Abstract
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.
Highlights
Nanopapers are thin sheets or films composed of selfassembled individual nanoparticles, generally obtained by filtration of a suspension in a solvent
They have gained increasing interest for their unique features, such as mechanical properties, gas barrier, and flame retardancy.[1−6] the above properties are mainly related to the highly concentrated nanoparticles, which are tightly packed in the thin films, because of their strong self-interactions[7,8] or mediated through a binding polymer, in the so-called brick and mortar structures.[9−14] Among the different lamellar nanoparticles which can be exploited in the preparation of nanopapers, graphene-related nanomaterials (GRMs), such as graphite nanoplates (GNP) and multilayer graphene, represent ideal systems for producing high-performance nanopapers, being characterized by ultrahigh strength, excellent electrical, and thermal conductivity.[15−19]
PCL appears strongly adhered to GNP (Figure 1b) and likely intercalated between thin galleries observed in pristine GNP nanopaper (Figure 1a)
Summary
Nanopapers are thin sheets or films composed of selfassembled individual nanoparticles, generally obtained by filtration of a suspension in a solvent They have gained increasing interest for their unique features, such as mechanical properties, gas barrier, and flame retardancy.[1−6] the above properties are mainly related to the highly concentrated nanoparticles, which are tightly packed in the thin films, because of their strong self-interactions[7,8] or mediated through a binding polymer, in the so-called brick and mortar structures.[9−14] Among the different lamellar nanoparticles which can be exploited in the preparation of nanopapers, graphene-related nanomaterials (GRMs), such as graphite nanoplates (GNP) and multilayer graphene, represent ideal systems for producing high-performance nanopapers, being characterized by ultrahigh strength, excellent electrical, and thermal conductivity.[15−19]. The above reduction step is essential to restore the conductivity properties of the material but represents a limitation of the method as the complete reduction of GO can hardly be achieved and various defects remain within the sp[2] carbon structure of reduced GO (rGO).[31−35]
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