Development of Solution-Processable, Optically Transparent Polyimides with Ultra-Low Linear Coefficients of Thermal Expansion

4.2.1. Problems in the PAA Polymerization

t

-CHDA system. This system is expected to lead to a low-CTE PI film because, as well as PMDA,

t

-CHDA has high structural linearity. However, it is challenging to obtain a homogeneous PAA solution in this system owing to the formation of an extremely “tight” salt. This is probably related to (1) the shorter molecular length (a consequent higher crosslinking density) than that of other cycloaliphatic diamines, (2) the relatively planar/rigid structure of

t

-CHDA (a decrease in the solubility of the yielded salt), and (3) the enhanced COOH acidity (an intensification in the salt linkages) induced by the adjacent electron-withdrawing acid anhydride group in the low-

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w amic acids, as shown in

t

-CHDA does not change, even after dilution and heating [

t

-CHDA [

Commercially available aliphatic diamines with cyclic structures, which are applicable for the present purpose, are shown in Figure 8 . Linear (non-cyclic) aliphatic diamines (e.g., 1,6-hexanediamine) are removed here because of the absence of sufficient heat resistance in the resultant PIs. In principle, the use of these cycloaliphatic diamines will result in colorless and heat-resistant PI films. However, the polyaddition between these diamines and tetracarboxylic dianhydrides is disturbed by the salt formation that occurs in the initial reaction stage. The formed salt probably consists of a crosslinked structure through acid–base salt bonding, as schematically depicted in Figure 9 . The basicity of cycloaliphatic diamines is not significantly affected by their structures. However, in fact, the “tightness” of the formed salt strongly depends on them, as mentioned later. The salt is poorly soluble in anhydrous amide solvents, so that precipitation occurs during the polyaddition. In some cases, the PAA polymerization reactions are completely terminated, as typically observed in the PMDA/-CHDA system. This system is expected to lead to a low-CTE PI film because, as well as PMDA,-CHDA has high structural linearity. However, it is challenging to obtain a homogeneous PAA solution in this system owing to the formation of an extremely “tight” salt. This is probably related to (1) the shorter molecular length (a consequent higher crosslinking density) than that of other cycloaliphatic diamines, (2) the relatively planar/rigid structure of-CHDA (a decrease in the solubility of the yielded salt), and (3) the enhanced COOH acidity (an intensification in the salt linkages) induced by the adjacent electron-withdrawing acid anhydride group in the low-amic acids, as shown in Figure 9 . The difficulty in the polyaddition between PMDA and-CHDA does not change, even after dilution and heating [ 17 ]. A similar situation is also observed in the reaction between CBDA and-CHDA [ 17 ].

t

-CHDA. For example, in the reaction of PMDA and MBCHA, a salt is similarly formed in the initial reaction stage, but it is gradually dissolved by simply stirring at room temperature for a prolonged period, and, finally, a homogeneous/viscous PAA solution is formed. This is probably due to the decreased crosslinking density in the salt resulting from the use of MBCHA with a distorted and long molecular structure compared to

t

-CHDA [method (a) listed below]. Consequently, the predicted “loosely formed” salt permits the penetration of the solvent molecules into the salt, which helps the dissolution of the salt. Similar behavior is also observed in the systems using isophoronediamine (IPDA), 2,5(2,6)-bis(aminomethyl)bicyclo[2.2.1]heptane (NBDA), and other flexible cycloaliphatic diamines shown in

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ws of the resultant PAAs (or more conveniently, their inherent viscosities, ηinh). This is a great obstacle particularly concerning mass-production for the commercialization of semi-cycloaliphatic colorless PIs using cycloaliphatic diamines.

