Preparation, microstructure, and properties of novel low-κ brominated epoxy/mesoporous silica composites

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European Polymer Journal 44 (2008) 1414–1427

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POLYMERJOURNAL

Preparation, microstructure, and properties of novel low-jbrominated epoxy/mesoporous silica composites

Jingjing Lin, Xiaodong Wang *

Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials,

School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Received 1 November 2007; accepted 24 February 2008Available online 4 March 2008

Abstract

Brominated epoxy resin (BER) composites containing various amounts of SBA-15 and SBA-16 types mesoporous sil-icas were prepared through the thermal curing with 3-methyl-tetrahydrophthalic anhydride, and their morphologies,dielectric constants (j), thermal properties and mechanical properties were studied. The investigation suggested thatthe dielectric constant could be reduced from 4.09 of the pure thermosetting BER to 3.74 and 3.7 by incorporating3 wt.% SBA-15 and SBA-16, respectively. The reduction of the dielectric constant is attributed to the incorporationof the air voids (j = 1) stored within the mesoporous silica materials, the air volume existing in the gaps on the inter-faces between the mesoporous silica and the BER matrix, and the free volume created by introducing large-sizeddomains. The BER/mesoporous silica composites also present stable dielectric constants across the wide frequencyrange. An improvement of thermal stability of the BER is achieved by incorporation of the mesoporous silica materials.The enhanced interfacial interaction between the surface-modified mesoporous silica and the BER matrix has also led toan improvement of the toughness.� 2008 Elsevier Ltd. All rights reserved.

Keywords: BER/mesoporous silica composites; Dielectric constant; Microstructure; Properties

1. Introduction

Epoxy resins have been commercially developedfor more than half a century, and have many majorindustrial applications owing to their attractivecharacteristics of high tensile strength and modulus,low shrinkage on curing, excellent moisture resis-tance, high adhesion to many substrates, good

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doi:10.1016/j.eurpolymj.2008.02.022

* Corresponding author. Tel.: +86 10 6441 0145; fax: +86 106442 1693.

E-mail address: [email protected] (X. Wang).

chemical and corrosion resistance, excellent dimen-sional stability, and superior electrical properties[1–3]. Many efforts have been made to incorporatevarious inorganic organic fillers into the epoxy res-ins so as to obtain epoxy-based composites withmuch higher performance and lower cost. In the lasttwo decades, the rapid development of epoxy-basedcomposites in electronic applications has beenachieved, which includes encapsulations for thesemiconductors, packaging for the integrated circuit(IC) chips and ball grad access, insulating materialsfor electric devices such as the copper-clad laminates

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J. Lin, X. Wang / European Polymer Journal 44 (2008) 1414–1427 1415

and solder mask resistant inks for the printed cir-cuit broads (PCB). In most cases, these electronicapplications require the good thermal [4] and elec-tric properties [5] for the epoxy-based composites.However, the fire risk of the epoxy resins is a majordrawback of these materials that have to meet aremarkable flame-retardant grade in most of theirapplications [6]. Therefore, as one of the mostimportant and widely employed non-flammableepoxy-based materials for the electronic applica-tions, brominated epoxy resins (BER) and theBER-based composites have also received a greatdeal of attention [7,8]. Nowadays, they have beenwidely used in copper-clad laminates and soldermask resistant inks for the PCB, laminated elementsfor the electrical appliances, encapsulation com-pounds for the IC chips, and the other areas, whichare expected to additionally offer flame-retardantperformance. Through incorporating suitable fillerssuch as aluminum nitride (AlN), silicon dioxide(SiO2) and montmorillonite (MMT), the BER-basedcomposites can be donated the excellent mechanical,thermal and electrical properties to cope with thetrend of the electronic epoxy-based materialstoward greater performance and lower cost placinga great challenge on the basic epoxy resins. Severalattempts have been made on the BER-based com-posites. Yung et al. investigated the effects of thesize and content of the AlN on the properties ofBER-based composites used as the PCB substrate.They found that the composites achieved very excel-lent properties when the loading content of the AlNwith the size of 2.3 lm is 50 wt.% [9,10]. Yung et al.also prepared the MMT-filled BER composites usedfor PCB by in situ polymerization and obtainedBER/MMT composites with low-dielectric con-stants and dissipation factors, low coefficient ofthermal expansion, good thermal stability, andgood resistance to moisture absorption [11]. Chiuet al. incorporated various inorganic fillers such asAl(OH)3, SiO2, TiO2 and Sb2O3 into the BER,and found that the comparative tracking index(CTI) of the composites was improved with increas-ing the amount of the fillers [12].

