Characterizing Cycloaliphatic-containing Polyimides

This article deals with the synthesis and characterization of polyimides that contain cycloaliphatic compounds, as shown below.

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Polymides are useful compounds for many applications which occur under harsh or demanding conditions, or involve compounds of high value. Polyimides such as PMDA-ODA (a copolymer of pyromellitic dianhydride with 4,4′-oxydianiline) tolerate operating temperatures as high as 450 °C, displaying very high levels of resistance to electrical, thermal and radiation energy. PMDA-ODA has a tensile strength of 2 GPa, for instance. Aromatic polyimides turn to carbon in the form of graphite at temperatures over 2300 °C.

Polyimides are used in applications such as:

• Satellite shielding material
• Flexible electronics
• Cryogenics
• High-vacuum conditions

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Image Credit: CABB

The range of applications is restricted by the structures of modern polyimides, because the presence of aromatic rings makes the chains very rigid.

This leads to a T_g > 300 ^oC, to produce a compound which is completely insoluble and does not melt before degrading. This limits their use in applications such as melt-pressing and injection molding which require fusible materials. Their insolubility means they have to first be processed in soluble polyamic acid precursor form before being finally casted into films and permanently imidized. Thus many of these are available only in 2D form, as films.

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The aromatic structure also enhances the rate of formation of charge transfer complexes (CTC) as the aromatic or imide segments take part in electron donor/acceptor interactions. In addition, it also gives the compound a color which prevents its use in many optical environments which require colorless compounds. The color can be lightened by substituting aromatic segments with aliphatic monomers in the compound, but this makes it less heat-stable.

A literature survey was performed to find other research projects which had incorporated cycloaliphatic monomers into polyimides. These used an equimolar mixture of cis and trans CHDA isomers, treated with BPDA according to classical polyimide methods. This generated a brittle transparent film with T_g = 225 °C. However, when trans-1,4-CHDA was used, its poor solubility impeded the synthesis of high molecular weight compounds. When dianhydride was added, a precipitate was formed.

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When cycloaliphatic diamines were introduced into the polyimides of the primary chain, the following changes occurred:

• The optical transparency is enhanced
• The dielectric constant is decreased
• This was because of reduced intermolecular CTCs formation

Ionic Aggregates Between Polyamic Acids

When polyamic acids are formed using alkyl amines, intermolecular salts are detected as a result of intermolecular ionic aggregates, causing precipitation. This is enhanced when alkyl amines are used rather than aryl amines. Thus some studies have resorted to thermal treatment in the initial stages to increase the solubility of the salts, but care must be taken to regulate the heating as overheating causes polyamic acids with very low molecular weight to be formed instead.

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Preparation of Polyimides from Diammonium Salts as Monomers

To avoid the formation of precipitates, this experiment uses diammonium salts in the form of N700 (Neximid 700)-CHDA monomers. This reduced the amount of oxidized impurity which gave traditionally prepared CHDA its high color. The use of ionic N700 with CHDA results in the formation of polyamic acids without precipitation. These are much purer and significantly more stable during storage as well as being less colored than CHDA.

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The figures below show the transmittance of recrystallized and commercial CHDA in polyamic acid solutions compared to just N700.

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How N700 Acts During the Synthesis of Polyamic Acid

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It is predicted that the anhydride will be under nucleophile attack by the neutral amine which is formed with the addition of N700, with the reaction rate being a function of the equilibrium between the ammonium salt and the free amine. The role of acetic acid is also not understood, but when present in the reaction chamber during the heating, it may also change the equilibrium and cyclodehydration may be enhanced.

N700 Cannot be Used with H-PMDA or CBDA

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With H-PMDA (hydrogenated pyromellitic dianhydride) the solution is heterogeneous after 5 days from the date of addition.

The compound forms a gel when the weight percent loadings are above 10 weight percent.

The compound does not form polyamic acid (PAA) or polyimide (PI) freestanding films.

With CBDA (cyclobutane dianhydride) the solution is homogeneous after 2 days.

The increase in viscosity is minimal.

The compound does not form freestanding films of PAA or PI.

Polyamic Acids of N700 with BPDA

This reaction involving BPDA (biphenyl dianhydride) generates a film which is transparent and can be creased, but is brittle. The viscosity increase is very little, over about 2 days, because of slow dissolution and reaction of the ammonium salts and change of equilibrium. Imidization, as shown in the second step below, results in the appearance of a deep color but retention of brittleness.

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Image Credit: CABB

The t_homo is approximately 5 hours.

When the percent loadings exceeds 20 weight percent, the solution is thick enough to require mechanical stirring.

It forms a brittle and creasable polyimide film.

T_g of PI is over > 300 °C.

BPDA and N700 Copolymerization with DDS

In an attempt to increase the mechanical robustness, copolymerization was done with BPDA and N700 with DDS (diaminodiphenyl sulfone), as the aro-amine. DDS was introduced at different mol% and in all cases the result was a freestanding film. The films shown in the lower two images were removed from the glass substrate before imidization was carried out, which caused the films to warp and curve. All these copolyimides showed a T_g of over 275 ^oC.

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Viscosity of 7.5% N700-BPDA-DDS Solutions on Rheology

Since the molecular weight of these systems could not be directly assessed, the solution was subjected to rheology after 2 days of reaction, which showed that all of the solutions attained a viscosity of 1 Pascal-second except for the 0.10 copolymer which was higher.

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These solutions produce strong PAA films which do not show shear thinning within the range of shear rate. Thus 7.5 wt% solutions are necessary to form freestanding films.

