Download - Polyurethane Networks Modified with Octa(propylglycidyl ether) Polyhedral Oligomeric Silsesquioxane

1842 DOI: 10.1002/macp.200600241 Full Paper

Summary:OpePOSS was incorporated into polyurethane tomake organic-inorganic hybrid composites and nano-com-posites containing up to 20 wt.-% POSS were prepared. Theformation of the hybrid polyurethane networks is ascribed totwo principal cross-linking reactions: i) the cross-linkingreaction between MOCA and the polyurethane prepolymerand ii) the inter-component reaction between the PU net-works and OpePOSS. The latter was confirmed by modelcompound reactions. TEM indicates that the POSS washomogeneously dispersed in the polymer matrix at the nano-meter scale. DSC showed that the nanocomposites displayedincreased glass transition temperatures compared to thecontrol polyurethane. In terms of TGA, the nanocompositesdisplayed improved thermal stability. Tensile tests indicatethat the organic-inorganic hybrid networks were signifi-cantly reinforced with the inclusion of POSS. Contact anglemeasurements show that the organic-inorganic nanocompo-sites displayed a significant enhancement in surface hydro-phobicity as well as a reduction in the surface free energy.The improvement in surface properties was ascribed to thepresence of the POSS moiety in place of the polar com-ponent of polyurethane. XPS shows enrichment with Si-containing moieties on the surfaces.

Organic-Inorganic Hybrid PU Networks.

Polyurethane Networks Modified with

Octa(propylglycidyl ether) Polyhedral

Oligomeric Silsesquioxane

Yonghong Liu, Yong Ni, Sixun Zheng*

Department of Polymer Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road,Shanghai 200240, ChinaFax: þ86 21 5474 1297; E-mail: [email protected]

Received: May 16, 2006; Revised: July 12, 2006; Accepted: August 16, 2006; DOI: 10.1002/macp.200600241

Keywords: morphology and properties; polyhedral oligomeric silsesquioxane; polyurethanes

[17–29]
Introduction
The purpose of incorporating inorganic or organometallic

segments into organic polymers is to afford improved

material properties via the synergism of the inorganic and

organic components.[1–6] During the past years, organic-

inorganic nanocomposites have been prepared via the sol-

gel process:[7–11] intercalation and exfoliation of layered

silicates by organic polymers.[12–16] Polyhedral oligomeric

silsesquioxane (POSS) macromers and POSS-containing

polymers are emerging as a new technology for accessing

Macromol. Chem. Phys. 2006, 207, 1842–1851

organic-inorganic nanocomposites. POSS molecules

possess nano-sized cage-like structures, derived from

hydrolysis and condensation of trifunctional organosilanes

with a formula of [RSiO3/2]n, with n in the range from 6 to

approximately 12, and where R are various types of organic

groups, one or more of which is reactive or polymeriz-

able. With the reactive R groups, POSS molecules can be

introduced onto the backbones of polymer chains via the

formation of covalent bonds between the POSS cages and

the polymer matrices.[17–29] During the past years, there

has been extensive work reported in the literature on

� 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Polyurethane Networks Modified with Octa(propylglycidyl ether) Polyhedral Oligomeric Silsesquioxane 1843

organic-inorganic hybrid thermoplastic (or thermoset)

nanocomposites containing POSS. However, the modifica-

tion of elastomeric materials via POSS remains largely

unexplored.

