Thermally induced deformations in multi-layered polymeric struts

Abstract

Stimulus activated structures that deform from a reference to target configurations are used in various fields such as soft robotics, smart materials, and actuators. Examples of stimuli include electric and magnetic fields, swelling, and temperature. The design of stimulus activated structures requires a comprehensive understanding of the relations between geometry, material properties, structure composition, and the influence of the chosen stimulus. With the aim of better understanding these relations, this work employs moderate rotations theory to model the thermo-mechanical response of multi-layered polymeric struts in a wide range of temperatures. In particular, the transition from a glassy to a rubbery state is explicitly discussed. Implicit equations that relate the material properties and the geometry to the thermally-induced deformation are developed. To illustrate the merit of the model, we investigate the thermo-mechanical behavior of homogeneous and heterogeneous multi-layered struts under two boundary conditions - free-standing and pinned. We show that multi-layered beams with a mismatch in the thermal and elastic properties bend as a result of temperature change. Interestingly, the glass-transition temperature can result in the reversal of bending direction. We follow with simulations of thermally-activated polymer-based bi-layer struts with typical properties to demonstrate the applicability of the proposed design. This framework can be used as a guideline and an inverse design tool for the selection of materials and geometries for thermally-activated shape-morphing lattice-based structures that are capable of achieving multiple shape-shifts with a single stimulus. Additionally, this work can be readily extended to other stimuli and thus serves as a foundation for the effective design of stimulus activated structures capable of polymorphism.