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

Abstract

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.

Crystallization kinetics, structure, and rheological behavior of poly(ethylene terephthalate)/multilayer graphene oxide nanocomposites

“…From the viscosity curves it is possible to notice that all the studied compositions show (a) shear‐thinning behavior; (b) mGO behaves as a lubricant agent, easing the polymer flow through the capillary and reducing its shear viscosity relatively to pure PET; and (c) the amount of mGO is not high enough to induce different viscosity levels in the composites. This trend of decreasing viscosity was already reported by some works in the literature [ 28,34,37,39,53 ] and it is industrially interesting, because this effect may act as a “processing aid” during the composites compounding and molding, allowing the use of less severe parameters, such as lower temperature, torque and pressure. In order to confirm that the reason for these results are due to lubrication effects, viscometric analyses were carried out and the relative viscosities ( η r ) were obtained for the processed neat PET and all nanocomposites, as presented in Table 3.…”

“…These phenomena allow the filler to enter its "superlubricity state," that is, internal friction in the material flow is reduced, consequently, reducing its viscosity. On the other hand, when less shear stress is generated, it might not be enough to produce the effect reported by Ferreira et al [34] In this case, instead of easing the flow, the filler/filler and filler/polymer interactions will restrict the molecules mobility, which in turn increases the material's viscosity. Figure 8 exhibits the shear stresses present during the capillary and steady shear tests for neat PET.…”

“…It was recently reported by Ferreira et al [34] that when higher shear stresses (τ s ) are produced and transferred to the nanofiller during flow, it enables the alignment and slippage of the filler flakes. These phenomena allow the filler to enter its "superlubricity state," that is, internal friction in the material flow is reduced, consequently, reducing its viscosity.…”

“…[30] In particular, rheometry is a powerful tool to investigate nanocomposites microstructures, and several studies have been carried out with the aim to identify changes in the viscoelastic behavior of polymers by the presence of two-dimensional nanofillers. [30][31][32][33][34][35][36][37][38][39] Therefore, this work aims to investigate not only the PET/mGO crystallization characteristics, but also its rheological behavior in two different conditions, that is, capillary and rotational rheometry.…”

Influence of Oxidation Degree of Graphene Oxide on the Shear Rheology of Poly(ethylene glycol) Suspensions

“…It can be observed that the d-spacing increase from 0.312 nm (Gr) to 0.895 nm (GrO 2 h) and 0.907 nm (GrO 96 h) due to the presence of oxygen functional groups, such as −OH and −COOH, obtained from the oxidation process [33]. The higher d-spacing obtained for the GrO 96 h is due to the higher oxidation time, which led to an increase in functional groups developed in the material structure [34]. As shown in Figure 3, the Raman spectrum can provide several bands, but the most important ones for carrying out the study and characterization of the load are the D, G, and 2D bands.…”