Phase Coherence and Microphase Separation Transitions in Diblock Copolymer Thin Films

In thin films of diblock copolymers it was found that microphase separation and the establishment of phase coherence can occur as two distinct events in diblock copolymer thin films, at widely separated temperatures. At temperatures far removed from the bulk microphase separation transition temperature increasing overlap of the decaying concentration profiles from the substrate and air interfaces leads to a sharp onset of phase coherence throughout a thin film and the discretization of the film thickness, despite only weak microphase separation in the center of the film. A further transition to a fully microphase separated state occurs at temperatures close to the microphase separation transition temperature, but this transition becomes broader and less distinct with decreasing film thickness. In thicker films, on the other hand, phase coherence is not established until the ordering transition temperature is reached, due to the first-order nature of the transition and the finite coherence length in the disordered state. In thick films, phase coherence throughout the film is established either simultaneously or after complete microphase separation has occurred.