Applied Clay Science 72 (2013) 1–8

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Applied Clay Science

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Research paper

Using an organically-modifiedmontmorillonite to compatibilize a biodegradable blend

Natália F. Magalhães, Karim Dahmouche, Gisela K. Lopes, Cristina T. Andrade ⁎Instituto de Macromoléculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bloco J, P.O. Box 68525, 21945-970 Rio de Janeiro, RJ, Brazil

⁎ Corresponding author. Tel.: +55 21 2562 7208; faxE-mail address: ctandrade@ima.ufrj.br (C.T. Andrade

0169-1317/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.clay.2012.12.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 March 2012Received in revised form 21 December 2012Accepted 27 December 2012Available online xxxx

Keywords:Clay polymer nanocompositeModified montmorilloniteStarchPoly(3-hydroxybutyrate-co-3-hydroxyvalerate)Physical propertiesSAXS

Glycerol-plasticized cornstarch and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) blends were pre-pared by melt-extrusion at a constant 70:30 (mass/mass) ratio in the presence of a commercial organoclay.The effect of increasing the organoclay content on themorphology and physical properties of the blends was in-vestigated soon after processing and after aging for 12 months. After processing, the materials were character-ized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and dynamic mechanical analysis(DMA). The results indicated that increasing clay mineral contents promoted significant improvements in thecompatibility between the components. After aging, the samples were characterized by SEM, XRD andsmall-angle X-ray scattering (SAXS). Unexpectedly, the reduced size of the PHBV dispersed phase and the de-creased crystallinity of both phases in the hybrids were maintained. XRD and SAXS results unambiguouslyproved the presence of both exfoliated layers and a small volume fraction of organoclay aggregates in the agedclay polymer nanocomposites.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Thedevelopment anduse of biodegradablematerialswere consideredas an alternative to reduce environment deterioration causedby syntheticpolymers waste (Harding et al., 2007; Maiti et al., 2007; Chanprateep,2010; Fukushima et al., 2011). Starch and poly(hydroxyalkanoates)might be included in a list of a few biodegradable polymer materials.The interest in starch as a thermoplastic is related to its worldwideavailability at a low cost. Granular starch can be transformed into athermoplastic material (TPS) by thermomechanical treatment inthe presence of some plasticizers. However, both the starch matrixand the plasticizer are hydrophilic, and water absorption causes sig-nificant changes in mechanical properties. The development ofmulticomponent systems, a conventional method for the productionof synthetic newmaterials, was suggested to improve TPS properties(Huneault and Li, 2007; Mondragón et al., 2009).

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a bio-compatible and biodegradable copolyester, produced as a reserve ma-terial by numerous microorganisms, under limited concentrations ofessential nutrients, such as nitrogen or phosphorus, in an excess ofa carbon source (Lenz and Marchessault, 2005; Arun et al., 2009;Rodgers and Wu, 2010). PHBV has a lower melting point and a higherflexibility than the homopolymer, poly(3-hydroxybutyrate) (PHB).However, high production costs and its high crystallinity preventedits extensive use (Avella et al., 2000). TPS/PHBV blending wouldlead to a biodegradable material, with balanced properties betweenthose of the individual components.

: +55 21 2270 1317.).

rights reserved.

With similar objectives, PHB/potato starch films at various composi-tionswere prepared by the casting technique from chloroform solution.A single glass transition temperature in the range 63.1 to 87.4°C wasfound for all compositions studied, and the optimum tensile strengthwas achieved for the PHB/starch ratio of 70/30 (mass/mass) (Godboleet al., 2003). PHB was also mixed with two types of starch, differing inamylose content, at a 70/30 (mass/mass) ratio. The blends were pre-pared in an internal mixer at the same conditions. Improvements inthermal, rheological and mechanical properties were observed, and at-tributed to the existence of hydrogen bonding between the components(Zhang and Thomas, 2010). On the other hand, immiscibility betweenstarch and PHBV was considered by other authors. To overcome thepoor interfacial adhesion between the matrix and the dispersed phase,bis(tert-butylperoxyisopropyl)benzene was used as the initiator in thereactive blending of PHBV and high-amylose cornstarch (Avella et al.,2002).

