Stress-transfer from polymer substrates to monolayer and few-layer graphenes.

In the present study, the stress transfer mechanism in graphene-polymer systems under tension is examined experimentally using the technique of laser Raman microscopy. We discuss in detail the effect of graphene edge geometry, lateral size and thickness which need to be taken under consideration when using graphene as a protective layer. The systems examined were composed of graphene flakes with a large length (over ∼50 microns) and a thickness of one to three layers simply deposited onto PMMA substrates which were then loaded to a tension of ∼1.60% strain. The stress transfer profiles were found to be linear while the results show that large lateral sizes of over twenty microns are needed in order to provide effective reinforcement at levels of strain higher than 1%. Moreover, the stress built up has been found to be quite sensitive to both edge shape and geometry of the loaded flakes. Finally, the transfer lengths were found to increase with the increase of graphene layers. The outcomes of the present study provide crucial insight into the issue of stress transfer from polymers to graphene nano-inclusions as a function of edge geometry, lateral size and thickness in a number of applications.

Wrinkling formation in simply-supported graphenes under tension and compression loadings.

Wrinkles in supported graphenes can be formed either by uniaxial compression or uniaxial tension beyond a certain critical load depending on the mode of loading. In the first case, the wrinkling direction is normal to the compression axis whereas in tension, wrinkles of the same pattern are formed parallel to the loading direction due to Poisson's (lateral) contraction. Herein we show by direct AFM observations that in simply-supported graphenes such instabilities appear as periodic wrinkles over existing stochastic undulations caused by the underlying-substrate-roughness. The critical strain for the generation of these wrinkles in both tension and compression is less than 1% which particularly for the former is far lower than the predicted tensile strain to fracture of suspended graphene estimated at ∼30%. Based on these findings, a constitutive model that provides the critical tensile strain for induced buckling in the lateral direction is proposed that depends only on the graphene-support interaction and not on the nature of the substrate. Understanding the wrinkling failure of graphenes under strain is of paramount importance as it leads to new threshold limits beyond which the physical-mechanical properties of graphene are impaired.