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Phthalonitrile prepolymer and PAN blends: New strategy for precursor stabilization and pyrolytic char yield enhancement
Journal Pre-proof Phthalonitrile prepolymer and PAN blends: New strategy for precursor stabilization and pyrolytic char yield enhancement Yu Shan Tay, Ming Liu, Jacob Song Kiat Lim, Hui Chen, Xiao Hu PII:
S0141-3910(19)30384-2
DOI:
https://doi.org/10.1016/j.polymdegradstab.2019.109056
Reference:
PDST 109056
To appear in:
Polymer Degradation and Stability
Received Date: 24 October 2019 Revised Date:
2 December 2019
Accepted Date: 16 December 2019
Please cite this article as: Tay YS, Liu M, Kiat Lim JS, Chen H, Hu X, Phthalonitrile prepolymer and PAN blends: New strategy for precursor stabilization and pyrolytic char yield enhancement, Polymer Degradation and Stability (2020), doi: https://doi.org/10.1016/j.polymdegradstab.2019.109056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Phthalonitrile prepolymer and PAN Blends: New Strategy for Precursor Stabilization and Pyrolytic Char Yield Enhancement
Yu Shan TAY1, Ming LIU2, Jacob Song Kiat LIM1,2, Hui CHEN2, Xiao HU 1,2,3*
1
School of Materials Science and Engineering, Nanyang
Technological University, Singapore 2
Temasek Laboratories@NTU
3
Environmental Chemistry and Materials Centre, NEWRI,
Nanyang Technological University, Singapore
* Corresponding author: School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798, Singapore. Tel.: + 65 6790 4610 E-mail address: [email protected].
1
Abstract New polymer blends of polyacrylonitrile (PAN) and resorcinolbased phthalonitrile prepolymer (pPN) are studied as superior carbon precursors. pPN has a hyperbranch-like structure with multiple terminal nitrile groups available for chemical interactions. The addition of pPN into PAN significantly lowers the cyclization temperature by more than 15 °C during the oxidative stabilization stage which is unprecedented and highly desirable giving rise to a multitude of advantages during carbonization. The presence of pPN also leads to large synergy in char yield by due to the specific interaction between the nitrile terminal groups in the hyperbranch-like pPN and PAN chains. The char yield at 600 °C increased from 57.7% to a remarkable 69.0% when 10 wt% of pPN is added into PAN even though under the same condition the char yield of neat pPN itself is only 48.8%. Additional advantages of this new approach, i.e., large shrinkage reduction and property enhancement, are also observed in the carbonaceous materials obtained from the pPN/PAN blends. Raman spectra reveal that the carbon structure is retained when 10 wt% or less pPN is used.
Keywords: Carbon precursors; Hyperbranch-like phthalonitrile prepolymer; Polyacrylonitrile; Carbonization; Stabilization; Char yield
2
1. Introduction
Carbon materials possess a wide diversity of structures and properties even though they are primarily made up of carbon atoms as the only element[1]. The highly desired properties such as light weight, high mechanical strength, high thermal and electrical conductivity of the carbon materials allow them to be widely used for many applications such as energy storage, separation and catalysis[1, 2]. An important factor that determines the properties of the final carbonized products is the nature of their precursors. Naturally occurring precursors such as wood, coal, lignite, coconut shell, and petroleum pitch are commonly used as a source of carbon precursor[2]. Organic precursors such as PAN and pitch are found to have relatively high char yield and give rise to ordered carbon structure upon carbonization. PAN by far is the most well-known carbon precursor[3-5]. Interesting attempts have been made to improve carbonization process particularly lowering the stabilization temperature and enhancing the char yield by various modifications of PAN. On the other hand, phthalonitrile-based resins are often studied as high temperature resistant polymers due to their high glass transition temperature and also as possible carbon precursor[6-8]. There is no report on the
3
carbonization of the blends of these two polymers although both contain large number of nitrile functional groups.
PAN is a thermoplastic with a melting point centered at 320 °C above its decomposition temperature and glass transition temperature around 110 °C[3]. The pyrolysis of acrylic PAN is a complex multi-step process[9-11]. The multiple reactions such as cyclization, crosslinking, oxidation, dehydrogenation and aromatization take place during the stabilization step of PAN[12-17]. The conversion process of PAN to carbon can be categorized
into
three
main
steps
namely,
oxidative
stabilization, carbonization and graphitization[3]. PAN is usually stabilized in air under a controlled low temperature heating (200 °C to 300 °C) to achieve a cyclized form of the precursor that can undergo further heat treatment without being melted[18].
The cyclization step is important[19] as it is
necessary for the fabrication of specific microstructure[20] which directly impacts the final quality of the resultant carbon materials[21, 22]. The suppression of cyclization exothermicity was found to improve the structure of the carbon material obtained [23]. It is advantageous to promote cyclization at a lower temperature or allowing it to occur over a wider range of temperature[24]. Reduction of exotherms during the oxidative stabilization was achieved through the alteration of the cyclization reaction pathway[25, 26]. For example, cyclization
4
reaction mechanism can be change from a free radical to an ionic reaction by copolymer or terpolymer approach. Acrylic ester, itaconic acid, and acrylamide were attempted as comonomers with acrylonitrile [10, 22, 27, 28]. Reduction of cyclization temperature could be achieved when the right comonomers are used [21]. However, when added as additive to PAN, it may not be effective in initiating the cyclization reaction[27]. Although, the claimed advantageous implication copolymer approach is higher carbon yield, due to the comonomers being sacrificial components, the absolute carbon yield is still low.
Phthalonitrile polymers were first developed by the Keller et al for its potential applications in aerospace, marine and electronic packaging[7] due to their high thermal stability and mechanical properties. Resorcinol-based phthalonitrile prepolymer (pPN) when fully cured is a thermoset with high crosslinked density that is essential for its exceptional thermal and thermooxidative stability[29, 30], superior fire resistance and low water absorptivity[6]. When carbonized at 1000 °C, fully cured pPN yields carbon materials with a high char yield[29]. However, pPN curing is lengthy and tedious multi-step process. Moreover, unlike PAN, neat pPN tends to melt in the initial stage of curing limiting its application as a versatile carbon precursor.
5
Superb properties and wide-ranging applications of carbon materials have spurred search for new precursors with further enhanced performance. While many of reported efforts serve to provide insights into relating the precursor design and stabilization/carbonization mechanisms, the limited extent of improvement does not justify the complexity of added chemical-physical modifications. In this work, a new approach to produce carbon materials by simply blending of PAN and pPN is investigated. Remarkable synergetic increase in pyrolytic char yield and significant reduction of stabilization exothermic temperature are achieved. These desirable results are supported by mechanistic discussion on the inter-molecular interaction between PAN and pPN prepolymer.
2. Experimental Figure 1 summaries the preparation of new precursors via the addition of pPN prepolymer carbon materials in various forms.
6
Figure 1. (a) PAN white powder; (b) in-house synthesized pPN prepolymer green powder (formulae indicating the PN monomer used); (c) solution of pPN/PAN blend in DMF; (d), (e), and (f) are casted film, aerogel and electrospun nanofiber derived from pPN/PAN blend prior to carbonization.
2.1. Materials Polyacrylonitrile (PAN, Mw=150,000 and the density is 1.184 g/cm3) and N, N-dimethylformamide (DMF) were purchased from Sigma Aldrich. Resorcinol-based phthalonitrile (PN) monomer was synthesized (yield ~ 80 %) through nucleophilic displacement of a nitrile-substituent from 4-nitrophthalonitrile by the dialkaline salt generated from resorcinol. Resorcinolbased phthalonitrile prepolymer (pPN) was synthesized by addition of 3.5 mol% of 1,4-Bis(4-aminophenoxy)benzene as curing additive and pre-polymerized at 210 °C under nitrogen
7
purge for 80 min. All materials were used as-received without any further treatment. Figure S1 shows the synthesis scheme and molecular structures of resorcinol-based phthalonitrile (PN) and its prepolymeric forms.
2.2. Preparation of film, aerogel and nanofiber of the pPN/PAN blends 1 g pPN/PAN blends of various amount of pPN (0, 5, 10, 15, 25 wt%) were dissolved in 9 g DMF under magnetic stirring at room temperature. Once it is completely dissolved, films were casted from the viscous solutions on an aluminum plate and dried at 80 °C under vacuum of 20 mbar overnight. Figure 1(d) shows a casted film of pPN(10 wt%)/PAN blend. To prepare porous aerogels, 9 g of each blend solution was placed in a glass petri dish. The mixtures were subjected to vapor-induced phase inversion in a desiccator[31]. Once the gels are formed and stabilized, they are immersed in DI water to washing out the residual DMF before freeze drying. Figure 1 (e) shows a pPN(10 wt%)/PAN aerogel. For nanofiber preparation, a solution of the pPN/PAN blend was loaded into a 10 ml syringe with a 22-gauge blunt tip needle that was subsequently mounted onto a syringe pump with a flow rate control. A positive voltage of 19 kV was applied between the needle and a grounded aluminium foil separated by a distance of about 20
8
cm. The electrospun nanofibers as shown in Figure 1(f) were collected.
