Structural materials combining high stiffness and damping capabilities are in high demand for passive damping applications in vibration control, precision manufacturing, and resilient buildings. However, the development of enhanced passive damping materials with high stiffness at low weight has been hindered by the mutually excluding nature of these mechanical properties. Motivated by the mechanical performance of biological materials such as bone and nacre, we exploit a simple casting and magnetically assisted manufacturing process to fabricate platelet-reinforced polymers with a stiff and damping staggered architecture. Dynamic mechanical analysis of our staggered architectures reveals an increase in the stiffness of the composites by a factor up to 4.5 while maintaining the strong damping response of the host polymer. Using an established micromechanical model, we can predict the damping figure of merit of such composites and provide guidelines for the creation of stiff and damping bio-inspired structures while considering boundary conditions of the manufacturing process. By reaching mechanical performance superior or comparable to bone and nacre, our bio-inspired strategy proves to be a promising pathway for the further development of low-power passive damping materials.

Natural materials such as wood exhibit high mechanical properties through cellulose structured at multiple length scales and embedded in a matrix of similar chemical structure. These hierarchical materials have inspired the design of lightweight composites composed of naturally occurring polymers. However, the close proximity of melt and decomposition temperature remain a challenge. In this work, cellulose propionate (CP) is modified to reduce its glass transition temperature and melt viscosity, allowing its use as a matrix in a natural fibre-reinforced composite. Through better impregnation, the modified CP matrix composites showed an increase in stiffness and strength of ∼10% and 20%, respectively, in comparison to unmodified CP matrix composites. The impact properties also increased by up to 28%, showing that modified CP is a credible matrix for realising sustainable all-cellulose natural fibre composites with high stiffness, strength and toughness.

Structural materials such as metals and fibre-reinforced plastics are stiff engineering materials used in many industrial fields. However, their high stiffness leads usually to the inability to dissipate energy via vibration damping. Contrarily, compliant materials such as rubbers and soft polymers exhibit an excellent damping behaviour, but lack in stiffness. Natural composites, such as wood, bone and seashells possess both high stiffness and high damping and thus combine properties that are generally mutually exclusive. In this study, the design principles of two biological composites, i.e. nacre and wood, were studied to design composites out of engineering materials which display a damping behaviour like the natural systems and are producible on a larger scale. The dynamic properties of nacre-inspired epoxy and flax fibre-reinforced composites were investigated and compared to structural engineering materials and to the biological materials served for the inspiration.

Natural fibre (NF) reinforced composites offer high specific mechanical properties and are an ecological alternative to synthetic fibre-reinforced composites. While having great potential, their use today is limited to non-structural applications, mostly with epoxy or polypropylene matrices. This work studies suitable high-performance thermoplastic matrices and characterises their bulk properties, fibre-wetting and composite mechanical behaviour. Thermoplastic polymers such as poly-L-lactide (PLLA) and polyoxymethylene (coPOM) are matrices with bulk properties similar to epoxy. The results show that PLLA matrix NF-composites have a longitudinal modulus and strength of 27 GPa and 308 MPa. The tougher coPOM matrix NF-composites show both high transverse stiffness and strength of 2.6 GPa and 41.5 MPa and show that even the drawback of creep can be overcome by the use of hierarchically structured coPOM. The developed NF-composites demonstrate in-plane properties comparable to those with epoxy matrices and can outperform them by up to 26% in the transverse direction.

Fibre-reinforced polymer structures are often used when stiff lightweight materials are required, such as in aircraft, vehicles and biomedical implants. Despite their very high stiffness and strength ¹ , such lightweight materials require energy- and labour-intensive fabrication processes ² , exhibit typically brittle fracture and are difficult to shape and recycle ^3,4 . This is in stark contrast to lightweight biological materials such as bone, silk and wood, which form by directed self-assembly into complex, hierarchically structured shapes with outstanding mechanical properties ^5–11 , and are circularly integrated into the environment. Here we demonstrate a three-dimensional (3D) printing approach to generate recyclable lightweight structures with hierarchical architectures, complex geometries and unprecedented stiffness and toughness. Their features arise from the self-assembly of liquid-crystal polymer molecules into highly oriented domains during extrusion of the molten feedstock material. By orienting the molecular domains with the print path, we are able to reinforce the polymer structure according to the expected mechanical stresses, leading to stiffness, strength and toughness that outperform state-of-the-art 3D-printed polymers by an order of magnitude and are comparable with the highest-performance lightweight composites ^1,12 . The ability to combine the top-down shaping freedom of 3D printing with bottom-up molecular control over polymer orientation opens up the possibility to freely design and realize structures without the typical restrictions of current manufacturing processes.

To evaluate compatibility between a substrate and a thermoplastic polymer, the established methodology is to estimate their surface composition in terms of surface energy components, utilizing the results of contact angle measurements of probe liquids onto substrate and polymer surfaces at room temperature. Using this methodology, polymer surfaces are studied in solid state, however, during spreading of polymers on a substrate, polymers are in molten state and at high temperature, having different surface energies and more complex polymer/substrate interactions due to polymer chain mobility. This paper presents a model study with practical relevance to predict polymer/substrate compatibility including contact angle measurements at high temperature directly performed between molten thermoplastics; polypropylene (PP), polyvinylidene fluoride (PVDF) and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and plates. The values of total surface energy of thermoplastics at high temperature (260 °C) are down to 57% of that measured at room temperature, which has a strong influence on the wetting prediction. Surface energies of both the polymer and the substrate were found not to be the only factor controlling the wetting behaviour of molten polymers and the level of adhesion with the substrate, but also some intrinsic characteristics of the polymer melt play a role. We also observed that the wetting behaviour of molten MAPP is affected by the maleic anhydride (MA) content, demonstrating dramatically different results to room temperature measurements, which is suspected to be due to the formation of covalent bonds of MA groups with the glass surface enhancing the interface strength beyond the shear strength of MAPP.

Direct contact angle measurements were performed between different molten thermoplastics, polypropylene (PP), polyvinylidene fluoride (PVDF), and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and smooth glass plates. The matrices were selected as model systems for the investigation of the fibre–matrix interphase, based on the difference in surface energies between PP and PVDF (physical interactions) and the effect of chemical bonding between PP and MAPP. In this way, physical and chemical adhesion were studied independently. The mechanical strength of the interfaces was then assessed by single fibre pull-out tests. The interfacial strength and the wettability of molten thermoplastics correspond well to the predictions based on the calculation of the theoretical work of adhesion between the matrices and the fibre surfaces for PP and PVDF; however, the wetting behaviour of molten MAPP is affected by the MA content if compared with the surface energy analysis made at room temperature. The hypothesis is that the wetting behaviour of molten MAPP is mainly related to chemical interaction of MA groups with the glass surface rather than a variation in surface energy.