Enthalpy relaxation of comb-like polymer analysed by combining activation energy spectrum and TNM models

In the enthalpy relaxation of poly(cyanobiphenyl ethylacrylate) (PCB2A), the decrease in enthalpy was measured as a function of ageing time and ageing temperature with DSC technique. Obtained data was analysed at first with the activation energy spectrum (AES) model, then compared with the prediction of a multiparameter phenomenological model which follows the evolution of the configurational entropy (SC) of the sample during the whole thermal history. In AES model, the decrease in enthalpy is controlled by molecular processes whose energies are distributed over a continuous spectrum. The spectrum gives the relation between the number and energy of processes. In consequence, AES of a single peak was obtained. The energy at which the peak maximum was located decreased with the increase in ageing temperature, which is a well-known behaviour as a distribution of relaxation processes. Furthermore, c_p(T), the heat capacity data obtained by the calculation based on SC model reproduced the experimental c_p(T) curve. Model parameters found in the calculation was discussed in relation to the energy value of AES maximum.     

Dynamic properties of solvent confined in silica gels studied by broadband dielectric spectroscopy

We report the results of a broadband (10⁻²–10⁷ Hz) dielectric spectroscopy study on a solvent system (glycerol–water solution) confined in a porous silica matrix. The dielectric relaxation of the system is studied as a function of both temperature (120–280 K) and solvent composition (0–36 glycerol molar percentage), at constant matrix composition. Our data show that glycerol–water systems confined inside silica gel are characterized by a very complex dynamics quite different from that observed in solution, thus indicating that confinement may deeply modify solvent dynamics. Indeed in addition to the relaxation processes similar to those occurring in bulk samples, new dielectric relaxations are detected: two non-collective relaxations, attributed to water molecules strongly interacting with pore surfaces and to the glycerol trapped within the matrix structure, respectively; a relaxation in the glycerol free sample (and in samples at very low glycerol content) almost coincident with that observed in other different confinement conditions and governed by geometrical confinement per se. Moreover, at high glycerol content we observe two non-Arrhenius processes at least 4 order of magnitude slower than solution-like main relaxation; at low glycerol content the two above relaxations merge and show a fragile to strong transition at about 200 K.     

Thermal characterization of a vinylidene fluoride-trifluorethylene (75–25) (%mol) copolymer film

Vinylidene fluoride-trifluorethylene (VDF-TrFe) copolymers – exhibits ferroelectric properties due to the special arrangement of the chain units in the crystalline phase. The co-polymers have attracted intense scientific and technological interest due to the possibility of studying the ferroelectric phase transition, as it occurs before the melting of the material. Being that the piezo- and pyroelectric properties of the material are the most interesting from a practical point of view, they have been intensively studied in the recent years. Nonetheless, there is a lack of information about the thermal behavior of other relevant characteristics of the samples. In this work, P(VDF-TrFE) (75/25) has been characterized by mechanical tests and a series of thermal analysis techniques, including thermal mechanical analysis (TMA) and differential scanning calorimetry (DSC). The films exhibit slight mechanical anisotropy, and in the longitudinal direction larger mechanical properties, due to the higher structural organization. DSC results show two well-defined peaks corresponding to the ferroelectric-paraelectric transition and the melting of the paraelectric phase at higher temperatures. Two relaxation processes were found (β and α_c) by dynamic mechanical analysis (DMA). The β process, attributed to segmental motions in the amorphous phase, appears at the same temperature in both directions, but its intensity was found to be higher for the films tested in the longitudinal direction. At higher temperature, the α_c relaxation was related to the thermal variation of the dimensions of the films in the longitudinal direction during heating, recorded by TMA.     

Dielectric investigations of organic–inorganic hybrid based on (2-hydroxypropyl) cellulose with nanosheet crystallites of quasi-TiO2

In this paper, dielectric spectroscopy investigations of organic–inorganic hybrid composite obtained from (2-hydroxypropyl)cellulose (HPC) with nanosheet crystallites of quasi-TiO₂ are presented. This hybrid belongs to the class of composites, which have no covalent bonds between organic and inorganic constituents and the behaviour of composite is determined mainly by interface of two constituents. The analysis of the molecular dynamics of the hybrid system reveals the influence of the nanosheet crystallites of quasi-TiO₂ on the relaxation processes of HPC matrix. The relaxation strength of γ process strongly increases in composite as compared to neat HPC. A shift to higher temperature has been evidenced in the case of α and β processes. A higher value of the slope of the maximum frequency vs. the inverse temperature for the α-relaxation of the composite has been observed. At high temperatures the mesomorphic behaviour of HPC has been drastically reduced in the composite. The results of our investigations are interpreted in terms of a presence of intermolecular forces between HPC and nanosheet crystallites of quasi-TiO₂.     

