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International audience; The kinetic fragility of a glass-forming liquid is an important parameter to describe its molecular mobility. In most polymers, the kinetic fragility index obtained from the glassy state by thermally stimulated depolarization current is lower than the one determined in the liquid-like state by dielectric relaxation spectroscopy, as shown in this work for neat polylactide (PLA). When PLA is plasticized to different extents, the fragility calculated in the liquid-like state progressively decreases, until approaching the value of fragility calculated from the glass, which on the other hand remains constant with plasticization. Using the cooperative rearranging region (CRR) concept, it is shown that the decrease of the fragility in the liquid-like state is concomitant with a decrease of the cooperativity length. By splitting the fragility calculated in the liquid, in two contributions: volume and energetic, respectively, dependent and independent on cooperativity, we observed that the slope of the fragility plot in the glass is equivalent to the energetic contribution of the fragility in the liquid. It is then deduced that the difference between the slopes of the relaxation time dependence calculated in both glass and liquid is an indicator of the cooperative character of the segmental relaxation when transiting from liquid to glass. As the main structural consequence of plasticization lies in the decrease of interchain weak bonds, it is assumed that these bonds drive the size of the CRR. In contrast, the dynamics in the glass are independent on plasticization structural effects.

Microgel suspensions of an interpenetrated Polymer Network (IPN) of PNIPAM and PAAc in D2O have been investigated through dynamic light scattering as a function of temperature, pH and concentration across the Volume Phase Transition (VPT). The dynamics of the system is slowed down under H/D isotopic substitution due to different balance states between polymer/polymer and polymer/solvent interactions suggesting the crucial role played by H-bonding. The swelling behavior, reduced with respect to PNIPAM and water, has been described by the Flory-Rehner theory, tested for PNIPAM microgel and successfully expanded to higher order for IPN microgels. Moreover the concentration dependence of the relaxation time at neutral pH has highlighted two different routes to approach the glass transition: Arrhenius and super-Arrhenius (Vogel Fulcher Tammann) respectively below and above the VPT and a fragility plot has been derived. Fragility can be tuned by changing temperature: across the VPT particles undergo a transition from soft-strong to stiff-fragile.

The temperature dependence of the optical non-linear response of dye molecules dissolved in a glass-forming liquid over a temperature range that includes the glass transition is investigated. Cooling down to temperatures below the glass transition dramatically slows the diffusive motion of the solvent molecules, while the bath retains its strong amorphous character. The results of ultrafast experiments such as echo-peak shift and heterodyne-detected echo are presented. The temperature dependence of the optical response is discussed within the context of the multimode Brownian oscillator model that is commonly used to describe dynamics in liquids. Freezing out a large part of the bath fluctuations allows for testing of the physical interpretation associated with this model. A temperature-dependent spectral density is proposed that can describe the results at all temperatures over the explored temperature range. The temperature dependence follows readily from the physical processes that are associated with the various parts of the density of states, with the exception of the fastest modes. The temperature dependence of these modes is inspected closely and the spectral shape is reinterpreted. (C) 2007 Elsevier B.V. All rights reserved.

International audience; Dielectric loss spectra of pure quinaldine and tert-butylpyridine and their mixtures with tri-styrene are presented. The pure systems present an excess wing and no secondary peaks in the temperature interval from above to well below the glass transition. However, when mixed in low concentration with tri-styrene the excess wing is replaced by a distinct secondary peak. This distinct process can be identified as a Johari-Goldstein relaxation within the Coupling Model interpretation. In the frame of the Coupling Model the transition from the relaxation scenario with the excess wing to that with a distinct secondary peak is related to the increase of intermolecular constraints. In our case this increase of constraints is due to the low mobility component of the mixture (tri-styrene).

