Fabio Zulli

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.

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.

Heterogeneities in the Dynamics of a Molecular Tracer in Mesogenic and Nonmesogenic Azobenzene Copolymers

We studied by electron spin resonance (ESR) spectroscopy the reorientational dynamics of the cholestane spin probe dissolved in the matrix of the PMA4 40/60 copolymer containing methyl methacrylate (MMA, 40 mol%) and azobenzene methacrylate (MA4, 60 mol%) counits. Heterogeneities in the probe dynamics at the nanometer length scale were evidenced and attributed to the existence of molecular sites with slow and fast spinning correlation times. Different dynamic regimes were also observed with a crossover temperature TC that was correlated with the glass transition temperature Tg of the copolymer, T C = 1.17T g . Even though the nematic-to-isotropic transition temperature could not be detected, the probe dynamics experienced the onset of nematic order at the molecular level. The results are compared with those of the closely related nematic PMA4 30/70 copolymer.

Unentangled Rheological Behavior of a Nematic Azobenzene Polymethacrylate

The rheological behavior of a nematic polymethacrylate containing an azobenzene side group was investigated. The time-temperature superposition principle was found to hold over a large temperature range, as no discontinuities were detected at the isotropic-nematic transition temperature, despite the relatively high molar mass of the polymer. A direct proof of the unentangled behavior of the polymer is provided, which stems from a dynamic model able to predict the relaxation mechanisms by simply knowing the molar mass distribution of the polymer sample.

Onset of entanglement and reptation in melts of linear homopolymers: consistent rheological simulations of experiments from oligomers to high polymers

The shear rheological behavior is investigated in this work for a series of poly(ethyl acrylate) samples, whose molar mass ranges from oligomers to high polymers. The focus was on studying the onset of entanglement effects over selected reptation models in order to ascertain their ability to reproduce the complex shear modulus of the polymers and to provide consistent values of the microscopic parameters driving the structural relaxation of the polymer system. Among ordinary reptation topological models, we found that the Doi–Edward model, implemented with contour length fluctuation and constraint release mechanism for the tube relaxation, better reproduced the rheological response of the materials. Most importantly, we were able to simulate material functions to obtain consistent microscopic information on the materials, such as Rouse time and entanglement molar mass, over the whole range of investigated molar masses, therefore overcoming the discrepancy usually found, mostly in the mass region of partial entanglement. Finally, descriptions of the polymer entanglement features, in agreement with the experimental and microscopic model findings, are provided in the framework of the packing-length phenomenological model and by means of analytical calculations of the polymer viscosity according to the Milner–McLeish–Likhtman model.

Length Scales and Dynamics in the Reorientational Relaxation of Tracers in Molecular and Polymeric Glass Formers via Electron Spin Resonance Spectroscopy

It is widely accepted that a temperature region exists above the glass transition temperature, playing a fundamental role in the physics of polymers and glass formers. In this region, several dynamic crossovers have been experimentally and numerically revealed in the past years and the onset of glassy behavior is generally located, because of cooperative and heterogeneous features exhibited by the dynamics. In this Article, the rotational dynamics of two different stiff molecular spin probes dissolved in poly(propylene glycol) has been investigated by electron spin resonance spectroscopy in a wide temperature range. Decoupling phenomena between macroscopic and microscopic transport properties have been observed in the crossover region. A comparison with previous studies carried out with the same tracers dissolved in different polymers and glass formers strongly supports the idea that the observed crossover signals the onset of spatial correlations of dynamics on the length scale probed by the tracers.

Effects of bismuth vandate and anthraquinone dye on the photodegradation of polycarbonate

Both inorganic and organic compounds, such as oxides or salts of metals and polycyclic and azo compounds, are frequently used as colorants in polymeric systems. Bismuth vanadate pigment has been used as an environmentally friendly alternative for cadmium containing pigments and anthraquinone dyes represent a polycyclic colorant class of wide use in polymers. Besides their coloring properties, both bismuth vanadate and anthraquinone present photocatalytic activity or photochemical properties that can influence the mechanism and the kinetics of the photodegradation of the polymer into which they are incorporated. In this paper the influence of bismuth vanadate pigment and anthraquinone dye on the photodegradation of polycarbonate was evaluated. For this purpose, samples of polycarbonate containing the colorants were submitted to photochemical aging following the recommendations of ASTM G53. Bismuth vanadate accelerates the photodegradation of polycarbonate, causing a faster drop in the mechanical properties and an increase in the hydroperoxide concentration during the aging process. The coloring ability of the pigment is also affected. The photodegradation behavior of polycarbonate in presence of bismuth vanadate may be caused by a possible direct chemical reaction between pigment and polymer, promoted by UV radiation. On the other hand, the presence of anthraquinone dye seems to stabilize the polycarbonate against photodegradation through a deactivation mechanism of the excited state.

Rheological Behavior of Azobenzene Nematic Homopolymer and Copolymer

ABSTRACT A rheological study of a methacrylate homopolymer containing azobenzene side groups and its 70/30 copolymer with methyl methacrylate was performed in the linear viscoelastic regime. The creep and oscillatory measurements enabled us to obtain the temperature dependence of the zero shear viscosity η. The time-temperature superposition principle (TTS) was found to hold over large temperature intervals; in particular no discontinuities were detected at the isotropic-nematic transition in either sample. However, the homopolymer evidenced a peculiar signature in the shear elastic complex modulus G* (ω,T) at a temperature T c in the nematic phase. This was attributed to a conformational transition driven by the nematic order.

Linear Viscoelastic Behavior of an Azobenzene Nematic Block Copolymer

We report on the rheological investigation of a nematic block copolymer of an azobenzene methacrylate (MA4) and methyl methacrylate (MMA). Relaxation processes are discussed in terms of chain architecture and viscoelastic response of the polymer. In contrast to analogous random copolymers of MMA and MA4, the present block copolymer did not show validity of the time-temperature superposition (TTS) principle for all material functions. TTS was found to work only for the storage modulus. Rheological steady-state and oscillatory measurements were thus compared in order to obtain further insight into such a peculiar phenomenon.