Ciro Autiero

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.

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.

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.

Probing the cooperative dynamics varying the side-chain length of poly(alkyl acrylate)s: ESR experiments

The rotational dynamics of the tracer cholestane dissolved in non-entangled nearly monodisperse poly(alkyl acrylate) melts has been investigated by means of electron spin resonance spectroscopy. Three samples with almost the same molecular weight were selected, poly(methyl acrylate), poly(ethyl acrylate) and poly(n-butyl acrylate) and their linear viscoelastic properties were also characterized. Large temperature intervals were found with power laws relating shear flow relaxation and probe rotational diffusion. The main aim of this paper was the study of the sharp crossover between two different fractional regimes that for all the samples was observed at a temperature TC  = 1.15 − 1.25 Tg . The decoupling from structural relaxation at T < TC was ascribed to the onset of cooperative effects on the length scale probed by the tracer. These results fit in the general scenario regarding the dynamical information extractable from ESR on cholestane, a probe with its long-axis dimension greater than the typical length characterizing the rigid segment of polymers.

On the Dynamics of the Cholestane Spin Probe in a Nematic Azobenzene Side Group Oligomer

ESR studies were carried out on the rotational dynamics of the cholestane spin probe dissolved in a nematic methacrylate oligomer containing azobenzene side groups from well above TNI to below Tg. Only one molecular site was detected in contrast to analogous high molar mass samples, for which distributions of molecular sites were found depending on their thermal histories. The probe dynamics was coupled to that of either the methacrylate main chain or the azobenzene side groups in different temperature regions.

Structural relaxation of an unentangled polymer in terms of a simple phenomenological approach

The enthalpy relaxation mechanism of a low molecular weight synthesis of polymethylmethacrylate was investigated by means of calorimetric experiments. The data were analyzed in terms of a kinetic approach treating nonlinearity in a different manner with respect to the Tool-Narayanaswamy-Moynihan model. The relaxation isotherms recorded at four different temperatures were well reproduced by this approach that, however, failed in describing the relaxation asymmetry towards the equilibrium after opposite temperature jumps. A modification of the model was proposed with an additional free parameter accounting for the stretching of the relaxation function. In this way all the experimental data were reproduced fairly well.