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Development of Effective Porosity in Carbon Nanofibers Based on Phase Behavior of Ternary Polymer Blend Precursors: Toward High-Performance Electrode Materials Seonmyeong Noh, Duong Nguyen Nguyen, Chul Soon Park, Yukyung Kim, Hye Jeong Kong, Saerona Kim, Semin Kim, Su-Mi Hur, and Hyeonseok Yoon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b02558 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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The Journal of Physical Chemistry

Development of Effective Porosity in Carbon Nanofibers Based on Phase Behavior of Ternary Polymer Blend Precursors: Toward High-Performance Electrode Materials Seonmyeong Noh,2 Duong Nguyen Nguyen,2 Chul Soon Park,2 Yukyung Kim,2 Hye Jeong Kong,2 Saerona Kim,2 Semin Kim,1 Su-Mi Hur,1,2 and Hyeonseok Yoon1,2,* 1

School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea. 2

Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea.

Corresponding Authors: *E-mail: [email protected]

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ABSTRACT Electrospinning and subsequent carbonization of ternary polymer blends led to many interesting and important composition-dependent characteristics. Ternary blends consisting of polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), and poly(N-vinyl-2-pyrrolidone) (PVP) exhibited unique phase separation behaviors, which were attributed to the disparities in intermolecular interactions among the constituent polymer pairs. In addition to this phase behavior, combination of the individual characteristics of each polymer as a carbon precursor led to unique morphologies and microstructural properties of the carbonized nanofiber products (CNPs). Representatively, use of PMMA as a continuous phase in the blend generated a bundle-like morphology, while the combination of PAN and PMMA/PVP as a continuous phase and dispersed phases, respectively, led to the development of open, effective porosity. These morphological and microstructural properties were correlated with the electrochemical properties of the CNPs. High specific discharge capacitances were achieved from the CNPs with open, effective porosity, even in highly viscous ionic liquid electrolytes of relatively large molecules. It is anticipated that precise control of the composition of ternary-blend precursors will result in further optimized properties for specific applications.


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Introduction The phase separation behavior of polymers has been one of the most important research topics in polymer science and technology.1,2 The combination of different polymers into a single material creates a diverse range of phase behaviors that affect the major properties of the material.3−5 In particular, the microphase separation of polymers at the nanoscale has significant influence on the properties of the resulting material.6,7 Block copolymers are representatively employed as models to investigate microphase separation behavior because they have well-defined segments with different sizes and chemical properties.8−13 The phase or self-assembly behaviors of block copolymers have been extensively studied for electronic,14−17 biological,18−21 emulsion,7,22,23 and membrane24−27 applications. However, the segregation behavior of polymer blocks is inherently different from the phase separation in homopolymer mixtures; thus, great demand remains for better understanding of the microphase behavior of homopolymer mixtures.28−30 From a technical perspective, various kinds of polymers can be combined into single materials in many ways, often generating unprecedented and unique products.31,32 Electrospinning is an efficient method that allows continuous production of sub-micrometer polymeric fibers.33 The microphase separation behavior of polymers has also been explored in the electrospinning process.34−36 As an example, polymer fibers having core-shell structures can be fabricated directly using a co-annular nozzle.37,38 However, recent efforts have developed a single-nozzle technique relying on the microphase separation of binary polymer blends, requiring a careful balance of the main variables such as viscosity and miscibility.39−42 Hence, an understanding of the microphase behavior of different polymers allows for the rational design and efficient fabrication of various functional materials with desired structures and properties.43,44 However, the investigation of more complex systems containing more than two polymers in the presence of solvents is limited, despite the meaningful progress made for binary polymer blends.45,46 In this work, we systematically investigate how the phase behaviors of electrospun nanofibers consisting of ternary polymer blends affect the morphologies of the final carbonized nanofiber products ACS Paragon Plus Environment


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(CNPs). Polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), and poly(N-vinyl-2-pyrrolidone) (PVP) were blended in an organic solvent, where upon phase segregation behaviors arose from different inter-polymer interactions. Electrospinning of the ternary blends facilitated the inspection of the phase segregation of the ternary polymer blend at the nanoscale. Along with the phase behaviors, the inherent characteristics of the individual polymers as carbon precursors had significant influence on the morphologies and microstructural properties of the CNPs. Finally, the electrochemical properties of the CNPs were explored, providing insight into the structural factors most important for the application of such materials.


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Experimental Section Materials. PAN (Mw: 150,000), PVP (Mw: 40,000), PMMA (Mw: 120,000) were purchased from Sigma-Aldrich. Dimethylformamide (DMF, ≥99.5%) and N-methyl-2-pyrrolidone (NMP, 99%) were obtained from Acros Organics. Sulfuric acid (95%) was obtained from OCI Co., Ltd. As an ionic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF6]) was purchased from C-TRI Co., Ltd. Whatman® cellulose membrane (pore size: 8 µm) was also used. Nanofiber preparation and carbonization. To prepare the precursor solutions for electrospinning, PAN, PMMA, and PVP were dissolved at different weight ratios in DMF (2.5 mL) and then vigorously mixed together for 2 h. The as-obtained mixed solution (7.5 mL) was electrospun at a flow rate of 1.2 mL h−1 with an applied voltage of 20 kV. The electrospun fibers were calcined at 280 °C for 2 h in air and then carbonized for 1 h at 800 °C in a nitrogen atmosphere, which yielded the final CNPs. Electrochemical measurements. The CNPs were immobilized on stainless steel (60 mm2) with the aid of a polyvinylidine fluoride (PVDF) binder (14 wt%, containing 7 wt% conductive filler). The CNPs (2 mg) were mixed with the PVDF/NMP solution (15 µL) containing the conductive filler. The resulting mixture was pasted on the stainless steel and allowed to dry in a vacuum oven at room temperature. Cyclic voltammetry (CV) and galvanostatic charge/discharge experiments were performed in a threeelectrode cell (100 mL) containing 1 M sulfuric acid solution (60 mL) as an electrolyte, and using a Pt counter electrode and Ag/AgCl reference electrode. For the ionic liquid electrolyte, a Ag/Ag+ reference electrode was used. All data were collected more than five times and average values were provided with error ranges. Calculations. All quantum chemistry calculations were performed with the ORCA 3.0.3 program package using the Avogadro interface.47 Full geometry optimizations of the model oligomers were performed using density functional theory (DFT) with the Becke ’88 exchange and Perdew ’86 correlation (BP86) generalized gradient approximation (GGA) functional and def2-SVP basis set. The Resolution-of-Identity approximation was employed to speed up the calculation without any significant error. A very tight self-consistent field (SCF) convergence and damping (slow convergence) were ACS Paragon Plus Environment


