Reference

NBS

Publications

82-2534

Deductions about the Morphology

of Wet and Wet Beaten

Celluloses from Solid State

NMR

NAT'L INST. OF STAND & TECH

AlllDS

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymer Science and Standards Division

Washington, DC 20234

Final Report Covering the Period

October 1, 1980 through September 30, 1981

Issued June 1 982

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1982

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. Department of Treasury

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NBSIR 82-2534

DEDUCTIONS ABOUT THE MORPHOLOGY

OF WET AND WET BEATEN

CELLULOSES FROM SOLID STATE

NMR

D. L. VanderHart

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymer Science and Standards Division

Washington, DC 20234

Final Report Covering the Period

October 1, 1980 through September 30, 1981

Issued June 1 982

Note: This document has been prepared for the use of the

Bureau of Engraving and Printing, responsibility

for its further use rests with that agency.

Prepared for Bureau of Engraving and Printing

U.S. Department of Treasury

Washington, DC 20401

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

TABLE OF CONTENTS

Page

Introduction 1

Experimental 7

Results 9

Discussion of the NMR Results 13

Conclusion 21

References 23

Table 24

Figures 25

I. Introduction

- 1 -

A. NMR Background - The Experiment

In late 1972 and early 1973 papers appeared [1,2] describing the combined

use of high power proton decoupling (HPPD) and cross-polarization (CP) techniques

13 in order to observe C spectra in organic solids. Resulting spectra were much

narrower than would otherwise be observed since HPPD eliminated the strong

1 13

'^C dipolar interactions. The CP techniques, which were used to generate

13 13

-^-’C magnetization indirectly via the protons, resulted in a C signal enhancement

13 circumventing the problem of large C relaxation time dispersions. That is to

say CP techniques implied a substantial gain in sensitivity. But in return for

13 the advantage, it was no longer necessarily true that relative C intensities

would be proportional to the corresponding populations of different kinds of

carbons. Although this assumption often held in CP spectra, there were exceptions.

Morphologically inhomogeneous systems (particularly those possessing molecules

with motions in the mid-kilohertz regime [3], or those containing rubbery domains

with highly mobile molecules) constituted some of these exceptions.

The -"C spectra obtained by HPPD and CP still have strongly overlapping

resonances associated with the anisotropic chemical shift (CS) interaction, which

in turn is the result of the orientation dependence of the electronic shielding

13 of the C nucleus. To eliminate this anisotropic CS interaction, it was

recognized [4-7] that so-called magic angle sample spinning (MAS), if fast

enough, could collapse each of these broad lines to a single line located at

the isotropic chemical shift position. What is meant by "fast enough" depends

on the strength of the magnetic field, Bq, and on the size of the anistropic C3.
- 2 -

For example, in cellulose, a 1 KHz spinning rate would be more than sufficient

at a magnetic field strength of 1 . 4T (T=Tesla) , whereas 3 KHz would be an ample

frequency at 4.7T. The term "magic angle" refers to the angle between the

spinning axis and and is defined to be arccos (/J) ^=54.7°. Experimentally it is

important to maintain the accuracy of this magic angle setting to within 0.1°.

Spectra obtained with the combined use of CP, HPPD, and MAS (CP-MAS spectra)

13 bear a strong relationship to proton-decoupled liquid state C spectra. In

13 both kinds of spectra C resonances appear at their isotropic chemical shift

positions. Nevertheless a substantial difference between solid-state and

13 liquid state spectra must be recognized. In liquids every C nucleus of a

given kind is likely to have the same average history of rotational motion and

intermolecular interactions over the time scales(~10ms-ls) of inverse linewidths.

In contrast, in solids, on time scales (3-lOOms) of inverse linewidtHs,

molecules are often confined to particular orientations, conformations, or

hydrogen bonding geometries. They may also be limited by aniS'otropic molecular

motion, and/or reside in only one region of a morphologically inhomogeneous

T system. These different environments produce a dispersion of isotropic CS s 13

for a particTilar kind of carbon [3]. Thus, in solids, C linewidths are

13 generally broader than those observed in liquid-state C NMR spectra. A

systematic analysis of C linewidth in CP-MAS spectra has been published [3]

and provides a basis for subsequent interpretation of cellulose spectra.

13

B. NMR Background - General Cellulose C NMR Spectra and Interpretation

13

A few studies applying CP-MAS C NIIR techniques to cellulose have appeared

in the literature [8-10]. In Figure 1, CP-MAS spectra of four different
preparations of cellulose are shown, illustrating the sensitivity of the

isotropic CS positions to differences in chain environment. Samples

corresponding to Figures lA and 1C are highly crystalline forms of cellulose I

and II respectively. Spectrum IB belongs to cotton which is a rather highly

crystalline form of Cellulose I, except that the lateral dimensions of the

cellulose crystallites are generally thought to be 3-5 times smaller in

Sample B than in Sample A [10-12]. Finally, Sample D is a ball-milled sample

of a Cellulose I material having a CP-MAS spectrum very similar to B.

13 Assignments of resonance regions to particular C sites in the glucose

unit have been made [8,9]. The usual numbering scheme is used to designate

the six different carbons, i.e.

