Thermal and dynamic mechanical properties of epoxy resin/poly(urethane-imide)/polyhedral oligomeric silsesquioxane nanocomposites

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<ul><li><p>pted: 5 March 2010, Published online in Wiley Online Library: 15 July 2010</p><p>echanical properties ofane-imide)/polyhedralne nanocompositesa,b,c, Gang Wua,b,c, Chunhua Caia,b,c,</p><p>(PUI) with combination of the advantages of polyurethane andetween polyurethane prepolymer and pyromellitic dianhydrideic silsesquioxane (OapPOSS) and PUI were incorporated into theorganicinorganic nanocomposites for the purpose of simul-ness of the epoxy resin. Their thermal degradation behavior,ermthturnifnd</p><p>re</p><p>INTRO</p><p>In recenthave recsynergistand inorgPOSS nandensity, aPOSS nanby eightorganic greactivenorborny</p><p>excellent chemical resistance, and simplicity in processing. In</p><p>Research Article</p><p>a J. Song, G. Chen, G. Wu, C. Cai, Q. Li</p><p>Contract/grant sponsor: Beijing Natural Science Foundation; contract/grant</p><p>number: 2072015.</p><p>2order to improved comprehensive properties of epoxy resin,many functionalized POSS compounds with epoxy group,</p><p>Contract/grant sponsor: National High Technology Research and Develop-</p><p>ment Program of China; contract/grant number: 2006AA03Z563.structural materials due to their high modulus and strength,tend 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 nanofillerpolymer interactionsand leads to a reduction of the interfacial energy, can enhancecomplementary properties of the hybrid materials comparedwith the simple blends. [1]</p><p>Epoxy resins are widely utilized as adhesives, electronicencapsulating compounds and matrices as well as other</p><p>State Key Laboratory of Chemical Resource Engineering, Beijing University of</p><p>Chemical Technology, Beijing 100029, China</p><p>b J. Song, G. Chen, G. Wu, C. Cai, Q. Li</p><p>Key Laboratory on Preparation and Processing of Novel Polymer Materials of</p><p>Beijing, Beijing 100029, China</p><p>c J. Song, G. Chen, G. Wu, C. Cai, Q. Li</p><p>College of Material Science and Engineering, Beijing University of Chemical</p><p>Technology, Beijing 100029, China</p><p>d P. Liu</p><p>Beijing Institute of Aeronautical Materials, Beijing 100095, China</p><p>Contract/grant sponsor: Polymer Chemistry and Physics, Beijing Municipal</p><p>Education Commission (BMEC); contract/grant number: XK100100640.mainly due to the existence of numerous nanofillernanofillerinteractions, those POSS nanofillers with unreactive corner groupPolym. AdvDUCTION</p><p>years, polyhedral oligomeric silsesquioxanes (POSSs)eived considerable attention as they possess aic combination of constituent properties of organicanic materials.[17] Unlike conventional inorganic fillers,ofillers offer the advantages of monodisperse size, lownd synthetically well-controlled functionalities. Typicallyoparticle is a 3D cage like siloxane structure surroundedorganic R groups (RSiO4), where R can be unreactiveroup, such as alphatics,[8] or phenyl,[9] as well as can beorganic group, such as epoxy,[10,11] methacrylate,[12]</p><p>l,[13,14] vinyl,[15] styryl,[16] amines[9], and so on. However,</p><p>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</p><p>* 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.cnReceived: 26 October 2009, Revised: 12 January 2010, Acce</p><p>Thermal and dynamic mepoxy resin/poly(uretholigomeric silsesquioxa</p><p>Jiangxuan Songa,b,c, Guangxin ChenPinggui Liud and Qifang Lia,b,c*</p><p>Linear isocyanate-terminated poly(urethane-imide)polyimide was directly synthesized by the reaction b(PMDA). Then octaaminophenyl polyhedral oligomerepoxy resin (EP) to prepare a series of EP/PUI/POSStaneously improving the heat resistance and toughdynamic mechanical properties, and morphology wmechanical analysis (DMA), and transmission electronand mechanical modulus was greatly improved winanocomposites had lower glass transition temperaassemble into strip domain which could switch to uresults could be ascribed to synergistic effect of PUI a&amp; Sons, Ltd.</p><p>Keywords: polyhedral oligomeric silsesquioxane; poly(u</p><p>(wileyonlinelibrary.com) DOI: 10.1002/pat.1722. Technol. 