Epoxy/POSS organic–inorganic hybrids: ATR-FTIR and DSC studies

European Polymer Journal 44 (2008) 3035–3045

Contents lists available at ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology-Review

Epoxy/POSS organic–inorganic hybrids: ATR-FTIR and DSC studies

C. Ramírez *, M. Rico, A. Torres, L. Barral, J. López, B. MonteroGrupo de Polímeros, E.U.P. Ferrol, Universidad de A Coruña, Avda 19 de Febrero s/n, 15405 Ferrol, Spain

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Article history:Received 29 November 2007Received in revised form 10 April 2008Accepted 17 July 2008Available online 24 July 2008

Keywords:SilsesquioxanesNanocompositesEpoxy–amine systemATR-FTIRKineticsDSC

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0014-3057/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.eurpolymj.2008.07.024

* Corresponding author. Tel.: +34 981337400; faxE-mail address: [email protected] (C. Ramírez).

a b s t r a c t

Organic–inorganic hybrid nanocomposites were prepared by reaction of an octaepoxy-sils-esquioxane, OECh, with an epoxy–amine system. OECh was used to partially replace thethermosetting resin, diglycidyl ether of bisphenol A, DGEBA, in its reaction with anaromatic diamine, 4,40-(1,3-phenylenediisopropylidene) bisaniline, BSA. The OECh wascharacterized by different techniques. The curing kinetics of ternary systems formed byDGEBA, OECh and BSA, was followed by Attenuated Total Reflectance-Fourier TransformInfrared Spectroscopy, ATR-FTIR. All the mixtures were prepared with a stoichiometricratio between epoxy and amine groups. The degree of reaction of glycidyl epoxy ring alongthe curing cycle selected was obtained from the infrared spectra. A peak-height methodbased on the ratio of the height of the characteristic to reference absorbance peak wasused. The curing kinetic of different blends was obtained by differential scanning calorim-etry, DSC. Three different methods, the differential of Kissinger, the integral of Flynn-Wall-Ozawa and the phenomenological model of Kamal, were used in order to obtain the kineticparameters of the cure reaction. It is observed that the presence of POSS accelerates therate of opening of glycidyl epoxy rings from DGEBA. The behaviour of the mixture duringthe curing process can be explained with an autocatalytical model, corrected with thecontribution of the diffusion of the molecules during the course of the reaction.

� 2008 Elsevier Ltd. All rights reserved.

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Contents

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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30362. Experimental section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3036

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2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30362.2. Sample preparation for ATR-FTIR scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30362.3. Sample preparation for DSC scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30372.4. Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3037
3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3037
3.1. Characterization of octaepoxy-POSS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30373.2. Study of curing cycle of ternary blends by ATR-FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30383.3. Kinetic analysis procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30403.4. Dynamic kinetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30403.5. Isothermal kinetic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3040
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3043Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3044References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3045

. All rights reserved.

: +34 981337401.

mailto:[email protected]

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http://www.elsevier.com/locate/europolj

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3036 C. Ramírez et al. / European Polymer Journal 44 (2008) 3035–3045

1. Introduction

The great interest of organic/inorganic hybrid nano-composites lies in the improved properties that they offer,which are enhanced by interfacial interactions betweenindividual components [1–3]. Composite properties arenormally higher than the sum of the properties of individ-ual composites. Working at nanometric scales, it is possibleto make materials with an important interphase, and suchmaterials may present non-linear changes in propertieswith respect to macroscopic composites [4].

The polyhedral oligomeric silsesquioxanos, POSS, be-long to the family of these new hybrid materials [5,6].The POSS monomer presents the structure of an inorganiccentral core with a diameter around 0.53 nm. The nano-sized cores are surrounded by eight organic groups, R,providing the empirical formula of (RSiO)1.5. The reactivegroup, which takes part in the formation of the hybridnanocomposite, is placed in the organic chains. POSS cagescan be introduced into polymer systems via copolymeriza-tion or physical blending. Copolymerization involves theformation of chemical bonds between POSS cages andpolymer matrices. Nanocomposites obtained by physicalblending were less frequently reported, possibly due tothe unfavourable miscibility of silsesquioxanes with poly-mers [7,8].

Epoxy resins are one of the most important thermoset-ting materials that have been widely used due to theirexcellent thermal, mechanical and electrical properties[9,10]. However, the current problems in applications ofepoxy thermosetting resins include low stiffness andstrength, and also the exothermic heat generated in curingreaction that causes difficulties in the processing.

