Twood is a common phenomenon in various

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SWST RESEARCH PAPER NO 7 ONE OF A SERIES PRODUCED IN COOPERATION WITH THE SOCIETY OF WOOD SCIENCE AND TECHNOLOGY Interaction of Wood with Polymeric Materials Penetration Versus Molecular Size By

1 SWST RESEARCH PAPER NO 7 ONE OF A SERIES PRODUCED IN COOPERATION WITH THE SOCIETY OF WOOD SCIENCE AND TECHNOLOGY Interaction of Wood with Polymeric Materials Penetration Versus Molecular Size By Harold Tarkow, W. C. Feist, and C. F. Southerland U.S. Forest Products Laboratory Madison, Wisconsin HE INTERACTlON of polymeric materials with Twood is a common phenomenon in various areas of wood technology; for example, in adhesion, finishing, dimensional stabilization, and microbiological utilization. In the first paper of this series, a type of interaction involving adsorption at the surface of wood substance was described (1). A second type of interaction involving penetration into wood substance is described in this paper. Wood substance behaves as a three-dimensional network of polymeric materials. The network expands to a maximum size at 100 percent relative humidity, and complete dissolution is prevented by "cross-links." The swollen network contains submicroscopic voids in which water is condensed. Some average or effective size of void probably imposes an upper limit to the size of molecule of a waterborne solute capable of diffusing into or through the cell walls. The significance of such limitations is illustrated by the following examples and the questions posed by them: Action of Wood-Destroying Organisms: Do enzymes act only in the immediate vicinity of the organism or do they diffuse through the wood substance? The importance of the relative sizes of enzyme molecule and effective void is obvious (2). Bonding of Adhesives: Although the interaction of adhesives with wood substance is generally believed to he a surface adsorption phenomenon, the actual interaction may occur within a "surface region." Effective void size may he important. Modification of wood: Certain properties of wood are altered by infusion with polymeric materials. The properties then depend on the relative amounts of polymer within the lumina and cell walls. This ratio is influenced by the relative sizes of penetrant molecules and of effective voids in the cell walls. Delignification : In reverse, delignification involves the diffusion of lignin fragments out of wood substance. What is the relationship between the size of the fragment and the effective size of the void? What is the relationship between the total void volume created and the quantity of lignin removed? What happens to the size of these voids when the delignified substance is dried and rewetted; or, practically, what happens when pulp lap is dried and rewetted? Stone and Scallan (3) have considered some of these points. Several procedures have been suggested for measuring the effective submicroscopic void size in green wood substance: 1) Microscopic. These voids are considerably under a micron in size; hence, the optical microscope is not applicable. Preston (4) and Rudman (5) have described procedures for depositing metals within the voids. Following treatment, the specimens are dried and examined with the electron microscope. Because of surface tension forces, however, the voids probably collapse on drying. (A 100-angstrom void, on losing its water, experiences a collapsing force of about 150 atmospheres.) The measured size, therefore, bears no relationship to the size of the void in the green condition. 2) Moisture adsorption. Under certain conditions, an adsorption isotherm can be analyzed with the help of the Kelvin equation (6). This relates the relative vapor pressure at which a vapor condenses in an existing capillary to some characteristic size of the capillary. Although the procedure is sound FOREST PRODUCTS JOURNAL VOl. 16, No

