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A bright future for cellulose
Prog. Polym. Sci. 24 (1999) 481–483
‘Trends in Polymer Science’ A bright future for cellulose J. Schurz Heinrichstrasse 28, A-8010 Graz, Austria Received 15 March 1999; accepted 22 March 1999
Cellulose is one of the oldest natural polymers. It is renewable, biodegradable, and can be derivatized to yield various useful products. It is fabricated by many plants as hairs (cotton) or as a structural polymer in cells. It is also produced enzymatically by bacteria (bacterial cellulose). Its main source, however, is wood, a natural composite, where cellulose is contained in combination with lignin in a texture which certainly represents a masterpiece of natural architecture. This natural polymer composite must be destroyed for the isolation of cellulose (pulping). Cellulose and wood are most abundant in nature; they are produced in a sustainable way and offer many possibilities for use. Its assets are that it is renewable, biodegradable, biocompatible and derivatizable. It is possible to base a whole chemistry on cellulose, which, however, today cannot compete with petrochemistry on the grounds of costs. Several disadvantages of cellulose include its expensive production, its sensibility to water, and its slow regeneration—a tree must have at least 30 years before it can be used for cellulose production. Regarding new directions in cellulose research, there are three main topics that we can identify. The first is the field of analytical problems and the routes of reactions. With regard to analytical questions, all structural levels must be considered: molecular, supramolecular, and morphological. Concerning cellulose reactions, we deal mainly with derivatization—which is amply possible due to the three hydroxyls. So far mainly esters and ethers have been produced. But due to new approaches we can search for new derivatives, including graft copolymers, and we can try to put the reactions on a higher level with regard of the stereoselectivity of the substituents. The second topic is the search for completely new products and uses. Here we cannot try to compete with mass products already in the market, as paper and regenerated fibers and films, but rather we look for special products with a high net value, so that it will be worthwhile to produce them even in rather small amounts. It is this area that is most active today. The third topic, finally, is that of mass products. We must not forget that cellulose is the basis of a large industry already, and it will be necessary and useful to look for improvements and optimization as well as for environmental safety. In particular, the role of biotechnology has to be explored, although at present it cannot yet compete on economic grounds. The first topic comprises analytical problems and reactions. The most important issue here is the determination of the stereoselectivity of a reaction and the guiding of a reaction in such a way that strictly stereoselective products are obtained. In this way, a regioselective functionalization with a 0079-6700/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0 0 7 9 - 6 7 0 0 ( 9 9 ) 0 0 01 1 - 8
482
J. Schurz / Prog. Polym. Sci. 24 (1999) 481–483
definite primary structure will be obtainable. Generally, the substitution along a cellulose chain takes place in a statistical way, in spite of the different reactivities of the three OH-carrying carbons C2, C3 and C6. Now it is increasingly possible to perform reactions taking place in only one of these carbons. More so, it will be possible to make derivatives resembling block copolymers, in which blocks of fully substituted cellulose chains alternate with completely unsubstituted ones. It is to be expected that such materials will show new properties, even in solution. Here cases of special aggregations will occur, which can be controlled by varying the lengths of the substituted blocks. Of course, for such new products it will be necessary to develop fast and reliable means of determining not only the degree of substitution, but also stereoselectively and in the case of block substitution, the block length. It will also be possible to substitute two or more different groups in such a way that different blocks alternate along the molecular chain. So far the most effective analytical tool is NMR combined with chromatographic methods. But since present research in this field is very active, other methods will be added, based on dielectric properties, zetapotential, and various optical methods. Of course, the detection of other analytical parameters on all three levels of organization must also be improved. Examples are crystallite shape, void system and inner surface, amorphous orientation, surface structure and surface charge. With solutions, the formation of supramolecular structures must be investigated in detail, from random gels to liquid crystals. Liquid crystal solutions are in the center of current research, and it has turned out that a very new process for cellulose regeneration (production of lyocell fibers) is characterized by the fact that the spinning solution has a kind of liquid crystal structure. It yields very good fibers—little wonder in view of the fact that spiders and silk worms also spin their high performance threads from liquid crystal solutions. Another area of research deals with new and better methods of fractionation with regard to both molecular weight and the degree of substitution. Efficient procedures not only for analytical purposes, but also for preparative ones, are being developed. So far the evaluation of analytical data has been done in a rather reductionistic, monocausal way. But in reality, the phenomena are multicausal. That means, we need a more sophisticated interpretation of data with the help of complex statistics. Finally, model calculations by means of computer simulations have been undertaken to help elucidate the fine structure of cellulose, its reaction mechanisms, and to better predict NMR