Plasticized Starch-Tunicin Whiskers Nanocomposites. 1. Structural Analysis

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P la sticized St a rch/Tunicin Whiskers N a nocomposites. 1. S tr uctura lAnalysis

M. Neus Angle `s and Alain Dufresne*

Cent re d e Recherches sur les M acromole ´cules Ve ge tales (CERM AV-CNRS), Uni versi te ´ Joseph Fourier,BP 53, F-38041 Grenoble Cedex 9, France

Received M ay 19, 2000

ABS TRACT: Nanocomposite ma terials w ere obtained using glycerol plasticized sta rch as t he ma trix a nda colloidal suspension of cellulose w hiskers a s the reinforcing pha se. The cellulose wh iskers, preparedfrom tunicin, consisted of slender parallelepiped rods with a high aspect ratio. After mixing the raw materials and gelatinization of starch, the resulting suspension was cast and evaporated under vacuum.The composites were conditioned at various moisture contents in order to evaluate the effect of thisparameter on the composite structure. The resulting films were characterized using scanning electronmicroscopy, differentia l scanning calorimetry, wa ter a bsorption experiments, a nd w ide-an gle X-ra yscat tering . An a ccumula tion of plas ticizer in the cellulose/a mylopectin interfa cial zones wa s evidenced.The specific behavior of a mylopectin chains locat ed nea r t he int erface in t he presence of cellulose probablyled to a tra nscrystalliza tion phenomenon of a mylopectin on cellulose whiskers surfa ce.

Introduction

There is a growing interest in the nonfood usage ofstarch-based products for applications in which syn-thetic polymers have traditionally been the materialsof choice. Especially, t he incorporation of gra nularsta rch as filler1- 3 or disrupted sta rch gran ules4- 8 intocommodity plastics has generated worldwide a consider-able a t tent ion in using s tarch to enhance biodegrad-abi li ty of plast ic ma ter ia ls . S tarch is the cheapestbiopolymer, and it is totally biodegradable. It is alsoavai lable in large qua nt i t ies f rom several renewa bleplant sources produced in abundance beyond availablemarkets. But, in starch-filled plastics, bacteria and fungidigest the starch fraction, and the remainder one is notdegraded by any biological activity, representing up to95%of the whole material.

A second generation of sta rch-based ma terials ha vebeen studied in which gra nular sta rch must be mixedwith enough nonaq ueous pla sticizer (generally polyols,such a s glycerol) to enable melting below the decompo-sition tempera ture of sta rch. This procedure yields aproduct in which starch forms a continuous polymericentangled phase or a completely disordered molecularstructure of the gra nular sta rch. This type of star ch isknown as thermoplastic sta rch (TP S)9 or destructuredstarch (DS),10 which can be manufactured using tech-nology a lread y developed for th e production of synth eticplast ics, t hus representing a minor investment. Thepotentia l adva nta ges of such mat erials, apart from theirenvironmental gains, are the abundant availability of

the raw materials from renewable resources, not de-pending on fossil sources, and also their low cost, whichrepresents both economic an d social benefits.

B y itself, sta rch is a poor choice as a replacement forany plast ic. I t i s most ly wa ter-soluble, diff icult toprocess , and br it t le when used without plast icizeraddition. Furthermore, i ts mechanical properties arevery sensitive to the moisture content, which is difficultto contr ol.

In previous works11,12 composite materials were ob-

tained from a potato pulp cellulose microfibrils suspen-sion and an aqueous suspension of gelatinized potatos ta rch as the mat r ix. Improved thermomechanica lproperties and a decrease of the water sensitivity ofthese systems were reported. However, the understa nd-ing of the phenomena involved in t hese improvementsrequires the processing and the chara cter izat ion ofmodel systems. Such model systems can be obtainedusing cellulose w hiskers a s a model cellulosic filler.

Ph ysical incorporation of cellulose whiskers, a ndespecially tunicin whiskers, as cellulosic model filler intopolymeric ma tr ix for th e processing of model compositeshas been largely used, since the first announcement ofusing cellulose whiskers as a reinforcing phase.13 Thisextensive use14 - 18 can be explained by the regularshape, high aspect ratio, and monocrystalline nature ofcellulose whiskers. The main problem associated withthe model ing of the s ta rch-based mate r ia l s i s thepresence of four components (starch, cellulose, mainplasticizer, a nd wa ter). These components can be foundin different phases (amorphous, crystalline, liquid). Inaddition, competitive interactions should occur betweenthese components. All t hese factors ma ke the systemlargely more complex than in the case of unplasticizedamorphous or semicrystalline matrix fil led with cel-lulose w hiskers.

In the present study, the structure of the complexsystem obta ined from plast icized sta rch reinforced withtunicin whiskers is analyzed as a function of celluloseand relative humidity content. From this knowledge, themecha nical beha vior of these ma terials w ill be an alyzedin the second part of the paper.19

Experimental SectionStarch Matrix. The starch gels were prepared by gelati-

n iza t ion of w axy maize s ta rch (a lmos t pure amylopect in ,am ylose content is lower th an 1%) kindly supplied by R oquetteS.A. (Lestrem, France), dispersed in a mixture of water andglycerol (Prolabo, 98%purity). Gels contained 10 wt %waxymaize sta rch, 5 wt %glycerol, an d 85 wt %wa ter. These ratioswere the average va lues found in t he l i t era tu re fo r theprocessing of TPS .20 - 28 The gelatiniza tion wa s performed in a

* To wh om correspondence should be ad dressed. E-ma il: Ala [email protected].

