A novel phosphorus-containing thermotropic liquid crystalline poly(ester-imide) with high flame retardancy

Research Article

378

Received: 29 March 2008, Revised: 15 June 2008, Accepted: 20 June 2008, Published online in Wiley InterScience: 13 January 2009

(www.interscience.wiley.com) DOI: 10.1002/pat.1237

A novel phosphorus-containing thermotropicliquid crystalline poly(ester-imide) with highflame retardancy

Mi Yanga, Li Chena, Cheng-Shou Zhaoa, Heng-Zhen Huanga,Jun-Sheng Wanga and Yu-Zhong Wanga,b*

A novel phosphorus–nitrogen thermotropic liq

Polym. Adv

uid crystalline poly(ester-imide) (PN-TLCP) derived fromp-acetoxybenzoic acid (ABA), terephthalic acid (TPA), acetylated 2-(6-oxide-6H-dibenz<c,e><1,2>oxa phosphorin-6-yl)-1,4-dihydroxy phenylene (DOPO-AHQ) and N,N’-hexane-1,6-diylbis(trimellitimide) was prepared by melt trans-esterification. The chemical structure, the mesophase behavior, and the thermal properties of the copolymer wereinvestigated with Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy(1H NMR), elemental analysis, wide-angle X-ray diffraction (WAXD), hot-stage polarized light microscopy (PLM),differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). PN-TLCP exhibited a nematic meso-phase with a strong birefringence at a low and broad mesomorphic temperature ranging from 220 to 350-C, an initialflow temperature as low as about 190-C and a glass transition temperature of about 160-C. PN-TLCP has also goodthermal stability, high char residues and excellent flame retardancy (limiting oxygen index, LOI¼ 71 and UL-94 V-0rating). Copyright � 2009 John Wiley & Sons, Ltd.

Keywords: thermotropic liquid crystal; poly(ester-imide); flame retardancy

* Correspondence to: Y.-Z. Wang, Center for Degradable and Flame-Retardant

Polymeric Materials, College of Chemistry, Sichuan University, Chengdu

610064, China.

E-mail: [email protected]

a M. Yang, L. Chen, C.-S. Zhao, H.-Z. Huang, J.-S. Wang, Y.-Z. Wang

Center for Degradable and Flame-Retardant Polymeric Materials, Key

Laboratory of Green Chemistry and Technology (MoE), College of Chemistry,

Sichuan University, Chengdu 610064, China

b Y.-Z. Wang

State Key Laboratory of Polymer Materials Engineering, Chengdu 610065,

China

Contract/grant sponsor: National Science Foundation of China; contract/

grant number: 20674053.

Contract/grant sponsor: National Science Fund for Distinguished Young

Scholars; contract/grant number: 50525309.

INTRODUCTION

Wholly aromatic thermotropic liquid crystalline polymers (TLCP)with good mechanical properties and excellent thermal stabilityattract a great deal of attention.[1,2] TLCP can also be used toprepare the in situ reinforced composites by blending them withconventional polymers; the research has attracted the interest ofmany scientists.[3–5] However, only a few researchers have paidattention to the flame retardation of TLCP. Wang et al.[6,7] havesuccessfully used TLCP to develop a novel method for resolving achallenging problem in the field of flame-retardant polymermaterials: flame retardation of polymers always accompanies adecrease in their mechanical properties.[8] The authors synthes-ized a phosphorus-containing wholly aromatic main-chainthermotropic liquid crystalline copolyester (P-TLCP), which hasa very high limiting oxygen index (LOI)[6] and blended P-TLCPwith poly(ethylene terephthalate) (PET) to prepare an in situcomposite at the mesophase temperature of P-TLCP (�2908C).[7]

