Block copolymer in aqueous solution: Micelle formation and hard-sphere crystallization

Abstract

The phase behavior of triblock-copolymers of poly(ethylene oxide) and poly(propylene oxide) dissolved in water shows unique characteristics: dissolved polymers at low temperature and low polymer concentration, spherical micellar aggregates at intermediate temperatures with a range in temperatures where both dissolved polymers and micelles are present, and prolate ellipsoidal or rod-like micelles at high temperatures. In the high polymer concentration regime, an apparent “inverse crystallization” characteristic is observed as the micellar volume fraction approaches the critical value φ=0.53 of hard-sphere crystallization. The crystalline phase is cubic, characterized by true long-range correlation in the bond-angle correlation, but with absence of long-range order in the bond-length correlation. At higher temperatures a crystalline phase of hexagonally packed rods is observed.