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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]

Macromol. Mater. Eng. 2010, 295, 846–858

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinel

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),


� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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.

www.mme-journal.de 847

M. Sanchez-Soto, S. Illescas, H. Milliman, D. A. Schiraldi, A. Arostegui

848

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



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

DOI: 10.1002/mame.201000064


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.



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



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).

850

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).



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

DOI: 10.1002/mame.201000064


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.



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



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/



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

DOI: 10.1002/mame.201000064


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



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



854

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



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


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-



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



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.



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


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

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