Relaxometric Study of Secondary Transitions in Aromatic Polymers

Abstract

An accurate description of minor transitions occurring at low temperatures in aromatic polymers can be obtained by a combination of relaxometric methods. In each technique, suitable contrast agents will be chosen, the most suitable for each detection. In this paper, the method will be applied to SAN (styrene−acrylonitrile) copolymers. Oxygen can be selectively absorbed on aromatic polymers. The selective absorption of paramagnetic oxygen causes numerous effects well observable by the 1H NMR relaxometric method since it acts as a relaxation contrast agent. In SAN copolymers, the above observations allow a precise measurement of γ transitions due to the phenyl ring libration. Another small molecule well adsorbed by aromatic rings is water. Due to its strong dipole moment, water acts as a contrast agent for dielectric relaxation measurement. In SAN copolymers, γ transitions, barely observable at low temperature with extreme uncertainty, appear strong and well resolved in the presence of water. Thus a combination of these two relaxometric methods allows a precise description of the interruption of phenyl ring libration, the true cause of the relaxometric behavior. Dielectric and NMR relaxations are also in agreement with weak transitions observed by dynamic mechanical tan δ measurements.