Benzene-Induced Crystallization of PPO: A Combined Thermodynamic and Vibrational Spectroscopy Study

Sorption of benzene vapor in amorphous poly(2,6-dimethyl-1,4-phenylene) oxide (PPO) has been investigated experimentally at 40 degrees C by gravimetric analysis and in situ FTIR spectroscopy. This analysis indicates the occurrence of polymer crystallization during sorption when experiments are performed at elevated benzene activities. In fact, absorbed benzene determines a depression of the glass transition temperature of the system and, in turn, an increase of macromolecular mobility that promotes a solvent induced crystallization. The event of crystallization is evidenced by the presence of an overshoot in the sorption curve, both in gravimetric and spectroscopic sorption experiments performed at a relative pressure of benzene vapor close to 0.80. A theoretical approach, based on the combination of a nonrandom compressible lattice fluid theory and of the Gibbs-Di Marzio criterion, has been used to predict the glassy-rubbery state domain map of the PPO-benzene system, in temperature-benzene vapor pressure coordinates, showing that the system should display a retrograde vitrification phenomenon. This modeling allowed the identification of values of benzene vapor pressure favoring the occurrence of solvent induced crystallization that are consistent with the indications emerging from the gravimetric sorption isotherm. In addition, relevant insight into the crystallization process occurring during sorption has been obtained from vibrational spectroscopy, evidencing a rather peculiar behavior that points to the formation of cocrystalline benzene-PPO structures.