High-temperature polymer nanocomposites represent a critical advancement in materials science, particularly for industries demanding exceptional thermal stability, mechanical robustness, and lightweight properties. Among the most prominent high-temperature polymers are polyimides and polyether ether ketone (PEEK), which exhibit excellent thermal resistance, chemical inertness, and mechanical strength. However, their performance can be further enhanced through the incorporation of nanofillers such as boron nitride (BN) nanotubes, carbon nanotubes (CNTs), and graphene, which significantly improve thermal conductivity, creep resistance, and overall durability under extreme conditions.

The thermal stability of high-temperature polymers is a key consideration for aerospace and automotive applications, where components are routinely exposed to temperatures exceeding 300°C. Polyimides, for instance, possess a glass transition temperature (Tg) ranging from 250°C to 400°C, while PEEK exhibits a Tg of approximately 143°C and a melting point around 343°C. Despite these inherent properties, prolonged exposure to high temperatures can lead to polymer chain scission, oxidative degradation, and loss of mechanical integrity. The addition of thermally conductive nanofillers mitigates these issues by dissipating heat more efficiently and reinforcing the polymer matrix.

Boron nitride nanotubes (BNNTs) are particularly effective in enhancing thermal stability due to their high thermal conductivity (up to 300 W/m·K) and exceptional oxidation resistance. When dispersed uniformly within a polyimide or PEEK matrix, BNNTs create a thermally conductive network that reduces hotspots and delays thermal degradation. Studies have shown that incorporating 10 wt% BNNTs into polyimide can increase its thermal conductivity by over 200%, while also improving its thermal decomposition temperature by 20-30°C. Similarly, carbon nanotubes contribute to thermal management, with multi-walled CNTs (MWCNTs) offering a balance between thermal conductivity (up to 3000 W/m·K for single-walled CNTs) and mechanical reinforcement.

Processing these nanocomposites presents significant challenges, primarily related to nanofiller dispersion and interfacial adhesion. High-temperature polymers often require processing at elevated temperatures, which can exacerbate nanofiller agglomeration. Solution casting, melt blending, and in-situ polymerization are common methods, but each has limitations. For example, melt blending of PEEK with CNTs at temperatures above 350°C can lead to polymer degradation if not carefully controlled. Functionalization of nanofillers with hydroxyl or carboxyl groups improves compatibility but may introduce defects that reduce thermal conductivity. Optimizing processing parameters such as shear rate, temperature, and residence time is critical to achieving homogeneous dispersion without compromising polymer integrity.

Creep resistance is another critical property for high-temperature applications, particularly in aerospace components such as engine housings, turbine blades, and structural brackets. Under constant mechanical stress at elevated temperatures, polymers exhibit viscoelastic deformation, leading to dimensional instability. Nanocomposites reinforced with BN or CNTs demonstrate superior creep resistance due to the restriction of polymer chain mobility by the nanofillers. Research indicates that adding 5 wt% BNNTs to PEEK reduces creep strain by up to 50% at 200°C compared to unfilled PEEK. The nanofillers act as physical crosslinks, hindering chain slippage and delaying failure.

In aerospace applications, the combination of lightweight properties and thermal stability makes these nanocomposites ideal for replacing metal components. For instance, PEEK-BNNT composites are used in aircraft engine components, where weight reduction directly translates to fuel efficiency gains. The nanocomposites also exhibit low outgassing under vacuum conditions, making them suitable for satellite and spacecraft applications. In the automotive sector, high-temperature polymer nanocomposites are employed in under-the-hood components such as turbocharger housings, bearing cages, and electrical insulation. The ability to withstand continuous exposure to engine heat and aggressive fluids while maintaining mechanical performance is a key advantage.

The long-term durability of these materials under cyclic thermal and mechanical loads is a critical area of study. Thermal cycling between extreme temperatures can induce microcracking and interfacial debonding in poorly designed nanocomposites. However, systems with strong polymer-filler interfaces, achieved through covalent bonding or optimized processing, exhibit remarkable resistance to thermal fatigue. For example, polyimide-graphene oxide nanocomposites subjected to 500 thermal cycles between -196°C and 300°C showed no significant loss in tensile strength, highlighting their potential for space applications.

Electrical properties can also be tailored through nanofiller selection. While CNTs enhance electrical conductivity, BNNTs preserve the insulating properties of the polymer, which is crucial for electrical components in high-temperature environments. This selectivity allows for the design of materials with customized conductive or insulating characteristics based on application requirements.

Environmental resistance is another consideration, as high-temperature polymers often encounter oxidative and hydrolytic degradation. Nanocomposites with well-dispersed fillers exhibit improved barrier properties, reducing the diffusion of oxygen and moisture into the polymer matrix. This is particularly important for automotive applications where exposure to engine oils and coolants is common.

Future developments in high-temperature polymer nanocomposites are likely to focus on multifunctional systems that combine thermal stability with additional properties such as self-healing, flame retardancy, or radiation shielding. Advances in nanofiller functionalization and processing techniques will further enhance performance, enabling broader adoption in extreme-environment applications.

In summary, high-temperature polymer nanocomposites reinforced with boron nitride or carbon nanotubes offer unparalleled thermal stability, creep resistance, and mechanical performance for demanding aerospace and automotive applications. Overcoming processing challenges and optimizing nanofiller dispersion are key to unlocking their full potential, paving the way for next-generation materials capable of withstanding the most extreme conditions.