carbon nanotube composite with high efficient electromagnetic interference shielding and favourable mechanical properties

Accepted Manuscript Large-scale preparation of segregated PLA/carbon nanotube composite with high efficient electromagnetic interference shielding and favourable mechanical properties Fang Ren, Zhen Li, Ling Xu, Zhenfeng Sun, Penggang Ren, Dingxiang Yan, Zhongming Li PII:

S1359-8368(18)32711-2

DOI:

10.1016/j.compositesb.2018.09.030

Reference:

JCOMB 6000

To appear in:

Composites Part B

Received Date: 22 August 2018 Revised Date:

11 September 2018

Accepted Date: 14 September 2018

Please cite this article as: Ren F, Li Z, Xu L, Sun Z, Ren P, Yan D, Li Z, Large-scale preparation of segregated PLA/carbon nanotube composite with high efficient electromagnetic interference shielding and favourable mechanical properties, Composites Part B (2018), doi: https://doi.org/10.1016/ j.compositesb.2018.09.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Graphical Abstract

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Large-scale preparation of segregated PLA/carbon nanotube composite with high efficient electromagnetic interference shielding and favourable mechanical properties

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Fang Rena, Zhen Lia, Ling Xub, Zhenfeng Suna, Penggang Ren a*, Dingxiang Yan b, Zhongming Lib

Segregated structure poly (lactic acid) (PLA) conductive composites with efficient electromagnetic interference (EMI) shielding performance and favourable mechanical properties were easily fabricated by injection molding from poly (L-lactid acid) (PLLA), poly (lactic acid) stereocomplex crystallite (PLASC) and multi-wall carbon nanotube (MWCNT). Due to the selective dispersion of conductive MWCNT filler in the continuous PLA phase,

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good conductivity and high EMI shielding effectiveness (EMI SE) were achieved under low MWCNT content.

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Large-scale preparation of segregated PLA/carbon nanotube composite with high efficient electromagnetic interference shielding and favourable

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mechanical properties Fang Rena, Zhen Lia, Ling Xu b, Zhenfeng Suna, Penggang Ren a*, Dingxiang Yanb*, Zhongming Lib

Shaanxi 710048, People’s Republic of China

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a. The Faculty of Printing and Packaging Engineering, Xi’an University of Technology, Xi’an

b. College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials

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Engineering, Sichuan University, Chengdu 610065, People’s Republic of China *

Corresponding Author. [email protected]; [email protected]

Abstract: Environment friendly conductive polymer composites (CPCs) with high electromagnetic interference (EMI) shielding performance and favourable mechanical properties

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were successfully produced on large scale from poly (L-lactid acid) (PLLA), poly (lactic acid) stereocomplex crystallite (PLASC) and multi-wall carbon nanotube (MWCNT). The morphology

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results indicated that the perfect segregated structure was easily manipulated by controlling the injection temperature. The prepared composite exhibits good conductivity and high EMI shielding

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effectiveness (EMI SE) under low MWCNT content due to the selective dispersion of the conductive MWCNT in the continuous PLLA phase. With the addition of only 3 wt% MWCNT, the composite exhibits high electrical conductivity of 6.42 S/m and outstanding EMI shielding performance of 31.02 dB at 8.2 GHz. Moreover, similar molecular structure between PLLA and PLAsc endowed excellent interfacial adhesion, thus results in favourable mechanical properties of composites. The work provides a novel idea for manufacting high EMI shielding materials with both toughness and degradation.

ACCEPTED MANUSCRIPT Keywords: Conductive polymer composites; Segregated structure; Poly (lactic acid); Electromagnetic interference shielding 1. Introduction

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With the miniaturization and increasing power of electronic instruments and telecommunication devices, the human health and the inherent performance of the nearby-devices are under serious threat for the severe electromagnetic radiation [1-4]. To mitigate the undesirable impact, one

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common approach is to shield the electromagnetic interference (EMI) with electric and/or

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magnetic materials, such as metal materials, magnetic coating, and conductive polymer composites (CPCs) [5-10]. The CPCs have been viewed as advanced candidates for EMI shielding due to their design flexibility, processing advantages, resistance to corrosion, low density and especially for the high electromagnetic absorption as compared to traditional metallic shielding

