Molecular dynamics insights into the interfacial interactions of natural rubber and nylon 6,6 with carbon nanotubes

The microscopic structure, interfacial energetics, and localized transport kinetics of natural rubber (NR) and nylon 6,6 (NY) matrices close to bare, hydroxylated (-OH), and carboxylated (-COOH) carbon nanotubes (CNTs) were examined using molecular dynamics simulations (MD). Surface functionalization significantly modifies interfacial polymer physics, as shown by radial and spatial distribution functions, which act through completely different mechanisms for polar thermoplastics and non-polar elastomers. The hierarchy of diffusivity for non-polar NR is D _COOH-CNT > D _OH-CNT > D _bare-CNT. On bare CNTs, hydrophobic wetting encourages dense packing and severe mobility suppression, while polar modifications increase the local free volume by introducing steric hindrance and thermodynamic de-wetting. On the other hand, transport kinetics are completely inverted in polar NY (D _bare-CNT > D _OH-CNT > D _COOH-CNT). Strong, multi-point interfacial hydrogen bonds and an intense nonbonded binding energy of -262.433 kcal mol^-1 are established by carboxyl groups, which energetically trap the polyamide segments and significantly limit molecular hopping. Additionally, surface functionalization causes both matrices to undergo a conformational change that results in a strictly parallel alignment along the nanotube axis. These insights provide crucial design rules for tailoring interfacial shear strength and load transfer in high-performance nanocomposites.