Self-assembly of crystalline–coil diblock copolymers in solution: experimental phase map

Abstract

We present the morphological phase map of semicrystalline poly(butadiene)-b-poly(ethylene oxide) (PB-b-PEO) diblock copolymers (BCP) in dilute n-heptane solutions. The solutions of the diblock copolymer have been quenched from 70 °C, that is above the melting temperature Tm of the PEO block, following two thermal pathways: (A) by direct immersion into liquid nitrogen and (B) by quenching to the crystallization temperature Tc of the PEO block. Scanning force microscopy (SFM) and transmission electron microscopy (TEM) allowed imaging of the dried crystalline morphologies, whereas in solution the micelles have been visualized by cryogenic transmission electron microscopy (cryo-TEM). The apparent hydrodynamic radii of the micellar structures in solution were determined by dynamic light scattering (DLS). The effects of the block length and block ratio, and the influence of the thermal history on the morphology have been systematically investigated for the series of PB-b-PEO BCP. In pathway A, different morphologies such as spheres, rods, worms and entangled cylinders have been observed; whereas in pathway B, as the weight fraction and the length of the PEO block increased, the morphology changed from spherical to cylindrical or to lamellar, and finally to large dendritic micelles. This tailoring of morphologies is fully reversible with temperature and was found to correlate to the crystalline state of the PEO chains.