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Crystallization Behavior of Polylactide Matrix Under the Influence of Nano-magnetite.

INTRODUCTION

In recent years, magnetic nanoparticles ([Fe.sub.3][O.sub.4], [gamma]-[Fe.sub.2][O.sub.3], Pt, Ni, Co), which are sensitive to a change of external magnetic field, have drawn much research attention. [Fe.sub.3][O.sub.4] nanoparticles possesses certain advantages, such as easy fabrication, low toxicity and cost effectiveness. As a transitional metal oxide with an inverse spinel structure, [Fe.sub.3][O.sub.4] exhibit superparamagnetism, and its surface easily allows chemical modifications. Therefore, different types of [Fe.sub.3][O.sub.4]/polymer nanocomposites have been developed, e.g. microspheres [1], films [2], and grafted polymers [3]. It has been reported that [Fe.sub.3][O.sub.4]/polymcr nanocomposites have great potential for the application for cell separation [4], targeted delivery [5] and pollution treatment in wastewater [6].

Polylactide (PLA), a natural-based polymer, has been extensively used for the fabrication of scaffolds for tissue engineering, implants and drug delivery carriers because of its biodegradability, biocompatibility, and bioresorbability [7-10]. As reported by Hosseini et al. [11], PLA/multiwalled carbon nanotubes/[Fe.sub.3][O.sub.4] composite nanofibers, loaded with daunorubicin, were found to be promising for inducing the death of leukemia cells. Li et al. [12] synthesized magnetic PLA nanocarriers using a layer-by-layer method for controlled drug delivery to breast cancer cells. Other than biomedical applications, incorporation of [Fe.sub.3][O.sub.4] in a carrier matrix has a wide range of applications. Recently, ferrofluid, which is a colloidal suspension of magnetic nanoparticles in a carrier fluid, has drawn considerable interest. With proper control of the ferrofluid response by external magnetic fields, new applications, such as magnetic lithography, magneto-optical birefringence and ferrofluidic sensing, can be undertaken [13-15]. Although many works have focused on the applications of PLA and [Fe.sub.3][O.sub.4], some basic properties such as the effect of [Fe.sub.3][O.sub.4] on the thermostability and the crystallization behavior of PLA under the influence of alternating magnetic field, have not been fully studied. Macromolecular motion affects the process of polymer crystallization. When an isothermal crystallization of PLA/[Fe.sub.3][O.sub.4] is carried out in an alternating magnetic field, the [Fe.sub.3][O.sub.4] are forced to vibrate and thereby changing the properties of solution.

This work aims to investigate such effects, which are of great influence on the morphology and properties (such as optical, mechanical and degradable properties) of the composites. We prepared nanocomposite films of PLA, with different concentrations of [Fe.sub.3][O.sub.4], via a solution casting method. The isothermal crystallization behaviors of PLA/[Fe.sub.3][O.sub.4] under the influence of magnetic field were also studied. Transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarized optical microscopy (POM), and thermogravimetric analysis (TGA) were used to investigate the microstructures, crystal structures and thermostability of as-prepared [Fe.sub.3][O.sub.4]/PLA nanocomposite films.

EXPERIMENTAL

Materials

Ferrous sulfate heptahydrate, ferric chloride hexahydrate, oleic acid, ammonia, absolute ethyl alcohol, chloroform (AR, Sinopharm Chemical Reagent Co., Ltd); n-octane (CP, Chemical Reagent Co., Ltd); PLA (Ingeo[TM] biopolymer 3052D, Nature-Works LLC) were needed in the study.

Preparation of [Fe.sub.3][O.sub.4]/PLA Nanocomposite Films

At room temperature, 3.24 g of ferric chloride hexahydrate, 2.50 g of ferrous sulphate heptahydrate, and 80 mL of deionized water were mixed in a three-necked round-bottomed flask to obtain a transparent and homogenous solution. The solution was then heated from room temperature to 80[degrees]C and maintained at 80[degrees]C for 2 h. During heating, when the temperature reached 40[degrees]C, 16 mL of ammonia and a further 5 mL of oleic acid was added at 80[degrees]C into the flask, respectively. The whole process was carried out under a nitrogen atmosphere. After the completion of the reaction, ethanol and deionized water were then alternately added to purify the [Fe.sub.3][O.sub.4] nanoparticles three times and remove unreacted chemicals. The [Fe.sub.3][O.sub.4] nanoparticles were collected and washed using a magnetic separation method. The oleic acid coated [Fe.sub.3][O.sub.4] was then dispersed in n-octane. The suspension was centrifuged at 15,000 rpm for 10 min and the supernatant was removed.

