Functionalised carbon nanotube/polymer composites (CNPC) have received significant interest as promising structural materials with applications in in the most demanding areas of industry such as ballistic protection. However, the current generation of CNPCs falls short of their theoretical limit for mechanical properties and improvement is desirable. Quality of the interface between CNTs and polymers are a key factor affecting the reinforcement in CNPCs and developing a fundamental understanding of critical failure at the interface is essential to optimise the properties of this class of materials.

Computer simulations are instrumental in improving our understanding CNPC interfacial failure but their execution can be challenging; the bond-breaking processes at the polymer-CNT attachment point that initiate failure are quantum-mechanical (QM) in nature, yet the mechanisms by which stresses are transferred through the disordered polymer occur on length-scales far in excess of anything that can be simulated quantum-mechanically. In this work, we address this issue with a novel, adaptive quantum mechanics/molecular mechanics (QM/MM) simulation method where the majority of the system is simulated with a classical forcefield, while areas of particular interest are identified on-the-fly and simulated quantum-mechanically. We demonstrate that this method’s results are in excellent agreement with fully-QM benchmark simulations and offer qualitative insights missing from classical simulations.

Using this approach, we investigate the effects of interfacial chemistry on the ISS in CNPCs by simulating carbon nanotube pull-out from a crosslinked polyethylene matrix. We find that the choice of the functional group linking the polymer matrix to the nanotube determines the effective interfacial shear strength (ISS), which can be increased up to the limit dictated by the strength of the polymer backbone by choosing groups with higher interfacial binding energy. We rank the functional groups based on the strength of the resulting interface and suggest broad guidelines for the rational design of nanotube functionalisation for CNPCs.