2.3. Carbonization Film, nanofiber and aerogel preforms of pPN/PAN were carbonized in a Lindberg tube furnace from Thermo Scientific. The pre-forms were first heated to 250 °C in air and hold for 1 hour to allow stabilization to occur, before further heating at 5 °C/min to 1000 °C in argon gas and held for an hour. More detailed stabilization and carbonization study under simulated conditions was also done using a Thermogravimetric Analyzer (TGA) Q500, TA Instruments. Samples typically about 7 to 10 mg were heated from 40 °C to 250 °C and held isotherm for 20 min, before ramping to above 650 °C at 10 °C/min. The char yields at 600 °C were compared. A differential scanning calorimeter, TA Instruments DSC SDT Q600 was also used to probe further the stabilization exotherms (heat rate 10 °C/min).
2.4. Characterisation A JEOL JSM 7600-F FESEM was used for all morphology examination. Fourier Transform Infra-red (FTIR) in the attenuated total reflection (ATR) mode was employed to determine chemical interaction between the two constituents in the blends. The blend samples in film form were first subject to oxidative isothermal treatment at 250 °C for 1 hour. After
9
cooling, ATR-FTIR spectra from 400-4000 cm-1 with spectral resolution of 4 cm-1 were collected using a Perkin Elmer Instruments Spectrum GX FTIR spectrometer.
Raman spectra of the carbonized films were collected using a Confocal Raman microscopy Witec alpha-300 (Excitation laser wavelength 488nm (2.54eV), gratings 300, 600, 1800 Grooves/mm, vacuum sealed thermoelectric cooled CCD). The argon laser beam was illuminated on samples through a 100X objective lens with a nominal spot size of ~0.5 µm and the laser power was tuned below 1 mw to avoid overheating. Raman peak of Si peak at 520.6 cm-1 was used as a reference for wavenumber calibration. All scans were carried out from 400 to 3500 cm-1 under ambient conditions.
Hardness of carbonized films were measured using an Agilent G200 Nanoindenter fitted with a diamond Berkovich type tip. 20 indents were done for each sample. Compression tests were carried out using an Instron 5567 fitted with a 10 N load cell at a crosshead speed of 0.1mm/min till 80% strain. Samples with dimensions 10 x 10 x 10 mm3 were used. At least 3 specimens were tested for each sample to ensure repeatability.
3. Results and discussion
10
Compatibility between any additive and PAN is important for the formation of defect-less carbon materials[32]. In order to avoid any phase separation or inhomogeneity of PN and PAN, initial screening of conditions was systematically carried out (see supporting document). Although a few separate domains, highlighted by the white circles in Figure S2, appeared when 50 wt% of pPN was blended in PAN, no obvious inhomogeneity was seen for blend samples up to 25 wt% pPN. Hence most subsequent studies were focused on samples up to 25 wt% pPN. Initial screening also revealed that pPN is a far better additive than PN monomer (Figure S3). As shown in Figure S3 (d), the overall surface morphology of carbonized PAN film is smooth with only a few visible defects. With the addition of 10 wt% PN monomer, a larger number of porous defects appeared in the carbonized film as shown in Figure S3 (e). This is likely due to the propensity of PN monomer to crystallize out of the mixture into small domains and subsequently melt when heated to 250 °C above its melting temperature (~186 °C ) of PN monomer, resulting the excess number of defect pores. In contrast, when 10 wt% pPN prepolymer was added, a uniform and defect-less surface morphology was observed Figure S3 (f). It is known that pre-polymerization of PN into pPN eliminates its propensity of crystallization resulting in less defect formation. In addition, the change in morphology indicates high degree of miscibility between the pPN and PAN. More
11
discussion on their interaction will be given later. Figure S4 showed that there was an increase in char yield when either PN monomer or pPN was added. Nevertheless, in order to reduce defects in the obtained carbon material, pPN is chosen for further investigation.
3.1.Char yield analysis Simulated carbonization profiles of the new pPN/PAN blends are revealed in the TGA traces in Figure 2. Figure 2 showed that the addition of pPN indeed resulted in higher char yield. The char yield of pure PAN is 53.9 % at 800 °C, similar to that reported by literature[23, 33]. At around 800 °C, the char yield of properly cured neat PN can reach above 80 %. However, when pPN is cured under the same condition of PAN, the char yield at 800 °C is just 41.8% as shown in Figure 2 (a). The sharp drop in weight at 250 °C for all samples is due to the isothermal oxidative stabilization process where cyclization reaction is taking place between the nitrile groups. Figure 2(b) summaries the difference between the experimental values and calculated values of char yield at 800 °C. Based on the rule of mixture calculation, it is very distinct that the experimental char yield at 800 °C greatly exceeded the theoretical char yield value as reflected in Figure 2 (b). The red line connecting the square data points in Figure 2 (b) shows that regardless of the char yield value of fully cured or partially cured pPN, there will
12
be synergistic effect on char yield enhancement. We believe that the synergy arose from the interaction between PAN and pPN. It is interesting to note that the absolute char yield reaches the maximum at 10 wt% pPN content and although further increase of pPN resulted in higher synergy, there is slight diminishing in the absolute char yield value as seen in Figure 2 (b). Cheng et al demonstrated that 14 hours of acid hydrolysis of homopolyacrylonitrile can increase the char yield of PAN by about 29%[10]. However, in our case, without additional synthetic steps, char yield of pPN (10 wt% )/PAN resulted in 21% higher than the neat PAN. The chemical composition of 0, 10 and 25 wt% pPN/PAN samples were summarized in Table S1. All three blended samples after carbonized at 1000 °C have around 80% of carbon content with pPN (10wt%)/PAN having the highest carbon content at 85%.
13
Figure 2: (a) TGA curves of the various compositions of pPN/PAN precursors. The step change at 250 °C is due to a 20 min isotherm. Arrow indicates precursors of increasing pPN content. (b) Residual char yield at 800°C for precursors with varying pPN contents. Open bars: experimental data. Crossline filled bars: calculated data according to rule of mixture. Square data points: level of synergy indicated by the difference between experimental and calculated char yield values. The best char yield was attained with 10 wt% pPN additive. The 14
char yields of neat PAN and pPN under same curing conditions are indicated by the two horizontal dashed lines.
3.2. Chemical Interaction Films of PAN with 0, 5, 10, 25 weight % of pPN were prepared and characterised using FTIR to determine the interaction between PAN and pPN. PAN has characteristic IR peaks 2937 cm-1, 2243 cm-1 (ߥ≡ே ), 1452 cm-1 and 1356 cm-1 as shown in Figure S5. pPN has unreacted nitrile groups which give rise to the IR peak at 2231 cm-1 as well as a small amount of phthalocyanine (1008 cm-1) and triazine (1520 cm-1 and 1360 cm-1)[29]. The physical blending of the PAN and pPN did not result in the appearance of new IR peaks. Hence, the physical blending does not promote any chemical reaction between the two. The interaction between PAN and pPN prepolymer as deduced from the TGA results is most likely to occur during the heat treatment process.
15
Figure 3: FTIR spectrum showing nitrile peak of 0, 5, 10, 25 wt% of pPN/PAN and neat PN (a) before and (b) after oxidation. In ATR mode. The full spectrum can be found in Figure S5. (c) DSC curves for 0, 5, 10, 15, 25 wt% pPN/PAN in air, with a table showing the temperature values of the main peak for cyclization. pPN (10 wt%)/PAN gives the lowest reaction temperature as reflected by left shift in peak.
16
The presence of PAN nitrile peak in Figure 3 (a) and disappearance of it in Figure 3 (b) shows that the nitrile groups in PAN were fully reacted to form conjugated C=N which corresponds to the peak at 1584 cm-1 (Figure S5) after oxidative stabilization in air. Untreated nitrile groups were observed from neat pPN when subjected to the same heat treatment conditions, as indicated by the IR peak at 2231 cm-1 in Figure 3 (b). However, the IR peak at 2231 cm-1 became less distinct when blended with PAN, even up to 25wt% of pPN. This further suggested reaction between PAN and pPN upon oxidation.
The nitrile group in PAN polymer cyclized to form stable ring structure during stabilization process. The neighbouring nitrile groups experience repulsive force from each other and instead attract nitriles of other polymer chains by dipole-dipole interaction into an ideal orientation for intermolecular interactions[3]. Synergistic increase in char yield were observed when pPN were added. From the FTIR result, it would most likely be the nitrile groups present in the pPN that participated in the chemical interactions between the two. The terminal nitrile groups of the hyperbranch-like pPN serve as alternative for intermolecular interaction with PAN chains which results in the lowering of the onset of cyclization during the stabilization step, whereas only one exotherm peak[34] was observed for neat PAN as shown in Figure 3 (c). As the amount
17
of pPN added increased, the cyclization temperatures were lowered more and optimized at 10 wt%. Further increase in pPN weight percentages resulted in less lowering of the cyclization temperature and even leading to the increase of the cyclization temperature.
