preparation, characterization and thermal properties of organic–inorganic composites involving...

ORIGINAL PAPER

Preparation, characterization and thermal propertiesof organic–inorganic composites involving epoxyand polyhedral oligomeric silsesquioxane (POSS)

Chung-Hwei Su & Yi-Pang Chiu & Chih-Chun Teng &

Chin-Lung Chiang

Received: 24 June 2009 /Accepted: 16 October 2009 /Published online: 7 November 2009# Springer Science + Business Media B.V. 2009

Abstract Organic–inorganic hybrids comprising epoxyresin and polyhedral oligomeric silsesquioxanes (POSSs)were prepared via in situ polymerization of the diglycidylether of bisphenol A (DGEBA) and 4,4′-diaminodiphenyl-methane (DDM). The POSSs have an active functionalgroup that takes part in the ring-opening reaction with theoxirane group. The organic and inorganic moieties arejoined by covalent bonds. These covalent bonds enhancethe compatibility of the inorganic and organic phases.Scanning electron microscope (SEM) analytical resultsindicate that there was no obvious phase separationbetween the inorganic and organic phases. The UV/VISspectrum of the epoxy hybrid demonstrates the excellentoptical transparency of the hybrids—the most importantcharacteristic for their application as protective coatings.Thermogravimetric analysis (TGA), X-ray photoelectronspectra (XPS), and nuclear magnetic resonance spectrosco-py (NMR) of the char showed that the incorporation of thePOSSs into epoxy resin improves the thermal stability ofthe hybrids.

Keywords Hybrid . Thermal property . Epoxy . Polyhedraloligomeric silsesquioxanes (POSSs)

Introduction

Organic–inorganic composites are typically considered anew generation of high-performance materials, as theycombine the advantages of inorganic materials (e.g., rigidityand high stability) with those of organic polymers (e.g.,flexibility, dielectric properties, ductility and processability)[1–7]. Polyhedral oligomeric silsesquioxanes (POSSs) arean interesting class of three-dimensional silsesquioxanesthat serve as soluble silica models. These POSSs arestructurally well-defined compounds that consist of asilicon–oxygen framework that has the general formulaRSiO3/2, and which can be easily functionalized using abroad range of organic groups commonly employed inpolymerization or grafting reactions [8].

Epoxies are a class of important thermosetting resins thatare widely used as matrices of composite materials,adhesives, and electronic encapsulating materials due totheir high mechanical strength, excellent chemical resis-tance, and processing simplicity [9–11]. Their extensiveapplication motivates the preparation of organic–inorganichybrid composites of epoxy resins with enhanced thermaland flame retardant properties. Modifying epoxy resin byadding POSSs can provide materials with superior charac-teristics such as improved thermomechanical properties,thermal and oxidative stability, and dielectric properties. ThePOSS silanols have a hybrid inorganic–organic three-dimensional structure that contains 1–4 stable silanol (Si–OH)groups. These POSS silanols can be incorporated into apolymer by copolymerization or grafting [12–14].

C.-H. SuDepartment of Fire Science, Wu-Feng Institute of Technology,117, Sec. 2, Jianguo Rd., Minsyong,Chiayi 621, Taiwan

Y.-P. Chiu : C.-L. Chiang (*)Department of Safety, Health and Environmental Engineering,Hung-Kuang University,34, Chungchi Rd., Sha-Lu,Taichung 433, Taiwane-mail: [email protected]

C.-C. TengDepartment of Chemical Engineering,National Tsing Hua University,Hsin-Chu 30043, Taiwan

J Polym Res (2010) 17:673–681DOI 10.1007/s10965-009-9355-y

This study describes the preparation of epoxy/tetrasila-nolphenyl POSSs (TSP-POSSs) hybrid copolymers withvarious proportions of POSSs synthesized via a ring-opening reaction between the hydroxyl group of the TSP-POSSs and the oxirane group of DGEBA epoxy monomers.The primary goal of this study was to describe thesynthesis, structure and properties of these hybrids byFourier transform infrared spectroscopy (FTIR), nuclearmagnetic resonance spectroscopy (NMR), scanning elec-tron microscopy (SEM), differential scanning calorimetry(DSC), UV/vis spectroscopy, thermogravimetric analysis(TGA), and X-ray photoelectron spectroscopy (XPS).

