Isothermal Crystallization Kinetics, Morphology, and Dynamic

Feb 6, 2012 - Scanning electron microscopy observation reveals not only the fine dispersion but also the crystallization of ovi-POSS in the PCL matrix...
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Isothermal Crystallization Kinetics, Morphology, and Dynamic Mechanical Properties of Biodegradable Poly(ε-caprolactone) and Octavinyl−Polyhedral Oligomeric Silsesquioxanes Nanocomposites Wen Guan 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: Biodegradable poly(ε-caprolactone) (PCL) and octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) nanocomposites were prepared at low ovi-POSS loadings via a solution casting method in this work. Scanning electron microscopy observation reveals not only the fine dispersion but also the crystallization of ovi-POSS in the PCL matrix. The overall isothermal melt crystallization kinetics, spherulitic morphology, crystal structure, and dynamic mechanical properties of neat PCL and its nanocomposites were studied with various techniques. The presence of ovi-POSS has enhanced the overall isothermal melt crystallization rates of PCL in the nanocomposites; however, the crystallization mechanism and crystal structure of PCL remain unchanged despite the ovi-POSS loading. In addition, the storage modulus of the nanocomposites has been enhanced significantly relative to neat PCL, while the glass transition temperature varies slightly irrespective of the presence of ovi-POSS.

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INTRODUCTION Poly(ε-caprolactone) (PCL) has recently received more and more attention as one of the biodegradable and biocompatible aliphatic polyesters from both academic and practical viewpoints.1−5 It should be noted that some disadvantages such as slow crystallization rate and poor mechanical properties have limited its wider practical application. The incorporation of nanofillers has been proven to be an efficient way of improving its physical properties and extending its practical end use. Till now, some nanofillers have been utilized to prepare the PCLbased biodegradable nanocomposites, including layered silicate, carbon nanotubes, graphite oxide, and graphene.6−11 It is interesting to find that quite a small amount of nanofillers may achieve a significant improvement of physical properties of the PCL matrix, thereby promoting its wider application. As a novel type of three-dimensional nanofiller, polyhedral oligomeric silsesquioxanes (POSS) have a structure of cubeoctameric frameworks consisting of an inorganic cube-like core and eight organic corner groups.12−15 POSS molecules may be incorporated into some polymer matrixes either by copolymerization or physical blending;16−27 consequently, enhancements on polymer properties may be achieved, such as increased thermal stability, mechanical properties, and reduced flammability, etc. In some cases, POSS may also act as nucleating agent for the crystallization of semicrystalline polymers, thereby accelerating the crystallization process.18−22 Few works have dealt with the preparation, structure, and properties of PCL/POSS nanocomposites.23−27 Most of the PCL/POSS nanocomposites were prepared via a copolymerization method; therefore, a fine dispersion of POSS may be achieved in the polymer matrix.23−26 In a previous work, we prepared the PCL and octaisobutyl-polyhedral oligomeric silsesquioxanes (oib-POSS) nanocomposites via a solution and casting method, and found that the crystallization of PCL was enhanced by the presence of oib-POSS.27 Moreover, we have © 2012 American Chemical Society

recently found that octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) may enhance the crystallization of poly(L-lactide) (PLLA) by acting as nucleating agent.21 To our knowledge, biodegradable PCL and ovi-POSS nanocomposites have not been reported so far in the literatures. In this note, the PCL/ovi-POSS nanocomposites at low ovi-POSS loadings were prepared via a solution mixing method; moreover, the effect of ovi-POSS on the overall isothermal melt crystallization kinetics, spherulitic morphology, crystal structure, and dynamic mechanical properties of PCL in the nanocomposites was investigated with various techniques. The aim of this work is to provide a better understanding of the structure and properties relationship of biodegradable polymer nanocomposites from both academic and practical viewpoints.

