Molecular mobility of poplar cell wall polymers studied by dielectric techniques

In this work, thermally stimulated currents (TSC) analyzes combined with dynamic dielectric spectroscopy (DDS) have been applied to the investigation of polymers’ molecular mobility involves in poplar cell wall. The molecular origin of the various dielectric relaxations has been determined. Cellulose and lignin effects in wood dielectric response were distinguished. The correlation between results obtained by both dielectric methods allows us to follow molecular mobility involved in delocalized movement as primary relaxation mode. For these two major components of wood, the evolution of relaxation times involve in α-relaxation mode is explained using the strong/fragile pattern. We compared the cellulose and lignin in situ and ex situ responses to interpret wood compound behavior. The importance of structural wood interactions which modified the molecular mobility of polymer components will be underlined.     

Study of dielectric relaxations in cellulose by combined DDS and TSC

In this work, thermally stimulated currents (TSC) analyses combined with dynamic dielectric spectroscopy (DDS) have been applied to the investigation of molecular mobility of cellulose. The correlation between results obtained by both methods allows us to attribute the low temperature DDS relaxation mode to the γ-mode resolved in TSC. The values of its activation parameters point out that the chain mobility remains localized. At high temperature, the various dielectric relaxation phenomena are separated by applying a recent analytical protocol. The comparison between the activation enthalpy values obtained by DDS and TSC leads to the assignment of the so-called α-mode to cooperative movements of polymeric sequences. The Arrhenius behavior of α-relaxation time is explained using the strong/fragile pattern. The influence of water content on secondary and primary relaxation modes was examined as well.     

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.     

Scaling of the local dynamics and the intermolecular potential

The experimental fact that relaxation times, τ, of supercooled liquids and polymers are uniquely defined by the quantity TV^γ, where T is temperature, V specific volume, and γ a material constant, leads to a number of interpretations and predictions concerning the dynamics of vitrification. Herein we examine means to determine the scaling exponent γ apart from the usual superpositioning of relaxation data. If the intermolecular potential can be approximated by an inverse power law, as implied by the TV^γ scaling, various equations are derived relating γ to the Grüneisen parameter and to a common expression for the pressure derivative of the glass temperature. In addition, without assumptions, γ can be obtained directly from pressure-volume-temperature data. These methods for determining γ from molecular or thermodynamic properties are useful because they enable the P- and V-dependences of τ to be obtained, and thereby various analyses of the dynamics to be explored, without the need to carry out relaxation measurements beyond ambient pressure.     

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.     

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.     

Recent advances in fundamental understanding of glass transition

Some examples of recent advances in experiment and theory that have impact on solution of the glass transition problem are briefly discussed. The fundamental importance of a special class of secondary relaxations called the Johari–Goldstein β-relaxations becomes clear from the recent advances. The main part of the paper addresses a recent research problem, which is the purported anomalous temperature dependence of the relaxation time of another secondary (γ) relaxation found by fitting dielectric relaxation data in several glass-formers. We show the anomalous T-dependence of this faster γ-relaxation is caused by the influence exerted by the slower Johari–Goldstein β-relaxation in the vicinity.     

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.     

Space and time scaling in glycerol–water mixtures

The dielectric response of glycerol/water (G/W) mixtures in a wide frequency and temperature ranges is considered in terms of spatial dynamic heterogeneities. The minimal cooperative relaxation time τ₀ extracted from the dielectric ‘excess wing’ is associated with the main structural mesoscopic size L_m obtained by X-ray diffraction thereby providing clarification of the minimal unit structure responsible for the cooperative dynamics. The space and time scaling can be interpreted in terms of anomalous diffusion in the spanned percolation H-bonded networks.     

Studies of molecular dynamics in polyurethane networks with hyperbranched polyester Boltorn®H30 as a crosslinker

Several non-conventional polyurethane networks crosslinked with hyperbranched polyester (Boltorn^®H30) were synthesised with an aim to determine an influence of the polyurethane linkage length on molecular relaxations in such systems. The polyurethane chain length was regulated by changing the macrodiol length or by changing the number of the repeating macrodiol-diisocyanate units. Molecular dynamics were investigated by dielectric spectroscopy and by dynamic mechanical analysis. It was found that the macrodiol length has a strong influence on the glass transition and the α-relaxation process correlated with it. By contrast, the changes of the repeating unit number practically did not affect the molecular relaxations. This effect was explained by the formation of a physical network by hydrogen bonds between urethane groups, controlling the molecular mobility. The distribution of the hydrogen bonds is controlled by the length of soft segment between the urethane groups. It means that in the polyurethanes based on the same macrodiol the distribution of the hydrogen bonds is similar, and what follows it, the molecular dynamics of such systems is very similar.     

