Reaction‐induced phase separation in hexamethylene diisocyanate‐based poly(propylene oxide)/poly(ethylene oxide) bi‐soft segment oligomers

Abstract

Oligomeric bi-soft segment isocyanate-terminated polyurethanes (ITPUs) are semi-finished materials crucial for the synthesis of various PU products like foams, thermoplastic parts, dispersions or elastomers. Incompatibilities and thus phase separation phenomena play an essential role in tailoring the properties of the final products. Therefore, a detailed knowledge of these phenomena is mandatory in order to design products with properties meeting the requirements of a given application. In this study the reaction-induced phase separation during the formation of ITPUs by application of two partially miscible soft segments is presented. The physicochemical basics of this process as well as the extent of the resulting phase separation are discussed on the basis of the initial phase diagram of the reactants. Reaction monitoring by NCO content titration and UV-visible spectroscopy reveals a dependency between the onset of phase separation and conversion. It is found that an increase of the initial content of hexamethylene diisocyanate delays the onset of phase separation. Differential scanning calorimetry reveals further that the phase separation is a direct consequence of the incompatibility of the soft segments. Overall, the findings support the hypothesis that the mechanism and the extent of phase separation are closely related to the ternary phase diagram of the reactants.

Evolution of Reaction‐Induced Phase Segregations in Bi‐Soft Segment Urethane Oligomers

“…RIPS observed in ternary bi‐soft segment polyurethane systems such as PPO/PEO/HDI mixtures is suggested to be a direct consequence of the incompatibility of the soft segments with the phase segregation following the ternary phase diagram of the reactants 16. Here, RIPS may be caused essentially by two different structural arrangements.…”

“…Isocyanate Terminated Polyurethanes (ITPUs) with mixed soft‐segments were prepared applying the procedure published elsewhere 16. Corresponding amounts of dried polyols ( Table 1 ) were introduced into a 250 mL reaction vessel that was heated to 80 °C.…”

“…Corresponding amounts of dried polyols ( Table 1 ) were introduced into a 250 mL reaction vessel that was heated to 80 °C. To minimize the difference in reaction time and to prevent undesired exothermic heat, DBTDL as the catalyst was dosed as reported earlier 16. Successively, HDI was added in a molar ratio of NCO to OH groups of 3:2 and the reaction was allowed to proceed until the theoretical NCO content (NCO% theo ) is reached.…”

“…The mixture was held at 80 °C and 50 mbar for 2 h to remove water traces. Then 18.5 µL of the catalyst solution according to ref 16. was added.…”

“…Therefore, a deep understanding of the underlying thermodynamics of RIPS is essential to tailor bi‐soft segment PUs. First approaches provide phase diagrams and initially miscibility of partially miscible ternary bi‐soft segment reaction mixtures 15,16…”