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Macromolecular crowding influences protein mobility and stability in vivo. A precise description of the crowding effect on protein thermal stability requires the estimate of the combined effects of excluded volume, specific protein-environment interactions, as well as the thermal response of the crowders. Here, we explore an ideal model system, the lysozyme protein in powder state, to dissect the factors controlling the melting of the protein under extreme crowding. By deploying state-of-the art molecular simulations, supported by calorimetric experiments, we assess the role of the environment flexibility and of intermolecular electrostatic interactions. In particular, we show that the temperature-dependent flexibility of the macromolecular crowders, along with specific interactions, significantly alleviates the stabilizing contributions of the static volume effect.

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

According to the Coupling Model (CM), secondary relaxation belonging to a special class has strong connection to the α-relaxation. These Johari-Goldstein (JG) β-relaxations are predicted to be ubiquitous, and its relaxation time is approximately equal to the primitive relaxation time and the surface diffusion time. These predictions cannot be tested directly in ethylcyclohexane (ECH) because the JG β-relaxation is unresolved and overlapping the α-relaxation to modify its dispersion. To capture the actual dispersion of the α-relaxation we considered surface diffusion data, adiabatic calorimetry data of the JG β-relaxation, and dielectric data of cyanocyclohexane (CNCH). We demonstrated the dispersion of the α-relaxation is actually narrower than known before. The primitive relaxation times calculated are now in agreement with τJG(T) and with τsurface(T), and in accord with the CM predictions.

Cangialosi and co-workers found two steps in the enthalpy recovery deep in the glassy state of high and low molecular weight bulk polystyrene (PS). We attribute the first step to the segmental α-relaxation and the second step to the sub-Rouse modes and explain the two-step enthalpy recovery by the dynamic properties of the two processes in bulk PS by the coupling model (CM). The two-step enthalpy recovery was found in nanometer thin films of polystyrene (PS). On decreasing film thickness h, the first step was shifted to much lower temperature than the second step, and the effect is explained by the segmental α-relaxation shifting to lower temperature much more than the sub-Rouse modes, as predicted by the CM. Furthermore, the dependences of the faster and slower processes of the two-step enthalpy relaxation on film thickness are analogues of two effects observed on decreasing h of freestanding polystyrene films. One effect is the observation of two transitions by ellipsometry. We associated the lower tran...

Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature T near 136 K, not much known are the spectral dispersion of the structural alpha-relaxation and the temperature dependence of its relaxation time tau_alpha-bulk(T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (JG) beta-relaxation is a question that has not been answered. By studying the structural alpha-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures T close to 136 K, we deduce the properties of the alpha-relaxation and the temperature dependence of tau_alpha(T) of bulk water. The frequency dispersion of the alpha-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of tau_alpha(T) at low temperatures as well as at high temperatures from neutron scattering and GHz-THz dielectric relaxation, and hence, there is no fragile to strong transition. The T-scaled VFT temperature dependence of tau_alpha(T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG beta-relaxation in bulk water is supported by its equivalent relaxation observed in water confined in spaces with lengths of nanometer scale and having Arrhenius T-dependence of its relaxation times tau_conf(T). The equivalence is justified by the drastic reduction of cooperativity of the alpha-relaxation in nanoconfinement and rendering it to become the JG beta-relaxation. Thus, the tau_conf(T) from experiments can be taken as tauJG_bulk(T), the JG beta-relaxation time of bulk water. The ratio tau_alphaBulk(Tg)/tau_betaBulk(Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch alpha-correlation function, exp[-(t/tau_alphaBulk)^(1-n)], having (1-n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with T higher than that of water show the presence of a secondary nu-relaxation from the water component. The nu-relaxation is strongly connected to the alpha-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time tau_nu(T) is longer than tau_betaBulk(T), but tau_nu(T) becomes about the same as tau_betaBulk(T) at sufficiently high water content. However, tau_nu(T) does not become shorter than tau_betaBulk(T). Thus, tau_betaBulk(T) is the lower bound of tau_nu(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is tau_betaBulk(T) but not tau_alpha(T) that is responsible for the dynamic transition of hydrated globular proteins.

Secondary relaxations with properties closely related to the α-relaxation have fundamental importance in glass-forming substances including polymers. To distinguish these secondary relaxations from those involving intramolecular degrees of freedom, they are called the Johari–Goldstein (JG) β-relaxations. Acting as the precursor of the α-relaxation, the JG β-relaxation is supposedly ubiquitous in all glass-formers, a thesis supported by experiments on a variety of glass-formers. Notwithstanding, the JG β-relaxation has not been identified definitively in the hydroxyl-terminated polypropylene glycols (PPGs) with various molecular weights, despite these polymers have been intensively studied experimentally in the last several decades. The difficulty of finding the JG β-relaxation is due to the presence of a faster intramolecular γ-relaxation and a slower relaxation originating from residual water. This is demonstrated in two recent papers by Gainaru et al. Macromolecules 2010, 43, 1907, and Kaminski et al. M...

