End-block-curing ABA triblock copolymer towards dielectric elastomers with both high electro-mechanical performance and excellent mechanical properties

Abstract

Dielectric elastomers (DEs) featured with fast reversible and large deformation excited by an external electric field show promising applications as leading artificial muscle materials in many emerging technologies. It remains a great challenge to synthesize a DE simultaneously with high electromechanical performance (large actuation strain, high energy density, free of pre-stretch and bracing) and excellent mechanical properties (low Tg, low viscous loss, low modulus, and high mechanical strength). Herein, we propose and demonstrate that end-block-curing ABA triblock copolymer elastomer with well-designed structures is a new strategy to address the challenge by taking advantage of the microphase separation nature of the block copolymer. Particularly, poly(styrene-b-butyl acrylate-b-styrene) (SBAS) with post-curable end blocks, i.e. polystyrene (PS) blocks, are designed and synthesized. Different levels of cyclohex-3-enylmethyl acrylate (CEA) that contains two asymmetric vinyl groups with very different reactivity are incorporated into the end blocks for end-block curing while the molecular weight for each block is fixed. It is found that curing the end blocks enhances the strain-hardening effect during stretch and thus significantly increases the electro-mechanical performance. Without pre-stretch, the DE actuator of the end block cured SBAS exhibits a maximum actuation strain up to 72%, dielectric breakdown strength up to 154 kV mm−1, and energy density up to 270 kJ/m3, which are three, four, and almost fourty times that of the SBAS free of CEA, respectively. The maximum energy density is over 30 times that of mammalina skeletal muscle while the maximum actuation strain is 3.4 times. Interestingly, the improvement in eletromechanical performance is fulfilled with little increase in glass transition temperature (Tg), mild increase in modulus, and excellent tensile strength due to the microphase structures. The end-block-cured SBAS retains a low Tg of the rubbery phase around −35 °C, relatively low viscous loss of 0.2 at room temperature, a modulus lower than 1 MPa, and tensile strength around 3 MPa, which are attractive in terms of DE device applications.