Influence of thermal treatment time on structural and physical properties of polyimide films at beginning of carbonization
Introduction
A good understanding of the carbonization mechanism of polyimides (PI) is critical for the development of improved carbon materials for gas separation, and electrically conducting surfaces for microelectronics devices [1], [2], [3]. According to Lua and Su [4], the release of heteroatoms, most in form of CO and CO2 from cleavage of the imide ring, and removing of aliphatic groups present in the PI chains up to 1023 K promote molecular rearrangements yielding chars. Such carbon-rich derivatives lead to graphite-like structures depending on the temperature and nature of the starting materials [4]. The structural rearrangement may also lead to formation of lamellae [5]. Important contributions concerning the carbonization of PI were made by Bourgerette and Oberlin [6] and Sazanov et al. [7]. Their studies revealed that carbonaceous materials are initially formed below 1273 K and basically composed of aromatic building blocks fused into planar 10-20 planar rings arranged in different configurations named basic structural units (BSU). Beyond 1273 K, after successive orientations and arrangements the BSU coalesce generating graphitic structures.
Aromatic PI [8], [9], [10] over the years have been used as suitable carbon precursors [11]. Shao et al. [12] observed pores after thermal treatment of 6FDA-based PI from degradation of the polymer chain and molecular rearrangements. That enhanced the selectivity for H2, He and CO2 of the samples treated at 773 K with heating rate of 1 K min−1. Partially carbonized PI films are also interesting for aerospace applications. The insulating nature of PI causes electrostatic accumulation and the carbonization process lead to thermally conductive surfaces avoiding local overheating and premature degradation. Heo and Chang [13] and Luong [14] et al. showed that is possible to enhance significantly the mechanical strength and the thermal and electrical conductivity of PI by the mere incorporation of up to 3 wt% of graphene sheets. However, the efficiency of any composite relies on the suitable dispersion of the additive into the matrix, hence, the PI-carbon compatibility could be improved if the graphene or graphene-like structures were generated in situ, providing a mixture at molecular level, as occurs in hybrid materials [15]. Thus, to get more insight regarding the carbonization process taking place at beginning of thermal decomposition, in this work we investigated the influence of thermal treatment time on poly(4,4′-oxydiphenylene-oxydiphthalimide), (POO) films treated at the very 773 K under argon atmosphere. The gases evolved during treatment and the changes on the molecular structure of POO and consequent changes on its physical properties were evaluated by thermoanalytical, spectroscopic and microscopy techniques.
Section snippets
Materials
POO was synthesized from 4,4′-Oxydiphthalic anhydride (ODPA, Sigma-Aldrich, 97%) and 4,4′-oxydianiline (ODA, Sigma-Aldrich, 98%), using N,N-Dimethylacetamide (DMAc, VETEC, 98%) as solvent. DMAc was treated overnight with molecular sieves with 3 Å pores, and under nitrogen (N2, White Martins 99.99%) to provide inert atmosphere. Argon (Ar, White Martins, 99.99%) was used in the thermal treatment.
Synthesis and carbonization procedure
In a flask, 1.5 mmol of ODA were dissolved with 8 mL of DMAc under stirring followed by addition of
PI conversion and thermal stability
The conversion of OO into POO was evaluated by ATR-FTMIR and the spectra are presented in Fig. 1a. The bands assigned to the amide group at 1652 cm−1 (axial deformation of CO bond), and in the range of 1560–1520 cm−1 assigned as angular deformation of CNH bonds were no longer observed after imidization. Instead, the characteristic absorption bands from the imide group emerged at 1778 and 1711 cm−1 assigned to the asymmetric and symmetric axial deformation of CO bond from carbonyl groups, and at
Conclusions
POO was successfully synthesized by polycondensation reactions and thermal imidization, as confirmed by FTIR. TGA under Ar atmosphere revealed higher thermal stability with residual mass yield about 50 wt% at 1253 K. Complementary, Py/MS-GC showed CO2 among the major product of thermal decomposition. The data helped to set a model to describe the thermal decomposition process of POO, which included the possible formation of BSU. The evaluation of the physical properties showed that even at the
Acknowledgements
This research work was supported by CNPq (Grant 142910/2010-4) and FAPESP (CEPID 2013/07793-6) and also under auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. The authors acknowledge the Brazilian Synchrotron Light Source (LNLS) for the WAXS experiments (Proposal: GAR-14024), especially Dr. Matheus Cardoso for all assistance on training on the equipment and data manipulation. The authors also acknowledge the Brazilian
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