The incorporation of a silsesquioxane in a compositebased in an epoxy resin causes improvement in propertiesof these composites such as increased oxygen permeabil-ity, reduced flammability, enhanced mechanical strengthand, sometimes, higher Tg [11–13].

For epoxy–amine systems, three main reactions maytake place, either simultaneously or sequentially, depend-ing on the reactivity of the reactants and the process tem-perature. These are the addition of primary and secondaryamine groups to the oxirane ring and the etherification ofthe epoxy group with a pendant hydroxyl group. This lastreaction causes the appearance of an ether bridge. The firstof two steps are catalysed by the presence of hydroxylfunctionalities added to the formulation as acceleratorsand/or created by the amine reactions. This effect accountsfor the autocatalytic nature of the curing process [14,15].

Vitrification has been observed in many DGEBA resinsbecause [9], when the glass transition temperature reachesthe curing temperature, the mobility of the molecules isreduced. From this point on, the curing process proceedsthanks to diffusion. The reaction mechanism and the kinet-ics of the curing process for an epoxy resin determinate themorphology of the three-dimensional network, thus defin-ing its physical and mechanical properties.

The techniques employed to monitor the curing reac-tion can be grouped into direct and indirect methods.The direct methods allow the evaluation of the concentra-

tion of one or more reactive groups as a function of time.They include Attenuated Total Reflectance-Fourier Trans-form Infrared Spectroscopy (ATR-FTIR). Indirect analysisgives only an assessment of the degree of cure by monitor-ing, as a function of time, a physical property directly cor-related with the conversion of the reactants. The mostusual technique in this area is the differential scanningcalorimetry (DSC), which is based on the assumption thatthe rate of heat evolution is proportional to the rate ofpolymerization [16,17].

In this paper, the kinetic behaviour that presents anepoxy network containing POSS-epoxy reinforcementsduring the polymerization was analyzed. The appearanceand disappearance of functional groups throughout thecuring process was followed by means of FTIR. The overallcuring conversion and the glass transition temperaturewere also evaluated by DSC. Isothermal and dynamicexperiments were performed by differential scanning calo-rimetry to study the cure kinetics of an epoxy system con-taining diglycidyl ether of bisphenol A as thermosettingresin and 4,40-(1,3-phenylenendiisopropylidene) bisanilineas curing agent, and its blends with the octa-epoxycyclohexyldimethylsilyl-POSS at different amounts.The kinetic studies of systems with POSS have become avery complex and difficult task to unravel [18–20].

The goal of this work is to report the curing of a newternary system formed by two epoxies (a conventionalDGEBA and a novel POSS) cured with a diamine by FTIRand DSC.

2. Experimental section

2.1. Materials

The studied system is based on a commercial diglycidylether of bisphenol A, DGEBA, Araldite GY 260, from Ciba-Geigy (Summit, NJ), with weight per epoxy equivalenceof 188 g eq�1 determined by hydrochlorination in our lab-oratory. Octaepoxy-cyclohexyldimethylsilyl-POSS, EP0430,hereafter designing by OECh, from Hybrid Plastics (Foun-tain Valley, CA), was added to partially replace the epoxyresin in the ternary mixture; its molecular weight is2011.41 g mol�1. 4,40-(1,3-phenylenediisopropylidene)bisaniline, BSA, from Aldrich Chemical (Milwaukee, WI),was used as hardener agent, with a molecular weight of344.5 g mol�1 and melting point of 114.5 �C.

All components were commercial products, and theywere used as received, without previous purification.Fig. 1 shows the structures of these components.

2.2. Sample preparation for ATR-FTIR scans

Samples of DGEBA and BSA with 2.5%, 5%, 7.5%, 10%,20%, 30% and 50% in weight of octaepoxy-POSS were pre-pared. As well as DGEBA/BSA and OECh/BSA binary blendsin stoichiometric ratio.

The DGEBA and OECh were stirred at 130 �C for 30 minin all cases in order to dissolve the POSS into the resin. Thismixture was cooled at room temperature and then, thesuitable amount of BSA was added.

Fig. 1. DGEBA, Octaepoxy-POSS and BSA structures.

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C. Ramírez et al. / European Polymer Journal 44 (2008) 3035–3045 3037

Twenty milligrams of each blend were place in a con-ventional oven and the curing cycle was performed. Differ-ent times and temperatures were used during the cureprocess and are indicated next. Firstly, zero hours at roomtemperature, in sample A. One hour and a half and threehours at 130 �C, for samples B and C, respectively, and fi-nally, three hours at 130 �C followed by two hours, sampleD, and four hours, sample E, at 150 �C. A portion of eachone of them was taken out of the oven and frozen until per-forming the FTIR analysis.

