IN-SITU NONLINEAR-OPTICAL STUDIES OF PHOTOPOLYMERIZATION OF GAS-PHASEACROLEIN ONTO METALLIC SUBSTRATES

The kinetics of the photopolymerization of gas phase acrolein onto gol d surfaces were studied by monitoring the surface reaction dynamics an d the gas phase kinetics in-situ and in real time. A technique based o n surface second harmonic generation (SSHG), a second-order nonlinear optical process, was developed to monitor the surface second-order non linear susceptibility, which was related to the fraction of unreacted monomer coverage on the surface during ultraviolet (UV) irradiation Th e adsorption of acrolein onto the gold surface was also monitored usin g SSHG. It was found that the second harmonic signal at the surface de creased 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 i n 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, on e-half-order in light intensity, and independent of the gas pressure i n the range of partial pressure studied (61-152 Torr). The results sug gested that polymerization took place initially at the surface-adsorbe d layer. The existence of a metal surface may enhance the formation of free radicals on the surface-adsorbed layer and result in the selecti ve photopolymerization on the metal surface. After the formation of a polymer layer on the gold surface, the photopolymerization was found t o take place mainly in the gas phase. The apparent rate of the gas pha se polymerization was found to be first-order in the gas pressure and one-half-order in light intensity.