Comprehensive NMR Analysis of Pore Structures in Superabsorbing Cellulose Nanofiber Aerogels

Abstract

Highly porous cellulose nanofiber (CNF) aerogels are promising, environmentally friendly, reusable, and low-cost materials for several advanced environmental, biomedical, and electronic applications. The aerogels have a complex and hierarchical 3D porous network structure with pore sizes ranging from nanometers to hundreds of micrometers. The morphology of the network has a critical role on the performance of aerogels, but it is difficult to characterize thoroughly with traditional techniques. Here, we introduce a combination of nuclear magnetic resonance (NMR) spectroscopy techniques for comprehensive characterization of pore sizes and connectivity in the CNF aerogels. Cyclohexane absorbed in the aerogels was used as a probe fluid. NMR cryoporometry enabled us to characterize the size distribution of nanometer scale pores in between the cellulose nanofibers in the solid matrix of the aerogels. Restricted diffusion of cyclohexane revealed the size distribution of the dominant micrometer scale pores as well as the tortuosity of the pore network. T 2 relaxation filtered microscopic magnetic resonance imaging (MRI) method allowed us to determine the size distribution of the largest, submillimeter scale pores. The NMR techniques are nondestructive, and they provide information about the whole sample volume (not only surfaces). Furthermore, they show how absorbed liquids experience the complex 3D pore structure. Thorough characterization of porous structures is important for understanding the properties of the aerogels and optimizing them for various applications. The introduced comprehensive NMR analysis set is widely usable for a broad range of different kinds of aerogels used in different applications, such as catalysis, batteries, supercapacitors, hydrogen storage, etc.

Time-Domain NMR Elucidates Fibril Formation in Methylcellulose Hydrogels

“…62,63 The mean free path λ of water, estimated using eq 1, is on the order of tens of microns, meaning that the tortuosity of the gel for these MC concentrations is low, and therefore, the gel network is very loose. 64 MC4 is endowed with the largest molecular weight, and thus, this result can be extended to the other MCs.…”

Accelerating Restricted Diffusion NMR Studies with Time-Resolved and Ultrafast Methods

“…Nuclear magnetic resonance (NMR) diffusion experiments have been widely exploited in the studies of structures and fluid transport properties of porous materials . The applications of the method include rocks, glass beads, wood, , ionic liquids, , cells, , sol–gel-made silica particles, polymers, aerogels, zeolites, metal organic frameworks, etc. The walls of pores restrict the diffusion of absorbed fluid molecules.…”

Recent Progress in Liquid State Electrochemistry Coupled with NMR Spectroscopy

“…However, when the molecules with overlapping signals diffuse at different rates, the signals can be resolved by adding the diffusion dimension to the NMR spectra [108–110] . Contrary to the pure‐shift methods, the NMR diffusometry does not require field uniformity and can be measured in the setup with a stray field [111] or with an inhomogeneous sample [112,113] . The problem, however, arises when a high chemical resolution is required.…”