Recently, some results were published concerning the role of clayminerals in modifying themorphology and behavior of immiscible syn-thetic polymer blends (Khatua et al., 2004; Ray et al., 2004; Si et al.,2006; Kelarakis et al., 2007; Vo, Giannelis, 2007; Filippone et al., 2010;Tiwari and Paul, 2011). For many systems, clay minerals were reportedto be responsible for the increase in compatibility between the compo-nents of binary blends. Similarly to conventional compatibilizers, clayminerals at interfaceswould reduce the interfacial tension and suppresscoalescence of the dispersed phase droplets (Si et al., 2006; Hong et al.,2007). However, for some blends, the morphology was not stable afterannealing (Khatua et al., 2004).

Investigation on the aging of biopolymeric blends and their claypolymer nanocomposites (CPN) under storage at room conditionsconstitutes an important issue of interest for scientific and industrial

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purposes. Stable properties are required before and during effectiveperformance, until the product is discarded. In a previous work, claymineral-starch hybrids were shown to be less hydrophilic, but to bio-degrade faster than the neat TPS, most probably because of its lowercrystallinity (Magalhães and Andrade, 2009). The aim of this workwas to investigate eventual changes in the structure and propertiesof a TPS/PHBV blend at a fixed 70:30 (mass/mass) composition, in-duced by adding increasing amounts of an organoclay, soon after pro-cessing and after aging for 12 months. To the authors' knowledge, nodetailed study was published on the effect of prolonged aging of ex-truded TPS/PHBV blends compatibilized by an organoclay.

2. Experimental section

2.1. Materials

Regular cornstarch (CS) composed of 26–30mass% amylose and 74–70 mass% amylopectin, with less than 0.5 mass% gluten, and 12 mass%moisture content was supplied by Corn Products Brazil (São Paulo, SP,Brazil). Analytical grade glycerol was purchased from Vetec QuímicaFina (Rio de Janeiro, RJ, Brazil) and was used as received. PHBV, with3.5mass% of hydroxyvalerate units (HV), andMw=426.000,wasdonat-ed by PHB Industrial (Serrana, SP, Brazil). Cloisite® 30B (C30B), a mont-morillonitemodifiedwith dihydroxyethyl alkyl methylammonium ions,was supplied by Southern Clay Products (Gonzales, TX, USA), and wasused as received. Results of C30B characterization were published else-where (Botana et al., 2010).

2.2. Preparation of samples

The mixture of CS and glycerol, added at 25 mass% based onstarch dry mass, was homogenized in a conventional mixer (IkaWorks, Wilmington, NC, USA) for 10 min, and maintained in tightlysealed bags for 2 days at 4°C. Before processing, glycerol-plasticizedCS and PHBV mixtures were also homogenized at a constant 70:30(mass/mass) ratio. C30B was added at 5, 7.5, and 10 mass% contents,over the total mass of the blend.

2.3. Processing

One-step extrusion processings were carried out for the neat plas-ticized CS/PHBV blend and for the plasticized CS/PHBV/C30B hybridsin a Coperion ZSK 18 (Werner & Pfleiderer, Stuttgart, Germany)co-rotating twin-screw extruder, with a L/D ratio of 40. The sevenheating zones were maintained at 150, 150, 155, 155, 155, 150,150°C, and the screw speed was set at 200 rpm. The same conditionswere used to extrude glycerol-plasticized CS and PHBV. To preparespecimens for analyses, the extruded materials were pelletized andcompression-molded by heating at 170°C under 68.9×106 N/m2 for6 min, and cooling for 5 min in a cold press.

2.4. Aging

The samples were conditioned in sealed polyethylene bags atroom temperature (~25°C) and 50% relative humidity for a periodof 12 months.

2.5. Phase morphology

The phase morphology of the samples was examined with a Jeolelectron microscope, model JSM-6460LV (Akishima-shi, Japan) at theacceleration voltage of 20 kV. Samples were cryogenically fractured be-fore observation. The fractured surfaces were vacuum-coated with goldbefore measurements. Images were obtained before and after aging.

