Fluorinated, Soluble Polyimides with High Glass-Transition

Chapter 34

Fluorinated, Soluble Polyimides with High Glass-Transition Temperatures Based on a New, Rigid, Pentacyclic Dianhydride

Downloaded by FUDAN UNIV on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch034

12,14-Diphenyl-12,14-bis-(trifluoromethyl)-12H,14H-5,7dioxapentacene-2,3,9,10-tetracarboxylic Dianhydride Brian C. Auman and Swiatoslaw Trofimenko Experimental Station, DuPont Electronics, P.O. Box 80336, E336/205, Wilmington, DE 19880-0336

Polyimides have experienced ever increasing use in the electronics industry due to their excellent combination of thermal, mechanical and electrical properties. In recent years, fluorinated polyimides have been investigated as materials with the potential for reduced dielectric constant and moisture absorption over conventional polyimides. Fluorinated polyimides based on 6FDA, have been shown to indeed yield low dielectric constant and lower moisture absorption (1), but typically are solvent sensitive and have undesirably high coefficient of thermal expansion (CTE). Recent work in our laboratory (2) has dealt with the combination of fluorinated monomers with a rigid, quasi rod-like structure as a method for producing polyimides which have not only low dielectric constant and moisture absorption, but low CTE and reduced solvent sensitivity. This work was based on new fluorinated dianhydrides, 9,9-bis(trifluoromethyl)-2,3,6,7xanthenetetracarboxylic dianhydride (6FCDA) and its 9-trifluoromethyl-9-phenyl– analog (3FCDA) which have a rigid, tricyclic structure. As an offshoot of this work, we have also been investigating analogous rigid, pentacyclic structures as monomers for polyimides. In this paper, we describe the synthesis and characterization of 12,14-diphenyl-12,14-bis(trifluoromethyl)-12H,14H-5,7-dioxapentacene-2,3,9,10-tetracarboxylic dianhydride (PXPXDA) and polyimides prepared therefrom.

cr

PXPXDA 0097~6156/94/0537-0494$06.00/0 © 1994 American Chemical Society

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

34.

A U M A N AND TROFIMENKO

495

Fluorinated Soluble Polyimides

EXPERIMENTAL


Starting materials The p-phenylene diamine (PPD) and 4,4'-oxydianiline (4,4'-ODA) were high purity commercial materials and were used as received. The diaminodurene (DAD) and 2,2'-bis(trifluoromethyl)-benzidine (TFMB) were obtained in high purity from sources within Du Pont. TFMB was sublimed under reduced pressure, then recrystallized from toluene prior to use. N-cyclohexylpyrollidinone (CHP, Aldrich) was dried and distilled from calcium hydride. N-methylpyrollidinone (NMP, anhydrous 99 + % grade, Aldrich) and tetrachloroethane (TCE, Kodak) were used as received. 1244-DiphenyI-1244-bis(trifluoromethyI).12H,14H-5,7-dioxa-2 3,9,10-tetrar

methylpentacene ( T M P X P X )

A mixture of 91.4 g (0.287 mole) l,3-bis(3,4-dimethylphenoxy)benzene prepared as in (3), 100 g (0.575 mole) trifluoroacetylbenzene, and 180 g (9 moles) HF was heated in an autoclave for 8 hrs at 130 °C. After venting excess HF, the contents were transferred into a polyethylene jar containing ice-water slurry and 300 ml 50% NaOH. The product was extracted with C H C 1 , the extracts were filtered through alumina, and stripped. The residue was stirred with a methanol/acetone mixture, and filtered, yielding 87 g (50%) of creamy solid, which was recrystallized from toluene. The initial melting range was wide (sintering around 245 °, melting 255-260 °C), implying presence of cis-trans isomers. This mixed product was used for subsequent reactions. It was possible to isolate a single isomer by successive recrystallizations from toluene. M.p. = 287-289 °C. NMR: m 7.07, s 6.98, s 6.94, s 6.55, s 6.17, s 2.25, s 2.06 in 10:2:1:2:1:6:6 ratio. Analysis: Calc. for C H F 0 : C 72.4; H 4.44; F 18.1; Found: C 72.5; H 4.88; F 18.6 %. 2

