Intrahelical empty channels of nanoporous-crystalline α and β forms of PPO

A linear dichroism study of vibrational peaks of amorphous and semi-crystalline axially stretched poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films shows an unusual behaviour of co-crystalline and nanoporous-crystalline (α and β form) phases. In fact, signs of dichroism for all vibrational peaks of α and β phases are opposite with respect to those observed for amorphous phases. This behaviour indicates the occurrence in PPO α and β forms of large polymer helices, whose thermodynamic stability can be rationalized by placement of guest molecules in intrahelical channels. A large diameter helix, with chain periodicity of 1.06 nm, is compatible with layer-line position in WAXD patterns of axially stretched α form films. The occurrence for both α and β NC forms of PPO of intrahelical empty channels, can rationalize the very high guest diffusivity of semi-crystalline PPO films, which becomes particularly high when crystalline helical axes are preferentially perpendicular to the film plane.