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It is found that a fine dispersion of ovi-POSS has been achieved in the PESA matrix. The effects of ovi-POSS on the nonisothermal melt and cold crysta...
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Effect of Low Octavinyl-Polyhedral Oligomeric Silsesquioxanes Loading on the Crystallization Kinetics and Morphology of Biodegradable Poly(ethylene succinate-co-5.1 mol % ethylene adipate) as an Efficient Nucleating Agent Kai Chen, Jing Yu, 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, People’s Republic of China ABSTRACT: In this work, a biodegradable polymer composite was prepared by mixing poly(ethylene succinate-co-5.1 mol % ethylene adipate) (PESA) and octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) at a very low ovi-POSS loading. It is found that a fine dispersion of ovi-POSS has been achieved in the PESA matrix. The effects of ovi-POSS on the nonisothermal melt and cold crystallization behaviors, overall isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure of PESA in the composite were studied in detail with various techniques. Relative to neat PESA, both the nonisothermal melt and cold crystallization behaviors and isothermal melt crystallization kinetics of PESA have been enhanced significantly in the composite. Ovi-POSS is an efficient nucleating agent for the crystallization of PESA, because the nucleation density of PESA spherulites is apparently higher in the composite than in neat PESA; however, the crystallization mechanism and crystal structure of PESA remain unchanged in the composite.

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INTRODUCTION As a commercially available chemosynthetic biodegradable polyester, poly(ethylene succinate) (PES) has recently received considerable attention.1−5 PES has a chemical structure of (OCH2CH2O2CCH2CH2CO)n. Until now, much research has been devoted to the study of PES, including its crystal structure, crystal polymorphism, crystallization kinetics, spherulitic morphology and growth kinetics, melting behavior, enzymatic degradation, and thermal degradation.3−15 In addition, some novel biodegradable PES-based copolyesters have also recently been developed via chemical copolymerization, such as poly(ethylene succinate-co-butylene succinate) (P(ES-co-BS)), poly(ethylene succinate-co-trimethylene succinate) (P(ES-co-PS)), poly(ethylene succinate-co-ethylene adipate) (P(ES-co-EA)), etc.16−21 In previous work, we prepared biodegradable PES and its novel copolyesters P(ES-co-EA) with the ethylene adipate (EA) comonomer composition ranging from 5.1 mol % to 15.3 mol %, and we studied the effect of different EA content on the basic thermal behaviors, crystallization behaviors, spherulitic morphology, and crystal structure of P(ES-co-EA).21 Relative to neat PES, the crystallization rate of P(ES-co-EA) was reduced, apparently because of the incorporation of the EA content. From the viewpoint of polymer processing, a relatively slow crystallization rate is not helpful to the wider application of these novel biodegradable copolyesters. However, among these novel P(ESco-EA) copolyesters, poly(ethylene succinate-co-5.1 mol % ethylene adipate) (PESA) has the highest melting point and fastest crystallization rate, because of its lowest EA content; therefore, similar to neat PES, PESA may find some more practical application fields than the other copolyesters. Polyhedral oligomeric silsesquioxanes (POSS) are a family of novel nanofillers, which have a structure of cube-octameric © 2013 American Chemical Society

frameworks consisting of an inorganic cube-like core and eight organic corner groups.22−24 Through physical blending or copolymerization, the incorporation of POSS molecules into some polymer matrixes has been achieved; moreover, many physical properties of polymeric materials have been improved by the presence of POSS, such as increased thermal stability and mechanical properties.25−29 It is interesting to note that POSS can also behave as nucleating agent for the crystallization of some biodegradable semicrystalline polymers and accelerate the crystallization process in some cases.30−36 For instance, the crystallization rates of poly(L-lactide) (PLLA) and poly(εcaprolactone) (PCL) have been enhanced by the presence of various contents of octaisobutyl-polyhedral oligomeric silsesquioxanes (oib-POSS), indicating that oib-POSS acts as nucleating agent for the crystallization of PLLA and PCL.30−32 In addition, both the nonisothermal melt crystallization peak temperature and isothermal melt crystallization rate of PLLA have been enhanced by the presence of octamethyl-polyhedral oligomeric silsesquioxanes (omePOSS) at different ome-POSS loadings.33 In addition to oib-POSS and ome-POSS, octavinylpolyhedral oligomeric silsesquioxanes (ovi-POSS) was also often used as nanofiller to prepare the POSS-based biodegradable polymer nanocomposites.34−36 In previous works, we prepared PLLA/ovi-POSS and PCL/ovi-POSS nanocomposites at low ovi-POSS loadings, and we found that the crystallization processes of PLLA and PCL have been accelerated by the presence of ovi-POSS, suggesting that oviReceived: Revised: Accepted: Published: 1769

