synthesis and characterization of 1,1-bis(3-methyl-4-hydroxy phenyl)cyclohexane...

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Acta Materialia 57 (2009) 782–794

Synthesis and characterization of 1,1-bis(3-methyl-4-hydroxy phenyl)-cyclohexane polybenzoxazine–organoclay hybrid nanocomposites

Chinnakkannu Karikal Chozhan, Muthukaruppan Alagar *, Periyannan Gnanasundaram

Department of Chemical Engineering, Anna University, Chennai 600 025, India

Received 1 August 2008; received in revised form 10 October 2008; accepted 12 October 2008Available online 27 November 2008

Abstract

Typical high-performance polybenzoxazines were prepared from benzoxazines based on 1,1-bis(3-methyl-4-hydroxy phenyl)cyclohexane, paraformaldehyde and three distinctive aromatic diamines, namely 4,4’-diaminodiphenylmethane, 4,4’-diaminodiphen-ylether and 4,4’-diaminodiphenylsulfone, through ring-opening self-polymerization upon heating. The formation of benzoxazineswas confirmed by Fourier transform infrared, 1H and 13C nuclear magnetic resonance spectra. Polybenzoxazine–clay hybrid nano-composites were prepared by a solvent method using polybenzoxazine precursors and organoclay (OMMT) (up to 5 wt.%). Thehybrid mixture was subjected to ultrasonication for effective blending. The thermal properties of the resulting polybenzoxazine–claynanocomposites were studied using differential scanning calorimetry and thermogravimetric analysis. The dispersion of OMMT inthe polybenzoxazine and nanostructure of the composites was confirmed by X-ray diffraction analysis. The d spacing of the organo-clay interlayers was found to be increased from 1.69 to 2.10 nm. Thermal decomposition temperatures of the nanocomposites werein the range 294–637 �C. These nanocomposites exhibited a high char yield relative to unfilled polybenzoxazines depicting interfa-cial interactions between the organic and inorganic phases. The homogeneous morphological behavior was studied by scanningelectron microscopy.� 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Polybenzoxazine; Montmorillonite; Ring-opening polymerization; Thermoset; Thermal properties

1. Introduction

Polybenzoxazine (PBZ) is a versatile class of thermosetresin derived from the reaction between an aromatic alco-hol and a primary amine using formaldehyde. PBZs pos-sess excellent characteristics, such as high heat resistance,flame resistance, good chemical and electrical resistance,low absorption of water because of their flexible moleculardesign nature, and well-balanced thermal and mechanicalperformances [1]. In addition, PBZ derivatives possessunique properties, such as dimensional stability and near-zero shrinkage or expansion upon curing, which overcomethe shortcomings of traditional phenolic resins. Polymeri-zation of PBZ proceeds through the ring-opening of the

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doi:10.1016/j.actamat.2008.10.023

* Corresponding author. Tel./fax: +91 44 22203543.E-mail address: [email protected] (M. Alagar).

heterocyclic monomers by thermal treatment alone, with-out any catalysts and without producing by-products orvolatiles, and thus excellent dimensional stability isobtained [2–4].

Polymer–clay nanocomposites, in which the layered sil-icates of the clay become dispersed in the polymer–matrix,are a new class of composite materials. As a result of thedispersion of the clay on the nanoscale, such nanocom-posites often exhibit outstanding improvements in theirproperties, including increased modulus, strength, thermalstability and solvent resistance, and decreased gas perme-ability and flammability [5–15] relative to those of con-ventional fibre or filler based composites. The layeredsilicate that is most widely used for the preparation ofpolymer–clay nanocomposites is montmorillonite (MMT).

MMT is a multilayer silicate mineral that naturallypossesses inorganic cations within its galleries to balance

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Scheme 1. Synthesis of 1,1-bis(3-methyl-4-hydroxy phenyl) cyclohexane.

