Two-dimensional ordering in block copolymer monolayer thin films upon selective solvent annealing

The morphology evolution of poly(styrene-ethylenelbutylene-styrene) monolayer thin films upon annealing in the vapor of cyclohexane, a selective solvent for majority poly(ethylene/butylene) block, was investigated by atomic force microscopy (AFM). The pathway information on the two-dimensional ordering from poorly ordered short cylinders, to aligned long cylinders, and then to well-ordered hexagonal spheres was revealed by repeatedly taking images of the same marked area of 2 x 2 mu m(2) on the sample surface after ex-situ annealing treatments. It was found that the whole ordering process consists of (1) the cyclic transitions between poorly ordered cylinders and semidisordered phase via poorly ordered spheres, during which the orderliness of cylinders gradually improves, and (2) the pinching-off from enough-ordered cylinders into hex-spheres. It was noticed that part of the ordering takes place in the way of nucleation and growth, but it seems that the remaining part of ordering involves spinodal and decomposition. Post-thermal annealing results indicated the well-ordered hexspherical structure is not in the state of thermal equilibrium. It was also observed that the ordering process slows down as the films thickness increases. Finally, it was demonstrated that this selective solvent annealing technique can be used to block copolymers at different molecular weight to tailor the size and period of monolayer hexspherical domains.