Influence of the Processing Conditions on the Rheology and Heat of Decomposition of Solution Processed Hybrid Nanocomposites and Implication to Sustainable Energy Storage

Abstract

This study investigates the properties of solution-processed hybrid polyimide (PI) nanocomposites containing a variety of nanofillers, including polyaniline copolymer-modified clay (PNEA), nanographene sheets (NGSs), and carbon nanotube sheets (CNT-PVDFs). Through a series of experiments, the flow behavior of poly(amic acid) (PAA) solution and PAA suspension containing polyaniline copolymer-modified clay (PNEA) is determined as a function of the shear rate, processing temperature, and polymerization time. It is shown that the neat PAA solution exhibits a complex rheological behavior ranging from shear thickening to Newtonian behavior with increasing shear rate and testing temperature. The presence of modified clay in PAA solution significantly reduced the viscosity of PAA. Differential scanning calorimetry (DSC) analysis showed that polyimide–nanographene sheet (PI NGS) nanocomposites processed at a high spindle speed (100 rpm) have lower total heat of decomposition, which is indicative of improved fire retardancy. The effect of processing temperature on the specific capacitance of a polyimide–CNT-PVDF composite containing electrodeposited polypyrrole is determined using cyclic voltammetry (CV). It is shown that the hybrid composite working electrode material processed at 90 °C produces a remarkably higher overall stored charge when compared to the composite electrode material processed at 250 °C. Consequently, the specific capacitance obtained at a scan rate of 5 mV/s for the hybrid nanocomposite processed at 90 °C is around 858 F/g after one cycle, which is about 6.3 times higher than the specific capacitance of 136 F/g produced by the hybrid nanocomposite processed at 250 °C. These findings show that the properties of the hybrid nanocomposites are remarkably influenced by the processing conditions and highlight the need for process optimization.

1. Introduction

Due to their unique characteristics and excellent thermomechanical properties, polyimides (PIs) are highly desired by scientists and engineers. Polyimides have excellent thermal and mechanical properties and are produced as thin films and coatings from soluble precursors. They have excellent dielectric properties, high optical transparency, a low refractive index, and a high glass transition temperature [1,2,3]. Due to their usefulness as membranes, a composite matrix material, films, fibers, foams, coatings, and adhesives in the microelectronics and photoelectric industries, they are among the most desirable high-performance and high-temperature polymers [4,5,6,7]. It is important to highlight that the right combination of polymer matrix and nanofiller reinforcement can produce polymer nanocomposites with novel structures and improved mechanical, electrical, and thermal properties [8,9,10]. Polyimides have less chain flexibility than aliphatic polymers, which results in a higher glass transition temperature, T_g, higher heat resistance, and good mechanical properties. Polyimides are made up of planar, rigid monomers joined end-to-end at set bond angles [11,12]. As a result, polyimides, which are used as high-temperature insulators and dielectrics, are one of the most widely used polymers in microelectronics [7,13,14,15,16,17]. A common polymer precursor utilized in the production of high-performance polyimides is poly(amic acid) (PAA).

PAA is made up of aromatic rings joined together by amide (-CONH-) functional groups [18]. Outstanding thermal resistance, superior chemical resistance, low creep, and outstanding mechanical strength are the characteristics of polyimides produced from poly(amic acid). These characteristics make them useful for a variety of applications, including high-temperature coatings, electronics packaging, aerospace fuselage, and automotive chassis [7,19]. Hybrid materials composed of a polyimide matrix with nanoclay reinforcement have recently been investigated [20,21,22,23,24]. The studies of polyimide–clay nanocomposites have primarily focused on determining the impact of the nanoclay filler on the barrier properties and coefficient of thermal expansion (CTE) of the polyimide films. In comparison to neat polyimide, the gas permeability of polyimide–clay nanocomposites is lower [25,26,27], and the CTE of the nanocomposite is significantly reduced [28,29,30]. The addition of nanoclay fillers to poly(amic acid) can influence the rheological properties of the poly(amic acid) suspension. The presence of clay can increase the viscosity of the polymer suspension and lead to increased densification and enhanced thermomechanical properties of the polyimide matrix. Nanofillers can also affect the flow behavior of a poly(amic acid) suspension, thereby altering its rheological characteristics [31,32,33]. Typically, rheological tools such as an oscillatory rheometer and a concentric cylinder Couette viscometer are used to investigate the rheological behavior of poly(amic acid) nanocomposites. The data obtained from such tests offer useful insights into the response of the material to shear deformation, temperature, and frequency [34,35]. The rheological behavior of poly(amic acid) nanocomposites can be influenced by various factors, including the nanofiller concentration, dispersion, polymer matrix properties, and processing conditions. Therefore, understanding and controlling these factors can help tailor the processability and mechanical properties of the nanocomposite for desired applications [36,37,38,39]. The nature of the solvent, processing temperature, and filler concentration affect the solution properties of a polymer suspension. At low concentrations, the polymer chains are isolated from one another, with each chain occupying a sphere with a radius R_g. The volume of the polymer coil is dictated by the thermodynamic interactions between the polymer and the solvent in the solution. The intrinsic viscosity, [η], a parameter that can be determined by measuring the viscosity of dilute polymer solutions, is the hydrodynamic volume occupied by a given polymer mass, which best represents the interaction of the polymer with the solvent [40,41]. The intrinsic viscosity is typically determined by measuring the viscosity of a polymer solution at different concentrations and extrapolating the data to zero concentration. A higher intrinsic viscosity indicates a larger polymer size and higher molecular weight [42].

