Wood Cell Interactions to Hemicellulose-Based

Review pubs.acs.org/CR

Transfer of Biomatrix/Wood Cell Interactions to Hemicellulose-Based Materials to Control Water Interaction Anas Ibn Yaich, Ulrica Edlund, and Ann-Christine Albertsson* Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ABSTRACT: The family of hemicelluloses stands out as a very promising natural resource that can be utilized as a biobased materials feedstock. An in-depth understanding of the hemicellulose inherent structural and property features as well as the structure−property relationships induced by the specific supramolecular hierarchical organization of lignocellulosic biopolymers will be a key enabling technology in the emerging biorefinery sector. This Review aims to give a perspective on these issues and demonstrate how the transfer of molecular wood cell interactions into hemicellulosebased materials may offer new design principles for material formulations.

CONTENTS 1. Introduction 2. Wood Cell-Wall Polymers 2.1. Hemicelluloses 2.2. Lignin 2.3. Cellulose 3. Hemicellulose Function in the Cell Wall 4. Extraction of Hemicelluloses from the Cell Wall 5. Recovery and Fractionation of Hemicelluloses 6. Hemicellulose Characterization 6.1. Compositional Analysis 6.2. Structural Analysis 6.3. Molar Mass Analysis 7. Chemical Modification Extends the Functionality of Hemicelluloses 7.1. Esterification 7.2. Etherification 7.3. Carbonate Ester Linkage 7.4. Oxidation 7.5. Amination 7.6. Amidation 7.7. Miscellaneous Linkages 7.8. Copolymerization (Grafting from/onto) 8. Hemicellulose-Based Films and Coatings 8.1. Oxygen Barriers 8.2. Water Vapor Permeability 8.3. Mechanical Properties 9. Outlook Associated Content Special Issue Paper Author Information Corresponding Author ORCID Present Address © 2017 American Chemical Society

Notes Biographies Acknowledgments References

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1. INTRODUCTION The global demand for biopolymers and renewable materials is growing fast with the growing world population and increasing environmental awareness. To meet this demand and contribute to a more sustainable society, new renewable feedstocks for the production of chemicals, fuels, and materials that do not compete with food supply can be exploited. Among these feedstocks, forest biomass stands out as one of the most promising natural resources that can be exploited for this purpose for several reasons that are related not only to the availability of the raw material and its contents and characteristics but also to the well-established forest management and processing infrastructures present in many parts of the world. Forests cover over 4 billion hectares worldwide, which is equivalent to one-third of the total land area on earth. Very recently, the global number of trees was estimated to be ∼3.04 trillion,1 generating ∼3900 million m3 of wood per year.2 These trees synthesize each yearin the most environmentally friendly mannerbillions of tons of wood-derived polymers, including mostly cellulose, hemicellulose, and lignin. Today we are using cellulose in high-value products, but in the near future crude as well as purified hemicelluloses may also generate products of higher value. Wood is a structurally complex material that exhibits hierarchical organization from the molecular level to the

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Received: December 21, 2016 Published: June 5, 2017 8177

DOI: 10.1021/acs.chemrev.6b00841 Chem. Rev. 2017, 117, 8177−8207

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Figure 1. (A) Model of the wood cell wall structure, depicting the various regions including the middle lamella (ML), the primary wall (P), and the secondary wall in its three layers, S1, S2, and S3. Sectional and face-on views of a bordered pit are shown in the lower portion of the illustration. The figure is reproduced from ref 3 with permission from the USDA Forest Service Forest Products Laboratory, 2010. (B) Example of the variation in the chemical leaf surface composition across the wood cell wall. Adapted with permission from ref 4. Copyright 1980 McGraw-Hill.

macroscale. At the microscopic level, the wood cell walls are typically divided into three regions or layers, the primary wall, the secondary wall, and the middle lamella, all of which are enfolded around an open space known as the lumen (Figure 1). Each layer shows a specific composition and supramolecular assembly, where semicrystalline cellulose fibril aggregates are embedded in an amorphous hydrated matrix of hemicelluloses and lignin.2 At the molecular level, the wood-based polymers feature distinct chemical and structural properties, which inherently enable them to perform a diverse set of functions to meet the needs of the living tree. By identifying these needs and their convergence points with the needs of engineering materials, one can better predict and understand the utility of these biopolymers in an analogue engineering context. In addition to providing mechanical support, an important role of wood cells in trees is in water and mineral transport from the root to the foliage. The cells (vessels and tracheids) are connected to each other through either channels or membranes referred to as pits, which pass water. In the field of plant physiology, the majority of researchers believe that water ascends through the xylem and that it is driven, to a large extent, by the subatmospheric pressure generated by water evaporation at the leaf surface (Figure 2).5,6 However, it is also well-known that the formation or infiltration of gas bubbles into the xylem disrupts the continuous stream of water, causing an interruption in the flow.7,8 The specific contributions of each wood constituent to controlling and managing the liquid/gas flux through the cell -wall tissues are not yet fully understood. Nevertheless, it is fairly evident that woodthrough its polymer constituents and porous structurenot only is able to conduct large quantities of water and minerals but also has a sufficiently high gas barrier in order to cope with low pressure and prevent gas diffusion.

Figure 2. Schematic description of the water transport mechanism in a tree, according the cohesion−tension theory. Figure 2 is reproduced with permission from OpenStax College licensed under a Creative Commons Attribution License 3.0 license (download for free at http://cnx.org/contents/e5aabc6f-71d9-40d5-99f0-0fb2d8d47317@ 5).

structures and biosynthetic mechanisms of polysaccharides were not yet well understood and hemicelluloses were thought to be prepolymers for the synthesis of cellulose. It is now known that hemicelluloses belong to a highly heterogeneous group of noncellulosic polysaccharides containing xylans, mannans and glucomannans, xyloglucans, and β-(1→3,1→4)glucans.9,10 However, this term continues to be used as a

2. WOOD CELL-WALL POLYMERS 2.1. Hemicelluloses

Hemicelluloses constitute between 25 and 35% of the wood dry mass. The term hemicellulose was coined at a time when the 8178


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Figure 3. Representative structural formula of O-acetyl-4-O-methylglucuronoxylan.20

Figure 4. Representative structural formula of O-acetyl galactoglucomannan (AcGGM).20

mol−1, has been reported. A degree of acetylation of 0.6 was determined for birch MeGlcp-xylan.18,19 In softwood, the most common hemicellulose is O-acetyl galactoglucomannan (AcGGM) with the structure suggested in Figure 4. AcGGM consists of a backbone chain containing β(1→4)-linked mannose and glucose units partially substituted with α-(1→6)-linked galactose units. The composition ratio of galactose/glucose/mannose can be either 1:1:3 or 3:1:0.1. Some of the mannose residues may also be partly acetylated at the C-2 or C-3 positions.14

convenient common designation for these polysaccharides. In their native state, hemicelluloses are amorphous and often consist of branched homo- or copolymers with relatively few types of carbohydrate repeating units. These units include anhydrous neutral sugars (D-xylose, D-mannose, D-galactose, Dglucose, L-arabinose, and L-rhamnose) and uronic acids (Dglucuronic, D-galacturonic, and 4-O-methyl-D-glucuronic acids). The repeating units can also be O-acetylated to various degrees, generally at the C-2 and/or C-3 position.11,12 Compared to cellulose, hemicelluloses are short-chained with an average degree of polymerization ranging from 100 to 200.10,13 One of the remarkable aspects of hemicelluloses is their great diversity, for instance, in molecular weight, type of side groups, branching site, degree and frequency of branching, etc., even within the same plant source and within the same hemicellulose category.14,15 The extent of such diversity varies from one plant to another and can be significantly altered by different extraction and upgrading steps. This variability of hemicellulose can be both an opportunity and a challenge: an opportunity in terms of versatility and the possibility of tailoring their properties for a given product and a challenge in terms of dealing with a certain degree of intrinsic heterogeneity and variation associated with the starting material to still be able to reproduce and predict the properties of the final product. The diverse population of molecules within a given hemicellulose category can hardly be represented by a single model compound with a well-defined chemical structure. In hardwood, O-acetyl-4-O-methylglucuronoxylan (MeGlcpxylan) is the most common type of hemicellulose. A typical segment of this molecule is shown in Figure 3.9,14,16 MeGlcpxylan consists of a β-(1→4)-linked partially acetylated D-Xyl unit backbone substituted with α-(1→2)-linked 4-O-methyl-Dglucopyranosyl uronic acid (MeGlcpA). Depending on the isolation and fractionation procedure, the MeGlcpA/xylose molar ratio can vary from 1:4 to 1:16.14 A clustered distribution of MeGlcpA substituents on the backbone has been observed.17 A degree of polymerization ranging from 100 to 220, corresponding to an average molar mass of 5 600−40 000 g

2.2. Lignin

Lignin is a cross-linked polymer with a very complex and irregular macromolecular structure containing both aromatic and aliphatic segments built up mainly from three basic monomers or monolignols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 5). The monolignols are

Figure 5. Chemical structures of phenylpropene units present in lignin: p-coumaryl (I), coniferyl (II), and sinapyl alcohol (III).20

mostly linked together via ether and carbon−carbon bonds. In many wood species, lignin is the third major macromolecular component of the cell wall after cellulose and hemicellulose; it is mainly located in the secondary wall (∼70% of the overall lignin), but it is also found in a high concentration in the middle lamella and primary wall. Due to its aromatic and crosslinked structure, lignin contributes significantly in the hydrophobization and stiffening of the cell wall, while acting as a glue in the middle lamella to hold the different cells together.11,20 8179


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2.3. Cellulose

The nature and the intensity of the hemicellulose−cellulose interactions are strongly influenced by the distribution and degree of substitution of hemicellulose, where the lesssubstituted domains are thought to have the highest affinity for cellulose. Moreover, it has been proposed that MeGlcpxylan with evenly spaced MeGlcpA substituents along the xylan chain would allow for better interaction with cellulose fibrils compared with MeGlcp-xylan without evenly spaced MeGlcpA substituents. This was explained by the fact that the former may be able to adopt a flat 2-fold screw ribbon configuration, which allows it to interact more effectively with the 2-fold helical screw structure of cellulose in the fibrils compared to the latter structure, which has a 3-fold left-handed helical screw conformation.40−42 In the cell wall, hemicellulose is the component that has the capacity to bind the largest amount of water due to its amorphous, hydrophilic, and occasionally charged nature. This may lead to hydrogel-like domains confined between lesshydrated hydrophobic lignin and semicrystalline cellulose domains, which could impart more toughness and flexibility to wood tissues. These features and functions of hemicelluloses in the cell wall may serve as a source of inspiration as to how to utilize such components in the design of functional materials.

