green nanocomposite films based on cellulose acetate and biopolymer-modified nanoclays: studies on...

Iran Polym JDOI 10.1007/s13726-014-0286-z

1 3Iran Polymer and

Petrochemical Institute

ORIGINAL PAPER

Green nanocomposite films based on cellulose acetate and biopolymer‑modified nanoclays: studies on morphology and properties

Hafida Ferfera‑Harrar · Nassima Dairi

Received: 29 April 2014 / Accepted: 28 September 2014 © Iran Polymer and Petrochemical Institute 2014

and TGA methods. The results highlighted a retarding effect of nanoclays, except for Ge-MMT that showed a cat-alytic role.

Keywords Nano-biocomposites · Cellulose acetate · Plasticizer · Montmorillonite · Thermal stability · Biodegradation

Introduction

Nowadays, excessive levels of plastic waste have caused the concern of scientific community to develop green poly-meric materials that would not involve the use of toxic or noxious component in their manufacture and could degrade in the natural environmental products.

Biopolymers derived typically from renewable to abun-dant resources are an innovating alternative to petroleum-based polymers due to their eco-friendliness resulting from their inherent properties such as biodegradability and bio-compatibility. Nevertheless, the major disadvantages are their brittleness, and insufficient mechanical and moisture barrier properties at higher humidity conditions. These drawbacks restrict their use in a wide range of applications.

Nanotechnology is a tool to improve properties of these biopolymers. In this context, their nano-reinforcement using layered silicates as nanofillers potentially resulted in an improvement of properties in terms of mechanical, gas and water vapor barriers, thermal, and other proper-ties at low filler content [1–3]. The nano-biocomposites are extraordinarily versatile as they could be formed from a large variety of biopolymers such as polysaccharides, poly-peptides, proteins and nucleic acids, among others. Their development is an efficient way toward innovative applica-tions in disposable plastics and medicine [4, 5].

Abstract Green nanocomposite films were elaborated from cellulose acetate (CA), three clay types as nanofillers, namely natural montmorillonite (Na-MMT) and organo-modified MMT with gelatin (Ge-MMT) or chitosan (Cs-MMT) and in the presence or absence of triethyl citrate (TEC) as an eco-friendly plasticizer, using solvent-casting method. The formation of the organoclays was confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The nanoclay dispersion within CA matrix was investi-gated by XRD analysis together with transmission electron microscopy (TEM). For unplasticized nano-hybrids, it was observed intercalated/exfoliated structures with a small clay tactoïds remaining, although a more aggregated struc-ture was obtained in the presence of unmodified MMT. The plasticized nano-hybrids exhibited mainly intercalated/aggregated structure, while some exfoliated layers were much labeled in the presence of Ge-MMT nanoclay. Glass transition (Tg) and melting (Tm) temperatures of CA, as attested by differential scanning calorimetry (DSC) analy-sis, were slightly affected by clay addition. Besides, the thermal stabilities and water vapor barriers properties of CA-based nano-hybrids were enhanced by increasing clay loading, while the optical clarity, assessed by UV–visible spectroscopy, was rather decreased. Better nanocomposite properties were reached in the presence of Ge-MMT at 5 wt%. The clay impact on CA biodegradation was also stud-ied by gravimetric, scanning electron microscopy (SEM)

H. Ferfera-Harrar (*) · N. Dairi Materials Polymer Laboratory, Department of Macromolecular Chemistry, Faculty of Chemistry, University of Sciences and Technology Houari Boumediene (USTHB), B.P. 32 El-Alia, 16111 Algiers, Algeriae-mail: [email protected]

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Cellulosic esters such as cellulose acetate (CA), cel-lulose acetate propionate (CAP) and cellulose acetate butyrate (CAB) are thermoplastics produced by the esterifi-cation of cellulosic raw materials such as cotton, wood and sugarcane. Cellulose acetate is of great interest owing to its biodegradability, excellent optical clarity and stiffness. Although it can be produced in a range of degrees of sub-stitution (DS), the most frequent level is a DS of 2.5 due to the good solubility in common solvents and melt prop-erties. CA has been widely used in a variety of consumer products including textiles fibers, filters, membranes, pack-ing films, coatings for paper and plastic products, electrical isolation, and drug delivery systems [6, 7]. Nonetheless, it is moisture sensitive, brittle and presents low dimensional stability under high temperature and humidity. To secure its foothold in the market, it is desirable to improve its prop-erties. The common approach of addressing these issues is the use of clays as nano-reinforcement for the prepara-tion of nanocomposites which could be destined for plastic devices and more impermeable packing films and coatings applications.

Among the layered silicates, the montmorillonite (MMT) is a promising nanofiller due to its easy availability, non-toxicity, low cost, high in-plane strength, stiffness and high aspect ratio.

Some studies on CA/clay nanocomposites have been found in the literature in the last 10 years, aiming mostly to the enhancement of CA properties [8–10]. Misra et al. [11–13] have prepared plasticized CA/organoclay nanocompos-ites in the presence and absence of maleic anhydride-graft-cellulose acetate butyrate (CAB-g-MA) as compatibilizer to substitute petroleum-based polypropylene/thermoplas-tic olefins composites (PP/TPO) for automotive applica-tions. Likewise, Gonçalves et al. [14] prepared CA/pristine montmorillonite nanocomposites by solution intercalation process and pointed out the effect of solvent type on the morphology and properties of the obtained materials. They have concluded that the choice of an adequate solvent such as acetic acid/water led to a good control of the homogene-ity and a highest clay delamination, resulting from the high solvents polarity and the formation of hydrogen bonds with the layered silicate platelets. Gonçalves and the co-work-ers [15] have also prepared CA/MMTO nanocomposites by melt extrusion using two different plasticizers: di-octyl phthalate (DOP) and triethyl citrate (TEC). These authors have highlighted the efficient effect of the eco-friendly plasticizer TEC in the formulation of cellulosic plastics and concluded that it could substitute DOP plasticizer.

