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Morphology and Thermomechanical Propertiesof Melt-Mixed Polyoxymethylene/PolyhedralOligomeric Silsesquioxane Nanocomposites
Miguel Sanchez-Soto, Silvia Illescas, Henry Milliman, David A. Schiraldi,Asier Arostegui*
The influence of the functionalization of fully condensed POSS cages on the properties of POM-based nanocomposites is studied. POSS with different organic substituents [glycidylethyl,aminopropylisobutyl, and poly(ethylene glycol)] are taken into account and melt mixed withPOM. Good dispersion was achieved upon theaddition of amino functionalized POSS, leadingto an increase on the thermal decompositiontemperature under nitrogen atmosphere up to50 8C. However, mm-size aggregates were observedfor other nanocomposites. There is no significantchange in other thermal properties of the nano-composites. The relationships among these effectsand the morphological characteristics of the sys-tems were analyzed.
Introduction
Polyhedral oligomeric silsesquioxanes (POSS) based mate-
rials have received great attention in recent years both
as polymer nanocomposites and as organic/inorganic
hybrids.[1,2] POSS molecules are characterized by a 3D
cage structure, i.e., by a polyhedral Si�O nanostructured
skeleton [general formula (SiO3/2)n], surrounded by several
organic groups linked to silicon atoms by covalent bonds.
M. Sanchez-Soto, S. IllescasCentre Catala del Plastic, Universitat Politecnica de Catalunya,Colom 114, 08222 Terrassa, SpainH. Milliman, D. A. SchiraldiDepartment of Macromolecular Science and Engineering, CaseWestern Reserve University, Cleveland, OH 44106-7202, USAA. ArosteguiMechanical and Industrial Production Department, MondragonUnibertsitatea, Loramendi, 4, 20500 Arrasate-Mondragon, SpainFax: þ34 943 79 1536; E-mail: [email protected]
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The functionality, solubility, polarity, and reactivity of
these molecules can be easily changed through modifying
the organic groups with a variety of functional groups. The
interactions and/or reactions of these organic functional
groups with a polymer may result in the nanometric
dispersion of POSS into the matrix.
In order to form stable polymer/nanoparticle systems,
different routes have been proposed. Thus, POSS molecules
can be either physically dispersed in polymer matrices
using traditional melt-blending processing techniques,[3–5]
reactive blending,[6,7] or linked to the polymer chains by
direct copolymerization or grafting, via suitably reactive
side groups of POSS.[8–13] POSS and its derivatives have been
successfully incorporated in various commodity,[3,6,14,15]
engineering[4,5,16–23] and high-performance[24,25] thermo-
plastic polymers, and some thermoset systems.[26–28] The
incorporation of POSS or its derivates can lead to dramatic
improvement of several properties such as increases in use
temperature, oxidation resistance, as well as reductions in
flammability, and viscosity during processing.
ibrary.com DOI: 10.1002/mame.201000064
Morphology and Thermomechanical Properties of Melt-Mixed . . .
The majority of the studies reported to date have focused
on the morphology and thermomechanical properties of
the achieved nanocomposites. However, Misra et al.[13]
reported recently detailed understanding and of chain
dynamics and conformations in polystyrene (PS)/POSS
nanocomposites solutions and films. They proved that
calculated solubility parameter was a useful tool for
predicting dispersion and segregation of POSS molecules
into polymer matrices. Thus, they showed using different
microscopic techniques that nanometric dispersion level
was achieved when the solubility parameters were very
similar, meanwhile surface segregation and POSS aggre-
gation took place with a significantly larger solubility
parameter difference. Similarly, Brus et al.[28] described
quantitatively and qualitatively the tendency to aggrega-
tion and self-organization of individual building blocks
using high-speed magic-angle spinning 1H/1H spin-diffu-
sion measurements.
Polyoxymethylene (POM) is one of the major engineering
thermoplastics commonly used to replace metal or alloy
products, owing to its high stiffness, dimensional stability,
and corrosion resistance. However, low impact toughness,
notch sensitivity, and especially low heat-resistance limit
its range of applications. The development of modified
POM by various strategies, such as copolymerization with
oxyethylene chain[29,30] and blending with elastomers[31,32]
are proposed in the literature. The hybridization with
inorganic fillers, especially organoclay, to form nanocom-
posites is another approach to improve the properties of
POM from the nanoscale structure.[33,34] The incorporation
of POSS molecules into POM matrix, by means of melt-
blending techniques, has recently been demonstrated as a
suitable way to improve the thermal stability of the
material.[23]
The current work focuses on the compatibility of three
different POSS molecules with a POM matrix and the effect
of POSS concentration on the structure, morphology, and
thermomechanical properties of POM/POSS composites.
