# Epoxy/Glass Fiber Nanostructured p- and n-Type Thermoelectric Enabled Model Composite Interphases
## 1 Introduction  
During the last decades, the growing demand for the utilization of lightweight, high-performance structural composite materials has rapidly increased in various commercial sectors, such as the aerospace and automotive industries. Fiber reinforced polymers constitute an extremely interesting field of research due to their potential for the exploitation of additional functionalities. Tailoring or \"architecting\" the material in relation to both its structural and non-structural services is an inherent ability of advanced composites and is rapidly emerging technology for their next generation. Understanding their mechanical behavior requires the study of the interface, the common physicochemical areas between the distinct phases (reinforcing and matrix materials) [1, 2]. This is a prerequisite for the design of a composite material, as its behavior is most of the times governed by its interfacial properties [3]. The approach of blending nano-scaled reinforcements into the matrix of micro-scale reinforced systems, with a view to enhancing both the matrix dominated properties as well as imparting multifunctionality, has been extensively studied in the last decade. Additional functionalities include both capabilities that relate to structural integrity, such as wear resistance, morphing, or self-healing, as well as non-structural ones, such as inherent structural health monitoring, sensing and actuation, power harvesting, and power storage [4; 5; 6; 7]. In an alternative approach, the nanomaterials are deposited directly onto the surface of reinforcing fibers, such as carbon, aramid, and glass fibers (GFs), and this has triggered a new class of functional materials, where the nanoscale functional reinforcement is following the architecture of the microscale reinforcement. This leads to a biomimetic 'hierarchical' approach towards multiscale reinforcements with improved wettability, increased adhesion, robust interlocking mechanisms, and enhanced interfacial stress transfer related to the introduction of nanophases such as carbon nanotubes (CNTs) [8; 9; 10; 11; 12]. The resultant hierarchical interphases except for the modified mechanical properties also construse extra functionalities [13; 14; 15; 16; 17]. [PERSON] and co-workers presented a synergistic strategy by taking advantage of polyetherimide (PEI) and ZIF-67 to enhance the interfacial strength of carbon fiber (CF) reinforced polyetheretherketone (PEEK) composites via a facile route. After modification, the tensile strength of single CFs showed no deterioration and the interfacial shear strength (IFSS) of the as-prepared modified CF/PEEK model composites revealed an increase of 40.5% [18]. [PERSON] et al. [19] developed nanostructured ordered \"brick-and-mortar\" interphases using layer by layer deposition of PDDA(PSS/LDH)\({}_{25}\) solution onto the surface of hydroxylated GFs. The resulting anisotropic morphology caused strain hardening phenomena to the produced nanostructured interphases and thus reduced the local stress concentrations arising from fiber breaks and increased the extent of stable fiber slippage. [PERSON] et al. presented a scalable method for growing CNTs during either the CF fabrication or the CF surface treatment stage, that could potentially further minimize their damage and offer a suitable replacement for the CF polymer sizing. Moreover, improvements in electrical conductivity in all three directions with significant enhancement in the surface (300%), thickness (450%) and volume (230%) were observed. Additionally, the thermal conductivity was improved by 107% in the through-thickness direction [20]. Another study documented the strongly orthotropic electrical properties of composite materials comprised of hierarchical UD GFs coated with single-walled carbon nanotubes (SWCNTs). The exceptional damage-sensing capability of the SWCNTs hierarchical composites was predicted by simulations and validated experimentally [21].