Functionally Graded Polymer Matrix Nano-Composites by Solid Freeform Fabrication: A Preliminary Report

A research program has been initiated to develop a Solid Freeform Fabrication (SFF) technology for combining nanosized particulate or fiber reinforcements with a photocurable thermoset matrix resin in order to produce functional graded composites. The composites that are being studied initially are optical components filled with nano-phase ceramic particles that form gradient refractive index lenses (GRIN). The Solid Freeform Fabrication (SFF) method employs an ink-jet deposition (IJD) process to form the composites. The IJD process has the advantage of incorporating nano-reinforcements into a low viscosity matrix resin that is relatively easy to process and rapidly photocures to produce functional polymeric parts. It also has the advantage that major modifications to the basic SFF processing methodology are not necessary. The emphasis in the program is on demonstrating the feasibility of this approach for fabrication of gradient refractive index lenses (GRIN), which are flat instead of the traditional spherical lens geometry. As a result these lenses will be less costly to produce than conventional curved lenses. SFF is an ideal technique for meeting the needs of GRIN lens fabrication because changes in composition can be made from layer to layer and even within each layer, allowing for the introduction of compositional and structural gradients. Thus it has the potential for creating the spatial material distributions required for designing computer optimized, custom made GRIN lenses. Integral to the SFF process are computer design procedures that specify the exact material deposition patterns that need to be employed in order to optimize the performance of the GRIN lens. The optical nano-composites will serve as a model system that we will use to work out the many challenges for implementing a viable SFF polymer composites technology. We then will make use of the information obtained and lessons learned from the work on optical composites and extend the development to structural composites that incorporate nano-particulate clays and carbon nanofibers. POLYMER NANO-COMPOSITES FOR GRIN OPTICAL APPLICATIONS Background Optical lenses are traditionally created from blocks of uniform material by grinding curved (usually spherical) surfaces. The spatial contour of the index of refraction change provides optical power when the shape and orientation are appropriately chosen. The non-planar surfaces (Figure 1) of this type of lens are detrimental to many commercial and military applications. The curvature is the source of the glint signature used for detecting covert observation. The curvature also will generate turbulence if the lens forms the window of a detector viewing out from a moving body, such as an aircraft. This situation often necessitates the use of an additional window fitted to the surface curvature, which may adversely affect optical clarity or light transmission. n (r ) r