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Crystallization Behavior of Poly(ethylene oxide) in Vertically Aligned Carbon Nanotube Array Jiadong Sheng, Shenglin Zhou, Zhaohui Yang, and Xiaohua Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00070 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Crystallization Behavior of Poly(ethylene oxide) in Vertically Aligned Carbon Nanotube Array

Jiadong Sheng, † Shenglin Zhou, † Zhaohui Yang,* †,‡,§ Xiaohua Zhang* †,‡ †

Center for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China

‡

College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China

§

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China

ABSTRACT: We investigate the effect of the presence of vertically aligned multiwalled carbon nanotubes (CNT) on the orientation of poly(ethylene oxide)(PEO) lamellae and PEO crystallinity. The high alignment of carbon nanotubes acting as templates probably governs the orientation of PEO lamellae. This templating effect might result in the lamella planes of PEO crystals oriented along a direction parallel to the long axis of nanotubes. The presence of aligned carbon nanotubes also gives rise to the decreases in PEO crystallinity, crystallization temperature and melting temperature due to the perturbation of carbon nanotubes to the crystallization of PEO. These effects have significant implications for controlling the orientation of PEO lamellae, and decreasing the crystallinity of PEO and thickness of PEO lamellae, which have significant

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impacts on ion transport in PEO/CNT composite and the capacitive performance of PEO/CNT composite. Both the decreased PEO crystallinity and the orientation of PEO lamellae along the long axes of vertically aligned CNTs give rise to the decrease of charge transfer resistance, which is associated with the improvements of ion transport and capacitive performance of PEO/CNT composite.

INTRODUCTION There have been exhaustive studies on inorganic/polymer nanocomposites to develop hierarchically ordered inorganic−polymer nanocomposites for potential applications in sensing, active drug delivery, energy storage and chemical separation.1 Tremendous interests have focused on the nucleation of crystalline polymers, structures of liquid crystals and selfassembling of block copolymers in inorganic nanoparticle systems.2-4 Polymer/CNT nanocomposites are being explored as promising materials because of the excellent electrical, mechanical and thermal properties of carbon nanotubes.5 The perpendicularly oriented nanostructures in polymer/CNT composites are often desired in many applications. However, a variety of challenges in the control of the highly oriented nanostructures of polymer/CNT composites emerged, including the perturbation of polymers to the formation of highly oriented nanostructures, losing the intrinsic excellent properties of individual CNTs, and low concentration of CNTs in nanocomposites. Numerous methods were utilized to achieve highly oriented nanostructures in polymer/CNT composites and to create functional polymer/CNT materials for energy storage and biomedical applications. These methods include the electrospinning of polymer/CNT mixture solution,6 nanotube cutting,7 mechanical stretch of polymer/CNT blend,8 the use of magnetic field.9 Single-walled carbon nanotubes (SWCNTs) in semicrystalline polymer matrices can template the crystallization of polymers through nucleating


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the polymer crystals.10-16 Li et al. reported the crystallization behavior of semicrystalline polymer in a SWCNT solution system.15 In this study, lamellae of polymers were oriented along a direction perpendicular to the long axis of SWCNT where carbon nanotubes act as nucleation agents. The formation of “shish-kebab” structures, where carbon nanotube and lamella are “shish” and “kebab”, respectively, is attributed to “soft epitaxial growth” of lamellae around rigid carbon nanotubes. CNTs acting as templates in a poly(ε-caprolactone) matrix direct the lamellae of poly(ε-caprolactone) growing parallel to the long axis of CNT(lamella normal perpendicular to the long axis of CNTs). In a polyethylene/SWCNT system, SWCNTs guide the growth direction of polyethylene crystals.16 In a separate work, Zhang and coworkers have observed the formation of orthorhombic unit cell structures of polyethylene crystals in the presence of multi-walled CNT (MWCNT) instead of monoclinic unit cell structures, which are well-known structures of polyethylene crystals observed in most cases.17 Subjecting a PEO/SWCNT composite film to mechanical drawing induces carbon nanotubes oriented along a draw direction, and the lamellae of PEO also show an orientation parallel to the draw direction.8 PEO is a semicrystalline polymer, a material showing great potentials in biomedical, pharmaceutical, and ion conductive applications. From an application standpoint, PEO has long been considered a viable solution to the development of solid electrolyte in energy storage device. 18,19

