Micelle formation by block copolymers in organic solvents

Abstract

It has been known for many years that amphiphiles such as synthetic surfactants aggregate reversibly in water to form micelles. More recently it has been recognised that when a block copolymer is dissolved in a liquid that is a ‘solvent’ for only one of its polymeric components, the block copolymer may also form stable colloidal particles. In this contribution aspects of the association behaviour of block copolymers in selective organic solvents are compared with the behaviour of synthetic surfactants in water. Evidence is presented from gel permeation chromatography and electron microscopy which shows that like amphiphiles block copolymers can form globular micelles having narrow size distributions. Another similarity is that both types of system can, under suitable conditions, form extended or worm-like micelles. However, the thermodynamic factor responsible for the association of block copolymers in organic liquids is different from that for amphiphiles in aqueous media. Micelle formation by amphiphiles is mainly due to a positive standard entropy of micellization. On the other hand, results obtained for a polystyrene-b-poly(ethylene/propylene) block copolymer in n-decane show that for this case it is the enthalpy contribution to the free energy change that is solely responsible for association taking place. © 1983 IUPAC