synthesis and characterization of soluble, blue-fluorescent polyamides and polyimides containing...

Synthesis and Characterization of Soluble, Blue-FluorescentPolyamides and Polyimides Containing Substitutedp-Terphenyl as Well as Long Aliphatic Segmentsin the Main Chain

JOHN A. MIKROYANNIDIS,1 GERASIMOS M. TSIVGOULIS2

1 Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece

2 Department of Polymer Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands

Received 9 March 1999; accepted 15 May 1999

ABSTRACT: A novel class of semiflexible polyamides and polyimides bearing substitutedp-terphenyl as well as long aliphatic segments in the main chain were synthesizedthrough pyrylium salts. Characterization of polymers was accomplished by inherentviscosity, elemental analysis, FT-IR, NMR, UV-vis luminescence spectroscopy, X-rayanalysis, differential scanning calorimetry, thermomechanical analysis, thermogravi-metric analysis, isothermal gravimetric analysis, and water uptake measurements.Polyamides displayed a degree of crystallinity and dissolved in polar aprotic solventscontaining lithium chloride, as well as in trichloroacetic acid. Polyimides were amor-phous and showed an excellent solubility, being soluble in various common solvents.The solutions of polyamides in DMF were blue-fluorescent with maxima at 362–370 nm.The emission maxima were not influenced appreciably upon the structure of thependent groups and the length of the aliphatic spacers of backbone. The polymerspossessed Tgs at 98–131°C and exhibited a satisfactory thermal stability. © 1999 JohnWiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3646–3656, 1999Keywords: polyamides; polyimides; semiflexible polymers; pyrylium; synthesis; p-terphenyl; blue-fluorescence.

INTRODUCTION

Rigid rod polymers have received much interestbecause of their superior behavior as compared toflexible random coil polymers. The material prop-erties of rigid rod polymers arise from the highdegrees of molecular orientation that can be ob-tained in fibers from lyotropic solutions.1–4 How-ever most of these polymers have high glass tran-sition temperatures and poor solubility in organicsolvents, leading to difficulties in processing. In

order to overcome this drawback, either flexiblemoieties have been introduced in the main chainthus interrupting the rigid character, or bulkyside substituents have been attached along thebackbone.5–13

Certain rigid rod polyamides and polyimideshave been recently synthesized through pyryliumsalts in our laboratory. They contained biphe-nyl,14 p-terphenyl,15–19 m-terphenyl,20 or p-quin-quephenyl18,19,21 segments in the main chain andbulky side groups. Most of them displayed anenhanced solubility and relatively high Tgs (higherthan 200°C). In addition, some of them behavedas blue-light emitting polymers.16,19,21

In this article, we report the synthesis andcharacterization of new soluble, semiflexible poly-

Correspondence to: J. A. Mikroyannidis (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 3646–3656 (1999)© 1999 John Wiley & Sons, Inc. CCC 0887-624X/99/183646-11

3646

amides and polyimides with improved process-ability. They were successfully prepared frompyrylium salts using a convenient syntheticroute. The polymers contained substituted p-ter-phenyl as well as long aliphatic spacers in themain chain. The aliphatic segments were of var-ious length to investigate their influence on thepolymer properties. Furthermore, the polymerscarried phenyl or 4-biphenylyl pendent groups toincrease their solubility. The relatively low Tgs,the enhanced solubility, and the blue-fluorescenceare some attractive features of these polymers.

EXPERIMENTAL

Characterization Methods

Melting temperatures were determined on anelectrothermal melting point apparatus IA6304and are uncorrected. IR spectra were recorded ona Perkin–Elmer 16PC FT-IR spectrometer withKBr pellets. 1H-NMR spectra were obtained usinga Brucker spectrometer (400 MHz). 13C-NMR(100 MHz) spectra were obtained with a Bruckerspectrometer. The NMR spectra were recordedusing DMSO-d6 as solvent. Chemical shifts (d val-ues) are given in parts per million with tetra-methylsilane as an internal standard. UV-visspectra were recorded on a Beckman DU-640spectrometer with spectrograde DMF. DSC andTGA were performed on a DuPont 990 thermalanalyzer system. Ground polymer samples ofabout 10 mg each were examined by TGA andisothermal gravimetric analysis (IGA), and theweight loss comparisons were made between com-parable specimens. The DSC thermograms wereobtained at a heating rate of 10°C/min in N2 at-mosphere at a flow rate of 60 cm3/min. DynamicTGA measurements were made at a heating rateof 20°C/min in atmospheres of N2 or air at a flowrate of 60 cm3/min. Thermomechanical analysis(TMA) was recorded on a DuPont 943 TMA usinga loaded penetration probe at a scan rate of 10°C/min in N2 with a flow rate of 60 cm3/min. TheTMA experiments were conducted at least in du-plicate to assure the accuracy of the results. TheTMA specimens were pellets of 8-mm diameterand 2-mm thickness prepared by pressing powderof polymer for 3 min under 5–7 kpsi at ambienttemperature. The inherent viscosities of polymerswere determined for solutions of 0.5 g/100 mL inCCl3COOH or DMAc at 30°C using an Ubbelohdesuspended level viscometer. Elemental analyses

