On a Biobased Epoxy Vitrimer from a Cardanol Derivative Prepared by a Simple Thiol-Epoxy "Click" Reaction.

The development of this work lies in the relevant interest in epoxy resins, which, despite their wide use, do not meet the requirements for sustainable materials. Therefore, the proposed approach considers the need to develop environmentally friendly systems, in terms of both the starting material and the synthetic method applied as well as in terms of end-of-life. The above issues were taken into account by (i) using a monomer from renewable sources, (ii) promoting the formation of dynamic covalent bonds, allowing for material reprocessing, and (iii) evaluating the degradability of the material. Indeed, an epoxy derived from cardanol was used, which, for the first time, was applied in the development of a vitrimer system. The exploitation of a diboronic ester dithiol ([2,2'-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane], DBEDT) as a cross-linker allowed the cross-linking reaction to be carried out without the use of solvents and catalysts through a thiol-epoxy "click" mechanism. The dynamicity of the network was demonstrated by gel fraction experiments and rheological and DMA measurements. In particular, the formation of a vitrimer was highlighted, characterized by low relaxation times (around 4 s at 70 °C) and an activation energy of ca. 48 kJ/mol. Moreover, the developed material, which is easily biodegradable in seawater, was found to show promising flame reaction behavior. Preliminary experiments demonstrated that, unlike an epoxy resin prepared from the same monomer and using a classical cross-linker, our boron-containing material exhibited no dripping under combustion conditions, a phenomenon that will allow this novel biobased system to be widely used.

Dispersion of Cellulose Nanofibers in Methacrylate-Based Nanocomposites.

Poly(methylmethacrylate-co-methacrylic acid) (PMMA-co-MAA) polymers were prepared via cobalt-mediated free radical copolymerization and were characterized after synthesis. The synthesis led to a 98.5% conversion and a final ratio between the two units, MMA/MAA, was equal to 63:37 mol%. PMMA-co-MAA was then used as a matrix for cellulose-based nanocomposites to tailor filler compatibility, thanks to the presence of carboxylic groups capable of generating strong H-bonds with the cellulose surface. Cellulose nanofibers (CNFs) were dispersed using a solution with a mixture of two solvents to tailor compatibility of both the components. For this purpose, CNFs were successfully re-dispersed in methanol using the solvent exchange method and tetrahydrofuran/methanol mixtures at different ratios were used for the preparation of the films. Fully transparent films of PMMA-co-MAA + CNF were prepared up to 15 wt% of CNF with a good dispersion in the matrix. This dispersion state leads to the reinforcement of the polymethacrylate matrix, increasing its tensile strength whilst preserving optical transparency.

On the Development of Nanocomposite Covalent Associative Networks Based on Polycaprolactone and Reduced Graphite Oxide.

In this work, the development of nanocomposite systems based on reduced graphite oxide (rGO) was combined with the development of crosslinked materials characterized by dynamic covalent bonds, i.e., a covalent associative network, starting from ad-hoc synthesized hydroxyl terminated polycaprolactone (PCL-OH). The crosslinking reaction was carried out using methylenediphenyl diisocyanate (MDI) to create systems capable of bond exchanges via transesterification and transcarbamoylation reactions, in the presence of stannous octoate as a catalyst. The above materials were prepared at two different temperatures (120 and 200 °C) and two PCL-OH:MDI ratios. FT-IR measurements proved the formation of urethane bonds in all the prepared samples. Crosslinking was demonstrated by contacting the samples with a solvent capable of dissolving the star-shaped PCL. These tests showed a significant increase in the crosslinked fraction with increasing the temperature and the PCL-OH:MDI ratio. In order to evidence the effect of crosslinking on rGO dispersion and the final properties of the material, a nanocomposite sample was also prepared using a linear commercial PCL, with the nanofiller mixed under the same conditions used to develop the crosslinked systems. The dispersion of rGO, which was investigated using FE-SEM measurements, was similar in the different systems prepared, indicating that the crosslinking process had a minor effect on the dispersibility of the nanofiller. As far as the thermal properties are concerned, the DSC measurements of the prepared samples showed that the crosslinking leads to a decrease in the crystallinity of the polymer, a phenomenon which was particularly evident in the sample prepared at 200 °C with a PCL-OH: MDI ratio of 1:1.33 and was related to the decrease in the polymer chain mobility. Moreover, rGO was found to act as a nucleating agent and increase the crystallization temperature of the nanocomposite sample based on linear commercial PCL, while the contribution of rGO in the crosslinked nanocomposite samples was minor. Rheological measurements confirmed the crosslinking of the PCL-OH system which generates a solid-like behavior depending on the PCL-OH:MDI ratio used. The presence of rGO during crosslinking generated a further huge increase in the viscosity of the melt with a remarkable solid-like behavior, confirming a strong interaction between rGO and crosslinked PCL. Finally, the prepared nanocomposites exhibited self-healing and recyclability properties, thus meeting the requirements for sustainable materials.