[Fe.sub.3][O.sub.4]/PLA nanocomposite samples were prepared in the form of films for different tests. PLA pellets were dissolved in chloroform at room temperature to obtain a completely transparent and homogenous solution. Different weight ratios of [Fe.sub.3][O.sub.4] nanoparticles were added to the PLA solutions and ultrasonicated for 20 min to ensure a uniform dispersion of [Fe.sub.3][O.sub.4]. The [Fe.sub.3][O.sub.4]/PLA mixtures were finally placed on a glass substrate and [Fe.sub.3][O.sub.4]/PLA composite films were obtained upon solvent evaporation at room temperature for 24 h of air-drying, followed by 48 h of vacuum-drying. [Fe.sub.3][O.sub.4]/PLA composite films with different content of [Fe.sub.3][O.sub.4] nanoparticles (0, 0.5, 1, 1.5, 2, and 3 wt %), were labeled as PLA, 05[Fe.sub.3][O.sub.4]/PLA, 1[Fe.sub.3][O.sub.4]/PLA, 15[Fe.sub.3][O.sub.4]/PLA, 2[Fe.sub.3][O.sub.4]/ PLA, and 3[Fe.sub.3][O.sub.4]/PLA, respectively.

Characterizations

The morphologies of the [Fe.sub.3][O.sub.4]/PLA membranes and [Fe.sub.3][O.sub.4] nanoparticles were studied by field emission TEM (JEOL Model JEM-2100F, Japan) with an operating voltage of 200 Kv. To prepare the TEM samples, the unfilled PLA and [Fe.sub.3][O.sub.4]/PLA were dissolved in a chloroform solution, and a small amount of the solution was dropped onto a copper net sample holder using a syringe. A drop of solution on a salt tablet of potassium bromide was used to study the crystalline phases of the [Fe.sub.3][O.sub.4]/PLA composite by FT-1R spectroscopy (Tensor LI, Bruker). XRD (D8 Advance, Bruker) was used to obtain information on the crystalline phases with Cu[K.sub.[alpha]] ([lambda] = 1.5406 nm) radiation at a scanning rate of 2[degrees]/min from 10[degrees] to 70[degrees]. The voltage was 40 kV and current was 30 mA. Crystallization information of the films was obtained by calorimetric measurements using DSC (Q800 TA) in the temperature range from 25[degrees] to 200 [degrees]C under a nitrogen atmosphere at a heating rate of 10[degrees]C/min. The weight of the specimens was in the range 5-10 mg. POM (DM2500P, LEICA) with a thermal platform (THMS600, Linkam) was used to study the crystalline morphology of the membrane. An alternating magnetic field was incorporated with the thermal platform (Fig. 1). A thermogravimetric instrument (SDTQ600, TA) was used to investigate the thermal degradation properties of the [Fe.sub.3][O.sub.4]/PLA nanocomposites in the temperature range 50[degrees]C-430[degrees]C at a heating rate of 10[degrees]C /min. Nitrogen was used as purge gas at a flow rate of 100 mL/min, and the specimens in the TGA each weighed ca. 8 mg.

RESULTS AND DISCUSSION

TEM micrographs of the [Fe.sub.3][O.sub.4] nanoparticles and the [Fe.sub.3][O.sub.4]/ PLA nanocomposites are presented in Fig. 2. 3[Fe.sub.3][O.sub.4]/PLA (Fig. 2b and c) was chosen as a representative sample. As shown in Fig. 2, the diameter of the [Fe.sub.3][O.sub.4] nanoparticles is around 8 nm and the nanoparticles are uniformly dispersed in the PLA without aggregation. This indicated that the interaction between the [Fe.sub.3][O.sub.4] nanoparticles and the PLA chains are very strong, and simple solution casting is an effective method to fabricate [Fe.sub.3][O.sub.4]/PLA nanocomposites.

