Oscillations in modulus in solutions of graphene oxide and reduced graphene oxide with grafted poly-N-isopropylamide

In a material consisting of graphene oxide or reduced graphene oxide and poly-N-isopropylamide (PNIPAM) in an aqueous solution, a new type of rheological behaviour is found. When subjecting the material to a short and relatively small deformation pulse, the modulus, which is observed by small deformations in the linear-viscoelastic or very slightly nonlinear range, oscillates with periodicities between 100 and several 1000 seconds; however, in many cases, it also increases systematically. The periodicity depends on the filler content and the sample preparation method (in situ polymerisation vs. blending). When subjecting the material to high nonlinear deformations (gamma(0) = 100-300%), the resulting linear viscoelastic behaviour changes from a periodic oscillation to a quick recovery of the original data, followed by a decrease and a subsequent increase beyond the value of the modulus of the material prior to the deformation pulse.