The influence of POSS nanoparticles on selected thermal properties of polyurethane-based hybrids
Journal of Thermal Analysis and Calorimetry
The influence of POSS nanoparticles on selected thermal properties of polyurethane-based hybrids
Tomasz M. Majka 0 1
Konstantinos N. Raftopoulos 0 1
Krzysztof Pielichowski 0 1
0 Department of Chemistry and Technology of Polymers, Cracow University of Technology , ul. Warszawska 24, 31-155 Krako ́w , Poland
1 & Krzysztof Pielichowski
Organic-inorganic hybrid materials, prepared via chemical synthesis route or physical blending of functionalized nanofillers within polymer matrix, have gained an increased attention in the recent years. Polyhedral oligomeric silsesquioxane (POSS) nanoparticles, due to their nanometer size and functionalization possibilities, are applied as effective modifiersboth chemical and physical, for polymer matrices, including polyurethanes (PU). In this work, we describe the synthesis and processing of polyurethane/POSS hybrid nanocomposites and discuss the influence of POSS moieties on the thermal properties of PU matrices. Glass transition, the crystallinity of the soft phase, as well as the order-disorder transitions are affected by the incorporation of POSS in the polyurethane structure. Direct polymer-POSS interaction, or -indirect-due to the suppression or enhancement of microphase separation by the POSS moieties are discussed in terms of topology of the polymeric structure with the key role of POSS functionalization.
Polyurethanes; Polyhedral oligomeric silsesquioxanes; POSS; Thermal properties; Nanocomposites
Polyurethanes (PUs) are widely used in different industrial
sectors due to their light weight and excellent thermal
insulation. However, thermoplastic polyurethanes (TPUs)
have lower modulus and strength when compared to metals
and ceramics [
], which may make PU plastics less
attractive as engineering materials. An effective way to
improve mechanical properties is by reinforcing the PU
matrix with fillers, such as platelets, fibers or nanoparticles.
Enhancement of a certain physical property of TPU by
incorporating macro-sized particles often increases
processing costs [
]. That’s why, a new class of hybrid
materials composed of polymers into which nanosized
inorganic particles are dispersed, could produce desirable
structural and functional properties without high processing
costs and without compromising other valuable properties.
Nanoparticles impart many functional properties, and PU
macromolecules provide structure and processability [
Numerous studies showed that nanometer-sized fillers have
a large surface-area-to-volume ratio and could be easily
dispersed in a PU matrix, hence facilitating the
enhancement of a desired property, such as barrier property, heat
resistance, modulus, strength and flammability [
In the technology of nanocomposites the degree of
dispersion is a critical factor. Moreover, in the case of
copolymers—as are polyurethanes—the targeted placing of
nanoparticles in one of the two phase-separated
components is often beneficial for the tailoring of properties. Both
issues are facilitated by the so-called nanobuilding block,
i.e., using functional reinforcing moieties which react with
the monomers or macromonomers, becoming thus parts of
the macromolecular chain itself.
Polyhedral oligomeric silsesquioxanes (POSS) are a
family of molecules, very appropriate for this approach.
They consist of a siliceous core with size less than a
nanometer, typically cubic with silicon atoms on the
vertices and oxygen on the edges. Their particularity is that on
every Si atom, an organic ligand or vertex group is
attached; in that respect POSS are organic–inorganic
materials. The vertex groups may contain one or more
reactive groups which allow covalent bonding to the host
matrix or may be non-reactive, properly chosen to tune
physical compatibility with the host polymer.
From the chemical point of view, polyurethanes are the
copolymers whose constituent units bind to each other with
the urethane bond. In the simplest case, the urethane bond
is formed by the addition reaction of a diisocyanate group
and a hydroxyl one [
]. This reaction is quite fast and very
intense, so polyurethane materials are easy to make from
the chemical point of view. In addition, recently
environmental and safety concerns motivated the design of
polyurethane materials from other kinds of monomers and
]. Elastomeric polyurethanes consist
of the so-called soft segments, typically an OH terminated
flexible polyether or polyester, and the hard segments,
typically a sequence of a diisocyanate and a short diol.
