Signatures of the fast dynamics in glassy polystyrene: First evidence by high-field Electron Paramagnetic Resonance of molecular guests

Abstract

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron paramagnetic resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). Two different regimes separated by a crossover region are evidenced. Below 180 K the rotational times are nearly temperature independent with no apparent distribution. In the temperature range of 180-220 K a large increase of the rotational mobility is observed with the widening of the distribution of correlation times which exhibits two components: (i) a deltalike, temperature-independent component representing the fraction of spin probes w which persist in the low-temperature dynamics; (ii) a strongly temperature-dependent component, to be described by a power distribution, representing the fraction of spin probes 1-w undergoing activated motion over an exponential distribution of barrier heights g(E). Above 180 K a steep decrease of w is evidenced. The shape and the width of g(E) do not differ from the reported ones for PS within the errors. For the first time the large increase of the rotational mobility of the spin probe at 180 K is ascribed to the onset of the fast dynamics detected by neutron scattering at T(f)=175+/-25 K.

Electron Paramagnetic Resonance

“…Buchenau confirmed that conclusion by comparing results from several techniques covering a wide time window from 1Hz up to about 100GHz [18]. The same result has been reached by HF-EPR [10,11,19].…”

“…B 0 ∼ = 3T corresponding to Larmor frequencies about 95GHz ( W band ), [37,38] or even larger frequencies [19,39,40]. HF-EPR is widely used in polymer science [10,11,19,[41][42][43][44]. One major feature is the remarkable orientation resolution [44] due to increased magnitude of the anisotropic Zeeman interaction leading to a wider distribution of resonance frequencies [45].…”

“…Particular interest and a current subject of strong controversy is the so called fast dynamics of glasses, occurring in the time window 1 − 10 2 ps with several studies carried out mainly by neutron [1][2][3][4][5], Raman scattering [6][7][8][9] and highfield Electron Paramagnetic Resonance ( HF-EPR ) [10,11]. It is observed that on heating in a temperature range below the glass transition temperature T g the dynamics of glass-forming systems deviates from the harmonic behavior and quasielastic scattering starts to accumulate in the low frequency range of the scattering function S(Q, ω).…”

“…The shape of the energy-barriers distribution g(E) in glasses has been extensively investigated via experiments [8,10,11,16,17,[19][20][21][22][23][24], theories [25][26][27][28][29] and simulations [30]. Basically, two different distributions are usually recovered, the gaussian distribution [8,16,20,21,24,25,30,31] and the exponential distribution [8,10,11,17,19,22,[26][27][28][29].…”

“…Basically, two different distributions are usually recovered, the gaussian distribution [8,16,20,21,24,25,30,31] and the exponential distribution [8,10,11,17,19,22,[26][27][28][29].…”