Design And Controllable Preparation Of Dielectric Elastomer With Locally Tunable Modulus

Dielectric elastomers(DEs)are a class of intelligent soft material which can be deformed significantly in response to an external electric field.Dielectric elastomers can be an energy transducer between electrical and mechanical energy with promising applications as actuators,sensors,and green energy harvesters.The combination of large deformations,very fast response,low noise,high energy densities and mechanical compliance makes dielectric elastomer actuators show promising applications in a wide range of fields like smart medical devices,bio-robotics,and aero-space engineering.When a thin film of dielectric elastomer is actuated by an external electric field.The Maxwell force compresses the extremely soft thin film,leading to a large symmetric stretch on the film plane(x-y plane)perpendicular to the electric field direction and a small contraction in the thickness direction(z direction).From the perspective of devices,a rigid frame is often necessary to break the symmetric stretch and thus guide the stretch deformation to reach a desirable actuation.The incorporation of the mechanical frame would make the device complicated,less compact,and apt to failure.To address those issues,we fabricated a new DE film,the local modulus of which can be tunable,by rational molecular design and controlled synthesis.The locally hardened DE film broke the symmetry of the on-plane deformation to guide an actuation,allowing to simplify the design of a DE actuator and significantly expand the device design freedom.The main results and new contributions are as follows:(1)Polystyrene-b-poly(n-butyl acrylate-ran-cyclohex-3-enylmethylacrylate)-b-polystyrene(S(BA-ran-CEA)S)triblock copolymer,which could be post-crosslinked was designed and synthesized via RAFT emulsion polymerization.The S(BA-ran-CEA)S films were fabricated by casting a tetrahydrofuran solution of S(BA-ran-CEA)S,dithiothreitol(DTT)and photoinitiator(Darocur 2959).Click crosslinking reactions were locally triggered by UV lighting on the films,leading to locally hardening.The highest gel fraction from crosslinking was about 87%.(2)The effect of CEA levels on the mechanical properties of crosslinked S(BA-ran-CEA)S films was investigated.With the increase of the CEA level,the elastic modulus of S(BA-ran-CEA)S films increased significantly.The modulus is 0.97 MPa for 2.5 mol%CEA-crosslinked S(BA-ran-CEA)S while the modulus is 0.34 MPa for the corresponding SB AS.In the meanwhile,with the increase of the CEA level,the tensile strength increased and the elongation at break decreased.(3)The effect of the CEA levels on the electrical actuation strain of the crosslinked S(BA-ran-CEA)S films were investigated.With the increase of the CEA level,the maximum actuation strain of the crosslinked S(BA-ran-CEA)S films decreased while the dielectric breakdown strength increased.The maximum actuation strain of the 2.5 mol%CEA-crosslinked S(BA-ran-CEA)S film was about 40%while the maximum actuation strain was 63%for the corresponding SB AS,For the 2.5 mol%CEA-crosslinked S(BA-ran-CEA)S film,the dielectric breakdown strength reached 112 kV/mm but the dielectric breakdown strength was 88 kV/mm for the corresponding SBAS.(4)For a membrane-expansion actuator of the S(BA-ran-CEA)S film(the thickness was 40 ?m)with the 1 mm width belts alternatively hardened every 1 mm,the maximum actuation strain in the direction parallel to the belt direction was 10.3%while 23%in the perpendicular direction with 300%X 300%bixial prestrech,showing a noticed anisotropic deformation.A bending actuator of the alternatively hardened S(BA-ran-CEA)S film was also fabricated.It was demonstrated that complicated but controllable bending behaviors could be fulfilled simply by tuning actuation voltage,showing the unique advantage from the anistropic DE films.