Preparation and Characterization of Water-Soluble Xylan Ethers

polymers

Article Preparation and Characterization of Water-Soluble Xylan Ethers

Kay Hettrich 1,*, Ulrich Drechsler 2, Fritz Loth 1 and Bert Volkert 1 1 Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany; [email protected] (F.L.); [email protected] (B.V.) 2 Salzenbrodt GmbH & Co KG, Hermsdorfer Str. 70, D-13437 Berlin, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-331-568-1514

Academic Editor: André Laschewsky Received: 30 September 2016; Accepted: 27 March 2017; Published: 31 March 2017

Abstract: Xylan is a predominant hemicellulose component that is found in plants and in some algae. This polysaccharide is made from units of xylose (a pentose sugar). One promising source of xylan is oat spelt. This feedstock was used for the synthesis of two xylan ethers. To achieve water soluble products, we prepared dihydroxypropyl xylan as a non-ionic ether on the one hand, and carboxymethyl xylan as an ionic derivative on the other hand. Different preparation methods like heterogeneous, pseudo-homogeneous, and homogeneous syntheses were compared. In the case of dihydroxypropyl xylan, the synthesis method did not signiﬁcantly affect the degree of substitution (DS). In contrast, in the case of carboxymethyl xylan, clear differences of the DS values were found in dependence on the synthesis method. Xylan ethers with DS values of >1 could be obtained, which mostly show good water solubility. The synthesized ionic, as well as non-ionic, xylan ethers were soluble in water, even though the aqueous solutions showed slight turbidity. Nevertheless, stable, transparent, and stainable ﬁlms could be prepared from aqueous solutions from carboxymethyl xylans.

Keywords: hemicellulose; xylan ether; solubility; etheriﬁcation

1. Introduction Native polysaccharides such as cellulose, starch, and most of hemicelluloses are not soluble in cold water without preceding chemical modification. To make them soluble in water, polysaccharide ethers and esters are produced. Such polysaccharide ethers are widely used in everyday water-based applications. Typical examples are carboxymethyl cellulose (CMC), which is the most-used cellulose ether; methyl cellulose (MC), hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC), as well as cationic starches, or hydroxyethyl starch (HES) and hydroxypropyl starch (HPS). Cellulose and starch ethers are used as film formers, protective colloids, stabilizers, thickening agents, adhesives or detergent additives in the food, pharmacy, paper and textile industries, as well as in the building and mining industry. Next to cellulose, hemicellulose is the second most abundant class of polysaccharides found in nature. It is the third key structural ingredient of the wooden cell-wall, besides cellulose and lignin. Furthermore, hemicellulose is found in vegetable fibers and in cell-walls of grass and corn. Thus, hemicellulose is an attractive raw renewable resource for new biobased materials and functional polymers that can be adapted by chemical reactions. In contrast to cellulose, hemicellulose is a non-crystalline heteropolysaccharide, which can be extracted from plant or wood by water and/or aqueous alkali [1–5]. Depending on the phylogeny, hemicellulose can be comprised of glucans, xyloglucans, xylans, arabinoxylans, mannans, gluco- or galactomannan with different substitution patterns [6]. Accordingly, hemicellulose may contain C5 or C6 sugar units.

Polymers 2017, 9, 129; doi:10.3390/polym9040129 www.mdpi.com/journal/polymers Polymers 2017, 9, 129 2 of 13

Xylans, for example, are a prevalent hemicellulose component found in plants and in some algae, whichPolymers are made 2017, 9,from 129 units of xylose (C5 sugar units) (Figure1). Xylans compose about 10–15%2 of 13 of softwoods, about 10–35% of hardwoods and 35–40% of the total mass in the residues of annual plants, Xylans, for example, are a prevalent hemicellulose component found in plants and in some e.g., in husks or oat spelt, which are the residues of oat flake production. Xylan shows characteristic algae, which are made from units of xylose (C5 sugar units) (Figure 1). Xylans compose about 10– differences in comparison with cellulose or starch, such as a lower degree of polymerization (DP); 15% of softwoods, about 10–35% of hardwoods and 35–40% of the total mass in the residues of this meansannual lowerplants, chaine.g., in length husks or and oat lowerspelt, which molar ar masses,e the residues and theof oat absence flake production. of the primary Xylan OH-groupshows in positioncharacteristic C6. Note differences that, in thein comparison case of C6 polysaccharides,with cellulose or thestarch, primary such OH-groupas a lower atdegree position of C-6 in thepolymerization anhydroglucose (DP); unit this (AGU) means lower is the chain most length reactive and group lower inmolar comparison masses, and with the the absence two secondaryof the OH-groupsprimary at OH-group position C-2in position and C-3. C6. Therefore, Note that, the in aim the of case this of study C6 polysaccharides, was to explore ifthe it isprimary possible to modifyOH-group xylan chemically at position by C-6 methods in the comparableanhydroglucose to those unit that(AGU) are establishedis the most forreactive the modification group in of polysaccharidescomparison with at an the industrial two secondary scale, onOH-groups the one hand;at position and C-2 to achieve and C-3. water Therefore, soluble the xylanaim ofproducts this similarstudy to cellulose was to explore or starch if it derivatives,is possible to on modify the other xylan hand. chemically by methods comparable to those that are established for the modification of polysaccharides at an industrial scale, on the one hand; The structure of xylan varies depending on its herbal origin. The different structures have and to achieve water soluble xylan products similar to cellulose or starch derivatives, on the other been summarized in several reviews on hemicelluloses in wood [7,8], grass [9], and cereals [10,11]. hand. Some aspectsThe aboutstructure the of structure xylan varies of xylan depending from oaton its spelts herbal were origin. discussed The different by Saake structures et al. [12have]. Oatbeen spelts containsummarized high amounts in several of xylanreviews and on havehemicelluloses relatively in low wood lignin [7,8], content.grass [9], and Hence, cereals this [10,11]. raw material Some is an interestingaspects about source the forstructure the isolation of xylan of from xylan. oat Aspelts proposal were discussed for the structure by Saake of et xylan al. [12]. in Oat annual spelts plants is showncontain in high Figure amounts1[ 9]. of The xylan monomer and have relatively units are low linked lignin together content. byHence,β-(1 this→ raw4) glycosidicmaterial is an bonds. Typicalinteresting for xylan source is the for presence the isolation of sideof xylan. chains A proposal made of for arabinofuranose, the structure of xylan xylopyranose, in annual plants rhamnose, is glucuronicshown acid,in Figure and 1 [9]. acetyl The groups.monomer Mostunits are of theselinked sugartogether residues by β-(1 → are 4) connectedglycosidic bonds. by acetal Typical bonds, whilefor the xylan acetyl isgroups the presence are connected of side bychains ester made bonds. of Typicalarabinofuranose, degreeof xylopyranose, substitution (DS)rhamnose, values of glucuronic acid, and acetyl groups. Most of these sugar residues are connected by acetal bonds, these side groups are between 0.02 and 0.2 in the case of arabinose, between 0.02 and 0.08 of glucuronic while the acetyl groups are connected by ester bonds. Typical degree of substitution (DS) values of acid, <0.02these side of all groups other sugarare between units, and 0.02 between and 0.2 0.02in the and case 0.15 of of arabinose, acetyl groups between [13]. 0.02 Usually, and 0.08 the contentof of acetylglucuronic groups acid, in extracted<0.02 of all xylan other is sugar much units, lower and because between the 0.02 ester and bonds0.15 of are acetyl cleaved groups during [13]. the alkalineUsually, extraction. the content of acetyl groups in extracted xylan is much lower because the ester bonds are Despitecleaved during promising the alkaline application extraction. possibilities, xylan derivatives have not been used in quantities large enoughDespite for promising industrial application processing possibilities, up until now.xylan derivatives Because of have the not possibility been used of in isolating quantities xylan fromlarge residues enough of annual for industrial plants, proces it is asing particularly up until cheapnow. Because biobased of rawthe possibility material whichof isolating can be xylan used for differentfrom applications. residues of annual A typical plants, example it is a particularly is oat spelt, cheap which biobased up until raw nowmaterial has which been mostlycan be used used for generationfor different of energy. applications. While A they typical have example been also is oat used spelt, as which additives up until for now animal has feedbeen similarmostly used to cereal for generation of energy. While they have been also used as additives for animal feed similar to straw, the EU Food Safety Regulation 178/2002 makes it increasingly difficult for the food industry cereal straw, the EU Food Safety Regulation 178/2002 makes it increasingly difficult for the food to deliverindustry mill to products,deliver mill such products, as oat such spelt, as oat to thespelt, animal to the feedanimal industry. feed industry. Hence, Hence, oat speltoat spelt is widelyis availablewidely for available material for utilization, material utilization, with approximately with approximately 150,000 150,000 t of oat t spelt of oat occurring spelt occurring just in just Germany in in 2014Germany [14]; their in 2014 use [14]; avoids their the use conflict avoids the of foodconflict vs. of raw food material vs. raw material for chemical for chemical industry. industry.

