Polymorphism of poly(2,6-dimethyl-1,4-phenylene) oxide (PPO): Co-crystalline and nanoporous-crystalline phases

Relevant information on PPO crystallization, with particular emphasis on nanoporous crystalline (NC) and related co-crystalline (CC) forms, is reviewed. Preparation methods, structures, properties and possible applications of NC and CC samples are discussed. Reviewed results mainly refer to films (unoriented or with axial or planar orientations) but also to NC powders and NC aerogels. Particular attention is given to the impressive solubility and diffusivity of organic guests, which can be achieved for NC PPO samples. X-ray diffraction patterns and infrared linear dichroism, as collected for axially oriented NC films, indicate the presence of large diameter crystalline helices with intrahelical location of the guest molecules. The final section is mainly devoted to perspectives of basic and industrial research that are expected in the near future for CC and NC PPO samples. As for research studies, particularly suitable appear NC films with orientation of the crystalline empty channels perpendicular to the film plane (c⊥ orientation), mainly due to their fast guest sorption and good optical transparency.