Aligning PEO lamellae and reducing the crystallinity of PEO in room temperature are

crucial to progress in this field. It is generally appreciated from previous studies on semicrystalline polymer/CNTs composites in which carbon nanotube can template the crystallization of semicrystalline polymers. For a PEO/CNT mixture system, previous studies have demonstrated that CNTs favor amorphous PEO domains and ions preferentially transport in amorphous PEO domains.8,20 These reports also demonstrated that the presence of CNT can lead


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to a reduction in the crystallinity of PEO. Many studies focus on the effect of CNTs randomly dispersed in polymer matrix on the crystallinity of semicrystalline polymer.21-25 In those polymer/CNT systems with extremely low concentration of CNTs, the crystallinity of semicrystalline polymer is relatively low as compared to pure polymer systems.23,25 The crystallization behavior of polymer, and concentration of CNTs and CNT orientation in polymer/CNT composites play crucial roles in the ion transport process of energy storage devices. The effects of CNT arrays oriented perpendicular to catalyst substrate on the crystallization of PEO and ion transport in PEO/CNT composites are not well investigated, although much progress in understanding the crystallization behavior of PEO in a PEO/CNT solution system has been achieved. Here, we perform a systematic study of the crystallization behavior of PEO in the presence of a highly oriented CNT array (CNT-A), and specifically focus on the effects of vertically aligned CNT arrays on the orientation of lamellae and PEO crystallinity, which are crucial material parameters in modulating the ion transport in PEO/CNT composites. We also explore the effects of these material parameters on the charge transfer resistance (Rct) and capacitive performance of PEO/CNT composites since these electrochemical properties are closely associated with the ion transport in PEO/CNT composites. EXPERIMENTAL DETAILS Sample preparation PEO (Mn = 2, 6, 10, and 20 kg/mol) was purchased from Aladdin Industrial Corporation. PEO with a total relative molecular mass of 35 kg/mol was purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.. MWCNT arrays were grown by a thermal chemical vapor deposition (CVD) method. The height of CNT array vertically grown on catalyst substrate is ca. 1mm. Detailed information about the growth of CNT array (CNT-A) is given in our previous


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studies.26 PEO/CNT array (PEO/CNT-A) composites were fabricated by immerging CNT arrays tethered to the catalyst surface of substrates in a 2.4% by mass PEO solution in water and then drying PEO/CNT-A at room temperature. The diameter of CNT obtained from transmission electron microscope (TEM) images (Figure S1) is 14±2 nm. After CNT arrays were picked up from the PEO solution, CNT arrays were removed from substrate. PEO/random CNT (PEO/CNT-R) composites were obtained by dispersing CNT power in a PEO solution and then drying PEO/CNT-R at room temperature. PEO/CNT-A and PEO/CNT-R composites were annealed at 100 °C for 10 min and then crystallized at 25 °C for 25 min in a vacuum oven. Characterization Optical images were carried out using a Zeiss (Axio. imager. A2) optical microscope and a Zeiss (AxioCam HRC) CCD. Transmission electron microscope (TEM, Tecnai G220 (FEI)) was utilized to capture CNT images. Scanning electron microscope (SEM) images were obtained using a Hitachi S-4700 (Hitachi Inc.). Small Angle X-ray Scattering (SAXS) Small angle X-ray scattering measurements were performed using a Bruker NANOSTAR SAXS instrument. Two-dimensional SAXS images were captured using a Hi-Star detector at a sample-detector distance of 105 cm. X-ray wavelength is 0.154 nm. X-ray incident beam is perpendicular to the growth direction of CNT arrays. The zero pixel of the 2D SAXS patterns was calibrated using silver behenate. Two-dimensional wide angle X-ray diffraction (WAXD) Two-dimensional X-ray diffraction experiments were conducted using a Bruker D8 Discover diffractometer with Cu Kα radiation (λ = 0.154 nm). The spectra were collected in a scan range of 10°< 2θ < 30°. 2D WAXD patterns were calibrated using R-Al2O3.


