ORIGINAL PAPER

Preparation and properties of chitosan/clay(nano)composites: a silanol quaternary ammoniumintercalated clay

F.-C. Chiu & S.-M. Lai & I.-C. Hsieh & T.-M. Don &

C.-Y. Huang

Received: 25 May 2011 /Accepted: 18 October 2011 /Published online: 22 February 2012# Springer Science+Business Media B.V. 2012

Abstract Chitosan/clay (nano)composites were preparedby using a special quaternary ammonium intercalating agentcoupled with a silanol group to facilitate the organic clayformation. Exfoliated clay in the chitosan matrix wasattained at the higher intercalant dosages through X-raydiffraction (XRD) and transmission electron microscope(TEM) analyses. Optical transmittance for the (nano)com-posites increased slightly with increasing the amount ofintercalants in the clays. In light of the hydrophobic com-ponent on the intercalant and the effective clay content, theinterfacial interaction between chitosan and modifiedclay may not be strong enough to render higher me-chanical properties, even though the partially exfoliatedclays were achieved to provide high interfacial area forthe dispersed phase and the matrix. An optimumYoung’s modulus was thus found for (nano)compositesusing modified clay at a medium dosage of intercalant,

which resulted from the balance of the dispersion statusand interfacial interaction. This outcome indicated highdispersion of modified clay may not guarantee highmechanical properties of (nano)composites. The antimi-crobial property of chitosan against Escherichia coli (E.coli) increased further with the addition of modifiedclays, in which the intercalant exhibiting the antimicrobialfunction. The modified clay at an optimum dosage ofmodifier to balance the mechanical properties and anti-microbial property was attained.

Keywords Chitosan .Montmorillonite . (Nano)composites .

Mechanical property . Antimicrobial property

Introduction

The application of polymer composites or blends based onmaterials possessed different characteristics has been one ofsimple approaches to obtain novel materials for years. How-ever, the micro-phase separation between dispersed fillerand polymer matrix often occurs due to a lack of substantialinteraction for conventional polymer composites. Recently,organic–inorganic hybrids with nano-structure have beendeveloped to form nanocomposites, which rendered ad-vanced performance in thermal stability, stiffness, and pro-cessibility with the high dispersion of nanoparticles, such asnanoclays. These outstanding material properties have led toprofound research on numerous polymer/clay nanocompo-site systems [1–6].

Biorenewable polymers featuring ecological advantagewithout depending on fossil resource toward sustainabledevelopment have been of prime green market trend [7].Chitosan, a major derived byproduct from abundant naturalfood sources, has been considered as a targeted biopolymer

F.-C. Chiu : I.-C. HsiehDepartment of Chemical and Materials Engineering,Chang Gung University,Tao-Yuan 333, Taiwan, Republic of China

S.-M. Lai (*)Department of Chemical and Materials Engineering,National I-Lan University,I-Lan 260, Taiwan, Republic of Chinae-mail: smlai@niu.edu.tw

T.-M. DonDepartment of Chemical and Materials Engineering,Tamkang University,Taipei 251, Taiwan, Republic of China

C.-Y. HuangInstitute of Materials Science and Nanotechnology,Chinese Culture University,Taipei 111, Taiwan, Republic of China

J Polym Res (2012) 19:9781DOI 10.1007/s10965-011-9781-5

due to its biocompatibility, non-toxicity, biodegradability,heat resistance, antimicrobial property, etc.

Recently, the results of researches on chitosan/clay nano-composites to further improve their performance in variousproperties have been reported [8–15]. Darder et al. [8]employed cationic biopolymer chitosan into Na+-montmo-rillonite (Na+-MMT), providing a -NH3

+ functional groupfor the anion exchange site as an anion sensor. The d-spacing of pristine clay increased from 1.20 nm (pristineclay) to 2.09 nm at the 10:1 chitosan-clay ratio. To evaluatethe antimicrobial property, Wang et al. [9] found that cetyl-trimethyl ammonium bromide modified organic rectoriteincreased the d-spacing up to 8.24 nm at the 12:1 weightratio of chitosan and modified rectorite. Their preparednanocomposites showed positive antimicrobial propertyagainst Gram-positive bacterial, but weaker effect for Gram-negative bacterial. Han et al. [10] reported the investigation ofchitosan–montmorillonite nanocomposite systems preparedby an ion exchange reaction between a water soluble oligo-meric chitosan and a Na+-MMT. The nanocomposite filmsshowed a synergistic effect for antibacterial property [9–11].Ngah et al. [12] conducted a brief review on the adsorption ofdyes and heavy metal ions by chitosan composites. Xu et al.[13] have compared the dispersion status effect on themechanical properties of chitosan/clay nanocomposite filmsusing two types of commercial clay, Na+-MMT and organo-clay (30B). It was found that Na+-MMTcase gave an interca-lation and/or exfoliation dispersed structure up to 5% ofloadings rather than micro-scale tactoids structure for 30Bcase, leading to enhanced tensile strength for Na+-MMT casebut not for 30B case. Nevertheless, the above interestingfinding was attributed and limited to the evolved structurebut not for a detailed discussion unfortunately.