Figure 3a shows the FT-IR spectra (3,200-700 [cm.sup.-1]) of the unfilled PLA and FesOyPLA nanocomposite films with various [Fe.sub.3][O.sub.4] content, and typical PLA characteristic peaks were obtained. For example, in Fig. 3, a strong absorption band at 1,759 [cm.sup.-1], which corresponds to the -C=0 stretching of the ester group, was found [16,17], Symmetric and asymmetric -C[H.sub.3] deformation vibrations, at 1,383 and 1,456 [cm.sup.-1] respectively, were observed. The symmetric and asymmetric C-H stretching vibrations of the -C[H.sub.3] group, respectively, were also found at 2,945 and 2,995 [cm.sup.-1]. The bands at 1,184 and 1,212 [cm.sup.-1] represent the symmetric C-O-C stretching of the ester groups. The absorptions at 1,091, 1,047, and 870 [cm.sup.-1] are attributed to the asymmetric C-O-C, C-C[H.sub.3] and C-COO stretching vibrations, respectively [18]. The absorption band at 1,130 [cm.sup.-1] belongs to the -C[H.sub.3] rocking mode [19]. We found that there is no significant difference between the unfiled PLA and the [Fe.sub.3][O.sub.4]/PLA nanocomposites. This may be due to the weak absorption peaks of the [Fe.sub.3][O.sub.4] being overshadowed by the strong absorption peaks of the PLA and a similar phenomenon has been reported previously [20,21]. As stated by Zhang et al. [22], the band belongs to the coupling of the backbone C-C stretching vibrations, with -C[H.sub.3] rocking is found at 921 [cm.sup.1] that is sensitive to the [10.sub.3] helix chain, conforming stable [alpha]-phase crystals of PLA. The absorption band of the PLA [beta]-phase crystal is found at 908 [cm.sup.1], while the band of the amorphous PLA is at 957 [cm.sup.1]. In our study, the band at 921 [cm.sup.1] is clear observed in the unfilled PLA and 05[Fe.sub.3][O.sub.4]/PLA. When [Fe.sub.3][O.sub.4] was added, its intensity slightly decreased (Fig. 3b), which may be an indication of a decrease in the nucleation rate and nucleation density.

PLA is very difficult to crystallize during the common processing method and is usually in the amorphous state [23]. The slow melt-crystallization behavior of PLA is associated with the rigid structure of the PLA molecules, as indicated by a high Tg near 60[degrees]C [24]. In this FT-IR characterization, the sample was prepared by adding a drop of the solution onto a salt tablet of potassium bromide. The pure PLA has high nucleation and growth rate during the process of chloroform volatilization for the plasticization of the solvent molecules. However, after adding [Fe.sub.3][O.sub.4], the intensity of 921 [cm.sup.-1] decreased with increasing content of [Fe.sub.3][O.sub.4]. Because of the high specific surface area of [Fe.sub.3][O.sub.4] and the strong interaction between the polymer chain and [Fe.sub.3][O.sub.4], many polymer chains were absorbed on the surface of [Fe.sub.3][O.sub.4] [25], hindering the PLA crystallization.

XRD was used to obtain information about the crystal structure of the nanocomposite films prepared by the solution casting method, without any annealing process. Figure 4 shows the XRD patterns of the [Fe.sub.3][O.sub.4] nanoparticles, unfilled PLA and [Fe.sub.3][O.sub.4]/PLA nanocomposites. It is clear that no peculiar diffraction peaks are found in the unfilled PLA samples. The two major diffraction peaks correspond to the (200)/(l 10) and (203) crystalline diffractions of PLA [26,27]. When the concentration of [Fe.sub.3][O.sub.4] increases, these two peaks are suppressed. This phenomenon indicates that the existence of [Fe.sub.3][O.sub.4] hampers the crystallization of PLA, i.e. a higher filling amount of [Fe.sub.3][O.sub.4] leads to a lower crystallinity of PLA, projecting a consistent outcome with the FT-IR. It is interesting that the Fe3C>4/PLA patterns exhibit no diffraction peaks of [Fe.sub.3][O.sub.4]. This can be due to the Bragg reflection signal of the [Fe.sub.3][O.sub.4] nanoparticles being too weak as compared with the PLA, because of the low [Fe.sub.3][O.sub.4] content (only 3 wt% in the PLA matrix) [28].

[X.sub.c] = [DELTA][H.sub.m] - [DELTA][H.sub.cc]/[DELTA][H.sup.0.sub.[psi]] x 100% (1)

where [X.sub.c], [DELTA][H.sub.m], and [DELTA][H.sub.cc] are the degrees of (melt) crystallinity, enthalpies of melting and enthalpies of cold-crystallization, respectively.