were carried out with a Hewlett–Packard model185 analyzer. The wide-angle X-ray diffractionpatterns were obtained for powder specimens on aX-ray PW-1840 Philips diffractometer.

To determine the equilibrium water absorp-tion, polymer samples were previously condi-tioned at 120°C in an oven for 12 h. They weresubsequently placed in a desiccator where 65%r.h. (relative humidity) was maintained by meansof an oversaturated aqueous solution of NaNO2 at20°C, and were periodically weighed.

Reagents and Solvents

4-Hydroxybenzaldehyde and 49-phenylacetophe-none were recrystallized from distilled water and95% ethanol, respectively. Acetophenone was pu-rified by distillation under reduced pressure.4-Nitrophenylacetic acid sodium salt was pre-pared by reacting equimolar amounts of 4-nitro-phenylacetic acid with aqueous sodium hydrox-ide. Terephthaloyl chloride was recrystallizedfrom n-hexane. Pyromellitic dianhydride (PMDA)and benzophenone tetracarboxylic dianhydride(BTDA) were recrystallized from acetic anhy-dride. N,N-dimethylformamide (DMF), dimethy-lacetyamide (DMAc), and 1,2-dichloroethanewere dried by distillation over CaH2. 1,6-Dibro-mohexane, 1,10-dibromodecane, boron trifluorideetherate, 1,4-dioxane, and hydrazine hydratewere used as supplied.

Preparation of Starting Materials (Scheme 1)

1,6-Bis(4-formylphenoxy)hexane (1a)

A mixture of 4-hydroxybenzaldehyde (2.00 g,16.40 mmol), 1,6-dibromohexane (2.00 g, 8.20mmol), potassium carbonate (2.49 g, 18.04 mmol)and DMF (30 mL) was refluxed overnight underN2. The solvent was removed by distillation. Wa-ter was added to the residue and the pale brownprecipitate was filtered, washed with water, anddried to afford 1a (2.30 g, 86%, mp 94–96 °C).

IR (KBr, cm21): 2,944, 2,912, 2,852 (COHstretching aliphatic); 1,686 (CHO); 1,600, 1,510(aromatic); 1,468 (COH bending aliphatic); 1,260,1,156 (COOOC).

1H-NMR (DMSO-d6) ppm: 9.86 (s, 2H, CHO);7.85–7.83 (d, 4H, aromatic ortho to CHO); 7.10–7.07 (d, 4H, other aromatic); 4.07 (s, 4H, OCH2);1.75 (m, 4H, OCH2CH2); 1.48 (m, 4H,O(CH2)2CH2).

BLUE-FLUORESCENT POLYAMIDES AND POLYIMIDES 3647

1,10-Bis(4-formylphenoxy)decane (1b)Compound 1b was prepared as a semisolid in 92%yield (3.02 g) according to the procedure describedfor 1a by reacting 4-hydroxybenzaldehyde (2.44 g,20.0 mmol) with 1,10-dibromodecane ((3.00 g,10.00 mmol) and potassium carbonate (3.04 g,22.00 mmol) in DMF (30 mL).

IR (KBr, cm21): 2,935, 2,856 (C-H stretchingaliphatic); 1,690 (CHO); 1,598, 1,510 (aromatic);1,472 (COH bending aliphatic); 1,262, 1,158(COOOC).

1H-NMR (DMSO-d6) ppm: 9.84 (s, 2H, CHO);7.84–7.82 (d, 4H, aromatic ortho to CHO); 7.08–7.06 (d, 4H, other aromatic); 4.08 (s, 4H, OCH2);

Scheme 1.

