Preparation And Characterization Of New Biodegradable Materials:
Poly(Lactic Acid)/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
E-mail: [email protected]
ABSTRACT
The poly(l-lactic acid) (PLLA)/layered double hydroxide (LDH) nanocomposites were obtained by solution intercalation of
PLLA into the PLA-COOH modified LDHs (P-LDHs) in tetrahydrofuran (THF) solution. The P-LDHs were prepared using
ion-exchange reaction with various ratio magnesium (Mg)/aluminum (Al) LDHs (MgAl-LDHs). For the PLLA/LDH
nanocomposites, the dispersion behaviors and physical properties of nanocomposites are characterized by wide angle
X-ray diffraction (WAXD), transmission electron microscopy (TEM), dynamic scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). Both WAXD data and TEM images indicated that P-LDHs were exfoliated in PLLA matrix.
The DSC profiles of nanocomposites indicate that exfoliated 3 wt% P-LDHs into PLLA increase the melting temperature by
about 3℃. The TGA data give evidence that the thermal stability of exfoliated PLLA/LDH namocomposites were reduced by
adding the P-LDHs, which is probably due to the dehydroxylation of the LDH layers and decomposition of intercalated
PLA-COOH.
Introduction
In recent years there has been a lot of interest in ecological materials, such as poly(lactic acid) (PLA) [1], poly(glycolic acid)
(PGA) [2], poly(ε-capronlate) (PCL) [3], poly(3-hydroxybutyrate) (PHB) [4], and chitosan[5], due to their biodegradable and
biocompatible properties are better than those of polymers made from petroleum products. Owing to these properties, they
have potential applications in medicine, surgical implants, packing materials, drug delivery system. Among of these
biodegradable materials, poly(lactic acid) (PLA) is a most interesting material because of its outstanding biocompatibility
and biodegradability as well as excellent physical properties. Moreover, PLA is a linear thermoplastic polyester which was
synthesized from lactic acid obtained from renewable crop. In general, PLA is made up of L-form and D-form lactic acid
monomer with optical isomer structures; moreover, the amount of D-form lactic acid can affect the melting point and degree
of crystallinity of PLA. Furthermore, it is known that the degree of crystallinity can influence mechanical and degradable
properties of PLA.
Layered double hydroxides (LDHs) are constituted by positive hydroxylated layers with anionic species and water
molecules in the interlayer. Furthermore, LDHs possess two stacking structures, such as hexagonal and rhomohedral. The
lattice parameter c of hexagonal or rhomohedral is about two times or three times of the interlayer spacing [6]. In general,
the hexagonal structure of LDHs is possibly obtained at high synthesis temperature, but this stacking structure is rare.
Therefore, the structures of LDHs are identified to be rhomohedral structure [7]. It is well-known that LDHs can be
synthesized by tailoring components in the laboratory using three routes, such as ion-exchange, co-precipitation and
reconstruction method. The common chemical formula of LDHs can be described by
Ⅲ
x
n[M1-xMx (OH)2] [Ax/n ‧mH2O]
where M is univalent or divalent metal cations (such as, Li+, Mg2+, Ni2+, Ca2+and Zn2+); MⅢ is trivalent metal cations (such as,
3+
3+
3+
3+
nAl , Cr , Ga and Fe ). A is an anion like NO3-, Cl , SO42- and CO32-. These anions can be replaced by biomolecule, like
DNA [8], via ion-exchange method; they also can be replaced by organical guests, such us acrylate [9], glycine [10], and
lactate [11], by employing co-precipitation method. Therefore, organo-modified LDHs containing unique physicochemical
properties have been extensively used in a lot of applications, such as filler used in the nanocomposites, DNA reservoirs,
drug delivery, catalysts and optical-electron materials.
Over the past decade, significant attention has been focused on polymer/clay nanocomposites due to their superior
properties, such as excellent physical and mechanical properties compared to those of neat polymers [12-14]. Among of
these, several investigations have focused on montmorillonite-type layered silicate nanocomposites because of the
advantage of easily ion-exchange ability. In contrast with polymer/MMT nanocomposites, polymer/LDH nanocomposites
have fewer studies as LDHs have smaller gallery than MMT because of their stronger electrostatic interaction between
highly charged hydroxide sheets and the interlayer anions. Up to now, although a lot of researches have been made on
PLA/MMT nanocomposites [15-20], PLA/LDH nanocomposites have never been studied.
