Heat of Decomposition and Fire Retardant Behavior of Polyimide-Graphene Nanocomposites

: Polyimide is a high-performance engineering polymer with outstanding thermomechan-ical properties. Because of its inherent ﬁre-retardant properties, polyimide nanocomposite is an excellent material for packaging electronic devices, and it is an attractive electrode material for batteries and supercapacitors. The ﬁre-retardant behavior of polyimide can be remarkably improved when polyimide is reinforced with multilayered graphene sheets. Differential scanning calorimetry and thermogravimetric analysis were used to study the heat of decomposition and gravimetric decomposition rate, respectively, of polyimide-graphene nanocomposites. Polyimide/graphene nanocomposites containing 10, 20, 30, 40, and 50 wt.% of multilayered graphene sheets were heated at a rate of 10 and 30 ◦ C/min in air and in nitrogen atmosphere, respectively. The rate of mass loss was found to remarkably decrease by up to 198% for nanocomposites containing 50 wt.% of graphene. The enthalpy change resulting from the decomposition of the imide ring was found to decrease by 1166% in nitrogen atmosphere, indicating the outstanding heat-shielding properties of multilayered graphene sheets due to their high thermal conductivity. Graphene sheets are believed to form a continuous carbonaceous char layer that protects the imide ring against decomposition, hence decreasing initial mass loss. The enthalpy changes due to combustion, obtained from differential scanning calorimetry, were used to calculate the theoretical heat release rates, a major parameter in the determination of ﬂammability of polymers. The heat release rate decreased by 62% for composites containing 10 wt.% of graphene compared to the neat polyimide matrix. Polyimide has a relatively lower heat of combustion as compared with graphene. However, graphene signiﬁcantly decreases the mass loss rates of polyimide. The combined interaction of graphene and polyimide led to an overall decrease in the heat release rate. It is noted that both mass loss rate and heat of combustion are important factors that contribute to the rate of heat released. The rate of degradation of the polymer char is also shown to decrease in the presence of graphene. From SEM and EDX analysis, it was ascertained that graphene does not undergo appreciable degradation in inert atmosphere. It, therefore, densiﬁes and fortiﬁes the polymer char, hence, decreasing its propensity to degrade. show that the char structure of neat polyimide and the composites degraded in nitrogen atmosphere to a temperature of 900 ◦ C. The neat polymer char shows a highly porous morphology, however, for composites with increasing graphene content, the porosity signiﬁcantly decreases and completely disappears, forming a strong dense char layer. The formation of a dense char layer and reduction in porosity inhibits the evolution of volatile components and slows down polyimide degradation.


Introduction
Polymer nanocomposites are a class of flame retardant technology that was discovered in the early 1990s and has been shown to result in dramatic decreases in peak heat release rates [1,2]. Nanocomposites also enhance other properties of the final system, such as mechanical properties, unlike most flame retardants that are known to decrease the mechanical properties. However, nanocomposites have to be used in combination with other flame retardants in order to meet flame retardancy regulatory requirements [3]. Nanocomposites enhance flame retardancy by forming a barrier around the underlying polymer, which slows down the mass degradation and heat release rates [4][5][6][7]. Polymers containing graphene and layered silicates have been the focus of extensive research [8][9][10]. Even at low loadings (<20 wt.%), these nanofillers significantly change the properties of the materials [11,12].
Graphene, a one atom thick, two-dimensional sheet of sp 2 bonded carbon atoms arranged in a honeycomb structure, has generated great interest amongst material scientists because of its exceptional intrinsic properties [13]. Single-layered graphene has been reported to have Young's modulus of 1 TPa and ultimate strength of up to 130 GPa [14]. Thermal conductivity of 5000 W/(mK) [15] and electrical conductivity of 6000 s/cm [16] have been reported. It also has an extremely high surface area and has permeability [17]. When used in the fabrication of polymer nanocomposites, it results in the improvement of mechanical, electrical, thermal, and gas barrier properties of the polymers.
