Studies of magnetic resonance phenomena in polymers. II. Molecular motions in poly(methyl methacrylate) as studied by spin‐probe and spin‐label methods

Abstract

Abstract Thin films of spin‐probed and spin‐labeled poly(methyl methacrylate) (PMMA) have been examined by electron spin resonance (ESR) in the temperature range of 77–520°K. The rotational correlation times of nitroxides used as spin probes and labels have been determined as a function of temperature from which activation energies are also determined. The nitroxide rotational times are found to strongly correlate with local segmental and side‐chain motions of the host PMMA matrix. Five discrete molecular motions are detected in PMMA along with their activation energies as measured: side‐chain CH 3 rotation (1 Kcal/mol), main‐chain CH 3 rotation (2 Kcal/mol), ester side‐chain COOCH 3 rotation around the CC bond (4 Kcal/mol), main‐chain CC bond rotation (6 Kcal/mol), and side‐chain OCH 3 rotation around the CO bond (18 Kcal/mol). The activation energies determined by ESR are consistent with the potential‐energy barriers calculated theoretically for various rotations in PMMA. It is concluded that the probe and label rotational motions do respond to localized, small‐scale segmental and side‐chain motions of host polymers but are relatively ineffective in response to the large‐scale segmental motion with an activation energy larger than 20 Kcal/mol in the case of PMMA.