Differences between composites containing fully con-
densed POSS molecules with different organic groups will
be examined. The structural changes of the composites
were assessed by infrared (IR) spectroscopy, the morphol-
ogy by scanning electron microscopy (SEM), energy-
dispersive X-ray analysis and atomic force microscopy
(AFM), and the thermomechanical behavior by differential
scanning calorimetry (DSC), thermogravimetric analysis
(TGA), and dynamic-mechanical analysis (DMA).
Figure 1. Chemical structures of ge-POSS (a), apib-POSS (b), andpeg-POSS (c) cage molecules.
Experimental Part
Materials
The ethylene/POM copolymer type used in this work as a matrix
was a Hostaform C13021 supplied by TICONA (Barcelona, Spain),
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and it was used as received. POM pellets were characterized by
a melt volume flow rate giving a value of 12 cm3 � (10 min)�1 at
190 8C and 2.16 kg. Prior to compounding the polymer pellets
were dried for at least 4 h under vacuum at 80 8C, and maintained in
desiccators before mixing.
Glycidylethyl-, aminopropylisobutyl, and poly(ethylene glycol)-
POSS (referred herein to as ge-, apib-, and peg-POSS, respectively)
were kindly supplied from Hybrid Plastics, and used as received.
The chemical structures of the different POSS used in this work are
shown in Figure 1. The molar mass of the PEG substituents is close to
650 g � mol�1.
POM/POSS Nanocomposite Preparation
Nanocomposites were obtained by mixing POM and different
contents of POSS (2.5, 5, and 10 wt.-%) in a DACA model 2000 twin-
screw co-rotating microcompounding extruder (13.75 mm screw
diameter and 108 mm length). The blending process was carried out
at 190 8C and a screw speed of 100 rpm for 5 min.
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M. Sanchez-Soto, S. Illescas, H. Milliman, D. A. Schiraldi, A. Arostegui
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The extruded materials were compression molded into films
(constrained inside of a mold) using a Carver model C press. The
samples were first heated at 190 8C for approximately 10 min,
and then a rapid compression was applied followed by release of
pressure to remove any trapped gas bubbles. The samples were
then molded at 190 8C under 4–5 t pressure for approximately 2 min
and cooled rapidly between two water-chilled aluminum plates.
POM/POSS composites films with thickness between 0.2 and
0.3 mm were finally obtained.
Fourier-Transform Infrared (FTIR) Spectroscopy
The chemical structures of the composites, and that of the reference
materials, were analyzed using a Nicolet 6700 spectrophotometer.
The spectral resolution was 1 cm�1, and the wavenumber interval
analyzed was between 4 000 and 400 cm�1. For each measurement,
50 scans were performed. A Smart Orbit high-performance dia-
mond single bounce attenuation total reflection (ATR) accessory
was used. The depth of penetration of the equipment is 2.03 mm at
1 000 cm�1. The resulting samples from the melt-mixing process
were exposed to the ATR by its direct placement upon the ATR
crystal.
Microscopic Analysis
SEM was used to analyze the microstructure of the composites and
the degree of dispersion of POSS molecules into the matrix. The
surfaces of cryogenically fractured specimens were observed in a
Jeol 5610 electron microscope at an accelerating voltage of 10 kV.
Prior to its observation the fractured surfaces were sputter-coated
with a thin gold layer of approximately 10 nm to make the surface
conductive.
Samples were also analyzed by energy-dispersive X-ray analysis
(EDAX) to identify the presence of POSS in the bulk. In such
case samples were sputtered with carbon to avoid interferences
between gold and silicon signals.
Surface morphology studies were conducted on MultiMode
scanning probe microscope with an electronic NanoScope IV cont-
roller from Veeco Instruments, Inc. A silicon probe with a 125mm
long silicon cantilever, nominal force constant of 40 N � m�1, and
resonance frequency of 330 kHz was used for tapping mode surface
topography studies. Surface topographies of film samples were
studied on 1�1mm2 scan size areas at an image resolution
256�256 pixels and a scan rate of 0.75 Hz. Multiple areas were
imaged and figures show representative morphology.
Differential Scanning Calorimetry (DSC)
The thermal behavior of the composites, and that of reference
materials, was analyzed using a Mettler Toledo DSC822e/700
instrument in hermetically sealed aluminum pans, under nitrogen
flow (60 mL � min�1). The apparatus was calibrated with a high
purity indium standard. Samples of approximately 5 mg were
tested at a heating rate of 10 8C � min�1 from –20 or –30 8C depend-
ing on POSS molecules toþ200 8C performing three successive runs:
heating-cooling-heating. The melting temperatures and enthalpies
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were determined from the maxima and the areas of the
corresponding peaks, respectively. The degree of crystallinity of
the composites was determined by means of the ratio between the
measured melting enthalpy and the melting enthalpy of a
completely crystalline POM (taken to be 251.8 J � g�1[35]).
