Curing dynamics and network formation of cyanate ester resin/polyhedral oligomeric silsesquioxane nanocomposites

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<ul><li><p>nte</p><p>e b</p><p>Cyanate ester resinPolyhedral oligomeric silsesquioxaneNanocomposite</p><p>wor(POtedspe</p><p>t andes, cyavariet</p><p>tance is low. Thus, reinforcement of cyanate ester resins for high-performance applications becomes necessary.</p><p>In the last decade, hybrid organic polymer-inorganic nano-composites have attracted considerable research interest forvarious applications, such as in mechanical, optical and electronic</p><p>epoxy cubic silsesquioxane into a bisphenol A dicyanate ester resinto form the highly crosslinked organic inorganic hybrid nano-composites at a molecular level. The introduction successfullymodied the local structure of the molecules. The initial decom-position temperature (Ti) increased from 411 C to 511 C with theincorporation of 50 mol% POSS. The LOI value increased from 32 to61 when 50 mol% POSS was added, which indicated that the ameretardancy of the resin was signicantly improved. Liang et al. [6a]reported that incorporation of TriSilanoPhenyl POSS into a cyanate</p><p>* Corresponding author. Tel.: 44 1509 223331; fax: 44 1509 223949.</p><p>Contents lists availab</p><p>Polym</p><p>ls</p><p>Polymer 52 (2011) 1716e1724E-mail address: (M. Song).structural aerospace composites to electronic insulation. Cyanateester resins, which properties are superior to conventional ther-mosetting materials, such as epoxies, polyimides, and bismalei-mide (BMI) resins, have been brought to popular attention due totheir excellent mechanical properties, high thermal stability, radi-ation and ame resistance, low out-gassing and minimal waterabsorption [1]. However, the high curing temperature required bycyanate esters restricts their current applications, making theinclusion of an appropriate catalyst highly desirable In addition tothis, and in common with other thermosetting materials, the highcross link density of cyanate esters means that their impact resis-</p><p>Incorporation of POSS reagents into organic polymers offersa unique opportunity to prepare nanocomposites with trulymolecular dispersions of the inorganic llers [6]. The enhancementof physical properties of polymeric materials by incorporation ofPOSS has been shown on a wide range of thermoplastics, such aspolystyrene [7], ethylene and propylene blends [8], ethylenecopolymers [9], and polycarbonate [10], as well as thermosettingmaterials, which are polyimide [11], phenolic resins [12], epoxyresins [13], polyurethane [14], polyimide-epoxy blends [15] and soon. Modication of cyanate ester properties with POSS has beenattempted by several groups [6,16]. Lu et al. [6b] introduced amulti-1. Introduction</p><p>With technological developmenmaterials with enhanced properticurrently in widespread use for a0032-3861/$ e see front matter 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.02.041cyanurate) ring in the PT-30 and its nanocomposites with the POSS. Raman spectra revealed that the PT-30 resin preferentially reacted with eOH group in the POSS rstly to form a eOe(C]NH)eOe bond,rather than react with itself to form the triazine rings, during the network formation of the PT-30/POSSnanocomposites. The strong catalytic effect of the POSS on the curing process of the PT-30 appears to bedue to the formation of this eOe(C]NH)eOe bond.</p><p> 2011 Elsevier Ltd. All rights reserved.</p><p>increasing demand fornate ester resins arey of applications from</p><p>elds [2]. The inorganic nanollers being studied include inorganicand organic nanollers, such as clays [3], carbon nanotubes [4],grapheme [5], and polyhedral oligomeric silsesquioxanes (POSS).POSS reagents, which consist of cage, or partial cage structures, areinteresting silicon based compounds with the formula (RSiO1.5)n.Keywords:energy of the PT-30 decreased with increasing POSS content. The most effective catalytic effect wasobserved at 5 wt% of the POSS. Both FTIR and Raman spectra monitored the formation of triazine (i.e.Available online 3 March 2011 (decrease in curing temperature and activation