Honeycomb films of cellulose azide: molecular structure and formation of porous films.

Development of value-added micropatterned porous materials from naturally abundant polymers, such as cellulose, are of growing interest. In this paper, regioselectively modified amphiphilic cellulose azide, 3-O-azidopropoxypoly(ethylene glycol)-2,6-di-O-thexyldimethylsilyl cellulose, with different degrees of substitution (DS) and degrees of polymerization (DP) of the poly(ethylene glycol) (PEG) side chain, was synthesized and employed in the formation of honeycomb-patterned films. With the variation of the DP and/or DS, the amphiphilicity of the polymer and the pore size of the formed films changed accordingly. It was found that amphiphilicity of the cellulose azide played a significant role in the formation of honeycomb films. Balanced amphiphilicity was of particular importance in the formation of uniform honeycomb films. Via the Cu(I)-catalyzed alkyne-azide [2 + 3] cycloaddition reaction, fluorescent avidin and quantum dots were attached to the films. By means of confocal microscopy, it was confirmed that the functional azido group was preferentially allocated inside the pores. This provides a platform for the development of advanced honeycomb materials with site-specific functionalities, such as biosensors.

Preparation and characterization of Kraft lignin-based moisture-responsive films with reversible shape-change capability.

Preparation of moisture-responsive Kraft lignin-based materials by electrospinning blends of Kraft lignin fractions with different physical properties is presented. The differences in thermal mobility between lignin fractions are shown to influence the degree of interfiber fusion occurring during oxidative thermostabilization of electrospun nonwoven fabrics, resulting in different material morphologies including submicrometer fibers, bonded nonwovens, porous films, and smooth films. The relative amount of different lignin fractions and degree of fiber flow and fiber fusion is shown to influence the tendency for the electrospun materials to be transformed into moisture-responsive materials capable of reversible changes in shape. Material characterization by scanning electron microscopy and atomic force microscopy as well characterization of the chemical and physical properties of Kraft lignin fractions by dynamic rheology, 1H and 13C NMR, and gel permeation chromatography combined with multiangle laser light scattering are presented. A proposed mechanism underlying moisture-responsiveness, shape change, and shape recovery is discussed based on the differences in chemical structure and physical properties of Kraft lignin fractions.

Design of functionalized cellulosic honeycomb films: site-specific biomolecule modification via "click chemistry".

Value-added materials from naturally abundant polymers such as cellulose are of significant importance. In particular, cellulosic open-framework structures with controlled chemical functionality of the internal surface have great potential in many biosensor applications. Although various cellulose derivatives can form porous honeycomb structured materials, solubility issues and problems with film formation exist. To address this, we have generated robust cellulosic open-framework structures that can be post-functionalized through site-specific modification. Regioselectively modified amphiphilic cellulose azides, 3-O-azidopropoxypoly(ethylene glycol)-2,6-di-O-thexyldimethylsilyl cellulosics, were synthesized, and honeycomb-patterned films were readily produced by the simple breath figures method. Changing the degree of polymerization (DP) of the pendent ethylene glycol (EG(DP)) groups from 22 to 4 increased the corresponding honeycomb film pore diameters from ~1.2 to ~2.6 µm, enabling the potential tuning of pore size. Moreover, these novel azido-functionalized honeycomb films were easily functionalized using Cu(I)-catalyzed alkyne-azide [2 + 3] cycloaddition reaction; biotin was "clicked" onto the azide functionalized cellulosic honeycomb films without any effect to the film structure. These results indicate this system may serve as a platform for the design and development of biosensors.

Synthesis of lignin nanofibers with ionic-responsive shells: water-expandable lignin-based nanofibrous mats.

A series of ionic responsive poly(N-isopropylacrylamide) (PNIPAM) surface-modified lignin nanofiber mats were prepared by aqueous surface-initiated atom transfer radical polymerization (SI-ATRP). PNIPAM brushes with various molecular weights, thickness, and grafting densities were immobilized on electrospun lignin nanofiber mats by adjusting initial monomer concentration and surface initiator density. ATR-FTIR, SEM, TGA, XPS, and water contact angle measurements confirmed successful surface modification. Analysis of the PNIPAM-modified lignin nanofiber mats (Lig-PN) found that the lower critical solution temperature (LCST) was similar to that of PNIPAM and demonstrated ionic responsive characteristics. With increasing ionic concentration, the water contact angles of the Lig-PN increased correspondingly. AFM images showed that the PNIPAM on the lignin nanofiber mat surface expanded in water and contracted in 0.5 M Na(2)SO(4).

