Article pubs.acs.org/IECR
Fabrication of High Toughness Poly(lactic acid) by Combining Plasticization with Cross-linking Reaction Zhongjie Ren,† Huihui Li,† Xiaoli Sun,† Shouke Yan,*,† and Yuming Yang*,‡ †
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China
‡
ABSTRACT: A new method to produce high toughness Poly(lactic acid) (PLA) was introduced in this study. The blends of low molecular weight triacetin (TAC) and oligomeric poly(1,3-butylene glycol adipate) (PBGA) were first used as multiple plasticizers to lubricate PLA and then the plasticized PLA cross-linked with triallyl isocyanurate (TAIC) through electron beam irradiation. In this way, a kind of stable PLA material with high elongation at break was obtained. The elongation at break of the prepared material is more than 260% with a tensile strength of 23.5 MPa when the irradiation dose is 10 Mrad. Detailed studies of Fourier transform infrared (FTIR) spectra and viscosity experiments revealed that both the self-cross-linking of the PLA molecular chains and the cross-linking between the molecular chains of PLA and PBGA could occur. This results in the remarkable improvement in the performance of the obtained materials.
1. INTRODUCTION Poly(lactic acid) (PLA) is a kind of aliphatic polyester, which has a number of interesting properties including biodegradability,1−3 biocompatibility,4,5 good mechanical properties,6 and processability.7−9 For these reasons, it is most promising for application in a great number of fields. On the other hand, the PLA is a comparably brittle and rigid material. Its low elongation at break limits its application.10−13 Therefore, great efforts have been made to improve the flexibility of PLA.14−18 Among many others, blending PLA with plasticizers is confirmed to be a simple and effective method to modify the properties of PLA.19 It has been reported that the low molecular weight plasticizers such as tributyl citrate20 and triactine (TAC)21 can drastically lower the glass transition temperature of PLA, thus creating the homogeneous and flexible materials. The good solubility of PLA in the plasticizers should be attributed to the polar interactions between them. In our previous study,22 low molecular weight TAC and the oligomeric poly(1,3-butylene glycol adipate) (PBGA) were chosen as plasticizers to improve the toughness of PLA. It has been shown that the elongation at break of PLA was improved significantly, whereas a considerable loss of tensile strength was found. So, in the process of improving the flexibility of PLA, both the stability and tensile strength should also be considered. It should be pointed out that the low molecular weight plasticizers had a tendency to migrate to the surface upon aging of the plasticized materials during usage of the products, which may result in a decrease in property. A possible way to prevent the migration of low molecular plasticizers would be to increase the molecular weight of the plasticizers. In this way the larger molecules would reduce their diffusion ability and decrease the tendency of migration to the surface. However, increasing the molecular weight too much would eventually decrease the solubility of the plasticizers in the PLA matrix, causing phase separation and forming two-phase system.23−25 It is assumed that cross-linked structure would be another effective way for stabilizing the multiple plasticizers. © 2012 American Chemical Society
Moreover, by introducing cross-linking into the macromolecular chains, the physical properties, such as the crystallinity, melting point, and glass transition temperature, are affected, following the improvement in mechanical and thermal properties of the polymer.26 It is well documented that there are a lot of methods to produce cross-linking between polymer chains. Ionizing radiation is a very convenient tool for modification of polymer materials through cross-linking, grafting, and degradation techniques. PLA undergoes predominantly degradation under direct ionizing radiation. When PLA is exposed to the a gamma ray or an electron beam, the mechanical and physical properties of PLA decrease due to reduction of the molecular weight of PLA.27 To avoid the degradation of PLA and create a crosslinked network, we employed polyfunctional monomers trially isocyanurate (TAIC). The TAIC has three functional groups (CC) and a cyclic unit (isocyanuric ring), so it has high reactivity that achieves a great three-dimensional network by irradiation.28 In the present study, we first chose the multiple plasticizers (TAC/PBGA = 1:1 weight ratio) to plasticize PLA and then added TAIC to form cross-linking under electron beam irradiation. In this paper, we focus first on the mechanism of cross-linking reaction and then on the influence of crosslinking on the thermal and mechanical properties of the fabricated PLA materials.