Proper selection of those monomers and macromonomers,
as well as small alterations, i.e., using multifunctional
polyols or isocyanates, allows for an enormous flexibility
in the final materials in terms of chemistry, chain
It is also obvious now that a proper selection of the
POSS substituents, e.g., with ligands with hydroxyl or
isocyanate functionality will allow for easy and effective
chemical incorporation of the POSS moieties in the
polyurethane structure and—most important—on the desired
location in the chain. The POSS particles could be
incorporated into the PU by different methods such as
copolymerization, grafting or melt blending methods. The melt
blending method with TPU has the advantages that
compounding could be carried out by using traditional
technology and thereby TPU/POSS nanocomposite with
significant properties could be obtained. Incorporation of
POSS nanoparticles in the polymer matrix could improve
the processability of TPU, the use temperature and
oxidation resistance, as well the thermooxidative stability [
Hence, in this review, we describe in detail the chemical
and processing methods by which POSS can be
incorporated in polyurethane matrices, in laboratory and industrial
scale. Moreover, we will explore the effects they have on
the thermal transitions of polyurethane matrices, paying
attention to the complex underlying mechanisms behind
Polyhedral oligomeric silsesquioxanes
(POSS)—definition and architecture
The term silsesquioxanes refer to molecules, which have
the basic composition of RnSinO1.5n, where R-group—the
so-called vertex group is a hydrogen atom, an inert organic
group such as alkyl, phenyl or isobutyl or even carry a
reactive group such as hydroxyl, carboxyl or an unsaturated
bond. POSS feature a Si atom at each vertex of a
polyhedral cage. The shape of the cage is typically cubic
(n = 8) but cages of 6–12 vertices are often noted. The Si
vertices are interconnected with–O–linkages. The R-group
is attached to each of the Si atoms and is pretty much
tailored to the specific application. The backbone of those
cages substituents determines to a large extent the
compatibility of these molecules to the polymer, possible
chemical reactions and thus the final structure of the POSS
composite. It will directly or indirectly alter, to an extent,
the thermal, barrier, flammability reduction and other
physical properties [
Polyhedral oligomeric silsesquioxane compounds with
well-defined cube-like structures have been extensively
researched as the nanoscale building blocks of organic–
inorganic hybrid materials. Optically transparent films of a
single POSS compound are rarely formed without
crosslinking reagents because of their high symmetry and
crystallinity. Araki and Naka reported that
dumbbellshaped POSS derivatives linked by simple aliphatic chains
can be used, depending on their aliphatic chains, to form
optically transparent films [
dumbbellshaped POSS compounds are the first examples of optical
transparent POSS films exhibiting thermoplastic properties.
As crystallinity and thermal characteristics of POSS
molecules are very much dependent on their organic
substituents, further studies are required to develop various
thermoplastic POSS derivatives with the desired properties.
Mitsudo and coworkers synthesized dendritic POSS
derivatives for application to homogeneous catalyst
supports . Wang et al. [
] also synthesized a star-shaped
POSS supramolecules. The examples of star- and
dumbbell-shaped molecular architecture are presented in Fig. 1.
Silsesquioxane cages are synthesized by hydrolytic
condensation, where the polyhedral Si–O core (of
completely condensed nanofiller cages) is formed by a
hydrolytic condensation of trifunctional monomers. These
monomers have usually structure of YSiX3, where Y is a
suitable organic substituent that will form the R-group, and
X is a highly reactive substituent (Cl or alkoxy group) [
Incompletely condensed cages with hydroxyl groups in the
so-called missing corner have also attracted some interest
both as standalone particles, to be introduced in polymer
matrices, and as precursors for fully condensed backbones
with a differing eighth vertex group.
It should be noted that reactions of trisilanols with
ligand-deficient trivalent-metal complexes, usually lead to
more complex structures because of the inability of these
trisilanols to support trigonal planar coordination
]. Silsesquioxanes, such as other highly
symmetric molecules, including dendrimers, interacts with the
matrix host in the three dimensions of the surrounding
space. The characteristic size of the POSS particle is
usually comparable to the dimensions of polymeric segments
but shows nearly double typical intermolecular spacing.
Incorporation of silsesquioxane moieties into linear
polyurethane chains or networks could modify the local
molecular interactions, segmental mobility and molecular
]. In that respect, POSS fill the gap between
conventional nanoparticles and large oligomers. In the
following, we will explore how this double nature affects in
an interesting and complicated way the properties of the
synthesis and processing
The building blocks used to obtain PUs, both
thermoplastics and thermosets, are diisocyanates and polyisocyanates,
as well as compounds with a broad array of molar masses
containing two or more hydroxyl groups. The isocyanate
(–N=C=O) and hydroxyl (–OH) groups react forming the
urethane group [
]. Two general methods are very
common for the production of PU/POSS hybrid systems:
melt blending and chemical synthesis.
According to the number of groups at the eight corners
of POSS molecules, chemical synthesis can be divided into
copolymerization, grafting, and crosslinking. POSS as
organic/inorganic hybrid material has the special nature of
surface effect and nano quantum size effects. When
incorporated into polyurethane, the nanoscale POSS
Si O Si
Si OO O Si OR O
Si O Si
O O O O
Si O Si
Si O Si
O RO Si OOO Si
Si O Si
O O O O
Si O Si
improves the thermal stability, water resistance,
flame-retardant behavior and mechanical strength of the
In melt blending, POSS and polyurethane are mixed by
physical blending through a solution method or melt
method. Usually, TPU pellets are mixed in blending device
in appropriate conditions such as high temperature above
melting temperature and with special selected speed of
screws. For example, in reference [
] melt method was
used to mix a TPU and POSS using a Brabender mixer
running in nitrogen flow at 50 rpm for 10 min. Although
physical blending is simple, the solubility parameters of
polyurethane and POSS are different. Nanoscale POSS
easily forms aggregates, which lead to macroscopic phase
separation and have a negative effect on the modification
As second option consists of POSS incorporation into
the main chain, with conventional chemical methods. This
is mainly carried out in a one or two step process,
following standard polyurethane chemistry procedures. In this
case, functionalized POSS, usually containing one or more
OH groups participate in the reaction and form urethane
bonds with diisocyanates leading to often complex
architectures which we are going to describe in the following.