12

Cellulose constitutes 40−45% of the wood dry mass. In contrast to hemicelluloses, cellulose is a linear semicrystalline homopolymer consisting of β-D-glucosyl repeating units. Each unit contains two anhydroglucose units, where every such unit contains three hydroxyl groups, which gives cellulose hydrophilic characteristics and suggests that this biopolymer should be readily soluble in water and polar solvents. However, this expectation is not fulfilled due to the strong hydrogen bonds between the cellulose chains, the relatively crystalline supramolecular structure, and the spatial arrangement of the chains with hydrophilic groups in one direction and those with hydrophobic groups in the other.21 To make cellulose soluble in common solvents such as water and process it for a broader range of applications, cellulose has to be chemically modified. Esterification and etherification of the cellulosic hydroxyl groups are the most common derivatization reactions. Over the past two decades, the production of nanosized particles from cellulose has attracted ever-increasing attention. Those nanoparticles with lateral dimensions in the nanometer range can be divided into two main categories, namely, cellulose nanocrystals (CNCs)22−24 and micro-/nanofibrillated cellulose (MFC/NFC).25 CNCs are obtained through acid hydrolysis of the amorphous regions of cellulosic fibers, producing rodlike crystals with lengths ranging from ∼100 to 500 nm,26 while MFC/NFC is produced though delamination of the fibers using different mechanical processes (i.e., homogenization,25,27 grinding,28 and blending29) in combination with either chemical (i.e., TEMPO oxidation,29 carboxymethylation,30 and cationization31) or enzymatic pretreatments32 that yield fibrils with lengths in the micron range.

4. EXTRACTION OF HEMICELLULOSES FROM THE CELL WALL Traditionally, the extraction of hemicellulose is conducted in small scale for analytical purposes through the alkaline extraction of holocellulose after delignification treatments with either chlorine or acidified sodium chlorite.11,43−45 The types and amounts of hemicelluloses extracted are determined by the severity of the alkaline treatment in terms of alkali concentration, temperature, and time. These treatments, however, are always more or less associated with deacetylation, oxidation, hydrolysis, and/or the dehydration of sensitive sugars, as well as the degradation of the backbone via peeling reactions.46 The resulting nonbranched hemicelluloses show lower water solubility and higher tendencies to aggregate compared to their native counterparts. Alternatively, dimethylsulfoxide (DMSO) and dimethylformamide (DMF) are considered to be neutral solvents for hemicelluloses and are able to extract them without major alteration or degradation of their native structure. Hemicelluloses can potentially be obtained in large quantities as byproducts from process water in many existing forestry processes, such as thermomechanical pulping (TMP)13,47 and fiberboard production,48 and a number of new and old extraction strategies are being investigated and developed in connection with the reemergence of forest biorefineries.49,50 Among these, hydrothermal pretreatment has significantly high potential for integration with the Kraft pulping process. Advantages of this approach come from the fact that water is the only reagent, which makes the process environmentally friendly and noncorrosive to the equipment. The hydrothermal treatment of wood is usually associated with the acid-catalyzed hydrolysis of the glycosidic bond, leading to better solubilization of hemicelluloses. The hydrolysis reactions also lead to a decrease in the hemicellulose molecular weight and to the formation of new reducing end groups. The extraction yield and the extent of degradation are highly dependent on the treatment intensity. When hydrothermal treatment is performed in pure water, acetic acid is first generated in situ from the cleavage of the most labile O-acetyl

3. HEMICELLULOSE FUNCTION IN THE CELL WALL The structural variations of hemicelluloses in terms of type, amount, and distribution of acetyl and/other side-group substituents allow it to have a wider range of segment polarity and molecular conformations. This qualifies hemicelluloses to act as compatibilizers at the interface between hydrophilic cellulose fibrils and the more hydrophobic lignin, thereby increasing the overall cohesiveness and compatibility within the cell wall. In such cases, hemicellulose chains adopt conformations favoring the contact of the acetylated segments with lignin and nonacetylated regions with cellulose based on the similarity of their polarity and chemical functions. The main types of interactions between hemicellulose and lignin are noncovalent dipole−dipole and hydrogen-bonding interactions, and covalent bonding has been suggested to occur in so-called lignin−carbohydrate complexes (LCCs).33 Furthermore, some hemicelluloses are also found imbedded within cellulose domains, tethering together adjacent celluloses fibrils via hydrogen and van der Waals bonding. The latter arises from the correlation between both the permanent and induced dipoles of the cell-wall molecules. The close contact of hemicelluloses with cellulose is believed to play an important role in establishing the orientation of the fibrils and regulating their aggregation.34−39 In a study mimicking the cellulose fibrils formation in softwood cell wall, it was shown that cellulose that is synthesized in the presence of acetylated glucomannan by Acetobacter xylinum is smaller of crystallite size than its counterpart being synthesized in the control medium, indicating that acetylated glucomannan coats the fibrils and prevents their coalescence.35 8180


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miscible antisolvents or precipitating agents to aqueous process liquors is a well-established and prospective method in this regard. Ethanol is the most extensively used and widely reported antisolvent for the precipitation of hemicelluloses. Acetone, methanol, and other organic solvents have also been reported for this purpose.56 The precipitation efficiency in terms of yield was demonstrated to be inversely proportional to the dielectric constant of the antisolvent in the following order: acetone > ethanol > methanol.57 Other influencing factors include pH, temperature, concentration, and amount of antisolvent added. In the case of alkali process liquors, acidification with, for example, hydrochloric or acetic acid prior to precipitation is needed to achieve quantitative yields. The addition of a large excess of antisolvent or repeated precipitations can increase the yield, but this usually comes at the expense of selectivity due to the coprecipitation of lower molecular weight carbohydrates and other dissolved constituents.58 For example, it was demonstrated that GGM with higher molecular weight can be recovered from spruce TMP process waters at lower antisolvent content.57 Larger hemicellulose molecules are generally more prone to precipitate at lower additions of antisolvent compared with smaller hemicelluloses due to entropic reasons. This, however, is not always the case; for instance, graded ethanol precipitation of hemicelluloses from Caragana korshinskii showed that the molecular weight of the precipitated fraction first increased from 29 000 to 59 000 g mol−1 with increasing ethanol concentration from 10% to 30% but that it then decreased with the further addition of antisolvent to reach 44 000 to 15 000 g mol−1 with 45% and 80% ethanol content, respectively.59 Furthermore, certain precipitation agents can be more preferential to certain types of hemicelluloses; for example, barium hydroxide is known to selectively precipitate mannans, while cetyltrimethylammonium bromide and hydroxide are more specific to xylans.12 Membrane separation technology is another promising approach to recovering hemicelluloses that is gaining increasing attention. Different membrane filtration techniques, such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis, can be implemented in a continuous process to provide a good fractionation range for hemicelluloses as well as other wood constituents. Basically, all these separation techniques involve filtration through either a ceramic or polymeric semipermeable membrane with a given molecular weight (or particle size) cutoff by separating the process liquor into a high molecular weight (or particle size) fraction, referred to as the retentate, and a low molecular weight (or particle size) fraction, known as the permeate. Such techniques have been successfully employed in various recovery procedures to fractionate and purify hemicelluloses from Masonite process water,60 softwood61 and hardwood53,62 hydrolysates, and TMP process waters.13,47,63 One drawback associated with this technology is the common occurrence of fouling when accumulated solid colloidal residues block the membrane. The maintenance costs and time to clean and replace membranes should be taken into account. Our group has evaluated and compared three different recovery pathways (ultrafiltration, ultrafiltration combined with diafiltration, and ethanol precipitation) to upgrade WH by the hydrothermal treatment of hardwood (birch mixed with a small fraction of aspen).53 Each of these pathways resulted in a WH fraction with modified molecular characteristics (composition, structure, and molar mass). In general, all the modified WHs

bonds in the acetylated polysaccharides, creating an acidic environment to further catalyze the hydrolysis reactions.51,52 The process water released from hydrothermal treatment, known also as wood hydrolysate (WH), contains hemicellulose as a major component and a fair amount of lignin fragments and other dissolved and/or dispersed wood constituents. The yield, molecular weight, structure, degree of acetylation, and branching of the hemicelluloses in the process water can be controlled through the suitable selection of raw materials and appropriate adjustment of the extraction conditions (pH, temperature, time, etc.) and recovery technique, which in turn could be exploited to adjust the suitability of WH for conversion to useful products.43 For instance, Figure 6 illustrates the variation of WH composition with the upgrading pretreatment.

