Double hydrophilic block copolymers (DHBC) in the presence of multivalent oppositely charged ions form polyion complexes (PIC) in the micellar form. We use PIC micelles as an ecofriendly structuring agents for templating ordered mesoporous materials [1,2] : after concomittant micellization and silica condensation, the porosity of the final material is revealed by simple elution in water and the polyions can be recycled in successive synthesis batches. Various mesoporous silica materials have been obtained from cubic to lamellar, although lyotropic behavior of PIC micelles on their own has seldom been reported if ever. Moreover, the control over the structure of the final material is not solely determined by the asymmetry ratio of the diblock copolymer : a single PIC system yields different mesostructured silica materials depending on the physico-chemical conditions. In an attempt to address the relation between the mesoporous material structure and the lyotropic behavior of these new templating agents, our attention was brought to the role of water which is actually a prominent constituent of PIC micelles.PIC micelles of PEO-b-PAA/oligoamines have been studied. The water content of the core of the micelles has been addressed by establishing the complete phase diagram of the polyelectrolyte complex PAA/oligoamine in its coacervate form. It amounts to 50%, varies non-monotonously with concentration and can be tuned by ionic strength, pH. Shape transition and phase transition are revealed by swelling laws of the correlation peak position obtained from X-ray scattering. From light scattering and osmometry we establish the equation of state  of a PEO-b-PAA/oligoamine electrostatic complex. Its lyotropic behavior is characterized by a transition of shape of the micelles at =0.16 and a transition to lamellar phase at =0.30 corresponding to various levels of deswelling of the corona and of the core.This study evidences the high compressibility of the core of the micelles as opposed to the core of amphiphilic copolymer micelles. This feature probably explains why lyotropic behavior of PIC micelles is less rich than the one of amphiphilic micelles. But it also brings versatility in the use of PIC micelles as templating agents for mesoporous materials.