Crystallization Behavior of Poly(sodium 4-styrenesulfonate

Article pubs.acs.org/IECR

Crystallization Behavior of Poly(sodium 4‑styrenesulfonate)Functionalized Carbon Nanotubes Filled Poly(ε-caprolactone) Nanocomposites Tong-Hui Zhao,† Kai-Li Yang,† Run-Tao Zeng,† An-Ke Du,*,‡ Ming Wang,† and Jian-Bing Zeng*,† †

School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China Chongqing Academy of Science and Technology, Chongqing 401123, China

‡

ABSTRACT: Multiwalled carbon nanotubes (MWCNTs) were modified with a noncovalent functionalization method via ultrasonication in the presence of poly(sodium 4-styrenesulfonate) (PSS) as a modifier. The PSS-functionalized MWCNTs dispersed well in PCL matrix and showed good interfacial adhesion with PCL after being incorporated by solution coagulation. The isothermal crystallization kinetics of neat PCL and PCL/MWCNT nanocomposites were comparatively investigated by DSC. It was found that the overall crystallization rate of PCL was enhanced significantly, while the crystallization mechanism was unchanged by incorporation of the functionalized MWCNTs. The addition of only 0.1 wt % MWCNT caused 23.4 times improvement in overall crystallization rates, ascribing to the efficient nucleating effect of well-dispersed PSS-functionalized MWCNTs toward crystallization of PCL in the nanocomposites. The spherulitic morphology observation by polarizing optical microscope confirmed the nucleating effect. X-ray diffraction investigation indicates that the crystal structure of PCL remained unchanged after incorporation of PSS-functionalized MWCNTs. structures.10,16,17 This method involves improving dispersion by surface physical absorption of modifier molecules, which are able to form some particular interactions, such as π−π, cation−π, and anion−π interactions, with carbon nanomaterials.10,13 Although many investigations have been reported on crystallization behaviors of carbon nanomaterials filled PCL composites, the carbon nanomaterials in most of those studies were either nonfunctionalized or covalently functionalized.6,9,18−29 Less attention has been paid to incorporate noncovalently functionalized carbon nanomaterials into PCL matrix and reveal their effects on the crystallization behaviors of PCL nanocomposites. In a previous study,30 we modified graphenes with the noncovalent functionalization method by surface absorption of poly(sodium 4-styrenesulfonate) (PSS) with the aid of ultrasound and incorporated the functionalized graphenes into PCL via a solution coagulation technique. Welldispersed modified graphenes were found to have good interfacial adhesion with PCL and accelerate the crystallization rate of PCL considerably with very small addition. It is worth noting that, although GNS and CNT are both carbon nanomaterials, they are very different in morphologies. GNS has sheet-like morphology, while CNT has tube-like morphology. The morphology of nanofillers such as shape and size also has a remarkable influence on the crystallization behaviors of polymer composites.5,31,32 In this study, we incorporate PSSfunctionalized CNTs into PCL and investigate the effect of loading functionalized CNTs and crystallization temperatures on the isothermal crystallization behaviors of PCL composites

1. INTRODUCTION Poly(ε-caprolactone) (PCL) has attracted considerable attention from both academic and practical viewpoints, due to its excellent biodegradability and biocompatibility.1−3 However, some properties of PCL, such as the crystallization rate, mechanical strength, and thermal stability, are insufficient for practical application. Incorporation of nanofillers has been proven to be efficient to reinforce its physical properties.4−9 Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphenes (GNS) or its derivatives, showing unique mechanical strength, thermal properties, electrical conductivity, and huge surface areas, are thought of as ideal reinforcing fillers for high performance polymer composites; therefore, a lot of work has been reported on employing those carbon nanomaterials to reinforce physical properties of various polymer matrixes.10−12 The dispersion of nanofillers and interfacial adhesion between fillers and polymer matrices play important roles in the final properties of the formed composites. It is worth noting that the tendency to aggregate that arose from strong π−π interaction or van der Waals intermolecular interaction of carbon nanomaterials usually causes poor dispersion of pristine carbon nanotubes or graphenes in matrix polymers.10,12 Functionalization of carbon nanomaterials through either covalent or noncovalent methods gives a useful way of reducing π−π interaction of carbon nanomaterials, thus improving and stabilizing their dispersions within polymer matrixes.10,13 It is reported that the translational symmetry of carbon nanomaterials was disrupted by changing carbon atoms from sp2 to sp3 during covalent functionalization, which usually reduces some excellent performance of the carbon nanomaterials, for example, lowering electronic and transport properties.14,15 In contrast, noncovalent functionalization is of particular importance as it provides an efficient way of enhancing dispersion of carbon nanomaterials without changing their © XXXX American Chemical Society