Thermal mobility of ß-O-4-type artificial lignin.

Several lignin model polymers and their derivatives comprised exclusively of ß-O-4 or 8-O-4' interunitary linkages were synthesized to better understand the relation between the thermal mobility of lignin, in particular, thermal fusibility and its chemical structure; an area of critical importance with respect to the biorefining of woody biomass and the future forest products industry. The phenylethane (C6-C2)-type lignin model (polymer 1) exhibited thermal fusibility, transforming into the rubbery/liquid phase upon exposure to increasing temperature, whereas the phenylpropane (C6-C3)-type model (polymer 2) did not, forming a char at higher temperature. However, modifying the Cγ or 9-carbon in polymer 2 to the corresponding ethyl ester or acetate derivative imparted thermal fusibility into this previously infusible polymer. FT-IR analyses confirmed differences in hydrogen bonding between the two model lignins. Both polymers had weak intramolecular hydrogen bonds, but polymer 2 exhibited stronger intermolecular hydrogen bonding involving the Cγ-hydroxyl group. This intermolecular interaction is responsible for suppressing the thermal mobility of the C6-C3-type model, resulting in the observed infusibility and charring at high temperatures. In fact, the Cγ-hydroxyl group and the corresponding intermolecular hydrogen bonding interactions likely play a dominant role in the infusibility of most native lignins.

Enhancing the enzymatic hydrolysis of lignocellulosic biomass by increasing the carboxylic acid content of the associated lignin.

To assess the effects that the physical and chemical properties of lignin might have on the enzymatic hydrolysis of pretreated lignocellulosic substrates, protease treated lignin (PTL) and cellulolytic enzyme lignin (CEL) fractions, isolated from steam and organosolv pretreated corn stover, poplar, and lodgepole pine, were prepared and characterized. The adsorption of cellulases to the isolated lignin preparations corresponded to a Langmuir adsorption isotherm. It was apparent that, rather than the physical properties of the isolated lignin, the carboxylic acid functionality of the isolated lignin, as determined by FTIR and NMR spectroscopy, had much more of an influence when lignin was added to typical hydrolysis of pure cellulose (Avicel). An increase in the carboxylic content of the lignin preparation resulted in an increased hydrolysis yield. These results suggested that the carboxylic acids within the lignin partially alleviate non-productive binding of cellulases to lignin. To try to confirm this possible mechanism, dehydrogenative polymers (DHP) of monolignols were synthesized from coniferyl alcohol (CA) and ferulic acid (FA), and these model compounds were added to a typical enzymatic hydrolysis of Avicel. The DHP from FA, which was enriched in carboxylic acid groups compared with the DHP from CA, adsorbed a lower mount of cellulases and did not decrease hydrolysis yields when compared to the DHP from CA, which decreased the hydrolysis of Avicel by 8.4%. Thus, increasing the carboxylic acid content of the lignin seemed to significantly decrease the non-productive binding of cellulases and consequently increased the enzymatic hydrolysis of the cellulose.

Tuning the morphology of cellulose acetate gels by manipulating the mechanism of phase separation.

The effect of dihydric alcohol (nonsolvent) addition on the rheological and microstructural behavior of cellulose acetate (CA) in a ternary CA, N,N-dimethylacetamide (DMA), nonsolvent system was investigated. Increasing the dihydric alcohol concentration led to enhanced steady shear viscosity and dynamic viscoelastic properties that were dependent on CA concentration. Changing the dihydric alcohol from 1,2-ethanediol to 1,4-butanediol and 1,6-hexanediol increased the moduli and decreased the concentration of nonsolvent at which the sol-gel transition occurred. At 10 wt % CA concentration the modulus and gel morphology of the 1,2-ethanediol and 1,4-butanediol systems were quite similar and distinctly different from that of 1,6-hexanediol. In the former, the gel morphologies were more heterogeneous, evident of more extensive coarsening, and likely obtained via nucleation and growth and spinoidal decomposition of off-critical mixtures. The latter exhibited more uniform dense network morphology, indicative of a spinoidal decomposition of near-critical mixtures. The gels were fractal in nature and exhibited different fractal dimensions in-line with the observed differences in microstructure; D ∼ 1.87 ± 0.02 (1,2-ethanediol and 1,4-butanediol) and D ∼ 1.97 ± 0.02 (1,6-hexanediol). However, at 15 wt % CA content, the gels exhibited more similar viscoelastic behavior and gel microstructures; D ∼ 1.97 ± 0.02 for all three dihydric alcohol systems.