Mixtures of methyltetrahydrofuran (MTHF) with tristyrene and high molecular weight polystyrene involve an exceptionally large difference in the glass transition temperatures of the two components not realized in other binary mixtures studied before. The extensive study of the molecular dynamics of these mixtures by various experimental techniques by Blochowicz et al. has revealed the presence of a new α'-relaxation not found before in other mixtures and also the more familiar α- and β-relaxations, but their properties are more extreme. Attention was focused on the new α'-relaxation by Blochowicz et al. in interpreting it to originate from MTHF in confinement and explaining its properties by the Mode Coupling Theory. In a different direction, we concentrate on the highly unusual properties of the α- and β-relaxations. Earlier, we had success in explaining the properties of these two relaxations and their connection in other mixtures by the coupling model. In this paper, we apply the same model to explain the highly unusual dynamics of the α- and β-relaxations found in the mixtures of MTHF with tristyrene and polystyrene. Possible relation between the α'- and the β-relaxations also is explored.

One fundamental challenge in biophysics is to understand the connection between protein dynamics and its function. Part of the difficulty arises from the fact that proteins often present local atomic motions and collective dynamics on the same time scales, and challenge the experimental identification and quantification of different dynamic modes. Here, by taking lyophilized proteins as the example, we combined deuteration technique and neutron scattering to separate and characterize the self-motion of hydrogen and the collective interatomic motion of heavy atoms (C, O, N) in proteins on the pico-to-nanosecond time scales. We found that hydrogen atoms present an instrument-resolution-dependent onset for anharmonic motions, which can be ascribed to the thermal activation of local side-group motions. However, the protein heavy atoms exhibit an instrument-resolution-independent anharmonicity around 200 K, which results from unfreezing of the relaxation of the protein structures on the laboratory equilibrium time (100-1000 s), softening of the entire bio-macromolecules.

In a series of papers on binary glass-forming mixtures of tripropyl phosphate (TPP) with polystyrene (PS), Kahlau et al. [J. Chem. Phys. 140, 044509 (2014)] and Bock et al. [J. Chem. Phys. 139, 064508 (2013); J. Chem. Phys. 140, 094505 (2014); and J. Non-Cryst. Solids 407, 88-97 (2015)] presented the data on the dynamics of the two components studied over the entire composition range by several experimental methods. From these sets of data, obtained by multiple experimental techniques on mixtures with a large difference ΔTg ≈ 200 K between the glass transition temperatures of two starting glass formers, they obtained two α-relaxations, α1 and α2. The temperature dependence of the slower α1 is Vogel-Fulcher like, but the faster α2 is Arrhenius. We have re-examined their data and show that their α2-relaxation is the Johari-Goldstein (JG) β-relaxation with Arrhenius T-dependence admixed with a true α2-relaxation having a stronger temperature dependence. In support of our interpretation of their data, we made dielectric measurements at elevated pressures P to show that the ratio of the α1 and α2 relaxation times, τα1(T,P)/τα2(T,P), is invariant to variations of T and P, while τα1(T,P) is kept constant. This property proves unequivocally that the α2-relaxation is the JG β-relaxation, the precursor of the α1-relaxation. Subsequently, the true but unresolved α2-relaxation is recovered, and its relaxation times with much stronger temperature dependence are deduced, as expected for the α-relaxation of the TPP component. The results are fully compatible with those found in another binary mixture of methyltetrahydrofuran with tristyrene and PS with ΔTg ≈ 283 K, even larger than ΔTg ≈ 200 K of the mixture of TPP with PS, and in several polymer blends. The contrast between the two very different interpretations brought out in this paper is deemed beneficial for further progress in this research area.

When the thickness is reduced to nanometer scale, freestanding high molecular weight polymer thin films undergo large reduction of degree of cooperativity and coupling parameter n in the Coupling Model (CM). The finite-size effect together with the surfaces with high mobility make the a-relaxation time of the polymer in nanoconfinement, tau(nano)(alpha)(T), much shorter than tau(bulk)(alpha)(T) in the bulk. The consequence is avoidance of vitrification at and below the bulk glass transition temperature, T-g(bulk), on cooling, and the freestanding polymer thin film remains at thermodynamic equilibrium at temperatures below T-g(bulk). Molecular dynamics simulations have shown that the specific volume of the freestanding film is the same as the bulk glass-former at equilibrium at the same temperatures. Extreme nanoconfinement renders total or almost total removal of cooperativity of the alpha-relaxation, and tau(nano)(alpha)(T) becomes the same or almost the same as the JG beta-relaxation time tau(bulk)(beta)(T) of the bulk glass-former at equilibrium and at temperatures below T-g(bulk). Taking advantage of being able to obtain tau(bulk)(beta)(T) at equilibrium density below T-g(bulk) by extreme nanoconfinement of the freestanding films, and using the CM relation between tau(bulk)(alpha)(T) and tau(bulk)(beta)(T), we conclude that the Vogel-Fulcher-Tammann-Hesse (VFTH) dependence of tau(bulk)(alpha)(T) cannot hold for glass-formers in equilibrium at temperatures significantly below T-g(bulk). In addition, tau(bulk)(alpha)(T) does not diverge at the Vogel temperature, T-0, as suggested by the VFTH-dependence and predicted by some theories of glass transition. Instead, tau(bulk)(alpha)(T) of the glass-former at equilibrium has a much weaker temperature dependence than the VFTH-dependence at temperature below T-g(bulk) and even below T-0. This conclusion from our analysis is consistent with the temperature dependence of tau(bulk)(alpha)(T) found experimentally in polymers aged long enough time to attain the equilibrium state at various temperatures below T-g(bulk).

