High-Temperature Polyimide Composites—A Review on Polyimide Types, Manufacturing, and Mechanical and Thermal Behavior

Abstract

Polyimide composites represent a class of advanced materials with remarkable mechanical robustness and thermal stability, making them highly suitable for applications in extreme environments. Their unique ability to maintain performance under high temperatures and corrosive conditions, combined with a favorable strength-to-weight ratio, positions them as critical components in aerospace, electronics, and automotive systems. Several leading aerospace and electronics corporations have made significant investments in incorporating polyimide composites into their products, indicating the material’s transformative potential. This review paper provides an overview of mechanical and thermal behaviors of polyimide composites, summarizing recent developments and research trends. It examines the influence of various reinforcements, processing techniques, and composite architectures on material performance under mechanical loading and thermal stress. The paper synthesizes findings from experimental studies and modeling efforts to highlight the critical factors affecting strength, durability, and thermal stability. Discussion and recommendations regarding applications in aerospace, electronics, and other high-temperature environments are provided, emphasizing the challenges and opportunities presented by these advanced materials. This review adopts a broad scope to reflect the interdisciplinary nature of polyimide research. Due to gaps in literature, this work aims to provide a foundational overview that supports future, more specialized investigations.

1. Introduction

Composite materials continue to gain prominence in advanced structural applications, particularly in the aerospace and automotive industries. These sectors are increasingly driven by the demand for reduced weight, enhanced mechanical performance, and improved fuel efficiency [1]. As a result, fiber-reinforced polymers have emerged as a widely utilized class of materials due to their high specific strength and stiffness combined with relatively low density [2]. A prominent example of this shift toward lightweight composites in aerospace design is Pratt & Whitney’s recent development of next-generation aircraft engines. To improve fuel efficiency, the company is incorporating advanced fiber-reinforced composites into key engine components, including the fan blades and fan case [3]. These structures, which replace traditional metallic parts, reduce overall engine weight while maintaining mechanical integrity. As illustrated in Figure 1, the integration of high-performance composites into jet engines introduces demanding thermal and mechanical conditions throughout the entire life cycle, from manufacturing and assembly to in-service operation and eventual recycling. This emphasizes a critical challenge, as fiber-reinforced composites, despite their excellent weight-to-strength ratios, often face performance limitations due to the thermal constraints of the conventional polymer matrices used in their fabrication.

Despite these advantages, organic matrix composites are limited in environments involving elevated temperatures [4]. Carbon fibers, for example, possess excellent thermal stability in inert atmospheres, yet are susceptible to oxidative degradation when exposed to air at high temperatures [5,6]. Basalt fibers, which are derived from volcanic igneous rock, also exhibit strong thermal resistance due to their inherent mineral composition [7,8,9,10]. However, the performance of these fibers in high-temperature applications is often constrained by the thermal limitations, including creep behavior, of conventional polymer matrices such as epoxy and polyester [4,7,11]. These resins begin to degrade or lose their mechanical integrity well below the operational limits required for many high-temperature systems [4,12,13].

To address this issue, thermally stable matrix systems such as polyimides have been developed. Polyimide-based composites offer significant advantages in environments where temperatures may reach or exceed 400 °C. Their thermal stability, combined with suitable mechanical performance, makes them promising candidates for structural applications where strength and heat resistance are essential.

High-temperature polyimides are a remarkable class of advanced polymers, distinguished by their exceptional thermal stability, chemical resistance, and mechanical strength [14,15]. Their molecular architecture, defined by robust aromatic imide rings, enables them to perform reliably under severe thermal conditions. This inherent durability has established polyimides as the material of choice for applications in aerospace, electronics, and other high-demand fields [16]. Moreover, their adaptability through chemical modification ensures that they can be precisely tailored to meet the evolving challenges of modern engineering [15,16,17].

Expanding upon the strengths of pure polyimides, polyimide composites integrate reinforcing agents such as carbon fibers, glass fibers, or particulate fillers to create materials with superior performance [18,19]. The strategic combination of a polyimide matrix with high-strength reinforcements results in composites that retain excellent thermal and chemical resilience and also exhibit enhanced stiffness, strength, and impact resistance [18,20]. These composites offer a balanced solution for the design of lightweight yet robust components, making them ideally suited for the next generation of aerospace, automotive, and electronic systems. As research progresses, polyimide (PI) composites are poised to play a pivotal role in advancing high-performance material systems in extreme environments [18,21].

As industries push for lightweight and durable materials, the demand for PI composites is set to expand markedly. Projections indicate that, within the aerospace sector alone, the market can grow at a compound annual rate of roughly 8% between 2024 and 2029 [22]. Fabricating these composites leverages state-of-the-art techniques, including hydrothermal polymerization and innovative recycling methods that boost performance and address pressing sustainability challenges. Nonetheless, the inherent complexity and high cost of production raise issues regarding their affordability in cost-sensitive applications [23]. Furthermore, traditional PIs suffer from limited biodegradability and recycling challenges, which are increasingly problematic amid global sustainability initiatives [24].

