoplastic polyurethane dielectric elastomer by carbon nanospheres

Carbon nanospheres (CNS) were used as a new conductive filler to improve the electromechanical properties of a thermoplastic polyurethane (TPU) dielectric elastomer (DE). The results showed that CNS with many hydroxyl groups can form hydrogen bonds with TPU molecules, leading to a good dispersion of CNS in the TPU matrix and an improved tensile strength of CNS/TPU composites. More interestingly, CNS disrupted the crystallization of TPU, resulting in the decrease in elastic modulus and hysteresis loss of the composites. The dielectric constant at 1000 Hz increased from 7.1 for pure TPU to 137.3 for the composite with 5 wt% of CNS. The great increase in dielectric constant and the decrease in elastic modulus result in the largely improved actuation strain at low electric field of CNS/TPU composites. In addition, all the as-prepared CNS/TPU composites have a low dielectric loss (<1) at 1000 Hz. Our study provides a simple and effective way to obtain CNS/TPU DE with good mechanical strength and largely improved actuation performance at low electric field.