Morphology of poly(ether ether ketone)/poly(ether imide) blends and their application as a matrix for carbon fiber-reinforced plastic

Polymer Journal https://doi.org/10.1038/s41428-026-01162-9 ORIGINAL ARTICLE Morphology of poly(ether ether ketone)/poly(ether imide) blends and their application as a matrix for carbon fiber-reinforced plastic Yusaku Oba1 Hideyuki Uematsu ● 1,2 ● Ayaka Yamaguchi1 Fumihiro Nishimura3 Shuichi Tanoue1,2 ● ● 1234567890();,: 1234567890();,: Received: 28 November 2025 / Revised: 19 January 2026 / Accepted: 7 February 2026 © The Author(s) 2026. This article is published with open access Abstract This study aims to develop composite materials that combine high chemical resistance with superior thermal stability by blending poly(ether ether ketone) (PEEK), known for its chemical resistance, and poly(ether imide) (PEI), recognized for its heat resistance. The interfacial adhesion between carbon fibers (CFs) and PEEK/PEI blends was systematically investigated from the perspective of the blend morphology. Compared with that between PEEK and CFs, the interfacial shear strength (IFSS) between PEI and CFs was greater, and the IFSS of the PEEK/CF composites increased with increasing PEI content. At 50 wt% PEI, the IFSS became nearly equivalent to that of the PEI/CF composite. The distribution of PEI and PEEK near the CFs in the PEEK/PEI (50/50) matrix was examined using micro-Raman spectroscopy and scanning electron microscopy (SEM). The results revealed a submicron-sized PEI-rich region near the CF surface. While PEI was completely soluble in chloroform, the solubility of the PEEK/PEI (50/50) blend was limited to 13%, indicating enhanced chemical resistance due to PEEK incorporation. The glass transition temperature (Tg) of PEEK also increased with the addition of PEI. These findings demonstrate that submicron-scale heterogeneous structures in PEEK/PEI blends enhance thermal and chemical resistance and interfacial adhesion between the matrix and the CFs. Introduction In recent years, addressing global environmental challenges such as climate change has become an urgent priority, and reducing carbon dioxide emissions remains one of the most effective strategies. Carbon fiber-reinforced plastics (CFRPs), which are lighter and stiffer than metals, are considered potential alternatives in the aerospace and automotive industries. One of the critical factors determining the mechanical performance of CFRPs is the interfacial adhesion between carbon fibers (CFs) and the polymer matrix [1]. Various approaches have been proposed to enhance interfacial adhesion depending on the type of * Hideyuki Uematsu 1 Graduate School of Engineering, University of Fukui, Bunkyo, Japan 2 Research Center for Fibers and Materials, University of Fukui, Bunkyo, Japan 3 Headquarters for Innovative Society–Academia Cooperation, University of Fukui, Bunkyo, Japan polymer used [2, 3]. Thermoplastics offer superior recyclability; however, their interfacial strength with CFs is generally weaker than that of thermosets, making them a major focus of ongoing research [4–6]. Poly(ether ether ketone) (PEEK), a super engineering plastic, has been widely used in aerospace and biomedical applications because of its excellent thermal stability, chemical resistance, wear resistance, mechanical strength, and biocompatibility [7, 8]. However, the high processing temperature of PEEK can cause thermal degradation of the sizing agent on the CF surface, leading to reduced interfacial adhesion [9, 10]. To address this issue, the development of thermally stable sizing agents compatible with PEEK has been actively pursued [11–15]. Furthermore, combining high-temperature-resistant sizing agents with nanoparticles such as carbon nanotubes (CNTs) has been reported to further enhance interfacial bonding [16–18]. In addition, thermoplastics with a glass transition temperature (Tg) higher than that of PEEK (Tg ≈ 140 °C), such as poly(ether imide) (PEI, Tg ≈ 220 °C) and poly(ether sulfone) (PES, Tg ≈ 225 °C), have been used as matrices for CFRPs to improve heat resistance [19–22]. However, the relatively poor chemical resistance of both PEI and PES limits their ability to achieve simultaneous thermal and chemical Y. Oba et al. stability in super engineering plastic-based CFRPs. Therefore, blending different thermoplastic polymers has emerged as a promising strategy to balance these properties effectively. Amorphous PEI is known to be miscible with the amorphous regions of semicrystalline PEEK; therefore, the addition of PEI increases the Tg of PEEK [23–25]. The combination of PEEK and PEI is unique because many polymer chain combinations are incompatible in various polymer blends [26, 27]. Leveraging this compatibility, PEI has been used as a sizing agent on CF surfaces for PEEK matrices [17, 28–30]. However, to the best of our knowledge, no studies have investigated PEEK/PEI blends as matrices for CFRPs. Previous studies on CFRPs with polymer blend matrices have reported that blending poly(phthalazinone ether sulfone ketone) (PPESK) with PEI improves both interfacial adhesion and impregnation between CFs [31]. Similarly, the addition of acid-modified poly(phenylene ether) (mPPE) to syndiotactic polystyrene (sPS) enhances the interfacial adhesion of CFs [32]. On the basis of the available literature, no studies have examined the use of polymer blends as matrices for CFRPs. As discussed above, polymer blends effectively compensate for the inherent disadvantages of individual polymers. Therefore, using polymer blends as matrices in CFRPs represents a promising concept for next-generation composites. Nevertheless, research in this area remains extremely limited. In the present study, the feasibility of using PEEK/PEI blends as matrix resins for CFRPs was explored to integrate the thermal stability of PEEK with the chemical resistance of PEI. This study systematically investigated both the solvent resistance of PEEK/PEI blends containing CFs and the interfacial adhesion between the PEEK/PEI matrix and the CFs, which is a key factor governing the CFRP performance. The results demonstrated that the strong interfacial bonding between PEI and CFs was effectively retained in the PEEK/PEI–CF composites. Furthermore, the chemical resistance of PEI was enhanced by the incorporation of PEEK. Microscopic Raman spectroscopy and electron microscopy revealed that the submicron-scale dispersion of PEEK and PEI around the CFs, including partially miscible components, contributed significantly to the improved interfacial adhesion and chemical durability. Experimental Materials and preparation PEEK (151G, Victrex) and PEI (Ultem 1000, SABIC) were used as matrix polymers. The polymers were melt-blended using a twin-screw extruder (ULTnano, Technovel) at 370 °C with a screw rotation speed of 200 rpm. The PEI content was varied from 10 to 50 wt% relative to PEEK, in increments of 10 wt%. Since the objective of this study was to achieve both heat resistance and solvent resistance, the maximum PEI content was limited to 50 wt%. Each sample is denoted (...truncated)