data. The second topic is concerned with new products or applications, preferably special uses, which require only small quantities of material, but still have a high potential for producing net value. Here completely new routes are pursued. One is the use of mono-or multilayered Langmuir–Blodgett (LB) films. By dipping hydrophilic carriers into solutions of stiff chain derivatives, 1–40 nm layers are obtained. Using charged derivatives, such as cellulose sulfates, electrolyte multilayers can be obtained. Such ultrathin films can be used as supramolecular structures at interfaces, and then reacted further to acquire various special properties. In this way, crosslinked gel layers can be formed. For instance, by using appropriate derivatives, hairy rod layers can be produced which form a planar supramolecular network in which other polymers will show retarded diffusion and should move in reptation-like fashion. Further, hemocompatible surfaces can be produced in this way. Redox active layers could be applied to electrodes. Another specialized application is the use of cellulose structures as a basis for biomolecular signal detection in biosensors. For instance, a glass surface can be coated with a suitable cellulose derivative and then reacted with an enzyme, e.g. glucose oxidase, having the desired recognition structure. Such devices can show higher sensitivity than the dissolved enzyme. LB-films with covalently bound proteins can be used as models for studying cell adhesion, and also as matrices for growing neuronal networks. Cellulose is a polycrystalline polymer. By means of partial hydrolyzation it is possible to obtain microcrystalline cellulose, consisting mainly of micro crystals in the nanometer
J. Schurz / Prog. Polym. Sci. 24 (1999) 481–483
483
range. It is possible to use this product for further reactions. In this way anisotropic block structures can be obtained by topochemical reactions at the reducing end groups, which can be further reacted, using graft copolymerization. Finally, these products can be used to construct supermolecular architectures in the colloidal range. Another new field is that of anisotropic solutions, mainly liquid crystal structures. Here interesting new results can be expected by the use of regioselective functionalized derivatives. In solution, lyotropic cholesteric phases are obtained which can be readily characterized by optical methods. Such solutions not only show remarkable phase behavior, but they can also be used for various supramolecular structures. It is expected that they will exhibit new effects in the technical process of fiber spinning. In the solid state, thermotropic cholesteric structures can be obtained. Here it is hoped that the search for selective niches for products of high value, even in small quantities, will go on and lead to completely new applications. The third major topic is that of mass products. These include mainly paper and regenerated cellulose fibers and films. In the very old technology of papermaking, little real progress is to be expected and problems tend to be focused around optimization and ecological issues. In fiber making, the search for new spinning systems is important on environmental grounds. This includes the search for new solvents for cellulose. At present, the lyocell process is trying to replace the familiar viscose technologies, but so far the breakthrough has not taken place. The graft copolymerization of cellulose to make it water repellent, fire retarding or even thermoplastic, has been attempted, but all these processes have been too costly, at least as long as crude oil, the basis of petrochemistry, remains cheap. However, there is one new option in this field, namely the use of biotechnology. Enzymes from wood destroying fungi can be used to destroy lignin selectively. This can allow for the isolation of cellulose in a “soft” and ecologically benign manner. The big disadvantage remains the low throughput of these processes, which at present, rules them out on economic grounds. The space–time efficiency is too low. As an example, a beech tree, about 10 m high, will produce about 14 g of cellulose per day. A chemical reactor, half a meter high, can easily produce 20 kg of polystyrene per hour. Therefore, though the biochemical processes operate at low temperature and normal pressure, they require much longer time and can, therefore, hardly compete with normal “hard” chemistry. However, they pose no ecological problems and generally produce no wastes. Still, there have been attempts to degrade cellulose and hemicelluloses by means of the enzyme cellulase to sugars, which can then be fermented to alcohol (biofuel for cars) or various other products. An advantage of this process is that refuse, cellulose like sawdust or wastepaper can be used. However, here too the existing processes are at present too expensive. But the use of enzymes in various technical reactions, including pulping and bleaching, and for analytical purposes, is still growing. Finally, a question can be raised as to whether it is always necessary to isolate cellulose from lignin, or if it will be possible to use whole wood, derivatized or in its native state, for certain technical applications. These might include ion exchangers or materials for sequestering heavy metal ions from solutions. The ultimate problem in cellulose biochemistry is certainly its biosynthesis. However, little real progress has been made in this field, and we are still far away from developing a technically feasible cellulose synthesis. But it is a hot field and high quality work is under way, using increasingly the methods of molecular biology and genetic engineering. Summarizing, we may state that today the potential of cellulose as a versatile, renewable natural product is by no means exhausted. It is very likely that it will be the main chemical resource of the future, which will still be available when other substances become increasingly scarce due to exhausted reserves or environmental difficulties.
