study of cellulose structure and its relation to reactivity

J . POLYMER SCI.: PART C NO. 36, PP. 305-320 (197 1)

STUDY OF CELLULOSE STRUCTURE AND ITS RELATION TO REACTIVITY

RUSSELL NELSON and DON W. OLIVER

The Buckeye Cellulose Corporation Memphis, Tenn.

Information is presented to show that the total volume of micropores in acetic acid swollen fibers correlates closely with acetylation reactivity. Pore size distribution measurements indicate that the pores with diameters of 25-75 a m a y be particularly influential for promoting reactivity since fibers which perform well in acetylation processing have a char- acteristically high volume of pores in this size range. Internal surface area also correlates with acetylation reactivity if allowances are made for the large fraction of surface area which is associated with pores that are too small for reagent penetration. This lower limit is calculated to be 20-25 A Based on estimates of void spaces within and between the cellulose structural building units, acetylation of acetic acid swollen fibers should occur on the external surfaces of the elementary fibrils, !he microfibrils and the lamellae as well as on the internal surfaces of the microfibrils and the lamellae. The overall effect of cellulose structure on reactivity seems to be the control of the inward flow of reagents to reaction surfaces but not the outward transport of reaction products. Internal void volume may function to accommodate reaction products as they swell inwardly and thus provide a means for allowing the reaction to go to completion. The relationship between cellulose structure and reactivity can be extended to chemical reactions other than acetylation.

INTRODUCTION

The fine structure of cellulose is generally considered to have a strong influence on the rate, extent and uniformity of its chemical reactions. Accessibility is believed t o play the major role, while other structure related factors such as fiber dimensions, purification process, crystalline form, and hydrogen bonding have lesser effects [l-31. The implied function of accessibility is to allow transport of reagents 'and catalysts inward to reaction sites, or the movement of reaction products outward from the reaction sites, or both. Accessibility must therefore depend on fiber microporosity and internal surface area. Methods for determining accessibility are designed to quantify either of these two fundamental fiber properties or a combination of both [4-71. Accessibility methods rank ribers according to reactivity in some cases, but they do not in others because measurements are usually made under conditions which are different from those which exist during

305

@ 197 1 by John Wiley & Sons, Inc.

306 NELSON AND OLIVER

the reaction. For example, accessibility of either dry or water wet fibers seldom correlates with reactivity of a derived alkali cellulose because of the large swelling effect of the lye on the fiber and therefore on accessibility. Hence, it is not surprising that interpretations of the influence of cellulose fine structure on reactivity are often subject to considerable speculation.

It is suggested that an alternate and more direct approach for studying the relationship between structure and reactivity would be to characterize microporosity and surface area of the fiber within the reaction environment. For practical reasons, this requires working with the fiber in the same physical state as it enters the reaction. In most cellulose reactions, the fiber is at or near equilibrium swelling as a result of contacting it with some pretreatment agent or solvent system. This situation presents a problem because very few methods for determining microporosity and surface area are suited to working with solvent swollen fibers. There are at least two cases reported in the literature where the problem has been recognized and overcome. Stone and co-workers [8] using the principle of “polymer exclusion” found that enzymatic degradation of pulp fibers is closely dependent on the porosity of the water swollen cell wall. Klenkova and co-workers [4-51 measured surface area and microporosity of various pulp fibers by acetic acid vapor adsorption and suggested that these two properties controlled pulp acetylation behavior.

The present studies were directed toward the characterization of the micro- porous nature of pulp fibers at equilibrium swelling in glacial acetic acid, and the investigation of the possible mechanisms by which microporosity influences acetylation reactivity. The “polymer exclusion” method as refined and used extensively by Stone and Scallan [9-111 is particularly suited to estimating the total pore volume and pore size distribution of water swollen fibers. The same basic method was employed in the present work, but several modifications were necessary including the substitution of glacial acetic acid as the solvent and the use of a series of polyethylene glycols of different molecular weights as the measuring polymers or feeler gages. Although the results from these studies are based on acetylation behavior in particular, the relationships found between fiber structure and reactivity are believed to be more general in nature.