Received: September 14, 2015 Revised: January 27, 2016 Accepted: February 2, 2016

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DOI: 10.1021/acs.iecr.5b03419 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research systematically. To the best of our knowledge, no investigation on the effect of noncovalently functionalized CNTs on the crystallization behavior of PCL has been reported in the literature.

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(ε-caprolactone) (PCL, Esun500C) and poly(sodium 4-styrenesulfonate) were purchased from Guanghua Weiye Co., Ltd. (Shenzhen, China) and Micxy Chemical Co., Ltd. (Chengdu, China), respectively. The molecular weights of PCL and PSS are 5.0 × 104 and 7.0 × 104 g/mol, respectively, given by suppliers. Multiwalled CNTs (NC 7000) were purchased from Nanocyl Corp. (Belgium). Tetrahydrofuran (THF) and other reagents with AR grades were procured from Kelong Chemical Co., Ltd. (Chengdu, China). All of the materials and chemicals were used as received. 2.2. Preparation of Noncovalently Functionalized CNTs. MWCNTs were modified via a noncovalent functionalization technique with PSS as a macromolecular modifier.33 MWCNTs and PSS with a weight ratio of 1:5 were added to a 500 mL beaker, and then deionized water was added into the beaker to obtain a mixture, in which the concentration of CNTs in water was 2.0 g L−1, and finally the mixture was treated with a probe sonicator (SCIENTZ-IID, Ningbo China) for 30 min to give rise to a uniform dispersion. 2.3. Fabrication of Noncovalently Functionalized MWCNTs Filled PCL Nanocomposites. PCL nanocomposites filled with noncovalently functionalized MWCNTs were fabricated via a solution coagulation method. Briefly, PCL in THF solution with the concentration of 5% (W/V) was prepared first with the aid of mild stirring at 50 °C for 2 h. After being cooled to room temperature, a predetermined amount of functionalized MWCNTs dispersion was dropwise added into the strongly stirred PCL solution, and then additional excessive water was dropped into the solution to precipitate the coagulated composites. When the products were precipitated, the solvents changed almost to colorless, as shown in Figure 1, indicating that almost all PSS-functionalized MWCNT was incorporated into PCL matrix with the coagulation method. The composites were obtained by filtration followed by vacuum drying for 2 days at 50 °C. Several composites with functionalized MWCNTs content varying from 0.1 to 1.0 wt % were prepared and abbreviated as PCL/MWCNT-x, for convenience, where x indicates the content of MWCNTs in weight percentage. For example, PCL/MWCNT-0.5 represents PCL nanocomposite containing 0.5 wt % MWCNTs. For property comparison, neat PCL was also treated with the same procedures. The sample sheets with thickness of 1 mm were prepared by hot mold pressing. 2.4. Characterization. Thermal properties under N2 atmosphere were performed on a TA TGA-Q600 from room temperature to 700 °C at a heating rate of 10 °C/min. Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) for pristine MWCNTs and PSS-functionalized MWCNTs were carried out on a JEM-2100F TEM an accelerating voltage of 200 kV. The MWCNTs and PSS-functionalized MWCNTs were dispersed in water with the aid of ultrasonication for 30 min. Samples were prepared by depositing a drop of the sample water dispersion onto a copper micro grid, which was then vacuum-dried at 80 °C for 24 h. The dispersion of PSSfunctionalized MWCNTs in PCL matrix was observed by scanning electron microscopy (SEM) and TEM. For SEM

Figure 1. Digital photos of mixtures of PCL with PSS-modified CNT (a) and the coagulated PCL/CNT nanocomposites (b) obtained by addition of excessive water.