Graphene, an atomic-scale carbon atom lattice, has attracted a great deal of interest due to its remarkable mechanical strength, electrical conductivity, and surface area [43,44]. Graphene-reinforced polyimide should have improved electrical and electrochemical properties. Polyimide–graphene nanocomposites, therefore, provide a special combination of properties appropriate for advanced electrode materials. The same is true for polyimide/carbon nanotube (CNT) composites [45,46,47]. Carbon nanotubes (CNTs) have attracted a lot of attention in energy storage applications due to their unique properties, such as high electrical conductivity, large surface area, and excellent mechanical properties [48]. Li et al. [49] demonstrated the use of CNTs in hybrid anodes for rechargeable lithium-ion batteries. In their study, tin-alloy heterostructures encapsulated in amorphous carbon nanotubes exhibited enhanced cycling stability and rate capability. The inherent high electrical conductivity of CNTs significantly improves the charge–discharge rates in energy storage devices, including supercapacitors and lithium-ion batteries, where rapid charge and discharge cycles are essential [50]. The high surface area of CNTs of about 1315 m²g⁻¹ provides more active sites for electrochemical reactions, resulting in increased capacitance and energy density in supercapacitors [51]. Additionally, CNTs enhance the structural integrity of composite materials and improve the longevity and reliability of energy storage devices [52]. The reinforcement of polymer matrices by CNTs results in enhanced electrical properties, and the resulting nanocomposites can be used in charge storage applications [53]. Their role as a filler material can be attributed in part to their very high aspect ratio. The dimensions of nanotubes are around 1–5 µm in length and 1–6 nm in diameter [54].

The electrical properties of polyimide can be greatly improved by the addition of CNTs because of the latter’s high electrical conductivity [55,56,57]. Polypyrrole (PPy) is a conductive polymer with the inherent ability to store energy through the formation of an electric double layer [58]. Polypyrrole (PPy) displays a pseudocapacitive behavior in addition to its electric double layer storage mechanism. Its structure, characterized by conjugated double bonds, enables electroactivity in both aqueous and organic media, with an electrical conductivity range of 10⁻⁴ to 10⁻² S/cm [59]. Doping PPy with inorganic and organic acids, as well as polymeric stabilizers, has been shown to enhance its electrical and electrochemical properties by influencing its morphology [59].

This study is focused on a novel approach to optimizing the rheological, thermal, and electrochemical properties of polyimide nanocomposites through the incorporation of various nanofillers, including polyaniline-modified montmorillonite clay and nanographene sheets. Unlike the traditional methods, our study systematically investigates how different additives and processing conditions influence the flow behavior, thermal stability, fire retardancy, and electrochemical properties of the nanocomposites. Key advances in our study include innovative material combinations, comprehensive rheological analysis, superior electrochemical performance, and the exploration of the effect of high-speed shearing and processing temperature. This integrated approach to material characterization and process optimization paves the way for the development of advanced polyimide-based materials for energy storage applications.

The purpose of this study is to comprehensively investigate the rheological, thermal, and electrochemical properties of polyimide and polyimide nanocomposites obtained from poly(amic acid) (PAA) solution and PAA suspensions containing 5 wt.% polyaniline-copolymer-modified montmorillonite clay and nanographene sheets. Specifically, this study aims to elucidate how various additives and processing conditions influence the flow behavior, thermal stability, fire retardancy, and electrochemical properties of the nanocomposites. This study seeks to optimize the processability and energy storage ability of PI and its nanocomposite electrode materials for application in high-temperature and safety-critical applications in a sustainable energy storage system.

