We studied by ESR the rotational motion of a paramagnetic tracer (a deuterated 15N-enriched nitroxide) in poly(vinyl acetate) (PVAc). The reorientation of the tracer occurs via jumps of about 50° with a heterogeneous distribution of correlation times. Depending on the temperature range, the average correlation time 〈τ〉 scales as the α, β, and γ relaxation times of PVAc, i.e., 〈τ〉 = C i 〈τ〉 i , i = α, β, γ. On cooling, 〈τ〉 tracks the α relaxation to about the α−β bifurcation temperature and then the β relaxation down to the glass transition. In glassy PVAc 〈τ〉/〈τ〉γ = C γ ≈ 1.

Heterogeneities induced in a nematic polymethacrylate by thermal annealing in the isotropic phase were investigated by studying the dynamics of the cholestane spin probe dissolved in the host matrix by electron spin resonance. The molecular site distribution was well described by a two δ-like distribution function. The temperature dependences of the dynamics of the slow and fast components were fully characterized in going from the isotropic state to the glassy state through the nematic phase. It was found that the behaviors of the spinning correlation times with temperature in the isotropic and nematic regions were well represented by the Vogel-Fulcher law. By comparing the temperature dependence of the probe dynamics with the R relaxation process of the polymer, we estimated the cooperativity degrees in the dynamics of the molecular probe in the different sites. The relative populations of the slow and fast sites were determined over the whole temperature range investigated. It was shown that such population was sensitive to conformational change of the polymer backbone.

The effects of the molecular weight on the physical aging of polymers were investigated by means of differential scanning calorimetry (DSC) experiments in five PMMA samples with mass values across the entanglement mass M e. The Tool−Narayanaswamy−Moynihan (TNM) model and a recent configurational entropy approach were systematically compared with the experiments. For the samples with the lowest molecular weights, the two approaches exhibited similar agreement with experiments; however, in the systems with higher molecular weights, the TNM model met major difficulties, and a clear improvement was obtained with the other model. The additional parameter introduced by the entropic model showed a strong molecular-weight dependence, with a sharp increase at the entanglements mass of PMMA. These results evidence the role of the chain entanglements on the physical aging of polymers. Finally, the influence of the molecular weight on the fragility was investigated and related to the recent literature debates on this topic.

In the present work, we characterize the rheological behavior of 10 nearly monodisperse poly-(ethyl acrylate) samples, whose molar mass ranges from 1200 to 150 000 g/mol. The poly(ethyl acrylate)s were obtained by means of a controlled/living radical polymerization technique. The time-temperature superposition principle works, and the T dependence of the horizontal shift factor a Tr (T) is fairly well described by the Williams-Landel-Ferry law. Furthermore, the zero-shear viscosity dependence on the temperature, for all the investigated samples, has resulted to be well described by means of Vogel-Fulcher laws. The mass dependence of thermal parameters such as the Vogel temperature T 0 and the pseudo-activation energy T b has been worked out and compared to the mass dependence of the glass transition temperature T g . This leads us to propose here a coherent way to describe their behavior and estimate several microscopic parameters in terms of free volume. Moreover, the molar mass dependence of material parameters has been investigated. The zero-shear viscosity η at different temperatures has been evaluated, and the critical mass value has been found to be M c ) 26 000 g/mol. The ratio between the critical M c and the entanglement mass M e has been found to be about 2.2 from the evaluation of the plateau modulus G N 0 . A mass dependence analysis of the steady-state compliance J e 0 has also been carried out from which the second critical mass M c ′ is inferred.