Recent investigations have concentrated on enhancing PI composites’ functional and environmental profiles. Research efforts are exploring biodegradable alternatives and advanced recycling technologies to mitigate waste issues, while the incorporation of nanoparticles into polyimide matrices has shown promise for significantly improving dielectric properties [25,26]. These advances are poised to broaden the application scope of PI composites, particularly within the electrical and electronics sectors [27]. Figure 2 illustrates the leading PI producers, such as DuPont, alongside the overall polyimide market size, which is projected to increase from USD 5.46 billion in 2024 to USD 7.6 billion in 2029, reflecting a compound annual growth rate (CAGR) of 6.83%.

Nevertheless, polyimides’ processing presents significant challenges. Most conventional polymer processing systems operate near 300 °C, which falls short for polyimide fabrication. Thermosetting formulations such as PMR 15 require curing above 350 °C [28], and thermoplastic polyimides often need melt temperatures beyond 400 °C [29]. These elevated requirements call for specialized equipment and precise thermal control. Thus, processing remains a barrier to wider implementations of polyimide composites in structural applications.

Given the limited number of studies specifically addressing the mechanical and thermal behavior of Bee Trees, the literature was identified primarily through Google Scholar, including sources from Elsevier, Springer, Sage, Taylor & Francis, and MDPI. This broad approach ensured the inclusion of all relevant and available research. This review paper discusses the chemical structures and modifications, mechanical and thermal behavior, the manufacturing techniques, and applications of PI and PI composites. In Section 2, an examination of various PI types, their composite formulations, and associated properties is presented. This section also explores the polymerization methodologies employed, the structural modifications, and the impact of fluorinated groups on the material characteristics. Section 3 outlines the manufacturing processes utilized in PI production and composite fabrication. In Section 4, the mechanical, thermal, and electrical behaviors of these composites are reviewed. Section 5 offers a synthesis of PI and PI composite materials, followed by a summary and a forward-looking perspective on emerging trends and future research directions.

2. Types of Polyimides

Polyimides (PIs) represent a versatile family of high-performance polymers, each tailored by its processing characteristics and application demands. NASA used the Polymerization of Monomeric Reactants (PMR) approach to develop polyimide PMR-15 [30]. Thermosetting polyimides, such as PMR-15, undergo an irreversible cure that forms a densely cross-linked network, endowing them with exceptional thermal stability, chemical resistance, and mechanical strength [30]. These attributes render them indispensable for high-temperature structural components in aerospace and other demanding environments. In contrast, thermoplastic PIs, exemplified by LARC-TPI, offer the advantage of remelting and reprocessing [31]. Their ability to combine robust thermal and mechanical performance with recyclability and compatibility with conventional melt-processing techniques makes them ideally suited for electronic packaging and high-performance applications that benefit from reworkability. Furthermore, PI films, epitomized by the renowned Kapton, are engineered as thin, flexible sheets that deliver high dielectric strength, remarkable thermal stability, and mechanical flexibility [32]. Such properties are crucial for applications in flexible electronics, insulation, and scenarios requiring lightweight, conformable materials. Ultimately, the selection of a specific polyimide type hinges on a careful consideration of the targeted performance requirements, processing parameters, and the intended application environment [32,33].

Table 1. Comparative overview of polyimide types along with their properties and applications [31,34].

Type of Polyimide	Description	Examples	Properties	Applications
Thermosetting Polyimides	Cure irreversibly to form a highly cross-linked network. They require controlled curing processes and do not melt upon reheating.	PMR-15	Exceptional thermal stability, chemical resistance, and mechanical strength. They maintain structural integrity in extreme environments.	High-temperature structural components in aerospace, military, and other demanding environments.
Thermoplastic Polyimides	They can be remelted and reprocessed using conventional melt techniques. Their formulation allows for recyclability and ease of processing compared to their thermosetting counterparts.	LARC-TPI	Excellent thermal and mechanical performance, with added benefits of reworkability and recyclability.	Electronic packaging and high-performance components where reshaping, reworking, or recycling is advantageous.
Polyimide Films	Produced as thin, flexible sheets rather than bulk materials. They are engineered for applications that demand both flexibility and high thermal resistance.	Kapton	High dielectric strength, impressive thermal stability, and mechanical flexibility, combined with lightweight properties.	Flexible electronics, insulation, and applications in aerospace and military sectors that require conformable, lightweight materials.

classifies PIs based on their chemical structure, physical properties, and application areas. Thermosetting and thermoplastic polyimides differ in their processing behavior and recyclability, while PI films offer unique advantages for flexible and lightweight applications.

