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layered double hydroxide nanocomposites
Composites Science and Technology 70 (2010) 110–115
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Synthesis and characterization of biodegradable poly(L-lactide)/layered double hydroxide nanocomposites Ming-Feng Chiang, Tzong-Ming Wu * Department of Materials Science and Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan
a r t i c l e
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Article history: Received 23 December 2008 Received in revised form 29 June 2009 Accepted 22 September 2009 Available online 26 September 2009 Keywords: A. Polymers A. Nanocomposites B. Thermal properties Layered double hydroxide
a b s t r a c t This study reports the preparation and physical properties of biodegradable nanocomposites fabricated using poly(L-lactide) (PLLA) and magnesium/aluminum layered double hydroxide (MgAl-LDH). The MgAl-LDH with molar ratio of Mg/Al = 2 were synthesized by the co-precipitation method. In order to improve the chemical compatibility between PLLA and LDH, the surface of LDH was organically-modified by polylactide with carboxyl end group (PLA–COOH) using ion-exchange process. Then, the PLLA/LDH nanocomposites were prepared by solution intercalation of PLLA into the galleries of PLA–COOH modified LDH (P-LDH) in tetrahydrofuran solution. Both X-ray diffraction data and Transmission electron microscopy images of PLLA/P-LDH nanocomposites indicate that the P-LDHs are randomly dispersed and exfoliated into the PLLA matrix. Mechanical properties of the fabricated 1.2 wt.% PLLA/P-LDH nanocomposites show significant enhancements in the storage modulus when compared to that of neat PLLA. Adding more P-LDH into PLLA matrix induced a decrease in the storage modulus of PLLA/P-LDH nanocomposites, probably due to the excessive content of PLA–COOH moleculars with low mechanical properties. The thermal stability and degradation activation energies of the PLLA and PLLA/P-LDH nanocomposites can also be discussed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The conventional synthetic polymers made from petrochemical products, such as polypropylene, polystyrene (PS), polyamide, and poly(ethylene terephthalate) (PET), have emerged environmental effects due to their low recovery/reproduction rates and poor degradability. In contrast, for the past few years, there has been renewal of interest in aliphatic polyesters [1–13], for instance poly(lactic acid) (PLA), poly(glycolic acid), poly(e-caprolate), and poly(3-hydroxybutyrate), because they are highly friendly for the environment and the human body. Among of these ecological materials, poly(L-lactide) (PLLA) has drawn intensive attention for their outstanding biocompatibility and biodegradability as well as physical properties which are comparable with PS and PET [1]. Normally, PLLA was synthesized from a cyclic monomer via ringopening polymerization or an L-lactic acid by direct condensation polymerization, and those monomers obtained from 100% renewable agricultural resources, such as corn, potato, and sugar beets. Owing to this ecological characteristic combined with its predominant biodegradation ability, PLLA is considered as a promising material in medicine fields, for instance wound closure, surgical implants [3–5], drug delivery system [6,7], and tissue culture [8]. * Corresponding author. Tel.: +886 4 2287 2482; fax: +886 4 2285 7017. E-mail address: [email protected] (T.-M. Wu). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.09.012
The properties of biodegradable polymer might be enhanced by the incorporation of nanoscale reinforcements [14–18]. In the last decade, the polymer/layered silicate nanocomposites have received a lot of attentions as a result of their exceptional physical and mechanical properties compared to those of neat polymer matrix [14–16]. Most investigations have been devoted to the study of montmorillonite (MMT)-type layered silicate nanocomposites due to the advantage of easily ion exchange efficiency of MMT. Therefore, there is only a few investigations focused on the polymer/layered double hydroxide (LDH) nanocomposites due to their strong electrostatic interaction between highly charged hydroxide layers and the intercalated anions to hinder the exfoliation of the LDH layers [19–25]. Nevertheless, polymer/LDH nanocomposites have become a new promising class of materials because of their exceptional thermal and mechanical properties. Moreover, to differ from MMT with negatively charged sheets, the LDH is constituted by positively charged metal oxide/hydroxide sheets with anionic species and water molecules in the interlayers [26]. It is well-known that LDH can be easily synthesized by tailoring chemical compositions in the laboratory using the co-precipitation method [22]. The general chemical formula of LDH can be repreyþ m sented by ½M1x MIII x ðOHÞ2 ½Ay=m nH2 O, where M is an univalent or a divalent metal ion such as Li+, Mg2+, Ni2+, Cu2+, Ca2+, or Zn2+, MIII is a trivalent metal ion such as Al3+, Fe3+, Cr3+, or Ga3+, y is the value of layer charge for octahedral unit, and Am is an exchangeable anion
M.-F. Chiang, T.-M. Wu / Composites Science and Technology 70 (2010) 110–115 2 2 with valence m like NO 3 , Cl , CO3 , or SO4 . Therefore y is equal to 2x 1 or x for M is an univalent or a divalent metal ion. These anions can be replaced by variety of organically anionic surfactants, such as glycine [27], acrylate [28], and lactate [29], by employing co-precipitation method. They also can be replaced by biomacromolecules, for example DNA [30], via ion exchange method. The organically-modified LDHs containing unique physicochemical properties could be extensively used in many potential fields of applications, for instance drug delivery, DNA reservoirs, and filler used in the polymer nanocomposites. Although the reports about the preparations and properties of PLLA/MMT nanocomposites have been extensively studied [31– 36], there is no investigation using LDH to fabricate the PLLA/layered silicate nanocomposites. To the best of our knowledge this is the first report of exfoliated LDHs within a PLLA matrix. In this report, we have used organically-modified LDH as the dispersed phase to prepare the PLLA/LDH nanocomposites by solution intercalation of PLLA into the swellable layered hosts, which were modified by PLA–COOH via anion exchange process. The microstructure, thermal, and mechanical properties of PLLA/LDH nanocomposites were examined by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, and dynamic mechanical analysis.