measurement, the cryo-fractured surfaces of PCL/MWCNT nanocomposites were observed by a XL-30s FEG SEM (Philips, Holland) with an accelerating voltage of 5 kV. The fractured surface was sputtered with a layer of gold prior to observation. For TEM measurement, an ultrathin section of ca. 70−80 nm in thickness was used. The ultrathin section was sliced with a Leica EM FC6 cryo-ultramicrotome. The experiment was carried out on a FEI Tecnai G2F20 S-TWIN TEM (Holland) with an accelerating voltage of 200 kV. A TA differential scanning calorimeter (DSC Q200) was used to study the thermal and crystallization behaviors of neat PCL and PCL/MWCNT composites. The sample with ∼6 mg in an aluminum pan was first melted at 80 °C for 3 min to erase any thermal history, then cooled to −70 °C at a cooling rate of 10 °C/min, and finally reheated to 80 °C at a heating rate of 10 B


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Industrial & Engineering Chemistry Research °C/min. All experiments were implemented under N 2 atmosphere. The cooling and the second heating scans were recorded for data analysis. Isothermal crystallization kinetics was carried out on the TA DSC Q200. The samples with ∼6 mg in aluminum pans were first melted at 80 °C for 3 min to eliminate thermal history, and then quickly cooled to isothermal crystallization temperature (Tc) at a cooling rate of 50 °C/min, and kept at Tc until crystallization was finished. The Tc varies from 40 to 54 °C depending on the composition of the samples. The operations were carried out under N2 atmosphere. The crystallization exothermal curves were recorded for analysis. After completion of crystallization at Tc, the samples were heated to 80 °C at a heating rate of 10 °C/min to investigate the melting behaviors and calculate the equilibrium melting temperatures. Spherulitic morphologies of neat PCL and PCL/MWCNT nanocomposites were investigated by a NIKON ECLIPSE LV100POL polarizing optical microscope (POM) with an HSC621 V temperature controller. The sample film in two microscopic cover glasses was first heated to 80 °C and kept at the temperature for 3 min to eliminate thermal history. The sample was then rapidly cooled to 46 °C and kept at this temperature until crystallization finished. X-ray diffraction (XRD) patterns of neat PCL and PCL/ MWCNT nanocomposites were recorded on a Philips X’Pert X-ray diffractometer with Cu Kα radiation. The measurement was carried out at room temperature with a rate of 2°/min scanning from 5° to 40°.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of PSS Wrapped MWCNTs. PSS wrapped MWCNTs were prepared by probe ultrasonication of MWCNTs water dispersion in the presence of PSS. Uniform dispersion was obtained, and no obvious precipitation can be observed after several hours, as shown by the digital photo in Figure 2a. In contrast, probe ultrasonication of MWCNTs in the absence of PSS led to an unstable dispersion, in which MWCNTs aggregated and precipitated rapidly in several minutes after unloading of ultrasoniction. Thermal gravimetric analysis (TGA) was used to study the thermal decomposition behaviors of the different MWCNTs. Precipitates of PSS wrapped MWCNT were obtained by addition of excessive ethanol into the dispersion followed by ultracentrifugation. The precipitates were extracted by water through Soxhlet extractor for 2 days to remove unwrapped PSS, and then vacuum-dried at 50 °C. Figure 2b shows the TGA curves of pristine MWCNT and PSS wrapped MWCNT. Pristine MWCNT showed weight loss of less than 5 wt % with temperature up to 670 °C, probably due to the presence of some impurities. For PSS wrapped MWCNT, about 8 wt % weight loss occurred before 150 °C, which should be attributed to the absorbed water by the moisture-sensitive PSS. The other weight loss of ∼12 wt % that occurred in the range of 400−670 °C was ascribed to the thermal decomposition of PSS that wrapped on the surface of MWCNT. The results suggest that the π−π interaction between carbon nanotubes and PSS was strong enough to suffer from water extraction. To further confirm the successful absorption of PSS on the surface of MWCNTs, TEM was carried out for pristine MWCNTs and PSS-functionalized MWCNTs. Figure 3a and b shows the TEM images of the two types of MWCNTs. The two MWCNTs seemed to show almost the same morphologies,

Figure 2. (a) Digital photos for water dispersions of pristine MWCNT and PSS wrapped MWCNT and (b) TG curves of pristine MWCNT and PSS wrapped MWCNT.

