Two-dimensional order-disorder (melting) transition in a diblock copolymer cylinder-forming thin-film system.

We have studied the phase diagram of a two dimensional smectic system, a monolayer film of di-block copolymer cylinders. Previous work was done on sphere-forming diblock polymer films. In this work we anneal PS-PEP 5-13 cylinder-forming diblock copolymer using a temperature gradient system, to investigate the orientational order-disorder (melting) transition. We find that on the high temperature side correlation lengths are very small, revealing complete lack of orientational order. This corresponds to annealing far above the order-disorder (or melting) transition. The disordered state in this case is a thermodynamic equilibrium state, continued annealing of the sample will not produce more ordering in this region of the sample. The order-disorder transition, ODT, occurred at $\approx $ 163 oC, higher than the bulk ODT of the same polymer (TODT =144 $^{o}$C). We present data on the temperature dependence of the correlation functions and the densities of topological defects. By lowing the temperature of the high T end of the gradient, we can also sweep the temperature gradient to study the effects of zone refining.