Functional nanomaterials through esterification of cellulose: a review of chemistry and application
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As the most abundant biopolymer in nature, cellulose has become a fascinating building block for the design of functional nanomaterials. Owing to the presence of numerous hydroxyl groups, cellulose provides a unique platform for the preparation of new materials via versatile chemical modifications. This critical review aims to present the advances about nanomaterials based on cellulose derivatives with the focus on cellulose esters within the last two decades, including the chemistry and application of these nanostructured materials. This review starts with the introduction on first fundamental aspects about diverse esterification techniques used up to now to modify cellulose. The in situ esterification for the isolation of nanocelluloses and diverse post esterification methods of nanocelluloses for the surface functionalization were highlighted in the following description. Various esterification strategies and further nanostructure constructions have been developed aiming to confer specific properties to cellulose esters, extending therefore their feasibility for highly sophisticated applications, which were summarized with respect to the categories of the introduced ester moieties. Thus, this review assembles and emphasizes the state-of-art knowledge of functional nanomaterials derived from diverse esterified cellulose compounds.
KeywordsCellulose Esterification Nanomaterials
Atom transfer radical polymerization
Cellulose 10-undecenoyl ester
Degree of substitution
Lower critical solution temperature
Rhodamine B isothiocyanate
In nature, cellulose is preferentially biosynthesized as fibers via assembly of individual cellulose chains through both intra- and intermolecular hydrogen bonds (Habibi 2014; Somerville 2006). These hydrogen bonds give rise to various three-dimensional (3D) arrangements of the cellulose chains, leading to coexisting of crystalline and amorphous regions within cellulose fibers (John and Thomas 2008; Klemm et al. 2005; Moon et al. 2011; Nada and Hassan 2003). To be more specific in the case of cellulose from plant sources, approximately 36 cellulose chains arrange as a basic fibrillar unit known as elementary fibrils, which have a characteristic lateral dimension of 1.5–3.5 nm with the length up to 100 nm (Chinga-Carrasco 2011; Klemm et al. 2005; Krassig 1990; Yuan and Cheng 2015). These elementary fibrils are further assembled as microfibrils with widths in the range of 10–30 nm, which in turn further assemble into the familiar cellulose macrofibers. However, cellulose from different sources may exhibit different assembling morphologies (Williamson et al. 2002). According to these morphological features, cellulose fibers can be dissociated transversely at the amorphous regions leading to nanoscaled and highly crystalline rod-like fragments, which are referred to as cellulose nanocrystals (CNCs). Similarly, cellulose fibers also can be laterally disintegrated by applying high shear force resulting in nanofibrillated cellulose (NFC) (Habibi 2014). Nanocellulose can also be obtained as bacterial nanocellulose (BNC) after the biosynthesis by bacterial species, such as Gluconoacetobacter xylinum (Brown and Montezinos 1976).
With the presence of three hydroxyl groups per AGU within cellulose chains and on the surface of nanocelluloses, cellulose represents a unique platform for versatile chemical modifications to introduce required functional groups using various techniques to extend their use in a wide range of highly sophisticated applications. All three hydroxyl groups in the AGU including primary hydroxyl group at C6 and secondary hydroxyl groups at C2 and C3 (Fig. 1a) can participate in almost all the reactions as the alcoholic hydroxyl groups do, such as esterification, etherification, oxidation, silylation and polymer grafting (Braun and Dorgan 2009; Braun et al. 2012; Coseri et al. 2013; Dong and Roman 2007; Duan et al. 2016; Filpponen and Argyropoulos 2010; Habibi et al. 2010; Hasani et al. 2008; Ma et al. 2010; Mormann and Demeter 1999; Mormann and Wagner 1997; Pang et al. 2016; Qiu and Hu 2013; Xu et al. 2010; Yoo and Youngblood 2016). Among diverse chemical modifications, esterification represents one of the most promising technique, which was first adopted to synthesize cellulose derivatives (Klemm et al. 1998a). Over the past several decades, there has been extensive research in esterification of cellulose at both polymeric backbone and surface of nancelluloses (Fig. 1b, c). The fundamental aspects of the cellulose esterification, together with highlights of the recent advances about the functionalization of nanocelluloses are considered at first. Then, the potential applications of cellulose esters in the fields of nanomaterials are described. They are by no means a comprehensive summary of all the vast number of research results available, but only of selected pertinent aspects relating to the attached ester moieties primarily of the last two decades.
During the esterification, the reaction either occurs on the whole cellulose polymer chains to form conventional cellulose esters or occurs at the outer of cellulose fibers leaving the cellulose crystalline structure in the interior intact. Both homogenous and heterogeneous esterification can be applied for the synthesis of a vast number of cellulose esters. Moreover, the reactions under heterogeneous conditions can be carried out almost exclusively for the surface modification of native cellulose, which also represents one of the main strategies for the isolation and chemical modification of nanocelluloses.
