The Effect of Plant Source on Lignin-based Polyurethanes and the Interdiffusion Dynamics of Semi-Rigid Polymers

The work reported in this thesis increases our understanding of the effect of lignin plant source on the mechanical and morphological properties of lignin-based polyurethanes and the interdiffusion of glassy/liquid bilayer thin films. The interdiffusion of glassy/liquid polymer pairs has received much less attention than liquid/liquid bilayers, leading to conflicting results and unresolved discrepancies. Therefore, the reported study of interdiffusion between polysulfone glassy/liquid polymer layers provides insight into the dynamics of these systems.

This thesis first reports the correlation between the mechanical properties of lignin-based polyurethanes and the lignin plant source. Lignin is a molecule that can be used as a polyol to synthesize polyurethanes, and the specific aromatic structure of the lignin is heavily reliant on the lignin’s plant source. Polyurethanes were synthesized reacting lignin with one of two diisocyanates that differ in length. The morphology and the mechanical properties were monitored. These results show that the longer cross-linker created polyurethanes using wheat straw lignin that exhibited better mixing and higher moduli than in the hardwood and softwood lignin polyurethanes, whose morphology was dominated by large aggregates. However, the shorter cross-linker created polyurethanes with a fairly uniform morphology and higher moduli from the hardwood and softwood lignin than that of the wheat straw lignin polyurethane. This demonstrates that the size of the cross-linker impacts the role of the lignin structure (and plant source) on the morphology and mechanical properties of lignin-based polyurethanes.

Neutron reflectivity was also employed to study the interdiffusion of polysulfone bilayers consisting of a small deuterated polysulfone and larger protonated polysulfone. These results demonstrate that the diffusion of the deuterated polysulfone is sterically hindered by the aromatic nature of the protonated polysulfone and that the structural differences between the two protonated polysulfones did not have a significant effect on the diffusive properties of the deuterated polysulfone. Additionally, when compared to other common polymers with the same molecular weight and at the same temperature relative to their glass transition, the diffusion of the deuterated polysulfone chains is ca. three orders of magnitude slower.