SWNT Nano-composite

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Interphase Development in Polyacrylonitrile/SWNT Nano-composite and its Effect on Cyclization and Carbonization for Tuning Carbon Structures Yinhui Li, Yuxiu Yu, Yaodong Liu, and Chunxiang Lu ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Nano Materials

Interphase Development in Polyacrylonitrile/SWNT Nano-composite and its Effect on Cyclization and Carbonization for Tuning Carbon Structures Yinhui Li,†‡ Yuxiu Yu,† Yaodong Liu,*† Chunxiang Lu*† †

National Engineering Laboratory for Carbon Fiber Technology, Institute of Coal Chemistry,

Chinese Academy of Sciences, 27 Taoyuan South Road, Taiyuan 030001, China ‡

University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China

E-mail:

[email protected](Yaodong Liu); [email protected] (Chunxiang Lu).

Keywords: polyacrylonitrile, nano-composite, self-stiffening, interphase, carbonization.

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Abstract In this study, the self-stiffening behavior of polyacrylonitrile (PAN)/CNT nano-composites is used to develop the interphase structure. Instead of MWNTs, SWNTs were used since they provide much more interfacial area with PAN matrix. The effects of SWNT content (0~1.5 wt%) and temperature on the structural development of PAN/SWNT nano-composites during dynamic straining were compared for processing optimization. Using this unique dynamic straining method, we were able to tune the structures, especially the interphase, of PAN/SWNT nano-composites. The degree of crystallinity of nano-composite could be improved from 54.3 to 58.5 % after dynamic straining for 12 hr, and the activation of the PAN glass transition increased from 434 to 1192 kJ/mol. The thermal behaviors of PAN and PAN/SWNT films with various degrees of crystallinity were compared by differential scanning calorimeter and thermal gravimetric analysis. The relationships among the structural parameters of a PAN film, its cyclization reaction, and carbonized structures were proposed. A higher degree of crystallinity of PAN would benefit a more completion of cyclization reaction, and led to a higher carbon yield. Additionally, the high resolution transmission electron microscope images of the carbonized PAN/SWNT nano-composites commonly show the formation of graphitic structures. Whereas, the carbonized PAN films only contained amorphous carbon structures. Our findings not only deepen the understanding on how the physical structures of PAN affect its cyclization and carbonized structures, but also provide a new way for making carbon materials with possibly much improved graphitic structure, mechanical performance and thermal/electrical conductivity.

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Introduction

Polyacrylonitrile (PAN) is the most important and primary precursor for high performance carbon fiber manufacturing, and the physical structures of polyacrylonitrile are essential to its carbonized structures and mechanical performances.1-3 In recent years, the technological development of high performance carbon fibers has become rapid. For the top-down approach, dry-jet gel spinning has been developed to fabricate carbon fibers with both high strength and high modulus.4-6 The other side, for the bottom-up approach, nano-fillers, especially 1-D CNTs, were found to be able to improve and template the micro-structures of PAN chains, as well as induce low temperature graphitization during carbonization.5, 7-9 With the addition of CNTs, the PAN chains are highly aligned and crystalized in the vicinity of CNTs;5, 9 the thermal and chemical reaction shrinkage of PAN are reduced;9-10 more conjugated structures are formed during cyclization and less chain scission reaction occurs;5, 10 and graphitized structures are formed during relatively low temperature carbonization.5, 7-8 So far, it has been experimentally proven that the incorporation of CNTs was able to improve the modulus,5, 11-12 thermal and electrical conductivity of PAN-based carbon fibers.8 In our previous review article focusing on polymer/CNT nanocomposite fibers,13 the effect of CNT on the structures of PAN is ascribed to the development of interphase. In this region, the PAN chain orientation and crystalline structure are strongly affected by the CNTs. With the development of interphase regions in PAN/CNT nano-composite, the structures and performances of the resulting carbon material are expected to be significantly improved. However, how to develop the interphase region is still in certain mysterious.

