Article pubs.acs.org/acsapm
Cite This: ACS Appl. Polym. Mater. 2019, 1, 1443−1450
Thermoplastic and Flexible Films from Arabinoxylan Mikaela Börjesson, Gunnar Westman, Anette Larsson, and Anna Ström*
Downloaded via BUFFALO STATE on August 1, 2019 at 10:11:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Chemistry and Chemical Engineering, Chalmers University of Technology, Kemivägen 10, SE-412 96 Gothenburg, Sweden ABSTRACT: Current interest in replacing fossil-fuel-derived polymers and materials in favor of renewable materials is high. An inherent difficulty with the use of biomass-derived polysaccharides and hemicelluloses in this context, however, is their stiffness and lack of flowability at temperatures relevant for thermal processing, which severely limits their capacity for thermal processing. Here, we present a modification that enables a heat-processable arabinoxylan (AX). The modification involves a ring-opening oxidation to a dialdehyde with subsequent reduction of the aldehydes to alcohol, to increase the number of OH groups, followed by an etherification with hydrophobic alkyl chains. The modified AX was successfully compression molded with heat into filmswhich become thermoplastic in behavior and highly flexibleand flows at temperatures above 130 °C. The films are stretchable up to 200%, and their strength and strain deformation are controlled by the degree of oxidation and substitution of the AX polymer. These findings are highly encouraging and open up the potential use of modified AX alone or as a composite in applications that include films, food packaging, and barriers via hot-melt processing techniques. KEYWORDS: hemicellulose, oxidation, etherification, wheat bran
■
Apart from starch5,6 and cellulose acetate,7 few polysaccharide materials have been reported to have thermoplastic properties or thermal processing ability. A thermoplastic starch material is achieved in the presence of water, plasticizers, high temperature, and shear. Despite a great deal of effort having been spent on investigating the effect of various formulation principles and processing parameters of a thermoplastic starch material, several drawbacks remain, such as the inherent retrogradation of starch, the unsatisfactory mechanical properties of the material in dry environments (where it is fragile and brittle), its hydrophilicity, and the migration of plasticizers.5,6 Other possible polysaccharides to find use in thermal processing available in large quantities are hemicelluloses such as xylans, which are a group of polysaccharides found in the primary cell wall of wood, cereals, and grasses. Xylan is a linear polymer composed of xylose units; when substituted with arabinose side groups, it is termed arabinoxylan (AX). AX is the primary hemicellulose in all major cereal grains and can be extracted from underutilized side streams from agriculture. Attempts to produce a hemicellulose material with a lowered Tg have been made with xylan derivatized with acetyl groups,8 propylene oxide,9 and glycidyl ethers,10 enabling a reduction of Tg from about 217 °C for AX11 to between 124 and 200 °C. Nevertheless, a thermally processed material has not yet been reported, although solvent-casted films from derivatized xylan
INTRODUCTION
An increasing amount of effort is being spent in both the public and private sectors to reduce society’s current dependence on fossil fuels and petroleum-based products. The driving forces behind this effort include an increased use of renewable sources and the efficient use of raw materials. The task of replacing fossil-fuel-derived polymers in the area of packaging and plastics, however, has proved to be challenging; the difficulties relate to the hydrophilicity of biomass-based material, the brittleness and lack of extensibility of the films produced by using such material, and the inherently high glasstransition temperature (Tg) of polysaccharides, which limits thermal processing. Thermal processing, such as injection molding, extrusion blow molding, extrusion, injection