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materials [11-15]. The electrical conductivity of CPCs is generally required to reach at least 1 S m-1 to satisfy commercially application in EMI shielding [13, 16]. Such conductivity can be only obtained under the high filler concentration, which seriously affects the cost, flowability,

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processing characteristics and toughness of composites [17, 18]. Although the utilization of

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nanofiller (especially for carbon nanotube or graphene) effectively reduced the electrical percolation threshold of the CPCs due to the easy formation of conductive networks, the filler loading of CPCs with electromagnetic interference shielding effectiveness (EMI SE) at least 20 dB is still high (10-20 wt%) [19, 20]. For example, about 15wt% graphene loading should be required for CPCs to obtain the EMI SE =21 dB [21] , and when EMI SE improved to 29.3 dB, the graphene loading need to reach 30 wt% [22]. Thus, preparing CPCs with superior EMI SE at low nanofiller loading remain a mission of shouldering heavy responsibilities.

ACCEPTED MANUSCRIPT The construction of segregated structure in CPCs can effectively reduce the electrical percolation threshold and improve electrical conductivity [23-29]. For instance, the segregated structure in a cross-linked carbon nanotube (CNT)/poly(ethylene-co-octene) (POE) composite resulted in a low

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threshold percolation of 1.5 vol % compared to 9.0 vol % in a conventional CNT/ POE composite [30]. Such segregated architectures also exhibited superior EMI SE to conventional CPCs due to the selective distribution of fillers in the polymer matrices. The thermally reduced

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graphene/ultrahigh molecular weight polyethylene (RGO/UHMWPE) composite with segregated

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structure realized a satisfactory EMI SE of 28.3-32.4 dB at only 0.66vol% filler loading, significantly lower than that of the uniform CPCs [22]. Up to now, the segregated CPCs are mainly prepared by high melt viscosity to prevent the diffusion of electrical filler into their interior, thus mainly limited to UHMWPE [31-33] , crosslinked polymer [11, 34], or amorphous polymers

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under low temperature condition [35, 36]. It is difficult to obtain complete segregated structure in crystalline or semi-crystalline polymers with low-melt-viscosity, such as PP, PA, PET and PLA. Furthermore, the common preparation methods of segregated CPCs, such as mold compressing or

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compression molding plus salt-leaching method [22, 37], are not suitable for industrial application

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due to the low productivity. To our knowledge, constructing the segregated structure in low-melt-viscosity crystalline polymer on large scale remains a daunting challenge. On the other hand, excessive consumption and discarding after their service life of fossil-based polymer will lead to serious environment pollution and energy crisis in the near future [38]. Thus, the use of eco-friendly polymer propagates aggressively for the growing need to minimize the carbon footprint in the environment. Compared with other biodegradable polymers (such as poly butylene succinate (PBS), poly-e-caprolactone (PCL) and poly glycolic acid (PGA)), poly(lactic

ACCEPTED MANUSCRIPT acid) (PLA) is considered as the most promising material to substitute for petroleum-based polymer because of its excellent mechanical properties, manipulative degradability and good processibility [39, 40]. However, PLA is a typical semi-crystalline polymer with

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low-melt-viscosity. It is an enormous challenge to create segregated structure with perfect conductive networks in PLA matrix because the electrical filler may migrate into the polymer interior when temperature exceeds its melting temperature, especially under the external forces.

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Fortunately, the PLA stereocomplex crystallite (PLASC) with high melting temperature (Tm) of

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220℃ is easily formed in the poly L-lactic acid (PLLA) and poly D-lactic acid (PDLA) blends because the multicenter hydrogen bonding interactions between L -lactyl and D -lactyl unit sequences could maintain the two helical chains with the opposite configuration together. While, the Tm of PLA homocrystallites (PLAHC) (about 160-180oC) is relative lower than PLASC.