The research of dynamics of protein is predominantly focused on the hydrated or solvated state, which is close to the physiological condition of biological relevance. Eclipsed by the hydrated and solvated counterparts, the dynamics of dry proteins are seldom studied in earnest. Studied occasionally, the data of dry proteins were not addressed, possibly due to the preconceived notion that dry proteins are bio inactive, and thus are uninteresting and unimportant. Actually lyophilized dry proteins are important for the pharmaceutical industry as the therapeutic proteins are often stored in lyophilized conditions to achieve long-term stability, which is of direct connection to the dynamics of protein in dry state. In order to uncover the intrinsic dynamics of dry proteins that previous studies overlooked or ignored, we analysed quasielastic neutron scattering, Mossbauer, and Brillouin light scattering data of various dry proteins. We demonstrate the existence of a novel transition of the dry protein dynamics at temperatures around 200 K, with the exact value dependent on the kind of protein. The transition temperature of dry proteins is found here independent of the time/frequency of the measurement, in stark contrast to the well-documented dynamical transition observed in hydrated proteins, the temperature of which is strongly depending on the time window probed experimentally. In addition we point out that this transition in dry proteins is analogous to those observed before in the glassy state of various glass-forming materials, the origin of which is the change of the caged molecule dynamics in response to the change of the temperature dependence of density when crossing the secondary glass transition temperature Tgβ for the Johari-Goldstein β-relaxation. We show that the transition of various dry proteins also occurs around Tgβ, thus suggesting that the underlying microscopic mechanism is the same as the one identified and explained before for many glass-formers of different types. The presence of the transition is independent of the trace amount of water in supposedly dry proteins, although the value of Tgβ can vary somewhat. The novel transition in dry proteins has impact on bio-pharmaceutical research and applications because of the connection to the Johari-Goldstein β-relaxation, which can affect long-term stability of lyophilized dry proteins.

The dynamic transition found by Mossbauer spectroscopy and neutron scattering in hydrated and solvated proteins has been an active research area for the past three decades. By now a consensus among some researchers has been reached that it originates exclusively from relaxation of the hydration water (HW) coupled to the protein. The dynamic transition temperature Td depends on energy resolution of the spectrometer and is higher than the glass transition temperature Tg. Recently demonstrated is the presence of yet another transition at Tg, which is independent of the resolution of the spectrometer and coexists with the dynamic transition at a higher temperature Td. The transition at Tg is similar to that found in various kinds of glass-formers by neutron and dynamic light scattering at short times when molecules are mutually caged via the intermolecular potential. Like in the case of conventional glass-formers, the transition at Tg of hydrated proteins has been explained by the sensitivity of the extent of the caged dynamics to change of specific volume and entropy on crossing Tg. The caged dynamics are terminated by the onset of relaxation of HW, which in turn gives rise to the dynamic transition at Td > Tg. Despite these important roles played by the caged dynamics and the HW relaxation in the observed dual transitions of the hydrated proteins, their exact nature is still unclear. In this paper we clarify their nature in hydrated proteins by use of various experimental data, with the assist of the results from studies of mixtures of water with hydrophilic solutes, taking advantage of the fact that the properties are similar in both systems.

The effect of confinement on structural relaxation in ultrathin poly(vinyl acetate) films has been studied by local dielectric spectroscopy. This scanning probe method allows the investigation of dielectric relaxation at nanometer scale of supported ultrathin films having a free surface. Measurements have been performed at ambient pressure and controlled atmosphere on films with decreasing thickness. A deviation of dynamic properties from the bulk behavior, showing up as an increase of the relaxation rate, was observed starting from film thickness of 35 nm, which corresponds to about 3 times the gyration radius of polymer chains. A 2-fold increase of relaxation rate was measured for the thinnest investigated film of 18 nm. Local dielectric spectroscopy is therefore an effective method to elucidate confinement effects on relaxation dynamics in ultrathin polymer films with a free upper surface.

Orientationally disordered (OD) mixed crystals formed between cyanoadamantane (CN-A) and chloroadamantane (Cl-A) have been thermodynamically characterized by means of thermal analysis and X-ray diffraction. The dynamics of these OD mixed crystals has been studied by means of broad band dielectric spectroscopy. Results evidence that dynamics of the α-relaxation process associated with the overall molecular tumbling slows down when molecules of CN-A are substituted by molecules of Cl-A.

In this Letter, we present results of dielectric measurements of water-propylene glycol oligomer mixtures at ambient and high pressure. The ? relaxation process that emerges with the addition of water exhibits unexpected behavior at elevated pressure. These features indicate local motions of water molecules as the origin of this relaxation process. In addition, we observe a pressure-induced change of the dynamics in the water-related relaxation; such a crossover occurs near the glass transition of the mixture and significantly differs from properties observed for confined water.

Our work focuses on determining the effect of crowded environment and different solvents on the thermal stability of the protein Lysozyme [1] placed in a dilute water solution, dehydrated protein powder, and in a protein powder glycerol solution, the latter two representing crowded environments.The ultimate goal of our work is to probe the validity of the Lindemann criterion for protein melting [2, 3]. We employ an enhanced sampling Molecular Dynamics technique, REST2 [4, 5], where mutually exchangeable protein replicas are simulated at different effective temperatures, achieved by rescaling the force-field potential energy terms. The simulations are paralleled with Elastic Incoherent Neutron Scattering experiments.We first estimate the in silico melting temperature of our systems and reconstruct the stability curves. Instructed by this information, we calculate the scaling of atomic fluctuation approaching melting. Our results show that the atomic fluctuations of different Lysozyme systems converge to similar values approaching the melting temperature, which agrees both with the experimental results [unpublished] and the Lindemann criterion [2, 3]. Furthermore, we show that the thermal stability of the hydrated Lysozyme is increased in the presence of crowders (powder) and even more so if glycerol is present in addition to the crowders. The molecular factors, excluded volume versus specific interactions, will be discussed as source of the stability shifts.[1] H.-X. Zhou, G. Rivas, A.P. Minton, Annu. Rev. Biophys. (2008) 37, 375-397.[2] C. Chakravarty, P.G. Debenedetti, F.H. Stillinger, J. Chem. Phys. (2007) 126, 204508.[3] Y. Zhou, D. Vitkup, M. Karplus, J. Mol. Biol. (1999) 285, 1371-1375[4] L. Wang, R.A. Friesner, B.J. Berne, J. Phys. Chem. B (2011) 115, 9431-9438.[5] G. Stirnemann and F. Sterpone, submitted to J. Chem. Theory Comput. (2015).