Polyurethanes (PU) are a class of important elastomers,

and their wide spread applications drive efforts to prepare

materials with improved properties. Several authors have

reported the modification of polyurethanes via POSS. Fu

et al.[30] and Hsiao et al.[31] investigated the structure and

properties of a linear polyurethane containing POSS. In

their work, a 3-(allylbisphenol-A) propyl(dimethylsilox-

ane) POSS was used as a chain extender and POSS mole-

cules were grafted onto the macromolecular backbone

through one of the corner groups of the POSS. Devaux

et al.[32] reported that the incorporation of POSS into

polyurethane gave materials with improved flame retar-

dance. To facilitate incorporation of silsesquioxane cages

into polyurethanes, Neumann et al.[33] reported the syn-

thesis of an octa(isocyanate)-functionalized POSS macro-

mer, which can subsequently be used to prepare organic-

inorganic hybrid polyurethanes. Liu et al.[34] used

octa(aminophenyl) POSS to replace a part of the aromatic

amine, the traditional extender or cross-linker, to cross-link

the polyurethane networks and found that even the pre-

sence of a small amount of POSS can result in a significant

improvement in the thermo-mechanical properties. More

recently, Turri et al.[35,36] and Oaten et al.[37] reported

studies on the surface properties of linear polyurethanes

modified with mono-functional POSS macromers and

noted that the presence of POSS can significantly reduce

the surface free energy and thus improve surface hydro-

phobicity.

To the best of our knowledge, all previous studies were

concerned mainly with linear polyurethanes modified

with POSS, while cross-linked polyurethane networks

remain largely unexplored.[33,34] In POSS-containing

linear polyurethanes, the POSS molecules were grafted

onto polyurethane chains with one (or two) covalent

bond(s). Nonetheless, there is continuing interest in intro-

ducing octa-functionalized POSS into polyurethanes to

obtain POSS-reinforced PU networks. In this protocol of

preparation, octa-functionalized POSS molecules will act

as nano-sized cross-linking sites and thus, the thermo-

mechanical properties of materials may be further im-

proved. In the present work, we present studies on the

modification of polyurethanes using octa(propylglycidyl

ether) polyhedral oligomeric silsesquioxane (OpePOSS).

In this system, additional nano-cross-linking reactions

could be formed apart from the cross-linking reaction

between the polyurethane prepolymers and aromatic ami-

nes. The additional cross-linking reaction occurs between

the amide moiety of the polyurethane and the epoxide

groups of the OpePOSS. The goal of this work is to explore

the efficiency of this approach by investigating the pro-

perties of the resulting materials. To this end, the cross-

Macromol. Chem. Phys. 2006, 207, 1842–1851 www.mcp-journal.de

linking reactions involved in the formation of hybrid net-

works are investigated via the reaction of model com-

pounds. The morphology and properties of the organic-

inorganic hybrid networks were investigated through

transmission electronic microscopy (TEM), thermal ana-

lyses (DSC and thermogravimetric analysis (TGA)) and

measurements of the mechanical and surface properties.

Experimental Part

Materials

Toluene-2,4-diisocyanate (TDI) and 4,40-methylenebis(2-

chloroaniline) (MOCA) were of analytical grade, purchasedfrom Shanghai Reagent Co., China. Poly(propylene oxide)glycol (PPG) was kindly supplied by the Polyurethane Divi-sion, Gaoqiao Petrochemical Co., Shanghai/China, under thetrade name GE-210. It has a quoted molecular weight ofMn ¼1 000. Diglycidyl ether of bisphenol A (DGEBA)was suppliedby Shanghai Resin Co., China and has a quoted epoxideequivalence of 185. OpePOSS was synthesized in our labo-ratory via the hydrosilylation reaction of octahydrosilses-quioxane (H8Si8O8) with allylglycidyl ether, catalyzed withthe Karstedt catalyst, as detailed elsewhere.[38,39] All of thesolvents such as toluene, tetrahydrofuran (THF), methanol andethanol were of chemically pure grade, obtained from com-mercial sources. Prior to use, toluene was purified via distil-lation over CaH2 under reduced pressure.

Reaction of Model Compounds

In order to investigate the additional cross-linking reactionbetween PU and OpePOSS, 2,4-di(ethyl carbamate)toluene(DEC) was designed and synthesized via the reaction betweenTDI and ethanol. Typically, TDI (1.00 g, 5.74 mmol) wasadded to a flask equipped with a condenser and a magneticstirrer, and then ethanol (1.10 g, 0.024 mol) was added withvigorous stirring in a dry nitrogen atmosphere. The reactionwas carried out at 85 8C for 2 h. The excess ethanol waseliminated via rotary evaporation and a white solid wasobtained with a yield of 90 wt.-%.

