Anisotropic Conductive Polymer Composites Based on High Density

Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3636−3647

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Anisotropic Conductive Polymer Composites Based on High Density Polyethylene/Carbon Nanotube/Polyoxyethylene Mixtures for Microcircuits Interconnection and Organic Vapor Sensor Lele Li,†,‡ Honghui Shi,† Zhongzhu Liu,§ Liwei Mi,§ Guoqiang Zheng,*,† Chuntai Liu,† Kun Dai,† and Changyu Shen†

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†

College of Materials Science and Engineering, Key Laboratory of Materials Processing and Mold, Ministry of Education, Henan Key Laboratory of Advanced Nylon Materials and Application, Zhengzhou University, Zhengzhou 450001, P. R. China ‡ College of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China § College of Materials and Chemical Engineering, Zhongyuan University of Technology, Zhengzhou 450007, P. R. China S Supporting Information *

ABSTRACT: Anisotropic conductive polymer composites (ACPCs) are of great significance; however it is still a huge challenge to prepare ACPCs via a continuous, low-cost but facile method. In this study, a facile “melt extrusion-calendering” method is proposed to prepare ACPCs based on high density polyethylene (HDPE)/carbon nanotubes (CNTs)/polyoxyethylene (PEO) composite system. An alternating microlayered structure with different polymer phases parallel to extrusion direction is developed, and CNTs are selectively localized in HDPE microlayers. Such interesting biphase structure endows the resulting composites with extraordinary anisotropic property. That is, both extrusion direction and width direction show good conductivity but thickness direction is insulative. The conductivity of such ACPCs in extrusion direction exhibits 6 orders of magnitude higher than that in thickness direction. Moreover, the anisotropy can be easily regulated from 104 to 106 by adjusting the stretching ratio. Interestingly, the as-prepared ACPCs can be used as reliable microcircuits interconnection and organic vapor sensor. This study will open the way to the facile preparation of anisotropic conductive nanomaterials and their wide potential applications. KEYWORDS: conductive anisotropy, alternating microlayered structure, extrusion molding, microcircuits interconnection, organic vapor sensor to assemble the conductive fillers into ordered stripe-like10,18−22 or layer-like assemblies8,9 in polymer matrix to construct the anisotropic conductive networks. Apart from the methods mentioned above, other sophisticated methods such as electrospinning,23 mechanical stretching,24 template-guided growth,25 and doctor blade technique26 have also been proposed to prepare ACPCs. Unfortunately, the aforementioned methods always require complicated manufacturing process or sophisticated fabricating facilities, resulting in relatively high cost and being time-consuming. Therefore, a universal, low-cost, and mass production method to prepare ACPCs is urgently necessary. Melt extrusion molding is one of the most important polymer melt processing techniques by which common thermoplastic polymers can be processed into macrosized but microstructure controllable products.27−33 More importantly, it shows facile,

1. INTRODUCTION Anisotropic functional materials have been extensively investigated due to their unique anisotropy as well as their promising applications in various fields.1−7 Particularly, anisotropic conductive polymer composites (ACPCs) with distinct electrical conductivity in a given direction are of great significance in electrical interconnection, electrochemical actuators, and miniaturized sensing devices,1,2,7−12 which can significantly expand the application of conductive polymer composites. Generally, successful fabrication of conductive fillers based ACPCs relies on the orientation of the conductive fillers or conductive fillers-filled polymer assemblies along one direction in the composite. Recently, electric-field-induced orientation1,13,14 and magnetic-field-induced orientation15−17 have been employed to fabricate ACPCs through control of the orientation of conductive fillers. However, these methods usually require extremely high electric or magnetic field, and they only work well for a very low viscosity thermosetting matrix to permit anisometric conductive fillers alignment and connectivity. Another strategy for the fabrication of ACPCs is © 2019 American Chemical Society

Received: March 29, 2019 Accepted: May 28, 2019 Published: May 28, 2019 3636

DOI: 10.1021/acsanm.9b00584 ACS Appl. Nano Mater. 2019, 2, 3636−3647

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ACS Applied Nano Materials

Figure 1. Chemical composition of the ACPCs and schematic diagram of the “melt extrusion-calendering” process for preparing ACPCs. The SEM micrographs in the top-right corner are the microlayered structure of ACPCs after PEO phase was leached by deionized water.

percolation mechanism, the as-prepared ACPCs possess good conductivity and remarkable conductive anisotropy. The mechanisms for the morphological evolution and the formation of anisotropic conductive network during the fabrication process were studied. Moreover, effects of CNTs content and stretching ratio on the anisotropic conductive behavior of ACPCs were also discussed in detail. Finally, the application of ACPCs as microcircuits interconnection was explored. Furthermore, the PEO phase can also serve as a template for sacrifice, and stacked microlayered conductive composites will be obtained after PEO was leached, enabling its application as organic vapor sensor with great stability and reliability.

solvent-free, low-cost, high-efficiency, and large-scale features. In practice, targeted for specific practical applications, two immiscible polymers are usually melt blended by extruder, resulting in an immiscible heterogeneous biphase system. Additionally, due to the complicated thermal28,30,32,33 and mechanical history8,10,29,31−33 during melt mixing process, immiscible blends can be changed into various morphology, e.g., droplets,34 fibrils,10,22,27 lamellae,8,35 and so on. Since the morphology of polymer blends can significantly determine many physical properties (e.g., electrical conductivity, thermal conductivity, mechanical properties, and barrier properties), it provides us a useful strategy to obtain ACPCs through tuning the phase morphology to form anisotropic conductive networks.8,10 Unfortunately, the resulting ACPCs fabricated by polymer melt processing techniques always show poor conductivity and low anisotropy (generally lower than 105).8,10,22 From this point, it is necessary to fabricate ACPCs with high anisotropy and conductivity via a common melt processing method. In our previous work,27 we have successfully fabricated oriented HDPE microfiber bundles based on HDPE/PEO biphase system, and such interesting biphase structure has inspired us to construct the anisotropic conductive networks by integrating the conductive fillers only into ordered HDPE phase. Herein, “melt extrusion-calendering” process was proposed, by which high-performance anisotropic conductive high density polyethylene (HDPE)/carbon nanotubes (CNTs)/polyoxyethylene (PEO) composites were fabricated. Such ACPCs were designed based on the following criteria: (1) conductive fillers (CNTs) must be selectively localized in aligned HDPE microlayers; (2) the conductive HDPE/CNTs phase and insulative PEO phase must be assembled into alternating microlayered architecture, and HDPE/CNTs phase must be continuous. The first requirement could be satisfied by setting proper blend feeding order. To satisfy the other one, a calendering process was employed to transform the microfiber-like HDPE/CNTs phase into microlayer-like structure. The as-prepared ACPCs demonstrate microlayered structure with alternating HDPE microlayer and PEO microlayer, but CNTs are selectively localized in HDPE microlayer. Owing to the alternating microlayered structure as well as the double