The freezing of the cooperative reorientational motions in orientationally disordered (OD) molecular crystals marks the so-called \glassy" transition, which may be considered a lower-dimensional version of the structural glass transition. While structural glasses display both positional and orientational disorder, in fact, in orientational glasses the disorder involves exclusively the orientational degrees of freedom of the constituent molecules, while the molecular centres of mass form an ordered lattice. We report here on a glass-forming system with even less degrees of freedom, namely the OD phase of a dipolar benzene derivative, pentachloronitrobenzene (C6Cl5NO2). We probe the orientational dynamics of PCNB as a function of temperature and pressure by means of dielectric spectroscopy at normal and high pressure and high-pressure density measurements, and show that the system exhibits a double primary relaxation feature associated with two distinct motions of the molecular dipole moment. After ruling out an interpretation in terms of primitive or intramolecular relaxations, we discuss an assignment of the double relaxation feature based on the material's anisotropy and on the comparison with discotic liquid crystals.

The presence of sub-Rouse modes in bulk polymers with length scale and relaxation times in between the segmental α-relaxation and the Rouse modes had long been justified by theoretical consideration, and found in many experimental studies. The sub-Rouse modes had been seen directly in creep compliance measurements of polymer thin films by McKenna and co-workers. On decreasing film thickness, the sub-Rouse modes shift to shorter times like the segmental α-relaxation, but the shift of the former is less than the latter. We had used the sub-Rouse modes and the segmental mode to explain the two transitions found by ellipsometry in freestanding high molecular weight PS films by Pye and Roth (PR). The upper transition at a higher temperature originates from the sub-Rouse modes, and the lower transition comes from the segmental α-relaxation. On the other hand, PR suggested that the upper and the lower transitions both came from the segmental α-relaxation, and the upper transition occurs in ~90% of the material. In this paper we use dielectric relaxation data of freestanding films to rule out their suggestion. Furthermore, we demonstrate by experimental evidences that the sub-Rouse modes are coupled to density, and respond to physical aging to validate our interpretation.

Three recently published papers have discovered a general property of the fast caged dynamics observed in the glassy states of polyalcohols (S. Capaccioli et al., J. Phys. Chem. B 119, 8800 (2015)), amorphous polymers (K. L. Ngai et al., J. Phys. Chem. B 119, 12502 (2015)), and van der Waals molecular glass-formers (K. L. Ngai et al., J. Phys. Chem. B 119, 12519 (2015)). The fast caged dynamics are manifested as nearly constant loss (NCL) in dielectric and mechanical susceptibility. Shown before in these papers is the intensity of the caged dynamics change temperature dependence at a temperature THF nearly coincident with the secondary glass transition temperature Tgβ, at which the Johari-Goldstein (JG) β-relaxation time τJG reaches ∼103 s. Evidently this finding indicates the coupling of the caged dynamics to the secondary glass transition. The glass-formers considered so far are all soft matters. However, the generality of the phenomenon and its explanation implies that the relation, THF ≈ Tgβ, should b...

The complex dynamics of a nearly alternating copolymer of vinylidene cyanide (1,1-dicyanoethylene, VCN) with 2,2,2-trifluoroethyl methacrylate (TFEMA), including two α-relaxations with diverging time scale in the glass transition temperature range, was thoroughly characterized by dielectric spectroscopy over wide temperature and frequency ranges and analyzed in the frame of the Ngai’s coupling model. The dielectric relaxation strength as well as the glass transition temperature, the temperature dependence of the α-relaxation time, and the corresponding distribution of relaxation times were all larger than those of a reference TFEMA homopolymer, as expected from the introduction of the stiffening VCN units all along the macromolecular chain. The effect of casting solvent and applied poling electric field on the copolymer dielectric strength suggests the onset of local orientational order involving the strong dipoles in the VCN units, a requirement for piezo- and pyroelectricity in amorphous polymers.

Most studies of the dynamics at the surface and in thin films of polystyrene (PS) are focused on the change of glass transition temperature from the bulk value. In this perspective, we highlight three new developments in research on the dynamics of PS in high molecular weight (MW) freestanding PS thin films and at the surface of low MW PS. Novel findings from these developments require consideration of viscoelastic mechanisms with length scales longer than the segmental α-relaxation. The first development is the creep compliance measurements of high-MW PS thin films, probing not only the segmental α-relaxation, but also the polymer chain modes at higher compliance levels, including the sub-Rouse modes and the Rouse modes. The compliance data indicate the relaxation times of the sub-Rouse modes are reduced in thinner films like that of the segmental α-relaxation but to a much less extent. The second development is the novel observation of two glass transitions in freestanding polystyrene thin films by elli...

