Rheological properties of suspensions of thermo-responsive poly(N-isopropylacrylamide) microgels undergoing volume phase transition

Abstract

The rheological properties of suspensions of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) microgels are investigated in the swollen and collapsed states using oscillatory and long-time creep measurements. In the swollen state with repulsive interparticle interaction, the zero shear viscosity and quasi-plateau modulus are solely governed by the effective volume fraction (φeff) and are independent of temperature, particle diameter and particle concentration. The long-time creep tests reveal that in the collapsed state with attractive interparticle interactions, the moderately concentrated suspensions with φeff≈0.5 become viscoelastic liquids with high viscosities of >105 Pas and long-terminal relaxation times of >103 s due to the formation of a network-like aggregation of particles, whereas the suspensions with the same φeff in the swollen state exhibit a Newtonian flow with low viscosities of less than 10−1 Pas. The network-like aggregation of particles is so fragile that it can be broken by a very small stress of several Pascals. The modulus and viscosity in the collapsed state depend on both φeff and temperature: the modulus and viscosity increase, as the temperature increases due to an increase in the hydrophobic nature of the PNIPAM chains, which govern the attractive interparticle interaction. The particle aggregations in the collapsed state considerably lower the threshold value of φeff (φc), above which the infinite percolation clusters are formed, as compared with the values of φc in the swollen state and for the random close packing of hard spheres. The rheological properties of the suspensions of thermo-responsive poly(N-isopropylacrylamide) microgels are investigated in the swollen and collapsed states using creep and oscillatory measurements. The results from the long-time creep tests reveal that in the collapsed state with attractive interparticle interactions, the suspensions with a particle volume fraction of approximately 0.5 become viscoelastic liquids with high viscosities due to the formation of a network-like aggregation of particles. However, the suspensions with the same volume fraction in the swollen state exhibit a Newtonian fluid behavior with low viscosities.