Aromatic polyimide is a high-performance thermosetting polymer with excellent thermal and thermooxidative stability, radiation and chemical resistance, and mechanical properties. Fiber-reinforced polyimide composites have found applications in the aerospace industries where they are used to replace metal counterparts, resulting in fuel savings due to reduced weight. It is also used to avoid corrosion issues often encountered by metals or ceramics. However, their use as replacement parts for metals or ceramics means that they have to withstand extreme operation temperatures [18]. Polyimide still suffers from thermal oxidative degradation that slowly wears off the materials, resulting in structural failure of the composites [19][20][21][22]. The addition of nanofillers, such as graphene, to organic polymers has resulted in the improvement of thermal stability because of the mass transport barrier to volatiles generated during thermal decomposition and thermal isolation effect of sheets [23,24].
The pyrolytic degradation of an organic polymer in the condensed phase to yield combustible volatile products can be regarded as the first stage in the flaming combustion of the polymer. The presence of flame retardants in the polymer can affect the rate and mode of pyrolysis of the polymer in the condensed phase [25]. The use of DSC and TGA techniques can provide insight into the stages of pyrolytic breakdown of polymers, their thermal stability, and the nature and amount of solid residues and volatile products [25]. The role played in combustion by the initial condensed phase burning of the polymer can, therefore, be well understood from these studies.
Direct measurement of the flammability and fire behavior of materials requires large quantities of materials and can be prohibitively expensive. In this work, the thermal stability and combustion properties of milligram-sized samples were obtained using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Materials and Methods
Graphene powder with the characteristics listed in Table 1 was purchased from Global Graphene Group, Dayton, OH, USA. As shown in Table 1, the graphene used is not single-layer and is relatively cheaper in terms of production cost than the single-layer graphene. The samples fabricated used up to 50 wt.% graphene powder, while maintaining good dispersibility within the polymer matrix with no need for chemical functionalization.
2.1. Synthesis of Graphene/Polyamic Acid 5.2 g of 4,4-oxydianiline (ODA) was added to 100 mL of N-methyl-pyrollidone (NMP) contained in a round-bottomed flask followed by mechanical stirring until the solid completely dissolved in the solvent. Graphene quantity corresponding to 10 wt.% of total solids was added to the solution and stirring continued for 8 h. Pyromellitic dianhydride Energies 2021, 14, 3948 3 of 12 (PMDA) was then added to the mixture, followed by stirring for 12 h, the temperature was maintained at 10 • C. Subsequent mixtures containing 20, 30, 40, and 50 wt.% graphene were synthesized.

Fabrication of Graphene-Polyimide Films
The mixture was solution casted on a glass substrate followed by thermal imidization in a vacuum oven at 120 • C for 2 h and 200 • C for 1 h to form polyimide-graphene composite films (PI-NGS).

Thermal Analysis
The films were subjected to heating rates of 10 • C/min from 25 • C to 725 • C in nitrogen and air atmospheres in a DSC Q20 thermal analyzer purchased from TA instruments, New Castle, Delaware, USA. In TGA, the films were heated from 25 • C to 1000 • C in nitrogen and air at a heating rate of 30 • C/min in a TGA Q50 Thermal Analyzer purchased from TA Instruments, New Castle, DE, USA. Figure 1 shows the TGA weight loss curves of neat polyimide obtained at 30 • C/min in air and in nitrogen. Degradation of polyimide in nitrogen occurs in a single step resulting in the formation of approximately 60% char, as has also been reported by Lua et al. [26]. Reaction mechanism proposed by Hatori et al. [27] and also reported by Cella [28] indicates that the imide ring is the site of initial degradation. This is evidenced by the production of carbon monoxide in large quantities. The cleavage of the imide ring leads to the formation of isocyanate groups as intermediates, with the release of carbon monoxide. With further heat treatment, the isocynate groups combine, leading to the release of carbon dioxide. Other fragments in the residue combine to form aromatic and heteroaromatic rings, with hydrogen released as a by-product. Volatile by-products detected by various groups are: Phenol, aniline, cyanobenzene, dicyanobenzenes, and pthalimide, in addition to CO, CO 2 , H 2 , and N 2 .

Results and Discussion
2.1. Synthesis of Graphene/Polyamic Acid 5.2 g of 4,4-oxydianiline (ODA) was added to 100 mL of N-methyl-pyrollidone (NMP) contained in a round-bottomed flask followed by mechanical stirring until the solid completely dissolved in the solvent. Graphene quantity corresponding to 10 wt.% of total solids was added to the solution and stirring continued for 8 h. Pyromellitic dianhydride (PMDA) was then added to the mixture, followed by stirring for 12 h, the temperature was maintained at 10 °C. Subsequent mixtures containing 20, 30, 40, and 50 wt.% graphene were synthesized.