To get good clay dispersion within polymer matrix that is required for high performance of these materials, alkyl quaternary ammonium salts are commonly used as interca-lants in the commercial organoclays. The intercalation of their organophilic cations intended to weaken the attractive

layers forces, increase the interlayer spacing, and improve polymer/clay compatibility. Unfortunately, these intercal-ants are not suitable for bio-applications due to their tox-icity [16]. For instance, higher amount of clay intercalated with hexadecyltrimethyl ammonium has shown an adverse effect on fibroblast skin cell growth [17].

The current study is oriented to develop eco-nanocom-posites based on cellulose acetate and bio-modified or pristine montmorillonites at room temperature by solvent-casting method. For this purpose, two green macromolecu-lar intercalants are selected as organo-modifiers of MMT, namely gelatin (Ge) and chitosan (Cs) that are widely used in bio-applications. Gelatin, a denatured derivative of struc-tural protein collagen with plenty of –NH2 and –COOH groups in its chains, is a typical amphoteric polyelectrolyte. Chitosan is a polycationic biopolymer of N-acetyl-glucosa-mine and N-glucosamine units distributed randomly or in blocks.

To our knowledge, the elaboration of such materials using the cellulose acetate as matrix and particularly the gelatin organo-modifier of an Algerian montmorillonite, originated from Maghnia bentonite, as nanofiller has not been reported in the literature yet. In the first step, we pre-pared and characterized the Ge- and Cs-modified MMT nanoclays. In second step, the as-prepared organoclays as well as the Na-MMT as nanofillers were used to elaborate CA-based nanocomposites at different clay contents in the absence and presence of 20 wt% of TEC plasticizer. We then reported on characterizations by several techniques (XRD, TEM, SEM, UV–visible, DSC and TGA). The results were discussed with particular emphasis toward the effects of clay (type and loading) and plasticization on the morphology and the properties (thermal, optical, water bar-rier, and biodegradation) of these bio-based materials.

Experimental

Materials

Cellulose acetate (CA, 39.8 wt% acetyl content, Mn ca. 30,000 g.mol−1, DS = 2.45) was supplied by Sigma-Aldrich. Chitosan (Cs, 75 % deacetylated, medium molecu-lar weight, viscosity 200–800 cps) was purchased from Sigma-Aldrich. Gelatin (Ge, type B extracted from bovine skin, isoelectric point (IEP) of 5.05) was purchased from Merck, Darmstadt. Triethyl citrate (99 % purity) was pro-vided by SAFC Inc. Glacial acetic acid of analytical grade was used as received. Deionized water was used in experi-ments. The layered silicate used in this study is an Alge-rian montmorillonite from a bentonite category in Maghnia Roussel, which was kindly supplied by ENOF Chemical Ltd. The unmodified MMT (size fraction < 2 μm) was

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extracted from bentonite using sedimentation method [18] and then homoionized with sodium cations (Na-MMT). Finally, its cation exchange capacity (CEC) was deter-mined to be about 86.16 mequiv/100 g by conductometric titration [19].

Preparation of organoclays

The organo-modification of Na-MMT with gelatin or chi-tosan was performed according to the following procedure: 1 g of Ge powder was soaked in 50 mL of deionized water and heated to 70 °C to obtain a homogeneous solution, fol-lowed by the addition of NaOH 0.5 N aqueous solution to adjust the pH value to 8 (beyond the isoelectric point, IEP). On the other hand, Cs solution was prepared by dis-solving 1 g of Cs in 150 mL of 1 % (v/v) acetic acid aque-ous solution, and then the pH was adjusted to 4.9 by NaOH (0.5 N) solution. Concurrently, 1 g of Na-MMT was swol-len in 50 mL of deionized water at room temperature under mechanical stirring for 24 h. The clay suspensions were then ultrasonically treated for 15 min to reach better clay particles’ dispersion. Subsequently, the Ge and Cs solutions were separately added dropwise to Na-MMT suspensions under vigorous mechanical stirring at 70 and 60 °C, respec-tively, and kept stirring for further 4 h. The organoclays so obtained using 1/1 weight ratio of MMT/Ge or Cs, denoted Ge-MMT and Cs-MMT, were harvested by centrifuge, washed with water to remove the residual bio-modifiers, centrifuged again and dried at 60 °C in a vacuum oven for 12 h. Finally, the clays were ground and stored away from moisture to heat.

Preparation of cellulose acetate/clay nano-biocomposite films

The CA-based nanocomposite films were prepared at room temperature in the presence of different types of clay by solvent-casting technique. It is well known that the dis-persion force of the solvent is the main factor determining whether the organoclay layers remain suspended in the sol-vent [14]. In this context, preliminary tests were performed on the suspension state of both organoclays after 5 h rest in acetone, acetic acid solvents, and their mixtures with water. The swelling effect was more pronounced in acetic acid/water solvents mixture. Thus, the efficient acetic acid/water solvent mixture was selected for dispersing organoclay and dissolving cellulose acetate.

