FULL PAPER 1700544 (1 of 8) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de Prediction of Initial Miscibility for Ternary Polyurethane Reaction Mixtures on Basis of Solubility Parameters and Flory–Huggins Theory Christian Wenning, Stéphan Barbe, Dirk Achten, Annette M. Schmidt, and Marc C. Leimenstoll* C. Wenning, Prof. S. Barbe, Prof. M. C. Leimenstoll Macromolecular Chemistry and Polymer Technology TH Köln, Kaiser-Wilhelm-Allee E39, 51368 Leverkusen, Germany E-mail: Marc.Leimenstoll@th-koeln.de Dr. D. Achten Coatings, Adhesives & Specialities Covestro Deutschland AG Kaiser-Wilhelm-Allee, 51368 Leverkusen, Germany Prof. A. M. Schmidt Department of Physical Chemistry Universität zu Köln Luxemburger Straße 116, 50939 Köln, Germany DOI: 10.1002/macp.201700544 induced in order to obtain tailored mor- phologies by phase separation of hard and soft segments. [4] Most commonly this is accomplished by conversion of a polar low molecular weight polyol (e.g., 1,4-butane- diol, chain extender) and a less polar mac- romolecular or oligomeric polyol (soft seg- ment forming component) with diisocy- anates. It was shown that the mechanical properties of such TPUs can be improved by addition of a second soft segment forming polyol leading to products known as bi-soft segment PUs. [5,6] For instance, mixtures of different soft segment struc- tures lead to increased tensile strengths of the resulting chain extended TPUs in comparison to single soft segment TPU analogs. [5] This finding was recently sup- ported and described by a coarse-grained simulation. [7] The results indicate that the beneficial effect relies on a better control over microphase sepa- ration due to application of soft segments with different polari- ties. Hence, tailored interactions between soft and hard seg- ments allow for specific morphologies and is used for a wide range of applications, e.g., to produce materials shape-memory polymers [8] to polymeric membranes. [9] Despite the vast number of published investigations aiming on single soft segment TPUs [3,10a,b] the kinetics and thermody- namics of phase separation in bi-soft segment PUs are quite unexplored. Blending different soft segment forming polyols creates further complexity in phase separation but allows for tailoring the properties of the resulting bi-soft segment PUs. In order to ensure a proper process control it is thus highly desir- able to provide an easily accessible approach that allow for the estimation of reactant miscibility. The synthesis of PUs is often achieved by a two-step pro- cess. [3] In the first step, soft segments react with a diisocyanate to form the so-called prepolymer. Subsequent chain extension leads to the desired high molecular weight polymer. To date there is little information about the predictability of phase behavior in diisocyanate/bi-soft segment mixtures. In our study we focused on the miscibility of polyether polyols (PEPs), namely, poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPG) with 1,6-hexamethylene diisocyanate (HDI). Aiming at a description of the ternary system PPG/PEO/HDI, it was essen- tial to start with the phase behavior of binary PEP mixtures. For Polyurethane Reaction Mixtures The initial miscibility in bi-soft segment polyurethane reaction mixtures is investigated on the basis of Flory–Huggins (FH) theory and solubility para- meters (SPs). Different group contribution theories are applied to estimate the SPs of polyether polyol (PEP) structures and 1,6-hexamethylene diisocy- anate (HDI). The PEP structure comprising starter molecule, ethylene oxide/ propylene oxide moieties, and end-functional hydroxyl groups is considered in the calculations. Binodal curves are determined computationally for binary PEP mixtures as well as for a ternary mixture consisting of HDI and a binary mixture of partially miscible PEPs. The computed phase equilibria are found to be in good agreement to experimental cloud points and binodal compositions. The prediction of miscibility for practically relevant ternary polyurethane systems on the basis of FH theory and SPs is thus feasible. Additionally, the study demonstrates that HDI is acting as a compatibilizer for partially miscible PEPs in the initial reaction mixture. 1. Introduction It is well known in polyurethane (PU) chemistry that unequal polarities and thus incompatibilities between the reactants (i.e., oligomeric polyols, low molecular weight polyols, and polyiso- cyanates) can lead to heterogeneous reaction mixtures. [1] Thus, compatibility and incompatibility are crucial phenomena in PU synthesis. [2] The properties of the resulting products can strongly ben- efit from the occurring phase separation and is technically used for instance in segmented PUs (e.g., thermoplastic PUs (TPUs)). [3] In case of TPUs, the incompatibility is purposely Macromol. Chem. Phys. 2017, 1700544