Pd

Ha

b

a

ARRA

KETMS

1

mayammaewr(r[

i

0d

Materials Chemistry and Physics 132 (2012) 950– 956

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

jo u rn al hom epage : www.elsev ier .com/ locate /matchemphys

reparation and characterization of high performance Schiff-base liquid crystaliepoxide polymer

uan Liua,b, Zi-en Fua,b, Kai Xua,∗, Hua-lun Caia,b, Xin Liua,b, Ming-Cai Chena

Key Laboratory of Organic Polymer Material for Electronics, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, P.O. BOX 1122, Guangzhou 510650, PR ChinaGraduate School of the Chinese Academy of Sciences, Beijing 100049, PR China

r t i c l e i n f o

rticle history:eceived 5 March 2011eceived in revised form 1 November 2011ccepted 19 December 2011

eywords:poxyhermal propertiesorphology

chiff-base

a b s t r a c t

A novel Schiff-base liquid crystal diepoxide polymer was prepared via a thermal copolymerization of aSchiff-base epoxy monomer (PBMBA) with a diamine co-monomer (MDA). We first proposed that specificeffects of highly conjugated Schiff-base moiety on thermal properties of the Schiff-base epoxy polymer(PBMBA/MDA). Thermal degradation behavior of the polymer was characterized using thermogravimet-ric analysis (TGA) under nitrogen and under air, respectively. Thermogravimetric data obtained from TGAunder nitrogen and under air reveal that PBMBA/MDA exhibits higher thermal stability compared withbisphenol-A type epoxy polymer (DGEBA/MDA) and other mesogene-containing epoxy polymer. It isworth pointing out that the outstanding residual char value for the Schiff-base epoxy polymers had beenrarely reported. For thermal degradation mechanism of PBMBA/MDA under nitrogen, thermogravimet-ric analysis/infrared spectrometry (TG-IR) were used to investigate volatile components, and scanningelectron microscopy/energy dispersive spectroscopy (SEM/EDS) was used to explore morphologies and

chemical components of the residual char. The effects of calcination temperature and calcination time onevolution of morphologies and chemical components of the residual char have been studied. It is proposedthat the highly �-conjugated Schiff-base moiety is not only involved in a formation of intramolecularhydrogen bonding increasing the onset thermal degradation temperature (Td), but also possesses aneffective charring ability retarding a further degradation of polymers. Due to the presence of the specificeffects, the thermal stability of the Schiff-base epoxy is improved.

. Introduction

As the important branch of polymer materials science, the newaterials and innovations in the field of epoxy polymer materi-

ls have gained great developments and advancements in recentears. However, due to the major limitations on thermo-stabilitynd flammability, higher-performance multifunctional epoxy poly-ers should be prepared for applying in more areas and underore special environmental conditions [1–4]. There exist two

pproaches to achieve higher-performance in epoxy polymers gen-rally known as the ‘additive-type’ and the ‘reactive-type’. Alongith the ‘additive-type’ approach, the ‘reactive-type’ approach, car-

ied out by chemical structure modification such as rigid groupsaromatic or imide) introduction, phosphorus and silicon incorpo-ation, can also improve the thermal stability of the epoxy polymers

5–9].

Schiff-bases (known as azomethines), containing rigid C Nmine groups and benzene rings at alternate positions in the

∗ Corresponding author. Fax: +86 20 85231058.E-mail address: xk@gic.ac.cn (K. Xu).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.12.040

© 2011 Elsevier B.V. All rights reserved.

main chain, exhibit potential as materials for wide spectrum ofapplications, particularly as corrosion inhibitors [10,11], liquidcrystal [12,13], thermo-stable materials [14,15], and a metal ioncomplexing agents [16,17]. It has been widely reported that thepoly(azomethine)s (known as polyimines or Schiff-base polymers)were synthesized by condensation polymerization of dialdehydes(or diketones) with diamines. Recently, liquid crystalline epoxyresins containing Schiff-base as mesogenic groups in their back-bone have been investigated. Mormann et al. [18] studied theeffect of polymerization reaction rate of mesogenic Schiff-basediepoxides on phase behavior. Lee et al. [19] investigated the cur-ing characteristics of diaromatic Schiff-base mesogenic diepoxidemonomer in the presence of aliphatic diamine. Ochi et al. [20] syn-thesized the mesogenic diepoxide and studied its adhesive bondingproperties. However, there are rare reports that dedicate to the spe-cific effect of the Schiff-base moiety on the thermal degradation ofthe Schiff-base epoxy.

