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Synthesis and Properties of Fluoro-Polyotherimides.

ROHITKUMAR H. VORA [1,2] [*]

SUAT HONG GOH [2]

TAI-SHUNG CHUNG [3]

A series of amorphous fluoro-polyethenmides based on 2,2'-bis(3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and di-ether-containing diamines 4,4'-bis(3-aminophenoxy)diphenyl sulfone (m-SED), 4,4'-bis(4-aminophenoxy)diphenyl sulfone (p-SED), 4,4'-bis(4-aminophenoxy)diphenyl propane (BPADE) were synthesized. These melt processable polyetherimide polymers from p-SED and BPADE showed excellent electrical properties. The dielectric constants, 2.74 and 2.65 at 10 MHz respectively, are lower than commercially available polyetherimide ULTEM(R) 1000, and polyimide Kapton(R) H films. In addition, we found that trifluoromethyl groups--containing polyimides not only show extraordinary electrical properties, but they also exhibit excellent long-term thermo-oxidative stability and reduced water absorption relative to non-fluorinated polyimides. The weight retention of these fluoro-polyetherimides at 315[degrees]C for 300 h in air varies from 93% to 98%. Whereas, their moisture absorption at 100 RH at 50[degrees]C was in the range of 0.3% to 1.05%, which is much lower than those of Ultem 1000 and Kapton H. In the case of fluoro-polyetherimides from p-SED and m-SED (para and meta isomers) diamines with 'ether' and sulfonyl (-[SO.sub.2]-) spacer groups, the d-spacing and [T.sub.g] values decreased from 4.72[dot{A}] to 4.56[dot{A}] and 293[degrees]C to 244[degrees]C respectively. Similarly, the transparency of these polymer films (in the range of 80% to 90%) at 500 nm solar wavelength was higher than Ultem 1000 and Kapton H.

1. INTRODUCTION

Aromatic polyimides possess outstanding thermal mechanical and electrical properties as well as excellent chemical resistance [1]. The search for new polyimides with improved processability and higher glass transition temperatures ([T.sub.g]) than the commercially available polyetherimide Ultem 1000 has received significant attention from both academia and industries. It is also known that structural rigidity of dianhydrides contributes to the increase in the glass transition temperatures of those polyimides in the range of 300 to 400[degrees]C. Therefore, because of their poor solubility in common organic solvents and high softening temperatures, uses of these polymers in industrial applications are limited. Because of these limitations and because of their high glass transition temperatures ([T.sub.g]), many researchers focused their research on the modification of the backbone structures of polyimides. Their approach such as an incorporation of a flexible ether linkage and meta oriented phenylene rings int o polymer backbone led to an increase in polymer chain flexibility and solubility, but lowered the effective upper use temperature of these polymers [2-7].

For some niche electronic applications the high cost may not be the determining factor. In fact, the premium performance dictates premium price. The continuous performance of a given polyimide at higher temperature processing conditions and specific electrical and thermomechanical properties it imparts may justify its higher price. We have focused our efforts to develop high-performance polyetherimides from commercially available monomers and study their structure-property relationships.

In the past it was shown that incorporation of fluorine-containing monomers lowers the dielectric constant and moisture absorption in the polyimide polymers [8-10]. Some of these fluoro-polyimides were either patented or reported in the literature. One fluoro-polyetherimide [6F-BDAF] based on 2,2'-bis(3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and 2,2'-bis [4-(4-aminophenoxy)diphenyl]hexafluoropropane (BDAF) was reported as [LARC.sup.TM] CP1 by NASA [9-10]. It was commercialized through licensing. However, it is not currently available commercially. Therefore, a series of polyetherimides based on aromatic dianhydrides, including a fluorinated dianhydride and di-ether linkage containing diamines having bis-trifluoromethyl group, sulfone and isopropyl groups were successfully synthesized by a simplified one-pot with two-step solution polymerization process. The inherent viscosity of these polyetheramic acids (PEA) and polyetherimide (PEI) was determined. The thermal, electrical, mechanical and rheologi cal properties, thermo-oxidative and processing stability of the selected fluoro-polyetherimide polymers were studied and the results are summarized here.

