ORIGINAL PAPER Triptycene poly(ether-imide)s with high solubility and optical transparency Sheng-Huei Hsiao & Hui-Min Wang & Ji-Shian Chou & Wenjeng Guo & Tzong-Ming Lee & Chyi-Ming Leu & Chun-Wei Su Received: 6 May 2011 /Accepted: 28 August 2011 # Springer Science+Business Media B.V. 2011 Abstract A series of new poly(ether-imide)s containing three- dimensional triptycene moieties were prepared from 1,4- bis(3,4-dicarboxyphenoxy)triptycene dianhydride with various aromatic diamines via a conventional two-stage process. The inherent viscosities of the amic acid pre- polymers were in the range of 0.44~0.91 dL/g. Most of the resulting poly(ether-imide)s presented good solubility in many organic solvents and could be solution-cast into transparent and strong films. They also showed good thermal stability with glass-transition temperatures of 238~302 °C and 10% weight loss temperatures in excess of 572 °C. These triptycene-based poly(ether-imide)s showed enhanced optical transparency, decreased color intensity and lowered dielectric constants as compared to conventional aromatic polyimides. The poly(ether-imide)s derived from trifluoromethyl-containing bis(ether amine)s could afford highly optically transparent and almost colorless films. Keywords Polyimides . Triptycene . Thermal properties . Optical transparency . Structure–property relations Introduction Wholly aromatic polyimides such as DuPont’ s Kapton and Ube’ s Upilex are well known for their high thermal stability and excellent mechanical properties [1–3]. However, most of the conventional aromatic polyimides are difficult to be processed because of their high melting or glass-transition temperatures (T g ) and limited solubility in most organic solvents, which are caused by their rigid backbones and strong interchain interactions. Thus, polyimide processing is generally carried out via poly(amic acid) precursor, and then converted to polyimide by vigorous thermal or chem- ical cyclodehydration. However, this process has inherent problems such as emission of volatile by-products and storage instability of poly(amic acid) solution. To overcome these problems, many attempts have been made to the syn- thesis of soluble and processable polyimides in fully imidized form while maintaining their excellent properties [4–11]. In addition, the aromatic polyimide films generally show consid- erable coloration from light yellow to dark brown due to their highly conjugated aromatic structures, intermolecular charge- transfer complexing, and electronic polarization interactions [12]. The development of soluble and colorless polyimides has been to attract great interesting because they can be incorporated into a variety of electronic devices or products that use flexible and transparent substrates [13]. Although soluble and colorless polyimides can be prepared from alicyclic dianhydride or diamine monomers [14–18], these materials may have limited long term thermal stability because of less stable aliphatic segments. The coloration of aromatic polyimides is mainly caused by the intra- and intermolecular charge transfer (CT) interactions between alternating electron-donating diamine and electron- accepting dianhydride components [19]. Incorporation of bulky, packing-disruptive groups such as tert-butyl and norborane groups into the diamine or dianhydride monomers reduces the amount of interchain electronic interactions and lessens CT complex formation, thereby leading to soluble and colorless polyimides [20, 21]. S.-H. Hsiao (*) : H.-M. Wang : J.-S. Chou : W. Guo Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan e-mail: [email protected]T.-M. Lee : C.-M. Leu : C.-W. Su Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan J Polym Res (2012) 19:9757 DOI 10.1007/s10965-011-9757-5
Transcript
ORIGINAL PAPER
Triptycene poly(ether-imide)s with high solubilityand optical transparency
Sheng-Huei Hsiao & Hui-Min Wang & Ji-Shian Chou &
Wenjeng Guo & Tzong-Ming Lee & Chyi-Ming Leu &
Chun-Wei Su
Received: 6 May 2011 /Accepted: 28 August 2011# Springer Science+Business Media B.V. 2011
Abstract A series of new poly(ether-imide)s containing three-dimensional triptycene moieties were prepared from 1,4-bis(3,4-dicarboxyphenoxy)triptycene dianhydride withvarious aromatic diamines via a conventional two-stageprocess. The inherent viscosities of the amic acid pre-polymers were in the range of 0.44~0.91 dL/g. Most ofthe resulting poly(ether-imide)s presented good solubility inmany organic solvents and could be solution-cast intotransparent and strong films. They also showed goodthermal stability with glass-transition temperatures of238~302 °C and 10% weight loss temperatures in excess of572 °C. These triptycene-based poly(ether-imide)s showedenhanced optical transparency, decreased color intensity andlowered dielectric constants as compared to conventionalaromatic polyimides. The poly(ether-imide)s derived fromtrifluoromethyl-containing bis(ether amine)s could affordhighly optically transparent and almost colorless films.
