Positron Lifetime Reveals the Nano Level Packing in Complex

Mar 20, 2012 - comprising a fair share of lignin coexisting with poly- and oligo-saccharides, have very low but variable oxygen permeability but diffe...
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Positron Lifetime Reveals the Nano Level Packing in Complex Polysaccharide-Rich Hydrolysate Matrixes Ulrica Edlund,† Yang Yu,‡ Yingzhi Zhu Ryberg,† Reinhard Krause-Rehberg,‡ and Ann-Christine Albertsson*,† †

Fiber and Polymer Technology, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle/S, Germany

‡

S Supporting Information *

ABSTRACT: Positron annihilation lifetime spectroscopy (PALS) was used to quantify the free volume and molecular packing in hydrolysate and hemicellulose-based barriers films, derived from process streams during wood processing operations. These hydrolysate films, comprising a fair share of lignin coexisting with poly- and oligo-saccharides, have very low but variable oxygen permeability but differ among themselves with respect to barrier performance as well as molecular weight, degree of branching, and monosaccharide residue main chain composition. From PALS measurements on hydrolysates, the free volume hole radius (rh), radius distributions (n(rh)), volume-weighted hole sizes (v), and hole volume distributions (g(vh)) were calculated showing that the hydrolysate matrixes are very densely packed with small holes. The results show a clear relationship between hydrolysate molecular architecture and composition, the nanolevel molecular packing, and the ability of suppressing the diffusion of oxygen through the film.

T

A low oxygen permeability is related to a low free volume in the matrix suppressing the diffusion of small permeants through the matrix and hence to the nanolevel packing of the macromolecular chain segments forming the matrix. The improved barrier performance of said wood hydrolysates, comprising a fair share of lignin coexisting with poly- and oligosaccharides, compared to the isolated hemicellulose, suggests that a key role in the barrier performance may be found in an improved molecular packing ability of the former. A driving force for packing is a strong interaction between the lignin and hemicelluloses components indicating that the origin, extraction, and recovery routes are tunable parameters by which the hydrolysate performance can be controlled. Revealing the relationships between the characteristics of the molecular packing arrangements and the hydrolysate components structure and ratio would hence serve as an integral toolbox in the future formulation of coatings. Positron annihilation lifetime spectroscopy (PALS) has previously been successfully used to model and provide an understanding of the molecular packing characteristics, hole volumes, and hole size distributions in a range of different matrixes, inorganic as well as organic.8,9 PALS assesses the change in lifetime of positronium in its ortho-state (spin 1), oPs, when colliding with the walls of holes constituting the free volume of amorphous solid matrixes. PALS has been applied to mono- and oligosaccharides, for instance, to analyze plasticization effects of water in the matrixes.10,11 Turning from small to macromolecular matrixes and turning from pure to mixed

he utilization of green materials and the formulation of products from green resources are becoming increasingly acknowledged alternatives for future material production. In our efforts toward a more sustainable lifestyle, renewable resources may to some extent replace fossil fuels as a feedstock for materials. Among the huge variety of available biopolymers, the noncellulosic yet poly- and oligo-saccharide rich fractions of wood stand out as resources of high potential thanks to their abundance and availability and being nonedible. The heteropolysaccharides referred to as hemicelluloses1 are typically found in such fractions, being released to the liquid stream during many forest and agricultural processing operations, such as pulping, together with other noncellulosic wood components such as lignin, monosaccharides, and extractives. The soluble and dispersed macromolecules in such process waters, hydrolysates, can be recovered through a number of different pathways and optionally refined to yield a more or less pure hemicellulose-rich biomass. Utilizing the hydrolysate macromolecular fractions as materials, in a less refined state, is of great potential, not in the least from an economic standpoint, and may offer excellent barrier properties typically associated with hemicelluloses, which are important in sustainable packaging applications.2−4 Noteworthy, recent findings show that the less refined, mixed component character of spruce hydrolysates performs superior to their highly purified hemicellulose counterpart, O-acetyl galactoglucomannan, in thin barrier coating formulations with respect to oxygen permeability.2 Various hemicellulose matrixes have over the years been amply characterized1,5−7 with respect to structure, molecular weights, and composition while the barrier performance is conventionally assessed through gas, water, and grease transmission.3 © 2012 American Chemical Society