2.6. X-ray diffraction (XRD)

XRD patterns were performed with a Miniflex diffractometer(Rigaku Corporation, Osaka, Japan) operating at the CuKα wave-length of 1.542 Å, at 30 kV and 15 mA. The scattered radiation wasdetected at ambient temperature in the angular region 2° to 35°(2θ) at 1°/min and a step size of 0.05°. The samples were also ana-lyzed with a Ultima IV diffractometer (Rigaku Corporation) in the an-gular region 0.6 to 10° (2θ), in reflection mode. The CuKα1 radiationwas generated at 40 kV and 20 mA. The samples were scanned at a0.01°/s rate. The d-values were calculated by the Bragg's equation.

2.7. Dynamic mechanical analysis (DMA)

Dynamic mechanical analyses were performed in a Q-800 DMAequipment from TA Instruments (New Castle, DE, USA) withcompression-molded samples with 35×10×2 mm3 dimension, condi-tioned at 25°C, and 50% relative humidity, for a period of at least 48 h.The experiments were carried out in bending mode from −100 to150°C, at a heating rate of 3°C/min, at 1 Hz. Prior to analysis, the visco-elastic region was determined by a strain sweep experiment.

2.8. Small-angle X-ray scattering (SAXS)

The nanostructure of the aged samples was investigated by SAXSexperiments, performed at the beam line of the Brazilian SynchrotronNational Light Laboratory (LNLS, Campinas, Brazil) with a fixed wave-length of 1.608 Å at room temperature. The scattering intensity I (q)was plotted as a function of the modulus of the scattering vectorq=4πsinθ/λ, where 2θ is the scattering angle. The scattering intensitywas normalized by subtracting the background scattering and takinginto account the sample thickness. Each SAXS pattern corresponded toa data collection time of 120 s.

3. Results and discussion

The TPS sample was flexible and translucent. However, flexibilityand transparency decreased with the addition of PHBV and increasingC30B content.

3.1. Phase morphology, XRD patterns and DMA after processing

The phase morphology of TPS/PHBV materials was investigated bySEM. As expected, for the neat blend (Fig. 1a), a coarse morphologywas visualized. Immiscibility was evidenced by the size, and thelack of adhesion between the two phases. For the TPS/PHBV/C30Bmaterials, the micrographs revealed an improving degree of interac-tion between the two polymers as the organoclay content was in-creased. At 5 mass% C30B (Fig. 1b), a rough surface composed ofirregular particles of the PHBV dispersed phase (indicated by arrows)were embedded in the smooth starch matrix. With higher C30B con-tents, spherical PHBV particles were clearly observed in largeramounts with reduced dimensions. With the addition of 10 mass%C30B (Fig. 1d), a homogeneous dispersion of PHBV droplets wasvisualized.

From the images obtained by SEM, some conclusions might bestated on the location of clay mineral particles. Since a reduction ofthe dispersed domain size was observed as the content of C30B wasincreased, the clay mineral particles might be assumed to be locatedwithin the starch matrix or at the interface. The presence of clay min-erals in the dispersed phase was reported to increase the disperseddomain size (Kontopoulou et al., 2007), and this possibility shouldbe discarded in the present case. Furthermore, clay mineral particleswould be incorporated preferentially into the polymer with the low-est melting temperature (Fenouillot et al., 2009). In this case, TPS hada lower melting temperature than PHBV.

Fig. 1. Scanning electron micrographs taken for the neat TPS/PHBV blend at 70:30 composition (a) and for the blends with the addition of 5 mass% (b), 7.5 mass% (c), and 10 mass%C30B (d).

Fig. 2. X-ray diffractograms soon after processing for TPS (trace I), PHBV (trace II), theTPS/PHBV neat blend at 70:30 composition (trace III), and the TPS/PHBV blends withthe addition of 5 mass% (trace IV), 7.5 mass% (trace V), 10 mass% C30B (trace VI),and for C30B alone (trace VII); (a) in the 2θ region of 2° to 35°; (b) in the 2θ regionof 0.6° to 10°.

