Material Properties of Plasticized Hardwood Xylans for Potential

May 11, 2004 - Malin Brodin , Mar?a Vallejos , Mihaela Tanase Opedal , Mar?a Cristina Area , Gary Chinga-Carrasco. Journal of Cleaner Production 2017 ...
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Biomacromolecules 2004, 5, 1528-1535

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Material Properties of Plasticized Hardwood Xylans for Potential Application as Oxygen Barrier Films Maria Gro¨ndahl, Lisa Eriksson, and Paul Gatenholm* Biopolymer Technology, Department of Materials and Surface Chemistry, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden Received February 6, 2004; Revised Manuscript Received March 24, 2004

Free films based on glucuronoxylan isolated from aspen wood were prepared by casting from aqueous solutions and drying in a controlled environment. Addition of xylitol or sorbitol facilitated film formation and thus examination of the material properties of these films. The mechanical properties of the films were evaluated using tensile testing and dynamic mechanical analysis in a controlled ambient relative humidity. The strain at break increased, and the stress at break and Young’s modulus of the films decreased with increasing amounts of xylitol and sorbitol due to plasticization. At high amount of plasticizer, it was found that films with xylitol gave lower extensibility. Wide-angle X-ray scattering analysis showed that xylitol crystallized in a distinct phase, which we believe contributes to the more brittle behavior of these films. The effect of the plasticizers on the glass transition temperature was determined using dynamic mechanical analysis and differential scanning calorimetry. An increased amount of plasticizer shifted the glass transition to lower temperatures. The effect of moisture on the properties of plasticized films was investigated using water vapor sorption isotherms and by humidity scans in dynamic mechanical analysis. Sorption isotherms showed a transition from type II to type III when adding plasticizer. The films showed low oxygen permeability and thus have a potential application in food packaging. Introduction The great majority of plastic materials in use today are based on fossilic raw materials. More than 40% of all plastic materials are used as packaging, and their disposal contributes to growing landfills and enhanced greenhouse effects when burned.1 Furthermore, the fossil resources on earth are limited. A sustainable development in the future requires the use of renewable materials. The use of plant biopolymers to produce plastic materials has several advantages. In addition to their being a renewable resource, the product may after use be composted, recycled or incinerated with minimal environmental impact.2 Food packaging requires materials with good mechanical properties so that the food remains undamaged during storage. Good barrier properties are also necessary to prevent gases such as oxygen from degrading the product. Such materials should furthermore be resistant to water and it is important that they do not emit poisonous substances since they are in contact with food.3 Starch films have been evaluated for possible use in food packaging applications.4-7 Starch is present in large quantities in potato and maize, for example, and has been proven to fulfill a number of necessary criteria for food packaging applications. The mechanical properties are excellent and the flexibility can be further improved by the addition of plasticizers. Glycerol is frequently added as a low molecular weight plasticizer,8-10 but xylitol and sorbitol have also been used.7 Starch exhibits * To whom correspondence should be addressed. Fax: +46 (0)317723418. E-mail: [email protected].

very good oxygen barrier properties,11-12 but a drawback of the material if used on a large scale is that it would then compete with the food industry. Hemicellulose may be an alternative material for such applications. It is one of the main constituents of wood and other plants and exists in major quantities in agricultural waste. Approximately 60 billion tons of hemicellulose is biosynthesized every year.13 So far, the commercial utilization of hemicellulose in nonfood applications has been very limited. In the pulp and paper industry, retaining the hemicellulose in the pulp has been shown both to improve the mechanical properties of the paper and the yield.14,15 Hemicellulose has also been used as sweetening agents, thickeners, and emulsifiers in food.16 There has been interest in the use of hemicellulose as a nutraceutical,17 in chiral separations,18 and as an HIV inhibitor.19 Hemicelluloses are heteropolysaccharides whose composition varies between different plant species. In hardwoods such as aspen, beech, and birch, however, the hemicellulose consists chiefly of O-acetyl-(4-O-methylglucurono)xylan.20 The backbone consists of β-(1f4)-linked D-xylopyranosyl residues substituted with one R-(1f2)-linked 4-O-methylD-glucuronic acid per approximately every 10th such residue. The xylopyranosyl residues are partially acetylated at the C-2 and/or C-3 positions with a degree of acetylation between 0.6 and 0.7 in aspen wood.21 In many aspects, the properties of glucuronoxylan lie between those of cellulose and starch. Glucuronoxylan is a cell wall polymer, like cellulose, but cellulose is a linear polymer with a rigid backbone designed to crystallize and