2.3. Sample preparation for DSC scans

Samples of DGEBA/BSA with 0%, 2.5%, 5%, 7.5% and 10%in weight of octaepoxy-silsesquioxane were prepared. Inall cases, the number of epoxy equivalents is equal to thenumber of amine equivalents.

The DGEBA and OECh were stirred at room temperatureand heated at 130 �C for 30 min. After that, they have beencooled and a suitable amount of the curing agent, BSA, wasadded.

2.4. Techniques

FTIR spectra were obtained on a Bruker Vector 22 FTIRSpectrometer equipped with a Specac Golden GateTM singlereflection ATR system.

The DSC measurements were made with a differentialscanning calorimeter (Perkin-Elmer DSC-7), (Norwalk,CT) equipped with an intracooler and supported by aPerkin-Elmer computer for data acquisition and analysis.The instrument was calibrated using the melting point ofhigh purity indium. All experiments were conducted un-der a dry nitrogen flow of 20 mL min�1 as purge gas.

Samples of about 5–10 mg were enclosed in aluminiumDSC capsules.

For dynamic heating experiments, heating rates of 5, 10,15 and 20 �C min�1 were investigated from 30 to 300 �C, tocalculate the total heat of cure reaction.

In isothermal experiments, the DSC was stabilized to30 �C with the sample introduced in the DSC cell. Then,it was heated at high rate to the curing temperature. Iso-thermal scans were conducted at three temperatures,110, 120 and 130 �C, in order to obtain both the curerate and the extent of cure as a function of time. Thereaction was considered complete when the signal re-turned to the baseline. The origin of the time, t = 0 s,was obtained by rerunning the experiment on the re-acted sample, under the same conditions, to obtain thebaseline. The heat of reaction was not recorded in sam-ples with a POSS content greater than 10%.

A scanning electron microscope (SEM, Hitachi S-2700)was used to study the morphologies of the octaepoxy-POSSat an accelerating voltage of 15 kV. The sample was sput-ter-coated with a thin layer of gold to obtain the differentimages.

3. Results and discussion

3.1. Characterization of octaepoxy-POSS

Two melting points were observed by DSC for OECh asoccur with other POSS [21], with values of 112 and124 �C. Two different morphologies for the same sampleof octaepoxy-POSS (Fig. 2), which must correspond withthe melting points obtained by DSC.

By TGA analysis the onset of POSS degradation, T0, wasobtained at 404 �C in argon and 361 �C in oxygen. Besides,

Fig. 2. SEM images of octaepoxy-silsesquioxane.

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the char yields were similar, 45.3% and 45.7%, in bothatmospheres, respectively.

3.2. Study of curing cycle of ternary blends by ATR-FTIR

Bands remaining invariable in curing, corresponding tolinkages not participant in the reaction, can be used as aquantitative reference in the analysis, because the absor-bance of these bands does not vary during polymerization.On the other hand, there are some bands associated togroups participating in the curing reaction [22]. The varia-tion in the absorbance of one of these bands allows us toquantify the progress of the reaction, having defined theconversion as a ¼ 1� ðA�t =A�0Þ, where A�t is the reducedabsorbance after a certain time t, and A�0, is the reducedabsorbance at the start of the curing process. Both of them,A�t and A�0, are corrected by using a reference band. Themethod consists on dividing the absorbance of the reactiveband by the absorbance of an invariant band during thecuring process, and it is used to correct the variation inthe mass and in the thickness of the sample. In our case,the reference peak used as an internal standard for the nor-malization of epoxy peaks absorbance was the tensionvibration band of aromatic C@C blends, associated to ben-cene rings, �1507 cm�1.

The spectra of binary DGEBA/BSA and OECh/BSA sam-ples, Fig. 3, subjected to different curing processes previ-ously described, were used to identify the bandscorresponding to the end epoxy groups that belong toDGEBA and OECh, respectively. To avoid crowding the dia-grams, only the samples A, C and E were plotted. From thespectra obtained, it can be observed that the vibrationband corresponding to the epoxy group of DGEBA appearsat �917 cm�1. In the case of the epoxy group from OECh,the vibration band appears at �745 cm�1. These two peakswere used to follow the reaction during the curing process.

The vibration bands considered for this study can beseen in Table 1 [23]. For the spectra of DGEBA/BSA mixture,it can be observed that the epoxy ring vibration band, tas

(917 cm�1) decreases during the curing cycle due to theopening of the epoxy rings of DGEBA until disappearingin process E. So, in the final step of the proposed curingcycle the epoxy rings are completely open and the reactionis supposed to be complete. As the curing reaction ad-vances, the ts,as (NAH) bands, 3400–3500 cm�1, diminishwhile the bands related with hydroxyl groups, ts, 3200–3500 cm�1, increase. It confirms that the opening of theepoxy rings occurs as mentioned above.