FTIR (KBr): 3 337, 3 288 (N–H in amide), 1 696 cm�1 (O––C< in urethane moiety).

1H NMR (chloroform-d): d¼ 1.33–1.28 (CH3CH2O–, 6.03H, m, J¼ 6.83 Hz); 2.2 (CH3–Ph, 3.23 H, s); 4.25–4.18(CH3CH2O–, 4.00 H, m, J¼ 6.83 Hz); 6.38, 6.58 and 7.09(protons of aromatic ring; 1.00 H, 0.96 H and 1.08 H; s, s andd, J¼ 7.80 Hz); 7.78 (–OCONH–, 2.45, s).

Equimolar amounts of DEC (2.66 g, 0.01 mmol) andDGEBA (3.91 g, 0.01 mmol) were added to a flask equippedwith a magnetic stirrer. In a nitrogen atmosphere, the mixturewas heated up to 100 and 150 8C, and reactions were carriedout with stirring for 4 h, respectively. It was observed that theviscosity of system did not display significant changes for thereaction at 100 8C. However, the system viscosity increasedsignificantly when the reaction was carried out at 150 8C, inthe same timescale, thus polymerization occurred under theseconditions. In the final stage of the reaction, the reactants were


1844 Y. Liu, Y. Ni, S. Zheng

converted into a thermoplastic product. Solubility tests showthat the resulting product is soluble in common solvents suchas THF and chloroform.

Preparation of POSS-Containing PolyurethaneNanocomposites

50.00 g PPG and 17.416 g TDI (i.e., [NCO]:[OH]¼ 2:1) wereadded to a dried, 250 ml, three-necked flask, equipped with acondenser capped with a CaCl2 drying tube, a mechanicalstirrer and a N2 inlet. The reactants were reacted at 85 8C withvigorous stirring for 2 h under a dry nitrogen atmosphere toproduce the PU prepolymer. The desired amount of OpePOSSwas added to the PU prepolymer under vigorous stirring toattain complete dissolution, and then an amount ofMOCAwasadded to system, equimolar with respect of the quality of the –NCO groups of the polyurethane prepolymers, under vigorousstirring to ensure that the MOCAwas completely dissolved inthe system. The mixtures were degassed under decreasedpressure, before the reactants were poured into a pre-heatedstainless steel mold. The samples were cured at 110 8C for 4 hand 150 8C for 4 h to access the complete cross-linking reac-tions. The compositions of organic-inorganic hybrid poly-urethanes containing POSS are summarized in Table 1.

Measurement and Techniques

Solubility Analysis

The organic-inorganic hybrid composites were subjected tosolubility tests using common solvents such as chloroform,acetone and THF. The extraction experiments were carriedout using a Soxhlet extractor for 48 h. After extraction withTHF as the solvent, the gel components were dried at 60 8C for4 h, followed by drying in a vacuum oven at 60 8C for another4 h to remove the residual solvent. The dried samples wereweighed to calculate the fraction of insoluble (or soluble)components.

Fourier Transform Infrared (FTIR) Spectroscopy

Infrared measurements were carried out on a Perkin-ElmerParagon 1000 Fourier transform spectrometer at roomtemperature (25 8C). The attenuated total reflection (ATR)accessories were used to measure the FTIR spectra of all the

Table 1. Compositions of organic-inorganic hybrid polyur-ethane containing POSS.

POSS content Molar ratio of reactantsa)

wt.-%

0 2.0:1.0:0.36:0.0005 2.0:1.0:0.36:0.15810 2.0:1.0:0.36:0.33315 2.0:1.0:0.36:0.52920 2.0:1.0:0.36:0.750

a) The reactants are TDI, PPG, MOCA, and OpePOSS, respect-ively.


specimens on fresh surfaces. In all cases, 64 scans at aresolution of 2 cm�1 were used to record the spectra.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR measurements were carried out on a Varian MercuryPlus 400 MHz NMR spectrometer at 25 8C. The samples weredissolved in deuterated chloroform and tetramethylsilane(TMS) was used as the internal reference.

