Thermal stability and decomposition kinetic studies of ...

Thermal stability and decomposition kinetic studies of antimicrobial PCL/ nanoclay packaging films Assia Siham Hadj-Hamou, Farid Metref & Farida Yahiaoui

Polymer Bulletin ISSN 0170-0839 Polym. Bull. DOI 10.1007/s00289-017-1929-y

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Author's personal copy Polym. Bull. DOI 10.1007/s00289-017-1929-y ORIGINAL PAPER

Thermal stability and decomposition kinetic studies of antimicrobial PCL/nanoclay packaging films Assia Siham Hadj-Hamou1 • Farid Metref1 Farida Yahiaoui1

•

Received: 15 April 2016 / Revised: 18 December 2016 / Accepted: 27 January 2017 Ó Springer-Verlag Berlin Heidelberg 2017

Abstract In our previous research, we had successfully developed antimicrobial PCL/nanoclay nanocomposite films with enhanced mechanical and water vapor barrier properties for food packaging field. Now, in this present work, we complete our study by investigating the thermal stability and decomposition kinetics of these materials using thermogravimetry technique (TGA). The TGA results revealed that the incorporation of 3% of organoclay in the PCL matrix reduced the thermal stability of PCL nanocomposite films. In addition, the decomposition kinetic study showed that the apparent activation energies Ea of the thermal degradation of the PCL nanocomposites are lower compared to those calculated for pristine PCL, confirming the catalytic effect of the organoclay upon the PCL decomposition. The master plots of g(a) function showed a similar thermal degradation mechanism of PCL and its PCL nanocomposites. All the studied samples followed Avrami–Erofeev, m = 1.5 model. Also, the thermodynamic parameters such as DH#, DS# and DG# were calculated and discussed. Keywords Poly(e-caprolactone) Packaging films Clay’s organomodifier Thermal stability Kinetics

Introduction The aliphatic polyesters have been the subject of increasing focus because of their biodegradability and biocompatibility [1, 2]. They are used in packaging, agriculture, medicine and other areas. Among them, poly(e-caprolactone) (PCL) & Assia Siham Hadj-Hamou [email protected] 1

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, Algeria

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Author's personal copy Polym. Bull.

has attracted a lot of attention owing to its exceptional properties [3–6] such as its biodegradability, its compatibility with a wide range of other polymers, its good processibility which enables fabrication of a variety of structures and forms, its ease of melt processing due to its high thermal stability and its relatively low cost [7, 8]. Unfortunately, the main use restrictions of PCL in a wide range of applications are due to its weak barrier properties and its poor mechanical performances as a result of its low melting point. A nano-reinforcement using layered silicates as nanofillers can lead to an improvement of its properties at low filler content [9–12]. Furthermore, the thermal stability of biodegradable polymers is important in the determination of their processing conditions and their further applications. It is well known that layered silicates improve the thermal stability of the polymer matrix because they act as a heat barrier, and this enhances the overall thermal stability of the system, as well as assists in the formation of char during thermal decomposition [13–15]. In other words, the thermal stability of polymeric nanocomposites is closely related to the nature and the content of the organoclay and its dispersion. Corres et al. [15] studied the influence of clays on the thermal decomposition pathway of phenoxy under both inert and oxidative atmosphere. Their findings showed that the organo-modified clays have two opposite effects on the thermal stability of the phenoxy matrix. For a low proportion of clay, there was a shielding effect that improved the thermal stability of the matrix. While, by increasing the organoclay content, the destabilization effect of the acidic sites generated by the early degradation of the modifier is more pronounced leading to an acceleration of the degradation process of the phenoxy polymer. Likewise, in our recent research on the effect of Cloisite 30B on the thermal and tensile behavior of the poly(butylene adipate-co-terephthalate)/poly(vinyl chloride) nanoblends, we found that the addition of C30B to PBAT/PVC matrixes reduced their thermal stability [16]. On the other hand, kinetic data obtained from TGA are very useful for understanding thermal degradation process and also to identify whether filler helps to improve the thermal stability of material. Previously, we investigated the effect of the incorporation of pure bentonite (PBT) or organically modified bentonite (OBT) into isotactic polypropylene (iPP) matrix on the thermal stability of iPP [17]. The activation energy (Ea) of thermal degradation of pure iPP and its iPP/PBT or iPP/ OBT nanocomposites was calculated by using the Ozawa, Flynn, and Wall and Tang equations and found out that the Ea of nanocomposites are higher than that of pure iPP. Our results also showed that the thermal stability improvement in iPP/PBT nanocomposite is not as important as in iPP/OBT nanocomposite, mainly due to the lower degree of dispersion of the PBT as compared to OBT. Achilias’s survey on the thermal degradation kinetics of nanocomposites based on the poly(3-hydroxybutyrate) (PHB) and organo-modified clays revealed that the thermal stability of the material is improved by the nanofiller addition [18]. In contrast, some authors found that the addition of the organoclay to neat polymer decreases its thermal stability. Surender et al. [19] studied the thermal degradation of bisphenol A-based polybismaleimide/Cloisite15a nanocomposites using model free kinetics and they found that the addition of Cloisite15a nanoclay particles decreased the thermal stability of the thermally cured bismaleimide system.

