Reaction-Induced Phase Separation of PPG/PEO/HDI based bi-Soft Segment Polyurethanes

The presented thesis explores the physicochemical background of reaction-induced phase separation (RIPS) in bi-soft segment isocyanate-terminated polyurethanes (ITPUs). ITPUs are a preliminary product in the synthesis of various PU products like foams, dispersions or elastomers. Typically, only one soft segment is used but it was shown in the literature that the application of two different soft segments can lead to an improvement of morphological properties. In this work, it is hypothesized that the combination of two soft segments can lead to RIPS during the ITPU formation process. It is suggested that the occurrence and the extent of phase separation is closely related to the initial phase diagram of the reactants. In order to study the proposed relationship, the initial ternary phase diagram of the reaction mixture, consisting of poly (ethylene oxide) (PEO), poly (propylene oxide) (PPG) and 1,6-hexamethylene diisocyanate (HDI) is determined at first. Secondly, detailed experimental observation of the RIPS is conducted to identify critical parameters. Furthermore, the composition and morphology of the phase separated products are studied briefly. The ternary phase diagram is determined by theoretical and experimental methods. Flory-Huggins theory and solubility parameters are applied in order to calculate equilibrium compositions of demixed phases in the binary (PEO/PPG) and ternary (PEO/PPG/HDI) mixtures. The obtained data is in qualitative agreement with experimental cloud points and equilibrium compositions. The results demonstrate that HDI acts as a solvent for PEO and PPG. Reaction monitoring by NCO%-content titration, FTIR- and UV-Vis spectroscopy revealed a dependency between the onset of phase separation and the reaction conversion. It is found that an increase of the initial HDI content leads to a delayed onset of phase separation. The competing second order kinetic of the reaction is studied by 1H-NMR analysis. DSC analysis reveals that the phase separation is a consequence of the incompatibility of the soft segment structures. The composition and molar weight distribution found in isolated phases of the ITPUs indicate that the phase separation is controlled thermodynamically. Overall, the findings support the hypothesis that mechanism and extent of the phase separation are closely related to the ternary phase diagram of the reactants. This work comprises a comprehensive description of the phase behaviour during the reaction towards bi-soft segment ITPUs and provides a basis for future studies on this topic.