Article pubs.acs.org/JPCB

Nucleation Ability of Thermally Reduced Graphene Oxide for Polylactide: Role of Size and Structural Integrity Yuan-Ying Liang, Su Yang, Xin Jiang, Gan-Ji Zhong, Jia-Zhuang Xu,* and Zhong-Ming Li* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China S Supporting Information *

ABSTRACT: Following our previous work on graphene oxide-induced polylactide (PLA) crystallization [Macromolecules 2010, 43, 5000−5008], in the current work, we further revealed the role of size and structural integrity of thermally reduced graphene oxide (RGO) in PLA crystallization. RGO nanoplatelets with different architectures were obtained via bath and probe ultrasound (RGOw and RGOp). The average size of RGO decreased substantially with ultrasound intensity and time, where the generation of RGO edges constituted the translocation of functional group sites from in-plane to edges. The formation of sp3-configuration dominated in RGOw, whereas the partial recovery of sp2configuration occurred in RGOp, giving rise to either the escalation of sp3/sp2 ratio for RGOw or retrogradation of that for RGOp. Isothermal crystallization kinetics of PLA nanocomposites containing RGOw and RGOp was determined by in situ synchrotron wide-angle X-ray diffraction. The induction period and overall crystallization rate of PLA/ RGOw nanocomposites were strengthened with diminishing platelet size because of more nucleation sites encouraged by redistribution of functional groups. However, the adverse situation was found in PLA/RGOp nanocomposites. The observed phenomenon was ascribed to the disruption of the internal structure, i.e., the CC sp2 π-bond network, which deteriorated the CH−π interaction between PLA and RGO. These results conclusively suggested that the size and structural integrity of RGO had a concerted effort to determine the final nucleation ability of RGO dispersed by ultrasound.

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INTRODUCTION Graphene nanoplatelet (GNP), a carbonaceous nanofiller with two-dimensional (2D) geometry, stimulates great excitement as a versatile material especially in the fields of polymer nanocomposites due to its unique merits, such as electrical, thermal, and mechanical properties.1,2 Addressing the issue about crystallization of semicrystalline polymers is of important scientific and practical interests, on account that the crystalline structure and morphology are responsible for the resultant properties of the polymeric materials.3,4 Excellent nucleation efficiency of GNP has been well documented in various semicrystalline polymers.5 For example, Xu et al. first reported the acceleration effect of GNP on the crystallization kinetics of polylactide (PLA), where the induction period of PLA isothermally crystallized at 124 °C was significantly reduced from 130 to 39 min at a very low GNP loading of 0.05 wt %.6 Their further study found that only incorporating 0.05 wt % GNP caused an augment of 8 °C in the crystallization peak temperature (Tp) of isotactic polypropylene (iPP) at a cooling rate of 2 °C/min.7 The increased crystallinity was observed in polyethylene (PE)/reduced graphene oxide (RGO) nanocomposites.8 On the other side, crystalline morphology of polymers could be altered by the presence of GNP as well.5 Cheng et al. realized PE crystal-decorated RGO via a controlled solution crystallization method, where edge-on lamellae were grown on the basal plane of RGO.8 Owing to high density of active nuclei on RGO surface, robust PLA transcrystallization © 2015 American Chemical Society

was successfully induced on the surface of RGO-coated glass fibers. Understanding the origins of GNP-induced polymer crystallization has attracted growing attention because of the theoretical and experimental significance. By employing Fourier-transform infrared spectroscopy, the physical image of polymer crystallization accelerated by GNP was disclosed as surface-induced conformational ordering.5,7,9 The surface of GNP was suggested to act as a template for polymer chains to landscape and to multiply long ordering segments which eventually became the precursors for lamellae growth, resulting in the considerable enhancement of overall crystallization rate.10 Recently, as revealed by selected area electron diffraction, lattice matching between polymers (i.e., polyethylene8 and polyaniline11) and RGO was clarified, indicative of the fact that epitaxial growth is another rational explanation for GNP-induced polymer crystallization. Further studies manifested that geometrical dimensionality,6,12,13 functionalization,12,14 specific surface,15 and surface grafting16,17 considerably affected the induction ability of GNP. For instance, the nucleation ability of 2D GNP was inferior to that of 1D carbon nanotubes.6 In contrast to pristine GNP, diphenylmethane diisocyanate-modified GNP advanced the melt crystallization rate of thermoplastic polyester elastomer Received: November 24, 2014 Revised: March 11, 2015 Published: March 12, 2015 4777

