lable at ScienceDirect

Polymer 52 (2011) 1716e1724

Contents lists avai

Polymer

journal homepage: www.elsevier .com/locate/polymer

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

Yue Lin a, Jie Jin a, Mo Song a,*, S.J. Shawb, C.A. Stone b

aDepartment of Materials, Loughborough University, Loughborough LE11 3TU, UKbDstl, Porton Down, Salisbury SP4 0JQ, UK

a r t i c l e i n f o

Article history:Received 29 October 2010Received in revised form4 February 2011Accepted 23 February 2011Available online 3 March 2011

Keywords:Cyanate ester resinPolyhedral oligomeric silsesquioxaneNanocomposite

* Corresponding author. Tel.: þ44 1509 223331; faxE-mail address: M.song@lboro.ac.uk (M. Song).

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.02.041

a b s t r a c t

Curing dynamics and network formation of cyanate ester resin (PT-30)/TriSilanolPhenyl polyhedraloligomeric silsesquioxane (POSS) nanocomposites were studied by means of differential scanningcalorimetry (DSC), modulated temperature differential scanning calorimetry MTDSC), Fourier transforminfrared (FTIR) and Raman spectroscopies. The incorporation of the POSS showed a strong catalytic effect(decrease in curing temperature and activation energy) on the curing reaction of PT-30. The activationenergy 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.cyanurate) 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 firstly 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.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

With technological development and increasing demand formaterials with enhanced properties, cyanate ester resins arecurrently in widespread use for a variety of applications fromstructural 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 flame 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-tance is low. Thus, reinforcement of cyanate ester resins for high-performance applications becomes necessary.

In the last decade, hybrid organic polymer-inorganic nano-composites have attracted considerable research interest forvarious applications, such as in mechanical, optical and electronic

: þ44 1509 223949.

All rights reserved.

fields [2]. The inorganic nanofillers being studied include inorganicand organic nanofillers, 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.Incorporation of POSS reagents into organic polymers offersa unique opportunity to prepare nanocomposites with trulymolecular dispersions of the inorganic fillers [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. Modification of cyanate ester properties with POSS has beenattempted by several groups [6,16]. Lu et al. [6b] introduced amulti-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 successfullymodified 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 flameretardancy of the resin was significantly improved. Liang et al. [6a]reported that incorporation of TriSilanoPhenyl POSS into a cyanate

Fig. 1. Schematic of molecular structure of (A) TriSilanolPhenyl POSS, (B) possible structure of the PT-30.

Y. Lin et al. / Polymer 52 (2011) 1716e1724 1717

ester resin, PT-15, can improve the storage modulus and thermalstability. The storage modulus E0 above Tg was significantlyimproved until the addition of 5 wt% POSS. The best improvementin flexural 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-performancepolymeric materials.

It is believed [6a] that the addition of functionalized POSS couldinfluence the cure dynamics and network formation of cyanateesters significantly. However, the mechanism of these influences isstill not clarified. 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 flexuralmoduli? 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. Theinfluence 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.

50 100 150 200 250 300 350

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.00wt%1wt%5wt%

10wt%

N2

)g

/W

(w

ol

Ft

ae

H

Temperature (o

C)

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

2.1. Materials

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 ¼ 931.34 g/mol) was purchased from Hybrid Plastics Inc. Fig. 1shows a schematic of the respective molecular structures.

2.2. Preparation of PT-30/POSS mixtures

1 wt%, 5 wt%, 10 wt% PT-30/POSS mixtures were prepared asfollows. The cyanate ester resin, PT-30, was firstly 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.

2.3. Characterization

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 cm�1 e 400 cm�1 using a Shimadzu FTIR-8400s spec-trophotometer with a 4 cm�1 resolution over 128 scans. Ramanspectrawere recorded from 100 cm�1 e3500 cm�1, on a Jobin YvonHoriba high-resolution LabRam 800 Raman microscope system,

Table 1DSC results of the PT-30/POSS systems in nitrogen atmosphere (60 ml/min) withheating rate of 10 �C/min.