2.1. Composition and Properties

Chemical Structure

Polyimides are a sophisticated class of high-performance polymers defined by incorporating imide groups within their backbone. Typically synthesized via a condensation reaction between a dianhydride and a diamine, the process yields repeating units that feature a cyclic imide motif, –CO–N–CO–, embedded in a rigid aromatic structure [14]. For instance, the reaction of pyromellitic dianhydride (PMDA) with 4,4′-oxydianiline (ODA) is a classic example that produces a polymer chain with PMDA-ODA repeating units. Selecting different dianhydrides and diamines can lead to fine-tuning the thermal stability, mechanical robustness, and chemical resistance of these polymers. Further modifications, such as the introduction of aliphatic segments or flexible linkers, allow for additional customization, broadening the applicability of PIs across various advanced technological domains [35].

The composition of PI films can be amorphous or crystalline, and this is contingent upon their synthetic chemistry [32]. The arrangement of the polymer chains and the presence of functional groups, such as imide and ether linkages, significantly influence their properties. The aromatic rings present in PI films exhibit distinct vibrational modes and contribute to their physical characteristics, including mechanical strength and thermal stability.

2.2. Polymerization Techniques

The synthesis of PIs is predominantly achieved through the polymerization of monomeric reactants, employing techniques such as step-polymerization and addition polymerization. The Polymerization of Monomeric Reactants (PMR) technology, pioneered in the mid-1970s at NASA’s Glenn Research Center, leverages an alcohol solution of PI monomers to produce “prepreg” materials. These prepreg composites, comprising graphite or glass fiber bundles infused with PI resins, can then be thermally cured to form structures with minimal void content [36,37,38]. This innovative method has been critical in overcoming the challenges of traditional condensation polymerization, particularly by eliminating the need to remove high-boiling solvents [37,39].

2.3. Structural Modifications

Recent advancements in PI chemistry have centered on the deliberate modification of the polymer backbone to achieve enhanced performance. Incorporating thermally stable yet flexible linkages or asymmetrical structures serves to improve the material’s rigidity while minimizing internal defects [40]. Moreover, introducing tailored functional groups and dendritic architectures leads to fine-tuning the properties of polyimides to meet the demands of specific applications, thereby broadening their utility across a diverse range of fields [40]. For example, highly conductive three-dimensional carbon/polyimide (3D-C/PI) composite films have been developed by integrating three-dimensional graphene with a PI matrix [41]. These composites exhibit remarkable long-term stability, as evidenced by rigorous ground-based simulated space environment tests. Detailed investigations into their electrical and thermal properties, along with assessments of aging under various bending and thermal cycles, have confirmed that the 3D-C/PI films effectively retain the 3-D carbon framework’s superior conductive properties. The material successfully passed space environment qualification tests, demonstrating its suitability as an electrostatic discharge (ESD) shielding material with proven long-term reliability [41].

Furthermore, incorporating 3-D carbon into the PI matrix enhances the electrical and thermal conductivities by up to 10 orders of magnitude, reaching approximately 3.5 Ω⁻¹ and 1.7 W·m⁻¹·K⁻¹, respectively [42].

Table 2. Comparative overview of the thermal conductivity of PI composite films with various reinforcements [41,42,46].

	Filler		Thermal Conductivity (W m−1 K−1)
	wt%	vol%	Neat PI	Filled PI
Sic nanowires on graphene [41,42]	7		0.25	0.577
	11		0.25	2.63
MWCNTs [42,43,44]	3		0.18	0.25
BN-c-MWCNTs [42,45]	3		0.18	0.38
AlN [46]		9	0.225	0.675
BN [42,47]	30		0.18	1.2
Silver particles [42,48]		45	0.2	15
BNNS [49]	7		0.25	2.95 (in-plane TC)
Aluminum nitride nanoparticles [42,50]		30	0.22	0.6
3D-C [41,42]	0.35	0.3	0.15	1.7

MWCNT: multi-walled carbon nanotubes; BN: boron nitride; 3-D C: three-dimensional carbon; BNNSs: boron nitride nanosheets; TC: thermal conductivity.

presents an analysis of the thermal conductivity of PI composites containing various fillers [42], including graphene [43,44], carbon nanotubes (CNTs) [45], and metal and ceramic particles [46,47,48], in contrast to that of the 3D-C/PI composite [41].

As Table 2 shows, adding fillers to neat PI can substantially enhance its thermal conductivity, as illustrated by comparing the thermal conductivity values of filled PI with those of neat PI. For instance, incorporating SiC nanowires on graphene at about 11 wt% can raise the thermal conductivity from 0.25 W·m⁻¹·K⁻¹ to 2.63 W·m⁻¹·K⁻¹, representing an increase in over 950% [44]. Meanwhile, incorporating multi-walled CNTs (MWCNTs) at 3 wt% increases the thermal conductivity from around 0.18 W·m⁻¹·K⁻¹ (neat PI) to about 0.25 W·m⁻¹·K⁻¹, representing an improvement of 39% [45,49]. Similarly, introducing boron nitride (BN) at 30 wt% can yield a thermal conductivity near 1.2 W·m⁻¹·K⁻¹, corresponding to an increase in 567% [47]. Additionally, aluminum nitride nanoparticles at 30 vol% elevate thermal conductivity to about 0.6 W·m⁻¹·K⁻¹, which is nearly 170% above that of neat PI [50]. Even low filler loadings can be highly effective: integrating 3D carbon (3D-C) at only 0.35 wt% (equivalent to 30 vol%) achieves a thermal conductivity of approximately 1.7 W·m⁻¹·K⁻¹, which shows an increase in of over 1000% compared to the neat PI [41]. These examples demonstrate how different fillers, depending on their properties and content levels, can significantly enhance the PI composites’ thermal conductivity.