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EXPERIMENTAL SECTION Materials and Preparation of PCL/POSS Nanocomposites. PCL (Mn = 8.0 × 104 g/mol) and ovi-POSS were both purchased from Sigma-Aldrich (Shanghai) Trading Co. Ltd. The PCL/ovi-POSS nanocomposites with 0.5 and 2 wt % ovi-POSS were prepared via a solution and casting method. Both PCL and ovi-POSS were first dissolved in chloroform separately. Then they were mixed together and stirred for 6 h. The mixture was poured into a dish to evaporate the solvent at room temperature for 24 h. The obtained film was further kept in a vacuum at 50 °C for 3 days to remove the solvent completely. For comparison, the neat PCL film was obtained in the same way. The nanocomposites containing 0.5 and 2 wt % oviPOSS are abbreviated as POSS-0.5 and POSS-2, respectively, from now on for brevity. Received: Revised: Accepted: Published: 3203

December 1, 2011 January 17, 2012 February 6, 2012 February 6, 2012 dx.doi.org/10.1021/ie202802d | Ind. Eng. Chem. Res. 2012, 51, 3203−3208

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Characterizations. A Hitachi S-4700 scanning electron microscope (SEM) was used to observe the morphology of the fractured surfaces of the PCL/POSS nanocomposites. The samples were pressed into film at 100 °C and fractured in liquid nitrogen. All the samples were coated with gold before examination. Thermal analysis was carried out using a TA Instruments differential scanning calorimetry (DSC) Q100 with a Universal Analysis 2000. All the operations were performed under nitrogen purge. The weight varied between 4 and 6 mg for all the samples. For the overall isothermal melt crystallization kinetics study, the samples were heated to 100 at 40 °C/min, held for 3 min to erase any thermal history, cooled to the desired crystallization temperature (Tc) at 60 °C/min, and held for a certain period of time until the crystallization was completed. Spherulitic morphology of neat PCL and its nanocomposites was studied with a polarized optical microscopy (POM) (Olympus BX51) equipped with a Linkam THMS 600 hot stage. The samples were first heated to 100 at 40 °C/min, held there for 3 min to erase any thermal history, and then cooled at 60 °C/min to 44 °C. Wide-angle X-ray diffraction (WAXD) experiments were performed on a Rigaku D/Max 2500 VB2t/PC X-ray diffractometer in the range of 5−40° with a scanning rate of 5°/min at room temperature. The CuKα radiation (λ = 0.15418 nm) source was operated at 40 kV and 200 mA. The samples were first pressed into films with a thickness of around 0.6 mm on a hot stage at 100 °C and then crystallized at 44 °C for 3 days in a vacuum oven. Dynamic mechanical analysis (DMA) measurements of neat PCL and the PCL/ovi-POSS nanocomposites were investigated on the samples of 10 mm × 5 mm × 0.2 mm in size with a dynamic mechanical analyzer from Rheometric Scientific Company in a temperature range of −90 to 40 °C at a heating rate of 3 °C/min under tension film mode with a frequency of 1 Hz.

in the following section also confirms that ovi-POSS may crystallize in the nanocomposites despite the ovi-POSS loading. In brief, the SEM observation reveals not only the fine dispersion but also the crystallization of ovi-POSS in the PCL matrix Isothermal Melt Crystallization Kinetics of Neat PCL and Its Nanocomposites. It is essential to study the crystallization behavior of biodegradable polymers, which affects the crystalline structure and morphology of semicrystalline polymers and thereby influences further their final physical properties and biodegradation. As introduced in the Experimental section, the overall isothermal melt crystallization kinetics of neat PCL and its nanocomposites were studied with DSC at different crystallization temperatures. Parts a and b of Figure 2 illustrate the development of relative crystallinity with crystallization time for both neat PCL and POSS-2 at different crystallization temperatures ranging from 36 to 42 °C, respectively, as examples. As shown in Figure 2, the crystallization time is found to prolong with increasing Tc for both the samples, indicating that the crystallization is retarded at high Tc. In addition, the crystallization time becomes shorter in the nanocomposite than in neat PCL. For example, the time required to finish crystallization is around 80 min for neat PCL at a given Tc of 42 °C; however, in the case of POSS-2, the crystallization time is reduced significantly to around 60 min at the same Tc. Such results clearly indicate that the presence of POSS has accelerated the isothermal melt crystallization of PCL apparently in the PCL/ovi-POSS nanocomposites. The overall isothermal melt crystallization kinetics of neat PCL and its two nanocomposites with different ovi-POSS loadings were further analyzed by the well-known Avrami equation. The Avrami equation is as follows:

