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We have measured the fracture toughness of PMMA slabs with carbon black and colloidal silica fillers placed at the interface. The results show that the fracture toughness decreases linearly with carbon black concentration. The fracture toughness scales as annealing time, t1/2 indicating that it is diffusion limited. Addition of Colloidal Silica drastically reduces the fracture toughness regardless of concentration and annealing time. These results indicate that carbon black introduces physical cross links which decrease dynamics whereas colloidal silica produces permanent chemical cross links which prevent diffusion across the interface.