8344 M acromolecul es 2000, 33 , 8344 - 8353

10.1021/ma 0008701 CC C: $19.00 © 2000 America n Chemica l SocietyP ubl ish ed on Web 10/07/2000



stirred a utoclave rea ctor operating a t 160 °C for 5 min. Thesecondit ions w ere optimized by varying systematical ly theprocessing temperature and duration in the ranges 150- 190° C a n d 5- 60 min, respectively. The criteria for the optimiza-tion of the process were the complete disappearance of ghostswithin the starch gel and the avoiding of starch degradation.The determinat ion of the disa ppear an ce of ghosts w as carriedout by optical microscopy, a nd the observat ion of s ta rchdegradation was checked by visual inspection of films appear-ance, degradation leading to a tanning of resulting films. Aftermixing, the suspension w as degassed under va cuum in orderto remove the rema ining air an d cast in a Teflon mold storedat 70 °C under vacuum to al low wa ter evaporat ion.

CelluloseWhiskers. Cellulose microcrystals, or whiskers,were extracted from tunicate (a sea a nimal). The ma ntle oftunicate is formed of cellulosic microfibrils (tunicin) particu-larly well organized and therefore highly crystalline. Colloidalsuspensions of wh iskers in wa ter w ere prepared a s describedelsewhere.13,14,29,30 Mant els were first cut into small fragmentsthat were deproteinized by three successive bleaching treat-ments , fol lowing the method of Wise et al .29 The bleachedmantle ( the tunicin) was then disintegrated in water with aWa ring blender (at a concentra tion of 5 wt %). The resultin gaq ueous tunicin suspension wa s mixed with H2S O4 to reach afina l a cid/wa ter concentra tion of 55 wt %. Hyd rolysis condi-t ions were 60 ° C for 20 min under s t rong s t i rr ing. Thesuspension was neutral ized and washed with water. Aftersonicat ion a dispersion of w ell individualized cellulose w his-kers resul ted, which did not sediment or f locculate a s aconsequence of surface sulfate groups created during t hesulfuric acid treatment.30

Film Processing . The starting products (starch + glycerol+ water + cellulose whiskers suspension) were mixed in orderto obtain composite films w ith a homogeneous dispersion a ndwith different compositions. The glycerol content was fixed at33% (dry basis of s tarch mat r ix). The cel lulose whiskerscontent wa s varied from 0 to 25 wt % (cellulose/sta rch +glycerol). Similar processing conditions as those described forthe unfilled starch matrix were used to gelatinize starch andto process nanocompos ite f ilms. Nat ive w axy maize wa sgelatinized directly in the presence of wa ter, plasticizer, andcellulose.

Film Conditioning. Starch and cel lulose a re highly hy-groscopic ma terials. The str ucture a nd t herefore th e propertiesof these mat erials are strongly related to the wa ter content.31- 36

The moisture content of th e na nocomposite films w a s a chievedby conditioning the samples at room temperature in desicca-tors at controlled humidities containing saturated salt solu-tions. Six relative hum idity (RH) conditions a t 20- 25 ° C w ereused, na mely 0, 35, 43, 58, 75, a nd 98%. The sa tura ted sa ltsolutions were P2O5, C a C l2‚6H 2O, K2CO3‚2H 2O, NaB r‚2H 2O,NaCl, and CuSO4‚5H 2O, respectively. Conditioning w as achievedfor a t least 2 weeks to ensure the equil ibrat ion of the w atercontent in t he films w ith th at of the atmosphere (sta bilizat ionof the sample weight).

Thermogravimetric Analysis. Thermogravimetric analy-sis wa s used to accura tely determine the w at er content of thefilms conditioned at different relative humidities. The mea-surements w ere achieved with a Perkin-Elmer TG A7 instru-

ment. A few milligrams of the sample was heated from roomtemperat ure up to 130 °C a t 5 ° C/min un der nitrogen flow (flow ra te 20 mL/min). The tempera ture w as subsequently st abilizedfor 1 h. The loss of w eight, ascribed to the w at er content, w asmeasured for different wa ter a ct ivi t ies (wa ter a ct ivi ty ) %condit ioning rela tive h umidit y/100) of th e sa mples.

Water Uptake. The k inet ics o f wa te r absorp t ion wa sdetermined for all compositions. The specimens used were thinrectangular strips with dimensions of 10 mm × 10 mm × 1mm. The films were t herefore supposed to be t hin enough sotha t the molecu la r d i ffusion wa s cons idered to be one-dimensional . Sa mples were f i rs t dr ied overnight a t 100 ° C.After w eighing, they were condit ioned a t 20- 2 5 ° C i n adesiccator containing sodium sulfate to ensure a RH ratio of98%. The condit ioning of sam ples in high moist ure a tm ospherewa s preferred to the classical technique of immersion in wa ter,

because s ta rch i s very sens it ive to l iqu id wat e r a nd canpart ial ly dissolve af ter long t ime exposure to wa ter. Thesamples were removed at specific intervals and weighed usinga four-digit bala nce. The wa ter content or wa ter upta ke (WU)of the samples was calculated as follows:

where M t a nd M 0 are the weights of the sample af ter t min

exposure to 98% RH an d before exposure t o high moisturecontent, respectively.The mean moisture uptake of each sample was calculated

for various conditioning times (t ). The mass of water sorbeda t t ime t , (M t - M 0), can be expressed as37

where M ∞ is the ma ss sorbed a t equilibrium, 2L t he thicknessof the polymeric film, and D t he diffusion coefficient. At sh ortt imes, eq 2 can be wri t ten as

At (M t - M 0/M ∞) e 0.5, the error in using eq 3 inst ead of eq 2to determ ine t he diffusion coefficient is on t he order of 0.1%.38

Microscopies . Tra nsmis sion electron microscopy (TEM)observations were achieved with a Philips CM200 electronmicroscope operating at 80 kV. A drop of a dilute suspensionof cellulose whiskers was deposited and allowed to dry on acarbon-coated grid, previously irradiated with a UV lamp.