3 8

2 8

6

2

2

12,14-Diphenyl-12,14-bis(trifluoromethyl)-12H,14H-5,7-dioxapentacene-2 3,9,10tetracarboxylic Dianhydride ( P X P X D A ) r

The oxidation of TMPXPX was carried out on a 31.5 g (0.05 mole) sample as described in (4). The tetraacid, PXPXTA, was isolated by filtration after acidification of the final oxidation filtrate with sulfuric acid, and was washed thoroughly with water. It was boiled briefly in 500 ml acetic anhydride, filtered through Celite, and stripped. The product was recrystallized from a mixture of anisole and acetic anhydride (300ml/50 ml for 35 g of crude product), using Darco, and filtering through Celite. After drying, the dianhydride exhibited a slightly broadened m.p. at about 340 °C owing to the mixture of isomers. This sample was considered pure as checked by elemental analysis and NMR and was used for polymerization studies. It was possible to isolate a single isomer by successive recrystallizations. NMR of single isomer: s 7.84, broader s 7.57, m 7.3-7.4, s 7.24, broad d 7.17 and broad s 6.37 ppm in 2:2:6:1:4:1 ratio. Calc. for 38 i4 6°8 °; Found: C 64.0; H 2.25. The second isomer was C

H

F

:

c

6 4

H


496

POLYMERS FOR MICROELECTRONICS

difficult to isolate due to its high solubility but a fraction containing ~ 65% was obtained (additional proton peak at 6.26 ppm). P X P X D A was also characterized as the p-tolyl, and n-butyl diimides, by stirring a solution of P X P X D A in THF with two equivalents of the appropriate amine for one hour. After evaporation of the solution, the residue was heated in vacuo at 240 ° for ten minutes. The crude product was dissolved in methylene chloride, filtered through alumina and, after evaporation of the solvent, was purified by reciystallization from toluene. Identity of these derivatives (only single isomer isolated) was established by NMR: PXPX-bis(p-tolyl)imide: s 7.78, s (broad) 7.52, m 7.3-7.1, including a sharp spike at 7.20, s (broad) 6.35 ppm in 2:2:19:1 ratio. IR: 1725 and 1775 (vs) cm* . PXPX-bis(n-butyl)imide: s 7.67, s (broad) 7.38, m 7.30, d 7.16, with a sharp spike at 7.15, s (broad) 6.34, t 3.65, m 1.64, m 1.35 and t 0.94 ppm in the correct 2:2:6:(4 + 1):1:4:4:4:6 ratio. IR: 1720,1775 (vs) cm' ; M.p. 291-294 °C.


1

1

Polymerization Two methods were used to synthesize the polyimides: synthesis of the poly(amic acid) with subsequent coating and thermal cure to the polyimide, or direct synthesis of the polyimide in one pot by solution imidization (5). These are illustrated in Scheme 1. Representative examples of both these methods are as follows: Poly(amic acid), PAA. Into a 100 ml reaction kettle fitted with a nitrogen inlet and outlet and a mechanical stirrer were charged 6.2488 g (8.746 mmol) of P X P X D A and 1.7512 g (8.746 mmol) of 4,4'-ODA followed by 32 ml of NMP. Both monomers dissolved quickly and after overnight stirring under nitrogen at RT, the solution built to a moderately high viscosity. Subsequently, the solution was pressure filtered through a 1 micron filter in preparation for spin coating. Soluble Polyimide, PL Similar quantities of the same monomers as above were weighed into a similar apparatus followed by 48 ml of NMP and 12 ml of CHP. After stirring for several hours at room temperature under nitrogen, a Dean-Stark trap with condenser was added and the reaction mixture was raised to a temperature of 180-190 °C and maintained overnight (~ 16 hrs.) to imidize the polymer. A viscous, homogeneous solution resulted at high temperature which remained so upon cooling. This polyimide solution was pressure filtered through a 1 micron filter in preparation for spin coating. Film Preparation Films were prepared by spin coating the filtered poly(amic acid) or polyimide solution onto 5" silicon wafers containing 1000 A of thermally grown oxide on the surface, followed by drying at 135 °C for 30 minutes in air, and then heating under nitrogen to 200 °C (2 °C/min) and holding for 30 minutes followed by heating to 350 °C (2 °C/min) and holding for 1 hour. Free standing films of about 10 fim thickness (goal) were obtained by etching the oxide layer of the silicon wafer in dilute aqueous H F to release the film.