Polyelectrolytes Enabled Reduced Graphite Oxide Water Dispersions: Effects of the Structure, Molecular Weight, and Charge Density.

The polyelectrolyte (PE)-based water dispersion of graphene-related materials (GRMs) represents an interesting intermediate for the development of advanced materials by sustainable processes. Although the proof of concept has been demonstrated, there is a lack of knowledge for what concerns the effects of parameters typical of PEs such as functionalization, molecular weight, and charge density. In this work, we evaluate the effects of such parameters on the quality and long-term stability of reduced graphite oxide (rGO) dispersion in aqueous media prepared by ultrasound sonication in the presence of different PEs. Four PEs were evaluated: polyacrylic acid (PAA), branched poly(ethylenimine) (BPEI), sodium carboxymethyl cellulose (CMC), and poly(sodium 4-styrenesulfonic acid) (PSS). The prepared dispersions were thoroughly characterized by means of UV-visible spectroscopy, thermogravimetric analysis, dynamic light scattering, and Raman spectroscopy. The highest concentrations of rGO were achieved by BPEI with a molecular weight of 25,000 and 270,000 Da (33 and 26 µg/mL, respectively). For other PEs, the rGO concentration was found to be independent of the molecular weight. The PAA-based dispersions displayed the best through-time stability while yielding homogeneous dispersion with a smaller average size and narrower size distribution.

Thermoreversible Cross-Linked Rubber Prepared via Melt Blending and Its Nanocomposites.

A covalent adaptable network based on the thermoreversible cross-linking of an ethylene-propylene rubber through Diels-Alder (DA) reaction was prepared for the first time through melt blending as an environmental-friendly alternative to traditional synthesis in organic solvents. Functionalization of the rubber with furan groups was performed in a melt blender and subsequently mixed with different amounts of bismaleimide in a microextruder. Cross-linking was confirmed by FT-IR spectroscopy and insolubility at room temperature, while its thermoreversible character was confirmed by a solubility test at 110 °C and by remolding via hot-pressing. Mechanical and thermomechanical properties of the obtained rubbers showed potential to compete with conventionally cross-linked elastomers, with stiffness in the range 1-1.7 MPa and strain at break in the range 200-500%, while allowing recycling via a simple melt processing step. Nanocomposites based on the thermoreversible rubber were prepared with reduced graphene oxide (rGO), showing significantly increasing stiffness up to ca. 8 MPa, ∼2-fold increased strength, and thermal conductivity up to ∼0.5 W/(m K). Results in this paper may open for industrially viable and sustainable applications of thermoreversible elastomers.

Flexible, Air-Stable, High-Performance Heaters Based on Nanoscale-Thick Graphite Films.

Graphite sheets are known to exhibit remarkable performance in applications such as heating panels and critical elements of thermal management systems. Industrial-scale production of graphite films relies on high-temperature treatment of polymers or calendering of graphite flakes; however, these methods are limited to obtaining micrometer-scale thicknesses. Herein, we report the fabrication of a flexible and power-efficient cm2-scaled heater based on a polycrystalline nanoscale-thick graphite film (NGF, ∼100 nm thick) grown by chemical vapor deposition. The stability of these NGF heaters (operational in air over the range 30-300 °C) is demonstrated by a 12-day continuous heating test, at 215 °C. The NGF exhibits a fast switching response and attains a steady peak temperature of 300 °C at a driving bias of 7.8 V (power density of 1.1 W/cm2). This excellent heating performance is attributed to the structural characteristics of the NGF, which contains well-distributed wrinkles and micrometer-wide few-layer graphene domains (characterized using conductive imaging and finite element methods, respectively). The efficiency and flexibility of the NGF device are exemplified by externally heating a 2000 µm-thick Pyrex glass vial and bringing 5 mL of water to a temperature of 96 °C (at 2.4 W/cm2). Overall, the NGF could become an excellent active material for ultrathin, flexible, and sustainable heating panels that operate at low power.

