Acetylation of Cellulose Nanowhiskers with Vinyl Acetate under Moderate Conditions

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Acetylation of Cellulose Nanowhiskers withVinyl Acetate under Moderate Conditions

Nihat Sami Cetin, Philippe Tingaut,* Nilgul Ozmen, Nathan Henry,David Harper, Mark Dadmun, Gilles Sebe*

A novel and straightforward method for the surface acetylation of cellulose nanowhiskers bytransesterification of vinyl acetate is proposed. The reaction of vinyl acetate with the hydroxylgroups of cellulose nanowhiskers obtained from cotton linters was examined with potassiumcarbonate as catalyst. Results indicate that during the first stage of the reaction, only thesurface of the nanowhiskers was modified, whiletheir dimensions and crystallinity remainedunchanged. With increasing reaction time, diffu-sion mechanisms controlled the rate, leading tonanowhiskers with higher levels of acetylation,smaller dimensions, and lower crystallinity. InTHF, a solvent of low polarity, the suspensionsfrom modified nanowhiskers showed improvedstability with increased acetylation.

Introduction

Cellulose is the main constituent of wood and plants and is

one of the most abundant renewable resources on earth.

N. S. Cetin, N. OzmenFaculty of Forestry, Kahramanmaras Sutcu Imam University,Kahramanmaras, TurkeyD. HarperTennessee Forest Products Center, The University of Tennessee,2506 Jacob Drive, Agriculture Campus, Knoxville, Tennessee37996-4570, USAN. Henry, M. DadmunChemistry Department, The University of Tennessee, Knoxville,Tennessee 37996-1600, USAG. SebeUnite Sciences du Bois et des Biopolymeres, Universite Bordeaux1, INRA, CNRS, UMR US2B, 351 Cours de la Liberation, TalenceF-33405, FranceFax: þ33 5 4000 6439; E-mail: [email protected]. TingautSwiss Federal Laboratories for Materials Testing and Research(EMPA), Uberlandstrasse. 129, CH-8600 Dubendorf, SwitzerlandFax: þ41 44 823 4007; E-mail: [email protected]

Macromol. Biosci. 2009, 9, 997–1003

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cellulose is composed of 1,4-b-glucopyranose units asso-

ciated by hydrogen bonding, which forms a semicrystalline

structure where highly ordered regions (crystallites) are

distributed among disordered domains (the amorphous

phase).[1] The crystallites (also called whiskers or nano-

whiskers) are nanometer-sized and can be recovered by

various methods and used as reinforcing agent in polymer-

based composite materials.[2–4] Because of their high specific

strength, modulus, and aspect ratio, cellulose nanowhiskers

can significantly improve the mechanical properties of the

composites at nanofiber loading levels as low as 6%.[5]

Transparent composites with improved mechanical and

thermal characteristics can also be prepared, even at high

nanofiber loading.[6–8] The other advantages of these

nanowhiskers stem from their low density, renewable

nature, biodegradability, and relatively low cost.

