Effect of rod-rod interaction on self-assembly behavior of ABC π-conjugated rod-coil-coil triblock copolymers

A new class of ABC pi-conjugated rod-coil-coil triblock copolymers of poly(diethylhexyloxy-p-phenylene vinylene)-b-poly (2-vinyl pyridine)-b-polystyrene (PPV-PVP-PS) was synthesized and its self-assembly behavior was explored. Three different triblock copolymers of PPV-PVP-PS1, PPV-PVP-PS2, and PPV-PVP-PS3, each with PPV, PS, and PVP, respectively, as the major species in the copolymers, were used to study the effects of copolymer composition and rod-rod interaction between PPV blocks on their morphology. Transmission electron microscopy (TEM), polarizing optical microscopy (POM), and simultaneously measured small-angle (SAXS) and wide-angle (WAXS) X-ray scattering experiments as a function of different annealing conditions revealed the details of the copolymer morphology, molecular packing, and their phase transitions. Despite their large differences in the rod volume fraction, f(PPV), from 0.43 to 0.18, all three triblock copolymers adopted a self-assembled lamellar structure, in sharp constrast with the observation of many non-lamellar structures typically exhibited by ABC coil-coil-coil triblock copolymers with similar segregation strength. For PPV-PVP-PS1 with its major species PPV rod coupled with a single-phase symmetric PVP-PS diblock precursor, PPV-PVP-PS1 self-organized to form a triple-lamellar phase with each domain corresponding to the three respective blocks. Investigation of the molecular packing of PPV rods within their domain through the analysis of the 1D electron density profile suggests the PPV rods adopted a smectic C monolayer organization below its order-disorder transition temperature (T-ODT). For PPV-PVP-PS2 with its PS-rich asymmetric PVP-PS diblock precursor that displayed a disordered micelle structure, PPV-PVP-PS2 with f(PPV) of only 0.19 still exhibited a triple-lamellar phase with PPV forming a broken lamellar layer, thus preventing the excessive chain stretching of the coil blocks on the otherwise long-range ordered PPV lamellar phase. A similar broken triple-lamellar phase can also be observed for the PVP-rich PPV-PVP-PS3 with a low f(PPV) of only 0.18. Simultaneous SAXS and WAXS measurements show that all three triblock copolymers undergo the ordered lamella-to-disorder transition and the smectic/isotropic transition at the same temperature, indicating that the rod-rod interaction between PPV rods plays a critical role in forming and stabilizing these lamellar structures. The observation of the phase transformations is in good agreement with a recent mean-field prediction of a rod-coil-coil triblock copolymer system.