Article pubs.acs.org/IECR
Crystallization Kinetics and Morphology of Biodegradable Poly(ethylene succinate)/Octavinyl-Polyhedral Oligomeric Silsesquioxanes Nanocomposites Lu Tang and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: One of the main drawbacks of biodegradable poly(ethylene succinate) (PES) is its slow crystallization rate. We prepared three nanocomposite samples in the present work by incorporating different small amounts of octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) into the PES matrix. The morphology and dispersion of ovi-POSS were first studied with a scanning electron microscope in the PES matrix. The nanofiller showed a nice dispersion throughout the polymer matrix with a regular structure, indicating the crystalline feature. The influences of different amounts of ovi-POSS on the nonisothermal and isothermal crystallization behaviors, spherulitic morphologies, and crystal structures of the nanocomposites were extensively investigated with several techniques. Ovi-POSS obviously enhanced the crystallization behaviors of PES as an efficient nucleating agent in the nanocomposites; moreover, the nanocomposites and neat PES had the same crystal structures and crystallization mechanisms.
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INTRODUCTION As a semicrystalline polyester, poly(ethylene succinate) (PES) is one of the most promising biodegradable polymers with high heat resistance and good mechanical properties, which are comparable to those of polyethylene and polypropylene; therefore, PES may find many practical applications.1−14 The synthesis, crystal structure, melting behavior, crystallization kinetics, and biodegradation of PES have been extensively reported until now;2−14 however, one of the main drawbacks of PES is its slow crystallization rate, restricting its wider application from a polymer processing viewpoint. By the incorporation of nanofillers, we can greatly improve the crystallization behavior of PES, as they may act as nucleating agents even at a very low content. Until now, PES has already been compounded with some kinds of nanofillers to prepare the nanocomposites, such as silica, clay, multiwalled carbon nanotubes, and graphene, since the nanofillers may improve both the crystallization behaviors of PES under different crystallization conditions and its physical properties simultaneously.15−20 Polyhedral oligomeric silsesquioxanes (POSS) with formula R8Si8O12 is characterized by its typical cube-octameric framework with an inorganic cubelike core surrounded by eight organic corner groups, which makes it a novel threedimensional organic−inorganic nanofiller. The preparation, structure, properties, and application of POSS have been well investigated.21−25 Through copolymerization or physical blending, POSS may be linked to a polymer chain or incorporated into a polymer matrix.25 Relative to copolymerization, physical blending is a convenient and effective way to prepare POSS incorporated polymer nanocomposites, because there may exist hydrogen bonding or other interactions between the functional groups of POSS and the polymer matrix. After the fabrication of POSS incorporated polymer nanocomposites, many physical properties of the nano© XXXX American Chemical Society
composites have been significantly improved compared with those of neat polymers, such as enhanced glass transition temperature, mechanical properties, and thermal stability. In addition, the nucleation agent effect of POSS may appear during the crystallization process of some crystalline polymers; especially, some kinds of POSS with different functional groups are efficient nucleating agents for some biodegradable polyesters, such as poly(ε-caprolactone) (PCL), poly(butylene succinate), and poly(L-lactide) (PLLA).26−36 In previous studies, octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS), a member of the POSS family, was introduced to a series of biodegradable polymers to prepare some novel biodegradable polymer nanocomposites, showing an efficient nucleation agent effect for the crystallization processes of PLLA, PCL, and poly(ethylene succiate-coethylene adipate) (PESA), a random copolyester of PES.37−40 To our knowledge, the preparation, crystallization kinetics, and morphology of PES/ovi-POSS nanocomposites have attracted no attention until now. The interaction between nanofiller and polymer matrix plays an important role for the improvement of properties of the nanocomposites. The vinyl group of ovi-POSS may show some interaction with the carbonyl group of PES, thereby not only restricting the aggregation of ovi-POSS in the PES matrix but also enhancing the nucleation activity of oviPOSS on the crystallization process of PES. We intended not only to prepare the PES/ovi-POSS nanocomposites but also to study the nucleating agent effect of ovi-POSS on the morphology and crystallization behaviors of PES in the nanocomposites. Therefore, through a solution and casting process, a series of ovi-POSS incorporated PES nanoReceived: May 8, 2014 Revised: June 16, 2014 Accepted: June 24, 2014
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In the following section, the WAXD study further verified the crystallization of ovi-POSS in the nanocomposites at high loadings. Similar results were also found in previous work.39 The SEM image suggested both a homogeneous dispersion and the crystallization of ovi-POSS. Crystallization Behaviors of Neat PES and Its Nanocomposites. The crystallization behaviors of neat PES and the PES/ovi-POSS nanocomposites were investigated by DSC in this section. Figure 2 displays the nonisothermal crystallization
composites with small amounts of ovi-POSS were successfully prepared; furthermore, the nonisothermal melt crystallization behaviors, isothermal melt crystallization kinetics, spherulitic morphologies, and crystal structures of the PES/ovi-POSS nanocomposites were extensively investigated with various techniques. The research results are expected to be interesting and important for the wider application of biodegradable polymer nanocomposites.
