Research Article

Received: 26 October 2009, Revised: 12 January 2010, Accepted: 5 March 2010, Published online in Wiley Online Library: 15 July 2010

(wileyonlinelibrary.com) DOI: 10.1002/pat.1722

Thermal and dynamic mechanical properties ofepoxy resin/poly(urethane-imide)/polyhedraloligomeric silsesquioxane nanocomposites

Jiangxuan Songa,b,c, Guangxin Chena,b,c, Gang Wua,b,c, Chunhua Caia,b,c,Pinggui Liud and Qifang Lia,b,c*

Linear isocyanate-terminated poly(urethane-imide)

Polym. Adv

(PUI) with combination of the advantages of polyurethane andpolyimide was directly synthesized by the reaction between polyurethane prepolymer and pyromellitic dianhydride(PMDA). Then octaaminophenyl polyhedral oligomeric silsesquioxane (OapPOSS) and PUI were incorporated into theepoxy resin (EP) to prepare a series of EP/PUI/POSS organic–inorganic nanocomposites for the purpose of simul-taneously improving the heat resistance and toughness of the epoxy resin. Their thermal degradation behavior,dynamic mechanical properties, and morphology were studied with thermal gravimetric analysis (TGA), dynamicmechanical analysis (DMA), and transmission electronmicroscope (TEM). The results showed that the thermal stabilityand mechanical modulus was greatly improved with the addition of PUI and POSS. Moreover, the EP/PUI/POSSnanocomposites had lower glass transition temperatures. The TEM results revealed that POSS molecules could selfassemble into strip domain which could switch to uniform dispersion with increasing the content of POSS. All theresults could be ascribed to synergistic effect of PUI and POSS on the epoxy resin matrix. Copyright� 2010 JohnWiley& Sons, Ltd.

Keywords: polyhedral oligomeric silsesquioxane; poly(urethane-imide); epoxy resin; thermal degradation; morphology

* Correspondence to: Q. Li, State Key Laboratory of Chemical Resource Engin-eering, Beijing University of Chemical Technology, Beijing 100029, China.E-mail: qflee@mail.buct.edu.cn

a J. Song, G. Chen, G. Wu, C. Cai, Q. Li

State Key Laboratory of Chemical Resource Engineering, Beijing University of

Chemical Technology, Beijing 100029, China

b J. Song, G. Chen, G. Wu, C. Cai, Q. Li

Key Laboratory on Preparation and Processing of Novel Polymer Materials of

Beijing, Beijing 100029, China

c J. Song, G. Chen, G. Wu, C. Cai, Q. Li

College of Material Science and Engineering, Beijing University of Chemical

Technology, Beijing 100029, China

d P. Liu

Beijing Institute of Aeronautical Materials, Beijing 100095, China

Contract/grant sponsor: Polymer Chemistry and Physics, Beijing Municipal

Education Commission (BMEC); contract/grant number: XK100100640.

Contract/grant sponsor: Beijing Natural Science Foundation; contract/grant

number: 2072015.

Contract/grant sponsor: National High Technology Research and Develop-

ment Program of China; contract/grant number: 2006AA03Z563. 2

INTRODUCTION

In recent years, polyhedral oligomeric silsesquioxanes (POSSs)have received considerable attention as they possess asynergistic combination of constituent properties of organicand inorganic materials.[1–7] Unlike conventional inorganic fillers,POSS nanofillers offer the advantages of monodisperse size, lowdensity, and synthetically well-controlled functionalities. TypicallyPOSS nanoparticle is a 3D cage like siloxane structure surroundedby eight organic R groups (RSiO4), where R can be unreactiveorganic group, such as alphatics,[8] or phenyl,[9] as well as can bereactive organic group, such as epoxy,[10,11] methacrylate,[12]

norbornyl,[13,14] vinyl,[15] styryl,[16] amines[9], and so on. However,mainly due to the existence of numerous nanofiller–nanofillerinteractions, those POSS nanofillers with unreactive corner grouptend to aggregate when they were simply blended withinpolymer matrices. In order to improve the dispersion, themiscibility of the nanofillers and polymer matrix should beenhanced. One of the most common and successful methods issurface organic modification of nanofillers. The creation of astrong chemical bond between the filler and the polymer matrix,which promotes more favorable nanofiller–polymer interactionsand leads to a reduction of the interfacial energy, can enhancecomplementary properties of the hybrid materials comparedwith the simple blends. [1]