Over the past several decades there has been extensive research in cellulose esterification. The cellulose esters are usually classified into inorganic and organic cellulose esters. Among the numerous inorganic acids known today, only a few have been employed to synthesize inorganic cellulose esters, such as cellulose nitrate, cellulose sulfate, cellulose phosphate and cellulose xanthate (Heinze et al. 2006, 2018).
Cellulose nitrate is by far the oldest and one of the most important inorganic cellulose esters, which have been produced on an industrial scale for more than one century (Klemm et al. 1998a). Cellulose nitrate is used in many application fields including plastics, lacquers, coatings, explosives and propellants (Heinze et al. 2006; Wertz et al. 2010). The industrial production of cellulose nitrate is generally based on the fast heterogenous equilibrium reaction between cellulose and the classical nitrating acid mixture containing nitric acid and sulfuric acid. The degree of substitution (DS) with rang from 1.8 to 2.8 can be controlled by adjusting the composition of the nitrating acid mixture to meet the various requirements (Klemm et al. 1998a). Using this technique, the maximum DS is limited to around 2.9 due to the side reaction of cellulose with sulfuric acid. Cellulose trinitrate can be achieved using nitrating agent systems of nitric acid/phosphoric acid/phosphorus pentoxide or nitric acid/acetic acid/acetic anhydride (Alexander and Mitchell 1949; Klemm et al. 1998a; Heinze et al. 2006). Furthermore, there are some other nitrating agent systems including dinitrogen pentoxide/tetrachloromethane, nitric acid aqueous and nitric acid/dichloromethane that can be used for the production of cellulose nitrates (Klemm et al. 1998a).
Cellulose sulfate is synthesized by the direct esterification of cellulose using sulfuric acid. Besides sulfuric acid, sulfur trioxide, chlorosulfonic acid, sulfuryl chloride, fluorosulfuric acid, ethyl chlorosulfonate and sulfoacetic acid were employed to produce cellulose sulfates (Klemm et al. 1998a). Cellulose sulfates generally can be prepared by three sulfation routes. The first is sulfation of hydroxyl groups from native cellulose. This usually occurs in a heterogeneous system, which results in non-uniformly distributed substitution, leading to poor solvability in water. To obtain uniformly distributed substitution, partially modified cellulose derivatives can be adopted as starting materials. Using this route, the primary substituent acts as a protecting group. During the sulfation under suitable conditions, the sulfating agents solely react with the free hydroxyl groups (Heinze et al. 2006; Zhang et al. 2010, 2011). Cellulose sulfates with a regioselective distribution of substituents have been synthesized via this rout by partial or complete displacement of a labeled group of a cellulose derivative, usually ester (e.g., nitrite) or ether (e.g., trimethylsilyl) (Fox et al. 2011; Klemm et al. 2005; Richter and Klemm 2003; Zhang et al. 2013). Moreover, the cellulose sulfates can also be synthesized by means of displacement of an ester or ether group already present in cellulose using sulfating agents. A wide variety of cellulose sulfates with regioselective substitution patterns also can be realized via this route (Klemm et al. 1998a; Fox et al. 2011).
The introduction of phosphoric acid ester moieties to form cellulose phosphates can be accomplished by means of pentavalent phosphorus reagents including phosphoryl chloride, phosphorus pentoxide and phosphoric acid (Illy et al. 2015). Similar to sulfation, phosphorylation of cellulose is usually carried out either by reaction with unmodified cellulose, or with cellulose derivatives containing specific substituents (Klemm et al. 1998a). Using the former route, the reaction usually occurs in a heterogeneous system or employs a cellulose solution in non-derivatizing solvent systems, such as N-methylmorpholine N-oxide, lithium chloride (LiCl)/dimethylacetamide (DMAc) and dinitrogen tetroxide/dimethylformamide (DMF) (Klemm et al. 1998a). In the latter route, a homogeneous system is generally preferred using completely or partially substituted cellulose esters or ethers in order to arrive at soluble products (Klemm et al. 1998a; Heinze et al. 2006). In comparison to sulfating agents, most of phosphorylating agents show a lower reactivity in esterification and lead to much less chain degradation. Moreover, cellulose phosphates tend to cross-linking due to the formation oligo-phosphate side chains, which impedes products solubility (Heinze et al. 2006).
Esterification of cellulose for the introduction of organic functional groups is among the most versatile transformations of chemical modifications of cellulose. It gives ready synthetic access to a wide range of valuable products. Esterification of cellulose is acylation procedure using carboxylic acids as acylating agents under strong-acid catalysis or by using an activated derivative such as an anhydride or acid chloride, either with base or with a Lewis acid (Heinze et al. 2006). Due to the low reactivity of carboxylic acids, it is not capable to esterify cellulose to a significant extent using the former esterification procedure. The most traditional method for the acylation of cellulose is the reaction with carboxylic acid anhydrides or acid chlorides.
