Phase Behavior and Molecular Aggregation in Bulk Poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene)

Abstract

Structural evolution and morphological development in films of poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, drop-cast from toluene solutions) upon isothermal heat treatment at elevated temperatures were studied by means of a combination of differential scanning calorimetry, polarized light microscopy, X-ray diffraction, transmission electron microscopy, ultraviolet−visible spectroscopy, and photoluminescence spectroscopy. Results indicated that MEH-PPV is mesomorphic in nature (optically biaxial, showing nematic-like texture under cross-polarization), with glass transition temperature Tg = ca. 80 °C and isotropization temperature Ti = ca. 290 °C. Upon short-term (i.e., 5 min) heat treatment at elevated temperatures (Ta) below Ti, MEH-PPV chains stack into boardlike entities (ca. 1.6 nm in thickness and ca. 0.4 nm in interbackbone spacing) within beadlike domains ca. 10−20 nm in size, which in turn aggregate transversely into wormlike features ca. 200 nm in length. Shearing at an elevated temperature results in disintegration of the wormlike agglomerates, leaving the beadlike domains arrayed into wavy lines transverse to the shear direction. Accompanying the morphological changes, ultraviolet−visible light absorption and photoexcited emission spectra vary systematically with improvement or disruption of the mesomorphic order.