2. EXPERIMENTAL

2.1 Material.

Electronic grade pyromellitic dianhydride (PMDA), 2,2'-bis(3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA), biphenyl dianhydride (BPDA), 3,3', 4, 4'-benzophenone tetracarboxylic dianhydride (BTDA), 4,4'-oxydiphthalic anhydride (ODPA), 4,4'-bis (3-axninophenoxy)diphenyl sulfone (m-SED) 4,4'-bis (4-aminophenoxy)diphenyl sulfone (p-SED), all received from Chriskev, KS, USA, 2,2'-bis[4-(4antinophenoxy) phenyl] propane (BPADE) was received from Wakayama Seika Kogyo Co. Ltd., Japan, and 2,2'-bis [4-(4-aminophenoxy)diphenyl ]hexafluoropropane (BDAF) was received from Chriskev. N-methyl pyrolidinone (NMP), tetrahydrofuran (THF), N,N-dimethylacetamide (DMAc), N,N-dimethyformamide (DMF), methylene chloride, [beta]-picoline, acetic anhydride and methanol, all received from Sigma-Aldrich, were used as received. Ultem 1000 Pellets were obtained from General Electric and its films were prepared by solution casting. Kapton H Film of 1 mil and 2 mil thickness were obtained from DuPont.

2.2 Polymerization

Several methods for the preparation of polyimides have been reported in the literature [11-28]. The most common procedure used in this investigation is a simplified two-step polymerization synthesis process as shown in the reaction scheme in Fig. 1b. For example, in the case of synthesis of [6F-mSED], a fluoro-polyetherimide based on 2,2'-bis (3,4-dicarboxyphenyl) hexafluropropane dianhydride (6FDA) and 4,4'-bis(3axninophenoxy) diphenyl sulfone (m-SED), accurately weighed 8.884 gm (0.02 mole) of solid 6FDA was added to an equimolar amount of diamine (8.65 gm) pre-dissolved in freshly distilled NMP to make 20% solid concentrations. The reaction mixture was stirred under nitrogen at room temperature for over 8 h to make polyether-amic acid (PEA) solution, which was then imidized to form polyetherimide (PEI). The cyclization can be achieved by either thermal or chemical means. In these experiments, the chemical imidization was carried out by addition of stoichiometric amounts of [beta]-picoline (catalyst) base (pKa 5.6) and acetic anhydride (dehydrating agent) [29]. The polymer and a small sample of the polyether-amic acid were then precipitated with methanol and de-ionized water respectively and dried at 100[degrees]C overnight in a air circulating oven. The polyether-amlc acid and polyetherimide polymers were characterized by FTIR.

2.3 Film Preparation

There are a number of commercial products produced by solvent casting with a fairly wide variety of applications ranging from electrical, electronics, solar films and adhesive coatings for automobile trim etc. [30]. For many polymeric materials that are not melt processable, but soluble in organic solvents, such as in the case of many polyimides, solvent casting is the only way to prepare polymer film. Solutions of selected fluoro-polyetherimides and Ultem 1000 were made at 15% solid concentration level in NMP and filtered through a 0.5 [mu]m filter under nitrogen pressure. The clear solution was then coated on a glass plate using a doctor blade. The films were dried in a nitrogen environment, then heated gradually in an oven up to 250[degrees]C stepwise and held at 300[degrees]C for 1 h. After heating, the films were allowed to cool down gradually to room temperature. The self-supporting flexible films were lifted up from the glass plate by soaking in water and dried with a paper towel and further dried in a n oven at 150[degrees]C for 30 mm. 1.0 to 1.4 mil thick films were obtained. The films of Kaption H (as received 2 mil thick) and Ultem 1000 were used as The control'.