Figure 6. Influence of the upgrading pretreatment on the composition of wood hydrolysate (WH) extracted from hardwood.53

5. RECOVERY AND FRACTIONATION OF HEMICELLULOSES The postextraction of hemicelluloses, subsequent fractionation recovery, and purification stages offer further opportunities for controlling the composition and molecular features of the extract. Various isolation and purification strategies have been suggested to upgrade wood process waters and to isolate the solubilized and/or dispersed compounds. Nonetheless, the complete separation and isolation of wood constituents is very challenging, which is attributed to the similar solubility and chemical affinity of certain components and/or to the strong interactions between them. For instance, it has been reported that even supposedly pure AcGGM will always contain substantial traces of other polysaccharides such as xylans or pectins.13,54 Furthermore, it has been suggested that hemicelluloses can be partly covalently bonded to lignin, forming socalled lignin−carbohydrates complexes (LCCs), making their separation even more difficult. Consequently, the production of highly purified hemicelluloses is typically conducted on small laboratory scales because it requires time-consuming multistep purification procedures involving precipitation, bleaching, enzymatic or solvent-extraction steps, and/or chromatographic techniques, resulting in a low yield, high energy consumption and production costs, and possibly undesirable alterations to the native structure and inherent properties of the hemicelluloses.55 The development of green and more practical, although less extensive, refining procedures is of priority for securing hemicellulose production on large industrial scales. The precipitation of hemicellulose-rich fractions by adding water8181


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that the methylated monosaccharides obtained from methanolysis can be analyzed only by GC.45,70

contained a larger fraction of less-branched and higher molar mass polysaccharides than crude WH. The most remarkable change in crude WH was achieved by ethanol precipitation, resulting in a substantial reduction in lignin content from 6% (w/w) to 1% (w/w) and an enrichment in the higher molecular weight fraction of Mw from 2100 to 3800 g mol−1.53 The combination of membrane filtration processes with either ethanol precipitation13 or enzymatic cross-linking64 treatments can lead to further increases in the molecular weight of the recovered hemicelluloses and better separation capability, as demonstrated for spruce TMP process waters.13,64

6.2. Structural Analysis

The characterization of hemicelluloses via hydrolysis and monosaccharide analysis discussed above is generally complemented by nondestructive methods that permit the analysis of hemicelluloses in polymer and/or oligomer form. Among these, over the past three decades, NMR spectroscopy has become the method of choice for a more precise and reliable structural characterization of these oligo- and polysaccharide fractions. For most wood hemicelluloses, one-dimensional 1Hand 13C NMR spectra and two-dimensional NMR spectra can be recorded from solutions of hemicelluloses in NMRcompatible solvents (i.e., D2O or DMSO-d6). The combination of these NMR spectroscopic techniques provides valuable information regarding the monosaccharide composition, anomeric configurations, acetylation, and sequences along the main chain, as well as the nature, content, and position of the different side groups. The efficacy of analysis is, however, conditioned by a certain number of constraints specific to the sample heterogeneity, molecular weight, and solubility in the NMR solvent.19,71,73−76 For low molar mass hemicelluloses with homogeneous and regular structures, almost all the proton signals are typically well-resolved and can be more easily assigned using onedimensional 1H NMR only. The chemical shift of each proton differs based on its local chemical environment, allowing for the identification of different residues (neutral sugars, uronic acids, and acetyl groups) by comparing the obtained chemical shifts with those described in the literature or with spectra of known model compounds. As an illustrative example, Figure 7A shows

6. HEMICELLULOSE CHARACTERIZATION 6.1. Compositional Analysis

As already described, hemicelluloses, which are inherently structurally heterogeneous, are generally extracted as mixtures with other wood constituents. Therefore, the compositional analysis of these mixtures can be carried out based on the standard protocols applied to wood samples. When the source (plant species) of the sample is known, the hemicelluloses can be quantified fairly accurately by analysis of the monosaccharaide composition of the extracted fractions. This analysis starts with the depolymerization of the hemicelluloses into monomeric units by cleaving the glycosidic bonds. The liberated monosaccharides are then identified and quantified by gas chromatography−mass spectrometry (GC−MS) or gas chromatography−flame ionization detection (GC-FID) after chemical modification (silylation or acetylation)65,66 or by highperformance anion exchange chromatography−pulsed amperometric detection (HPAEC-PAD).67 Alternatively, the monosaccharides can also be analyzed by capillary electrophoresis (CE).68,69 Once the monosaccharide composition is determined, the amount of hemicelluloses can then be calculated based on the known monosaccharide ratio of different types of hemicellulose.70 The depolymerization of hemicelluloses can be performed via different methods, namely, acid hydrolysis, enzymatic hydrolysis, and acid methanolysis. Acid hydrolysis is by far the most commonly used method due to its simplicity and to the fact that it can be applied to both delignified and lignincontaining samples. In the latter case, acid hydrolysis also results in the precipitation of Klason lignin (acid-insoluble lignin), which can be isolated by filtration and gravimetrically quantified, while UV spectrometry can determine the acidsoluble lignin content. However, one difficulty encountered in acid hydrolysis is the incomplete cleavage of the glycosidic bonds, causing an underestimation of the measured sugars. This is particularly relevant for the MeGlcp-xylan containing samples due to the high stability against acidic hydrolysis of the bond connecting MeGlcp side groups to xylose.67,71 Increasing the amount of hydrolysis will entirely depolymerize the hemicelluloses, but it might also cause the decomposition of neutral sugars and the decarboxylation of uronic acids. In comparison, enzymatic hydrolysis is conducted under mild conditions that do not cause any undesirable reactions. This method can lead to a complete liberation of monosaccharides, given that a suitable combination of the enzymes specific to each bond is used and that the samples are free of lignin.68,72 Alternatively, methanolysis is less sensitive to the presence of lignin and very effective in liberating both uronic acid and neutral sugars, which makes it optimal for MeGlcp-xylan rich samples. One limitation of this technique compared with acid or enzymatic hydrolysis is

Figure 7. (A) 1H NMR spectrum of the acidic oligosaccharide fraction obtained from xylanase hydrolysis of pine pulp and (B) 1H NMR spectrum of crude pine xylan precipitated from pulp liquor. Adapted with permission from ref 76. Copyright 1995 Elsevier.

the 1H NMR spectrum of the acidic oligosaccharide fraction (DP ≈ 9) obtained from the xylanase hydrolysis of pine pulp that was first separated from the neutral components and then further fractioned by size-exclusion chromatography.76 More than seven main signals at 5.35 (hexenuronic acid, HexA), 5.41 (arabinose, Ara), 5.82 (HexA H-4), 5.29 (MeGlcpA), 5.2 (xylose at the reducing end in the α-conformation, Xyl red α), 4.75 (hydrogen−deuterium−oxygen (HDO)), and 4.5−4.7 8182


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ppm (xylose) are clearly shown. The latter, i.e., the signal at 4.5−4.7 ppm, reflects the multiple chemical environments of the xylose residues, including internal segments with and without side groups, terminal (nonreducing end) segments, and reducing end segments in the β-conformation. In addition, other signals not shown in the figure corresponding to the nonanomeric region are usually observed at 3−4 ppm, along with a well-resolved peak characteristic of O-acetyl groups at ∼2 ppm. Integration of the peaks attributed to the different residues relative to an internal standard provides information about the number of protons; hence, the molar composition can be deduced. On the other hand, Figure 7B shows the 1H NMR spectrum of crude pine xylan precipitated from pulp liquor (lignin content < 10%). This xylan fraction contains longer chains with large molecular polydispersity, which resulted in broader peaks and many overlapping signals that do not allow for the accurate integration of individual peaks.76 Moreover, 13C NMR spectra can provide well-resolved and quantifiable signals due to the larger range of 13C chemical shifts compared to 1H NMR and to special methods allowing for the elimination of the carbon−proton spin coupling. However, 13C NMR measurement requires long times and high sample concentrations. The latter is sometimes detrimental due to the associated increase in viscosity, which leads to a high noise-to-signal ratio.74 Furthermore, studies of two-dimensional homo- and heteronuclear NMR, including COSY, TOCSY, NOESY, and HSQC, provide additional information based on the different coupling constants along with less ambiguous signal assignment, which allows for a more detailed and accurate determination of the complex structure and number of hemicelluloses.74,77,78 Notably, the NMR analysis of hemicelluloses is not necessarily affected by the presence of lignin fragments that are not covalently connected to the carbohydrates. In fact, the protons connected to the aromatic structure resonate further downfield and do not interfere with the carbohydrate signals. Hydrophobic lignin fragments are relatively less soluble in polar NMR solvents such as D2O and DMSO-d6, meaning that their corresponding signals are typically weaker. Nevertheless, NMR analysis has been used in a number of studies to investigate the linkages between lignin and hemicelluloses in LCCs.79,80 In the same context, recent advances in high-resolution 2D solutionstate NMR have allowed the study of the whole cell wall without the need to isolate fractions of individual components.77,81,82 In addition to structural elucidation, NMR spectroscopy is also commonly used as a characterization tool to study the chemical changes resulting from the chemical modification of hemicelluloses.83−88 Solid-state NMR CP/MAS spectroscopy is particularly useful for characterizing samples that cannot be dissolved properly, including cellulose, linear hemicelluloses, and cross-linked hemicelluloses derivatives.89,90 It can also provide additional information regarding intermolecular interactions, chain conformation, the degree of crystallinity, and the nature of allomorphs in the semicrystalline hemicelluloses.91,92 In one of the few studies applying solid-state NMR to wood-based hemicelluloses, it was shown that the organization of xylan chains is quite sensitive to the surrounding environment. A shift in the resonance peaks assigned to carbons C-1 and C-4 of the xylose repeating units was observed upon drying and when water in the xylan paste was exchanged by methanol or isopropanol.93

Fourier transform infrared (FT-IR) is a classical spectroscopic technique that is easy to perform, is nondestructive, and permits small sample volumes. The analysis is based on the fact that different chemical bonds absorb IR radiation at characteristic frequencies matching, the transition energies of the various vibrations (waggling, stretching, bending, etc.) of the bonds. The mid-infrared region between 4000−400 cm−1 is the most commonly used region to analyze the structure of organic compounds.94 In general, the FT-IR spectrum of a hemicellulosic polysaccharide shows characteristic bands, including the stretching of hydrogen bonds between OH groups at 3200−3570 cm−1, CH2 stretching at 2850−2980 cm−1, CO stretching at 1725−1730 cm−1 in acetyl and carboxyl groups, CH2 bending at 1416−1430 cm−1, OH bending at 1630−1640 cm−1 in adsorbed water, CH deformation at 1374−1375 cm−1, and C−O stretching at 1040 and 1050 cm−1 in C−O−C glycosidic bonds.94−96 The occurrence of bands at approximately 1510 and 1600 cm−1, assigned to the aromatic skeletal vibration, would indicate the presence of residual lignin.97 Furthermore, specific band maxima can evidence the occurrence of certain anhydrosugar units that constitute hemicelluloses. For example, galactose was reported to exhibit a strong band at ∼1080 cm−1, as was mannose at ∼1070 cm−1, glucose at ∼1035 cm−1, and xylose at ∼1040 cm−1.98,99 Uronic acids are typically distinguished by the asymmetric stretching band of COO− groups at 1425 cm−1.96 Additionally, in the anomeric region (700−950 cm−1), absorption bands for αanomers at ∼845 cm−1 and β-anomers at ∼898 cm−1 clearly differentiate between the two types of glycosidic linkages between pyranose anhydrosugars.99 As in the case of NMR, FT-IR has been widely used to verify and study the chemical modification of hemicelluloses by tracking the appearance of new bands associated with the introduction of functional groups or the disappearance of certain bands related to the unmodified structure.87,100−102 Furthermore, the shift in the peak absorbance upon physically blending with other compounds can provide information about noncovalent interactions in the blends. This has been used to study inter-/intramolecular hydrogen-bonding interactions in films based on crude wood hydrolysates/CMC through the stretching of the hydrogen-bond band.103 Polarized FT-IR microspectroscopy has been proved to be an adequate method to elucidate aspects of ultrastructural arrangement and orientation of hemicelluloses among the other wood polymers in the cell wall.2,104 With a few exceptions, however, FT-IR spectroscopy is less reliable for the quantitative analysis of hemicellulose samples due to the multiple overlaying bands, which makes the assignment and height measurement of single bands far from trivial. 6.3. Molar Mass Analysis

