Morphology of cross-linked cellulose nanocrystal aerogels: cryo-templating versus pressurized gas expansion processing
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Cellulose nanocrystal (CNC)-based aerogels are often produced through cryo-templating, followed by either critical point drying or freeze drying. While cryo-templating gives aerogels with a bimodal pore size distribution, better morphological control may be needed for certain applications. This work compares CNC aerogels prepared using a new processing method, called pressurized gas expansion (PGX) technology, to aerogels produced via cryo-templating. In all cases, CNCs were surface-modified with orthogonal functional groups to produce covalently cross-linked aerogels which are flexible and do not disperse in water. The aerogels were imaged by scanning electron microscopy and X-ray micro-computed tomography and further characterized by nitrogen sorption isotherms, X-ray diffraction, X-ray photoelectron spectroscopy, and compression testing. PGX aerogels appeared expanded and fibrillar at high magnification, with small mesopores and macropores less than 7 µm, but with large mound-like porous aggregates. Conversely, cryo-templated aerogels were comprised of denser CNC sheets surrounding macropores of 10–950 μm. Overall, PGX aerogels had a lower density, higher porosity, and a higher specific surface area than cryo-templated aerogels; they were also less stiff due to their morphology and reduced number of chemical cross-links. Scale-up of aerogel processing and understanding of the tunability of such methods may extend the use of CNCs in applications including insulation, separations, flexible supports, drug delivery, and template materials.
The authors would like to thank Ceapro Inc. for their cooperation, useful discussions, and access to PGX equipment. Funding is from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants RGPIN 402329 and RGPIN 588778. NSERC Engage Grant 492456-15 with Ceapro Inc., Alberta Innovates project ABI-15-001, and support from the Faculty of Engineering at McMaster University are gratefully acknowledged. We also thank N. Kudeba and L. Delgado at Ceapro for help with the production of PGX samples. We thank Professors T. Hoare, R. Pelton, A. Guarne, the Canadian Center for Electron Microscopy (CCEM), the McMaster Automotive Resource Center (MARC), McMaster Biointerfaces Institute, and the University of Waterloo for shared equipment. A. Kacheff, K.A. Johnson, I. Shahid, J. Tedesco, M. Reid, V. Jarvis, N. Belsito, T. Stimpson, and Dr. D. Covelli are gratefully acknowledged for training, sample analysis, and expertise. D. O. would like to extend a special thanks to X. Yang, for his pioneering work on CNC aerogels and guidance throughout this work.
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