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Significant difference between Tm of SC and HC makes it possible to establish segregated conductive networks in PLA matrix by controlling the processing temperature. Herein, the PLASC was firstly fabricated from PLLA/PDLA melt blend and then coated with PLLA particles and

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multi-wall carbon nanotube (MWCNT) (abbreviate as PLASC@PLLA-MWCNT). Finally, a

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segregated structure PLA composite with perfect conductive networks was successfully prepared by injection-molding technique under various temperatures. Such segregated structure endowed the PLA composite with a much high conductivity of 6.42 Sm-1 and EMI SE value of 27.4 dB at the 3.0 wt% MWCNT loading. More exhilaratingly, the as-prepared composite exhibits steadily increased strength and elongation at break with the MWCNT filler increasing. Furthermore, the effects of injection temperature on morphology, electrical conductivity and the EMI shielding performance of PLASC@PLLA-CNT composite were also investigated.

ACCEPTED MANUSCRIPT 2. Experimental section

2.1 Materials

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PLLA 4032D and PDLA were purchased from Nature works Co. Ltd. (USA). The weight-average molecular weight (Mw), polydispersity index (PDI) and melting temperature (Tm) of PLLA are 2.23×105 g/mol, 2.10, and 167.4oC, respectively. The corresponding values of PDLA are 8.5×104

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g/mol, 1.82, and 176.0oC, respectively. MWCNT was supplied by Nanocyl S.A., Belgium (Nanocyl NC 7000), with average diameter of 9.5nm and average length of 1.5µm. Alcohol and

Reagent Factory (Chengdu, China).

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dichloromethane (DCM) with analytical grade were purchased from Chengdu Kelong Chemical

2.2 Preparation of PLASC@PLLA-MWCNT segregated composites

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The schematic diagram of the fabrication of PLASC@PLLA-MWCNT segregated composites is shown in Fig.1. Firstly, PLLA and PDLA particles (with weight ratio 1:1) were blended with

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Haake torque rheometer (Rheomix 600) at 180oC for 10min, the PLAsc particles with high melting temperature were obtained. 12g PLLA particles were dissolved in 150ml DCM with

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vigorous stirring at room temperature. After completely dissolved, 150ml alcohol was dripped slowly into the PLLA/DCM solution mixture to adjust the solubility parameter. Then 8g PLAsc particles were added and mixed for 10min. Subsequently, another 150ml alcohol was added into the mixture and large amounts of white precipitate appeared immediately. After filter and dry at 50oC for 48h, the PLASC coated with PLLA hybrid particles (PLASC@PLLA) were obtained. Finally, the PLASC@PLLA particles and appropriate amount of MWCNT (weight fraction 0.5, 1.0, 2.0 and 3.0%) were mixed with high-speed mixer and then injected into PLASC@PLLA-MWCNT

ACCEPTED MANUSCRIPT segregated composite specimens. The injection temperature in our experiment fluctuated from 210 to

230oC

to

explore

the

optimal

processing

parameter

for

the

preparation

of

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PLASC@PLLA-MWCNT segregated composite.

molding method.

2.3 Characterization

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Scanning electron microscopy (SEM)

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Fig.1 Schematic of the fabrication of the PLASC@PLLA-MWCNT segregated composites using injection

The surface morphologies of the PLAsc and PLASC@PLLA-MWCNT particles were observed by using a field emission scanning electron microscope (Inspect-F, FEI, USA), at an accelerating

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voltage of 5.0 kV. The particles were coated with a thin layer of gold beforehand. Cross-sections

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for the SEM observations were obtained by cryo-fracturing the specimens with liquid nitrogen for 30 min.

Differential scanning calorimetry (DSC) Crystallization behavior of PLAsc and PLAsc@PLLA particles was investigated by differential scanning calorimetry on a TA Q2000 instrument. The samples (5-6 mg) were heated from 40 °C to 250 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Crystallinitie (χ) of PLAsc was

ACCEPTED MANUSCRIPT calculated by normalizing their enthalpy to the equilibrium enthalpy of completely crystals, taken as142 J/g for completely crystals PLAsc. The crystallinity (χ) of PLAsc can be calculated as follows:

Where ∆H

and ∆H

∆ ∆

(1)

× 100%

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χ=

are the enthalpy of 100% PLAsc crystal and the melting enthalpy

measured in experiment, respectively.

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For convenience of comparison, the Crystallinitie (χ) of PLAsc in PLAsc@PLLA blend was

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converted to relative crystallinity (χr) of PLAsc in the pure PLAsc matrix as follows: = Where

is the weight content.