Received: January 16, 2012 Accepted: March 20, 2012 Published: March 20, 2012 3676

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four PLgel 20 μm Mixed-A columns was used. The eluent flow rate was 0.5 mL/min at 80 °C. Pullulan standards with narrow molecular weight distributions were used for calibration. All samples were dissolved in DMAc containing LiCl (0.5% w/w) and filtered prior to analysis (0.45 μm, Millipore). The injection volume was 200 μL. Shimatzu software was used for data acquisition and calculations. Carbohydrate and Lignin Content of Hydrolysates. Carbohydrate analysis of hydrolysates SW and HW was performed by acid hydrolysis. Each hydrolysate was then diluted with a mixture of 6 mL of 72% H2SO4 (aq) in 84 mL of deionized water and kept in an autoclave at 125 °C for 1 h. The samples were filtered through a fiber glass filter, washed with deionized water, diluted with deionized water to a volume of 100 mL, and finally analyzed by ion-exchange chromatography using a high-performance anion exchange chromatograph (HPAEC-PAD, Dionex ICS-3000) equipped with a gradient pump (Dionex, GP50), an electrochemical detector (Dionex, ED40), and a CarboPac PA1 separation column. A NaOH/ acetate gradient was used as an eluent. The filters used in the acid hydrolysis as described above were used to determine the Klason lignin content by washing the filters two times with 100 mL of deionized water and then drying them at 105 °C for about 12 h. The Klason lignin content was determined gravimetrically by comparing the filter weight prior to filtration and after drying. The degree of substitution of acetyl groups (DSAc) was calculated from the quantity of acetyl residues, which was determined by keeping the hydrolysates at 80 °C for 1 h in 2 mL of NaOH (1.0 M) and then filtered through Teflon filters (0.2 μm, Millipore). A volume of 0.20 mL of the filtrate was diluted with water to 10 mL and analyzed with respect to acetate ion content using a Dionex ICS-2000 ion chromatography system (with an electrochemical detector), Dionex, GA15 guard column, SA15 separation column, and a potassium hydroxide (35 mM) buffer eluent. Positron Annihilation Lifetime Spectroscopy (PALS). A fast−fast coincidence system8,9 with a time resolution of 253 ps (fwhm, 22Na) and an analyzer channel width of 26.1 ps was applied in PALS measurements. The positron source used is 22 Na2CO3 with an activity of 1.2 MBq deposited between two 2 mg/cm2 thin aluminum foils. This source was covered by two identical sample sheets of a thickness of 1.1−1.2 mm sufficiently thick to stop all positrons. The measurements were conducted at room temperature in air. For each spectrum 4.7−4.9 × 106 counts were collected. The source correction was determined by measuring a defect-free p-silicon reference sample (τSi = 219 ps). Lifetimes of 0.160 ns (Al-foil) and 0.385 ns (22Na2CO3) with a relative sum intensity of 18.94% appeared for the reference sample. Because polymers have a lower average atomic number than Si and therefore a lower back-reflection of positrons toward the source, the sum intensity was lowered to 7.9% following a literature calibration procedure.13 This source component was subtracted from the spectrum before the final data analysis.

matrixes presents new challenges but also a range of opportunities of assessing and understanding the assembly of macromolecular segments and the morphology down to the nanolevel scale. Our hypothesis is that PALS can shed light on the molecular packing arrangements in hydrolysate and hemicellulose-based barriers and serve as a powerful analytical tool contributing to the strategic choice of components in barrier formulations where a dense structure contributes to preventing the diffusion of small permeants through the matrix. The aim of this work is to reveal the nanolevel packing in hydrolysate matrixes recovered from the process or wastewater of the wood refining process. A PALS methodology is elaborated to assess such complex noncellulosic polysaccharide-rich matrixes.

