Properties of segmented polyurethane zwitterionomer elastomers

Four series of polyurethane zwitterionomers based on different soft segment polyols [polyethylene oxide (PEO), polypropylene oxide (PPO), polytetramethylene oxide (PTMO), and polybutadiene (PBD)] were synthesized, and their properties were investigated using differential scanning calorimetry, dynamic mechanical spectroscopy, infrared dichroism, and stress-strain testing. Two different molar ratios of hard segment [4,4′-diphenylmethane diisocyanate (MDI)] to chain extender [N-methyl diethanol-amine (MDEA)] to soft segment polyol, each with three different levels of ionization, were prepared based on the four different polyols. Zwitterionization was accomplished by quaternizing the tertiary amine of MDEA with γ-propane sultone. For the PBD materials, the un-ionized samples exhibited a high degree of phase separation, and ionization served mainly to improve domain cohesion. Increasing ionic content improved material strength while sacrificing extensibility. For the polyether materials the un-ionized samples were not highly phase separated, and ionization improved both the degree of phase separation and hard segment domain cohesion. These morphological changes led to greatly improved mechanical properties. The tensile properties of the ionized PEO and PTMO materials were superior to the tensile properties of the PPO and PBD materials. This was attributed to the ability of the PEO and PTMO segments to crystallize under strain. The effects of segment length (by comparison with a previous study) and hard segment content on mechanical properties are also discussed.