Experimental

Materials

The epoxy resin used was the diglycidyl ether of bisphenol A(DEGBA, NPEL-128) with an epoxide equivalent weight of180, which was generously provided by Nan-Ya PlasticsCorporation, Taiwan. The tetrasilanolphenyl POSSs (TSP-POSSs, C48H44O14Si8) were purchased from Hybrid PlasticsCo., Hattiesburg, MS, USA. 4,4′-Diaminodiphenylmethane(DDM) and triethylamine (TEA) were of analytical grade, andwere obtained from Acros Organics Co. Janssens Pharmaceu-ticalaan, Geel, Belgium. Tetrahydrofuran (THF) was reagentgrade and supplied by Echo Chemical Co. Ltd., Taiwan.

Preparation of epoxy/TSP-POSS/DDM hybrids

To prepare the nanocomposites containing the POSSs, aspecific amount of the TSP-POSSs was dissolved in aminimal amount of THF. The resulting solution was addedto a pre-weighed amount of DGEBA via vigorous stirringto create a homogeneous solution. The contents of the TSP-POSSs in the nanocomposites were adjusted to be 10, 20,30, and 40 wt%. The curing agent, DDM, was then addedrelative to the amount of DGEBA. The equivalent weightratio of DGEBA to DDM was 1:1. TEA (0.2 wt% DGEBA)was then added to the mixture. The TEA was used as acatalyst for the DGEBA and TSP-POSS reaction. Themixtures were stirred to obtain transparent solutions andthen poured into aluminum discs, and most of the solventwas evaporated at 60 °C for 24 h. To remove any residualsolvent, all of the samples were dried in vacuo at 60 °C fora minimum of 4 h. The systems were then cured at 80 °Cfor 2 h and 180 °C for 2 h to achieve complete curing.

Reaction schemes

Scheme 1 shows the reaction scheme for the preparation oforganic–inorganic DGEBA involving TSP-POSSs.

Measurements

FTIR

The FTIR spectra of the materials were recorded between4,000 and 400 cm−1 on a Nicolet Avatar 320 FT-IRspectrometer, Madison, WI, USA. Thin films were preparedby the solution-casting method. A minimum of 32 scanswere signal-averaged with a resolution of 2 cm−1 in the4,000–400 cm−1 range.

29Si NMR

29Si NMR was performed using a Bruker DSX-400WB,Karlsruhe, Germany. The samples were treated at 180 °Cfor 2 h and then ground into fine powder.

Differential scanning calorimetry (DSC)

The glass transition temperatures (Tgs) of the sampleswere measured using a differential scanning calorimeter(DSC) (DuPont, Wilmington, DE, USA; model 10). Theheating rate was 10 °C min−1 within a temperature rangeof 50~200 °C. The measurements were made with 3~4 mgsample on a DSC plate after the specimens had beenquickly cooled to room temperature following the firstscan. Tgs were determined at the midpoint of thetransition point of the heat capacity (Cp), and thereproducibility of the Tg value was estimated to be within2 °C.

Thermogravimetric analysis (TGA)

The thermal degradation of the composite was examined by athermogravimetric analyzer (PerkinElmer, Wellesley, MA,USA; TGA 7) from room temperature to 800 °C at 10 °C/minin an atmosphere of nitrogen. Measurements were made using6–10 mg samples. Weight loss/temperature curves wererecorded.

Scanning electron microscopy (SEM)

The morphology of the fractured surface of the polysilses-quioxanes was examined using a scanning electron micro-scope (SEM) (JSM 840A, JEOL, Tokyo, Japan). Thedistribution of Si atoms in the hybrid was obtained bySEM EDX mapping (JSM 840A, JEOL).

UV/vis

UV/vis spectra were tested on a Hitachi (Tokyo, Japan) U-3300 spectrophotometer, and the sample was prepared as athin film on a glass substrate by spin coating.

674 C.-H. Su et al.

Transmission electron microscopy (TEM)

The TEM study was carried out using a Hitachi H-7500 withan accelerating voltage of 100 kV during measurements. Thesamples were microtomed with a Leica (Solms, Germany)Ultracut and cut into 60 nm-thick slices. These slices wereplaced on mesh 200 copper nets for TEM observation.