Fabrication of Graphene-Polyimide Films
The mixture was solution casted on a glass substrate followed by thermal imidization in a vacuum oven at 120 °C for 2 h and 200 °C for 1 h to form polyimide-graphene composite films (PI-NGS).

Thermal Analysis
The films were subjected to heating rates of 10 °C/min from 25 °C to 725 °C in nitrogen and air atmospheres in a DSC Q20 thermal analyzer purchased from TA instruments, New Castle, Delaware, USA. In TGA, the films were heated from 25 °C to 1000 °C in nitrogen and air at a heating rate of 30 °C/min in a TGA Q50 Thermal Analyzer purchased from TA Instruments, New Castle, DE, USA. Figure 1 shows the TGA weight loss curves of neat polyimide obtained at 30 °C /min in air and in nitrogen. Degradation of polyimide in nitrogen occurs in a single step resulting in the formation of approximately 60% char, as has also been reported by Lua et al. [26]. Reaction mechanism proposed by Hatori et al. [27] and also reported by Cella [28] indicates that the imide ring is the site of initial degradation. This is evidenced by the production of carbon monoxide in large quantities. The cleavage of the imide ring leads to the formation of isocyanate groups as intermediates, with the release of carbon monoxide. With further heat treatment, the isocynate groups combine, leading to the release of carbon dioxide. Other fragments in the residue combine to form aromatic and heteroaromatic rings, with hydrogen released as a by-product. Volatile by-products detected by various groups are: Phenol, aniline, cyanobenzene, dicyanobenzenes, and pthalimide, in addition to CO, CO2, H2, and N2.  Burger et al. [29] concluded that the residual char comprised mostly planar polyaromatics. Their proposed degradation mechanism is shown in Figure 2 Evolved gas analysis was used to predict the degradation mechanism. Burger et al. [29] concluded that the residual char comprised mostly planar polyaromatics. Their proposed degradation mechanism is shown in Figure 2 Evolved gas analysis was used to predict the degradation mechanism. Two main weight loss stages occur in degradation in air ( Figure 1). The first one begins around 550 °C and stops around 660 °C . This initial reaction stage overlaps with the stage observed in nitrogen. The second reaction in air appears between 660 and 770 °C . This corresponds to the degradation of the pre-graphitic char at the initial stages and degradation of the more crystalline and highly graphitized graphene char at higher temperatures (for the composites) as reported by Akinyi et al. [30]. The modes of degradation in air and nitrogen are similar, and oxygen only serves to further oxidize the products that were initially formed by the thermal bond-breaking process. Elemental analysis of the char shows that for degradation in air, H and C decreases while O increases. The reverse trend is observed in nitrogen, indicating that the residues obtained in the two atmospheres are vastly different [31]. Figure 3a,b shows the derivative mass loss profiles of PI and PI-NGS composites. Three major decomposition peaks can be identified in Figure 3a. The first decomposition peak occurs between 600 and 650 °C and is attributed to the decomposition of the imide ring. The second and third decomposition peaks occur around 700 and 800 °C , due to the decomposition of polyimide char and graphene, respectively. The first peak shown in Figure 3b, corresponding to the degradation of the cyclic imide ring, decreases in height in the presence of graphene. The rate of degradation of this peak decreases drastically in the presence of multilayer graphene sheets, as shown in Figure 4. At 50 wt.% of graphene, a 198% decrease in rate was calculated. This decrease can be attributed to the high thermal conductivity of graphene that makes it effective as a heat sink, hence, transporting heat away from the flame and slowing down the initial degradation of the material. Its layered structure also makes it effective as a barrier, blocking the diffusion of pyrolysis by-products and the transfer of oxygen [17]. Two main weight loss stages occur in degradation in air ( Figure 1). The first one begins around 550 • C and stops around 660 • C. This initial reaction stage overlaps with the stage observed in nitrogen. The second reaction in air appears between 660 and 770 • C. This corresponds to the degradation of the pre-graphitic char at the initial stages and degradation of the more crystalline and highly graphitized graphene char at higher temperatures (for the composites) as reported by Akinyi et al. [30]. The modes of degradation in air and nitrogen are similar, and oxygen only serves to further oxidize the products that were initially formed by the thermal bond-breaking process. Elemental analysis of the char shows that for degradation in air, H and C decreases while O increases. The reverse trend is observed in nitrogen, indicating that the residues obtained in the two atmospheres are vastly different [31]. Figure 3a,b shows the derivative mass loss profiles of PI and PI-NGS composites. Three major decomposition peaks can be identified in Figure 3a. The first decomposition peak occurs between 600 and 650 • C and is attributed to the decomposition of the imide ring. The second and third decomposition peaks occur around 700 and 800 • C, due to the decomposition of polyimide char and graphene, respectively. The first peak shown in Figure 3b, corresponding to the degradation of the cyclic imide ring, decreases in height in the presence of graphene. The rate of degradation of this peak decreases drastically in the presence of multilayer graphene sheets, as shown in Figure 4. At 50 wt.