The correspondences between carbons and resonance positions (relative to

liquid tetramethylsilane) in these spectra are approximately: C6958-68ppm)

,

C2,C3,C5 (68-79ppm) , C4(77-91 ppm), and Cl (96-109ppm) . A small chip of linear

polyethylene has been added to each of these samples to serve as a secondary

chemical shift reference. Its crystalline resonance position is 33.63ppm
- 4 -

From the spectra of Figure 1 it appears that spectral resolution is

degraded significantly for the amorphous cellulose sample. This result is

expected [3]. Secondly, a given carbon, say Cl of Spectrum 1C, can give

rise to a multiplicity of sharp lines, thereby indicating magnetic

inequivalence within the unit cell. Thirdly, in the cotton spectrum, (13),

large broad wings are associated with the C4 and C6 resonances. These wings

constitute approximately 40-50% of the total intensities of these resonances,

which is much higher than any x-ray estimates of amorphous cellulose in this

material [10].

13 C Based on C transverse relaxation time (T

2

) measurements performed in

this laboratory, some of which have been published [9], the dominant broadening

in the spectra of Figure 1 is dispersion of isotropic OS's. Relaxation, at

most, accounts for 0.5 ppm of broadening, except in the region of

the C6 wing (~63 ppm). For this wing, relaxation broadening may provide up

to 1 ppm of broadening.

Broadening of resonance lines, possibly accompanied by CS displacements

such as are seen in the C4 and C6 resonances of cotton, imply a dispersion of

cellulose chain environments. However, we do not feel justified in interpreting

the observed shifts in terms of specific models of, say, hydrogen bonding

changes, distortions of bond angles, or changes in crystal packing or chain

conformation. Rather, the strongest argument in interpreting the C4 and C6

wings in cotton has been made [10] based on the CP-MAS spectra of a series of

native cellulose I samples having increasingly greater lateral crystalline

perfection as judged by X-ray and electron microscopy studies. The series

chosen was cotton, the pellicle of the bacterium, Acetobacter xylinum , and
5

the cell wall of the alga, Valonia ventricosa. Spectra A and B of Figure 1

correspond to the extremae in this series. In Spectrum lA, the wings of

C4 and C6, which are so prominent in Spectrum IB, have almost vanished.

Thus, it was concluded [10] that the wings of C4 and C6, appearing at

approximately 85 and 63 ppm respectively, were associated with cellulose

chains on the lateral crystal surfaces. In cotton, where so-called

elementary fibrils of approximately 3.5 nm lateral dimension are often

observed, nearly half of the cellulose chains are surface chains, consistent

with the observed wing intensities for C4 and C6. Thus NMR appears to be

an excellent tool for probing the integrity of the elementary fibril and/or

the lateral surface area of the cellulose crystallites. This is the

important observation which forms the basis for the XMR project reported

herein.

Three final points can be made based on the spettra of Figure 1. First,

in Spectrum lA, resonances of Cl and C4 show emerging multiplicity (The

13 multiplicity is more convincing for C spectra taken at 50 MHz) , which

implies that the unit cell of cellulose I contains at least 4 glucose moieties

[10] . Secondly, the shape of the resonances for Cl and C4 in the amorphous

sample (Spectrum ID) suggests that in a sample of cotton (Spectrum IB), there

is, at most, 20% of the cellulose which is amorphous in the sense of Spectrum

ID. Finally, one can easily distinguish between cellulose I and cellulose II,

particularly in the Cl and C6 regions.

C. Cellulose background - Definition of the cellulose problem to be

13 investigated by C NMR.

Wetting of cellulosic materials followed by a process called wet beating,

produces fibers which achieve a plasticity suitable for making paper. Without
- 6 -

wet beating, the fibers are too stiff and consequently do not bond to one

another sufficiently to make a strong paper.

In a previous report [14], various cellulosic preparations were wetted

and then beaten. Two materials in particular were investigated, Norway

Spruce Kraft -pulp (NSKP) and purified cotton (PC) . The results of this

report, in which fiber saturation points and bound (non-freezing) water

fractions were measured, demonstrated a significant difference between the

NSKP and the cotton. In NSKP, wet beating did not increase the fraction of

bound water whereas it increased by 55% the amount of free water. (Free

water plus bound water equals the amount of water measured by the fiber

saturation point method.) In contrast, in cotton wet beating increased the

amount of bound water by 46%; at the same time, the free water doubled.

Although the absolute amount of bound and free water in NSKP was always

larger than in cotton, it was an intriguing question whether the increase in

bound water observed in cotton with wet beating could be related to an increase

in the surface a^ea of the elementary fibrils. The bound water, in order to

be non-freezing, should interact strongly with the cellulose chains.

13 Therefore the ob j ective of the C NMR studies in FY 81 was to look for

spectral changes between wet and wet beaten cellulose preparations as a

monitor of changes in crystal surface areas accessible to water.

Three cellulose samples were chosen for this study, purified cotton,

NSKP, and microcrystalline cellulose, the latter being a highly crystalline

form of cellulose. The former two materials are commonly used in paper production
II. Experimental

A. NMR Parameters

CP-MAS spectra were obtained on a homebuilt spectrometer. The magnetic

13

field strength is 1.4T; the corresponding C radiofrequency (rf) is 15.08 MHz.

Cross-polarization was performed via the spin-lock method [2]. Amplitudes

of the rf field for both protons and carbons [15] corresponded to nuclear

precession frequencies of 60 KHz. Contact times for cross-polarization were

13 typically 2 ms. Proton decoupling amplitudes during C signal observation

13

were maintained at 60 ICHz. The C free induction decays were Fourier

transformed to obtain the spectra. Signal transients were accumulated in a

computer using the sequence [16] for eliminating artifacts. Under these

conditions, no signals from the rotor material, polychlorotrifluoroethylene,

are present.