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POSS is an effective nucleating agent for PLLA and PCL.34−36 It is very interesting to investigate whether ovi-POSS is an efficient nucleating agent for the crystallization of P(ES-co-EA), because a slow crystallization rate must influence the wider practical application of these novel biodegradable copolyesters. To our knowledge, such research has not been reported so far. In this research note, a biodegradable PESA/ovi-POSS composite was prepared via a solution and casting method at a very low ovi-POSS loading; furthermore, the effects of oviPOSS on the nonisothermal melt and cold crystallization behaviors, overall isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure of PESA in the composite were investigated with various techniques. It is expected that the research reported herein will be of interest and help for a better understanding of the structure and property relationship of biodegradable composites, from both academic and practical viewpoints.

Research Note

RESULTS AND DISCUSSION Dispersion of Ovi-POSS in the PESA matrix. It is of great importance to study the dispersion of ovi-POSS in the PESA matrix, which will influence the physical properties of biodegradable polymers. Therefore, the dispersion of ovi-POSS in the PESA matrix was first investigated via SEM in this work. Figure 1 shows an overview of the fracture surface of PESA/

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EXPERIMENTAL SECTION PESA (Mw = 4.60 × 104 g/mol) was synthesized in our laboratory,21 and ovi-POSS was purchased from Shenyang Amwest Technology Company, China. Neat PESA and a PESA/ovi-POSS composite containing 0.1 wt % content of ovi-POSS were prepared through a solution and casting method, using chloroform as the solvent.34−36 For the sake of brevity, the PESA/ovi-POSS composite is abbreviated as PESA/POSS0.1 hereafter in this work. The fracture morphology of PESA/POSS0.1 was observed using a Hitachi Model S-4700 scanning electron microscopy (SEM) system. The specimen was coated with gold before examination. The nonisothermal melt and cold crystallization behaviors and overall isothermal melt crystallization kinetics of neat PESA and the PESA/POSS0.1 composite were studied with a TA Instruments Model Q100 differential scanning calorimetry (DSC) device with a Universal Analysis 2000 system. For the nonisothermal melt crystallization behavior study, the samples were heated to 140 °C at 20 °C/min, held for 3 min to erase any thermal history, and then cooled from the melt at a cooling rate of 5 °C/min. For the nonisothermal cold crystallization behavior study, the samples were heated to 140 °C at 20 °C/ min, held for 3 min to erase any thermal history, quenched to −60 °C at a cooling rate of 60 °C/min, and heated to the melt again at 10 °C/min. For the isothermal melt crystallization kinetics study, the samples were heated to 140 °C at 20 °C/ min, held for 3 min to erase any thermal history, cooled to the desired crystallization temperature (Tc) at 40 °C/min, and held for a period of time until the isothermal crystallization was complete. The exothermal traces were recorded for the later data analysis. The spherulitic morphology of neat PESA and PESA/ POSS0.1 was investigated with a polarized optical microscopy (POM) system (Olympus Model BX51) equipped with a firstorder retardation plate and a temperature controller (Linkam, Model THMS600). The samples were first annealed at 140 °C for 3 min to erase any thermal history and then cooled to 64 °C at 60 °C/min. Wide-angle X-ray diffraction (WAXD) experiments were performed on a Rigaku Model D/Max 2500 VB2t/PC X-ray diffractometer at 40 kV and 200 mA in the range of 5°−40° at a scanning rate of 5°/min. The samples were first pressed into films on a hot stage at 140 °C and then transferred into a vacuum oven at 64 °C for 3 days.

Figure 1. SEM image showing an overall morphology of fracture surface for PESA/POSS0.1.

