Transitions between Nanoporous crystalline α and β forms of poly(2,6-dimethyl-1,4-phenylene) oxide (PPO)

Abstract

Nanoporous-crystalline (NC) α and β forms of Poly(2,6-dimethyl-1,4-phenylene) oxide (PPO), exhibit two different large helix conformations, both being suitable for intrahelical low-molecular-mass guest inclusion. α → β and β → α transitions can be achieved by sorption of liquid β-guests and α-guests (i.e., of guest inducing β and α forms by solution casting and by sorption in amorphous PPO), respectively. Both α → β and β → α transitions occur with complete loss of possibly present crystalline phase orientations (both axial and planar). Moreover, the reported data show that transitions between the two phases and the associated loss of crystalline phase orientation occur only for guests being good PPO solvents. These observations rule out the possibility of solid–solid transitions while can be rationalized by mechanisms involving a local dissolution of the NC phases. This local polymer dissolution is followed by host/guest co-crystallization with the helical conformation being different from the starting one and more suitable to host the used guest. These data fully confirm complete separation between the two NC phases while rule out the hypothesis of a continuum of NC phases between two limit ones.