2. EXPERIMENTAL SECTION 2.1. Materials. The HDPE/CNTs master batch (see Figure S1 in Supporting Information) containing 20 wt % multiwalled carbon nanotubes was bought from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Science. In the master batch, HDPE (5000S) with a weight-average molecular weight (MW) of 5.28 × 105 g/mol was provided by Daqing Petroleum Chemical Co., Ltd., China, and CNTs with a diameter of 5−15 nm and a length of 10−30 μm were grown via chemical vapor deposition (CVD) method. In the process of diluting, the same commercial HDPE granules (5000S) were used. PEO (Polyox N10) with MW of 1 × 105 g/mol was bought from Dow Chemical Company. Deionized water was obtained by a water purifier (MingcheD 24UV, Millipore). 2.2. Sample Preparation. First, HDPE/CNTs master batch was diluted into different concentrations by blending with pure HDPE in a mini twin-screw extruder (SJSZ-10A, Wuhan Ruiming Plastic and Mechanical Co., Ltd., China) with screw speed of 40 rpm at 160 °C. For convenience, the obtained diluted pellets were labeled as PE-x, where x represents the weight content of CNTs in HDPE based composite. The PEO powder was dried and then extruded to obtain PEO pellets by a single-screw extruder (CSJ-20, Shanghai Changkai Mechanical Co., Ltd., China) with a length/diameter ratio of 25. The temperature profile from hopper to die was respectively 50, 80, 80, and 80 °C and the screw speed was 60 rpm. Subsequently, PE-x and PEO pellets were melt blended in this mini twin-screw extruder to prepare HDPE/CNTs/ PEO pellets. The weight ratio of PE-x and PEO was fixed at 1:1. The temperature of melt was 160 °C, and the screw speed was 40 rpm. Finally, the dried HDPE/CNTs/PEO pellets were extruded by the same single-screw extruder through a slit die (2 × 30 mm2). The temperature profile was respectively 50, 110, 160, and 160 °C from hopper to die, and the screw speed was 60 rpm. Then, the extrudate was 3637


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Figure 2. SEM micrographs of cryofractured surfaces of ACPCs: (a) B-1.5 composite, (b) B-3.5 composite, (d) B-7.5 composite, and (c) TEM micrograph of B-3.5 composite. The CNTs and the aggregated ones are respectively pointed out with blue arrows or circle. (e) Raman spectra of pristine materials and (f) polarized Raman spectra of B-3.5 composite. 2.4. Rheological Measurement. PE-x pellets were hot pressed into disks with a thickness of 1 mm and diameter of 25 mm using a vacuum-assisted compression machine (Y002, Zhengzhou Craftsman Machinery Equipment Co., Ltd., China). The machine and its major parameters were respectively shown in Figure S2 (Supporting Information) and Table S1 (Supporting Information). The rheological measurements were carried out on a rheometer (Bohlin Gemini 2, Malvern, U.K.) with parallel-plate geometry (diameter of 25 mm) at 160 °C in air atmosphere. Oscillatory frequency was swept from 0.025 to 100 rad/s with a fixed strain amplitude of 1% to ensure that the rheological behavior was located in the linear viscoelasticity regime. 2.5. Mechanical Property Measurement. The mechanical measurement was performed on a universal tensile testing machine (UTM2203, Shenzhen Suns Technology Stock Co., Ltd., China) with a 100 N load cell at room temperature. The tested samples with dimensions of 20 (X) × 4 (Y) × 1.2 (Z) mm3 or 20 (Y) × 4 (X) × 1.2 (Z) mm3 were cut from the extrusion sheets. The crosshead speed was 1 mm/min, and the gauge length was 10 mm. The reported values were calculated as averages of at least five samples. 2.6. Two-Dimensional Wide-Angle X-ray Diffraction (2DWAXD) Measurement. Two-dimensional wide-angle X-ray diffraction (2D-WAXD) was performed on a Bruker D8 Discover X-ray diffractometer. The radiation wavelength of X-ray was 0.154 nm, and a two-dimensional detector MAR 165 CCD (1024 × 1024 pixel with pixel size 80 mm) placed 84 mm away from the sample was used to collect 2D-WAXD patterns. The exposure time was 200 s for every 2DWAXD image. 2.7. Electrical Measurement and Applications. The volume electrical conductivity of HDPE/CNTs/PEO composites in three directions was measured by two-point method. For convenience, three directions, i.e., parallel to extrusion direction, width direction, and thickness direction, are respectively marked as X, Y, and Z directions (see Figure 1). The volume electrical conductivity lower than 10−6 S/m

stretched at the die exit at different line speeds by two water-cooled rollers having a polished chrome surface (see Figure 1). Temperature of the calender rollers was controlled at around 20 °C by circulating water. When the hot polymer extrudate was stretched into the gap between the two adjacent squeezing rollers, it would be immediately pressed into designed thickness and cooled down. The distance between the die exit and rollers was 25 cm. The cooled composite sheets were finally taken up by a winder. The as-prepared HDPE/CNTs/PEO composites were marked as B-y, where y represents the weight content of CNTs in the composites. If without a specification, B-y sample represents the composite processed at a fixed stretching ratio (viz., the area of cross section of die to that of extrudate) of 1.5. 2.3. Dispersion State of CNTs. The samples were first cryogenically fractured after immersing in liquid nitrogen for 1 h. Subsequently, PEO phase of the samples was leached by deionized water for 9 h. The microstructure of HDPE/CNTs/PEO composites and the dispersion status of CNTs were observed by SEM (MERLIN Compact, Zeiss, Germany) at an accelerating voltage of 5 kV. Meanwhile, the localization of CNTs in some samples without leaching was also observed. Before observation, all the samples were coated with a thin layer of gold for better conductivity. The transmission electron microscope observation was performed on a JEOL JEM-1230 with an accelerating voltage of 90 kV. Before observation, samples were ultramicrotomed into ultrathin films with a thickness of about 100 nm in liquid nitrogen by a microtome (Leica UC-7) equipped with a glass knife. Raman spectra and polarized Raman spectra were performed by using a micro-Raman spectrometer (Renishaw) with laser excitation at 532 nm. In order to characterize the orientation degree of CNTs in polymer matrix, polarized Raman spectra were recorded along two directions normal between each other, which were parallel (0°) and perpendicular (90°) to the extrusion direction (i.e., X direction) of B3.5 composite, respectively. 3638


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ACS Applied Nano Materials was measured by a resistance meter (TH2683, Tonghui, China) while the volume electrical conductivity higher than 10−6 S/m was measured by a digital multimeter (DMM4050, Tektronix, USA). Silver paint was coated on the surfaces of both ends of the sample to ensure good contact. The volume electrical conductivity of pure PEO was measured by a high resistance meter (ZC-36, Anbiao, China). As for the microcircuits interconnection application, B-3.5 composite was cut into small pieces with dimensions of 5 (X) × 5 (Y) × 1.2 (Z) mm3 and chosen as measured sample. The measured sample was connected with copper foils by conductive silver paint. The red and blue wires were two independent cycle circuits with two separated dc voltage supplies (3 V). With respect to vapor sensing test, B-3.5 composite was first cut into small pieces with dimensions of 20 (X) × 4 (Y) × 1.2 (Z) mm3. Then, after PEO phase is leached by deionized water, the resulting composite was chosen as testing sample. The vapor sensing behaviors were investigated by in situ monitoring of their chemoresistive responses upon exposure to different organic solvent vapors (e.g., cyclohexane, dichloromethane, and ethyl acetate) for 10 successive immersiondrying runs (IDRs). For a complete IDR, the sample was immersed in saturated organic vapor (25 °C) for 150 s and then dried in air for another 150 s.

evaluate the orientation degree of CNTs, the depolarization factor (D) is calculated by the following equation: D = G0°/G90°