In recent years, mesoporous silica materials haveattracted considerable interests in applications ofmolecular sieves, catalysts, adsorbents, opticaldevices, and sensor devices due to their highlyordered and uniform mesoporosity [13]. Mostimportantly, the mesoporous silica materials synthe-sized in a surfactant-templating process have largepore sizes (5–30 nm), high porosities (45–75%) and

controlled pore structures [14]. This makes it possi-ble to introduce voids into the bulk so that thelow-dielectric air (j = 1) can be utilized to reducethe dielectric constant (j) of the materials. Manystudies showed that the mesoporous silica materialshave low-dielectric constants in the range of 1.42–2.1[15–17], which are lower than most of other low-dielectric materials, such as silisequioxane baseddielectric, fluorine doped silica film, carbon dopedsilica film, and organic polymer dielectric [18]. Thelow-dielectric mesoporous silica materials can meetthe requirements of the new dielectric films withlow-dielectric constant (j < 2.5) for the applicationsin the microelectronics and the insulations. Based onthe low-dielectric constant of the mesoporous silicamaterials, incorporation of the mesoporous silicamaterials into the epoxy resins would be expectedto reduce their dielectric constants.

In present work, we reported a new approach tothe BER-based composites by incorporating themesoporous silica materials into the resin matrixto combine the low-dielectric constant of mesopor-ous silica and the excellent properties of the BER.A significant reduction of the dielectric constantsfor the BER-based composites can be achieved viathis method. We also evaluate the effects of the mes-oporous silica materials on the BER-based compos-ites in term of the dielectric, thermal, andmechanical properties. The present work providesa sample and effective means to obtain the low-dielectric epoxy-based composites.

2. Experimental part

2.1. Materials

Poly(ethylene oxide–b-propylene oxide–b-ethyl-ene oxide) (EOn–POm–EOn) triblock copolymers,EO20PO70EO20 (Pluronic P123) and EO106PO70-EO106 (Pluronic F127) were commercially obtainedfrom BASF Company. Tetraethoxysilane (TEOS)used as silica source and 3-glycidyl-oxypropyl tri-methoxysilane (GOTMS) used as coupling agentwere purchased from Aldrich Chemical Company.The BER with an EEW of 420–450 g/equiv. andbromine weight content of 18–22% was kindly sup-plied by Wuxi Dic Epoxy Co., Ltd. 3-Methyl-tetra-hydrophthalic anhydride (MeTHPA) and 2-methylimidazole were purchased from Beijing ChemicalReagent Company. All chemicals were of reagentquality and used as received without furtherpurification.

1416 J. Lin, X. Wang / European Polymer Journal 44 (2008) 1414–1427

2.2. Synthesis of mesoporous silica materials

Two types mesoporous silica materials, SBA-15and SBA-16 were synthesized by using theEO20PO70EO20 and the EO106PO70EO106 surfactantas template, respectively, in a sol–gel process. A typ-ical synthesis was carried out as follows: the amountof surfactant template was dissolved in deionizedwater with stirring in a beaker at 35 �C to form atransparent template solution containing the surfac-tant micelles. In another beaker, a mixture of theamount of TEOS and aqueous HCl solutionrequired to give the desired pH was stirred at35 �C until a homogeneous solution was obtained,indicating that the hydrolysis of the TEOS was com-plete. Then, the above two solutions were mixedwith stirring for 24 h. Subsequently, the mixturewas aged at 100 �C for another 24 h. The solid pow-ers were collected by filtration, washed with water,and dried at room temperature in air. The resultingproducts were calcined at 500 �C for 5 h to removethe template.

2.3. Preparation of BER/mesoporous silica

composites

The mesoporous silica materials were first sur-face-modified with coupling agent before use ina following way: the mesoporous silica materialswere dispersed in dry acetone under ultrasonicagitation for 30 min, and then the calculatedamount of GOTMS was added to the mixture,refluxed 12 h under vigorous stirring. After themixture was filtrated with Buchner funnel, theproduct was washed several times using ace-tone and dried in the vacuum oven at roomtemperature.

The BER/mesoporous silica composites wereprepared as follows: the BER was dissolved inacetone in a beaker. The calculated amount ofmesoporous silica was dispersed in the solutionunder ultrasonic agitation in another beaker for30 min, and then the MeTHPA as curing agentand the 2-methyl imidazole as catalyst were addedinto this beaker. The above two solutions weremixed with stirring for 30 min. Then, the solventwas removed at 50 �C under 0.05 MPa pressurein a vacuum oven. Subsequently, the mixturewas poured into the preheated molds, which weresurface treated with the mold-releasing agent. Allthe samples were cured at 100 �C for 1 h and at178 �C for 2 h.

