Effect of layered double hydroxides on the thermal degradation behavior of biodegradable poly(l -lactide) nanocomposites

Polymer Degradation and Stability 96 (2011) 60e66

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Effect of layered double hydroxides on the thermal degradation behavior of biodegradable poly(L-lactide) nanocomposites Ming-Feng Chiang a, Mei-Zhen Chu a, b, Tzong-Ming Wu a, * a b

Department of Materials Science and Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan Plastics Industry Development Center, 193, Gongyequ 38th Road, Taichung 407, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 August 2010 Received in revised form 2 November 2010 Accepted 7 November 2010 Available online 13 November 2010

This study elucidates the thermal degradation behavior of biodegradable poly(L-lactide) (PLLA)/layered double hydroxide (LDH) nanocomposites was explored using thermogravimetric analysis (TGA) and pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) in an inert atmosphere. PLLA/LDH nanocomposites were fabricated using PLLA and organically-modified magnesium/aluminum layered double hydroxide (P-LDH) in tetrahydrofuran solution. According to the TGA results, the thermal stability of PLLA/P-LDH nanocomposites was significantly lower than that of pure PLLA matrix, perhaps because P-LDH provides thermal acceleration of the degradation of the underlying polymer from the heat source. The identification of the thermal degradation products by Py-GC/MS evidently shows that introducing P-LDH into PLLA leads to a remarkable change during the thermal degradation process. The main reaction route of neat PLLA was through inter- and intra-transesterification to generate lactides and oligomer. The primary volatile products obtained from PLLA/P-LDH nanocomposites were lactides regardless of the temperature of degradation. These results suggest that the thermal degradation behavior of PLLA/P-LDH nanocomposites is governed by the preferential formation of lactide by the unzipping depolymerization reaction, which is catalyzed by Mg and Al components in P-LDH. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Poly(L-lactide) Layered double hydroxide Nanocomposite Thermal degradation Unzipping depolymerization reaction

1. Introduction Environmental concerns have led to increased interest in biodegradable and biocompatible polymers, including poly(L-lactide) (PLLA), polyglycolide, polyhydroxyalkanoate and poly(3-caprolactone). Among these biodegradable polymers, used in both basic and applied research, PLLA and its related copolymer are of particular commercial interest as promising replacement of petroleum-derived plastics, since they are extracted entirely from renewable agricultural products. The main limitations on the industrial application of PLLA are its poor mechanical properties and relatively high gas permeability, which inhibit its use in such industrial applications as packaging. However, because of the enormous advantages and wide range of applications of PLLA, many groups are attempting to enhance its properties. The literature includes studies on the incorporation of nanoscale reinforcements, especially montmorillonite (MMT), as additives, to overcome the above shortcomings of PLLA and also improve its physicochemical properties, even with the addition of a few percent of inorganic fillers [1e4]. It is well known that adding inorganic materials into

* Corresponding author. Tel.: þ886 4 22840500x806; fax: þ886 4 22857017. E-mail address: [email protected] (T.-M. Wu). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.11.002

a PLLA matrix changes its physical properties and degradability [5]. Nevertheless, the thermal stability and degradation behaviors of PLLA nanocomposites must be better understood to improve their thermal processing because the PLLA aliphatic ester structure easily undergoes breakdown by hydrolysis, photolysis and pyrolysis. Numerous studies of the thermal degradation and thermal stability of PLLA have been published [6e15]. Factors that influence thermal behavior of PLLA include the molecular weight, the residual metal catalyst, the amounts of remaining and hydrolyzed monomers and oligomers, the moisture and others [6,7]. Additionally, suggested thermal degradation mechanisms for PLLA involve intra- and intermolecular transesterifications, cis-elimination, unzipping depolymerization, random chain scission, and other radical and non-radical side reactions [8e15]. In our previous study [16], the disorderly exfoliated PLLA nanocomposites with organically-modified layered double hydroxide (P-LDH) were successfully fabricated by directly inserting PLLA molecular chains from the solution in the presence of PLA-COOH intercalated magnesium/aluminum LDH. The mechanical properties of the fabricated nanocomposites include a storage modulus that is much better than that of pure PLLA. Before PLLA/P-LDH nanocomposites can be used in thermal processing, their thermal degradation behaviors and mechanisms of PLLA/P-LDH nanocomposites must be well understood. However, little information is