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XRD was used to investigate the crystallinity of the extruded ma-terials in the 2–35° (2θ) region (Fig. 2a). For TPS alone (trace I), thereflections at 13° and 20° (2θ) were assigned to the VA structure, at-tributed to processing-induced crystallinity of single helical amylose,found in extruded and compression-molded TPS with less than 10%water (van Soest et al., 1996). In trace II, the reflections located at13° and 17° (2θ) were assigned to the 020 and 110 reflections ofthe orthorhombic PHB crystal lattice (Bloembergen et al., 1986). Thehydroxybutyrate and the hydroxyvalerate repeating units wereshown to co-crystallize in a single hydroxybutyrate unit cell, in copol-ymers at compositions from 0 to 37 mol% hydroxyvalerate units(Bloembergen et al., 1986; Bluhm et al., 1986; Kunioka et al., 1989).

The diffractogram for the neat TPS/PHBV sample (trace III)showed the same reflections observed for the individual components,which corroborated the immiscibility between the polymers. The VA

and the PHBV reflections were also present in the diffractograms ofTPS/PHBV/C30B materials. However, their intensities were reducedas the C30B loading was increased in the hybrids (traces IV, V, andVI), indicating a corresponding decrease of crystalline fractions ofTPS and PHBV. Moreover, the PHBV reflections were shifted tolower 2θ values, which revealed an alteration of the local order insidethe crystalline lamellae of this polymer. Particularly for the blendwith 10 mass% C30B, the intensity of those reflections was significant-ly decreased. This result indicated that this hybrid was formed by alow volume fraction of very imperfect TPS and PHBV crystals, embed-ded in a high volume fraction of an amorphous phase, in which bothpolymers were compatibilized.

In trace VII, the diffractogram for the neat C30B showed the 001reflection at 5° (2θ). No reflection within the 2–10° (2θ) range wasobserved for the hybrids. However, the lack of resolution of this tech-nique at low angles contributed to the featureless curve in this angu-lar region. Using a diffractometer with a higher resolution at lowangles, the 001 reflection for the neat C30B (Fig. 2b, trace VII) was ob-served at 2θ=4.6°, and revealed a d001 value of 1.9 nm. The intensityof this reflection was significantly reduced for the hybrids, with the

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increase of added C30B, and disappeared completely for the hybridwith 10 mass% C30B. Since such reflection is associated with the exis-tence of some pure C30B aggregates in the samples, this result indi-cated that the compatibilization between both polymers is relatedto the presence of individually dispersed organoclay particles in thepolymer mixture; namely, exfoliated organoclay particles.

For the hybrid material with 5 mass% C30B (trace IV), a shallow re-flection around 2.3° (2θ) was visualized. The presence of this reflection,which corresponds to a d001 value of 3.8 nm, indicated the existence ofsome C30B aggregates, in which the polymer phase was intercalated.The intensity of this reflection decreased by increasing the C30B contentto 7.5% (trace V), and this reflection disappeared completely for the hy-brid preparedwith 10mass% C30B (trace VI). This result confirmed thatthe compatibilization between the TPS matrix and PHBV occurred byencirclement of the dispersed PHBV droplets by exfoliated organoclaylayers or very small organoclay aggregates. At 5 and 7.5 mass%organoclay content, the amount of C30B present in the hybrids seemedto be not enough to achieve the droplets encirclement and, consequently,the organoclay dispersion and compatibilization process were lesspronounced.

