2014 surface modification of cellulose nanocrystals by grafting with poly(lactic acid)

8/17/2019 2014 Surface Modification of Cellulose Nanocrystals by Grafting With Poly(Lactic Acid)

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Research Article

Received: 26 May 2013 Revised: 14 August 2013 Accepted article published: 28 August 2013 Published online in Wiley Online Library: 2 October 2013

(wileyonlinelibrary.com) DOI 10.1002/pi.4610

Surface modification of cellulose nanocrystals

by grafting with poly(lactic acid)Mercedes Peltzer,a∗ Aihua Pei,b,c Qi Zhou,b,c Lars Berglundc

and Alfonso Jimeneza

Abstract

The use of biopolymers obtained from renewable resources is currently growing and they have found unique applicationsas matrices and/or nanofillers in ‘green’ nanocomposites. Grafting of polymer chains to the surface of cellulose nanofillerswas also studied to promote the dispersion of cellulose nanocrystals in hydrophobic polymer matrices. The aim of this studywas to modify the surface of cellulose nanocrystals by grafting from L-lactide by ring-opening polymerization in order toimprove the compatibility of nanocrystals and hydrophobic polymer matrices. The effectiveness of the grafting was evidencedby the long-term stability of a suspension of poly(lactic acid)-grafted cellulose nanocrystals in chloroform, by the presenceof the carbonyl peak in modified samples determined by Fourier transform infrared spectroscopy and by the modificationin C1s contributions observed by X-ray photoelectron spectroscopy. No modification in nanocrystal shape was observed inbirefringence studies and transmission electron microscopy.c 2013 Society of Chemical Industry

Keywords: cellulose nanocrystals; surface modification; grafting-from; ring-opening polymerization; L-lactide

INTRODUCTION The use of biopolymers obtained from renewable resources is

currently growing and they have found unique applications as

matricesand/ornanofillersin ‘green’nanocomposites.The positive

impact of using natural nanofibers as fillers and as reinforcement

material in nanocomposites provides several advantages suchas their full biodegradability, low density, relatively low cost

and enhanced mechanical properties.1–3 The main idea of the

addition of cellulose nanofillers in this approach is to obtain new

and efficient nano-biocomposites with improved properties while

keeping their environmental character.4,5 Cellulose nanocrystals

(CNs) or nanowhiskers have been proposed as convenient nano-

reinforcements,sincetheyareconsideredoneofthebestmodifiers

for some biopolymers and they are obtained from an abundant

and renewableresource, cellulose. CNs are nanoparticles obtained

from highly crystalline cellulose extracted from cellulose fibers

such as bleached wood pulp, cotton or ramie fibers by acid

hydrolysis. The use of cellulose crystallites as nanofillers for

reinforcement in poly(lactic acid) (PLA) formulations has shownimportant advantages over their inorganic counterparts (i.e.

laminated silicates) because of their unique characteristics, suchas

biodegradability and biocompatibility, high stiffness, high aspect

ratioandlowdensity.6 Nevertheless,the mostimportant drawback

of using nanocrystals from polysaccharides as reinforcement in

nanocomposites is thepoor compatibilitybetween thehydrophilic

CNs and the hydrophobic polymer matrices, such as PLA.7

A stable dispersion of cellulose nanofibers and/or nanocrystals

in non-polar media, such as solvents or polymer matrices,

can be accomplished by the addition of surfactants8,9 or by

chemical modification of the nanocrystal surface.10 Other authors

have proposed functionalization of CNs by an esterification

reaction with organic acids with aliphatic chains of different

sizes, in order to improve their dispersity in low density

polyethylene.11 These graftingreactionsand furtherblendingwith

low density polyethylene led to homogeneous nanocomposites

with a significant improvement in their elongation at break with

increasing length of thegraftedchains.On theother hand,Siqueira

etal. grafted cellulose nanoparticles with long chain isocyanates,obtaining nanoparticles with good dispersion in organic solvents

and allowing processing of nanocomposite films for a broad range

of polymeric matrices.5 In addition to these modifications, surface

acetylation has also been considered to reduce the polarity of

cellulose and to improve the dispersion and adhesion of the

CNs to polymer matrices.4,12,13 This is a simple and fast method,

where cellulosehydroxyl groups react withacetic anhydride. It was

reported that this modification of CNs resulted in good dispersion

in organic solvents and lower polarity in comparison with non-

modifiednanocrystals.Goodmechanicalpropertieswereobserved

when modified CNs were blended with PLA. Another method for

CN modification was based on their partial silylation through

reaction with n-docecyl-dimethylchlorosilane in toluene in order

to improve their dispersion in PLA. Silylated cellulose showed

∗ Correspondence to: M. Peltzer, Analytical Chemistry, Nutrition and Food

Sciences Department, University of Alicante, PO Box 99, 03080 Alicante, Spain.