To elucidate the detailed role on the compatibilizer effectin assisting the clay dispersion to the mechanical propertiesof nanocomposites, Szazdi et al. [16] pointed out that a highexfoliation extent may not guarantee high strength for lay-ered silicate nanocomposites as demonstrated by PP/claynanocomposites using polypropylene (PP)-g-maleic anhy-dride (MA) as a compatibilizer. Their proposed model foryield strength was evaluated using a survey of about fortyPP/clay nanocomposite systems, suggesting the significanceof the compatibilized inter-phase between the matrix andclay. In our previous study to justify the above finding, thePP-g-MA compatibilized styrene-ethylene-butylene-sytrenecopolymer (SEBS)/clay system conferred higher Young’smodulus than the SEBS-g-MA compatibilized system, eventhough a higher dosage of SEBS-g-MA was capable offurther expanding the interlayer spacing of the clay [17,18]. To the authors’ best knowledge, no available literaturehas been reported on the formation of chitosan/clay (nano)composites using a novel intercalating agent (octadecyltrimethoxysilane trimethylammonium chloride) endowed

with antimicrobial property. A special quaternary ammoniumintercalant agent (Q-guard) with a silanol group was chosen tomodify the pristine clay in order to prepare chitosan/organo-clay (nano)composites equipped with antimicrobial function.Either ammonium group or silanol group could act as anintercalating moiety [19], though the higher efficiency of theammonium group was often suggested for an ion-exchangereaction in the clay gallery. The properties of the (nano)composites with a typical clay content of 3 wt.% achievingdifferent degrees of intercalation were thus discussed. Inparticular, this organic intercalating agent with its hydrophobicsegment may diminish the interfacial interaction between clayand hydrophilic chitosan. Thus, this work aims to furtheremploy the above concept to investigate the role of this novelintercalating agent on the clay dispersion status, antimicrobialproperty, and mechanical properties of prepared (nano)composites.

Experimental

Materials

The materials used in this study were chitosan, clay, andorganic modifier. The chitosan (HCS) was reagent grade andsupplied from Aldrich. Its degree of deacetylation was higherthan 75% according to supplier information. A commercialclay of Na+- MMT (Cloisite Na+) obtained from SouthernClay Products, Inc. was used as received. The organic modi-fier received from Taiwan Surfactant Inc. is trimethoxysilyl-propyl octadecyl dimethyl ammonium chloride (Q-guard),whose molecular structure is listed below

All reagents and solvents, such as acetic acid and sodiumhydroxide, were used as received.

Sample preparations

First, the clay modified with Q-guard was prepared by thefollowing procedure. Two grams of Na+-clay was thoroughlymixed with various amounts of Q-guard (2, 4, and 8 g) in100 mL de-ionized water for 2 h using a homogenizer at amixing speed of 6,500 rpm at room temperature. The filtrateof homogenized solution subjected to vacuum filtration waskept in the frozen state at -30°C for 6 h, which was followedby the freeze-drying for 18 h. Afterwards, modified clays wereobtained by further vacuum drying at 40°C for 1 day. Thesample code of cxQy represents the weight ratio of clay (x)

Page 2 of 11 F.-C. Chiu et al.

and Q-guard (y) for modified clays. To prepare the chitosan/clay (nano)composite films, chitosan was first dissolved in a 1wt.% acetic acid solution to form a 1wt.% chitosan solution.The 3wt.% of Na+-clay or modified clays with respect tochitosan base was then swelled for 2 h in another 1 wt.%acetic acid solution. Both solutions were thenmixed, followedby stirring at a rate of 800 rpm using a magnetic stirrer for1 day at room temperature, to form a homogeneous solution.Then, 50 mL of the above mixture was cast on PP plasticround dish with 15 cm diameter and dried at 40°C for 6 h. Thedried films were soaked in 1 M NaOH for 1 h to neutralizeacetic acid, and then rinsed by de-ionized water to neutralstate. The thin film samples were then obtained after a furtherair drying for 4 h and vacuum drying at 40°C for 2 h. A samplecode of HCS-cxQy denotes the chitosan/modified clay (nano)composites with cxQy inclusion. The prepared samples werekept in a vacuum desiccator for 12 h at room temperature priorto further measurements.