RESULTS AND DISCUSSION Dispersion and Morphology of ovi-POSS in the PCL Matrix. In this note, the dispersion of ovi-POSS in the PCL matrix was studied first with SEM on the fractured surfaces of the nanocomposites. For example, Figure 1 illustrates an overview

where Xt is the relative crystallinity at crystallization time t, n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is crystallization rate constant involving both nucleation and growth rate parameters.28,29 Figure 3 shows the related Avrami plots for neat PCL and POSS-2, from which a serials of almost parallel lines are obtained. It is obvious from Figure 3 that the overall isothermal melt crystallization process of neat PCL and its nanocomposites can be described very well by the Avrami equation. From Figure 3, the n and k values can be obtained from the slopes and intercepts of the Avrami plots, respectively. They are listed in Table 1 for both neat PCL and its two nanocomposites at different Tc values for comparison. Table 1 clearly shows that the values of n are varied between 2.2 and 2.5 for neat PCL and its two nanocomposites within the crystallization temperature range investigated in this work, suggesting that the crystallization mechanism of PCL may correspond to a threedimensional truncated growth with heterogeneous nucleation.30 The almost unchanged n values indicate that the crystallization mechanism may not change for neat PCL and the PCL/oviPOSS nanocomposites despite crystallization temperature and the ovi-POSS loading. It is also interesting to investigate the effects of Tc and the ovi-POSS loading on the overall isothermal crystallization rate of PCL in the nanocomposites. However, it is not suitable to compare the overall crystallization rate from the k values directly, because the unit of k is min−n and n is not constant. Therefore, the crystallization half-time (t0.5), the time required to achieve 50% of the final crystallinity of the samples,

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1 − X t = exp( −kt n)

Figure 1. SEM image of POSS-0.5.

on the fractured surface of POSS-0.5. As shown in Figure 1, oviPOSS are randomly dispersed in the PCL matrix. Moreover, it is interesting to find that ovi-POSS actually crystallize in the PCL matrix, showing regular cubic crystals with dimensions ranging from 80−120 nm. Similar morphology was also found for POSS-2. For brevity, the result is not shown. The WAXD study 3204

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Figure 2. Development of relative crystallinity with crystallization time at selected crystallization temperatures for (a) neat PCL and (b) POSS-2.

Figure 3. Avrami plots for (a) neat PCL and (b) POSS-2.

values for both neat PCL and its two nanocomposites with different ovi-POSS loadings at different Tcs. It is clear from Table 1 that 1/t0.5 decreases while t0.5 increases with increasing Tc for all the three samples, indicating a slow-down of the overall isothermal crystallization rate at higher Tc. The reduction of the overall isothermal melt crystallization rate is reasonable because the crystallization kinetics is controlled by the nucleation process in the present work, which becomes more difficult with increasing Tc. The effect of ovi-POSS loading on the overall isothermal melt crystallization rate of PCL was further investigated. Table 1 obviously shows that, at a given Tc, 1/t0.5 is greater in the nanocomposites than in neat PCL; moreover, 1/t0.5 is greater in POSS-2 than in POSS-0.5. Therefore, it can be concluded that the overall isothermal melt crystallization process of PCL has been accelerated gradually in the nanocomposites with an increase in the ovi-POSS loading because of the heterogeneous nucleation effect. Effect of ovi-POSS on the Spherulitic Morphology and Crystal Structure of PCL in the PCL/ovi-POSS Nanocomposites. In the above section, the presence of ovi-POSS is found to accelerate the overall isothermal melt crystallization process of PCL in the PCL/ovi-POSS nanocomposites relative to neat PCL, suggesting that ovi-POSS may act as a nucleation agent for the crystallization of PCL. In this section, the spherulitic morphology of neat PCL and its nanocomposites was further studied with POM. Figure 4 shows the spherulitic morphology of neat PCL and its two nanocomposites crystallized at 44 °C. As shown in Figure 4a, the well developed spherulites have a size of roughly 100−200 μm in diameter with distinct boundaries for neat PCL. Parts b and c of Figure 4