The molecular weights and the molecular weights distribution are important molecular features that govern many of the properties of polymers. Traditionally, several methods have been applied to measure the molecular weight parameters of hemicelluloses, including membrane osmometry,105,106 viscosimetry,107,108 ultracentrifugation,107 light scattering,106,108 and reducing end-group analysis109,43 However, the most widely applied method is size-exclusion chromatography (SEC), which is also known as gel permeation chromatography (GPC).110,111 Notably, the values reported in the literature for the various hemicelluloses obtained from the different methods show 8183


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generally acknowledged as a good matrix.123,124 The high degree of sensitivity, the rapidity with which results are acquired, and its applicability to both pure compounds and mixtures are some advantages of this method. Nonetheless, accurate average molecular weight and dispersity values can be deduced from the recorded spectra only if the sample exhibits low dispersity (Đ), typically below 1.1. Consequently, it is very common that polydisperse hemicellulose samples are first separated into several fractions with narrow dispersity by SEC or another fractionation technique prior to MALDI-MS analysis.124,125 Additionally, from the other point of view, MALDI-MS has been frequently used to calibrate the SEC columns.123,126 In addition to the molecular weight determination, this technique can provide more detailed structural information of the analyzed hemicellulose samples and their derivatives. For instance, MALDI-MS has proven to be a very useful analytical tool to evaluate the distribution of MeGlcpA residues along the xylan chain from various soft- and hardwood species.78,127−129 This method has also been applied to study acetyl substitution patterns in xylan and glucomannan samples.128,130 Furthermore, MALDI-MS has been utilized in a number of studies to analyze the transformation of the hemicellulose structure after chemical and enzymatic modifications.83,131−134

considerable variations, and therefore a direct comparison between them can be misleading. Generally, the values obtained by ultracentrifugation are lower than those determined by SEC, which in turn are lower than those obtained by light scattering. This discrepancy can be related to multiple reasons, not only to differences in the measuring principles and assumptions relative to each method but also to the effect on the measurements by other intrinsic properties of hemicelluloses, such as the content and distribution of side-chain units (acetyl, uronic acid, and neutral sugars). Moreover, a well-recognized source of artifact is the partial insolubility of certain hemicelluloses in the utilized solvents/eluents, leading to the formation of clusters or small aggregates in solution prior to or during analysis. These aggregates would be coeluted with single molecules and detected as such, which results in an overestimation of the molecular weight.108 Filtration or centrifugation of the hemicellulose solutions prior to analysis is a common practice to remove the aggregates and any impurities, but the determined values will not be representative of the whole sample. To overcome the solubility problem, molecular weight measurements were performed using more soluble derivatives of the investigated sample.112 In SEC, molecules are separated in a column according to their hydrodynamic volume, which is proportional to their molecular weight but depends also on the conformation and substitution of the chains. The columns are often calibrated by a series of nearly monodisperse dextran or pullulan standards with well-known molecular weights. The eluted samples are then analyzed by differential refractive index (RI) and/or ultraviolet−visible (UV−vis) spectroscopy as a function of retention time. One obvious disadvantage of this approach is that the measured values are relative to the standards and that the accuracy of the measurement is highly dependent on the degree of structural similarities between the measured sample and the standards. However, this problem can be eliminated by coupling SEC to both a viscometer and an RI detector, which permits the measurement of absolute molecular weight based on a universal calibration curve, i.e., irrespective of the chemical structure of the standards employed in the calibration.62,113−116 Alternatively, new SEC systems equipped with a multiangle laser light scattering (MALLS) detector were also used to determine absolute molecular weight values of hemicellulose fractions independently of both the elution time and the external calibration.113,116−120 For crude hemicellulose fractions, comparison of the elution profiles obtained by a UV−vis detector set at 290 nm with those obtained via other detectors allows for the differentiation between the aromatic (lignin) and nonaromatic (carbohydrate) fractions. If the two elution curves do not overlap, the molecular weight of each fraction can be determined directly without fractionation and separation pretreatment.121 Matrix-assisted laser desorption ionization (MALDI) is an ionization method developed in the late 1980s to enable the rough analysis of polar macromolecules (i.e., oligo- and polysaccharides, proteins, etc.) with molecular weights ranging from 1 000 to 500 000 g mol−1 using mass spectrometry (MS). In MALDI-MS experiments, the samples are typically cocrystallized within a matrix and then irradiated with laser light. During the ionization step, the matrix absorbs the energy transmitted by the laser beam and induces the desorption/ionization of the macromolecules. The choice of the matrix is therefore of utmost importance for success experiments.122 In the case of hemicellulose samples, 2,5-dihydrobenzoic acid (DHB) is

7. CHEMICAL MODIFICATION EXTENDS THE FUNCTIONALITY OF HEMICELLULOSES Chemical modification of wood hemicelluloses offers numerous possibilities to control and tailor the properties of hemicellulose, enhance their processability and introduce new functionalities such as thermoplasticity, hydrophobicity, conductivity, and stimuli-responsiveness. This aims to extend the applicability of hemicelluloses and facilitate their introduction into the market value chain. Hemicelluloses, like other polysaccharides, are highly functionalized polymers that contain a large number of hydroxyl groups, which are often used as the starting reactive site for most chemical modifications. The yields of such hydroxyl-mediated reactions are usually expressed in terms of the degree of substitution (DS), which corresponds to the number of reacted hydroxyls per sugar unit. The number of synthetically available hydroxyl groups per sugar unit may vary between 2 and 3 depending on whether the hemicellulose is mostly composed of pentoses, as is the case for xylans, or hexoses, as is the case for GGM. This number is further lowered when considering native substitutions such as acetylation and branching. The reactivity of the different OH-groups on hemicelluloses may vary to some extent, not only due to the difference in their position, electronegativity, and accessibility but also depending on the type, medium, and conditions of the reaction. For anhydroglucose units, it is generally expected that the reactivity of OH-groups decreases in the following order: C-1 ≥ C-2 > C3 (Figure 8). However, most studies report DS as an average value, without specifying the DS relative to each position. Targeted and regioselective modifications are usually difficult to achieve for most reactions, as they require multiple protection/ deprotection steps with reagents such as tosylate. However, a few exceptions exist; for instance, TEMPO-mediated oxidation selectively targets the primary alcohol at the C-6 position. Hemicelluloses contain hemiacetal groups in equilibrium with the ring-opened aldehyde at the reducing end of the chain, as well as carboxylic acid functionalities at the uronic acid side groups. Such functionalities, as well as the glycol bonds in the 8184


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homogeneous esterification.211 While DMAc/LiCl is able to dissolve cellulose, it has been shown that DMF/LiCL is a better solvent for hemicelluloses. However, full acetylation of aspen MeGlp-xylan (DS = 1.9) was achived after reaction with acetic anhydride in formamide/pyridine for 4 h.135 The MeGlp-xylan acetate presented good solubility in DMSO and chloroform as well as themoplastic propreties that permitted film production using hot-press processing technique.135 Similarly, ester moieties with long carbon chains were also attached in order to increase the hydrophobicity of hemicellulose (Table 1, entries 2−10). The esterification of poplar MeGlcp-xylan with different acyl chlorides bearing alkyl (or alkene) chains with 2− 18 carbons in DMF/LiCl occurred in the presence of a catalytic amount of 4-dimethylaminopyridine and triethylamine (NEt3) as a neutralizing agent.140,141 The reaction was studied as a function of the molar ratio of acyl chloride/anhydroxylose (AXU), various NEt3 concentrations, reaction temperatures varying from 25 to 75 °C, and times varying from 25 to 45 min. The maximum degree of esterification ≈ 1.5 was achieved for 3:1 acyl chloride/AXU, NEt3 280% based on hemicellulose weight, 45 min, and 75 °C. Molecular weight analysis indicated that a longer reaction time (75 min) resulted in an increased hemicellulose chain degradation.140,141 Moreover, esterification of hemicellulose with moieties bearing functional reactive groups such as alkenes, carboxyls, thiols, controlled radical polymerization initiating groups, and azides has also been largely studied to broaden the reactivity of hemicellulose. Succinoylation is an esterification method that allows for introduction of groups with a carboxylic acid function by reacting the hemicelluloses with succinic anhydride.101,144 The introduced carboxylic functionalities increase the hydrophilicity of the hemicelluloses and introduce a pH responsiveness. The esterification of hemicellulose was also performed using a sulfuric acid derivative in DMF/LiCl in order to introduce negatively charged sulfate groups. Sulfation of beechwood xylan (Table 1, entry 26) was primarily conducted in a dinitrogen tetroxide-N,N-dimethylformamide (N2O4-DMF) system that is a derivatizing solvent system.212 This system has been shown to form and solvate an unstable hemicellulose derivative. Subsequently, the simultaneous elimination of the unstable hemicellulose derivative and the introduction of the desired acyl groups are achieved by means of an active sulfating agent (NOSO4H). The formation of the sulfating agent can be performed via a redox process using N2O4 and SO2. The secondary OH-groups of beechwood xylan were only partially modified using this procedure with a low degree of substitution as determined by 13C NMR spectroscopy (DS = 0.17−0.55). In recent studies, sulfation was conducted with sulfur trioxide pyridine complex or SO3DMF in DMF/LiCl at various sulfating agent/AXU unit molar ratios varying from 0.5 to 10, and a DS as high as 1.9 was reached for xylan extracted from eucaluptus using nickeltrisethylenediamine (Ni(en)3), SO3/pyridine sulfating agent, and 3 in molar ratio to AXU.157