2. Materials and Methods

2.1. Materials

4,4-oxydianiline (ODA) (97% purity), pyromellitic dianhydride (PMDA) (99% purity), and N-methylpyrrolidone (NMP) (99% purity) were the reagents used in this investigation. They were bought from Sigma-Aldrich St. Louis, MO, USA. The Cloisite 30B clay used in this study is natural montmorillonite (MMT) modified with a polyaniline copolymer. Nanographene sheets (NGSs, 98.48% purity) were purchased from Angstron Materials, Inc., Dayton, OH, USA. All the materials were used as received.

The selection of materials and their respective percentages in this study was guided by several key factors aimed at optimizing the performance of polyimide nanocomposites for high-temperature and safety-critical applications in sustainable energy storage systems. These factors include the processing temperature, which is adjusted to produce structures ranging from dense to porous, and the high spindle speed, which is set to produce uniform and aligned nanofillers. The CV scan rate was varied to adjust the time scale for electrochemical processes. The CNT sheet used in this study is composed of 90% carbon nanotubes (CNTs) and 10% polyvinylidene fluoride (PVDF), as specified by the manufacturer; however, a 10 wt.% polyimide (PI) thin coating was applied onto it in order to improve the thermal stability and mechanical robustness. The incorporation of polyaniline-modified montmorillonite clay (PNEA) and nanographene sheets (NGSs) aimed to improve both the mechanical and electrochemical properties, making the composites suitable for energy storage applications.

2.2. Synthesis and Fabrication of the Nanocomposites

2.2.1. Poly(amic Acid)/Clay Nanocomposites

Poly(amic acid) was synthesized by the reaction of pyromellitic dianhydride (PMDA) with 4,4′-diaminodiphenyl ether (ODA). The resulting poly(amic acid) was dissolved in N-methyl-2-pyrrolidone (NMP) to form a viscous solution. To prepare neat PAA solutions, 0.025 mol of ODA was dissolved in 100 mL of NMP in a three-necked flask, and the solution was maintained at 5 °C with stirring for 30 min. Then, an equimolar amount of PMDA was added to the solution. Polymerization was carried out for 15 h with continuous stirring. To prepare the polyimide–clay nanocomposite, clay was dispersed in the ODA solution and stirred for 6 h, after which an equimolar amount of PMDA was added and stirring continued for another 15 h. Both the neat poly(amic acid) and the poly(amic acid)–clay suspensions were ultrasonicated for 5 min before they were cast onto a glass substrate and rectangular steel (Fe₃C) coupons, respectively (Figure 1a), and thermally treated to form a PI thin film (Scheme 1). Dilute solution viscometry of the poly(amic acid) and the PAA solution containing polyaniline copolymer-modified clay was performed using an Ubbelohde viscometer Cannon instrument Inc., State College, PA, USA. The solvent used to carry out dilute solution viscometry was N-methylpyrrolidone (NMP), and the solution concentrations ranged from 1 to 0.4 g/dL. The time taken by the solvent and the respective solutions to pass through the marked section of the viscometer was measured, and each run was repeated until a deviation of less than 2 s was obtained. The sequential addition and ultrasonication steps were designed to ensure uniform dispersion of the nanofillers. The thermal treatment conditions were chosen to produce PI with a range of structures and porosities.

2.2.2. Preparation of Polyimide–Carbon Nanotube/Poly(vinylidene Fluoride) (PI/CNT-PVDF) Nanocomposites

A carbon nanotube (CNT) sheet containing up to 90 wt.% of CNTs dispersed onto a polyvinylidene fluoride (PVDF) matrix was used to prepare the polyimide/CNT-PVDF composite. PAA solution synthesized by reacting equimolar amounts of PMDA and ODA in N-methyl-2-pyrrolidone (NMP) was used. The PAA solution was uniformly applied onto the nanotube sheet by the solution casting method. The coated CNT sheet was initially cured in a vacuum oven at 90 °C for several hours to remove the solvent. This was followed by a stepwise thermal treatment process, raising the temperature to 250 °C for an additional six hours under a vacuum of 28 in.Hg. The thermal treatment converts the PAA into polyimide, resulting in a stable and robust composite material (Figure 1b). After cooling, the composite sheet was carefully peeled off the substrate. The final product was a CNT-PVDF sheet uniformly covered with polyimide. The composite sheet was subsequently used as the working electrode for the electrochemical deposition of polypyrrole (PPy). This meticulous process ensures the film’s stability and integrity, preventing shrinkage and defects.

