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Deformation/Perforation Mechanisms and Energy Absorption of Glassy Polymer and Multiwall Carbon Nanotube Thin Films at Supersonic Strain Ratesarticle
Edwin L. Thomas
Department of Materials Science and NanoEngineering Rice University, Houston, Texas USA
Abstract
We investigate the energy absorption characteristics and associated deformation behavior of free standing thin films of multiwall carbon nanotubes (MWCNTs) and glassy polymers using a micro-projectile impact test. Target films with thicknesses between 30-300 nm are impacted by micron size silica spheres at projectile velocities ranging from 300 m/s to 900 m/s, corresponding to kinetic energies up to 20 nJ. The deformation features are characterized by electron microscopy to deduce energy dissipation mechanisms operative at the extremely high strain rates ( 107 s-1). The quasi-static properties of the 2D isotropic network of meandering MWCNT nanofibers are quite modest but at the extreme strain rates and large strains of ballistic impact, the deformation behavior of the mat results in unprecedented energy absorption per unit mass of the target mat. As the projectile moves forward, the MWCNT tubes and tube bundles are straightened and pulled into the impact region. The increased friction associated with the amplified surface interactions as well as secondary and covalent bonding occurring between the translating principal tubes distributes the load around the impact region and raises the load on those portions of the tubes adhering to the sphere surface. The subsequent large back-deflection of the impact region slows the advancing projectile as KE is converted into elastic stretching energy of the network and ultimately fracture of many principal tubes. The deformation results in very high specific energy absorption of 7-9 MJ/kg, greater than any other material under micro-ballistic impact.
Freestanding glassy polystyrene (PS) films also show unexpectedly large energy absorption at extreme rates of loading. The more mobile and less entangled near-surface regions of the PS facilitate crazing and dramatically increase craze multiplication and subsequent growth with accompanying large adiabatic temperature rise of the highly deforming film. By grafting the PS chains to nanoparticle (NP) surfaces creating several hundred covalently anchored polymer chains to individual silica NPs, both the well-entangled coronal regions between NPs and the giant “NP crosslinks” improve the stress transfer through the composite. The single component nanocomposite PS grafted NP films ( 1% v/v, 16nm diameter SiO2 NPs) show 25% enhanced high kinetic energy absorption per unit mass of the target film over the previous high specific energy absorption of the thin, freestanding homopolymer PS films ( 3 MJ/kg).