Karacan et al demonstrated a 3 °C lowering of exothermic peak temperature by using ferric chloride to assist in the thermal stabilization of PAN prior to carbonization[26]. Figure 3 (c) shows that the PAN cyclization temperature was lowered by 15.5 °C when 10 wt% pPN was added. Since cyclization would enhance the formation of carbonaceous material when subjected to further heat treatment, lowering the cyclization temperature would allow more effective ring formation which would translate to higher char yield as observed by TGA.
3.3.Properties of the carbonaceous materials PAN derived carbon materials can be highly ordered when carbonized at high temperatures. Even though addition of pPN helps to promote the formation of carbonaceous materials, the order of the resulting carbon material may be affected since the chemical interaction between pPN and PAN may affect the structure regularity of ladder structure derived from neat PAN.
18
XRD analysis were also done on pre-carbonized films to measure crystallinity. The 2θ peak at 17.3° and the amorphous halo at around 26.9° is the characteristic of PAN[10, 35, 36]. The largest area under the curve at 17.3° corresponds to the pPN (10 wt%)/PAN casted film (Figure S6(a)). This suggests that the addition of pPN promotes the alignment of chains probably due to the pPN structure being more hyperbranch-like compared to monomer. No dominant phase separation was observed between PAN and pPN and remained compatible for further heat treatment. XRD analysis were also carried out on films carbonized to 1000°C (Figure S6(b)). The interlayer distance corresponding to the 002 reflection was calculated as summarized in Table S2. The interlayer distance increases as amount of pPN increases.
19
Figure 4: Raman spectra of carbonized films and (insert) ID/IG ratio against amount of pPN added. At 5 wt% pPN, the carbonaceous material is more ordered and at 10wt% pPN the order of carbonaceous material is similar to that fabricated from neat PAN.
Figure 4 shows the Raman spectra of the carbonized films from different blends mixture. There were mainly two bands, D band at 1350 cm-1 which corresponds to the disorder carbon and G band at 1586 cm-1 which corresponds to the ordered graphitic carbon. There would be also a 2D band at 2800 cm-1 that corresponds to disordered sp2 carbon. D’’ band, associated with amorphous sp2-bonded carbons or interstitial defects[37], at 1508 cm-1 was observed when more than 15 wt% pPN was added. R-value, also referred to as the ID/IG ratio, provides an indication to the amount of ordered graphite-like crystal 20
structures in carbonaceous materials[23]. From Figure 4 (insert), the calculated ID/IG ratio showed an increasing trend generally. When amount of pPN added is 10 wt% or below, the order of carbonaceous material is not significantly different from that of neat PAN. At low pPN, e.g., 5 wt%, there is even a slight improvement in the order of the carbonized product. However, when more than 15wt% of pPN is added, the order of carbonaceous material is affected. This is because at higher pPN concentrations, the availability of the terminal nitrile groups on the hyperbranch-like pPN largely increased, promoting
chemical
reactions
such
as
intermolecular
trimerization to form triazine rings and in situ cyclization of the PAN main chain as shown in Figure 5. Hence, the cyclization reaction can take place at a lower temperature. Nevertheless, too much pPN addition will result in the distortion of ladder structures due to heavy crosslinks between pPN and PAN leading to a more disordered carbonaceous material.
21
Figure 5: Proposed interaction between PAN and pPN. The high concentration of terminal nitrile groups in hyperbranch-like pPN facilitates the crosslinking via nitrile cyclization between the chains of PAN and pPN.
Besides films, different forms of carbonaceous materials can be fabricated from PAN/pPN blends demonstrating its versatility. The mechanical properties of two forms namely films and porous aerogels were measured. Another property of concern is
22
shrinkage after carbonization. The thickness of each nanofiber under SEM can serve as an indication for shrinkage after carbonization.
Figure 6: (a) Hardness measured using nanoindentation (solid line) of carbonized films obtained from precursors with different pPN concentrations. Also shown is the corresponding shrinkage (dotted line) measured using SEM on nanofibers 23
before and after carbonization. (b) Stress-strain curves of carbonized aerogels from precursors with different pPN concentrations. SEM images (10k magnification) of carbonized aerogels with 0 and 10 wt% pPN are shown as insets, the scale bar measures 1μm.
Nanoindentation was done on carbonized films to give a trend on the changes in hardness as pPN is added. As shown in Figure 6 (a), carbonized films fabricated from neat PAN is very brittle and fragile (Figure S7 (a)). When small amount of pPN is added, the robustness of carbonized films increased allowing it to be easily handled (Figure S7 (b)). However, when too much pPN is added, films become very fragile again.
Nanofibers can be electrospun from pPN/PAN blends. With addition of pPN, thinner as-spun nanofibers are produced which lead to thinner carbon fibers. There is also less shrinkage in pPN/PAN fiber during carbonization in comparison to neat PAN derived nanofibers. Thinner nanofibers give rise to higher porosity and surface area which can be an advantage for applications. Morphology of carbonized electrospun nanofibers can be found in Figure S7. Shrinkage of the fiber during carbonized decreases steadily with increasing amount of pPN as shown in Figure 6 (a).
24
For porous aerogels, compression test was done and the improvement in mechanical properties is also optimized at carbonaceous material fabricated from pPN(10 wt%)/PAN as shown in Figure 6 (b). With 10 wt% pPN added, the resulting carbon aerogel exhibits higher modulus and increased yield stress compared to neat PAN.
A dense monolith can be formed from PAN, however defects will be formed during heat treatment and it can result in mechanical failure when heated to high temperature (Figure S8 (a)). Addition of pPN allows the formation of smooth and crack-free dense carbon monolith (Figure S8 (b)). Heat treated pPN (10wt%)/PAN dense monolith showed different burning behavior from that of PAN (Figure S8 (c), video).
4. Conclusion
The blend of pPN and PAN proves to be a good polymeric precursor for forming carbon structures with enhanced properties and in various dimensional form factors such as nanofibers, films, aerogels and monoliths. Efforts to prepare carbonaceous material using neat PAN in monolithic form has limited success as PAN monolith tends to crack during the heat treatment process. However, when pPN is added to PAN, a
25
smooth and crack free monolith was formed and the interaction between pPN and PAN gave sintering properties.
In summary, the optimised pPN/PAN blend was shown to be a new generation of carbon precursor with enhanced pyrolytic yield through synergistic interaction induced during the thermal oxidative stabilization step in the carbonization process, enhance thermal stability by reducing cyclization temperature without compromising the order of carbonaceous material, produce carbonaceous material with lower shrinkages and better mechanical properties. This work managed to overcome the agglomeration problems of fillers and molecular defects of commoner incorporation, while simultaneously augmenting the physicochemical properties without any additional synthesis or preparative steps. This study serves as a basis for the fundamental study of polymer blends or interpenetrating network morphology to produce carbon materials with enhanced processability and microstructures for various applications.
Acknowledgements This paper was supported by Temasek Laboratories@NTU and School of Materials Science and Engineering, Nanyang Technological
University,
Singapore.
26
References [1] M. Inagaki, F. Kang, M. Toyoda, H. Konno, Advanced materials science and engineering of carbon, ButterworthHeinemann 2014. [2] S. Zhang, K. Dokko, M. Watanabe, Direct synthesis of nitrogen-doped carbon materials from protic ionic liquids and protic salts: structural and physicochemical correlations between precursor and carbon, Chemistry of Materials 26(9) (2014) 2915-2926. [3] E. Cipriani, M. Zanetti, P. Bracco, V. Brunella, M.P. Luda, L. Costa, Crosslinking and carbonization processes in PAN films and nanofibers, Polymer degradation and stability 123 (2016) 178-188. [4] A.K. Gupta, D.K. Paliwal, P. Bajaj, Acrylic precursors for carbon fibers, Journal of Macromolecular Science, Part C: Polymer Reviews 31(1) (1991) 1-89. [5] G. Henrici-Olivé, S. Olivé, The chemistry of carbon fiber formation from polyacrylonitrile, Springer Berlin Heidelberg, Berlin, Heidelberg, 1983, pp. 1-60. [6]
T.M.
Keller,
D.D.
