epoxy resin containing octamaleimidophenyl polyhedral oligomeric silsesquioxane

9
Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane Yong Ni, Sixun Zheng* Department of Polymer Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China Fax: (þ86) 21 54741297; E-mail: [email protected] Received: June 24, 2005; Revised: August 12, 2005; Accepted: August 15, 2005; DOI: 10.1002/macp.200500267 Keywords: epoxy; nanocomposites; polyhedral oligomeric silsesquioxane Introduction Organic-inorganic nanocomposites have received consid- erable attention since they can combine the advantages of inorganic materials (e.g., rigidity, stability) and organic polymers (i.e., flexibility, ductility, and processibility). [1–4] During the past decades, polymeric-inorganic nanocompo- sites have been prepared via the sol-gel process, [2,5–8] intercalation, and exfoliation of layered silicates by organic polymers. [9–13] Polyhedral oligomeric silsesquioxane (POSS) macromer and POSS-containing polymers have been emerging as a new technology for the preparation of organic-inorganic nanocomposites; POSS-containing nanocomposites are becoming the focus of many studies due to the excellent comprehensive properties of this class of hybrid materials and the simplicity in processing. [14–16] Polyhedral oligomeric silsesquioxanes are a class of important nanosized cage-like compounds (Scheme 1), derived from hydrolysis and condensation of trifunctional organosilanes and possess a formula of [RSiO 3/2 ] n , n ¼ 6– 12, where R can be various types of organic groups, one (or more) of which is reactive or polymerizable. The copoly- merization of POSS macromers with organic monomers has been proved to be an efficient approach to afford the nanocomposites due to the formation of covalent bonds between POSS cages and polymer matrices. Epoxy resins are a class of important thermosets, which have been widely used as matrices of composite materials, adhesives, and electronic encapsulating materials due to their high mechanical strength, excellent chemical resist- ance, and simplicity in processing. The extensive applica- tion motivates to prepare the organic-inorganic hybrid composites of epoxy resins with improved properties. The modification of epoxy resin via POSS could endow the materials with some superior properties such as impro- ved thermomechanical properties, thermal and oxidative Summary: Octamaleimidophenyl polyhedral oligomeric silsesquioxane (OmipPOSS) was synthesized via the imid- ization reaction between octaaminophenyl polyhedral oligo- meric silsesquioxane (OapPOSS) and maleic anhydride, and it was characterized by means of Fourier transform infrared (FTIR) and NMR spectroscopies. OmipPOSS was further employed to prepare epoxy hybrids. The thermosetting hybrids containing OmipPOSS up to 10 wt.-% were obtained via in situ polymerization of diglycidyl ether of bisphenol A (DGEBA) and 4,4 0 -diaminodiphenylmethane (DDM) in the presence of OmipPOSS. High-resolution transmission electronic microscopy (TEM) indicates that the nanometer- scaled dispersion of POSS molecules was obtained, suggest- ing that the nanocomposites were successfully prepared. The results of DSC showed that the glass transition temperatures (T g ’s) of the POSS-containing nanocomposites are dependent on the content of POSS in the nanocomposites. When the contents of POSS are less than 5 wt.-%, the nanocomposites displayed the enhanced glass transition temperatures (T g ’s) in comparison with control epoxy. Thermogravimetric analysis (TGA) showed that all the nanocomposites containing POSS displayed improved char yield, suggesting the flame retar- dance of the materials is improved. Macromol. Chem. Phys. 2005, 206, 2075–2083 DOI: 10.1002/macp.200500267 ß 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper 2075

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Page 1: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

Epoxy Resin Containing Octamaleimidophenyl

Polyhedral Oligomeric Silsesquioxane

Yong Ni, Sixun Zheng*

Department of Polymer Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240,P. R. ChinaFax: (þ86) 21 54741297; E-mail: [email protected]

Received: June 24, 2005; Revised: August 12, 2005; Accepted: August 15, 2005; DOI: 10.1002/macp.200500267