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EXPERIMENTAL SECTION Materials. The highly purified hemicellulose birch xylan (xylose residues >90%) was used as received from Sigma (CAS number: 9014-63-5). The weight average molecular weight (Mw) was ∼10 290 g/mol (polydispersity index, PDI = 3.7) as measured by size exclusion chromatography (SEC). Carboxymethyl cellulose sodium salt (CMC), having a degree of substitution of 0.6−0.9 and a medium viscosity of 400−1000 mPa s in a 2% H2O solution at 25 °C, was purchased from Sigma-Aldrich (CAS number: 9004-32-4). Hydrolysates. A softwood hydrolysate, SW, was kindly supplied by Södra Cell AB. The hydrolysate was recovered from process water and was produced from spruce chips according to a recently invented upgrading procedure12 involving the recovery of wastewater generated in a pulping process. Briefly, the spruce chips were subjected to hydrothermal treatment at 165 °C in preheated water in a liquid/ wood ratio of 6:1 (volume/mass ratio). The water phase was then membrane filtered using a cellulose membrane with a cutoff of 1000 Da. The retentate was diluted with water in a ratio of retentate/water of 1:10 and diafiltered. The retentate was finally lyophilized (LyoLab 3000 instrument). A hardwood hydrolysate, HW, was recovered from birch chips which were first pretreated at 100 °C for 15−20 min. Then, sodium sulphite cooking chemicals were added and the temperature raised to 160−170 °C so that a red liquor was produced under alkaline conditions (pH = 8−11). A process water sample was removed from the cooking liquor in the dewatering step of the pulp. This liquid phase was subjected to membrane filtration using a ceramic membrane with a cutoff of 5000 Da and then precipitated in methanol. The light yellow precipitate was dissolved in water and lyophilized. Film Preparation. Films were prepared from hydrolysates or the highly purified hemicellulose xylan, in each case mixed with 50% (w/w) CMC according to the following protocol: Solutions of each component in deionized H2O were first prepared separately on a shaking board, then combined under stirring, and additionally homogenized ultrasonically. The resulting mixtures, having a volume of 28 mL, were cast on Petri dishes (diameter 8.7 cm) and dried for 3 days in a controlled humidity room (55% relative humidity (RH) at 23 °C). For PALS analysis, each type of film was cut into 0.8 cm × 0.8 cm film squares, and the squares were stacked into a thickness of 1.1−1.2 mm. Size Exclusion Chromatography (SEC). SEC was used for molecular weight determinations of the hemicelluloses and the hydrolysates. A Shimadzu SEC system based on N,Ndimethylacetamide (DMAc) as the eluent and equipped with

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RESULTS AND DISCUSSION Hydrolysates and Hemicellulose Based Films. The hydrolysates recovered from process waters in the hydrothermal treatment of wood were rich in hemicelluloses. The major sugar residues detected in the softwood hydrolysate, SW, were galactose, glucose, and mannose. This is consistent with the hemicellulose structure O-acetylated galactoglucomannan, AcGGM, reported to be the major constituent of noncellulosic 3677