A molecular dynamics simulation study of the α- and β-relaxation processes in 1,4-polybutadiene

We have conducted molecular dynamics simulation studies of melts of 1,4-polybutadiene (PBD) using a quantum chemistry-based force field where the rotational energy barriers between conformational states have been reduced (LB-PBD model, where LB indicates lowered barrier). Segmental relaxation in the LB-PBD melts was investigated over a wide range of temperature by monitoring the decay of the torsional autocorrelation function (TACF). The decay of the TACF could not be well-represented by a single stretched (Kohlrausch–William–Watts) exponential indicating the presence of resolvable α- and β-relaxation processes even at the highest temperatures investigated. We found that all or nearly all dihedrals undergo conformational transitions on the time scale of the β-relaxation process. The β-relaxation process was observed to weaken with decreasing temperature due to an increasingly heterogeneous population of conformational states with decreasing temperature on time scales longer than the β-relaxation time but shorter than the α-relaxation time. The heterogeneity in conformational populations is imposed by the matrix which biases the conformational states of individual dihedrals (but not the average over all dihedrals) on time scales shorter than the α-relaxation time. Complete segmental relaxation (α-relaxation) occurred on longer time scales over which all dihedrals are able to populate each conformational state with near-equilibrium probability, a process that requires cooperative motion of the matrix. The β-relaxation process as monitored by the TACF was also found to broaden with decreasing temperature, consistent with an increasingly broad distribution of times required for individual dihedrals to visit each conformational state.     

High-temperature relaxation mechanisms in inorganic glasses

At temperatures below T_g two relaxation processes are observed in sheet glass (200–500°C) and low-alkali glass (300–600°C): the fast R₁ and the slow R₂ processes which are not connected with the viscous flow, and the structural relaxation occurring R₃ above T_g. The processes R₁ and R₂ proceed at an invariable structure and are characterized by activation energies as high as 5 kcal mol^?1 and 13–15 kcal mol^?1, respectively. The contribution of R₂ amounts to 70–80%. The process R₃, observed near and above T_g, is accompanied with structural variations and, therefore, its activation energy depends on temperature; at T_g it is equal to 60 kcal mol^?1.The processes R₁ and R₂ are due to the mobility and rearrangement of large kinetic units. On the contrary, R₃ is characterized by a low volume of kinetic units. This shows that the ions of silicon and oxygen are involved in this process. The relaxation process R₁ is assumed to be connected with the local fluctuation deformationsof the glass network as in the case of reverse glass deformation under high pressures, and the process R₂ with the mobility of microscopic areas of the glass micro-inhomogeneous structure (structural complexes, microblocks). The continuous spectra corroborate the existence of several high-temperature relaxation processes in silicate glasses.Thus, three relaxation processes are observed in alkali-silicate glasses in the temperature range 200–600°C: the processes R₁ and R₂ are mechanical relaxations, whereas the process R₃ is a structural relaxation determining the viscous flow of glass. The contribution of R₃ to stress relaxation amounts to 5%.There exists a temperature T_k (20–30° below T_g) which is the upper limit of the process R₂. At higher temperatures beginning from T_k the stress relaxation is first determined by the two processes R₁ and R₃, and then by one process R₃. At temperatures below T_k all three processes determine the stress relaxation, but with the decreasing temperature the rate of R₃ becomes negligible and, therefore, in the glass annealing range (below T_k) the mechanical relaxation R₂ and R₁ are mainly responsible; their contribution to the whole relaxation process is as high as 95%.     

Relaxations in polystyrene below the glass transition studied by viscosity measurement

The shear viscosity of organic glass polystyrene has been determined under pure shear deformation mode from room temperature up to the glass transition temperature. A mechanical model of series connection of anelasticity and viscosity was used to determine the viscosity of the material. Relaxation time for the viscous flow was determined as a function of temperature. The relaxation was composed of two thermal-activation type relaxation processes: the high temperature relaxation (HTR) and the low temperature relaxation (LTR). In both relaxations the relaxation time was represented as τ=τ_o exp(E/k_BT), and the values of τ₀ and E were different in specimens treated differently – aged, loaded, and annealed. The observed τ₀ and E were not independent of each other but a compensation effect, a linear decrease of logτ₀ with increased E, was seen. The results were explained using the idea of cooperative relaxations of relaxing elements. HTR and LTR were considered to correspond to the structural and the slow relaxations, respectively, and the relaxing elements could respectively be a single atom or molecule and a segment in molecular chains.     