To realize property improvements, the cellulose nano-

whiskers must be homogeneously dispersed in the poly-

meric matrix, which is a non-trivial task. Because of their

high surface area and hydrophilic nature, the nanowhiskers

tend to flocculate by hydrogen bonding. Stable suspensions

of cellulose nanowhiskers can, however, be prepared in

water by acid hydrolysis of the biomass. The cellulose

nanowhiskers do not flocculate as they are stabilized by

DOI: 10.1002/mabi.200900073 997

N. S. Cetin et al.

998

the electrostatic repulsion resulting from the negative

surface charges imparted by the treatment.[2,3] These

suspensions have interesting characteristics since they

are birefringent and can self-organize into stable chiral

nematic phases when a critical concentration is reached

(typical of liquid crystals).[2] Recently, it has been shown

that stable suspensions with similar properties can also

be obtained in polar aprotic solvents such as N,N-

dimethylformamide (DMF)[9–11] or dimethyl sulfoxide

(DMSO).[11] Cellulose-reinforced nanocomposites can then

be prepared by casting a mixture of the stable suspensions

with hydrosoluble polymers, latexes, or DMF-soluble

polymers.[2,3,9,10] However, since cellulose nanowhiskers

cannot be easily dispersed in apolar solvents or hydro-

phobic non-polar monomers, it has until now been difficult

to efficiently reinforce most of the classical non-polar

polymer matrices, such as polyolefins, thermoplastic

hydrocarbons, etc. To overcome this problem, several

methods have been proposed recently, which involve the

modification of the chemical surface of the cellulose

nanowhiskers.[2,3] One approach is to use a surfactant that

screens the steric interactions between the cellulose

chains.[12–14] Other approaches involve chemical modifica-

tion of the surface cellulose hydroxyl groups with various

reagents such as acetic anhydride,[7,8,15–17] alkenyl succinic

anhydrides,[18] chlorosilanes,[19–21] or hexamethyldisila-

zane.[22] For example, the partial acetylation of cellulose

nanowhiskers with acetic anhydride allows their disper-

sion in solvents of medium polarity such as acetone or

acetic acid.[16] The suspensions obtained are stable and

maintain their birefringent characteristics. Similar results

have been obtained in tetrahydrofuran (THF), toluene, or

chloroform using silylation.[19,20]

It has been reported that polysaccharides such as starch,

cyclodextrins, or cellulose can be successfully esterified in

homogeneous or heterogeneous conditions by reaction

with vinyl esters.[23–28] More recently, we showed that

lignocellulosic materials such as wood can be easily

esterified in mild conditions by this method.[29–31] The

by-product of this reaction is acetaldehyde, which is not

acidic and can be easily removed from the reaction medium

because of its low boiling point (b.p. [760 mm Hg]¼ 21 8C). To

our knowledge, this reaction has not been exploited for

the modification of cellulose nanowhiskers. Accordingly, in

this study, we examined the surface acetylation of cellulose

nanowhiskers by reacting them with vinyl acetate under

moderate conditions. We monitored the progress of the

acetylation reaction spectroscopically and documented

the evolution over time of the nanowhiskers structure

as the acetylation reaction proceeded. The progress of

the reaction was then correlated to the dispersion of the

cellulose whiskers in non-polar solvents, which provides

insight into their ability to disperse in non-polar polymeric

matrices.



Experimental Part

Materials and Chemicals

Microcrystalline cellulose (MCC) Avicel (PH 101), was purchased

from Fluka. Vinyl acetate, DMF, potassium carbonate (K2CO3),

acetone, and toluene were purchased from Sigma-Aldrich. Sulfuric

acid (95–98%) was purchased from Fischer Scientific. All chemicals

were used as received without further purification. Deionized

water was used in all experiments.

Preparation of Cellulose Nanowhiskers

Cellulose nanowhiskers were prepared by acid hydrolysis of MCC

with sulfuric acid (64 wt.-%). Ten grams of MCC and 49 g of water

were placed in a beaker, and the suspension was stirred until a

homogeneous dispersion was obtained (10 min). The beaker was

immersed in an ice bath and 94 g of sulfuric acid was added

dropwise (the temperature did not exceed 20 8C during addition of

the acid). After the addition was complete, the suspension was

heated at 44 8C for 3 h, under strong magnetic stirring. The excess

sulfuric acid was then removed by repeated centrifugation with

deionized water (10 min, 4 400 rpm) until the supernatant became

turbid. The final suspension of nanowhiskers was sonicated

overnight (Branson 2510) at 10 8C and freeze-dried.

Chemical Modification of Cellulose Nanowhiskers

Acetylation reactions were performed at 94 8C, under nitrogen flow,

in a round-bottomed flask equipped with a condenser. For each

reaction, 0.3 g of cellulose nanowhiskers (presenting �5.55 mmol

total hydroxyl groups) was introduced in the reagent solution

consisting of 10 mL of DMF, 1 mL of vinyl acetate (10.8 mmol), and

0.1 g K2CO3 (0.7 mmol). Different reaction times were investigated:

1, 2, 3, 6, and 24 h. After reaction, the modified nanowhiskers were

filtered on a 4-mm isopore polycarbonate filter. To eliminate all non-

bonded chemicals (i.e., unreacted compounds and by-products

formed), the modified material was subsequently rinsed in 50 mL of

hot water (60 8C) for 3 h, then in 45 mL of hot toluene/ethanol/

acetone (4:1:1 by vol., at 90 8C) for 3 h. Samples were finally oven-

dried at 80 8C for 16 h under vacuum.

