Strategies for Cellulose Fiber Modification
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Strategies for Cellulose Fiber Modification Per Persson Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie doktors- examen fredagen den 23 april 2004 kl. 10.00 i Kollegiesalen KTH, Valhalla- vägen 79, Stockholm. Avhandlingen försvaras på engelska. This work was performed at STFI-Packforsk AB, Stockholm, Sweden. STFI-Packforsk AB is a center for research and development across the whole field of fiber, packaging and print. The Royal Institute of Technology (Kungliga Tekniska Högskolan), KTH, was founded in 1827 and is the largest of Sweden’s universities of technology. The university has over 11,000 undergraduate students, 1,500 active postgraduate students and a staff of 3,100 people. © 2004 Per Persson Trita-FPT-Report 2004:7 ISSN 1652-2443 ISRN/KTH/FPT/R-2004/7-SE ABSTRACT This thesis describes strategies for and examples of cellulose fiber modification. The ability of an engineered biocatalyst, a cellulose-binding module fused to the Candida antarctica lipase B, to catalyze ring-opening polymerization of ε- caprolactone in close proximity to cellulose fiber surfaces was explored. The water content in the system was found to regulate the polymer molecular weight, whereas the temperature primarily influenced the reaction rate. The hydrophobicity of the cellulose sample increased as a result of the presence of surface-deposited polyester. A two-step enzymatic method was also investigated. Here, Candida antarctica lipase B catalyzed the acylation of xyloglucan oligosaccharides. The modified carbohydrates were then incorporated into longer xyloglucan molecules through the action of a xyloglucan endotransglycosylase. The modified xyloglucan chains were finally deposited on a cellulose substrate. The action of Candida antarctica lipase B was further investigated in the copolymerization of ε-caprolactone and D,L-lactide. Copolymerizations with different ε-caprolactone-to-D,L-lactide ratios were carried out. Initially, the polymerization was slowed by the presence of D,L-lactide. During this stage, D,L- lactide was consumed more rapidly than ε-caprolactone and the incorporation occurred dimer-wise with regard to the lactic acid units. Morphological studies on wood fibers were conducted using a sol-gel mineralization method. The replicas produced were studied, without additional sample preparation, by electron microscopy and nitrogen adsorption. Information concerning the structure and accessibility of the porous fiber wall was obtained. Studies of never-dried kraft pulp casts revealed micro-cavities and cellulose fibrils with mean widths of 4.7 (±2) and 3.6 (±1) nm, respectively. Finally, cationic catalysis by simple carboxylic acids was studied. L-Lactic acid was shown to catalyze the ring-opening polymerization of ε-caprolactone in bulk at 120 °C. The reaction was initiated with methyl β-D-glucopyranoside, sucrose or raffinose, which resulted in carbohydrate-functionalized polyesters. The regioselectivity of the acylation was well in agreement with the corresponding lipase-catalyzed reaction. The polymerization was also initiated with a hexahydroxy-functional compound, which resulted in a dendrimer-like star polymer. The L-lactic acid was readily recycled, which made consecutive reactions using the same catalyst possible. Keywords: Candida antarctica lipase B, cationic catalysis, cellulose-binding module, dendrimer, enzymatic polymerization, fiber modification, silica-cast replica, sol-gel mineralization, organocatalysis, xyloglucan endotransglycosylase LIST OF PUBLICATIONS This thesis is based on the following publications, which in the text are referred to by their Roman numerals: I Polyester coating of cellulose fiber surfaces catalyzed by a cellulose- binding module – Candida antarctica lipase B fusion protein M.T. Gustavsson, P.V. Persson, T. Iversen, K. Hult, M. Martinelle Biomacromolecules 2004, 5, 106-112 II A two-step strategy for the enzymatic modification of cellulose fiber surfaces using Candida antarctica lipase B and the Populus tremula x tremuloides xyloglucan endotransglycosylase M.T. Gustavsson, P.V. Persson, T. Iversen, M. Martinelle, K. Hult, T.T. Teeri, H. Brumer Manuscript III Structural characterization of a lipase-catalyzed copolymerization of ε-caprolactone and D,L-lactide J. Wahlberg, P.V. Persson, T. Olsson, E. Hedenström, T. Iversen Biomacromolecules 2003, 4, 1068-1071 IV Silica-cast replicas for morphology studies on spruce and birch xylem P.V. Persson, A. Fogden, J. Hafrén, G. Daniel, T. Iversen IAWA Journal 2004, 25, in press (issue 3) V Silica nanocasts of wood fibers: a study of cell wall accessibility and structure P.V. Persson, J. Hafrén, A. Fogden, G. Daniel, T. Iversen Biomacromolecules 2004, 