Dominguez,
High
temperature
resorcinol-based phthalonitrile polymer, Polymer 46(13) (2005) 4614-4618. [7] F. Zhao, R. Liu, C. Kang, X. Yu, K. Naito, X. Qu, Q. Zhang, A
novel
high-temperature
naphthyl-based
phthalonitrile
27
polymer: synthesis and properties, RSC Advances 4(16) (2014) 8383-8390. [8] L. Zhang, M. Liu, S. Roy, E.K. Chu, K.Y. See, X. Hu, Phthalonitrile-based carbon foam with high specific mechanical strength and superior electromagnetic interference shielding performance, ACS applied materials & interfaces 8(11) (2016) 7422-7430. [9] K. Yan, L.-B. Kong, Y.-H. Dai, M. Shi, K.-W. Shen, B. Hu, Y.-C. Luo, L. Kang, Design and preparation of highly structure-controllable mesoporous carbons at the molecular level and their application as electrode materials for supercapacitors, Journal of Materials Chemistry A 3(45) (2015) 22781-22793. [10] R. Cheng, Y. Zhou, J. Wang, Y. Cheng, S. Ryu, R. Jin, High char-yield in an-am copolymer by acidic hydrolysis of homopolyacrylonitrile, Carbon letters 14(1) (2013) 34-39. [11] S. Chand, Review Carbon fibers for composites, Journal of Materials Science 35(6) (2000) 1303-1313. [12] Z. Bashir, A critical review of the stabilisation of polyacrylonitrile, Carbon 29(8) (1991) 1081-1090. [13] T.J. Xue, M.A. McKinney, C.A. Wilkie, The thermal degradation of polyacrylonitrile, Polymer Degradation and Stability 58(1-2) (1997) 193-202. [14] M. Zhong, E.K. Kim, J.P. McGann, S.-E. Chun, J.F. Whitacre, M. Jaroniec, K. Matyjaszewski, T. Kowalewski,
28
Electrochemically active nitrogen-enriched nanocarbons with well-defined morphology synthesized by pyrolysis of selfassembled block copolymer, Journal of the American Chemical Society 134(36) (2012) 14846-14857. [15] K.T. Cho, S.B. Lee, J.W. Lee, Facile synthesis of highly electrocapacitive nitrogen-doped graphitic porous carbons, The Journal of Physical Chemistry C 118(18) (2014) 9357-9367. [16] B. Saha, G.C. Schatz, Carbonization in polyacrylonitrile (PAN) based carbon fibers studied by ReaxFF molecular dynamics simulations, The Journal of Physical Chemistry B 116(15) (2012) 4684-4692. [17] B. Saha, A.o. Furmanchuk, Y. Dzenis, G.C. Schatz, Multistep mechanism of carbonization in templated polyacrylonitrile derived fibers: ReaxFF model uncovers origins of graphite alignment, Carbon 94 (2015) 694-704. [18] P. Gutmann, J. Moosburger-Will, S. Kurt, Y. Xu, S. Horn, Carbonization of polyacrylonitrile-based fibers under defined tensile load: Influence on shrinkage behavior, microstructure, and mechanical properties, Polymer degradation and stability 163 (2019) 174-184. [19] Z. Fu, B. Liu, L. Sun, H. Zhang, Study on the thermal oxidative stabilization reactions and the formed structures in polyacrylonitrile
during
thermal
treatment,
Polymer
Degradation and Stability 140 (2017) 104-113.
29
[20] B.S. Kim, J.H. Park, N. Hong, J. Bae, C.-S. Yang, K. Shin, Ultrathin carbon film from carbonization of spin-cast polyacrylonitrile film, Journal of Industrial and Engineering Chemistry 19(5) (2013) 1631-1637. [21] J. Hao, Y. Liu, C. Lu, Effect of acrylonitrile sequence distribution on the thermal stabilization reactions and carbon yields of poly (acrylonitrile-co-methyl acrylate), Polymer Degradation and Stability 147 (2018) 89-96. [22] D.W. Cho, S.C. Hong, Synergistic effect of comonomers on the thermal oxidative stabilization of polyacrylonitrile copolymers for carbon materials, Polymer degradation and stability 161 (2019) 191-197. [23] S. Nataraj, K. Yang, T. Aminabhavi, Polyacrylonitrilebased nanofibers—A state-of-the-art review, Progress in polymer science 37(3) (2012) 487-513. [24] A. Shiedlin, G. Marom, A. Zilkha, Catalytic initiation of polyacrylonitrile stabilization, Polymer 26(3) (1985) 447-451. [25] S. Martin, J. Liggat, C. Snape, In situ NMR investigation into the thermal degradation and stabilisation of PAN, Polymer degradation and stability 74(3) (2001) 407-412. [26] I. Karacan, G. Erdoğan, Ferric Chloride Assisted Thermal Stabilization of Polyacrylonitrile Precursor Fibers Prior to Carbonization, Journal of Inorganic and Organometallic Polymers and Materials 22(5) (2012) 1016-1027.
30
[27] Q. Ouyang, L. Cheng, H. Wang, K. Li, Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile, Polymer Degradation and Stability 93(8) (2008) 1415-1421. [28] Q. Ouyang, X. Wang, X. Wang, J. Huang, X. Huang, Y. Chen, Simultaneous DSC/TG analysis on the thermal behavior of
PAN
polymers
prepared
by
aqueous
free-radical
polymerization, Polymer Degradation and Stability 130 (2016) 320-327. [29] M. Liu, Aromatic heterocyclic resin: precursor for carbon materials and high temperature foams, School of Materials Science and Engineering, Nanyang Technological University, Singapore, 2012. [30] X. Chen, S. Shan, J. Liu, X. Qu, Q. Zhang, Synthesis and properties of high temperature phthalonitrile polymers based on o, m, p-dihydroxybenzene isomers, RSC Advances 5(98) (2015) 80749-80755. [31] X. Chen, Y.N. Liang, X.-Z. Tang, W. Shen, X. Hu, Additive-free poly (vinylidene fluoride) aerogel for oil/water separation and rapid oil absorption, Chemical Engineering Journal 308 (2017) 18-26. [32] E. Jo, J.G. Yeo, D.K. Kim, J.S. Oh, C.K. Hong, Preparation of well‐controlled porous carbon nanofiber materials by varying the compatibility of polymer blends, Polymer International 63(8) (2014) 1471-1477.
31
[33] P. Morgan, Carbon fibers and their composites, CRC press2005. [34] K. Tse-Hao, Y. Ching-Chyuan, C. Wen-Tong, The effect of stabilization on the properties of PAN-based carbon films, Carbon 31(4) (1993) 583-590. [35] D. Papkov, A. Goponenko, O.C. Compton, Z. An, A. Moravsky, X.Z. Li, S.T. Nguyen, Y.A. Dzenis, Improved graphitic structure of continuous carbon nanofibers via graphene oxide templating, Advanced Functional Materials 23(46) (2013) 5763-5770. [36] J. Miao, M. Miyauchi, T.J. Simmons, J.S. Dordick, R.J. Linhardt, Electrospinning of nanomaterials and applications in electronic components and devices, Journal of nanoscience and nanotechnology 10(9) (2010) 5507-5519. [37] C. Kim, S.H. Park, J.I. Cho, D.Y. Lee, T.J. Park, W.J. Lee, K.S.
Yang,
Raman
polyacrylonitrile‐based
spectroscopic carbon
nanofibers
evaluation prepared
of by
electrospinning, Journal of Raman Spectroscopy 35(11) (2004) 928-933.
32
Figure Captions Figure 1. (a) PAN white powder; (b) in-house synthesized pPN prepolymer green powder (formulae indicating the PN monomer used); (c) solution of pPN/PAN blend in DMF; (d), (e), and (f) are casted film, aerogel and electrospun nanofiber derived from pPN/PAN blend prior to carbonization.
Figure 2: (a) TGA curves of the various compositions of pPN/PAN precursors. The step change at 250 °C is due to a 20 min isotherm. Arrow indicates precursors of increasing pPN content. (b) Residual char yield at 800°C for precursors with varying pPN contents. Open bars: experimental data. Crossline filled bars: calculated data according to rule of mixture. Square data points: level of synergy indicated by the difference between experimental and calculated char yield values. The best char yield was attained with 10 wt% pPN additive. The char yields of neat PAN and pPN under same curing conditions are indicated by the two horizontal dashed lines.
Figure 3: FTIR spectrum showing nitrile peak of 0, 5, 10, 25 wt% of pPN/PAN and neat PN (a) before and (b) after oxidation. In ATR mode. The full spectrum can be found in Figure S5. (c) DSC curves for 0, 5, 10, 15, 25 wt% pPN/PAN in air, with a table showing the temperature values of the main peak for
33
cyclization. pPN (10 wt%)/PAN gives the lowest reaction temperature as reflected by left shift in peak.
Figure 4: Raman spectra of carbonized films and (insert) ID/IG ratio against amount of pPN added. At 5 wt% pPN, the carbonaceous material is more ordered and at 10wt% pPN the order of carbonaceous material is similar to that fabricated from neat PAN.
Figure 5: Proposed interaction between PAN and pPN. The high concentration of terminal nitrile groups in hyperbranch-like pPN facilitates the crosslinking via nitrile cyclization between the chains of PAN and pPN.
Figure 6: (a) Hardness measured using nanoindentation (solid line) of carbonized films obtained from precursors with different pPN concentrations. Also shown is the corresponding shrinkage (dotted line) measured using SEM on nanofibers before and after carbonization. (b) Stress-strain curves of carbonized aerogels from precursors with different pPN concentrations. SEM images (10k magnification) of carbonized aerogels with 0 and 10 wt% pPN are shown as insets, the scale bar measures 1μm.