3648 MIKROYANNIDIS AND TSIVGOULIS

1.72 (m, 4H, OCH2CH2); 1.47 (m, 4H,O(CH2)2CH2); 1.26 (m, 8H, O(CH2)3CH2CH2).

1,6-Bis[(4-phenyloxy-2,6-diphenyl)pyryliumtetrafluoroborate]hexane (2Ka)

A flask was charged with a mixture of 1a (2.15 g,6.59 mmol), acetophenone (3.17 g, 26.36 mmol)and 1,2-dichloroethane (20 mL). Boron trifluorideetherate (4.13 mL, 32.95 mmol) was added por-tionwise to the stirred mixture at room tempera-ture and it was refluxed for 5 h under N2. Etherand petroleum ether (2:1 v/v) were added to thesolution and the red precipitate was filtered,washed with ether, and dried to afford 2Ka. Itwas recrystallized from a mixture of chloroform/ether (2:1 v/v) (4.42 g, 74%, mp 87–89°C).

IR (KBr, cm21): 2,942, 2,850 (COH stretchingaliphatic); 1,262, 1,588, 1,494, 1,460 (pyryliumstructure and aromatic); 1,056 (BF4

2); 1,244, 1,184(COOOC).

1H-NMR (DMSO-d6) ppm: 9.12 (s, 4H, aromaticmeta to O1); 7.49–7.00 (m, 24H, other aromaticexcept those ortho to O); 7.10 (m, 4H, aromaticortho to O); 4.02–3.74 (m, 4H, OCH2); 1.58–1.10(m, 8H, other aliphatic).

In a procedure similar to 2Ka, compounds2Kb, 2La, and 2Lb were prepared and character-ized as follows:

Compound 2Kb

Mp 73–75 °C; Reaction yield 72 %.IR (KBr, cm21): 2,940, 2,853 (COH stretching

aliphatic); 1,627, 1,585, 1,488, 1,458 (pyryliumstructure and aromatic); 1,057 (BF4

2); 1,244, 1,184(COOOC).


Compound 2La

Mp 71–72°C; Reaction yield 75%.IR (KBr, cm21): 2,932, 2,858 (COH stretching


2); 1,244, 1,184(COOOC).


Compound 2Lb

Mp 79–80°C; Reaction yield 69%.IR (KBr, cm21): 2,924, 2,852 (C-H stretching


2); 1,244, 1,184(COOOC).


1,6-Bis[(4(-oxo-2*,6*-diphenyl-4-nitro)p-terphenyl]hexane (3Ka)

A mixture of 2Ka (1.00 g, 1.10 mmol), 4-nitrophe-nylacetic acid sodium salt (1.12 g, 5.50 mmol), andacetic anhydride (2.5 mL) was stirred and refluxedfor 4 h. It was cooled at 0°C and the pale yellow–brown precipitate was filtered, washed firstly withwater, then with methanol, and dried to afford 3Ka.It was recrystallized from a mixture of chloroform/ether (1:1 v/v) ( 0.98 g, 92%, mp 108–110°C).

IR (KBr, cm21): 2,932, 2,857 (COH stretchingaliphatic); 1,598 (aromatic); 1,518, 1,346 (NO2);1,244, 1,180 (COOOC).

1H-NMR (DMSO-d6) ppm : 8.25–8.16 (m, 4H,aromatic ortho to NO2); 7.93–7.80 (m, 4H, aro-matic meta to NO2); 7.60–7.14 (m, 28H, otheraromatic except those ortho to O); 6.81 (m, 4H,aromatic ortho to O); 4.13–3.82 (m, 4H, OCH2);1.59–1.10 (m, 8H, other aliphatic).


Compound 3Kb


aliphatic); 1,602 (aromatic); 1,512, 1,346 (NO2);1,242, 1,176 (COOOC).


Compound 3La





Compound 3Lb




1,6-Bis[(4(-oxo-2*,6*-diphenyl-4-amino)p-terphenyl]hexane (4Ka)

A flask was charged with a mixture of 3Ka (1.20g, 1.24 mmol), 1,4-dioxane (20 mL) and a cata-lytic amount of palladium 10% on activated car-bon. Hydrazine hydrate (5 mL) was added drop-wise to the stirred mixture at 101°C and reflux-ing was continued for 70 h. The mixture wassubsequently filtered, and the filtrate was con-centrated under vacuum. Water was added tothe residue and 4Ka was obtained as a paleyellow– brown precipitate. It was recrystallizedfrom a mixture of 1,4-dioxane/water (1:2 v/v)(0.82 g, 73%, mp 151–152°C).