The characteristic bands of the asymmetrical stretchingof CAOAC groups from ether at �1100 cm�1, appear inprocess B. This fact indicates that some homopolymeriza-tion of DGEBA molecules occurs even from the start ofthe polymerization. So, some amine groups must remainunreacted during the curing process.

For the spectra of OECh/BSA mixture, the asymmetric(SiAOASi) stretching vibration band, �1075 cm�1, belong-ing to the strong cage of the POSS appears. This band doesnot vary during the polymerization because the cages ofPOSS do not break in the reaction [24]. A degraded cubewould produce broad asymmetric peaks. The indicativeband of the reaction is, in this case, the cycloaliphaticepoxy ring vibration band, tas (745 cm�1). This band is soweak that it can hardly be followed during the curingprocess.

The band corresponding to the NAH deformation vibra-tion of primary aromatic diamines, ds (NAH), �1621 cm�1,shows a small reduction for curing process C.

The bands associated with hydroxyl groups, ts (3200–3500 cm�1), do not increase as much as had been expected,so, the cycloaliphatic epoxy does not seem to react withthe amine groups of BSA and, therefore, the octaepoxy-POSS will not act as a reactant agent, but rather, as a diluterin the course of the polymerization.

Fig. 4 shows the FTIR spectra obtained for ternary sam-ples with 5, 30 and 50 wt.% of OECh for different steps ofthe curing cycle proposed. The sample with 5 wt.% is repre-sentative of the behaviour observed for mixtures with min-or content of POSS (2.5–10 wt.%), so only this spectra willappear. Also, the spectra for mixtures with 20 and30 wt.% are similar, so only one of them is illustrated. Inall cases, the glycidyl epoxy ring band, tas (�914 cm�1),belonging to DGEBA disappears during the curing processC for sample with 5 wt.% of OECh and during process Bfor 30 and 50 wt.% of POSS. In order to maintain the stoi-chiometric ratio, the amount of DGEBA added is minorwhen the content of POSS increases. This fact causes a

5001000150020002500300035004000

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DGEBA/BSA A

bsor

banc

e

Polyether

as (DGEBA epoxy ring)

(C=C) DGEBA

OECh/BSA

Wavenumber (cm-1)

Wavenumber (cm-1)

Abs

orba

nce

s (SiOSi) cage

s,as (N-H) and s(O-H)

s (Cycloaliphatic epoxy OECh)

s(N-H)

υυ

υ

υ

υ

s,as (N-H) and s(O-H)υ

ϕ

υ

b

a

δ

Fig. 3. Spectra obtained for (a) DGEBA/BSA mixture and (b) OECh/BSA mixture. MA

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decrease in the intensity of glycidyl epoxy band. Moreover,as the curing process only applies to DGEBA epoxy, this iscompleted in a shorter time.

As occurs in DGEBA/BSA spectra, the ts,as (NAH) bands,3400–3500 cm�1, weaken and the ts bands related withhydroxyl groups, 3200–3500 cm�1, increase, confirmingthe opening of glycidyl epoxy rings. The vibration band(745 cm�1) of cycloaliphatic epoxy ring of OECh can notbe seen in the spectra of ternary samples, so it can not befollowed, but, as was observed in the spectra of OECh/BSA sample, no reaction of cycloaliphatic epoxy ring withamine groups is expected. The band of SiAOASi cage at�1084 cm�1, belonging to the molecule of OECh, isobserved in all cases. This band already appears in thesample without reaction and its intensity is higher whenthe content of POSS increases. The band of asymmetrical

stretching vibration of polyether appears during the courseof the reaction at �1100 cm�1. This band overlaps theSiAOASi cage band, so, the monitoring of this band is hin-dered. This fact indicates that the homopolymerizationprocesses occurs in all cases.

The polymerization of DGEBA/BSA sample occurs in thehigher part during the curing process C. It seems to beautoaccelerated for the hydroxyl groups generated in thefirst part of the reaction (curing process B) from the openingof epoxy rings of DGEBA by the primary amine groups of BSA.In process D, the temperature of cure is increased from 130to 150 �C, which is close to the glass transition temperatureof the system. In this case, the degree of conversion hardlychanges, but in the last step the epoxy band disappearsand a conversion degree close to unity is reached. Thus, thiscuring process, D, is appropriate for this system.

