M. Faetti

Scaling between the rotational diffusion of tracers and the relaxation of polymers and glass formers

The rotational dynamics of subnanometric molecular tracers in glass formers with low molecular weight and polymers is discussed by presenting electron spin-resonance and time-resolved fluorescence studies. It is shown that the reorientation of the tracer is fully or partially coupled to the host relaxation processes. The degree of coupling is well described by the fractional scaling between the correlation time of the tracer and the host timescales . The crossovers between the different regimes are sharp. Remarkably, one crossover occurs systematically very close to the splitting point of the dielectric relaxation or the critical temperature predicted by the mode-coupling theory of the glass transition.Abstract:Conservation equations are written for surface flows (either fluid or granular). The particularity of granular surface flows is then pointed out, namely that the depth of the flowing layer is not a priori fixed, leading to open equations. It is shown how some hypothesis on the flowing layer allows to close the system of equations. A possible hypothesis, similar to that made for a fluid layer, but inspired from granular flow experiments, is presented. The force acting on the flowing layer is discussed. Averaging over the flowing depth, as in shallow water theory, then allows to transform these conservation laws into equations for the evolution of the profile of a granular pile. Apart from their interest for building models, these conservation laws can be used to measure experimentally the effective forces acting on a flowing layer.

Rotational probe relaxation and scaling in fragile glass formers

Abstract The rotational dynamics of a stiff paramagnetic tracer dissolved in supercooled SALOL is investigated via electron spin resonance spectroscopy. The study shows that the molecular rotation follows different dynamical regimes as the temperature is lowered. In particular, on cooling through the critical temperature TC of the SALOL, the coupling between rotational relaxation and viscosity weakens and enhanced rotational diffusion is observed. In this temperature interval, the relationship between rotational correlation times and viscosity is fairly well described by a power law τ∝ηξ (Fractional Debye–Stokes–Einstein law). Activated reorientation is observed in the temperature region around the glass transition of the SALOL. The rotational dynamics of the tracer dissolved in SALOL are compared with its rotation in ortotherphenyl (OTP) investigated in previous studies, and a scaling procedure is proposed.

On the scaling in the rotational dynamics of molecular probes in salol and ortho-terphenyl: a possible role of the energy landscape basins

The reorientational dynamics of a stiff paramagnetic tracer dissolved in the glass former salol is investigated by means of electron spin resonance over a broad temperature range. The Debye–Stokes–Einstein law describing rotational diffusion in simple liquids is found to break down in the supercooled region where the diffusion is less temperature dependent than the viscosity. Over a large temperature interval a simple power law relates diffusion and viscosity, whereas at lower temperatures the decoupling is stronger and an activated dynamics is observed. These experiments are discussed together with previous data concerning other tracer/glass former couples. Starting from some observed common features, an attempt is made to obtain a unifying interpretation of the data in the framework of the energy landscape picture.

Crossover region and entanglement in nearly monodisperse poly(ethyl acrylates) studied with electron spin resonance spectroscopy

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) (Mn  = 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.

Dynamics, fragility, and glass transition of low-molecular-weight linear homopolymers

Analysis of the dynamics of low-molar-mass linear homopolymers is important when trying to understand the mechanisms driving structural relaxation properties from molecular liquids to polymers. Theoretical models assume, in general, the same mobility of the different moieties of the macromolecule, but this hypothesis fails for low-molar-mass polymers. Therefore, isofrictional comparisons are necessary to interpret experimental data with theoretical models. In this work, the molar-mass dependence of the glass transition temperatures T g, fragility indexes, and dynamic moduli of unentangled poly(ethyl acrylate) (PEA) melts are studied and discussed. All the results are interpreted in a coherent framework developed from the free-volume concept. The predictive power of this framework is also tested to fit theoretical and experimental master curves, introducing an ‘inverse’ isofrictional correction.

Molar mass and molar mass distribution in a nematic azobenzene polymethacrylate: Effect on optical recording

The mass contents in five samples of a nematic azobenzene polymethacrylate having different molar masses and molar mass distributions were evaluated starting from either Logarithmic Normal (LN) or Zimm-Schulz (ZS) distribution functions. Reliability of these two approaches was verified by comparing the experimental T g with those predicted (Fox-Flory) on the basis of the LN and ZS mass contents. A birefringent bit was induced in the various samples, and its stability on time was monitored at constant temperature. The optical bit was found to be quite stable at temperatures below that at which conformational disorder was frozen in the nematic phase.

On the influence of the backbone conformations of a side chain liquid crystal polymer on the fast rotational dynamics of a probe molecule

Dynamical processes effective on the nanometer length scale are of interest in polymeric systems due to their importance on the structural properties of the materials. In this study, a nm scale molecular probe is dissolved in a polymeric liquid crystal and its reorientation is studied by non-linear Electron Spin Resonance spectroscopy. Different dynamic behaviours as a function of the temperature are observed. Moreover it is found that the rotational motion of the molecular probe is sensitive to the different conformations of the polymer chain, which strongly depend on the thermal history of the material.

Effects of Thermal Annealing on the Heterogeneities in the Dynamics of a Paramagnetic Probe Dissolved in Azobenzene Polymethacrylates

ABSTRACT The rotational dynamics of the cholestane spin probe dissolved in nematic azobenzene polymethacrylates (homopolymer and copolymer) was studied by electron spin resonance spectroscopy. Upon isothermal annealing in the isotropic phase, dynamic nanoscale heterogeneities were induced in both polymers, with slow and fast sites being available for reorientation of the spin probe. The stability with temperature of the different sites was found to be very different in the homopolymer and copolymer. In any case, the rotational correlation time in both the isotropic and nematic phases was well represented by the Vogel-Fulcher law. By comparing the probe dynamics with the polymer structural relaxation, it was possible to locate the reorientation sites at different positions of the polymer structure. Finally, the annealing of the copolymer in the isotropic phase resulted in a dynamically very homogeneous sample in the nematic phase.

Enthalpy relaxation of low molecular weight PMMA: a strategy to evaluate the Tool–Narayanaswamy–Moynihan model parameters

The enthalpy recovery mechanism of a low molecular weight synthesis of polymethylmethacrylate is investigated by means of differential scanning calorimetry (DSC) experiments. The experimental results can be described satisfactorily in terms of the Tool–Narayanaswamy–Moynihan theory. This work is mainly focused on developing a strategy for evaluation of the best set of parameters for the model. The approach starts with a simultaneous fitting procedure of several experimental DSC traces. Sets of parameters are obtained which exhibit agreement with experiments. The enthalpy lost on ageing of the sample in the glassy state as a function of the annealing time is then compared with the predictions provided by using the different sets of parameters. We show that this procedure is able to single out the best set of parameters and to provide a good estimation of the Adam–Gibbs temperature.

Optical Bit Stability and Relaxation Processes in a Liquid Crystal Polymer with a Photosensitive Azo Dye Molecule as Side Group

We present a study of optically-induced molecular relaxation and optical writing processes in a photosensitive liquid crystalline polymer, containing the azobenzene moiety in the side chain. Linear and non linear ESR spectroscopies, thermal analysis and rheology investigations carried on azo polymethacrylate (PMA) samples subjected to different thermal treatments have allowed to single out a suitable procedure to get stable, homogeneous substrates. Optical studies on micrometer length scale, adopting the selected procedure, have confirmed the stability conditions in both unaligned and aligned samples. The temperature dependence of the bit stability has been determined. The writing threshold power and writing rate have been measured. The results are discussed within the perspective of extension to the nanometer length scale.