Formation of a Segregated Electrically Conductive Network Structure

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Formation of segregated electrically conductive network structure in low-melt-viscosity polymer for highly efficient electromagnetic interference shielding Cheng-Hua Cui, Ding-Xiang Yan, Huan Pang, Xin Xu, Li-Chuan Jia, and Zhong-Ming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00526 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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ACS Sustainable Chemistry & Engineering

Formation of segregated electrically conductive network structure in low-melt-viscosity polymer for highly efficient electromagnetic interference shielding Cheng-Hua Cui, Ding-Xiang Yan,* Huan Pang, Xin Xu, Li-Chuan Jia and Zhong-Ming Li* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, No.24 South Section 1, Yihuan Road, Chengdu 610065, Sichuan, People’s Republic of China. E-mail: [email protected] ABSTRACT Segregated electrically conductive network structure has been well demonstrated to efficiently improve electrical and electromagnetic interference (EMI) shielding performance owing to the controllable assembling of conductive additives in polymer matrices, however, up to now, the polymer matrices are mainly limited to high-melt-viscosity polymers (e.g., ultra-high molecular weight or cross-linked polymers). In the current work, we proposed a strategy to form typical segregate structure in a low-melt-viscosity polymer, i.e., poly(lactic acid) (PLA), making use of melting temperature difference of crystallites. Poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were first melt mixed, and then crystallized, herein the resultant blend consisted of homocrystallites (hc) and stereocomplex crystallites (sc), and interestingly, spontaneously granulated into 100 μm particles during melt mixing and crystallizing. The mixture of PLA crystallite granules and carbon nanotubes (CNTs) was compression molded at a temperature between the melting temperatures of hc and sc, where the survived sc, still in solid state, acts as the physical cross-linking points to confine PLA chain motion, forcing CNTs to localize only at

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the interfaces of PLA domains. The morphological observation indicates the successful formation of the typical segregated structure, resulting in an ultra-low percolation threshold of 0.040 vol% CNT. The segregated CNT/PLA composite with only 0.60 vol% (1.0 wt%) of CNT loading achieved high electrical conductivity of 12.0 S/m and outstanding EMI shielding effectiveness of 35.5 dB. This special structure provides numerous interfaces to reflect, scatter and absorb the incident microwaves many times, endowing an absorption dominated EMI shielding mechanism. Our work reveals a major breakthrough in creating segregated conductive network

structure

in

low-melt-viscosity

polymers,

further

developing

economical,

environmentally friendly, and highly efficient EMI shielding composites. KEYWORDS: segregated structure, conductive polymer composites, poly(lactic acid), stereocomplex crystallites, electromagnetic interference shielding

INTRODUCTION In the past decade, the formation of segregated structure has been well demonstrated to be an efficient approach to improve the electrical and electromagnetic interference (EMI) shielding performance of conductive polymer composites (CPCs),1 due to the selective distribution of conductive fillers at the interfaces of polymer regions which is beneficial to construct dense conductive networks. For instance, the segregated structure in cross-linked carbon nanotube (CNT)/poly(ethylene-co-octene) (POE) composite resulted in a low threshold percolation of 1.5 vol% compared to 9.0 vol% in conventional CNT/POE composite.2 In another work, only 2.0 wt% carbon nanotube (CNT) loading endowed a segregated CNT/polycarbonate (PC) composite with an EMI shielding effectiveness (SE) of 23.1 dB, already exceeding the commercially applicable EMI SE value (20 dB).3 Our previous work generated a segregated structure in CNT/cross-linked polyethylene (c-PE) composite, which realized an EMI SE of 46.4 dB, about 46% higher than

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that of conventional CNT/PE composite, at 5.0 wt% CNT loading.4 Very Recently, higher EMI SE of 50 dB was achieved in 10 wt% CNT loaded ultrahigh molecular polyethylene (UHMWPE) composite with a typical segregated structure.5 Up to now, it can be seen that the segregated structures are basically constructed in polymer matrices with high melt viscosity, mainly UHMWPE,5-10 cross-linked polymers (e.g., cross-linked PE, natural rubber, etc.),4,

