Synthesis and characterization of birch wood xylan succinoylated in 1-n-butyl-3-methylimidazolium chloride

Abstract

The chemical modification of birch wood xylan was undertaken in the ionic liquid 1-n-butyl-3-methylimidazolium chloride (C₄mimCl) using three different long-chain succinic anhydrides: n-octenyl succinic anhydride (n-OSA), n-dodecenyl succinic anhydride (n-DDSA) and n-octadecenyl succinic anhydride (n-ODSA). Reactions were performed at temperatures of 80–100 °C and reaction times of 0.5–22 hours resulting in a degree of substitution (DS) of 0.08–0.27. The formation of the desired products was confirmed through the use of ¹H NMR and FT-IR, while DS was determined by titration. Thermogravimetric analysis of the modified and native xylans showed a slight lowering of thermal stability with functionalization. Contact angle measurements on spin-coated surfaces of modified xylan films showed a significant increase in hydrophobicity with the introduction of the alkenyl-functionalized succinic anhydride moieties.

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Introduction

In response to the increasingly limited nature of fossil fuels, lignocellulosic biomass from trees, grasses, cereals and other plants has become the focus of the developing biorefining industry.¹ Plant materials are primarily made up of three biopolymers: cellulose, lignin and hemicellulose and, of these, cellulose and lignin have received by far the most attention in terms of material applications. Traditionally, hemicellulose is defined as the alkali-soluble material after the removal of pectic substances from plant cell walls² and xyloglycans (xylans) are a sub-group within hemicelluloses consisting of β(1→4)-D-xylopyranose backbone with side groups on the 2- or 3-position.

The use of renewable resources for the production of food packaging in particular has recently received increased interest.^3–5 Depending upon the application, low oxygen permeability as well as mechanical strength and flexibility can be important target properties for packaging films. Although there has been development of hemicellulose coatings for paperboard,⁶ research has shown that free-standing films produced from unmodified hemicelluloses are hygroscopic, resulting in poor properties in high humidity.⁷ Consequently, chemical modification and/or the addition of plasticizers has been explored as a route to obtaining coherent films with better properties. For example, surface fluorination of arabinoxylan films⁸ with trifluoroacetic anhydride has been used as a method to produce hydrophobic films, in which dramatic changes in water contact angles were seen with the introduction of even small amounts of fluorine. In another example, O-acetylgalactoglucomannan was treated with benzyl chloride and cast as films, resulting in products with low oxygen permeability (8 cm³ μm m⁻² d⁻¹kPa⁻¹) and increased resistance to solubility in water.⁹

A major complication during chemical modification of hemicellulose in traditional organic solvents is poor solubility resulting in heterogeneous reaction conditions. Since the complete solution of xylan in ionic liquids has been reported,^10,11 this problem may potentially be overcome by utilizing ionic liquids as reaction media. The use of ionic liquids for homogeneous reactions in carbohydrate chemistry has gained interest and the modification of polysaccharides, in particular cellulose, in these solvents, has been studied extensively over the last 20 years. Ionic liquids are thermally and chemically stable salts with a melting point below the boiling point of water and are often referred to as “green” chemicals on account of their high stability and low vapour pressure, making indefinite recycling possible in principle. There are, however, also concerns regarding the “greenness” of ionic liquids^12–14 and toxicity studies of the most commonly used (i.e., imidazolium-type) ionic liquids have shown that the effect on algae,^15,16 bacteria^17,18 and mammalian cells^19,20 is in some cases comparable to traditional solvents such as methanol, chloroform and acetonitrile in EC₅₀ tests. Furthermore, the generation of unwanted and toxic side-products may take place under certain reaction conditions.²¹ Nevertheless, as recently documented, research on ionic liquids as solvents and reaction media for polysaccharides and other polymers continues to attract interest.^22–26 The latest research on cellulose in ionic liquids includes modification with succinic anhydride in 1-butyl-3-methylimidazolium chloride^27–30 and 1-allyl-3-methylimidazolium chloride,³¹ while research on dissolution or ionic liquids as reaction media for carbohydrates and other macromolecules has been reviewed.^22,24 Functionalization of carbohydrates with succinic anhydride and related anhydrides has been performed by a number of groups (e.g. the utilization of long-chain alkyl succinic anhydrides to increase the hydrophobicity of starch^32–35). Maleic anhydride was recently used for functionalization of xylan-rich hemicelluloses from bamboo in 1-butyl-3-methylimidazolium chloride.¹¹ Modification of hemicellulose with succinic anhydride has previously been performed by Sun et al.^36,37 Similarly, xylan has been acetylated¹⁰ in ionic liquids and a number of studies have been completed on dissolving wood, including native hemicelluloses, in ionic liquids (e.g., Fort et al.,³⁸ Kilpelainen et al.,³⁹ and Zavrel et al.⁴⁰); however, the formation of hydrophobic hemicelluloses through reaction with widely available long-chain alkenyl succinic anhydrides (ASAs) in ionic liquids has, to our knowledge, not been attempted. The ASAs described here are widely used in the paper industry and therefore available in large quantities.