FigureFigure 1. Structure 1. Structure ofa of xylan a xylan from from annual annual plant plant [[9]9] inin comparison to to cellulose; cellulose; in both in both polymers polymers the the monomermonomer units units are linkedare linked together together by byβ -(1β-(1→ →4) 4) glycosidicglycosidic bonds; bonds; AGU AGU = anhydroglucose = anhydroglucose unit. unit.

Polymers 2017, 9, 129 3 of 13

Xylans with a low DP of approximately <30 are water soluble. For the preparation of water soluble xylan with a higher DP, derivatization is necessary, e.g., by incorporation of hydrophilic groups. High DP values are, in turn, necessary for some interesting applications, such as film forming. Hence, the wish to replace petrochemical-based plastics—for example, replacing food packaging films with biodegradable ones—motivates the study of hemicellulose derivatives and their properties. Previously, the gel- and film-forming properties of xylans were investigated by several studies. For instance, the nature of oxidative cross-linking of arabinoxylan was shown by Ng et al. [15]. The film-forming properties were examined for various mixtures of this hemicellulose and chitosan [1,16]. The stability of the films can be influenced by addition of various plasticizers, such as propylene glycol, glycerol, or sorbitol, in concentrations up to 22% in an arabinoxylan solution [17]. The synthesis of hemicellulose methyl ether was described by Fang et al. [18]. The ether was prepared by methylation with methyl iodide in dimethyl sulfoxide, using sodium hydride as a catalyst. However, different from methylcellulose, the described hemicellulose ethers were not soluble in water. A detailed report on the extraction of xylanes, their chemical modification, and their applicability, was given by Ebringerova and Hromadkova [19]. Water soluble amphiphilic derivatives of beech wood xylan and its sulfoethyl derivatives were obtained by the introduction of low amounts of long alkyl chains. The derivatives showed excellent emulsifying and foam-stabilizing properties. The synthesis of water soluble xylan sulfate ester has been reported in detail in a previous paper [20]. The preparation of the xylan sulfates succeeded at a broad DS range, from 0.58 to 1.95, by controlling the synthesis parameters. Other reviews summarize the chemical functionalization of xylan by different authors [20–22]. Nypelö and coworkers specified the synthesis of hydroxypropyl and 3-butoxy-2-hydroxypropyl xylan recently, with DS values in the range of 0.28–0.73 and 0.2–1.0, respectively [23]. In the case of the hydroxypropyl xylan, water solubility increased with higher DS values. More hydrophobic derivatives were obtained by synthesis of 3-butoxy-2-hydroxypropyl xylan. Both xylan derivatives can be used as interfacial modifiers in oil-in-water emulsions. The derivatization of xylan can be carried out under typically heterogeneous conditions, similar to those associated with most of the cellulose derivatives; or under homogeneous ones, because xylan is soluble in alkaline conditions. The homogenous method implies derivatization after dissolution in a suitable solvent. In contrast, the reaction occurs in slurry without dissolution of the polymer in a heterogeneous synthesis. These two synthesis strategies were compared to a synthesis under pseudo homogeneous conditions in a kneader with the reaction as an emulsion. The focus of this study is the preparation of carboxymethyl and novel xylan dihydroxypropyl ethers by these different methods. For all the derivatives, their water solubility behavior and their film-forming properties were examined. Fundamentally, it is interesting to discover how the absence of the primary OH-group will influence the reactivity of xylan and the water solution behavior of xylan ethers. Different synthesis possibilities of xylan ethers were thus investigated regarding the potential for future industrial-oriented procedures. Also, the properties of the synthesized xylan ethers, as typical C5 polysaccharide representatives, are discussed in comparison with the properties of cellulose and starch ethers.