Thermogravimetric Analysis (TGA)
The thermal stability of the composites was measured by
thermogravimetric analysis using a Mettler Toledo TGA/SDTA851e
instrument. Samples sizes of about 10 mg were loaded in
aluminum pans and heated at a rate of 10 8C � min�1. Weight
loss curve was traced as samples were heated from room tem-
perature to 600 8C under a dry nitrogen purge of 60 mL � min�1.
Dynamic Mechanical Analysis (DMA)
A Thermal Analysis Instruments Q800 DMA was used in tensile
mode at an oscillatory frequency of 1 Hz with applied 1% strain for
all samples. The temperature scan was performed at 2 8C � min�1
heating rate in the range from –100 to around þ150 8C. Sample
dimensions were typically 4.9 mm long, 9.8 mm wide, and 0.2–
0.3 mm thick.
Results and Discussion
Infrared Spectroscopy
The possible reactions and/or interactions between POM
and different POSS molecules, and therefore, chemical
structural changes, were studied by means of IR. The
different characteristic absorption bands corresponding
to neat POM and different POSS functional groups are
collected in Table 1. The most characteristic absorption
band of POM is located at 1 090 cm�1, corresponding to the
symmetric stretching vibration of C�O�C ether groups. As
the functional groups of the POSS molecules were different,
they had only a common intense and wide absorption band
at 1 084 cm�1 that corresponds to the stretching of the
Si�O�Si group. Thus, the absorption bands at 1 260 (epoxy
stretching), 1 020 (epoxy stretching), and 760 cm�1
(deformation of C�H of epoxy) are attributed to the func-
tional groups of ge-POSS molecule; the absorption bands
at 1 620 (N�H in-plane deformation), 840 (N�H out-of-
plane deformation), and 1 230 cm�1 (C�N stretching) are
attributed to apib-POSS molecule; whereas the absorption
bands at 1 470 (CH2 deformation) and 2 920 (C�H
stretching) are attributed to peg-POSS molecules.
Figure 2 shows the FTIR spectra of POM/POSS composites
with 10 wt.-% POSS, and that of neat POM and different
POSS molecules as reference. The additional spectra for
other composites with different POSS contents are not
included herein because of their similarity to that shown in
Figure 2. As it can be observed, the most characteristic band
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Morphology and Thermomechanical Properties of Melt-Mixed . . .
Table 1. Assignment of FTIR spectra characteristic absorption bands of POM, ge-POSS, apib-POSS, and peg-POSS.
Material Wavenumber Assignment
cm�1
POM 1 090 C�O�C symmetric stretching vibration
POSS 1 084 Si�O�Si symmetric stretching vibration
ge-POSS 1 260 epoxy symmetric stretching vibration
1 020 epoxy asymmetric stretching vibration
760 C�H in epoxy deformation vibration
apib-POSS 1 620 N�H in-plane deformation vibration
840 N�H out-of-plane deformation vibration
1 230 C�N stretching vibration
peg-POSS 1 455 CH2 deformation vibration (scissoring)
2 870 CH2 asymmetric stretching vibration
of POM at 1 090 cm�1 remained at the same wavenumber,
whatever the POSS added. This constancy can be attributed
to the coincidence of C�O�C and Si�O�Si (1 084 cm�1)
absorption bands of POM and POSS, respectively. Moreover,
as the amount of POSS added to POM was small, any
absorption band corresponding to POSS molecules can be
masked by POM absorption bands.[23]
As it can be also observed in Figure 2, other absorption
bands of POM remained at the same wavenumbers
and with similar peak height ratios upon the addition of
10 wt.-% ge-POSS or peg-POSS. This fact indicates that
ge-POSS or peg-POSS molecules did not interact substan-
tially with POM matrix, as they are not detectable by FTIR;
if functionalized POSS molecules linked to the polymeric
chain of POM, changes or displacements of the absorption
bands of POM should have been observed.
Figure 2. FTIR spectra of POM/POSS composites with 10 wt.-%POSS, and that of neat POM and different POSS molecules as areference. To aid clarity, the curves are shifted on the vertical axis.
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In view of the structures of POM and apib-POSS, it is
expected that there could be intermolecular hydrogen
bonding between the ether oxygen atoms of POM and
hydrogen atoms of apib-POSS. These interactions can
readily be detected by means of FTIR in the range of 3
000–3 800 cm�1, corresponding to the amine and hydroxyl
stretching vibrations that are hydrogen bonded. No new
absorption bands were detected for the 10 wt.-% apib-POSS
composite system (Figure 2), indicating that hydrogen
bonding interactions would be very small – if they exist at
all. It should be pointed out that the presence of these
intermolecular interactions could affect the degree of
association between the POM matrix and apib-POSS
molecules by affecting the molecular dynamics in the
composite system.[36,37] However, it has been proved that
the presence of hydrogen bonding between POM and POSS
nanoparticles increases their mutual compatibility, but
these interactions do not prevent POSS aggregation phase
separation.[38]
In this sense in a recent paper Misra et al.[13] working
with PS consistently demonstrated that enhanced disper-
sion and miscibility between POSS molecules and polymers
are achieved when the solubility parameters are similar.
