Molecular Relaxation Phenomena Research Articles

Impact of sol-gel transition on the acoustic properties of complex model foods: Application to agar/gelatin gels and emulsion filled gels

Quantitative Ultrasound techniques are good candidates for the in situ and real-time mechanical characterization of tongue-food-palate system, and thus to improve the understanding of the determinants of texture perception of food. Different model foods (consisting in gels and emulsion filled gels composed of agar and/or gelatin) have been designed for their contrasting properties in terms of texture perceptions. Prior to the feasibility study of a Quantitative Ultrasound method to monitor their mechanical breakdown during a compression, the aim of this study is to determine the respective roles of structure and mechanical properties of the different model foods in the variations of ultrasonic wave properties. Ultrasonic velocity, reflectivity, and attenuation were monitored during the sol-gel transition (from 50°C to 20°C) at 1 MHz in pulse-echo mode, and were confronted to visco-elastic moduli and mass density measurements. The results put in evidence the role of biopolymer concentration (independently from Young’s and shear moduli) on the variations of velocity and reflectivity, resulting from joint variations of mass density and bulk modulus. Moreover, the ultrasonic attenuation was confirmed to depend on molecular relaxation phenomena of water, which are important in high-concentration gelatin samples.

The determination of the phase transition temperatures of a semifluorinated liquid crystalline biphenyl ester by impedance spectroscopy as an alternative method

Dielectric spectroscopy (DS) is a very powerful and important for better understanding of the molecular dynamics and relaxation phenomena in liquid crystals. The dielectric and impedance characteristics Ethyl 4-(7,7,8,8,9,9,10,10,10-nonafluorodecyloxy)biphenyl-4′-carboxylate (ENBC) liquid crystal have been analyzed over the frequency range of 100Hz to MHz in the temperature region from room temperature to 180°C. The compound ENBC shows enantiotropic a smectic mesophase in a wide temperature range. The phase transition temperatures T (°C) of the liquid crystal ENBC, which were characterized by Differential Scanning Calorimetry (DSC), have been verified by the dielectric measurements and conductivity mechanisms of the ENBC. The activation energies for some selected angular frequencies have also been calculated.

Thermomechanical effects of co-solute on the structure formation of bovine serum albumin

The effect of glucose syrup on the structural properties of bovine serum albumin has been addressed in preparations from low to high solids. Fifteen percent protein was mixed with the co-solute at concentrations up to 65% and subjected to thermal treatment to examine the changes in phase and state transitions. Thermomechanics were the working protocol being carried out with micro differential scanning calorimetry and small deformation dynamic oscillation. Results argue that protein molecules have been extensively stabilised by the addition of a co-solute, recorded via a delayed thermal denaturation. Further, increasing the glucose syrup enhanced polymer–polymer interactions leading to stronger networks following thermal denaturation of the globular protein. Condensed BSA/glucose syrup mixtures, i.e. at 80% solids, were cooled at subzero temperatures to exhibit a considerable state of vitrification. Molecular relaxation phenomena were successfully followed using theoretical concepts from synthetic polymer research to yield the mechanical glass transition temperature.

Single-ion-conducting nanocomposite polymer electrolytes based on PEG400 and anionic nanoparticles: Part 2. Electrical characterization

Molecular relaxation and polarization phenomena of twelve single-ion-conducting nanocomposite polymer electrolytes (nCPEs) are studied using Broadband Electrical Spectroscopy (BES). The electrolytes are obtained by combining PEG400 oligomers with increasing amounts of anionic nanofiller comprised of fluorinated-TiO2 associated with Li+ cations (LiFT®), resulting in [PEG400/(LiFT)y] systems with 0 ≤ y ≤ 26.4. This new class of [PEG400/(LiFT)y] electrolytes allows us to achieve a significant single-ion conductivity (1.1·10−5 S cm−1 at 30 °C for nLi/nO = 0.113) without the addition of lithium salts. To the best of our knowledge, this is the highest conductivity value reported for this class of electrolytes. This study, in conjunction with the results reported in Part 1, leads us to hypothesize a conduction mechanism in terms of two types of long-range charge-transfer processes. The first charge-transfer occurs at the interface between the filler nanoparticles and filler-PEG domains, while the second occurs through the PEG400 matrix with the assistance of polymer segmental motion. The measured Li+ transference numbers confirm that the studied materials are single-ion conductors.

Algorithm for capturing primary relaxation processes in excitable gases by two-frequency acoustic measurements

It is still a challenge to employ the acoustic method to investigate the molecular relaxation phenomena in excitable gases. Here we present an algorithm to capture the primary relaxation processes by only measuring the sound absorption and sound speed of two operating frequencies at a single pressure, without the necessity of detecting the gas density. This algorithm is developed from the fact that the frequency-dependent sound absorption curve due to a single-relaxation process can be reconstructed from the two values of the relaxation frequency and the maximum relaxational absorption, and they can be synthesized by the acoustic measurements at two frequencies. Moreover, by acquiring the high-frequency sound speed, those two synthesized values can be used to reconstruct the sound dispersion curve. The simulations demonstrate the validity of the proposed algorithm and its robustness against errors of acoustic measurements.

