Received: 11 December 2017 Revised: 12 January 2018 Accepted article published: 24 January 2018 Published online in Wiley Online Library: 22 February 2018
(wileyonlinelibrary.com) DOI 10.1002/pi.5539
Reaction-induced phase separation
in hexamethylene diisocyanate-based
poly(propylene oxide)/poly(ethylene oxide)
bi-soft segment oligomers
Annette M Schmidt
and Marc C Leimenstoll
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 monitoringby 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.
© 2018 Society of Chemical Industry
Keywords: polyurethanes; phase behaviour; reaction-induced phase separation; soft segment
Since their discovery, polyurethanes (PUs)
all ﬁelds of polymer applications like isolating foams, thermoplas-
tic parts, adhesives, elastomers and coatings, and are even being
considered as potential materials for life-science applications.
The versatility of PUs is essentially a result of the huge variabil-
ity in combining raw materials such as aliphatic or aromatic
polyisocyanates, diﬀerent oligomeric or polymeric polyols (e.g.
polyethers, polyesters, polycarbonates, polysiloxanes) and/or
low-molecular-weight polyols (e.g. 1,4-butanediol, glycerol).
combination of reactants may result in speciﬁc morphologies,
primarily determining the material characteristics like mechanical
properties or optical appearance.
Thus, a manufacturer has
the opportunity to tailor the properties of PU materials simply by
choosing appropriate combinations of educts.
It is well known that dissimilar polarities of reactants have a
marked impact on their miscibility and may cause heterogeneous
If such inhomogeneity appears during the
progress of a reaction, the phenomenon is commonly referred to
as reaction-induced phase separation (RIPS). In fact, RIPS is dis-
cussed as an eﬀective process for producing unique morpholo-
gies like co-continuous or spherical domain structures in resulting
In such cases the reaction starts usually as a homo-
geneous mixture and remains homogeneous until the proceeding
reaction forces the system to enter metastable/unstable regions
of the phase diagram, eventually leading to phase separation. In
the case of segmented PUs, incompatibility is purposely induced
in order to obtain domain structures by phase separation of hard
and soft segments.
Most commonly this is accomplished by
conversion of diisocyanates with a polar low-molecular-weight
polyol (e.g. 1,4-butanediol, chain extender) and a less polar macro-
molecular or oligomeric polyol as the soft segment forming
and is often designated as a balance between com-
patibility and incompatibility.
PUs consisting of at least two soft segment forming polyols
are commonly referred to as bi-soft segment
or mixed soft
PUs. These systems show improved mechanical prop-
erties (e.g. tensile strength)
and enhanced control of microphase
separation can be achieved by utilization of soft segments with
Such materials are potentially employable
as shape-memory polymers
the resulting properties of bi-soft segment PU-based membranes
are based essentially on the interaction between the two soft
Correspondence to: MC Leimenstoll, Macromolecular Chemistry
and Polymer Technology, TH Köln, 51368 Leverkusen, Germany.
a Macromolecular Chemistry and Polymer Technology, TH Köln, Leverkusen,
b Department of Physical Chemistry, University of Cologne, Cologne, Germany
Polym Int 2018; 67: 481–489 www.soci.org © 2018 Society of Chemical Industry