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 DOI: 10.1177/0954008311415301

2011 23: 457 originally published online 5 August 2011High Performance PolymersSilvia Illescas, Miguel Sánchez-Soto, Henry Milliman, David A. Schiraldi and Asier Arostegui

compositesThe morphology and properties of melt-mixed polyoxymethylene/monosilanolisobutyl-POSS

  

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Article

The morphology and properties ofmelt-mixed polyoxymethylene/monosilanolisobutyl-POSS composites

Silvia Illescas1, Miguel Sanchez-Soto1, Henry Milliman2,David A. Schiraldi2, and Asier Arostegui3

AbstractIn this study, the morphology and thermo-mechanical behavior of composites formed by a polyoxymethylene (POM)matrix and monosilanolisobutyl polyhedral oligomeric silsesquioxane (msib-POSS) filler have been studied. The msib-POSS molecules were added to the POM by direct melt blending at loadings between 0 and 10 wt.%. Hydrogen bondinginteractions were detected between POM and msib-POSS Si–OH groups, increasing their mutual compatibility and leadingto nanometer-size dispersion of some msib-POSS molecules. These interactions do not prevent POSS aggregation duringblending, but lead to micron-scale msib-POSS domains. The thermal decomposition temperature of the compositesremained practically constant under inert and oxidative conditions. The low temperature thermal transition (g) and glasstransition temperature (Tg) of POM were found to move to higher temperatures only when 2.5 wt.% of msib-POSS wasadded, indicating that POSS is physically linked to the POM chains, restricting their motion under those conditions. Lowcontent (2.5 wt.%) of msib-POSS results in antiplastization, whereas higher levels of POSS lead to a decrease in the storagemodulus of the polymer. The relationships among these effects and the morphological characteristics of the systems willbe discussed herein.

KeywordsPolyhedral oligomeric silsesquioxane (POSS), melt-mixed, morphology

Introduction

Polymer composites in which the filler has at least one

dimension in the nanometer range, that is, polymer nano-

composites, have attracted much interest in recent years for

the development of high-performance materials. It is well

recognized that the behavior of composites largely depends

on interfacial interactions, the smaller in size the compo-

nents, the greater the contribution of interfacial interactions

to the material properties. Thus, a major challenge in the

development of high performance nanocomposites is the

control of nanoparticle dispersion. The mechanical, ther-

mal and physical properties are greatly enhanced by the

incorporation of nanoparticles of different sizes and aspect

ratios, such as layered silicates,1 carbon nanotubes2 and

polyhedral oligomeric silsequioxanes molecules3 into vari-

ous polymeric matrices.

Polyhedral oligomeric silsesquioxane (POSS) mole-

cules have a basic polyhedral silicon-oxygen nanostruc-

tured skeleton or cage described by the general chemical

structure R(SiO1.5)n, where n ¼ 8, 10 or 12. This cage is

surrounded by a corona of organic groups linked to silicon

atoms by covalent bonds and may be fully ‘condensed’ or

exist in an ‘open’ structure. The size of the POSS nanopar-

ticles range from 1 to 3 nm,3 and they can be successfully

incorporated into various polymers by physical blending4–8

or chemical tethering.8–14 Thus, a range of POSS grades

have been incorporated in various commodity,4,15–17 engi-

neering6–8,13,14,18,19 and high-performance20–22 thermo-

plastic polymers, and some thermoset systems.23,24 The

1 Centre Catala del Plastic, Universitat Politecnica de Catalunya, Terrassa,

Spain2 Department of Macromolecular Science and Engineering, Case Western

Reserve University, Cleveland, OH, USA3 Mechanical and Industrial Production Department, Mondragon

Unibertsitatea, Arrasate-Mondragon, Spain

Corresponding Author:

Miguel Sanchez-Soto, Centre Catala del Plastic, Universitat Politecnica de

Catalunya, Colom 114, 08222 Terrassa, Spain

Email: m.sanchez-soto@upc.edu

High Performance Polymers23(6) 457–467ª The Author(s) 2011Reprints and permission:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954008311415301hip.sagepub.com