Mechanical and ultrasonic mixing modes were com-bined, which provided the contribution of both dispersive and distributive mixing mechanisms, to promote a good dispersion of the clay particles within matrix. Typically, a required amount of natural or bio-modified clays was suspended in acetic acid/water solvents mixture, and then

stirred mechanically at room temperature overnight. The ultrasonically pretreated clay suspension was added drop-wise to CA solution. In all mixtures, the weight ratio of CA/acetic acid/water was kept at 11/66/23. This suspension mixture was stirred mechanically for another 2 days, and then ultrasonically treated for 15 min. The nano-hybrids films were obtained after solution casting on rectangular glass molds and evaporating at room temperature, and then drying in oven under air at 80 °C for 4 days. Finally, the films were stored in polyethylene bags to avoid contamina-tion. These nanomaterials were prepared so as to give final 98/2 and 95/5 wt% for CA/clay ratio.

The plasticized nano-hybrids films containing 5 wt% of clays were also prepared using 5 wt% of each clay and 20 wt% of TEC (with respect to CA). The plasticiser was added to the CA solution before the formation of the nano-composite, according to previously mentioned method. Finally, reference films of CA were prepared with and without TEC.

Characterizations

FTIR spectra of nanoclays, gelatin and chitosan were recorded on a Perkin-Elmer Spectrum one spectrometer, with a spectral resolution of 2 cm−1 and 62 scans. All sam-ples were prepared as KBr pellets at 2 wt% of product.

X-ray diffraction patterns of pristine MMT, organoclays and their CA-based biocomposites were carried out with an automated powder diffractometer (Bruker D8 advance dif-fractometer) in the reflection mode at 40 kV with incident CuKα monochromatic radiation (λ = 1.5406 Å) and in 2θ range 1–30° using 0.01°/s scan rate.

The morphology of the nano-hybrids was observed through TEM microscopy in a JEM-2000EX oper-ated at an acceleration voltage of 200 kV and equipped with a MORADA SIS numerical camera. TEM images were taken from cryogenically ultrathin sections that were cut perpendicular to sample using a Leica EM FC6 cryo-ultramicrotome.

Thermogravimetric analysis was conducted to evaluate the bio-modifier content in nanoclay and the thermal sta-bility of CA and its nano-hybrids. The measurements were performed on a Q500 analyzer (TA instruments) under nitrogen atmosphere from 30 to 600 °C at 10 °C/min.

Glass transition (Tg) and melting (Tm) temperatures of CA and its nanocomposites were determined by DSC using a Q100 calorimeter (TA instruments). Each sample was heated and subsequently cooled twice in temperature ranges 30–200 °C and 30–240 °C under nitrogen flow at 10 °C/min. The Tg’s were determined at the midpoint of the second scan.

The optical properties of virgin CA and its nanocom-posite films were investigated by monitoring their light

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absorption at wavelengths ranging from 200 to 800 nm using high-resolution UV–visible spectrophotometer model JASCO V-650. The film samples were cut into a rectangu-lar piece and attached to the wall of the spectrophotometer test cell directly. Film transparency was evaluated as per-centage of transmittance at 700 nm.

The water vapor transmission rates (WVTR) of film specimens were studied following a gravimetric technique according to the standard test ASTM E96/E96 M-05. This methodology is based on the weight increase (wt–w0) of anhydride calcium chloride placed inside an aluminum capsule coated with the sample film. The analyses were carried out in a controlled environment chamber at constant temperature of 38 °C and 90 % relative humidity (RH) for 24 h. The thickness of the films (without physical defects such as cracks or bubbles) was measured by a Mitutoyo micrometer model IDC 112. The WVTR values [g/(m2 day)] were calculated according to Eq. (1)

where, A is the film area and t is the time, t = 24 h. The reported and discussed value for each sample was the aver-age of five permeation experiments.

Biodegradation of virgin CA and its CA/clay nano-hybrids was investigated by gravimetric method, in which the weight loss (%) was measured through 6 months of incubation in compost. All sample films were uniformly cut into small rectangles (3.5 × 0.8 cm), accurately weighed and then put into the compost. After compost tests, samples were recovered by deionized water, washed with methanol in an ultrasonic bath for 5 min, and then dried in oven at 60 °C for 24 h.

(1)WVTR =Wt − W0

A × t

SEM images of the surfaces and cross-sections films were taken from environmental scanning electron micro-scope ESEM type FEI Quanta 200 to observe their mor-phological changes after burial in compost.

Results and discussion

Organoclays’ characterization

Figure 1 shows FTIR spectra of Na-MMT, both organo-clays and their corresponding intercalants (gelatin or chi-tosan). The Ge spectrum (Fig. 1a) displays the bands of the amino acids mainly: amide A at 3,315 cm−1 (νN–H coupled with hydrogen bonding), amide B at 2,930 cm−1 (νC–H, νN–H), amide I at 1,654 cm−1 (νCOO, νN–H), amide II at 1,540 cm−1 [δN–H(sciss),νC–N], carboxylic at 1,448 cm−1 (δC–

O–H), amide III at 1,240 cm−1 (νC–N, δN–H) and at 620 cm−1 [δN–H(wagg)] [20].

In the spectrum of Ge-modified MMT, besides the absorption bands of MMT (νO–H ~3,360 and δO–H ~1,643 cm−1 of interlayer water; νSi–O ~1,034 and δSi–O ~470 cm−1 of Si–O–Si; δAl–O ~527 cm−1 of Si–O–Al) [20, 21], week additional bands due to gelatin are detected in regions not disturbed by MMT bands at 1,540 cm−1 (amide II), 1,450 cm−1 (δC–O–H of the carboxylic group) and 600 cm−1 [δN–H(wagg)]. As well, the band of δO–H ~ of absorbed H2O that appears at 1,643 in pristine MMT spec-trum overlaps by that of the amide I, resulting in the shift-ing of this band to 1,662 cm−1 in Ge-MMT spectrum. Also, the band at 3,426 cm−1 in the Na-MMT spectrum due to νO–H shifts to 3,370 cm−1 after the organophilic treatment. This suggests that the chemical environment of MMT

Fig. 1 FTIR spectra of Na-MMT with gelatin and Ge-MMT (a) and with chitosan and Cs-MMT(b)

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layers changed owing to hydrogen interactions developed between the hydroxyl clay groups and Ge groups (amino, hydroxyl and carboxylic groups).