Several images from different references have been gathered to represent how PIs have been modified with fillers and reinforcers to form advanced PI composites. Figure 3 presents a series of images that elucidate the structure and morphology of these graphene-PI systems [42]. In panel (a), a neat 3D-C foam is shown alongside a uniformly distributed PI matrix, as demonstrated in several research studies [41,42]. Panel (b) features high-resolution scanning electron microscopy (HRSEM) cross-sectional images of both the pristine 3D-C and the PI-infiltrated 3D-C, clearly illustrating that the porous network has been filled with PI [41,42]. An atomic-resolution scanning tunneling microscopy (STM) image in panel (c) reveals a distinct hexagonal symmetry with a periodicity of 0.246 nm within the 3D-C structure [41,42]. Panel (d) displays a scanning electron microscopy (SEM) image capturing the interconnected macro-porous architecture of a 3D-C/PI electrode [42,51]. Finally, panel (e) presents a low-magnification transmission electron microscopy (TEM) image that highlights the dispersion of graphene layers and stacks within the nanocomposite at a concentration of 1.23 vol%, emphasizing the high particle density of these graphene structures [42,52].

2.4. Influence of Fluorinated Groups

With high integration and rapid signal propagation in miniaturized electronic devices, speed is critical in the microelectronics industry. Lowering the dielectric constant of insulation materials directly enhances signal propagation. Fluorine-containing polymers are particularly attractive in this context, as they exhibit low dielectric constants, high optical transparency, low refractive indices, and exceptionally low water absorption [53]. Moreover, the incorporation of 6F (1,3-ditrifluoromethyl-2-isopropyl) groups into the polymer backbone improves solubility, which is a phenomenon known as the “fluorine effect”, without sacrificing thermal stability. Additionally, the presence of bulky trifluoromethyl (–CF3) groups increases the free volume of the polymer, thereby enhancing both gas permeability and electrical insulation properties [54].

The chemical structure of polyimides can also be altered by the introduction of fluorinated groups, which have been shown to significantly reduce the dielectric constant (Dk) of PI materials. This modification is particularly advantageous in applications requiring low dielectric properties, as fluorinated groups exhibit low polarization ability, thereby enhancing the polymer’s electrical performance [54,55]. Such modifications enhance the electrical insulation characteristics and contribute to the overall robustness of PI composites in demanding environments.

3. Manufacturing Processes

Manufacturing PI polymers typically relies on sophisticated chemical processes that employ the unique reactivity of dianhydrides and diamines. A principal method is the Polymerization of Monomeric Reactants (PMR) process, wherein an alcohol solution of polyimide monomers is used to form prepreg materials that are subsequently thermally cured to yield dense, void-free structures [56]. In addition to PMR, techniques such as solution casting and film deposition are used to produce PI films with uniform thickness and excellent dielectric properties [57,58].

For PI composites, the fabrication process is tailored to integrate reinforcing fillers, such as carbon fibers, graphene, and various nanoparticles, into the polyimide matrix. In situ polymerization is commonly used to achieve a homogeneous dispersion of fillers and to secure strong interfacial adhesion. This way, mechanical strength and thermal conductivity can be optimized. Other methods, including melt blending [59], solution mixing [60], and the impregnation of preformed fiber networks [60], are also widely utilized. Emerging techniques, such as 3D printing and layer-by-layer assembly, further expand the potential of PI composites, which enable the design of advanced structures for modern applications [61].

PI materials are predominantly manufactured using conventional processing techniques rather than additive manufacturing (AM). While AM has gained attention in recent years, particularly for specialized or custom applications, it remains a relatively limited method in the broader context of PI production. Traditional approaches continue to dominate due to their reliability, scalability, and compatibility with the chemical and thermal characteristics of PIs.

One of the most widely used methods is film casting, also referred to as solvent casting. In this process, a polyamic acid precursor is cast onto a substrate and then subjected to thermal imidization, producing high-performance PI films such as Kapton^®. For thermoplastic PIs, including polyamide-imide (PAI) materials like Torlon^®, compression molding and injection molding are commonly employed. These molded parts are subsequently sintered at elevated temperatures to achieve the necessary consolidation and mechanical integrity [62].

In electronic applications, solution-based processing is frequently used. Here, PI precursor solutions are spin-coated or dip-coated onto substrates, followed by thermal curing. This technique is especially relevant in flexible printed circuit boards and as insulating layers. Additionally, PIs are often machined from preformed stock shapes such as rods, blocks, or sheets using CNC or other traditional subtractive manufacturing methods. This allows for the fabrication of precise components when direct forming is impractical. Laminating and bonding are also common, particularly in composite systems or multilayer electronics, where PI films are integrated with other materials to create multifunctional structures [63].

