To design polymer blend nanocomposites for a given application, it is paramount to understand the different interfacial interactions within the complex system. For polymer-polymer interaction, due to the macromolecular nature of polymers polymer blends are mostly immiscible, thus it is important to improve the interfacial adhesion. Blending of two or more polymers is the most cost-effective route to make new product as it utilizes the individual properties of each constituent polymer, and we can make different morphological structures. For polymer-filler systems, modification of the interface is necessary for efficient load transmission. Therefore, design of the interface is required to tune the physical and mechanical properties of polymer blend nanocomposites (PBNs). In this thesis, compatibilization process was used to improve the polymer blend adhesion and induce nanofiller migration. The two main compatibilization processes include, 1) reactive in-situ compatibilization and 2) addition of premade copolymer during melt processing. In the first approach, we added reactive polystyrene maleic anhydride (PSMA) polymer, into a biphasic polymer matrix of amorphous polyamide (aPA)/ polystyrene (PS) blend, taking advantage of the chemical reaction between the maleic anhydride and amine functional groups to form in-situ grafted aPA-PSMA copolymer. At the chosen filler concentration of 1.5 vol% MWCNT and at 3 vol% PSMA, the conductivity of the reactive blend was three and a half orders of magnitude higher than that of non-reactive PS/aPA/CNT composites. The enhanced conductive properties correlate well with the improved rheological properties, and both effects are a result of the improved network formation due to the interfacial reaction. In the second approach, we added different concentrations of premade block copolymers with two different architectures, diblock and triblock. Both copolymers contributed to morphology refinement and an enhanced conductivity, particularly at lower concentrations of copolymer (1 vol%). We achieved up to 4 orders of magnitude increase in the iii 80:20/PP/PS, from 5.1510-7S/cm for the uncompatibilized blend to 1.0710-2 S/cm for the PBN with diblock copolymer SEP, and 1.5110-3 S/cm for that with triblock copolymer, SEBS. These increases were due to MWCNT migration and MWCNT selectively localizing in the droplet phase (PS), which resulted in dispersed domain deformation and interconnection. The highest conductivities were found for the diblock SEP in 80:20 PP:PS blend, where we achieved four orders of magnitude increase in conductivity at 1 vol% copolymer (1.07x10-2 S/cm), while for SEBS we attained three order of magnitude increase (1.5110-3 S/cm) and saw a similar increase in rheological properties with this copolymer. The 50:50 PP:PS blend nanocomposites at 3 vol% SEBS copolymer gave increased conductivities near 1x10-2 S/cm. Moreover, we determined that the extent of MWCNT migration and localization is correlated to the difference in melt viscosity between the constituent polymers, as we specifically selected two polymers with significant viscosity difference, PPE and HDPE. The addition of premade SEBS triblock copolymer substantially increased the conductivity by almost 4 orders of magnitude for PPE/HDPE/80:20 blend nanocomposites with 1 wt% MWCNT and 2 wt% SEBS. Similarly, the mechanical properties were enhanced for the compatibilized PBNs (with MWCNT) with 38.8% increase for the 10 min mixing and 28.5% increase for the 5 min mixing time, compared to uncompatibilized PBNs (without compatibilizer), and the pristine blend (without compatibilizer and CNT), demonstrating the synergistic effect of nanofillers and compatibilizer on a multiphase system. We achieved a substantial increase in ductility at 20:80 PPE/HDPE blend composition of about 339.8% with the addition of SEBS block copolymer (i.e., the elongation at break value).