2.4 Compression Molding

The thermoplastic fluoro-polyetherimides based on 6FDA and 4,4'-bis(4-amino phenoxyphenyl) sulfone (m-SED) [i.e. 6F-mSED], 4,4'-bis(3-aminophenoxyphenyl) sulfone (m-SED) [i.e. 6F-mSED], 2,2'-bis[4-(4-amino phenoxy)phenyl] propane (BPADE) [i.e. 6F-BPADE] and 2,2'-bis[4-(4-aminophenoxy)diphenyl] hexafluoropropane (BDAF) [i.e. 6F-BDAF] were compression molded in a 4" X 4" square mold between heated platens of a Karver press at 320[degrees]C at 500 psi for 7 min. Light yellowish to yellow color transparent rectangle plate of 1.5 mm thickness was obtained. The densities [rho] (gm/[cm.sup.3]) of polymer samples were determined and reported.

2.5 Characterization

The solubility of solid polyetherimide was determined by preparing a 2% wt. solution in a small capped vials. The solutions were stirred vigorously with magnetic stirrer bars at room temperature and at slightly warm ([sim]35[degrees]C) temperature. The level of solubility was evaluated after 24 h.

The inherent viscosity of the polyether-amic acids and polyetherimides were measured in NMP at a concentration of 0.5 g/dl at 20[degrees]C using a modified Cannon Fenske viscometer.

% Moisture absorption measurements of thin films of selected fluoro-polyetherimide were calculated from the difference in weight of pre-dried and wet film with an ultra microbalance from Satorius, model YDP-03-OCE with weight reading range of 0.00 1 mg to 5.1 gm. The film samples were pre-dried at 150[degrees]C for 1 h before weighing accurately and dipped in de-ionised water in a vial. The vials were sealed immediately and maintained at 50[degrees]C for 100 h. After which the samples were taken out of vials and dried by tissue paper and re-weighed immediately.

% Transparency of visible light at 500 nm of the fluoro-polyetherimides, Ultem 1000 and Kapton H films was determined by a Shimadzu UV-VIS Spectrophotometer model UV-2501 (PC)S with RS658 photomultiplier tube. The films were scanned from 200 nm to 550 nm at ambient condition. The spectral data were recorded automatically by a programmed computer.

Wide-angle X-ray diffraction (WAXD) measurement of compressed disks of average thickness 1 mm of solid fluoro-polyetherimide polymers, Ultem 1000, and Kapton H films were carried out on an X-ray defractor unit (Phillips model PW 1729-10) fitted with Cu-K[alpha] radiation (30 kV, 20 mA) with wavelength [lambda] of 1.54. The scanning rate was 0.5[degrees]C /min. at ambient temperature. The spectral window ranged from 2[theta] = 6[degrees] to 2[theta] = 40[degrees]. The corresponding d-spacing value was calculated from the diffraction peak maximum, using the Bragg equation, d = [lambda]/2 sin[theta].

FTIR spectra of the polyether-amic acid and polyetherimide solids were obtained using Perkin Elmer FTIR model with Spectrum 2000 software. The scanning range was from 4000 to 400 [cm.sup.-1]. The Glass Transition temperatures ([T.sub.g]) of polyetherimide solids and films were determined using Perkin Elmer DSC-7 differential scanning calorimeter with Pyris software. Scans were run at a heating rate of 20[degrees]C/min in flowing nitrogen atmosphere (10 cc/min). Thermal decomposition temperatures (5% wt. loss) of polymers films were examined using dynamic TGA [model Perkin Elmer TGA-7 with Pyris software]. Scans were run at a heating rate of 20[degrees]C/min. in flowing air atmosphere (10 cc/min). Also % char yield was determined in flowing nitrogen atmosphere (10 cc/min) at a heating rate of 20[degrees]C/min from 100[degrees]C to 1000[degrees]C.

Long-term isothermal, thermo-oxidative stability (TOS) studies of fluoro-polyetherimides and commercially available Ultem 1000 and Kapton H were also performed on film samples in air for 300 h at 315[degrees]C (600[degrees]F) in a Blue-M programmable, forced air oven.

The storage modulus G', tan [delta] and loss modulus E' were determined on a dynamic mechanical analyzer (DMA) [model TA-DMA-2980 from TA Instruments]. Polyetherimide film samples of size 10 mm in length, 2 mm in width subjected to temperature scan mode at a heating rate of 3[degrees]C/min in air at a frequency of 1 Hz and an amplitude of 0.2 mm. The coefficient of thermal expansion (CTE) values (m./m. [degrees]C) were determined by means of thermal mechanical analyzer (TMA) [model TMA-2940 from TA instruments] at heating rate of 5[degrees]C/min air.