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polysaccharides in sprucewood.14,15 A representative structure of AcGGM is available in the Supporting Information (Scheme S1). AcGGM is typically acetylated in the C-2 and/or C-3 positions, and accordingly, the hemicellulose detected in SW had a degree of acetylation (DSAc) of about 0.5. In addition, SW contains 10% of lignin and trace amounts of arabinan and arabinoxylan. The weight average molecular weight (Mw) of SW was ∼2900 g/mol (polydispersity index, PDI = 1.6) as measured by size exclusion chromatography (SEC). The hydrolysate HW, derived from birchwood, had a carbohydrate composition dominated by xylose, which was expected since O-acetyl-(4-O-methylglucurono)xylan is known to be the chief component of birchwood hemicelluloses.15,16 Mw was ∼9770 g/mol (PDI = 1.6) as measured by SEC. The xylan in HW was not acetylated which may be explained by the somewhat harsh, acidic conditions to which the biomass was subjected during the pulping process where the hydrolysate was produced. A representative structure of 4-O-methylglucurono)xylan is available in the Supporting Information (Scheme S1). While upgraded through membrane filtration and precipitation, this fraction is not pure in hemicellulose but contains about 9% of lignin and trace amounts of pectic substances. Films consisting of hemicellulose-rich hydrolysates, either from softwood or hardwood, together with carboxymethyl cellulose (CMC) were prepared by casting from the water solution. A film based on highly pure commercial birch xylan mixed with CMC was also prepared for comparison. This commercial xylan from birchwood consisted of >90% xylose residues and was nonacetylated. CMC was selected as a cocomponent based on our recent findings17,18 showing that films comprising hydrolysates mixed with CMC had very low OP values and good mechanical properties. While CMC is typically semicrystalline in its pristine state, the films composed of CMC mixed with hemicellulose-rich hydrolysates showed no sign of crystallinity according differential scanning calorimetry (DSC) analyses. (DSC thermograms are shown in Figure S1 in the Supporting Information.) All films were macroscopically and microscopically homogeneous, free-standing, smooth, and transparent with a yellow to light brown color and a thickness in the range of 40−60 μm. Positron Lifetime Spectrum (PALS). Positronium in its ortho-state (spin 1), o-Ps, is used as a probe for subnanometersized local free volumes in a positron lifetime experiment. In polymers, a fraction of the positrons injected from a radioactive source can form positronium (Ps). It is the size of a hydrogen atom but only the mass of two electrons. This light quantum mechanical particle can be trapped by the subnanometer-sized holes that constitute the hole free volume of amorphous materials. Holes with a diameter larger than about 0.25 nm can be detected. Because of collisions with the walls of the holes, the positron in o-Ps annihilates with an electron of opposite spin other than its bound partner (pick-off annihilation), decreases its lifetime from 142 ns in infinitely large holes (selfannihilation of o-Ps in a vacuum) to the low nanosecond-range in subnanometer-sized holes (free volume holes). Therefore, the pick-off annihilation time reflects the path of o-Ps and hence directly relates to the size of the free volume hole which traps it, regardless of the void shape. Free volume holes in a macromolecular structure have an irregular geometry. Furthermore, the hole geometries are constantly changing slightly due to the dynamic character of the macromolecules which rearrange through the main chain bond rotations, making it impossible to decide any exact shape for free volume holes.

Using PALS, we seek to measure and quantify the size of such irregular geometries. Imagining the holes as spherical is then an assumption made to allow for feasible calculations where only one parameter is necessary to determine in the threedimensional space, the radius, rather than three individual length scale parameters associated with other geometries. Here, the classic Tao-Eldrup model, where the shape of free volume holes is assumed to be spherical, is adopted19,20 so that the free volume size and distribution can be calculated from the o-Ps lifetime distribution, using a quantum-mechanical model with an empirical parameter.19−21 It was shown for polymers that these distributions correlate well with those from molecular modeling when the probing of the free volumes by Ps is properly taken into account.22 PALS and Its Analysis. The positron lifetime spectrum usually consists of three exponential-like components with the characteristic lifetimes τi and relative intensities Ii. In the spur/ blob model, the thermalized positron (e+) combines with an inblob electron to form Ps and annihilates as para-Ps (p-Ps) with the mean lifetime τ1 or as o-Ps with τ3.23,24 Because of its short lifetime, the p-Ps will not have much interaction with the material and annihilates with a lifetime close to its intrinsic lifetime of 0.125 ns (p-Ps self-annihilation in a vacuum). The oPs annihilates with a lifetime in the low nanosecond-range as a consequence of the pick-off process (in a vacuum, 142 ns). Positrons which have left the blob annihilate without Ps formation as e+ with a lifetime τ2 (τ1 < τ2 < τ3). For the analysis of the positron lifetime spectra, the routine LifeTime, version 9.0 (LT9.0)25,26 was employed. Disordered (amorphous) materials show a size and shape distribution of local free volumes which leads to a distribution of the lifetimes around their mean. The positron lifetime spectrum s(t) is given by the Laplace transformation of the function α(λ)λ, where α(λ) is the probability density function (pdf) of the annihilation rate λ = 1/τ and τ the corresponding lifetime. The parameters of these functions are obtained from a nonlinear least-squares fit of the model function to the spectrum (after convoluting with the resolution function). Taking into account that the p-Ps lifetime is assumed to appear discrete, the decay spectrum can now be expressed by eqs 1a and 1b: s(t ) = I1λ1exp( −λ1t ) +