Scanning electron microscopy (SEM) wa s performed toinvestigate t he morphology of the na nocomposite films w ith aJ EOL J SM-6100 instrument. The specimens were frozen underliquid nitrogen, then fractured, mounted, coated with gold/pal ladium on a J EOL J FC-1100E ion sputter coater, andobserved. SE M micrographs w ere obta ined using 7 kV second-ary electrons.

Differential Scanning Calorimetry (DSC). Differentialscanning calorimetry (DSC) w as performed with a Perkin-Elmer DS C7 equipment, f i t ted with a cooler system usingliquid ni trogen. I t was cal ibrated with an indium standard.Conditioned samples were placed in pressure-tight DSC cells,and a t l eas t th ree ind ividua l measurements were made t oensure reproducibility. Each sample was heated from - 100to + 250 °C a t a hea t ing ra te of 10 °C/min . The mel t ingtemperature (T m) was taken as the peak temperature of themelting endotherm w hile the glass tra nsition temperature (T g)wa s ta ken as t he inflection point of the specific heat incrementat the glass- rubber transition.

Contact Angle Measurements. Contact a ngle measure-ments were achieved in order to evaluate the selective affinityof the na tura l polymers used (amy lopectin a nd cellulose) withthe pla sticizers (wa ter a nd glycerol). S olid pure am ylopectinan d tunicin whisker films were obtained by evaporation. Drops

of glycerol or water were deposited on the solid polymericsurface. The conta ct angles were measured wit h a C CD cam eraand processed by an image analysis video card which calcu-lated θ (contact angle) automatically using an image analysissetup. This image analyzer determines the diameter, D , andthe height, h , of the solvent droplet in order to evaluate theconta ct a ngle following eq 4.

Measurements were performed by static mode on the starchsurface and by dynamic mode on the cellulose surface.39,40

The Owens - Wendt approach 41 was used to es t imate thesurface energy (polar and dispersive components) of theam ylopectin a nd cellulose surface. It is w orth noting tha t th e

WU (%) )M t - M 0

M 0× 100 (1)

M t - M 0

M ∞) 1 - ∑

n ) 0

∞ 8

(2n + 1)2π 2exp[- D (2n + 1)2π 2t

4L 2 ] (2)

M t - M 0M ∞

) 2L(D

π )1/2

t 1/2 (3)

ta n(θ2))2h D

(4)

M acromolecul es, Vol. 33, N o. 22, 2000 St ar ch/Tunicin Whiskers Na nocomposites 8345





both unfilled a nd filled with t unicin whiskers plast icizeds ta rch . We ascerta in tha t for a l l the samples thedesorbed wa ter content increases as th e wa ter a ctivityof conditioning saturated salt solutions (a w ) increases,

but following different behaviors.Three well-separated zones are displayed in Figure2. At low water act ivi ty (0 < a w < 0.35), the watercontent increases slightly, and the curves correspondingto the different loading levels ar e very similar a nd tendto merge into a single curve. The w a ter content is lowerthan 10 wt % wha tever the composi t ion may be. Atintermediate water activity (0.35< a w < 0.75), th e wa tercontent increases more rapidly. This is ascribed to thefact tha t for this wa ter activity ra nge the glass- rubbertransi t ion of the plast ic ized s tarch matr ix probablybecomes lower than room temperature a t which thesamples were conditioned. The concomitant increase inthe free volume generates an increased mobility of watermolecules within the entangled amylopectin network.In t his wa ter a ctivity ra nge aga in, no obvious differenceof behavior is observed a s a function of t he cellulosewhiskers content , a nd the w ater content var ies f rom∼ 10 to ∼ 25 wt %as t he wa ter a ctivity var ies from 0.35up to 0.75. However, for highly filled sam ples th e wa tercontent seems to be lower than for poorly filled ones.This observation is more pronounced for a w ) 0.75. Thiscan be a scribed to either a n increase of the crysta llinityor an increase of the glass- rubber tra nsition temper-ature of starch material in the presence of cellulosewhiskers. In both cases, i t should result in a decreaseof the mobility of wa ter molecules. At high w a ter a ctivity(a w > 0.75), t he desorbed w at er content continues toincrease more rapidly, and the previously differencereported between highly and poorly filled compositestones down.

Water Uptake . In sorption kinetics experiments, themass of sorbed penetra nt is measured as a function oftime. The water uptake during exposure to 98%RH ofthe va rious cellulose wh iskers/plast icized st a rch com-posites versus time wa s evaluat ed. It w a s observed tha teach composition absorbed wa ter dur ing th e experiment.The diffusivity of water is strongly influenced by themicrostructure of the ma terial, such as t he porosity t ha tcan develop during drying a nd a lso by the wa ter a ffinityof the polymer components.42 Moreover, the a ddition ofplast icizers generally increases ga s, wa ter, a nd solutepermeability of the film.43 The change in w eight dur ingconditioning at 98%RH is plotted against time in Figure

3. These swelling da ta are means of severa l trials, a ndthe reliability of measurements was very good. Twowell-separ a ted zones ar e displayed in Figur e 3. At lowertimes (zone I: t < 100 h), the kinetics of absorption isfast, whereas at extended times the kinetics of a bsorp-tion is slow a nd leads to a plat eau (zone II). In zone I,no clear trend is observed w ith r espect to t he cellulosewhiskers content. In zone II, t he wa ter upta ke reachesa plat eau, which corresponds to the wa ter uptake a tequilibrium.