In the research described here, birch wood xylan was modified with alkenyl chain-functionalised succinic anhydrides (ASAs) (Fig. 1) to yield novel hydrophobic polymers as a step towards new bio-derived packaging film materials. 1-Butyl-3-methylimidazolium chloride (C₄mimCl) was used as the reaction medium.

Fig. 1 Modification of xylan with long chain alkenyl succinic anhydrides. Substitution may take place at either hydroxyl group (or both) and is shown here at C2 as an example. The structure of xylan is simplified and here represented by a D-xylopyranose ring.

Experimental

Materials

Birch wood xylan and 1-n-butyl-3-methylimidazolium chloride (C₄mimCl) were purchased from Sigma Aldrich (Steinheim, Germany) and used without further purification. The bottle containing C₄mimCl was stored in a desiccator over silica gel after opening. n-Octenyl succinic anhydride (n-OSA), n-dodecenyl succinic anhydride (n-DDSA) and n-octadecenyl succinic anhydride (n-ODSA) were kindly supplied by Pentagon Chemicals (Workington, UK). Laboratory-grade dimethylsulfoxide (DMSO) was purchased from Sigma Aldrich (Steinheim, Germany) and anhydrous ethanol (99.9%) was obtained from Kemetyl (Køge, Denmark).

Succinoylation of hemicelluloses

In a typical succinoylation, 1.00 g (7.6 mmol) of xylan and 10 g of C₄mimCl (57 mmol) were placed in a round-bottomed flask equipped with a magnetic stirrer and heated at 80 °C until the ionic liquid had melted. In the next step 1.35 g (6.4 mmol) of n-OSA was added and reaction proceeded for the chosen reaction time. The reaction was stopped by quenching in ethanol. The formed precipitate was isolated by filtration, rinsed in ethanol and then dried overnight in a vacuum oven at ambient temperature. A series of such experiments was carried out in which reactant ratios (n_xylan : n_ASA), reaction temperatures and times were varied in the ranges of 1 : 0.8 to 1 : 2.5, 80–100 °C and 0.5–22 hours respectively. Molar amounts of xylan were calculated on the basis of xylose sub-units (M = 132 g mol⁻¹) with two free OH-groups.

The modified xylan yield was calculated from the following equation:

In eqn (1)m_product is the weight of the end-product, n_HC and m_HC are the initial amounts of hemicellulose in terms of mol and g, respectively, M_ASA is the molecular weight of the ASA and DS is the degree of substitution determined by titration. The theoretical maximum value of DS was estimated as two based on the number of hydroxyl groups on the predominant xylose unit.

Characterization

Sugar analysis. Total sugar content of xylan was determined by HPLC analysis using the following procedure.⁴¹Xylan (0.12 g) was dissolved in 20 ml 4% H₂SO₄ and autoclaved for 10 minutes at 121 °C. 5 ml of the autoclaved sample was then neutralized with 0.4 g CaCO₃ and filtered for HPLC analysis. 40 μl samples were injected at a temperature of 63 °C and flow rate of 0.6 ml min⁻¹ using 4 mM H₂SO₄ as eluent on an Aminex HPX-87H column equipped with a Shimadzu refractive index (RI) detector. By this method, the selected birch wood xylan was shown to contain xylose (78.0%), glucose (2.0%), arabinose (0.0%) and Klason lignin (0.6%).

Titration . The degree of substitution (DS) in the reaction products was determined by titration of the formed ester groups with 0.02 M NaOH. DS was calculated using the following equation (modified from Stojanovic et al.⁴²):

In eqn (2), m_sample is the weight of the titrated sample, V_NaOH and c_NaOH are the volume and concentration of NaOH used for titration, M_ASA is the molecular weight of the ASA and 132 g mol⁻¹ is the molecular weight of the D-xylopyranose backbone unit. Blind tests were performed with the native xylan to determine the amount of acid present. These experiments showed that the contribution from acid groups in the unmodified xylan was negligible (less than 0.8% substitution). Four samples of each modified xylan were titrated and DS was calculated as an average of the obtained values.

NMR . ¹H NMR was performed on a Bruker 250 MHz apparatus using DMSO-d₆ as solvent (15 mg sample in 0.7 ml solvent).

FT-IR . FT-IR spectra of the reaction products were recorded on a Perkin & Elmer Spectrum One spectrometer with an STI Thunderdome ATR system in transmission mode. Samples were prepared as KBr pellets and spectra were then collected in the range of 4000–450 cm⁻¹ at a resolution of 2 cm⁻¹.