Table 1Characteristic absorption of epoxy and siloxane functional groups

Functional group Wavenumber (cm�1) Vibration type

NAH 3400–3500 ts,as doubletsNAH 1580–1650 ds

OAH 3200–3500 ts

SiAOASi 1030–1110 ts

SiAH �2200 ts

SiAH 800–950 ds

CAN 1250–1360 ts

C@C, aromatic 1400–1500 ts

C@C, aromatic 1580–1600 ts

CAOAC, ether 1100

O 1250 ts

O 810–950 tas

O 750–840 ts 12 lband

CH in O

2990–3050 ts

ts: Symmetric vibration.tas: Assymetric vibration.ds: In-plane bending (scissoring).

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When OECh is added to DGEBA/BSA mixture, the degreeof conversion increases from the first step of the curingprocess, and at a content of POSS higher than 10 wt.% themaximum conversion is already reached during the curingprocess B. The autoacceleration phenomenon occurs fromthe start of the reaction of polymerization. The content ofDGEBA is minor, and the reaction of glycidyl epoxy groupswith BSA is favoured because there are an excess of aminegroups.

The octaepoxy-POSS does not react with BSA, as it hadbeen expected, but the reaction is quicker when thecontent of OECh increases.

3.3. Kinetic analysis procedures

Kinetic analysis was performed using three kineticmodels: Kissinger [25] and Flynn-Wall-Ozawa [26,27]methods, which can be used to calculate the activation en-ergy without any consideration given to the reactionmechanism; and the phenomenological model developedby Sourour and Kamal [28] which allows us to obtain anexpression to describe the cure reaction of this epoxysystem. It was necessary to introduce the diffusion factorin the Kamal expression obtained to successfully fit exper-imental data to the theoretical model.

3.4. Dynamic kinetic methods

The overall heat evolved in the reaction has been deter-mined as the average value of reaction heats, DH0, calcu-lated in the thermograms for each blend. Thetemperatures of the exotherms peaks, Tp, and the individ-ual values of reaction heat are shown in Table 2. The tem-

peratures of the peaks increase with the heating rate, butthey do not vary significantly with the content of POSS.The total heat of reaction does not appear to be influencedby the presence of OECh. The normalized heat of the reac-tion is similar in all blends, and slightly smaller, but closeto the theoretical value [29] of 107 kJ mol�1, expressedwith respect to epoxy equivalents. This fact indicates thatthe reaction mechanism does not change due to the pres-ence of the OECh in the system.

The glass transition temperature values, Tg, of the differ-ent epoxy resin systems were recorded from second dy-namic scans, Fig. 5. They were acquired at the midpointof the capacity change and they can be seen in Table 2.The Tg values decrease from 152.0 to 127.9 �C at a heatingrate of 10 �C min�1, with the content of OECh. This diminu-tion can be attributed to a higher hinder of the chains dur-ing the polymerization due to the size of POSS molecules[7].

The Kissinger and Flynn-Wall-Ozawa methods wereapplied to the dynamic experiments. The Kissinger methodis a differential method that let us obtain the activationenergy from the maximum reaction rate, where da/dt iszero, under a constant-heating rate condition.

The activation energy is obtained from the slope of theplot of lnðq=T2

pÞ versus reciprocal of peak temperature. Theneat mixture gives a value for Ek of 54.7 kJ mol�1. Thisvalue hardly varies with the presence of OECh.

An integral method was developed by Flynn-Wall-Oza-wa for calculation of activation energy. A plot of logarithmof heating rate, q, as a function of the reciprocal of peaktemperature yields the values of the activation energy,E0. For the neat mixture, the value obtained was59.1 kJ mol�1. The plots and the activation energiesobtained from Kissinger and Flynn-Wall-Ozawa methodscan be seen in Fig. 6 for some blends. In both cases, theresults show that the activation energy does not changewith the content of OECh. It can be observed that thevalues obtained by the Flynn-Wall-Ozawa method areslightly higher than the values obtained by Kissinger meth-od for all blends as occurs in other epoxy systems [17].

3.5. Isothermal kinetic analysis

The autocatalytic model is a phenomenological ap-proach developed by Sourour and Kamal. The followingequation is assumed [30] for the curing process of epoxyamine systems,

dadt¼ ðk1 þ k2aÞmð1� aÞn ð1Þ

where a is the conversion, m and n are the kinetic orders ofthe reaction, and m + n provides the overall reaction order.The kinetic rate constants for the reaction, k1 and k2, areconsidered as a constant at a given temperature, theydepend on the temperature according to an Arrheniuslaw and they have associated two different activation ener-gies, E1 and E2, and two preexponential factors [29].

