Abstract
Light absorption of conjugated polymers (such as MEH-PPV) not only promotes excitation of the electronic states, but concomitantly changes the backbone conformation, resulting reduction of molecular entropy and free energy increase. Long-range lateral molecular diffusion can thus be incited to form fine patterns with large height contrast emulating those on the optical mask through which a light is exposed to the molecular assembly mobilized by permeated solvent vapor. This entropy effect, however, produces a just exactly reverse pattern when the film contains a blending component of an inert polymer, such as polystyrene (PS), by engenders an opposite molecular flow transferring polymer molecules from the dark regions to the lighted regions which has apparently overwhelmed the MEH-PPV exodus from the lighted regions. This opposite molecular flow was found to be exclusively composed of the optically inert PS molecules that were driven by the concentration gradient resulted from enhanced solvent absorption in the lighted region prompted by the entropy reduction of the MEH-PPV molecules therein. Clearly, the Feakean driving force, however, is smaller than that given rised by the light-induced entropy effect for the conjugated polymer MEH-PPV. Interestingly, the Feakean PS flow was later dominated by a capillary flow when the thickness difference between the dark and lighted regions became large enough to trigger capillary dewetting that evacuates from the dark regions all PS molecules but leaves no changes of the MEH-PPV polulation there, forming a unique phase separation that may be controlled by an operation using optical masks. The constant distribution of the MEH-PPV seems to imply that the net MEH-PPV flows resulted from the competition between those by the light-induced entropy reduction and capillary forces are miniscule. The excess solvent absorption in the lighted regions and the correction of the X interaction parameter due to entropy change (Xs)in the lighted regions were calculated using the Flory-Huggins model on both the binary and tertially systems that have yielded excellent agreement with the observations. This prominent effect of light exposure on the molecular motions is consistent with the strong electron-phonon coupling operating in the dramatic photoluminescence enhancements by mechanical stresses observed elsewhere and may be used for precision molecular motion controls and fine patterning. The solvent annealing process under the exposure condition could control conjugated polymer distribution and diffusion within the film, in addition to the polymer can be used to explore the film configuration, entropy, molecular packing, and the molecular reactions with photon absorption. It can be used to form a fine pattern and a concentration gradient, which has the potential applications of nanotechnology in the manufacturing of optoelectronics.