Dynamic Personality of Proteins and Effect of the Molecular Environment.

Protein dynamics display distinct traits that are linked to their specific biological function. However, the interplay between intrinsic dynamics and the molecular environment on protein stability remains poorly understood. In this study, we investigate, by incoherent neutron scattering, the subnanosecond time scale dynamics of three model proteins: the mesophilic lysozyme, the thermophilic thermolysin, and the intrinsically disordered ß-casein. Moreover, we address the influence of water, glycerol, and glucose, which create progressively more viscous matrices around the protein surface. By comparing the protein thermal fluctuations, we find that the internal dynamics of thermolysin are less affected by the environment compared to lysozyme and ß-casein. We ascribe this behavior to the protein dynamic personality, i.e., to the stiffer dynamics of the thermophilic protein that contrasts the influence of the environment. Remarkably, lysozyme and thermolysin in all molecular environments reach a critical common flexibility when approaching the calorimetric melting temperature.

Pressure Scanning Volumetry of Physically Aged Polymer Glasses, Fictive Pressure, and Memory Effect.

Physical aging of a glass decreases its volume, V, entropy, and enthalpy, H, toward the equilibrium state values. For glasses usually formed by cooling a melt, the effect is modeled in terms of non-exponential, nonlinear structural relaxation by using a plot of the heat capacity, Cp = (dH/dT)p, against T obtained from differential scanning calorimetry (DSC) cooling and heating scans. A melt becomes glass also on isothermal pressurizing and the glass formed becomes liquid on depressurizing, showing a hysteresis of the sigmoid-shape plot of -(dV/dp)T against p, which resembles the thermal hysteresis observed in the Cp against T plots. By analogy with DSC, it was named pressure scanning volumetry (PSV). Here, we use the known values of non-exponential and nonlinearity parameters ß and x and volume of activation for structural relaxation time, ΔV*, of atactic poly(propylene) to investigate the effect of aging pressure, page, of aging time, tage, and of the pressurizing rate on aging features in PSV scans. The scans show a post-pg→l feature on depressurizing before the -(dV/dp)T overshoot peak appears. We provide quantitative plots (i) of the monotonic decrease of V and increase of fictive pressure, pf, with tage and (ii) of the memory (Kovacs) effect in V and pf of the polymer and (iii) provide generic plots of -(dV/dp)T against p for different combinations of ß, x, and ΔV*. The study is of academic significance because PSV scans show a change in the density fluctuation response. It is of technological significance in polymer-extrusion processing and it may stimulate the commercial development of computer-controlled, high-pressure equipment.

Dielectric Characterization of Core-Shell Structured Poly(vinylidene fluoride)-grafted-BaTiO3 Nanocomposites.

Dielectric properties of poly(vinylidene fluoride)-grafted-BaTiO3 (PVDF-g-BT) core-shell structured nanocomposites obtained from Reversible Addition Fragmentation chain Transfer (RAFT) polymerization of VDF were investigated by Broadband Dielectric Spectroscopy (BDS). The dielectric constant increased along with the BT content, about +50% by addition of 15 vol% of BT, which was around 40% more than expected from predictions using the usual dielectric modeling methods for composite materials, to be ascribed to the effect of the interfacial core-shell structure. The known dielectric relaxations for PVDF were observed for the neat polymer as well as for its nanocomposites, not affected by the presence of nanoparticles. A relaxation process at higher temperatures was found, due to interfacial polarization at the amorphous-crystalline interface, due to the high crystallinity of materials produced by RAFT. Isochronal BDS spectra were exploited to detect the primary relaxation of the amorphous fraction. Thermal analysis demonstrated a very broad endotherm at temperatures much lower than the usual melting peaks, possibly due to the ungrafted fraction of the polymer that is more easily removable by repeated washing of the pristine material with acetone.

Endothermic Effects on Heating Physically Aged Sucrose Glasses and the Clausius Theorem Violation in Glass Thermodynamics.