The reaction may take place in the presence of small
amounts of a suitable solvent such as tetrahydrofuran
], N,N-dimethylformamide (DMF) [
ethyl ketone (MEK) [
], dimethylacetamide (DMAC)
] or even in solvent-free environments [
] in which
case the polyol dissolves the other components
(diisocyanate and POSS). The selection of the solvent is
obviously crucial for the effective reaction and good dispersion
of the POSS moieties. Following the standard PU synthesis
procedures, the reaction should be carried out in an inert
gas to avoid side reactions of the diisocyanates with water.
According to the type and number of functional groups
at the eight corners of POSS, the structures of the hybrids
formed by copolymerization can be divided into four
architectures of PU/POSS composites [
Non-reactive POSS molecules dispersed in PU matrix
as filler, POSS monomer containing only one reactive
group. The suspension type of POSS/PU
nanocomposites mainly used the T8X7Y type of POSS, where X is
an inert functional group, Y is a functional group with
two hydroxyl groups or two amino groups and POSS
suspends in the polyurethane segment by the
Bead type—POSS core with two reactive functional
groups is incorporated in the backbone of PU.
Pendant type—POSS molecules with a single reactive
functional group and can be polymerized as a monomer
Network or crosslinked type—synthesized by the
polymerization of a POSS cage containing
multifunctional polymerizable groups which will form
threedimensional networks. To obtain these type of
structure, it could be used amino-POSS, hydroxy-POSS,
isocyanate-POSS, epoxy-POSS or vinyl-POSS
In Fig. 2, architectures classification of PU/POSS
composites, depending on the type of POSS, was presented.
Among various works on PU/POSS materials, it’s worth
mentioning the Hsiao et al. work [
]. This time
polyurethane/POSS nanocomposites were prepared through the
reactions of POSS derivative featuring one corner group
substituted by either a 3-(allylbisphenol-A)
propyldimethylsiloxy or a hydridomethylsiloxy group. A
polyurethane system using an organic biodegradable PDLA soft
block and an inorganic diol-POSS hard block was obtained
by Mather et al. [
]. Pielichowski’s group in a series of
35, 39, 43–46
] synthesized and studied
polyurethane/POSS nanocomposites using a methylene
diphenyl isocyanate (MDI)—poly(tetramethylene ether
glycol) PTMEG-based polyurethanes with 1,4-butanediol
as chain extender. In this series, POSS particles with
different number and configuration of functional groups were
used leading to a plurality of chain architectures.
Turri et al. [
] synthesized a linear ionomeric PU/
POSS nanocomposite through a diol-functionalized POSS
macromer. More recently Huang et al. [
] reported on
systems based on natural resources, i.e., castor oil and
soycastor oil, combined with isophorone diisocyanate and a
double-decker POSS moiety.
Hydroxyl functionalized POSS moieties are far more
common in the literature; however, systems with
isocyanate functionalized POSS have been also reported:
Neumann et al. [
] and Mya et al. [
] synthesized a
POSS macromer with eight reactive isocyanate groups
((NCO)8-POSS) through hydrosilylation of
m-isopropenylR,R-dimethylbenzyl isocyanate with Si–H bonds of
Q8M8H. The moieties were then reacted and bound with
each other, with appropriate polyethers.
The biggest obstacle during the preparation of PU/POSS
nanocomposites is the aggregation tendency of the
silsesquioxanes. As they possess high specific surface area,
POSS may aggregate between themselves very easily, and
the aggregation may act as defect center and sometimes
leads to worsen the thermal and mechanical properties of
the polyurethane. The main question is, how we should
disperse POSS nanoparticles especially at higher loading
up to 10 wt.%. Nowadays, different methods are used to
disperse the silsesquioxanes into the polymer matrix, such
as shear mixing, mechanical mixing, in situ polymerization
and sonication, but some of them could be very
complicated. In order to get the necessary dispersion of POSS, the
surface of nanoparticles should be modified and/or
suitable compatibilizer should be used. High dispersion is not
easily achieved, but when the POSS nanoparticles are
homogeneously dispersed in the PU matrix, the full
potential of these nanomaterials could be exploited.
Regardless of the preparation approach, the dispersion and
self-assembly of POSS moieties inside the PU host are the
key factors affecting the final physicochemical properties.
