Research Note pubs.acs.org/IECR
Enhanced Thermal Stability and Crystallization Rate of Biodegradable Poly(butylene adipate) by a Small Amount of Octavinyl-Polyhedral Oligomeric Silsesquioxanes Shuo Huang and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Through a solution and casting procedure, a biodegradable poly(butylene adipate) (PBA)-based nanocomposite was prepared by the incorporation of 0.25 wt % of octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) as the nanofiller in the present research. On the basis of the scanning electron microscopy investigation, the ovi-POSS nanofillers not only crystallize but also disperse evenly in the PBA matrix. Compared with PBA, the thermal stability and isothermal crystallization rate of the nanocomposite are obviously improved. The accelerated crystallization process of the nanocomposite should arise from the nucleating effect of ovi-POSS, as the spherulite nucleation density has become greater in the nanocomposite than in PBA; however, the nanocomposite and PBA show the unmodified crystallization mechanisms and crystal structures, despite ovi-POSS.
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INTRODUCTION From academic and industrial viewpoints, poly(butylene adipate) (PBA) has been attracting much research interest as an important biodegradable polyester.1−25 Depending on the different crystallization conditions, PBA can crystallize in two different modifications, i.e., α- and β-form.2−4 The α-form presents a monoclinic unit cell, while the β-form is characterized by an orthorhombic unit cell.2−4 Crystallization temperature (Tc) is a decisive factor that affects which crystal modification of PBA will occur.5 When PBA is crystallized at Tc > 31 °C, it crystallizes into the pure α-form crystals. When PBA is crystallized at Tc < 29 °C, it crystallizes into the pure β-form crystals. When PBA is crystallized between 29 and 31 °C, its crystals are a mixture of two modifications. The α-form crystals have a higher equilibrium melting point than the β-form crystals; moreover, the former degrades faster than the latter.6,10 To improve the physical properties, the blending of PBA with other polymers is one of the most effective ways.12−19 For instance, PBA forms miscible blends with poly(vinylidene fluoride), poly(ethylene oxide), poly(butylene succinate), and poly(vinylphenol).12−17 On the other hand, PBA shows phase separation with poly(3-hydroxybutyrate) and poly(L-lactic acid) (PLLA).18,19 In addition, the physical properties and the crystal modifications may be adjusted by the addition of some nucleating agents or some nanofillers.20−25 The polymorphic behaviors and crystallization rates of PBA may be regulated by carbon nanotubes and uracil in previous works.23−25 In the PBA-based composites, increasing the carbon nanotubes and uracil loading favors the α-form crystals formation at faster cooling rates, which should be very meaningful from a viewpoint of practical application; moreover, the crystallization rate of the composites have been increased during the isothermal melt crystallization processes, relative to neat PBA.23−25 Polyhedral oligomeric silsesquioxanes (POSS) can usually improve the physical properties of some polymers, including © 2014 American Chemical Society
improved mechanical properties, thermal stability, reduced flammability, etc., after their incorporation into the matrix because of the inorganic−organic hybrid feature.26−29 In addition, the crystallization rates of some semicrystalline polymers, especially some biodegradable polymers, have been obviously enhanced by various types of POSS molecules.30−39 POSS may usually exist as the crystals during the crystallization process of the polymer matrix, thereby providing a number of sites for the matrix to nucleate and acting as efficient nucleating agents.30−39 Depending on the substitution group, many types of POSS molecules have been developed. Among the big POSS family, octavinyl-polyhedral oligomeric silsesquioxanes (oviPOSS) have already found many applications in biodegradable polymer/POSS nanocomposites, as the vinyl group of the nanofiller may show some interactions with the carbonyl group of the matrix.35−39 In previous works, the crystallization rates of PLLA, poly(ε-caprolactone) (PCL), poly(ethylene succinate) (PES), and poly(ethylene succinate-co-ethylene adipate) (PESA) have been increased by the addition of ovi-POSS.35−39 The preparation and properties of PBA/ovi-POSS nanocomposite have not been studied until now; however, such research should be interesting and essential, from fundamental and practical viewpoints. In this research, through a solutioncasting process, a nanocomposite sample consisting of PBA being the matrix and 0.25 wt % of ovi-POSS being the nanofiller was prepared. The objectives of this work are to explore the effects of the nanofiller on the thermal stability and crystallization behaviors in the nanocomposite. Consequently, ovi-POSS may not only enhance the thermal stability but also accelerate the crystallization process of the nanocomposite at a low loading, which, in turn, will be interesting and important for the practical end use of PBA. Received: Revised: Accepted: Published: 15296
July 13, 2014 August 31, 2014 September 15, 2014 September 15, 2014 dx.doi.org/10.1021/ie502794a | Ind. Eng. Chem. Res. 2014, 53, 15296−15300
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Research Note
EXPERIMENTAL SECTION The PBA sample was purchased from Sigma−Aldrich (Shanghai, China), which had a molecular weight (Mw) of 1.4 × 104 g/mol. The ovi-POSS sample was provided by Amwest Technology Company (Shenyang, China). The PBA/ovi-POSS nanocomposite was prepared by using a similar process, which has been described elsewhere.37 The fracture surface of the nanocomposite was examined using a scanning electron microscopy (SEM) system (Hitachi, Model S-4700). The thermal stability was performed with a TA Instruments thermogravimetric analysis device (Model Q50), and the crystallization kinetics was studied with a TA Instruments differential scanning calorimeter (Model Q100). The spherulites were observed using a polarized optical microscopy (POM) system (Olympus, Model BX51) with a hot stage (Linkam, Model THMS 600). The crystal structures were studied with an X-ray diffractometer (Rigaku Model D/Max 2500).
