Large friction anisotropy of a polydiacetylene monolayer

Friction force microscopy measurements of a polydiacetylene monolayer film reveal a 300% friction anisotropy that is correlated with the film structur e. The film consists of a monolayer of the red form of N-(2-ethanol)-10,12- pentacosadiynamide, prepared on a Langmuir trough and deposited on a mica s ubstrate. As confirmed by atomic force microscopy and fluorescence microsco py, the monolayer consists of domains of linearly oriented conjugated backb ones with pendant hydrocarbon side chains above and below the backbones. Ma ximum friction occurs when the sliding direction is perpendicular to the ba ckbones. We propose that this effect is due to anisotropic film stiffness, which is a result of anisotropic side chain packing and/or anisotropic stif fness of the backbone itself. Friction anisotropy is therefore a sensitive, optically-independent indicator of polymer backbone direction and monolaye r structural properties.