Lamellae Evolution of Stereocomplex-Type Poly(Lactic Acid)/Organically-Modified Layered Zinc Phenylphosphonate Nanocomposites Induced by Isothermal Crystallization

Stereocomplex-type poly(lactic acid) (SC-PLA)/oleylamine-modified layered zinc phenylphosphonate (SC-PLA/m-PPZn) nanocomposites are successfully fabricated using a solution mixing process. Wide-angle X-ray diffraction (WAXD) analysis reveals that the structural arrangement of the oleylamine-modified PPZn exhibits a large interlayer spacing of 30.3 Å. In addition, we investigate the temperature effect on the real-time structural arrangement of PPZn and m-PPZn. The results indicated that the lattice expansion of m-PPZn with increasing temperature leads to an increase in the interlayer spacing from 30.3 to 37.1 Å as the temperature increases from 30 to 150 °C. The interlayer spacing decreases slightly as the temperature further increases to 210 °C. This behavior might be attributed to interlayer oleylamine elimination, which results in hydrogen bonding destruction between the hydroxide sheets and water molecules. As the temperature reaches 240 °C, the in situ WAXD patterns show the coexistence of m-PPZn and PPZn. However, the layered structures of m-PPZn at 300 °C are almost the same as those of PPZn, after the complete degradation temperature of oleylamine. The morphology of the SC-PLA/m-PPZn nanocomposites characterized using WAXD and transmission electron microscopy (TEM) demonstrates that most partial delamination layered materials are randomly dispersed in the SC-PLA matrix. Small-angle X-ray scattering reveals that higher crystal layer thickness and lower surface free energy is achieved in 0.25 wt% SC-PLA/m-PPZn nanocomposites. These results indicate that the introduction of 0.25 wt% m-PPZn into SC-PLA reduces the surface free energy, thereby increasing the polymer chain mobility.


Fabrication of the SC-PLA/m-PPZn Nanocomposites
The preparation of oleylamine intercalation into zinc phenylphosphonate (m-PPZn) was previously reported [41]. Separately prepared solutions of PLLA and PDLA in dichloromethane (0.2 g¨mL´1) were mixed for 3 h. The weight ratio of PLLA/PDLA in the blend was fixed at 1:1, and solution-mixing was applied for 3 h to obtain the SC-PLA solution. The m-PPZn dispersion suspended in 10 mL dichloromethane was obtained by sonication for 3 h. Immediately after sonication, various compositions (0.25, 0.5 and 1 wt%) of the m-PPZn suspension were added to the SC-PLA solution, followed by further mechanical stirring for 12 h. Various concentrations of SC-PLA/m-PPZn solution were cast on a glass Petri dish at 25˝C and dried in a vacuum oven at 40˝C for 24 h. All samples were heated to the pre-melting temperature (T max ) of 240˝C at a heating rate of 10˝C/min, held for 2 min to erase the thermal history, cooled to the proposed crystallization temperatures (T cs ) at a cooling rate of 100˝C/min and held for 1 h. According to the results of previous studies, the SC-PLA crystallization temperatures range between 180 and 200˝C [15,[31][32][33][34]. Because the addition of m-PPZn to SC-PLA could induce heterogeneous nucleation at a 180 to 200˝C crystallization temperature, the crystallization behavior of SC-PLA is very difficult to observe using polarized optical microscopy at lower crystallization temperatures. Thus, the proposed T cs values were selected between 192 and 198˝C. Subsequently, the samples were rapidly cooled to room temperature using liquid nitrogen and, then, used for the following analysis.

Wide-Angle X-Ray Diffraction
WAXD was performed on a Bruker D8 diffractometer (BRUKER AXS, Inc., Madison, WI, USA) equipped with Ni-filtered Cu K α radiation in the reflection mode. X-ray diffraction patterns were recorded between 2θ = 1.5˝and 40˝at a scan rate of 1˝/min. In situ WAXD experiments were performed using a temperature-attachment assembly under vacuum; the applied temperature increasing rate was 10˝C/min, and the temperature was stabilized for 5 min before each measurement.

Small-Angle X-Ray Scattering
The small-angle X-ray scattering (SAXS) experiments were performed using the same Bruker D8 Discover equipped with Ni-filtered Cu K α radiation in the transmission mode. The distance between the sample and the detector was~30 cm. The scattering vector (q, nm´1) was defined as q = (4πsinθ)/λ, where λ and 2θ are the wavelength and scattering angle, respectively.