This result confirms the presence of both compounds in organoclay. In Fig. 1b, the Cs spectrum shows mainly a broad absorption band around 3,389 cm−1 due to νO–H and νN–H, the bands at 2,923 and 2,857 cm−1 relative to νC–H

(asym, sym) of methyl groups. Also, the bands at 1,657, 1,598, 1,379, 1,320 and 1,080 cm−1 are assigned to νC=O (amide I) of acetyl groups, δN–H of NH2 and NHCO groups, δO–H of hydroxyl groups, δCO–NH (amide III) and skeletal νC–O–C in anhydroglucose units, respectively [22].

The presence of Cs within MMT is evidenced by the appearance of new bands in the spectrum of Cs-modified MMT as compared to that of natural MMT at 2,923, 1,532 and 1,383 cm−1 corresponding to νC–H of CH3 groups, δN–H in both acetyls (amide II) and NH2 groups, and δO–H of hydroxyl groups, respectively. Moreover, the band of νO–H, which appeared at 3,426 cm−1 in Na-MMT spectrum, shifted slightly to 3,406 cm−1, indicating most likely the occurrence of hydrogen bonding interactions between –OH structural groups of clay and Cs functional groups (hydrox-yls and carbonyls).

From FTIR analysis, all these new peaks indicated that the Ge and the Cs have been introduced onto pristine MMT successfully, and the MMT surface modification had been realized.

To check the successful organo-modification treatment of the natural MMT, tests of clay dispersion in water are also performed. Hence, 1 g of each sample of Na-MMT, Ge-MMT and Cs-MMT particles is dispersed in 40 mL of deionized water into flask. After vigorous stirring for 4 h, the clay suspension was ultrasonically treated for 20 min, and then left to set undisturbed. As depicted in Fig. 2a (inset), both organo-modified montmorillonites aggregated after not more than 10 min, while Na-MMT well dispersed and forms a stable suspension owing to its hydrophilic character originated from its hydroxyl groups and exchangeable Na+ cations. This observation sup-ports the hydrophobic character of the synthesized clays, and thus the presence of organic component in their structures.

To further assess intercalation of the bio-modifiers between MMT layers, the XRD analysis was performed. Figure 2b exhibits XRD patterns of pristine MMT, Ge-MMT and Cs-MMT. As it can be seen, Na-MMT exhibits a single sharp diffraction peak at 2θ = 6.76° correspond-ing to d-spacing (d001) of 1.30 nm. After organo-modifi-cation with Ge or Cs as organo-modifiers, this peak shifts to lower 2θ values. It appears at 4.23° for Ge-MMT and 5.48° for Cs-MMT matching to d001 of 2.09 and 1.61 nm, respectively. This result indicates the incorporation of each biopolymer inside the clay gallery. Likewise, Kabiri

et al. [23] have prepared chitosan-intercalated bentonite using a combination of ultrasound and microwave irradia-tion, where XRD patterns exhibited analogous increase in d-spacing from 1.23 to 1.64 nm. The XRD patterns also show a broadening of these diffraction peaks, indicating a partial disruption of parallel stacking of the pristine MMT. This could be due to the polymeric nature of the bio-mod-ifiers that induced different dispositions of their chains inside of the interlayer space, in contrast with small mol-ecules such as surfactants. Besides, the Cs-MMT diffrac-tion peak is less intense than that of Ge-MMT due to prob-ably a lowest incorporation of organic component inside the clay.

According to Zheng et al. studies on gelatin/pristine MMT nanocomposite [24, 25], the intercalation of gela-tin chains is closely related to pH of its solution. At IEP, the amounts of NH3

+ and COO− groups are equal but the charge changes when the pH media is higher or lower than IEP, as below:

Hence, in basic medium at pH > IEP, Ge chains with more carboxylic–COO− groups could interact strongly with hydroxyl groups on MMT sheets via hydrogen bond and intercalate between MMT interlayers. In our previous study [26], the organo-modification of Na-MMT with gela-tin was achieved in acidic environment at pH < IEP. The DRX patterns of the obtained organoclay showed a higher

NH+

3 −Ge−COOHH+

←−−−−pH<IEP

NH+

3 −Ge−COO

OH−

−→pH>IEP

NH2−COO−

+ H2O

Fig. 2 Photograph of dispersibility in water (a) and XRD patterns of Na-MMT, Ge-MMT and Cs-MMT (b)

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shifting of the corresponding diffraction peak to lower angles at 2θ = 3.55° (d001 = 2.49 nm) as compared to Ge-MMT prepared in this study in basic medium.

Similar difference in behavior was also reported accord-ing to the pHs of gelatin solutions (just below, equal to, and above IEP) [24, 25]. The higher increase in d-spacing observed in the case of acidified gelatin highlights a bet-ter level of intercalation. This result is obviously related to the interaction type established between the Ge and MMT. Indeed, at pH < IEP, the Ge chains rich in NH3

+ groups can interact with the MMT layers charged negatively and thus intercalate readily via electrostatic interactions. Hence, an efficient intercalation was reached in the pres-ence of this type of interactions that seems to be stronger than hydrogen bond interactions developed in the case of basified gelatin.

