Critical micellization density: A small-angle-scattering structural study of the monomer-aggregate transition of block copolymers in supercritical CO2

In this paper we report a small-angle neutron-scattering investigation of m icelle formation by the fluorocarbon-hydrocarbon block copolymer, polyvinyl acetate-b-poly (1,1,2,2-tetrahydroperfluoro-octyl acrylate) in supercritic al CO2 (scCO(2)) at 313 K. At high pressure the copolymer is in a monomeric state with a random coil structure, while at low pressure the polymer form s spherical aggregates stable in a wide range of thermodynamic conditions. By profiling pressure, a sharp monomer-micelle transition is obtained due t o the tuning of the solvating ability of scCO(2). We confirm the previous f inding that this aggregate-monomer transition is driven by the gradual pene tration of CO2 molecules toward the core of the aggregate and is critically related to the density of the solvent, thus giving additional support to t he concept of a critical micellization density reported earlier on a simila r polymer.