Dynamic mechanical analysis (DMA)

The dynamic mechanical tests were carried out on adynamic mechanical thermal analyzer (DMTA MK III,Polymer Laboratories, Church Stretton, UK) between 30 °Cand 200 °C with a heating rate of 2 °C/min and a frequencyof 2 Hz. The rectangular bending mode was chosen and thedimensions of the specimen were 40×7×3 mm3.

Results and discussion

FTIR of the reaction processes

Infrared spectroscopy was performed to characterize thestructures of the composites via the ring-opening reaction

between the oxirane functional group of epoxy resin andthe –OH functional group of the POSSs. Scheme 1 showsthe reaction process and the structure of the epoxy/POSScomposite. Figure 1 presents FTIR results for the reaction

Scheme 1 Preparation of organic–inorganic DGEBA involving TSP-POSSs

Fig. 1 FTIR of the reaction process for: a, the epoxy prepolymer; b,pure POSS monomer; c, epoxy/POSS composites after the curingreaction

Preparation, characterization and thermal properties of organic–inorganic composites 675

process of the epoxy monomer (a), the pure POSSsmonomer (b), and the epoxy/POSS composites after curing(c). The characteristic peak of the oxirane group of theepoxy resin is at 910 cm−1 in curve (a). The Si–O–Si groupwas detected in the hybrid at 1,112 cm−1, which corre-sponds to the cage structure of POSSs in curve (b). In curve(c), the oxirane group at 910 cm−1 has disappeared, and theintensity of the hydroxyl group at 3,450 cm−1 hasincreased, meaning that the epoxy resin has reacted withthe POSSs. Connections are formed between the organicand inorganic phases. Covalent bonds exist between theorganic and inorganic moieties. These covalent bondsenhance the compatibility of the two phases. The hybridshave networks that enhance their thermal properties.

Morphological properties of composites

The compatibility of organic polymers and silica markedlyaffects the thermal, mechanical and optical properties of the

composites. The morphology of the fractured compositesurface was observed by SEM. A mapping technique wasused to elucidate the silica distribution and the separation ofthe microphases in the hybrid matrix. Figure 2a shows anSEM micrograph of the morphology of a composite. Thefracture surface was very dense and no obvious phaseseparation or gaps existed between the organic andinorganic phases. Figure 2b shows an EDX Si map of anepoxy nanocomposite. The particles were uniformly dis-persed throughout the polymer matrix. Aggregation ofinorganic particles was not observed in the SEM micro-graphs. This analytical result demonstrates that the nano-composites exhibit good miscibility between the organicand inorganic phases. The compatibility of POSS moleculesand polymers is key to achieving a well-dispersed polymernanocomposite. Generally, POSS molecules show betterdispersion than conventional additives. This enhanceddispersion can be attributed to the existence of covalentbonds between POSS particles and the epoxy resin. SomePOSS molecules can easily disperse into the polymermatrix at the molecular level, which may result incomposite properties that are very different from those ofconventional polymer composites.

TEM was employed to study the internal morphologiesof the composites at a fine scale. Sections about 75 nmthick were found to contain entire small-sized domains andportions of larger-sized domains. Figure 3 presents a TEMmicrophotograph of a fractured composite surface. Basedon this figure, the size of the particles in the composites isabout 100 nm. Since each POSS molecule has a three-dimensional inorganic core covered with four organic sidegroups and four hydroxyl groups, it is believed that thebetter dispersion of these composites may result from thechemical bonding of the hydroxyl groups and increasedinteraction between compatible side groups and thenetwork of the epoxy resin.

Fig. 3 TEM microphotograph of a fractured surface of a compositecontaining 2 wt% of POSS

Fig. 2 a SEM micrograph of an epoxy/POSS composite. b EDX Simap of an epoxy/POSS composite

676 C.-H. Su et al.

UV/vis spectroscopy of the hybrids

Figure 4 shows UV/vis spectra of epoxy/POSS compositeswith different inorganic contents. No obvious absorbanceoccurred in the range 400–800 nm (the visible light region),meaning that visible light can pass through the composites,and there was no obvious phase separation between theorganic and inorganic phases. These results show thatthe hybrids have excellent optical transparency and that theinorganic particles are smaller than the wavelength ofvisible light. Hybrids with different inorganic contentsshow similar transparencies. This optical transmittance canbe utilized as a criterion for the formation of a homoge-neous phase. Figure 5 presents photographs of epoxy/POSScomposites with different inorganic contents. Analyticalresults demonstrate that the hybrids exhibit excellent opticaltransparency, which is the most important characteristic fortheir application as protective coatings.