On the other hand, surface segregation occurs when the
incorporation of POSS occurs with unfavorable enthalpic
interactions and high entropic penalty due to the incor-
poration of large POSS aggregates to the polymer chain.
Morphology
The cryofractured surfaces of POM/POSS composites were
studied by SEM with the aim of establishing the micro-
structure of the nanocomposites and the degree of
dispersion of the nanoparticles into the matrix, and
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Figure 3. SEMmicrographs of the cryofractured surfaces of neat POM (a and b) and POM/POSS composites with 10 wt.-% ge-POSS (c and d),and peg-POSS (e and f).
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consequentially, the interaction level between them.
Figure 3 shows the cryofractured surfaces of neat POM as
a reference (Figure 3a and b) and that of POM/POSS
composites with 10 wt.-% ge-POSS (Figure 3c and d) and peg-
POSS (Figure 3e and f).
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A poor dispersion of the ge-POSS and peg-POSS molecules
within the polymeric matrix was achieved since several
particles with sizes between 1 and 20mm were found. As
POSS cage molecules are characterized to have diameters
between 1.5 and 3 nm,[1] these sub-mm particles observed in
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Morphology and Thermomechanical Properties of Melt-Mixed . . .
Figure 4. SEMmicrographs of the cryofractured surfaces of POM/apib-POSS composites with 2.5 (a), 5 (b), and 10 wt.-% (c) apib-POSS content.
the micrographs are the result of nanospherical aggregates.
The spherical morphology of the aggregates was indicative
that the phase separation took place during the processing
of both types of POM/POSS composites. It must be taken
into account that the possibility of obtaining phase
separation is greater during the processing by melt-mixing
than in other processes, such as copolymerization or
solution blending. It can also be observed, the size of the
aggregates was clearly higher in peg-POSS composites
(Figure 3e) than in ge-POSS composites (Figure 3c), indicat-
ing lower compatibility between POM and peg-POSS.
We propose that the aggregates in the ge-POSS and peg-
POSS composites are generated by different mechanisms.
Despite the structural similarity between the polymeric
chain of POM and the glycidyl groups of the ge-POSS
molecules,[39] the development of aggregates in this system
could be due to POSS/POSS interactions, as it has been
observed during the melt-mixing of other POSS-based
composites.[12,13,23,40,41] The interfacial adhesion level
between POM and ge-POSS is quite poor, resulting in
well-defined and regular holes observed in Figure 3d; the
number of detached particles indicates lack of adhesion/
compatibility with the POM matrix.
In the case of POM/peg-POSS composites, some com-
patibility between the POM matrix and the peg-POSS
molecules could be expected, as a consequence of the
solubility of low-molecular-weight ethylene chains into
the POM matrix.[42] As it can be observed in Figure 3f, the
interfacial adhesion between the POM matrix and peg-POSS
aggregates is indeed good because most of the peg-POSS
aggregates appeared to be effectively linked to the POM
matrix, and no voids appeared at the interface. This high
level of matrix-filler adhesion is expected to be a con-
sequence of the similarity between the chemical structures
of POM and peg-POSS.
Figure 4 shows the cryofractured surfaces of POM/apib-
POSS composites with 2.5 (Figure 4a), 5 (Figure 4b), and
10 wt.-% (Figure 4c) of apib-POSS contents. As it can be
observed, there is a good dispersion of apib-POSS molecules
into the POM matrix because only few sub-mm size aggre-
gates could be detected, evidencing an almost fully homo-
geneous microstructure. Whatever the POSS content, the
size of the aggregates is always of sub-micrometric size
(approximately 0.3mm), although the number of POSS
aggregates seems to little increase with the amount of POSS
added. The absence of mm-size aggregates is indicative of a
high level of miscibility or compatibility between the POM
matrix and apib-POSS, due to the similar polarity between
the amine end-groups of the POSS molecules and the ether
groups of the POM polymeric chain. Moreover, as previously
it had been mentioned, some hydrogen bonding interac-
tions can be expected between ether oxygen and hydrogen
atoms of POM and apib-POSS, respectively,[36,43,44] leading
to good dispersion of apib-POSS molecules into the matrix.
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The dispersion and morphology of apib-POSS into the
POM matrix has been studied by means of EDAX analysis.
Figure 5 shows the SEM-EDAX micrographs of POM with the
addition of 10 wt.-% apib-POSS. Additional micrographs for
other apib-POSS loadings are not shown because of their
similarity to that shown in Figure 5. As it can be seen, there
was a good dispersion of apib-POSS at nanometric level into
the POM matrix, without observing high concentration of
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Figure 5. SEM-EDAX micrographs of the cryofractured surfaces ofPOM with 10 wt.-% apib-POSS content.
