Effect of clay treatment on the thermal degradation of PHB based nanocomposites

Applied Clay Science 163 (2018) 146–152

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Research paper

Effect of clay treatment on the thermal degradation of PHB based nanocomposites

T

⁎

Romina P. Olliera, , David A. D'Amicob, Walter F. Schroederc, Viviana P. Cyrasb, Vera A. Alvareza a

Grupo de Materiales Compuestos Termoplásticos (CoMP) - Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Av. Colón 10890, 7600 Mar del Plata, Argentina b Grupo de Ecomateriales - Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Av. J. B. Justo 4302, 7600 Mar del Plata, Argentina c Grupo de Polímeros Nanoestructurados - Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA), Universidad Nacional de Mar del Plata – Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Av. J. B. Justo 4302, 7600 Mar del Plata, Argentina

A R T I C LE I N FO

A B S T R A C T

Keywords: Thermal stability PHB Clay Chemical modification Processing

A detailed understanding of the thermal degradation processes taking place during the melt processing of bionanocomposites is crucial in order to increase the processing window of these materials. In this work, the influence of the content of neat clay and modified-clay on the thermal degradation of a biodegradable bacterial poly(3-hydroxybutyrate) (PHB) matrix, was studied. The modified clay consisted in a multi-treated organobentonite, which was first acid-activated, then silylated and further modified by cationic exchange treatment. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) tests were carried out to investigate the thermal behavior of the different nanocomposites as a function of temperature, whereas size exclusion chromatography (SEC) runs were performed to analyze the changes on the molecular weight distribution of the PHB. The obtained results reveal that the organic modifiers of the muti-treated clay promote the thermal degradation process leading to a dramatic decrease in the molecular weight of PHB. It was demonstrated that the degradation mechanism of PHB was not modified by the incorporation of neat clay or modified-clay, and that the process can be well described by the Avrami–Erofeev random nucleation model (m = 4), in which the reaction is controlled by initial random nucleation followed by overlapping growth.

1. Introduction Biodegradable polymers, in particular polyhydroxyalkanoates (PHA), represent an interesting alternative to synthetic polymers due to many advantages related with their biodegradability and biocompatibility, and also because they are produced from renewable resources (Hablot et al., 2008). Nevertheless, they have several disadvantages related to their mechanical and barrier properties. In order to overcome such disadvantages one alternative is the preparation of bio-nanocomposites, (Pandey et al., 2005; Chivrac et al., 2006) since the nanofillers are able to enhance the material properties. There are several studies in the literature focalized on PHB or PHBV/Montmorillonite (Mt) based bio-nanocomposites (Chen et al., 2002; Lim et al., 2003; Mook Choi et al., 2003; Chen et al., 2004; Wang et al., 2005). The main drawback of PHB is its thermal instability during melt processing. Due to this limitation, most of the works use the environmentally unfriendly route of solvent intercalation (Chen et al., 2002; Lim et al., 2003; Chen et al., 2004; Wang et al., 2005). Therefore, ⁎

great interest has been shown in the thermal degradation of PHB and other related PHA. Since degradation is assumed to occur by random chain excision with or without surfactants, as recently demonstrated for PHB (Hablot et al., 2008), the lower the initial Mw, the faster the short chains are obtained, and therefore, the less thermally stable is the PHA. In the last years, it has been proved that PHB is a chemically recyclable polymer with several end products such as crotonic acid, linear oligomers, having a crotonate end group (Morikawa and Marchessault, 1981), and a cyclic trimer (Melchiors et al., 1996). Many studies have been dedicated to thermal (Grassie et al., 1984c; Grassie et al., 1984b; Grassie et al., 1984a; Kunioka and Doi, 1990; Aoyagi et al., 2002; Li et al., 2003; Abe, 2006; Carrasco et al., 2006) and thermomechanical (Melik and Schechtman, 1995; Renstad et al., 1997) degradation of neat PHB and polyhydroxybutyrate-co-valerate (PHBV). These studies have revealed that the degradation occurs rapidly near the melting point mainly through a random chain scission process based on typical structures of pyrolysis products, i.e. crotonic acid and oligomers with a crotonate end group (Morikawa and Marchessault, 1981; Kawalec et al.,

Corresponding author. E-mail address: rominaollier@fi.mdp.edu.ar (R.P. Ollier).

https://doi.org/10.1016/j.clay.2018.07.025 Received 9 March 2018; Received in revised form 27 June 2018; Accepted 16 July 2018 0169-1317/ © 2018 Published by Elsevier B.V.

