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carbon nanotube materials
Polymer 137 (2018) 346e357
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Polymer journal homepage: www.elsevier.com/locate/polymer
Low-temperature graphitic formation promoted by confined interphase structures in polyacrylonitrile/carbon nanotube materials Navid Tajaddod, Heng Li, Marilyn L. Minus* Department of Mechanical and Industrial Engineering, Northeastern University, 360 Huntington Avenue, 334 Snell Engineering Center, Boston, MA 021155000, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2017 Received in revised form 14 December 2017 Accepted 2 January 2018 Available online 6 January 2018
In this work polyacrylonitrile (PAN)/single wall carbon nanotube (SWNT) composites with 50 wt% SWNT loading were studied. A previously studied solvent-based phase separation process was used to enhance PAN-SWNT crystallization conditions and interphase formation within the composites. Two types of SWNT species were used here to produce two composites materials with well-ordered PAN-SWNT interphase structures. All precursor composite films were stabilized and subsequently carbonized at temperatures ranging from 900 to 1500 C under various carbonization times. Wide-angle X-ray diffraction and electron microscopy were used to analyze the structural change in these precursors during carbonization. It was found that ordered graphitic structures were formed using specific temperature/time conditions as early as 900 C. This low-temperature graphitic structure can be tracked directly to the PAN interphase structure within the precursors. This paper discusses the linkage between precursor PAN-SWNT interphase formation and subsequent low-temperature graphitic formation. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Carbon fiber Interphase Heat treatment
1. Introduction Polyacrylonitrile (PAN) is widely used as a precursor for carbon fiber production [1e7]. Typically, a three-step heat treatment process is performed in order to convert PAN precursors to carbon fibers. These steps are: (i) thermal stabilization, (ii) carbonization, and (iii) optional graphitization. During stabilization between a temperature range of 200e300 C, a ladder structure will be formed due to cyclization of PAN molecules [8,9]. Carbonization processes increase the carbon content through formation of a nearamorphous microcrystal structure [10]. Carbonization is usually performed within a temperature range from 1000 to 1700 C [5,6]. Optional graphitization will further enhance the ordering of the carbon structure by formation of graphite and this process occurs at temperatures greater than 2000 C and up to 3000 C [5,6]. By increasing the heat treatment temperatures and applying tension to the fibers during pyrolysis, ordering of the carbon structure increases [11e13]. Semi-crystalline carbon is referred to turbostratic, while highly ordered carbon is graphitic. The inter-layer distance between turbostratic carbon planes varies and is on
* Corresponding author. E-mail address: [email protected] (M.L. Minus). https://doi.org/10.1016/j.polymer.2018.01.007 0032-3861/© 2018 Elsevier Ltd. All rights reserved.
average > 0.34 nm [14,15], while the distance between perfect graphite planes is 0.3345 nm [15,16]. Conversion to this graphitic structural form in the material improves the mechanical properties of carbon fibers. New research on carbon fiber development and processing have been of interest toward decreasing the gap between commercial and theoretical tensile strength properties for the materials [17,18]. An increase in tensile strength properties is expected to occur in the presence of oriented graphitic regions which can be increased throughout the fibers, while minimizing defective carbonaceous regions. To this end many studies have focused on the use of filler particles such as carbon black [19,20], graphene [21,22], and carbon nanotubes (CNTs) [23e27] to modified the PAN matrix in order to improve precursor properties and subsequent carbon fiber performance. In addition, the use of these carbonaceous materials may also enhance the conductive properties of these materials broadening their applications [19e33]. There have been several recent studies focused on understanding the role of controlling the precursor polymer structure toward producing a specific variation of carbonaceous structure upon pyrolysis by addition of CNTs to the PAN matrix [18,34e37]. CNTs have been shown to induce ordered interphase [38,39] which can promote the ordering of the PAN ladder structure after stabilization [18,35]. This interphase structure leads to PAN conversion to graphitic structures at ~1100 C
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[18,35,37,40,41]. However, while it has been shown that graphitic structure can form at 1100 C at the interphase, studies focused on understanding the mechanism behind this conversion process and subsequent structure are still needed. In this paper, PAN/CNT composites (with 50 wt% CNT total loading) were prepared to purposely isolate interphase regions in the composite. Several studies have looked at much lower CNT loading in the composite [18,35,37,40,41]. However, the direct linkage between filler content and PAN conversion during carbonization heat treatment has been difficult to observe. The choice of 50 wt% CNT loading in this work is based on the previously studied method of preparing PAN/CNT films by phase separation [42]. This technique allows for the development of regions in the composite with high interphase content [42]. At 50 wt% CNT the level of interfacial content is sufficient to directly study carbonization behavior. In addition, the bulk PAN and CNT interaction can also be distinguished from the interphase. This work is performed to confirm the value of introducing ordered PAN interphase regions to the precursor to control graphitic forms in the final carbon material. This work also shows the role of polymer confinement effects in early carbonaceous and graphitic formation. 2. Materials and method 2.1. Materials The PAN used in this work is a poly(acrylonitrile-co-methacrylic acid) random copolymer with methacrylic acid content of 4 wt% (Mw ~513,000 g/mol), obtained from Exlan Co. Japan. Two types of CNT materials were used, (i) PT (purified single wall carbon nanotubes (SWNT), ~94.5%, Continental Carbon Nanotechnologies, Inc.) and (ii) SW (SWNT, ~90%, SouthWest Nano Technologies, Inc. batch SG76). 2.2. Solution processing and film fabrication 2.2.1. Dispersion preparation The materials used in this study are PAN/CNT composite films. For hybrid polymer/CNT buckypaper (hPBP) fabrication, PAN powders were first dissolved in 90 C DMF (obtained from Sigma Aldrich) at a concentration of 250 mg/L. CNTs of equal amount were then dispersed in the polymer solution for 24 h via a bath sonicator (Fisher FS30, frequency 43 kHz, power 150 W). For control buckypaper (BP) fabrication, CNTs were dispersed in DMF using similar sonication conditions. 2.2.2. BP and hPBP films preparation After sonication, the PAN/CNT dispersion was subjected to a solution-based shear crystallization process by stirring the dispersion at 90 C. Simultaneously vacuum distillation was applied to remove half the volume of solvent at a controlled time and temperature. Subsequently, this overall system was cooled down to room temperature (~25 C). A non-solvent for PAN (i.e., water [42]) was added into dispersion at various solvent:non-solvent (S:NS) ratios. The two ratios chosen in this work were 7:1 and 1:2, respectively. This S:NS treatment during solution processing of the PAN/CNT films help to isolate regions, where PAN has enhanced interfacial interaction with the CNT. The isolation of this interphase region is important for studying subsequent carbonization process effects. The final PAN/CNT dispersions were then filtered through a nylon membrane (0.45 mm pore size obtained from Millipore) to form the PAN/CNT hPBP. The films made using PT-SWNT with S:NS ratios of 7:1 and 1:2, respectively, were named PT-1 and PT-2, accordingly. The films made with SW-SWNT were named SW-1 and SW-2, respectively at similar S:NS ratios of 7:1 and 1:2. A
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free-standing hPBP was removed from the filter paper after drying in a vacuum oven and further characterized to understand the effect of processing on the structure of the hybrid films. Control CNT BPs were also fabricated using a similar filtration process for comparison. 2.2.3. Heat-treatment processes A Lindberg/Blue M™ Mini-Mite™ tube furnace equipped with a quartz tube (diameter 1 inch, obtained from Quartz Scientific Inc.) was used for heat-treatment to conduct both stabilization and carbonization procedures. Two flow meters were used to control the inlet and outlet gas flow through the furnace. The entire system was sealed and the outlet gas flow was directly released into a venting system. The film samples were held in compression using a customized sample holder throughout both heat treatment processes. For stabilization (in air), the temperature was (i) ramped up at 1 C/min from room temperature (~25 C) to 250 C or 300 C; (ii) maintained isothermally for 10 h; and (iii) gradually decreased to room temperature. For carbonization (in argon), the temperature was (i) ramped up at 5 C/min from room temperature to 900, 1000 or 1100 C; (ii) maintained isothermal for 20 or 40 min at 900 C, 5 or 20 min at 1000 C, and 5 min at 1100 C; and (iii) gradually decreased to room temperature (~25 C). In both processes, the required air or argon gas flow was maintained constant at 3000 ccm until the chamber was cooled down to room temperature (~25 C). To confirm the existence of structural features the early onset graphite (i.e., formed between 900 and 1100 C), the pyrolysed composite films were further heat-treated at up to 2100 C for 40 min in vacuum using high temperature furnace (Red Devil vacuum furnace WEBB 124). Fig. 1 summarizes the time-temperature profile for the stabilization, carbonization, and graphitization procedures. 2.2.4. Sample characterization Morphology characterization was performed using a Zeiss Supra 25 field emission scanning electron microscope (SEM) (operating voltage 5 kV). All film samples were fractured and mounted to a 90 pin stub with the fractured end facing up for SEM observation. Precursor (non-carbonized) samples were coated with a thin gold/ palladium layer (15e20 nm) for image purposes using a Gatan high-resolution ion beam coater. Wide-angle X-ray diffraction (WAXD) was performed on a Rigaku RAPID II equipped with a curved detector X-ray diffraction (XRD) system with a 3 kW sealed tube X-ray source (operating voltage 40 kV and current 30 mA). XRD curve fitting and analysis was performed using software PDXL 2 (version 2.0.3.0) and 2DP (version 1.0.3.4). Raman spectroscopy was conducted on a Jobin Yvon LabRam HR800 (laser wavelength 532 nm). Differential scanning calorimetry (DSC) analysis was conducted by heating samples from 40 to 400 C at a heating rate of 1 C/min. DSC curves were examined using the TA Instrument Universal Analysis software. 3. Results and discussion 3.1. Precursor films A non-solvent (water) is added during filtration to process the hPBPs in order to isolate PAN-CNT interactions. PT-1 and SW-1 hPBPs are fabricated using a 7:1, S:NS ratio, while PT-2 and SW-2 materials utilize a 1:2, S:NS ratio. Based on previous computational and experimental studies, it was found that as the nonsolvent increases, PAN-CNT isolated interactive regions also increase and these changes are observed in the composite morphology. The presence of the water leads to higher interaction between the PAN and CNT leading to a ‘trapping’ of the CNT by the
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Fig. 1. Graphical representation for the heat treatment procedures used in this work for (a) stabilization, (b) carbonization, and (c) graphitization of the samples. (dotted lines illustrate the lower time or temperature heat treatments that were used for this research).
