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

Introducing SuFEx click chemistry into aliphatic polycarbonates: a novel toolbox/platform for post-modification as biomaterials

As a biodegradable and biocompatible biomaterial, aliphatic polycarbonates (APCs) have attracted substantial attention in terms of post-polymerization modification (PPM) for functionalization. A strategy for the introduction of sulfur(VI)-fluoride exchange (SuFEx) click chemistry into APCs for PPM is proposed for the first time in this work. 4'-(Fluorosulfonyl)benzyl 5-methyl-2-oxo-1,3-dioxane-5-carboxylate (FMC) was designed as a SuFEx clickable cyclic carbonate for APCs via ring-opening polymerization (ROP), and an operational and nontoxic synthetic route was achieved. FMC managed to undergo both ROP and PPM through the SuFEx click chemistry organocatalytically without constraining or antagonizing each other, using 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD) as a co-organocatalyst here. Its ROP was systematically investigated, and density functional theory (DFT) calculations were performed to understand the acid-base catalytic mechanism in the anionic ROP. Exploratory investigations into PPM by SuFEx of poly(FMC) were conducted as biomaterials, and the one-pot strategies to achieve both ROP and SuFEx were confirmed.

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.

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.

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.

Heat Transfer at the Interface of Graphene Nanoribbons with Different Relative Orientations and Gaps

Because of their high thermal conductivity, graphene nanoribbons (GNRs) can be employed as fillers to enhance the thermal transfer properties of composite materials, such as polymer-based ones. However, when the filler loading is higher than the geometric percolation threshold, the interfacial thermal resistance between adjacent GNRs may significantly limit the overall thermal transfer through a network of fillers. In this article, reverse non-equilibrium molecular dynamics is used to investigate the impact of the relative orientation (i.e., horizontal and vertical overlap, interplanar spacing and angular displacement) of couples of GNRs on their interfacial thermal resistance. Based on the simulation results, we propose an empirical correlation between the thermal resistance at the interface of adjacent GNRs and their main geometrical parameters, namely the normalized projected overlap and average interplanar spacing. The reported correlation can be beneficial for speeding up bottom-up approaches to the multiscale analysis of the thermal properties of composite materials, particularly when thermally conductive fillers create percolating pathways.