The wa ter uptake at equilibrium versus cellulosecomposition is plotted in Figure 4. It is observed thatunfilled sta rch a bsorbs around 62% wa ter. I t corre-sponds t o ∼ 1.6 g of wat er per gram of star ch. The wa terupta ke at equilibrium decreas es as the tun icin whiskercontent increases. It is only ∼ 40% for the 25 wt %tunicin whiskers filled composite. Therefore, the swell-ing of the m a teria l is reduced in t he presence of cellulosewhiskers within t he plasticized st arch syst em. Similarresults were reported with cellulose microfibril-filledstarch.11,12 This phenomenon was ascribed to the forma-t ion of a microf ibr il network, which prevented theswelling of the star ch and therefore its wa ter a bsorption.However, i t i s w or th not ing tha t the s t ructu re o fcellulose whiskers completely differs from that of themicrofibrils. The former occur a s rigid a nd geometr ica lly

Figure2. C omparison of wa ter content determined from TG Aexperiments versus water activity for glycerol plasticized waxyma ize starch filled with 0 (b ), 3.2 (O ), 6.2 (9 ), 16.7 (0 ), a nd 25w t % (( ) tunicin w hiskers.

Figure 3. Water uptake during condit ioning at 98% RHversus time for glycerol plasticized waxy maize starch filledwi th 0 (b ), 3.2 (O ), 6.2 (9 ), 16.7 (0 ), and 25 wt %(( ) tunicinwhiskers. Results are the average values of triplicates and 95%confidence int ervals are reported.

Figure 4. Maximum relat ive wa ter uptake, or water uptakeat equilibrium, during conditioning at 98% RH for glycerolplast icized waxy maize s tarch f i l led with tunicin whiskers

versus w hiskers content. The solid line serves to guide th e eye.Results are the average values of triplicates, and 95%confi-dence intervals are reported.

M acromolecul es, Vol. 33, N o. 22, 2000 St a rch/Tunicin Whiskers Na nocomposites 8347



well-defined rods, whereas the la tt er consist of soft an droughly individualized ha iry-shaped fillers. The de-creased w a ter sens itivity of cellulose filled sta rch shouldconsequent ly resul t , a t least par t ia l ly, f rom anotherphenomenon. This could include a decrease of t heam ylopectin cha ins mobility, resulting from a n increaseof the glass- rubber t ransi t ion temperature or an in-crease of the crysta llinity.

The water diffusivity or diffusion coefficient, D , ofwa ter in the sta rch-based mat erial w as estimat ed usingeq 3. The plots of (M t - M 0)/M ∞ as a function of (t /L 2)1/2

were performed for all the compositions and for (M t -M 0)/M ∞ e 0.5. The diffus ion coefficients wer e calcula tedfrom the s lope of these plots . The wa ter diffusion

coefficients of the unfilled matrix and reinforced com-posites a re collected in Ta ble 1. The un filled plas ticizedsta rch ma trix displays t he highest diffusion coefficient.Adding cel lulose whiskers within the s tarch matr ixresults first in a decrease of D va lue from 1.76 × 10- 9

cm 2 s - 1 for unfilled st a rch up to 1.47× 10 - 9 cm 2 s - 1 forthe 6.2 wt %filled system. This observation agrees withprevious result s obta ined for cellulose microfibrils/sta rchcomposites . This phenomenon wa s ascr ibed to thepresence of a three-dimensional intertwined cellulosemicrofibrils network within the matrix, resulting fromthe establishment of strong hydrogen bonds betweencellulose microfibrils which can develop during theevaporation step. This network tends to stabilize thestarch matrix when it is submitted to strong moisture

conditions. At higher load ing level, th e previous t rendis no t clea r, and an inverse dependence wi th thewhiskers addition is observed.

Thermal Analysis . Differential scanning calorimetry(DSC ) measurements were performed on plast icizedsta rch ma trix a nd r elat ed tunicin w hiskers fil led com-posites conditioned at various relative moisture con-tents.

Sta rch / Glycero l M at r ix . F igure 5 shows the DSCtra ces of glycerol plast icized sta rch ma trix conditionedat 0 and up to 75%RH. All samples display t wo distinctill-defined (as least for this heat capacity scale) specificheat increments. Expended views (not shown) of theapparently flat low-tempera ture DS C t ra ces were per-formed to precisely analyze these events. The temper-a tu res associa ted wi th the midpoint s of these twocalorimetric transitions are plotted in Figure 6 (filledcircles) as a function of the water content.

The low-temperature specific heat increment is lo-cated betw een - 4 7 a n d - 98 °C depending on themoisture content . A relaxat ion process in glycerolplasticized potato starch was observed by Lourdin eta l.44 in this temperature ra nge using dynamic mechan-ical analysis. It was assigned to the combination of asecondary relaxat ion of starch an d the ma in relaxa tionof the water- glycerol mix. Secondary relaxation pro-cesses have been observed by several a uthors in a ra ngeof gelatinized and granular solid starches. They wereassigned to either an increase in mobility of water in

starch45 or small motions of the chain backbone androtation of methylol groups.36 However, i t i s w orthnoting that secondary relaxations are not detectable byDS C. It w as r eported th a t a t high glycerol content (12%and up) th i s relaxa t ion p rocess was main ly due toglycerol.45 We suggest assigning this low-temperaturetra nsition to the glass- rubber tra nsition of glycerol-richdomains.