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Differential Scanning Calorimetry (DSC) A differential scanning calorimetry (DSC2010, TA Instrument, USA) is employed to measure the crystallization enthalpy, crystallization temperature and melting temperature of samples. The heating/cooling rate is 10 °C/min. Electrochemical measurements The electrochemical properties of PEO/CNT composites were obtained using a two-electrode "sandwich" configuration as shown in Figure S2. The size of PEO/CNT composites is 7.5 × 7.5 × 0.5 mm (length × width × height). Both Ag/AgCl reference electrode and a Pt counter electrode were connected with one piece of thin carbon cloth (current collector). PEO/CNT composite is a working electrode. 1-Butyl-3-MethyliMidazolium tetrafluoroborate is used as electrolyte. The PEO crystallinities in PEO/CNT-A and in PEO/CNT-A/1-Butyl-3MethyliMidazolium tetrafluoroborate are 48% and 43%, respectively. It indicates that the presence of 1-Butyl-3-MethyliMidazolium tetrafluoroborate did not have a significant influence on the crystallinity of PEO. Two PEO/CNT composite units are glued together by a layer of polyvinyl alcohol (PVA) hydrogel (PVA, 10.9% w/w) as a separator. Cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a RST4800F electrochemical workstation (Suzhou Risetest Electronic Co., Ltd., China). CV scans were obtained in a potential range from 0-0.8 V and the scan rate is 50 mV/s. EIS measurement frequency is from 0.01 Hz to 100 kHz. RESULTS AND DISCUSSION The morphological observations on pure PEO film and PEO/CNT-A composite are shown in Figure 1. Samples are annealed at 25 °C for 25 min. The diameter of CNT obtained from TEM images is 14±2 nm (Figure S1). Representative optical microscope images of pure PEO film and


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PEO/CNT-A composite indicate that the presence of CNTs has a significant influence on the morphologies of PEO crystals. For a pure PEO film (Figure 1a), the optical microscope image exhibits an isotropic morphology of PEO crystal, which is a characteristic of PEO crystals obtained during an isothermal crystallization processing. Well-defined boundary lines between adjacent PEO crystals are clearly observed. It is natural given the relatively fast growth rate of PEO crystals. For PEO/CNT-A composites (Figure 1b), we observe the formation of small PEO crystals in vertically aligned CNT arrays. The formation of large PEO spherulites is significantly suppressed. In the presence of CNT arrays, only small crystals are observed probably because relatively small intertube distances in CNT arrays suppress the formation of large PEO crystals. The presence of CNTs has a strong impact on the growth of PEO crystals. (a)

100μm (b)

50μm

Figure 1. The optical microscope images of (a) pure PEO film and (b) PEO/CNT-A composite. In our studies, PEO/CNT composites were fabricated by immerging CNT arrays tethered to the catalyst surface of substrates in a 2.4% by mass PEO solution in water for 5 min, and then drying CNT arrays at room temperature. The incorporation of PEO significantly increases the flexibility of PEO/CNT-A composites. Without any rigorous acid or sonication treatment, the PEO/CNT-A composites are easily removed from catalyst substrates. It is of practical importance in the applications of fragile CNT arrays. The scanning electron microscope (SEM) images (Figure 2) of pure CNT array, PEO/random CNT (PEO/CNT-R) and PEO/CNT-A composites show that the vertical orientation and integrity of CNT arrays retain in PEO/CNT-A composite. It is natural


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because the alignment of carbon nanotubes is an ideal condition for the overlap of van der Waals forces between nanotubes.27

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Figure 2. The SEM images of (a) pure CNT array, (b) PEO/CNT-R composite and (c) PEO/CNT-A composite. The Mn of PEO is 20 kg/mol. Next we specifically focus on the influence of vertically aligned CNT arrays on the crystallization of PEO. We consider whether CNTs can induce the orientation of PEO lamellae and influence the crystallinity of PEO. There is of great interest in investigating the impact of CNTs on the orientation of PEO lamellae and crystallinity in PEO/CNT composites because of the importance of the crystallinity and morphology of PEO crystals in applications, where PEO is being considered a solid electrolyte for various energy storage devices. We obtained information about PEO lamellae by using a small angle X-ray scattering technique. In our studies, an edge-on experimental setup, in which the growth direction of CNT arrays (the long axis direction of CNT) is perpendicular to X-ray incident beam (Figure 3), is employed to investigate the orientation of PEO lamellae. The representative SAXS observation of pure CNT array is showed in Figure 3a. The azimuthal angle, φ, anticlockwise increases with φ=0o at right equator (see Figure 3a). Two distinct clouds near φ = 0o and 180o at higher scattering vector are the signature of the aligned CNTs with the long axis normal to the catalyst substrate.28 The growth direction of CNT arrays (the long axis direction of CNT) is perpendicular to catalyst substrate. However, many CNTs are not perfectly oriented normal to catalyst substrate and some CNTs are tilt or curved as shown in Figure 2. Those curved or tilt CNTs give rise to a wide distribution of nanotube-to-nanotube repeat distance, which leads to two diffuse scattering peaks near φ = 0o


