The interplay between bi-modal molecular weight distribution polystyrene/methyl ethyl ketone solution and humidity induces self-assembly of islands/honeycomb structure in the thin polymer film

Segregation of polymer chains of different molecular weights is a well-known process. A traditional experimental approach of studying phase segregation in thin films composed of polymer blends with identical chemical compositions but different molecular weights was focused on functionalizing chemical groups or modifying end groups. In this study, however, a different approach was proposed. Polystyrene with bimodal molecular weight distribution, but no additional chemical modification was used. The films were prepared by spin-coating, and the segregation between polystyrene phases was investigated by force spectroscopy. The solubility of bimodal polystyrene was explored. At the proper molecular weight distribution and soluted in methyl ethyl ketone, phase separation occurred. The introduction of moist airflow induced the secondary phase separation of the lower molecular weight into islands and the heavier fraction self-organized into a honeycomb. As a result, an easy, fast, and effective method of obtaining island/honeycomb morphologies was demonstrated. The possible mechanisms of the formation of such structures were discussed.