The resulting PET/ P-TLCP composites realized simultaneousimprovements in both mechanical properties and flameretardancy. However, this method is not efficient enough forsome polymers having lower melting temperatures due to thehigher mesophase temperature of P-TLCP (�2908C), which doesnot match the processing temperatures of most of theconventional polymers. Therefore, the design and synthesis ofTLCP with lower mesophase and melting temperature and highflame retardancy become the key to addressing this problem.Many methods such as the incorporation of flexible

spacers,[9–11] bulky substituents,[12–14] and nonlinear links[15] into

. Technol. 2009, 20 378–383 Copyright � 200

the main chains of TLCP have been utilized to lower themelting-transition temperatures of TLCP. In this article, on thebasis of our previous work,[6,7] we introduced the imide unit intoP-TLCP to synthesize a novel phosphorus–nitrogen thermotropicliquid crystalline poly(ester-imide) (PN-TLCP). The introduction ofthe monomer N,N’-hexane-1,6- diylbis(trimellitimide) in the mainchain is expected to lower the mesophase and meltingtemperature due to the existence of its six-methylene segment,whereas the existence of the imide group can provide a goodthermal stability. We also expect a phosphorus–nitrogen (P–N)synergistic flame retardant effect.[16]Fourier transform infraredspectroscopy (FTIR) and proton nuclear magnetic resonance

9 John Wiley & Sons, Ltd.

PHOSPHORUS-CONTAINING POLY(ESTER-IMIDE)

spectroscopy (1H NMR) are used to identify the chemicalstructures of the products, whose thermal properties,morphology, and optical textures are investigated by thermo-gravimetric analysis (TGA), differential scanning calorimetry(DSC), wide-angle X-ray diffraction (WAXD) and polarized lightmicroscopy (PLM), respectively.

EXPERIMENTAL

Materials

Acetylated 2-(6-oxide-6H-dibenz<c,e><1,2>oxa phosphorin-6-yl)-1,4-dihydroxy phenylene (DOPO-AHQ) was prepared fromDOPO (Weili Flame Retardant Chemicals Co. Ltd. Chengdu, China)as described in a previous publication.[6] N,N’-Hexane-1,6-diylbis(trimellitimide) was synthesized according to a methodpreviously described by Kricheldorf and Pakull.[17] Terephthalicacid (TPA) was supplied by Zhenghao AdvancedMaterials Co. Ltd.(Jinan, China). p-Acetoxybenzoic acid (ABA) was obtained fromWulian Chemical Co. All other materials were obtained from acommercial source and were used as received.

Polycondenzation

Melting transesterification was utilized to prepare PN-TLCP undera nitrogen atmosphere. All poly(ester-imide)s were prepared in asimilar manner. A mixture of DOPO-AHQ, ABA, and the twodiacids, TPA and N,N’-hexane-1,6-diylbis(trimellitimide), was putinto a 100mL, three-necked glass flask equipped with amechanical stirrer and nitrogen-inlet and outlet tubes. Themolar contents of p-ABA and DOPO-AHQ are constant at 60 and20%, respectively, and the total molar content of the diacids(TPA and N,N’-hexane-1,6-diylbis(trimellitimide)) was 20%. Theflask was placed in a salt bath preheated at 210–2308C, andthe polycondenzation started under nitrogen. Acetic acid, thebyproduct, was taken off by the nitrogen flow. The temperaturewas then raised in a stepwise manner to 270–2908C within 3–4 h.Then, the flow of nitrogen was halted and a vacuum was appliedin the last 1–1.5 h. The pressure in the flask fell to 20–40 Pa.After that the flask was cooled to room temperature and theproduct was dried at 808C in vacuo. The inherent viscosity ofthe obtained copolymers ranged from 0.44 to 0.69 dL/g. Beforetesting, the crude product was dissolved in the phenol/1,1,2,2-tetrachloroethane (1:1w/w) solution. Then the solution

Scheme 1. Synthesis

Polym. Adv. Technol. 2009, 20 378–383 Copyright � 2009 John Wiley

was filtered and precipitated by methanol, and the precipitatewas filtered and dried under vacuum at 808C till a constantweight was reached.The general synthetic scheme is presented in Scheme 1.