Fourier Transform Infrared Spectroscopy
The in situ Fourier transform infrared (in situ FTIR) spectra were obtained on a Perkin-Elmer spectrometer (PerkinElmer, Waltham, MA, USA). One spectrum in the transmission mode from 650 to 4000 cm´1 was obtained after 20 scans at a 4 cm´1 resolution using the standard KBr disk method (1 mg of sample in 100 mg of KBr). The spectra were recorded every 10˝C, and the heating rate was 10˝C/min.

Transmission Electron Microscopy
Ultrathin sections of SC-PLA/m-PPZn nanocomposites were mounted on a carbon-coated copper grid and observed on a TE microscope (Hitachi HF-2000 at 200 kV, JEOL Ltd., Tokyo, Japan). A Reichert Ultracut ultramicrotome equipped with a diamond knife was used to prepare the ultrathin films.

Polarized Optical Microscopy
Polarized optical microscopy (POM) was performed using a Zeiss optical microscope (Carl Zeiss Microscopy GmbH, Jena, Germany) equipped with crossed polarizers. To observe the SC-PLA/m-PPZn nanocomposites' crystallization process, nanocomposite samples were heated to melting at T max = 240˝C for two minutes on a Mettler FP-82 hot stage to eliminate the thermal history. Subsequently, the samples were cooled quickly to the proposed T cs . POM data were repeatedly recorded at the proposed T cs .

Thermogravimetric Analysis
Thermogravimetric analysis of the samples was performed on a Perkin-Elmer TG/DTA 6300 (PerkinElmer, Waltham, Massachusetts, USA). Five-milligram powdered samples were mounted in an aluminum crucible. The TG/DTA analysis was performed in flowing nitrogen at a heating rate of 10˝C/min from 30 to 800˝C.

PPZn and m-PPZn Characterization
The organically-modified layered zinc phenylphosphonate was prepared by a facile chemical intercalation method. According to the previously literature [41], we knew that oleylamine is successfully intercalated into the interlayer distances of PPZn. Applying Bragg's equation, the interlayer spacing d 010 in PPZn and m-PPZn was determined at 14.5 Å and 30.3 Å, respectively.

In Situ Study of the Intercalation Behavior and the Thermal Properties of PPZn and m-PPZn
Typical thermogravimetric analysis (TGA) curves of PPZn, m-PPZn and oleylamine are presented in Figure 1. For PPZn, the first weight loss begins at approximately 80˝C, when the monohydrate molecules in the interlayer gallery of PPZn are removed [33,34]. The second weight loss begins at approximately 570˝C and is attributed to the aromatic ring degradation. This result indicates that the PPZn exhibits remarkable thermal stability. For m-PPZn, three main regions of weight loss are observed. The first loss is attributed to the removal of the monohydrate molecules in the interlayer gallery of m-PPZn, beginning at approximately 80˝C. The second weight loss occurs at 180˝C because of the removal of oleylamine from the interlayer gallery of PPZn. This behavior is continued with increasing temperature, and then, complete decomposition of oleylamine occurs at approximately 300˝C. For the comparison, the TGA data of oleylamine is also presented in this figure. The aromatic ring removal of m-PPZn is also characterized by mass loss at approximately 570˝C.

Thermogravimetric Analysis
Thermogravimetric analysis of the samples was performed on a Perkin-Elmer TG/DTA 6300 (PerkinElmer, Waltham, Massachusetts, USA). Five-milligram powdered samples were mounted in an aluminum crucible. The TG/DTA analysis was performed in flowing nitrogen at a heating rate of 10 °C/min from 30 to 800 °C.

PPZn and m-PPZn Characterization
The organically-modified layered zinc phenylphosphonate was prepared by a facile chemical intercalation method. According to the previously literature [41], we knew that oleylamine is successfully intercalated into the interlayer distances of PPZn. Applying Bragg's equation, the interlayer spacing d010 in PPZn and m-PPZn was determined at 14.5 Å and 30.3 Å , respectively.