Wholly aromatic polyimides such as DuPont’s Kapton andUbe’s Upilex are well known for their high thermal stability
and excellent mechanical properties [1–3]. However, mostof the conventional aromatic polyimides are difficult to beprocessed because of their high melting or glass-transitiontemperatures (Tg) and limited solubility in most organicsolvents, which are caused by their rigid backbones andstrong interchain interactions. Thus, polyimide processingis generally carried out via poly(amic acid) precursor, andthen converted to polyimide by vigorous thermal or chem-ical cyclodehydration. However, this process has inherentproblems such as emission of volatile by-products andstorage instability of poly(amic acid) solution. To overcomethese problems, many attempts have been made to the syn-thesis of soluble and processable polyimides in fully imidizedform while maintaining their excellent properties [4–11]. Inaddition, the aromatic polyimide films generally show consid-erable coloration from light yellow to dark brown due to theirhighly conjugated aromatic structures, intermolecular charge-transfer complexing, and electronic polarization interactions[12]. The development of soluble and colorless polyimideshas been to attract great interesting because they can beincorporated into a variety of electronic devices or productsthat use flexible and transparent substrates [13]. Althoughsoluble and colorless polyimides can be prepared fromalicyclic dianhydride or diamine monomers [14–18], thesematerials may have limited long term thermal stability becauseof less stable aliphatic segments.
The coloration of aromatic polyimides is mainly caused bythe intra- and intermolecular charge transfer (CT) interactionsbetween alternating electron-donating diamine and electron-accepting dianhydride components [19]. Incorporation ofbulky, packing-disruptive groups such as tert-butyl andnorborane groups into the diamine or dianhydridemonomers reduces the amount of interchain electronicinteractions and lessens CT complex formation, therebyleading to soluble and colorless polyimides [20, 21].
S.-H. Hsiao (*) :H.-M. Wang : J.-S. Chou :W. GuoDepartment of Chemical Engineering and Biotechnology,National Taipei University of Technology,Taipei, Taiwane-mail: [email protected]
T.-M. Lee : C.-M. Leu : C.-W. SuMaterial and Chemical Research Laboratories, IndustrialTechnology Research Institute,Hsinchu, Taiwan
J Polym Res (2012) 19:9757DOI 10.1007/s10965-011-9757-5
Polyimides bearing trifluoromethyl (–CF3) substituents havebeen the subject of considerable interest to material chemistssince the presence of a bulky –CF3 group in the polyimidestructure has been shown to cause significant improvements intheir properties over the corresponding non-fluorinatedpolyimides [22–27]. It has been demonstrated that the incor-poration of bulky –CF3 group onto the polyimide backbonesresulted in an enhanced solubility and optical transparencytogether with a lowered dielectric constant, which was attrib-uted to low polarizability of the C–F bond and the increase ofthe fractional free volume. Soluble and colorless polyimidescould be obtained from –CF3 substituted diamines and ether-linked dianhydrides because this combination led to reducedintra- and intermolecular CT interactions [28, 29].
Iptycenes are a class of structurally unique compoundsthat consist of a number of arene rings joined together toform the bridges of [2.2.2] bicyclic ring systems [30]. Thename iptycene originated from the basic unit triptycene,which was first synthesized and named by Bartlett and co-workers in 1942 [31]. Later, triptycene has become readilyavailable thanks to the well known work of Wittig [32, 33]on the preparation of dehydrobenzene (benzyne) and itsinteraction with anthracene. The chemistry of triptycenehas been studied in comparative detail and has been sum-marized in an earlier review paper reported by the Russianresearchers [34]. Triptycene is a rigid molecular unit withthree blades each composed of a benzene ring. Its rigid,three-dimensional framework has been incorporated inmolecular rotors, [35, 36] molecular cages, [37, 38] andsupramolecular architectures [39, 40]. Perhaps the earliestefforts in the study of triptycene polymers were made atEastman Kodak and DuPont in the late 1960s wherein bifunc-tional, bridgehead-substituted triptycenes were synthesizedand used to prepare a series of triptycene polymers, includingpolyesters, polyamides, polyurethanes, and a polyoxadiazole[41, 42]. However, the synthesis and properties of triptycene-containing polymers have only begun to attract attention since1998 [43–49]. The use of triptycene moiety as a rigid andshape-persistent component is a method to introducemolecular-scale free volume into a polymer film. Polymerswith high triptycene content were found to be interesting low-k dielectric materials owing to the high degree of internal freevolume [44]. Incorporation of triptycene units into polyesterswas also found to significantly enhance the mechanical per-formance of film samples through molecular threading andinterlocking [45, 46]. Additionally, it was shown thattriptycene-containing polycarbonates displayed improve-ments in mechanical properties at both low and high strainrates [48].
