Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99804

99804 - Photo-Induced Spatiotemporal Bending of Shape Memory Polymers 

Stimuli-responsive soft materials (SSMs) can change their shapes in response to external stimuli such as heat, light, magnetic fields, and electric fields. They show promising applications in soft robotics and biomedical devices. In response to external stimuli, SSMs can change their current configuration to a new equilibrium state through non-equilibrium kinetic processes, including reaction, diffusion, and viscoelastic relaxation, which generates novel spatiotemporal shape-morphing behavior. Achieving programmable and predictable spatiotemporal shape morphing of SSMs requires a deep understanding of their non-equilibrium kinetic processes. However, the study of non-equilibrium kinetic processes of SSMs is currently limited, and more works have focused on their equilibrium states.

By using a photothermal shape memory polymer (SMP) cantilever beam as a model system, this work analytically, numerically, and experimentally studies its non-equilibrium kinetic processes, including heat transfer and viscoelastic relaxation. An SMP beam is first pre-stretched above its glass transition temperature and then cooled to below the glass transition temperature to fix the deformed shape. When light is shone from one side of the SMP beam, the top layer absorbs light and converts it to heat, which is gradually diffused into the bottom layer or lost on the boundaries. Once the temperature reaches above the glass transition temperature, the stored pre-stretch is released. Therefore, a temperature gradient in the sample can generate an inhomogeneous strain inside the beam and results in bending. If the temperature in the entire sample reaches above the glass transition temperature, the deformation recovers homogeneous, and the sample unbends.

This work focuses on the spatiotemporal bending behavior of the SMP beam. Depending on the diffusion, convection, absorption, and viscous relaxation parameters, we theoretically and experimentally observe three different types of bending behaviors: the bending curvature monotonically increases, the bending curvature non-monotonically increases and then decreases to zero, and the bending curvature non-monotonically increases and decreases to a non-zero plateau value. When heat generation is fast, the maximum bending curvature increases and the bending duration decreases. As the absorption decay length increases, the maximum bending curvature changes non-monotonically, and the bending duration decreases. Moreover, in the analytical model and numerical simulations, we consider the effect of the incident angle of light on beam bending. When a beam subjected to perpendicular illumination bends significantly, most parts are far away from being perpendicular to the light incident, so the bending process can be dramatically delayed due to the reduction of the effective light intensity. Using one case of perpendicular illumination and another case of tilted illumination, we show the effect of the incident angle on the spatiotemporal bending of an SMP beam. This work demonstrates programmable and predictable spatiotemporal morphing of SMPs and provides design guidelines for SMP morphing structures and robots.

Presenting Author: Boliang Wu UCLA

Presenting Author Biography: Boliang Wu is a Ph.D. student in Prof.Lihua Jin's group at the University of California, Los Angeles from 2020. His research area at UCLA is focused on autonomous soft robots made from stimuli-responsive materials.

Authors:

Boliang Wu UCLA
Tianzhen Liu Southeast University
Yuzhen Chen UCLA
Lihua Jin UCLA