Influence of Anions on Proton-Conducting Membranes Based on Neutralized Nafion 117, Triethylammonium Methanesulfonate, and Triethylammonium Perfluorobutanesulfonate. 2. Electrical Properties

The electrical properties of a Nafion proton exchange membrane change dramatically when neutralized and then doped with a proton-conducting ionic liquid (PCIL). Broadband electric spectroscopy elucidates the molecular relaxation and polarization phenomena of neutralized Nafion (nN117) doped with triethylammonium methanesulfonate (TMS) and triethylammonium perfluorobutanesulfonate (TPFBu) ionic liquids. These data, coupled with those of part 1 suggest proton conduction mechanisms for both the pure PCILs and in PCIL-doped nN117. At 130 °C, the PCILs have conductivities of σTMS = 1.4 × 10–2 S/cm and σTPFBu = 9 × 10–3 S/cm, while correspondingly doped nN117 have conductivities of σNTMS = 6.1 × 10–3 S/cm and σNTPFBu = 1.8 × 10–3 S/cm. The pure PCILs show three interfacial polarizations associated with proton transfer mechanisms above the melting point. PCIL-doped nN117 also has three interfacial polarizations that depend on the nanostructure characteristics of the PCIL sorbed within the nN117 polar domains. Below the PCIL melting point, doped nN117 has two dielectric relaxations, α and β, associated with dipolar relaxations involving both the sorbed PCILs and the ionomer matrix. The data indicate a long-range charge transfer process that occurs through proton exchange between cationic clusters. Segmental motion of the polymer chains and the molecular dimensions of the ionic liquid nanoaggregates mediate this charge transfer.

Molecular relaxation phenomena during accelerated weathering of a polyurethane coating

Results from a polyester-urethane (PU) coating system under accelerated weathering showed that crosslink density, obtained from high-temperature modulus data, diminished due to chain scission. However, T g and room-temperature tensile modulus both increased with weathering. Molecular relaxation phenomena in polymers have long been investigated to explore changes occurring in a polymer at temperatures below its glass transition and were explored for an explanation to supplement chemical degradation for these observations. Relaxation was quantified using “enthalpy recovery” which first increased with exposure, then diminished. The concurrent physical and chemical aging effects were characterized by tracking nonexponentiality in the spectrum of relaxation times, and the size of “co-operatively relaxing regions” deduced from relaxation around the glass transition. Mechanical relaxation in this coating extended longer than cycle periods typical of accelerated weathering, suggesting that frequency effects might be important when comparing accelerated to natural weathering.

Mesoscale Dynamics and Cooperativity of Networking Dendronized Nonlinear Optical Molecular Glasses

First, molecular scale insight into the mobility of a novel class of organic materials for photonic applications with electro-optical activities larger than 300 pm/V is presented. A representative second order nonlinear optical (NLO) material of this class of self-assembling molecular glasses involving quadrupolar phenyl-perfluorophenyl (Ph-PhF) interactions is analyzed based on its molecular relaxation phenomena and phase behavior. Thereby, a new and straightforward nanoscale methodology, involving shear modulation force microscopy and intrinsic friction analysis is introduced. It provides both the submolecular enthalpic and entropic dynamics in nanoconstrained systems (e.g., ultrathin films), and thus, insight into local motion of single molecules due to dissociation of Ph-PhF pairs as well as the cooperative dynamics of the assembled network. This nanoscale model-independent thermomechanical methodology is shown to be very effective in fundamentally evaluating appropriate poling conditions of organic NLO materials. It promises to be a straightforward analysis tool to guide organic material synthesis from a molecular mobility perspective, particularly for applications that impose nanoscale constraints on the system.

Effect of SiO2 on Relaxation Phenomena and Mechanism of Ion Conductivity of [Nafion/(SiO2)x] Composite Membranes

