all-aromatic liquid crystalline thermosets with high glass transition temperatures

All-Aromatic Liquid Crystalline Thermosets with High GlassTransition Temperatures

MAZHAR IQBAL,1 BEN NORDER,2 EDUARDO MENDES,2 THEO J. DINGEMANS1

1Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg1, 2629 HS Delft, The Netherlands

2Section Nanostructured Materials, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

Received 4 November 2008; accepted 9 December 2008DOI: 10.1002/pola.23245Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: We have synthesized and characterized a new family of low melting all-aromatic ester-based liquid crystal oligomers end-capped with reactive phenylethynylend groups. In a consecutive, high-temperature step, the reactive end groups werethermally activated and polymerization was initiated. This reactive oligomerapproach allows us to synthesize liquid crystal thermosets with outstanding mechan-ical and thermal properties, which are superior to well-known high-performancepolymers such as PPS and PEEK. We have modified an intractable LC formulationbased on hydroquinone and terephthalic acid, with Mn ¼ 1000, 5000, and 9000 gmol�1, and varied the backbone composition using isophthalic acid, resorcinol, 4-hydroxy-benzoic acid, 6-hydroxy-2-naphthoic acid, and chlorohydroquinone. All fullycured polymers showed glass transition temperatures in the range of 164–275 �C,and high storage moduli at room temperature (� 5 GPa) and elevated temperature(� 2 GPa at 200 �C). All oligomers display nematic mesophases and in most cases,the nematic order is maintained after cure. Rheology experiments showed that thephenylethynyl end group undergoes predominantly chain extension below 340 �Cand crosslinking above this temperature. Highly aligned fibers could be spun fromthe nematic melt, and we found that the order parameter hP2i was not affected bythe chain extension and crosslink chemistry. VVC 2009 Wiley Periodicals, Inc. J Polym Sci

Part A: Polym Chem 47: 1368–1380, 2009

Keywords: liquid-crystalline polymers; polyesters; reactive oligomer; thermosets

INTRODUCTION

Wholly aromatic thermoplastic liquid crystallinepolymers (TLCPs) are in use now for more than30 years and are found in demanding applica-tions, such as high-strength and high-modulusfibers, chemically resistant coatings and films,precision-molded electronic components, connec-

tors, and electronic switches.1–3 A major draw-back of all-aromatic main-chain rigid rod-likepolymers is that they tend to have melting pointstypically above their decomposition temperature(Tm � Td) and low solubilities in all but aggres-sive solvents. These factors make them difficult toprocess and have significantly limited theirapplicability. Several different types of structuralmodifications have been demonstrated over theyears, which were aimed at reducing the crystal-line melt transition and improve the melt process-ability. For example, introducing flexible aliphaticspacers or side-chain substitutions will reduce the

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 1368–1380 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: T. J. Dingemans (E-mail: [email protected])

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melt transition considerably, but at the same timethe thermal and mechanical properties, such asthe Tg and storage E-modulus, are reduced aswell.4–8 Introducing nonlinear or ‘‘kinked’’ mono-mers such as 2,5-thiophene, or even nonmeso-genic 1,3-phenylenes, will lower the melt temper-ature effectively but here the mesophase stability(i.e., DN) might become compromised or disappearaltogether.9,10

Recently, we reported our results on a reactiveTLCP concept, where we modified a well-knownTLCP based on 4-hydroxybenzoic acid (HBA) and6-hydroxy-2-naphthoic acid (HNA) (VectraTM),with reactive phenylethynyl end groups.11,12 Byreducing the molecular weight of the polymerbackbone, the melt viscosity could be reducedfrom 10,000 Pa s�1, which is typical for high-mo-lecular weight Vectra, to 100 Pa s�1 for a 5000 gmol�1 analog, and at the same time Tm could bereduced by as much as 100 �C for the 1000 gmol�1 analog. After processing, polymerizationvia the phenylethynyl end groups was initiated,and films and plaques with excellent thermal andmechanical properties were obtained, which sur-passed the original parent polymer.

Motivated by these results, we realized thatthis concept will provide a route toward highlyrigid LCP backbone chemistries, which were pre-viously not accessible because such materialswould not melt and could therefore not be synthe-sized and processed. Increasing the concentrationof all para-substituted aromatic monomers isexpected to increase the thermal and mechanicalproperties of this class of polymers and result in anew family of high-performance materials. To fur-ther explore this concept, we selected a well-known LCP concept based on readily availableand cheap terephthalic acid (TA) and hydroqui-none (HQ). Frosini et al.13 showed that this poly-ester, that is, poly(p-phenylene terephthalate) (I)has a Tm of � 500 �C and a glass transition tem-perature (Tg) of 267

�C.

Herein, we will present the synthesis and char-acterization of processable highly rigid polyestersbased on readily available TA and HQ with cross-linkable end groups having molecular weights in

the range of 1000–9000 g mol�1. In addition, thepolymer backbone composition was varied usingdifferent molar ratios of isophthalic acid (IA), res-orcinol (RS), 6-hydroxy-2-naphthoic acid (HNA),4-hydroxy-benzoic acid (HBA), and chlorohydro-quinone (ClHQ). All oligomers were terminatedwith phenylethynyl reactive end groups and poly-merized in a successive high temperature step.We studied the effect of molecular weight andbackbone composition on the thermal and me-chanical properties of these oligomers.

