Enhanced order and electronic delocalization in conjugated polymers oriented by gel processing in polyethylene

Abstract

We report the polarized absorption ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{\ensuremath{\theta}}}$(\ensuremath{\Elzxh}\ensuremath{\omega}) and photoluminescence ${\mathit{L}}_{\mathrm{\ensuremath{\theta}}}$(\ensuremath{\Elzxh}\ensuremath{\omega}) spectra of gel-processed blends of poly(2-methoxy,5-(2'-ethyl-hexoxy)-p-phenylenevinylene), MEHPPV, in ultrahigh-molecular-weight polyethylene. Both ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{\ensuremath{\theta}}}$(\ensuremath{\Elzxh}\ensuremath{\omega}) and ${\mathit{L}}_{\mathrm{\ensuremath{\theta}}}$(\ensuremath{\Elzxh}\ensuremath{\omega}) are highly anisotropic, with preferred direction polarized parallel to the draw axis, demonstrating that gel processing and subsequent tensile drawing results in the orientation of the conjugated polymer guest. In contrast to cast films of the pure conjugated polymer (MEHPPV), the oriented blends display a sharpening of the vibronic structure and a redistribution of spectral weight into the zero-phonon line in both absorption and emission for light polarized parallel to the draw axis. In the most highly oriented MEHPPV-polyethylene blends, the induced order is sufficient to enable the determination of the intrinsic line shape of the absorption: we find the band-edge square-root singularity characteristic of a one-dimensional semiconductor. The changes in the spectral features resulting from materials processing are quantified using a Franck-Condon analysis and interpreted in terms of a modification of the ground- and excited-state configurational manifolds. We conclude that processing the blends via the gel intermediate state and subsequently orienting by tensile drawing yields a system with significantly reduced disorder.