DOI: 10.1021/jp511742b J. Phys. Chem. B 2015, 119, 4777−4787

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The Journal of Physical Chemistry B slightly.14 The GNP with larger specific surface area prepared by ultrasonic treatment exerted better nucleation efficiency on iPP compared with pristine GNP.15 The Tp of iPP containing longchain alkylamine-modified GNP was on the rise by extending the chain length of grafted alkylamines.18 Despite this, the role of architecture of GNP in inducing polymer crystallization, however, is still a bit elusive. The foremost factor to convert the potential merits of graphene into reality is to achieve its homogeneous dispersion in the polymer matrix. Solution processing is normally adopted to reach the above goal.19−22 Hereinto, ultrasound as a straightforward and clean route is widely utilized to delaminate and to disperse GNP.23 The effects of ultrasound on materials arise from the generation of cavitation (growth and collapse of micrometer sized bubbles) and shear forces, between which ultrasonic cavitation is more important.23,24 The release of high mechanical energy stemming from the eventual collapse of bubbles (cavities) hits GNP surface with great impact and removes the top layers, thus facilitating the delamination and dispersion of GNP.24−26 By being transmitted directly to the crystals and accumulating in phonon mod, the intense acoustic wave energy was prone to change the architecture of GNP.23,26 It was reported that the reduced size of GNP due to prolonging ultrasound time endowed nanocomposites with improved mechanical properties.27 These studies give a straightforward indication that GNP architecture is closely related to the ultrasound conditions, which may allow us to associate the architecture with the ability of GNP to induce polymer crystallization. The aim of this study is to investigate the influence of the architecture of RGO on its nucleation ability to polymer crystallization. In order to feed different ultrasonic intensity and frequency, bath and probe ultrasound were utilized to disperse RGO, and thermally RGO was employed because of less defect sites stemming from the recovery of CC sp2 network during thermal reduction.8,28 PLA, an increasingly popular biodegradable polymer from renewable resources, was chosen as the model polymer. Our previous studies shed light on the physical image of GNP-induced PLA crystallization, which is conductive to understand the effects of GNP architecture on its nucleation ability.6 Discerning the nucleation ability of RGO with different architectures is of critical importance to build the relationship between the structures and properties of polymer nanocomposites.

the bath ultrasound (SB 5200DT, 40 kHz, 300 W), the RGO/ ethanol suspension was subjected to ultrasonic wave as well as mechanical stirring for the selected time (0.5, 1, and 2 h). For the sake of clarity, the resultant samples were denoted as RGOw0.5, RGOw1.0, and RGOw2.0, respectively. For the other method, the suspension was dispersed under probe ultrasound (JY 92IIN, 20 kHz, 650 W) for 0.33, 0.67, and 1 h. And the obtained RGO was termed as RGOp0.3, RGOp0.6, and RGOp1.0, separately. The PLA nanocomposites containing RGOw and RGOp (0.1 wt %) were prepared via solution coagulation. A 10 g sample of PLA was completely dissolved in 200 mL of CH2Cl2. By pouring the predispersed RGO (10 mg)/ethanol suspension into PLA/CH2Cl2 solution, coagulated materials precipitated continuously. Then, a large amount of ethanol was poured into the mixture until no more coagulations precipitated. The asprepared PLA/RGO (PLGN) nanocomposites were transferred to evaporating dishes, left overnight at room temperature, and dried in a vacuum oven for 48 h at 60 °C to remove residual solvent. Characterization of Ultrasonically Treated RGO. X-ray diffraction (XRD) curves of the samples were collected by a DX1000 diffractometer with Cu Kα radiation (λ = 0.154 nm) under a voltage of 40 kV and a current of 40 mA. Samples were scanned over the range of diffraction angle 2θ = 2°−45°, with a scan speed of 3°/min at ambient temperature. The microstructure of RGO was characterized using transmission electronic microscopy (TEM) (JEM-100CX, JEOL, Japan) with an acceleration voltage of 80 kV. Raman spectra were recorded on a Labram HR spectrometer (Horiba, France) using 532 nm laser excitation with a power of 1 mW. The intensity of characteristic bands was obtained by calculating the height of peak. X-ray photoelectron (XPS) spectra were recorded on an ESCA LAB 250 spectrometer (VG Scientific) with an Al Kα radiation (1486.6 eV) to determine the surface composition of RGO. Crystallization Kinetics of PLA/RGOw (PLGNw) and PLA/RGOp (PLGNp) Nanocomposites. Synchrotron 2D wide-angle X-ray diffraction (WAXD) was carried out by using beamline X27C at National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The wavelength was 0.1371 nm, and the diffraction data were collected by using a MAR CCD X-ray detector with a resolution of 1024 × 1024 pixels (pixel size = 158 μm). The data acquisition time was 30 s for each scattering pattern. The scattering angle was calibrated by Al2O3, and air scattering was subtracted. A Linkam CSS-450 high-temperature shear stage modified for in situ X-ray scattering measurements was employed to control the thermal history of samples. The samples were heated from 40 to 200 °C at a heating rate of 30 °C/min and were held for 5 min to erase any thermal history. Then the samples were cooled at a rate of 30 °C/min to the preset isothermal crystallization temperature (Tc = 135 °C). Linear WAXD profile was obtained by circularly integrating intensities of the corresponding 2D-WAXD pattern. By deconvoluting the peaks of linear WAXD profiles, overall crystallinity (Xc) was calculated by the following equation: Xc = ∑Acryst/(∑Acryst + ∑Aamorp), where Acryst and Aamorp are the fitted areas of crystal and amorphous phases, respectively. Relative crystallinity (Xr) was used to estimate the crystallization rate of neat PLA and its nanocomposites according to the equation Xr = Xc(t)/Xc(∞), where Xc(t) and Xc(∞) are the fitting crystallinity of PLA at the holding time (t) and after the completion of crystallization, respectively.

