ORIGINAL PAPER

Synthesis of soluble and thermally stable triptycene-basedpoly(amide-imide)s

Sheng-Huei Hsiao & Wenjeng Guo & Teh-Hua Tsai &Yu-Ting Chiu

Received: 6 January 2014 /Accepted: 13 February 2014# Springer Science+Business Media Dordrecht 2014

Abstract A series of new triptycene-based poly(amide-imide)s(PAIs) were prepared by the phosphorylation polyamidationreactions from 1,4-bis(4-trimellitimidophenoxy)triptycene witharomatic diamines. Most of the resulting PAIs were readilysoluble in many organic solvents and could be solution-castinto tough and flexible polymer films. These PAIs exhibitedglass-transition temperatures (Tgs) in the range of 274–305 °C,and they did not show significant weight-loss before 500 °C. Incomparison with the structurally similar PAIs without thetriptycene unit, the present PAIs exhibited higher solubility,lower crystallization tendency, and better film-forming ability.

Keywords Triptycene . Poly(amide-imide)s .

Diimide-diacid . Trimellitic anhydride . Phosphorylationpolyamidation

Introduction

Aromatic polyimides are known for their outstanding properties,such as unique high thermal stability, good mechanical andelectrical properties as well as excellent chemical resistance[1–4]. They are commercially important materials used exten-sively in photoresists, alignment layers in liquid crystal displays,and high-temperature coatings. However, wholly aromaticpolyimides are difficult to process because of limited solubilityand extremely high softening temperatures that stem from therigidity of the backbone and strong interchain interactions. Toovercome these drawbacks, various attempts have been made todevelop structurally modified polymers existing increased solu-bility with retention of their good thermal stability [5–18].

Several major approaches include the incorporation of flexiblelinkages, non-coplanar moieties, asymmetric units, or bulkypendant groups into the polyimide backbones. On the otherhand, poly(amide-imide)s (PAIs) have been developed as analternative material offering a compromise between excellentthermal stability and processability [19–22]. A number of PAIshave been synthesized through two main routes: via amide–imide-forming reaction from trimellitic anhydride, and throughamide-forming reaction from imide-containing monomers suchas imide-preformed dicarboxylic acid or their derivatives [19].Since we reported the successful synthesis of high-molecular-weight PAIs from the imide ring-preformed dicarboxylic acidsand aromatic diamines [23–25] by the phosphorylationpolyamidation technique [26], this efficient synthetic route hasproved to exhibit significant advantages in preparing operationsas compared with conventional acid chloride or isocyanatemethods. Thus, many novel PAIs have been readily preparedby this convenient technique in our and other laboratories[27–33]. Furthermore, this synthetic procedure can offer us theoption of the incorporation of specific functionalities on theamide or imide side in the PAI backbone [34–36].

Iptycenes refer to rigid molecules with the aromatic ringsattached to a bicyclo[2.2.2]octane bridgehead system [37].The name iptycene originated from the basic unit triptycene,which was first synthesized and named by Bartlett and co-workers in 1942 [38]. Triptycene is a rigid molecular unit withthree blades each composed of a benzene ring. Its rigid, three-dimensional and noncompliant scaffolds have demonstratedspecial utility in the design of materials structure, includingmolecular rotors [39, 40], molecular cages [41], and supramo-lecular architectures [42, 43]. The use of triptycene moiety asa rigid and shape-persistent component is a method tointroduce molecular-scale free volume into a polymer film.Polymers with high triptycene content were found to beinteresting low-k dielectric materials owing to the highdegree of internal free volume [44]. Triptycene incorporation

S.<H. Hsiao (*) :W. Guo : T.<H. Tsai :Y.<T. ChiuDepartment of Chemical Engineering and Biotechnology,National Taipei University of Technology, Taipei 10608, Taiwane-mail: shhsiao@ntut.edu.tw

