The rotational dynamics of the tracer cholestane dissolved in non-entangled nearly monodisperse poly(alkyl acrylate) melts has been investigated by means of electron spin resonance spectroscopy, Three samples with almost the same molecular weight were selected, poly(methyl acrylate), poly(ethyl acrylate) and poly(n-butyl acrylate) and their linear viscoelastic properties were also characterized. Large temperature intervals were found with power laws relating shear flow relaxation and probe rotational diffusion. The main aim of this paper was the study of the sharp crossover between two different fractional regimes that for all the samples was observed at a temperature T-C = 1.15 - 1.25 T-g. The decoupling from Structural relaxation at T < T-C was ascribed to the onset of cooperative effects on the length scale probed by the tracer. These results fit in the general scenario regarding the dynamical information extractable from ESR on cholestane, a probe with its long-axis dimension greater than the typical length characterizing the rigid segment of polymers
The rotational dynamics of the spin probe cholestane dissolved in a narrow distribution poly(n-butyl acrylate) sample has been investigated via electron spin resonance (ESR) spectroscopy. The measurements were carried out in a wide temperature range: different dynamic regions have been recognized, and the coupling of the probe dynamics to the α and secondary relaxations has been revealed. In particular, the coupling with the structural relaxation is ruled by two fractionary Vogel-Fulcher laws (VF). The crossover from one VF region to the other occurs at the temperature T(C) = 1.17T(g), signalling the onset of the cooperativity in the dynamics and confirming a behaviour previously observed in ESR studies carried out on polymeric glass-formers. Furthermore, in this work we discuss the activated regime at the highest temperatures and show that the activation energy does not depend on the length of the polymer main- and side-chains, while its onset temperature linearly depends on the chain length.
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