POSS0.1. As shown in Figure 1, some ovi-POSS particles are randomly dispersed within the PESA matrix; moreover, the dimensions of these particles range from 200 nm to 400 nm. The formation of submicrometer aggregates of ovi-POSS particles indicates that a fine dispersion of ovi-POSS has been achieved in the PESA matrix. Similar results have also recently been found in the PCL/ovi-POSS nanocomposites.36 Effect of ovi-POSS on the Nonisothermal Melt and Cold Crystallization Behaviors and Overall Isothermal Melt Crystallization Kinetics of PESA in the Composite. It is of great interest to investigate the crystallization behavior of biodegradable polymers, which affects not only the crystalline structure and morphology but also their final physical properties and biodegradation. As introduced in the Experimental Section, the effect of ovi-POSS on the nonisothermal melt and cold crystallization behaviors of PESA was first studied with DSC. Figure 2a shows the nonisothermal melt crystallization behavior at a cooling rate of 5 °C/min for neat PESA and PESA/POSS0.1. As shown in Figure 2a, neat PESA has a melt crystallization peak temperature (Tp) of 34.7 °C with a melt crystallization enthalpy (ΔHc) of 6.7 J/g. For the PESA/ POSS0.1 sample, it shows a Tp value of 51.7 °C with ΔHc = 60.5 J/g. It is clear that the values of both Tp and ΔHc are significantly greater in PESA/POSS0.1 than in neat PESA. The nonisothermal melt crystallization of PESA has been enhanced apparently by the presence of ovi-POSS, even at very low loading, indicating that ovi-POSS acts as an efficient nucleating agent for the crystallization of PESA. The nonisothermal cold crystallization behaviors of neat PESA and PESA/ovi-POSS were also studied with DSC. It should be noted that neat PESA did not crystallize and reached the completely amorphous state, while PESA/POSS0.1 crystallized during the quenching process, even if a fast cooling rate of 60 °C/min was used in this work, because of the nucleating agent effect of ovi-POSS. Figure 2b shows the nonisothermal cold crystallization behaviors at a heating rate of 10 °C/min after quenching from the melt at a cooling rate of 60 °C/min for neat PESA and PESA/POSS0.1. As shown in Figure 2b, neat PESA has a cold 1770

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Figure 2. (a) Nonisothermal melt crystallization behaviors at a cooling rate of 5 °C/min and (b) nonisothermal cold crystallization behaviors at a heating rate of 10 °C/min after quenching from the melt at a cooling rate of 60 °C/min for neat PESA and PESA/POSS0.1.

Figure 3. Development of relative degree of crystallinity with crystallization time for (a) neat PESA and (b) PESA/POSS0.1 at different Tc values.

Figure 4. Avrami plots of (a) neat PESA and (b) PESA/POSS0.1.

crystallization peak temperature (Tch) of 39.1 °C with a cold crystallization enthalpy (ΔHch) of 49.0 J/g. For the PESA/ POSS0.1 sample, it shows a Tch at 25.6 °C with a ΔHch of 18.2 J/g. The Tch values are apparently smaller in PESA/POSS0.1 than in neat PESA, suggesting that the nonisothermal cold crystallization of PESA has been enhanced, obviously because of the efficient nucleating agent of ovi-POSS for the crystallization of PESA. The effect of ovi-POSS on the overall isothermal melt crystallization kinetics of PESA was further studied with DSC. Figure 3 illustrates the development of relative degree of crystallinity with crystallization time for neat PESA and PESA/

POSS0.1. The crystallization time is found to prolong with increasing Tc for both neat PESA and PESA/POSS0.1, indicating that the crystallization is retarded at higher Tc, because of small supercooling. In addition, the corresponding crystallization time becomes shorter in PESA/POSS0.1 than in neat PESA at the same Tc value. For instance, it took neat PESA ∼72 min to finish crystallization at 67 °C, but for the PESA/POSS0.1 sample, the time required to finish crystallization was only ∼6 min. It is obvious that the presence of oviPOSS has significantly accelerated the isothermal melt crystallization of PESA in the composite. 1771

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PESA/POSS0.1 at different Tc values for comparison. As shown in Table 1, the t0.5 values increase as Tc increases for both neat PESA and PESA/POSS0.1; moreover, the t0.5 values are smaller in PESA/POSS0.1 than in neat PESA at the same Tc value. Figure 5 shows the Tc dependence of 1/t0.5 for neat PESA and PESA/POSS0.1. It is clear from Figure 5 that the 1/t0.5

The overall isothermal melt crystallization kinetics was further analyzed by the well-known Avrami equation for both neat PESA and PESA/POSS0.1. For the Avrami equation, the relative degree of crystallinity (Xt) develops with crystallization time (t) as 1 − X t = exp( −kt n)