2 Figure 1. Schematic diagrams of a cross section of a green wood cell (at top) and a cell acting as an osmometer. for gels with pre-existing capillaries, it is questionable whether the method is applicable to wood substance that has a very small content of truly pre-existing capillaries. The voids are best considered as being created when the substance goes into "partial solution." Stone and Scallan (3) attempted to circumvent the problem of interpretation by "freezing-in" the voids in green wood substance by solvent replacement with methanol followed by pentane and driving off the pentane. The resulting aerogel was then examined by nitrogen adsorption and the average void size computed. It was tacitly assumed that no shrinkage occurred on driving off the pentane. Actually, considerable shrinkage occurs when the final replacement fluid is evaporated from swollen wood below the critical temperature of the fluid (7). 3) Bulking experiments. The ovendry dimensions of whole wood are enlarged (bulked) if foreign materials are introduced into the wood substance. Suggestions have been made to relate the degree of bulking to the molecular size of the penetrating chemical (8). The procedure is very crude, at best, because it assumes constancy of lumen volume. High concentration of chemical is necessary. This imposes osmotic shocks (9) with complications analogous to those of the plasmolysis of living cells. 4) X-ray scattering. X-rays are scattered from a medium containing regions of fluctuating electron density. From the angular dependence of scattering intensity, under certain conditions, the radius of gyration of the voids (or that of the solid islands ) can he determined (10). The method has not been used with green wood substance, but it deserves serious consideration. Aggebrandt and Samuelson (11) described an interesting procedure for measuring the nonsolvent-water content of water-saturated cotton and rayon. With slight modification, the procedure can be adapted to measure the effective void size in green wood substance. A cross section of green wood is considered. The circle in the upper drawing of Figure 1 rep. resents, schematically, the totality of lumen cross sections. The green cell walls are around the circle. The section is immersed in a dilute aqueous solution. It is assumed that the solute encounters no difficulty in penetrating the lumen and attaining essentially the same concentration as that in the external solution. If it also encounters no resistance in penetrating the cell wall, then at equilibrium, the ratio of water to solute in the entire section is the same as that in the external solution. (If, in addition, some adsorption of solute occurs onto the wood substance, the ratio may he slightly smaller.) If resistance to penetration into the cell wall is encountered, the ratio should be larger. With increasing molecular weight of solute, the ratio should increase (relative to that of the external solution) to some value at a critical molecular weight, which is then essentially independent of further increases in molecular weight. The solute now is completely excluded from the wood substance and is present only in the lumen. The effective void size is related to the molecular size of solute at the critical molecular weight. From the values of the two ratios (that in the section and that in the external solution), one can compute the nonsolvent-water content per gram of dry tissue. Above the critical molecular weight, the nonsolvent-water content should not change significantly with molecular weight or concentration of solute (refer to the lower part of Figure 1.) Above the critical molecular weight of solute, the specimen behaves as an ideal osmometer. One chamber, the cell wall, contains only water; the second chamber, the lumen, contains the solution. The cell wall acts as its own semipermeable membrane (it is permeable only to water). Figure 2. Gel permeation chromatogram of a mixture of polyethylene glycols on Sephadex, showing clean separation of glycols. 62 OCTOBER 1966

3 Figure 3. Gel permeation chromatograms of PEG-9000, A. PEG-9000 containing 2.5 percent PEG-4000, B; and PEG containing 5.0 percent PEG-4000, C. Since the water in the lumen is at a lower chemical potential, moisture will leave the cell wall and enter the lumen at an initial rate given by where R is the gas constant, T is the absolute temperature, is the molar concentration of solute in the lumen, is the difference in hydrostatic pressure in the two chambers, and LP is a permeability constant. is the osmotic pressure of the solution. At time zero, the hydrostatic pressure on both chambers is the same; that is, is zero. As water leaves the cell wall, the remaining moisture is placed under increasing tension increases. When numerically equals net flow, is zero. For a highly crossof water that must occur from the cell wall before linked, rigid gel like wood substance, the loss the net flow is zero is extremely small. A good approximation is that no significant amount of moisture leaves the cell wall at low molar concentrations. Experimental Wood Green Sitka spruce heartwood cross sections (2- by 2- by inch) were extracted with boiling water. Solute Aggebrandt and Samuelson (11) used radioactive polyphosphates and polyethylene glycols for the solutes. For our purpose, the solute should be fairly monodisperse or convertible into narrow molecular weight classes by fractionation techniques. Its molecular configuration io solution should he that of a random coil to simulate a sphere. Polyethylene glycol meets these requirements. Different molecular weight classes were supplied by the Dow Chemical Co. Repented fractionations by liquid-liquid or gel permeation chromatography procedures showed they were fairly monodisperse. This does not imply that all commercial polyethylene glycols ace equally monodisperse. Figure 2 is a gel permeation chromatogram (on Sephadex) of a mixture of polyethylene glycols. It shows that PEG-7000 and PEG-4000 do not contain any low molecular weight species that are present in PEG The curve found for Blue Dextran indicates the void volume of the column. Figure 3 shows the effect of adding small amounts of PEG-4000 to PEG This indicates how sensitive gel permeation chromatography is in detecting polydispersity in polymers whose molecular weight class is of the order of several thousand. molecular weights were measured by vapor pressure osmometry and end-group acetylation (Table 1). The structure of polyethylene glycolmonomeric units joined by either linkages provides for a very flexible chain; hence, a random coil in solution. Analysis The only analyses are for concentration of polyethylene glycol (by percent) in aqueous solutions. These were made quickly with an interferometer with a standard deviation of the order of percent from three replicates. Treatment and Calculation The extracted specimens were equilibrated in aqueous polyethylene glycol solution (1 to 4 percent concentration) at 22 C. Where penetration into wood substance could occur, 4 to 5 days were required. Where penetration did not occur, 2 days were adequate. The specimens were removed from the solution, blotted (nonquantitatively), and weighed (A, in grams, for calculation). They were then extracted with water to remove all polyethylene glycol present in the wood. (Separate experiments confirmed that quantitative separation can be made.) The solution was weighed (B, grams) and analyzed for polyethylene glycol (C, percent). The extracted sections were ovendried and weighed (D, grams). The external treating solution was analyzed for water-to-polyethylene glycol ratio (E) Table 1. RELATIONSHIP BETWEEN MOLECULAR WEIGHT OF POLYETHYLENE GLYCOL AND THE NONSOLVENT-WATER CONTENT OF GREEN WOOD Standard Molecular (gram of water per deviation PEG No. weight gram of wood) (gram per gram) , , USP 3, , , ,000 18, FOREST PRODUCTS JOURNAL Vol. No

