Optical Properties of a Fluorinated Polyimide as Related to Ethanol and Water-Vapor-Sensing Capability

Thin films of a fluorinated polyimide derived from 4,4´-(hexafluoroisopropylidene) diphthalic anhydride and 2,3,5,6-tetramethyl-1,4-phenylenediamine have been obtained by spin-coating a solution of polyimide powder dissolved in chloroform. The synthesized polyimide has been characterized by a variety of techniques including Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis, differential scanning calorimetry, solubility tests, optical transmission spectroscopy, and fluorescence spectroscopy. Changes in the intensity, peak shape, and position of the intrinsic fluorescence features as observed under exposure to ethanol and water vapors have been correlated to specific chemical interactions between the analytes and the macromolecules. Interactions stronger than dipole-dipole forces such as hydrogen bonds, evidenced by FTIR, are thought to play an important role in the changes of the fluorescence features. Single-wavelength fluorescence versus time measurements have also been performed by exposing the samples to alternating pulses of N ₂ and different organic-vapor concentrations in order to evaluate the response and recovery times and the quantitative determination capability of the material. The polymer proved to be sensitive to both the tested analytes, with response and recovery times in the order of the tens of seconds