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Structure and dynamics in soft colloidal systems

Final Activity Report Summary - DYCOSYS (Structure and dynamics in soft colloidal systems)

We investigated the rheological behaviour of block copolymer micelles with cross linked cores and varying core-to-shell ratio. These stable systems represent model soft colloids where the core can deform, swell or shrink under the influence of external stimuli (such as osmotic forces or solvent quality). We focused on the concentration, temperature, solvent quality and core-shell ratio dependence of the rheological properties of core-shell micelles with covalently cross-linked polybutadiene or polyisoprene core and polystyrene corona, suspended in an intermediate solvent (dibutyl -phthalate (DBP) or dioctyl-phthalate (DOP)).

A tuneable non-monotonic behaviour of the relative zero-shear viscosity as a function of temperature was found. In particular, we first observed a viscosity increase with increasing the temperature, which was attributed to the swelling of the core coming from the solvent quality improve: at enough high concentrations, the swelling of the particles leads to a dynamic arrest of the system, and a state transition is observed, from a liquid-like to a glass behaviour.

However, based on the rheological data of core-shell micelles having a very small core-shell ratio, we showed that this viscosity increase with temperature could be counterbalanced by another process: the increase of the mobility of the corona with temperature. Indeed, having one extremity capable to freely explore the surrounding, the branches of the corona are able to reorganise and change their configuration through time. So, when moving fast enough, they cannot constrain the cores anymore, which can be seen as moving in a "sea" of dangling branches. In particular, we showed the importance of the role plaid by the core-shell ratio of the micelles in these two opposite processes: the dynamic response of short shell micelles is mainly governed by the cores, which swell with temperature. Therefore, a temperature increase leads to a viscosity increase of the suspensions. While, on the other hand, the dynamic response of long-shell micelles is mainly governed by the shell, which has a higher mobility when the temperature increases. Therefore, in this case, a temperature increase leads to a viscosity decrease of the suspensions.

Playing with these two consequences of the temperature, we presented a suspension of short-shell micelles where a solid-to-liquid transition followed by a liquid-to-solid transition was observed by increasing the temperature, defining a new way for tuning the rheological properties of core-shell micelles, towards smart materials.

For the systems presented in this work, both the mass concentration and the temperature increase can lead to a larger effective volume fraction and thus induced a kinetic arrest. Therefore, we investigated the universality of the relationship between their relative zero-shear viscosity and their effective volume fraction. While a master-curve can be obtained with hard spheres or microgel systems, this universality was not observed here, pointing to the quantitative different effect of mass concentration and temperature on the system's response.

Then, we investigated the flow behaviour of these particles, looking at the influence of the temperature and the concentration on their yielding stress or yielding strain.

At the present, several points are still under consideration, as the influence of these different parameters (temperature, concentration, core-shell ratio and solvent quality) on the non-linear response of the micelles or the meaning of being able to build a master-curve by time-concentration superposition of the viscoelastic data.