J Polym Res (2014) 21:391DOI 10.1007/s10965-014-0391-x

into certain polyester backbones has revealed dramatic enhance-ments in polymer mechanical properties including increasedstiffness, toughness, and ultimate tensile strength throughmolecular threading and interlocking [45–48]. Insertion ofrigid, packing-disruptive triptycene units into polyimide back-bones was found to improve the solubility of polyimides incommon organic solvents while retaining high Tg and ther-mal stability [49–51]. It has also been demonstrated thattriptycene-containing polyimides can be considered as poten-tial polymeric membrane materials for gas separation due toa high internal free volume induced by the three-dimensionalrigid triptycene unit [52]. As reported in our previous work[53], 1,4-bis(4-aminophenoxy)triptycene was synthesized andreacted with aromatic dianhydrides leading to a series oftriptycene polyimides. However, most of these triptycenepolyimides were insoluble in conventional organic solvents,probably due to the rigid nature of the polymer backbonesand strong intermolecular interactions. In view of the factthat aromatic poly(amide-imide)s possess balanced character-istics between polyamides and polyimides such as highthermal stability and good mechanical properties togetherwith ease of processability, this work deals with the synthesisand characterization of a series of new triptycene-basedpoly(amide-imide)s derived from a triptycene diimide-diacid,namely 1,4-bis(4-trimellitimidophenoxy)triptycene, and aro-matic diamines. The solubility, film-forming ability, tensileproperties, crystallinity, and thermal properties of the poly(amide-imide)s were investigated and compared with those ofanalogous counterparts without the triptycene groups.

Experimental

Materials

N,N-Dimethylformamide (DMF, Tedia), N-methyl-2-pyrrol-idone (NMP, Tedia), and pyridine (Py, Wako) were dried over

calcium hydride for 24 h, distilled under reduced pres-sure, and stored over 4 Å molecular sieves in a sealedbottle. Commercially available aromatic diamines includingp-phenylenediamine (2a, TCI), benzidine (2b, TCI), 4,4′-oxydianiline (2c, Wako), 1,4-bis(4-aminophenoxy)benzene(2d, TCI), bis(4-aminophenoxy)biphenyl (2e, TCI) wereused as received. According to the procedure reportedpreviously [53], 1,4-bis(4-aminophenoxy)triptycene (2f)(mp=254–255 °C) was prepared by the aromatic nucleo-philic substitution reaction of p-chloronitrobenzene with1,4-dihydroxytriptycene in the presence of potassium car-bonate and by the subsequent reduction of the intermedi-ate bis(p-nitrophenoxy) compound with hydrogen as thereducing reagent and palladium on carbon (Pd/C) as thecatalyst. Trimellitic anhydride (Wako) and calcium chloride(CaCl2, Wako) was dried under vacuum at 250 °C for 3 hprior to use. Triphenyl phosphite (TPP, TCI) was usedwithout further purification.

Synthesis of 1,4-bis(4-trimellitimidophenoxy)triptycene (1)

A 500 mL round-bottom flask was charged with 5.0 g(0.01 mol) of 1,4-bis(4-aminophenoxy)triptycene, 4.1 g(0.02 mol) of trimellitic anhydride, and 230 mL of acetic acid.The heterogeneous mixture was stirred at 120 °C for 8 h. Afterthat, the reaction mixture was poured into 750 mL of stirredmethanol to precipitate a yellow powder, and the product wascollected by filtration, washed thoroughly with water, anddried in vacuo at 150 °C. Yield 7.4 g (85 %); mp=385–387 °C. IR (KBr) (see Fig. 1): 3300 (O-H stretch), 1700(C=O stretch), 1388 (C-N stretch), 1241 cm−1 (C-O stretch).1H NMR (500 MHz, DMSO-d6, δ, ppm) (Fig. 2): 5.76(s, 2H, Hb), 6.89 (s, 2H, Ha), 7.01 (dd, J=5.5 and3.5 Hz, 4H, Hc), 7.06 (d, J=9.0 Hz, 4H, He), 7.33(dd, J=5.5 and 3.5 Hz, 4H, Hd), 7.47 (d, J=9.0 Hz, 4H, Hf),8.09 (d, J=7.5 Hz, 2H, Hi), 8.33 (d, J=1.5 Hz, 2H, Hg), 8.43