C.K. Chozhan et al. / Acta Materialia 57 (2009) 782–794 783

the charge of the oxide layers in a hydrophilic environ-ment. The ion exchange of these cations with organicammonium ions affords a hydrophobic environmentwithin the galleries of the organically modified MMT(OMMT) [16]. The organophilic galleries of OMMTenhance the compatibility of the clay with polymers[16], improve the dispersion of the silicate layers intothe matrix [5] and assist the penetration of monomersor polymers into the galleries [11]. In addition, theorganic ammonium cations can provide functional groupsthat react or interact with the monomer or polymer unitsto improve the interfacial strength between the reinforce-ment and the polymer–matrix [17]. The degree of disper-sion of clay nanolayers and the resulting morphology ofthe nanocomposites depend on a number of factors,including the mixing method (melt or solvent), tempera-ture, time, choice of solvent and its concentration, stericsize of the monomer or polymer, choice of intercalatingagent and yield of the ion exchange process.

Benzoxazines derived from aromatic biphenols, mono-amines, difunctional/multifunctional aromatic aminesand formaldehyde or their derivatives have already beenreported to provide desired properties like a high Tg, bet-ter thermal stability with high char yield, excellent flameretardancy, low water absorption and reduced dielectricconstant. Due to limitations in solubility, only benzoxa-zines based on some of the aromatic diamines havehitherto been prepared. Thus, in the present work, anattempt has been made to synthesize three novel benzox-azines from 1,1-bis(3-methyl-4-hydroxy phenyl)cyclohex-ane and three typical aromatic diamines (DDM,DDE and DDS), and to develop PBZ–clay-basednanocomposites.

In this study, we have synthesized characteristicbifunctional benzoxazine monomers and used them sub-sequently with organo-modified nanoclay to developPBZ–clay hybrid nanocomposites. We expected that thisorgano-nanoclay-filled PBZ would undergo ring openingpolymerization with the benzoxazine matrix to promotethe formation of an exfoliated nanocomposite structure.The influencing effect of the molecular structure andtypes of functional aromatic diamines used in the prepa-ration of distinctive PBZs and the effect of the natureand content of organo-modified nanoclay on the proper-ties of the hybrid nanocomposites have also beeninvestigated.

The disadvantages of PBZs are the high temperatureneeded for complete cure and the brittleness of the curedPBZ. Further, an improvement in its thermal propertiesis also required for a wide range of applications in the formof coatings in the harsh thermal conditions experienced inheavy industrial environments. Herein we have designednovel benzoxazine monomers and high-molecular-weightPBZ precursors, and hybridized them with inorganics toimprove the performance of PBZ. By so doing, a loweringof the cure temperature and an enhancement of thermalproperties were achieved.

2. Experimental

2.1. Materials

Cyclohexanone and o-cresol were obtained from SRL,India. Aniline, chloroform and 1,4-dioxane (from SRL,India), diaminodiphenylsulfone (DDS) and diaminodi-phenylether (DDE) (from Alfa Aesar, Sigma, USA) anddiaminodiphenylmethane (DDM) (from Aldrich Chemi-cals, USA) were all used as received. Montmorillonite clay(MMT) was purchased from Aldrich Chemicals, USA.Cetyltrimethylammonium bromide (CH3(CH2)15N(CH3)3Br)was procured from SRL, India.

2.2. Synthesis of 1,1-bis(3-methyl-4-hydroxy

phenyl)cyclohexane

Cyclohexanone (0.05 mol), o-cresol (0.1 mol) and a mix-ture of conc. HCl and glacial acetic acid (2:1 v/v) werereacted at 50 �C. The resulting pink-colored product [18]was isolated, washed with water to remove acid and dis-solved in 2 M NaOH solution. The following day, the solu-tion was filtered to remove resinous material, acidified withdilute HCl, filtered, washed with distilled water and driedat 90–100 �C to yield the purified product (81% m.p.186 �C). Infrared (KBr, cm�1): 3536, 3407 (O–H), 2926,1600, 1506, 1450, 1268, 1174, 1119, 1032, 895. 1H nuclearmagnetic resonance (NMR) (ppm): d 1.24–2.03 (m, 4H),2.31 (s, 6H), 3.24–3.45 (m, 6H), 6.67 (d, J = 8.3 Hz, 2H),6.86 (d, J = 8.3 Hz, 2H), 8.54 (s, 2H).

13C NMR (ppm, d6-CDCl3): d 154, 139.6, 129.4, 125.0,123.6, 114.3 (C aromatic); 44.3, 37.0, 26.3, 22.8, 16.5(C aliphatic) (Scheme 1).