Keywords: epoxy; nanocomposites; polyhedral oligomeric silsesquioxane

Introduction

Organic-inorganic nanocomposites have received consid-

erable attention since they can combine the advantages of

inorganic materials (e.g., rigidity, stability) and organic

polymers (i.e., flexibility, ductility, and processibility).[1–4]

During the past decades, polymeric-inorganic nanocompo-

sites have been prepared via the sol-gel process,[2,5–8]

intercalation, and exfoliation of layered silicates by organic

polymers.[9–13] Polyhedral oligomeric silsesquioxane

(POSS) macromer and POSS-containing polymers have

been emerging as a new technology for the preparation

of organic-inorganic nanocomposites; POSS-containing

nanocomposites are becoming the focus of many studies

due to the excellent comprehensive properties of this class

of hybrid materials and the simplicity in processing.[14–16]

Polyhedral oligomeric silsesquioxanes are a class of

important nanosized cage-like compounds (Scheme 1),

derived from hydrolysis and condensation of trifunctional

organosilanes and possess a formula of [RSiO3/2]n, n¼ 6–

12, where R can be various types of organic groups, one (or

more) of which is reactive or polymerizable. The copoly-

merization of POSS macromers with organic monomers

has been proved to be an efficient approach to afford the

nanocomposites due to the formation of covalent bonds

between POSS cages and polymer matrices.

Epoxy resins are a class of important thermosets, which

have been widely used as matrices of composite materials,

adhesives, and electronic encapsulating materials due to

their high mechanical strength, excellent chemical resist-

ance, and simplicity in processing. The extensive applica-

tion motivates to prepare the organic-inorganic hybrid

composites of epoxy resins with improved properties. The

modification of epoxy resin via POSS could endow the

materials with some superior properties such as impro-

ved thermomechanical properties, thermal and oxidative

Summary: Octamaleimidophenyl polyhedral oligomericsilsesquioxane (OmipPOSS) was synthesized via the imid-ization reaction between octaaminophenyl polyhedral oligo-meric silsesquioxane (OapPOSS) and maleic anhydride, andit was characterized by means of Fourier transform infrared(FTIR) and NMR spectroscopies. OmipPOSS was furtheremployed to prepare epoxy hybrids. The thermosettinghybrids containing OmipPOSS up to 10 wt.-% were obtainedvia in situ polymerization of diglycidyl ether of bisphenolA (DGEBA) and 4,40-diaminodiphenylmethane (DDM) inthe presence of OmipPOSS. High-resolution transmissionelectronic microscopy (TEM) indicates that the nanometer-scaled dispersion of POSS molecules was obtained, suggest-ing that the nanocomposites were successfully prepared. Theresults of DSC showed that the glass transition temperatures(Tg’s) of the POSS-containing nanocomposites are dependenton the content of POSS in the nanocomposites. When thecontents of POSS are less than 5 wt.-%, the nanocompositesdisplayed the enhanced glass transition temperatures (Tg’s) in

comparison with control epoxy. Thermogravimetric analysis(TGA) showed that all the nanocomposites containing POSSdisplayed improved char yield, suggesting the flame retar-dance of the materials is improved.

Macromol. Chem. Phys. 2005, 206, 2075–2083 DOI: 10.1002/macp.200500267 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper 2075

Page 2: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

stability, and dielectric properties.[17–32] Lee et al.[21]

reported that the molecular level reinforcement of POSS

cages could significantly retard the physical aging process

of epoxy resin in the glassy state. Using a series of

octasilsesquioxanes with various R groups such as amino-

phenyl, dimethylsiloxypropyl glycidyl ether groups, Laine

et al.[18,19,28–31] investigated the dynamic mechanical

properties, fracture toughness, and thermal stability of the

epoxy nanocomposites and concluded that the modifica-

tions of epoxy resins via POSS were quite dependent on the

types of R groups, tether structures between epoxymatrices

and POSS cages, and the defects in silsesquioxane cages,

etc. Williams et al.[24] reported that a primary liquid-liquid

phase separation occurred at the time of adding the cuing

agent to epoxy due to the incompatibility between epoxy

and isobutyl POSS glycidyl. Matejka et al.[25,26] inves-

tigated the effects of POSS-POSS interactions on the

thermal properties of the nanocomposites. More recently,

Zheng et al.[20,27] studied the correlations between

morphology and thermomechanical properties of POSS-

containing epoxy hybrid composites.