Fig. 1 IR spectrum of diimide-diacid 1

391, Page 2 of 10 J Polym Res (2014) 21:391

(dd, J=7.5 and 1.5 Hz, 2H, Hh), 11.5 (br, 2H, –COOH).13C NMR (125 MHz, DMSO-d6, δ, ppm) (Fig. 2): 47.3 (C4),117.3 (C9), 119.2 (C1), 123.4 (C14), 123.8 (C17), 124.0 (C7),

125.4 (C6), 126.3 (C13), 129.2 (C10), 132.1 (C18), 135.0 (C15),135.4 (C16), 136.4 (C11), 139.0 (C3), 144.3 (C5), 146.7 (C2),157.7 (C8), 165.8 (−COOH), 166.4 (C12 + C19).

Fig. 2 a 1H, b 13C, c H-H COSY, and d C-H HSQC NMR spectra of diimide-diacid 1 in DMSO-d6

1

2AcOH

O

O

O

COOH

2AcOH

O

O

O

COOH

1'

OOH2N NH2 + OON N

O

O

O

O

HOOC COOH

OO NHOOC COOH

O

O

O

O

OOH2N NH2

Scheme 1 Synthesis of diimide-diacid monomers 1 and 1′

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Synthesis of PAIs

The synthesis of PAI 3c was used as an example to illustratethe general synthetic route used to produce the PAIs. A mix-ture of 0.490 g (0.6 mmol) of the diimide-diacid monomer 1,0.120 g (0.6 mmol) of 4,4′-oxydianiline (2c), 0.1 g of anhy-drous calcium chloride, 0.6 mL of triphenyl phosphite (TPP),0.2 mL of pyridine, and 0.6 mL of NMP was heated withstirring at 120 °C for 3 h. The resulting polymer solutionwas poured slowly into 150 mL of methanol producing a

tough fiber-like precipitate that was washed thoroughlywith methanol and hot water, collected on a filter, anddried. The inherent viscosity of the polymer was 0.40 dL/g,measured in DMAc (containing 5 wt % LiCl) at a concentra-tion of 0.5 g/dL at 30 °C. The IR spectrum of 3c film exhibitedcharacteristic amide and imide absorption bands at 3290 cm−1

(amide N–H str.), 1670 cm−1 (amide carbonyl str.), 1780 and1720 cm−1 (imide carbonyl stretching). 1H NMR (500 MHz,DMSO-d6, δ, ppm): 5.76 (s, 2H, Hb), 6.89 (s, 2H, Ha), 7.02(br. s, 4H, Hc), 7.06 (br. s, 8H, He + Hk), 7.34 (br. s, 4H, Hd),

Ar

2a-f

TPP, Py

NMP, CaCl2

H2N NH2

3a-f

Ar =

O O O

O O

a b

e

dc

f

1'

1

3'a-f

O O

ON

O

O

N

O

O

NH

NHOO

n

OAr

ON

O

O

N

O

O

NH

NHOO

n

OAr

OON NHOOC COOH

O

O

O

O

OON NHOOC COOH

O

O

O

O

Scheme 2 Synthesis of PAIs

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7.48 (d, J=8.5 Hz, 4H, Hf), 7.85 (d, J=9.0 Hz, 4H, Hj), 8.12(d, J=7.5 Hz, 2H, Hi), 8.46 (d, J=7.5 Hz, 2H, Hh), 8.56(s, 4H, Hg), 10.65 (amide protons).

Preparation of PAI films

A solution of the polymer was made by dissolving about0.6 g of the PAI sample in 8 mL of hot DMAc. The

homogeneous solution was poured into a 7-cm glassPetri dish, which was placed in a 90 °C oven overnightfor evaporation of the solvent, and then the film wasstripped off from the glass substrate and further dried invacuum at 160 °C for 6 h. The obtained films wereabout 70 μm thick and were used for solubility andtensile tests, X-ray diffraction measurements, and thermalanalyses.