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.     

Molecular dynamics of poly (oxybutylene)s confined in pores of Vycor glass

We have investigated the relaxation dynamics of poly (oxybutylene), POB, chains of various lengths, with different end groups (OH and CH₃) in bulk and in confinement using dielectric relaxation spectroscopy. It is known that POB chains exhibit, apart from the segmental (main) relaxation process, the dielectric normal mode process which reflects the global chain motions. The comparative study of OH- and methyl-ended POB chains in bulk reveals the marked effect of association of hydroxyl end groups and indicates that the decrease in the density of the methylated specimens results in the acceleration of both relaxation processes, main and normal mode relaxations. For the confined POB chains the data indicate that the segmental relaxation process becomes faster (the effect being more pronounced for the methylated POB chains). A decrease in T_g of about 8 K for the methyl- and 2 K for OH-ended POB chains is estimated. Regarding the average relaxation rates of the normal mode process our data indicate that global chain mobility is not affected by the confinement.     

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.     

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

During electric polarization charge is injected into the material. The structure is decorated with space charge and during the subsequent heating an apparent peak and the genuine peaks that are related to dipole randomization and charge detrapping are observed. The method is used here to analyze the molecular movements in polyimide in the temperature range from 293 to 623 K. Two weak relaxations have been observed around 337 K and around 402 K. The electrical conductivity changes with temperature in agreement with the Arrhenius law only below (W = (0.84 ± 0.03) eV ) and above ( W = (0.82 ± 0.03) eV) the temperature range where the β relaxation is observed. The variation of the electrical conductivity with temperature, in the range of the β relaxation, is controlled by the variation of the charge currier mobility with temperature and it shows a non-Arrhenius behavior. We suggest that the β₁ sub-glass relaxation is related to the rotation or oscillation of phenyl groups and the β₂ sub-glass relaxation is related to the rotation or oscillation of the imidic ring. At higher temperatures an apparent peak was observed. The relaxation time of the trapped charge, at 573 K, is high than 8895 s.     

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.     

Molecular dynamics in hyperbranched polyimides cross-linked with ethylene glycol diglycidyl ether

Chain dynamics in hyperbranched polyimides based on 2,4,6-triaminopyrimidine and 4,4′-oxydiphthalic anhydride (molar ratio 1:1) cross-linked with ethylene glycol diglycidyl ether was investigated by means of broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents techniques. The secondary γ relaxation increases in magnitude and slows down with increasing degree of cross-linking, whereas the α relaxation, associated with the glass transition, becomes faster. These effects were explained in terms of increase of free volume and of constraints to the motion imposed by cross-links.     

From small molecules to polymers: Relaxation behavior of n-butyl methacrylate based systems

Calorimetric and dielectric data for a series of n-butyl methacrylate (nBMA) based systems are presented. Dynamic glass transition (α) and secondary relations (β, β′, γ) are studied for monomer, dimer, oligomers as well as polymers with a narrow molecular weight distribution. Fragility m and cooperativity N_α increase with decreasing degree of polymerization P indicating that the cooperative character of the α motions increases. All glass transition parameters change smoothly with P without clear peculiarities that would indicate a transition from small molecules to polymers. Relations between α dynamics and the different secondary relaxations in these series are discussed. The results are compared with the predictions of different approaches aimed to understand the nature of the dynamic glass transition and the interaction between localized and cooperative dynamics in glass-forming liquids.     

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.     

Viscosity, relaxation and elastic properties of photo-thermo-refractive glass

Viscometry, ultrasonic echography and mechanical spectroscopy were applied to explore the thermo-mechanical properties of a photo-thermo-refractive (PTR) glass. Newtonian shear viscosity and ultrasonic velocity were used to determine the Maxwell time of structural relaxation. Two fast relaxation modes are found in PTR glass below glass transition temperature, which are attributed to alkali and mixed cation mobility. Respective activation energies, E_γ = 72 ± 4 kJ mol⁻¹ and E_β = 117 ± 4 kJ mol⁻¹, are considerably lower than that of viscous flow at the glass transition E_α ≈ 465 kJ mol⁻¹ as quantified from mechanical spectroscopy. Decoupling ratios of E_γ/E_α = 0.155 and E_β/E_α = 0.255 are similar to those of sodium trisilicate glass but at considerably longer timescales. These observations imply that β- and γ-relaxations are delayed as local structural arrangements are more complex for the multi-component PTR glass.     

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.