Shear-thickening behaviour of concentrated polymer dispersions under steady and oscillatory shear

The rheological behaviour of a 58 vol.% dispersion of styrene/acrylate particles in ethylene glycol was investigated using a plate-on-plate rheometer. Experimental results showed that the concentrated polymer dispersion exhibited a strong shear-thickening transition under both steady shear and dynamic oscillatory conditions. The low-frequency dynamic oscillatory behaviour could be reasonably interpreted in terms of the steady shear behaviour. Accordingly, the critical dynamic shear rate \( \dot{\gamma }_{{{\text{c\_d}}}} , \) agreed well with the critical shear rate obtained in steady flow \( \dot{\gamma }_{{{\text{c\_s}}}} , \) where \( \dot{\gamma }_{{{\text{c\_d}}}} \) was calculated as the maximum shear rate by the critical dynamic shear strain γc and the frequency ω, i.e. \( \dot{\gamma }_{{{\text{c\_d}}}} = \omega \gamma_{\text{c}} . \) However, during high-frequency dynamic oscillation, it was observed that the shear thickening occurred only when an apparent critical shear strain was reached, which could not be fully explained by the wall-slipping effect. Based on freeze fracture microscopic observations, the effect of the micro-sized flocculation of particles on the rheology of concentrated dispersions was also discussed.