In this work, we have synthesized the liquid crystal diepox-

ide oligomer (PBMBA), alternately having imine groups (C N)and benzene rings in the main chain, and being �-conjugatedthrough rigid molecular scaffolds, in a manner similar to themethod reported in Ref. [20]. To our knowledge, this liquid

H. Liu et al. / Materials Chemistry and Physics 132 (2012) 950– 956 951

and t

chsmtfPtAbu(Poeomnwcfwwcstlb

2

2

aTS4raCaC

Fig. 1. Synthetic routes of PBMBA

rystal diepoxide was investigated since the early 1998s [18–20],owever, we noted that these studies did not investigate thepecific effects of highly conjugated Schiff-base moiety on ther-al properties of the polymer, but rather explore its LC phase

ransitions. Via the thermal copolymerization method, being dif-erent from it reported by others [18–20], the novel polymer forBMBA/MDA was prepared. TGA, TG-IR and SEM/EDS were usedo investigate the thermal degradation behavior of PBMBA/MDA.s one aim of this study, the thermal and thermal-oxidative sta-ilities of the thermosets were studied by TGA under nitrogen andnder air. In comparison to the bisphenol-A type epoxy polymerDGEBA/MDA) and other mesogene-containing epoxy polymer,BMBA/MDA exhibits higher thermal stability. In particular, theutstanding residual char value under nitrogen for the Schiff-basepoxy polymer had been rarely reported. Therefore, a second aimf this study was to further understand the thermal degradationechanism. With the help of TG-IR technique the volatile compo-

ents from thermal degradation of PBMBA/MDA under nitrogenere investigated. Via an ingenious design, PBMBA/MDA was

alcined by TGA under different calcination temperatures and dif-erent calcination time, respectively. Two series of residual charere obtained by quenching in liquid nitrogen. The SEM/EDSas used to explore the evolution of morphologies and chemi-

al components of the residual char. From the point of view oftructure-properties relationship, it can be considered that owingo the presence of the Schiff-base group, the high performanceiquid crystal diepoxide polymer displayed improved thermal sta-ility.

. Experimental

.1. Materials

All reagents and solvents were purchased as reagent grade,nd were purified or dried by standard methods before use.he following reagents were purchased from Aladdin-reagent Co.hanghai, China: terephthalaldehyde, p-aminophenol, aniline and, 4′-diaminodiphenylmethane (MDA). Analytical grade zinc chlo-ide obtained from Shanghai Chemical Reagent, Ltd. China was used

s catalyst. Epichlorohydrin (ECH) was purchased from Lingfenghemical Reagent Co. Shanghai, China. DGEBA with epoxy equiv-lent weight (EEW) of 185 g equiv.−1 was purchased from Shellhemical Co.

he chemical structures of DGEBA.

2.2. Synthesis of PBMBA

The synthetic pathways of PBMBA are shown in Fig. 1. Theprocedures were carried out in a four-neck round-bottom flask,equipped with a nitrogen inlet and stirrer under controlled tem-perature and pressure. Schiff-base type diphenol (DPBA) wassynthesized from terephthalaldehyde 20.0 g (0.15 mol) and p-aminophenol 30.0 g (0.3 mol) in a solvent mixture [dimethylsulfoxide (DMSO) 150 mL, ethanol (EtOH) 300 mL] using zinc chlo-ride (0.5 g) as the catalyst. The reaction mixture was stirred at85 ◦C for 4 h and then poured into 1000 mL of H2O. A yellowsolid was collected by filtration, recrystalized, and dried. PBMBAwas synthesized from 0.1 mol of DPBA, 5 mol of ECH, and 20 mLof DMSO as a solvent using 22.0 g of a 40 wt% NaOH solution asthe catalyst.

DPBA: yield: 42.0 g, 90%. mp: 275 ◦C. 1H NMR: (400 MHz, DMSO-d6): ı 9.57 (1H, single, OH), 8.57 (1H, single, N CH), 7.80 (2H, single,Ph), 7.24–7.26 (2H, multiplet, Ph), 6.80–6.82 (2H, multiplet, Ph).13C NMR: (400 MHz, DMSO-d): ı 160.0, 157.0, 144.6, 138.9, 129.5,122.5, 117.2. FT-IR (KBr cm−1): 3384 (OH), 1619 (CN). MS: m/z 316[M+]. EA (%): found: C, 75.77; H, 5.15; N, 8.81; O, 10.27. Calc. forC20H16N2O2: C, 75.93; H, 5.10; N, 8.86; O, 10.11.