∑ i = 2,3

with

∑

Ii

∫0

∞

αi(λ)λexp( −λt )dλ

Ii = 1

i = 1,2,3

(1a)

and α i (λ )λ d λ =

⎡ (ln λ /λ )2 ⎤ i0 ⎥ exp⎢ − dλ ⎢⎣ 2σi*2 ⎥⎦ σi*(2π)1/2 1

(1b)

Table 1 summarizes the most important parameter, the o-Ps intensity I3, the mean pick-off lifetime time, ⟨τ3⟩ and standard deviation of the o-Ps lifetime distribution α3(τ) of average value from repeated experiments on the hydrolysate and xylan films. Size Distribution of Local Free Volumes (Holes). From the analyzed lifetime parameters, the distribution of hole sizes can be calculated. The quantum-mechanical (Tao-Eldrup) standard model19,20 relates the o-Ps pick-off annihilation rate 3678

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Table 1. Ortho-Ps Lifetime Parameters of Films Consisting of Blends of a Softwood Hydrolysate (SW), a Hardwood Hydrolysate (HW), or Commercial, Pure Xylan (XYL) with 50% (w/w) CMC quantity unit stat. error

I3 % ±1

τ3 ns ±0.04

σ3 ns ±0.04

SW HW XYL

18.8 17.7 19.6

1.204 1.260 1.302

0.276 0.408 0.345

Figure 2. Hole volume distribution g(vh) in films consisting of blends of a softwood hydrolysate (SW), a hardwood hydrolysate (HW), or commercial, pure xylan (XYL) with 50% (w/w) CMC. This distribution gives the volume fraction of holes of sizes between vh and vh + dvh and has been here normalized to ∫ g(vh) dvh = 1.

λpo = 1/τpo = 1/τ3 to the hole (assumed spherical) radius (rh) via eq 2: ⎡ ⎛ 2πr h ⎞⎤ rh 1 sin⎜ λ po = 2 ns−1⎢1 − + ⎟⎥ ⎢⎣ r h + δr 2π ⎝ r h + δr ⎠⎥⎦

be approximated by a log-normal function, a Γ-function, or more roughly by a Gaussian in agreement with the theoretical considerations. Table 2 shows the various hole volume

(2)

The parameter δr = 1.66 Å describes the penetration of the Ps wave function into the hole walls and was determined empirically.19,20 Since λpo follows a distribution and the relation between λpo and the hole radius rh is nonlinear (eq 2), we have estimated the mean hole volume as the mass center of the hole size distribution. The radius distribution, n(rh), can be calculated from n(rh) = −α3(λ) dλ/drh, via eq 3:27 n(r h) =

⎡ ⎛ 2πr h ⎞⎤ ⎢1 − cos⎜ ⎟⎥α3(λ) ⎝ r h + δr ⎠⎥⎦ (r h + δr )

Table 2. Hole Volume Parametersa

2δr

2 ⎢⎣

⟨rh⟩ Å 0.025

vh(⟨τ3⟩) Å3 2

⟨vh⟩v Å3 3

σhv Å3 3

SW HW XYL

1.930 1.972 2.050

33.9 37.8 40.8

33.7 39.5 41.2

19.6 31.0 26.2

a Mean of the hole radius distribution, n(rh) − ⟨rh⟩; mean and standard deviation of the volume-fractional hole-size distribution, g(vh), ⟨vh⟩v and σhv. vh(⟨τ3⟩) gives the hole volume calculated directly from the Tao-Eldrup equation.

