Concentration-Dependent Double-Step Transition of the Diffusion Coefficient in Dilute Solutions of Styrene-Butadiene Diblock Copolymer in Selective Solvent

Abstract

A poly(styrene)-block-poly(butadiene) copolymer dissolved in n-decane exibits a double-step transition in the diffusion coefficient D(c) at the extremely dilute polymer concentration ranging from l×10^-6 to 7.24×10^-3 gcm^-3 at 25°C. The copolymer disperses monomolecularly only in the extremely dilute solution region limited to c<c₁ with c₁ = 3.8×10^-6 gcm^-3. At c>c₁ (first onset concentration), micellization of some kind occurs due to the strong attractive interaction between polystyrene (PS) subchains, the copolymers associating intermolecularly. The micelle is huge and nearly monodisperse in size and is constructed from loose packing of PS subchains. At c>c₂ (second onset concentration with c₂ = 1.1×10^-4 gcm^-3), the PS subchains collapse into a hard and compact core, while the polybutadiene (PB) subchains swell well and cover the core, forming approximately a “two-phase” concentric sphere. This micelle exibits an internal motion which may be caused by the concentration fluctuation of the highly extended PB chains relative to the hard core of PS chains.