EXPERIMENTAL

Pulp Preparation and Extraction Mechanically defibered pulp samples were conditioned to 6-7% moisture

and extracted with glacial acetic acid by vigorously shaking until liquors from successive extractions reached a constant refractive index. Usually 10- 15 extractions with 12-15 parts of acid per part of pulp were required. Specially treated samples such as aqueous activated or never-dried pulps were first dewatered by displacement with several portions of glacial acetic acid.

CELLULOSE STRUCTURE 307

Polymer Exclusion Method

The experimental procedure for determining inaccessible solvent or pore volume by polymer exclusion has been reported previously in detail [9-I 11. Consequently, only those differences in procedure occasioned by the use of polyethylene glycol as the measuring polymer and glacial acetic acid as the solvent are discussed in the following paragraphs.

The polymer-solvent combination of polyethylene glycol-glacial acetic acid meets the requirements for measuring pore characteristics:

1. The polymer must not be adsorbed on the surfaces of the fibrous substrate. This has been experimentally demonstrated by the fact that solutions ranging in concentration from 0.5-1070 gave the same value of inaccessible solvent.

The polymer should have a narrow molecular weight distribution and be available over a wide range of molecular weights. Commercial grades of polyethylene glycol, E-200, E-300, E-400, E-600, E- 1000, E-9000, and E-20,000, manufactured by the Dow Chemical Company, were used. These have been shown by Tarkow and co-workers [ 121 to have satisfactorily narrow molecular weight distributions.

The polymer must not be an electrolyte.

The polymer molecule must be spherically shaped in solution, and its size must be known. The polyethylene glycols are basically linear or only slightly branched molecules according to the manufacturer. There are some indications that the glycols deviate slightly from a spherical configuration over the molecular range of 200-20,000 [ 131, but this was not considered to be a significant factor. The number average molecular weights of the poly glycols were determined osmometrically and found to agree closely with the manufacturer’s published values shown in Table I. Molecular diameters in acetic acid solution were calculated from hydrodynamic volumes using the Flory relation shown in the footnote in Table I . According to Moore [ 141, hydrodynamic volume correlates closely with pore penetration.

The relationship between the diameter of the polymer and the diameter of the pore which can be penetrated is not simple and straightforward. Permeability has been shown to be a function of the shape of the pore opening [ 141. If the opening is circular, the polymer can penetrate only if the pore is at least four times as large. If the opening resembles parallel slits, the polymer can penetrate a pore which is twice as large or larger. Cellulose is well known to be aligned preferentially in linear structures such as elementary fibrils and microfibrils. Lateral openings or “pores” between and perhaps within these building units can reasonably be expected to assume an elliptical shape. Therefore, the penetration factor for such a shape should be approximately the average of the circular and

2.

3.

4.


TABLE I

Relationship between Polyethylene Glycol Molecular Diameter and Pore Diameter

P E G PORE

MOL. WT. A 0 ++ A 0 PEG DIAMETER, DJAMETER,

2 0 , 0 0 0 130 390

9 , 0 0 0 90 2 70

4,000 54 162

1 , 0 0 0 27 81

600 21 6 3

400 18 5 4

300 16 40

2 00 1 3 3 9

*Calculated from relation: radius = 3.2 ([q] X MW) 1/3. **Based on elliptical pore opening: pore diameter = 3 X molecular diameter.

slit factors, or three. This assumption accounts for the three-fold difference between diameters of the polyethylene glycols and the pores shown in Table I. There are undoubtedly other factors such as molecular conformation and entropy effects which further complicate the relationship between the size of the pore and that of the permeating polymer. The full importance of these factors is difficult to assess in practice, but their net effect is probably to reduce the penetration factor to somewhat less than three. This would suggest that the pore diameters shown in Table I are slightly on the high side.