In this work, we reported the synthesis of a novel octa-

functional POSS, octamaleimidophenyl POSS (Omip-

POSS) (Scheme 2), which was subsequently incorporated

into epoxy system. From the structural point of view,

OmipPOSS can be taken as a POSS-modified maleimide

and thus can offer high temperature performance of

polyimides and maintain the epoxy-like processing. It is

expected that theMichael addition between aromatic amine

and maleimide can be involved with the crosslinking

reaction among epoxy, aromatic amine, and OmipPOSS,

which will facilitate the fine dispersion of POSS macromer

in the composite system. The goal of this work is to

report the synthesis of OmipPOSS and to investigate the

morphology structure and thermal properties of the POSS-

containing nanocomposites by means of transmission

electronic microscopy (TEM), differential scanning calo-

rimetry (DSC), and thermogravimetric analysis (TGA),

respectively.

Experimental Part

Materials

Epoxy monomer, diglycidyl ether of bisphenol A (DGEBA)with epoxide equivalent weight 185–210 was purchased fromShanghai Resin Co., China. Phenyltrichlorosilane (PhSiCl398%) was purchased from Changzhou Chemical Factory,Jiangsu, China. Maleic anhydride, 4,40-diaminodiphenyl-methane (DDM), acetic anhydride, and sodium acetatetetrahydrate were of analytical grade, obtained from ShanghaiReagent Co., Shanghai, China. The bismaleimide (BMI) basedon DDM and maleic anhydride was prepared in this labaccording to the literature methods.[33] The solvents such asacetone, chloroform, triethylamine, methanol, N,N0-dimethyl-formamide (DMF), and tetrahydrofuran (THF)were purchased

Si O Si

O O

SiSi O

O O

O O

Si Si

Si

O

O

O O

Si

R

R

R

R

R

R

R

R

Scheme 1. Structure of octameric framework POSS molecule.

H2N

NH2

Si O Si

O

SiOSi

O

Si O Si

OOO O

O OSiOSi

NH2

NH2

NH2

NH2H2N

H2N

Si O Si

O

SiOSi

O

Si O Si

OOO O

O OSiOSi

N

OC

CO

N

OC

CO

N

OC

CO

N

OC

CO

N

OC

CO

N

OC

CO

N

OC

CO

N

OC

CO

(1) Maleic anhydride

(2) Sodium acetate tetrahydrate + acetic anhydride

Scheme 2. Synthesis of OmipPOSS from OapPOSS.

2076 Y. Ni, S. Zheng

Macromol. Chem. Phys. 2005, 206, 2075–2083 www.mcp-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Page 3: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

from commercial sources and they were further purified ingeneral ways prior to use.

Synthesis of Octamaleimidophenyl POSS

A multi-step approach was employed to synthesize Omip-POSS, which was based on the syntheses of octaphenyl POSS[(C6H5)8Si8O12], octanitrophenyl POSS [(O2NC6H4)8Si8O12],and octaaminophenyl POSS (OapPOSS) [(H2NC6H4)8Si8O12].The preparations of the later three POSS have been detailedpreviously;[20,34–36] therefore, only the general scheme isgiven here. The octaphenyl POSS was synthesized viahydrolysis and condensation of phenyltrichlorosilane andsubsequent rearrangement reaction catalyzed by benzyltri-methylammonium hydroxide, which was described by Brownet al. in 1964.[36] The nitrification of octaphenyl POSS wasemployed to prepare octanitrophenyl POSS, which was furtherreduced to afford OapPOSS.[20,34,35]