DSC heating curves of the [Fe.sub.3][O.sub.4]/PLA nanocomposites are shown in Fig. 5. Eq. 1 was used to conduct an analysis of the thermograms. Table 1 shows the parameters of interest such as the thermal-transitions and the degrees of (melt) crystallinity ([X.sub.c]). From the DSC curves of the unfilled PLA and [Fe.sub.3][O.sub.4]/PLA nanocomposites, the glass transition temperature (Tg) of the unfilled PLA is around 60[degrees]C, which is very close to the values given in the literature [22,29]. The [T.sub.g] of PLA was not affected by the existence of [Fe.sub.3][O.sub.4], and the crystallization temperature of the unfilled PLA was 130.3[degrees]C. [Fe.sub.3][O.sub.4] has a heterogeneous nucleation effect on the crystallization of PLA when 0.5 wt% of [Fe.sub.3][O.sub.4] was incorporated. PLA has a strong nucleation capacity at higher temperatures, which results in an increase of [T.sub.cc]. The higher crystallization temperature implies a higher degree of crystal perfection, which subsequently improves the [X.sub.c] and [T.sub.m]. On further increase, the concentration of oleic acid capped [Fe.sub.3][O.sub.4], the distance between the PLA chains became larger and the motion freedom of PLA chains was restricted by the strong interaction between the [Fe.sub.3][O.sub.4] nanoparticles and PLA chains. Therefore, the [T.sub.cc] of PLA shows a decreasing trend with an increase of [Fe.sub.3][O.sub.4] filling content. Crystal defects in the PLA increase as [T.sub.cc] decreases and can contribute to the decrease of [T.sub.m] and [X.sub.c].

As an effective approach to visualize the crystal patterns, POM is a powerful tool to observe the crystallization behavior of the PLA that was affected by [Fe.sub.3][O.sub.4]. In order to verify the experimental results shown in Table 1, POM was used to study the effect of [Fe.sub.3][O.sub.4] on the isothermal crystallization behavior of PLA. Figure 6 shows the POM images of the unfilled PLA and [Fe.sub.3][O.sub.4]/PLA nanocomposite films with different [Fe.sub.3][O.sub.4] content (0.5%, 1%, 1.5%, 2%, and 3%), after melting at 200[degrees]C for 5 min and crystallizing at 120 [degrees]C for 20 minutes. Under the same magnification, [Fe.sub.3][O.sub.4]/PLA composite films with 0.5%, 1%, and 1.5% of [Fe.sub.3][O.sub.4] (i.e., Fig. 6b-d) exhibited smaller sizes of spherocrystals (Fig. 6a) as compared with the unfilled PLA. As [Fe.sub.3][O.sub.4] can act as an effective heterogeneous nucleating agent to facilitate the melt crystallization of PLA during the isothermal crystallization, smaller sizes of spherocrystals were observed in the [Fe.sub.3][O.sub.4] incorporated samples. However, the spherocrystal size became bigger when the [Fe.sub.3][O.sub.4] concentration was >1.5%, as shown in Fig. 6e and f. It can be concluded that nucleation is more favorable when the [Fe.sub.3][O.sub.4] concentration is <1.5% in the samples treated in the above heating regime.

In order to further investigate the effect of heat treatment, another regime (i.e., melting at 200[degrees]C for 5 min and crystallizing at 123[degrees]C for 60 min.) was applied and the POM images are presented in Fig. 7. As seen in Fig. 7, the spherocrystal size of the FesOyPLA composite films is bigger than those obtained in Fig. 6, with the same concentration of [Fe.sub.3][O.sub.4]. Moreover, the same isothermal crystallization behavior was observed that is, when the incorporation of [Fe.sub.3][O.sub.4] was <1.5%, the nucleation of PLA composite was facilitated because the motion of PLA chains was confined by the strong interaction between PLA chains and [Fe.sub.3][O.sub.4] nanoparticles (Scheme la). However, when a higher [Fe.sub.3][O.sub.4] content was incorporated, the nucleation of the PLA was mostly induced by two or more [Fe.sub.3][O.sub.4] nanoparticles (Scheme lb), which significantly reduced the space between the [Fe.sub.3][O.sub.4] nanoparticles and thereby decreased the number of PLA spherocrystals.