Experimental heat capacity, Cp,app, of some physically aged polymers had shown an endothermic peak or a shoulder on heating when the material was still in the glassy state. In the first part of this paper, we report observation of a similar feature in a molecular glass, sucrose, indicating increase in the enthalpy and entropy from kinetic unfreezing of molecular motions in the solid state. Aging decreases Cp,app of glass. Increase in the aging time, tage, or aging temperature, Tage, interferes with the onset temperature of Cp,app increase toward the liquid state value. When the endothermic feature is not obvious in the Cp,app-T plots of the glassy state, its presence may be discerned in a plot of dCp,app/dT against T. Molecular motions producing this feature have implications for the state point in a potential energy landscape of an aging glass. In the second part of the paper, we use the Cp,app data to examine how much our violation of the Clausius theorem affects the entropy determined from the Cp,app d ln T integral. In addition to calculating this integral for a closed cycle of (irreversible) cooling and heating paths, we suggest an analysis which uses the δCp,app d ln T integrals (δCp,app is the difference between the Cp,app of the aged and the unaged glass) measured only on the heating paths. The closed cycle Cp,app d ln T integral value is negligibly small. The δCp,app d ln T integral value increases with tage. It is equal to the enthalpy lost on aging divided by Tage. Clausius theorem violation has no significant effect on determination of the entropy from Cp,app d ln T integral of an aged glass.

Cooperativity in plastic crystals.

A statistical mechanical model previously adopted for the analysis of the α-relaxation in structural glass formers is rederived within a general theoretical framework originally developed for systems approaching the ideal glassy state. The interplay between nonexponentiality and cooperativity is reconsidered in the light of energy landscape concepts. The method is used to estimate the cooperativity in orientationally disordered crystals, either from the analysis of literature data on linear dielectric response or from the enthalpy relaxation function obtained by temperature-modulated calorimetry. Knowledge of the specific heat step due to the freezing of the configurational or conformational modes at the glass transition is needed in order to properly account for the extent to which the relaxing system deviates from equilibrium during the rearrangement processes. A number of plastic crystals have been analyzed, and relatively higher cooperativities are found in the presence of hydrogen bonding interaction.

Cooperativity at the glass transition: A perspective from facilitation on the analysis of relaxation in modulated calorimetry.

The glass transition region in nonconfined polymeric and low-molecular-weight supercooled liquids is probed by temperature-modulated calorimetry at a frequency of 3.3 mHz. From the distribution of relaxation times derived by analyzing the complex heat capacity, the number N_{α} of cooperatively rearranging units is estimated. This is done by resorting to a method in which cooperative motion is viewed as a result of a spontaneous regression of energy fluctuations. After a first, local, structural transition occurs, the energy threshold for the rearrangement of adjacent molecular units decreases progressively. This facilitation process is associated to a corresponding evolution of the density of states in a canonical representation and may be considered as a continuous spanning through different dynamic states toward a condition in which configurational constraints disappear. A good agreement is found with the N_{α} values obtained from the same calorimetric data within the framework of Donth's fluctuation theory. It is shown that, at variance from previous treatments, N_{α} can be estimated from just the relaxation function, without resorting to the knowledge of the configurational entropy. Examples point to a modest dependence of the N_{α} estimates on the experimental method used to derive the relaxation function.

Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view.

