Biomacromolecules 2003, 4, 1232-1237
1232
Load Distribution in Native Cellulose Barbara Hinterstoisser,*,† Margaretha Åkerholm,‡ and Lennart Salme´ n*,‡ BOKU - University of Natural Resources and Applied Life Science, Muthgasse 18, A-1190 Vienna, Austria, and Swedish Pulp and Paper Research Institute (STFI), Box 5604, SE-114 86 Stockholm, Sweden Received February 20, 2003; Revised Manuscript Received May 13, 2003
The properties of cellulose materials are dependent on interactions between and within the cellulose chains. To investigate the deformation behavior of cellulose and its relation to molecular straining, sheets with fibers oriented preferably in one direction were studied by dynamic FT-IR spectroscopy. Celluloses with different origins (spruce pulp, Cladophora cellulose, cotton linters) were used. The sheets were stretched sinusoidally at low strains and small amplitudes while being irradiated with polarized infrared radiation. The cellulose fibers showed mainly an elastic response. The cellulose fibers showed mainly an elastic response. The glucose rings and the C-O-C bridges connecting adjacent rings, as well as the O(3)H‚‚‚O(5) intramolecular hydrogen bonds are the components mainly deformed under stress, whereas the O(2)H‚‚‚ O(6) intramolecular hydrogen bonds play a minor role. The load distribution was also found to be different in the different allomorphic forms of cellulose I, namely, IR and Iβ. Introduction Cellulose is the most widespread biopolymer. It is made up of poly-β(1,4)-D-glucose chains and plays a significant role in the structural support of the cell walls of plants as well as of bacteria. This is a result of, on one hand, its high degree of polymerization and the linear orientation of the molecules and, on the other hand, its ability to form hydrogen bonds which stabilize the molecule itself and connect it to neighboring ones and to form microfibrils. On a molecular level, strength is related to the covalent as well as to intramolecular hydrogen bonds,1 so that cellulose is an ideal component giving strength to the cell wall. Despite the new approaches and new insights obtained in the past decade, there is still a gap in knowledge concerning the relationship between structural features and the mechanical behavior of cellulose.2-4 Cellulose contains several types of hydrogen bonds within the molecule and between the different molecules. The main ones, according to the generally accepted structure of cellulose I, are the intramolecular hydrogen bonds O(3)H‚‚‚O(5) and O(2)H‚‚‚O(6), present on both sides of the chain, and a O(6)H‚‚‚O(3) intermolecular hydrogen bond, which builds a bridge between two neighboring molecules (cp. Figure 1). Native cellulose (cellulose I) exists in two different crystalline forms: cellulose IR and Iβ.5 The secondary structure of native cellulose is a ribbonlike conformation with an approximately 2-fold helical structure.3 The cellulose molecular chain itself is considered to be not very stiff, but * To whom correspondence should be addressed. (B.H.) Phone: +431-36006 6074. Fax: +43-1-36006 6079. E-mail: barbara.hinterstoisser@ boku.ac.at. (L.S.) Phone: +46-8-6767 340. Fax: +46-8-411 55 18. E-mail:
[email protected]. † University of Natural Resources and Applied Life Science. ‡ Swedish Pulp and Paper Research Institute.
Figure 1. Schematic drawing of cellulose molecules including the hydrogen bonds (- - -): Intramolecular hydrogen bonds O(3)H‚‚‚O(5) and O(2)H‚‚‚O(6); intermolecular hydrogen bond O(6)H‚‚‚O(3).
rather semiflexible. Although it is generally known and accepted that the hydrogen bonds play an important role for the conformational and mechanical properties of cellulosic materials,2 several questions concerning the structure and the mechanical behavior of native cellulose are still left unanswered.3 Cellulose is known to be extremely resistant to tensile stress because of the covalent bonding within the pyranose ring and between the individual units. Tashiro and Kobayashi4 calculated that the strain energy should be distributed mainly via deformation of the glucose rings (∼30%), bending of the ether linkages connecting the adjacent rings (∼20%), and deformation of O(3)H‚‚‚O(5) hydrogen bonds (∼20%). Infrared spectroscopy is widely used as a tool to investigate the chemistry and chemical structure of molecules.6-13 To record dynamic responses of molecules and submolecular groups, a special technique has been applied: An FT-IR spectrometer, working in a step-scan mode and providing a polarized IR-beam, is coupled to a mechanical stretcher, which applies a sinusoidal strain of very low amplitude and constant frequency to the sample. The parameters are chosen so that the sample is periodically and reversibly stretched over the whole recording time of the spectra. The external perturbation, produced by the polymer modulator (stretcher), induces selectively time-dependent reorientations of the electric dipole-transition moments that are associated with the individual normal vibrational modes in the sample. The
10.1021/bm030017k CCC: $25.00 © 2003 American Chemical Society Published on Web 07/10/2003
Load Distribution within Native Cellulose
Figure 2. Schematic drawing of a spectrometer performing dynamic FT-IR measurements.
altered orientation distribution of dipole-transition moments is detected as a variation of the directionally sensitive IRabsorbance of the molecular system. The dynamic spectra obtained (in phase and out of phase) provide information about the interactions within and between the polymer chains and the elastic and viscous behavior of the molecules. This coupling of dynamic mechanical analysis (DMA) and FT-IR spectroscopy (cf. Figure 2) has turned out to be very useful for assigning and interpreting spectra of polymer molecules and, subsequently, for investigating intra- and intermolecular interactions.13-15 The technique was introduced for studies on cellulose molecules by Hinterstoisser and Salme´n.16, 17 The present work focuses on the inter- and intramolecular interactions within different kinds of native celluloses and tries to draw conclusions about the load distribution within and between the molecules. Experimental Section Cellulose Samples. A spruce dissolving pulp, provided by Borregaard, with a cellulose content of 98%, a cellulose from Cladophora (i.e., algae), and cotton linters were used to make oriented sheets. The thickness of the sheets was 40 µm for the spruce dissolving pulp, 20 µm for the Cladophora cellulose, and 50 µm for the cotton linters. The pulps were homogenized by pumping them several times through a slit (0.3-0.4 mm) in a laboratory homogenizer (Gaulin Corp.) in order to fibrillate the fibers for better bonding in the sheet formation. Sheets with a grammage of 13-20 g/m2 were made on a Formette Dynamique to obtain sheets with an oriented fiber direction. Dynamic FTIR Spectroscopic Experiments. A Bio-Rad FTS 6000 spectrometer equipped with a Manning Polymer Modulator (Manning Applied Technologies, Inc., Troy, ID) was used. For each measurement, a sample sheet with a length of 35 mm and a width of 25 mm was mounted between the jaws of the polymer modulator. A sinusoidal strain with a frequency of 16 Hz and a very small amplitude (