TEM

TEM was performed on a JEM 2010 high-resolution TEM atan accelerating voltage of 200 kV. The samples were trimmedusing an ultra-thin microtome and the specimen sections of 70nm approximate thickness were placed in 200-mesh coppergrids for observation.

DSC

Calorimetric measurements were performed on a PerkinElmer Pyris-1 DSC in a dry nitrogen atmosphere. The instru-ment was calibrated with standard indium. All the samples(about 10 mg) were heated from 20 to 200 8C and the DSCcurves were recorded at a heating rate of 20 8C �min�1. Theglass transition temperature (Tg) was taken as the midpoint ofthe curve at the change in heat capacity.

TGA

A Perkin-Elmer thermal gravimetric analyzer (TGA-7) wasused to investigate the thermal stability of the hybrid elasto-mers. Under an air atmosphere, the samples (about 10 mg)were heated from ambient temperature to 800 8C at a heatingrate of 20 8C �min�1. The initial thermal degradation tem-perature was taken as the onset temperature at which a massloss of 5 wt.-% occurs.

Tensile Mechanical Tests

Tensile mechanical tests were carried out according to rele-vant ASTM standards. Tensile strengths were measured usinga tensile testing machine (Instron 4465, United Kingdom) at astrain rate of 0.5% at room temperature. All the reportedresults are an average of at least five successful measurementsfor the tensile determinations.

Contact Angle Measurements

The static contact angle measurements with ultra-pure waterand diiodomethane were carried out using a JC2000A contactangle measurement instrument (Powereach Digital Instru-ments Ltd., Shanghai, China) at room temperature. Thespecimens were dried in a vacuum oven for 12 h prior tomeasurement.

X-Ray Photoelectron Spectroscopy (XPS)

XPS experiments were carried out on an RBD upgradedPHI-5000C ESCA system (Perkin Elmer Co.) with Mg and Alanode radiations. The X-ray anode was run at 250 W and thehigh voltage was kept at 14.0 kV with a detection angle at 548.The pass energy was fixed at 93.90 eV to ensure sufficientresolution and sensitivity. The base pressure of the analyzerchamber was about 10�8 Torr. The sample was directly



pressed to a self-supported disk (10 � 10 mm2), mounted on asample holder and then transferred into the analyzer chamber.The whole spectra (0 to 1 200 eV), and the narrow spectra ofall the elements with much higher resolution, were bothrecorded by using an RBD 147 interface (RBD Enterprises,USA) and using the AugerScan 3.21 software. The bindingenergies were calibrated by using the containment carbon(C1s¼ 284.6 eV). The data was analyzed using the RBDAugerScan 3.21 software.

Results and Discussion

Preparation of Inorganic-OrganicNanocomposites

The synthesis of organic-inorganic hybrid polyurethanes

containing POSS is illustrated in Scheme 1. For the control

PU networks, the reaction between PPG and TDI was set

at the molar ratio of isocyanate to epoxide groups of 2:1 to

produce the PU prepolymer terminated by isocyanate

groups. The PU prepolymer was further reacted with

the MOCA amino groups to form elastomeric networks

bearing a great number of amide moieties with urethane

structural units. For the hybrid composites containing

POSS, the cross-linking steps could include reactions be-

tween the PU networks and OpePOSS and between Ope-

NCO

O

Si

O

Si

OHO

O

HO

O

OH

OHO

C

O

N

NCO

NCO

CH3

+ OHHO

PU PPPG

OCN

N

TDI

: *

:

NHC

Scheme 1. Synthesis of the organic-inorganic


POSS and MOCA except for that between the prepolymer

and MOCA. It has been shown that cross-linking or chain

extending of the PU prepolymer by MOCA can readily

occur to completion at 110 8C for 4 h.[34] A still higher

temperature (e.g., 150 8C) is required for the reaction bet-

ween the PU networks and OpepOSS, which was con-

firmed via reactions of model compounds. In this work, the

organic-inorganic hybrid networks were prepared with

POSS contents up to 20 wt.-% (see Table 1).

