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Recent efforts, fostered by a pioneering work by us, have shown the of multiple steps in the recovery of equilibrium of glasses. Jin and McKenna raise concerns regarding the validity of such scenario alleging that the multiple recovery steps would be an artifact arising from poor temperature control in the oven used for isothermal glass equilibration. We critically discuss Jin and McKenna arguments from both the viewpoints of scrutinizing previous literature data and that of the temperature control in the oven. In doing so, we provide compelling arguments that Jin and McKenna conjectures are unjustified and point out the need for efforts to describe glass dynamics significantly below the glass transition temperature, Tg, by accounting for the presence of different relaxation mechanisms active in glass equilibration. [ABSTRACT FROM AUTHOR]

We investigate the kinetics of enthalpy recovery in stacked glassy polystyrene (PS) films with thickness from 30 to 95 nm over a wide temperature range below the glass transition temperature (Tg). We show that the time evolution toward equilibrium exhibits two mechanisms of recovery, in ways analogous to bulk PS. The fast mechanism, allowing partial enthalpy recovery toward equilibrium, displays Arrhenius temperature dependence with low activation energy, whereas the slow mechanism follows pronounced super-Arrhenius temperature dependence. In comparison to bulk PS, the time scales of the two mechanisms of recovery are considerably shorter and decreasing with the film thickness. Scaling of the equilibration times at various thicknesses indicates that the fast mechanism of recovery is compatible with the free volume holes diffusion model. Conversely, the slow mechanism of recovery appears to be accelerated with decreasing thickness more than predicted by the model and, therefore, its description requires additional ingredients. The implications, from both a fundamental and technological viewpoint, of the ability of thin polymer films to densify in relatively short time scales are discussed. [ABSTRACT FROM AUTHOR]

The use of an atomic force microscope for studying molecular dynamics through dielectric spectroscopy with spatial resolution in the nanometer scale is a recently developed approach. However, difficulties in the quantitative connection of the obtained data and the material dielectric properties, namely, frequency dependent dielectric permittivity, have limited its application. In this work, we develop a simple electrical model based on physically meaningful parameters to connect the atomic force microscopy (AFM) based dielectric spectroscopy experimental results with the material dielectric properties. We have tested the accuracy of the model and analyzed the relevance of the forces arising from the electrical interaction with the AFM probe cantilever. In this way, by using this model, it is now possible to obtain quantitative information of the local dielectric material properties in a broad frequency range. Furthermore, it is also possible to determine the experimental setup providing the best sensitivity in the detected signal. [ABSTRACT FROM AUTHOR]

The behavior of water dynamics confined in hydrated calcium silicate hydrate (C-S-H) gel has been investigated using broadband dielectric spectroscopy (BDS; 10-2-106 Hz) in the low-temperature range (110-250 K). Different water contents in C-S-H gel were explored (from 6 to 15 wt%) where water remains amorphous for all the studied temperatures. Three relaxation processes were found by BDS (labeled 1 to 3 from the fastest to the slowest), two of them reported here for the first time. We show that a strong change in the dielectric relaxation of C-S-H gel occurs with increasing hydration, especially at a hydration level in which a monolayer of water around the basic units of cement materials is predicted by different structural models. Below this hydration level both processes 2 and 3 have an Arrhenius temperature dependence. However, at higher hydration level, a non-Arrhenius behavior temperature dependence for process 3 over the whole accessible temperature range and, a crossover from low-temperature Arrhenius to high-temperature non-Arrhenius behavior for process 2 are observed. Characteristics of these processes will be discussed in this work. [ABSTRACT FROM AUTHOR]

Using a simple mathematical formalism, we show that temperature dependent nonexponential relaxation found in glass-forming liquids and amorphous polymers, often resulting in a decrease in the stretching exponent when decreasing temperature, can be suitably described assuming the combination of an intrinsic stretched response and the existence of temperature independent heterogeneities. The effect of the latter is incorporated by assuming a Gaussian distribution of Vogel temperatures. Comparison with experimental data of a large number of glass formers showed that this approach is able to quasiquantitatively describes the temperature dependence of the stretching exponent using the width of the distribution as the single fitting parameter. According to this approach, the rapidity of the decrease in the stretching exponent with decreasing temperature depends not only on the magnitude of the standard deviation of Vogel temperatures but also on the value of the intrinsic stretching exponent and on the fragility of the glass former. The latter result is able to rationalize, at least partially, the empirical correlation between the fragility and the stretching exponent at Tg. [ABSTRACT FROM AUTHOR]