Table 1 Inherent viscosity andsolubility behavior of PAIs

a Inherent viscosity measured at aconcentration of 0.5 g/dL inDMAc-5 wt % LiCl at 30 °Cb The qualitative solubility wastested with 10 mg of a samplein 1 mL of stirred solvent. +,soluble at room temperature; +h,soluble on heating; –, insolubleeven on heating. Solvent: NMPN-methyl-2-pyrrolidone; DMAcN,N-dimethylacetamide; DMFN,N-dimethylformamide; DMSOdimethyl sulfoxide; THFtetrahydrofuranc Insoluble in DMAc

Polymer code ηinha (dL/g) Solubility in various solventsb

NMP DMAc DMF DMSO m-Cresol THF

3a 0.43 +h - – – – –

3b -c – – – – – –

3c 0.40 + + + + +h –

3d 0.46 + + + + +h –

3e 0.61 + + + + +h –

3f 0.38 + + + + +h –

3′a –c – – – – – –

3′b – – – – – – –

3′c – – – – – – –

3′d – – – – – – –

3′e – – – – – – –

3′f 0.31 + + + + +h –

Fig. 3 IR spectra of a typical setof isomeric PAIs 3d and 3′f

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Measurements

IR spectra were recorded on a Horiba FT-720 Fourier trans-form infrared (FTIR) spectrometer. 1H and 13C NMR spectrawere measured on a Bruker AVANCE 500 MHz FT-NMRsystem with DMSO-d6 as the solvent and tetramethylsilane asan internal standard. The inherent viscosities were determinedwith a Cannon-Fenske viscometer at 30 °C.Wide-angle X-raydiffraction (WAXD) measurements were performed atroom temperature (ca. 25 °C) on a Shimadzu XRD-6000X-ray diffractometer (40 kV, 20 mA), using graphite-monochromatized Cu-Kα radiation (λ=1.5418 Å). Themechanical properties of the polymer films were determinedby a Bose ElectroForce 3220 test instrument with a 5 kg loadcell at a crosshead speed of 5 mm/min on strips approximately70 μm thick and 0.5 cm wide with a 2 cm gauge length. An

average of at least five individual determinations was used.DSC analyses were performed on a Perkin-Elmer Pyris 1 DSCat a scan rate of 20 °C/min in flowing nitrogen. Glass-transition temperatures (Tg) were read as the midpoint tem-perature of the heat capacity jump and were taken from thesecond heating scan after a quick cooling down from 400 °Cto room temperature. Thermogravimetric analysis (TGA) wasperformed with a Perkin-Elmer Pyris 1 TGA. Measurementswere carried out on 3–5 mg film samples heated in flowingnitrogen or air (90 cm3/min) at a heating rate of 20 °C/min.Thermomechanical analysis (TMA) was conducted with aPerkin-Elmer TMA 7 at a scan rate of 10 °C/min with apenetration probe of 1.0 mm diameter under an appliedconstant load of 10 mN. Softening temperatures (Ts) weretaken as the onset temperature of probe displacement onthe TMA traces.

Fig. 4 a 1H NMR and b H-HCOSY NMR spectra of PAI 3c inDMSO-d6

Fig. 5 Wide-angle X-raydiffractograms of PAIs

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Results and discussion

Monomer synthesis

The triptycene diimide-diacid monomer 1was prepared by thecondensation of 1,4-bis(4-aminophenoxy)triptycene with twomolar equivalents of trimellitic anhydride in refluxing glacialacetic acid (Scheme 1). The FTIR spectrum diimide-diacid 1is illustrated in Fig. 1. The absorption bands at around 2700–3700 (carboxyl O-H stretching), 1778 (asymmetric imideC=O stretching), 1749 (symmetric imide C=O stretching),1700 (carboxyl C=O stretching), 1388 (imide ring C-Nstretching), and 730 cm−1 (imide ring deformation) confirmthe presence of imide ring and carboxylic acid groups in thestructure. Figure 2 shows the 1H NMR and 13C NMR spectraof compound 1. Assignments of each carbon and protonassisted by the two-dimensional NMR spectra are also indi-cated in these spectra, and they are in good agreement with theproposed molecular structure of compound 1. The diimide-diacid 1′ was prepared from the condensation of 1,4-bis(4-aminophenoxy)benzene and trimelliric anhydride by asimilar synthetic procedure of 1.