The high-tempera ture specific heat increment isobserved from 27 to - 13 °C depending on the moisturecontent . It is a scribed to the glass- rubber transition ofamylopectin-rich domains. The wa xy maize sta rch-glycerol matr ix appears therefore to be a complexsystem composed of glycerol-rich and amylopectin-richdomains. The avera ge composition of these t wo dist inctdomains can be es t imated, a t l eas t for wa te r-f reemat erial, using the Fox equa tion. Taking t he extrapo-lated va lue of Orford et al.46 for T g (∼ 500 K) of starchand the experimental va lue for T g of glycerol as deter-mined from DSC measurements (∼ 194 K), i t leads toglycerol-rich domains containing around 20 wt %amy-lopectin a nd to a mylopectin-rich doma ins composed ofabout 65 wt % amy lopectin. These values ha ve to be

Table 1. Water Diffusion Coefficients in CelluloseWhiskers/Plasticized Starch Composites Conditioned at

98% RH

whiskerscontent (wt %)

waterdiffusion coeff(cm 2/s × 109)

whiskerscontent (wt %)

wate rdiffusion coeff(cm 2/s × 10 9)

0 1.76 16.7 1.693.2 1.53 25 1.596.2 1.47

Figure 5. DSC thermograms of glycerol plast icized waxymaize s tarch for various moisture contents . The relat ivehumidity conditions are indicated in the figure.

Figure 6. Glass- rubber transition temperatures associatedwith the midpoints of the tra nsitions versus wat er content forglycerol plast icized wa xy ma ize s tarch f i lled with 0 (b ), 3.2(O ), 6.2 (9 ), 16.7 (0 ), and 25 wt %(( ) tunicin wh iskers. Solidlines serve to guide t he eye.

8348 Angles a nd D ufresn e M acromolecul es, Vol. 33, N o. 22, 2000



compar ed to the mean va lue, a ssuming a homogeneousd is t r ibut ion of g lycerol wi th in s ta rch , of 67 wt %amylopectin. This obvious aberration clearly shows thatth e Fox equa tion is unsuit a ble for th is polymer/solventsystem. This inadequacy results from expected stronginteractions between both components. Dynamic me-chanical measurements performed on this system in t hesecond part of this paper19 support the idea that thisheterogeneous system should be visualized as a blendcomposed of glycerol-rich domains included in an amy-lopectin-rich matrix.

Both events strongly depend on the water content,and their temperature decreases as the moisture con-tent increases, displaying a classical plasticizing effectof water. This phenomenon tends to stabilize at highmoisture content, because no significant evolution ofboth t ra nsitions is observed betw een 15 and 25%wa tercontent.

For low water content plasticized starch (up to 35%RH), the f la t shape of the DS C trace (Figure 5) is a nindication of the amorphous state of the material. Atincreasing moisture content, a n endotherma l peak a p-pears . I ts temperature posit ion f irs t increases withwater content, from ∼ 130 °C for samples conditioneda t 43%RH up to ∼ 155 ° C for samples conditioned a t58% RH. At increasing moisture content, i t tends tosta bilize. This first-order tra nsition is att ributed to themelting of w at er-induced crystalline amylopectin do-mains. Indeed, i t i s wel l -known that during s toragegelatinized starch can convert from a noncrystallizedform to a crys ta ll ine form. This event , known asretrogradation, results from the reassociation of amor-phous starch or starch with a low degree of orderinginto a more ordered state. This phenomenon includesthe formation of short-range order ing such as theformation of single and double helices, gelation, theformation of entanglements or juncture points, and thecrystallization of aggregates of helical structures. This

reorganization and crystallization of the amylopectinmolecules is favored by the plasticization effect inducedby water.47,48 This increase in crystallinity of starcheswhen submitted to increased moisture conditions wa salso supported by X-ray diffraction experiments.31,49,50

At high wa ter contents, th e amylopectin is thought toform both inter- a nd int ra molecular double helices. Thedisplacement of the endothermal peak toward highertemperatures w hen t he w ater content increases isproba bly due to the forma tion of larger cryst a l doma insas a result of increased mobility of amorphous chains.In addition, increasing crystallinity of the amylopectinlowers the mobility of the amy lopectin, r esulting in areinforcement of the network by the formation of physi-cal cross-links and a stabilization of the retrogradationphenomenon.

Tunicin Whisker/ Plasticized Starch Composites . Thetherma l behavior of tun icin w hiskers-based compositeswa s also cha ra cterized by DS C measur ements. The DSCtraces aga in show two g lass- rubber t ransi t ions as ,already, observed for t he sta rch ma trix. The tempera-ture a ssociat ed with the midpoints of these tw o calori-metric transitions are plotted in Figure 6 as a functionof water content for the different compositions. Fromthe knowledge of the t hermal behavior of the unfilledglycerol plasticized starch matrix, the low- and high-temperature events were a ssociated w ith the glass-rubber tra nsition of glycerol- and amy lopectin-richdomains, respectively.

