What type of molecule is lignin

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Google Scholar 2 Nord, F. Article Google Scholar 3 Cowling, E. Google Scholar 5 Meier, H. Author information Author notes W. View author publications. Rights and permissions Reprints and Permissions. Copy to clipboard. Further reading Proof testing of ceramic materials? Fine Metallurgical Transactions A Toughness, fracture markings, and losses in bisphenol-A polycarbonate at high strainrate R.

Kai, M. Tan, P. Chee, Y. Chua, Y. Yap, and X. Zeng, M. Himmel, and S. Zhou, Q. Li, V. Chiang, L. Lucia, and D. Jung, M. Foston, U. Kalluri, G. Tuskan, and A. Ragauskas, G. Beckham, M. Biddy et al. Dodd, J. Kadla, and S. Giummarella, L. Zhang, G. Henriksson, and M. Wang, H. Ben, H.

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Patti, and K. Long, W. Lou, L. Wang, B. Yin, and X. Khokarale, T. Le-That, and J. Mittal, R. Katahira, B. Donohoe et al. Yuan, T. You, W. Wang, F. Xu, and R. Sun, M. Wang, and R. Nitsos, R. Stoklosa, A. Karnaouri et al. Yang, D. Wang, D. The handicap of the limiting solubility and thermal deformability of isolated commercial Kraft lignins has been demonstrated to be reversible to different extents by chemical modification. Lignins from non-commercial, experimental sources, however, such as organosolv or steam explosion lignins, with an abundance of T g -lowering intermonomer ether bonds, can offer lower T g 's as well as more compatible chemistry Glasser, Other applications in thermosetting polymeric materials have included a range of network-forming polymers crosslinked using isocyanate, polyamine, polyacrylate, epoxy etc.

Several examples of such applications are illustrated in Figure 5. In applications that rely on resin formulations using non-alkaline or non-aqueous conditions, chemical modification or molecular fractionation becomes mandatory since most industrial lignin sources are insoluble in most common solvents.

The same is true for applications in thermoplastic materials, where thermal processability requires the restoration of moderate T g 's. Figure 5. Early lignin-containing structural materials—Polyurethane foams using bleached and unbleached HPL see Figure 4 according to Hsu and Glasser A , and foams according to Lora and Glasser B ; polyacrylate sheet C according to Wang and Glasser ; and solvent-cast polyurethane coating S and injection-molded test specimens E according to Ciemniecki and Glasser D.

The potential recovery of lignin as a solvent-soluble polymer with improved solubility and reduced T g by chemical modification must be considered the first challenge to the reclamation of its performance attributes Wang et al. It has long been established that the branched nature of lignin combined with its abundance of phenolic OH groups following isolation involving alkyl-aryl ether cleavage that give rise to strong internal hydrogen-bonds, is most effectively changed by oxyalkylation as shown in Figure 6A Hsu and Glasser, , ; Glasser et al.

Non-phenolic hydroxyalkyl lignin derivatives are recognized as virtually uniformly functional with aliphatic OH-groups branched molecules with good solubility and thermal properties. Their aromatic character offers many opportunities to contribute to the properties of multi-phase materials that resemble those of natural lignocellulosic materials.

Figure 6. Benefits of lignin modification by propoxylation— A Derivatization reaction is oxyanion-dependent; B T g of lignin esters is related to the size of the acyl substituent i. Following chemical modification with alkylene oxides Figure 6 , lignins become sufficiently soluble to perform well in wood adhesives when crosslinked with isocyanates Newman and Glasser, as well as in fire resistant foams Glasser and Leitheiser, The former has been accomplished, for example, with rapidly curing wood adhesives using blocked diisocyanates as crosslinking agents Gillespie, Blends of hydroxypropyl lignin with soy protein have resulted in materials with significantly increased tensile strength and undiminished elongation at break due the formation of supramolecular domains and strong inter-molecular adhesion Wei et al.

Combining non-phenolic lignin derivatives with such inherently low-modulus polymer components as polyether glycols mimics the role lignin plays in the amorphous component of wood, where it adds modulus by the formation of copolymer structures with gum-like non-crystalline hetero-polysaccharides Kelley et al. This has been examined repeatedly for the case of both lignin-containing mixtures with, and copolymers of, polyaliphatic glycols Figure 6.

