Theoretical Studies for Improving the Dispersion of Carbon Nanotubes in Caprolactam Polymers by Enhancing the Intermolecular Interactions with Functionalization: Implications for Reinforced Polymer Composites

The inhomogeneous dispersion caused by carbon nanotube (CNT) agglomeration in composites still needs to be explored, though mechanical properties of the polymer materials can be improved significantly if CNTs were added as a reinforcement. In this work, to explore an effective way to solve the agglomeration problem of CNTs within polycaprolactam (Nylon66), a functionalization strategy to enhance the intermolecular interaction between the caprolactam (CPL) unit molecule and (7,3) single-walled carbon nanotube (SWCNT) was investigated theoretically at the molecular level. Molecular moving behavior and intermolecular interaction energy indicate that the surface functionalization, especially with the –NH2 group, is an effective way to strengthen the interaction between CPL and (7,3) SWCNTs, which highlights the key role of controlled surface functionalization of CNTs in improving its dispersion degree in polycaprolactam. This can be explained by a small interfacial charge-transfer amount, which occurs from the occupied valence band maximum of (7,3) SWCNTs to the occupied highest occupied molecular orbital of the CPL molecule. In this case, the dipole moment of involved molecules plays an important role in the interfacial interaction strength and then increased intermolecular interaction between CPL and functionalized (7,3) SWCNTs, especially incorporating the –NH2 group with larger dipole moment, can be understood. Our results for the functionalization effects on the physical adsorption behavior of organic molecules around (7,3) SWCNTs can provide more useful information for further design and optimization of a SWCNT-based reinforcement.