(1)

where G0° and G90° are respectively the peak intensity for the G band in the parallel direction and perpendicular direction. Because the value of D of B-3.5 composite is 1.05, which is very close to unity, the CNTs are randomly distributed in HDPE phase. The results agree well with the aforementioned morphological observation by SEM and TEM. To further understand the migration behavior of CNTs in the biphase system, a theoretical analysis was conducted. As is well documented,37 the selective localization behavior is mainly determined by the combined action of thermodynamic and kinetic factors. The former factor depends on the wetting coefficient (ωa) which can be calculated by the following equation: − γC‐PEO γ ωa = C‐HDPE γHDPE‐ PEO (2) where γC‑HDPE, γC‑PEO and γHDPE‑PEO respectively represent the interfacial free energy between polymer−particles and polymer−polymer. If ωa < −1, CNTs tend to localize in HDPE phase. If ωa > 1, CNTs tend to localize in PEO phase; otherwise, CNTs will localize at the interface between HDPE phase and PEO phase. The interfacial free energy can be estimated by the harmonic-mean equation and geometric-mean equation shown as follows:38

3. RESULTS AND DISCUSSION 3.1. Dispersion State of CNTs in HDPE/PEO Blends. For a conductive filler-filled immiscible polymer composite system, there usually exists a selective localization of conductive filler because of the different interfacial interactions between polymer components and filler particles.36 In order to ensure CNTs selectively localize in HDPE phase, the CNTs were premixed with HDPE. To further verify the strategy of premixing CNTs with HDPE, the localization of CNTs in HDPE/CNTs/PEO composites was characterized by SEM and TEM. On the whole, there is a clear boundary between HDPE phase and PEO phase because of their notable difference in polarity as well as high interfacial free energy (see Figure 2). For both B-1.5 and B-3.5 composites (see Figure 2a,b), no CNTs are found in PEO phase while a few CNTs can be observed on the surface of HDPE phase (shown by the blue arrows). In other words, most CNTs cannot migrate from HDPE phase to PEO phase but are selectively localized in HDPE phase during the sample preparation process. This can be also verified by the TEM micrograph shown in Figure 2c, where CNTs are selectively localized in HDPE phase (gray domain) forming dense networks while no CNTs can be seen in PEO phase (bright domain). Interestingly, for the CNTs at HDPE/PEO interface, their one end is embedded in HDPE phase while the other one is in PEO phase (shown by the blue arrows in Figure 2c). As for B7.5 composite (see Figure 2d), both single CNTs and their aggregated ones (shown by the blue arrows and blue circle) can be seen in PEO phase, indicating that some CNTs have migrated from HDPE phase to PEO phase during the sample preparation process. On the whole, aforementioned SEM and TEM micrographs show that CNTs have no obvious orientation in the polymer matrix. Then, in order to further characterize the orientation degree of CNTs in the polymer matrix, Raman spectra and polarized Raman spectra were performed. The Raman spectra of pristine HDPE, PEO, and CNTs are shown in Figure 2e, and the characteristic D band (disorder band) and G band (graphite band or TM-tangential mode) of CNTs can be clearly observed. Combined with Figure 2e, with respect to B-3.5 composite, the characteristic Raman peak of CNTs D band located at 1348 cm−1 has been superposed by the HDPE and PEO signals while the G band at 1586 cm−1 can be distinguished clearly from the polymer matrix (see Figure 2f). In order to

ij γ dγ d γ pγ p yz γ12 = γ1 + γ2 − 4jjjj d 1 2 d + p 1 2 p zzzz jγ + γ γ1 + γ2 z 2 k 1 {

(3)

γ12 = γ1 + γ2 − 2( γ1dγ2d +

(4)

γ1pγ2p )

where γi represents the surface free energy of component i. γdi and γpi are respectively the dispersive and polar parts of the surface free energy of component i. It is important to note that the surface free energy is temperature dependent. In order to evaluate the surface free energy more accurately, eq 5 is used to calculate the surface free energy at elevated temperature:39 ij T yz γ = γ0jjj1 − zzz j Tc z{ k

11/9

(5)

where γ0 is the surface free energy at 0 K, T and Tc are respectively the temperature of the polymer and the critical temperature (for most polymers, Tc = 1000 K). Table S2 (Supporting Information) shows the corresponding surface free energy data of HDPE, PEO, and CNTs reported in the literature.38,40,41 Because the reported surface free energy data for CNTs are different, two groups of surface free energy data with different polarity proposed by Nuriel et al.40 and Barber et al.41 were adopted. Moreover, the surface free energy of CNTs is temperature independent. Therefore, according to eqs 3 and 4, the corresponding interfacial free energy was calculated and listed in Table S3 (Supporting Information). According to the data of interfacial free energy listed in Table S3 (Supporting Information) and eq 2, ωa was calculated as 2.0 or 1.2 via harmonic-mean equation and as 2.5 or 1.2 via geometricmean equation. In other words, whatever equation is employed, ωa is always larger than 1. Therefore, the theoretical thermodynamic calculation demonstrates that CNTs have a 3639


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Figure 3. SEM micrographs of ACPCs in outer region. The cryofractured surfaces parallel to X−Z plane (top column) and corresponding parallel to Y−Z plane (bottom column): (a, a′) B-1.5 composite, (b, b′) B-3.5 composite, (c, c′) B-7.5 composite. The histograms in (a′, b′) are the thickness distributions of HDPE/CNTs microlayers. The inset in (c) shows the partial enlarged micrograph of (c).

Figure 4. SEM micrographs of ACPCs in inner region. The cryofractured surfaces parallel to X−Z plane (top column) and corresponding parallel to Y−Z plane (bottom column): (a, a′) B-1.5 composite, (b, b′) B-3.5 composite, (c, c′) B-7.5 composite. The histograms in (a′, b′) are the thickness distributions of HDPE/CNTs microlayers. The inset in (c) shows the partial enlarged micrograph of (c).

Obviously, the aforementioned theoretical analysis suggests that both thermodynamic and kinetic factors are beneficial for the selective localization of CNTs in PEO phase. Nevertheless, it should be pointed out that the sample preparation process also plays a significant role in determining the selective localization of conductive fillers,36 providing a strategy to regulate the localization of CNTs in HDPE phase by premixing CNTs with HDPE. This strategy can be realized because of the following reasons: (1) CNTs were premixed with HDPE and then melt mixed with PEO; it would be very difficult for CNTs to migrate from HDPE phase to PEO phase due to the obstruction originated from higher viscosity of HDPE; (2) migration of CNTs from HDPE phase to PEO phase would be restrained because of its large length/diameter ratio and high specific surface area; (3) it had small probability for CNTs to diffuse into PEO phase because of the relatively low shear rate and the short melt mixing time during the sample preparation process. However, once the content of CNTs exceeds the maximum packing density in HDPE phase (e.g., 15 wt % in HDPE phase for B-7.5 composite), CNTs are prone to being agglomerated or protruded out of HDPE phase to reduce their surface energy.45,46

tendency to preferentially localize in PEO phase during the melt mixing process. As for the kinetic factors, the melt viscosity is another key factor that can affect the dispersion of conductive fillers.36,38,42 Figure S3 (Supporting Information) shows the complex viscosity of pure HDPE and PEO as a function of angular frequency at the processing temperature (i.e., 160 °C). It can be clearly seen that the complex viscosity of HDPE is much higher than that of PEO within the whole angular frequency range. The shear rate (γ̇) applied in the melt extrusion process could be approximatively evaluated by eq 6:43 γ̇ =

6Q L0D0 2

(6)

where Q is the volumetric output of the single-screw extruder, L0D0 is the cross-sectional area of die. In this study, when screw speed reached 60 rpm, the volumetric output was about 30 cm3/ min. Thus, the shear rate calculated according to eq 6 was 25 s−1. Moreover, according to the Cox−Merz rule (η*(ω) = η(γ̇)),44 the shear viscosity at the shear rate of 25 s−1 corresponds to the complex viscosity at an angular frequency of 25 rad/s. Hence, from the kinetic viewpoint, CNTs are prone to localizing in the PEO phase with lower viscosity. 3640


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Figure 5. (a) Storage modulus (G′) vs angular frequency (ω) of the HDPE/CNTs composites with different CNTs contents; (b) viscosity ratio between HDPE/CNTs phase and PEO phase as a function of CNTs content.