Figure 6. AFM images of pristine POM (a) and 10 wt.-% loadedPOM/apib-POSS composite (b). Scan size: 1� 1mm2.
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silicon. This behavior suggests good compatibility and/or
interaction between POM matrix and apib-POSS molecules,
as was deduced from SEM observations.
The apparently homogeneity of POM/apib-POSS nano-
composites has also been analyzed by means of AFM.
Tapping mode AFM phase images of the neat POM (as
reference) and that of POM with the addition of 10 wt.-%
apib-POSS are shown in Figure 6. The images of POM/POSS
composites with peg-POSS and ge-POSS are not shown as a
consequence of the aggregates sizes (approximately 6mm)
were larger than the scan size (1� 1mm2).
As it can been observed in Figure 6a, pristine POM
exhibits a rough surface as a consequence of compression
molded films were water-chilled between two aluminum
plates. However, the POSS samples showed raised features
attributed to POSS aggregates and crystallites. Thus, POM/
apib-POSS nanocomposites show relatively large raised
surface features with an average particle diameter of
approximately 62 nm with broad particle size distribution.
This fact indicates that the apparently homogeneous POM/
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apib-POSS nanocomposites consist of aggregates of POSS
nanoparticles.[13,24]
Differential Scanning Calorimetry
The influence of the POSS molecules addition on the melting
and crystallization behavior of the POM matrix was
evaluated by means of DSC analysis. The melting tempera-
ture and crystallinity content values for POM/POSS
composites and that of neat materials as reference, as
determined by DSC second heating scans are reported in
Table 2. It should be pointed out that no significant
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Table 2. DSC second-heating melting temperatures (Tm) andcrystallinities (xc) values for POM/POSS composites.
Material [Substituent] Tm,POM Tm,POSS xc,POM
wt.-% -C -C %
POM – 169.1 58.6
ge-POSS – 147.0
2.5 168.4 – 57.1
5 167.5 – 56.8
10 167.5 146.0 53.0
apib-POSS – 48.6
2.5 168.5 – 59.5
5 168.4 – 58.3
10 168.1 – 57.5
peg-POSS – 22.1
2.5 170.2 17.3 60.2
5 168.9 19.3 59.2
10 167.6 19.3 61.7
Figure 7. DSC second heating curves for POM/peg-POSS compo-sites, and that of pure peg-POSS as reference. To aid clarity, thecurves are shifted on the vertical axis.
differences were observed between first and second heating
scans, so the thermal behavior of the second scan will be
only presented herein.
The second heating scan of POM showed a narrow
melting peak at approximately 169 8C. Neat ge-POSS and
apib-POSS are white crystalline powders at room tempera-
ture, with measured melting temperatures at 147 and 49 8C,
respectively. peg-POSS is a viscous liquid at room tempera-
ture with a melting temperature of approximately 22 8C.
The three different POSS molecules would be expected to be
in the molten state during the processing with POM, leading
to an improvement in the mixing efficiency and minimiz-
ing their domain size within the composites.
As it can be seen in Table 2, the melting temperature of
POM in the composites remained practically constant, with
the maximum change (1.5 8C with 10 wt.-% ge-POSS) being
within the estimated experimental variability (2–3 8C).
These results indicate that the addition of POSS molecules
did not affect the crystalline structure of POM, in contrast to
some other POSS-based composites.[3,15,41]
With respect to the melting temperature of the POSS
molecules, no melting peak corresponding to ge-POSS (at 2.5
and 5 wt.-%) and apib-POSS could be observed during the
second heating scan of the POM/POSS composites. These
observations suggest that the POSS molecules are well
dispersed within the POM matrix leading to the suppres-
sion of crystallization of neat POSS, and/or that the size of
formed crystals is too small for being detected by DSC
measurements[4] The POSS molecules were observed to
crystallize to some extent, however, during the processing
of the composites at high ge-POSS contents, showing a small
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melting peak at approximately 146 8C, suggesting that the
maximum solubility of ge-POSS into POM matrix had been
exceeded.[45]
Figure 7 shows DSC second heating scan for POM/peg-
POSS composites and that of neat peg-POSS as reference in
the interval of POSS molecules melting. As it can be
observed, the crystallization of peg-POSS molecules took
place at all POSS levels. The melting temperature of peg-
POSS molecules in the composites (Table 2) increased
from approximately 17 to 19 8C with increasing peg-POSS
loadings, indicating that separate POSS crystalline domains
are present in POM/peg-POSS composites.