compression molding, and so forth, is currently a standard part of manufacturing with fossil-fuel-derived polymers. Thermal and melt rheology properties are linked to the molecular structure of the polymer,1,2 including backbone flexibility, steric hindrance, and chain−chain polar interactions.3 Polysaccharides are inflexible polymers due to the rigidity of the carbohydrate rings, fixed bond angles, and often severely restricted rotation around the glycosidic bonds.4 Furthermore, their strong inter- and intramolecular hydrogen bonds give rise to high Tg, often above their thermal decomposition temperature. Chemical modification of a polysaccharide is thus required to enable the thermal processing ability of the material and to control the hydrophilicity/hydrophobicity of the final material. © 2019 American Chemical Society
Received: March 5, 2019 Accepted: May 17, 2019 Published: May 17, 2019 1443
DOI: 10.1021/acsapm.9b00205 ACS Appl. Polym. Mater. 2019, 1, 1443−1450
Article
ACS Applied Polymer Materials
room temperature. The extraction was neutralized by the addition of 1 M HCl, as the cellulose precipitate and the hemicellulose remained in the aqueous solution. The cellulose precipitate was separated from the hemicellulose fraction through centrifugation at 4300 rpm for 10 min. The hemicellulose fraction obtained after isolation contained high amounts of glucose (∼18% of the carbohydrate content), probably originating from starch and β-glucan.29 The starch was removed from the hemicellulose fraction by the addition of 3 mL of α-amylase (Termamyl 120) to an AX/water solution (5 g/L) and heated at 60 °C for 20 h. Dialysis against deionized water then followed. The carbohydrate composition of the extracted and purified hemicellulose was found to be as follows: 47% arabinose, 46% xylose, and small residues of galactose and glucose. Thus, the ratio of arabinose to xylose was 1:1. An arabinose:xylose ratio of 1:1 for wheat bran is a rather high arabinose substitution compare to AX from oat (1:5) and barley husk (1:3) which has been extracted with the same extraction procedure. Previous studies propose that oxidation occurs primarily on arabinose prior to the xylose backbone26 which is why the arabinose:xylose ratio of the original material is of importance.30 The wheat bran consisted of ∼36% hemicellulose, and the yield of the isolated and destarched AX was 11% in total, corresponding to 31% of the available hemicellulose in wheat bran. Periodate Oxidation and Sodium Borohydride Reduction of AX. The ring-opening oxidation of AX was performed through a periodate oxidation following earlier procedures published in the literature.19,31 Four grams (30.3 mmol) of arabinoxylan was dissolved in 165 mL of deionized water and 10 mL of isopropanol. Different mole equivalents (equiv)/anhydroxylose unit (AXU) of NaIO4 were added to the AX solution to give different degrees of oxidation (DO). Two different amounts of NaIO4 were used: 0.25 mol equiv/AXU (7.6 mmol) and 0.125 mol equiv/AXU (3.8 mmol). The NaIO4 was dissolved in 25 mL of water and then added to the AX solution. The concentration of AX in the final mixture was 2%. The reaction was magnetically stirred at room temperature, and the reaction flask was covered with aluminum foil and placed in a dark room to avoid side reactions caused by light. The progress of the reaction was followed with ultraviolet spectroscopy (UV−vis) by studying the absorption band at λ = 290 nm corresponding to the concentration of IO4−.32 UV−vis showed that the periodate consumption reached a plateau after 7.5 h; however, the reaction was left for 24 h. At the end of that time, a reduction with sodium borohydride was immediately followed. The reduction of the dialdehyde groups into dialcohol groups was performed according to the procedure described by Larsson and coworkers.16 Two grams of sodium borohydride and 0.3 g of monobasic sodium phosphate were first dissolved in 50 mL of water and then added to the 200 mL oxidized AX solution (described above). The reduction