The work reported by Kasashima et al. seems to be the firstarticle dealing with the incorporation of triptycene moietiesinto the backbone of the polyimides [50]. Recently, some newtriptycene polyimides have been developed [51–53]. Due to
the presence of rigid, packing-disruptive triptycene unit, thepolyimides exhibited good solubility in common organic sol-vents while preserving moderately high Tg and good thermalstability [51, 52]. The polyimide incorporating a three-dimensional, rigid triptycene framework has a high internalfree volume to allow fast molecular diffusion, leading to highgas permeability [53]. In addition, a facile synthesis oftriptycene-based hyperbranched polyimides from a triptycenetriamine has recently been reported by Cheng and coworkers[54]. In view of the structural feature of triptycene, synthesiz-ing and characterizing new polyimides bearing triptycenemoieties in the backbone are interesting enough to be inves-tigated further. This work deals with the synthesis and prop-erties of a series of triptycene containing poly(ether-imide)sderived from a triptycene bis(ether anhydride), namely 1,4-bis(3,4-dicarboxyphenoxy)triptycene dianhydride, and variousaromatic diamines. The rigid, three-dimensional triptyceneunits may decrease interchain interactions and hinder closechain packing. Thus, it was expected that the introduction oftriptycene structure into the polymer backbone could improvethe solubility, decrease the color intensity, and preserve mod-erately high Tg values of the poly(ether-imide)s. The poly(ether-imide) derived from the triptycene bis(ether anhydride)with a typical trifluoromethyl-substituted bis(ether amine),1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene, couldafford an essentially colorless, highly optically transparentfilm.
Experimental
Materials
Anthracene (Alfa Aesar), p-benzoquinone (Acros), 4-nitrophthalonitrile (Acros), N,N-dimethyl formamide(DMF, Tedia), potassium carbonate (K2CO3, Wako), potas-sium hydroxide (KOH, Wako), pyridine (Py) (Wako) andacetic anhydride (Acros) were used as received. N,N-Dimethylacetamide (DMAc) (Tedia) was dried over calciumhydride for 24 h, distilled under reduced pressure, and storedover 4 Å molecular sieves in a sealed bottle. Commerciallyavailable aromatic diamines such as p-phenylenediamine (5a,TCI), benzidine (5b, Wako), 4,4′-oxydianiline (5c, TCI),3,4′-oxydianiline (5e, TCI), 1,4-bis(4-aminophenoxy)benzene(5f, TCI) were used without further purification. 2-Trifluoromethyl-4,4′-diaminodiphenyl ether (5d; mp: 112–113 °C) [55] and 1,4-bis(4-amino-2- trifluoromethylphenoxy)benzene (5g; mp: 132–133 °C) [24] were synthesized bythe nucleophilic chloro-displacement reaction of 2-chloro-5-nitrobenzotrifluoride with 4-nitrophenol and hydroquinone,respectively, in the presence of potassium carbonate, followedby the hydrazine Pd/C-catalyzed reduction of the intermediatedinitro compounds. By a similar synthetic procedure, bis
Page 2 of 12 S. Hsiao et al.
[4-(4-aminophenoxy)phenyl] ether (5h; mp: 128–129 °C)[56] and 1,4-bis(4-aminophenoxy)triptycene (5i; mp: 254–255 °C) [57] were prepared starting from the aromatic nucle-ophilic substitution reaction of p-chloronitrobenzene with 4,4′-dihydroxydiphenyl ether (TCI) and 1,4-dihydroxytriptycene,respectively, in the presence of potassium carbonate, followedby the hydrazine Pd/C-catalyzed reduction of the intermediatedinitro compounds.
Monomer synthesis
1,4-Dihydroxytriptycene (1)
p-Benzoquinone (12.1 g, 112.0mmol) and anthracene (10.0 g,56.0 mmol) were refluxed in dry toluene (100 mL) undernitrogen for 4 h. The reaction mixture was cooled to roomtemperature, filtered, and the residue was washed with toluene(2×5mL). The resulting crude quinone (14.2 g) was dissolvedin hot acetic acid (350 mL) and 48% aq. HBr (4 mL) wasadded dropwise to the solution. An off-white precipitate wasdeveloped immediately. The product was collected by filtra-tion and washed with acetic acid (15 mL), hexane (30 mL) anddried. The yield was 12.8 g (80%); mp0340–342 °C. IR(KBr) (see Fig. 1): 3,260 cm−1 (O-H stretch), 1,240 (C-Ostretch). 1H NMR (500 MHz, DMSO-d6, δ, ppm): 5.81 (s,2H, Hb), 6.92 (s, 2H, Ha), 6.96 (dd, J05.3 and 3.2 Hz, 4H, Hc),7.38 (dd, J05.3 and 3.2 Hz, 4H, Hd), 8.81 (s, 2H, -OH). 13CNMR (125 MHz, DMSO-d6, δ, ppm): 46.6 (C4), 112.8 (C1),123.4 (C7), 124.5 (C6), 131.9 (C3), 144.7 (C5), 145.7 (C2).