PBMBA: mp: 198–203 ◦C. 1H NMR: (400 MHz, CDCl3-d3): ı 8.51(1H, single, N CH), 7.97 (2H, single, Ph), 7.24–7.26 (2H, multi-plet, Ph), 6.94–6.96 (2H, multiplet, Ph), 4.23–4.27 (2H, multiplet,CH2), 3.95–4.00 (2H, multiplet, CH2), 3.53–3.38 (1H, multiplet, CH),2.90–2.91 (2H, multiplet, CH2), 2.76–2.78 (2H, multiplet, CH2). 13CNMR: (400 MHz, CDCl3-d): ı 157.5, 157.4, 145.1, 138.6, 128.9, 122.4,115.2, 77.3, 77.0, 76.7, 69.0, 50.1, 44.7. FT-IR (KBr cm−1): 3384 (OH),1610 (CN), 914 (epoxy ring). EEW: 225 g equiv−1.

2.3. Preparation of samples

2.3.1. Thermal copolymerization of PBMBA with MDAPBMBA was dissolved completely in chloroform, and then stoi-

chiometric amount of MDA was added. Afterward, the solvent wasremoved by rotary evaporator under reduced pressure. A mixturecontaining the stoichiometric amounts of DGEBA and MDA was pre-pared in the same way as mentioned above. The mixture of PBMBA

and MDA was rapidly melted at 200 ◦C, then cured for 2 h at 180 ◦C,for 2 h at 220 ◦C, for 2 h at 260 ◦C and for 1 h at 290 ◦C. The DGEBAand MDA mixture was cured at 100 ◦C for 2 h, at 150 ◦C for 2 h, at180 ◦C for 2 h, at 190 ◦C for 1 h and at 260 ◦C for 1 h.

952 H. Liu et al. / Materials Chemistry and Physics 132 (2012) 950– 956

polym

2

bow7prgTaPhrs

2

(cdaEsTt5as(FfscQat

3

3

at(

Fig. 2. TG (a) and DTG (b) thermograms of the epoxy

.3.2. Preparation of SEM/EDS samplesThe samples prepared for SEM/EDS observation were obtained

y TGA calcination under nitrogen. One series of residual char werebtained at the different calcination temperatures. PBMBA/MDAas heated from room temperature to 400 ◦C, 500 ◦C, 600 ◦C and

00 ◦C at a heating rate of 10 ◦C min−1, respectively. At the end-oints of these given calcination temperatures, the samples wereapidly taken out from TGA pool and quenched in liquid nitro-en. The corresponding samples for this purpose were named as-400, T-500, T-600 and T-700. In isothermal calcination process,nother series of residual char were obtained by rapidly heatingBMBA/MDA to 440 ◦C (Tmax for PBMBA/MDA under nitrogen) andolding at that temperature for 1 min, 2 min, 3 min and 5 min,espectively, named as Tim-1, Tim-2, Tim-3 and Tim-5. All of theamples were dried in vacuum before reading for imaging.

.4. Characterization

1H NMR spectrum was recorded on a Bruker AM-400S400 MHz) spectrometer using TMS as internal standard andhloroform-d3 (at 7.26 ppm), DMSO-d6 (at 2.50 ppm) and acetone-6 (at 2.05 ppm) as solvents. FT-IR spectrum was recorded withn Analect RFX-65A infrared spectrophotometer using KBr discs.A was performed on a PE 2400 Series II CHNS/O Analyzer. Masspectrum was determined on a VG-7070E spectrometer (EI, 70 eV).he TGA measurement was carried out with a Perkin-Elmer TGA-6hermogravimetric analyzer over a temperature range of between0 ◦C and 700 ◦C at a heating rate of 10 ◦C min−1 under nitrogennd under air, respectively. Thermogravimetric analysis/infraredpectrometry (TG-IR) of the samples was recorded using a TG-209Netzsch, Germany) instrument that was interfaced to a Vector 22T-IR (Bruker, Germany) spectrometer. The samples were heatedrom 20 ◦C to 700 ◦C at a heating rate of 10 ◦C min−1 (nitrogen atmo-phere, flow rate of 40 mL min−1). The morphologies and chemicalomponents of the residual char were characterized using a FEIuanta 400 FEG scanning electron microscope (SEM) connected ton EDAX Genesis energy dispersive spectroscopy (EDS). The elec-ron beam energy was 20 KeV in all cases.