Figure 3. TEM images (a,b) and HRTEM images (c,d) for pristine MWCNT (a,c) and PSS-functionalized MWCNT (b,d).

maybe due to that the absorbed PSS was relative low after suffering from water extraction. To observe the surface C


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Industrial & Engineering Chemistry Research morphologies of the two different MWCNTs in detail, a highresolution transmission electron microscope (HRTEM) was carried out for the two samples. Figure 3c and d shows the HRTEM images of pristine MWCNT and PSS-functionalized MWCNTs. It is obvious that pristine MWCNT showed a smooth surface, while the surface of PSS-functionalized MWCNTs was wrapped with an about 3 nm thickness of polymers. All of the above evidence indicates that PSS can be successfully absorbed onto the surface of MWCNT to form excellent water dispersion, which then can be used to fabricate composites with polymers through solution coagulation if the solvents for the polymers are miscible with water. Because of the poor dispersity of pristine MWCNT in water, it was hard to incorporate MWCNT into PCL matrix via solution coagulation. Therefore, PCL nanocomposite with pristine MWCNTs was not prepared. In the following sections, the properties of PCL composites with different loadings of PSS wrapped MWCNTs were discussed in detail and compared to those of neat PCL. 3.2. Morphology of PCL/CNT Nanocomposites. It is well-known that the final properties of nanofillers reinforced polymer composites depend strongly on the dispersion of fillers in the polymer matrix. Figure 4 shows the SEM and TEM

Figure 5. DSC cooling scans (a) and the second DSC heating scans (b) for neat PCL and its nanocomposites.

Figure 4. SEM image for cryo-fractured surfaces (a) and TEM image for ultrathin section (b) of PCL/MWCNT-0.3 nanocomposites.

Table 1. Data for Thermal Transition and Crystallization of Neat PCL and Its Nanocomposites

images for PCL/MWCNT-0.3. It can be seen from the SEM image (Figure 4a) that MWCNTs dispersed homogeneously in PCL matrix without obvious aggregation, and no pulling out of CNTs occurred, which indicates that there is good interfacial adhesion between PSS modified MWCNT and PCL matrix. Although PCL is water insoluble, it showed good affinity with water as it is able to absorb up to 1.0 wt % water molecules.30,34,35 Therefore, PCL may have some compatibility with hydrophilic PSS-functionalized MWCNTs to show good interfacial adhesion. TEM images (Figure 4b) further showed that PSS-functionalized MWCNTs dispersed randomly in PCL matrix without obvious aggregation. 3.3. Basic Thermal Properties. The basic thermal properties including melting temperature (Tm), fusion enthalpy (ΔHm), nonisothermal crystallization temperature (Tc), and crystallization enthalpy (ΔHc) of PCL and PCL/MWCNT nanocomposites were studied by DSC. Figure 5 shows the DSC cooling and the second heating scans of PCL and its nanocomposites at scanning rates of 10 °C/min, and the corresponding data are listed in Table 1. For the nonisothermal crystallization of the samples, neat PCL showed a Tc of 26.6 °C with a ΔHc of 69.35 J/g; the Tc was significantly increased to 39.2 °C with incorporation of only 0.1 wt % MWCNTs; with further increasing MWCNTs content, the increase in Tc

sample

Tc (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

neat PCL PCL/MWCNT-0.1 PCL/MWCNT-0.3 PCL/MWCNT-0.5 PCL/MWCNT-1.0

26.6 39.2 41.4 42.1 42.7

69.35 69.09 70.67 70.35 70.74

56.9 57.9 59.3 59.3 59.2

69.73 69.85 70.73 70.68 70.91

became less prominent or even leveled off, and the values were 41.4, 42.1, and 42.7 °C for PCL/MWCNT-0.3, PCL/ MWCNT-0.5, and PCL/MWCNT-1.0, respectively. Higher Tc in cooling scan usually means a faster crystallization rate. Therefore, we can conclude that the nonisothermal crystallization rate of PCL was accelerated by incorporation of PSSfunctionalized MWCNTs, due to the nucleation effect. Meanwhile, the ΔHc almost remained unchanged regardless of the content of MWCNTs, as the values were all around 70 J/ g. The increase in Tc during nonisothermal condition indicates accelerated nonisothermal crystallization rate. As compared to PSS-modified graphenes, PSS-functionalized CNTs caused more significant improvement in Tc at the same filler contents. The Tc of PCL increased by 7.8 °C when 0.1 wt % PSS modified graphenes was loaded, while the same content of PSSfunctionalized MWCNTs caused a 12.6 °C increase in Tc. The D