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70 °C for 4 h. The clay was then filtered and washed several times with distilled water. The exchanged clay was dispersed in ethanol and dried in an oven at 80 °C for 24 h. The resulting sample was designated E-S-ABent.

2007). On the other hand, they have also shown that the parameters of melt processing have to be optimized in order to avoid or restrict this phenomenon that reduces the processing window. During the last decades, the development of nanocomposites by incorporating nanoscaled fillers into a polymer matrix has been believed to become a key technology on advanced composite materials. It is known that nanocomposites obtained by the incorporation of low amounts of clay into polymeric matrices displayed, among others, enhanced thermal and oxidative barrier properties as compared with traditional composites (Huang et al., 2005; Cyras et al., 2008; Bordes et al., 2009b). Kinetic data obtained from thermogravimetric (TGA) measurements are very useful for understanding thermal degradation processes and also to identify whether the filler helps to improve the thermal stability of the material. There are several reports about the thermal degradation of PHB-based bio-nanocomposites filled with Mt. Maiti et al. (Maiti and Batt, 2003) have reported that the presence of aluminum Lewis acid sites in the octahedral sheet of the silicate layers enhances the thermal degradation of PHB by catalyzing the hydrolysis of ester linkages. (Xie et al., 2001) have pointed out the complex degradation reactions that could exist in organically modified Mt. and consequently, in nanocomposite with that kind of clays. The aim of this work was to investigate the influence of the incorporation of the clay, the processing technique and the addition of clay modifiers (silane and phosphonium salts) on the thermal degradation of PHB. A kinetic analytical model (Criado et al., 1989) was applied to non-isothermal TGA measurements trying to identify the possible mechanism of PHB degradation in the presence of neat and modified bentonite.

2.3. Preparation of the PHB/organoclay nanocomposites PHB films and PHB/organoclay nanocomposites were prepared by melt compounding method. Different clay contents (2, 4 and 6 wt%) and 20 wt% of a natural hydrophobic plasticizer, TBL, was incorporated to facilitate the polymer processability. TBL is a natural triglyceride present in fats and oils that has shown to be a good candidate as plasticizer for bio-based polymer were added to the PHB (D'Amico et al., 2016). In order to obtain the nanocomposites, raw PHB pellets were mechanically mixed with TBL and the clay prior to melt them in a Haake mixer at 185 °C and a rotation speed of 50 rpm for 3 min. Blends were then molded into films at 190 °C in a hot press. The materials were kept between the plates at atmospheric pressure for 1 min until melting and then for 2 min at 5 MPa. Both clays (original and modified) and PHB pellets were previously dried in a vacuum oven at 80 °C overnight. 2.4. Characterization techniques X-ray diffraction (XRD) analysis was performed on the clay powders using an X-Pert pro diffractometer, operating at 40 kV and 40 mA, with CuKα radiation (λ = 1.5406 Å), at a scanning speed of 1.5°/min. To calculate the interplanar basal distance (d001), the 2θ angle corresponding to the maximum value of the 001 reflection was used in the Bragg equation. The X-ray diffraction pattern of the raw Bent was analyzed using the X'Pert HighScore 2.2d software. Size exclusion chromatography (SEC) was used to determine the molar mass distribution and the average molar mass. The tests were performed in a LKB-2249 instrument at 25°C. A series of four μStyragel® columns, ranging in pore size 105, 104, 103, 100 Å, was used with chloroform as an eluent. The sample concentration was 4–5 mg/ mL and the flow rate was 0.5 mL/min. The polymer was detected by the carbonylic absorption of the ester group (5.75 μm), using an infrared detector (Miram 1A Infrared Analyzer) and the calibration was done with poly(methyl methacrylate) (PMMA) standards supplied by Polymer Laboratories and Polysciences. Differential Scanning Calorimetry (DSC) measurements were performed in a Perkin Elmer Pyris calorimeter, operating from −50 to 200 °C at a heating rate of 10 °C/min using nitrogen as a purge gas flow (ASTM D3417–83). The samples (average weight 10 mg) were placed in standard aluminum pans. The degree of crystallinity (Xcr) of each sample was calculated from the following equation, with the assumption that the heat of fusion is proportional to the crystalline content:

