Study of colloidal suspensions of multi-responsive microgels

Abstract

Microgels are aqueous dispersions of nanometre- or micrometre-sized hydrogel particles which have attracted great interest as repulsive-soft colloids due to the possibility of changing their effective volume fraction by tuning their response to the external stimuli, such as pH, temperature, electric field, ionic strength, solvent, external stress or light. Fundamental studies on this class of systems, have highlighted the richness of microgel properties, since they allow to modulate the interaction potential through easily accessible experimental parameters and give rise to unusual transition between di erent arrested states. On the other hand they are particularly attractive smart materials largely used for many applications in a lot of different fields. The system investigated in this thesis is a colloidal suspension of Interpenetrated Polymer Network (IPN) microgels, composed by two interpenetrated homopolymeric networks of a thermo-sensitive polymer, the poly(N-isopropylacrylamide), usually known as PNIPAM, and a pH-sensitive one, the poly(acrylic acid), usually known as PAAc. Its dynamics and local structure have been investigated as a function of temperature, pH and concentration both in H2O and D2O solutions, in order to provide a preliminary picture of the PNIPAM-PAAc IPN microgel phase diagram as a function of temperature, pH and concentration. The H/D isotopic substitution has allowed to understand the role played by the H-bonding in the dynamics of the system and also to get enough contrast in neutron scattering experiments. A systematic characterization of the typical swelling behavior has been performed through Dynamic Light Scattering (DLS) in the high dilution regime in the temperature range T=(293 313) K at both acidic and neutral pH. A thermoreversible transition around 305 􀀀 307 K corresponding to the swollen-shrunken volume phase transition (VPT) has been highlighted, such as an interesting pH-sensitivity, which leads to complex phase behaviors mainly at the highest concentrations where a transition from an ergodic to a non-ergodic state is observed. Moreover a sharper VPT is observed at pH 7 than at pH 5, with a discontinuous and a continuous transition respectively. The intraparticle structure during the cross-over from the fully swollen to the completely shrunken state has been probed through Small-Angle Neutron Scattering (SANS) as a function of temperature, pH and concentration. SANS data have been described by taking into account the presence of both interpenetrated polymer networks and cross-linkers, highlighting a structural transition from a water-rich open inhomogeneous structure to a homogeneous porous solid-like one, associated to the collapse of the polymer chains across the VPT, with a sharp change at neutral pH and an almost continuous transition at acidic pH. The combination of these results has allowed to draw preliminary temperature- concentration phase diagrams as a function of pH and solvent conditions and has opened the way to further investigations in the formation of arrested states in this smart IPN microgel belonging to the intriguing class of soft colloids.