4 B x C = weight of polyethylene glycol in the wood 100 D = weight of water in the wood Assuming that all the polyethylene glycol in the wood is associated with water in the same ratio as in the external solution, the weight of such water (solvent water) is: E x B x C grams 100 The weight of nonsolvent water is: The weight of nonsolvent water per gram of wood is: An analysis of errors suggested that the maximum instrumental error is under gram of water per gram of wood. The term nonsolvent-water content is a fictional one when dealing with penetrating solutes. It has a definite physical meaning when nonpenetrating solutes are used. This will be discussed in a future paper as well as values found for delignified wood. Results The values of are given in Table 1. External solution concentrations were approximately 2.5 percent. If were zero, the solute would have complete access to the wood substance; that is, it would distribute itself in the cell wall in the same ratio with water as it does in the external solution. Obviously, this does not occur, even for PEG-200. This is not surprising, since the molar volumes of water and PEG-200 are appreciably different. Nonsolvent-water content increases with molecular weight of solute and then becomes constant when the molecular weight exceeds about 3,000, as shown in Figure 4. Thus, the effective submicroscopic void size in green wood substance is that of a PEG-3000 molecule dissolved in water. Figure 4. Relation of nonsolvent water content to molecular weight polyethylene glycol for green Sitka spruce at 22 C. An approximation of the radius of gyration of such a molecule, based on intrinsic viscosity measurements (12), suggests it is about angstroms. A more reliable value is being obtained by measuring the diffusion constant of the polyethylene glycol in water (13). This effective size is in serious conflict with conclusions made by Stamm (14), who reported penetration of PEG into wood substance. The two procedures are radically different, and experiments are being designed to rationalize the difference. Discussion The limiting size for the penetration of a watersoluble material into green wood substance is that of a PEG-3000 molecule. This size, of course, must he related to some anatomical characteristic of the cell wall. Which one? The cross sections were 500 microns thick. The maximun distance a penetrating molecule would have to diffuse longitudinally would be 250 microns. On the other hand, the maximum distance a molecule would have to diffuse transversely (for example, from the lumen) would be only 2 to 10 microns. At first, one would expect diffusion to occur predominantly along the transverse pathway. There is evidence, however, that the tertiary layer across which diffusion would have to occur from the lumen is inert and perhaps able to swell only slightly. Bücher has discussed this (15). Figure 5 shows one of his photographs in which the fiber was treated with a swelling agent. Note that, whereas, the secondary wall is highly swollen, the tertiary layer seems intact. Pew has shown that linerlike materials may also be present at the lumen-cell wall interface (16). Conceivably, the tertiary layer may be so highly impermeable that in a chip, sliver, or board which has relatively little exposed secondary wall, the upper limit for the penetration of a water-soluble molecule may he determined by the permeability of the tertiary layer. Although the effective submicroscopic void size seems to be equivalent to that of a PEG-3000 molecule, it should be recalled that the work was done at 22 C. At elevated temperatures, the resistance offered by the cell wall (or tertiary layer) may diminish, allowing solutes of higher molecular weight to penetrate. Goring (17) has measured the softening points of isolated lignin and hemicellulose in the green condition. Both softening points can be considerably below 100 C. Thus, if penetration is influenced by micromotions in the polymer network of the wood substance, the critical molecular weight may exceed that of PEG-3000 if the temperature is considerably above room temperature. The observation that the maximum size of a penetrating molecule corresponds to a PEG-3000 molecule confirms an interpretation made in the previous paper (1). It was then noted that the 64 OCT O B E R 1966

interaction capacity of swollen wood substance toward polyvinyl acetate from carbon tetrachloride solution was greater than the adsorption capacity of nonswollen wood substance.

