Multiscale mechanical and structural characterizations of Palmetto wood for bio-inspired hierarchically structured polymer composites

There has been a great deal of effort focused on engineering polymer composites with hierarchical microstructures consisting of one or more ingredients that can be organized differently across multiple length scales. However, there are hierarchical microstructures that have evolved over eons in biological materials. These unique structure–property relationships may serve as templates for engineering hierarchically structured polymer composites with tailored properties. One such biological material is the Palmetto wood of South Carolina, which was successfully used as a protective structure during the Revolutionary and Civil Wars to absorb cannon shot. Through an assembly of microfibers into macrofibers embedded in a cellulose matrix, the Palmetto wood has optimized its ability to resist failure when subjected to extreme dynamic loading events, such as hurricanes. Understanding of the dynamic and static structure–property relationship in Palmetto wood can facilitate the development of new hierarchically structured polymer composites with increased resistance to failure. Therefore, the structure–property relationship in Palmetto wood has been studied using novel multiscale microstructural and mechanical characterization techniques. Models have been developed that indicate that the hierarchical structure of Palmetto wood obeys the linear Rule-of-Mixtures across multiple length scales. This understanding has led to the development of new polymer composite structures that exhibit properties similar to Palmetto wood using conventional laminated carbon fiber–epoxy composites and new polymer nanocomposites consisting of carbon nanofibers. The use of the nanofibers appears to enhance the interaction between the composite components in a manner similar to the interaction between fibers in the Palmetto wood that enables the laminated composite to behave more like the individual layers by resisting the tendency to delaminate and increasing the Weibull statistical parameters closer to those observed in Palmetto wood.