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EXPERIMENTAL SECTION The PES sample (Mw = 5.3 × 104 g/mol) was synthesized by our group.13 The ovi-POSS sample was supplied by Shenyang Amwest Technology Co., Shenyang, China. In this work, a solution and casting process was applied for the preparation of the PES/ovi-POSS nanocomposites with low ovi-POSS contents from 0.25, 0.5, to 1 wt %. The detailed preparation process was similar to our previous works.39 For simplicity, the three nanocomposite samples were abbreviated as PES/ovi-POSS0.25, PES/ovi-POSS0.5, and PES/ovi-POSS1, respectively, and the number indicated the ovi-POSS content. The morphology and dispersion of ovi-POSS were observed using a JEOL JSM-7600F field emission scanning electron microscope (SEM), and the samples were all coated with gold before examination after fracturing in liquid nitrogen. A differential scanning calorimeter (DSC; TA Instruments Q100) was used to perform the thermal analysis under nitrogen purge. The flow rate of the nitrogen purge was 50 mL/min. Before any detailed thermal analysis, previous thermal history should be erased by annealing the samples for 3 min at 130 °C. The spherulitic morphologies of all of the samples were studied by an Olympus BX51 polarized optical microscope (POM) equipped with a temperature controller (Linkam THMS 600). Wide-angle X-ray diffraction (WAXD) patterns were obtained for all of the samples from a Rigaku D/Max 2500 VB2t/PC X-ray diffractometer at room temperature with a scanning rate of 5 deg/min. The samples were crystallized for 3 days at 82.5 °C.
Figure 2. DSC cooling curves at 2.5 °C/min from the melt.
exotherms at a cooling rate of 2.5 °C/min after erasing any previous thermal history for all of the samples. The crystallization peak temperature (Tp) was 63.5 °C for neat PES, and it further increased to 68.6, 69.9, and 71.8 °C for PES/ovi-POSS0.25, PES/ovi-POSS0.5, and PES/ovi-POSS1, respectively. Such results indicated that an obvious upward shift of Tp to a high temperature range could be induced by oviPOSS. The upward shift of Tp exhibited an obvious ovi-POSS content dependence in the nanocomposites. For PES/oviPOSS0.25, the increase of Tp was over 5 °C, compared with neat PES; however, with increasing the ovi-POSS content from 0.25 to 0.5 and 1 wt %, the further increments of Tp were only about 1 and 3 °C for PES/ovi-POSS0.5 and PES/ovi-POSS1, indicating an almost saturated effect. Despite the ovi-POSS content, the crystallization enthalpy values were all around 60 J/g, indicating that the crystallinity values of PES were not apparently affected by the ovi-POSS content. It is clear that oviPOSS obviously enhanced the nonisothermal melt crystallization behaviors of the nanocomposites, showing a small level of increment with the increase of ovi-POSS content and its obvious nucleation agent effect. The aforementioned study indicated that ovi-POSS favored the nonisothermal crystallization process of the nanocomposites, and DSC was also applied to investigate the isothermal crystallization kinetics of the nanocomposites and neat PES. The isothermal crystallization temperature (Tc) values were from 77.5 to 85 °C. Figure S1 in the Supporting Information illustrates the evolution of heat flow with crystallization time at 82.5 °C for all of the samples, and the crystallization time of the nanocomposites became shorter with increasing ovi-POSS loading. Figure 3 demonstrates the evolution of relative crystallinity with crystallization time at chosen Tc values. From Figure 3, the crystallization time became longer at higher Tc for all of the samples; thus, the crystallization process of PES was impeded at
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RESULTS AND DISCUSSION Distribution and Morphology of ovi-POSS in the Nanocomposites. The distribution and morphology of nanofillers in the polymer matrix is vitally important to the physical properties of the prepared nanocomposites. Figure 1 is an SEM image, showing a typical fractured surface of PES/oviPOSS1. It could be distinctly seen that ovi-POSS particles not only dispersed but also crystallized in the PES matrix with dimensions from 150 to 200 nm, exhibiting a regular structure.
