Dynamic light scattering, rheology and simulations of Interpenetrating network microgels

Soft colloids have recently attracted great attention both in fundamental studies and applications thanks to their deformability, elasticity and interpenetrable nature, providing a very rich phenomenology. Among soft colloids, microgels, particles composed of chemically crosslinked polymer networks, are very enthralling due to their hybrid nature which combines the properties of polymers and colloids. Herein we focus on a colloidal suspension of poly(N-isopropylacrylamide) (PNIPAM) microgel [1] and on colloidal suspensions of interpenetrated polymer network (IPN) microgels made of PNIPAM and poly(acrylic acid) (PAAc) [2]. PNIPAM microgels change their affinity with the solvent as the temperature changes; the addition of the second network of PAAc makes IPN microgels sensitive also to pH. Moreover, these systems undergo to a phase transition passing from a liquid to an arrested state as a function of concentration. The study aims to verify the influence that different contents of N,N'-methylbisacrylamide (BIS), the crosslinker used to polymerize the PAAc network, have on the swelling behaviour, the dynamics and the viscoelastic properties of the systems. To this purpose, concentration, temperature and pH-dependence of the dynamic of four different IPN microgel solutions is carefully investigated through Dynamic Light Scattering (DLS) and rheology. The swelling behaviour of PNIPAM and IPN microgels is also studied performing molecular dynamics simulations. This in silico study on microgel particles is divided in three phases. As a first step the swelling behaviour of a microgel particle composed of single-network is studied. Then it is analysed how the swelling capability is affected by the introduction of a second neutral interpenetrated network that does not respond to temperature changes. Eventually the effects that a second charged network introduces on the swelling is investigated. Both experimental and simulation data allow to conclude that the different concentrations of BIS have a strong effect on the swelling capability of IPN microgels and on the softness of their structure. Particularly a higher concentration of BIS reduces the swelling capability of microgels particles as a function of temperature. Moreover, also the slowing down of the dynamics up to the formation of an arrested state is influenced by the BIS concentration. Results suggest that a higher concentration of BIS shifts the transition to an arrested state to higher concentrations of IPN solutio ns. These findings open the way to further investigation on the role that topological constrains have on the interactions between microgel particles and how they influence the transition to arrested states.