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Manipulating the Filler Network Structure and Properties of. Polylactide/Carbon Black Nanocomposites with the Aid of. Stereocomplex Crystallites. Huil...
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Article Cite This: J. Phys. Chem. C 2018, 122, 4232−4240

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Manipulating the Filler Network Structure and Properties of Polylactide/Carbon Black Nanocomposites with the Aid of Stereocomplex Crystallites Huili Liu, Dongyu Bai, Hongwei Bai,* Qin Zhang, and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China S Supporting Information *

ABSTRACT: Construction of various filler networks is an important issue for developing high-performance and multifunctional polymer nanocomposites. In this work, we report a facile and effective strategy to manipulate the filler network structure and properties of poly(L-lactide)/carbon black (PLLA/CB) nanocomposites with the aid of stereocomplex (SC) crystallization between PLLA matrix and small amounts of poly(Dlactide) (PDLA). The results reveal that the incorporation of only 1 wt % PDLA can facilitate the formation of CB network in PLLA/CB nanocomposites because SC crystallites induced enhancement in the melt viscosity of PLLA matrix could depress CB aggregation, finally leading to an evident decease in the electrical percolation threshold (φc). However, with further increasing PDLA concentration to 10 wt %, the SC crystallites could organize into a dense network in the matrix and then serve as physical barrier for the networking of CB nanoparticles. As a result, the φc of the nanocomposites increases sharply. The outstanding nucleating and strengthening effects of such SC crystallites on the nanocomposites are also highlighted. These findings suggest that the formation of SC crystallites could be a promising solution to create PLLA-based nanocomposites with tunable filler networks and properties.

1. INTRODUCTION Over the past few decades, growing concerns about resource crisis and environmental sustainability have driven tremendous efforts to develop new eco-friendly materials from renewable resources.1,2 As the most promising alternative to some conventional petroleum-based and nonbiodegradable polymers in current scenario, polylactide (PLA) exhibits enormous potential in diverse applications because it possesses not only complete biorenewability from plant resources (e.g., corn) and full biodegradability in soil but also prominent advantages in transparency, mechanical strength and stiffness, melt-processability, and compostability.3,4 With the remarkable drop of industrial-scale production costs, the commercial application of PLA has been rapidly expanded from high-value biomedical sector to more general fields, particularly in packaging and textile industries.5−9 Recently, PLA-based conductive nanocomposites have attracted considerable research interest because the compositing of PLA with conductive nanofillers makes it possible to fulfill the requirements for its potential applications, such as antistatic packaging or fibers, sensors, and electromagnetic interference shielding.10−13 Carbon black (CB) is one of the widely used conductive and reinforcing fillers in industry due to its abundant supply, low cost, permanent conductivity, low density, as well as unique self-networking capability (namely, CB nanoparticles have a strong tendency to self-organize into three-dimensional continuous networks14) in © 2018 American Chemical Society

various polymer matrices. The properties of CB-filled polymer nanocomposites are strongly dependent on the CB network structure or percolating particle structure,15−20 especially for electrical conductivity and mechanical properties. Nevertheless, most CB nanoparticles are often aggregated into large clusters before networking, thus leading to relatively high percolation threshold (φc).21 More importantly, the existence of the aggregated CB clusters is not beneficial to improving or maintaining other properties of the nanocomposites such as strength and toughness. Thus, simultaneously manipulating the network structure and dispersion level of CB nanoparticles becomes an important issue for developing high-performance PLA/CB nanocomposites. Until now, many endeavors have been devoted to regulate the filler network structure of CB-filled polymer nanocomposites,22−26 such as the synthesis of CB with tailor-made particle size and specific surface area. In general, small particle size and large specific surface area are favorable for the networking of CB clusters.27,28 Wu et al.29 reported that the buildup of CB network can be promoted by thermal annealing at a temperature above the melting temperature (Tm) of polymer matrix because of the Brownian motion of CB particles Received: January 13, 2018 Revised: February 9, 2018 Published: February 12, 2018 4232