Measurements

FTIR characterization

Infrared spectra of the (nano)composite films were recordedon a Fourier Transform Infrared Spectrophotometer (HORIBA,FT-730) at a resolution of 4 cm−1 for 32 scans from 4,000 to400 cm−1. For clay powder samples, KBr pellet wasmixedwithclay at 100:1 ratio to form pressed disks.

X-ray Diffraction (XRD) technique

A Siemens D5005 X-ray unit operating at 40 kV and 40 mAwas used to carry out the XRD experiments at room temper-ature. The X-ray source was CuKα radiation at a wavelengthof 0.154 nm. The samples were scanned in the 2θ range from1.5° to 30° at a rate of 0.01°/sec.

Transmission Electron Microscopy (TEM)

For TEM observation, the samples were cut into several thinstrips, which were then directly embedded in epoxy resinunder vacuum drying at 70°C for 24 h. The TEM observationwas performed on ultrathin sections of microtomed thincomposite films with a JEOL JEM-1230 system using anacceleration voltage of 100 kV.

Optical transmittance

The optical transmittance of the films with the thickness of 21±3 μm was evaluated using a UV-Visible spectrophotometer(Varian, 50 Conc) at a scanning wavelength from 200 to800 nm.

Thermal stability

Thermogravimetric analysis (TGA, TA Q 50) was used toevaluate the thermal stability of the samples with a heatingrate of 20°C/min under a nitrogen environment from 30°Cto 700°C.

Tensile properties

Tensile property measurements were conducted on samplesaccording to ASTM-D638 [20] with a gauge length of25 mm at a crosshead speed of 5 mm/min using an Instron4469 machine. Tensile strength, elongation at break andYoung’s modulus were recorded. Five specimens were con-ducted for each measurement.

Contact angle

Contact angle was determined using First Ten Angstrom(FTA125) under the static mode with distilled water placedonto the surface of (nano)composite films. Three repetitionswere conducted for each case.

Antimicrobial property

For the antimicrobial property measurements, eight gram ofnutrient broth (Difco) was dissolved into 1,000 mL sterilede-ionizedwater at 120°C for 15mins, followed by sterilizing.Gram-negative bacteria, E. coli (ATCC25922), were first cul-tivated at 37°C for 24 h. Thirty mL of nutrient broth and E.coli were aseptically activated in each flask. One mL ofinoculum of cell suspension in a flask without the followingrotating procedure was used as a control value (Ao). Theturbidity of 1 mL of inoculum of cell suspension added intoa flask containing 30 mL nutrient broth without samples afterrotating at 200 rpm on an orbital shaker at 37°C for 24 h wasdetermined as A. The similar procedure was conducted for theflask containing 0.15 g sample and the value was recorded as

B. The antimicrobial degree (%)0[1- B�A0A�A0

]×100 was then

determined according to ASTM E2149-01 [21]. Five repeti-tions were conducted in most cases.

Results and discussion

Structure characterization and clay dispersibility assessment

The major FTIR spectra regions of pure chitosan and chito-san/clay (nano)composites are depicted in Fig. 1 for com-parison. Control samples of Q-guard and Na+-clay werepresented as well. Typical absorption signals of C0O(1,654 cm−1) and N-H (1,585 cm−1) were illustrated for

Preparation and properties of chitosan/clay (nano)composites Page 3 of 11

chitosan. The broad absorption regions of –CO (1,190~960 cm−1) and hydroxyl group (3,600~3,000 cm−1) werealso observed [22]. With the introduction of Na+-clay, thebroad absorption regions of asymmetric Si-O-Si stretching(1,031 cm−1), and –OH (3,630 cm−1) are indiscernible dueto the overlapping with previous signals and the existence oflattice water in carbohydrate polymer (1,000–1,200 cm−1)[23]. Yet, a visible peak of Mg-O bending vibration at465 cm−1 from clays was identified. Further introductionof various kinds of modified clays into chitosan did notshow much difference in light of a related chemical structurefound in chitosan (NH and CH) and silicates (Si-O).Accordingly, there is a difficulty in identifying the interactionthrough the ammonium site or silane site on the modifier tointeract with chitosan, although some literature observed wid-ening of FTIR bands or multiplicity that may be attached tothe electrostatics interactions between clay and other biobasedpolymers such as soy protein or gelatin [24, 25]. Alternativecharacterizations such as the antimicrobial test would be ofhelp to provide a suggestive molecular interpretation withinthese (nano)composites and will be discussed later.

