{ The rotational dynamics of nearly spherical and cylindrical tracers dissolved in supercooled o-terphenyl is studied via the Electron Spin Resonance spectroscopy. On decreasing the t emperature, a sharp crossover is observed from the regime w h ere the D e bye-Stokes-Einstein law (DSE) holds to a n ew one which is accounted for by a fractional DSE (FDSE) Dr / ( = T) ; with 1. Dr is the rotational di usion constant, the s h ear viscosity a n d T the t emperature. Tumbling a n d spinning o f t he c y l i n drical tracer exhibit distinct crossovers. A simple model of FDSE based on the presence of dynamic heterogeneities is presented.

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.

Dye-sensitized solar cell (DSSC) is one of the promising photovoltaic (PV) technologies for applications requiring high aesthetic features combined with energy production such as building integration PV (BIPV). In this context, DSSCs have the ability to be wavelength selective, thanks to the development of new sensitizers by molecular engineering. The long history of dye research has afforded is technology different colorations for reaching panchromatic light absorption. However, nearly 45% of radiation from sunlight lies in the near-infrared (NIR) region, where human cones are not sensitive. This review provides the reader with key information on how to selectively exploit this region to develop colorless and transparent PV based on DSSC technology. Besides selective NIR absorbers, the triptych photoanode, counter-electrode, and redox mediator are together contributing to reach high aesthetic features. Details of all the components, interplay, and an opinion on the technological limitations to reach colorless and transparent NIR-DSSC are herein discussed in relationship with BIPV applications.

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

The enthalpy recovery mechanism of polymers is usually described in terms of the Tool-Narayanaswamy-Moynihan model (TNM). Even if it is able to qualitatively reproduce all the peculiar features of the aging process, clear discrepancies have been found in quantitative analysis. In this work, differential scanning calorimetry experiments are performed in two different polymers, and data are used in order to compare the predictions of a recently developed configurational approach with possible extensions of the TNM model. All the approaches add a free parameter with respect to the original TNM model. In this study we have found that appreciable improvements between experimental and calculated DSC traces can be observed if the configurational entropy approach is adopted. Furthermore, the longterm annealing experiments performed in one of the polymers clearly support the basic hypothesis of the entropic approach concerning the limit state of the structural relaxation.