Figure 1. SEM image of PES/ovi-POSS1. B
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Figure 3. Crystallization time dependence of relative crystallinity at indicated Tc values.
Figure 4. Related Avrami plots.
higher Tc values. Moreover, the PES/ovi-POSS nanocomposites needed shorter crystallization time than neat PES, and the crystallization time became shorter with the increment of oviPOSS loading. At a given Tc of 77.5 °C, the time for neat PES to finish crystallization was 24.2 min, and it dropped to 11.3, 10.8, and 7.9 min for PES/ovi-POSS0.25, PES/ovi-POSS0.5, and PES/ovi-POSS1, respectively. The introduction of ovi-
POSS into the PES matrix promoted the isothermal crystallization process, and the higher ovi-POSS content further accelerated the crystallization process. The isothermal crystallization kinetics was analyzed by the well-known Avrami equation for neat PES and its nanocomposites as follows: 1 − X t = exp( −kt n) C
(1)
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where k and n are the Avrami parameters, reflecting the crystallization rate and crystallization mechanism, respectively.41,42 The related Avrami plots of all of the samples are illustrated in Figure 4, and all of the n and k data were obtained from these almost parallel lines. All of the Avrami parameters are listed in Table 1. The average n values were around 2.7 for neat PES and its
because the nucleation process mainly controlled the whole crystallization process, and at higher Tc, the nucleation of the samples was more difficult. Moreover, the significant increment of the 1/t0.5 values between the neat sample and the nanocomposites suggested that ovi-POSS may obviously accelerate the crystallization process of its nanocomposites. As a result, ovi-POSS should play a nucleating agent role during the isothermal crystallization process of the nanocomposites. The spherulitic morphologies were further studied by POM for the nanocomposites and neat PES and are demonstrated in Figure 6. They were isothermally crystallized at 82.5 °C. From
Table 1. Summary of the Avrami Parameters sample neat PES
PES/ovi-POSS0.25
PES/ovi-POSS0.5
PES/ovi-POSS1
Tc (°C)
n
77.5 80 82.5 85 77.5 80 82.5 85 77.5 80 82.5 85 77.5 80 82.5 85
2.8 2.5 2.8 2.7 2.8 2.9 2.7 2.5 2.5 2.7 2.6 2.5 2.6 2.6 2.5 2.4
k (min‑n) 1.2 9.6 1.6 5.1 8.3 1.3 4.4 1.6 2.0 3.3 7.2 2.2 4.8 1.2 3.8 9.1
× × × × × × × × × × × × × × × ×
10−3 10−4 10−4 10−5 10−3 10−3 10−4 10−4 10−2 10−3 10−4 10−4 10−2 10−2 10−3 10−4
t0.5 (min) 9.8 13.9 20.0 34.0 4.9 8.8 15.3 28.3 4.2 7.2 14.0 24.9 2.8 4.7 8.0 15.9
Figure 6. Spherulitic morphologies of (a) neat PES, (b) PES/oviPOSS0.25, (c) PES/ovi-POSS0.5, and (d) PES/ovi-POSS1.
