We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine-sheath structure observed by Space-VLBI with RadioAstron in 3C 273.

Context. Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated. Aims. We study the innermost jet morphology and magnetic field strength in the AGN 3C 345 with an unprecedented resolution using images obtained within the framework of the key science programme on AGN polarisation of the Space VLBI mission RadioAstron. Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016 March 30 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode. Results. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300 μas angular resolution, corresponding to a projected linear scale of about 2 pc or a few thousand gravitational radii. We identify the synchrotron self-absorbed core at the jet base and find the brightest feature in the jet 1.5 mas downstream of the core. Several polarised components appear in the Space VLBI images that cannot be seen from ground array-only images. Except for the core, the electric vector position angles follow the local jet direction, suggesting a magnetic field perpendicular to the jet. This indicates the presence of plane perpendicular shocks in these regions. Additionally, we infer a minimum brightness temperature at the largest (u,  v)-distances of 1.1  ×  1012 K in the source frame, which is above the inverse Compton limit and an order of magnitude larger than the equipartition value. This indicates locally efficient injection or re-acceleration of particles in the jet to counter the inverse Compton cooling or the geometry of the jet creates significant changes in the Doppler factor, which has to be > 11 to explain the high brightness temperatures.

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.

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.

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.