2.3. Syntheses of benzoxazines

Three typical benzoxazine monomers were synthesizedby reacting 1,1-bis(3-methyl-4-hydroxy phenyl)cyclohexaneand paraformaldehyde with three aromatic diamines hav-ing distinct functionalities, as shown in Schemes 2–4. Thebenzoxazines were abbreviated as BZDDM, BZDDE andBZDDS according to the diamines (DDM, DDE andDDS) used in the reactions. As a model reaction, the ben-zoxazine monomer BZAni was synthesized using distilledaniline.

Synthesis of BZDDM was performed as follows. To 100 mlof chloroform, 4,4’-diaminodiphenylmethane (40.0 mmol,7.92 g), 1,1-bis(3-methyl-4-hydroxy phenyl)cyclohexane(40.0 mmol, 11.84 g) and paraformaldehyde (172.0 mmol,

Scheme 2. Syntheses of benzoxazines.

Scheme 3. Synthesis of BZAni and PBZAni.

Scheme 4. Structures of PBZs.

784 C.K. Chozhan et al. / Acta Materialia 57 (2009) 782–794

5.15 g) were added and refluxed for 5 h. The reaction mixturewas filtered and washed once with 1 M NaHCO3 aqueoussolution (200 ml), then dried with anhydrous sodium sulfatefor 12 h. Removal of solvent by evaporation and dryingunder vacuum afforded BZDDM as a yellow powder. Theyield was 87%.

Synthesis of BZAni, BZDDE and BZDDS were performedin a similar way as reported elsewhere [19], yielding precur-

sors as yellow, dark yellow and light yellow powder in 81,84 and 57% yield, respectively.

2.4. Preparation of PBZ films

The PBZ precursor (2 g) was dissolved in 1,4-dioxane(4 g), and the solution was cast on a glass plate. The solventwas removed by drying at 50 �C for 5 h, giving yellowtransparent precursor films. Heat treatment of the precur-sor films at 100, 120, 160, 200 and 240 �C for 1 h each gavebrown transparent PBZ films.

2.5. Preparation of organophilic MMT clay

For the preparation of nanocomposites, hydrophilicnature of clay was changed into organophilic using cetyltri-methylammonium bromide (CH3 (CH2)15 N (CH3)3 Br). A15 g quantity of the purified Na-montmorillonite clay wasdispersed into 1200 ml of distilled water at 80 �C. Cetyltri-methylammonium bromide, 5.7 g, in 300 ml of distilledwater was poured into the hot montmorillonite/water solu-tion and stirred vigorously for 1 h at 80 �C. A white precip-itate formed, which was isolated by filtration and washedseveral times with hot water/ethanol (1:1) mixture untilno chloride was detected in the filtrate upon the additionof one drop of 0.1 M AgNO3 solution. The cetyltrimethyl-ammonium ion exchanged montmorillonite was then driedfor about 15 days at 75 �C, before being ground with amortar and pestle. The fraction collected was <50 lm.The organophilic nanoclay was stored in a desiccator [20]for further use.


2.6. Preparation of PBZs–organophilic clay hybrid

nanocomposites

Polybenzoxazine–clay hybrid nanocomposites were pre-pared by the solvent method according to the reported pro-cedure [21]. A 10 g aliquot of benzoxazine monomer wasfirst dissolved in 20 ml of 1,4-dioxane at 25 �C. The desiredamount of powdered organophilic clay (1, 3 and 5 wt.%)was slowly added to the monomer solution. Instead ofmechanical stirring, the homogeneous mixture was sub-jected to ultrasonication for effective collision and stirringat 70 �C for 2 h. The blends were casted on glass plate thatwas pre-treated with dichlorodimethylsilane. After dryingat 50 �C for 5 h, the films were cured at 100, 150, 200 and240 �C for 1 h each in an air oven. The obtained PBZ–orga-nophilic clay nanocomposites (Scheme 5) were transparentand red-wine colored, with thickness ranging from 0.3 to0.5 mm.

2.7. Instrumentation

2.7.1. SpectroscopyThe infrared spectra were recorded on a Perkin-Elmer

(Model RX1) Fourier transform infrared (FTIR) spec-trometer, with KBr pellets, for solid samples. The 1H and13C NMR spectra of benzoxazines were measured with aJEOL 500 MHz NMR spectrometer. Samples were dilutedby deuterated chloroform (CDCl3) and tetramethylsilanewas used as an internal standard.

