Stress Transfer Mechanisms at the Submicron Level for Graphene/Polymer Systems

TL;DR

This study investigates stress transfer at the submicron scale in graphene/polymer systems, revealing deviations from classical models near edges due to residual stresses and edge effects, and quantifying interfacial shear stress.

Contribution

It provides the first detailed Raman mapping analysis of stress distribution at the edges of monolayer graphene flakes at submicron resolution, highlighting factors affecting stress transfer.

Findings

Deviations from classical shear-lag predictions near edges.

Maximum interfacial shear stress of 0.4 MPa before slipping.

Edge effects and residual stresses influence stress distribution.

Abstract

The stress transfer mechanism from a polymer substrate to a nano-inclusion, such as a graphene flake, is of extreme interest for the production of effective nanocomposites. Previous work conducted mainly at the micron scale has shown that the intrinsic mechanism of stress transfer is shear at the interface. However, since the interfacial shear takes its maximum value at the very edge of the nano-inclusion it is of extreme interest to assess the effect of edge integrity upon axial stress transfer at the submicron scale. Here, we conduct a detailed Raman line mapping near the edges of a monolayer graphene flake which is simply supported onto an epoxy based photoresist (SU8)/poly(methyl methacrylate) (PMMA) matrix at steps as small as 100 nm. We show for the first time that, the distribution of axial strain (stress) along the flake deviates somewhat from the classical shear-lag prediction for a region of about 2 um from the edge. This behavior is mainly attributed to the presence of residual stresses, unintentional doping and/or edge effects (deviation from the equilibrium values of bond lengths and angles, as well as different edge chiralities). By considering a simple balance of shear-to-normal stresses at the interface we are able to directly convert the strain (stress) gradient to values of interfacial shear stress for all the applied tensile levels without assuming classical shear-lag behavior. For large flakes a maximum value of interfacial shear stress (ISS) of 0.4 MPa is obtained prior to flake slipping.