% of graphene, a 198% decrease in rate was calculated. This decrease can be attributed to the high thermal conductivity of graphene that makes it effective as a heat sink, hence, transporting heat away from the flame and slowing down the initial degradation of the material. Its layered structure also makes it effective as a barrier, blocking the diffusion of pyrolysis by-products and the transfer of oxygen [17].      The rate of degradation of the polymer char is also shown to decrease in the presence of graphene. From SEM and EDX analysis, it was ascertained that graphene does not undergo appreciable degradation in inert atmosphere. It, therefore, densifies and fortifies the polymer char, hence, decreasing its propensity to degrade.

Results and Discussion
The SEM images in Figure 5 show that the char structure of neat polyimide and the composites degraded in nitrogen atmosphere to a temperature of 900 • C. The neat polymer char shows a highly porous morphology, however, for composites with increasing graphene content, the porosity significantly decreases and completely disappears, forming a strong dense char layer. The formation of a dense char layer and reduction in porosity inhibits the evolution of volatile components and slows down polyimide degradation.
OR PEER REVIEW 6 of 12 The rate of degradation of the polymer char is also shown to decrease in the presence of graphene. From SEM and EDX analysis, it was ascertained that graphene does not undergo appreciable degradation in inert atmosphere. It, therefore, densifies and fortifies the polymer char, hence, decreasing its propensity to degrade.
The SEM images in Figure 5 show that the char structure of neat polyimide and the composites degraded in nitrogen atmosphere to a temperature of 900 °C . The neat polymer char shows a highly porous morphology, however, for composites with increasing graphene content, the porosity significantly decreases and completely disappears, forming a strong dense char layer. The formation of a dense char layer and reduction in porosity inhibits the evolution of volatile components and slows down polyimide degradation.  Table 2 presents the summary of EDX area analyses from selected areas of the SEM images for the char residue. Based on the molecular weight of one repeat unit of polyimide, the percentage of carbon, nitrogen and oxygen in the non-degraded polymer is 70.97%, 7.53%, and 21.5%, respectively. The EDX analysis of neat polyimide char shows that the carbon percentage increased by 26% to 98.2%, while the oxygen percentage decreased by 20% to 1.8%. Following the degradation mechanism proposed in Figure 2, the planar polyaromatics structures in the char are expected to comprise mainly of carbon with negligible quantities of oxygen, nitrogen, and hydrogen. The fact that hydrogen and nitrogen are not observed in the EDX analysis of the neat PI char may indicate that their quantities are below the lower limit of detection of the instrument.   Table 2 presents the summary of EDX area analyses from selected areas of the SEM images for the char residue. Based on the molecular weight of one repeat unit of polyimide, the percentage of carbon, nitrogen and oxygen in the non-degraded polymer is 70.97%, 7.53%, and 21.5%, respectively. The EDX analysis of neat polyimide char shows that the carbon percentage increased by 26% to 98.2%, while the oxygen percentage decreased by 20% to 1.8%. Following the degradation mechanism proposed in Figure 2, the planar polyaromatics structures in the char are expected to comprise mainly of carbon with negligible quantities of oxygen, nitrogen, and hydrogen. The fact that hydrogen and nitrogen are not observed in the EDX analysis of the neat PI char may indicate that their quantities are below the lower limit of detection of the instrument. The composites containing graphene show some detectable levels of nitrogen, indicating that there is some protection of the polymer char from the extensive degradation observed in the neat polymer. This explains the higher levels of nitrogen-containing groups in the composite. The composite containing 10 wt.% graphene has a higher percentage of nitrogen compared to the composites with higher loadings of graphene. This indicates better protection of the polymer char at lower loadings, which can be attributed to better dispersion of the graphene in the polymer matrix at lower loadings.
