Structure, Dynamics and Phase Behaviour of Charged Soft Colloidal Dispersions

Sammanfattning: Soft and deformable ionic microgels such as poly(N-isopropylacrylamide-co-acrylic acid), PNIPAM-co-AA, microgels have shown to be a versatile alternative to the already well-established hard sphere model systems. Their tuneable interaction potential makes them well suited as model systems to study the phase behaviour found for particles interacting via a soft isotropic potential. This stems from the microgels ability to respond to changes in the environment, such as changes in temperature, pH or particle concentration. Additionally, subjecting these particles to an alternating electric field induces a dipolar contribution to the interaction potential, which strongly depends on the amplitude and frequency of the applied field. This additional, tuneable and directional attraction allows us to explore and deepen our understanding of systems interacting via an even more complex and anisotropic potential.At low number densities particles are shown to be aligning into strings along the direction of the field. These strings assemble into crystal structures as the field strength is further increased. At concentrations above freezing the parent face-centred cubic, FCC, structure is found to melt and diffusively transforms into a BCT crystal via nucleation and growth but in the reverse direction, the BCT phase transforms cooperatively into a metastable body-centred orthorhombic, BCO, phase, which only relaxes back to the parent FCC phase as the temperature is increased. The kinetics is consequently either diffusive or martensitic depending on the path and is believed to be due to the interpenetrable nature of the microgel particles. In order to learn more about the origin of this puzzling path dependence, we studied the shape and size of the particles as a function of packing fraction and field strength by performing a combination of scattering experiments, both in the absence and presence of the electric field. We found that the particle size is dramatically altered by a small increase in particle concentration to reach a plateau value at intermediate concentration. In the over-packed state the particle size is again seen to decrease due to shell overlap. The applied electric field however was shown to only slightly alter the particle size, thus confirming the interpenetration of particles in field-induced structures. As a last step we performed dielectric spectroscopy measurements to obtain information about the polarisation mechanisms present in the system.In the future the collected information will be used to derive a theoretical model that will provide us with the intrinsic and field-induced interaction potential at the relevant concentrations and field-strengths. This potential will be compared to obtained data in the absence and presence of the alternating electric field.