2.4. Characterizations

2.4.1. X-ray diffraction measurements

X-ray powder diffraction (XRD) was carried outwith a Rigaku D/max-r C diffractometer (40 kV,50 mA) with Cu-Ka radiation (k = 0.154 nm), thediffraction patterns being collected in the 2h range0.5–8.0� at a scanning rate of 0.2�/min.

2.4.2. Transmission electron microscopy

Transmission electron microscopy (TEM) imageswere obtained by a Hitachi H-800 transmission elec-tron microscope operating at 200 kV. Samples forTEM observation were prepared by dispersing adrop of a suspension of a ground sample in ethanolon a copper grid covered by carbon film.

2.4.3. N2 adsorption–desorption measurements

N2 adsorption–desorption isotherms wereobtained at 77 K using a Nova 4200e gas-adsorp-tion analyzer. Before the adsorption measurements,the samples were degassed under vacuum at 200 �Cfor 12 h. The mean pore diameter and the diameterdistribution were calculated from the adsorptionbranch of the isotherm using the Barrett–Joyner–Halenda (BJH) method. The specific surface areawas calculated using the Brunauer–Emmett–Teller(BET) model.

2.4.4. Fourier transform IR spectroscopy

Fourier transform IR (FTIR) spectra of the mes-oporous silica materials and the surface-modifiedones were obtained using a Bruker Tensor-27 FTIRspectrometer with 30 scanning numbers.

2.4.5. Scanning electron microscopy

Scanning electron microscopy (SEM) observa-tion was performed on a Hitachi S-4700 scanningelectron microscope. The morphologies of the mes-oporous silica powders and the fracture surfaces ofthe BER/mesoporous silica composites were deter-mined from SEM images. The composites were frac-tured firstly in liquid nitrogen and mounted on thesample stud by means of a double-sided adhesivetape for cross-sectional view study. A thin layer ofgold was sputtered onto the cross-sectional surfaceprior to SEM observation.

2.4.6. Measurements of electrical properties

The dielectric constants of the composites weremeasured on a WY2851-type LCR bridge meter(Shanghai Wuyi Electronics Co., Ltd.) in the

Fig. 1. X-ray diffraction patterns of the mesoporous silicamaterials.


frequency range of 50 kHz to 1 MHz. The dielectrictesting plats with a diameter of 30 mm and a thick-ness of 2 mm were obtained through cast molding.All the tests were done at room temperature and fivemeasurements were carried out for each data point.

2.4.7. Measurements of thermal properties

Differential scanning calorimetry (DSC) was per-formed with a Perkin–Elmer Pyris-1 differentialscanning calorimeter. All measurements were madeunder an N2 atmosphere at a heating rate of 10 �C/min on samples weighing about 10 mg. Thermalgravimetric analysis (TGA) was carried out on aPerkin–Elmer Pyrid-1 thermal gravimetric analyzerat a heating rate of 10 �C/min from 50 to 800 �Cunder an N2 atmosphere.

2.4.8. Measurements of mechanical properties

The impact and tensile test bars were fabricatedvia a cast molding. Charpy impact strength wasmeasured with a SUMITOMO impact machine tes-ter according to a Chinese national standard of GB/T1043-98. The thickness of the Charpy impact spec-imen was 4 mm, and impact energy was 4 J. Thetensile properties were determined with an Instron-1185 universal testing instrument using a 1000 Nload transducer according to a Chinese nationalstandard of GB/T1040-98. Small dumb-bell speci-mens with waist dimensions of 20 � 4 mm were usedfor tensile mechanical tests. All the tests were doneat room temperature and five measurements werecarried out for each data point.

3. Results and discussion

3.1. Microstructure and physical properties ofmesoporous silica materials

The SBA-15 and SBA-16 type mesoporous silicamaterials have greater mesoporosity and higherhydrothermal stability in comparison with the othertype ones, indicating their significant advantage foruse in the low-dielectric BER-based composites [19].However, the commercial types of these mesopor-ous silica materials are currently not available, sothey were synthesized in our laboratory throughthe micelle-templating technique by seeding rodmicelles of the surfactant aggregate accompaniedwith the condensation of a silica sol performed bythe hydrolysis of TEOS. Fig. 1 shows the XRD pat-terns of the SBA-15 and the SBA-16, in which twowell-resolved single diffraction peaks at 2h of 0.76

and 0.79 could be observed, respectively, and a ser-ies of broad diffraction peaks with low intensity canalso be observed in the XRD patterns of SBA-15and SBA-16. The corresponding interplane dis-tances for two types mesoporous silica materialswere resolved and shown in the inset of Fig. 1.Through the Bragg’s law and the correspondingrelationship between the cubic unit cell parameterand interplanar distance at different Miller indices,a two-dimensional (2D) hexagonal structure(p6mm) and a 3D body-centered cubic structure(Im3m) could clearly be identified and assignedrespectively to the SBA-15 type and the SBA-16type mesoporous silica materials.