nanocomposites; thus, neither the loading of ovi-POSS nor the variation of Tc values influenced the crystallization mechanism. Because the n values in the unit of k were different, the crystallization rate may not be compared from the k values directly. Hence, for a direct comparison of crystallization rates, the crystallization half-time (t0.5) was utilized, which was obtained by the following equation and also summarized in Table 1:
t0.5 =
⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠
Figure 6, the spherulites of neat PES were around 100−150 μm in diameter; moreover, the clear boundaries could be easily observed between neighboring spherulites. As also shown in Figure 6, after adding more ovi-POSS in the nanocomposites, the boundaries of neighboring spherulites became obscure; furthermore, the size of them became smaller. Consequently, the nucleation density of the nanocomposites became larger, especially at higher POSS content, suggesting its efficient nucleating agent effect. Moreover, the spherulitic growth rates were determined to be around 0.54 μm/min for all of the samples. Both PES and its nanocomposites are semicrystalline polymers; therefore, it is necessary to discuss the evolution of the crystallinity of them on the basis of primary nucleation and subsequent crystal growth mechanism. The aforementioned results shown in Supporting Information Figure S1 and Figure 3 indicate that at the same Tc the degree of relative crystallinity of the nanocomposites developed faster than that of neat PES. It is clear that their increased primary nucleation values favored the faster evolution of crystallinity, because the subsequent spherulitic growth rates were almost the same. The crystal structures were further investigated for the nanocomposites, the nanofiller ovi-POSS, and the polymer matrix PES. Some strong diffraction peaks shown in Figure 7 indicated a highly crystalline structure of neat ovi-POSS. In addition, a diffraction peak of 2θ = 9.7° from ovi-POSS could even be observed in the PES/ovi-POSS0.5 and PES/ovi-POSS1 nanocomposites, revealing that ovi-POSS also crystallized in the PES matrix. Such results were consistent with the regular structure of ovi-POSS shown in the SEM image of Figure 1. For neat PES, three strong characteristic diffraction peaks appeared at 20.0°, 22.7°, and 23.4° and were ascribed to (021), (121), and (200) planes, respectively.2 The nanocomposites displayed similar diffraction peaks as neat PES, indicating that they had
(2)
Figure 5 shows the plots of 1/t0.5 versus Tc for all of the samples. As illustrated in Figure 5, 1/t0.5 decreased with increasing Tc for each sample, suggesting a slower crystallization rate at small supercooling. The results were rational,
Figure 5. Plots of 1/t0.5 versus Tc. D
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the same crystal structures. Consequently, ovi-POSS exhibited no influence on the crystal structures of the polymer matrix in the nanocomposites. From the WAXD patterns, all of the samples exhibited a similar degree of crystallinity values of around 70 ± 5%.
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CONCLUSION In this work, three different small amounts of ovi-POSS incorporated biodegradable PES nanocomposites were successfully fabricated through a solution and casting process. SEM images indicated that crystalline ovi-POSS particles were homogeneously dispersed at higher POSS content in the PES matrix. Regardless of crystallization conditions, ovi-POSS could promote the crystallization behaviors of PES without changing its crystallization mechanism. The nucleating effect of ovi-POSS was further clearly proved by the spherulitic morphology study. With increasing ovi-POSS loading, the PES spherulites became smaller in the nanocomposites, indicative of enhanced nucleation density values. Despite the ovi-POSS content, the nanocomposites and neat PES had the same crystal structures. ASSOCIATED CONTENT
S Supporting Information *
Figure showing the evolution of heat flow with crystallization time for neat PES and its nanocomposites crystallized at 82.5 °C. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 7. WAXD patterns of ovi-POSS, neat PES, and their nanocomposites.
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Article
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 are grateful to Miss Huina Wu for synthesizing the PES sample and the National Natural Science Foundation, China (Grants 51221002 and 51373020) for the financial support of this research. E
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