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Studies on the thermal degradation of PCL have provided different interpretations regarding its thermal degradation mechanism. The degradation of PCL was first investigated isothermally in sealed tubes in nitrogen atmosphere [20]. It has been found that PCL degrades by three different mechanisms occurring at the same temperature, around 220 °C, in nitrogen atmosphere and reduced pressure. Persenaire et al. [21] and Draye et al. [22] proposed a two-step sequential mechanism by random chain cleavage via cis-elimination and followed by unzipping. Afterwards, Aoyagi et al. [23] and Ruseckaite et al. [24] suggested a single-step unzipping depolymerization from the hydroxyl end of the polymer chains. Besides, Sivalingam et al. [25] also investigated the thermal degradation of PCL under non-isothermal and isothermal heating by TGA in a nitrogen flow environment. A single step of thermal degradation behavior has been obtained in both modes. Sivalingam et al. suggested that PCL undergoes both random chain scission and specific chain end scission simultaneously, but these two steps cannot be separated as consecutive steps and should be considered as parallel steps on non-isothermal heating and degrades by pure unzipping of the monomer from the hydroxyl end of the polymer chain on isothermal heating. Although the thermal degradation kinetics of PCL has been studied in the literature, only a few articles have been published so far on the non-isothermal degradation kinetics of PCL nanocomposites [26]. In our previous work [12], we have successfully developed antimicrobial PCL nanocomposites (based on two different organically modified montmorillonites) films with enhanced mechanical and water vapor barrier properties for food packaging applications. Now, in this paper, we will investigate the thermal stability of PCL-based nanocomposites using three different clay types: Cloisite 30B, trimethylhexadecyl ammonium and cetylpyridinium chloride monohydrate modified montmorillonite. For this purpose, the kinetics of the thermal degradation of neat PCL and its nanocomposites will be thoroughly studied using the Tang method [27]. We will attempt to propose a kinetic model relative to the degradation mechanism for each sample and also will evaluate the thermodynamic parameters which will be compared and discussed.

Experimental Materials Poly(e-caprolactone) (CAPAR650) was supplied by Solvay Chemicals sector-SBU. The number average molar mass was 49,000 with a polydispersity of 1.4, as determined by size exclusion chromatography. As we have previously reported [12], a bentonite originated from Algeria kindly supplied by Bentonite Company of Algeria and analyzed by the central laboratory of the ENOF [SiO2 (55–65%), Al2O3 (12–18%), Fe2O3 (1–3%), Na2O (1–3%), CaO (1–5%), K2O (0.76–1.75%) and MgO (2–3%)] was at a first stage, purified by sedimentation technique and then organically modified by a cation exchange method with trimethyl hexadecyl ammonium chloride (OMMT1) or pyridiniumhexadecyl chloride monohydrate (OMMT2).

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Cloisite 30B (alkyl quaternary ammonium bentonite) C30B was supplied from Southern Clay Products, Inc. and used as received. Composite preparation Melt blending of biodegradable polymer PCL with 3% by weight of organoclay (OMMT1, OMMT2 or C30B) was conducted using a twin-screw micro-extruder (5 &15 Micro Compounder DSM Xplore). The clay concentration was fixed at 3 wt% to potentially minimize the effect of the viscosity ratio on the particle size reduction [28]. The mixing process parameters (60 rpm screw speed, 10 min mixing time, and temperature profile: 125–130–130 °C) were modulated to optimize material final properties. Pure PCL was also processed in the same way as the composites and was used as a control. To obtain the desired specimens for characterization, all samples of PCL, PCL/OMMT1, PCL/OMMT2 and PCL/C30B materials are compressed to produce thin films using a hydraulic press, equipped with two heated plates at 80 °C with a pressure of 200 bars for 1 min. Characterizations XRD patterns of organically modified montmorillonites (OMMT1, OMMT2 and C30B), virgin PCL and its PCL/OMMT1, PCL/OMMT2 and PCL/C30B composites were recorded on a Phillips PW 1710 diffractometer in the in 2h range 2°–50° using CuKa monochromatic radiation (k = 0.15405 nm), operating at 40 kV and 40 mA at scanning rate of 0.12° min-1. The morphology of PCL-based nanocomposites was examined by TEM as a complementary technique to XRD analysis on HF2000 Hitachi (cold field emission gun 100 kV, point to point resolution = 0.23 nm). The ATR/FTIR spectra of PCL and its PCL nanocomposites were recorded at room temperature in the range 4000–500 cm-1 on a Thermo Nicolet, Nexus 670 spectrometer with a spectral resolution of 2 cm-1 and 60 scans were signal averaged. Differential scanning calorimetry (DSC) measurements were performed under nitrogen atmosphere by using a TA Instruments DSC Q100. Each sample was heated and subsequently cooled twice in temperature ranges -80 to 100 °C under nitrogen flow at 10 °C/min. The melting temperature (Tm) was determined from the first heating cycle while crystallization temperature (Tc) and glass transition temperature (Tg) were calculated by the second heating–cooling cycle. The area under the curve corresponding to the enthalpy was calculated from the instrument software. DSC scans for each sample were carried out in triplicate and the average values are reported in this study. Tests specimens for tensile measurements were prepared from 1-mm-thick plates. The tensile modulus, strength and elongation at break were measured in a Tensile Instron Zwick/Roell Z 100. Tester, at a strain rate of 200 mm min-1. The films were conditioned in a desiccator under 50% RH, at 25 °C, for 48 h before being characterized. The values of tensile strength and elongation at break were obtained based on the average tensile results of at least five tensile specimens and the results were provided with mean ± SD (standard deviation) values.