2. Materials and Methods Xylan with a molar mass of about 30,000 g/mol, which was isolated by alkaline extraction of oat spelts, was kindly provided by Peter Koelln KGaA, Elmshorn, Germany. A detailed description of the procedures for the isolation and the determination of the molar mass are given in the paper of Puls et al. [24]. The reactants for the modiﬁcation of xylans and the solvents used for product puriﬁcation were obtained from Sigma Aldrich (Taufkirchen, Germany) with puriss grade and used as received. Subsequently, a typical example of the reaction procedure is described for every synthesis variant. Polymers 2017, 9, 129 4 of 13

2.1. Synthesis of Dihydroxypropyl Xylan

2.1.1. Variant 1A—Pseudo Homogeneous Synthesis in a Kneader The synthesis was carried out in a temperate Duplex kneader of the company IKA (Staufen, Germany). A dispersion of 39.6 g of xylan (0.30 mol, 35%, w/w) in 73.5 g of a 4% NaOH solution (w/w) was prepared at 25 ◦C, and homogenized in the kneader for 1 h. After increasing the temperature to 60 ◦C, 44.5 g (0.60 mol) of 2,3-epoxy-1-propanol was added, and mixing was continued for 4 h. After cooling to room temperature, 50 mL of methanol was added and a pasty slurry was yielded. The material was transferred to a beaker. Adding 200 mL of methanol, the mixture was homogenized by treatment with an Ultra Turax at 12,500 r.p.m. for 15 min. Finally, the pH was adjusted to 8 with 50% acetic acid-methanol mixture (v/v). After ﬁltration and washing with methanol, the solid product was dried at 60 ◦C and 75 mbar.

2.1.2. Variant 2A—Reaction as Emulsion in 2-Propanol or in Cyclohexanone The synthesis was carried out in a 100 mL three-neck flask with dropping funnel, reflux condenser and KPG stirrer. Firstly, 3.3 g (25 mmol) of xylan was dispersed in 51.7 g of 4% NaOH solution (w/w) and stirred for 18 h at 25 ◦C. The emulsion was prepared by addition of 2-propanol (or of cyclohexanone) in ratio 1 to 1 in relation to the xylan dispersion (v/v) during strong stirring. Then, 3.3 mL (50 mmol) of epoxy propanol was dropped into the emulsion. The mixture was stirred for 4 h at 60 ◦C. Then 400 mL of methanol was added to the mixture, which was homogenized by treatment with an Ultra Turrax at 12.500 r.p.m. for 15 min. Finally, the pH was adjusted to 8 with 50% acetic acid-methanol mixture (v/v). After filtration and washing with methanol, the solid product was dried at 60 ◦C and 75 mbar.

2.1.3. Variant 3A—Homogeneous Syntheses The synthesis was carried out in a 100 mL ﬂask with stirrer and dropping funnel. Firstly, 3.3 g (25 mmol) of xylan was dispersed in 84 g of 4% NaOH solution (w/w) for 24 h at room temperature. Then, 3.3 mL (50 mmol) of epoxy propanol was dropped into the solution. The mixture was stirred for 24 h at room temperature, before it was precipitated into 400 mL of methanol. Finally, the pH was adjusted to 8 with 50% acetic acid-methanol mixture (v/v) and after ﬁltration and washing with methanol, the solid product was dried at 60 ◦C and 75 mbar.

2.2. Syntheses of Carboxymethyl Xylan

2.2.1. Variant 1B—Pseudo Homogeneous Synthesis in a Kneader A dispersion of 13.2 g (100 mmol) of xylan in 73.4 g of 27% NaOH solution (w/w) was prepared at 25 ◦C. The dispersion was homogenized in a temperature-controlled Duplex kneader of the company IKA for 18 h. Then, 18.9 g (200 mmol) of monochloroacetic acid (MCA) was added. After increasing the temperature to 60 ◦C, the mixture was kneaded for 4 h. The temperature was cooled down to room temperature and 50 mL methanol was added, where a pasty material resulted. The slurry was transferred to a beaker. After further addition of 150 mL methanol, the mixture was homogenized by treatment with Ultra Turrax at 12,500 r.p.m. for 15 min. Then, the pH was adjusted to 8 with 50% acetic acid-methanol mixture (v/v). After ﬁltration, the solid product was washed with a mixture of methanol–water (80/20, v/v) until the silver nitrate test became negative. Finally, the product was washed with methanol and dried at 60 ◦C and 75 mbar.

2.2.2. Variant 2B—Reaction as Emulsion in 2-Propanol or in Cyclohexanone The synthesis was carried out in a 100 mL three-neck flask with dropping funnel, reflux condenser and KPG stirrer. Firstly, 3.3 g (25 mmol) of xylan was dispersed in 51.7 g of 18% NaOH solution (w/w) and stirred for 18 h at 25 ◦C. The emulsion was prepared by addition of 2-propanol (or of Polymers 2017, 9, 129 5 of 13 cyclohexanone) in ratio 1 to 1 in relation of the xylan dispersion (v/v) during strong stirring. Then, 4.7 g (50 mmol) of solid mono chloroacetic acid (MCA) was added into the emulsion. The mixture was stirred for 4 h at 70 ◦C. Afterwards, the pH was adjusted to 8 with 50% acetic acid-methanol mixture (v/v). After filtration, the solid product was washed with a mixture of methanol–water (80/20, v/v) until the silver nitrate test became negative. Finally, the product was washed with methanol and dried at 60 ◦C and 75 mbar.

2.2.3. Variant 3B—Homogeneous Synthesis The synthesis was carried out in a 100 mL flask with stirrer and dropping funnel. Firstly, 3.3 g (25 mmol) of xylan was dispersed in 29.2 g of 10% NaOH solution (w/w) for 24 h at room temperature. The reaction was started by dropping 3.3 mL (50 mmol) of epoxy propanol into the solution. Then, 1.5 g (37 mmol) of NaOH as 45% aqueous solution was added. After 30 min, 4.7 g (50 mmol) of monochloroacetic acid (MCA) as an aqueous solution (80%, w/w) was added via a dropping funnel. The mixture was stirred for 72 h at room temperature. The reaction was finished by precipitation into 400 mL of methanol. Then, the pH was adjusted to 8 with 50% acetic acid-methanol mixture (v/v). After filtration, the solid product was washed with a mixture of methanol–water (80/20, v/v) until the silver nitrate test became negative. Finally, the product was washed with methanol and dried at 60 ◦C and 75 mbar.

2.2.4. Variant 4B—Heterogeneous Method (Slurry) The synthesis was carried out in a cylindrical double coat flask with reflux condenser, dropping funnel and KPG stirrer. Firstly, 3.3 g (25 mmol) of xylan was dispersed in 40 mL of 2-propanol. Then, 1.0 g (25 mmol) of solid NaOH was added to the mixture and stirred intensively. After 10 min, 5.33 g of 45% NaOH solution was added and stirred for 2 h at room temperature. Then, 1.0 g (25 mmol) of solid NaOH and 4.7 g (50 mmol) of solid mono chloroacetic acid (MCA) was added. The mixture was stirred for 4 h at 50 ◦C. After cooling to room temperature, the pH was adjusted to 8 with a 50% acetic acid-methanol mixture (v/v). The raw product was filtered and washed with methanol–water (80/20, v/v) until the silver nitrate test became negative. Finally, the product was washed with methanol and dried at 60 ◦C and 75 mbar.