IR (KBr, cm21): 3,444, 3,370 (NOH stretching);2,926, 2,854 (COH stretching aliphatic); 1,623(N-H deformation); 1,602, 1,512 (aromatic); 1,248,1,178 (COOOC).

1H-NMR (DMSO-d6) ppm : 7.60 – 6.80 (m,36H, aromatic except those ortho to NH2); 6.49(m, 4H, aromatic ortho to NH2); 4.85 (br, 4H,NH2); 3.82–3.60 (m, 4H, OCH2); 1.38 (m, 8H,other aliphatic).

13C-NMR (DMSO-d6) ppm : 150.06, 147.76,143.05, 141.16, 132.65, 131.75, 129.70, 128.84,126.01, 123.86, 122.92, 119.30, 115.84, 114.86,113.14, 112.56, 111.29, 31.53, 29.78, 28.41, 27.78,26.40, 24.36.

Anal. Calcd. for C66H56N2O2 : C, 87.19; H, 6.21;N, 3.08. Found : C, 86.85; H, 6.38; N, 2.93.


Compound 4Kb

Mp 179–180°C; Reaction yield 78%.IR (KBr, cm21): 3,456, 3,365 (NOH stretching);

2,922, 2,852 (COH stretching aliphatic); 1,618(NOH deformation); 1,600, 1,512 (aromatic);1,280 (C-N stretching); 1,44, 1,178 (COOOC).

1H-NMR (DMSO-d6) ppm : 7.66–6.75 (m, 36H,aromatic except those ortho to NH2); 6.51 (m, 4H,aromatic ortho to NH2); 4.87 (br, 4H, NH2); 3.70–3.41 (m, 4H, OCH2); 1.23 (m, 8H, other aliphatic).

13C-NMR (DMSO-d6) ppm: 148.07, 147.19,143.13, 141.14, 139.79, 135.15, 133.87, 132.70,132.44, 132.10, 130.41, 129.62, 128.35, 125.83,115.09, 114.12, 69.11, 68.23, 30.45, 29.60, 27.43,26.17, 25.58, 24.88.


Compound 4La


2,930, 2,856 (COH stretching aliphatic); 1,619(NOH deformation); 1,608, 1,512 (aromatic);1,281 (C-N stretching); 1,244, 1,178 (COOOC).


13C-NMR (DMSO-d6) ppm : 151.32, 147.97,142.80, 141.39, 140.72, 139.84, 138.92, 132.70,132.09, 131.04, 129.87, 128.21, 128.01, 127.34,126.73, 126.11, 115.93, 115.06, 68.32, 30.56,29.58, 27.53, 26.18.


Compound 4Lb


2,926, 2,850 (COH stretching aliphatic); 1,620(NOH deformation); 1,606, 1,512 (aromatic);1,244 (CON stretching); 1,244, 1,178 (COOOC).


13C-NMR (DMSO-d6) ppm : 142.26, 142.14,141.46, 140.74, 140.56, 140.43, 140.16, 139.85,139.51, 138.54, 132.66, 131.07, 129.63, 128.01,127.33, 124.92, 123.93, 116.20, 115.82, 114.99,68.44, 33.48, 29.80, 27.73, 26.40, 22.61.



Preparation of Polymers

Polyamides MKa, MKb, MLa, and MLb (Chart 1)

As a typical example for the preparation of poly-amides, the synthesis of MKa is given: A flaskwas charged with a solution of 4Ka (0.5500 g, 0.61mmol) in DMAc (15 mL) containing 5 wt % LiCl.To the mixture 0.5 mL of propylene oxide wasadded. Terephthaloyl chloride (0.1228 g, 0.61mmol), dissolved in DMAc (8 mL), was addeddropwise to the stirred solution at 210°C underN2. Stirring of the mixture was continued at thistemperature for 5 h and then at room tempera-ture overnight in a stream of N2. It was pouredinto water, and the pale yellow–brown precipitatewas filtered, washed firstly with water, then withhot acetone, and dried to afford MKa (0.49 g,78%).