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Over the past few years, nano-materials have been found to be capable of stimulating biomimetic actuation, when some nano-composites were subjected to a periodic external dynamic strain, the modulus of the material was improved instead of decaying, such as polydimethylsiloxane/CNT14-15 and poly(methyl methacrylate)/nano-silica.16 In our previous study, we observed same phenomenon for PAN/MWNT nano-composites, and the improvement of the modulus of the nano-composites was ascribed to the development of interphase structure mainly perpendicular to the applied dynamic strain direction.17 In this study, we focused on how to further develop interphase structure in PAN/CNT nano-composites and how the structural development affects the thermal cyclization and carbonization. To better develop interphase PAN, we used SWNTs instead of MWNTs, since SWNTs could possibly provide much higher interfacial area than MWNTs.13 Because the high aspect ratio and surface area of SWNTs, they tend to aggregate and form bundles.18-19 Here, we aggressively oxidized the SWNT to chop down the length and functionalized the SWNTs according to a reported method to improve the SWNT dispersion20. Additionally, we systematically studied the effects of SWNT content, temperature and frequency on the dynamic strain hardening behaviors of PAN/SWNT nano-composites, and observed an increase of storage modulus up to 67 % when the nano-composite was subjected to a relatively high temperature of 80 °C under a dynamic strain of 1 Hz and 0.2 % amplitude for 12 hr. We characterized structural changes before and after dynamic straining, and correlated these structural changes to the cyclization reaction and carbonization yield of PAN. Although there are many reports on how the comonomer types,21-26 compositions,27-29 and chemical tacticity30 affect the thermal stabilization of PAN, there are few reports and many 4


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uncertainties on how the structures of PAN chains affects its pyrolysis and carbonization. In this paper, how the structure of PAN develops during dynamic straining and how the PAN structural changes affect its cyclization and carbonization will be discussed in detail. With the development of interphase PAN, its carbonized material is expected to possess a higher graphitic structure, mechanical performances and thermal/electrical conductivity. Experimental section Materials: PAN (atactic, Mw ~150,000 g/mol) was purchased from Sigma-Aldrich Company (USA). N,N-Dimethylformanide (DMF, AR) was obtained from Tianjin Reagent company (Tianjin, China) and was vacuum distilled before use. Single-walled Carbon Nanotubes (SWNTs, purity > 95 wt%, diameter < 2 nm, length 1-3 µm) were purchased from Chengdu Time Nano Material Technology Co., Ltd. Acid Treatment of SWNTs: According to a reported method,20 the as-received SWNTs (100 mg) were stirred vigorously with a glass stirring rod in oleum (120 %, 100 ml) at room temperature inside a double glass reactor under a nitrogen atmosphere for 72 hr. Concentrated nitric acid (68 %, 50 mL) was mixed with 35 ml oleum, and the acid mixture was slowly added into the SWNT flask while the solution temperature was maintained at 40 °C, and the mixture was stirred for 2 hr followed by quenching over ice. The material was filtered over Teflon membrane (pore diameter 0.22 µm), and the collection was rinsed by DI water for few times. The SWNTs were then re-suspended in a minimal amount of methanol, then precipitated in ethyl ether (250 mL), followed by vacuum filtration by Teflon membrane. The re-suspension, precipitation and filtration were repeated for few times until the methanol/ether filtrate was neutral as determined by pH test paper. The resulting SWNTs were dried before 5