Octamaleimidophenyl POSS was synthesized via theimidization reaction betweenOapPOSS andmaleic anhydride.The synthetic method used in the present work is slightlydifferent from that reported by Krishnan et al.[37] In this work,sodium acetate (NaAc) was used as the catalyst to facilitate theimidization.[38–41] Typically, in a 250 ml three-necked flaskequippedwith amagnetic stirrer and a reflux condenser,maleicanhydride (1.359 g, 13.872 mmol) dissolved in 100mlanhydrous DMF was charged at 80 8C with vigorous stirringand then OapPOSS [(H2NC6H4)8Si8O12] (2.0 g, 1.734 mmol)that was pre-dissolved in 50 ml anhydrous DMF was droppedto the system within 30 min in nitrogen atmosphere. Thereactionwas allowed to be carried out at 80 8C for 24 h and thenboth sodium acetate tetrahydrate (NaAc � 4H2O) (0.1180 g,0.867 mmol) and acetic anhydride (2.124 g, 20.808 mmol)were added. The reactive mixture was held at 60 8C withstirring for additional 48 h. After the system was cooled toroom temperature, the majority of solvents were removed viarotation evaporation and the concentrated solution was precipi-tatedwith1 000mlofdeionizedwater.Thepowderyproductwasre-dissolved in the mixture of THF and ethyl acetate, and wasprecipitated into 1 000 ml of hexane. The product was collectedby filtration and dried in vacuo at room temperature to give2.341 g of slightly brown powdery solids (yield: 75.2%).

FTIR (KBr powder): 1 145 (Si–O–Si), 1 381 (C–N),1 603 (C C), 1 716 (C O in phase), 1 775 cm�1 (C O outof phase).

1H NMR (DMSO-d6): 8.0–6.6 (m), 6.1 (s).13C NMR (DMSO-d6): 172.1, 169.7, 167.2, 162.3, 135.3,

134.5, 131.3, 128.4.29Si NMR: �78.2, �81.4.

Preparation of Nanocomposites

To prepare the nanocomposites containing POSS, the desiredamount of OmipPOSSwas dissolved with the smallest amountofDMF, and the solutionwas charged to pre-weightedDGEBAwith vigorous stirring to afford homogenous solution. Afterthat, the curing agent, DDM was added with respect to theamount of DGEBA. The mixtures were poured into Teflonmould and the majority of solvent was evaporated at 60 8C

overnight. To remove the residual solvent, all the samples weredried in vacuo at 60 8C for at least 24 h. The contents ofOmipPOSS in the nanocomposites were controlled to be 2.5, 5,7.5, and 10 wt.-%, respectively. The systems were cured at80 8C for 2 h, 160 8C for 2 h, and 200 8C for 2 h to access acomplete curing reaction.

Reaction Between DDM and OmipPOSS

To investigate the reaction between DDM and OmipPOSS,OmipPOSS (2.24 g, 1.25mmol) andDDM (0.99 g, 5.00mmol)were dissolved with a small amount of DMF and the solutionwas poured into a Teflon mould. The major solvent wasevaporated at 60 8C overnight, and the films were further driedinvacuo at 60 8C for at least 24 h to remove the residual solvent.The mixture was cured at 80 8C for 2 h, 160 8C for 2 h, and200 8C for 5 h to attain a complete curing reaction. The curedsample was subject to FTIR measurement.

Measurement and Techniques

Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR measurements were conducted on a Perkin-ElmerParagon 1000 Fourier transform spectrometer at roomtemperature (25 8C). The samples were mixed with the powderof KBr and then pressed into small flakes. The specimens weresufficiently thin to be within a range where the Beer-Lambertlaw is obeyed. In all cases, 64 scans at a resolution of 2 cm�1

were used to record the spectra.