Measurement

Chemical structures of the copolymers were characterized byFTIR and 1H NMR spectroscopy and were recorded on an FTIR170SX spectrometer and a Bruker 600MHz spectrometer, respect-ively.The intrinsic viscosities of the copolymers were measured in a

mixed solution of phenol/1,1,2,2-tetrachloroethane (1:1w/w) at308C at a concentration of 0.5 g/dL with an Ubbelohde capillaryviscometer.DSC was conducted on a TA Q-20 instrument under a nitrogen

atmosphere at a heating rate of 108Cper min. Indium was used asa reference for temperature calibration.Thermogravimetric analysis (TGA) was performed with a TA

SDT-Q600 instrument at a heating rate of 208Cper min from roomtemperature to 7008C in a nitrogen atmosphere at a flowing rateof 100mL/min.WAXD was performed at room temperature on a DX-1000

diffractometer using Ni-filtered Cu Ka radiation. The rotatedvelocity of the goniometry was 28/min.The liquid crystalline texture of PN-TLCP was observed by a

Leitz Model Laborlux 12 Pols equipped with a Mettler FP-2 hotstage.The combustion behavior of P-TLCP was evaluated by LOI and

UL-94 tests. LOI values were determined according to ASTM D2863-97, and the size of the samples is 120� 6.5� 3.2mm3,where UL-94 rating was obtained according to ASTM D 3801 teststandard with a sample size of 125� 12.7� 3.2mm3.Elemental analyses were performed on a Perkin Elmer EA2400

II elemental analyzer. The content of phosphorus was evaluatedby Perkin Elmer 2100 Inductive Coupled Plasma EmissionSpectrometer.

RESULTS AND DISCUSSIONS

Chemical structure

The structures of the resulting copolymers were characterizedwith FTIR and 1H NMR. A representative IR spectrum of sample P-2

route of PN-TLCP.

& Sons, Ltd. www.interscience.wiley.com/journal/pat

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Figure 1. FTIR spectrum of P-2.

Figure 2. 1H NMR of P-3 (in CDCl3 and CF3COOD).

M. YANG ET AL.

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(Fig. 1) shows the characteristic peaks: 2800–3100 cm�1 (–CH2–),1739 cm�1 (C––O), 1178 cm

�1 (P–O–Ar), and 1475 cm�1 (P–Ar).The results of the characteristic bands in FTIR spectra correlatesufficiently with the proposed structures of the copolymer, whichwere further proved by 1H NMR.The 1H NMR spectrum of sample P-3, measured in a mixed

solution of chloroform (CDCl3) and trifluoroacetic acid(CF3COOD), is shown in Fig. 2. In general, the 1H NMR spectrum

Table 1. TPA content of PN-TLCP and results of elemental analysi

Sample TPA content (%) C

P-2 15 66.26 (65.35)��

P-3 10 66.3 (65.28)P-4 5 65.63 (65.22)P-5 0 65.84 (65.17)

� Evaluated by Perkin Elmer 2100 ICP spectrometer.��Calculated values.

www.interscience.wiley.com/journal/pat Copyright � 2009 John

of the polymer is divided into two parts: one showing thearomatic (phenyl) protons in the down field region in the range of6.8–9.0 ppm, and another showing the methylene protons in therange of 1–4 ppm. This also confirms that the flexible methylenespacers were introduced into the main chain of PN-TLCPssuccessfully, and were not lost duringmelt polymerization at hightemperature.Table 1 presents the results of elemental analyses of the

resulting PN-TLCP. The experimental contents of various elementsare very close to the calculated values, which further confirmedthat the resulting polymer is what we expected.

Thermal stability and flame retardancy

The thermal behavior of these copolymers was evaluated by TGA.The thermal decomposition curves of the copolymers withdifferent compositions and different inherent viscosities areshown in Fig. 3, and the TGA data are listed in Table 2. All the TGAcurves are quite similar and show one main decomposing stage.Just as we expected, the introduction of a six-methylene segment(as a flexible spacer of PN-TLCP) from N,N’-hexane-1,6-

s along with calculated values

Elemental analysis (wt%)

H P� N

3.74 (3.26) 3.26 (3.31) 1.04 (0.75)3.85 (3.36) 3.08 (3.06) 1.5 (1.38)3.58 (3.45) 2.87 (2.86) 1.93 (1.93)3.6 (3.53) 2.68 (2.67) 2.82 (2.41)

Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 378–383

Figure 3. TGA curves of the copolymers with different TPA contents.

