The unconventional behaviour of responsive microgels has attracted great interest in the last years since their effective volume fraction and their elastic properties can be tuned by changing external parameters leading to novel phase diagrams, drastically different from those of conventional hard-spheres-like colloids. However their phase behaviour is far from being understood and it may be even more complex and exotic by interpenetrating two homopolymeric networks with independent responsiveness to different external stimuli. Indeed Interpenetrated Polymer Network (IPN) microgels of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc) exhibit unusual behaviours, since their thermo- and pH-sensitivity allows to tune the delicate balance between polymer/polymer and polymer/solvent interactions by changing pH or by varying the PAAc content. This offers a useful tool to directly control the microgel softness and to explore how the elastic properties affect the dynamic arrest.
By combining different experimental techniques, such as Dynamic Light Scattering, Differential Scanning Calorimetry, Rheometry, Small-Angle Neutron Scattering and Small-Angle X-Ray Scattering we have observed that IPN microgels elasticity is strongly affected by the topological inhomogeneities , making them useful model systems for investigating the “strong-to-fragile” transition . Moreover we propose a novel phase diagram for IPN microgels: across the typical swollen-shrunken volume phase transition at T≈305 K [3,4], an ergodic to non-ergodic transition and different arrested states (colloidal crystals, gels and/or glasses) show up, depending on concentration, pH and PAAc content. More insight on the interparticle interaction potential will be provided through a comparison with numerical simulation, as previously done for other colloidal systems systems [5,6].
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