In this study, we prepared PLLA/LDH nanocomposites by solution intercalation of PLLA into the organo-modified LDHs
(P-LDHs). The LDHs with different magnesium (Mg)/aluminum (Al) molar ratio were synthesized by co-precipitation method.
In order to improve the interaction between the PLLA matrix and LDHs, the interlayer surfaces of neat MgAl-LDHs were
modified by using poly(DL-lactide) with carboxyl end group (PLA-COOH). The microstructures of MgAl-LDHs and
organo-modified LDHs have been determined by X-Ray diffraction (XRD), transmission electron microscopy (TEM) and
Fourier transform infrared spectroscopy (FTIR). The physical characteristic of PLLA/LDH nanocomposites was examined
by XRD, TEM, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).
Experimental
Materials
Mg(NO3)2‧6H2O (97%), Al(NO3)3‧9H2O (98%) and sodium hydroxide were supplied by Showa chemical company, Japan.
The powders of Poly(l-lactide) (PLLA) and Poly(DL-lactide) with carboxyl end group (PLA-COOH) were obtained from Bio
Invigor Corporation (Taipei, Taiwan). Acetone and tetrahydrofuran (THF) were purchased from TEDIA (USA) and ECHO
(Taiwan), respectively. All the chemicals used without further purification in this research. Magnesium-Aluminum layered
double hydroxides with magnesium/aluminum ratio of 2 and 3, (designated as 21LDHs and 31LDHs) were prepared by the
co-precipitation method according to the previous literature [21]. In a typical experiment method, Mg(NO3)2‧6H2O and
Al(NO3)3‧9H2O were dissolved in the 200 ml deionized water. The solution was vigorously stirred at 60℃ for 24 h and
maintained pH value at 10.0±0.2 by drop-wise addition of 2N NaOH solution. The white slurry was filtered and washed
three times with deionized water. In order to avoid the pollution of carbonate, all of experimental processes proceeded
under the nitrogen atmosphere. 0.5 g 21LDHs and 0.5 g PLA-COOH was prepared by dissolving in 50 and 100 ml acetone,
respectively. Then, the solution was mixed in the 4-neck vessel and refluxed at 70℃ for 6, 12, 24, 36, and 48 h under
nitrogen atmosphere. The obtained slurry was filtered and washed three times with acetone. The products were obtained
by lyophilizing in vacuum. The organo-modified layered double hydroxides (P-LDHs) was dispersed in 20 ml
tetrahydrofuran and treated with ultrasound for 48 h. At the same time, the powder of PLLA was dissolved in 30 ml
tertahydrofuran and stirred at 60℃. The 1 wt%, 3wt%, 5wt%, and 10 wt% homogeneous dispersedly P-LDHs was mixed
with PLLA solution and stirred for 24 h. The final solution was poured into culture dish and dried in ventilator for 48 h.
Characterization techniques
The wide angle X-ray diffraction (WAXD) was performed on a Rigaku Ⅲ diffractometer equipped with Ni-filtered Cu Kα
radiation. The data of samples were collected from 2θ=1.5o-40o in increment of 1o/min. The transmission electron
microscopy (TEM) images were obtained from JEOL JEM-1200 CX Ⅱ at an accelerating voltage of 120 KV. The
nanocompoistes samples were ultramicrotomed with diamond knife, and then the thickness of 100 nm of slices was
transferred from water to copper grid. The Fourier transform infrared (FTIR) spectroscopy was examined using a
Perkin-Elmer Spectrum One with an average of eight spectra recorded of the specimens. Thermal properties of PLLA and
PLLA/LDH nanocomposites were performed using Perkin-Elmer PYRIS diamond DSC calibrated using indium and
measured under nitrogen atmosphere. All samples were weighted 3 mg. Thermogravimetric analysis (TGA) was carried out
using Perkin-Elmer TGA/DTA 6300 thermoanalyzer at heating rate of 10℃/min under air atmosphere; all of the specimens
were weighted about 8 mg.
Results
The structure of 21LDHs and 31LDHs were confirmed by XRD diffraction pattern shown in Fig. 1. The XRD results revel
that there are three strong diffraction peaks at 2θ=11.2o, 22.3o, 34.5o, corresponding to (003), (006) and (009) three orders
of diffractions for 21LDHs. These diffraction peaks will slightly shift to smaller angles as Mg/Al ratio increases to 3. These
results indicate the order of lamellar structure in 21LDHs and 31LDHs is well packed. In addition to these, the (003)
diffraction peak shifted to lower diffraction angle by raising the molar ratio of Mg/Al, which is probably due to the decrease of
the electrostatic interaction between positive hydroxylate layers and negative interlayer anions as increasing Mg/Al molar
ratio. The TEM micrograph of neat 21LDHs is presented in Fig. 2. From this photograph, it indicates that LDHs exist in the
form of agglomerate layered structure. This phenomenon can be attributed to the electrostatic force occurred between
positive hydroxylate sheets and interlayer anion species.
