Chapter 1
High-Modulus Oriented Cellulose Nanopaper
Downloaded by COLUMBIA UNIV on August 23, 2012 | http://pubs.acs.org Publication Date (Web): August 17, 2012 | doi: 10.1021/bk-2012-1107.ch001
Wolfgang Gindl-Altmutter,*,1 Stefan Veigel,1 Michael Obersriebnig,1 Christian Tippelreither,1 and Jozef Keckes2 1Department
of Materials Scvience and Process Engineering, BOKU-University of Natural Resources and Life Science, Konrad Lorenz Stzrasse 24, A-3430 Tulln, Austria 2Erich Schmid Institute of Materials Science, Austrian Academy of Sciences and Institute of Metal Physics, University of Leoben, Jahnstrasse 12, A-8700 Leoben, Austria *E-mail:
[email protected] Thin sheets of nanopaper were prepared from cellulose nanofibrils (CNF) obtained by means of high-pressure homogenisation of dissolving-grade beech pulp. Untreated pulp and pulp pre-treated by TEMPO-mediated surface oxidation were used, which resulted in significant differences in structure and mechanical performance of the nanopaper specimens. Overall, surface-oxidized CNF were characterized by reduced diameter, reduced crystallinity, and reduced crystallite thickness compared to untreated CNF. Nanopaper produced from surface-oxidized CNF showed better mechanical performance than untreated nanopaper with regard to tensile strength and modulus of elasticity, and also exhibited higher failure strain indicating better toughness. While stretching experiments with the aim of inducing preferred orientation failed with untreated nanopaper, surface-oxidized nanopaper could be stretched up to 30% elongation. Stretching resulted in a high degree of preferred orientation of CNF parallel to the direction of induced elongation as shown by wide-angle x-ray scattering and AFM. The mechanical performance of nanopaper improved to a tensile strength of up to 380 MPa and a modulus of elasticity up to 46 GPa parallel to the direction of preferred orientation.
© 2012 American Chemical Society In Functional Materials from Renewable Sources; Liebner, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
Downloaded by COLUMBIA UNIV on August 23, 2012 | http://pubs.acs.org Publication Date (Web): August 17, 2012 | doi: 10.1021/bk-2012-1107.ch001
Introduction The tensile properties along the chain of the cellulose macromolecule are excellent. With a chain elastic modulus in the order of 140 GPa and a tensile strength of 7.5 GPa, as obtained from theoretical predictions and molecular simulations, cellulose I, the crystal allomorph which occurs naturally in plants, is well competitive with technical fibers such as glass, carbon and polyaramide (1). Plant tissue is porous and exhibits a complex hierarchical arrangement of its constituent polymers cellulose, hemicellulose, pectin, and lignin. Because of the inferior properties of non-cellulosic cell-wall building blocks such as lignin and hemicellulose compared to cellulose, the mechanical properties of plant fibers are obviously well below the maximum values inherent to pure cellulose (2). In order to exploit the full mechanical potential of cellulosic plant fibers, it is therefore necessary to remove non-cellulosic components. Subsequent mechanical disintegration of cellulosic plant material down to nano-scale further improves its suitability for reinforcement. Nano-scale cellulosic objects obtained in this procedure are termed cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC). CNF are long and slender, ideally with diameters from 4-20 nm, but frequently also up to 100 nm, and have an aspect ratio in the order of 100 or more (1). The diameter of CNF, which depends on the source of cellulose and on the type of processing, is critical for the performance of materials produced thereof (3, 4). The modulus of cellulose I nanofibrils (bacterial cellulose) was measured directly by means of a three-point bending experiment in the AFM (5). For fibrils with diameters ranging from 35 to 90 nm, a value of 78 ± 17 GPa was obtained. By Raman spectroscopy, the modulus of elasticity of bacterial cellulose fiber networks was determined and an estimate of 114 GPa was inferred for single cellulose fibrils (6). In view of a reference value of 140 GPa for crystalline cellulose I (7), this value seems plausible considering the fact that CNF consist of both highly ordered crystalline domains and less ordered non-crystalline domains. Through an additional processing step involving hydrolysis of non-crystalline domains by strong acid, CNC are obtained (8). With the exception of CNC from tunicates, which can be up to 4 µm in length, CNC exhibit significantly lower aspect ratio than CNF. AFM three-point bending experiments showed a modulus of elasticity of 145 - 150 GPa for tunicate CNC with cross-sectional dimensions of 8 × 20 nm and several micrometers in length (9). The excellent mechanical reinforcement potential of CNF and CNC, as well as other interesting features like low thermal expansion