All of the POSS-containing polyurethane networks were

homogeneous and transparent, implying that no macro-

scopic phase separation occurred, at least on the scale

exceeding the wavelengths of visible lights. The solubility

tests show that the materials can be swollen without being

dissolved in common solvents such as chloroform, acetone,

and THF, suggesting the formation of crosslinked net-

works. The as-prepared networks were further subjected to

extraction experiments with THF as the solvent. The fact

that no soluble components were extracted indicates that

all of the starting components participated in the formation

of the crosslinked networks. FTIR spectroscopy was used

to examine the degree of reaction and the expanded FTIR

spectra in the range of 600 to 1 850 cm�1 are shown in

Figure 1. For OpePOSS, the bands at 910 and 840 cm�1 are

ascribed to the stretching vibration of C–O–C moieties in

the epoxide groups, and can be used as probes to examine

CN

O

Si

O

Si

Si

O

Si

Si

O

SiO

OO

O

O OO

O

O OH

O

OH

O

OH

O

HOOCN

O

repolymer

NCO1) OpePOSS

O

O*

CH3

O

2) MOCA

hybrid PU networks.



Figure 1. The expanded FTIR spectra of the control poly-urethane and its hybrid composites containing POSS in the rangefrom 600 to 1 850 cm�1.

the reaction of OpePOSS in the composite systems.

Unfortunately, there are some overlaps of the band at

910 cm�1 between OpePOSS and the control polyurethane,

and thus the band at 840 cm�1 was used to provide infor-

mation about the degree of reaction of OpePOSS. It was

seen that the band at 840 cm�1 almost vanished for all the

hybrid composites containing OpePOSS, indicating that

the epoxide groups of OpePOSS participated in the cross-

linking reactions.

It is proposed that the following reactions could be

involved in the formation of the resulting cross-linking

networks in the composite system: i) the reaction between

the PU prepolymer and MOCA (that is, the reaction of

isocyanate with the amino groups); ii) the reaction between

OpePOSS and MOCA (that is, the reaction of epoxide and

amine) (this reaction leads to secondary hydroxyl groups in

the hydroxyether structural units [–O–CH2–CH(OH) –

CH2–]); iii) the reaction of the secondary hydroxyl groups

with the terminal isocyanate groups of the prepolymer. The

first reaction can readily occur to completion at 110 8C in

4 h[34] whereas the occurrence of the second reaction at a

considerable scale requires an elevated temperature (e.g.,

150 8C). In order to verify the above reactions a model

reaction was carried out under identical reaction condi-

tions. 2,4-Di(ethyl carbamate) toluene (DEC) was prepared


(Scheme 2) and used as a model compound for the urethane

moiety of the PU networks, to react with DGEBA, a

compound with epoxide groups. In order to compare the

degree of reaction at different curing stages, a two-stage

reaction was carried out at 110 and 150 8C, respectively. Itwas observed that the viscosity of system did not change at

110 8C. The FTIR spectra (not shown here) further indicate

that the majority of reactants remain unreacted. However,

the viscosity of system increased significantly at 150 8C. Inthe final stage of reaction the reactants were converted into

a thermoplastic product, implying that polymerization

occurred under these conditions. Figure 2 presents the

FTIR spectra of DEC, DGEBA and the reacted product

from their stoichiometric mixture after being reacted at

150 8C for 4 h. DEC was characterized by the stretching

vibration band of the urethane moiety at 1 685 cm�1

and the stretching vibration bands of N–H at 3 337, and

3 288 cm�1 (Curve A). For Curve B, the stretching vibra-

tion band at 910 cm�1 was assigned to the epoxide groups of

OpePOSS. Under these conditions, it was seen that the

epoxide band of DGEBA was depleted, indicating that the

inter-component reaction between DGEBA and DEC was

performed to completion. It is of interest to note that, for the

reacted product, a broad band attributable to the stretching

vibration of hydroxyl groups appears at 3 313 cm�1. The

reaction of the model compounds verified that a significant

inter-component reaction between the PU networks and

epoxide groups of POSS occurred when held at 150 8C for 4

h.

