In-Situ Nonlinear Optical Studies of Photopolymerization of Gas Phase Acrolein onto Metallic Substrates

Abstract

The kinetics of the photopolymerization of gas phase acrolein onto gold surfaces were studied by monitoring the surface reaction dynamics and the gas phase kinetics in-situ and in real time. A technique based on surface second harmonic generation (SSHG), a second-order nonlinear optical process, was developed to monitor the surface second-order nonlinear susceptibility, which was related to the fraction of unreacted monomer coverage on the surface during ultraviolet (UV) irradiation. The adsorption of acrolein onto the gold surface was also monitored using SSHG. It was found that the second harmonic signal at the surface decreased as a result of adsorption of acrolein and that the adsorption was an irreversible process. Upon UV irradiation, the second harmonic signal was found to decrease immediately as a result of the decrease in the surface monomer coverage, while an induction period was observed in the gas phase by vapor pressure measurements. The surface reaction rate was found to be first-order in surface monomer concentration, one-half-order in light intensity, and independent of the gas pressure in the range of partial pressure studied (61-152 Torr). The results suggested that polymerization took place initially at the surface-adsorbed layer. The existence of a metal surface may enhance the formation of free radicals on the surface-adsorbed layer and result in the selective photopolymerization on the metal surface. After the formation of a polymer layer on the gold surface, the photopolymerization was found to take place mainly in the gas phase. The apparent rate of the gas phase polymerization was found to be first-order in the gas pressure and one-half-order in light intensity.