Morphology and properties evolution upon ring-opening polymerization during extrusion of cyclic butylene terephthalate and graphene-related-materials into thermally conductive nanocomposites

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.

Extrusion of Polymer Nanocomposites with Graphene and Graphene Derivative Nanofillers: An Overview of Recent Developments

This review is focused on the recent developments of nanocomposite materials that combine a thermoplastic matrix with different forms of graphene or graphene oxide nanofillers. In all cases, the manufacturing method of the composite materials has been melt-processing, in particular, twin-screw extrusion, which can then be followed by injection molding. The advantages of this processing route with respect to other alternative methods will be highlighted. The results point to an increasing interest in biodegradable matrices such as polylactic acid (PLA) and graphene oxide or reduced graphene oxide, rather than graphene. The reasons for this will also be discussed.

Facile and Low Environmental Impact Approach to Prepare Thermally Conductive Nanocomposites Based on Polylactide and Graphite Nanoplatelets

In this work, the preparation of nanocomposites based on poly(l-lactide) PLLA and graphite nanoplatelets (GNP) was assessed by applying, for the first time, the reactive extrusion (REX) polymerization approach, which is considered a low environmental impact method to prepare polymer systems and which allows an easy scalability. In particular, ad hoc synthesized molecules, constituted by a pyrene end group and a poly(d-lactide) (PDLA) chain (Pyr-d), capable of interacting with the surface of GNP layers as well as forming stereoblocks during the ring-opening polymerization (ROP) of l-lactide, were used. The nanocomposites were synthesized by adding to l-lactide the GNP/initiator system, prepared by dispersing the graphite in the acetone/Pyr-d solution, which was dried after the sonication process. DSC and X-ray diffraction measurements evidenced the stereocomplexation of the systems synthesized by using the pyrene-based initiators, whose extent turned out to depend on the PDLA chain length. All the prepared nanocomposites, including those synthesized starting from a classical initiator, that is, 1-dodecanol, retained similar electrical conductivity, whereas the thermal conductivity was found to increase in the stereocomplexed samples. Preferential localization of stereocomplexed PLA close to the interface with GNP was demonstrated by scanning probe microscopy (SPM) techniques, supporting an important role of local crystallinity in the thermal conductivity of the nanocomposites.

Supernucleation and Orientation of Poly(butylene terephthalate) Crystals in Nanocomposites Containing Highly Reduced Graphene Oxide

The ring-opening polymerization of cyclic butylene terephthalate into poly(butylene terephthalate) (pCBT) in the presence of reduced graphene oxide (RGO) is an effective method for the preparation of polymer nanocomposites. The inclusion of RGO nanoflakes dramatically affects the crystallization of pCBT, shifting crystallization peak temperature to higher temperatures and, overall, increasing the crystallization rate. This was due to a supernucleating effect caused by RGO, which is maximized by highly reduced graphene oxide. Furthermore, combined analyses by differential scanning calorimetry (DSC) experiments and wide-angle X-ray diffraction (WAXS) showed the formation of a thick α-crystalline form pCBT lamellae with a melting point of ∼250 °C, close to the equilibrium melting temperature of pCBT. WAXS also demonstrated the pair orientation of pCBT crystals with RGO nanoflakes, indicating a strong interfacial interaction between the aromatic rings of pCBT and RGO planes, especially with highly reduced graphene oxide.