Polyhedral oligomeric silsesquioxanes nanoparticles
exhibit a tendency to aggregate in the form of big particles
that may retain the crystalline structure. If they can form
crystals, POSS particles tends to aggregate together to form
a crystalline structure with increasing POSS concentra-
tion.[46] Specifically, the affinity between POSS units causes
these particles to aggregate and closely pack into a
crystalline lattice. The organic polymer, covalently con-
nected to each POSS unit, limits the crystallization into a
two-dimensional lattice as demonstrated by Zheng et al.[47]
When POSS spheres are attached along polymer chains, the
crystal packing of the cubic silsesquioxane units is
constrained by their covalent attachment to the polymer
chain. As a result, a bilayer or lamella-like structure is
formed where POSS spheres are packed hexagonally within
planes and two planes are stacked together. In other words,
the covalently connected chains serve as the source of
crystallization confinement, preventing the development
of spherical packing in three dimensions, and therefore
results in a two-dimensional raft-like structure.
The crystallinity for all POM/POSS composites was also
evaluated by DSC, and the corresponding values are
collected in Table 2. The crystalline content of POM at
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low ge-POSS contents was very similar to that of neat
POM, while at high loadings its value decreased from 58%
(neat POM) up to approximately 53%, indicating that
high concentrations of ge-POSS slightly inhibited the
crystallization of the POM matrix.[3,6,23] In POM/apib-POSS
composites the crystalline content remained practically
constant at all apib-POSS contents tested.[3] And finally,
the degree of crystallinity in POM/peg-POSS composites
slightly increased, indicating that the peg-POSS molecules
may act as a nucleating agent that affects the POM
matrix.[7,39,41]
Thermal Stability
The thermal stability of POM/POSS composites was studied
by means of thermogravimetric analysis under inert
(nitrogen) conditions. Use of nitrogen atmosphere prevents
auto-oxidation and reactions with secondary products, and
so the chain stability and the efficiency of POSS molecules as
heat stabilizers can be tested. The most important features
such as the temperature at 5 wt.-% weight loss (T5%), the
temperature of the maximum weight loss rate (Tmax) and
the fraction of the solid residue at 600 8C of the thermo-
grams for POM/POSS composites, as well as neat POM and
POSS molecules, under nitrogen are presented in Table 3.
The weight loss of POM took place in a single step in the
temperature range of 250–350 8C, with an abrupt drop at
270 8C (T5%). The thermal degradation of POM is thought to
be due to chain scission of the C�O�C bonds.[48] The weight
loss in neat ge-POSS and peg-POSS also took place in a single
Table 3. Temperatures of 5 wt.-% weight loss (T5%), of maximumweight loss rate (Tmax), and the fraction of the solid residue at600 8C for POM/POSS composites.
Material [Substituent] T5% Tmax Solid residue
wt.-% -C -C wt.-%
POM 268 321 0
ge-POSS – 239 296 15.4
2.5 286 341 0.1
5 291 344 1.4
10 285 350 2.4
apib-POSS 261 306 19.5
– 321 369 2.3
512.5 322 376 3.5
10 323 379 3.8
peg-POSS – 244 299 10.6
2.5 279 320 2.6
5 281 328 2.8
10 284 330 3.6
Macromol. Mater. Eng. 2010, 295, 846–858
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
step in the temperature ranges of 200–360 and 190–470 8C,
respectively. The degradation process of pristine apib-POSS
had two stages in the temperature ranges of 220–380 and
490–535 8C, with weight losses in the first and second stages
of approximately 70% and 4–5%, respectively.
As it can be seen in Table 3, the neat POSS molecules
showed lower T5% values (239, 261, and 244 8C for ge-POSS,
apib-POSS, and peg-POSS, respectively) than the POM
matrix despite the stability of Si�O structure of the cage.
This difference in stability was probably due to the ability of
the three filler molecules to decompose via partial loss of
organic substituents at lower temperatures, as was observ-
ed with octaisobutyl-POSS.[49] Another possibility for the
partial loss of POSS at low temperatures may be caused by
the presence of impurities as all POSS types were used as
received without any further purification (separation of
POSS structures is exceedingly difficult). The solid residue
amount at 600 8C was approximately 15, 20, and 11% for ge-
POSS, apib-POSS, and peg-POSS, respectively.
Once the thermal stability of neat constituent materials
was analyzed, the thermal stability of POM/POSS compo-
sites was examined. The weight loss (a) and the derivate
weight (b) thermograms of POM/POSS composites with
5 wt.-% POSS are shown in Figure 8, and the data extracted
from these thermograms are given Table 3. The weight loss
of the composites occurred in a single, almost complete
step, leaving a residue amount of below 4% that increased
with POSS content and was consistent with POSS degrada-
tion products. All the POSS molecules used in this work
increased the T5% and Tmax of the composites, showing a
considerable stabilization of the POM matrix. In all cases a
synergistic effect between POM and POSS molecules was
found to occur. The thermal stability of the POM/POSS
composites was, however, affected to different degrees
depending on the organic group of the POSS molecules.