It should be noted that the introduction of more complex carboxylic acid moieties including fatty acid moieties and aromatic groups, anhydrides are not reactive enough. In this case, acid chlorides in combination with a tertiary base, i.e. pyridine and triethylamine, are applied (Heinze et al. 2006). This procedure is widely used for the preparation of cellulose fatty acid esters with different lengths of aliphatic chains (Crepy et al. 2011; de Menezes et al. 2009; Granstrom et al. 2011; Kulomaa et al. 2015; Zhang et al. 2015a, b). In the case of the esterification in pyridine, pyridine not only acts as the solvent, but also acts as a catalyst via forming a reactive intermediate driving the reaction forward. Cellulose esters with aromatic groups are basically accessible via the same path, but the relating studies are still rare (Garces et al. 2003).
It should be noted that a few new synthesis pathways have been developed over the past years for more effective esterification to introduce new functional groups with more complex chemical structures. One of these synthetic approaches is the in situ activation for the conversion of cellulose with carboxylic acids (Heinze et al. 2006). These reactions are normally carried out in the mild reaction conditions, which avoids the common side reactions including pericyclic reactions, hydrolysis, and oxidation. During these reactions, the carboxylic acids are activated by a reagent, which leads to an intermediately formed highly reactive carboxylic acid derivative. The activation of carboxylic acids with p-toluenesulfonyl chloride (TosCl) (Heinze and Liebert 2001; Heinze et al. 2003; Shimizu et al. 1991; Tosh et al. 2000; Xu et al. 2011; Zheng et al. 2015) and N,N′-dicyclohexylcarbodiimide in combination with 4-pyrrolidinopyridine or 4-dimethylaminopyridine (Fujisawa et al. 2011; Samaranayake and Glasser 1993; Wang et al. 2014; Wu et al. 2004; Yue and Cowie 2002) are typical examples of this synthetic technique (Grabner et al. 2002; Heinze et al. 2018). Homogeneous esterification of cellulose was carried out via in situ activation with TosCl in DMAc/LiCl for the synthesis of 3-(hydroxyphenylphosphinyl)-prop-anoic acid esters of cellulose (Zheng et al. 2015). It was found that the DS range from 0.62 to 1.42 could be adjusted by changing the reaction conditions. While the high toxicity and presence of cellulose-degrading side reactions impeded the wide application of these activating agents. Hasani and Westman reported a new commercially available, non-toxic activating agent, namely 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methyl-morpholinium chloride (DMT-MM), for the esterification of cellulose via in situ activation (Fig. 2b) (Hasani and Westman 2007). The resulting cellulose ester has a low DS of 0.67 due to the low activation efficiency. Among others, N,N′-carbonyldiimidazole (CDI) is the most frequently used non-toxic activating agent for the in situ activation esterification of cellulose (Boufi et al. 2008; Heinze and Liebert 2001; Heinze et al. 2006; Liebert and Heinze 2005; Peng et al. 2016). In this case, the acylating agent is N-acylimidazol that readily reacts with cellulose for the synthesis of cellulsoe ester and regeneration of to imidazole (Heinze et al. 2018).
Moreover, transesterification under the presence of catalysts has been used for the formation of cellulose esters. The preparation of cellulose esters with long aliphatic chains via transesterification with methyl esters have been studied (Antova et al. 2004). In the new transesterification approach, the vinyl esters of the carboxylic acids are predominantly investigated (Cao et al. 2014; Cetin et al. 2009; Ding et al. 2017). Heinze et al. (2000) reported that cellulose could dissolve in dimethyl sulfoxide (DMSO)/tetrabutylammonium fluoride and reacted with vinyl esters homogeneously with or without catalyst. Cetin et al. (2009) demonstrated that CNCs could react with vinyl acetate in DMF under the catalysis of K2CO3, producing acetylated CNCs. However, long pretreatment and/or reaction times from hours to days were required for most of the above-mentioned transesterification reactions, leading to relatively low DS of lower than 2 for many of them even under homogeneous conditions. Cao et al. (2013) developed a new reaction system composed of DMSO, aqueous NaOH or KOH, and vinyl esters to rapidly synthesize cellulose esters by transesterification (Fig. 2c). Remarkably, cellulose could react with vinyl acetate, vinyl propionate, and vinyl butyrate, leading to corresponding cellulose acetate, cellulose propionate, and cellulose butyrate with a high DS of higher than 2 in 5 min under heterogeneous conditions. The authors claimed that the fast reaction is due to the volatile acetaldehydes formed by tautomerization of the produced vinyl alcohol. This can effectively prevent the occurrence of the reverse reaction and, promote the formation of cellulose esters. This type of reaction with very short reaction time is in sharp contrast to the required reaction time of hours in previously existing methods. With the development of ionic liquids, transesterification is also applied in diverse ionic liquid systems heterogeneously for the modification of cellulose (Brand et al. 2017; Hufendiek et al. 2016; Söyler and Meier 2017; Schenzel et al. 2014; Wen et al. 2017).
