Two Nanoporous Crystalline Forms of Poly(2,6-dimethyl-1,4-phenylene)oxide and Related Co-Crystalline Forms

Extensive characterizations, mainly by wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) techniques, are reported for co-crystalline (CC) poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) films with many different low-molecular-mass guest molecules. These characterizations are also reported for the corresponding nanoporous crystalline (NC) phases, as obtained by suitable guest-removal procedures. Two well-separated NC forms, hereafter named α and β, are obtained by guest removal from two well-separated groups of CC forms. α and β NC forms can be easily recognized by reflections in WAXD patterns as well as by suitable FTIR crystalline peaks. Density and degree of crystallinity measurements confirm that both NC phases exhibit a density definitely smaller than for the amorphous phase (ρam = 1.04 g/cm3 > ρβ = 0.95 g/cm3 > ρα = 0.93 g/cm3). Density functional theory calculations combined with geometrical analysis on PPO model systems indicate conformations suitable to fit the observed chain periodicities of the two crystalline forms (c = 5.28 and 5.47 Å, for α and β NC forms, respectively)