The self-assembly of bis aromatic ureas and their applications in gels and healable polymer networks

This thesis is focused on the study of the bis aromatic urea unit shown in Figure i. Particular attention is given to the self-assembly capabilities of the units as a consequence of aromatic ring functionality. The ease of synthesis, explained and developed throughout this thesis, and the properties of materials containing the units make them suitable for many industrial and biological applications (as described in Chapter 1). Chapter 2 describes a study on chemical structure variations oflinked bis aromatic urea units (a variation of RI -4, Figure i) and the correlation of these to self-assembly capabilities. The self¬assembly studies are focused around the gelating capabilities of the formed compounds. Variations of the covalent linkers and outer aromatic functionalities (see R x and RI-4 in Figure i respectively) afforded a series of organo and hydrogelators. It was found that those molecules with electron withdrawing moieties in the meta position relative to the urea bond (R2 in Figure i) formed the most effective self-assembly units. Chapter 3 details several low molecular weight hydrogelators based on mono, bi and tri-armed bis aromatic urea units. The Chapter specifically focuses upon their applications as water purification and drug delivery agents., Hydrogelators described in Chapter 2 are employed and expanded upon to increase water purification properties. It was found that the formed hydrogelators were capable of removing a range of industrial dyes from aqueous environments. Furthermore the biocompatibility of the systems and drug removal as well as release capabilities demonstrate the possibility of such systems in biomedical applications. Chapter 4 explores the results of the introduction of the bis aromatic urea self-assembly units into polymers as low molecular weight additives to enhance both the mechanical and healable properties of the bulk phase. This effect is achieved via supramolecular interactions between the units and polymers and specific self-assembly between the additives. Initial explorations were performed with poly( ethylene-co-acrylic acid) and low molecular weight mono and diacid additives. After this proof of concept stage was completed a range of bis aromatic ureas were then synthesized and blended with the polymers. These bis aromatic ureas additives had the dual functionality ofpromoting system toughness as well as lowering healing temperatures. The chemistries reported in Chapter 5 employed the results from Chapters 2 and 4 to link the most successful self-assembly units, via covalently bonds, to telechelic polyethylene glycol (PEG) oligomers to create healable polymeric coatings. Control over the systems mechanical and healable properties were realized via synthesis and blending of tri-armed polymeric units to the telechelic PEG derivatives. A significant advantage of employing a water absorbing polymeric backbone in the form of PEG was demonstrated in the ability ofthe formed systems to close punctures in coatings via swelling phenomena. Finally Chapter 6 reports an approach based upon the data described in the previous Chapters whereby polypropylene glycol oligomers (which are more hydrophobic in character when compared to analogies PEG systems) were endcapped with the self-assembling bis aromatic urea units and the physical properties of the resultant supramolecular networks assessed. These supramolecular networks exhibited remarkable self-healing properties