DOI: 10.1021/acs.jpcc.8b00417 J. Phys. Chem. C 2018, 122, 4232−4240

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The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION 2.1. Materials. All materials used in this work are commercially available. PLLA pellets with an Mw of 1.70 × 105 g/mol and an Mw/Mn of 1.74 were purchased from Nature Works Co. LLC, USA. PDLA powders (Mw = 1.67 × 105 g/ mol, Mw/Mn = 1.58) were obtained from Changchun Sino Biomaterials Co. Ltd., China. CB (grade Printex XE2B) with a specific surface area of 1000 m2/g was supplied by Evonik Degussa Specialty Chemicals Co., Ltd., China. The average particle size and n-dibutyl phthalate (DBP) adsorption number are 30 nm and 420 mL/100g, respectively. Prior to use, both the PLAs and CB were vacuum-dried at 60 °C for at least one night. 2.2. Sample Preparation. PLLA/PDLA/CB nanocomposites containing different amounts of CB nanoparticles (0.0−5.0 wt %, with respect to the total weight amount of the nanocomposites) were prepared by directly melt mixing in a Haake internal mixer (Rheomix 600, Germany) at 190 °C for 5 min with a rotation speed of 60 rpm. Since the critical PDLA concentration required for the formation of continuous SC crystallite networks in PLLA melt has been reported to be ca. 2 wt %,52 three typical PDLA concentrations (1, 2, 10 wt % on the basis of actual weight of the PLAs) were selected to reveal the effect of the SC crystallites on the self-networking behavior of CB nanoparticles. For convenience, the obtained PLLA/ PDLA/CB nanocomposites were denoted as L/yD/xCB, where x and y represent the amounts of the CB and PDLA, respectively. For comparison, PLLA/CB nanocomposites without PDLA (denoted as L/xCB) were also prepared using the same mixing procedure. The specimens used for rheological and electrical conductivity measurements were fabricated by compression-molding of these nanocomposites into sheets with a thickness of ca. 1.0 mm at 190 °C under a pressure of 10 MPa, followed by quenching in ice−water. In addition, injection molding, which was conducted on the HAAKE MiniJet II (Germany) at a barrel temperature of 200 °C and a mold temperature of 130 °C, was utilized to fabricate the standard dumbbell-shaped specimens for tensile tests. To obtain the nanocomposites with the same amorphous PLLA matrix, the isothermal time of the nanocomposite melts in the hot mold was set as 10 s. To highlight the role of SC crystallites in tailoring the filler network structure of PLLA/PDLA/CB nanocomposites, annealing treatments were carried out on the compressionmolded specimens at 190 and 240 °C (below and above the melting temperature of SC crystallites) for 30 min, respectively. Please note that the annealing treatment was performed under a dry nitrogen atmosphere to prevent possible thermal degradation of PLLA at 240 °C. On the basis of the thermogravimetric analysis (TGA) curves (shown in Figure S1), no obvious weight loss can be detected during the annealing treatment process. Gel permeation chromatography (GPC) results (not shown here) indicate that the annealing treatment only induces a slight decrease in the Mw of PLLA (from 1.70 × 105 g/mol to 1.55 × 105 g/mol). 2.3. Characterizations and Measurements. 2.3.1. WideAngle X-ray Diffraction (WAXD). WAXD measurements were performed on a PANalytical X’Pert pro MPD diffractometer (Holland) equipped with a Cu Kα radiation source (λ = 0.154 nm, 40 kV, 40 mA). For each measurement, WAXD pattern was recorded in the diffraction angle (2θ) range of 5−30° at a scanning speed of 3°/min. The content of SC crystallites (XSC)