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and 180o at higher scattering vector. One-dimensional SAXS intensity as a function of scattering vector for CNT array obtained by integrating over rectangular slices with width × length = 50 pixels × 400 pixels around φ = 0o is shown in Figure 3e. A broad and weak peak is located at q = 0.65 nm-1. For randomly oriented PEO lamellae, an isotropic ring of scattering would be generated. As a reference case, SAXS measurement on a pure PEO sample is performed. For a pure PEO sample, 2D SAXS pattern shows two intense isotropic scattering rings. In the plot of 1D SAXS intensities as a function of scattering vector, two strong peaks, which arise from the first order ( 1q* = 0.29 nm-1) and second order (2q* = 0.57 nm-1) of PEO lamellae, are observed. It is indicative of randomly oriented lamellae in a pure PEO sample. The long period of PEO lamellae, Lo, is calculated from the relation, Lo = 2π/q*. For a PEO sample annealed at 25 oC for 25 min, the long period of PEO lamellae is 22 nm. A 2D SAXS pattern of PEO/CNT-A composite is shown in Figure 3c. We see two relatively intense clouds near φ = 0o and 180o at higher scattering vector. Corresponding 1D SAXS intensities as a function of scattering vector (Figure 3g) show a weak peak at 0.65 nm-1, which corresponds to the SAXS signature of CNTs oriented normal to catalyst substrate. We take a closer look at the 2D SAXS pattern of PEO/CNT-A composite. No obvious isotropic scattering rings from random PEO lamellae are observed in 2D SAXS pattern. If a large number of PEO lamellae are oriented with lamella normal parallel to the long axis of CNTs, periodic structures would be formed along the long axis direction of CNTs due to relatively long CNTs (ca.1 mm) and scattering peaks from periodic structures formed by PEO lamella and amorphous PEO would be observed at φ = 90o and 270o in 2D SAXS pattern. In our studies, no obvious peaks from PEO lamellae perpendicular to the long axis of CNTs (lamella normal parallel to the long axis of CNTs) are observed at φ = 90o and 270o in 2D SAXS pattern of PEO/CNT-A composite. It suggests that no significant amount of PEO


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lamellae are oriented with lamella normal parallel to the long axis of CNTs. The azimuthal (φ) dependence of intensity of SAXS pattern between φ = 90o and φ = 270o at q = (0.75 ±0.25) nm-1 for PEO/CNT-A is shown in Figure 3h. The anisotropic scattering intensities of SAXS are observed in this azimuthal plot, where the scattering intensities are suppressed at φ = 90o and φ = 270o. It is also indicative of no large number of PEO lamellae perpendicular to the long axis of CNTs. Previous studies have demonstrated that CNTs favor amorphous PEO domains and the amorphous segments of PEO chains preferentially surround carbon nanotubes in a PEO/CNT mixture system.8,13,20 It suggests that PEO lamellae are probably oriented with lamella normal perpendicular to the long axis of CNTs since amorphous PEO and PEO lamella are alternately arranged. However, the presence of CNTs has a strong impact on the growth of PEO crystals. The relatively small intertube distances in CNT arrays suppress the formation of periodic structures with amorphous PEO and PEO lamella.Therefore, no obvious scattering peaks associated with periodic structures of PEO with lamella normal perpendicular to the long axis of CNTs are observed at φ = 0o and φ = 180o. We also performed WAXS and DSC measurements to check the formation of PEO lamellae. In the representative 2D WAXS pattern of PEO/CNT-A composite (Figure 4a), we see two strong signature reflections of PEO crystals. 1D WAXS intensities show these two signature peaks located at 2θ = 19.04°and 24.39o, which arise from (021) and (032) reflections of PEO crystals with monoclinic unit cells, respectively.29 The heating and cooling DSC curves of PEO/CNT-A composite are shown in Figure 5. Distinct melting and crystallization peaks of PEO lamellae are observed. It is indicative of the formation of PEO crystals in PEO/CNT-A composite. WAXS and DSC measurements on PEO/CNT-A composites provided evidence for the formation of PEO crystals (Figure 4 and 5). The combination of the experimental results from SAXS, DSC and WAXS measurements suggests