. Results and discussion

.1. Thermogravimetric analysis (TGA) of the polymers

TG and DTG curves of PBMBA/MDA and DGEBA/MDA in nitrogenre presented in Fig. 2a and b. From the TG curve, an onset degrada-ion temperature (Td) evaluated by temperature of 5% weight lossT5%) and temperature of 10 wt% weight loss (T10%) of PBMBA/MDA

ers under nitrogen at a heating rate of 10 ◦C min−1.

were 389 ◦C and 432 ◦C; the residual char at 700 ◦C of PBMBA/MDAwere 70%. From the DTG curve, the temperature of the maximumweight loss rate (Tmax) of PBMBA/MDA was 439 ◦C. The values ofT5% and T10%, important parameters for evaluating the thermal sta-bility of polymer materials, for DGEBA/MDA and for tetramethylstilbene-based epoxy/MDA system reported by Lin et al. [21] wereall lower than those for PBMBA/MDA.

Usually, thermal degradation of any epoxy resin starts fromthe dehydration of the secondary alcohol, leading to the forma-tion of vinylene ethers [22–24]. However, due to rigid C N iminegroups and benzene rings in the main chain alternately being�-conjugated, the secondary alcohols as hydrogen donors mayform a network of intramolecular hydrogen bonding assisted by�-electron delocalization [16,25,26]. The intramolecular hydrogenbonding network may limit the movement of secondary alcohol,thus decreasing the reactivity of dehydration of the group. Thiseffect is inferred to be responsible for the increase in the onsetthermal degradation temperature. As shown in Fig. 2b, the epoxypolymers exhibited one stage of weight loss, and accordingly therewere only one differential thermogravimetric (DTG) peak. The Tmax

and residual char of PBMBA/MDA were significantly higher thanthose of DGEBA/MDA. Noteworthily, PBMBA/MDA presented a suf-ficiently slow weight loss rate and a dramatic high residual char at700 ◦C. These results have not been reported in literature reviewedfor an epoxy polymer. Generally, at high temperatures, the thermaldegradation products of the thermosets with highly aromatic bridg-ing groups, such as naphthalene and phenanthrene, are obtainedby ‘pyro-synthesis’ and aromatization reactions [27]. The highlyconjugated Schiff-base moiety with imine groups (C N) and ben-zene rings in the main chain alternately possesses the significantcharring tendency, implying a significant amount of carbonaceousresidues probably left. Presumably, the Schiff-base moiety appearsto have made important contribution to promote the formationof residual char and the formed char layer with relatively highpyrolysis resistance retards the further degradation of the ther-mosets. In summary, the Schiff-base moiety, possessing the abilitiesto increase the hydrogen bonding ability and promote the forma-tion of residual char, can improve the thermal degradation stabilityof the Schiff-base epoxy polymer.

3.2. TG-IR analysis of PBMBA/MDA

Under nitrogen, the volatile components from the thermal

degradation of PBMBA/MDA were investigated by TG-IR. The TG-IRspectra of the gas phase products obtained from the thermal degra-dation of PBMBA/MDA at the different temperatures are presentedin Fig. 3. At the onset degradation temperature 389 ◦C, the bands

H. Liu et al. / Materials Chemistry an

Fig. 3. FT-IR spectra of volatilized products during the thermal degradation ofPBMBA/MDA at various temperatures by TGA under nitrogen.

oi2p((rabctt5wCs

aoamtcp

components of the residual char, it is helpful to further under-stand the mechanism of thermal degradation of polymers. The

bserved at 1508 cm−1 and 1610 cm−1 were related to character-stic absorption peaks of aromatic ring. The bands at 1172 cm−1,360 cm−1 and 3250 cm−1 suggested the formation of volatileroducts containing hydrocarbons (C–H stretching), CO2 and –OHsuch as H2O, phenol). When the temperature was raised to 432 ◦CTmax), the intensity of peaks at 1508 and 1610 cm−1 becameather strong and the peak at 3015 cm−1 for the N–H stretchingppeared, implying that the MDA units had decomposed. This maye attributed to the fact that due to the further decomposition thehain scission of the secondary amine group (C–NH) may result inhe amine volatilizing as a part of the chain fragments. In relationo the absorption bands described above, at higher temperatures,18 ◦C and 683 ◦C, the main absorption bands were associatedith C–H (1172 cm−1), aromatic ring (1508 cm−1 and 1610 cm−1),O2 (2270 and 2360 cm−1), H2O and phenol (3250 cm−1) and N–Htretching (3015 cm−1).