The use of glacial acetic acid or almost any other volatile organic solvent in the polymer exclusion method presents certain problems with experimental technique which are not encountered when water is used as the solvent. The refractive index of acetic acid is sufficiently different from the index for water such that special precautions to prevent moisture contamination of the acetic acid solutions by the starting pulp, the atmosphere, and the glassware are absolutely necessary. Further precautions are needed to minimize evaporation. An additional problem which is unique to the particular polymer-solvent combination used is the gradual deterioration of polyethylene glycol by acetic acid as evidenced by solution discoloration. Stock solutions must therefore be prepared and used within 2-3 days. The refractive index of glacial acetic acid, like most organic solvents, is extremely sensitive to small variations in temperature. For this reason, measurements of indexes by interferometry were very unsatisfactory in spite of great precautions t o maintain the liquid cell temperature constant within ? 0.02"C. Successful measurements were made with a Brice-Phoenix differential refractometer fitted with a 1 cm cell, giving a

CELLULOSE STRUCTURE 309

reproducibility of +- 5% for the average of three replicates. individual samples of pulp fiber pre-extracted with glacial acetic acid were

equilibrated with 12-15 parts of 2-3% polymer solution by vigorously shaking for a minimum of 4 hours at room temperature. After working up the equilibrated solutions in the usual manner, the pore volume is calculated from the following formula:

weight of inaccessible solvent - A-BD weight of pulp C

where A = weight of original solvent on the pulp, B = weight of polymer solution added t o the pulp, C = oven dry weight of pulp, D = dilution factor.

The dilution factor D is calculated from a calibration curve which relates the instrument reading to the degree of dilution of the stock polymer solution. The calibration curve is constructed by plotting the dilution factor versus the change in instrument reading resulting from dilution of the stock polymer solution with known quantities of glacial acetic acid. For the known, the dilution factor D equals the ratio weight of acetic acid/weight of stock polymer solution. A linear plot is obtained for instrument reading versus the expression 1 -l/(l+D) for three known dilutions of each measuring polymer.

Note that inaccessible solvent is expressed in units of weight rather than volume. This is preferred on the basis that the density of solvent in intimate contact with the cellulose is believed to be indeterminately greater than the density of free solvent [ 151 . Therefore, the actual volume occupied by a certain weight of inaccessible solvent within the fiber is unknown. It is convenient, however, to refer to this weight of solvent as “pore volume.”

Acetylation Reactivity

The adiabatic acetylation rate of pulp fiber was determined as described by Malrn and co-workers 1161. Pulp fiber was pre-extracted with glacial acetic acid to a constant refractive index in the usual manner. The amount of acetic acid in the acetylation solution was reduced to compensate exactly for the acid in the pre-extracted pulp. The calculated rate constant K is the rate at which temperature rises with respect to time during reaction. Since most of the heat evolved is due to esterification and all other conditions are futed, K provides a relative measure of reactivity.

RESULTS AND DISCUSSION

Relationship between Total Pore Volume and Reactivity

The total pore volumes and acetylation rate constants of several pulps are listed in Table 11. Note that the adiabatic rate constant increases regularly with

3 10 NELSON AND OLIVER

TABLE 11

Effect of Pore Volume on Acetylation Reactivity

TOTAL PORE VOLUME ADIABATIC RATE PULP G. HOAC/G.CELLULOSE K X 104

COTTON LINTERS 0 .30 97

SULFATE PINE A 0 . 4 7 118

SULFATE PINE B 0 .48 153

SULFITE SPRUCE 0. 56 160

SULFITE PINE 0 . 5 7 159

SULFATE GUM 0. 68 175

SULFATE PINE C 0. 70 183

SULFATE PINE A 0 . 7 7 (WATER ACTIVATED)

SULFATE PINE A 1 .09 (NEVER DRIED)

SULFATE PINE C 1 .20 (NEVER DRIED)

243

293

366

increasing pore volume. The same data are shown in Figure 1 where reactivity is plotted as a linear function of total pore volume. The two variables are closely correlated as evidenced by a 0.954 correlation coefficient. These data strongly suggest but do not necessarily prove that a cause and effect relationship exists.