Additive manufacturing (AM) techniques, though still in the early stages for PI materials, have shown promise in specific applications. Methods such as fused filament fabrication (FFF) use thermoplastic PI filaments, including PEEK-polyimide blends [64]. Other approaches include stereolithography (SLA) and direct ink writing (DIW), which utilize photosensitive or reactive precursors [65,66]. Selective laser sintering (SLS) with PI powders has also been explored, though it presents significant processing challenges and remains less commonly used [61,67]. As an example, Figure 4 shows how the ink was printable using various inner diameters of nozzles (1.60 to 0.21 mm) [66]. The printed structures demonstrated stability, effectively resisting slumping and spreading across varying nozzle sizes. This behavior suggests that the heat treatment process successfully modified the ink’s rheological properties, making it suitable for direct ink writing (DIW) printing [67].

In the AM field, Ga refers to the gauge number, which describes the diameter of the nozzle. Smaller Ga values correspond to larger nozzle diameters, while higher Ga values indicate smaller nozzle sizes. The unit mm⁻¹ suggests an inverse relationship to nozzle size, where larger Ga values result in finer extrusions. This system is particularly relevant in processes like direct ink writing (DIW), where nozzle size influences the precision and resolution of printed structures.

As another example of a PI manufacturing method, Figure 5a illustrates the initial step in which the polyetherimide (PEI) substrate was fabricated via fused filament fabrication (FFF) to produce a rectangular geometry. Subsequently, three sequential layers of poly(ethylene glycol) diacrylate (PEGDA) were deposited using inkjet printing and immediately cured under ultraviolet light (Figure 5b). This crosslinked PEGDA layer served as a chemical barrier, protecting the underlying PEI from degradation by the N-methyl-2-pyrrolidone (NMP) solvent present in the PI ink [66]. A single layer of PI dielectric ink was then applied by inkjet printing onto the PEGDA-coated substrate, as shown in Figure 5c. Finally, a silver conductive trace was printed using direct ink writing (DIW) on the modified surface, as depicted in Figure 5d.

Table 3. Common manufacturing methods for polyimides, including processes, applications, and key characteristics.

Manufacturing Method	Description	Applications	Notes
Film Casting/Solvent Casting	Polyamic acid is cast onto a substrate and thermally imidized into a film	Kapton® films, electronics insulation	Precise thickness control, used for flexible films
Molding and Sintering	Thermoplastic polyimides are molded, then sintered at high temperatures	Structural parts (e.g., Torlon®)	Suitable for melt-processable polyimides
Solution Processing/Coating	Precursor solution is spin- or dip-coated, followed by thermal curing	Flexible PCBs, insulating layers	Ideal for thin films and coatings in electronics
Machining from Stock Shapes	Rods, blocks, or sheets are machined using CNC or conventional techniques	Precision parts, custom shapes	Allows high-dimensional accuracy; subtractive process
Laminating and Bonding	Polyimide films are bonded with other materials in layered configurations	Multilayer electronics, composites	Enables integration of different functional materials
Additive Manufacturing (AM)	3D printing using thermoplastic or reactive polyimide materials	Prototyping, complex geometries, custom parts	Emerging; includes FFF, SLA, DIW, and SLS; not yet widely adopted

FFF: Fused Filament Fabrication; SLA: Stereolithography; DIW: Direct Ink Writing; PEGDA: Poly(ethylene glycol) Diacrylate; SLS: Selective Laser Sintering.

presents an overview of common manufacturing methods used for processing PIs. Each method is suited for specific forms and end-use applications, ranging from thin films and coatings to molded or machined structural parts. Additive manufacturing is emerging as a versatile approach for producing complex geometries.

4. Mechanical and Thermal Behavior

Polyimides (PIs) are a class of high-performance polymers known for their exceptional thermal stability and robust mechanical properties. Their molecular structure, characterized by imide linkages within an aromatic backbone, imparts significant rigidity to the polymer chain. This structural feature results in neat PI films exhibiting tensile strengths ranging from 94 to 120 MPa, elongations at break between 7% and 15%, and tensile moduli approximately between 1.85 and 2.18 GPa [68]. These properties make PIs suitable for applications requiring materials that can withstand elevated temperatures without significant degradation.

However, while neat PIs offer commendable thermal and mechanical stability, their inherent brittleness limits their applicability in structural components subjected to dynamic or impact loading. The fracture toughness of neat PIs is relatively low, making them susceptible to crack initiation and propagation under stress. To enhance their mechanical performance, especially in terms of toughness and strength, PIs are often reinforced with various fillers, leading to the development of PI composites.