(3)

where α3(λ) is the o-Ps annihilation rate distribution defined by eq 1b.

parameters. The volume-weighted mean holes size ⟨vh⟩v agrees almost with that calculated directly from eq 2, vh(⟨τ3⟩). Comparison of the PALS Results for the Various Film Types. The most important parameter indications of the nanolevel molecular arrangements of the hydrolysate and hemicellulose based films are the o-Ps lifetime parameters, shown in Table 1, and the local free volume parameters derived from these shown in Table 2. The o-Ps lifetimes are in the range of τ3 = 1.2−1.3 ns and rather small. Usually, the o-Ps lifetime in amorphous polymers lie in the range between 1 and 8 ns, depending on the type of polymer and temperature. The hydrolysate and hemicellulose based matrixes assessed herein are hence very densely packed and have rather small holes. The hole radius distribution ranges from 1 to 4 Å with mean values between ⟨rh⟩ =1.9−2.0 Å. These results indicate that strong interaction and thus a denser amorphous packing is favorable for such hemicellulose-rich formulations. This dense morphology is in line with a reported free volume size is about 1.5 Å for chitosan, in the untreated, hydrated, or cross-linked state.30 Being polysaccharides, chitosan as well as the hemicellulose-rich hydrolysates are very hydrophilic and have many pendant hydroxyl groups giving rise to strong inter/intra molecular interactions which may effectively lead to an average smaller hole volume. In comparison, the more hydrophobic polymer polycarbonate has a free volume size in the range of 8−11 Å at room temperature.31 The aromatic/ester group ratio of PC influences its chain flexibility. However, it will be still more difficult for PC than chitosan to fold chains with aromatic groups which may leave bigger voids and leads to larger free

Figure 1. Hole radius distribution n(rh) (probability density function, pdf) measured for films consisting of blends of a softwood hydrolysate (SW), a hardwood hydrolysate (HW), or commercial, pure xylan (XYL) with 50% (w/w) CMC. The distributions were normalized to the unity area below the curve.

Figure 1 shows the hole radius distributions for the three film types under investigation. For this calculations, the parameters ⟨τ3⟩ and σ3 shown in Table 1 were used. From eq 3, the hole volume distribution can be calculated according to eq 4: g (v h) = n(r h)/4pr h 2

quantity unit stat. error ±

(4)

This distribution is considered as a volume-weighted function, i.e., g(vh) gives the volume fraction of holes of sizes between vh and vh + dvh.28,29 Figure 2 shows these distributions which may 3679

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shows that the origin and the recovery route of wood hydrolysates, both affecting the composition and molecular structure of the constituent hemicelluloses, are important parameters to consider when utilizing more or less upgraded hydrolysates as green resources in barrier materials design.

volumes. There are no reported data of the size of free volume in films composed only of CMC in the literature. One would however expect very small free volume due to semicrystallinity and from the fact that CMC is a polysaccharide with pendant hydroxyl groups, just like the hydrolysates studied here, where strong inter/intra molecular interactions drive the formation of a densely packed structure. For hydrolysate based matrixes, SW and HW, the volumeweighted hole size amounts to ⟨vh⟩v = 33.7 and 39.5 Å3, respectively, while the corresponding volume for the pure hemicellulose XYL is 41.2 Å3. The holes in XYL are larger than in the SW and HW samples. This is consistent with and can explain why hydrolysate based barriers are superior to corresponding formulations based on highly purified hemicellulose with respect to oxygen permeability (OP).2,17 OP values of hydrolysate coatings are tabulated in Table 3.

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CONCLUSIONS Hydrolysates recovered from process waters in the hydrothermal treatment of wood were used to produce barrier films. Positron annihilation life time spectroscopy was shown to be a useful tool to revealing the nanolevel molecular packaging in these renewable matrixes. Matrixes derived from softwood (SW) and hardwood (HW) hydrolysates, as well as a corresponding matrix based on the highly purified hemicellulose xylan (XYL) show o-Ps formation at local free volumes (holes) with relative intensities and lifetimes of I3 = 17.7−19.6% and τ3 = 1.2−1.3 ns, respectively. The hole radius distribution ranges from 1 to 4 Å with mean values around ⟨rh⟩ = 2.0 Å. The mean volume-weighted hole volume range from ⟨vh⟩ = 34 and 41 Å3 showing that the matrixes are very densely packed and with small holes and that the hydrolysate based matrixes are more densely packed than the XYL matrix. There is also a tendency of larger holes with broader size-distributions for the hardwood derived matrixes compared to SW. The branched and shorter chain structure of SW gives rise to a denser packing than the longer and linear hardwood based HW and XYL. PALS quantification of the nanolevel packing in macromolecular matrixes has the potential to serve as a powerful analytical tool to indirectly measure the barrier properties and guide future strategic choices of components in packaging formulations.