The TEM image in Fig. 2a confirms the orderedstructure of the SBA-15, and shows that the cylin-drical pores are arranged in an ordered hexagonalarray. From Fig. 2b, the arrays of the ordered anduniform pores interconnection can be observed forthe SBA-16. N2 adsorption–desorption isothermsfor the two types mesoporous silica materials areshown in Fig. 3. The SBA-15 exhibits a classicalLangmuir IV-type isotherm with H1 type verticalhysteresis loop, characteristic of cylindrical channelsin good agreement with the mesostructure of thereported SBA-15 [14]. The SBA-16 demonstrates atypical Langmuir IV-type isotherm with a H2 hys-teresis loop, which has a strong steep desorptionbranch and a sloping adsorption branch assigned

Fig. 2. TEM images of the mesoporous silica materials: (a) SBA-15 and (b) SBA-16.

Fig. 3. N2 adsorption–desorption isotherms and pore size distri-bution (the inset) plots of the mesoporous silica materials; theisotherm for SBA-16 is offset vertically by 170 cm3/g STP.


to the their 3D interconnected mesoporous struc-ture [20]. The physical properties of the two typesmesoporous silica materials obtained by analysisof the N2 adsorption–desorption data demonstrate

that both of these mesoporous silica materials havesmall pore diameters of 5.3–7.5 nm with very nar-row pore size distributions. Total pore volumes forSBA-15 and SBA-16 can also be calculated, respec-tively, to be 1.23 and 0.65 cm3/g in Brunauer–Emmett–Teller model. These important parametersindicate the amount of air voids stored in the meso-porous silica materials [21].

3.2. FTIR spectroscopy

The BER/mesoporous silica composites wereobtained through thermal curing of the BER con-taining various amount of SBA-15 or SBA-16 withthe MeTHPA. However, the surface modificationshould firstly be performed for mesoporous silicamaterials because poor interfacial adhesion betweenthe inorganic mesoporous silica and the organicBER matrix. In this study, we employed the GOT-MS as coupling agent, which was capable of linkingmesoporous silica covalently to the cross-linkingBER molecular network through silylation of themesoporous silica [22]. Fig. 4 shows the typicalFTIR spectra of silica for the two unmodified mes-oporous silica materials, in which two intensiveabsorption peaks at 1094 and 838 cm�1 belongingto the asymmetric and symmetric stretching vibra-tions, respectively, corresponding to the Si–O–Si

Fig. 4. FTIR spectra of the mesoporous silica materials and theirsurface-modified products.


framework could be observed. A wide absorptionband at around 3460 cm�1 and the weak band at968 cm�1 represent Si–OH stretching and bendingvibrations, respectively. As both of the mesoporoussilica materials we synthesized have been synthe-sized through synergistic self-assembly between sur-factant (triblock polymer) and silica source(hydrolysate of the TEOS) to form mesoscopicallyordered composites, they were formed via a conden-sation of silanols along the micellar surface of thesurfactant in an acidic media. It is expected that alarge quantity of silanol groups should be detectedon their inorganic walls. It is evident that the FTIRspectra of the mesoporous silica materials confirmthese silanol groups (Si–OH). From the IR spectraof two surface-modified mesoporous silica materialsas shown in Fig. 4, a series of new absorption peaksare observed. The peaks at 2928 cm�1 and2866 cm�1 are attributed to the –CH2 stretchingabsorption and the –CH2 scissor vibration, respec-tively, but the peak at 914 cm�1 corresponding tothe absorption of the oxirane ring is overlappedby a series of intensive peaks at 838–1094 cm�1

due to the absorption of the Si–O–Si and Si–OHstretching vibrations. These results indicate thatthe GOTMS has been chemically grafted onto the

mesoporous silica materials through condensationbetween the silanol groups of the hydrolyzed GOT-MS and those on the surface of the mesoporous sil-ica. These surface-linking GOTMS may furtherreact with the MeTHPA during the curing process(the possible linking way is seen in Fig. 5) [22],and thus enhance the interfacial adhesion betweenthe mesoporous silica materials and the BERmatrix.

3.3. Morphology

Fig. 6 shows the SEM images of the externalmorphologies for SBA-15 and SBA-16. It can beclearly observed that the SBA-15 displays morphol-ogy of node-rod-like shape with around 20 lm inlength. The node-rod-like sub-particles agglomerateeach other and form clusters. This is the character-istic morphology of the typical SBA-15 obtainedfrom the condensation of TEOS around the adja-cent micelles of the P123 template as preciouslyreported [23]. On the other hand, the SBA-16 pre-sents regular and homogeneous spherical particleswith uniform sizes of 2–3 lm. The original morphol-ogies are determined by the colloidal phase separa-tion mechanism, which results in the differentexternal morphologies for two types mesoporoussilica materials in terms of their individual templateused for synthesis [24].