polymer chains [42]. This is a size dependent process, and as a result smaller diameter bundles are isolated by the polymer. Crosssections of fabricated precursor PT-1 and PT-2 hPBP films are shown in Fig. 2a and b. PT-1 film cross-sections show a heterogeneous structure, where CNTs are randomly distributed throughout the films. However, the PT-2 films show a two-layered structure; A PAN-rich layer (PR-L) which is dominated by PAN and contains a lower concentration of CNTs, and a CNT-rich layer (CR-L) that contains little polymer but a very high concentration of CNT. From a morphological point of view, the structure of the CR-L in the PT-2 sample is similar to the overall structure for the PT-1 film, while the PR-L is much denser and shows even distributions of the CNT. Although for both films that same amount of PAN and CNT is used, in general, the PT-2 film is thicker than the PT-1 samples. This thickness difference may be related to the loose and compact structures present in the CR-L and PR-L regions, respectively. As mentioned, hPBPs from two CNT types were fabricated. Similar non-layered and layered structures are observed for SW-1 and SW2 films and are shown in Fig. 2c and d. The SW-2 two-layered films also exhibit a PR-L and CR-L and film thickness trends are similar to those for the PT-based hPBPs. Based on electron microscopy analysis, the formation of the layered structures in the PT-2 and SW-2 hPBPs were found to affect PAN morphology. It was also observed that during film processing (by filtration) the approximate PAN weight loss percentage for PT-1, SW-1, PT-2 and SW-2 was found to be 27.3, 25.0, 11.1 and 10.0%, respectively. Therefore, in general, the concentration of PAN in the PT-2 and SW-2 films is slightly higher than for the PT-1 and SW-1 samples. Considering the presence of PAN in all films, WAXD analysis was performed in order to investigate the actual structural features of the PAN polymer in the composites. The WAXD micrographs for the PT-1, PT-2, SW-1 and SW-2 films are shown in Fig. 3. It is observed that the PT-2 and SW-2 films show a narrow and predominate peak at 2q of 16.7 associated with crystalline PAN (dotted box in Fig. 3). The broad peak observed for the PT-1 and SW-1 films is consistent with a more amorphous PAN morphology (dotted box in Fig. 3). This WAXD data suggest that for the single layered films the PAN predominately retains an amorphous structure, whereas in the two-layered films the polymer is more crystalline. Based on the electron microscopy analysis, it is clear that PAN in the two-layered films is isolated in the PR-L, this WAXD data also suggests that the PAN in this region is highly ordered. As shown in Fig. 4, the SWNT in the PR-L are more homogenously distributed as compared to the CR-L region. This WAXD analysis shows that the improved dispersion of CNT in the PR-L region allows for PAN to form more ordered domains. Previous research has shown that well-dispersed SWNT can promote interphase formation in PAN [34,39]. These PT-2 and SW-2 hPBPs allow for the isolation of such ordered regions in the composite material, which enables a close study of how ordered interphase PAN affects heattreatment toward turbostratic carbon and graphitic formations. Based on both microscopy and WAXD analysis a cross-sectional schematic was developed to illustrate the significance of the layered structure in the hPBPs (Fig. 5). The schematic shows that the distribution and subsequent ordered (interphase) formation of PAN in the vicinity of the CNT differs for the one-layer films (PT-1 or SW1) as compared to the two-layered films (PT-2 or SW-2). These structural features ultimately affect the overall PAN structural distribution within the precursor films, and this is expected to influence the formation of the carbonized structures upon heat-treatment. Isolation of the interphase regions is important for tracking the transformation of this region during carbonization, and its eventual role in mechanical and electrical properties of the overall composite. In addition, the different types of CNT used here (i.e., PT and SW) are found to slightly affect the ordered (interphase) structure formed
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Fig. 2. SEM images of cross-sections of fabricated films: (a1) PT-1 film, (a2) zoom in area of the boxed region in a1 image; (b1) PT-2 film, (b2) zoom in area of boxed region shows CRL, (b3) zoom in area of boxed region shows PR-L; (c1) SW-1 film, (c2) zoom in area of boxed region in c1 image, (d1) SW-2 film, (d2) zoom in area of boxed region shows CR-L, (d3) zoom in area of boxed region shows PR-L.
Fig. 3. WAXD spectra of the PT-BP, PT-1, PT-2, SW-BP, SW-1 and SW-2 films. PT-1 and SW-1 films show a more broad PAN (110) peak, PT-2 and SW-2 materials exhibit a sharper crystalline PAN (110) peak. Several broad peaks pertaining to the SWNT are also observed (see arrows).
(Fig. 3), which will in turn influence carbonization [34]. Based on the schematic in Fig. 5, the one-layered films exhibit a large distribution of CNT bundle size with surrounding PAN matrix. While some interphase PAN may be present, the matrix is
dominated by amorphous PAN as indicated by WAXD. The twolayer schematic show both CR-L and PR-L regions. It should be noted that for the film processing conditions used, the transition from the CR-L to PR-L layer is abrupt. The CR-L region is dominated by CNT of varying bundle size exhibiting an average of ~20.48 nm [42]. Based on previous study of these films, the CR-L exhibits more open structure as compared to the PR-L region, where voids are present and a lower content PAN is found [42]. These structural characteristics can be observed in Fig. 4. Comparatively, the PR-L was found to exhibit much smaller average bundle size (~6.25 nm) with less variation, lower void content, and higher presence of PAN [42]. Based on WAXD analysis this polymer is also found to be ordered (crystalline), and this is associated with interphase PAN. Some disordered PAN matrix is also expected to be present. However, unlike the one-layered films (no CR-L or PR-L distinction), the ordered interphase PAN dominated based on WAXD analysis. By using this visual scheme, the heat-treatment of the one-layer and two-layered films are tracked as a function of morphology, in order to understand the effects of pyrolysis. Due to the structure of the PAN hPBPs films produced here, DSC analysis was first performed to anticipate stabilization effects. DSC thermographs for the PT-1 and PT-2 films under air show that the heat evolved for the PT-1 films is higher than that of the PT-2 samples (Fig. 6). The focus of the DSC study is to understand differences in stabilization for the interfacial PAN-SWNT regions (PT2) as compared to regions of PAN in the general vicinity of the SWNT as a part of the composite (PT-1). This DSC results suggests that the CNT in the PT-2 films interact with PAN in a way that delays the PAN stabilization process. In other words, the PAN present in the PT-2 films exhibits higher thermal stability in comparison to the PAN in the PT-1 films. This effect may be related to the increase of
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Fig. 6. DSC thermographs of PT-1 and PT-2 films stabilized at the heating rates of 1 C/ min.
processing (cyclization) at this stage. Completion of exothermic reaction is found to occur at ~300 C and higher, while the reaction begins at ~250 C and 300 C to examine the effect of PAN to ladder structure transformation. Completion of this transformation is important for subsequent carbonization steps. Fig. 4. Magnified SEM images for a two-layered hPBP showing the (a) CR-L and (b) PRL, regions.
ordered packing in PAN for the PT-2 films as compared to PT-1 (Fig. 3). Previous work [18] has shown that with increasing presence of interfacial structure in PAN/CNT composites, due to compact crystal formation of PAN in this region, stabilization is less complete at similar heat treatment conditions used for the sample which shows no interphase formation. Therefore, in this work, the stabilization temperatures are examined to promote complete
3.2. Film stabilization WAXD data of the stabilized films at 250 and 300 C for both PT1 and PT-2 samples are shown in Fig. 7. The PAN peak at 2q of 16.7 diminishes for all the films except the PT-2 sample stabilized at 250 C. This indicated that stabilization is incomplete for the PT-2 films as mentioned early, where DSC curves (Fig. 6) show lower heat flow. This is not the case for films stabilized at 300 C. WAXD graphs for the SW-1 and SW-2 films stabilized at 300 C are also shown in Fig. 7. Similar to the PT films stabilized at 300 C, the PAN peak is not observed in these samples. For this reason, all samples
Fig. 5. Cross section schematic for the fabricated films. The interphase region is assumed to be larger around smaller diameter SWNT bundles due to prior studies [39,42].
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3.3. Film carbonization The stabilized films are carbonized at different temperatures and times (i.e., 900 C for 20 and 40 min, 1000 C for 5 and 20 min, 1100 C for 5 min, and 1500 C for 5 min). Previous work by this research group has shown that the presence of interphase PAN can lead to early onset of graphitic structures at 1100 C [34]. In this work, the time and temperature effects on PAN to carbon evolution during carbonization is explored further. WAXD spectra for these carbonized films are shown in Fig. 9a and b. The values of d-spacing, full width at half maximum (FWHM), and crystal size for (002) graphitic peak at 2q of ~26 are provided for all films in Table 1. Layered structure, as well as time and temperature dependence of heat treatment on the structure of the carbonized films, is also discussed in the following sub-sections.