NMR Spectroscopy

The 1H and 13C NMR measurements were carried out on aVarian Mercury Plus 400 MHz NMR spectrometer at 25 8C.The samples were dissolved with deuterated dimethylsulfoxide (DMSO-d6) and the solutions were measured withtetramethylsilane (TMS) as the internal reference. The high-resolution 29Si NMR spectra were obtained using the cross-polarization (CP)/magic angle spinning (MAS) together withthe high-power dipolar decoupling (DD) technique. The908-pulse width of 4.1 ms was employed with free inductiondecay (FID) signal accumulation, and the CP Hartmann-Hahncontact time was set at 3.5 ms for all experiments. The rate ofMAS was 4.0 KHz for measuring the spectra. The Hartmann-Hahn CP matching and DD field was 57 KHz. The chemicalshifts of all 29Si spectra were determined by taking the siliconof solid Q8M8 relative to TMS as an external referencestandard.

Differential Scanning Calorimetry (DSC)

The DSC measurement was performed on a Perkin ElmerPyris-1 thermal analysis apparatus in a dry nitrogen atmo-sphere. The instrument was calibrated with standard indium.All samples (about 10 mg in weight) were heated from�20 to250 8Cand the thermogramswere recorded using a heating rateof 20 8C �min�1. The glass transition temperatures were takenas the midpoint of the capacity change.

Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane 2077

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Page 4: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

Transmission Electronic Microscopy (TEM)

The TEM was performed on a JEM 2010 high-resolutiontransmission electronmicroscope at the accelerating voltage of200 kV. The samples were trimmed using an ultramicrotomeand the specimen sections (ca. 70 nm in thickness) were placedin 200 mesh copper grids for observation.

Thermogravimetric Analysis (TGA)

APerkin-Elmer TGA-7 thermal gravimetric analyzer was usedto investigate the thermal stability of the nanocomposites. Allthe thermal analysis was conducted in nitrogen atmospherefrom ambient temperature to 800 8C at the heating rate of20 8C �min�1. The thermal degradation temperature was takenas the onset temperature at which 5 wt.-% of weight lossoccurs.

Results and Discussion

Syntheses of OmipPOSS

The imidization reaction between OapPOSS and maleic

anhydride was utilized to synthesize OmipPOSS, which is

described as Scheme 2. Shown in Figure 1 are the FTIR

spectra of OmipPOSS andOapPOSS. In the FTIR spectrum

of OmipPOSS, the five major bands characteristic of

Si–O–Si stretching vibration, C–N–C, double bond (C C),

and carbonyl (C O in phase and out of phase) were seen at

1 138, 1 378, 1 603, 1 716, and 1 775 cm�1, respective-

ly.[42,43] In the FTIR spectrum of OapPOSS, the character-

istic bands at 3 365 and 1 138 cm�1 are assigned to the

stretching vibration of N–H and Si–O–Si, respectively.

Both disappearance of the OapPOSS bands and appearance

of the characteristic bands of OmipPOSS indicate a

complete conversion of amine groups into maleimide

groups.

The 1H NMR spectra in the range of 4.0–10.0 ppm for

OmipPOSS and OapPOSS were presented in Figure 2. In

the 1H NMR spectrum of OmipPOSS, the single and sharp

resonance at 6.1 ppm is ascribed to the protons in carbon

double bonds (C C) of maleimide groups. It is seen that

upon imidization, the resonance of the protons in aromatic

rings was seen to shift low field possibly due to deshielding

effect of maleimide groups. At the same time, the broad

resonance at 5.3–3.5 ppm, which is assigned to the protons

of amino groups of OapPOSS disappeared, suggesting that

all the amino groups of OapPOSS were virtually converted

into maleimide groups. In the 29Si NMR spectrum, the

appearance of two peaks at�78.2 and�69.2 ppm indicates

that the OmipPOSS is the combined isomers containing

meta- and para-position substitutions, which is just like the

case ofOapPOSS (see Figure 3). The 29SiNMR result is in a

good agreement with that reported by Tamaki et al.[35]

Morphology and Thermal Properties ofNanocomposites

Morphology

To prepare the epoxy nanocomposites containing Omip-

POSS, the miscibility (or solubility) of OmipPOSS with

epoxy monomers (viz. DGEBA and DDM) is critical. All

the ternary mixtures composed of DGEBA, DDM, and

OmipPOSS were homogenous and transparent, suggesting

that all the components are miscible in the concentration

ranges investigated. The ternary mixtures were subject to

the curing reaction at elevated temperature to prepare

the POSS-containing composites. The organic-inorganic

epoxy composites containing 2.5, 5, 7.5, and 10 wt.-% of

OmipPOSS were prepared.

