Onset and manipulation of self-assembled morphology in freely standing polymer trilayer films

We have used reflected light microscopy to study the lateral morphology which self-assembles at elevated temperatures in films consisting of a polyisoprene (PI) layer capped on both sides by polystyrene (PS) layers: freely standing PS/PI/PS trilayer films. Heating of the trilayer films causes the formation of a periodic, lateral morphology which is driven by the attractive dispersion interaction acting across the film. In our studies of the temperature dependence of the morphology, we find that the onset temperature for the formation of the morphology increases with increasing heating rate. By heating the films to temperatures greater than the glass transition temperature of the PS-capping layers, the morphology is removed. By heating and then cooling the films, the morphology formed upon heating disappears and reforms at right angles to the original morphology with a larger periodicity characteristic of the lower temperature. These results can be explained by considering the time and temperature dependence of Young's modulus of PS.