E-mail: [email protected]

a Analytical Chemistry, Nutrition and Food Sciences Department, University of

Alicante, PO Box 99, 03080 Alicante, Spain

b School of Biotechnology, Royal Institute of Technology, AlbaNova University

Centre, SE-106 91 Stockholm, Sweden

c Department of Fiber and Polymer Technology, Royal Institute of Tecnology,

SE-100 44 Stockholm, Sweden

PolymInt 2014; 63: 1056–1062 www.soci.org c 2013 Society of Chemical Industry



Surface modification of cellulose nanocrystals www.soci.org

very good dispersion in the polymer matrix and it was reported

that, after the addition of only 1 wt% of modified nanocellulose,

the crystallization rate of PLA increased significantly. However,

the interfacial adhesion between PLA and the nanocrystals was

not improved since mechanical properties of the nanocomposites

were not enhanced by the CN modification.6

Graftingof polymer chains to thesurface of cellulose nanofillers

wasalso studied to promote thedispersion of CNs in hydrophobic

polymermatrices. The catalyticring-openingpolymerization(ROP)of lactones is the most common route for the synthesis of

polyesters based on bio-based monomers, such as lactic acid. This

reaction can be carried out in the melt or in solution by cationic,

anionic or coordination-insertion mechanisms, depending on the

catalyst. Stannous octoate (Sn(Oct)2) is the most common catalyst

in ROP processes due to its high effectiveness and low toxicity. 7

Lonnberg et al .12 and Lin et al.14 grafted PCL from the surfaces

of cellulose nanofibers and nanocrystals, respectively, with good

dispersion of the cellulose nanofillers in hydrophobic polymer

matrices. PCL-grafted CNs were incorporated into a PLA matrix to

produce nano-biocomposites with high mechanical performance.

Recently,PLA chains were also chemically grafted on the surface of

CNs to enhance the compatibility between them and biopolymermatrices such as PLA, resulting in a general improvement in the

physicochemical properties of the nano-biocomposites.15

The aim of this study was to modifythe surfaceof CNs by grafting

from L-lactide by ROP in order to improve the compatibility of

nanocrystals and hydrophobic polymer matrices, such as PLA.

EXPERIMENTALMaterials

Microcrystalline cellulose (MCC) Avicel PA-200 was purchased

from Sigma-Aldrich (Mostoles, Spain). Sulfuric acid was supplied

by Merck (Darmstadt, Germany). Chloroform and toluene were

also obtained from Sigma-Aldrich. L-Lactide 98% (Sigma-Aldrich)

and tin(II) ethylhexanoate (Sn(Oct)2) (95%, Aldrich) were used as

received.

Nanocrystal preparation

CNswereprepared by acid hydrolysis of MCCwith H2SO4 .MCC(20

g)wasdispersedin175mLof64wt%H2SO4 and then stirredat 600

rpm for 30 min at 45 ◦C. The reaction was stopped by diluting the

suspension with 20-fold of distilled water and washed by succes-

sive centrifugations at 4500 rpm for an extra 20 min. Furthermore

nanocrystals were dialysed against deionized water for 7 days,

followed by an ultrasonic treatment. Mixed bed ion exchange

resin (Dowex Marathon, Sigma-Aldrich, Stockholm, Sweden

MR-3 hydrogen and hydroxide form) was added to the cellulose

suspension for 48 h and then it was removed by filtration. This

procedure ensured that all ions linked to the cellulose molecules

were removed except the H+ counter ions associated with the

sulfate groups on the CN surfaces. A small amount of NaOH (1%)

wasaddedin order to increase thepH until neutrality andto avoid

degradation of CNs during surface modification and processing athigh temperatures. The resultant CN aqueous suspensions were

approximately 0.5% (w/w) and the yield was ca 20%. Finally, CN

suspensions were lyophilized to ensure safe storage before use.