Contraction of amorphous CoP alloys during structural relaxation

The structural relaxation of amorphous alloys is accompanied by an increase of their mechanical density. This is shown by measurements of the thermal expansion of electrodeposited CoP alloys between room temperature and crystallization. Above 80°C, the temperature of preparation, the linear thermal expansion coefficient is apparently reduced as a consequence of a superimposed shrinkage, whereas the true thermal expansion coefficient is not affected by any preceding thermal treatment. The velocity of contraction increases with temperature when the material is continuously heated. But the value of the contraction per degree increase in temperature is constant above a certain temperature T₁. The higher the heating rate, the higher T₁. The mathematical treatment of these experimental results is difficult because the relaxation property, the contraction, depends not only on temperature but also on time and on the preceding thermal treatment.     

Optical and broadband dielectric investigations of photochromic polymethacrylates

The molecular dynamics and optical switching behavior (photo alignment) of novel azobenzene containing side-group (co)polymers were studied by time-resolved optical spectroscopy and broadband dielectric spectroscopy (BDS). To elucidate the effect of the molecular structure on the photochromic properties, two series of poly(methacrylates) with different aliphatic spacer units and four different concentrations of the chromophoric monomer were investigated. For both series, an inverse relation between the switching (retardation and relaxation) times and the chromophore content was found as well as a strong correlation between switching times and alignment efficiency. Dielectric spectroscopy on all materials revealed up to three relaxation processes (α, α*, δ) above the glass transition temperature that were assigned to the dynamic glass transition of the polymer backbone and the fast and slow fluctuation of the chromophores around their long and short molecular axes. In the glassy state, occasionally two Arrhenius-type relaxations were observed that were identified as local motions of the butyl side group and the chromophore in its anisotropic environment. Both materials series showed a monotonic increase in T_g and the dynamic fragility with increasing chromophore concentration, which was explained by an increasing effect of physical crosslinking provided by the increasingly dense packed side groups.     

Defect model for the mixed mobile ion effect revisited: An importance of deformation rates

The progress in understanding the behavior of glassy mixed ionic conductors within the concept of the defect model for the mixed mobile ion effect [V. Belostotsky, J. Non-Cryst. Solids 353 (2007) 1078] is reported. It is shown that in a mixed ionic conductor (e.g., mixed alkali glass) containing two or more types of dissimilar mobile ions of unequal size sufficient local strain arising from the size mismatch of a mobile ion entering a foreign site cannot be, in principle, absorbed by the surrounding network-forming matrix without its damage. Primary site rearrangement occurs immediately, on the time scale close to that of the ion migration process, through the formation of intrinsic defects in the nearest glass network. Neither anelastic relaxation below glass transition temperature, T_g, nor viscoelastic or viscous behavior at or above T_g can be expected being observed in this case because the character of the stress relaxation in a wide temperature range is dictated above all by the deformation rates employed locally to the adjacent network-forming matrix. Since the ion migration occurs on the picosecond time scale, the primary rearrangement of the glass network adjacent to an ionic site occurs at rates orders of magnitude higher than those of the critical minimum values, so the matrix demonstrates brittle-elastic response to the arising strain even at temperatures well above T_g, which explains, among other things, why mixed alkali effect is observable in glass melts.     

Dielectric studies of segmental dynamics in poly(dimethylsiloxane)/titania nanocomposites

Differential scanning calorimetry, thermally stimulated depolarization currents and dielectric relaxation spectroscopy techniques, covering together a broad frequency range of 10⁻⁴ to 10⁶ Hz, were employed to investigate the effects of in situ synthesized titania nanoparticles on thermal transitions, segmental dynamics and interfacial interactions in poly(dimethylsiloxane)/titania nanocomposites. Titania particles (TiO₂, 20-40 nm in diameter) were prepared and well dispersed into the polymer network through sol-gel technique, aiming at stable and mechanically reinforced systems. The interactions between polymer and fillers were found to be strong, supressing crystallinity and affecting the temperature development of the glass transition. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (α_c relaxation), and from the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α? relaxation). The thickness of the interfacial layer was estimated to be in the range of 3-5 nm. Measurements using different thermal protocols proved very effective in analyzing the origin of each relaxation and the respective effects of filler addition.     

