Cellulose nanofibril-based aerogels have promising applicability in various fields; however, developing an efficient technique to functionalize and tune their surface properties is challenging. In this study, physically and covalently crosslinked cellulose nanofibril-based aerogel-like structures were prepared and modified by a molecular layer-by-layer (m-LBL) deposition method.
Carbohydrate Polymers 282 (2022) 119098 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Surface tailoring of cellulose aerogel-like structures with ultrathin coatings using molecular layer-by-layer assembly Zhaleh Atoufi a, Michael S Reid a, Per A Larsson a, Lars Wågberg a, b, * a b Division of Fibre technology, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56–58, SE-100 44 Stockholm, Sweden KTH Royal Institute of Technology, Department of Fiber and Polymer Technology, Wallenberg Wood Science Center (WWSC), Stockholm, Sweden A R T I C L E I N F O A B S T R A C T Keywords: Cellulose nanofibril Aerogels Molecular layer-by-layer deposition Surface functionality Wet strength Oil absorption Cellulose nanofibril-based aerogels have promising applicability in various fields; however, developing an effi cient technique to functionalize and tune their surface properties is challenging In this study, physically and covalently crosslinked cellulose nanofibril-based aerogel-like structures were prepared and modified by a mo lecular layer-by-layer (m-LBL) deposition method Following three m-LBL depositions, an ultrathin polyamide layer was formed throughout the aerogel and its structure and chemical composition was studied in detail Analysis of model cellulose surfaces showed that the thickness of the deposited layer after three m-LBLs was approximately nm Although the deposited layer was extremely thin, it led to a 2.6-fold increase in the wet specific modulus, improved the acid-base resistance, and changed the aerogels from hydrophilic to hydrophobic making them suitable materials for oil absorption with the absorption capacity of 16–36 g/g Thus, demon strating m-LBL assembly is a powerful technique for tailoring surface properties and functionality of cellulose substrates Introduction Substituting petroleum-derived, non-biodegradable materials with sustainable and biodegradable materials from renewable sources is of great scientific and economic interest (Aalbers et al., 2019) In this re gard, bio-based, sustainable aerogels are considered as promising ma terials for a wide range of applications, including thermal and acoustic insulations (Eskandari et al., 2017; Nguyen et al., 2020), adsorption of liquids and gases (Jatoi et al., 2021), energy storage (Sun et al., 2021), drug carriers (Liu et al., 2021) and catalyst supports (Gu et al., 2021) Aerogels can be prepared from many different sources, such as carbon nanomaterials (Khoshnevis et al., 2018; Pruna et al., 2019), cellulosic nanomaterials such as cellulose nanofibrils (CNFs) and cellulose nano crystals (CNCs) (Cervin et al., 2012; De France et al., 2017), synthetic polymers (Wu et al., 2019), and silica-based materials (He et al., 2018) Among these, CNF-based aerogels, here defined as lightweight materials derived from deaerated CNF gels, have attracted great attention due to the intrinsic properties of CNFs, such as high mechanical strength, high aspect ratio, bio-degradability, and their green preparation process Moreover, due to the numerous hydroxyl groups, CNFs can be func tionalized with various functional groups, leading to diverse properties that can be tailored to the desired application (Tavakolian et al., 2020) In order to achieve CNF-based aerogels with particular physical and chemical properties, CNFs can either be pre-functionalized before forming the aerogel, or aerogels can be post-functionalized to target a specific application CNFs can be modified via chemical modification of hydroxyl groups, such as TEMPO-oxidation (Kim et al., 2021), carbox ymethylation and phosphorylation (Patoary et al., 2021), or via chem ical modifications via ring opening reactions, which typically breaks the C–C bonds of carbons at C2 and C3 position These reactions can further involve chemical grafting of polymers or single molecules to the CNF surface (Abushammala & Mao, 2019; Rol et al., 2019) Although these modifications have become relatively common, with acceptable yields, implementation of these reactions while maintaining dispersion stability can be challenging Specifically, it is difficult to avoid aggregation of the system or to properly remove excess reagents, which can subsequently inhibit aerogel formation Thus, it is often favorable to perform a post functionalization of the aerogels Generally, CNF-based aerogels can be modified via vapor- or liquid-based methods Commonly, silanes, organosilanes or other fluorine-containing chemicals are evaporated and allowed to diffuse through the aerogel (Liao et al., 2016; Yu et al., 2021; Zhu et al., 2020) However, these chemical vapor deposition * Corresponding author at: Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56–58, SE-100 44 Stockholm, Sweden E-mail addresses: zhaato@kth.se (Z Atoufi), mreid@kth.se (M.S Reid), perl5@kth.se (P.A Larsson), wagberg@kth.se (L Wågberg) https://doi.org/10.1016/j.carbpol.2022.119098 Received 27 September 2021; Received in revised form 29 December 2021; Accepted January 2022 Available online 10 January 2022 0144-8617/© 2022 The Authors Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Z Atoufi et al Carbohydrate Polymers 282 (2022) 119098 methods usually involve toxic and expensive chemicals, are diffusion limited and can result in non-uniform modification Another method is Cold plasma which is more environmentally friendly, but similarly limited and require lengthy processing and high discharge energy (Schroeter et al., 2021) As a result, it can be more effective to use liquid modification methods, whereby the capillary forces uniformly wet the aerogel surfaces Although these chemical grafting methods are usually very efficient, they can require complex reactions and processing con ditions which are not favorable for large-scale production (Xu et al., 2019) Therefore, there is a demand for an efficient and fast modifica tion technique which is applicable for cellulose aerogels Molecular layer-by-layer (m-LBL) deposition is a unique technique which allows for the deposition of ultra-thin layers onto a surface through sequential covalent reactions The result is the formation of a precise molecular-scale coating, essentially via surface oligomerization, which cannot be achieved by bulk synthesis methods (Chan et al., 2012; Johnson et al., 2012) With m-LBL, it is possible to precisely control the surface chemistry, roughness and the thickness of the deposited film (Chan et al., 2013) The presence of numerous hydroxyl groups on the surface of CNF-based aerogels make them suitable substrates for m-LBL modification However, CNF-based aerogels must be prepared such that they are mechanically robust and can maintain their structure through the modification process CNF-based aerogels are typically prepared through freeze drying or critical point drying of CNF dispersions (Mulyadi et al., 2016) However, due to the hydrophilicity of CNFs, unmodified aerogels are generally not wet-stable and disintegrate, (Kim et al., 2017) making them unsuitable for m-LBL modification Additionally, scale-up of these aerogels is challenging due to the high energy consumption of freeze drying and critical point drying techniques One way to prepare wet-stable CNFbased aerogel-like materials with scale-up possibility is the so-called freeze-linking technique (Erlandsson et al., 2016), whereby aerogellike structures are prepared through molding and freezing of CNF dispersion followed by solvent exchange and ambient drying During freezing, ice crystal growth forces CNFs together to form a strong micrometer thick interconnected network structure By solvent exchanging to acetone capillary forces are significantly reduced such that the aerogel-like structure is maintained during ambient drying Although freeze-linked aerogel-like structures usually have larger pores and lower specific surface area compared to conventional aerogels, the fact that they can be prepared without using critical point drying or freeze drying is indeed a huge advantage Additionally, by incorporating calcium ions or using aldehyde-containing CNFs, the aerogel-like structure can be physically or covalently crosslinked during freezelinking to obtain wet stability (Erlandsson et al., 2018; Franỗon et al., 2020) We hypothesized that m-LBL modification is an efficient method to tune the surface properties and functionality of CNF-based aerogels In this study, two types of wet-stable CNF-based aerogel-like structures were prepared through a freeze-linking technique: i) hemiacetal cross linked aerogel-likes, where dialdehyde-containing CNFs are chemically crosslinked through hemiacetal bonds formed between the individual CNFs during freezing, and ii) calcium ion cross-linked aerogel-likes, in which calcium ions physically crosslink the structure Both aerogel-likes (hereafter called aerogel) were then modified through m-LBL deposition of trimesoyl chloride (TMC) and m-xylylene diamine (MXD), resulting in an ultrathin polyamide film covering all surfaces throughout the aero gels The chemical structure, atomic composition, thickness and roughness of the deposited film was characterized and the effect of mLBL modification on morphology, wet and dry mechanical properties, acid-base resistance, thermal degradation and surface properties of the aerogels was investigated Finally, the modified aerogels were examined for potential use as oil-water separators for water purification Experimental section 2.1 Materials Carboxymethylated CNFs with a total charge of 600 ± 50 ueq/g were produced according to a previously reported method (Wågberg et al., 2008) and provided by RISE Bioeconomy AB in the form of 20 g/l