Esterification of nanocelluloses
Summary of main esterification of nanocelluloses
Sulfuric acid, phosphoric acid
Stable dispersing in water due to the presence of negatively charged ester groups
Acetic acid, butyric acid, citric acid, malic acid, malonic acid
Applied for the one-pot isolation of CNCs with desired functions
Succinic anhydride, n-dodecyl succinic anhydride, hexanoyl
Together with mechanical shearing for the one-pot isolation of CNF
Rao et al. (2015)
Vinyl acetate, vinyl cinnamate, canola oil fatty acid methyl ester
Under mild conditions with long reaction time
Using palmitoyl chloride vapor
Iso-octadecenyl succnic anhydride, n-tetradecenyl succinic anhydride
Esterified in solid state
Yuan et al. (2006)
Using esterifying agents as solvents
Ramirez et al. (2017)
Aromatic carboxylic acids
During the esterification of nanocelluloses, the reaction either solely occurs on the surface of nanocelluloses or occurs inside crystal as bulk reaction, which highly depends on the esterification strategies and reaction conditions. Sassi and Chanzy studied the structural aspects of acetylation of cellulose using a mixture of acetic acid and acetic anhydride, and using toluene as non-swelling agent to stop swelling and dissolution of acetylated chains (Sassi and Chanzy 1995). With the presence of the toluene as non-swelling agent, acetylated chains remain insoluble and surrounded the crystalline core of unreacted cellulose chains, leading to great degrees of acetylation without imparting the morphological features. In contrast, without the presence of non-swelling agent, acetylated chains are stripped from the surface of the crystal into solution, leading to severe morphological change (Sassi and Chanzy 1995). Eyley and Thielemans applied a quantitative strategy using a term of surface degree of substitution with the maxium value of 1.5 to assess the level of modification carried out on CNCs (Eyley and Thielemans 2014). In contrast to CNCs, quantification of surface modification on NFC and BNC is more challenging using this mehod due to diverse of the crystalline structures and difficulty to measure the size of the naofibers precisely. Furthermore, the modification level can be, to some extent, qualitatively verified by examining the changes of crystallinity structure and morphology before and after the modification reactions. This review emphasizes more particularly on the diverse functional groups introduced on nanocelluloses via esterification routes in order to confer to specific properties. While, it will not be discussed specificity whether the reaction solely occurs on the surface of nanocelluloses.
In situ esterification during the isolation of nanocelluloses
The main in situ esterification reactions for the isolation of CNCs are sulfation and phosphorylation that occur during the hydrolysis process (Chen et al. 2014; Espinosa et al. 2013; Klemm et al. 2011; Lu et al. 2015b; Revol et al. 1994). During the isolation of CNCs via hydrolysis, sulfuric acid or phosphoric acid reacts with the surface hydroxyl groups via an esterification process allowing the introduction of anionic sulfate ester groups or phosphate ester groups. The sulfation and phosphorylation levels depend highly on diverse parameters including temperature, acid concentration, reaction time, and ratio of acid to cellulose. Compared with phosphorylation, sulfation results in a much higher content of sulfate groups on the surface of resulting CNCs (Espinosa et al. 2013).
This one-pot strategy also has been applied to produce surface-esterified NFC together with mechanical shearing. Herrick et al. (1983) presented a method for the isolation of acetylated NFC using a mixture of acetic acid and acetic anhydride with sulfuric acid as catalyst. Huang and coworkers reported a similar one-step procedure via mechanochemical strategy in an organic solvent aiming to esterify and defibrillate cellulose fibers simultaneously (Huang et al. 2012, 2013, Huang et al. 2016; Kang et al. 2017; Rao et al. 2015). The method consists of ball milling solid cellulose in a non-aqueous solvent loaded with an esterifying agent. The authors claim that the organic solvents and esterifying agents have dramatic effects on nanoscale dispersion and surface derivatization of NFC. Milling cellulose with hexanoyl chloride in DMF gave hexanoylated NFC with excellent dispersibility in several organic solvents according to the redispersing results in diverse solvents, and milling cellulose with pentafluorobenzoyl chloride in the mixture of pyridine and DMF resulted in hydrophobic fluorinated NFC (Huang et al. 2012, 2013; Rao et al. 2015). Water-dispersible succinylated CNF was also obtained by milling cellulose fiber with succinic anhydride in DMSO for 20 h (Huang et al. 2012, 2016). The produced CNFs are around 20 nm wide and several micrometers long.
