Melting of nanoporous-crystalline and co-crystalline solution cast films of poly(2,6-dimethyl-1,4-phenylene) oxide

Molecular Features Behind Formation of α or β Co-Crystalline and Nanoporous-Crystalline Phases of PPO

Guest molecular features determining the formation of α and β phases of poly(2-6-dimethyl-1,4-phenylene) oxide (PPO) are explored by collecting literature data and adding many new film preparations, both by solution casting and by guest sorption in amorphous films. Independently of the considered preparation method, the α-form is favored by the hydrophobic and bulky guest molecules, while the hydrophilic and small guest molecules favor the β-form. Furthermore, molecular modeling studies indicate that the β-form inducer guests establish stronger dispersive interactions with the PPO units than the α-form inducer guests. Thus, the achievement of co-crystalline (and derived nanoporous crystalline) α- and β-forms would result from differences in energy gain due to the host–guest interactions established at the local scale.

Dependence on Film Thickness of Guest-Induced c Perpendicular Orientation in PPO Films

For poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films exhibiting nanoporous-crystalline (NC) phases, c⟂ orientation (i.e., crystalline polymer chain axes being preferentially perpendicular to the film plane) is obtained by crystallization of amorphous films, as induced by sorption of suitable low-molecular-mass guest molecules. The occurrence of c⟂ orientation is relevant for applications of NC PPO films because it markedly increases film transparency as well as guest diffusivity. Surprisingly, we show that the known crystallization procedures lead to c⟂ oriented thick (50-300 μm) films and to unoriented thin (≤20 μm) films. This absence of crystalline phase orientation for thin films is rationalized by fast guest sorption kinetics, which avoid co-crystallization in confined spaces and hence inhibit formation of flat-on lamellae. For thick films exhibiting c⟂ orientation, sigmoid kinetics of guest sorption and of thickening of PPO films are observed, with inflection points associated with guest-induced film plasticization. Corresponding crystallization kinetics are linear with time and show that co-crystal growth is poorly affected by film plasticization. An additional relevant result of this study is the linear relationship between WAXD crystallinity index and DSC melting enthalpy, which allows evaluation of melting enthalpy of the NC α form of PPO (ΔHmο = 42 ± 2 J/g).