We present a study of isotropic and uniaxially oriented binary blend films comprising 1 wt % of the conjugated polymer poly(9,9-dioctylfluorene) (PFO) dispersed in both ultrahigh molecular weight (UHMW) and linear-low-density (LLD) polyethylene (PE). Polarized absorption, fluorescence and Raman spectroscopy, scanning electron microscopy, and X-ray diffraction are used to characterize the samples before and after tensile deformation. Results show that blend films can be prepared with PFO chains adopting a combination of several distinct molecular conformations, namely glassy, crystalline, and the so-called b-phase, which directly influences the resulting optical properties. Both PFO concentration and drawing temperature strongly affect the alignment of PFO chains during the tensile drawing of the blend films. In both PE hosts, crystallization of PFO takes place during drawing; the resulting ordered chains show optimal optical anisotropy. Our results clarify the PFO microstructure in oriented blends with PE and the processing conditions required for achieving the maximal optical anisotropy. INTRODUCTION Molecular semiconductors are distinct from elemental and compound inorganic semiconductors in a number of important ways. One key feature is the option of low temperature processability that derives from relatively weak (typically van der Waals) intermolecular bonding; this allows the molecules in solid form to be readily separated in solution or melt and then reassembled in, for example, films on a device substrate. It is an enabling characteristic for printable/plastic electronics and has attracted major interest since the first report of solution-processed conjugated polymer light-emitting diodes (LEDs). 1 A consequence of relatively weak inter-molecular bonding is, however, that the solid-state microstructure can be sensitive to processing conditions and, therefore, complex. As a result, a range of amorphous/glassy, liquid-crystalline (LC) and (semi-)crystalline phases are found for many conjugated polymers.