The viscoelastic properties of polystyrene particles bearing poly(ethylene oxide)-poly(propylene oxide) ABA block copolymers

Abstract

The viscoelastic properties of concentrated polystyrene latices stabilised by ABA block copolymers of poly(ethylene oxide)-poly(propylene oxide) have been measured using oscillatory and steady-state shear measurements and osmotic pressure measurements. For stable systems, highly concentrated polystyrene latices showed non-Newtonian flow behaviour from steady-state shear measurements. From the oscillatory shear measurements it was noted that the elastic moduli started to increase dramatically when the interparticle separation was such that the polymer layers were touching. The osmotic pressures of these latex systems were measured and correlated to the elastic moduli measurements and to the simple scaling theory for adsorbed polymers where a qualitative agreement was found between the osmotic pressures obtained from experiment and the scaling theory. In a complementary series of experiments, the oscillatory rheological behaviour of these systems is they were flocculated (brought about by an increase in temperature) was measured. At the onset of flocculation both the elastic and loss moduli of the dispersion increased by many orders of magnitude. It was noted that the temperature at which flocculation occurred remained constant below a volume fraction Φ of 0.5 but decreased sharply at higher volume fractions, in accord with simple statistical-mechanical theories. Moreover, the change in the elastic modulus G′ with the volume reaction after flocculation was found to be in agreement with a fractal analysis for percolating networks.