Precise prediction of photothermally induced irreversible bending deformation based on non-uniform thermal expansion of layer-structure films

Although photothermally induced self-bending films based on nonuniform thermal expansion are created, heat transfer and shape-deformation procedures are not investigated very well, limiting the understanding of such complicated behavior and the achievement of precise shape control. Here, thermally expanded microspheres (TEMs) were added into polydimethylsiloxane (PDMS) to create active layers as PDMS-TEM single-layer films that were attached onto pure PDMS passive layer to create PDMS/PDMS-TEM bi-layer films subsequently. After heating, TEMs in the film undergo phase transition and present irreversible thermal expansion, driving deformation of films. Combined with steady-state (uniform heating) deformation simulations and experiments, a deformation model was established before a non-steady state (light irradiation) heat transfer model was set up to simulate the heat transfer process of films under light irradiation. Then the temperature distribution was coupled with the deformation model to simulate the photothermally induced deformation of the samples. The PDMS-TEM single-layer films also presented bending deformation under illumination due to the non-uniform thermal expansion caused by temperature gradient. For bi-layer films, different degrees of thermal expansion between two layers lead to bending deformation. The films with TEM concentration of 30 wt% can achieve great deformation, and the bending curvature was 0.07 mm⁻¹. The geometric parameters including thickness, layer thickness ratio and aspect ratio had little effect on deformation degrees of the films. Deformation mode was varied with aspect ratio. A long side bending and diagonal bending were achieved in bi-layer films at aspect ratios of 7:1 and 1:1. In combination with experimental results, the heat transfer model and deformation model were constructed to demonstrate the complex deformation process, which provides guidance for the structural design of the photothermally induced shape-changing films in practical applications.