Table 1. Summary of the Related Crystallization Kinetics Parameters for Neat PCL and the PCL/Ovi-POSS Nanocomposites samples

Tc (°C)

n

neat PCL

36 38 40 42 36 38 40 42 36 38 40 42

2.4 2.4 2.5 2.2 2.5 2.3 2.4 2.3 2.5 2.5 2.4 2.3

POSS-0.5

POSS-2

k (min−n) 2.13 4.75 6.77 3.19 2.07 7.28 1.25 3.71 2.96 1.03 1.78 4.95

× × × × × × × × × × × ×

10−2 10−3 10−4 10−4 10−2 10−3 10−3 10−4 10−2 10−2 10−3 10−4

t0.5 (min) 4.16 7.86 16.4 31.1 4.03 7.38 13.2 26.8 3.45 5.50 11.8 22.6

1/t0.5 (min−1) 2.40 1.27 6.11 3.22 2.48 1.35 7.57 3.73 2.90 1.82 8.47 4.42

× × × × × × × × × × × ×

10−1 10−1 10−2 10−2 10−1 10−1 10−2 10−2 10−1 10−1 10−2 10−2

is introduced in this work for discussing the crystallization kinetics of neat PCL and the PCL/ovi-POSS nanocomposites. The t0.5 value can be calculated by the following equation based on the Avrami equation: t0.5 =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(2)

The crystallization rate can thus be described by the reciprocal of t0.5 (1/t0.5). The effect of Tc on the overall isothermal melt crystallization rate was first studied for both neat PCL and its nanocomposites. Table 1 summarizes all the t0.5 and 1/t0.5 3205

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Figure 4. Optical micrographs of spherulites morphology of neat PCL and its nanocomposites crystallized at 44 °C: (a) neat PCL, (b) POSS-0.5, and (c) POSS-2.

a number of strong diffraction peaks are found for pure oviPOSS, indicative of their highly crystalline structure. In the nanocomposites, a diffraction peak at 2θ = 9.7° is also observed, which is similar to the WAXD pattern of pure POSS. The WAXD results indicate that the crystallization of POSS may occur when they are dispersed in the PCL matrix. Similar results are also found in the PLLA/ovi-POSS nanocomposites in our previous work.21 Dynamic Mechanical Properties of Neat PCL and Its Nanocomposites. In the above sections, the effect of oviPOSS on the overall isothermal melt crystallization kinetics, spherulitic morphology, and crystal structure of PCL in the PCL/ovi-POSS nanocomposites was investigated with DSC, POM, and WAXD. In this section, the dynamic mechanical properties of neat PCL and the PCL/ovi-POSS nanocomposites were further studied. Figure 6a illustrates the temperature dependence of storage modulus (E′) of neat PCL and its nanocomposites. It is clear from Figure 6a that the values of E′ are significantly higher in the nanocomposites than in neat PCL within a wide temperature range of −90 to 40 °C; moreover, they further increase with increasing the ovi-POSS loading. For instance, the E′ value at −80 °C is around 715 MPa for neat PCL, while in the nanocomposites the E′ values are improved to be around 1295 and 2685 MPa for POSS-0.5 and POSS-2 at the same temperature, respectively. Relative to neat PCL, the E′ values are significantly improved by about 81% and 275% in the nanocomposites with incorporation of only 0.5 and 2.0 wt % ovi-POSS, respectively. It is also interesting to study the enhancement of the storage modulus of the nanocomposites at room temperature. The E′ values are determined to be around 91, 183, and 388 MPa for neat PCL, POSS-0.5, and POSS-2, respectively. Compared with neat PCL, the values of E′ of POSS-0.5 and POSS-2 are improved by about 101% and 327%, respectively. The increment of E′ in the nanocomposites indicates that the presence of ovi-POSS provides a reinforcement effect to the PCL matrix because of high performance and fine dispersion of ovi-POSS in the PCL matrix. Figure 6b shows the temperature dependence of tan δ (the ratio of loss modulus to storage modulus) for neat PCL and the PCL/ovi-POSS nanocomposites. As shown in Figure 6b, the tan δ peak is at around −50 °C for neat PCL, which remains almost unchanged in the nanocomposites despite the ovi-POSS loading. Figure 6b clearly shows that the presence of ovi-POSS imposes less constraint on the polymer chains of PCL and does not significantly influence the segmental motion of PCL in the nanocomposites; consequently, the glass transition temperature of PCL remains almost unchanged in the nanocomposites. In brief, the incorporation of ovi-POSS shows little influence on the glass transition but increases the storage modulus in the nanocomposites significantly.