Epoxy resins are widely utilized as adhesives, electronicencapsulating compounds and matrices as well as otherstructural materials due to their high modulus and strength,excellent chemical resistance, and simplicity in processing. Inorder to improved comprehensive properties of epoxy resin,many functionalized POSS compounds with epoxy group,

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hydroxyl groups and amino groups have been employed toprepare a wide variety of EP/POSS hybrids. Results showed thatPOSS improved the performances of epoxy resin, especially forthe thermal stability and mechanical performance. But as aconsequence of their highly cross-linked structure, thesematerials tended to suffer from brittle behavior, poor crackresistance, and low fracture toughness. A well-known procedureto toughen such brittle polymers was to incorporate flexiblepolymer into the rigid matrix, such as polyurethane.[17,18] This

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method had been applied with great success to enhance thetoughness of epoxy resins but usually scarifying other usefulproperties, especially thermal resistance and mechanical per-formance.In this paper, we synthesized the –NCO terminated poly-

(urethane-imide) (PUI), which not only has a high reactivelyterminated function group but also combine the advantages ofthe polyurethane and polyimide. Then PUI together with POSSwas introduced into epoxy resin matrix to prepare EP/PUI/POSScomposites. The thermal stability and dynamic mechanicalproperties were studied with aim to investigate the effect of POSSand PUI on the properties of the modified epoxy system.

Scheme 1. Preparation of poly(urethane-imide).

EXPERIMENTAL

Materials

Di-hydroxyl-terminated polytetrahydrofuran glycol oligomer(PTMEG, Mn¼ 1000) was purchased from BASF. Pyromelliticdianhydride (PMDA) and isophorone diisocyanate (IPDI) werereceived from Aldrich without further purification. The diglyci-dylether of bisphenol A type epoxy resin (DGEBA, epoxyequivalent �500 g/Eq) was obtained from Huntsman. Theoctaaminophenyl polyhedral oligomeric silsesquioxane (Oap-POSS) was synthesized in this laboratory through the methoddescribed in the literature,[7] 4,40-diaminodiphenylmethane(DDM) was used as curing agent obtained form Beijing RegentCo., China. All other organic solvents, such as toluenetetrahydrofuran (THF), N,N-dimethylformamide (DMF) and othersolvents were analytical reagents and freshly distilled prior to use.

Characterization

Infrared spectroscopic measurements were performed in therange 4000–400 cm�1 at a resolution of 4.0 cm�1 using a BrukerTensor 27 FT-IR spectrometer. The samples were dissolved in THFand evaporated on KBr pellet under IR lamp.The thermal stability of these hybrids was assessed by

thermogravimetric analysis using a TG 209 C analyzer operatedat a heating rate of 10 8C/min under a continuous flow of nitrogenfrom room temperature to 6008C.Dynamic mechanical thermal analysis measurements were

performed using a DMA Rheometric scientific V in a singlecantilever bending mode over a temperature range from 50 to2008C. Heating rate and frequency were fixed at 2 8C/min and1Hz, respectively.A H800 transmission electron microscope (TEM) (HITACHI JAP,

INC.) was used to characterize the phase morphology of EP/PUI/POSS nanocomposites. The samples were ultramicrotomed to70–90 nm thickness with a diamond knife and mounted on CuTEM Grids for observation.

Table 1. Composition of EP/PUI/POSS composites

Sample Epoxy resin (g) PUI (g)

EP 10.00 0EP/PUI 5.00 5.00EP/PUI/POSS-1 5.00 5.00EP/PUI/POSS-2 5.00 5.00

wileyonlinelibrary.com/journal/pat Copyright � 2010 John Wiley

Synthesis of poly(urethane-imide) prepolymer

The –NCO terminated PUI prepolymer was prepared in a two-stepprocess as shown in Scheme 1. First –OH terminated soft segmentPTMEG was degassed and dried in around flask under highvacuum at 1058C for 2 hr; after the temperature was cooled to708C, IPDI was charged into the flask in a molar ratio of 2:1 (NCO/OH), then dried toluene solvent was charged into the flask andthe reaction system was continued at 708C for 2 hr under stirring,giving a 50wt% solution of NCO-terminated PU prepolymer.In the next step, the solution of PMDA in DMF (50wt%) was

