Polymer nanocomposites with nanosized inorganic fillers are useful in many commercial applications due to their improved rheological and mechanical properties. Similar improvements can also be realized when flexible polymers are used as fillers instead of inorganic nanoparticles. In this poster, I will describe how adding a linear polymer chain (i) improves domain segregation and long-range order in block copolymer thinfilms, and (ii) results in orders-of-magnitude improvement in toughness of an otherwise rigid but brittle hydrogel network. In both cases, addition of flexible polymer serves to reduce the available interfacial free-volume. In the former, filling the free-volume at the interface with small, flexible molecules improves domain packaging and long-range order. In the latter, the flexible chains absrob the critical strain required for crack propagation thus increasing the toughness of hydrogels. The block copolymer templates with improved domain segregation can be used in many optoelectronic, photonic, or magnetic applications. On the other hand, the toughness of composite hydrogels is similar to that of articular cartilage found in the bone joints of a human body and can be used as a model system to understand the root cause of osteoarthritis.