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of 48 °C. For the effect of content of MWCNT, it can be seen that less time was required for PCL nanocomposites with higher content of MWCNT at a given Tc. For instance, the time for PCL/MWCNT-0.1 to finish crystallization at 50 °C was ∼33.3 min, and those for PCL/MWCNT-0.3 and PCL/ MWCNT-0.5 were ∼8.4 and ∼5.1 min, respectively. The results indicate that PSS-functionalized CNTs worked as a very efficient nucleating agent for crystallization of PCL, and the nucleating effect increased with the content of PSS-functionalized CNTs. The isothermal crystallization kinetics of both neat PCL and PCL/MWCNT nanocomposites were analyzed by the Avrami equation, which assumes that the relationship between relative crystallinity (Xt) and crystallization time (t) corresponds to

increasing extent was also more than some other PCL nanocomposites filled with pristine MWCNTs.22,36 The increased Tc was ascribed to the nucleation effect of the incorporated MWCNTs. The melting point of neat PCL was 56.9 °C, and increased slightly to 57.9−59.3 °C upon addition of PSS-functionalized MWCNTs, which may be ascribed to that the crystallization of PCL in nanocomposites happened at higher temperatures, which could form more thermally stable crystals, thus showing relatively high Tm. The incorporation of MWCNTs would also not change the fusion enthalpy apparently, as all of the samples showed close fusion enthalpy of around 70 J/g. 3.4. Isothermal Crystallization Kinetics. To study the effect of PSS-functionalized MWCNTs on the crystallization behavior of PCL in detail, the isothermal crystallization kinetics of neat PCL and PCL/MWCNTs nanocomposites was further investigated in a wide range of crystallization temperatures. Because of the approaching crystallization temperatures between PCL/MWCNT-1.0 and PCL/MWCNT-0.5, the isothermal crystallization kinetics of PCL/MWCNT-1.0 was not shown for brevity. Figure 6 shows the exothermic curves for isothermal crystallization of neat PCL at different temperatures. It is

1 − X t = exp( −kt n)

(1)

where Xt is the relative crystallinity at crystallization time t, k is a rate constant depending on nucleation and crystalline growth, and n is the Avrami exponent, which denotes the nature of the nucleation and growth process.37,38 Double logarithm of Avrami equation gives rise to log[− ln(1 − X t )] = log k + n log t

(2)

The Avrami equation can be reasonably used to analyze isothermal crystallization kinetics of samples if plotting log[−ln(1 − Xt)] versus log t results in a straight line, and from which both the rate constant and the Avrami exponent can be calculated from the intercept and slope of the line. Figure 8 shows the Avrami plots of neat PCL and its nanocomposites at different crystallization temperatures. A series of almost parallel straight lines are obtained for all samples, indicating that the Avrami equation can be suitably applied to deal with isothermal crystallization kinetics of both neat PCL and its nanocomposites. Accordingly, the values of k and n for all samples at different crystallization temperatures could be calculated. Table 2 summarizes the isothermal crystallization kinetics parameters for the isothermal crystallization of neat PCL and its composites at various crystallization temperatures. From Table 2, the averaged n value for neat PCL was 2.29, and those for PCL/MWCNT-0.1, PCL/MWCNT0.3, and PCL/MWCNT-0.5 were 2.21, 2.09, and 2.10, respectively. All of the values are between 2 and 2.5, which indicates that the incorporation of PSS-functionalized MWCNTs into PCL does not change its isothermal crystallization mechanism and the isothermal crystallization of PCL in all samples may correspond to a mechanism of threedimensional truncated spherulitic growth with athermal nucleation.37 The crystallization half-life time (t1/2), the time required to achieve 50% of the final crystallinity, is an important parameter for discussing isothermal crystallization kinetics,39−41 and the overall crystallization rate can be described by the reciprocal of t1/2. The value of t1/2 is given by

Figure 6. Exothermic curves for isothermal crystallization of neat PCL at different temperatures.