Infrared Spectroscopy

Infrared spectra of the modified and unmodified cellulose

nanowhiskers were recorded using a Perkin-Elmer Spectrum One

FT-IR spectrometer. For each sample, the diamond crystal of an

attenuated total reflectance (ATR) accessory was brought into

contact with the area to be analyzed. The contact area was a circle of

about 1.5 mm in diameter. All spectra were recorded between 4 000

and 600 cm�1 with a resolution of 4 cm�1 with 32 scans per sample.

For comparison, spectra were adjusted to the same baseline and

were normalized to the C�O stretching vibration of the glucopyr-

anose ring at about 1 060 cm�1.[15] The peak height ratio of 1 740 to

1 060 cm�1 vibrations (I1740/I1060) in each spectrum was calculated

using a baseline constructed by extrapolating two lines between

DOI: 10.1002/mabi.200900073

Acetylation of Cellulose Nanowhiskers with Vinyl Acetate under . . .

Figure 1. Acetylation of cellulose nanowhiskers by the transesterification reactionbetween cellulose hydroxyl groups and vinyl acetate (VA).

the valleys at 1 790 and 1 700 cm�1 and between the valleys at 1 500

and 860 cm�1.[32,33]

13C CP-MAS NMR Spectroscopy

Solid state 13C cross polarization-magic angle spinning (CP-MAS)

NMR spectra of modified and unmodified cellulose whiskers were

recorded at room temperature on a Varian Inova 400 NMR

spectrometer, using a MAS rate of 5 kHz, a contact time of 500ms, at

a frequency of 100.61 MHz for 13C NMR. Samples were packed in

MAS 4-mm-diameter zirconia rotors. All spectra were run for 3 h

(3 000 scans).[34]

X-Ray Diffraction Analysis

Wide-angle X-ray diffraction (WAXD) patterns were collected on a

Panalytical Materials Research diffractometer working in reflec-

tion mode, from 2u¼ 5 to 608. Cu Ka radiation (l¼0.15418 nm) was

generated at a voltage of 45 kV and a current of 40 mA and was

monochromated with a diffracted beam monochromator. Modified

and unmodified cellulose whiskers were packed on top of a glass

plate and the surface was analyzed. All spectra

were normalized to the (004) plane at 2u¼ 348,which was not affected by the chemical

modification.[15]

Figure 2. FT-IR absorbance spectra of unmodified cellulose nanowhiskers and nano-whiskers reacted with vinyl acetate for 1, 2, 3, 6, and 24 h.

Atomic Force Microscopy (AFM)

AFM measurements were carried out on a

Nanoscope IIIa, multimode scanning probe

microscope (Digital Instruments, Plainview,

NY), mounted on a vibration isolation system.

All measurements were performed at room

temperature using Si scanning probe micro-

scope tips having a nominal spring constant

of 20–80 N �m�1, and a resonance frequency of

approximately 300 kHz. Height images were

obtained in tapping mode. The scan rate was

1 line � s�1 for all images. Before each AFM

experiment, a 5� 10�6 wt.-% suspension of

unmodified (in water), or modified (in THF)

nanowhiskers, was sonicated overnight at 10 8C.

One droplet of the suspension was then

deposited on a freshly cleaned silicon wafer

surface [before each experiment, the silicon

wafer was immersed in a mixture of sulfuric



acid and hydrogen peroxide (3:1 v/v) for 1 h,

thoroughly washed with deionized water, and

dried under nitrogen flow] and allowed to dry

at room temperature for 1 day. Particle

dimensions were determined using the section

analysis tool provided with the AFM software

(Digital Instruments, Nanoscope v5.30r2).