(1)

where n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is a composite rate constant involving both nucleation and growth rate parameters.37,38 Figures 4a and 4b illustrate the related Avrami plots for neat PESA and PESA/POSS0.1, respectively. As shown in Figure 4, a serials of almost parallel lines were obtained for both neat PESA and PESA/POSS0.1, indicating that the Avrami equation is suitable to describe the overall isothermal melt crystallization process. Therefore, the n and k values may be calculated from the slopes and intercepts of the Avrami plots, respectively. The Avrami parameters are summarized in Table 1 for neat PESA and its composite, crystallized at different Tc values. For Table 1. Summary of the Isothermal Melt Crystallization Kinetics Parameters of Neat PESA and PESA/POSS0.1 at Different Tc Values Based on the Avrami Equation Neat PESA Tc (°C)

n

58 61 64 67

2.2 2.2 2.3 2.3

k (min−n) 6.31 2.00 7.94 1.58

× × × ×

10−3 10−3 10−4 10−4

Figure 5. Tc dependence of 1/t0.5 for neat PESA and PESA/POSS0.1.

values decrease as the Tc values increase for the two samples, indicating that the overall crystallization rates are reduced at higher Tc, because of small supercooling. Moreover, the 1/t0.5 values are apparently greater in PESA/POSS0.1 than in neat PESA at a given Tc value of 64 or 67 °C, indicating again that the addition of ovi-POSS has accelerated the crystallization of PESA in the composite as an efficient nucleating agent. Effect of ovi-POSS on the Spherulitic Morphology and Crystal Structure of PESA in the Composite. To study the crystallization mechanism and the effect of ovi-POSS on the crystalline morphology of PESA, POM was used to observe the crystalline morphology of neat PESA and PESA/POSS0.1. Figure 6 displays the spherulitic morphology of neat PESA and

PESA/POSS0.1 t0.5 (min)

Tc (°C)

n

8.5 12.7 19.0 28.6

64 67 70 73

2.6 2.7 2.6 2.7

k (min−n) 1.58 3.98 2.00 2.51

× × × ×

10−1 10−2 10−2 10−3

t0.5 (min) 1.8 2.9 3.9 8.0

neat PESA, the n values are varied between 2.2 and 2.3, despite the Tc value. For PESA/POSS0.1, the n values are varied between 2.6 and 2.7, despite the Tc value. For each of the samples, the n values remain almost unchanged at different Tc values, indicating that the crystallization mechanism does not change, despite the variation of Tc. It should be noted that it will be questionable to deduce the crystallization mechanism based only on the n values. In the following section, the crystallization mechanism of both neat PESA and PESA/ POSS0.1 will be discussed further, based on the crystalline morphology study by POM. It is also of interest to investigate the effect of ovi-POSS on the overall isothermal melt crystallization rate of PESA in the composite; however, it is not acceptable to compare the overall crystallization rate directly from the k values, because the unit of k is min−n, and n is not constant for neat PESA and PESA/ POSS0.1 at different Tc values. Thus, the crystallization half-life time (t0.5) is used to discuss the crystallization kinetics of neat PESA and PESA/POSS0.1 in the present work. (The crystallization half-life time is the time required to achieve 50% of the final crystallinity of the samples.] The t0.5 value can be calculated by the following equation:

t0.5 =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

Figure 6. Spherulitic morphologies of (a) neat PESA and (b) PESA/ POSS0.1 crystallized isothermally at 64 °C. (The calibration bar is same for both images.)

PESA/POSS0.1 isothermally crystallized at 64 °C as an example. As shown in Figure 6, the size of PESA spherulites becomes apparently smaller in PESA/POSS0.1 than in neat PESA. For PESA/POSS0.1, the diameter of PESA spherulites is only ∼100 μm; however, the diameter of spherulites is ∼400− 500 μm for neat PESA. In addition, the number of PESA spherulites is significantly greater in the composite than in neat PESA, suggesting that the presence of ovi-POSS may provide more nucleation sites as an efficient nucleating agent for the crystallization of PESA. Similar results have also recently been found in the PCL/ovi-POSS nanocomposites.36 It should also be noted that new PESA spherulites started growing

(2)

where n and k have the same meanings in the Avrami equation. The overall isothermal melt crystallization rate can thus be described by the reciprocal of t0.5, i.e., 1/t0.5. The t0.5 values were calculated and are also listed in Table 1 for neat PESA and 1772

