Chain Extension of Blocked Isocyanates for the Creation of Covalent Adaptable Networks

From mattresses to car coatings, polyurethanes and polyureas (PUs) are ubiquitous thermoset materials with tunable thermo-mechanical performance. This tunability is afforded through the extent of crosslinking or covalent linkages established by the formation of urea bonds. Traditionally, urea bonds are highly stable yet through the blocked isocyanate, hindered urea bond (HUB) mechanism this bond is thermally labile allowing for re-processing of normally non-recyclable PU materials. The overarching focus of my dissertation is the exploration synthetic pathways to access secondary diamines to enable re-processing of covalent adaptable networks (CANs) with tunable re-processing through the diamine moiety and the molecular weight of the diamine. Given aromatic and aliphatic secondary amine structure as well as modularity of monomer length via chain extension, initial step-growth cure and subsequent re-processing performance of the PU CANs were able to be quantified and explored relative to these structure-property relationships. Chapter III of this dissertation addressed the secondary amine structure in terms of accessing azole terminated diamines and characterizing their ability to polymerize and re-process. Chapter IV explored chain extension and azole secondary amines were abandoned due to their poor solubility and amphiphilic reactivity. Chapter V explored chain extension and aliphatic secondary amines were realized for their uncomplicated incorporation facilitating chain extension over their aromatic counterparts. In Chapter VI, an aliphatic, chain extended monomer from the previous chapter was employed to establish a testing scaffold for lightly crosslinked, dissociative CANs. Comparisons against commercial standards and re-processing techniques allowed for transition from thermoset to thermoplastic to be visualized. Chapter VII employed another monomer from Chapter V and the testing scaffold from Chapter VI to directly compare the effect of secondary diamine molecular weight on thermo-mechanical performance relative to a commercial standard and then against the larger monomer from Chapter II.