34.


\>


497


+

H N 2

Ar

—NH

2

PJ:

NMP/CHP 4/1 @RT

NMP

Poly(amic acid) 1. Solution Imidize

1. Coat 2. Dry, Thermally Imidize, 350°C

or

180-190°C 2. Coat 3. Dry, 350°C

o

Polyimide

o

Scheme 1. Synthesis of poly(amic acid)s and soluble polyimides based on P X P X D A H N - A r - N H represents aromatic diamines such as those listed in Table 1 2

2


498


Techniques Gel permeation chromatography (GPC) was performed either on a Waters G P C 2 at 35 °C with 4 linear Phenogel columns in the D M A C / L i B r / H P 0 / T H F solvent system (6), or on a Waters instrument (150C) with a Zorbax TMS precolumn and 2 Shodex AD80M/S columns in D M A C at 135 °C. Flow rate was 1 ml/min, detection was by RI, and calibration was based on polystyrene standards. Mechanical properties of the films were measured in accordance with ASTM D-882-83 (Method A) on an Instron model 4501 tensile tester (crosshead speed = 0.2Vmin). Linear coefficient of thermal expansion (CTE) was obtained from a P-E TMA-7 thermo-mechanical analyzer (5 °C/min, -10 to 225 °C, 30 mN tension). The value (0-200 °C) was recorded after an initial conditioning step (heat to 250 °C, hold 5 min, cool). The onset of weight loss and the temperature of 5% weight loss in air were measured on a Du Pont 951 T G A at 15 °C/min from 50 to 600 °C. The measurements were taken after an initial 150 °C/5 min. drying step. Glass transition temperatures (T^) were obtained from a Rheometrics RSA-II dynamic-mechanical analyzer in tension (freq = 10 rads). Dielectric constant was measured by the parallel plate capacitor method in the frequency range 10 KHz-10 MHz on thin (10-20 /Am) films. Gold electrodes were vacuum deposited on both surfaces of dried films, followed by thorough drying (min. 48 hrs.) at 150 °C under vacuum/N prior to measurement in a sealed humidity chamber at 0% R H . Moisture absorption measurements were made by the Quartz Crystal Microbalance technique (QCM) (7, 8) on thin (~ 3 /im) films spin coated and cured (as above) onto electroded quartz crystals. Measurements were taken at various humidity settings in a controlled humidity chamber and are reported at 85% R H .


3

4

2

II O HF

1. Oxid.

O

2. Dehydr.

O

PXPXDA

O

TMPXPX Scheme 2. Synthesis of PXPXDA


34.