The planar polyaromatic structures formed as residue in the initial degradation stage undergo complete degradation at the second stage in air atmosphere, followed by the degradation of the more crystalline and highly graphitized graphene char at higher temperatures ( Figure 3a).
DSC thermograms (Figure 6) show the heat flow during the degradation of the samples in nitrogen. This is the heat released because of cleavage of the imide ring followed by recombination reactions of the intermediate products leading to a stable char comprised of planar polyaromatics. The composites containing graphene show some detectable levels of nitrogen, indicating that there is some protection of the polymer char from the extensive degradation observed in the neat polymer. This explains the higher levels of nitrogen-containing groups in the composite. The composite containing 10 wt.% graphene has a higher percentage of nitrogen compared to the composites with higher loadings of graphene. This indicates better protection of the polymer char at lower loadings, which can be attributed to better dispersion of the graphene in the polymer matrix at lower loadings.
The planar polyaromatic structures formed as residue in the initial degradation stage undergo complete degradation at the second stage in air atmosphere, followed by the degradation of the more crystalline and highly graphitized graphene char at higher temperatures (Figure 3a).
DSC thermograms (Figure 6) show the heat flow during the degradation of the samples in nitrogen. This is the heat released because of cleavage of the imide ring followed by recombination reactions of the intermediate products leading to a stable char comprised of planar polyaromatics. Total enthalpy change for the degradation process was obtained by numeric integration of the DSC peak over the temperature interval and expressed in kJ/g. At constant pressure, the heat capacity equals the change in enthalpy, as expressed in Equation (1) [32].

= ( )
The total enthalpy change over the temperature range (T1, T2) is given by Equation  Total enthalpy change for the degradation process was obtained by numeric integration of the DSC peak over the temperature interval and expressed in kJ/g. At constant pressure, the heat capacity equals the change in enthalpy, as expressed in Equation (1) [32]. The total enthalpy change over the temperature range (T 1 , T 2 ) is given by Equation (2).
PI-NGS composites release significantly less heat upon degradation than the neat polymer, a 1100% decrease is observed between the neat PI and the composite containing 50 wt.% graphene. The trend is shown in Figure 7. This trend suggests that graphene acts by shielding the imide ring, leading to a decreased extent of degradation of the matrix, hence, preserving its structural integrity. The retention of substantial amounts of carbonaceous protective layer due to the presence of graphene leads to lower rates of heat transfer and significantly less mass loss of polymers [33,34].
Energies 2021, 14, x FOR PEER REVIEW hence, preserving its structural integrity. The retention of substantial amounts o naceous protective layer due to the presence of graphene leads to lower rates of he fer and significantly less mass loss of polymers [33,34].  Figure 8, show two distinct peaks for th and slightly broadened and staggered peaks for the PI-NGS composites. Multip indicate that the mechanism of degradation in air is different from that in iner phere. The intermediates formed from the cleavage of the imide ring react with o form an oxidized product that is more stable and requires higher activation energ decomposition, hence, it decomposes at higher temperatures. Pramoda et al. [3 kinetic study of the degradation of polyimide in air and in nitrogen atmospher reported that the activation energies were stable in nitrogen and air for the initia loss. However, there was a jump at 40% weight loss in air, which they postulated to the formation of a more stable oxidation product that required higher activatio for its decomposition.  Figure 8, show two distinct peaks for the neat PI and slightly broadened and staggered peaks for the PI-NGS composites. Multiple peaks indicate that the mechanism of degradation in air is different from that in inert atmosphere. The intermediates formed from the cleavage of the imide ring react with oxygen to form an oxidized product that is more stable and requires higher activation energy for its decomposition, hence, it decomposes at higher temperatures. Pramoda et al. [35] did a kinetic study of the degradation of polyimide in air and in nitrogen atmospheres. They reported that the activation energies were stable in nitrogen and air for the initial weight loss. However, there was a jump at 40% weight loss in air, which they postulated was due to the formation of a more stable oxidation product that required higher activation energy for its decomposition. decomposition, hence, it decomposes at higher temperatures. Pramoda et al. [3 kinetic study of the degradation of polyimide in air and in nitrogen atmospher reported that the activation energies were stable in nitrogen and air for the initia loss. However, there was a jump at 40% weight loss in air, which they postulated to the formation of a more stable oxidation product that required higher activatio for its decomposition.  Onset degradation temperature is a measure of ignitibility of a sample. Fuel consumption and volatiles generation begin at this temperature. This is determined in DSC by the first detected deviation from the baseline. Graphene is shown to have little to no effect on the ignitability of the polymer. This is reflected in the nearly constant onset of degradation temperature both in air and in nitrogen atmospheres. The imide bond, which is the site for initial degradation, has a lower bond enthalpy (average bond enthalpy = 293 kJ/mol), and therefore, easily degrades at elevated temperatures.