The mechanical property of compression molded fluoro-polyetherimide plates and films of Ultem 1000 and Kapton H were determined using Instron Mechanical Analyzer Model 5542.

The dielectric constant ([varepsilon]') of polyetherimide film was measured between two-parallel plate of dielectric analyzer [model DEA-2970) from TA instruments] at a frequency of 10 mega-Hertz (MHz) at and at 50% relative humidity at temperature of 25[degrees]C in a flowing nitrogen atmosphere condition.

A Rheometric Mechanical Spectrometer model ARES was used to determine melt rheology and processing stability at 50[degrees]C above [T.sub.g] between two parallel plates. Fluoro-polyetherimide samples were first molded into disk and melted in the parallel plate test fixture of the Rheometer with minimum exposure to air. The polymer's complex viscosity was measured as a function of time for 30 min at a frequency of 1 radian per second (rad/s).

3. RESULTS AND DISCUSSION

The properties of a series of polyetherimides prepared from the monomers (Fig. 1a) by the one-pot-two-step method (Fig. 1b) are listed in the given Tables 1 through 5. The chemical structures of the repeat units of the fluoro-polyetherimides of interest, Ultem 1000 and Kapton H are shown in Fig. 2. FTIR Spectra in Fig. 3 clearly indicated the imide ring formation and disappearance of the amide peak during the chemical cyclization step. For example, in the case of fluoro-polyetherimide, based on 6FDA and m-SED, (i.e. [6F-mSED]), the FTIR spectra of polyether-amic acid and polyetherimide showed distinct features. The characteristic absorption bands of amides and carboxyl groups in the spectra at 3240 to 3320 [cm.sup.-1] and 1500 to 1730 [cm.sup.-1] region disappeared and those of imide ring appeared near 1784 [cm.sup.-1] (asym. C=O stretching) and 1728 [cm.sup.-1] (sym. C=O stretching), 1376 [cm.sup.-1] (C-N stretching) 1063 [cm.sup.-1] and 744 [cm.sup.-1] imide (ring deformation). Also the aryl-ether absorpti on band around 1250 [cm.sup.-1] for both amic-acid and imide was very strong indicating stability of the structure and successful conversion of polyether-amic acid to polyetherimide.

The solubility of Ultem 1000 pellet, Kapton H film and precipitated solid polymers were tested in NMP, THF, DMAc, DMF, methylene chloride and conc. [H.sub.2][SO.sub.4]. The solubility and inherent viscosity values of solid polymers are listed in Table 1. It shows that polymers containing PMDA and BPDA were insoluble at room temperature or partially soluble upon heating. Others were soluble in NMP, THF, DMAc, and DMF, at room temperature and partially soluble on heating in methylene chloride. All of these polymers were soluble in conc. [H.sub.2][SO.sub.4] except Kapton H, which showed disintegration. All of the fluoro-polyetherimides based on 6FDA were soluble in almost all the solvents tested at room temperature.

The glass transition ([T.sub.g]) values, thermal decomposition, thermo-oxidative stability and thermo-mechanical properties of the fluoro-polyetherimide film samples are reported in Table 2. The glass transition temperatures of all 20 solid polymers were plotted against dianhydride structure as shown in Fig. 4. The wide range in [T.sub.g] values reflects the large variation in molecular structures. The plot clearly reflects that the polymer [T.sub.g] increased with the rigidity of the dianhydride structure. Rigidity of the dianhydride structure can be controlled via incorporation of various "separator or spacer" groups, such as -CO -, -O-, -[SO.sub.2]-, -C [([CF.sub.3]).sub.2]- [9, 31, 32]. The observed [T.sub.g] increases, based on the dianhydride structures in this study, are in the following order: PMDA [greater than] BPDA [greater than] 6FDA [greater than] BTDA [greater than] ODPA [32]. There was a significant reduction in [T.sub.g] of the polyetherimide polymers prepared from meta-oriented 4,4'(3-amino phenoxyphenyl) sulfone (m-SED) as compared to 4,4'(4-amino phenoxyphenyl) sulfone (p-SED) isomers. This was due to the distortion of the linearity of the polyimide chain by meta linked bond angle, thereby reducing rotational energy [9, 31-33].

