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The form factor of partially sulfonated polystyrene PSSNa (degree of sulfonation f =1, 0.72, 0.64 and 0.36), at polymer concentration 0.17M and 0.34M, without or with added salt (0 M, 0.34M, & 0.68M), is measured by Small Angle Neutron Scattering using the Zero Average Contrast method. The total scattering function is also measured, allowing us to extract the distinct interchain function and an apparent structure factor. The main result is the behavior of the form factor which shows contributions of spherical entities as well as extended chain parts. This is striking for 0.64, while for f = 0.36 the sphere contribution is more dominant. The conformation does not depend on polymer concentration. When salt is added, the sphere sizes do not vary, but the contribution Spiteri et al. Form factor of partially sulfonated polystyrene 2 attributed to the stretched parts does vary very much like for fully sulfonated PSSNa. Discussion of the interchain contribution permits to establish the level of interpenetration: solutions are interpenetrated for f= 0.64, and at the limit for f=0.36. The theoretical pearl necklace model appears very suitable to modeling the results. Comparisons are made with analytical calculation and simulation data of pearl necklace. While the respective roles of Rayleigh transition, heterogeneous architecture, and strong hydrophobicity of non sulfonated PS monomers remain under discussion, the data give an accurate three dimensional image of the pearl necklace in solution.