On the other hand, when flexible cycloaliphatic diamines (e.g., 4,4′-methylenebis(cyclohexylamine) (MBCHA, a mixture of isomers, Figure 8 )) were used, the situation is different from the case using-CHDA. For example, in the reaction of PMDA and MBCHA, a salt is similarly formed in the initial reaction stage, but it is gradually dissolved by simply stirring at room temperature for a prolonged period, and, finally, a homogeneous/viscous PAA solution is formed. This is probably due to the decreased crosslinking density in the salt resulting from the use of MBCHA with a distorted and long molecular structure compared to-CHDA [method (a) listed below]. Consequently, the predicted “loosely formed” salt permits the penetration of the solvent molecules into the salt, which helps the dissolution of the salt. Similar behavior is also observed in the systems using isophoronediamine (IPDA), 2,5(2,6)-bis(aminomethyl)bicyclo[2.2.1]heptane (NBDA), and other flexible cycloaliphatic diamines shown in Figure 8 . However, even when these flexible cycloaliphatic diamines are chosen, a crucial problem remains, i.e., the insufficient reproducibility with respect to thes of the resultant PAAs (or more conveniently, their inherent viscosities, η). This is a great obstacle particularly concerning mass-production for the commercialization of semi-cycloaliphatic colorless PIs using cycloaliphatic diamines.

To monitor the

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w-reproducibility, the measurements of the reduced viscosity (ηred) of PAAs at a fixed low polymer concentration (usually, 0.5 wt %), for example, by an Ostwald viscometer is often more useful in terms of speed than gel permeation chromatography (GPC). That is because PAAs decompose with a prompt decrease in the

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w as soon as the PAA solutions are diluted to a low concentration (e.g., 0.05 wt % as for GPC measurements). Thus, the ηred measurements can be carried out much more rapidly than GPC. In addition, it is not necessary to determine the inherent viscosity (ηinh) by extrapolation to zero concentration because the extrapolation is disturbed by an unusual increase in the ηred values in the low concentration range by the polyelectrolyte effect. Therefore, the ηred values measured at 0.5 wt % can be regarded, for all practical purpose, as the ηinh values. This parameter is also useful for soluble PI systems; however, it is difficult to obtain the viscosity data after imidization for PIs using cycloaliphatic diamines owing to their insolubility. The ηinh values give a useful criterion; PIs with a ηinh value lower than 0.3–0.5 dL·g−1 often have an insufficient film-forming ability. When the PIs of interest are highly soluble, it is desirable to conduct not only the GPC measurements for the PIs but also a set of viscosity measurements for both the PAAs and the corresponding PIs because an undesirable molecular weight decrease during imidization, if any, can be monitored by comparing between these ηinh values.

Other methods [(b)–(e)] for preventing or suppressing the salt formation have also been reported:

(a)

Choice of higher-

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w cycloaliphatic diamines with bulkier and more distorted structures.

(b)

Choice of a higher-

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w tetracarboxylic dianhydride with bulkier and more distorted structures.

(c)

Optimization of the reaction conditions.

(d)

One-pot polymerization method (only for highly soluble PI systems).

(e)

Addition of acetic acid.

(f)

Use of silylated cycloaliphatic diamines.

t

-CHDA, as in the use of aromatic diamines, without prominent salt precipitation. A cardo-type tetracarboxylic dianhydride (TA-BPFL,

t

-CHDA film maintains excellent solubility in contrast to the fact that

t

-CHDA-derived PIs are usually quite insoluble in common organic solvents [

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ws of the PAAs because the tetracarboxylic dianhydrides dissolved in advance suffer partial hydrolysis by a small quantity of water contained in the solvents. Method (d) is applicable when the imidized forms are highly soluble, and it is expected to be effective in suppressing salt formation.

Here, method (b) has a similar effect to method (a). For example, in a rare case, a sulfone-containing tetracarboxylic dianhydride (DSDA, Figure 10 ) reacts smoothly with-CHDA, as in the use of aromatic diamines, without prominent salt precipitation. A cardo-type tetracarboxylic dianhydride (TA-BPFL, Figure 10 ) possessing a bulky fluorenyl side group is also useful in preventing the formation of a tight salt [ 64 ]. Incidentally, the molecular bulkiness of TA-BPFL also contributes to the excellent solubility of the resultant PIs; surprisingly, the thermally imidized TA-BPFL/-CHDA film maintains excellent solubility in contrast to the fact that-CHDA-derived PIs are usually quite insoluble in common organic solvents [ 64 ]. Method (c) also sometimes contributes to the rapid dissolution of the salt by optimizing the reaction conditions [the monomer content, the type of solvents, and reaction temperature]. An unusual monomer feeding procedure (the addition of diamine to tetracarboxylic dianhydride solutions [ 65 ]) is also a possible method, although it is not always effective in accelerating the salt dissolution. However, this method is not suitable in terms of enhancing thes of the PAAs because the tetracarboxylic dianhydrides dissolved in advance suffer partial hydrolysis by a small quantity of water contained in the solvents. Method (d) is applicable when the imidized forms are highly soluble, and it is expected to be effective in suppressing salt formation.