Qens/wins 2014 - 11th International Conference on Quasielastic Neutron Scattering and 6th International Workshop on Inelastic Neutron Spectrometers / editado por:Frick, B; Koza, MM; Boehm, M; Mutka, H Motivated by the proposition of a new theoretical ansatz [V.N. Novikov, K.S. Schweizer, A.P. Sokolov, J. Chem. Phys. 138, 164508 ( 2013)], we have revisited the question of the characterization of the collective response of polyisobutylene at intermediate length scales observed by neutron spin echo (NSE) experiments. The model, generalized for sublinear diffusion -as it is the case of glass-forming polymers- has been successfully applied by using the information on the total self-motions available from MD-simulations properly validated by direct comparison with experimental results. From the fits of the coherent NSE data, the collective time at Q -> 0 has been extracted that agrees very well with compiled results from different experimental techniques directly accessing such relaxation time. We show that a unique temperature dependence governs both, the Q. 0 and Q -> infinity asymptotic characteristic times. The generalized model also gives account for the modulation of the apparent activation energy of the collective times with the static structure factor. It mainly results from changes of the short-range order at inter-molecular length scales.

We report a study of the dynamic crossover in two different glass-forming materials: an adhesive hard-sphere (AHS) colloidal system and the molecular liquid o-terphenyl. Systems that present a fragile-to-strong dynamical crossover in their transport parameters. A combination of light scattering, viscosity and nuclear magnetic resonance (NMR) is used to study the dynamical arrest in these systems. In both systems the data analysis is made in the conceptual framework of the Mode Coupling Theory in its extended version. For the AHS colloid, which is characterized by clustering process, we give evidence that the evolution toward the arrested phase can be described by using both the temperature and the concentration as the order parameter. The light scattering studies conducted on the o-terphenyl confirm the suggestion of recent statistical mechanical approaches that such a molecular system remains ergodic also below the calorimetric glass-transition temperature.

The effect of nanoscale confinement on the dynamics of,I series of poly(n-octadecyl methacrylate)-b-poly(tert-butyl acrylate)-b-poly(n-octidecyl methacrylate) (pODMA-b-ptBA-b-pODMA) triblock copolymers, synthesized through atom transfer radical polymerization, has been investigated with dielectric spectroscopy as a function of temperature and pressure. The main effects of confinement were found in the triblocks with the more antisymmetric compositions when the radius of gyration of the confined block was comparable to the cylinder radius. It was found that the dynamics of the amorphous block accelerate upon confinement. Furthermore, confinement of the crystallizable block within cylindrical nanodomains results in the depression of the crystallization temperature. Macromolecules

Opposite effects on the width of the dispersion of the alpha-relaxation and the separation on the time scale of the Johari-Goldstein beta-relaxation from the alpha-relaxation are predicted for guest molecules dissolved in a host, depending on whether the glass transition temperature of the host is higher or lower than that of the guest. These predictions are tested by dielectric relaxation measurements on mixtures of two glass-forming liquids. The results are in agreement with the predictions.

Several remarkable dynamic properties of glass-forming materials have recently been discovered experimentally by the application of pressure. These properties have had a great impact on the research field of glass transition because they are general and fundamental, and not easy to explain. We review some of these experimental facts and show that they originate from the intermolecular interactions and many-body relaxation dynamics of the structural a-relaxation. While these properties are either not explained or not explainable by conventional theories and models, they can be rationalized by the coupling model.

"Glass transition is still an unsolved problem in condensed matter physics and chemistry. In this paper, we critically reexamine experimental data and theoretical interpretations of dynamic properties of various processes seen over a wide time range from picoseconds to laboratory time scales. In order of increasing time, the ubiquitous processes considered include (i) the dynamics of caged molecular units with motion confined within the anharmonic intermolecular potential and where no genuine relaxation has yet taken place; (ii) the onset of the Johari-Goldstein secondary relaxation involving rotation or translation of the entire molecular unit and causing the decay of the cages, to be followed by the cooperative and dynamically heterogeneous motions participated by increasing number of molecules or length scale; and (iii) the terminal primary alpha-relaxation with the maximum cooperative length-scale allowed by the intermolecular interaction and constraints of the glass former. Some general and important properties found in each of these processes are shown to be interrelated, indicating that the processes are connected, with one being the precursor of the other following it. Thus, a theory of glass transition is neither complete nor fundamental unless all of these processes and their inter-relations have been accounted. In addition to published data, new experimental data are reported here to provide a limited collection of critical experimental facts having an impact on current issues of glass transition research and serving as a guide for the construction of a complete and successful theory in the future."

