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The “Superlubricity State” of Carbonaceous Fillers on Polyethylene-Based Composites in a Molten State

It is very well known that the incorporation of fillers into polymers increases their viscosity in a molten state; however, results contrary to this have been reported when graphite (Gr), graphite oxide (GrO), and graphene oxide (GO) among other carbonaceous fillers are used. Many authors have justified the reduction in polymer viscosity due to the slipping interlayers of the Gr, GrO, and GO fillers. The slipping interlayer cannot explain the reduction in polymer viscosity since the shear stresses produced or applied during rheological tests are lower than the interlayer shear strength of these fillers. Here, rheological experiments with two types of polyethylenes and Gr, GrO, and GO as fillers were carried out to elucidate this phenomenon. Remarkably, it was observed that the viscosity reduction occurs due to the presence of agglomerates in the composites. The agglomerates contain many flake–flake contacts out of registry (superlubricity state), which have very low slipping resistance. The slipping of these contacts promotes the reduction in polymer viscosity during rheological tests. The elucidation of the superlubricity state phenomenon proposed here may contribute to the understanding of the carbonaceous filler influence during the deformation of polymer composites.