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The water vapor permeations were realized using the ‘‘cups methods’’, referring to the standard ISO 7783. The experimental setting consists in a cylindrical vessel filled with a desiccant powder and sealed with the investigated film. For our test, 10 g of CaCl2 were used as the desiccant powder, while the temperature was set to 23 ± 1 °C with a relative humidity of 48 ± 2%. This method consists of monitoring the water mass uptake of the desiccant powder with time. The water vapor transmission rate (WVTR) is then calculated from the slope of the mass uptake profile versus time as soon as the steady state is reached using Eq. (1): WVTR e ; ð1Þ WVP ¼ DP where WVP is the water vapor permeability coefficient, WVTR is the water vapor transmission rate, e is the film thickness and DP is the water vapor partial pressure difference. From the experimental conditions, the water vapor DP is 1400.3 Pa calculated for a temperature of 23 °C and a relative humidity of 48%. Four films of each sample were tested and the results are arithmetically averaged. The antibacterial activity of virgin PCL, PCL/C30B, PCL/OMMT1, PCL/ OMMT2 nanocomposite films was tested against Staphylococcus aureus CIP 4.83 (Gram-positive) and Escherichia coli CIP 53.126 (Gram-negative) using the bacterial enumeration method in sample. This procedure has been thoroughly described in our previous paper [12]. Thermogravimetric analysis (TGA) was conducted on a TA Instruments TGAQ500 using an ultrahigh purity (UHP) nitrogen atmosphere, heated from room temperature to 600 °C at the rate of 10, 20, 30 and 40 °C per min.

Theoretical background In order to analyze comprehensively the effect of the organoclay type on the decomposition mechanism of PCL upon heating, we attempted to evaluate the kinetic parameters (activation energy Ea and pre-exponential factor A) and the mechanism-dependent function g(a) described by the following relationships: da=dt ¼ AexpðEa=RTÞf ðaÞ

gðaÞ ¼ A=b

ZTa expðEa=RTÞdt:

ð2Þ

ð3Þ

T0 Ea , Eq. (3) can be written in the form By substituting u ¼ RT

gðaÞ ¼ ½AEa=bRPðuÞ:

ð4Þ

Because the exponential integral, P(u), has no analytical solution, an approximation formula of high accuracy [25] was used ln PðuÞ ¼ 0:37773896 þ 1:89466100 ln u þ 1:00145033u

ð5Þ

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and Eq. (4) can be transformed into (6) b ln 1:89466100 ¼ þ½lnðAa EaÞ=ðRgðaÞÞ þ 3:63504095 1:89466100 ln Ea T 1:00145033Ea=RT;

ð6Þ

where b is the heating rate, a is the extent of conversion (weight of materials volatilized/initial weight of materials), R is the universal gas constant and g(a) is a mathematical function whose form is related to the reaction mechanism. Ea, Aa, and Ta are, respectively, the activation energy, the pre-exponential factor and the absolute temperature related to a given extent of conversion. The apparent activation energy Ea can be deduced from the slope of ln b=ðT 1:89466100 Þ versus 1/T. Inserting a = 0.5 into Eq. (4), it can be written: gð0:5Þ ¼ ½AEa=bRPðu0:5 Þ;

ð7Þ

where u0:5 ¼ RTEa0:5 , T0.5 is the temperature when a equals to 0.5. By dividing Eqs. (4) by (7), one can get: gðaÞ=gð0:5Þ ¼ PðuÞ=Pðu0:5 Þ;

ð8Þ

where gðaÞ=gð0:5Þ ¼ f ðaÞ plots correspond to different theoretical model functions [29] and PðuÞ=Pðu0:5 Þ ¼ f ðaÞ are the experimental master plots obtained from the experimental data. gðaÞ=gð0:5Þ ¼ PðuÞ=Pðu0:5 Þ indicates that, when an appropriate kinetic model is used, the experimental value PðuÞ=Pðu0:5 Þ and theoretically calculated values of gðaÞ=gð0:5Þ are equivalent. Finally, according to Eq. (7), the pre-exponential factor A can be obtained from the slope of the plot of g(a) versus EaPðuÞ=bR at different heating rates.

