Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) conjugated polymer domains in a thermoplastic polyurethane matrix

Abstract

Self-sustained and spin-casting films formed from blends of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) conjugated polymer and thermoplastic poly-urethane (TPU) at different concentrations have been investigated. Scanning electron microscopy images of self-sustained films show the formation of circularlike domains of MEH-PPV in the TPU matrix, characterizing the blend material as heterogeneous. The circularlike domains in the spin-casting films were also observed by electric force microscopy (EFM) and by atomic force microscopy. The EFM, in particular, has allowed the differentiation of the MEH-PPV domains from the TPU matrix due to the electrical properties of the conjugated polymer. The MEH-PPV domains in the spin-casting films are much smaller in size than in the case of self-sustained films. The glass transition temperatures for the TPU and for the self-sustained blended films were obtained by temperature modulated differential scanning calorimetry. A very small variation in the glass transition temperature (Tg) has been observed for the blends. This fact further corroborates that MEH-PPV and TPU form immiscible blends. Intermolecular interactions between the MEH-PPV and the TPU materials were proposed to occur at the interfaces of the domains. These interfacial interactions have not affected the Raman modes, which pointed out that they are relatively weak interactions. The formation of small conjugation segments of the MEH-PPV molecules due to these interfacial interactions with the TPU molecules at the interfaces of the domains was assigned to be responsible by the appearance of a large tail and clear shoulders to the high energy side of the photoluminescence (PL) spectra of self-sustained samples. This was also assigned to be responsible by the blueshift observed in the PL spectra of spin-casting films. The redshift of the pure electronic transition peak and the first vibronic band (which presents an inhomogeneous broadening) were observed in the self-sustained blended films and were related, respectively, to the increase of the internal intermolecular and intramolecular interactions due to the conformational disorder of the MEH-PPV molecules imposed by the casting spread technique.