Thermally stable, flexible substrates for electronic devices

Abstract

A flexible substrate with a high optical transparency (>80% from 400 to 750 nm) that is retained after exposure to 300° C., near-zero birefringence (<±0.001), and a relatively low CTE (<60 ppm/° C.) is disclosed. The substrate may be manufactured as single layer, polyimide films and as a multi-layer laminate comprising a polyimide layer and a thin glass layer. The polyimides may include alicyclic dianhydrides and aromatic, cardo diamines. The films formed of the polyimides can serve as flexible substrates for optical displays and other applications that require their unique combination of properties.

Claims

1. A substrate for use in or with electronic devices, wherein the substrate comprises: an organo-soluble polyimide film having an out of plane birefringence of less than about ±0.002, wherein the organo-soluble polyimide has a glass transition temperature greater than about 300° C. and is comprised of: an alicyclic dianhydride selected from the group consisting of: 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride; and at least one aromatic cardo diamine selected from the group consisting of 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, and 9, 9-bis(4-amino-3-methylphenyl)fluorene; and wherein: the film is flexible and has a thickness of less than about 50 microns and a transmittance of greater than about 80% at about 400 nm to about 750 nm both before the film is heated and after the film is heated to 300° C. for no longer than 60 minutes.

2. The substrate of claim 1, wherein the electronic device is a flat panel display, a lighting device, a photovoltaic device, or an input/output device.

3. The substrate of claim 1, wherein the film has a thickness of at least 25 microns.

4. The substrate of claim 1, wherein the film has a coefficient of thermal expansion of less than about 60 ppm/° C.

5. The substrate of claim 1, wherein the polyimide further comprises an aromatic dianhydride.

6. The substrate of claim 1, wherein the polyimide further comprises a non-cardo, aromatic diamine.

7. A substrate for use in or with electronic devices, wherein the substrate consists essentially of: a single polymer layer having a thickness of less than about 50 microns comprising: an organo-soluble polyimide having a glass transition temperature greater than about 300° C. and an out of plane birefringence of less than about ±0.002, wherein the organo-soluble polyimide comprises at least one alicyclic dianhydride selected from the group consisting of 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride; and at least one aromatic cardo diamine selected from the group consisting of 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-amino-3-fluorophenyl)fluorene, and 9, 9-bis(4-amino-3-methylphenyl)fluorene; and a glass layer; wherein the single polymer layer has a transmittance of greater than about 80% at about 400 nm to about 750 nm both before and after it has been exposed to 300° C. for no longer than 60 minutes, and is flexible.

8. The substrate of claim 7, wherein the single polymer layer has a thickness of at least 2 microns.

9. The substrate of claim 7, wherein the glass layer has a thickness of at least 20 microns.

10. The substrate of claim 7, wherein the single polymer layer has a coefficient of thermal expansion of less than about 60 ppm/° C.

Description

DETAILED DESCRIPTION

(1) Surprisingly, it has been discovered that soluble polyimides with Tgs>300° C. can be used in the manufacture of transparent flexible substrates with near zero birefringence that maintain excellent transparency after exposure to 300° C. for 10 minutes in air, or for 30 minutes under reduced pressure or in an inert atmosphere. In particular, it has been found that polyimides that are prepared from alicyclic dianhydrides and aromatic, cardo dianhydrides can be used in the manufacture of transparent substrates (transmittance of greater than 80% at 400 nm to 750 nm) with an out-of-plane birefringence of less than ±0.001 and a CTE of less than 60 ppm/° C. The substrates can be used in the manufacture of flexible, electronic devices where they are exposed to high temperatures. The polyimides may generally be prepared from alicyclic dianhydrides and aromatic diamines. Alicyclic dianhydrides may be selected from the group:

(2) ##STR00007##

(3) wherein Ac is selected from the group:

(4) ##STR00008##
and particularly useful dianhydrides include:

(5) 1,2,3,4-Cyclobutanetetracarboxylic dianhydride (CBDA);

(6) ##STR00009##

(7) 1,2,3,4-Cyclopentanetetracarboxylic dianhydride (CPDA);

(8) ##STR00010##

(9) 1,2,4,5-Cyclohexanetetracarboxylic dianhydride (HPMDA); and

(10) ##STR00011##

(11) Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BODA).