further use and characterizations. PAN/SWNT Nano-composite Film Preparation: PAN powder was dispersed in DMF at room temperature by mechanical stirring, and then the dispersion was slowly heated to 80 °C and isothermal for 2 hr to get a homogeneous solution (0.94 wt%). The acid treated SWNTs were dispersed in DMF at a concentration of 100 mg/L by bath ultra-sonication for 8~10 hr, and then the SWNT/DMF dispersion was poured into PAN solution under vigorous stirring; next, the excess DMF was removed by vacuum evaporation to prepare PAN/SWNT nano-composite solutions at various CNT contents of 0, 0.1, 0.5, 1, and 1.5 wt%. The nano-composite solution was poured on a flat-bottom glass bottle and then vacuum dried at 40 °C for 72 hr to remove all the solvent. After the complete solvent evaporation, PAN/SWNT nano-composite films with various concentrations of SWNTs were prepared for further characterizations. The PAN/SWNT nano-composite film was prepared from acid-treated SWNT and PAN via solvent evaporation method illustrated in Figure 1.

Figure 1. Schematic of PAN/SWNT nanocomposite film preparation. Characterizations: Transmission electron microscope (TEM) was performed on a JEOL-2100F microscope operating at 200 kV. For SWNTs, the TEM sample was prepared by dispersing SWNTs in DMF by ultra-sonication; for carbon samples, the carbon materials were 6


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grinded using an agate mortar and the tiny broken pieces were dispersed in methanol by ultra-sonication. The suspension was deposited onto a holey carbon grid for further TEM observations. PAN/SWNT nano-composite films were cut into rectangular strips (12 mm long and 4 mm wide). Dynamic mechanical analysis (DMA, Netzsch 242-E, German) was used to apply external dynamic strain at a frequency of 1 Hz with a pretension of 20 MPa. The structural parameters of the films were measured by X-ray diffraction (XRD, Bruker D8 Advance, German) at a 2θ scanning speed of 2 °/min. The obtained XRD curves were analyzed by Jade 5.0 software. The morphologies of PAN/SWNTs composite films were observed by a field emission scanning electron microscope (FE-SEM, model JSM-7001F, made in JEOL Ltd., Japan). The cross-sections of the PAN nano-composite films were prepared by fracturing the films inside liquid nitrogen. All SEM samples were sputter coated by gold particles before observation. Raman spectra were recorded using a Holoprobe Kaiser optical spectrometer (VV mode, Renishaw, U.K.) with 638 nm laser excitation source. Differential scanning calorimetry (DSC) was carried out by DSC-Q2000 (TA Instruments, USA) in nitrogen atmosphere (flow rate 50 ml/min), and samples were scanned from 35 to 350 °C at a heating rate of 5 °C/min. Cyclization and carbonization weight losses were recorded by thermo-gravimetric analysis (TGA, Mettler Toledo, USA) at a heating rate of 5 °C/min from 35 to 800 °C in nitrogen atmosphere (flow rate 60 ml/min). The reported values of cylization and carbonization are averaged from at least 3 independent tests and the errors presented are standard errors. Result and Discussion 7



Characterizations of Acid Treated SWNTs Raman spectra of the SWNTs before and after acid treatment are showed in Figure 2a. The unique radial breathing mode (RBM) in between 100 and 400 cm-1 is the characteristics of SWNTs. The peak at 1590 cm-1 is G-band which is ascribed to the tangential vibration mode of graphitic structure, and the peak at ~1320 cm-1 is D-band which belongs to defects, such as SP3 carbon and voids.31-32 The D/G ratio of the as-received SWNTs is 0.141. After acid treatment, the D/G ratio increases to 0.566. This change indicates that more defects are created on SWNTs during acid treatment. The EDS spectra of SWNTs before and after acid treatment are included in Figure S1 in ESI. After acid treatment, oxygen containing functional groups are introduced on SWNTs and the oxygen content increases a lot. These results indicate a high level of sidewall functionalization of the SWNTs after the acid treatment. The photos of the as-received and acid-treated SWNT dispersions at a concentration of 100 mg/L are shown in Figure 2b, and their optical images are shown in Figure S2 in ESI. The as-received SWNTs cannot be well dispersed in DMF at a concentration of 100 mg/L after 10 hr of bath sonication, and become sedimented after standing for few days (in figure S2d). Whereas the acid treated SWNTs can be homogeneously dispersed in DMF after ultra-sonication, and the dispersion is stable even after standing for 15 days without visible agglomerations. The HR-TEM images of acid treated SWNTs are summarized in Figure 2c-d. In Figure 2c, there are some SWNTs bundles which may be caused by the solvent evaporation during TEM sample preparation. Also, there are many individual SWNTs as seen in Figure 2d. Most observed individual SWNTs have a length of less than 200 nm. Also, the rough edges of the acid treated SWNTs in the TEM image suggest a high level of functionalization 8


