Harnessing reversible dry adhesion using shape memory polymer microparticles

Abstract

Reversible adhesion switching on the micron scale greatly extends the functionality of shape memory polymers. Herein, we report the first usage of polystyrene microparticles for the reversible dry adhesive of the on/off switch between bonding and debonding. The reversible dry adhesive property is attributed to the stiffness change under the varying temperature of the polystyrene microparticle, as well as its ability to lock a temporary shape and recover to its original shape. The decrease in the modulus/viscosity of polystyrene microparticles at high temperature improves the surface wetting/contact and enhances the adhesive bond by contact pressure. Then, when heating above its glass transition temperature after bonding, the adhesive recovers to its initial shape, resulting in almost a zero adhesion strength. Besides, adhesion tests reveal that the magnitude of adhesion variations depends on substrates, contact pressures, and particle sizes. Therefore, as a thermotropic-induced shape memory material, the adhesive (polystyrene microparticles) can be used to create joints and can be heated to achieve its own restoration.

Fig. 1. Schematic illustration of the bonding/debonding using SMPS microparticles by TE-NIL.

Fig. 2. SEM images showing the SMPS particles with 30 μm diameter after (a) compression, and (b) shape recovery.

Fig. 3. SEM images of flattened PS particles after deformation at 140 °C and bonded in the deformed state by cooling below T_g with different adhesive area. (a) Adhesive area: 5.5 × 10⁻⁵ cm². (b) Adhesive area: 7.3 × 10⁻⁵ cm². (c) Adhesive area: 9.2 × 10⁻⁵ cm².

Fig. 4. Test method of adhesive performance of the SMPS microparticles using a universal testing machine.

Fig. 5. Demonstration of adhesive performance for the 30 μm SMPS microparticles with different particle numbers using the Si wafers. (a) 183 particles. (b) 42 particles. (c) 19 particles.

Fig. 6. SEM images of flattened PS particles after deformation at 140 °C and bonded in the deformed state by cooling below T_g with different adhesive area. (a) Adhesive area: 1.0 × 10⁻³ cm². (b) Adhesive area: 1.1 × 10⁻³ cm². (c) Adhesive area: 1.6 × 10⁻³ cm².

Fig. 7. Demonstration of adhesive performance for the 200 μm SMPS microparticles with different particle numbers using the Al sheets. (a) 171 particles. (b) 122 particles. (c) 76 particles.