Such mixtures of aromatic and aliphatic polyols have been demonstrated to become the basis of a variety of adhesives, coatings, foams, etc. These examples demonstrate that lignin as isolated by separation from woody biomass performs like a normal polymer in a predictable manner and continues to present the option of re-use in a form it was initially designed for by nature.

In addition to thermosetting materials, lignin can also contribute important properties to thermoplastic mixtures polyblends with other polymers as long as it meets thermal and compatibility needs. Lignin in native state, such as in solid wood, will readily undergo thermal deformation if a the plasticizing effect of water on amorphous hetero-polysaccharides is assured, and b the T g of water-plasticized lignin is exceeded Ito et al. Thermoplastic blend materials with lignin derivatives having acceptable T g 's have been widely demonstrated.

Ester formation has been studied in terms of the effect on thermal properties as early as Lewis and Brauns, ; Glasser and Jain, , and etherifications by ring opening reactions with aliphatic oxirans have been pursued by Glasser et al.

Wu and Glasser, ; Jain and Glasser, For reasons of biodegradability and sustainability, thermally processable natural polymers have attracted attention also in the field of melt-processed materials. Since ester-type lignin derivatives meet those requirements, their feasible incorporation into blend materials has been examined Ghosh et al. Lignin esters were found to be readily miscible with cellulose esters, starch esters and poly hydroxybutyrate. Because of its potential contribution to the modulus of rigid multi-phase materials and its resistance to thermal degradation, lignin has also been considered as an attractive source for carbon fibers Fukuoka, Lignin appears to be a relatively small molecule, both in vivo and in vitro , that serves lignocellulosic multi-phase materials i.

Figures 1C and 7 represent widely—used illustrations of the molecular architecture of the amorphous component of wood. Since thermal and solubility characteristics of the two components, hetero-polysaccharides and lignin, are very different, the copolymer behaves like a crosslinked, three-dimensional network structure in native unplasticized wood.

The polysaccharides provide stability and immobility in dry as well as high-temperature state, and lignin anchors the branched gum-like hetero-polysaccharides in place under high-moisture conditions. It can therefore not be expected that lignin by itself will qualify as stand-alone polymeric material.

It is most likely performing best as partner with a molecular entity that complements its properties, as it does in wood. This partnership must, by design, experience different levels of stress that results in the separation of molecular components into individual phases.

The molecular response involves a lignin-carbohydrate separation by hydrolysis, and b relocation of the separated lignin within the fibrous tissue Debzi et al. However, it is obvious that disruption of the block copolymer architecture of the amorphous component of wood results in spontaneous phase-separation of carbohydrates and lignin within the limits of mobility. This results in the opportunity to extract lignin partially with aqueous alkali or organic solvents without further pretreatment Wright and Glasser, The re-incorporation of lignin into polymeric materials structures i.

Phases separate on either or both, kinetic and thermodynamic grounds. Figure 7. Lignin's potential as component in structural materials, both network-forming and thermally processable, is limited by its solution and thermal properties. Restricting the solubility of isolated Kraft lignin to aqueous alkali, and its thermal deformability i. The degree to which such limitations apply to thermosetting materials has been analyzed by Gillham using a time-temperature-transition TTT -diagram Figure 8 Gillham, ; and thermoplastic blend compatibility is often analyzed in terms of thermodynamic solubility characteristics Rials and Glasser, a , b , Both approaches have been applied to lignin-containing polymeric materials.

Figure 8. Basis of phase separation in lignin-containing thermosets— A Schematic time-temperature-transformation TTT -diagram identifying phase separation preceding gelation and vitrification on a time scale after Gillham, ; B Dynamic mechanical thermal analysis DMTA diagram of a lignin-containing thermosetting resin mixture undergoing cure with identification of gelation gel , and vitrification vit on the temperature-scale Toffey and Glasser, ; C SEM picture of cured lignin-containing thermoset with phase separation of presumably lignin-rich particulate inclusions Hsu and Glasser, ; D Schematic phase separation of gel-forming molecules illustrating the clustering that produces regions of greater mass density embedded in a continuous phase of lower-density gel after Kinloch, The TTT-diagram records molecular changes of resin mixtures under cure by isothermal heat exposure on a time scale.