For the wet and wet beaten cellulose samples, a specially designed rotor was

used. This rotor assembly, which provided a 6.8 mm diameter sample chamber, included

13 an 0-ring seal for containing the water. The seal produced no interfering C

resonances. In some cases, tiie original amount of water was partially depleted during

the several hours of spinning required for taking the spectra. In every case, the

samples were unloaded, weighed, dried in a vacuum oven overnight at 100°C, and reweighed in order to determine the final moisture contents for the wet samples. It

might be expected that MAS would act as a centrifuge so that during spinning,

the actual water content of the observed wet cellulosic materials would be less

than that measured afterward. This possibility cannot be dismissed in principle;

however, only in the case of wet beaten microcrystalline cellulose was there a

noticeable radial material gradient when the sample was opened after spinning.

It was in a large part concern about this centrifugation which directed us to

perform all of these experiments at 1.4T even though a spectrometer operating
- 8 -

at 4.7 T was available. At 1.4T, rotation frequencies for the wet sample

were in the range of 1500-1800 Hz. A small chip of linear polyethylene (LPE)

was added to most samples as a secondary chemical shift reference. The

sharp feature in its resonance arises from the crystalline regions which

have a chemical shift relative to tetramethyl silane (TMS) of 33.63 ppm [13].

The linewidth of this reference ranged from 8-13 Hz (.53-. 87 ppm). Variation

in LPE linewidth is probably due to magnetic field drift and/or a slight

misadjustment of the magic angle (less than 0.35°). Precise adjustment of

the magic angle presented some problem because the wet samples spun with

greater difficulty compared to the dry samples.

B. Materials and Their Preparation

Purified cotton (PC) was received from Southern Regional Research

Center. The cotton was grown in the Mississippi Delta region (Delta Pine,

smooth leaf variety). The cotton was purified by a modification of the Conrad

method [17] with the addition of an 8 hour boil in 1% NaOH under atmosphere,

and was washed successively with distilled water, 1% acetic acid. 1% NH^ OH

and finally distilled water. It was then dried in a vacuum oven at 60C. The

white, finely-powered microcrystalline cellulose (MC) is a commercial product,

PH-101, of the EMC Corporation [18]. It is considered to be a highly

crystalline form of cellulose I. The Norway Spruce kraft pulp (NSKP) was

obtained from a single log of Norway Spruce. The spruce was pulped to a yield

of 47.5% and a Kappa No. of 41.8. All of the above samples of cellulose were

stored in a freezer prior to use. The storage containers were polyethylene

bags inside of sealed metal cans.
Other materials are mentioned in this study although they have not

been studied in connection with wet beating. Spectra of two amorphous,

ball-milled cellulose specimens are shown. One specimen [19] (cf. Figure 2D)

was obtained by ball-milling the commercial microcrystalline cellulose,

Sigmacel (American Viscose) [18], for 72 hours under argon. A second sample

[20] of ball-milled cellulose (cf. Figure ID) was prepared from Whatman

CFl powder and was preserved in a dessicated environment until after the

spectrum was run. The final sample (cf. Figure 1C) is a highly crystalline,

special preparation of cellulose II [20]. It is a low DP material regenerated

from Whatman CFl powder. This material should not be regarded as a typical

commercial regenerated cellulose II material, since the latter materials

have much less definition in their NMR spectrum.

Beating was done in a laboratory mill at a 10% consistency with distilled

water. The relative velocity of roll to bedplate was 6 m/s at a force of 3.4

Kg. The beating cycle was 10000 revolutions for PC and nSKP and 5000 revolutions

for MC.

III. Results

A. General Spectral Observations About the Dry Celluloses

Figure 2 shows CP-MAS spectra of the original materials, equilibrated

at room temperature and a relative humidity of approximately 30%. The

materials are identified on the Figure. The lower spectrum of the ball-milled

cellulose is shown for comparison. Note the substantial intensity at 80 and

100 ppm in the amorphous cellulose spectrum. If one argues that the three

upper spectra are first of all a combination of crystalline and amorphous

cellulose lineshapes, then, if the lower spectrum is taken as the definition

of the amorphous lineshape, upper limits on the amorphous content of the three
- 10 -

celluloses may be obtained by spectral subtraction. Such subtraction yields

upper limits of 0.20, 0.15, and 0.30 respectively for the amorphous fraction

of cellulose in PC, MC and NSKP. In all cases, further subtraction causes

the region near 100 ppm to go negative. Moreover, after subtraction substantial

wing intensities remain at 85 and 63 ppm which are associated with the C4 and

C6 carbons respectively. The persistence of these wings even after subtraction

of the amorphous lineshape fraction is further support for the interpretation

that these wings originate primarily from glucose moieties on the surfaces of

the cellulose crystallites.

In Figures 3 and 4 comparisons are made between the spectra of PC and

MC (Figure 3) and PC and NSKP (Figure 4) . The top three spectra of Figure 2

are reproduced in these Figures along with difference spectra which illustrate

variations in the starting materials. After normalizing spectra to the same

total intensities, differences are taken (those marked 'A-B’) as well as

linear combinations of the top two spectra. The linear combinations

approximate the "pure component spectra" assuming that each spectrum is a

different linear combination of the same two lineshapes. The 'A-rich'

(Figures 3D,4C) linear combination spectra generally resemble one another

and may approximate the spectrum of large cellulose crystals. If that is

true, these spectra should also resemble the Valonia spectrum (Figure lA)

That agreement is pretty good with the notable exception that the splitting of

the Cl resonance at 105ppm in the 'A-rich' spectra of Figures 3 and 4 is about

twice that observed in Valonia. The 'B-rich' linear combination spectra

(Figures 3E and 4D) crudely resemble the amorphous spectra (Figure ID and 2D)
11 -

A notable region of difference is the C2,3,5 region (68-79 ppm) which is

not split in the amorphous spectrum but has well-defined structure in the

linear combination spectra. The linear combination spectra, therefore,

suggest either that the cellulose lineshapes are made up of more than two

component lineshapes or that mixing of the components is on such an intimate

level that the component lineshapes do not apply. A three component system

may indeed be indicated, i.e. amorphous, interior crystalline, and surface

crystalline chains. The validity of this suggestion is discussed in the

Discussion section.