Enthalpy change due to degradation of the imide ring in nitrogen was compared to that in air, as shown in Figure 9. There is a 70% decrease in the heat of degradation of the imide ring in nitrogen compared to that in air. This might be due to the formation of more stable oxidation products that protect the imide bond from further degradation. The heat of degradation of the PI-NGS composites is slightly higher in air than in nitrogen, but constant across the different compositions, as is observed in nitrogen atmosphere too. Polymer degradation occurs faster in air or oxygen than in an inert atmosphere. Reactions between oxygen and alkoxy radicals (RO • ) released from initial degradation products accelerate the degradation of the polymer matrix. This leads to an increased concentration of polymer alkyl radicals (R • ) that lead to high levels of scission and cross-linked products. [36].
Mass loss rates obtained from TGA measurements and enthalpy changes calculated from DSC thermograms were used to estimate the theoretical heat release rates. With the knowledge of the enthalpy changes due to degradation in air, the theoretical heat release rate is calculated by: where ∆H is the enthalpy change due to combustion and . m f uel is the mass loss rate [37]. The uncertainty analysis of this method leads to an error of <10%, assuming complete combustion. This method has few parameters, hence, less propagation of errors.
From Equation (3), the theoretical heat release rates of neat polyimide, grapheme, and polyimide-graphene composites were obtained and graphed in Figure 10. The trend shows that the heat release rate decreases by 62% at 10 wt.% graphene. constant across the different compositions, as is observed in nitrogen atmosphere too. Pol ymer degradation occurs faster in air or oxygen than in an inert atmosphere. Reaction between oxygen and alkoxy radicals (RO • ) released from initial degradation products ac celerate the degradation of the polymer matrix. This leads to an increased concentration of polymer alkyl radicals (R • ) that lead to high levels of scission and cross-linked products [36]. Figure 9. Enthalpy changes of neat PI and PI-NGS composites due to the degradation of the imid bond as measured by DSC in nitrogen and air at a heating rate of 10 °C /min. Mass loss rates obtained from TGA measurements and enthalpy changes calculated from DSC thermograms were used to estimate the theoretical heat release rates. With the knowledge of the enthalpy changes due to degradation in air, the theoretical heat release rate is calculated by: where ∆ is the enthalpy change due to combustion and ̇ is the mass loss rate [37]. The uncertainty analysis of this method leads to an error of <10%, assuming complete combustion. This method has few parameters, hence, less propagation of errors. From Equation (3), the theoretical heat release rates of neat polyimide, grapheme, and polyimide-graphene composites were obtained and graphed in Figure 10. The trend shows that the heat release rate decreases by 62% at 10 wt.% graphene. Figure 10. Predicted heat release rates as a function of wt.% graphene.
The enthalpy change due to the combustion of graphene is reportedly higher than that of neat polyimide [38,39]. Both graphene and polyimide contribute to the decreased heat release rate of the composites. Graphene significantly decreases the mass loss rate of the composites, while the polymer contributes to a lower net enthalpy change of the composites because of its relatively lower individual enthalpy change.