to form more thermodynamically stable structures. Moreover, the build-up process of the CB network is accelerated dramatically with increasing annealing temperature or decreasing thermodynamic interactions between CB and polymer matrix. Very interestingly, Starý et al.30 found that the CB network build-up under shear stress is much more stable than that formed under quiescent conditions due to the preferential orientation of CB particle structures in the shear direction. Recently, considerable attention has been paid to the conductive polymer composites (CPCs) based on the heterogeneous distribution of conductive fillers in single polymer31,32 or cocontinues polymer blends.33−35 Specially, the selective localization of the fillers in one phase or at the interface of the polymer blend enables the formation of conductive networks with dramatically decreased filler loadings.36−39 However, as far as we are aware, none of the reported works have addressed the poor dispersion of CB nanoparticles within the conductive networks probably because it is a challenge to depress the inherent aggregation without affecting their self-networking capability. For example, surface modification can substantially improve the dispersion of CB nanoparticles, but inevitably causes the loss of their ability to self-organize into networks.20 In this work, therefore, we will report our attempt to simultaneously improve the dispersion and network structure of CB nanoparticles in PLA/CB nanocomposites with the aid of stereocomplex (SC) crystallites, which can be readily formed through cocrystallization between enantiomeric poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA).40 Compared with PLLA or PDLA homocrystallites, the SC crystallites possess denser chain packing density and thus endow PLA products with superior properties, including higher mechanical strength and better hydrolysis resistance.41−45 Most notably, the extremely high Tm of SC crystallites (∼230 °C, about 50 °C higher than that of the homocrystallites) makes it possible for them to be reserved in asymmetric PLLA/PDLA blend melts and, subsequently, serve as efficient rheological modifier and nucleating agent to drastically enhance melt viscoelasticity and crystallization rate of the PLLA or PDLA matrix, respectively.46−49 It was expected that the formation of SC crystallites in the PLA/CB nanocomposites could prevent CB nanoparticles from aggregating during melt-mixing process because a stronger shear stress could be generated to break the aggregated CB clusters with the enhancement in the melt viscosity of PLA matrix. On the other hand, because the inherent characters of the CB nanoparticles remain unchanged, the depressed aggregation was expected to be favorable for the networking of the CB clusters at lower percolation thresholds. On the basis of these considerations, small amounts (1−10 wt %) of PDLA were incorporated into PLLA/CB nanocomposites through melt mixing at 190 °C, which is a suitable processing temperature for the SC crystallization between the incorporated PDLA chains and their surrounding PLLA matrix chains.50,51 The role of the SC crystallites in regulating the filler network structure and properties of the PLLA/CB nanocomposites has been investigated. Also, the excellent nucleating and reinforcing effects of such SC crystallites on the nanocomposites are confirmed. We believe that this work is very significant in developing high-performance and multifunctional PLLA-based nanocomposites. 4233

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Figure 1. WAXD patterns of the melt-quenched PLLA/PDLA/CB nanocomposites containing various amounts of CB: (a) PLLA/CB, (b) PLLA/ 1PDLA/CB, (c) PLLA/2PDLA/CB, and (d) PLLA/10PDLA/CB. The content of SC crystallites (XSC) is presented in the figure profiles.

σ = σo(φ − φc)t

was evaluated by comparing the diffraction peak area of the SC crystallites with the total peak area. 2.3.2. Dynamic Rheological Analysis. Dynamic rheological measurements were carried out on a HAKKE MARS rotational rheometer (Germany) with parallel plate−plate geometry (25 mm in diameter). For each measurement, oscillatory frequency sweep was performed from 0.01 to 100 rad/s at 190 °C under a dry nitrogen atmosphere and the strain was set as 1% to ensure a linear viscoelastic response. 2.3.3. Scanning Electron Microscopy (SEM). Dispersion state of CB nanoparticles was observed with an FEI Inspect F SEM (USA) at an accelerating voltage of 10 kV. The specimens were quickly fractured after being immersed in liquid nitrogen for 1 h, and then the fractured surface was sputter-coated with a gold layer for the SEM observations. 2.3.4. Transmission Electron Microscopy (TEM). Morphological structure was visualized by a FEI Tecnai G2 F20 TEM (USA) with an accelerating voltage of 200 kV. The ultrathin sections with a thickness of ca. 80 nm were prepared by ultracryomicrotomy at −140 °C using a Leica UCT ultramicrotome (Germany). 2.3.5. Electrical Conductivity. Electrical conductivity was measured by using a Keithley 6487 picoammeter (China) under a constant voltage of 1 V at room temperature. Before the measurements, both the two ends of the compressionmolded rectangular specimens were coated with a thin layer of silver paint in order to ensure good contact between the specimen surfaces and copper electrodes. For each sample, the reported value was averaged from at least three independent specimens. The electrical percolation threshold (φc) was estimated by fitting the relationship between the electrical conductivity (σ) and CB loading (φ) of each PLLA/PDLA/CB nanocomposite based on the classical percolation scaling law:

(1)

Here σo is a scaling factor and t is a critical exponent relative to the dimensionality of percolated CB network. 2.3.6. Differential Scanning Calorimetry (DSC). Nonisothermal and isothermal crystallization behaviors were studied by using a TA Q200 instrument (USA) under a dry nitrogen atmosphere. For nonisothermal crystallization, the specimen (ca. 5 mg) was cooled to 30 °C at a rate of 2 °C/min after being held at 200 °C for 3 min to erase any thermal history, and subsequently heated to 200 °C at a rate of 10 °C/min. While for isothermal crystallization, the completely melted specimen (at 200 °C for 3 min) was rapidly cooled (100 °C/min) to a predetermined crystallization temperature (i.e., 125, 130, 135, and 140 °C) and then held at this temperature until the isothermal crystallization is finished. 2.3.7. Mechanical Tests. Tensile properties were tested at room temperature (23 ± 2 °C) by using an Instron 5567 universal testing machine (UK) with a cross-head speed of 5 mm/min, according to the ISO 527-3 standard. For each sample, at least six specimens were tested. Please note that the specimens were conditioned at 23 °C for 48 h before tests.

3. RESULTS AND DISCUSSION 3.1. Formation of SC Crystallites in PLLA/PDLA/CB Nanocomposites. PLLA/PDLA/CB nanocomposites were prepared by melt mixing at a low temperature of 190 °C, where SC crystallites could be readily formed through cocrystallization between the incorporated PDLA chains and PLLA matrix ones.50 WAXD patterns provide direct evidence for the in situ formation of the high-Tm SC crystallites in the PLLA matrix during the melt-mixing process. As shown in Figure 1a, no diffraction peaks can be detected in the WAXD patterns of PLLA/CB nanocomposites, implying that the PLLA matrix is 4234

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Figure 2. Frequency dependences of (a) storage modulus (G′), (b) loss modulus (G″), (c) loss tangent (tan δ), and (d) complex viscosity (η*) for the PLLA/PDLA blends containing various amounts of PDLA.

10 wt % (Figure 2, parts a and b). For the blends with lowconcentration PDLA (1 wt %), the formed SC crystallites could be dispersed in the PLLA melt as isolated solid particles and thus only slight increases in the G′ and G″ are observed at low frequencies. In contrast, much higher G′ and G″ in the low frequency region are obtained in the PLLA/5PDLA and PLLA/ 10PDLA blends, suggesting that a SC crystallite network has been formed in the PLLA/PDLA blend melts and such network structure becomes much denser in the PLLA/10PDLA blend. The loss tangent (tan δ = G″/G′) is believed to be more sensitive to the viscoelasticity changes of polymer relative to G′ and G″. As presented in Figure 2c, the tan δ decreases dramatically with increasing PDLA concentration. Different from the monotonous decrease of the tan δ with increasing frequency for neat PLLA, a frequency-independent tan δ appears for PLLA/2PDLA blend at low frequencies and the plateau of tan δ becomes much wider with further increasing PDLA concentration to 10 wt %. These results indicate that the SC crystallite network could be formed in the melts of PLLA/ PDLA/CB nanocomposites when the PDLA concentration reaches to 2 wt %. Furthermore, the incorporation of PDLA induces a notable enhancement in η* of PLLA (Figure 2d), confirming that the SC crystallites can serve as efficient rheological modifier to greatly reinforce the PLLA melt. On the basis of the above results, it is clear that the SC crystallites can be readily formed in the PLLA melts of PLLA/ PDLA/CB nanocomposites during melt-mixing process and they could undergo a significant change of dispersion state from isolated particles to continuous crystallite network when the PDLA concentration increases from 1 to 2 wt %. Moreover, further increasing PDLA concentration (up to 10 wt %) can induce the formation of a much denser crystallite network. 3.2. Effect of SC Crystallites on the CB network and Electrical Properties of PLLA/PDLA/CB Nanocomposites.