was performed with stirring for 4 h at room temperature. The samples were then purified through dialysis against deionized water for at least 2 days with repeated changes of the deionized water. The ring-opening oxidation to a dialdehyde with subsequent reduction of the aldehydes to alcohol will hereafter be called DiolAX. Etherification of AX and DiolAX. Arabinoxylan was grafted with BGE following a method described by Nypelö and co-workers, with minor modifications as described herein.10 The BGE was grafted to both the native AX and the ring-opened DiolAX solution, described in previous step. The dry native AX powder was dissolved in water to give a concentration of 4% while the DiolAX solution from previous step was reduced in volume on a rotary evaporator, resulting in a concentration of ∼4% AX in water. NaOH was added to the 4% AX or DiolAX solution (assumed average anhydrous molecular weight is 132.1 g/mol, giving here an AXU of 30.3 mmol) in a 250 mL three-necked round-bottom flask equipped with a reflux condenser and nitrogen inlet. The amount of NaOH added was either 3 mol equiv/AXU (3.6 g) or 5 mol equiv/ AXU (6.1 g). The NaOH was dissolved in the solution at 45 °C for 1 h before 3 or 5 mol equiv/AXU (corresponding to 13 or 22 mL) of the BGE (in a 1:1 molar ratio with NaOH) was slowly added to the solution through a needle. The reaction was stirred overnight at 45 °C. After the reaction, the solutions were neutralized with 1 M sulfuric
and AX have shown low oxygen permeability and mechanical properties similar to those of films from poly(lactic acid) and cellulose acetate.8,12 Periodate oxidation is a common treatment for carbohydrates and has long been used as a routine technique to study the structure of polysaccharides; it is also used as a route to alter polysaccharide flexibility via the oxidation of carbohydrate rings in the polysaccharide backbone. Periodate oxidation has been performed on polysaccharides such as alginate,13,14 chitosan,15 cellulose,16−19 starch,20−22 mannans,23 xylans, and AX.24,25 We have shown in a previous study that periodate oxidation of AX from wheat bran does not increase its overall chain flexibility as oxidation appears to occur primarily on the arabinose side groups rather than in the backbone composed of xylose.26 Subsequently, the Tg of the oxidized AX did not decrease, and the oxidized AX films remained brittle. However, although the backbone flexibility is not increased upon oxidation of the arabinose groups, dialdehyde functionalization of the side groups was enabled. The dialdehydes can thereafter be transformed into the more stable dialcohol functionalization upon reduction with sodium borohydride. Kochumalayil et al.27 showed in their study that periodate oxidized xyloglucan followed by subsequent reduction to dialcohols improves the elongation of the ring-opened xyloglucan. It is possible that oxidation followed by reduction of dialdehydes to dialcohols improves the flexibility also for highly substituted AX. In this study we propose a sequential process of ringopening oxidation of AX followed by derivatization with butyl glycidyl ether (BGE). The AX used was obtained from wheat bran, which is a major agro-industrial byproduct produced during the dry milling of common wheat into flour. Global production figures for wheat bran have been estimated as ranging from 45 to 90 million tons,28 and the wheat bran byproduct is produced in much larger quantities than utilized for human consumption today. BGE is a high-production-volume chemical that is primarily used as a reactive diluent or chemical intermediate in epoxy resins. It is also widely used in electronics, construction, and coating materials. The hemicellulose material obtained after this stepwise sequence of ring-opening oxidation followed by etherification can undergo thermal processing, leading to a highly deformable BGE-functionalized AX film. Possible applications for a material with this qualification, alone or as a composite, include packaging, barriers, or coating materials.