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improvement in thermo-oxidative resistance and reduction

in flammability observed in some POSS-based nanocom-

posites has made these systems excellent candidates for

high-temperature and fire-resistance applications.25,26

The structure/morphology and the properties of the

POSS–polymer systems were found to depend on the

degree of bonding and on the concentration of the POSS

units. Thus, POSS–polymer interactions dominate these

systems when the POSS units are covalently linked to the

polymer chains.8,17,19,27–29. On the other hand, POSS–

POSS interactions become more important in physical

POSS–polymer systems and give rise to aggregation and

crystallization of POSS molecules.8,18,30,31

Polyoxymethylene (POM) is a thermoplastic polymer

with a basic molecular structure consisting of repetitive

carbon–oxygen linkages. POM is highly crystalline and is

characterized by a high stiffness, dimensional stability and

corrosion resistance. However, its low impact toughness

and mainly low heat resistance limit its range of applica-

tions. To improve its behavior POM has been modified

with elastomers,32 inorganic fillers such as organoclay,33

and recently with POSS molecules.8,19 Given the relatively

high price of most commercial POSS, the use of these

nanoparticles as fillers is restricted to cases in which a low

amount is used or the advantages of the application over-

come the increase in total cost.

The current work focuses on the effect of

monosilanolisobutyl-POSS (msib-POSS) addition on the

structure, morphology and thermo-mechanical behavior

of POM/POSS composites. This filler was selected because

it possesses an ‘open’ hybrid inorganic–organic three-

dimensional structure which contains one silanol (Si–OH)

group capable of reacting with the POM backbone. The

structural changes of the composites were assessed by

infrared spectroscopy (IR), the morphology by scanning

electron microscopy (SEM) and energy-dispersive X-ray

analysis, the thermal properties by differential scanning

calorimetry (DSC) and thermogravimetric analysis (TGA)

and the thermo-mechanical behavior by dynamic-

mechanical analysis (DMA).

Experimental

Materials

The ethylene-polyoxymethylene copolymer (POM) type

used in this work as a matrix was a Hostaform C13021 sup-

plied by TICONA (Barcelona, Spain), with a melt volume

rate of 12 cm3/10 min at 190 �C and 2.16 kg. Prior to com-

pounding the polymer pellets were dried for at least 4 h

under vacuum at 80 �C, and maintained in desiccators prior

to mixing.

Monosilanolisobutyl-POSS (msib-POSS) was kindly

supplied from Hybrid Plastics, and used as received. The

chemical structure of msib-POSS used in this work is

shown in Figure 1.

POM/POSS composites preparation

Composites were obtained by mixing POM and different

contents of msib-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 �C at a screw

speed of 100 rpm for 5 min prior to extrusion. As well as

low POSS contents (2.5 wt.%), high contents (5–10 wt.%)

were selected in order to clearly show the effects of POSS

addition, which are otherwise difficult to observe if very lit-

tle quantities are used.

The extruded materials were compression molded into

films (constrained inside a mold) using a Carver model C

press. The samples were first heated at 190 �C for approx-

imately 10 min and a rapid compression (4–5 ton), fol-

lowed by release of pressure to remove any trapped gas

bubbles. The samples were then molded at 190 �C under

4–5 ton pressure for approximately 2 min and cooled rap-

idly between two water-chilled aluminum plates. POM/

msib-POSS composites films with thickness between 0.2

and 0.3 mm were finally obtained.

Fourier transform infrared spectroscopy

The chemical structure of the composites, and that of the

reference materials, was analyzed using a NICOLET

6700 spectrophotometer. The spectral resolution was of

1 cm�1, and the wavenumber interval analyzed between

4000 and 400 cm�1. A Smart Orbit high-performance dia-

mond single bounce Attenuated total reflection (ATR)

Figure 1. Chemical structure of msib-POSS molecule.

458 High Performance Polymers 23(6)

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accessory was used. The depth of penetration of the equip-

ment is 2.03 mm at 1000 cm�1. The resulting samples from

the melt-mixing process were exposed to the ATR by its

direct placement upon the ATR crystal.

Morphological analysis

Scanning electron microscopy was used to analyze the

microstructure of the composites and the degree of disper-

sion 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 palladium layer to make the sur-

face conductive.

Samples were also analyzed by energy dispersive X-ray

analysis (EDAX) scan to identify the presence of msib-

POSS in the bulk. In such case, after being cryogenically

fractured, the samples were sputtered with carbon to avoid

interferences between gold and silicon signals.

Differential scanning calorimetry

The thermal behavior of the composites, and that of refer-

ence 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 indium of high purity. Samples of

approximately 5 mg were tested at a heating rate of

10 �C min�1 from 30 to 200 �C 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 crystal-

linity level 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)34.

Thermogravimetric analysis

Thermogravimetry (TGA) was performed on a Mettler

Toledo TGA/DSC 1 Star System. Samples sizes ranging

from 5 to 10 mg were loaded in alumina pans and heated

at a rate of 10 �C min�1 Weight loss was traced as samples

were heated from room temperature to 600 �C under a dry

nitrogen or oxygen purge of 60 mL min�1.