Figure 2a shows the XRD patterns of Na+-clay andmodified clays. A visible diffraction peak of 2θ at 7.6°(corresponding to the interlayer spacing of 1.16 nm) forNa+-clay was present. When an equivalent amount oforganic modifier agent was employed to intercalate the clay,the spacing increased to 2.10 nm as revealed by the diffractionpeak shifted to 2θ04.2° (c1Q1). With a further incorporationof the modifier (2-fold of clay content), a broad diffractionwith weaker intensity was observed, indicating a partial exfo-liation or swollen tactoids was attained through the treatment(c1Q2). To improve the exfoliation degree, a higher dosage ofmodifier was incorporated to form c1Q4. A much weakerdiffraction intensity was observed, implying the Q-guardwas an effective intercalant for Na+-clay. However, the

effectiveness in intercalating Na+-clay from either quaternaryammonium ion to exchange metal ion in the clay gallery orsilanol group to form a bonding with clay moiety was still notclear at present. To examine the clay dispersibility in thechitosan matrix, Figure 2b shows the XRD patterns of chito-san/clay (nano)composites. When Na+-clay was employed, aslight increment toward a higher d-spacing of 0.2 nm wasfound, which was attributed to the enthalpy decrementthrough possible hydrogen bonding between OH on clayand NH2 or OH on chitosan to overcome the entropy decreasefrom the confinement of chitosan. On the other hand, notmuch improvement in the interlayer spacing increment wasobserved while c1Q1 was added into the chitosan matrix,which was attributed to the limited compatibility betweenhydrophobic component on the modifier and hydrophilic seg-ment on chitosan. Furthermore, the partially exfoliated clay(c1Q2 and c1Q4) remained in a disorder state within thechitosan matrix, implying a certain interaction was still possi-ble. Note that the characteristic diffraction peaks of 2θ at 10.0and 22.6° for neat chitosan [26] were not varied with theaddition of clays, indicating the undisturbed lattice structureand the insignificant effect on the variation of mechanicalproperties discussed in the later sections.

In addition to the XRD investigations, TEM experimentswere carried out to elucidate the clay dispersion status in the(nano)composites in detail. Figure 3 illustrates the TEMmicrographs of chitosan/clay (nano)composites containingNa+-clay and cxQy. The dark lines (or stacked dark platelets)represented clay tactoids and the gray base represented thechitosan matrix. In Fig. 3a, layers of wrinkles were observedfor Na+-clay dispersed in the chitosan matrix. Figures 3b–dillustrate the dispersion status of cxQy. The dispersion statusof c1Q1 resembled to that of Na+-clay except less c1Q1wrinkles. As for c1Q2 and c1Q4 clays, the agglomerates werepartially intercalated/exfoliated into a multi-layered structureexcept for some observed stacked platelets. A relativelythinner layered-structure was evident within these samples.The higher amount of Q-guard modifiers appeared to be moreeffective in assisting clay dispersion, which was in closeagreement with the earlier XRD results. Literature works[27, 28] also suggested a convenient way to further enhancethe clay dispersion within the polymer matrix using a sonica-tion technique, and this technique is worth carrying out in ourfuture study.

Optical properties

In general, several factors including the matrix properties,the interfacial refractive index difference between matrixand dispersed organic/inorganic domains, and the size ofthe dispersed organic/inorganic domains govern the opticalproperties of polymer composites [29, 30]. Figure 4 showsthe optical transmittance of the prepared chitosan and (nano)

Wavenumber (cm-1)

5001000150020002500300035004000

Tra

nsm

ittan

ce

HCS

HCS-Na+-clay

HCS-c1Q1

HCS-c1Q2

HCS-c1Q4

-OH

3500~3000 cm-1

N-H

1585 cm-1

-CH2

2921 cm-1

-CH3

2851 cm-1

C=O

1654 cm-1

Mg-O

465 cm-1

Al-O

520 cm-1

Na+-clay

Q-guard

2918cm-1

-CH2

2850cm-1 -CH31468cm-1 -CH3

H-O-H

1639 cm-1

Fig. 1 FT-IR spectra of chitosan and chitosan/clay (nano)composites

Page 4 of 11 F.-C. Chiu et al.

composites. The decrement of optical transmittance for allinvestigated samples in the wavelength shorter than 400 nmwas mainly due to the absorption effect. The optical trans-mittance of chitosan/Na+-clay composite exhibited the low-est value in comparison with the highest value for unfilledchitosan in the shorter wavelength range of visible light,reflecting a slight aggregation of the Na+-clay. As for thesystems of clays with various amounts of modifiers, theincrement in the optical transmittance compared with Na+-clay system was observed with the exploit of the organicmodifier. Note that the c1Q4 system composed of the leastamount of effective inorganic content as shown in the follow-ing TGA section could contribute the higher transmittance.