In Situ Study of the Intercalation Behavior and the Thermal Properties of PPZn and m-PPZn
Typical thermogravimetric analysis (TGA) curves of PPZn, m-PPZn and oleylamine are presented in Figure 1. For PPZn, the first weight loss begins at approximately 80 °C, when the monohydrate molecules in the interlayer gallery of PPZn are removed [33,34]. The second weight loss begins at approximately 570 °C and is attributed to the aromatic ring degradation. This result indicates that the PPZn exhibits remarkable thermal stability. For m-PPZn, three main regions of weight loss are observed. The first loss is attributed to the removal of the monohydrate molecules in the interlayer gallery of m-PPZn, beginning at approximately 80 °C. The second weight loss occurs at 180 °C because of the removal of oleylamine from the interlayer gallery of PPZn. This behavior is continued with increasing temperature, and then, complete decomposition of oleylamine occurs at approximately 300 °C. For the comparison, the TGA data of oleylamine is also presented in this figure.
The aromatic ring removal of m-PPZn is also characterized by mass loss at approximately 570 °C.   (020), reflections observed at 2θ = 6.31° and 12.48° slightly shift toward a lower 2θ angle with increasing temperature. This finding indicates that the PPZn lattice is expanded as the temperature increases. Consequently, the interlayer spacing of PPZn slightly increases. As observed in Figure 2b, the (010) and (020) diffractions of m-PPZn occur at a lower 2θ angle accompanied by simultaneous broadening from 90 to 120 °C. Therefore, m-PPZn expands the lattice with increasing temperature. The in situ WAXD patterns of the (010) and (020) diffractions   (020), reflections observed at 2θ = 6.31˝and 12.48˝slightly shift toward a lower 2θ angle with increasing temperature. This finding indicates that the PPZn lattice is expanded as the temperature increases. Consequently, the interlayer spacing of PPZn slightly increases. As observed in Figure 2b, the (010) and (020) diffractions of m-PPZn occur at a lower 2θ angle accompanied by simultaneous broadening from 90 to 120˝C. Therefore, m-PPZn expands the lattice with increasing temperature. The in situ WAXD patterns of the (010) and (020) diffractions for m-PPZn shifted slightly to a higher 2θ angle from 180 to 210˝C. These results indicate that the oleylamine is gradually removed in the interlayer gallery of PPZn during this heating process. As the temperature reached 240˝C, a significant amount of oleylamine was removed. The in situ WAXD patterns show the coexistence of m-PPZn and PPZn. When the temperature continuously increases to 300˝C, the XRD curve presents the profile of PPZn. This result indicates that the oleylamine was completely removed in the interlayer gallery of m-PPZn, a hypothesis consistent with the finding of TGA data. The results suggest that the structure of m-PPZn at 300˝C is almost the same as that of PPZn.
Materials 2016, 9,159 for m-PPZn shifted slightly to a higher 2θ angle from 180 to 210 °C. These results indicate that the oleylamine is gradually removed in the interlayer gallery of PPZn during this heating process. As the temperature reached 240 °C, a significant amount of oleylamine was removed. The in situ WAXD patterns show the coexistence of m-PPZn and PPZn. When the temperature continuously increases to 300 °C, the XRD curve presents the profile of PPZn. This result indicates that the oleylamine was completely removed in the interlayer gallery of m-PPZn, a hypothesis consistent with the finding of TGA data. The results suggest that the structure of m-PPZn at 300 °C is almost the same as that of PPZn. To investigate the change of the interlayer distance further of PPZn and m-PPZn, the d spacings of the (010) diffraction obtained for various temperatures are summarized in Figure 3. The interlayer distance of PPZn increases slightly from 30 to 240 °C. In contrast, the d spacing of the (010) diffraction for m-PPZn significantly increases from 90 to 150 °C and then slightly decreases to 210 °C. As the temperature reached 240 °C, the structure presents the coexistence of PPZn and m-PPZn. Both d spacings of the (010) diffraction for PPZn and m-PPZn are obtained. When the temperature reaches 300 °C, the structure reveals the presence of PPZn only. The d spacing significantly decreases to 1.48 nm as the temperature increased to 300 °C. To interpret the in situ WAXD results during the heating process, in situ FTIR was employed to explain the difference between PPZn and m-PPZn.  To investigate the change of the interlayer distance further of PPZn and m-PPZn, the d spacings of the (010) diffraction obtained for various temperatures are summarized in Figure 3. The interlayer distance of PPZn increases slightly from 30 to 240˝C. In contrast, the d spacing of the (010) diffraction for m-PPZn significantly increases from 90 to 150˝C and then slightly decreases to 210˝C. As the temperature reached 240˝C, the structure presents the coexistence of PPZn and m-PPZn. Both d spacings of the (010) diffraction for PPZn and m-PPZn are obtained. When the temperature reaches 300˝C, the structure reveals the presence of PPZn only. The d spacing significantly decreases to 1.48 nm as the temperature increased to 300˝C.