Dynamic mechanical analysis

A Thermal Analysis Instruments Q800 DMA was used in

tensile mode at an oscillatory frequency of 1 Hz at 20 mm

displacement for all samples. The temperature scan was

performed at 3 �C min�1 heating rate in the range from

�100 to around 150 �C. Sample dimensions were typically

4.9 mm long, 9.8 mm wide and 0.2–0.3 mm thick.

Results and discussion

External appearance

The external appearance of the POM/msib-POSS compo-

sites after blending and compression molding was similar

to that of pure POM samples. The opacity of the POM and

POM/msib-POSS composites disallowed further conclu-

sions about the compatibility/miscibility between the

matrix and filler to be drawn. In fact, the external appear-

ance of the matrix was mostly unaffected by the addition

of POSS. When the amount of POSS was lower than

10 wt.%, all samples showed a similar white color and

smooth appearance. At the highest POSS level, the color

seems to slightly turn to yellow. Nevertheless, external sig-

nals of incompatibility, such as the migration of POSS to

the surface of the films, were not detected. This indicated

that nano-particles were, at the very least, well dispersed

within the POM copolymer matrix.

Infrared spectroscopy

The possible reactions and/or interactions between POM

and msib-POSS molecules were studied by means of infra-

red spectroscopy, because of Fourier transform infrared

(FT-IR) absorption changes, in terms of strength and posi-

tions of characteristic functional groups, are indicative of

the existence of specific intermolecular and/or intramole-

cular interactions.7,13,20,29 Thus, FT-IR can be used to study

the mechanisms of interpolymer miscibility, through the

formation of hydrogen bonds or dipole–dipole interactions,

as interactions in composites can lead to considerable dif-

ferences between the spectra of the starting polymer matrix

and that of the polymer within a composite.

The different characteristic absorption bands corre-

sponding to neat POM and msib-POSS functional groups

are presented in Table 1. The most characteristic absorption

band of POM is located at 1090 cm�1, corresponding to the

Table 1. Assignment of FT-IR spectra characteristic absorptionbands of POM and msib-POSS.

MaterialWavenumber(cm�1) Assignment

POM 1090 C–O–C symmetric stretchingvibration

msib-POSS 1080 Si–O–Si symmetric stretchingvibration

3530 O–H stretching vibration ofSi-O-H

895 Si–O stretching vibration2945 CH2 asymmetric stretching

vibration1465 CH2 deformation vibration

(scissoring)

Illescas et al 459

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symmetric stretching vibration of C–O–C ether groups. The

FT-IR spectrum of the pristine msib-POSS shows several

characteristic absorption bands: the intense and wide band

at 1084 cm�1 assigned to Si–O–Si stretching vibration; the

Si–O–H absorption bands including the broad strong band

at 3530 cm�1 assigned to O–H stretching vibration (inter-

molecular hydrogen bonds) and the band at 895 cm�1

assigned to Si–O stretching vibration. Finally, associated

to the CH2 group, the more characteristics bands are the

ones at 2945 cm�1 assigned to asymmetric stretching vibra-

tion and the band at 1465 cm�1 assigned to deformation

vibration (scissoring).

The FT-IR spectra of POM composite with 10 wt.%msib-POSS, and that of neat POM and msib-POSS mole-

cules as reference over the range 400–4000 cm�1 and

3000–4000 cm�1 are shown in Figure 2(a) and (b), respec-

tively. The additional spectra for other composites with dif-

ferent msib-POSS contents are not included herein because

of their similarity to that shown in Figure 2. As it can be

seen in Figure 2(a), the FT-IR spectrum of POM with

10 wt.% msib-POSS shows the main absorption bands that

appeared in neat POM and msib-POSS. This spectrum does

not show any new apparent band. This constancy can be

attributed to the coincidence of C–O–C and Si–O–Si

(1084 cm�1) signals of POM and msib-POSS, respectively.

Moreover, as the amount of msib-POSS added to POM was

small, any absorption band corresponding to POSS mole-

cules can be masked by POM absorption bands.8

Due to its chemical structure, hydrogen bonding interac-

tions between the oxygen of POM and the hydrogen of

msib-POSS may be expected. Therefore, Figure 2(b) pre-

sents scale-expanded FT-IR spectra (4000–3000 cm�1) of

the composite with 10 wt.% msib-POSS, and that of neat

materials as reference. As can be observed, pure msib-

POSS shows a strong band located at 3530 cm�1 assigned

to O–H vibrations, while POM does not show any signifi-

cant signal at these wavenumbers. When the spectrum of

the 10 wt.% msib-POSS/POM composite is analyzed, a

slightly broad band located between 3600 and 3100 cm�1

can be observed. This is attributed to hydrogen bonding

interactions between POM and msib-POSS. The presence

of these intermolecular interactions could modify the

degree of association between POM matrix and msib-

POSS molecules by affecting the molecular dynamics in

the composite system.29,35,36 No new absorption bands

were in fact detected for the 2.5 and 5 wt.% msib-POSS

composite systems, however, indicating that if existing,

hydrogen bonding interactions would be below the detec-

tion limit of the ATR equipment.