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required to reduce the noise in the gradients. The single-point energy for the optimized structures was evaluated using basis set superposition error (BSSE). In addition, the solvent effect of DMF was also considered using the continuum solvation model COSMO. To calculate the intermolecular interactions between the model oligomers, the potential energy curve scan was performed with a scan rate of 0.2 Å/step in the Z-matrix. Characterization. The morphological characteristics of the CNPs were analyzed using a JEOL JSM7500F scanning electron microscope (SEM) and a TECNAI F20 ST transmission electron microscope (TEM). The phase separation of the blends was observed using a LEICA DM750P optical microscope. Argon sorption experiments were conducted at 77.4 K with ASAP 2020 micromeritics. The viscosity of the polymer blend was measured using a Brookfield DV-II+ viscometer. Raman spectroscopy analysis was performed using a Nanobase XperRam 200 spectrometer.


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Results and Discussion

(Figure 1)

To investigate the phase behavior of ternary polymer blends in a solvent, the polymers of PAN, PMMA, and PVP were dissolved with DMF solvent. The polymers were blended at different weight ratios (Figure 1) and the resulting mixtures were electrospun under an optimized condition in order to produce nanofibers. During the electrospinning, nanofibers were continuously formed with solvent evaporation. The microstructures of the nanofibers were determined by the phase behaviors of the three polymers. The nanofibers were subject to heat treatment (max. 800 °C) in an inert atmosphere to obtain carbonized nanofibers. PAN is a typical carbon precursor with a high char yield (>50%).48 PVP can be converted to carbon species under the appropriate conditions (char yield ~10%),49 while PMMA experiences thermal degradation at high temperatures. The carbonization of the ternary polymer nanofibers allowed estimations of the microphase behaviors of the complicated polymeric systems because the PMMA domains in the nanofibers were pyrolyzed. The PVP-rich domains could also be identified after washing the electrospun polymer nanofibers with water (Figure S5).

(Figure 2)

Figure 2 displays SEM images of the CNPs obtained from each polymeric blended sample. The diameters of the nanofibers range from 100 nm to 700 nm and several remarkable morphologies are observed that are not found in the non-carbonized electrospun polymer nanofibers. Prior to carbonization, the electrospun products revealed a strong dependence of nanofiber diameter on the weight percentages of the components in the ternary polymer blends. As clearly shown in Figures S3 and S4, the diameters of the resulting polymer nanofibers increased with increasing PVP or PMMA concentrations, probably because of the variations in the viscosity of the blend solution depending on ACS Paragon Plus Environment


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the polymer concentration. However, the changing profiles of the nanofiber surfaces were quite different in each case. Increasing the PVP content in the ternary polymer blend decreased the surface roughness, whereas increasing the PMMA content created corrugated surface patterns along the long axes of the fibers. Importantly, carbonization caused morphological evolutions in the electrospun polymer nanofibers consisting of ternary blends. In particular, as seen in the image of product d, the electrospun polymer nanofibers containing high PMMA content show a unique bundle-like morphology, in which individual nanofibers consist of aggregates of many thinner nanofibers, after carbonization. Remarkably, the diameters of the thinner nanofibers are as small as a few tens of nanometers. PMMA would serve a continuous phase while other polymers would be dispersed phases. Considering the morphologies of products h and l, it is clear that the thermal decomposition of PMMA contributes to the formation of this bundle-like morphology. On the other hand, it is found that PVP inhibits the function of PMMA as a kind of porogen. In products f, k, and p, the weight ratio of PMMA-to-PVP is maintained at 1:1 and the bundle-like morphology is not observed. Additionally, the amount of PVP relative to PMMA is increased in the order of products d < h < l < p, where the bundle-like morphology of product d finally disappears in product p. These results indicate that PVP prevents the microphase separation of PMMA in the ternary blend. Compared to products b and c, however, product g shows elongated large pores at higher density (see high-magnification images in Figure S1 and S2), indicating that the use of an appropriate, small amount of PVP in the ternary blend further facilitates the development of the porous structure. Products a, e, i, and m are made with PAN/PVP binary blends without PMMA. The content of PVP is increased in the order of products a < e < i < m. No remarkable porous structures are found, implying that PAN and PVP are compatible with each other and well converted to solid carbon structures at high temperatures. Finally, the CNPs prepared from the ternary blends with high PVP contents, namely products m, n, and o, feature inter-connected fiber structures in which fibers are fused together at contact points, as well as relatively smooth surfaces. At high PVP contents, the surfaces of the electrospun fibers mainly consist of PVP (see Figure S5). Therefore, it is


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considered that the surface morphologies of products m, n, and o originate from the characteristics of PVP as a carbon precursor.

(Figure 3)

Figure 3 exhibits high-magnification SEM and TEM images of the key CNPs, allowing further interpretation of the microphase behavior mentioned earlier. The ternary polymer blends are viscous, so the phase behavior would not reach thermodynamic equilibrium within a short period. In principle, therefore, the phase behavior determining the microstructures of the electrospun products would be governed by both thermodynamic and kinetic factors. In this work, all polymer blends and electrospun nanofibers were made under the same blending and electrospinning conditions to exclude differences in kinetic effects. From a thermodynamic point of view, it is known that PMMA is incompatible with PAN in dilute solution, while PVP is miscible with either PAN or PMMA.50,51 It is therefore anticipated that PVP acts to lower the interfacial tension between PAN and PMMA, at least at a macroscopic level. However, microphase separation at the scale of less than 100 nm can occur even between PVP/PAN and PVP/PMMA. Complex, multiple-phase separations can occur at different length scales of a few nanometers to micrometers; this complicates the prediction of phase behaviors of the ternary blend. Several remarkable carbonized structures were selected from the products exhibited in Figure 1, and their high-magnification SEM and TEM images are given in Figure 3. The electron microscopic analysis of the sub-micrometer fibers with nanometer feature sizes provides insight into the phase separation of the ternary polymer blend. Considering the observation of the electrospun polymer nanofibers and CNPs, the following hypotheses can be suggested. i) In the case of PAN/PMMA blending, the polymers can be segregated into spherical domains of one phase dispersed in the matrix of another by one variety of high interfacial tension. Each phase can be co-axially oriented during the ACS Paragon Plus Environment


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electrospinning process, thus creating the bundle-like morphologies shown in products d and h. ii) In the presence of PVP, the interfacial tension between PAN and PMMA is reduced. Thus, the degree of phase segregation in the fiber tends to decrease with increasing PVP (see products d, h, and p).