The population of glucose moieties found in the interior of cellulose

crystallities can be ranked from Figures 3C and 4E. Since the sharper

features of the C4 and C6 resonances, at 89 and 64 ppm respectively, are

believed [10] to measure the number of interior crystalline glucose moieties,

it can be concluded that PC has slightly more interior crystalline chains

than MC which, in turn has more than NSKP. Moreover, if the numbers for the

maximum amorphous fraction, given earlier in this section, are assumed »

correct, it can also be concluded that the ratio of surface chains to

interior chains in the cellulose crystallities increases in the same order, i.e.

PC, MC, and NSKP. In other words, this would be the order of descending

lateral crystallite dimensions.

A final remark is that the NSKP exhibits broad weak resonaces (not shown)

which are just visible, particularly in the aromatic region (110-155 ppm).

These signals are typical of lignins [21] thereby indicating that such

residues remain after the pulping process. Crude intensity estimates suggest

a lignin content of 10% in NSKP. However, for purposes of spectral subraction
12

in Figure 4, the NSKP spectrum was normalized based on the cellulose

intensity fraction. Morphologically, the presence of the lignin probably

increases the complexity of the cellulose spectra, particularly if lignin

is a molecular dispersion as opposed to an aggregate. The presence of

dispersed lignin could easily account for an increased surface to volume

ratio in NSKP crystallites relative to PC and MC.

B. Spectral Effects Upon Wetting and Wet Beating Celluloses

Figures 5,6, and 7 show CP-MAS spectra of the PC, MC, and NSKP

respectively. Each figure consists of five spectra which correspond to

the wet beaten sample (A) , the sample wet with distilled water (B) , the

dry sample (C) , and two difference spectra (D=A-B and E=B-C) . In Table I,

data corresponding to these spectra are given. Included in these data are

the final weight ratios of water to cellulose, following the CP-MAS runs.

Similar weight ratios defining the previously measured [14] fiber saturation

points and bound water fractions are also given. Included are the LPE

reference linewidths and the percentages of the observed intensity relative to

13 the total expected C intensity (based on cellulose weight)

The weight ratios of water to cellulose are somewhat erratic due to the

fact that the rotors were not uniformly leak-proof. The wet beaten MC sample

was the least hydrated after the CP-MAS run. That was also the sample where

the cellulose had centrifuged to the outer walls of the rotor cavity. The

fiber saturation point, which measures[14] the sum of the "bound” (non-freezing)

and 'free’ (freezing) water absorbed by celluloses, corresponds to the ideal

minimum weight ratio of H2 O to cellulose for the CP-MAS experiments. This

weight ratio was not always exceeded. However, no final weight ratio was
- 13 -

lower than the bound water fraction. Although the level of hydration is

rather erratic no attempt was made to redo these experiments since the

results, to be discussed, show no spectral evidence of free water content.

Spectra 5D, 6D, and 7D indicate that there are no significant changes

in crystal surface area in going from wet to wet beaten cellulose preparations

Indeed, within the digital resolution of 0.16 ppm, there is virtually no

change. The largest spectral changes are observed when one goes from dry to

wet cellulose (cf. Spectra 5E, 6E, and 7E) . The principal effects here

consist of line sharpening. In particular, the tail of the C2,3,5 resonance

line near 76-78 ppm is pulled into the main body of this line. It is

difficult to say whether the fraction of surface crystalline chains, i.e.

whether the total intensities of the wings of CA and C6, have diminished

slightly upon wetting since the original lineshapes cannot be deconvoluted

exactly. The main conclusion, however, is that 'wet beating produces no

observable spectral changes with respect to the wet cellulose preparations.

IV. Discussion of the NMR Results

A. Intensity Anomalies in CP-MAS Spectra

In terms of the stated objective of this study, the most significant

conclusion of this work is that wet beating does not change crystallite

structure, judging by the near superposition of the wet and wet beaten

spectra of each of the celluloses studied. There is a problem, however,

which needs to be addressed before this conclusion can be stated unambiguously

The problem is that the CP-MAS spectra may possess distorted relative

intensities

Table I gives the ratios of the observed to expected intensities in

the CP-MAS experiments. A uniform cross polarization time of 2 ms was used in
- 14 -

all of these spectra. The value of unity was assigned to the dry

cotton spectrum in order to establish a signal strength psr unit weight of

cellulose. This signal strength compared very favorably with that of LPE

taking into account the different carbon densities in the two materials and

the weaker intensity of the non-crystalline signal in LPE. Therefore, within

experimental error in the dry cotton, all of the carbons are observed with

minor, if any, intensity distortion.

The intensity ratios of Table I indicate that many samples yielded

intensities less than unity which raises the possibility that relative

intensities are distorted or signals are even missing. The question of

intensities in CP-MAS spectra is a broad question and only a qualitative

presentation will be given, starting with a brief description of the CP

process itself.