499

RESULTS AND DISCUSSION The synthesis of P X P X D A was similar to that of 3FDA (2), involving double bridging condensation of l,3-bis-(3,4-dimethylphenoxy)benzene with trifluoroacetylbenzene, using H F as solvent and catalyst (Scheme 2). The sequential double ring closure to form TMPXPX was more difficult than the case of a single ring closure, in the reaction of 3,4,3',4'-tetramethylphenyl ether, as manifested by lower product yield in the present reaction. The reason for this is that after the first -C(Ph)(CF )- bridge is formed at the 4- or 6-positions of the central ring, the second ring closure may take place either at the 2- or 6-positions, only one of which yields the desired product. The by-product arising from condensation at the 2-position may have been present in the reaction mixture, but it was not isolated. The oxidation of TMPXPX and dehydration of the resulting tetraacid, PXPXTA, to the dianhydride, PXPXDA proceeded easily, and in good yields. While the pentacyclic ring system in an analog of TMPXPX, containing -C(CF ) bridging units instead of -C(Ph)(CF )- units, was perfectly planar (and crescentshaped), as was demonstrated by an X-ray crystallographic structure determination (9), TMPXPX, and P X P X D A were obtained were obtained as a mixture of two isomers. Both were crescent-shaped (due to the difference of C-O and C-C bond lengths: 1.36 and 1.54 A, respectively), so that the imide links derived from this dianhydride would deviate by 22 from linearity. In addition, they exhibited folding along the 0"C(Ph)(CF ) axis . The reason for this is the steric bulk of the CF group which strives to remain outside the molecular fold, while the planar phenyl group fits inside the fold. In this manner, the pentacyclic fra/w-system has an over-all S-shape, while the cw-isomer has a C-shape (schematically illustrated in Figure 1). The same cis- and fra/w-isomers were found in the parent pentacyclic heterocycle, 12,14-diphenyl-12,14-bis(trifluoromethyl)-12H,14H-5,7-dioxapentacene, obtained by copper-catalyzed decarboxylation of PXPXTA. According to G C / M S data, the major isomer is trans. Table 1 shows the GPC molecular weight characterization of PAAs and Pis prepared from P X P X D A and several diamines. Although the molecular weight values were only relative (based on polystyrene), they were comparable on a relative basis with commercial P M D A / O D A (PI-2540) and thus indicated acceptable monomer purity. All samples listed gave good quality, creasable films with the exception of the P A A with PPD which under the chosen processing conditions gave a brittle film. Although most of the Pis were soluble in NMP at RT, TCE was sometimes chosen as coating solvent due to lower hygroscopicity vs. NMP. Interestingly, while the PI with rigid PPD precipitated from solution upon solution imidization, Pis with rigid D A D and TFMB were fully soluble, despite the fair amount of extended chain character of the backbone. This is likely due to the combination of bulky phenyl and C F groups, cis/trans isomerism which changes the conformation, and the 22 ° bend within the P X P X D A unit, along with the solubilizing groups of the diamines. A similar solubility effect was noted with structurally similar 3FCDA (2, 10) containing polyimides. The solubility was even further enhanced with PXPXDA. The


3

3

3

0

3

3

3


2



500


34.


501


Table 1. Molecular Weight Characterization and Film Forming Performance of PXPXDA-Based Poly(amic acid)s and Soluble Polyimides


P A A CMDiamine

PI

4,4'-ODA 4,4'-ODA PPD PPD DAD TFMB PMDA/ODA

PAAA PI

PAA PI PI PI

PAA

%F

Mn

Mw

Mw/Mn

13.0 44000 90900 13.0 68400 190000 14.5 53600 108000 ppt. — 14.5 13.5 *38100 nioooo 22.8 68400 181000 0.0 50800 125000

2.02 2.78 2.02 —

•2.87 2.66 2.48

Film Qual./ Coating Solv Good/NMP Good/NMP Brittle/NMP — Good/TCE Good/TCE Good/NMP

*GPC in DMAC @135 °C, others in DMAC mixture at 35 °C ppt. - precipitated upon solution imidization

combination of a very stiff structure with solubility is unusual in polyimides and in polymers in general. The fact that the P X P X D A molecule had such a stiff but irregular structure allowed this unusual combiniation of properties. It is likely that other rigid diamines, even those with less bulky substituents than D A D or TFMB, would also give soluble structures. Table 2 shows the characterization of ~ 10 /xm films prepared from the samples in Table 1. Mechanical properties were found to be typical of polyimides with a higher modulus and lower elongation for the stiffer D A D and TFMB materials. CTEs of the more extended chain D A D and TFMB materials were also advantageously lower than 20 ppm/°C while the O D A materials showed higher CTEs typical of increased chain flexibility. A stiff, more extended chain structure allows for better orientation during the film forming process and thereby lower in-plane CTE. CTE's lower than 20 ppm/°C are very desirable in order to match those of copper and ceramic substates often used in electronic devices. There are very few examples, if any, noted in the literature of soluble Pis which give low in-plane CTE films. The 3FCDA-based Pis were another of

