Decoupling Temperature-Volume Effects on Poly[2-Methoxy-5-(2'-Ethylhexyloxy)-1,4-Phenylene-Vinylene] Films at the β-Relaxation Temperature.

In this paper, we report a simple procedure to decouple effects from temperature and volume on the emission properties of thin films of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). This procedure consists of applying a positive pressure close to the β-relaxation temperature, Tβ (∼220 K), of MEH-PPV, which controls the molecular movement to retain a disordered state for the polymer chains even after the sample is cooled from room temperature. Such decoupling could be confirmed by calculating the photoluminescence (PL) spectra line shape using a semi-empirical model based on molecular exciton and Franck-Condon transitions, and with electron-vibrational modes coupling being parameterized with the Huang-Rhys factor. We also show that the decoupling between temperature and volume effects does not occur if the molecular movement is restricted either by thermal annealing or by depositing the MEH-PPV film on a rigid substrate. This latter finding may be exploited in designing thermally stable devices.