Polyimides PLa, PLb, BLa, and BLb (Chart 2)

The synthesis of PLA is given as a typical exam-ple for the preparation of polyimides: PMDA(0.0719 g, 0.33 mmol) was added to a stirred so-lution of 4La (0.4000 g, 0.33 mmol) in DMAc (15mL) at 0°C. The mixture was stirred at that tem-perature for 2 h and then at room temperature for6 h under N2. Acetic anhydride (5 mL) and pyri-

dine (2 mL) were added to the stirred solution,and it was heated at 100°C overnight. It wassubsequently poured into water, and the brownprecipitate was filtered, washed firstly with wa-ter, then with hot acetone, and dried into a vac-uum oven at about 150°C to afford PLa (0.43 g,93%).

The reaction yields and the inherent viscositiesfor all polymers are given in Table I.

RESULTS AND DISCUSSION

Synthesis of Monomers and Polymers

Diamines 4, which contain aliphatic ether spacersas well as substituted p-terphenyl segments, weresynthesized according to Scheme 1. Specifically,4-hydroxybenzaldehyde reacted with half molaramount of a,v-dibromoalkanes to afford com-pounds 1. The latter reacted with acetophenoneor 49-phenylacetophenone to yield pyrylium salts2.22 They reacted with 4-nitrophenylacetic acidsodium salt in the presence of acetic anhydride toafford dinitro compounds 3.23 Finally, the corre-sponding diamines 4 were obtained by catalytichydrogenation of 3 using hydrazine hydrate.

The structures of monomers were confirmed byelemental analyses as well as 1H- and 13C-NMRspectroscopy. The 13C-NMR spectra of monomersshowed a characteristic peak approximately at 68

Chart 1.


ppm assigned to the OCH2 segments whereas theother aliphatic units appeared at the region of33–22 ppm.

Starting from diamines 4, a new series of aro-matic-aliphatic polyamides and polyimides weresynthesized (Charts 1 and 2). They were obtainedin high yields (75–98%) and their inherent viscos-ities ranged from 1.03–1.73 dL/g (Table I).

The polymers were characterized by elementalanalyses, FT-IR as well as 1H- and 13C-NMR spec-troscopy. The elemental analyses of polymerswere consistent with their chemical structures.The FT-IR spectrum of a typical polyamide MLa

showed characteristic absorptions at 3,444, 3,371(NOH stretching); 2,930, 2,858 (COH stretchingaliphatic); 1,630 (CAO); 1,606 (aromatic); 1,510(NOH deformation); 1,320 (CON stretching); and1,242, 1,176 cm21 (ether). In addition, the FT-IRspectrum of a typical polyimide PLb displayedpeaks at 2,924, 2,852 (COH stretching aliphatic);1,776, 1,726, 1,368, 1,116 (imide structure); and1,244, 1,178 cm21 (ether).

The 1H-NMR spectra of polymers showedrather broad signals typical of polymeric materi-als. The recorded peaks in the 1H -NMR spectrumof polyamide MLa from a DMSO-d6- solution are

Chart 2.

Table I. Yields, Inherent Viscosities, and Glass Transition Temperatures (Tg) of Polymers Determined from theTMA and DSC Methods

Polymer MKa MKb MLa MLb PLa PLb BLa BLb

Yield (%) 78 75 84 89 93 98 96 90ninh (dl/g) 1.12a 1.03a 1.65a 1.73a 1.62b 1.67b 1.43b 1.54b

Tg from TMA (°C) 123 112 118 107 127 119 107 98Tg from DSC (°C) 131 120 125 115 130 123 116 109

a Inherent viscosity in CCl3COOH (0.5 g/dL) at 30°C.b Inherent viscosity in DMAc (0.5 g/dL) at 30°C.


given as a typical example: 10.30 (br, 2H, NHCO);8.10 (m, 4H, aromatic of terephthalic acid seg-ment); 7.66–6.80 (m, 56H, other aromatic); 3.70(m, 4H, OCH2) and 1.41 ppm (m, 8H, other ali-phatic). The 13C-NMR spectrum of MLa showed aweak but characteristic peak at 165 ppm associ-ated with the ONHCOO moiety, whereas theother peaks were almost the same with those ofthe parent diamine 4La.