■
EXPERIMENTAL SECTION
Materials. AX was isolated from wheat bran provided by Lantmännen AB (Stockholm, Sweden). All chemicals used for the isolation process and the oxidation, reduction, and etherification reactions were purchased from Sigma-Aldrich (Schnelldorf, Germany), except the sodium (meta)periodate (NaIO4) which were purchased from Riedel-de Haën AG (Seelze, Germany). The chemicals were used without further purification. A dialysis membrane (Spectra/Por 3) with a molecular weight cutoff of 3.5 kDa was purchased from Spectrum Laboratories (Rancho Dominguez, CA). Isolation of AX. The isolation of AX from wheat bran was performed via an alkaline extraction process, as described in earlier work.29 As prehydrolysis, 500 g of wheat bran flakes was stirred in 10 L of 0.05 M HCl at room temperature over one night. The wheat bran flakes were then filtered, and the solid residue was mixed with 2 L of deionized water and 75 g of sodium chlorite. Delignification occurred at 80 °C for 3 h at a pH of 3.1, adjusted with 1 M NaOH. After delignification, the sample was filtered; the solid residue was dispersed in 1 M NaOH solution (5 L) containing 50 g of sodium thiosulfate as a reducing agent. Alkaline extraction was performed overnight at 1444
DOI: 10.1021/acsapm.9b00205 ACS Appl. Polym. Mater. 2019, 1, 1443−1450
Article
ACS Applied Polymer Materials
Figure 1. Reaction scheme for the (i) oxidation of arabinoxylan followed by reduction from dialdehyde to dialcohol groups and (ii) etherification of DiolAX with BGE. spectrometer (Agilent Technologies, St. Clara, CA) by studying the absorption band at λ = 290 nm corresponding to the concentration of IO4−. A linear standard curve for the periodate solution at λ = 290 nm was obtained using concentrations of NaIO4 between 0.1 and 5 mM, in accordance with the work of Maekawa and co-workers.32 The samples measured in UV−vis were diluted to be within in the range of the linear standard curve (i.e., 0.1−5 mM), filtrated through a 0.45 μm filter, and analyzed in quartz cuvettes between 800 and 200 nm with a scanning rate of 300 nm/min. The DO was then obtained from the consumption of periodate, as measured by UV−vis, which was multiplied with the mol equiv/AXU of NaIO4 that had been added (i.e., if 76% periodate was consumed and 0.25 mol equiv/AXU was added, this gives a DO of 0.25 × 76% = 19%).26 Fourier-Transform Infrared Spectroscopy. Fourier-transform infrared (FTIR) spectroscopy of the dried AX samples was recorded on a PerkinElmer Frontier FT-IR spectrometer (Waltham, MA) equipped with an attenuated total reflection (ATR) device. The FTIR was analyzed between 4000 and 400 cm−1, and 16 scans were collected. Molar Substitution of BGE. The molar substitution (MS) was evaluated using nuclear magnetic resonance (NMR) on the etherified samples. Proton (1H) and COSY (correlation spectroscopy) NMR spectra were recorded on a Varian MR-400 MHz spectrometer (Agilent Technologies) at 25 °C. Deuterated water was used as the solvent, and all samples were hydrolyzed with 72% sulfuric acid and neutralized with barium hydroxide prior to the measurements.33 An estimation of MS was calculated from eq 1, which compares the intensity of the signal from the BGE methyl group (0.9 ppm) to the intensity of the proton signals from the α-H1 (5.2 ppm) and β-H1 (4.5 ppm) from the carbohydrate units. The methyl group in the BGE substitution contributes with three protons while the −CH in the carbon ring contributes with one proton for α and one for the β configuration; therefore, α-H1 and β-H1 are are multiplied by 3. Equation 1 is a rough estimation of MS and does not take into account that the BGE, beside the hydroxyl groups in the carbohydrates, also will react with the grafted BGE substituents in the polymer. The MestreNova software (version 11.0, Mestrelab Research, Santiago de Compostela, Spain) was used for integration of the peaks.