Polycaprolactone Adsorption and Nucleation onto Graphite Nanoplates for Highly Flexible, Thermally Conductive, and Thermomechanically Stiff Nanopapers.

Free-standing nanopapers based on graphene and its related materials have been widely studied and proposed for flexible heat spreader applications. Given that these materials are typically brittle, this work reports the exploitation of polycaprolactone (PCL) as a polymer binder to enhance resistance and flexibility of nanopapers based on graphite nanoplates (GNP), while maintaining a high thermal conductivity. Properties of nanopapers appear to correlate with the excellent PCL adhesion and strong nucleation of the surface of GNP flakes. Furthermore, different crystalline populations were observed for PCL within the nanopaper and were investigated in detail via differential scanning calorimetry advanced techniques and X-ray diffraction. These demonstrated the coexistence of conventional unoriented PCL crystals, oriented PCL crystals obtained as a consequence of the strong nucleation effect, and highly stable PCL fractions explained by the formation of crystalline pre-freezing layers, the latter having melting temperatures well above the equilibrium melting temperature for pristine PCL. This peculiar crystallization behavior of PCL, reported in this paper for the first time for a tridimensional structure, has a direct impact on material properties. Indeed, the presence of high thermal stability crystals, strongly bound to GNP flakes, coexisting with the highly flexible amorphous fraction, delivers an ideal solution for the strengthening and toughening of GNP nanopapers. Thermomechanical properties of PCL/GNP nanopapers, investigated both on a heating ramp and by creep tests at high temperatures, demonstrated superior stiffness well above the conventional melting temperature of PCL. At the same time, a thermal conductivity > 150 W/m·K was obtained for PCL/GNP nanopapers, representing a viable alternative to traditional metals in terms of heat dissipation, while affording flexibility and light weight, unmatched by conventional thermally conductive metals or ceramics. Besides the obtained performance, the formation of polymer crystals that are stable above the equilibrium melting temperature constitutes a novel approach in the self-assembly of highly ordered nanostructures based on graphene and related materials.

Molecular Junctions Enhancing Thermal Transport within Graphene Polymer Nanocomposite: A Molecular Dynamics Study.

Thermal conductivity of polymer-based (nano)composites is typically limited by thermal resistances occurring at the interfaces between the polymer matrix and the conductive particles as well as between particles themselves. In this work, the adoption of molecular junctions between thermally conductive graphene foils is addressed, aiming at the reduction of the thermal boundary resistance and eventually lead to an efficient percolation network within the polymer nanocomposite. This system was computationally investigated at the atomistic scale, using classical Molecular Dynamics, applied the first time to the investigation of heat transfer trough molecular junctions within a realistic environment for a polymer nanocomposite. A series of Molecular Dynamics simulations were conducted to investigate the thermal transport efficiency of molecular junctions in polymer tight contact, to quantify the contribution of molecular junctions when graphene and the molecular junctions are surrounded by polydimethylsiloxane (PDMS) molecules. A strong dependence of the thermal conductance was found in PDMS/graphene model, with best performances obtained with short and conformationally rigid molecular junctions. Furthermore, the adoption of the molecular linkers was found to contribute additionally to the thermal transport provided by the surrounding polymer matrix, demonstrating the possibility of exploiting molecular junctions in composite materials.

Polyelectrolyte-Assisted Dispersions of Reduced Graphite Oxide Nanoplates in Water and Their Gas-Barrier Application.

Dispersion of graphene and related materials in water is needed to enable sustainable processing of these 2D materials. In this work, we demonstrate the capability of branched polyethylenimine (BPEI) and polyacrylic acid (PAA) to stabilize reduced graphite oxide (rGO) dispersions in water. Atomic force microscopy colloidal probe measurements were carried out to investigate the interaction mechanisms between rGO and the polyelectrolytes (PEs). Our results show that for positive PEs, the interaction appears electrostatic, originating from the weak negative charge of graphene in water. For negative PEs, however, van der Waals forces may result in the formation of a PE shell on rGO. The PE-stabilized rGO dispersions were then used for the preparation of coatings to enhance gas barrier properties of polyethylene terephthalate films using the layer-by-layer self-assembly. Ten bilayers of rGOBPEI/rGOPAA resulted in coatings with excellent barrier properties as demonstrated by oxygen transmission rates below detection limits [<0.005 cm3/(m2 day atm)]. The observed excellent performance is ascribed to both the high density of the deposited coating and its efficient stratification. These results can enable the design of highly efficient gas barrier solutions for demanding applications, including oxygen-sensitive pharmaceutical products or flexible electronic devices.