Scheme 5. Schematic diagram of 1,1-bis(3-methyl-4-hydroxy phenyl)cyclohexane PBZ–clay intercalation.

2.7.2. Thermal properties

Glass transition temperature (Tg) of the samples wasdetermined using a DSC 200 PC differential scanning calo-rimeter (Netzsch Gerateban GmbH) in the temperaturerange between 50 and 250 �C at a heating rate of 10 �Cper min in nitrogen atmosphere. Thermogravimetricanalysis (TGA) was carried out using a DSTA 409 PCanalyzer (Netzsch Gerateban GmbH) at a heating rate of10 �C min�1 in nitrogen atmosphere.

2.7.3. X-ray diffraction studies

X-ray diffraction (XRD) patterns were recorded at roomtemperature by monitoring the diffraction angle 2h from0.5 to 40o as the low angle on a Rich Seifert (Model3000) X-ray powder diffractometer. The diffractometerwas equipped with Cu target (k = 0.154 nm) radiationusing a Guinier type camera employed as a focusing geom-etry and solid-state detector. Curved Ni crystal was used asa monochromator. The step width (scanning speed) usedwas 2h = 0.04�min�1. Bragg’s law (k = 2dsinh) was usedto compute the spacing.

2.7.4. Morphology

The surface morphology of fractured surface of the sam-ples was examined using a scanning electron microscope(SEM; JEOL JSM Model 6360). An Auto Fine Coater(JEOL JFC-1600) platinum sputtered the fractured surfaceof the samples at a vacuum pressure of 8 Pa/20 mA for95 s. The fractured surface of the samples was coated withplatinum before scanning.

3. Results and discussion

3.1. Spectral analysis

The structure of the synthesized benzoxazines wasexamined by FTIR, 1H and 13C NMR spectra.

3.1.1. FTIR (KBr)

BZAni: 947 cm�1 (N–C–O stretch), 1027 cm�1 (Ar–O–Csymmetric stretch), 1252 cm�1 (Ar–O–C asymmetricstretch), 1370 cm�1 (C–N stretch), 1501 cm�1 (substitutedbenzene ring), 2856 and 2927 cm�1 (symmetric and anti-symmetric CH2 bands of the cyclohexane group).

BZDDM: 943 cm�1 (N–C–O stretch), 1032 cm�1 (Ar–O–Csymmetric stretch), 1223 cm�1 (Ar–O–C asymmetricstretch), 1328 cm�1 (CH2 stretch of DDM), 1383 cm�1 (C–N stretch), 1509 cm�1 (substituted benzene ring), 2854 and2925 cm�1 (symmetric and anti-symmetric CH2 bands ofthe cyclohexane group).

BZDDE: 945 cm�1 (N–C–O stretch), 1026 cm�1 (Ar–O–Csymmetric stretch), 1224 cm�1 (Ar–O–C asymmetricstretch), 1070 and 1280 cm�1 (symmetric and asymmetricstretch of C–O–C), 1385 cm�1 (C–N stretch), 1504 cm�1

(substituted benzene ring), 2856 and 2929 cm�1 (symmetricand anti-symmetric CH2 bands of the cyclohexanegroup).


BZDDS: 939 cm�1 (N–C–O stretch), 1027 cm�1 (Ar–O–Csymmetric stretch), 1171 and 1267 cm�1 (SO2 asymmetricstretch), 1343 and 1387 cm�1 (C–N stretch), 1455, 1506and 1597 cm�1 (aromatic ring group), 2855 and 2922 cm�1

(symmetric and anti-symmetric CH2 bands of the cyclohex-ane group).

3.1.2. 1H NMR (ppm)

BZAni: d 1.45–1.51 (methyl proton of cresol, m,p-methy-lene protons of cyclohexane); 2.08–2.19 (Ar–CH coupledwith oxazine carbon); 4.19–4.53 (Ar–CH2–N of oxazinering); 5.29–5.30 (O–CH2–N); 6.60–7.23 (o-protons of cyclo-hexane, aromatic protons) (Fig. 1).

