Synthesis And Properties Of Poly(Styrene-b-butyl Acrylate-b-styrene)Thermoplastic Dielectric Elastomer

Dielectric elastomers(DEs)are a kind of soft active materials which can generate large deformations excited by an external electric field.DE-based devices can be an energy transducer between electric and mechanical energies,thus are promising as actuators,sensors and green energy collectors.Dielectric elastomer actuators(DEAs)own many benefits like soft,large actuation strain,fast response,low noise and high energy conversion efficiency.They are attractive in many application fields such as bionics robotics,medical devices,and aerospace.High performance dielectric elastomer materials should own the following properties:low modulus,strong strain hardening,high elongation at break,high fracture strength,high elasticity,low viscosity,high dielectric constant,high breakdown strength,and good processability.The up-to-date dielectric elastomers are difficult to balance those demanding on properties.In particular,most DE materials have to use stiff frames to maintain pre-stretch in order to achieve good actuation performance,which increases the complexity of the actuator and significantly limits their application.Additionally,most dielectric elastomers are thermoset,which are difficult to process and repair.To address the problems in exploiting the up-to-date DE materials,this thesis designed and synthesized thermoplastic poly(styrene-b-butyl acrylate-b-styrene)triblock polymer(SBAS)as a new type of DE materials using the newly developed reversible addition fragmentation chain transfer(RAFT)emulsion polymerization.A series of SBAS with different molecular chain structures were synthesized.The influence of the molecular chain structures on mechanical properties,dielectric properties,and electrical actuation performance of this new material were systematically studied.The main new results obtained were as follows:(1)SBAS with a suitable structure(molecular weights for each block:15-120-15,kg/mol)was found to be a high performance DE.It owns a higher dielectric constant(?4.8)than that of widely used DE materials.After bi-axial pre-stretch(200%×200%),the circle actuator of SBAS exhibits a large static actuation area strain(?98%),a high breakdown field strength(>180 MV/m),a large output force(up to 300 times its own weight),high actuation performance at high exciting frequency(25%area strain at 400 Hz),high power density(1200 W/kg),long continuous actuation life(?500,000 cycles).The actuation performance at high frequency(?400 Hz)and power density is the highest among all reported materials.(2)The thermoplastic property was found to be advantageous in the fabricating thin DE films and repairing a break-down DE actuator.By reducing the thickness of a DE membrane,the actuating voltage was reduced.Actuation area strain could reach 14%at 600 V for a SBAS actuator of a 20 ?m thick SBAS membrane.Bonding a SBAS patch on the broken area turned out to be an effective method to repair a physical breakdown or electric breakdown SBAS actuator.The repair would extend the actuation life of the DEA.(3)It was found,for the first time,that the stretch of a SBAS membrane was able to be partially frozen by heat treating the stretched membrane at temperature around the glass transition temperature(Tg)of polystyrene for 20 min,and then cooling down to room temperature.Based on this new finding,a new technique to frozen pre-stretch of a thermoplastic triblock copolymer was invented.The bi-axial pre-stretch up to 105%×105%was able to be frozen while the modulus of the stretched SBAS remained low and could be adjusted within a certain range(0.34 to 0.84 MPa).A DEA of the stretch-frozen SBAS exhibited very large actuation strain as high as 210%without any rigid frame,which would significantly expand the design space for a DE device.For the first time,different frozen pre-stretchs were built on the same membrane to make bump actuators with the response time less than 10 ms,demonstrating a 3D actuation mode.(4)A series of high performance actuators of SBAS was fabricated like a mcro-rotary actuator with a rotation speed of up to 30,000 rpm,a pump with a transferring rate of 110 mL/min,and a flapper actuator with maximum static flapping angle of 120° and flapping angle of 50° at 5 Hz.It is suggested that the new SB AS DE materials should have broad impacts on a large variety of applications.