34
•
pPN/PAN blends as new precursors for carbon films, nanofibers and aerogels
•
presence of pPN significantly lowers the cyclization temperature of PAN
•
presence of pPN simultaneously leads to synergy on char yield enhancement
•
proposed mechanism for the chemical interaction between pPN and PAN nitrile groups
•
Carbon materials from blends have lower shrinkage and better mechanical properties
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S0141-3910(19)30384-2
DOI:
https://doi.org/10.1016/j.polymdegradstab.2019.109056
Reference:
PDST 109056
To appear in:
Polymer Degradation and Stability
Received Date: 24 October 2019 Revised Date:
2 December 2019
Accepted Date: 16 December 2019
Please cite this article as: Tay YS, Liu M, Kiat Lim JS, Chen H, Hu X, Phthalonitrile prepolymer and PAN blends: New strategy for precursor stabilization and pyrolytic char yield enhancement, Polymer Degradation and Stability (2020), doi: https://doi.org/10.1016/j.polymdegradstab.2019.109056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Phthalonitrile prepolymer and PAN Blends: New Strategy for Precursor Stabilization and Pyrolytic Char Yield Enhancement
Yu Shan TAY1, Ming LIU2, Jacob Song Kiat LIM1,2, Hui CHEN2, Xiao HU 1,2,3*
1
School of Materials Science and Engineering, Nanyang
Technological University, Singapore 2
Temasek Laboratories@NTU
3
Environmental Chemistry and Materials Centre, NEWRI,
Nanyang Technological University, Singapore
* Corresponding author: School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798, Singapore. Tel.: + 65 6790 4610 E-mail address: [email protected].
1
Abstract New polymer blends of polyacrylonitrile (PAN) and resorcinolbased phthalonitrile prepolymer (pPN) are studied as superior carbon precursors. pPN has a hyperbranch-like structure with multiple terminal nitrile groups available for chemical interactions. The addition of pPN into PAN significantly lowers the cyclization temperature by more than 15 °C during the oxidative stabilization stage which is unprecedented and highly desirable giving rise to a multitude of advantages during carbonization. The presence of pPN also leads to large synergy in char yield by due to the specific interaction between the nitrile terminal groups in the hyperbranch-like pPN and PAN chains. The char yield at 600 °C increased from 57.7% to a remarkable 69.0% when 10 wt% of pPN is added into PAN even though under the same condition the char yield of neat pPN itself is only 48.8%. Additional advantages of this new approach, i.e., large shrinkage reduction and property enhancement, are also observed in the carbonaceous materials obtained from the pPN/PAN blends. Raman spectra reveal that the carbon structure is retained when 10 wt% or less pPN is used.
Keywords: Carbon precursors; Hyperbranch-like phthalonitrile prepolymer; Polyacrylonitrile; Carbonization; Stabilization; Char yield
2
1. Introduction
Carbon materials possess a wide diversity of structures and properties even though they are primarily made up of carbon atoms as the only element[1]. The highly desired properties such as light weight, high mechanical strength, high thermal and electrical conductivity of the carbon materials allow them to be widely used for many applications such as energy storage, separation and catalysis[1, 2]. An important factor that determines the properties of the final carbonized products is the nature of their precursors. Naturally occurring precursors such as wood, coal, lignite, coconut shell, and petroleum pitch are commonly used as a source of carbon precursor[2]. Organic precursors such as PAN and pitch are found to have relatively high char yield and give rise to ordered carbon structure upon carbonization. PAN by far is the most well-known carbon precursor[3-5]. Interesting attempts have been made to improve carbonization process particularly lowering the stabilization temperature and enhancing the char yield by various modifications of PAN. On the other hand, phthalonitrile-based resins are often studied as high temperature resistant polymers due to their high glass transition temperature and also as possible carbon precursor[6-8]. There is no report on the
3
carbonization of the blends of these two polymers although both contain large number of nitrile functional groups.
PAN is a thermoplastic with a melting point centered at 320 °C above its decomposition temperature and glass transition temperature around 110 °C[3]. The pyrolysis of acrylic PAN is a complex multi-step process[9-11]. The multiple reactions such as cyclization, crosslinking, oxidation, dehydrogenation and aromatization take place during the stabilization step of PAN[12-17]. The conversion process of PAN to carbon can be categorized
into
three
main
steps
namely,
oxidative
stabilization, carbonization and graphitization[3]. PAN is usually stabilized in air under a controlled low temperature heating (200 °C to 300 °C) to achieve a cyclized form of the precursor that can undergo further heat treatment without being melted[18].
The cyclization step is important[19] as it is
necessary for the fabrication of specific microstructure[20] which directly impacts the final quality of the resultant carbon materials[21, 22]. The suppression of cyclization exothermicity was found to improve the structure of the carbon material obtained [23]. It is advantageous to promote cyclization at a lower temperature or allowing it to occur over a wider range of temperature[24]. Reduction of exotherms during the oxidative stabilization was achieved through the alteration of the cyclization reaction pathway[25, 26]. For example, cyclization
4
reaction mechanism can be change from a free radical to an ionic reaction by copolymer or terpolymer approach. Acrylic ester, itaconic acid, and acrylamide were attempted as comonomers with acrylonitrile [10, 22, 27, 28]. Reduction of cyclization temperature could be achieved when the right comonomers are used [21]. However, when added as additive to PAN, it may not be effective in initiating the cyclization reaction[27]. Although, the claimed advantageous implication copolymer approach is higher carbon yield, due to the comonomers being sacrificial components, the absolute carbon yield is still low.
Phthalonitrile polymers were first developed by the Keller et al for its potential applications in aerospace, marine and electronic packaging[7] due to their high thermal stability and mechanical properties. Resorcinol-based phthalonitrile prepolymer (pPN) when fully cured is a thermoset with high crosslinked density that is essential for its exceptional thermal and thermooxidative stability[29, 30], superior fire resistance and low water absorptivity[6]. When carbonized at 1000 °C, fully cured pPN yields carbon materials with a high char yield[29]. However, pPN curing is lengthy and tedious multi-step process. Moreover, unlike PAN, neat pPN tends to melt in the initial stage of curing limiting its application as a versatile carbon precursor.
5
Superb properties and wide-ranging applications of carbon materials have spurred search for new precursors with further enhanced performance. While many of reported efforts serve to provide insights into relating the precursor design and stabilization/carbonization mechanisms, the limited extent of improvement does not justify the complexity of added chemical-physical modifications. In this work, a new approach to produce carbon materials by simply blending of PAN and pPN is investigated. Remarkable synergetic increase in pyrolytic char yield and significant reduction of stabilization exothermic temperature are achieved. These desirable results are supported by mechanistic discussion on the inter-molecular interaction between PAN and pPN prepolymer.
2. Experimental Figure 1 summaries the preparation of new precursors via the addition of pPN prepolymer carbon materials in various forms.
6
Figure 1. (a) PAN white powder; (b) in-house synthesized pPN prepolymer green powder (formulae indicating the PN monomer used); (c) solution of pPN/PAN blend in DMF; (d), (e), and (f) are casted film, aerogel and electrospun nanofiber derived from pPN/PAN blend prior to carbonization.
2.1. Materials Polyacrylonitrile (PAN, Mw=150,000 and the density is 1.184 g/cm3) and N, N-dimethylformamide (DMF) were purchased from Sigma Aldrich. Resorcinol-based phthalonitrile (PN) monomer was synthesized (yield ~ 80 %) through nucleophilic displacement of a nitrile-substituent from 4-nitrophthalonitrile by the dialkaline salt generated from resorcinol. Resorcinolbased phthalonitrile prepolymer (pPN) was synthesized by addition of 3.5 mol% of 1,4-Bis(4-aminophenoxy)benzene as curing additive and pre-polymerized at 210 °C under nitrogen
7
purge for 80 min. All materials were used as-received without any further treatment. Figure S1 shows the synthesis scheme and molecular structures of resorcinol-based phthalonitrile (PN) and its prepolymeric forms.
2.2. Preparation of film, aerogel and nanofiber of the pPN/PAN blends 1 g pPN/PAN blends of various amount of pPN (0, 5, 10, 15, 25 wt%) were dissolved in 9 g DMF under magnetic stirring at room temperature. Once it is completely dissolved, films were casted from the viscous solutions on an aluminum plate and dried at 80 °C under vacuum of 20 mbar overnight. Figure 1(d) shows a casted film of pPN(10 wt%)/PAN blend. To prepare porous aerogels, 9 g of each blend solution was placed in a glass petri dish. The mixtures were subjected to vapor-induced phase inversion in a desiccator[31]. Once the gels are formed and stabilized, they are immersed in DI water to washing out the residual DMF before freeze drying. Figure 1 (e) shows a pPN(10 wt%)/PAN aerogel. For nanofiber preparation, a solution of the pPN/PAN blend was loaded into a 10 ml syringe with a 22-gauge blunt tip needle that was subsequently mounted onto a syringe pump with a flow rate control. A positive voltage of 19 kV was applied between the needle and a grounded aluminium foil separated by a distance of about 20
8
cm. The electrospun nanofibers as shown in Figure 1(f) were collected.