EXPERIMENTAL

Materials

All chemicals were obtained from the indicatedsources and used as received. 3-Aminobenzoicacid, 3-aminophenol, hydroquinone, terephthalicacid, isophthalic acid, 4-hydroxybenzoic acid, andchlorohydroquinone were obtained from Aldrich.Resorcinol and 6-hydroxy-2-naphthoic acid werereceived from Fluka and Ueno Fine Chemicals,respectively. 4-Phenylethynylphthalic anhydride(PEPA) was purchased from Daychem. The syn-thesis of the para-substituted phenylethynyl reac-tive end groups, that is, N-(4-carboxyphenyl)-4-phenylethynylphthalimide (p-PE-COOH) and N-(4-acetoxyphenyl)-4-phenylethynylphthalimide(p-PE-OAc) was reported elsewhere.11

Characterization

The mesophase behavior of our oligomers wasinvestigated with a Leica DMLM polarizing opti-cal microscope (POM) equipped with a Linkhamhot stage. Samples were investigated usinguntreated glass slides in air. The thermal proper-ties of the reactive oligomers and cured polymerswere determined using a Perkin–Elmer SapphireDSC with a heating rate of 10 �C min�1. All meas-urements were conducted under a nitrogenatmosphere. The thermal stability of the curedpolymers was investigated by thermogravimetricanalysis (TGA) using a PerkinElmer Pyris Dia-mond TG/DTA. Samples were investigated usinga heating rate of 10 �C min�1, in either nitrogenor air atmosphere. The dynamic mechanical anal-yses were performed on a PerkinElmer DiamondDMTA, using thin films (10 mm � 5 mm � 0.25mm) under a nitrogen atmosphere and at a heat-ing rate of 2 �C min�1. The thin films were pre-pared using standard melt pressing techniques.The reactive oligomers were ground into a

ALL-AROMATIC LIQUID CRYSTALLINE THERMOSETS 1369

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

powder and this powder was placed between twoKaptonTM foils and consolidated for 1 h at 370 �Cin a Joos press to allow the oligomers to polymer-ize completely. The obtained films showed nolong-range preferred order, that is, during meltpressing and subsequent crosslinking, no effortwas made to align the liquid crystal phase. Themelt and polymerization behavior of the reactiveoligomers were investigated using a RheometricRMS 800 rheometer equipped with a force-rebalanced transducer in a parallel plate geome-try. Parallel plates of 8 mm diameter were usedand suitable samples, 8 mm in diameter and 0.2mm thick, were prepared by compression mold-ing. The samples were investigated under inertatmosphere using a heating rate of 2 �C min�1. X-ray diffraction (XRD) analysis was conducted on aBruker AXS D8 Discovery diffractometer, using aCu-Ka X-ray tube (k ¼ 1.54 A) at 40 keV. Thealigned LC fibers were placed in an quartz capil-lary and heated without the presence of an exter-nal electric or magnetic field.

Synthesis of the Meta-Substituted PhenylethynylEnd Groups

A 2-L Erlenmeyer flask equipped with a mechani-cal stirrer and reflux condenser was charged with800 mL glacial acetic acid and PEPA (0.25 mol,62.06 g). The mixture was slowly heated to� 110 �C andwhen all solids were dissolved, 3-ami-nobenzoic acid (0.25 mol, 34.29 g) was added. Athick suspension formed almost immediately andwas stirred for 2 h at reflux temperature. Aftercooling the reactionmixture to� 70 �C, the precipi-tated crystals were collected by filtration andwashed with acetic acid (2�) and ethanol (2�). Thebright yellow N-(3-carboxyphenyl)-4-phenylethy-nylphthalimide (m-PE-COOH) was dried undervacuumat 150 �C for 48 h. Yield: 89 g (97%).

1H NMR (DMSO-d6, 300 MHz): d 7.45–7.49 (m,3H), 7.63–7.75 (m, 4H), 7.98–8.08 (m, 5H), 12.05–13.85 (s, COOH). 13C NMR (DMSO-d6, 75.46MHz): d 88.71, 94.17, 122.14, 124.55, 126.52,128.70, 129.21, 129.57, 129.91, 130.31, 131.53,132.15, 132.37, 132.45, 132.82, 132.94, 137.94,166.83, 166.92, 167.34. IR (KBr): 1680, 1704 (im-ide), 2207 (acetylene) cm�1.

N-(3-hydroxyphenyl)-4-phenylethynylphthali-mide (m-PE-OH) was prepared in a similar fash-ion. After the initial work-up, however, the phenolend group was acetylated by refluxing m-PE-OHin anhydrous acetic anhydride for 2 h. N-(3-acetoxyphenyl)-4-phenylethynylphthalimide (PE-

OAc) precipitated as off-white crystals uponcooling. The title compound was washed with ace-tic acid (2�) and ethanol (2�) and dried undervacuum at 150 �C for 48 h. Yield: 84.5 g (94%).