Thus, when POM was blended with 5 wt.-% ge-POSS the
T5% increased from 268 to 291 8C then it decreased slightly to
285 8C with 10 wt.-% ge-POSS. This slight decrease obtained
at high ge-POSS contents can be related with the crystal-
lization of POSS molecule due to the maximum solubility
level had reached,[45] as it has been seen in DSC measure-
ments.
In the case of blending with peg-POSS the degradation
onset temperature of POM increased continuously with
peg-POSS content from 279 (2.5 wt.-%) up to 284 8C (10 wt.-
%). This behavior can be principally attributed to the
dissolution of low molecular ethylene chains into POM
that promoted excellent adhesion between both phases.
Although some large aggregates were observed by SEM and
peg-POSS molecules showed a definite crystallization peak,
indicating phase separation; the continuous improvement
of the thermal decomposition temperature is a clear hint of
the presence of peg-POSS molecules dispersed also at the
nanometric scale in the POM matrix.
DOI: 10.1002/mame.201000064
Morphology and Thermomechanical Properties of Melt-Mixed . . .
Figure 8. Thermogravimetric curves for POM/POSS compositeswith 5 wt.-% POSS, and that of neat POM as reference, showingthe evolution of weight loss (a) and the derivative weight loss (b).
Table 4. DMA results for POM/POSS composites.
Material [Substituent] Tg Ta E(25 -C
wt.-% -C -C MPa
POM –60.8 111.4 1 849
ge-POSS
2.5 –59.4 113.9 1 656
5 –57.9 112.4 1 861
10 –60.8 122.9 1 724
apib-POSS
2.5 –58.6 112.2 1 669
5 –62.4 114.2 1 901
10 –61.7 115.5 1 467
peg-POSS
2.5 –58.5 115.6 1 832
5 –59.8 114.1 1 831
10 –59.3 112.5 1 562
And finally, as it can be seen in Table 3 and Figure 8, the
greatest stabilization of POM was achieved with the addi-
tion of apib-POSS where theT5% of the composites increased
more than 50 8C with the addition of only 2.5 wt.-%, re-
maining constant with further POSS molecules contents.
Despite the inert conditions used in TGA analysis, the
thermal stabilization of POM obtained upon the addition of
apib-POSS molecules can be considered of great importance
taking into account the small amount of apib-POSS molec-
ules added. This extremely high level of stabilization was
probably due to hydrogen bonding interactions between
POM and apib-POSS, and in consequence, to the excellent
dispersion of apib-POSS obtained that promotes a more
robust polymer network.
Dynamic-Mechanical Analysis
The possible changes in thermal transitions of POM upon
the addition of POSS were studied by means of DMA
analysis. The corresponding results collected from the loss
tangent (tan d) and the storage moduli (E’) versus tem-
Macromol. Mater. Eng. 2010, 295, 846–858
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
perature for POM/POSS composites, and pristine POM as a
reference, are shown in Table 4.
As it can be seen in Table 4, the neat POM showed two
main transitions.[31,39] The first is a narrow g-transition at
low temperatures, approximately at –60 8C, that is related
with the glass transition temperature (Tg) of POM. This
transition is associated with the motion of short segments
in the disordered regions of the polymer chain. The second is
a broad a-transition in the range of 50–150 8C, with a
maximum value closes to 111 8C, being characteristic of
highly crystalline POM. This transition is associated with
translational motions of the crystalline structure along
the chain. Moreover, the POM of this study showed a
b-transition of low intensity between –40 8C and 30 8C and
with a maximum value close to –8 8C, that is related with
motion of long segments in the disordered regions of the
polymer chain. This transition is usually associated with
POM polymers of low crystallinity and it is a function of the
absorbed water and the thermal history of the polymer.[50]
Given its low intensity and relative unimportance in highly
crystalline POM polymers, this transition is not further
considered in the present study.
The values of the two main transitions and that of the E’
measured at 25 8C for POM/POSS composites are also
presented in Table 4. The Tg of POM remained practically
constant at –60 8C, independent of the organic substituent
of the POSS molecules added and its concentration. This
constancy of the Tg can be attributed to the low chain
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M. Sanchez-Soto, S. Illescas, H. Milliman, D. A. Schiraldi, A. Arostegui
856
mobility at these temperatures and/or to the high crystal-
linity of the POM used in this work, reducing the possible
changes in the Tg.
Figure 9 shows the plots of log(tan d) as a function of
temperature for POM/apib-POSS (a), POM/ge-POSS (b), and
POM/peg-POSS (c) composites compared to neat POM.
The DMA spectrum of POM/POSS composites exhibited a
well-defined relaxation peak near 110 8C that is ascribed to
Figure 9. DMA log(tan d) for POM/apib-POSS (a), POM/ge-POSS(b), and POM/peg-POSS (c) composites.