(Figure 4)

Optical microscopy allowed the direct observation of macro- to intermediate-scale phase separation in the ternary polymer blends. Figure 4 presents optical micrographs of the PMMA/PVP blend solution used for making CNPs. It is clear that there is a high interfacial tension between PAN and PMMA, creating the spherical dispersed phase. Notably, the size of the segregated spherical domains is decreased with increases in the amount of PVP (see the images for products d, h, and n in Figure 4). The blend for product d shows spherical PMMA segregated phases approximately twice as large (domain size ~47 µm) as those in the blend for product h (domain size ~24 µm). In the case of product n, PVP acts as a continuous phase, and spherical segregated domains with diameters of 7−18 µm are found as dispersed phases. In both the PAN/PVP (product m) and PMMA/PVP blends, at the optical microscopic level, phase separation is not observed, supporting the hypothesis of PVP reducing the interfacial tension between PAN and PMMA. No remarkable microstructures are observed in product m from the blend without PMMA, as seen in Figure 4 (the 50/50 wt% PMMA/PVP blend was incompatible with the electrospinning process to yield nanofibers). Comparison of the optical micrographs for products j and g demonstrates that an increase in PMMA content increases the sizes of the spherical segregated domains. Considering the morphologies of products j and g in Figure 3, the component of the segregated domain corresponds to PMMA and produces porous microstructures, depending on the PMMA content. Product g shows many pores elongated along the long axis of the fibers, whereas it is difficult to find pores in product j.


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(Figure 5) (Table 1)

The phase behavior of ternary polymer blends, especially microphase separation, is strongly affected by intermolecular interactions among the constituent polymers. DFT was employed to calculate the intermolecular interactions, for which pentamer and octamer molecules were constructed as miniature models for the three polymers of PAN, PMMA, and PVP. A potential energy scan was performed for each pair of oligomers to calculate the dependence of the interaction energy on the distance between the model oligomers. The interaction energy was calculated using the following equation: E(r)int = E(A/B)a – [E(A)a + E(B)a]

(1)

where E(r)int is the interaction energy at the distance r, and E(A)a, E(B)a, and E(A/B)a are the absolute energies of the model oligomers A and B, and of the A/B ensemble, respectively, taken from a singlepoint energy calculation on the geometries optimized at the distance r. If the energy is negative, attractive interaction is dominant; in the opposite case, repulsive interactions are dominant. The potential energy curve profile for all models (Figure 5) shows clearly how the interaction between model oligomers varies as a function of the separation distance. Pentamers and octamers reveal similar potential energy curve profiles. When far apart (more than 10 Å), the two oligomers do not interact and ଴ the initial potential energy of the system is zero (‫ܧ‬௜௡௧ ≅ 0). As the oligomers approach each other, the

repeated units of the individual oligomers freely rotate around the molecule backbone and the constituent atoms are attracted to each other because of their differences in electronegativity. Such attractions may be considered long-range electrostatic interactions. In response to these attractions, the interaction energy is decreased, becoming more negative as the distance between the oligomers ௔௧௧௥௔௖ decreases. In other words, attraction is stronger than repulsion (‫ܧ‬௜௡௧ < 0). This trend continues until the ௠௜௡ interaction energy reaches a minimum value (‫ܧ‬௜௡௧ ). At this distance (rmin), the two-oligomer system is

the most stable with the lowest energy. However, if the distance between the oligomers decreases further, the interaction energy increases steeply and eventually becomes positive, indicating that ACS Paragon Plus Environment


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௥௘௣௨௟ significant repulsion (‫ܧ‬௜௡௧ > 0) occurs between the oligomers. This description is suitable for

interactions between PMMA/PVP and PAN/PVP, but is inadequate for the PAN/PMMA pair. In other words, the PAN/PMMA pair shows completely different profiles compared to those of the PAN/PVP ௠௜௡ (PAN/PMMA) occurs at distances of approximately 9 and PMMA/PVP pairs. In detail, it appears that ‫ܧ‬௜௡௧

Å; Eint fluctuates around zero at closer distances, indicating equilibrium between attractive and repulsive interactions. Of course, finally, the interaction energy reaches positive values, although the distances are ௠௜௡ slightly different between the pentamer and octamer pairs. Table 1 summarizes the ‫ܧ‬௜௡௧ and rmin derived ௠௜௡ value at the shortest rmin, followed by from Figure 5. The PAN/PVP oligomer pair has the lowest ‫ܧ‬௜௡௧ ௠௜௡ the PMMA/PVP oligomer pair. The PAN/PMMA oligomer pair has the highest ‫ܧ‬௜௡௧ value at the longest

rmin, indicating that this pair is the most unstable.

(Figure 6) (Table 2)

The electron density was calculated to identify hydrogen bonding in the oligomer pair, as a typical intermolecular interaction. Possible hydrogen bonds were predicted by considering the electronegativity and separation distance of constituent atoms, as illustrated in Figure 6 (see green dotted lines). The PMMA/PVP and PAN/PVP oligomer pairs exhibit overlapping electron clouds for components that can participate in hydrogen bonds. First, the carbonyl group of PVP can form two concurrent hydrogen bonds with the α- and β-hydrogens of PAN at distances of 2.12 and 2.16 Å. The carbonyl group of PVP also forms a hydrogen bond with the ester-methyl group of PMMA at a distance of 2.29 Å. Meanwhile, the PAN/PMMA oligomer pair has no overlapping electron clouds between candidate components that can interact. Interaction between the nitrile group of PAN and the ester-methyl group of PMMA may be possible. However, the separation distance is as far as 2.55 Å, indicating that the possible intermolecular interaction between PAN and PMMA is weak. As a result, PVP can make hydrogen bonds with PAN ACS Paragon Plus Environment

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and PMMA, where the strength of the hydrogen bonding proceeds in the order of PMMA/PVP < PAN/PVP. These results well support the data obtained from the potential energy scan (Figure 5). Namely, the thermodynamic stability of the polymer pairs is proportional to the strength of the intermolecular interactions, causing the different miscibility behaviors of the polymers. The phase behavior of different polymers in a blend can be determined by the intermolecular interactions. The phase separation in the ternary polymer blends occurs by the disparity in intermolecular interactions among the PAN/PMMA, PAN/PVP, and PMMA/PVP pairs, creating the composition-dependent morphologies of the post-carbonization products, as exhibited in Figure 2. The intermolecular interactions are experimentally estimated by measuring the viscosity of the polymer blends. Table 2 presents the viscosities of the precursor solutions used for fabricating the key CNPs. The molecular weight of PAN was similar with that of PMMA. The ternary blends for products d and h revealed high viscosities. The viscosity of the PAN/PVP blend solution (e.g., the blend for product m) was also higher than that of the PAN-only solution (e.g., the blend for product a) even though the molecular weight of PVP was 73.3% and 66.7% lower than those of PAN and PMMA. Compared with homopolymer PAN solution, the enhanced viscosities of the polymer blends demonstrated the intermolecular interaction between the polymers.