Polymer synthesis

A series of novel PAIs 3a-f were synthesized from thetriptycene diimide-diacid 1 with various aromatic diamines(2a-f) by the Yamazaki-Higashi phosphorylation polyamidationreaction [26] using TPP and pyridine as condensing agents(Scheme 2). Except for PAI 3b, all the polymerizationsproceeded homogeneously throughout the reaction. Early pre-cipitation of the product occurred in the case of PAI 3b becauseof its insolubility in the reaction medium. As shown in Table 1,the resulting PAIs had inherent viscosities ranged from 0.38 to0.61 dL/g. The solution-cast film of PAI 3a cracked uponfingernail creasing possibly due to its highly rigid backbone.No attempt was made to cast the polymer film from 3b becauseof insolubility in available solvents. However, the other PAIs(3c-f) withmore flexible chain could be solution-cast into strongand tough films. For comparative studies, structurally relative 3′series PAIs without the triptycene group were also prepared andcharacterized. Except for 3′f, all the 3' series PAIs precipitated

from the reactionmedium and couldn’t be dissolved in availableorganic solvents for film casting.

The structures of the PAIs were confirmed by FTIR and 1HNMR spectroscopy. Figure 3 shows a typical set of FTIRspectra for PAIs 3d and 3'f with an isomeric repeating unit.

Table 2 Tensile properties of PAI films

Polymercode

Tensile strength(MPa)

Elongation atbreak (%)

Initial modulus(GPa)

3c 64 4 2.8

3d 69 5 2.7

3e 68 9 2.5

3f 86 4 3.2

3′f 67 5 2.8

Table 3 Thermal properties of PAIs

Polymer code Tgb (°C) Ts

c (°C) Td, 10d (°C) Char yielde (%)

In N2 In air

3a 305 –g 558 539 66

3b –f – 550 538 60

3c 279 264 550 549 65

3d 274 265 578 549 65

3e 275 255 571 559 68

3f 298 276 565 543 68

3′a – – 557 517 57

3′b – – 568 527 61

3′c – – 553 501 51

3′d – – 544 516 61

3′e – – 548 526 63

3′f 275 260 577 547 67

a The polymer film samples were heated at 300 °C for 30 min before allthe thermal analysesb The samples were heated from 50 to 400 °C at a scan rate of 20 °C/minfollowed by rapid cooling to 50 °C at −200 °C/min in nitrogen. Themidpoint temperature of baseline shift on the subsequent DSC trace(from 50 to 400 °C at heating rate 20 °C/min) was defined as 0054gc Softening temperature measured by TMAwith a constant applied loadof 10 mN at a heating rate of 10 °C/minndDecomposition temperature at which a 10 % weight loss was recordedby TGA at a heating rate of 20 °C/min and a gas flow rate of 20 cm3 /mine Residual weight percentages at 800 °C under nitrogen flowfNo discernible transition was detectable by DSCgNo available specimens for the TMA testing

Fig. 6 DSC curves of PAIs 3f and 3′f

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All of the PAIs showed the characteristic absorption bandsof the imide ring near 1720 (symmetric C=O stretching)and 1780 cm−1 (asymmetric C=O stretching), and the char-acteristic amide absorption bands at near 3300 (N–H) and1670 cm−1 (amide C=O). Figure 4 depicts a typical 1HNMR spectrum of the representative PAI 3c in DMSO-d6,in which all the protons in the repeating unit can beassigned. The signal appearing at 5.76 ppm is assigned tothe bridgehead protons of the triptycene units. The aromaticprotons resonated in the region of 6.89~8.56 ppm. Assign-ments of each proton, assisted by the H-H COSY NMRspectroscopy, are in good agreement with the structure of

the repeating unit. The resonance peak at 10.65 ppm sup-ports the formation of amide linkage.

