Maria Mar Bernal

The outstanding properties of carbon nanotubes (CNTs) present some limitations when introduced into rubber matrices, especially when these nano-particles are applied in high-performance tire tread compounds. Their tendency to agglomerate into bundles due to van der Waals interactions, the strong influence of CNT on the vulcanization process, and the adsorptive nature of filler–rubber interactions contribute to increase the energy dissipation phenomena on rubber–CNT compounds. Consequently, their expected performance in terms of rolling resistance is limited. To overcome these three important issues, the CNT have been surface-modified with oxygen-bearing groups and sulfur, resulting in an improvement in the key properties of these rubber compounds for their use in tire tread applications. A deep characterization of these new materials using functionalized CNT as filler was carried out by using a combination of mechanical, equilibrium swelling and low-field NMR experiments. The outcome of this research revealed that the formation of covalent bonds between the rubber matrix and the nano-particles by the introduction of sulfur at the CNT surface has positive effects on the viscoelastic behavior and the network structure of the rubber compounds, by a decrease of both the loss factor at 60 °C (rolling resistance) and the non-elastic defects, while increasing the crosslink density of the new compounds.

We introduce oligo-(1,6-pyrene terephthalamide, oPyrTA) as an oligoamide model for Kevlar®. Through comparison with an oligo-(p-phenylene terephthalamide, oPTA) of similar molecular weight, this work shows that the incorporation of polycyclic aromatic pyrene moieties improves drastically the mechanical properties of the structure, increasing elastic nanoindentation-determined modulus and hardness by factors of 1.9 and 4.3, respectively. Liquid deprotonated dispersions of oPyrTA nanofibers were used as nanoscale building blocks for producing large-surface, free-standing oligomer macroscopic nanofilms. This 2D assembly leads to further significant improvements in reduced modulus and hardness (more than twice) compared to the starting oligomer macroscale fibres, due to a better re-organizational arrangement of the oPyrTA nanofibers in the nanofilms, formed under 2D spatial confinement.

The tuning of covalently bound molecular junctions could increase heat transfer between graphene platelets inside nanocomposites materials.

The control of nanostructuration of graphene and graphene related materials (GRM) into self-assembled structures is strictly related to the nanoflakes chemical functionalization, which may be obtained via covalent grafting of non-covalent interactions, mostly exploiting π-stacking. As the non-covalent functionalization does not affect the sp² carbon structure, this is often exploited to preserve the thermal and electrical properties of the GRM and it is a well-known route to tailor the interaction between GRM and organic media. In this work, non-covalent functionalization of graphite nanoplatelets (GnP) was carried out with ad-hoc synthesized pyrene-terminated oligomers of polylactic acid (PLA), aiming at the modification of GnP nanopapers thermal properties. PLA was selected based on the possibility to self-assemble in crystalline domains via stereocomplexation of complementary poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) enantiomers. Pyrene-initiated PLLA and PDLA were indeed demonstrated to anchor to the GnP surface. Calorimetric and X-ray diffraction investigations highlighted the enantiomeric PLAs adsorbed on the surface of the nanoplatelets self-organize to produce highly crystalline stereocomplex domains. Most importantly, PLLA/PDLA stereocomplexation delivered a significantly higher efficiency in nanopapers heat transfer, in particular through the thickness of the nanopaper. This is explained by a thermal bridging effect of crystalline domains between overlapped GnP, promoting heat transfer across the nanoparticles contacts. This work demonstrates the possibility to enhance the physical properties of contacts within a percolating network of GRM via the self-assembly of macromolecules and opens a new way for the engineering of GRM-based nanostructures.