In addition, the appearance of alkane formed in a wide temper-ture range from 380 ◦C to 680 ◦C may result from the degradationf the residual char accumulating at the surface of the epoxy matrixnd acting as a barrier to inhibit the further degradation of the ther-osets with the temperature increasing. With the appearance of

he main degradation products such as CO2, phenol, and hydro-

arbons, the process of thermal degradation associated with therocess of depolymerization of the thermoset was confirmed.

Fig. 4. TG (a) and DTG (b) thermograms of the epoxy po

d Physics 132 (2012) 950– 956 953

3.3. Thermal oxidation

Fig. 4a and b illustrates the TG and DTG thermograms of the ther-mosets under air. The T5% and T10% of PBMBA/MDA being 362 ◦C and415 ◦C were higher than those of DGEBA/MDA. However, comparedwith the tetramethyl stilbene-based epoxy polymer, PBMBA/MDAfailed to exhibit the higher values of T5% and T10% as it did undernitrogen [21]. As seen in Fig. 4b, PBMBA/MDA performed onedegradation stage and a relatively high rate of weight loss from450 ◦C to 650 ◦C and the corresponding Tmax was 584 ◦C, whileDGEBA/MDA showed two degradation stages and the correspond-ing Tmaxs were 400 ◦C and 632 ◦C. By comparing the values ofresidual char of these two thermosets at the temperature rangeof between 50 ◦C and 630 ◦C, it was found that PBMBA/MDA pre-sented higher residual char values than DGEBA/MDA. At 400 ◦C, thetemperature for first degradation stage of DGEBA/MDA, the resid-ual char of PBMBA/MDA and DGEBA/MDA were 91.6% and 83.2%; at584 ◦C, the temperature for first degradation stage of PBMBA/MDA,the residual char of PBMBA/MDA and DGEBA/MDA were 41.2% and31.4%. From 630 ◦C to 700 ◦C, PBMBA/MDA showed a relative lowresidual char as well as DGEBA/MDA.

Basically, the mechanism for the thermal oxidation of theamine-cured epoxies is that the oxygen attacks on the nitrogen,leading to the formation of carbonyl groups (isomerization of theamine-cured epoxy). The carbonyl groups are further decomposed,which can result in chain splitting [22,28,29]. Additionally, the ther-mal oxidation process depends on the strengths of the bonds withinthe network functional groups. In the case of PBMBA, due to thenetwork of intramolecular hydrogen bond formed between the sec-ondary alcohols and the conjugated Schiff-base moiety, the thermalmotion of the network chains is suppressed, resulting in the rela-tively high onset degradation temperature. Besides, the Schiff-basemoiety possessing the high charring ability may promote the for-mation of residual char which can serve as a ‘polymer-protector’during combustion and retard the thermal oxidative decomposi-tion. It is probably that the residual char containing amount ofcarbon is burned away in air with the increase of temperature,leading to a nearly complete thermal oxidative degradation of thethermoset.

3.4. SEM/EDS analysis

Via the investigation of surface morphologies and chemical

SEM/EDS observation is a novel method for investigation of thethermal degradation behavior of polymers that provides detailed

lymers under air at a heating rate of 10 ◦C min−1.

954 H. Liu et al. / Materials Chemistry and Physics 132 (2012) 950– 956

Fe

inrio4tpafacpcoTcmoawtTs5tr

Fd

ig. 5. SEM morphologies (a: magnification 400×) and EDS spectra (I and II fornergy spectral acquisition domains) for T-400.

maging information about the morphologies and chemical compo-ents of residue [30,31]. With a review of early works, few literatureeported that the process of thermal degradation of epoxy was stud-ed by means of the SEM/EDS. As described above, the Td and Tmax

f PBMBA/MDA detected by TGA under nitrogen were 389 ◦C and39 ◦C. According to the result, the sample obtained at the givenemperature 400 ◦C was chose as the initial one to observe the mor-hology and chemical components of the residual char. For T-400,

macropore with some micro-cracks appeared in the compact sur-ace of the residual char (see Fig. 5). The chemical components of

compact surface domain (I) and a light part of the sample (II)onsisted of carbon, nitrogen and oxygen. With the increase of tem-erature, at 500 ◦C, the residual char surface was highly porous andontains micro-cracks (see Fig. 6 for T-500). With the appearancef more macropores, the sizes of the macropores were increased.he result of chemical components of T-500 showed the carbonontent of a domain of compact surface (I) was higher than that ofacropores domain (II). For T-600, Fig. 7 shows that the macrop-

re grew into a large crack, which contained many micro-cracksnd macropores and presented the tendency to rupture. Comparedith domain II, domain I possessed the higher carbon content and

he lower oxygen and nitrogen contents. The morphology of the-700 showed the compact surface and cracks evolved into mas-ive amorphous powders with a mean size ranging from 10 �m to

0 �m. As can be seen from Fig. 8, these powders accumulated onhe surface, leading to the formation of inter-particle distance withegions up to 1-20 �m. The EDS spectra of this sample showed that

ig. 6. SEM morphology and EDS spectra (I and II for energy spectral acquisitionomains) for T-500.

