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.

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The topic of dynamic changes undergone by glass-forming materials in the supercooled region is addressed in this study. Crossover regions and temperatures are generally considered as key features in order for the glass transition phenomenon to be understood. The attention is here focused on the crossover region of polymers and its dependence on the polymeric entangled dynamics. To avoid the superposition of possible dependence on the polymeric polydispersity, nearly monodisperse syntheses of poly(ethyl acrylates) have been used. Rotational dynamics have been investigated with electron spin resonance spectroscopy, dissolving the cholestane molecular tracer in a poly( ethyl acrylate) (M-n = 7500 amu). Comparison is carried out with the findings obtained in the case of an almost monodisperse poly(ethyl acrylate) with M-n = 58 200 amu. Different dynamic regimes and crossover regions were recognized in the temperature dependence of the molecular rotation. The crossover temperatures T-c were found to be dependent on the molecular weight. Moreover, the dynamics in non-Arrhenius regions were satisfactorily described as a fractionary law of structural relaxation