Williams-Landel-Ferry Equation (Eq. 2) can be used to calculate the relaxation time of polymer chains at different temperatures.

Log([[alpha].sub.T])=-[C.sub.1](T - [T.sub.g])/[C.sub.2] + (T - [T.sub.g]) (2)

where [[alpha].sub.T] is the superposition parameter, [c.sub.1] and [c.sub.2] are empirical constants, T is temperature, and [T.sub.g] is the glass transition temperature. For most polymers, [c.sub.1] is 17.44 and [c.sub.2] is 51.6 [30]. Based on Eq. 2, 1/[tau] of PLA chains is 5.2 x [10.sup.7] Hz under 100[degrees]C (x is the relaxation time of PLA chains). To investigate the effect of [Fe.sub.3][O.sub.4] on the isothermal crystallization behaviors of [Fe.sub.3][O.sub.4]/PLA, the thermal platform was enclosed in an alternating magnetic field (Fig. 1). The frequency range of the alternating magnetic field is 60-200 Hz. Therefore, the motion of PLA chains can be synchronized with the vibration of [Fe.sub.3][O.sub.4] particles during the isothermal crystallization. When the vibrational amplitude and frequency are higher, the motion of PLA chains will be bigger and faster.

The isothermal crystallization of l[Fe.sub.3][O.sub.4]/PLA samples was studied at 123[degrees]C under the influence of magnetic field. For unfilled PLA, the alternating magnetic field has no effect on the crystallization behaviors. In Fig. 8, the POM images of the 1[Fe.sub.3][O.sub.4]/PLA with different alternating magnetic field condition melting at 200[degrees]C for 5 min and crystallizing at 123[degrees]C for 60 min are displayed. When the alternating magnetic field was 165 Hz, the size of spherocrystals decreased with the increase of vibrational amplitude and it was hard to maintain the spherical shape of the crystals when the vibrational amplitude increased to 0.64 mT (Fig. 8a). When the frequency was swept from 60 to 200 Hz under a strength of 0.48 mT, the size of spherocrystals increased firstly, and then decreased at the frequency of 165 Hz (Fig. 8b). These results showed that the crystallization behavior of PLA was also affected by the vibrational motion of PLA chains. When the frequency was swept from 60 to 130 Hz, the number of crystal nuclei decreased and the size of spherocrystals increased. However, the size of spherocrystals decreased and the crystal was deformed from spherical to heteromorphic shape after the frequency surpassing 165 Hz due to the intensive vibrational of PLA chains.

These results showed when the amplitude of the alternating magnetic field is stronger, the effect of vibration on the crystallization behaviors of PLA are more remarkable. Moreover, the frequency of the alternating magnetic field also affects the nucleation rate of PLA. The PLA chains are easy to form crystal nuclei under a particular vibration of PLA chains. The number of crystal nuclei decreases and the size of spherocrystals increases when the frequency is increased from 60 to 165 Hz. However, the size of spherocrystals decreases after the frequency 165 Hz. The effect of alternating magnetic field on the degree, structure and rate of crystallinity will be investigated in the on-going research work.

TGA curves can provide information on the thermal stability of PLA under the influence of [Fe.sub.3][O.sub.4]. Unfilled PLA and the [Fe.sub.3][O.sub.4]/PLA nanocomposites show similar weight loss (%) in the TGA curves shown in Fig. 9a. A decrease of onset temperature, which is the initial weight loss and maximum degradation rate of PLA (i.e., the peaks in the derivative Differential Thermal Gravity (DTG), Fig. 9b), was obtained when more [Fe.sub.3][O.sub.4] was added, indicating that the incorporation of [Fe.sub.3][O.sub.4] can affect the thermal stability. Because of the large surface area and high chemical activity of [Fe.sub.3][O.sub.4] [31,32], degradation of PLA occurs due to the local heat generated from the inelastic electron-phonon collisions [33,34], The thermal degradation of PLA can be catalyzed by this current-induced heat from [Fe.sub.3][O.sub.4] due to the strong interactions between the [Fe.sub.3][O.sub.4] and the PLA matrix. When the temperature reaches 430[degrees]C, the residual weights of [Fe.sub.3][O.sub.4]/PLA films are close to the weight of the initially added [Fe.sub.3][O.sub.4]. As shown in Fig. 9b, only a single peak is observed in all samples, implying that all materials are thermally degraded. The earlier onset corresponds to the lower thermal stability of PLA due to the embedded [Fe.sub.3][O.sub.4] nanoparticles.