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low-

temperature (or low-pressure) processed amorphous polymers (e.g., PC, polystyrene, polymethyl methacrylate, etc.).12-14 Use of the high-melt-viscosity polymers would prevent the migration of conductive fillers into their interior during melt processing and thus benefit the construction of segregated structure. Unfortunately, the development of segregated structure in conventional semi-crystalline polymer matrices (raw PE, polypropylene, polyethyleneterephthalate, etc.) with low melt viscosity is still a great challenge because of the strong molecular motion favouring the diffusion of conductive fillers into polymer domains. In the current work, we proposed a new strategy to fabricate segregated structure in semicrystalline poly(lactic acid) (PLA), a typical low-melt-viscosity polymer, making use of melting temperature (Tm) discrepancy of homocrystallites (hc) (160-180

o

C) and stereocomplex

crystallites (sc) (~220 oC).15-19 PLA granules containing hc and sc were spontaneously obtained via the melt blending of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) at relatively low temperature and subsequent crystallizing. Under such condition, the formation of abundant sc locked the dissociative PLLA and PDLA chains, and thus made the PLLA/PDLA blends in granule form. CNTs were subsequently coated on the surfaces of the PLA granules and the CNT/PLA complex granules were compression molded to prepare the segregated CNT/PLA composites (denoted as s-CNT/PLA composites). As the compression temperature is between the Tm of hc and sc, the survived sc could act as the physical cross-linking points to restrict the

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movement of PLA chains, which is conducive to prohibiting the penetration of CNTs into PLA domains and forming segregated structure in the final composite. The resultant s-CNT/PLA composite with only 0.60 vol% (1.0 wt%) CNT achieves an outstanding EMI SE up to 35.5 dB, the highest value ever reported in CNT/polymer composites at such a low CNT loading. Moreover, it is worth noting that the matrix of the fabricated EMI shielding materials is based on renewable and biodegradable PLA rather than the previously reported petroleum-based polymers suffering from the depletion of non-renewable petroleum resources and ecological hazards.

EXPERIMENTAL SECTION Materials PLLA (trade mark 4032D) with the weight-average molecular weight (Mw) of 2.23 × 105 g/mol was purchased from Nature Works, containing around 2% D-LA. PDLA with the Mw of 8.5 × 104 g/mol) was used. Tm of PLLA and PDLA are 167.4 and 176.0 oC, measured using differential scanning calorimetry (DSC) at a heating rate of 10 oC/min. Polydispersity index values of PLLA and PDLA are 2.10 and 1.82, respectively. CNTs (NC7000) were supplied by Nanocyl S.A., Belgium, with an average diameter of 9.5 nm and length of 1.5 μm. Preparation of the s-CNT/PLA composites Fabrication of the s-CNT/PLA composite is schematically depicted in Figure 1a. Initially, PLLA and PDLA (1/1 w/w) were mixed in a HAAKE internal mixer (Rheomix 600) to prepare PLA granules (Figure 1b), at 180 oC for 10 min, with a mixer rotation speed of 40 rpm. As the temperature was between the Tm of hc and sc, only sc can grow during the mixing process, which locked the PLA chains and led to the blends in granule form (average size of 100 μm, shown in Figure S1). The PLA granules were then mechanically mixed with CNTs to prepare CNT coated PLA complex granules (Figure 1c). Subsequently, the CNT/PLA complex granules were

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compression molded into disks at 190 oC for 5 min, with a pressure of 10 MPa, after preheating for 10 min. Because the compression temperature is much higher than the glass transition temperature of PLA and the Tm of hc crystallites, PLA chains (amorphous and molten hc) could move and penetrate into the CNT layers between PLA domains, forming a monolithic sample. The s-CNT/PLA composites with various CNT loadings from 0.03 to 0.60 vol% (0.05 to 1 wt%) were prepared. The CNT/PLA composites with constant CNT content of 0.60 vol% were also molded at different temperatures (190, 200, 210, 220 and 230 oC). As a contrast, the control sample with CNT randomly distributed through PLA (denoted as r-CNT/PLA composite) was fabricated by the direct melt compounding of PLLA, PDLA and CNTs, with the subsequent compression molding process. Characterizations The melting behavior of PLA granules was characterized using a TA Q200 at a heating rate of 10 oC/min from 40 to 250 oC. The diameter distribution of PLA granules was measured using the laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd, Britain) by the means of dispersing PLA granules in alcohol media. Wide-angle X-ray diffraction (WAXD) measurement was performed by a Philips X’Pert pro MPD diffractometer with Cu Kα radiation (λ = 0.154 nm) operating at 40 kV and 40 mA. The scanning range was from 5 to 35° at a 2θ scanning rate of 10°/ min. Scanning Electron Microscopy (SEM) analysis was taken on a field emission SEM (Inspect-F, FEI, Finland) at an accelerating voltage of 5 kV. The CNT/PLA complex granules and the fractured surface of the s-CNT/PLA composite were sprayed with gold before SEM observation. The composite sample was initially immersed in liquid nitrogen for 30 min and then quickly