A facile approach for the development of high mechanical strength 3D neuronal network scaffold based on chitosan and graphite nanoplatelets.

In this work, novel composite microparticles based on chitosan (CHI) and graphite nanoplatelets (GNP) were developed as 3D scaffolds for neuronal cells. The aim is to improve the scaffold strength while maintaining its ability to sustain cell adhesion and differentiation. An air-assisted jetting technique followed by physical crosslinking is employed to obtain CHI/GNP microparticles. Optical and Field Emission Scanning Electron Microscopy micrographs showed a uniform distribution of GNP within the CHI porous matrix. The presence of GNP turned out to improve the strength of the microparticles while conferring good electrical conductivity and ameliorating their stability in aqueous environment. The morphological and immunocytochemical characterization, combined with a preliminary electrophysiological analysis, evidenced the effectiveness of the developed composite microparticles as a scaffold for neuron growth. These scaffolds could be employed for the development of advanced 3D neuronal in vitro models for networks dynamics analysis and drug screening.

A Multiscale Investigation on the Thermal Transport in Polydimethylsiloxane Nanocomposites: Graphene vs. Borophene.

Graphene and borophene are highly attractive two-dimensional materials with outstanding physical properties. In this study we employed combined atomistic continuum multi-scale modeling to explore the effective thermal conductivity of polymer nanocomposites made of polydimethylsiloxane (PDMS) polymer as the matrix and graphene and borophene as nanofillers. PDMS is a versatile polymer due to its chemical inertia, flexibility and a wide range of properties that can be tuned during synthesis. We first conducted classical Molecular Dynamics (MD) simulations to calculate the thermal conductance at the interfaces between graphene and PDMS and between borophene and PDMS. Acquired results confirm that the interfacial thermal conductance between nanosheets and polymer increases from the single-layer to multilayered nanosheets and finally converges, in the case of graphene, to about 30 MWm-2 K-1 and, for borophene, up to 33 MWm-2 K-1. The data provided by the atomistic simulations were then used in the Finite Element Method (FEM) simulations to evaluate the effective thermal conductivity of polymer nanocomposites at the continuum level. We explored the effects of nanofiller type, volume content, geometry aspect ratio and thickness on the nanocomposite effective thermal conductivity. As a very interesting finding, we found that borophene nanosheets, despite having almost two orders of magnitude lower thermal conductivity than graphene, can yield very close enhancement in the effective thermal conductivity in comparison with graphene, particularly for low volume content and small aspect ratios and thicknesses. We conclude that, for the polymer-based nanocomposites, significant improvement in the thermal conductivity can be reached by improving the bonding between the fillers and polymer, or in other words, by enhancing the thermal conductance at the interface. By taking into account the high electrical conductivity of borophene, our results suggest borophene nanosheets as promising nanofillers to simultaneously enhance the polymers' thermal and electrical conductivity.

On the Development of an Effective Method to Produce Conductive PCL Film.

The aim of this work was to develop an effective approach to improve the graphite dispersion and, consequently, the electrical conductivity of nanocomposites based on polycaprolactone (PCL) and graphite nanoplates (GNP). With this aim, a polymeric additive was designed to be compatible with the polymer matrix and capable of interacting with the graphite layers. Indeed, the compound consists of a low molecular mass PCL ending with a pyrene group (Pyr-PCL). The exploitation of such a molecule is expected to promote from one side specific interactions of the pyrene terminal group with the surface of graphite layers and from the other to guarantee the compatibility with PCL, having a chain with the same nature as the matrix. The features of the nanocomposites prepared by directly blending PCL with GNP were compared with those of the same systems also containing the additive. Moreover, a neat mixture, based on PCL and PCL-Pyr, was prepared and characterized. The specific interactions between the ad hoc synthesized compound and graphite were verified by UV measurements, while SEM characterization demonstrated a finer dispersion of GNP in the samples containing Pyr-PCL. GNP nucleating effect, proved by the increase in the crystallization temperature, was observed in all the samples containing the nanofiller. Moreover, a significant improvement of the electrical conductivity was found in the systems based on the pyrenyl terminated PCL. This peculiar and interesting phenomenon was related to the optimized nanofiller dispersion and to the ameliorated compatibility with the polymer matrix.

Bispyrene Functionalization Drives Self-Assembly of Graphite Nanoplates into Highly Efficient Heat Spreader Foils.