acid and dialyzed against deionized water. The modified AX was obtained as a solid material after centrifugation at 4000 rpm for 10 min; it was then air-dried. Figure 1 shows the stepwise sequence of the oxidation and reduction of AX followed by etherification with BGE on DiolAX. Hemicellulose Films. Two different methods were used to prepare the hemicellulose films: (i) from solution casting and (ii) through a hot-melt compression-molding technique. Native AX does not flow at temperatures much lower than its degradation temperature; therefore, this material could only be made into a film through solution casting. The solubility of all the BGE grafted AX samples had changed in character and were not soluble in water (unlike the native AX). For the solution casting of BGE grafted AX samples either a high concentration of ethanol (96%) or a mixture of ethanol and water (75 vol % ethanol and 25 vol % water) was found to be a good solvent. The BGE-AX sample and the two BGE grafted DiolAX samples were cast from 20 g/L ethanol solutions (50 mL) in Petri dishes (d = 9 mm). A reference sample with the native AX was cast from a water solution (50 mL, 20 g/L). The thickness of the cast films was approximately 0.1−0.16 mm under these conditions. The air-dried BGE-AX and BGE-DiolAX samples were compression-molded into films by pressing between two hot plates. The samples were placed in a squared mold (50 × 50 × 0.5 mm3) between two metal plates in a hot press and heated at 140 °C for 3 min with no pressure; thereafter, they were heated for another 3 min at 140 °C with 50 kPa pressure. The compression molding was repeated until a homogeneous film without air bubbles was achieved. The AX film showed no visible changes upon repeated heating and remolding. The thickness of the film was between 0.35 and 0.47 mm after the film was cooled to room temperature. Characterization. Carbohydrate Composition. The neutral carbohydrate composition was analyzed in three replicates of the hydrolyzed AX (native) by means of high-performance anionexchange chromatography with pulsed amperometric detection (HPAEC-PAD), using an ion chromatography system ICS 3000 (Dionex, Thermo Fisher Scientific) equipped with a CarboPac PA1 (4 × 250 mm2) analytical column, a gradient pump, an isocratic post column pump, a column oven, and an autosampler. The different sugars detected with this system were arabinose, galactose, glucose, xylose, and mannose. Prior to analysis, the samples were hydrolyzed with 72% sulfuric acid according to Theander and Westerlund and diluted to a concentration of 200 mg/L.33 Degree of Oxidation. As mentioned earlier, the progress of the oxidation reaction was followed using a Cary 60 UV−vis ultraviolet
MS NMR =
1445
∫ (3 methyl protons of BGE)
(
)
3 × ∫ α H1 + ∫ β H1
(1)
DOI: 10.1021/acsapm.9b00205 ACS Appl. Polym. Mater. 2019, 1, 1443−1450
Article
ACS Applied Polymer Materials Thermal Properties. Thermogravimetric analysis (TGA) was used to detect thermal degradation in the samples. A TGA/DSC 3+ Star System (Mettler Toledo, Columbus, OH) was used. A sample with a mass of ∼10 mg was placed in an open aluminum pan and heated from 25 to 500 °C under a nitrogen flow of 50 mL/min with a heating rate of 5 °C/min. The TGA measurements give the percent weight loss and the thermal degradation temperature, measured from two replicates using the STARe Excellence Software (Mettler Toledo). Tensile Test. Tensile testing on the AX films was performed on an Instron 5565A (Instron, Norwood, MA). Solution-casted AX films with a gauge dimension of 20 × 5.6 × 0.1 mm3 or compressionmolded films with a gauge dimension of 20 × 5.6 × 0.4 mm3 were attached between two clamps, where the lower clamp was fixed and the upper clamp moved apart until breakage of the films occurred. A load cell of 100 N was used with a cross-head speed of 30 mm/min. All samples were conditioned in a climate chamber at 23 °C and 50% relative humidity for at least 24 h before measurement. The tensile test was performed at room temperature under ambient conditions; therefore, the samples were taken out from the climate chamber just before measurement to reduce the influence of the moisture in the air. All samples were also measured within a short period of time to ensure as similar conditions as possible. Six replicates were done for each sample, and Instron Bluehill 2 software was used to test and evaluate the samples. Rheology. The small-amplitude rheology of the films at 130 °C was measured using a DHR-3 rheometer (TA Instruments, New Castle, DE). The geometry used was plate−plate with a diameter of 25 mm and gap of 0.8 mm. The samples were held at 130 °C for 60 s (γ̇ = 10 s−1) followed by a frequency sweep between 0.01 and 80 Hz. The temperature was controlled under air by an Environmental Test Chamber kit-DHR provided by TA Instruments. The moduli and dynamic viscosity at 130 °C was only measured for the BGE grafted DiolAX films, since native AX and BGE grafted AX without prior ringopening do not flow or melt at this temperature with no external pressure.