10.1021/bm049925n CCC: $27.50 © 2004 American Chemical Society Published on Web 05/11/2004

Properties of Plasticized Hardwood Xylans

be loadbearing. Glucuronoxylan is, as mentioned above, substituted with glucuronic acid and acts as a matrix material in the cell wall. Starch, which is an energy reserve in the plant, has an extensively branched component, amylopectin. Cellulose and starch are semicrystalline in their native state, whereas glucuronoxylan is amorphous in the native state but can crystallize after isolation using alkali.22 The crystalline structure incorporates water, as in starch.23 However, glucuronoxylan is not as easily soluble in water as starch,24 which could be an advantage in using it in food packaging. The aim of this study was to evaluate glucuronoxylan isolated from hardwood for potential application as food packaging. This work focuses on investigating the effect of addition of xylitol and sorbitol on film formation of glucuronoxylan. The effect on the mechanical and barrier properties of the films and the interactions with water were evaluated. Materials and Methods Materials. The aspen glucuronoxylan used in this study was isolated by alkali extraction at the Department of Wood Sciences and Forest Products at Virginia Tech, Blacksburg, VA. The isolated material contained 83 wt. % xylose, 14 wt. % methyl glucuronic acid, 2 wt. % mannose, and less than 1 wt. % of other sugars. The weight average molecular weight was 15 000 g/mol, determined using size exclusion chromatography (SEC) in DMSO. The isolation procedure and characterization are described in detail elsewhere.22,25 Xylitol and sorbitol were purchased from Fluka (article no X-3375) and Sigma (article no 85529), respectively. Preparation of Films. Films were prepared by mixing glucuronoxylan, xylitol, or sorbitol and deionized water during magnetic stirring at 95 °C for 15 min. The total amount of dry substance, glucuronoxylan and an additive, in each film was kept constant at 1 g and 35 mL of water was added. The additive content was varied between 20 and 50 wt. % of the dry weight (20, 27.5, 35, 42.5, and 50 wt. % of xylitol and 20, 35, and 50 wt. % of sorbitol). The solutions were poured onto polystyrene Petri dishes with a diameter of 14 cm, and films were allowed to form upon drying in a temperature of 23 °C and a relative humidity (RH) of 50%. The dried films were stored in these conditions until analysis. Films of poly(vinyl alcohol) were prepared to compare the oxygen permeability with that of the glucuronoxylan films. 1 g of poly(vinyl alcohol) (Fluka 81382) was dissolved in 35 mL of deionized water at 95 °C for 15 min during magnetic stirring. The solutions were poured onto dishes similar to those used for the glucuronoxylan solutions and dried under the same conditions. Tensile Testing. The mechanical properties of the conditioned films were measured with a tensile testing machine (Lloyd L2000R) with a load cell of 100 N capacity. The samples were cut into dog-bone-shaped strips with a width of 7 mm. The thickness of the samples, measured with a micrometer, was 35 ( 5 µm. The initial distance between the grips was 20 mm and the separation rate of the grips was kept constant at 5 mm per minute. Fifteen replicates of