2.2.3. Polyimide–Nanographene Composite (PI-NGC)

Around 10 mL of the PAA-NGC mixture was evenly cast onto a glass substrate and thermally treated in a vacuum oven. The thermal treatment process involved heating at 120 °C for 2 h, followed by increasing the temperature to 200 °C for 1 h to form a polyimide–nanographene sheet composite (Figure 1c). This stepwise approach ensured that the film remained stable without shrinkage. Subsequent mixtures containing 10, 20, 40, and 60 wt.% NGS were prepared, and the samples were designated based on the graphene sheet content in the composites as PI-NGC-10, PI-NGC-20, PI-NGC-40, and PI-NGC-60. The fully cured films had a thickness ranging from 100 to 200 µm.

2.2.4. Polypyrrole Electrodeposition and Doping

Pyrrole (Py) was dissolved in 100 mL of water to make a 0.5 M solution, to which 0.0225 M p-Toluene sulfonic acid was added. The mixture was stirred until it completely dissolved, resulting in a clear solution. The CNT-PVDF/PI film was used as the working electrode. Each electrode was immersed in 0.5 M pyrrole solution in a three-electrode cell configuration connected to a Gamry 3000 Potentiostat, Gamry Instruments, Warminster, PA, USA (Figure 1d). A glassy carbon counter electrode and a Ag/AgCl reference electrode were used to perform potentiostatic electrochemical polymerization by applying 2 V for 60 s. After the deposition of polypyrrole was completed, each electrode was rinsed with ethanol, dried in a vacuum oven at 100 °C to remove moisture, and weighed.

2.3. Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) was used to investigate the thermal behavior of the samples. DSC was conducted at a heating rate of 5 °C/min in two temperature ranges of 25 to 350 °C and 25 to 725 °C for the samples designated based on the nanographene sheet composition as PI-NGC-10, PI-NGC-10 (100 RPM), and PI-NGC-40 (100 RPM).

2.4. Cyclic Voltammetry (CV)

Electrochemical characterization was carried out using the Gamry 3000 potentiostat in a 3-electrode cell configuration with a Ag/AgCl reference electrode. Cyclic voltammetry (CV) was carried out at a voltage range between 0 and 1 V using two different scan rates of 5 and 25 mV/s for 1 to 10 cycles to determine the peak current, total charge stored, and specific capacitance of the nanocomposite electrode.

3. Results and Discussion

3.1. Rheology

A concentric cylinder rotary viscometer (Brookfield Viscometer) AMETEK Brookfield, Middleboro, MA, USA was used to measure the viscosity of the poly(amic acid) (PAA) solution and the PAA suspension containing 5 wt.% polyaniline copolymer-modified (PNEA) Cloisite 30B clay. The spindle speed was varied from 0.5 to 100 rpm. The viscometer measured the viscosity in centipoise (cP) and torque percentage as functions of temperatures and spindle speed. The simplified version of the constitutive equation (Equation (1)) for the concentric cylinder viscometer was used to calculate the shear stress at the bob surface. The Calderbank and Moo-Young equation (Equation (2)) was used to calculate the shear rate at the bob surface.

$${\tau }_b={\displaystyle \frac{T}{2\pi R_b^{3}l}}$$

(1)

$${\left({\displaystyle \frac{dV}{dr}}\right)}_b={\displaystyle \frac{4\pi N}{1-{\left(\frac{R_c}{R_b}\right)}^2}}{C}_R$$

(2)

The rheological behavior of the poly(amic acid) (PAA) solution and its suspension containing 5 wt.% polyaniline copolymer-modified (PNEA) clay was fitted using the power law model (Equations (3) and (4)). The power law model, also known as the Ostwald–de Waele model, is used to describe the flow behavior of fluids that exhibit non-Newtonian behavior:

$$\eta =K{\dot{\gamma }}^{\left(n-1\right)}$$

(3)

where η is the viscosity, K is the consistency index, $\dot{\gamma }$ is the shear rate, n is the flow behavior index, ${\tau }_b$ is the shear stress at the bob, N is the spindle speed, $R_b$ is the bob radius, $R_c$ is the cup radius, and T is the torque.

Viscosity measurements were taken at temperatures between 25 and 95 °C and at spindle speeds between 0.5 and 100 rpm.