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available concerning the thermal stability of PLLA composites with one-dimensional and two-dimensional inorganic fillers, and the effect of these inorganic fillers on the thermal degradation behaviors of PLLA has rarely been examined as the layered silicates are usually thought to prevent thermal degradation of the polymer component [17]. Until now, although there are few reports on thermal degradation of PLLA/Al(OH)3 and PLLA/MgO composites [18,19], no report has been published to discuss the thermal degradation of the PLLA/ LDH nanocomposites. Nishida et al. found that an unknown particle size of Al(OH)3 caused significant thermal degradation of PLLA and selectively converting to L-form lactide as temperatures lower than 300  C [18]. Motoyama et al. reported that the nanoscale particle of MgO had a significant influence on the thermal degradation of PLLA. The metals, such as Mg and Al, can be effectively reduced the thermal degradation of PLLA. They had proposed a thermal degradation mechanism that the MgO might cause an unzipping depolymerization in PLLA pyrolysis [19]. However, the thermal degradation behavior of PLLA composites investigated previously may not be able to demonstrate the pyrolysis behavior of PLLA/P-LDH system due to the Mg and Al metal ions are interacted with P-LDH molecules. In this study, thermogravimetric analysis (TGA) and pyrolysisgas chromatography/mass spectroscopy (Py-GC/MS) are applied in an inert environment to investigate the thermal degradation and stability of PLLA/P-LDH nanocomposites. The possible thermal degradation pathway of PLLA/P-LDH nanocomposites is also discussed, with emphasis on the role of P-LDH in PLLA matrix. 2. Experiment 2.1. Preparation of the PLLA/P-LDH nanocomposites Poly(L-lactide) (PLLA) and Poly(DL-lactide) with carboxyl end group (PLA-COOH) were purchased from Bio Invigor Corporation. The L-content in PLLA sample is >99% and L/D enantiomeric ratio of PLA-COOH is 51/49 as reported by the manufacturer. Mg (NO3)2$6H2O, Al (NO3)3$9H2O, and NaOH were obtained from Showa Chemical Company. The PLLA/P-LDH nanocomposites with different P-LDH loadings reported fabricated by solution mixing process as previously reported [16]. Various weight ratios of P-LDH were added into 20 ml Tetrahydrofuran (THF) and ultrasonicated for 48 h to form the stable dispersion. Simultaneously, the powder of PLLA was dissolved in 30 ml THF and mechanically stirred at 60  C for 12 h. The PLLA/P-LDH nanocomposites were prepared by solution-direct intercalation process using various weight ratios of well-dispersed P-LDH and PLLA in THF solution and mechanically stirred for 24 h. The obtained solutions were poured into glass Petri dish and dried under vacuum for 48 h.

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Table 1 Molecular weight and metal contents of PLLA/P-LDH samples. Sample

Mna

Mwa

Mg (ppm)b

Al (ppm)b

PLLA 1% PLLA/P-LDH 3% PLLA/P-LDH 5% PLLA/P-LDH 10% PLLA/P-LDH

161,000 169,000 198,000 197,000 193,000

181,000 185,000 201,000 201,000 199,000

e

e 270 1000 3200 5740

200 1370 5790 11,100

a Mw and Mn are the weight-average and number-average molecular weight. Both data were determined by GPC measurement. b The ratios of Mg and Al were measured by ICP experiment.