1H NMR (DMSO-d6, 300 MHz): d 2.30 (s, 3H),7.26–7.41 (m, 3 H), 7.47–7.49 (m, 3H), 7.55–7.61(t, J ¼ 8.1 Hz, 1 H), 7.63–7.66 (m, 2H), 8.01–8.08(m, 3H). 13C NMR (DMSO-d6, 75.46 MHz): d21.53, 88.69, 94.19, 121.39, 122.12, 122.43,124.58, 125.29, 126.54, 129.24, 129.58, 130.29,131.43, 132.45, 132.86, 133.36, 137.98, 151.17,166.72, 166.81, 169.76. IR (KBr): 1708, 1768 (im-ide), 2212 (acetylene) cm�1.

Synthesis of the PhenylethynylEnd-Capped Oligomers

All reactive oligomers were synthesized usingstandard melt condensation techniques.14,15 Inthe first series, four reactive oligomers based onTA and HQ were prepared with a target Mn of1000, 5000, and 9000 g mol�1. The samples werelabeled, TA/HQ-1K, TA/HQ-5K, and TA/HQ-9K,respectively, where TA/HQ refers to the polymerbackbone composition, and the integers refer tothe polymer molecular weight, that is, 9K ¼ 9000g mol�1. A reference polymer, without reactiveend groups, was also synthesized by us and waslabeled TA/HQ. Because we investigated two dif-ferent phenylethynyl end groups in the HQ/TA se-ries, that is, para- or meta-substituted, a p or msubscript refers to the substitution pattern, forexample, TA/HQ-9Km.

In successive series, we modified the TA/HQbackbone with different concentrations comono-mers, including IA, RS, HNA, HBA, or ClHQ.These oligomers were labeled in a similar fashion,for example, TA/HQ/IA(50)-5K refers to a 5000 gmol�1 random copolymer comprised of TA, HQ,and 50 mol % IA.

Synthesis of TA/HQ-1Kp

As a representative example, we describe the syn-thesis of a 1000 g mol�1 reactive oligomer with aTA/HQ backbone composition, that is, TA/HQ/-1Kp. To synthesize this oligomer, TA (0.1 mol,16.61 g), HQ (0.1 mol, 11.01 g), p-PE-OAc (0.0639mol, 24.37 g), p-PE-COOH (0.0639 mol, 23.47 g),and potassium acetate (0.1 mmol, 10 mg) werecharged to a 250 mL three-neck round bottomflask. The flask was equipped with a nitrogen gasinlet, an overhead mechanical stirrer, and a refluxcondenser. The reactor was purged with nitrogen

1370 IQBAL ET AL.


for 0.5 h prior to the start of the reaction, and aslow nitrogen flow was maintained throughoutthe synthesis. Acetic anhydride (45 mL, 0.44 mol)was added for the in situ acetylation of HQ. Thereaction mixture was slowly stirred under a nitro-gen atmosphere and heated to 140 �C to allowacetylation to take place. After a 1-h isothermalhold, the temperature of the reaction mixture wasslowly increased to 310 �C using a heating rate of1 �C min�1. During this process, acetic acid wascollected as a condensation by-product. At 300 �C,the nitrogen flow was stopped and a vacuum wasapplied to remove the remaining acetic acid. Thereaction flask was allowed to cool-down overnightunder a nitrogen flow, and the final product wasremoved from the flask and ground into a powderusing a high-speed MicroMill grinder. The productwas placed in a vacuum oven at 250 �C for 48 h tocomplete the polymerization and remove traces ofacetic acid and other undesirable side products.Yields for all syntheses were generally above 95%.

RESULTS AND DISCUSSION

Synthesis of the Reactive End Groups

In addition to our para-substituted phenylethynylreactive end groups, that is, p-PE-COOH and p-PE-OAc,11 we synthesized two meta-substitutedanalogs as well. By placing the phenylethynyl re-active functionality at the meta position, thiswould further lower the oligomer backbonesymmetry and hence reduce the crystal-nematic(K-N) melt transition. Both m-PE-COOH andN-(3-acetoxyphenyl)-4-phenylethynyl phthalimide(m-PE-OAc) were successfully prepared using anacid-catalyzed imidization method, as shown inScheme 1.

Both compounds indeed showed a much lowermelting point when compared with their para-substituted analogs, that is, p-PE-COOH and p-PE-OAc, and melt at 303 and 204 �C, respectively,as shown in Figure 1. The melting endothermsare followed by a broad exothermic event, whichis due to the chain extension and crosslinkingchemistry of the phenylethynyl end groups. TheDSC results for both compounds are summarizedin Table 1.

Synthesis of Reactive End-Capped Oligomers

All phenylethynyl-terminated oligomers weresynthesized using a one-pot melt condensation po-lymerization. The synthesis is represented inScheme 2, and most oligomers could be synthe-sized in high yield without difficulty.

Scheme 1. Synthesis of the meta-substituted phenylethynyl reactive end groups,that is, m-PE-COOH and m-PE-OAc.

Figure 1. DSC traces of the m-PE-OAc and m-PE-COOH reactive end groups, first heat, recorded at10 �C min�1.