Fig. 7. SEM morphology and EDS spectra (I and II for energy spectral acquisitiondomains) for T-600.

the chemical components of the T-700 surface consisted of carbonand oxygen (I).

Observations of the residues recovered after calcination usingTGA clearly reveal the presence of a continuous dark layer coveringthe residues of the Schiff-based epoxy matrix from the beginning ofcalcination. The morphological structures and the chemical compo-nents of the residual char surfaces are different and variable duringthe thermal degradation process. The evolution of the morpholo-gies of the residual char surfaces apparently suggests that manyintricate reactions and some physical processes occur during thethermal degradation process. The chemical reaction results in theformation of the carbon-rich char and the volatile components. Thestructured carbon-rich char layer acts as a shield and re-emits muchof the incident radiation back into the gas phase, decreasing thethermoset degradation rate. As can be observed, with the increaseof temperature, some macropores with micro-cracks appearing inthe compact surface of the samples evolve into large and contin-uous cracks with many micro-cracks and macropores. When thecontinuous cracks are broken down, discontinuous residues formand accumulate on the surface in the form of powder. This behavioris ascribed to the disruption and breakdown of the compact surfaceand cracks. With the mean size ranging from 10 �m to 50 �m, thepowders possessing high specific surface area give a good disper-sion in polymer matrix. The increased surface area expose to the

heat implies that an increase in fuel for combustion is available fordegradation. The dispersion of the powders is believed to improvethe residue strength. Furthermore, the high specific surface area

Fig. 8. SEM morphologies (a: magnification 4000×) and EDS spectra (I for energyspectral acquisition domains) for T-700.

H. Liu et al. / Materials Chemistry and Physics 132 (2012) 950– 956 955

Fd

ih

ostvcdItdpitDdtr

mvTbpckw(swaFaIifiedndacn

t

Fig. 10. SEM morphology and EDS spectra (I and II for energy spectral acquisitiondomains) for Tim-2.

obviously evolution observed in the surface morphologies, whichis also an important piece of evidence for the retardation effectof carbon-rich char during the thermal degradation. It is probably

ig. 9. SEM morphology and EDS spectra (I and II for energy spectral acquisitionomains) for Tim-1.

mproves heat transfer performance of the polymer matrix, andence results in an overall improvement in residue yield.

Based on the analysis of chemical components of the samplesbtained at these different temperatures, it is found that the mas-ive carbon, nitrogen and oxygen aggregate on the surfaces and arehe main chemical components of the residual char surfaces. Theariation in the chemical components in residual char formulationonfirms the occurrence of chemical reaction during the thermalegradation. Furthermore, the volatile components detected by TG-

R can also confirm that the process of the thermal degradation ofhe Schiff-base epoxy polymer is associated with the process ofepolymerization. Note that the weight loss of about 30% com-osites at 700 ◦C performs nitrogen with high content strongly

ncreases the amount of residual char. The result is probably due tohe mechanism of intumescent flame retardant of nitrogen [32–35].uring the thermal degradation process, the disruption and break-own of many tiny intumescent structures in the surface lead tohe formation of powders, thus improving the ability of thermalesistance.

At the temperature of maximum mass loss rate during the ther-al degradation process of thermoset, no significant amount of the

olatile compounds released, only 11%, which was evidenced byGA measurement. This is not a uniquely indicative of thermal sta-ility for the Schiff-base epoxy, because the thermal degradation ofolymers does not always accompany with the release of volatileompounds. The Schiff-base epoxy polymer was heated at Tmax andept for different time interval. For Tim-1, some nub protrusion thatas illustrated in the bright parts of the morphologies appeared

see Fig. 9). The major chemical components of the nub protru-ion (II) were carbon and oxygen without nitrogen being detected,hich indicated that the carbon-rich char formed and accumulated