illustrate the POM images of PCL spherulites for POSS-0.5 and POSS-2, respectively, from which some smaller and imperfect spherulites are observed without clear boundaries. The spherulitic morphology study indicates that the nucleation density of PCL spherulites is higher in the nanocomposites than in neat PCL. Therefore, the presence of POSS has significantly influenced the spherulitic morphology and the overall crystallization process of PCL in the nanocomposites by acting as a nucleating agent. Wu et al. have also found the similar results in the PCL and multiwalled carbon nanotubes (MWCNTs) nanocomposites.31 They found that the nonisothermal melt crystallization of PCL was enhanced significantly by the presence of MWCNTs; however, the variation of the MWCNTs loading from 1 to 5 wt % showed no apparent influence on the crystallization peak temperature of PCL in the nanocomposites. It is interesting to investigate the presence of ovi-POSS on the crystal structure of PCL in the PCL/POSS nanocomposites. Figure 5 shows the WAXD patterns of pure ovi-POSS,

Figure 5. WAXD patterns for ovi-POSS, neat PCL, and their nanocomposites.

neat PCL, and their two nanocomposites with different oviPOSS loadings. As shown in Figure 5, neat PCL shows three main diffraction peaks at 2θ = 21.27°, 21.87°, and 23.51°, which may be ascribed to (110), (111), and (200) planes, respectively.1 For both POSS-0.5 and POSS-2, they present the diffraction peaks at almost the same locations, suggesting that the presence of POSS does not modify the crystal structures of PCL in the nanocomposites. In addition, the crystallinity values are estimated to be around 45% for both neat PCL and its nanocomposites. In brief, the addition of ovi-POSS does not modify the crystal structure and change the crystallinity of PCL apparently in the nanocomposites. It should also be noted that 3206

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Figure 6. Temperature dependence of (a) storage modulus and (b) tan δ for neat PCL and its nanocomposites.