Solubility and Inherent Viscosity

To confirm the molecular weight of our oligomers,we attempted to find suitable solvents or solventmixtures, for example, trifluoroacetic acid, penta-fluorophenol, or phenol/1,1,2,2-tetrachloroethane(70:30). All oligomers, however, appeared insolu-ble at room temperature and elevated tempera-ture, which precludes size exclusion chromatogra-phy (SEC) and inherent viscosity measurementsof these oligomers.

Polarizing Optical Microscopy

Polarizing optical microscopy was used to deter-mine the phase behavior of our reactive oligomersbefore and after crosslinking. All oligomersshowed classic low viscous nematic textures,which could easily be aligned using shear. It wasobserved that the TA/HQ oligomers modified withkinked structures, that is, IA and RS exhibitedlower nematic melt viscosities when comparedwith their more linear modification, that is, basedon HBA and HNA, respectively. All reactiveoligomers tend to form stable nematic phases andno accessible isotropic phases (N-I). Upon rapidheating to 500 �C, the samples under investigationcrosslink and their textures become fixed around400 �C and impossible to shear, that is, a nematicnetwork was obtained and continued heating([500 �C) resulted in sample decomposition.

In this class of reactive LCPs, the reactive endgroups appear to be fully compatible with meso-phase formation, which was clearly demonstratedby the melt behavior of our 1000 g mol�1 TA/HQsample. This is the only oligomer which melts inthis series, and has a relatively short backbonecomprised of � 4 repeat units. End capping withp-PE-COOH and p-PE-OAc results in a reactiveoligomer, which melts into a stable nematic phaseat 291 �C and no N-I transition could be observed.To further investigate the effects of mesogenshape on mesophase stability, we replaced thepara-substituted reactive end groups with meta-substituted phenylethynyl functionalities, that is,

m-PE-COOH and m-PE-OAc, as shown in Scheme2, series 2. The overall mesogenic shape becomessignificantly compromised, and this results in areduction of the crystalline melting point to 128 �C(DT ¼ 163 �C). In addition, the liquid crystal phaseis initially lost but upon curing, at � 350 �C, theformation of a nematic texture becomes apparent.From these results, we may conclude that thechemical functionalities,16 formed during chainextension and crosslinking, are able to increasethe molecular aspect ratio and restore mesophaseformation in this family of polymers.

A nematic to isotropic transition (N-I) was alsoobserved for TA/HQ/IA(75)-5K, an oligomer with arelatively high concentration of IA. It is well-known that such nonlinear moieties disrupt thelinear progression of the main chain, and hencereduce the mesogenicity of the polymer.9 Despitethe high concentration of IA, TA/HQ/IA(75)-5Kshows a stable, but short lived, nematic melt from321 to 360 �C, after which the melt becomesbiphasic, that is, the nematic melt coexist withthe isotropic melt. At 375 �C, the melt is com-pletely isotropic and crosslinking takes place inthe isotropic phase. Typically, all our sampleswere cured at 370 �C, and the viscosity of the meltincreases rapidly and the samples solidify after a1-h isothermal hold, while maintaining their ne-matic order. Representative nematic textures ofour reactive oligomers before and after crosslink-ing are shown in Figure 2.

Wide Angle X-Ray Scattering

To observe the effect of curing on the orientationof the reactive liquid crystals and their networks,we prepared melt-spun fibers from TA/HQ/HNA(25)-5K. On a hot plate, this oligomer washeated to 310 �C, which is well above the K-Ntransition and well below the temperature, wherethe reactive end groups start to react. Severalfibers were rapidly drawn from the homogenousnematic melt in air using tweezers. The averagefiber diameter was � 250 lm, and these fiberswere used for our XRD experiments without any

Table 1. Transition Temperatures (�C) and Enthalpies (kJ mol�1) of the Meta-Substituted Reactive End Groups

Name

Tm (�C)/DHf (kJ mol�1) Exotherm (�C)

Peak Onset Peak End DHexo (kJ mol�1)

m-PE-COOH 303 (43) 340 390 413 �46m-PE-OAc 204 (46) 355 405 425 �52

1372 IQBAL ET AL.


Scheme 2. Synthesis of the terephthalic acid (TA)- and hydroquinone (HQ)-basedliquid crystalline oligomers with phenylethynyl end groups. The oligomers in series 1are terminated with the more linear para-substituted reactive end groups, whereasthe series 2 are end capped with the meta-substituted phenylethynyl functionalities.



posttreatment. The diffraction pattern, recordedat room temperature, is displayed in Figure 3(A).

Several fibers of TA/HQ/HNA(25)-5K werecured in a sealed quartz capillary by heating to350 �C followed by an isothermal hold at this tem-perature for 60 min. Diffraction patterns wererecorded in successive intervals of 2 min each. Af-ter the sample was fully cured, the capillary wascooled to 50 �C for 10 min, after which the temper-ature was increased again from 50 to 350 �C.

The scattering intensity, as a function of theAzimuthal angle, v, was obtained from integrat-ing the scattering data on a ring along the 2hdirection containing the scattering peaks [Fig.3(A)]. The intensity profiles of Figure 3(B) werefitted using a Maier-Saupe-type function (1) fol-lowing a procedure described elsewhere.17,18

I ¼ I0 þ Aea cos2ðx�x0Þ (1)

where I0 is the free base line, v0 is the position of thepeak maximum, and alpha (a) determines thewidth of the peak. The value of the alpha (a)parameter was used to calculate the average valueof the orientational order parameter hP2i, whichcan be calculated using eq 2.