4000 3600 3200 2800 2400 2000 1600 1200 800 400

B

A

Wavenumber (cm-1)

Abs

orba

nce

3358 cm-1

3219 cm-1

1138 cm-1

1775 cm-1

1716 cm-1

1378cm-1

1603 cm-1

Figure 1. The FTIR spectra: (A) OmipPOSS; (B) OapPOSS.

10 9 8 7 6 5 4

Chemical shift (ppm)

OmipPOSS

OapPOSS

Figure 2. The 1H NMR spectra of OmipPOSS and OapPOSS.

2078 Y. Ni, S. Zheng

Macromol. Chem. Phys. 2005, 206, 2075–2083 www.mcp-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Page 5: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

It is observed that all the cured composites were

transparent, suggesting that the composites are homoge-

neous, i.e., no phase separation occurs on the scale at least

more than the wavelength of visible light. In the present

work, TEMwas employed to examine the degree of disper-

sion of POSS cages in the composite system. Figure 4 repre-

sentatively presents the TEM micrographs of the sectioned

composites containing 10 wt.-% of OmipPOSS. To contrast

with the background, the TEM image was taken at the edge

of the sectioned sample. It is seen that the dark area (the

portion of the hybrid composite) was virtually homogenous

and no localized domains were detected at this scale,

implying that the POSS component was homogenously

dispersed in the continuous epoxy matrix at the nanoscale.

The result of TEM indicates that the nanocomposites were

obtained. It is proposed that it is crucial for the nano-

composites that the chemical bonds are formed between the

crosslinked networks and OmipPOSS since the polymer-

ization-induced phase separation of OmipPOSS could be

suppressed from the in situ polymerization system.[20,27]