Results and discussion Structure and morphology The XRD pattern of Na-MMT showed a peak at approximately 2h = 6.5° which corresponds to a basal spacing of 1.35 nm (Fig. 1). After the organo-modification reaction, this diffraction peak shifts towards lower angles values at 2h = 3.7° (d001 = 2.40 nm) and 2h = 4.2° (d001 = 2.10 nm) for OMMT1 and OMMT2, respectively. The increase in the d-spacing is an evidence of the intercalation of the quaternary ammonium or pyridinium surfactants via cationic exchange reactions. The Cloisite 30B exhibited a broad peak at 2h = 4.6° corresponding to a d-spacing of d = 1.90 nm. Figure 2 illustrates the XRD patterns of PCL, and its PCL/OMMT1, PCL/OMMT2 or PCL/C30B nanocomposites in 2h range 2°–50°. As can easily be seen, the nanoclay incorporation in the PCL matrix does not cause any changes in its crystal structure [12]. However, the XRD patterns of the three nanocomposites exhibit a significant

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Author's personal copy Polym. Bull. Fig. 1 X-ray diffraction patterns at low angle of NaMMT and three organoclays

Fig. 2 XRD patterns of PCL, and its PCL/OMMT1, PCL/OMMT2 or PCL/C30B nanocomposites in 2h range 2°–50°

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increase of the intensity of the crystalline peaks compared to those of PCL, indicating that the crystallinity in PCL-based nanocomposite increases. This behavior is mainly due to the well-known nucleating effect of the nanoclay. The most significant features are encountered in the lower angle range, which gives indication of the clay dispersion. Indeed, the characteristic diffraction peak intensities of OMMT1, OMMT2 and C30B are not only significantly reduced due to their dilution within the PCL matrix but also shifted to 2h = 2.20° (d001 = 4.01 nm) for OMMT1, 2.40° (d001 = 3.67 nm) for OMMT2 and 2.80° (d001 = 3.15 nm) for C30B. These peak shifts towards the lower angle side indicate the intercalation of the PCL chains inside the clay galleries. Nevertheless, the broadening and intensity decrease of the C30B characteristic peak observed, in the PCL/C30B nanocomposite case, reveal most likely the presence of disordered intercalated or mixed intercalated/partially exfoliated structures. The presence of mixed structure will be further confirmed with TEM as a complementary technique. Luduena et al. [30] reported similar observation for PCL/Cloisite 30B nanocomposite that the d001 for the PCL nanocomposite was increased to 3.31 nm after the nanocomposite preparation, indicating the formation of intercalated nanocomposites. Moreover, in the case of complete exfoliated nanocomposite, no diffraction peaks were detected due to the disorderly distributed clay platelets with the separation between single clay particles of 20–50 nm [30]. While, Di et al. [31] did not observe any noticeable peaks of Cloisite 30B in the low angle range for the 2 and 5 wt% C30B in PCL/C30B nanocomposite, which confirmed the exfoliated structure of silicate layers of Cloisite 30B in the PCL matrix. The dispersion of the organoclay (OMMT1, OMMT2 or C30B) into PCL matrix was examined by TEM analysis as displayed in Fig. 3. In agreement with XRD observations, TEM images confirmed the formation of mainly intercalated nanocomposite structures for both PCL/OMMT1 and PCL/OMMT2 nanocomposites, as evidenced by the presence of some intercalated layered stacks of relatively small thickness randomly dispersed within PCL matrix [12]. In the case of nanocomposite prepared with C30B, higher density of individual platelets of C30B together with lower tactoids is noticed, reflecting a better dispersion of C30B within PCL matrix as compared to OMMT1 and OMMT2 organoclays [12]. Similar results were reported in literature by Miltner et al. [32] who conducted a morphological characterization of PCL/C30B nanobiocomposite by means of atomic force