If the POSS–PU interaction is favorable compared to the
POSS–POSS interaction, silsesquioxane moieties will
disperse well; otherwise, nanoparticles will aggregate. Unlike
a filled hybrid system or blend, however, proper
functionalization of POSS may limit aggregation due to
covalent attachment to the PU backbone, or an appropriate
choice of non-reactive R-groups, may prevent aggregation
beyond a scale of ca. one radius of gyration [
resulting effect on physicochemical properties will vary
with the silsesquioxane moieties dispersion or the
aggregation level. Therefore, it is of crucial importance to
understand the nanostructure—property—processing
relationships for given hybrid systems in order to successfully
tailor properties to intended applications [
inorganic ceramic or silica particles are immiscible in
organic systems because of poor specific interactions
within these organic–inorganic hybrid systems and the
negligibly small combined entropy contribution to the free
energy of homogenization. Specific intermolecular
interactions such as: interactions include hydrogen bonding,
dipole–dipole interactions and acid/base complexation are
required to enhance the miscibility of PU matrix with
inorganic particles [
The processing routes of typical polymer
nanocomposites reinforced with polyhedral oligomeric silsesquioxanes
were described briefly in reference [
Depending on the form of occurrence of polyurethane
matrix (resin, liquid substrates and thermoplastic pellets)
we distinguish several methods of processing.
Polyurethanes and its composites could be processing with
some main techniques such as:
• direct skinning,
• reaction injection molding (RIM),
• rebounded foam and spray foam.
The direct skinning process is mainly a combination of
TPU/POSS processing and injection molding. It brings
injection molding and reaction molding together in one
device to give the surface of the component a more opulent
appearance. The clue of this technique is that the
thermoplastic substrate produced conventionally is not demolded
after injection molding but remains on the mold core after
the mold is opened. The highly crosslinked thermoset of
PU/POSS composites obtained in this way displays not
only excellent heat stability but also very good chemical
17, 60, 61
Lamination production process is based on special
slicing devices long PU/POSS blocks which are cut to
sheet-products with the desired thicknesses of 2–5 mm at
different length and then rolled for semi-finished product.
In the next step, semi-finished product is used to the
lamination of the foams with the contour cutting and sewing.
This technique is used mainly to combination with
different textiles, leather or other trim materials [
Pultrusion is a major manufacturing technology for the
continuous production of POSS reinforced thermoplastics
polyurethanes with structural shapes despite that these
techniques are often used for PU hybrids with glass fiber.
The pultrusion process is achieved by a series of steps that
are geared toward creating a quality composite material
• continuous filler reinforcement, where rolls of filament
or fabric work are used to keep strength across the
profile of the product and the material is fed into the
machinery to begin the process,
• preforming guides where the composites material
spools and reinforcements are threaded into a machine
known as a tension roller,
• impregnation and resin bath where in impregnating
stage of the pultrusion may utilize different types of
resin, such as polyester and vinylester. In this step
pigments could be added into mixture to enhance the
product’s appearance. Catalysts that could also assist in
curing or solidifying the profile also find their way into
• exposure to heat source where the product enters a hot,
steel-forming die. This hot die is pivotal to the
pultrusion process as it creates the hard shape of the
• caterpillar pull mechanism and cutting saw there the
cured profile is now advanced along a pull mechanism.
The profile meets the cutting saw, where it is cut into
appropriate lengths [
Injection molding or blending is a common process for
polyurethane/silsesquioxanes composites. In many
applications, the mold release agent is applied to the mold, and
then a paint coating is applied over the mold release agent.
Then, the mold is closed and the injection take place. The
final hybrid product is removed fully painted. A closed
mold usually contains some form of channels after the
mixer gate designed to improve the laminar flow of
injected POSS systems at the entrance to the mold.
Injection molding differs from the open pour in contact angle
and mixing motion, which leads to different stresses on the
mold release agents. Other type of injection molding
processing is reaction injection molding (RIM). In RIM
process, a high-pressure impingement-mixed two component
stream is used into a mold or cavity, where nanoparticles
are dispersed in one of them with lower viscosity. These
liquid components—an isocyanate and a polyol—are
developed in two-part formulations, which are often called
polyurethane RIM systems. As it is presented in Fig. 3,
when injection of the liquids into the mold begins, the
valves in the mix head open. The liquid reactants enter a
chamber in the mix head at pressures between 100 and
200 bar, and they are intensively mixed by high-velocity
impingement. From the mix chamber, the liquid then flows
into the mold at approximately atmospheric pressure.