Values are given in Table I1 for two of the pulps in the never-dried form and for one pulp after water activation. Although these pore volumes were determined on the fibers in glacial acetic acid, the values fall in the same general range of published values for pulps analyzed in water [9]. This does not mean that the pulps have the same pore volume in both solvents. Quite the contrary, a pulp will usually exhibit a higher pore volume if it is analyzed in the presence of a solvent with good swelling power or if it is properly exchanged from such a solvent and then analyzed. For example, regular dried sulfate pine pulp A has a pore volume of 0.47 g/g in acetic acid, but when the same pulp is first swollen with water and then exchanged to acetic acid, it has 1.6 times as much pore volume or 0.77 g/g.

Relationship between Surface Area and Reactivity

The acetylation of cellulose is well accepted as being a heterogeneous reaction. Therefore, surface area, in addition to pore volume, should be expected to play an important part in the reaction. Huff [17] demonstrated that acetylation reactivity can indeed be correlated with surface area as determined by nitrogen adsorption after solvent exchanging the fibers from acetic acid. The data are plotted in Figure 2 for several pulps. The same worker found, however,

CELLULOSE STRUCTURE 31 1

I

I I b I , L I

o .lo .20 .30 .40 .so .BO .70 .m .w 1.00 1.10 1.20 L

TOTAL P O R E V O L U M E - G R A M S A C E T I C / G R A M C E L L U L O S E

FIG. 1. Adiabatic acetylation rate divided by total pore volume relationship.

200

0 * 180

- 2

160

I- 5 140

k 0" 120

: 100

z

w I-

0, 4. 8 0 m I 4 6 0

l-

0

SULFITE SO. PINE

SULFITE SPRUCE A.

SULFATE sa PINE A

SULFATE SO. PINE B

SULFATE SO. PINE D

SULFATE HARDWOOD

1 I I

I 2 3 4 5 6 7 8

SPECIFIC SURFACE A R E A ( M 2 / O ) A F T E R A C E T I C A C I D SWELLING

FIG. 2. Adiabatic reaction rate versus specific surface area.

31 2 NELSON AND OLIVER

that the correlation suprisingly breaks down when surface areas of 50-200 m2/g are generated in the fiber by activation with water or 80% aqueous acetic acid or by other special means. This lack of correlation at the higher levels of surface area is shown by the data in Table 111 for a single pulp given three different pretreatments.

TABLE 111

Nonlinear Correlation between Surface Area and Reactivity

SURFACE ADIABATIC AREA R A T E

PR E TR EA T ME NT M2/G K x 104

STANDARD 2 . 5 6 60

EXHAUSTIVE HOAC 1 1 . 1 1 1 5 EXTRACTION

WATER ACTIVATION 140 243

The fact that a very large surface area does not assure a proportionately high reactivity can be explained on the basis of the manner in whch surface area is distributed relative to pore size. Sommers has shown that the surface area bound by pores in cellulose fibers is inversely related to the pore diameter [18]. This relationship is illustrated by the curve in Figure 3 for a hypothetical fiber with pore diameters up to 400 A. The surface area of the 100 A pores is assigned a value of So as a matter of convenience. Pores with larger diameters have relative surface areas which are fractions of S o , and those with smaller diameters have relative areas which are multiples of So. The curve is essentially an accessibility profile for the hypothetical fiber, showing the relative surface area available to a penetrating molecule of a given size. If the molecular diameters of the acetylation reagents are calculated from bond radii and these values are increased by a pore penetration factor of three as discussed under Experimental, it is estimated that the reagents cannot penetrate pores less than 20-25 A in diameter. This means that the surface area associated with pores less than 20-25 A is unavailable for reaction. Since the nitrogen molecule has a diameter of only 3.6 A, it can penetrate pores down to about 10-1 1 A having surface areas which in the case of the hypothetical fiber are 8-10 times as large as the reference value of So for the 100 A pores. It is reasonable to suggest, therefore, that the major portion of the total surface area generated in real fibers by activation with water and other such means must be attributed to pores with diameters which are too small to be penetrated by the reagents but large enough to admit the nitrogen molecule. In other words, the correlation between surface area and reactivity should be linear if allowances are made for the fraction of total surface area which is inaccessible to the reagents. According to this reasoning, approximately 120 M2/g or 86% of the surface area listed in Table I11 for the water activated pulp is unavailable for reaction.


f

a , a a

z

W

W V

a 3 u)

W

c -1 W

z a

a

40 80 120 160 200 24.0 280 320 360 400 PORE DIAMETER, A

FIG. 3. Relative surface area versus pore diameter for hypothetical fiber assuming a fixed pore shape.