The CP process can be discussed in terms of a thermal analogue [22]

13 where the protons and the C nuclei each represent respectively, a large

and a small thermal mass, each initially at ambient temperature. Spinlocking of the protons is then equivalent to making the protons suddenly

13

very cold. The CP process follows this sudden cooling by placing the C

nuclei in thermal contact with the cold protons. If we add to this picture

that both thermal masses are imbedded in an infinitely large thermal mass

(the lattice) which is maintained at ambient temperature, then we are ready

to discuss the problem of intensities. The objective in this analogue is

13 1 ? to produce cold C nuclei in thermal equilibrium with the protons (-^^C signal

strength is proportional to the inverse temperature)

The analogue suggests several things. First, the cold protons cannot be

kept cold indefinitely since they are imbedded in the warm, 'infinite* lattice.
15

TJ Thus, there is a time constant called T, which governs the rate by

-'-P which the lattice heats up the cold protons. This time constant need not

be uniform, especially in a morphologically inhomogeneous solid. Secondly,

there is a time constant, T^^ which depends on the dipolar couplings among

13 the protons and between the protons and C nuclei. This time constant

determines the rate of thermal transfer between the protons and the

13 nuclei. If the objective is to produce uniformly cold C nuclei, then cne

H requires that T <<T . Moreover, if one wishes to insure equilibrium between

Ctl -Lp 13

protons and C nuclei during CP, then the time chosen for CP, T , must be

L/ Jr 13

at least 5 T_„. For rigid molecules and protonated C nuclei, T„„ is less

Ln Lri than 75 ys, so T^p need only be 400 ys. However, as molecular motion

increases, T^^ increases with the consequence that T^^ increases. Thus a CP

T^p of 2ms was chosen in these studies to include, if possible, an^^ partially

mobile molecules. (In the extreme limit where molecules reorient isotropically

and diffuse rapidly over nanometer distances, T^p becomes very long and the

corresponding resonances do not appear.)

It is not necessarily true that relative intensities are distorted in a

spectrum simply because the signal strength for a given amount of material

u H

decreases. For example, if T^" for dry cotton were 100 ms and T^

^ for the

H, wet cotton were 6.4 ms then since both ’ s are still significantly longer

13 than 5T (for T^„ less than, say, 200ys), the C spectra will have proper

Lrl Ltl relative intensities. Nevertheless, the total intensity of the wet cotton

would only be 0.75 times that of the dry cotton (cf. Table 1).

More serious problems arise in morphologically heterogeneous systems where

H

one region might have a shorter T^^ (less than a few tens of ms) and another

H H

a longer • l^i that case, signals from the region of shorter T^^ would be
- 16 -

preferentially suppressed.

H

Molecular mobility and molecular geometry usually determine .

For the conditions of these experiments, values would be minimum

when the correlation time, t , for molecular motion peaks in the region

c

10 ^ T ^ 10 °s. In this case, can become as short as 10 ^ for

c ip

large amplitude motions. In principle, molecules like cellulose need not

have their values determined simply by cellulose chain motions. For

example, in wet cellulosic systems, the surface crystalline chains as well

as the amorphous chains (if the latter exist) can be in intimate contact

with water. The transient interactions of cellulose and water molecules,

-4 -7

if lifetimes in the range 10 - 10 s exist, could strongly influence the

behavior of these chains. On the other hand, the presence of water may

also result in increased molecular mobility of surface and amorphous chains,

facilitated by the breaking of interchain hydrogen bonds. Therefore it is

H certainly possible that T^^ will not be uniform throughout the cellulose

system; i.e. there is little assurance at this*point that relative intensites

are undistorted.

There is a facet of proton spin behavior, called 'spin diffusion’ [23],

which tends to average out local differences in T^^ behavior. This phenomenon

involves the transport, i.e. the diffusion, of proton magnetization in a

solid, proton-rich system. The diffusion constants are not large, however.

A reasonable estimate for the diffusion constant in cellulose under proton spin

locking conditions might be 2x10 cm"^/s [23]. Such a small diffusion constant

would transport proton magnetization only about Inm in a time of 2ms (=T^p)
17

So, for a system where all inhomogeneities in structure have domain

sizes larger than 1 nm, signal distortions will probably persist provided

H

is short in one of the domains. If on the other hand, the heterogeneous

structures have minimum dimensions less than Inm, T, behavior will be

Ip

13 quite uniform and C CP signal intensities will have reasonably undistorted

relative intensities.

With that introduction to signal distortions, does the data suggest

anything about signal distortions in the three cellulose samples? Unfortunately

the measurement of was not a part of this study. For only one sample,

the wet PC, T, was crudely determined to be 31 ms based on intensities at

Ip

0.8, 2.0, and 10.0 ms for This implies that the observed intensity

of the wet cotton spectrum, using 2ms = T^pj should be no less than exp (-2/31)=

0.94. However, the observed value is 0.75. At the same time, no lineshape

changes were observed in any of these spectra as a function of T . Thus the

Stability of the lineshape for varying T^p suggests that relative intensities

H

are correct; meanwhile the total intensity is much lower than the T, would

Ip

predict.

In essence, the data at this point are not sufficient to answer the question

about the missing intensities. Other parameters such as the longitudinal

H H relaxation time of the protons, T^, could play a role although T^ would have

to exceed 2.5s in the wet materials in order to explain the observations since

H repetition times between 4 and 6s were used. Reported T^ values [24] for

cotton linters as a function of moisture content indicate that T^ increases at

higher moisture levels; however, these T^ measurements were only performed

up to moisture levels of 0.25 cellulose. Moreover, it is difficult to
- 18 -

Imagine that would exceed the value for free water, v/hich, especially

with some dissolved oxygen present, xrould not be as long as 2.5s. Other

possible explanations for the missing intensity could be regions which give

rise to very broad lines due to large amplitude motion of the C-H bond

vectors at frequencies comparable to the 60 KHz decoupling field amplitude [3].