2. Materials and methods 2.1. Materials The natural clay employed in this work was a bentonite (Bent), supplied by Minarmco S.A. (Neuquén, Argentina). It consisted predominantly of montmorillonite as evidenced by X-ray diffraction (XRD) analysis (D'Amico et al., 2014). It contained quartz and feldspar as major impurities, as well as traces of gypsum and sepiolite. Dimethyloctadecylchlorosilane (DMOCS) and tributylhexadecylphosphonium bromide (TBHP) organic modifiers as well as pyridine (anhydrous, 99.8%) were purchased from Aldrich and used as received. PHB polymer (Mw = 246,000 g.mol−1), was supplied by PHB Brazil and tributyrin (TBL) was provided by Fluka. 2.2. Preparation of the organoclay Bent was firstly activated by treatment with mineral acid, then it was silylated with DMOCS and finally it was modified by cationic exchange reaction with TBHP (D'Amico et al., 2014). A 5 g portion of clay was dispersed in 200 mL of water. Then 10 mL of 98% (w/w) H2SO4 was added and the mixture was stirred at room temperature for 6 h. The wet acid activated clay was separated by centrifugation, washed with distilled water and centrifuged at 10,500 rpm for 10 min. This procedure was repeated 5 times. Finally, the wet product was frozen for 24 h and then lyophilized at 100 mTorr and − 50 °C for 72 h. Subsequently, the activated-Bent was silylated with DMOCS. For this treatment, 1.5 g of A-Bent was dispersed in 350 mL of anhydrous n-butanol, together with an excess of DMOCS (5 g). The mixture was heated at 100 °C and kept under stirring. After 15 min, 3.5 mL of pyridine was added and it was refluxed for 24 h. The product was separated by filtration and then washed three times with n-butanol and once with ethanol. Finally, the product (S-A-Bent) was dispersed in 20 mL of ethanol and dried in an oven at 80 °C for 24 h. Finally, ion exchange was performed by dispersing a 2.5 g S-A-Bent in 100 mL of distilled water and 1.1 g of TBHP was added. The dispersion was kept under stirring at the temperature of

Xcr (%) =

ΔHm, × 100 wPHB × ΔH100

(1)

where ΔHm is the experimental heat of fusion, wPHB is the PHB weight fraction and ΔH100 is the heat of fusion of 100% crystalline PHB and its value is 146 J/g (Barham et al., 1984). Thermogravimetric analysis (TGA) was carried out on a TA Q500 thermogravimetric analyzer, the mass of each sample was 5–7 mg and the carrier gas was nitrogen at a flow rate of 50 mL/min. Each sample was heated from room temperature to 700 °C at various heating rate values (5, 10, 15, 20, and 25 °C/min). The organic content was determined as the mass loss in the temperature range between 200 and 500 °C, where no additional thermal events occur for the pristine clay. 3. Theoretical Background 3.1. Isoconversional methods Any solid state decomposition kinetic can be expressed as a single 147

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temperature integral (Eq. 9) is numerically solved.

step kinetic equation:

dα = k (T ) f (α ) dt

n

ς (E∞) =

(2)

n

∑∑ i−1 i=j

where t is the time, T is the temperature, k is the rate constant, f(α) is the differential form of the kinetic model and α is the conversion defined as:

