Supramolecular Modifications of Semicrystalline Polymers

Supramolecular interactions play an important role in defining the structure and the resulting mechanical properties of materials. For instance, interchain hydrogen-bonding in PAs gives them superior strength and stiffness in engineering materials, while the predominance of van der Waals interactions in PE materials is one of the factors responsible for their ductility. In the present thesis, we have strived to achieve novel materials with a useful combination of mechanical properties by incorporating both types of supramolecular interactions into the same material in the form of micro- or nanophase-segregated domains. The first section addresses the challenge of increasing the ductility of a semiaromatic polyamide without compromising strength or stiffness. To this end, we have synthesized amino-telechelic PE and subjected it to high-temperature melt compounding with the polyamide PA6TI. We show that, under these conditions, rapid transamidation reactions between the two materials result in the formation of PA-PE block copolymers, which stabilized the interfaces of the PE particles and resulted in reduced PE domain sizes in the approximate regime where particulate toughening is most efficient (0.1-1 µm). Furthermore, because the PE domains were shown to remain semicrystalline, their presence only induced minimal losses of stiffness and strength. This translated to improved toughness and ductility over blends with non-functional PE at an optimized concentration of 20 wt% modified PE, which was also observed in corresponding glass fiber-reinforced composites. The second section started from the same amino-telechelic PE, aiming to create novel materials comprising hydrogen-bonded supramolecular networks. To this end, we synthesized PE end-modified with ligands capable of forming extended one-dimensional nanostructures by ditopic, multivalent hydrogen bonding, such as oligopeptides or benzene-1,3,5-tricarboxamide (BTA) derivatives. These polymer end groups were designed to form well-defined nanofibrils by co-assembly with low molecular weight oligopeptide or BTA additives. We studied additives and end groups with different core structures and solubilizing substituents and compared the resulting materials to reference systems based on non-modified PE at various additive concentrations, in order to analyze the role of these different molecular parameters on the formation of well-defined one-dimensional nanostructures versus phase separation into crystalline additive precipitates. The presence of the nanofibrils served as reversible physical crosslinks above the PE melting temperature, giving rise to increased melt strength and elasticity, tailorability of rheological properties, and a drastically increased extensibility in the polymer melt, which is relevant for polymer processing procedures that require large degrees of deformation.