Polymorphism of Poly(2,6-dimethyl-1,4-phenylene)oxide in Axially Stretched Films

Axial orientation of poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films is achieved by stretching in a narrow temperature range (220–230 °C) close to the polymer glass-transition temperature (Tg). Stretching at high draw ratios of PPO films exhibiting the nanoporous-crystalline (NC) α and β forms leads to the formation of a new crystalline form (thereafter named γ), which reaches a very high degree of axial orientation (up to 0.98). This transition from α and β forms toward the γ form occurs with a partial loss of crystallinity, as pointed out by DSC and FTIR measurements. Wide-angle X-ray diffraction (WAXD) patterns of the γ form show only two intense and broad peaks: an equatorial peak, suggesting the presence of short-range pseudohexagonal packing of conformationally disordered parallel PPO chains, and a layer line peak, indicating a coarse chain axis periodicity of c = 1.62 ± 0.02 nm. FTIR and polarized FTIR spectra clearly show vibrational peaks associated with the new γ crystalline phase. Density measurements and sorption tests of low-molecular-mass guest molecules suggest that the γ form, differently from the starting NC α and β forms, does not exhibit crystalline cavities. This hypothesis is confirmed by the absence of dichroism of infrared peaks of benzene molecules absorbed by γ form films, which indicates that guest sorption occurs only in the amorphous phase.