2
1
34
5
6
7
ab
c
OH
HO
d
1,4-Bis(3,4-dicyanophenoxy)triptycene (2)
In a 250-mL flask, 5.9 g (20.0mmol) of 1,4-dihydroxytriptycene,5.9 g (41.0 mmol) of 4-nitrophthalonitrile, and 2.8 g(20.0 mmol) of K2CO3 were dissolved/suspended in 50 mLof DMF. The suspension was stirred at room temperature forabout 24 h. Then, the reaction mixture was poured into400 mL of water, and the precipitated pale white solid wascollected and washed thoroughly with methanol and water.The yield of bis(ether dinitrile) 2 was 10.3 g (96%), mp>400 °C. IR (KBr): 2,231 cm−1 (C≡N stretch). 1H NMR(500 MHz, DMSO-d6, δ, ppm): 5.66 (s, 2H, Hb), 7.02(dd, J05.3 and 3.2 Hz, 4H, Hc), 7.03 (s, 2H, Ha), 7.29
In a 250-mL flask, a suspension of bis(ether dinitrile) 2(10.3 g, 19.1 mmol) in an ethanol/water mixture(100 mL/100 mL) containing 20 g (0.36 mol) of dissolvedKOH was boiled under reflux. Refluxing was continued forabout 48 h until the evolution of ammonia had ceased. Theresulting clear solution was filtered hot to remove any possibleinsoluble impurities. The filtrate was allowed to cool and wasthen acidified by hydrochloric acid to a pH of 2–3. The white
Fig. 1 IR spectra of the synthesized compounds
Triptycene poly(ether-imide)s with high solubility and transparency Page 3 of 12
precipitated product was filtered off, washed with water untilit was neutral, and dried. The yield of bis(ether diacid) 3 was10.6 g (95%), mp0376–381 °C. IR (KBr): 2,500–3,500 (O-Hstretch), 1,718 cm−1 (C0O stretch). 1H NMR (500 MHz,DMSO-d6, δ, ppm): 5.72 (s, 2H, Hb), 6.91 (s, 2H, Ha), 6.97(dd, J08.6 and 2.4 Hz, 2H, Hg), 7.00 (dd, J05.3 and 3.2 Hz,5H, Hc), 7.04 (d, J02.4 Hz, 2H, He), 7.31 (dd, J05.3and 3.2 Hz, 4H, Hd), 7.76 (d, J08.6 Hz, 2H, Hf), 13.08(s, 4H, -COOH). 13C NMR (125 MHz, DMSO-d6, δ, ppm):47.1 (C4), 115.8 (C10), 117.7 (C12), 119.6 (C1), 124.0 (C7),125.4 (C6), 125.6 (C8), 131.4 (C13), 136.7 (C3), 139.5 (C5),144.0 (C2), 146.0 (C9), 159.7 (C11), 167.4 (C14), 168.5 (C15).
In a 200-mL flask, a mixture of 10.0 g (16.2 mmol) of bis(ether diacid) 3 dissolved in 50 mL of acetic anhydride washeated at a reflux temperature for 3 h. The resulting clearsolution was filtered to remove any insoluble impurities.
The off-white crystallized product was filtered, washed withtoluene, and dried at 150 °C in vacuo. The yield of bis(etheranhydride) 4 was 8.6 g (90%), mp0387–390 °C. IR (KBr):1,847 (asymmetric C0O stretch), 1,778 cm−1 (symmetricC0O stretch). 1H NMR (500 MHz, DMSO-d6, δ, ppm)(for the peak assignments, see Fig. 2a): 5.72 (s, 2H, Hb),6.90 (s, 2H, Ha), 6.97 (dd, J08.6 and 2.6 Hz, 2H, Hf), 7.00(dd, J05.3 and 3.2 Hz, 5H, Hc), 7.03 (d, J02.6 Hz, 2H, He),7.31 (dd, J05.3 and 3.2 Hz, 4H, Hd), 7.76 (d, J08.6 Hz, 2H,Hg).
Synthesis and film preparation of poly(ether-imide)s
A typical polymerization procedure is as follows. Thediamine 5f (0.3356 g, 1.14 mmol) was dissolved in 9.5 mL ofanhydrous DMAc in a 50-mL round-bottom flask. Then bis(ether anhydride) 4 (0.6642 g, 1.14 mmol) was added to thediamine solution in one portion. Thus, the solid content of thesolution is approximately 10 wt.%. The mixture was stirred atroom temperature for about 12 h to yield a viscous poly(amicacid) solution. The inherent viscosity of the resulting poly(amic acid) was 0.91 dL g−1, measured in DMAc at a
Fig. 2 a 1H, b 13C, c H-HCOSY and d C-H HMQCspectra of triptycene bis(etheranhydride) 4 in DMSO-d6
Page 4 of 12 S. Hsiao et al.