During isothermal heating of homogeneous thermosetting mixtures at different temperature-levels, three events take place in sequence: a separation of molecular phases representing differences in chemical or molecular structures incl.

Using these analytical principles, it becomes apparent that an excessive separation of phases prior to gelation can produce particulate-filled polymer networks in which the included particles are rubbery or glassy; and the sizes are on the nano-, micron-, or millimeter-scale.

In resin mixtures that contain molecules with different character i. This is illustrated in Figure 8D Kinloch, In the case of a resin mixture with lignin, phase separation likely results in the formation of glassy because of high T g particles disconnected from the continuous network polymer.

It must be recognized that this formation of an inhomogeneous resin phase depends on both lignin functionality and molecular size, with lower molecular fractions preserving phase compatibility for a greater period during the cure cycle.

The employment of low molecular weight lignin fractions is therefore preferable in phenol-formaldehyde PF resin products Gillespie, An example of a phase-separated polyurethane network with lignin-rich particulate enclosures is given in Figure 8C. Even though the component mixture, polyol and isocyanate, started the curing process as homogeneous mixture, it quickly experienced phase separation as gelation advanced.

This phenomenon is observed universally with network forming mixtures, regardless of chemistry. In the case of lignin, it can be retarded by chemical modification aimed at the development of structural compatibility, or by the addition of functionality chemical modification , that promotes the participation in crosslinking gelation reactions before phases separate. Different resins have different tolerances in terms of molecular size and chemical functionality as well as cure cycle times to the formation of separate phases during normal gelation.

Similar arguments can be applied to the analysis of phase separation in thermoplastic polymer mixtures i. This has previously been approached experimentally with lignins modified in different ways and to different extents prior to solution-blending with non-crystalline thermoplastic cellulose derivatives Figures 9B,C Rials and Glasser, , The results of this study demonstrated that a difference in solubility parameter between lignin and cellulose derivatives was responsible for the formation of regions with greater or lesser molecular compatibility.

Calculations based on experimental observations of the shift in T g with blend composition Figure 9C resulted in the conclusion that greatest phase compatibility between the derivatives of cellulose and lignin i.

Figure 9. Analysis of phase compatibility— A Theoretical range of solubility parameters of several Kraft lignin esters as predicted by the Hoy model, compared to the solubility range of styrene from Thielemans and Wool, Only the points representing the lignin derivatives fitting into the gray sphere can be considered phase compatible with polystyrene. Phase compatibility rises with the negative magnitude of the B-parameter. Maximum compatibility is reached with lignin derivatives having OH-contents of 0.

In this graph, only lignin butyrate is predicted to be miscible with polystyrene from Thielemans and Wool, Phase compatibility of lignin derivatives with polystyrene has also been assessed using a theoretical approach involving solubility parameter calculations Figure 9A. Various chemical modification treatments of lignin were found to allow predicting the compatibilization of lignin derivatives with polystyrene Thielemans and Wool, Esters of lignin with selected acids were found to contribute to both compatibility and reduced thermal transitions Figure 9D.

Creating single-phase network structures with lignin requires both component miscibility and complementary functionality. The desire to use lignin in phenol-formaldehyde PF resins has an extremely long history Lambuth, It resurfaces any time the price of phenol rises Lake, Both, lignin sulfonates and kraft lignins, are amenable to substituting phenol; but this requires either molecular fractionation or chemical modification of the parent lignin if high substitution degrees are to be reached.

Lignin content in epoxy resins can be achieved using several approaches. The functionalization of lignin with oxiran functionality produces resins crosslinkable with amines or phenols Nieh and Glasser, ; Hofmann and Glasser, ; Lora and Glasser, , whereas bis-phenol A-based epoxies are capable of serving as crosslinking agents for phenolated or aminated lignin derivatives.

Figure The introduction of functionality useful for participation in chemical reactions is limited only by sufficient molecular compatibility to prevent premature phase separation, as pointed out above. In polyblend applications, in which a lignin component is mixed in homogeneous melt-state with a thermoplastic polymer, both thermodynamic and kinetic factors dictate the degree of phase separation.