2. EXPERIMENTAL SECTION 2.1. Materials. The used PLA was supplied by Mitsui Chemicals, Inc. (Tokyo, Japan). The weight-average molecular weight (Mw) of PLA is 2.07 × 105 with a polydispersity index of 1.71, as determined by gel permeation chromatography. TAC was obtained from Jiangsu Yixing Fenshui Chemical-Auxiliary Received: Revised: Accepted: Published: 7273
March 6, 2012 May 2, 2012 May 8, 2012 May 8, 2012 dx.doi.org/10.1021/ie3006098 | Ind. Eng. Chem. Res. 2012, 51, 7273−7278
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Fourier transform infrared (FTIR) spectra were obtained with a Bio-Rad FTS-7 spectrophotometer. A total of 32 scans were taken with a resolution of 4 cm−1 in all cases. Thermal stability of the materials was analyzed with a TA Instruments model Perkin-Elmer TGA7 thermogravimetric analyzer by heating from room temperature to 600 °C at a rate of 10 °C/min under nitrogen gas. Dynamic mechanical analysis (DMA) was carried out on a DMA-983 (Dupont). Test bars were cut from the tensile bar specimens (dimensions W × H × L = 1 × 4 × 5 in millimeters). The experimental temperature ranged from −40 to 140 °C, at a heating rate of 3 °C/min. The amplitude was set at 5 μm, with a frequency of 1 Hz. Storage modulus and tan δ were recorded as a function of temperature. Viscosity measurements of the plasticizers were carried out in an NDJ-8S digital display viscometer, the measurement temperature was 21 °C, which is constant within ±0.5 °C, and the rotation speed of the rotor was 60 turns/min.
Factory, China. PBGA was obtained from Tianjin Epoch Chemical Co., Ltd., China. The Mw of PBGA is 3000 with a polydispersity index of 1.03. TAIC was obtained from Shanghai Fangruida Chemical Co., Ltd., China. All substances were used as received. Their structural formulas are shown as Scheme 1. Scheme 1. Structural Formulas of TAIC, TAC, PBGA, and PLA
3. RESULTS AND DISCUSSION 3.1. Gel Fraction of the Cross-linked PLA. We presented first the gel fraction of the prepared PLA composites to show the cross-linking degree. It has previously been reported that TAIC is biodegradable and the cross-linked PLA is also biodegradable, even the gel fraction of cross-linked PLA reaches 100%.30 Therefore, TAIC was chosen as a cross-linking agent for PLA. As shown in Figure 1, the cross-linking reaction of
2.2. Preparation of the Mixture Plates. 30% multiple plasticizers of TAC and PBGA (1:1) and 70% PLA were dissolved in chloroform. Then, TAIC was added to the solution at different weight percentages (0.8 wt %, 2.3 wt %, and 3.7 wt %); after blending formed a homogeneous solution, the solvent was allowed to evaporate at 40 °C under vacuum oven for 24 h. The blends were then cut into small granules. The mixture was further compressed at 160 °C to obtain a 1 mm thick plate. 2.3. Electron Beam Irradiation of the Mixture Plates. The obtained mixture plates were then irradiated with electron beam at a beam current of 18 mA and an acceleration energy of 1.8 Mev generated by a Dynamitron-type accelerator. Different doses of electron beam (2, 4, 6, 8, 9, 10, 15, and 20 Mrad) were chosen for producing different cross-linking structure and investigating the influence of cross-linked structure on the properties of PLA. 2.4. Gel Fraction of the Cross-linked PLA. To characterize the degree of cross-linking, the gel fraction of the cross-linked PLA was measured. The gel fraction was obtained by dividing the weight of dried gel by the initial weight of the PLA. The weight of dried gel is the weight remaining (dry gel component) of the cross-linked PLA after dissolved in chloroform at room temperature for 24 h. 2.5. Analytical Measurement. Tensile properties of the materials were measured in accordance with ASTM D638 with testing machine (Instron 1211) at room temperature. Thermal properties of the materials were measured with a Perkin-Elmer DSC7 differential scanning calorimetry (DSC) using aluminum oxide as the standard. Heating rate is 10 °C/ min the crystallinity of the materials was calculated by the following equation: ⎡ ΔH ⎤ f ⎥ × 100 Cr = ⎢ PLA ⎢⎣ W ΔH 0f ⎥⎦
Figure 1. Gel fraction of PLA with different irradiation doses.