Reinforcement strategies include the incorporation of carbon fibers, CNTs, and other nanomaterials. For instance, carbon fiber-reinforced PI composites can achieve flexural strengths up to 1552 MPa at room temperature, with flexural moduli around 119 GPa [69]. Similarly, the addition of CNTs to PI matrices has been shown to improve mechanical properties, with composites exhibiting tensile strengths up to 1.4 GPa and elongations at break of 14.3% [70]. These enhancements are attributed to the improved load transfer between the matrix and the reinforcement, as well as the increased energy dissipation mechanisms within the composite structure.

The mechanical properties of PI composites are also influenced by the degree of cure and the molecular weight of the PI matrix. Studies have demonstrated that increasing the molecular weight of the PI leads to higher tensile strengths and moduli, up to certain limits. Additionally, the interfacial bonding between the reinforcement and the PI matrix plays a crucial role in determining the overall mechanical performance. Optimizing the interfacial adhesion can significantly enhance the strength and toughness of the composite material [70]. The enhancement of interfacial strength between fibers and the matrix is a well-established strategy for improving the overall mechanical performance of fiber-reinforced composites. This principle is not unique to PI-based systems but is broadly applicable across a wide range of fiber-reinforced composite materials [71,72,73]. For example, the incorporation of short fibers into polymer matrices has been widely reported to enhance the mechanical performance of the resulting composites, including improvements in tensile stiffness and strength, flexural stiffness, heat deflection temperature, and fracture toughness [74,75].

Table 4. Comparative overview of neat polyimide (PI) and composite PIs, along with their tensile and flexural properties.

Material Type	Tensile Strength (MPa)	Elongation at Break (%)	Tensile Modulus (GPa)	Flexural Strength (MPa)	Flexural Modulus (GPa)
Neat PI	94–120	7–15	1.85–2.18	-	-
CF-PI	-	-	-	1552	119
CNT-PI	1400	14.3	-	-	-

CF-PI: Carbon fiber-reinforced polyimide composites; CNT-PI: Carbon nanotube-reinforced polyimide composites.

summarizes the tensile and flexural properties of neat PI, carbon fiber-reinforced PI (CF-PI), and CNT-reinforced PI composites (CNT-PI).

Thus, while neat PIs possess desirable thermal and mechanical properties, their brittleness can be a limitation in certain applications. The development of PI composites, through the incorporation of various reinforcements, has led to materials with enhanced mechanical performance, making them suitable for a broader range of structural applications.

Considering the central role that fiber-matrix interfacial strength plays in governing composite performance, particular emphasis has been placed on its influence under time-dependent loading conditions. Thus, several investigations have focused on the creep behavior of short fiber-reinforced PI composites to evaluate their long-term structural integrity in elevated-temperature environments. For instance, Panin et al. [76] evaluated creep behavior under cyclic loading for PI and polyetherimide (PEI) matrix composites reinforced with short carbon fibers. Under low-cycle fatigue conditions, PI-based systems exhibit significantly enhanced durability, with markedly lower rates of cyclic creep accumulation compared to their PEI counterparts. This difference is attributed to the inherent molecular structure of the matrices; the greater chain mobility in PEI facilitates microstructural reorganization, thereby accelerating cyclic deformation processes in the composite.

Thermoset PIs, such as PMR-15, likely demonstrate low creep rates even at temperatures up to 250 °C, with minimal plastic deformation over extended periods. Reinforcement with materials like carbon fibers further enhances this property, resulting in composites that maintain their mechanical integrity at temperatures up to 260 °C [34,77,78]. In contrast, other engineering polymers exhibit higher creep rates under similar conditions [4]. Epoxy resins, though commonly used for their adhesive properties, tend to exhibit higher creep rates, particularly at elevated temperatures. Polyester resins, including unsaturated polyesters, also demonstrate higher creep rates compared to PIs, which limit their use in high-temperature applications. Phenolic resins, known for their high creep resistance, likely maintain their mechanical properties up to approximately 150 °C, but their performance diminishes at higher temperatures [79]. By and large, few studies have directly examined the creep behavior of PIs and PI composites. However, indirect estimates may be obtained from related measurements, such as thermogravimetric analysis, which provide insight into thermal stability and degradation. The thermal endurance of PI composites can be assessed through several methods. Thermogravimetric analysis of PMR-15 shows notable weight loss at 343, 316, and 288 °C within 100 h, indicating thermal degradation. Although creep was not directly studied, it is reasonable to infer that stable performance is expected at temperatures 10 to 15% below these values, where material integrity is typically maintained [34].

Table 5. Comparative overview of creep behavior for neat polyimide (PI), PI composite, epoxy, polyester, and phenolic resins [4,13,34,77,78].

Polymer Type	Max Service Temp.	Creep Resistance at Elevated Temperature	Notes
Neat Polyimide	Up to 260 °C	Excellent	Low creep rates even at high temperatures; suitable for demanding applications.
Polyimide Composites	Up to 260 °C	Excellent	Reinforcement with materials like carbon fibers enhances creep resistance.
Epoxy Resin	Up to 120 °C	Moderate	Higher creep rates at elevated temperatures; commonly used for adhesive properties.
Polyester Resin	Up to 100 °C	Moderate	Higher creep rates compared to polyimides; limited use in high-temperature applications.
Phenolic Resin	Up to 150 °C	Good	Maintains mechanical properties up to approximately 150 °C; performance diminishes at higher temperatures.

shows a comparative overview of creep behavior for neat PI, PI composite, epoxy, polyester, and phenolic resins. Not all studies cited in Table 5 focus on creep behavior. However, the thermal data they report may provide a basis for estimating creep response under relevant conditions.

