Microstructure and material modification of polymer films induced by femtosecond optical thermal treatment

In an effort to induce surface microstructure and material modification, spin-coated poly (9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) films on a glass substrate were irradiated by a femtosecond laser. The surface morphology, micro-Raman, and microscopic photoluminescence (PL) spectral characteristics of the F8BT films being irradiated by various laser doses were analyzed. The results demonstrate that the droplet-like microstructure and material modification including broadening and red-shift of the PL spectra were induced by femtosecond optical thermal treatment. This can be attributed to the competition between intrachain coupling, which leads to J-favoring interactions and interchain coupling, which leads to H-favoring interactions. When the thermal effect of the laser melts the polymer molecules into a liquid phase, the degree of freedom of the molecular chains increases and the molecular chains move in multiple directions. This process makes the molecular chains to rearrange themselves more orderly with a lower energy configuration. Consequently, the J-aggregation state transforms to the H-aggregation state at the molecular level, and the PL spectra become broader and red-shifted to longer wavelengths, macroscopically. The Raman spectroscopic evidence indicates that neither photooxidization nor degradation of the conjugated polymer material has occurred.