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that in PEO/CNT-A composite, the PEO lamellae are indeed formed and probably oriented with lamella plane normal perpendicular to the long axis of CNTs. (a)

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Figure 3. 2D SAXS patterns of CNT array (a), pure PEO film (b) and PEO/CNT-A composite (c); (d) the schematic of PEO/CNT-A composite; 1D SAXS intensities of CNT array (e), pure PEO film (f) and PEO/CNT-A composite (g); (h) the azimuthal dependence of intensity of SAXS pattern between φ = 90o and φ = 270o at q = (0.75 ±0.25) nm-1 for PEO/CNT-A composite. The Mn of PEO is 20 kg/mol.

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Figure 4. 2D WAXS pattern (a) and 1D WAXS intensities (b) of PEO/CNT-A composite. The Mn of PEO is 20 kg/mol.


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(b) PEO2000

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Pure PEO PEO/CNT-A

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Figure 5. DSC heating and cooling curves of pure PEO (a,b), and PEO/CNT composites (c,d) with PEO molecule weights of 2, 6, 10, 20 and 35 kg/mol (denoted as PEO2000/CNT-A, PEO6000/CNT-A, PEO10000/CNT-A, PEO20000/CNT-A, PEO20000/CNT-R and PEO35000/CNT-A, respectively); (e)The crystallinity of PEO and (f) the lamellar thicknesses of PEO crystals in pure PEO samples and PEO/CNT-A composites. We perform DSC measurements to investigate the crystallinity of PEO in PEO/CNT-A composite, which is another crucial parameter in modulating the ion transport. Figure 5 shows the representative cooling and heating DSC curves of pure PEO samples, PEO/CNT-R and PEO/CNT-A composites. The crystallization and melting temperatures increase with the increase of PEO molecular weight. However, the crystallization and melting temperatures of PEO crystals in PEO/CNT-A and PEO/CNT-R composites decrease as compared to the corresponding values of pure PEO samples. The crystallization and melting temperatures of PEO crystals in PEO/CNT composites have shown 8.1±2.5 oC and 2.5±1.1

o

C decreases, respectively. From the

endothermic melting peaks of heating DSC curves, we obtain the PEO crystallinity of pure PEO samples and PEO/CNT composites shown in Figure 5e. The PEO crystallinity is evaluated from


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the relation, X = Δhm/Δh0  100%, where Δh0 is the melting enthalpy of a 100% crystalline PEO. A value of Δh0 = 197 J/g is used for the calculation.30,31 As compared to pure PEO samples, PEO/CNT composites show a significant reduction in the crystallinity. In a molecular weight range from 2 to 35 kg/mol, the crystallinity of pure PEO samples is between 84% and 88%. The corresponding values for PEO/CNT composite is between 48% and 60%. The reduction of PEO crystallinity in PEO/CNT-A composite with PEO molecular weight in a range from 2 to 35 kg/mol is 25%-37%. Some interesting studies have shown that the presence of CNTs can disrupt the crystallization process of semicrystalline polymers.8,20,32 The incorporation of CNTs has an evident impact on the crystallization kinetics of semicrystalline polymers.8,20 The CNTs acting as transport barriers significantly slow down the crystallization process of semicrystalline polymers. It explains the relatively low crystallinity of PEO in PEO/CNT composites. In the presence of CNT arrays, the crystallization and melting temperatures obviously decrease. The decrease of melting temperature of PEO crystals can be associated with a reduction in the thickness of PEO lamellae. As pointed out before by Kripotou et al.,30 the crystallization and melting temperatures of PEO crystals are closely related to the thickness of PEO lamellae. In our studies, the lamellar thickness of PEO is obtained from DSC measurement. Specifically, the lamellar thickness, L, of the PEO crystals is evaluated from the Gibbs−Thomson equation, L = (2σe  Tm°)/(ΔT  ΔH° ρc), where Tm°is the equilibrium melting temperature of PEO crystals (Tm°= 349 K), σe the surface free energy of lamellae (σe = 0.065 J/m2), ΔH° the specific bulk heat of fusion (ΔH° = 197 J/g), and ρc the density of PEO crystalline phase (ρc = 1.239 g/cm3), and ΔT the degree of supercooling, (ΔT = Tm° − Tm).30,33-35 The lamellar thicknesses of PEO crystals with PEO molecular weights in a range from 2 to 35 kg/mol are 9.19 nm -16.55 nm as shown in Figure 5f. It is noteworthy that if the crystallinity of PEO is known, the lamellar thickness can also be


