Crystal nucleation in poly(ether ether ketone)/carbon nanotube nanocomposites at high and low supercooling of the melt

Engineering the Crystalline Architecture for Enhanced Properties in Fast-Rate Processing of Poly(ether ether ketone) (PEEK) Nanocomposites

Rapid cooling in fast-rate manufacturing processes such as additive manufacturing and stamp forming limits the development of crystallinity in semicrystalline polymer nanocomposites and, therefore, potential improvements in the mechanical performance. While the nucleation, chain mobility, and crystallization time from rapid cooling are known competing mechanisms in crystallization, herein we elucidate that the crystalline morphology and architecture also play a key role in tuning the mechanical performance. We explore how modifying the spherulite morphology via a cellulose nanocrystal (CNC) and graphene nanoplatelet (GNP) hybrid system in their pristine form can improve or preserve the mechanical properties of poly(ether ether ketone) (PEEK) nanocomposites under two extreme cooling rates (fast -460 °C/min and slow -0.7 °C/min). A scalable manufacturing methodology using water as the medium to disperse the powder system was developed, employing a CNC as a dispersing agent and stabilizer for PEEK and GNP. Despite the expected limited mechanical reinforcement due to thermal degradation, CNCs significantly impacted PEEK's crystalline architecture and mechanical performance, suggesting that surface interactions via lattice matching with PEEK's (200) crystallographic plane play a critical role in engineering the microstructure. In fast cooling, the CNC and CNC:GNP systems reduced the crystallinity, respectively, yet led to minimizing the reduction in the tensile strength and maintaining the tensile modulus at the Neat level in slow cooling. With slow cooling, crystallinity remained relatively unchanged; however, the addition of CNC:GNP improved the strength and modulus by ∼10% and ∼16%, respectively. These findings demonstrate that a hybrid nanomaterial system can tailor PEEK's crystalline microstructure, thus presenting a promising approach for enhancing the mechanical properties of PEEK nanocomposites in fast-rate processes.

Advanced 3D Printing of Polyetherketoneketone Hydroxyapatite Composites via Fused Filament Fabrication with Increased Interlayer Connection

Additively manufactured implants, surgical guides, and medical devices that would have direct contact with the human body require predictable behaviour when stress is applied during their standard operation. Products built with Fused Filament Fabrication (FFF) possess orthotropic characteristics, thus, it is necessary to determine the properties that can be achieved in the XY- and Z-directions of printing. A concentration of 10 wt% of hydroxyapatite (HA) in polyetherketoneketone (PEKK) matrix was selected as the most promising biomaterial supporting cell attachment for medical applications and was characterized with an Ultimate Tensile Strength (UTS) of 78.3 MPa and 43.9 MPa in the XY- and Z-directions of 3D printing, respectively. The effect of the filler on the crystallization kinetics, which is a key parameter for the selection of semicrystalline materials suitable for 3D printing, was explained. This work clearly shows that only in situ crystallization provides the ability to build parts with a more thermodynamically stable primary form of crystallites.

Fundamentals of Crystalline Evolution and Properties of Carbon Nanotube-Reinforced Polyether Ether Ketone Nanocomposites in Fused Filament Fabrication

As fused filament fabrication (FFF) continues to gain popularity, many studies are turning to nanomaterials or optimization of printing parameters to improve the materials' properties; however, many overlook how materials formulation and additive manufacturing (AM) processes cooperatively engineer the evolution of properties across length scales. Evaluating the in-process evolution of the nanocomposite using AM will provide a fundamental understanding of the material's microstructure, which can be tailored to create unique characteristics in functionality and performance. In this study, the crystallinity behavior of polyetheretherketone (PEEK) was studied in the presence of carbon nanotubes (CNTs) as a nucleation aid for improved crystallization during FFF processing. Using various characterization techniques and molecular dynamics simulations, it was discovered that the crystallization behavior of extruded filaments is very different from that of 3D printed roads. Additionally, the printed material exhibited cold crystallization, and the CNT addition increased the crystallization of printed roads, which were amorphous without CNT addition. Tensile strength and modulus were increased by as much as 42 and 51%, respectively, due to higher crystallinity during printing. Detailed knowledge on the morphology of PEEK-CNT used in FFF allows gaining a fundamental understanding of the morphological evolution occurring during the AM process that in turn enables formulating materials for the AM process to achieve tailored mechanical and functional properties, such as crystallinity or conductivity.

