Functional polymers featuring molecular auxeticity and covalent dynamic bonds

Functional polymers are a class of polymers that possess specialized chemical functionalities or structural features that enable them to perform specific functions or exhibit unique properties. These polymers are designed and synthesized to serve various applications across a wide range of industries. They can be tailored to have specific properties such as conductivity, biocompatibility, stimuli-responsiveness, adhesion, or biodegradability, among others. The versatility and tunability of functional polymers make them indispensable in numerous technological advancements and applications, driving ongoing research and development in polymer chemistry and materials science. This PhD thesis is focused on the study and development of auxetic materials and reprocessable thermosets for adhesive application. After an overview on auxetic materials, an interesting class of materials with Negative Poisson’s ratio values, reported in in Chapter 1, the attention is focused on the development of molecular auxetic system, using a quinoxaline cavitand (QxCav) as expandable unit. In Chapter 2, the first synthetic molecular auxetic polymer is reported, obtained by incorporating a conformationally expandable cavitand as a crosslinker into a rigid polymer with intrinsic microporosity (PIM1). For this purpose, a QxCav bearing eight hydroxyl groups on the quinoxaline walls (Cv8H) is synthesized and reacted via multiple nucleophilic aromatic substitution reaction with the commercial tetrafluoroterephtalonitrile and 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethyl-1,1’- spirobiindane to obtain the corresponding crosslinked polymer. The rigidity and microporosity of the polymeric matrix are pivotal to maximize the expansion effect of the cavitand by directly transfer the mechanical stress, resulting in sizable NPR values. This effect could be reached thanks to the direct covalent linking of the quinoxaline wings to the PIM structure. A theoretical micromechanical model is developed to predict the auxetic behavior, whereas experimentally, the NPR was verified via the digital image correlation (DIC) technique performed during the mechanical tests on films obtained by blending the auxetic crosslinked polymer with pristine PIM-1. In chapter 3, QxCav is introduced as crosslinker in highly oriented main chain liquid crystal elastomeric (LCE) networks. For this purpose, the four quinoxaline walls are decorated with terminal alkenes and the resulting cavitand is reacted via metathesis Finally, the complete reversibility of the auxetic response is assessed upon five repeated loading cycles. 2 with a proper mesogenic unit in the presence of a Hoveyda-Grubbs 2nd generation catalyst. Monodomain LCE are achieved by mechanical alignment, and then characterized via differential scanning calorimetry (DSC) and polarized optical microscopy (POM). All materials retain the liquid crystalline properties, showing a range of mesophase of around 30°C. However, the inhomogeneity of the samples hampered the observation of auxetic behavior. Moving to Chapter 4, a general introduction on covalent adaptable network is given, providing different examples of the main used dynamic exchange mechanism. These new materials bridge the gap between thermoplastics and thermosets. In Chapter 5 a new dissociative exchange mechanism is reported. It is based on b- amino amide moieties mechanism synthetized using a two temperature dependence steps: a first Michael addition between methyl acrylate and an amine at 50°C and a subsequent amidation reaction at 100°C using a commercially available amine. Model study shows a dissociation from temperature beyond 160°C. The materials are reprocessed multiple times without compromising the properties. They exhibit also creep resistance and exceptional hydrolytic resistance in acid, basic and neutral environment. Finally, in Chapter 6 a potential application as reversible adhesive using the chemistry discussed in Chapter 5 is tested. More in detail, a commercial liquid phenoxy-like resin is crosslinked with a dynamic hardener containing b-amino amide functionalities. Three different formulations are prepared and characterized using two different amines to tailor the Tg of such materials. All the materials give good lap shear strength results (up to 13 MPa) even after the first recycle (≈ 8 MPa). Subsequently, debonding is tested performing lap shear test at 180 °C, which resulted in complete detachment of the two aluminum joints. The broken specimens are then joined together, and the debonding is tested and confirmed after the first recycling showing zero force necessary for the detaching of the specimens.