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fractured. For optical microscopy (OM) measurement, the s-CNT/PLA specimens were cut into films in 20 μm thickness using a microtome and observed with an Olympus BX51 polarizing optical microscope (Olympus Co., Tokyo, Japan) with a Micro-Publisher 3.3 RTV CCD camera. The electrical resistance of the s-CNT/PLA composites (with CNT loadings lower than 0.30 vol%) and the r-CNT/PLA composite were measured using a Keithley electrometer model 4200SCS (USA). The samples were cut into rectangular sheets and the opposites were coated with silver paste to eliminate the contact resistance. The electrical conductivity was calculated using the equation resistance,

= /( ∙ ). Where and

and

represent the electrical conductivity and electrical

are the length and cross-sectional area of the rectangular sheet. The electrical

conductivity of the s-CNT/PLA composites with 0.30 and 0.60 vol% CNT was conducted on a four-point probe instrument (RTS-8, Guangzhou Four-Point Probe Technology Co., Ltd., China). EMI shielding measurements were conducted in the frequency range of 8.2-12.4 GHz using Agilent N5230 vector network analyzer. The measured scattering parameters (S11 and S21) were used to calculate the power coefficients of reflectivity (R), transmissivity (T) and absorptivity (A). Then the EMI SE can be obtained using the flowing equations. SEtotal = SE + SE + SE = −10logT (1) SE = −10log(T⁄(1 − R))

(2)

SE = −10log(1 − R)

(3)

where the EMI SE ( SEtotal ) is described as the summation of reflection shielding ( SE ), absorption shielding ( SE ) and multiple reflections shielding ( SE ). SE

usually can be

neglected when SEtotal > 10 dB.

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Figure 1. (a) Schematic showing the fabrication of the s-CNT/PLA composite. SEM images of (b) PLA granules, (c) 0.30 vol% CNT coated PLA granule, (d) the fractured surface of the 0.30 vol% s-CNT/PLA composite, (e) the magnified micrographs of the rectangle area in (d).

RESULTS AND DISCUSSION The present of hc and sc in the PLA granules is crucial to the formation of segregated structure in the s-CNT/PLA composites to achieve high electrical and EMI shielding performance, thus the crystalline information of the PLA granules were first detected using DSC and 2D-WAXD measurement. The DSC curve of the PLA granules in Figure 2a displays a strong endothermic peak around 217 oC and two nearby weak peaks around 164 and 175 oC, indicating the existence of sc, PLLA hc and PDLA hc, respectively. The crystallinity of sc ( DSC data is 34.9%, while the

(sc)) estimated by the

(hc) is only 4.0%. It means that nearly 90% crystallites in the

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PLA granules belong to sc. The WAXD measurement in Figure 2b exhibits the similar results. Three obvious characteristic peaks at 2θ values of 11.9, 20.7 and 24.0º correspond to (110), (300)/(030) and (220) planes of sc, while the weak characteristic peak at 2θ value of 16.9º corresponds to the (200)/(110) planes of hc,15 demonstrating that sc rather than hc is predominantly formed in the PLA granules. After the PLA granules were compression molded into a disk, sc still maintains a high concentration but hc disappears (the melting peaks of hc in Figure 2a are contributed to the cold crystallization of PLA). .Since sc possesses the Tm of about 50 oC higher than hc, they can be well reserved upon the compression molding when the temperature is between the Tm of sc and hc. It is believed that the saved sc could act as physical cross-linking points to confine the PLA chain motion to suppress the intense diffusion between PLA domains and CNTs.19 Under such condition, CNTs, originally coated at the surfaces of PLA granules by mechanical mixing would be effectively prevented penetrating into the PLA domains, guaranteeing the formation of segregated structure in the composite.

Figure 2. (a) DSC curves (appropriate compression temperature for the formation of segregated structure marked) and the (b) WAXD profiles for the PLA granules and PLA samples.