Materials 2016, 9,159 for m-PPZn shifted slightly to a higher 2θ angle from 180 to 210 °C. These results indicate that the oleylamine is gradually removed in the interlayer gallery of PPZn during this heating process. As the temperature reached 240 °C, a significant amount of oleylamine was removed. The in situ WAXD patterns show the coexistence of m-PPZn and PPZn. When the temperature continuously increases to 300 °C, the XRD curve presents the profile of PPZn. This result indicates that the oleylamine was completely removed in the interlayer gallery of m-PPZn, a hypothesis consistent with the finding of TGA data. The results suggest that the structure of m-PPZn at 300 °C is almost the same as that of PPZn. To investigate the change of the interlayer distance further of PPZn and m-PPZn, the d spacings of the (010) diffraction obtained for various temperatures are summarized in Figure 3. The interlayer distance of PPZn increases slightly from 30 to 240 °C. In contrast, the d spacing of the (010) diffraction for m-PPZn significantly increases from 90 to 150 °C and then slightly decreases to 210 °C. As the temperature reached 240 °C, the structure presents the coexistence of PPZn and m-PPZn. Both d spacings of the (010) diffraction for PPZn and m-PPZn are obtained. When the temperature reaches 300 °C, the structure reveals the presence of PPZn only. The d spacing significantly decreases to 1.48 nm as the temperature increased to 300 °C. To interpret the in situ WAXD results during the heating process, in situ FTIR was employed to explain the difference between PPZn and m-PPZn. Figure 4 presents the in situ FTIR spectra of PPZn and m-PPZn from 30 to 300 °C. For PPZn (Curve (a) in Figure 4), the intensity of O-H stretching at 3470 and 3430 cm −1 decreases with increasing temperature. The peaks of the O-H stretching disappeared after 90 °C, indicating that the monohydrate molecules in the interlayer gallery of PPZn are completely removed and are consistent with TGA data. In addition, the To interpret the in situ WAXD results during the heating process, in situ FTIR was employed to explain the difference between PPZn and m-PPZn. Figure 4 presents the in situ FTIR spectra of PPZn and m-PPZn from 30 to 300˝C. For PPZn (Curve (a) in Figure 4), the intensity of O-H stretching at 3470 and 3430 cm´1 decreases with increasing temperature. The peaks of the O-H stretching disappeared after 90˝C, indicating that the monohydrate molecules in the interlayer gallery of PPZn are completely removed and are consistent with TGA data. In addition, the intensity of H-O-H bending at 1645 cm´1 shows a similar tendency using the same experimental conditions. As observed in Figure 4b, the intensity of the N-H stretching vibration at 3376 and 3297 cm´1 decreases when the temperature reaches 210˝C because of the gradual removal of oleylamine. The N-H stretching peaks completely disappeared at 300˝C. In addition, the intensity of the absorption bands at 1620 cm´1 for N-H bending decreases when the temperature increases from 150 to 300˝C. It can be assumed that oleylamine is removed at this range of temperature. Subsequently, the intensity of N-H bending slightly shifts to a higher wavenumber (approximately 1662 cm´1), because the interlayer oleylamine of m-PPZn is completely removed from 180 to 300˝C (Figure 4c). According to the in situ WAXD and FTIR data, the interlayer spacing of m-PPZn with the incorporation of oleylamine can be significantly enhanced from 90 to 120˝C, which corresponds to a lattice expansion with increasing temperature. The interlayer spacing of m-PPZn drastically decreases at 300˝C, close to the degradation temperature of oleylamine and consistent with the TGA results.
Materials 2016, 9,159 intensity of H-O-H bending at 1645 cm −1 shows a similar tendency using the same experimental conditions. As observed in Figure 4b, the intensity of the N-H stretching vibration at 3376 and 3297 cm −1 decreases when the temperature reaches 210 °C because of the gradual removal of oleylamine. The N-H stretching peaks completely disappeared at 300 °C. In addition, the intensity of the absorption bands at 1620 cm −1 for N-H bending decreases when the temperature increases from 150 to 300 °C. It can be assumed that oleylamine is removed at this range of temperature. Subsequently, the intensity of N-H bending slightly shifts to a higher wavenumber (approximately 1662 cm −1 ), because the interlayer oleylamine of m-PPZn is completely removed from 180 to 300 °C (Figure 4c). According to the in situ WAXD and FTIR data, the interlayer spacing of m-PPZn with the incorporation of oleylamine can be significantly enhanced from 90 to 120 °C, which corresponds to a lattice expansion with increasing temperature. The interlayer spacing of m-PPZn drastically decreases at 300 °C, close to the degradation temperature of oleylamine and consistent with the TGA results.

