Andrew Anstey1,2, Mayesha Binte Mahmud1,2, Chul B. Park1*, Patrick C. Lee1,2* 1 Microcellular Plastics Manufacturing Laboratory (MPML), Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Canada. 2 Multifunctional Composites Manufacturing Laboratory (MCML), Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Canada. * Corresponding author: patricklee@mie.utoronto.ca Abstract In situ nanofibrillation via melt-spinning processes have heavily investigated and developed in recent years as an efficient, scalable method for generating nanostructured polymer composites. Extreme elongational flows are used to deform micro-scale droplets within a molten immiscible polymer blend, forming nanofibrils of the dispersed phase within the matrix fibre. A significant hindrance to both the dispersion of the secondary phase and stress transfer from matrix to fibre is the interfacial tension between the two phases. In this work, we approach this problem by introducing an in situ nanofibrillated blend that also incorporates the “self-reinforced composite” concept. In this case, we develop a blend in which polyamide 6,6 (PA66) nanofibrils are generated within a polyamide 6 (PA6) matrix. The resulting composites demonstrate extremely interesting behaviour, in terms of thermal, mechanical, and optical properties. The PA66 nanofibrils act as an extremely efficient nucleating agent, allowing the PA6 matrix to crystallize rapidly and with a dramatic increase in nuclei density. Due to the incredibly high nuclei density, the crystallite size does not exceed the nano-scale, which imparts a high degree of optical clarity which is unprecedented for highly crystalline polyamide systems. We further elaborate the underlying mechanism of this phenomenon, as well as the modified crystallization kinetics in the composite material.