2.3. Measurements

2.3.1. Solid State 13C-NMR Spectroscopy for the Determination of the Dihydroxypropyl Content 13C-NMR spectra were recorded by a Varian UNITY 400 NMR spectrophotometer (frequency 100.58 MHz, Varian, Darmstadt, Germany). High-resolution solid-state spectra were recorded with the cross-polarization/magic angle spinning (CP/MAS) method with spinning frequencies of 5–6 kHz. The contact time was 1–2 ms and the repetition time of the experiments was 3 s. For 13C-CP/MAS NMR measurements, the samples, which were wetted with lye, were mechanically squeezed. About 0.1 cm3 of the xylan derivatives were filled into a sample rotor. Depending on the sample and the specific objective, the measuring time ranged from 1 to 15 h. For the calculation of the DS values of 13 dihydroxypropyl xylans from C-NMR spectra, the intensity of the signals from the CH2 groups (60 ppm to 67 ppm) was used. At first, the whole spectrum was normalized by setting the integral of C-1 signal (98 to 107 ppm) equal to 1. Then, the integral of the superposed CH2 signal of C-5 of xylan and CH2–OH group from the substituent introduced (at 63 ppm) was corrected accordingly, to yield the intensity due to the substituent in order to obtain the DS. The relative error of the determination is in the range of ±0.05.

2.3.2. Determination of the Carboxymethyl Content by ICP-OES The sodium content was determined by ICP-OES (inductive coupled plasma—optical emission spectrometry, Perkin Elmer, Rodgau, Germany) after a nitric acid digestion by assisted microwave Polymers 2017, 9, 129 6 of 13 irradiation. The analysis was performed as double determination. The reproducibility was better than 1%. The degree of substitution (DS) was calculated as follows [25]:

132 × %Na DS = (1) 2300 − 80 × %Na

2.3.3. Solubility The solubility of the xylan derivatives was characterized by preparing aqueous solutions (2%, w/w) and stirring for 18 h at room temperature. The solution turbidity was measured with a nephelometer 2100AN (Hach, Loveland, CO, USA). The calibration of the instrument was carried out with a formazin standard.

3. Results and Discussion The aim of the investigation was to synthesize water soluble xylan derivatives by etherification with an ionic and with a non-ionic reagent. In contrast to the derivatization of cellulose or starch, the more reactive primary group at position C-6 is absent in the case of xylan, and there are only two secondary OH-groups in position C-2 and C-3 which can be substituted. Thus, the maximum DS value which can be obtained is 2. In this study, two possibilities of etherification to introduce functional groups into xylan were examined. On the one hand, we investigated the incorporation of an additional OH-group by using epoxy propanol as reagent. On the other hand, we investigated the incorporation of an anionic group by using chloro acetic acid. The synthesis and the water solving properties, as well as film forming properties, of dihydroxypropyl xylan and carboxymethyl xylan are described below.

3.1. Synthesis of Dihydroxypropyl Xylan The advantage of the preparation of dihydroxypropyl xylan, in comparison to hydroxypropyl xylan, is an extra hydroxyl group, which is incorporated by using epoxy propanol (glycidol) instead of epoxypropan. By using this reagent, a new type of xylan ether is prepared. Additionally, the used etherification reagent is easier to handle because of its higher boiling point. The synthesis of dihydroxypropyl xylan is carried out according to the scheme in Figure2. The molar ratio of n2,3 epoxy-1-propanol/nxylan as well as ratio of nNaOH/nxylan was changed by using different methods of preparation. Due to the poor water solubility of the dihydroxypropyl xylan prepared under heterogeneous standard slurry conditions, the following derivatives were prepared under more homogeneous conditions, because a more uniform distribution of the substituents along the polymer chain was expected. It is known that the solubility of polysaccharide derivatives is improved when the substituent distribution is more uniform. The results of the various syntheses are shown in Table1. Dihydroxylpropyl xylans with DS values between 0.4 and 1.2 were obtained. Figure3 provides an overview of the DS values. Only small differences were observed for the DS values in dependence on the used synthesis method. A DS value of 0.7 was obtained already at a molar ratio n2,3 epoxy-1-propanol/nxylan = 1:1 by using the emulsion method in i-propanol (DHPX4). Surprisingly, a further increase of the reagent 2,3-epoxy-1-propanol, e.g., a molar ratio n2,3 epoxy-1-propanol/nxylan = 4:1, did not lead to a significantly higher DS value. The use of cyclohexanone instead of i-propanol as the emulsion medium did not improve the extent of substitution either. Even the use of a high excess of reagent (n2,3 epoxy-1-propanol/nxylan= 10:1; DHPX8) yielded virtually the same DS value. Only under homogeneous conditions, this high excess of epoxy-1-propanol did result in DS values >1 (DPHX12). A clear tendency regarding the influence of the sodium hydroxide concentration under pseudo-homogeneous conditions cannot be recognized (DPHX2 in comparison to DHPX3, in Table1). The reagent efficiency varied depending on the amount of used reagent; the higher the excess of reagent, the lower is the reagent efficiency (Figure4). Surprisingly, the reagent efficiency did not show any clear difference with regard to the used preparation method. Polymers 2017, 9, 129 7 of 13

Figure5 depicts 13C-NMR (CP/MAS) spectra of the dihydroxypropyl xylans in comparison with unsubstitutedPolymers 2017, 9, 129 xylan. In general, three characteristic changes in the spectra are seen with increasing7 of DS: 13 ﬁrst, two rather narrow signals at 70 and 72 ppm emerge due to a shift of the C-2 signal and to the newlyto 62Polymers ppm. formed 2017Third,, CH-OH 9, 129 a new group signal of due the substituent.to the CH2–OH Second, group the of peak the ofglycerol the C-5 substituent signal shifts is fromseen7 of 13 64at 64 to 62ppm. ppm. Third, a new signal due to the CH2–OH group of the glycerol substituent is seen at 64 ppm. to 62 ppm. Third, a new signal due to the CH2–OH group of the glycerol substituent is seen at 64 Noteworthy is the fact fact that that all all products products in in Ta Tableble 11 showedshowed good solubility in water. water. Still, Still, a ppm. adependence dependenceNoteworthy on on thethe preparation is preparationthe fact that method allmethod products is observed. is inobserved. Table The 1 showed more The homogeneous moregood solubilityhomogeneous the in reaction water. the Still, conditions, reaction a theconditions, moredependence soluble the more areon thesoluble products.preparation are the products.method is observed. The more homogeneous the reaction conditions, the more soluble are the products.