Aging kinetics of a glass is currently modeled in terms of slowing of its α-relaxation dynamics, whose features are interpreted in terms of dynamic heterogeneity, i.e., formation and decay of spatially and temporally distinct nm-size regions. To test the merits of this view, we studied the calorimetric effects of aging an orientational glass of levoglucosan crystal in which such regions would not form in the same way as they form in liquids, and persist in structural glasses, because there is no liquid-like molecular diffusion in the crystal. By measuring the heat capacity, Cp, we determined the change in the enthalpy, H, and the entropy, S, during two aging-protocols: (a) keeping the samples isothermally at temperature, Ta, and measuring the changes after different aging times, ta, and (b) keeping the samples at different Tas and measuring the changes after the same ta. A model-free analysis of the data shows that as ta is increased (procedure (a)), H and S decrease according to a dispersive rate kinetics, and as Ta is increased (procedure (b)), H and S first increase, reach a local maximum at a certain Ta, and then decrease. Even though there is no translational diffusion to produce (liquid-like) free volume, and no translational-rotational decoupling, the aging features are indistinguishable from those of structural glasses. We also find that the Kohlrausch parameter, originally fitted to the glass-aging data, decreases with decrease in Ta, which is incompatible with the current use of the aging data for estimating the α-relaxation time. We argue that the vibrational state of a glass is naturally incompatible with its configurational state, and both change on aging until they are compatible, in the equilibrium liquid. So, dipolar fluctuations seen as the α-relaxation would not be the same motions that cause aging. We suggest that aging kinetics is intrinsically dispersive with its own characteristic rate constant and it does not yield the α-relaxation rate. In this view, thermodynamic and other properties define the fictive temperature; the real or imaginary components of a dynamic property do not define it. While particles' overall motions may still play a crucial role in (structural) glass physics, we conclude that translational diffusion alone is not a requirement for structure stabilization on aging of a kinetically frozen state.

Structural fluctuations and orientational glass of levoglucosan--High stability against ordering and absence of structural glass.

To investigate whether a non-exponential relaxation always indicates 2-4 nm-size regions of dynamic heterogeneity, we studied the kinetic freezing and unfreezing of structural fluctuations involving the rotational modes in orientationally disordered crystal (ODIC) of levoglucosan by calorimetry. The heat capacity, Cp, of levoglucosan measured over the 203 K-463 K range shows that its low-temperature, orientationally ordered crystal (ORC) transforms to ODIC phase on heating, which then melts to a low viscosity liquid. On cooling, the melt transforms to the ODIC which then does not transform to the ORC. Instead, the ODIC supercools. Fluctuations resulting from hindered (random) rotations of levoglucosan molecules confined to the lattice sites and from their conformational changes become progressively slower on cooling and an orientational glass (O-G) forms showing the sigmoid shape decrease in Cp characteristic of structural arrest like that of a glass. On heating the O-G state, rotational fluctuations begin to contribute to Cp at To-g of 247.8 K and there is an overshoot in Cp and thermal hysteresis (characteristic of physical ageing) in the temperature range of 230-260 K. The non-exponential relaxation parameter, ß(cal), determined by fitting the Cp data to a non-exponential, nonlinear model for relaxation of a glass is 0.60, which is similar to ß(cal) found for polymers, molecular liquids, and metal-alloy melts in which Brownian diffusion occurs. Such ß(cal) < 1 are seen to indicate 2-4 nm-size dynamically heterogeneous domains in an ultraviscous liquid near the glass formation, but its value of 0.60 for ODIC levoglucosan, in which Brownian diffusion does not occur, would not indicate such domains. Despite the lack of Brownian diffusion, we discuss these findings in the potential energy landscape paradigm. Levoglucosan melt, which is believed to vitrify and to stabilize a protein's disordered structure, did not supercool even at 200 K/min cooling rate. The findings have consequences for reports on the dielectric relaxation studies that indicated that levoglucosan melt supercools to form a structural glass of Tg of ∼245 K, and for computer simulation of its dynamics. Levoglucosan is the ninth ODIC that forms O-G. It does so more easily than the other eight.

Change in entropy in thermal hysteresis of liquid-glass-liquid transition and consequences of violating the Clausius theorem.