Figure 2. Microphotographs of TA/HQ/HNA(25)-5K(5000 g mol�1 oligomer); A: low viscous nematic tex-ture at 370 �C (t ¼ 0), B: solidified nematic thermosetafter a 1-h isothermal hold at 370 �C.

Figure 3. A: Diffraction pattern recorded on a two-dimensional detector. Fibers were aligned diagonallywith respect to the 2D detector, from top left to bot-tom right in the figure, at 30 �C. B: Scattering inten-sity as a function of the Azimuthal angle (v) fromdata of (A) as grouped around the scattering peaks.The angle, v, was arbitrarily set equal to zero in thedirection of the fiber.

1374 IQBAL ET AL.


P2h i ¼R 1�1 P2ðcos/Þea cos2 /d cos/

R1

�1

ecos2 /d cos/

(2)

We can now plot the order parameter hP2i ofthe TA/HQ/HNA(25)-5K fibers as a function oftemperature and time as shown in Figure 4.

The as-spun fibers have an initial order param-eter, hP2i of 0.87, which is a typical value foraligned LC fibers. Increasing the temperature to350 �C results in melting of the fibers and a smallbut measurable drop of hP2i to 0.84, which can beattributed to the relaxation of local nematicdomains with respect to one another as the sys-tem relaxes. During the isothermal cure at 350 �Cfor 1 h, hP2i increases to 0.87 and remains virtu-ally the same from this point on. Cooling the sam-ple to 50 �C and reheating to 350 �C does not havea measurable effect on the order parameter. Thisexperiment shows that the chain extension andcrosslink chemistry associated with the phenyl-ethynyl functionality are indeed fully compatiblewith mesophase formation. In addition, thisexperiment shows that it is possible to producehighly aligned all-aromatic nematic networks andretain this order above and below Tg.

DSC Analysis

Differential scanning calorimetry was used tostudy the thermal behavior of our reactive oligo-mers. The first heating cycle, that is, from 25 to370 �C, was used to detect the Tg, melt transition(Tm), and possibly a reaction exotherm due to the

high temperature chain extension/crosslink chem-istry of the terminal phenylethynyl end group.The samples under investigation were kept at 370�C under an N2 atmosphere for 1 h to fully cross-link the polymers, after which the samples werequenched to room temperature and heated again.None of reactive oligomers in series 1 and 2, how-ever, displayed a clearly identifiable Tg upon suc-cessive heating or cooling scans. The low-molecu-lar weight reactive TA/HQ oligomers, that is, TA/HQ-1Kp, TA/HQ-1Km, and TA/HQ-5Km showed amelt transition upon the first heat only. After the1 h isothermal cure, this melt transition could nolonger be detected, confirming the fact that fullycrosslinked thermosets were obtained. A repre-sentative DSC thermogram of TA/HQ-1Kp beforeand after curing is shown in the Figure 5.

Replacing the para-substituted phenylethynylreactive end group (TA/HQ-1Kp) with a meta-sub-stituted phenylethynyl end group (TA/HQ-1Km)results in a significant reduction of the meltingtemperature by � 160 �C for the 1000 g mol�1

oligomer. Although TA/HQ-5Kp does not melt atall, TA/HQ-5Km shows a Tm around 353 �C fol-lowed by immediate crosslinking. Despite thenonlinear meta-substituted phenylethynyl endgroups, TA/HQ-9Km remained intractable andcould not be processed into films or fibers. Themelting temperatures (Tm) for the TA/HQoligomer series end-capped with para- and

Figure 4. Nematic order parameter hP2i of the TA/HQ/HNA(25)-5K fibers as a function of time and tem-perature.

Figure 5. DSC trace of our TA/HQ-1Kp reactiveoligomer measured at a heating rate of 10 �C min�1

under an N2 atmosphere. The upper trace representsthe first heat and shows a Tm � 290 �C, and thelower trace shows the second heat after curing for 1 hat 370 �C.



meta-substituted phenylethynyl reactive function-alities are summarized in Table 2.

The use of chloro-substituted HQ also reducesthe melting point. Although Cai et al.10 showedthat the Tm of poly(chloro-p-phenylene terephtha-late) is [500 �C, we observed that the Tm of our5000 g mol�1 oligomer (TA/ClHQ-5K) is reducedto 264 �C. We prepared this reactive LC model-compound for our high temperature rheologyexperiments.

To lower the Tm of this class of polymers evenfurther and access stable liquid crystal melts, wemodified our TA/HQ copolymer with linear andnonlinear comonomers such as IA, RS, HBA, and

HNA, a well-known approach to lower the transi-tion temperatures in all-aromatic LCPs.9 To mini-mize the sample size, we only explored reactiveoligomer formulations with Mn � 5000 g mol�1.The melt (K-N) transitions for these compoundsare summarized in Table 3.

