EPR Application to Polymers. 3. EPR Studies of Poly(phenylacetylene) Using Different Nitroxide Spin Probes

EPR line-shape analysis of spin-probed poly(phenylacetylene) in the slow-motional region has been performed in the temperature range 77–258 K where the outermost hyperfine extrema are separable. Two approaches were used to obtain the rotational correlation time, τ_R. In the first approach, the rotation is assumed to be isotropic, and τ_Rwas calculated by using the empirical formula τ_R = α(1-S)^b for the three models of rotation, namely Brownian, moderate jump, and strong jump. The results showed that, depending on the model of rotation, activation energy of rotation decreased by a factor of three in going from the Brownian model to the strong jump model. In the second approach, the line shapes were simulated by using the stochastic Liouville theory of slow-motional effects on EPR spectrum. A systematic method has been developed to determine the axis of rotation, the anisotropy of rotation, and the model of rotation. All four spin probes used (Tempone, doxylcyclohexane, doxylcholestane, and Tempo palmitate) were found to undergo anisotropic reorientation. The model of rotation for Tempone was found to be moderate jump, while the models for the other three were Brownian. The apparently low activation energies (0.3-1.1 kcal/mol) calculated from EPR line-shape simulations in the low-temperature region have been explained in terms of the spin probes rotating in the “holes” of dimensions comparable to those of the probe in the polymer matrices. A comparison of the two approaches for calculating τ_Remphasizes the fact that accurate τ_Rvalues of nitroxide spin probes in poly(phenylacetylene) could best be obtained from a simulation of EPR line shapes employing magnetic parameters determined from their rigid-limit spectra at 77 K.