(Table 3)

Considering the results obtained so far, control of phase separation in the polymer blends by changing the composition can effectively create CNPs with desirable morphologies and properties. As observed earlier via electron microscopy, the CNPs from the ternary polymer blends show compositiondependent morphologies. To provide deeper insight into the microstructural characteristics, the textural properties of the CNPs were examined using argon sorption experiments. The measured adsorption– desorption isotherms (see Figure S7) exhibited forms combining those of type I and type IV isotherms as defined by IUPAC, indicating that the CNPs were micro- and meso-porous materials (see Figure S7). ACS Paragon Plus Environment


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Specifically, a steep rise in the adsorbed amount was observed at low relative pressures of less than approximately 0.5 × 10−1, indicating the presence of micropores. The hysteresis loop originating from capillary condensation in mesopores was then observed. Major textural parameters that affected the physical properties, such as specific surface area and pore volume, were derived from the argon adsorption isotherm. Table 3 summarizes the textural parameters of the key CNPs. The effective surface area and pore volume depend on the diameter of the CNPs. The highest-concentration polymers in the blends for products a, d, and m were PAN, PMMA, and PVP, respectively, indicating that these polymer served as continuous phases in the respective blends. The specific surface area of product a obtained from only PAN precursor mainly arose from the large micropore volume, rather than from the mesopore volume. In fact, it is well known that the heat treatment of PANs under inert atmospheres yields microporous carbons. Thus, products g and j prepared with high PAN contents also showed high micropore volume portions in their total pore volumes. The textural properties of product d provide information on the role of PMMA as a porogen. Product d showed a high mesopore volume portion in the total pore volume, which was consistent with product h obtained from a high-PVP-content blend. Consequently, it is clear that PMMA contributed to the generation of mesopores in the CNPs. Next, product m revealed an interesting characteristic of PVP as a carbon precursor (Figure S6). The mesopore volume of product m was much smaller than that of product a, indicating that the high content of PVP against PAN caused a serious decrease in the mesopore volume. The individual characteristics of the three polymers as carbon precursors were reflected in the textural properties of the other CNPs. When a small amount of PMMA was added to the blend for product m, the portion of mesopores increased, as seen in the textural properties of product n. Compared with product d, product h had an increased mesopore volume due to the small amount of PVP, despite the decreased PMMA content. In Figure 4, the segregated spherical PMMA domains as the dispersed phase were observed from the blends for products g, h, j, m, and n. The comparison of products g (domain size ~25 µm in the blend solution) and j (domain size ~6 µm in the blend solution) provided that an increase in PMMA amount relative to PVP resulted in a higher surface area and larger mesopore volume. The comparison of ACS Paragon Plus Environment

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products m (no PMMA, no segregated domain) and n (domain size 7–18 µm in the blend solution) more clearly provided that PMMA contributed to the enhanced surface area and mesopore volume. The specific surface area increased in the order of products j < g < a, although the diameters of products a, g, and j were similar. However, the pore volume increased in the order of products j < g ≈ a, due to changes in the mesopore volume by PMMA and PVP.

(Figure 7)

CV analysis allows another interpretation of the microstructures of the CNPs. CV curves were recorded in the range of −0.2 to 0.8 V (vs. Ag/AgCl) using 1 M sulfuric acid as the electrolyte. The CV curves of all CNPs exhibited distorted rectangular shapes (see Figure S8), indicating that electric double-layer capacitance served as a major electrochemical component. The area of the CV curve was proportional to the effective surface area of the CNP. Figure 7 plots the calculated integrated surface areas of the CV curves. Considering simply the key CNPs, the integrated areas of the CV curves increase in the order of products m < n < p < h < d < j < a < g. Notably, the CV curve areas are inconsistent with the trend (p < h < m < d < j < n < g < a) in the specific surface areas of the products, as suggested in Table 3. Specifically, the CV curve area of product a is only 80% that of product g, while the specific surface area of product a is 1.3 times larger than that of product g. In addition, product m has the smallest CV curve area despite having a specific surface area of 4.2 and 2.0 times larger than those of products p and h, respectively. These results can be interpreted in terms of the effective surface areas that the electrolyte molecules can access. The CNPs may have open and closed pores, of which only the open pores can contribute to the effective surface area. Clearly, PAN and especially PVP are unfavorable to the generation of open pores in the CNPs as carbon precursors. However, the combination of PAN and/or PVP with PMMA enables the increase of the proportion of open pores in the total pores. Therefore, the disparity between the trends in the BET surface area and the ACS Paragon Plus Environment


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CV curve area originate from the composition-dependent effective porosity. The CNPs were further characterized by Raman spectroscopy (Figure S9). The relative intensity of “G” peak was found to increase in the order a < j < carbonized PVP-only ≈ p ≈ n < m ≈ d < g < h (Table S2). The overall trend in the Raman spectroscopy data indicated that the use of PVP-only as a carbon precursor can product higher crystalline carbonized products than that of PAN-only. In addition, it was found that the formation of the unique nanostructured morphology by using the ternary blend precursor was favorable to make the CNPs more crystalline.