The 5% weight loss in nitrogen was observed at a temperature on an average about 21[degrees]C higher than in air. The % char yield (residue) in nitrogen was higher and in the range of 52% to 55% at 800[degrees]C. The 5% weight (loss) decomposition temperature does not fully indicate the thermal performance of polymer at a given upper temperature limits. Therefore, isothermal thermo-oxidative stability (TOS) measurements of selected fluoro-polyetherimides, i.e. [6F-pSED], [6F-mSED], [6F-BPADE] and [6F-BDAF] from the series and Ultem 1000 and Kapton H films were carried out in a programmable oven at 315[degrees]C for 300 h in air environment. All these film samples were preheated at 150[degrees]C for one hour and their weights at this point taken as the reference or 100% weight value. During the test, the crucibles with the samples were removed from the oven simultaneously at appropriate times, and immediately sealed for cooling. They were weighed and then returned to the oven immediately for further aging. Neg lecting the initial weight loss of all the samples tested, which is thought to be associated with solvent and absorbed moisture removal, an approximate weight loss of only 2 to 3% occurred for [6F-pSED] and Ultem 100. Whereas 4.4% weight loss for Kapton H, 5.5% weight loss for [6F-mSED], and 7% weight loss for [6F-BDAF] and 16% weight loss for [6F-BPADE] polymer occurred over 300 h. The 97.7% weight retention of [6F-pSED] over 300 h is the best observed among the materials tested (Fig. 5).

The thermo-mechanical properties of fluoro-polyetherimide films such as storage modulus G', tan [delta] max, and loss modulus E' values are also tabulated in Table 3. The peak of tan [[delta].sub.Max] was identified as the glass transition temperature, because a large decrease of the storage modulus G' occurred at that temperature. While comparing the fluoro-polyetherimide film [T.sub.g] of 293[degrees]C to 266[degrees]C measured by DSC with those of tan [delta] value of 294[degrees]C to 274[degrees]C measured by DMA, it was noted that the [T.sub.g] values obtained from DMA were slightly higher. The thermo-mechanical properties of polyimides are directly related to the inter- and intramolecular chain conformation rotation flexibility beside the chemical structure [2, 34]. They also depend on the previous heat history of the polymer samples. The results could be compared with decreasing order of rigidity and stiffness of polymer back bone as indicated by the varying value of glass transition temperature.

The coefficient of thermal expansion (CTE) values of fluoro-polyetherimides were higher (5.95 X [10.sup.-5] to 9.07 X [10.sup.-5] m/m.[degrees]C) than Kapton H (3.15 X [10.sup.-5] m/m.[degrees]C), but consistent with that of flexible polyimides. The rigid rod type polyimides possess very low CTE value similar to metals. On the other hand polyetherimides containing flexible 'spacer' links, such as -O-, -[CH.sub.2]-, -C[([CH.sub.3]).sub.2]-, -C[([CF.sub.3]).sub.2]-, -[SO.sub.2]-, and meta substituted aromatic ring tend to have higher CTE values. In addition, the polyimide film prepared from its solution generally has a higher CTE value than that from the corresponding polyamic acid due to its high temperature curing (imidization)/heating history [28, 35].

The fluoro-polyimides are known for their excellent transparency, low moisture absorption, and low dielectric properties and high thermal stabilty. The films thickness of fluoro-polyetherimides [6F-pSED], [6F-mSED] and [6F-BDAF] were almost identical as reported in Table 3. Films were colorless with transparency in the range of 80 to 90% at 500 nm solar wavelength as compared to 72 and 27% for light amber to amber colored Ultem 1000, and Kapton H films respectively. (Fig. 6). Koton et al. [35] explained that the transparency of visible light by polyimide films is closely associated with the electronic characteristic of the monomer used in the synthesis. Many non fluorinated polyimides films are known to have yellow to dark amber colors whereas for fluorinated polyimides films are almost colorless. Also optical transparency to visible light is affected by the intra-and inter-molecular interaction of [pi] electrons between the monomer moieties in the polymer chain inducing a charge transfer complex (OTO). The [pi] electron transfers from the electron donating diamine to the electron acceptor dianhydride moiety. The polyimide chain is basically composed of an alternating donor and acceptor moieties, which can interact with each other inducing inter-chain CTC. It is possible to reduce CTC by incorporating electronegative fluorine groups on the polymer backbone or incorporating bulky electron withdrawing substituent groups, as they restrict the inter-chain conformational mobility and thus lower the CTC [9, 35-37].