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the films with 20, 35, and 50 wt. % additive and five replicates of the films containing 27.5 and 42.5 wt. % xylitol were tested. The stress-strain curve was recorded for each sample, and Young’s modulus, strength at break, and strain at break were calculated. The measurements were performed at 50% RH and 23 °C after conditioning for at least one week. Wide-Angle X-ray Scattering. The crystallinity of the materials was determined with wide-angle X-ray scattering (WAXS). The films were milled in liquid nitrogen, and the samples were investigated with a Siemens D5000 goniometric diffractometer. Cu KR radiation with a wavelength of 1.54 Å was used, and 2θ was varied between 5 and 30° at a rate of 1° (2θ) per minute and a step size of 0.1° (2θ). A relative crystallinity was calculated to compare the crystallinity of the samples. The background was subtracted from the diffractogram by drawing a baseline tangentially to the curve minimum. The relative crystallinity was determined by comparing the area of the crystalline peak at about 18° (2θ) with the total area under the peak.26 Dynamic Mechanical Analysis. Temperature scans in dynamic mechanical analysis (DMA) were performed in tension at 1 Hz in a Rheometrics RSA-II (Rheometrics Scientific, Piscataway, NJ) in order to determine the glass transition temperature at 50% RH. 4 × 20 mm strips of the conditioned films were covered with a hydrophobic grease (Stabox 9415 provided by AB Axel Christiernsen in Nol, Sweden) and heated from temperatures of -70 up to +110 °C with a heating rate of 5 °C per minute. The mechanical behavior of the films as a function of the surrounding relative humidity was studied with DMA humidity scans. A Perkin-Elmer DMA7 with extension assembly connected to a humidity controller accessory was used. Strips of the films, 5 × 20 mm in size, were mounted between the two holders. The samples were then loaded in tensile mode with a frequency of 1 Hz and an amplitude of about 4 µm. The temperature was 30 °C, and the relative humidity in the sample chamber during testing was changed according to a humidity program. The program was initiated with 30 min at 1% RH, followed by a ramp of 1% RH per minute until 90% RH was reached, and was finished with five minutes at 1% RH. The air humidity at which the transition took place was taken from the onset of the decrease in the storage modulus. This is not the transition at equilibrium water content, wg, but the relative humidity at which softening takes place under the described conditions, providing a comparison between the films. Differential Scanning Calorimetry. The thermal transitions of the conditioned films were studied using differential scanning calorimetry (DSC). The instrument used was a Perkin-Elmer Pyris. Pieces of the RH conditioned films were heated from -110 to +110 °C in sealed steel cups with heating and cooling rates of 10 °C per minute. All measurements were made in at least triplicate. The inflection point of the heat flow step change, close to the half Cp change of the second heating scan was taken as the operative definition of the glass transition temperature Tg. Before and after the measurements, the cups were weighed in order to ensure that no evaporation had taken place.