(12) ##STR00012##

(13) The aromatic diamines may have the following structure:

(14) ##STR00013##

(15) wherein n=1-4 and R is selected from the group comprising hydrogen, halogen (fluoride, chloride, bromide, and iodide), alkyl, substituted alkyl such as halogenated alkyls, nitro, cyano, thioalkyl, alkoxy, substituted alkoxy such as halogenated alkoxy, aryl, or substituted aryl such as halogenated aryls, ethynyl, phenylethynyl, alkyl ester and substituted alkyl esters, and combinations thereof. It is to be understood that when n is less than 4 the other positions on the aromatic ring are assumed by hydrogen atoms. It is also to be understood that each R can be different.

(16) Particularly useful diamines include:

(17) 9,9-Bis(4-aminophenyl)fluorene (FDA);

(18) ##STR00014##

(19) 9,9-Bis(4-amino-3-fluorophenyl)fluorene (FFDA); and

(20) ##STR00015##

(21) 9,9-Bis-(4-amino-3-methylphenyl)fluorene (MeFDA).

(22) ##STR00016##

(23) In one embodiment, the polyimide may be prepared from monomer mixtures containing one or more alicyclic dianhydrides and one or more cardo, aromatic diamines. In some cases, it may be desirable to substitute some of the cardo diamine with a non-cardo, aromatic diamine, and/or some of the alicyclic dianhydride with an aromatic dianhydride. In particular, it may be useful to include a monomer containing a group, such as a carboxyl group, that can be used in crosslinking the polymer so as to induce solvent resistance. When this group is present crosslinking can be induced by heating near 300° C. or by heating above 200° C. in the presence of a multi-functional epoxide. The monomers can be polymerized in high boiling solvents, such DMAC, NMP or m-cresol, which can contain an imidization catalyst, such as isoquinoline, at elevated temperatures to directly yield the imidized polymer. The polymerization mixture may also contain a dehydration reagent such as toluene.

(24) In another embodiment, the monomers can be polymerized in polar aprotic solvents below 100° C. to yield a low molecular weight polyamic acid that is imidized either chemically or thermally imidized. In yet another embodiment, imidization can also be carried out by a combination of these two methods. In fact, a combination of the two methods may be useful for the continuous casting of film directly from the polyamic acid polymerization mixture.

(25) The flexible substrates are prepared as single layer, films and as multi-layer laminates comprising a polyimide layer and a thin glass layer. The single layer films can be prepared by solution casting techniques known to those skilled in the art from solutions of the imidized polyimides in common organic solvents. Both batch and continuous processes, such as a roll-to-roll process, may be used. In both techniques, the viscosity of the solution is adjusted by adjusting the solids concentration and the polymer molecular weight so that optimum films may be produced with the equipment used. The multi-layer laminates can be prepared in one step by solution casting a layer of the polyimide on thin glass films. Additives may be used to increase the adhesion of the polyimide to the glass. The laminates may also be prepared in a multi-step process, wherein a polyimide layer is first solution cast on a carrying tape such as a polyester film. The combination is then laminated to the glass film so that the polyimide layer adheres to the glass. The carrying tape is removed prior to or during the construction of the flat panel display.

(26) The substrates can also be prepared by solution casting techniques from solutions of the polyamic acid precursors to the polyimides. In this case, the conversion of the polyamic acid to the polyimide is carried out chemically and/or thermally during or subsequent to the casting process. A continuous, roll-to-roll process whereby the polyamic acid is mixed with a chemical imidization mixture immediately prior to casting on an endless belt, which passes through heated zones, can be used to prepare single layer substrates or to form a polyimide layer on a glass film.