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on the side-wall of the SWNTs. In this study, the acid treated SWNTs were used for preparing PAN/SWNT nano-composites.

Figure 2. (a) Raman spectra of as-received and acid-treated SWNTs, (b) as-received and acid-treated SWNT dispersion; TEM image of (c) acid-treated SWNTs SWNTs bundles and (d) individual SWNTs. Development of Interphase PAN

Our previous study has proven the development of interphase PAN during external dynamic straining17. Here, we further investigated the influence of CNT contents, temperatures and relaxation on the structural developments of PAN/SWNT nano-composites, and optimized the external stimulation conditions. The dynamic straining experiments were carried out in a DMA equipment. The storage modulus (E’) of the nano-composites under dynamic straining was measured. E’ can be regarded as the Young’s modulus of the nano-composite films, and the frequency is proportional to the strain rate. First, PAN/SWNT nano-composite films with various SWNT contents of 0, 0.1, 0.5, 1.0 and 1.5 wt% were prepared. The time dependent E’ of these films under a dynamic frequency of 1 Hz and strain amplitude of 0.2 % were 9



measured at 35 °C for 12 hr, and the corresponding curves are plotted in Figure 3a. By comparison with the pristine MWNT used in our previous study17, the acid treated SWNTs were used here. Normally, the functionalization occurs on the defects of CNTs which mainly locates in the ends and defects of the tubes. The Figure 3a shows that the E’ monotonously increases along with dynamic straining, which is consistent to our previous study17. We believe that the acid treatment of SWNT mainly improves its dispersion, but has little or no detectable effect on the interphase PAN development during dynamic straining. The E’ of the films before and after dynamic straining are summarized in Table S1 in ESI. It can be seen that PAN/SWNT nano-composite films have higher initial E’ than PAN films which suggests that the addition of SWNTs improves the mechanical properties of PAN matrix. When SWNT content increases from 0 to 0.1 and to 0.5 wt%, the improvement percentage of E’ after 12 hr of dynamic straining increases from 0.9 to 3.9 and to 13.5 %, respectively. With the addition of more SWNTs, the interfacial area between SWNTs and PAN increases and more fractions of PAN matrix are affected. Additionally, when SWNT content increases from 0.5 to 1.0, and to 1.5 wt%, the improvement percentage of E’ decreases from 13.5 to 9.7, and to 5.4 %, respectively. This decrease could be ascribed to the aggregation of SWNTs at a higher weight fraction, and the aggregation becomes more visible and severe when the content of SWNTs is higher (Figure S3 in ESI). In Table S1, similar trend could be found for the initial E’ of PAN/SWNT nano-composite films. When excess amount of SWNTs is introduced to PAN matrix, the SWNT agglomeration would occur during nano-composite preparation which counteracts the addition of more SWNTs.

