On the overall viscoelastic behavior of graphene/polymer nanocomposites with imperfect interface

Determination of the time-dependent behavior of graphene/polymer nanocomposites is a highly complicated issue, for there are many factors that could affect the effectiveness of graphene reinforcement. In particular, it is greatly known that condition of imperfect load transfer at the graphene–polymer interface can have a significant impact on the overall response of nanocomposite. In this paper, we intend to uncover how the interface condition affects the fundamental viscoelastic characteristics of graphene/polymer composites. To do so, we first call upon a homogenization theory for determining the viscoelastic response of nanocomposites with perfect interface. Next, we develop two different models to address the issue of imperfect interface condition within the framework of our effective medium theory. In the first approach, a diminishing layer of interphase with weakened viscoelastic properties is introduced between the graphene filler and surrounding matrix to build a thinly-coated inclusion system. Through the second methodology, the condition of imperfect load transfer is modeled by a displacement jump, say interfacial sliding, at the graphene–polymer interface. At the end, a comparison of our theoretical results with experimental data for real examples of graphene/polymer nanocomposites (i.e. graphene/Polystyrene and graphene/Polylactide materials) serves to verify the applicability and robustness of our effective medium theory at various concentrations and aspect ratios of graphene fillers. In the process of numerical studies, we show that the experimental creep data cannot be satisfactorily modeled under assumed condition of perfect interface; however, the measured creep curves can be well captured by adopting the imperfect interface models developed in this paper. The calculated results also demonstrate that the interface condition has a profound influence on the stress relaxation and stress–strain relation of graphene/polymer nanocomposites under a constant strain-rate loading. While the surface functionalization for the improvement of interface condition is currently a very active area of research, all of our findings point to the urgency of this process in order to fully realize the potential of graphene nanocomposites.