‘Trends in Polymer Science’ A bright future for cellulose J. Schurz Heinrichstrasse 28, A-8010 Graz, Austria Received 15 March 1999; accepted 22 March 1999
Cellulose is one of the oldest natural polymers. It is renewable, biodegradable, and can be derivatized to yield various useful products. It is fabricated by many plants as hairs (cotton) or as a structural polymer in cells. It is also produced enzymatically by bacteria (bacterial cellulose). Its main source, however, is wood, a natural composite, where cellulose is contained in combination with lignin in a texture which certainly represents a masterpiece of natural architecture. This natural polymer composite must be destroyed for the isolation of cellulose (pulping). Cellulose and wood are most abundant in nature; they are produced in a sustainable way and offer many possibilities for use. Its assets are that it is renewable, biodegradable, biocompatible and derivatizable. It is possible to base a whole chemistry on cellulose, which, however, today cannot compete with petrochemistry on the grounds of costs. Several disadvantages of cellulose include its expensive production, its sensibility to water, and its slow regeneration—a tree must have at least 30 years before it can be used for cellulose production. Regarding new directions in cellulose research, there are three main topics that we can identify. The first is the field of analytical problems and the routes of reactions. With regard to analytical questions, all structural levels must be considered: molecular, supramolecular, and morphological. Concerning cellulose reactions, we deal mainly with derivatization—which is amply possible due to the three hydroxyls. So far mainly esters and ethers have been produced. But due to new approaches we can search for new derivatives, including graft copolymers, and we can try to put the reactions on a higher level with regard of the stereoselectivity of the substituents. The second topic is the search for completely new products and uses. Here we cannot try to compete with mass products already in the market, as paper and regenerated fibers and films, but rather we look for special products with a high net value, so that it will be worthwhile to produce them even in rather small amounts. It is this area that is most active today. The third topic, finally, is that of mass products. We must not forget that cellulose is the basis of a large industry already, and it will be necessary and useful to look for improvements and optimization as well as for environmental safety. In particular, the role of biotechnology has to be explored, although at present it cannot yet compete on economic grounds. The first topic comprises analytical problems and reactions. The most important issue here is the determination of the stereoselectivity of a reaction and the guiding of a reaction in such a way that strictly stereoselective products are obtained. In this way, a regioselective functionalization with a 0079-6700/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0 0 7 9 - 6 7 0 0 ( 9 9 ) 0 0 01 1 - 8
482
J. Schurz / Prog. Polym. Sci. 24 (1999) 481–483
definite primary structure will be obtainable. Generally, the substitution along a cellulose chain takes place in a statistical way, in spite of the different reactivities of the three OH-carrying carbons C2, C3 and C6. Now it is increasingly possible to perform reactions taking place in only one of these carbons. More so, it will be possible to make derivatives resembling block copolymers, in which blocks of fully substituted cellulose chains alternate with completely unsubstituted ones. It is to be expected that such materials will show new properties, even in solution. Here cases of special aggregations will occur, which can be controlled by varying the lengths of the substituted blocks. Of course, for such new products it will be necessary to develop fast and reliable means of determining not only the degree of substitution, but also stereoselectively and in the case of block substitution, the block length. It will also be possible to substitute two or more different groups in such a way that different blocks alternate along the molecular chain. So far the most effective analytical tool is NMR combined with chromatographic methods. But since present research in this field is very active, other methods will be added, based on dielectric properties, zetapotential, and various optical methods. Of course, the detection of other analytical parameters on all three levels of organization must also be improved. Examples are crystallite shape, void system and inner surface, amorphous orientation, surface structure and surface charge. With solutions, the formation of supramolecular structures must be investigated in detail, from random gels to liquid crystals. Liquid crystal solutions are in the center of current research, and it has turned out that a very new process for cellulose regeneration (production of lyocell fibers) is characterized by the fact that the spinning solution has a kind of liquid crystal structure. It yields very good fibers—little wonder in view of the fact that spiders and silk worms also spin their high performance threads from liquid crystal solutions. Another area of research deals with new and better methods of fractionation with regard to both molecular weight and the degree of substitution. Efficient procedures not only for analytical purposes, but also for preparative ones, are being developed. So far the evaluation of analytical data has been done in a rather reductionistic, monocausal way. But in reality, the phenomena are multicausal. That means, we need a more sophisticated interpretation of data with the help of complex statistics. Finally, model calculations by means of computer simulations have been undertaken to help elucidate the fine structure of cellulose, its reaction mechanisms, and to better predict NMR data. The second topic is concerned with new products or