Crosslinkable polyimides obtained from a reactive diamine and the effect of crosslinking on the thermal properties

This study presents novel thermally crosslinkable polyimides (PIs) obtained using a diamine (3,5-DABA) containing an amide (CONH2) group. This diamine was highly reactive with pyromellitic dianhydride (PMDA), and the equimolar polyaddition in N-methyl-2-pyrrolidone at room temperature led to a high molecular weight poly(amic acid) (PAA) without causing gelation or precipitation. The results suggest that the amide side group of 3,5-DABA does not participate in PAA polymerization at room temperature. A PI system derived from PMDA and 2,2′-bis(trifluoromethyl)benzidine (TFMB), which has an ultralow coefficient of thermal expansion in the X–Y direction, was modified by copolymerization with 3,5-DABA. The FT-IR spectra suggested that the amide side groups in the 3,5-DABA-modified PIs can react with adjacent imide C=O groups to form crosslinks on heating above 350 °C, even in the absence of sufficient molecular fluidity. This behavior is attributed to the presence of a very high concentration of imide C=O groups distributed in the vicinity of the amide reactive groups. A crosslinking mechanism is proposed in this paper. An effect of crosslinking on the volumetric coefficient of thermal expansion (β) was investigated by comparing the 3,5-DABA-modified PIs with their crosslinker-free counterparts, i.e., a PI derived from PMDA with TFMB and m-phenylenediamine (m-PDA). At a 3,5-DABA content of 20 mol%, the 3,5-DABA-modified copolymer film showed a lower β value than that of the corresponding m-PDA-containing copolymer. The difference in the β values of these copolymers increased with increasing comonomer (3,5-DABA or m-PDA) content. Thus, the use of 3,5-DABA is effective in reducing the β values through crosslinking. Dynamic mechanical analysis showed that crosslinking also contributed to enhancing the storage modulus and broadening the glass transition. 3,5-DABA was also applied to modify the properties of a thermoplastic poly(ester imide) (PEsI). The thermal crosslinking of the 3,5-DABA-modified PEsI caused a Tg enhancement of about 30 °C and an appreciable decrease in the thermoplasticity. Thus, the present approach is also effective in improving the low-Tg character of thermoplastic PIs.