Curing Reactions

In the ternary reactive systems composed of DGEBA,

DDM, and OmipPOSS, two major reactions are involved

with the formation of thermosetting composites including

the polymerization betweenDGEBA andDDM that affords

the tightly crosslinked networks (Reaction 1) and the

Michael addition between aromatic amine (viz. DDM) and

the maleimide groups of OmipPOSS (Reaction 2) as shown

in Scheme 3. TheMichael addition under the present curing

conditions was evidenced by the curing reaction between

OmipPOSS and DDM. With the Michael addition,

OmipPOSS is incorporated into the crosslinked networks

of epoxy via the formation of covalent bonds. FTIR was

used to examine the degree of curing reaction after the

POSS cages were introduced to the systems. Shown in

Figure 5 are the FTIR spectra of DGEBA, the control epoxy

and the nanocomposites containing 2.5, 5, 7.5, and 10wt.-%

of POSS. The pure DGEBA is characterized by the

stretching vibration band of epoxide groups at 915 cm�1

(see curve F). Under the present condition the curing

reaction of the control epoxy was quite complete, which

was evidenced by the disappearance of the epoxide band

(see curve A). It is noted that all the epoxide bands were

virtually vanished for the POSS-containing nanocompo-

sites under the identical curing conditions, indicating that

the curing reactions in the nanocomposites were carried out

to completion. In order to confirm the occurrence of the

Michael addition between OmipPOSS and DDM, the

stoichiometric mixture of the two compounds were cured

using the identical condition with the preparation of epoxy

composites. The solubility test of the cured sample in some

common solvents such as DMF indicates that the cross-

linked products were obtained. The DDM-cured Omip-

POSS was subject to the FTIR analysis, and the FTIR

spectrum of DDM-cured OmipPOSS is presented in

Figure 6, and the FTIR spectra of OmipPOSS and DDM

were also incorporated into this figure (curves A and B) for

comparison. It is seen that the intensity of the stretching

vibration band at 1 716 cm�1, attributable to carbonyl in

unreacted maleimide moieties of OmipPOSS, was sig-

nificantly decreased. In the maleimide moiety of Omip-

POSS, the C C double bond and the carbonyl of imide

could constitute a conjugated system. The conjugated

system could be interrupted with the occurrence ofMichael

addition between N–H bonds in amino groups of DDM

and the C C double bonds. Therefore, the characteristic

carbonyl band of maleimide at 1 716 cm�1 was drastically

reduced. It should be pointed out that in the range of 4 000–

3 000 cm�1, the stretching vibration bands of amino groups

100 50 0 -50 -100

Chemical shift (ppm)

OapPOSS

OmipPOSS

Figure 3. The 29Si NMR spectra of OmipPOSS and OapPOSS.

Figure 4. The TEMmicrograph of the nancomposite containing10 wt.-% of OmipPOSS.

Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane 2079

Macromol. Chem. Phys. 2005, 206, 2075–2083 www.mcp-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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O CH2 HC CH2

O

NH2

DGEBA

+ O CH2 HC CH2

OH

N

DDM

NH2+N

C

C

HC

CH2

N

+

OmipPOSS DDM

(2)

(1)

NC

C

O

O

O

O

Scheme 3. Preparation of the POSS-containing nanocomposites.

4000 3600 3200 2800 2400 2000 1600 1200 800 400

Wavenumber (cm-1)

915 cm-1

Abs

orba

nce

A

B

C

D

E

F

Figure 5. The FTIR spectra of the nanocomposites containing:(A) 0, (B) 2.5, (C) 5.0, (D) 7.5, and (E) 10 wt.-% of OmipPOSS;(F) DGEBA.

4000 3600 3200 2800 2400 2000 1600 1200 800 400

Wavenumber (cm-1)

Tra

nsm

ittan

ce

A

B

C

1132

33663203

1716

Figure 6. The FTIR spectra of: (A) OmipPOSS, (B) DDM, and(C) DDM-cured OmipPOSS.

2080 Y. Ni, S. Zheng

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Page 7: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

(N–H) at 3 366 and 3 203 cm�1 are still discernible,

suggesting that not all of amino groups (N–H) participated

in the Michael addition under the present curing condition.

Nonetheless, the FTIR results imply that under the identical

curing reaction the Michael addition can occur between

DDM and OmipPOSS.

Glass Transition Behavior

All the nanocomposites were subject to thermal analysis.

The DSC curves of control epoxy resin, POSS-containing

nanocomposites are presented in Figure 7. All the DSC

thermograms displayed single glass transition temperatures

(Tg’s) in the experimental temperature range (�20–

250 8C). It is noted that the composites containing 2.5 and

5 wt.-% of OmipPOSS displayed the enhanced Tg’s (178

and 174 8C) in comparison with the control epoxy resin

(172 8C), while the hybrids containing 7.5 and 10 wt.-% of

OmipPOSS have the lower Tg’s than the control epoxy. The

enhancement in glass transition temperatures could be

ascribed to the nanoreinforcement effect of POSS on the

polymer matrix. It is proposed that in POSS-modified

polymers there could be the two competitive factors to

determine the glass transition temperatures of resulting

materials. On the one hand, POSS cages on the segmental

level could restrict the motion of macromolecular chains,

and thus the glass transition temperatures are enhanced. On

the other hand, the presence of the bulky POSS cages could

act as the internal plasticizer, which gives rise to decreased

Tg’s. In the present system, the Michael addition between

DDMandOmipPOSS could be an additional factor to lower

the glass transition temperatures of nanocomposites. In the

present work, the curing agent, DDM was stoichiome-

trically added with respect to the amount of DGEBA.

Therefore, it is proposed that the tightly crosslinked

networks would not be formed in the POSS-containing

nanocomposites as in the control epoxy since a part of

curing agent (viz. DDM) could be consumed by OmipOSS

through the Michael addition reaction.[44–46] This effect is

quite pronounced for the nanocomposites containing the

higher amount of OmipPOSS.