3.2. Morphology of Alternating Microlayered ACPCs. In order to improve the phase contrast and make the microlayered HDPE/CNTs phase be clearly observed by SEM, PEO phase was leached by deionized water. As shown by the SEM micrographs in Figure 1, two regions can be clearly distinguished along the thickness direction (i.e., Z direction) of ACPCs: outer region and inner region. The detailed morphology of the outer region and the inner region in the composites with different CNTs contents is illustrated respectively in Figure 3 and Figure 4. For all composites, a continuous structure is observed in both outer and inner regions. According to the morphology parallel to the X−Z plane and Y− Z plane, HDPE/CNTs phase in B-1.5 and B-3.5 demonstrates an interesting microlayered feature. Meanwhile, the microlayers in the inner region (∼350 μm) are much more compact than those in the outer region. In the outer region, HDPE/CNTs microlayers definitely orient along the X direction (see Figure 3a,b) while it is discontinuous in the Y direction (see Figure 3a′,b′), indicating that the HDPE/CNTs microlayered structure in outer region is incomplete. Compared with the phase morphology in the outer region, well-defined HDPE/CNTs microlayers in the inner region are of high length/thickness ratio and regularly arranged (see Figure 4a,b,a′,b′). There are many gaps between the adjacent HDPE/CNTs microlayers in inner region, which result from the fact that PEO phase was completely dissolved into deionized water. As a result, both the composites containing 1.5 and 3.5 wt % CNTs exhibit an alternating microlayered structure. Additionally, the thickness distribution of HDPE/CNTs microlayers is evaluated, which can be seen in the histograms insert in Figures 3 and 4. In the outer region, the average thickness of HDPE/CNTs microlayers of B-1.5 and B-3.5 composites is respectively 3.1 and 2.8 μm, while it is around 0.32 and 0.39 μm in inner region. The thickness of HDPE/CNTs microlayers generally remains unchanged when the CNTs content increases from 1.5 wt % to 3.5 wt %, whereas when CNTs content reaches 7.5 wt %, the alternating microlayered structure becomes unclear. Moreover, it can be clearly seen that many spherical or ellipsoidal CNTs aggregations attach to the HDPE/CNTs microlayers or bridge the gap between two adjacent microlayers (see insets in Figures 3c and 4c). In other words, B-7.5 composite loses the feature of alternating microlayered structure. Such structural transformation can be ascribed to the migration of CNTs as well as the increased viscosity of the composite system, which will be investigated in detail in the following section. Such alternating microlayered structure is seldom found in immiscible polymer blends, so it is necessary to investigate the formation mechanism of such interesting biphase structure

developed by the method proposed in this study. To investigate the morphological evolution, B-3.5 composite is selected as an example. Figure S4 (Supporting Information) presents the cryofractured surfaces of ACPCs containing 3.5 wt % CNTs just at the exit of the die without calendering. It is clear that in the X− Z plane, HDPE/CNTs phase solely subjected to shear in the die presents continuous microfibrillar network structure orienting in the X direction. It should be noted that, no matter in the outer region or the inner region, the microfibrillar network generally shows the similar feature except those in the outer region are slightly compressed along the Z direction. However, the microfibrillar network structure will be evolved into microlayered structure once the squeezing effect is applied (see Figure S5 in Supporting Information). In view of this, this interesting morphological evolution during the “melt extrusion-calendering” process essentially results from two driving factors: one is the shear generated in die, upon which the HDPE/CNTs phase can be transform into microfibrillar network structure evidently oriented in the X direction; the other one is the squeezing effect produced by the calender rollers which can squeeze the microfibrils into thinner sheets parallel to the X−Y plane. Even more importantly, once squeezing effect is applied, sheets will be further extended in the X−Y plane and coalesced with each other to form the microlayers with larger area. It is worth noting that once the extrudate leaves the die exit and contacts two running calender rollers, the outer region of the composites will be immediately cooled by water-cooled rollers. Therefore, the melt in the outer region only undergoes a relatively weak deformation. Because of temperature gradient from the outer region to the inner region, a delay of solidification is expected in the inner region, which gives a wider time window for the polymer melt to be solidified. As a result, it is easier for the polymer melt in the inner region to be squeezed into a thinner and finer microlayer than that in the outer region. It is well-known that rheological properties of polymer melt components (especially, viscosity ratio) play a significant role in determining the morphological evolution.43,47 Figure 5a shows the storage modulus G′ of HDPE/CNTs composites with different CNTs content as a function of angular frequency at 160 °C (i.e., the extrusion temperature). At a fixed frequency, G′ increases with increasing CNTs content. At fixed CNTs content, G′ increases with elevating frequency. In addition, when CNTs content reaches 5 wt %, G′ shows a “plateau” at low frequencies. It has been well documented that such “plateau” effect is derived from the interconnected structures of anisometric fillers.47,48 Therefore, the low-frequency “plateau” in G′ can be attributed to the formation of CNTs network, suggesting that the rheological percolation threshold is in the range of 3−5 wt %. 3641


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Figure 6. Schematic diagram of the morphological evolution: (a) just at the die exit; (b) after calendering and cooling. Mechanical properties of B-3.5 composite before and after leaching: (c) tensile strength; (d) elongation at break. (e) 2D-WAXD scattering pattern of B-3.5 composite. (f) Azimuthal scans of (110) plane of HDPE and (120) plane of PEO.

lead to severe aggregation of CNTs in HDPE phase, hindering the deformation as well as coalescence of HDPE phase in the X− Y plane.52 The rheological analysis is well in line with the morphological evolution presented above. In addition, interfacial free energy is another key factor influencing the phase morphological evolution under shear. The interfacial free energy has been well documented that it can not only resist the deformation of dispersed phase but also restore the original morphology during processing.8,53 Because the interfacial free energy between the HDPE phase and PEO phase is relatively high (large than 9 mJ/m2, see Table S3 in Supporting Information), the deformation of dispersed phase will be hindered and the original microfibrillar structure can be maintained before the extrudate undergoes squeezing effect. And then, the microfibrillar network structure will be deformed into microlayered structure and frozen before it is relaxed during the calendering process. On the basis of aforementioned discussion, the development of microlayered structure can be schematically diagramed in Figure 6a,b. Mechanical properties are important prerequisites for the practical application of ACPCs, whereas they are seldom tested due to the limited size and low yield of the resulting ACPCs in

It has been well believed that CNTs content is a crucial obstacle to the development of alternating microlayered structure. As shown in Figures 3c and 4c, when the content of CNTs is excessive (e.g., 15 wt % in HDPE phase), the alternating microlayered structure cannot be developed. It has been reported in open literature that the viscosity of the polymer melt could increase significantly with the incorporation of excessive CNTs,47−49 which in turn determines the phase morphological evolution.43,50,51 In order to investigate the effect of viscosity on the development of alternating microlayered structure, viscosity ratio (λ) between HDPE/CNTs phase and PEO phase was calculated by the following equation: λ=

ηHDPE/ CNTs ηPEO

(7)

where ηHDPE/CNTs and ηPEO are respectively the melt viscosity of HDPE/CNTs phase and PEO phase at shear rate of 25 s−1. As shown in Figure 5b, when CNTs content increases from 0 to 10 wt %, viscosity ratio only shows a slight increase. However, it demonstrates a sharp rise when CNTs content increases from 10 to 15 wt %, which will lead to a dramatic transformation of phase morphology. As is well documented, the excessive CNTs will 3642


