Conjugated Polymers Confined and Sheared: Photoluminescence and Absorption Dichroism in a Surface Forces Apparatus

Abstract

Within a surface forces apparatus newly equipped for spectroscopic measurements, the photoluminescence and absorption spectra were studied of conjugated polymer films confined to less than the random coil dimension. A dilute solution of MEH-PPV, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene), was exposed to atomically smooth surfaces of muscovite mica and allowed to adsorb from toluene, and then the surface spacing was reduced to ≈2 nm, which is less than the unperturbed radius of gyration of the polymer chains. Subsequent unidirectional shear with amplitude ≈20 times the surface spacing provided preferential alignment while the solvent evaporated. Chain alignment was quantified from both photoluminescence and absorption spectra. Curiously, a bimodal distribution of chain alignment was observed, parallel to the shear direction in 2/3 of the cases but perpendicular to the shear direction in 1/3 of the cases. On the basis of analysis of the photoluminescence spectra, confinement and shear-induced changes of the electronic structure are also discussed. This is considered to be the first study of polymer absorption and photoluminescence spectra within the contact area of a surface forces apparatus.