Relationship between Pore Sue Distribution and Reactivity

There is nothing to suggest from the preceding discussion that pores of a particular size larger than the 20-25 A minimum are any more influential in promoting reactivity than pores of another size. Whether this is actually the case can only be determined by comparing pore size distributions of various pulps which are known to differ in their reactivity behavior. Accordingly, pore size distributions were determined for a high purity dissolving pulp prepared from Southern pine by the prehydrolyzed sulfate process with a cold caustic extraction in the bleaching sequence. Four variations of the same basic pulp


were investigated: never-dried, regular dried, regular dried, and then activated with 80% aqueous acetic acid, and regular dried, dampened with glacial acetic acid and then mechanically refined in a manner similar to the one described by Holloway and co-workers [ 191 and Geurden [20].

The experimentally determined values for the pore volumes are shown in Table IV along with the corresponding pore diameters. The data were plotted to give a cumulative pore volume distribution for each of the four pulp treatments. Smooth curves were drawn and differentiated to give the differential distributions shown in Figure 4. Any point on these curves represents the

TABLE IV

Cumulative Pore Size Distributions for Four Treatments of Prehydrolyzed Southern Sulfate Pine Pulp

POW DIAMETER N E W I I I): lIEJ REGULAR DHIGI] AQUEOUS ACTIVArED A CID REFINLD

3 Y O I; 1.09 q/q 0.49 g/g 0.60 g/g 0.56 g/g

2 7 0 0.86 0.47 0.55 0.52

16 2 0.73 0.34 0.40

01 0.50 0.22 0.33 0.46

63 0.31 0.19 0.16 0.31

54 .). 16 J.16 0.19

4 0 0.10 0.08 0.09

3 9 0.09 0.11 0.06 0.05

15.0-

10.0-

AV 103 APD

5.0-

0 I00 150 200 2%

P O R E D I A M E T E R , A*

FIG. 4. Pore size distribution curves. Sulfate southern pine. - - - Acid mechanical r e fined. - . - . - Never dried. . . . . . Aqueous acetic activated.- Standard.


volume of pores of a given diameter. The area underneath each curve in Figure 4 is equal to the total volume of pores ranging in diameter from 400 A down to about 20 A, below which the acetic acid molecule cannot penetrate. Therefore, the distribution curves at pore diameters less than 20 A may or may not have the configuration shown in Figure 4. From the discussion above, the area beneath each curve must also be equal to the pore volume values listed in Table IV for the 20,000 M.W. polyethylene glycol.

The never-dried pulp, represented in Figure 4 by the dot-dash curve, has excellent acetylation properties including reactivity and filterability and clarity of acetone solutions. This pulp is distinguished by a sharp peak between 25 and 75 A or about 50 A with a long, drawn out tail in the region of larger pore sizes. Machine drying the pulp to 743% moisture results in a severe deterioration of acetylation properties. As shown by the solid curve, the distribution of pores is changed remarkably by drying. The peak at 50 A has disappeared, apparently as the result of collapse or shrinkage of pores with diameters of 25 A and larger. This is in close agreement with Stone and Scallan’s observations on drying paper pulp [21]. When the machine dried or standard pulp is slurried in 80% aqueous acetic acid to simulate certain commercial activations, acetylation properties are substantially improved but not to the same level as those of the never-dried pulp. As indicated by the dotted curve, the peak at 50 A is only partially restored by the activation step while a large share of the volume of the larger pores is restored. When the standard pulp is dampened with about one-third part of glacial acetic acid and subjected to a rigorous mechanical shredding, the acetylation properties compare very favorably with those of aqueous acetic acid activated pulp. Acetylation pretreatment by mechanical refining pulp in the presence of acetic acid has been studied extensively and found to have potentially important practical advantages over most other methods [22]. According to the dashed curve, the peak at 50 A is fully recovered. There are indications of a slight decrease in the volume of pores with diameters above 75 A. If the decrease is real, it could result from a mechanical compaction of the larger pores to smaller sizes. It could also result from a disorganization of structures involving the larger pores or an upward shift in the distribution.