Table 1. Degradation Temperature for Native AX, Oxidized AX (with Dialdehyde Groups), and the DiolAX (with Dialcohol Groups)a sample native AX 7% oxidized AX 19% oxidized AX 7% DiolAX 19% DiolAX a
degradation temperature (°C) 259 ± 242 ± 212 ± 256 ± 250b
1.1 0.7 4.6 7.7
The measurements were repeated twice. bNo replicate performed.
increase the number of primary −OH/−ONa groups via a ring-opening reaction and hypothesize that this raises the number of possible reaction sites for the etherification reaction. We chose to determine the MS instead of the DS of BGE. BGE is an epoxide that forms terminal alkoxides, which can become more reactive than the hemicellulose hydroxyl groups or the −ONa groups, especially as the terminal alkoxides are more distant from the bulky polymer backbone. This leads to a competition between substitution on the hemicellulose polymer and chain extension on an already substituted grouphence, the reaction may lead to a higher MS than the number of hydroxyl groups in a carbohydrate.34,35 The MS for the different reactions was calculated via eq 1 after 1H NMR analysis of the BGE grafted AX. Table 2 shows the obtained MS for three BGE grafted AX samples with different DO and two different mol equiv of BGE to AXU. For samples with 3 mol equiv of BGE, it can be observed that the MS increases with DO. This finding aligns with our hypothesis that increasing the amount of primary −OH/−ONa groups (i.e., the 7% and 19% DiolAX samples) increases the possible amount of substituted BGE. A similar trendalbeit weaker in absolute termscan be observed in the case of 5 mol equiv of BGE/AXU. The etherified samples were further analyzed using FTIR to study the introduction of BGE to the AX samples. Figure 2 shows the spectra for the different samples. In all of the etherified samples (Figure 2b−d), the absorption band around 3400 cm−1, which corresponds to O−H interactions, was shifted to higher wavenumbers due to changes in the −OH groups, where part of the −OH groups in the carbohydrates are substituted with BGE. The intensity of the absorption band around 2900 cm−1, which correlates to the −CH2 and −CH3 groups, is augmented for the BGE-modified samples. The intensity at these wavenumbers is higher for the two DiolAX samples grafted with BGE (Figure 2c,d compared to Figure 2b). The new absorption bands at 1465 cm−1 (CH2 bending) and 740 cm−1 (C−H, long-chain methyl rocking) originate from the BGE reagents, and the same trend is seen here: The sample with the higher MS has a higher intensity. The FTIR results are in line with the results found by NMR; that is, the BGE substitution is higher for the two DiolAX samples and increases with DO. Film Preparation. Films from BGE grafted samples were made via solution casting or hot compression molding. Figure 3 shows two samples that were solution cast (Figure 3a,d) and two samples compression molded into a film (Figures 3b,c and 3e,f). Figure 3a−c shows an AX film treated with BGE, for which the AX had not undergone oxidation prior to derivatization with BGE. Figure 3d−f shows the 19% DiolAX film grafted with BGE. It is worth noting that the 7% DiolAX and the 19% DiolAX derivatized with BGE have the same
■