The nanocrystals obtained by following this procedure were

examined by SEM (FE-SEM, Hitachi S-4800) at an acceleration

voltage of 1 kV. A droplet of dilute CN suspension (0.1 wt%) was

deposited on a mica disc with a drop of poly-L-lysine solution

before testing.

Surface modification of CNsby grafting from PLA via ROPof L-lactide

Grafting was carried out by the following procedure. 50 mg of

lyophilized CNs were dispersed in 10 mL of toluene under stirringin a reaction flask for 24 h. Thereafter, L-lactide and toluene (15

mL) were added and the flask was immersed in an oil bath at

95 ◦C. A catalytic amount of Sn(Oct2) (1 wt% of the monomer)

was added to the reaction mixture under a nitrogen flux. The

polymerizationreactionwasthenallowedtoproceedfor20−24h.

The ratio between monomer (L-lactide) and CNs was 300:1 and

400:1; these samples were calledCN-g-PLA300 and CN-g-PLA 400,

respectively. In order to remove the adsorbed but not chemically

bonded polymer, the grafted CNs were washed with chloroform

with further centrifugation in the same experimental conditions.

Fourier transform infrared (FTIR) analysis of the supernatant was

carried out in order to confirm the efficiency of the washing

procedure until no polymer was detected in the supernatant.

Characterization

FTIR was performed on a Bruker Analitik IFS 66 equipped with a

Golden Gate single reflection attenuated total reflectance (ATR)

system. ATR infrared spectra were obtained in the 4000−600

cm−1 region, using 128 scans and 4 cm−1 resolution, in order to

compare thesignificant absorption bandsfor graftedcellulose and

the original CNs. Birefringence of the suspensions containing CNs

wasstudied with a setup containing a lamp, a magnetic stirrer and

two polarizing filters.

(a) (b)

Figure 1. Scanning electron micrographs of cellulose nanocrystals: (a) 50 000×; (b) 100 000× .

PolymInt 2014; 63: 1056–1062 c 2013 Society of Chemical Industry wileyonlinelibrary.com/journal/pi



www.soci.org M Peltzer et al.

Figure2. Suspensions of unmodified cellulose nanocrystals(right vial) andCN-g-PLA 400 (left vial) in chloroform after 24 h.

Transmission electron microscopy (TEM) images were recorded

with a JEOLJEM-2010(Tokyo, Japan) usingan accelerating voltage

of 100 kV. X-ray photoelectron spectroscopy (XPS) (K-ALPHA,

Thermo Scientific Walthlm (MA), USA) was used to analyse the

CN and CN-g-PLA surfaces. All spectra were collected using Al-K

radiation (1486.6 eV), with a twin crystal monochromator, yielding

a focused X-ray spot with a diameter of 400 nm, at 3 mA and 12

kV. The alphahemispherical analyser was operatedin the constant

energy mode with survey scan pass energiesof 200 eV to measure

the whole energy band and 50 eV in a narrow scan to selectively

measure particular elements. Charge compensation was achieved

with the system flood gun providing low energy electrons and

low energy argon ions from a single source. Thus, XPS was used

to provide the chemical bonding state as well as the elemental

composition of the CNs and CN-g-PLA surfaces.

RESULTS AND DISCUSSION The homogeneity of the suspensions obtained after treatment

by acid hydrolysis of MCC was studied by obtaining the SEMimages of the CNs (Fig. 1). It can be observed that the presence of

CNs leads to individualized rod-like nanocrystals with high aspect

ratio. In addition, no agglomerates were observed because of the

dilution of the nanocrystals before lyophilization, the electrostatic

repulsion between surface-grafted sulfate ester groups resulting

from the sulfuric acid hydrolysis and the CNs repelling each other

and not flocculating.16

The mainpurposeof the surfacemodification of CNs is to achieve

their full dispersion in non-polar solvents and/or hydrophobic

polymers to make their blending with biopolymers such as

PLA easier. Figure 2 shows a photograph of suspensions of

unmodified CNs andgraftednanocrystals withPLA, bothdispersed

in chloroform. As can be seen, the dispersion after the grafting

reaction was significantly improved due to the covalent bonding

of PLAchains with CNs. This resultis an indication of theimproved

performance of grafted CNs to obtain homogeneous blends with

hydrophobic polymers such as PLA.

However, it is important to maintain the CN structure after

grafting. At low concentrations, CN particles were randomly

oriented in aqueous suspensions as an isotropic phase. At higher

(a) (b)

Figure 3. Birefringence of the cellulose nanocrystals dispersed in chloroform: (a) CN; (b) CN-g-PLA 400.

