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

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.

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

“…The peculiar behavior of the above composite system might be ascribed to the effect of rGO on the polymer structuring. Indeed, as widely reported, the GRM can act as a nucleating agent for PCL [ 35 ], an effect that is particularly evident in the systems characterized by a high affinity for the graphitic surface. In the case of the PCL-OH_MDI_1:1_200_G and PCL_MDI_1:1.33_200_G, the crosslinking was found to have a slight effect on the polymer structuring and on T c , the decrease in crystallinity was not as severe as in the neat crosslinked samples described previously.…”

3D-printed versatile biliary stents with nanoengineered surface for anti-hyperplasia and antibiofilm formation

“…The melting temperature (T m ) analyzed using DSC was also slightly increased from 62.2 °C to 64.5 °C (i.e., 2.3 °C higher) for the pristine PCL and fPCL Si30 ( Fig. 1 E), attributed to a marginal change in the crystallization kinetics of fPCL [ 70 ]. Generally, reinforcement in a polymer matrix can cause heterogeneous crystallization with incomplete crystals, consequently increasing T m [ 71 ].…”

Flexible and High Thermal Conductivity Composites Based on Graphite Nanoplates Paper Impregnated with Polydimethylsiloxane

“…Bispyrene molecules have indeed been found to drive self-assembly of GNP and improve contacts between those, eventually enhancing the thermal conductivity of nanopapers [31]. The inclusion of macromolecules into the nanopapers has also been explored to enhance their toughness and deformability [32][33][34], but the amount of polymer and its crystalline organization [35] should be carefully controlled to avoid a dramatic decrease in the thermal conductivity of the nanostructure.…”