Moreover, a new method leading to novel surface-esterified cellulose nanoparticles (NPs) after a one-step esterification of cellulose fibers under heterogeneous conditions was developed using fatty acid chlorides in pyridine and a follow-up purification process (Fig. 3b) (Wang et al. 2015b, 2017). The obtained surface-stearoylated cellulose NPs and surface-undecenoated cellulose NPs have sphere-like morphology with a relatively high size distribution from a few dozens to hundreds of nanometer. Both surface-esterified cellulose NPs have high DS of around 1.4, which would result in significant depletion of the interchain hydrogen bonds, leading to complete conversion of surface hydroxyl groups to esters. With the presence of numerous fatty acid ester groups on the surfaces, they were well dispersible in various non-polar organic solvents, such as, tetrahydrofuran, dichloromethane, cyclohexane, which significantly promoted their compatibility with non-polar compounds for the formation of functional composites.
Post esterification of nanocelluloses
Owing to its ease and straightforwardness, modification of hydroxyl groups present at the surface of nanocelluloses through esterification is widely used. Sulfation has been conducted to introduce stable electrostatic charges on the surface of nanocelluloses for more stable aqueous dispersions. In addition to the in situ sulfation during the isolation of nanocelluloses via sulfuric acid-catalyzed hydrolysis, CNCs produced by hydrochloric acid hydrolysis could also be post-sulfated using sulfuric acid to introduce sulfate moieties in a controlled fashion (Araki et al. 1999, 2000).
Nanocelluloses and functionalized nanocelluloses are excellent reinforcing components for the construction of materials with diverse shapes, such as films, fibers and aerogels (Eichhorn 2011; Klemm et al. 2011; Lam et al. 2012; Moon et al. 2011; Olsson et al. 2010; Walther et al. 2011). The dispersibility of nanocelluloses within the matrix and their interfacial interaction with other matrix components play pivotal roles for the final properties of the obtained nanocomposite materials (Fujisawa et al. 2013). The poor dispersibility of nanocelluloses in non-polar solvents and weak interactions with non-polar synthetic polymers are the main drawbacks limiting the full performance of nanocelluloses. In order to improve all these issues, nanocelluloses are generally surface-modified with functional groups, such as alkyl groups, synthetic polymer chains via “grafting to” or “grafting from” techniques (Fujisawa et al. 2011; Habibi et al. 2008; Johnson et al. 2011; Kan et al. 2013; Siqueira et al. 2009).
Surface-modified nanocelluloses by alkylacyl chains are supposed to be well miscible with other synthetic polymers and exist as reinforcing nanofillers in diverse materials, including films and foams (Blaker et al. 2009; Fujisawa et al. 2011; Habibi et al. 2010; Johnson et al. 2011; Siqueira et al. 2009). Generally, a post esterification of hydroxyl groups on nanocelluloses surface has been used for the immobilization of alkylacyl groups on nanocelluloses surface. Among diverse post esterification reactions for the introduction of alkyl groups, acetylation of nanocelluloses is the most widely investigated approach. The acetylation of nanocelluloses could be conducted using acetic anhydride in the presence of catalyst such as sulfuric acid, perchloric acid and pyridine. These procedures have been applied to produce surface-acetylated nanocelluloses using CNCs (Kim and Song 2016; Naeli et al. 2017; Sassi and Chanzy 1995; Yang et al. 2013), NFC (Fahma et al. 2014; Mashkour et al. 2015; Rodionova et al. 2011) and BNC (Kim et al. 2002; Tome et al. 2011). Furthermore, a novel straightforward route using citric acid as catalyst for the surface esterification of CNCs was proposed (Ramirez et al. 2017). Only the acetic anhydride was used in sufficient excess to allow CNCs dispersion and proper suspension agitation, while no additional solvent was required. By tuning the amount of loaded catalyst, surface-acetylated CNCs with different DS (i.e. DS = 0.18 and 0.34) were obtained. Under the moderate conditions at 120 °C for 3 h, only the surfaces of CNCs were esterified, while the initial crystalline structure of CNCs remained unaffected during the chemical treatment.