Inside the mold, the liquid undergoes an exothermic
chemical reaction, which forms the polyurethane polymer
in the mold. Shot and cycle times vary, depending on the
part size of filler and other components and the
polyurethane system used. An average mold for an elastomeric
part may be filled in one second or less and be ready for
demolding in 30–60 s. Special extended gel-time
polyurethane RIM systems allow the processor enough time to
fill very large molds using equipment originally designed
for smaller molds [
Release agents used in RIM processes need to be able to
resist movement and disruption during injection and still be
mobile enough to produce a slippery surface, once the PU/
POSS system has been cured. Additionally, the material
composition of the polyurethanes can interact differently
with the mold release agents. That’s why concentration of
POSS particles and its homogeneity come into play with
respect to the amount of material that is left behind by the
mold release agent. A low amount of residual material left
in the mold is highly desirable to reduce the cleaning
frequency of the mold. Depending on how the PU or PU/
POSS RIM system is formulated, the parts molded with it
can be a foam or a solid, and they can vary from flexible to
extremely rigid. Thus, polyurethane RIM processing can
produce virtually anything from a very flexible foam-core
part to a rigid solid part, with specific gravities ranging
from 0.2 to 1.6 [
Incorporation of silsesquioxanes into polyurethane
matrix give these composites barrier properties, thus these
hybrid materials have much better thermal insulation than
conventional PU foams. The PU/POSS line of spray
polyurethane foam insulation and roofing systems could
provide a high R-value and the ability to reduce air
intrusion, accumulation, radiative heat transfer and air
movement. The application technology allows the spray PU/
POSS foam to expand, filling cracks, crevices and voids.
This creates a ‘‘seamless’’ air barrier system that provides
exceptional building envelope performance. In order to
utilization of PU wastes, there is some type of recycling
process used called rebounded foam processing. In this
technique, PU wastes are shredded into flakes, mixed with
nanofiller and then sprayed with an isocyanate-terminated
prepolymer and chemically bound to the required grade of
foam using a superheated steam injection process. Despite
intense efforts to develop alternative technologies such as
hydrolysis, glycolysis, adhesive pressing or even grinding
into powder producing hybrids rebounded foams is the
Return line 1
Return line 2
Stirrer motor 1
Nanofiller feed tank
Stirrer motor 2
method of material recycling with the greatest technical
and economic potential [
62, 63, 70
Polyurethane formations could be monitored in situ to
obtain the structural information during the gelation
process. This technique investigates the connectivity of
polyurethane as a function of time, temperature and
composition by measuring storage and loss modulus during
gelation. The gel point of polyurethane during the gelation
process could also be obtained by rheological experiments.
This characteristic moment is reached at a critical degree of
crosslinking, when the largest connected cluster diverges to
For the preparation of PU nanocomposites reinforced
with POSS by processing in melt, mainly a Brabender-type
mixer was applied [
]. According to this reference,
thermoplastic polyurethane was mixed in melt with 10 wt.% of
poly(vinylsilsesquioxane) using a Brabender device
operating at 50 rpm for 10 min at 180 C under nitrogen flow.
Mass loss calorimetry results revealed a large reduction of
the maximum peak of heat release rate (PHRR) in
thermoplastic composites as compared to pure TPU. The
intumescent material has been found to be composed of
ceramified char made of silicon network in a polyaromatic
structure. This intumescent PU nanocomposite acts as a
thermal barrier at the surface of the substrate limiting thus
the heat and mass transfer as evidenced by lowered HRR.
There is also known novel method to prepare PU/POSS
hybrids by melt reactive blending, proposed by
Monticelli et al. [
]. This practical approach is based on
the reaction between OH groups in functionalized
oligomeric silsesquioxanes molecules and the isocyanate
functional groups, which could formed during the melt blending
Polyol feed tank
Supply line 2
Hydraulic Drive 2
Heat Exchanger 2
Water in 2
Water out 2
through a controlled scission of the thermoplastic
polyurethane. These nanosystems were then prepared by
mixing at 220 C under inert gas atmosphere for 10 min the
neat polymer and trans-cyclohexanediolisobutyl or
octaisobutyl POSS at concentrations up to 20 wt.%. Results
showed an increase in glass transition temperature and
better surface water wettability—water contact angle
decreased from 95 for pure matrix to 70 in nanosystems
with 10 wt.% of nanofiller.
Lopes et al. [
] also proposed novel method to
processing PU/POSS composites and additionally explained
how changes in the thermal behavior reflect in rheological
properties of dispersions systems. The addition of
silsesquioxanes nanoparticles showed a clear tendency
toward higher intrinsic viscosities as the amount of the
chain modifier was increased. Samples tended toward a
linear pseudo-plastic behavior, with a higher amount of
nanofiller leading to higher intrinsic viscosities at all shear
rates evaluated. Nanda et al. [
] confirmed similar
behavior in polyurethane hybrid systems, as well. They
observed that the incorporation of POSS into the TPU
backbone produced a significant change in the viscosity.
The viscosity increases linearly with nanoparticles
concentration, corroborating the reinforcing efficiency of
nanofiller in the matrix backbone. Authors observed that
the viscosity of PU/POSS composites at low frequencies
was significantly higher than that obtained at high
frequencies. These results suggested that the viscosity
strongly depends on frequency, revealing the
non-Newtonian behavior of TPU/POSS hybrid systems, and POSS
particles restraint the movement of flexible domain
probably by the formation of silicate layers. Besides,
Nanda et al. [
], noted that the incorporation of POSS
with eight OH functional vertex groups in polyurethane
films resulted in higher thermal stability and crosslink
density. Aqueous polyurethane dispersions with
functionalized POSS were prepared through homogeneous solution
polymerization by the use of acetone as the initial
polymerization solvent enabled the facile incorporation of both
diamine- and diol-functional POSS monomers
2,3propanediol propoxy-hep-taisobutyl-POSS, respectively).