BZDDM: d 1.31–1.54 (methyl proton of cresol, m,p-meth-ylene protons of cyclohexane); 2.14–2.97 (Ar–CH coupledwith oxazine carbon); 3.82–3.92 (Ar–CH2–N of oxazinering, methylene proton of DDM); 4.53 (NH2 of DDM);5.31 (O–CH2–N); 6.68–7.32 (aromatic protons) (Fig. 3).

BZDDE: d 1.17–1.43 (methyl proton of cresol, m,p-meth-ylene protons of cyclohexane); 1.99–2.16 (Ar–CH coupledwith oxazine carbon); 4.14, 4.38 (NH2 of DDE); 5.14 (O–CH2–N); 6.45–6.91 (aromatic protons); 7.13 (CH protonsof DDE attached to N of oxazine ring) (Fig. 5).

BZDDS: d 1.27–1.85 (methyl proton of cresol, m,p-meth-ylene protons of cyclohexane); 2.07–2.31 (CH3, Ar–CHcoupled with oxazine carbon); 2.91–2.97 (o-protons of

Fig. 1. 1H NMR sp

cyclohexane); 4.60–4.87 (NH2 of DDS); 5.34 (O–CH2–N);6.67–6.73, 6.86–7.13, 7.26–7.41 (aromatic protons) (Fig. 7).

3.1.3. 13C NMR (ppm, d6-CDCl3)

BZAni: d 16.2–16.28 (methyl carbon); 23.06 (m-carbon ofcyclohexane); 26.51 (p-carbon of cyclohexane); 37.48 (aro-matic carbon attached to oxazine ring); 44.8, 49.2, 50.6,55.8 (Ar–C–N); 79.1 (O–C–N); 114.6–119.6, 120.6–129.7,140.1–152.3 (aromatic carbons) (Fig. 2).

BZDDM: d 16.2 (methyl carbon); 22.9 (m-carbon ofcyclohexane); 26.4 (p-carbon of cyclohexane); 37.4 (aro-matic carbon attached to oxazine ring); 40.6 (o-carbon ofcyclohexane); 44.8 (O–C–N); 50.7 (Ar–C–N); 79.4 (methy-lene carbon of DDM); 114.6–119.5, 121.1–129.7, 130.0–137.4, 140.4–162.6 (aromatic carbons); 146.7 (carbonattached to primary amine) (Fig. 4).

BZDDE: d 16.20–16.29 (methyl carbon); 23.03 (m-carbonof cyclohexane); 26.4, 29.7 (p-carbon of cyclohexane); 36.6,37.4 (aromatic carbon attached to oxazine ring); 44.8(O–C–N); 50.0 (Ar–C–N); 114.5–114.6, 116.4–116.5,117.2–117.3, 118.8–120.3, 121.7–129.6 (aromatic carbons);141.07–142.7 (carbon attached to primary amine);150.0–152.6 (aromatic carbon attached to ether of DDE)(Fig. 6).

BZDDS: d 15.8–16.6 (methyl carbon); 22.6–23.2 (m-car-bon of cyclohexane); 26.1–26.9, 29.7 (p-carbon of cyclohex-

ectra of BZAni.

Fig. 3. 1H NMR spectra of BZDDM.

Fig. 2. 13C NMR spectra of BZAni.


ane); 34.77–37.5 (aromatic carbon attached to oxazinering); 43.7–44.8 (O–C–N); 114.5–152.4 (aromatic carbons)(Fig. 8).

FTIR analysis was also performed to identify the self-polymerization of benzoxazines after curing thermally.The ring opening of benzoxazines confirms the formation

Fig. 4. 13C NMR spectra of BZDDM.

Fig. 5. 1H NMR spectra of BZDDE.


Fig. 6. 13C NMR spectra of BZDDE.


of PBZs. It is clearly shown that the peaks at 1161 cm�1, dueto asymmetric stretching of C–N–C, and at 1328 cm�1, dueto CH2 wagging in the benzoxazine structure [1], have com-pletely disappeared after curing at 240 oC. Instead, a peakappears at 1489 cm�1 for a substituted benzene ring, whichsuggests that the ring opening of benzoxazine precursoraffords PBZ [1]. In addition, the IR spectra shows that theabsorption bands at 522 and 1043 cm�1 observed beforeand after curing are due to the Si–O vibration of OMMT,indicating the presence of a layered silicates framework inthe hybrid PBZ–clay [22].