αT =

t∞

J [E∞ , Ti (t∞)] ≡

dα −E ⎞ f (α ) = A exp ⎛ dt ⎝ RT ⎠

(4)

∂ ln(f (α )) ⎤ ∂ ln(k (T )) ⎤ =⎡ +⎡ −1 −1 ⎣ ∂T ⎦α ⎣ ∂T ⎦α

( ) ⎤⎥

⎥ ⎦α

=−

Eα R

(5)

α

g (α ) =

∫ fdα(α )

(10)

0

g (α ) θ = g (0.5) θ0.5 (6)

(11)

When a linear heating rate is employed during non-isothermal experiments, the right-hand side of Eq. 11 can be calculated by:

In Eq. 6 the temperature dependence of the isoconversional rate can be used to evaluate isoconversional values of the activation energy, Eα without assuming or determining any particular form of the reaction model. For this reason, isoconversional methods are frequently called “model-free” methods. Nevertheless, one should not take this term literally. While the methods do not need to identify the reaction model, they assume that the conversion dependence of the rate obeys some f(α) model. The temperature dependence of the isoconversional rate can be obtained experimentally by performing a series of runs with different temperature programs, typically a series of 5 runs at different heating rates. The Eα dependence is important for detecting and treating the multistep kinetics (Vyazovkin et al., 2011). In this work two integral isonversional methods were employed, in order to obtain the Eα. For a constant heating rate program, the integral form of Eq. (4) does not have an analytical solution. Consequently, there are a number of integral isoconversional methods that differ in approximations of the temperature integral. By using the Coats–Redfern (Vyazovkin et al., 2011) and considering that 2RT/Eα is much lower than unity, the Kissinger–Akahira–Sunose (KAS) (Akahira, 1971) equation can be written in the form:

β AR ⎞ Eα ln ⎛ 2 ⎞ = ln ⎛⎜ ⎟ − RTα ⎝ T ⎠α ⎝ g (α ) Eα ⎠

(9)

The aforementioned model-free method allows one to evaluate the activation energy without determining the reaction model. However, this should not be understood as implying that the model-free methods cannot be used to determine the reaction models. An alternative to explain it is the master plot method (Criado et al., 1989; Gotor et al., 2000; Perez-Maqueda et al., 2002). According to this method, master plots can be drawn based on either the integral or the differential form of the kinetic equation describing degradation by using the concept of the generalized time, θ. From the integral kinetic equation, the following equation can be obtained using a reference point at θ = 0.5:

where the subscript α indicates isoconversional values, i.e., the values related to a given extent of conversion. Being at α = const, f(α) is also constant, therefore the second term in the right hand side of Eq. (5) is zero. Thus:

⎡ ∂ ln dα dt ⎢ ⎢ ∂T −1 ⎣

−E∞ ⎤ exp ⎡ . dt ⎢ ⎣ R. Ti (t ) ⎥ ⎦

3.2. Determination of the reaction mechanism

where A is a pre-exponential factor, R is the gas constant and E is the activation energy. When an isoconversional method is used, the reaction rate at constant extent of conversion should be only a function of temperature. By taking the logarithmic derivative of the reaction rate (Eq. (1)) at α = const:

⎥ ⎦α

(8)

where i and j denote the different thermal experiments, Δα is the conversion increment, and J[Eα,Ti(tα)] is the temperature integral which can be well approximated with a numeric integral. Subsequently, these data are used to minimize Eq. (7) by seeking an appropriate Eα value. Repeating this minimization procedure for each α of interest, Eα as a function of α can be obtained.

