UC San Diego

We report on a new type of carbon nanotube (CNT)-polymer nanocomposite with enhanced electrical, electromagnetic and mechanical properties. Such composites were synthesized through a new methodology for integrating CNTs with polymers, where functional groups on CNTs were made to interact with select polymer groups, e.g., epoxy linkages, enabling uniform dispersion over a very wide range of CNT loading. Such composites, e.g., single-walled CNT- RET (reactive ethylene terpolymer), incorporate good CNT dispersion with low electrical percolation volume fractions ( ̃0.1 volume %), yielding outstanding microwave shielding efficiency (SE) of ̃30 dB for electromagnetic interference (EMI) applications. The SE and dielectric properties were characterized for both single-walled and multi-walled CNTs and were seen to be much enhanced in the former. The specific roles of absorption and reflection in determining the total SE, as a function of the CNT filling fraction was probed in detail.. It was also seen that CNT- RET composites possess a complex dielectric permittivity twenty times larger than composites composed of pristine single walled CNTs and three hundred times larger than functionalized multi-walled CNT-RET composites. We seek to understand such an enhancement, both in terms of uniform nanotube dispersion and through a parallel resistor- capacitor model. We subsequently show that the AC electrical conductivity is a good predictor of the EMI shielding characteristics of nanocomposites. It was also seen that homogeneous nanostructure dispersion leads to a concomitant two-fold increase in the tensile strength and elastic modulus, E. We have then proposed, for the first time, a simple model to explain the variation of E with CNT filling fraction, even considering the effects of agglomeration. On the other hand, an initial increase in the toughness was observed at low CNT filling fractions, presumably due to incipient crack bridging of the polymer matrix by the CNTs. From a comparison of the properties of single-walled and multi-walled CNT impregnated composites, we ascribe such effects due to the aspect ratio of the nanostructures. We have also used, for the first time, novel CNT morphologies incorporating helical nanostructures, which seem to yield even better EMI shielding performance compared to linear nanostructures