hard to crystallize upon quenching from melt. However, with the incorporation of 1 wt % PDLA, three typical diffraction peaks appear in the patterns of PLLA/PDLA/CB nanocomposites at 2θ values of around 11.9°, 20.8°, and 24.0° (Figure 1b), ascribed to the (110), (300)/(030), and (220) planes of SC crystal structure.53 Moreover, the intensity of these characteristic diffraction peaks enhances gradually with the increase of PDLA concentration up to 10 wt % (Figure 1, parts c and d), clearly indicating an evidently increased content of SC crystallites (XSC) in the melts of PLLA/PDLA/CB nanocomposites. It should be noted that, for the PLLA/PDLA/ CB nanocomposites with the same PDLA concentrations, the incorporation of CB nanoparticles into PLLA/PDLA blends has no apparent effect on the XSC, as confirmed by the constant intensities of their characteristic peaks (Figure 1b−d). It has been reported that the SC crystallites dispersed in asymmetric PLLA/PDLA blend melts can act as efficient rheological modifier to drastically reinforce the melt because isolated SC particles can organize into crystallite network (i.e., the SC particles are connected by interparticle PLA chains).52,54,55 The rheological behaviors of PLLA/PDLA/CB nanocomposites containing different concentrations of PDLA were measured at 190 °C (lower than the Tm of SC crystallites), with an aim to reveal the dispersion state of the SC crystallites formed in the melts. Considering that CB nanoparticles could also enhance the melt viscoelasticity of the nanocomposites,56 special attention has been paid to the dynamic viscoelasticity of PLLA/PDLA binary blends. Figure 2 shows the frequency dependences of storage modulus (G′), loss modulus (G″), loss tangent (tan δ), and complex viscosity (η*) for PLLA/PDLA blends with different concentrations of PDLA. Clearly, neat PLLA displays a typical terminal behavior following the scaling law of G′ ∝ ω2 and G″ ∝ ω, while such terminal behavior disappears gradually with increasing PDLA concentration up to 4235

DOI: 10.1021/acs.jpcc.8b00417 J. Phys. Chem. C 2018, 122, 4232−4240

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The Journal of Physical Chemistry C In order to explore the role of SC crystallites in regulating the CB network structure, the morphologies of the PLLA/PDLA/ CB nanocomposites with different CB loadings (1.0−5.0 wt %) were investigated by SEM and some representative SEM images are shown in Figure 3. For the nanocomposites with 2.0

Figure 4. Representative TEM images of (a) PLLA/2.5CB, (b) PLLA/ 1PDLA/2.5CB, (c) PLLA/2PDLA/2.5CB, and (d) PLLA/10PDLA/ 2.5CB nanocomposites.

although the CB clusters are found to be broken into much smaller ones with further increasing the amounts of SC crystallites in the PLLA/PDLA/CB nanocomposites, the networking of CB nanoparticles becomes difficult (Figure 4, parts c and d). In particular, for the PLLA/10PDLA/2.5CB nanocomposite possessing perfect SC crystallite network, no trace of the CB networking can be observed (Figure 4d). The roles of the SC crystallites in determining the CB network structure of the PLLA/PDLA/CB nanocomposites containing different PDLA concentrations could be tentatively illustrated by the schematic representation presented in Figure 5. For the PLLA/CB nanocomposites, most CB nanoparticles are

Figure 3. SEM images showing the dispersion state of CB nanoparticles in the PLLA/PDLA/CB nanocomposites containing various amounts of CB: (a−c) PLLA/CB, (d−f) PLLA/1PDLA/CB, (g−i) PLLA/2PDLA/CB, and (j−l) PLLA/10PDLA/CB.

