Spin Probe Mobility in Relation to Free Volume and Relaxation Dynamics of Glass-Formers: A Series of Spin Probes in Poly(isobutylene)

We present the dynamics of a series of three paramagnetic molecules of different volume, mass, and shape in amorphous glass-forming polymer poly(isobutylene) (PIB) as investigated by means of electron spin resonance (ESR) technique. The reorientation behavior of spin probes is related to the ortho-positronium (o-Ps) annihilation in PIB from positron annihilation lifetime spectroscopy (PALS) and the extracted free volume information. It is also related to the dynamic data of PIB from broadband dielectric spectroscopy (BDS), neutron scattering (NS), and nuclear magnetic resonance (NMR) spectroscopy from literature. In the case of the smallest spin probe, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), a discontinuous course of the spectral parameter 2A(zz') versus T dependence was observed and the subsequent phenomenological model-free analyses of the spectral parameter, 2A(zz') versus T, as well as of the correlation time, tau(c), versus 1/T plots provided the characteristic ESR temperatures (T-X1(2Azz), T-50G, T-X2(2Azz)) and (T-X1(tau), T-c(tau), T-X2(tau)). These characteristic ESR temperatures were found to be consistent with the characteristic PALS temperatures: T-g(PALS), T-b1(L) = T-b1(sigma) from temperature dependences of the mean o-Ps lifetime, tau(3), or the width of o-Ps lifetime distribution, sigma(3), respectively. In addition, the relationships between the spin probe size, V-Tempo(W), and the free volume hole size distributions g(n)(V-h) at the characteristic ESR temperatures indicate the significant influence of the free volume fluctuation at the crossover from slow to rapid regime as well as within the rapid motional regime. On the other hand, the two larger spin probes exhibit a rather continuous 2A(zz')-T plots with the respective T-50G's lying in the vicinity of T-b2(sigma) independently of their volume, mass and shape, suggesting the common origin of underlying process controlling this T-50G transition. Finally, these mutual PALS and ESR findings were compared with the known dynamic behavior of PIB which suggest that the dynamics of the TEMPO and the larger spin probes are related to free volume fluctuation associated with primary alpha - and secondary beta processes, respectively. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1058-1068, 2009