Fig. 7. WAXD spectra for the stabilized PT-1, PT-2, SW-1 and SW-2 films. A broad peak between 2q from 23 to 25 is related to PAN ladder structure [34].
were subsequently stabilized at 300 C before further carbonization and graphitization. The stabilization process converts PAN to a cyclized ladder and cross-linked structure. This new structure is observed in WAXD by the presence of a broad scattering peak between 2q from 23 to 25 (Fig. 7) [34]. SEM images for the cross-sections of all stabilized films are shown in Fig. 8. Like the precursor films, PT-1 shows a non-layer structure (Fig. 8a) while PT-2 films maintain its bi-layer structure (Fig. 8b). The thickness of PR-L in the PT-2 films is decreased in comparison to the precursor sample. A similar trend is observed for SW-1 and SW-2 films (Fig. 8c and d). It is expected that some weight loss will occur in the PAN matrix during heat-treatment and conversion to the cyclized form [1,6].
3.3.1. Effect of layer structure Dark boxes in Fig. 9a and b shows the (002) graphitic peak region at 2q of ~26 . For PT-1 and PT-2 films carbonized at 900 C for 20 min, 900 C for 40 min, 1000 C for 20 min, 1100 C for 5 min, and 1500 C for 5 min the (002) graphitic peak at 2q of ~26 is visible. No graphitic peak is observed for films carbonized at 1000 C for 5 min. In general, the (002) graphitic peak for the PT-2 film is broader than for the PT-1 sample carbonized at similar conditions. At this condition, the FWHM for PT-2 film at 2q of ~26 is 3.267, while for the PT-1 film it is 1.296. The WAXD results indicate, based on the average 2q, d-spacing, and crystal size parameters for the graphitic (002) plane, that the PT-1 films generally exhibit a more ordered carbon structure as compared to PT-2 samples. This result seems somewhat counterintuitive to what was observed for the precursor materials, where PT-2 films show more crystalline PAN. Previous studies also suggest that this ordered PAN helps to promote ordered carbon formation [34]. However, the PT-1 films (showing less interphase) exhibit higher degree of graphite ordering in terms of crystal size. This result may be due to two features. First, in general, PT-2 films are found to be thicker than PT-
Fig. 8. SEM images of cross sections for (a1) stabilized PT-1 film, (a2) zoom in of boxed region in the PT-1 film; (b1) PT-2 film, (b2) zoom in of boxed region in the PR-L area in the PT-2 film, (b3) zoom in of boxed region in the CR-L area in the PT-2 film; (c1) stabilized SW-1 film, (c2) zoom in of boxed region in the SW-1 film, (d1) SW-2 film, (d2) zoom in of boxed region in the PR-L area in the SW-2 film, (d3) zoom in of boxed region in the CR-L area in the SW-2 film.
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1 films. For this reason, less diffusion is anticipated in PT-2 film, which may delay the carbonization process. By comparing PT-2 films which were heat treated at the same temperature (i.e., 900 C) but for longer times (20 vs. 40 min), more complete carbonization was observed as time increases. This is shown by the presence of a more intense and narrower (002) graphitic peak. This result also confirms the role of thickness on the heat-treatment of the films. Second, WAXD and electron microscopy analysis of the precursor film indicate that the PT-2 film has a predominate presence of interphase regions in the PR-L, this region may not be very confined due to the lower content of CNT in this area. On the other hand, for PT-1 films, the PAN is more confined by surrounding rigid CNT even though there may be lower overall interphase content. Therefore, these carbonization results suggest that, while the presence of the PAN interphase is important for the formation of graphitic structure at low carbonization temperature, this region also requires some additional confinement of the PAN to allow for more ordered development early on. Previous work has shown the influence of precursor and confinement on the early onset of ordered carbonaceous species [34]. Analyses of the carbonized SW-1 and SW-2 films show no obvious graphitic peak. Instead, the peak at 2q of ~26 from WAXD is broad indicating a predominance of amorphous or turbostratic carbon (Fig. 9a and b). This result confirms that the CNT type also plays a role not only to nucleate and promote formation of the interphase PAN, but also to provide confinement during the carbonization process [34]. 3.3.2. Effect of time and temperature While the onset of graphitic phase transformation was found to occur at temperature as low as 900 C, sufficient time for the graphitization transition to occur is variable. Films carbonized at different temperatures and keeping time constant were compared to understand its effect on changes in crystal structure of the films. It was found that the graphitic peak intensity increased with temperature. These results suggest that a higher degree of graphitization occurs in films carbonized at lower temperature. For example, PT-1 and PT-2 films carbonized at 900 C for 40 min show graphitic (002) peak, but films carbonized at higher temperature using less time (i.e., 1000 C for 5 min) do not show any graphitic (002) peak. By comparing films carbonized at 1100 C for 5 min and 1000 C for 20 min, it was found that crystal size is smaller and FWHM is broader for samples carbonized at higher temperature but less amount of time (i.e., 1100 C for 5 min). WAXD curves (Fig. 9 and Table 2) also show higher order peaks
Fig. 9. WAXD spectra for the carbonized films (a) at temperatures ranging from 900 to 1100 C and (b) at 1500 C. (002) graphitic peak occurs at 2q of ~26 . AC is the amorphous carbon peak. Peaks 1,2,3,4 and 5 are related to higher order graphitic planes (100) of hexagonal form with ABAB stacking, (101) of hexagonal form with ACBACB stacking or (100) of rhombohedral form, (101) of hexagonal form, (102) of hexagonal form with ACBACB stacking or (110) of rhombohedral form and (102) of hexagonal form with ABAB stacking or (103) of hexagonal form with ACBACB stacking, respectively [34].
Table 1 Effect of carbonization time and temperature on (002) plane crystal structure development in carbonized films. Layer(s)
1
2
a
Sample
PT
Temperature ( C)
900
Time (min)
20
40
2q ( ) d-Spacing (nm) FWHM Crystal size (nm) I(101)H/(100)R:I(101)Ha 2q ( ) d-Spacing (nm) FWHM Crystal size (nm) I(101)H/(100)R:I(101)Ha
26.0 0.342 1.148 7.4 0.89 ̶ ̶ ̶ ̶ 0.89
26.1 0.341 1.296 6.6 1.37 26.1 0.342 3.267 2.6 1.48
I: intensity of peak, H: hexagonal, R: rhombohedral.
SW 1000
1100
5
20
5
̶
26.1 0.341 1.181 7.2 1.76 26.0 0.342 2.358 3.6 2.14
26.1 0.341 1.318 6.5 1.43 26.1 0.341 2.444 3.5 2.10
̶ ̶ ̶ 0.89 ̶ ̶ ̶ ̶ 0.74
1500
1100
1500
26.0 0.343 1.153 7.4 ̶ 26.0 0.342 1.510 5.6 ̶
25.9 0.344 0.992 8.6 ̶ 26.0 0.342 4.067 2.1 ̶
26.0 0.342 4.868 1.7 ̶ 25.9 0.343 4.626 1.8 ̶
N. Tajaddod et al. / Polymer 137 (2018) 346e357 Table 2 Higher order WAXD peaks associated with the hexagonal and rhombohedral forms of layered graphite [34]. Peak
1 2 3 4 5
Experimental 2q ( )
42.8 43.7 44.7 46.3 51.0
Indexed planes for different crystal unit cells (ABAB)
(ACBACB)
Hexagonal
Hexagonal
Rhombohedral
(1 0 0) ̶ (1 0 1) ̶ (1 0 2)
̶ (1 0 1) ̶ (1 0 2) (1 0 3)
̶ (1 0 0) ̶ (1 1 0) ̶
related to the crystalline form of the graphitic structure formed. Peaks at 2q of ~43.7 are related to the (101) crystal plane for hexagonal (i.e., with ACBACB stacking) or (100) of rhombohedral carbon structures. Peaks at 2q of ~44.7 are related to (101) crystal plane of hexagonal form (i.e., with ABAB stacking). The intensity ratio between the peaks at 2q of 43.7 and 44.7 is increased by carbonizing at higher temperature (Table 1). This implies that for these samples, carbonization at higher temperature increases the amount of (101) of hexagonal with ACBACB stacking or (100) of rhombohedral structures which causes an increase in interlayer distance between the graphitic structure. For this reason, carbonization at lower temperature (i.e., 900 C) but longer time (i.e., 40 min) may lead to a more ordered graphitic structure. The intensity ratio for the peaks at 2q of 43.7 and 44.7 in the PT-2 film is higher than PT-1 at similar carbonization condition. This suggests that the presence of more confined PAN in PT-1 films, which
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decreases the formation of crystal planes with larger spacing between graphitic layers (i.e., ACBACB stacking of hexagonal and rhombohedral structure). To investigate the microstructure of carbonized films, SEM images of PT films carbonized at 1000 C for 5 min, 1100 C for 5 min, and 1500 C for 5 min were examined and compared (Fig. 10). The films all show evidence of shrinkage buckling and waviness in the PAN matrix during heat-treatment and conversion to carbon (which is expected). For the two-layered films, this effect is most pronounced considering that the PR-L would undergo more shrinkage as compared to the CR-L. As discussed, WAXD results show the presence of a (002) peak (i.e., graphitic formation). It is expected that this graphitic structure forms in the near vicinity of the CNT. Based on previous work [34], high-resolution transmission electron microscopy analysis has shown graphitic formation near the CNT at low carbonization temperature of 1100 C. It is assumed for this work that the graphitic structure formed at low temperature (i.e., 900e1100 C) also forms most probably at the PANprecursor/CNT interphase or in confined PAN regions. 3.4. Graphitization treatment WAXD curves for the graphitized films are shown in Fig. 11. All graphitized films exhibit an increase for the (002) plane at 2q of 26 in terms of crystal size and d-spacing. This result is expected for both SW and PT materials regardless of the initial presence of graphitic structure during carbonization since heat-treatment above 2000 C leads to graphite formation and perfection. At high heat-treatment temperatures, all turbostratic and amorphous
Fig. 10. SEM of carbonized films. (a) PT-1 and (b1) PT-2 films carbonized at 1000 C for 5 min, (b2) and (b3) zoom in of boxed region in (b1); (c) PT-1 and (d1) PT-2 films carbonized at 1100 C for 5 min, (d2) and (d3) zoom in of boxed region in (d1); (e) SW-1 and (f1) SW-2 films carbonized at 1100 C for 5 min, (e2) and (e3) zoom in of boxed region in (e1); and (g) PT-1 and (h1) PT-2 films carbonized at 1500 C for 5 min, (h2) and (h3) zoom in of boxed region in (b1).
