Method for Simultaneously Improving the Thermal Stability and

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Method for Simultaneously Improving the Thermal Stability and Mechanical Properties of Poly(lactic acid): Effect of High-Energy Electrons on the Morphological, Mechanical, and Thermal Properties of PLA/MMT Nanocomposites De-Yi Wang,*,†,‡,∥ Uwe Gohs,‡ Nian-Jun Kang,‡ Andreas Leuteritz,‡ Regine Boldt,‡ Udo Wagenknecht,‡ and Gert Heinrich‡,§ †

Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China ‡ Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, D-01069 Dresden, Germany § Technische Universität Dresden, Institut für Werkstoffwissenschaft, D-01069 Dresden, Germany S Supporting Information *

ABSTRACT: Nanocomposites derived from poly(lactic acid) (PLA) and organically modified montmorillonite (oMMT) have been cross-linked by high-energy electrons in the presence of triallyl cyanurate (TAC). The morphology of untreated and cross-linked PLA/MMT nanocomposites was characterized by wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). This treatment can improve both the thermal stability and the glass-transition temperatures of the PLA nanocomposites (e.g., PLA-MMTTAC 30kGy, 50kGy, and 70kGy) because of the formation of cross-linking structures in the nanocomposites that will considerably reduce the mobility of polymers. Interestingly, at relatively low irradiation doses (e.g., 30 and 50 kGy) a good balance between tensile strength and elongation at break for the PLA nanocomposites could be achieved. These mechanical properties are superior to those of pure PLA. Therefore, combining nanotechnology and electron beam cross-linking is a promising new method of simultaneously improving the mechanical properties (toughness and tensile strength) and thermal stability of PLA.

1. INTRODUCTION It is well-known that poly(lactic acid) (PLA) is one of the most promising candidates in the class of environmentally friendly polymers. This is because it is produced from renewable resources and has the advantage of high biodegradability. It is even considered that PLA may replace some petrochemical polymers.1 However, there are still some disadvantages that limit its further application and development, such as low toughness, low thermal stability, and high flammability. Thus, the simultaneous improvement of some of these properties is a very important target in academic research and industrial applications. To improve the flame retardance of PLA, several methods have been reported in patents2−7 and publications.8−13 In our recent studies, organomodified layered double hydroxides (LDH) synthesized via a one-step route and a series of polymer/LDH nanocomposites have been investigated.14−17 Among these investigations, it is noted that the addition of LDH can significantly improve the flame retardance of PLA17 whereas its mechanical properties are decreased because of poor interfacial compatibility between the nanofiller and © 2012 American Chemical Society

polymer. In fact, in recent years many efforts have been made to improve the mechanical properties of PLA, especially its toughness. For example, many polymers such as polycaprolactone,18 poly(ethylene glycol),12 polyhydroxyalkanoate copolymers,19 polyamide elastomer,20 poly(butylene succinate),21 polyethylene,22 poly(ether)urethane elastomer,23 and PLA-g-polymer24 have been used to improve this property. However, though the rubbery component can provide supplementary energy dissipation mechanisms during deformation and improve the toughness of PLA, it still has some limitations in toughening PLA without compromising other mechanical responses, such as the modulus and the strength of a composite material. The technique of modifying polymers with high-energy electrons (via an electron beam) has been well established in several industrial applications because of its unique characteristics and advantages. The most relevant industrial application is Received: January 2, 2012 Revised: July 26, 2012 Published: July 30, 2012 12601

dx.doi.org/10.1021/la3025099 | Langmuir 2012, 28, 12601−12608

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Figure 1. TEM patterns of PLA nanocomposites showing the nature of the dispersion of nanoclay in the matrix: (A, B) PLA-MMT-TAC 0kGy and (C, D) PLA-MMT-TAC 70kGy.

morphology of nonirradiated and irradiated PLA/MMT nanocomposites was characterized by wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). Their thermal behavior and tensile properties in different absorbed doses has been investigated in detail. The purpose of this research was to test a promising new method for improving the mechanical properties (specifically, toughness, tensile strength, and thermal stability) of PLA simultaneously by combining nanotechnology and electron beam irradiation.