energy) on the curing reaction of PT-30. The activationCuring dynamics and network formatiooligomeric silsesquioxane nanocomposi</p><p>Yue Lin a, Jie Jin a, Mo Song a,*, S.J. Shawb, C.A. StonaDepartment of Materials, Loughborough University, Loughborough LE11 3TU, UKbDstl, Porton Down, Salisbury SP4 0JQ, UK</p><p>a r t i c l e i n f o</p><p>Article history:Received 29 October 2010Received in revised form4 February 2011Accepted 23 February 2011</p><p>a b s t r a c t</p><p>Curing dynamics and netoligomeric silsesquioxanecalorimetry (DSC), modulainfrared (FTIR) and Raman</p><p>journal homepage: www.eAll rights reserved.of cyanate ester resin/polyhedrals</p><p>k formation of cyanate ester resin (PT-30)/TriSilanolPhenyl polyhedralSS) nanocomposites were studied by means of differential scanningtemperature differential scanning calorimetry MTDSC), Fourier transformctroscopies. The incorporation of the POSS showed a strong catalytic effect</p><p>le at ScienceDirect</p><p>er</p><p>evier .com/locate/polymer</p></li><li><p>ester resin, PT-15, can improve the storage modulus and thermalstability. The storage modulus E0 above Tg was signicantlyimproved until the addition of 5 wt% POSS. The best improvementin exural moduli is 28% increase when the 5wt% POSS wasincorporated. Furthermore, the POSS was covalently bonded withthe cyanate ester resin through imino bond. These improvementssuggest that POSS/cyanate esters nanocomposites can be one of thesolutions to the ever-increasing demand for high-performance</p><p>Fig. 1. Schematic of molecular structure of (A) TriSilanolPhenyl POSS, (B) possible structure of the PT-30.</p><p>Y. Lin et al. / Polymer 52 (2011) 1716e1724 1717polymeric materials.It is believed [6a] that the addition of functionalized POSS could</p><p>inuence the cure dynamics and network formation of cyanateesters signicantly. However, the mechanism of these inuences isstill not claried. For example, what is the role of the POSS duringthe cure of the cyanate ester? Why does the incorporation of 5 wt%POSS show best improvement on storage modulus and exuralmoduli? How and when does the POSS react with the cyanate esterin curing process? In order to develop high-performance cyanateester resin/POSS nanocomposites, it is necessary to answer thesequestions. Furthermore, a clear understanding of network forma-tion in the cyanate ester resin during cure is essential, so that theproperties of the cyanate ester resin/POSS nanocomposites can becontrolled. In this paper, the catalytic effect of a functionalizedPOSS, TriSilanolPhenyl POSS, on curing dynamics and networkformation of a cyanate ester resin, PT-30, were investigated. Theinuence of weight percentage of the POSS incorporated on thecuring dynamics and network formation of the cyanate ester/POSSnanocomposites and the origin of catalytic effect were analyzed.This approach could be applied to other cyanate ester-based resinsystems as well.</p><p>2.0</p><p>2.5</p><p>3.0 0wt%1wt%5wt%</p><p> 10wt%</p><p>N2</p><p>)g</p><p>/W50 100 150 200 250 300 350</p><p>-0.5</p><p>0.0</p><p>0.5</p><p>1.0</p><p>1.5</p><p>(w</p><p>ol</p><p>Ft</p><p>ae</p><p>H</p><p>Temperature (o</p><p>C)</p><p>Fig. 2. DSC plots of the PT-30/POSS nanocomposites in nitrogen atmosphere (60 ml/min) with heating rate of 10 C/min.2. Experimental</p><p>2.1. Materials</p><p>Cyanate ester resin, PRIMASET PT-30, was provided by LONZALTD. The PT-30 has a low viscosity (80 c.p.s.) at its processingtemperature (120 C) andhas less than0.5%volatiles andgeneratesnogaseousby-productsduringcure. TriSilanolPhenylPOSS (C42H38O12Si7MW 931.34 g/mol) was purchased from Hybrid Plastics Inc. Fig. 1shows a schematic of the respective molecular structures.</p><p>2.2. Preparation of PT-30/POSS mixtures</p><p>1 wt%, 5 wt%, 10 wt% PT-30/POSS mixtures were prepared asfollows. The cyanate ester resin, PT-30, was rstly held at 100 C for30 min with magnetic stirring to remove moisture. Next, the PT-30was heated to 120 C, and calculated amounts of the TriSilanol-Phenyl POSS were added into the low viscosity resin to prepare thePT-30/POSS mixtures. These mixtures were stirred at 120 C for80 min. After mixing, all the POSS/PT-30 resins prepared weresealed in glass bottles and stored at 20 C for further use.