2. Experiment 2.1. Materials Poly(L-lactide) (PLLA) and poly(lactide) with carboxyl end group (PLA–COOH) were supplied by Bio Invigor Corporation, Taipei (Taiwan). Magnesium nitrate hexahydrate (Mg(NO3)26H2O), aluminum nitrate-9-hydrate (Al(NO3)39H2O) and sodium hydroxide were purchased from Showa Chemical Company. Acetone and tetrahydrofuran (THF) were obtained from Tedia and Echo Chemical Industries, Ltd., respectively. All the chemicals were used without further purification. 2.2. Synthesis of the Mg/Al layered double hydroxides The magnesium/aluminum layered double hydroxides (hereafter designated as MgAl-LDH) with molar ratio of Mg/Al = 2 were synthesized by the co-precipitation method [37]. The reactants of Mg(NO3)26H2O and Al(NO3)39H2O were dissolved in the 200 ml deionized water at room temperature. Then, an aqueous solution containing reactant was vigorously stirred at 60 °C for 16 h and simultaneously maintained pH value at 10.0 ± 0.2 by drop-wise addition of 2 N NaOH solution. In order to avoid the contamination of carbon dioxide, all experimental processes were carried on under the nitrogen atmosphere. The obtained precipitates were filtered and washed three times systematically with deionized water. 2.3. Preparation of the organically-modified MgAl-LDH The organically-modified MgAl-LDH (henceforth designated as P-LDH) was treated by PLA–COOH using the anion exchange method. In a typical fabrication experiment, an acetone solution containing 0.5 g MgAl-LDH and 0.5 g PLA–COOH was made in a four-neck vessel and refluxed at 70 °C for 48 h under nitrogen atmosphere. Then, the fabricated P-LDH rinsed three times via the dispersion and filtration process with acetone was obtained after lyophilizing in vacuum. 2.4. Preparation of the PLLA/P-LDH nanocomposites The PLLA/P-LDH nanocomposites with different P-LDH loadings were fabricated by solution mixing process. Various weight ratios
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of P-LDH were added into 20 ml 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. 2.5. Characterization X-ray diffraction (XRD) scan was performed on a Rigaku D/MAX 2000 diffractometer equipped with Ni-filtered Cu Ka radiation in the reflection mode. The scan ranges of specimens were collected from 2h = 1.5°–40° with the increment of 1°/min. The chemical component of P-LDH was obtained using JEOL JSM-6700F fieldemission scanning electron microscopy (FESEM) with energy dispersive X-ray analysis (EDX) at 3 keV. The morphology of P-LDH in PLLA matrix was obtained using JEOL JEM-1200 CX II transmission electron microscopy (TEM) with an accelerating voltage of 120 keV. The samples were prepared with an ultramicrotome equipped with a diamond knife, and the slices were collected on carbon-coated copper grid. Fourier transform infrared (FTIR) measurements were recorded on a Perkin–Elmer Spectrum One spectrometer in the transmission mode from 400 to 4000 cm1 with an average of twenty spectra record of the samples. The ratio of Mg and Al was measured using an inductively coupled plasmaatomic emission spectrometer (ICP-AES) equipped with a simultaneous using a Jarrell-Ash ICAP 9000. The storage modulus (E0 ) of the PLLA/P-LDH nanocomposites was performed on a Perkin Elmer DMA 7e dynamic mechanical analyzer in a temperature range of 20–100 °C at 5 °C/min heating rate and 5 Hz constant frequency. The thermal degradation behaviors of PLLA and PLLA/P-LDH nanocomposites were carried out using Perkin Elmer TG/DTA 6300 thermoanalyzer. All of the samples were examined from room temperature to 600 °C under nitrogen atmosphere at a heating rate of 10 °C/min. For non-isothermal degradation kinetics, the specimens of PLLA and PLLA/P-LDH nanocomposites were heated from room temperature to 100 °C for 3 min to remove the residual water. Then, the samples were employed to 600 °C using four varied heating rates (b) at 5, 10, 20, and 40 °C/min. 3. Results and discussion 3.1. Characterization of the MgAl-LDH and P-LDH The structures of MgAl-LDH synthesized by the co-precipitation method and P-LDH were identified by XRD diffraction pattern shown in Fig. 1. From curve a in Fig. 1, there is a strong diffraction peak at 2h = 11.2°. According to Bragg’s equation, the interlayer spacing of MgAl-LDH, corresponding to the interlayer distance of (0 0 3) plane, is equal to 7.9 Å. For the P-LDH (curve b in Fig. 1), the XRD diffraction data contains two diffraction peak at 2h = 6.3° and 2h = 10.1°, revealing that the interlayer spacing of the LDH was increased to 14.03 and 8.76 Å by adding PLA–COOH anion. A rough estimation of the molecular size of PLA–COOH anion using the Materials Studio software package is about 4 Å in thickness and 59.8 Å in length and the thickness of hydroxide layer for LDH is estimated to be 4.8 Å. Those results indicate that the intercalated PLA–COOH into the LDH gallery could possibly form a structure of bilayer and monolayer in which the molecules are expected to lay on or tilt at a fixed angle to enlarge the interlayer spacing. Fig. 2 presents the FTIR spectrum of MgAl-LDH and P-LDH. The FTIR data of MgAl-LDH shown in Fig. 2a contains the intense characteristic peak at 1384 cm1 due to the asymmetric stretching
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Fig. 3. TGA profiles of: (a) MgAl-LDH and (b) P-LDH. Fig. 1. X-ray diffraction data of: (a) MgAl-LDH and (b) P-LDH.
weight loss for the MgAl-LDH sample is attributed to the evaporation of the physically absorbed and intercalated water [42]. The second step of weight loss can be ascribed to the dehydroxylation of the hydroxide layers and elimination of intercalated nitrate anion [37]. The TGA data of P-LDH exhibits similar tendency, and the temperature of weight loss is almost at the same range as that of MgAl-LDH. But there is additional weight loss between the temperature ranges of 240 and 340 °C, this data may be ascribed to the presence of un-exchanged nitrate [37]. The main weigh loss step occurred at the temperature range of 340–550 °C is due to the thermal decomposition of intercalated PLA–COOH and dehydroxylation of the hydroxide layers. The weight ratio of the inorganic compositions of the MgAl-LDH and P-LDH were 56% and 41%, respectively. The real content of P-LDH in the fabricated 1, 3, 5 and 10 wt.% PLLA/P-LDH nanocomposites is about 0.4, 1.2, 2 and 4 wt.%. 3.2. Morphology of the PLLA/P-LDH nanocomposites
Fig. 2. FTIR spectra of: (a) MgAl-LDH and (b) P-LDH.
vibration of interlayer nitrates. A broad adsorption band at 3500 cm1 can be attributed to the O–H groups stretching vibration of hydroxide sheets in the MgAl-LDH. Another peak at 1640 cm1 is due to the bending mode of water molecules in the interlayer gallery [38]. The lattice vibration peaks in the range of 400–800 cm1 region are assigned to M–O and O–M–O group, which M represents Mg or Al [39]. The FTIR spectrum of P-LDH (Fig. 2b) containing four additional absorption peaks at 1095 cm1, 1190 cm1, 1384 cm1, 1760 cm1, and 2990 cm1, respectively, is significantly different to that of MgAl-LDH. The absorption peaks at 1095 cm1 and 1190 cm1 may correspondingly result from symmetric and asymmetric C–O–C group of the PLA–COOH. In addition, characteristic absorption bands at 1384 cm1 and 1760 cm1 is related to the symmetrical deformation mode of CH3 and C=O group of the PLA–COOH [40–41]. The weak absorption peak at 2990 cm1 is associated with the stretching vibration of the –CH3 band arising from interlayer molecules of PLA–COOH. This result indicates that there are several characteristic peaks of PLA–COOH molecules observed in the FTIR spectrum of P-LDH, suggesting that PLA–COOH molecules successfully intercalated into the interlayer spacing of the MgAl-LDH. Fig. 3 reveals the TGA curves of MgAl-LDH and P-LDH at a heating rate of 10 °C/min. Two distinguishable weight loss of the MgAlLDH occurred in the range 100–240 °C and 340–550 °C. The first
X-ray diffraction method is an effective technique to determine the interlayer spacing of the inorganic layered materials for the fabricated PLLA/P-LDH nanocomposites. Fig. 4 shows the X-ray diffraction data for the PLLA/P-LDH nanocomposites with various contents of P-LDH. All X-ray diffraction scans of PLLA/P-LDH nanocomposites
Fig. 4. X-ray diffraction data of: (a) neat PLLA matrix, (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.