Second, we studied the temperature effects on the dynamic strain hardening of PAN/SWNT 10


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nano-composite films (0.5 wt%). The time dependent E’ measured at 50, 60, 80, 90 and 95 °C were compared, and the corresponding curves are shown in Figure 3b. When the temperature increases from 50 to 60, and to 80 °C, the improvement percentage of E’ after 12 hr of dynamic straining increases from 9.5 to 14.3, and to 64.4 %, respectively (Table S2 in ESI). This increase could be ascribed to the improvement of PAN chain mobility at a higher temperature. It is known that PAN has three transitions in the ranges of 0 ~ 20 °C (γ transition), 80 ~ 100 °C (βc or glass transition) and 130 ~ 160 °C (α/αc transition).33-37 The γ transition is caused by the molecular motion in the syndiotactic rich sequences as well as the local mode motions of conformationally disordered sequences.36 When temperature increases, the local molecular motion of PAN chains is enhanced which leads to faster development of interphase structure. If the temperature further increases from 80 to 90 °C, the improvement percentage of E’ decreases from 64.4 to 36.9 %. The glass-transition temperature of PAN/SWNTs films is in the range of 80 ~ 100 °C. When the temperature is equal or higher than PAN glass transition, the oriented PAN chains would relax,38 which weakens the dynamic strain hardening effect.

Figure 3. (a) Time dependent E’ of nano-composite films with various SWNT contents during dynamic strain testing for 12 hr at 35 °C; (b) time dependent E’ of PAN/SWNT (0.5 wt%) nano-composite films measured at 50, 60, 80, 90 and 95 °C for 12 hr. 11



In order to understand the structural recovery after dynamic straining, a PAN/SWNT (0.5 wt%) nano-composite film was subjected to alternative dynamic straining and standstill periods. The time dependent E’ is shown in Figure 4. The E’ increases for 67, 19 and 15 % during the first, second and third dynamic strain periods, respectively. Also, it is noted that a small drop of E’ in the range of 6 ~ 8 % occurs after standstill for 12 hr. This small drop suggests that there is structural recovery in a small extend, and the major structural changes of PAN/SWNT films are maintained after dynamic straining.

Figure 4. Time dependent E’ of PAN/SWNT (0.5 wt%) nano-composite films during alternative dynamic straining and standstill testing at 80 °C. XRD curves of PAN/SWNT films after dynamic straining at various temperatures are summarized in Figure 5a, and the plots of calculated PAN crystal size and crystallinity versus testing temperature are plotted in Figure 5b. The changes of PAN crystal size and crystallinity are consistent to the changes of E’ during dynamic strain testing. The PAN(110, 200) crystal size increases from 4.1 to 4.3, and to 4.8 nm, PAN crystallinity increases from 51.1 to 54.9, and to 58.4 % when testing temperature increases from 50 to 60, and to 80 °C, respectively. At higher testing temperatures, such as 90 and 95 °C, the PAN crystal size and crystallinity are smaller than PAN/SWNT treated at 80 °C. 12


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Based on above results, the optimal conditions for the interphase development of PAN/SWNT nano-composites are 0.5 wt% SWNT content and 80 °C treatment temperature. The XRD curves of PAN and PAN/SWNT nano-composite films subjected to dynamic straining at 80 °C for 0, 6 and 12 hr are included in Figure S4 in ESI. The corresponding structural parameters were calculated from XRD curves and are plotted in Figure 5c-d. Along with dynamic straining, both PAN crystal size and crystallinity decreases for neat PAN film. The crystallization of PAN film decrease for 5.0 %, and the PAN(110,

200)

crystal size slightly

decreases from 4.2 to 4.1 nm. Whereas, for PAN/SWNT nano-composite films, the PAN(110, 200) crystal

size increases from 4.3 to 4.6, and to 4.8 nm after dynamic straining at 80 °C for 0,

6 and 12 hr, respectively. The increment of PAN(110, 200) crystal size indicates that the external dynamic straining induces PAN crystallization. Also, the PAN crystallinity increases from 54.3 to 58.5 % after dynamic straining for 12 hr.