Crystallinity, Hydrophilicity, and Solubilityof Polymers

An attempt was made to evaluate the crystallin-ity of polymers by means of X-ray diffractograms.Figure 1 presents the X-ray diffraction patterns ofthe as prepared powders at room temperature.Most polyamides showed diffraction peaks at theregion of 29–50°, indicating a degree of crystal-

Figure 1. Wide-angle X-ray scattering curves of polymers.


linity. In contrast, all polyimides were amorphouswith some low-level ordering. It is obvious thatthe more rigid imide structure caused looserchain packing. The higher crystallinity of poly-amides was in agreement with their poorer solu-bility (see below). It is noteworthy that the corre-sponding rigid rod polyamides carrying phenyl or4-biphenylyl pendent groups, which have beenpreviously prepared in our laboratory, were gen-erally amorphous.15–19 The crystallinity of thepresent polyamides was attributed to the pres-ence of the long aliphatic segments in backbonethat increased the flexibility and probably al-lowed a higher ordering of chains.

The isothermal water absorption of polyamideswas determined to estimate their hydrophilicity.Polyamides MKa, MKb, MLa, and MLb dis-played water uptake 1.35, 1.03, 0.71, and 0.84%,respectively, after an exposure time of 50 h. Thecorresponding numbers of moles of absorbed wa-ter per amide equivalent weight (NMAW) were0.39, 0.31, 0.27, and 0.33. Note that the respectiverigid rod polyamides without the aliphatic spac-ers in the main chain15–19 exhibited NMAW val-ues of 0.15–0.57. The semicrystalline nature ofthe present polyamides, which was established bytheir X-ray diffractograms, reflects to a densechain packing that decreased the water accessi-bility.

One of the objectives of this investigation wasthe enhancement of solubility by reducing therigidity of the main chain. Table II summarizesthe qualitative solubility of polymers. It seemsthat polyamides displayed lower solubility thanpolyimides owing to the combination of their

semicrystalline feature and the strong intermo-lecular hydrogen bonding. More particularly, allpolyamides dissolved in hot polar aprotic solvents(DMF, NMP) containing LiCl, whereas polyim-ides dissolved in these solvents at room tempera-ture (except PLb which dissolved upon heating)without the addition of LiCl. All polymers werealso soluble in CCl3COOH. Furthermore, polyim-ides dissolved in halogenated solvents (TCE,DCB) as well as in pyridine, 1,4-dioxane, m-cresol, and cyclohexanone, while polyamides wereinsoluble or partially soluble in these solvents.

Optical Properties of Polymers in Solution

The optical properties of the synthesized poly-mers are of primary importance for our study, andtherefore the absorption and the emission fea-tures in the DMF solution of polyamides wereexamined. The UV-vis spectra in DMF solution ofpolyamides were recorded, and only those of MLaand MLb are presented in Figure 2 because oftheir similarity. As it can be seen, they showedabsorption maxima at 285 nm. Upon irradiatingwith a UV lamp, these solutions were blue-fluo-rescent. MLa and MLb were remarkably morefluorescent than MKa and MKb. Figure 2 pre-sents also typical emission spectra of polyamidesMLa and MLb in a DMF solution. They displayedmaxima at 370 and 366 nm, respectively, whenexcited at 285 nm. The emission maxima wereindependent of the wavelength of the excitation.Since all polyamides showed almost identical ab-sorption and emission maxima, they had approx-imately the same effective conjugation length.

Table II. Solubilities of Polymersa

Polymer

Solventsb

DMF NMP CCl3COOH TCE DCB Pyridine 1,4-Dioxane m-Cresol CH THF

MKa 1c 1c 1 2 2 2 2 2 2 2MKb 1c 1c 1 2 2 2 2 2 2 2MLa 1c 1c 11 6 6 6 6 6 6 6MLb 1c 1c 11 6 6 6 6 6 6 6PLa 11 11 1 1 1 11 1 1 1 6PLb 1 1 1 1 1 11 1 1 1 6BLa 11 11 1 1 1 11 1 1 1 1BLb 11 11 1 1 1 11 1 1 1 11

a Solubility: 11, soluble at room temperature; 1, soluble in hot solvent; 6, partially soluble; 2, insoluble.b DMF, N,N-dimethylformamide; NMP, N-methylpyrrolidone; TCE, 1,1,2,2-tetrachloroethane; DCB, 1,2-dichlorobenzene; CH,

cyclohexanone; THF, tetrahydrofurane.c Solvent containing 5% wt/wt LiCl.