Dielectric spectroscopy of low-losses sugar lyophiles: III: The influence of moisture on the dielectric response of freeze-dried lactose

This paper presents the results of a dielectric spectroscopy study of freeze-dried lactose with a range of moisture contents. Dielectric properties were measured over a wide range of frequency (10⁻¹ to 10⁶ Hz) and temperature (−120 °C to 120 °C). Four relaxation processes were analysed with respect to moisture content and corresponding relaxation mechanisms were suggested. Two processes (γ and β) were observed in the sub-T_g range of temperature and another two processes were observed near to and above the glass transition temperature, T_g. The relatively high-frequency γ-process was ascribed to the mobility of pendant hydroxymethyl groups and exhibited only a weak dependence on moisture content. The most moisture sensitive process was the second sub-T_g (Johari-Goldstein) β-process, whereby the relaxation time changed by 2 orders of magnitude as the moisture was increased by 7%. Also the third process (α-relaxation, near T_g) was sensitive to moisture content and was in good agreement with DSC data measured for freeze-dried lactose. The fourth process was a proton percolation process at the micro-crystals formed at the surface of amorphous particles during heating at the temperatures higher than T_g and shows the moisture dependence.     

The Johari-Goldstein β-relaxation of glass-forming binary mixtures

The present paper shows, by means of broadband dielectric measurements, that the primary α- and the secondary Johari-Goldstein (JG) β-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range.We found that the coupling parameter n = 1 − β_KWW and the ratio between α- and β-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant α-relaxation time are well superposed in both the α- and β-frequency ranges. Experimental results can be rationalized by Coupling Model equation.     

Effect of orientation and crystallization on dielectric and mechanical relaxations in uniaxially stretched poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films

Microstructural analysis (crystallinity, orientation) have been performed on stretched and isothermally crystallized poly(ethylene naphthalene 2,6 dicarboxylate)(PEN) films. The crystallinity ratios are higher at drawing temperatures below T_g than above T_g, and for similar draw ratios, a higher orientation can be obtained at drawing temperature below T_g than above T_g. The molecular mobility study by dielectric relaxation spectroscopy (DRS) and mechanical relaxation spectroscopy (MRS) was carried out as a function of draw ratio (λ) and drawing temperature (T_draw). Drawing in glassy state apparently splits the α-relaxation into two components: an α-low component which exhibits a strongly accelerated dynamics but this relaxation process is masked by microstructural rearrangements occurring while heating followed by an α-high relaxation process. The two sub-glass processes (β and β¹) are influenced by the drawing in the glassy state. This can be observed from the increase of relaxation amplitudes and was related to a high disorder in the amorphous phase that is induced by drawing below glass transition temperature. Drawing at 160 °C induces the opposite trend associated with crystallization and confinements effects. Differences in viscoelasticity behaviour were found by MRS in tensile mode parallel and perpendicular to stretching direction. As a result, when comparing oriented and crystallized samples with the same crystallinity ratios, a strong effect of morphology on the location and amplitude of the three relaxations of PEN can be found.     

Effect of structural relaxation on helium diffusion and solubility in vitreous B2O3

Helium permeation, diffusion, and solubility in vitreous B₂O₃ were measured as a function of thermal history and as a function of time at constant temperature. Volume relaxation measurements were also made on similar specimens. The correspondence between the effective relaxation times for gas mobility and molar volume suggests that the changes observed in both properties results from the same changes in the glass structure. This relaxation mechanism is described by the expression τ′ = 10^?4 exp(-18 000/RT), where τ′ is in seconds and the activation energy is in cal/mole. It is concluded that helium mobility is a function of the molar volume of the glass.     

Glass transition,thermal expansion and relaxation in B2O3 glass measured by time-resolved X-ray diffraction

In situ heating experiments using high-energy, high-intensity synchrotron radiation, can be successfully designed to study structural evolution with temperature of glassy materials. Coherent diffraction from glassy materials forms a succession of halos or diffraction maxima in reciprocal space and the variation with temperature, of the wave-vector Q_max or angular position of the first diffracted intensity I(Q_max) maximum below T_g can be used to determine the iso-structural volume expansion. In the present work we have obtained synchrotron X-ray diffraction patterns in transmission during in situ heating of a B₂O₃ glass. Samples were obtained by melting the B₂O₃ glass rods which were then air-cooled or liquid nitrogen-cooled. The evolution with temperature (and time) of the position of the first diffraction maximum of the diffraction pattern accurately reflected the thermal expansion coefficient and the relaxation behavior of the B₂O₃ glass. Such results allowed determination by diffraction of the glass transition temperature, T_g, at 580 K, as well as information on the structural relaxation during thermal annealing. The total volume changes due to relaxation were measured to be about 1.5 vol.% and 2.5 vol.%, for the air-cooled and the liquid nitrogen-cooled B₂O₃ glass, respectively.     