Poly(Butylene Succinate) Hybrid Multi-Walled Carbon Nanotube/Iron Oxide Nanocomposites: Electromagnetic Shielding and Thermal Properties

To address the ever-increasing electromagnetic interference (EMI) pollution, a hybrid filler approach for novel composites was chosen, with a focus on EMI absorbance. Carbon nanofiller loading was limited to 0.6 vol.% in order to create a sustainable and affordable solution. Multiwall carbon nanotubes (MWCNT) and iron oxide (Fe₃O₄) nanoparticles were mixed in nine ratios from 0.1 to 0.6 vol.% and 8.0 to 12.0 vol.%, respectively. With the addition of surfactant, excellent particle dispersion was achieved (examined with SEM micrographs) in a bio-based and biodegradable poly(butylene succinate) (PBS) matrix. Hybrid design synergy was assessed for EMI shielding using dielectric spectroscopy in the microwave region and transmittance in the terahertz range. The shielding effectiveness (20-52 dB) was dominated by very high absorption at 30 GHz, while in the 0.1 to 1.0 THz range, transmittance was reduced by up to 6 orders of magnitude. Frequency-independent AC electrical conductivity (from 10^-2 to 10⁷ Hz) was reached upon adding 0.6 vol.% MWCNT and 10 vol.% Fe₃O₄, with a value of around 3.1 × 10^-2 S/m. Electrical and thermal conductivity were mainly affected by the content of MWCNT filler. The thermal conductivity scaled with the filler content and reached the highest value of 0.309 W/(mK) at 25 °C with the loading of 0.6 vol.% MWCNT and 12 vol.% Fe₃O₄. The surface resistivity showed an incremental decrease with an increase in MWCNT loading and was almost unaffected by an increase in iron oxide loading. Thermal conductivity was almost independent of temperature in the measured range of 25 to 45 °C. The nanocomposites serve as biodegradable alternatives to commodity plastic-based materials and are promising in the field of electromagnetic applications, especially for EMI shielding.

Non-Isothermal Crystallization Kinetics of Polyether-Ether-Ketone Nanocomposites and Analysis of the Mechanical and Electrical Conductivity Performance

High-performance polyether-ether-ketone (PEEK) is highly desirable for a plethora of engineering applications. The incorporation of conductive carbon nanotubes (CNTs) into PEEK can impart electrical conductivity to the otherwise non-conductive matrix, which can further expand the application realm for PEEK composites. However, a number of physical properties, which are central to the functionalities of the composite, are affected by the complex interplay of the crystallinity and presence of the nanofillers, such as CNTs. It is therefore of paramount importance to conduct an in-depth investigation to identify the process that optimizes the mechanical and electrical performance. In this work, PEEK/CNTs composites with different carbon nanotubes (CNTs) content ranging from 0.5 to 10.0 wt% are prepared by a parallel twin-screw extruder. The effects of CNTs content and annealing treatment on the crystallization behavior, mechanical properties and electrical conductivity of the PEEK/CNTs composites are investigated in detail. A non-isothermal crystallization kinetics test reveals a substantial loss in the composites’ crystallinity with the increased CNTs content. On the other hand, mechanical tests show that with 5.0 wt% CNTs content, the tensile strength reaches a maximum at 118.2 MPa, which amounts to a rise of 30.3% compared with the neat PEEK sample after annealing treatment. However, additional annealing treatment decreases the electrical conductivity as well as EMI shielding performance. Such a decrease is mainly attributed to the relatively small crystal size of PEEK, which excludes the conductive fillers to the boundaries and disrupts the otherwise conductive networks.