3.1.1. Occurrence of Two Regions of Rheological Behavior

Equation (4) was used to fit the data obtained and calculate the power law parameters K and n.

$$\ln \left(\eta \right)=\ln K+\left(n-1\right)\ln \dot{\gamma }$$

(4)

From Figure 2, it is shown that at 25 °C, the viscosity of the neat PAA shows two important trends: an initial increase in viscosity up to a shear rate of about 100 s⁻¹, followed by a leveling-off region with a constant viscosity of around 1500 cP. Figure 2 shows that the PAA solution behaved as a dilatant fluid at a low shear rate and transitioned into a Newtonian fluid at moderate shear rates ≥ 100 s⁻¹ (Figure 2). By fitting the first region of the rheological behavior with the power law model, an exponent n value of around 2 was obtained, which is indicative of rheological dilatancy behavior. As the testing temperature was increased to 45 °C, 60 °C, and 95 °C, the shear rate at which the transition from dilatant to Newtonian behavior occurred increased from 100 s⁻¹ at 25 °C to 400 s⁻¹ at 95 °C. The transition from power law to Newtonian behavior is associated with both the critical shear rate and the leveling-off viscosity (Figure 2). At 45 °C, the transition occurred at a critical shear rate of around 200 s⁻¹ and a leveling-off viscosity of around 800 cP. At 60 °C, the transition occurred at a critical shear rate and viscosity of around 240 s⁻¹ and 544 cP, respectively. As shown in Figure 2, higher testing temperatures increased the critical shear rate for the onset of Newtonian behavior and decreased the steady-state Newtonian viscosity. The increasing steepness of the curves at higher temperatures signifies more pronounced shear-thickening behavior, due to faster evaporation of the solvent.

Figure 3 shows the variation in the critical shear rate and Newtonian (plateau) viscosity with the testing temperature. The steady-state plateau viscosity decreases while the critical shear rate increases with increasing temperature. The intersection of the critical shear rate versus temperature curve and the steady-state viscosity versus temperature curve occurred at 60 °C, corresponding to a critical shear rate of 220 s⁻¹ and viscosity of 600 cP, which is indicative of the optimal rheological condition. Figure 3 also shows that as the testing temperature increased, the steady-state viscosity decreased from approximately 1000 cP at 25 °C to 300 cP at 95 °C, indicating that the polymer fluid flows better at higher temperatures. Concurrently, the critical shear rate, marking the transition from power law flow behavior to Newtonian flow behavior, increased from around 120 s⁻¹ at 25 °C to 365 s⁻¹ at 95 °C. These observations indicate that the PAA solution shows two distinctive regions of rheological behavior: a rheological dilatancy region at low shear rates and a steady-state Newtonian region characterized by invariant viscosity at moderate to higher shear rates.

Figure 4 shows the dependence of viscosity on the shear rate and testing temperature for the PAA-PNEA suspension. It shows nearly Newtonian behavior at low testing temperatures of 25 °C and 40 °C and a transition from shear thickening to Newtonian flow behavior at higher testing temperatures of 60 °C and 95 °C. Analysis of the first region of rheological behavior of the PAA/PNEA clay suspension at 60 °C and 95 °C indicates a power law index of around 2, which is consistent with shear thickening behavior. The inclusion of PNEA clay into the PAA solution significantly reduced the steady-state viscosity. This suggests that the modified clay particles disrupt the polymer matrix, facilitating polymer chain mobility and reducing resistance to flow. The addition of PNEA clay reduced the viscosity of the PAA solution across all temperatures and significantly modified the rheological behavior of the solution. This behavior can be attributed to the modified clay particles’ ability to facilitate poly(amic acid) chain mobility and reduce chain entanglement, leading to decreased flow resistance. Figure 4 also shows that as the testing temperature increases, the PAA solution’s transition from power law flow to Newtonian flow behavior becomes more pronounced. At higher temperatures (60 °C and 95 °C), both the neat PAA solution and the PAA/PNEA clay suspension are characterized by lower plateau Newtonian viscosity and higher critical shear rates, indicating compliance with both the Arrhenius law and the Oswald–de Waele law.

3.1.2. Effect of Polymerization Time

Figure 5 shows the dependence of viscosity on the polymerization time and shear rate. The viscosity of the polymerizing solution increased with both polymerization time and shear rate. Figure 5 also compares the rheological behavior of the neat poly(amic acid) (PAA) polymerizing solution at two different polymerization time intervals of 30 and 1440 min. It provides a useful insight into how the viscosity of the PAA polymerizing solution changes over the reaction time. It is shown that the viscosity of the PAA reaction solution increased significantly after 30 min of polymerization. This is evident from the higher steady-state viscosity of 2440 cP obtained after 24 h of polymerization when compared to that obtained at 30 min of polymerization of 735 cP. The increase in viscosity with polymerization time suggests that the molecular weight of the PAA polymerizing solution increases with polymerization time in accordance with the step growth polymerization kinetic model. The Carothers kinetic model for step growth polymerization shows that the molecular weight (M_w) of the polymer formed will increase with increasing polymerization time (t): $M_w=M_0\left(1+k{\left[M\right]}_{0}t\right)$, where $M_0$ is the molar mass of the repeat unit, k is the rate constant for polycondensation, ${\left[M\right]}_0$ is the initial monomer concentration, and t is the polymerization time. The variation in viscosity with shear rate shows a common pattern of an initial increase in viscosity with shear rate up to a critical shear rate, beyond which the viscosity remains constant and does not change significantly with increasing shear rate. The first region of rheological behavior may be due to a combination of factors, including increasing molecular weight and solvent removal as a result of increasing shear rate. The second region, where Newtonian flow behavior is observed, could be associated with the existence of some form of structural equilibrium in the system.