aqueous gel Branched polyethylenimine (PEI) with the molecular weight of 25,000 Da, poly(allylamine hydrochloride) (PAH) with the molecular weight of 17,500 Da, molecular sieve beads (pore diameter of Å and Å, 8–12 mesh), trimesoyl chloride, m-xylylene diamine, oil red O, toluene (ACS reagent, ≥99.5%), calcium chloride, and sodium hy droxide were purchased from Sigma Aldrich Sodium metaperiodate (99%) was purchased from Acros Organic, Belgium Acetone was pur chased from VWR International (Radnor, PA, USA) Prior to use, water was carefully removed from acetone and toluene with the aid of the molecular sieves All other chemicals were used without further purification 2.2 Sample preparation 2.2.1 Synthesis of calcium ion crosslinked aerogels CNF gels with a concentration of 7.5 g/l were mixed with CaCl2 using an Ultra Turrax (IKA Werke GmbH & Co KG, Staufen, Germany) at 12000 rpm for Total concentration of CaCl2 in the CNF gel was 13.5 mM to achieve a 3:1 ratio of calcium ions to CNF charge groups The gels were then placed into cylindrical polystyrene molds and frozen in a freezer (− 18 ◦ C) overnight to ensure a complete freezing The frozen samples were then thawed and solvent exchanged in acetone followed by ambient drying Calcium ion crosslinked aerogels are abbreviated as Ca-AG in the text 2.2.2 Synthesis of hemiacetal crosslinked aerogels Hemiacetal crosslinked aerogels (HA-AG) were prepared according to an earlier described method (Erlandsson et al., 2016) Briefly, dia ldehyde CNFs were prepared by mixing a 7.5 g/l CNF gel with sodium metaperiodate to a concentration of 60 mM, using an Ultra Turrax disperser at 12000 rpm for The mixture was covered with aluminum foil to prevent exposure to light After h of reaction, the obtained gel was quickly transferred to a mold and frozen (− 18 ◦ C) overnight, thawed and solvent exchanged to acetone, dried under ambient conditions and stored for further use Before use, the aerogels were thoroughly rinsed with water until the conductivity of the washing water was below μS/cm 2.2.3 Preparation of CNF dispersions CNF dispersions were prepared according to an earlier described method (Erlandsson et al., 2018) A 20 g/l CNF gel was diluted to g/l and homogenized with an Ultra Turrax at 12000 rpm for 10 The dispersion was then ultrasonicated at 300 W for 10 using an ul trasonic probe (Sonics VCX 750, Newton, USA) followed by centrifu gation at 4500 rpm for h The stable CNF supernatant was collected and stored in the fridge for further use The dry content was obtained gravimetrically 2.2.4 Molecular layer-by-layer deposition on cellulose aerogels CNF-based aerogels were modified by three cycles of m-LBL depo sition of TMC and MXD molecules to form a thin polyamide film on the surface Each bilayer was deposited using a four-step procedure as shown schematically in Fig First, aerogels were soaked in a solution of wt% TMC in toluene for to ensure that the reactants have penetrated the entire aerogel and reacted with the cellulose surface The aerogels were then soaked in toluene with being the solvent replaced every a total of three times The aerogels were then dried by vacuum filtration to remove excess toluene as to not dilute the reactants in the next step The dried aerogels were then soaked in a wt% solution Z Atoufi et al Carbohydrate Polymers 282 (2022) 119098 Fig a) Molecular layer-by-layer deposition of TMC and MXD on a CNF-based aerogel b) Schematic illustration of the formation of polyamide film by sequential reaction of TMC and MXD monomers with the cellulose surfaces during the deposition The term lamellae in the figure refers to the internal walls of the aerogels that were formed during the preparation of the aerogels of MXD, in toluene, for The MXD modified aerogel was then soaked in acetone with the solvent being replaced every a total of three times, followed by vacuum filtration These steps repeated three times to achieve three bilayers on the surface of the aerogels deposition and scratched and imaged by atomic force microscopy (AFM) Solutions of wt% TMC and MXD in toluene were prepared CNF model surfaces were soaked in TMC solution for 30 s followed by excessive washing with toluene The CNF surfaces were then soaked in MXD solution for 30 s, washed excessively with acetone followed by drying under a gentle flow of nitrogen gas These steps were repeated to achieve the desired number of bilayers The same procedure was used to adsorb TMC/MXD bilayers on the free standing CNF films 2.2.5 Preparation of CNF films CNF films were prepared by vacuum filtration of 75 ml of g/l CNF dispersion through a 0.45 μm membrane (Durapore, Merck Millipore) After filtration, the obtained wet film was dried for 15 at 93 ◦ C and 95 kPa using the dryer of a Rapid-Kă othen sheet former (PTI, Austria) 2.3 Analysis 2.2.6 Preparation of CNF model surfaces The formation of m-LBL layers inside the three-dimensional CNFbased aerogel structures is difficult to study due to the thin nature of the deposited film (