Introducing kinked structures such as IA andRS, both monomers exhibit an exocyclic bondangle of 120�, appeared to be a very successfulapproach in lowering the melt transitions of theTA/HQ oligomers to acceptable levels, that is,between 300 and 325 �C. However, high concen-trations of IA were required to lower the Tm.When 25 mol % IAwas used, the oligomer TA/HQ/

Table 2. Thermal and Mechanical Properties of the Liquid Crystalline TA/HQ Oligomers Series as Function ofOligomer Molecular Weight and Reactive End-Group Substitution

SampleTm (�C)Oligomer

Td (�C)N2/Air

aChar

Yield (wt %)E0 (GPa)(25 �C)b

E0 (GPa)(100 �C)

E0 (GPa)(200 �C) Tg (�C)

TA/HQ refc – 520/494 40 – – – –TA/HQ-1Kp 291 498/460 61 4.6 3.8 3.0 –TA/HQ-5Kp

d – 498/477 50 – – – –TA/HQ-9Kp

d – 505/484 47 – – – –TA/HQ-1Km 128 491/477 50 3.3 3.1 2.6 309TA/HQ-5Km

d 353 492/467 50 – – – –TA/HQ-9Km

d – 491/472 46 – – – –TA/ClHQ-5Kp 264 486/483 40 5.1 3.8 1.7 275

aTGA experiments performed at a heating rate of 10 �C min�1

b The storage modulus (E0) and Tg data were obtained from DMTA experiments using fully crosslinked films. Measurementswere conducted at a frequency of 1 Hz, and a heating rate of 2 �C min�1.

cHigh-molecular weight TA/HQ, or poly(p-phenylene terephthalate), was prepared for reference purposes. The melt solidifiedat 350 �C and could not be processed.

dCould not be processed into thin films.

Table 3. Thermal and Mechanical Properties of the Liquid Crystalline Reactive TA/HQ Oligomers Series withDifferent Backbone Compositions Having a Molecular Weight of 5000 g mol�1

Sample Tm (�C)Td (�C)N2/Air

aChar

Yield (wt %)E0 (GPa)(25 �C)b

E0 (GPa)(100 �C)

E0 (GPa)(200 �C) Tg (�C)

TA/HQ/IA(25)-5K 340c 486/458 51 – – – –TA/HQ/IA(50)-5K 315 470/451 52 4.2 3.5 1.2 220TA/HQ/IA(75)-5K 321 468/457 58 3.2 2.8 1.1 243TA/HQ/RS(25)-5K 318c 479/458 52 – – – –TA/HQ/RS(50)-5K 316 469/459 56 4.9 3.9 1.5 259TA/HQ/HBA(25)-5K 335c 496/478 47 – – – –TA/HQ/HBA(50)-5K 328 486/475 50 3.2 2.5 1.8 225TA/HQ/HNA(15)-5K 317 493/470 54 4.1 3.1 1.0 164TA/HQ/HNA(25)-5K 290 490/478 52 3.9 2.6 0.7 186

All oligomers were end-capped with para-substituted phenylethynyl end-groups.a TGA experiments performed at a heating rate of 10 �C min�1.

b The storage modulus (E0) and Tg data were obtained from DMTA experiments using fully crosslinked films. Measurementswere conducted at a frequency of 1 Hz, and a heating rate of 2 �C min�1.

c Solid-to-solid (K–K0) transition only, no Tm observed. Could not be processed into films.

1376 IQBAL ET AL.


IA(25)-5K remained intractable, and showed asolid-to-solid (K-K0) transition at 340 �C only. Anoptimum was found at 50 mol % IA, where the re-active oligomer TA/HQ/IA(50)-5K melts at 315 �C,exhibiting a broad nematic melt and no N-I tran-sition, whereas an increase to 75 mol % (TA/HQ/IA(75)-5K) resulted in an increase in Tm, that is,321 �C, and at the same time the high concentra-tion of this nonlinearmonomer limited the stabilityof the nematic phase and the nematicmelt becomesisotropic during cure. Replacing IA for RS has asimilar effect on Tm, that is, at least 50 mol % RS isneeded to access a stable nematicmelt.

Modifying TA/HQ with HBA appears equallyeffective as using IA or RS. Again, at least50 mol % HBA is needed to reduce Tm to accepta-ble levels, that is, TA/HQ/HBA(50)-5K melts at328 �C and forms a stable nematic melt withoutan N-I transition. The use of HNA, a well-knowncrankshaft structure, is the most effective when itcomes to reducing the K-N transition. Two reac-tive oligomers were prepared with 15 mol % HNA(TA/HQ/HNA(15)-5K) and 25 mol % HNA (TA/HQ/HNA(25)-5K), and display K-N transitions at317 and 290 �C, respectively. Again, no N-I transi-tion could be observed.