wt % CB, no apparent CB network structures can be detected in the matrix and only isolated CB clusters can be observed in the PLLA matrix. However, it is interesting to find that the size of the CB clusters decreases with increasing PDLA concentration from 1 wt % to 10 wt % due to the formation of SC crystallites (Figure 3d, g, j). This phenomenon becomes much more obvious in the nanocomposites with 2.5 wt % CB. For example, much smaller CB clusters are observed to be uniformly dispersed in the PLLA/10PDLA/2.5CB nanocomposite (Figure 3k). More interestingly, a continuous CB network is found to be formed in the PLLA/1PDLA/2.5CB nanocomposite (Figure 3e), suggesting that the SC crystallites induced depression of CB aggregation can facilitate the networking of CB nanoparticles in PLLA matrix. With further increasing CB loading to 5.0 wt %, an effective CB network is already formed in the PLLA/5CB nanocomposite and thus the SC crystallites plays no significant role in the manipulation of the CB network structure (Figure 3, parts c, f, i, and l). To get a more clear-cut evidence for the role of SC crystallites in manipulating CB network, the filler network structure of PLLA/PDLA/2.5CB nanocomposites was further observed using TEM (Figure 4). As expected, the CB nanoparticles exhibit a strong tendency to self-organize into large clusters in the matrix of PLLA/2.5CB nanocomposite (Figure 4a). Clearly, the formation of small amount of SC crystallites in the PLLA/1PDLA/2.5CB nanocomposite induces an evident reduction in the size of CB clusters, which is favorable for the formation of continuous CB network (as highlighted by the dotted lines in Figure 4b). However,

Figure 5. Schematic representations showing the possible roles of SC crystallites in manipulating the CB network structures of (a) PLLA/ CB, (b) PLLA/1PDLA/CB, (c) PLLA/2PDLA/CB, and (d) PLLA/ 10PDLA/CB nanocomposites. 4236

DOI: 10.1021/acs.jpcc.8b00417 J. Phys. Chem. C 2018, 122, 4232−4240

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The Journal of Physical Chemistry C aggregated into large clusters before networking and thus no continuous CB network can be formed via self-networking of the aggregated clusters (Figure 5a). By contrast, the rigid SC particles formed in the PLLA/1PDLA/CB nanocomposites could function as rheological modifier to enhance the melt viscosity of PLLA matrix (Figure 2d). In this way, a stronger shear stress could be generated in the matrix melt to prevent the CB nanoparticles from seriously aggregating. As a result, less CB nanoparticles are required to form continuous CB network (Figure 5b). However, when the PDLA concentration reaches to 2 wt % or above, the dramatically enhanced melt viscosity can improve the dispersion of CB nanoparticles but the SC crystallite network could serve as physical barrier for the CB networking (Figure 5, parts c and d). The effects of the SC crystallites tailored CB network structure on the electrical properties of PLLA/PDLA/CB nanocomposites were investigated and the results are displayed in Figure 6a. With increasing CB loading from 1.0 wt % to 5.0

Figure 7. Electrical conductivity values and WAXD patterns of PLLA/ PDLA/CB nanocomposites after being annealed at 190 °C (a, a′) and 240 °C (b, b′) for 30 min.