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revealed no d003-reflection peak in the relevant region, thus indicating the presence of interlayer distances at least larger than 48 Å or no regular periodicity. These results suggest that the molecular chains of PLLA could have been successfully inserted and well-dispersed in the P-LDH galleries even if the loading of P-LDH was as high as 4 wt.%. The neat PLLA and PLLA/P-LDH nanocomposites exhibit a very strong crystalline peak at 2h ffi 16.6°, corresponding to the (2 0 0) and/ or (1 1 0) plane of typical orthorhombic crystal. Although XRD is a powerful and essential apparatus for evaluating the microstructure of nanocomposites, TEM instrument can be directly employed to visualize the exact intercalation or exfoliation degree of filler in the polymer matrix. Fig. 5a and b illustrate the bright-field TEM micrographs for 2 wt.% and 4 wt.% PLLA/P-LDH nanocomposites, respectively. The TEM images of PLA/P-PLH nanocomposites show completely different morphology and the original stacked lamellar structure of LDH layers can be completely destroyed to form the disorderly dispersed morphology in the PLLA matrix. Both XRD and TEM results demonstrate that most of the hydroxide layers are exfoliated and randomly dispersed in the PLLA matrix. Therefore the preparation of exfoliated PLLA/P-LDH nanocomposites has been successfully by solution mixing process. 3.3. Mechanical properties of the PLLA/P-LDH nanocomposites Fig. 6 displays the temperature dependence of storage modulus E0 of neat PLLA and the corresponding nanocomposites over a temperature range of 20–90 °C. The obtained data of storage
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modulus at 20 and 80 °C are summarized in Table 1. At 20 °C, the storage modulus of PLLA is 459 MPa, which decreases with the increasing temperature; at 80 °C it drastically drops to 105 MPa. This phenomenon is attributed to insufficient thermal energy to overcome the potential barrier for transitional and rotational motions of chain segments of the PLLA molecules in the glassy region, whereas above the glass transition temperature, the thermal stability becomes comparable to the potential energy barriers to the segmental motions [35,36]. With 0.4 wt.% P-LDH loading, the storage modulus of the PLLA/P-LDH nanocomposites at 20 °C is 750 MPa, which is 63% higher than neat PLLA. The storage modulus continuously increases to 865 MPa by adding 1.2 wt.% of P-LDH into PLLA matrix. The remarkable increase of the storage modulus at the lower temperature range may be attributed to the reinforcement effect of the presence of the rigid LDH layers as well as the superior interaction between PLLA and P-LDH, leading to the prominent improvement on the stiffness of the PLLA matrix. By adding more P-LDH into PLLA matrix, the storage modulus slightly decreases by incorporating 2 wt.% P-LDH into PLLA matrix and then continuously decreases with increasing the content of P-LDH to 4 wt.%. This finding can be ascribed to the higher P-LDH contents of PLLA/LDH nanocomposites containing more PLA–COOH molecules with low mechanical properties may reduce the storage modulus of these nanocomposites [43]. 3.4. Thermal stability of the PLLA/P-LDH nanocomposites In order to better understand the effect of P-LDH on the thermal stability of PLLA matrix, TGA analysis was operated to investigate the thermal degradation of PLLA and PLLA/P-LDH nanocomposites. Typical thermogravimetric profiles of weight loss for neat PLLA and PLLA/P-LDH nanocomposites at a heating rate of 10 °C/min are illustrated in Fig. 7. All experimental results of PLLA and PLLA/PLDH nanocomposites exhibit similar tendency, and the onset temperature of degradation (Tonset) can be determined from these curves by extrapolating from the curve at the peak of degradation back to the initial weight of the polymer. The Tonset of pristine PLLA is 321.1 °C and unexpectedly decreases to 288.4 °C as the loading of 1 wt.% P-LDH contents. As the addition of more P-LDH into PLLA matrix, the Tonset continuously decreases to 272.6, 252.7 and 244.4 °C for 3, 5, 10 wt.% PLLA/P-LDH nanocomposites, respectively. These results indicate that the incorporation of organically-modified LDH did not always improve the thermal stability of the nanocomposites as those reported in the literatures [19–22,28]. From these experimental data, it can be clearly seen that the presence of P-LDH in PLLA
Fig. 5. TEM micrographs of: (a) 5 wt.% PLLA/P-LDH and (b) 10 wt.% PLLA/P-LDH nanocomposites.
Fig. 6. Storage modulus of: (a) neat PLLA matrix, (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.
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Table 1 Dynamic storage modulus, degradation activation energies (Ea) and Mg and Al residues of PLLA and PLLA/P-LDH nanocomposites. Sample code
PLLA 0.4% PLLA/P-LDH 1.2% PLLA/P-LDH 2% PLLA/P-LDH 4% PLLA/P-LDH a b c
Storage modulus (MPa) (20 °C)
(80 °C)
459 750 865 718 651
105 175 231 164 158
Rb
Mgc (wt.%)
Alc (wt.%)
158.04 140.50 129.52 105.17 102.92
0.9998 0.9993 0.9902 0.9978 0.9999
– 0.020 0.137 0.579 1.110
– 0.027 0.100 0.320 0.574
The degradation activation energies (Ea) were obtained from the Kissinger equation. R: correlation coefficient. Mg and Al ratio were obtained from ICP-AES experiment.
Fig. 7. TGA patterns of: (a) neat PLLA matrix, (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.
induced the worse thermal stability which the degradation starting temperature obviously shifted to lower temperatures. Similar phenomenon was also reported for synthetic biodegradable aliphatic polyester/montmorillonite nanocomposites [44]. The degradation starting temperature of the 4 wt.% PLLA/P-LDH is about 80 °C lower than that of neat PLLA. These results imply that the thermal stability of PLLA significantly decreases as the weight percentage of P-LDH increases. Above results suggest that the thermal stability of PLLA was predominated by the presence of P-LDH even though the addition of high thermal stability of inorganic LDH. Possible degradation behavior will be discussed below. According to the analytical method of Kissinger [45], the degradation activation energies (Ea) of thermal degradation for PLLA and PLLA/P-LDH nanocomposites were estimated from TGA experimental data recording at different heating rates in the ranges from 5 to 40 °C/min. The Kissinger expression is
ln
Eaa (kJ/mol)
b T 2m
!
