In recent years, the in situ nanofibrillation method for the production of polymerpolymer composites has been explored with great interest. In this technique, large extensional flows are applied to a well-dispersed immiscible polymer blend to inducea highly elongated nanofibril morphology in the dispersed phase. It has been demonstrated that highly desirable thermomechanical and viscoelastic properties can be imparted to the composite material with only a small weight fraction of dispersed polymer nanofibrils. The in situ nanofibrillation technique presents an especially interesting avenue for research and development due to its high degree of scalability, as well as its relatively inexpensive nature compared to other techniques for the production of polymeric nanocomposites.

The property changes in in situ nanofibrillated composites originate predominantly from the extremely high aspect ratio and specific surface area of the nanofibril phase. There is a large degree of interaction between the matrix and nanofibril phase due to their large interface, which induces structural changes at the nanoscale that dramatically change the behaviour of the bulk material. This thesis investigates the formation and behaviour of nanoscale structures within nanofibrillated polymer-polymer composites, and builds structure-property relationships to correlate these observations to the macroscopic behaviours of the material.

The present work contributes substantially to our fundamental understanding of the physical chemistry of in situ nanofibrillated composites. The primary emphasis of this work is on the kinetics of crystal nucleation and growth at the nanofibril interface, which largely governs the structural development of the crystalline and amorphous phases. We also provide new insights into mechanical behaviours of the investigated systems, demonstrating the application of in situ nanofibrillation to induce unique functionalities in semicrystalline polymers without sacrificing mechanical performance. Within the scope of the thesis work, we also develop a novel technique employing fast scanning calorimetry to probe the kinetics of homogeneous nucleation in supercooled polymer melts.