(3)

where m0 and mf denotes the initial and residual mass, respectively, and m(T) refers to the actual mass of the sample. In general, the Arrhenius equation expresses the explicit temperature dependence of the rate constant, therefore Eq. 2 results:

( ) ⎤⎥

∫ t∞− Δ∞

m 0 − m (T ) m 0 − mf

⎡ ∂ ln dα dt ⎢ ⎢ ∂T −1 ⎣

J [E∞ , Ti (t∞ )] J [E∞ , Tj (t∞ )]

p θ = θ0.5 p0.5

(12)

where the function p(x) can by expressed by the fourth rational approximation of Senumand Yang corrected by Flynn was used (Eq. 12), which allows an accuracy of better than 10–5% (Gotor et al., 2000):

p (x ) =

exp(−x ) . π (x ) x

(13)

and

π (x ) =

x4

x 3 + 18x 2 + 86x + 96 + 20x 3 + 120x 2 + 240x + 120

(14)

where x = Eα/RT. Algebraic expressions of g(α) for the most frequently used mechanisms appear in Table 1. By overlaying the experimental data with the different mechanisms, the model that better describes the process can be found. Table 1 - Algebraic expressions of g(α) for the most frequently used mechanisms.

(7)

where g(α) is the integral conversion function and β is the linear constant heating rate. It is also interesting to calculate the effective activation energy by means of advanced integral isoconversional method developed by Vyazovkin (Vyazovkin and Sbirrazzuoli, 2006). For a series of experiments performed at different heating rates, the Eα value can be determined by minimizing the function expressed by Eq. 8. The 148

Mechanism

Symbol

g(α)

1D diffusion 2D diffusion

D1 D2

3D diffusion Random nucleation and growth of nuclei (Avrami-Erofeev, JMA model)

D3 Am(0.5 ≤ m ≤ 4)

α2 (1- α)ln(1α) + α [1-(1- α)1/3]2 [−ln(1- α)]1/m

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strong reflection at 2θ = 6.8°. In the PHB/Bent nanocomposites the same reflection appears near to 2θ = 6.3°, regardless the clay content. There is almost no change in the position of the 001 reflection for E-S-A bent (2θ = 2.6°) and PHB/E-S-A-Bent nanocomposites (2θ = 2.4°). Similar results were found in PHB/organomodified clay nanocomposites, where the interplanar basal distance was independent of the clay concentration (D'Amico et al., 2012). These results would indicate that the interlayer region was expanded. That is to say, the polymer chains entered into the interlayer space forming intercalated PHB/clay nanocomposites. Regarding thermal properties of clay polymer nanocomposites, the organoclay may have two opposite effects: a barrier effect, which should improve the thermal stability, or a catalytic effect on the degradation of the polymer matrix, which should decrease their thermal resistance (Zhao et al., 2005). The clay acts as a heat barrier improving the overall thermal stability of the system. The presence of the filler restricts the polymer chain mobility and assists the char formation, during the thermal decomposition. Besides, the silicate layers could hold accumulated heat that can act as a heat source to accelerate the decomposition process. In addition, the presence of aluminum Lewis acid sites in the silicate layers would promote the thermal degradation of polyesters by catalyzing the hydrolysis of ester linkages, and then it would produce a diminution of the thermal degradation temperature (Davis et al., 2003; Maiti and Batt, 2003; Bordes et al., 2009a; Bordes et al., 2009b). The thermal stability of the PHB-based systems has been investigated through TGA and the characteristic temperatures were determined (Fig. 2). Fig. 3 summarizes the peak temperature from DTG curves of the different studied systems obtained by melt intercalation. The peak centered at 290 °C in the DTG curves at 10 °C/min (Fig. 3a) that represents the decomposition of PHB, was slightly shifted to higher temperatures when 2 and 4 wt% bentonite were incorporated but to lower temperatures when 6 wt%. Bent was added. The same effect was also observed at 15, 20 and 25 °C/min. An optimum clay loading for the enhancement of thermal stability was also reported for PS and PETbased nanocomposites (Beyer, 2002). It was suggested that when a low clay fraction is added to the polymer, the thermal barrier effect is predominant because the clay is better dispersed (Lim et al., 2003). However, the catalytic effect becomes dominant with increasing loading, which results in a decrease in the thermal stability of the nanocomposite. On the other hand, the decrease in thermal stability of the nanocomposites containing E-S-A-Bent (see Fig. 3b) reflects that the modifiers of the multi-treated clay have a promotor effect on the thermal degradation of PHB that predominates over the thermal barrier effect produced by the clay platelets. In addition, aluminum Lewis acid sites could catalyze the hydrolysis of ester linkages at high temperatures (Davis et al., 2003; Maiti and Batt, 2003). It has been also argued that after the early stages of thermal decomposition the stacked silicate layers could hold accumulated heat, acting as a heat source to accelerate the decomposition process, in conjunction with the heat flow supplied by the outside heat source (Bordes et al., 2009b). Table 3 includes the thermal properties (obtained from DSC) for different studied systems: melting temperature (Tm), cold crystallization temperature (Tcc) and degree of crystallinity (Xcr). Tcc clearly decreased when the plasticizer (TBL) was incorporated. However, no significant changes were registered with the addition of the original and modified bentonite. Moreover, the molar mass of PHB was monitored by SEC measurements (Fig. 4). Whereas the molar mass of the polymer is slightly lowered in PHB/Bent nanocomposite, PHB/E-S-A-Bent presents a notorious decrease. This result clearly shows that the organic modifiers of the multi-treated clay promoted the thermal decomposition of PHB during the melt processing of the nanocomposite.