Effect of hydrophilic and hydrophobic interactions on the rheological behavior and microstructure of a ternary cellulose acetate system.

The effect of hydrophilic and hydrophobic interactions on the rheological and microstructural behavior of cellulose acetate (CA) in a ternary CA, N,N-dimethylacetamide (DMA), nonsolvent (alcohol) system was examined. Increasing nonsolvent concentration increased the viscosity and dynamic viscoelastic properties of the system. At a critical nonsolvent concentration, a sol-gel transition was observed, which was dependent on nonsolvent structure. Increasing the available hydrogen bonding groups within the nonsolvent led to higher modulus (stronger gels) and a sol-gel transition at lower nonsolvent concentration. Likewise, increasing the alkyl chain length (hydrophobicity) of the nonsolvent also enhanced the viscoelastic properties; however, hydrogen bonding, specifically the ability to hydrogen bond donate was critical for gel formation. For all gels studied, the elastic modulus shifts to higher values with increasing hydrophilicity and hydrophobicity of the nonsolvent and exhibits a power-law behavior with nonsolvent content. All of the gels exhibit similar fractal dimensions; however, confocal images of the different systems reveal distinct differences. Increasing the hydrophilicity of the nonsolvent led to a more uniform denser gel microstructure, whereas increasing the hydrophobicity resulted in a larger more heterogeneous network structure despite the increase in moduli.

Preparation of a thermoresponsive lignin-based biomaterial through atom transfer radical polymerization.

Copolymerization of N-isopropylacrylamide (NIPAM) with technical hardwood kraft lignin (HWKL) was achieved by atom transfer radical polymerization (ATRP) using a selectively modified lignin-based macroinitiator. The degree of polymerization (DP) of polyNIPAM graft side chains was affected by varying the ratio of the DMF/water solvent system from 5:0 to 0:5, and an estimated DP(NIPAM) of >40 was obtained using a ratio of 1:4 (v/v). The thermal decomposition temperature of the lignin-g-polyNIPAM copolymers significantly increased with increasing DP(NIPAM). Likewise, the solubility of the lignin-g-polyNIPAM copolymers in water changed depending on copolymer structure. In both the water-soluble and suspended copolymers, at temperatures above 32 degrees C, the g-polyNIPAM component underwent the typical hydrophilic-to-hydrophobic transition, resulting in the precipitation of the copolymer.

Preparation of a thermosensitive highly regioselective cellulose/N-isopropylacrylamide copolymer through atom transfer radical polymerization.

Regioselective copolymerization of N-isopropylacrylamide (NIPAM) onto cellulose was achieved by atom transfer radical polymerization (ATRP) using a regioselectively modified 6- O-bromoisobutyryl-2,3-di- O-methyl cellulose macroinitiator. Varying the ratio of NIPAM to macroinitiator to ligand to transition metal in a Cu(I)Br/ N, N, N', N'', N'''-pentamethyldiethylenetriamine (PMDETA) catalyst system affected graft yield and degree of polymerization. ATRP proceeded to completion without any trace of the macroinitiator, and a degree of polymerization (DP) of polyNIPAM up to 46.3 was obtained. Increasing the DP of the NIPAM component increased both the thermal decomposition temperature and the glass transition temperature of the copolymer. The grafting of NIPAM also affected the solubility properties of the methylcellulose. The 6- O-polyNIPAM-2,3-di- O-methyl cellulose formed a stable suspension in water at room temperature and underwent a hydrophillic-to-hydrophobic transition and copolymer precipitation when the temperature was raised above 30 degrees C.

Organosolv ethanol lignin from hybrid poplar as a radical scavenger: relationship between lignin structure, extraction conditions, and antioxidant activity.

Twenty-one organosolv ethanol lignin samples were prepared from hybrid poplar (Populus nigra xP. maximowiczii) under varied conditions with an experimental matrix designed using response surface methodology (RSM). The lignin preparations were evaluated as potential antioxidants. Results indicated that the lignins with more phenolic hydroxyl groups, less aliphatic hydroxyl groups, low molecular weight, and narrow polydispersity showed high antioxidant activity. Processing conditions affected the functional groups and molecular weight of the extracted organosolv ethanol lignins, and consequently influenced the antioxidant activity of the lignins. In general, the lignins prepared at elevated temperature, longer reaction time, increased catalyst, and diluted ethanol showed high antioxidant activity. Regression models were developed to enable the quantitative prediction of lignin characteristics and antioxidant activity based on the processing conditions.