Examples of blends of lightly modified lignin with thermoplastic polymers in need of stiffening are shown in Figures 10B—D , which represent an injection-molded chair seat with polypropylene Figure 10B , and a melt-blown waste bag made with a biodegradable polyester Figures 10C,D Glasser et al. Improving the solubility and thermal properties of isolated Kraft lignin thereby helps to compatibilize the two components sufficiently but incompletely to assure mutual support.

This is a prerequisite for opening important pathways to lignin's use in melt-blended polymeric materials based exclusively on secondary molecular interactions. Using nature's approach to the design of greater component compatibility required from inherently thermodynamically incompatible polymer types employs the creation of co-polymers that help in the dispersion of otherwise incompatible but structurally complementary molecular phases.

In the case of natural lignocellulosic materials, kinetic phase compatibilization factors are eliminated, and thermodynamic factors become prevalent. Mimicking the approach of compatibilizing an aromatic polymer lignin with a crystalline polysaccharide i.

In all cases, co-polymer synthesis was achieved by reacting mono-functional with terminal NCO-groups synthetic polymer segments of variable sizes with lignin derivatives to produce star-like block-copolymer structures resembling native lignin-carbohydrate complexes. These block-copolymer preparations were solution-blended with the linear polymer partner de Oliveira and Glasser, , a , b.

The response of changes in blend properties to composition was remarkable in terms of mechanical, thermal, crystallinity, and morphological characteristics as is illustrated for PVC blends in Figure 11 de Oliveira and Glasser, b. The compatibilizing power of caprolactone attached to lignin resulted in blend compositions that revealed either miscibility or near-miscibility of the aromatic and linear phases.

Whereas, blends of polycaprolactone-free lignin derivatives with PVC generated structures with macro-phase separation on the scale of 0. Similarly astonishing was the consequence on mechanical properties following aging Figure 11A , and on stress- and strain-levels Figure 11B de Oliveira and Glasser, b. C Illustrations by SEM of the degree of compatibility achieved by block copolymer formation of HPL and caprolactone segments a-c are caprolactone-free, and d-f reveal the impact of the presence of caprolactone as copolymer component.

The compatibilization of lignins for use in thermoplastic polymer systems has found a significant following in recent years. The effect of derivatization with fatty acids of different sizes was found to enhance both phase morphology and melt rheology Koivu et al.

Thermoplastic blends of acrylonitrile-butadiene rubber with various lignins in different blend ratios were found to produce tough materials with high failure strains Bova et al. The gradual build-up of lignin-containing phases on cellulose surfaces mimicking a hydrophobization effect of plant fibers required by native plants using copolymers of lignin with hetero-polysaccharides has been studied in aqueous environments Gradwell et al.

This suggests that lignin-carbohydrate copolymers serve as phase compatibilizers between cellulose and lignin. It is obvious that this parallels the multi-phase molecular architecture of wood, where a lignin-rich copolymer phase of water-soluble or gum-like non-crystalline polysaccharides becomes attached to cellulose crystals via secondary bonds. The principle of phase compatibilization of lignin and PVC via caprolactone parallels that of lignin-co-hetero-polysaccharides and cellulose.

The principle produces non-covalently-linked blends of molecular entities with widely different characteristics on the nano-scale. The principle is adoptable for the development of a virtually unlimited range of sustainable and biodegradable materials.

Differences are detectable between middle lamella and secondary cell wall location, among other factors. Resistance to dissolution is a consequence of re-attachment to polysaccharides incl. Properties can readily be tailored for uses that require special solubility, thermal and compatibility behaviors. The author confirms being the sole contributor of this work and approved it for publication. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

This is to acknowledge the contributions of the many students and research associates that have contributed to the many phases of the research reported here. Many are named in text and references. Xerxes Steirer, Michael F. Advances in Multiscale Modeling of Lignocellulosic Biomass. Hammond , Caixia Wan. Revealing the role of hydrogen bonding interactions and supramolecular complexes in lignin dissolution by deep eutectic solvents.

Journal of Molecular Liquids , , Stochastic model of lignocellulosic material saccharification. Lignin-based monolithic carbon electrode decorating with RuO2 nanospheres for high-performance chlorine evolution reaction.

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