Sample OnsetTemperature(�C)

PeakTemperature(�C)

Endtemperature(�C)

Duration(min)

6HT

(J/g)

Pure PT-30 257 303 345 8.8 3461 wt%

POSS/PT-30240 303 328 8.8 363

5 wt%POSS/PT-30

199 283 332 13.3 415

10 wt%POSS/PT-30

220 277 314 9.4 352

100 150 200 250 300 350

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

In air In N2

gW

(w

ol

ft

ae

H1

-

)

Temperature (o

C)

278 oC 303 oC

225 oC264 oC

Fig. 3. Heat flow vs. temperature for PT-30 resin with (60 ml/min) and withoutnitrogen atmosphere. Heating rate of 10 �C/min was used.

Y. Lin et al. / Polymer 52 (2011) 1716e17241718

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).

3. Results and discussion

Fig. 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

0 50 100

0.00

0.05

0.10

0.15

0.20

0.25

)g/

W(

w

ol

F

ta

eH

v

er

no

N

Time (min)

215 C

220 C

225 C

A B

0 20 40 60 80

0.00

0.05

0.10

0.15

0.20

0.25

)g

/W

(

wo

lF

ta

eH

v

er

no

N

Time (min)

200 C 205 C 210 C

C D

Fig. 4. Isothermal DSC plots for the PT-30/POSS nanocomposites at different isothermal temPOSS/PT-30.

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 significantly 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.

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.

For cure of a thermoset resin, the conversion at time, t, can bedefined as follows:

at ¼ DHt

DHT(1)

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.

The conversion rate can be defined as follows:

dadt

¼ dDHT=dtDHT

(2)

where, da/dt is the conversion rate at time t.

0 20 40 60 80 100

0.00

0.05

0.10

0.15

0.20

0.25

)g/

W(

w

ol

F

ta

eH

v

er

no

N

Time (min)

210 C 220 C 230 C

0 50 100 150

0.00

0.05

0.10

0.15

)g

/W

(

wo

lF

t

ae

H

ve

rn

oN

Time (min)

185 C 195 C 200 C

peratures (A) pure PT-30, (B) 1 wt% POSS/PT-30, (C) 5 wt% POSS/PT-30, and (D) 10 wt%

0.0 0.2 0.4 0.6 0.8 1.0

0.0000

0.0002

0.0004

0.0006

0.0008

td

/a

d

a

215 C 220 C 225 C

A

0.0 0.2 0.4 0.6 0.8 1.0

0.0000

0.0002

0.0004

0.0006

0.0008

td

/a

d

a

210 C 220 C 230 C

B

0.0 0.2 0.4 0.6 0.8 1.0

0.0000

0.0002

0.0004

0.0006

td

/a

d

a

200 C 205 C 210 C

C

0.0 0.2 0.4 0.6 0.8 1.0

0.0000

0.0002

0.0004

0.0006

td

/a

d

a

185 C 195 C 200 C

D

Fig. 5. da/dt vs. a for the PT-30/POSS nanocomposites at different isothermal temperatures (A) pure PT-30, (B) 1 wt% POSS/PT-30, (C) 5 wt% POSS/PT-30, and (D) 10 wt% POSS/PT-30.

Y. Lin et al. / Polymer 52 (2011) 1716e1724 1719

An 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:

dadt

¼ ðk1 þ k2amÞð1� aÞn ¼ kkinð1� aÞn (3)

where, k1 and k2 are the rate constants, m and n are the reactionorders, and kkin is the kinetic rate constant under chemicallycontrolled condition.

Furthermore, the temperature dependence of any rate constantis given by the Arrhenius relationship:

k ¼ Aexp��EaRT

�(4)

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.