Ea nARW n1 m ¼ þ ln RT m Ea
into PLLA matrix. As the addition of more P-LDH into PLLA matrix, the Ea continuously decreases to 129.5, 105.2 and 102.9 kJ/mol for 1.2, 2, 4 wt.% PLLA/P-LDH nanocomposites, respectively. These results further confirm that the Ea of the PLLA decreases as the amounts of MgAl-LDH increases. From these kinetic analyses, the thermal stability of the samples would be in the following order: PLLA > 0.4 wt.% PLLA/P-LDH > 1.2 wt.% PLLA/P-LDH > 2 wt.% PLLA/ P-LDH > 4 wt.% PLLA/P-LDH nanocomposites. Therefore, a reasonable explanation for the reduction in the Ea values with the presence of high thermal stability of inorganic MgAl-LDH needs to be reached. Actually, Cam et al. [46] had reported that the presence of residual metals, such as Al, Fe, Zn and Sn, could cause apparent thermal degradation of PLLA. Fan et al. [47] had studied the effect of MgO on the thermal degradation behavior of the PLLA/MgO composites, and the results revealed that MgO could act as an effective catalyst for reducing the degradation temperature of PLLA. From above investigations, the presence of Mg and Al metals in the inorganic layered silicate LDH could decrease the thermal stability of PLLA. Therefore, the contents of Mg and Al residues in PLLA/PLDH nanocomposites were further examined by ICP-AES and their corresponding data are also listed in Table 1. All data indicate that the thermal stability of PLLA/P-LDH nanocomposites decrease with increasing the amounts of Mg and Al. These obtained data are sufficient to prove that the presence of Mg and Al residues in PLLA/ P-LDH nanocomposites plays a key role to reduce the thermal stability of PLLA. Therefore, the possible explanation for reducing thermal degradation temperature might be caused by the presence of Mg and/or Al metals in PLLA matrix that can catalyze the depolymerization and/or inter- and intra-molecular transesterification reactions of the PLLA resulting in their worse degradation stability [46].
! ð1Þ
where Tm is the absolute temperature at the maximum rate of weight reduction, b is the heating rate in the TGA experiment, Ea is the degradation activation energy, R is the gas constant, A is the pre-exponential factor, Wm is weight of the sample at the maximum rate of weight loss, and n is the apparent kinetic order of the degradation reaction. Based on Kissinger equation, the plots of log ðb=T 2m Þ versus 1/RTm for the degradation of PLLA/P-LDH nanocomposites are shown in Fig. 8. The determined Ea values of these PLLA/PLDH nanocomposites calculated from the slopes of the straight lines in Fig. 8 are illustrated in Table 1. The Ea of PLLA is 158.0 kJ/mol and decreases to 140.5 kJ/mol as the loading of 0.4 wt.% P-LDH content
Fig. 8. Plots of ln(b/T2) versus 1/RT for PLLA and PLLA/P-LDH nanocomposites.
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4. Conclusions The fabricated MgAl-LDH was successfully modified with PLA– COOH molecules using ion-exchange process. Both FTIR and XRD data indicate that PLA–COOH molecules were absorbed onto the gallery between hydrotalcite sheets to enlarge the interlayer spacing. Moreover, the PLLA and organically-modified LDH has been prepared through the direct insertion of PLLA polymer chains from the solution into the surface-treated layered silicate and contained disorderedly intercalated layered silicate layers within a PLLA matrix. Mechanical properties of 1.2 wt.% PLLA/P-LDH nanocomposites measured by DMA possess significant enhancements when compared with that of pure PLLA. The degradation starting temperature and degradation activation energies of PLLA/P-LDH decreases as the amounts of P-LDH increases. These results is probably due to the presence of Mg and/or Al metals in PLLA matrix that can catalyze the depolymerization and/or inter- and intra-molecular transesterification reactions of the PLLA resulting in their worse degradation stability. Acknowledgement The financial support provided by National Science Council through the project NSC96-2212-E-005-049-MY3 is greatly appreciated. References [1] Lunt J. Large-scale production, properties and commercial applications of polylactic acid polymers. Polym Degrad Stabil 1998;59:145–52. [2] Jain RA. The manufacturing techniques of various drug loaded biodegradable poly(lactide-co-glycolide) (PLGA) devices. Biomaterials 2000;21:2475–90. [3] Hofmann GO, Wagner FD. New implant designs for bioresorbable devices in orthopaedic surgery. Clin Mater 1993;14:207–15. [4] Anselme K, Flautre B, Hardouin P, Chanavaz M, Ustariz C, Vert M. Fate of bioresorbable poly(lactic acid) microbeads implanted in artificial bone defects for cortical bone augmentation in dog mandible. Biomaterials 1993;14:44–50. [5] Russias J, Saiz E, Nalla RK, Gryn K, Ritchie RO, Tomsia AP. Fabrication and mechanical properties of PLA/HA composites: a study of in vitro degradation. Mat Sci Eng C-Bio S 2006;26:1289–95. [6] Park TG, Cohen S, Langer R. Poly(L-lactic acid)/Pluronic blends: characterization of phase separation behavior, degradation, and morphology and use as protein-releasing matrixes. Macromolecules 1992;25:116–22. [7] Mi FL, Shyu SS, Lin YM, Wu YB, Peng CK, Tsai YH. Chitin/PLGA blend microspheres as a biodegradable drug delivery system: a new delivery system for protein. Biomaterials 2003;24:5023–36. [8] Mikos AG, Lyman MD, Freed LE, Langer R. Wetting of poly(L-lactic acid) and poly(dl-lactic-co-glycolic acid) foams for tissue culture. Biomaterials 1994;15:55–8. [9] Wang ZG, Hsiao BS, Zong XH, Yeh F, Zhou JJ, Dormier E, et al. Morphological development in absorbable poly(glycolide), poly(glycolide-co-lactide) and poly(glycolide-co-caprolactone) copolymers during isothermal crystallization. Polymer 2000;41:621–8. [10] Calandrelli L, Immirzi B, Malinconico M, Volpe MG, Oliva A, Ragione FD. Preparation and characterisation of composites based on biodegradable polymers for ‘‘in vivo” application. Polymer 2000;41:8027–33. [11] Hurrell S, Cameron RE. Polyglycolide: degradation and drug release. Part I: Changes in morphology during degradation. J Mater Sci-Mater M 2001;12:811–6. [12] Lepoittevin B, Pantoustier N, Alexandre M, Calberg C, Jerome R, Dubois P. Polyester layered silicate nanohybrids by controlled grafting polymerization. J Mater Chem 2002;12:3528–32. [13] Sato H, Nakamura M, Padermshoke A, Yamaguchi H, Terauchi H, Ekgasit S, et al. Thermal behavior and molecular interaction of poly(3-hydroxybutyrateco-3-hydroxyhexanoate) studied by wide-angle X-ray diffraction. Macromolecules 2004;37:3763–9. [14] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8:29–35. [15] Xu R, Manias E, Synder AJ, Runt J. New biomedical poly(urethane urea)layered silicate nanocomposites. Macromolecules 2001;34:337–9. [16] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28:1539–641. [17] Paiva MC, Zhou B, Fernando KAS, Lin Y, Kennedy JM, Sun YP. Mechanical and morphological characterization of polymer–carbon nanocomposites from functionalized carbon nanotubes. Carbon 2004;42:2849–54.