To determine the cure kinetics by means of an autocat-alytic model, first, isothermal cured is performed at severaltemperatures, and later, experimental reaction rate isobtained in all ranges of times as shown in Fig. 7. A

5001000150020002500300035004000

A

C

E

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C

E

5001000150020002500300035004000

A

C

E

Wavenumber (cm–1)

Wavenumber (cm–1)

Wavenumber (cm–1)

Abs

orba

nce

Abs

orba

nce

Abs

orba

nce

Polyether Si-O-Si cage

s(C=C)s,as (N-H) and s(O-H)

as(DGEBA epoxy ring)

s,as (N-H) and s(O-H)

s,as (N-H) and s(O-H)s(C=C)

s(C=C)

Polyether/ Si-O-Si cage



Polyether/ Si-O-Si cage

υ

υ υυ

υ

υ

υυυ

υυ υ

Fig. 4. Spectra obtained for mixture with (a) 5 wt.% in OECh, (b) 30 wt.% in OECh and (c) 50 wt.% in OECh.

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Tabl

e2

Hea

tsof

reac

tion

,tem

pera

ture

sof

exot

herm

spe

aks

and

T gva

lues

obta

ined

for

epox

ybl

ends

wit

hva

riou

sO

ECh

cont

ents

atdi

ffer

ent

heat

ing

rate

s

%O

ECh

02.

55

7.5

10

q(�

Cm

in�

1)

T p(�

C)

DH

0(J

g�1)

T g(�

C)

T p(�

C)

DH

0(J�g�

1)

T g(�

C)

T p(�

C)

DH

0(J

g�1)

T g(�

C)

T p(�

C)

DH

0(J

g�1)

T g(�

C)

T p(�

C)

DH

0(J

g�1)

T g(�

C)

515

1.4

350.

916

6.3

150.

333

8.8

147.

415

0.2

379.

214

3.7

150.

639

2.7

143.

215

1.8

377.

413

8.6

1017

0.7

376.

215

2.0

168.

336

3.1

146.

417

0.5

395.

714

5.2

170.

735

1.3

138.

716

7.7

357.

912

7.9

1518

0.1

355.

115

0.9

178.

936

9.9

140.

118

7.7

363.

414

0.2

182.

232

5.3

139.

717

7.5

368.

712

9.6

2018

8.2

362.

615

0.1

188.

234

8.2

143.

418

6.6

383.

813

5.7

191.

834

5.5

139.

218

8.2

388.

913

0.0

Ave

rage

valu

e36

7.2

355.

038

0.5

353.

737

3.2

Stan

dard

devi

atio

n10

.324

.411

.624

.511

.4Q

0[k

J(m

olep

oxy

equ

iv)�

1]

100.

897

.910

5.5

98.8

105.

0

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maximum rate of the system can be observed after thebeginning of the reaction, this fact shows its autocatalyticeffect. This behaviour demonstrates that the presence ofPOSS in the system does not vary the curing mechanism.By integrating da/dt values the conversion that the systempresents in each time of cure reaction, is obtained. Conver-sion increases quickly at the beginning of the cure, to reacha maximum value that is always less than that of the unit.

The experimental rate equation (a, da/dt) is determinedfor the complete course of the reaction, and the experi-mental results are adjusted to Eq. (1). Thus, the values ofthe rate constants and the reaction orders are obtainedfrom this fit and they are shown in Table 3. The values ofk1 and k2 increase with the cure temperature, but do notdepend on the content of POSS. Reaction orders have val-ues about 1 for m, and about 2 for n. The overall reactionorder m+n is in the range of 3–4. These values are similarfor the other epoxy systems cured with diamines [31].The activation energies, E1 and E2, associated with param-eters k1 and k2 are obtained for the neat amine-cured sys-tem and its blends are shown in Table 4. These values donot depend on POSS content, similar to the behaviour ofk1 and k2. Octaepoxy-POSS seems to react with the diaminein the same way that the DGEBA does, because they do notshow differences in the kinetic parameters of the reactionwhen the OECh is added in substitution of DGEBA fordifferent blends.