were performed under a nitrogen atmosphere at a purge rate of 100 ml/min. All specimens weighed about 8 mg were heated from room temperature to 600  C under a flowing nitrogen atmosphere at a heating rate of 10  C/min. For dynamic thermal degradation, each sample was heated from room temperature to 600  C at various heating rates of 5, 10, 20 and 40  C/min. Volatile products analyses during dynamic thermal degradation of PLLA and PLLA/P-LDH nanocomposites were measured using pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) analysis, which is recorded on a Froniter Lab double-shot pyrolyzer PY-2020iD with a Hewlett Packard 6890 GC/5973 MSD chromatograph/mass spectrometer. The volatile products were analyzed with an Ultra Alloy 5-30M-0.25F capillary column. High purity helium was used as carrier gas at a purge rate of 100 ml/min. For dynamic thermal degradation, samples of PLLA and PLLA/P-LDH nanocomposites were put into the pyrolyzer and heated from 60  C to a predetermined temperature at a heating rate of 10  C/min. The volatile pyrolysis products were introduced into the GC through the selective sampler. The temperature of column oven was first set at 40  C. After the pyrolysis process had finished, the column was heated according to the following program: 50  C for 1 min; 50e320  C at a heating rate of 10  C/min; 320  C for 6 min. Mass spectrum measurements were recorded 2 times per second during this period. 3. Results and discussions 3.1. Thermal degradation behavior of the PLLA/P-LDH nanocomposites Table 1 presents the molecular weight of fabricated nanocomposites used herein to clarify the effect of sample preparation

2.2. Characterization of the PLLA/P-LDH nanocomposites Molecular weights were measured by gel permeation chromatography (GPC) on a Waters GPC system at 40  C using a Styragel HR 4 column and THF as mobile phase at a flow rate of 1 mL/min. The PLLA/ P-LDH nanocomposites were also dissolved in THF and the solutions were filtered through a 0.45 mm filter before injected into the column. The molecular weights were calibrated using narrow dispersity polystyrene standards. The ion content of Mg and Al in PLLA/P-LDH nanocomposites was measured using Jarrell-Ash ICAP 9000 Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). 2.3. Thermal degradation of the PLLA/P-LDH nanocomposites The degradation behaviors of pure PLLA and PLLA/P-LDH nanocomposites were performed using a Perkin Elmer Thermogravimetric/Differential Thermal Analyzer (TG/DTA) and all experiments

Fig. 1. TGA analysis of (a) PLLA, (b) 1 wt% PLLA/P-LDH, (c) 3 wt% PLLA/P-LDH, (d) 5 wt% PLLA/P-LDH and (e) 10 wt% PLLA/P-LDH nanocomposites Insert is Tonset and T50% versus the weight ratio of P-LDH for PLLA and nanocomposites with different loading of P-LDH.

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Fig. 2. Apparent Ea values of PLLA and PLLA/P-LDH nanocomposites at various residual weight fractions (W).

on the structure of PLLA/P-LDH nanocomposites. The numberaverage molecular weights (Mn) and weight-average molecular weights (Mw) of PLLA/P-LDH nanocomposites were almost the same as those of the neat PLLA sample. This fact allows the effect of the additional P-LDH content on the thermal degradation of PLLA to be evaluated independently of the influence of the molecular weight of these samples on the subsequent thermal degradation experiments. Thermogravimetric analysis (TGA) is a useful method for rapidly evaluating the thermal stability of the polymer. Fig. 1 plots the TGA curves for pure PLLA and PLLA/P-LDH nanocomposites at a heating rate of 10  C/min. All of the TGA data for PLLA and PLLA/P-LDH nanocomposites exhibit similar tendencies, and the onset temperature of degradation (Tonset) can be determined from these curves by extrapolation from the peak of degradation back to the initial weight of the polymer. The Tonset of PLLA is 321  C and unexpectedly declines to 288  C as the loading increases to 1 wt% P-LDH content. When 3, 5 and 10 wt% P-LDH are added to the PLLA matrix, the Tonset falls further to 273, 253 and 244  C, respectively. These results reveal that the incorporation of organically-modified LDH did not always improve the thermal stability of the nanocomposites as those reported in the literatures [20e23]. These experimental results clearly demonstrate that P-LDH in PLLA

Fig. 4. Py-GC/MS chromatograms of (a) PLLA, (b) 1 wt% PLLA/P-LDH, (c) 3 wt% PLLA/PLDH, (d) 5 wt% PLLA/P-LDH and (e) 10 wt% PLLA/P-LDH. nanocomposites pyrolyzates measured in a temperature range of 60e400  C at a constant heating rate of 10  C/min.

