This study has shown that ultrafiltration allows the selective extraction from industrial black liquors of lignin fraction with specific thermo-mechanical properties, which can be matched to the intended end uses. Ultrafiltration resulted in the efficient fractionation of kraft lignin according to its molecular weight, with an accumulation of sulfur-containing compounds in the lowmolecular weight fractions. The obtained lignin samples had a varying quantities of functional groups, which correlated with their molecular weight with decreased molecular size, the lignin fractions had a higher amount of phenolic hydroxyl groups and fewer aliphatic hydroxyl groups. Depending on the molecular weight, glass-transition temperatures (T g ) between 70 and 170 C were obtained for lignin samples isolated from the same batch of black liquor, a tendency confirmed by two independent methods, DSC, and dynamic rheology (DMA). The Fox-Flory equation adequately described the relationship between the number average molecular masses (M n ) and T g 's-irrespective of the method applied. DMA showed that low-molecular-weight lignin exhibits a good flow behavior as well as high-temperature crosslinking capability. Unfractionated and high molecular weight lignin (M w >5 kDa), on the other hand, do not soften sufficiently and may require additional modifications for use in thermal processings where melt-flow is required as the first step.

Full utilization of plant-based feedstocks for sustainable societies must include the valorization of lignin, an abundant aromatic component of the cell wall of plant stems, in processes that follow green chemistry principles. However, one of the major issues limiting lignin utilization is the chemical nonuniformity of the isolated polymer, along with the thermal sensitivity of the free phenolics which hinders processing at elevated temperatures. To address these issues, free phenolics and carboxylic acids of industrial lignins were hydroxyalkylated through the use of ethylene carbonate, which served as both the reagent and the solvent. This facile and safe reaction resulted in near complete conversion of phenolics and carboxylic acids into aliphatic hydroxyls resulting in uniform chemical functionality and enhanced thermal stability. However, the increased reaction temperature decreased the total hydroxyl content and increased the molecular weight of the lignin; the work identified a narrow processing window that achieved derivitazation without extensive structural modification, which plagued earlier work with carbonate modification of lignin. 13C NMR spectroscopy of hydroxyalkylated lignin showed a low degree of condensation and limited to no copolymerization of the ethylene carbonate when reacted under modest conditions. The hydroxyethylated derivative had enhanced solubility in propionic acid, which was used as a solvent and reagent in order to directly esterify the lignin. The reaction achieved over 95% substitution of the lignin hydroxyls creating ethylpropionate derivatives where excess propionic acid could be recycled under vacuum. The hydroxyalkylation followed by direct esterification provided a route toward the development of greener lignin esters by avoiding added solvents, carcinogens such as ethylene oxide, and halogens like acid chlorides for lignin-based polymeric materials synthesized utilizing green chemistry principles.

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.

Oxidative thermal stabilization is considered a critical process before carbonization to prevent fusion of fibers, while aiding in the formation of homogeneous fiber cross sections during carbon fiber manufacturing. In this study, we investigated the impact of nanocrystalline cellulose (NCC) on the thermal, electrical, and mechanical properties of electrospun lignin-derived carbon nanofibers when the oxidative thermal stabilization step was skipped. Results showed that by adding small amounts of NCC (up to 5 wt %), uniform lignin-based carbon nanofibers were prepared with direct carbonization processes without oxidative thermal stabilization. SEM images revealed that NCC filled lignin carbon nanofibers retained their fibrous morphology after the heat treatment, dependent upon the carbonization rate. Further, carbonization conditions were exploited to form a unique interconnected structure, which increased the electrical conductivity of carbon nanofiber mats from 5 to 35 S/cm. Dynamic thermomechanical analysis of NCC/lignin nanofiber mats showed a reduction of the tan δ peak during the glass transition indicating NCC restricted the molecular mobility of lignin’s chains. Through thermal rheological evidence, this study revealed significant interaction of NCC and lignin blends that prevented the fusion of nanofibers during heat treatment. This study is unique that it provides a method to reduce processing time and energy cost associated with carbon fiber production, while controlling fiber mat structure.