Results and Discussion

Acetylation of Cellulose Nanowhiskers with VinylAcetate

Cellulose nanowhiskers were acetylated according to the

reaction scheme in Figure 1. The vinyl alcohol formed

during the process tautomerized to the acetaldehyde and

the equilibrium was naturally shifted towards ester

formation.

The characteristic vibrations of the grafted acetyl groups

were easily identified in the FT-IR spectra of modified

nanowhiskers (Figure 2): namely the carbonyl stretching

vibration at 1 740 cm�1 (nC¼O), the methyl in-plane bending

at 1 375 cm�1 (dC�H), and the C�O stretching at 1 235 cm�1

(nC�O). The intensity of these bands increased gradually

with reaction time, indicating that nanowhiskers were

increasingly modified. The kinetics of acetylation was

determined by calculating the peak height ratio of I1740/

I1060 in each spectrum and plotting this ratio as a function of

reaction time (Figure 3). It has been reported that this

method is a valid method of quantifying the extent of

www.mbs-journal.de 999

N. S. Cetin et al.

Figure 3. Peak height ratios of the 1 740 to 1 060 cm�1 vibrations(I1740/I1060) as a function of reaction time for nanowhiskersreacted with vinyl acetate.

1000

esterification and investigating reaction kinetics of cellu-

lose modified with acetic anhydride or valeryl chlor-

ide.[32,33] This method compares the intensity of the

carbonyl stretching vibration of the grafted acyl group

with the 1 060 cm�1 vibration associated with C�O

Figure 4. 13C CP-MAS NMR spectra of unmodified cellulose nanowhiskers and nano-whiskers reacted with vinyl acetate for 1, 2, 3, 6, and 24 h.

stretching of the cellulose backbone,

which is used as an internal standard.

Acetylation was relatively fast during the

first 2 h, suggesting that easily accessible

surface hydroxyl groups were first mod-

ified. After 2 h, the reaction rate slowed

down. This result may be explained in

terms of steric hindrance induced by

the grafted acetyl groups at the whisker

surface, or by the need for vinyl acetate

to diffuse into the nanowhiskers, as has

been reported for cellulose modified with

acetic anhydride.[32]

Acetylated whiskers were further char-

acterized by 13C CP-MAS NMR spectro-

scopy (Figure 4). The carbons of the

unmodified whiskers were assigned as

follows:[34] C1 (105 ppm), C4 crystalline

(89 ppm), C4 amorphous (84 ppm), C2/C3/

C5 (72 and 75 ppm), C6 crystalline

(65 ppm), and C6 amorphous (63 ppm).

After acetylation, the carbons of the

acetyl moieties were clearly identified

at 171 and 20 ppm (carbons a and b,

respectively, according to the nomencla-

ture in Figure 1), confirming the success of

the reaction. The chemical shift at

121.5 ppm was identified as a spinning

sideband arising from the grafted carbo-

nyl group (identified by varying the

spinning velocity). The intensities of the

a and b carbon resonances increased



gradually with reaction time, indicating that nanowhiskers

were increasingly modified, as was observed by FT-IR

spectroscopy.

X-Ray Diffraction Analysis

The impact of chemical modification on the crystallite

structure of the cellulose nanowhiskers was further

evaluated using WAXD analysis. The diffraction profiles

of unmodified and acetylated nanowhiskers are presented

in Figure 5. Unmodified cellulose nanowhiskers display the

typical X-ray diffraction pattern of cellulose I, with char-

acteristic diffraction maxima at 2u¼ 14.6, 16.3, 22.3, and 348[(101), (101), (002), and (004) planes, respectively).[35]

The diffraction pattern of cellulose I remained

unchanged after 1 h of acetylation, indicating that nano-

whiskers maintained their original crystalline structure

(Figure 5). This result is in agreement with our earlier

suggestion that easily accessible surface hydroxyl groups

were first modified. At longer reaction times, the X-ray

diffraction spectra of acetylated nanowhiskers changed

gradually. The intensity of the peak corresponding

DOI: 10.1002/mabi.200900073


Figure 5. WAXD spectra of unmodified cellulose nanowhiskers and nanowhiskers reactedwith vinyl acetate for 1, 2, 3, 6, and 24 h.

