Responsive materials via vinylogous urethane chemistry and host-guest interactions.

Abstract

Responsive materials exhibit remarkable macroscopic responses upon specific external stimuli, providing solutions to today’s progress challenges. The growing demand for more sustainable technological development is becoming a necessity in many fields, prompting the scientific community to push the boundaries of the progress even further. The goal is to provide ever more performing materials with a well-defined end-of-life management strategy. For this purpose, polymers incorporating dynamic bonds or weak interactions represent the best tradeoff between a well-consolidated platform of synthetic polymers and a new generation of materials, more durable or, alternatively, able to easily be reintroduced in a circular economy. The envisaged dynamicity in these new polymeric materials can be imparted by the incorporation of weak interactions, such as hydrogen bonding, host-guest interactions, metal-ligand coordination, or more robust reversible covalent bonds, giving arise to covalent adaptable networks. This PhD thesis reports design, synthesis, and properties of responsive materials for application in a wide variety of fields: from promoter for interlayer adhesion in composites until the tailoring of new recyclable insulators, as well as the generation of new self-reporting polymers. The main investigated covalent adaptable networks (CANs) were those based on the vinylogous urethane chemistry. In Chapter 2, a systematic study of the thermal, rheological, and mechanical properties of phenoxy-based vitrimers was presented. They relied on the transamination of vinylogous urethane. The aforementioned vitrimers were obtained by a two-steps synthesis from a commercial phenoxy resin via partial conversion of hydroxyl groups to acetoacetates (AcAc), followed by network formation by reaction with a 20% molar excess of m-xylylendiamine (XYDIA) as crosslinker. Three different vitrimers with variable crosslinking density were obtained by tuning the density of AcAc moieties along the phenoxy resin scaffold (5%, 10% and 15% conversion of hydroxyl groups). The conversion of linear polymers to dynamic crosslinked networks was confirmed by DMTA and rheology measurements, followed by stress relaxation tests to investigate the kinetics of bond exchanges. The calculation of activation energies for the relaxation process showed the negligible existence of additional relaxation modes, compared to the relaxation due to vinylogous bond exchanges, especially for higher vinylogous urethane crosslinking %. Tensile tests as a function of reprocessing cycles revealed an increase of the maximum elongation and stress at break and proved the good recyclability of the vitrimers. Enhanced adhesive properties compared to pristine phenoxy resins were demonstrated, including the possibility to thermally re-join the assembly after its mechanical failure. Finally, the solvent-free preparation of vitrimer was exploited for 5% crosslinked vitrimer via melt reactive blending, providing a valuable alternative to the less environmentally sustainable synthesis in solution. As adhesion promoters for carbon fiber-reinforced epoxy matrices, phenoxy resins and their functionalized counterparts with acetoacetate units were studied in Chapter 3. Multilayer objects are known to be susceptible to failure in a variety of modes in the thickness direction since their main weakness is the fiber/matrix interface. This phenomenon is called interlaminar delamination. Delamination is detrimental for composites because it is capable to worsen drastically their mechanical properties, therefore the enhancement of the interlaminar adhesion becomes a crucial factor. Our purpose was to address this issue playing on different factors, including the type of bonds introduced to the polymeric component of the composite, the topology of connection between the epoxy resin of the composite, the polymeric chains of the adhesion promoter and the ability to dissipate mechanical stresses. We proposed the application of two commercial phenoxy resins with different average molecular weights (PKHB and PKHP). They are amorphous thermoplastic mixable with epoxy resin, ensuring mutual diffusion of the relevant polymeric chains, hence able to give arise to a physical entanglement, which is one of the enhancing factors of interlaminar adhesion. A further step was the introduction of a properly functionalized phenoxy resin (PKHP-AcAc 20% and PKHB-AcAc 20%) It was thought that the presence of acetoacetate units could be crucial for the formation in situ of a vitrimeric network, once crosslinked with the amines of the crosslinker present in the epoxy matrix of the composite. The interlaminar adhesion would also be enhanced due to the development of a stitch-stitch topology entanglement, which entails the diffusion of acetoacetylated