Water absorption and diffusion properties are important with regards to their practical use in microelectronics and separation membranes. The absorbed water in polymer structures affects their performance and long term stability [38]. Very significant lower moisture absorption values were noted for fluoro-polyetherimide films at 100 RH at 50[degrees]C. The values were in the range of 0.3% to 1.05% for these fluoro-polyetherimides containing hydrophobic -C[([CF.sub.3]).sub.2]- groups. These values were lower than those of nonfluorinated polyetherimides, such as Ultem 1000 (1.52%) and Kapton H (3.0%) films, which were used as controls in this study. The percent absorption values measured in our study for Ultem 1000 and Kapton H films were found to be similar to those reported in the product brochures (1.25% at 23[degrees]C for 50% RH and 2.9% at 23[degrees]C for 100% RH) respectively [39-41]. The highly transparent polyimides with low moisture absorption are suitable for wave-guide applications in opto-electronics.

The X-ray diffraction spectra of the fluoro-polyetherimide solids as shown in Fig. 7 were broad and without any significant or obvious peaks indicating that the polyetherimides were amorphous. Polymers showed an amorphous pattern in the XRD spectral window range from 2[theta] = 10[degrees] to 2[theta] = 30[degrees]. The result was consistent with that of the solubility behavior of polymers and also with the glass transition temperature (DSC) result. This could be explained in terms of the presence of 'spacer' link such as ether, isoropylidene, and sulfonyl group. This 'spacer' link reduces the rigidity of the polymer chain which inhibits its packing [42]. However, in our study, the XRD spectrum of Kapton H film showed no peak in the scanning region. It is likely that the film sample was not thick enough to get some meaningful reading. An increase in d-spacing was observed when bulkier group -C[([CF.sub.3]).sub.2]- was substituted for -C[([CH.sub.3]).sub.2]- in the structure of the diamine moiety of polymer ch ain segment. As shown in the case of fluoro-polyetherimides [6F-BPADE] with d spacing = 5.08 [dot{A}] and 6F-BDAF with d spacing = 5.48 [dot{A}]. The increased inhibition of the conformational rotation is also reflected in a slight increase in [T.sub.g] values of [6F-BDAF] with [T.sub.g] = 266[degrees]C as compared to [6F-BPADE] with = 259[degrees]C. The meta oriented monomers also greatly reduces polymer chain segmental mobility and thus lowers d spacing and [T.sub.g]s [43-45]. In the case of [6F-pSED] and [6F-mSED], which are made from para and meta isomers of "ether" containing diamine monomer with sulfonyl (-[SO.sub.2]-) spacer group, the d-spacing and [T.sub.g] values lowered from 4.72A to 4.56[dot{A}] and 293[degrees]C to 244[degrees]C respectively. The Utem 1000 (d spacing = 5.24[dot{A}] and [T.sub.g] = 218[degrees]C) was completely amorphous.

The mechanical properties of fluoro-polyetherimide films are summarized in Table 4. The films possessed a tensile strength in the range of 11.5 to 14.0 Kpsi and elongation at break of 6.65 to 14.0% and modulus of 339 to 424 Kpsi. These polymers have comparable tensile properties, even when the diamine structures are different. The only exception was that the % elongation at break values for control samples of Ultem 1000 and Kapton H films were higher (60% and 72% respectively) than the fuoro-polyetherimides in our study.