Polarizing Optical Microscopy (POM)

×

(2)

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The segregated structure of composite film was observed by an Olympus BX51 polarizing optical microscopy (Olympus Co., Tokyo, Japan). The film samples were prepared with a thickness of 8 µm by a Leica EMUC6/FC6 microtome.

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Measurement of electrical conductivity

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The electrical conductivity of the composite samples was measured using a Keithley electrometer Model 4200-SCS (USA) according to a two-point method. Before the test, both ends of the rectangular specimens were coated with silver paste to reduce the contact resistance between specimens and electrodes.

EMI shielding characteristics EMI shielding measurements were performed with a coaxial test cell (APC-7 connector) in conjunction with an Agilent N5247A vector network analyzer, according to ASTM ES7-83 and

ACCEPTED MANUSCRIPT ASTM D4935-99. The intermediate frequency bandwidth was set as 1 kHz during the measurement and 201 points were collected for each specimen. Samples with a 10 mm diameter and 2 mm thickness were placed in the specimen holder. The scattering parameters (S11 and S21) in

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the frequency range of 8.2–12.4 GHz were recorded to calculate the reflected power (R), transmitted power (T), absorbed power (A), EMI SE ( SET ), microwave reflection ( SER ) and microwave absorption ( SE A ), using the following equations: 2

2

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R = S11 , T = S21 A = 1− R −T

(3)

(4) (5)

SET (dB ) = SER + SE A + SEM

(6)

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SER (dB ) = −10 log(1 − R ), SE A (dB ) = −10 log(T / (1 − R ))

Where SEM is the microwave multiple internal reflections, which can be negligible when SET

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is higher than10 dB [41]. Mechanical properties testing

The mechanical performance measurements were conducted on a test instrument (Model 5576,

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Instron Instruments, USA). For tensile test, the specimens with dimension of 45×5 ×2mm3 were

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conducted with a speed of 20 mm/min. The tensile strength and elongation at break were averaged from 5 samples.

3. Results and discussions

3.1 Morphology and structure of PLASC@PLLA-MWCNT particle

Fig.2 shows the surface SEM morphologies of the PLASC and PLASC@PLLA-MWCNT particles. The as-prepared PLASC particle has irregular and rough geometrical morphology due to the

ACCEPTED MANUSCRIPT melting mixing (Fig.2a and 2b). This particular structure is beneficial to the subsequent coating by PLLA and MWCNT particles. Compared with PLASC particle, PLASC@PLLA-MWCNT particle exhibits larger dimension and roughness (Fig.2c), suggesting the successful coating of the PLLA

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and MWCNT particles on PLAsc. Abundant and uniformly dispersed MWCNT can be observed from the magnified SEM morphology of PLASC@PLLA-MWCNT particle (Fig.2d). It ensures the

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formation of conductive networks in as-prepared PLASC@PLLA-MWCNT segregated composite.

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Fig.2 SEM morphologies of PLAsc ((a) and (b)) and PLAsc@PLLA-MWCNT ((c) and (d)).

To further demonstrates the successful coating of the PLLA particles, melting behavior of the PLAsc and PLAsc@PLLA particles was investigated and their DSC curves are listed in Fig.3.

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strong melting peak at about 220.0oC and two small peaks at 166.5oC and 174.6oC appeared in the PLAsc curve are corresponded to melting endothermic of PLAsc, PLLA and PDLA crystallite, respectively. It indicates that the overwhelming stereocomplex crystallite with high melting temperature is successfully constructed in the PLLA/PDLA melting blend system, in spite of the

ACCEPTED MANUSCRIPT existence of a small quantity of PLLA and PDLA homocrystallites. On the contrary, the strong peak at 167.4oC and relatively weak peak at 220.0oC can be observed from the PLAsc@PLLA curve. The crystallinity of PLAsc calculated from PLAsc and PLAsc@PLLA curves are 34.48%

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and 12.31%, respectively. Combining the weight ratio of PLLA/PLAsc (3:2), the relative crystallinity of PLAsc in the PLAsc@PLLA blend was transformed to 30.78%, which is closed to that of PLAsc in pure matrix. Unchanged crystallinity of PLAsc suggested that the PLAsc is not

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dissolved in DCM solution during the coating process, which ensures the formation of segregated

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molding condition (215oC).