T g of the glycerol-r ich fract ion decreases as themoisture content increases , s imilar to what was ob-served for the unfil led plast icized matr ix . For theamylopectin-rich fraction, tw o distinct trends wereobserved depending on the tunicin whiskers content. Atlow loa ding level (up to 3.2 wt %tun icin w hiskers), theclassical plasticization effect of water is reported, andT g decreases a s t he w at er content increases followingthe behavior of the unfilled matrix. For higher cellulosecontent (6 .2 wt % and up), T g of a mylopectin-richdomains significa ntly increases a s th e moisture contentincreases. The a ppar ent unaltera tion of the evolutionof T g of the glycerol-rich fraction versus water contentin the presence of cellulose whiskers resul ts mostprobably from the fact that these domains occur asinclusions in the continuous phase constituted of amy-lopectin-rich domains. Therefore, the tunicin whiskersare most probably in direct contact with amylopectin-rich domains ra ther tha n glycerol-rich domains w hendispersed in th e plasticized st ar ch ma trix.

Three phenomena could expla in the a ntipla sticizat ioneffect of amylopectin-rich domains in the presence ofcellulose whiskers.

(i) The first one is due t o the likely str ong a ffinity ofamylopectin molecules with the reactive cellulose sur-face. Both components exhibit a high densit y of hydroxylgroups. This coupling effect could result in a restrictedmolecular mobility of amylopectin molecules in contactwi th the whiskers su r face . Owing to the very h ighspecific surface of tunicin whiskers, this hinderedmobility could be strong enough to affect the globalflexibility of the starch matrix.

(ii) The second expla na tion could be t he selectivepart i t ioning of glycerol within the mater ia l in thepresence of cellulose w hiskers. One can imagine t ha tglycerol can present higher affinity for the cellulosesurface tha n for the sta rch-based ma trix. A migrat ionof the main plasticizer from the amylopectin-rich do-

mains towa rd the filler/mat rix interface could result ,decreasing the plast icizing efficiency of the glycerol forthe sta rch ma trix. This phenomenon should result inan increase of T g and could be emphasized in moistcondit ions. The selective pa rt itioning of g lycerol in poly-(vinyl a lcohol) (P VA) a nd hyd roxypropylmethylcellulose(HPMC) blends was reported elsewhere.51

(iii) Another explan at ion could be the most l ikelydifferent crystallization conditions of t he glycerol-water- sta rch syst em in the presence of the omnipresentcellulose surfa ce in tun icin w hiskers -rich composites. Atra nscrysta llizat ion phenomenon wa s reported for poly-(hydroxyoctanoate) in the presence of tunicin whis-kers.18 If such a transcrystallization occurs in the tuni-cin whisker/sta rch composites, one can think tha t i tcould result in a restr icted mobility of a morphous amy lo-pectin chains in the vicinity of the crystalli te-coatedfiller surfa ce, because crysta lline doma ins of a mylopec-tin act as physical cross-links. This hindered molecularmobility of amorphous chains in the amylopectin-richdomains should be emphasized w ith th e wa ter contentas a result of the wa ter-induced crystallizat ion.

Though the three mentioned explanations could beinvolved simulta neously in th e observed increas e of T gof the amylopectin-rich fraction of the plasticized starchmat rix, an experimental feature supports the last one.Figure 7 shows t he DS C t ra ces of moist tunicin whisker/plast icized sta rch composites (58% RH conditionedsamples) for different loading levels. S imilar to t he




unfilled ma trix, an endothermic peak a ttributed to themelt ing of wa ter-induced crystal l ites grows as themoisture content increases. This melting end otherm isobserved wha tever the w hiskers content may be. Themelting t emperatures of these endothermic peaks arecollected in Table 2 for all the samples. The heats offusion were not calculated because of the strong dubi-ousness for the determination of the baseline.

It is worth noting tha t t he melting endotherm of filledmat erials exhibits a shoulder on the low-tempera tureside (Figure 7). This sh oulder wa s a lso observed for 75%RH conditioned samples. This splitting of the meltingendotherm resul ts f rom the presence of a bimodaldistribution of crysta llite size. Cellulose most proba blyacts a s a nu cleat ing agent for amylopectin, producing atra nscrystalline r egion a round t he cellulose w hiskers.Orientat ed crysta llizat ion of am ylose from a solution oncellulose was previously observed.52,53 It wa s shown tha tthe la mella r crysta ls of amylose grew exclusively on thecellulose to give a “shish kebab” morphology, consistingof a regular system of edge-on amylose crystals orga-nized perpendicular to t he cellulose microfibrilla r d irec-tion. Such a behavior was assigned to a row nucleationphenomenon rather than to a t rue epi taxial growth.However, t he high viscosity of a mylopectin-rich doma inssurrounding the tunicin w hiskers limits t his phenom-enon and restr ic ts the growth of the “shish kebab”structure.

Contact Angle Measurements . The contact angletechnique wa s used in order to qua ntita tively chara cter-ize the affinity of wa ter a nd glycerol for t he a mylopectinand cellulose pha ses. The conta ct angles measureda ccording to the t echnique described in the experimenta lpart are collected in Table 3 for the two liquids used

(wa ter a nd glycerol) an d th e tw o surfaces (am ylopectinan d t unicin wh isker films). Conta ct an gle values clearlyshow that both glycerol and water display a higheraffinity for the cellulose whisker film surface than forthe a mylopectin one. The surfa ce energy, a s well as thepolar a nd d ispersive component values, of the t wo filmswa s calculated a ccording to t he Owens- Wendt ap-proach41 (Table 4). For both substrates , the polarcomponent is much h igher t ha n th e dispersive one. This

is ascribed to the high d ensity of hydr oxyl groups of bothpolysaccharides. The surface energy is higher for cel-lulose than for amylopectin.