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peaks cannot be observed for PESA/POSS0.1, because the oviPOSS content is only 0.1 wt % in this work.

throughout the crystallization, and resulted in spherulites of varying size for both neat PESA and PESA/POSS0.1, indicative of thermal nucleation.39 Based on the POM study, the crystallization mechanism of PESA may correspond to spherulitic growth with thermal nucleation for both neat PESA and PESA/ovi-POSS, regardless of the presence of oviPOSS.39 In the above sections, the effects of ovi-POSS on the nonisothermal melt crystallization behavior, overall isothermal melt crystallization kinetics, and spherulitic morphology of PESA were investigated with DSC and POM. It is found that the nonisothermal melt crystallization peak temperature and crystallization enthalpy, overall isothermal melt crystallization rate, and spherulites density of PESA have been enhanced apparently in PESA/POSS0.1 compared with neat PESA, indicating that ovi-POSS may act as an efficient nucleating agent for the crystallization of PESA in the composite, even at a low loading (0.1 wt %). However, note that the crystallization kinetics and morphology of PESA must be influenced by different ovi-POSS loadings. Generally, below a critical loading, the nucleation density will increase as the ovi-POSS loading increases; however, the nucleation density will level off or even decrease with further increases in the ovi-POSS loading when it reaches the critical value. The effect of concentration of oviPOSS on the nucleation of PESA will be clarified in the forthcoming research. The exact nucleation mechanism of oviPOSS for the crystallization of PESA is still not clear at present. There may be some relationship between the POSS structure and polymer structure. For instance, one possibility is that there is somewhat of an interaction between the vinyl group of oviPOSS and the carbonyl group of PESA. Further research is currently underway and will be reported in the forthcoming work. The influence of ovi-POSS on the crystal structure of PESA in the composite was also studied in this work. As introduced in the Experimental Section, the crystal structures were studied via WAXD for both neat PESA and PESA/POSS0.1. Figure 7

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CONCLUSIONS A novel biodegradable PESA/ovi-POSS composite was prepared via a solution and casting method in this work at a very low content of ovi-POSS. The SEM observation displays a fine dispersion of ovi-POSS in the PESA matrix. The nonisothermal melt and cold crystallization behaviors, overall isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure of PESA in the composite were studied and compared with those of neat PESA. During the nonisothermal melt crystallization, the melt crystallization peak temperature of PESA was increased to 51.7 °C with a crystallization enthalpy of 60.5 J/g for the composite from 34.7 °C with a melt crystallization enthalpy of 6.7 J/g for neat PESA at 5 °C/min. During the nonisothermal cold crystallization, the cold crystallization peak temperature of PESA was shifted downward to 25.6 °C with a cold crystallization enthalpy of 18.2 J/g for the composite from 39.1 °C with a cold crystallization enthalpy of 49.0 J/g for neat PESA. The overall isothermal melt crystallization rates of PESA are faster in the composite than in neat PESA at the same crystallization temperature. Both the nonisothermal melt and cold crystallization behaviors and isothermal melt crystallization kinetics of PESA have been improved apparently in the composite, with respect to neat PESA, indicating that ovi-POSS acts as an efficient nucleating agent for the crystallization of PESA, even at a very low content. The spherulitic morphology study reveals that the nucleation density of PESA spherulites is greater in the composite than in neat PESA, again indicative of the nucleating agent effect of ovi-POSS. However, the crystallization mechanism and crystal structure of PESA have not changed, despite the presence of ovi-POSS in the composite.

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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 Part of this research was financially supported by the National Natural Science Foundation, China (Grant No. 51221002).

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REFERENCES

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Figure 7. WAXD patterns of neat PESA and PESA/POSS0.1.

displays the WAXD patterns of neat PESA and PESA/POSS0.1. As shown in Figure 7, three main diffraction peaks are observed, at ∼20.1°, 22.7°, and 23.2°, corresponding to (021), (121), and (200) planes, respectively, for neat PESA and its composite.5,6 Figure 7 clearly shows that both the samples present the main diffraction peaks at almost the same locations, indicating that the crystal structures of PESA are not altered by the presence of ovi-POSS in the composite. Pure ovi-POSS is highly crystalline, and the diffraction peaks corresponding to ovi-POSS may be found for the PLLA/ovi-POSS and PCL/oviPOSS nanocomposites, because the ovi-POSS loading is ≥0.5 wt % in the polymer matrix.34−36 However, such diffraction 1773

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Research Note

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