Table 2 Characteristics of original and E-S-A bentonites. Clay

d001 (nm)

Organic content (%)

Moisture24

Bent E-S-A-Bent

13.1 26.2

0.0 27.8

17.41 2.62

h

(%)

H2O content TGA (%)

Tdeg TGA (°C)

4.27 0.45

575 404

4. Results and discussions In a previous work (D'Amico et al., 2014), a multi-treated organobentonite, which was first acid-activated, then silylated and further modified by cationic exchange treatment with tributylhexadecylphosphonium bromide, was prepared (TBHP). The characteristics of the obtained bentonite (E-S-A-Bent) and the original one (Bent) are summarized in Table 2. From TGA curves the organic content was determined, and from the XRD pattern the interplanar basal distance was calculated. This last parameter significantly increased as compared with the original bentonite; in addition the equilibrium water uptake percentage was remarkably lower than for the bentonite (D'Amico et al., 2014). X-ray diffraction patterns, shown in Fig. 1, evidence that PHB matrix was partially intercalated in E-S-A-Bent and Bent. Bent exhibits a

Fig. 1. XRD spectra of clay alone and PHB based nanocomposites with different contents of Bent(a) and E-S-A-Bent obtained melt intercalation. 149

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Fig. 2. TGA curves of the PHB matrix and nanocomposites with different contents of Bent (a) and E-S-A-Bent (b) obtained at a heating rate of 10 °C/min.

Fig. 3. Peak temperature from DTGA curves as a function of clay content of the PHB/Bent (a) and PHB/E-S-A Bent (b) composites.

4.1. Modelling of the thermal degradation process Table 3 Thermal properties of PHB and PHB/clay nanocomposites.

In order to comprehensively analyze the effect of the nanoclay type on the decomposition mechanisms of PHB upon heating, the effective activation energy (Ea) and the mechanism-dependent function g(α) were investigated. Initially, Ea was estimated using isoconversional methods (Fig. 5). In this way, the complexity of the process could be established. As it has been reported, the PHB is approximately constant during the entire process indicating that the degradation mechanism of PHB remains unchanged throughout degradation (Vyazovkin et al., 2011). Based on the results shown in Fig. 5, it can be assumed that the nonisothermal degradation of PHB/Bent and PHB/E-S-A Bent nanocomposites proceeds also by a one step process for all the relative amounts of nano-filler, since the estimated values of Ea are similar to those of PHB and also approximately constant during the whole conversion range. The average values of activation energies in the conversion range between 20 and 95% are shown in Table 4. This similarity in the thermal behavior of PHB and its PHB nanocomposites reflects the similarity in their thermal degradation process. As shown in Table 4, Ea increased for the PHB/Bent nanocomposites, which could be related with the higher thermal stability of original clay. Also, the Ea values for PHB/E-S-A-Bent were lower than for PHB/ Bent and neat PHB for the temperature range in which the thermal degradation took place. These results demonstrate that the modifiers of