Molecular dynamics of phenylbutazone in the supercooled liquid and glassy state is studied using broadband dielectric spectroscopy for test frequencies 1 kHz, 10 kHz and 100 kHz over a wide temperature range. Above the glass transition temperature Tg, the presence of the structural α-relaxation peak was observed which shifts towards lower frequencies as the temperature decreases and kinetically freezes at Tg. Besides the structural α-relaxation peak, a β-process which arises due to the localized molecular fluctuations is observed at lower temperature.

We investigated the microscopic origin of the excess wing through isothermal and isobaric dielectric relaxation measurements for the Quinaldine/tristyrene mixture. Our results show that the excess wing, characteristic of the high frequency side of the structural loss peak in neat Quinaldine, becomes a well resolved Johari-Goldstein secondary relaxation on mixing with the apolar tristyrene. Analyzing the temperature and pressure behavior of the two processes, a clear correlation has been found between the structural relaxation time, the Johari-Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). These results support the idea that the Johari-Goldstein relaxation acts as a precursor of the structural relaxation and therefore of the glass transition phenomenon. (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).

Most glass-forming systems are composed of basic units interacting with each other with a nontrivial anharmonic potential. Naturally, relaxation and diffusion in glass formers is a many-body problem. Results from recent experimental studies are presented to show the effects of many-body relaxation and diffusion manifested on the dynamic properties of glass formers. Considering that the effects are general and critical, the problem of glass transition will not be solved until the many-body nature of the relaxation process has been incorporated fundamentally into any theory.

The different nature of multiple secondary (β- and γ-) relaxations for the glass-formers dipropyleneglycol-dibenzoate and benzoin-isobutylether has been investigated by varying temperature and pressure in the equilibrium liquid and glassy states, as well as by monitoring the structural recovery after a rapid quenching of the liquid to form a glass. For both systems the behavior of the slower β-relaxation mimicked that of the α-relaxation: its time scale was strongly affected by pressure variation and its behavior below T g was sensitive to thermal history and to aging. On the contrary the γ-relaxation time was negligibly dependent on pressure and on aging. The correlation between the dynamic properties of the α-process and those of the slower β-process indicates that the latter was likely the genuine Johari–Goldstein (JG) intermolecular relaxation. Moreover, a good agreement was found between the β-relaxation time and the independent relaxation time τ 0 of the coupling model (CM), leading to predictive quantitative relations between the JG and the structural relaxation, supported also by the experimental data of many other systems, in both liquid and glassy state.

We analyse the slowing down of the structural relaxation dynamics of polymers in terms of the Adam and Gibbs theory. We consider a previously derived general relation between the configurational and the excess entropy, which was used to derive an analytical equation for the dependence of the structural relaxation time from the pressure and temperature. The model proved to successfully fit the relaxation dynamics of poly(methyl methacrylate), poly(propylene glycol) and poly(propylene glycol dimethylether), of different molecular weights, over a wide region of temperature and pressure values above the glass transition.

The dielectric relaxation experimental data on the α- and Johari−Goldstein β-relaxation dynamics of sorbitol in the neat state and in mixtures with glycerol published by Duvvuri and Richert (J. Phy...

Dielectric relaxation times τ, over a broad range of temperatures and pressures for simple molecular glass-forming systems reveal good agreement with the pressure-extended Adam–Gibbs equation derived from the Adam–Gibbs model, relating directly the dynamics to the thermodynamics through the configurational entropy S c, assuming it to be proportional to the excess entropy S exc of the melt with respect to the crystal. In the present study by making use of both calorimetric and expansivity measurements it was possible to make an accurate determination of the pressure and temperature dependences of S exc over the whole investigated range, and the expected proportionality between S c and S exc was experimentally checked. Results show that different proportionality factors g P and g T exist between S c and S exc in isothermal and isobaric conditions and the ratio g T/g P of these is nearly 0.7 for all the systems.

The d.c. conductivity σdc and the a.c. electric response of poly(3-decylpyrrole) prepared with different dopants and synthesis conditions were studied over a broad temperature (75–300 K) and frequency (100 Hz–40 MHz) range, both in bulk state (films) and in solution. Concerning films, the temperature dependence of σdc followed Mott's law, with the hopping parameters strongly doping dependent. A.c. conductivity σac was strictly coupled to d.c. transport, so that a master curve resulted from a plot of normalised complex permittivity versus a critical frequency proportional to σdc. Similar results to the bulk case were observed for non-dilute solutions, whereas for very dilute solutions σdc scaled with temperature as the reciprocal of solvent viscosity. Moreover, a steeper bilogarithmic slope of σac versus frequency was found. Inter-chain charge transport in dilute solutions can be attributed to dissociated counterions diffusing through the solvent, whereas intra-chain hopping can eventually contribute only at high frequencies. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using broadband dielectric spectroscopy the molecular mobility of dibucaine is investigated in the supercooled liquid and gassy states, over a wide temperature range for some test frequencies. Above the glass transition temperature Tg, the presence of structural α- relaxation peak was observed due to the cooperative motions of the molecule and upon cooling frozen kinetically to form the glass. The secondary relaxation process was perceivable below Tg due to localized motions. The peak loss frequency of α-relaxation process shows non-Arrhenius behavior and obeys Vogel-Fulcher-Tammann equation over the measured temperature range whereas the β- process shows Arrhenius behavior.