Figure 5. (a) XRD curves of PAN/SWNT (0.5 wt%) nano-composite films after dynamic straining for 12h at various temperatures, and (b) corresponding PAN crystallinity (Xc) and crystal size (XS) of PAN(110, 200); (c) Plots of PAN(110, 200) crystal size and crystallinity versus dynamic straining durations for PAN and (d) PAN/SWNTs (0.5 wt%) films at 80 °C. 13



The SEM images of the cross-sections of PAN and PAN/SWNT films after dynamic straining at 80 °C for 0, 6 and 12 hr are show in Figure 6. The PAN film shows granular-like structures with an average diameter of ~60 nm, and the morphology hardly changes after dynamic straining for 6 and 12 hr. The granular-like PAN structure could be formed due to solvent evaporation and phase separation. By comparison, for PAN/SWNT nano-composite films, the cross-section exhibits fibril structures due to the addition of SWNTs, which is in good agreement with our previous report17. The average fibril diameter increases from 29.7 to 56.3, and to 66.5 nm when dynamic strain duration increases from 0 to 6, and to 12 hr, respectively. It is obvious that the diameters of these fibrils become larger, which suggests the crystallization of PAN along the radial direction of SWNTs or bundles during dynamic straining.

Figure 6. SEM images of the cross-sections of PAN film after dynamic straining for (a) 0 hr, (b) 6 hr and (c) 12 hr, and PAN/SWNT nano-composite films for (d) 0 hr, (e) 6 hr and (f) 12 hr. The corresponding diameter histogram of the fibril structures are shown in (d-1) 0 hr, (e-1) 6 hr and (f-1) 12 hr. 14


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The frequency dependent Tan(δ) versus temperature for PAN and PAN/SWNT films before and after dynamic straining at 80 °C for 12 hr were measured by DMA, and the curves are plotted in Figure S5 in ESI. The activation energy (Ea) of PAN glass transition can be calculated from Arrhenius plots as shown in Figure 7. The glass transition temperature of the PAN/SWNT film measured at 10 Hz is 99 °C, which is 4 °C higher than PAN film since the addition of CNTs restrict the mobility of PAN chains in their vicinities.9-10 For neat PAN films, the glass transition temperature decreases from 95 to 92 °C after dynamic straining. By comparison, the glass transition temperature of PAN/SWNT films increases from 99 to 103 °C after the same dynamic straining. The glass transition activation energies (Ea) were calculated from Arrhenius plots. With the addition of 0.5 wt% of SWNTs, the Ea increases from 421 to 434 kJ/mol. The slightly increment could be ascribed to reduce chain mobility of PAN by the addition of SWNTs. After dynamic straining at 80 °C for 12 hr, the Ea of neat PAN films decreases from 421 to 360 kJ/mol due to the chain relaxation during heating treatment. Whereas, the Ea of PAN/SWNT films significantly increases from 434 to 1192 kJ/mol. This dramatic increase is caused by the development of interphase structure in PAN/SWNT nano-composite films.

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Figure 7. Arrhenius plots of PAN films (a) before and (b) after dynamic straining, and PAN/SWNT films (c) before and (d) after dynamic straining at 80 °C for 12 hr. Effect of PAN Structural Developments on its Cyclization and Carbonization After dynamic straining, the physical structures of PAN and PAN/SWNT films changes. These structural changes of PAN chains are expected to affect their thermal cyclization and carbonization. The DSC heating curves of PAN and PAN/SWNT films after dynamic strain testing at 80 °C for 0, 6 and 12 hr are plotted in Figure 8a-b, and the peak temperature (Tpeak) and cyclization enthalpy (∆HCyclization) are summarized in Table S3. The Tpeak increases from 287.5 to 291.1 °C with the addition of 0.5 wt% of SWNTs. SWNTs restrict and reduce the PAN polymer mobility of the chain in its vicinity regions, which will retard the initiation of cyclization and shift Tpeak to higher temperature. Also, the ∆HCyclization slightly increases from 445.9 to 459.4 J/g. Along with the dynamic straining at 80 °C, the Tpeak of PAN films decreases from 287.5 to 286.8, and to 285.8 °C when the testing duration increases from 0 to 6, and to 12 hr, respectively. The decrease of the Tpeak could be attributed to the PAN chain relaxation. The cyclization of PAN in its amorphous regions is believed to occur at a lower temperature than in crystalline regions.39-40 During the treatment at 80 °C, the PAN chain relaxation leads to more free volume. For the cyclization of PAN, the helical chains have to 16


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transform into a planar configuration. The increase of free volume makes this configuration transformation easier, and thus lowers cyclization peak temperature. Same trend could be found for PAN/SWNT nano-composite films.