These comparisons suggest that a general relationship exists between reactivity and pores having diameters between 25 and 75 A, with an average of about 50 A. A pulp with good acetylation properties is distinguished by a high volume of pores in this range. Pores below this range do not likely affect reactivity to any significant degree because of their inability to be penetrated by the reactants. Pores above this range certainly must participate in the reaction, but their overall contribution to reactivity cannot be appreciable. This is evidenced by the fact that the standard pulp is shown in Figure 4 to have a higher volume of pores above 75 A than the acid-mechanical refined pulp, but yet the standard pulp does not acetylate nearly as well. To eliminate the possibility that the indicated importance of pores in the 27-75 A range was simply a coincidence of using the same base pulp in these comparisons, the pore size distributions shown in Figure 5 were determined for three other acetate wood pulps. These pulps exhibit the same characteristic peak at about 50 A, and


0 50 100 150 200 250 300 550 400

PORE D I A M E T E R , A'

FIG. 5 . Pore size distribution curves. - - - Sulfite spruce. . . . . . . Sulfate hardwood. - Sulfite Southern pine.

their acetylation properties are close to what would be predicted on the basis of the performance of the four pulps described in Figure 4.

Since total pore volume is shown in Figure 1 to correlate closely with reactivity, and since the total volume of pores in the 25-75 A range may be particularly influential on reactivity, the possibility exists that the total pore volume may be simply another measure of the effective pores, i.e., those having a diameter of 25-75 A. In other words, the total pore volume may be more of a reflection of the 25-75 A pores than of the pores on either side of this range, at least as far as their effect on reactivity is concerned. The volume of pores in the 0-25, 25-75, and 75-400 A size classes was determined from the corresponding areas underneath the pore size distribution curves. These values are listed in Table V along with the total pore volumes and adiabatic rate constants for the various pulps. Reactivity seems to improve as the volume of the 25-75 A pores increases up to a level of 0.40-0.45 g/g. There is no clear-cut indication, on the other hand, of a relationship between pores on either side of this range and reactivity.

Reactivity has been related in the preceding discussion to the total pore volume, the volume of pores between 25 and 75 A and the surface area of pores larger than 20-25 A. The relationship between reactivity and these structural characteristics must involve the cellulose fine structure and the manner in which pores and associated surface areas are distributed throughout the cellulose matrix. The dry fiber cell wall appears to be a nonporous solid since it has a specific surface area of about 1 M2/g corresponding closely with the external fiber surface. Therefore, the void spaces within the cell wall can be no larger than 10-1 1 A which is about the minimum size opening that can be penetrated


