Hardwood Kraft Lignin-Based Hydrogels: Production and Performance Alyssa Zerpa, Leila Pakzad, and Pedram Fatehi* Chemical Engineering Department, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1 ABSTRACT: In this study, hydrogels were synthesized through the radical polymerization of hardwood kraft lignin, N-isopropylacrylamide, and N,N′-methylenebisacrylamide. Statistical analyses were employed to produce lignin-based hydrogels with the highest yield and swelling capacity. The success of the polymerization reactions was confirmed by NMR and Fourier infrared spectroscopy. The lignin-based hydrogel was more thermally and rheological stable, but exhibited less swelling affinity, than synthetic hydrogel. The rheological studies indicated that the swollen hydrogels were predominantly elastic and exhibited a critical solution temperature that was between 34 and 37 °C. Compared with the synthetic hydrogel, lignin-based hydrogel behaved less elastic as temperature increased. In addition to inducing a green hydrogel, the results confirmed that hardwood lignin-based hydrogel would have different properties than synthetic-based hydrogels, which could be beneficial for some applications. ■ INTRODUCTION Hydrogels are often described as three-dimensional polymeric networks formed from cross-linked hydrophilic homopolymers, copolymers, or macromers. 1-4 They are insoluble polymer matrices capable of retaining a large amount of water in their swollen state; in some cases, up to a thousand times of their dry weight. Their swelling capability allows them to obtain the shape of their surroundings when confined. 2,5,6 Depending on the source of polymers, hydrogels may be synthetic, natural, or hybrid. 2 Hydrogels may be degradable in aqueous environments, making them biocompatible in most cases and a good carrier for nutrients to cells and their metabolic products. 5 They have been deemed to be efficient in the protection of cells and fragile drugs, such as peptides and proteins. 5 Some hydrogels have exhibited stimuli-induced swelling and deswelling capabilities without disintegration. 7-9 The advantage of tunable properties has given hydrogels attention for biomedical and environmental applications. 10,11 Poly (N-isopropylacrylamide) (PNIPAAm) is a thermores- ponsive hydrogel. 12-14 However, the applications of PNI- PAAm hydrogels are limited by their fragility. 15,16 These applications can be extended via enhancing their rigidity. Recently, lignin (LGN) has been incorporated into the production of flocculants, dispersants, and hydrogels because lignin is biocompatible and biodegradable with low tox- icity. 17-20 At a relatively low production cost, 21 lignin presents the greatest available aromatic renewable resource worldwide, as well as a primary supplier of soil’s organic matter. 6,22 Lignin provides the structural strength of plants by nature, which makes it a potential candidate for PNIPAAm hydrogel modifications. Previous studies have been conducted on synthesizing hydrogels using different types of lignin through free radical polymerization. Acetic acid lignin was incorporated into N- isopropylacrylamide (NIPAAm) for hydrogel production. 23 The kinetics studies indicated that the lignin-containing hydrogel retained 7.2% more water than the lignin-free hydrogel over 10 min of swelling. 23 In another work, wheat straw alkali lignin was cross-linked with acrylic acid and N,N′- methylenebisacrylamide (MBAAm) via free radical polymer- ization. 24 The addition of acid hydrolysis lignin was found to greatly improve the surface morphology and swelling affinity of the hydrogel. 25 Yu et al. developed lignosulfonate-g-acrylic acid hydrogels by grafting acrylic acid onto lignosulfonate with MBAAm as a cross-linker and lacase/tert-butyl hydroperoxide as the initiator. 26 It is well known that the type of lignin (i.e., hardwood vs softwood vs nonwood) and its production process (i.e., enzymatic hydrolysis vs kraft vs sulfite treatment) affect its structure and chemical properties, which, in turn, affect its polymerization performance and end-use applications. 27-32 There is currently limited research on the use of hardwood kraft lignin for hydrogel production following free radical polymerization. As hardwood is vastly available for end-use applications, it is of great importance to investigate the performance of hardwood lignin for hydrogel produc- tions. 17-21 The first objective of this study was to evaluate the cross- linking of hardwood-based kraft lignin with NIPAAm as the Received: May 29, 2018 Accepted: July 16, 2018 Published: July 24, 2018 Article Cite This: ACS Omega 2018, 3, 8233-8242 © 2018 American Chemical Society 8233 DOI: 10.1021/acsomega.8b01176 ACS Omega 2018, 3, 8233-8242 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Downloaded via 3.239.57.10 on February 14, 2022 at 02:04:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.