These observat ions t end to show t ha t t he localiza tionof the two plast ic izers (glycerol and water) is mostobviously not homogeneous in the tunicin whiskers/sta rch composites. They probably redistr ibute wit hin th emat rix, diffusing towa rd the cellulose surfa ce. Thisreloca lizat ion d ecrea ses th e plast icizing effect of glyceroland water in the bulk amylopectin matrix. The mainconsequence of this redistribution should be the shiftof the glass- rubber transition of the amylopectin-richphase toward higher temperatures in the composites.The a ccumula tion of plas ticizer in the cellulose/a my-lopectin interfa cial zones could, in turn, improve t heability of amy lopectin chains t o crystallize, leading t othe formation of a t ranscrystal line zone a round thewhiskers.

Wide-Angle X-ray Scattering (WAXS). The n a no-composite films r esulting from t he casting an d eva pora -tion of gelat inized sta rch were cha ra cterized by WAXS.X-ray diffractograms were collected for the differentwa ter content s a nd loading levels in order to determinethe evolution of the crystallinity.

Starch/ Glycerol M atr i x . Wide-a ngle X-ra y d iffra ctionpatt erns of th e unfilled plasticized sta rch ma trix con-ditioned at 35 and up to 75%RH a re presented in Figure8. The diffra ctogra m r ecorded for a film of pure celluloseobtained from the evaporation of a tunicin whiskerssuspension is added in Figure 8. At low moisturecontent, t he sta rch film shows no diffra ction peak an ddisplays t ypica l behavior of a fully a morphous polymer.It is chara cterized by a broad hump locat ed around 2θ) 18°. This result agrees with DS C mea surements.

As the wa ter content increases, the amorphous broadhump shades off progressively, and three il l-defineddiffraction peaks, which grow with moisture content, areobserved. Their angula r locations ar ound 2θ ) 17.2°,22.1°, a nd 23.9° a re typical of B-star ch structure.54 Thecrystalline regions consist therefore of double heliceswith a loose packing density in the unit cell, water beingan integra l pa r t of th is polymorph . The d valuesa ssociat ed w ith t hese peaks a re 5.20, 4.01, a nd 3.72 Å,respectively. The semicrysta lline na tur e of moist plas-

Figure 7. DS C th ermograms of 58%RH conditioned tunicinwh isker/glycerol plas ticized wa xy ma ize sta rch composites. Thetunicin whisker contents are indicated in the figure.

Table 2. Melting Temperatures ( °C) of Tunicin Whiskers/

Plasticized Starch Nanocomposite Films Conditioned atDifferent Moisture Contents

rela tive hum idity (%)whisker content(w t %) 0 35 43 58 75

0 132.6 156.9 156.23.2 132.2 158.4 156.26.2 131.2 165.1 155.9

16.7 132.0 160.0 156.725 134.7 169.5 158.4

Table 3. Contact Angle (deg) Values for Water andGlycerol on Amylopectin and Cellulose Whiskers Surface

Films

subst ra t e films w a ter glycerol

a mylopect in 64 72cellulose w hiskers 24 55

Table 4. Total Surface Energy ( γ S), Polar ( γ p ), andDispersive ( γ d ) Components of Amylopectin and

Cellulose Whisker Films Calculated from the

Owens - Wendt Approachγ S (mJ /m 2) γp (mJ /m 2) γ d (mJ /m 2)

a mylopect in 44.0 42.2 1.8cellulose w hiskers 94.9 94.9 4.9 × 10 - 6




t icized waxy maize starch films evidenced by WAXSexperiments agrees with DSC resul ts . The tunicinwhiskers film displays t hree well-defined pea ks a round2θ ) 14.6°, 16.4°, and 22.7°. The d values associatedwit h t hese peaks a re 6.06, 5.40, an d 3.91 Å, respectively.They a re t ypica l of cellulose I. The relative ma gnitudeof these peaks depends on the whiskers orientation inthe film.

Tunicin Whisker/ Plasticized Starch Composites . Dif-fra ctogra ms of t he highly moist (75%RH) na nocompos-ite materia ls are shown in Figure 9. Diffraction patternsof unfilled pla sticized sta rch a nd t unicin w hiskers filmsare added as references. The diffractograms of theva rious cellulose/st a rch composites consist in a super-imposit ion of the diffractograms of the two parentcomponents bala nced by the composition.

However, i t is worth noting that for the films filledwith 16.7 and 25 wt %of cellulose whiskers, a new well-defined diffraction peak located around 2θ ) 21.15°(corresponding to the vertical line) is observed, whoseintensity increases w ith cellulose content . The d valueas sociat ed w ith t his peak is 4.2 Å. This diffra ction pea karises neither from the unfilled plast icized sta rch norfrom the tunicin. Because tunicin whiskers are well-defined objects, which crystallinity should not be changedwhen dispersed in th e mat rix, this new peak a rises mostprobably from amylopectin. This specific crystallinityoccurs, or at least is detectable, in amylopectin only inhighly moist conditions a nd in th e presence of a hightunicin whiskers content.