Morphology

The degree of dispersion of POSS moieties in polymer

matrices has a great effect on both thermal and mechanical

properties of the resultant composites.6–8,14 For this reason,

the cryofractured surfaces of POM/msib-POSS composites

were studied by SEM with the aim of establishing the level

of dispersion of msib-POSS molecules into the matrix, and

in consequence, the interaction level between them.

Figure 3 shows the cryofractured surfaces of (a) neat POM

as a reference and that of POM/msib-POSS composites

with (b) 2.5, (c) 5 and (d) 10 wt.% msib-POSS contents.

A poor dispersion of msib-POSS molecules within the

POM matrix was achieved as the filler particle size

increased from approximately 3 mm (2.5 wt.% msib-POSS)

up to 10–20 mm (10 wt.% msib-POSS). As POSS cage

molecules are characterized to have a maximum diameter

of approximately 3 nm,3 these micron size particles

observed in the micrographs are the result of nano-

spherical aggregates. The presence of aggregates at low

msib-POSS contents suggests that the solubility limit for

msib-POSS in POM is lower than 2.5 wt.%.

In the authors’ previous study, POM was melt-mixed

with trisilanolphenyl-POSS (tsp-POSS), leading to a

5001000150020002500300035004000

POM

10 wt%

(a) (b)

msib-POSS

Abs

orba

nce

Wavenumber (cm–1) Wavenumber (cm–1)

300032003400360038004000

Abs

orba

nce

POM

10 wt% msib-POSS

msib-POSS

Figure 2. FT-IR spectra of POM/POSS composites with 10 wt.% msib-POSS, and that of neat POM and different POSS molecules as areference over the range (a) 400–4000 cm�1 and (b) 3000–4000 cm�1. To aid clarity, the curves are shifted on the vertical axis.

460 High Performance Polymers 23(6)

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molecular level dispersion of the nanoparticles.8 This dis-

persion was successful because the similar polarities of the

three hydroxyl groups of the nanoparticles and the ether

groups of the polymeric chain of POM, and the possible

reaction between both components during melting. Among

two kinds of POSS silanols (msib-POSS and tsp-POSS),

msib-POSS has lower enhancement effect on the morphol-

ogy than that of tsp-POSS, because each msib-POSS mole-

cule has only one reactive Si–OH group and seven inert

isobutyl groups creating strong steric hindrance.37

Another possibility for this behavior could be due to

hydrogen bond interactions, which increase the compatibil-

ity between POM and msib-POSS. However, these interac-

tions are not strong, so they do not prevent msib-POSS

aggregation and phase separation during processing. This

leads to micrometric size dispersion of POSS.29

The spherical morphology of msib-POSS aggregates

was indicative that the phase separation took place dur-

ing the cooling of POM/msib-POSS composites. Similar

to other POSS-based composites, the development of

aggregates could be due to some POSS/POSS interac-

tions,8,18,30,31 as small particles are usually obtained

when effective interactions between matrix and POSS

molecules are formed.9,17,19,27,29 Nevertheless, the main

reason seems to be the low solubility of msib-POSS in

the POM matrix. As can also be observed in Figure 3,

the adhesion level between POM and msib-POSS was

quite poor, because the surfaces of the majority of par-

ticles were very clear and regular, indicating a low level

of adhesion and/or compatibility between the POM

matrix and msib-POSS moieties.

The dispersion and morphology of msib-POSS into

the POM matrix has been also studied by means of

EDAX analysis. Figure 4 shows the SEM–EDAX micro-

graphs of POM/msib-POSS composites with the addition

of (a) 2.5 and (b) 10 wt.% msib-POSS. The micrograph

for 5 wt.% composite is not shown because of its simi-

larity to that shown in Figure 4. As it can be seen, a

poor dispersion of the msib-POSS is detected by the

accumulation of silicon signals (white dots) indicating

POSS aggregation. Moreover, as has been previously

observed, the size of the aggregates clearly increase with

msib-POSS content from approximately 5 mm for 2.5 wt.%to 20 mm for 10 wt.%.

