Characterization of the motion of spin probes and spin labels in amorphous polymers with two‐dimensional field‐step ELDOR

Abstract

SYNOPSISThe motion of nitroxide spin probes and spin labels in amorphous polymers is studied below the glass transition temperature with a two-dimensional pulsed electron doubleresonance experiment. Polystyrene and a liquid crystalline side group polymer are studied using both spin probes and spin labels covalently bound to specific sites along the polymer chain. Two methyl acrylic polymers differing only in their side group structure and polyvinylacetate are compared and large differences in the molecular dynamics deduced from both the nuclear and the electron spin relaxation rates are observed as the glass transition is approached. The results demonstrate the complexity of small amplitude motion in simple polymers below the glass transition temperature and show that it is very sensitive to the packing in the polymer. 0 1996 John Wiley & Sons, Inc. Keywords: EPR spin label spin probe spin relaxation correlation time distributions electron double resonance amorphous polymers side group liquid crystalline polymer

Simulation of Rigid-Limit and Slow-Motion EPR Spectra for Extraction of Quantitative Dynamic and Orientational Information

“…Characteristic widths of the distributions lie in the range of 0.6-1.0 decades. The obtained data conform to the literature data on the width of distribution of correlation times in polymers [56,[58][59][60]. The distribution narrows when temperature rises.…”

Molecular mobility of nitroxide spin probes in glassy polymers. Quasi‐libration model

“…In spite of considerable progress in this field, there is some uncertainty regarding which models of molecular mobility should be used for simulation of ESR spectra of nitroxide spin probes in glassy polymer media, and what are quantitative characteristics of corresponding probe movements. The rotation correlation times obtained by analysis of ESR spectra in glassy state well below T g in the framework of widely used model of rotational diffusion are incorrect as these values significantly contradict data obtained by other techniques 5–7, 10, 18–24, 41–44. Even in the case of most commonly used and investigated X‐band (∼ 10 Hz) spin probe technique, there are no well‐defined guidelines for analysis of experimental results.…”

“…Two main trends in development of spin probe method have aimed to obtain more precise and reliable data and clearer interpretation. One of these is further technical development of experimental ESR technique: application of high‐frequency ESR, echo‐detected ESR (ED ESR), two‐dimensional ESR, and others 2, 13–24. The other trend involves developing of advanced theoretical models of molecular mobility.…”

“…The frequency of libration motion is high enough to result in partial averaging of magnetic parameters. Fast motional dynamics in glassy polymers was evidenced by high‐frequency ESR,23, 24 magnetization transfer,18 spin‐echo experiments,41–44 and other techniques 15–22, 46, 47. Despite that the model of libration motions is successfully used for interpretation of different experimental results, simulation of spectra and exact determination of parameters in the framework of libration model are used rather rarely 10, 48, 49.…”

“…Despite that the model of libration motions is successfully used for interpretation of different experimental results, simulation of spectra and exact determination of parameters in the framework of libration model are used rather rarely 10, 48, 49. The amplitudes of orientation displacements in polymer glasses are found to be in the range of 5–15° 10, 19–21, 48. Movements characterized by so large amplitudes seem to be inharmonic.…”