The vibrational dynamics of protein hydration water has been studied by incoherent neutron scattering. Experiments on a sample of fully deuterated maltose binding protein allowed us to single out the hydration water susceptibility. The main inelastic features, corresponding to hydrogen-bond bending, hydrogen-bond stretching and librational excitations, have been followed over a temperature range extending from 50 to 300 K. It turns out that the temperature dependence of the hydrogen-bond stretching contribution is quite similar to that of the mean square displacements deduced by the quasielastic signal, thus suggesting a close relationship between the anharmonicity of longitudinal phonon-like motions and the onset of diffusive molecular dynamics. On the other hand, both hydrogen-bond bending and librational excitations show a temperature dependence consistent with a harmonic character over the whole temperature range.

Secondary relaxations have been classified into two types, depending on whether they are related to the structural α-relaxation in properties or not. Those secondary relaxations that are related to the α-relaxation may have fundamental importance, and are called the Johari–Goldstein (JG) β-relaxations. Two polar molecular glass-formers, one flexible and another rigid, dissolved in apolar host with higher glass transition temperature are studied by broadband dielectric spectroscopy at ambient and elevated pressure. The neat flexible glass-former diethylphthalate (DEP) has a resolved secondary relaxation which, unlike the α-relaxation, is insensitive to pressure and hence is not the JG β-relaxation. In the solution, the JG β-relaxation of DEP shows up in experiment and its relaxation time τ β is pressure and temperature dependent like τ α . The result supports the universal presence of the JG β-relaxation in all glass-formers, and the separation between τ α and τ β is determined by intermolecular interaction. The rigid glass-former is cyano-benzene (CNBz) and its secondary relaxation involves the entire molecule is necessarily the JG β-relaxation. The dielectric relaxation spectra obtained at a number of combinations of pressure and temperature at constant τ α show not only unchanged is the frequency dispersion of the α-relaxation but also τ β . The remarkable results indicate that the JG β-relaxation bears a strong connection to the α-relaxation, and the two relaxations are inseparable when considering the dynamics of glass-forming systems. Experimentally, τ α has been found to be a function of the product variables, T / ρ γ , where ρ is the density and γ is a material constant. From the \invariance of the ratio, τ α /τ β , to change of thermodynamic conditions seen in our experiment as well in other systems, it follows that τ β is also a function of T / ρ γ , with the same γ at least approximately. Since the JG β-relaxation is the precursor of the α-relaxation, causality implies that the T / ρ γ -dependence originates from the JG β-relaxation and is passed on to the α-relaxation.

Polymer chains can adsorb onto a solid substrate without the formation of chemical bonds. Because this mechanism of adsorption is driven by the weak dipolar interactions and requires simultaneous pinning of many repeating units of the chain, its kinetics can be extremely slow, especially for polymer melts. As a consequence, polymer chains at the interface with a substrate can lie for very long times in non-equilibrium states, before reaching their equilibrium configuration. Remarkably, recent work verified that the deviations from the bulk behavior in the dynamics of nanoconfined polymers are strongly affected by those non-equilibrium configurations assumed in adsorbed layers. In this Chapter, we report experimental evidences on the existence of an irreversibly adsorbed layer in poly(vinyl acetate) (PVAc) films in contact with different substrates. The presence of such a layer is proved through atomic force microscopy imaging of the residual layer remaining on the substrate after washing the polymer film in a good solvent. Moreover, we demonstrate that the evolution of the irreversibly adsorbed layer is unambiguously related to the change in relaxation dynamics of polymer films under annealing at a high temperature (~ Tg + 60 K). Finally, we demonstrate the direct effect of this adsorbed layer on the maximum moisture uptake of supported ultrathin PVAc films in various amounts of ambient relative humidity, hence providing a simple approach for controlling the moisture absorption of nanosized polymer films.

The relaxational dynamics of poly(propyleneglycol) (PPG) and poly(propyleneglycol)-diglycidylether (PPGDE) compounds, the latter with different average number of repeating units, was studied by broadband dielectric spectroscopy (10 mHz to 10 GHz) from above to below the glass transition temperature in the range 115–350 K. The dielectric response of all systems showed a high temperature single relaxation while below a crossover temperature TB it split in a main (α-) and a secondary (β-) relaxation. The effect on the main and secondary relaxation of different groups terminating the polymer chain was analyzed: in particular, a decrease of the secondary relaxation time was observed for PPGDE with respect to PPG with the same molecular weight. Moreover the change of the Vogel temperature T0 paralleled what previously found for similar systems. The relationship between main and secondary relaxation and the microscopic nature of this latter were analyzed in the framework of the coupling model.

The pressure and temperature behaviours of the structural relaxation time in glass-forming systems can be well described by a new equation, based on the Adam-Gibbs model, where the configurational entropy has been written in terms of the excess heat capacity and the molar thermal expansion. An experimental check of this equation was performed on the dynamics of an epoxy compound, investigated by wide-band dielectric spectroscopy (10−2–1010Hz). Six different sets of relaxation data, corresponding to two different conditions, were analysed: firstly, isobaric (atmospheric pressure and 95.5 MPa) with various temperatures down to the supercooled and glassy phases and, secondly, isothermal (at four different temperatures) with changing pressure in the range 0.1–270 MPa. Moreover, the thermal expansion was independently determined from data on the pressure and temperature variations of the volume of the material. From this analysis, the pressure-extended Adam-Gibbs model resulted, which is so suitable f...

