High Performance Dielectric Elastomer Films Of Block Copolymer For Sub-kV Soft Actuators

Dielectric elastomers(DEs)as a kind of electroactive polymers are able to transform electric energy to mechanical energy.As DEs can generate large deformation in a reversiable way excited by an external electric field,the DE can be used as key material to fabricate soft actuators and soft robotics,showing the promising applications in the fields of bionics,medicine,aerospace and military.However,there are some key isssues in the application of DE actuators,such as the high driving voltage(usually over kV),easy to electric breakdown failure and hard to make good utilization of the actuation deformation.To address the above issues,we have firstly studied the fabricating techniques for high quality DE films and anisotropic DE films from thermoplastic triblock copolymer poly(styrene-b-butyl acrylate-b-styrene)(SBAS).Then,soft actuators and robotics excited by driving voltage lower than 1 kV have been fabricated.The specific research contents and new results are as follows:(1)We have developed a two-step method to fabricate high quality DE films,i.e.firstly,a pristine DE film was fabricated by a solution blading method,then,the film was treated by prestretching and thermal relaxing(the PSTR treatment).The processing conditions in the solution blading were optimized.It was found that remaining low relative humidity(<20%)was crucial to fabricate a high qualified pristine SBAS film.One can conveniently fabricate a high qualified and ultrathin SBAS DE film(thickness lower than 20 μm)with the PSTR treatment of the pristine SBAS film.(2)It was found that a symmetric multi-axial PSTR treatment of the pristine SBAS film could not only reduce the driving voltage to lower than 1 kV via decreasing the thickness of SBAS film,but also significantly improve the actuation performance through improving the initial modulus,tensile strength and electric breakdown strength of PSTR treated SBAS films.The higher pre-stretch ratio of the PSTR treatment leads to higher enhancing effect in both mechanical and electromechanical performance.Symmetric PSTR treating pristine SBAS films of different thicknesses at different pre-stretch ratios(R/r=1-3.5),we fabricated SB AS films with close thickness(12 μm).With increase of the pre-stretch,the electric breakdown strength of actuation was improved from 43 to 82 V/μm,accompanying with increased actuated area strain(from 12 to 22%).Thus,the energy density was increased by a factor of 4.7(from 4.5 to 25.8 kJ/m3).The enhancement effect was ascribed to the change of nanomorphology of polystyrene domains from pristine sphere to nano-rods with higher ellipticity.The higher aspect ratio leads to higher enhancement effect in molulus.The dielectric breakdown strength was found to be directly proportional to the initial modulus.(3)Anisotropic SBAS ultrathin films were conveniently fabricated from the uni-axial PSTR treatment of the pristine SBAS films,which exhibited anisotropic actuation behaviors.The anisotropic SBAS films show mechanically self-strengthening effect in pre-stretch direction(during PSTR treatment)(D//S)and the higher pre-stretch ratio of uni-axial PSTR treatment leads to higher strengthening effect.But,the mechanical properties of the anisotropic SBAS films in the orthogonal direction(D⊥S)remain as the same as those of the pristine film.After the uni-axial PSTR treatment(L/1=6),the initial modulus of anisotropic SBAS film in the pre-stretch direction(D//S)was increased from 0.3 to 2.9 MPa,and the ultimate tensile strength was increased from 3 to 12 MPa.The anisotropic SBAS films could generate anisotropic actuation(the actuation strain at D⊥S direction is larger than D//S direction)under external electric field,which is of great interest to fabricate stacked DE actuators with directional output.The actuation performance of a stacked actuator(10 layers,each layer 22 μm,L/1=4,800 V)in isotonic configuration loaded with an external object could be modulated via the loading mass and the orientation of the anisotropic SBAS film.The higher energy output was observed when loading and prestretch directions were orthogonal.The maxisum discharge strain and largest output specific energy density of stacked actuator in D⊥S isotonic configuration are 4.5%(10 g)and 9 J/kg(10 g),respectively.(4)Finally,we have fabricated stacked cantilever bending actuators and soft robots with low driving voltage(<1 kV)based upon the anisotropic SBAS films(L/l=4,22μm).Without any rigid frame to guide the deformation,the anisotropy endows soft DE actuators with the direction-preferred response to an applied electric field,which could be programmed to fabricate stacked bending actuators with multiple actuation morphs(bend to a full roll or spiral shape,etc).Further,we have assembled a crawling robot by assembling two stacked bending actuators(30 layers,body length:5.7 cm).The highest crawling speed reached about 0.9 cm/s and the crawling direction could be instantly switched by swapping the actuated frequency applied on the two bending actuators.