Figure 2. Thermal stability study of PBA and the nanocomposite.
restricted the permeation of combustion gas from the matrix.40,41 As a result, the thermal decomposition should be retarded, and the thermal stability may be improved. In the literature, POSS has also enhanced the thermal stability of some biodegradable polymers. For example, the Td values were increased from 374 °C to 382 °C after the incorporation of 2 wt % of ovi-POSS into a 70/30 PLLA/poly(butylene succinateco-adipate) (PBSA) blend.40 The Td values were increased from 340.6 °C to 347.4, 353.7, 354.5, and 363.2 °C in the presence of 3 wt % of four types of POSS molecules with different substitution groups for a 90/10 PLLA/poly(ethylene glycol) (PEG) blend.41 In brief, the thermal stability has been apparently enhanced by a small amount of ovi-POSS in the nanocomposite. The crystallization kinetics studies of biodegradable polymers are of great importance and interest, as they not only determine their crystalline structures and morphologies but also affect their final physical properties, thereby further influencing their biodegradation behaviors. The isothermal crystallization kinetics studies were performed for PBA and the nanocomposite in the present work. Figure 3 depicts the plots of relative crystallinity versus time of both of the samples during the crystallization process at indicated Tc values. The total crystallization time is increased from ∼7.3 min to 73.5 min with an increase in Tc from 39 °C to 45 °C for PBA, and in the case of the nanocomposite, it is increased from ∼5.8 min to 67.8 min with increasing Tc from 41 °C to 47 °C. For PBA and the nanocomposite, the crystallization time becomes longer with increasing Tc, indicative of a slower crystallization rate at higher Tc. At the same Tc, the crystallization time of the nanocomposite becomes shorter. For instance, at a Tc value of 43 °C, the crystallization time of the nanocomposite is ∼10 min, while that of PBA is ∼35.7 min, which suggests an accelerated crystallization process of the nanocomposite by the incorporation of ovi-POSS. In this research, the crystallization kinetics was further investigated for PBA and the nanocomposite by using the Avrami equation, which is shown as follows:
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RESULTS AND DISCUSSION The fracture surface of the nanocomposite was examined via SEM, and the SEM image is illustrated in Figure 1. It is obvious
Figure 1. Scanning electron microscopy (SEM) image of the nanocomposite.
from Figure 1 that three ovi-POSS crystals exist randomly in the PBA matrix. The three ovi-POSS crystals exhibit a regular rectangular shape, with dimensions in the range of ∼200−300 nm, indicating the formation of the crystals of ovi-POSS. The crystallization and homogeneous dispersion of ovi-POSS have been found in some other biodegradable polyesters, such as PLLA, PCL, PES, and PESA, indicative of the crystalline feature of ovi-POSS.35−39 The crystals of ovi-POSS, in turn, may influence the physical properties of the PBA matrix, including its thermal stability and crystallization behaviors. The thermal stability study of PBA and the nanocomposite is of great importance from a processing viewpoint. Figure 2 illustrates the thermogravimetric analysis results for PBA, oviPOSS, and the nanocomposite at 20 °C/min under nitrogen. All of them exhibit one-step decomposition behavior. OviPOSS has a decomposition temperature at a weight loss of 5 wt % (Td) of 218.2 °C. PBA displays a Td of ∼357.2 °C, while the nanocomposite presents a Td of ∼374.4 °C. It is obvious that the Td value of the nanocomposite has been improved by ∼17 °C in the presence of ovi-POSS. The enhanced thermal stability may be attributed to the fine dispersion of ovi-POSS in the matrix. During the thermal decomposition process, the formation of silica layers actually acted as physical barriers, which not only limited the heat flux to the matrix but also
1 − X t = exp( −kt n)
(1)
where k is the crystallization rate constant, n is the Avrami exponent, and Xt is relative crystallinity at crystallization time (t).42,43 The typical Avrami plots of both of the samples are illustrated in Figure 4. Regardless of the Tc value, each sample presents four almost-parallel lines. 15297
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Figure 3. Plots of relative crystallinity versus crystallization time for (a) PBA and (b) the nanocomposite.
Figure 4. Avrami plots for (a) PBA and (b) the nanocomposite.