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WAXD data presented here provides information for the whole films. However, coupled with the SEM characterization, the analysis helps to define the regions for more prominent PAN to graphite transitions. The PT-2 film, which is carbonized for the same time but lower temperature (1000 C), does not show a layer structure in the PR-L region (Fig. 12c3). This suggests that for low-temperature graphite formation, the heat-treatment temperature-time conditions do play a role in the film morphology. The distinct observation of SWNT in the cross-sections of PT-1 and PT-2 regions of the PT-2 graphitized film is also not obvious. This makes sense as all the material is now graphitic in nature. Regardless, layered formation is most developed and continuous in nature for the PT-2 films. The PT-2 CR-L region shows a web-like SWNT morphology confirming the porous nature of this layer and the initial lack of precursor PAN matrix. 3.5. Raman spectroscopy
Fig. 11. WAXD curves for the PT-1, PT-2, SW-1 and SW-2 films graphitized at
2100 C.
carbon forms transform to graphite. In addition, any graphitic structures formed at lower temperature continue to heal (increase in crystalline perfection) leading to narrowing (decrease of FWHM) for the (002) peak. In general, the intensity of the (002) peak for the PT-2 and SW-2 films is higher than for the PT-1 and SW-1 samples. Considering the similar nature of the components present in both film types (1 versus 2), this change in intensity may be related to enhanced development of graphite structures in PR-L region at higher temperature. This development is more apparent after microscopy analysis (Fig. 12). The average (002) d-spacing is 0.342 nm and the crystal size is ~6.0 nm. Healing of the graphitic structures formed at low temperature (900e1100 C) during graphitization is evidenced by the changes in the peaks show in the WAXD curves (Figs. 9 and 11). This is an important finding because it confirms that the ordered structure formed at low temperature is likely graphitic rather than a hard turbostratic phase [14,43]. This work shows the evidence that the PAN/CNT interphase, as well as CNT confinement of the matrix, are both key precursor morphology features which aid early formation of graphite. Hard turbostratic phases have previously been shown to form in PAN-precursor materials at lower temperatures with the use of pressure [43]. Under such conditions, the carbon form appears to exhibit a graphitic structure at low temperatures, [43] but then undergoes no additional structural changes during graphitization heattreatment processes. For this reason, such ordered carbon is referred to as ‘hard turbostratic’ in nature. The opposite effect is observed here. The order carbon forms found as early as 900 C continue to undergo healing as graphitization heat treatment is applied, and this confirms the phase is graphitic. These results confirm that CNT can play a role in PAN to graphitic transformations at low temperature-time combinations (~900 C). SEM images of graphitized PT-1, PT-2 and SW-2 films previously carbonized at 1100 C for 5 min and PT-2 previously carbonized at 1000 C for 5 min are shown in Fig. 12. The PR-L in PT-2 and SW-2 which was previously carbonized at 1100 C for 5 min both show a distinct layered structure (Fig. 12b3 and 12d3) in comparison to the CR-L region and PT-1 film (non-layered) (Fig. 12a2, 12b2, and 12d2). The layered morphology observed for the graphitized films is consistent for graphitize materials. The fact that this morphology is isolated to the PR-L region suggests that the PAN-CNT interaction enhanced by the S:NS treatment within precursor material (Fig. 4) is mostly responsible for the eventual graphitic presence. The
To further understand the role of PAN interphase to graphite transformation, Raman spectroscopy analysis was performed on the PT-2 films carbonized at 900 C for 20 min and 1100 C for 5 min (Fig. 13). D-band intensity of the PR-L region is higher than the CR-L. This is likely related to the presence of the graphitic layer edge defects [44]. Since there is more PAN in this PR-L region, carbonization transformations during pyrolysis likely occur here as compared to the CR-L regions. CNT structures within the overall films also undergo annealing during processing. For this reason, the CR-L is expected to exhibit a lower D-band intensity. In general, the D-band to G-band intensity ratio of the PR-L and CR-L regions for the films carbonized at 1100 C for 5 min and 900 C for 20 min are 2.5 and 2, respectively. The higher ratio of the PR-L and CR-L intensity for film carbonized at 1100 C for 5 min indicates more graphitic defects are present in the PR-L regions. This data is in accordance with WAXD data (Fig. 8a). As mentioned, WAXD data showed that films carbonized at 1100 C for 5 min had more hexagonal structure with ACBACB stacking coupled with rhombohedral stacking in comparison to films treated at 900 C for 20 min. Films carbonized at 900 C for 20 min show more hexagonal ABAB stacking with little rhombohedral presence. The coupling of hexagonal and rhombohedral structures leads to more defects present in the graphitic planes, and this is also confirmed through the Raman analysis. Raman spectroscopy combined with the WAXD and microscopy analysis all confirm that the PR-L exhibits a distinctive graphitic layered morphology stemming from the initially ordered morphology found in the precursor PR-L. The formation of the two-layered precursor also shows the selectivity of PAN-CNT interactions and its influence on interphase growth. The analysis of these carbonized materials clearly shows the linkage between precursor morphology and final carbonized film structure. Overall, these works show that CNT do play a role in indicating ordered PAN formation and this structure is important for forming order carbon at low temperatures during pyrolysis. The work also confirms that confinement of the polymer is also important since its inherent shrinkage is not helpful for carbon formation. When comparing both effects of interphase formation and confinement through analysis of the two-layered composite films, it is clear that interphase formation is most influential for early graphitic formation and subsequent healing during graphitization (Fig. 12). The type of CNT used in these materials is also a key parameter and shows that polymer-nanotube interactions leading to precursor interphase formation are not equal. Therefore, multiple parameters should be carefully controlled during composite precursor formation, stabilization, carbonization, and graphitization, as these PAN/ CNT materials are explored for use as the next-generation carbon
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Fig. 12. SEM image of graphitized (a1) PT-1 film (previously carbonized at 1100 C for 5 min), (a2) zoom in of boxed area in a1; (b1) PT-2 film (previously carbonized at 1100 C for 5 min), (b2) zoom in of boxed region in CR-L area in b1, (b3) zoom in of boxed region in PR-L area in b1, (c1) PT-2 film (previously carbonized at 1000 C for 5 min), (c2) zoom in of boxed region in CR-L area in c1, (c3) zoom in of boxed region in PR-L area in c1; (d1) SW-2 film (previously carbonized at 1100 C for 5 min), (d2) zoom in of boxed region in CR-L area in d1, (d3) zoom in of boxed region in PR-L area in d1.
precursor. The advantage of using these systems is the potential ability to control morphology throughout all processing steps, as shown in this work. 4. Conclusion PAN/CNT films containing 50 wt% CNT were made by enhancing polymer crystallization conditions and processing parameters. Films were formed by filtration methods in the presence of a PAN non-solvent. This processing method led to the formation of different PR-L and CR-L regions in the composite films made by changing S:NS ratios before filtration processing. WAXD analysis reveals that PR-L regions consist of highly ordered/crystalline PAN. Two type of SWNT were used in this work. For the formed composite films it was found that the PAN crystalline structure differed as a function of the CNTs types. For this reason the interfacial polymer was mostly present in the system containing purified p-
type (PT) tubes. The difference in PAN ordering in the presence of these SWNT types may be related to tube purity and purification routes as well as tube length after dispersion. Regardless, both films exhibited layered morphology after phase separation and filtration processes. Isolation of the PR-L and CR-L regions in the films show that PAN has a preferential size interaction with the SWNT bundles [42], while nucleation of the ordered interphase structure may be related to tube surface features/chemistry. PT-2 films allow for the exploration of interphase and confinement effects on PAN conversion to graphite, while SW-2 films allow for tracking the effect of PAN confinement only. All films were stabilized at 250 and 300 C, where 300 C was found to be most favorable. Stabilized films were carbonized at temperatures ranging from 900 to 1500 C. Carbonization times were also viewed. WAXD and SEM analysis were used to understand the effect of stabilization temperature and carbonization conditions. It was found that graphitic structure formed at specific
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References
Fig. 13. The Raman spectra of PT-2 films carbonized at 1100 C for 5 min and 900 C for 20 min.