obvious that prolonged time is required for the sample to complete crystallization at elevated temperatures. The PCL/ MWCNT composites show similar exothermic curves after isothermal crystallization; for brevity, the curves are not shown. From the exothermic curves, the development of relative crystallinity with time can be derived, and the corresponding plots are shown in Figure 7. It can be seen that all of the plots displayed a similar sigmoid shape, and the crystallization time was prolonged with increase in crystallization temperature for both neat PCL and its nanocomposites. It is interesting to find that the crystallization finished in much shorter time for PCL/ MWCNT nanocomposite as compared to neat PCL at the same crystallization temperature. For example, it took ∼69 min for neat PCL to finish crystallization at 46 °C, while the time signigicantly reduced to ∼2.5 min for PCL/MWCNT-0.1. It is worth pointing out that with further increase in content of MWCNT, due to the drastically accelerated crystallization rate, it is hard to obtain an integrated crystallization exothermic peak at low temperature. So the study on isothermal crystallization of PCL/MWCNT-0.3 and PCL/MWCNT-0.5 started from the Tc

t1/2 = (ln 2/k)1/ n

(3)

According to the values of n and k, the values of 1/t1/2 were calculated and are graphically shown in Figure 9. It can be seen that 1/t1/2 decreased with increase in Tc for both neat PCL and its composites, indicative of decreased crystallization rate. It is obvious that the crystallization rates of PCL/MWCNT composites were higher than neat PCL and increased with increasing content of MWCNT at given Tc’s. For example, the E


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Figure 7. Plots of relative crystallinity (Xt) versus crystallization time at various crystallization temperatures for neat PCL (a), PCL/MWCNT-0.1 (b), PCL/MWCNT-0.3 (c), and PCL/MWCNT-0.5 (d).

Figure 8. Avrami plots for isothermal crystallization at various crystallization temperatures for neat PCL (a), PCL/MWCNT-0.1 (b), PCL/ MWCNT-0.3 (c), and PCL/MWCNT-0.5 (d).

1/t1/2 for neat PCL at 46 °C was 0.034 min−1, while the value was increased by ∼23.4 times to 0.796 min−1 when only 0.1 wt % MWCNTs was filled; the 1/t1/2 for PCL/MWCNT-0.1 at 50

°C was 0.111 min−1, and those for PCL/MWCNT-0.3 and PCL/MWCNT-0.5 were enhanced by 3.29 and 4.93 times to 0.365 and 0.525 min−1, respectively. The results could be F


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Industrial & Engineering Chemistry Research Table 2. Parameters for Isothermal Crystallization Kinetics of Neat PCL and Its Nanocomposites sample

Tc (°C)

n

40 42 44 46 46 48 50 52 48 50 52 54 48 50 52 54

2.24 2.24 2.26 2.42 2.22 2.20 2.22 2.20 2.08 2.08 2.09 2.11 2.15 2.01 2.13 2.12

neat PCL

PCL/MWCNT-0.1

PCL/MWCNT-0.3

PCL/MWCNT-0.5

K (min−1) 2.11 6.67 1.56 1.87 0.42 5.46 5.21 4.92 0.40 8.52 7.35 4.86 0.51 0.19 1.23 6.94

× × × ×

10−2 10−3 10−3 10−4

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

× 10−2 × 10−4

Figure 10. (a) Melting curves of neat PCL after isothermal crystallization at various temperatures and (b) Hoffman−Weeks plots for calculating Tm° of neat PCL and its nanocomposites. Figure 9. 1/t1/2 of neat PCL and its nanocomposites at various crystallization temperatures.

crystallization at higher temperature formed larger and less defective crystals, which showed higher thermal stability and thus higher melting temperature. The other three samples showed similar melting behaviors; for brevity, their melting curves were not shown. Figure 10b shows the Hoffman−Weeks plots of neat PCL and its nanocomposites. The Tm° of neat PCL was calculated to be 70.7 °C, which was close to that reported by Liang et al.43 The addition of PSS-functionalized MWCNTs caused enhancement in Tm° of PCL, and the value of Tm° increased gradually to 73.1, 75.0, and 78.6 °C with MWCNT loading increasing to 0.1, 0.3, and 0.5 wt %, respectively, which is in agreement with the change of melting points of PCL/MWCNT nanocomposites versus loadings of MWCNT as shown in section 3.4. 3.5. Spherulitic Morphology and Crystal Structure. To observe the nucleation effect of PSS-functionalized CNTs on crystallization of PCL intuitively, spherulitic morphologies of neat PCL and its nanocomposites were observed by POM. Figure 11 shows the POM images of neat PCL and its nanocomposites after being isothermally crystallized at 46 °C. Neat PCL as shown in Figure 6a displayed well-developed spherulites with large size and clear boundaries. However, the size of spherulites of PCL containing only 0.1 wt % MWCNTs was reduced so extensively that integrated spherulites were not observed and the boundaries were not discriminated, which