Dynamic Thermogravimetric Analysis

The decomposition temperatures (Td) of the fullycrosslinked LC thermosets were investigated byTGA using a heating rate of 10 �C min�1. Thethermal stability was evaluated in terms ofweight loss (5 wt %) in both air and nitrogen. Allreactive oligomers were cured at 370 �C for 1 h ina nitrogen atmosphere and cooled to room temper-ature prior to use. The Td and the char yield at600 �C for all liquid crystal thermosets (LCTs) aresummarized in Tables 2 and 3. In general, it canbe stated that all our polymers showed excellentthermal stabilities under an air or nitrogenatmosphere. Our TA/HQ reference material (Ta-ble 2), with no phenylethynyl reactive end groups,displays the highest decomposition temperatures,that is, 520 and 494 �C in nitrogen and air, respec-tively. Reducing the molecular weight and intro-ducing reactive end groups result in a smalldecrease in thermal stability. All crosslinkedoligomers either from the para-substituted ormeta-substituted phenylethynyl functionality dis-play thermal stabilities in the range of 490–505 �C and 460–484 �C in nitrogen and air,respectively. All crosslinked polymers show highchar yields, that is, 40–60% at 600 �C, which is

typical for all-aromatic polymers. Copolymeriza-tion of TA/HQ with IA, RS, HBA, or HNA hasonly a minimal effect on the thermal stability andchar yield, as can be seen in Table 3. These resultsstrongly suggest that the substitution pattern ofthe phenylethynyl end groups and the backbonecomposition has little to no effect on the thermalstability of the fully crosslinked LC thermosets.

Rheology

Rheology experiments were performed to studythe effect of temperature and time on the curingbehavior of our reactive oligomers. The curingchemistry of the phenylethynyl group was studiedby Roberts et al.16 using solid-state NMR, andthey found that phenylethynyl reactive function-alities form a variety of chemical functionalitiesupon curing and depends on factors such as endgroup concentration, that is, oligomer length,temperature, and time.

Most of our oligomers have a melting tempera-ture above 300 �C and could not be investigatedbecause of the temperature limitation of our rhe-ology equipment. As the end group chemistry forall our oligomers is presumed to be the same, wewill discuss the melt behavior of a low meltingmodel compound, that is, TA/ClHQ-5Kp, and con-sider the results as representative for our otherreactive LC oligomers. The first experiment, asshown in Figure 6, depicts the complex melt vis-cosity as a function of temperature. The viscositydrops rapidly when the temperature was in-creased, and at 362 �C, a minimum viscosity of� 4 Pa s�1 was obtained. At this point, chain exten-sion and crosslinking result in an increase in

Figure 6. The complex melt viscosity (|g*|) of TA/ClHQ-5Kp as function of temperature (�C) and time(min). The applied heating rate was 2 �C min�1.



molecular weight, and hence a rapid increase in|g*|. After a 60-min hold, chain extension and cross-linking are completed and the viscosity levels off.

A second series of experiments were performed,where we investigated the dependence of the stor-age modulus (G0) and loss modulus (G00) on thecuring temperature. In four separate experi-ments, we cured our model compound, TA/ClHQ-5Kp, for 1 h at 310, 340, 370, or 400 �C. Theresults are summarized in Figure 7. Figure 7(A)shows a typical viscosity profile for a thermoset-ting polymer under isothermal curing conditions.The sample is held at 310 �C for 1 h, and the ini-tial increase in viscosity is due to the chain exten-

sion chemistry, which predominates at this tem-perature. After 131 min., however, crosslinkingbecomes the dominant reaction, which is identi-fied by the gelation point, that is, where the lossmodulus (G00) crosses over with the storage modu-lus (G0), or G0 ¼ G00 and tan d ¼ 1. When the iso-thermal hold temperature was increased to 340,370, and 400 �C, the gel point was reached in 54,10, or 5 min, respectively. The crossover times aresummarized in Table 4.

From the data, we can also conclude that thechain extension and crosslink chemistry arerather slow at 310 and 340 �C. After 100 min, theviscosity increases steadily, whereas the viscosity

Figure 7. The storage modulus (G0) and loss modulus (G00) of p-TA/ClHQ-5Kp; A: at310 �C, G0 ¼ G00 at t ¼ 131 min; B: at 340 �C, G0 ¼ G00 at t ¼ 54 min; C: at 370 �C,G0 ¼ G00 at t ¼ 10 min; and D: at 400 �C, G0 ¼ G00 at t \ 5 min. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com.]

1378 IQBAL ET AL.


seems to level off after a 60-min isothermal holdat 370 or 400 �C, indicating that the system isclose to being fully cured.

Dynamic Mechanical Thermal Analysis

The thermomechanical properties, that is, storagemodulus (E0) and loss modulus, of our fully curedLCTs were measured using DMTA in the temper-ature range of �100 to 500 �C at 1 Hz. The reac-tive oligomers based on HQ/TA alone, either withthe para- or meta-substituted phenylethynyl endgroups, were difficult to process into thin films.Only the low molecular weight (low melting) TA/HQ-1K oligomers, and TA/ClHQ-5Kp could beprocessed into thin films, which could be used forour DMTA experiments. All of these films showhigh storage moduli up to 200 �C, that is, 3, 2.6,and 1.7 GPa for TA/HQ-1Kp, TA/HQ-1Km, and TA/ClHQ-5Kp, respectively. All films show brittle frac-ture behavior, which is a result of the high cross-link density in these materials. The high degree ofcrosslinking is also reflected in the high Tgs ofthese networks, that is, 309 �C for TA/HQ-1Km and275 �C for TA/ClHQ-5Kp. The storage moduli (E0),and Tgs for the TA/HQ series are reported in Table2. The storagemodulus (E0) as a function of temper-

ature for a fully cured TA/HQ-1Km film is shown inFigure 8.