To provide insights into the structural formation and the evolution of conductive networks, the OM images of the s-CNT/PLA composites with various CNT loadings are shown in Figure 3. 8


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The selective distribution of CNTs surrounding the PLA domains is observed regardless of CNT loading, illustrating the typical segregated structure. This means that CNTs were indeed restricted to penetrate into the PLA granules interior during the molding process. When the CNT content is only 0.03 vol% (Figure 3a and d), the conductive paths feature low density and thickness, indicative of defective CNT conductive networks. As the CNT content rises to 0.06 and 0.30 vol%, denser and thicker conductive paths develop, showing well defined CNT conductive networks (Figure 3b, e and c, f). The segregated structure of the s-CNT/PLA composite can be also detected using SEM measurement. The fractured surface of the sCNT/PLA composites in Figure 1d showed that polyhedral PLA domains are squeezed together with interfacial lines. The interfacial region was magnified to manifest more detailed microstructure in the composite, as shown in Figure 1e. Clear CNT blanket is observed at the boundary of PLA domain, which further demonstrates that the CNTs were difficult to diffuse into the PLA granules interior and only located at their interfaces.

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Figure 3. OM images of the s-CNT/PLA composites with CNT contents of (a, d) 0.03 vol%, (b, e) 0.06 vol% and (c, f) 0.30 vol%.

In order to guarantee the formation of segregated structure, polymer granules should be maintained in a high-viscosity state during processing to prevent the penetration of conductive filler into their interior. The melt viscosity of conventional PLA (regardless of PLLA or PDLA) is always very low once molded at normal processing temperature, restricting molecular chain motion becomes the main difficulty to create segregated structure. Recently, Wei et al demonstrated that sc could obviously increase the viscosity of PLA melt even at a low

(sc) of

only 2.6%, through confining the long-range motions of PLA chains.19 In this work, the reserved sc with very high

(sc) of 34.9% can confine the long-range motions of PLA chains to a

greater extent and develops a physical gel state for the PLA granules. Thus the PLA granules only showed plastic deformation under pressure to form irregular polyhedrons and CNTs are difficult to penetrate into their interior, leading to the microstructure where ordered segregated CNT networks wraps PLA polyhedrons. The establishment of perfect CNT conductive networks in the s-CNT/PLA composites is beneficial for achieving high performances in electrical conductivity and EMI SE. Figure 4 shows the volume electrical conductivity of the s-CNT/PLA composites as a function of CNT loading. The electrical conductivity exhibits a drastic increase by six orders of magnitude as the CNT loading increases from 0.03 to 0.042 vol%, indicating a typical percolation threshold behavior. According to classical percolation theory, the power-law equation σ =

( − ) is employed to evaluate the relationship between the electrical

conductivity of s-CNT/PLA composites and the CNT content, where σ represents the composite conductivity,

is a constant related to the intrinsic conductivity of CNT,

is the volume

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fraction of CNT,


is the percolation threshold and

is a critical exponent revealing the

dimensionality of the conductive networks. As inserted in Figure 4, the best fitting result presents a percolation threshold of 0.040 vol%, which is a very low filler content to endow the composites conductive and also considerably lower than the values reported in CNT/polymer composites (Table 1). Such a low threshold is mainly attributed to the large aspect ratio of CNT and the dense CNT conductive networks among the interfaces of PLA polyhedrons. The fitted is 1.88, indicative of a three-dimensional CNT conductive network in the s-CNT/PLA composites.20 Once the CNT loading exceeding the threshold, the electrical conductivity of the s-CNT/PLA composite increases gradually and satisfies the target value (1 S/m) for commercial EMI shielding application.21 It is exciting that the composite with only 0.30 vol% CNT already achieves a satisfied electrical conductivity of 4.2 S/m and increasing CNT content to 0.60 vol% results in an outstanding electrical conductivity of 12.0 S/m, already exceeding the highest value in CNT/polymer composites at such a low CNT content (Table 1). To further establish the superiority of segregated structure in the s-CNT/PLA composite, we also prepared the rCNT/PLA composite (with randomly distributed CNTs in the whole system) and estimated the electrical conductivity for comparison. The r-CNT/PLA system exhibits much lower value than that of s-CNT/PLA system among all the CNT loadings due to the inefficiency of CNT on constructing conductive networks. The conductivity of the r-CNT/PLA composite with 0.60 vol% CNT is only 3.62 × 10-3 S/m, nearly four orders of magnitude below the s-CNT/PLA composite at the same CNT loading. This demonstrates that the preferred distribution of CNTs developed an advanced conductive network in the s-CNT/PLA composite compared to the r-CNT/PLA composite with random distributed CNTs. With the aid of preconceived sc, the PLA domains expressed as high-viscosity physical gels that squeezed CNTs along their interfaces in the s-