Fig. 3 TEM micrographs of PCL/OMMT1, PCL/OMMT2 and PCL/C30B nanocomposites

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microscopy (AFM), TEM and XRD scattering. From AFM and TEM images of PCL containing 3 wt% of C30B filler, these authors confirmed the formation of mainly exfoliated nanocomposites. Several authors have linked the variation of the degree of intercalation with the polar or apolar nature of the montmorillonite’s organomodifier. Indeed, in the presence of relatively hydrophilic clay such as the Cloisite 30B, polymer-filler interactions would be more intense than in the case of hydrophobic clays [33–35]. DSC characterization Table 1 summarizes the DSC results of the PCL and its different nanocomposites. The glass temperature of the neat PCL is -66 °C. This value seems to be affected only in the case of PCL/C30B, for which a slight increase in Tg is observed. This reflects a chain stretching restriction which is probably due to the presence of hydrogen bond interaction between the carbonyl group of PCL and the hydroxyl group of bis-2hydroxyl ethyl surfactant used in C30B nanoclay [36]. Moreover, there is no substantial difference in the melting temperature of nanocomposites compared to the neat matrix. This means that the PCL crystalline structure is not affected by the presence of the clay [11, 30, 31]. The degree of crystallinity of the PCL matrix (vc ) in the samples was obtained from the values of enthalpies of fusion, by normalizing them to the PCL content and comparing to the heat of fusion of the 100% crystalline PCL (139 J/g) [37]. Judging from obtained values for the degree of crystallinity of the nanocomposites, the amount of the crystalline PCL fraction is increased by the presence of the clay especially with OMMT1 or OMMT2 organoclay. The enhancement in PCL crystallinity suggests that the clay acts as a heterophase nucleating agent [38]. From Table 1, it can be also noticed that the addition of the organoclay to PCL matrix increases its crystallization temperature due to the nucleating effect of the clay [31]. Among the nanocomposites studied, PCL/C30B is characterized by the lowest value of Tc. This behavior would be mainly attributed to the hydrogen bonding interactions between C30B and PCL causing a restriction on the stretched chains which impedes the crystallization. We have subsequently used FTIR/ATR spectroscopy, a technique of choice, to study the specific interactions of hydrogen bonding type that occurred between the PCL matrix and the nanoclay in the 4000–400 cm-1 region. For instance, we illustrate in Fig. 4 comparative FTIR spectra between pristine PCL and its PCLTable 1 DSC results of PCL and its PCL nanocomposites Sample

Tg (°C)

Tm (°C)

Tc (°C)

v (%)

PCL

-66.1 ± 0.3

55.1 ± 0.4

23.2 ± 0.5

38.0 ± 1.1

PCL/C30B

-68.4 ± 0.3

56.5 ± 0.3

25.4 ± 0.2

43.2 ± 1.1

PCL/OMMT1

-67.2 ± 0.2

57.6 ± 0.3

26.7 ± 0.4

50.9 ± 1.4

PCL/OMMT2

-67.4 ± 0.2

57.7 ± 0.2

27.9 ± 0.3

51.3 ± 1.2

Values are reported as mean ± standard deviation

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Author's personal copy Polym. Bull. Fig. 4 Comparative FTIR spectra between pristine PCL and its PCL-based nanocomposites in the 1800–1600 cm-1 carbonyl stretching region

based nanocomposites in the 1800–1600 cm-1 carbonyl stretching region. As shown for PCL/C30B nanocomposite, the carbonyl stretching of the PCL (1723 cm-1) tends to shift to lower wavenumbers (1720 cm-1). This change may be induced by specific interactions between the carbonyl groups of PCL and the hydroxyl group of C30B owing to their compatibility. Tensile properties analysis Tensile properties of the PCL nanocomposites were studied and data (tensile modulus, tensile strength at break, and percent elongation at break) are summarized in Table 2. Compared to pure PCL, the obtained nanocomposites exhibited superior mechanical properties. An increase in the Young’s modulus of about 29% for PCL/OMMT1, 23% for PCL/OMMT2 and 34% for PCL/C30B in comparison with pure PCL has been noted. This result is mainly attributed to the intercalated structure which was the dominant state in these nanocomposites. In addition, due to its partially exfoliated structure, the PCL/C30B nanocomposite displayed the highest value of Young’s modulus. This result is related to the good C30B dispersion which is accompanied by a high surface area and a strong adhesion between the C30B and the PCL matrix [39]. Table 2 Tensile tests results and water vapor permeability values of PCL and PCL nanocomposites Sample

E (MPa)

r (MPa)

e (%)

WVP (g/m 24 h atm)