Figure 3. SEM micrographs of the cryofractured surfaces of POM/msib-POSS composites with (a) 0 (b) 2.5 (c) 5 and (d) 10 wt.%msib-POSS content.

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Some msib-POSS particles are found to be well dis-

persed into POM matrix as deducted from the isolated sili-

con signals observed all around the micrographs with

independence of the msib-POSS content. As explained, this

behavior is thought to be a consequence of weak interac-

tions between the ether and hydroxyl groups of POM and

msib-POSS, respectively.

Differential scanning calorimetry

Differential scanning calorimetry analysis was used to

study the influence of the msib-POSS molecules on the

melting and crystallization behavior of thePOM matrix.

The melting temperature and crystallinity content values

for POM/msib-POSS composites, and that of neat POM

and msib-POSS materials as reference, as determined by

the first and second DSC heating scans are reported in

Table 2. As can be seen, the melting temperature and

crystallinity content of POM was slightly higher in the sec-

ond heating scan than in the first one. This was because the

samples were slowly cooled between both heating scans.

However, these differences were negligible; then the ther-

mal behavior of the first DSC heating scans will be only

presented herein.

Figure 5 shows the first DSC heating scan for POM/

msib-POSS composites, and that of neat POM and msib-

POSS as reference. The first DSC heating scan of POM

only showed a narrow melting peak at around 167 �C. No

crystallization exotherm was observed during the DSC

heating scan, indicating that POM was fully crystallized

upon the processing conditions used. Neat msib-POSS is

a white crystalline powder at room temperature with a

melting peak at approximately 140 �C. Therefore, the

msib-POSS will be in the molten state during the POM

processing, making possible the mutual interaction.

As shown in Table 2 and Figure 5, the melting

temperature of POM in the composites remained constant

approximately at 166 �C independently of the amount of

msib-POSS added. These results indicate that msib-POSS

Figure 4. SEM-EDAX micrographs of the cryofractured surfaces of POM/msib-POSS composites with (a) 2.5 wt.% and (b) 10 wt.%msib-POSS.

Table 2. First and second DSC melting temperature (Tm) andcrystalline level (xc) values for POM/msib-POSS composites.

Tm,POM

(�C)Tm,msib-POSS

(�C)xc,POM

(%)

First heating POM 167.1 – 55.02.5 wt.% 166.2 – 53.65 wt.% 165.7 – 51.410 wt.% 166.4 -– 52.2msib-POSS – 139.1 –

Secondheating

POM 169.1 – 58.62.5 wt.% 168.4 – 57.15 wt.% 166.5 – 55.410 wt.% 166.9 – 57.0msib-POSS – 138.6 – 60 80 100 120 140 160 180 200

POM

5.0 wt.%

2.5 wt.%

10 wt.%

msib-POSS(E

xo)

Hea

t fl

ow:

(Wg–1

)

Temperature (˚C)

Figure 5. First DSC heating scans for POM/msib-POSS compo-sites, including that of pure POM and msib-POSS for reference.

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did not affect the crystalline structure of POM. However,

crystalline disruption has been observed in other POSS-

based composites with polypropylene,4 poly(ethylene ter-

ephthalate),38 POM19 and nylon 6.30

It is also interesting to note that POM composites do not

show a melting peak for msib-POSS in the DSC heating

scans. This observation suggests that the size of formed crys-

tals is too small for being detected by DSC measurements.5

The crystallinity content of POM/msib-POSS compo-

sites are collected in Table 2. The crystallinity slightly

decreased upon the addition of msib-POSS from 55% (neat

POM) up to approximately 52% (with 10 wt.% msib-

POSS). The slightly lower degree of crystallinity in the

POM/msib-POSS composites could be attributed to the

presence of hydrogen bond interactions that hinders the for-

mation of ordered domains.10 This small effect on the crys-

tallinity also hints at a low hydrogen bonding interaction

between POM and msib-POSS.

Thermal stability

The thermal stability of POM/msib-POSS composites was

studied by means of thermogravimetric analysis under inert

(nitrogen) and oxidative (air) conditions. The chain stabi-

lity and the efficiency of POSS molecules as heat

stabilizers were tested under nitrogen, because this atmo-

sphere prevents auto-oxidation and reactions with second-

ary products. Otherwise, air conditions were used because

this environment represents the most common conditions

used in the processing of thermoplastics.