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Figure 7. Electrical properties of HDPE/CNTs/PEO composites: (a) electrical conductivity in three directions as a function of CNTs content; (b) intensity of anisotropy as a function of CNTs content; (c) electrical conductivity in three directions of HDPE/CNTs/PEO composites containing 3.5 wt % CNTs as a function of stretching ratio; (d) intensity of anisotropy of HDPE/CNTs/PEO composites containing 3.5 wt % CNTs as a function of stretching ratio. (e) Application of ACPCs in functional LED integrated circuits. The ACPCs can be easily cut (f) or drilled (g) into a “ZZU” shape.

which is indicative of obvious molecular orientation along flow direction (i.e., X direction).8 Furthermore, the azimuthal scans of (110) plane of HDPE and (120) plane of PEO are obtained from 2D-WAXD pattern, as presented in Figure 6f. Clearly, the diffraction peak of (110) plane is narrower and steeper while the diffraction peak of (120) plane is relatively gentle, indicating the higher molecular orientation of HDPE and lower molecular orientation of PEO, respectively. This should be ascribed to the shear and stretching flow fields during the sample preparation process. Moreover, the continuity of microlayers along the Y direction is weaker than that along the X direction, which leads to poor ability of exterior force resistance. As a result, from a “microstructure−property” relation perspective, higher tensile strength along the X direction can be attributed to the orientation of polymer molecular and the continuous microlayers along the X direction. Additionally, due to the incomplete microlayer structure in the outer region of ACPCs, the ACPCs will break more easily along the X direction than the Y direction, showing the relatively weaker toughness. To sum up, the ACPCs as well as their leached composite are solid enough which can meet the mechanical requirements of its further applications.

most studies. Because of our efficient processing method, the tensile tests can be respectively performed along the X direction and the Y direction of the ACPCs before and after leaching of PEO phase. As shown in Figure 6c, with regard to B-3.5 composites that possessed alternating multilayered structure, it is clear that both the tensile strength along the X direction and the tensile strength along the Y direction are approximately twice higher than those of the corresponding leached composites. The decrease of tensile strength in leached composites is caused by the absence of PEO phase which acts as the role of mechanical support. Meanwhile, no matter before or after leaching of PEO phase, tensile strength along the X direction is always twice than that along the Y direction. After leaching, the elongation at the break along the X direction or the Y direction decreases in a certain degree, and the elongation at the break along the Y direction is higher than that along the X direction (see Figure 6d). Such anisotropy of mechanical properties is closely related to the microstructure of ACPCs. Hence, the molecular chain orientation of B-3.5 composite was characterized using 2DWAXD measurement. As shown in Figure 6e, in the equatorial direction, apparent arc-like diffraction is seen on the (110) plane of HDPE while the arc on the (120) plane of PEO is attenuated, 3643


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ACS Applied Nano Materials Table 1. Electrical Anisotropy and Conductivity Reported in Previous Literature conductive filler

method

intensity of anisotropy

conductivity in X direction (S/m)

ref

graphene/MWCNTs MWCNTs γ-Fe2O3/MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs

electrospinning electric field-induced alignment magnetic field-induced alignment magnetic field-induced alignment doctor blade shear-induced self-assembly stretch-induced alignment injection molding extrusion molding extrusion molding extrusion molding

105 105 4 10 104 105 102 >105 103 105 106

10 10−3 10−7 10−2 1 10−4 10−1 10−2 10−4 10−3 10−2

23 13 15 17 26 18 24 8 10 22 this work

3.3. Electrical Properties of ACPCs and Its Applications. One of the most essential characteristics for the ACPCs is their anisotropic conductive property. The electrical conductivity σ was calculated by the following equation: σ=

L RS

formed, whereas when the content of CNTs is excessive (e.g., 7.5 wt %), the alternating microlayered structure cannot be developed. On the other hand, some CNTs can migrate from HDPE phase to PEO phase, and many CNTs aggregations bridge the gap between the two adjacent HDPE/CNTs microlayers (see Figures 3c and 4c), which can trigger the formation of conductive networks in the Z direction. Moreover, it should be noted that the conductivity in the X direction is always slightly higher than that in the Y direction. This should be ascribed to the fact that the continuity of HDPE/CNTs phase along the X direction is better than that along the Y direction in the X−Y plane, which can be confirmed by Figure 3b,b′. To intuitively estimate the conductivity anisotropy, the intensity of anisotropy (I) was calculated by eq 9:

(8)

where L is length of the sample along measurement direction and R and S are respectively the electrical resistance and cross section area of the sample. The relationship between electrical conductivity and CNTs content is illustrated in Figure 7a. Clearly, the composite of B-0.5 or B-1.5 is nonconductive in all directions. However, the conductivity in the X direction shows a sharp rise of about 2 orders of magnitude (from 1.7 × 10−6 to 7.7 × 10−4 S/m) when CNTs content increases from 1.5 to 2.5 wt %. This indicates that a good conductive network can be formed in HDPE/CNTs/PEO composites if CNTs content exceeds a critical value. Additionally, it suggests that the percolation threshold is between 1.5 and 2.5 wt %. It should be noted that the CNTs content here represents the weight content of CNTs in the whole HDPE/CNTs/PEO composites. As a result, this result agrees well with the rheological percolation threshold, which can be explained by the double percolation mechanism:36,37 on one hand, CNTs can form effective conductive network in HDPE phase when its content exceeds the percolation threshold; on the other hand, the conductive pathways can be achieved through building the continuous HDPE/CNTs phase. With further increase of CNTs content, the conductivity in X and Y directions shows the same increasing trend. Nevertheless, the conductivity in the Z direction exhibits a different trend with the increase of CNTs content. When CNTs content is less than or equal to 3.5 wt %, the conductivity in the Z direction is in the range of 10−9−10−8 S/m (i.e., the composite is insulative in thickness direction), which is almost at the same level with pure PEO (2.3 × 10−9 S/m). With further increase of CNTs content, the conductivity in the Z direction shows a significant rise and the composite becomes conductive (2.4 × 10−2 S/m at 7.5 wt %). In view of this, this interesting phenomenon is closely related to the microstructure of the HDPE/CNTs/PEO composite. As is well demonstrated by the aformentioned morphological observation (see Figures 3 and 4), when CNTs content is less than or equal to 3.5 wt %, the composites exhibit alternating microlayered structure. As a result, electrons can easily transfer in HDPE/CNTs microlayer plane when the interconnected CNTs network is formed in HDPE/CNTs microlayers. However, due to the separation of insulative PEO microlayers between the adjacent HDPE/CNTs microlayers, valid conductive path in the Z direction cannot be