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structure in samples because PLAsc particles (Tm=220oC) are not melted under normal injection

Fig.3 DSC curves of PLASC and PLASC @PLLA particles.

3.2 Effect of injecting temperature on the performances of composites

Melting behavior of PLA is mainly depended on the injecting temperature. The preferable injecting temperature should be ranged from the Tm of PLLA to the Tm of PLAsc. In this case, only the PLLA particles exhibit melt fluidity, while the PLAsc particles still maintain solid state.

ACCEPTED MANUSCRIPT Thus the "sea-island" can be formed naturally in the composites and the MWCNT dispersed only in the continuous PLLA phase, rather than the island PLAsc phase, resulting in the significant improvement of electrical conductivity due to volume exclusion effect. Polarizing microscope

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images of PLAsc@PLLA-MWCNT composites at different temperature are shown in Fig.4. Perfect segregated structure was established in the composites prepared at relative low temperature (<220oC). The scattered PLAsc phase is bright, while the continuous phase (PLLA)

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looks more black and opaque. It indicates the MWCNT fillers selectively dispersed into the

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continuous phase, rather than uniformly dispersed into all polymer matrices. No apparent difference of segregated structure existed between composites prepared at 210oC and 215oC (Fig.4a and 4b). However, when temperature rises to 220oC (about Tm of PLAsc), the size and total area of the dispersed phase are decreased obviously (Fig.4c). It means that the PLAsc crystal

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begins to melt, which consistent with the DSC analysis. As the temperature rises further to 230oC, "sea-island" structure completely disappeared, and MWCNT fillers uniformly dispersed in whole polymer matrix (Fig.4d). Thus, it concluded that the perfect segregated structure can be obtained

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only below the Tm of PLAsc (220oC). At this temperature, MWCNT fillers dispersed in the melted

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PLLA phase, unable to penetrate into the solid PLAsc interior, thus formed perfect segregated structure with conductive networks.

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Fig.4 POM images of PLAsc @PLLA-MWCNT (3wt%MWCNT) composites at different temperature.

The SEM images of composites at 215oC and 230oC are showed in Fig.5. Obvious two phase

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sea-island structure exists in the composite injected at 215oC (Fig.5a). A closer inspection reveals that the island phase exhibits a relative rough surface as same as the PLAsc particles (Fig.2),

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suggesting the PLAsc particles are not melted in 215oC. Despite the existence of the obvious sea-island structure, the blurry interface between two phases in composite indicates the excellent

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interfacial interaction and adhesion due to the similar molecular structure of PLAsc and PLLA. Homogeneous polymer matrix with relative smooth surface observed in composite prepared at 230oC (Fig.5b) was further proved the melting behavior of PLAsc crystal and the good compatibility between PLAsc and PLLA matrices.

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Fig.5 SEM images of the fractured surface of injection molding composites at different temperature (a): 215oC, (b)

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230oC

Fig.6 shows the effect of injection temperature on the electrical conductivity of

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PLAsc@PLLA-MWCNT composites. The conductivity of the composites firstly increases and then decreases with the injection temperature increasing, and achieves the maximum value of 6.42 S/m at the 215oC. In addition, the fluctuation of conductivity below 215oC is relatively smaller than that above 215oC. The conductivity of composites only increased by 32% (from 4.84 to 6.42

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S/m) when temperature rise from 210 to 215oC. Below the Tm of PLASC (Tm=220℃), MWCNT selectively dispersed in the continuous PLLA phase. With the increase of temperature, the fluidity

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of PLLA phase is further enhanced, making MWCNT more evenly dispersed in PLLA, which facilitates to form conductive channels. Thus, conductivity of composites increased. However,

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almost 2.7 times decrease in conductivity was observed when temperature rise from 215 to 220oC. It can be explained by construction and destruction of the segregated structure with conductive networks in composites. When injection temperature below 220oC, segregated structure with good conductive networks was formed due to the infusibility of PLAsc particles, the difference at various injection temperatures is merely the perfection of segregated structure. Therefore, the conductivity fluctuation is relatively small. Whereas, the conductive MWCNT fillers will disperse into the PLAsc particles and the segregated structure gradually destructed at high injection

ACCEPTED MANUSCRIPT temperature (>220oC), results in sudden drop of conductivity. This is consistent with the

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observation from Fig.4 and Fig.5.

