Alexandre Velhinho

tInspired by chitin based hierarchical structures observed in arthropods exoskeleton, this work reportsthe capturing of chitin nanowhiskers’ chiral nematic order into a chitosan matrix. For this purpose, highlycrystalline chitin nanowhiskers (CTNW) with spindle-like morphology and average aspect ratio of 24.9were produced by acid hydrolysis of chitin. CTNW were uniformly dispersed at different concentrationsin aqueous suspensions. The suspensions liquid crystalline phase domain was determined by rheologicalmeasurements and polarized optical microscopy (POM). Chitosan (CS) was added to the CTNW isotropic,biphasic and anisotropic suspensions and the solvent was evaporated to allow films formation. The Films’morphologies as well as the mechanical properties were explored. A correlation between experimentalresults and a theoretical model, for layered matrix’ structures with fibers acting as a reinforcement agent,was established. The results evidence the existence of two different layered structures, one formed bychitosan layers induced by the presence of chitin and another formed by chitin nanowhiskers layers.By playing on the ratio chitin/chitosan one layered structure or the other can be obtained allowing thetunning of materials’ mechanical properties.

This paper reports the application of additive manufacturing technology to fabricate bidimensional
lightweight composite meshes capable of demonstrating auxetic properties
(negative Poisson’s ratio (NPR)) in combination with negative thermal expansion (NTE)
behaviour, using as constituent materials polymers that do not exhibit NTE behaviour. To
describe the combination of NPR and NTE characteristics, the designation of ‘anepectic’ is being
proposed. Each mesh, obtained from varying either the material combination or the design
parameters, was tested on a heated silicone bath to study the effects of the different combinations
on the coefficient of thermal expansion (CTE). It was found that all meshes studied demonstrated
a successful combination of NPR and NTE behaviours, and it was revealed that there is a
possibility to tailor the meshes to activate the NTE behaviour within a chosen range of
temperatures. For an extreme case, a Poisson’s ratio of −0.056, along with a CTE of
−1568×10^−6 K−1 has been achieved.

Accumulative Roll Bonding (ARB) was used to fabricate Graphene Oxide-reinforced Al-matrix composites.
Graphene Oxide reinforcement was suspended in a stabilized aqueous solution and applied, prior to each ARB
cycle, through airgun spraying. Different concentrations (graphene oxide/milipore water) were used and for
each concentration, samples produced have undergone up to 5 rolling cycles.
Optical and electron scanning microscopies were used for microstructural characterization which revealed a
non-homogenous deformation of the layers across the composite's thickness.
Although the presence of graphene-oxide promoted an increase in the microhardness, higher values were
obtained with its lowest concentration for similar samples. The number of ARB cycles and the direction of the
tested sections also influenced the microhardness results since the 5-cycle samples and the rolling direction
sections for all the samples achieved higher hardness results. Graphene Oxide revealed to be a major contributor
to the increase of stiffness during bending of the tested samples.