Defence of doctoral thesis: Hanieh Mianehrow – Two-dimensional Nanocomposites Based on Cellulose Nanofibrils and Graphene Oxide

Two-dimensional (2D) nanocomposites based on cellulose nanofibrils (CNF) and 2D nanomaterials are of interest as sustainable materials combining functional and structural properties. To achieve reinforcement effects from 2D nanomaterials, their orientation and dispersion state in the CNF matrix are important. In this thesis, nanocomposites based on CNF and graphene oxide (GO) platelets are investigated. The focus is on understanding nanostructure-property relationships, reinforcement mechanisms and interfacial molecular interactions. For this purpose, experimental investigations are combined with molecular dynamics (MD) simulations. 

CNF-GO nanocomposite nanopapers with different GO content are prepared using vacuum filtration of stable hydrocolloid dispersions, followed by drying. Nanostructure and physical and mechanical properties are investigated. Even a small, “homeopathic” amount of large aspect ratio GO platelets (0.07 vol%) is shown to induce ordering in the CNF matrix resulting in strong property improvement. In order to add an additional functionality to such nanocomposites, the GO in CNF-GO wet cake (after vacuum filtration) is chemically reduced to reduced graphene oxide (RGO). The main idea is to preserve the homogeneous distribution of GO in the CNF matrix and then reduce GO to RGO to achieve electrical conductivity together with mechanical reinforcement. The mechanical properties are very high. Effects from moisture on the mechanical performance of CNF-RGO nanocomposite are also studied and compared to CNF-GO and neat CNF films. Although CNF-GO adsorbs more moisture, it shows higher tensile strength at 90% relative humidity (RH) compared to CNF-RGO and neat CNF. Moisture effects on molecular interactions at CNF-GO interface were further studied by MD simulations. Dry interfaces are formed even in water-soaked conditions. The reason is that the system gains entropy as trapped interfacial water diffuses to form bulk water. The CNF-GO interface shows higher interfacial shear strength than CNF-graphene or RGO, because of higher hydrogen bond density. This may contribute to the higher strength for CNF-GO compared with CNF-RGO, despite higher moisture content for CNF-GO at 90% RH.

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