Thermal Stability

Thermogravimetric analysis was applied to evaluate the

thermal stability of the POSS-containing epoxy nano-

composites. Shown in Figure 8 are the TGA curves of the

control epoxy, OmipPOSS, DDM-cured OmipPOSS, and

the POSS-containing nanocomposites, recorded at 20 8C �min�1 in nitrogen atmosphere. Within the experimental

temperature range, all the TGA curves displayed similar

degradation profiles. This observation indicates that the

existence of POSS did not significantly alter the degrada-

tion mechanism of the matrix polymers. The temperatures

of degradation (Td) were taken as the onset temperatures at

which 5wt.-% ofmass loss occurred. For the control epoxy,

the initial decomposition occurred at 413 8C, and no

residual of decomposition was detected when the degrada-

tion was carried out at 740 8C as expected. For OmipPOSS,

the residue of decomposition was 53.7%, which is much

higher than theoretical ceramic yield of 23.6%, suggesting

that a great amount of char exists in the residues of decom-

position of silsesquioxane-containing compound. This

observation was further confirmed by the degradation of

0 50 100 150 200 250

163oC

170oC

174oC

178 oC

172oC

10

7.5

5

2.5

0

OmipPOSS wt%

End

o

Temperature ( oC)

Figure 7. The DSC curves of the control epoxy resin and of thePOSS-containing nanocomposites.

200 400 600 8000

20

40

60

80

100

OmipPOSS wt%

10.07.55.02.5

0

Wei

ght (

%)

Temperature ( oC)

OmipPOSS

DDM-cured POSS

Figure 8. The TGA curves of the control epoxy, DDM-curedBMI, and POSS-containing nanocomposites.

Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane 2081

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Page 8: Epoxy Resin Containing Octamaleimidophenyl Polyhedral Oligomeric Silsesquioxane

DDM-cured OmipPOSS, in which the theoretical ceramic

yield is even lower than OmipPOSS. From Figure 8, it is

seen that both OmipPOSS and DDM-cured OmipPOSS

have the lower Td’s than the control epoxy. The Td’s of the

nanocomposites are intermediate between those of the

control epoxy and DDM-cured OmipPOSS. More impor-

tantly, the incorporation of OmipPOSS into epoxy resin

networks results in retarded weight loss rates and enhanced

char yield of the materials at high temperatures. This effect

was increasingly pronounced with increasing the concen-

tration of OmipPOSS. In this system, the cubic POSS cages

and the maleimidophenyl groups were connected onto the

crosslinking networks via the formation of covalent bonds,

which thus hamper the continuous decomposition of the

epoxy matrix. The nanocomposites exhibited very high

char yields, implying that there are fewer volatiles that were

released from the nanocomposites during heating and thus

the flame retardance is improved.

Conclusion

In this work, OmipPOSS was synthesized via the imidiza-

tion reaction between OapPOSS and maleic anhydride.

The octafunctional POSS was characterized by means of

FTIR and NMR spectroscopies. OmipPOSS was employed

to prepare the nanocomposites with epoxy resin. The

thermosetting nanocomposites containing OmipPOSS up

to 10 wt.-% were obtained via in situ polymerization of

DGEBA and DDM in the presence of OmipPOSS. High-

resolution TEM indicates that the nanometer-scaled dis-

persion of POSS molecules was obtained, suggesting that

the nanocompositeswere successfully prepared. The results

of DSC showed that the glass transition temperatures (Tg’s)

of the POSS-containing nanocomposites are dependent on

the content of POSS in the nanocomposites. When the

contents of POSS are less than 5wt.-%, the nanocomposites

displayed the enhanced glass transition temperatures (Tg’s)

in comparison with control epoxy. TGA showed that the

nanocomposites displayed improved flame retardance in

terms of char yield in the materials.

Acknowledgements: The financial support from ShanghaiScience and Technology Commission, China was acknowledgedunder a key project (No. 02DJ14048). We thank the NaturalScience Foundation of China (GrantNo. 50390090 and 20474038)for the partial support.

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