Fig. 7 shows the SEM images of the fracture sur-face of the pure thermosetting BER and the thermo-setting BER/mesoporous silica composites. It isnoted that the pure thermosetting BER as a refer-ence illustrates a glazed fracture surface with veryslight plastic deformation, indicating a typical brit-tle fracture behavior. Although it is difficult to viewthe dispersed domains in the SEM images of thefracture surface of the composites due to the power-ful adhesive ability of the BER, the mesoporous sil-ica materials embedded in the composites can stillbe distinguished as marked with circles in theSEM images. From Fig. 7b, the longer node-fiber-shaped particles of the SBA-15 can be partiallyobserved, which are dispersed uniformly in theBER matrix. One can see the great deformation ofthe surface during fracture occurred as shown inFig. 7c. In addition, the integrated node-fibershaped SBA-15 domain covered by a thin layer ofthe resin is also clearly observed from a magnifiedSEM image (as shown in Fig. 7d). This means thatthe delamination between the interfaces does notoccur and the crack propagated through the matrix

Fig. 5. Schematic linking formation of the BER with the mesoporous silica through the interfacial reaction.

Fig. 6. SEM images of the mesoporous silica materials: (a) SBA-15 and (b) SBA-16.


around the mesoporous silica particles because ofthe good interfacial adhesion as a result of the sur-face modification of the mesoporous silica materi-als. The similar phenomena can be observed fromthe SEM image of the fracture surfaces of theBER/SBA-16 composites, in which the SBA-16 isuniformly dispersed almost as individual particlesin the matrix as shown in Fig. 7e and f, and noagglomerated particles appear on the fracture sur-faces. The magnified SEM image (the inset ofFig. 7f) of a dispersed SBA-16 domain shows clearlyits coarse surface covered with a thin layer of the

resin, indicating a good interfacial adhesionbetween the SBA-16 and the BER matrix. Theseresults also confirm the effective surface treatmentof the mesoporous silica particles, and reveal thatthe two types of mesoporous silica particles havegood bonding interfaces with the matrix.

3.4. Electrical properties

Total pore volumes of the SBA-15 and SBA-16obtained from N2 adsorption–desorption isothermshave implied the amount of air voids stored in the

Fig. 7. SEM images of the BER/mesoporous silica composites: (a) pure thermosetting BER, (b) containing 3 wt.% SBA-15, (c, d)containing 7 wt.% SBA-15, (e) containing 3 wt.% SBA-16, and (f) containing 7 wt.% SBA-16.


mesoporous silica materials, which were expected toinduce the reduction of the dielectric constants ofthe composite materials. Fig. 8 displays the varietyof the dielectric constants measured at the frequencyof 1.0 MHz as a function of the mesoporous silicacontent. It is noticed that the dielectric constant ofthe composites reduced from 4.09 of the pureBER to 3.79 and 3.84, respectively, with incorporat-ing 1 wt.% SBA-15 and SBA-16 into the composite,and the dielectric constant gradually decreases withincreasing the amount of the mesoporous silicamaterials continuously until the percentage reaches

3 wt.%, at which both the composites achieve theminimum dielectric constants. After exceeding thischaracteristic content, the dielectric constants ofall the composites begin to increase with increasingthe amount of mesoporous silica continuously. Fur-thermore, the BER/SBA-15 composite presents aslightly greater decrement in the dielectric constantof the BER/SBA-16 composite at the content ofthe mesoporous silica (1 wt.%). All of the measure-ments have been repeated five times and the devia-tions of the testing data were controlled with±0.05. Therefore, it is believable that the reduction

Fig. 8. Dielectric constants of the BER/mesoporous silicacomposites at a frequency of 1 MHz as a function of compositionof the mesoporous silica materials.


of the dielectric constant is resulted from the incor-poration of air voids (j = 1) when the mesoporoussilica materials are introduced into the BER.