Morphology of SC-PLA and SC-PLA/m-PPZn Nanocomposites
X-ray diffraction effectively determines the interlayer spacing of the inorganic layered materials and the crystalline structure of fabricated SC-PLA/m-PPZn nanocomposites. The above WAXD and FTIR study demonstrate that most of the m-PPZn is very stable below 210 °C. Therefore, the temperature for isothermal crystallization selected in this study is between 192 and 198 °C, which is below 210 °C. The WAXD data of SC-PLA and SC-PLA/m-PPZn nanocomposites isothermally crystallized at 192 and 196 °C, respectively, are shown in Figure 5. The diffraction

Morphology of SC-PLA and SC-PLA/m-PPZn Nanocomposites
X-ray diffraction effectively determines the interlayer spacing of the inorganic layered materials and the crystalline structure of fabricated SC-PLA/m-PPZn nanocomposites. The above WAXD and FTIR study demonstrate that most of the m-PPZn is very stable below 210˝C. Therefore, the temperature for isothermal crystallization selected in this study is between 192 and 198˝C, which is below 210˝C. The WAXD data of SC-PLA and SC-PLA/m-PPZn nanocomposites isothermally crystallized at 192 and 196˝C, respectively, are shown in Figure 5. The diffraction peaks of m-PPZn disappear in all nanocomposite samples. The diffraction peaks at 2θ = 11.91˝, 20.69˝and 23.95 can be observed, and they are attributed to the crystalline structure of SC-PLA [13,42,43]. The complete disappearance of the m-PPZn diffraction peaks may be because of the low content of m-PPZn, leading to the formation of a disordered and exfoliated nanostructure within the SC-PLA matrix, in which the gallery height of intercalated layers is large enough and the layer correlation is not detected by the X-ray diffractometer. The result suggests that the polymer chains are successfully intercalated into the m-PPZn galleries, thereby leading to partial delamination of the m-PPZn with tactoid reduction.
Materials 2016, 9, 159 7 20.69° and 23.95 can be observed, and they are attributed to the crystalline structure of SC-PLA [13,42,43]. The complete disappearance of the m-PPZn diffraction peaks may be because of the low content of m-PPZn, leading to the formation of a disordered and exfoliated nanostructure within the SC-PLA matrix, in which the gallery height of intercalated layers is large enough and the layer correlation is not detected by the X-ray diffractometer. The result suggests that the polymer chains are successfully intercalated into the m-PPZn galleries, thereby leading to partial delamination of the m-PPZn with tactoid reduction. Furthermore, the morphology of 1 wt% m-PPZn in the SC-PLA matrix isothermally crystallized at 196 °C is observed through the TEM analysis (Figure 6a). Figure 6b presents high magnification TEM images of this sample. The gray areas represent the SC-PLA matrix, whereas the dark lines correspond to the PPZn layers. It is clear that the m-PPZn layers follow the morphology of the partially delaminated SC-PLA matrix and retain little of their stacking order. Accordingly, the m-PPZn layers' partial delamination morphology in the SC-PLA matrix is in good agreement with the WAXD results.

