Synthesis, Characterization and Application of Polycaprolactone Based Systems

The PhD research program focused on the development of new formulations based on polycaprolactone (PCL) using polymers with linear and star geometry, commercially and ad hoc synthesized. For all these systems, the influence of the chemical nature of PCL as well as its chain terminal groups, introduced by different types of chemical functionalization, on the final properties of the studied formulations was investigated. The interest in PCL lies particularly in its intrinsic characteristics of biodegradability and biocompatibility. In addition, the use of this polymer can be promising with regard to the development of new sustainable materials, as it can potentially be obtained from renewable sources. Of particular relevance in this research project was the use of polymers characterized by star molecular topology, commonly defined as “star-shaped”, with low molecular weight. In general, this type of molecular structure allows to influence some important physical properties of polymers, such as crystallinity, hydrodynamic and gyration radius, solubility, viscosity of the melt and the resulting solutions. In particular, it is worth noting that these systems have a greater number of terminal functional groups per unit mass than linear polymers of equivalent molecular weight. Indeed, one of the main problems of linear PCL is the lack of chain functionalities that can be exploited for chemical modification by conventional methods, which is only possible at the hydroxyl end groups. The combination of these peculiar features therefore makes star-shaped systems very interesting in terms of their use as additives in polymer systems, due to their miscibility and ability to confer functionality through the high number of groups. Specifically, the research activities during the PhD were articulated through the development of seven different systems containing PCL. In particular, two projects focused on the preparation of linear and star-shaped pyrenyl-terminated PCLs and their application in the development of nanocomposites and nanopapers containing graphene nanoplates (GNPs). The exploitation of pyrene functionality aimed at promoting PCL/GNP interactions by introducing π-π stacking interactions between the polymer functionalities and the surface of the graphite layers of GNP. In one work, the above property was applied to promote the dispersion of GNP in polymer nanocomposites and consequently their electrical conductivity, while in another system, namely a nanopaper, the specific PCL/GNP interactions were used to improve its thermomechanical properties by the physical cross-linking of GNP. The PhD work also dealt with the application of linear and star-shaped PCLs with different end functionalities in the preparation of porous and dense films as well as hydrogels and vitrimers. Specifically, one work focused on the development of porous films with properties suitable for their use as sensors and absorbers, exploiting a high molecular weight linear PCL in combination with polylactic acid (PLA) and GNP to increase the ductility and manipulability of the films. In another study on the preparation of dense PLA-based films, three types of star-shaped PCLs with hydroxyl (PCL-OH), carboxyl (PCL-COOH) and pyrenic (PCL-Pyr) end groups were used. Indeed, the influence of the different functionalities on the final properties of the materials was investigated as well as the improvement of some properties of the films, such as elongation at break and the adsorption capacity towards cationic species. A related work also dealt with the development of dense films based on high molecular mass PCL and GNP. For this purpose, a star-shaped PCL with furoyl-type terminal functionalities (PCL-Fur) was added to the system to promote the GNP dispersion within the PCL matrix. This was achieved by the occurrence of covalent bonds through Diels-Alder reaction between the furan rings of PCL-Fur and the edges of the graphene systems. In the application of PCL in the preparation of hydrogels, star-shaped polymers with reactive end functionalities against radical polymerization reactions were used, namely acrylic (PCL-TA) and maleic (PCL-COOH), in combination with hydroxyethyl acrylate and N-isopropyl acrylamide, respectively. These functionalities were exploited to promote cross-linking with consequent formation of copolymers during the frontal polymerization process used. In particular, the effect of the above-mentioned copolymers on the compabilization of the two polymer phases and consequently on the hydrogel mechanical properties was investigated. At the same time, the use of maleic functions has in turn enabled the adsorption capacity of hydrogels towards cationic species. Finally, in a last work, three star-shaped PCLs with acrylic functionalities and different mass and/or number of arms were used for the development of catalyst-free biodegradable vitrimers. This was possible by exploiting a thiol-acrylate cross-linking reaction between the terminal PCL functionalities and the thiol groups of a dynamic bifunctional crosslinker containing boronic esters. In conclusion, the results reported in this thesis highlight the versatility of PCL, and prove that, almost a century after its discovery, this bioplastic has promising characteristics for the development of new sustainable materials in systems where its potential has not yet been fully exploited.