We compare the specific heat and dielectric relaxation (T) data of two epoxy resins, poly[(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol A, with the predictions of the entropy theory of Adam and Gibbs. Specific heat data from temperature-modulated differential scanning calorimetry are used to evaluate configurational entropy S C(T) data experimentally. The systems studied show two secondary (β and γ) relaxations inside the experimentally accessible frequency window of dielectric spectroscopy related to two crossover regions along the trace of the dynamic glass transition. The analysis, bypassing the use of the Vogel-Fulcher-Tamman equation, supports a good description of the structural relaxation time made in terms of the Adam-Gibbs model in the temperature range T g < T < Tβ (or T B), with Tβ the temperature where the structural and secondary β-relaxation times tend to merge, and T B the temperature of deviation from a high-temperature Vogel-Fulcher-Tamman τ(T) depend...

The dynamics of an epoxy resin, poly(phenyl glycidyl ether-co-formaldehyde), has been investigated in the supercooled and glassy phases by wideband dielectric spectroscopy (10/sup -2/ to 3/spl times/10/sup 9/ Hz) and compared with that of the previously investigated epoxy resin, diglycidyl ether of bisphenol-A. The temperature evolution of the dynamics of the system is monitored through the characteristic parameters of the relaxations, namely the relaxation times, the relaxation strengths and the shape parameters. Two transition regions are revealed: the glass transition and the split between structural and fast secondary relaxation, where the onset of the structural relaxation is located also.

The dielectric properties of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin and n-butylamine have been studied in the frequency range 100 Hz to 3 GHz at 25 °C. The unreacted mixture showed a single relaxation which gradually split in two relaxations, the separation of which increased as the reaction progressed. The low and high frequency relaxations are attributed to the structural dynamics of the growing macromolecules and to the motions of the dipoles associated with the monomers, respectively. The increase of the structural relaxation time is described by a phenomenological relationship, similar to the Williams–Landel–Ferry equation, that allows estimation of the conversion at vitrification. A marked decrease of the electrical conductivity has also been observed, as a result of the increasing viscosity of the system. The conductivity and the structural relaxation time are correlated by means of a generalized Debye–Stokes–Einstein model. It has also been found that both components of the complex dielectric permittivity, measured at microwave frequencies, yield information on the advancement of the polymerization process. © 2001 Society of Chemical Industry

The application of electrical impedance spectroscopy to gas sensors and light emitting devices (LEDs) based on Langmuir–Blodgett (LB) films is reviewed. The sensing material was a co-ordination polymer formed by reaction of the bifunctional amphiphilic ligand 5,5′-methylenebis ( N -hexadecylsalicylideneamine)) (MBSH) and copper ions in an interfacial reaction at the water surface. Changes of the device capacitance and conductance during exposure to ethanol, acetonitrile and benzene were related to the polarity of the organic vapour. Impedance measurements on LEDs incorporating a substituted polypyridine derivative, poly(6-hexyl-2,5-pyridinediyl), provided an understanding of the equivalent circuit of the device structure.

This work concerns some electrochemical polymerizations of pyrrole derivatives in order to obtain intrinsically conducting polymers: the study was mainly concerned with the electrochemical polymerization of 3-alkyl substituted pyrroles with different chain lengths (3-hexylpyrrole, 3-decylpyrrole, 3-hexadecylpyrrole). Different experimental conditions were adopted with 3-alkylpyrroles: different solvents (propylene carbonate, acetonitrile), different counterions (ClO4−, BF4−, NO3−, PF6−, TsO−) and different current densities (0.05, 0.1, 0.2, 0.4 mA/cm2). The synthesized conducting polymers were characterized through Fourier transform infrared spectroscopy, 13C cross-polarization magic angle spinning solid-state nuclear magnetic resonance and electrical conductivity measurements to study the relations connecting the experimental conditions of synthesis with the structure and electrical properties of the polymers. Thermogravimetric analysis, differential scanning calorimetry and isothermal heatings were performed in order to evaluate the polymer's thermal stability. Copyright © 2000 John Wiley & Sons, Ltd.

Dielectric measurements of an epoxy resin, N, N-diglycidyl-4-glycidyloxyaniline, have been carried out in the supercooled and glassy phase over a broad frequency range (10 2-6 109 Hz). The measurements reveal electrical transport due to ionic impurities as well as three different dipolar relaxations—in addition to the - and -relaxation, a slower 0 -relaxation is recognized, whose loss peak is disclosed after subtraction of the dc conductivity contribution. The glass transition is found to affect markedly the secondary relaxation, whose strength and shape parameters change across Tg. The major inference from the results concerns the existence of a transition in the dynamics, occurring some tens of degrees above Tg, in the vicinity of the temperature TS where the peaks of the - and -relaxations merge. Evidence in favour of such a transition is given by: (i) the change in the temperature dependence of the -relaxation time; (ii) independently, the change in the temperature dependence of the dc conductivity; (iii) the breakdown of the Debye-Stokes-Einstein model, replaced at lower temperatures by a fractional regime. Concerning the 0 -process, it shows a Vogel-Fulcher behaviour with the same temperature T0 as the -relaxation but, unlike this last, it is not involved in a splitting phenomenon with the -relaxation. Several hypotheses concerning the nature of the 0 -process are explored.