t the surface of the polymer matrix at the onset degradation stage.ig. 10 shows that some bright and tiny dark domains appearedlternately on the sample surface. The chemical components of the

and II domains of Tim-2 consisted of carbon and oxygen. With thencrease of calcination time, for 3 min, a continuous bright domainormed, partly covering the sample surface without any major vis-ble cracks in the Tim-3 surface (see Fig. 11). Comparison of thelements content of the different domains of the surface, the brightomain (II) contained the higher carbon content and the loweritrogen content. For Tim-5, Fig. 12 presents the continuous brightomain broaden out slightly and the residual char surface exhibits

thicker carbon ash deposit and less cracks. The major chemical

omponents of the I and II domains of Tim-5 were also carbon,itrogen and oxygen.

These results show that a series of relatively uniformly struc-ured network surfaces, without any cracks or holes are generated

Fig. 11. SEM morphology and EDS spectra (I and II for energy spectral acquisitiondomains) for Tim-3.

during the thermal decomposition. This is probably due to thatat the onset degradation stage the carbon-rich char acting as a‘polymer-protector’, forms and accumulates at the surface of thepolymer matrix. With long-playing heat treatment, the carbon-richchar starts to decompose and the residual char surface exhibits athicker carbon ash deposit and less cracks. Likewise, there is no

Fig. 12. SEM morphology and EDS spectra (I and II for energy spectral acquisitiondomains) for Tim-4.

9 try an

tracaedliafn

4

agtrmhostqtddt

docbtlcpsditorot

[

[[[

[[[

[

[[[[[[[

[

[

[[[[

[[

56 H. Liu et al. / Materials Chemis

hat the thermal degradation forms a coherent and strong carbon-ich char with no visible cracks, which is able to increase themount of thermal energy needed to raise the temperature of theomposition to the pyrolysis level and decrease the amount of avail-ble fuel. The retardation effect of carbon-rich char can also bexplained by the evolution of the major chemical components of theifferent domains. When the thermoset undergone the relatively

ong-playing heat treatment, the formed carbon-rich char consist-ng of carbon and oxygen aggregate at the surface. In addition, theppearances of the bright domains and dark domains reveal theormation of the intumescent structures due to the presence ofitrogen compounds.

. Conclusions

Via the study of the thermal degradation behavior and mech-nism of the Schiff-base epoxy, the highly conjugated Schiff-baseroups have shown some specific effects on the thermal degrada-ion of the Schiff-base epoxy polymer. PBMBA/MDA showed theelatively high Tds and Tmaxs, during the investigation of ther-al and thermal-oxidative stabilities. The interpretation of the

igh Td values in terms of structure–property relationship is that,wing to the presence of �-conjugated Schiff-base moiety, theecondary alcohols as hydrogen donors may form a network ofhe intramolecular hydrogen bonding with the �-electron. Conse-uently, the intramolecular hydrogen bonding network may limithe movement of secondary alcohol, decreases the reactivity of theehydration of the thermosets, thus increasing the onset thermalegradation temperature. For the higher Tmaxs, it may be attributedo the retardation effect of the dramatic high residual char.

The processes of degradation and char-formation that occurreduring the process of calcination by TGA were studied in termsf the volatile components and the morphologies and chemicalomponents of the residual char surfaces. TG-IR technique hadeen used to identify and characterize volatile components. Thehermal degradation process was also demonstrated by the evo-ution of morphologies and chemical components of the residualhar surfaces using the SEM/EDS. During the thermal degradationrocess, the disruption and breakdown of many tiny intumescenttructures in the surface lead to the formation of powders. The pow-ers possessing high specific surface area give a good dispersion

n polymer matrix, which improves the ability of thermal resis-ance. With long-playing heat treatment, the thermal degradation

f Schiff-base epoxy polymer forms a coherent and strong carbon-ich char with no visible cracks, thus able to increase the amountf thermal energy needed to raise the temperature of the composi-ion to the pyrolysis level and decrease the amount of available

[[

[

d Physics 132 (2012) 950– 956

fuel. These results suggest that during the thermal degradationprocesses being dehydration, depolymerization and carbonization,the highly conjugated Schiff-base moiety can serve as a promoterfor ‘char-formation’. Due to the presence of the char, the thermaldegradation of the thermoset is retarded. Therefore, it is probablydue to the presence of the Schiff-base group, the high performanceliquid crystal diepoxide polymer displayed the improved thermalstability.

