A significant improvement in the electroluminescence (EL) properties was observed for a poly{5-methoxy-2-[(2'-ethyl-hexyl)-oxy]-p-phenylenevinylene} (MEH-PPV)/poly(2,3-diphenyl-5-octyl-p-phenylenevinylene) (DPO-PPV) blend after a thermal treatment at 200 degrees C for 2 h in vacuo to furnish the chemical bonding between polymer chains. H-1 NMR spectroscopy and two-photon excitation microscopy revealed that the chemical bonding turned the immiscible polyblend into a system more like a block copolymer with a vertically segregated morphology. Because both the lowest unoccupied molecular orbital and highest occupied molecular orbital levels of MEH-PPV in the wetting layer were higher than those of DPO-PPV in the upper layer, the heterojunction between the two layers of the polymers fit the category of so-called type II heterojunctions. As a result, the turn-on voltage of the polymer light-emitting diode prepared with the thermally treated polyblend decreased to similar to 0.6 V, and the EL emission intensities and quantum efficiencies increased to about 4 times those of the untreated polyblend. (c) 2005 Wiley Periodicals, Inc.