Use of heterocomplementary hydrogen bonding motifs for supramolecular materials chemistry

Hydrogen bonding is one of the most useful of the non-covalent interactions. Highly directional and easily tuneable, the strength of hydrogen bonded arrays enable controlled assembly of macromolecular structures. Because association can be designed to be selective, self-assembly involving low-molecular-weight amides and ureas has been expanded to higher order polymeric structures, so called ‘supramolecular polymers’. Chapter 1 introduces and develops upon the current themes of research in small-molecule hydrogen bonding, and the subsequent application towards the assembly of supramolecular polymers, in particular polyurethanes. The Wilson group is focused on the development of orthogonal recognition pathways, and their future application in the controlled assembly of polymers. The work presented in this thesis, therefore focuses on the development of self-sorting cascades- where molecules capable of hydrogen bonding have defined partners at specific stages of the cascade. Selecting two heterocomplementary hydrogen bonding arrays, and using them to form supramolecular polymers then advance this. Chapter 2 introduces the design and investigation of these self-sorting pathways involving hydrogen bonding arrays reported both in the literature and from within the Wilson group. The application of two of these hydrogen bonding motifs to assemble supramolecular polyurethanes is described in Chapter 3. The effect of the thermal history of supramolecular polyurethanes is then investigated, highlighting the change in response to thermal stimuli dependent on previous processing and treatment. The latter part of Chapter 3 introduces a ‘toolbox’ for supramolecular chemists, whereby components of the supramolecular polymer are changed systematically to gauge effect on subsequent mechanical properties. The synthetic route to supramolecular polymers is then discussed in Chapter 4, and the evolution of a solvent-free route to this particular class of polyurethanes is realised

A systematic study of the effect of the hard end-group composition on the microphase separation, thermal and mechanical properties of supramolecular polyurethanes

This paper reports a systematic study on a series of supramolecular polyurethanes that possess microphase separated morphologies which afford elastic materials at room temperature. Combinations of urea and/or urethane linkers in addition to a phenyl spacer have been used to study the effect of the rigidity of the hard end group segments as well as the hydrogen bonding capability of the urethane-urea linker units. Small angle X-ray scattering (SAXS) experiments have revealed characteristic microphase separated morphologies. Wide angle X-ray scattering (WAXS) was used to probe the lateral packing of the urethane and/or urea within the hard segments. Differential scanning calorimetry (DSC) analysis confirmed that unsymmetrical soft/hard segment phases have been achieved by varying the urethane/urea content. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) determined that a 1-D fibrillar structure was obtained when the hard segment featured ureas whereas a 3-D structure was achieved when a combination of urea and urethane groups was used, giving rise to enhanced elongation properties. Finally, we present mechanical testing data in which oscillatory rheology at a range of frequencies and temperatures has revealed the effect of the connectivity of the hard segments on the relaxation times of the supramolecular chains. Tensile tests showed that end groups with ureas or a combination of a urea and urethane yielded elastic materials with strengths of ca. 5 MPa at room temperature