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Synthesis and characterization of biodegradable poly(L-lactide)/layered double hydroxide nanocomposites Ming-Feng Chiang, Tzong-Ming Wu * Department of Materials Science and Engineering, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan
a r t i c l e
i n f o
Article history: Received 23 December 2008 Received in revised form 29 June 2009 Accepted 22 September 2009 Available online 26 September 2009 Keywords: A. Polymers A. Nanocomposites B. Thermal properties Layered double hydroxide
a b s t r a c t This study reports the preparation and physical properties of biodegradable nanocomposites fabricated using poly(L-lactide) (PLLA) and magnesium/aluminum layered double hydroxide (MgAl-LDH). The MgAl-LDH with molar ratio of Mg/Al = 2 were synthesized by the co-precipitation method. In order to improve the chemical compatibility between PLLA and LDH, the surface of LDH was organically-modified by polylactide with carboxyl end group (PLA–COOH) using ion-exchange process. Then, the PLLA/LDH nanocomposites were prepared by solution intercalation of PLLA into the galleries of PLA–COOH modified LDH (P-LDH) in tetrahydrofuran solution. Both X-ray diffraction data and Transmission electron microscopy images of PLLA/P-LDH nanocomposites indicate that the P-LDHs are randomly dispersed and exfoliated into the PLLA matrix. Mechanical properties of the fabricated 1.2 wt.% PLLA/P-LDH nanocomposites show significant enhancements in the storage modulus when compared to that of neat PLLA. Adding more P-LDH into PLLA matrix induced a decrease in the storage modulus of PLLA/P-LDH nanocomposites, probably due to the excessive content of PLA–COOH moleculars with low mechanical properties. The thermal stability and degradation activation energies of the PLLA and PLLA/P-LDH nanocomposites can also be discussed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The conventional synthetic polymers made from petrochemical products, such as polypropylene, polystyrene (PS), polyamide, and poly(ethylene terephthalate) (PET), have emerged environmental effects due to their low recovery/reproduction rates and poor degradability. In contrast, for the past few years, there has been renewal of interest in aliphatic polyesters [1–13], for instance poly(lactic acid) (PLA), poly(glycolic acid), poly(e-caprolate), and poly(3-hydroxybutyrate), because they are highly friendly for the environment and the human body. Among of these ecological materials, poly(L-lactide) (PLLA) has drawn intensive attention for their outstanding biocompatibility and biodegradability as well as physical properties which are comparable with PS and PET [1]. Normally, PLLA was synthesized from a cyclic monomer via ringopening polymerization or an L-lactic acid by direct condensation polymerization, and those monomers obtained from 100% renewable agricultural resources, such as corn, potato, and sugar beets. Owing to this ecological characteristic combined with its predominant biodegradation ability, PLLA is considered as a promising material in medicine fields, for instance wound closure, surgical implants [3–5], drug delivery system [6,7], and tissue culture [8]. * Corresponding author. Tel.: +886 4 2287 2482; fax: +886 4 2285 7017. E-mail address: [email protected] (T.-M. Wu). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.09.012
The properties of biodegradable polymer might be enhanced by the incorporation of nanoscale reinforcements [14–18]. In the last decade, the polymer/layered silicate nanocomposites have received a lot of attentions as a result of their exceptional physical and mechanical properties compared to those of neat polymer matrix [14–16]. Most investigations have been devoted to the study of montmorillonite (MMT)-type layered silicate nanocomposites due to the advantage of easily ion exchange efficiency of MMT. Therefore, there is only a few investigations focused on the polymer/layered double hydroxide (LDH) nanocomposites due to their strong electrostatic interaction between highly charged hydroxide layers and the intercalated anions to hinder the exfoliation of the LDH layers [19–25]. Nevertheless, polymer/LDH nanocomposites have become a new promising class of materials because of their exceptional thermal and mechanical properties. Moreover, to differ from MMT with negatively charged sheets, the LDH is constituted by positively charged metal oxide/hydroxide sheets with anionic species and water molecules in the interlayers [26]. It is well-known that LDH can be easily synthesized by tailoring chemical compositions in the laboratory using the co-precipitation method [22]. The general chemical formula of LDH can be repreyþ m sented by ½M1x MIII x ðOHÞ2 ½Ay=m nH2 O, where M is an univalent or a divalent metal ion such as Li+, Mg2+, Ni2+, Cu2+, Ca2+, or Zn2+, MIII is a trivalent metal ion such as Al3+, Fe3+, Cr3+, or Ga3+, y is the value of layer charge for octahedral unit, and Am is an exchangeable anion
M.-F. Chiang, T.-M. Wu / Composites Science and Technology 70 (2010) 110–115 2 2 with valence m like NO 3 , Cl , CO3 , or SO4 . Therefore y is equal to 2x 1 or x for M is an univalent or a divalent metal ion. These anions can be replaced by variety of organically anionic surfactants, such as glycine [27], acrylate [28], and lactate [29], by employing co-precipitation method. They also can be replaced by biomacromolecules, for example DNA [30], via ion exchange method. The organically-modified LDHs containing unique physicochemical properties could be extensively used in many potential fields of applications, for instance drug delivery, DNA reservoirs, and filler used in the polymer nanocomposites. Although the reports about the preparations and properties of PLLA/MMT nanocomposites have been extensively studied [31– 36], there is no investigation using LDH to fabricate the PLLA/layered silicate nanocomposites. To the best of our knowledge this is the first report of exfoliated LDHs within a PLLA matrix. In this report, we have used organically-modified LDH as the dispersed phase to prepare the PLLA/LDH nanocomposites by solution intercalation of PLLA into the swellable layered hosts, which were modified by PLA–COOH via anion exchange process. The microstructure, thermal, and mechanical properties of PLLA/LDH nanocomposites were examined by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, and dynamic mechanical analysis.