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Acknowledgments Funding support for this work is provided by DARPA and the Army Research Office (ARO) (W911NF-13-1-0190), as well as the Air Force Office of Scientific Research (AFOSR) (FA9550- 11-1-0153). The authors would also like to thank Professor Philip Bradford at North Carolina State University for his assistance in performing high-temperature graphitization treatment for the PAN/CNT samples.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Low-temperature graphitic formation promoted by confined interphase structures in polyacrylonitrile/carbon nanotube materials Navid Tajaddod, Heng Li, Marilyn L. Minus* Department of Mechanical and Industrial Engineering, Northeastern University, 360 Huntington Avenue, 334 Snell Engineering Center, Boston, MA 021155000, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2017 Received in revised form 14 December 2017 Accepted 2 January 2018 Available online 6 January 2018
In this work polyacrylonitrile (PAN)/single wall carbon nanotube (SWNT) composites with 50 wt% SWNT loading were studied. A previously studied solvent-based phase separation process was used to enhance PAN-SWNT crystallization conditions and interphase formation within the composites. Two types of SWNT species were used here to produce two composites materials with well-ordered PAN-SWNT interphase structures. All precursor composite films were stabilized and subsequently carbonized at temperatures ranging from 900 to 1500 C under various carbonization times. Wide-angle X-ray diffraction and electron microscopy were used to analyze the structural change in these precursors during carbonization. It was found that ordered graphitic structures were formed using specific temperature/time conditions as early as 900 C. This low-temperature graphitic structure can be tracked directly to the PAN interphase structure within the precursors. This paper discusses the linkage between precursor PAN-SWNT interphase formation and subsequent low-temperature graphitic formation. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Carbon fiber Interphase Heat treatment
1. Introduction Polyacrylonitrile (PAN) is widely used as a precursor for carbon fiber production [1e7]. Typically, a three-step heat treatment process is performed in order to convert PAN precursors to carbon fibers. These steps are: (i) thermal stabilization, (ii) carbonization, and (iii) optional graphitization. During stabilization between a temperature range of 200e300 C, a ladder structure will be formed due to cyclization of PAN molecules [8,9]. Carbonization processes increase the carbon content through formation of a nearamorphous microcrystal structure [10]. Carbonization is usually performed within a temperature range from 1000 to 1700 C [5,6]. Optional graphitization will further enhance the ordering of the carbon structure by formation of graphite and this process occurs at temperatures greater than 2000 C and up to 3000 C [5,6]. By increasing the heat treatment temperatures and applying tension to the fibers during pyrolysis, ordering of the carbon structure increases [11e13]. Semi-crystalline carbon is referred to turbostratic, while highly ordered carbon is graphitic. The inter-layer distance between turbostratic carbon planes varies and is on
* Corresponding author. E-mail address: [email protected] (M.L. Minus). https://doi.org/10.1016/j.polymer.2018.01.007 0032-3861/© 2018 Elsevier Ltd. All rights reserved.
average > 0.34 nm [14,15], while the distance between perfect graphite planes is 0.3345 nm [15,16]. Conversion to this graphitic structural form in the material improves the mechanical properties of carbon fibers. New research on carbon fiber development and processing have been of interest toward decreasing the gap between commercial and theoretical tensile strength properties for the materials [17,18]. An increase in tensile strength properties is expected to occur in the presence of oriented graphitic regions which can be increased throughout the fibers, while minimizing defective carbonaceous regions. To this end many studies have focused on the use of filler particles such as carbon black [19,20], graphene [21,22], and carbon nanotubes (CNTs) [23e27] to modified the PAN matrix in order to improve precursor properties and subsequent carbon fiber performance. In addition, the use of these carbonaceous materials may also enhance the conductive properties of these materials broadening their applications [19e33]. There have been several recent studies focused on understanding the role of controlling the precursor polymer structure toward producing a specific variation of carbonaceous structure upon pyrolysis by addition of CNTs to the PAN matrix [18,34e37]. CNTs have been shown to induce ordered interphase [38,39] which can promote the ordering of the PAN ladder structure after stabilization [18,35]. This interphase structure leads to PAN conversion to graphitic structures at ~1100 C
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[18,35,37,40,41]. However, while it has been shown that graphitic structure can form at 1100 C at the interphase, studies focused on understanding the mechanism behind this conversion process and subsequent structure are still needed. In this paper, PAN/CNT composites (with 50 wt% CNT total loading) were prepared to purposely isolate interphase regions in the composite. Several studies have looked at much lower CNT loading in the composite [18,35,37,40,41]. However, the direct linkage between filler content and PAN conversion during carbonization heat treatment has been difficult to observe. The choice of 50 wt% CNT loading in this work is based on the previously studied method of preparing PAN/CNT films by phase separation [42]. This technique allows for the development of regions in the composite with high interphase content [42]. At 50 wt% CNT the level of interfacial content is sufficient to directly study carbonization behavior. In addition, the bulk PAN and CNT interaction can also be distinguished from the interphase. This work is performed to confirm the value of introducing ordered PAN interphase regions to the precursor to control graphitic forms in the final carbon material. This work also shows the role of polymer confinement effects in early carbonaceous and graphitic formation. 2. Materials and method 2.1. Materials The PAN used in this work is a poly(acrylonitrile-co-methacrylic acid) random copolymer with methacrylic acid content of 4 wt% (Mw ~513,000 g/mol), obtained from Exlan Co. Japan. Two types of CNT materials were used, (i) PT (purified single wall carbon nanotubes (SWNT), ~94.5%, Continental Carbon Nanotechnologies, Inc.) and (ii) SW (SWNT, ~90%, SouthWest Nano Technologies, Inc. batch SG76). 2.2. Solution processing and film fabrication 2.2.1. Dispersion preparation The materials used in this study are PAN/CNT composite films. For hybrid polymer/CNT buckypaper (hPBP) fabrication, PAN powders were first dissolved in 90 C DMF (obtained from Sigma Aldrich) at a concentration of 250 mg/L. CNTs of equal amount were then dispersed in the polymer solution for 24 h via a bath sonicator (Fisher FS30, frequency 43 kHz, power 150 W). For control buckypaper (BP) fabrication, CNTs were dispersed in DMF using similar sonication conditions. 2.2.2. BP and hPBP films preparation After sonication, the PAN/CNT dispersion was subjected to a solution-based shear crystallization process by stirring the dispersion at 90 C. Simultaneously vacuum distillation was applied to remove half the volume of solvent at a controlled time and temperature. Subsequently, this overall system was cooled down to room temperature (~25 C). A non-solvent for PAN (i.e., water [42]) was added into dispersion at various solvent:non-solvent (S:NS) ratios. The two ratios chosen in this work were 7:1 and 1:2, respectively. This S:NS treatment during solution processing of the PAN/CNT films help to isolate regions, where PAN has enhanced interfacial interaction with the CNT. The isolation of this interphase region is important for studying subsequent carbonization process effects. The final PAN/CNT dispersions were then filtered through a nylon membrane (0.45 mm pore size obtained from Millipore) to form the PAN/CNT hPBP. The films made using PT-SWNT with S:NS ratios of 7:1 and 1:2, respectively, were named PT-1 and PT-2, accordingly. The films made with SW-SWNT were named SW-1 and SW-2, respectively at similar S:NS ratios of 7:1 and 1:2. A
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free-standing hPBP was removed from the filter paper after drying in a vacuum oven and further characterized to understand the effect of processing on the structure of the hybrid films. Control CNT BPs were also fabricated using a similar filtration process for comparison. 2.2.3. Heat-treatment processes A Lindberg/Blue M™ Mini-Mite™ tube furnace equipped with a quartz tube (diameter 1 inch, obtained from Quartz Scientific Inc.) was used for heat-treatment to conduct both stabilization and carbonization procedures. Two flow meters were used to control the inlet and outlet gas flow through the furnace. The entire system was sealed and the outlet gas flow was directly released into a venting system. The film samples were held in compression using a customized sample holder throughout both heat treatment processes. For stabilization (in air), the temperature was (i) ramped up at 1 C/min from room temperature (~25 C) to 250 C or 300 C; (ii) maintained isothermally for 10 h; and (iii) gradually decreased to room temperature. For carbonization (in argon), the temperature was (i) ramped up at 5 C/min from room temperature to 900, 1000 or 1100 C; (ii) maintained isothermal for 20 or 40 min at 900 C, 5 or 20 min at 1000 C, and 5 min at 1100 C; and (iii) gradually decreased to room temperature (~25 C). In both processes, the required air or argon gas flow was maintained constant at 3000 ccm until the chamber was cooled down to room temperature (~25 C). To confirm the existence of structural features the early onset graphite (i.e., formed between 900 and 1100 C), the pyrolysed composite films were further heat-treated at up to 2100 C for 40 min in vacuum using high temperature furnace (Red Devil vacuum furnace WEBB 124). Fig. 1 summarizes the time-temperature profile for the stabilization, carbonization, and graphitization procedures. 2.2.4. Sample characterization Morphology characterization was performed using a Zeiss Supra 25 field emission scanning electron microscope (SEM) (operating voltage 5 kV). All film samples were fractured and mounted to a 90 pin stub with the fractured end facing up for SEM observation. Precursor (non-carbonized) samples were coated with a thin gold/ palladium layer (15e20 nm) for image purposes using a Gatan high-resolution ion beam coater. Wide-angle X-ray diffraction (WAXD) was performed on a Rigaku RAPID II equipped with a curved detector X-ray diffraction (XRD) system with a 3 kW sealed tube X-ray source (operating voltage 40 kV and current 30 mA). XRD curve fitting and analysis was performed using software PDXL 2 (version 2.0.3.0) and 2DP (version 1.0.3.4). Raman spectroscopy was conducted on a Jobin Yvon LabRam HR800 (laser wavelength 532 nm). Differential scanning calorimetry (DSC) analysis was conducted by heating samples from 40 to 400 C at a heating rate of 1 C/min. DSC curves were examined using the TA Instrument Universal Analysis software. 3. Results and discussion 3.1. Precursor films A non-solvent (water) is added during filtration to process the hPBPs in order to isolate PAN-CNT interactions. PT-1 and SW-1 hPBPs are fabricated using a 7:1, S:NS ratio, while PT-2 and SW-2 materials utilize a 1:2, S:NS ratio. Based on previous computational and experimental studies, it was found that as the nonsolvent increases, PAN-CNT isolated interactive regions also increase and these changes are observed in the composite morphology. The presence of the water leads to higher interaction between the PAN and CNT leading to a ‘trapping’ of the CNT by the
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Fig. 1. Graphical representation for the heat treatment procedures used in this work for (a) stabilization, (b) carbonization, and (c) graphitization of the samples. (dotted lines illustrate the lower time or temperature heat treatments that were used for this research).