3.2. Glass transition temperatures and curing behavior

Table 1 shows the data of differential scanning calorim-etry (DSC) thermograms of unfilled PBZ and the PBZ–claynanocomposites. The DSC of unfilled PBZ was carried outto examine the effect of the OMMT and the type of organicmodifier used for the modification of the MMT clay sur-face on the curing of PBZ. DSC thermograms of unfilledPBZ and 1, 3 and 5 wt.% organophilic nanoclay-filledPBZ are shown in Table 1. PBZAni, PBZDDM, PBZDDE

and PBZDDS are represented as XA, XB, XC and XD,respectively. The onset of the exotherm due to the curingof the unfilled XA, XB, XC and XD starts at 222, 228,213 and 251 �C, respectively, with respective maxima at238, 244, 225 and 258 �C. However, in the presence of 5%OMMT, the onset of the exotherm starts at 198, 204, 191

and 226 �C, respectively, with respective maxima at 212,219, 201 and 235 �C, respectively. The decrease in onsetof the ring opening polymerization is due to cetyltrimethylammonium ions on the OMMT surface and can readilyparticipate in the curing reaction which favors the partialexfoliation of the clay platelets [23]. The glass transitiontemperature, Tg, decreases with increasing clay content[24]. The main reason for this phenomenon is that the con-version of PBZ decreases with increasing organophilicnanoclay content. The data further indicated that reachinga high value for the conversion was difficult at a high claycontent. In other words, the value of Tg is lower when theconversion is low. A single exothermic peak was observedfor each curing system, but the sharp exothermic peakfor the unfilled PBZ system occurred at a relatively highertemperature. From the above observation, it is found thatthis heterocyclic ring opening curing reaction involves asingle exothermic chemical process. For PBZ–clay systems,the exothermic peak was smooth and broad, and it shiftedto a lower temperature upon increasing the clay content.This shows that the curing reaction was delayed in theunfilled PBZ system and required a high temperature. Thisfurther supports the finding that the higher curing temper-ature increased the curing rate and decreased the curingtime.

From the values of Tg, it is observed that, of the fourtypes of PBZs, the XD has the highest Tg (258 �C) andXC has the lowest (225 �C); the values of Tg for XA and

Fig. 7. 1H NMR spectra of BZDDS.


XB lie in between. The values of Tg for PBZs follow theorder XD > XB > XA > XC. The presence of the sulphonylgroup of DDS in XD accounts for its high Tg. The ethergroup of DDE in XC accounts for its low Tg. The methy-lene group of DDM in XB means that it has a higher valueof Tg (244 �C) than XA (238 �C), where a single aromaticgroup is present.

Comparing the three characteristic diamines (DDM,DDE and DDS are referred to as B, C and D, respectively)used in the preparation of the PBZs, B shows lower reactiv-ity than D and higher reactivity than C. This variation inreactivity could be due to the electronic effect. The electronwithdrawing group –SO2– in D reduced the electron den-sity of the amine nitrogen and consequently showed lowerreactivity. The electron-donating group –CH2– in B wouldenrich the electron density of the amine group and enhancethe reactivity of the amine. The strong electron-donatingether group –O– in C shows much higher reactivity thanB. Since the pair of single electrons in the amine group ofaniline is in resonance with the aromatic ring, the reactivityof A will be lower than that of C. Though it has one aro-matic group less than the other three diamines, the tightpacking of stable single units and its high crosslink densityrestricts the movement of the polymer chains. This wouldprobably be predominant in C, and the flexible behaviorof the ether group in C would also account for its lower

Tg. The higher reactivity accelerates the rate of the reactionand reduces the curing temperature, which in turndecreases the value of Tg.

3.3. Thermal stability

The PBZ–clay nanocomposites displayed higher decom-position temperatures than unfilled PBZ and the thermalstability of the nanocomposites was improved by the pres-ence of dispersed organophilic nanoclay layers, which actas barriers to minimize the permeability of the volatile deg-radation products from clay-filled PBZ nanocomposites[18]. Table 2 summarizes the TGA values for these nano-composites. From the degradation temperatures for 20,40 and 60 wt.% losses, the TGA values for the PBZ–claysamples show a clear trend of improved thermal stabilityupon increasing the clay content. The decomposition ofthe samples takes place in single stage.