Fig. 4 shows the plots of heat flow versus time, recorded byMTDSC for the PT-30/POSS systems at different isothermal

Table 2Autocatalytic model constants for the PT-30/POSS nanocomposites.

Content of POSS (wt%) Temperature (�C) k1 (� 104 s�1) k2 (� 104 s�1

0 215 2.41 36.6220 2.87 45.0225 5.93 72.4

1 210 1.27 116220 2.77 289230 6.09 402

5 200 2.27 421205 3.64 556210 4.52 705

10 185 1.44 82.1195 2.07 140200 3.19 226

experimental 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 A1reveal 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.

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 significantly 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 on the curingprocess of the PT-30. However, excessive addition hindered thecross-linking reaction of the PT-30.

) m n lnA1 lnA2 E1 (kJ/mol) E2 (kJ/mol)

1.45 2.11 36.34 28.28 181.65 137.721.35 1.951.55 2.711.83 4.80 30.45 27.00 158.39 125.982.15 4.862.30 4.322.41 6.55 24.97 21.75 131.06 98.012.41 5.672.41 5.861.93 4.15 14.99 26.06 90.94 117.682.00 4.862.19 4.22

0 2 4 6 8 10

80

100

120

140

160

180

)lo

m/

Jk

(

yg

re

nE

n

oi

ta

vi

tc

A

POSS content (wt%)

E1 E2

A

0 2 4 6 8 10

15

20

25

30

35

40

A

nl

POSS content (wt%)

ln A1 ln A2

B

Fig. 6. Activation energy (A) and pre-exponential factor (B) vs. POSS content.

Y. Lin et al. / Polymer 52 (2011) 1716e17241720

From Eq. (3), the following equation can be deduced:

kkin ¼ ðk1 þ k2amÞ (5)

Table 3 and Table 4 list the activation energy and the pre-exponential factor for different conversions for the POSS/PT-30nanocomposites.

Fig. 7 shows changes of the activation energywith conversion forthe PT-30/POSS systems. For the pure PT-30, the activation energyand the pre-exponential factor decreased with increasing degree ofconversion until they reached a minimum value at a conversionfactor of 0.4. When 1% POSS was added, the activation energy andthe pre-exponential factor decreased dramatically belowadegree ofconversion of 0.2,which indicated a strong catalytic effect. However,after the conversion of 0.2, the activation energy and the pre-exponential factor increased significantly, which may be due to theexhaustion of POSS in the reaction. The activation energy at 5wt% ofPOSS decreased throughout the whole curing process. The pre-exponential factor showed just a slight increase after the conversionof 0.2. However, for 10 wt% POSS/PT-30 system, similar phenom-enon with 1 wt% POSS/PT-30 was observed. It is clear that theappropriate POSS content could continue to catalyze the cure of thePT-30. Although the excessive addition of the POSS showed bettercatalytic effect at the very beginning, it hindered the chemicalreaction of the PT-30 when the conversion reached a certain level.

Fig. 8 (A) shows FTIR spectra for the pure PT-30 resin with timecured at 225 �C in nitrogen atmosphere. The characteristicabsorption bands of the OeChN cyanate ester functional groupwere observed in the infrared spectrum between 2200 and

Table 3Activation energy at different conversions for the POSS/PT-30 nanocomposites.

a 0 0.1 0.2 0.3 0.4

POSS content (wt%) Activation energy (kJ/mol)

0 181.7 154.5 139.9 134.7 133.41 158.4 100.9 78.5 80.9 88.45 131.1 117.9 105.3 101.2 99.710 90.9 77.1 77.7 84.7 91.6

Table 4Pre-exponential factor at different conversions for the POSS/PT-30 nanocomposites.

a 0 0.1 0.2 0.3 0.4

POSS content (wt%) Pre-exponential factor (Ln A)

0 36.34 30.14 27.02 26.13 24.141 30.45 16.99 12.36 13.63 16.005 24.97 22.15 19.98 19.76 20.0210 14.99 11.90 12.82 15.26 17.56