2. Experiment 2.1. Materials Poly(L-lactide) (PLLA) and poly(lactide) with carboxyl end group (PLA–COOH) were supplied by Bio Invigor Corporation, Taipei (Taiwan). Magnesium nitrate hexahydrate (Mg(NO3)26H2O), aluminum nitrate-9-hydrate (Al(NO3)39H2O) and sodium hydroxide were purchased from Showa Chemical Company. Acetone and tetrahydrofuran (THF) were obtained from Tedia and Echo Chemical Industries, Ltd., respectively. All the chemicals were used without further purification. 2.2. Synthesis of the Mg/Al layered double hydroxides The magnesium/aluminum layered double hydroxides (hereafter designated as MgAl-LDH) with molar ratio of Mg/Al = 2 were synthesized by the co-precipitation method [37]. The reactants of Mg(NO3)26H2O and Al(NO3)39H2O were dissolved in the 200 ml deionized water at room temperature. Then, an aqueous solution containing reactant was vigorously stirred at 60 °C for 16 h and simultaneously maintained pH value at 10.0 ± 0.2 by drop-wise addition of 2 N NaOH solution. In order to avoid the contamination of carbon dioxide, all experimental processes were carried on under the nitrogen atmosphere. The obtained precipitates were filtered and washed three times systematically with deionized water. 2.3. Preparation of the organically-modified MgAl-LDH The organically-modified MgAl-LDH (henceforth designated as P-LDH) was treated by PLA–COOH using the anion exchange method. In a typical fabrication experiment, an acetone solution containing 0.5 g MgAl-LDH and 0.5 g PLA–COOH was made in a four-neck vessel and refluxed at 70 °C for 48 h under nitrogen atmosphere. Then, the fabricated P-LDH rinsed three times via the dispersion and filtration process with acetone was obtained after lyophilizing in vacuum. 2.4. Preparation of the PLLA/P-LDH nanocomposites The PLLA/P-LDH nanocomposites with different P-LDH loadings were fabricated by solution mixing process. Various weight ratios
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of P-LDH were added into 20 ml 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. 2.5. Characterization X-ray diffraction (XRD) scan was performed on a Rigaku D/MAX 2000 diffractometer equipped with Ni-filtered Cu Ka radiation in the reflection mode. The scan ranges of specimens were collected from 2h = 1.5°–40° with the increment of 1°/min. The chemical component of P-LDH was obtained using JEOL JSM-6700F fieldemission scanning electron microscopy (FESEM) with energy dispersive X-ray analysis (EDX) at 3 keV. The morphology of P-LDH in PLLA matrix was obtained using JEOL JEM-1200 CX II transmission electron microscopy (TEM) with an accelerating voltage of 120 keV. The samples were prepared with an ultramicrotome equipped with a diamond knife, and the slices were collected on carbon-coated copper grid. Fourier transform infrared (FTIR) measurements were recorded on a Perkin–Elmer Spectrum One spectrometer in the transmission mode from 400 to 4000 cm1 with an average of twenty spectra record of the samples. The ratio of Mg and Al was measured using an inductively coupled plasmaatomic emission spectrometer (ICP-AES) equipped with a simultaneous using a Jarrell-Ash ICAP 9000. The storage modulus (E0 ) of the PLLA/P-LDH nanocomposites was performed on a Perkin Elmer DMA 7e dynamic mechanical analyzer in a temperature range of 20–100 °C at 5 °C/min heating rate and 5 Hz constant frequency. The thermal degradation behaviors of PLLA and PLLA/P-LDH nanocomposites were carried out using Perkin Elmer TG/DTA 6300 thermoanalyzer. All of the samples were examined from room temperature to 600 °C under nitrogen atmosphere at a heating rate of 10 °C/min. For non-isothermal degradation kinetics, the specimens of PLLA and PLLA/P-LDH nanocomposites were heated from room temperature to 100 °C for 3 min to remove the residual water. Then, the samples were employed to 600 °C using four varied heating rates (b) at 5, 10, 20, and 40 °C/min. 3. Results and discussion 3.1. Characterization of the MgAl-LDH and P-LDH The structures of MgAl-LDH synthesized by the co-precipitation method and P-LDH were identified by XRD diffraction pattern shown in Fig. 1. From curve a in Fig. 1, there is a strong diffraction peak at 2h = 11.2°. According to Bragg’s equation, the interlayer spacing of MgAl-LDH, corresponding to the interlayer distance of (0 0 3) plane, is equal to 7.9 Å. For the P-LDH (curve b in Fig. 1), the XRD diffraction data contains two diffraction peak at 2h = 6.3° and 2h = 10.1°, revealing that the interlayer spacing of the LDH was increased to 14.03 and 8.76 Å by adding PLA–COOH anion. A rough estimation of the molecular size of PLA–COOH anion using the Materials Studio software package is about 4 Å in thickness and 59.8 Å in length and the thickness of hydroxide layer for LDH is estimated to be 4.8 Å. Those results indicate that the intercalated PLA–COOH into the LDH gallery could possibly form a structure of bilayer and monolayer in which the molecules are expected to lay on or tilt at a fixed angle to enlarge the interlayer spacing. Fig. 2 presents the FTIR spectrum of MgAl-LDH and P-LDH. The FTIR data of MgAl-LDH shown in Fig. 2a contains the intense characteristic peak at 1384 cm1 due to the asymmetric stretching
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Fig. 3. TGA profiles of: (a) MgAl-LDH and (b) P-LDH. Fig. 1. X-ray diffraction data of: (a) MgAl-LDH and (b) P-LDH.
weight loss for the MgAl-LDH sample is attributed to the evaporation of the physically absorbed and intercalated water [42]. The second step of weight loss can be ascribed to the dehydroxylation of the hydroxide layers and elimination of intercalated nitrate anion [37]. The TGA data of P-LDH exhibits similar tendency, and the temperature of weight loss is almost at the same range as that of MgAl-LDH. But there is additional weight loss between the temperature ranges of 240 and 340 °C, this data may be ascribed to the presence of un-exchanged nitrate [37]. The main weigh loss step occurred at the temperature range of 340–550 °C is due to the thermal decomposition of intercalated PLA–COOH and dehydroxylation of the hydroxide layers. The weight ratio of the inorganic compositions of the MgAl-LDH and P-LDH were 56% and 41%, respectively. The real content of P-LDH in the fabricated 1, 3, 5 and 10 wt.% PLLA/P-LDH nanocomposites is about 0.4, 1.2, 2 and 4 wt.%. 3.2. Morphology of the PLLA/P-LDH nanocomposites
Fig. 2. FTIR spectra of: (a) MgAl-LDH and (b) P-LDH.
vibration of interlayer nitrates. A broad adsorption band at 3500 cm1 can be attributed to the O–H groups stretching vibration of hydroxide sheets in the MgAl-LDH. Another peak at 1640 cm1 is due to the bending mode of water molecules in the interlayer gallery [38]. The lattice vibration peaks in the range of 400–800 cm1 region are assigned to M–O and O–M–O group, which M represents Mg or Al [39]. The FTIR spectrum of P-LDH (Fig. 2b) containing four additional absorption peaks at 1095 cm1, 1190 cm1, 1384 cm1, 1760 cm1, and 2990 cm1, respectively, is significantly different to that of MgAl-LDH. The absorption peaks at 1095 cm1 and 1190 cm1 may correspondingly result from symmetric and asymmetric C–O–C group of the PLA–COOH. In addition, characteristic absorption bands at 1384 cm1 and 1760 cm1 is related to the symmetrical deformation mode of CH3 and C=O group of the PLA–COOH [40–41]. The weak absorption peak at 2990 cm1 is associated with the stretching vibration of the –CH3 band arising from interlayer molecules of PLA–COOH. This result indicates that there are several characteristic peaks of PLA–COOH molecules observed in the FTIR spectrum of P-LDH, suggesting that PLA–COOH molecules successfully intercalated into the interlayer spacing of the MgAl-LDH. Fig. 3 reveals the TGA curves of MgAl-LDH and P-LDH at a heating rate of 10 °C/min. Two distinguishable weight loss of the MgAlLDH occurred in the range 100–240 °C and 340–550 °C. The first
X-ray diffraction method is an effective technique to determine the interlayer spacing of the inorganic layered materials for the fabricated PLLA/P-LDH nanocomposites. Fig. 4 shows the X-ray diffraction data for the PLLA/P-LDH nanocomposites with various contents of P-LDH. All X-ray diffraction scans of PLLA/P-LDH nanocomposites
Fig. 4. X-ray diffraction data of: (a) neat PLLA matrix, (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.
