Colorless Poly(amide-imide)s with Low Dielectric Constants and Enhanced Mechanical Properties through Tri-Armed Cross-Linkers

Colorless polyimides (PIs) have attracted a lot of attention because of the prosperous development of flexible devices. Traditional aromatic PIs possess great thermal stabilities and mechanical properties but lack high transparency. Aliphatic PIs provide great optical transparency, but the poor film formability, low mechanical strengths, and high coefficient of thermal expansion (CTE) make them hard to utilize practically. In this work, we present a series of colorless poly(amide-imide) films consisting of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), trimetallic anhydride chloride (TMC), terephthalic chloride (TCL), and 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFMB) to overcome the coloration of aromatic PIs. In addition, the introduction of tri-armed cross-linker 1,3,5-benzenetricarbonyl trichloride (BTC) enhances the thermal properties and mechanical strengths. The resulting poly(amide-imide) films exhibit great thermal stabilities with the glass transition temperatures (T_g) of 342 to 351 °C and the CTE of 37 to 28 ppm/°C. In addition, attributed to the cross-linker BTC, these films possess excellent mechanical strengths from 90.1 to 130 MPa and low dielectric constants of 3.06 to 2.82. Most importantly, the cross-linked films still maintain high transparency. The optical properties of these films are marvelous in that the T₄₁₀ is higher than 80% (82-86%) and the yellow indexes (YI) are lower than 2.5 (1.89-2.37). We believe these results can offer useful instructions for making colorless poly(amide-imide) films with a low CTE and high mechanical strength.