2.3. Carbonization Film, nanofiber and aerogel preforms of pPN/PAN were carbonized in a Lindberg tube furnace from Thermo Scientific. The pre-forms were first heated to 250 °C in air and hold for 1 hour to allow stabilization to occur, before further heating at 5 °C/min to 1000 °C in argon gas and held for an hour. More detailed stabilization and carbonization study under simulated conditions was also done using a Thermogravimetric Analyzer (TGA) Q500, TA Instruments. Samples typically about 7 to 10 mg were heated from 40 °C to 250 °C and held isotherm for 20 min, before ramping to above 650 °C at 10 °C/min. The char yields at 600 °C were compared. A differential scanning calorimeter, TA Instruments DSC SDT Q600 was also used to probe further the stabilization exotherms (heat rate 10 °C/min).
2.4. Characterisation A JEOL JSM 7600-F FESEM was used for all morphology examination. Fourier Transform Infra-red (FTIR) in the attenuated total reflection (ATR) mode was employed to determine chemical interaction between the two constituents in the blends. The blend samples in film form were first subject to oxidative isothermal treatment at 250 °C for 1 hour. After
9
cooling, ATR-FTIR spectra from 400-4000 cm-1 with spectral resolution of 4 cm-1 were collected using a Perkin Elmer Instruments Spectrum GX FTIR spectrometer.
Raman spectra of the carbonized films were collected using a Confocal Raman microscopy Witec alpha-300 (Excitation laser wavelength 488nm (2.54eV), gratings 300, 600, 1800 Grooves/mm, vacuum sealed thermoelectric cooled CCD). The argon laser beam was illuminated on samples through a 100X objective lens with a nominal spot size of ~0.5 µm and the laser power was tuned below 1 mw to avoid overheating. Raman peak of Si peak at 520.6 cm-1 was used as a reference for wavenumber calibration. All scans were carried out from 400 to 3500 cm-1 under ambient conditions.
Hardness of carbonized films were measured using an Agilent G200 Nanoindenter fitted with a diamond Berkovich type tip. 20 indents were done for each sample. Compression tests were carried out using an Instron 5567 fitted with a 10 N load cell at a crosshead speed of 0.1mm/min till 80% strain. Samples with dimensions 10 x 10 x 10 mm3 were used. At least 3 specimens were tested for each sample to ensure repeatability.
3. Results and discussion
10
Compatibility between any additive and PAN is important for the formation of defect-less carbon materials[32]. In order to avoid any phase separation or inhomogeneity of PN and PAN, initial screening of conditions was systematically carried out (see supporting document). Although a few separate domains, highlighted by the white circles in Figure S2, appeared when 50 wt% of pPN was blended in PAN, no obvious inhomogeneity was seen for blend samples up to 25 wt% pPN. Hence most subsequent studies were focused on samples up to 25 wt% pPN. Initial screening also revealed that pPN is a far better additive than PN monomer (Figure S3). As shown in Figure S3 (d), the overall surface morphology of carbonized PAN film is smooth with only a few visible defects. With the addition of 10 wt% PN monomer, a larger number of porous defects appeared in the carbonized film as shown in Figure S3 (e). This is likely due to the propensity of PN monomer to crystallize out of the mixture into small domains and subsequently melt when heated to 250 °C above its melting temperature (~186 °C ) of PN monomer, resulting the excess number of defect pores. In contrast, when 10 wt% pPN prepolymer was added, a uniform and defect-less surface morphology was observed Figure S3 (f). It is known that pre-polymerization of PN into pPN eliminates its propensity of crystallization resulting in less defect formation. In addition, the change in morphology indicates high degree of miscibility between the pPN and PAN. More
11
discussion on their interaction will be given later. Figure S4 showed that there was an increase in char yield when either PN monomer or pPN was added. Nevertheless, in order to reduce defects in the obtained carbon material, pPN is chosen for further investigation.
3.1.Char yield analysis Simulated carbonization profiles of the new pPN/PAN blends are revealed in the TGA traces in Figure 2. Figure 2 showed that the addition of pPN indeed resulted in higher char yield. The char yield of pure PAN is 53.9 % at 800 °C, similar to that reported by literature[23, 33]. At around 800 °C, the char yield of properly cured neat PN can reach above 80 %. However, when pPN is cured under the same condition of PAN, the char yield at 800 °C is just 41.8% as shown in Figure 2 (a). The sharp drop in weight at 250 °C for all samples is due to the isothermal oxidative stabilization process where cyclization reaction is taking place between the nitrile groups. Figure 2(b) summaries the difference between the experimental values and calculated values of char yield at 800 °C. Based on the rule of mixture calculation, it is very distinct that the experimental char yield at 800 °C greatly exceeded the theoretical char yield value as reflected in Figure 2 (b). The red line connecting the square data points in Figure 2 (b) shows that regardless of the char yield value of fully cured or partially cured pPN, there will
12
be synergistic effect on char yield enhancement. We believe that the synergy arose from the interaction between PAN and pPN. It is interesting to note that the absolute char yield reaches the maximum at 10 wt% pPN content and although further increase of pPN resulted in higher synergy, there is slight diminishing in the absolute char yield value as seen in Figure 2 (b). Cheng et al demonstrated that 14 hours of acid hydrolysis of homopolyacrylonitrile can increase the char yield of PAN by about 29%[10]. However, in our case, without additional synthetic steps, char yield of pPN (10 wt% )/PAN resulted in 21% higher than the neat PAN. The chemical composition of 0, 10 and 25 wt% pPN/PAN samples were summarized in Table S1. All three blended samples after carbonized at 1000 °C have around 80% of carbon content with pPN (10wt%)/PAN having the highest carbon content at 85%.
13
Figure 2: (a) TGA curves of the various compositions of pPN/PAN precursors. The step change at 250 °C is due to a 20 min isotherm. Arrow indicates precursors of increasing pPN content. (b) Residual char yield at 800°C for precursors with varying pPN contents. Open bars: experimental data. Crossline filled bars: calculated data according to rule of mixture. Square data points: level of synergy indicated by the difference between experimental and calculated char yield values. The best char yield was attained with 10 wt% pPN additive. The 14
char yields of neat PAN and pPN under same curing conditions are indicated by the two horizontal dashed lines.
3.2. Chemical Interaction Films of PAN with 0, 5, 10, 25 weight % of pPN were prepared and characterised using FTIR to determine the interaction between PAN and pPN. PAN has characteristic IR peaks 2937 cm-1, 2243 cm-1 (ߥ≡ே ), 1452 cm-1 and 1356 cm-1 as shown in Figure S5. pPN has unreacted nitrile groups which give rise to the IR peak at 2231 cm-1 as well as a small amount of phthalocyanine (1008 cm-1) and triazine (1520 cm-1 and 1360 cm-1)[29]. The physical blending of the PAN and pPN did not result in the appearance of new IR peaks. Hence, the physical blending does not promote any chemical reaction between the two. The interaction between PAN and pPN prepolymer as deduced from the TGA results is most likely to occur during the heat treatment process.
15
Figure 3: FTIR spectrum showing nitrile peak of 0, 5, 10, 25 wt% of pPN/PAN and neat PN (a) before and (b) after oxidation. In ATR mode. The full spectrum can be found in Figure S5. (c) DSC curves for 0, 5, 10, 15, 25 wt% pPN/PAN in air, with a table showing the temperature values of the main peak for cyclization. pPN (10 wt%)/PAN gives the lowest reaction temperature as reflected by left shift in peak.
16
The presence of PAN nitrile peak in Figure 3 (a) and disappearance of it in Figure 3 (b) shows that the nitrile groups in PAN were fully reacted to form conjugated C=N which corresponds to the peak at 1584 cm-1 (Figure S5) after oxidative stabilization in air. Untreated nitrile groups were observed from neat pPN when subjected to the same heat treatment conditions, as indicated by the IR peak at 2231 cm-1 in Figure 3 (b). However, the IR peak at 2231 cm-1 became less distinct when blended with PAN, even up to 25wt% of pPN. This further suggested reaction between PAN and pPN upon oxidation.
The nitrile group in PAN polymer cyclized to form stable ring structure during stabilization process. The neighbouring nitrile groups experience repulsive force from each other and instead attract nitriles of other polymer chains by dipole-dipole interaction into an ideal orientation for intermolecular interactions[3]. Synergistic increase in char yield were observed when pPN were added. From the FTIR result, it would most likely be the nitrile groups present in the pPN that participated in the chemical interactions between the two. The terminal nitrile groups of the hyperbranch-like pPN serve as alternative for intermolecular interaction with PAN chains which results in the lowering of the onset of cyclization during the stabilization step, whereas only one exotherm peak[34] was observed for neat PAN as shown in Figure 3 (c). As the amount
17
of pPN added increased, the cyclization temperatures were lowered more and optimized at 10 wt%. Further increase in pPN weight percentages resulted in less lowering of the cyclization temperature and even leading to the increase of the cyclization temperature.