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CNT/PLA composites. The PLA domains without CNTs act as “excluded volume” to significantly increase the efficient utilization of CNTs to establish conductive networks, contributing to the superior electrical conductivity. Moreover, compared with the volume exclusion using crystalline regions as “excluded volume” in CNT/PLLA system, our special segregated structure shows obvious advantages in enhancing electrical conductivity.22

Figure 4. The electrical conductivity of the s-CNT/PLA and r-CNT/PLA composites as a function of CNT loadings. The inset shows a log-log plot of electrical conductivity versus j-jc for the s-CNT/PLA composite.

The high-performance electrical conductivity of the s-CNT/PE composite would help develop efficient EMI shielding material, especially at ultra-low CNT loading. EMI SE of the sCNT/PLA composite is shown in Figure 5a, as a function of CNT loading over the frequency of X-band. EMI SE indicates a material’s ability to attenuate microwaves intensity and is represented as the logarithm of the ratio of incident power ( ) to transmitted power ( ), i.e., EMI SE = 10log( ⁄ ). EMI SE of 20 dB means the blocking of 99% incident microwave energy, which is the target value for commercially applicable in EMI shielding devices. For the s-CNT/PLA composites with 0.12 and 0.18 vol% CNT, the EMI SE is found to be 10-19 and 1222 dB, exhibiting frequency dependence across the measured frequency range. The frequency-

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determined behavior can be attributed to the losses from the dielectric polarizations of CNT in the composite with inferior conductivity (lower than 1 S/m).23 As the CNT content is 0.30 and 0.60 vol%, the EMI SE of the composites exhibits weak frequency dependence and the average EMI SE increases up to 27.0 and 35.5 dB, respectively, corresponding to 99.80% and 99.97% blocking of microwave radiation. The improved EMI shielding performance is mainly originated from the enhanced electrical conductivity of the s-CNT/PLA composites and the increased absorption CNT volume fraction.24 The EMI SE of r-CNT/PLA composites with various CNT loadings was also shown in Figure 5a, which exhibit frequency dependence similar to the sCNT/PLA composites with lower CNT contents. The low electrical conductivity because of the randomly distributed CNT in the r-CNT/PLA composite should be responsible for the frequencydependent EMI SE performance. Additionally, the r-CNT/PLA composites own much poorer ability to attenuate electromagnetic microwaves than the s-CNT/PLA composites because of their lower electrical conductivity. It can be seen that the EMI SE of the r-CNT/PLA composite is lower than 20 dB even at the largest CNT loading, not reaching the shielding level for commercial applications. The largest EMI SE of the r-CNT/PLA composite is 18.4 dB, only half of that achieved in the s-CNT/PLA composite (35.5 dB), testifying that the great superiority of segregated structure over randomly distributed structure in improving EMI SE. Again, the preformed sc in PLA granules help to develop the segregated structure in the s-CNT/PLA to construct highly conductive networks that contribute to efficient EMI SE. To explore the EMI shielding mechanism in the s-CNT/PLA composites, SE

, SE and SE

at the 8.2 GHz are calculated, and their changes as a function of CNT loadings are plotted in Figure 5b. It is clear that SE increases greatly with CNT loading while SE undergoes a slight decrease, and the contribution of SE to SE

is negligible regardless of the CNT contents. For

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instance, the SE

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, SE and SE of the 0.60 vol% s-CNT/PLA composite are 33.5, 32.5, and