As for the dynamic mechanical behavior, the variation of the lossfactor (tan δ) with temperature (Fig. 3a) showed two peaks for TPSalone, at −57°C and 19°C (trace I). The transition at 19°C was asso-ciated with a starch-rich phase, whereas that at−57°C was associat-ed with a glycerol-rich phase (Forssell et al., 1997). For PHBV alone(trace II), the peak at −51°C was attributed to the γ-relaxation ofPHBV, and that at 21°C was attributed to the β-relaxation, relatedto the Tg of PHBV (Boyd, 1985). The β-relaxation peaks for the poly-meric components were very close to each other and, for the neatblend (trace III), a single broad peak appeared around 22°C. For thehybrids with 5 mass% and 7.5 mass% C30B (traces IV and V, respec-tively), the broad glass transition region had the maxima displacedto even higher temperatures, respectively at 28°C and 25°C. For

Fig. 3. Tan delta (a) and storage modulus (b) variation with temperature for TPS (traceI), PHBV (trace II), for the neat TPS/PHBV blend at 70:30 composition (trace III), and forthe TPS/PHBV blends with the addition of 5 mass% (trace IV), 7.5 mass% (trace V), and10 mass% C30B (trace VI).

the blend with 10 mass% clay (trace VI), broadening associated withincreasing values of tan δ was visualized at temperatures above0°C, with no evident maximum. This behavior, observed for at leastanother CPN system (Kumar et al., 2010), suggested restricted seg-mental motions in the amorphous region, and confirmed that claymineral particles were essentially located in the amorphous phaseat the interface between both polymers, as suggested by XRD results.

In Fig. 3b, a significant increase in the dynamic storage modulus (E′)values was observed for the blends in relation to TPS alone, particularlyfor the CPN with 10 mass% organoclay. Although no synergy was ob-served, blending TPS with PHBV in the presence of C30B at 10 mass%composition led to the stiffening of the TPSmatrix, especially at temper-atures above Tg up to 100°C. The role of C30B at 10 mass% compositionon themodulus enhancementwas better evidenced when the behaviorof this hybrid was compared to that observed for the neat blend. Signif-icant improvements in E′ were observed (Table 1), reaching 251% at25°C, and 275% at 50°C.

3.2. Phase morphology, XRD patterns and SAXS results after aging

After aging at room conditions for 12 months, the samples wereanalyzed by SEM and XRD. For the neat blend (Fig. 4a and a1) andthe hybrid with addition of 5 mass% C30B (Fig. 4b and b1), the lackof adhesion between the two phases was visualized by SEM. A homo-geneous morphology, in which a larger number of small droplets(smaller than those observed after processing), was observed forthe hybrids as the clay mineral content was increased to 7.5 and 10mass% (Fig. 4c, c1 and d, d1, respectively).

As expected, the diffractogram for the aged TPS (Fig. 5a, trace I) re-vealed a new reflection at 2θ=17°. This reflection, observed forstarch materials submitted to aging, was attributed to the B-type dou-ble helical crystal structures of amylose and amylopectin (van Soestet al., 1994). Such process (retrogradation) occurs because of humid-ity absorption, which favors molecular mobility, and time-induced re-crystallization of starch molecules (van Soest et al., 1994; Magalhãesand Andrade, 2009). For the aged TPS, a reflection at 2θ=19° wasalso observed. The appearance of this reflection, observed forprocessed starches with water contents higher than 10 mass%, andascribed to the VH-type single helical crystal structure of amylose(van Soest et al., 1996), confirmed that the TPS sample absorbed hu-midity during aging. Contrarily, neither B-type nor VH-type reflec-tions were observed for the neat blend (trace III) and for thehybrids (traces IV, V and VI). For these samples, the reflection at 2θaround 20° characterized the VA-type crystallinity (van Soest et al.,1996). The less hydrophilic character of PHBV compared to TPS, andthe barrier effect against water molecules, promoted by the organoclaylayers, explain this result.

Although aging led to no changes in the XRD pattern for PHBV(trace II), for the neat blend (trace III), the PHBV reflections werebroadened and shifted to higher 2θ values. This result revealed aPHBVmore compact crystalline structure in the neat blend, comparedto PHBV alone. This trend was also observed for the aged hybrids(traces IV, V and VI). However, the decrease in intensity and the in-crease in width of PHBV and TPS characteristic reflections by adding

Table 1Temperature dependence of the storage modulus (E′) for the TPS/PHBV neat blend(70:30 composition) and for the TPS/PHBV/C30B (70:30 composition with 10 mass%C30B) hybrid.