Thermal properties of the hybrids

The epoxy/POSSs hybrids were subjected to thermalanalysis. Figure 6 shows DSC curves for the pure epoxy

and the hybrids. Pure epoxy had a glass transitiontemperature of 152 °C. All of the DSC curves for thecomposites containing POSSs had single glass transitiontemperatures in the experimental temperature range (roomtemperature to 200 °C). Notably, the presence of a singleglass transition temperature indicates that the hybrids arehomogeneous [15]. The epoxy/POSS hybrids had slightlylower Tg values than pure epoxy (Fig. 6). It is proposedthat the decreased Tg values may be responsible for theincrease in the free volume of the system through theinclusion of some of the bulky POSS cages at the nanoscalelevel. In other words, there is an effect comparable to aplasticization effect of the low molecular weight com-pounds on the polymer matrix. Nevertheless, the hybridscontaining 10, 20, 30 and 40 wt% POSSs had very similarglass transition temperatures. No clear trend in glasstransition temperatures is discernible in Fig. 6. Thus, wepropose that two competing factors determine the glasstransition temperatures of POSS-modified polymers. Clear-ly, the POSS cages hinder polymer chain motion, increasingthe glass transition temperature. Conversely, the inclusion

Fig. 6 DSC of epoxy/POSSs composites with various POSS contents

Fig. 5 Photographs of epoxy/POSS composites with differentinorganic contents

Fig. 4 UV/vis spectra of epoxy/POSS composites with differentinorganic contents

Fig. 7 TGA of epoxy/POSS composites with different inorganiccontents at 10 °C/min in a nitrogen atmosphere


of the bulky POSS groups may increase the free volume ofthe system, resulting in a decrease in Tg. Therefore, thebehavior of the glass transition in hybrids containingPOSSs may reflect these two competing factors, dependingon the characteristics of the composite system. Although allof the glass transition temperatures of the POSS hybridswere lower than that of the pure epoxy, they did not varysignificantly as the POSS concentration increased, espe-cially for TSP-POSS hybrids, in which the POSS cages actas crosslinking units. This behavior can be attributed toinhibitory effect of POSS cages on molecular motion, and itcontrasts with the observed behavior of polymer systemsplasticized with low molecular weight compounds.

Thermogravimetric analysis (TGA) is typically used toaccurately track the in situ weight changes of a sampleduring heating, thereby providing thermal degradation data.Figure 7 shows TGA thermograms for epoxy with variousinorganic contents at room temperature to 800 °C in anitrogen atmosphere. Table 1 shows the weight losscharacteristics of epoxy with various POSS contents. Thethermal stability of the hybrids was higher than that of thepure polymer considering the char yields of the hybrids andthe pure polymer. The values of the char yields of thehybrids increased as the inorganic content increased. Thechar yields of the hybrids were higher than those of the copolymers at high temperatures. The TGA results

show that the char contents were higher than the amounts ofinorganic filler in different samples, meaning that thepresence of inorganic content can prevent thermal degra-dation of the polymer matrix and retain significant amountsof char in order to stop fires from spreading.

The integral procedural decomposition temperature(IPDT) [16] has been correlated with the volatile parts ofpolymeric materials, and is used to estimate the inherentthermal stabilities of polymeric materials. The IPDT wascalculated using

IPDT �Cð Þ ¼ A*K* T f � T ið Þ þ T i ð1Þ

A* ¼ S1 þ S2ð Þ= S1 þ S2 þ S3ð Þ ð2Þ

K* ¼ S1 þ S2ð Þ=S1 ð3ÞFig. 8 Schematic representations of S1, S2 and S3, which are used tocalculate A* and K*

Table 1 Thermal characteristics of epoxy/POSS composites

Sample no. Residue (%) at 800°C IPDT (°C)

DGEBA 19 702

DGEBA/10 wt% TSP-POSS 24 802

DGEBA/20 wt% TSP-POSS 32 1,009



Fig. 9 a SEM of the char of an epoxy/POSS composite. b Si map ofthe char of an epoxy/POSS composite

678 C.-H. Su et al.

where A* is the area ratio of the total experimental curvedefined by the total TGA thermogram, Ti is the initialexperimental temperature, and Tf is the final experimentaltemperature. Figure 8 shows S1, S2 and S3, which aresurface areas related to the weight retention versustemperature curve that are used to calculate A* and K*.

