In Situ Polycondensation for Rigid–Flexible Nanofibrillar Composites of Aramid Nanofibers and Polyamide 66

Abstract Polyamide 66 (PA66) is restricted by the inherent conflict between strength and toughness. Adding rigid aromatic reinforcements often causes poor dispersion and weak interfacial bonding. In this paper, we develop an in situ solution polycondensation strategy to construct a rigid–flexible nanofibrillar composite. In this method, the PA66 solution in hexafluoroisopropanol (HFIP) acts as both the reaction medium and a dispersing medium. The entangled PA66 chains are proposed to influence the aggregation of the growing poly(p-phenylene terephthalamide) (PPTA) chains, thereby favoring the formation of dispersed nanofibers. These nanofibers form interfacial interactions consistent with hydrogen bonding and physical entanglements with the surrounding PA66 matrix, creating a nanofibrillar composite. As a result, the optimized composite (PA66/A-3) achieves a 68.9% increase in tensile strength and a 164% improvement in toughness, due to the efficient load transfer and energy dissipation provided by the dispersed PPTA nanofibrils and their interfacial interactions with the PA66 matrix. In addition, this structure increases surface wettability while reducing the equilibrium water uptake from 14.14 ± 0.14% to 8.75 ± 0.16%. This mild, acid-free approach overcomes the thermodynamic incompatibility and degradation issues associated with conventional solution blending, offering a scalable route to high-performance rigid–flexible nanofibrillar composites.