Linewidths could become as large as 2 KHz, in an unfavorable case. Such a wide

line would probably be missed completely. In addition, such motion would

H -4

also cause T. to be of the order of 10 s in which case CP intensities in

Ip

these regions would also be suppressed.

The experimental evidence which argues against the plausibility of

such linebroadening is that in both NSKP and PC, the wet beaten samples

regain most of the intensity they were missing in the wet preparations with

no change in lineshape .

In summary, with respect to the question ^f possible distortions of

relative intensities in the CP-MAS spectra, the data are not sufficient to

dismiss the possibility of distortion; there is, however, strong indirect

evidence that the spectra have no major distortions. Therefore the conclusion

that wet beating does not increase crystallite surface area is probably valid.

One final observation which perpetuates the confusion is that a strong

correlation exists between the intensity and the water content. Intensities

increase monotonically for moisture contents in the range 0.48 to 0.88g H20/g

cellulose. If this increase is strictly due to a dilution effect (assuming

bound and free water molecules in rapid exchange [25]) then it is likely

that a proton magnetization gradient would exist between surface and interior

crystalline cellulose chains. Then, however, it would be difficult to justify
- 19 -

simultaneously both the stability of the C CP-MAS lineshape as a function

of cross-polarization time and the 31ms observed in wet cotton. An

H H understanding of these problems certainly awaits combined T^, , and

linewidth data in these systems as a function of moisture content.

B. The Sharpening of Resonances in Wet Cellulosic Materials

It has been reported [24] that cellulose proton spin-spin relaxation

times in cotton linters, are lOys at 0, 22ys at 0.06, and 32ys at 0.23g H^O/g

cellulose. These T

2 values, attributed solely to the cellulose protons,

would therefore suggest that all of the cellulose chains were undergoing some

large amplitude molecular motion on a time scale of 10 ^s or less. The role

of chemical exchange with the H^O protons is not known in influencing

nevertheless, only 3 of 10 protons in the anhydroglucose unit are exchangeable

protons. Therefore, it seems reasonable that some large amplitude motions

are occurring.

The motion referred to above, need not be the motion of an individual

molecule. Linenarrowing could also occur if, for example, an entire

elementary fibril (diameter -3.5nm) would rotate as a unit about its long

13 axis. The observation in the C CP-MAS spectra that resonances sharpen but

do not shift substantially supports the idea that each chain does not move

13 independently. Considering that C isotropic chemical shift values are

sensitive to hydrogen bonding configurations, any breaking of intermolecular

hydrogen bonds x^rould have to be accompanied by the rapid formation of similar

13 bonds in order to maintain the C shifts. This seems unlikely.

The sharpening of both the interior and surface crystalline resonances

in wet samples could therefore be understood to arise partly from motional

averaging. More important in the resonance sharpening is that water could
20 -

break up interf ibrillar cellulose hydrogen bonds, thereby relieving the

crystallites of possible strains. Hydrogen bonding of water to the surface

cellulose chains would also improve the uniformity of their environment

resulting in a sharpening of surface carbon resonances as is observed.

The claim that wet celluloses have higher crystallinity than dry

preparations has been made by Morosoff [26] based on X-ray results. Although

the wet /dry difference spectra (Figures 5E, 6E, and 7E) show positive

intensities in the C4 and C6 interior-crystalline resonance region (66 and 90

ppm) , remaining ambiguities in lineshape and relative intensities prevent

a definite statement on this point. Suffice it to say that if there are not

relative intensity distortions, these spectra would support the idea that

there is a small increase in crystallinity upon wetting. Figure 8 shows

proof that wetting is capable of producing a gain 'in crystallinity in an

amorphous sample. In this case, after 36 hours of soaking in distilled water

at ambient temperature, resonances associated with the interior regions of

cellulose I crystals are evident. Moreover, just as in the other cellulosic

materials, one of the principal changes in the amorphous cellulose in going

from dry to wet is the pulling in of the shoulder at 77-79 ppm.

One of the explanations offered by Morosoff [26] for an increase in

crystallinity upon wetting, is that drying sets up stress in cellulosic

materials, mediated presumably by interchain hydrogen bonds. The immobilizing

H

effect of these hydrogen bonds is reflected in the sudden lowering of

in going from 0.06 to 0 gH^O/g cellulose [241. With respect to our results.

only two vacuum oven dried samples have been run in a desiccated condition.

One is the amorphous cellulose (cf. Figure ID); the other (not shown) is a

cotton sample. In each case, the linewidths are broader and in cotton, this
21

is particularly true for the crystalline surface chain resonances. The

broadness of these resonances does not, of itself, prove the existence of

internal stress. Nevertheless, broadness (in this case it is known to be

chemical shift dispersion) is an indication of non-uniformities among surface

chain resonances. The idea that stress should accompany such non-uniformity

is reasonable. It would also follow that in paper making, fibril damage

would be minimized by mechanical deformation in the wet state.

C. The Legitimacy of Decomposing Lineshapes into Component Lineshapes.

In Figures 3 and 4 linear combinations of different cellulose spectra

were taken in the hopes that a "pure component" spectrum could be constructed.