Thermal properties of PU/POSS hybrid
Up to now, it should be clear that polyurethanes, even in
themselves, are quite complex systems and the possible
combinations of macromolecular components, flexible
segments, diisocyanates and chain extenders, can give a
large number of possible systems that differ in chemical
properties, architecture, rigidity, mechanical modulus, and
of course molecular mobility. The situation gets even more
complex given the extended number of available POSS
moieties and the various techniques used for their
incorporation in the matrices.
Before discussing the effects of POSS on the thermal
properties, we would like to comment briefly on some
principles that govern the thermal behavior of
polyurethanes, and apply to virtually all members of the family.
We remind here that as a general rule, the hard segments
tend to segregate and form hard microdomains, often
crystalline. Those hard microdomains are distributed in a
soft phase consisting predominantly of soft segments, but
also contain a non-negligible amount of diluted hard
segments. The final micromorphology depends strongly on the
chemical nature of the components, the geometry of the
chain, and the thermal history of the material. The resulting
degree of microphase separation is crucial to the thermal
properties to be discussed immediately.
In principle, the thermal transitions expected in a
polyurethane system are the glass transitions of the soft and
hard microdomains, the melting/crystallization of the soft
component where possible, and the order–disorder
transition, i.e., the dissolution of hard segments in the soft matrix
Being essentially copolymers, phase-separated
polyurethanes exhibit in principle two glass transitions: one of
the soft phase and in principle one of the hard
microdomains. The latter though may be very weak, practically
undetectable, especially in the case of aromatic
0.05 Wg endo
Fig. 4 Reversing (continuous lines) and non-reversing (dashed lines)
components of the modulated DSC signal of a model polyurethanes
and its hybrids with crosslinking POSS moieties, in a broad
temperature range. The curves are typical of segmented
polyurethanes: The glass transition of the soft domains is observed as a step
in the reversing component at subambient temperatures, while at
higher temperatures the order disordered transition is evident as a
series of endotherm peaks, mainly in the non-reversing component
]. Copyright 2015. Reproduced with permission from American
diisocyanates because of the rigidity of the chains and
possible strong crystallinity. This one has rarely been
studied in the literature [
]. However, a broad endotherm
peak between the glass transitions of the bulk hard and soft
segments is often related to the glass transition or softening
of the hard microdomains.
The soft phase glass transition though has attracted a lot
of attention because it is essentially the main transition of
the system. Its temperature Tg lies, as expected, above the
Tg of the soft segments. The temperature rise of Tg is
discussed usually in terms of slowing of dynamics because
of the diluted hard segments, according to the mixing laws
which apply to blends and copolymers, i.e., a poorly
phaseseparated polyurethane will have a higher soft phase Tg
than a well phase-separated one, all other parameters
(chemistry—composition) being the same [
The effect on POSS on the molecular dynamics of the
soft phase is a challenging question. So far, investigations
by us and others, on which we are going to refer in detail in
the following, has shown rather increasing trend of the soft
matter glass transition temperature Tg with increasing
POSS content. An increasing Tg is associated with slowing
of dynamics. However, the extent of this increase and the
mechanisms driving it depend strongly on the type of the
incorporated POSS, the chain architecture and of course the
amount of POSS diluted in the soft phase, and their degree
of dispersion. That said, we would like to point out that the
mechanisms by which POSS affect the dynamics of any
polymer—not only copolymers like PUs—are very
]. The reason is that POSS themselves are moieties
with size smaller than a nanoparticle and comparable to an
oligomer: their siliceous core is of size * 0.5 nm [
smaller than that of a conventional filler. It is thus expected
and observed that moieties which cannot crystalize present
their own glass transition [
]. So, if the moieties dilute
well in a polymer matrix they behave as simple diluents in
the sense of the mixing models like that of Fox . On the
other hand, most of the moieties are strongly crystalline. In
this case, they tend to form crystallites, often of the size of
nm which essentially behave as nanoparticles, and affect
dynamics in the same way conventional nanofillers do.
Returning to the specific question of polyurethanes with
POSS, Fig. 5 summarizes our investigations on a
polyurethane matrix with methylene diphenyl isocyanate (MDI)
and butanediol (BD) forming the hard domains, and
poly(tetramethylene ether glycol) with Mw * 1400
forming the soft domains. It shows the rise of calorimetric Tg on
addition of various POSS with respect to the matrix. In this
series of investigations, the POSS substituents have been
incorporated covalently in three distinctively different
chain architectures. Namely, they have been bonded as (1)
pendent groups on the hard domains, using the POSS
substituents with one bifunctional ligand [
(2,3Propanediol)propoxy-heptaisobutyl POSS), (2) as part of
the main chain, forming a bead-like copolymer, using an
open cage with OH groups attached to the two
‘‘disconnected’’ Si atoms  (disilanol octaisobutyl POSS), and
(3) as crosslinks, forming a polyurethane network, using an
octafunctional moiety [
methylbutyldimethylsiloxy) POSS). Note that all inert vertex
groups in these cases are isobutyl, which in principle drives
to strong crystallization. A fourth system was prepared by
blending a non-functional moiety during the polyurethane
Fig. 5 Tg increase in a common polyurethane on addition of four
different POSS moieties resulting on different chain architectures.
