Recoverable Lightweight Nanocomposites

A.B. Huckleby, J. Li, M. Shahrizan Jamal, M. Zhang
Florida State University,
United States

Keywords: nanocomposite, carbon nanotube, structure recovery

Summary:

The demand for lightweight and strong structures continues to rise especially for weight critical applications such as aerospace and automotive. It is also desired that the materials can recover after being deformed. In this work, we fabricated a special lightweight material, nanocarbon foam, and then reinforced the foam by infiltrating the thermoplastic polymer into the foam to form a nanocomposite. Such nanocomposite is lightweight, strong, and recoverable in both structure and property after deformation. Nanocarbon foam is a porous carbon material based on carbon nanotubes (CNTs). Different from various methods used to produce CNT foams, we made nanocarbon foams by using polymer spheres as template and achieved the foams with regular cell shape and controllable pore size. The CNT networks form the struts of the foam. We used polyacrylonitrile (PAN) as precursor to reinforce the CNT network by locking the contacted CNTs and generating connections among CNTs because PAN is converted into nano-graphitic structures through carbonization process. The produced nanocarbon foam is lightweight, conductive, and exceptionally elastic. The nanocomposite is made by infiltrating thermoplastic polymer into the nanocarbon foam. Unlike the infiltration process reported in literatures, our process only coats the polymer on to the surface of the CNTs in the cell walls. The polymer coatings reinforce the wall of the cells in the foam effectively. The compressive strength is increased tens of times while the electrical property is kept intact. The nanocomposites are not elastic. When compressed, the permanent deformation occurs due to the plastic deformation undergone by the polymer. We found that the permanent deformation can be recovered by simply using heat. Heating the deformed nanocomposite to the temperature around the glass transition temperature (Tg) range of polymer, results in the full strain recovery of the nanocomposite. It is because that the polymer assumes a soft and rubbery behavior beyond Tg, while the intrinsically elastic CNTs, which are still surrounded by polymer, are able to rebound and exhibit their elasticity by overcoming the force applied by the enclosing polymer upon the reduction in its Young’s modulus. At the temperature higher than polymer’s Tg, the nanocomposite is elastic, while it becomes rigid again when its temperature is lower than Tg. It is remarkable that the recovery in both structure and property can be repeated.