(a) (b)

Figure 4. TEM of CNs (a) before and (b) after grafting.

wileyonlinelibrary.com/journal/pi c 2013 Society of Chemical Industry Polym Int 2014; 63: 1056–1062




Figure 5. FTIR spectra of unmodified and grafted CNs with PLA: (A) full spectra; (B) carbonyl peak area enlarged.

80013001800230028003300

2nd wash

3rd wash

4th wash 5th wash

Wavenumber (cm-1)

Figure 6. FTIR spectra of the chloroformic supernatant from washing steps.





concentrations, suspensions of CNs should suffer a change in

their optical behaviour, from an isotropic to an anisotropic chiral

nematic liquid crystalline phase.17 Therefore, for further increases

in CN concentration, suspensions of CNs showed thebirefringence

phenomenon.18 It has been suggested that a study of this

birefringencecould give an indication of the degree of association

or isolation of CNs in a suspension.19 Figure 3 shows the clear

flow birefringence of CNs and grafted CNs. In both cases, strong

birefringencewas observed. This result indicates the existence of anematic liquid crystalline alignment in both samples showing the

presence of CNswith theusual characteristics. This is an additional

indication of the good dispersion of CNs in chloroform, without

agglomeration and preserving the intact crystal shape.20

TEM images were recorded for neat and modified CNs. Dilute

suspensions (0.01 wt%) in chloroform for both samples were

prepared and a microdrop was deposited on a TEM grid for

further analysis. Figure 4 shows TEM images of CNs and CN-g-PLA

samples. Individualnanoparticles wereobserved in bothcases and

the rod-like shape of the nanocrystals was also maintained after

grafting.

The surface modification of CNs was evaluated by FTIR for all

samples.Figure5showsthemostrelevantresultsobtainedinthesetests. The main difference between spectra for non-modified and

graftedCNs was thepresence of thepeakat 1758 cm−1 (seeFigure

5 (b)), corresponding to the stretching frequency of the carbonyl

group in PLA and/or lactic acid oligomers.14 The presence of this

peak in PLA suspensions has already been reported by Rincon-

Lasprill et al.21 These authors studied the synthesis and further

characterization of PLA in specific conditions. They reported that

the stretching band related to the carbonyl group shifted from

1727.1 to 1757.9 cm−1 after polymerization of the monomer

(lactide) to polymer (PLA). As expected, the intensity of that

peak increased for the sample with a higher initial concentration

of monomer (CN-g-PLA-400). This result could indicate that the

amountof PLA grafted from theCN surface could be controlled by

the added monomer to nanocrystal ratio. Lonnberg et al. alsoreported that the variation of the -caprolactone to initiator

ratio conditioned the length of PCL chains grafted to CNs

and consequently the amount of PCL grafted from the surface

increased.12 In addition, the intensity of the broad band around

3300 cm−1 corresponding to the hydroxyl group present in the

original cellulose structure clearly decreased with increase in

PLA concentration and the consequently higher efficiency in the

polymer grafting. These results confirm thesuccess of thegrafting

procedure of CNs by ROP of L-lactide, initiated from the hydroxyl

groups available on the CN surface and catalysed by tin octoate.

FTIR was also used to evaluate theefficiency of thewashing and

purification steps after the grafting reaction. The main indication

of the high efficiency of these processes was obtained from thespectra of chloroform supernatants aftercentrifugation in order to

determine the absence of free PLA, polymerized during reaction

but not grafted to CNs. These spectra were obtained after every

washing step andtheyare shownin Fig. 6. It shouldbe noticed that

the intensity of the characteristic peak of the PLA carbonyl group

decreased withthe successivewashing and purification steps until

complete disappearance after the fifth step. Therefore, it can be

stated that no free PLA was present after five purification steps

andconsequently all PLA present in thematerials is grafted to CNs

forming the CN-g-PLA samples.

The presence of PLA grafted on the CN surface was also

confirmed by XPS tests. The main chemical elements and

particularly the carbon based bonds were detected using this

Figure 7. General XPS spectra for (a) unmodified CN; (b) CN-g-PLA 300; (c)CN-g-PLA 400.

technique and this led to a study of the chemical composition

of all materials. In particular this technique helped to elucidate

whether PLA was grafted from CNs and to evaluate whether

the grafting reaction was successful. Spectra of unmodified and

modifiedCNs areshown in Figure7 with carbonand oxygen atoms

being the main components.