References

[1] C. Sanchez, B. Lebeau, F. Chaput, J.P. Boilot, Adv. Mater. 15 (2003) 1969.[2] X. Wang, L. Song, W.Y. Xing, H.D. Lu, Y. Hu, Mater. Chem. Phys. 125 (2011) 536.[3] I.A. Rousseau, T. Xie, J. Mater. Chem. 20 (2010) 3431.[4] K. Xu, M.C. Chen, K. Zhang, J.W. Hu, Polymer 45 (2004) 1133.[5] H. Liu, K. Xu, H. Ai, L.L. Zhang, M.C. Chen, Polym. Adv. Technol. 20 (2009) 753.[6] M. Fujiwara, K. Kojima, Y. Tanaka, R. Nomura, J. Mater. Chem. 14 (2004) 1195.[7] A. Strachota, I. Kroutilova, J. Kovarova, L. Matejka, Macromolecules 37 (2004)

9457.[8] I. Park, T.J. Pinnavaia, Adv. Funct. Mater. 17 (2007) 2835.[9] Y.T. Xu, Y.Y. Ma, Y.M. Deng, C.J. Yang, J.F. Chen, L.Z. Dai, Mater. Chem. Phys. 125

(2011) 536.10] N.A. Negm, Y.M. Elkholy, M.K. Zahran, S.M. Tawfik, Corros. Sci. 52 (2010)

3523.11] I. Ahamad, R. Prasad, M.A. Quraishi, Mater. Chem. Phys. 124 (2010) 1155.12] D. Ribera, A. Mantecón, A. Serra, J. Polym. Sci. A: Polym. Chem. 40 (2002) 4344.13] N.J. Thompson, J.L. Serrano, M.J. Baena, P. Espinet, P. Dzygiel, Chem. Eur. J. 2

(1996) 214.14] M.W. Sabaa, R.R. Mohamed, E.H. Oraby, Eur. Polym. J. 45 (2009) 3072.15] S. Destri, I.A. Khotina, W. Porzio, Macromolecules 31 (1998) 1079.16] J. Rosenthal, J.M. Hodgkiss, E.R. Young, D.G. Nocera, J. Am. Chem. Soc. 128 (2006)

10474.17] S. Sarkar, Y. Aydogdu, F. Dagdelen, B.B. Bhaumik, K. Dey, Mater. Chem. Phys. 88

(2004) 357.18] W. Mormann, M. Bröcher, Polymer 40 (1999) 193.19] J.Y. Lee, Y.W. Song, M.J. Shim, J. Ind. Eng. Chem. 10 (2004) 601.20] M. Ochi, H. Takashima, Polymer 42 (2001) 2379.21] C.H. Lin, J.M. Huang, C.S. Wang, Polymer 43 (2002) 2959.22] S.V. Levchik, E.D. Weil, Polym. Int. 53 (2004) 1901.23] C.S. Chen, B.J. Bulkin, E.M. Pearce, J. Appl. Polym. Sci. 28 (1983) 1077.24] S.V. Levchik, G. Camino, M.P. Luda, L. Costa, B. Costes, Y. Henry, E. Morel, G.

Muller, Polym. Adv. Technol. 6 (1995) 53.25] F.H. Beijer, H. Kooijman, A.L. Spek, R.P. Sijbesma, E.W. Meijer, Angew. Chem.

Int. Ed. 37 (1998) 75.26] F.J.M. Hoeben, P. Jonkheijm, E.W. Meijer, A.P.H.J. Schenning, Chem. Rev. 105

(2005) 1491.27] L. Becker, D. Lenoir, G. Matuschek, A. Kettrup, J. Anal. Appl. Pyrol. 60 (2001) 55.28] B.L. Burton, J. Appl. Polym. Sci. 47 (1993) 1821.29] P. Musto, Macromolecules 36 (2003) 3210.30] M. Gryta, J. Grzechulska-Damszel, A. Markowsk, K. Karakulski, J. Membr. Sci.

326 (2009) 493.31] J.F. Lin, C.F. Ho, S.K. Huang, Polym. Degrad. Stabil. 67 (2000) 137.32] S.Y. Lu, I. Hamerton, Prog. Polym. Sci. 27 (2002) 1661.

33] Z.G. Huang, W.F. Shi, Eur. Polym. J. 42 (2006) 1506.34] S. Bourbigot, M.L. Bras, S. Duquesne, M. Rochery, Macromol. Mater. Eng. 289

(2004) 499.35] P.A. Song, H. Liu, Y. Shen, B.X. Du, Z.P. Fang, Y. Wu, J. Mater. Chem. 19 (2009)

1305.

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具