</p><p>2.3. Characterization</p><p>A TA Instruments DSC 2920 calorimeter was employed forDifferential Scanning Calorimetry (DSC) and Modulated Tempera-ture Differential Scanning Calorimetry (MTDSC) measurements.Nitrogen was used as the purge gas (60 ml/min). All the dynamicexperiments were carried out using DSC. Samples were heatedfrom room temperature to 350 C at a heating rate of 10 C/min. Forall the quasi-isothermal experiments, MTDSC was employed.Samples were held at selected temperatures with modulationamplitude of 0.5 C and a period of 60 s. Fourier Transform Infrared(FTIR) spectra of the sample coated on KBr pellet were recordedfrom 4000 cm1 e 400 cm1 using a Shimadzu FTIR-8400s spec-trophotometer with a 4 cm1 resolution over 128 scans. Ramanspectrawere recorded from 100 cm1e3500 cm1, on a Jobin Yvon</p><p>Horiba high-resolution LabRam 800 Raman microscope system,</p><p>Table 1DSC results of the PT-30/POSS systems in nitrogen atmosphere (60 ml/min) withheating rate of 10 C/min.</p><p>Sample OnsetTemperature(C)</p><p>PeakTemperature(C)</p><p>Endtemperature(C)</p><p>Duration(min)</p><p>6HT(J/g)</p><p>Pure PT-30 257 303 345 8.8 3461 wt%</p><p>POSS/PT-30240 303 328 8.8 363</p><p>5 wt%POSS/PT-30</p><p>199 283 332 13.3 415</p><p>10 wt%POSS/PT-30</p><p>220 277 314 9.4 352</p></li><li><p>which contains an optical microscope adapted to a double gratingspectrograph and a CCD array detector. The laser excitation wasprovided by a Spectra-Physics model 127 heliumeneon laseroperating at 35 mWof 633 nm output. All the samples for FTIR andRaman experiments were cured in a DSC cell in a nitrogen atmo-sphere (60 ml/min).</p><p>3. Results and discussion</p><p>systems. The onset temperature fell dramatically from 257 C forthe pure PT-30 until it reached a minimum of 199 C for theincorporation of 5 wt% POSS. In contrast, the peak temperaturecontinued to decrease with increasing POSS content. A 10 wt%loading of POSS, resulted in a reduction in the peak temperature byup to 26 C, compared with the pure PT-30. The total reactionenthalpy increased with increasing POSS content until 10 wt% ofPOSS was incorporated. These results indicate that the curingtemperature of the PT-30 resin can be reduced signicantly withthe addition of the POSS. The incorporation of POSS catalyzed thecuring reaction of the PT-30. However, excessive addition of thePOSS leads to an increase of onset temperature, and reduction inthe total reaction enthalpy.</p><p>Fig. 3 shows the DSC plots of the PT-30 cured with and withouta nitrogen atmosphere. Compared with the reaction in nitrogenatmosphere, the samples cured in air displayed a 39 C lower onsettemperature and a 25 C lower peak temperature, which revealedthe catalytic effect of oxygen to the curing process of the pure PT-30.</p><p>For cure of a thermoset resin, the conversion at time, t, can bedened as follows:</p><p>at DHtDHT(1)</p><p>where, at is the conversion at time t, Ht is the reaction heat at time t,and 6HT is the total reaction heat shown on a typical non-isothermal experiment. In this experiment, the 6HT was deter-mined by scanning of uncured samples with a heating rate of 10 C/min, as shown in Table 1.</p><p>The conversion rate can be dened as follows:</p><p>100 150 200 250 300 350</p><p>-0.5</p><p>0.0</p><p>0.5</p><p>1.0</p><p>1.5</p><p>2.0</p><p>2.5</p><p>3.0</p><p>In air In N2</p><p>gW</p><p>(w</p><p>ol</p><p>ft</p><p>ae</p><p>H1</p><p>-</p><p>)</p><p>Temperature (o</p><p>C)</p><p>278 oC 303 oC</p><p>225 oC 264oC</p><p>Fig. 3. Heat ow vs. temperature for PT-30 resin with (60 ml/min) and withoutnitrogen atmosphere. Heating rate of 10 C/min was used.