Fig.6 Electrical conductivity of PLAsc@PLLA-MWCNT at different injection temperature.

The EMI SE of PLAsc @PLLA-MWCNT composites injected at different temperature are shown in Fig.7. The variation of average EMI SE is essentially in accord with the conductivity of

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composite. The average EMI SE of composite firstly increased from 25.07 dB (210oC) to 27.44 dB (215oC), then rapidly decreased to 17.44 dB (220oC) and 15.61 dB (230oC). This phenomenon

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is easily explained by the construction and deconstruction of segregated structure in composites at various temperatures. Interestingly, the EMI SE of composites with perfect segregated structure

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(210oC and 215oC) exhibits a weak dependence on frequency, whereas the transitional (220oC) and homogeneous structure composites (230oC) show strong dependence on frequency. The resonance absorption peak shifts toward the higher frequency with the increase of temperature. This could be understood in terms of the factor that the employment of the electrical loss spacers would essentially vary the dielectric constant of the transport medium, which results in the changes in wave length and velocity. Such phenomena is in accordance with the some previous researches [42-44]. In addition, the comparison of SET (total EMI SE), SEA (microwave

ACCEPTED MANUSCRIPT absorption) and SER (microwave reflection) for various composites at the frequency of 8.2GHz (Fig.7b) indicates that the contribution of absorption to the total EMI SE is much larger than that from reflection in all prepared composites. To give a comprehensive understanding of the

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microwave attenuation mechanism, the power coefficient of reflectivity (R), transmissivity (T), and absorptivity (A) are obtained from the measured scattering parameters (S11, S21) as shown in Figure 7c. With increasing the injected temperature, T value increases noticeably, R value

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significantly decreases in initial and then slightly increases, and A value exhibits small changes.

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This is mainly attributed to the destruction of the segregated structure in composites at high injection temperature (>220oC). Whatever, the composite prepared at 215oC exhibits high EMI SE, broad shielding frequency range and excellent absorption characteristics. Therefore, the optimal

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injection temperature was fixed at 215oC.

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Fig. 7 EMI SE properties of PLAsc@PLLA-MWCNT composites (3wt%MWCNT) injected at different

temperature. (a) EMI SE as a function of frequency; (b) Comparison of SET, SEA and SER at the frequency of 8.2 GHz; (c) Coefficients of reflection (R), absorption (A), and transmittance (T) at 8.2 GHz.

3.3 The effects of MWCNT contents on the performances of composites

The POM images of PLAsc@PLLA-MWCNT composites with various MWCNT contents injected at 215oC are shown in Fig.8. As a dispersed phase, the infusible PLAsc particles are

ACCEPTED MANUSCRIPT surrounded by the opaque PLLA+MWCNT phase. Because the MWCNT filler are merely dispersed in the continuous PLLA phase, the conductive network is easily constructed in PLLA phase due to the excluded volume effect. Therefore, segregated structure can be observed in all

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composite samples and the conductive paths are constantly improved with the MWCNT filler increasing. Nevertheless, the perfect conductive networks are still not constructed in Fig.8a because of the low filler contents. The conductivities of composites with various MWCNT

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contents are shown in Fig.9. Rapid increase of the conductivity of composite at relative low

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MWCNT loading implies the establishment of the conductive paths. The conductivity of PLA increases from 10-15 S/m to 2.84×10-9 S/m at 0.5 wt% and 0.09 S/m at 1.0 wt% MWCNT loading, respectively, almost 6 and 13 orders of magnitude increasement. Thereafter, the conductivity of composites increases slowly with the MWCNT filler increasing. It suggests that the prepared

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composites exhibit a typical percolation behavior and is usually evaluated with power-law equation, i.e. σ = σ 0 (φ − φc ) . Where t

σ , σ 0 , φ and φc is the electrical conductivity of

composite, constant conductivity associated with the intrinsic characteristic of MWCNT, volume