(Figure 8)

A comparative experiment was designed to provide further insight into the porosities of the CNPs. The electrochemical properties of products a, d, g, and h were measured using electrolytes of different molecular sizes and viscosities, namely the small-molecule sulfuric acid and the large-molecule ionic liquid [HMIM][PF6], as displayed in Figure 8. First, the integrated area of the CV curves recorded in sulfuric acid increased in the order of h < d < a < g, while that of the CV curves recorded in [HMIM][PF6] increased in the order of a < d < h < g. The charge transfer resistance of the CNPs in the electrolytes was also examined using electrochemical impedance spectroscopy, the result of which was commensurate with that of the CV curve area (see Figure S12 and Table S3). A similar trend was observed in the charge/discharge behaviors of the CNPs. The highly microporous product a showed the smallest CV curve area and specific capacitance in the ionic liquid, in contrast with its behavior in the sulfuric acid electrolyte. On the other hand, product g maintained the best electrochemical properties for both electrolytes.40,52-54 It was also found that product g had excellent capacitance retentions of more than 95% over 10,000 cycles in both electrolytes (Figure S13). The sizes of the hydrated cation and anion of sulfuric acid are as small as 2.8 and 3.8 Å, respectively.55 The ionic liquid is highly viscous (560 cP at 25 °C) and the sizes of HMIM+ and PF6− are estimated as 11.4 and 5.1 Å, respectively.56,57 Therefore, it is believed that the excellent electrochemical properties of product g arose from the high ACS Paragon Plus Environment

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proportions of open micro- and meso-pores. The unique bundle-like morphologies of products d and h also permitted better electrochemical behaviors in the ionic liquid than in the sulfuric acid solution. Despite its small specific surface area (only ~33% of the specific surface area of product g), in particular, product h yielded a specific capacitance reaching ~90% that of product g.



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Conclusions Interesting and important composition-dependent characteristics were found through the electrospinning and subsequent carbonization of ternary polymer blends. First, from a structural perspective, the morphologies of the CNPs depended on the components serving as continuous phases. Notably, when PMMA was employed as a major component in the ternary blend, a unique bundle-like morphology was generated (e.g., product h). It was calculated that the phase separation of the ternary polymer blends arising from disparities in intermolecular interactions caused these compositiondependent characteristics. The characteristics of the individual polymers as carbon precursors also affected the microstructures of the CNPs. PAN yielded microporous carbons with ineffective porosities, while PMMA acted as a porogen that formed mesopores in the CNP. PVP itself yielded nonporous, solid, and smooth carbons. However, the appropriate combination of PAN and/or PMMA with PVP caused the development of open and effective porosity (e.g., product g). These structural properties were directly correlated with the electrochemical properties of the CNPs. Products with open and effective porosities yielded good specific discharge capacitances, even in highly viscous and relatively largemolecule ionic liquid electrolytes. Ultimately, this study can be extended to combinations of various carbon precursor polymers, which can lead to unprecedented, unique properties for the advanced application of carbon nanofibers.


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ASSOCIATED CONTENT Supporting Information. Low-/high-magnification SEM and TEM images of all polymer nanofiber precursors and CNPs (Fig. S1 to S5), SEM images of polymer nanofiber precursors before and after PVP removal (Fig. S5), morphology and textural properties of carbonized PVP-only nanofibers (Fig. S6 and Table S1), argon sorption isotherms for CNPs (Fig. S7), CV curves used for plotting Fig. 7 (Fig. S8), Raman spectra (Fig. S9 and Table S2), charge/discharge curves used for plotting Fig. 8c and 8f (Fig. S10 and S11), impedance data (Fig. S12 and Table S3), and capacitance retention plots (Fig. S13). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (NRF-2015R1A2A2A01007166).



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REFERENCES (1) Robensin, L. Historical Perspective of Advances in the Science and Technology of Polymer Blends. Polymers 2014, 6, 1251−1265. (2) Barner-Kowollik, C.; Goldmann, A. S.; Schacher, F. H. Polymer Interfaces: Synthetic Strategies Enabling Functionality, Adaptivity, and Spatial Control. Macromolecules 2016, 49, 5001−5016. (3) Schmitt, M.; Zhang, J.; Lee, J.; Lee, B.; Ning, X.; Zhang, R.; Karim, A.; Davis, R. F.; Matyjaszewski, K.; Bockstaller, M. R. Polymer Ligand–Induced Autonomous Sorting and Reversible Phase Separation in Binary Particle Blends. Sci. Adv. 2016, 2, e1601484. (4) Eastwood, E.; Viswanathan, S.; O’Brien, C. P.; Kumar, D.; Dadmun, M. D. Methods to Improve the Properties of Polymer Mixtures: Optimizing Intermolecular Interactions and Compatibilization. Polymer 2005, 46, 3957−3970. (5) Anselmo, A. S.; Lindgren, L.; Rysz, J.; Bernasik, A.; Budkowski, A.; Andersson, M. R.; Svensson, K.; van Stam, J.; Moons, E. Tuning the Vertical Phase Separation in Polyfluorene: Fullerene Blend Films by Polymer Functionalization. Chem. Mater. 2011, 23, 2295−2302. (6) Lee, Y.; Choi, H.; Kim, M. -S.; Noh, S.; Ahn, K. -J.; Im, K.; Kwon, O. S.; Yoon, H. NanoparticleMediated Physical Exfoliation of Aqueous-Phase Graphene for Fabrication of ThreeDimensionally Structured Hybrid Electrodes. Sci. Rep. 2016, 6, 19761; DOI: 10.1038/srep19761. (7) Ahn, K. -J.; Lee, Y.; Choi, H.; Kim, M. -S.; Im, K.; Noh, S.; Yoon, H. Surfactant-Templated Synthesis of Polypyrrole Nanocages as Redox Mediators for Efficient Energy Storage. Sci. Rep. 2015, 5, 14097; DOI: 10.1038/srep14097. (8) Ruzette, A. -V; Leibler, L. Block Copolymers in Tomorrow’s Plastics. Nat. Mater. 2005, 4, 19−31.


