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We report a microscopic observation of the time-dependent dynamic tube dilation process on isofrictional bidisperse melts. By applying neutron spin echo (NSE) and dielectric techniques on blends of long polyisoprene (PI) chains with short PI additives with different topology, we access the dynamics of the tube dilation process on a molecular scale. The time-dependent tube dilation is directly revealed by NSE as an additional time dependence of the dynamic structure factor in the local reptation regime. We identify the characteristic time of tube dilation as the terminal time of the additive. [ABSTRACT FROM AUTHOR]

We investigated the kinetics of enthalpy recovery of several glass-forming polymers at temperatures significantly below the glass transition temperature (Tg) and for aging times up to one year. We find a double-step recovery at relatively low aging temperatures for the longest investigated aging times. The enthalpy recovered after the two-step decay approximately equals that expected by extrapolation from the melt. The two-step enthalpy recovery indicates the presence of two time scales for glass equilibration. The equilibration time of the first recovery step exhibits relatively weak temperature dependence, whereas that of the second step possesses pronounced temperature dependence, compatible with the Vogel- Fulcher-Tammann behavior. These results, while leaving open the question of the divergence of the relaxation time and that of a thermodynamic singularity at a finite temperature, reveal a complex scenario of glassy dynamics. [ABSTRACT FROM AUTHOR]

The aging-time dependence of the segmental relaxation time of polyvinyl acetate) (PVAc) in the glassy state is investigated in the bulk polymer and its nanocomposites with silica (SiO2). These systems present identical segmental dynamics, when this is probed in the equilibrium supercooled liquid by broadband dielectric spectroscopy. An acceleration of the physical aging process of PVAc with SiO2 was detected by monitoring the enthalpy recovery through differential scanning calorimetry. The segmental relaxation time during physical aging, followed by means of BDS, has been shown to increase more rapidly the higher the SiO2 concentration in PVAc is. Thermally stimulated depolarization current experiments show that this is the case over the whole probed glassy state. This means that nanocomposites displaying a relatively slow segmental mobility evolve toward equilibrium more rapidly than the bulk. Furthermore, despite the faster increase in the relaxation time with aging time, so-called self-retardation, the nanocomposites and their bulk counterpart reach the same values of equilibrium relaxation time. These findings not only confirm the assumption of identical equilibrium dynamics even in the aging regime for all nanocomposites and bulk polymers, proposed in previous works, but also highlight the fact that the physical aging rate is not determined solely by the polymer segmental dynamics, the amount of interface being an additional relevant parameter. [ABSTRACT FROM AUTHOR]

We report a microscopic observation of the time-dependent dynamic tube dilation process on isofrictional bidisperse melts. By applying neutron spin echo (NSE) and dielectric techniques on blends of long polyisoprene (PI) chains with short PI additives with different topology, we access the dynamics of the tube dilation process on a molecular scale. The time-dependent tube dilation is directly revealed by NSE as an additional time dependence of the dynamic structure factor in the local reptation regime. We identify the characteristic time of tube dilation as the terminal time of the additive.

Homogeneous diethyl phthalate/phenylphthalein-dimethylether (DEP/PDE) mixtures have been investigated by means of broadband dielectric spectroscopy. Contrarily to the widespread view that homogenous binary mixtures should give rise to a single glass transition, the mixture displays two dynamics giving rise to two glass transitions. Such a finding can be rationalized invoking the self-concentration concept that relies on the localized nature of the glass transition phenomenon. In such a way, the analogy with miscible polymer blends, for which this concept has been introduced, is highlighted. A model based on the combination of the Adam-Gibbs (AG) theory of the glass transition and the self-concentration concept resulted to be fully predictive once the only unknown variable, namely, the glass-former specific parameter (alpha) connecting the characteristic length for the glass transition to the configurational entropy, is extracted applying the model itself to DEP/toluene and DEP/PDE solutions highly concentrated, respectively, in DEP and PDE. The alpha parameter obtained in such a way allows the precise determination of the most probable relaxation time even for those DEP/PDE mixtures displaying a strong overlap of the dielectric response. The model incorporating the self-concentration concept to the AG theory also provides the characteristic length scale for the glass transition for both DEP and PDE. Such a length scale was found to be on the order of 1-2 nm. This is comparable to that obtained for other glass formers.