concentration of 0.5 g dL−1 at 30 °C. The poly(amic acid) filmwas obtained by casting from the reaction polymer solutiononto a glass Petri-dish and drying at 90 °C overnight. The poly(amic acid) in the form of solid film was converted to poly(ether-imide) 7f by successive heating under vacuum at150 °C for 30 min, 200 °C for 30 min, and then 250 °C for1 h. The inherent viscosity of poly(ether-imide) 7f was0.53 dL g−1, measured at a concentration of 0.5 gdL−1
in DMAc at 30 °C. The IR spectrum of 7f (Fig. 3)exhibited characteristic imide absorption bands at 1,778 cm−1
For the chemical imidization method, 4 mL of aceticanhydride and 2 mL of pyridine were added to the poly(amic acid) solution obtained by a similar process as above,and the mixture was heated at 100 °C for 1 h to effect acomplete imidization. The homogenous polymer solutionwas poured slowly into 200 mL of stirring methanol givingrise to pale yellow precipitate that was collected by filtra-tion, washed thoroughly with hot water and methanol, anddried. A polymer solution was made by the dissolution ofabout 0.5 g of the polyimide sample in 10 mL of hot DMAc.The homogeneous solution was poured into a 9-cm glassPetri dish, which was placed in a 90 °C oven overnight for
the slow release of the solvent, and then the film wasstripped off from the glass substrate and further dried invacuum at 160 °C for 6 h.
Measurements
IR spectra were recorded on a Horiba FT-720 Fouriertransform infrared (FTIR) spectrometer. 1H and 13C NMRspectra were measured on a Bruker AVANCE 500 MHz FT-NMR spectrometer. The inherent viscosities were determinedat a 0.5 g/dL concentration with an Ubbelohde viscometer at30 °C. Weight-average molecular weights (Mw) and number-average molecular weights (Mn) were obtained via gel perme-ation chromatography (GPC) on the basis of polystyrenecalibration using Waters 2410 as an apparatus and THF asthe eluent. The mechanical properties of the films were mea-sured with an Instron model 1130 tensile tester with a 5 kgload cell at a crosshead speed of 5 mm min−1 on stripsapproximately 40–60 μm thick and 0.5 cm wide with a 2 cmgauge length. An average of at least five individual determi-nations was used. Wide-angle X-ray diffraction (WAXD)measurements were performed at room temperature (ca.25 °C) on a Shimadzu XRD-6000 X-ray diffractometer(40 kV, 20 mA), using graphite-monochromatized Cu-Kαradiation. Ultraviolet–Visible (UV–Vis) spectra of thepolymer films were recorded on Agilent 8453 UV-Visiblediode array spectrophotometer. Color intensity of the poly-mers was evaluated on an Admesy Brontes colorimeter.Measurements were performed for the films at an observa-tional angle of 45° and a standard D65 light source with theCIE LAB values. DSC analyses were performed on a Perkin-Elmer Pyris 1 DSC at a scan rate of 20 °C/min in flowingnitrogen. Glass-transition temperatures (Tgs) were read as themidpoint temperature of the heat capacity jump and weretaken from the second heating scan after a quick cooling downfrom 400 °C to room temperature. Thermogravimetric analy-sis (TGA) was performed with a Perkin-Elmer Pyris 1 TGA.Measurements were carried out on 3–5 mg film samplesheated in flowing nitrogen or air (90 cm3/min) at a heatingrate of 20 °C/min. Thermomechanical analysis (TMA) wasconducted with a Perkin-Elmer TMA 7 at a scan rate of10 °C/min with a penetration probe of 1.0 mm diameter underan applied constant load of 10 mN. Dielectric property of thepolymer films was tested using an Agilent 4291B RFImpedance/Material Analyzer.
Results and discussion
Monomer synthesis
The new triptycene-based bis(ether anhydride) 4 was syn-thesized via the synthetic route shown in Scheme 1.
Fig. 3 IR spectra of poly(ether-imide) 7f and its poly(amic acid)precursor
Triptycene poly(ether-imide)s with high solubility and transparency Page 5 of 12
According to a reported method, [31] the triptycene-hydroquinoe (TPHQ) 1 was obtained in a good yield startingfrom the Diels-Alder reaction of p-benzoquinone andanthracene and subsequent rearrangement reaction usingacetic acid/HBr as catalyst. The intermediate compound,1,4-bis(3,4-dicyanophenoxy)triptycene (2), was obtainedfrom the nitro-displacement of 4-nitrophthalonitrile withthe potassium salt of TPHQ formed in situ by treatment
of K2CO3 in DMF. Alkaline hydrolysis of the bis(etherdinitrile) 2 with aqueous potassium hydroxide in ethanolgave 1,4-bis(3,4-dicarboxyphenoxy)triptycene (3), whichwas subsequently cyclodehydrated with acetic anhydrideto generate the target bis(ether anhydride) monomer 4.The yields in each step were reasonable, and the molecularstructures of the synthesized compounds could be affirmed byIR and NMR spectroscopic techniques. The FTIR spectra of
Fig. 4 a 1H, b 13C, c H-HCOSYand d C-H HMQC NMRspectra of poly(ether-imide) 7fin CDCl3 (* solvent peak)
intermediate compounds 1–3 and the target bis(etheranhydride) monomer 4 are illustrated in Fig. 1. Figure 2illustrates the 1H NMR and 13C NMR spectra of 4. Thesespectra are in good agreement with its proposed molecularstructure. The 1H NMR spectrum of 4 is essentially identicalto that reported in literature, [51] where a different route wasused to its synthesis.