The changes of ∆HCyclization show discrepancy between PAN and PAN/SWNT films. At prolonged dynamic straining durations, the ∆HCyclization of PAN films decreases. Whereas, the ∆HCyclization of PAN/SWNT nano-composite film increases. The ∆HCyclization versus the crystallinity of PAN and PAN/SWNT films are plotted in Figure 8c. The ∆HCyclization has a good dependency with the crystallinity of PAN. While PAN crystallinity increases, the ∆HCyclization increases which indicates that the cyclization reaction becomes more completed. These results prove that the cyclization of PAN in crystallized regions has higher degree of completion than amorphous regions.

The TGA curves of PAN and PAN/SWNT films before and after dynamic straining for various durations were measured by TGA in N2 atmosphere, and the weight loss curves are shown in Figure 8d-e. The corresponding weight loss parameters are summarized in Table S4, where Tini is the on-set temperature of PAN cyclization, Tmax is the temperature when the maximum weight loss occurs, DTG is the maximum weight loss rate, and char yield is the residual char percentage at 800 °C. From Table S4, similar to DSC data in Table S3, the Tini and Tmax of both PAN and PAN/CNT films slightly decrease at a longer dynamic straining duration. Whereas, the DTG value moderately increases. The weight loss in the temperature range of 200 to 300 °C is caused by cyclization reaction, and in the temperature range of 300 to 450 °C is due to denitrification and inter-molecule crosslinking. The weight loss of PAN

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films in nitrogen after dynamic strain testing for 0, 6 and 12 hr does not change much or slightly decreases. By comparison, the char yield of PAN/SWNT films moderately increases from 37.8 to 39.2, and to 39.9 wt%, when the dynamic straining duration increases from 0 to 6, and to 12 hr, respectively. In our previous research,41 we observed that the ∆HCyclization or the degree of cyclization completion had a direct correlation with the carbon yields of PAN. Here, the char yields measured by TGA versus the ∆HCyclization measured by DSC are plotted in Figure 8f. During dynamic straining at 80 °C, the crystalline structure of PAN films is slightly destructed; whereas, the interphase crystallization occurs for PAN/SWNT films. A higher crystallized PAN structure benefits a more completion of PAN cyclization reaction, leads to a higher cyclization enthalpy and a higher char yield. Here, the stabilization was carried out in nitrogen. By comparison, if the stabilization was carried out in air, the stabilized structure would have better thermal stability and the char yield would be higher.

Figure 8. DSC curves of (a) PAN and (b) PAN/SWNT films after dynamic straining at 80 °C for various durations; (c) Plot of ∆HCyclization versus the crystallinity of PAN and PAN/SWNT films after dynamic straining for various durations; TGA weight loss curves of (d) PAN and 18