A true pure component, by NMR standards, is one in which a chain is surrounded

in all three directions by molecules configured in a way typical of that

component. Thus it might be reasonable to isolate a spectral component

corresponding to the interior crystalline chains. However, to isolate the

pure surface crystalline chain lineshape makes less sense because the details

of the surface and the availability and geometry of other hydrogen-bonding

sites could easily in^uence the lineshape. Finally, the lineshape

determination of the amorphous cellulose component also depends heavily on the

dimensions of the amorphous region.

V. Conclusions

1. Three cellulose preparations were studied; purified cotton (PC),

microcrystalline cellulose (MC) , and Norway Spruce kraft pulp (NSKP) . Based

on the CP-MAS NMR spectra wet beating does not alter the cellulose crystal

surface area in these samples, even though, in cotton, the fraction of bound

water increases with beating. However, this conclusion may be misleading due

to possible intensity distortions in the CP-MAS spectra. The possibility that

such distortions exist accompanies the observation of deficient intensity in

some spectra. Indirect evidence presented here is weighted in favor of true

relative intensities.
22 -

I

2. On the basis of the NMR spectra of PC, MC, NSKP, and an amorphous ball-milled cellulose, it was argued that the order of

decreasing crystallinity was MC>PC>NSKP and that the order of decreasing

lateral crystallite dimensions was PC>MC>NSKP.

3. Comparison of ^^C NMR spectra of wet versus dry celluloses supports

the idea that wetting breaks up interfibrillar hydrogen bonds involving the

surface cellulose chains. Moreover, in this process, stresses may be

relieved allowing for slight increases in crystallinity. Restricted

reorientation of entire elementary fibrils may also be occurring; however,

the evidence for this latter claim depends solely on cellulose proton linewidth data reported in the literature [27].

4. CP-MAS spectra have been presented which confirm the utility of

NMR for distinguishing between various cellulose preparations as well as

distinguishing morphological features within a cellulose preparation. For

example, in cellulose I, the interior crystalline chain lineshape can be

isolated. It is not as easy to distinguish lineshapes arising from amorphous

chains from lineshapes due to crystal surface chains, but the existence of

significant numbers of crystal surface chains is not in doubt. The spectra of

a highly crystalline cellulose II preparation as well as a ball-milled

amorphous cellulose preparation have also been shown to give very distinct

spectra.
- 23 -

References

1. A. Pines, M. G. Gibby, and J. S. Waugh, J. Chem.Phys. 56 ,1776 (1972).

2. A. Pines, M. G. Gibby, and J. S. Waugh, J. Chem.Phys. 59, 569 (1973).

3. D. L. VanderHart, William L. Earl, J.Magn.Res.^, 361 (1981).

4. E.R. Andrew, Arch.Sci. (Geneva) 12, 103 (1959).

5. J. Schaefer, E. 0. Stejskal, and R. Buchdahl, Macromolecules 8,291 (1975).

6. E. Lippmaa, M. Alla, and T. Tuherm, in ''Proceedings, XIX Congress Ampere,

Heidelberg, 1976," p.ll3.

7. A. N. Garroway, W. B. Moniz, and H. A. Resing, Prepr.Div.Org. Coatings

Plastics Chem. 36, 133 (1976).

8. R. H. Atalla, J. C. Cast, D. W. Sindorf, V. J. Bartuska, G. E. Maciel,

J Chem.Soc 102, 3249 (1980).

9. W. L. Earl and D. L. VanderHart, J. Am. Chem. Soc . 102, 3251 (1980).

10. W. L. Earl and D. L. VanderHart, Macromolecules 1^, 570 (1981).

11. J. Blackwell and F. J. Kolpak, Appl . Polym. Symp . 751 (1976) and

references cited therein.

12. I. Nieduszynski and R. D. Pres Nature (London) 225 , 273 (1970).

13. W. L. Earl and D. L. VanderHart, J. Magn.Res. 48,35 (1982).

14. E. L. Graminski and E. E. Toth, The Cellulose-Water Interaction in Rag and

Wood Pulps, NBSIR 80-1970.

15. S. R. Hartmann and E. L. Hahn, Phys.Rev. 128 , 2042 (1962).

16. E. 0. Stejskal and J. J. Schaefer, Magn.Reson. 1975, ]^, 560.

17. J. E. Stone, A. M. Scallan, and B. Abrahamson, Svensk Papperstidn, 71 , 687 (1964).

18. Certain commercial companies are named in order to adequately specify the

experimental procedure. Such identification does not imply recommendation

by the National Bureau of Standards of the U. S. Department of Commerce, nor

does it imply that the equipment and material are the best available for the

purpose.

19. This sample was prepared by William L. Earl at NBS.

20. This sample was prepared by R. H. Atalla of the Institute for Paper

Chemistry, Appleton, Wisconsin.

21. G. E. Maciel, D. J. O'Donnell, J. J. H. Ackerman, B. H. Hawkins, and V. J.

Bartuska, Makromol. Chem. 182 , 2297 (1981).

22. M. Mehring "NMR Basic Principles and Progress", Vol.ll, Chap. 4, 1976.

23. D. C. Douglass and G. P. Jones, J. Chem. Phys. 4^, 956 (1966).

24. M. F. Froix and R. Nelson, Macromolecules 726 (1975).

25. J. E. Carles and A. M. Scallan, J. Appl. Polym. Sci. '\J_, 1855 (1973).

26. N. Morosoff, J. Appl. Polym. Sci. 3^, 1837 (1974).

27. Note added in proof: Recent experiments performed in our laboratory discount

the idea that either the chains or the fibrils undergo large amplitude motions

on a timescale less than lO'^s.