2300 cm�1. The band was split into a doublet of partially resolvedpeaks and was separated by approximately 35 cm�1. The cure of thePT-30 cyanate ester resins can be followed by monitoring the cor-responding increase in the absorbance bands of the triazine (i.e.,cyanurate) ring near 1360 and 1570 cm�1. With the curing time, thecyanate bands at 2250 cm�1 decreased and new bands appeared at1565 cm�1 (nC]NeC) and 1368 cm�1 (nNeCeO) due to triazineformation. Fig. 8 (B) shows FTIR spectra for the PT-30/POSS (5wt %)nanocomposite with time cured at 210 �C in nitrogen atmosphere.From the characteristic absorption bands of the OeChN cyanateester functional group, it seems that the cure of cyanate ester resinsand the formation of the triazine ring in the PT-30/POSS nano-composites are the same as that of the PT-30.

For clarification, the FTIR spectra of the TriSilanolPhenyl POSS(A), and the comparison of neat PT-30 with 5 wt% POSS/PT-30nanocomposite (B) in the range from 2000 cm�1 e1000 cm�1 weregiven in Fig. 9. A broad band near 3200 cm�1 indicating stretchingof Si-OH group, was observed in the infrared spectra of the TriSi-lanolPhenyl POSS. The aromatic CeH stretching at 3050 cm�1 andthe SieC6H5 stretching at 1430 cm�1 indicated the presence ofphenyl group of the POSS. Furthermore, a narrow band appearing at1075 cm�1 indicated the asymmetric stretching of SieOeSi group.The same band showed as a shoulder, can also be observed in theinfrared spectra of the 5 wt% POSS/PT-30 nanocomposite in Fig. 9(B). From Fig. 9 (B), no difference in the cure between the neatPT-30 and the 5 wt% POSS/PT-30 nanocomposite was observed.Raman spectra could reveal the difference.

Fig. 10 (A) and (B) shows the Raman spectra for the pure PT-30and its nanocomposites with 5 wt% POSS at different curing times.

0.5 0.6 0.7 0.8 0.9 1.0

133.8 134.9 136.3 137.8 139.3 140.796.4 103.7 110.3 116.2 121.6 126.499.0 98.7 98.5 98.3 99.1 98.297.6 102.6 107.0 110.9 114.2 117.3

0.5 0.6 0.7 0.8 0.9 1.0

26.50 26.99 27.52 28.06 28.58 29.0818.37 20.53 22.45 24.17 25.71 27.1120.37 20.71 21.02 21.31 21.57 21.8119.52 21.18 22.62 23.87 24.98 25.97

0.0 0.2 0.4 0.6 0.8 1.0

70

80

90

100

110

120

130

140

150

160

170

180

190 0wt% 1 wt% 5 wt% 10 wt%

)l

om

/J

k(

y

gr

en

E

no

it

av

it

cA

conversion

A

0.0 0.2 0.4 0.6 0.8 1.0

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

A

nl

Conversion

0wt% 1 wt% 5 wt% 10 wt%

B

Fig. 7. Activation energy (A) and pre-exponential factor (B) vs. conversion for the PT-30/POSS nanocomposites.

Fig. 8. FTIR spectra of the pure PT-30 resin (A) with time cured at 225 �C and 5 wt% POSS/PT-30 nanocomposite (B) with time cured at 210 �C in nitrogen atmosphere (60 ml/min).For clarification, the spectra were shifted parallel, and were calibrated basing the absorption band of phenyl ring symmetric breathing vibration near 1500 cm�1. The peaks near2350 cm�1 are due to the presence of CO2.

