The structure and thermodynamic aspects of symmetric poly(styrene-block-acrylicacid)(PS-b-PAA) copolymer micelle in salt-free aqueous solution as a function of ionization (f) of PAA was probed by molecular dynamics simulations. Quantitative comparison of micelle radius shows that the GROMOS parameter set gives the best agreement with experiment. As f increases, Micelle size increases, attains spherical shape, PS surface area increases, and PAA water hydrogen-bonds increases. Pair correlation functions and solvation enthalpy show that PS interactions are insensitive to f. Density profiles and solvation enthalpy of PAA-Na+, water-Na+ pairs confirm the micelle being in the "osmotic regime" experiments. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.