Besides, it is well established from chitosan/pristine MMT nanocomposite studies [27, 28] that in acidic diluted solution the NH2 groups of Cs get protonated and the chains display an extended structure owing to NH3

+–NH3+

electrostatic repulsions. Then, these chains bind with the negatively charged layers surfaces and intercalate via cati-onic exchange reactions.

TGA analysis provides another evidence for the inter-calation of the bio-modifiers and allows evaluating their amounts as well as the thermal stability of their corre-sponding organoclays. The TGA and derivative d (TG) thermograms of Ge and Cs, their corresponding organo-clays, and the unmodified MMT are regrouped in Figs. 3 and 4, respectively.

The Ge curves (Fig. 3) exhibit two thermal decomposi-tion steps. The first step occurs up to 180 °C due to mois-ture weight loss. The second one took place in the tempera-ture range of 200–500 °C attributed to entire breakdown of protein chains [29]. It can also be noticed that the appear-ance of slight weight loss at the beginning of this main step (1.97 wt%) is due to high water retention capacity of Ge. The onset and maximum degradation temperatures (Tonset and Tmax) are observed at 260 and 311 °C, respectively.

In TGA and d (TG) curves of Cs, besides the weight loss up to 180 °C due to the adsorbed water, the main ther-mal degradation occurs in one step and starts at 270 °C that is initiated by the random chain break and deacety-lation [30]. The maximum degradation temperature is reached at 298 °C. From ATG to d (TG) curves of Na-MMT, Ge-MMT and Cs-MMT represented in Fig. 4, it can be noticed that all clays showed dehydration under 200 °C and dehydroxylation of the aluminosilicate from 520 to 700 °C [31]. As expected, there is no weight loss for unmodified MMT between 200 and 500 °C, while one step weight loss appeared for both organoclays. This dif-ference is evidently attributed to the main decomposition of the bio-modifiers in Ge-MMT and Cs-MMT samples.

These results highlight once again the organo-modification of MMT layers with Ge or Cs. The Tonset and Tmax val-ues are 298 and 347 °C for Ge-MMT as well as 235 and 257 °C for Cs-MMT, respectively. Therefore, the contents of the organic agents within MMT galleries are calculated in this temperature range and are about 14.94 for Ge-MMT and 10 wt% for Cs-MMT. The Cs-MMT presents the lowest incorporation of organic component, which is in agreement with the value reported by Rodríguez et al. [32] about 8.9 wt%.

Additionally, it can be noticed that the Cs-MMT is less stable than the chitosan, but Ge-MMT is more stable than gelatin. This behavior is due to the fact that dry Cs-MMT

Fig. 3 Thermograms of TGA (a) and derivative (TG) (b) of gelatin and chitosan

Fig. 4 Thermograms of TGA (a) and derivative (TG) (b) of Na-MMT, Ge-MMT and Cs-MMT

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still contains residue of acetic acid between chitosan chains, which forms chitosonium acetate, and hence pro-motes the thermal degradation of the confined Cs chains into MMT sheets. A similar phenomenon was also reported by Wang et al. [33].

The thermal stability of an organoclay is an important property for the elaboration of polymer/clay nanocompos-ites by melt method, since it should be stable to degradation at the processing temperature. It is interesting to mention that both Ge-MMT and Cs-MMT display higher thermal stability than the commercial organoclay Cloisite 30B that the corresponding Tonset and organic content values, as given by the manufacturers, are 220 °C and 15.8 wt%,

respectively [34]. Thus, these so-prepared bio-modified nanoclays could be good candidates to develop CA-based nanocomposites by melt process.

CA-based nanocomposite study

Morphological analysis

Figure 5 a, b, and c displays the XRD patterns of unfilled CA and its nano-hybrids, prepared with and without TEC, together with that of the corresponding clays (Na-MMT, Cs-MMT and Ge-MMT). As it can be seen, the CA dif-fractogram shows no diffraction peak in the 2θ range 1°–7°.

Fig. 5 XRD patterns of CA and its nanocomposites prepared with and without TEC in the presence of a Na-MMT, b Cs-MMT and c Ge-MMT

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The peak observed around 2θ = 8.7° is attributed to its semi-crystalline structure.

For unplasticized CA-based nanocomposites, all curves exhibit incredibly weak diffraction peak at lower angles compared to the original peak of clay, which correspond to 2θ value of clay before its incorporation into matrix. The low intensity and the broadening of the basal reflection could mean exfoliation and/or a structure with irregular distances in a way that X-ray no longer detects interfer-ences. The presence of mixed structure will be further dis-cussed in view of TEM images. Additionally, the original diffraction peak almost disappears at 2 wt% of clay con-tent, while an increase in its intensity is observed at higher clay loading, and mainly in the presence of Na-MMT. This suggests the coexistence of more aggregated platelets for

nanocomposite prepared using unmodified MMT compared to both organoclays.

These results give evidence that the modification of MMT surface polarity by bio-intercalants has not only increased the d-spacing, but also induced a better matrix/nanofiller interface affinity. The formation of these nanocomposites may be mainly controlled by specific interactions between the polar groups of CA (i.e., carbonyl and hydroxyl groups) and those of clay bio-modifiers. Plasticized nanocomposite films were also elaborated with 20 wt% of TEC and 5 wt% of clay loading to examine the impact of plasticizer on intercalation/exfoliation process.