charged into a flask containing 50wt% NCO-terminated PUprepolymer in a molar ratio of 2:1 (NCO/PMDA), the mixture wasrigorously stirred under N2 atmosphere. The reaction between anisocyanate and a hydride yields imide and carbon dioxide. Theevolved carbon dioxide was monitored to study the course of thereaction. The carbon dioxide was passed into a saturated calciumoxide to precipitate calcium carbonate. The calcium oxide waschanged every half an hour. The reaction time was determinedfrom the cessation of carbon dioxide evolution.

Preparations of composites

The prescribed amounts of epoxy resin solution, as shown inTable 1, were mixed with PUI prepolymer, and then the chemicalstoichiometric DDM and POSS were added. The mixture wasthoroughly stirred to ensure mixing well. After evaporatingthe solvent, a series of polymer films containing PUI and POSSwere obtained. These formed films were cured at ambienttemperature for 2 weeks under laboratory humidity condition.Finally a series of transparent composites were obtained.

POSS (g) DDM (g) POSS content (wt%)

0 0.96 00 0.69 00.19 0.58 1.80.81 0.48 7.2

& Sons, Ltd. Polym. Adv. Technol. 2011, 22 2069–2074

Figure 2. TGA curves (a) and first derivative curves of TGA curves (b) of

EP, EP/PUI, and EP/PUI/POSS composites. This figure is available in color

online at wileyonlinelibrary.com/journal/pat

THERMAL AND DYNAMIC MECHANICAL PROPERTIES

RESULTS AND DISCUSSION

FTIR analysis

As previously reported,[19–21] synthesis of –OH terminated PUprepolymer was directly carried out via nucleophilic additionbetween –OH groups of polydiol (PTMEG) and –NCO groups ofisocyanate (IPDI). Figure 1 shows the FTIR spectra of the PTMEG,PU, and PUI. It is clearly seen that the broad –OH peak at3400–3600 cm�1 of PTMEG changed to 3330 cm�1 assigned to–NH– of –CONH– groups in PU segment, and some importantspectroscopic features appeared, such as the absorbance peak at1730 cm�1 due to the conversion of –NCO groups into –CONH–groups. Also, the sharp peak at 2270 cm�1 of NCO appeared in PUspectra. These indicated –NCO terminated PU prepolymer wassuccessfully synthesized under experimental condition.The absorption bands at about 1780 and 725 cm�1 that are

characteristic bands of imide bond verify that the imide grouphas been introduced to PU successfully. 1380, 1120, and 725 cm�1

are assigned to axial, transverse, and out of plane vibrations ofcyclic imide structure.

Thermal properties

The thermal stability of EP/PUI/POSS composites was evaluatedby thermal gravimetric analysis (TGA) and DTGA (1st derivative ofthermal gravimetric analysis curve versus temperature) as shownin Fig. 2. Within the experimental temperature range, the TGAcurves of neat epoxy resin cured by DDM displayed differentdegradation profiles from others. Three-stage degradation wasobserved obviously from the DTGA curves: an early stage withmaximum rate at about 203.78C, assigned to thermal decompo-sition the species related to epoxy rings in structures, the secondstep at 389.28C might correspond to the thermal decompositionof the remaining organic structure; the third at 451.88C wasattributed to the carbonization of epoxy structure. But for the PUImodified epoxy resin, very different degradation behavior wasobserved in TGA curve, which has only one stage. Moreover EPmodified by both PUI and POSS (EP/PUI/POSS) also has only onestage. This observation indicates that the introduction of PUIprepolymer can alter the degradation mechanism of the matrixpolymer. The temperature T5%, at which 5% weight loss occurred,

Figure 1. FTIR spectra of PTMEG, PU, and PUI.

