Laura Andreozzi

Near-field optical writing on azo-polymethacrylate spin-coated films

We report on near-field optical writing and topographic reading of spin-coated azo-polymethacrylate films with subwavelength resolution. The sample is functionalized with photosensitive side chain azobenzene groups. These photochromes are locally illuminated at 325 and 488 nm through the aperture of a metalized tapered optical fiber. The printing on subwavelength scale is due to optically induced cooperative rearranging of the polymeric chain. It is read in situ in the surface topography imaged by shear-force microscopy. The stability of the optically produced pattern and the cycling characteristics of the photoisomerization of the azobenzene molecules make these samples suitable for optical nanolithography and high density optical data storage.

Jump reorientation of a molecular probe in the glass transition region of o-terphenyl

The reorientation process of a molecular probe dissolved in the fragile glass former o-terphenyl is studied via electron spin-resonance spectroscopy. Owing to the good resolution of the ESR lineshape in the so-called slow-motion regime, we are able to discriminate between diffusive and jump rotations. In the region of the glass transition ( the results are at variance with the diffusion models and are fairly well reproduced by assuming that the probe jumps with steps . The temperature dependence of the correlation time is well described, in the temperature range investigated, by a double Arrhenius law broken at . The findings are compared with previous NMR, dielectric, fluorescence and molecular dynamics studies.

Anisotropic jump model of the rotational dynamics in glasses

Anisotropic jump reorientation occurs in glasses, polymers, and plastic crystals. A general theoretical frame to describe such processes is presented. It generalizes previous work by Ivanov [Sov. Phys. JETP 18, 1041 (1964)]. A tractable model is given by a simple phenomenological assumption on the jump distribution. Analytical results and fast numerical methods to evaluate the relevant quantities are derived. The model is validated by comparing the predictions with ESR experiments on stiff, cylindrical tracers dissolved in the glassformer o-terphenyl.

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.

Linear and non-linear electron spin resonance study of the rotational diffusion of a molecular tracer in supercooled o-terphenyl

The long-time and the short-time rotational relaxation of a molecular tracer in supercooled o-terphenyl (OTP) have been studied by linear and non linear electron spin resonance (ESR) spectroscopy, respectively. At higher temperatures the related correlation times. τ 1 and τ s , are equal within the experimental errors, i.e. the relaxation is exponential, and well coupled to the viscosity. η. On cooling. τ s , and, at lower temperatures, τ 1 , are partially coupled to thc viscosity in agreement with the fractional Debye-Stokes-Einstein (DSE) law τ i α (η/T), i = 1. s with ξ i constant. For T < 330 K the rotational correlation function is not exponential.

Efficient characterization of the orientational ordering of ESR-active probes in supermolecular fluids

Supermolecular fluids often exhibit organized arrangements which order the orientations of solved ESR-active probe molecules. Due to the high microviscosity, the ESR line-shape of the probe virtually reflects the distribution of the resonating frequencies of the involved spin packets and then the degree of orientational order of the probe. An efficient strategy to get precise estimates of both the magnetic parameters of the probe molecule and the width of the spin packet from the ESR spectrum has been developed. This allows us to adjust only the strength of the effective orienting potential in the fit procedure. A study of the orientational ordering of cholestane probe molecule dissolved in side-chain polymeric mesophases provides accurate testing of the procedure.

A study of the Debye-Stokes-Einstein law in supercooled fluids

The reorientation of a nearly spherical paramagnetic tracer dissolved in supercooled o-terphenyl is studied by electron spin resonance in the range 200 K 300 K the rotational correlation time follows the Debye - Stokes - Einstein law. On decreasing the temperature, the rotational motion of the probe and the viscosity decouple. In particular, in the region around T = 290 K it is found that with .

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.