5 interaction capacity of swollen wood substance toward polyvinyl acetate from carbon tetrachloride solution was greater than the adsorption capacity of nonswollen wood substance. This was interpreted in terms of a more reactive external surface in the swollen wood rather than to a penetration of the polymer into the swollen wood substance. Since the molecular weight of the polyvinyl acetate was about 75,000, the correctness of the interpretation is established. The surface of the water-swollen wood substance is thus extremely reactive, a point of great significance in papermaking. A number of questions are raised by the findings reported here. Enzyme molecules are generally larger than this critical molecular weight found with polyethylene glycol (PEG-3000). Does this mean that enzymes from cellulose or wood-attacking organisms do not act by direct diffusion into the cell wall? Is one of the requirements for the removal of lignin, as in pulping, that of degrading it into fragments which can diffuse through the effective channels? Assuming the density of lignin is 50 percent greater than that of polyethylene glycol, a crude interpretation is that the critical molecular size of the lignin fragment is 4,000 to 5,000. The technique described here lends itself for studies of other aspects of the submicroscopic structure of green wood substance, These include the size of voids in delignified wood, as well as a technique for a more conclusive measurement of fiber-saturation point. Such studies are now under way and will be described in future publications. Literature Cited 1. Tarkow, H., and C. Southerland Interaction of wood with polymeric materials. I: Nature of the adsorbing surface. For. Prod. Jour. 14(4): Cowling, E. B Structural features of cellulose that influence its susceptibility to enzymatic hydrolysis. In Advances in enzymatic hydrolysis of cellulose and related materials. Pergamon Press, New York. 3. Stone, J. E., and A. M. Scallan Effect of component removal upon the porous structure of the cell wall of mood. Jour. Polymer Sci. C11: Preston, R. D The fine structure of wood with reference to impregnation-2. Timber Technology 67: Rudman, P Fine structure of wood. Nature 208: Emmett, P. H Measurement of surface areas of solids. In Advances in colloid science. E. O. Kraemer, ed. Interscience Publishers, New York, p Stamm, A. J., and L. A. Hansen Minimizing wood shrinkage and swelling. Replacing water in wood with nonvolatile materials. Ind. Eng. Chem. 27: Figure 5. A swollen secondary wall showing differentiated resistant tertiary layer (X 110), by permission of H. Bucher. 8. Cowling, E. B., and A. J. Stamm An approach to the measurement of solid-solution structures in wood and other cellulosic materials. Jour. Polymer Sci. C2: Kenaga, D Dimensional behavior of wood during polyethylene glycol soak treatments. For. Prod. Jour. 13(8): Hermans, P. H., D. Heikens, and A. Weidinger A quantitative investigation on the X-ray small angle scattering of cellulose fibers. Part II. The Scattering power of various cellulose fibers. Jour. Polymer Sci. 35: Aggebrandt, L. G., and O. Samuelson Penetration of water-soluble polymers into cellulose fibers. Jour. Appl. Polymer Sci. 8: Flory, P. J Principles of polymer chemistry. Cornell University Press, Ithaca, N.Y., p Benko, J Measurement of the relative molecular weight of lignosulfonates by diffusion. I. Methods for molecular weight determinations and for separation of interferring substances by ion exclusion or by exhaustive dialysis. Tappi 44: Stamm, A. J Factors affecting bulking and dimensional stabilization of wood with PEG solutions. For. Prod. Jour. 14(9): Bücher, H Die Struktur der Tertiärwand von Holzfasern. Holzforschung 11(4): Pew, J. C Membranous substances in common heartwoods. Tour. Forestry 47: Goring, D. A. I Thermal softening of lignin, hemicellulose, and cellulose. Pulp and Paper Map. of Canada 64: T517-T527. FOREST PRODUCTS JOURNAL Vol. 16, No

Sem /2007. Fisika Polimer Ariadne L. Juwono

Sem /2007. Fisika Polimer Ariadne L. Juwono Chapter 8. Measurement of molecular weight and size 8.. End-group analysis 8.. Colligative property measurement 8.3. Osmometry 8.4. Gel-permeation chromatography 8.5. Ultracentrifugation 8.6. Light-scattering