Phase behavior in thin films of cylinder-forming diblock copolymer: Deformation and division of heptacoordinated microdomains

The microdomain areas in a 2D hexagonal array of vertically oriented PS-b-PMMA diblock copolymer cylinders have been studied, and we have found that their area increases as a function of their coordination number. For the 7-fold coordinated cylinders, the unit cell could adopt an anisotropic configuration due to the stress of the lattice. In this case, in order to minimize the free energy of the system, the PMMA domain deforms commensurately with the unit cell and adopts an elliptical or a lozenge-like cross section, in good agreement with strong segregation considerations. However, these strained configurations reveal to be metastable so that thermal fluctuations could induce a transition from a distorted column to two other circular columns. This phenomenon has been used to explain the motion of dislocations.