Design And Controllable Preparation Of SBAS Triblock Dielectric Elastomers With High Elasticity

As a new type of smart material,dielectric elastomer(DE)has received wide attention in the past two decades due to its large actuation strain,large Maxwell stress,high acutation speed and high energy density.A basic actuation unit of DE is composed of an elastomer film sanwitched with two flexible electrodes.When an electric field is applied,a reversible large deformation can be generated to realize mutual conversion between electric and mechanical energies.Widely studied dielectric elastomers include acrylic VHB and silicone rubber,while no DE is qualified in all the aspects.For instance,VHB owns a relatively high dielectric constant,very high acutation strain but high viscoelastity.Previously,our group had synthesized a triblock copolymer poly[styrene-b-(n-butyl acrylate)-b-styrene](SBAS)used as dielectric elastomer.It had been found that SBAS showed low viscoelastity while remained the other's advantages of VHB However,SBAS exhibited relatively strong creep and poor performance at high temperature.In this thesis,we designed and synthesized SBAS with cycloalkenyl functionalized in the end blocks,which could be post-crosslinked.The end-crosslinked SBAS was used as a dielectric elastomer.The relaxation in the long term was found to be significantly improved by introducing chemical crosslinking in the PSt blocks.The stress relaxation behaviors of the SBAS triblock polymers before and after chemical cross-linking were systematically studied.The generalized Maxwell model was used to describe the stress relaxation behaviors of the end-crosslinked SBAS with different crosslinking degrees and at different temperatures.The main new results obtained are as follows:(1)The copolymerization of styrene with 3-cyclohexene-1-methyl acrylate was successfully achieved by amphiphilic macromolecular RAFT emulsion polymerization.The ratio of CEA(the ratios of St to CEA are 20:1,10:1 and 5:1,respectively)was controlled to embed different proportions of cycloalkenyl in the PSt segment,and synthesized triblock copolymer P((St140-ran-CEAn)-b-nBA940-P(St140-ran-CEAn))with styrene segments containing different proportions of cycloalkenyl groups,thereby achieving post-crosslinking curing properties of the material.The UV curing conditions of the CEA-incorporated SBAS were optimized.The effect of different CEA content on the degree of cross-linking was studied.It was found that with the increase of the number of cycloalkenyl functional groups in the molecular chain,the cross-linking density of the polymer after UV-curing increased,and the gel content also increased,up to 96.5%.(2)Studying the viscoelasticity of the material before and after chemical crosslinking of the cycloalkenyl group,it was found that by introducing cycloalkenyl functional groups to the PSt segment,the ralative stress of the end-crosslinked SBAS at the equilibrium state was increased at room temperature with increasing crosslink density,and the stress relaxation performance is obviously improved.At 150?,the presence of chemical cross-linking makes the end-crosslinked SBAS still have a certain rubber elasticity,which improves the high-temperature performance of SBAS.The stress relaxation properties of different cross-linking degrees were studied.It was found that the seven-element Maxwell model can well describe the stress relaxation behavior of different samples.After the introduction of chemical cross-linking,the relative stress of the end-crosslinked SBAS at room temperature(25?)can reach 57%,while the relative stress of non-crosslinked SBAS is 34%;at 150?,the relative stress of end-crosslinked SBAS can still maintain 67%of the initial stress,which is three times higher than that of the non-crosslinked SBAS(20%).(3)The tensile properties of SBAS were studied.It was found that with the introduction of chemical cross-linking,the elastic modulus of the material gradually increased with the increase of cross-linking density,while the elongation at break gradually decreased,and the chemical cross-linking could effectively inhibit the polystyrene segments from being pulled out during the stretching process,significantly strengthening the strain hardening effect of the material,which is expected to reduce creep.(4)After chemical cross-linking,due to the strong strain hardening effect,the maximum actuation area strain of SBAS film without prestretch is as high as 72%,the breakdown field strength is 153.8 kV mm-1.The actuation performance has significantly improved compared with that of the non-crosslinked SBAS(24.6%,37.3 kV mm-1).The implementation of non-prestretching device avoids damage to actuation performance caused by long-term stress relaxation.