3.1.3. Effect of Testing Temperature

Figure 6 shows the relationship between viscosity and temperature for the neat poly(amic acid) (PAA) solution and the PAA solution mixed with 5 wt.% polyaniline copolymer-modified (PNEA) clay, respectively. Both systems show an inverse relationship between viscosity and temperature in accordance with the Arrhenius law: $\ln \eta =\ln {\eta }_0+E_a/RT$, where h is the temperature-dependent viscosity, ${\eta }_0$ is the zero-shear rate viscosity, $E_a$ is the activation energy, R is the gas constant, and T is the absolute temperature. This behavior is typical for polymer solutions, where higher temperatures reduce the viscosity, facilitating easier flow. The neat PAA solution showed a higher initial viscosity at lower temperatures and a gradual decrease in viscosity as the temperature increased. The zero-shear rate viscosity for the neat PAA solution was around 1480 cP. The PAA/PNEA clay suspension had a slightly lower initial viscosity compared to the neat PAA solution and showed a sharper decrease in viscosity with increasing temperature. The zero-shear rate viscosity for the PAA/PNEA clay suspension was 990 cP. The activation energy for viscous flow for the neat PAA solution was 23 J/mole, which is lower than that for the PAA solution containing 5 wt.% polyaniline copolymer-modified clay of 51 J/mole, suggesting that the presence of 5 wt.% polyaniline modified clay may reduce the processability of PAA.

3.1.4. Intrinsic Viscosity of PAA Solution and PAA Suspension Containing Modified Clay

Figure 7a,b show the dependence of the reduced viscosity and inherent viscosity on solution concentration for the neat poly(amic acid) solution (a) and the poly(amic acid) suspension containing 5 wt.% polyaniline copolymer-modified clay (b). The intercept of the reduced viscosity versus concentration curve and that for the inherent viscosity versus concentration curve gives the intrinsic viscosity, [h]. The intrinsic viscosity for the PAA solution is around 1.52 dL/g. This value reflects the hydrodynamic volume occupied by the polymer chains in the solution. Figure 7b shows the analysis of the dilute solution viscometry of the PAA suspension containing 5 wt.% PNEA. The intrinsic viscosity for the PAA/5 wt.% PNEA clay suspension is around 1.46 dL/g, which is slightly lower than that for the neat PAA solution. The presence of PNEA modified clay in the PAA solution slightly decreased the latter’s intrinsic viscosity. This result suggests that the modified clay filler may interfere with the solvent–polymer interaction, resulting in a slightly lowered hydrodynamic volume of polymer chains.

3.2. Differential Scanning Calorimetry (DSC)

Figure 8a,b show the DSC thermograms of polyimide–nanographene sheet composites (PI-NGC) containing 10 and 40 wt.% graphene produced following the standard solution casting method and those produced by initially shearing the mixture in a Brookfield viscometer at a spindle speed of 100 rpm for 30 min before solution casting. Figure 8a shows the DSC thermograms for the polyimide nanocomposite containing a 10 wt.% nanographene sheet, PI-10 wt.%. The composite film obtained after subjecting the mixture to high-speed shearing at 100 rpm in the Brookfield viscometer had a lower decomposition energy peak height and lower total heat of decomposition than the nanographene sheet/polyimide nanocomposite of the same composition, PI-10 wt.% graphene, produced without high-speed shearing. This indicates that the exposure of the mixture to high-speed shearing after the standard processing protocol improved the composite thermal properties by enhancing solvent removal, resulting in a dense and oriented film structure. Similar to Figure 8a, the DSC thermogram for the nanocomposite containing a 40 wt.% graphene sheet, PI-40 wt.% graphene, subjected to high spindle speed shearing has a lower decomposition energy peak height and lower energy of decomposition than the PI-40 wt.% nanocomposite processed without the additional high spindle speed shearing step (Figure 8b). This improvement in the thermal stability of the high-speed sheared composites is believed to be due to enhanced graphene sheet distribution and orientation and improved densification of the composite material. Comparing the two figures, it is evident that the nanocomposite with the higher graphene content (40 wt.%) had higher thermal stability (Figure 8b). The combination of high graphene concentration and high spindle speed shearing improved the thermal stability and fire-retardant properties of the composites. The neat PI matrix had a higher decomposition energy peak height, indicating lower thermal stability compared to the polyimide–nanographene sheets composites. The presence of nanographene sheets significantly increased the total decomposition time, lowered the heat of decomposition, and therefore enhanced the thermal stability of the PI matrix.

