Thesis title: Study of the Phase Behaviour of Interpenetrated Polymer Networks of Poly(N-isopropylacrylamide) and Poly(acrylic acid) Microgels
Soft materials such as colloids, polymeric suspensions, emulsions, foams, gels, micelles, membranes, biological materials and similar show both liquid-like and solid-like behaviours and are involved in every day life.
Among them, soft colloids can display intriguing phase behaviours since they are characterized by many fascinating properties: the most remarkable is their softness, that depends
on the underlying macromolecular architecture in systems such as microgels, star polymers, core-shell particles, liposomes etc. and can also be tuned during the synthesis process.
Particle softness is strictly related to the interparticle interaction potential, that can be suitably modified and can vary from repulsive interactions over long distances to short-range attractive interactions. Soft colloids have long been the focus of research either in technological applications as well as in fundamental studies. Thanks to the peculiarity of their particles to be deformable, elastic and interpenetrable, they provide a very rich phenomenology. Particles can reach non spherical shapes and can be packed in all the available volume, giving rise to a large variety of new physical states and phenomenologies. In particular, at very high volume fractions, different arrested states can be reached such as the colloidal glass state, that shares similarities with the glass observed decreasing temperature in molecular supercooled liquids. A great advantage of soft colloids is that, due to the
typical size of the constituent particles, they are experimentally accessible through many techniques.
Among soft colloidal systems, microgels, aqueous dispersions of nanometer- or micrometer sized particles of almost spherical shape made of crosslinked polymer networks, represent an interesting class of glass-forming systems. One of the main advantages of microgels is their ability to respond to environmental stimuli. In particular, depending on the nature of the constituent monomers, microgel particles could manifest high sensitivity to changes of temperature, pH, electric field, ionic strength, solvent and light pulses. Interestingly, they offer manifold possibilities in several research fields thanks to their polymer/colloid duality. Indeed, the responsiveness of microgels, coupled with their versatility and relatively easy synthesis methods, makes them attractive for several applications, such as drug delivery, tissue engineering in artificial muscle, bone and cartilage fabrications, for organ-on-chip and microlenses device development. Of note, microgels also find applications in cultural heritage conservation for cleaning of modern and ancient paper.
In this work, colloidal suspensions of Interpenetrated Polymer Network (IPN) microgels composed by interpenetration of a thermo-sensitive polymer, the poly(N-isopropylacrylamide), known as PNIPAM, and a pH-sensitive one, the poly(acrylic acid), known as PAAc have been investigated. Adding the PAAc to the well-known PNIPAM microgel, provides an additional pH-sensitivity to the system and a more complex phase behaviour. Varying different parameters during the microgel synthesis allows to obtain systems with different particle structure, dynamics and phase behaviours. At variance with previous works on these systems, different
PNIPAM and IPN microgels have been synthesized by varying fundamental control parameters. By changing the amount of surfactant (SDS) it is possible to tune particles dimension,
in particular we focused on "small" (R about 40 nm) and "large" (R about 200 nm) PNIPAM particles that give rise to "small" (R about 100 nm) and "large" (R about 300 nm) IPN particles. Moreover, the crucial role of initiator (KPS) addition time on particles morphology has been assessed. All synthesized microgels have been systematically investigated at different temperatures, concentrations, pH and PAAc content, using different experimental techniques that have provided new insights in their understanding.
In particular, the hydrodynamic radius and the dynamics of the systems have been investigated through Dynamic Ligth Scattering (DLS), the structure has been assessed through Small Angle Neutron Scattering (SANS), viscoelastic properties have been studied through rheology, both in rotational and oscillatory regime and the thermal properties have been evaluated through calorimetric measurements.
Finally, the use of all these complementary techniques has permitted to draw a temperature-concentration phase diagram.