based on the cross-linking of cables and pipes via high-energy electrons at ambient temperature. As a result of the interaction of high-energy electrons with the material, radicals are produced by inducing complex chemical reactions. These reactions result in a modified structure of the polymer as well as altered chemical, mechanical, and thermal properties of the resulting polymer. Thus, environmentally friendly electron beam processing is a valued resource for polymer modification and offers significant advantages over traditional methods of modification. For example, electron-beam-modified atactic polypropylene (aPP) was used as a compatibilizer for nonpolar aPP and organically modified montmorillonite (oMMT).25 It has been found that the addition of electron-beam-modified aPP improved the dispersion of oMMT in the aPP matrix and increased the thermal stability. Electron beam irradiation also has been used in the modification of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) to improve the thermal stability, but this leads to a decrease in toughness.26,27 In this study, we have investigated the effect of electron beam irradiation on the thermal and tensile properties of a PLA/ organically modified montmorillonite (oMMT) clay nanocomposite in the presence of triallyl cyanurate (TAC). The

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. 2.1.1. Materials. Organically modified montmorillonite (oMMT, EXM 1485) was obtained from Süd-Chemie AG (Germany). Triallyl cyanurate (TAC) was supplied by Cytec Surface Specialties, The Netherlands. Type4042D PLA was provided by Nature Works (U.S.). 2.1.2. Processing of PLA/oMMT Nanocomposite and Injection. PLA-MMT-TAC composites in a 95:3:2 weight ratio were prepared by a batch process in a Haake rheomix (internal mixer), having a mixing chamber volume of 50 cm3, at a rotor speed of 60 rpm and at 190 °C in air. The friction ratio was maintained at 2:3. The total time of mixing amounted to 13 min. An injection molding machine (BOY 12602


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22D, Dr. Boy, Neustadt Fernthal, Germany) was used to prepare tensile specimens (ISO 5271, specimen type 1A). 2.2. Electron Beam Irradiation. PLA-MMT-TAC composites were irradiated at room temperature in an air atmosphere with dose values of 30, 50, 70, and 100 kGy using an ELV-2 electron accelerator (BINP, Novosibirsk, Russia). The electron energy was 1.5 MeV, and the electron current amounted to 4 mA. 2.3. Characterization. Wide-angle X-ray scattering (WAXS) was performed using an XRD 3003 θ/θ two-circle diffractometer (GE Inspection Technologies/Seifert-FPM, Freiberg, Germany) with Cu Kα radiation (k = 0.154 nm) generated at 30 mA and 40 kV in the range of 2θ = 0.5−12° using 0.05° as the step length. The Fourier transform infrared spectra (FTIR) of the irradiated PLA nanocomposites was obtained using a Bruker Vertex 80 V spectrometer (Karlsruhe, Germany) over the range of 4000−400 cm−1. The morphological analysis was carried out using transmission electron microscopy (TEM) with a Libra200MS microscope (Carl Zeiss SMT, Jena, Germany, field-emission cathode, point resolution 0.2 nm). The conditions used during analysis were room temperature, a 200 kV acceleration voltage, and bright-field illumination. Ultrathin sections of the samples with a thickness of about 50 nm were prepared by ultramicrotomy at −100 °C. The thermal stability of the samples was investigated by thermogravimetric analysis (TGA, TA Instruments Q 5000) from room temperature to 800 °C at a heating rate of 10 K/min in a nitrogen atmosphere. The thermal properties of the samples were measured by differential scanning calorimetry (DSC, TA Instruments Q1000) from −80 to 230 °C at a heating rate of 10 K/min in a nitrogen atmosphere. The thermal history of the samples was erased by a preliminary heating−cooling cycle at a heating and cooling rate of ±10 K/min. Tensile tests were carried out according to ISO 527/1BA/10 on dumbbell-shaped specimens using a Zwick 8195.05 universal tensile testing machine (Ulm, Germany) at a constant cross-head speed of 10 mm/min. The E modulus was determined in between 0.1 and 0.25% tensile strain. The structure of radicals in PLA after irradiation was studied by electron paramagnetic resonance (EPR) spectroscopy. All spectra were recorded at room temperature in air using a MiniScope MS 200 (Magnettech Limited, Berlin, Germany) operating in the X band with a TE102 rectangular cavity. PLA pellets were put into a cell that was positioned identically in the cavity for all measurements. The cell that was used exhibits no background signal within the experimental uncertainty of the EPR measurement. The weight of the samples was in the range of 340−343 mg. The operating conditions of the EPR spectrometer were as follows: 9.81 GHz microwave frequency, 100 kHz modulation frequency, 0.1 mT magnetic field modulation, 1 mW microwave power, 12 s scan time, four scans, 334.3 mT central magnetic field, and 9.8 mT sweep range. Each sample (0, 30, 50, and 70 kGy) was measured three times for accuracy. The average EPR spectra were normalized to the sample weight. Finally, the measured EPR spectra were approximated using normalized Gaussians.