4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

)%

(n

oi

ss

im

sn

ar

T

Wavenumber (cm )

A

2400 2200 2000 1800 1600 1400 1200 1000

5wt% POSS/PT-30 320 min

5wt% POSS/PT-30 0 min

neat PT-30 320 min

)%

(n

oi

ss

im

sn

ar

T

Wavenumber (cm )

B

neat PT-30 0 min

Fig. 9. FTIR spectra of the TriSilanolPhenyl POSS (A), and the comparison of neat PT-30 with 5 wt% POSS/PT-30 nanocomposite (B).

Y. Lin et al. / Polymer 52 (2011) 1716e1724 1721

The curves recorded at 40 min became quite noisy, and it becamehard to distinguish the peaks. As the curing reaction proceeded,some vibrations of bonds in the PT-30were hindered by the rigid 3-dimensional cross-linking structure formed. Consequently, thevibrations can no longer be clearly detected by Raman scattering. Inthis case, we have to focus on the early stages of the cure.

Fig. 10 (C) and (D) shows the details of Raman spectra for thepure PT-30 and its nanocomposites with 5 wt% POSS in the earlystages of the cure. Table 5 lists the analysis of the Raman spectrashown in Fig. 10. From Fig. 10 (C), it can be seen that the intensity ofthe absorption band at 2250 cm�1 indicating the stretching ofeOeChN bond, and the intensity of absorption band at 340 cm�1

indicating the scissoring of eOeChN bond, decreased with

increasing curing time. This observation indicated that the cyanategroup of the resin started to react in the very early stages of thecure. The cure of the pure PT-30 cyanate ester resin can be followedby monitoring the corresponding increase in the absorbance bandsof the triazine ring at 1004 cm�1, 1304 cm�1, 1445 cm�1, and1668 cm�1. As the curing reaction proceeded, the cyanate bands at2250 cm�1 decreased and some new bands appeared due tostretching of eOeC]Ne at 1668 cm�1. This result is correspondedwith that obtained in the FTIR experiments. Furthermore, thesignificant increase in the intensity of absorption band at2921 cm�1 indicating stretching of in the region of aliphatic CeHbond may be caused by the formation of hydrogen bonding, whichis due to the presence of triazine ring.

Fig. 10. Raman spectra of the pure PT-30 resin (A) cured at 225 �C, 5% POSS/PT-30 resin (B) cured at 210 �C up to 100 min, (C) A-enlarged, and (D) B-enlarged in nitrogen atmosphere(60 ml/min).

Y. Lin et al. / Polymer 52 (2011) 1716e17241722

From Fig. 10 (D), it can be seen that the major reaction of the5 wt% POSS/PT-30 system during cure is still the formation of thetriazine ring, which is the same as that of the pure PT-30. The mainreaction can be followed by monitoring the corresponding increase

Table 5Analytical results of the Raman spectra of the pure PT-30 resin cured at 225 �C andthe 5 wt% POSS/PT-30 system cured at 210 �C

RamanShift(cm�1)

Assignment Bond Tendency

340 d�C^N ReChN Decrease2250 n-ChN Decrease627 Ring in-plane bending Constant1042 dCeH 1,2-disubstituted

benzene ringConstant

711 dCeC for 1,2,4-trisubstitutedbenzene ring and nSkeletalfor 1,2-disubstitutedbenzene ring

Constant

1200 nSkeletal for substitutedbenzene ring

Constant

1605 nC]C of aromatic ring Constant3055 nCeH CeH stretching

for benzeneConstant

1304 nCeN Increase1668 n�O�C]Ne usually strong

doublet due to rotationalisomerism

Increase

1445 Triazine ring stretching Increase1004 Triazine ring “breathing”

vibrationIncrease

2921 nas CH2eCH2 Increase

in the absorbance bands of the triazine ring at 991 cm�1,1300 cm�1,1445 cm�1, and 1675 cm�1, and monitoring the correspondingdecrease in absorption band at 2235 cm�1 indicating stretchingof eOeChN bond, and the absorption band at 345 cm�1 indicatingscissoring of eOeChN bond. Compared with the pure PT-30, theintensity of absorption band at 345 cm�1 was almost the same,and the intensity of absorption band at 2235 cm�1 decreased justa little at the very beginning of the curing reaction. This phenom-enon revealed that the cyanate group of the resin reacted veryslowly and did not form the triazine ring in the very early stage ofthe cure.