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EXPERIMENTAL SECTION Materials. Commercially available PLA (trade name 4032D) was purchased from NatureWorks LLC (USA), the weightaverage molecular weight and number-average molecular weight of which was 2.23 × 105 and 1.06 × 105 g/mol, respectively. Graphene oxide (GO) were synthesized from expanded graphite by the modified “Hummers” method as reported previously.6 Then, RGO with C/O atomic ratio of 13.2 was then obtained by thermal reduction of graphene oxide at 900 °C for 30 s under atmosphere pressure (see Figure S1 in the Supporting Information). The as-prepared RGO, taking the RGO subjected to bath ultrasound for 0.5 h for example, had an average thickness of 1 nm, corresponding to a single-layer graphene (Figure S2).29 Ethanol and dichloromethane (CH2Cl2) were purchased from Chengdu Kelong Chemical Reagent Company (China), which were used without further purification. Fabrication of PLA/RGO Nanocomposites. The asprepared RGO powder was dispersed in ethanol by two ultrasound methods, namely bath and probe ultrasound. For 4778

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Figure 1. XRD patterns of (a) RGOw and (b) RGOp. The XRD patterns of pristine graphite and GO are also depicted for comparison.

Figure 2. Typical TEM images and the corresponding size distribution of (a, a′) RGOw0.5, (b, b′) RGOw1.0, and (c, c′) RGOw2.0.

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RESULTS

shifts to 10.4°, suggesting that the d-spacing increases to 0.85 nm for the GO, which is attributed to the formation of oxygencontaining groups, such as epoxy, hydroxyl, carboxyl, and carbonyl groups.30 Compared to the pristine graphite and GO, the intensity of the ultrasonically treated RGO is quite weak. The characteristic peak in the RGO is almost imperceptible when these samples are plotted together, which is typical for the randomly ordered graphitic platelets and the formation of graphene single layers.31 Additionally, the homogeneous dispersion of RGOw and RGOp in PLA matrix is identified by

Structure and Morphology of Ultrasonically Treated RGO. To elucidate the role of RGO architecture in inducing PLA crystallization, morphological and structural characterization of RGO after ultrasound dispersion was initially implemented. The exfoliation degree of RGO dispersed by the bath and probe ultrasound is evaluated by XRD (Figure 1). It could be observed that a distinct peak assigned to the (002) lattice plane appears at 26.6° in pristine graphite, manifesting that a d-spacing of approximately 0.34 nm.30 After chemical oxidation, the peak 4779

DOI: 10.1021/jp511742b J. Phys. Chem. B 2015, 119, 4777−4787

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Figure 3. Typical TEM images and the corresponding size distribution of (a, a′) RGOp0.3, (b, b′) RGOp0.6, and (c, c′) RGOp1.0.