Sample E′ (MPa)

25°C 50°C 75°C 100°C

TPS/PHBV neat blend 367.4 157.9 107.9 71.9TPS/PHBV/10% C30B 1291.0 593.0 314.1 192.5

(251) (275) (191) (167)

The values in parentheses represent the percent in improvement.

Fig. 4. Scanning electronmicrographs taken after aging for 12 months for the TPS/PHBVneat blend at 70:30 composition (a, a1) and for the blendswith the addition of 5mass% (b, b1), 7.5mass% (c, c1), and 10 mass% C30B (d, d1) at two different magnifications.

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more clay were less pronounced for the aged samples. These results in-dicated that the loss of blends crystallinity induced by clay additionwaslower in the aged samples, and that the role of C30B nanoparticles as ef-fective compatibilizers was reduced by aging.

Solid particles within polymer blends may migrate and aggregatewith time, and this particular point was investigated by XRD at lowerangles (Fig. 5b). For the hybrid materials after aging (traces IV, V, andVI), the same shallow reflection around 2° (2θ) was envisaged, and

was more pronounced for the sample prepared with 10 mass%organoclay. Note that this reflection was observed after processingonly for the hybrids prepared with 5 and 7.5% C30B. This reflection,which corresponds to a d-value of 3.8 nm, indicated the existence inthe aged hybrids of some aggregates, in which the polymer phasewas intercalated. Furthermore, the reflection corresponding to thetypical interlayer space of C30B was also present in the hybrids pat-terns, revealing the existence of some organoclay aggregates in the

Fig. 5. X-ray diffractograms after aging for 12 months for TPS (trace I), PHBV (trace II),the TPS/PHBV neat blend at 70:30 composition (trace III), for the TPS/PHBV blendswiththe addition of 5 mass% (trace IV), 7.5 mass% (trace V), 10 mass% C30B (trace VI), and forC30B alone (trace VII); (a) in the 2θ region of 2° to 35°; (b) in the 2θ region of 0.6° to 10°.

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samples. However, the large width and the low intensity of the tworeflections suggested the presence of only a small volume fractionof both intercalated and nonintercalated particles in the materials.

To better investigate the influence of clay mineral dispersion on thestructure and properties of the hybrids, SAXS measurements wereperformed. Differently from XRD, which is sensitive to the total volumeof crystallinematter in amaterial, the SAXS technique is sensitive to theelectronic density contrast between the nanodomains and the matrix,inwhich such domains are dispersed. For the aged TPS (Fig. 6), two scat-tering regimes were observed, which included a tendency to a Guinierplateau at low q-range, and two scattering peaks located at 3.8 nm−1

and 5.3 nm−1. By fitting the experimental curve up to 0.65 nm−1 bythe Guinier equation (Guinier and Fournet, 1955; See Supportingmate-rial), the presence of a diluted system of starch nanocrystallites with an

Fig. 6. SAXS pattern for aged TPS. The continuous line represents the experimental fitof the curve at low q-range given by the Guinier equation.

average radius of gyration (Rg) of 4.5 nm dispersed in the amorphousmatrix was evidenced. The peak centered at 3.8 nm−1 was already ob-served by Perry and Donald (2000) for B-type nanocrystals of potatogranular starch and, in the present case, was attributed to TPS retrogra-dation (retrogradation leads to starch B-type crystallinity). To theauthor's knowledge, no other work reported until now the presenceof the peak located at 5.3 nm−1 for TPS samples. Nevertheless, thispeak could be assigned to the internal structure of the VH-type crystal-lites detected by XRD. Note that no peak associated to lamellar crystal-linity was observed in the SAXS pattern for TPS. Such peak, which wasproposed to result from the alternating crystalline and amorphous la-mellae of starch granules, was observed around 0.6 nm−1 for wet gran-ular starch (Perry, Donald, 2000). Therefore, the absence of such peakcan be attributed to the complete rupture of the granular structure byprocessing.