SEC . Xylan molecular weights were determined by size exclusion chromatography (SEC). Samples for analysis were dissolved in 1 M NaOH (4 mg ml⁻¹) by stirring overnight, diluted four times in eluent (0.01 M NaOH, 50 mM NaCl, pH 12), and 200μl aliquots were then injected on a Superose 12 HR 10 × 300 mm column (GE Healthcare). The eluent flow rate was 0.5 ml min⁻¹. Chromatograms were monitored using light scattering (Model 270 from Viscotek,), refractive index (Shimadzu) and UV-VIS photodiode array (Shimadzu) detectors employing pullulan standards in the molecular weight range of 5600–380 000 g mol⁻¹.

Solubility. Solubility experiments were performed using an n-OSA-modified xylan with DS = 0.15 (experiment 2). Samples of 10 mg were placed in small glass vials with 0.5 ml solvent and heated to 80 °C (for lower boiling solvents to 10 °C below boiling point) for six hours. The presence of undissolved product could be observed with the naked eye.

DSC . A differential scanning calorimetry (DSC) Q1000 system (TA Instruments) was used to observe phase changes in the range of −50 °C to 200 °C in nitrogen atmosphere. Samples were heated to 200 °C at a rate of 10 °C min⁻¹ and cooled to −50 °C to remove any effects induced by prior treatment. Phase transitions were then observed by reheating from −50 °C to 200 °C at 10 °C min⁻¹.

TGA . Thermogravimetric analysis (TGA) was performed using a TGA Q500 (TA Instruments) and heating samples from ambient temperature to 600 °C at a rate of 10 °C min⁻¹ under nitrogen atmosphere.

Contact angle measurements. Samples for contact angle measurements were dissolved in DMSO by heating and stirring at 50 °C at a concentration of 40 mg ml⁻¹ and then spin coated onto glass slides (2.5 cm × 7.5 cm). Prior to spin coating the glass slides were cleaned by sonication for 5 min in isopropanol and rinsed with DMSO. Solution samples (400 μl) were spin coated at 800 rpm for 1 min followed by 0.5 min at 2000 rpm to remove excess solvent. Spin-coated samples were dried overnight under vacuum at 60 °C to remove residual solvent. Measurements were performed using an OCA20 contact angle system apparatus (Dataphysics, Filderstadt, Germany) and water contact angles were obtained at 25 °C by the sessile drop method. Initial water droplet contact angles were determined at seven different positions on each film and a mean value was calculated.

Results and discussion

Succinoylation of xylan

Xylan modification reaction conditions were varied for the three ASAs with respect to the ratio of reactants, reaction temperature and reaction time. Most experiments were carried out using n-OSA but selected experiments to determine the effect of chain length were also conducted with n-DDSA and n-ODSA. An overview of the experiments is shown in Table 1, in which DS determined by titration is reported.

Table 1 Overview of succinoylation reactions. m(xylan) : m(C₄mimCl) = 1 : 10

Experiment	Succinic anhydride	Temperature/°C	Time/h	Yield (%)	n xylan  : nASA^a	DS
a Molar ratio of xylose units (M = 132 g mol^−1) to ASA.
1	n-OSA	80	0.5	98	1 : 0.8	0.08
2	n-OSA	85	0.5	96	1 : 0.8	0.15
3	n-OSA	90	0.5	98	1 : 0.8	0.18
4	n-OSA	95	0.5	98	1 : 0.8	0.24
5	n-OSA	100	0.5	96	1 : 0.8	0.27
6	n-OSA	80	1	94	1 : 0.8	0.12
7	n-OSA	80	2	94	1 : 0.8	0.17
8	n-OSA	80	6	90	1 : 0.8	0.23
9	n-OSA	80	12	70	1 : 0.8	0.27
10	n-OSA	80	18	82	1 : 0.8	0.27
11	n-OSA	80	22	81	1 : 0.8	0.24
12	n-OSA	80	2	50	1 : 1.7	0.16
13	n-OSA	80	2	62	1 : 2.5	0.10
14	n-DDSA	80	2	79	1 : 0.8	0.16
15	n-DDSA	80	2	86	1 : 1.7	0.17
16	n-DDSA	80	2	70	1 : 2.5	0.19
17	n-ODSA	80	2	86	1 : 0.8	0.16

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The strategy of using long-chain alkenyl anhydrides instead of acetic anhydride or other similar low molecular weight anhydrides was chosen so as to avoid carboxylic acid by-products while at the same time introducing hydrophobic character. However, it must also be noted that the selected method results in the presence of a carboxylic acid group on the introduced side chain (see Fig. 1). As indicated in Table 1, the DS values were moderate with reaction efficiencies ranging from 0.2 to 0.8. This could be due to unwanted side reactions including hydrolysis of the succinic anhydride function of the ASAs, degradation of the xylan, and crosslinking (e.g., by reaction between the new side chains and the backbone functionalities).