PCL

527 ± 25

29 ± 2

713 ± 14

0.35 ± 0.07

PCL/C30B

705 ± 46

36 ± 2

627 ± 18

0.19 ± 0.03

PCL/OMMT1

680 ± 36

34 ± 3

631 ± 50

0.15 ± 0.02

PCL/OMMT2

650 ± 63

33 ± 2

623 ± 33

0.18 ± 0.06


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Furthermore, tensile strength exhibited a slight increase (about 17% PCL/OMMT1, 14% for PCL/OMMT2 and 24% for PCL/C30B). This reflects the presence of moderate interfacial interactions between the clay layers and PCL matrix [40–42]. While the elongation at break was lightly decreased (*12%), this reduction would be attributed to the possible aggregates of non-intercalated silicates which might embrittle the PCL matrix. Many authors have studied the mechanical properties of the nanocomposite PCL (PCL/OMMT) and led to similar results [11, 30, 43–45]. Water vapor barrier properties The effect of the nanoclay on the nanocomposite water vapor permeability was estimated by means of water vapor permeability, as given in Table 2. A reduction in WVP values of nanocomposites compared to virgin PCL can be seen. This behavior could be explained by an increase in tortuous path generated by the dispersion of the impermeable clay platelets characterized by a high aspect ratio in the PCL matrix, thereby increasing the effective path length for diffusion [12]. In addition, this decline in the water vapor permeability can also be attributed to the higher number of crystallites into PCL matrix, leading to a high tortuosity, as already revealed by DSC analysis; the organoclay, whatever its type, acted as nucleating agents increasing the rate of the formation of the crystallites. It should also be noted that the partially exfoliated PCL/C30B nanocomposite has the highest water vapor permeability. This could suggest that the C30B clay may interact with the water vapor via the hydroxyl functional groups, thus increasing the water diffusion [46, 47]. Antimicrobial activity of PCL/nanoclay nanocomposites The antibacterial activity of virgin PCL, PCL/OMMT1, PCL/OMMT2 and PCL/ C30B was tested against Staphylococcus aureus CIP 4.83 (Gram-positive) and Escherichia coli CIP 53.126 (Gram-negative), using inhibition zone method [12]. Figure 5 depicts the results of inhibitory zone tests obtained for each sample, after 24 h of incubation at 37 °C.

Fig. 5 Photographs of antibacterial tests of PCL and its PCL nanocomposites against E. coli and S. aureus bacteria

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Author's personal copy Polym. Bull. Table 3 Growth of S. aureus and E. coli in the presence of PCL and PCL nanocomposites S. aureus (CFU/ml) Control (strain)

(1.9 ± 0.4) 10

9

PCL

(2.1 ± 0.2) 109

PCL/C30B

(7.8 ± 0.4) 109

Reduction (%)

E. coli (CFU/ml)

Reduction (%)

–

9

(2.1 ± 0.5) 10

–

–

(2.8 ± 0.3) 109

–

96

(2.6 ± 0.1) 109

91

PCL/OMMT1

(1.2 ± 0.1) 10

9

94

9

(3.1 ± 0.1) 10

89

PCL/OMMT2

(1.7 ± 0.2) 109

92

(3.8 ± 0.1) 109

86


An antibacterial activity is manifested by the appearance of clear inhibitory zones around PCL-based nanocomposites films, revealing that the presence of the organoclay into PCL matrix inhibit the growth of both bacteria. The number of viable bacteria expressed as CFU/ml and the reduction percent calculated based on the decrease of final counts from untreated controls in PCL and its nanocomposites are gathered in Table 3. As seen from Table 3, after the addition of 3 wt% of the organoclay to the PCL matrix, a significant decrease in the number of bacteria is noticed. A maximum of 96% of growth inhibition was reached. The antimicrobial performance of PCL/ organoclay composite films could be attributed to the migration of free ammonium (or pyridinium) surfactant from PCL nanocomposite films to culture medium because intercalation of PCL inside the organoclay structure could cause the release of part of the ammonium surfactant associated to the negatively charged part of the clay [48]. Owing to its positive charge, the ammonium or pyridinium group can bind to anionic groups on cell surface and form polyelectrolyte complexes with the bacterial surface, causing structural changes in the cell wall and also in the nuclear membranes provoking cell death [49–55]. Additionally, it also appears that the antibacterial action of all samples is better against S. aureus than against E. coli. This is due to the structure of the cell wall of E. coli (Gram-negative strain) which is much more complicated than that of Gram-positive species. This is due to the structure of the cell wall of E. coli (Gram-negative strain) which is much more complicated than that of S. aureus (Gram-positive species). Indeed, the cell wall of Gram-negative strain has another layer (called outer membrane) outside the peptidoglycan layer, which is composed mainly of lipopolysaccharides and phospholipids forming an effective resistive barrier against foreign compounds attack. Thermal stability Thermal stability of PCL-based nanocomposites was assessed by thermogravimetric analysis. Figure 6 represents TGA and respective d (TG) curves of virgin PCL and its nanocomposites. The weight losses of PCL and its PCL/nanoclay nanocomposites occur in a single main decomposition step in the temperature range of 250–420 °C that is due to the degradation of PCL chains.