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content of the MWCNT filled in composites, and the percolation threshold (the minimum contents

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of filler to construct the electrical path in composite), respectively. t represents a critical exponent related to dimensionality. The fitted curve of Logσ vs Log (φ − φc ) is listed in the inset of Fig.9. 0.44 vol% percolation threshold obtained from the fitted curve indicates the conductive networks are cost-effectively established in segregated composites with high efficiency. Thus, it means that the perfect electrical networks exist in composites when the filler loading exceed 0.44 vol% (about 0.8 wt%). This is consistent with the observation from Fig.8. Once formed the conductive network, the improvement of composite conductivity is indistinctive with the filler

ACCEPTED MANUSCRIPT increasing. Even so, the conductivity of prepared PLA segregated composite with 3 wt% MWCNT

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reaches 6.42 S/m, completely meet the practical application in EMI shielding.

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Fig.8 POM images of PLAsc@ PLLA-MWCNT composites with different MWCNT contents. (a) 0.5 wt%; (b)1.0

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wt%; (c) 2.0 wt%; (d) 3.0 wt%.

Fig.9 Electrical conductivity of the PLAsc@ PLLA-MWCNT composites as a function of MWCNT contents.

ACCEPTED MANUSCRIPT Fig.10 shows the EMI SE properties of segregated composites with different filler loading. The EMI SE continues to increase with the MWCNT filler increasing, especially for the composites with relative high filler loading. With the addition of 3 wt% MWCNT, the maximum and the

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average EMI SE of composites reach up to 31.02 and 27.44dB. More importantly, the EMI SE beyond 25 dB in frequency range of 8 to 12GHz suggested that the prepared composite can meet the requirements of traditional electromagnetic shielding materials [13]. To further understand the

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mechanism of electromagnetic shielding, SET, SEA and SER of composites with various filler

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loading at the frequency of 8.2 GHz are calculated and were listed in Fig.10b. The SET and the SEA are obviously enhanced with increasing MWCNT filler content. However, the SER exhibits a strong independence on MWCNT filler contents. With the 3wt% MWCNT loading, the value of SEA reaches 28.11dB, almost 90.62% of SET, indicating the absorption dominant shielding

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mechanism rather than reflection in the prepared composites. According to the power balance of T, R, and A versus incident power (as shown in Figure 10c), T value decrease significantly, R value increases noticeably, and A value first slightly increased and then decreased as the filler content

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increasing. It is attributed to the unique segregated structure. At the low filler contents, the perfect

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conductive networks are not constructed, which allows more electromagnetic waves penetrating through the composite and therefore results in a high T value. Thereafter, segregated structure can be constructed and the conductive paths are constantly improved with the filler increasing. Thus, T value decreases sharply and the composites exhibits excellent shielding characteristics. The PLAsc particles without MWCNT filler can be considered as a “skin” of shielding materials, and PLLA-MWCNT phase with high conductivity act as inner absorption and reflecting layer. The electromagnetic microwaves are easily propagated into the “skin” due to the impedance matching

ACCEPTED MANUSCRIPT of non-conductive PLAsc particles. Subsequently, the incident electromagnetic microwaves will be attenuated in the PLLA-MWCNT phase by reflecting, adsorption and scattering many times. Electromagnetic microwaves are “trapped” in the closed conductive network and is difficult to

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penetrate this reflecting layer. This shielding mechanism has been reported in our previous

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researches [22, 25, 26, 37].

Fig.10 EMI SE properties of PLAsc@PLLA-MWCNT composites molded at different melt temperature. (a) EMI

SE as a function of frequency; (b) Comparison of SET, SEA and SER at the frequency of 8.2 GHz. (c) Coefficients of R, A and T at 8.2 GHz.