A low degree of hemicellulose substitution has been noted in previous studies of similar reactions. Succinoylation of wheat straw hemicelluloses in alkaline solution (pH = 8.5–9.0) at temperatures of 28–45 °C yielded products with DS of 0.017–0.21 at reaction times of 0.5–16 hours.³⁶ The authors stated that hydrolysis of succinic anhydride and the formed hemicellulose derivatives could explain the low degree of substitution. Peng et al.¹¹ functionalized hemicellulose with maleic anhydride in C₄mimCl, obtaining products with higher substitution degrees (DS of 0.095–0.75) than reported here; however, reactions were in all cases catalyzed with LiOH, while varying reaction times (40–100 min), reaction temperatures (60–100 °C) and maleic anhydride to anhydroxylose unit reactant ratios (1 : 1 to 8 : 1). Xie et al.³⁵ compared acetylation and succinoylation of starch in C₄mim and found that the degree of substitution was significantly lower for the latter reaction. While DS of 0.01–0.18 could be obtained for both esters without using a catalyst, the introduction of pyridine increased DS to 0.7–2.35 for acetylated starch and 0.46–0.91 for succinylated starch.

Cellulose and starch are similar in nature to hemicelluloses in terms of being high molecular weight polysaccharides and the reactions of these biopolymers give an indication of what may be expected for xylan. Two studies on succinoylation of cellulose with succinic anhydride in ionic liquids demonstrated that the expected degree of substitution is modest. Cellulose was succinoylated in C₄mimCl/DMSO by Liu et al.²⁷ who varied reaction temperatures (85–100 °C), reaction times (5–120 min) and reactant ratios (2 : 1 to 14 : 1), resulting in DS of 0.038–0.53 (100% substitution of cellulose being DS = 3). Similar results were obtained in a study of succinoylation of cellulose in 1-allyl-3-methylimidazolium chloride,³¹ where temperatures of 60–110 °C, reaction times of 30–120 min and reactant ratios of 1 : 1 to 12 : 1 yielded a DS of 0.07–0.22. Liu et al.³⁰ improved their results by catalysing the succinoylation reaction of cellulose in C₄mimCl/DMSO with iodine and increased the DS from 0.24 to 0.56–1.54 by varying reaction temperatures (85–110 °C), reaction times (30–120 min) and the reactant ratios of I₂ : succinic anhydride (2 : 1 to 15 : 1). Succinoylation of cellulose in C₄mimCl/DMSO catalyzed with 4-dimethylaminopyridine (DMAP)²⁸ increased the DS from 0.24 to 0.94–2.34 by varying reaction temperatures (60–110 °C), reaction times (30–120 min) and the ratios of DMAP : succinic anhydride (1–15%). A parallel study utilizing N-bromosuccininide (NBS) as a catalyst for the formation of cellulose succinylates in C₄mimCl/DMSO²⁹ yielded similar results with DS improving from 0.24 to 0.92–2.31 by varying reaction temperatures (90–120 °C), reaction times (30–240 min) and the ratios of NBS : succinic anhydride (1–20%).

In the following section the effect of varying the reaction conditions in the present study is discussed.

Effect of reaction time. Fig. 2 illustrates the calculated yield and DS as a function of reaction time when xylan was reacted with n-OSA in C₄mimCl at varying reaction times. The reaction temperature of 80 °C was chosen as this is just above the melting temperature of the ionic liquid and is therefore the lowest practical operating temperature. It was desirable to keep the temperature as low as possible as degradation of xylan has previously been observed at temperatures of 75–85 °C in organic solvents.⁴³ The calculated DS increased as a function of reaction time until ∼12 hours, after which no further increase was observed and indeed a slight decrease in DS may have occurred at the longest reaction time. At longer reaction times (i.e., six hours or longer) discoloration of the modified products was observed and reaction times of two hours or less were therefore preferred. Discoloration could be due to unwanted side-reactions such as crosslinking at the OSA side-chain double bonds, crosslinking from the carboxylic acid to other side groups³⁷ and partial degradation of xylan as well as modified xylan.^11,43

Fig. 2 Yield and DS of n-OSA-modified xylan in C₄mimCl. T = 80 °C, [xylan] : [n-OSA] = 1 : 0.8, m(xylan) : m(C₄mimCl) = 1 : 10.