(27) In order to simplify the construction of a flexible electronic device such as a display, other functional and non-functional layers may be cast on or laminated to the substrate. For example, a gas barrier layer might be added.

(28) General Polymerization Procedures

(29) The following general procedure may be used to prepare a polyimide from a cardo diamine and an alicyclic dianhydride in a high boiling solvent:

(30) To a three-neck, round-bottom flask (250 mL) that was equipped with an overhead stirrer, a nitrogen inlet, and a short path distillation apparatus, 0.040 mol of the cardo diamine and 60 ml of m-cresol was added to form a mixture. The mixture was heated to about 60° C. under nitrogen with stirring until all the diamine dissolved. The alicyclic dianhydride (about 0.040 mol) was added to the mixture to form a reaction mixture, which was heated at about 100° C. until all the dianhydride dissolved. The heating bath was then removed, and the reaction mixture was allowed to cool to room temperature and was then stirred for about 4 hours. After several drops of isoquinoline were added, the reaction mixture was heated to about 200° C. for 12 hours. During this process, water and some m-cresol were removed by distillation. The reaction mixture was then diluted with 50 ml of m-cresol, allowed to cool to room temperature and then added to 500 ml of methanol. The fibrous precipitate that formed was collected by filtration, soaked in methanol to remove the majority of the solvent (process was repeated three times) and then dried under reduced pressure at 100° C.

(31) Polymer solubility. The solubility of the polymer was determined in N-methyl-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc) and cyclopentanone (CPN). The results are shown in Table 1. The polymer was considered soluble if a 5 wt % solution could be prepared.

(32) Preparation of film for substrate qualification. The dry polymer was dissolved in cyclopentanone with a solids content between 5-20%. (Films could also be prepared from solutions of the polymers in polar aprotic solvents.) After filtration, the solution was poured on a glass substrate. The solvent was allowed to evaporate at ambient temperature. The glass substrate containing the film was dried at 100° C. under reduced pressure. The polymer film was removed from the glass by dipping the substrate glass in water.

(33) Film birefringence. The birefringence of the freestanding film (25 μm) was determined on a Metricon Prism Coupler 2010/M.

(34) Film transparency. Transparency was measured by determining the transmittance of a 25 μm thick film from 400 to 750 nm with a UV-Visible spectrometer (Shimadzu UV 2450). The transmittance was determined before and after the film was heated at 300° C. for 10 minutes in air or under reduced pressure or an inert atmosphere for 30 minutes. A plot of transmittance vs. wavelength was nearly identical to that of commercial PEN substrates. The minimum transmittance of the films, which is given in Table 1, was at 400 nm. If the film was heated under these conditions for longer than 60 minutes the transmittance at 400 nm was less than 80%.

EXAMPLES

Examples 1a-c

(35) BODA was polymerized by the general polymerization procedure with FDA (1a), FFDA (1b), and MeFDA (1c) (Table 1).

Comparative Examples 1a-c

(36) The general polymerization procedure was carried out with the aromatic dianhydride 3,3′,4,4′-Biphenyltetracarboxylic acid dianhydride (BPDA), which was used in place of the alicyclic dianhydride and FDA (1a), FFDA (1b), and MeFDA (1c) (Table 1).

Comparative Examples 2a-c

(37) The general polymerization procedure was carried out with the aromatic dianhydride bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BisADA), which was used in place of the alicyclic dianhydride and FDA (2a), FFDA (2b), and MeFDA (2c) (Table 1).

(38) TABLE-US-00001 TABLE 1 Polymerization of BODA, BPDA and BisADA Out of CTE Example Plane ppm/ T % at 400 nm Dianhydride Diamine No. NMP DMAc CPN Δn ° C. Tg° C. Initial Final* BODA FDA 1a Yes Yes No −0.0002 47 445 85.7 80.6 FFDA 1b Yes Yes Yes +0.0003 45 MeFDA 1c Yes Yes Yes +0.0009 47 440 83.4 81.0 BPDA FDA Comparative Yes No No −0.0380 32 1a FFDA Comparative Yes Yes Yes −0.0276 38 57.0 1b MeFDA Comparative Yes Yes Yes −0.0240 36 1c BisADA FDA Comparative Yes Yes Yes −0.0115 55 2a FFDA Comparative Yes Yes Yes −0.0099 56 83.1 2b MeFDA Comparative Yes Yes Yes −0.0073 62 2c *After thermal treatment at 300° C. for 30 minutes under reduced pressure.