BPDA-N700-DDS Copolymer Thermogravimetric Analysis

Thermogravimetric analysis was performed on solutions of BPDA-N700-DDS copolymer concerning varying degrees of imide formation and T_d5% of each, as shown in the graph below. The aromatic character increases with the percentage loading.

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TGA Q50, RT- 600 °C, 10 °C/min, N₂ Fill gas

Robust PI Film Formation with 6FDA-N700 Reaction

The following reaction uses hexafluorodianhydride (6FDA) with N700 to produce homopolymerization.

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Image Credit: CABB

t_Homo is approximately 5 hours, and the viscosity increased significantly after about 12 hours.

A colorless PAA film with robust mechanical properties was formed.

Full imidization at 150-300 ^oC was shown by the formation of a slightly yellowish colored PI compound.

Viscosity of 6-FDA-N700 PAA Solutions over Time

Solution rheology methods were used to determine the change in viscosity of 6-FDA-N700 PAA solutions over the course of time. The instrument used was a AR-G2 rheometer with a concentric cylinder, possessing a steady state frequency sweep at 25 °C.

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Image Credit: CABB

The findings are summarized below:

t_homo is approximately 5 hours.

The determination is performed at percent weight loadings of 10 wt.% in NMP.

The viscosity shows a large linear increase over the first 18 hours of reaction.

Copolymerization of H-PMDA and 6-FDA with N700

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Image Credit: CABB

In order to produce a clear film, H (hydrogenated)-PMDA and 6-FDA are copolymerized in a 50:50 mol% ratio with N700, it forms a clear, colorless and brittle copolyimide film. The t_homo is about 2 days, and there is a macroscopic increase in viscosity.

However, the polyimide film is very brittle and has to be vacuum-cast between 150-300 ^oC as it has very poor shear quality.

When the solution contains 75 mol% H-PMDA, the copolymer does not produce a film.

H-PMDA is Poorly Polymerized

A literature search showed that H-PDMA is poorly polymerized and the resultant PAA formed has low molecular weight because of steric hindrance:

• The cis structure shields the chain from anhydride attack
• Even when it reacts with highly reactive diamines such as ODA (4,4-deoxydianiline) low MW compounds are the result, as shown in the viscosity results in the table

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Image Credit:CABB

The trans-isomer, H’-PDMA, reacts to produce PI films with robust mechanical properties.

Exceptionally, cyclobutane dianhydride (CBDA) polymerizes to very high levels when reacted with diamines despite the cycloaliphatic character, because of the presence of strain in the anhydride ring.

CBDA-N700 Copolymerization with 6FDA

When CBDA and N700 are copolymerized with 6FDA, the result is a lightly colored film in about half a day less.

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Image Credit: CABB

In this case, the following characteristics are observed:

t_homo is about 1.5 days.

There is a macroscopic increase in viscosity.

The polyamic acid film shows greater brittleness compared to the PI.

No film is formed with the 75 mol% CBDA copolymer.

Imidization Dynamics for CHDA/CBDA-6FDA-Nex7 Copolymers

As heating progresses imidization occurs in increasing amounts by cyclodehydration. Temperatures of above 300 ^oC are necessary for 100% imidization.

TGA-Q50, RT- 600 °C, 10 °C/min, N₂ Fill gas

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N700 Facile Deprotonation

N700 is readily deprotonated to give a very pure form of t-CHDA as shown below and thus serves as an excellent medium for the storage of this polyamic acid precursor without oxidation as would otherwise occur when stored.

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The procedure was as follows:

N700 was stirred in 3 M KOH solution for 20 minutes, and then extracted with DCM. The extracted compound was dried at low pressure for one day at 35 °C. This yielded a fine white powder, at about 83% yield, with a melting point of approximately 70 °C.

Thus the N700 vehicle was shown to store t-CHDA very well.

Spectroscopy of Deprotonated N700

The spectra below are of the molecules N700 and t-CHDA, and help understand their deprotonation by showing the disappearance of specific peaks following the steps of the procedure.

Agilent U4-DD2: 400 MHz, 25 °C, D2O/CDCl3

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t-CHDA and PIs resistant to N700

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It was found that even when t-CHDA was used instead of N700 polymerization remained poor with previously inaccessible PIs.

In the case of R = CBDA (fully aliphatic) / H-PMDA there was rapid gel formation, and the diluted solution was heterogeneous after 5 days

With R = ODPA / BTDA/ BPDA (benzophenone dianhydride), a precipitate was formed and this showed slow dissolution to become homogeneous solution at about 12 hours.

With R= CBDA-6FDA / H-PMDA-6FDA the T_homo remained unchanged even when sublimed N700 was used

The traditional PAA procedure was used and it was found that t-CHDA is dissolved in NMP, DMAc, DMF, DMSO. Following the addition of dianhydride, the viscosity increased quickly leading to precipitation which hindered further work. The films formed in this reaction had the same mechanical properties as before.

Conclusion

When N700 is used, robust PI films are synthesized. The formation of PAA is due to the maintenance of acid-base equilibrium, and a reduced number of ionic intermolecular aggregates as opposed to that which occurs when only non-ammonium salts or diamines are used.

The viscosity was always observed to increase steadily when PAA was forming. The rapid conversion of N700 to t-CHDA failed to result in any increase in the rate or amount of PAA synthesis. If CBDA formed a copolymer with 6FDA and N700, the result was a mechanically robust polyimide film with a light color. However, copolymerization of H-PMDA with 6FDA and N700 was not possible because of steric hindrance within the molecules of the H-PMDA anhydride system.

This information has been sourced, reviewed and adapted from materials provided by CABB Group GmbH and Nexam Chemical AB.

For more information on this source, please visit CABB.