An investigation of photopolymerization of gas-phase acrolein onto metallic substrates using real-time nonlinear optics

Gas phase photopolymerizations are commercially used for a variety of coating and device applications such as corrosion resistance and electronic insulation. Acrolein forms a crosslinked polymer onto a metal readily upon ultraviolet irradiation. The reaction kinetics and processing, however, are poorly understood. In the present work, the dynamics of the photopolymerization of acrolein on Au surface were studied by monitoring the gas phase kinetics and the surface dynamics in-situ and in real time. Surface second harmonic generation (SSHG), a second order nonlinear optical process, was developed to determine the fraction of the unreacted monomer concentration on the surface during the polymerization. The SSHG was also used to determine the adsorption of acrolein on Au. It was found that adsorption of acrolein was a chemisorption process and can be characterized as a Langmuir adsorption process. Upon UV irradiation, the surface reaction rate was found to be first-order in the surface monomer concentration, 1/2-order in light intensity, and independent of the gas pressure at the operational pressure range. The comparison of the macroscopic rate laws with the microscopic rate equations suggested that the initiation and propagation took place initially at the surface adsorbed layer. The existence of a metal surface may enhance the free radical formation on the surface and led the photopolymerization occur selectively on the surface. After the formation of a polymer layer on Au, 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 1/2 order in light intensity. This mechanism was further confirmed by studying the effects of the surface steric hindrance, the nature of substrate, and the volume of the irradiated gas molecules on the polymerization dynamics, as well as the evolution of polyacrolein topography. Moreover, it was found that the final film thickness increased with ultraviolet light intensity, but was insensitive to the initial pressure. The polyacrolein film formed was partially crosslinked and had a glass transition temperature of 118$\sp\circ$C. This research demonstrated the effectiveness of using SSHG in studying surface dynamics in-situ. Most significantly, it demonstrated the importance of the metal/monomer interaction that leads to the enhancement of the surface reaction and showed an ability to control the deposition of high integrity thin dielectric films onto metallic substrates