Considering other reports, an increase in DS with reaction time, with a maximum at two hours within the time range of 0.5–16 hours, was found in the modification of wheat straw hemicelluloses with succinic anhydride in an aqueous alkaline system.³⁶ For the succinoylation of hemicelluloses in an N,N′-dimethylformamide/lithium chloride system catalyzed with pyridine, an increase in DS was observed up to 12 hours, after which a decrease was seen.³⁷ The same tendency was seen in the succinoylation of cellulose in C₄mimCl/DMSO catalyzed by DMAP²⁸ and NBS²⁹ respectively. Reactions of maleic anhydride and hemicellulose in C₄mimCl showed an increase in DS when proceeding for up to 80 minutes, while the DS decreased for longer reaction times.¹¹ In contrast to the mentioned studies the DS of sugar cane bagasse cellulose reacted with succinic anhydride in 1-allyl-3-methylimidazolium chloride for 30–160 min was observed to increase with increasing reaction time and temperature.³¹

Effect of reaction temperature. Fig. 3 shows the calculated yield and DS as a function of reaction temperature when n-OSA was reacted with xylan in C₄mimCl in the range 80–100 °C. As the reaction rate was expected to increase as a function of temperature, a relatively short reaction time of 30 minutes was chosen for this study. As shown, there is a clear increase in DS as a function of reaction temperature. Increased discoloration was noted at the higher temperatures as shown in Fig. 4.

Fig. 3 Yield and DS of n-OSA-modified xylan in C₄mimCl. Reaction time = 0.5 hours, [xylan] : [n-OSA] = 1 : 0.8, m(xylan) : m(C₄mimCl) = 1 : 10.

Fig. 4 Appearance of final products from reaction of birch wood xylan with n-OSA at various temperatures.

As with the findings reported here, reaction temperature increased the DS in a seemingly linear fashion in the n-DDSA modification of corn starch performed in an aqueous slurry under alkaline conditions.³³ The same tendency was seen for both NBS-catalyzed²⁹ and non-catalyzed²⁷ reactions of cellulose with succinic anhydride in C₄mimCl/DMSO. Increased functionalization at elevated temperatures is mainly due to increased diffusion of the ASA species^11,27 and may also stem from increased solubility of this compound in the solvent.^28,29,33 For maleic anhydride functionalization of hemicellulose an increase in DS was seen in the temperature range of 60–80 °C, while a decrease in DS was observed for elevated temperatures (80–120 °C), most likely due to enhanced solubility of all species leading to more unwanted side reactions.¹¹

Effect of reactant ratios. Experiments were performed for n-OSA and n-DDSA with varying ratios of xylan : ASA. The chosen ratios were 1.0.8, 1 : 1.7 and 1 : 2.5. For n-OSA the DS decreased with increased amount of n-OSA from 0.17 to 0.10, while a slight increase in DS from 0.16 to 0.19 was seen for n-DDSA. Furthermore, an experiment using xylan : n-ODSA of 1 : 0.8 was carried out, which yielded a product with DS of 0.16. Contrary to earlier observations on other systems there was no clear dependency on the ratio of ASA to xylan in the studied range of reactant ratios, indicating that DS ∼0.3 was probably the limit of the reaction under the given conditions. A more pronounced variation in the DS might have been observed in the current study with a larger excess of the anhydride.

In the literature, the efficiency of the reaction of xylan and maleic anhydride increased with addition of the anhydride until a maximum in DS was achieved with a four-fold excess of anhydride (relative to anhydroxylose units).¹¹ In the functionalization of carboxymethyl starch with octenyl succinic anhydride in a solvent system of dimethylsulfoxide and p-toluenesulfonic acid, the DS increased with increasing ratio of anhydride to starch.³⁴ A similar tendency was seen for acylation of both starch⁴⁴ and cellulose⁴⁵ in C₄mimCl using acetic anhydride, in which the DS increased with increasing anhydride : polysaccharide ratio. Succinoylation reactions of cellulose^27,30,31 and starch³⁵ under similar conditions as those used in the current study showed the same tendency. In these latter studies, the ratio of anhydride to polymer was varied to a greater extent than in the present study with a ratio of anhydride to hydroxyl functionality of up to 14 : 1.

Effect of alkenyl chain length. As mentioned earlier the change in DS as a function of xylan : ASA ratio did not show the same tendency for n-OSA and n-DDSA. The DS values obtained for these two ASAs are, however, in the same range (0.10–0.19) under the chosen reaction conditions at 80 °C. The DS appears to be independent of chain length, as functionalization with n-DDSA and n-ODSA reached the same levels as with n-OSA. This may indicate that the rate-determining step is the opening of the cyclic anhydride rather than diffusion of this species to the reactive sites (i.e., the OH-groups on xylan). Otherwise, the larger and more sterically hindered n-DDSA and in particular n-ODSA would exhibit lower modification of xylan under the same conditions as used for n-OSA.

Considering relevant literature reports, in the esterification of starch with alkenyl succinates using an aqueous system, the length of the alkenyl chain has been shown to have an influence on the ease of substitution due to differences in hydrophobicity and therefore differences in diffusion in the chosen media.³² The behavior of the ASAs in an ionic liquid is expected to differ from that in an aqueous system and therefore steric factors may outweigh the diffusion of the reactive species in the chosen system. The effect of steric hindrance was seen by Xie et al.³⁵ in their study, which compared the efficiency of starch esterification with acetic anhydride and succinic anhydride respectively. DS was significantly higher when using acetic anhydride, as opposed to succinic anhydride, which requires a more sterically hindered ring-opening of the anhydride for reaction with the hydroxyl groups on the carbohydrate to take place.