PLA started already at a low weight ratio (0.8%) of TAIC and low electron beam irradiation dose. A high gel fraction was observed at higher weight ratio of TAIC and elevated electron beam dose. When the weight ratio of TAIC reaches 3.7%, the gel fraction of PLA reaches almost 100% with 20 Mrad of irradiation. TAIC proved to be a good cross-linking agent for PLA under electron beam irradiation. 3.2. Reaction Mechanism. Babanalbandi et al.31 have suggested that H abstraction occurred predominantly from −CH groups in PLA chains when irradiated by electron beam to form radical 1, as shown in Scheme 2. In addition, if TAIC molecules coexisted with PLA, the double bonds of allyl groups in TAIC were broken to form a pair of radicals shown as 2 in Scheme 2, with one of them recombined with abstracted H radical, while the other one combined with radical 1. In this way, two (or three) allyl groups of TAIC combined with radical
(1)
ΔH0f is the thermodynamic enthalpy of fusion per gram of a completely crystallized PLA (ΔH0f = 93.6 J g −1),29 the ΔHf is the apparent enthalpy of fusion of pure PLA and blends, and WPLA is the percentage of the PLA content. 7274
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shown in Scheme 2. The −CH2 groups by the side of carboxyl groups of PBGA change into −CH groups after cross-linking. This leads to a peak intensity decrease of the 1409 cm−1 band. On the other hand, the intensity of peak at 1460 cm−1 remains constant. Therefore, we can indicate the cross-linking extent of PBGA by the ratio of peak areas of bands at 1409 cm−1 and 1460 cm−1. As shown in Table 2, we found that the ratio of peak area between the bands at 1409 cm−1 and 1460 cm−1 decreased with
Scheme 2. Structural formulas of Three Kinds of Radicals
1 in PLA chains, and the cross-linkings between PLA molecules were produced.32 To investigate whether the multiple plasticizers (TAC/ PBGA) have involved in the cross-linking reaction, we devised adding 3.7% TAIC to TAC, PBGA, and TAC/PBGA (1:1) respectively, and then irradiating them by electron beam under the different doses. The viscosities of the irradiated blends at 21 °C were determined. As shown Table 1, the viscosity of TAC
Table 2. Area Ratio of the Peak at 1409 and 1460 cm−1 with Different Irradiation Doses
Table 1. Viscosities of the Plasticizers at 21 °C with Different Irradiation Doses 21 °C
0 Mrad
10 Mrad
20 Mrad
TAC + 3.7% TAIC PBGA + 3.7% TAIC TAC/PBGA + 3.7% TAIC
0.20 Pa s 5.91 Pa s 3.72 Pa s
0.21 Pa s 7.55 Pa s 4.19 Pa s
0.21 Pa s 7.78 Pa s 4.60 Pa s
irradiation dose (Mrad)
A1409 /A1460
0 10 20
0.454 0.333 0.308
increasing irradiation dose, and the value fell from 0.454 with unirradiation to 0.308 with a 20 Mrad irradiation dose. Obviously, PBGA could cross-link with TAIC under irradiation. Therefore, we could deduce a conclusion that PBGA also could cross-link with PLA and TAIC under irradiation. 3.3. Thermal Properties of the Cross-linked PLA. The thermal properties of the cross-linked PLA were evaluated by DSC first. The DSC results of the PLA containing 3.7% TAIC with different cross-linking density are shown in Figure 3. The
with 3.7% TAIC is invariable under the different doses, and it is about 0.21 Pa s. However, the viscosity of PBGA with 3.7% TAIC is gradually increased from 5.91 Pa s unirradiated to 7.78 Pa s with a 20 Mrad irradiation dose. The viscosity of the multiple plasticizers (TAC/PBGA 1:1) also increased with increasing irradiation doses. The value of viscosity increased from 3.72 Pa s unirradiated to 4.60 Pa s with a 20 Mrad irradiation dose. From these results, it is concluded that TAC cannot react with TAIC under electron beam irradiation while PBGA can. The viscosity of the multiple plasticizers increased because PBGA occurred the cross-linking reaction with TAIC under irradiation. To further confirm that the PBGA participate in the crosslinking reaction, FTIR spectra of the multiple plasticizers under the different irradiation doses were invetigated. As shown in Figure 2, two characteristic C−H in-plane bending vibration bands are displayed, a C−H bond of −CH2 or −CH3 groups at 1460 cm−1 and a C−H bond of −CH2 groups by the side of the carboxyl groups at 1409 cm−1. Under irradiation, PBGA could change into radicals through abstracting hydrogen of −CH2 groups by the side of the carboxyl groups, that is, radical 3
Figure 3. DSC curves of the plasticized PLA with 3.7% TAIC under different irradiation doses.