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari-Goldstein process, originating by a local motion of the whole molecule. (c) 2007 Elsevier B.V. All rights reserved.

We investigated, by means of dielectric spectroscopy, the relaxation dynamics of glass forming binary mixtures composed by the quite rigid polar molecules tert-butylpyridine dissolved in the apolar solvent tristyrene. By changing the relative concentration of the components we observed a transition from a relaxation scenario with a structural process and an excess wing to that with a structural process and a well resolved secondary process. Another relaxation process, slower than the latter, was observed, well below T-g. Our detailed analysis evidenced that the secondary relaxation with shorter relaxation time can be identified as the Johari-Goldstein relaxation for all the mixtures, whereas the new relaxation process was attributed to a different type of motion of tert-butylpyridine needing a larger amount of free volume for the molecular rotation. (c) 2007 American Institute of Physics.

We report dielectric relaxation measurements of the mixture 25% w/w cyclohexane in decahydroisoquinoline. Cyclohexane is a nonpolar liquid that acts as an external parameter influencing relaxation dynamics of pure decahydroisoquinoline. Two different secondary relaxation processes dominate the relaxation dynamics below the glass transition temperature, as previously observed in the pure decahydroisoquinoline. Based on the coupling model analysis we identified the fastest secondary process as the genuine secondary Johari Goldstein process, reflecting the motion of the whole molecule. On the other hand, the microscopic origin of the slowest secondary process still remains unknown. (c) 2006 Elsevier B.V. All riehts reserved.

Relaxation times for different temperatures, T, and specific volumes, V, collapse to a master curve versus TV^g, with g a material constant. The isochoric fragility, m_V, is also a material constant, inversely correlated with g. From these we obtain a 3-parameter function, which fits accurately relaxation times of several glass-formers over the supercooled regime, without any divergence below Tg. Although the 3 parameters depend on the material, only g significant varies; thus, by normalizing material-specific quantities related to g, a universal power law for the dynamics is obtained., Comment: 12 pages, 4 figures

Broadband dielectric spectroscopy measurements were performed on glass-forming binary mixtures, composed of rigid polar molecules dissolved at low concentration in apolar viscous solvent (tristyrene). Dielectric spectra were dominated by the polar molecule contribution, so enabling the study of its dynamic behavior. A well resolved secondary relaxation, not attributable to internal degrees of freedom, was visible in both the liquid and glassy states: for its intermolecular nature, it can be called a 'genuine' Johari-Goldstein (JG) relaxation, in the sense that it is a local and non-cooperative process but entailing the motion of the molecule as a whole. Among our results, the following ones are noteworthy: (a) polar systems in the neat state showed an excess wing that became a well resolved JG-peak on mixing with the apolar solvent; (b) the time-scale distance between structural and JG loss peak increased with the apolar solvent fraction; (c) broader the structural loss peak was, larger was the separation in the frequency scale between structural and JG peak; (d) the JG relaxation time showed a non-Arrhenius temperature behavior above T, paralleling that of the structural relaxation time. All the results can be rationalized in the framework of Coupling Model. (c) 2006 Elsevier B.V. All rights reserved.