FigureFigure 2. Reaction2. Reaction scheme scheme of of thethe preparation of of dihydroxypropyl dihydroxypropyl xylan. xylan.xylan.

TableTableTable 1.1. ResultsResults 1. Results ofofsynthesis ofsynthesis synthesis of of dihydroxypropylof dihydroxypropyl dihydroxypropyl xylan, xylan,xylan, dependant dependant dependant onon on thethe the methodmethod method (EProH (EProH = = 2,3- = epoxy-1-propanol).2,3-epoxy-1-propanol).2,3-epoxy-1-propanol).

Sample nEPrOH/nxylan nNaOH/nxylan DS Method SampleSample nnEPrOH/n/nxylan nNaOH//nnxylan DS DSMethod Method DHPX1EPrOH 1:1xylan NaOH 0.3:1xylan 0.40 pseudo-homogeneous, 60 °C, 4 h DHPX1 1:1 0.3:1 0.40 pseudo-homogeneous, 60 °C, 4◦ h DHPX1DHPX2 1:1 2:1 0.3:10.23:1 0.90 0.40 pseudo-homogeneous, pseudo-homogeneous, 60 °C, 4 60h C, 4 h DHPX2 2:1 0.23:1 0.90 pseudo-homogeneous, 60 °C, 4◦ h DHPX2DHPX3 2:1 2:1 0.23:1 1:1 0.90 0.90 pseudo-homogeneous, pseudo-homogeneous, 60 °C, 4 60h C, 4 h DHPX3DHPX3 2:1 2:1 1:1 1:1 0.90 0.90 pseudo-homogeneous, pseudo-homogeneous, 60 °C, 60 4◦C, h 4 h DHPX4 1:1 2.05:1 0.70 emulsion, i-propanol, 25 °C, 18 h ◦ DHPX4DHPX4 1:1 1:1 2.05:1 2.05:1 0.70 0.70 emulsion, emulsion, i-propanol, i-propanol, 25 °C, 25 18C, h 18 h DHPX5 2:1 2.05:1 0.80 emulsion, i-propanol, 25 °C, 18 h ◦ DHPX5DHPX5 2:1 2:1 2.05:1 2.05:1 0.80 0.80 emulsion, emulsion, i-propanol, i-propanol, 25 °C, 25 18C, h 18 h DHPX6DHPX6 4:1 4:1 2.05:1 2.05:1 0.85 0.85 emulsion, emulsion, i-propanol, i-propanol, 25 °C, 18 25 h ◦C, 18 h DHPX7DHPX6DHPX7 2:1 4:1 2:1 2.05:1 2.05:1 2.05:1 0.850.77 0.77 emulsion, emulsion, emulsion, cyclohexanone, i-propanol, cyclohexanone, 25 25 °C, °C, 18 2518 h ◦ hC, 18 h DHPX8DHPX7DHPX8 10:1 2:1 10:1 2.05:1 2.05:1 2.05:1 0.770.70 0.70 emulsion, emulsion, emulsion, cyclohexanone, cyclohexanone, cyclohexanone, 25 °C, 25 18°C, 25 h 18◦C, h 18 h DHPX9DHPX8DHPX9 2:110:1 2:1 2.05:1 1.2:1 1.2:1 0.700.85 0.85 emulsion, homogeneous, cyclohexanone, homogeneous, 25 °C, 24 25 25h ◦ C,°C, 24 18 h h DHPX10DHPX10 4:1 4:1 2.05:1 2.05:1 0.88 0.88 homogeneous, homogeneous, 25 °C, 24 25 h ◦C, 24 h DHPX9 2:1 1.2:1 0.85 homogeneous, 25 °C, 24◦ h DHPX11DHPX11 4:1 4:1 2.05:1 2.05:1 0.93 0.93 homogeneous, homogeneous, 25 °C, 95 25 h C, 95 h DHPX10 4:1 2.05:1 0.88 homogeneous, 25 °C, 24◦ h DHPX12DHPX12 10:1 10:1 2.05:1 2.05:1 1.22 1.22 homogeneous, homogeneous, 25 °C, 95 25 h C, 95 h DHPX11 4:1 2.05:1 0.93 homogeneous, 25 °C, 95 h DHPX12 10:1 2.05:1 1.22 homogeneous, 25 °C, 95 h

FigureFigure 3. Effect3. Effect of of the reaction reaction conditions conditions on the on degree the degree of substitution of substitution (DS) values (DS) of xylan, values achieved of xylan, achievedwith with2,3-epoxy-1-propanol. 2,3-epoxy-1-propanol.

Figure 3. Effect of the reaction conditions on the degree of substitution (DS) values of xylan, achieved

with 2,3-epoxy-1-propanol.

Polymers 2017, 9, 129 8 of 13 Polymers 2017, 9, 129 8 of 13

Polymers 2017, 9, 129 8 of 13

Figure 4. Reagent efﬁciencyefficiency in dependence on the amountamount of epoxypropanol engaged for the various methodsFigure used. 4. Reagent efficiency in dependence on the amount of epoxypropanol engaged for the various methodsmethods used. used. C-3 C-3

C-2C-2 C-1C-1 C-4 C-4 C-5C-5 DS:DS:

XylanXylan 0 0

n/nEPrOH XYL DHP4 n/nEPrOH XYL 0.7 DHP4 1:1 0.7 1:1 DHP5 0.8 2:1 DHP5 0.8 2:1Emulsion Emulsion DHP6 0.85 4:1 DHP6 0.85 4:1 -CH2 -OH

