Shear Thickening Behaviour of Polyborosiloxane

Polyborosiloxane (PBS) is often referred to as a "shear thickening/stiffening gel" or "dilatant compound", sometimes actually meaning an increase in modulus with frequency/strain rate rather than the rheological definition of "dilatant compound", i.e. an increase in viscosity with shear rate. The shear behaviour of bulk PBS and PBS solution has been studied here by steady shear experiments, and it is observed that PBS exhibits shear thickening behaviour only under certain conditions. The shear behaviour of bulk PBS depends on the degree of reaction of the PBS (i.e. the content of B in the PBS). For PBS solution, the shear behaviour is influenced by the concentration of the solution and the type of solvent (with or without oxygen atom in the solvent). The shear behaviour of PBS systems is related to shear induced structural changes, where the formation of new elastically active B:O dynamic dative bonds (interchain cross-linking) contributes to shear thickening.