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Water Vapor Sorption Isotherms. Pieces of the films were conditioned in closed vessels containing saturated salt solutions. The salts used were lithium chloride (SigmaAldrich R13013), magnesium chloride hexahydrate (SigmaAldrich R31413), magnesium nitrate hexahydrate (SigmaAldrich F63079), sodium chloride (Sigma-Aldrich F71381), and potassium sulfate (Sigma-Aldrich R31270), resulting in relative humidities of 11, 33, 54, 75, and 97%, respectively, at ambient temperature according to ASTM E104-85. The films were first conditioned at the lowest relative humidity and then moved successively to the highest relative humidity. Finally, the samples were dried at 130 °C overnight. The equilibrium water content was measured gravimetrically on a Mettler AE260-DR balance and calculated as the weight of water in the sample at equilibrium compared to the total weight. The points displayed are average values calculated from triplicates. Oxygen Permeability. The oxygen transmission of the films was measured in accordance with ASTM D3985-81 using Mocon Oxtran 2/20 equipment (Modern Controls Inc., Minneapolis, MN) with a coulometric oxygen sensor. The area of the samples was 5 cm2, and the analysis was made in 50% RH after conditioning of the samples. Data were collected for 24 h. The permeability was calculated from the transmission and the measured thickness of the films and is presented in units of (cm3 µm)/(m2 d kPa), where 1 d ) 24 h. Results and Discussion Film Formation. Films prepared from glucuronoxylan alone became very brittle and fragmented upon drying. Film formation conditions, such as temperature and surrounding humidity, were kept constant and varying the drying time did not result in any change of the film forming ability. Possible explanations of the poor film forming ability would be either insufficient chain length of the polymer, high glass transition temperature, or poor solubility. We have seen that slight carboxymethylation of this very aspen glucuronoxylan improves film formation.22 Thus, the molecular weight requirements should be fulfilled. The glass transition temperature of hardwood xylan is difficult to determine due to thermal degradation at elevated temperatures. The decomposition temperature of hemicellulose is lower than that of cellulose and lignin27-28 but can be increased by esterification.29-30 The glass transition temperature has been estimated to be around 180 °C,31-32 which might explain the poor film formation at room temperature. To suppress the glass transition temperature, we have added xylitol and sorbitol as potential plasticizers. After the addition of xylitol or sorbitol in the amounts described in the experimental part, transparent films with good mechanical cohesion were formed, which was a prerequisite for the evaluation of material properties. Mechanical Properties. The evaluation of material properties started off with the determination of tensile properties at room temperature and 50% RH. Parts a and b of Figure 1 show typical stress-strain curves for the glucuronoxylan films with various concentrations of xylitol and sorbitol,

Figure 1. a. Stress-strain behavior of xylitol-plasticized films at 23 °C and 50% RH, from left to right: 20, 35, and 50 wt. % xylitol. b. Stress-strain behavior of sorbitol-plasticized films at 23 °C and 50% RH, from left to right: 20, 35, and 50 wt. % sorbitol.

respectively. Films with 20 wt. % xylitol or sorbitol are strong, with a stress at break above 40 MPa, but are brittle, with an elongation at break of around 2%. The strength and brittleness of these films are in agreement with tensile testing results of the hydroxypropyl derivative of xylan.33 The addition of more xylitol or sorbitol results in reduced strength but an increase in elongation at break. Pay attention to the difference in elongation at break with increasing amount of sorbitol compared to xylitol, which contributes to a larger work of fracture of the sorbitol-plasticized films. Parts a-c in Figure 2 illustrate how the amount of plasticizer affects the stress at break, Young’s modulus, and strain at break of the films. The effect on strength at break and Young’s modulus with increasing amount of plasticizer is similar for xylitol- and sorbitol-containing films. However, as seen in Figure 2c, the xylitol-plasticized films are less extensible than the films plasticized with sorbitol when 50 wt. % of plasticizer is added. Wide-Angle X-ray Scattering. The morphology of the films was investigated using WAXS. The diffractograms of the xylitol-containing films and xylitol itself are shown in

Properties of Plasticized Hardwood Xylans

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Figure 3. a. X-ray diffractograms of samples containing xylitol, from bottom to top: 0 (pure glucuronoxylan), 20, 27.5, 35, 42.5, 50, and 100 (pure xylitol, rescaled with the factor 1/3) wt. % xylitol. The diffractograms are displaced in the y direction. b. X-ray diffractograms of samples containing sorbitol, from bottom to top: 0, 20, 35, 50, and 100 (rescaled with the factor 1/2) wt. % sorbitol. The diffractograms are displaced in the y direction.

Figure 2. a. Effect of the amount of plasticizer on the stress at break at 23 °C and 50% RH, b, xylitol; O, sorbitol. The error bars represent the standard deviation. b. Effect of the amount of plasticizer on Young’s modulus at 23 °C and 50% RH, b, xylitol; O, sorbitol. The error bars represent the standard deviation. c. Effect of the amount of plasticizer on the strain at break at 23 °C and 50% RH, b, xylitol; O, sorbitol. The error bars represent the standard deviation.

