Concentration Effect on the Oriented Microstructure in Tensile Drawn Polyfluorene−Polyethylene Blend

We report on the segregation and microstructure of poly[9,9-bis(2-ethylhexyl)-fluorene] (PF2/6) blended in polyethylene (PE) aligned by tensile drawing at elevated temperature. X-ray diffraction and photoluminescence data indicate that when the PF2/6 fraction is high (7−15 wt %), PF2/6 assembles in uniaxially aligned crystallites (the coherence length ∼32 nm) segregated within the PE matrix. The full-width-half-maximum of the PF2/6 crystallite orientation distribution about the stretching direction is ≤10°. No significant macrophase separation is observed with AFM or optical and fluorescence microscopy. When the PF2/6 fraction is low (0.25 wt %), aligned PF2/6 forms a frozen-in nematic-like phase. The aligned polymers remain closely located but their coherence length is significantly decreased (∼8 nm), which is attributed to the decreased crystallite size. This behavior differs from that of poly(phenylene-vinylene) type polymers and the difference is rationalized by compatibility and entanglement arguments.