Microstructure of SC-PLA and SC-PLA/m-PPZn Nanocomposites
The above WAXD findings suggest that the microstructure of SC-PLA/m-PPZn nanocomposites is strongly affected by the loading of m-PPZn. The Lorentz-corrected small-angle X-ray scattering (SAXS) profiles of the SC-PLA and SC-PLA/m-PPZn nanocomposites isothermally Furthermore, the morphology of 1 wt% m-PPZn in the SC-PLA matrix isothermally crystallized at 196˝C is observed through the TEM analysis (Figure 6a). Figure 6b presents high magnification TEM images of this sample. The gray areas represent the SC-PLA matrix, whereas the dark lines correspond to the PPZn layers. It is clear that the m-PPZn layers follow the morphology of the partially delaminated SC-PLA matrix and retain little of their stacking order. Accordingly, the m-PPZn layers' partial delamination morphology in the SC-PLA matrix is in good agreement with the WAXD results.  Furthermore, the morphology of 1 wt% m-PPZn in the SC-PLA matrix isothermally crystallized at 196 °C is observed through the TEM analysis (Figure 6a). Figure 6b presents high magnification TEM images of this sample. The gray areas represent the SC-PLA matrix, whereas the dark lines correspond to the PPZn layers. It is clear that the m-PPZn layers follow the morphology of the partially delaminated SC-PLA matrix and retain little of their stacking order. Accordingly, the m-PPZn layers' partial delamination morphology in the SC-PLA matrix is in good agreement with the WAXD results.

Microstructure of SC-PLA and SC-PLA/m-PPZn Nanocomposites
The above WAXD findings suggest that the microstructure of SC-PLA/m-PPZn nanocomposites is strongly affected by the loading of m-PPZn. The Lorentz-corrected small-angle X-ray scattering (SAXS) profiles of the SC-PLA and SC-PLA/m-PPZn nanocomposites isothermally

Microstructure of SC-PLA and SC-PLA/m-PPZn Nanocomposites
The above WAXD findings suggest that the microstructure of SC-PLA/m-PPZn nanocomposites is strongly affected by the loading of m-PPZn. The Lorentz-corrected small-angle X-ray scattering (SAXS) profiles of the SC-PLA and SC-PLA/m-PPZn nanocomposites isothermally crystallized at 192 and 196˝C are presented in Figure 7. To obtain the morphological parameters, such as the long period (L p ), the crystal layer thickness (l c ) and the amorphous layer thickness (l a = L p -l c ), we utilized a single-dimensional correlation function. The single-dimensional correlation function, which is the Fourier transform of the corrected SAXS data, is written as follows [44,45]: where z is the correlation distance; Q is a scattering invariant; and I(q) is the experimental SAXS intensity corrected for thermal fluctuations. According to the pseudo-two-phase model, the linear degree of crystallinity within the lamellar stacks can be estimated from the correlation function [40]: where γ 0 is the ordinate corresponding to the first zero of the abscissa and X c is the linear crystallinity.
Notably, Equation (2) only stands for X c > 0.5. For X c < 0.5, X c is substituted by (1-X c ) in the above expressions.
Materials 2016, 9,159 8 crystallized at 192 and 196 °C are presented in Figure 7. To obtain the morphological parameters, such as the long period (Lp), the crystal layer thickness (lc) and the amorphous layer thickness (la = Lp-lc), we utilized a single-dimensional correlation function. The single-dimensional correlation function, which is the Fourier transform of the corrected SAXS data, is written as follows [44,45]: where z is the correlation distance; Q is a scattering invariant; and I(q) is the experimental SAXS intensity corrected for thermal fluctuations. According to the pseudo-two-phase model, the linear degree of crystallinity within the lamellar stacks can be estimated from the correlation function [40]: where γ0 is the ordinate corresponding to the first zero of the abscissa and Xc is the linear crystallinity. Notably, Equation (2) only stands for Xc > 0.5. For Xc < 0.5, Xc is substituted by (1-Xc) in the above expressions.  Table 1.
The q-value cannot be observed because the SC-PLA degree of crystallinity is lower. Therefore, the values of Lp, lc and la in SC-PLA are not obtained. On the contrary, the q-value is obtained for the SC-PLA/m-PPZn composites. This result indicates that the incorporation of m-PPZn into SC-PLA can significantly enhance the crystallization behavior of composites. However, the Lp and lc values at the same crystallization temperature decrease as the m-PPZn content increases, suggesting that   Table 1. The q-value cannot be observed because the SC-PLA degree of crystallinity is lower. Therefore, the values of L p , l c and l a in SC-PLA are not obtained. On the contrary, the q-value is obtained for the SC-PLA/m-PPZn composites. This result indicates that the incorporation of m-PPZn into SC-PLA can significantly enhance the crystallization behavior of composites. However, the L p and l c values at the same crystallization temperature decrease as the m-PPZn content increases, suggesting that the crystal layer thickness of SC-PLA/m-PPZn nanocomposites decreases with increasing m-PPZn content. In addition, the l a -values increase at the same crystallization temperature for SC-PLA/m-PPZn nanocomposites. These results reveal that the m-PPZn oleylamine chains inhibit the SC-PLLA crystalline chain packing, which leads to a decrease in crystal layer thickness and an increase of the amorphous layer thickness. In addition, the la-values increase at the same crystallization temperature for SC-PLA/m-PPZn nanocomposites. These results reveal that the m-PPZn oleylamine chains inhibit the SC-PLLA crystalline chain packing, which leads to a decrease in crystal layer thickness and an increase of the amorphous layer thickness.   To obtain the equilibrium melting temperature (T 0 m ) of SC-PLA and SC-PLA/m-PPZn nanocomposites, the linear Hoffman-Weeks equation is determined [46,47]:

