Materiali Termoindurenti Riprocessabili con Proprietà Autodiagnostiche e Cristalli Liquidi Supramolecolari Elastomerici

Polymeric materials can be categorized into two main classes: thermoplastics and thermosetting polymers, each possessing distinct advantages and limitations. Thermoplastics, characterized by non-crosslinked polymer chains, derive their mechanical properties from chain entanglement. While they exhibit good mechanical properties, often inferior to those of thermosetting polymers, their non-crosslinked nature enables melting and reprocessing. In contrast, thermosetting polymers possess a covalent crosslinked network, conferring superior mechanical properties but compromising their reprocessability. Once cured, thermosetting materials cannot be melted or reshaped. Consequently, the end-of-life disposal options for thermosetting materials are limited to landfill disposal, incineration for energy recovery, or grinding into fine particles for use as fillers in new materials. In the modern era, characterized by a circular economy perspective which takes into account the entire product lifecycle, these options are clearly unsustainable. In recent years, covalent adaptable networks (CANs) have emerged as an intriguing solution to the limitations of traditional thermosetting polymers. These materials behave like thermosets at their working temperature but can undergo dynamic crosslink exchange reactions at elevated temperatures, enabling reprocessing. However, CANs are susceptible to the formation of microfractures due to minor mechanical stresses, which can compromise their long-term mechanical properties leading to catastrophic failures. Therefore, developing materials provided by self-diagnostic properties able to detect micro-damages at their early stage represents a precious tool. This thesis aims to develop innovative smart materials that incorporate probes to achieve self-diagnostic properties within dynamic covalent matrices. This approach enables the autonomous detection of microfractures caused by minor mechanical stresses, while the matrix made of a covalent adaptable network allows the material reprocessing extending the material lifespan. The work begins with the study of the main matrices characterized by a dynamic crosslinking network and the primary methods used for self-diagnostics. This allows for careful selection of the best combination between dynamic polymer matrix and the method for identifying mechanical stress. Chapter 2 presents the development of a self-diagnostic and reprocessable system characterized by core-shell nanocapsules containing crystal violet lactone (CVL) as fluorescent probe. CVL emission can be turned on through interaction with a hydrogen donor group and subsequently switched off via thermal treatment. The work involved the synthesis of core-shell polymethyl methacrylate (PMMA) nanocapsules encapsulating CVL, followed by their characterization using dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The ability of these nanocapsules to release the fluorophore upon rupture was subsequently demonstrated. The released CVL interacts with carboxylic acid groups on graphene oxide (GO) nanostructures, triggering the transition from the non-emissive CVL to the emissive form of CVL+. To assess the self-diagnostic capabilities of this system within a solid matrix, the nanocapsule-GO system was initially incorporated as additives into an epoxy resin model system. Upon successful demonstration of damage detection, the additives were introduced into a matrix characterized by a dynamic covalent crosslinking network based on disulfide bonds. This integration enabled the material to both identify microfractures and undergo reprocessing and reuse. Nanocapsuels and GO were dispersed in the polymer matrix, then the application of a mechanical stress caused capsule rupture releasing CVL. The interaction of CVL with graphene-oxide triggered the switch on of the fluorophore emission thus detecting the mechanical damage. Subsequent heating deactivated the fluorophore and initiated the healing process which resulted to be only partial. The material could then be subjected to further mechanical stress, triggering another round of damage detection. The developed system, however, presents a limitation: each capsule can only provide a single damage detection. Upon rupture and fluorophore release, the capsule becomes non-functional. Chapter 3 introduces a system that addresses this limitation by utilizing mechanophores as damage detection probes. Mechanophores are molecules that undergo a reversible transition between a non-emissive and an emissive form through the breaking and reforming of a labile bond. When incorporated as crosslinkers within a polymeric matrix, mechanophores respond to mechanical stress applied to the material, leading to bond breakage and subsequent emission switch on. Thermal treatment can reverse this process, restoring the non-emissive form and enabling repeated cycles of damage detection and repair. The chapter demonstrates how, by appropriately functionalizing a derivative of rhodamine 6G with silyl ether groups, it is possible to obtain a mechanophore that can be used as a crosslinker to create a material that is both self-diagnostic and characterized by a dynamic covalent crosslinking network (CAN), making it reprocessable. Upon mechanical solicitation, the labile lactam ring of the rhodamine breaks, triggering fluorescence emission. Subsequent thermal treatment quenches the fluorescence and repairs the material. This cycle of damage detection, repair, and reuse can be repeated multiple times without compromising the material’s self-diagnostic or mechanical properties. Another approach to achieve high mechanical properties while maintaining reprocessability involves the use of supramolecular crosslinking through hydrogen bonds. At operating temperatures, these hydrogen bonds provide sufficient crosslinking to enhance the material’s mechanical properties. However, upon heating, the H-bonds break, enabling the material to be reprocessed like a thermoplastic. Cooling the material facilitates the reformation of hydrogen bonds, restoring the crosslinked state. Chapter 4 explores the synthesis of photoactivated liquid crystal elastomers suitable for electrospinning, incorporating a reversible supramolecular crosslinking via hydrogen bonding. Initial efforts focused on side-chain liquid crystalline materials, where the mesogen, photoactuator, and hydrogen bond crosslinker were attached as side groups to the polymer backbone. However, these approaches did not yield materials with the desired properties. Subsequently, the focus shifted to polyurethane-based materials with a main-chain geometry. In this design, the mesogen and the photoactuator are incorporated into the polymer backbone, while the urethane groups act as hydrogen bond donors and acceptors. The abundance of hydrogen bonds ensures strong interchain interactions, facilitating electrospinning and locking in the orientation of liquid crystalline monomers during fiber formation. The first synthesized material, utilizing a mesogen with a two-ring aromatic core, exhibited excellent electrospinnability and photoactuation properties but lacked the desired liquid crystalline behavior. To address this, a mesogen with a three-ring aromatic core was employed. This modification led to materials with improved liquid crystalline properties. Further investigation is underway to evaluate the electrospinnability and photoactuation properties of these materials. Per- and polyfluoroalkyl substances (PFAS) are a class of emerging pollutants known for their environmental persistence and detrimental health effects. The scientific community has actively developed innovative methodologies to detect and remove these substances from various environmental matrices, including soil, wastewater, and drinking water. Chapter 5 explores the potential of supramolecular organic receptors, quinoxaline cavitands, to complex small organic molecules, enabling their application in PFAS detection and removal. To develop a PFAS sensor, the cavitand was incorporated into a conductive polymer matrix. This approach aimed to leverage the host-guest complexation within the cavity as a trigger for generating a sensor response. To achieve this goal two novel quinoxaline cavitands, functionalized with thiophene and pyrrole units at the upper rim, were synthesized for electropolymerization. While the thiophene-functionalized cavitand resulted unsuitable for the polymerization reaction, the pyrrole-functionalized cavitand was successfully copolymerized with N-methyl pyrrole. However, attempts to create a functional sensor were unsuccessful, as exposure to both blank and pollutant-containing matrices resulted in inconsistent signal variations. Subsequently, the pyrrole-functionalized cavitand was copolymerized with N-methyl pyrrole to create a membrane for PFOA removal. The strategy involved exploiting the oxidation of the conductive polymer matrix to generate positive charges, enabling dual interactions with the pollutant: ion pair formation between the positively charged matrix and negatively charged PFOA, coupled with host-guest complexation of the PFOA lipophilic tail within the hydrophobic cavity. While the material demonstrated good PFOA removal capabilities its performances resulted to be lower when compared with control material poly-N-methyl pyrrole alone.