Furthermore, transesterification has been adopted in the modification of nanocelluloses using vinyl esters for the attachment of diverse acyl moieties including acetyl (Cetin et al. 2009; Sebe et al. 2013), cinnamoyl (Sebe et al. 2013) and maleyl (Yuwawech et al. 2017). The group of Gilles Sèbe studied the effect of reaction time on the acetylation of CNCs by transesterification of vinyl acetate in DMF at 94 °C using potassium carbonate as catalyst (Cetin et al. 2009). During the first stage of the reaction (less than 2 h), only the surface of the CNCs was modified, while their dimensions and crystallinity remained unchanged. By increasing the reaction time, the inner crystallites were increasingly attacked by the vinyl acetate, leading to an erosion of the CNCs structure and loss of crystallinity. But, the DS of the acetylated CNCs under these reactions has not been reported. Wei et al. (2017) esterified the CNCs successfully by a sustainable and green transesterification approach using vegetable oil fatty acid methyl ester for the first time. After transesterification, the degree of crystallinity and crystalline structure of nanocrystals were not changed, but the esterified CNCs showed higher thermal stability and smaller particle size than unmodified CNCs.
Peng et al. (2016) demonstrated a comparative study of various surface esterification methods of CNCs via acid anhydrides, acid chlorides, acid catalyzed carboxylic acids and in situ activated carboxylic acids to introduce acetyl-, hexanoyl-, dodecanoyl-, oleoyl-, and methacryloyl-functions. Acid anhydrides exhibited better grafting efficiency than other reagents as low molecular weight moieties with short aliphatic chains. In addition, utilizing in situ activated carboxylic acids was more viable approach for long aliphatic chain grafts. The preservation of structural morphology and crystallinity of grafted CNCs were confirmed using transmission electron microscopy and X-ray diffraction. The dispersibility of such surface-esterified CNCs in organic solvents was generally improved. In addition, the surface hydrophobization of CNCs by fatty acids, biodiesel, or plant oils was conducted via a green process using an organic solvent as a one-pot method (Yoo and Youngblood 2016).
An environmental friendly surface esterification route was presented by Yuan et al. (2006) using alkenyl succinic anhydride emulsions in water to compatibilize the CNCs with non-polar media. The emulsions were simply mixed with CNCs aqueous suspensions, freeze dried, and the resulting solid was heated to 105 °C. Due to the low DS of around 0.02, the obtained surface-esterified CNCs retained their morphological and crystalline integrity. They were also well dispersible solvents with widely different polarities, such as, DMSO with a very high dielectric constant of 46.45 and 1,4-dioxane with a quite low dielectric constant of 2.21. Another environmental friendly and simple approach, named SolReact, has been developed for a solvent-free esterification of CNCs using aromatic carboxylic acids (Fig. 4b) (Espino-Perez et al. 2014, 2016). In this process, the critical point is the use of an in situ solvent exchange strategy for utilizing the aromatic carboxylic acids as grafting agent as well as solvent media. Furthermore, the reactant can be easily recycled and does not suffer from chemical degradation due to moderate reaction temperatures.
Grafting of polymer chains on the surface of nanocelluloses can be achieved by esterification directly or indirectly via “grafting to” or “grafting from” approaches. The “grafting to” approach was used to graft maleated polypropylene by esterification onto the surface of CNCs in the suspension of toluene (Ljungberg et al. 2005). The resulting grafted CNCs showed very good compatibility and high adhesion when dispersed in atactic polypropylene. A similar approach was described by Mulyadi et al. who grafted maleated styrene block copolymers on the surface of NFC through esterification (Mulyadi and Deng 2016). The grafted polymer fraction of 25 wt% by gravimetric measurement was obtained. The presence of the grafted polymer promoted the surface hydrophobicity and better thermal stability. Furthermore, a significant number of surface esterification reactions were used as precursors for further polymerization on the surface of nanocelluloses via “grafting from” approach. For instance, 2-bromoisobutyryl bromide (BriBB) as the initiator agent was attached to the hydroxyl groups of nanocellulose by esterification for further polymerization (Majoinen et al. 2011; Wu et al. 2015; Yi et al. 2008). This strategy has been used extensively for the creation of initiating sites for the polymerization on the surface of nanocellulose. Moreover, Wang et al. (2016a) developed a multi-step approach using esterification as first step to attach bis(acyl)phosphane oxide photoinitiators on CNCs surface for polymer grafting.
Esterification represents one of the most versatile transformation strategy of cellulose as it provides easy access to a variety of functional cellulose-based materials. In the past few decades, the investigation and utilization of esterified cellulose compounds in functional nanomaterials have attracted a tremendous level of attention because of their exceptional properties. Various functional nanomaterials using esterified cellulose compounds have been developed for a broad range of applications, which include but not limited to sensors, mechanical reinforcement, biomedical materials and interfacial materials (e.g. superhydrophobic surfaces) (Dong et al. 2014; Geissler et al. 2013; Heinze et al. 2006; Mulyadi and Deng 2016; Sehaqui et al. 2014; Zhang et al. 2015b). The properties and potential applications mainly depend on the introduced ester moieties.
