Solution-processable CF3-substituted ductile polyimides with low coefficients of thermal expansion as novel coating-type protective layers in flexible printed circuit boards

• This work proposes novel solution-processable copolyimides (PIs).
• A series of fluorinated diamines were synthesized to overcome a trade-off between low thermal expansion property and excellent film ductility.
• The PIs were highly soluble in common organic solvents.
• Solution casting of the PIs led to films with relatively low coefficients of thermal expansion, excellent heat resistance, and sufficient elongation at break.
• The high-temperature polymers developed in this work can be applied as novel coating-type protective layer materials in flexible printed circuits.

This work proposes practically useful novel coating-type cover layer materials. A copolyimide (PI) system derived from pyromellitic dianhydride (PMDA, 50 mol%), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA, 50 mol%) with 2,2′-bis(trifluoromethyl)benzidine (TFMB) showed a low coefficient of thermal expansion (CTE) by thermal imidization. However, this system was not compatible to chemical imidization process because of gelation. A structural modification of this copolymer by 4,4′-(hexafluoroisopropylidene)dianiline (HFIDA) significantly improved the solubility, as a result, allowed for chemical imidization in a homogeneous state and the formation of a stable PI solution from less hygroscopic solvents such as triglyme (TriGL). The TriGL-cast PI film at an HFIDA content of 25 mol% displayed excellent combined properties; a low CTE (22.9 ppm K−1), a very high Tg (327 °C), and a common level of tensile properties [modulus (E) = 3.6 GPa and the elongation at break (εbmax) = 10.4%]. The PMDA(50);s-BPDA(50)/TFMB copolymer system was also modified with other ether-linked fluorinated diamines; i.e., 2-trifluoromethyl-4,4′-diaminodiphenylether (3FODA) and 4,4′-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (6FBAPB). At a 3FODA content of 25 mol%, the NMP-cast PI film displayed the lowest CTE (20.7 ppm K−1) among the PIs examined in this work. The modification using 6FBAPB was most effective to enhance the solubility. Thus, the structural modifications of PMDA(50);s-BPDA(50)/TFMB copolymer by these fluorinated diamines (HFIDA, 3FODA, and 6FBAPB) were an effective way to obtain almost satisfying target properties except for a high level of εb. A mechanism was proposed for explaining why the effect of the CF3-substituted ether-linked diamines on the toughness improvement was not so prominent. The approach simultaneously using some CF3-free ether-linked monomers enabled us to significantly enhance the film toughness (εbmax > 50%) while maintaining low CTE characteristics and good solution-processability. Thus, some of the PI systems can be promising candidates as novel coating-type cover layer materials.

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