Figure 3a and the corresponding data for the sorbitolcontaining films is shown in Figure 3b. The diffractograms

of the pure low molecular weight plasticizers give rise to sharp distinct peaks, whereas the polymer has broader peaks and a halo, resulting from scattering from the amorphous parts. Xylan crystallizes in a helical structure with a 3-fold screw axis along the chain direction. The crystallinity and the interplanar distances increase with an increasing amount of water. The hydrated form has a layer spacing of about 14.84 Å.34-35 In the left-handed structure, an intramolecular hydrogen bond O5‚‚‚O3′ is possible, whereas there is a steric conflict for a right-handed helix between O2 and H4′.36 The O5‚‚‚O3′ intramolecular hydrogen bond is weaker in xylan than in cellulose, and the xylan helix is stabilized mainly by van der Waal forces. The absence of strong intramolecular hydrogen bonds makes the xylan less rigid than cellulose.37 There are six monomer units in two antiparallel chains and six water molecules in the unit cell of the hydrated form. The water molecules probably form a column. The empty lattice site is able to accommodate water as well as side

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groups such as glucuronic acid. Xylan seems to depend on the presence of water to stabilize the structure.38 The O2-H and O3 atoms of each xylose residue point toward one of the unoccupied lattice sites in the structure, the expected locations of the water molecules. In the proposed water chain, each water molecule accepts from the xylose O2-H and donates to an O3. The same water molecule also accepts and donates to its neighboring water molecules, giving two strong hydrogen bonds in addition to the weak intramolecular hydrogen bond O3′‚‚‚O5.23 The crystalline peak at about 2θ ) 18° can be used to compare the relative crystallinity of the samples. The addition of plasticizer did not affect the degree of crystallinity of the films, which was calculated as the area under the crystalline peak at about 2θ ) 18° divided by the total area at those angles (amorphous and crystalline scattering). The effect of the plasticizer on the crystallinity is complex and involves two opposite factors. On one hand, the plasticizer increases the mobility of the polymer chains and therefore the ability to crystallize. On the other hand, an increase in the amount of plasticizer leads to more interactions between the plasticizer and the glucuronoxylan and therefore to a decrease in the possible interactions between the glucuronoxylan chains. The relative crystallinity was at the same level for all films, between 44 and 47%. The crystalline peak at 2θ ) 18° is displaced in the diffractograms in the plasticized samples as compared to the pure glucuronoxylan. The distance between the lattice planes, d, can be related to the Bragg angle, θ, using Bragg’s law: 2 d sin θ ) nλ, where n is an integer and λ is the wavelength of the X-ray radiation. A lower angle correlates to a larger distance between the lattice planes. The spacing between the crystal lattice planes of the glucuronoxylan increases when plasticizer is added, and large amounts of plasticizer give rise to greater separation. It seems as though the low molecular weight plasticizer molecules can penetrate the crystalline parts of the glucuronoxylan. Small peaks from crystalline xylitol can be observed in the films containing 42.5 and 50 wt. % xylitol. This indicates crystallization of xylitol in a distinct phase. The crystallized xylitol does not plasticize the glucuronoxylan, which may contribute to the lower elongation at break of the films with 50 wt. % xylitol compared to films with a corresponding amount of sorbitol, observed by tensile testing. Furthermore, the crystallized xylitol can induce fracture of the films. Dynamic Mechanical Analysis. Temperature scans in DMA were carried out at 50% RH. During heating, the films lose water and become brittle. Shrinking of the films was also observed upon evaporation of water. To prevent this, the films were sealed with a hydrophobic grease that has been carefully studied and shown to contribute to the modulus only to a small extent and to give a tight encapsulation.39 The films with 20 wt. % plasticizer have a transition above room temperature at about 40-50 °C, see Figure 4a, which correlates well with the high Young’s modulus and low elongation at break observed during tensile testing. Transitions are observed as a rapid decrease in storage modulus and a peak in loss modulus and phase angle (tan δ). The films with higher plasticizer content have consider-

Gro¨ndahl et al.

