Thermal stability of gamma-irradiated polyurethane/POSS hybrid materials
Journal of Thermal Analysis and Calorimetry
Thermal stability of gamma-irradiated polyurethane/POSS hybrid materials
Traian Zaharescu 0 1 2
Virgil Marinescu 0 1 2
Edyta Hebda 0 1 2
Krzysztof Pielichowski 0 1 2
0 Department of Chemistry and Technology of Polymers, Cracow University of Technology , ul. Warszawska 24, 31-155 Krako ́w , Poland
1 & Krzysztof Pielichowski
2 & Traian Zaharescu
In this work, new version: we report essential data on the stability of gamma-irradiated polyurethanes chemically modified by octa(3-hydroxy-3-methylbutylmethylsiloxy) POSS (o-POSS) which varies from 2, 4, 6, 8 to 10 mass%. These hybrid materials were tested by isothermal (190 C) and nonisothermal (b = 2, 3.7, 5 and 10 K min-1) chemiluminescence, and the thermal stability of gamma-radiation-aged samples was correlated with the change in the nanofiller loading and absorbed dose. The compositions where inorganic phase is less than 6% show an increasing thermal strength as o-POSS concentration enhances. The other samples with higher nanoparticle content present less stability in respect of inferior homologous composition. The nonisothermal chemiluminescence profiles are changing from one type of sample to the other where nanofiller induces different effects. The considerations on mechanistic aspects are discussed, too.
Polyurethane; POSS; Chemiluminescence; Thermal stability
The hybrid materials are essential in all economical fields
where the improved endurance is required. The enlarging
knowledge areas of polyhedral oligomeric silsesquioxanes
(POSS)-modified polymers have received deep attention
] because the presence of this inorganic filler allows
the manufacture of long-life engineering products. The
diversity of polymer materials such as PMMA [
], epoxy resins [
] and polyurethanes [
] was studied,
where POSS filler acts as a compound suitable for new
resistant structures. These papers emphasize additive
contribution to the polymer functionality. An intimate
interaction between POSS nanoparticles and polyurethane
structure was previously presented [
Department of Advanced Materials, INCDIE ICPE CA, 313
Splaiul Unirii, 030138 Bucharest, Romania
The degradation of polyurethanes has been amply
discussed because these materials have several applications in
medical wear, chemical engineering, aircraft industry,
nuclear areas [
9, 11, 12
]. The excellent radiation stability
of polyurethanes [
] and their large processing dose range
] recommends them for long-term applications. The
degradation of polyurethanes accelerated by their exposure
to high-energy radiation [
] occurs somewhat slowly
because they show an evident tendency to cross-link [
The chemiluminescence (CL) examination on the thermal
stability of polyurethane composite reveals the contribution
of bond dissociation and the elimination of carbon dioxide
to the evolution of thermal degradation in polyurethane
]. The spectroscopic (ATR-FTIR) analysis
identified oxygenated products as main degradation
products formed during natural and artificial aging of
polyurethane foams .
The degradation mechanism of polyurethanes and their
POSS composites was previously analyzed. The thermal
stability study on the degradation of rigid polyurethanes
foams modified with polyhedral oligomeric silsesquioxane
on whose structure propanediolizobutyl or
(3-hydroxy-3methylbutyldimethylsiloxy) moieties were grafted has
pointed out the changes in physical and structural features
caused by the interaction between polymer matrix and
inorganic particles [
However, the studies on the stabilization of
polyurethanes are scarcely published [
9, 12, 21
underline the delay of oxidative degradation by the additive
activities in respect of the scavenging free radicals, the
efficient adsorption on particle surface or the penetration of
radicals through channels existing in POSS morphology.
The telechelic behavior of PU in the presence of modified
POSS nanoparticles [
] confirms the remarkable thermal
resistance of these compositions proving the further
capacity of material for the oxidation prevention of
creating radicals. The proofs on the improved endurance PU/
POSS systems are the results of phase stability
investigation over large temperature range (50–400 C) [
In this paper, the stability investigations by
chemiluminescence on radiation processes of MDI-based
polyurethane modified with
octa(3-hydroxy-3methylbutyldimethylsiloxy) polyhedral oligomeric
silsesquioxane (o-POSS) are analyzed.