The dynamics of homogenous polychlorinated biphenyl (PCB54)/toluene mixtures have been investigated by means of broadband dielectric spectroscopy. The mixture presents dynamical heterogeneity, alike miscible polymer blends, which is manifested with the presence of two relaxational processes. The relatively slow one has been attributed to the motion of PCB54 in the mixture, whereas the relatively fast one was related to the toluene dynamics in the mixture. These results have been interpreted according to the self-concentration concept, first introduced to describe the dynamics of miscible polymer blends, which relies on the limited size of the cooperative length scale in glass-forming liquids. The self-concentration concept has been incorporated in the Adam–Gibbs theory of the glass transition relating the characteristic relaxation time and the length scale for structural relaxation to the configurational entropy. This allowed the determination of the cooperative length scale of PCB54 and toluene both in mixture and alone through the fitting of a single parameter, namely, that connecting the cooperative length scale to the configurational entropy. This length scale resulted to be in the range of 1–2 nm for all systems. Finally, the out-of-equilibrium dynamics of toluene induced by the selective freezing in of PCB54 has been examined and its relation to the Johari–Goldstein relaxation of pure toluene was critically discussed. [ABSTRACT FROM AUTHOR]

We have studied the influence of plasticization on the microscopic dynamics of a glass-forming polymer. For this purpose we studied polyvinylchloride (PVC) with and without the commercially used plasticizer dioctylphthalate (DOP). We used dielectric spectroscopy and inelastic neutron scattering employing the neutron spin echo (NSE) technique. For both kinds of spectra the α relaxation could be consistently described by a model involving a distribution of individual relaxations of the Kohlrausch type. In contrast to earlier studies it turned out that an asymmetric distribution is necessary to fit the data at the lower temperatures investigated here. The shape parameters of the distribution (width, skewness) for PVC and PVC/DOP turned out to coincide when the characteristic relaxation times were the same. This means that the plasticizer only induces a remapping of the temperature dependence of the α relaxation. Comparison of NSE spectra S(Q,t)/S(Q) at different scattering vectors Q gave the result that the slowing down at the structure factor peak Qmax is surprisingly small for PVC while it is in the normal range for PVC/DOP. [ABSTRACT FROM AUTHOR]

We have investigated the dynamics of poly(vinly acetate) using broadband dielectric spectroscopy (DS) covering over 14 decades in frequency up to 20 GHz and high-resolution quasielastic neutron-scattering (QENS) technique. The dielectric results have been interpreted in terms of the phenomenological Kohlrausch–Williams–Watts [G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970); F. Alvarez, A. Alegría, and J. Colmenero, Phys. Rev. B 47, 125 (1993)] description. Because of the wide frequency range covered by DS, it provides a precise determination of dynamics over a wide temperature range and it revealed a crossover in polymer dynamics at 387 K through different dielectric parameters, viz., characteristic times, asymmetric shape parameter, and dielectric strength. Moreover, shape parameter is found to be higher in comparison to other typical polymeric systems. The characteristic relaxation times observed by QENS displayed an anomalous dependence of momentum transfer, indicating the possible existence of heterogeneities in the system even at the high temperatures. In addition, spin-lattice relaxation times, T1, were found to be decoupled from dielectric characteristic times. Based on these results, a model was proposed to account for heterogeneities where we consider coexistence of different regions with standard polymeric behavior but with different characteristic times, leading to a distribution of relaxation times. The model is found to account for the anomalous behavior and an inherent shape parameter is found to account for the shape of α relaxation. This model is also found to predict the T variation of T1 characteristic time scales at all temperatures. The origin of the heterogeneous domains is believed to lie in the microstructure of polymer chains. [ABSTRACT FROM AUTHOR]

Searching for the ideal glass transition, we exploit the ability of glassy polymer films to explore low energy states in remarkably short time scales. We use 30 nm thick polystyrene (PS) films, which in the supercooled state basically display the bulk polymer equilibrium thermodynamics and dynamics. We show that in the glassy state, this system exhibits two mechanisms of equilibrium recovery. The faster one, active well below the kinetic glass transition temperature (Tg), allows massive enthalpy recovery. This implies that the ‘fictive’ temperature (Tf) reaches values as low as the predicted Kauzmann temperature (TK) for PS. Once the thermodynamic state corresponding to Tf = TK is reached, no further decrease of enthalpy is observed. This is interpreted as a signature of the ideal glass transition. [ABSTRACT FROM AUTHOR]

The folding/collapse of linear single chains to single-chain nanoparticles (SCNPs) is a field of intense activity. Current synthetic methods allow obtaining from sparse SCNP morphologies in solution - resembling those observed in intrinsically disordered proteins - to compact SCNP conformations, more akin to those displayed by globular enzymes. A useful characterization technique in the field of SCNPs is 'conventional' size-exclusion chromatography (SEC) with differential refractive index (DRI) detection, providing rapid and convincing evidence of SCNP formation through comparison of the SEC traces of the precursor and the SCNPs. Upon SCNP formation, a noticeable shift of the SEC curve toward longer retention times (i.e., lower hydrodynamic sizes) is observed. However, quantitative information embedded in the SEC curve about the actual 'compaction degree' of the SCNPs has not yet been extracted to distinguish between sparse and globular SCNPs. Here, a useful methodology for quantifying the compaction degree of SCNPs by SEC with conventional DRI detector is introduced allowing: i) construction of theoretical SEC curves corresponding to the collapse of a linear precursor having a log-normal molecular weight distribution to either sparse or globular SCNPs and ii) analysis of experimental SEC curves to estimate the actual morphology of synthetic SCNPs in solution. [ABSTRACT FROM AUTHOR]