-CH2 -OH C-5

DHP1 C-5 0.4 1:1 DHP1 0.4 1:1Kneader 0.9 DHP2 2:1 Kneader 11 0 100 90 80 70 60 ppm 0.9 DHP2 2:1 13 Figure 5. Solid-state13 C-NMR (CP/MAS) spectra of selected dihydroxypropyl xylans. Figure 5. Solid-state C-NMR11 0 (CP/MAS)100 90 spectra80 of selected70 60 dihydroxypropylppm xylans. 3.2. SynthesesFigure of 5.Carboxymethyl Solid-state 13 C-NMRXylan (CP/MAS) spectra of selected dihydroxypropyl xylans. 3.2. Syntheses of Carboxymethyl Xylan Generally, the synthesis procedure of carboxymethyl xylan was chosen to be very similar to the 3.2. SynthesesonesGenerally, commonly of theCarboxymethyl synthesis applied for procedure Xylan the carboxymethylation of carboxymethyl of xylancellulose. was First, chosen the to polysaccharide be very similar is to the ones commonly applied for the carboxymethylation of cellulose. First, the polysaccharide is alkalized alkalizedGenerally, with the sodium synthesis hydroxide, procedur followede of carboxymethyl by the reaction xylan with waschloroacetic chosen acidto be (Figure very similar 6). The to the withsynthesis sodium hydroxide,of carboxymethyl followed xylan by was the investigated reaction with by varying chloroacetic the preparation acid (Figure method,6). The molar synthesis ratio of ones commonly applied for the carboxymethylation of cellulose. First, the polysaccharide is carboxymethylof reagents, temperature xylan was investigated and reaction time by varying (Table 2). the High preparation DS values method,of >1 were molar achieved ratio by of using reagents, alkalized with sodium hydroxide, followed by the reaction with chloroacetic acid (Figure 6). The temperatureheterogeneous and reaction reaction timeconditions (Table in2). a Highstirring DS re valuesactor or ofunder >1 were pseudo-homogeneous achieved by using conditions heterogeneous in synthesis of carboxymethyl xylan was investigated by varying the preparation method, molar ratio reactiona kneader conditions (Figure in7). aThe stirring reagent reactor efficiencies or underunder heterogeneous pseudo-homogeneous and under pseudo-homogeneous conditions in a kneader of reagents,conditions temperature were higher and in comparison reaction time to the (Table preparation 2). High under DS valueshomogeneou of >1 swere conditions achieved or in by an using (Figure7). The reagent efﬁciencies under heterogeneous and under pseudo-homogeneous conditions heterogeneousemulsion (Figure reaction 8). Underconditions the latter in a twostirring conditions reactor, a or higher under excess pseudo-homogeneous of reagent was necessary conditions to in were higher in comparison to the preparation under homogeneous conditions or in an emulsion a kneader (Figure 7). The reagent efficiencies under heterogeneous and under pseudo-homogeneous (Figure 8). Under the latter two conditions, a higher excess of reagent was necessary to yield conditions were higher in comparison to the preparation under homogeneous conditions or in an emulsion (Figure 8). Under the latter two conditions, a higher excess of reagent was necessary to

Polymers 2017, 9, 129 9 of 13 Polymers 2017, 9, 129 9 of 13 comparableyield comparable DS values. DS values. Also, Also, neither neither an increasean increase of theof the reaction reaction time time from from 24 24 to to 72 72 h, h, nor nor ofof thethe ◦ temperaturetemperaturePolymers 2017 from , 9, 129 25 25 to to 50 50 °C,C, showed showed any any effect effect on on the the DS DS (CMX9 (CMX9 and and CMX10). CMX10). Nevertheless, Nevertheless,9 of 13 we wenote note that thatthe homogeneously the homogeneously synthesized synthesized carboxym carboxymethylethyl xylans xylans with comparatively with comparatively low DS low values DS valuesof aboutyield of comparable about0.3 displayed 0.3 displayed DS values. good good Also,solubility solubilityneither in an water. inincrease water. Amidst of Amidst the reactionthe the pr products oductstime from obtained obtained 24 to 72 byh, by nor the of various variousthe temperature from 25 to 50 °C, showed any effect on the DS (CMX9 and CMX10). Nevertheless, we synthesissynthesis methods,methods, thethe carboxymethylcarboxymethyl xylansxylans preparedprepared underunder homogeneoushomogeneous conditionsconditions showedshowed thethe note that the homogeneously synthesized carboxymethyl xylans with comparatively low DS values bestbest solubility, even even though though a aslight slight turbidity turbidity was was also also observed observed (turbidity (turbidity in the in range the range from from30 to 3060 of about 0.3 displayed good solubility in water. Amidst the products obtained by the various toNTU; 60synthesis NTU;nephelometric nephelometric methods, turbidity the carboxymethyl turbidity unit) at unit) dry xylans matter at dry prep mattercontentared under content of 2% homogeneous (w of/w 2%). As (w expected,/ conditionsw). As expected, the showed DS increased the DS increasedwithbest increasing solubility, with increasing of even the thoughratio of the n aMCA ratioslight/nxylann turbidityMCA. In/ thenxylan was same. also In theorder, observed same the order, (turbidityreacti theon reaction inefficiency the range efﬁciency decreased, from 30 decreased, to 60while whilesolubilityNTU; solubility nephelometricwas improved. was improved. turbidity unit) at dry matter content of 2% (w/w). As expected, the DS increased with increasing of the ratio nMCA/nxylan. In the same order, the reaction efficiency decreased, while solubility was improved.

Figure 6. Reaction scheme of the preparation ofof carboxymethylcarboxymethyl xylan.xylan. Figure 6. Reaction scheme of the preparation of carboxymethyl xylan. Table 2.2. Effect ofof thethe syntheticsynthetic procedureprocedure onon thethe synthesissynthesis ofof carboxymethylcarboxymethyl xylan.xylan.