Table 1. Average Size and Platelet Number per 25 μM2 of RGOw and RGOp bath ultrasound av size (μm2) number/25 μm2

probe ultrasound

RGOw0.5

RGOw1.0

RGOw2.0

RGOp0.3

RGOp0.6

RGOp1.0

5.9 18

3.4 24

1.8 36

2.9 30

2.5 38

0.7 60

μm2. The same tendency is also found in RGOp (Figure 3). With prolonging ultrasound time, the average size of RGOp plummets from 2.9 to 0.7 μm2 (Table 1), and the dimension less than 1 μm2 accounts for a majority of RGOp1.0 (84.8%). The overall tendency in the size of both RGOw and RGOp is in agreement with the morphology presented in PLA matrix (Figure S3). The decrease of platelet size could be ascribed to the mechanical shockwaves and shear forces created by the collapse of bubbles.34 It is noteworthy that high-intensity probe ultrasound sharply decreases platelet size in contrast to bath ultrasound.35 Fixing the ultrasound time for 1 h, the average size of RGOw1.0 (3.4 μm2) is 5 times larger than that of RGOp1.0 (0.7 μm2). The substantial decrease of RGO size after probe ultrasound is probably on account of the severe etching elicited by the extremely high power of probe generator. Raman spectroscopy is a powerfully nondestructive technique to study the ordered and disordered structures of carbonaceous materials, such as carbon nanotubes and graphene. The typical features of the Raman spectra of carbonaceous materials are the

observing the cryo-fractured surface of the nanocomposites (Figure S3). The results mentioned above isolates the effects of exfoliation degree of RGO specifically, ensuring an important prerequisite for the following study on the crystallization kinetics of PLA nanocomposites containing RGOw and RGOp. The visual evidence about the resultant morphology and size distribution of RGOw and RGOp is detected by TEM. As illustrated in Figures 2 and 3, some wrinkles and crumples in both RGOw and RGOp sheets could be observed owing to the structural disorder in the graphene lattice introduced by the oxidation process.29,32,33 The size distribution of RGOw and RGOp is recorded by measuring about 100 platelets of each sample. Conspicuously, extension of the ultrasound time decreases lateral size and shrinks size distribution of RGO. For RGOw0.5, the size distribution is relatively wide, with area of 18.8% over 10 μm2, 31.3% for 5−6 μm2, and 49.9% for 1−5 μm2, whereas ultrasound for 2 h results in all platelets with the lateral size less than 5 μm2 and even only ca. 0−1 μm2. The average size obtained by calculating size distribution declines from 5.9 to 1.8 4780

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Figure 4. Raman spectra of (a) RGOw0.5, (b) RGOw1.0, and (c) RGOw2.0 for the excitation at 532 nm.

Figure 5. Raman spectra of (a) RGOp0.3, (b) RGOp0.6, and (c) RGOp1.0 for the excitation at 532 nm.

Figure 6. Peak deconvolution of C(1s) XPS core level of (a) pristine RGO, (b) RGOw0.5, (c) RGOw2.0, (d) RGOp0.3, and (e) RGOp1.0.

G band at 1590 cm−1 which is usually assigned to the vibration of sp2 carbon atoms in a graphitic 2D hexagonal lattice and the D band at 1355 cm−1 corresponding to the vibrations of sp3 carbon atoms arising from defects and disorder especially at the edge of graphene nanoplatelet.36 The intensity ratio of D to G band (ID/ IG) generally reflects the extent of defects. As can be seen in Figure 4, the ID/IG value varies from 0.86 for RGOw0.5 to 0.90 for RGOw2.0, which infers the introduction of sp3-hybridized carbon atoms into the sp2-hybridized graphene layers during ultrasound.37 Combined with the TEM results, the RGO nanoplatelets are radically broken into smaller pieces once imposed ultrasound treatment, thereby causing an upward trend of RGO edge. As the D band is assigned to local defects of graphene especially the disorder at the edges,38 the increase of ID/IG value of RGOw thus could be ascribed to the appearance of new edges. On the contrary, the ratio of ID/IG unexpectedly runs counter to the trend of increased platelet edges in RGOp, that is, declines from 0.92 for RGOp0.3 to 0.87 for RGOp1.0 (Figure 5).

