Complex Self-Assembled Morphologies of Thin Films of an Asymmetric A3B3C3 Star Polymer

An asymmetric nine-arm star polymer, (polystyrene)(3)-(poly(4-methoxystyrene))(3)-(polyisoprene)(3) (PS3-PMOS3-PI3) was synthesized, and the details of the structures of its thin films were successfully investigated for the first time by using in situ grazing incidence X-ray scattering (GIXS) with a synchrotron radiation source. Our quantitative GIXS analysis showed that thin films of the star polymer molecules have very complex but highly ordered and preferentially in-plane oriented hexagonal (HEX) structures consisting of truncated PS cylinders and PMOS triangular prisms in a PI matrix. This HEX structure undergoes a partial rotational transformation process at temperatures above 190 degrees C that produces a 30 degrees-rotated HEX structure; this structural isomer forms with a volume fraction of 23% during heating up to 220 degrees C and persists during subsequent cooling. These interesting and complex self-assembled nanostructures are discussed in terms of phase separation, arm number, volume ratio, and confinement effects.