1250 1500 1750 2000

5 wt% POSS/PT-30 at 210 o

C at 20 min

5 wt% POSS/PT-30 at 210 o

C at 0 min

pure PT-30 at 225 o

C at 30 min

pure PT-30 at 225 o

C at 10 min

pure PT-30 at 225 o

C at 0 min

).

u.

a(

yt

is

ne

tn

I

raman shift (cm-1)

Fig. 11. Raman spectra of the pure PT-30 resin cured at 225 �C and 5% POSS/PT-30 resincured at 210 �C from 1250 to 2000 cm�1 in nitrogen atmosphere (60 ml/min).

Fig. 12. Scheme of crosslinked network formation through triazine ring and eOe(C]NH)eOe bond for the POSS/PT-30 nanocomposites.

Y. Lin et al. / Polymer 52 (2011) 1716e1724 1723

Enlarged Raman spectra for the pure PT-30 resin cured at 225 �Cand the 5% POSS/PT-30 system cured at 210 �C from 1250 to2000 cm�1 were shown in Fig. 11 for clarification. For the neat PT-30, a band near 1668 cm�1 indicating the formation of triazine ringappeared at the very beginning of the curing process. On contrary,for the 5 wt% POSS/PT-30 system, the band appearing at 1668 cm�1

is much weaker. Another band which is quite sharp showed at1615 cm�1 indicated the formation of eOe(C]NH)eOe bond.These results implied that the cyanate ester resin reacted witheOHgroup of the POSS firstly to form eOe(C]NH)eOe bond. Thus, thecuring mechanism of the PT-30/POSS nanocomposites is quitedifferent from that of the pure PT-30. The strong catalytic effect ofthe POSS to the curing process of PT-30 should be originated fromformation of the eOe(C]NH)eOe bond. Fig. 12 shows the schemeof crosslinked network formation through triazine ring and eOe(C]NH)eOe bond for the POSS/PT-30 nanocomposites.

4. Conclusions

The incorporation of the POSS catalyzed the reaction of the PT-30. However, excessive addition leads to an increase of onsettemperature, and reduction in the total reaction enthalpy. Theresults showed that the activation energy of the PT-30/POSSsystems decreased with increasing POSS content, up to 5 wt%. Themost pronounced catalytic effect was observed with 5 wt% POSS.Although the excessive addition of the POSS showed catalyticbehavior in the initial stages, it hindered the chemical reaction ofthe PT-30 when the conversion reached a certain level. Both FTIRand Raman spectra can be applied to monitor the reaction of thePT-30/POSS nanocomposites through the formation of triazinering. Raman spectra revealed that the PT-30 resin preferred to react

witheOH group on the POSS firstly to formeOe(C]NH)eOe bondin PT-30 and its nanocomposites with the POSS. The strong catalyticeffect of the POSS to the curing process of the PT-30 should beoriginated from formation of the eOe(C]NH)eOe bond.

References

[1] (a) Simon SL, Gillham JK. J Appl Polym Sci 1993;47:461e5;(b) Snow AW, Buckley LJ, Armistead JP. J Polym Sci Part A Polym Chem1999;37:135e50;(c) Hamerton I. Chemistry and technology of cyanate ester resins. London ;New York: Blackie Academic & Professional; 1994. p. xviii,p. 357;d) Hamerton I, Hay JN. Polym Int 1998;47:465e73.