Thermally conductive nanopapers fabricated from graphene and related materials are currently showing great potential in thermal management applications. However, thermal contacts between conductive plates represent the bottleneck for thermal conductivity of nanopapers prepared in the absence of a high temperature step for graphitization. In this work, the problem of ineffective thermal contacts is addressed by the use of bifunctional polyaromatic molecules designed to drive self-assembly of graphite nanoplates (GnP) and establish thermal bridges between them. To preserve the high conductivity associated to a defect-free sp2 structure, non-covalent functionalization with bispyrene compounds, synthesized on purpose with variable tethering chain length, was exploited. Pyrene terminal groups granted for a strong π-π interaction with graphene surface, as demonstrated by UV-Vis, fluorescence, and Raman spectroscopies. Bispyrene molecular junctions between GnP were found to control GnP organization and orientation within the nanopaper, delivering significant enhancement in both in-plane and cross-plane thermal diffusivities. Finally, nanopapers were validated as heat spreader devices for electronic components, evidencing comparable or better thermal dissipation performance than conventional Cu foil, while delivering over 90% weight reduction.

Effects of Graphite Oxide Nanoparticle Size on the Functional Properties of Layer-by-Layer Coated Flexible Foams.

The exploitation of self-assembled coatings comprising graphite oxide (GO) nanoplates has been recently demonstrated as a promising route to improve the fire safety of flexible polyurethane (PU) foams. However, limited knowledge has been gathered on the correlations between the physical and chemical properties of different GO grades and the performance obtained in this application. This work addresses the effects of the nanoparticle dimensions on the layer-by-layer (LbL) assembly and flame-retardant properties of GO-based coatings deposited on PU foams. To this aim, three GO bearing different lateral sizes and thicknesses were selected and LbL-assembled with chitosan (CHIT). Coating growth and morphology were evaluated by FTIR and FESEM, respectively. The resulting CHIT/GO assemblies were demonstrated to be capable of slowing down the combustion of the PU both in flammability and forced combustion tests. In addition, compressive stress/strain tests pointed out that the LbL-coated foams (22-24 kg/m3) could easily replace denser commercial PU foam (40-50 kg/m3) with weight reduction potentials in the transport field. These results are correlated with the properties of the employed GO. The production of assemblies characterized by a high density of CHIT/GO interfaces is identified as the main parameter controlling the FR efficiency and the mechanical properties of the coatings.

Properties of Graphene-Related Materials Controlling the Thermal Conductivity of Their Polymer Nanocomposites.

Different types of graphene-related materials (GRM) are industrially available and have been exploited for thermal conductivity enhancement in polymers. These include materials with very different features, in terms of thickness, lateral size and composition, especially concerning the oxygen to carbon ratio and the possible presence of surface functionalization. Due to the variability of GRM properties, the differences in polymer nanocomposites preparation methods and the microstructures obtained, a large scatter of thermal conductivity performance is found in literature. However, detailed correlations between GRM-based nanocomposites features, including nanoplatelets thickness and size, defectiveness, composition and dispersion, with their thermal conductivity remain mostly undefined. In the present paper, the thermal conductivity of GRM-based polymer nanocomposites, prepared by melt polymerization of cyclic polybutylene terephtalate oligomers and exploiting 13 different GRM grades, was investigated. The selected GRM, covering a wide range of specific surface area, size and defectiveness, secure a sound basis for the understanding of the effect of GRM properties on the thermal conductivity of their relevant polymer nanocomposites. Indeed, the obtained thermal conductivity appeares to depend on the interplay between the above GRM feature. In particular, the combination of low GRM defectiveness and high filler percolation density was found to maximize the thermal conductivity of nanocomposites.

Strong Reinforcement Effects in 2D Cellulose Nanofibril-Graphene Oxide (CNF-GO) Nanocomposites due to GO-Induced CNF Ordering.

Nanocomposites from native cellulose with low 2D nanoplatelet content are of interest as sustainable materials combining functional and structural performance. Cellulose nanofibril-graphene oxide (CNF-GO) nanocomposite films are prepared by a physical mixing-drying method, with focus on low GO content, the use of very large GO platelets (2-45µm) and nanostructural characterization using synchrotron x-ray source for WAXS and SAXS. These nanocomposites can be used as transparent coatings, strong films or membranes, as gas barriers or in laminated form. CNF nanofibrils with random in-plane orientation, form a continuous non-porous matrix with GO platelets oriented in-plane. GO reinforcement mechanisms in CNF are investigated, and relationships between nanostructure and suspension rheology, mechanical properties, optical transmittance and oxygen barrier properties are investigated as a function of GO content. A much higher modulus reinforcement efficency is observed than in previous polymer-GO studies. The absolute values for modulus and ultimate strength are as high as 17 GPa and 250 MPa at a GO content as small as 0.07 vol%. The remarkable reinforcement efficiency is due to improved organization of the CNF matrix; and this GO-induced mechanism is of general interest for nanostructural tailoring of CNF-2D nanoplatelet composites.