Karacan et al demonstrated a 3 °C lowering of exothermic peak temperature by using ferric chloride to assist in the thermal stabilization of PAN prior to carbonization[26]. Figure 3 (c) shows that the PAN cyclization temperature was lowered by 15.5 °C when 10 wt% pPN was added. Since cyclization would enhance the formation of carbonaceous material when subjected to further heat treatment, lowering the cyclization temperature would allow more effective ring formation which would translate to higher char yield as observed by TGA.
3.3.Properties of the carbonaceous materials PAN derived carbon materials can be highly ordered when carbonized at high temperatures. Even though addition of pPN helps to promote the formation of carbonaceous materials, the order of the resulting carbon material may be affected since the chemical interaction between pPN and PAN may affect the structure regularity of ladder structure derived from neat PAN.
18
XRD analysis were also done on pre-carbonized films to measure crystallinity. The 2θ peak at 17.3° and the amorphous halo at around 26.9° is the characteristic of PAN[10, 35, 36]. The largest area under the curve at 17.3° corresponds to the pPN (10 wt%)/PAN casted film (Figure S6(a)). This suggests that the addition of pPN promotes the alignment of chains probably due to the pPN structure being more hyperbranch-like compared to monomer. No dominant phase separation was observed between PAN and pPN and remained compatible for further heat treatment. XRD analysis were also carried out on films carbonized to 1000°C (Figure S6(b)). The interlayer distance corresponding to the 002 reflection was calculated as summarized in Table S2. The interlayer distance increases as amount of pPN increases.
19
Figure 4: Raman spectra of carbonized films and (insert) ID/IG ratio against amount of pPN added. At 5 wt% pPN, the carbonaceous material is more ordered and at 10wt% pPN the order of carbonaceous material is similar to that fabricated from neat PAN.
Figure 4 shows the Raman spectra of the carbonized films from different blends mixture. There were mainly two bands, D band at 1350 cm-1 which corresponds to the disorder carbon and G band at 1586 cm-1 which corresponds to the ordered graphitic carbon. There would be also a 2D band at 2800 cm-1 that corresponds to disordered sp2 carbon. D’’ band, associated with amorphous sp2-bonded carbons or interstitial defects[37], at 1508 cm-1 was observed when more than 15 wt% pPN was added. R-value, also referred to as the ID/IG ratio, provides an indication to the amount of ordered graphite-like crystal 20
structures in carbonaceous materials[23]. From Figure 4 (insert), the calculated ID/IG ratio showed an increasing trend generally. When amount of pPN added is 10 wt% or below, the order of carbonaceous material is not significantly different from that of neat PAN. At low pPN, e.g., 5 wt%, there is even a slight improvement in the order of the carbonized product. However, when more than 15wt% of pPN is added, the order of carbonaceous material is affected. This is because at higher pPN concentrations, the availability of the terminal nitrile groups on the hyperbranch-like pPN largely increased, promoting
chemical
reactions
such
as
intermolecular
trimerization to form triazine rings and in situ cyclization of the PAN main chain as shown in Figure 5. Hence, the cyclization reaction can take place at a lower temperature. Nevertheless, too much pPN addition will result in the distortion of ladder structures due to heavy crosslinks between pPN and PAN leading to a more disordered carbonaceous material.
21
Figure 5: Proposed interaction between PAN and pPN. The high concentration of terminal nitrile groups in hyperbranch-like pPN facilitates the crosslinking via nitrile cyclization between the chains of PAN and pPN.
Besides films, different forms of carbonaceous materials can be fabricated from PAN/pPN blends demonstrating its versatility. The mechanical properties of two forms namely films and porous aerogels were measured. Another property of concern is
22
shrinkage after carbonization. The thickness of each nanofiber under SEM can serve as an indication for shrinkage after carbonization.
Figure 6: (a) Hardness measured using nanoindentation (solid line) of carbonized films obtained from precursors with different pPN concentrations. Also shown is the corresponding shrinkage (dotted line) measured using SEM on nanofibers 23
before and after carbonization. (b) Stress-strain curves of carbonized aerogels from precursors with different pPN concentrations. SEM images (10k magnification) of carbonized aerogels with 0 and 10 wt% pPN are shown as insets, the scale bar measures 1μm.
Nanoindentation was done on carbonized films to give a trend on the changes in hardness as pPN is added. As shown in Figure 6 (a), carbonized films fabricated from neat PAN is very brittle and fragile (Figure S7 (a)). When small amount of pPN is added, the robustness of carbonized films increased allowing it to be easily handled (Figure S7 (b)). However, when too much pPN is added, films become very fragile again.
Nanofibers can be electrospun from pPN/PAN blends. With addition of pPN, thinner as-spun nanofibers are produced which lead to thinner carbon fibers. There is also less shrinkage in pPN/PAN fiber during carbonization in comparison to neat PAN derived nanofibers. Thinner nanofibers give rise to higher porosity and surface area which can be an advantage for applications. Morphology of carbonized electrospun nanofibers can be found in Figure S7. Shrinkage of the fiber during carbonized decreases steadily with increasing amount of pPN as shown in Figure 6 (a).
24
For porous aerogels, compression test was done and the improvement in mechanical properties is also optimized at carbonaceous material fabricated from pPN(10 wt%)/PAN as shown in Figure 6 (b). With 10 wt% pPN added, the resulting carbon aerogel exhibits higher modulus and increased yield stress compared to neat PAN.
A dense monolith can be formed from PAN, however defects will be formed during heat treatment and it can result in mechanical failure when heated to high temperature (Figure S8 (a)). Addition of pPN allows the formation of smooth and crack-free dense carbon monolith (Figure S8 (b)). Heat treated pPN (10wt%)/PAN dense monolith showed different burning behavior from that of PAN (Figure S8 (c), video).
4. Conclusion
The blend of pPN and PAN proves to be a good polymeric precursor for forming carbon structures with enhanced properties and in various dimensional form factors such as nanofibers, films, aerogels and monoliths. Efforts to prepare carbonaceous material using neat PAN in monolithic form has limited success as PAN monolith tends to crack during the heat treatment process. However, when pPN is added to PAN, a
25
smooth and crack free monolith was formed and the interaction between pPN and PAN gave sintering properties.
In summary, the optimised pPN/PAN blend was shown to be a new generation of carbon precursor with enhanced pyrolytic yield through synergistic interaction induced during the thermal oxidative stabilization step in the carbonization process, enhance thermal stability by reducing cyclization temperature without compromising the order of carbonaceous material, produce carbonaceous material with lower shrinkages and better mechanical properties. This work managed to overcome the agglomeration problems of fillers and molecular defects of commoner incorporation, while simultaneously augmenting the physicochemical properties without any additional synthesis or preparative steps. This study serves as a basis for the fundamental study of polymer blends or interpenetrating network morphology to produce carbon materials with enhanced processability and microstructures for various applications.
Acknowledgements This paper was supported by Temasek Laboratories@NTU and School of Materials Science and Engineering, Nanyang Technological
University,
Singapore.
26
References [1] M. Inagaki, F. Kang, M. Toyoda, H. Konno, Advanced materials science and engineering of carbon, ButterworthHeinemann 2014. [2] S. Zhang, K. Dokko, M. Watanabe, Direct synthesis of nitrogen-doped carbon materials from protic ionic liquids and protic salts: structural and physicochemical correlations between precursor and carbon, Chemistry of Materials 26(9) (2014) 2915-2926. [3] E. Cipriani, M. Zanetti, P. Bracco, V. Brunella, M.P. Luda, L. Costa, Crosslinking and carbonization processes in PAN films and nanofibers, Polymer degradation and stability 123 (2016) 178-188. [4] A.K. Gupta, D.K. Paliwal, P. Bajaj, Acrylic precursors for carbon fibers, Journal of Macromolecular Science, Part C: Polymer Reviews 31(1) (1991) 1-89. [5] G. Henrici-Olivé, S. Olivé, The chemistry of carbon fiber formation from polyacrylonitrile, Springer Berlin Heidelberg, Berlin, Heidelberg, 1983, pp. 1-60. [6]
T.M.
Keller,
D.D.