2011, 22 20692074 Copyright 2e studied with thermal gravimetric analysis (TGA), dynamicicroscope (TEM). The results showed that the thermal stabilitythe addition of PUI and POSS. Moreover, the EP/PUI/POSSes. The TEM results revealed that POSS molecules could selform dispersion with increasing the content of POSS. All thePOSS on the epoxy resin matrix. Copyright 2010 JohnWiley</p><p>thane-imide); epoxy resin; thermal degradation; morphology010 John Wiley &amp; Sons, Ltd.</p><p>069</p></li><li><p>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.</p><p>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 mechanical</p><p>POSS nanocomposites. The samples were ultramicrotomed to</p><p>Preparations of composites</p><p>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.</p><p>POSS (g) DDM (g) POSS content (wt%)</p><p>0 0.96 00 0.69 00.19 0.58 1.80.81 0.48 7.2</p><p>J. SONG ET AL.</p><p>20707090 nm thickness with a diamond knife and mounted on CuTEM Grids for observation.</p><p>Table 1. Composition of EP/PUI/POSS composites</p><p>Sample Epoxy resin (g) PUI (g)</p><p>EP 10.00 0EP/PUI 5.00 5.00EP/PUI/POSS-1 5.00 5.00EP/PUI/POSS-2 5.00 5.00properties were studied with aim to investigate the effect of POSSand PUI on the properties of the modified epoxy system.</p><p>EXPERIMENTAL</p><p>Materials</p><p>Di-hydroxyl-terminated polytetrahydrofuran glycol oligomer(PTMEG, Mn 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.</p><p>Characterization</p><p>Infrared spectroscopic measurements were performed in therange 4000400 cm1 at a resolution of 4.0 cm1 using a BrukerTensor 27 FT-IR spectrometer. The samples were dissolved in THFand evaporated on KBr pellet under IR lamp.</p><p>The thermal stability of these hybrids was assessed bythermogravimetric 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.</p><p>Dynamic mechanical thermal analysis measurements wereperformed 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.</p><p>A H800 transmission electron microscope (TEM) (HITACHI JAP,INC.) was used to characterize the phase morphology of EP/PUI/wileyonlinelibrary.com/journal/pat Copyright 2010 John WileySynthesis of poly(urethane-imide) prepolymer</p><p>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.</p><p>In the next step, the solution of PMDA in DMF (50wt%) wascharged 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.</p><p>Scheme 1. Preparation of poly(urethane-imide).&amp; Sons, Ltd. Polym. Adv. Technol. 2011, 22 20692074</p></li><li><p>RESULTS AND DISCUSSION</p><p>FTIR analysis</p><p>As previously reported,[1921] 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 at34003600 cm1 of PTMEG changed to 3330 cm1 assigned toNH of CONH groups in PU segment, and some importantspectroscopic features appeared, such as the absorbance peak at1730 cm1 due to the conversion of NCO groups into CONHgroups. Also, the sharp peak at 2270 cm1 of NCO appeared in PUspectra. These indicated NCO terminated PU prepolymer wassuccessfully synthesized under experimental condition.</p><p>The absorption bands at about 1780 and 725 cm1 that arecharacteristic bands of imide bond verify that the imide grouphas been introduced to PU successfully. 1380, 1120, and 725 cm1</p><p>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.</p><p>In addition, the TGA curves of all the EP/PUI/POSS compositesplayed similar degradation profiles, suggesting that the existenceof the POSS did not significantly alter the degradation</p><p>Figure 2. TGA curves (a) and first derivative curves of TGA curves (b) of</p><p>EP, EP/PUI, and EP/PUI/POSS composites. This figure is available in color</p><p>online at wileyonlinelibrary.com/journal/pat</p><p>THERMAL AND DYNAMIC MECHANICAL PROPERTIESare assigned to axial, transverse, and out of plane vibrations ofcyclic imide structure.</p><p>Thermal properties</p><p>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,</p><p>Figure 1. FTIR spectra of PTMEG, PU, and PUI.Polym. Adv. Technol. 2011, 22 20692074 Copyright 2010 John Wiley &amp; Sons, Ltd. wileyonlinelibrary.com/journal/pat</p><p>2071</p></li><li><p>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.</p><p>Dynamic mechanical properties</p><p>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.</p><p>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 a...</p></li></ul>