Thermal enhancement of AFM phase contrast for imaging diblock copolymer thin film morphology

A simple and effective means of increasing the morphological detail in AFM phase micrographs of microphase separated block copolymer films is presented. Effective AFM phase imaging of microphase separated systems hinges upon the existence of appropriate contrast mechanisms such as differences in elasticity between the microphase separated domains. For some systems, AFM phase imaging at room temperature results in low contrast images due to a paucity of differential mechanical behavior between the microphase domains, e.g. at room temperature both species are glassy. Through the use of a heating apparatus custom-designed for AFM, an elastic contrast mechanism can be created in some systems by raising the specimen to a temperature between the glass transitions of the constituent polymer species. This serves to preferentially soften one species with respect to the other, thus enhancing the phase contrast mechanism, which results in micrographs with superior detail. This simple technique is demonstrated using films of a series of polystyrene-b-poly(n-alkyl methacrylate) diblock copolymers and both commercial and custom-built heating stages. By choosing appropriate measurement temperatures, AFM phase contrast could be greatly enhanced, or indeed created, when compared to room temperature images of these specimens. For these materials, contrast enhancement required that the sample be heated roughly 20 degrees C above the glass transition of the lower-Tg species.