Figure 8. Structural representation of the anhydroglucose unit.

units containing vicinal secondary hydroxy groups, can serve as additional reactive sites for many chemical modifications, which extends the chemical versatility of these molecules. A variety of chemical modification reactions have been studied and reported in the literature for various wood-derived hemicelluloses, as summarized in Table 1. 7.1. Esterification

Esterification is commonly used for the chemical modification of polysaccharide hydroxyl groups. Acyl halide, anhydrides, and carboxylic acids have all been reported in the literature for the formation of ester linkages to hemicelluloses. A typical first step is to activate the hydroxyl groups with a base such as TEA, DMAP, or pyridine. Thereafter, the activated hydroxyl groups react with either acyl halide or the anhydride to form the ester bond. Another approach is the formation of activated carboxylic acid species by CDI coupling prior to nucleophilic reaction with the hydroxyl groups (Table 1, entries 10, 15, and 20−23). The esterification of hemicelluloses with carboxylic acids after CDI activation offers accessibility to a broad variety of ester groups, specifically when the corresponding acyl halide or anhydride derivatives are unstable. The esterification is typically conducted under homogeneous or heteregenous conditions in either DMF, DMAc/LiCl, DMSO/THF, or various ionic liquids by acyl chlorides using an alkaline catalyst (pyridine, 4-dimethylaminopyridine, or Nbromosuccinimide) and in the presence of TEA to neutralize the hydrochloric acid generated during the reaction. A classic reaction is the acetylation (Table 1, entry 1) of the hemicellulose backbone via esterification using acetic anhydride or acetic chloride. To study the influence of structural features of hemicelluloses on the acetylation, various hemicelluloses recovered from kraft and sulfite processes were acetylated using acetic acid and acetic anhydride under the same conditions used for the industrial production of cellulose acetate.209 The study showed that kraft glucomannans, sulfite xylans which are branched, yielded clear acetate solutions, while linear kraft xylans and sulfite galactomannans resulted in crystalline acetates and hazy solutions. Beech MeGlcp-xylan was acetylated under heterogeneous conditions in chloroform and dichloromethane with acetic anhydride and in the presence of perchloric acid as a catalyst. The hemicellulose derivatives progressively dissolved in the organic solvents as the reaction processed, becoming completely soluble when the full degree of acetylation was reached. On the other hand, beech MeGlcpxylan was acetylated under homogeneous condition with ethanoic anhydride in trifluoroacetic acid for 15 min at room temperature. The maximum degree of substitution of 0.6 was achieved. It was claimed based on the molecular weight analysis that the quantity of trifluoroacetic acid used did not have a considerable effect on the hemicelluloses degradation.210 Alternatively, N,N′-dimethylacetamide (DMAc)/lithium chloride (LiCl), N,N′-dimethylformamide (DMF)/LiCl systems are commonly used as the reaction medium for

7.2. Etherification

Etherification is another commonly used reaction for the chemical modification of hemicelluloses. Ether bonds are chemically more stable and less susceptible to cleavage by hydrolysis than ester bonds. Etherification of hemicelluloses has been used in order to increase the stability against microorganisms, the solubility, and the film formability. Nevertheless, one major drawback is that the formation of ether linkages 8185


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Table 1. Overview of Hemicellulose Chemical Modificationsa

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Table 1. continued

a

Abbreviations in table: GX, glucuronoxylan; AGX, arabinoglucuronoxylan; GGM, galactoglucomannan; AcGGM, O-acetylgalactoglucomannan; Eucal xylan, eucalyptus xylan; WH, wood hydrolysate; AG, arabinogalactan; Py, pyridine; DMF, dimethylformamide; DMAP, 4-(dimethylamino)pyridine; MSA, methanesulfonic acid; GMA, glycidyl methacrylate; PMDETA, N,N,N′,N″,N″-pentamethyldiethylenetriamine; MMA, methyl methacrylate; TEA, triethylamine; DMSO, dimethyl sulfoxide; THF, tetrahydrofuran; EPTMAC, (2,3-epoxypropyl)trimethylammonium chloride; CHMAC, 3-chloro-2-hydroxypropyl trimethylammonium chloride; aq., aqueous; KPS, potassium peroxydisulfate; CDI, 1,1′-carbonyldiimidazole; TEMPO, 2,2,6,6-tetramethyl-1-piperidinyloxy; EDC, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide; NHS, N-hydroxysuccinimide; METAC, methacryloyloxy ethyl trimethyl ammonium chloride; TBD, triazabicyclodecene; NIPAM, N-isopropylacrylamide; MeDMA, [2-(methacryloyloxy)ethyl]trimethylammonium chloride.

requires high pH in order for the hydroxyl groups to be deprotonated into alkoxides that will react with the etherifying agent (typically epoxide- or halide-bearing compounds) and that this can lead to partial degradation of the hemicelluloses.

To reduce the extent of such degradation, the reaction can be carried out under heterogeneous conditions in mixtures of water and organic solvents (i.e., ethanol or toluene). One typical example of etherification is methylation, which is 8193


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7.5. Amination

traditionally used to make volatile monosaccharide derivatives prior to analysis by gas chromatography (see section 6.1). Methylation of birch xylan was performed in an aqueous medium with methyl halide in the presence of sodium/ potassium hydroxide (Table 1, entry 29). Another example is the benzylation of AcGGM with benzyl chloride in an aqueous system in the presence of tetrabutylammonium iodide as a phase-transfer agent (Table 1, entry 31). The DS value was ∼1.3, and the water solubility of substituted AcGGM was considerably reduced. Lamination of unmodified acetylated galactoglucomannan with benzyl AcGGM was employed in order to produce flexible, water-resistant transparent films with good oxygen barrier properties. Sulfoalkylation of hemicelluloses obtained from beech GX has been carried out in aqueous media (Table 1, entries 37 and 39) using 2-chloroethanesulfonic acid or 3-chloro-2-hydroxypropylsulfonic acid, and a DS of 0.5 or from 0.1 to 0.3 was obtained, respectively, depending on the procedure conditions. Sulfoethyl beech GX derivative was further modified via alkylation using 1-bromododecane in dimethyl sulfoxide (Table 1, entry 30) with the aim to provide an amphiphilic character and to decrease the surface tension of GX. These multifunctional GX derivatives can be used as potential surfactants derived from renewable resources. Carboxymethylation of hemicelluloses is another etherification reaction that can be conducted in aqueous media. Xylan from birch was carboxymethylated using sodium monochloroacetate in alkaline 2-propanol/water mixture to a degree of substitution varying from 0.1 to 1.6 (Table 1, entry 40). Etherification has also been used to introduce various functional groups including ionic groups, amines, azides, thiols, and double bonds to the hemicellulose structure.

Reductive amination is a common method used in polysaccharide chemistry to immobilize different types of aminofunctionalized moieties using NaBH3CN. This reaction is conducted in aqueous media and under mild conditions and represents a robust and highly selective method for the chemical modification of hemicellulose. In a typical reaction, the aldehydes, either synthesized by oxidation and subsequent ring opening or at the reducing end, are reduced and a new amine linkage is formed (Table 1, entries 62−71). Using this approach, different functionalities have been attached to hemicellulose, such as alkyls, amines, azides, controlled radical polymerization initiating groups, carboxylic acids, and vinyl groups. 7.6. Amidation

Amide formation between hemicellulose and amine-functionalized entities is mainly based on N-(3-dimethylaminopropyl)N′-ethylcarbodiimide (EDC)/N-hydroxysuccinimide (NHS) coupling. In this system, activated O-acylisourea is first formed from the reaction of EDC with a carboxylic acid. Then, the subsequent nucleophilic attack of this intermediate by a primary amino group leads to an amide linkage with the carboxylic acid and the formation of a soluble urea-derivative byproduct. NHS is usually added to the mixture to improve the efficiency of EDC coupling. This reaction is performed after the oxidation of primary alcohols of hemicellulose to carboxylate groups (Table 1, entries 72−74, obtained from modification of entries 59 and 61). 7.7. Miscellaneous Linkages

Other molecules attached to the hemicellulose backbone include fluorescein isothiocyanate and organostannane chloride (Table 1, entries 75 and 80, respectively). Furthermore, the indium-mediated allylation of hemicellulose derivatives has been conducted. This approach is employed for the formation of C−C bonds and has many advantages. It can be performed under mild conditions and in aqueous systems, and it is reported to be a diastereoselective reaction. The attached allyl functionalities can be used for further postfunctionalization of hemicellulose.

7.3. Carbonate Ester Linkage

The reaction of unsaturated alcohols with CDI results in the formation of imidazoyl-activated coupling species. The subsequent nucleophilic reaction of the activated hydroxyl groups of hemicellulose with the coupling species leads to the formation of carbonate ester linkages. This approach was used mainly to attach pendant functional groups such as vinyl alcohols, vinyl ethers, and acrylates to the hemicellulose backbone (Table 1, entries 50−56).