Polymer synthesis
A series of triptycene-containing poly(ether-imide)s 7a–7iwere prepared from bis(ether anhydride) 4 with various aro-matic diamines (5a–5i) by a conventional two-step procedurevia the formation of poly(amic acid)s 6a–6i, followed bythermal or chemical cyclodehydration (Scheme 2). In the firststep, the viscosities of the reaction mixtures became very highas poly(amic acid)s were formed, indicating the formation ofhigh molecular weight polymers. As shown in Table 1, thepoly(amic acid) precursors had inherent viscosities in therange of 0.44~0.91 dL/g. The resulting viscous poly(amicacid) solutions were poured into a clean glass Petri-dish anddried to form thin solid films. The thermal conversion topolyimides was carried out by step-by-step heating of thepoly(amic acid) films to 250 °C. The poly(amic acid) precur-sors also could be chemically dehydrated to the polyimides bytreatment with acetic anhydride and pyridine. All poly(ether-
imide)s could afford flexible and tough films via thermalimidization of their poly(amic acid) films. For the organo-soluble polyimides such as 7e–7i could be solution cast toflexible films in the fully imidized form. The poly(ether-imide)s soluble in THF were characterized by GPC, and therelevant data are also included in Table 1. The weight-averagemolecular weights (Mws) and number-average molecularweights (Mns) were recorded in the range of 27,000~55,500and 12,000~58,000, respectively, relative to polystyrenestandards.
All the poly(ether-imide)s showed the characteristicabsorption bands of the imide ring near 1780 (asymmet-ric C0O stretching) and 1,720 cm−1 (symmetric C0O stretch-ing). Typical IR spectra of a representative poly(ether-imide)7f and its poly(amic acid) precursor are included inFig. 3. The 1H and 13C NMR spectra of poly(ether-imide) 7fare illustrated in Fig. 4. Assignments of each proton andcarbon, assisted by the 2-D NMR spectra, are in goodagreement with the structures of the repeating unit. All thearomatic protons and carbons of 7f resonated in the region of δ6.82–7.92 and 112.0–163.7 ppm, respectively. The signalsappearing at 5.59 ppm in the 1H NMR spectrum and48.2 ppm in the 13C NMR spectrum were respectivelyassigned to the bridgehead protons and carbons of the tripty-cene units. The imide carbonyl carbons resonated at around167 ppm.
Triptycene poly(ether-imide)s with high solubility and transparency Page 7 of 12
Solubility and film property
The solubility properties of these triptycene-containing poly(ether-imide)s synthesized by both of thermal and chemicalimidization methods are reported in Table 2. The poly(ether-
imide)s 7a and 7b derived from more rigid diamines such asp-phenylenediamine and benzidine are insoluble in all thetest solvents. The poly(ether-imide)s 7e–7i derived frommore flexible diamine components exhibited good solubilityin organic solvents; they are soluble not only in polar
Table 1 Inherent viscosity andGPC data of the poly(ether-imide)s
aT: samples prepared via thermalimidization; C: via chemicalimidizationbMeasured at a polymer concen-tration of 0.5 g/dL in DMAc at30 °CcDetermined in THF relative topolystyrene standardsdInsoluble in DMAceInsoluble in THF
aThe qualitative solubility wastested with 10 mg of a sample in1 mL of stirred solvent. ++, sol-uble at room temperature; +,soluble on heating; +−, partiallysoluble; −, insoluble even onheating
Polymer code Solubility in various solventsa
NMP DMAc DMF DMSO m-Cresol THF CHCl3 Toluene
7a-T − − − − − − − −
7a-C − − − − − − − −
7b-T − − − − − − − −
7b-C − − − − − − − −
7c-T + − − − + − + − −
7c-C + + − + − + − + + − + − + −
7d-T + + − + − + − + − + − −
7d-C + + − + − + − + + − + − + −
7e-T + + + + + + + + + + + + + −
7e-T + + + + + + + + + + + + −
7f-T + + + + + + + + + + + + + −
7f-C + + + + + + + + + + + + + −
7g-T + + + + + + + + + + + + + −
7g-C + + + + + + + + + + + + + −
7h-T + + + + + + + + + + + + + −
7h-C + + + + + + + + + + + + + + −
7i-T + + + + + + + + + + + + −
7i-C + + + + + + + + + + + + −
Page 8 of 12 S. Hsiao et al.