CB and PLLA/1PDLA/CB nanocomposites exhibit a substantially enhanced σ upon annealing at 190 °C because the isolated SC particles cannot prevent the Brownian motion of CB nanoparticles from forming a more stable network. However, the annealing has no apparent effect on the σ of PLLA/10PDLA/CB nanocomposites, clearly demonstrating the physical barrier effect of the SC crystallite network on the CB networking. With regard to the PLLA/2PDLA/CB nanocomposites, the enhanced σ upon annealing can be ascribed to the weak SC crystallite network (namely, the SC crystallites are not strongly connected by the interparticle PLA chains). With increasing annealing temperature from 190 to 240 °C, the σ of PLLA/PDLA/CB nanocomposites enhances significantly but one cannot observe the strong dependence of σ on the PDLA concentration. All the nanocomposites with the same CB loading share the similar σ because the complete melting of SC crystallites significantly promotes the CB networking. 3.3. Crystallization Behavior and Mechanical Properties of PLLA/PDLA/CB Nanocomposites. Besides the significant roles in the tailoring of CB network structure, the formed SC crystallites are also expected to serve as highly active nucleating agent to remarkably accelerate the matrix crystallization of PLLA/PDLA/CB nanocomposites. Thus, both the nonisothermal and isothermal crystallization behaviors of the nanocomposites were investigated using DSC. Figure 8 shows the DSC cooling curves, subsequent heating curves, and the isothermal half-crystallization time (t0.5) of the PLLA/PDLA/ 2.5CB nanocomposites with different concentrations of PDLA. Noticeably, with the incorporation of 1 wt % PDLA into the PLLA/2.5CB nanocomposite, the crystallization temperature of PLLA matrix (Tc,PLLA) shifts to a much higher temperature and the broad crystallization peak becomes much sharper (Figure 8a). Moreover, the area of the second heating peak is increased (Figure 8b), indicating that the SC crystallites can serve as nucleating agent for PLLA crystallization. For semicrystalline polymers, t0.5 is one of the most useful parameters to estimate crystallization rate. As shown in Figure 8c, the incorporation of PDLA gives rise to a sharp decrease in the t0.5 value of PLLA/ 2.5CB nanocomposite, further demonstrating that the for-

Figure 6. (a) Effect of SC crystallites on the electrical conductivity (σ) of PLLA/PDLA/CB nanocomposites containing various amounts of CB, and (b) log−log plots of the σ as a function of reduced CB content.

wt %, a dramatic enhancement in the electrical conductivity (σ) can be clearly observed in all nanocomposites. However, the electrical percolation threshold (φc) of CB, i.e., the critical CB loading required to form effective conductive network, is strongly dependent on the PDLA concentration. The doublelog plots of σ vs (φ − φc) are presented in Figure 6b. Compared with the PLLA/CB nanocomposites, an evidently decreased φc (from 2.70 to 2.23 wt %) is obtained for the PLLA/1PDLA/CB nanocomposites because the formation of small amounts of isolated SC particles facilitates the CB networking at lower CB loadings. However, the φc of the PLLA/2PDLA/CB nanocomposites shifts to a higher value of 2.73 wt %, which suggests that the formation of SC crystallite network is rather harmful to the networking of CB nanoparticles. Especially, the φc of the PLLA/10PDLA/CB nanocomposites is as high as 3.64 wt %. The electrical properties of the PLLA/PDLA/CB nanocomposites are in good agreement with the CB network structures illustrated in Figure 5. To further confirm the significant roles of the SC crystallites in determining the CB network structure and resulting electrical properties of PLLA/PDLA/CB nanocomposites, a series of annealing experiments were performed. As mentioned in the introduction, thermal annealing at a temperature above the Tm of polymer matrix can promote the buildup of CB network, finally leading to a great enhancement in the σ. Figure 7 shows the σ values of the PLLA/PDLA/CB nanocomposites after being annealed at 190 and 240 °C (below and above the Tm of SC crystallites) for 30 min. As expected, both the PLLA/ 4237