Furthermore, the ID/IG value of RGO subjected to ultrasound treatment is lower than that of the pristine RGO (0.95, Figure S4). Local restoration of the pristine planar sp2 carbon network by the repair of ethanol with prolonging ultrasound time is conjectured to contribute to the decreased ID/IG ratio.37,39 The variation of surface functional groups of RGOw and RGOp is ascertained by deconvolving the C (1s) XPS spectra (Figure 6). Five peaks located at binding energies of 284.4, 285.7, 286.7, 288.0, and 289.1 eV are typically assigned to the C−H (C−C, CC), C−OH, epoxide, CO (carbonyl C), and O C−OH (carboxylate C) groups.40 The absolute content of individual functional group (the area of the peak divided by the total area of all peaks) displays slight variation of chemical structure of RGO after sonication (Table 2). It is clear that the content of C−C group for both RGOw and RGOp increases while the content of C−OH and C−O−C groups decreases compared to the pristine RGO. This is explained by the fact that 4781

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The Journal of Physical Chemistry B Table 2. Peak Area Ratios of Individual Group to the Total Groups for Pristine RGO, RGOw, and RGOp C−C(H), CC

C− OH

C− O−C

CO

pristine RGO

66.5

16.1

10.1

5.5

1.8

6.5

RGOw0.5 RGOw2.0

71.6 71.2

4.5 4.3

2.0 2.1

6.6 6.8

RGOp0.3 RGOp1.0

70.2 70.6

5.2 6.0

2.4 2.3

6.3 6.1

bath ultrasound 12.6 9.4 13.5 8.9 probe ultrasound 13.0 9.2 11.2 9.8

O−CO C/O

the stability of −C−O−C is inferior to −COOH and −CO groups, resulting in the diminished content.41 Crystallization Behavior of the PLA/RGOw and PLA/ RGOp Nanocomposites. To evaluate the influence of RGO architecture on its induction ability for PLA crystallization, in situ synchrotron 2D-WAXD is employed to follow the isothermal crystallization of neat PLA, PLGNw, and PLGNp. Figures 7 and

Figure 8. Selected 2D-WAXD patterns of (a) neat PLA, (b) PLGNp0.3, (c) PLGNp0.6, and (d) PLGNp1.0 isothermally crystallized at 135 °C.

Unexpectedly, an opposite tendency for ta is detected in PLGNp nanocomposites (Figures 8 and 10). The ta increases from 2 min for PLGNp0.3 to 5 min for PLGNp1.0. Additionally, at the same predispersed time of 1 h, the accelerating effect of RGOp1.0 is inferior to that of RGOw1.0. These results indicate that besides ultrasound time, the ultrasound method should also be taken into account for the diverse inducing ability of RGOw and RGOp. Combined TEM and Raman results (Figures 2−5) with the mechanism of surface-induced conformational order,5 it could be extrapolated that the variation of RGO structure driven by different ultrasound methods may influence the formation of conformational order, thus affecting the final crystallization process. In addition, the absence of characteristic diffraction peak at 24.4° corresponding to α′-crystal signifies the formation of αcrystal alone in the aforementioned systems.42 Relative crystallinity of PLA and its nanocomposites loaded with RGOw and RGOp as a function of crystallization time is plotted in Figure 11. A typical sigmoidal evolution is observed in all the samples. Incorporation of either RGOw or RGPp leads to the advancement in the overall crystallization rate of PLA. The induction time (ti) is defined as the time at which the relative crystallinity reaches 5%. As depicted in Figure 11a, PLGNw2.0 yields the shortest ti (1.5 min), followed by PLGNw1.0 (ti = 4 min) and PLGNw0.5 (ti = 4.5 min), while the situation is conspicuously different for PLGNp (Figure 11b), where the ti of PLGNp1.0 is 5 min, behind that of PLGNp0.3 (2 min) and PLGNp0.6 (4.5 min). Half-crystallization time (t1/2) is also presented to assess the inducing ability of RGOw and RGOp. The prolonged ultrasound time declines t0.5 from 13.3 min for PLGNw0.5 to 4.8 min for PLGNw2.0, whereas the accelerating effect of RGOp0.3 (t0.5 = 6 min) prominently outperforms that of RGOp0.6 (t0.5 = 13.8 min) and RGOp1.0 (t0.5 = 14.0 min). In other words, the extension of ultrasound time improves the nucleation ability of RGOw but depresses that of RGOp. This variation tendency coincides with the isothermal crystallization

Figure 7. Selected 2D-WAXD patterns of (a) neat PLA, (b) PLGNw0.5, (c) PLGNw1.0, and (d) PLGNw2.0 isothermally crystallized at 135 °C.