to 2u¼ 228 progressively decreased after 2, 3, 6, and 24 h

acetylation, indicating that the inner crystalline regions

were increasingly modified by the reaction progress. The

modification of these crystallites is concomitant with

the decrease in reaction rate noted by FT-IR ATR spectro-

scopy (Figure 3). The peak at 2u¼ 348 remained constant in

all spectra, suggesting that the 004 lattice was not affected

by the chemical modification. This behavior has been

previously reported for the acetylation of cellulose with

acetic anhydride.[15]

Dispersion Characteristics and Morphology ofNanowhiskers

A stable aqueous suspension was obtained with the

unmodified nanowhiskers, as expected, at the 1 wt.-%

concentration tested. This aqueous suspension showed

flow birefringence when observed between crossed polar-

Figure 6. Suspensions in THF (1 wt.-%) of unmodified nanowhiskers and nanowhiskersreacted with vinyl acetate for 1, 2, 3, 6, and 24 h.

izers. The behavior of acetylated nano-

whiskers was then examined in THF, a

solvent of low polarity. For this purpose,

1 wt.-% suspensions of unmodified and

acetylated nanowhiskers were sonicated

overnight at 10 8C and allowed to stand

for 15 min, before a photograph was

taken. The unmodified nanowhiskers

sedimented quickly, as can be seen in

Figure 6. The THF suspensions of acety-

lated nanowhiskers showed varying

degrees of stability, which were depen-

dent on reaction time. The suspensions



prepared from nanowhiskers acetylated

for 2, 3, 6, and 24 h remained dispersed

for more than 8 h (Figure 6). At lower

degrees of acetylation, the suspensions

were less stable; for instance, total

flocculation occurred in less than 2 h

for the sample that was acetylated for

1 h. The phase separation observed in

that case (Figure 6) may be due to the

incomplete acetylation of the nanowhis-

ker surfaces. Our results suggest that

stable THF suspensions were obtained

only when the surface was totally

modified (i.e. for reaction times above

2 h). Flow birefringence was not clearly

demonstrated for these experimental

conditions, although it may become

apparent at higher concentrations.

Cellulose nanowhiskers were also

analyzed by AFM, before and after

acetylation (Figure 7a,c,d). A droplet of

the aqueous or THF suspension was

deposited on a freshly cleaned silicon

wafer surface and allowed to dry at room temperature. In all

cases, the micrographs show the presence of both

aggregated and isolated nanowhiskers. It is not clear

whether these aggregates reflect the state of the suspen-

sion, or if they were formed during drying of the droplet

deposited on the silicon surface. Larger aggregates were

observed with acetylated nanowhiskers in Figure 7c,

indicating that the size of these aggregates depends on

the solvent used for the suspension and the nature of the

nanocrystal surface. After 24 h of acetylation, the aggre-

gates were found to be generally smaller and more well

packed (Figure 7d).

The dimensions of individual nanowhiskers were

estimated, assuming a cylindrical shape, by scanning a

line along individual whiskers (length) or in the transversal

direction (diameter) as illustrated in Figure 7b. The length

was measured parallel to the surface, while the diameter

was estimated from the height difference between the


N. S. Cetin et al.

Figure 7. AFM images of (a) unmodified nanowhiskers after drying from a watersuspension, (c) 3-h-acetylated nanowhiskers after drying from a THF suspension and(d) 24-h-acetylated nanowhiskers after drying from a THF suspension. The diameter andlength of the nanowhiskers was estimated by scanning a line across individualnanowhiskers as illustrated in (b).

1002

silicon surface and the nanocrystals. The width of the

nanowhiskers was not accessible from this AFM scan

because of broadening due to the convolution of the

nanowhiskers and the AFM tip geometry, but should

be equal to the height.[36,37] The average length and

diameter of unmodified individual nanowhiskers were

estimated to be 311� 69 and 7� 1 nm, respectively

(Figure 8). This result is in agreement with the dimensions

Figure 8. Length and diameter of individual nanowhiskers beforeand after acetylation at different reaction time (AFM evaluation).



previously reported for nanowhiskers

obtained from cotton linters.[38] The

dimensions of the nanowhiskers were

not affected during the first 2 h of

the treatment, suggesting once again that

only the surface was modified during

this stage of the reaction. The length and

diameter of the nanowhiskers decreased

after 2 and 3 hours, respectively, indicat-

ing that the crystalline structure of the

nanowhiskers began to erode at this time.

