Synthesis and properties of novel colorless and thermostable polyimides containing cross-linkable bulky tetrafluorostyrol pendant group and organosoluble triphenylmethane backbone structure

A colorless and high temperature resistant fluorinated polyimide (PI) film based on a novel cross-linkable diamine monomer 4,4′-diamino-4″-(2,3,5,6-tetrafluoro-4-ethylenephenoxy) triphenylmethane (DFPTM) was successfully prepared. The special triphenylmethane backbone and the bulky tetrafluorostyrol pendant group in DFPTM gave the PI material eximious solubility in organic solvents, high tensile properties, good thermal stability, and excellent optical transparency (cutoff wavelength for 331 nm and light transmittance above 94% at 450 nm). Furthermore, the styryl could initiate self-crosslinking reaction to form a dense three-dimensional network structure by thermal curing. Hence, the cross-linked PI film showed superior combination property (including excellent resistance to solvents). It's T_g increased by 57.1–306.0°C, the 5% thermal weight-loss temperature improved by 10.2% to 500.0°C, and the tensile strength enhanced by 17.5% to 39.7 MPa. Moreover, after thermal curing, the cross-linked PI attained a lower elongation at break from 8.1 to 3.9%, and the thermal expansion coefficient reduced from 82.2 to 68.8 ppm/°C without obvious loss of optical transparency (cutoff wavelength for 347 nm and light transmittance remained 93.0% at 450 nm).