Design And Controllable Preparation Of Dielectric Elastomer Composites With High Electromechanical Performances

Dielectric elastomers(DEs)as new smart materials are able to be deformed significantly in response to an external electric field.Thus,dielectric elastomer actuators(DEAs)can be used as an electromechanical transducer.DEs show promising applications in many emerging fields such as sensors,flexible actuators,biomimetic robots,next-generation smart medical devices,and green energy harvesters due to the merits of high energy density,high efficiencies,fast response,light weight and excellent flexibility.Typical DEs such as silicones,acrylics,and polyurethanes suffer from either low permittivity or high elastomer modulus.Consequently,the maximum actuation strain of the corresponding DEAs remains lower than 10%without pre-stretch.The high permittivity inorganic fillers and conductive fillers have often been used as functional fillers to increase the permittivity.However,those DE composites with increased permittivity have often suffered from much increased elastic modulus,higher dielectric loss,and decreased dielectric breakdown strength.As a result,the maximum actuation strain was often increased a little or even decreased.In the current thesis,we fabricate a novel soft DE composite.We design and synthesize polystyrene-b-poly(n-butyl acrylate)-b-polystyrene(SBAS)triblock copolymer with ultra-low modulus.The graphene oxide(GO)/SBAS composite films are fabricated by simply drying the mixture of the as-prepared SBAS latex and the GO aqueous dispersion at room temperature.The in situ thermal reduction of the GO/SBAS composite films is conducted to achieve the partially thermally reduced graphene oxide(RGO)/SBAS composite film.At the same time,the effects of RGO levels on the properties of RGO/SBAS composites are also investigated.The main acheivements are as follows:(1)The nanostructured GO/SBAS composite films are able to be fabricated by simply drying the mixture of the as-prepared SBAS latex and the GO aqueous dispersion at room temperature.The GO sheets uniformly cover the surface of SBAS particles to form a segregated network of the GO sheets at a low percolation threshold,leading to a composite with low modulus.During the composite film formation,the depletion effect,which may lead to particle coagulation,is avoided due to the presence of the strong ?-? stacking interactions of the multiple aromatic regions between GO sheets and polystyrene.(2)The effects of RGO levels on the mechanical properties of RGO/SBAS composites are investigated.With the increase in the content of RGO(0 wt%?2.0 wt%),the tensile strength of RGO/SBAS composites first significantly increases and then steadily decreases reaching a peak of 10.74 MPa at 0.5 wt%RGO;the elongation at break slightly decreases from 1075%for pure SBAS to 1006%for 0.5 wt%RGO/SBAS and then moderately decreases to 572%for 2.0 wt%RGO/SBAS;the elastic modulus of the composite increases but the elastic modulus of the RGO/SBAS composites still remains as low as 0.51 MPa at 1.5 wt%RGO.(3)The permittivity of the SBAS composites is significantly increased with the increase in the content of RGO.The relative permittivity of the pure SBAS is around 4.3 at 103 Hz.Interestingly,the relative permittivity of the thermally reduced composites with 1.5 and 2.0 wt%RGO is significantly increased to 11 and 58,respectively.The dramatic increase is ascribed to the good dispersion of the RGO nanosheets,the formation of the RGO network structure in the SBAS matrix,and the interfacial polarization between RGO and SBAS matrix.(4)The RGO/SBAS composite represents a novel class of soft DE composite featuring high permittivity,low dielectric loss,low elastic modulus,and high dielectric breakdown strength,exhibiting high actuation strain without pre-stretch at relative low electric field.The actuation strains sharply increase with the increase in the content of RGO at the same electric field.The DE composite film of RGO/SBAS(1.5 wt%RGO)exhibits 3.51%area actuation strain at 7 kV/mm and 21.3%maximum area actuation strain at 33 kV/mm,which is 5.5 times higher than that of the pure SBAS.To the best of our knowledge,21.3%presents the highest actuation strain so far for ceramic or conductive fillers/DE composites,and the electric field to reach the same actuation strain is 30%lower than that of the reported composites with similar maximum actuation strain.