The diffraction patterns of CA/TEC/Na-MMT and CA/TEC/Cs-MMT nanocomposites (Figs. 5a and b) exhibit a sharp diffraction peaks located at 5.28 (d001 = 1.67 nm)

Fig. 6 TEM micrographs of typical nano-biocomposites with 5 wt% of clays loading at 200 and 100 (inset) nm scale bar. a CA/Na-MMT, b CA/Ge-MMT, c CA/TEC/Na-MMT and d CA/TEC/Ge-MMT

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and 5.22° (d001 = 1.70 nm), respectively. Considering a d001 = 1.30 nm for Na-MMT and a d001 = 1.61 nm for Cs-MMT, clear intercalation of the TEC alone or plasticized CA chains inside the interlayer gallery of clay could be recognized. The competitive penetration of TEC between MMT layers may be more favorable than CA chains owing to its smaller molecular size that facilitates the d-spacing expansion, and also since its hydroxyl and carbonyl groups can interact strongly through hydrogen bonding with MMT surface. As reported by Park et al. [35] and Gonçalves et al. [36], TEC is able to intercalate within clay layers in TEC/natural or organo-modified MMT mixture; thus, its diffraction peak appears around 2θ = 5°. However, in this work, CA was pre-plasticized with TEC, followed by mixing with clay. Thus, the most plausible interpreta-tion is related to the intercalation of plasticized CA chains. The confined CA/TEC chains interact with the clay surface through hydrogen bonding between the polar groups in CA and the hydroxyls groups of silicate layers or Cs-modifier. The coexistence of some exfoliated layers is, however, not excluded.

In Fig. 5c, the XRD pattern of CA/TEC/Ge-MMT shows a lack of any obvious peak in the 2θ range below 4.23°, i.e., for distances greater then 2.09 nm. The loss of the pure Ge-MMT peak suggests that the intercalation of CA/TEC chains into Ge-modified MMT, matching to the highest d-spacing as compared to Na-MMT and Cs-MMT, promotes the layers delamination and thus leads to mostly exfoliated MMT platelets.

Further information on the morphology of the elabo-rated nano-hybrids is provided by TEM analysis as com-plementary technique. Figure 6 displays TEM images at high magnification (200 or 100 nm scale bar) of

typical nanocomposites CA/Na-MMT, CA/Ge-MMT, CA/TEC/Na-MMT and CA/TEC/Ge-MMT with 5 wt% of clay loading. In agreement with XRD results, TEM images of unplasticized nano-hybrids in Fig. 6a and b confirm the formation of mixed intercalated/exfoliated structures, as evidenced by the presence of intercalated layered stacks of relatively small thickness randomly dispersed within CA matrix, coexisting with disorderly exfoliated platelets. Some agglomerated platelets are also observed mainly in the presence of unmodified MMT. For CA/TEC/Na-MMT nanocomposite (Fig. 6c), mainly intercalated platelets as well as aggregated structure are observed. However, higher density of individual platelets of Ge-MMT together with

Fig. 7 UV-visible spectra of virgin CA and its CA/clay nanocompos-ites films

Fig. 8 Thermograms of TGA (a) and derivative (TG) (b) of virgin CA and its CA/clay nanocomposites

Fig. 9 Thermograms of TGA (a) and derivative (TG) (b) of plasti-cized CA and its CA/TEC/clay nano-biocomposites

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lower tactoids is noticed in Fig. 6d, reflecting a better dis-persion of this organoclay within plasticized matrix.

The information gathered by combining XRD and TEM results reveals that good clay dispersion is achieved in nanocomposites prepared with organoclays and high-lights the efficient organic treatment of MMT layers by gelatin that promotes CA/Ge-MMT interface affinity owing to favor interactions between functional groups in CA and bio-modifier clay. Also, upon plasticization, the TEC groups (hydroxyl and carbonyl) as well as those of CA can interact with the structural oxygen and the Si–OH groups of the pristine MMT or with its bio-modifier (Ge or Cs).

Optical clarity

Cellulose acetate films are characterized by their outstand-ing optical clarity. One important issue is the increasing demand for transparency for certain applications where product visibility is required.

The effect of nanoclays on this property is examined for CA and its CA/clay nanocomposites, as shown in Fig. 7. In the visible region at 700 nm the unfilled CA film transmits 82 % of the light, while all CA-based nanocomposite films show a decrease in transmittance with an increase of the clay content, similar to others studies reported on polymer/clay nanocomposites [37, 38].

This result indicates that the compatibility between the clays and the matrix is not significantly enough. There-fore, the clay platelets are not completely exfoliated within the CA matrix and some heterogeneous distribution of the agglomerated platelets is formed, as confirmed by DRX and TEM characterization. Additionally, the film transpar-ency determined by transmittance is affected by the clay types used. This difference in the transmittance of the nano-hybrids may be due to limited miscibility between clay and matrix. In that sense, Ge-MMT seems to be more

compatible with CA as compared to other clays, since its nanocomposite containing 5 wt% can still retain an opti-cal transmission of approximately 70 % at 700 nm. The large decline in transmittance is noticed in the film contain-ing 5 wt% of Na-MMT, reflecting a lower dispersion and agglomeration within matrix, in agreement with XRD pat-terns and TEM images.

Thermal properties

Thermal stability of CA-based nanocomposites is assessed by thermogravimetric analysis. Figures 8 and 9 represent TGA and respective d (TG) curves of virgin CA and CA/TEC together with their nano-hybrids, respectively. From Fig. 8, the weight losses of CA and its CA/clay nano-biocomposites occur in a single main decomposition step in the tempera-ture range of 255–420 °C that is due to the degradation of CA chains. Table 1 summarizes the thermogravimetric data of neat CA and its nanocomposites. For further comparison of their thermal stabilities, the reported data include the tem-perature at which 5 % weight loss occurs (T5 %) that is consid-ered as the Tonset, the Tmax and the residue at 575 °C.

