New Dielectric Elastomers Prepared By Click Chemistry And Their Electro-mechnical Properties

Owing to their light weight,large actuated strains,fast responses and good processability,dielectric elastomer actuator(DEA)finds applications in many fields including artificial muscles,micro-robotics,and loudspeakers.However,the potential application of DEA is predominantly limited by the high voltage that is required to drive them.Because high voltage could be potentially harmful to human body and also to the equipment.Thus,the design and preparation of dielectric elastomer(DE)which are capable of achieving large actuated strain at a low electric field is one priority for the development of next generation DEAs.Up to now,achieving DEs with large actuated strains by synthesizing new DEs or chemically modify existing ones is largely desired.In this study,we prepared a series of homogeneous DE with high dielectric constant(?')and low modulus(Y)by grafting dipoles onto polymer chains of polystyrene-block-polybutadiene-block-polystyrene(SBS)or polymethylvinylsiloxane(PMVS).We seek to reveal the relationships between molecular structure-intrinsic properties-actuation behaviors.In the meantime,aiming at the problems faced by DE during practical application,we tailored the molecular structure and crosslinking network to realize the functionalization of silicone-based DE.The above work includes several aspects as follows:(1)Homogeneous dielectric elastomers with dramatically improved actuated strain by grafting dipoles onto SBS using thiol-ene click chemistry.An approach to the preparation of a homogeneous SBS DE by using thiol-ene click reaction was set up.The polybutadiene(PB)domain of SBS was grafted with ester groups to increase the polarizability.The results showed the grafting degree of dipoles can be controlled by irradiation time.After modification,the phase mixing of PB and PS blocks occurs and the size of PS domains largely decreases,leading to the obvious decrease in the tensile strength and Y.More importantly,the ?' largely increases.The simultaneous enhanced ?' and reduced Y lead to a largely increased electromechanical sensitivity(?).The actuated strain(SA)at 15 kV/mm of SBS is 0.01%,and that of modified SBS with 81%grafting degree of ester groups can reach 2.67%.(2)The role of dipole structure and their interaction on the electromechanical and actuation performance of homogeneous SiR DEs.Three kinds of polar groups including carboxyl,hydroxyl,and ester were grafted onto PMVS by using thiol-ene reaction to prepare PMVS-COOH,PMVS-OH and PMVS-COOCH3 DE.Three grafting degrees were prepared for each modified PMVS.The results showed that a higher grafting degree,a larger dipolar moment,a lower dipolar interaction,a higher dipolar mobility and a higher mobility of chain segments lead to a larger ?'.The effects of the presence of side chain dipoles on the mechanical behavior of modified PMVS are determined by two factors:(1)the interaction of polar dipoles could increase intermolecular attractive forces thus increasing the cohesive energy density,increasing Y,and(2)the steric hindrance of the bulky dipoles can reduce the entanglement of polymer chains,resulting in a less dense material with a lower Y.The actuated train at 15 kV/mm sharply increases from 0.2%for PMVS to 9.1%for PMVS-OH with the grafting degree of 95%,much higher than that of the commercial silicone DE(2.7%).(3)Simultaneously improved dielectric and mechanical properties of silicone DE.The application of SiR DE as an actuator is limited by its low properties in terms of ?',mechanical strength and actuated strain in a low electric field.In this study,carboxyl modified PMVS(PMVS-COOH)was synthesized.Then PMVS-COOH was blended with a commercial PDMS-OH followed by the crosslinking of PDMS-OH using an amino(NH2)bearing crosslinking agent to form a PDMS-NH2 crosslinked network,forming a PMVS-COOH and PDMS-NH2 dual-crosslinking network.The as prepared DN structure includes the chemical crosslinked PDMS-NH2 network and the network formed by hydrogen bonds between PMVS-COOH and PDMS-NH2.SiR-DN 1.5/1(with the PMVS-COOH/PDMS-OH mass ratio of 1.5/1)exhibited the best comprehensive performance.Its ?' at 102 Hz,tensile strength,tensile toughness and maximum SA were 21.9,0.7 MPa,1.08 MJ m-3 and 9.3%,respectively.Those of the reference SiR were only 3.2,0.2 MPa,0.14 MJ m-3 and 1.8%,respectively.(4)Self-healing supramolecular silicone DE based on ionic bonding.DE suffer from cracks and breaks that occur during repetitive use and stress.The ability of self-healing of DE material would greatly promote the reliability,durability and service life.In this work,carboxyl modified PMVS(PMVS-COOH)was synthesized.Then iron ion was introduced into PMVS-COOH.Ionic bonds could form between iron ion and carboxyl groups and aggregate to form ion clusters.Ion clusters could act as physical crosslinking points which leads to the assemble of supramolecular SiR(SiR-Fe).The results showed that the electromechanical properties can be controlled by adjusting the content of Fe3+.For SiR-Fe with a Fe3+ content of 8%,a ?' of 7.5(@104 Hz)can be achieved,which is much higher than commercial silicone DE(3.2@104 Hz).A high self-healing efficiency in both tensile strength and tensile toughness of 100%could be achieved in room temperature.(5)Self-healing dielectric elastomer of silicone supramolecular network based on hydrogen/ionic bonding.A self-healing DE which is a supramolecular silicone network(SiR-SN)was designed and prepared.The SiR-SN was assembled by two components:carboxyl modified PMVS(PMVS-COOH)and amino terminated polydimethylsiloxane(PDMS-NH2).Both hydrogen bonds and ionic bonds can form between PMVS-COOH and PDMS-NH2.The results showed that the actuated strain increases significantly with increasing PMVS-COOH content,this is ascribed to the largely increased ?caused by the simultaneous increase in ?' and the decrease in Y with increasing the content of PMVS-COOH.The SiR-SN can self-heal through different self-healing mechanisms under different conditions:for those samples treated at 80?,the self-healing process is essentially the reformation of hydrogen bonds;for samples treated at 100?,the self-healing process is the combination of the reformation of hydrogen bonds,the reformation of ionic bonds and the conversion from hydrogen bonds into ionic ones.The SiR-SN 0.2/1(the mass ratio of PMVS-COOH to PDMS-OH=0.2/1)showed improved dielectric constant(4.4 at 1000 Hz),actuated strain at 15 kV mm-1(8.3%)and self-healing efficiency in tensile toughness(85%).