Characterization of modified xylans

¹H NMR . ¹H NMR spectroscopy was used to verify the formation of the functionalized xylans as well as to estimate the DS (data not shown). Polymeric peaks deriving from the ASAs could be observed in the ¹H NMR spectra at 2.5–0.5 ppm depending on ASA structure. Fig. 5 and 6 show examples of modified xylan NMR spectra with characteristic peaks of the side-group from ASAs at ∼1.2 and ∼0.8 ppm. Characteristic peaks of each ASA could be used to approximate DS values. The values of DS estimated from ¹H NMR were, however, lower than those found by titration in all cases. This may partly be explained by the difference in solubility in the chosen solvent of the xylan backbone and the ASAs, introducing an element of uncertainty in determining the ratios between the two. This effect would be especially pronounced for the longer chain ASAs, n-DDSA and n-ODSA. Furthermore, there will invariably be water present during the ¹H NMR analysis⁴⁶ when analyzing a hygroscopic compound in a likewise hygroscopic solvent and, as the water signal overlaps with the xylan signals, this could also be a source of error. In the calculations the degree of branching and side chains with their varying hydroxyl group substitution are not taken into account. The values of DS obtained by titration are therefore considered to be the most reliable and all DS data reported here were determined by this method.

Fig. 5 ¹H NMR spectra of birch wood xylan (A) and n-OSA-modified xylan (B) (experiment 10, Table 1). Signals from the xylan ring are found at δ 3.0–5.4 ppm (∼6H) and signals from the terminal part of the alkenyl chain of n-OSA are seen at δ 1.23 ppm (6H) and δ 0.84 ppm (3H). (The prominent peak at δ 2.5 is from the solvent, DMSO.)

Fig. 6 ¹H NMR spectra of ASA-modified xylans. A: n-ODSA-modified xylan (experiment 17, Table 1). Signals from the xylan ring are found at δ 3.0–5.4 ppm (∼6H) and the signal from the terminal part of the alkenyl chain of n-ODSA are seen at δ 1.21 ppm (26H) and δ 0.82 ppm (3H). B: n-DDSA-modified xylan (experiment 14, Table 1). Signals from the alkenyl chain of n-DDSA are seen at δ 1.22 ppm (14H) and δ 0.82 ppm (3H). (The prominent peak at δ 2.5 is from the solvent, DMSO.)

FT-IR . As shown in Fig. 7, FT-IR spectra of the birch wood xylan showed peaks associated with native hemicelluloses^{10,43,47–51} at 1611, 1416, 1252, 1164, 1047, 989 and 896 cm⁻¹. The signal at 3429 cm⁻¹ is assigned to hydroxyl stretching, while the peak at 2920 cm⁻¹ derives from C–H bond vibrations. The peak at 1047 cm⁻¹ is due to C–O–C bond stretching in glucosidic linkages,^36,48,52 whereas the signal at 896 cm⁻¹ is characteristic of β-glucosidic linkages between xylose rings.^48,50 The peaks at 1164 and 989 cm⁻¹ signals may be assigned to arabinosyl side chains,⁴⁹ while absorbed water is responsible for the prominent peak at 1611 cm⁻¹.^10,47FT-IR spectroscopy was used to confirm the formation of a new ester bond in the hemicellulosic product, indicated by the presence of a new peak in the carbonyl area (1745–1735 cm⁻¹) and the absence of the anhydride peaks from unreacted ASAs (1781 and 1862 cm⁻¹ for n-OSA, 1782 and 1860 cm⁻¹ for n-DDSA, see Fig. 7, and 1784 and 1863 cm⁻¹ for n-ODSA). The intensity of the C=O ester peak increased with DS (see Fig. 8). The signal from the new carboxylic acid may overlap with the signals from the inherent carboxylic acid groups³⁶ as well as from absorbed water. The intensity of the signal at 2920 cm⁻¹ was increased and also shifted slightly due to the presence of the long carbohydrate chains in the new side groups.^10,36,52

Fig. 7 FT-IR spectra of xylan (top), n-DDSA-modified xylan (middle) with DS = 0.16 (experiment 14, Table 1), and n-DDSA (bottom).

Fig. 8 FT-IR spectra of xylan (top) and n-OSA-modified xylans with DS = 0.08 (middle) and DS = 0.27 (bottom) (experiments 1 and 9, Table 1 respectively).