Properties of the PAIs

Solubility of the polymer is a critical factor that shows itsprocessability for different applications. The solubility behaviorof these PAIs was determined by stirring 0.01 g of polymersin 1 mL of solvent, including NMP, DMAc, DMF, DMSO,m-cresol, and THF for 24 h at room temperature (25 °C) or uponheating in tightly closed vials. As in Table 1, PAIs 3a and 3bderived from more rigid diamines such as p-phenylenediamine

Fig. 7 TMA curves of PAIs3f and 3′f

Fig. 8 TGA curves of PAIs3d and 3′f in both air andnitrogen atmospheres

391, Page 8 of 10 J Polym Res (2014) 21:391

and benzidine were insoluble in most of the test solvents;however, the other PAIs 3c-f were readily soluble in aproticdipolar solvents such as NMP, DMAc, DMF, and DMSO atroom temperature. This result indicates that insertion of etherlinkages into the polymer chains of these PAIs reduced thechain rigidity and led to an increased solubility. Except for thetriptycene-containing 3′f, the 3′ series PAIs were insoluble in allthe tested solvents. The insolubility of these PAIs can be attrib-uted to their high crystallinity as evidenced by their WAXDpatterns (Fig. 5). The enhancement in solubility of 3c-e ascompared to the 3′c-e analogs is clearly attributed to the effectarising from the bulky triptycene segments, which decreased thebackbone symmetry and regularity, weakened intermolecularforces such as hydrogen bonding between the polymer chains,and hindered close chain packing, subsequently causing adecrease in crystallinity. Thus, the amorphous nature ofthese triptycene PAIs also reflected in their good solubilityin common organic solvents and good film-forming ability.

The tensile properties of the PAI films are summarized inTable 2. The polymer films had strengths at break of 64–86 MPa, an elongation at break of 4–9 %, and an initialmodulus of 2.5–3.2 GPa. All of these PAI films exhibitedmoderately high tensile strengths, indicating that they arestrong polymeric materials. The higher tensile strength andinitial modulus associated with PAI 3fmay be explained by itshigher triptycene content in the backbone.

The thermal properties of all the PAIs were evaluated byDSC, TMA, and TGA techniques. Table 3 summarizes thethermal behavior data of all the PAIs. DSC experiments wereconducted at a heating rate of 20 °C/min in nitrogen. The glasstransition temperatures (Tgs) were read from the second DSCheating traces after rapid cooling from 400 °C to 50 °C. Theamorphous PAIs such as 3a, 3c-3f, and 3′f showed obviousglass transitions in the range of 274–305 °C. As expected,the highest Tg of 305 °C was observed for PAI 3abecause of its rigid diamine component, and the lowestTg of 274 °C was observed for PAI 3d due to its flexiblediamine residue. Typical DSC traces for PAIs 3f and 3′fare illustrated in Fig. 6. However, the crystalline PAIs 3band 3′a-3′e didn’t show any discernible glass transitionson their DSC thermograms.

The softening temperatures (Ts; may be referred toapparent Tg) of the polymer films were determined by theTMA method with a penetration mode. They were read fromthe onset temperature of the probe displacement on the TMAtrace. As a representative example, the TMA traces of PAIs 3fand 3′f are depicted in Fig. 7. The Ts values obtained by TMAwere recorded in the 255–276 °C range, and the trend of Tsvariation with the chain stiffness is similar to that of Tgobserved in the DSC experiments. The Ts values of the PAIfilms were lower than the Tg values determined by DSC. Thismay indicate that a higher degree of plasticity near Tg due tothe increased free volume by the triptycene unit in these PAIs.

The thermal and thermo-oxidative stabilities of thePAIs were evaluated by TGA in both nitrogen and airatmospheres. Typical TGA thermograms for PAIs 3dand 3′f in both nitrogen and air atmospheres are shownin Fig. 8. The temperatures for 10 % weight loss (Td,10)of the 3 series PAIs in nitrogen and air atmospheresstayed within 538–578 °C and 539–559 °C, respectively.They left more than a 60 % char yield at 800 °C in nitrogen.The TGA data indicated that these PAIs had high thermalstability.

Conclusions

The diimide-diacid 1 was used as a new building block ofPAIs with triptycene units in the main chain. Most of theresulting PAIs are amorphous and easily soluble in polarorganic solvents and can be solution-cast into flexible andstrong films. They also show high thermal stability. In com-parison with their counterparts without the triptycene units,the present PAIs revealed an enhanced solubility to organicsolvents and better film-forming ability. Thus, these propertiesmade these triptycene-based PAIs promising high-performancepolymeric materials.

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