Effect Of Pre-Stretch On The Micro Structure And Electromechanical Properties Of Silica Gel-based Dielectric Elastomer Composites

Dielectric elastomer(DE),as a kind of electroactive polymer,can change its shape significantly under electrical excitation,which is considered to be the most promising material to simulate artificial muscles.However,the practical application of DE seriously limited by its high driving voltage.How to obtain larger actuated strain under low driving voltage is an urgent problem in DE field.Interestingly,the actuated stain of DE with pre-stretch at low driving voltage is close to or even more than that of the DE without any pre-stretch at high driving voltage.However,most of the materials used to research the effect of pre-stretch on the electromechanical properties of DE are usually pure rubber systems such as commercial VHB tape or pure silicone rubber(SiR),and systematic research on the effect of pre-stretch on DE composites was rarely reported.In addition,the research on pre-stretch only focuses on the macro phenomenon of pre-stretch on the electromechanical properties of DE,and the mechanism of the effect of pre-stretch on the electromechanical properties of DE from the perspective of microstructure was also rarely reported.In this work,Polymethylvinylsiloxane(PMVS)with high elasticity,high electromechanical stability and low viscous loss was employed as the DE matrix.Traditional nano rutile titanium dioxide ceramic filler(TiO2)and new high dielectric silicon dioxide(SiO2)are filled into PMVS to increase the dielectric constant of PMVS,the prepared TiO2/PMVS composite materials and SiO2/PMVS composite materials are further pre-stretched to further improve the electromechanical properties of PMVS.The effect of pre-stretching on the microstructure and electromechanical properties of two dielectric elastomer composites was systematically studied,and the mechanism of the effect of pre-stretching on electromechanical properties was revealed.The results of the study are as follows:(1)The effect of pre-stretching on the microstructure and electromechanical properties of pure PMVS and TiO2/PMVS composites:Using silane coupling agent KH570 to modify TiO2,TiO2/PMVS composite materials with high dielectric constant,high breakdown strength,and high electro-deformation was prepared by mechanical blending method combined with pre-stretching.The orientation degree of polymer chain in PMVS and TiO2/PMVS composites was calculated quantitatively by infrared dichroism method.The results show that the polymer chain orientation occurs during the pre-stretch process,and it can be further increased by the slip of PMVS chain adsorbed on the filler due to the "bridging" effect of TiO2.For the molecular chain with low pre-stretch(<50%),it is more easily stretched under the electric field,which makes its actuated strain increase.As for the materials with high pre-stretch(>50%),it cannot produce greater actuated strain under electric field because the molecular chain has already produced large deformation,which make its actuated strain lower than that of molecular chain with low pre-stretch,but still higher than that of molecular chain without any pre-stretch.As a result,the optimum pre-stretch value of pure PMVS and TiO2/PMVS composites is 50%.The actuated stain at 30 kV/mm of TiO2/PMVS composites only can be increased from 1.46%of pure PMVS to 7.34%of 40wt%TiO2/PMVS composite.Comparatively,the actuated stain at 30 kV/mm of 40wt%TiO2/PMVS composites with 50%pre-stretch reaches 12.68%and a large actuated stain(21.20%)was obtained by 40wt%TiO2/PMVS composites with 100%pre-stretch at 46.5 kV/mm.In addition,the breakdown strength of TiO2/PMVS composite with pre-stretch can be increased due to the improvement of pull in instability.Compared with that of DE without any pre-stretch,the breakdown strength of the composite with 200%pre-stretch from 1.6 times to 2.2 times as the content of TiO2 filler increases from 10 wt%to 40 wt%.(2)The effect of pre-stretching on the microstructure and electromechanical properties of SiO2/PMVS composites:Compared with TiO2/PMVS composite materials,adding a new type of high-dielectric filler SiO2 can produce greater deformation of PMVS at a smaller and lower dosage.For example,the finally produced driving strain of 5wt%SiO2/PMVS composite and 2.5wt%SiO2/PMVS composite reaches 7.35%at 30 kV/mm and 12.33%at 53.5 kV/mm,respectively.The large decrease of filler content can conducive to improve the processing performance of materials,reduce product defects and enhance product stability.The orientation degree of polymer chain in SiO2/PMVS composites was calculated quantitatively by infrared dichroism method.The results show that the polymer chain orientation occurs during the pre-stretch process,and it can be further increased by the slip of polymer chain adsorbed on the filler due to the "bridging" effect of SiO2.Compared with that of TiO2,the "bridging"effect of SiO2 can more promote the stretching orientation of molecular chain.In addition,the breakdown strength of SiO2/PMVS composite with pre-stretch can be increased due to the improvement of pull in instability,such as the breakdown strength of 5wt%SiO2/PMVS composite reaches to 61.5 kV/mm under 200%pre-stretch.As a result,the actuated strain first increases and then decreases with the increase of pre-stretch value,and the optimal pre-stretch value of SiO2/PMVS composite is 50%.