A wide-band (102 − 2 × 1010Hz) dielectric study of epoxy compounds was carried out under isobaric conditions (atmospheric pressure) by changing the temperature down to the supercooled and glassy phases. One of these systems (diglycidyl ether of bisphenol A (DGEBA)) was also measured under isothermal conditions at 293 K by changing the pressure from 0.1 up to 235 MPa. The analysis of variable-temperature measurements gave evidence of a connection between the α β-splitting phenomenon, the breakdown of the Debye-Stokes-Einstein (DSE) relation which turns into a fractional DSE law, and the transition of the α-relaxation dynamics between two different temperature regimes. The variable-pressure measurements revealed that the pressure dependence of the α-relaxation time in DGEBA is better described by a second order polynomial function rather than a Vogel-Fulcher-like function. The perfect scaling observed between couples of isobaric and isothermal spectra with the same value of the α-relaxation time su...

The d.c. conductivity and the electric a.c. response from 100 Hz up to 40 MHz of poly(3n-decylpyrrole) were measured in the 80-330 K interval to characterize the charge transport behaviour of the system. The d.c. conductivity well fitted the variable range hopping model, and the loss factor, after having deducted the d.c. contribution, showed a relaxation peak when the conductivity versus frequency started to rise. The strength of this relaxation increased with temperature and became too large to be related to a dipolar relaxation; moreover, the temperature dependence of the loss peak frequency and d.c. conductivity coincided. The observed relaxation was attributed to the hopping charge transport, as further confirmed by the temperature behaviour of the relaxation strength and by the frequency dependence of the exponents of the power law which locally approximate the conductivity behaviour. As the activation energy of the d.c. conductivity differed from the frequency of the loss peak, the theoretical prediction concerning the selfsimilarity of the a.c. conductivity was roughly verified.

From ellipsometry measurements, Pye and Roth [Phys. Rev. Lett. 107, 235701 (2011)] presented evidence of the presence of two glass transitions originating from two distinctly different and simultaneous mechanisms to reduce the glass transition temperature within freestanding polystyrene films with thickness less than 70 nm. The upper transition temperature T-g(u)(h) is higher than the lower transition temperature T-g(l)(h) in the ultrathin films. After comparing their data with the findings of others, using the same or different techniques, they concluded that new theoretical interpretation is needed to explain the two transitions and the different dependences of T-g(u)(h) and T-g(l)(h) on film thickness and molecular weight. We address the problem based on advance in delineating the different viscoelastic mechanisms in the glass-rubber transition zone of polymers. Theoretical considerations as well as experiments have shown in time-scales immediately following the segmental alpha-relaxation are the sub-Rouse modes with longer length scale but shorter than that of the Rouse modes. The existence of the sub-Rouse modes in various polymers including polystyrene has been repeatedly confirmed by experiments. We show that the sub-Rouse modes can account for the upper transition and the properties observed. The segmental alpha-relaxation is responsible for the lower transition. This is supported by the fact that the segmental alpha-relaxation in ultrathin freestanding PS films had been observed by dielectric relaxation measurements and photon correlation spectroscopy. Utilizing the temperature dependence of the segmental relaxation times from these experiments, the glass transition temperature T-g(alpha) associated with the segmental relaxation in the ultrathin film is determined. It turns out that T-g(alpha) is nearly the same as T-g(l)(h) of the lower transition, and hence definitely segmental alpha-relaxation is the mechanism for the lower transition. Since it is unlikely that the segmental alpha-relaxation can give rise to two very different transitions simultaneously, a new mechanism for the upper transition is needed, and the sub-Rouse modes provide the mechanism. (C) 2014 AIP Publishing LLC.

The conductivity and dielectric relaxation behaviour of a glass-forming monoepoxide, cresyl glycidyl ether (CGE), were studied from to by impedance spectroscopy, in the temperature interval 83 - 333 K. Data analysis indicated the existence of a transition temperature marking the separation between two different relaxation regimes, in accordance with previous results given by Stickel and co-workers. The temperature is well above the glass transition temperature , and coincides with the temperature at which the timescales of the main and secondary relaxation processes coincide, so at there is a unique overall relaxation process. The splitting layout of CGE differs from that of the very common diepoxide diglycidyl ether of bisphenol-A (DGEBA), studied previously, and also the relaxation at high temperature has a correspondingly different dynamical behaviour. The relaxation strength behaviour is also analysed and compared with that of DGEBA; the similarities are discussed.

In polymer blends where the glass transition temperatures of the two components differ greatly, the segmental α-relaxation and the chain dynamics of the faster component exhibit a number of anomalous properties not seen before in homopolymers, and not explainable by conventional theory of polymer dynamics. In the first part of this paper, these anomalous properties are collected altogether and made known. We show their interconnections and emphasize the necessity of explaining all of them together if the objective is to fully solve the problem. In the second part, the predictions from a single theoretical framework, namely, the coupling model, are applied to explain the anomalous properties in toto.

The Comment of Colmenero asserts no change in Fs(Q,t) of the poly(ethylene oxide) (PEO) chains in blends with poly(methyl methacrylate) on crossing times of about 1–2 ns in data obtained by neutron scattering experiments and simulations. The assertion is opposite to that reported in the original papers where the neutron data and simulations were published. To make this point clear, we cite the data and the very statements made in the original papers concluding that indeed in the time interval from 60 ps to 1–2 ns the dynamics of PEO chain follows approximately the Rouse model, but becomes slower and departs from the Rouse model in the dependencies on time, momentum transfer, and temperature at longer times past tc = 1–2 ns. It is noteworthy that similar crossover of chain dynamics in entangled homopolymers at the ns time scale was found by neutron scattering.