shapes of DSC curves are similar, displaying the glass transition, cold crystallization, and melting peaks. As shown in Table 3, there is a decrease in crystallinity of PLA with increasing crosslinking density of PLA, because the cross-linking structure confined greatly the molecular chains of PLA. When the Table 3. Thermal Data of the Plasticized PLA with 3.7% TAIC under Different Irradiation Doses Tg (°C)
Figure 2. FTIR spectra of the multiple plasticizers after irradiation with different irradiation doses. 7275
irradiation dose (Mrad)
method of DSC
method of DMA
Tm (°C)
crystallinity (%)
0 4 6 8 10 15 20
20.0 18.3 18.0 17.9 22.6 23.5 24.6
37.5 35.3 34.3 34.1 38.2 40.5 44.2
154.3 152.1 151.0 150.2 148.1 142.4 139.3
46 42 39 38 30 25 23
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irradiation dose increased from 10 Mrad to 20 Mrad. This result sufficiently indicated that the cross-linked structure should make the multiple plasticizers more stable. The irradiation could be effective to prevent the migration of the plasticizers from material to the surface. This observation may be attributed to the cross-linking reaction between PBGA and PLA, which produce confinement by the cross-linked structure of PLA. 3.4. Mechanical Properties the Cross-linked PLA. The dynamic mechanical properties of the cross-linked PLA were determined by dynamic mechanical analysis (DMA). Figure 5
irradiation dose increased to 20 Mrad, the crystallinity of PLA fell to 23% from 46%. In addition, there is a slight decrease and then an increase in glass transition temperature with increasing cross-linking density of PLA. This observation should be attributed to the coaction of the crystallization and cross-linking of PLA. When the irradiation dose is low, the cross-linked structure does not influence the chain movement of the PLA. However, the crystallization of PLA is, to some extent, depressed. Therefore, the confinement of the amorphous PLA chains by the crystal phase is declined, and it causes a low glass transition temperature; On the other hand, with increasing cross-linking, the cross-linked structure confined the movement of polymer chains and consequently causes a higher glass transition temperature. This is to say that the increased glass transition temperature at high dose irradiation is attributed to the confinement of the polymer chains by the high density cross-linked structure. As seen from Figure 3, the melting endotherm peaks and the crystallization exotherm peaks are gradually diminishing with increasing the irradiation dose. This indicated that the crystallization of PLA becomes more and more difficult. As shown in Table 3 and Figure 3, the melting temperature of PLA has an apparent decrease with increasing irradiation dose. The melting temperature falls to 139.3 °C with a 20 Mrad irradiation dose from 154.3 °C with unirradiation. The lower melting temperature is also caused by the imperfect crystal of PLA, which resulted from the confinement of the cross-linking structure. The thermal properties of the other PLA with the different TAIC have the same change tendency with the increase of the irradiation doses. The thermal stability of the cross-linked PLA was determined by TGA. Figure 4 shows the thermal degradation and its
Figure 5. tan δ-temperature and storage-temperature curves of the cross-linked PLA with 3.7% TAIC under the different irradiation doses.
shows tan δ curves of the cross-linked PLA with 3.7% TAIC under different irradiation doses versus temperature. The αrelaxation peak of the tan δ curve could be assigned as the glass transition temperature (Tg). Not surprisingly, Tg data obtained by DMA is higher than that obtained by DSC. However, they have the similar tendency with increasing the irradiation dose. The tan δ peaks are sharp and high when the irradiation dose is low, and wide and low tan δ peaks are observed when the irradiation dose increased, which is caused by the cross-linked network. Moreover, the peaks of α-relaxation have a trivial decrease when the irradiation dose is low, and then shift to a higher temperature with increasing irradiation doses. The peak position increases from 37.5 °C with unirradiation to 44.2 °C with a 20 Mrad irradiation dose. The results may be attributed to the difficult relaxation of molecular chains restricted by the increasing cross-linking density of PLA. As shown in Figure 5, the storage modulus of the crosslinked PLA increased following the irradiation dose because of the increasing cross-linking density. In addition, the increase of storage modulus above the Tg region resulted from cold crystallization. It is also observed that the temperature of the ascending storage modulus above Tg region shifts to a higher value with increasing irradiation dose. This indicates the cold crystallization become more and more difficult because the cross-linkings prevent the molecular motion or rearrangement.33 We also found that there is not any rise in storage modulus caused by cold crystallization above Tg region when the irradiation dose is as high as 10 Mrad, which resulted from the high cross-linked density of PLA. The major function of a plasticizer is to improve the elongation at break and increase the toughness of a polymeric material. The trade-offs include reduced tensile strength. After electron beam irradiation, the tensile strength could increase because of the cross-linked structure. Therefore, we could obtain a kind of high toughness materials of PLA. Figure 6 shows the plots of tensile strength and elongation at break of
Figure 4. TGA and derivative curves of the plasticized PLA with 3.7% TAIC under different irradiation doses.