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in Situ Prepared Poly(ε-caprolactone) Nanocomposites. Compos. Sci. Technol. 2007, 67, 2165−2174. (7) Zeng, H.; Gao, C.; Yan, D. Poly(ε-caprolactone)-Functionalized Carbon Nanotubes and Their Biodegradation Properties. Adv. Funct. Mater. 2006, 16, 812−818. (8) Chen, E.; Wu, T. Isothermal Crystallization Kinetics and Thermal Behavior of Poly(ε-caprolactone)/Multiwalled Carbon Nanotube Composites. Polym. Degrad. Stab. 2007, 92, 1009−1015. (9) Qiu, Z.; Wang, H.; Xu, C. Crystallization, Mechanical Properties, and Controlled Enzymatic Degradation of Biodegradable Poly(εcaprolactone)/Multiwalled Carbon Nanotubes Nanocomposites. J. Nanosci. Nanotechnol. 2011, 11, 7884−7893. (10) Lei, H.; Kai, W.; Liang, Z.; Inoue, Y. Polyester/Organo-Graphite Oxide Composite: Effect of Organically Surface Modified Layered Graphite on Structure and Physical Properties of Poly(ε-caprolactone). J. Polym. Sci. Polym. Phys. 2010, 48, 294−301. (11) Zhang, J.; Qiu, Z. Morphology, Crystallization Behavior and Dynamic Mechanical Properties of Biodegradable Poly(ε-caprolactone)/Thermally Reduced Graphene Nanocomposites. Ind. Eng. Chem. Res. 2011, 50, 13885−13891. (12) Wu, J.; Mather, P. POSS Polymers: Physical Properties and Biomaterials Applications. Polym. Rev. 2009, 49, 25−63. (13) Cordes, D.; Lickiss, P.; Rataboul, F. Recent Developments in the Chemistry of Cubic Polyhedral Oligosilsesquioxanes. Chem. Rev. 2010, 110, 2081−2173. (14) Fina, A.; Monticelli, O.; Camino, G. POSS-Based Hybrids by Melt/Reactive Blending. J. Mater. Chem. 2010, 20, 9297−9305. (15) Kuo, S.; Chang, F. POSS Related Polymer Nanocomposites. Prog. Polym. Sci. 2011, 36, 1649−1696. (16) Xu, H.; Kuo, S.; Lee, J.; Chang, F. Preparations, Thermal Properties and Tg Increase Mechanism of Inorganic/Organic Hybrid Polymers Based on Polyhedral Oligomeric Silsesquioxanes. Macromolecules 2002, 35, 8788−8793. (17) Hato, M.; Sinha Ray, S.; Luyt, A. Nanocomposites Based on Polyethylene and Polyhedral Oligomeric Silsesquioxanes, 1. Microstructure, Thermal and Thermomechanical Properties. Macromol. Mater. Eng. 2008, 293, 752−762. (18) Yu, J.; Qiu, Z. Preparation and Properties of Biodegradable Poly(L-lactide)/Octamethyl−Polyhedral Oligomeric Silsesquioxanes Nanocomposites with Enhanced Crystallization Rate via Simple Melt Compounding. ACS Appl. Mater. Interfaces 2011, 3, 890−897. (19) Qiu, Z.; Pan, H. Preparation, Crystallization and Hydrolytic Degradation of Biodegradable Poly(L-lactide)/Polyhedral Oligomeric Silsesquioxanes Nanocomposites. Compos. Sci. Technol. 2010, 70, 1089−1094. (20) Pan, H.; Qiu, Z. Biodegradable Poly(L-lactide)/Polyhedral Oligomeric Silsesquioxanes Nanocomposites: Enhanced Crystallization, Mechanical Properties, and Hydrolytic Degradation. Macromolecules 2010, 43, 1499−1506. (21) Yu, J.; Qiu, Z. Effect of Low Octavinyl−Polyhedral Oligomeric Silsesquioxanes Loadings on the Melt Crystallization and Morphology of Biodegradable Poly(L-lactide). Thermochim. Acta 2011, 519, 90−95.

CONCLUSIONS Biodegradable PCL and ovi-POSS nanocomposites have been successfully prepared via a solution and casting method in this work. SEM observation reveals not only a fine dispersion but also perfect crystallization of ovi-POSS throughout the PCL matrix. The overall isothermal melt crystallization kinetics, spherulitic morphology, crystal structure, and dynamic mechanical properties of PCL in the nanocomposites were investigated with various techniques and compared with those of neat PCL. The overall isothermal melt crystallization rates of PCL are faster in the nanocomposites than in neat PCL, indicative of a nucleation agent effect of ovi-POSS, which is also supported by the spherulitic morphology study. However, the presence of ovi-POSS does not change the crystallization mechanism and crystal structure of PCL in the nanocomposites. The DMA experimental results indicate that the storage modulus of the nanocomposites has been significantly improved by about 101% and 327%, respectively, with only incorporating 0.5 and 2.0 wt % ovi-POSS at room temperature as compared with neat PCL; however, the glass transition temperature varies slightly around −50 °C despite the presence of ovi-POSS.

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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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REFERENCES

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