Figure 4. DSC curves of copolymers: (a) second heating and (b) cooling.

Table 2. Thermal analysis data of the copolymers withdifferent TPA contents

P-1� P-2 P-3 P-4 P-5

hint (dL/g) 1.53 0.55 0.69 0.44 Insol.Tg (8C) 183 164 161 161 161Tm1 (8C)

�� 290 185 195 192 201Tm2 (8C) — — — — 330TLC (8C) 290 220 220 220 220Td, 5% (8C) 440 413 434 440 437Td, max (8C) — 485 492 488 485Char residues (%) at 6408C 41.1 39.3 45.6 42.8 39.8

� Reported in our previous work.[6]��Obtained from a hot-stage polarized light microscope at aninitial flow temperature.


3

diylbis(trimellitimide) monomer does not reduce the thermalstability of the copolymer. The 5%-weight-loss decompositiontemperatures ranged from 413 to 4408C, indicating that thesecopolymers had good thermal stability. The char residues at6908C were 38–44%. In some cases, there is a linear relationshipbetween the amount of char formed in the thermal decompo-sition and the LOI values of halogen-free polymers.[18] Theformation of char could lower the exothermicity of the pyrolysisreaction, limit the production of combustible gases, and decreasethe thermal conductivity of the burning materials, which cause areduction in the flammability of polymers. The LOI values of thecopolymers with different compositions are about 71, which ismuch higher than those of conventional aromatic polyesters andpolyimides without phosphorus. Also, all the copolymers showedno burning or dripping during the vertical burning test, and thusobtained a V-0 rating by the UL-94 test. Interestingly, there existno significant differences among the flame retardancy ofPN-TLCPs with different compositions. Compared with theP-TLCP synthesized previously by the authors,[6] PN-TLCP didnot have, evidently, improved flame retardancy; although thenitrogen element was incorporated into the macromolecule


there was no significant P–N synergistic effect, which had beenexpected.

Thermal transition behavior

The thermal and phase-transition behavior of the copolymerswas investigated with TGA, DSC, and hot-stage PLM. TGA dataand thermal transitions obtained from DSC are presented inTable 2.DSC measurements were conducted at a heating and cooling

rate of 10 8C/min in the temperature range of 50–3508C, as shownin Fig. 4, from which we can see that all the copolymers havesimilar glass transition temperatures (Tg), ranging from 161 to1648C, which are not very sensitive to the compositions of thecopolymers. However, from DSC curves, we cannot findthe mesophase transition peak and melting peak except insample P-5, which exhibits an endothermic peak in the heatingtrace and an exothermic peak in the cooling trace in its DSCcurves. The endothermic peak appears at 3308C, whichsuggested a dependence to the melting temperature (Tm2) ofthe crystalline region with an enthalpy (DHm) value of 6.7 J/g. Theexothermic peak appears at 2828C, which should correspond tothe crystallization temperature (Tc), with an enthalpy (DHc) valueof 8.9 J/g. Other copolymers showed no significant endothermicpeaks but only the one corresponding to glass transition. Theresults indicated that the main parts of the copolymers wereamorphous, though the crystallinity of sample P-5 was muchhigher than those of others, which was also confirmed by WAXD


81

Figure 6. WAXD patterns of Sample P-2 (a) and Sample P-5 (b) beforeannealing and after annealing at 2508C for 1–2 h.

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measurements. Although samples P-2–4 have no melting peakson their DSC curves, the flow temperatures (Tm1) can be observedfrom PLM, which are summarized in Table 2. It has been notedthat, for sample P-5, the flow temperature (Tm1¼ 2018C) fromPLM, which is related to the flow process of the amorphousregion, is much lower than the melting temperature(Tm2¼ 3308C) from DSC curves, which is related to the meltingprocess of the partially crystalline region.