This report describes a study of the effect of SiO2 nanopowders on the mechanism of ionic motion and interactions taking place in hybrid inorganic-organic membranes based on Nafion. Five nanocomposite membranes of the formula [Nafion/(SiO2)x] with SiO2 ranging from 0 to 15 wt % were prepared by a solvent casting procedure. TG measurements demonstrated that the membranes are thermally stable up to 170 degrees C but with the loss water it changes the cluster environments and changes the conductivity properties. MDSC investigations in the 90-300 degrees C temperature range revealed the presence of three intense overlapping endothermal peaks indicated as I, II, and III. Peak I measures the order-disorder molecular rearrangement in hydrophilic polar clusters, II corresponds to the endothermic decomposition of -SO3 groups, and III describes the melting process in microcrystalline regions of hydrophobic fluorocarbon domains of the Nafion moiety. ESEM with EDAX measurements revealed that the membranes are homogeneous materials with smooth surfaces. DMA studies allowed us to measure two relaxation modes. The mechanical relaxation detected at ca. 100 degrees C is attributed to the motion of cluster aggregates of side chains and is diagnostic for R-SO3H...SiO2 nanocluster interactions. DMA disclosed that at SiO2/-SO3H (psi) molar ratios lower than 1.9, the oxoclusters act to restrict chain mobility of hydrophobic domains of Nafion and the dynamics inside polar cages of [Nafion/(SiO2)x] systems; at psi higher than 1.9, the oxoclusters reduce the cohesiveness of hydrophilic polar domains owing to a reduction in the density of cross-links. FT-IR and FT-Raman studies of the [Nafion/(SiO2)x] membranes indicated that the fluorocarbon chains of Nafion hydrophobic domains assume the typical helical conformation structure with a D(14pi/15) symmetry. These analyses revealed four different species of water domains embedded inside polar cages and their interconnecting channels: (a) bulk water [(H2O)n]; (b) water solvating the oxonium ions directly interacting with sulfonic acid groups [H3O+...SO3(-)-].(H2O)n; (c) water aggregates associated with H3O+ ions [H3O+.(H2O)n]; and (d) low associated water species in dimer form [(H2O)2]. The conductivity mechanism and relaxation events were investigated by broadband dielectric spectroscopy (BDS). [Nafion/(SiO2)x] nanocomposite membranes were found to possess two different molecular relaxation phenomena which are associated with the alpha-relaxation mode of PTFE-like fluorocarbon domains and the beta-relaxation mode of acid side groups of the Nafion component. Owing to their strong coupling, both these relaxation modes are diagnostic for the interactions between the polar groups of the Nafion host polymer and the (SiO2)x oxoclusters and play a determining role in the conductivity mechanism of the membranes. The studies support the proposal that long-range proton charge transfer in [Nafion/(SiO2)x] composites takes place due to a mechanism involving exchange of the proton between the four water domains. This latter proton transfer occurs owing to a subsequent combination of domain intersections resulting from the water domain fluctuations induced by the molecular relaxation events of host Nafion polymer.

Dielectric and mechanical relaxation behavior in poly(butyl methacrylate) isomers

Dielectric and mechanical spectroscopies have been used to study the dynamics in poly(butyl methacrylate) isomers (namely PnBMA, PiBMA and PtBMA). This analysis has revealed four relaxation processes: δ, γ, β and α and has allowed the observation of the evolution of molecular relaxation phenomena with the lateral chain steric hindrance. The α relaxation is associated with the glass transition of the isomers and depends strongly on the lateral chain structure. The secondary relaxation phenomena are very similar for the three isomers. Arrhenius diagrams have been plotted for the three isomers. The activation parameters have been calculated from Arrhenius equation for the sub-glass processes δ and γ whereas the dynamics are characterized by a Vogel–Fulcher–Tamman law for the α relaxations. With a deconvolution procedure for the α and β processes, a departure from the Arrhenius law of the β process in the vicinity of the merging between the α and β phenomena is observed for each isomer.

Dynamic phenomena in caseinate–monoglyceride mixed films at the air–water interface

The aim of this work is to perform a comparative study of the long-term relaxation phenomena and surface dilatational characteristics in protein and monoglyceride (monopalmitin, monoolein and monolaurin) mixed monolayers spread at the air–water interface as a function of processing variables (surface composition, surface pressure and temperature) and aqueous phase pH. The study has been centred on a real protein (caseinate), to see if the relaxation phenomena observed in protein–monoglyceride mixed films with a model protein (β-casein) are generic. Different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial dilatational rheology) have been used to analyse the static (structure, reflectivity, miscibility, and interactions) and dynamic characteristics (long-term relaxation phenomena and surface dilatational properties) of protein–monoglyceride mixed films spread on the air–water interface. The static and dynamic characteristics of the mixed films depend on the protein–monoglyceride ratio and the surface pressure. At higher surface pressures, collapsed protein residues may be displaced from the interface by monoglyceride molecules with important repercussions on the interfacial characteristics of the mixed films. From the frequency dependence of the surface dilatational modulus we have deduced the relationships between interfacial dilatational rheology and changes in molecular structure, interactions, miscibility, and relaxation phenomena.

Structure, miscibility, and rheological characteristics of beta-casein-monoglyceride mixed films at the air-water interface.