Hybrid polyurethane/o-POSS materials were synthesized
using 4,40-diphenylmethane diisocyanate (MDI,
SigmaAldrich), poly(tetramethylene glycol) (Invista),
1,4-butanediol (Sigma-Aldrich) and o-POSS (Hybrid Plastics)
(Fig. 1) in a two-step process [
Radiation processing was accomplished in air at room
temperature in an irradiation device (Ob Servo Sanguis,
Hungary) provided with 60Co source. The doses were 25,
50 and 100 kGy. The dose rate was 1 kGy h-1. The
samples were measured immediately after the end of each
Isothermal and nonisothermal chemiluminescence
spectra were recorded by means of LUMIPOL 3 (Slovak
Academy of Sciences, Bratislava). Thermal regimes were
as follows: for isothermal investigations, the temperature
was performed at 100, 110 and 120 C, and for
nonisothermal determinations the four heating rates, 2, 3.7, 5
and 10 C min-1. Small square pieces weighing around
3 mg were placed on aluminum pans which do not
influence the oxidation profile.
Results and discussion
The radiation processing, which is an accelerated
procedure for polymer modification, provides high
concentration of reactive intermediates (free radicals as former
entities). In the opposition with PP/POSS, PP being
degraded by c-irradiation in air [
hybrids exhibit higher oxidation resistance at small filler
]. As a result of polypropylene radiolysis, the
values of decomposition temperature fall significantly
down for low absorbed dose (less than 20 kGy), while the
stability of polyurethane is improved for the concentration
up to 4% [
The modification of POSS molecular configuration by
the grafting of branched moiety brings about a
supplementary effect on the efficiency in oxidation delay. The
contribution of this kind of grafted POSS was previously
reported for the promotion of cross-linking [
classical reactions, but high-energy exposure involves more
intimately the changing amplitude. The molecular
architecture would be defined by the competition between
oxidation and radical scavenging [
]. If the substitute is not
enough active for the slowing down degradation rate,
polyurethane molecules are dominantly converted into
alcohol by-products [
Isothermal CL investigation on pristine polyurethane
shows curves with a minimum value (Fig. 2).
They demonstrate fragmentation process accompanied
by a slight oxidation on the descendant part followed by an
advanced oxidation on the ascendant part of curves. An
accelerated degradation is obtained as the working
temperature increases because the quicker diffusion of oxygen
feeds this process. The related mechanism was previously
]. The temperature of 110 C would be
considered as an acceptable value for reliable experiments.
The presence of
octa(3-hydroxy-3-methylbutyldimethylsiloxy)/polyhedral oligomeric silsesquioxane
in the studied polyurethane matrix changes profoundly the
shape of oxidation curves (Fig. 3).
The nonirradiated samples show CL curves with
pronounced descending former part followed by an
equilibrium plateau or slight monotone ascending portion. This
last tendency is characteristic for pristine and 2% modified
polyurethane, where the filler loading is minimal. It can be
easy noticed that the increase in the o-POSS concentration
improves significantly the oxidation strength of basic
polymer till it reaches a reasonable figure (6%). This
feature was also found for polyurethane samples containing
neat POSS [
], but the stability threshold was only 4%.
Although the content of filler becomes higher, up to 10%
the sample stabilities remain higher than that was observed
for pristine polyurethane. The CL curves recorded for
superior concentrations (6 and 8%) of o-POSS are placed
in the upper region in respect of the most stable material
(6%) because the oxidation takes place faster. The highest
content of o-POSS delays oxidation only on the first step of
degradation. It would be explained by the saturation of
o-POSS reacting positions; consequently, the competition
between adsorption and oxidation of free radicals is gained
by the oxidation aging.
The most suggestive comparison between the
degradation of irradiated samples may be made on pristine and 6%
loaded polyurethane (Fig. 4).
In the case of pristine material, the irradiation causes a
former stabilization at 25 kGy, while higher doses promote
oxidation. The curve shapes over the first 10 min of
oxidation are sharper in the case of modified polymer because
the scavenging of free radicals is efficient. The higher CL
intensities recorded on irradiated samples especially at
100 kGy prove the availability of high radical
concentration for oxidation in respect of unprocessed polymer.
However, the values of CL intensities after the
accomplished degradation tend to similar figures that characterize
each filler concentration (Fig. 5).