Dominguez,
High
temperature
resorcinol-based phthalonitrile polymer, Polymer 46(13) (2005) 4614-4618. [7] F. Zhao, R. Liu, C. Kang, X. Yu, K. Naito, X. Qu, Q. Zhang, A
novel
high-temperature
naphthyl-based
phthalonitrile
27
polymer: synthesis and properties, RSC Advances 4(16) (2014) 8383-8390. [8] L. Zhang, M. Liu, S. Roy, E.K. Chu, K.Y. See, X. Hu, Phthalonitrile-based carbon foam with high specific mechanical strength and superior electromagnetic interference shielding performance, ACS applied materials & interfaces 8(11) (2016) 7422-7430. [9] K. Yan, L.-B. Kong, Y.-H. Dai, M. Shi, K.-W. Shen, B. Hu, Y.-C. Luo, L. Kang, Design and preparation of highly structure-controllable mesoporous carbons at the molecular level and their application as electrode materials for supercapacitors, Journal of Materials Chemistry A 3(45) (2015) 22781-22793. [10] R. Cheng, Y. Zhou, J. Wang, Y. Cheng, S. Ryu, R. Jin, High char-yield in an-am copolymer by acidic hydrolysis of homopolyacrylonitrile, Carbon letters 14(1) (2013) 34-39. [11] S. Chand, Review Carbon fibers for composites, Journal of Materials Science 35(6) (2000) 1303-1313. [12] Z. Bashir, A critical review of the stabilisation of polyacrylonitrile, Carbon 29(8) (1991) 1081-1090. [13] T.J. Xue, M.A. McKinney, C.A. Wilkie, The thermal degradation of polyacrylonitrile, Polymer Degradation and Stability 58(1-2) (1997) 193-202. [14] M. Zhong, E.K. Kim, J.P. McGann, S.-E. Chun, J.F. Whitacre, M. Jaroniec, K. Matyjaszewski, T. Kowalewski,
28
Electrochemically active nitrogen-enriched nanocarbons with well-defined morphology synthesized by pyrolysis of selfassembled block copolymer, Journal of the American Chemical Society 134(36) (2012) 14846-14857. [15] K.T. Cho, S.B. Lee, J.W. Lee, Facile synthesis of highly electrocapacitive nitrogen-doped graphitic porous carbons, The Journal of Physical Chemistry C 118(18) (2014) 9357-9367. [16] B. Saha, G.C. Schatz, Carbonization in polyacrylonitrile (PAN) based carbon fibers studied by ReaxFF molecular dynamics simulations, The Journal of Physical Chemistry B 116(15) (2012) 4684-4692. [17] B. Saha, A.o. Furmanchuk, Y. Dzenis, G.C. Schatz, Multistep mechanism of carbonization in templated polyacrylonitrile derived fibers: ReaxFF model uncovers origins of graphite alignment, Carbon 94 (2015) 694-704. [18] P. Gutmann, J. Moosburger-Will, S. Kurt, Y. Xu, S. Horn, Carbonization of polyacrylonitrile-based fibers under defined tensile load: Influence on shrinkage behavior, microstructure, and mechanical properties, Polymer degradation and stability 163 (2019) 174-184. [19] Z. Fu, B. Liu, L. Sun, H. Zhang, Study on the thermal oxidative stabilization reactions and the formed structures in polyacrylonitrile
during
thermal
treatment,
Polymer
Degradation and Stability 140 (2017) 104-113.
29
[20] B.S. Kim, J.H. Park, N. Hong, J. Bae, C.-S. Yang, K. Shin, Ultrathin carbon film from carbonization of spin-cast polyacrylonitrile film, Journal of Industrial and Engineering Chemistry 19(5) (2013) 1631-1637. [21] J. Hao, Y. Liu, C. Lu, Effect of acrylonitrile sequence distribution on the thermal stabilization reactions and carbon yields of poly (acrylonitrile-co-methyl acrylate), Polymer Degradation and Stability 147 (2018) 89-96. [22] D.W. Cho, S.C. Hong, Synergistic effect of comonomers on the thermal oxidative stabilization of polyacrylonitrile copolymers for carbon materials, Polymer degradation and stability 161 (2019) 191-197. [23] S. Nataraj, K. Yang, T. Aminabhavi, Polyacrylonitrilebased nanofibers—A state-of-the-art review, Progress in polymer science 37(3) (2012) 487-513. [24] A. Shiedlin, G. Marom, A. Zilkha, Catalytic initiation of polyacrylonitrile stabilization, Polymer 26(3) (1985) 447-451. [25] S. Martin, J. Liggat, C. Snape, In situ NMR investigation into the thermal degradation and stabilisation of PAN, Polymer degradation and stability 74(3) (2001) 407-412. [26] I. Karacan, G. Erdoğan, Ferric Chloride Assisted Thermal Stabilization of Polyacrylonitrile Precursor Fibers Prior to Carbonization, Journal of Inorganic and Organometallic Polymers and Materials 22(5) (2012) 1016-1027.
30
[27] Q. Ouyang, L. Cheng, H. Wang, K. Li, Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile, Polymer Degradation and Stability 93(8) (2008) 1415-1421. [28] Q. Ouyang, X. Wang, X. Wang, J. Huang, X. Huang, Y. Chen, Simultaneous DSC/TG analysis on the thermal behavior of
PAN
polymers
prepared
by
aqueous
free-radical
polymerization, Polymer Degradation and Stability 130 (2016) 320-327. [29] M. Liu, Aromatic heterocyclic resin: precursor for carbon materials and high temperature foams, School of Materials Science and Engineering, Nanyang Technological University, Singapore, 2012. [30] X. Chen, S. Shan, J. Liu, X. Qu, Q. Zhang, Synthesis and properties of high temperature phthalonitrile polymers based on o, m, p-dihydroxybenzene isomers, RSC Advances 5(98) (2015) 80749-80755. [31] X. Chen, Y.N. Liang, X.-Z. Tang, W. Shen, X. Hu, Additive-free poly (vinylidene fluoride) aerogel for oil/water separation and rapid oil absorption, Chemical Engineering Journal 308 (2017) 18-26. [32] E. Jo, J.G. Yeo, D.K. Kim, J.S. Oh, C.K. Hong, Preparation of well‐controlled porous carbon nanofiber materials by varying the compatibility of polymer blends, Polymer International 63(8) (2014) 1471-1477.
31
[33] P. Morgan, Carbon fibers and their composites, CRC press2005. [34] K. Tse-Hao, Y. Ching-Chyuan, C. Wen-Tong, The effect of stabilization on the properties of PAN-based carbon films, Carbon 31(4) (1993) 583-590. [35] D. Papkov, A. Goponenko, O.C. Compton, Z. An, A. Moravsky, X.Z. Li, S.T. Nguyen, Y.A. Dzenis, Improved graphitic structure of continuous carbon nanofibers via graphene oxide templating, Advanced Functional Materials 23(46) (2013) 5763-5770. [36] J. Miao, M. Miyauchi, T.J. Simmons, J.S. Dordick, R.J. Linhardt, Electrospinning of nanomaterials and applications in electronic components and devices, Journal of nanoscience and nanotechnology 10(9) (2010) 5507-5519. [37] C. Kim, S.H. Park, J.I. Cho, D.Y. Lee, T.J. Park, W.J. Lee, K.S.
Yang,
Raman
polyacrylonitrile‐based
spectroscopic carbon
nanofibers
evaluation prepared
of by
electrospinning, Journal of Raman Spectroscopy 35(11) (2004) 928-933.
32
Figure Captions Figure 1. (a) PAN white powder; (b) in-house synthesized pPN prepolymer green powder (formulae indicating the PN monomer used); (c) solution of pPN/PAN blend in DMF; (d), (e), and (f) are casted film, aerogel and electrospun nanofiber derived from pPN/PAN blend prior to carbonization.
Figure 2: (a) TGA curves of the various compositions of pPN/PAN precursors. The step change at 250 °C is due to a 20 min isotherm. Arrow indicates precursors of increasing pPN content. (b) Residual char yield at 800°C for precursors with varying pPN contents. Open bars: experimental data. Crossline filled bars: calculated data according to rule of mixture. Square data points: level of synergy indicated by the difference between experimental and calculated char yield values. The best char yield was attained with 10 wt% pPN additive. The char yields of neat PAN and pPN under same curing conditions are indicated by the two horizontal dashed lines.
Figure 3: FTIR spectrum showing nitrile peak of 0, 5, 10, 25 wt% of pPN/PAN and neat PN (a) before and (b) after oxidation. In ATR mode. The full spectrum can be found in Figure S5. (c) DSC curves for 0, 5, 10, 15, 25 wt% pPN/PAN in air, with a table showing the temperature values of the main peak for
33
cyclization. pPN (10 wt%)/PAN gives the lowest reaction temperature as reflected by left shift in peak.
Figure 4: Raman spectra of carbonized films and (insert) ID/IG ratio against amount of pPN added. At 5 wt% pPN, the carbonaceous material is more ordered and at 10wt% pPN the order of carbonaceous material is similar to that fabricated from neat PAN.
Figure 5: Proposed interaction between PAN and pPN. The high concentration of terminal nitrile groups in hyperbranch-like pPN facilitates the crosslinking via nitrile cyclization between the chains of PAN and pPN.
Figure 6: (a) Hardness measured using nanoindentation (solid line) of carbonized films obtained from precursors with different pPN concentrations. Also shown is the corresponding shrinkage (dotted line) measured using SEM on nanofibers before and after carbonization. (b) Stress-strain curves of carbonized aerogels from precursors with different pPN concentrations. SEM images (10k magnification) of carbonized aerogels with 0 and 10 wt% pPN are shown as insets, the scale bar measures 1μm.
34
•
pPN/PAN blends as new precursors for carbon films, nanofibers and aerogels
•
presence of pPN significantly lowers the cyclization temperature of PAN
•
presence of pPN simultaneously leads to synergy on char yield enhancement
•
proposed mechanism for the chemical interaction between pPN and PAN nitrile groups
•
Carbon materials from blends have lower shrinkage and better mechanical properties
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