Nitroxide Spin Probe/Label Study of Hydrogen Bonding and Probe Size Effects in a Linear Epoxy Polymer

Abstract

An electron spin resonance (ESR) nitroxide spin probe/spin label study was performed on a linear diglycidyl ether Bisphenol A (DGEBA)-cyclohexylamine (CHA) epoxy polymer over a 30-200 °C temperature range. Measurements of correlation time and a T 50G parameter on two series of tetramethylpiperidyloxy-based spin probes occluded in an epoxy matrix and on the spin-labeled epoxy matrix itself have demonstrated the effects of probe size and hydrogen bonding. The probes in the series TEMPO, TEMPOL, and TAMINE (respectively, unsubstituted and 4-hydroxy- and 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl) display large increases in hydrogen-bonded effects, as indicated by T 50G measurements, while the probe size remains small and nearly constant. These increases correlate with an increasing hydrogen bond acceptor strength. The enhanced hydrogen-bonded effect of TAMINE over TEMPO was directly observed for these probes in methanol and cyclohexane solvents where free volume and viscosity effects of the polymer matrix do not interfere. As the probe size becomes larger, as in the series of probes BZONO, PGETA, and CPGETA (respectively, 4-(benzoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl, 2 :1 phenyl glycidyl ether :4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, and 2 :1 4-cumylphenyl glycidyl ether :4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl), the T 50G measurement gradually increases and approaches that of the spin-labeled epoxy matrix. These effects were analyzed using established theoretical relationships of T 50G to probe volume.