The pattern obtained for PHBV (Fig. 7a) revealed a tendency to aGuinier plateau, and by fitting the experimental curve with theGuinier equation up to 0.5 nm−1, crystalline nanodomains with aver-age Rg of 3.5 nm were found. The peak that appeared at 1.2 nm−1

was related to the lamellar structure of PHBV. The PHBV lamellarlong period, D=2π/qmax, was determined as 5.2 nm, and was similarto the value reported in the literature (Sawayanagi et al., 2007).

The pattern obtained for PHBV also revealed a shoulder at0.85 nm−1 and, by applying the Lorentz correction (Cser, 2001), abroad peak was revealed (Fig. 7b). A similar broad peak was observedin Lorentz-corrected SAXS pattern reported by Caballero et al. (1995)for poly(hydroxybutyrate-co-hydroxycaproate) (PHBC) containinga minor amount of hydroxycaproate (HC) repeating units. Theauthors demonstrated that such peak originated from a HC-richpolymer, rejected from the PHB crystallites. Therefore, the peak at0.85 nm−1 was attributed to the existence of a partially crystallized

Fig. 7. SAXS pattern for PHBV without the Lorentz correction (a), and after Lorentz cor-rection (b). The continuous line represents the experimental fit of the curve (a) at lowq-range given by the Guinier equation.

Fig. 9. SAXS patterns for aged TPS/PHBV neat blend (trace I), and for TPS/PHBV/C30Bnanocomposites with 5 mass% C30B (trace II), 7.5 mass% C30B (trace III), and 10mass% C30B (trace IV). The curves were vertically displaced for clarity.

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poly(hydroxyvalerate) PHVphase dispersed in the PHBV polymer. Notethat this interpretation is also consistent with the presence of the broadreflections centered around 2θ=22° and 26° in the XRD patterns ofboth aged and nonaged PHBV (Figs. 5a and 2a, respectively). Thevalue of the long period D inside such not well-organized crystals isaround 7.3 nm, which is larger than inside the well-ordered lamellarPHBV crystallites, probably because of the large thickness of amorphouspoly(hydroxyvalerate) (PHV) regions present in this partially orderedPHV phase. The presence of a phase separation at the nanometer scalein PHBV was not surprising because it was established that, while HVunits can cocrystallize into the PHB lattice, they do so with some diffi-culty (Bluhm et al., 1986; Kunioka et al., 1989).

For the TPS/PHBV neat blend (Fig. 8a), the peak associated withthe lamellar PHBV phase exhibited a larger width and was shifted to-wards lower q-values (~0.9 nm−1) compared to pure PHBV, whereasthe shoulder associated with the partially organized PHV phasedisappeared. This result corroborated XRD data on the fact that,even after aging, PHBV had a more amorphous character whenmixed with TPS than alone. The long period D, associated with the la-mellar PHBV phase, and determined from the peak position in theLorentz-corrected curve (Fig. 8b), was around 7.4 nm. Since XRDevidenced a contraction of the crystalline structure inside lamellae pro-moted by aging, the observed increase inD, compared to the pure PHBV,was attributed to the larger thickness of the amorphous PHBVinterlamellar region. As a consequence of the less crystalline characterof PHBV phase in the blend, the estimated PHBV average crystallitessize (24 nm), determined from the Scherrer's equation (Lc=4π/Δq),was smaller than that obtained for pure PHBV (63 nm). Such inhibitionof crystallization of PHBV was previously observed for other polymerblends with low contents of HV (less than 25 mass%) (Yang et al.,2010). On the other hand, no structural information about TPS in theblend could be extracted from the SAXS curve because the low

Fig. 8. SAXSpattern for aged TPS/PHBV neat blendwithout the Lorentz correction (a), andafter the Lorentz correction (b).

q-regime could be originated from both TPS-rich and PHV-richnanodomains.

