NANOPHASE SEPARATION IN MONODISPERSE RODCOIL DIBLOCK POLYMERS

We have observed nanophase-separated structures in rodcoil polymers synthesized in our laboratory. Each polymer consists of a perfectly monodisperse, rodlike segment covalently bonded to a coillike segment of polyisoprene Such that both rod and coil share the same molecular backbone. The polyisoprene was prepared by living anionic polymerization, and thus the rodcoil diblock molecules in the systems studied are of fairly uniform molar mass. Annealing of solution-cast thin films appro;dmately 5-10 nm thick leads to ordered nanoscale morphologies that depend upon the volume fraction of rod segments in these diblock molecules. We find by transmission electron microscopy that the morphology varies from alternating strips of rod- and coil-rich domains to discrete aggregates of rods arranged in a hexagonal superlattice as rod volume fraction is decreased. At an intermediate volume fraction We see a coexistence of strips and aggregates. This breakup of rod domains may be the result of entropic coil stretching penalties analogous to those postulated recently;by Williams and Fredrickson in lamellar assemblies of rodcoils.