Irradiation of poly(perfluoropropylene oxide) by a 25-kV electron beam: Electron beam induced chemistry of poly(perfluoropropylene oxide) in the absence of oxygen

Abstract

Studies are reported on the 25-kV electron beam exposure of poly(perfluoropropylene oxide), PPFPO, when enclosed in a vacuum system; contrary to electron beam exposures conducted in air, where the viscosity decreased as a function of absorbed dose, the viscous polymeric liquid solidified to a point where a film thickness could be measured by a profilometer. Concomitant with the radiation induced physical change from a liquid to a solid, a rapid mass loss of the material was observed. Specular reflection infrared spectroscopy was used to follow the electron beam induced changes and film thickness loss in PPFPO as a function of incident charge density and absorbed energy. The slope of semilog plots for the normalized thickness loss, h/h0, vs. incident charge density decreased as higher molecular weights of PPFPO were taken for analysis. In order to understand the spectroscopic analysis of the electron beam induced changes, a study was made for the reflectance of the vacuum-PPFPO-gold system. After a classical dispersion analysis was performed on PPFPO to obtain the real and imaginary parts of its refractive index, n2, and κ2, Maxwell's equations were numerically solved on a computer to obtain the reflectance spectra for the three-layered system. The calculations not only provided insight for the reflectance measurements but enabled the electron beam induced etch rate and G value for polymer-to-monomer degradation to be determined. A mechanism, proposed to explain the radiation induced etching and gelation within the vacuum system, is based on a vacuum assisted mass loss that depends on the pumping action of the vacuum system. In essence, the removal of low molecular weight material by the vacuum pump relaxes the conditions for gelation and permits the formation of gel even when r, the probability for scission per monomer per unit dose, is 4 times greater than c, the probability that a monomer has an active site for cross-linking per unit dose. © 1987 American Chemical Society.