organic solvents such as NMP, DMAc, DMF, and m-cresolbut also in less polar solvents such as THF and CHCl3.These triptycene-containing polymers exhibit excellentsolubility characteristics in comparison with their non-triptycene-containing counterparts [58]. The good solubilityof these poly(ether-imide)s can be attributed to the presenceof three-dimensional triptycene units, together with the flexiblesegments along the polymer backbone. In addition, the chem-ically imidized samples generally revealed a higher solubility
than those prepared by the thermal imidization method. Thelower solubility of the thermally cured polyimide may beattributed to the presence of partial interchain crosslinking oran aggregation of the polymer chains during thermal curingprocess. As evidenced by theirWAXDpatterns, these polymersexhibited an amorphous nature. This can be attributed in part tothe bulky, packing-disruptive triptycene unit along the polymerbackbone, which does not favor their close chain packing.
Thermal properties
The thermal properties of all the poly(ether-imide)s wereinvestigated by TGA, DSC, and TMA techniques, and therelevant thermal behavior data are summarized in Table 3.Typical TGA curves of representative poly(ether-imide) 7din both air and nitrogen atmospheres are illustrated in Fig. 5-inset. All the polymers exhibited good thermal stability withinsignificant weight loss up to 550 °C in both air andnitrogen atmospheres. The decomposition temperatures(Td) at a 10% weight-loss of the poly(ether-imide)s innitrogen and air were recorded in the range of 572~634and 565~618 °C, respectively. The polymer char weights at800 °C are also shown as percentages and range from 57% to67%. The high char yields provide an indication of the prac-ticality of these polymers as a high-temperature coating. TheTgs of all the polymers were measured to be in the range of238~302 °C by DSC. The decreasing order of Tg generallycorrelated with that of chain flexibility. As expected, thepolymers 7a and 7b showed a relatively higher Tg due to thepresence of rigid diamine components. Polymer 7h exhibiteda lower Tg value of 247 °C because of the presence of multipleflexible ether linkages in its diamine residues. A lower Tg forpolymer 7g as compared with its non-fluorinated analog 7fmay be a result of reduced interchain interactions andincreased free volume due to the bulky pendant -CF3side groups. All the polymers indicated no clear melting
Table 3 Thermal properties of the poly(ether-imide)s (the polymerfilm samples were heated at 300 °C for 30 min before all the thermalanalyses)
Polymercode
Tg(°C)a
Ts(°C)b
Td at 10%weight loss (°C)c
Char yield(wt%)d
In N2 In air
7a 302 289 606 600 66
7b 295 288 617 598 67
7c 279 273 594 610 63
7d 273 262 634 614 67
7e 262 251 595 606 62
7f 262 252 589 592 62
7g 238 232 620 618 64
7h 247 236 572 588 57
7i 280 270 588 565 65
aMidpoint temperature of the baseline shift on the second DSC heatingtrace (rate 0 20 °C/min) of the sample after quenching from 400 °C to50 °C (cooling rate 0 −200 °C/min) in nitrogenb Softening temperature measured by TMA with a constant appliedload of 10 mN at a heating rate of 10 °C/minc Decomposition temperature at which a 10% weight loss wasrecorded by TGA at a heating rate of 20 °C/min and a gas flowrate of 20 cm3 /mind Residual weight percentage at 800 °C in nitrogen
Fig. 5 TMA and TGA curves ofpoly(ether-imide) 7d with aheating rate of 10 and 20 °C/min,respectively
Triptycene poly(ether-imide)s with high solubility and transparency Page 9 of 12
Table 4 Optical properties and dielectric constants of the poly(ether-imide)s
Polymer code Film thickness (μm) Color coordinatesa λ0 (nm)b Λ at 80%transmission(nm)
a The color parameters were calculated according to a CIE LAB equation. L* is the lightness, where 100 means white and 0 implies black. Apositive a* means a red color, and a negative a* indicates a green color. A positive b* means a yellow color, and a negative b* implies a blue colorb Absorption edge from the UV–vis spectra of the polymer thin films
Fig. 6 Transmission UV–visspectra of some poly(ether-imide)s and PMDA/ODA(Kapton) films. The data shownin the parentheses are the cut-off wavelengths of the polymerfilms
Page 10 of 12 S. Hsiao et al.
