Investigating the Mechanism of Temperature-Induced Graphene Self-Folding in Water

The popularity of graphene origami - eyed for applications in electronics, energy harvesting, molecular storage and membrane development - has been on the rise for the last few years, with the world’s tiniest microchips created through the same process just in 2021. With hopes of making graphene origami easier, temperature-induced self-folding of graphene using thermosensitive polymers like poly(N-isopropylacrylamide) (PNIPAM) has been demonstrated experimentally. This process, attributed to the coil-to-globule transition of PNIPAM, is however, poorly understood, making the creation and control of these self-folded three dimensional (3D) structures difficult. For this reason, we employ coarse-grained (CG) molecular dynamics (MD) simulations to develop an accurate molecular-level mechanism for the self-folding of PNIPAM-grafted graphene in presence of an explicit solvent. Our analyses of contacts, energetics and structure of PNIPAM-solvent interface indicate that the process is actually initiated by the weakening of PNIPAM-water interactions at higher temperatures, resulting in strong hydrophobic graphene-PNIPAM interactions. Densely grafted PNIPAM chains above LCST are observed to stabilize through chain aggregation and a coil-to-globule transition may not always be observed. By further varying system parameters like graphene width and polymer coverage, we show that graphene folding only occurs when the total energy of the final conformation is lower than the total energy of the initial conformation, indicating the stabilization of the structure after folding. We also find that PNIPAM-PNIPAM interactions compete with graphene-PNIPAM interactions and folding occurs when the latter dominates. Through the molecular-level insights obtained in this study, we thus provide energy criteria for graphene-folding that can be essential for controlling and tuning this process via temperature control.