The liquid–solid transition in a micellar solution of a diblock copolymer in water

Abstract

The structure of a diblock copolymer solution in the vicinity of the transition between micellar liquid and solid phases was investigated using small-angle x-ray scattering (SAXS). An amphiphilic poly(oxyethylene)–poly(oxybutylene) diblock was studied in water. Static and dynamic light scattering techniques were used to provide an independent measure of micelle dimensions and aggregation number. Dynamic shear rheometry and mobility measurements were used to locate phase transitions. A micellar liquid phase was identified at low concentration and a cubic micellar phase at higher concentration, the transition between the two occurring at higher temperature as the concentration increased. The cubic micellar phase behaves rheologically as a solid and SAXS confirmed a face-centered cubic structure. Intermediate between these two phases, a viscoelastic soft solid was observed, with a finite yield stress but with a much lower dynamic modulus than the crystalline solid. Several distinct suggestions have been put forward for the structure of the solution in this region. In a poly(oxyethylene)–poly(oxypropylene)–poly(oxyethylene) Pluronic triblock, small-angle neutron scattering and rheology provided evidence for a percolation transition between micellar liquid and solid phases [L. Lobry et al., Phys. Rev. E. 60, 7076 (1999)], indicating a fractal structure of micelles aggregated due to attractive interactions. Alternatively, a defective solid phase has been proposed. We analyzed the structure of solutions of our diblock copolymer via detailed model fits to the SAXS data for concentrations spanning the liquid–solid transition. The micellar form factor was modeled as a homogeneous micellar core with attached Gaussian chains; and the intermicellar structure factor could be described using the hard sphere model. Thus there is no evidence for percolation induced by effective attractive interactions between micelles in our system. In contrast SAXS data indicates there is a coexistence region between hard sphere fluid and solid crystal phases, in which small grains of close-packed crystal coexist with fluid. It is apparent that block copolymer micelles acts as model colloidal systems in which it is possible to investigate the influence of attractive and repulsive interactions between spherical particles by varying the copolymer composition