Relative Intensity
(003)
(006)
(009)
(b)
(a)
5
10
15
20
25
30
35
40
2θ (degree)
Figure 1. The XRD diffraction pattern for (a) 21LDHs and (b) 31LDHs
Figure 2. Transmission electron micrograph of 21LDHs
LDHs are a family of hydrophilic layered structures composed of anion in the interlayer. Moreover, the distance between
hydroxide sheets is less than 1 nm. In this condition, it is very difficult for the polymer chain segment to diffuse into small
gallery space between hydroxylate layers. Besides, compatibility between hydrophilic clay and hydrophobic polymer is
another most significant factor for promoting the extent of exfoliation in polymer nanocomposites [22]. In order to increase
interlayer distance and chemical compatibility between LDHs and PLLA matrix, we used PLA-COOH to modify the LDH
surface by ion-exchange method with different reaction times. The XRD patterns of 21LDHs and organo-modified LDHs
o
(P-LDHs) for various treated times are shown in Fig. 3. It is clear that the diffraction peak at 2θ=11.24 (trace a in Fig. 3),
corresponding to the (003) crystalline plane of a layered structure, shifts to lower angle after modified by PLA-COOH.
Moreover, it is necessary to point out that there is a small diffraction peak observed at 2θ=6.42o as 21LDHs was modified by
PLA-COOH for 6 hours (trace b in Fig. 3) and the intensity of diffraction peak gradually increase with increasing the treated
times. At the same times, diffraction peak of (003) plane slightly shifts to smaller angle. According to Bragg’s law, the
interlayer spacing of LDHs increases from 0.78 nm to 1.41 nm as treated times achieved at 48 hours (Fig. 3, trace f). This
result indicates that molecular chain of PLA-COOH probably intercalated into the interlayer of LDHs, causing the increase
in the interlayer distance between hydroxides sheets. It is noteworthy that (009) diffraction peak of 21LDHs dramatically
disappear when 21LDHs are modified by PLA-COOH in acetone. The phenomenon is probably reasonable as original
stacking order of layered structure of LDHs was affected after treated with PLA-COOH.
14.1 A
Relative Intensity
(f)
(e)
(d)
(c)
(b)
(003)
5
10
(009)
(006)
15
20
25
30
35
(a)
40
2θ (degree)
Figure 3. The XRD pattern of (a) 21LDHs and P-LDHs for different reaction times, (b) 6 h, (c) 12 h, (d) 18 h, (e) 24 h and (f)
48 h
Fig.4 shows the FTIR spectrum of 21 LDHs and P-LDHs for 48 h treated times. The trace a in Fig .4 shows that 21LDHs
-1
sample has a broad adsorption band at 3500 cm due to the O-H groups stretching of hydroxylate sheets in the LDHs and
the water molecules in the interlayer. The intense absorption peak at 1387 cm-1 can be attributed to asymmetric stretching
-1
vibration of nitrates. The characteristic peaks around 400-800cm are assigned to M-O and O-M-O group, where M
represents Mg or Al. The FTIR spectrum of the P-LDHs for 48 h treated periods (Fig. 4 (b)) shows different characteristic
peak at 1203cm-1, 1102cm-1, 1762 cm-1 and 2994cm-1, expect for the strong characteristic peak at 1387 cm-1 (nitrates
-1
stretch overlapped with COO group) which is not easily separated from the result. The absorption peaks at 1203cm and
-1
1102cm are related to the asymmetric and symmetric C-O-C group of the PLA-COOH, respectively. Moreover, the
-1
characteristic peak at 1762 cm is assigned to C=O group of the PLA-COOH. In addition to these, the stretching vibration
of O-H band in PLA-COOH occurs at about 2994cm-1. These FTIR results demonstrate that PLA-COOH was absorbed onto
the interlayer of LDHs.
Transmittance
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Figure 4. FTIR spectra of samples: (a) 21LDHs, (b) P-LDHs for 48 h treated times
o
o
The XRD patterns at the range of 2θ=1.5 -40 for these PLLA/LDH nanocomposites are shown in Fig. 5. These diffraction
patterns reveal the structural changes of the P-LDHs after blending with PLLA. It is clear that original diffraction peaks of
LDHs at 2θ=6.27o and 11.24o were disappeared. These results indicate that the P-LDH layers have been completely
dispersed into the PLLA matrix.