randomly dispersed in the PLA matrix, providing direct evidence that most nanoplatelets are exfoliated. Besides the exfoliation state, TEM images of PLA-MMT-TAC 0kGy also show few ordered stacks of MMT, which correspond to intercalated nanostructures. In the case of Figure 1c, the TEM image of PLA-MMT-TAC 70kGy reveals a well-distributed, randomly dispersed MMT in the matrix. The images at higher magnification (Figure 1d) show some exfoliated MMT sheets next to larger stacks of MMT layers. On the other hand, it is well known that X-ray scattering provides a way to determine the interlayer issue of the MMT in the polymer/MMT nanocomposite. To monitor the dispersion state of MMT in the PLA, PLA-MMT-TAC 0kGy and PLAMMT-TAC 70kGy samples were measured by X-ray scattering, as shown in Figure 2.

Figure 2. X-ray scattering patterns of PLA-MMT-TAC 0kGy and PLA-MMT-TAC 70kGy.

Both samples in Figure 2 show the characteristic peak of the (001) reflection of MMT, indicating the presence of agglomerates in the nanoclay. The decreased intensity of this reflection peak in the nonirradiated sample (PLA-MMT-TAC 0kGy) shown in the TEM image (Figure 1a) suggests that there exist texture effects in the sample. 3.2. Thermal Behavior of the Irradiated PLA-MMT-TAC Nanocomposite. The thermal decomposition behavior of irradiated PLA-MMT-TAC nanocomposites was investigated by thermogravimetric analysis (TGA), as presented in Figure 3. The thermal degradation behavior of all of the samples showed a one-stage weight loss. However, the initial decomposition temperatures and the char residues at 800 °C of all of the samples investigated are significantly different. In the case of the nonirradiated sample (PLA-MMT-TAC 0kGy), the onset decomposition temperature (T2 wt %) amounts to 232 °C. In contrast, for irradiated samples with absorbed doses of 30, 50, and 100 kGy, the onset decomposition temperature (T2 wt %) increased to 297, 296, and 291 °C, respectively. The increase in the onset decomposition temperature may correspond to the cross-linked structure in the nanocomposites formed during electron beam irradiation in the presence of TAC. The formed network in the PLA nanocomposite retards the decomposition. This suggests that the cross-linking of PLA in the presence of TAC by electron irradiation can improve the thermal stability of the PLA-MMT-TAC nanocomposite. Moreover, for the untreated PLA-MMT-TAC nanocomposite, the char residue at

3. RESULTS AND DISCUSSION 3.1. Morphological Analysis. The morphological analysis of layered nanofillers in the polymer nanocomposite was generally characterized by transmission electron microscopy (TEM) analysis and X-ray scattering patterns. On one hand, TEM shows a qualitative understanding of the internal structure of composites, such as the dispersion state of the nanofiller in the polymeric matrix through direct visualization. TEM micrographs for PLA-MMT-TAC 0kGy and PLA-MMTTAC 70kGy nanocomposites are presented in Figure 1. In the overview image of PLA-MMT-TAC 0kGy (Figure 1a), we observe a layered orientation of the MMT particles, which could be a result of the injection molding. From Figure 1b, it is clearly visible that most of the MMT nanoplatelets are 12603


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Figure 3. Thermogram showing the thermal decomposition behavior of PLA-MMT-TAC (0, 30, 50, and 100 kGy) nanocomposites.

800 °C amounts to 1.77 wt % whereas the char residues of PLA-MMT-TAC 30kGy, PLA-MMT-TAC 50kGy, and PLAMMT-TAC 100kGy at the same temperature are 2.57, 2.80, and 2.50 wt %, respectively. The reason for this difference may correspond to the cross-linking structure formed during electron beam irradiation because the cross-linking structure in polymers will lead to more char residue remaining during thermal decomposition. It also can be noted that different absorbed doses have an effect on both the thermal stability and char residues at high temperature. The 30 kGy dose could be the optimum absorbed dose. At higher dose values, degradation becomes more pronounced in comparison to cross-linking. For example, the results clearly show that a dose of 100 kGy is too high for this nanocomposite system, showing the relatively low initial decomposition temperature and char residues caused by the decomposition behaviors of the PLA chain at a high absorbed dose. The DSC results of PLA and PLA-MMT-TAC (0, 30, 50, and 70 kGy) nanocomposites are shown in Figure 4. First, the degree of crystallinity of PLA and PLA-MMT-TAC has been determined by subtracting ΔHc from ΔHm and by considering a melting enthalpy of 93 J/g for 100% crystalline PLA. By calculation, the degree of crystallinity of melt-mixed PLA amounts to 0.2% and that of PLA-MMT-TAC without electron beam irradiation amounts to 0.3%. This means that there is no significant difference in the degree of crystallinity of PLA after introducing MMT and TAC into the PLA system, suggesting that the PLA and PLA nanocomposite studied are almost completely amorphous. In the case of the nonirradiated PLAMMT-TAC sample, the glass transition, cold crystallization, and melting peaks can be clearly observed in Figure 4. In comparison, it is noted that the glass-transition temperatures of irradiated nanocomposites (PLA-MMT-TAC 30, 50, and