The OMMT alone starts to degrade at 219 �C. Forunfilled PBZ alone the weight loss starts at 280 �C; thisgradually increases with the incremental addition of theorganophilic nanoclay up to 343 �C. For example, the ini-tial degradation temperature for unfilled XA, XB, XC

and XD is 280, 309, 298 and 288 �C, respectively, whereas5% organoclay-filled XA, XB, XC and XD start to decom-pose at 319, 343, 336 and 316 �C, respectively. The thermal

Fig. 8. 13C NMR spectra of BZDDS.

Table 1DSC data of Polybenzoxazines-clay hybrid nanocomposites.

Polybenzoxazines–clay Exotherm (�C)

Onset Max End

XA0 222 238 246XA1 213 226 235XA3 207 220 228XA5 198 212 221XB0 228 244 252XB1 218 233 239XB3 211 228 235XB5 204 219 229XC0 213 225 237XC1 203 214 224XC3 196 208 219XC5 191 201 213XD0 251 258 263XD1 241 249 256XD3 233 240 249XD5 226 235 243

X – PBZ; XA – PBZAniline; XB – PBZDDM; XC – PBZDDE; XD – PBZDDS

0,1,3,5 – organophilic nanoclay content (wt.%).

Table 2TGA data of Polybenzoxazines-clay hybrid nanocomposites.

Polybenzoxazines-clay

Initialdecompositiontemperature (�C)

Temp. atcharacteristicWeight loss (�C)

Char yield(%) 800 �C

20% 40% 60%

OMMT clay 219 410 – – 65.52XA0 280 374 402 452 33.57XA1 294 397 428 491 35.80XA3 305 423 452 534 37.76XA5 319 440 495 586 40.81XB0 309 396 454 552 42.13XB1 322 418 488 590 43.73XB3 335 448 537 621 46.53XB5 343 463 561 637 48.12XC0 298 390 432 492 33.61XC1 311 418 448 558 34.80XC3 322 438 478 592 36.92XC5 336 472 532 624 39.60XD0 288 364 388 420 29.48XD1 295 373 403 444 31.02XD3 303 392 424 471 32.46XD5 316 421 473 549 35.77

X – PBZ; XA – PBZAniline; XB – PBZDDM; XC – PBZDDE; XD – PBZDDS

0,1,3,5 – organophilic nanoclay content (wt.%).


decomposition of those nanocomposites shifts towards thehigher temperature range, which confirms the enhancementof thermal stability of the organoclay nanocomposites.This shows that the organophilic nanoclay resists weightloss better than unfilled PBZ. Above 700 �C, almost allthe curves became smooth and straight, indicating the pres-ence of inorganic residue (MgO, Al2O3 and SiO2) [25].

Hence it is observed that the decomposition product ismainly due to the polymer–matrix and not from the clayparticles. The char yield of OMMT is 65.5% at 800oC.The char yield for unfilled XA, XB, XC and XD is 33.6,


42.1, 33.6 and 29.5%, respectively. For 5% organophilicnanoclay-filled XA, XB, XC and XD, the char yield is40.8, 48.1, 39.6 and 35.8%, respectively. The PBZ–claynanocomposites clearly have a greater char yield, whichincreases upon increasing the clay content, as expected.The increase in char yield suggests the reduction of thepolymer’s flammability [15] and implies good thermal sta-bility. The incorporation of silicate layers into PBZenhances its thermal stability by acting as a superior insu-lator and mass transport barrier to the volatile productsproduced during decomposition [26,27].

Among the four types of PBZs, the initial decomposi-tion temperatures for unfilled and clay-filled PBZs in XB

and XC are high due to their high reactivity. The presenceof electron donating and the flexible behavior of the meth-ylene and ether groups retard their degradation. The lowreactivity, rigidity and electron-withdrawing nature of thesulfonyl group in XD causes its initial decomposition tem-perature to be lower than those of XB and XC. XA showslower thermal stability owing to its low aromatic ring con-tent. The thermal stability of unfilled and clay-filled PBZsfollows the order XB > XC > XD > XA.

