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Polyether-ether-2-ketone (PEKK) is a high-performance thermoplastic polymer used in various fields, from aerospace to medical applications, due to its exceptional mechanical and thermal properties. Nonetheless, the mechanical behavior of 3D-printed PEKK still deserves to be more thoroughly investigated, especially in view of its production by 3D printing, where mechanical properties measured at different scales are likely to be correlated to one another and to all play a major role in determining biomechanical properties, which include mechanical strength on one side and osteointegration ability on the other side. This work explores the mechanical behavior of 3D-printed PEKK through a multiscale approach, having performed both nanoindentation tests and standard tensile and compression tests, where a detailed view of strain distribution was achieved through Digital Image Correlation (DIC) techniques. Furthermore, for specimens tested up to failure, their fractured surfaces were analyzed through Scanning Electron Microscopy (SEM) to clearly outline fracture modes. Additionally, the internal structure of 3D-printed PEKK was explored through Computed Tomography (CT) imaging, providing a three-dimensional view of the internal structure and the presence of voids and other imperfections. Finally, surface morphology was analyzed through confocal microscopy. The multiscale approach adopted in the present work offers information about the global and local behavior of the PEKK, also assessing its material properties down to the nanoscale. Due to its novelty as a polymeric material, no previous studies have approached a multiscale analysis of 3D-printed PEKK. The findings of this study contribute to a comprehensive understanding of 3D-printed PEKK along with criteria for process optimization in order to customize its properties to meet specific application requirements. This research not only advances the knowledge of PEKK as a 3D-printing material but also provides insights into the multifaceted nature of multiscale material characterization. [ABSTRACT FROM AUTHOR]

Polymer materials are increasingly widely used in high-fire-risk applications, such as aviation interior components. This study aimed to compare the tensile, thermal, and flame-retardant properties of test samples made from ultra-performance materials, polyetherimide (PEI) and polyetherketoneketone (PEKK), using the fused filament fabrication process (FFF). The tensile tests were performed for these materials at different raster angles (0, 45, and 90°). The thermomechanical tests were done in the axial, perpendicular, and through-thickness directions to the extruded filaments. The impact of printing parameters on the flame retardancy of 3D-printed samples was investigated in vertical burn tests with varying specimen thicknesses and printing directions. Experimentally, it was testified that PEKK had better isotropic behaviour than PEI for mechanical performance, thermal expansion, and fire-resistant properties, which are essential in fabricating intricately shaped products. [ABSTRACT FROM AUTHOR]

Polymer materials are increasingly widely used in high-fire-risk applications, such as aviation interior components. This study aimed to compare the tensile, thermal, and flame-retardant properties of test samples made from ultra-performance materials, polyetherimide (PEI) and polyetherketoneketone (PEKK), using the fused filament fabrication process (FFF). The tensile tests were performed for these materials at different raster angles (0, 45, and 90°). The thermomechanical tests were done in the axial, perpendicular, and through-thickness directions to the extruded filaments. The impact of printing parameters on the flame retardancy of 3D-printed samples was investigated in vertical burn tests with varying specimen thicknesses and printing directions. Experimentally, it was testified that PEKK had better isotropic behaviour than PEI for mechanical performance, thermal expansion, and fire-resistant properties, which are essential in fabricating intricately shaped products.

With the development of high-density electronics, polymer-based thermal conductive composites with excellent thermal and mechanical performance are receiving great attention. For the development of polymer-based thermal conductive composites with superior properties, it is essential to reduce the agglomeration of fillers and enhance the interfacial interactions between thermal conductive fillers and the polymer matrix. Among the several thermal conductive materials, graphene nanoplatelets (GNPs) and cellulose nanofiber (CNFs) are attracting attention. However, poor interfacial interaction between GNP and CNF leads to interfacial separation, which in turn, deteriorates its properties. Herein, we synthesized a pyranine-functionalized polyether dispersant (PyPE), which can significantly improve the interfacial interactions between GNPs and CNFs. The effect of PyPE on the dispersion behavior of GNPs and the thermomechanical properties of GNP-CNF composite were monitored. Furthermore, we also performed molecular dynamics (MD) simulations to investigate the impact of applying PyPE dispersant on the interfacial interactions between the GNPs and CNF matrix. The application of PyPE significantly enhanced the dispersibility of GNPs within the CNF matrix, resulting in a 15.5 % increase in-plane thermal diffusivity and a 24.5 % enhancement in tensile strength of the composite, demonstrating their potential applications in high power density electrical equipment and electronic devices. [Display omitted] [ABSTRACT FROM AUTHOR]

• A laser spot periodic heating technique based on sub-region phase fitting is developed in measuring the anisotropic thermal diffusivity of thin films. • The spatial distribution of phase is divided into linear and nonlinear regions for fitting in-plane and out-of-plane thermal diffusivity, respectively. • The thermal diffusivity of the tested films ranges from 5.5 × 10−8 m²/s to 9.6 × 10−4 m²/s. • The present method can accomplish anisotropic thermal diffusivity measurements for thin films with thicknesses ranging from a few to hundreds of microns. The thermal transport properties of thin films have garnered significant research interest due to their diverse applications in new energy sources and thermal management. As the thickness of anisotropic thin films decreases to a few microns, conventional laser flash analyzer and infrared lock-in thermography encounter difficulties in characterizing their thermophysical properties. In response to this challenge, we propose a laser spot periodic heating technique based on sub-region phase fitting for measuring anisotropic thin films. In this method, a thermocouple monitors the temperature response on the sample back surface, and the amplified voltage is subsequently measured using a lock-in amplifier to record the spatial distribution of the phase signal. Finite element simulations indicate that the impact of contact thermal resistance between the thermocouple and the sample can be negligible when measuring in variable locations. The anisotropic thermal diffusivity is then fitted using one-dimensional and three-dimensional unsteady-state heat conduction models, with the spatial distribution of phase lag being partitioned into linear and non-linear regions, respectively. We measure the in-plane and out-of-plane thermal diffusivity of a stainless-steel film, demonstrating the reliability and accuracy of the method. As a showcase study demonstrating the capabilities of our methodology, we test four anisotropic thin films, covering a large spectrum of thermal diffusivity, ranging from 5.5 × 10−8 m2/s to 9.6 × 10−4 m2/s. It is verified that the present method can accomplish anisotropic thermal diffusivity measurements for thin films with thicknesses ranging from a few to hundreds of microns, a capability crucial for advancing the characterization of the thermophysical properties of materials. [ABSTRACT FROM AUTHOR]