CONCLUSIONS

[Fe.sub.3][O.sub.4]/PLA nanocomposite films containing different concentrations of [Fe.sub.3][O.sub.4] were successfully fabricated via the solution casting method. TEM, XRD, and FT-IR were used to study the microstructures and crystalline phases of [Fe.sub.3][O.sub.4]/PLA films. TEM images showed that [Fe.sub.3][O.sub.4] nanoparticles of size around 8 nm were uniformly dispersed in the PLA matrix. According to the results of FT-IR and XRD, we found that [Fe.sub.3][O.sub.4] affects the crystallization of PLA during the precipitation process. The DSC results indicate that [Fe.sub.3][O.sub.4] can promote the crystallization of PLA when a small amount of [Fe.sub.3][O.sub.4] is incorporated in the PLA. Further increase in the concentration of [Fe.sub.3][O.sub.4] resulted in an impediment to the crystallization of PLA. According to POM images of the [Fe.sub.3][O.sub.4]/PLA composite films with different content of [Fe.sub.3][O.sub.4] nanoparticles that the melted at 200[degrees] C for 5 min and crystallized at 120[degrees]C or 123[degrees]C, it was found that [Fe.sub.3][O.sub.4] could promote the nucleation of PLA when 1.5 wt% of [Fe.sub.3][O.sub.4] particles were added into the PLA. The morphology and nucleation rate of PLA crystallization can be adjusted by the vibration amplitude and frequency of [Fe.sub.3][O.sub.4] particles. The results of TGA indicated that the onset temperature and the thermal decomposition time for the weight loss of PLA decreased with an increase of [Fe.sub.3][O.sub.4] content, suggesting that [Fe.sub.3][O.sub.4] can reduce the thermal stability of PLA.

ACKNOWLEDGMENTS

We greatly appreciate the staff of Industrial Centre of The Hong Kong Polytechnic University for technical advice, and substantial support from the Research Committee of The Hong Kong Polytechnic University (Project Account Code: 1-ZVJ5) and the National Natural Science Foundation of China (NSFC) (No.51173039).

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Xinghou Gong, (1,2) Cheng Cheng, (1) Chak Yin Tang, (2) Wing-Cheung Law (iD), (2) Xueting Lin, (1) Yehuang Chen, (1) Ling Chen, (2) Gary Chi Pong Tsui, (2) Nanxi Rao (2)

(1) Hubei Provincial Key Laboratory of Green Materials for Light Industry, Collaborative Innovation Center of Green Lightweight Materials and Processing, School of Materials and Chemistry Engineering, Hubei University of Technology, Wuhan, Hubei Province, 430068, China

(2) Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Horn, Kowloon, Hong Kong, SAR, China

Correspondence to: W.-C. Law; e-mail: roy.law@polyu.edu.hk

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 51173039. contract grant sponsor: Hong Kong Polytechnic University; contract grant number: 1-ZVJ5.

DOI 10.1002/pen.24976

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. The alternating magnetic field producer with a thermal platform. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. (a) TEM image of [Fe.sub.3][O.sub.4] nanoparticles; (b) TEM image of 3[Fe.sub.3][O.sub.4]/PLA composite; (c) high magnification of selected area in Fig 1b.