attributed to the improved nucleating effect with increasing content of MWCNT. The effect of PSS-functionalized MWCNTs on the equilibrium melting temperature (Tmo) of PCL was investigated by DSC. Tm°’s of the samples were determined by the Hoffman−Weeks equation, which is given as follows:42 ⎛ 1⎞ T Tm = Tm° ⎜1 − ⎟ + c γ⎠ γ ⎝

where Tm is the melting temperature of a crystal formed by isothermal crystallization temperature Tc, and γ is the ratio of final to initial lamellar thickness. Plotting the Tm as a function of Tc gives a straight line. Tm° is then obtained from the intersection of this line with the Tm = Tc line. As mentioned in the Experimental Section, the subsequent heating curves of both PCL and its nanocomposites after isothermal crystallization at various Tc’s were recorded for Tm° analysis. Figure 10a shows the melting curves of neat PCL after isothermal crystallization at a Tc of 40−46 °C as an example. It can be seen that the melting endothermic peaks shifted gradually to a higher temperature range with increasing Tc, which should be ascribed to the fact that isothermal G


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lattice parameters of PCL in the presence of PSS-functionalized MWCNTs.

4. CONCLUSIONS PSS-functionalized MWCNTs were incorporated into PCL matrix by solution coagulation method. The PSS-functionalized MWCNTs were found dispersed uniformly without obvious aggregation and showed good interfacial adhesion with PCL matrix. Well-dispersed MWCNTs acted as a very efficient nucleating agent for crystallization of PCL, which then significant enhanced both the nonisothermal and the isothermal crystallization of PCL. The nonisothermal crystallization temperature of PCL was improved by 12.6 °C, and the isothermal crystallization rate at 46 °C was increased by 23.4 times by incorporation of only 0.1 wt %, as compared to neat PCL. It is worth noting that the presence of PSS-functionalized MWCNTs did not change the crystallization mechanism and crystal structure of PCL. Both neat PCL and its nanocomposites followed a mechanism of two-dimensional circular growth with heterogeneous nucleation.

Figure 11. Spherulitic morphologies formed by isothermal crystallization at Tc of 46 °C for neat PCL 1/t1/2 of neat PCL (a), PCL/ MWCNT-0.1 (b), PCL/MWCNT-0.3 (c), and PCL/MWCNT-0.5 (d).

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indicates that a large number of sites have been formed for PCL to nucleate by incorporation of even only 0.1 wt %, due to the well-dispersed state of PSS-functionalized MWCNTs in PCL matrix. With the increase in the content of MWCNTs, the morphologies of the crystals of PCL in PCL/MWCNT-0.3 and PCL/MWCNT-0.5 nanocomposites were similar to those of PCL/MWCNT-0.1. It can be found that the size of the crystals was further reduced by careful observation, which was attributed to the further increased nucleation sites that resulted from the further increasing MWCNT contents. In a word, the well-dispersed PSS-functionalized MWCNTs work as a very efficient nucleating agent for crystallization of PCL, therefore influencing both the spherulitic morphology and the overall crystallization kinetics of PCL in PCL/MWCNT nanocomposites. XRD was performed to study the effect of PSS-functionalized MWCNTs on the crystal structure of PCL. Figure 12 shows the XRD patterns of neat PCL and its nanocomposites. Neat PCL showed three typical diffraction peaks at 2θ of 21.27°, 21.87°, and 23.55°, corresponding to (110), (111), and (200) planes,9 respectively. It is obvious that the incorporation of PSSfunctionalized MWCNTs did not apparently alter the positions of diffraction peaks, indicative of unchanged crystal structure

AUTHOR INFORMATION

Corresponding Authors

*Phone/fax: +86-23-68254000. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Fundamental Research Funds for the Central Universities (XDJK2015C022, SWU115006) and the Natural Science Foundation Project of Chongqing (CSTC2013JCYJA20017).

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Figure 12. XRD patterns of neat PCL and its nanocomposites. H


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