The IPDT of pure polymer was 702 °C (Table 1). TheIPDTs of the composites were higher than that of the purepolymer; the values for hybrids with different POSScontents were 802, 1,009, 1,306, and 1,559 °C, respective-ly. High-temperature thermal stability was increased byadding silicon compounds, because the product is silicondioxide, which cannot be degraded further. These resultsindicate that the thermal stability of the composite increaseswith its inorganic content. Thus, inorganic components canimprove the thermal stability of epoxy resin.

Char analysis

Many researchers have asserted that improvements inthermal oxidation resistance can be attributed to theformation of an inert silica-like layer on the surface of thepolymer system during the thermal oxidation process [17].In this study, SEM and Si mapping were used to identifythe mechanism by which POSSs enhance thermal oxidationresistance. SEM was used to observe the microstructures ofthe hybrid materials after thermal oxidation at 600 °C for10 min in air. The results are shown in Fig. 9a. Figure 9bshows that the char contains silicon components; the whitedots in this figure represent silicon atoms. This means thatthe char is SiO2. An inert layer formed on the compositesurface after thermal oxidation. The inert layer, which had asilica-like structure, was very stable, and can prevent thediffusion of substances and matrix oxidation. Thus, theoxidation of materials was retarded, and thermal oxidationresistance was enhanced.

29Si NMR provides structural information on the type ofsilicon present in the resin network. Figure 10 shows the29Si NMR spectra of 20 wt% POSS-reinforced epoxy atroom temperature and after pyrolysis in air for 10 min at

Fig. 10 29Si NMR spectra of an epoxy/20 wt% POSS composite atdifferent temperatures

Fig. 11 Si 2p XPS of POSS-reinforced epoxy with different POSSscontents: a 10 wt%; b 20 wt%; c 40 wt%


600 °C. At room temperature, the only peak occurred at−82.77 ppm, which corresponds to the T3 structure, as allsilicon atoms have the same chemical environment in thePOSS cages. When the temperature was increased to 600 °Cfor 10 min, the POSS cage structures were destroyed and theT3 structure changed into T2, Q2, Q3 and Q4 structures,where Tn and Qn are the siloxane unit structures RSi(OSi)nX3-n and RSi(OSi)nX4-n, respectively. The Qs are themajor structures, and have a similar effect to SiO2 inpreventing the thermal degradation of epoxy resin.

Peak shape analysis by XPS provided further informa-tion on the chemical changes involved. Figure 11 shows theSi 2p peaks obtained upon the pyrolysis of POSS-reinforced epoxy at 600 °C for 10 min. Table 2 summarizesthe peak analysis results for the C 1s and Si 2p peaks. TheC 1s data indicate that the peaks from highly oxidizedcarbon content at BE=288.8 and 286.5 eV decrease as thepeaks from the POSS content increases (from 24.6% to13.7%), meaning that the antioxidation resistance of theepoxy resin was improved by introducing the POSS cages.On the other hand, these peaks occur at a BE of 102.0 eV,which corresponds to RSiO3/2 in the POSS cages. However,a new component in the Si 2p region appeared afterpyrolysis at 600 °C for 10 min. In the literature [18], the Si

2p peak at 103.4 eV was assigned to an “inorganic silica-like phase.” A change was observed in the Si 2p spectrumafter oxidation at 600 °C, which indicates that the silicalayer formed a protective barrier on the surface of thePOSS-reinforced epoxy, preventing further degradation ofthe epoxy chain.