To yield the specific cha ra cter of this peak, a modelingof the X-ray diffractograms was performed from thecombination of the diffractograms of the pure parentcomponents. A simple mixing r ule wa s used to build upthe t heoretical diffra ctograms. B oth experimenta l a ndtheoretical wide-angle X-ray diffraction patterns areshown in pa rt s a , b, c, an d d of Figure 10 for highly filledna nocomposites (25 wt %tun icin w hiskers), conditioned

a t 35, 43, 58, an d 75%RH, respectively. The th eoreticalcurve fits very well the experimental data at 35%RH(Figure 10a). At higher moisture content (43 an d 58%RH, Figure 10, b and c), t he fit is sa tisfactory, exceptfor the cellulose peak located around 2θ ) 14.6°, forwhich the predicted magnitude is much higher tha n theexperimental one. This probably results from the dif-ferent orientat ion of tunicin w hiskers within the filmobtained from only cellulose and from the composite.The wh iskers distribution is most likely ra ndom in th e

composite contrarily to the reference whisker films,which is composed of in-plane oriented filler. The sametrend is reported between the experimental and pre-dicted X-ray diffraction patterns of composites condi-tioned at 75%RH (Figure 10d), but the most significantdifference between these tw o diffra ctogra ms is the tota labsence of the peak at 2θ ) 21.15° (d ) 4.2 Å) in thepredicted diffra ctogra m. Similar differences betweenexperimental and predicted X-ray diffra ction pat ternswere obtained for the glycerol plasticized starch filledwith 16.7 wt %t unicin wh iskers.

This observat ion could probably be interpret ed a s a ninterfa cia l effect in r elation w ith t he shoulder observedon the low-temperature side of the melting endothermby DSC for s imilar condi t ions. This a gain could beinterpreted a s a tra nscrystallization phenomenon ofamylopectin on t he surface of the cellulose w hiskers.The wh iskers could act a s nucleation points, a nd owingto the st eric obstacles due to the high w hiskers content ,distortion of amylopectin crystallites occurs. However,the experimental evidence of the t ranscrystal linitycannot be displayed by classical techniques, such asoptical microscopy because of the dimensions of thetunicin whiskers.

Regarding the nature of the crystal l ized s t ructuregiving th e diffra ction peak a t 2θ ) 21.15°, it is d oubtful.However, it is w ell-known t ha t am ylopectin cryst allizesreadily and forms complexes with glycerol detectable byX-ra y diffra ction or other methods. The peak at 2θ )

Figure 8. Wide-angle X-ray diffraction patterns of glycerolplasticized waxy maize starch for various moisture contentsand tunicin whiskers film. The relative humidity conditionsare indicated in the figure.

Figure 9. Wide-an gle X-ra y diffra ction pa tterns of 75%R Hconditioned t unicin wh isker/glycerol plast icized w axy ma izestarch composites. The tunicin whisker contents are indicatedin the figure.




21.15° could probably be a ssigned t o a glycerol- starchV structure, but i t does not correspond to not anyreported stru cture. Since the V form is a single helix, itappears tha t the 4.2 Å is not from the double-helix Bform. Further experiments are necessary to confirm thishypothesis.

ConclusionsNanocomposite materials were obtained from glycerol

p las t i c ized waxy maize s ta rch as the mat r ix and asuspension of tun icins an a nimal celluloses whiskers asa model reinforcing pha se. The unfilled mat rix appear sa s a complex heterogeneous syst em composed of glycerol-rich domains dispersed in an amylopectin-rich continu-ous phase. Each phase exhibits i ts own glass- rubbertra nsition, for w hich t he temperat ure decreases a s th emoisture content increases owing to the plasticizingeffect of water. This lowering of T g induces the crystal-lization of the matrix at room temperature (retrograda-tion phenomenon) when the water content increases.

Significant changes occur in the composite systemswhen tunicin whiskers a re homogeneously dispersed inthis complex mat rix. All results lead to t he conclusionthat both plasticizers (glycerol and water) redistributewithin the matrix, diffusing toward the cellulose surface.This relocalization effect decreases the plasticizing effectof glycerol and wa ter in the bulk a mylopectin mat rix,resulting in an increase of the T g of amylopectin-richdomains. The accumulation of plasticizer in the cel-

lulose/a mylopectin int erfa cial zones improves th e abilityof amylopect in chains to crystal l ize, leading to theforma tion of a possible tra nscrysta lline zone a round thewhiskers. These specific crystallization conditions havebeen evidenced at high moisture content and highwhiskers content (> 16.7 wt %) by DS C a nd WAXS. Itis displa yed thr ough a sh oulder on the low-temperat ureside of the melting endotherm a nd t he observa tion of anew peak in the X-ra y diffraction pa ttern. This tra ns-crysta lline zone could originat e from a glycerol- starchV structure. This inherent restricted mobility of amy-lopectin chains most likely accounts for the lower waterupta ke of cellulose/st a rch composites for increa sing fillercontent.

Acknowledgment. The a uthors gra tefully a cknowl-edge Roquette S .A. for supplying w axy maize sta rch, Dr.H. Chanzy for stimulating discussions, Dr. M. Pailletand Y. Minot for their help in film processing, Mr. L.Fouchet for his help in contact angle, Mrs. D. Dupeyreand Mr. R. Vuong for their help in S EM a nd TEM mi-croscopy, r espectively, a nd Zebda for mora l support. Theaut hors ar e indebted t o ADE ME (Agence Fra nc¸aise del’Environnement e t de la Maıtrise de l’En ergie) forfinan cia l support (ADE ME/CNRS convention # 99 01033).

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Figure 10. Experimenta l an d predicted w ide-an gle X-ra y diffra ction patt erns of 25 wt %filled tun icin w hiskers/glycerol plasticizedwaxy maize starch composites conditioned at (a) 35, (b) 43, (c) 58, and (d) 75%RH.




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Plasticized Starch-Tunicin Whiskers Nanocomposites. 1. Structural Analysis