This supports that the optical properties of poly-amides in solution were mainly governed uponthe rigid rod segment of backbone and they wereaffected neither by the structure of the pendentgroups nor the length of the aliphatic spacers inbackbone. The fluorescent behavior of the studiedpolymers can be compared with that of our anal-ogous polymers containing substituted oligophe-nyl moieties in the main chain16,19,21 as well aswith other substituted poly(p-phenylene)s.24,25

The observed maxima are in agreement withthose of the previously mentioned polymers,which appeared in the range of 350–430 nm.

Thermal and Thermomechanical Propertiesof Polymers

Thermal and thermomechanical characterizationof polymers was accomplished by DSC, TMA,TGA, and isothermal gravimetric analysis (IGA).The DSC traces for all polymers did not showendotherm associated with melting. This meansthat all polymers, and even polyamides, behavedgenerally as amorphous. A step drop in eachcurve was observed above 100°C during the firstheating scan of DSC, which can be assigned to theoccurrence of a glass transition. Figure 3 depictstypical DSC (first heating scan) as well as TMAtraces (second heating scan) for polymers MKb,MLa, BLa, and BLb. The Tg values were ob-tained from the onset temperature of the TMAtransition, as well as from the center of the DSCtransition, and are listed for all polymers in TableI. The Tgs of TMA were usually lower than those

of DSC. The Tg values ranged from 98–131°C. Itseems that the Tg was reduced with increasingthe length of the aliphatic segment in backbone.In addition, the polymers bearing 4-biphenylylpendent groups displayed lower Tgs than thosecarrying phenyl pendent groups due to the higherfree volume of the former. Since the Tgs followedthe order PLa . BLa and PLb . BLb, obviouslythe polyimides of PMDA were more rigid thanthose of BTDA. Generally, the present polymersshowed remarkably lower Tgs than the analogousones without the aliphatic segments in back-bone,15–19 the Tgs of which were above 230°C.

The thermal stability of polymers was ascer-tained by TGA and IGA. As expected, the poly-mers did not show an outstanding thermal stabil-ity due to the presence of the thermally unstablealiphatic moieties. Figure 4 illustrates typicalTGA thermograms of polymers MKb, MLa, and

Figure 2. UV-vis absorption spectra, as well as emis-sion spectra, of polyamides MLa and MLb in diluteDMF solution.

Figure 3. DSC thermograms (first heating; solid line)as well as TMA thermograms (second heating; dashedline) of polymers MKb, MLa, BLa, and BLb. Condi-tions: N2 flow, 60 cm3/min; heating rate, 10°C/min.


PLa in N2 and air. Generally, the polymers werestable up to 330–367°C in N2 and 320–350°C inair. They afforded anaerobic char yields of 60–69% at 800°C. It was shown that the polymerswith shorter aliphatic spacers in backbone weremore thermally stable. Finally, the weight losswas 21.7–29.6%, following 20 h isothermal agingat 300°C in static air.

CONCLUSIONS

Starting from pyrylium salts, a new series ofsemiflexible polyamides and polyimides carryingsubstituted p-terphenyl, as well as long aliphaticsegments in the main chain, were synthesized.Polyamides showed higher crystallinity andpoorer solubility than polyimides. Polyamides dis-solved in polar aprotic solvents and trichloroace-tic acid. Polyimides dissolved readily in variouscommon solvents and even in halogenated organicones. The optical properties in solution of poly-amides were investigated. They were blue-fluo-rescent with emission maxima at 362–370 nm.The structure of the pendent groups, as well asthe length of the aliphatic spacers in the mainchain, did not affect noticeably the fluorescencemaxima. Despite the presence of the aliphaticmoieties, the polymers were stable up to 330–367°C in N2 and afforded anaerobic char yields of60–69% at 800°C. In conclusion, the synthesizedpolymers possess relatively low Tg values, en-hanced solubility, satisfactory thermal stability,

and are potential candidate materials for fabri-cating blue-light-emitting devices.

The authors are indebted to chemist D. Vachliotis (Cen-ter of Instrumental Analysis, University of Patras,Greece) for recording the NMR spectra.

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Figure 4. TGA traces in N2 and air of polymers MKb,MLa, and PLa. Conditions: Gas flow, 60 cm3/min; heat-ing rate, 20°C/min.


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