Polym. Adv. Technol. 2011, 22 2069–2074 Copyright � 2010 John Wil

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was also investigated to further assess the thermal stability ofthe composite materials. For the modified system, the initialdecomposition T5% which occurred at 331.18C, was 141.38Chigher than that of neat epoxy. It is well known that a molecularunit with a higher aromatic ring content has a higher rigidity, heatresistance, and a higher steric hindrance to molecular motion.Here PUI combine with the advantages of polyurethane andpolyimide. Thus the introduction of imide moieties increases thethermal stability of the PU. Generally the thermal stability of apolymer will be determined by the strength of its weakest bond.When compared to PU, PUI copolymer consumes part of theisocyanate groups after imidization for forming the imide groups,which is highly thermally stable. The thermal stability of EP/PUIcomposites prepared by this method was almost the same as theEP/PI composites based on polyimide (6FDA/AHBP) and epoxyresin.[22] Moreover the terminated –NCO of PUI can easily reactwith –OH of epoxy resin forming –NHCO– bond, which canenhance the interaction of epoxy resin and PUI. Due to thesynergistic effect of these resins, the composites have a goodthermal stability property.In addition, the TGA curves of all the EP/PUI/POSS composites

played similar degradation profiles, suggesting that the existenceof the POSS did not significantly alter the degradation

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mechanism of the matrix polymers. The incorporating of POSSinto EP/PUI networks showed a significant effect in improving thethermal stability, resulting in a retarded weight loss rate and anenhanced char yielded in the higher temperature region. Thiseffect was observed to be increasingly significant whenincreasing the concentration of POSS cages. The improvementin weight retention was ascribed to the POSS constituent, whichparticipated in the formation of a homogeneous hybrid network.The higher char yield for EP/PUI/POSS implied that there werefewer volatiles released from the nanocomposites duringheating.

Dynamic mechanical properties

Dynamic mechanical thermal analysis is a good method to studythe relaxation behavior and to detect the glass transitiontemperature as well as the change in storage and loss moduluswith temperature. This provides information on the phaseseparation and the mechanical behavior of a polymer. Figure 3shows the DMA curves for storage modulus and tan d of neat EPand EP/PUI/POSS nanocomposites.

Figure 3. Storagemodulus and tan dprofiles of EP, EP/PUI, and EP/PUI/POSS

composites. This figure is available in color online at wileyonlinelibrary.com/

journal/pat

wileyonlinelibrary.com/journal/pat Copyright � 2010 John Wiley

It is interesting to note that within the glass state the dynamicstorage modulus of EP/PUI is lower than pure EP, but theintroduction of POSS gave rise to a significant increase indynamic storagemodulus, i.e. the storagemodulus of compositescontaining 7.2wt% POSS reached to 2.45MPa, higher than pureEP. It is also worth noticing that the storage modulus of therubbery plateau showed the same change trend as the above.There are several competitive factors which affect the modulus ofthe composites. First, PUI has a long soft segment which willreduce the stiffness of the nanocomposites; on the other hand,POSS hasmultifunctional groups (eight amino groups), which caneasily react with epoxy and –NCO, resulting in high crosslinkingdensity than that cured by DDM which has two amino groups. Sothe nanoreinforcement of the POSS on the polymer matrices willgive rise to an increase in modulus. Moreover, the silsesquioxanecages have the ability to reinforce the epoxy matrix.Figure 3 also shows plots of tan d as a function of temperature

for epoxy resin and its modified composites. The loss factor tan dis very sensitive to the structural transformation of the materialsand can be used to identify Tg of these nanocomposites. It is seenthat the neat epoxy resin exhibits a well-defined relaxation peakcentered at 79.18C, which is ascribed to the glass transition ofcross linked epoxy resin. As we all know, PU has a flexible chainand a low Tg. It clearly displayed that the PUI modified epoxy resinshowed a broad and lower Tg (68.58C) with contrast to epoxyresin. It is also clearly seen that there is only one peak for the EP/PUI composite, which indicates that the –NCO terminated PUI has

