In this work, we established the relationship between the thermally induced microstructure development and the photoluminescence efficiency (E p ) of the poly(9,9-dioctylfluorene) (PF8) fibers prepared from the semidilute PF8/chloroform solutions by direct electrospinning. The as-spun (AS) fibers were found to be composed of an amorphous phase and a significant fraction of mesomorphic β-phase, which may promote the E p of PF8. The temperature-dependent wide-angle X-ray scattering (WAXS) measurement revealed that the PF8 fibers exhibited complex structural transformation on heating from the AS state, and there existed a strong correlation between E p and the microstructure developed. The AS fibers exhibited the highest E p value. The E p decreased progressively on heating at temperatures below the cold crystallization temperature (T cc ), where the microstructure remained unperturbed. The β-phase transformed to γor α′-phase (C γ -/C α′ -crystals) above T cc , and such a structural transformation led to a reduction of E p . With further heating, the C γ -/C α′ -crystals transformed into the more stable C α -crystals, presumably as the premelting−reorganization−remelting proceeded at the temperatures over the crystal transformation temperature (T ct ). The crystal transformation process reduced the E p markedly. Finally, above the melting temperature (T m ), the C α -crystals were disrupted into the disordered molten state, and the E p value remained largely unchanged up to 200 °C.

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