Transition Behavior of Block Copolymer Thin Films on Preferential Surfaces

The phase transitions in block copolymers (BCPs), like the order-to-disorder transition, occur when the enthalpic term of free energy of mixing is equal to the entropic term. In thin films, interactions at the substrate/polymer and polymer/air interfaces influence this free energy balance, leading to a change in the transition behavior. Here, we report on the transition behavior of BCP thin films on a substrate modified by polymer chains that preferentially interact with one component of the BCP. The thickness dependence of the transition temperature shows that interfacial interactions enhance the orientation of the lamellar microdomain parallel to the film surface even in 40 L-0 in thickness, where L-0 is the equilibrium period of the BCP in the bulk. This phenomenon can be attributed to the fact that, in a thin film geometry with a preferential interaction of one component with the substrate, a high concentration of one component is pinned at the substrate leading to an amplification of a periodic variation in the composition and a shift of transition temperature.