(39) As shown in Table I, examples 1a-c produced polymers that formed films with a near zero birefringence and excellent transparency. Specifically, examples 1a-c had a transmittance of greater than 80% from 400 nm to 750 nm with an out-of-plane birefringence of less than ±0.001 and a CTE of less than 60 ppm/° C.

Examples 2a-h

(40) The general polymerization procedure was carried out with HPMDA and various mixtures of FFDA (98 mol % to 88 mol %) and PFMB (2 mol % to 12 mol %) (Table 2).

Example 3

(41) The general polymerization procedure was carried out with HPMDA and a mixture of FFDA (90 mol %) and ODA (10 mol %) (Table 2).

Example 4

(42) The general polymerization procedure was carried out with HPMDA (90 mol %) and 6FDA (10 mol %) and FFDA (Table 2).

Example 5

(43) The general polymerization procedure described was carried out with HPMDA and FDA (Table 2).

(44) TABLE-US-00002 TABLE 2 Polymerization of HPMDA Out of CTE Example Plane ppm/ Tg T % at 400 nm Dianhydride Diamine No. NMP DMAc CPN Δn ° C. ° C. Initial Final* HPMDA FFDA 2a Yes Yes Yes 0.0018 59 426 86.8 82.5 100% 100% FFDA PFMB 2b Yes Yes Yes 0.0012 57 427 87.5 82.5 98% 2% FFDA PFMB 2c Yes Yes Yes 0.0008 55 431 87.3 82.9 97% 3% FFDA PFMB 2d Yes Yes Yes 0.0005 60 428 87.6 85.2 96% 4% FFDA PFMB 2e Yes Yes Yes 0.0004 60 427 87.4 84.5 95% 5% FFDA PFMB 2f Yes Yes Yes 0.0002 54 433 87.2 82.0 92% 8% FFDA PFMB 2g Yes Yes Yes −0.0005 59 422 90% 10% FFDA PFMB 2h Yes Yes Yes −0.0009 55 422 88% 12% HPMDA FFDA ODA 3 Yes Yes No −0.0003 55 428 85.7 82.0 100% 90% 10% HPMDA 6FDA FFDA 4 Yes Yes Yes 0.0001 58 422 85.0 80.6 90% 10% 100% HPMDA FDA 5 Yes Yes No 0.0009 45 83.3 100% 100% *After thermal treatment at 300° C. for 10 minutes in air.

(45) As shown in Table II, the selected dianhydrides and diamines, and mixtures thereof, along with the percentages used may be varied in the reaction mixtures to form polyimides that can be used to form acceptable flexible substrates.

(46) To the extent that the term “includes” or “including” is used in the specification or the claims, it is intended to be inclusive in a manner similar to the term “comprising” as that term is interpreted when employed as a transitional word in a claim. Furthermore, to the extent that the term “or” is employed (e.g., A or B) it is intended to mean “A or B or both.” When the applicants intend to indicate “only A or B but not both” then the term “only A or B but not both” will be employed. Thus, use of the term “or” herein is the inclusive, and not the exclusive use. See, Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1995). Also, to the extent that the terms “in” or “into” are used in the specification or the claims, it is intended to additionally mean “on” or “onto.” Furthermore, to the extent the term “connect” is used in the specification or claims, it is intended to mean not only “directly connected to,” but also “indirectly connected to” such as connected through another component or components.

(47) While the present application has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Therefore, the application, in its broader aspects, is not limited to the specific details, the representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant's general inventive concept.