Fluorine Substitution Effect on the Material Properties in Transparent Aromatic Polyimides

The recent development of flexible display technology raised additional requirements for optical and electric properties of polyimides, accelerating the structure and property tunning of transparent polyimides. The unique electronic effect and steric hindrance effect of fluorine substitutions make fluorine-containing polyimides occupy an important position in the transparent polyimide family. In this work, a series of transparent aromatic polyimides were prepared from a fixed 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and biphenyl diamines with different substitute groups at the 2,2′,6,6′-positions. We systematically studied the effects of pendant groups on the thermal, mechanical, optical and dielectric properties of these 6FDA-based polyimides with the aid of density function theory (DFT) calculation. In particular, we paid special attention to the simple but compact fluoro group substitution. The simple fluoro substitution brought the advantages of maintaining the linearity of the backbone and dense polymer chain packing, which would minimize the weakening of polyimides’ inherent thermal, dimensional and mechanical properties. Comparing with trifluoromethyl substituted polyimides with the best optical transparency, polyimides containing fluoro substitutes exhibited slightly decreased optical transparency, but increased thermal and dimensional stability and higher mechanical strength. These results could shed light on the ultimate transparent polyimide film development toward the application in extreme working condition, e.g., the colorless polyimide substrate film for the flexible display technology.