The dynamic miscibility of poly(4-vinylphenol)/poly(vinyl ethyl ether) (PVPh/PVEE) blends, with a T-g contrast of 186 K, as well as the PVEE segmental dynamics have been investigated by temperature- and pressure-dependent dielectric spectroscopy. In PVEE the pressure coefficient of T-g amounts to 0.215 KMPa, and its apparent activation volume displays the usual T dependence. Although both temperature and volume contribute to the segmental dynamics, the former has a stronger influence within the T and P investigated. In the blends, dynamic heterogeneity is suppressed because of hydrogen bonds that couple the components segmental dynamics. In the PVEE-rich blends, increasing temperature and pressure results in the broadening of the distribution of relaxation times through the weakening of hydrogen bonds and the associated decoupling of the segmental dynamics. A central result of the present study is the identification of a critical temperature above which the system becomes increasingly heterogeneous. Macromolecules

The dynamics of poly(2-vinylpyridine) (P2VP) have been studied as a function of temperature (in the range from 123 to 453 K), pressure (0.1-270 MPa), and molecular weight (1.1 x 10(3)-3.0 x 10(4) g/mol), using dielectric spectroscopy (DS) within the frequency range from 10(-2) to 10(6) Hz. Structural methods (wide-angle X-ray scattering) have been employed in parallel with thermodynamic methods (pressure-volume-temperature). Three relaxation processes were found: two above the glass temperature (T-g) associated with the segmental (alpha-) process and a slower process with an apparent activation volume comparable to the monomer volume and another well below T-g, with an Arrhenius T dependence (beta-process). The results from the dynamic study combined with the thermodynamic results revealed that both decreasing thermal energy and insufficient volume lead to glass formation at lower temperatures, with the former having the stronger effect at temperatures near T-g (i.e., values of the ratio of apparent activation energies at constant volume and constant pressure in the range Q(V)/Q(P) similar to 0.6-0.85 for the different temperatures and pressures investigated). Macromolecules

The dynamics of poly(methyl-p-tolyl-siloxane) (PMpTS) have been studied as a function of temperature (in the range from 143 to 413 K), pressure (0.1-300 MPa), frequency (10(-2)-10(6) Hz), and molecular weight. Independent pressure-volume-temperature (PVT) measurements (for temperatures in the range from 293 to 393 K and for pressures in the range from 10 to 200 MPa) allowed calculation of the relevant thermodynamic parameters. Two dielectrically active channels of relaxation were found, one in the glassy state reflecting a localized motion of the substituted phenyl ring and one at higher temperatures reflecting the usual segmental (alpha) relaxation. In PMpTS, there are two dominant control variables; both density and temperature have a strong influence on the segmental dynamics. The PVT results allowed us to follow distinct thermodynamic (T,P) paths resulting in states bearing the same density. These isodensity states are characterized by an apparent activation energy (Q(V)) that is not very different from the corresponding activation energy under isobaric conditions (Q(V)/Q(P)similar to0.55) reflecting the importance of thermal effects. At temperatures above the glass temperature (T(g)), strong orientation correlations exist above some critical pressure that depends on temperature. This state extends from T(g) up to 1.08 T(g) and separates a normal liquid at higher temperatures from an oriented liquid at lower temperatures. Using the "phase diagram" we discuss separately the influence of the temperature and density on the PMpTS dynamics. Physical Review E

The molecular dynamics of a side-chain polymer liquid crystal with a poly(methyl acrylate) backbone and a (p-alkoxy-phenyl)-benzoate mesogenic group have been studied in the unaligned state as a function of temperature and pressure using dielectric spectroscopy. Polarizing optical microscopy, differential scanning calorimetry, and pressure-volume-temperature $(\mathrm{PVT})$ measurements revealed three transition temperatures separating four phases (glass, smectic, nematic, and isotropic). Different dynamic processes have been identified reflecting librational modes (\ensuremath{\gamma} process), local relaxation of the mesogenic group (\ensuremath{\beta} process), the segmental mode (\ensuremath{\alpha} process) associated with the dynamic glass transition, and a slower process (\ensuremath{\delta} process) reflecting the side-chain dynamics within the liquid crystal order. Pressure exerts a stronger influence on the \ensuremath{\alpha} as compared to the \ensuremath{\delta} process. Starting from the nematic phase, pressure was found to induce the nematic-to-smectic transformation. The associated dynamic changes were in excellent agreement with the $\mathrm{PVT}$ results implying that the dynamics are directly coupled to the thermodynamic state. Pressure was found to enhance the stability of the smectic order within the P-T phase diagram.