Sample

Tcc (°C)

Tm (°C)

Xcr (%)

PHB melt blended PHB 20% TBL PHB-TBL-2%Bent PHB-TBL-4%Bent PHB-TBL-6%Bent PHB-TBL-2%ESA-Bent PHB-TBL-4% ESA-Bent PHB-TBL-6% ESA-Bent

49.4 27.9 25.6 25.4 26.6 20.6 23.2 16.4

169.5 163.2 164.7 163.9 165.3 163.1 161.4 161.4

51.7 56.3 57.7 58.2 57.0 52.2 53.2 51.1

the multi-treated clay have a promoter effect on the thermal decomposition of the PHB matrix. 4.2. Initial estimation of the kinetic of the degradation mechanism 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. 6. The kinetics of the degradation mechanism of the neat PHB is best described by the Avrami–Erofeev random nucleation model (A4) in which the reaction is controlled by initial random nucleation followed by overlapping growth. Fig. 6 also evidences that the thermal degradation mechanism 150

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Fig. 4. SEC measurements of neat PHB and PHB/clay nanocomposites.

Fig. 6. Masterplots for PHB and nanocomposites with 4 wt% of clay (a) Bent and b) E-S-A-Bent.

Fig. 5. Activation energy (Ea) as a function of temperature for neat PHB and PHB/clay nanocomposites.

macromolecules could possibly enter into the interlayer space forming intercalated PHB/clay nanocomposites. No important changes in melting temperature and cold crystallization temperature were observed with the incorporation of different bentonites. In addition, a decrease on the cold crystallization temperature when the plasticizer was incorporated was observed. The influence of the addition of original and modified bentonites (combined reactions of cationic exchange, silylation and acid activation of bentonite) on the thermal degradation of a biodegradable bacterial PHB was determined. While the nanocomposites containing Bent presented a maximum of thermal stability for 4 wt% clay, those with modified-Bent showed a continuous decrease in the degradation temperature with the content of E-S-ABent. This behavior was attributed to the organic modifiers of the E-S-ABent that promote the thermal decomposition of PHB. Isoconversional methods were applied to evaluate the activation energy of the thermal degradation process of PHB. The obtained results and model's parameters clearly show that the organic modifiers have a promotor effect on the thermal degradation leading to a dramatic decrease in the PHB molecular weight as corroborated by GPC determinations. The Ea values in the nanocomposites were similar to that of neat PHB, which suggests that the degradation mechanism of PHB was not modified by the incorporation of Bent or E-S-A-Bent. It has been demonstrated that the degradation process can be well described by the Avrami–Erofeev random nucleation model (m = 4), in which the reaction is controlled

Table 4 Average values of Ea (kJ/mol) in the conversion range between 20 and 95%. Clay content (wt%)

Bent KAS

0 2 4 6

103.3 113.4 108.7 108.9

E-S-A-Bent KAS

97.5 104.0 95.8

Bent Vyazovkin

89 103.6 90.4 102.2

E-S-A-Bent Vyazovkin

93.4 95.6 95.5

of the three PHB-based nanocomposites follows the Avrami-Erofeev, with m = 4. In view of these results, it can be said that the incorporation of Bent and E-S-A-Bent to PHB matrix did not modify the degradation mechanism of PHB in the nanocomposites. 5. Conclusions Nanocomposites based on PHB containing different amounts of Bent or E-S-A-Bent were prepared by melt compounding method, with 2, 4 and 6 wt% clay contents and 20 wt% of TBL. For these materials, the effects of the content and type of clay on the thermal degradation of the PHB matrix were investigated. XRD data revealed that PHB chains were partially intercalated in both Bent and E-S-A-Bent and that the basal spacing was independent of the clay concentration. The PHB 151

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by initial random nucleation followed by overlapping growth. The obtained behavior is of interest for people working in biodegradable clay polymer nanocomposites for packaging applications.

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