Additionally, thickness effect may also play a role in thedetermination of optical transmittance. Yet, the current obser-vations in the optical transmittance were still in line with theevaluated dispersion status of various modified clays in the(nano)composites as seen in TEM morphology and XRDanalysis. A further study is necessary to elucidate the influen-ces of other factors in detail.

Thermal stability

To evaluate the effect of added clays on the thermal stabilityof chitosan, TGAwas employed. Figure 5a shows the thermalscans of Q-guard, Na+-clay and cxQy samples. As expected,

2 (degree)

0 5 10 15 20 25 30

Inte

nsity

(a.

u.)

c1Q4

c1Q2

c1Q1

Na+-clay

d001= 1.80 nm

d001= 2.10 nm

d001= 1.16 nm

(a)

2 (degree)

0 5 10 15 20 25 30 35

Inte

nsity

(a.

u.)

HCS

HCS-Na+-clay

HCS-c1Q1

HCS-c1Q2

HCS-c1Q4

d001= 2.10 nm

d001= 1.36 nm

(b)

Fig. 2 X-ray diffractionpatterns of a Na+-clay andmodified clay, b chitosan andchitosan/clay (nano)composites

Preparation and properties of chitosan/clay (nano)composites Page 5 of 11

Na+-clay showed a relatively high thermal stability up to 700°C. The residual ash content was ca. 91.1 wt.%. The treatedc1Q1clay gave a slightly lower residue at 68.4 wt.%, a similarorder to that of a commercial organoclay (Cloisite® 20A)manufactured by Southern Clay Products, Inc. [31]. If onetakes the residual ash content as an index, it implied that theeffective clay content for c1Q1 was ca. 25% lower than that ofNa+-clay. In addition, the c1Q4 with the highest loading of Q-guard gave the lowest value of residue at 47.6 wt.%,corresponding to the effective clay content of ca. 52.4 wt.%.

The effective clay content was often not considered in theevaluation on the final performance of (nano)compositeswhen modified clays were loaded into the polymer matrix.Further, Q-guard showed a measurable loss around 200°C, butnot for modified clays. Thus, the amounts of residual modi-fiers were limited. For the self-condensation of silyl groups,Q-guard would also self-condensate to give a certain amountof ash content based on TGA results. Yet, the XRD and TEMresults didn’t reveal the significant amount of derived silicawhen clays were modified with Q-guard, which should beattributed to the less available silanol group on Q-guardcompared with tetraethoxy silane (TEOS) commonly used forthe formation of silica. These observations should be kept inmind for a later comparison on the performance of (nano)composites.

The TGA thermal scans of chitosan and its (nano)com-posites were shown in Fig. 5b. Two main features wereobserved. Firstly, for (nano)composites containing a certainamount of pristine clay, weight loss continued to decreasewith increasing temperature up to ca. 260°C, which wasascribed to the evaporation of water and dehydration ofthe saccharide rings [32]. The incorporated clays hardlyimproved the thermal stability of chitosan as much as nor-mally seen in the clay reinforced (nano)composite systems.Secondly, ash content for chitosan was kept at about 32.3wt.%. This might be associated with its chemical structureprone to form a thermal resistance layer to give a highdegree of residual carbon [33]. In addition, the residual ash

HCS-Na+-clay 1Q1c-SCH

(b)

4Q1c-SCH

(d)

2Q1c-SCH

(c)

(a) Fig. 3 TEM micrographs of aHCS-Na+-clay, b HCS-c1Q1, cHCS-c1Q2, d HCS-c1Q4 (scalebar: 50 nm)

Wavelength (nm)

200 300 400 500 600 700 800

Tra

nsm

ittan

ce (

%)

0

20

40

60

80

100

HCSHCS-Na+-clay HCS-c1Q1HCS-c1Q2 HCS-c1Q4

Fig. 4 Optical transmittance of chitosan and chitosan/clay (nano)composites

Page 6 of 11 F.-C. Chiu et al.

content for (nano)composites containing either Na+-clay orcxQy did not varymuch in the range of 24.2 to 31.9wt.%. Thefactors including complex thermal reaction, the effective claycontent in the (nano)composites, the intercalant loss within theclay gallery, and the aforementioned thermal resistance layerformation should be taken into account for this observation. Ingeneral, the loaded clays appeared to have a limited effect onenhancing thermal stability of chitosan.

