Polybutadiene-based Dielectric Elastomer: Preparation And Performance

Dielectric elastomers(DEs)are a kind of typical intelligent materials,which can undergo large deformation under the stimulation of external electric field and thus enables the conversion of electrical energy and mechanical energy.By virtue of its outstanding deformability,fast response speed,high energy density,light weight and low cost,DEs have exhibited tremendous potentials in many applications,including intelligent robots,artificial muscle,actuators,sensors and power generators.In practice,increasing the driving voltage is the most direct and effective methods to improve the efficiency of electromechanical conversion of DE materials.However,it is also easy to lead to the electrical breakdown of the elastomeric matrix.Therefore,the integration of high dielectric constant,low conductivity,low mechanical modulus and hysteresis into DE materials,by rational molecular design of the network,is crucially important to improve the efficiency of electromechanical conversion of DE materials.In this thesis,the 1,4-polybutadiene rubber(BR)with excellent flexibility and low mechanical hysteresis was adopted as the basic elastomer.Upon the incorporation of dielectric solvent or in situ generated polar polymer,the dielectric properties of the elastomers were significantly improved while their low conductivity,mechanical modulus and hysteresis were maintained.The main contents of the thesis are as follows.(1)Aldehyde group-terminated polybutadiene(APB)oligomer was prepared by the epoxidation and the subsequent chain cleavage of BR.Then,the resultant APB was admixed with the polyethylamine(D230)as the chain extender,the polyfunctional aldehyde as the crosslinker,and propylene carbonate(PC)as the dielectric solvent.Upon one-step aldehyde-amine condensation reaction,the polarity of the APB-based networks is significantly improved,which efficiently enhances the compatibility between PC and the networks.The effects of the molecular weight of APB and the content of D230 and PC on the mechanical and dielectric properties of the resultant elastomers were disclosed.With the decrease of the molecular weight of APB,the crosslinking density of the prepared elastomer increases,leading to an increase of the Young's modulus and a decrease of elongation at break.Meanwhile,by increasing the content of D230 and PC,the dielectric and mechanical properties of the prepared elastomers are effectively improved due to the largely improved polarity of network.For instance,with the incorporation of 25 phr D230 and 60 phr PC,the tensile strength of the sample based on APB oligomer with the molecular weight of?18000g/mol increases from 3.4 MPa to 9.2 MPa,which achieves approximately 2-fold improvement compared to that of the control sample and a sight compromise on Young's modulus.Moreover,the permittivity of the sample increases from 2.8 to 4.6,while the corresponding dielectric loss and conductivity increase slightly.These results can be attributed to the fact that the increase of D230 and PC content significantly enhance the interaction of the network chains,thus effectively improved the mechanical and dielectric properties of the prepared elastomers.(2)By utilizing acrylonitrile-butadiene rubber(NBR)as a compatibilizer,diallyl phthalate(DAP)was employed as a polar monomer for the modification of BR by solution mixing.Accordingly,the polarity of the resultant BR-based networks significantly are improved as the polar NBR and DAP can co-crosslink with BR by using diisopropyl peroxide(DCP)as the initiator.The influences of the NBR,DAP,and DCP concentrations on the mechanical performance and dielectric properties of the resultant elastomers were investigated.With increasing NBR loading,the dielectric permittivity of the resultant elastomer is effectively improved while the mechanical performance is essentially unchanged.Although the conductivity of the sample is also increased,it still remains at a relatively low level(?10^-14S/cm).As the incorporated DAP monomers can undergo radical-initiated self-cyclization,the formation of small cyclic oligomers can serve as the plasticizer for the network.Therefore,with increasing DAP loading,the elongation at break of the prepared elastomer is remarkably increased while the Young's modulus and ultimate strength are decreased.It should be emphasized that the loading-unloading hysteresis during tensile test of all the above samples are very low,indicating the excellent elasticity of the networks.Moreover,the incorporation of DAP can also improve the network polarity of the samples.For instance,the dielectric permittivity of the sample with incorporating 40 phr DAP is up to4.5,while the conductivity only increases by one order of magnitude(?10^-13S/cm).In addition,the mechanical properties of the samples can be tuned by changing the DCP loading,while the dielectric performance is essentially maintained.It turns out that the mechanical and dielectric properties of the prepared samples can be rationally manipulated by adjusting the ratio of different components and the crosslinking density of samples.Consequently,the combination of effectively improved dielectric and high elasticity has been achieved.