Figure 4. a. DMA heating scan of film with 20 wt. % xylitol at 50% RH. b Storage modulus (E′); O, loss modulus (E′′); 1, tan δ. b. DMA heating scan of film with 35 wt. % xylitol at 50% RH. b, Storage modulus (E′); O, loss modulus (E′′); 1, tan δ. c. DMA heating scan of film with 50 wt. % xylitol at 50% RH. b, Storage modulus (E′); O, loss modulus (E′′); 1, tan δ.

ably lower transition temperatures at 50% RH, see Figure 4, parts b and c. Films with 35 wt. % xylitol have a transition

Properties of Plasticized Hardwood Xylans

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Figure 5. DSC second heating scans of the plasticized films. From bottom to top: film with 20 wt. % sorbitol, 35 wt. % sorbitol, 50 wt. % sorbitol, 20 wt. % xylitol, 35 wt. % xylitol, and 50 wt. % xylitol. The thermograms are displaced in the y direction.

Figure 6. Water sorption isotherms at 20 °C for glucuronoxylan films with different amounts of xylitol. b, Pure glucuronoxylan; O, 20 wt. %; 1, 27.5 wt. %; 3, 42.5 wt. %.

at about -30 °C and films with 50 wt. % xylitol at -45 °C. The storage modulus at room temperature corresponds well with data obtained during tensile testing for all samples. The reason why the modulus in the rubbery plateau is still in the range of 1 Gpa is that it is a semicrystalline material. The high values of the modulus at -50 to -60 °C is due to that the material is frozen. Differential Scanning Calorimetry. DSC was used for further study of the thermal transitions of the glucuronoxylans. The measurements were performed at 50% RH using sealed cups. Figure 5 shows that when the plasticized glucuronoxylan films are heated a glass transition is detected as an increase of the heat flow to a higher level. The glass transition detected in the second heating scan was at the same level as in the first heating scan for all samples. The glass transition occurs below room temperature for all plasticized films and there is a tendency toward a decrease of the calorimetric glass transition temperature with increasing amount of plasticizer. The film with 35 wt. % xylitol has a transition at -35 °C, the film with 50 wt. % xylitol at -43 °C, the film with 35 wt. % sorbitol at -26 °C, and the film with 50 wt. % sorbitol at -31 °C. This is in good agreement with the transitions observed during DMA heating scans. Furthermore, a melting endotherm was observed in the thermogram of the film with 50 wt. % xylitol, supporting the presence of crystallized xylitol. However, we cannot explain the low glass transitions detected for the films with 20 wt. % of plasticizer. Effect of Water. It is well-known that the properties of polysaccharides are affected by water. We have therefore performed our material investigations in a controlled environment at 50% RH. The ternary system polymer-plasticizerwater is very complex and thus deserves to be a subject for extensive study. A first step is to examine the effect of the surrounding humidity on the water content. We have measured the water vapor sorption isotherms, and they are shown in Figure 6. The standard deviations were small. Water vapor sorption isotherms of pure hemicellulose have