Tensile properties

As discussed earlier, a higher intercalant content tended toachieve a better dispersibility of cxQy clay in the chitosan

matrix in comparison with Na+-clay through XRD, TEM,and optical analyses. However, due to the hydrophobiccomponent on the modifier (Q-guard), the interfacial inter-action between chitosan and cxQy may not be strong torender evident improvements of chitosan mechanical prop-erties. Our recent studies [17, 18] and Szazdi [16] showedthat high dispersion of clay did not guarantee high tensilestrength of nanocomposites. It is interesting to see how bothcompetitive factors (interfacial interaction and clay disper-sion status) perform in terms of the mechanical propertiesfor the prepared chitosan/clay (nano)composites.

To understand the basic phenomena on interfacial inter-action, a contact angle measurement was carried out and the

Temperature ( oC )

0 200 400 600 800

Wei

ght (

%)

0

20

40

60

80

100

Q-guardNa+-clayc1Q1c1Q2c1Q4

(a)

Temperature ( oC )

0 200 400 600 800

Wei

ght (

%)

0

20

40

60

80

100

HCSHCS-Na+-clayHCS-c1Q1HCS-c1Q2HCS-c1Q4

(b)

Fig. 5 Thermogravimetriccurves of a Q-guard, Na+-clay,and modified clay; b chitosan/clay (nano)composites

Preparation and properties of chitosan/clay (nano)composites Page 7 of 11

results were shown in Fig. 6. Although this investigation isbased on the surface property of prepared sample films andmay not completely reflect their bulk properties in additionto possible effects of residual modifiers and surface rough-ness, the results are still informative as important references.A higher contact angle implied a more hydrophobic charac-ter. For chitosan, the value was about 83.6±2.0°. The intro-duction of Na+-clay led to the decrease of contact angle to77.5±1.0°. The intercalated clay (c1Q1) filled chitosan alsoexhibited a similar value of 76.9±0.4°. On the other hand,with increasing modifier content, contact angle continued toincrease up to 88.0±2.0°, which in turn appeared to increasea more hydrophobic character of modified clays and toreduce available specific interaction between organic mod-ifier and functional group on chitosan.

For the tensile properties (Table 1), Young’s modulus ofthe samples is shown in Fig. 7a. Young’s modulus for theNa+-clay filled composite showed slightly higher value thanthat of chitosan due to a conventional filler reinforcingeffect. Moreover, the additions of cxQy clays gave evenhigher reinforcements than that of Na+-clay in general. Asseen earlier in the dispersion status assessment, a higherdosage of modifier in cxQy gave higher contact surface ofclay with the chitosan matrix. However, the maximum in-crement of Young’s modulus was reached for intercalatedcases (c1Q1) ca. 45% in comparison with neat chitosan.Most likely, the interfacial interaction dominated throughthe increased contact area here. As pointed out in Fig. 6, thec1Q1-filled chitosan exhibited a similar value of contactangle to chitosan/Na+-clay composite, which implied theprevious maximum increment of Young’s modulus wasattributed to a high dispersion status of clay instead ofreduced compatibility for chitosan matrix with relativelylow dosage of organic modifier. On the other hand, withfurther increasing modifier content, the hydrophobic character

of modified clays continued to increase, which in turnappeared to reduce available specific interaction betweenorganic modifier on cxQy and functional group on chitosan.The Young’s modulus then dropped slightly for both highdosage modifier-incorporated (nano)composites, even thoughthey exhibited a high extent exfoliation of dispersed clay.

A similar study of chitosan/organoclay (30B) nanocom-posites also indicated a similar behavior but even gave alower Young’s modulus than that using untreated clay,which was attributed to the limited interaction between the30B and chitosan [13]. Another study provided a differentphenomenon that chitosan/organoclay nanocomposites gavea higher Young’s modulus than that using untreated clay,instead [34]. Apparently, there was a debate on the perfor-mance of chitosan-based nanocomposites in terms of theseobservations. Note that the effective clay content oftenneglected in the literature was slightly different for theNa+-clay and c1Q1 clay as evidenced by the above TGAanalysis. Nevertheless, the Na+-clay system with the highesteffective clay content still gave the lowest values in Young’smodulus. And, fine adjustment of clays with different dosagesof modifiers provided a better understanding on the abovecompeting factors from interfacial interaction and contactsurface area in our study here.