Figure 6 shows the weight loss curves as a function of

temperature under (a) nitrogen and (b) air conditions of

POM/msib-POSS composites, and that of neat materials

as reference. Whatever the conditions used, the onset

degradation temperature was evaluated by the temperature

of 5 wt.% weight loss (T5) and the end degradation tem-

perature was evaluated by the temperature of 90 wt.%(T90) and 95 wt.% weight loss, (T95) from the weight loss

curve. The temperature of the maximum weight loss rate

(Tmax) was taken from the derivative weight loss curve.

In addition, the fraction of the solid residue at 600 �C was

obtained. From the thermograms of Figure 6 the activation

energy of the thermal decomposition, E, was obtained. This

parameter was calculated applying the general solution for

the non-isothermal decomposition of a solid-state mate-

rial.45 The order of the reaction, n, was obtained through

the least squares method. The corresponding values are col-

lected in Table 3.

As shown in Figure 6(a), the weight loss of POM under

nitrogen conditions took place in a single step in the tem-

perature range of 300–425 �C, with an abrupt drop at

323 �C (T5). The thermal degradation of POM is thought

to be due to results of random initiated bond cleavage in the

carbon–oxygen backbone.39,40 The depolymerization or

‘unzipping’ yields formaldehyde (CH2O), which is known

to accelerate the reaction, especially in the presence of an

oxidizing atmosphere. In nitrogen up to 323 �C (T5), the

rate of degradation is low, because the stabilizers incorpo-

rated in commercial POM are being consumed. After this

point, a fast sublimation of the polymer takes place. The

degradation process of neat msib-POSS had two stages in

the temperature ranges of 220–350 �C and 360–540 �C,

with weight losses in the first and second stages of approx-

imately 40 and 35 wt.%, respectively. According to the T5

values, the msib-POSS molecules showed lower stability

than the POM matrix. This is probably due to the tendency

of msib-POSS molecules to decompose via partial loss of

organic substituents.19,41

Once the thermal stability of neat constituent materials

was analyzed, the thermal stability of POM/msib-POSS

composites under nitrogen conditions was examined. As

can be seen in Figure 6(a), the weight loss of the compo-

sites occurred in a single, almost complete step, leaving a

residue amount below 3 wt.% that increased with msib-

POSS content and was consistent with its degradation prod-

ucts. The T5 of the composites increased approximately

0

20

40

60

80

100

0 100 200 300 400 500 600 700

POM2.5 wt.%5.0 wt.%10 wt.%msib-POSS

Wei

ght

(%)

0

20

40

60

80

100

0 100 200 300 400 500 600

POM2.5 wt.%5.0 wt.%10 wt.%msib-POSS

Wei

ght

(%)

Temperature (˚C) Temperature (˚C)

(a) (b)

Figure 6. Thermogravimetric curves for POM/msib-POSS composites, along with that of neat POM for reference, showing the evolutionof weight loss under (a) nitrogen and (b) air conditions.

Illescas et al 463

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8 �C with the addition of 2.5 wt.% msib-POSS, giving a

synergistic behavior. For greater msib-POSS contents the

T5 remained practically constant at the same values of pris-

tine POM.

The thermal stability enhancement exhibited by the

composite upon the addition of 2.5 wt.% msib-POSS in

inert atmosphere could be ascribe to the promotion of a

more robust polymer network due to the restriction of the

thermal motions of the tethered polymer/POSS macromo-

lecular chain structure.13,14,19 Moreover, the slight thermal

stability improvement obtained at low msib-POSS contents

was also a consequence of nanometric dispersion, enhan-

cing interaction of msib-POSS with the POM matrix. This

is evident from Figure 4(a). At high loadings, the thermal

stability remained unchanged because of the formation of

aggregates. Pure POSS and POSS aggregates decomposed

earlier than neat POM. On the other hand, they did not

interact with polymer chains; as a result, the decomposition

temperature of POM prevails.

The present results are consistent with a previous study

when POM was melt-mixed with aminopropylisobutyl-

POSS (apib-POSS),19 the decomposition temperature

increased up to 50 �C upon the addition of 2.5 wt.% apib-

POSS, as a consequence of hydrogen bonding interactions

that led to nanometric dispersion of apib-POSS molecules

into POM matrix. In the present study the hydrogen bonding

interactions do not prevent msib-POSS phase separation,

then the improvement in thermal stability was much lower

than in POM/apib-POSS nanocomposites.