The electrical properties, measured on the dried film samples of two fluoro-polyetherimides [6F-pSED] and [6F-BPADE]. and control samples of Kapton H and Ultem 1000 of thickness ranging from 1 to 2 Mils are reported in Table 5. The dielectric constants at 10 MHz ranged from 2.65 for fluoro-polyetherimide [6F-BPADE] of the series to 3.2 for the commercial DuPont polyimide Kapton H film. Overall, the incorporation of fluorine atoms into polymer backbone via trifluoromethyl group attachment has produced low dielectric properties [17, 32, 46] as compared to Ultem 1000 and Kapton H, which do not have trifluoromethyl groups in the polymer backbone.

The melt rheology/processing stability of the fluoro-polyetherimide solid samples [6F-mSED], 16F-pSED], [6F-BPDAE] and [6F-BDAF] were compared against Ultem 1000 by measuring the complex viscosity as a function of time [47, 48]. The complex viscosity of the sample plotted as a function of time as shown in Fig. 8, indicates that 6FDA containing fluoro-polyetherimide (PEI) [6F-mSED], [6F-pSED] and [6F-BPDAE] behaved similarly to Ultem 1000 and had stable melt viscosity at 50[degrees]C above [T.sub.g] for 30 min. Whereas the melt viscosity of [6F-BDAF] increased by 30% in the given time scale, may be due to the onset of molecular weight increase or crosslinking in the melt processing condition used. These values are within manageable processing parameters. Fluoro-polyetherimides of this study therefore can be melt processed by conventional methods.

3. CONCLUSION

A series of high temperature stable polyetherimides has been synthesized by solution polymerization. The FTIR study confirmed that polyether-amic acid was successfully converted to polyetherimide by chemical immidization method. Of these polyetherimides studied, all of the fluoro-polyetherimides based on 6FDA were soluble in almost all the organic solvents tested in this study at room temperature. The films of fluoro-polyetherimides [6F-pSEd], [6F-mSED] and [6F-BDAF] were almost colorless and had transparency in the range of 80 to 90% at 500 nm solar wavelength. Very significant lower moisture absorption (in the range of 0.3% to 1.05%) was noted for these films at 100 RH at 50[degrees]C. These fluoro-polyetherimides are amorphous as determined from XRD measurements. The polymer, [6F-pSED] (d spacing = 4.72[dot{A}], and [T.sub.g] 293[degrees]C) had higher thermo-oxidative stability (TOS) and an exceptionally low dielectric constant 2.74. This dielectric constant value was significantly lower than those of Ult em 1000 and Kapton H. The glass transition temperatures ([T.sub.g]) of all fluoro-polyetherimides synthesized were in excess of 240[degrees]C and well above the [T.sub.g] of Ultem 1000, but they could be processed readily into thin film and molded articles via conventional processing technique. It was demonstrated that the glass transition temperature ([T.sub.g]) and dielectric constants ([varepsilon]') of polyetherimides also could be tailored by controlling the rigidity of the chain through the introduction of rigid or flexible "separator" groups [37, 49]. These fluoro-polyetherimides are ideal candidates for applications such as high temperature insulators and dielectrics for micro-electronic packaging, coating and adhesive as well as substrates for electronic flex circuit and matrices for high performance composites for aerospace and advanced aircraft and materials for gas separation membranes.

ACKNOWLEDGMENT

The authors would like to acknowledge the timely support of Dr. Pramoda Kumari, Dr. Liu Jun-Ming for X-ray diffraction analysis and Mr. S. Veeramani for thermal and mechanical property characterization of the polymer samples. We also would like to express our gratitude for the material (Diamine) support provided by the Wakayma Seika Kogyo Co. Ltd. of Japan.

(1.) Institute of Materials Research and Engineering 3 Research Link, Singapore 117602

(2.) Department of Chemistry

(3.) Department of Chemical Engineering National University of Singapore 10 Kent Ridge Crescent, Singapore 119260

(*.) To whom correspondence should be addressed. Fax: 65-872 0785: Email address: rh-vora@ixnre.org.sg

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Author:VORA, ROHITKUMAR H.; GOH, SUAT HONG; CHUNG, TAI-SHUNG
Publication:Polymer Engineering and Science
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Date:Jun 1, 2000
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