The evolution of oxidation as the temperature increases
is presented in Fig. 6.
On the low-temperature range (room temperature
-180 C), the emission intensities are comparable
because the concentration of free radicals is not high
enough, at elevated temperatures, for example 250 C.
The relative position of specific intensities confirms the
behavior found by isothermal investigation. The neat
polyurethane is the most unstable material, and the
difference between pristine and compounded materials
appears evidently during for advanced degradation state
20 40 60
Fig. 6 Evolution of CL intensity for all unirradiated polyurethane
samples at three temperatures. The meaning of coloring is the same as
in Fig. 3. (Color figure online)
when the filler acts as intermediate scavenger. As it was
previously reported [
], the degradation process occurs
by the secondary peroxyl radicals which are further
involved in different reactions like hydrogen and radical
abstraction, disproportionation or partial recombination.
Their feeding is practically delayed by o-POSS
nanoparticles which retain the oxidation initiators.
The comparison of nonisothermal CL spectra for
nonirradiated samples (Fig. 7) reveals the delay of oxidation
by o-POSS nanoparticles.
This kind of materials reveals a limiting concentration
threshold at 6%, while the delimitation of protection
regime in other engineering polymers like polyethylene,
ethylene-propylene elastomer or any other thermoplastic
material was never demonstrated.
The detailed analysis of the radiation degradation of
polyurethanes based on infrared spectroscopy [
the tendency to cross-linking is proved by Charlesby–
Pinner representation and the degradation features is
justified by the changes in characteristic spectral
absorption. The most sensitive parts of polyurethane molecules
are found in amorphous zone as soft fragments and
ethylene glycol moieties. In our cases, the scavenging activities
shown by the polyurethane/o-POSS formulations irradiated
at low dose (25 kGy) characterizing the sterilization
operation are similar on the low- and medium-temperature
ranges and they do not strictly depend on material
composition (Fig. 8).
The more intensive heating brings about an accelerated
oxidation in the samples with medium amount of o-POSS.
It may be ascribed to the availability of substituted
octa(3hydroxy-3-methylbutyldimethylsiloxy) to hinder the
detachment of scavenged intermediates due to its large
The history of materials gets influence on the
degradation manner by which polymer items are degrading. The
irradiation and the features related to the radiochemical
scission and cross-linking yields are permanently in
competition during c-exposures. The comparison between
Figs. 8 and 9a, higher emission intensities for longer
exposure of polyurethane/o-POSS samples, can be
accounted, but the likeness of each composition for the two
doses does not confirm similarity in the progress of
The differences in the radical concentrations and in the
filler loadings cause the shift of intensity peaks toward
higher temperatures. The simultaneous molecular
fragmentation, radical scavenging by filler and their reactions
change the oxidation profile. The most relevant behavior
for the both situation is the preservation of stability
Figure 9b, c illustrates the thermal behavior of similarly
irradiated samples by CL measurements at two different
heating rates. The higher the testing rate, the greater the
emission intensities. The most stable composition is
polyurethane/6% o-POSS which shows this favorable
characteristic even at higher temperatures. The analysis of
intimate mechanism of this material response leads to the
conclusion that this filler content corresponds to a
saturation threshold for radical scavenging. Further, the increased
o-POSS amount does not sustain superior stability; by
contrary, the oxidation progresses faster because of the
greater concentration of blocked initiators.
The thermal behavior of
silsesquioxane presents a stability threshold at the
concentration of o-POSS of 6%. The contribution of this filler
in the nanosize state in polyurethane matrix is based on its
availability for the scavenging radicals formed by
molecular fragmentation. Similar stability limit was also shown
by polyurethane loaded with pristine POSS, but the highest
stability was noticed at 4%. The investigations
accomplished by isothermal and nonisothermal
chemiluminescence revealed the increase in thermal and radiation
stabilities up to limit concentration followed by the more
accelerate oxidation. The prented results are useful for the
assessment of polyurethane based medical wear which
have to resist over long operation term. The accelerated
degradation carried out under c-irradiation is a proper
manner to demonstrate the availability of these hybrid
materials for nuclear application.
Acknowledgements Authors (EH and KP) are grateful to the National
Science Center in Poland for financial support under Contract No.
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