Polyhedral Oligomeric Silsesquioxane (POSS) Surface Grafting: A Novel Method to Enhance Polylactide Hydrolysis Resistance.

Abstract: This work considers the development of an easy and scalable approach to change the features of poly(l-lactide) (PLLA) films, which is based on the application of a surface treatment with an amino-functionalized polyhedral oligomeric silsesquioxane (POSS). Indeed, the developed approach is based on the potential reactivity of POSS amino group towards the polymer functionalities to produce an aminolysis reaction, which should promote the direct grafting of the silsesquioxane molecules on the polymer surface. Neat and treated films were studied by infrared spectroscopy and X-ray photoelectron spectroscopy, which proved the effectiveness of POSS grafting. Moreover, scanning electron microscopy measurements demonstrated the homogeneous distribution of Si on the film surface treated with the silsesquioxane. The influence of the film treatment on the surface wettability was evidenced by contact angle measurements. These findings demonstrated a relevant enhancement of the surface hydrophobicity, which increase turned out to depend on the conditions applied, as it increased by increasing the reaction temperature and the contact time. Finally, in order to evaluate the stability of neat and of the treated PLLA films the surface morphology of the samples treated with pH 7.4 buffer at 50 °C was studied.

Chemical-Vapor-Deposited Graphene as a Thermally Conducting Coating.

We performed scanning thermal microscopy measurements on single layers of chemical-vapor-deposited (CVD) graphene supported by different substrates, namely, SiO2, Al2O3, and PET using a double-scan technique to remove the contribution to the heat flux through the air and the cantilever. Then, by adopting a simple lumped-elements model, we developed a new method that allows determining, through a multistep numerical analysis, the equivalent thermal properties of thermally conductive coatings of nanometric thickness. In this specific case we found that our CVD graphene is "thermally equivalent", for heat injection perpendicular to the graphene planes, to a coating material of conductivity k eff = 2.5 ± 0.3 W/m K and thickness t eff = 3.5 ± 0.3 nm in perfect contact with the substrate. For the SiO2 substrate, we also measured stacks made of 2- and 4-CVD monolayers, and we found that the effective thermal conductivity increases with increasing number of layers and, with a technologically achievable number of layers, is expected to be comparable to that of 1 order of magnitude-thicker metallic thin films. This study provides a powerful method for characterizing the thermal properties of graphene in view of several thermal management applications.

Stereocomplexation of Poly(Lactic Acid)s on Graphite Nanoplatelets: From Functionalized Nanoparticles to Self-assembled Nanostructures.

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.

Hydrated Salt/Graphite/Polyelectrolyte Organic-Inorganic Hybrids for Efficient Thermochemical Storage.

Hydrated salt thermochemical energy storage (TES) is a promising technology for high density energy storage, in principle opening the way for applications in seasonal storage. However, severe limitations are affecting large scale applications, related to their poor thermal and mechanical stability on hydration/dehydration cycling. In this paper, we report the preparation and characterization of composite materials manufactured with a wet impregnation method using strontium bromide hexahydrate (SBH) as a thermochemical storage material, combined with expanded natural graphite (G). In addition to these fully inorganic formulations, an organic polyelectrolyte (PDAC, polydiallyldimethylammonium chloride) was exploited in the structure, with the aim to stabilize the salt, while contributing to the sorption/desorption process. Different formulations were prepared with varying PDAC concentration to study its contribution to material morphology, by electron microscopy and X-ray diffraction, as well as water sorption/desorption properties, by thermogravimetry and differential calorimetry. Furthermore, the SBH/G/PDAC powder mixture was pressed to form tabs that were analyzed in a climatic chamber, which is evidence for an active role of PDAC in the improvement of water sorption, coupled with a significant enhancement of mechanical resistance upon hydration/dehydration cycling. Therefore, the addition of the polyelectrolyte is proposed as an innovative approach in the fabrication of efficient and durable TES devices.