Relative Intensity
(e)
(d)
(c)
(b)
(a)
5
10
15
20
25
30
35
40
2θ (degree)
Figure 5. X-ray diffraction pattern of (a) PLLA and PLLA/LDH nanocomposites with various P-LDHs content, (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
Although XRD is a powerful analytic technology for determining the microstructure of intercalated or exfoliated polymer/clay
nanocomposites, TEM instrument can be used to visualize the exact microstructure of polymer/clay nanocomposites
directly. Fig. 6 (a) and (b) present the bright-field TEM of 10 wt% PLLA/LDH nanocomposites at low and high magnification,
respectively. From this result, the LDHs are exfoliated and dispersed disorderly into the PLLA matrix. Moreover, there are
some single layers of LDHs perpendicular to the cutting section of the 10 wt% PLLA/LDH nanocomposites specimen shown
in Fig. 6(b). The dispersion behaviors observed by TEM agree with those obtained by XRD.
(a) (b) Figure 6. Transmission electron micrograph of 10 wt% PLLA/P-LDH at (a) low magnification and (b) high magnification (the
arrows indicate the single layer of LDHs)
The thermogravimetric profiles of neat PLLA and PLLA/LDH nanocomposites are shown in Fig. 7. It is necessary to point
out that T50%, defined as the temperature of 50% weight loss, of PLLA is 351℃ and shifts into 281℃ as the addition of 10
wt% P-LDHs. These results indicate that the thermal stability of PLLA decreases with increasing the P-LDH contents. These
results are in contract with the previous researches about polymer/LDH nanocomposites [9, 23-25] which show the stability
of those polymers were enhanced via adding the LDHs. The main reasons for the decreasing thermal stability might be
attributed to the dehydroxylation of the LDH sheets and decomposition of intercalated PLA-COOH.
100
PLA
1% PLLA/P-LDH
3% PLLA/P-LDH
5% PLLA/P-LDH
10% PLLA/P-LDH
Weight (%)
80
60
40
20
0
100
200
300
400
500
o
Temperature ( C)
Figure 7. The TGA profiles for neat PLLA and PLLA/LDH nanocomposites with different contents of P-LDHs
nd
Fig. 8 illustrates the 2 heating scan of DSC for PLLA and PLLA/LDH nanocomposites. It is clear that the recrystallization
temperature of PLLA is about 108℃ and increases to 116℃ as P-LDHs content increases up to 10 wt%. This phenomenon
can be attributed to the mobility of PLLA chains restricted by the homogeneous dispersion of LDHs in the PLLA matrix.
Therefore, higher temperature is needed to supply enough energy to allow molecular chain of PLLA re-arranged in this
condition. The melting temperature of PLLA is slightly enhanced as the contents of P-LDHs were added to 3 wt%. However,
the temperature decreased when P-LDH loads＞5 wt% in the nanocomposites (Fig. 8(d), Fig. 8(e)). It is not clear about this
variation of melting point in the high loading LDHs nanocomposites and the detail investigation of this system will be studied
later.
)
(e)
Heat Flow ( Endo
(d)
(c)
(b)
(a)
40
60
80
100
120
140
160
180
200
O
Temperature ( C )
nd
Figure 8. The 2 heating curves of DSC for (a) PLLA and PLLA/LDH nanocomposites with different contents of P-LDHs, (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
Conclusions
The co-precipitation method was used to synthesize various Mg/Al molar ratio LDHs with well stacked layered structure.
The MgAl-LDHs were modified by the intercalation of the PLA-COOH into the LDHs interlayer to improve the compatibility
between the hydrophilic MgAl-LDHs and hydrophobic PLLA matrix. The FTIR results suggested that PLA-COOH could be
absorbed onto the spacing between hydroxylate sheets by ion-exchange reaction. Moreover, the XRD results and TEM
images indicate the well exfoliated PLLA/LDH nanocomposites were successfully prepared by solution casting. The TGA
profiles of the PLLA/LDH nanocomposites show the less thermal stability than neat PLLA. The decreasing thermal stability
of PLLA/LDH nanocomposites could be attributed to the dehydroxylation of hydroxide layers and decomposition of
PLA-COOH in the interlayer. Besides, the DSC analysis implied exfoliated LDH sheets increase the melting temperature of
PLLA by about 3℃ as the loading of 3 wt% P-LDHs.
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