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Fig. 6 TG and d(TG) curves of neat PCL and PCL nanocomposites Table 4 Thermogravimetric parameters of PCL and PCL nanocomposites


Samples

T0.05 (°C)

T0.5 (°C)

Tmax (°C)

Res (%)

PCL

362 ± 2

400 ± 2

404 ± 2

0.43 ± 0.08

PCL/OMMT1

360 ± 2

393 ± 2

393 ± 2

6.00 ± 1.00

PCL/OMMT2

358 ± 2

395 ± 3

395 ± 2

8.00 ± 0.08

PCL/C30B

336 ± 2

384 ± 2

391 ± 2

5.00 ± 0.05

Table 4 summarizes the thermogravimetric parameters of neat PCL and its nanocomposites. For further comparison of their thermal stabilities, the reported data include the temperature at which 5% weight loss occurs (T0,05) that is considered as the Tonset, the temperature at 50% mass loss (T0,5), the temperature at which the PCL decomposition rate is the highest (Tmax) and the residue at 550 °C. Table 4 clearly shows that all nanocomposites are less thermally stable than the neat PCL. The decreased thermal stability of the nanocomposites is the consequence of the presence of thermally unstable organic ammonium or pyridinium ions which accelerate the degradation of the matrix [56]. Since, it has been reported [57] that the alkyl pyridinium salts decompose via Hofmann elimination, in the same way that the quaternary alkylammonium salts. Among the clays used, C30B leads to the lowest thermal resistance of the corresponding nanocomposite. Our results are consistent with those previously reported [58], that a lower thermal stability is noticed for PCL/C30B due to PCL hydrolysis caused by the presence of hydroxyl groups in the modifier compared to nanocomposites with organoclays, where the modifier contains only nonpolar groups. On the other hand, it should be noted that the residue mass increases in the case of the PCL-based nanocomposites compared to that of the neat PCL. This could be due to the trapping of the volatile products, issued from the decomposition of the polymer, by the

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lamellar particles of the nanoclay. In others words, the weight loss of the organomodified clay was counterbalanced by a remaining charring fraction of the degradation of the PCL. A similar behavior was observed with EVA filled with OMMT [59]. Thermal decomposition kinetics The thermal degradation process for all the samples was studied through nonisothermal measurements at different heating 10, 20, 30 and 40 °C/min. The Ea values for all the studied samples are obtained from the slope of the b lnðT 1:89466100 Þ versus 1/Ta plot with a correlation coefficient greater than 0.996. Figure 7 displays the dependence of the activation energies of PCL and its PCL/ organoclay nanocomposites with conversion degree. The activation energy of PCL is approximately constant during the entire process indicating that the degradation mechanism of PCL remains unchanged throughout its decomposition (Fig. 4). The average Ea is 172 kJ/mol is close to that calculated by Marquez et al. [29]. Two different explanations about the PCL thermal degradation process were found in the literature. In agreement with our results, Aoyagi et al. [23] and Ruseckaite et al. [25] obtained only a single degradation step and proposed a unique unzipping mechanism for the PCL thermal degradation. Whereas, Persenaire et al. [21] and Draye et al. [22] proposed a two-stage mechanism with a cis-elimination followed by an unzipping depolymerization with production of the monomer and other oligomers. Furthermore, the Ea values of PCL nanocomposites varied with conversion in a similar way compared to the neat PCL (Fig. 7). This similarity in the thermal behavior of PCL and its PCL nanocomposites reflects the similarity in their thermal degradation

Fig. 7 Dependence of activation energy (Ea) on the degree of conversion (a) of the mass loss, as calculated with Tang method for PCL and PCL nanocomposites

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process [26]. Nevertheless, it should be noted that the Ea values of PCL nanocomposites are lower than those of neat PCL, thus reflecting that all the surfactants have a significant degrading effect on the thermal decomposition of PCL matrix. Subsequently, the activation energy calculated for each sample by applying the Tang method has been used for the construction of the generalized master plots for obtaining the kinetic model followed by the decomposition process. Eighteen different kinetic models were tested (Table 5) [60]. Generalized master plots constructed using the average value of the activation energy of each sample, are plotted together with the master plots corresponding to the ideal kinetic models in Fig. 8. This figure clearly illustrates that the kinetics of the degradation mechanism of the neat PCL is best described by the Avrami–Erofeev random nucleation model (A1.5) with g(a) = [-ln(1-a)]2/3, in which the reaction is controlled by initial random nucleation followed by overlapping growth. This is in good accordance with the results reported by Marquez [29]. Figure 5 also shows that the thermal degradation mechanism of the three PCL nanocomposites follows the Avrami–Erofeev, n = 1.5 (A1.5) model. In view of these results, we can say that the incorporation of 3% wt of OMMT1, OMMT2 or Cloisite 30B into PCL matrix does not modify the degradation process of PCL in nanocomposites but only the activation energies and the decomposition rate. The pre-exponential factor A values of each sample were also determined by plotting g(a) against EaPðuÞ=bR at different heating rates. For instance, we illustrate in Fig. 9, the linear variation of g(a) versus EaP(u)/bR for PCL/OMMT1 and PCL/ Table 5 Kinetic models for thermal degradation of polymers No.