The weight loss curves for POM/msib-POSS compo-

sites in air are shown in Figure 6(b) and the correspond-

ing values collected in Table 3. As it can be seen, the

TGA curves of the composites, whatever the msib-

POSS content, showed very limited differences as com-

pared neat POM in terms of T5 and Tmax, indicating that

msib-POSS do not influence the thermal stability of

POM in air conditions. This can be due to the simulta-

neous effect of heat and reaction with oxygen which

induces chain scission and rapid formation of formalde-

hyde.39,46 If the unstable hemi-acetal (hydroxy) end

groups (–O–CH2–OH) of POM are not blocked, rapid

depolymerization takes place at only moderate tempera-

tures leading to an earlier breakdown of the POM chain

than only high temperatures in an inert atmosphere. The

lowering in the onset temperature in air indicates that the

stabilizing system has been consumed earlier. In sum-

mary, the presence of an oxidizing atmosphere is favor-

able to the earlier POM chain scission and formation of

compounds which accelerate the degradation.

The values of the activation energy of thermal decompo-

sition, E, are shown in Table 3. Parameter E reflects the

energy to destroy the molecular structure of the polymer.

The higher the E value, the higher the thermal stability.

From the table it can be appreciated that the stability of the

POM/msib-POSS composites increased in nitrogen with

the addition of msib-POSS whereas in air the stability is

always lower than the one of the pure POM.

In regard to char, it is worth noting that char formation

was higher under air than under nitrogen atmosphere. At

low POSS content, differences were hardly seen because

POM decomposes. This yields almost no residue. If any

residue was observed, it would be formed by processing

aids and stabilizers. At the higher POSS levels, char signif-

icantly increases in air. This is the result of the formation of

a SiO2 protective layer on the surface of POM/msib-POSS

composites. This layer acts as a protective barrier, prevent-

ing further degradation of the polymer. The organic part of

the msib-POSS can undergo Si–C bond cleavage. This

Table 4. The results of dynamic mechanical analysis (DMA) forPOM/msib-POSS composites.

Tg (�C) Ta (�C) E025 �C (MPa)

POM �61.3 111.4 1849msib-POSS2.5 wt.% �51.9 125.5 19135 wt.% �52.4 124.5 176810 wt.% �52.1 124.5 1497

Table 3. The temperature of 5 wt.% (T5%), 90 wt.% (T90%) and 95 wt.% (T95%) weight loss, the temperature of the maximum weight lossrate (Tmax), the activation energy (E), the reaction order (n) and the fraction of the solid residue at 600 �C for POM/POSS compositesunder nitrogen and air conditions.

T5% (�C) T90% (�C) T95% (�C) Tmax (�C) E (kJ mol�1) n Residue (wt.%)

Nitrogen conditions POM 323 406 414 366 356 2 1.72.5 wt.% 331 410 416 368 375 2 0.45 wt.% 320 402 414 354 390 2 2.810 wt.% 320 407 418 356 372 2 2.2msib-POSS 264 516 – 313 94 2 8.5

Air conditions POM 274 324 326 320 397 1 02.5 wt.% 273 327 329 323 390 1 0.45 wt.% 272 325 329 318 395 1 2.810 wt.% 273 345 – 330 324 1 6.0msib-POSS 248 – – 347/436 62 1 52.4

464 High Performance Polymers 23(6)

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cleavage is immediately followed by fusion of POSS cages,

which gives a stable silica layer.47,48

Dynamic-mechanical thermal analysis

Dynamic-mechanical thermal analysis permits the molecu-

lar relaxation behavior of small chain segments to be

detected, and, as a result, the phase heterogeneity can be

detected on smaller scales than by using DSC. For this rea-

son, DMA analysis has been performed to further investi-

gate the molecular interactions of POM/msib-POSS

composites. The corresponding results collected from the

loss tangent (tan d) and the storage moduli (E0) versus tem-

perature for composites, and pristine POM as a reference,

are shown in Table 4. All transition temperatures were

taken as the peak of the log(tan d) curves.

The neat POM had two main transitions.42,43 A narrow gtransition at low temperatures, approximately at �60 �C,

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 and

a broad a transition in the range of 50–150 �C, with a max-

imum value closes to 111 �C, being characteristic of the

crystalline fraction. This transition is associated with trans-

lational motions of the crystalline structure along the chain.

Moreover, the POM of this study showed a b transition of

low intensity between �40 and 30 �C and with a maximum

value close to�8 �C, that is related with motion of long seg-

ments 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.43 Given its low intensity and

relative unimportance in highly crystalline POM polymers,

this transition is not further considered in the present study.