Macroscopic volume as well as capacitive dilatometry (CD) measurements have been performed on polystyrene/silica nanocomposites during the course of physical aging below the glass transition temperature ( Tg). Our results show that the macroscopic volume recovery during physical aging is not affected by the presence of nanofillers, whereas the CD measurements, delivering also information on the polymer matrix density, show acceleration of the recovery with increasing the silica content. Hence, the main outcome of the present work is that the evolution of macroscopic and matrix densities are markedly different in polymer nanocomposites. We interpret these results invoking an equilibration mechanism based on volume holes diffusion. According to this model, excess free volume migration at the polymer/filler interface only modifies the matrix density, thereby explaining the faster recovery detected by CD measurements in comparison to the macroscopic volume one. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys., 2013 [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical-like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1332-1338, 2011 [ABSTRACT FROM AUTHOR]

The reliability of tripeptide glutathione as an excellent model for protein–water interactions is tested by means of broadband dielectric spectroscopy. Measurements performed on aqueous solutions with different water contents show a surprisingly rich relaxation map that strongly resembles those observed for more complex protein macromolecules. At variance with what is normally observed for solutions of hydrophilic compounds with similar molecular weights, the presence of at least two water-related processes is detected. The faster one is symmetric, has an Arrhenius temperature dependence with an activation energy EA= 0.45 ± 0.05 eV and is attributed to water dipole reorientation. The slower one undergoes a clear dynamical change from a non-Arrhenius to an Arrhenius temperature dependence when crossing the calorimetric glass transition temperature of the solution from high to low values. This last process is proposed to be due to the dynamics of strongly-hydrated glutathione components, such as carboxyl or aminic groups. [ABSTRACT FROM AUTHOR]

The dielectric relaxation of cis-1,4 Polyisoprene [PI] is sensitive not only to the local and segmental dynamics but also to the larger scale chain (end-to-end) fluctuations. We have performed a careful dielectric investigation on linear PI with various molecular weights in the range of 1 to 320 kg/mol. The broadband dielectric spectra of all samples were measured isothermally at the same temperature to avoid utilizing shift factors. For the low and medium molecular weight range, the comparisons were performed at 250 K to access both the segmental relaxation and normal mode peaks inside the available frequency window (1 mHz–10 MHz). In this way, we were able to observe simultaneously the effect of molecular mass on the segmental dynamics—related with the glass transition process—and on the end-to-end relaxation time of PI and thus decouple the direct effect of molecular weight on the normal mode from that due to the effect on the monomeric friction coefficient. The latter effect is significant for low molecular weight ( Mw < 33 kg/mol), i.e., in the range where the crossover from Rouse dynamics to entanglement limited flow occurs. Despite the conductivity contribution at low frequency, careful experiments allowed us to access to the normal mode signal for molecular weights as high as Mw = 320 kg/mol, i.e., into the range of high molecular weights where the pure reptation behavior could be valid, at least for the description of the slowest chain modes. The comparison between the dielectric relaxations of PI samples with medium and high molecular weight was performed at 320 K. We found two crossovers in the molecular weight dependence of the longest relaxation time, the first around a molecular weight of 6.5 ± 0.5 kg/mol corresponding to the end of the Rouse regime and the second around 75 ± 10 kg/mol. Above this latter value, we find a power law compatible with exponent 3 as predicted by the De Gennes theory. [ABSTRACT FROM AUTHOR]

The dynamics of n-propanol confined in regular MCM-41matrix with the pore size Dpore = 40 Å, under various matrix conditioning and sample confining conditions, using broadband dielectric spectroscopy (BDS), is reported. First, various drying procedures with the capacitor filling under air or N2 influence the BDS spectra of the empty MCM-41 and the confined n-PrOH/MCM-41 systems, but have a little effect on the maximum relaxation time of the main process. Finally, various filling factors of n-PrOHmedium in the optimally treated MCM-41system lead to unimodal or bimodal spectra interpreted in terms of the two distinct dynamic phases in the confined states. [ABSTRACT FROM AUTHOR]

Access to completely deuterated single‐chain nanoparticles (dSCNPs) has remained an unresolved issue. Herein, the first facile and efficient procedure to produce dSCNPs is reported, which comprises: i) the use of commercially available perdeuterated cyclic ether monomers as starting reagents, ii) a ring‐opening copolymerization process performed in bulk to produce a neat dSCNP precursor, iii) a standard azidation reaction to decorate this precursor with azide moieties, and iv) a facile intramolecular azide photodecomposition step carried out under UV irradiation at high dilution providing with highly valuable, completely deuterated soft nano‐objects from the precursor. dSCNPs are used to investigate by means of neutron‐scattering measurements the form factor (radius of gyration, scaling exponent) of polyethylene oxide (PEO) chains in nanocomposites with different amounts of dSCNPs. Moreover, to illustrate the possibilities offered by the synthetic route disclosed in this communication for potential applications, the significant reduction in viscosity observed in a pure melt of polyether‐based single‐chain nanoparticles when compared to a melt of the corresponding linear polymer chains is shown. [ABSTRACT FROM AUTHOR]