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revealed no d003-reflection peak in the relevant region, thus indicating the presence of interlayer distances at least larger than 48 Å or no regular periodicity. These results suggest that the molecular chains of PLLA could have been successfully inserted and well-dispersed in the P-LDH galleries even if the loading of P-LDH was as high as 4 wt.%. The neat PLLA and PLLA/P-LDH nanocomposites exhibit a very strong crystalline peak at 2h ffi 16.6°, corresponding to the (2 0 0) and/ or (1 1 0) plane of typical orthorhombic crystal. Although XRD is a powerful and essential apparatus for evaluating the microstructure of nanocomposites, TEM instrument can be directly employed to visualize the exact intercalation or exfoliation degree of filler in the polymer matrix. Fig. 5a and b illustrate the bright-field TEM micrographs for 2 wt.% and 4 wt.% PLLA/P-LDH nanocomposites, respectively. The TEM images of PLA/P-PLH nanocomposites show completely different morphology and the original stacked lamellar structure of LDH layers can be completely destroyed to form the disorderly dispersed morphology in the PLLA matrix. Both XRD and TEM results demonstrate that most of the hydroxide layers are exfoliated and randomly dispersed in the PLLA matrix. Therefore the preparation of exfoliated PLLA/P-LDH nanocomposites has been successfully by solution mixing process. 3.3. Mechanical properties of the PLLA/P-LDH nanocomposites Fig. 6 displays the temperature dependence of storage modulus E0 of neat PLLA and the corresponding nanocomposites over a temperature range of 20–90 °C. The obtained data of storage
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modulus at 20 and 80 °C are summarized in Table 1. At 20 °C, the storage modulus of PLLA is 459 MPa, which decreases with the increasing temperature; at 80 °C it drastically drops to 105 MPa. This phenomenon is attributed to insufficient thermal energy to overcome the potential barrier for transitional and rotational motions of chain segments of the PLLA molecules in the glassy region, whereas above the glass transition temperature, the thermal stability becomes comparable to the potential energy barriers to the segmental motions [35,36]. With 0.4 wt.% P-LDH loading, the storage modulus of the PLLA/P-LDH nanocomposites at 20 °C is 750 MPa, which is 63% higher than neat PLLA. The storage modulus continuously increases to 865 MPa by adding 1.2 wt.% of P-LDH into PLLA matrix. The remarkable increase of the storage modulus at the lower temperature range may be attributed to the reinforcement effect of the presence of the rigid LDH layers as well as the superior interaction between PLLA and P-LDH, leading to the prominent improvement on the stiffness of the PLLA matrix. By adding more P-LDH into PLLA matrix, the storage modulus slightly decreases by incorporating 2 wt.% P-LDH into PLLA matrix and then continuously decreases with increasing the content of P-LDH to 4 wt.%. This finding can be ascribed to the higher P-LDH contents of PLLA/LDH nanocomposites containing more PLA–COOH molecules with low mechanical properties may reduce the storage modulus of these nanocomposites [43]. 3.4. Thermal stability of the PLLA/P-LDH nanocomposites In order to better understand the effect of P-LDH on the thermal stability of PLLA matrix, TGA analysis was operated to investigate the thermal degradation of PLLA and PLLA/P-LDH nanocomposites. Typical thermogravimetric profiles of weight loss for neat PLLA and PLLA/P-LDH nanocomposites at a heating rate of 10 °C/min are illustrated in Fig. 7. All experimental results of PLLA and PLLA/PLDH nanocomposites exhibit similar tendency, and the onset temperature of degradation (Tonset) can be determined from these curves by extrapolating from the curve at the peak of degradation back to the initial weight of the polymer. The Tonset of pristine PLLA is 321.1 °C and unexpectedly decreases to 288.4 °C as the loading of 1 wt.% P-LDH contents. As the addition of more P-LDH into PLLA matrix, the Tonset continuously decreases to 272.6, 252.7 and 244.4 °C for 3, 5, 10 wt.% PLLA/P-LDH nanocomposites, respectively. These results indicate that the incorporation of organically-modified LDH did not always improve the thermal stability of the nanocomposites as those reported in the literatures [19–22,28]. From these experimental data, it can be clearly seen that the presence of P-LDH in PLLA
Fig. 5. TEM micrographs of: (a) 5 wt.% PLLA/P-LDH and (b) 10 wt.% PLLA/P-LDH nanocomposites.
Fig. 6. Storage modulus of: (a) neat PLLA matrix, (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.
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Table 1 Dynamic storage modulus, degradation activation energies (Ea) and Mg and Al residues of PLLA and PLLA/P-LDH nanocomposites. Sample code
PLLA 0.4% PLLA/P-LDH 1.2% PLLA/P-LDH 2% PLLA/P-LDH 4% PLLA/P-LDH a b c
Storage modulus (MPa) (20 °C)
(80 °C)
459 750 865 718 651
105 175 231 164 158
Rb
Mgc (wt.%)
Alc (wt.%)
158.04 140.50 129.52 105.17 102.92
0.9998 0.9993 0.9902 0.9978 0.9999
– 0.020 0.137 0.579 1.110
– 0.027 0.100 0.320 0.574
The degradation activation energies (Ea) were obtained from the Kissinger equation. R: correlation coefficient. Mg and Al ratio were obtained from ICP-AES experiment.
Fig. 7. TGA patterns of: (a) neat PLLA matrix, (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.
induced the worse thermal stability which the degradation starting temperature obviously shifted to lower temperatures. Similar phenomenon was also reported for synthetic biodegradable aliphatic polyester/montmorillonite nanocomposites [44]. The degradation starting temperature of the 4 wt.% PLLA/P-LDH is about 80 °C lower than that of neat PLLA. These results imply that the thermal stability of PLLA significantly decreases as the weight percentage of P-LDH increases. Above results suggest that the thermal stability of PLLA was predominated by the presence of P-LDH even though the addition of high thermal stability of inorganic LDH. Possible degradation behavior will be discussed below. According to the analytical method of Kissinger [45], the degradation activation energies (Ea) of thermal degradation for PLLA and PLLA/P-LDH nanocomposites were estimated from TGA experimental data recording at different heating rates in the ranges from 5 to 40 °C/min. The Kissinger expression is
ln
Eaa (kJ/mol)
b T 2m
!