Change in the entropy, dS, with change in the temperature T, with or without phase transformation, is determined only if the thermal path is reversible. For an irreversible thermal path, dS > dqirrev/T, an expression known as inequality in the Clausius theorem. In the glass formation range, Cp and enthalpy show a time-dependent hysteresis between the cooling and heating paths and the two Cp paths cross each other. We provide new data on Cp of poly(styrene) and use the previous Cp data [E. Tombari, C. Ferrari, G. Salvetti, and G. P. Johari, Phys. Rev. B 78, 144203 (2008)], both data obtained from measurements performed on cooling from melt to glass and heating from glass to melt at unusually slow rates, and show that violation of the Clausius theorem in such cases has insignificant consequences for determining the entropy of glass. We also report Cp of the samples annealed for different times at different temperatures in which the enthalpy spontaneously decreased. These measurements also show that violation of the Clausius theorem is relatively inconsequential for interpreting the entropy of the glassy state.

The role of the crystallization temperature on the nanophase structure evolution of poly[(R)-3-hydroxybutyrate].

The nanophase structure of semicrystalline polymers, which determines the mechanical, thermal, and gas permeability behavior, can be quantified by thermal methods. A detailed investigation of the nanophase structure of poly[(R)-3-hydroxybutyrate] (PHB) was performed under conditions of isothermal, quasi-isothermal, and nonisothermal crystallizations. The experimental analyses revealed that the establishment of the nanophase rigid amorphous fraction (RAF) in PHB depends on the temperature at which crystallization occurs. The RAF grows in parallel with the crystal phase during quasi-isothermal crystallization at 30 °C, whereas during nonisothermal crystallization at higher temperatures, RAF starts to develop at 70 °C, in correspondence with the final stages of the crystallization process. The influence of crystallization temperature on the nanophase structure was rationalized taking into account the effect of the mobility of the entangled chain segments during the phase transition. The melting behavior was found to change after isothermal crystallization at 70 °C, revealing that complete RAF mobilization is achieved approximately at this temperature. The temperature of 70 °C could be the limit for the formation and the disappearance of rigid amorphous fraction in the PHB analyzed in the present study.

Specific heat of hydrated lysozyme, water's contribution to its dynamics, and criteria for glass formation of biomaterials.

Previous studies of the dynamics of hydrated proteins had shown a feature resembling an exceptionally broad glass-softening endotherm. Its onset temperature, denoted as T(g), was indefinable in one calorimetric study of hydrated lysozyme and was in the 148-218 K range in another study, depending upon hydration. Other methods reported this T(g) as ~170 K. We argue that glass-formation of biomaterials should be studied by measuring a property on both the cooling and heating paths and it should be ascertained (i) that there is thermal hysteresis of the measured property, (ii) that the real and imaginary components of a dynamic property obey the Kramers-Kronig relations, and (iii) that there is an effect of annealing that is consistent with the glass phenomenology. We report the real and imaginary components of the dynamic specific heat, C(p)' and C(p)", of dry and two hydrated lysozyme samples on the cooling and the heating paths as well as the effects of annealing and changing the frequency. For the most hydrated (34.6 g water per 100 g lysozyme) sample, C(p,app) does not show thermal hysteresis in the 160-230 K range, C(p)' varies in a sigmoid-shape manner with T while C(p)" remains close to zero, and there is no effect of annealing. We interpret these findings in terms of continuous development of ice-like aggregates of immobile H2O as more H-bonds form on cooling, and continuous deterioration of the aggregates on heating. As the equilibrium constant between the aggregates and mobile H2O increases on cooling, configurational degrees of freedom of H2O molecules and lysozyme segments decrease. Consequently, the net change in enthalpy is small but the change in C(p) is large. Mobility of the lysozyme segments still depends upon the mobility of H2O molecules.

On the state of water in 2.4 nm cylindrical pores of MCM from dynamic and normal specific heat studies.