Method (e) has some effect on shortening the reaction period [ 66 ]. In this case, the added acetic acid probably acts as an end-capping reagent for the unreacted aliphatic amino groups that are the origin of crosslinking via salt linkages ( Figure 9 ).

In principle, the silylation method (f) can completely inhibit salt formation because the reactions between trialkyl-silylated cycloaliphatic diamines and tetracarboxylic dianhydrides yield the trialkylsilylester forms of PAAs, which do not contain the COOH groups necessary for salt formation [ 67 ]. However, in our experience, even these methods ((e) and (f)) are not always universally effective because there are several systems where no homogeneous precursor solutions are obtained.

At present,

t

-CHDA is a very limited cycloaliphatic diamine, suitable for achieving simultaneously excellent transparency, high

T

g, and low-CTE characteristics. In contrast, isomeric

cis

-CHDA, as well as as-hydrogenated

p

-PDA (a mixture of the

trans

- and

cis

-isomers), is not useful for obtaining a low CTE because of the significant decrease in the chain linearity on using the

cis

-form.

t

-CHDA and a variety of tetracarboxylic dianhydrides in DMAc can be classified into three categories based on the “tightness” of the initially formed salt, as shown in

t

-CHDA system, the salt-containing reaction mixture in DMAc was heated at 100–150 °C for a very short period (< a few minutes) to induce partial dissolution of the salt in association with an exothermic reaction. The reaction mixture was stirred without additional heating, using only the heat of reaction, until the reaction mixtures homogenized. This set of processes yield a high-

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w PAA (ηinh = 1.5–2.5 dL·g−1) [37,

t

-CHDA system (category-III) and the 1,3-dimethyl-CBDA (DM-CBDA)/

t

-CHDA system (category-I) suggests that the dimethyl substituents (highlighted in

We have also found that the reaction behavior between-CHDA and a variety of tetracarboxylic dianhydrides in DMAc can be classified into three categories based on the “tightness” of the initially formed salt, as shown in Figure 10 . The results correspond well to the assumption mentioned above that the “tightness” of the salt is dominated by the structural rigidity and bulkiness of the monomers used, the strength of the salt linkages, and the predicted crosslinking density in the salt. The tetracarboxylic dianhydrides in category-I result in homogeneous PAA solutions after prolonged stirring at room temperature via gradual dissolution of the initially formed salt. These tetracarboxylic dianhydrides commonly have distorted or bulky structures, corresponding to method (b) mentioned above. However, the systems using semi-rigid tetracarboxylic dianhydrides belonging in category-II require heating to obtain homogeneous PAA solutions because of the formation of a relatively tight salt. For example, in the s-BPDA/-CHDA system, the salt-containing reaction mixture in DMAc was heated at 100–150 °C for a very short period (< a few minutes) to induce partial dissolution of the salt in association with an exothermic reaction. The reaction mixture was stirred without additional heating, using only the heat of reaction, until the reaction mixtures homogenized. This set of processes yield a high-PAA (η= 1.5–2.5 dL·g) [ 45 ]. A solvent effect was also observed in this system; for example, the replacement of DMAc by NMP enables the formation of a homogeneous PAA solution by continuous stirring at room temperature (without heating), probably owing to slightly increased salt solubility in NMP. However, the use of NMP is not desirable because it tends to cause slight coloration in the resultant PI film compared to DMAc, as mentioned above. On the other hand, the systems in category-III are not polymerizable under any conditions [ 17 46 ]. The difficulty in the PAA polymerization probably reflects the formation of an extremely tight salt, which corresponds well to the extremely rigid monomer structures in category-III. A comparison between the CBDA/-CHDA system (category-III) and the 1,3-dimethyl-CBDA (DM-CBDA)/-CHDA system (category-I) suggests that the dimethyl substituents (highlighted in Figure 10 ) play a great role in reducing the tightness of the salt. Thus, the promising systems (category-III) in terms of the target properties inevitably involve polymerization problems.