Ea nARW n1 m ¼ þ ln RT m Ea
into PLLA matrix. As the addition of more P-LDH into PLLA matrix, the Ea continuously decreases to 129.5, 105.2 and 102.9 kJ/mol for 1.2, 2, 4 wt.% PLLA/P-LDH nanocomposites, respectively. These results further confirm that the Ea of the PLLA decreases as the amounts of MgAl-LDH increases. From these kinetic analyses, the thermal stability of the samples would be in the following order: PLLA > 0.4 wt.% PLLA/P-LDH > 1.2 wt.% PLLA/P-LDH > 2 wt.% PLLA/ P-LDH > 4 wt.% PLLA/P-LDH nanocomposites. Therefore, a reasonable explanation for the reduction in the Ea values with the presence of high thermal stability of inorganic MgAl-LDH needs to be reached. Actually, Cam et al. [46] had reported that the presence of residual metals, such as Al, Fe, Zn and Sn, could cause apparent thermal degradation of PLLA. Fan et al. [47] had studied the effect of MgO on the thermal degradation behavior of the PLLA/MgO composites, and the results revealed that MgO could act as an effective catalyst for reducing the degradation temperature of PLLA. From above investigations, the presence of Mg and Al metals in the inorganic layered silicate LDH could decrease the thermal stability of PLLA. Therefore, the contents of Mg and Al residues in PLLA/PLDH nanocomposites were further examined by ICP-AES and their corresponding data are also listed in Table 1. All data indicate that the thermal stability of PLLA/P-LDH nanocomposites decrease with increasing the amounts of Mg and Al. These obtained data are sufficient to prove that the presence of Mg and Al residues in PLLA/ P-LDH nanocomposites plays a key role to reduce the thermal stability of PLLA. Therefore, the possible explanation for reducing thermal degradation temperature might be caused by the presence of Mg and/or Al metals in PLLA matrix that can catalyze the depolymerization and/or inter- and intra-molecular transesterification reactions of the PLLA resulting in their worse degradation stability [46].
! ð1Þ
where Tm is the absolute temperature at the maximum rate of weight reduction, b is the heating rate in the TGA experiment, Ea is the degradation activation energy, R is the gas constant, A is the pre-exponential factor, Wm is weight of the sample at the maximum rate of weight loss, and n is the apparent kinetic order of the degradation reaction. Based on Kissinger equation, the plots of log ðb=T 2m Þ versus 1/RTm for the degradation of PLLA/P-LDH nanocomposites are shown in Fig. 8. The determined Ea values of these PLLA/PLDH nanocomposites calculated from the slopes of the straight lines in Fig. 8 are illustrated in Table 1. The Ea of PLLA is 158.0 kJ/mol and decreases to 140.5 kJ/mol as the loading of 0.4 wt.% P-LDH content
Fig. 8. Plots of ln(b/T2) versus 1/RT for PLLA and PLLA/P-LDH nanocomposites.
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4. Conclusions The fabricated MgAl-LDH was successfully modified with PLA– COOH molecules using ion-exchange process. Both FTIR and XRD data indicate that PLA–COOH molecules were absorbed onto the gallery between hydrotalcite sheets to enlarge the interlayer spacing. Moreover, the PLLA and organically-modified LDH has been prepared through the direct insertion of PLLA polymer chains from the solution into the surface-treated layered silicate and contained disorderedly intercalated layered silicate layers within a PLLA matrix. Mechanical properties of 1.2 wt.% PLLA/P-LDH nanocomposites measured by DMA possess significant enhancements when compared with that of pure PLLA. The degradation starting temperature and degradation activation energies of PLLA/P-LDH decreases as the amounts of P-LDH increases. These results is probably due to the presence of Mg and/or Al metals in PLLA matrix that can catalyze the depolymerization and/or inter- and intra-molecular transesterification reactions of the PLLA resulting in their worse degradation stability. Acknowledgement The financial support provided by National Science Council through the project NSC96-2212-E-005-049-MY3 is greatly appreciated. References [1] Lunt J. Large-scale production, properties and commercial applications of polylactic acid polymers. Polym Degrad Stabil 1998;59:145–52. [2] Jain RA. The manufacturing techniques of various drug loaded biodegradable poly(lactide-co-glycolide) (PLGA) devices. Biomaterials 2000;21:2475–90. [3] Hofmann GO, Wagner FD. New implant designs for bioresorbable devices in orthopaedic surgery. Clin Mater 1993;14:207–15. [4] Anselme K, Flautre B, Hardouin P, Chanavaz M, Ustariz C, Vert M. Fate of bioresorbable poly(lactic acid) microbeads implanted in artificial bone defects for cortical bone augmentation in dog mandible. Biomaterials 1993;14:44–50. [5] Russias J, Saiz E, Nalla RK, Gryn K, Ritchie RO, Tomsia AP. Fabrication and mechanical properties of PLA/HA composites: a study of in vitro degradation. Mat Sci Eng C-Bio S 2006;26:1289–95. [6] Park TG, Cohen S, Langer R. Poly(L-lactic acid)/Pluronic blends: characterization of phase separation behavior, degradation, and morphology and use as protein-releasing matrixes. Macromolecules 1992;25:116–22. [7] Mi FL, Shyu SS, Lin YM, Wu YB, Peng CK, Tsai YH. Chitin/PLGA blend microspheres as a biodegradable drug delivery system: a new delivery system for protein. Biomaterials 2003;24:5023–36. [8] Mikos AG, Lyman MD, Freed LE, Langer R. Wetting of poly(L-lactic acid) and poly(dl-lactic-co-glycolic acid) foams for tissue culture. Biomaterials 1994;15:55–8. [9] Wang ZG, Hsiao BS, Zong XH, Yeh F, Zhou JJ, Dormier E, et al. Morphological development in absorbable poly(glycolide), poly(glycolide-co-lactide) and poly(glycolide-co-caprolactone) copolymers during isothermal crystallization. Polymer 2000;41:621–8. [10] Calandrelli L, Immirzi B, Malinconico M, Volpe MG, Oliva A, Ragione FD. Preparation and characterisation of composites based on biodegradable polymers for ‘‘in vivo” application. Polymer 2000;41:8027–33. [11] Hurrell S, Cameron RE. Polyglycolide: degradation and drug release. Part I: Changes in morphology during degradation. J Mater Sci-Mater M 2001;12:811–6. [12] Lepoittevin B, Pantoustier N, Alexandre M, Calberg C, Jerome R, Dubois P. Polyester layered silicate nanohybrids by controlled grafting polymerization. J Mater Chem 2002;12:3528–32. [13] Sato H, Nakamura M, Padermshoke A, Yamaguchi H, Terauchi H, Ekgasit S, et al. Thermal behavior and molecular interaction of poly(3-hydroxybutyrateco-3-hydroxyhexanoate) studied by wide-angle X-ray diffraction. Macromolecules 2004;37:3763–9. [14] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater 1996;8:29–35. [15] Xu R, Manias E, Synder AJ, Runt J. New biomedical poly(urethane urea)layered silicate nanocomposites. Macromolecules 2001;34:337–9. [16] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28:1539–641. [17] Paiva MC, Zhou B, Fernando KAS, Lin Y, Kennedy JM, Sun YP. Mechanical and morphological characterization of polymer–carbon nanocomposites from functionalized carbon nanotubes. Carbon 2004;42:2849–54.
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