been reported previously.29,40 The shape of the curve for pure xylan is characteristic for systems with strong polymerpolymer and polymer-solvent interactions (type II isotherm).41 The shape arises from partial replacement of polymer-polymer hydrogen bonds with polymer-water hydrogen bonds. The equilibria involve bound and unbound water and free and associated polymer. The addition of plasticizer results in a transition from a type II to a type III isotherm, which is typical for plasticized systems. This difference in shape has also been observed for amylose and amylopectin films with or without glycerol.12 It is interesting to note that all plasticized films had a similar water content at 50% RH at which the mechanical tests were carried out. The plasticized films have a higher water content than the unplasticized films at ambient RH above 70%. Humidity scans in DMA show how the modulus of the materials depends on the air humidity. A softening of the material occurs at a certain RH, which leads to a rapid decrease in the storage modulus, see Figure 7. The films with higher amounts of plasticizer have a lower onset RH for the decrease in modulus. The same trend was observed in the sorbitol-plasticized films. When the relative humidity rises, the materials take up water. Water incorporated in the crystalline regions does not affect the mechanical properties, whereas water present in the amorphous segments acts as a plasticizer and reduces the stiffness of the material. A greater amount of plasticizer reduces the intermolecular interactions between the glucuronoxylan chains, thus facilitating interactions between water and the hydroxyl groups of the glucuronoxylan. Oxygen Permeability. Polysaccharides are generally good oxygen barriers because hydrogen bonds contribute to good packing of the material and thus a low permeability. The oxygen transmission of the glucuronoxylan film with 35 wt. % sorbitol was measured at 50% RH. The average oxygen permeability was 0.21 (cm3 µm)/(m2 d kPa). The measured oxygen permeability is lower than literature values for glycerol-plasticized starch and comparable with the often

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Table 1. Oxygen Permeability Data on Plasticized Glucuronoxylan Film and Comparable Values for Plasticized Starch Polymers, Ethylene Vinyl Alcohol, and Low Density Polyethylene material

oxygen permeability [(cm3 µm)/(m2 d kPa)]

source and conditions

glucuronoxylan with 35 wt. % sorbitol amylose with 40 wt. % glycerol amylopectin with 40 wt. % glycerol poly(vinyl alcohol) (PVA) ethylene vinyl alcohol (EVOH) low-density polyethylene (LDPE)

0.21 7 14 0.21 0.1-12 1870

present study, 50% RH 42 50% RH 42 50% RH present study, 50% RH, 43 70% VOH, 0-95% RH 43 50% RH

isotherms. The storage modulus of the plasticized films decreased when the surrounding RH was increased in a humidity scan in DMA. The higher the amount of plasticizer, the lower the onset RH for the decrease in modulus. The films with 35 wt. % sorbitol exhibited excellent oxygen barrier properties at 50% RH. The oxygen permeability was lower than that of plasticized starch and in the same range as that of the commercially used synthetic polymeric material EVOH. Thus, glucuronoxylan has a potential in film applications, such as food packaging.

Figure 7. DMA of xylitol-plasticized films, humidity scans between 0 and 90% RH. From left to right: 50, 42.5, 35, 27.5, and 20 wt. % xylitol.

Acknowledgment. This work was funded by Vinnova, the Swedish agency for innovation systems, through the Prohem program. Ingela Gangby at SIK, the Swedish institute for food and biotechnology, is acknowledged for her assistance in the oxygen transmission measurements and AnneMari Olsson and Lennart Salme´n at STFI, the Swedish pulp and paper research institute, as well as Mats Stading at SIK for letting us use their DMA equipment. References and Notes

used barrier plastic ethylene vinyl alcohol (EVOH), see Table 1. Conclusions Plasticization of glucuronoxylan with xylitol or sorbitol enables the formation of transparent, continuous, and flexible films by lowering the glass transition temperature, determined by using DMA and DSC. The stress at break and Young’s modulus of the films decreased, whereas the strain at break increased with increasing plasticizer content. Sorbitol and xylitol gave rise to films exhibiting similar mechanical properties, but the xylitol plasticized films were slightly less extensible at high plasticizer levels. Analysis with WAXS showed that the glucuronoxylan in all samples was semicrystalline. The presence of peaks from crystalline xylitol in the samples with large amounts of xylitol as well as the melting endotherm in the DSC heating scan revealed that xylitol crystallized in a distinct phase. The crystallized xylitol does not plasticize the glucuronoxylan but can induce fracture of the films. This might explain the difference in strain at break between the films with 50 wt. % xylitol and 50 wt. % sorbitol. The xylitol and sorbitol molecules are also possibly intercallants in the crystalline lattice. The amount of plasticizer in the films affected the water content of the materials. The addition of plasticizer resulted in a transition from type II to type III water vapor sorption

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