Low-temperature and high-temperature DMA scans are

shown in Figure 7(a) and (b), respectively. The DMA spec-

trum of POM/msib-POSS composites, as pristine POM,

exhibited two well-defined relaxation peaks. As it can be

observed, the Tg that is ascribed to the g transition tempera-

ture of POM were found to increase with the addition of

only 2.5 wt.% msib-POSS from �61 �C (neat POM) to

approximately �52 �C, remaining constant with further

msib-POSS contents. Similar shifts to higher temperatures

on the Tg values of polymers upon POSS molecules addi-

tion have been observed in composites in which intermole-

cular interactions or chemical reactions took place;7,13,28

meanwhile Tg constancy took place in unreacted POSS-

polymer systems.19

With respect to the a transition temperature of the com-

posites (Figure 7(b)), its behavior was similar to that of Tg.

Thus, Ta that is associated with the crystalline structure

increased from 111 �C for POM up to approximately

125 �C for POM/msib-POSS composites, whatever the

msib-POSS content. The increase achieved for the Ta is

thought to be due to relatively rigid POSS which is physi-

cally linked to the POM macromolecular chain structure

restricting the motion of the crystalline POM in the

POM/msib-POSS structure.19 Thus, the free volume

decreased and may consequently hinder the phase separa-

tion, being two factors that lead to increasing transition

temperatures of synthetic polymers and blends.28 The con-

stancy of Ta with the amount of POSS added indicates that

maximum interactions with the polymer are achieved at

low POSS levels. Moreover, higher quantities of POSS are

detrimental because of the formation of aggregates.

The difference between DSC and DMA results, in terms

of the transition associated with the crystalline structure,

may be understood by considering the different experimen-

tal probe sizes. DMA is capable of identifying composi-

tional heterogeneity on the scale of approximately 5 nm,

while DSC is sensitive to heterogeneity only on a scale

of larger than 20 nm; that is, heterogeneities on smaller

dimensions will be averaged out by this probe.13,44 Thus,

the effect of nano-size dispersed msib-POSS molecules

(as evident from the morphology of the composites) into

POM transitions are detected by DMA.

−80 −70 −60 −50 −40 −30−1.7

−1.6

−1.5

−1.4

−1.3

−1.2

−1.1

−1.0

−1.7

−1.6

−1.5

−1.4

−1.3

−1.2

−1.1

−1.0−0.9(a) (b)

40 60 80 100 120 140 160

POM2.5 wt.%5.0 wt.%10 wt.%

POM2.5 wt.%5.0 wt.%10 wt.%

Temperature(°C) Temperature(°C)

log

(tan

δ )

log

(tan

δ )

Figure 7. DMA log(tan d) for POM/msib-POSS composites with different msib-POSS contents: (a) low and (b) high temperaturetransitions.

Illescas et al 465

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The values of E0 at 25 �C for POM/msib-POSS compo-

sites are also collected in Table 4. The most significant dif-

ferences were observed at the lowest content, because

while the value of E0 increased slightly from 1849 MPa

(pristine POM) up to 1913 MPa with the addition of

2.5 wt.% msib-POSS; it significantly decreased with the

addition of higher msib-POSS contents. The increase at low

msib-POSS contents could be attributed to the finer disper-

sion achieved at low contents. This maximum reinforce-

ment of a matrix polymer by POSS grades occurring at

about 2.5 wt.% has been observed by several authors, and

probably indicates that excess POSS within the matrix will

merely serve as a plasticizer, eventually exceeding the

solubility limit of POSS in most polymers, leading to exud-

ing of filler from the composites typically above approxi-

mately 10 wt.% filler.

Conclusions

Polyoxymethylene has been modified by the incorporation

of a partially condensed POSS cage (msib-POSS) in con-

centrations between 0 and 10 wt.% through direct melt

blending. Hydrogen bonding interactions between POM

and msib-POSS have been detected. As a consequence of

hydrogen bonding, a nanoscale dispersion of msib-POSS

molecules up to *2.5 wt.% loading occurred. Phase

separation of POSS and polymer was observed above

2.5 wt.% filler, producing aggregates up to 20 mm in dia-

meter, and likely resulting in plasticization of the matrix,

and a general decrease in polymer properties.

The melting temperature and the crystallinity level of

POM remained practically constant with POSS addition,

indicating that the crystalline structure of POM is not

affected upon the addition of msib-POSS. The thermal sta-

bility of POM under inert conditions at low msib-POSS

contents slightly increased. However, the thermal decom-

position of POM under an inert or oxidative atmosphere

remained practically constant for further msib-POSS con-

tents indicating a threshold level for POSS addition.

DMA testing showed that the glass and a transition tem-

perature of POM increased with the addition of msib-

POSS. The addition of POSS at levels higher than 2.5 wt.%was detrimental to the thermo-mechanical properties of the

composites.

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