I = σX /σZ

(9)

in which σX and σZ are the conductivity of composites in X and Z directions, respectively. As shown in Figure 7b, when CNTs content is 2.5 or 3.5 wt %, it can be seen that the corresponding conductivity in the X direction is respectively 3.6 × 104 or 1.4 × 106 times higher than that in the Z direction, showing a remarkable conductive anisotropy. In fact, the remarkable conductive anisotropy performance depends on the ordered CNTs-filled HDPE microlayers and the unique alternating microlayered structure, and it has nothing to do with the orientation degree of CNTs (see Figure 2) and polymer matrix (see Figure 6). With further increasing CNTs content (more than 3.5 wt %), the intensity of anisotropy (see Figure 7b) begins to evidently decrease. This is mainly ascribed to the fact that the interconnection between adjacent HDPE/CNTs microlayers will occur once CNTs content is excessive, which can trigger the formation of conductive networks in the Z direction. Figure 7c shows the electrical conductivity of composites with 3.5 wt % CNTs content in three directions as a function of stretching ratio. The conductivity in X and Y directions gradually decreases with the increasing stretching ratio, while the conductivity in the Z direction remains a constant value (i.e., 10−8 S/m). At the same stretching ratio, the conductivity in the Y direction is steadily lower than that in the X direction. Moreover, intensity of anisotropy decreases with the increase of stretching ratio, which can be adjusted in a large range from 106 to 104 (see Figure 7d). When the stretching ratio is 1.5, the intensity of anisotropy is 1.4 × 106. Such a high anisotropy is remarkable among the reported ACPCs. For comparison, Table 1 summarizes the intensity of anisotropy and conductivity of CNTs-filled ACPCs fabricated using different methods. From the aspects of intensity of anisotropy, it can be clearly seen that the intensity of anisotropy in our work is extraordinary. 3644


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Figure 8. Vapor sensing behaviors of the stacked microlayered structure composite toward cyclohexane (a), dichloromethane (b), and ethyl acetate (c). Maximum responsivity in 10 IDRs (d). Black dashed lines in (d) are the fitted lines of the experimental data.

Furthermore, the conductivity in the X direction is higher than most reported ACPCs. More importantly, in our case, ACPCs were fabricated only by common processing equipment, which is suitable for large-scale production. Therefore, compared with other methods, this is a facile, efficient, low-cost, environmentally friendly, and large-scale method to prepare ACPCs. Due to the extraordinary conductive anisotropy and good conductivity in the X direction, such ACPCs can be directly employed for microcircuits interconnection. Figure 7e shows the application of such ACPCs in LED integrated circuits. As shown in Figure 7e and Movie S1 (Supporting Information), when the two circuits were switched on, LED was well operational only in the red circuit, indicating that it is conductive only along the X direction while insulative along the Z direction. The results are mainly ascribed to the alternating microlayered structure and the resulting extraordinary conductive anisotropy. Therefore, the ACPCs may be employed as reliable electrical interconnects. Moreover, the ACPCs can be easily processed into various shapes to integrate with desired applications or systems. For example, it can be cut into a “ZZU” shape (see Figure 7f) or drilled to form a “ZZU” shape (see Figure 7g). Furthermore, as shown in Figure 4b,b′, after PEO phase is removed (this can be confirmed by TGA results shown in Figure S6, Supporting Information), the resulting composite (with CNTs content of 7 wt %) consists of numerous stacked microlayers. Reasonably, the stacked microlayered structure composite possesses large specific surface area and abundant interconnected microchannels, which can offer potential access to application as vapor sensor. To confirm its vapor sensing ability, the vapor sensing tests were conducted by in situ monitoring of their chemoresistive responses upon exposure to three representative organic solvent vapors (i.e., cyclohexane, dichloromethane, and ethyl acetate). Figure 8a−c shows the responsivity of the stacked microlayered composite against each organic vapor for 10 successive immersion-drying runs (IDRs). The responsivity is defined as the ratio of ΔR/R0. Here, R0 represents the original resistance of the sample, and ΔR is the resistance change during the IDRs. Clearly, the responsivity

increases quickly once it is exposed to organic vapor, followed by decreasing to initial value when the sample was in air. Moreover, the stacked microlayered composite shows superior stability and reversibility when exposed to afromentioned organic vapors. As shown in Figure 8d, the ranking of the average maximum responsivity in 10 IDRs is as follows: cyclohexane (2.97) > dichloromethane (1.43) > ethyl acetate (0.40). The corresponding maximum responsivity against each vapor is a significant parameter to detect different organic vapor. The superior organic vapor sensing behavior of the composite can be ascribed to the unique stacked microlayered structure which has better adsorption and desorption properties to organic vapor. When the sample was immersed in organic vapors, the matrix would absorb organic vapors, leading to the swelling of matrix, which could change the conductive paths. During the drying process, the organic vapors would desorb from the matrix, so nearly all conductive paths would return to their initial state.54 Therefore, our results provide a new facile strategy to fabricate low-cost vapor sensors.

4. CONCLUSIONS In this study, ACPCs with alternating microlayers have been successfully fabricated through a “melt extrusion-calendering” method. The morphology of HDPE/CNTs/PEO blends is strongly dependent on the rheological properties and the fabrication process. When the CNTs content is less than or equal to 3.5 wt %, the CNTs are selectively localized in HDPE phase. Meanwhile, the alternating microlayered conductive networks parallel to the X−Y plane is observed in ACPCs. Therefore, they show outstanding conductive anisotropy; i.e., the electrical conductivity in X direction is 6 orders of magnitude higher than that in the Z direction, which is the highest conductive anisotropy among these CNTs-filled ACPCs. Moreover, the intensity of anisotropy in conductivity can be controlled in a wide range by regulating stretching ratio. Additionally, the as-prepared ACPCs show reliable interconnection capability and good organic vapor sensing ability, 3645


