22
and PMMA, respectively. Formation of graphene from (b) PAA and (c) PMMA according to the corresponding temperature of the graph shown above. Evaporated and re-adsorbed molecules are expressed by color: oxygen (orange), hydrogen (blue), carbon (grey), and nitrogen (green).
Finally, we discuss how the quality of the PI-GR products was dependent on the type of PI precursor employed. From the Raman spectra shown in Figure 2.2(d), we concluded that the PI-GRs grown from the BTDA-based PIs were of higher quality than those prepared from the PMDA-based PIs.
It is known that BTDA-based PIs and PMDA-based PIs have different glass temperatures (Tg 280 ℃ ~ 320 ℃ for the former and 400 ℃ ~ 420 ℃ for the latter) but a similar decomposition temperature (Td
500 ℃). The relatively large temperature range between the Tg and the Td displayed by the BTDA-based PIs may have enabled sufficient rearrangement during heat treatment and resulted in the formation of a well-ordered structure.99 Likewise, the small temperature window between the Tg and the Td exhibited by the PMDA-based PIs was insufficient to enable proper rearrangement. In support of this conclusion, annealing PMDA-based PIs at 450 ℃ improved the quality of graphene that was formed (Figure C.5).
23 2.4 Experimental Method & Materials
General: Unless otherwise specified, all reagents were purchased from commercial sources and used without further purification. 1H NMR spectra were recorded in DMSO-d6 (δ = 2.50 ppm) using Bruker 400 MHz spectrometers, respectively. Infrared (FT-IR) spectra were recorded on a 670/620 Varian spectrometer. Thermogravimetric analysis (TGA) data were recorded under argon at a flow rate of 60 mL min-1 on a TA Instruments TGA Q500 module at a heating rate of 100 ℃ min-1 and using a platinum sample pan. The Langmuir-Blodgett (LB) experiment was performed in a KSV mini-trough. The optical image was obtained using a Carl Zeiss Axio scope.A1 and the scanning electron microscope (SEM) images were obtained with a Hitachi S-4800 instrument. The atomic force microscope (AFM) images were collected using a Veeco Dimension 3100 microscopy under the tapping mode. Raman spectra were measured using a WITEC Alpha 300s micro Raman spectrometer equipped with a 532 nm laser.
Transmission electron microscopy (TEM) images were obtained using a FEI Titan Cube G2 60-300.
Synthesis of PAA: An oven-dried of schlenk flask was charged with dianhydride (1.01 equiv.), diamine (1.00 equiv.) and DMAc (0.45 M). The reaction mixture was stirred at room temperature for 23 hrs under nitrogen atmosphere. The solution was poured into large amount of MeOH. The resultant precipitated solid was filtered, washed with MeOH and dried to afford the corresponding PAA. To synthesis PAA salt, Hexadecylamine was added to the aforementioned PAA solution in DMAc and then stirred for 16 hrs. The resultant solution was directly used to LB method without further purification.
LB assembly: The resulting PAA salt solutions were then diluted with a mixture of DMAc:benzene (1:1 v:v) such that the final concentration was 0.8 mg/mL. Afterward, 50 µL of the solution was spread on the trough. After the solvent had evaporated (approximately 4 h), a barrier speed of 7.0 mm/min was used to deposit the PAA salt onto the Cu substrate at 22 mN/m with an up speed of 1.5 mm/min. Before deposition, the Cu substrate was treated with an oxygen plasma for 10 minutes to render the surface hydrophilic. To prepare multilayer-coated films of PAAs, 15 minute delays were employed after each layer of film was deposited. Afterward, the PAA salt coated Cu substrate was dried in an oven thermostatted to 80 ℃ for 1 hour to evaporate the residual solvent. To prepare an LB film of PMMA, a 0.1% of a chlorobenzene solution of PMMA (MicroChem Corp., 950 PMMA C4, 4 % in chlorobenzene) was used; all other steps were unchanged.
Conversion reaction: Samples were loaded under vacuum in a CVD system (1.0 × 10-3 torr) and then heated to 240 ℃ for 30 minutes under a flow of Ar (100 sccm). After increasing the temperature to 1000 ℃ over the course of 5 minutes, the sample was exposed to a stream of H2 (100 sccm) for 15 minutes. Afterward, the sample was cooled upon removal from the furnace and transferred to a Si substrate prepared by depositing 300 nm of SiO2 onto a Cu substrate followed by etching with ammonia persulfate (0.1 M).
24 2.5 Acknowledgements
Portions of this chapter were reproduced from Jo, H. J.; Lyu, J. H.; Ruoff, R. S.; Lim, H.; Yoon, S. I.;
Jeong, H. Y.; Shin, T. J.; Bielawski, C. W.; Shin, H. S. 2D Materials 2016, 4 (1), 014005.
Copyright 2016 IOP Publishing Ltd.
Acknowledges the contributions of Jo, Hye Jin in the synthesis and analysis of graphene and also grateful to Lim, Hyunseob, Yoon, Seong In, C.W. Bielawski, and Shin, Hyeon Suk, for their kind assistance.
25
Chapter 3: Synthesis of aromatic polymers (PPX, BrPPX and PPV) and deciphering their structure
3.1 Introduction
While the CVD method under the gas phase was commercially used to synthesis PPX, researchers explore other methods to synthesis PPX since solution state reaction often gives easy condition and an efficient understanding of its reaction. As previously mentioned, reducing halogenated compounds with metal was typically used to form a carbon-carbon single bond. However, metal removing problem after polymerize and side reaction to form cross-linked product especially with an alkali metal makes insoluble polymer and hard to characterize its property. Researchers explore organic reagents to synthesis PPX and preparing the suitable monomer was rising another issue.
Even those reactions are well developed; researchers explore alternative reagents, organic sources to remove the remaining metal problem, and unexpected side reaction. The organic electron donor is a type of reducing agent, which often forms a radical anion from organic halide by single electron transfer (SET). TDAE, which is representative of OED, can form carbon anion and TDAE2+
through two times of SET, and its redox potentials were measured (E1/2 (CH3CN) = -0.78 and -0.61 V vs SCE),which have a similar reduction potential of zinc (E1/2 = -0.76 V vs SCE).32 Nishiyama and coworkers explore reductive dehalogenation reaction of α-bromo ketones and treated with TDAE in THF solution afforded diketones and diesters with carbon-carbon single bond linkage.33