Table 1. Crystallization Kinetics Parameters of PBA and the Nanocomposite
From Figure 4, the n and k values were acquired. Table 1 lists them, for comparison. The n values are between 2.1 and 2.8 for both samples, regardless of the Tc value, indicative of a possible three-dimensional truncated growth with athermal nucleation mechanism.44 The k values in Table 1 were not used for a thorough comparison of the crystallization rates of PBA and its nanocomposite in the investigated Tc range, because of the different n values in their units (min−n). The time at 50% of the final crystallinity, i.e., the crystallization half-time (t0.5), was acquired according to the following equation:
t0.5 =
⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠
(2)
Thus, 1/t0.5 (the reciprocal of t0.5) may represent the crystallization rate. Table 1 lists all the t0.5 and 1/ t0.5 values of both of the samples after isothermal crystallization. Each sample shows an increase in t0.5 and a decrease in 1/t0.5 with increasing Tc, indicating a reduction of crystallization rate at higher Tc, because of small supercooling. In addition, the nanocomposite displays a smaller t0.5 value and a greater 1/t0.5 value than PBA at the same T c , suggesting a faster crystallization rate in the nanocomposite than in PBA. The enhanced crystallization rate of the nanocomposite may arise from the nucleating effect of ovi-POSS.
Tc (°C)
n
39 41 43 45
2.1 2.3 2.1 2.1
41 43 45 47
2.8 2.7 2.7 2.6
k (min−n)
t0.5 (min)
PBA Sample 2.2 1.3 × 10−1 1.6 × 10−2 5.3 5.3 × 10−3 10.0 1.4 × 10−3 18.7 PBA/ovi-POSS Sample 8.7 × 10−2 2.1 1.7 × 10−2 4.0 1.6 × 10−3 9.6 3.5 × 10−4 19.7
1/t0.5 (min−1) 4.5 1.9 1.0 5.3
× × × ×
10−1 10−1 10−1 10−2
4.8 2.5 1.0 5.1
× × × ×
10−1 10−1 10−1 10−2
The spherulitic morphologies of PBA and the nanocomposite were further observed by POM after crystallizing at 45 °C for ∼80 and 45 min, respectively. In Figure 5a, PBA presents several spherulites of ∼100 μm in diameter, whereas, in Figure 5b, the nanocomposite exhibits several spherulites smaller than 50 μm in diameter. Figure 5 shows that the nucleation density value of the spherulites has been obviously increased in the nanocomposite, compared with that of PBA, which further favors the enhanced overall crystallization rate of the nanocomposite. The ovi-POSS crystals do not melt; thus, 15298
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ovi-POSS as the nanofiller. The scanning electron microscopy (SEM) image illustrates both the occurrence of the crystallization of octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) and its homogeneous dispersion. The thermal stabilities and crystallization behaviors were studied for both PBA and the nanocomposite to explore the effect of ovi-POSS. The thermal stability of the nanocomposite has been obviously enhanced, as evidenced by an increase of ∼17 °C in the decomposition temperature than PBA. The nanocomposite and PBA show the same crystallization mechanism, regardless of ovi-POSS and crystallization temperature. The crystallization rates of both of them decrease with increasing crystallization temperature; however, the nanocomposite crystallizes faster than PBA at the same crystallization temperature, indicating the nucleating effect of ovi-POSS. The spherulitic morphology study reveals the spherulites nucleation density of the nanocomposite is greater than that of PBA. In addition, PBA and the nanocomposite present the same crystal structures and similar crystallinity values.
Figure 5. Spherulites morphology of (a) PBA and (b) the nanocomposite.
they may act as the nucleating agent for PBA during the crystallization process of the nanocomposite, thereby resulting in a higher nucleation density and a faster crystallization rate. The possible nucleation mechanism of ovi-POSS for the crystallization of PBA was further discussed in this work according to their crystal structures. Ovi-POSS has a rhombohedral structure with dimensions of a = b = 1.353 nm, c = 1.422 nm, and β = 120°,45 while PBA has a monoclinic structure with dimensions of a = 0.673 nm, b = 0.794 nm, c = 1.420 nm, and β = 45.5° for the α-form crystals.2,3 It is clear that ovi-POSS and PBA have almost the same c-axis values (∼1.42 nm); therefore, an epitaxial nucleation mechanism may exist between the two components, because of the good matching. The crystal structures of PBA and the nanocomposite were further investigated. Figure 6 demonstrates the wide-angle X-
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-10-64413161. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation, China (Nos. 51373020 and 51221002) for the support of this work.
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REFERENCES
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Figure 6. Crystal structures study of PBA and the nanocomposite.
ray diffraction (WAXD) profiles of PBA and the nanocomposite after complete crystallization at 45 °C. Under this crystallization condition, they should crystallize in α-form. In Figure 6, the three characteristic peaks are present at 2θ = 21.62°, 22.36°, and 23.94° for PBA, corresponding to the (110), (020), and (021) planes of the α-form crystals.4 The three diffraction peaks shift slightly to 2θ = 21.70°, 22.42°, and 24.06° for the nanocomposite, indicating a slight variation of the lattice parameters of the unit cell. It is obvious that the nanocomposite and PBA have the same crystal structures. The crystallinity values were determined to be ∼50% ± 5% for both samples from the WAXD patterns.
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CONCLUSIONS In this research, we prepared a nanocomposite consisting of biodegradable poly(butylene adipate) (PBA) as the matrix and 15299
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