Preparation And Dielectric Properties Of Polysiloxane Modified Multi-walled Carbon Nanotubes/Silicone Rubber Composites

As a new kind of stretchable elastomeric smart materials,dielectric elastomers(DEs),when sandwiched between two compliant electrodes,can expand in in-plane and shrink in thickness directions under an applied electric field and further returned to their original shape when the electric field is removed.Due to their low modulus,fast response and wide application temperature range,silicone rubbers(SR)have become the most promising polymeric materials for DEs.However,the inherent low relative permittivity(?)of SR has limited their applications in DEs materials.This paper aims to improve the?of SR and maintain other superior properties of SR.Therefore,this paper adopted a filling method of adding conductive multi-walled carbon nanotubes(MWCNTs)to improve the?of SR.Meanwhile,MWCNTs were first modified by silanes to improve their dispersion state in SR.This paper first investigated the influence of modification conditions and functional degree of silanes toward the structure of MWCNTs.Specifically,methyltriethoxysilane(MTES),dimethyldiethoxysilane(DMDES),and trimethylethoxysilane(TMES)were used to modify the surface of multi-walled carbon nanotubes(MWCNTs)under the hydrous and anhydrous conditions.The chemical structures of MWCNTs before and after modification were characterized by fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS),scanning electron microscope(SEM),and transmission electron microscopy(TEM).The results showed that the modification of MWCNTs by MTES under hydrous conditions(MWCNTs-MTES-B)was the most successful.The coating degree of MTES modified MWCNTs was calculated by thermogravimetric analysis(TGA),and the encapsulated polysiloxane was about 15wt%.MWCNTs-MTES-B with mass fractions of 0,0.5,1.0,1.5,and 2.0 wt%were filled in SR by mechanical blending.SEM images showed that the surface coating of polysiloxanes could reduce the agglomeration of MWCNTs and increased their dispersibility in SR.Tensile test results showed that the after modification with MTES the interaction between MWCNTs and SR could be improved,and it had little effect on the Young's modulus(Y)of SR.Dielectric test results showed that when 2wt%of MWCNTs-MTES was filled in SR,the?of the MWCNTs-MTES/SR composite material reached 5.02 at 10~4 Hz,which was 57%higher than that of the pure silicone rubber,and the dielectric loss was still below 0.01 and remained at a very low level.In order to further explore the effects of silane structure on structures of MWCNTs and further their effects on properties of MWCNTs/SR composites,three kinds of silanes containing different groups,namely MTES,vinyltriethoxysilane(VTES)and phenyltrimethoxysilane(PTMS)were also used to modify the surface of the MWCNTs under hydrous conditions in this paper.The obtained MWCNTs were denoted as:MWCNTs-MTES,MWCNTs-VTES and MWCNTs-PTMS.The FTIR and XPS results together showed that the modification of MWCNTs by these three silanes was all successful.TEM images directly showed that the surface of MWCNTs were successfully coated by polysiloxanes.TGA results showed that the amount of polysiloxanes coated on the surface of MWCNTs were 11 wt%,17 wt%,and 36 wt%,coated respectively,corresponding to MWCNTs modified by MTES,VTES,and PTMS y.1.0 wt%and 2.0%of MWCNTs-MTES,MWCNTs-VTES,and MWCNTs-PTMS were filled in SR by mechanical blending.The interaction between MWCNTs and the SR matrix,tensile properties and dielectric properties of the composites were characterized.Tensile test results showed that the polysiloxane surface of MWCNTs produced by the three kinds of silanes could all improve the interaction between MWCNTs and SR.The surface polysiloxane structures of MWCNTs modified by MTES were more similar to SR molecular chains,so the tensile strength of composites was the highest when the modification agent was MTES,while that of the composites containing the PTMS modified MWCNTs was the worst.In addition,all three kind of silane modified MWCNTs have little effect on the Y of SR.Dielectric test results showed that when 2wt%of MWCNTs-MTES and MWCNTs-VTES were filled,the?of the composite reached 5.03 and 5.07,respectively,which was about 59%higher than the pure SR,and the dielectric loss was less than 0.01 and remained at a very low level.