SEC . SEC was used to determine whether xylan degradation took place during the reactions and in all cases the amount of low-molecular weight product was low. This could indicate that the side reactions of the ASAs are predominant in limiting the reaction with respect to DS values. This is in contrast to observations for cellulose, which as reported elsewhere is degraded extensively in C₄mimCl.³¹ The peak-average molecular weight of the birch wood xylan, M_p, was 35 000 g mol⁻¹ as determined by SEC giving D_p ∼265. Fig. 9 shows SEC-traces of an n-OSA-modified xylan, an n-DDSA-modified xylan and the unmodified xylan. SEC data for modified xylans are shown in Table 2. In most cases a slight increase in molecular weight was found, which was most pronounced for the n-ODSA- and n-DDSA-modified xylans. This is not unexpected, as the introduction of side-groups should only give a minor change in conformation of the xylan molecule, thereby affecting the hydrodynamic volume only to a small degree. Limited degradation could also reduce molecular weights slightly, as seen for maleic anhydride-functionalized xylans where, as reported, the modified products exhibited lower molecular weights than the initial species.¹¹

Fig. 9 SEC-traces for xylan (black), n-OSA-modified xylan (red) and n-DDSA-modified xylan (dashed). The modified xylans are products from experiment 9 (DS = 0.27) and experiment 14 (DS= 0.16) respectively. The peak at retention volume near 15 ml is an artifact from the solvent.

Table 2 Molecular weights obtained by SEC analysis of ASA-modified xylans

Experiment No	Succinic derivative	DS	M̄ w/g mol^−1	M̄ n/g mol^−1	M peak/g mol^−1
Xylan	—	—	39 500	24 200	35 000
13	n-OSA	0.10	39 700	23 300	36 300
2	n-OSA	0.15	42 100	25 500	34 700
12	n-OSA	0.16	40 900	22 600	37 500
7	n-OSA	0.17	38 500	23 800	33 600
4	n-OSA	0.24	40 000	26 100	35 900
11	n-OSA	0.24	41 800	24 700	37 500
9	n-OSA	0.27	39 400	22 300	37 500
10	n-OSA	0.27	42 400	24 100	38 700
5	n-OSA	0.27	38 300	25 500	33 600
14	n-DDSA	0.16	41 400	24 000	35 200
15	n-DDSA	0.17	44 800	24 500	37 500
16	n-DDSA	0.19	48 900	25 100	38 700
17	n-ODSA	0.16	50 500	24 400	36 300

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DSC . DSC was applied to detect glass transition temperatures for the functionalized hemicelluloses. All samples exhibited a broad endothermic peak in the temperature range from ambient to approx. 150 °C. This is attributed to water loss and has previously been observed for glucomannans⁵³ and cellulose acetate.⁵⁴ This peak was more pronounced in terms of both intensity and broadening for the native xylan than the modified xylans, which could indicate the retention of less water in the modified samples. No distinct glass transitions were observed for the samples. However, values were expected to be difficult to observe due to overlap with the water retention peak compared to T_g values found in the literature,^9,53,55,56 which range from 55 °C to 115 °C.

TGA . In the TGA analyses, all samples were seen to contain some moisture as observed during heating from ambient temperature to 100 °C. The chemical modification of xylan resulted in reduced thermal stability, when comparing onset of degradation (T_initial) and peak degradation (T_peak) temperatures of the modified and non-modified xylans (Table 3, Fig. 10 and 11). Onset of degradation was in most cases observed at lower temperatures for substituted xylans (187–213 °C) than for the unmodified xylan (202 °C). Peak degradation temperatures ranged from 229 to 295 °C for the modified xylans, while it was 299 °C for non-modified xylan. However, for end-use purposes these differences may not be detrimental. Xylans with lower substitution degrees were generally more thermally stable than more highly substituted xylans, while the n-ODSA substituted xylan was more thermally stable than the others, exhibiting the highest initial degradation temperature (213 °C) and one of the highest peak degradation temperatures (264 °C) (Fig. 11).

Table 3 Data from TGA analysis of n-ASA-modified xylans

Exp. No	Succinic derivative	DS	T initial/°C	T peak/°C
Pure xylan	None	—	202	299
1	n-OSA	0.08	206	282
13	n-OSA	0.10	210	295
2	n-OSA	0.15	198	266
12	n-OSA	0.16	196	242
7	n-OSA	0.17	199	242
3	n-OSA	0.18	193	242
8	n-OSA	0.23	187	229
4	n-OSA	0.24	193	254
5	n-OSA	0.27	198	244
9	n-OSA	0.27	203	247
10	n-OSA	0.27	208	249
14	n-DDSA	0.16	199	254
15	n-DDSA	0.17	193	239
16	n-DDSA	0.19	193	239
17	n-ODSA	0.16	213	264

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Fig. 10 TGA plots of birch wood xylan and n-OSA-modified xylans with DS of 0.08 and 0.27, respectively (experiments 1 and 9, Table 1).

Fig. 11 TGA plots of birch wood xylan, n-DDSA and n-ODSA-modified xylans both with DS of 0.16 (experiments 14 and 17, Table 1 respectively). The corresponding first derivative of the weight loss of n-ODSA-modified xylan is also shown.