An improvement in thermal stabilities of these cellu-losic nano-hybrids compared to CA matrix is evidenced by the Tonset increase of CA from 260 °C with clay loading, whatever clay type. Among the clays used, Ge-MMT leads to the best thermal resistance of the corresponding nano-hybrid. The highest Tonset value is reached at 5 wt%, about 38 °C higher than that of virgin CA. This behavior is likely related to better dispersion state within matrix driving from favorable interactions between its Ge-modifier and CA groups, according to both XRD and TEM analyses.

The addition of clay affects slightly the Tmax of matrix, but increases the performance of the char formed at 575 °C, which acts as a superior insulator and efficient barrier to reduce the permeability to the volatile decomposition prod-ucts [39, 40]. The impaired diffusion of the volatile gas

Table 1 TGA and DSC data of CA and its CA-based nano-biocomposite films prepared without and with plasticizer

a Corresponding to the Tonset of the thermal degradation at the third stepb Obtained from second scan of the DSC thermogram

Sample Clay content (wt%) TGA analysis DSC analysis

T5 % (°C) Tmax (°C) Char (%) at 575 °C Tg (°C)b Tm (°C)b

CA 0 260 360 13.73 193 235

CA/Na-MMT 2 286 362 14.44 196 232

5 289 362 16.63 197 233

CA/Ge-MMT 2 290 362 13.84 197 234

5 298 365 14.97 198 233

CA/Cs-MMT 2 288 362 13.85 196 234

5 290 364 15.64 195 233

CA/TEC 0 314a 360 11.15 98 175

CA/TEC/Na-MMT 5 316a 363 14.52 101 173

CA/TEC/Ge-MMT 5 327a 365 11.58 102 172

CA/TEC/Cs-MMT 5 320a 365 11.42 101 173

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enhances the thermal resistance of the elaborated nanoma-terials compared to unfilled CA.

The plasticized matrix CA/TEC and its nanocompos-ites (Fig. 9) display similar three-step thermal degradation. The first step is below 240 °C and the second one is located in 240–300 °C range. These two steps are ascribed to the weight losses of moisture and the TEC plasticizer as free or/and associated with several compound groups (moisture, CA, clays) via hydrogen bondings. The third step above 300 °C represents substantial CA chain degradation, in which the corresponding Tmax values are nearly consistent, compared to the unplasticized counterparts.

Other thermal performance, such as the Tg and Tm temper-atures of CA and its nanocomposites obtained without and with plasticizer, was evaluated by DSC analysis. The corre-sponding data are also regrouped in Table 1. The unfilled CA shows Tg and Tm values of 193 °C and 235 °C, respectively [41, 42]. For unplasticized CA/clay nanocomposites, the Tg’s are shifted but only slightly to higher values when compared with CA, suggesting that the addition of clay does not appre-ciably restricts the segmental motion of the CA chains. No obvious difference is observed in Tm value of CA after clay addition. The explanation is confused since several factors may be involved such as clay dispersion degree, and interac-tions nature at CA/clay platelets interface.

The values regrouped in Table 1 give also evidence of plasticizing effect of TEC since its addition reduces the Tg and Tm values of CA and CA-based nanocomposites. Moreover, the Tm value of virgin CA underwent significant decrease upon plasticization, indicating the shift of CA property from rigid to ductile. Similar decrease in Tm val-ues was obtained for CA/TEC/clay nano-hybrids, resulting in not completely amorphous materials.

Water vapor barrier properties

The effects of clay and plasticizer on the nanocomposite water vapor permeability were estimated by means of water vapor transmission rates determination, as given in Fig. 10.

A reduction in WVTR values of nanocomposites com-pared to virgin CA and with an increase in clay load-ing, whatever the clay type, can be seen. This decrease in water vapor permeability would be explained by an

Fig. 10 Effects of clay and plasticizer on water vapor transmission rate of films of CA and its nano-biocomposites

Fig. 11 Weight losses of CA and its nanocomposites films during 6 months of incubation in compost

Fig. 12 Visual appearance of films samples recovered from com-post after 6 months: 1 virgin CA, 2 CA/TEC, 3 CA/Na-MMT5 %, 4 CA/TEC/Na-MMT5 %, 5 CA/Ge-MMT2 %, 6 CA/Ge-MMT5 %, and 7 CA/Cs-MMT5 %

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increase in tortuous path caused by the impermeable platelets having a large aspect ratio [43]. The unfilled CA presented a WVTR value of 128 ± 3 gwater/m

2.day, while the nanocomposite with 5 wt% of Ge-MMT displayed the lowest WVTR value of 84 ± 2 gwater/m

2.day. This behavior is due to a better dispersion of this organoclay within matrix. The decline in water vapor transmission rates suggests a potential of these CA/organoclay films in food packaging application. Apart from this, it can be noticed that WVTR values of plasticized CA and its nano-composites were slightly greater than unplasticized coun-terparts. Here, the water affinity of the hydrophilic TEC that is originated from its hydroxyl and carbonyl groups may be more efficient than the barrier effect of the layered silicates.

Biodegradation study

Although the biodegradation of cellulose acetate has deserved a particular attention in natural or seminatu-ral environment [44–46], the study of CA/silicate layered nanocomposite biodegradation is still limited. Thus, the evaluation of clay effect on CA biodegradation is an inter-esting aspect of research. Figure 11 reports the progressive weight losses under compost of CA together with its CA/clay nano-hybrids films prepared with 2 and 5 wt% of clay, which were monitored till 6 months. The impact of TEC on biodegradability of CA and a typical nanocomposite CA/TEC/Na-MMT5 % was also examined.