7.8. Copolymerization (Grafting from/onto)

Chemical modification of hemicellulose can be achieved not only by attaching small molecular weight molecules but also by covalent grafting of long polymeric chains. Polymers can be attached either along the hemicellulose backbone as pendant side chains by graft copolymerization or from one or both ends of the hemicellulose chain via block copolymerization. Copolymers of hemicellulose can be obtained either by “grafting from” or “grafting to” approaches. In the grafting from approach, polymerization is initiated from the backbone of the hemicellulose, and the subsequent propagation of monomer molecules forms the growing polymer. The grafting to approach is based on the covalent attachment of preformed polymers on hemicellulose through reactive end groups. The main advantage of this method is the possibility to synthesize and characterize the polymer separately prior to the grafting to hemicellulose. Yet, to be able to conduct the grafting under homogeneous conditions, both the hemicellulose and the polymer to be grafted need to be soluble in the same solvent, which is not the case for hydrophobic polymers. In contrast, more combinations are possible for the grafting from approach as only the monomer of the polymer to be grafted has to be

7.4. Oxidation

Unlike the previously mentioned chemical modification methods, where molecules are covalently attached to hemicellulose, oxidation is a reaction that transforms the alcohol groups of hemicellulose to aldehydes or carboxylic acids. The oxidation of vicinal diols of hemicellulose using phthalocyanine/H2O2 leads to ring opening and the formation of aldehyde and carboxylic functionalities (Table 1, entry 57). Periodate oxidation is also specific to vicinal diols for the formation of aldehydes at low periodate concentration (Table 1, entry 58). Another regioselective reaction is the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation of the primary alcohol on C-6 to a carboxylate group (Table 1, entry 59). Oxidation of the primary alcohol on the galactose unit to an aldehyde can also be performed by using specific enzymes such as galactose oxidase and horseradish peroxidase (Table 1, entry 59). The aldehyde can be further oxidized to a carboxylate group using NaClO2 (Table 1, entry 60). All of the previously mentioned oxidation reactions can be performed under mild conditions and in aqueous media. 8194


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weight, etc.) of hemicellulose itself, while others rely on the addition of one or several components to the film and coating. Here, we can distinguish among blends, composites, and plasticized hemicelluloses depending on the type and function of the added cocomponent. Many of these approaches have been applied separately or together to achieve a synergetic effect to different types of hemicelluloses. Each approach affects both the barrier and the mechanical properties in different ways. For these different film/coating systems, the nature of hemicelluloses and the resulting oxygen barrier, water vapor permeability, and mechanical properties are collected in Tables 2, 3, and 4, respectively.

soluble in the same solvent for hemicellulose. For both approaches and in most cases, a suitable initiating species must either be attached to the hemicellulose or generated by radiation. This makes the modification more tedious and requires a multistep approach. Early works pioneered the grafting of living polystyrene and poly(2-vinylpyridine) to methylated MeGlcp-xylan, or grafting of methyl acrylate, via cerium-induced polymerization. However, these reactions suffered from many drawbacks such as degradation, lack of control over the grafting density, and the molecular weight of the grafts, and yielded homopolymers in parallel. Recent studies applied more controlled grafting polymerization approaches to hemicellulose such as ringopening polymerization (ROP) and controlled radical polymerization (CRP). In ROP, the polymerization is usually initiated directly from the hydroxyl groups of hemicellulose; however, the attachment of a bromo ester moiety on the hemicellulose backbone is required prior to CRP. For the grafting to approach, azide-alkyne Huisgen cycloaddition and thiol-ene click chemistry were both reported in the literature to graft preformed polymers on hemicellulose. Another approach for the preparation of block copolymers is chain extension through reductive amination. Diblocks and triblocks of hemicellulose-based copolymers were prepared using di- or triamines. This approach was used in order to either graft another polymer such as polydimethylsiloxane (PDMS) (Table 1, entries 108−110) or increase the molecular weight of hemicellulose by linking at least two chains head-tohead (Table 1, entries 106 and 107).

8.1. Oxygen Barriers

The oxygen barrier properties can be quantified by oxygen permeability (OP) analysis.213 The OP value describes the volume of oxygen that passes through a film with a defined area and specific thickness over a set time frame under a given pressure.213 The gas permeability (GP) can be expressed in several units. In this Review, OP is expressed in cm3 μm/m2 day kPa. A suggested maximum OP value of 38.9 cm3 μm/m2 day kPa or below indicates that the material is considered to be a sufficiently good oxygen barrier for food-packaging applications.213,214 This value should be taken as a rough estimation as it very much depends on other considerations, such as the storage conditions and type of contained food. Hemicellulose-based films and coatings are usually well below the suggested max value and, therefore, are considered to be excellent oxygen barrier materials. To put this in perspective, the OP values of hemicellulose-based films and coatings are, in general, 2−3 orders of magnitude lower than the OP of highdensity polyethylene (HDPE) or polypropylene (i-PP) with OP values around 300−400 cm3 μm day−1 m−2 kPa−1. Another conventional fossil-based packaging plastic, polyethylene terephthalate (PET), performs better, with an OP of ∼15 cm3 μm day−1 m−2 kPa−1. Poly(vinyl alcohol) (PVOH) is considered an excellent oxygen barrier with an OP ≈ 0.3 cm3 μm day−1 m−2 kPa−1 at 0% RH, but this value increases with increasing RH. Nanofibrillated cellulose have been reported to offer even better oxygen barrier values than this, as low as 0.001 cm3 μm day−1 m−2 kPa−1 at 0% RH. Like other hygroscopic materials, this value increases significantly as the RH increases;215 still nanofibrillated cellulose constitutes excellent oxygen barriers at RH 50% and deteriorates only at RH higher than 70%. Poly(L-lactide) is a popular renewable and degradable packaging plastic candidate but is not favored for its oxygen barrier properties; OP is typically ∼50 cm3 μm day−1 m−2 kPa−1. Decreasing the OP even further would mean that the same barrier performance could be achieved with a thinner barrier layer, which could represent significant savings in terms of the material used and the weight of the package. The high oxygen barrier capability of hemicellulose films stems from the strong intermolecular hydrogen-bonding interactions, dense chain-packing ability, and reduced chain mobility. These, in turn, are attributed to the large amount of polar hydroxyl groups and the high chain stiffness because they are made of rigid sugar rings with restricted rotation around the forming bonds, as well as the high glass transition temperatures (Tg) of hemicelluloses. The latter are usually so high that they approach or even overlap with their thermal degradation temperatures. For some xylans, Tg has been estimated to be in the range of 167−180 °C. This has been further confirmed by Hansen’s solubility parameter calculations, as well as by the free

8. HEMICELLULOSE-BASED FILMS AND COATINGS Hemicelluloses, like many other polysaccharides have some outstanding properties including low oxygen permeability, water solubility, renewability, and biodegradability. Furthermore, wood-derived hemicelluloses are extracted from an abundant and sustainable feedstock that does not conflict with food supply. Together, these properties make wood hemicellulose an interesting starting material for the production of biobased barrier films and coatings. These barriers can be used for different applications, notably in food-packaging materials. Indeed, many food products require storage under specific atmospheric conditions with low oxygen content in order to extend the food shelf life and retain its nutritious value and safety. To maintain this low oxygen level inside the package, the packaging material needs to be highly impermeable to oxygen. At present, the oxygen barrier function of a package is usually provided by aluminum foil or a petroleum-based polymer such as ethylene vinyl alcohol (EVOH) or polyvinylidene chloride (PVDC), but these materials are associated with drawbacks related to their nonrenewable sources, persistence in the environment, and high costs. Although hemicelluloses are suitable and attractive as environmentally friendly and efficient oxygen barrier materials, they suffer from some inherent difficulties, mostly related to the high production costs to attain a high degree of purity, brittleness, water/moisture sensitivity, and high water vapor permeability. Thus, different approachesalthough sometimes complementaryto overcome these limitations have been proposed. Some of these approaches, for instance, selective recovery and chemical and enzymatic modification, are intended to achieve a desired property change by modifying the molecular characteristics (chemical composition, structure, molecular 8195


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Table 2. Oxygen Permeability of Hemicellulose-Based Films and Coatings RH (%) hemicellulose blend alginate + 65% AcGGM 70% AcGGM CMC + 65% AcGGM 70% AcGGM 50% AcGGM 50% birch xylan coated on PET 50% spruce WH on PET 60−20% spruce WH 60% hardwood WH fractions 50% hardwood WH 50% hardwood WH Finnfix + 60−20% spruce WH chitosan + 50% spruce AcGGM 50% WH coated on PET

50

50 80 50 50 80 50 80 50

oxygen permeability (cm3 μm/m2 day kPa)

0.55 0.55

ref hemicellulose-based composite NFC + 15% GGM (prepared either by sorption or mixing) 20% AGX (prepared either by sorption or mixing) 80−50% birch xylan 40 or 10% xylan + 10 or 30% glycerol 40 or 10% xylan + 10 or 30% MPEG 40 or 10% xylan + 10 or 30% sorbitol CNC + 57% spruce GGM + 40% sorbitol MMT + 59−40% hardwood WH + 40% CMC

216 83, 216

1.28 1.28 8.7 13.9 5.0

216 83 217

1.6 11.5 1.0−2.7 11.1−22.5 1.3−6.19 3.8 2.5

217

218

219 53 218 218

50 80

0.3−1.5 8.8−25.4

219

50 80 50 80

9.4 13.8 3.3 10.3

217

PLLA + 5−50% softwood WH 50 quaternized cellulose (3 different DS) + 60% birch WH 50 80 plasticized hemicellulose sorbitol + 65% aspen GX 50 65% softwood AcGGM 50 60% spruce GGM between 50 and 75 15% spruce GGM + 45% between 50 KGM and 75 45% spruce GGM + 15% between 50 KGM and 75 15% spruce GGM + 45% between 50 PVOH and 75 45% spruce GGM + 15% between 50 PVOH and 75 xylitol + 65% softwood AcGGM 50 glycerol + 75% softwood AcGGM 50 65% softwood AcGGM + 50 17.5% alginate

RH (%)

203

0.1−0.3 1.3−6.2

220

0.21 2.0 ∼7

221 216 222

∼6

222

∼22 ∼3

216

4.60 4.56

216 216

50

1.2−1.7

223

50

1.6−1.8

223

50 50

0.19−0.24 0.69−9.70

224 224

50

1.81−206

224

50

0.048−0.083

224

between 50 and 75

∼7

222

50

1.5−5.7

225

80

2.42−9.36

50 1.4−2.7 225 59−40% hardwood WH + 80 3.98−10.82 40% CMC modified hemicellulose AcGGM-g-PLLA (entry 90, Table 1) + 95−50% softwood WH/ 50 ∼230−400 203 PLLA benzylated AcGGM (entry 50 130−559 83 31, Table 1) 83 170−546 laminated benzylated AcGGM (entry 31, Table 1) and GGM + 30% CMC 73 8 83 chain-extended WH (entries 106−107, Table 1) + 15−30% CMC 50 0.2−5.9 194 80 10.6−14.9 styrene vapor-phase grafting of benzylated AcGGM (entry 31, Table 1) + 30% CMC 50 1.75 83 hydroxypropyl xylan (entry 32, Table 1) + 50−75 24 164 10−20% sorbitol 0 4.7−10 163 10% sorbitol + 10% glycerol 0 1487 5% citric acid 50 15 14 cross-linked hemicellulose spruce WH + +5−50% glyoxal 50 1.6−7.7 226 80 1.3a