Despite the uniquely high thermal conductivity of graphene is well known, the exploitation of graphene into thermally conductive nanomaterials and devices is limited by the inefficiency of thermal contacts between the individual nanosheets. A fascinating yet experimentally challenging route to enhance thermal conductance at contacts between graphene nanosheets is through molecular junctions, allowing covalently connecting nanosheets, otherwise interacting only via weak Van der Waals forces. Beside the bare existence of covalent connections, the choice of molecular structures to be used as thermal junctions should be guided by their vibrational properties, in terms of phonon transfer through the molecular junction. In this paper, density functional tight-binding combined with Green’s functions formalism was applied for the calculation of thermal conductance and phonon spectra of several different aliphatic and aromatic molecular junctions between graphene nanosheets. Effects of molecular junction length, conformation, and aromaticity were studied in detail and correlated with phonon tunnelling spectra. The theoretical insight provided by this work can guide future experimental studies to select suitable molecular junctions, in order to enhance the thermal transport by suppressing the interfacial thermal resistances. This is attractive for various systems, including graphene nanopapers and graphene polymer nanocomposites, as well as related devices. In a broader view, the possibility to design molecular junctions to control phonon transport currently appears as an efficient way to produce phononic devices and controlling heat management in nanostructures.

The edge‐functionalization of graphene nanoplatelets (GnP) is carried out exploiting diazonium chemistry, aiming at the synthesis of edge decorated nanoparticles to be used as building blocks in the preparation of engineered nanostructured materials for enhanced heat transfer. Indeed, both phenol functionalized and dianiline‐bridged GnP (GnP‐OH and E‐GnP, respectively) are assembled in nanopapers exploiting the formation of non‐covalent and covalent molecular junctions, respectively. Molecular dynamics allow to estimate the thermal conductance for the two different types of molecular junctions, suggesting a factor 6 between conductance of covalent vs noncovalent junctions. Furthermore, the chemical functionalization is observed to drive the self‐organization of the nanoflakes into the nanopapers, leading to a 20% enhancement of the thermal conductivity for GnP‐OH and E‐GnP while the cross‐plane thermal conductivity is boosted by 190% in the case of E‐GnP. The application of chemical functionalization to the engineering of contact resistance in nanoparticles networks is therefore validated as a fascinating route for the enhancement of heat exchange efficiency on nanoparticle networks, with great potential impact in low‐temperature heat exchange and recovery applications.

In this study, we report a novel strategy to prepare graphene nanopapers from direct vacuum filtration. Instead of the conventional method, i.e., thermal annealing nanopapers at extremely high temperatures prepared from graphene oxide (GO) or partially reduced GO, we fabricate our graphene nanopapers directly from suspensions of fully reduced graphene oxide (RGO), obtained after RGO and thermal annealing at 1700 °C in vacuum. By using this approach, we studied the effect of thermal annealing on the physical properties of the macroscopic graphene-based papers. Indeed, we demonstrated that the enhancement of the thermal and electrical properties of graphene nanopapers prepared from annealed RGO is strongly influenced by the absence of oxygen functionalities and the morphology of the nanoflakes. Hence, our methodology can be considered as a valid alternative to the classical approach.

In the present work we show the effect of graphene loading on the restoration of the mechanical properties and thermal and electrical conductivity of a self-healing natural rubber nanocomposite. The graphene loading led to a minimal enhancement of mechanical properties and yielded a modest increase in thermal and electrical conduction. The polymer nanocomposites were macroscopically damaged (cut) and thermally healed for 7 h in a healing cell. Different healing trends as function of the graphene content were found for each of the functionalities: (i) thermal conductivity was fully restored independently of the graphene filler loading; (ii) electrical conductivity was only restored to a high degree above the percolation threshold; and (iii) tensile strength restoration increased more or less linearly with graphene content but was never complete. A dedicated molecular dynamics analysis by dielectric spectroscopy of the pristine and healed samples highlighted the role of graphene-polymer interactions at the healed interphase on the overall restoration of the different functionalities. Based on these results it is suggested that the dependence of the various healing efficiencies with graphene content is due to a combination of the graphene induced lower crosslinking density, as well as the presence of strong polymer-graphene interactions at the healed interphase.