A Rubber Rubber blends Carbon black C Distribution D Transmission electron microscopy Dynamic mechanical thermal analysis A B S T R A C T Rubber-rubber blends [RRB's] of natural rubber [NR] and butadiene rubber [BR] filled with carbon black [CB] were compounded on a two-roll mixing mill. Partition of CB in RRB's is a challenging problem in the rubber industry. Therefore, the present study aims to quantify CB's partitioning in NR/BR RRB's at different blending ratios using relative damping height measurements from the dynamic mechanical analysis [DMA]. To support the findings from the DMA, structural and morphological analyses were carried out using solid state nuclear magnetic resonance [Solid State NMR] and transmission electron microscope [TEM] respectively. Constrained regions in neat (unfilled) NR/BR RRB's were analysed for the first time by dynamic mechanical analysis and this was ascribed to the entanglements between NR and BR chains at the interface. The tanδ max analysis confirmed the presence of 80% of CB partitioned towards BR phase in 70/30 NR/BR blends. For 50/50 NR/BR blends, 6.4% of CB is partitioned towards BR phase and 38% of CB was partitioned towards BR phase in 30/70 NR/BR blends. This indicated preferential migration of CB to one of the phases based on rubber blend composition. Solid State NMR studies were in agreement with DMA analysis. TEM images of CB filled rubber blends represent a near to homogenous state of dispersion, where in the identification of individual rubber phases becomes more difficult at the nanometre scale. The CB networking and its migration to the dispersed phase was evident from the TEM analysis.