Structural relaxation of Ti40Zr25Ni8Cu9Be18 bulk metallic glass

Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass has a unique quenched-in nuclei/amorphous matrix structure. The crystallization of quenched-in nuclei, when the experimental isothermal annealing time is within its incubation time, may not disturb the enthalpy relaxation, which makes it have the accordingly common enthalpy relaxation behavior with amorphous materials. The alloy's annealing time dependence of recovery enthalpy follows a stretched exponential function with the mean relaxation time obeying an Arrhenius law. The equilibrium recovery enthalpy ΔH_T^eq, mean relaxation time τ and stretching exponent β are all dependent on the annealing temperature, and generally, a higher annealing temperature comes with a lower value of ΔH_T^eq, τ and a higher value of β. Two parameters, β_g and τ_g, representing the stretching exponent and the mean structural relaxation time at the calorimetric glass transition temperature, respectively, are correlated with glass forming ability and thermal stability, respectively. For Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG, the high value of β_g, which is much higher than 0.84 and approaches unity, reveals its good glass forming ability, while, on the other hand, the low value of τ_g indicates a worse thermal stability compared with typical BMGs.     

Invariance of the local segmental relaxation dispersion in polycyclohexylmethacrylate/poly-α-methylstyrene blends

Dielectric spectroscopy was carried out on polycyclohexylmethacrylate (PCHMA) and its blend with poly-α-methylstyrene (PaMS) as a function of temperature and pressure. When measured at conditions whereby the local segmental relaxation time for the PCHMA was constant, the dispersion in the loss spectra had a fixed shape; that is, the relaxation time determines the breadth of the relaxation time distribution, independently of T and P. This result is known for neat materials and could be observed for the blend herein due to the nonpolar character of the PaMS and the degree of thermodynamic miscibility of the blend.     

The two-step scenario of the protein dynamical transition

The “protein dynamical transition” (PDT) characterizes the abrupt loss of structural flexibility at a particular temperature and time scale in response to the glass transition of protein hydration water. The water-coupled structural degrees of freedom interact with the protein via hydrogen bonds, causing fluctuations, which can be probed by dynamic neutron scattering experiments. To emphasize the properties of hydration water a perdeuterated protein C-PC hydrated with H₂O is investigated together with native myoglobin. The respective intermediate scattering function of hydration water displays a two-step decay involving fast local re-orientational fluctuations and a slow collective relaxation. The anharmonic onset in the mean squared displacements, which is generally used to identify the PDT, is derived from the properties of the intermediate scattering function at the time given by the resolution of the spectrometer. It is shown that the onset temperature depends on the shape of the relaxation time spectrum. A shape-independent transition temperature T_Δ is defined, associated with the main structural relaxation, which decreases with increasing resolution. A second onset is identified near the glass temperature T_g, which is related to the initial decay of the intermediate scattering function. This onset is independent of the instrumental resolution and causes a change in molecular elasticity and thermal expansion. With this approach a more precise definition of the PDT is given, providing answers to the critical questions about the nature and the mechanism of the effect.     

Unified application of the coupling model to segmental, Rouse, and terminal dynamics of entangled polymers

The temperature dependence and relaxation function breadth of segmental dynamics (α-relaxation) for 1,2-polybutadiene and 1,4-polybutadiene are used to predict their respective temperature-dependent terminal relaxation times by unified application of the Ngai coupling model. Literature results for the terminal flow of near-monodisperse linear polybutadienes having widely varying molecular weights are successfully represented using the coupling model by variation of only a single parameter, C, which is the proportionality constant between the longest Rouse relaxation time and the primitive relaxation time which underlies the cooperative segmental process. The value of C varies with molecular weight (M) according to C ∝ M^b where b is found to range from 1.8 to 2.1, in close agreement with the expected exponent of 2. Contrary to experimental data and coupling model predictions, reptation theory predicts identical influences of temperature on Rouse and terminal relaxation processes; we suggest that invoking a temperature dependence for contour length fluctuations, and hence number of effective entanglements per chain, may resolve this deficiency of the tube model.