Table 2. Characterization of PXPXDA-Based Polyimide Films

Diamine 4,4'-ODA* 4,4'-ODA DAD TFMB PMDA/ODA*

%H20 Film Tens. abs. @ Thick. Str. Mod. CTE % /xm MPa Elong. GPa ppm/°C 85% R H 12.2 11.1 10.5 11.2 12.2

123 133 135 204 168

34 32 9 18 82

1.9 2.0 2.6 4.5 1.3

46 34 17 10 31

1.3 na 3.4 na 3.5

*prepared from PAA solution, others from soluble PI solution


Dielec. Const. dry® 1 MHz 2.6 2.8 2.5 2.6 3.2

502


Table 3. Thermal Characterization of PXPXDA-Based Polyimide Films Diamine

Tg-DMA E" max

Td-onset in air

Td-5% wt. loss

439 435 388 436 425

421 419 408 430 454

485 485 446 486 565

4,4'-ODA* 4,4'-ODA DAD TFMB PMDA/ODA*


•prepared from PAA solution, others from soluble PI solution

these examples (2, 10)> while those polyimides reported by Hsu and Harris (11) may also yield low CTEs in films (film CTE data not reported). The stiff, orientable nature of these materials may also prove useful for the preparation of high modulus, high strength fibers, while the unique structure/conformations of the monomer may make useful membranes. Dielectric constants were all notably lower than P M D A / O D A , and moisture absorption for the PXPXDA-ODA material was very low, while that based on D A D was fairly high (possibly due to the free volume effect). Table 3 gives the thermal characterization of the various samples. The very stiff structure of P X P X D A gave very high Tg. A typical D M A trace is given in Figure 2. It should be noted that the modulus curve was very flat and that the Tg occurred in the range of the onset of weight loss, such that the modulus changed little over its entire useful temperature range. At Tg, the modulus drops very little owing to the likely onset of crosslinking reactions that occur at this temperature. On the whole, the T G A thermal stability in air of these polyimides, although high, was somewhat less than that of the conventional P M D A / O D A polyimide. Figure 3 shows a typical ATR-FTIR spectrum of the PXPXDA-based polyimide film indicating the standard imide bands at about 1720 and 1780 cm" . In Figure 4, a typical proton N M R spectrum (CDC1 ) is presented with assignments. Interestingly, signal c was split into two resonances due to the cis/trans arrangement of phenyl and CF groups of the P X P X D A unit, both isomers being present in the monomer used for polymerization. The integrals indicated that one isomer (the more insoluble one, and probably trans-) dominated (2/1) the monomer sample. Also of interest was the fact that the methyl groups of the D A D unit existed as an apparent doublet, possibly due to restricted mobility of the very stiff chains, leading to non-equivalent environments. A similar effect was noted for related 3FCDA polyimides (2, 10). 1

3

3

CONCLUSIONS A new fluorinated rigid dianhydride has been prepared which has yielded soluble, high Tg polyimides. When this dianhydride was paired with rigid "rod-like" diamines, low thermal expansion was realized in films and an unusual combination of solubility and low themal expansion was found. In addition to





34.

( o) fewo/uAp] .g

( ) [gUJ3/uAp] V

,3


503



504

Figure 3. ATR-FTIR spectrum of P X P X D A / O D A polyimide film


34.



505

α


•ι 1

I PH

"8 0

gε 2

ο

Fluorinated, Soluble Polyimides with High Glass-Transition

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