Page 21 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Epps, T. H. III; O’Reilly, R. K. Block Copolymers: Controlling Nanostructure to Generate Functional Materials – Synthesis, Characterization, and Engineering. Chem. Sci. 2016, 7, 1674−1689. (10) Wang, Z.; Li, T.; Schulte, L.; Almdal, K.; Ndoni, S. Photocatalytic Nanostructuring of Graphene Guided by Block Copolymer Self-Assembly. ACS Appl. Mater. Interfaces 2016, 8, 8329−8334 (11) Nandan, B.; Horechyy, A. Hairy Core−Shell Polymer Nano-objects from Self-Assembled Block Copolymer Structures. ACS Appl. Mater. Interfaces 2015, 7, 12539−12558. (12) He, C.; Stoykovich, M. P. Photopatterning of Cross-Linkable Epoxide-Functionalized Block Copolymers and Dual-Tone Nanostructure Development for Fabrication Across the Nano- and Microscales. Small 2015, 11, 2407−2416. (13) Kato, T.; Hillmyer, M. A. Functionalized Nanoporous Polyethylene Derived from Miscible Block Polymer Blends. ACS Appl. Mater. Interfaces 2013, 5, 291−300. (14) Kim, H. -C.; Park, S. -M.; Hinsberg, W. D. Block Copolymer Based Nanostructures: Materials, Processes, and Applications to Electronics. Chem. Rev. 2010, 110, 146−177. (15) Guo, C.; Lin, Y. -H.; Witman, M. D.; Smith, K. A.; Wang, C.; Hexemer, A.; Strzalka, J.; Gomez, E. D.; Verduzco, R. Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation. Nano Lett. 2013, 13, 2957−2963. (16) Lee, K. H.; Zhang, S.; Gu, Y.; Lodge, T. P.; Frisbie, C. D. Transfer Printing of Thermoreversible Ion Gels for Flexible Electronics. ACS Appl. Mater. Interfaces 2013, 5, 9522−9527. (17) Khan, M. A.; Bhansali, U. S.; Almadhoun, M. N.; Odeh, I. N.; Cha, D.; Alshareef, H. N.; HighPerformance Ferroelectric Memory Based on Phase-Separated Films of Polymer Blends. Adv. Funct. Mater. 2014, 24, 1372−1381.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

(18) Pitto-Barry, A.; Barry, N. P. E. Pluronic® Block-Copolymers in Medicine: from Chemical and Biological Versatility to Rationalization and Clinical Advances. Polym. Chem. 2014, 5, 3291−3297. (19) Blanazs, A.; Armes, S. P.; Ryan, A. J. Self-Assembled Block Copolymer Aggregates: From Micelles to Vesicles and their Biological Applications. Macromol. Rapid Commun. 2009, 30, 267−277. (20) Obermeyer, A. C.; Olsen, B. D. Synthesis and Application of Protein-Containing Block Copolymers. ACS Macro Lett. 2015, 4, 101−110. (21) Duan, Z.; Zhang, Y.; Zhu, H.; Sun, L.; Cai, H.; Li, B.; Gong, Q.; Gu, Z.; Luo, K. StimuliSensitive Biodegradable and Amphiphilic Block Copolymer-Gemcitabine Conjugates SelfAssemble into a Nanoscale Vehicle for Cancer Therapy. ACS Appl. Mater. Interfaces 2017, 9, 3474−3486. (22) Muñoz-Bonilla, A.; Ali, S. I.; del Campo, A.; Fernández-García, M.; van Herk, A. M.; Heuts, J. P. A. Block Copolymer Surfactants in Emulsion Polymerization: Influence of the Miscibility of the Hydrophobic Block on Kinetics, Particle Morphology, and Film Formation. Macromolecules 2011, 44, 4282−4290. (23) George, S. R.; Champagne-Hartley, R.; Deeter, G. A.; Campbell, J. D.; Reck, B.; Urban, D.; Cunningham, M. F. Amphiphilic Block Copolymers as Stabilizers in Emulsion Polymerization: Effects of the Anchoring Block Molecular Weight Dispersity on Stabilization Performance. Macromolecules 2017, 50, 315−323. (24) Shim, H. W.; Ahn, K. -J.; Im, K.; Noh, S.; Kim, M. -S.; Lee, Y.; Choi, H.; Yoon, H. Effect of Hydrophobic Moieties in Water-Soluble Polymers on Phyiscal Exfoliation of Graphene. Macromolecules 2015, 48, 6628−6637.


Page 23 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Lee, J. E.; Lee, Y.; Ahn, K. -J.; Huh, J.; Shim, H. W.; Sampath, G.; Im, W. B.; Huh, Y. -I.; Yoon, H. Role of Co-Vapors in Vapor Deposition Polymerization. Sci. Rep. 2015, 5, 8420; DOI: 10.1038/srep08420. (26) Bhalani, D. V.; Bera, A.; Chandel, A. K. S.; Kumar, S. B.; Jewrajka, S. K. Multifunctionalization of Poly(vinylidene fluoride)/Reactive Copolymer Blend Membranes for Broad Spectrum Applications. ACS Appl. Mater. Interfaces 2017, 9, 3102−3112. (27) Panapitiya, N. P.; Wijenayake, S. N.; Nguyen, D. D.; Huang, Y.; Musselman, I. H.; Balkus, Jr. K. J.; Ferraris, J. P. Gas Separation Membranes Derived from High-Performance Immiscible Polymer Blends Compatibilized with Small Molecules. ACS Appl. Mater. Interfaces 2015, 7, 18618−18627. (28) Salim, N. V.; Hameed, N.; Hanley, T. L.; Guo, Q. Microphase Separation Induced by Competitive Hydrogen Bonding Interactions in Semicrystalline Triblock Copolymer/Homopolymer Complexes. Soft Matter 2013, 9, 6176−6184. (29) Tirumala, V. R.; Pai, R. A.; Agarwal, S.; Testa, J. J.; Bhatnagar, G.; Romang, A. H.; Chandler, C.; Gorman, B. P.; Jones, R. L.; Lin, E. K. et al. Mesoporous Silica Films with Long-Range Order Pre-pared from Strongly Segregated Block Copolymer/Homopolymer Blend Templates. Chem. Mater. 2007, 19, 5868−5874. (30) Liu, G.; Thomas, C. S.; Craig, G. S. W.; Nealey, P. F. Integration of Density Multiplication in the Formation of Device-Oriented Structures by Directed Assembly of Block Copolymer– Homopolymer Blends. Adv. Funct. Mater. 2010, 20, 1251−1257. (31) Herwig, G.; Hornung, C. H.; Peeters, G.; Ebdon, N.; Savage G. P. Porous Double-Layer Polymer Tubing for the Potential Use in Heterogeneous Continuous Flow Reactions. ACS Appl. Mater. Interfaces 2014, 6, 22838−22846.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 41

