Non-covalent polymeric networks: via hydrogen bonds and electrostatics interactions.

Abstract

“Supramolecular chemistry has been defined as chemistry beyond the molecule, as it aims at designing and implementing functional chemical systems based on molecular components held together by noncovalent intermolecular forces”. This definition provided by the Nobel Price, Jean-Marie Lehn, lays the foundation of a new branch of chemistry based on the non-covalent interactions and the effect of such bonds on a system. Even though these noncovalent interactions are generally weak, they have a significant impact on the behavior of self-assemble complex structures. Supramolecular interactions include electrostatic interactions, hydrogen bonds, ion-dipole and dipole-dipole interactions, metal coordination, hydrophobic forces, and van der Waals forces. This thesis describes the use of electrostatic interactions and hydrogen bonds as tools for the formation of polymeric networks in polyolefins, featuring new thermal, mechanical, and electric properties. Polyethylene (PE) is the most widely used thermoplastic polymer and the leader in most applications in the global plastic industry. It exhibits strong resistance to solvents, exceptional flexibility, low cost, lightness, and simplicity of processing. However, its use is limited by its low strength and stiffness, low upper service temperature, stress cracking, and poor UV resistance. Additionally, its apolar nature makes it unsuitable for any coating and adhesive application unless a surface treatment is applied (like flame and corona treatment). To overcome those issues, and drastically improve its properties, crosslinking of polyethylene is performed. Chapters 1 and 2, report the synthesis of a new type of polyethylene-based ionomer, where a few mole percent of positively and negatively charged groups, are covalently bound to the polymer backbone. Amino-terminated methacrylates and methacrylic acid, present in the form of ion pairs, are used as comonomers (ion pair comonomer, IPC) together with ethylene, in a high-temperature/high-pressure process. Specifically, Chapter 1 will present a study of the polymerization reaction of a polyethylene-based ionomer using different reaction conditions, such as temperature, IPC content, in the presence/absence of a chain transfer agent (CTA). The introduction of ionic groups into the PE matrix is an effective way to increase its hydrophilicity and thus its adherence properties to aluminum. In Chapter 2 polyethylene-based ionomers are demonstrated to feature a thermo-mechanical and dielectric property portfolio that is comparable to crosslinked polyethylene (XLPE), which may enable the design of more sustainable high-voltage direct-current (HVDC) power cables, a crucial component of future electricity grids that seamlessly integrate renewable sources of energy. Chapters 3 and 4, present the use of multiple hydrogen bonding as a tool to compatibilize immiscible polymers. Chapter 3 introduces two-photon microscopy (2PM) as an original technique to investigate the compatibilization between PE-HEMA and EVOH at the sub-micrometer level, both on the surface and in the bulk. 2PM allows to visualize polymer blending through 3D images of the obtained films. Compatibilization was performed in solution, upon functionalization of PE-HEMA with 1.4% molar of ODIN, a fluorescent molecule able to form multiple hydrogen bonding with EVOH and to act as fluorescent probe. Different blends were synthesized, and the obtained films analyzed by 2PM. For all compositions, it was demonstrated that ODIN is evenly distributed both on the surface and in the bulk. 2PM analysis of the thermally reprocessed specimen revealed that repeated reprocessing allows the reformation of ODIN dimers as the most stable H-bonding array in the solid state, partially reversing the compatibilization. Chapter 4 discusses the possibility to create polymer blends acting as oxygen barrier, to evaluate the applicability of a monolayer instead of a multilayer as a solution to produce monolayer packaging films to substitute triple-layer films. Ethylene-co-vinyl alcohol (EVOH) which is already used in the food and pharmaceutical industry for its oxygen barrier property, and polyethylene-co-hydroxyethyl methacrylate (PE-HEMA), which acts as a water barrier, have been utilized. The study involved the permeability characterization of the blends produced in Chapter 3 as well as the production of new binary mixtures produced by functionalizing PE-HEMA with phenyl-urea and combining it with EVOH. The following assessment of the blends, paying special attention to the oxygen permeability tests, shows an unequal distribution of the two polymers within the blends. The PE-HEMA fraction is mainly present at the surface, while EVOH is concentrated in the inner bulk. This self-segregation in the films leads to improved oxygen barrier properties.