Figure 4. The TEM micrograph of the EP/PUI/POSS nanocompositeswith different content: (a) 1.8wt% (b) 7.2wt%. This figure is available

in color online at wileyonlinelibrary.com/journal/pat

& Sons, Ltd. Polym. Adv. Technol. 2011, 22 2069–2074

THERMAL AND DYNAMIC MECHANICAL PROPERTIES

strong interactions with epoxy resin through the reactionbetween –NCO and –OH of EP, and form homogeneous phase.For POSS modified EP/PUI systems, the Tg decreases at first andthen increases with the increasing of POSS content in modifiedhybrids systems. As the POSS content is low, the crosslink densityis decreasing while destroying the EP-DDM networks in someextent, and as the POSS content increases, the adding of POSSleading to the high crossing density, the star-like structure retardsthe motion of the polymer chain.[23,24] In addition, it was notedthat the tan d peaks were significantly broadened when theincorporation of POSS into the epoxy matrix. The width ofthe tan d peaks could reflect the structural homogeneity of thecrosslinked networks. The incorporation of PUI and POSS, whichhave high reactive isocyanate and amino functional group, willdisrupt the epoxy resin matrix due to the multiple curingreactions. The interaction of EP, PUI, and POSS will result indiversity of crosslinking network, e.g. the length between twojoints, the segmental motion of molecular chains and networkjunctions are different from those of pure epoxy resin. It has beenproposed that the broadening of the loss peaks could prefiguresuperior mechanical properties, such as damping properties.[25]

Morphology of the EP/PUI/POSS composites

It is observed that all the cured composites were transparent,suggesting that no phase separation occurs on the scale at leastmore than the wavelength of the visible light. The morphology ofthe EP/PUI/POSS hybrids was further investigated by TEM toobserve the dispersion of POSS particles in EP/PUI matrix (Fig. 4).It is seen that the two-phase morphology with branching stripshaped domain distributes in continuous matrix appears in EP/

Scheme 2. Schematic illustration of the preparation of EP/PUI/POSS conta

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Polym. Adv. Technol. 2011, 22 2069–2074 Copyright � 2010 John Wil

PUI/POSS-1. Moreover with the POSS content increasing, themorphology is quite different from the above. The POSScomponent was homogenously dispersed in the continuousEP/PUI matrix at the nanoscale level, indicating that thenanocomposites were obtained.In the multi-component mixing system, miscibility is governed

by nature of constituents, which can be thermodynamicallyinterpreted as the contribution of both mixing enthalpy (DHm)(e.g. intermolecular interactions) and mixing entropy (DSm) Asdepicted in paper,[26–28] POSS particles are highly hydrophobicand tend to bind together to form a cluster of agglomerationduring the curing process. With the curing reaction proceeding,the systems experienced a series of changes in topologicalstructures, such as chain growth, branching, gelation, and so on.The obvious microscopy phase change of POSS modified EP/PUIcomposites could be ascribed by the following factors. Theincreased molecular weight of the systems owing to thecrosslinking reaction will result in the decrease of entropiccontribution to miscibility, even phase separated, i.e. POSS canself assemble into some special morphology during the curingprocess. In this study DDM is used as curing agent which has onlytwo amino groups. Furthermore, the curing process wasperformed under room temperature, and resulted in the curingrate is so slow that POSS tend to bind together forming stripdomain. Corresponding to DDM, POSS has much higher reactivefunction groups (eight amino groups); therefore, with POSScontent increasing the viscosity of curing system increasesquickly and the gel point would go ahead, which hinder POSSmolecular aggregate. So when the content of POSS increased to7.2wt%, the POSS was found to be homogeneous at nanometerscale (shown in Scheme 2). In addition, it is interesting to note

ining different content POSS. This scheme is available in color online at

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that the different morphology of the resulting EP/PUI/POSShybrids can be obtained by controlling the content of POSS.

CONCLUSION

In this work, isocyanate-terminated PUI oligomer, whichcombines the advantages of polyurethane and polyimide, wassuccessfully synthesized and characterized. Furthermore, toimprove the thermal and mechanical properties of epoxy resin,PUI together with POSS was introduced to epoxy matrix toprepare EP/PUI/POSS composites. Corresponding to the pureepoxy resin which has a three-stage thermal decomposition, theresultant EP/PUI/POSS composite exhibits only one stage and amuch higher T5% (331.18C) than that (189.88C) of the epoxy resin.The DMA results showed EP/PUI/POSS has a lower glass transitiontemperature and higher mechanical modulus. All the aboveresults could be ascribed to the nanoscale reinforcement effectof POSS on the EP/PUI matrix and the synergistic effect of PUI,epoxy resin, and POSS. The morphology of these compositeswas also further investigated to define the structure-propertiesrelationship.

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