The polymerization of epoxy resins is characterized bytwo phenomena which occur in the course of the reac-tion: the gelation and the vitrification. Any further pro-gress after vitrification in the cure reaction is stopped,and so, the extent of the cure is limited. This event indi-cates that the cure kinetics in the later stage was sub-jected to diffusion control as a result of thevitrification. Experimental data did not fit with predic-tion models in this stage of the reaction. To solve thisproblem Khanna and Chanda successfully applied asemi-empirical relationship based on free volume consid-erations [32]. In order to consider diffusion control in thereaction a new function is applied to Eq. (1). This func-tion is called diffusion factor, f(a), and can be repre-sented in the following form,

f ðaÞ ¼ ke

kc¼ 1

1þ exp½Cða� acÞ�ð2Þ

where ke is the overall effective rate constant, kc is the rateconstant of the process controlled by chemical factors, C isa fitted parameter and ac is the critical conversion, wherediffusion becomes controlling. As a is much smaller thanthe critical value, a << ac, the f(a) approximates to unityand the reaction is kinetically controlled. When aapproaches the value of ac, f(a) begins to decrease andapproaches zero as the reaction stops. The effective reac-tion rate at any conversion, (da/dt)eff, is equal to the chem-ical reaction rate multiplied by f(a).

Applying a non-linear regression to f(a) versus a datafollowing the Eq. (2), values of C and ac can be determined(Table 3). In all cases, the values of ac are about 80–90% ofconversion.

As the transition to the diffusion regime is gradual,the critical conversion is not an observable quantity

41.0

41.5

42.0

42.5

43.0

43.5

44.0

44.5

45.0

100 110 120 130 140 150 160 170 180

0%

2.5%

5%

7.5%

10%

Temperature (ºC)

Hea

t fl

ow (

mW

) E

ndo

Fig. 5. Glass transition temperatures in different blends, indicated withan arrow, at the heating rate of 10 �C min�1.

0.0

0.2

0.4

0.6

0.8

1.0

2 10-4

4 10-4

6 10-4

8 10-4

1 10-3

0 500 1000 1500 2000 2500 3000

110ºC120ºC130ºC

110ºC

120 ºC

130 ºC

0

d/dt

Time (s)

α

α

Fig. 7. Plot of reaction rate and conversion versus time for blends of 2.5%of POSS at three cure temperatures.

ROM

OLE

CULA

RN

AN

OTE

CHN

OLO

GY


[33]. As cure proceeds, the mobility of the reacting spe-cies is reduced and this leads to diffusional effects.Hence, ac would reflect the stage of the cure of the sys-tem rather than the temperature of cure. ac values de-crease when the temperature of curing is increased to130 �C for blends with content of POSS equal or higherthan 5 wt.%. This fact can be attributed to a hindranceduring the polymerization as a result of the huge sizeof POSS molecules.

Fig. 8 displays the plot of experimental reaction rate,adjusted to Kamal and corrected by diffusion for samplewith 2.5 wt.% of OECh cured at 130 �C over the entire rangeof conversion. A deviation of experimental data is observedin the higher values of conversion. However good agree-ment between experimental data and the autocatalyticmodel corrected with diffusion was found over the wholecuring temperature range for each amount of OECh andfor all temperatures studied.

After isothermal analysis, dynamic runs were made tosamples cured at different temperatures. In the first

-12

-11

-10

-9

-8

2.10 2.15 2.20 2.25 2

0% EK = 54.7 kJ mol-1

5 % EK = 54.4 kJ mol-1

10 % EK = 55.7 kJ mol-1

ln (

q/T

p2 )

1000/Tp (K

Fig. 6. Plots obtained applying Kissinger and Flynn-Wall-Ozawa methods

C

dynamic run, a relaxation peak, TR, appears in all cure tem-peratures, whose values are shown in Table 3. A secondscan was made to determinate the glass transition temper-atures, Tg. In general, Tg values decrease with the content ofOECh, as a consequence of the minor mobility of moleculesin presence of POSS cages [7,34].

4. Conclusion

An octaepoxy-silsesquioxane, OECh, was used to modifyan epoxy–amine system. The OECh results in a crystallineproduct that shows two melting points in DSC scans. Thisfact is attributed to the presence of two different morphol-ogies that can be seen when OECh is observed by SEM.

The presence or absence of the vibration bands fromdifferent functional groups participating in the polymeri-zation were followed by ATR-FTIR along the curing cycleproposed. In all cases, it is observed that the amine groupsof BSA open the epoxy ring of DGEBA. In the other hand, itis checked, as the cycloaliphatic epoxy ring from OECh

0

1

2

3

4

5

6

.30 2.35 2.40

0 % Eo = 59.1 kJ mol-1

5 % Eo = 58.6 kJ mol-1

10 % Eo = 59.9 kJ mol-1

log (q)

-1)

to dynamic experimental data from samples with 10 wt.% of POSS.

