Neutron reflection study on soluble and insoluble poly[2-(2’-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene) films

Neutron reflection (NR) has been utilized to study the physical structure of the light-emitting polymer poly[2-(2’-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene) (MEHPPV). Films of soluble MEHPPV (S-MEHPPV) and insoluble MEHPPV (I-MEHPPV) prepared via a chloro precursor polymer were investigated. For S-MEHPPV spin-coated films were found to contain two layers, one of low-density and thickness of ≈1360 Å with the second layer being thinner, ≈240 Å, but denser. The thicker layer only covered about 30% of the substrate surface. In contrast, I-MEHPPV formed uniform films across the substrate. We found that during the thermal conversion of the chloro precursor polymer the film thickness was maintained and it was only on cooling that the films contracted. Importantly, the thermal expansion and contraction of I-MEHPPV was reversible. NR showed that thermal annealing of I-MEHPPV had little affect on the films physical structure while for S-MEHPPV only the thinner layer was changed and became more dense. Photoluminescence spectra of S-MEHPPV before and after annealing showed that the densification observed in the neutron reflection measurements was mirrored in the emission properties of the polymer film.