Data compiled from [
34, 35, 39, 45
]. Copyright Cracow University of
Technology 2015. Reproduced with permission from 
reaction. A PEG (Mw * 600) substituted moiety was
chosen for this investigation, because
octaisobutyl-substituted POSS were immiscible to the system. In all the
systems, the POSS content varied up to 10 wt.% with respect
to total polyurethane mass.
The pendent POSS cause a moderate but consistent
increase in Tg (Fig. 5) by a few degrees. A comparative
study of the specific heat increase at Tg as well as dielectric
and morphological properties [
] showed that both direct
and indirect effects drove this slowing of dynamics. For
polyurethane with shorter segments (PU1000), POSS
increased miscibility of the hard domains in the soft phase,
while at larger segment lengths, POSS crystallites,
suppressed the chains which were anchored on them and even
formed a rigid amorphous fraction.
In the PU copolymer with POSS incorporated on the
chain as ‘‘beads’’, the soft calorimetric Tg remained
practically unaffected (Fig. 5). However, a closer investigation
by dielectric methods showed that a slower segmental
relaxation, with weak, if not nonexistent, thermal footprint
was severely enhanced on addition of POSS (Fig. 6). This
relaxation present (but weaker) in all the systems, including
the matrices, is due to soft chains anchored on bulky
structures (hard domains, POSS crystallites, soft segment
crystallites). The morphological investigation indeed
showed the existence of domains consisting of PU chain
extended practically only with POSS.
The only prominent increase occurred with the
crosslinking moiety. At a first glance, this is hardly
surprising: Crosslinking is expected to restrict dynamics.
However, the Young’s modulus at the rubbery phase
decreased by an order of magnitude despite the
10–2 10–1 100 101 102 103 104 105 106
Fig. 6 Dielectric spectra of a polyurethane and its hybrids with the
POSS cage along the chain contour, as ‘‘beads,’’ at - 15 C. A slow
relaxation a0 (10 Hz), attributed to segmental dynamics of anchored
chains accompanies the slower main a relaxation (10.000 Hz)
corresponding to the dynamic glass transition. The locations of the
two relaxations are shown for a representative sample [
2013. Reproduced with permission from American Chemical Society
crosslinking. This was attributed to the severe reduction of
microphase separation, which was confirmed also by SAXS
and other methods [
]. In this view, it is concluded that
the hard domains are better reinforcing agents as compared
to the POSS moieties.
Finally, the PEG-POSS blended in the system, caused
even an acceleration of dynamics as manifested by a
decrease in Tg. We believe here, that this was the effect of
the flexible PEG chains well diluted in the soft phase
according to the mixing laws [
The aforementioned studies show the crucial role of the
chain architecture and the mode of incorporation of POSS.
Despite the chemical similarity of all the systems the
behavior of dynamics with incorporation of POSS is much
different, not only in magnitude but also in the underlying
mechanisms. A point to stress though is that the
aforementioned slow dynamics (a0 relaxation), with the strong
dielectric but weak thermal response, seem to be very
similar for chains anchored on much different rigid
In addition, the role of the chemistry of the bonding was
demonstrated in [
]. In this work, POSS were bonded with
urea bonds as opposed to urethane. Urea groups have a
much stronger tendency to aggregation; hence, the
microphase separation was promoted to some extent, and
counteracted the slowing down of dynamics. The counteracting
phenomenon was so strong that for longer segments, the
calorimetric Tg even decreased by a few degrees.
Interestingly, the slow dynamics a0 were more intense in this
In a very similar PTMG-MDI-BD polyurethane, pendent
POSS, but with cyclopentyl inert vertex groups, caused an
increase in the calorimetric Tg very similar in magnitude to
the one by POSS with isobutyl inert groups. Here again, a
weakening of microphase separation was observed [
Slightly more intense increase in Tg was reported for a
polycaprolactone-based PU with pendent POSS
substituting their macrodiol with pendent POSS similar to those in
]. More prominent increase in Tg was reported for
systems where POSS with a cyclohexyl ring on their sole
reactive vertex group, were pendent on a polypropylene
oxide (PPO)–MDI–BD system [
]. The inert vertex
groups were isobutyl, as in the previous works. The POSS
here substituted part of the macrodiol as opposed to the
previous system where POSS substituted part of the chain
extender. TEM here showed an excellent dispersion, and
DMA a weakening of the hard segment-related
phenomena; hence, we believe that the increase here is rather due
to improved microphase mixing.
