Diego Palazzuoli

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.

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.

Heterogeneity in the dynamics of a molecular tracer dissolved in liquid crystal homopolymer and copolymer

ESR studies were carried out on the rotational dynamics of cholestane molecular tracer dissolved in liquid crystal homopolymer and copolymer from well above TNI down to below Tg. Rheological measurements also provided the flow relaxation properties of the polymer matrices. The distribution functions of molecular sites were determined, and their stability was monitored as a function of thermal history. The different molecular site distributions and cooperativity degrees were compared and discussed.

Fractionary couplings of spin probe to backbone and side group dynamics of a liquid crystal polymer

The cholestane spin probe dynamics in a nematic polymeric matrix after annealing at a temperature value slightly above the T NI was investigated by ESR spectroscopy in a wide temperature interval. Careful lineshape analysis showed that a two δ-like distribution better represented the experimental spectra. The dependence on temperature of the population in the different sites was evaluated and compared with the analogous one previously found for the same polymer/spin probe system that had been annealed at a temperature well above the T NI . [1] The temperature dependence of the spinning correlation time of the spin probe rearranging in slow and fast sites led us to locate them in the backbone and side groups of the polymer. The cooperativity degree in dynamics was quantified. A signature in temperature scale at the onset of cooperativity in backbone dynamics was recognized.

Rotational dynamics ESR investigation of the cholestane spin probe dissolved in a narrow dispersity poly(ethyl acrylate): effect of the presence of trace amounts of free monomer

The dynamics of cholestane spin probes dissolved in poly(ethyl acrylate) (PEA) matrices was fully investigated over a wide temperature range by means of ESR spectroscopy. A PEA sample with narrow molecular weight dispersion (PEA1) and a second one (PEA2) doped with about 2.7 wt.-% free ethyl acrylate were employed. Depending on temperature, different dynamical regimes were identified. The coupling between the spin probe dynamics and structural, β, and γ relaxation processes was recognised. At higher temperatures, Vogel-Fulcher laws well described the temperature dependence of the spin probe rotation, which tracks the backbone dynamics with different coupling degrees. The experiments provide the chance of evaluating the onset of cooperativity in the different PEA matrices. The information is obtained by comparing the temperature dependence of probe rotation with that of rheological measurements. Moreover, the dynamics of cholestane in PEA2 shows an Arrhenius regime in the highest temperature region, which exhibits a complete decoupling from the backbone dynamics. Cholestane spin probe.

Dynamics heterogeneity and optical writing in a liquid crystalline azobenzene copolymer

The dynamics of the cholestane spin probe dissolved in a nematic azobenzene copolymer was studied by ESR following a suitable thermal procedure. Heterogeneities in the dynamics of the guest molecule resulted to occur only in the high temperature region, slightly above the nematic-isotropic transition temperature. A temperature region above the glass transition was signalled, in which stable optical writing could be obtained. Optical birefringence and topological writing experiments were carried out on polymer films at the macroscopic and nanometer length-scales, respectively. SNOM analysis showed a very homogenous substrate that could be suitable for optical nanowriting.

On the simulation of inhomogeneous ESR lineshape of a spin probe in a liquid crystalline polymer

A procedure for the analysis of the electron spin resonance (ESR) lineshape of a cholestane paramagnetic tracer dissolved in a liquid crystalline polymer is described. Due to its rodlike geometry, the cholestane probe is subjected to an anisotropic reorientational motion in the polymer matrix. On the basis of the generalized Mori theory, simulation approaches to the lineshape have been developed. The ESR lineshape has been calculated for a molecule reorienting in a homogeneous molecular site according to diffusion or jump motion. Moreover, the case of inhomogeneous sites with continuous or discrete distribution functions was also accounted for. Systematic comparison with experimental spectra has been carried out, and a two-δ distribution function of the molecular sites has been singled out as best description of the experiment. The temperature dependence of the two species has been fully investigated and the spatial character of the heterogeneity has been proved.

A calorimetric study of structural relaxation in the glassy state of a liquid-crystal polymer

Abstract The physical ageing of a side-chain liquid-crystal polymer is investigated by means of differential scanning calorimetry, which allows one to follow the enthalpy relaxation of the glass. Measurements are performed for different thermal treatments and compared with the predictions of two phenomenological models where the nonlinearity of the ageing process is accounted for by multiparameter expressions. The experimental results can be described with the model based on the Narayanaswamy expression for nonlinearity, although it seems that the model based on the Hodge expression fits experimental data more satisfactorily. In particular, a single set of parameters is determined which describes different experiments. However, the value of the enthalpy loss as a function of the ageing time is greatly overestimated by both models.