Macromol. Mater. Eng. 2010, 295, 846–858
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
the a-transition temperature of POM. However, the evolu-
tion of this transition was strongly affected depending on
the organic substituent of the POSS molecules added. As it
can be seen in Figure 9a, the intrinsic value of Ta of POM/
apib-POSS composites increased monotonously with the
addition of POSS molecules up to 115 8C. This behavior can
be attributed to a decrease of the molecular mobility of POM
chains in the rubbery state by the network structure of
POSS.[7,36,40] Moreover, the slight increase of the Ta is likely
another consequence of the hydrogen bonding interactions
between POM matrix and apib-POSS molecules.
The a-transition of POM substantially changed upon the
addition of ge-POSS, independent of filler content
(Figure 9b). The composites with 2.5 and 10 wt.-% ge-POSS
contents showed a new transition that did not appear in
pristine POM; the transition with 5 wt.-% ge-POSS was
broader than that observed in neat POM. The Ta value of
POM moved to higher temperatures up to approximately
123 8C when a 10 wt.-% ge-POSS is added. This behavior
could be ascribed to interactions between POM and ge-POSS
that reduce the mobility of the polymeric matrix. In our
other study the value of the a-transition temperature
decreased up to approximately 8 8C with the addition of
5 wt.-% glycidylisobutyl-POSS (gib-POSS) likely because the
dispersion of gib-POSS was hindered by the formation of
aggregates.[23] This behavior suggests that POSS molecules
behave as particles having a siliceous hard-core surrounded
by a hydrocarbon soft-shell, which limit the stress transfer
from the matrix to the core depending on the strength of the
alkyl group/matrix compatibility, as has been observed in
other POSS-based composites.[7,22]
With respect to POM/peg-POSS composites (Figure 9c),
the Ta increased to a maximum value of approximately
116 8C with the addition of 2.5 wt.-% peg-POSS, and then
it slightly decreased as the peg-POSS amount decreased up
to 112 8C. The increase observed at low contents can be
attributed to the good adhesion seen in SEM micrographs
between POM and peg-POSS. However, the decrease of
the Ta of POM as the peg-POSS content increase can be due
to the increased size of peg-POSS aggregates, hinting to a
certain level the interaction between POM and peg-POSS at
nanoscale level.[23]
The values of E’ at 25 8C for POM/POSS composites are
also collected in Table 4. The most significant differences
were observed at the lowest POSS content, because mean-
while the value of E’ decreased from 1 849 MPa (neat POM)
up to approximately 1 660 MPa with the addition of ge-POSS
and apib-POSS; this value remained practically constant
with the addition of peg-POSS. When blends with 5 wt.-%
POSS were considered, the value of E’ remained constant
with the addition of ge-POSS and peg-POSS, while the
composite with apib-POSS exhibited a slight increase in
modulus, probably as a consequence of hydrogen bonding
interactions.
DOI: 10.1002/mame.201000064
Morphology and Thermomechanical Properties of Melt-Mixed . . .
Conclusion
Polyoxymethylene has been modified by the incorporation
of three different fully condensed POSS cages (ge-POSS,
apib-POSS, and peg-POSS) in concentrations between 0 and
10 wt.-% through direct melt blending. The morphologies of
POM/POSS composites varied from molecular dispersion,
formation of cluster of nanoparticles to mixed morpho-
logies where both molecular dispersion and aggregates
coexists.
The melting temperature and the crystallinity level of
POM remained practically constant, indicating that the
crystalline structure of POM is not affected upon the
addition of different POSS molecules. Crystallites of peg-
POSS and ge-POSS were observed at the highest loadings
tested, suggesting a solubility limit for these fillers within
the matrix.
The thermal stability under inert atmosphere of POM
dramatically increased with the addition of any of the three
POSS grades tested. The highest increase in thermal
stability was achieved upon the addition of apib-POSS
(up to 50 8C higher than that of POM), principally due to the
good dispersion of apib-POSS that promotes formation of a
more robust polymer/filler network. This increase can be
considered of great importance taking into account the
small amount of apib-POSS added (2.5 wt.-%).
Dynamic-mechanical analysis testing showed that the
Tg of POM remained constant, across the POSS grades and
concentrations tested. The behavior of the a-transition
strongly depends on the organic substituent of POSS molec-
ules. Thus, Ta increases when interaction between POM
and POSS molecules are present and it decreases upon the
presence of aggregates.
Acknowledgements: A. Arostegui thanks the Departamento deEducacion, Universidades e Investigacion of the Basque Govern-ment for the award of a grant for the development of this work.The authors also acknowledge the financial support given by theMinisterio de Educacion y Ciencia of Spanish Government (projectno. PSE-020400-2006-1). Material support from Hybrid Plastics Inc.is also gratefully acknowledged.
Received: February 18, 2010; Revised: May 19, 2010; Publishedonline: July 19, 2010; DOI: 10.1002/mame.201000064
Keywords: infrared spectroscopy; morphology; nanoparticles;POSS; thermal properties
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DOI: 10.1002/mame.201000064