Relaxation phenomenon, thermodynamics, and phase transformation of water in nanopores has been studied by differential scanning calorimetry, usually on heating a precooled sample. Interpretation of such results is ambiguous, because the data do not indicate whether or not there is a thermal hysteresis between the heating and cooling paths. We argue that measurements on both the cooling and heating paths are needed, particularly for complex systems, and also measurements of the complex quantity are needed to ascertain that the Kramers-Kronig relations for a relaxation process are obeyed. We report a study of the real and imaginary components of the complex specific heat, C(p)' and C(p)", and the time-dependent C(p,app) of water confined to 2.4 nm diameter cylindrical pores on both the cooling and heating paths, and use different thermal histories. C(p,app) of nanoconfined water shows two exothermic peaks during cooling below 255 K at 12 K/h and only one endothermic peak on heating, and the enthalpy change determined from the exotherm is more than that determined from the endotherm. C(p,app) and C(p)' of the partially crystallized water is higher at 240 K than at 275 K on the cooling path, and C(p,app) and C(p)' of the partially crystalline water are lower at 240 K than at 275 K on the heating path, thus showing a thermal hysteresis in this range. Studies by using 60 K/h cooling and heating rates and the effect of heat treatment at selected temperatures confirm that the features observed are due to slow crystallization and slow melting. The endotherm observed on the heating path with onset at 220 K and peak at 227 K is due to gradual melting of the ice in nanopores, and not due to glass-softening transition, a liquid-liquid transition, or an approach toward the conjectured critical point of the supercooled water in the 2.4 nm pores.

Does water need a lambda-type transition?

After ascertaining that the C(p) of water confined to 1.1 nm diameter pores had been used to reach the conjecture for a lambda-transition in supercooled bulk water, we argue that only three H(2)O molecules can fit across the 1.1 nm diameter pore. Two of these form a (one-molecule thick) nanoshell hydrogen bonded to the SiO(2) pore wall. Hydrogen bonding or cooperative motions of the remaining one H(2)O molecule would not produce a "lambda-type transition."

The low-temperature endotherm in poly(ethylene terephthalate): partial melting and rigid amorphous fraction mobilization.

A detailed investigation of the low-temperature endotherm of poly(ethylene terephthalate) (PET) performed by temperature-modulated differential scanning calorimetry is presented. The origin of the small endotherm, generally observed a few degrees above the crystallization temperature in PET and in many other polymers, is a widely discussed matter. The most frequent interpretation considers it the result of partial fusion with superposition of a recrystallization process even if it has also been proposed that it can originate from enthalpic recovery connected to mobilization of the rigid amorphous fraction. In an attempt to resolve the question, a new method for the interpretation of the modulated heat-flow-rate curve resulting from a temperature modulation program is proposed. The procedure consists of the analysis of the initial points of the steady-state heat-flow-rate signals in the heating and cooling semiperiods with the temperature modulation being performed with a sawtooth profile. The study conducted in parallel on the reversing specific heat capacity and the heat-flow-rate curves, observed on heating after isothermal crystallization at various temperatures, showed that multiple processes, involving both the crystalline and the rigid amorphous fraction, overlap in the temperature range in which the low-temperature endotherm is observed. The origin of the endotherm under investigation is therefore connected with both partial fusion of the crystalline portions and enthalpy recovery subsequent to structural relaxation of the rigid amorphous fraction. An estimation of the relative percentages of the two different processes is presented and discussed.

Photorefractivity of poly-N-vinylindole-based materials as compared with that of poly-N-vinylcarbazole-based blends.

The asymmetric two-beam coupling technique has been employed to measure the photorefractivity of thin films of polymer blends containing 2,5-dimethyl-4-(p-nitrophenylazo)anisole as the nonlinear optical component. Poly-(1-vinylindole) and poly-(2,3-dimethyl-1-vinylindole) were the photoconductive polymer counterparts. The values of the photorefractivity are reported. It appears that they are comparable with those of similar blends based on the well-known poly-(9-vinylcarbazole) (PVK), here considered as a reference standard. Careful differential scanning calorimetry analyses have been accomplished on the different blends taken into account to rationalize the significantly longer shelf lifetime of the indolyl-based films with respect to the PVK-based blends.