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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