Wide-angle X-ray diffraction

Figure 5 shows the WAXD patterns of the series of copolymerswith different TPA contents at room temperature. The peak atlarge scattering angles is rather broad, indicating that it is aone-dimensional liquid crystalline structure (nematic meso-phase), which was confirmed by polarized light microscopy, asmentioned below. The most significant peak observedat 2u¼ 208, according to the earlier conclusion,[6,19] revealedthat the bulky substituent and random polymerization by themelt transesterification method will destroy the ability to formliquid crystals. As there existed an evident degree of crystallinityin the DSC curve of P-5, two diffraction peaks can be found in therange of 2u¼ 198–258 in the WAXD patterns. There was also aweak diffraction peak at about 2u¼ 198 for sample P-3, and aweak diffraction peak at lower angles (2u¼ 78) in the series ofcopolymers except sample P-5, but no peak can be observed intheir DSC curves.Figure 6 shows the effects of annealing at nematic temperature

on the WAXD patterns of the copolymers (samples P-2 and P-5).The peaks at about 198 and 448 for both the samples, at about 248and 288 for sample P-5, and at 78 for sample P-2, becamesomewhat sharper and clearer with the increase of annealingtime. In principle, the crystallization occurs between the Tg andTm, and therefore, for both the samples, annealing at 2508C (atemperature between the Tg and Tm of the copolymers) for 1–2 hcould improve crystallization slightly. However, additional work isrequired for a further explanation of the crystal structure and itstransition.

Mesophase identification

After samples were quenched in an ice-water bath from the melt,a high degree of birefringence with threaded nematic texturewas clearly observed under the PLM. Typical photographs of two

Figure 5. WAXD patterns of as-synthesized polymers.

www.interscience.wiley.com/journal/pat Copyright � 2009 John

samples (P-3 and P-5) at different temperatures are shown inFig. 7. The initially isotropic samples were heated from roomtemperature to 3508C, which was the limiting temperature of thisPLM. The copolymer was not found to exhibit clearly thethreaded schlieren texture characteristic of the nematicmesophase until the temperature was raised to about 2208C.The birefringence and texture became clearer as the temperatureincreased. When the temperature reached 2808C, the clearestnematic texture (Fig. 7(d)) could be observed. However, at 3008Cthe texture became partially extinct. The isotropic temperatures,Ti, of the copolymers, were very high and above the limitingtemperature (3508C) of the hot-stage PLM, and therefore, the Tivalues could not be observed by PLM. Thus, the nematicmesophase temperature range of this series of copolymers wasvery broad, which might be attributed to the low degree offreedom of molecule movement. In the application of highperformance engineering plastics, this is an important advantagefor polymer processing.[20]

As mentioned above, the series of copolymers wereamorphous, glassy and difficult to crystallize even at hightemperature, except sample P-5. The copolymers changed fromthe glassy state to nematic state at approximately 2208C.However, the melt of sample P-5, as observed from DSC, is still inthe liquid crystalline state, though birefringence is partiallyextinct as the temperature passed 3208C. Then the dark areabecame larger as the temperature increased. However, thenematic–isotropic phase transition was not completed, as therewas still birefringence in the main area of the sample.

Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 378–383

Figure 7. Polarized lightmicroscope photographs of sample P-3 and P-5 at different temperatures (magnification 500�): (a) P-5 (250 8C), (b) P-5 (340 8C),(c) P-3 (230 8C) and (d) P-3 (280 8C). This figure is available in color online at www.interscience.wiley.com/journal/pat


CONCLUSIONS

Novel PN-TLCPs were synthesized successfully by melt transes-terification. PN-TLCPs with different compositions exhibit a lowand broadmesomorphic temperature ranging from 220 to 3508C,and a low initial flow temperature (about 1908C). They also have avery good thermal stability (decomposition temperature>4108C)and high char residues at high temperatures. Although nodistinct P–N synergistic flame retardant effect was found,PN-TLCPs have excellent flame retardancy (LOI¼ 71, UL-94 V-0rating) and mechanical properties when they act as macromol-ecular flame retardants and in situ ‘‘reinforced fillers’’.

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A novel phosphorus-containing thermotropic liquid crystalline poly(ester-imide) with high flame retardancy