The thermodynamics of micelle formation by a polystyrene-b-polyisoprene block copolymer inN,N′-dimethylacetamide

Abstract

The thermodynamics of micelle formation by a polystyrene-b-polyisoprene block copolymer in N,N′-dimethylacetamide, which is a selectively bad solvent for polyisoprene, has been investigated. The micelles have a very narrow size distribution enabling the association process to be treated as a single-stage equilibrium. The temperature dependence of the critical micelle concentration was determined from measurements of scattering intensity against temperature carried out at six concentrations within the range 2.3 × 10−7 to 1.2 × 10−4 mol dm−3. A plot of 1n (c.m.c.) against T−1 gave a value for the standard enthalpy of micellisation (per mole of copolymer chains in the micelle) of − 153 ± 6 kJ mol−1. The standard entropy contribution (-TΔSΘ) to the standard free energy of micellisation was found to be positive. These results are in strong contrast to those reported for synthetic surfactants in aqueous media for which a favourable standard entropy change is the factor mainly responsible for micelle formation.