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(7) Zhao, K.; Li, S.; Huang, M.; Shi, X.; Zheng, G.; Liu, C.; Dai, K.; Shen, C.; Yin, R.; Guo, J. Z. Remarkably Anisotropic Conductive MWCNTs/Polypropylene Nanocomposites with Alternating Microlayers. Chem. Eng. J. 2019, 358, 924−935. (8) Yu, F.; Deng, H.; Zhang, Q.; Wang, K.; Zhang, C.; Chen, F.; Fu, Q. Anisotropic Multilayer Conductive Networks in Carbon Nanotubes Filled Polyethylene/Polypropylene Blends Obtained through High Speed Thin Wall Injection Molding. Polymer 2013, 54 (23), 6425− 6436. (9) Zhu, J.; Shen, J.; Guo, S.; Sue, H.-J. Confined Distribution of Conductive Particles in Polyvinylidene Fluoride-based Multilayered Dielectrics: Toward High Permittivity and Breakdown Strength. Carbon 2015, 84, 355−364. (10) Gao, J.; Yan, D.; Yuan, B.; Huang, H.; Li, Z. Large-scale Fabrication and Electrical Properties of an Anisotropic Conductive Polymer Composite utilizing Preferable Location of Carbon Nanotubes in a Polymer Blend. Compos. Sci. Technol. 2010, 70 (13), 1973−1979. (11) Baek, D. H.; Park, J. S.; Lee, E. J.; Shin, S.; Moon, J. H.; Pak, J. J.; Lee, S. H. Interconnection of Multichannel Polyimide Electrodes using Anisotropic Conductive Films (ACFs) for Biomedical Applications. IEEE Trans. Biomed. Eng. 2011, 58 (5), 1466−1473. (12) Liu, S.; Liu, Y.; Cebeci, H.; de Villoria, R. G.; Lin, J. H.; Wardle, B. L.; Zhang, Q. M. High Electromechanical Response of Ionic Polymer Actuators with Controlled-morphology Aligned Carbon Nanotube/ Nafion Nanocomposite Electrodes. Adv. Funct. Mater. 2010, 20 (19), 3266−3271. (13) Oliva-Aviles, A. I.; Aviles, F.; Sosa, V.; Oliva, A. I.; Gamboa, F. Dynamics of Carbon Nanotube Alignment by Electric Fields. Nanotechnology 2012, 23 (46), 465710. (14) Wu, S.; Ladani, R. B.; Zhang, J.; Bafekrpour, E.; Ghorbani, K.; Mouritz, A. P.; Kinloch, A. J.; Wang, C. H. Aligning Multilayer Graphene Flakes with an External Electric Field to Improve Multifunctional Properties of Epoxy Nanocomposites. Carbon 2015, 94, 607−618. (15) Kim, I. T.; Tannenbaum, A.; Tannenbaum, R. Anisotropic Conductivity of Magnetic Carbon Nanotubes Embedded in Epoxy Matrices. Carbon 2011, 49, 54−61. (16) Shen, J.; Zhu, Y.; Zhou, K.; Yang, X.; Li, C. Tailored Anisotropic Magnetic Conductive Film Assembled from Graphene-encapsulated Multifunctional Magnetic Composite Microspheres. J. Mater. Chem. 2012, 22 (2), 545−550. (17) Kimura, T.; Ago, H.; Tobita, M.; Ohshima, S.; Kyotani, M.; Yumura, M. Polymer Composites of Carbon Nanotubes Aligned by a Magnetic Field. Adv. Mater. 2002, 14 (19), 1380−1383. (18) Huang, J.; Zhu, Y.; Jiang, W.; Yin, J.; Tang, Q.; Yang, X. Parallel Carbon Nanotube Stripes in Polymer Thin Film with Remarkable Conductive Anisotropy. ACS Appl. Mater. Interfaces 2014, 6 (3), 1754− 1758. (19) Mao, C.; Huang, J.; Zhu, Y.; Jiang, W.; Tang, Q.; Ma, X. Tailored Parallel Graphene Stripes in Plastic Film with Conductive Anisotropy by Shear-Induced Self-Assembly. J. Phys. Chem. Lett. 2013, 4 (1), 43− 47. (20) Huang, J.; Zhu, Y.; Jiang, W.; Tang, Q. Parallel Carbon Nanotube Stripes in Polymer Thin Film with Tunable Microstructures and Anisotropic Conductive Properties. Composites, Part A 2015, 69, 240− 246. (21) Huang, J.; Xu, J.; Sheng, Y.; Zhu, Y.; Jiang, W.; Xu, D.; Tang, Q.; Nie, X. Fabrication of Polymer Film with Extraordinary Conductive Anisotropy by Forming Parallel Conductive Vorticity-Aligned Stripes and Its Formation Mechanism. Macromol. Mater. Eng. 2016, 301, 743− 749. (22) Li, B.; Zhang, Y.; Li, Z.; Li, S.; Zhang, X. Easy Fabrication and Resistivity-temperature Behavior of an Anisotropically Conductive Carbon Nanotube-polymer Composite. J. Phys. Chem. B 2010, 114 (2), 689−696. (23) Liu, M.; Du, Y.; Miao, Y. E.; Ding, Q.; He, S.; Tjiu, W. W.; Pan, J.; Liu, T. Anisotropic Conductive Films Based on Highly Aligned Polyimide Fibers Containing Hybrid Materials of Graphene Nanoribbons and Carbon Nanotubes. Nanoscale 2015, 7 (3), 1037−1046.

providing a range of potential applications based on alternating microlayered structure. More importantly, this study provides a continuous but scale method to prepare ACPCs through an efficient industrialized processing process, opening a new pathway toward the wide application of anisotropic materials.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00584. SEM micrograph of the HDPE/CNTs master batch; photograph of the vacuum assisted compression machine and its major parameters and accessories; complex viscosity of HDPE and PEO; SEM micrographs of HDPE/PEO/CNTs composites containing 3.5 wt % CNTs without calendering; SEM micrographs of B-3.5 composite parallel to X−Y plane; TGA curves and DTG curves of the ACPCs before and after leaching; and surface free energy parameters and interfacial free energy parameters of each component (PDF) Movie S1, the application in microcircuits interconnection (MP4)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liwei Mi: 0000-0001-9239-6599 Guoqiang Zheng: 0000-0001-7595-8270 Chuntai Liu: 0000-0001-9751-6270 Kun Dai: 0000-0002-9877-8552 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Grants 11572290 and 51873199) for financial support for this work.

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REFERENCES

(1) Li, X.; Cai, J.; Shi, Y.; Yue, Y.; Zhang, D. Remarkable Conductive Anisotropy of Metallic Microcoil/PDMS Composites Made by Electric Field Induced Alignment. ACS Appl. Mater. Interfaces 2017, 9 (2), 1593−1601. (2) Wan, J.; Song, J.; Yang, Z.; Kirsch, D.; Jia, C.; Xu, R.; Dai, J.; Zhu, M.; Xu, L.; Chen, C.; Wang, Y.; Wang, Y.; Hitz, E.; Lacey, S. D.; Li, Y.; Yang, B.; Hu, L. Highly Anisotropic Conductors. Adv. Mater. 2017, 29 (41), 1703331. (3) Tawfick, S.; De Volder, M.; Copic, D.; Park, S. J.; Oliver, C. R.; Polsen, E. S.; Roberts, M. J.; Hart, A. J. Engineering of Micro- and Nanostructured Surfaces with Anisotropic Geometries and Properties. Adv. Mater. 2012, 24 (13), 1628−1674. (4) Wang, Y.; Wang, W.; Wo, Y.; Gui, T.; Zhu, H.; Mo, X.; Chen, C. C.; Li, Q.; Ding, W. Orientated Guidance of Peripheral Nerve Regeneration Using Conduits with a Microtube Array Sheet (MTAS). ACS Appl. Mater. Interfaces 2015, 7 (16), 8437−8450. (5) Deng, H.; Dong, Y.; Zhang, C.; Xie, Y.; Zhang, C.; Lin, J. An Instant Responsive Polymer Driven by Anisotropy of Crystal Phases. Mater. Horiz. 2018, 5 (1), 99−107. (6) Wang, J.; Xie, J.; Zong, C.; Han, X.; Zhao, J.; Jiang, S.; Cao, Y.; Fery, A.; Lu, C. Light-modulated Surface Micropatterns with Multifunctional Surface Properties on Photodegradable Polymer Films. ACS Appl. Mater. Interfaces 2017, 9 (42), 37402−37410. 3646