(32) Li, Y.; Shi, J. Hollow-Structured Mesoporous Materials: Chemical Synthesis, Functionalization and Applications. Adv. Mater. 2014, 26, 3176−3205. (33) Nguyen, D. N.; Yoon, H. Recent Advances in Nanostructured Conducting Polymers: from Synthesis to Practical Applications. Polymers 2016, 8, 118. (34) Lee, J. E.; Park, S. J.; Kwon, O. S.; Shim, H. W.; Jang, J.; Yoon, H. Systematic Investigation on Charge Storage Behaviour of Multidimensional Poly(3,4-ethylenedioxythiophene) Nanostructures. RSC Adv. 2014, 4, 37529−37535. (35) Choi, H.; Yoon, H. Nanostructured Electrode Materials for Electrochemical Capacitor Applications. Nanomaterials 2015, 5, 906−936. (36) Liu, W.; Zhu, L.; Huang, C.; Jin, X. Direct Electrospinning of Ultrafine Fibers with Interconnected Macropores Enabled by in Situ Mixing Microfluidics. ACS Appl. Mater. Interfaces 2016, 8, 34870−34878. (37) Nayani, K.; Katepalli, H.; Sharma, C. S.; Sharma, A.; Patil, S.; Venkataraghavan, R. Electrospinning Combined with Nonsolvent-Induced Phase Separation to Fabricate Highly Porous and Hollow Submicrometer Polymer Fibers. Ind. Eng. Chem. Res. 2012, 51, 1761−1766. (38) Jiang, Y.; Fang, D.; Song, G.; Nie, J.; Chenc, B.; Ma, G. Fabrication of Core–Shell Nanofibers by Single Capillary Electrospinning Combined with Vapor Induced Phase Separation. New J. Chem. 2013, 37, 2917−2924. (39) Song, C.; Du, J.; Zhao, J.; Feng, S.; Du, G.; Zhu, Z. Hierarchical Porous Core-Shell Carbon Nanoparticles. Chem. Mater. 2009, 21, 1524−1530. (40) Abeykoon, N. C.; Bonso, J. S.; Ferraris, J. P. Supercapacitor Performance of Carbon Nanofiber Electrodes Derived from Immiscible PAN/PMMA Polymer Blends. RSC Adv. 2015, 5, 19865−19873. ACS Paragon Plus Environment

Page 25 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Peng, M.; Li, D.; Shen, L.; Chen, Y.; Zheng, Q.; Wang, H. Nanoporous Structured Submicrometer Carbon Fibers Prepared via Solution Electrospinning of Polymer Blends. Langmuir 2006, 22, 9368−9374. (42) Wei, Q.; Xiong, F.; Tan, S.; Huang, L.; Lan, E. H.; Dunn, B.; Mai, L. Porous One-Dimensional Nanomaterials: Design, Fabrication and Applications in Electrochemical Energy Storage. Adv. Mater. 2017, 29, 1602300. (43) Wan, Y.; Shi, Y.; Zhao, D. Supramolecular Aggregates as Templates: Ordered Mesoporous Polymers and Carbons. Chem. Mater. 2008, 20, 932−945. (44) Kumar, K.; Boonstra, M.; Loos, K. Synthesis of Carbon Microrings Using Polymer Blends as Templates. RSC Adv. 2015, 5, 33294−33298. (45) Kolahchi, A. R.; Ajji, A.; Carreau, P. J. Surface Morphology and Properties of Ternary Polymer Blends: Effect of the Migration of Minor Components. J. Phys. Chem. B 2014, 118, 6316−6323. (46) Wang, J.; Lessard, B. H.; Maric, M.; Favis, B. D. Hierarchically Porous Polymeric Materials from Ternary Polymer Blends. Polymer 2014, 55, 3461−3467. (47) Neese, F. The ORCA Program System. WIREs Comput. Mol. Sci. 2012, 2, 73−78. (48) Kwon, O. S.; Kim, T.; Lee, J. S.; Park, S. J.; Park, H. -W.; Kang, M.; Lee, J. E.; Jang, J.; Yoon, H. Fabrication of Graphene Sheets Intercalated with Manganese Oxide/Carbon Nanofibers: Toward High-Capacity Energy Storage. Small 2013, 9, 248−254. (49) Krishnamoorthy, V.; Elumalai, G.; Rajiv, S. Environment Friendly Synthesis of Polyvinylpyrrolidone Nanofibers and Their Potential Use as Seed Coats. New. J. Chem. 2016, 40, 3268. (50) Ellis, B.; Smith, R. Polymers: A Property Database, 2nd ed. 2009, CRC Press, pp 390.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

(51) Guo, Q.; Huang, J.; Li, X. Miscibility of Poly(N-vinyl-2-pyrrolidone) with Poly(hydroxyether of phenolphthalein) and Polyacrylonitrile. Eur. Polym. J. 1996, 32, 423−426. (52) Wang, G.; Qian, B.; Wang, Y.; Dong, Q.; Zhan, F.; Qiu, J. Electrospun Porous Hierarchical Carbon Nanofibers with Tailored Structures for Supercapacitors and Capacitive Deionization. New. J. Chem. 2016, 40, 3786−3792. (53) Tran, C.; Kalra, V. Fabrication of Porous Carbon Nanofibers with Adjustable Pore Sizes as Electrodes for Supercapacitors. J. Power Sources 2013, 235, 289−296. (54) Le, T. H.; Yang, Y.; Yu, L.; Gao, T.; Huang, Z.; Kang, F. Polyimide-Based Porous Hollow Carbon Nanofibers for Supercapacitor Electrode. J. Appl. Polym. Sci. 2016, 133, 43397. (55) Zhong, C.; Deng, Y.; Hu, W.; Qiao, J.; Zhang, L.; Zhang, J. A Review of Electrolyte Materials and Compositions for Electrochemical Supercapacitors. Chem. Soc. Rev. 2015, 44, 7484−7539. (56) Espinosa-Marzal, R. M.; Arcifa, A.; Rossi, A.; Spencer, N. D. Ionic Liquids Confined in Hydrophilic Nanocontacts: Structure and Lubricity in the Presence of Water. J. Phys. Chem. C 2014, 118, 6491−6503. (57) Feng, G.; Qiao, R.; Huang, J.; Dai, S.; Sumpter, B. G.; Meunier, V. The Importance of Ion Size and Electrode Curvature on Electrical Double Layers in Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 1152−1161.


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Table Titles Table 1. Summary of main parameters derived from Figure 5. Table 2. Viscosities of the precursor solutions used for fabricating the selected CNPs (products a, d, m, g, j, h, n, and p). Table 3. Major textural properties of the key CNPs.