8 illustrate the representative 2D-WAXD patterns of PLGNw and PLGNp nanocomposites isothermally crystallized at Tc of 135 °C. A diffuse scattering ring is observed in the first pattern (t = 0 min) for all the samples, indicating no crystals are survived in the melt. With increasing crystallization time, the isotropic lattice planes, i.e., (200)/(110) reflection, can be observed and gradually heighten until the completion of crystallization, correlating with the formation and growth of the PLA crystals. In contrast to neat PLA, the advent of ultrasonically treated RGO shortens the appearance time (ta) of the diffraction peak remarkably, demonstrating the superb nucleation ability of RGO. A striking phenomenon observed in Figures 7 and 9 is that ta for the (200)/(110) diffraction peak at 16.7° plummets from 4.5 min for PLGNw0.5 to 1.5 min for PLGNw2.0, which is speculated to the increase of nucleation sites. 4782

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Figure 9. Initial 30 min evolution of the 1D-WAXD profiles of (a) PLA, (b) PLGNw0.5, (c) PLGNw1.0, and (d) PLGNw2.0.

the RGO surface, impinging the growth of adjacent spherulites at the relatively early stage of crystallization.

results measured by differential scanning calorimetry (Figure S5). Therein, the peak time of exothermic heat flow curves shifts to lower values for PLGNw but delays for PLGNp with increasing ultrasound time. The Avrami equation 43 was employed to study the crystallization kinetics of PLA and its nanocomposites as follows: ln[− ln(1 − X (t ))] = ln K + n ln t

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DISCUSSION Ultrasound treatment fragments RGO platelets into smaller size and disrupts their graphitic basal structure, thus influencing the nucleation ability of RGO, either promotion for RGOw or degeneration for RGOp. Our previous work by tracking the conformational changes of the nanocomposites at the early stage of crystallization delineated the dynamic process of grapheneinduced polymer crystallization, that is, surface-induced conformational ordering (SICO).5,6,9 Clearly ultrasound dispersion of RGO changes its architecture and further affects its nucleation ability, which inspires us to build the structure− nucleation ability relationship of RGO. In this regard, the structure characterization of RGO subjected to different ultrasound methods is crucial, which, unfortunately, was less noticed by the previous studies.14,16 The combination of TEM, Raman, and XPS results allows us to conceive a perspective of the effects of ultrasound treatment on the architecture of RGO, as schematically portrayed in Figure 13. Extension of ultrasound treatment breaks RGO palates into smaller particles due to cavitation, increasing the number of RGO nanoplatelets per unit volume (Table 1).44 Both theoretical and experimental results in the existing literatures supported that −COOH and −CO groups were mainly attached on the edges of GNP, while −OH and −C−O−C−

(1)

where X(t) is the relative crystallinity calculated as the ratio of the heat of fusion at isothermal time (t) to the total heat of fusion of the whole crystallization process; n is the Avrami exponent and K is crystallization rate constant. Values of n and K tabulated in Table 3 are determined by using the initial linear part of the Avrami plot (Figure 12). It can be observed that the K value of PLGN nanocomposites is 1−3 orders of magnitude higher than that of neat PLA, exhibiting outstanding induction ability of RGO. With prolonging ultrasound time, K value increases in succession for the PLGNw, while it declines for the PLGNp, which is in accordance with ti and t1/2 mentioned above. The reasons for discrepant nucleation efficiency of RGOw and RGOp with ultrasound time will be discussed later. General trends about the growth dimension of polymer crystals could be normally gained by comparing the n value, albeit n is affected by many factors in analyzing the crystallization behavior.8 In the present case, incorporation of RGO makes the n decrease from 4.3 to around 3 for the nanocomposites (Table 3). This is attributed to the numerous spherulites growing at or near 4783

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Figure 10. Initial 30 min evolution of the 1D-WAXD profiles of (a) PLA, (b) PLGNp0.3, (c) PLGNp0.6, and (d) PLGNp1.0.