This interesting result was in line with the work of Szazdi etal. [16] on studying PP/clay nanocomposites using 20 vol% ofPP-g-MA as a compatibilizer. It was indicated that yieldstrength of PP/PP-g-MA/clay was significantly lower thanthose of PP/pristine clay and PP/organoclay nanocompositesdue to the different matrix properties of PP/PP-g-MA fromthose of PP as well as the complex interphase interaction,although the silicate reflection almost completely disappearedfrom the XRD pattern of PP/PP-g-MA/clay system. In ourprevious study to justify the above finding, the PP-g-MAcompatibilized SEBS/clay system conferred a higher Young’smodulus than the SEBS-g-MA compatibilized system, eventhough a higher dosage of SEBS-g-MAwas capable of furtherintercalating/exfoliating the clays in the matrix [17, 18]. Notethat the effective clay content often neglected in the literaturewas slightly different for the Na+-clay and cxQy clays includ-ing various amounts of modifiers as evidenced by the aboveTGA analysis. Nevertheless, the Na+-clay system with the

Sample type

Ang

le (

degr

ee)

75

80

85

90

95

HCS HCS-c1Q1HCS-Na+-clay HCS-c1Q2 HCS-c1Q4

Fig. 6 Contact angle measurement of chitosan and chitosan/clay(nano)composites

Table 1 Mechanical properties of chitosan and chitosan/clay (nano)composites

Sample name Tensile strength (MPa) Young’s modulus (MPa)

HCS 61.5±5.9 1430.9±55.7

HCS-Na+-clay 49.6±2.3 1560.0±57.0

HCS-c1Q1 57.5±2.6 2140.4±29.7

HCS-c1Q2 70.8±6.6 1887.0±99.2

HCS-c1Q4 57.1±5.5 1950.8±12.0

Page 8 of 11 F.-C. Chiu et al.

highest effective clay content still gave the lowest values inYoung’s modulus among the investigated (nano)composites,which signified the unique feature of interfacial interactionbetween the polymer matrix and dispersed clays.

To further compare the effectiveness of modified clays onthe tensile properties of the (nano)composites, tensile strengthof the samples is listed in Fig. 7b as well. As discussedpreviously, the interfacial interaction between the polymermatrix and dispersed clays was a dominant factor in thoseinvestigated cases as demonstrated by the higher Young’smodulus for cxQy-included (nano)composites in comparisonwith the composite with Na+-clay inclusion. Likewise, the

lowest value in tensile strength was found for the system withNa+-clay. However, tensile strength for the (nano)compositesreached the maximum increment 70.8±5.4 MPa for partiallyexfoliated case (c1Q2), reflecting the important role ofmodifier’s dosage in terms of tensile strength for a largedeformation in comparison to a small deformation in thecase of Young’s modulus. Literature indicated that tensilestrength is more sensitive to a defect within the materials[35]. For the c1Q4-included (nano)composite, the tensilestrength did not further increase, which might stem from itslowest effective clay content, higher residual modifiers, andlimited compatibility between the chitosan and clay through

Sample type

You

ng's

mod

ulus

(M

Pa)

1200

1400

1600

1800

2000

2200

2400

HCS HCS-Na+-clay HCS-c1Q1 HCS-c1Q2 HCS-c1Q4

(a)

Sample type

Ten

sile

str

engt

h (M

Pa)

30

40

50

60

70

80

HCS HCS-Na+-clay HCS-c1Q1 HCS-c1Q2 HCS-c1Q4

(b)

Fig. 7 Mechanical propertiesof chitosan and chitosan/clay(nano)composites: a Young’smodulus b tensile strength

Preparation and properties of chitosan/clay (nano)composites Page 9 of 11

increasing hydrophobic component on the clay. The currentresults suggest that dispersion status and interfacial interactionare the important factors in attaining the best performance of(nano)composites in terms of tensile properties here. Adetailed work on the dispersion status was evaluated byFornes et al. [36] who specifically estimated the numberof platelets per clay particle, clay particle length, etc. toexplain how the filler aspect ratio and orientation influencedthe extent of reinforcement in nylon 6/clay nanocomposites.Correspondingly, a further study to take the above factors partis needed to contrast the detailed roles of the modifiersemployed in this study.