PI-based composites are often reinforced with continuous carbon fibers, which impart high stiffness and strength while allowing the material system to retain its integrity at elevated temperatures. The unfilled PI matrix typically exhibits tensile strengths between 90 and 130 MPa, and elastic moduli in the range of 2.5 to 4 GPa. With fiber reinforcement, these values increase significantly. Tensile strengths can exceed 1200 MPa, and flexural moduli often surpass 120 GPa, depending on fiber content and laminate architecture.

One of the most critical attributes of polyimide composites is their ability to function under high thermal loads. Many systems possess glass transition temperatures above 400 °C and can operate continuously in the range of 260 to 315 °C without significant degradation in mechanical properties. These composites also exhibit favorable creep resistance at elevated temperatures, making them suitable for sustained loading conditions. The fatigue behavior is generally stable, supported by strong fiber-matrix adhesion and low crack growth rates under cyclic stress.

Despite these advantages, fracture toughness remains a limiting factor. A study on the fracture toughness of polyimide films reported values ranging from 1.65 to 5.4 MPa√m for various PI films, including neat samples [80] (See

Table 6. Fracture toughness of polyimide materials [80].

Material	Thickness (µm)	Number of Tests	Kc (MPa m0.5)
LARC-TPI	25	12	1.98 ± 0.37
LARC-TPI	45	16	2.79 ± 0.37
Polyamidie-imide	16	16	4.28 ± 0.26
Kapton	25	37	1.65 ± 0.14

).

This relatively low toughness introduces vulnerability in impact or damage-prone environments. Moisture absorption is another concern, as it can lead to plasticization of the matrix and reductions in stiffness and strength. To address these challenges, several strategies have been developed, including the use of modified resin chemistries, optimized fiber surface treatments, and hybrid reinforcement configurations.

As an example, it is possible to fabricate porous and actively deformable shape memory polyimide aerogels. Traditional PI aerogels suffer from suboptimal rheological behavior and limited printing resolution, which impair their processability via 3D printing. In particular, the polyamic acid (PAA), which is the PI’s precursor, shows fluid-like characteristics, impeding precise molding and 3D printing advancement of polyimide aerogels [81]. These limitations can be addressed by formulating a composite aerogel that combines polyimide with silica aerogel particles (SAP). Various PAA/SAP inks were prepared by mixing SAP and PAA with different mass fractions of SAP in the ink, with 10 wt%, 20 wt%, and 30 wt%. For example, PAA/SAP20 represents SAP with 20 wt% (See Figure 6). These particles serve as rheological modifiers to optimize the ink’s flow and print fidelity. Additionally, the particles contribute to refining the pore architecture to restrict heat conduction, thereby sustaining superior thermal insulation. The resulting PI/SAP composites exhibit shape fixation and recovery ratios above 93% with exceptional thermal insulation and fire-retardant properties [82].

Furthermore, to assess the flame-retardant efficacy and thermal insulating capabilities of the PI/SAP composite aerogels, combustion tests were conducted on expanded polyethylene (EPE), neat PI aerogels, and PI/SAP20 composites using an alcohol flame source, as illustrated in Figure 7 [82]. Upon exposure to the flame, EPE exhibited rapid ignition and sustained combustion, undergoing complete thermal degradation within seconds, demonstrating negligible flame-retardant resistance (Figure 7a). In contrast, the neat PI aerogel displayed localized ignition and carbonization at the point of flame contact. Although the sample self-extinguished after flame removal (60 s), it exhibited substantial structural loss, with approximately 70% volumetric shrinkage (Figure 7b). Remarkably, the PI/SAP20 composite displayed a distinctly improved response: only minor surface reddening and slow carbonization occurred during flame exposure, with immediate cessation of visible thermal effects upon flame withdrawal. No observable smoke, melting, or deformation was noted, and the structural integrity was largely preserved (Figure 7c). This enhanced thermal stability is attributed to the in situ formation of a silica-rich surface layer, which acts as an effective barrier against oxidative degradation and volatile diffusion. Additionally, the retention of a carbonaceous char layer within the interior serves to preserve the porous network and impede thermal conduction, thereby contributing to the overall flame retardancy and insulation performance of the composite aerogel system [82].