Morphology of the Organic-InorganicPolyurethane Networks

The morphology of the POSS-containing polyurethane

networks was investigated by TEM. Figure 3 representa-

tively shows the TEM micrograph of the sectioned hybrid

elastomer containing 20 wt.-% of POSS. For comparison,

TEM images were taken at the edge of the sectioned

sample and the background. It is seen that the dark area (the

portion of the hybrid composite) is homogenous and no

localized domains were detected at the nanometer scale,

suggesting that the POSS component was homogenously

dispersed in the continuous polyurethane matrix at the

nanometer scale. A similar observation was also reported

for POSS-epoxy system by Choi et al.[40] The TEM results

indicate that the organic-inorganic PU nanocomposites

were obtained, based on the above preparation approach.

Thermal Properties

Glass Transition Behavior

The POSS-containing hybrid PU networks were subjected

to thermal analysis. The DSC thermograms of the control

PU and the organic-inorganic hybrid nanocomposites are



NCO

NCO

CH3

2CH3CH2OH

Reflux, 3h

CH3

HN

C O

O

C2H5

NH C

O

C2H5

DGEBA

150 oC, 4h

CH3

N

C O

O

C2H5

N C

O

C2H5

CH2 CH

OH

CH2 O C

CH3

CH3

O CH2 CH

OH

CH2

n

Scheme 2. Reaction of the model compounds.

presented in Figure 4. The control polyurethane displayed

a glass transition at �34 8C, indicating that this material is

a typical elastomer. For the POSS-containing hybrid

nanocomposites, all of the DSC curves exhibited single

glass transition temperatures (Tg) in the experimental

temperature range (�70 to 150 8C). In comparison with the

control PU, the hybrid nanocomposites showed an

enhanced Tg, which increased with increasing POSS

content. In POSS-containing hybrid nanocomposites, the

increase in the glass transition temperature is generally

interpreted on the basis of nano-reinforcement of the POSS

cages in the polymer matrices, which is able to restrict the

motions of the macromolecular chains.[41–44] In the present

case, the Tg enhancement is not only ascribed to the nano-

reinforcement of the POSS cage but also attributed to

additional cross-linking between the PU networks and

OpePOSS. It should be noted that although the nanocom-

Figure 2. The FTIR spectra of the model compounds. A) DEC,B) DGEBA, C) the product of a stoichiometric mixture ofDGEBA and DEC after reaction at 150 8C for 4 h.


posite containing 20 wt.-% POSS possessed a Tg of

�14 8C, the organic-inorganic network still retained its

elastomeric character.

Thermal Stability

TGA was used to evaluate the thermal stability of the

POSS-containing hybrid nanocomposites. The TGA

curves, recorded in air at 20 8C �min�1, of the control

PU and the nanocomposites are shown in Figure 5A.

Within the experimental temperature range, all of the

samples displayed similar degradation profiles, suggesting

that the POSS component did not significantly alter the

degradation mechanism of the matrix polymers. It is seen

that the initial thermal decomposition temperatures were

enhanced for the nanocomposites, which increased with

increasing POSS content. The observation can further be

illustrated by the derivatives of the TGA curves as function

Figure 3. The TEM micrograph of the hybrid polyurethanecontaining 20 wt.-% of POSS.



Figure 4. The DSC curves of the control PU and its hybridcomposites with POSS at a heating rate of 20 8C �min�1.

of temperature, which are shown in Figure 5B. It is noted

that the temperatures for the maximum rate of decom-

position (Tmax) increased with an increasing POSS content.

In addition, the yields of ceramic increased with an

increasing POSS content as expected. On the basis of the

above observations, it is adjudged that the thermal stability

of hybrids was enhanced with the inclusion of the inorganic

component.

It has been proposed that the tether structure is crucial

for the improvement of the thermal stability of POSS-

containing nanocomposites.[45,46] In the present case,

POSS cages participated in the formation of the cross-

linked network, that is the POSS cages were tethered into

PU networks. It is plausible that mass loss from thermal

decomposition via gaseous fragments could be suppressed

by well-dispersed POSS cubes at the molecular level.[47,48]

Therefore, the improved thermal stability of the nano-

composites could be a result of the combined effects of the

formation of tether structures between the PU matrix and

the POSS cages and the nano-scaled dispersion of POSS

cages in the PU matrix.

