Toward functionally graded cellular microstructures

The design of multifunctional materials offers great potential for numerous applications in areas ranging from biomaterial science to structural engineering. Functionally graded microstructures (e.g., polymeric foams) are those whose porosity (i.e., ratio of the void to the solid volume of a material) is engineered to meet specific requirements such as a superior mechanical, thermal, and acoustic behavior. The controlled distribution of pores within the matrix, as well as their size, wall thickness, and interconnectivity are directly linked to the porous materials properties. There are emerging design and analysis methods of cellular materials but their physical use is restricted by current manufacturing technologies. Although a huge variety of foams can be manufactured with homogeneous porosity, for heterogeneous foams there are no generic processes for controlling the distribution of porosity throughout the resulting matrix. This paper describes work to develop an innovative and flexible process for manufacturing engineered cellular structures. Ultrasound was applied during specific foaming stages of a polymeric (polyurethane) melt, and this affected both the cellular architecture and distribution of the pore size, resulting in a controlled distribution that can be designed for specific purposes, once the polymeric foam solidified. The experimental results demonstrate that porosity (i.e., volume fraction) varies in direct proportion to the acoustic pressure magnitude of the ultrasonic signal.