∼4

4.4

ref

talc +

217

230−300

oxygen permeability (cm3 μm/m2 day kPa)

a

volume size and distribution revealed by positron annihilation lifetime spectrum (PALS) measurements.154,155 Hemicellulose blend OP values reported in the literature vary between 0.1 and 200 cm3 μm/m2 day kPa (Table 2) depending on the type, content, and degree of miscibility and affinity of the cocomponent with hemicellulose. Blends with polar cocomponent such as PVOH, alginate, chitosan, CMC, or quaternized celluloses do usually form homogeneous films and

For 50% glyoxal with MPEG (methoxypolyethylene glycol).

coatings with OP values below 10 at 50% relative humidity (RH) and below 25 cm3 μm/m2 day kPa at 80% RH (Table 2). The increase in permeability at higher RH can be explained by the hygroscopic character of the cocomponents and hemicelluloses. The absorbed water molecules disrupt the hydrogen bonding between the hemicelluloses chains and increase the free volume within these materials. Interestingly, the presence of residual lignin in hemicellulose-rich WH-based films 8196


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Table 3. Water Vapor Permeability and Water Vapor Transmission Rate of Hemicellulose-Based Films conditions hemicellulose blend CMC + 50% spruce WH coated on 50/100% PET RH chitosan + 50% spruce WH coated on 50/100% PET RH quaternized cellulose (3 different DS) + 60% WH 50/100% RH plasticized hemicellulose sorbitol + 15% GGM + 45% KGM 0/54% RH 32/86% RH 45% GGM + 15% KGM 0/54% RH 32/86% RH 15% GGM + 45% PVOH 0/54% RH 32/86% RH 45% GGM + 15% PVOH 0/54% RH 32/86% RH composite NFC + 80−50% xylan 0/50% RH 45−30% xylan + 10−40% 0/50% glycerol RH 45−30% xylan + 10−40% 0/50% MPEGa RH

WVP (g mm/m2 day kPa)

WVTR (g/day m2)

ref

0.168

103

0.159

103

59.4−64.3

NFC + 45−30% xylan + 10−40% sorbitol CNC + 57% GGM + 40% sorbitol

MMT + 59−40% WH + 40% CMC talc + 59−40% WH + 40% CMC

222

TiO2 + 74.25−72% PVA + 24.75− 24% xylan modified hemicellulose CMX (entry 40, Table 1) DS = 0.36−0.58 WH (entry 106, Table 1) +

45 1.7 43 0.3 22

37

111−152

1.52−8.17

86.0−370

2.43−10.5

124−476

0/50% RH

0.32−0.65

14.7−20.4

0/54% RH 32/86% RH

2.0

50/100% RH

58.5−148.2

50/100% RH

97.8−185.8

0/75% RH

2.90−5.3

225

n.s.b 50/100% RH

19−38 79.2−82.6

228 194

227

ref

222

37

15−30% CMC hydroxypropyl xylan (entry 32, Table 1) + 0/53% 6.6 RH 10−20% sorbitol 1.3−3.3 cross-linked citric acid + 22.5−12.5% PVAc + 67.5− 0/75% 2.35−2.95c 37.5% xylan (beech RH wood)

1.0

1.6−2.4

WVTR (g/day m2)

conditions

220

1.9

WVP (g mm/m2 day kPa)

225

182

164

41−104

229

a

MPEG = methoxypolyethylene glycol. bn.s. = not specified. cg/(mm2 h) × 10−7.

Hemicellulose films have been reinforced with organic and inorganic fillers including sepiolite, nanoclay (MMT), talc, microfibrillated cellulose (MFC), cellulose nanocrystals (CNC), bacterial cellulose and nanofibrillated cellulose (NFC). The reported OP values for hemicellulose-based films with NFC are in the range of 0.048−206 cm3 μm/m2 day kPa at 50% RH depending on the type and percentage of the filler as well as on whether a plasticizing agent or cocomponent was used in the formulation (Table 2). It is noteworthy that introducing a layered silicate, such as talc or MMT, into the formulation of hemicellulose-based films improves the OP (2.42−10.82 cm3 μm/m2 day kPa) even at a high RH (80% RH) (Table 2). A wide range of structures prepared by chemical modification of wood hemicelluloses have been reported in the literature. However, only a few of these studies report OP values of the corresponding modified hemicelluloses. Grafting with hydrophobic entities, such as benzyl groups, fatty acids, or PLLA, substitutes the hemicellulose hydroxyl groups. This results in a reduction of the hydrogen-bonding density and in turn changes the molecular packing ability of hemicellulose, which will disturb its oxygen barrier capacity. For instance, films based on benzylated AcGGM have an OP value in the range of 130−560 cm3 μm/m2 day kPa. Other chemical modifications that do not change the polar nature of hemicellulose and/or lead to a cross-

decreases to some extent their water and moisture sensitivity, leading to OP values as low as 1.3 cm3 μm/m2 day kPa at 80% RH. Thus, the utilization of a less-purified hemicellulose grade, rather than highly pure hemicellulose, is considered to be a very promising approach. On the other hand, films based on poly (Llactide) (PLLA)-grafted hemicellulose show fairly high OP values ranging from 200 to 300 cm3 μm/m2 day kPa. This is mainly due to the intrinsic low barrier properties of PLLA (610 cm3 μm/m2 day kPa), as well as the inhomogeneous morphology that is caused by the incompatibility of the two components. Plasticized hemicellulose films have OP values ranging from 0.2 to 22 cm3 μm/m2 day kPa (Table 2). Plasticizers including sorbitol, glycerol, or xylitol have been used to enhance the cohesive film formation and reduce their brittleness. Similar to water, these polyols are known to increase the molecular mobility and free volume in the films, which in turn tend to impair the barrier performance. As an exception, films made from 65% GX and 35% sorbitol exhibited a rather low OP (0.21 cm3 μm/m2 day kPa). This may be explained by the fact that some plasticizers phase separate and form crystals at the surface of the films due to the low miscibility. Furthermore, when the amount of the plasticizer in the films is 13

method

ref 130

225

Tensile test

50% RH, 23 °C

203

50% RH, 23 °C

228

194

tensile test

50% RH, 23 °C

DMA at various RH

0%

>11 >5 a

conditions

164

50% 90%

n.s. = not specified.

decreased by over 70% compared to hemicellulose-based films without LS. This was related to the tortuous path effect imparted by the plate-like geometry and tightly packed crystalline structure of LS, making the diffusion path through the films more tortuous. The chain extension of hemicellulose chains through reductive amination also led to a decrease in the WVP by ca. 50%, even with a reduced amount of CMC cocomponent, providing a water barrier of the same order of magnitude as the hemicellulose-based films reinforced with LS. Typically, hemicelluloses without any added additives or cocomponents do not form coherent films that allow for analysis of WVP/WVTR; hence, there are no available data for pure hemicellulose films in Table 3.

linked system usually lead to low OP. A spruce WH coating cross-linked by adding 5−50% glyoxal has OP values of 1.6−7.7 cm3 μm/m2 day kPa at 50% RH. Films based on a mixture of chain-extended WH and CMC (15−30 wt %) exhibited an OP in the range of 0.2−5.9 cm3 μm/m2 day kPa at 50% RH and 10.6−14.9 cm3 μm/m2 day kPa at 80% RH (Table 2). 8.2. Water Vapor Permeability

As mentioned earlier, the polar and hydrophilic character of hemicellulose, the sensitivity to water/moisture and the moisture barrier properties are major challenges to be considered when extending the use of hemicellulose-based films under humid conditions for packaging moist products. The conventional fossil fuel-based polymers, such PE and PP, used today in the food-packaging industry have a water vapor transmission rate (WVTR) in the order of 10−1 g/m2 day. The barrier requirement, however, varies depending on the food product to be packaged and on the desired shelf life. The water vapor permeability (WVP) and WVTR are usually determined by the cup test according to the ASTM standard E 96/E 96M-05. The results strongly depend also on the specific RH at each side of the cup (Table 3). Films of a hemicellulose blend with quaternized cellulose show a WVP in the range of 59.4−64.3 g mm/m2 day kPa (Table 3). Plasticized hemicellulose with sorbitol and cross-linked hemicellulose with citric acid exhibit WVP values of 0.3−37 g mm/m2 day kPa and 2.35−2.95 g/(mm2 h) × 10−7, respectively. The incorporation of fillers has shown a positive effect in the lowering of WVP. For instance, the WVP and WVTR of hemicellulose-based films with NFC are in the range of 0.32−10.5 g mm/m2 day kPa and 14.7−476 g/day m2 at 0 and 50% RH, respectively. Another example is the addition of layered silicates (LSs), i.e., MMT and talc. Depending on the type and amount of LS, the WVP can be

8.3. Mechanical Properties

Mechanical properties are key parameters for many applications. A certain degree of strength and flexibility of the hemicellulose-based films/coating is necessary to be able to bear the stresses and strains that the material might be subjected to. Yet, wood-derived hemicelluloses alone are usually unable to form coherent free-standing films. The films are generally very delicate to handle and can easily crack or break into small pieces during the manufacturing process. This is mainly due to the low molecular weights, low chain mobility, and high chain stiffness of hemicelluloses, which do not allow the formation of a sufficient number of chain entanglements that will help formation of a cohesive and continuous network. Plasticization is the classical approach to enhance the film formability and extensibility of polysaccharides. Commonly used plasticizers for hemicelluloses include sorbitol, glycerol, xylitol, or even water. These molecules are able to disrupt the hydrogen bonding between the chains and increase the chain mobility and the free volume in the material. Hence, the 8199