[2] (a) Hussain F, Hojjati M, Okamoto M, Gorga RE. J Compos Mater 2006;40:1511e75;(b) Huang ZM, Hang YZ, Kotaki M, Ramakrishna S. Compos Sci Technol2003;63:2223e53;(c) Beecroft LL, Ober CK. Chem Mater 1997;9:1302e17;(d) Bockstaller MR, Mickiewicz RA, Thomas EL. Adv Mater 2005;17:1331e49;(e) Gangopadhyay R, De A. Chem Mater 2000;12:608e22;(f) Sanchez C, Lebeau B, Chaput F, Boilot JP. Adv Mater 2003;15:1969e94.

[3] (a) Wen JY, Wilkes GL. Chem Mater 1996;8:1667e81;(b) Usuki A, Hasegawa N, Kato M. Inorgan Polym Nanocompos Membran2005;179:135e95;(c) Pan YZ, Xu Y, An L, Lu HB, Yang YL, Chen W, et al. Macromolecules 2008;41:9245e58;(d) Kim DS, Lee KS. J Appl Polym Sci 2003;90:2629e33.

[4] (a) Lau KH, Hui D. Compos Part B Eng 2002;33:263e77;(b) Du FM, Scogna RC, Zhou W, Brand S, Fischer JE, Winey KI. Macromolecules2004;37:9048e55;(c) Dominguez DD, Laskoski M, Keller TM. Macromol Chem Phys2009;210:1709e16.

[5] (a) Cai D, Yusoh K, Song M. Nanotechnology 2009;20:1e5;(b) Eda G, Chhowalla M. Nano Lett 2009;9:814e8;(c) Fang M, Wang KG, Lu HB, Yang YL, Nutt S. J Mater Chem 2009;19:7098e105;(d) Li D, Kaner RB. Science 2008;320:1170e1;(e) Ramanathan T, Abdala AA, Stankovich S, Dikin DA, Herrera-Alonso M,Piner RD, et al. Nat Nanotechnology 2008;3:327e31;

Y. Lin et al. / Polymer 52 (2011) 1716e17241724

(f) Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA,et al. Nature 2006;442:282e6.

[6] (a) LiangKW,LiGZ,ToghianiH,Koo JH,PittmanCU.ChemMater2006;18:301e12;(b) Lu TL, Liang GZ, Guo Z. J Appl Polym Sci 2006;101:3652e8.

[7] Yei DR, Kuo SW, Su YC, Chang. Polymer 2004;45:2633e40.[8] Fu BX, Gelfer MY, Hsiao BS, Phillips S, Viers B, Blanski R, et al. Polymer 2003;

44:1499e506.[9] Zheng L, Farris RJ, Coughlin EB. Macromolecules 2001;34:8034e9.

[10] Zhao YQ, Schiraldi DA. Polymer 2005;46:11640e7.[11] Leu CM, Chang YT, Wei KH. Macromolecules 2003;36:9122e7.[12] Zhang YD, Lee SH, Yoonessi M, Liang KW, Pittman CU. Polymer 2006;47:

2984e96.[13] Liu HZ, Zhang W, Zheng SX. Polymer 2005;46:157e65.

[14] Kannan RY, Salacinski JH, Odlyha M, Butler PE, Seifalian AM. Biomaterials2006;27:1971e9.

[15] Huang JC, Xiao Y, Mya KY, Liu XM, He CB, Dai J, et al. J Mater Chem 2004;14:2858e63.

[16] (a) Pittman CU, Li GZ, Ni HL. Macromol Symposia 2003;196:301e25;(b) Cho HS, Liang KW, Chatterjee S, Pittman HU. J Inorgan Organomet PolymMater 2005;15:541e53;(c) Liang KW, Toghiani H, Li GZ, Pittman CU. J Polym Sci Part A Polym Chem2005;43:3887e98.

[17] (a) Kamal MR. Polym Eng Sci 1974;14:231e9;(b) Reading M, Hourston DJ. Modulated temperature differential scanningcalorimetry: theoretical and practical applications in polymer character-isation. Dordrecht: Springer; 2006. 131.