Crystallization Behavior of SC-PLA and SC-PLA/m-PPZn Nanocomposites
where γ is a factor depending on the final laminar thickness; the T 0 m values are obtained from the intersection between the T m = T c line and the extrapolation of T m as a function of T c in the isothermal crystallization range of 192 to 198˝C ( Table 2)  The growth rate (G) of spherulite for SC-PLA and SC-PLA/m-PPZn nanocomposites can be estimated from the POM data. Figure 9 shows the G-values of SC-PLA and its nanocomposites with m-PPZn crystallized at different T cs . The spherulite growth rate at the same crystallization temperature of the SC-PLA/m-PPZn nanocomposites is greater than that of SC-PLA, but increasing the m-PPZn content leads to a decrease in the growth rate of SC-PLA/m-PPZn nanocomposites.
where γ is a factor depending on the final laminar thickness; the   The growth rate (G) of spherulite for SC-PLA and SC-PLA/m-PPZn nanocomposites can be estimated from the POM data. Figure 9 shows the G-values of SC-PLA and its nanocomposites with m-PPZn crystallized at different Tcs. The spherulite growth rate at the same crystallization temperature of the SC-PLA/m-PPZn nanocomposites is greater than that of SC-PLA, but increasing the m-PPZn content leads to a decrease in the growth rate of SC-PLA/m-PPZn nanocomposites.  The thermodynamic parameters related to the crystallization process can be established by the Lauritzen and Hoffman theory Equation (4).
where G 0 is a pre-exponential term; U˚is the activation energy for the segment's diffusion to the crystallization site (U˚= 6300 J mol´1) [48]; T 8 is the hypothetical temperature, below which viscous flow ceases (T 8 = T g -30 K) [48]; K g is a nucleation constant; f = 2T c /(T 0 m + T c ) is a correction factor; and T = T 0 m -T c is the degree of supercooling. K g is a nucleation constant, as given by: where b is the crystal layer thickness; σ and σ e are the lateral and fold surface free energies, respectively; k is the Boltzmann constant; ∆H 0 f is the heat of fusion per unit volume. The parameter n applied in this equation is four in Regimes I and III and two in Regime II. The where G0 is a pre-exponential term; U * is the activation energy for the segment's diffusion to the crystallization site (U * = 6300 J mol −1 ) [48]; T∞ is the hypothetical temperature, below which viscous flow ceases (T∞ = Tg -30 K) [48]; Kg is a nucleation constant; f = 2Tc/( 0 m T + Tc) is a correction factor; and △T = 0 m T -Tc is the degree of supercooling. Kg is a nucleation constant, as given by: where b is the crystal layer thickness; σ and σe are the lateral and fold surface free energies, respectively; k is the Boltzmann constant;   [40,49], and b of SC-PLA is 14.98 Å [13,50]. In order to define the crystallization regimes at the different Tcs, the Lauritzen Z-test equation is utilized as follows [51]: where L is the effective lamellar width and ao is the lattice constant. In the test, if X = Kg, then Z ≤ 0.01, and the Regime I kinetics are obeyed. The Regime II kinetics are followed if X = 2 Kg leads to Z ≥ 1. As pointed out by Lauritzen and Hoffman [48], it is more convenient to use the known value for Kg and the inequalities for Z to obtain the values of L in both Regimes I and II and to estimate whether such L values are realistic. Testing whether the X = Kg data conform to Regime I reveals that the L-value is 15.43 nm for SC-PLA. These results are reasonable. If we assume Z ≥ 1 and substitute X = 2 Kg into the Z test, then the L-values are about 2.5 × 10 5 nm for SC-PLA, which is unrealistic. Consequently, the crystallization regime of SC-PLA and SC-PLA/m-PPZn nanocomposites proceeds corresponding to Regime I. The ∆H 0 f of SC-PLA is 142 J/g [40,49], and b of SC-PLA is 14.98 Å [13,50]. In order to define the crystallization regimes at the different T cs , the Lauritzen Z-test equation is utilized as follows [51]: where L is the effective lamellar width and a o is the lattice constant. In the test, if X = K g , then Z ď 0.01, and the Regime I kinetics are obeyed. The Regime II kinetics are followed if X = 2 K g leads to Z ě 1.
As pointed out by Lauritzen and Hoffman [48], it is more convenient to use the known value for K g and the inequalities for Z to obtain the values of L in both Regimes I and II and to estimate whether such L values are realistic. Testing whether the X = K g data conform to Regime I reveals that the L-value is 15.43 nm for SC-PLA. These results are reasonable. If we assume Z ě 1 and substitute X = 2 K g into the Z test, then the L-values are about 2.5ˆ10 5 nm for SC-PLA, which is unrealistic. Consequently, the crystallization regime of SC-PLA and SC-PLA/m-PPZn nanocomposites proceeds corresponding to Regime I. Because m-PPZn loading is low, the values of ∆H˝f and b of nanocomposites can be assumed to be the same as those of SC-PLA. The surface free energy (σσ e ) data are obtained from Equation (5) for SC-PLA and SC-PLA/m-PPZn nanocomposites. For the comparison, the obtained σσ e -values are listed in Table 2. The σσ e -values of the SC-PLA matrix are 0.25, 0.5 and 1 wt% SC-PLA/m-PPZn nanocomposites calculated at about 222.1, 104.7, 122.3 and 179.5 erg 2 /cm 4 , respectively. Clearly, the σσ e -value first decreases and, then, increases with the m-PPZn content. Consequently, the introduction of 0.25 wt% m-PPZn into SC-PLA causes a decrease in the surface free energy and, thus, an increase in the polymer chain mobility. However, with 0.5 to 1 wt% m-PPZn, the higher σσ e -values indicate the presence of existing constraints on the mobility of the polymer chains. Because the oleylamine serves as an organo modifier for PPZn intercalation, m-PPZn can hinder the diffusion and migration of either PLLA or PDLA chains to the packing of stereocomplex crystals.