Apparently, the weight loss started similarly after 1 week for both CA and its nanocomposites, while an

Fig. 13 SEM micrographs of films cross-sections of CA (a and b) and CA/Ge-MMT 5 wt% (c and d) before and after burial in compost, respec-tively

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obvious deviation was observed after 1 month of exposure. The CA film presents a slow degradation, since the weight loss does not exceed 40 % after 6 months of burial. Various studies [47] have demonstrated that it is required to remove the additional acetyl groups on cellulose acetate before it can be attacked by the microorganisms, which can degrade the cellulose. Thereby, the key mechanism for the CA deg-radation is an initial deacetylation step either by chemical hydrolysis or by esterases that are common in many micro-organisms. Once the deacetylation is accomplished, the cel-lulose backbone became readily biodegradable by cellulase enzymes.

Additionally, the rate of CA biodegradation depends upon the degree of substitution (DS). The biodegradability is reduced as the degree of substitution is increased, but it is not inhibited at any DS value. Thus, there is a less con-tent of hydroxyls groups in the CA used in this study with a DS about 2.45, which promote the water uptake required for the chemical hydrolysis from compost. As a result, the biodegradation process that occurs by hydrolysis of the ester groups is retarded [48].

All nanocomposites present a retarded rate of weight losses compared to virgin CA, except for the nano-hybrids prepared with Ge-MMT. Additionally, the more load of clay, the lower the weight loss. Such seemingly retarding in CA biodegradation could be attributed to the improved barrier properties of the nanocomposites developed by clay layers, which protected the matrix from the attack of microorganisms and hinder their diffusion into the bulk of the films through more tortuous paths. Maiti et al. [49] and Wu et al. [50] also reported on the decrease in biodegrada-tion of PHB/clay and PLA/chitosan-modified MMT nano-composites, respectively.

Overall, the weight losses of formulations containing unmodified MMT are higher than those obtained with Cs-MMT, indicating a less retarding effect of Na-MMT due most likely to its higher hydrophilicity. The slight enhance in the biodegradation of CA/Ge-MMT nanocomposite indi-cates a catalytic role of this organoclay. This behavior is probably due to its better dispersion within matrix, result-ing from interactions between the polar groups of gelatin modifier and CA, according to other studies on polymer/clay nanocomposites [51, 52].

Figure 12 shows the results of compost burial tests in terms of changes in the appearance. After 6 months of incu-bation, all surfaces are damaged with rougher, more heter-ogeneous surfaces and cracks. These fractures are advan-tageous by creating much more surface area for further microorganisms’ attack. Also, a whitening of their surfaces is observed that can be attributed to the chemical hydroly-sis of the matrix [52].

SEM micrographs of cross-sections films at high magni-fication (×3,000) of CA and a typical nanocomposite with

5 wt% of Ge-MMT taken before burial and after 6 months of exposure to compost are observed in Fig. 13. The films illustrate the extent of morphological changes after incuba-tion. Also, the nanocomposite exhibited particularly more defects with larger and deeper pits compared to unfilled CA, reflecting a better biodegradation and confirming the gravimetric deductions.

On the other hand, the burials tests on CA/TEC and a typical nanocomposite CA/TEC/Na-MMT 5 % are also shown in Fig. 11. The biodegradation of CA and its nano-hybrid is greatly enhanced in the presence of TEC, as evi-denced by their higher weight loss values compared to unplasticized counterparts. However, the gap between the weight loss values of plasticized and unplasticized films decreased after about 3 months. This effect is probably due to the first attack of TEC by microorganisms. To ascer-tain the quicker elimination of TEC, the TGA analysis is performed for these plasticized materials before and after 3 months of exposure to compost, as illustrated in Fig. 14. The comparison between curves before and after burial for each sample displays the disappearance of the two first steps after 3 months of burial related mainly to TEC decomposition, and only a single step due to CA decompo-sition is observed. This result confirms once again the pre-ferred consumption of TEC by microorganisms.

Conclusion

Two organo-modified MMT nanoclays based on gelatin and chitosan as green intercalants were prepared and their formation was confirmed by FTIR, XRD and TGA. Then,

Fig. 14 Thermograms of TGA (a) and derivative (TG) (b) of plasti-cized CA and its CA/TEC/Na-MMT5 % nanocomposite before and after 3 months of burial in compost

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these organoclays and natural MMT were used to elaborate cellulose acetate nanocomposites at different clay loadings and in the absence or presence of TEC plasticizer. These nanocomposites were characterized by several analyti-cal techniques. XRD patterns together with TEM images revealed the formation of mixed intercalated/exfoliated structures with small clay agglomerates remaining mostly in the presence of unmodified MMT. The effect of the plas-ticizer on the intercalation/exfoliation process has been highlighted, in which better clay dispersion was obtained for nanocomposite prepared with Ge-MMT. The addition of a small amount of clay to matrix resulted in obvious enhancement in thermal stabilities and water vapor barrier properties, whereas they cause a slight decrease in optical clarity and biodegradation. The better physical properties and biodegradability were reached in nano-hybrids pre-pared with gelatin-modified MMT due to its well disper-sion within CA. This investigation will be extended in the forthcoming paper to the preparation of green CA/TEC/clay nanocomposites using these nanoclays by melt extru-sion, with the aim to study the effect of fabrication route (solution casting versus extrusion) on morphology and properties of these materials. By adding Ge-MMT or Cs-MMT organoclays into plasticized CA matrix under high shear force and by optimizing processing conditions, we hope to get better exfoliated and/or intercalated clays inside the matrix.

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