Finally, the effect of C30B on the structure of the aged blendswas in-vestigated in the q-range between 0.2 and 3 nm−1 (Fig. 9). For compar-ison, the SAXS pattern for the TPS/PHBV neat blend was also presented.For all samples prepared with C30B, three scattering regimes were ob-served: i) a broad peak centered around 1.6 nm−1, ii) a very weakshoulder at medium q-range centered around 0.55 nm−1, iii) a linearregime at low q-range. Because of its absence in the SAXS pattern forthe neat blend, the broad peak was attributed to the presence of someorganoclay aggregates. The d-value, determined from the position ofthe peak maximum (d=2π/qmax) was approximately 4 nm for all hy-brids. Note that the broadness of the peak and the much larger basalspacing compared to pure C30B (d=1.86 nm) was consistent with ahigh degree of organoclay dispersion in the hybrids even after aging,as already suggested by XRD.

The most interesting point was related to the evolution of the ex-tension and shape of the two other scattering regimes observed in theSAXS curves. A decrease of the shoulder intensity at medium q-rangeand an increase of the extension of the linear regime at low q-rangewere observed by adding more clay mineral to the blends. Since thecenter of the shoulder was located at a lower q-range (0.55 nm−1)compared to the position of the peak associated to the PHBV lamellarphase, detected for the neat TPS/PHBV blend (0.85 nm−1), the shoul-der was attributed to the pronounced decrease of PHBV crystallinity,promoted by the organoclay incorporation.

It is also worth noting that the progressive disappearance of theshoulder was accompanied by an increase in extension of the shortlinear regime detected at low q-values. Such linear regime was presentup to 0.27 nm−1 for the hybrid prepared with 5 mass% C30B and up to0.38 nm−1 for the hybrid prepared with 10 mass% organoclay. Thismeans that the destruction of the PHBV crystallites in the hybridspromoted an additional X-ray scattering regime at low q-range.Therefore, this regime could be attributed to the scattering of asmall number of C30B aggregates.

Information about the aggregation state of the organoclay plate-lets could be obtained from the α exponent, related to the powerdecay function observed at low q-range. For a diluted system of ran-domly oriented set of plate-like particles, α was expected to bearound −2 (Guinier and Fournet, 1955), whereas for an openthree-dimensional network of linked clay mineral platelets, α wasfound to be around −3 (Higgins and Benoit, 1994). The intermediatevalue of α (−2.5), calculated for the hybrid prepared with 10 mass%C30B confirmed the presence of both exfoliated layers and aggregatesin this sample.

8 N.F. Magalhães et al. / Applied Clay Science 72 (2013) 1–8

4. Conclusions

This work revealed the role of C30B organoclay as a compatibilizerfor the TPS/PHBV blend after processing. The size of the PHBV dis-persed phase was reduced, increasing amounts of droplets withsmaller diameters were observed, and improved interfacial adhesionwas clearly visualized by the addition of increasing contents oforganoclay. The level of dispersion of the organoclay was sufficientto decrease the crystallinity of the blends components, change PHBVcrystal lattice and promote significant increases in the Tg and the dy-namic storage modulus values. The disappearance of the C30B basalreflection in the XRD pattern obtained for the blend prepared with10 mass% organoclay strongly indicated the location of the organoclaynanoparticles at the interface between the components. The surfac-tant in C30B, with two 2-hydroxyethyl groups linked to the nitrogenatom, might be involved in physical interactions with the polargroups of both components of the blends, starch hydroxyl groupsand PHBV carbonyl groups. After aging for 12 months, although thesize of the dispersed phase seemed to be reduced, the number ofdroplets was diminished. A slow phase separation phenomenon be-tween TPS and PHBV occurred after prolonged storage, attributed tomigration and aggregation of organoclay nanoparticles. However,the compatibilization of this TPS/PHBV blend was preserved by thehigh amount of exfoliated organoclay, as evidenced by XRD andSAXS experiments.

Acknowledgments

The authors acknowledge Conselho Nacional de DesenvolvimentoCientífico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa doEstado do Rio de Janeiro (FAPERJ), and Coordenação deAperfeiçoamentode Pessoal de Nível Superior (CAPES) for the financial support. The au-thors also acknowledge the Brazilian Synchrotron Light National Labora-tory (LNLS) for the support on SAXS experiments.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.clay.2012.12.008.

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