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calculated from SAXS data by the relation, L=XLo. In our studies, the crystallinity of PEO samples with a molecular weight range from 2 to 35 kg/mol is between 84% and 88%. The lamellar thicknesses obtained from SAXS data are 18 nm - 19 nm. The lamellar thicknesses evaluated from the Gibbs−Thomson equation, where ΔT = Tm° − Tm, are relatively small as compared to the corresponding values obtained from SAXS. Also the lamellar thicknesses of PEO obtained from the Gibbs−Thomson equation are slightly smaller than the reported values obtained from SAXS measurement.36 As pointed out before by Zhou et al., the use of DSC as a tool to determine lamellar thickness is sensitive to the heating rate in DSC experiments and the parameters in the Gibbs-Thomson equation.37 The disagreement between the lamellar thicknesses obtained from these two approaches might be caused by the heating rate of DSC experiment due to the effect of thermal lag during DSC scan.37 The values of the thickness of PEO lamella reveal that relatively thin PEO lamellae are formed in PEO/CNT composite as compared to pure PEO samples. The presence of vertically aligned CNTs suppresses the thickening of PEO lamellae during a PEO crystallization process, and gives rise to decreases in crystallization and melting temperatures and a reduction in PEO crystallinity, which is of great interest from a standpoint using PEO as a solid electrolyte. We also consider the crystallization of PEO in PEO/CNT-R composite (PEO, Mn = 20kg/mol), the PEO crystallinity and thickness of PEO lamella are 74% and 13.72 nm, respectively. The presence of random CNTs also leads to decreases in PEO crystallinity and lamella thickness. The lamella thickness in PEO/CNT-R composite is slightly higher than the corresponding value in PEO/CNT-A composite. As compared to vertically aligned CNTs, the PEO crystallinity in PEO/CNT-R composite is slightly high.


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For PEO/CNT composites, the alignment of CNTs, PEO crystallinity and orientation of PEO lamellae are of practical importance in many applications, where PEO is utilized as a solid electrolyte. These factors are relevant to improve ion transport in PEO/CNT composites. Ions preferentially diffuse in amorphous PEO domains,13 an effect of great significance in the solid electrolyte of electronic materials. The decrease in the crystallinity of PEO can enhance ion transport. The amorphous segments of PEO chains preferentially surround carbon nanotubes.13 The high alignment of CNTs, low crystallinity of PEO and orientation of PEO lamellae along the long axis of CNTs are especially beneficial for ion transport. We next restrict our attention primarily to the ion transport of PEO/CNT composites. A continuous ion transport path through electrolyte is crucial for ion diffusion. The orientation of PEO lamellae along the long axes of vertically aligned CNTs favors ion transport because of a relatively low ion transport barrier. To demonstrate the effects of the alignment of CNTs and orientation of PEO lamellae on the ion transport, we measure the charge transfer resistance, Rct, of PEO/CNT composite, which is obtained by an electrochemical impedance spectroscopy measurement in a frequency range from 100 kHz to 0.01 Hz. We are particularly interested in EIS measurements at high frequencies, which provide detailed information about the Rct of PEO/CNT composites. The value of charge transfer resistance is extracted from a Nyquist plot and corresponds to the diameter of a semicircle at high frequencies in a Nyquist plot.38 The representative Nyquist plots of PEO/CNT composites are shown in Figure 6a. In order to gain insight into the effect of the orientation of CNTs on the ion transport of PEO/CNT composite, we compare the charge transfer resistance of vertically oriented CNT array incorporated with PEO to a PEO/CNT-R composite after an identical thermal treatment (at 25 °C for 25 min). The charge transfer resistance for PEO/CNT-R composite is 264.3 Ω, whereas the Rct of PEO/CNT-A composite is 25.3 Ω. The high alignment