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Figure Captions Figure 1. Ternary plot showing the compositions of the PVP/PMMA/PAN blend systems dissolved in DMF. The compositions are given as weight ratios. Figure 2. SEM images of CNPs (products a to p) obtained through the heat treatment of electrospun polymer nanofibers prepared at different weight ratios of PAN/PMMA/PVP: a) 1:0:0, b) 0.75:0.25:0, c) 0.5:0.5:0, d) 0.25:0.75:0, e) 0.75:0:0.25, f) 0.74:0.13:0.13, g) 0.62:0.3:0.08, h) 0.25:0.63:0.12, i) 0.5:0:0.5, j) 0.62:0.08:0.3, k) 0.5:0.25:0.25, l) 0.3:0.55:0.15, m) 0.25:0:0.75, n) 0.25:0.12:0.63, o) 0.3:0.15:0.55, and p) 0.33:0.33:0.33 (scale bar 500 nm). Figure 3. SEM and TEM images of selected CNPs with remarkable structural characteristics (products a, d, m, g, j, h, n, and p) (scale bar 100 nm). The ternary plot displays the corresponding compositions (red) for the selected CNPs. Figure 4. Optical micrographs of PMMA/PVP (0.5:0.5) blend and the polymer blends used for fabricating products d, g, h, j, m, n, and p (scale bar 25 µm). Images were tinted green for clarification of phases. Figure 5. Calculated interaction energy of the model oligomer pairs as a function of separation distance. Potential energy scan is run from points 4.0 Å to 10.0 Å for each pair of two model oligomers: (a) pentamer pairs and (b) octamer pairs. Figure 6. Geometries of the model pentamer pairs, showing possible hydrogen bonds (green dotted lines), optimized at rmin (blue, red, dark gray, and pale gray indicate nitrogen, oxygen, carbon, and hydrogen atoms, respectively). The electron densities (below) were illustrated using the ORCA program with the isosurface value of 0.01 (red, white, and blue indicate negative, positive, and neutral charges, respectively). Figure 7. Ternary plot of the integrated areas of the CV curves of all CNPs measured at a scan rate of 25 mV s−1.


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Figure 8. Electrochemical characteristics of CNPs in different electrolytes, namely (a-c) 1 M sulfuric acid solution and (d-f) [HMIM][PF6]: (a,d) CV curves recorded at a scan rate of 25 mV s−1, (b,e) galvanostatic charge/discharge curves at a current density of 0.2 A g−1, and (c,f) specific capacitances plotted against current density.



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Table 1. Model oligomers Pentamer Pair

Octamer

rmin

௠௜௡ ‫ܧ‬௜௡௧

rmin

௠௜௡ ‫ܧ‬௜௡௧

[Å]

[10−2 eV]

[Å]

[10−2 eV]

PAN/PMMA

9.2

−2.2

8.8

−2.2

PAN/PVP

5.6

−12.5

6.4

−22.4

PMMA/PVP

6.8

−6.8

7.0

−9.5


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Table 2. Viscosity Product [cP] a

42.0 ± 5.6

d

504.0 ± 13.7

g

76.0 ± 3.9

h

446.4 ± 17.6

j

66.9 ± 4.4

m

140.3 ± 12.0

n

193.1 ± 6.9

p

186.0 ± 7.3



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Table 3.

Sareaa Product

a

Vmicrob

Vtotal −2

3

−2

[m2 g−1]

[10 cm g−1]

[10 cm g−1]

a

416.7

17.9

15.3

d

235.2

13.9

g

316.4

h

Vmesoc 3

−2

[10 cm g−1]

Vmicro/Vtotal Vmeso/Vtotal 3

[10−1]

[10−1]

2.6

8.5

1.5

8.9

5.0

6.4

3.6

17.8

11.9

5.9

6.7

3.3

105.5

9.3

4.1

5.2

4.4

5.6

j

266.0

12.7

10.0

2.7

7.9

2.1

m

212.9

8.6

8.0

0.6

9.3

0.7

n

293.3

14.6

11.2

3.4

7.7

2.3

p

50.7

6.1

1.9

4.2

3.1

6.9

Specific surface area calculated by the Brunauer–Emmett–Teller (BET) method.

b c

Micropore volume calculated by Horváth–Kawazoe (HK) method.

Mesopore (2−50 nm) volume calculated by Barrett–Joyner–Halenda (BJH) adsorption method.


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ⓐ1.00

0.00

ⓔ

0.25

P PV

ⓚ

0.50

0.75

ⓜ

0.00

ⓝ

ⓖ

N

ⓙ

ⓘ

ⓑ0.75

ⓕ

PA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ⓒ0.50

ⓟ

ⓞ 0.25

PMMA

ⓛ 0.50

ⓗ

ⓓ0.25

0.75

Figure 1



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Figure 2


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Figure 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4


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a)

0.6

PAN/PMMA PMMA/PVP PAN/PVP

0.5

Eint (eV)

0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 4

5

6

7

8

9

10

r( ) b)

0.6

PAN/PMMA PMMA/PVP PAN/PVP

0.5 0.4

Eint (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 4

5

6

7

8

9

10

r( )

Figure 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6


Page 38 of 41

ⓐ1.00

0.00

V A g−1

ⓔ

0.25

ⓙ

ⓘ

ⓜ

0.00

ⓝ

ⓑ0.75 ⓖ

ⓚ

0.50

0.75

ⓕ

ⓒ0.50

ⓟ

ⓞ 0.25

6.76 6.23 5.70 5.17 4.64 4.11 3.58 3.05 2.52

N PA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PV P

Page 39 of 41

ⓛ 0.50

ⓗ

ⓓ0.25

0.75

PMMA

Figure 7



b)

E (V vs. Ag/AgCl)

2 −1

I (A g )

c) 0.8

4

0 -2 -4

a d g h

-6

a d g h

0.6

200

0.4 0.2

150

100

50

0.0 0

-8 0.2

0.4

0.6

0.8

0

200

400

E (V vs. Ag/AgCl)

600

800

1000

1200

e)

+

2 1 −1

0 -1

a d g h

-2 -3 0

1

+

E (V vs. Ag/Ag )

2

2

3

4

5

−1

f) 80

a d g h

1

0

-1

-2 -1

1

Current density (A g )

2

E (V vs. Ag/Ag )

3

-2

0

Time (s)

a d g h

−1

0.0

Capacitance (F g )

-0.2

d)

a d g h

−1

6

Capacitance (F g )

a)

I (A g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 41

60

40

20

0 0

500

1000

1500

2000

2500

Time (s)

Figure 8


0

1

2

3

4 −1

Current density (A g )

5

Page 41 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Development of Effective Porosity in Carbon Nanofibers Based on

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