As we known, there are mainly three models topredict the dielectric constant of the dual-compo-nent composite: Maxwell-Garnett, Bargeman, andYamada models. The Maxwell-Garnett equationwas widely used to calculate the dielectric constantof polymeric composites. And the Yamada model[25] was mainly used to calculate the dielectric con-stant at different electric field strength, so the depo-

Table 1Measured and theoretical dielectric constants, and air volume percentagsilica materials calculated by corresponding models

Materials Measured dielectricconstant at 1 MHz

Theoretical dielectricconstanta

BER 4.09 –Silica (SiO2) 4.00 –Air 1.00 –BER/1 wt.% SBA-15 3.79 3.91BER/3 wt.% SBA-15 3.74 3.61BER/5 wt.% SBA-15 3.90 3.35BER/7 wt.% SBA-15 4.01 3.14BER/10 wt.% SBA-15 4.06 2.86BER/1 wt.% SBA-16 3.84 3.96BER/3 wt.% SBA-16 3.70 3.74BER/5 wt.% SBA-16 3.88 3.55BER/7 wt.% SBA-16 3.94 3.37BER/10 wt.% SBA-16 4.04 3.13

a The theoretical dielectric constants were calculated by using Maxwb The air volume stored within the mesoporous silica in the composite

adsorption–desorption measurements.c Total air volume in the composites (volume fraction) was calculate

larization factor should be considered based on theelectric field effect with a complicated equation. It isobvious that the Yamada model is outside of thetarget of our research on the BER/mesoporous sil-ica composites. Although there are three compo-nents including BER, silica (SiO2), and air in thecomposites, the reduction of the dielectric constantis mainly attributed to the implantation of air alonewith the mesoporous silica materials. Therefore, thecomposites can be firstly considered as a dual-com-ponent system (the BER and the mesoporous silica).The theoretic dielectric constants of the compositescan be predicted by using Maxwell-Garnett equa-tion [26]

j� jp

jþ 2jp

¼ /m

jm � jp

jm þ 2jp

ð1Þ

where jp and jm are the dielectric constants of theBER and the mesoporous silica, respectively, j thepredicted dielectric constant of the composite, and/m the volume fraction of the mesoporous silica.The dielectric constants of mesoporous silica mate-rials can also be calculated by Eq. (1) based onthe total pore volume, corresponding to the air vol-ume stored within mesoporous silica. All of the cal-culated dielectric constants as well as the measuredones for reference are listed in Table 1. It can benoted that the measured dielectric constants of thecomposites differ remarkably from the theoreticdata, which are much greater for the composite withthe same amount of SBA-15 or SBA-16. The results

es in the thermosetting BER and its composites with mesoporous

Air volume stored within mesoporoussilica in compositesb (v/v %)

Total air volume incompositesc (v/v %)

– –– –– –2.86 7.318.27 8.51

13.30 5.5417.99 2.8424.46 –1.96 6.085.77 9.509.43 5.09

12.96 3.5618.02 –

ell-Garnett equation.s (volume fraction) was calculated by using the data from the N2

d according to Bruggeman’s model.


show that the difference of the pore volumes in themesoporous silica exactly plays an important rolein decreasing the dielectric constant of the compos-ites. Furthermore, the theoretic data decrease con-tinuously with increasing the amount of themesoporous silica, but the measured values presenta minimum at a characteristic concentration of themesoporous silica. This implies that the reductionof the dielectric constants is not only attributed tothe air volume stored within the mesoporous silicamaterials.

In order to explore the reason why this deviationoccurs, we calculated the air volume percentages ofthe composite by only considering the air volumewithin the mesoporous silica materials obtainedfrom the N2 adsorption–desorption measurement,and total air volume in the composite by usingBruggeman’s model. Assuming there are only threephases in the composite as mentioned preciously,the total volume percentage in the composite canbe calculated by using Bruggeman’s equation [27]for three-component composite

/p

jp � jjp þ 2j

� �þ /s

js � jjs þ 2j

� �þ /a

ja � jja þ 2j

� �¼ 0

ð2Þ

where jp, js, and ja, are the dielectric constants ofthe BER, the silica and the air, respectively, j themeasured dielectric constant of the composite, and/p, /s and /a the volume fractions of the BER,the silica and the air, respectively. All of the calcu-lated results are also listed in Table 1. It is clearlynoticed that the actual total air volume in the com-posite is much greater than the air volume storedwithin mesoporous silica materials. These air voidsmay exist in the gaps on the interfaces between themesoporous silica and the BER matrix, and the freevolume created by introducing macro-sized domains[28]. Therefore, the reduction of the dielectric con-stant is contributed by the whole air voids createdby incorporating the mesoporous silica, which in-clude the air volume stored within the mesoporoussilicas, the air voids coming from the gaps on theinterfaces between the mesoporous silicas and theBER matrix, and the free volume created by intro-ducing macro-sized domains. It is also found thattotal air volume presents a maximum at a character-istic concentration of the mesoporous silica, andbegins to decrease with increasing the amount ofthe mesoporous silica continuously in terms of theBruggeman’s model. This result indicates that the