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(e) PAN/SWNT films after dynamic straining for various durations in N2 atmosphere; (f) Plot of char yield versus ∆HCyclization of PAN and PAN/SWNT films after dynamic straining for various durations. The PAN and PAN/SWNT films after dynamic straining for 12 hr were carbonized at 1100 °C under nitrogen. HR-TEM images of these carbonized films were included in Figure 9, and lattice spacing was measured from the images. The fragments of carbonized PAN films are completely amorphous, no graphitic structure was observed. By comparison, the graphitic structures in the carbonized PAN/SWNT films are commonly observed. The average lattice spacing in these graphitic structures is in the range of 0.340 ~ 0.345 nm (Figure 9d). The low temperature graphitization of PAN is caused by the addition of CNTs, and this template effect has been widely discussed in many previous studies.2, 4, 6-8 The formation of these graphitic structures is expected to significantly reinforce the mechanical properties, improve the thermal and electrical conductivities of carbonized PAN films or fibers. It is noted that the films and fibers are dramatic different on their chain orientations. We are carrying more detailed studies on the stiffening behavior of PAN/CNT fibers and observed 22% improvement of storage modulus after dynamic straining for 12 hr. The application of dynamic straining effect on PAN/CNT nano-composite fibers needs more investigations. A schematic of dynamic strain induced interphase development in PAN/SWNT nano-composite films and the resulting carbon structures is shown in Figure 10. The external dynamic straining is found to stimulate the growth of interphase PAN in the vicinity of CNTs, and the development of interphase structure leads to more graphitic structures in the carbonized products. That is why the Raman ID/IG value of the carbonized PAN/SWNT film decreases when the precursor film is subjected to dynamic straining for a longer time (Figure S6b). 19



Figure 9. HR-TEM images of (a) carbonized PAN and (b) carbonized PAN/SWNT films.

Figure 10. Schematic of dynamic strain induced interphase PAN development and the resulting carbonized structures. Conclusions In summary, we compared the effects of SWNT content, temperature and time on the interphase developments of PAN/SWNT nano-composite films under external dynamic straining. SWNT agglomeration and a temperature higher than the glass transition temperature of PAN slow down the interphase development. Thus, the optimal SWNT content and environmental temperature is 0.5 wt% and 80 °C, respectively. The dynamic straining is a

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unique method to stimulate the development of interphase structure in PAN/CNT nano-composites. During dynamic straining at 80 °C, the interphase PAN in the vicinity of CNTs grows while matrix PAN relaxes. For PAN/SWNT nano-composites, the dynamic straining improves PAN crystallinity and crystal size. The cyclization and carbonization studies on PAN and PAN/SWNT films with various physical structures suggest that highly crystallized PAN structures benefit a more completion of cyclization reaction which leads to a higher char yield. Additionally, with the addition of CNT, the interphase PAN forms graphitic structures after carbonization at 1100 °C, while the carbonized matrix PAN is totally amorphous. The dynamic straining behavior of PAN/CNT nano-composites could stimulate the growth of ordered interphase structures. It provides a facial tool to modify the structures of polymer nano-composites and study the structural relationships between polymer precursor and the resulting carbon materials. The interphase development caused by the dynamic strain hardening behavior of PAN/CNT nano-composites could be an effective tool to develop carbon and graphitic films or fibers with ultra-high mechanical performance, thermal and electrical conductivities.

Associated Content ESI. The ESI is available free of charge on the ACS Publications website at DOI: The elemental analysis of SWNT before and after acid-treatment, dispersion behavior of as-received and acid-treated SWNTs, the optical images of PAN/SWNT composite films, storage modulus of PAN and PAN/SWNT composite films before and after dynamic straining, XRD of PAN and PAN/SWNT composite films dynamic certain durations, Frequency dependent Tan(δ) versus temperature curves for PAN and PAN/SWNT composite film before 21



and after dynamic straining, thermal parameters of PAN and PAN/SWNT composite film dynamic straining for certain time characterized by DSC and TGA, Raman results of PAN and PAN/SWNT films after carbonized. Figure S1-S6, Table S1-S4.

Author Information Corresponding Author *E-mail：[email protected]; [email protected].

Notes The authors declare no competing financial interest. Acknowledgement The authors acknowledge the financial support by One Hundred Person Project of the Chinese Academy of Sciences, Hundred Person Project of Shanxi Province, Science Foundation of Shanxi Province (Grant No. 2016011D011020), and National Engineering Laboratory for Carbon Fiber Technology, Institute of Coal Chemistry, Chinese Academy of Science, China.

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SWNT Nano-composite

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