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plasticizing agent is covalently bound to the hemicellulose backbone. Analogy can also be drawn between the polymer grafting and physical blending with macromolecular components as they both impact the mechanical performance of hemicelluloses in an almost similar way. Then again, the fact that the polymer is covalently attached to hemicelluloses in the case of grafting makes it possible to obtain a more homogeneous material with a category of polymers that are not necessarily miscible or compatible with hemicelluloses. The grafted hemicellulose copolymers can also be used in smaller proportions as a compatibilizing agent in hemicelluloses/polymer blends, as was demonstrated in the case of softwood hydrolysate/PLLA blends.203 Reductive amination derivatization chemistry was used as a tool for attaching very small amounts of a coupling agent to the reducing end of each hemicellulose chain present in a hardwood hydrolysate, subsequently connecting the hemicellulose chains in a head-to-head arrangement.194 Here the chemical modification increases the hemicelluloses molecular weight while preserving the interchain interaction as the native hydroxyl groups were not altered. This makes it possible to increase the proportion of WH in the formulations up to 85% (w/w) without compromising the formability and strength of the films.194 Cross-linking, on the other hand, results in stiffer and stronger films/coatings, as the force required to move the chains relative to each other and ultimately break the material becomes significantly higher due to the introduced covalent cross-links between the chains. Unlike hydrogen bonding, covalent bonding is not sensitive to moisture, which explains why the cross-linked hemicelluloses based films still retain a relatively high modulus even at a RH as high as 90%.

hemicellulose-based material would be able to stretch more before cracking. As seen in Table 4, the tensile strain-to-break of plasticized hemicelluloses films can be as high as 27%. Such an improvement in extensibility of the hemicellulose is often obtained at the expense of strength and stiffness. With increasing amounts of xylitol (20−50 wt %) in aspen GX films, the tensile strength decreases from 40 to 3 MPa and the modulus drops from 2.5 GPa to 90 MPa.221 Moreover, due to their small size, migration of the plasticizing molecules inside and/or outside the hemicellulose films is likely to occur, which could result in a phase separation or loss of these molecules. Ultimately, this would lead to loss of the plasticizing effect with time. In barrier applications, it is important to note that the addition of hydrophilic plasticizers such as the aforementioned sorbitol, glycerol, and xylitol may significantly impair the barrier performance at higher RH. Alternatively, blending hemicelluloses with compatible macromolecular components may offer a better compromise between ductility and strength, while avoiding the migration and aging issues of plasticization. Macromolecular cocomponents that have been used include carboxyl methyl cellulose (CMC), quaternized cellulose, chitosan, alginates, carrageenan, and poly(vinyl alcohol). The tensile strain-to-break of these hemicellulose-based blends varies between 0.9 and 9.6%. The tensile strength can vary between 9.8 and 112 MPa while moduli range from 2.9 MPa to 2.9 GPa (Table 4). The mechanical properties of the blends depend not only on the composition and the inherent physical proprieties of the constituent polymers but also on the degree of miscibility, compatibility, and mutual affinity between hemicelluloses and cocomponent. A few other studies have considered the reinforcement of hemicelluloses with organic and inorganic fillers including sepiolite, nanoclays, microfibrillated, nanocrystals, and nanofibrillated celluloses. The hemicellulose-based composites present a generally high modulus and tensile strength reaching up to 16.2 GPa and 319 MPa, respectively.230 Here again, mechanical properties depend strongly on the content and inherent properties of the reinforcing particles, i.e., morphology, dimension, and physicochemical proprieties as well as their affinity with hemicelluloses and other constituents of the matrix. This highlights the need for additional studies that systematically assess the structure−property relationships and relate several important properties to each other. A common strategy is to focus on structure modification, mechanical properties, or some other property independently, depending on the intended application. Yet, modifications in one end can affect the various properties differently, and optimization with regard to one property, e.g., oxygen permeability, may at the same time prove inferior to the flexural strength or ductility. In some applications, such as in packaging materials, sufficient mechanical integrity is needed in combination with an appropriate barrier performance. Chemical modification is a versatile approach, as different effects can be obtained depending on the type and extent of the chemical changes introduced. Grafting with small entities such as hydroxypropyl groups leads to a more flexible and less-strong material.164 This type of modification is also referred to as internal plasticization. Substitution of hydroxyl groups results also in a reduction of the hydrogen bonding between the chains and increases the segment mobility. Unlike the external plasticization discussed earlier, the internal plasticization does not suffer from the migration and aging problems as the

9. OUTLOOK The family of hemicelluloses stands out as a very promising natural resource that can be utilized as a biobased materials feedstock for several reasons that are related not only to the availability of the raw material and its contents and characteristics but also to the well-established forest management and processing infrastructures. There are a number of interesting streams in the paper and pulp industrial processes that have great potential to provide value-added products alongside the core product of the mill. Enabling value-adding utilization of all wood biopolymers is an important step toward realizing pulping processes as biorefineries. Hemicelluloses feature distinct chemical and structural properties, which inherently enable them to perform a diverse set of functions in wood. In addition, the specific spatial molecular organization of the wood biopolymers and the hierarchical buildup of the wood matrix contribute to the complex and advanced performance of the wood composite. Understanding these features and preserving the native molecular interactions are key strategies to better understand and mimic the utilization of these biopolymers in an analogue engineering context. The liberation of hemicelluloses from wood, subsequent fractionation recovery, and purification stages offer opportunities for controlling the composition and molecular features of the extract. The production of highly purified hemicelluloses typically requires time-consuming multistep purification procedures and may not be beneficial for the resulting property profile of the hemicellulose product. We have shown that material formulations can be made directly from the non8200


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instance, with heavy metal ion adsorbants or chelating moieties. Gels may also serve as matrixes for slow release. In agriculture, hemicellulose gels may serve as moisture buffers in dry soil or systems for slow release of fertilizers or pesticides. Adequately functionalized, hemicelluloses constitute a new class of watersoluble or water-dispersable polyelectrolytes. Societal awareness of sustainability issues is a strong driver for the implementation of such biobased materials on the market; still, commercial applicability and success depend also on the price and efficiency of extraction and production processes. The applicability of cheap and not highly purified hemicellulose fractions is a key step in this direction.

purified hemicellulose-rich fractions, with better tensile and barrier performance than their highly purified counterparts. The development of green and more practical, although less extensive, refining procedures is of priority for securing hemicellulose production on large industrial scales. Such hemicellulose fractions, albeit never offering the structural and molecular weight reproducibility of synthetic polymers, are believed to have strong potential for a range of material applications. The successful design and development of hemicellulosebased products require a solid understanding of the composition−structure−property relationships making the physicochemical characterization of hemicelluloses essential in the design process as well as for optimizing and monitoring extraction, recovery, and modification processes. Characterization methods that assess the structural and physicochemical properties of organic molecules and synthetic polymers have been widely applied but may not always be sufficient to reveal the implications of specific structural variations (branching, substitution patterns, cocomponent interactions, etc.) on the physical and mechanical property profiles. We believe that a more in-depth understanding of the structure−property relationships induced by the specific supramolecular hierarchical organization of lignocellulosic biopolymers mediated by the development of more advanced and biopolymer-specific characterization techniques will be a key enabling technology in the emerging biorefinery sector. Chemical modification of wood hemicelluloses offers numerous possibilities to control and tailor the properties of hemicellulose for product development. Historically, reactions developed with cellulose modifications in mind were directly applied to hemicelluloses, although the structural diversity, branching patterns, and solubility of the latter sometimes rendered the modification pathways less suitable. Today, there is a growing interest in designing chemical pathways directly adapted for the hemicellulose-specific chemistry and properties. Recent advancements in polymer synthesis allow for better selectivity and efficiency and enable targeted hemicellulose modification such as grafting, copolymerization, and substitution. Still, further efforts are needed: to realize hemicelluloses as sustainable alternatives to fossil-based materials in the future, we need new synthesis tools and modification strategies, certainly running under much more green and water-based conditions than the conventional methods could offer. Such synthesis schemes should be robust enough to accommodate a hemicellulose feedstock that is heterogeneous with respect to molecular weight and purity. Transferring molecular wood-cell interactions into hemicellulose-based materials offers new design principles for material formulations. Using molecular modeling is a first step toward improving our understanding and control of structure−property relationships. More advanced characterization and formulation strategies will make it possible to control the structural and compositional heterogeneity and produce materials with targeted properties, although only a few applications are so far explored and suggested. Hemicellulosebased barriers are already in the spotlight, which is not surprising in view of the excellent barrier properties. However, the products have to be protected from direct contact with too high humidity. Hemicelluloses may also have a future as more or less cross-linked hydrogels. A high capacity for absorption and retention of water merits use as membranes for removal of water or purification of water if functionalized accordingly, for

ASSOCIATED CONTENT Special Issue Paper

This paper is an additional review for Chem. Rev. 2016, volume 116, issues 3 and 4, “Frontiers in Macromolecular and Supramolecular Science”.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Ann-Christine Albertsson: 0000-0001-8696-9143 Present Address

Anas Ibn Yaich is presently at BillerudKorsnäs Skog & Industri AB, Fröviforsvägen, SE-7180 Frövi, Sweden. Notes

The authors declare no competing financial interest. Biographies Anas IbnYaich received his Ph.D. degree from the KTH Royal Institute of Technology under the supervision of Profs Ulrica Edlund and Ann-Christine Albertsson in 2015. His research has focused on the development of barrier materials derived from renewable resources for packaging applications. Ulrica Edlund is a professor at Fiber- and Polymer Technology, KTH Royal Institute of Technology. After receiving her Ph.D. from KTH and postdoctoral training at Department of Chemistry at University of Pennsylvania, U.S.A., she joined the faculty at KTH. Her research expertise comprises synthesis, surface modification, characterization, and materials design of polymers, with special focus on the development of functional and advanced formulations and materials from renewable resources from terrestrial and marine biomass including forestry biomass, algae, and agricultural residues. Ann-Christine Albertsson is professor em at Fiber- and Polymer Technology, KTH Royal Institute of Technology. She was appointed Professor in Polymer Technology in 1989. Her honors and awards include, e.g., the Arnberg Award from the Royal Swedish Academy of Sciences (1992), the Giulio Natta Award, Italy (2009), and the ERC Advanced Grant (EU Top Scientists Award) 2010. Her research includes design, synthesis, modification, and characterization of polymers, mainly well-organized, tailor-made degradable polymers for medical applications as well as sustainable polymers with controlled environmental interaction.

ACKNOWLEDGMENTS The authors thank VINNOVA (Project no. 2009-04311) for financial support. 8201


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