incorporation of too much mesoporous silica resultsin its aggregation and poor dispersion, a decrease ofthe air volume, and thus an increase of the dielectricconstant. It is also noteworthy that both the internalstructures and the external morphologies of themesoporous silicas reveal a significant effect on thevariation trends of the dielectric constant as a func-tion of the content of the mesoporous silica. Asmentioned preciously, the SBA-15 has a larger porevolume than the SBA-16, so it results in a lowerdielectric constant for the composites than theSBA-16. However, the SBA-15 does not exhibitany superiority than the SBA-16 at a higher contentin the composites, as shown in Fig. 8. Apparently,the external morphologies of two kinds of mesopor-ous silicas dominate the dielectric constants of thecomposites with much higher content of mesopor-ous silica, because the characteristic morphologyof the mesoporous silica affects its distribution inthe BER matrix significantly, and then determinesthe free volume and the gaps on the interface be-tween the BER and the mesoporous silica. As a re-sult, the composites containing with higher contentof SBA-15 exhibit a higher dielectric constant thanthose containing SBA-16.

Fig. 9 shows the frequency dependence of thedielectric constant of the pure BER and its compos-ites in the frequency range of 50–1000 kHz.Although the variation of the dielectric constantsof the BER composites as a function of the SBA-15 or SBA-16 content is identical to the trend shownin Fig. 8, the dielectric constants for the pure BERand its composites are almost independent of thefrequency except at very low frequency, where somedielectric constants are fairly higher. One can findthat the dielectric constant only decreases slightlywith increasing frequency while temperature is keptconstant at room temperature. It is well known thatthe dielectric constant of materials tends to decreasegradually with increasing the frequency, because theresponse of the electronic, atomic, and dipolarpolarizable units varies with frequency. This behav-ior can be attributed to the frequency dependence ofthe polarization mechanisms [29]. Therefore, themagnitude of the dielectric constant for a polymerlike BER is determined by the ability of the polariz-able units to orient fast enough to keep up with theapplied alternating current electric field. The orien-tational polarization decreases while the frequencyincreases, as the orientation of dipole momentsneeds a longer time than the electronic and ionicpolarization. This results in a reduction of the

Fig. 9. Frequency dependence of dielectric constants for (A) BER/SBA-15, and (B) BER/SBA-16 composites with mesoporous silicacontent of (a) 0 wt.%, (b) 1 wt.%, (c) 3 wt.%, (d) 5 wt.%, (e) 7 wt.%, and (f) 10 wt.%.


dielectric constant. The BER/mesoporous silicacomposites prepared in this study exhibit the stabledielectric constants across the wide frequency range,which are highly preferred for many microelectronicapplications.

3.5. Thermal properties

The glass-transition temperature (Tg) is a veryimportant parameter for the thermosetting epoxyresins, because it established the service environ-ment for the epoxy-based materials. In most cases,the epoxy resins are only used well at a temperaturebelow Tg. Therefore, identification of the mecha-nisms responsible for Tg changes and prediction ofTg depression are critical for the engineering designand the application of the epoxy resins and theircuring systems. Tgs of the thermosetting BER andits composites with two types mesoporous silicamaterials were obtained by DSC measurements,and the data are listed in Table 2. It is observed thatthe Tgs of the composites containing 3 wt.% meso-

Table 2Thermal analysis experimental data of the thermosetting BER and its

Samples Tg (�C) Temperature at the characteristic

1 wt.% 10 wt.%

BER 118.3 226.3 344.1BER/3 wt.% SBA-15 119.7 228.5 346.2BER/7 wt.% SBA-15 121.6 232.2 351.6BER/3 wt.% SBA-16 120.2 228.9 345.8BER/7 wt.% SBA-16 122.4 231.5 348.2

porous silica are higher than that of the thermoset-ting BER and the Tgs increase with furtherincreasing the concentration of the mesoporous sil-ica. Considering the chemical linkage between theinorganic and organic domains, incorporation ofthe rigid mesoporous silica into the BER mainchains increases the motional and rotational barri-ers of the BER molecules and, therefore, improvesthe Tgs of the composites. The higher concentrationof the mesoporous silica can result in a greater bar-rier effect, and thus in a higher Tgs [30].