polymer chains [42]. This is a size dependent process, and as a result smaller diameter bundles are isolated by the polymer. Crosssections of fabricated precursor PT-1 and PT-2 hPBP films are shown in Fig. 2a and b. PT-1 film cross-sections show a heterogeneous structure, where CNTs are randomly distributed throughout the films. However, the PT-2 films show a two-layered structure; A PAN-rich layer (PR-L) which is dominated by PAN and contains a lower concentration of CNTs, and a CNT-rich layer (CR-L) that contains little polymer but a very high concentration of CNT. From a morphological point of view, the structure of the CR-L in the PT-2 sample is similar to the overall structure for the PT-1 film, while the PR-L is much denser and shows even distributions of the CNT. Although for both films that same amount of PAN and CNT is used, in general, the PT-2 film is thicker than the PT-1 samples. This thickness difference may be related to the loose and compact structures present in the CR-L and PR-L regions, respectively. As mentioned, hPBPs from two CNT types were fabricated. Similar non-layered and layered structures are observed for SW-1 and SW2 films and are shown in Fig. 2c and d. The SW-2 two-layered films also exhibit a PR-L and CR-L and film thickness trends are similar to those for the PT-based hPBPs. Based on electron microscopy analysis, the formation of the layered structures in the PT-2 and SW-2 hPBPs were found to affect PAN morphology. It was also observed that during film processing (by filtration) the approximate PAN weight loss percentage for PT-1, SW-1, PT-2 and SW-2 was found to be 27.3, 25.0, 11.1 and 10.0%, respectively. Therefore, in general, the concentration of PAN in the PT-2 and SW-2 films is slightly higher than for the PT-1 and SW-1 samples. Considering the presence of PAN in all films, WAXD analysis was performed in order to investigate the actual structural features of the PAN polymer in the composites. The WAXD micrographs for the PT-1, PT-2, SW-1 and SW-2 films are shown in Fig. 3. It is observed that the PT-2 and SW-2 films show a narrow and predominate peak at 2q of 16.7 associated with crystalline PAN (dotted box in Fig. 3). The broad peak observed for the PT-1 and SW-1 films is consistent with a more amorphous PAN morphology (dotted box in Fig. 3). This WAXD data suggest that for the single layered films the PAN predominately retains an amorphous structure, whereas in the two-layered films the polymer is more crystalline. Based on the electron microscopy analysis, it is clear that PAN in the two-layered films is isolated in the PR-L, this WAXD data also suggests that the PAN in this region is highly ordered. As shown in Fig. 4, the SWNT in the PR-L are more homogenously distributed as compared to the CR-L region. This WAXD analysis shows that the improved dispersion of CNT in the PR-L region allows for PAN to form more ordered domains. Previous research has shown that well-dispersed SWNT can promote interphase formation in PAN [34,39]. These PT-2 and SW-2 hPBPs allow for the isolation of such ordered regions in the composite material, which enables a close study of how ordered interphase PAN affects heattreatment toward turbostratic carbon and graphitic formations. Based on both microscopy and WAXD analysis a cross-sectional schematic was developed to illustrate the significance of the layered structure in the hPBPs (Fig. 5). The schematic shows that the distribution and subsequent ordered (interphase) formation of PAN in the vicinity of the CNT differs for the one-layer films (PT-1 or SW1) as compared to the two-layered films (PT-2 or SW-2). These structural features ultimately affect the overall PAN structural distribution within the precursor films, and this is expected to influence the formation of the carbonized structures upon heat-treatment. Isolation of the interphase regions is important for tracking the transformation of this region during carbonization, and its eventual role in mechanical and electrical properties of the overall composite. In addition, the different types of CNT used here (i.e., PT and SW) are found to slightly affect the ordered (interphase) structure formed
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Fig. 2. SEM images of cross-sections of fabricated films: (a1) PT-1 film, (a2) zoom in area of the boxed region in a1 image; (b1) PT-2 film, (b2) zoom in area of boxed region shows CRL, (b3) zoom in area of boxed region shows PR-L; (c1) SW-1 film, (c2) zoom in area of boxed region in c1 image, (d1) SW-2 film, (d2) zoom in area of boxed region shows CR-L, (d3) zoom in area of boxed region shows PR-L.
Fig. 3. WAXD spectra of the PT-BP, PT-1, PT-2, SW-BP, SW-1 and SW-2 films. PT-1 and SW-1 films show a more broad PAN (110) peak, PT-2 and SW-2 materials exhibit a sharper crystalline PAN (110) peak. Several broad peaks pertaining to the SWNT are also observed (see arrows).
(Fig. 3), which will in turn influence carbonization [34]. Based on the schematic in Fig. 5, the one-layered films exhibit a large distribution of CNT bundle size with surrounding PAN matrix. While some interphase PAN may be present, the matrix is
dominated by amorphous PAN as indicated by WAXD. The twolayer schematic show both CR-L and PR-L regions. It should be noted that for the film processing conditions used, the transition from the CR-L to PR-L layer is abrupt. The CR-L region is dominated by CNT of varying bundle size exhibiting an average of ~20.48 nm [42]. Based on previous study of these films, the CR-L exhibits more open structure as compared to the PR-L region, where voids are present and a lower content PAN is found [42]. These structural characteristics can be observed in Fig. 4. Comparatively, the PR-L was found to exhibit much smaller average bundle size (~6.25 nm) with less variation, lower void content, and higher presence of PAN [42]. Based on WAXD analysis this polymer is also found to be ordered (crystalline), and this is associated with interphase PAN. Some disordered PAN matrix is also expected to be present. However, unlike the one-layered films (no CR-L or PR-L distinction), the ordered interphase PAN dominated based on WAXD analysis. By using this visual scheme, the heat-treatment of the one-layer and two-layered films are tracked as a function of morphology, in order to understand the effects of pyrolysis. Due to the structure of the PAN hPBPs films produced here, DSC analysis was first performed to anticipate stabilization effects. DSC thermographs for the PT-1 and PT-2 films under air show that the heat evolved for the PT-1 films is higher than that of the PT-2 samples (Fig. 6). The focus of the DSC study is to understand differences in stabilization for the interfacial PAN-SWNT regions (PT2) as compared to regions of PAN in the general vicinity of the SWNT as a part of the composite (PT-1). This DSC results suggests that the CNT in the PT-2 films interact with PAN in a way that delays the PAN stabilization process. In other words, the PAN present in the PT-2 films exhibits higher thermal stability in comparison to the PAN in the PT-1 films. This effect may be related to the increase of
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Fig. 6. DSC thermographs of PT-1 and PT-2 films stabilized at the heating rates of 1 C/ min.
processing (cyclization) at this stage. Completion of exothermic reaction is found to occur at ~300 C and higher, while the reaction begins at ~250 C and 300 C to examine the effect of PAN to ladder structure transformation. Completion of this transformation is important for subsequent carbonization steps. Fig. 4. Magnified SEM images for a two-layered hPBP showing the (a) CR-L and (b) PRL, regions.
ordered packing in PAN for the PT-2 films as compared to PT-1 (Fig. 3). Previous work [18] has shown that with increasing presence of interfacial structure in PAN/CNT composites, due to compact crystal formation of PAN in this region, stabilization is less complete at similar heat treatment conditions used for the sample which shows no interphase formation. Therefore, in this work, the stabilization temperatures are examined to promote complete
3.2. Film stabilization WAXD data of the stabilized films at 250 and 300 C for both PT1 and PT-2 samples are shown in Fig. 7. The PAN peak at 2q of 16.7 diminishes for all the films except the PT-2 sample stabilized at 250 C. This indicated that stabilization is incomplete for the PT-2 films as mentioned early, where DSC curves (Fig. 6) show lower heat flow. This is not the case for films stabilized at 300 C. WAXD graphs for the SW-1 and SW-2 films stabilized at 300 C are also shown in Fig. 7. Similar to the PT films stabilized at 300 C, the PAN peak is not observed in these samples. For this reason, all samples
Fig. 5. Cross section schematic for the fabricated films. The interphase region is assumed to be larger around smaller diameter SWNT bundles due to prior studies [39,42].
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3.3. Film carbonization The stabilized films are carbonized at different temperatures and times (i.e., 900 C for 20 and 40 min, 1000 C for 5 and 20 min, 1100 C for 5 min, and 1500 C for 5 min). Previous work by this research group has shown that the presence of interphase PAN can lead to early onset of graphitic structures at 1100 C [34]. In this work, the time and temperature effects on PAN to carbon evolution during carbonization is explored further. WAXD spectra for these carbonized films are shown in Fig. 9a and b. The values of d-spacing, full width at half maximum (FWHM), and crystal size for (002) graphitic peak at 2q of ~26 are provided for all films in Table 1. Layered structure, as well as time and temperature dependence of heat treatment on the structure of the carbonized films, is also discussed in the following sub-sections.
