Effect of ionic group content on thermal and structural properties of polycaprolactone-based shape memory polyurethane ionomers

In this work, the effect of ionic group content on thermal, structural, and shape memory properties of segmented polyurethane ionomers based on polycaprolactone diol, diphenylmethane diisocyanate, 1,4-butanediol, dimethylol propionic acid, and triethyl amine was investigated. Fourier transform infrared spectroscopy was used to check the accomplishment of the synthesis. Thermal, structural, and shape memory properties of the synthesized shape memory polyurethanes were measured using different techniques such as differential scanning calorimetry, thermogravimetric analysis (TGA), wide-angle X-ray diffraction, and tensile cyclic tests. It was found that the ionomers with higher amount of ionic groups have relatively stronger crystallization tendency. In addition, the glass transition temperature of soft segments was influenced by the degree of phase separation. It was observed that the degree of phase separation improved when the amount of ionization was higher than a critical value. The increased polarity difference of soft and hard phase upon ionization could be the driving force for increased phase separation. The results obtained from TGA showed that the thermal stability of the ionomers was lower than that of the non-ionomer sample. This can be attributed to the reduction of hydrogen bond formation and decrease of cohesion in the hard domains of ionomers. In addition, ionic group content affected the shape recovery of the material, where this parameter increased with the increase of ionic groups.