The effect of the two types mesoporous silicamaterials on the thermal degradation of the BERwas studied using TGA in the temperature rangeof 50–800 �C. The thermal degradation behaviorsof the thermosetting BER and its composites areshown in Fig. 10, and the obtained data are summa-rized in Table 2. It can be found that the thermaldegradation of the thermosetting BER and its com-posites occurs through one degradation step in thistemperature range, which indicates that a goodphase interconnection between the mesoporous

composites with mesoporous silica materials

weight loss (�C) Temperature at therapid weight loss (�C)

Char ratiosat 700 �C (wt.%)

436.5 7.8441.3 13.6445.1 15.4439.5 13.3443.2 15.2

Fig. 10. TGA thermograms of the thermosetting BER and its composites with the mesoporous silica materials.


silica and the BER matrix, and successful chemicalgraft of the GOTMS onto the mesoporous silicamaterials. The TGA thermograms also indicate thatwater or solvent has been successfully removed fromthe resins and the composites because there is noweight loss below 100 �C. It is clearly suggested thatin Fig. 10 and Table 2 that the thermal stability ofthe BER is increased by incorporation of the meso-porous silica materials in terms of the temperaturesat the rapid weight loss and the weight percent ofthe residues above 700 �C. The increase in thesetwo parameters suggests that the incorporation ofthe mesoporous silica into the BER result ultimatelyin an improvement in thermal stability. Thisimprovement in the thermal stability of compositesmainly comes from the enhanced interactionbetween the BER and the mesoporous silicathrough chemical bonding [31]. Furthermore, itmay also be assumed that the thermal stability oforganic materials can be improved by introducinginorganic components like silica, on the basis ofthe fact that these materials have inherently goodthermal stability [32].

3.6. Mechanical properties

The impact strength, tensile strength, and elonga-tion at break of the BER/mesoporous silica com-

posites as a function of composition of themesoporous silica materials are displayed in Figs.11–13, indicating that the incorporation of meso-porous silica materials has significant effects on themechanical properties. The thermosetting BER sug-gests a typical brittle plastic behavior with a lowimpact strength of 4.3 kJ/m2 and a high tensilestrength of 87.7 MPa. It is noteworthy that theimpact strength of the two types compositesincreases with introducing 1 wt.% mesoporous silicainto the BER, and reach a maximum value withaddition of 3 wt.% mesoporous silica. It is evidentthat the toughening effect of the mesoporous silicamaterials could be explained by the crack frontbowing mechanism [33]. For the toughening poly-mer system using inorganic particles, the rigid parti-cles can resist the propagation of the crack, so theprimary crack has to bend between the neighboringparticles. However, the impact strength begins todecrease, as the weight percent of mesoporous silicaincreases continuously, suggesting that the highconcentration of mesoporous silica dominates thenatural incompatibility of the inorganic and organicphases, and thus results in a reduction in toughness[34,35]. Furthermore, it is also found that the tensilestrength and elongation at break decrease rapidlywith incorporation of the mesoporous silica (seeFigs. 12 and 13). This is a result of the natural

Fig. 11. Izod impact strength of the BER/mesoporous silicacomposites as a function of composition of the mesoporous silicamaterials.

Fig. 12. Tensile strength of the BER/mesoporous silica compos-ites as a function of composition of the mesoporous silicamaterials.

Fig. 13. Elongation at break of the BER/mesoporous silicacomposites as a function of composition of the mesoporous silicamaterials.


incompatibility between the inorganic silica and theorganic BER, leading to a poor stress transfer. Onthe other hand, although the mechanical propertiesof the two types composites present similar trend ofvariation as a function of composition of the meso-porous silica, the BER/SBA-16 composites indicatea much better toughening effect and a less deteriora-tion of the tensile properties than the BER/SBA-15one due to the much more regular morphology andthe much smaller particle size for the SBA-16.

4. Conclusion

The novel thermosetting BER composites con-taining various amounts of the SBA-15 and theSBA-16 type mesoporous silica were successfullyprepared through the thermal curing with theMeTHPA. The dielectric constants of the BER/mes-oporous silica composites can be reduced from 4.09of the pure thermosetting BER to 3.74 and 3.7 byincorporating 3 wt.% SBA-15 and SBA-16, respec-tively. The reduction of the dielectric constant isattributed to the incorporation of the air voids(j = 1) stored within the mesoporous silica materi-als, the air volume existing in the gaps on the inter-faces between the mesoporous silica and the BERmatrix, and the free volume created by introducinglarge-sized domains. The SBA-16 has a more signif-icant effect on reduction of the dielectric constantthan the SBA-15 because of its smaller domain sizeand more uniform distribution in the matrix. TheBER/mesoporous silica composites prepared in thisstudy also present stable dielectric constants acrossthe wide frequency range and a good phase inter-connection. The thermal stability of the BER canbe improved by incorporation of the mesoporoussilica materials. The enhanced interfacial interactionbetween the surface-modified mesoporous silica andthe BER matrix can also lead to the improvement ofthe toughness. The incorporation of the mesopor-ous silica materials is a promising approach toreduce the dielectric constant of the BER.


Acknowledgement

The authors greatly appreciate financial supportfrom the National Natural Science Foundation ofChina (Grant No.: 50573006).

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