worsened thermal stability, and obviously shifted the degradation starting temperature downward. The data of T50% defined as the temperature at which 50 wt% decompositions occurs is also shown in Fig. 1. The temperature of 50% weight loss of PLLA considerably exceeds those of PLLA/P-LDH nanocomposites. Notably, the decomposition temperatures at 50% weight loss for pure PLLA matrix, 1, 3, 5 and 10 wt% PLLA/P-LDH nanocomposites are 351, 327, 312, 284 and 280  C, respectively. All results show that introducing P-LDH into PLLA matrix cannot improve the thermal stability of PLLA, perhaps because P-LDH thermally accelerates the degradation of the underlying polymer from the heat source. These unexpected results differ completely from those in the literature for composites that contain aliphatic polyesters and organicallymodified MMT, which exhibited enhanced thermal stability [24e26]. This phenomenon is attributable to the components of PLDH, such as organic modifier, Mg and/or Al ions, which effectively catalyze and accelerate the thermal degradation of PLLA, reducing its thermal stability. To clarify the thermal degradation behavior of PLLA/P-LDH nanocomposites, the amounts of Mg and Al residues in the nanocomposites were precisely determined by ICP-AES and the results presented in Table 1. The amounts of Mg and Al in pure PLLA matrix were below the limit of detection under the experimental conditions herein. The Mg and Al contents of 1% PLLA/P-LDH nanocomposite were 200 and 270 ppm, respectively. Loading with 3 wt% P-LDH correspondingly increased the Mg and Al ratios of the PLLA/P-LDH nanocomposites to 1370 and 1000 ppm. Mg and Al ratios reached 11100 and 5740 ppm in the 10 wt% PLLA/P-LDH

Table 2 Relative content of pyrolysis products for PLLA and PLLA/P-LDH nanocomposites obtained by the Py-GC/MS chromatograms. Sample

PLLA 1% PLLA/P-LDH 3% PLLA/P-LDH 5% PLLA/P-LDH 10% PLLA/P-LDH Fig. 3. Activation energy (Ea) values plot against temperature for thermal degradation of PLLA and PLLA/P-LDH nanocomposites.

a

Relative content of pyrolysis products (%)a meso-lactide

L- and D-form lactides

Cyclic oligomers

11.2 17.5 17.3 17.8 20.9

43.5 56.5 56.4 62.5 61.5

45.3 26.0 26.3 19.7 17.6

Determined by relative peak intensities from correlated pyrolysate.

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Fig. 5. Volatile products on (a) PLLA, (b) 1 wt% PLLA/P-LDH, (c) 3 wt% PLLA/P-LDH, (d) 5 wt% PLLA/P-LDH and (e) 10 wt% PLLA/P-LDH nanocomposite in different thermal degradation temperature ranges. Content ratio (%) is composed of the meso-lactite, L- and/or D-form lactides and cyclic oligomers.

nanocomposite. These results suggest that the thermal stability of PLLA/P-LDH nanocomposites fell as the P-LDH contents and amounts of Mg and Al increased. In a further investigation of the effect of P-LDH on the thermal degradation of PLLA/P-LDH nanocomposites, the apparent activation energies (Ea) of thermal degradation of these disorderly exfoliated PLLA/P-LDH nanocomposites are calculated from the weight loss data according to the OzawaeFlynneWall equation [27,28], which yields a quantitative parameter that can be used to estimate the thermal stability of these nanocomposites. Fig. 2 plots changes in the apparent Ea values during the pyrolysis of PLLA and PLLA/

P-LDH nanocomposites as a function of changes in the residual fractional weight (W). The apparent Ea value of PLLA increased drastically from 120 kJ/mol to 180 kJ/mol as weight loss increased. Continuous increases in apparent Ea value from 120 to 150 kJ/mol were also observed for the pyrolysis of PLLA that contained 1 wt% P-LDH. The apparent Ea value of 3 wt% PLLA/P-LDH and 5 wt% PLLA/ P-LDH nanocomposites were relatively constant at 113e118 kJ/mol throughout the pyrolysis. A similar value was obtained for 10 wt% PLLA/P-LDH nanocomposites, whose apparent Ea value declined further to 105 kJ/mol during degradation. The significant difference between Ea of PLLA and that of PLLA/LDH nanocomposites reveals