Figure 5. A) TGA curves and B) derivative TGA curves for thecontrol PU and its hybrid composites with POSS, recorded at aheating rate of 20 8C �min�1 in air.

Mechanical Properties

The mechanical properties of the control polyurethane and

the POSS-containing hybrid composites were evaluated by

tensile tests at room temperature. The stress-strain curves

are shown in Figure 6 and the results are summarized in

Table 2. The tensile tests indicate that all of the POSS-

containing hybrid polyurethanes behave like elastomeric

materials. In comparison with the control PU networks, the


POSS-containing nanocomposites exhibited an increased

Young’s modulus with increasing POSS content. This

increase can be interpreted on the basis of two factors: i)

the nano-reinforcement of POSS cages on the polymeric

matrices, and ii) the increase in cross-link density of

the networks. The nano-reinforcement of POSS could be

ascribed to the restriction of nano-sized POSS cages on the

deformation of the macromolecular chains, which can be

reflected by the increase in glass transition temperature (Tg)

of the materials. Both the model reaction and the glass



Figure 6. The stress-strain curves for the control polyurethaneand its hybrid composites for the tensile mechanical tests.

Table 3. Static contact angles and surface free energy of POSS-containing polyurethanes.

POSScontent

uH2O uCH2I2 gsd gsp gs

wt.-% mN �m�1 mN �m�1 mN �m�1

0 93.8� 1.3 58.0� 0.7 1.7 28.2 29.95 95.4� 1.8 63.1� 0.4 1.8 25.3 27.010 100.1� 1.9 70.2� 0.7 1.4 21.6 23.015 106.3� 0.9 80.4� 0.8 1.0 16.4 17.420 108.3� 1.2 82.5� 1.2 0.9 15.5 16.3

transition behavior showed that therewas an additional inter-

component reaction (viz. additional cross-linking) between

OpePOSS and PU networks. The increased cross-link den-

sity could be responsible for the decrease in the elongation at

break of the materials (See Figure 6 and Table 2).

Surface Properties

Static Contact Angle Analysis

Contact angles were measured with water and diiodo-

methane as probe liquids to investigate the surface

behavior of the inorganic-organic networks. The results

are summarized in Table 3. The contact angle of the control

PU was estimated with water to be approximately 948.Upon adding POSS to the system, the contact angles

increased with an increasing POSS content of up to 20 wt.-%

(See Figure 7). The surface free energies of the hybrid

nanocomposites containing various percentages of POSS

Table 2. Tensile mechanical properties of POSS-containingpolyurethanes.

POSS content Tensilemodulus

Tensilestrength at break

Elongationat break

wt.-% MPa MPa %

0 7.5� 0.7 2.1� 0.1 290� 35 9.7� 0.9 4.1� 0.4 210� 710 13.8� 1.1 4.1� 0.2 200� 515 19.6� 1.3 4.8� 0.3 160� 1220 36.3� 1.9 6.3� 0.1 90� 8


were calculated according to the geometric mean

model:[49,50]

Figureliquid,

cos u ¼ 2

gL

ðgdLg

ds Þ

12 þ ðgp

Lgps Þ

12

h i� 1 (1)

gs ¼ gds þ gps (2)

where u is contact angle and gL is the liquid surface tension;

gds and gps are the polar and dispersive components of gL,

respectively. From Table 3 it is seen that the total surface

free energies of the composites were reduced from 30 to

16 mN �m�1 on increasing the POSS content. It is noted

that the polar component is very sensitive to the POSS

concentration, suggesting that the inclusion of the POSS

moiety significantly decreased the contribution of the polar

groups (the urethane moieties) to the surface energy of

materials, that is to say, the POSS moiety on the surface

7. The plot of contact angles with water as the probeas a function of POSS content.



Figure 8. Wide-scan XPS of the hybrid PU composites contain-ing 10 wt.-% of OpePOSS.

acts as a screening agent to the polar groups of poly-

urethane. Similar results were reported in several mono-

functionalized POSS-containing linear polymer sys-

tems.[35,36,51]

XPS

The surface elemental compositions of the organic-

inorganic hybrid composites were investigated by means

of XPS. Figure 8 shows a representative XPS spectrum of

the hybrid polyurethane containing 10 wt.-% of OpePOSS.