endotherms up to the decomposition temperatures on theDSC thermograms. This result also supports the amor-phous nature of these polymers. The softening temper-atures (Ts) (may be referred as apparent Tg) of thepolymer film samples determined by the TMA methodwith a loaded penetration probe are also listed inTable 3. They were obtained from the onset temperatureof the probe displacement on the TMA trace. As arepresentative example, the TMA trace of poly(ether-imide) 7d is illustrated in Fig. 5. In all cases, the Tsvalues of the poly(ether-imide)s obtained by TMA areslightly lower than the Tg values measured by the DSCexperiments. This may indicate that these polymersexhibited a higher degree of plasticity near Tg becauseof the increased free volume caused by the triptyceneunits. Overall, the thermal analysis results reveal thatthese poly(ether-imide)s exhibit excellent thermal stability,indicating that the thermal stabilities of these materials werenot compromised upon incorporation of the triptycene moie-ties or the pendant trifluoromethyl substituent groups..
Optical and dielectric properties
The color intensities of the poly(ether-imide)s were elu-cidated from the yellowness (b*) or redness (a*) indicesobserved by a colorimeter. For comparison, a standard poly-imide from PMDA and ODA (the same components to thecommercial Kapton film) was also prepared and characterizedby its color intensity. The color coordinates of the poly(ether-imide) films are given in Table 4. In general, the cast films ofchemically treated polyimide samples showed a lower yellow-ness index (b*) of 5.9~28.9. The slightly higher yellownessindex of the thermally imidized poly(ether imide)s filmsmightbe a result of thermal oxidation of chain-end aminogroups or a denser chain packing. In addition, thin filmswere measured for optical transparency using UV–visspectroscopy. Figure 6 shows the UV-visible transmittancespectra of some representative poly(ether-imide) films, andthe cut-off wavelengths (absorption edge; λ0) from the UV–vis spectra are also listed in Table 4. All poly(ether-imide)films exhibited cut-off wavelengths shorter than 400 nm andwere entirely transparent and near colorless. Figure 7 showsthe photographs of the films of poly(ether-imide)s 7g-C and7i-C and a standard Kapton (PMDA-ODA) polyimide. Ingeneral, the films made from the 7 series poly(ether-imide)sexhibited much lighter color than the standard Kapton
polyimide film. The low color of these poly(ether-imide)scould be explained by the decreased intermolecular electronicinteractions. The bulky and three-dimensional triptyceneunits in the bis(ether anhydride) component were effec-tive in decreasing charge-transfer complexing (CTC)between the polymer chains through a steric hindranceeffect. To determine whether the optical transparencywas retained upon aging at elevated temperatures, theyellowness index of the polymer films were recordedafter being heated at 300 °C in an air-circulating ovenfor a period of time. As shown in Fig. 8, the b* valuesof these polymer films just showed a slight increase. Asa typical example, the photographs of the film of poly-mer 7g-C after different aging time are shown in theinset of this figure. This polymer film retained its col-orlessness and good optical transparency after 6 h heat-ing at 300 °C.
The dielectric constants of the thermally imidized poly(ether-imide) films are also summarized in Table 4. Thesepolymers exhibited the dielectric constants at 1 MHz in therange of 2.22–3.40. Almost all the triptycene poly(ether-imide)s showed a lower dielectric constant than the standardPMDA/ODA polyimide at the same measuring conditions.In comparison, the fluorinated poly(ether-imide)s 7d and 7gdisplayed a lower dielectric constant than the correspondingnon-fluorinated 7c and 7f. The results suggested that theCF3 groups could improve the dielectric performancebecause of less efficient chain packing and increased
Fig. 7 Photographs of Kaptonfilm and the cast films obtainedfrom poly(ether-imide)s 7g-Cand 7i-C (~50μm thick)
Fig. 8 Yellowness index changes of the poly(ether-imide) films heatedat 300 °C in an air-circulating oven. The photographs show the imagesof poly(ether-imide) 7g-C film after the indicated heating time
Triptycene poly(ether-imide)s with high solubility and transparency Page 11 of 12
free volume, together with the very low polarizability ofthe C—F bonds.
Conclusions
A series of triptycene-based poly(ether-imide)s weresynthesized from 1,4-bis(3,4-dicarboxyphenoxy)triptycenedianhydride with various aromatic diamines by two-step thermal or chemical imidization method. All the poly(ether-imide)s afforded transparent, flexible, and strong films.Most of poly(ether-imide)s exhibited very good solubility inorganic solvents and could be solution-cast to pale yellow tonearly colorless films directly in their fully imidized form. Thehigh solubility, low color intensity and low dielectric constantsin these poly(ether-imide)s have been achieved at little sacri-fice in thermal stability. Thus, these properties suggest thepotential usefulness of these poly(ether-imide)s in microelec-tronics and optoelectronics applications.
Acknowledgements Financial support from National Science Coun-cil and Industrial Technology Research Institute of Taiwan is gratefullyacknowledged.
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