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 (alpha and beta form) phases. In fact, signs of dichroism for all vibrational peaks of alpha and beta phases are opposite with respect to those observed for amorphous phases. This behaviour indicates the occurrence in PPO alpha and beta 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 alpha form films. The occurrence for both alpha and beta 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.