Abstract
The thesis aims to measure the molecular stresses operative in conjugated polymer molecules and explore their packing in the ultrathin films prepared by spin coating. The conjugated polymer MEH-PPV (poly[2-methoxy-5-((2’-ethylhexyl)oxy)-1,4-phenylenevinylene]) was used as the model polymer in comparison to the non-conjugated polymer polystyrene (PS). The molecular stresses were determined from the thin film instability of dewetting, by measuring the stress release at the edge of an incipient hole or the depression in the soft elastic substrate underneath a dewetting hole. The molecular stresses of the conjugated polymer confined in thin films were found substantially smaller than those of PS. The observation is attributed to the rigid-rod molecular structure that exhibits a longer persistence length and hence a smaller entropy variation for the transition of the molecules from solute to solid film. In addition, the local mass density in the thin (~2nm) interfacial layer next to the substrate was found to be only one tenth of that of PS, believably resulted from the smaller molecular recoiling forces during solvent evaporation. Moreover, in contrast to the double-exponential behavior of flexible-chain polymers such as PS, the molecular stress demonstrated a single exponential decay with film thickness, indicating the absence of skin layer during the rapid solidification of spin coating, which is also attributed to the rigid-rod structure that allows easier passing of the solvent molecules. Since the optoelectronic quantum efficiencies are highly dependent on the segmental stress due to the robust electron-phonon interactions of the linear chains, good control over the residual stress of thin conjugated polymer films plays a fundamental role for the development of polymer-based devices.