Efficiently thermal management in electronic devices calls for creating polymer composites that have superior thermal transport abilities. Compared with single-filler composites, hybrid-filler composites have gained significant attention as they not only utilize the advantages of each filler individually but also allow for the potential cooperation in constructing filler networks between them. Differing from the previous research on hybrid-filler size, geometry, composition, and content and their impact on thermal conductivity (TC) in composites, this study aims to examine the spatial distribution effects of hybrid-fillers. A binary-filler strategy comprising of anisotropic 2D h- BN and 0D isotropic S–Al 2 O 3 has been proposed, with the primary h -BN filler aligned to filly utilize its anisotropy and intrinsic TC properties. Furthermore, positioning a small quantity of S–Al 2 O 3 between adjacent parallel h -BN considerably improves TC in the non-oriented direction of silicon rubber (SiR) composites. A special focus is given to the influence of the interstitial S–Al 2 O 3 filler on the individual through-plane TC (λ ⊥) and in-plane TC (λ ⫽) variation of the binary-filler hybrid SiR composites, which are perpendicular and parallel to the oriented h -BN. When incorporating 40 vol% h -BN and 5 vol% S–Al 2 O 3 , the binary SiR composites exhibit the highest concurrent λ ⫽ and λ ⊥ values of 14.581 and 2.132 W/m·K, respectively. The corresponding effects on the ductility, hardness, and dielectric properties of the SiR composites were explained. Moreover, the use of commercially available TC fillers, along with the effortless preparation method, enables the potential to produce h -BN based TIMs materials on a large scale for thermal management usage. [Display omitted] • A binary filler system consisting in-plane oriented 2D h -BN and 0D isotropic S–Al 2 O 3 is designed. • The addition of S–Al 2 O 3 filler between adjacent parallel h -BN flakes facilitates vertical heat transfer. • The binary structure of h -BN/S–Al 2 O 3 exhibits an anisotropic feature in the filler contact thermal resistances. • The binary filler strategy mitigates the trade-off between in-plane and through-plane thermal conductivity. [ABSTRACT FROM AUTHOR]

The determination of the thermoelastic properties of polyimide (PI) fibers is important for their applications, however, these properties are difficult to measure directly, especially the transverse thermoelastic properties. Here, the transverse thermoelastic properties of PI fibers, including transverse Young's modulus (2.12 GPa), shear modulus (0.94 GPa), Poisson's ratio (0.05), and coefficient of thermal expansion (45.68 μm/m°C), were estimated by combining dynamic and static thermo-mechanical techniques, as well as various relevant micromechanical models. The transverse Young's modulus of PI fibers was only 1/46th of the longitudinal one, and the transverse coefficient of thermal expansion of PI fibers was positive, unlike the longitudinal one, which was negative, showing the typical anisotropy of PI fibers. Finally, the thermoelastic properties of the PI fibers were in turn used to predict the thermoelastic behavior of the PI fiber-reinforced composites, thus validating their effectiveness. [Display omitted] [ABSTRACT FROM AUTHOR]

• Despite relatively weak hydrogen bonds, tetracarboxylic-based frameworks (TCFs), a three-dimensional (3D) HOF, show relatively high thermal conductivity. • Electrostatic interactions in TCFs dominate thermal conductivity and anisotropy but central atoms in the monomers show negligible impact. • Electrostatic interactions sustain phonon modes with frequencies of 5–25 THz to carry heat and are key to maintaining high crystallinity in TCFs. Hydrogen-bonded organic frameworks (HOFs), an emerging class of porous molecular materials assembled through hydrogen bonds (H-bonds), have shown intriguing latent for multifunctional materials or devices. Understanding thermal transport in HOFs is crucial to guiding the functional material design with desired thermal properties. Here, we report thermal transport properties in HOFs and reveal the significance of electrostatic interactions (H-bonds and π-π interactions) to heat conduction with molecular dynamics simulations. It is found that despite relatively weak H-bonds, tetracarboxylic-based frameworks (TCFs), a three-dimensional (3D) HOF, show relatively high thermal conductivity compared with known 3D porous covalent organic frameworks and metal-organic frameworks. Electrostatic interactions in TCFs dominate thermal conductivity and anisotropy but central atoms in the monomers show negligible impact. The structural analysis and x-ray diffraction patterns highlight the importance of electrostatic interactions in maintaining the high crystallinity of TCFs. The vibrational density of states and spectral thermal conductivity further demonstrate that phonon modes with frequencies below 25 THz contribute about 90 % of total thermal conductivity in TCFs, and electrostatic interactions sustain phonon modes with frequencies of 5–25 THz to carry heat. These findings provide a fundamental understanding of structure-thermal conductivity relation in HOFs and advance their applications for multifunctional materials and devices. [ABSTRACT FROM AUTHOR]