3.4. X-ray diffractions

XRD was employed to characterize the layered struc-tures of the organoclay and PBZ–clay nanocomposites;changes in the value of 2h (the angle between the diffractedand incoming X-ray waves) reflect changes in the gallerydistance of the clay. Fig. 11a–e shows the XRD patternof the PBZ-intercalated and partially exfoliated clay nano-composites, which shows the d-spacing. The interlayer dis-

Fig. 9. SEM photographs of PBZs: (a) PBZAni;

tance is obtained from Bragg’s law 2dsinh = nk, where h isthe (angle of diffraction) Bragg angle, k is the wavelengthused and n is the order of the crystalline plane. The organo-clay exhibits two peaks, at 5.2 and 7.52o, which correspondto a basal space of 1.69–1.73 nm. The insertion of the PBZbetween the galleries of the OMMT [28,29] increased the dspacing from 1.69 to 2.10 nm. The data obtained from theresults indicate that the PBZ was effectively intercalatedand partially exfoliated into the galleries of the clay nano-particles. Fig. 11b–e presents the XRD patterns for 3%OMMT filled PBZ nanocomposites.

3.5. Morphology

SEM micrographs of the fractured surfaces of the purePBZ systems indicate rough and glassy microstructures(Fig. 9a–d). This supports the brittle nature and inferiorimpact strength of the pure PBZ system. The micrographof the fractured surface indicates the homogeneous struc-ture of PBZs. Fig. 10a–d shows SEM micrographs of theorganoclay-filled PBZ hybrid nanocomposites. Theorganoclay-filled PBZ hybrid nanocomposites show asmooth and homogeneous surface morphology, indicatingthat the PBZ is compatible with layered silicate clay havingan organically modified surface. However, the clay layersdo not occupy the full volumes of the PBZ matrix andare separated over the whole of the matrix, which confirmsthe intercalation [30,31] of organoclay with the PBZ sys-tems. The efficient adhesion arises due to the influence ofintermolecular specific interactions between cetyltrimethyl-ammonium ion-modified nanoclay and PBZ matrix sys-tems, and hence leads to the formation of nanocomposites.

(b) PBZDDM; (c) PBZDDE; and (d) PBZDDS.

Fig. 10. SEM photographs of PBZ–organoclay nanocomposites: (a) PBZAni/3% organoclay; (b) PBZDDM/3% organoclay; (c) PBZDDE/3% organoclay;and (d) PBZDDS/3% organoclay.

Fig. 11. XRD patterns of PBZ–organoclay nanocomposites: (a) PBZAni/3% organoclay; (b) PBZDDM/3% organoclay; (c) PBZDDE/3% organoclay;and (d) PBZDDS/3% organoclay.


4. Conclusions

Novel high-performance benzoxazines and PBZs wereprepared from 1,1-bis(3-methyl-4-hydroxy phenyl)cyclo-

hexane, characteristic aromatic diamines and paraformal-dehyde. Polybenzoxazines filled with organoclay (up to 5wt.%) showed higher thermal stability than that of unfilledPBZs. The introduction of 3 wt.% organoclay into PBZshad a significant effect on the improvement of thermaland morphological properties. The char yields of PBZ–claynanocomposites were increased with increasing concentra-tion of OMMT clay content in PBZ. DSC traces of PBZ–clay nanocomposites indicated that the onset of the ringopening of the benzoxazine units in the presence of organo-clay occurred at comparatively lower temperatures thanthat of unfilled PBZ. This suggests that the clay surfacehas a catalytic effect on the ring-opening self-polymeriza-tion of PBZ–clay nanocomposites. The glass transitiontemperature (Tg) was decreased upon increasing theorganoclay content. The PBZ–clay nanocomposites exhib-ited partly exfoliated and intercalated structures for theincorporation of 3 wt.% organoclay into unfilled PBZs.The assessment of the mechanical properties of the PBZ–clay nanocomposites is currently being carried out. Thedesigning of novel monomers with advantageous charac-teristics is a promising approach to preparing high-perfor-mance PBZs. Thermal and morphological properties werecharacterized to design PBZs with excellent high-tempera-ture properties. The PBZ resins and clay nanocompositesdeveloped in this study could be good candidates for coat-ings, sealants, adhesives and matrix resins of advancedcomposite materials. They could also be used in the formof coatings for the fabrication of industrial componentsused in heavy thermal industrial and engineeringenvironments.


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