We have investigated the effect of cold drawing on the motion of phenylene rings in bisphenol-A polycarbonate by neutron scattering. The intensity scattered by isotropic and stretched polycarbonate is different, the quasielastic broadening being larger for the isotropic sample. This difference can be well accounted for by considering that preferentially oriented rings in the stretched polymer have their motion sterically hindered. The extent of the effect of stretching on the phenylene-ring motion obtained from this neutron-scattering investigation is in good agreement with that obtained when studying the effect of cold drawing on the gamma relaxation by dielectric spectroscopy.

A systematic investigation by dielectric spectroscopy of 18 different water-rich mixtures with very different hydrophilic substances shows universal features for the water dynamics. The temperature dependence of the relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior at the T(g) range of the mixtures. Furthermore, the temperature dependence of the relaxation times presents a universal behavior both above and below the crossover temperature. We also show that these features suggest that the observed crossover is associated with the emergence of confinement effects.

We have studied the dynamics of a trimer of vinyl acetate (3VAc) using broadband dielectric spectroscopy over a wide temperature range to cover its primary and secondary relaxations. The dielectric spectra for the primary process over the entire temperature range have been described in terms of the Kohlraush-Williams-Watts approach [Williams and Watts, Trans. Faraday Soc. 66, 80 (1970)]. The dielectric relaxation is found to show a characteristic crossover in the dynamics at 261 K through the temperature dependence of different dielectric parameters, viz., the relaxation time, shape parameter, and dielectric strength. Unlike in the polymer counterpart, this crossover temperature has been found to be different from the apparent merging temperature of the primary and secondary processes. Moreover, a direct comparison between the shape parameters in the case of 3VAc and poly(vinyl acetate) (PVAc) reflected a near absence of chain connectivity effects on the relaxation shape at the high-temperature side, which is interpreted as the decoupling of side group (containing the dipolar entity) motions from the main chain in the case of PVAc. A secondary relaxation for 3VAc was observed below its glass transition temperature, which is found to show the same activation energy as the beta process in PVAc, for which new measurements have been made. However, their shapes showed different temperature dependences, which are discussed in terms of the molecular structure.

We have investigated the dynamics of phenylene rings in the engineering thermoplastic bisphenol-A poly(hydroxyether) -- phenoxy -- below its glass transition temperature by means of neutron scattering techniques. A relatively wide dynamic range has been covered thanks to the combination of two different types of neutron spectrometers, time of flight and backscattering. Partially deuterated samples have been used in order to isolate the phenylene ring dynamics. The resulting neutron scattering signal of phenoxy has been described by a model that considers pi flips and oscillation motions for phenylene rings. The associated time scales are broadly distributed with mean activation energies equal to 0.41 and 0.21eV , respectively. Finally, a comparative study with the literature shows that the dielectric and mechanical gamma relaxation in phenoxy exhibit good correlation with the characteristic times of the aliphatic chain published elsewhere and with the characteristic times observed for the motion of phenylene rings by neutron scattering. These findings are discussed in a more general framework that considers, in addition, previous results on other polymers, which also contain the bisphenol-A unit.

The local dynamics of the low-T(g) component in a polymer blend, dynamically asymmetric poly(styrene)-poly(vinyl methyl ether) (PS-PVME), is studied below the glass transition, via dielectric relaxation spectroscopy. A particular attention has been paid to blends with a high PS content (PS weight fraction higher than 50%). A relaxation process, slower than the localized motions inducing the PVME secondary relaxations, is detected. Even though these blends fall out of equilibrium in this temperature regime, the structural recovery process is not efficient on the time scale of this PVME motional process. This relaxation is attributed to rather localized, weakly cooperative PVME motions resulting from the topological constraints imposed by the frozen PS chains.