3.3. Cyclic Voltammetry

Figure 9 and Figure 10 show the cyclic voltammograms (CVs) for PI/carbon nanotube (CNT) sheet hybrid nanocomposites processed at 90 °C and 250 °C and coated with electropolymerized polypyrrole (PPy). The CV results show that the hybrid nanocomposite electrode processed at 90 °C is pseudosupercapacitive, characterized by both redox reactions and double layer capacitive behavior. It also produced a higher stored charge and, consequently, a higher specific capacitance than that processed at 250 °C. Specifically, at a scan rate of 5 mV/s, the specific capacitance for the PPy-coated sample processed at 90 °C was around 858 F/g after one cycle. The specific capacitance for this composite decreased slightly to 802 F/g after 10 cycles. However, at a scan rate of 25 mV/s, the specific capacitance for the PPy-coated electrode processed at 90 °C was 345 F/g after one cycle but increased to 374 F/g after 10 cycles. Interestingly, the composite processed at 250 °C produced a specific capacitance of 136 F/g after one cycle at a scan rate of 5 mV/s, which increased to 156 F/g after 10 cycles of testing. However, tests performed at a scan rate of 25 mV/s produced a specific capacitance of 74 F/g after one cycle, which decreased slightly to 61 F/g in 10 cycles. These results indicate that increasing the scan rate resulted in decreased specific capacitance. Furthermore, a slight decrease in the specific capacitance with the increasing number of cycles of CV testing was observed. The observed increase in the total stored charge and specific capacitance at the low scan rate is attributed to the longer time available for electrochemical processes. Conversely, the low specific capacitance obtained at the higher scan rate is attributed to the shorter time that is available for electrochemical processes. It is suggested that controlling the processing temperature and time is crucial for optimizing the performance of composite electrode materials for application in pseudosupercapacitors and batteries, where a high specific capacitance and efficient charge–discharge cycles are desirable. The complex electrochemical processes observed, including electrochemical double layer formation and redox reactions, significantly impact device performance and life cycle. At 90 °C, the thicker PPy coating with a weight gain of 0.0017 g (0.7%) and a lower bulk resistance of 4.95 ohms (

Table 1. Theoretical porosity, resistance obtained using EIS, and electrode resistance from the i–t curve.

Material	Bulk Resistance (Ω), (EIS)	Electrode Resistance (Ω), i–t Transient Curve	Porosity, (EIS)
PI/CNT-PVDF (90 °C)	4.95	20	8.41
PI/CNT-PVDF (250 °C)	15	41.24	4.33

) led to pseudocapacitive behavior due to higher porosity, the presence of more active redox sites, and higher conductivity. The anodic peak current for this electrode material is higher, indicating more active redox processes during oxidation, and the presence of a cathodic peak current also shows significant activity during reduction. Conversely, at 250 °C, a thinner PPy coating was formed with a weight gain of 0.0006 g (0.45%) and higher bulk resistance of 15 ohms (Table 1), resulting in fewer active sites, poorer conductivity, and poor cycling stability. The anodic and cathodic peak currents for the sample processed at 250 °C were lower, reflecting minimal redox activity. Thus, a lower processing temperature provided better capacitance, conductivity, and redox activity, while processing at a higher temperature (250 °C) led to mostly supercapacitive behavior.

Figure 9c,d as well as Figure 10c,d show the effect of the number of cycles of voltammetry on the electrochemical properties. The CV for the electrode material processed at 90 °C followed by electrodeposition of PPy shows pseudocapacitive behavior, with an anodic current peak at 0.55V vs. Ag/Ag+ and a cathodic current peak at 0.45V vs. Ag/Ag+. These redox peaks are believed to be due to the oxidation and reduction of PPy during cyclic voltammetry. The anodic peak current height was slightly higher than the cathodic peak current height; however, the peak position and peak height remained unchanged after 10 cycles at a low scan rate of 5 mV/s.