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T I [ 1 1 1 I 1 I 1 I I i I 120 80 40 PPM

Figure 1. CP-MAS C spectra of four different cellulose preparations. Samples,

except for the desiccated amorphous cellulose, have been equilibrated at a RH of

approximately 30%. Gross polarization times are 2ms for all spectra, and all

spectra are normalized to the same total intensity. Pulse repetition times and

total scans are respectively as follows: (A) 4.5s, 48K; (B) 6.05s,3.9K; (C) 7.0s,

9.7K; and (D) 6.0s, 7.2K. Linear polyethylene is used as a chemical shift

reference. Baselines have been added for visual clarity. Signal to noise varies

' primarily because of sample quantity variations.
i i 1 1 1 1 I I 1 1 1 120 80 40 PPM

Figure 2. Normalized CP-MAS Spectra of the three samples studied in this report , along with a spectrum of ball milled cellulose. All samples have equilibrated at a RH of approximately 30%. Pulse repetition times and total scans are respectively as follows^ (A) 6.0s, 3.9K; (B) 6.0s, 8. OK; (C) 6.0s, 7.9K and (D) 4.0s, 15. IK. For

additional explanation, see Figure 1.
1 1 1 1 I I I 1 1 I 120 80 40 PPM

Figure 3. Comparison of normalized CP-MAS spectra of 'dry' purified cotton and

microcrystalline cellulose. Upper spectra are the same as Figure 2A and 2B. Linear

combination spectra (D and E) are attempts to resolve "pure component" spectra

assuming a 2-component system.
120 80 40 PPM

13

Figure 4. Comparison of normalized CP-MAS C spectra of 'dry* cotton and

Norway Soruce kraft pulp. Upper spectra are the same as Figure 2A and 2C.

Linear combination spectra (C and D) are attempts to resolve pure component

spectra assuming a 2-component system.
120 80 40 PPM

Figure 5. Comparison of normalized CP-MAS spectra of wet beaten (A), wet (B)

,

and dry (C) cotton spectra. Difference spectra are given in (D) and (E) . All cross

polarization times are 2ms. Pulse repetition times and total scans are as follows:

(A) 4.0s, 14. 4K, (B) 4.0s, 6.8K; and (C) 6.0s, 3.9K.
MICROCRYSTALLINE

CELLULOSE

Figure 6. Spectra of microcrystalline cellulose (see explanation in Figure 5).

Pulse repetition times and total scans are as follows: (A) 6.0s, 12. 6K; (B) 6.0s,

8K; and (C) 6.0s,8K.
Figure 7. Spectra of Norway Spruce kraft pulp (see explanation in Figure 5) . Pulse

repetition times and total scans are as follows: (A) 4.0s, 16K; (B) 4.0s, 14. 3K; and

(C) 6.0s, 7.9K.
120 80 40 PPM

Figure 8. Normalized CP-MAS spectra of amorphous cellulose; the dry sample has

|

n been equilibrated at 30% RH and the wet sample was soaked in distilled water for 36 | hours prior to taking the spectrum. The difference spectrum is also shown indicating | that crystallinity has increased after wetting. Pulse repetition times and total

|

, scans are as follows: (A) 4.0s, 15. IK and (B) 4.0s, 14. 3K. |

,
U.3. CEPT. OP COMM.

‘ BIBLIOGRAPHIC DATA.

, SHEET (See instructions)

1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date REPORT NO. Nat. Bur. Stds.

NBSIR 82-2534 . NBSIR 82-2534 June 1982

i. TITLE AND SUBTITLE

i Deductions about the Morphology of Wet and Wet-Beaten Celluloses from Solid

i State ^^C.NHR.

!

'

;. AUTHOR(S)

D. L. VanderHart

'j. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/’Grant No. national bureau of standards

DEPARTMENT OF COMMERCE S. Type of Report & Period Covered

i WASHINGTON, D.C. 20234

10/1/80 - 9/30/81

i SPONSORING organization NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

Bureau of Engraving and Printing • Treasury Department

; Washington , D.C. 20401

1. SUPPLEMENTARY NOTES

1 i Document describes a computer program; SF*IS5, FIPS Software Summary, is attached.

. ABSTRACT (A 2G0~>‘rord or less fociual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The Cfllulosic preparations, purified cotton (PC), microcrystalline

cellulose (MC) , and Non^ay Spruce Kraft pulp (NSKP) , were examined by

solid-state MI'IR methods. Based on previously published work, NMR

spectra can indicate the fraction of anhydroglucose moieties in interior

crystalline regions; moreover, the combined fraction of moieties in amorphous

regions .and at the surface of the crystalline regions can also be determined.

In this v7ork, changes in crystallite size resulting from wet beating (a papermaking processing step) are probed. Results show that no detectable changes

^ occur in going from v;et to wet beaten samples, although possibilities that

intensity distortions may be present in the spectra cannot be entirely dismissed,

Other observations made in this investigation are as follows: (1) The order of

decreasing crystallinity is MC>PC>NSKP; (2) The order of decreasing lateral

crystallite dimension is PC>MC>KSK?. (3) Tnis data in conjunction with other

published proton data raises the possibility that restricted reorientation of

entire elementary fibrils can take place in wet samples; finally (4) Spectra of

several different cellulose preparations are included in order to underscore the

ability of these solid-state NMR techniques to distinguish between different

crystallographic as well as different morphological preparations of cellulose.

1 <EY 'AOROS(Sjx to twe/ve entries; alohcbeticat order; cop/to/* 2 e onJ/ proper names; and separate key

13 NMR, C, cellulose; wet .beating; crystallinity; morphology.

words by semicolon s)

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