Differences between lignin in unprocessed wood, milled wood, mutant wood, and extracted lignin detected by 13C solid-state NMR.

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was applied to intact and isolated loblolly pine wood samples to identify potential structural changes induced by tree age, milling, lignin extraction, or naturally occurring mutations. Special attention was paid to ketone and aldehyde as well as nonpolar alkyl groups, which could be observed at low concentrations (<2 in 1000 C) using improved spinning-sideband suppression with gated decoupling. Carbonyl structures were present in intact wood, and there are more keto groups than aldehydes. Their concentrations increased from juvenile to mature wood and with milling time, whereas extraction did not alter the C=O fraction. Significant amounts of aldehyde and dihydroconiferyl alcohol residues were present in coniferyl aldehyde dehydrogenase-deficient wood, confirming solution-state NMR spectra of the corresponding lignin. These results demonstrate the utility of solid-state NMR as an assay for changes in the lignin structure of genetically modified plants.

Utilization of polar metabolite profiling in the comparison of juvenile wood and compression wood in loblolly pine (Pinus taeda).

Juvenile wood (JW) of conifers is often associated with compression wood (CW), with which it is sometimes believed to be identical. To determine whether JW and CW can be distinguished metabolically, we compared gas chromatographic profiles of 25 polar metabolites from rooted cuttings of a single loblolly pine (Pinus taeda L.) clone raised in controlled environment chambers and subject to three treatments: (1) grown erect with minimal wind sway (control); (2) swayed by wind from oscillating fans; and (3) with 30-cm growth increments successively bent at an angle of 45 degrees to the vertical. Profiles were compared by principal component analysis. Substantial increases in abundances of coniferin and p-glucocoumaryl alcohol separated immature JW-forming xylem tissues of the control trees from the CW-forming xylem of the bent and swayed trees.

Quantitative characterization of a hardwood milled wood lignin by nuclear magnetic resonance spectroscopy.

The structure of Eucalyptus grandis milled wood lignin (MWL) was investigated by 2D 1H-13C HSQC, HMQC, and 1H-1H TOCSY correlation NMR techniques and by quantitative 13C NMR as well as by the permanganate oxidation degradation technique. The combination of 2D NMR and quantitative 13C NMR spectroscopy of nonacetylated and acetylated lignin preparations allowed reliable identification and calculation of the amount of different lignin structures. About 85% of side-chain moieties were estimated on the structural level. This information was substantiated by data on the quantity of various functional groups and interunit linkages as a whole. A modified method for calculation of the h:g:s ratio has been suggested and compared with previously suggested approaches. E. grandis MWL has been determined to have an h:g:s ratio of 2:36:62. The amounts of various phenolic/etherified noncondensed/condensed guaiacyl and syringyl moieties were approximately estimated. E. grandis MWL contained approximately 0.60/Ar of beta-O-4 moieties along with small amounts of other structural units such as pino/syringyresinol (0.03/Ar), phenylcoumaran (0.03/Ar), and spirodienone (0.05/Ar). The degree of condensation was estimated at approximately 21%; the main condensed structures are 4-O-5 moieties (approximately 0.09/Ar). The structure of E. grandis MWL was compared with those of other lignin preparations isolated from various hardwoods.

Rapid screening of wood chemical component variations using transmittance near-infrared spectroscopy.

A rapid transmittance near-infrared (NIR) spectroscopy method was developed to predict the variation in chemical composition of solid wood. The effect of sample preparation, sample quantity (single versus stacked multiple wood wafers), and NIR acquisition time on the quantification of alpha-cellulose and lignin content was investigated. Strong correlations were obtained between laboratory wet chemistry values and the NIR-predicted values. In addition to the experimental protocol and method development, improvements in calibration error associated with utilizing stacked multiple wood wafers as opposed to single wood wafers are also discussed.

A comprehensive approach for quantitative lignin characterization by NMR spectroscopy.

A detailed approach for the quantification of different lignin structures in milled wood lignin (MWL) has been suggested using a combination of NMR techniques. 1H-13C heteronuclear multiple quantum coherence and quantitative 13C NMR of nonacetylated and acetylated spruce MWL have been found to have a synergetic effect, resulting in significant progress in the characterization of lignin moieties by NMR. About 80% of side chain moieties, such as different beta-O-4, dibenzodioxocin, phenylcoumaran, pinoresinol, and others, have been identified on the structural level. The presence of appreciable amounts of alpha-O-alkyl and gamma-O-alkyl ethers has been suggested. Although the quantification of various condensed moieties was less precise than for side chain structures, reliable information can be obtained. Comparison of the calculated results with known databases on spruce MWL structure shows that the suggested approach is rather informative and comparable with the information obtained from the combination of various wet chemistry methods. Discrepancies between the results obtained in this study and those previously published are discussed.