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of CNTs and orientation of PEO lamellae along the long axis of CNTs improve the ion transport in PEO/CNT-A composites. Because the high alignment of CNTs, low crystallinity of PEO and orientation of PEO lamellae along the long axis of CNTs allow for efficient charge transport, they should have a large impact on the electrochemical performance of PEO/CNT composites. We carried out cyclic voltammogram measurements on PEO/CNT composites. The representative CV curves of PEO/CNT composites in a potential range from 0-0.8 V are shown in Figure 6b. We see a rectangular shape CV curves, which is a characteristic of an electric double-layer capacitive performance. The electrostatic double-layer capacitance from CNTs contributes to the total capacitance value of PEO/CNT composites. We obtain the specific capacitance of the PEO/CNT composite, Cv, using the relation, Cv = S/(2 × V × ΔU × v), where S, V, ΔU and v are the area of CV curve, scan rate, voltage window, and total volume of the device (0.75 cm× 0.75 cm× 0.28 cm = 0.158 cm3). The specific capacitances of PEO/CNT-A and PEO/CNT-R composites are 155 mF/cm3, and 33 mF/cm3, respectively. The high alignment of CNTs and orientation of PEO lamellae along the long axis of CNTs result in an increase in the specific capacitances of PEO/CNT-A composite. The electrochemical performance of PEO/CNT composites is consistent with our studies on the charge transfer resistance.

(a)

(b)

PEO20000/CNT-R PEO20000/CNT-A

Current (A)

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-Z''(Ohm)

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60

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PEO20000/CNT-R PEO20000/CNT-A

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Rct

20

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Z'(Ohm)

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250

500

Z'(Ohm)

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1000

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Figure 6. (a) Impedance spectroscopy of PEO/CNT composite in a frequency range from 0.01 Hz to 100 kHz and (b) typical CV curves of PEO/CNT composite at a scan rate of 50 mV/s.


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CONCLUSIONS We investigate the influence of CNTs on the orientation of PEO lamellae and PEO crystallinity. Our results indicate that vertically oriented CNTs acting as physical templates to direct the orientation of PEO lamellae. In the presence of oriented CNT arrays, PEO lamellae probably grow along a direction parallel to CNT axis. The vertically aligned CNTs play an important role in the decrease of PEO crystallinity. Their presence also results in the remarkable decreases of the crystallization and melting temperatures. Correspondingly, the thickness of PEO lamellae shows a reduction. There are positive consequences of this oriented PEO lamella and relatively low crystallinity. We learn from comparing our results on PEO/CNT-A to PEO/CNTR composites that the orientation of CNTs is actually important in improving the ion transport and capacitive performance. The PEO/CNT-A composite with PEO lamellae oriented parallel to the long axis of CNTs shows a much lower charge transfer resistance and better capacitive performance, which are of great practical importance in applications where the PEO is sought as a solid electrolyte for energy storage devices. It is also possible that these highly aligned CNT arrays incorporated with PEO might be useful as a nanocomposite with controlled directionality properties. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: TEM image of CNT and the schematic of electrochemical measurements setup. AUTHOR INFORMATION *Corresponding Author E-mail: [email protected].


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E-mail: [email protected]; Phone: +86-51265884716 ORCID Xiaohua Zhang: 0000-0002-3996-702X Zhaohui Yang: 0000-0003-3329-5311 ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21274103) and State Key Laboratory of Separation Membranes and Membrane Processes (Tianjin Polytechnic University, No.M2-201501). The authors also thank the Specially Appointed Professor Plan in Jiangsu Province (No. SR10800312 and SR10800215) and Project for Jiangsu Scientific and Technological Innovation team (2013). REFERENCES (1) Balazs, A. C.; Emrick, T.; Russell, T. P. Nanoparticle Polymer Composites:Where Two Small Worlds Meet. Science 2006, 314, 1107-1110. (2) Wang, W. D.; Huang, Z. Y.; Laird, E. D.; Wang, S. J.; Li, C. Y. Single-Walled Carbon Nanotube Nanoring Induces Polymer Crystallization at Liquid/Liquid Interface. Polymer 2015, 59, 1-9. (3) Yaroshchuk, O. V.; Dolgov, L. O. Electro-Optics and Structure of Polymer Dispersed Liquid Crystals Doped with Nanoparticles of Inorganic Materials. Opt. Mater. 2007, 29, 10971102. (4) Zhu, L. F.; Wang, H.; Shen, X. S.; Chen, L. Y.; Wang, Y. W.; Chen, H. Y. Developing Mutually Encapsulating Materials for Versatile Syntheses of Multilayer Metal-Silica-Polymer Hybrid Nanostructures. Small 2012, 8, 1857-1862.


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The schematic of PEO/CNT-A composite.


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Crystallization Behavior of Poly(ethylene oxide) in ... - ACS Publications

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