Mechanical Response of Holographic Photopolymers

Two-chemistry polymer systems are attractive platforms for a wide range of optical and mechanical applications due to the orthogonal chemistries of the initial thermoset matrix and the subsequent photo-initiated polymerization. This scheme allows the mechanical and optical properties of the materials to be individually addressed. However, the mechanical properties of both the initial matrix and the photopolymer system affect the performance of these materials in many applications from holography to optically-actuated folding. We present a mechanical model along with experimental demonstrations of a two-chemistry holographic photopolymer system. A three-dimensional finite element model is used to simulate the mechanical and chemical responses in time. The model uses standard material measurements to predict both large-scale deformation and more localized stress and strain. To demonstrate the magnitude of mechanical stresses possible in these materials, we show bending of thin strips with UV light activation using an optical absorber to create an intensity gradient in depth. The resulting non-uniform polymerization causes shrinkage and bending toward the light followed by swelling and bending away from the light caused by monomer diffusion. In addition to this large-scale bending, we demonstrate that the model can be used to qualitatively predict surface deformations that can be used for surface relief optical elements. The mechanical model enables understanding of shrinkage and swelling properties of a material system that affect the performance of that system over a wide range of illumination conditions.