This erosion accompanies the decrease in

the reaction rate observed by FT-IR ATR

spectroscopy and the decrease in crystal-

linity noted by WAXD analysis. After 24 h,

the nanowhiskers lost about one third of

their length and their crystallinity was

highly reduced.

One interpretation that is consistent

with this suite of results is that under

these reaction conditions, the acetylation

initially proceeds by reacting with the

readily available surface hydroxyl groups

on the nanowhisker surfaces, but after 2 h

of reaction, the inner hydroxyl groups

become accessible to the vinyl acetate.

This increased accessibility may be due to

diffusion of vinyl acetate into the nano-

whiskers or the result of the dissolution of

the modified cellulose acetate chains that

are sufficiently substituted to become soluble in the

reaction media. Either way, this continued reaction results

in a degradation of the nanowhiskers to create a smaller,

less anisotropic nanoparticle. Two hours is quite a long

time to modify the nanowhisker surfaces in industry,

but this time can be probably shortened by optimizing the

reactions conditions (temperature, vinyl acetate concen-

tration, catalyst, etc.). We have shown in previous studies

that this functionalization method can be used to graft

varied saturated and unsaturated moieties into lignocellu-

losic materials such as wood.[29–31] Therefore, the method

could be applied to the design of nanocellulose-based

functional materials with unique mechanical, optical,

electronic, or selective permeation properties.

Conclusion

Cellulose whiskers were easily acetylated in DMF under

moderate conditions by reaction with vinyl acetate, with

potassium carbonate as a catalyst. The degree of acetylation

of cellulose was easily monitored as a function of reaction

time, but the nanostructure of the whiskers was preserved

only when short reaction times were used (less than 2 h in

DOI: 10.1002/mabi.200900073


our experimental conditions). In these conditions, only

the surfaces of the whiskers were modified and their

dimensions and crystallinity remained unaffected by the

treatment. By increasing the reaction time, the inner

crystallites were increasingly attacked by the vinyl acetate,

leading to a higher substitution, but also to an erosion of

the nanowhisker structure and loss of crystallinity. Stable

suspensions in THF, a solvent of low polarity, could be

obtained from whiskers that were sufficiently acetylated

(reacted for at least 2 h). In these conditions, the surfaces of

whiskers were totally acetylated and erosion was limited.

We conclude that esterification with vinyl esters shows

promise as a straightforward method to modify the surface

of cellulose nanowhiskers with various functionalities and

opens up new opportunities for using cellulose nanowhis-

kers as reinforcement in non-polar polymer matrices or as a

vector for the improvement of optical, electronic, or

selective permeation properties. We have already success-

fully grafted a series of saturated and unsaturated vinyl

esters on lignocellulosic materials such as wood[29–31] and

we will further apply the method to the functionalization of

cellulose nanowhiskers.

Acknowledgements: The National Science Foundation and theU.S. Department of Energy financially supported this work throughgrant DMR-0706323 and contract DEFG3605GO85014, respec-tively. The Division of Materials Sciences and Engineering, U.S.Department of Energy, provided further support, under contractwith UT-Battelle, LLC. The authors thank TUBITAK for the award ofBIDEB-2219 fellowship to N.O. The authors also acknowledge theassistance of Mr. Tim Stortz.

Received: February 14, 2009; Revised: April 7, 2009; Accepted:April 8, 2009; Published online: July 13, 2009; DOI: 10.1002/mabi.200900073

Keywords: acetylation; atomic force microscopy (AFM); cellulosenanowhiskers; 13C CP-MAS NMR spectroscopy; FT-IR ATR spectro-scopy; vinyl acetate; wide-angle X-ray analysis

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Acetylation of Cellulose Nanowhiskers with Vinyl Acetate under Moderate Conditions