Figure 11. Relative crystallinity of (a) PLGNw and (b) PLGNp as a function of crystallization time, calculated from their linear WAXD curves in Figures 9 and 10, respectively. The inset picture magnifies the time axis at the beginning of crystallization.

groups were distributed in plane.45 Hence, the unsubstantial sites in carbon framework especially those occupied by −C−OH and −C−O−C groups, were first fractured once imposed ultrasound,46 resulting in increasing the RGO edges and transferring the functional groups from in-plane to edge (Figure 13b,c). On the other hand, RGOp platelets with much smaller size (Figure 3 and Table 1) display the decreased sp3/sp2 ratio with ultrasound time even though sp3-configuration also generates at RGO edge. The factors that affect ultrasonic cavitation include ultrasonic intensity, frequency, surface tension and viscous coefficient of liquid, and liquid temperature.47 As opposed to high frequency, high ultrasonic intensity and liquid temperature are in favor of

the formation of cavitation.26 It is thus easier to form cavitation under probe ultrasound than bath ultrasound due to the high intensity (650 W), low frequency (20 kHz), and localized hot spots and high pressures.47 The RGO monolayers comprised defect areas dominated by clustered pentagons as well as heptagons that bonded to three neighbors maintaining a planar sp2-configuration.48 Thereby, the slight decline of sp3/sp2 ratio could be ascribed to the increase of sp2-configuration with smaller overall size of aromatic domains in RGOp.40 Moreover, the high-powered probe ultrasound could cause bond fracture in the carbon framework and the formation of the pores (Figure 13c).24,26,39 It would probably engender lots of disordered 4784

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The Journal of Physical Chemistry B Table 3. t1/2, n, and K of PLA and Its Nanocomposites Containing RGOw and RGOp Isothermally Crystallized at 135 °C t1/2 (min) PLA PLGNw0.5 PLGNw1.0 PLGNw2.0 PLGNp0.3 PLGNp0.6 PLGNp1.0

n

20.3 4.3 bath ultrasound 13.3 3.3 13.0 2.5 4.8 2.7 probe ultrasound 6.0 2.7 13.8 3.0 14.0 3.2

Additionally, the variation amplitude of the overall crystallization rate of PLA nanocomposites containing RGOw and RGOp should be addressed. The ti (4.5 min) and t1/2 (13.5 min) of PLGNw0.5 approximate those of PLGNw1.0 (ti = 4.0 min and t1/2 = 13.0 min) and are much longer than those of PLGNw2.0 (ti = 1.5 min and t1/2 = 4.8 min). Combined with TEM results, it is speculated that the distribution of functional groups is sensitive to the bath ultrasound when ultrasound time is longer than 1 h. Nevertheless, this interpretation is not applied to the PLA/ RGOp nanocomposites. There is remarkable downturn of crystallization kinetics from PLGNp0.3 to PLGNp0.6 albeit the similar average size of RGOp0.3 (2.9 μm) and RGOp0.6 (2.5 μm). Meanwhile, with dramatic decrease in the average size of RGOp, the overall crystallization rate of PLGNp0.6 is commensurate with that of PLGNp1.0. This further implies that structure integrity, rather than the size effect, is crucial in determining the nucleation efficiency of RGOp. Furthermore, it is noteworthy that the t1/2 of PLGNw2.0 nanocomposites (t1/2 = 4.8 min) was close to that of the PLGNp0.3 (t1/2 = 6.0 min). It gives an indication that compared to mild bath ultrasound, highintensity probe ultrasound in a short time caters for the sharp decrease of platelets size and the reservation of structural integrity, also assuring the nucleation ability of RGO. As discussed above, size and structural integrity of RGO platelets conspire to control the nucleation ability of RGO. For the low-intensity bath ultrasound, the translocation of functional groups arising from the escalation of RGO platelets endows the PLA/RGOw nanocomposites with the enhanced crystallization rate. For the high-intensity probe ultrasound, the inducing ability of RGO is counterbalanced by size reduction and structural integrity. The size reduction dominates at short ultrasound time, while both of the factors inevitably reach a compromise when further increasing the ultrasound time. In sum, as the final nucleation efficiency of RGO could be tailored by manipulating its architecture, it opens up a new route to obtain PLA products with different crystallinity.