Esterified cellulose containing charged moieties
The charged ester moieties, such as sulfate ester groups, can be introduced in cellulose chains using inorganic acids. The cellulose sulfates show excellent rheological and gel-forming properties, allowing themselves for potential applications as film-forming materials, anionic polyelectrolytes, and biologically active compounds (Klemm et al. 2005; Klemm et al. 1998a). Thus, over the past few decades, cellulose sulfates have undergone intensive study (Kamide and Saito 1994; Mestechkina and Shcherbukhin 2010; Zhang et al. 2015c). Due to the presence of negative charged sulfate ester groups, cellulose sulfates exhibit unique biological properties (Zhang et al. 2015c), leading to a wide use in biotechnology and pharmaceutics to encapsulate enzymes and cells (Bucko et al. 2005; Vikartovska et al. 2007), as inhibitors for HIV viruses and anticoagulant effectors (Agarwal et al. 2010; Van Damme et al. 2008).
Moreover, the negative charge of sulfated cellulose provides accessibility to electrostatic adsorption and conjunction with cationic groups or molecules (Huang et al. 2014). Horikawa et al. prepared a highly conductive poly(3,4-ethylene-dioxythiophene) (PEDOT) system using sulfated cellulose as dopants (Horikawa et al. 2015). PEDOT/sulfated cellulose composite films were prepared via in situ oxidative polymerization of 3,4-ethylene-dioxythiophene in an aqueous solution of sulfated cellulose, followed by the formation of films via spin-coating. It was confirmed that the electrical conductivity of PEDOT was enhanced by doping with sulfated cellulose. Novel oriented surfaces were prepared to promote skeletal muscle myogenesis (Dugan et al. 2013). The orientation was achieved by depositing a monolayer of negatively charged sulfated CNCs on a positively charged polyelectrolyte surface using a flexible and facile spin-coating method. Wang et al. (2016b) reported that sulfated BNC is a promising material for the preconcentration and separation of heavy metals. Furthermore, Thielemans et al. (2009) prepared nanostructured thin films of sulfated CNCs using a simple drop-coating procedure. The negatively charged sulfate groups inhibit the transfer of negatively charged species through the film, while the diffusion of neutral species is only slightly hindered. More specifically, the positively charged species including Ru(NH3) 6 3+ was adsorbed by the film, whereas the negatively charged species, such as, IrCl63−, were excluded by the film.
The other charged ester group is phosphate ester group. The introduction of phosphate groups to cellulose chains via the formation of ester bonds significantly decreases the inflammability of cellulose. During combustion, phosphorus generates a polymeric form of phosphoric acid as a char layer, which acts as a shield protecting the material from oxygen (van der Veen and de Boer 2012). Thus, cellulose phosphates have potential to be used as flame-retardant materials (Aoki and Nishio 2010; Cullis et al. 1992; Ghanadpour et al. 2015; Pan et al. 2014). Ghanadpour et al. (2015) prepared thermal stable and flame-retardant nanopaper sheets using phosphorylated NFC (Fig. 5b). The resulting nanopaper sheets showed self-extinguishing properties after consecutive applications of a methane flame for 3 s and did not ignite under a heat flux of 35 kW/m2. By introducing the anionic phosphate groups into the cellulose backbone, cation-exchange properties are conveyed to the polymer chains, showing excellent chelating properties. Thus, cellulose phosphates were used as metal-chelating polymers, as cation exchange materials and as adsorbents for the treatment of pollution (Bezerra et al. 2014; Illy et al. 2015; Li et al. 2002; Oshima et al. 2008; Padilha et al. 1995). Furthermore, phosphorylated BNC was found to be effective as an adsorbent for proteins with a high adsorption capacity via electrostatic interaction (Oshima et al. 2011).
Moreover, cationic amino groups were introduced to cellulose backbone by ring-opening esterification reaction using various lactams (Zarth et al. 2011). The resulting cationic esters are capable of forming polyelectrolyte complexes as capsules for drug delivery. Cationic pyridinium groups were grafted onto CNCs via a one-pot simultaneous esterification using 4-(bromomethyl)benzoic acid or 4-(1-bromoethyl)benzoic acid and TosCl in pyridine (Vandamme et al. 2015). Resulting positively charged CNCs were relatively insensitive to the inhibition of flocculation by algal organic matter showing potential application for microalgae harvesting. Imidazole-grafted CNCs with a low DS of 0.06 were successfully synthesized by in situ esterification with 4-(1-bromo-methyl)benzoic acid activated by TosCl (Eyley et al. 2015). The resulting imidazole-grafted CNCs were shown to have a pH-responsive flocculation property due to switching of the surface charge, which can be adjusted using CO2.