RESULTS AND DISCUSSION Oxidation and Reduction of Arabinoxylan. The oxidation of xylan and arabinoxylan has been evaluated in an earlier study by the authors, and the same oxidized AX material is used in this study.26 Different degrees of oxidation were obtained by adding different amounts of NaIO4 to water solutions containing 2 wt % AX. Two different NaIO4 concentrations were used to obtain two different DOs, based on the concentration of NaIO4 measured by UV−vis. The final DOs obtained were DO = 19% (for 0.25 mol equiv added) and DO = 7% (for 0.125 mol equiv added), corresponding to a consumption of 79% for the higher DO and 56% consumption of NaIO4 for the lower DO; for further details see an earlier publication.26 The reduction from dialdehyde to dialcohol was seen as a change in thermal degradation in TGA measurements. The carboxyl groups in the aldehydes are less stable than the hydroxyl groups in the native AX and in the DiolAX. Therefore, a decrease in degradation temperature was first seen for the oxidized samples with a dialdehyde functionalization, and thereafter the degradation temperature increased again when stabilized to a dialcohol functionalization (see Table 1). Molar Substitution of Butyl Glycidyl Ether. Nypelö and co-workers reported a lowering in Tg from 184 to 124 °C for softwood xylan (degree of substitution (DS) = 1, where DS refers to the number of substituted hydroxyl groups in a carbohydrate unit) after introducing BGE alkyl chains to the xylan polymer; however, thermal processing of the BGEderived xylan was not performed.10 In the present work, we 1446
DOI: 10.1021/acsapm.9b00205 ACS Appl. Polym. Mater. 2019, 1, 1443−1450
Article
ACS Applied Polymer Materials
Table 2. Integrated Peak Value of the α-H1, β-H1, and −CH3 from the BGE Grafted AX Samples and Their Estimated MS Value Calculated from eq 1a sample AX, DO = 0% DiolAX, DO = DiolAX, DO = AX, DO = 0% DiolAX, DO = DiolAX, DO =
7% 19% 7% 19%
mol equiv/AXU added
α-H1 (5.2 ppm)
β-H1 (4.5 ppm)
−CH3 (0.9 ppm)
MS
3 3 3 5 5 5
0.48 0.71 0.88 0.59 0.80 0.82
1.0 1.0 1.0 1.0 1.0 1.0
4.34 10.43 16.81 11.30 15.69 16.84
0.98 2.03 2.98 2.37 2.91 3.08
The integral for β-H1 was assigned as 1.0 in all samples using the MestreNova software.
a
solution; however, for the BGE-AX without prior oxidation (Figure 3a), a mixture of ethanol (75 vol %) in water (25 vol %) was found to be a good solvent for obtaining a clear, homogeneous solution. The differences in color between the solvent-casted film and the compression-molded film (Figure 3) can partly be related to variation in the thicknesses of the films, which were 0.1 and 0.4 mm, respectively. However, we cannot rule out initial degradation of the carbohydrates, carbohydrate−protein reactions, known to create browning of material, or possible residues of dialdehyde groups which contribute to the yellowing of the hot compression-molded samples. Hemicellulosic films are routinely made from solvent casting, but to our knowledge, no literature has been published on hot compression-molded hemicellulose films. Thermal Properties of BGE-Treated AX Films. The thermal degradation properties of the BGE grafted AX and DiolAX films were evaluated using TGA (Figure 4). All of the BGE grafted samples possessed two different slopes on the weight loss curve, with change in the slope at T1 of ∼220 °C, which is lower than that of the native AX (T1 = 259 °C). However, the second change in the slope (T2) appeared around 320 °C as a result of the BGE derivatization of the samples. The two slopes can be clearly seen in the first derivative of the curves (Figure 4b). The weight loss for three different temperature ranges is listed in Table 3; the weight loss at the highest temperature range (290−400 °C), around T2, increases with substitution of
Figure 2. FTIR spectra of the different samples reacted with BGE: (a) native AX from wheat bran; (b) BGE-grafted AX without prior oxidation; (c) 7% DiolAX grafted with BGE; (d) 19% DiolAX grafted with BGE (3 mol equiv of BGE/AXU was used in samples b−d).
