Citation: Khumalo, N.L.; Mohomane, S.M.; Motaung, T.E. Effect of Acetylation on the Morphology and Thermal Properties of Maize Stalk Cellulose Nanocrystals: A Comparative Study of Green-Extracted CNC vs. Acid Hydrolysed Followed by Acetylation. Crystals 2024, 14, 636. https:// doi.org/10.3390/cryst14070636 Academic Editor: Guanying Chen Received: 7 June 2024 Revised: 2 July 2024 Accepted: 7 July 2024 Published: 10 July 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). crystals Article Effect of Acetylation on the Morphology and Thermal Properties of Maize Stalk Cellulose Nanocrystals: A Comparative Study of Green-Extracted CNC vs. Acid Hydrolysed Followed by Acetylation Nduduzo Lungisani Khumalo 1, *, Samson Masulubanye Mohomane 1 and Tshwafo E. Motaung 2,3 1 Department of Chemistry, KwaDlangezwa Campus, University of Zululand, Empangeni 3886, South Africa; mohomanes@unizulu.ac.za 2 Department of Chemistry, School of Science, College of Science Engineering and Technology, University of South Africa, Preller Street, Muckleneuk Ridge, P.O. Box 392, Tshwane 0003, South Africa; motaungte@live.com 3 Department of Chemistry, Sefako Makgatho Health Science University, P.O. Box 94, Medunsa 0204, South Africa * Correspondence: khumalonl@unizulu.ac.za Abstract: This study highlights the advantages of employing acetylation to enhance the morphology and thermal properties of cellulose nanocrystals (CNCs) derived from maize stalks. Utilizing a green synthesis approach for CNC extraction, this research presents a novel comparison between green extracted CNCs, and their acid hydrolysed, followed by their acetylated counterparts (ACCNCs). This comparison reveals significant improvements in the properties of acetylated CNCs over those produced through conventional acid hydrolysis. The study employs advanced characterization techniques, including Fourier Transform Infrared (FTIR) spectroscopy, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Thermogravimetric Analysis (TGA), to analyze untreated maize stalk extracted cellulose, green extracted CNCs, and acetylated CNCs. FTIR spectroscopy identifies changes in functional groups, underscoring the efficacy of the extraction and modification processes. XRD analysis demonstrates a beneficial transformation from cellulose I to cellulose II allomorphs post-acetylation, with increased crystallinity index values indicating effective removal of amorphous regions. SEM imaging reveals the preservation of rod-like structures in CNCs, while acetylated CNCs exhibit advantageous morphological changes, such as reduced nanocrystal length and increased branching. TGA results show superior thermal stability in green extracted CNCs and favorable thermal degradation behavior in acetylated CNCs. Overall, this study underscores the potential of acetylation to develop sustainable nanomaterials with tailored properties, offering significant advancements for various applications. Emphasizing the advantages of the prepared ACCNCs and the green synthesis method over traditional acid hydrolysis extraction, this research paves the way for innovative applications in diverse fields. Keywords: maize stalk; cellulose; nanocrystals; acetylation 1. Introduction Cellulose nanocrystals (CNCs) have emerged as highly versatile and sustainable nanomaterials, captivating interest across diverse industries including food packaging, cosmetics, polymers, and biomedical applications [1–5]. Their abundant availability, re- newable nature, exceptional mechanical strength, low density, biodegradability, and potent reinforcing capabilities render them appealing for a broad spectrum of uses [6,7]. Crucially, the extraction process and the source of cellulose significantly influence the properties of CNCs, with acid hydrolysis standing out as the predominant extraction Crystals 2024, 14, 636. https://doi.org/10.3390/cryst14070636 https://www.mdpi.com/journal/crystals