Nacre-like GNP/Epoxy composites: Reinforcement efficiency vis-`a-vis graphene content

Nanocomposites based on the biomimetic brick and mortar architecture are gathering great attention recently due to the outstanding properties of the natural analogues. Thanks to the very high in-plane orientation of nanoplatelets and the low matrix content, these materials exhibit good mechanical performance combined with excellent functional properties based on the nanoplatelets characteristics. Key feature of these materials is the presence of a regular nanostructure that consists of alternated nanoplatelet and matrix layers. Such architecture is commonly attained by means of bottom-up manufacturing processes able to control the alternation of the two phases at the nanoscale. This differs markedly from the top-down manufacturing processes, which are suitable for an industrial scale up but fail to meet the expected requirements. In the present work, nacre-like graphene nanoplatelets (GNPs)/epoxy films at different filler content have been prepared by a top-down manufacturing technology and their mechanical properties in tension have been experimentally evaluated. The elastic modulus has been found to exhibit a maximum between 53 and 67 vol% filler content and then it starts dropping at higher loadings. This was attributed to the depletion of the polymeric matrix which disrupts the continuity of the matrix layer thus inducing incomplete GNP surface coverage. As a result, the effective area for stress-transfer is considerably reduced and this impairs the reinforcement efficiency. In order to explain this behaviour, a model is proposed for predicting the stress transfer characteristics in brick and mortar systems by paying attention to possible non-uniform matrix distribution over the nanoplatelets. The proposed analysis captures well the observed effects and paves the way for the development and further improvement of this new class of engineering materials.