Single-monolayer insitu modulus measurements using a SAW device Photocrosslinking of a diacetylenic thiol-based monolayer

Abstract

We report direct measurement of the modulus change that accompanies the crosslinking of a single molecular monolayer. We measured a change in elastic modulus of 5×10¹⁰ dyn cm^-2 as a result of UV-induced photocrosslinking of a single surface-confined monolayer of the conjugated diacetylenic thiol HS(CH₂)₁₀Câ–·CCâ–·C(CH₂)₁₀CO₂H (DAT). The modulus measurement was made on a monolayer of DAT chemisorbed upon a gold film on the surface of a 97-MHz ST-quartz surface acoustic wave (SAW) delay line. The ratio of the changes recorded in SAW velocity and attenuation, ca. 4:1, suggests that the measured effect is mainly a change in the elastic (real) component of the complex shear modulus, viscous changes playing a lesser role. In relation to typical polymer modulus values, the change of 5×10¹⁰ dyn cm^-2 is consistent with a change from a rubbery material (G′≈10⁷–10⁸ dyn cm^-2) to a fairly rigid, glassy material (G′≈10¹⁰ dyn cm^-2), reasonable for comparison of the monolayer in its as-adsorbed and crosslinked forms. This report of the direct measurement of the modulus change that accompanies the crosslinking of a single molecular monolayer is consistent with previous insitu measurements of this process using thickness-shear mode resonators