Enhancing interfacial interactions in fiber-reinforced polymer composites (FRPCs) is crucial for improving their mechanical properties. This can be achieved through the incorporation of nanomaterials or chemically functional agents into FRPCs. This study reports the tailoring of the fiber–matrix interface in FRPCs using non-functionalized graphene nanoplatelets (GNPs) in combination with a waterborne, highly branched, multi-functional polyurethane dispersion (HBPUD). A unique ultrasonic spray deposition technique was utilized to deposit aqueous mixtures of GNP/HBPUDs onto the surfaces of carbon fiber fabrics, which were used to prepare epoxy-prepreg sheets and corresponding FRPC laminates. The influence of the polyurethane (PU) and GNP content and their ratio at the fiber–matrix interface on the tensile properties of resulting high-performance composites was systematically investigated using stress–strain analysis of the produced FRPC plates and SEM analysis of their fractured surfaces. A synergistic stiffening and toughening effect was observed when as low as 20 to 30 mg of GNPs was deposited per square meter of each side of the carbon fiber fabrics in the presence of the multi-functional PU layer. This resulted in a significant improvement in the tensile strength from 908 to 1022 MPa, while maintaining or slightly improving the initial Young’s modulus from approximately 63 to 66 MPa.

One-component epoxy resins (OCERs) are proposed to overcome the energy inefficiency and processing difficulties of conventional two-component epoxy resins by employing latent curing agents, specifically thermal latent curing agents (TLCs). Despite recent progress, the need for TLCs with a simple preparation method for different curing agents, epoxy resins, and process conditions remains. Here, tailorable TLCs were prepared by forming complexes between imidazole (Im) and amphiphilic polyoxazoline copolymers with tunable structures and properties by a solvent evaporation method. The obtained TLCs were manually mixed with DGEBA to prepare OCERs. The miscibility of the complexes with DGEBA was studied, considering the functionalities of copolymers. The curing behaviors of TLCs were compared using dynamic Differential Scanning Calorimetry (DSC) studies considering the side chain and composition of the copolymers, copolymer:Im ratio, and concentration of Im in DGEBA. The curing behavior of the promising OCERs was studied by isothermal DSC studies to investigate their stability at different temperatures and curing rate at elevated temperatures revealing the stability of these OCERs.

Poly(2-oxazoline)s (POZs) are widely investigated for their applications in various fields due to their unique properties. To exploit and combine different characteristics of the POZ family, 2-oxazoline monomers can be copolymerized to prepare tailor-made copolymers with the desired glass transition temperature (T _g), melting temperature (T _m), amphiphilicity, and functionality. Here, we report the synthesis and characterization of 2-oxazoline monomers and a range of POZ copolymers produced, thereof. 2-Propyl-2-oxazoline (PrOZ) and 2-pentyl-2-oxazoline (PeOZ) monomers were synthesized by two different methods starting from nitriles or carboxylic acids. A number of POZ copolymers were synthesized by copolymerization of 2-ethyl-2-oxazoline (EOZ) with either one of PrOZ, PeOZ, or 2-phenyl-2-oxazoline (PhOZ) at three different compositions (25:75, 50:50, and 75:25) and three molecular weights (1000, 2000, and 5000 Da). The successful synthesis of the monomers and copolymers was demonstrated through their structural analysis by ¹H NMR and FTIR. SEC results confirmed the targeted molar masses of the copolymers and living nature of the polymerization by showing low dispersity values. Thermal properties of the copolymers were studied using DSC and TGA. DSC studies revealed the amorph and random state of the copolymers with obtained T _g values for the copolymers in the range of -3 to 84 °C depending on their molecular weight and type of the side chain. While the presence of longer aliphatic side chains resulted in lower T _g values, the presence of 2-phenyl substituents on the polymer led to higher T _g values. The decomposition temperatures determined by TGA were in the range of 328 to 383 °C depending on the molecular weight, composition, and side chain of the copolymers. It was observed that higher molecular weights led to higher T _g values and decomposition temperatures. While copolymers with aliphatic side chains exhibited a single-step decomposition profile, the decomposition of copolymers having aromatic side chains occurred in multiple steps. The variations in the molecular weight, composition, and side chains of the copolymers resulted in a library of tailorable amphiphilic copolymers suitable for multiple applications ranging from biomedical applications to composite manufacturing.

One-component epoxy resins (OCERs) with improved stability (shelf life) and controlled curing temperatures were prepared using epoxy resins and polyoxazoline-imidazole (POZ-Im) based thermal latent curing agents (TLCs). POZ homopolymers with molar masses of 1000, 2000, and 5000 g/mol were obtained via cationic ring-opening polymerization (CROP) of 2-ethyl-2-oxazoline, 2-propyl-2-oxazoline, 2-pentyl-2-oxazoline, and 2-phenyl-2-oxazoline. TLCs were prepared by physically entrapping imidazole, the curing agent, in the POZ matrix at the homopolymer/Im (HP/Im) ratios of 1:1 and 5:1 and characterized by FTIR and TGA. TLCs were then mixed with bisphenol A diglycidyl ether (DGEBA) to obtain OCERs with Im concentrations of 1, 3, and 5 wt %. Dynamic DSC tests were performed to determine the effect of the pendant group and molar mass of POZ, the POZ/Im ratio, and Im concentration on the curing behavior of the OCERs, whereas isothermal DSC tests were carried out to examine their thermal stability and optimal curing temperatures. Optical microscopy was performed to study the compatibility of the TLCs with DGEBA. This study showed that the dispersion quality of TLCs is highly associated with the compatibility of POZs and DGEBA, which affected the release of Im, thus left limit temperatures of curing. In addition, higher left limit temperatures were obtained when the POZ/Im ratio increased. Isothermal DSC results conformed to the improved stability and better thermal latency of the samples with a POZ/Im ratio of 5:1. Moreover, the higher left limit temperatures were obtained with the lowest molar mass of POZ due to the better interaction between the -OH end group of POZ and Im. The shelf life of PEOZ 1K-Im 5:1 1% OCER was predicted at -20, 0, and 20 °C, with an estimated 15.3 days at 20 °C using isothermal DSC and rheology at 50, 60, and 70 °C. Overall, this research contributes to the development of OCERs by introducing POZ-Im complexes as novel TLCs. The findings shed light on the importance of compatibility in achieving optimal dispersion and release of Im and the role of the POZ/Im ratio and POZ molar mass in controlling left limit temperatures, ultimately influencing the curing behavior of OCERs.