Nanophase separation in monodisperse rodcoil diblock polymers

Abstract

This work presents a study on the self-organization in condensed phases of rodcoil diblock polymers synthesized in our laboratory. Each macromolecule consists of a rigid and monodisperse rodlike segment covalently attached to a flexible coillike segment of narrow molecular weight distribution such that both segments share the same molecular backbone. By transmission electron microscopy (TEM) of solution-cast thin films we observe that the polymers "nanophase" separate into rod and coil-rich domains with dimensions of less than 10 nm, not common in typical coil-coil block copolymers. Highly periodic morphologies of rod and coil domains develop upon annealing and show a dependence on the relative volume fraction of segments. A series of polymers varying in rod volume fraction show long, continuous strip-like rod domains aligned in parallel and periodically separated by coils at the largest rod volume fraction studied, discrete aggregates of rods arranged on a hexagonal superlattice at the smallest volume fractions, and a coexistence of both structures at an intermediate volume fraction. This break up of rod domains may result from entropic coil-stretching penalties. Both strip and aggregate domains have a thickness and width of 6 to 8 nm, roughly the length of the extended rod segment. Thicker regions of films develop terraces upon annealing that give step-wise increments in thickness of 5 to 10 nm and show more complex, yet periodic, morphologies. Electron tomography reveals that layers of 2D superlattices containing aggregate or strip domains stack with specific registration, which varies with volume fraction and the rod segment's aspect ratio. An interesting arrangement in particular is a rotation of adjacent superlattices and a translation of aggregates resulting in the appearance of a "honeycomb" morphology. We also probe the order within rod domains and conclude that rod segments form interdigitated monolayers and pack with nematic-like order based on electron diffraction results. Polarized ATR-FTIR spectroscopy further suggests that rod segments are preferentially oriented in the plane of the film.