Article

ACS Applied Nano Materials (24) Deng, H.; Bilotti, E.; Zhang, R.; Loos, J.; Peijs, T. Effect of Thermal Annealing on the Electrical Conductivity of High-strength Bicomponent Polymer Tapes Containing Carbon Nanofillers. Synth. Met. 2010, 160 (5−6), 337−344. (25) Park, S. H.; Shin, H. S.; Kim, Y. H.; Park, H. M.; Song, J. Y. Template-free and Filamentary Growth of Silver Nanowires: Application to Anisotropic Conductive Transparent Flexible Electrodes. Nanoscale 2013, 5 (5), 1864−1869. (26) Lanticse, L. J.; Tanabe, Y.; Matsui, K.; Kaburagi, Y.; Suda, K.; Hoteida, M.; Endo, M.; Yasuda, E. Shear-induced Preferential Alignment of Carbon Nanotubes Resulted in Anisotropic Electrical Conductivity of Polymer Composites. Carbon 2006, 44 (14), 3078− 3086. (27) Wang, Y.; Liu, X.; Lian, M.; Zheng, G.; Dai, K.; Guo, Z.; Liu, C.; Shen, C. Continuous Fabrication of Polymer Microfiber Bundles with Interconnected Microchannels for Oil/Water Separation. Applied Materials Today 2017, 9, 77−81. (28) Chang, B.; Xie, M.; Dai, K.; Zheng, G.; Wang, S.; Liu, C.; Chen, J.; Shen, C. Pre-shear Induced Anomalous Distribution of β-form in Injection Molded iPP. Polym. Test. 2013, 32 (3), 545−552. (29) Pan, Y.; Shi, S.; Xu, W.; Zheng, G.; Dai, K.; Liu, C.; Chen, J.; Shen, C. Wide Distribution of Shish-kebab Structure and Tensile Property of Micro-injection-molded Isotactic Polypropylene Microparts: a Comparative Study with Injection-molded Macroparts. J. Mater. Sci. 2014, 49 (3), 1041−1048. (30) Huang, L.; Wang, Z.; Zheng, G.; Guo, J. Z.; Dai, K.; Liu, C. Enhancing Oriented Crystals in Injection-molded HDPE through Introduction of Pre-shear. Mater. Eng. 2015, 78, 12−18. (31) Willemse, R. C.; Ramaker, E. J. J.; van Dam, J.; Posthuma de Boer, A. Morphology Development in Immiscible Polymer Blends Initial Blend Morphology and Phase Dimensions. Polymer 1999, 40, 6651− 6659. (32) Liu, Z.; Liu, X.; Zheng, G.; Dai, K.; Liu, C.; Shen, C.; Yin, R.; Guo, Z. Mechanical Enhancement of Melt-stretched β-nucleated Isotactic Polypropylene: The Role of Lamellar Branching of β-crystal. Polym. Test. 2017, 58, 227−235. (33) Chang, B.; Schneider, K.; Heinrich, G.; Li, Y.; Zheng, G.; Häußler, L.; Liu, C.; Shen, C. Competition Effect of Shear-Induced Nuclei and Multiwalled Carbon Nanotubes (MWCNT) on β-Isotactic Polypropylene (iPP) Formation in Preshear Injection Molded iPP/ MWCNT Nanocomposites. Polym. Compos. 2018, 39, E1149−E1158. (34) Tol, R. T.; Mathot, V. B. F.; Groeninckx, G. Confined Crystallization Phenomena in Immiscible Polymer Blends with Dispersed Micro- and Nanometer Sized PA6 Droplets, part 2: Reactively Compatibilized PS/PA6 and (PPE/PS)/PA6 Blends. Polymer 2005, 46 (2), 383−396. (35) Cheng, X.; Shi, H.; Wang, Z.; Zheng, G.; Liu, P.; Dai, K.; Liu, C.; Shen, C. Bioinspired Concentric-cylindrical Multilayered Scaffolds with Controllable Architectures: Facile Preparation and Biological Applications. ACS Appl. Mater. Interfaces 2018, 10, 43512−43522. (36) Shi, Y.; Yang, J.; Huang, T.; Zhang, N.; Chen, C.; Wang, Y. Selective Localization of Carbon Nanotubes at the Interface of Poly(Llactide)/Ethylene-co-vinyl Acetate Resulting in Lowered Electrical Resistivity. Composites, Part B 2013, 55, 463−469. (37) Sumita, M.; Sakata, K.; Asai, S.; Miyasaka, K.; Nakagawa, H. Dispersion of Fillers and the Electrical Conductivity of Polymer Blends Filled with Carbon Black. Polym. Bull. 1991, 25, 265−271. (38) Mural, P. K. S.; Madras, G.; Bose, S. Positive Temperature Coefficient and Structural Relaxations in Selectively Localized MWNTs in PE/PEO Blends. RSC Adv. 2014, 4 (10), 4943. (39) Wu, S. Surface and Interfacial Tensions of Polymers, Oligomers, Plasticizers, and Organic Pigments; John Wiley & Sons, Inc., 2003; pp 521−541. (40) Nuriel, S.; Liu, L.; Barber, A. H.; Wagner, H. D. Direct Measurement of Multiwall Nanotube Surface Tension. Chem. Phys. Lett. 2005, 404 (4−6), 263−266. (41) Barber, A. H.; Cohen, S. R.; Wagner, H. D. Static and Dynamic Wetting Measurements of Single Carbon Nanotubes. Phys. Rev. Lett. 2004, 92 (18), 186103.

(42) Gubbels, F.; Blacher, S.; Vanlathem, E.; Jérôme, R.; Deltour, R.; Brouers, F.; Teyssié, P. Design of Electrical Composites Determining the Role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends. Macromolecules 1995, 28 (5), 1559−1566. (43) Wang, L.; Feng, L.; Gu, X.; Zhang, C. Influences of the Viscosity Ratio and Processing Conditions on the Formation of Highly Oriented Ribbons in Polymer Blends by Tape Extrusion. Ind. Eng. Chem. Res. 2015, 54 (44), 11080−11086. (44) Malkin, A.; Ilyin, S.; Roumyantseva, T.; Kulichikhin, V. Rheological Evidence of Gel Formation in Dilute Poly(acrylonitrile) Solutions. Macromolecules 2013, 46 (1), 257−266. (45) Chen, D.; Liu, T.; Zhou, X.; Tjiu, W. C.; Hou, H. Electrospinning Fabrication of High Strength and Toughness Polyimide Nanofiber Membranes Containing Multiwalled Carbon Nanotubes. J. Phys. Chem. B 2009, 113 (29), 9741−9748. (46) Wang, H.; Xiao, R. Preparation and Characterization of CNTs/ PE Micro-nanofibers. Polym. Adv. Technol. 2012, 23 (3), 508−515. (47) Huang, J.; Mao, C.; Zhu, Y.; Jiang, W.; Yang, X. Control of Carbon Nanotubes at the Interface of a Co-continuous Immiscible Polymer Blend to Fabricate Conductive Composites with Ultralow Percolation Thresholds. Carbon 2014, 73, 267−274. (48) McNally, T.; Pötschke, P.; Halley, P.; Murphy, M.; Martin, D.; Bell, S. E. J.; Brennan, G. P.; Bein, D.; Lemoine, P.; Quinn, J. P. Polyethylene Multiwalled Carbon Nanotube Composites. Polymer 2005, 46 (19), 8222−8232. (49) Yang, J.; Wang, C.; Wang, K.; Zhang, Q.; Chen, F.; Du, R.; Fu, Q. Direct Formation of Nanohybrid Shish-kebab in the Injection Molded Bar of Polyethylene/multiwalled Carbon Nanotubes Composite. Macromolecules 2009, 42 (18), 7016−7023. (50) Delaby, I.; Ernst, B.; Germain, Y.; Muller, R. Droplet Deformation in Polymer Blends during Uniaxial Elongational Flow: Influence of Viscosity Ratio for Large Capillary Numbers. J. Rheol. 1994, 38 (6), 1705−1720. (51) Tsebrenko, M. V.; Rezanova, N. M.; Vinogradov, G. V. Rheology of Molten Blends of Polyoxymethylene and Ethylene-vinyl Acetate Copolymer and the Microstructure of Extrudates as a Function of their Melt Viscosities. Polym. Eng. Sci. 1980, 20 (15), 1023−1028. (52) Xu, X.; Li, Z.; Yu, R.; Lu, A.; Yang, M.; Huang, R. Formation of in Situ CB/PET Microfibers in CB/PET/PE Composites by Slit Die Extrusion and Hot Stretching. Macromol. Mater. Eng. 2004, 289 (6), 568−575. (53) Puyvelde, P. V.; Moldenaers, P. Rheology and Morphology Development in Immiscible Polymer Blends. Rheol. Rev. 2005, 101− 145. (54) Kumar, B.; Castro, M.; Feller, J. F. Poly(lactic acid)-multi-wall Carbon Nanotube Conductive Biopolymer Nanocomposite Vapour Sensors. Sens. Actuators, B 2012, 161 (1), 621−628.

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