TABLE V

Relationship between Pore Size, Total Pore Volume, and Reactivity

TOTAL PORE ADIABATIC ~~ ~~ -

VOLUME OF PORES BETWEEN: VOLUME RATE PULP VARIATION 0-25 A 25-75 1 75-400 G. IG. K x 104

SULFATE PINE. 0 . 0 5 0. 1 3 0 . 2 9 0.47 118 STANDARD

SULFITE PINE 0. 0 3 0 . 2 4 0. 30 0.57 127

SULFITE SPRUCE 0. 03 0.43 0.10 0. 56 160

SULFATE PINE, 0.02 0 .40 0. 16 0 .58 165 ACID MECH. REFINED

SULFATE HARDWOOD 0.07 0. 41 0.20 0 .68 175

SULFATE PINE, 0.04 0 . 4 5 0 .60 1.09 293 NEVER DRIED

by the nitrogen molecule. This value of 10-11 A must then be the maximum diameter of voids within and between the fundamental cellulose structural building units of the dry fiber cell wall as indicated in Table VI. The water swollen fiber cell wall, in contrast, is a highly porous gel having a specific surface area of 100-200 M2/g after solvent exchanging. Not all of the pores and surface area generated by water swelling the fiber are caused by rearrangements between the three structural building units. The structural units themselves expand more or less with an attending creation of pores and internal surfaces within them. The size of these pores in water swollen fibers has been measured by a number of investigators using a variety of methods and species [7, 11, 231, and the ranges of their values are given in Table VI. Acetic acid should be expected to swell cellulose, although less than water, and create a definite system of pores and surface area. The third set of values in Table VI are estimates of the average diameter of pores in acetic acid swollen fiber based on the dry and water swollen values.

If one postulates that reaction can occur only on the surfaces of those pores which are penetrated by the reactants, the basis for the relationship between surface area of pores larger than 20-25 A and reactivity is immediately apparent from the values in Table VI for acetic acid swollen fiber. Reaction can occur on the external surfaces of all three building units and on the internal surfaces of the microfibirils and the lamellae, but not on the surfaces of the elementary fibrils.

A pulp with good acetylation properties has a large volume of the 25-75 A pores, but these same pores almost certainly have to be as uniformly distributed throughout the cellulose matrix as the natural cell wall structure will allow. Whether a given pulp will have these porosity characteristics available at the time of reaction seems to depend on the particular combination of the wood species and the process used in pulp manufacture. In the case of certain pulps, these characteristics are dormant and can only be vitalized by special pretreatment


TABLE VI

Estimated Diameter of Pores within and between Cellulose Structural Building Units

ACETIC ACID DRY WATER SWOLLEN SWOLLEN

WITHIN BETWEEN WITHIN BETWEEN wrr HIN BETWEEN

ELEMENTARY FIBRIL 10-11; l o - l l d 12-15; 1 5 - 5 0 1 13; 3511

MICROFIBRIL 10-11 1 0 - 1 1 15-50 100+ 35 50+

LAMELLA 10-11 1 0 - 1 1 1 5 - 5 0 1 5 - 3 0 0 35 loo+

methods before the acetylation reaction. From the discussion thus far, cellulose structure appears to govern reactivity

by determining the inward flow of reagents to reaction surfaces. The same reasoning can be used to show that the outward transport of the cellulose triacetate chains into the reaction medium cannot be a rate determining factor. It is only necessary to recognize that the largest pore in the fiber cell wall having a diameter of about 400 A is many times smaller in diameter than the average size cellulose triacetate molecule and could not possibly function as a passage- way. This situation presents an anomaly, at least under conditions of zero or very low shear rate during the reaction. If the product does not dissolve as it is formed [24], how do the reagents reach the unreacted cellulose chains under- lying the surfaces? It is suggested that the chain segments of the product expand into the void space immediately adjacent to the reaction surface and thereby create a new void into which reagents can penetrate for reaction on the freshly exposed surface. This sequential process is repeated until all of the cellulose chains have been reacted, and the gel swelling pressure eventually disintegrates the vestigial fiber structure. It is reasonable to believe that such a process of stepwise penetration plus accommodation of inward swelling would utilize the immediately available space within the fiber cell wall. This space could be either the 25-75 A pores or the total pore volume.

Relationship between Reactivity and Other Structural Factors

The present work addresses itself to the effect of porosity and surface area on reactivity independently of the gross structure of individual fibers. Yet, substantial reactivity differences may be expected to exist with respect to soft- woods and hardwoods, springwood and summerwood, normal wood and reaction wood, fibers and parenchyma cells, and even with respect to the


position in the tree. Bell and Kuiken [25], for example, have demonstrated that the “node” regions acetylate much faster and perhaps more uniformly than those other portions of the fiber wherein structure has not been mechanically disrupted. The radial distribution of pores and internal surface area across the cell wall could be another factor influencing reaction behavior of individual fibers. This should be a fruitful area of research when the necessary experimental techniques and tools are developed.