Rapid prediction of solid wood lignin content using transmittance near-infrared spectroscopy.

A rapid transmittance near-infrared (NIR) spectroscopic method has been developed to characterize the lignin content of solid wood. Using simple, multiple regression, and partial least-squares statistical analysis the lignin contents of wood wafers, taken from increment cores, and synthetic wood, prepared by blending milled wood lignin and holocellulose, were compared and quantified. Strong correlations were obtained between the predicted NIR results and those obtained from traditional chemical methods. In addition to the experimental protocol and method development, NIR results from wood samples with different particle sizes and various lignin contents are discussed.

Solution-state nuclear magnetic resonance study of the similarities between milled wood lignin and cellulolytic enzyme lignin.

The structures of milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL) have been analyzed using traditional chemical methods and solution-state NMR techniques. Comparisons of the results obtained reveal that subtle differences exist between the two lignin preparations. Thioacidolysis produced higher monomer yields from CEL than MWL, suggesting MWL has a more condensed structure. Quantitative (13)C NMR determined the degree of condensation in MWL to be 0.43 unit per aromatic moiety as compared to 0.36 in CEL. The MWL also contained a lower amount of beta-O-4' substructures per aromatic ring than CEL, 0.41 versus 0.47, respectively. Carbohydrate analysis revealed that the MWL may contain a higher proportion of middle lamella material as compared to the CEL. Because the middle lamella is considered to have a more condensed lignin structure, on the basis of the bulk polymerization theory, these results could explain the differences in beta-O-4' and degree of condensation.

Elucidation of lignin structure through degradative methods: comparison of modified DFRC and thioacidolysis.

Milled wood and milled wood lignin (MWL) samples were subjected to DFRC and thioacidolysis. Despite the fact that both methods selectively cleave aryl ether bonds, substantial differences in results were obtained. Lignin thioacidolysis gave total molar yields of degradation monomer products in the range of 3.5-7 mol % higher than DFRC. GPC analysis showed that the thioacidolysis-treated lignin was degraded to a lower average molecular weight than that treated by DFRC. Contrary to results reported for lignin model compounds, these results indicate that the DFRC method does not completely or efficiently degrade the lignin polymer. In fact, the DFRC-degraded lignin retained much of the characteristics of the original MWL. Elemental analysis revealed the presence of bromine in the DFRC-treated lignin, and two-dimensional (1)H-(13)C HMQC NMR spectroscopy showed the presence of beta-O-4 linkages in the DFRC-treated lignin. No beta-O-4 interunit linkages were detected in the thioacidolysis-treated lignin. These results are consistent with the lower monomer yields and the higher average molecular weight of the DFRC-treated lignin and indicate inefficiency in the chemistry of the method, probably due to steric constraints of the polymeric nature of lignin.

Studies on the effect of ball milling on lignin structure using a modified DFRC method.

The structures of milled wood lignin (MWL), cellulolytic enzyme lignin (CEL), and residual lignin (REL) from a loblolly pine were analyzed using a modified derivatization followed by reductive cleavage (DFRC) method developed to allow the quantitative determination of three different structural monomeric products originating in lignin: phenolic beta-O-4, alpha-O-4, and etherified beta-O-4 structures. Results show that MWL and CEL are structurally identical, with an increased phenolic beta-O-4 content compared to that of the original Wiley milled wood. These results indicate that the portion of lignin linked to carbohydrates and that not linked to carbohydrates are structurally the same. Modified DFRC analysis of the effect of ball milling on the structure of lignin in wood, MWL, CEL, and REL indicate that vibratory ball milling does not change the lignin structure provided certain precautions are taken. Specifically, dry vibratory ball milling under a nitrogen atmosphere causes substantial structural changes including condensation, whereas vibratory ball milling in toluene had little effect on the lignin structure. This indicates that the structural differences observed in MWL and CEL arise because of the extraction procedure, which preferentially extracts phenolic lignin structures. MWL and CEL are representative of the total lignin in wood; however, due primarily to the solvent extraction process, higher phenolic hydroxyl contents are observed. Nitrobenzene oxidation showed structural results similar to those from the modified DFRC method.