We report the formation of -phase nanofibers of poly(9,9-dioctylfluorene) (PF8) in aged toluene solution at low temperature and link their photophysical properties and rheological behaviors with the solution structures. By cooling the PF8 toluene solution and subsequently aging, the PF8 chains develop into nanofiber morphology and the effects of aging time, temperature, and solution concentration on the nanofiber formation are investigated. The formation and development process of PF8 -phase in aged solution captured by TEM has not been reported in previous studies. Further increasing the solution concentration leads to the gelation of PF8 after aging at low temperature. It is interesting to show that a scaling law correlating specific viscosity with the polymer concentration and intrinsic viscosity originally proposed for flexible polymer solutions also exists for this semiflexible PF8 polymer solution. This study may offer an effective way to improve the understanding of the formation mechanism of PF8 -phase and facilitate the ongoing exploration of using such nanofiber networks in electronic devices. (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 633-639