Application to Other Cellulose Reactions

The validity of the concept of characterizing the fiber within the reaction environment to develop meaningful relationships between cellulose structure and reactivity seems to be firmly established by the experimental results of the present work. Exploratory work in our laboratories indicates that the concept can also be applied to study the influence of structure on nitration, etherifica- tion, xanthation, and rayon biodegradability.

SUMMARY

Acetylation reactivity has been correlated with microporosity of acetic acid swollen fibers which approximates their condition in the reaction environment. Pores with diameters ranging from 25 to 75 A seem to have special importance. These can probably be identified as void spaces within the microfibrils and the lamellae, as well as between the elementary fibrils, the microfibrils and the lamellae. These particular pores appear to control the inward flow of reagents to reaction sites but not the outward transport of products into the reaction medium.

Acetylation reactivity has also been correlated with the surface area of pores larger than 20-25 A, which is estimated to be the minimum pore diameter for reagent penetration. There is probably an optimum average surface area: volume ratio for a pulp to perform at its best in acetylation, but it also seems necessary for the pores participating in the reaction to be uniformly distributed throughout the cellulose matrix.

Whether a given pulp possesses the required porosity-surface area characteristics for good reactivity seems to depend on the wood species and process used in pulp manufacture. In certain cases, these structural characteristics are dormant and can only be vitalized by special treatments before reaction.

Acknowledgment

The authors are grateful to Dr. B. J. L. Huff for permission to publish the surface area data, to Dr. D. F. Durso and Mr. J. C. Williams for their many helpful suggestions during the course of the work, and to Mr. J. B. Coker for his assistance in the laboratory.

REFERENCES

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2. H. M. Spurlin, in Cellulose and Cellulose Derivatives, E. Ott, H. M. Spurlin, and M. W. Grafflin, Eds., High Polymer Series, Vol. V, Part 11, 2nd ed., Interscience, New York, 1954, pp. 673-712.

3. M. A. Ivanov and I. A. Strelyugina, Z. Prikladnoi Khim’i, 43, No. 7, 1548 (1970). 4. N. I. Klenkova, Z. Prikladnoi Khimii, 36, No. 4,836 (1963). 5. N. I. Klenkova and G. P. Ivashkin, Z. Prikladnoi Khimii, 36, No. 2, 398 (1963). 6. J. E. Stone, Pulp Paper Mag. Can., 65, 3 (1964). 7. J. E. Stone and A. M. Scallan, h l p Paper Mag. Can., 66,407 (1965). 8. J. E. Stone, A. M. Scallan, E. Donefer, and E. Ahlgren, Pulp Paper Res. Inst. Can., PPR.

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17. B. J. L. Huff, unpublished results. 18. R. A. Sornmers, Tappi, 46,562 (1963). 19. J. H. Holloway, R. S . Tabke, and A. N. Parrett, US. 2,415,949 and U.S. 2,546,749. 20. J. Geurden, IUPAC 1966 Dissolving Pulps Symposium, Helsinki, Roceedings, Butter-

21. J. E. Stone, A. M. Scallan, and B. Abrahamson, Svensk Papperstidn., 71,687 (1968). 22. R. Nelson and J. C. Williams, to be published. 23. a. A. Frey-Wyssling, Protoplasma, 27, 372 (1937).

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b. A. N. J. Heyn, in The Physics and Chemistry of Wood Pulp Fibers, D. H. Page, Ed., TAPPI STAP No. 8, New York, 1970, pp. 27-49. c. G. W. Davis, Appita, 21, 117 (1968).

b. K. Kanamaru, Helv. Chem. Acta, 17, 1436 (1934). c. Also see ref. No. 2, pp. 698-700 and H. M. Spurlin, Trans. Electrochem. Soc., 73, 95 (1938).

24. a. K. H. Staudinger and M. Staudinger, Makromol. Chem., 9, 148 (1953).

25. W. R. Bell and K. A. Kuiken, unpublished results.

study of cellulose structure and its relation to reactivity

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