Hydrogen-Bonding Interactions in Hard Segments of Shape Memory Polyurethane: Toluene Diisocyanates and 1,6-Hexamethylene Diisocyanate. A Theoretical and Comparative Study

In this study, the hydrogen-bonding interactions of three widely used diisocyanate-based hard-segment (HS) models in polyurethane, 2,4-toluenediisocyanate–methanol (2,4-TDI-MeOH), 2,6-toluenediisocyanate–methanol (2,6-TDI-MeOH), and 1,6-hexamethylenediisocyanate–methanol (HDI-MeOH), were investigated theoretically by density functional theory (DFT). The B3LYP/6-31G* method was used to calculate the equilibrium structures, Mulliken charges, hydrogen-bonding energies, and infrared (IR) spectra, in good agreement with previous experimental data. The HS models with benzene ring have much longer hydrogen bonds (HB), due to steric hindrance of benzene ring, whereas the aliphatic model forms much shorter hydrogen bonds. Different positions of the methyl group on the benzene ring for 2,4-TDI-MeOH and 2,6-TDI-MeOH result in different types of hydrogen bonds with various strengths. The style of hydrogen bonding for HDI-MeOH is more flexible due to simple aliphatic chemical structure without the benzene ring. The ch...