applications, preferably special uses, which require only small quantities of material, but still have a high potential for producing net value. Here completely new routes are pursued. One is the use of mono-or multilayered Langmuir–Blodgett (LB) films. By dipping hydrophilic carriers into solutions of stiff chain derivatives, 1–40 nm layers are obtained. Using charged derivatives, such as cellulose sulfates, electrolyte multilayers can be obtained. Such ultrathin films can be used as supramolecular structures at interfaces, and then reacted further to acquire various special properties. In this way, crosslinked gel layers can be formed. For instance, by using appropriate derivatives, hairy rod layers can be produced which form a planar supramolecular network in which other polymers will show retarded diffusion and should move in reptation-like fashion. Further, hemocompatible surfaces can be produced in this way. Redox active layers could be applied to electrodes. Another specialized application is the use of cellulose structures as a basis for biomolecular signal detection in biosensors. For instance, a glass surface can be coated with a suitable cellulose derivative and then reacted with an enzyme, e.g. glucose oxidase, having the desired recognition structure. Such devices can show higher sensitivity than the dissolved enzyme. LB-films with covalently bound proteins can be used as models for studying cell adhesion, and also as matrices for growing neuronal networks. Cellulose is a polycrystalline polymer. By means of partial hydrolyzation it is possible to obtain microcrystalline cellulose, consisting mainly of micro crystals in the nanometer
J. Schurz / Prog. Polym. Sci. 24 (1999) 481–483
483
range. It is possible to use this product for further reactions. In this way anisotropic block structures can be obtained by topochemical reactions at the reducing end groups, which can be further reacted, using graft copolymerization. Finally, these products can be used to construct supermolecular architectures in the colloidal range. Another new field is that of anisotropic solutions, mainly liquid crystal structures. Here interesting new results can be expected by the use of regioselective functionalized derivatives. In solution, lyotropic cholesteric phases are obtained which can be readily characterized by optical methods. Such solutions not only show remarkable phase behavior, but they can also be used for various supramolecular structures. It is expected that they will exhibit new effects in the technical process of fiber spinning. In the solid state, thermotropic cholesteric structures can be obtained. Here it is hoped that the search for selective niches for products of high value, even in small quantities, will go on and lead to completely new applications. The third major topic is that of mass products. These include mainly paper and regenerated cellulose fibers and films. In the very old technology of papermaking, little real progress is to be expected and problems tend to be focused around optimization and ecological issues. In fiber making, the search for new spinning systems is important on environmental grounds. This includes the search for new solvents for cellulose. At present, the lyocell process is trying to replace the familiar viscose technologies, but so far the breakthrough has not taken place. The graft copolymerization of cellulose to make it water repellent, fire retarding or even thermoplastic, has been attempted, but all these processes have been too costly, at least as long as crude oil, the basis of petrochemistry, remains cheap. However, there is one new option in this field, namely the use of biotechnology. Enzymes from wood destroying fungi can be used to destroy lignin selectively. This can allow for the isolation of cellulose in a “soft” and ecologically benign manner. The big disadvantage remains the low throughput of these processes, which at present, rules them out on economic grounds. The space–time efficiency is too low. As an example, a beech tree, about 10 m high, will produce about 14 g of cellulose per day. A chemical reactor, half a meter high, can easily produce 20 kg of polystyrene per hour. Therefore, though the biochemical processes operate at low temperature and normal pressure, they require much longer time and can, therefore, hardly compete with normal “hard” chemistry. However, they pose no ecological problems and generally produce no wastes. Still, there have been attempts to degrade cellulose and hemicelluloses by means of the enzyme cellulase to sugars, which can then be fermented to alcohol (biofuel for cars) or various other products. An advantage of this process is that refuse, cellulose like sawdust or wastepaper can be used. However, here too the existing processes are at present too expensive. But the use of enzymes in various technical reactions, including pulping and bleaching, and for analytical purposes, is still growing. Finally, a question can be raised as to whether it is always necessary to isolate cellulose from lignin, or if it will be possible to use whole wood, derivatized or in its native state, for certain technical applications. These might include ion exchangers or materials for sequestering heavy metal ions from solutions. The ultimate problem in cellulose biochemistry is certainly its biosynthesis. However, little real progress has been made in this field, and we are still far away from developing a technically feasible cellulose synthesis. But it is a hot field and high quality work is under way, using increasingly the methods of molecular biology and genetic engineering. Summarizing, we may state that today the potential of cellulose as a versatile, renewable natural product is by no means exhausted. It is very likely that it will be the main chemical resource of the future, which will still be available when other substances become increasingly scarce due to exhausted reserves or environmental difficulties.