Fig. 7. WAXD spectra for the stabilized PT-1, PT-2, SW-1 and SW-2 films. A broad peak between 2q from 23 to 25 is related to PAN ladder structure [34].
were subsequently stabilized at 300 C before further carbonization and graphitization. The stabilization process converts PAN to a cyclized ladder and cross-linked structure. This new structure is observed in WAXD by the presence of a broad scattering peak between 2q from 23 to 25 (Fig. 7) [34]. SEM images for the cross-sections of all stabilized films are shown in Fig. 8. Like the precursor films, PT-1 shows a non-layer structure (Fig. 8a) while PT-2 films maintain its bi-layer structure (Fig. 8b). The thickness of PR-L in the PT-2 films is decreased in comparison to the precursor sample. A similar trend is observed for SW-1 and SW-2 films (Fig. 8c and d). It is expected that some weight loss will occur in the PAN matrix during heat-treatment and conversion to the cyclized form [1,6].
3.3.1. Effect of layer structure Dark boxes in Fig. 9a and b shows the (002) graphitic peak region at 2q of ~26 . For PT-1 and PT-2 films carbonized at 900 C for 20 min, 900 C for 40 min, 1000 C for 20 min, 1100 C for 5 min, and 1500 C for 5 min the (002) graphitic peak at 2q of ~26 is visible. No graphitic peak is observed for films carbonized at 1000 C for 5 min. In general, the (002) graphitic peak for the PT-2 film is broader than for the PT-1 sample carbonized at similar conditions. At this condition, the FWHM for PT-2 film at 2q of ~26 is 3.267, while for the PT-1 film it is 1.296. The WAXD results indicate, based on the average 2q, d-spacing, and crystal size parameters for the graphitic (002) plane, that the PT-1 films generally exhibit a more ordered carbon structure as compared to PT-2 samples. This result seems somewhat counterintuitive to what was observed for the precursor materials, where PT-2 films show more crystalline PAN. Previous studies also suggest that this ordered PAN helps to promote ordered carbon formation [34]. However, the PT-1 films (showing less interphase) exhibit higher degree of graphite ordering in terms of crystal size. This result may be due to two features. First, in general, PT-2 films are found to be thicker than PT-
Fig. 8. SEM images of cross sections for (a1) stabilized PT-1 film, (a2) zoom in of boxed region in the PT-1 film; (b1) PT-2 film, (b2) zoom in of boxed region in the PR-L area in the PT-2 film, (b3) zoom in of boxed region in the CR-L area in the PT-2 film; (c1) stabilized SW-1 film, (c2) zoom in of boxed region in the SW-1 film, (d1) SW-2 film, (d2) zoom in of boxed region in the PR-L area in the SW-2 film, (d3) zoom in of boxed region in the CR-L area in the SW-2 film.
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1 films. For this reason, less diffusion is anticipated in PT-2 film, which may delay the carbonization process. By comparing PT-2 films which were heat treated at the same temperature (i.e., 900 C) but for longer times (20 vs. 40 min), more complete carbonization was observed as time increases. This is shown by the presence of a more intense and narrower (002) graphitic peak. This result also confirms the role of thickness on the heat-treatment of the films. Second, WAXD and electron microscopy analysis of the precursor film indicate that the PT-2 film has a predominate presence of interphase regions in the PR-L, this region may not be very confined due to the lower content of CNT in this area. On the other hand, for PT-1 films, the PAN is more confined by surrounding rigid CNT even though there may be lower overall interphase content. Therefore, these carbonization results suggest that, while the presence of the PAN interphase is important for the formation of graphitic structure at low carbonization temperature, this region also requires some additional confinement of the PAN to allow for more ordered development early on. Previous work has shown the influence of precursor and confinement on the early onset of ordered carbonaceous species [34]. Analyses of the carbonized SW-1 and SW-2 films show no obvious graphitic peak. Instead, the peak at 2q of ~26 from WAXD is broad indicating a predominance of amorphous or turbostratic carbon (Fig. 9a and b). This result confirms that the CNT type also plays a role not only to nucleate and promote formation of the interphase PAN, but also to provide confinement during the carbonization process [34]. 3.3.2. Effect of time and temperature While the onset of graphitic phase transformation was found to occur at temperature as low as 900 C, sufficient time for the graphitization transition to occur is variable. Films carbonized at different temperatures and keeping time constant were compared to understand its effect on changes in crystal structure of the films. It was found that the graphitic peak intensity increased with temperature. These results suggest that a higher degree of graphitization occurs in films carbonized at lower temperature. For example, PT-1 and PT-2 films carbonized at 900 C for 40 min show graphitic (002) peak, but films carbonized at higher temperature using less time (i.e., 1000 C for 5 min) do not show any graphitic (002) peak. By comparing films carbonized at 1100 C for 5 min and 1000 C for 20 min, it was found that crystal size is smaller and FWHM is broader for samples carbonized at higher temperature but less amount of time (i.e., 1100 C for 5 min). WAXD curves (Fig. 9 and Table 2) also show higher order peaks
Fig. 9. WAXD spectra for the carbonized films (a) at temperatures ranging from 900 to 1100 C and (b) at 1500 C. (002) graphitic peak occurs at 2q of ~26 . AC is the amorphous carbon peak. Peaks 1,2,3,4 and 5 are related to higher order graphitic planes (100) of hexagonal form with ABAB stacking, (101) of hexagonal form with ACBACB stacking or (100) of rhombohedral form, (101) of hexagonal form, (102) of hexagonal form with ACBACB stacking or (110) of rhombohedral form and (102) of hexagonal form with ABAB stacking or (103) of hexagonal form with ACBACB stacking, respectively [34].
Table 1 Effect of carbonization time and temperature on (002) plane crystal structure development in carbonized films. Layer(s)
1
2
a
Sample
PT
Temperature ( C)
900
Time (min)
20
40
2q ( ) d-Spacing (nm) FWHM Crystal size (nm) I(101)H/(100)R:I(101)Ha 2q ( ) d-Spacing (nm) FWHM Crystal size (nm) I(101)H/(100)R:I(101)Ha
26.0 0.342 1.148 7.4 0.89 ̶ ̶ ̶ ̶ 0.89
26.1 0.341 1.296 6.6 1.37 26.1 0.342 3.267 2.6 1.48
I: intensity of peak, H: hexagonal, R: rhombohedral.
SW 1000
1100
5
20
5
̶
26.1 0.341 1.181 7.2 1.76 26.0 0.342 2.358 3.6 2.14
26.1 0.341 1.318 6.5 1.43 26.1 0.341 2.444 3.5 2.10
̶ ̶ ̶ 0.89 ̶ ̶ ̶ ̶ 0.74
1500
1100
1500
26.0 0.343 1.153 7.4 ̶ 26.0 0.342 1.510 5.6 ̶
25.9 0.344 0.992 8.6 ̶ 26.0 0.342 4.067 2.1 ̶
26.0 0.342 4.868 1.7 ̶ 25.9 0.343 4.626 1.8 ̶
N. Tajaddod et al. / Polymer 137 (2018) 346e357 Table 2 Higher order WAXD peaks associated with the hexagonal and rhombohedral forms of layered graphite [34]. Peak
1 2 3 4 5
Experimental 2q ( )
42.8 43.7 44.7 46.3 51.0
Indexed planes for different crystal unit cells (ABAB)
(ACBACB)
Hexagonal
Hexagonal
Rhombohedral
(1 0 0) ̶ (1 0 1) ̶ (1 0 2)
̶ (1 0 1) ̶ (1 0 2) (1 0 3)
̶ (1 0 0) ̶ (1 1 0) ̶
related to the crystalline form of the graphitic structure formed. Peaks at 2q of ~43.7 are related to the (101) crystal plane for hexagonal (i.e., with ACBACB stacking) or (100) of rhombohedral carbon structures. Peaks at 2q of ~44.7 are related to (101) crystal plane of hexagonal form (i.e., with ABAB stacking). The intensity ratio between the peaks at 2q of 43.7 and 44.7 is increased by carbonizing at higher temperature (Table 1). This implies that for these samples, carbonization at higher temperature increases the amount of (101) of hexagonal with ACBACB stacking or (100) of rhombohedral structures which causes an increase in interlayer distance between the graphitic structure. For this reason, carbonization at lower temperature (i.e., 900 C) but longer time (i.e., 40 min) may lead to a more ordered graphitic structure. The intensity ratio for the peaks at 2q of 43.7 and 44.7 in the PT-2 film is higher than PT-1 at similar carbonization condition. This suggests that the presence of more confined PAN in PT-1 films, which
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decreases the formation of crystal planes with larger spacing between graphitic layers (i.e., ACBACB stacking of hexagonal and rhombohedral structure). To investigate the microstructure of carbonized films, SEM images of PT films carbonized at 1000 C for 5 min, 1100 C for 5 min, and 1500 C for 5 min were examined and compared (Fig. 10). The films all show evidence of shrinkage buckling and waviness in the PAN matrix during heat-treatment and conversion to carbon (which is expected). For the two-layered films, this effect is most pronounced considering that the PR-L would undergo more shrinkage as compared to the CR-L. As discussed, WAXD results show the presence of a (002) peak (i.e., graphitic formation). It is expected that this graphitic structure forms in the near vicinity of the CNT. Based on previous work [34], high-resolution transmission electron microscopy analysis has shown graphitic formation near the CNT at low carbonization temperature of 1100 C. It is assumed for this work that the graphitic structure formed at low temperature (i.e., 900e1100 C) also forms most probably at the PANprecursor/CNT interphase or in confined PAN regions. 3.4. Graphitization treatment WAXD curves for the graphitized films are shown in Fig. 11. All graphitized films exhibit an increase for the (002) plane at 2q of 26 in terms of crystal size and d-spacing. This result is expected for both SW and PT materials regardless of the initial presence of graphitic structure during carbonization since heat-treatment above 2000 C leads to graphite formation and perfection. At high heat-treatment temperatures, all turbostratic and amorphous
Fig. 10. SEM of carbonized films. (a) PT-1 and (b1) PT-2 films carbonized at 1000 C for 5 min, (b2) and (b3) zoom in of boxed region in (b1); (c) PT-1 and (d1) PT-2 films carbonized at 1100 C for 5 min, (d2) and (d3) zoom in of boxed region in (d1); (e) SW-1 and (f1) SW-2 films carbonized at 1100 C for 5 min, (e2) and (e3) zoom in of boxed region in (e1); and (g) PT-1 and (h1) PT-2 films carbonized at 1500 C for 5 min, (h2) and (h3) zoom in of boxed region in (b1).
