Structure and diffusion of copolymer micelles in entangled homopolymer chains

Abstract

Self-diffusion coefficients of spherical polystyrene-block-polyisoprene (PS-b-PI) micelles, with PS core and PI corona, in an entangled matrix of linear homopolyisoprene (PI) chains have been investigated by Forced Rayleigh Scattering (FRS). The micellar architecture has been varied from wet brush, i. e. micelles with coronae strongly swollen by the matrix PI, to dry brush conditions, using PI matrix chains of various molecular weights. The concentration of micelles has been adjusted from 10 to 50 wt.-% block copolymer. At high block copolymer concentrations c = 50 wt.-%, the micelles form a cubic lattice for wet, intermediate and dry brush systems as confirmed by small angle X-ray scattering (SAXS). This lattice becomes more distorted with increasing molecular weight of the matrix PI, which is explained by strongly repulsive interactions between wet brush micelles in comparison to weaker repulsive interactions of dry brush micelles. Decreasing the amount of block copolymer, a liquid-like interparticle structure is found. The sample temperature surprisingly shows a strong influence on FRS diffusion measurements for samples with a very high micellar concentration. The signal intensity strongly increases above the glass transition temperature of the polystyrene cores (Tg (PS)). Further, the relaxation process becomes seemingly slower at high temperatures. In some cases, even non-monotonous decay-grow-decay FRS signals are found for those highly concentrated crystalline systems. These signals could be analyzed according to a complementary grating scenario. For samples with lower micellar concentrations, and, correspondingly, a less crystalline structure, no temperature effects on the FRS signal shape have been found. Also, all signals showed a monotonous biexponential decay for the more dilute systems at low temperatures. The slower of the two relaxation processes has been identified as micellar diffusion, the faster process as rotation of individual micelles.