At a higher scan rate of 25 mV/s, both the anodic current peak position and peak height increased with the number of cycles. Multiple redox peaks are also shown on Figure 10a–c. The prominent anodic and cathodic peaks shown at 0.65 and 0.35 V vs. Ag/Ag+, respectively, are due to the oxidation of PPy, while the minor anodic and cathodic current peaks shown at 0.4 and 0.2 V vs. Ag/Ag+, respectively, may be due to the oxidation and reduction of doped PPy. The CVs obtained by applying a faster scan rate (25 mV/s) show lower cycling stability; however, both the anodic and cathodic peak current intensities increased with the increasing number of cycles.

Table 1 compares the porosity of the electrode material processed at 90 °C followed by deposition of PPy with that of the material processed at 250 °C. It is shown that the former has a lower bulk resistance and higher porosity than the latter. The electrode resistance obtained from the transient i–t curves obtained during the potentiostatic deposition of PPy was higher for the electrode material processed at 250 °C than that for the electrode processed at 90 °C, in agreement with the EIS finding.

Table 2. Comparison of the specific capacitance obtained in this study with those from other studies.

Material	Specific Capacitance (F/g)	Reference
PI/CNT-PVDF (90 °C)	858	This study
PI-CNT-PVDF (250 °C)	136	This study
PI-SWCNTs	127	[59]
Graphene nanosheets (GNSs), carbon nanotubes (CNTs), and PANI	1035	[60]
Polythiophene (PTh)-CNT composites	125	[61]

compares the electrochemical properties of the composite electrode materials prepared in the present study with those reported elsewhere. It is shown that the electrochemical properties of the electrode materials prepared in this study are very competitive.

3.4. Current–Time, i–t, Transient Curves

Figure 11 shows the transient current–time curves obtained during the potentiostatic electrochemical polymerization of pyrrole onto the composite PI/CNT-PVDF working electrodes processed at 90 °C and 250 °C. The composite electrode processed at 90 °C had a peak and steady-state current of 0.15 A and 0.10 A, respectively, which are higher than the peak current and steady-state current of 0.06 A and 0.04 A, respectively, obtained for the composite working electrode processed at 250 °C. This finding suggests that the electropolymerization of pyrrole is favored on the composite electrode processed at a lower temperature of 90 °C than the electrode material processed at 250 °C. The potentiostatic polymerization of pyrrole onto the hybrid PI/CNT-PVDF electrode at an applied voltage of 2 V for 60 s resulted in a lower electrode resistance for the composite processed at 90 °C than that processed at 250 °C (Table 1). The current–time transient curve for polymerization conducted using the hybrid electrode processed at 90 °C has a higher peak current and higher steady-state current than that for the hybrid working electrode processed at 250 °C. These results, therefore, underscore the importance of processing temperature in controlling the electrochemical properties of the electrode material.

3.5. Discussion of the Effect of Protocol Choices

The impact of procedure choices in this study is substantial, influencing the structure and properties of polyimide nanocomposites in multiple ways. Using an optimal material selection and composition, including polyaniline-modified montmorillonite clay and nanographene sheets, significantly enhanced the thermal and electrochemical properties. Additionally, the processing temperature and high shear rate mixing played crucial roles. A high shearing force improved the dispersion and orientation of nanofillers within the polyimide matrix, while optimal processing temperatures further enhanced the thermal stability and electrochemical performance of the composites.

Processing the hybrid composites at 90 °C prior to electrodeposition of PPy yielded a higher specific capacitance and better cycling stability in the nanocomposite electrodes. High-speed shearing improved the distribution of nanofillers, resulting in a lower total heat of decomposition and better fire retardancy. Detailed rheological testing provided insights for optimizing processability, while varying the CV scan rate and number of cycles affected the electrochemical properties. DSC analysis confirmed the improved fire retardancy with a lower heat of decomposition for composites subjected to additional high-speed shearing. These procedure choices collectively led to a better understanding of the behavior of polyimide nanocomposites for use in high-temperature and safety-critical applications in sustainable energy storage systems.

4. Conclusions

This study shows that both PAA solution and PAA dispersions containing PANI copolymer-modified clay particles have two regions of rheological behavior: (i) an initial region of increasing viscosity with increasing shear rate (power law region), followed by (ii) a second region where the viscosity remains constant with increasing shear rate (Newtonian region).

Polyimide–nanographene sheet composites processed by adding an additional high-speed shearing step have a lower total heat of decomposition, longer decomposition time, and lower heat of decomposition peak height and are therefore more thermally stable and fire retardant.

PI/CNT-PVDF composite electrode materials processed at 90 °C followed by potentiostatic deposition of PPy are (i) more conductive, (ii) more porous, and (iii) pseudocapacitive and have (iv) a higher specific capacitance and (v) higher cycling stability (at low scan rate) than electrode materials processed at 250 °C followed by potentiostatic deposition of PPy.