MA

Table 3Kinetic parameters obtained by Kamal model and applying the diffusion factor, relaxation peaks, and glass transition temperatures of samples cured at 110, 120and 130 �C at the heating rate of 10 �C min�1

% OECh k1 10�4 (s�1) k2 10�3 (s�1) m n C ac TR (�C) Tg (�C)

Tc/110 (�C)0 1.9 3.9 1.4 2.2 35.3 0.8 131.1 140.12.5 2.1 4.1 1.4 2.0 39.9 0.8 122.6 144.95 1.6 3.6 1.2 2.1 23.8 0.8 121.5 135.57.5 1.6 2.3 1.1 1.7 52.4 0.8 119.0 140.310 1.8 3.6 1.4 2.2 31.7 0.8 120.6 133.1

Tc/120 (�C)0 2.8 5.4 1.3 2.3 31.0 0.8 135.9 141.42.5 2.6 5.2 1.2 2.1 26.8 0.8 135.9 149.05 3.3 5.5 1.3 2.0 45.2 0.9 139.8 143.37.5 2.7 3.8 1.1 1.6 125.6 0.9 137.1 139.510 3.3 3.6 1.2 1.6 418.6 1.0 133.4 135.2

Tc/130 (�C)0 5.1 6.4 1.2 2.0 53.0 0.9 147.3 153.02.5 4.2 6.6 1.1 2.0 20.1 0.9 145.7 148.15 3.9 6.5 1.1 2.4 19.7 0.7 144.0 145.37.5 4.5 8.5 1.3 2.4 39.6 0.8 142.2 141.110 4.4 8.0 1.3 2.3 29.5 0.8 132.8 137.1

0

0.0002

0.0004

0.0006

0.0008

0.001

0 0.2 0.4 0.6 0.8 1

KAMALEXPERIMENTALEFFECTIVE

Tc

= 130ºC

2.5% OECh

α

dα/d

t

Fig. 8. Experimental, Kamal and effective reaction rate in all range ofconversion for mixtures with 2.5 wt.% of OECh cured at 130 �C.

Table 4Activation energies calculated from Arrhenius relation of k1 and k2 withtemperature of cure

% OECh 0 2.5 5 7.5 10

E1 (kJ mol�1) 69.0 49.3 49.2 58.6 53.0E2 (kJ mol�1) 28.8 26.7 35.0 92.9 66.0

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does not open during the reaction. However, the presenceof POSS accelerates the rate of opening of glycidyl epoxyrings of DGEBA.

The kinetic parameters that explain the curing reactionof the system with OECh were obtained by different treat-ments of dynamic and isothermal DSC data. An overallreaction heat close to that expected for an epoxy–aminesystem was calculated in dynamic runs. The presence ofOECh does not significantly influence the value of totalheat. Values of glass transition temperature decreaseswith the content of OECh because of the hindrance thatthis molecule produces. The activation energy obtained

from Kissinger method is less than the value obtainedfrom Flynn-Wall-Ozawa method in all cases, as occursin various epoxy–amine systems. An autocatalytic mecha-nism was observed for amine-cured epoxy blends over arange of composition. The maximum reaction rates ofthe blends did not experience important changes withthe content in OECh, so the kinetic mechanism of theepoxy blends remained the same as that of the neatepoxy.

Kinetic parameters were obtained for the epoxy blendsand the kinetic model proposed by Kamal was found todescribe the cure behaviour of the epoxy and its blends.

The values of k1 and k2 increase with the cure tempera-ture but they do not depend on the content of POSS. Fromvalues of kinetic constants, the activation energies, E1 andE2, associated to process of isothermal curing were obtained.

Diffusion control was incorporated to describe the curein the latter stages of the reaction. With the introduction ofthis factor it is possible to fit the experimental data to themodel quite well, and to calculate the degree of conversionover the whole range of cure temperature fairly precisely.It was observed from the values of ac obtained that thehighest contents of OECh lead to a hindrance during poly-merization, which are attributed to the size of POSSmolecule.

The relaxation process and the glass transition wereobserved for each blend at each temperature of cure. Thevalues of the relaxation peaks, TR, and the glass transitiontemperatures, Tg, seem to increase with the cure tempera-ture because of the higher crosslinking obtained butdecreases with the content of OECh due to the hindrancethat POSS molecules produce in the system.

Acknowledgments

Financial support for this work has been provided byComisión Interministerial de Ciencia y Tecnología (CICYT)and FEDER through grant MAT2007-61677 and by GrantXUGA-PGIDIT05PXIC17201PN.

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Epoxy/POSS organic–inorganic hybrids: ATR-FTIR and DSC studies

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Transcript of Epoxy/POSS organic–inorganic hybrids: ATR-FTIR and DSC studies