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polymers, the nanocomposites with the same amorphous PLLA matrix were tested. Clearly, no notable enhancement in tensile strength of PLLA can be achieved after incorporating various loadings of CB nanoparticles (Figure 9a), which should be mainly ascribed to the severe agglomerations of CB nanoparticles in the matrix. However, for the PLLA/PDLA blends, the incorporation of CB nanoparticles gives rise to an evident increase in the tensile strength because the SC crystallites can improve the dispersion of CB nanoparticles (as confirmed by the TEM images shown in Figure 4b−d). More interestingly, the tensile strength of the PLLA/PDLA/CB nanocomposites exhibits a strong dependence on the PDLA concentration. At the same CB loadings, the tensile strength of PLLA/1PDLA/ CB nanocomposites is only slightly higher than that of the PLLA/CB ones, but a remarkably increased tensile strength is obtained in the PLLA/2PDLA/CB and PLLA/10PDLA/CB nanocomposites, indicating that the SC crystallites network has a good reinforcing effect on the nanocomposites. Interestingly, the incorporation of rigid CB particles can significantly enhance the tensile modulus of PLLA/PDLA/CB nanocomposites as expected, but the formation of SC crystallites has no apparent effect on the modulus (Figure 9b). On the contrast, the elongation at break of the nanocomposites decreases greatly with increasing CB loading probably due to the CB aggregation (Figure 9c). Nevertheless, it is surprising to find that the elongation at break of the PLLA/10PDLA/CB nanocomposites is much higher than that of PLLA/CB nanocomposites at the same CB loadings, indicating that the formation of dense SC crystallites network can enhance the ductility of the PLLA/CB nanocomposites.

Figure 8. (a) DSC cooling curves, (b) subsequent heating curves, and (c) half-crystallization time (t0.5) as a function of isothermal crystallization temperature for the PLLA/PDLA/CB nanocomposites.

mation of small amounts of SC crystallites can greatly enhance crystallization rate of PLLA matrix. Furthermore, it is worth noting that no significant enhancement in the nucleating efficiency can be obtained with further increasing PDLA concentration to 2 wt % or above due to the confining effect of the SC crystallite network on the motion of PLLA matrix chains.52 Because excellent mechanical properties are essential for the practical applications of conductive polymer composites, the tensile properties of PLLA/PDLA/CB nanocomposites were measured via uniaxial tensile tests. Figure 9 gives the tensile strength, tensile modulus, and elongation at break of these nanocomposites as a function of CB loadings (the stress−strain curves of some representative nanocomposites are shown in Figure S2). Considering the crystallization plays a significant role in determining the mechanical properties of semicrystalline

4. CONCLUSIONS In summary, a facile strategy to manipulate CB network structure and resulting properties of PLLA/CB nanocomposites by using SC crystallites as efficient rheological modifier has been demonstrated for the first time. The SC crystallites can be formed in the nanocomposites by incorporating small amounts (1−10 wt %) of PDLA through simple melt mixing at 190 °C. For the nanocomposites containing 1 wt % PDLA, the formation of SC crystallites can obviously enhance the melt viscosity of PLLA matrix and subsequently facilitate the networking of CB nanoparticles by depressing their aggregation, finally leading to an evident decrease in the electrical percolation threshold. However, with the increase of PDLA content up to 10 wt %, the isolated SC crystallites could organize into a rigid network capable of serving as physical barrier for the networking of CB nanoparticles. As a result, although the dispersion level of CB nanoparticles is improved substantially, a sharply increased percolation threshold is obtained in the nanocomposites with the formation of SC crystallite network. Compared with the long-term (at least 0.5 h) and tedious annealing treatment, the utilization of SC crystallites is more simple and economic to manipulate the CB network structure. Furthermore, these SC crystallites can act as nucleating agent and reinforcer to significantly enhance the matrix crystallization rate and mechanical properties of PLLA/ CB nanocomposites. Overall, this work could provide a promising and industrially meaningful solution for fabricating various high-performance PLLA-based nanocomposites via manipulating filler network with the aid of SC crystallites.

Figure 9. (a) Tensile strength, (b) tensile modulus, and (c) elongation at break of the PLLA/PDLA/CB nanocomposites as a function of CB loadings. 4238

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00417. TGA data and stress−strain curves of some PLLA/ PDLA/CB nanocomposites (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86 28 8546 1795. E-mail: hongweibai@scu. edu.cn, [email protected] (H.W.B.). * E-mail: [email protected] (Q.F.). ORCID

Hongwei Bai: 0000-0003-4927-6422 Qiang Fu: 0000-0002-5191-3315 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Nos. 21404075 and 51421061) and Science Foundation for The Excellent Youth Scholars of Sichuan University (No. 2015SCU04A28).

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REFERENCES

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