Dynamic mechanical properties

Dynamic storage moduli are shown in Fig. 12 as plots ofstorage modulus versus temperature for the control epoxyand for the composites. It is interesting to note that in theglass state, the dynamic storage moduli of all of the POSS-containing hybrids were significantly higher than that of thecontrol epoxy, and the moduli increased with the concen-tration of POSS. In the hybrid systems, the POSS cageswere homogeneously dispersed in the epoxy matrices onthe nanoscale, and thus the increased modulus could beattributed to the nanoreinforcing effect of POSS cages onthe epoxy matrix. Nonetheless, it should be noted that themoduli of the nanocomposites did not exhibit a monoto-nous increase as a function of POSS concentration,although all of the nanocomposites investigated showedan increased storage modulus in the glassy state. Theexplanation for this observation could be based on the twoopposing effects of POSS cages on the matrices of thematerials. The nanoreinforcing effect of the cubic silses-quioxane cages on the polymer matrix will result in anincreased modulus in these materials.

Conclusions

Novel epoxy composites containing POSSs were preparedsuccessfully using the ring-opening reaction. The POSScage structure was incorporated into the networks of thehybrids, increasing the thermal stability of the epoxy resin.The following are the principal conclusions of this study:

1. The POSSs can react with the polymer matrix, therebyimproving the compatibility of the organic and inor-ganic phases.

Fig. 12 Storage modulus as a function of temperature for the controlepoxy and for the composites

Sample Peak area (%)

C 1s (eV) Si 2p (eV)

288.8 286.5 285.0 283.7 103.4 102.0C = O C-O C-C/C-H C-Si SiO2 RSiO3/2/Si-C

At 600 °C 10 wt% 13.6 11.0 19.7 55.7 11.4 88.6

20 wt% 14.8 3.0 16.7 65.5 8.2 91.8

40 wt% 4.9 8.8 10.3 76.0 6.1 93.9

Table 2 Peak area analysisresults from the C 1s and Si 2pXPS spectra for epoxy/POSScomposite char

680 C.-H. Su et al.

2. The hybrids are transparent, and no phase separationwas observed between the organic and inorganic phases.

3. The thermal stability of the hybrids increased as the contentof inorganic components increased. The inorganic compo-nents improved the thermal stability of the epoxy resin.

4. The antioxidation resistance of the epoxy resin wasimproved by introducing the POSS cages.

Acknowledgments The authors would like to thank the NationalScience Council of the Republic of China, Taiwan, for financiallysupporting this research under Contract No. NSC. 97-2622-E-241-001-CC3. Ted Knoy is appreciated for his editorial assistance.

References

1. Mark JE (1996) Polym Eng Sci 36:2905–29202. Sanchez C, Lebeau B, Chaput F, Boilot JP (2003) Adv Mater

15:1969–19943. Nicole L, Boissiere C, Grosso D, Quach A, Sanchez C (2005) J

Mater Chem 15:3598–3627

4. Rao Y, Chen S (2008) Macromolecules 41:4838–48445. Daimatsu K, Sugimoto H, Nakanishi E, Yasumura T, Inomata K

(2008) J Appl Polym Sci 109:1611–16176. Marini M, Pilati F, Saccani A, Toselli M (2008) Polym Degrad

Stabil 93:1170–11757. Burgos-Asperilla L, Darder M, Aranda P, Vazquez L, Vazquez M,

Ruiz-Hitzky E (2007) J Mater Chem 17:4233–42388. Eisenberg P, Erra-Balsells R, Ishikawa Y, Lucas JC, Nonami H,

Williams RJJ (2002) Macromolecules 35:1160–11749. Pham HQ, Marks MJ (2005) Epoxy resins. Wiley-VCH, Weinheim

10. Ellis B (1993) Chemistry and technology of epoxy resins. BlackieAcademic & Professional, London

11. Lee H, Neville K (1967) Handbook of epoxy resins. McGraw-Hill, Inc., New York

12. Lichtenhan JD, Vu NQ, Carter JA, Frank WG, Feher J (1993)Macromolecules 26:2141–2142

13. Liu YR, Huang YD, Liu L (2007) Compos Sci Technol 67:2864–2876

14. Liu YR, Huang YD, Liu L (2006) Effects of trisilanolisobutyl-POSS on thermal stability of methylsilicone resin. Polym DegraStab 91:2731–2738

15. Ni Y, Zheng S, Nie K (2004) Polymer 45:5557–556816. Doyle CD (1961) Anal Chem 33:7717. Cardoen G, Coughlin EB (2004) Macromolecules 37:5123–512618. Toth A, Bertoti I, Blazso M, Banhegyi G, Bognar A, Szaplonczay

P (1994) J Appl Polym Sci 52:1293–1307


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