visual appearance, which is why only the 19% DiolAX-BGE sample is shown here. The solubility of AX in polar solvent was reduced upon substitution with the hydrophobic BGE, in agreement with the reported hydrophobicity of BGE substituted xylan.10 As a consequence, the solvent casting was performed using different solvents depending on MS. For most substituted samples (Figure 3d), the films were therefore cast from a 96% ethanol
Figure 3. Comparison between AX and DiolAX samples grafted with BGE without (a−c) and with (d−f) oxidation pre-treatment. Images (a) and (d) show films from solution casting, images (b) and (e) show films made with compression molding at 140 °C, and images (c) and (f) give an indication of the flexibility of the two compression-molded films. 1447
DOI: 10.1021/acsapm.9b00205 ACS Appl. Polym. Mater. 2019, 1, 1443−1450
Article
ACS Applied Polymer Materials
compression-molded films showed a large variation in behavior depending on whether or not the films had been ring-opened (and to what DO) prior to BGE substitution (Table 4 and Figure 5). Figure 5a shows the influence of the BGE substitution on native AX where it clearly shows that the BGE reduces the brittleness of the modified AX. Figure 5b shows the effect of oxidation prior to the BGE substitution, and the more oxidized the samples (19% DiolAX), the larger the strain until break. In Figure 5b two different BGE molar ratios are compared for the 19% DiolAX samples where it shows that both a high DO and a high substitution of BGE affect the tensile strain of the material the most. The BGEsubstituted films on 19% DiolAX show a large reduction in stress at breakage as well as a large increase in tensile strain of up to 185% (±32%). While the tensile stress is high for an arabinoxylan film (that of externally plasticized arabinoxylan films with glycerol is ∼10% and with sorbitol is up to 30%36,37), the strength at fracture is lower by a factor of 10.38 An increase in tensile strain is often obtained at the expense of tensile strength, which indeed is observed for the substituted samples and especially for the BGE grafted DiolAX samples. Shear Rheology of BGE Grafted AX Films. The BGE grafted DiolAX samples were observed to easily fill the mold used during compression molding (T = 140 °C and P = 50 kPa), and even lower temperature would be possible. The theological properties of the material were measured at T = 130 °C. It is notable that although the nonoxidized AX sample treated with 5 mol equiv of BGE/AXU could be pressed into a film through compression molding, it did not flow at a this temperature without applying pressure. Similar results have been observed in our lab using BGE-derivatized galactoglucomannan; that is, while a hot-melt film can be pressed from the BGE-derivatized galactoglucomannan, a high pressure is necessary in order for the material to flow. The 7% and 19% DiolAX with BGE substitutions were, however, able to flow at 130 °C and under a low normal force. Figure 6 shows the dynamic viscosity (η*), storage modulus (G′), and loss modulus (G″) as a function of the frequency of the BGEsubstituted DiolAX with a DO of 7% and 19%. The moduli of both samples increase with frequency, typical of entangled but not cross-linked polymer fluid. G′ is larger than G′′ over the frequency range studied for the 7% DiolAX sample while the crossover occurs at a frequency of 2 rad/s for the sample with 19% DiolAX. Further studies are required to verify whether the difference in crossover between the samples is related to variations in molecular weight and flexibility of the DiolAXBGE of different DO. Few or no data on shear rheology of hemicelluloses have been reported at 130 °C, which iswhy comparisons of the values to other hemicelluloses are hard to find. The η* of the DiolAX with DO of 7% and 19% is,
Figure 4. TGA curve for native AX and the BGE grafted AX and DiolAX samples where (a) shows the weight loss (%) and (b) shows the first derivative of the curves in part (a). All BGE grafted samples shown here were made with 5 mol equiv/AXU.
BGE. The weight loss between 25 and 150 °C corresponds to moisture evaporation and is highest for the native AX sample and lower for all the BGE grafted samples. This result can be due to the treated samples possessing a more hydrophobic character than the native AX, which is why less water interacts with the polymer. Mechanical Properties of BGE Grafted AX Films. Bioplastics, including polysaccharide and polylactic acid films, typically demonstrate very brittle behavior with tensile strains at a breakage of