Caption: FIG. 3. FT-IR spectra for the unfilled PLA and [Fe.sub.3][O.sub.4]/PLA nanocomposites of different [Fe.sub.3][O.sub.4] contents as indicated on the ends of FT-IR traces: (a) in the range of 3,200-700 [cm.sup.-1] and (b) a magnification in the sub-range of 1,000-850 [cm.sup.-1] [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. XRD patterns of [Fe.sub.3][O.sub.4] nanoparticles, the unfilled PLA and the [Fe.sub.3][O.sub.4]/PLA nanocomposites with different [Fe.sub.3][O.sub.4] concentrations indicated on the right ends of XRD curves. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. DSC heating curves of unfilled PLA and [Fe.sub.3][O.sub.4]/PLA composites. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. POM images of [Fe.sub.3][O.sub.4]/PLA composite films with different contents of [Fe.sub.3][O.sub.4] melting at 200[degrees]C for 5 min and crystallizing at 120 [degrees]C for 20 minutes: (a) PLA, (b) 05[Fe.sub.3][O.sub.4]/PLA, (c) 1[Fe.sub.3][O.sub.4]/PLA, (d)15[Fe.sub.3][O.sub.4]/PLA, (e) 2F630VPLA and (f) 3[Fe.sub.3][O.sub.4]/PLA (Scale bars: 50 [micro]m) [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. POM images of [Fe.sub.3][O.sub.4]/PLA composite films with different contents of [Fe.sub.3][O.sub.4] melting at 200[degrees]C for 5 min and crystallizing at 123[degrees]C for 60 min: (a) PLA, (b) 05[Fe.sub.3][O.sub.4]/PLA, (c) l[Fe.sub.3][O.sub.4]/PLA, (d) 15[Fe.sub.3][O.sub.4]/PLA, (e) 2[Fe.sub.3][O.sub.4]/PLA and (f) 3[Fe.sub.3][O.sub.4]/PLA (Scale bars: 50 [micro]m) [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. POM images of 1[Fe.sub.3][O.sub.4]/PLA composite films with different alternating magnetic field condition melting at 200[degrees]C for 5 min and crystallizing at 123[degrees]C for 60 min: (a) 165 Hz, (b) 0.48 mT (scale bar: 100 pm) [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. TGA results of the PLA and [Fe.sub.3][O.sub.4]/PLA nanocomposites with different [Fe.sub.3][O.sub.4] contents: (a) TGA curves and (b) DTG curves. [Color figure can be viewed at wileyonlinelibrary.com]

Caption : SCH 1. Schematic diagram of PLA nucleation induced by (a) appropriate amount of [Fe.sub.3][O.sub.4] nanoparticles and (b) excess amount of [Fe.sub.3][O.sub.4] nanoparticles. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Thermal transitions (a) and degrees of (Melt)
Crystallinity (b) for the neat PLA and the [Fe.sub.3][O.sub.4]/
PLA nanocomposites with different concentration of
[Fe.sub.3][O.sub.4](0.5, 1,2, and 3 wt%).

Sample          [T.sub.g]([degrees]C)    [T.sub.cc]([degrees]C)

PLA                      59.8                     130.3
05[Fe.sub.3]             60.8                     132.3
[O.sub.4]/PLA
l[Fe.sub.3]              60.3                     126.0
[O.sub.4]/PLA
2[Fe.sub.3]              60.5                     128.8
[O.sub.4]/PLA
3[Fe.sub.3]              60.3                     119.5
[O.sub.4]/PLA

Sample           [T.sub.m]([degrees]C)    [DELTA][H.sub.cc]
                                                (J/g)

PLA                      164.6                  -23.2
05[Fe.sub.3]             166.2                  -19.0
[O.sub.4]/PLA
l[Fe.sub.3]              165.3                  -34.4
[O.sub.4]/PLA
2[Fe.sub.3]              165.5                  -33.2
[O.sub.4]/PLA
3[Fe.sub.3]              163.8                  -29.3
[O.sub.4]/PLA

Sample          [DELTA][H.sub.m]   [X.sub.c] (%)
                     (J/g)

PLA                   29.1              6.3
05[Fe.sub.3]          27.2              8.8
[O.sub.4]/PLA
l[Fe.sub.3]           44.7             11.1
[O.sub.4]/PLA
2[Fe.sub.3]           41.9              9.4
[O.sub.4]/PLA
3[Fe.sub.3]           33.3              4.3
[O.sub.4]/PLA

(a) Thermal transitions include glass transition temperature
([T.sub.g]), cold-crystallization temperature ([T.sub.cc]),
enthalpy of cold-crystallization ([DELTA][H.sub.cc]), melting
temperature ([T.sub.m]), and enthalpy of melting
([DELTA][H.sub.m]).

(b) Degree of (melt) crystallinity ([X.sub.c]) is estimated using
Equation 1, where [DELTA][H.sup.0] is the enthalpy of melting of
the 100% perfectly crystalline PLA, 93.0 J/g, and [phi] the weight
fraction of PLA in the [Fe.sub.3][O.sub.4]/PLA nanocomposite.
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Author:Gong, Xinghou; Cheng, Cheng; Tang, Chak Yin; Law, Wing-Cheung; Lin, Xueting; Chen, Yehuang; Chen, Li
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2019
Words:5678
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