Antimicrobial property

It is well known that chitosan shows antimicrobial propertydue to its cationic nature. In particular, chitosan wasreported to be more effective on Gram-positive bacteriarather than on Gram-negative bacteria [9]. It is thus inter-esting to see how the formation of (nano)composites withinclusion of Na+-clay or cxQy affects the antimicrobialproperty of chitosan against Gram-negative bacteria. Theresults are shown in Fig. 8. Pristine chitosan shows theantibacterial property at 79.2±4.0% against E. Coli. Theorganic modifier displayed the highest antibacterial propertyamong all the samples. This may be due to the quaternaryammonium group bearing alkyl groups to disrupt the bacterialcell membranes and cause cell lysis for the similar agent as anintercalant a commercial organic modified clay, Cloisite 30B,in the literature [34]. Some reports [9–11] also found theincreased antimicrobial effect for chitosan nanocompositefilms, which was attributed to the absorption of microbes ontoclay to allow chitosan to function effectively, even though

natural clay didn’t show much antimicrobial property. In ourwork, the addition of Na+-clay into chitosan didn’t increasethe antimicrobial property. Perhaps, the loss of antibacterialamine group on chitosan participating the intercalation mayalso play a different role. Yet, on behalf of this inter-calant with antibacterial property, the incorporation ofcxQy into the chitosan matrix slightly increased the antibac-terial property of chitosan, especially at higher dosage ofmodified clay.

Conclusions

This study investigated the dispersion of 3 wt.% claymodifiedwith a special quaternary ammonium intercalating agent witha silanol group within the chitosan matrix to develop chitosan/clay (nano)composites with antimicrobial function. Theresults from XRD and TEM experiments revealed thatexfoliated clay was attained at the higher intercalantdosages. Optical transmittance for the (nano)compositesincreased slightly with increasing the amount of intercalants.The incorporated clay did not improve thermal stability ofchitosan/clay (nano)composites as much as normally seen inthe clay reinforced nanocomposite systems. The treated claygave higher reinforcement than untreated clay in general. Lowdosage of modifier treated clay (c1Q1) filled chitosanexhibited a similar value of contact angle as untreated hydro-philic clay (Na+-clay) filled system. Additionally, the effectiveclay content often neglected in the literature was slightlydifferent for the Na+-clay and c1Q1 clay. The Na+-clay systemwith the highest effective clay content still gave the lowestvalues in Young’s modulus. This implied the maximum incre-ment of Young’s modulus for c1Q1 clay systemwas attributedto a high dispersion of clay instead of reduced compatibilityfor chitosan matrix with relatively low dosage of organicmodifier. With increasingmodifier content, hydrophobic char-acter of modified clay continued to increase, which in turnappeared to reduce available specific interaction betweenorganic modifier and functional group on chitosan and toresult in a slight drop of Young’s modulus, even though theyexhibited a high extent of dispersed clay. This is in line withour other work [17, 18] to suggest that dispersion status andinterfacial interaction are the major factors in attaining the bestperformance of (nano)composites in terms of tensile proper-ties here. High dispersion of modified clay may not guaranteehigh mechanical properties of (nano)composites. On behalf ofthis intercalant with antibacterial property, the incorporationof organic modified clay into the chitosan matrix slightlyincreased the antibacterial property of (nano)composites.The modified clay at an optimum dosage of organic modifierto balance mechanical properties and antimicrobial propertywas attained.

Sample type

HCS Na+-clay HCS-c1Q1 HCS-c1Q2 HCS-c1Q4 Q-guard

Ari

tibac

teri

al a

ctiv

ity (

%)

70

80

90

100

HCS-Na+-clay

Fig. 8 Antimicrobial properties of chitosan and chitosan/clay (nano)composites

Page 10 of 11 F.-C. Chiu et al.

Acknowledgments The grant-in-aid from R.O.C government underNSC 95-2622-E-197-005-CC3 is greatly acknowledged. Financial sup-port from Taiwan surfactant is acknowledged. Helpful discussions onTEM discussion from Prof. J.-Y. Lai and Dr. Y.-H. Su in Chung-YuanChristian University and on clay modification procedure from Prof. C.-S.Wu at the Department of Chemical and Biochemical Engineering, KaoYun University, TAIWAN are greatly appreciated. We are also grateful toProf. Li-Chien Chang at School of Pharmacy, National Defense MedicalCenter, Taiwan for his discussion on antimicrobial properties.

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