The general carbon XPS signal in Fig. 7 could be resolvedinto several peaks corresponding to different chemical structures,

which reflect the local environment of the carbon atoms.

Figure 8 shows the deconvolution of the general C1 peaks for

all the studied materials. The C1 high resolution spectra could

be resolved into four different peaks corresponding to C1, C2,

C3 and C4 and their corresponding binding energies. It should

be noted that the C1 peak corresponds to C−C/C−H linkages,

the C2 peak corresponds to C−O bonds in alcohol and ether

functional groups, the C3 peak corresponds to O−C−O and C O

structures fromacetalmoieties andfinally theC4 peakcorresponds

to O−C O bonds representing the ester carbon contribution.5,11

The quantification of the relative intensity of each peak after

deconvolution is shown in Table 1.





0

5000

10000

15000

20000

25000

280282284286288290292294

Binding Energy (eV)

Total

C1

C2

C3

C4

0

5000

10000

15000

20000

25000

30000

280282284286288290292294

Binding Energy (eV)

Total

C1C2

C3C4

0

10000

20000

30000

40000

50000

60000

70000

280282284286288290292

Binding Energy (eV)

Total

C1

C2

C3

C4

(a)

(b)(c)

Figure 8. Deconvolution of the C1 signal into its constituent contributions for unmodified and modified CNs: (a) unmodified CN; (b) CN- g-PLA 300; (c)CN-g-PLA 400.

Table 1. XPS analysis of cellulose nanocrystals before and aftergrafting

Samples

C−C/C

−H x (%) C−O (%)

O−C

−O (%)

O−C

O (%)

CN 8.5 40.1 9.2 1.3

CN-g-PLA 300 14.0 27.1 5.2 8.1

CN-g-PLA 400 26.0 25.8 2.5 9.7

The presence of carbon atoms with three of their four typical

bonds linked to oxygen atoms in unmodified CNs could be due to

the presence of residual cell wall polysaccharides with carboxylic

groups that could remain linked to the CNs or could be the result

of the oxidation of the end groups of cellulose. 7 In addition,

the presence of carbon atoms without any oxygen bond couldbe explained only by the presence of some contaminants, since

cellulose structures do not contain any carbon without oxygen

bonds.7 In Figure 8 and Table 1, it can be noted that considerable

changes were found in the proportion of each type of C atom,

especially in theC1 peakreferringto aliphaticchains (C−H)present

in grafted samples in comparison with the pristine cellulose. In

addition, a decrease in the intensity of the C2 peak with the

modification was observed due to the decrease in the number of

C−OH bonds, since cellulose −OH groups are considered to be

initiators of the ROP reaction. C3 and C4 peaks were shifted to

higher energies and their intensities were increased in modified

CNs due to the increase in the quantity of C O and O−C O

bonds by the presence of PLA in these materials. This shift to

higher energies could also be caused by oxidation of the hydroxyl

groups. This behaviour could be attributed to the chain grafting

to the nanocrystal surface providing an additional proof of the

successful grafting of PLA to cellulose after following the process

proposed in this study.

CONCLUSIONSA ‘grafting-from’ method to get CNs with active surfaces to be fur-

therblendedwith hydrophobicpolymerswas developed.CNs were

prepared by acid hydrolysis of MCC and further modified by the

ROP of L-lactide, initiated from the hydroxyl reactive groups avail-

ableintheCNs.Birefringencewasobservedinsolutionsofmodified

nanocrystals in chloroform showing their intact crystal shape with

no apparent agglomeration. TEM images also evidenced that the

crystal shapewas maintained after modification. The effectiveness

of the polymerization reaction was evidenced by spectroscopictechniques and by the dispersion in non-polar solvents, showing

the reduction of the polar character of the nanocrystals. In sum-

mary, the ‘grafting-from’ CNs with L-lactide is a very promising

way to achieve good dispersion of nanocrystals through polymer

matrices in order to obtain nano-biocomposites with improved

properties. Further studies on blending of these new modifiedCNs

with common biopolymers such as PLA are currently ongoing.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the

Spanish Ministry of Economy and Competitiveness (project Ref.

MAT2011-28468-C02-01).





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2014 surface modification of cellulose nanocrystals by grafting with poly(lactic acid)

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