1.0 dB, that is, absorption (97%) makes a much larger contribution to the total EMI SE than reflection (3.0%), indicating an absorption dominated shielding mechanism. Thanks to the specific segregated structure, continuous CNT conducting blankets among PLA domains (see Figure 1e) certainly provide numerous interfaces to reflect, scatter, and absorb the incident electromagnetic microwaves many times and thus the microwaves were difficult to escape from the sample before being absorbed. The similar shielding mechanism was also detected in previous reported CPCs with segregated structure.4, 10, 14 The sample thickness is also a key factor in determining the EMI shielding performance of a material for the practical use, which means material and energy savings. In this section, the effect of sample thickness on EMI SE of the 0.60 vol% s-CNT/PLA composite is evaluated in order to determine the critical thickness satisfying the commercial shielding application (20 dB). The EMI SE exhibits a significant increase with the sample thickness, which can be easily understood since a thicker sample supplies more CNTs to interact with the incident electromagnetic waves. For the composite samples with the thickness of 0.9 and 1.5 mm, the average EMI SE is 17 dB and 27.5 dB, respectively, and thus the composite possesses a critical thickness between 0.9 and 1.5 mm to show commercial EMI SE. The critical thickness can be also estimated by the skin depth ( ) of a shielding material, as speculated and confirmed by the previous experimental results.24-28 For example, for the 60 vol% graphene/ethylene–vinyl acetate composite with a

of

335 μm, the EMI SE was only 17 dB as the sample thickness was 150 μm and the EMI SE increased to 23 dB once the sample thickness (350 μm) beyond the .26 In the present work, the of the 0.60 vol% s-CNT/PLA composite was calculated to be 1.4 mm (shown in Figure S2), according with the EMI SE results for the composites with sample thicknesses of 0.9 mm and 1.5

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mm. Figure 5d presents the SEtotal , SE and SE as a function of sample thickness at 8.2 GHz to further illustrate the shielding mechanism of the composites. SER maintains constant since the reflection only occurred at the incident surface and is independent of the sample thickness. SEA increases significantly with sample thickness and is the main contribution to SETotal, demonstrating the adsorption dominated shielding mechanism again.

Figure 5. (a) The EMI SE as a function of frequency (X-band range) for the s-CNT/PLA and r-CNT/PLA composites with different CNT loadings; (b) Comparison of SEtotal , SE and SE at the frequency of 8.2 GHz for the s-CNT/PLA composites with various CNT contents; (c) The EMI SE as a function of frequency for the 0.60 vol% s-CNT/PLA composites with different thicknesses; (d) Comparison of SEtotal , SE and SE at 8.2 GHz for the 0.60 vol% s-CNT/PLA composites with different thicknesses.

The EMI SE achieved in the s-CNT/PLA composites is also outstanding compared to the

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previously reported CNT/polymer for EMI shielding, as listed in Table 1. It can be seen that realizing the commercial EMI shielding level (20 dB) is quite difficult for common CNT/polymer composites, with relative low CNT contents (below 5 wt%) and sample thickness of ~2 mm. Fortunately, the s- CNT/PLA composites with only 0.60 vol% (1 wt%) CNT and 1.5 mm sample thickness exhibits a relatively high EMI SE of 27.5 dB. The exceptional EMI SE must lie in the highly interconnected CNT networks due to the formation of the segregated structure that significantly increasing the effective CNT concentration to participate in constructing conducting pathways. Table 1. Comparison of electrical properties and EMI SE (in the X-band) of the sCNT/PLA composite and reported CNT/polymer composites. CNT Conductivity EMI SE Sample Ref. content [S/m] [dB] thickness [mm] PLA 0.60 vol% 35.5 2.7 0.040 vol% 12.0 This work PLA 27.5 1.5 (1.0 wt%) -5 29 --PLA 0.35 wt% 2.0 wt% 10 24 PP ~ 1.0 vol% -4.6 2.8 30 PS ~ 5.0 wt% -25.4 2.0 31 PS ~ 5.0 wt% 7.1 17.2 2.0 32 PU ~ 3 wt% 0.1 ~18 1.5 33 PTT 0.48 vol% 4.76 vol% 4.5 23 2.0 25 ABS ~0.5 wt% 1.0 wt% -~12 1.1 28 PPCP 0.19 vol% 1.8 vol% -~23 2.0 4 PE 0.013 vol% 1 wt% 1.24 ~17 2.1 34 PVDF/ABS 3 wt% -~20 dB 5.6 PP: polypropylene; PS: polystyrene; PU: polyurethane; PTT: poly(trimethylene terephthalate); ABS: acrylonitrile-butadiene-styrene; PPCP: polypropylene random copolymer; PVDF: poly(vinylidene fluoride); --: not provided. Polymer

Percolation threshold

As mentioned above, the formation of segregated structure in the s-CNT/PLA composite is significant for the outstanding electrical and EMI shielding performance, and the reserved sc in PLA granules during compression process is prerequisite for forming such special structure. To further demonstrate the determinative effect of sc, CNT/PLA composites with various concentration of reserved sc were prepared via adjusting the compression temperature, according to the melting range (from 190 to 230 oC) of sc (Figure 2a). The corresponding microstructures of the CNT/PLA composites are shown in Figure 6. Perfect segregated structures with highly interconnected CNT networks are formed as the compression temperatures range from 190 to