K × 106 (min−n) 2.26 150.7 370.7 9095.3 4305.5 283.9 24.9

structures and defects in the CC sp2 π-bond network in RGOp,39 but this detrimental effect could be overlooked for the RGOw due to less cavitation elicited by gentle bath ultrasound. In this perspective, the variation of structural integrity and size of RGO platelets after ultrasound treatment should be highlighted. The prominent architecture divergence of RGOw and RGOp leads to the discrepant nucleation efficiency for PLA crystallization. The ti and t1/2 decline from 4.5 and 13.3 min for PLGNw0.5 to 1.5 and 4.8 min for PLGNw2.0, respectively. However, they increase from 2.0 and 6.0 min for PLGNp0.3 to 5.0 and 14.0 min for PLGNp1.0. Building upon SICO, the threshold for PLA chains adsorbing on GNP surface is through the interaction between −CH group in PLA and π bond in GNP, which is interfered by the presence of surface functional groups on GNP.5 Since the reduced size of RGO leads to the translocation of functional groups from in-plane to edge, it is suitable for PLA chains to landscape and to adjust their conformation. As a result, the increased platelet number accompanying with fewer functional groups on RGO surface is envisioned to codetermine the enhancement of nucleation ability of RGOw. Given the analogous variation tendency of platelet size, a phenomenon concerned to the depression of inducing ability of RGOp is worthwhile to be mentioned. It is unambiguous that other factors, rather than size variation, control the nucleation activity of RGOp. As aforementioned, high-intensity probe ultrasound cause the destruction of the C C sp2 π-bond network in RGOp interspersed with large pores,26 which is adverse to induce the ordered conformation, thus providing a hint for the unexpected deterioration of nucleation ability of RGOp with prolonged ultrasound time. Therefore, the structural integrity of RGO cannot shed its responsibility in surface-induced PLA crystallization.

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CONCLUSIONS The effects of the architecture of RGO dispersed by bath and probe ultrasound on the crystallization behavior of PLA were thoroughly investigated by using in situ synchrotron WAXD measurements. The architecture of RGO was proved to be dependent on intensity and time of ultrasound source. The average size of RGOw and RGOp both decreased with ultrasound time accompanied by the translocation of the functional groups from the in-plane to the edge. The augment

Figure 12. Avrami plots of the isothermal crystallization data of (a) PLGNw and (b) PLGNp. 4785

DOI: 10.1021/jp511742b J. Phys. Chem. B 2015, 119, 4777−4787

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Figure 13. Schematic representation for structure characteristic of (a) pristine RGO, (b) RGOw, and (c) RGOp.

*Tel +86-28-8540-6866; Fax +86-28-8540-6866; e-mail jzxu@ scu.edu.cn (J.-Z.X.).

of RGO edges led to the generation of the sp3-configuration as reflected by the increased sp3/sp2 ratio of RGOw, while the sp3/ sp2 ratio of RGOp was unexpectedly in decline, which may result from the partial restoration of sp2-configuration. It was suggested that cavitation could crush the RGO platelets (reduction of the basal size) and even spoil π-bond network, especially under the circumstance of intense probe ultrasound. Both RGOw and RGOp could serve as heterogeneous nucleation agents for PLA, curtailing the induction period of crystallization and accelerating the overall crystallization rate. Prolonged ultrasound time gradually enhanced the nucleation ability of RGOw but suppressed that of RGOp. As far as the mechanism of SICO was concerned, surface functional groups and structural integrity of RGO were suggested to have a counteractive effect on inducing the crystallization of PLA. Thereinto, the redistribution of functional groups arising from the increased platelets provided extra nucleation sites to curtail the induction period dramatically. However, the demolition of CC sp2 π-bond network deteriorated CH-π interaction between PLA and RGOp, leading to adverse situation. Thus, it is suggested that the nucleation ability of ultrasonically treated RGO is counterbalanced by its average size and structural integrity. The above results provided better understanding of the relationship between the architecture and nucleation ability of RGO, which will render important guidance to tune the properties of PLA/RGO nanocomposites for given applications.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge for financial support from National Natural Science Foundation of China (Grants 51120135002, 51421061), the Doctoral Program of the Ministry of Education of China (Grant 20130181130012), and Sichuan Youth Science & Technology Foundation (Grant No. 2014TD0002) and the startup fund of Sichuan University (Grant No. 2015SCU11006). We are also indebted to the help form the National Synchrotron Radiation Laboratory, Shanghai, China, and Dr. Yan-Hui Chen in Synchrotron Light Source, Brookhaven National Laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

XPS spectra for GO and RGO; AFM images of the RGOw0.5; SEM images of PLGNw and PLGNp; Raman spectra for pristine RGO; DSC heat flow curves of isothermal crystallization of neat PLA, PLGNw, and PLGNp. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel +86-28-8540-6866; Fax +86-28-8540-6866; e-mail zmli@ scu.edu.cn (Z.-M.L.). 4786

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