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WAXD data presented here provides information for the whole films. However, coupled with the SEM characterization, the analysis helps to define the regions for more prominent PAN to graphite transitions. The PT-2 film, which is carbonized for the same time but lower temperature (1000 C), does not show a layer structure in the PR-L region (Fig. 12c3). This suggests that for low-temperature graphite formation, the heat-treatment temperature-time conditions do play a role in the film morphology. The distinct observation of SWNT in the cross-sections of PT-1 and PT-2 regions of the PT-2 graphitized film is also not obvious. This makes sense as all the material is now graphitic in nature. Regardless, layered formation is most developed and continuous in nature for the PT-2 films. The PT-2 CR-L region shows a web-like SWNT morphology confirming the porous nature of this layer and the initial lack of precursor PAN matrix. 3.5. Raman spectroscopy
Fig. 11. WAXD curves for the PT-1, PT-2, SW-1 and SW-2 films graphitized at
2100 C.
carbon forms transform to graphite. In addition, any graphitic structures formed at lower temperature continue to heal (increase in crystalline perfection) leading to narrowing (decrease of FWHM) for the (002) peak. In general, the intensity of the (002) peak for the PT-2 and SW-2 films is higher than for the PT-1 and SW-1 samples. Considering the similar nature of the components present in both film types (1 versus 2), this change in intensity may be related to enhanced development of graphite structures in PR-L region at higher temperature. This development is more apparent after microscopy analysis (Fig. 12). The average (002) d-spacing is 0.342 nm and the crystal size is ~6.0 nm. Healing of the graphitic structures formed at low temperature (900e1100 C) during graphitization is evidenced by the changes in the peaks show in the WAXD curves (Figs. 9 and 11). This is an important finding because it confirms that the ordered structure formed at low temperature is likely graphitic rather than a hard turbostratic phase [14,43]. This work shows the evidence that the PAN/CNT interphase, as well as CNT confinement of the matrix, are both key precursor morphology features which aid early formation of graphite. Hard turbostratic phases have previously been shown to form in PAN-precursor materials at lower temperatures with the use of pressure [43]. Under such conditions, the carbon form appears to exhibit a graphitic structure at low temperatures, [43] but then undergoes no additional structural changes during graphitization heattreatment processes. For this reason, such ordered carbon is referred to as ‘hard turbostratic’ in nature. The opposite effect is observed here. The order carbon forms found as early as 900 C continue to undergo healing as graphitization heat treatment is applied, and this confirms the phase is graphitic. These results confirm that CNT can play a role in PAN to graphitic transformations at low temperature-time combinations (~900 C). SEM images of graphitized PT-1, PT-2 and SW-2 films previously carbonized at 1100 C for 5 min and PT-2 previously carbonized at 1000 C for 5 min are shown in Fig. 12. The PR-L in PT-2 and SW-2 which was previously carbonized at 1100 C for 5 min both show a distinct layered structure (Fig. 12b3 and 12d3) in comparison to the CR-L region and PT-1 film (non-layered) (Fig. 12a2, 12b2, and 12d2). The layered morphology observed for the graphitized films is consistent for graphitize materials. The fact that this morphology is isolated to the PR-L region suggests that the PAN-CNT interaction enhanced by the S:NS treatment within precursor material (Fig. 4) is mostly responsible for the eventual graphitic presence. The
To further understand the role of PAN interphase to graphite transformation, Raman spectroscopy analysis was performed on the PT-2 films carbonized at 900 C for 20 min and 1100 C for 5 min (Fig. 13). D-band intensity of the PR-L region is higher than the CR-L. This is likely related to the presence of the graphitic layer edge defects [44]. Since there is more PAN in this PR-L region, carbonization transformations during pyrolysis likely occur here as compared to the CR-L regions. CNT structures within the overall films also undergo annealing during processing. For this reason, the CR-L is expected to exhibit a lower D-band intensity. In general, the D-band to G-band intensity ratio of the PR-L and CR-L regions for the films carbonized at 1100 C for 5 min and 900 C for 20 min are 2.5 and 2, respectively. The higher ratio of the PR-L and CR-L intensity for film carbonized at 1100 C for 5 min indicates more graphitic defects are present in the PR-L regions. This data is in accordance with WAXD data (Fig. 8a). As mentioned, WAXD data showed that films carbonized at 1100 C for 5 min had more hexagonal structure with ACBACB stacking coupled with rhombohedral stacking in comparison to films treated at 900 C for 20 min. Films carbonized at 900 C for 20 min show more hexagonal ABAB stacking with little rhombohedral presence. The coupling of hexagonal and rhombohedral structures leads to more defects present in the graphitic planes, and this is also confirmed through the Raman analysis. Raman spectroscopy combined with the WAXD and microscopy analysis all confirm that the PR-L exhibits a distinctive graphitic layered morphology stemming from the initially ordered morphology found in the precursor PR-L. The formation of the two-layered precursor also shows the selectivity of PAN-CNT interactions and its influence on interphase growth. The analysis of these carbonized materials clearly shows the linkage between precursor morphology and final carbonized film structure. Overall, these works show that CNT do play a role in indicating ordered PAN formation and this structure is important for forming order carbon at low temperatures during pyrolysis. The work also confirms that confinement of the polymer is also important since its inherent shrinkage is not helpful for carbon formation. When comparing both effects of interphase formation and confinement through analysis of the two-layered composite films, it is clear that interphase formation is most influential for early graphitic formation and subsequent healing during graphitization (Fig. 12). The type of CNT used in these materials is also a key parameter and shows that polymer-nanotube interactions leading to precursor interphase formation are not equal. Therefore, multiple parameters should be carefully controlled during composite precursor formation, stabilization, carbonization, and graphitization, as these PAN/ CNT materials are explored for use as the next-generation carbon
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Fig. 12. SEM image of graphitized (a1) PT-1 film (previously carbonized at 1100 C for 5 min), (a2) zoom in of boxed area in a1; (b1) PT-2 film (previously carbonized at 1100 C for 5 min), (b2) zoom in of boxed region in CR-L area in b1, (b3) zoom in of boxed region in PR-L area in b1, (c1) PT-2 film (previously carbonized at 1000 C for 5 min), (c2) zoom in of boxed region in CR-L area in c1, (c3) zoom in of boxed region in PR-L area in c1; (d1) SW-2 film (previously carbonized at 1100 C for 5 min), (d2) zoom in of boxed region in CR-L area in d1, (d3) zoom in of boxed region in PR-L area in d1.
precursor. The advantage of using these systems is the potential ability to control morphology throughout all processing steps, as shown in this work. 4. Conclusion PAN/CNT films containing 50 wt% CNT were made by enhancing polymer crystallization conditions and processing parameters. Films were formed by filtration methods in the presence of a PAN non-solvent. This processing method led to the formation of different PR-L and CR-L regions in the composite films made by changing S:NS ratios before filtration processing. WAXD analysis reveals that PR-L regions consist of highly ordered/crystalline PAN. Two type of SWNT were used in this work. For the formed composite films it was found that the PAN crystalline structure differed as a function of the CNTs types. For this reason the interfacial polymer was mostly present in the system containing purified p-
type (PT) tubes. The difference in PAN ordering in the presence of these SWNT types may be related to tube purity and purification routes as well as tube length after dispersion. Regardless, both films exhibited layered morphology after phase separation and filtration processes. Isolation of the PR-L and CR-L regions in the films show that PAN has a preferential size interaction with the SWNT bundles [42], while nucleation of the ordered interphase structure may be related to tube surface features/chemistry. PT-2 films allow for the exploration of interphase and confinement effects on PAN conversion to graphite, while SW-2 films allow for tracking the effect of PAN confinement only. All films were stabilized at 250 and 300 C, where 300 C was found to be most favorable. Stabilized films were carbonized at temperatures ranging from 900 to 1500 C. Carbonization times were also viewed. WAXD and SEM analysis were used to understand the effect of stabilization temperature and carbonization conditions. It was found that graphitic structure formed at specific
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References
Fig. 13. The Raman spectra of PT-2 films carbonized at 1100 C for 5 min and 900 C for 20 min.
temperature/time conditions (i.e., 40 min at 900 C or 20 min at 1000 C). This low temperature graphitic structure can be directly correlated to the PAN interphase formation as well as PAN confinement. Both are observed to be equally effective in allowing for early onset of graphitic forms. High-temperature graphitization confirms that graphite, and not a ‘hard turbostratic’ phase, is formed in this study. High-temperature treatment also confirms that interphase PAN is more important in inducing uniform layered graphitic forms throughout the material. Raman analysis confirms the presence of layered graphitic features in the PR-L as compared to the CR-L for the two-layered precursors. This showed the selectivity of PAN to only prefer interfacial interaction with smaller bundles.
Acknowledgments Funding support for this work is provided by DARPA and the Army Research Office (ARO) (W911NF-13-1-0190), as well as the Air Force Office of Scientific Research (AFOSR) (FA9550- 11-1-0153). The authors would also like to thank Professor Philip Bradford at North Carolina State University for his assistance in performing high-temperature graphitization treatment for the PAN/CNT samples.
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