In an effort to advance the dynamic mechanical analysis (DMA) of very small biomass specimens, and/or specimens having poor mechanical integrity, the functional equivalent of pendulum-torsion (tensile-torsion) DMA was compared to parallel-plate compressive-torsion DMA. The solvent-saturated lignin glass transition in yellow-poplar (Liriodendron tulipifera) was generally similar determined by both modes; however, direct data comparisons should be avoided or carefully considered. First-heat glass transition temperatures (T gs) were relatively similar; however, specimen densification elevated subsequent cooling-mode T gs by 5–8°C in compressive-torsion. Both modes revealed a first-heat tan δ shoulder; it was more prominent and had grain dependency in compressive-torsion. Below fiber saturation, subambient tensile-torsion DMA was superior; compressive-torsion resulted in an anomalous response, obscuring subambient secondary relaxations. With these differences and limitations in mind, compressive-torsion offers specific advantages. Solvent-submersion studies are simplified because solvent cups are easily devised for torsional rheometers. Specimens lacking mechanical integrity are more easily analyzed. Heavily biodegraded spruce (Picea sp.) was analyzed in the solvent-submersion mode as fibrous mats and the different actions of Gloeophyllum trabeum and Postia placenta were revealed. Very small specimens are easily analyzed in compressive-torsion; tissue maturity effects were revealed in minute sections of switchgrass (Panicum virgatum) stems. Applied appropriately, parallel-plate compressive-torsion DMA will provide new research opportunities.

This work continues the introduction of the dynamic mechanical analysis (DMA) of wood in a novel stress mode, compressive-torsion. The glass/rubber transition in yellow-poplar (Liriodendron tulipifera) wood was measured under saturation in four organic solvents: N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), ethylene glycol, and glycerol. Systematic measurement of the linear viscoelastic response (LVR) revealed a complex relationship of LVR with grain orientation, temperature and solvent. This indicates that special care is required when analysis within the LVR is critical. Glass transition temperatures (Tgs) of swollen wood showed an inverse relationship with solvent induced volumetric swelling. DMF and NMP both caused volumetric swelling of ≥25%, and the corresponding Tgs were near 50°C. Ethylene glycol and glycerol caused volumetric swelling <25% and the corresponding Tgs were ≥100°C. Glycerol showed significantly higher solvolytic power than ethylene glycol. Glycerol plasticized wood underwent irreversible swelling when heated beyond the Tg, causing a reduction in the effective crosslink density and in the Tg. Consistent with the literature, the observed Tg grain dependency was highly varied. The weakest plasticizer, glycerol, produced a prominent Tg grain dependency, whereas grain dependency was variable in the stronger swelling agents. This study demonstrates the importance of researching lignocellulose rheology in a greater variety of organic liquids, to achieve new perspectives into the structure/property relationships of lignocellulose, and aspects of processing in organic media.

It is demonstrated that plasticized lignocellulose fails to satisfy classic criteria normally required to validate time/temperature superposition (TTS) in dynamic mechanical analysis (DMA). However, insightful relaxation behavior is available and dismissing it would be a mistake. TTS was applied to Liriodendron tulipifera wood using parallel plate compressive-torsion DMA with specimens immersed in different organic liquids, ranging from weak to strong swelling power. While all storage moduli shifted smoothly, thermorheological complexity was detected in loss modulus shift failures, which themselves must reflect unknown structural features. Storage modulus shift factors clearly distinguished solvent specific relaxation behavior and interpretations through the WLF model or through fragility (cooperativity) analysis are useful. However, it is demonstrated that fragility analysis is preferred and solvents of different swelling power are compared. Coupled with other methods, TTS and fragility analysis warrant further development as a means to improve the understanding of structure/property relationships in plasticized lignocellulose.

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