Beyond those capabilities, PI composites have shown promising applications in 5G technology. Flexible circuit boards are essential in modern electronics due to their lightweight structure, mechanical compliance, and ability to support complex geometries. With the rise of 5G technology, there is an increasing demand for materials capable of high-frequency, high-speed signal transmission. Since signal propagation is strongly influenced by dielectric properties, lowering the dielectric constant (ε_r) of insulating layers has become critical. Although PI is widely used in electronic devices for its thermal and mechanical stability, its relatively high dielectric constant (3.1–3.5) limits its use in advanced applications. Recent efforts to reduce ε_r have focused on two strategies: chemical modification to reduce polarizability (e.g., via fluorinated or alicyclic groups), and structural approaches to increase free volume or introduce porosity. These methods have yielded PI systems with ε_r as low as 2.1–2.8 and, in porous configurations, as low as 1.3 [83].

However, introducing porosity often degrades mechanical strength, thermal stability, and dielectric loss performance. To overcome these trade-offs, Zhang et al. [83] developed a sandwich-structured PI film composed of a porous low-ε_r core and dense fluorinated graphene/PI outer layers. This multilayer architecture achieves a balance of low dielectric constant, mechanical robustness, and thermal stability, making it suitable for flexible electronics, 5G devices, and aerospace applications.

A multilayer polyimide (PI) film was fabricated using a sandwich architecture, consisting of a porous PI core flanked by dense fluorinated graphene/polyimide (FG/PI) composite outer layers. The porous intermediate layer significantly reduced the overall dielectric constant, while the FG/PI surface layers enhanced mechanical integrity and dielectric breakdown resistance relative to the porous PI alone. The resulting three-layer system exhibited a tensile strength of 65.76 MPa, a modulus of 2.96 GPa, and a dielectric breakdown strength of 170.49 kV/mm. Dielectric characterization revealed a relative permittivity of 1.92 and a loss tangent below 0.003. These combined properties position the sandwich-structured PI film as a strong candidate for use in flexible circuit substrates for next-generation electronic and microelectronic devices.

Fractographic analysis via cross-sectional SEM imaging (Figure 8) reveals that, following tensile failure, the interfaces between the top and middle layers, and between the bottom and middle layers, remain well bonded, with no evidence of interfacial delamination. The fracture traverses the entire three-layer architecture uniformly, with no discernible layer displacement or pull-out. This cohesive failure mode underscores the structural integrity achieved through the sequential layer-by-layer deposition process, indicating strong interfacial adhesion. Such integrity across the laminate directly contributes to the enhanced mechanical performance of the composite system.

5. Discussion, Summary, and Outlook

Polyimide (PI) polymers and PI composites have established themselves as a cornerstone in high-performance polymer science, particularly for high-temperature applications and extreme environments. Their unique molecular architecture, typically produced through condensation reactions of dianhydrides and diamines or innovative techniques such as the Polymerization of Monomeric Reactants (PMR) process, enables these materials to perform reliably under extreme conditions. At the same time, the development of PI composites has earned significant attention to further improve performance by integrating reinforcing fillers such as carbon fibers, graphene, and other nanomaterials into the PI matrix.

Leading aerospace manufacturers, including Pratt & Whitney and General Electric, are actively pursuing the integration of composite materials throughout jet engine assemblies, with a particular emphasis on organic matrix composites for jet engines’ cold and medium sections. Jet engine components operate across a broad range of service temperatures, and, in regions where the temperature ranges from approximately 90 °C to 150 °C, conventional metallic alloys such as steel, titanium, and aluminum have historically been employed without necessitating thermal and creep analysis, due to their inherently superior creep resistance within this regime. However, the ongoing transition from metallic to polymer matrix composites has introduced new considerations, as creep deformation likely becomes a significant concern under long-term thermal and mechanical loading. In this context, PI-based composites have emerged as more suitable candidates than conventional epoxy-based systems, owing to their markedly superior resistance to time-dependent deformation. Compared to epoxy matrices, polyimides demonstrate enhanced thermal stability and mechanical performance under sustained load, rendering them particularly attractive for structural applications within the intermediate temperature range of jet engines.

In this paper, we provided an overview of PI and PI-based composites, focusing on the various types of polyimides, their molecular structures, and the resulting thermal and mechanical properties. The discussion includes synthesis routes, processing techniques, and performance characteristics under thermal and mechanical loading. Particular attention has been given to their suitability for advanced applications, including aerospace structures and electronic circuit components, where high thermal stability, mechanical strength, and chemical resistance are critical. As the range of potential applications for PI-based materials continues to grow, particularly in microelectronics, aerospace systems, and emerging sustainable energy technologies, understanding their behavior and design considerations remains of central importance.

Further investigation into the chemical structure, synthesis, and modification of polyimides remains important. A focused review on these aspects would offer significant value to both materials scientists and engineers working with high-performance polymer systems. Several additional focused studies can be considered for future reviews, including, but not limited to, fluoro-containing polyimide matrices, which would examine their chemistry, processing, and performance in demanding environments for a chemistry-focused study, as well as the mechanical behavior of PI and PI composites for a mechanical-focused study. Moreover, biodegradability and recycling pose key challenges and opportunities for polyimides. Emerging recovery methods and biodegradable alternatives deserve closer examination. Thus, targeted reviews on these topics could advance sustainable polyimide technologies.