The same particle was introduced in a polybutadiene–
isophorone diisocyanate–BD system by substitution of the
chain extender by a sol–gel technique [
]. Although the
dispersion was in the order of micrometers, the glass
transition temperature increased significantly. Increase in
Tg was reported also for a system moderately crosslinked
by a trisilanol open-cage moiety [
]. Despite the
crosslinking, the authors based on the relative stability of
the activation energy of the dynamic glass transition
conclude that the reduction of mobility is related to
nanocrystals formed by the POSS moieties.
Recently, Huang et al. showed that the mechanisms of
POSS may be concentration dependent: In a castor
oilbased system incorporating a double-decker POSS, less
than 1.5 wt.% of POSS reinforced the system, leading to an
increase in Tg by more than 30 K, as compared to the
matrix; however, beyond that concentration aggregates
were formed and acted as plasticizers, increasing the
mobility of the system and decreasing Tg [
Crystallization of the soft phase
Similar to the glass transition the degree of microphase
separation governs to some extent the ability of the soft
segments to crystallize, provided that they are crystalline in
their bulk form. The soft segment polyethers or polyesters
in bulk have melting temperatures below 100 C. If the PU
segments are short, then the hard segments that intervene in
the chain, introduce a disorder and crystallization is not
possible. However, longer segments, typically of molar
mass greater than 2000, can fold and eventually crystalize
]. The melting temperature and degree of
crystallinity increase with the molar mass of the segments,
reflecting an increase in the quality and size of the
In our work, we have observed that POSS pendent on a
PTMG-based polyurethane affect crystallinity in a way
very much dependent on the nature of the chemical bond
used for their incorporation on the chain [
]. The PU
chain in this case contained a PTMEG chain of
Mw * 2000, which in bulk form crystalized 25 K below
the melting temperature of the bulk PTMEG. POSS
tethered on the chain with urethane linkage suppressed
completely the crystallinity at content as low as 4 wt.%, as a
result of the increased disorder—disruption of microphase
separation. The POSS linked with urea bond though,
improved the microphase separation, increased the purity
of the soft phase and as a result, the crystallinity was
On the other hand, a new component in the system,
unavoidably would introduce some disorder and therefore
reduce the ability of chains to fold and organize
themselves. In that respect, a suppression of crystallinity was
observed also in polycaprolactone-based polyurethane
elastomers, when the POSS substituents moiety with
isobutyl as inert vertex groups was pendent on it [
happened, albeit the polyurethane in this case was not
phase separated, and no significant effect on glass
transition by the POSS was observed.
Maybe the most common transition of polyurethanes is the
order–disorder transition observed at temperatures well
above the glass transition of the hard domains. During this
transition, the hard domains vanish and dissolve in the soft
phase forming a homogeneous material. This transition is
rather complex and happens in more than two steps [
is usually discussed in terms of dissolution of the hydrogen
bonding network between the urethane groups of the chain,
and therefore it resembles a lot melting, albeit it is not
always the case that the hard domain are actually
crystalline. In calorimetric experiments, it manifests itself as a
broad endothermic peak with multiple components.
In our investigation, we have observed that POSS
bonded to the chain as pendent groups, using urethane linkages
] did not impose any significant change in the
temperature of the transition, but its enthalpy was moderately
suppressed, indicating that the quality of the hard domains
did not change, i.e., the POSS likely did not penetrate the
hard domains. However, urea bonds were used for the
covalent attachment, the hydrogen bonding increased and
led to hard domains with higher dissolution temperature.
Crosslinking POSS caused an increase in the temperature
of one of the components of the transition but the other one
remained unaffected [
]. The strength of the two however
decreased with increasing POSS content. This can be
attributed to the formation of less but sturdier hard
domains. Finally, when the POSS where introduced as
beads on the macromolecular chain, the components of the
peak did not change position in the temperature domain but
their relative intensity seemed to change significantly.
We reviewed in this work, the most common methods for
the production of polyurethane–POSS hybrid elastomers,
either by chemical synthesis or by melt blending. In this
process, the importance of the degree and length scale of
dispersion of the moieties, in the molecular level or in
nanosized-crystals, was stressed. The methods for
improving it were also discussed.
Several transitions occur in polyurethanes, namely the
glass transitions of the hard and soft components, the
crystallinity of the soft phase, as well as the order–disorder
transition (also related to the melting of the hard domains).
All of them—with the exception possibly of the hard
domain glass transition—are effected by the incorporation
of POSS in the structure. The effects are either due to direct
polymer–POSS interaction, or -indirect—due to the
suppression or enhancement of microphase separation by the
POSS moieties. In this very complex interplay, the
topology of the polymeric structure plays a key role and is
governed by the functionality of POSS moieties.
Acknowledgements This work has been financed by the National
Science Centre of Poland under Contract No. DEC-2011/02/A/ST8/
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
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appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
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