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that the thermal degradation procedure causes stepwise changes in degradation behavior which are determined by the P-LDH ratio and weight loss of each specimen [28]. The small change in the Ea value for the nanocomposites with high P-LDH content demonstrates that degradation proceeds via a simple route. Fig. 3 shows Ea as a function of temperature. The data reveals that the thermal degradation temperature and Ea value of PLLA exceed those of PLLA/P-LDH nanocomposites. Adding P-LDH to PLLA moves the degradation temperature to a lower temperature range as the Ea value declines. These results are in good agreement with prior TGA results, which suggest that the addition of P-LDH that contains Mg and Al ions effectively catalyzes and accelerates the thermal degradation of PLLA, worsening its thermal stability. To understand the role of Mg and Al ions in P-LDH, a possible mechanism of thermal degradation of PLLA/P-LDH nanocomposites is discussed below. 3.2. Dynamic thermal degradation of the PLLA/P-LDH nanocomposites To elucidate the effect of Mg and Al ions in P-LDH on the thermal degradation behavior of PLLA/P-LDH nanocomposites, the changes in the composition of pyrolyzates of PLLA/P-LDH nanocomposite as a function of temperature must be determined. Therefore, the pyrolyzates that were evolved in different temperature ranges were collected and analyzed using Py-GC/MS. Total ion current (TIC) profiles of the Py-GC/MS measurement for PLLA and its nanocomposites were obtained in a temperature range of 60e400  C and were shown in Fig. 4, in which the content of each component was calculated from the corresponding peak intensity in the Py-GC/ MS chromatogram. The peaks that appeared at retention time of 10.0e10.3 and 11.1e11.4 min were assigned to meso-lactide and Land/or D-form lactides. A series of peaks that appeared periodically in groups at 15e30 min were attributed to the production of higher cyclic oligomers than lactide, each of which comprised a group of diastereoisomers [10,29,30]. The multiple peaks for each cyclic oligomer reveal that the racemization reaction, in which diastereoisomers are derived from the asymmetric C atom in L-lactate unit, occurred during thermal degradation, since the PLLA is composed of almost entirely L-lactide units [10,31].

To identify the volatile products of the thermal degradation of PLLA and PLLA/P-LDH nanocomposites, the relative meso-lactide, Land/or D-form lactides and cyclic oligomers contents in each sample were evaluated from the TIC peak intensity and presented in Table 2. Cyclic oligomers that were formed by intra-transesterification were the main volatile products of the thermal degradation of the pure PLLA sample [9]. In contrast, the analysis of the thermal degradation products revealed that all PLLA/P-LDH nanocomposites contained 74e82% lactides. Interestingly, the thermal degradation of PLLA that contains P-LDH formed less of the cyclic oligomers, to an extent that was significant decreased with a loading of 1 wt% P-LDH; moreover, the proportion of cyclic oligomers in PLLA/P-LDH nanocomposites gradually decreased as the P-LDH loading increased, and ultimately approaching into a similar range. Mori et al. compared the products of pyrolysis of three types of PLLAs that had Sn contents from 23 to 1006 ppm. They found similar changes in the proportions of volatile products of PLLA samples in which the residual Sn contents exceeded 689 ppm [32]. Hence, the results herein suggest that the thermal degradation route of PLLA may be strongly affected by the presence of P-LDH, which included Mg and Al metal compounds, and that the ratios of volatilized degradation products of PLLA/P-LDH nanocomposites slightly changed as the P-LDH content increased. The effect of P-LDH additives on the amount and distribution of the products of PLLA/P-LDH nanocomposites evolved during thermal degradation was studied using non-isothermal pyrolysis coupled with GC/MS to gain insight into their degradation mechanisms. This approach provides information regarding the decomposition behavior of a polymer when exposed to a temperature that is higher than its onset temperature of degradation. The proportions of the amounts of the volatile pyrolysis products were calculated from the TIC value of each peak in the Py-GC/MS spectra. Fig. 5 displays the results of the analyses of the products of the pyrolysis of PLLA/P-LDH nanocomposites at temperature from 60  C to predetermined temperatures. In Fig. 5a, pure PLLA yields about 10% meso-lactide at 300  C, and the weight of the sample decreases beyond this temperature. As the predetermined temperature is increased from 320 to 380  C, PLLA generates 12e18% meso-lactide. These results also indicate that the thermal degradation products that were evolved from neat PLLA throughout the thermal

Scheme 1. Thermal degradation mechanism of PLLA.