Reaction model

Symbol

g(a)

1

Avrami–Erofeev, m = 4

A4

[-ln(1-a)]1/4

2


A3

[-ln(1-a)]1/3

3


A2

[-ln(1-a)]1/2

4

Avrami–Erofeev, m = 1.5

A1.5

[-ln(1-a)]2/3

5

Phase boundary reaction, n = 1

R1

a

6


R2

1-(1-a)1/2

7


R3

1-(1-a)1/3

8

One-dimensional diffusion

D1

a2

9

Two-dimensional diffusion

D2

[(1-a) ln(1-a)] ? a

10

Three-dimensional diffusion

D4

1-2 a/3-(1-a)2/3

11

Jander’s type diffusion

D3

[1-(1-a)1/3]2

12

Power law, n = 1/4

a1/4

13

Power law, n = 1/3

a1/3

14

Power law, n = 1/2

a1/2

15

Power law, n = 3/2

a3/2

16

First order

A1, F1

-ln(1-a)

17

Second order

F2

(1-a)-1-1

18

Third order

F3

1/2[(1-a)-2-1]

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Fig. 8 Theoretical and experimental master plots at different heating rates for PCL and PCL nanocomposites

OMMT2 nanocomposites. The kinetic triplets of the thermal degradation of PCL and its PCL nanocomposites are gathered in Table 6. In the last part of this contribution, the values of thermodynamic parameters such as DS#, DH# and DG# were calculated at T = Tmax at which the PCL decomposition rate is the highest. According to the theory of the activated complex (transition state) of Eyring [61–64], the activation entropy (DS#) is calculated using the following equations: DS# ¼ R ln

Ah ; Tmax

ð8Þ

where k is Boltzmann constant, h is Plank constant. The activation enthalpy DH# is determined from Eq. (9) DH # ¼ Ea RTmax :

ð9Þ

The variation of Gibbs free energy DG# is expressed by [65, 66] DG# ¼ DH # Tmax DS# :

ð10Þ

The variation of Gibbs free energy DG# reflects the total energy increase of the system at the approach of the reagents and the formation of the activated complex. DG# is influenced by two thermodynamic functions, the variations of the activation enthalpy DH# and the activation entropy DS#. The variation of the reaction enthalpy represents the energy differences between the activated complex and the reagents.

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Fig. 9 Variation of g(a) vs (Ea/b)P(u) for PCL/OMMT1 and PCL/OMMT2 nanocomposites Table 6 Kinetic triplet (Ea, A and g(a)) and thermodynamic parameters of the non-isothermal decomposition of PCL and its nanocomposites Sample

Ea (kJ/mol)

A (1010 min-1)

g(a)

DH# (kJ/mol)

-DS# (J/ K mol)

DG# (kJ/mol)

PCL

172

680

g(a) = [-ln(1-a)]2/3

178

40

205

PCL/OMMT1

157

63

g(a) = [-ln(1-a)]2/3

162

60

204

PCL/OMMT2

154

37

g(a) = [-ln(1-a)]2/3

159

64

203

PCL/C30B

148

18

g(a) = [-ln(1-a)]2/3

153

70

201

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Obtaining a small variation of the activation DH# value reflects the favored formation of activated complex. On the other hand, the variation of reaction entropy represents the position of the material with respect to its own thermodynamic equilibrium. Indeed, low activation entropy means a low reactivity. In this case, the material takes a long time to form the activated complex. On the contrary, high activation entropy means that the material is far from its own thermodynamic equilibrium. The reactivity is high and the system can react faster to produce the activated complex [66, 67]. All thermodynamic parameters of the non-isothermal decomposition of PCL and its nanocomposites are listed in Table 6. As seen, all Ea, DH # and DG# values are positive and DS# values are negative, indicating non-spontaneous processes of decomposition for all the samples. Table 6 also shows that in the case of PCL nanocomposites the Ea and the DH # values are lower than those of the neat PCL, while the DS# values are higher compared to those of PCL. These results confirm again that the addition of 3% of organoclay (whatever its type) in PCL matrix accelerates its thermal degradation due to the degrading effect of the surfactants. On the other hand, the DG# values remain almost constant after the nanocomposite preparation. This is attributed to the fact that the organoclay has no influence on the decomposition mechanism of PCL. This later acts as a catalyzing agent during the decomposition reaction and consequently does not change its Gibbs free energy.

Conclusion Thermal stability and decomposition kinetic studies of antimicrobial PCL/nanoclay packaging films were investigated by thermogravimetry technique. The results showed a reduction in their thermal stability due to the presence of thermally less stable clay organic modifier (ammonium or pyridinium) ions which accelerate the degradation process of the PCL matrix. The Ea values of PCL nanocomposites were lower than those of neat PCL, reflecting that all the surfactants have a significant degrading effect on the thermal decomposition of PCL. The kinetic study showed that the incorporation of 3% wt of OMMT1, OMMT2 or Cloisite 30B into PCL matrix did not modify the degradation mechanism of PCL in nanocomposites but only the activation energy and the decomposition rate. Acknowledgements The authors are grateful to Mrs. Bouchra Benkhaldoun for her contribution in the development of master plots programs.

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