derivative curves of the cross-linked and uncross-linked PLA with 3.7% TAIC. The decomposition of the plasticized PLA with unirradiation occurred in two steps. It started to decompose at around 149.4 °C, which may correspond to the structural decomposition of the multiple plasticizers PBGA/ TAC. A higher decomposition peak around 363.1 °C was observed for the decomposition of PLA. However, the decomposition of the cross-linked PLA occurred in three steps. Besides the decomposition of the plasticizers and PLA, there was also a decomposition peak around 439.4 °C, which was assigned to the degradation of TAIC. In addition, comparing the uncross-linking PLA, a higher decomposition temperature of the multiple plasticizers was obtained with the higher irradiation dose. The decomposition temperature of the multiple plasticizers rose from 168.2 to 181.2 °C when the 7276
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Figure 6. Tensile properties of the cross-linked PLA with 3.7% TAIC under different irradiation doses.
Figure 7. Elongation at break of the blends with 3.7% TAIC before and after aging for 13 months under different irradiation doses.
the cross-linked PLA with 3.7% TAIC versus the irradiation dose. We found that there is, at first, an increase in tensile strength and then a slight decrease as the irradiation dose increased. This observation may also be attributed to the coaction of cross-linking and crystallization. On one hand, the low crystallinity gives a low tensile strength. On the other hand, the high cross-linked density gives a higher tensile strength. While the irradiation dose increase, the crystallinity of the cross-linked PLA declines and the cross-linked density increases simultaneously. Therefore, the tensile strength of the crosslinked PLA get a peak value (ca. 23.5 MPa) when the irradiation dose is 10 Mrad. Contrary to the tensile strength, the low crystallinity gives a high elongation at break and the high cross-linked density gives a lower elongation at break due to the cross-linked network constrains the mobility of molecular chains.19 The elongation at break also rests with the coaction of the cross-linking and crystallization. A higher elongation at break around 268% is obtained with 10 Mrad irradiation dose. The elongation at break of pure PLA used in this study is 3% with a tensile strenth of 57.4 MPa. So we obtained a high toughness PLA material with 10 Mrad irradiation dose, which has a elongation at break of 268% and a tensile strength of 23.5 MPa. The morphological and performance stability of polymer/ plasticizers blends is of great importance in most of the applications. PLA/plasticizers blends with 3.7% TAIC under the different irradiation doses were naturally aged at ambient temperature (approximately 20 °C) for 13 months to investigate the morphological and performance stability of the materials. After aging under those conditions, no the plasticizaers were found to migrate to the surface of the cross-linked PLA. In addition, Figure 7 compared the elongation at break of the unaged cross-linked PLA with those aged for 13 months with the different irradiation doses. When irradiation dose is low, a drastic shift to the lower value in elongation at break can be seen in Figure 7. However, the elongation at break of the PLA materials keeps nearly invariable when the irradiation dose is high, that is, over 10 Mrad. This indicated that the morphology and performance of the PLA materials are stable with a higher irradiation dose.
viscosity experiment. It indicated that molecular chains of PLA could cross-link with TAIC, and PBGA also takes part in the cross-linking reaction. The tensile study shows that PLA material with high elongation at break was obtained through combination of the plasticization and cross-linking. Investigations on the mechanical and thermal properties of PLA by TGA and the aging experiments indicate that the prepared materials with cross-linking networks had a higher stability and strength compared with the only plasticized PLA.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Tel.: +86-10-6445 5928. Fax: +86-10-6445 5928. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of the NSFC [Grant Nos. 20974011, 21004003, 21104002] and the fundamental research funds for the central universities [No. ZY1204] are gratefully acknowledged.
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REFERENCES
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