Structure and thermodynamics of weakly segregated rod-coil block copolymers

The self-assembly of a series of monodisperse rod-coil block copolymers is studied in the weak segregation limit. This unusual weakly segregated system consists of polyisoprene (PI) coil blocks and poly(alkoxyphenylene vinylene) (PPV) rod blocks solubilized with alkoxy side groups. The order to microphase disorder transition (ODT) and nematic isotropic (NI) transition are experimentally investigated to produce a rod-coil block copolymer phase diagram in a system that follows polymeric scaling relationships. Small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), polarized optical microscopy, depolarized light scattering, and wide-angle X-ray scattering (WAYS) are used to map the phase diagram. As the symmetric diblock copolymer is heated, a series of transitions from lamellar to nematic to isotropic phases are observed. The NI transition temperature decreases with increasing coil fraction, and at high coil fractions only an isotropic phase is observable. The phase behavior is in qualitative agreement with weak segregation calculations based on Landau expansions reported by other groups. Transmission electron microscopy (TEM) reveals unusual grain structures in the low-temperature lamellar phase. The high bending energy of the rod microdomains results in lamellae with long persistence lengths and grain boundaries defined by broken lamellae. Changes in domain spacing with temperature suggest rod rearrangements within the lamellar phase.