Figure 4. DSC heating curves of PLA-MMT-TAC (0, 30, 50, and 100 kGy) nanocomposites.

70kGy) have increased from 56.4 to 59.6, 59.5, and 59.2 °C, respectively. This is caused by the cross-linking structures in the nanocomposites that reduce the mobility of the polymer chains. In contrast, the melting temperatures (Tm) shift to lower values with increased absorbed dose, with the reason being that electron beam irradiation also induces a slight decomposition in PLA as the cross-linking structures formed in the presence of TAC. Thus, the PLA segments with low molecular weight lead to the Tm shift to low temperature. These results are similar to the previous reports on cross-linked PLA,26,27 although the interpretations of the phenomenon are dissimilar. 3.3. Mechanical Properties. The tensile properties of the PLA-MMT-TAC nanocomposite without and with electron beam irradiation (30, 50, and 70 kGy) are shown in Table 1 12604


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Table 1. Mechanical Properties of PLA-MMT-TAC (0, 30, 50, and 70 kGy) Nanocomposites samples PLA PLA-MMT PLA-MMT-TAC 0 kGy PLA-MMT-TAC 30 kGy PLA-MMT-TAC 50 kGy PLA-MMT-TAC 70 kGy

tensile strength (MPa)

tensile modulus (GPa)

elongation at break (%)

60.7 ± 1.1 59.5 ± 0.5 56.1 ± 1.4

2.60 ± 0.07 1.51 ± 0.03 2.09 ± 0.02

9.2 ± 0.3 80.8 ± 7.0 66.6 ± 8.0

63.6 ± 0.5

2.08 ± 0.02

47.8 ± 5.6

62.3 ± 0.5

2.11 ± 0.13

32.9 ± 3.3

62.4 ± 1.2

2.15 ± 0.05

16.1 ± 5.1

and Figure 4. For comparison, pure PLA and PLA-MMT are also measured under the same conditions, and their tensile properties are listed in Table 1. In the case of the nanocomposites, the results reveal the decrease in elongation at break with increasing absorbed dose. This effect is well known because of cross-linked polymers and from polymer degradation. The tensile modulus appears to be independent of the absorbed dose. In contrast to this, the tensile strength of irradiated nanocomposites is higher but does not depend on the absorbed dose within the experimental uncertainty. Interestingly enough, in comparison to the tensile properties of pure PLA it is noted that both tensile strength values and elongation at break values of irradiated nanocomposites (30, 50, and 70 kGy) are higher than those of pure PLA. Thus, the electron beam irradiation of PLA nanocomposite could be a promising new way to improve the tensile strength and toughness of PLA simultaneously. In most reports of irradiated or chemically cross-linked PLA, the toughness (elongation at break) of the PLA materials deteriorated after forming the cross-linking structure. Although elongation at break values of the present PLA nanocomposites also decrease with increasing absorbed dose, at an absorbed dose of 30 kGy a good balance between tensile strength and elongation at break was achieved, which is still much better than those of pure PLA. PLA-MMT-TAC 30 kGy has an average elongation at break of 47.8 ± 5.6% and an average tensile strength of 63.6 ± 0.5 MPa. Thus, in comparison to pure PLA the average elongation at break and average tensile strength increase to about 520 and 105%, respectively. Furthermore, the average tensile modulus of PLA-MMT-TAC 30 kGy was only slightly lower than that of pure PLA. However, the tensile strength of composites without TAC cannot be improved by electron beam treatment, indicating that PLA does not form cross-linked structures in PLA-MMT-composites (sFigure 2 and s-Table 1). To investigate the effect of TAC on the mechanical properties of PLA composites, the results shown in s-Figure 3 reflect the fact that the addition of TAC cannot improve the elongation at break of the PLA composite but can improve only the tensile strength of the PLA composite by electron beam treatment. 3.4. Possible Cross-Linking Mechanism. Herein, an attempt has been made to explain the mechanism of crosslinking reactions of PLA nanocomposites in the presence of TAC by electron beam irradiation. The FTIR spectra of both nonirradiated and irradiated PLA TAC-based nanocomposites reflect a sharp band at around 1560 cm−1, which has remained unchanged after electron beam irradiation. This band is related to the cyclic CN groups in TAC. However, the band around