Table 3. Molecular Weight (Mw) and PDI from Size Exclusion Chromatography (SEC) As Well As the Oxygen Permeability (OP) Values for the Coatings on PET from Hydrolysates SW, HW, and XYL SEC

SW HW XYL

OP (50% RH)

Mw (g/mol)

PDI

cm3 μm m−2 day−1 kPa−1

Barrer = 10−11 (cm3 O2) cm cm−2 s −1 mmHg−1

2900 9770 10290

1.6 1.6 3.7

1.6 1.3 8.7

2.4 × 10−5 2.0 × 10−5 13.2 × 10−5

Coatings based on SW and HW (with the same cocomponent as in this work, CMC, at 40% (w/w)) had OPs of 2.4 × 10−5 and 2.0 × 10−5 Barrer at 50% relative humidity (RH), respectively. Formulations based on highly purified XYL or AcGGM, otherwise prepared identically, had OPs of 13.2 × 10−5 and 4.8 × 10−5 Barrer, respectively. The larger holes in XYL are hence reflecting a higher permeability of XYL compared to SW. These results indicate that the component interactions in the mixed hydrolysates favor densely packed matrixes. Both HW and SW contain a considerable amount of lignin (9−10%) while XYL is pure. The lignin fraction is a highly irregular aromatic structure and may contain a range of molecular sizes from oligomeric to polymeric. The wood hydrolysates, being mixed fractions, contain compounds with complex chemical structures and different molecular weights. Therefore, it is possible that in the hydrolysate itself, some small molecules may fill in the gaps (holes) between long-chain molecules. This “filling effect”32 leads to a decrease of the overall hole volume, while the volume distribution should be larger. The hydrolysates differ in molecular weight, degree of branching, as well as monosaccharide residue main chain composition, which allows for a comparative assessment of the relationships between composition and nanolevel molecular packing in the hydrolysate amorphous solid state structures. HW and XYL are very similar in molecular weight (Mw around 10 000 g/mol), while SW consists of shorter chains (Mw almost 3000 g/mol. SW is acetylated and thus somewhat branched, while HW and XYL are not. The different molecular architecture may contribute to a filling effect32 and contribute to lowering the free volume. In addition, the branched SW has a lower free volume hole distribution than either HW rich in the linear xylan and the pure linear xylan containing XYL. This

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ASSOCIATED CONTENT

S Supporting Information *

Schemes showing representative structures of the major hemicelluloses found in SW, O-acetyl galactoglucomannan, and HW, 4-O-methyl-glucuronoxylan This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors from KTH thank VINNOVA (Project Number 2009-04311) for financial support.

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REFERENCES

(1) Albertsson, A.-C.; Edlund, U.; Varma, I. K. In Biopolymers: new materials for sustainable films and coatings; Plackett, D., Ed.; Wiley: Chichester, U.K., 2011; pp 133−150. (2) Ryberg, Y. Z.; Edlund, U.; Albertsson, A.-C. Biomacromolecules 2011, 12, 1355−1362. (3) Hansen, N. M. L.; Plackett, D. Biomacromolecules 2008, 9, 1493− 1505. (4) Mikkonen, K. S.; Heikkinen, S.; Soovre, A.; Peura, M.; Serimaa, R.; Talja, R. A.; Helen, H.; Hyvonen, L.; Tenkanen, M. J. Appl. Polym. Sci. 2009, 114, 457−466. (5) Timell, T. E. Wood Sci. Technol. 1967, 1, 45−70. (6) Bjarnestad, S.; Dahlman, O. Anal. Chem. 2002, 74, 5851−5858. 3680

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dx.doi.org/10.1021/ac300152g | Anal. Chem. 2012, 84, 3676−3681