phenoxy chains into the epoxy networks of the two adherends, crosslinking into a third covalent adaptable-based polymer, in topological entanglement with both pre-existing polymer networks. The third polymer network acts like a molecular suture. To separate, at least one of the three polymer networks must break. These polymeric adhesion promoters were applied to a commercial prepreg (DT120), as thin films or dissolved in a suitable solvent. Mechanical characterization of bonded joint was evaluated in single lap shear configuration and fracture toughness configuration. The application of vitrimeric networks as recyclable alternative to the wide-spread thermosets-based insulators was analyzed in Chapter 4. The replacement of crosslinked polyethylene is becoming a prominent issue to relieve the environmental imprint due to the end-life high voltage cables disposal. Thereby, the design of a new vitrimer which intrinsically presenting high free volume was identified as the best solution to address this problem. As building blocks, triptycene-based molecules were chosen thanks to the ability to create interstitial space around them. We were interested to develop a vinylogous transamination-based CAN, attainable by the reaction between aminic crosslinkers and acetoacetylated amorphous thermoplastics. Several routes were explored to generate triptycene-based phenoxy resins, in which the triptycene unit ideally replaces the bisphenol A. The synthetic attempts concerned the reactions between the diglycidylated triptycene (9,10-benzenoanthracene-1,4-diol diglycidyl ether) with the hydroquinone homologous (9,10-benzenoanthracene-1,4-diol) or primary amines. The polymerization experiments showed only the formation of oligomers even varying the conditions in terms of temperature, base/catalyst, and time. To overcome the limited growth of the polymeric chain, a more straightforward approach was also studied, thus avoiding the challenging synthesis of the linear polymer and its functionalization. The 9,10-benzenoanthracene-1,4-diol was properly acetoacetylated, becoming a suitable raw material for the reaction with tris(2-aminoethyl) amine (TREN), to generate a vinylogous urethane-based vitrimer. In Chapter 5 cavitand-based host-guest chemistry has been tested to produce self-diagnostic polyurethanes. The purpose was to introduce a supramolecular unit in a polyurethane matrix, designed for the non-destructive and non-invasive detection of micro–damages. A nonemissive supramolecular complex was prepared and embedded into a rigid PU for the assessment of the mechanical-induced stress in the polymeric matrix. Indeed, internal stress detection in polymers used for high performance applications is critical in preventing structural failures when the integrity is crucial. The investigated system is based on the complex between a fluorescent N-methyl pyridinium salt (PyPyr-OH) and a tetraphosphonate cavitand, CavPOPh (mono -OH). Both the host and the guest have a peripheral OH group to be embedded in the polyurethane matrix. In the complex the radiation emitted by the fluorophore is immediately quenched upon the complexation and theoretically restored by the mechanical stress-induced dissociation of the supramolecular complex. The photophysical behavior of the host-guest probe was investigated. Absorption and fluorescence emission titrations were performed, proving the emission quenching of PyPyr-OH in the complex. The spectroscopic characterization in solution was preparatory to perform investigation when the system was chemically linked to a polyurethane matrix. A polyol, in which a solution of the probe was dispersed, was polymerized with an isocyanate. The non-fluorescent behavior of the complex was retained in the polymer matrix. Tensile tests were carried out on dog-bone specimens to evaluate how the fluorescent response of the probe changed as stress level increased. The fluorescence of PyPyr-OH remained quenched in the specimens after a mechanical stress application. Finally, the effect of the host-guest interaction between cucurbit[8]uril (CB[8]) and a model trimethine indocyanine (Cy3) on dye spectral properties and aggregation in water was investigated in Chapter 6. Inclusion of polymethine cyanine dyes in the cavity of macrocyclic receptors is an effective strategy to alter their absorption and emission behavior in aqueous solution. Solution studies, performed by a combination of spectroscopic and calorimetric techniques, indicate that the addition of CB[8] disrupted Cy3 aggregates, leading to the formation of a 1:1 host-guest complex with an association constant of 1.5×106 M-1. At concentrations suitable for NMR experiments, the slow formation of a supramolecular polymer was observed, followed by precipitation. Single crystals X-ray structure elucidation confirmed the formation of a polymeric assembly with 1:1 stoichiometry in the solid state.