Esterified cellulose containing aliphatic moieties
Furthermore, as a commercial product, cellulose acetate can be used as a starting material for further modification to form advanced materials. For example, superhydrophobic nanofibrous mat is obtained via electrospinning technique of surface-modified cellulose acetate using perfluoroalkoxysilanes (Arslan et al. 2016). The introduction of the perfluoroalkyl groups tailored their chemical and physical features as oil–water separation materials (Fig. 6b). Chen et al. (2009) found that asymmetric ultrafiltration membranes of cellulose acetate-graft-polyacrylonitrile copolymers exhibited remarkably high water permeability of about 100 times higher than the pure cellulose acetate membranes, leading to excellent oil/water separation performance.
Esterified cellulose containing aromatic moieties
Espino-Perez et al. (2016) esterified CNCs surfaces with aromatic functions using phenylacetic acid or hydrocinnamic acid. These CNCs with aromatic functionalities at surface showed macroscopically hydrophobic and water-repellent characters, while the water vapour sorption isotherms were only slightly affected. Moreover, such CNCs were able to reversibly take up large quantities of the volatile aromatic compound anisole, while the non-aromatic compound cyclohexane was much less absorbed. Furthermore, diverse hydrophobic dye molecules could be incorporated into NPs prepared by self-assembly of hydrophobic cellulose acetate phthalate (Schulze et al. 2016). The thermal reactive carboxyl moieties in phthaloyl groups were further employed for coupling C-reactive protein anti-bodies. These composite NPs based on cellulose acetate phthalate were well suitable as dye labels in immunoassay applications.
Esterified cellulose containing terminal active moieties
In addition, Navarro et al. have chemically modified NFC with furan and maleimide groups through esterification with 2-furoyl chloride and a Diels–Alder cycloaddition with 1,1′-(methylenedi-4,1-phenylene)bismaleimide (Navarro et al. 2015). The modified NFC fibers were selectively labeled with fluorescent probes, i.e. 7-mercapto-4-methylcoumarin and fluorescein diacetate 5-maleimide, through two specific click chemistry reactions as Diels–Alder cycloaddition and Thiol-Michael reaction. These two luminescent dyes could be selectively labeled onto NFC, yielding a multicolored NFC that could be imaged using a confocal laser scanning microscope (Fig. 10b). In addition, Kim et al. (2015) prepared a novel group of robust aerogels based on maleic acid-grafted NFC, which exhibited good network stability in water and springiness after compression. Such advantageous mechanical properties are derived from the grafted maleic acid that reacted with hypophosphite forming a chemically cross-linked network.
Esterified cellulose containing other more complex structures
Esterification of cellulose is among the most versatile modifications leading to a wide range of structural and functional types with valuable properties. The current review attempts to provide a general overview of chemical transformations of cellulose via esterification for the functional applications. We emphasized various methodologies, materials and achievements for esterified cellulose compounds and provided an overview of their applications as functional materials on a large scale. From the scientific point of view, esterification can yield a broad spectrum of cellulose ester derivatives with DS in the range of 0–3, which were promoted and expanded continually due to the introduction of new esterification methodologies. The maintained challenges are the precise esterification for the introduction of functionalities onto cellulose in diverse size scales including cellulose polymeric chains, nanocellulose, and cellulose microfibers and at the same time the persistence of cellulose polymeric chains or supramolecular morphologies.
Remarkably, nanocelluloses including CNCs, NFC, BNC and other unconventional nanocelluloses are currently the objects of intense scientific curiosity and have been intensively studied over the last 10 years. A wide variety of esterification approaches have been carried out on nanocelluloses ranging from simple in situ esterification to sophisticated post surface modifications, which all impart desired functions to the surface of nanocelluloses. Most of these approaches carried out in nanocelluloses have concentrated on the compatibilization of nanocelluloses with other matrices via turning their hydrophilic nature for the formation of composite functional materials. Esterification has leads to the highest reported surface DS at around 1.5, but the average DS is usually much lower to avoid any damage either to the morphology or to the native crystalline structure of nanocelluloses.
To achieve a broad understanding of the application of esterified cellulose compounds, the review touched upon selected important ester moieties that can lead to advanced materials in many fields including drug delivery, tissue engineering, water purification, catalysis, electrical devices, sensing and more. To be more specific, the conventional cellulose esters, such as cellulose sulfates, cellulose acetates and cellulose fatty acid esters, have been intensively studied to develop new advanced functional materials. Meanwhile, a wide variety of new functional ester moieties, such as pyridinium, chromene, coumarin, rhodamine spiroamide and polymeric chains, have been introduced by esterification in cellulose directly or indirectly to import new properties.
Y.W. thanks the Northeast Forestry University for a startup grant with the project number of 000/41113296. K.Z. thanks German Research Foundation (DFG) with the Project Number of ZH546/2-1 for the financial support. X.W. thanks the China Scholarship Council (CSC) for financial support.
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