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The mechanical property is a major parameter for evaluating the application of CPCs shielding materials. Generally, the mechanical properties of composite, including tensile strength and

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elongation at break, significantly deteriorated in the presence of the segregated structure due to the weak interfaces. Therefore, the tensile performance of the prepared segregated composites with

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various MWCNT loading was measured and the results are plotted in Fig.11. Pure PLAsc matrix exhibits a typical brittle fracture behavior, without obvious yielding phenomenon, which is attributed to the special crystal texture of PLAsc [45]. The tensile strength and elongation at break of PLAsc matrix are only 39.14MPa and 1.78%, respectively. While an apparent two yielding existed in the strain-stress curve of PLLA matrix. The tensile strength and elongation at break can reach 70.78MPa and 5.13%, respectively. Interestingly, the strength and elongation at break of the PLLA@PLAsc-MWCNT composites increased with the MWCNT filler content increasing. With

ACCEPTED MANUSCRIPT the addition of 3wt% MWCNT, increments of 32.70% and 67.17% were obtained for the tensile strength and elongation at break; increased from 43.73MPa (PLAsc@PLLA) to 58.03MPa and from 1.98% (PLAsc@PLLA) to 3.31%, respectively. The improvement of strength and elongation

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of composites may be ascribed to the excellent mechanical performance of MWCNT, toughening modification of PLAsc particles and excellent interfacial adhesion between PLLA and PLAsc (as shown in Fig5a). As continuous phase, PLLA matrix bears most of the external load and therefore

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some microcracks initiated and propagated in PLLA matrix when the applied load exceeds its

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breaking strength. Once the propagated microcrack encountered the stiff PLAsc particles, the crack propagation path may be changed. Particle reinforcement, crack deflection and particle pullout are the main toughening mechanisms. Due to the excellent interfacial adhesion between PLLA and PLAsc, the PLAsc@PLLA composite exhibits superior strength and toughness to

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PLAsc. Furthermore, with the increase of MWCNT concentration, the strength and deformability of the PLLA-MWCNT phase was gradually increased. The breaking point of the PLAsc @PLLA-MWCNT composites lied on the strain-stress curves of PLLA matrix and the special two

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yielding appeared in the composites containing high MWCNT loading. This indicates that the

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tensile strength and elongation at break is mainly determined by PLLA–MWCNT phase. In other words, the brittle rupture is not occurred in PLAsc@PLLA-MWCNT composite because the stress concentration on crack tip eliminated by dispersed PLAsc particles. The schematic diagram of the toughening and reinforcing mechanism is shown in Fig.12. Regardless, this large-scale produced composite exhibits not only the good electromagnetic shielding effectiveness, but also excellent mechanical properties with low MWCNT loading. Together with the PLA degradation performance, this approach provides a novel idea for manufacting new high EMI shielding

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materials with both toughness and degradation.

Fig.11 Strain-stress curves of PLLA, PLAsc and PLAsc@ PLLA-MWCNT composites with various MWCNT

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content.

Fig.12 The schematic diagram of the toughening and reinforcing mechanism.

4. Conclusion

Segregated structure PLA conductive composites with efficient EMI shielding performance and favourable mechanical properties were easily fabricated by injection molding from PLAsc, PLLA and MWCNT particles. Due to the selective dispersion of conductive MWCNT filler in the

ACCEPTED MANUSCRIPT continuous PLLA phase, good conductivity and high EMI SE were achieved under low MWCNT content. Moreover, enhanced tensile strength and toughness was simultaneously obtained because of the excellent interfacial adhesion between PLLA and PLAsc. With the additional of 3 wt%

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MWCNT, the EMI SE reached 31.02 dB at the frequency of 8.2 GHz, the tensile strength increased from 43.73 to 58.03 MPa and elongation at break increased from 1.98 to 3.31%. Our work reveals a major breakthrough in creating a segregated conductive network structure in

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friendly, and highly efficient EMI shielding composites.

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low-melt-viscosity polymers on large scale, further developing economical, environmentally

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 21706208, 51773167, 21704070), the Natural Science Foundation of Shaanxi Province, China (Grant No.

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2017JQ5065), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme2017-4-13).

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ACCEPTED MANUSCRIPT Highlights: 1. Segregated structure PLA conductive composites were easily fabricated by injection molding from PLASC, PLLA and MWCNT particles.

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2. The prepared composite exhibited good conductivity and high EMI SE under low MWCNT content due to the selective dispersion of the conductive MWCNT in the continuous PLLA

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phase.

3. Similar molecular structure between PLLA and PLASC endowed excellent interfacial

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adhesion, thus results in favourable mechanical properties of composites.