Effect of different itaconic acid contents of poly(acrylonitrile-co-itaconic acid)s on their carbonization behaviors at elevated temperatures
Graphical abstract
Introduction
Polyacrylonitrile (PAN)-based (co)polymers are recognized as revolutionary materials that have been of great interest for decades due to their utilization as precursors to produce carbon materials [1]. PAN (co)polymers are the leading precursor materials for carbon fibers with high strength, light weight, and excellent thermal/electrical conductivity, comprising approximately 90% of all carbon production worldwide [2,3]. PAN (co)polymer precursors with desirable characteristics for preparing better carbon materials have therefore been the focus of continuous interest [1,[4], [5], [6]].
To transform PAN (co)polymers to carbon materials, several heat treatment steps of stabilization, carbonization, and graphitization are involved [7]. During thermal oxidative stabilization (TOS), PAN (co)polymers form thermally stable ladder structures to withstand the subsequent high-temperature pyrolysis processes [8]. The carbonization process comprises a series of cascade reactions in an inert atmosphere, such as crosslinking, condensation reactions and elimination of elements as volatile gases, resulting in reorganization and coalescence of the cyclized sections [9].
Over the past decades, the characteristics of the structural transformations of PAN-based precursors during TOS have been well investigated [10]. In particular, the effects of the structural and compositional factors of the precursors, such as architecture, tacticity, and type/content/distribution of the comonomers, on their TOS characteristics have been investigated in depth by our group [11], [12], [13], [14], [15]. However, unlike the TOS process, the relationship between the structural characteristics of precursors and their carbonization behaviors has not been explored enough. Some examples include studies to understand the effect of parameters such as the heating rate [16] and carbonization atmosphere [17] on the carbonization behaviors. The chemical transformation of functional groups during carbonization was also investigated by Fourier transform infrared (FT-IR) spectroscopy [18,19], which, however, afforded limited information due to insufficient isolation and low intensities of the characteristic peaks.
X-ray photoelectron spectroscopy (XPS) is a nondestructive method that enables us to study the chemistry and functionality of materials through quantified spectral deconvolutions. Qian et al. tried to track the structural evolution of PAN precursors to carbon fibers by XPS [20]. Bernardo et al. demonstrated a quantitative study on the surface chemistry of carbon fibers [21]. Weidenthaler et al. used XPS to study the development of functional groups on the surface of mesoporous carbons during the pyrolysis of PAN [22]. XPS is obviously helpful for tracking the structural and compositional evolution of precursors during the formation of carbon materials [23], [24], [25], [26], [27].
In line with our extended efforts to provide information on desirable precursors for improved carbon materials, in this study, the effects of the compositional characteristics of PAN (co)polymer precursors on their carbonization behaviors were investigated. Poly(acrylonitrile-co-itaconic acid)s (PAIs) with controlled compositions were carefully prepared through free radical polymerization (FRP), and their structural transformations during the carbonization processes were investigated with FT-IR and XPS. The aim of our present work was to provide an understanding and information on the carbonization characteristics of the precursors, which may allow us to select a better precursor to fabricate carbon materials with expected characteristics such as electrical conductivity and catalytic activity.
Section snippets
Materials
Acrylonitrile (AN, 99%, Aldrich, St. Louis, MO, USA) was dried over CaH2, distilled under reduced pressure, and stored in a freezer at -20°C under nitrogen before use. Itaconic acid (IA, 99%, Aldrich, St. Louis, MO, USA), 2,2’-azobisisobutyronitrile (AIBN, 98%, Aldrich, St. Louis, MO, USA) and dimethyl sulfoxide (DMSO, 99%, Samchun Chemicals, Seoul, Korea) were used as received. All other chemicals were used without further purification.
Preparation of PAN and PAI
PAN and PAI were prepared by FRP in DMSO using a
Results and discussion
Acidic comonomers in PAN copolymers generally reduce the initiation temperature of the cyclization reaction and enhance the oxygen uptake reaction, facilitating ionic TOS and moderating sudden and uncontrolled heat release. IA has been acknowledged as one of the most efficient comonomers for this purpose [29,30]. In this context, PAN and PAIs with different IA contents were selected as model precursors and carefully prepared through FRP under controlled conditions (Fig. S1; see Supplementary
Conclusion
The aim of this study was to provide technical insight into PAN-based precursors for the preparation of carbon materials with desired characteristics by investigating their chemical transformations during carbonization procedures. PAN and PAIs with 1 ~ 6 mol% IA were successfully prepared through well-controlled FRP. The thermally stabilized PAN and PAIs were further subjected to thermal treatments at different carbonization temperatures (400 ~ 1200 °C). As confirmed by FT-IR analyses, –CN,
Supplementary material
Detailed supplementary data (5 Tables and 11 Fig.s) associated with this article are provided as a separate file. mmc1.docx.
CRediT authorship contribution statement
Ravindra V. Ghorpade: Conceptualization, Investigation, Writing - original draft. Sungho Lee: Methodology, Resources, Investigation. Sung Chul Hong: Conceptualization, Writing - original draft, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2019R1A2C1003735). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2020R1A6A1A03043435).
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