Phase coherence and microphase separation transitions in diblock copolymerthin 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 tempe ratures far removed from the bulk microphase separation transition temperat ure increasing overlap of the decaying concentration profiles from the subs trate and air interfaces leads to a sharp onset of phase coherence througho ut a thin film and the discretization of the film thickness, despite only w eak microphase separation in the center of the film. A further transition t o a fully microphase separated state occurs at temperatures close to the mi crophase separation transition temperature, but this transition becomes bro ader and less distinct with decreasing film thickness. In thicker films, on the other hand, phase coherence is not established until the ordering tran sition temperature is reached, due to the first-order nature of the transit ion 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.