Conclusions
SC-PLA/oleylamine-modified PPZn nanocomposites were successfully fabricated using the solution-mixing technique. The structural arrangement of the m-PPZn determined using WAXD exhibited a large interlayer spacing of 30.3 Å. In addition, the temperature effect on the real-time structural arrangement of PPZn and m-PPZn was investigated using in situ WAXD and FTIR. The results indicated that the lattice expansion of m-PPZn with increasing temperature leads to an increase in the interlayer spacing from 30.3 to 37.1 Å as the temperature increases from 30 to 150˝C. The interlayer spacing decreases slightly as the temperature further increases to 210˝C. This behavior might be attributed to interlayer oleylamine elimination, which results in hydrogen bonding destruction between the hydroxide sheets and water molecules. As the temperature reaches 240˝C, the in situ WAXD patterns show the coexistence of m-PPZn and PPZn. However, the layered structures of m-PPZn at 300˝C are almost the same as those of PPZn, after the complete degradation temperature of oleylamine. The structure and morphology of the SC-PLA/m-PPZn nanocomposites characterized using WAXD and TEM demonstrate that most of the layered materials of partial delamination are randomly dispersed in the SC-PLA matrix. The crystal layer thickness of SC-PLA decreases, whereas the amorphous layer thickness increases with m-PPZn content, suggesting that the oleylamine chains of m-PPZn inhibit the SC-PLLA crystalline chain packing, which causes a decrease in the crystal layer thickness and an increase in the amorphous layer thickness. In the presence of m-PPZn, the spherulite size of SC-PLA nuclei decreases while their number increases substantially. Therefore, adding m-PPZn to SC-PLA induces heterogeneous nucleation. The product of the surface free energies of the SC-PLA/m-PPZn nanocomposites is considerably higher than that of SC-PLA because the incorporation of m-PPZn can hinder the diffusion and migration of either PLLA or PDLA chains to the packing of stereocomplex crystals.