SampleTable 2. EffectnMCA /ofn xylanthe syntheticnNaOH/ nprocedurexylan DS on the synthesis of Methodcarboxymethyl xylan. Sample nMCA/nxylan nNaOH/nxylan DS Method CMX1Sample nMCA 1:1 /nxylan nNaOH 4.9:1/n xylan 0.46DS pseudo-homogeneous, Method 70 °C, 3 h ◦ CMX1CMX2CMX1 1:1 1.5:1 1:1 4.9:14.7:1 4.9:1 0.730.46 0.46 pseudo-homogeneous, pseudo-homogeneous, pseudo-homogeneous, 70 70°C, °C, 3 h 4 h 70 C, 3 h ◦ CMX2CMX3CMX2 1.5:1 2:1 1.5:1 9.25:1 4.7:1 4.7:1 1.400.73 0.73 pseudo-homogeneous, pseudo-homogeneous, pseudo-homogeneous, 70 70°C, °C, 4 h 4 h 70 C, 4 h ◦ CMX3CMX4CMX3 2:1 2:1 2:1 9.25:1 9.25:1 0.421.40 1.40 pseudo-homogeneous, emulsion, pseudo-homogeneous, i-propanol, 70 60 °C, °C, 4 h 4 h 70 C, 4 h ◦ CMX4CMX5CMX4 2:1 4.2:1 2:1 9.25:1 9.25:1 0.730.42 0.42 emulsion, emulsion, emulsion,i-propanol, i-propanol, i-propanol,60 60°C, °C, 4 h 4 h 60 C, 4 h ◦ CMX5CMX5 4.2:1 4.2:1 9.25:1 9.25:1 0.73 0.73 emulsion, emulsion,i-propanol, i-propanol,60 °C, 4 h 60 C, 4 h CMX6 4.2:1 9.25:1 0.55 emulsion, cyclohexanone, 60 °C, 4 h ◦ CMX6CMX6 4.2:14.2:1 9.25:1 9.25:1 0.55 0.55emulsion, emulsion,cyclohexanon cyclohexanone,e, 60 °C, 4 h 60 C, 4 h CMX7 2:1 4.4:1 0.60 homogeneous, 50 °C, 3.5 h ◦ CMX7CMX7 2:1 2:1 4.4:1 4.4:1 0.60 0.60 homogeneous, homogeneous, 50 °C, 3.5 h 50 C, 3.5 h CMX8 2:1 4.4:1 0.41 homogeneous, 50 °C, 8 h ◦ CMX8CMX8 2:1 2:1 4.4:1 4.4:1 0.41 0.41 homogeneous, homogeneous, 50 °C, 8 h 50 C, 8 h CMX9 2:1 4.4:1 0.30 homogeneous, 25 °C, 24 h ◦ CMX9CMX9 2:1 2:1 4.4:1 4.4:1 0.30 0.30 homogeneous, homogeneous, 25 °C, 24 h 25 C, 24 h CMX10 2:1 4.4:1 0.29 homogeneous, 50 °C, 72 h ◦ CMX10CMX10 2:1 2:1 4.4:1 4.4:1 0.29 0.29 homogeneous, homogeneous, 50 °C, 72 h 50 C, 72 h CMX11CMX11 4.2:1 4.2:1 9.25:1 0.86 0.86 homogeneous, homogeneous, 50 °C, 3.5 50 h ◦C, 3.5 h CMX11 4.2:1 9.25:1 0.86 homogeneous, 50 °C, 3.5 h ◦ CMX12CMX12CMX12 0.75:1 0.75:10.75:1 0.9:1 0.9:1 0.51 0.51 he heterogeneous,terogeneous, heterogeneous, 50 50°C, °C, 4 h 4 h 50 C, 4 h CMX13CMX13 1:1 1:1 2.2:1 0.86 0.86 heterogeneous, heterogeneous, 50 °C, 4 h 50 ◦C, 4 h CMX13 1:1 2.2:1 0.86 heterogeneous, 50 °C, 4 h ◦ CMX14CMX14 1.5:11.5:1 2.2:1 0.76 0.76 heterogeneous, heterogeneous, 50 °C, 4 h 50 C, 4 h CMX14 1.5:1 2.2:1 0.76 heterogeneous, 50 °C, 4 h ◦ CMX15CMX15CMX15 2:1 2:1 2:1 4.4:1 4.4:1 1.28 1.28 heterogeneous, heterogeneous, heterogeneous, 50 50°C, °C, 4 h 4 h 50 C, 4 h

Figure 7. Effect of the reaction conditions on the DS values of xylan, achieved with monochloro acetic Figure 7. Effect of the reaction conditions on the DS values of xylan, achieved with monochloro acetic acid (MCA). acidFigure (MCA). 7. Effect of the reaction conditions on the DS values of xylan, achieved with monochloro acetic acid (MCA).

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Figure 8. Reagent efficiency efﬁciency in dependence on the am amountount of used monochloroacetic acid (MCA) engaged for the various methods used.

3.3. Dissolution Behaviour of the Xylan Ether The dissolution behavior of the xylan derivatives was estimated by turbidity measurements. The aqueous solutions of the various xylan ethers showed a slight turbidity in contrast to aqueous The aqueous solutions of the various xylan ethers showed a slight turbidity in contrast to aqueous solution of commercial cellulose ether, e.g., carboxymethyl and hydroxypropyl cellulose or starch solution of commercial cellulose ether, e.g., carboxymethyl and hydroxypropyl cellulose or starch ether. ether. In the case of dihydroxypropyl xylan, the solubility depends on the synthesis method. The In the case of dihydroxypropyl xylan, the solubility depends on the synthesis method. The products products prepared under pseudo-homogeneous conditions in a kneader are less soluble in prepared under pseudo-homogeneous conditions in a kneader are less soluble in comparison to the comparison to the ones synthesized by the emulsion and homogeneous methods. This may be due to ones synthesized by the emulsion and homogeneous methods. This may be due to the lower DS the lower DS value in the case of DHPX1 (Table 1), but also due to a more equal distribution of the value in the case of DHPX1 (Table1), but also due to a more equal distribution of the substituents substituents along the polymer chain in the case of a homogeneous synthesis. along the polymer chain in the case of a homogeneous synthesis. Also, in the case of carboxymethyl Also, in the case of carboxymethyl xylan, the water solubility of the homogeneously synthesized xylan, the water solubility of the homogeneously synthesized products was best, while the solubility products was best, while the solubility of products with comparable DS values from the of products with comparable DS values from the heterogeneous and pseudo-homogeneous syntheses heterogeneous and pseudo-homogeneous syntheses did not differ from each other. Generally, the did not differ from each other. Generally, the solubility was improved with increasing DS. In spite of solubility was improved with increasing DS. In spite of high DS values of >1 of the prepared high DS values of >1 of the prepared carboxymethyl xylans, totally transparent aqueous solutions carboxymethyl xylans, totally transparent aqueous solutions were not obtained. We presume that were not obtained. We presume that the side chains of the pure xylan (e.g., arabino, glucuronic the side chains of the pure xylan (e.g., arabino, glucuronic or rhamnose units), and the formation of or rhamnose units), and the formation of small gel particles, are responsible for this effect. Also, small gel particles, are responsible for this effect. Also, the distribution of the carboxymethyl groups the distribution of the carboxymethyl groups along the chain influenced the solubility of the products. along the chain influenced the solubility of the products. As it is known for carboxymethyl cellulose As it is known for carboxymethyl cellulose with identical average DS values, products synthesized with identical average DS values, products synthesized under homogeneous conditions show better under homogeneous conditions show better solubility in comparison to the heterogeneously prepared solubility in comparison to the heterogeneously prepared ones [26]. This effect was also observed for ones [26]. This effect was also observed for the carboxymethylated xylans. Similarly, turbidity was the carboxymethylated xylans. Similarly, turbidity was also observed for dihydroxypropyl xylan also observed for dihydroxypropyl xylan solutions in water. Nevertheless, we note that nearly all solutions in water. Nevertheless, we note that nearly all xylan ethers prepared were soluble in water, xylan ethers prepared were soluble in water, and that no precipitation was detected after extended and that no precipitation was detected after extended storage over months or after centrifugation. storage over months or after centrifugation. The turbidity declined slightly after heating the xylan The turbidity declined slightly after heating the xylan ether solution, and did not change after ether solution, and did not change after subsequently cooling the samples. Temperature-induced subsequently cooling the samples. Temperature-induced phase separation with a lower or upper phase separation with a lower or upper critical solution temperature (LCST or UCST), as frequently critical solution temperature (LCST or UCST), as frequently encountered for synthetic water-soluble encountered for synthetic water-soluble polymers bearing hydroxyl groups [27–31], was not observed. polymers bearing hydroxyl groups [27–31], was not observed. Slight turbidity was also detected in Slight turbidity was also detected in alkaline or acidic solutions. We noted the carboxymethyl xylan alkaline or acidic solutions. We noted the carboxymethyl xylan solution showed yellow color and an solution showed yellow color and an increase of viscosity after increasing the pH. increase of viscosity after increasing the pH. Nevertheless, the prepared carboxymethyl xylans were well dispersible in water. Also, clear and Nevertheless, the prepared carboxymethyl xylans were well dispersible in water. Also, clear transparent xylan ether films could be prepared from aqueous dispersion. When a dye was added to and transparent xylan ether films could be prepared from aqueous dispersion. When a dye was the dispersion, homogeneously colored clear films were obtained (Figure9). added to the dispersion, homogeneously colored clear films were obtained (Figure 9).