Referring to previous reports, maleic anhydride-functionalized hemicelluloses exhibited similar thermal stability behavior as the functionalization led to less thermally stable products.¹¹ A range of wheat straw hemicellulose esters were also observed by Xu et al.⁵⁰ to have lower thermal stability than the native hemicelluloses and this was also the case for wheat straw hemicellulosic succinates synthesized by Sun et al.³⁷Xylan succinylates obtained by microwave irradiation⁵⁷ were less thermally stable than the native xylan, although this in part could be due to degradation of the polymer as a result of the chosen modification process. Ren et al.⁵¹ found that lauroylated hemicelluloses with low DS (<0.78) were less thermally stable than the native biopolymer, while higher DS (>1.56) lead to compounds which were more thermally stable than the starting material. Stearoylated⁴³ and acetylated^10,48 hemicelluloses have previously been synthesized and found to be more thermally stable than the non-functionalized hemicelluloses. Succinylated celluloses modified in ionic liquids were less thermally stable than the native cellulose,^27,31 while starch succinates (DS < 0.7) from various plant sources were more thermally stable than the native compound.⁵⁸

Solubility. n-OSA-modified samples (DS = 0.15) were soluble in dimethylsulfoxide under heating. Products were not soluble in the following: tetrahydrofuran, pyridine, hexane, ethyl acetate, toluene, methylene chloride and methanol. The solubility of xylan in methylene chloride/methanol mixtures has previously been reported;⁵⁹ however, 90/10 and 50/50 (v/v) methylene chloride/methanol mixtures were not efficient solvents for the modified xylans.

Film surface properties. Contact angles of water droplets were measured as sessile drops on spin-coated films of selected modified xylans. The results are shown in Table 4 and the influence of the chemical modification of xylan on the surface properties is clearly shown. A slight increase in contact angle as a function of DS is observed for the n-OSA-modified samples. The impact of the n-OSA substituent on the contact angle of water even at low DS may in part be due to rearrangements on the film surface, resulting in orientation of the hydrophobic alkenyl chains away from the bulk of the material. This type of surface ordering has previously been observed for spin-coated fluorinated copolymers,^60,61 but has also been noted for systems similar to the one studied here. For example, organization of the long chain alkyl hydrophobic palmitate grafts on a hydrophilic poly(vinyl alcohol) backbone resulted in water contact angles of up to ∼125° at a molar grafting ratio of less than 5%.⁶¹ This was a very significant increase in water contact angle compared to the value obtained for pure PVA of 55°.

Table 4 Contact angles of water on film surfaces of xylan and ASA-modified xylans

Exp. No	Succinic derivative	DS	θ/°
Pure xylan	None	—	28 (±1.7)
1	n-OSA	0.08	55 (±1.4)
7	n-OSA	0.17	60 (±2.2)
10	n-OSA	0.27	65 (±3.3)
14	n-DDSA	0.16	81 (±3.0)
17	n-ODSA	0.16	76 (±1.6)

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The longer alkenyl chains of n-DDSA and n-ODSA appear to have a considerable influence on surface properties yielding more hydrophobic xylan films even at low DS. It would be expected for n-ODSA to give the most hydrophobic surface, as it has the longest alkenyl chain, which is however not the case. This could be due to differences in surface roughness of the samples, which is in part dependent on the solubility of the polymer in the chosen solvent. However, the difference between the contact angle results obtained for the n-DDSA- and n-ODSA-modified xylans is not significant with the method employed.

Conclusions

Chemical modification of birch wood xylan in C₄mimCl using long-chain alkenyl succinic anhydrides was confirmed through use of ¹H NMR and FT-IR spectroscopy, while titration was used to determine DS. Products with low DS (0.08–0.27) were obtained in good yield. The low DS values are consistent with previous reports in the literature for various hemicellulose modification methods. An increase in temperature and reaction time increased the DS; however product discoloration also occurred, which was attributed to unwanted side reactions. There was no obvious effect of chain length on the DS obtained for the three ASAs under similar reaction conditions. The thermal stability of the ASA-modified xylans was found to be lower than that of the unmodified xylan; however, this difference may not be of significance for application purposes. The introduction of the alkenyl side chains on the xylan backbone induced higher hydrophobicity as seen from increased contact angles of water on the surface of spin-coated modified xylan films.

Acknowledgements

The authors wish to thank the Danish Agency for Science Technology and Innovation for funding this project (# 09-065804). We are grateful to Kim Chi Szabo (Danish Polymer Centre, Department of Chemical and Biochemical Engineering, Technical University of Denmark) for technical assistance with DSC and TGA measurements, Lotte Nielsen (Department of Micro- and Nanotechnology, Technical University of Denmark) for performing the SEC experiments, and Zsuzsa Sarossy (Biosystems Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark) for performing the acid hydrolysis and sugar analyses. The supply of ASAs by Pentagon Chemicals is also gratefully acknowledged.

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