Figure 5. Stress−strain curves of PLA and PLA-MMT-TAC (0, 30, 50, and 70 kGy) nanocomposites.

1650 cm−1 corresponding to the CC groups in TAC are changed significantly, with increasing absorbed dose, because of the consumption by grafting and the cross-linking reaction induced by electron beam irradiation. It can be found that the CC peak totally disappears at absorbed doses of 50 and 70 kGy. Scheme 1 describes the mechanism of the cross-linking reaction in the PLA TAC-based nanocomposite by electron beam irradiation. Thus, at absorbed dose values higher than 50 kGy the degradation of PLA dominates because all CC bonds are utilized. First, electron beam treatment results in the generation of macroradicals. Electron paramagnetic resonance (EPR) spectroscopy studies on PLA have rarely been documented in the literature. However, information on the EPR spectra of lactid acid,28 poly(L-lactic acid),29,30 does exist in the literature. Thus, the EPR spectra of PLA irradiated at 30, 50, and 70 kGy were measured in air at room temperature and are shown in Figure 7 in comparison to nonirradiated PLA. The EPR spectra were approximated using four normalized Gaussians in order to determine the coupling constants of the surrounding nuclear spins. The coupling constant amounts to about 2.2 mT and is in good agreement with the coupling constant of protons for carbon-centered radicals. Because of the structure of PLA, the radical is localized at the tertiary carbon atom (Scheme 1). It is generated by hydrogen abstraction from C−H. The three 12605


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Scheme 1. Possible Cross-Linking Mechanism of the PLA-MMT-TAC Nanocomposite by Electron Beam Irradiation

and 70 kGy), it has been found that the thermal stability was improved and the glass-transition temperature increased because of the cross-linked structures under the electron beam treatment. With absorbed doses of 30, 50, and 100 kGy, the onset decomposition temperatures (T2 wt %) of PLA-MMT nanocomposites were all increased from 232 °C (pure PLAMMT sample) to 297, 296, and 291 °C, respectively. The glasstransition temperatures of these irradiated nanocomposites increased by about 3 °C compared to that of the nonirradiated sample. The elongation at break values of irradiated PLA nanocomposites decreased with the increase in the absorbed dose. However, at an absorbed dose of 30 kGy (PLA-MMTTAC 30 kGy) a good balance between tensile strength and elongation at break was achieved, showing an average

surrounding protons of the methyl group result in the observed four-line spectrum. In the following text, the macroradical will attach to the C C group present in the TAC to realize the first grafting reaction. The same reaction would happen in other CC groups by the macroradicals that were formed during the electron beam treatment. Finally, the cross-linked structure is generated in the PLA-MMT nanocomposite, as shown in Scheme 1.

4. CONCLUSIONS In this article, an electron beam treatment has been used to improve the thermal stability and mechanical properties of PLA-MMT-TAC nanocomposites. In the case of irradiated PLA-MMT-TAC nanocomposites (PLA-MMT-TAC 30, 50, 12606


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PLA-MMT-TAC nanocomposites under the electron beam treatment.

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ASSOCIATED CONTENT

S Supporting Information *

DSC heating curves of PLA and PLA-MMT-TAC nanocomposites without irradiation. Stress−strain curves and mechanical properties of PLA and PLA-MMT-Na+ composites. Stress−strain curves of PLA and PLA-TAC. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Present Address ∥

IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid, Spain. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the AvH Foundation, the National Science Foundation of China (50703026), the International Foundation for Science (IFS, F/4285-2), and OPCW. We are grateful to Ms. Isabel David from Michigan State University for helpful discussions.

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REFERENCES

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Figure 6. FTIR spectra of PLA-MMT-TAC (0, 30, 50, and 70 kGy) nanocomposites.

Figure 7. Electron paramagnetic resonance (EPR) spectrum of PLA irradiated at 0, 30, 50, and 70 kGy in air at room temperature.

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