5, in press (issue 3) VI Selective organocatalytic ring-opening polymerization: a versatile route to carbohydrate-functionalized poly(ε-caprolactones) P.V. Persson, J. Schröder, K. Wickholm, E. Hedenström, T. Iversen Manuscript VII Direct organocatalytic synthesis of dendrimer-like star polymers P.V. Persson, T. Iversen, A. Córdova Manuscript TABLE OF CONTENTS 1 INTRODUCTION 7 2 ENZYMATIC STRATEGIES FOR FIBER MODIFICATION 9 2.1 Lipase catalysis 9 2.2 Xyloglucan endotransglycosylase catalysis 10 2.3 Polyester modification of cellulose fiber surfaces 11 2.4 Fiber modification using modified xyloglucan oligosaccharides 14 2.5 Copolymerization of ε-caprolactone and D,L-lactide 16 3 SILICA MINERALIZATION OF WOOD 18 3.1 Chemistry and structure of wood 18 3.2 Silica sol-gel 21 3.3 Mineralization of wood fibers 22 3.4 Silica casts studied by electron microscopy 23 3.5 Image analysis 25 3.6 Nitrogen adsorption 26 3.7 Image gallery 27 4 ORGANOCATALYTIC POLYMERIZATION 31 4.1 Organocatalysis 31 4.2 Ring-opening polymerization catalyzed by L-lactic acid 31 4.3 Selective acylation of carbohydrates 33 4.4 Synthesis of dendrimer-like poly(ε-caprolactone) 37 5 CONCLUDING REMARKS 39 LIST OF ABBREVIATIONS 40 ACKNOWLEDGEMENTS 41 REFERENCES 42 1 INTRODUCTION Composites of natural fibers and man-made polymers are desirable from an environmental point of view. Aliphatic polyesters are often biodegradable and biocompatible, and have good mechanical properties, and they are therefore particularly interesting. Polylactide (PLA) and poly(ε-caprolactone) (PCL) are two examples of such aliphatic polyesters. PLA is produced from lactic acid or lactide, and has mechanical properties that make the polymer promising as a matrix component in natural fiber composites. Some applications, such as automotive interior components, are already commercially viable.1 PCL, made from petroleum-derived ε-caprolactone (ε-CL), is a hydrophobic polyester with a low glass transition temperature.2 It is also notable that this cyclic monomer is liquid at room temperature and a good solvent for many compounds. Ring-opening polymerization (ROP) of cyclic esters is carried out mainly in bulk or in solution, and is normally catalyzed/initiated by an organometallic compound.3 However, ROP performed in the presence of cellulose fibers is somewhat delicate. The fibers are moist and carry numerous reactive hydroxyl groups. Tin (II) 2-ethylhexanoate, which is otherwise a common and efficient initiator/catalyst for ring-opening polymerization, is hygroscopic, hydrolyzes easily to stannous hydroxide, and is susceptible to undesired reactions with hydroxyl groups.4 A more suitable choice is therefore a biocatalyst. Lipases have been shown to efficiently catalyze transesterification reactions.5,6 The conditions need not be extremely dry; in fact, small amounts of water can be favorable for the polymerization. In Chapter 2 (papers I–III), lipase catalysis in fiber-modifying processes is summarized. Paper I describes the use of an engineered biocatalyst – the Candida antarctica lipase B equipped with a cellulose-binding module (CBM- CALB)7,8 – in the polymerization of ε-CL in close proximity to cellulose fibers. Paper II elaborates on this with a different approach; here xyloglucan oligosaccharides are acylated in a lipase-mediated step using a vinyl ester or a thiolactone, and are then incorporated into longer xyloglucan chains through the action of a xyloglucan endotransglycosylase. In the final step, the modified xyloglucan is deposited on a cellulose substrate. In paper III, the CALB- catalyzed copolymerization of ε-CL and D,L-lactide is examined with respect to mechanism and product pattern. The study also provides a hint that CBM- CALB would also be capable of catalyzing the ROP of D,L-lactide. 7 The wood ultrastructure, including the pore system, is altered during the pulping process. It is important to take these changes into account, especially those affecting the accessibility to reactants and catalysts, when designing systems for fiber modification. In Chapter 3 (IV,V), a casting method based on silica sol-gel is described. This technique allows the nanostructure of wet fibers to be transferred to a hard silica cast, which can be studied under the electron microscope or by adsorption measurements. Paper IV is primarily a reference study to demonstrate the usefulness and versatility of the casting method. Paper V deals mainly with delignified fibers, and provides a rationale for the results of the biocatalytic modifications (I,II). One important insight is that polymerization inside the fiber cell wall is unlikely to be accomplished using enzymes such as lipases due to the restricted pore size. A natural step forward is to search for a catalyst by which this could be achieved.