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210 oC (Figure 6a-c). Similarly, Bai et al. also fabricated the high-performance stereocomplexed PLA products by the compression molding of PLA powders under low temperature (180-210 o

C).35 As the compression temperature increases to 220 oC, some CNTs penetrate into PLA

domains and parts of conductive paths are not connected (Figure 6d). Further increasing the compression temperature to 230 oC, results in a disturbed distribution of CNTs and seriously damaged conductive paths (Figure 6e). The sc in PLA granules would gradually melt as the compression temperature increased and rapidly melt as the temperature exceeding the melting peak (217 oC) of sc. The evolution of microstructure of the CNT/PLA composite with compression temperature is related to the amount of reserved sc during processing. As calculated from the DSC curves in Figure S3, the

( ) of reserved sc is higher than 35% at 190-210 oC

range, and decreases to 16.8% at 220 oC and 0% at 230 oC. As the compression temperature below the melting peak, the reserved sc is sufficient to crosslink the PLA chains to make the PLA granule as a high-viscosity gel, guaranteeing the formation of segregated structure. Once the compression temperature beyond melting peak, sc is difficult to be reserved and the PLA chains are activated. In such condition, the interdiffusion of PLA chains and CNTs easily occurs, which should be responsible for the destructive segregated structure.

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Figure 6. The OM images of CNT/PLA composites with 0.60 vol% CNT content at different compression temperature.

The microstructure behavior directly influences the EMI SE the CNT/PLA composites, as shown in Figure 7. The composites molded at the temperature range of 190-210 oC exhibit high EMI SE over 35.5 dB, while the composites molded at 220 and 230 oC exhibit the EMI SE of 22.4 and 18.0 dB, respectively. The decreased EMI SE is in line with the depressed electrical conductivity, primarily attributed to the deteriorated conducting CNT networks. Moreover, these results suggest a wide processing temperature window (190-210 oC) to form segregated structure in the CNT/PLA composite and establish outstanding EMI SE.

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Figure 7. The average EMI SE and electrical conductivity of the 0.60 vol% CNT/PLA composites at different compression temperature.

CONCLUSION PLA granules with plenty of sc and slight hc were prepared through the melt blending of PLLA and PDLA at low temperature. As the Tm of sc is much higher than that of hc, sc would be easily reserved when appropriate temperature was chosen for compression molding. During the molding process, the reserved sc could act as the physical cross-linking point to confine PLA chain motion, which prohibits the penetration of CNTs into PLA granules and makes the CNTs selectively located among their interfaces. The formation of segregated structure significantly enhances the efficient CNT concentration to develop perfect conductive networks in the sCNT/PLA composite, leading to an ultra-low percolation threshold of 0.040 vol% and a high electrical conductivity of 12.0 S/m. Only 0.60 vol% CNT endows the segregated composite with a high EMI SE of 35.5 dB, almost the highest value for CNT/polymer composites at such a low active material loading. The influence of compression temperature on the EMI SE was performed to demonstrate that the reserved sc plays an important role in forming segregated structure and gaining efficient EMI shielding performance in the CNT/PLA composite.

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ASSOCIATED CONTENT Supporting Information Electronic supplementary information available: The size distribution of PLA granules; the skin depth of s-CNT/PLA composites composites at 8.2 GHz; the DSC curves of the CNT/PLA composites. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Ding-Xiang Yan), [email protected] (Zhong-Ming Li) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51421061, 51533004, 51120135002 and 51473102), the Innovation Team Program of Science and Technology Department of Sichuan Province (Grant No. 2014TD0002), and the China Postdoctoral Science Found (Grant No. 2015M572474).

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For Table of Contents Use Only

Formation of segregated electrically conductive network structure in low-melt-viscosity polymer for highly efficient electromagnetic interference shielding Cheng-Hua Cui, Ding-Xiang Yan,* Huan Pang, Xin Xu, Li-Chuan Jia and Zhong-Ming Li*

Using renewable and biodegradable poly(lactic acid) as the matrix of high-efficiency electromagnetic interference shielding materials.

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Formation of a Segregated Electrically Conductive Network Structure

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