HEAT-INDUCED TRANSITION OF POLYSTYRENE-BLOCK-POLY(ETHYLENE-CO-PROPYLENE) MICELLES IN DECANE AND IN DIOXANE

Dissolution of polystyrene-block-poly(ethylene co-propylene) (PS-PEP) in decane, a selective solvent for the PEP block, and dioxane, a selec tive solvent for the PS block, at room temperature leads to formation of metastable micellar structures, which convert into stable micelles with molar mass reduced several times after heating above similar to 5 0 degrees C. By comparing small-angle X-ray scattering (SAXS) and ligh t scattering (LS) data, the following models of micellar structures ar e suggested: At room temperature, dissolution of PS-PEP in decane yiel ds aggregates of micelles. These are detected by LS, while SAXS reflec ts the molar mass of individual micellar cores formed by PS blocks. At elevated temperature, these compound particles disaggregate and then reorganize into equilibrium micelles of reduced molar mass. Solutions of PS-PEP in dioxane at room temperature have a different character. T he molar masses of the observed particles determined by SAXS are in a good agreement with LS results, so we can exclude aggregation of micel les in this solvent. Nevertheless, also here, after heat treatment, bo th SAXS and LS yield a molar mass that is about 5 times lower than tha t determined for unheated solutions. Metastable structures surviving f rom the solid-state morphology after dissolution in a selective solven t at room temperature may convert to classical equilibrium micelles on ly after heating.