Dioctyl-substituted polyfluorene (PF) is especially well known for the presence of an unusual spectral feature identified as the beta phase, known to originate in regions of enhanced chain planarity. Although this phase appears as a minority constituent it dominates the emission, resulting in a red shift of the luminescence. We present Raman scattering studies of poly[9,9'-(di n, octyl) fluorene] as a function of thermal cycling that establish a connection between the conformational isomers and chain morphology. Density-functional theory calculations of the vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the incompatibility of the beta phase with the overall alpha crystalline phase. Further, electroluminescence and photoluminescence measurements from PF-based light-emitting diodes are presented and discussed in terms of the crystalline phases and chain morphologies in the PFs.