pH‐Dependent Morphology Control of Cellulose Nanofiber/Graphene Oxide Cryogels

Abstract

The precise control of the ice crystal growth during a freezing process is of essential importance for achieving porous cryogels with desired architectures. The present work reports a systematic study on the achievement of multi‐structural cryogels from a binary dispersion containing 50 wt% 2,2,6,6‐tetramethylpiperidin‐1‐oxyl, radical‐mediated oxidized cellulose nanofibers (TOCNs), and 50 wt% graphene oxide (GO) via the unidirectional freeze‐drying (UDF) approach. It is found that the increase in the sol's pH imparts better dispersion of the two components through increased electrostatic repulsion, while also causing progressively weaker gel networks leading to micro‐lamella cryogels from the UDF process. At the pH of 5.2, an optimum between TOCN and GO self‐aggregation and dispersion is achieved, leading to the strongest TOCN‐GO interactions and their templating into the regular micro‐honeycomb structures. A two‐faceted mechanism for explaining the cryogel formation is proposed and it is shown that the interplay of the maximized TOCN‐GO interactions and the high affinity of the dispersoid complexes for the ice crystals are necessary for obtaining a micro‐honeycomb morphology along the freezing direction. Further, by linking the microstructure and rheology of the corresponding precursor sols, a diagram for predicting the microstructure of TOCN‐GO cryogels obtained through the UDF process is proposed.

Aqueous Dispersion of Carbon Nanomaterials with Cellulose Nanocrystals: An Investigation of Molecular Interactions

“…Without the covalent reactions, the CNCCNT/GnP interactions are limited to physical interactions. [52,53] The Gibbs energy difference (ΔΔG b ), calculated by DFT analysis, for pCNT-pCNT, pGnPpGnP, and CNCCNT/GnP shows that the agglomeration remains highly probable if the CNC molecules, are attached by side-to-side physical (e.g., van der Waals) interactions, (Table 1; Figure S2, Supporting Information). CNC particles were modeled with glucan chains, labeled CNC nano particles, with similar interacting properties (see the Experimental Section and the Supporting Information for more details).…”

Current international research into cellulose as a functional nanomaterial for advanced applications

“…For example, Karageorgiou and Kaplan reported that the optimal pore size to promote osteogenesis was [300 µm [119], while the optimal pore diameter for neuronal cells has often been reported to be \100 µm [120][121][122]. However, strategies to control pore size and morphology in nanocellulosic cryogels are known, including regulation of the gel composition [123][124][125]; freeze-casting temperature and/or rate [123,126]; sol pH [127]; and nanocellulose morphology [126]. Furthermore, Tetik et al [128] have recently incorporated directional freezing with 3D printing, which enabled the production of controlled 3D geometries with aligned micropores.…”

Fabrication of graphene-based porous materials: traditional and emerging approaches

“…Nevertheless, sponges made using dispersions at low pH are known to possess higher storage modulus. 93 Relative humidity is found to affect the compression load afforded by freeze-dried sponges; the higher the relative humidity, the weaker the sponge would be, so it can withstand a smaller load and for a shorter period of time. 94 Furthermore, it is reported that GO loses some of its oxygencontaining moieties during the freeze-drying process (increasing the C : O and I D : I G ratios from $1.6 to $2.1 and 0.84 to 0.99, respectively).…”

“…5f , different regions of the aerogel possess varied spacing between the GO nanosheets and thus, pore density differs locally within the sample. In addition to rate and direction of freezing, the properties of these sponges are also governed by GO nanosheets' size, 92 pH, 93 and humidity. 94 Ultra-large GO nanosheets are reported to yield a better alignment with ultralow percolation threshold, which when reduced, would make better electrical conductors.…”