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Figure 9. Cold water soluble films of dyed carboxymethylated xylan, cast from aqueous solutions. Figure 9. Cold water soluble ﬁlms of dyed carboxymethylated xylan, cast from aqueous solutions.

These films can be easily redissolved by cold water. Hence, a water soluble film-forming materialThese based films on can a bebiopolymer easily redissolved was obtained by cold withou water.t any Hence, additives, a water such soluble as plasticizers, film-forming etc. material Such basedwater onsoluble a biopolymer films or was coating obtained materials without are any useful additives, for pharmaceutical, such as plasticizers, cosmetic, etc. Such or waterother solubleapplications. films Further or coating investigations materials areregarding useful the for properties pharmaceutical, of these cosmetic, films are orin progress. other applications. Further investigations regarding the properties of these films are in progress. 4. Conclusions 4. Conclusions Xylan which was isolated from oat spelts and its derivatives are attractive biobased polymers, Xylan which was isolated from oat spelts and its derivatives are attractive biobased polymers, and water soluble polymers can be generated by incorporation of functional ether groups. Thus, and water soluble polymers can be generated by incorporation of functional ether groups. Thus, xylan xylan ethers can be a low-cost alternative to the use of low viscous cellulose ethers. ethers can be a low-cost alternative to the use of low viscous cellulose ethers. Two different water soluble xylan ethers—one ionic and one non-ionic—were successfully Two different water soluble xylan ethers—one ionic and one non-ionic—were successfully prepared by using various synthesis methods in this study. In particular, novel hemicellulose ethers prepared by using various synthesis methods in this study. In particular, novel hemicellulose were synthesized by using epoxypropanol as a reagent. For the synthesis of the xylan ethers, ethers were synthesized by using epoxypropanol as a reagent. For the synthesis of the xylan ethers, heterogeneous, pseudo-homogeneous, and homogeneous preparation methods were used. These heterogeneous, pseudo-homogeneous, and homogeneous preparation methods were used. These synthesis methods, especially the heterogeneous ones, are typical for the industrial production of the synthesis methods, especially the heterogeneous ones, are typical for the industrial production of analogous functional cellulose ethers. When preparing dihydroxypropyl xylan, no significant the analogous functional cellulose ethers. When preparing dihydroxypropyl xylan, no significant influence of the synthesis method on the DS was found. In contrast, clear differences were observed influence of the synthesis method on the DS was found. In contrast, clear differences were observed concerning the resulting DS values in the case of carboxymethyl xylan in dependence on the concerning the resulting DS values in the case of carboxymethyl xylan in dependence on the synthesis synthesis method. Carboxymethyl xylans with the highest DS were obtained by using kneader or method. Carboxymethyl xylans with the highest DS were obtained by using kneader or heterogeneous heterogeneous methods. Xylan ethers with DS values of >1 could be synthesized in both cases. These methods. Xylan ethers with DS values of >1 could be synthesized in both cases. These xylan ethers xylan ethers showed interesting dissolution behavior. In spite of the lack of the primary OH-group showed interesting dissolution behavior. In spite of the lack of the primary OH-group in comparison in comparison to cellulose or starch, the synthesized xylan ethers were soluble in water, and to cellulose or starch, the synthesized xylan ethers were soluble in water, and produced freely flowing produced freely flowing aqueous solutions. Moreover, stable, transparent, and dyeable biobased aqueous solutions. Moreover, stable, transparent, and dyeable biobased polymer films could be polymer films could be prepared from carboxymethyl xylans. prepared from carboxymethyl xylans. Acknowledgments: This research project was realized through cooperation between the Peter Kölln KGaA Acknowledgments: This research project was realized through cooperation between the Peter Kölln KGaA (Elmshorn, Germany), Institute of Wood Wood Chemistry and Chemical Technology of Wood in the Federal Research Centre of Forestry and Forest Products (Hamburg, Ge Germany),rmany), Wolff Cellulosics Gm GmbHbH & Co. KG KG (Walsrode, (Walsrode, Germany) andand Fraunhofer Fraunhofer Institute Institute for Appliedfor Applied Polymer Po Researchlymer (Potsdam-Golm,Research (Potsdam-Golm, Germany). WeGermany). acknowledge We theacknowledge help of B. the Saake help (BFH, of B. Institute Saake (BFH, for Wood Institute Chemistry, for Wood Hamburg, Chemistry, Germany) Hamburg, by molarGermany) mass by determination molar mass of the xylan starting material. The financial support from the Agency of Renewable Resources (FNR, project determination of the xylan starting material. The financial support from the Agency of Renewable Resources No. 00NR098) is acknowledged. (FNR, project No. 00NR098) is acknowledged. Author Contributions: Fritz Loth, Ulrich Drechsler, and Kay Hettrich conceived and designed the experiments; UlrichAuthor Drechsler Contributions: and Kay Fritz Hettrich Loth, Ulrich performed Drechsler, the experiments; and Kay Hettr Ulrichich conceived Drechsler, and Kay designed Hettrich the and experiments; Bert Volkert analyzedUlrich Drechsler the data; and Kay Kay Hettrich Hettrich and perf Bertormed Volkert the wrote experiments; the paper. Ulrich Drechsler, Kay Hettrich and Bert Volkert Conflictsanalyzed the of Interest: data; KayThe Hettrich authors and declare Bert Volkert no conflict wrote of interest.the paper. Conflicts of Interest: The authors declare no conflict of interest. References

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