TA/HQ reactive oligomers copolymerized witha third AA- or AB-type monomer, that is, IA, RS,HBA, or HNA, result mostly in melt processablereactive LC oligomers. When IA or RS are used, aminimum concentration of 25 mol % is required tolower the Tm to acceptable levels, that is, belowthe temperature where chain extension and cross-link chemistry become a factor. The oligomer with50 mol % IA, that is, TA/HQ/IA(50)-5K, remainsliquid crystalline during curing and formed afixed nematic thermoset after 1 h isothermal cure.On the other hand, high concentrations ([75mol %) of IA or RS tend to suppress the meso-phase. Although the liquid crystal phase is lost,the Tg of this polymer is 70 �C higher when com-pared with its high-molecular weight linear ana-log.9 It is quite remarkable to observe that TA/HQ/IA(75)-5K is able to form a stable nematicphase despite the high IA concentration. Uponcuring, however, the nematic phase disappearedand an isotropic thermoset was obtained. Withrespect to the thermomechanical properties, thereis very little difference between the IA- or RS-modified thermosets. All films exhibit excellentstorage moduli at room temperature (� 3–5 GPa)and elevated temperature (� 1.1–1.5 GPa at 200�C). In this series, we noticed that replacing IAfor RS has a pronounced effect on the Tg, that is,an increase by � 40 �C was observed going fromTA/HQ/IA(50)-5K (Tg ¼ 220 �C) to TA/HQ/RS(50)-5K (Tg ¼ 259 �C). The storage modulus (E0) as afunction of temperature for a fully cured TA/HQ/IA(50)-5K film is shown in Figure 9.

Table 4. The Crossover Point Measured at DifferentIsothermal Hold Temperatures

IsothermalHold

Temperature (1 h) 310 �C 340 �C 370 �C 400 �C

G0 ¼ G00 (min) 131 54 10 �5

Figure 8. The storage modulus (n) and tan d (l) of afully curedTA/HQ-1Kmfilmasmeasured byDMTAusinga heating rate of 2 �Cmin�1 andmeasured at 1Hz.

Figure 9. The storage modulus (n) and tan d (l) ofa fully cured TA/HQ/IA(50)-5K film as measured byDMTA using a heating rate of 2 �C min�1 and mea-sured at 1 Hz.



HBA and HNA can be used as comonomers aswell. Again, at least 50 mol % HBA has to be incor-porated to obtain a melt processable LC oligomer.Fully cured thermosets maintain their LC orderand show similar thermomechanical properties astheir IA and RS analogs, that is, E0 ¼ 1.8 GPa at200 �C and Tg ¼ 225 �C.When HNA is used, on theother hand, the storage modulus (E0) and Tg aresomewhat reduced to � 1 GPa and 186 �C, respec-tively. HNA-based LC oligomers display the lowestmelt transition (K-N), and hence provide a broadprocessing window well below the onset of chainextension and crosslinking. The storage moduli(E0), and Tgs for the backbone modified TA/HQ se-ries are reported in Table 3.

CONCLUSIONS

We have successfully demonstrated the synthesisof a new family all-aromatic ester-based LCTswith excellent thermomechanical properties. TA-and HQ-based nematic thermosets could be pre-pared when the molecular weight (Mn) was lim-ited to 1000 g mol�1. The substitution pattern ofthe phenylethynyl end group, that is, para versusmeta, can be used to lower the backbone symme-try, and hence suppress the K-N transition. Allnematic thermosets exhibit Tgs in the range of275–309 �C and high storage moduli ([1.0 GPa at200 �C). Because of the high crosslink density, thefilms are brittle and difficult to handle. To obtainLCTs with useful mechanical properties and highTgs, we modified the TA/HQ backbone with differ-ent AA- and AB-type comonomers, that is, IA, RS,HBA, and HNA. Although these modificationsreduced the Tg somewhat, that is, in the range of186–259 �C, the thermomechanical properties areexcellent. Thin, flexible films could be made usingconventional film processing techniques. Storagemoduli at room temperature vary from 2.6–4.9GPa and 0.7–1.8 GPa at 200 �C. Rheology experi-ments showed that the phenylethynyl end groupundergoes both chain extension and crosslinkchemistries, and these reactions can easily be con-trolled using temperature and time. All reactiveoligomers display nematic mesophases and inmost cases, the nematic order is maintained aftercure. Aligned fibers could be melt spun from ourreactive 5000 g mol�1 TA/HQ/HNA-5K oligomer,and we found that the order parameter hP2i wasnot affected by the chain extension and crosslinkchemistry. Preliminary results show that thinLCT films exhibit very useful mechanical proper-ties, that is, flexural strengths of � 110 MPa, flex-

ural moduli of � 8 GPa, and an elongation atbreak of 9%. The outstanding physical and thermo-mechanical properties suggest that these LC ther-mosets offer significant improved properties overexistingmain-chain LCPs and other high-perform-ance polymers such as PPS, PEEK, and PEKK.

This work was sponsored in part by the NIVR (Nether-lands Agency for Aerospace Programmes), TiconaGmbH, and Ten Cate.

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all-aromatic liquid crystalline thermosets with high glass transition temperatures

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