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Scheme 2. Scheme of the possible thermal degradation mechanism of PLLA/P-LDH nanocomposites.

degradation were 65e30% L- and/or D-form lactides and 26e53% cyclic oligomers, as shown in Fig. 5a. The generation of cyclic oligomers increased significantly with the predetermined temperature. However, the yield of L- and/or D-form lactides declined as the predetermined temperature increased. The PLLA/P-LDH nanocomposite samples selectively yielded 60e70 wt% L- and/or D-form lactides during the thermal degradation. The products of the thermal degradation of PLLA/P-LDH nanocomposites comprised less of the cyclic oligomers than those of the pure PLLA matrix. These quantitative results suggest that P-LDH containing Mg and Al ions can effectively catalyze the depolymerization of PLLA, whose degradation yields mainly L- and/or D-form lactides. 3.3. Thermal degradation mechanism of the PLLA/P-LDH nanocomposites TGA measurements reveal that the rate of thermal degradation of PLLA increases monotonically with temperature. However, the thermal degradation of PLLA does not proceed by a simple degradation reaction. Scheme 1 proposes a mechanism of the pyrolysis of neat PLLA, based on our analytic results. According to the TGA and Py-GC/MS results, the thermal degradation reaction of pure PLLA occurred in a region of high temperature and Ea values can be deduced from several well-known thermal degradation reactions, including cis-elimination, inter- and intra-molecular transesterification, to give not only L- and/or D-form lactide but also mesolactide, cyclic oligomers, and linear oligomers having carboxyl, hydroxyl, and/or acrylic chain end groups [9,33]. Especially, diastereoisomers including meso-lactide and cyclic oligomers are produced by intra-molecular transesterification at higher temperatures [19]. In contrary, the thermal degradation mechanism of PLLA/P-LDH nanocomposites, illustrated in Scheme 2, involved catalysis by integrants of P-LDH. The results of Py-GC/MS demonstrate that large amounts of lactides formed as the volatile products of thermal degradation of PLLA/P-LDH nanocomposites under nonisothermal conditions. Moreover, the weight percentage of lactides in the volatile products increased with the P-LDH content in the PLLA/P-LDH nanocomposites. As shown in Fig. 5, P-LDH reduced

both Ea values and thermal degradation temperature of PLLA. These results show that the weight loss of PLLA/P-LDH nanocomposites during thermal degradation is dominated by the preferential formation of lactide via an unzipping depolymerization reaction, which occurs at a low temperature with low Ea values, and was catalyzed by the Mg and/or Al components in P-LDH. The previous investigations suggested that the MgeOH group and the AleOH group might react with the carbonyl group of PLLA [19,34,35]. Therefore, in the initial stage, some degradation reactions may occur, as shown in Scheme 2, in which Mg and Al components of PLDH in the PLLA matrix are in the form of alkoxide and/or are positioned at the carboxyl end. Additionally, the formation of a large amount of meso-lactide implies that the racemization (configuration inversion) should follow the SN2 depolymerization reaction, which involves nucleophilic attack by a carboxyl anion end on an asymmetrical methine carbon atom in the penultimate unit [31].

4. Conclusions The thermal degradation behaviors and mechanisms of PLLA/ P-LDH nanocomposites were determined by TGA and Py-GC/MS analyses in an inert atmosphere. According to the TGA results, the thermal stability of PLLA/P-LDH nanocomposites was significantly lower than that of a pure PLLA matrix, perhaps because P-LDH thermally accelerates the degradation of the underlying polymer. The products of thermal degradation, identified by Py-GC/MS, demonstrate that the introduction of P-LDH into PLLA considerably changes the thermal degradation process. The main reaction route of neat PLLA involved inter- and intra-transesterification and ciselimination by high Ea values, producing large amounts of lactide and oligomers. The primary volatile products obtained from PLLA/ P-LDH nanocomposites were lactide, regardless of the temperature of degradation. These results suggest that the thermal degradation behavior of PLLA/P-LDH nanocomposites is governed by the preferential formation of lactides via unzipping depolymerization reaction with low Ea values, catalyzed by Mg and Al components in P-LDH.

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