The C 1s peak observed at approximately 286.8 eV is

responsible for the contribution of several types of carbon

atom from the PU and OpePOSS components, and the O ls

peak at approximately 535 eV is ascribed to oxygen in the

system. It should be pointed out that Cl and N signals are

too weak to be shown in this spectrum. The Si 2s and Si 2p

signals appeared at 156 and 105 eV, respectively, and are

ascribed to the Si atom in OpePOSS. The relative

Table 4. The elemental compositions of the surfaces of PU and its h

POSS content Experimental composition

wt.-% C O Si Cla)

mol-% mol-% mol-% mol-% m

5 56.08 26.31 16.77 0.14 010 54.30 24.42 20.95 0.03 015 52.59 25.15 22.07 0.05 020 51.00 25.13 23.83 ND 0

a) ND¼ not detected.


concentrations of C, O, N, Cl and Si on the surfaces of

the organic-inorganic hybrid composites, calculated from

the corresponding photoelectron peak areas are listed in

Table 4. It is noted that carbon and oxygen were dominant,

detected on the surfaces of all samples; a small amount of

nitrogen and chlorine in the hard segments of PU was also

detected. An examination of Table 4 reveals that the

concentrations of elemental Si on the surface of the hybrid

composites are significantly higher than the theoretical

values calculated in terms of the feed ratios. This obser-

vation suggests that an enrichment of Si-containing moiety

on the surfaces exists.[51,52] This result is important in

understanding the improvements in surface hydrophobicity

for the POSS-modified polyurethane composites. It is

worth noting that Si enrichment on the surfaces of the

hybrid composites does not increase significantly when the

concentration of OpePOSS is 10 wt.-% or more, implying

that thereafter the concentration of POSS moiety on the

surface was no longer increased.

Conclusion

OpePOSS was incorporated into polyurethane to prepare

organic-inorganic hybrid composites. Two principal cross-

linking reactions were involved with the formation of the

hybrid polyurethane networks. The first one is responsible

for the cross-linking reaction between MOCA and the

polyurethane prepolymer. It was identified that there was

an inter-component reaction between the PU networks and

OpePOSS, which was confirmed by model compound

reactions. TEM indicated that POSS was homogeneously

dispersed in the polymer matrix at the nanometer scale.

DSC showed that that the nanocomposites displayed

increased glass transition temperatures compared to the

control polyurethane. In terms of TGA, the nanocompo-

sites displayed an improved thermal stability. Tensile tests

indicated that the organic-inorganic hybrid networks were

significantly reinforced with the inclusion of POSS. Con-

tact angle measurements showed that the organic-inorganic

nanocomposites displayed a significant enhancement in

surface hydrophobicity, as well as a reduction in surface

ybrid composites containing POSS determined by means of XPS.

Theoretical composition

N C O Si Cl N

ol-% mol-% mol-% mol-% mol-% mol-%

.70 74.41 21.31 1.14 0.65 2.47

.20 72.98 22.23 2.09 0.57 2.13

.13 71.78 22.99 2.88 0.49 1.85

.03 70.76 23.65 3.55 0.43 1.61



free energy. The improvement in surface properties was

ascribed to the presence of the POSS moiety in the place of

the polar component of polyurethane. XPS showed enrich-

ment with Si-containing moieties on the surfaces.

Acknowledgements: Financial support from the ShanghaiScience and Technology Commission, China under a key project(No. 02DJ14048) is acknowledged. The authors also thank theNatural Science Foundation of China for partial support (No.20474038 & 50390090). One of the authors (S. Z.) would like tothank the Shanghai Educational Development Foundation,China for the ‘‘Shuguang Scholar’’ Award (2004-SG-18).

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