</p><p>B</p><p>Y. Lin et al. / Polymer 52 (2011) 1716e17241718Fig. 2 shows DSC plots of the curing process of PT-30/POSSsystems in a nitrogen atmosphere (60 ml/min) with a heating rateof 10 C/min Table 1 lists the non-isothermal curing temperature,curing period and heat of reaction for the different PT-30/POSS</p><p>0.10</p><p>0.15</p><p>0.20</p><p>0.25</p><p>)g/</p><p>W(</p><p> w</p><p>ol</p><p>F </p><p>ta</p><p>eH</p><p> 215 C</p><p> 220 C</p><p>225 C</p><p>A0 50 100</p><p>0.00</p><p>0.05</p><p> v</p><p>er</p><p>no</p><p>N</p><p>Time (min)</p><p>0 20 40 60 80</p><p>0.00</p><p>0.05</p><p>0.10</p><p>0.15</p><p>0.20</p><p>0.25</p><p>)g</p><p>/W</p><p>( </p><p>wo</p><p>lF </p><p>ta</p><p>eH</p><p> v</p><p>er</p><p>no</p><p>N</p><p>Time (min)</p><p> 200 C 205 C 210 C</p><p>C D</p><p>Fig. 4. Isothermal DSC plots for the PT-30/POSS nanocomposites at different isothermal temPOSS/PT-30.dadt</p><p> dDHT=dtDHT</p><p>(2)</p><p>where, da/dt is the conversion rate at time t.</p><p>0 20 40 60 80 100</p><p>0.00</p><p>0.05</p><p>0.10</p><p>0.15</p><p>0.20</p><p>0.25</p><p>)g/</p><p>W(</p><p> w</p><p>ol</p><p>F </p><p>ta</p><p>eH</p><p> v</p><p>er</p><p>no</p><p>N</p><p>Time (min)</p><p> 210 C 220 C 230 C</p><p>0 50 100 150</p><p>0.00</p><p>0.05</p><p>0.10</p><p>0.15</p><p>)g</p><p>/W</p><p>( </p><p>wo</p><p>lF</p><p> t</p><p>ae</p><p>H </p><p>ve</p><p>rn</p><p>oN</p><p>Time (min)</p><p> 185 C 195 C 200 Cperatures (A) pure PT-30, (B) 1 wt% POSS/PT-30, (C) 5 wt% POSS/PT-30, and (D) 10 wt%</p></li><li><p>0.0 0.2 0.4 0.6 0.8 1.0</p><p>0.0000</p><p>0.0002</p><p>0.0004</p><p>0.0006</p><p>0.0008</p><p>td</p><p>/a</p><p>d</p><p>a</p><p> 215 C 220 C 225 C</p><p>A</p><p>0.0 0.2 0.4 0.6 0.8 1.0</p><p>0.0000</p><p>0.0002</p><p>0.0004</p><p>0.0006</p><p>0.0008</p><p>td</p><p>/a</p><p>d</p><p>a</p><p> 210 C 220 C 230 C</p><p>B</p><p>0</p><p>D</p><p>res (</p><p>Y. Lin et al. / Polymer 52 (2011) 1716e1724 1719An autocatalytic model was employed for analysis of the cure ofthe PT-30/POSS nanocomposite. An empirical rate equationproposed by Kamal [17] can be applied for thermosetting cureshowing autocatalytic behavior:</p><p>0.0 0.2 0.4 0.6 0.8 1.</p><p>0.0000</p><p>0.0002</p><p>0.0004</p><p>0.0006</p><p>td</p><p>/a</p><p>d</p><p>a</p><p> 200 C 205 C 210 C</p><p>C</p><p>Fig. 5. da/dt vs. a for the PT-30/POSS nanocomposites at different isothermal temperatudadt</p><p> k1 k2am1 an kkin1 an (3)</p><p>where, k1 and k2 are the rate constants, m and n are the reactionorders, and kkin is the kinetic rate constant under chemicallycontrolled condition.</p><p>Furthermore, the temperature dependence of any rate constantis given by the Arrhenius relationship:</p><p>k AexpEaRT</p><p>(4)</p><p>Where, Ea is the activation energy, R is the gas constant, T is abso-lute temperature, and A is the pre-exponential or frequency factor.</p><p>Fig. 4 shows the plots of heat ow versus time, recorded byMTDSC for the PT-30/POSS systems at different isothermal</p><p>Table 2Autocatalytic model constants for the PT-30/POSS nanocomposites.</p><p>Content of POSS (wt%) Temperature (C) k1 ( 104 s1) k2 ( 104 s1</p><p>0 215 2.41 36.6220 2.87 45.0225 5.93 72.4</p><p>1 210 1.27 116220 2.77 289230 6.09 402</p><p>5 200 2.27 421205 3.64 556210 4.52 705</p><p>10 185 1.44 82.1195 2.07 140200 3.19 226experimental temperatures. Fig. 5 shows da/dt versus a for thePT-30/POSS systems at different isothermal temperatures. Table 2lists the results of kinetic analysis, based on the autocatalyticmodel (Eq. (3)). According to Eq. (3), the parameters k1, E1, and A1</p><p>0.0 0.2 0.4 0.6 0.8 1.0</p><p>0.0000</p><p>0.0002</p><p>0.0004</p><p>0.0006</p><p>td</p><p>/a</p><p>d</p><p>a</p><p> 185 C 195 C 200 C</p><p>A) pure PT-30, (B) 1 wt% POSS/PT-30, (C) 5 wt% POSS/PT-30, and (D) 10 wt% POSS/PT-30.reveal the effect of the POSS on the PT-30 at the very beginning ofthe curing process in the presence POSS. In contrast, the parametersk2, E2, and A2 are more important, as they show the effect of thePOSS on network formation of the PT-30 throughout the wholecuring process.</p><p>Fig. 6 illustrates the effect of the incorporation of POSS on theactivation energy and the pre-exponential factor. Activation energy,E1 and E2, and the pre-exponential factor, A1 and A2, decreased withincreasing POSS content, up to 5 wt%. Furthermore, the reactionorder (see Table 2) increased signicantly with increasing POSScontent, up to 5wt%. However, over 5 wt% POSS leads to an increasein the activation energy, E2, and the pre-exponential factor, A2, anda decrease in reaction order. This result revealed that the incor-poration of the POSS had a strong catalytic effect...</p></li></ul>