Interference Effects in Sum Frequency Vibrational Spectra of Thin Polymer Films: An Experimental and Modeling Investigation

Abstract

Sum frequency (SF) vibrational spectra in the C−H stretching region of polydimethyl siloxane (PDMS) and of the comb copolymer cetyl dimethicone copolyol (CDC), consisting of a PDMS backbone with grafted poly(ethylene oxide) and cetyl side chains, have been recorded in air after deposition onto a gold-coated substrate. The polymers were deposited over a range of thicknesses (up to 70 nm) by spin coating from chloroform solutions of different polymer concentrations. Film thicknesses were determined by ellipsometry. The methyl symmetric (r+) stretching modes appeared as peaks in the SF spectra of both polymers at all film thicknesses investigated, indicating that the constituent methyl groups have a net orientation toward the air. However, the phase of the methyl anti-symmetric (r-) stretching mode displayed a dependence upon film thickness, changing from a peak (positive phase) to a dip (negative phase) as film thickness was increased. The phase behavior of the r- methyl resonance has been successfully mode...