In this work we have used different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial dilatational rheology) to analyze the static (structure, morphology, reflectivity, miscibility, and interactions) and dynamic characteristics (surface dilatational properties) of beta-casein and monoglyceride (monopalmitin and monoolein) mixed films spread on the air-water interface. The static and dynamic characteristics of the mixed films depend on the interfacial composition and the surface pressure. At higher surface pressures, collapsed beta-casein residues may be displaced from the interface by monoglyceride molecules with important repercussions on the interfacial characteristics of the mixed films. From the frequency dependence of the surface dilatational properties, we have elucidated the relationships between interfacial dilatational rheology and changes in molecular structure, interactions, miscibility, and relaxation phenomena in protein-monoglyceride mixed films.

Static and Dynamic Properties of a Whey Protein Isolate and Monoglyceride Mixed Films at the Air−Water Interface

The static and dilatational properties of mixed emulsifiers are of interest due to their importance in relation to dispersion formation and stability. In this work, we have used different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial dilatational rheology), to analyze the static (structure, morphology, reflectivity, and miscibility) and dynamic (surface dilatational properties) characteristics of whey protein isolate (WPI) and monoglyceride (monopalmitin or monoolein) mixed films spread on the air−water interface. The static and dynamic characteristics of the mixed films depend on monolayer composition and the surface pressure. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the interfacial characteristics of the mixed films. A close relationship between interfacial dilatational rheology and changes in molecular structure, interactions, miscibility, and relaxation phenomena has been established from the frequency dependence of the surface dilatational properties.

Dielectric relaxation behaviour of poly(ethylene naphthalene 2,6 dicarboxylate) (PEN)

Dielectric spectroscopy has been used to study poly(ethylene naphthalene 2,6 dicarboxylate) (PEN) samples of different morphologies obtained by thermally treating bi-axially stretched PEN films. Neat and thermally treated samples of PEN films have been characterised by differential scanning calorimetry in order to measure the glass transition and melting temperatures as well as the degrees of crystallinity. Dielectric analysis has allowed the observation of the evolution of molecular relaxation phenomena with morphology changes and has revealed three relaxation processes: β-, β *- and α-relaxation (increasing temperature). The β-relaxation is associated with local motions of ester and the β *-relaxation with partially cooperative motions of naphthalene groups. The latter has been shown to be related to the morphology of the materials under study. The α-relaxation associated to the glass transition of PEN corresponds to cooperative motions induced by conformational rearrangements of the main chain and depends also on the morphology of the PEN films. Dielectric relaxation behaviours were compared using the activation energies calculated from Arrhenius equation formalism for the two sub-glass processes. Vogel–Fulcher–Tammann fits were performed on the α-relaxation. Our important contribution for the bi-axially stretched PEN films study is related to the assignment of the β *-relaxation that can be attributed to naphthalene aggregates.

Molecular relaxations in the composites of liquid crystalline cellulose derivatives with poly(acrylic acid)

The molecular relaxation phenomena of the specific polymer composites obtained by photopolymerisation of the oriented lyotropic liquid-crystalline systems composed of cellulose derivatives dissolved in photo-polymerisable acrylic acid are studied. We have investigated the composites based on two cellulose derivatives, which differ by the length of their side-chains and consequently by their physical properties.In this work, the molecular relaxations of such aniso- tropic composites were studied by dielectric relaxation spectroscopy and by thermooptical analysis. In the dielectric relaxation spectroscopy two representations were analysed: temperature dependences of dielectric loss ϵ“(T) and of electric modulus M“(T). The electric modulus representation is especially convenient to monitor the relaxations in a high temperature range where the ionic conductivity dominates the dielectric response.

What is the maximum yield in the solid state cinnamic acid dimerisation? A combinatorial mathematical approach

Recurrence relations have been employed to obtain the maximum possible theoretical conversion in a topochemically controlled organic solid state reaction. Experimental deviations from this analytical value of ≈82% reflect the importance of defects, molecular relaxation and surface phenomena in real crystals.

Non-maxwellian velocity distributions and molecular beam intensities in sonic nozzle expansions

The thermal conduction model is used to calculate the second moment of the kinetic energy distributions in both the parallel and perpendicular degrees of freedom of sonic nozzle expansions. Comparisons with the first moment affords a measure of the departure from maxwellian distributions in these coordinates. Agreement between the computed parallel velocity distribution and measurements described in the literature is good. A Monte Carlo simulation is then used to get further information on the perpendicular velocity distribution at large distances and, in particular, on the intensity of molecular beams which can be skimmed from these expansions. The sonic nozzle expansion constitutes a potentially useful medium for the study of molecular relaxation phenomena. Our understanding of the evolution of the parallel temperature is now virtually complete. The present study has also shown how Monte Carlo calculations can supplement the thermal conduction model to get information on the less tractable problem of the perpendicular velocity distribution. The methodology can be readily extended to polyatomic media [5]. We have focused on the central portion of the perpendicular velocity distribution, as this is the regime of greatest interest. It is disappointing to f

Study by microwave spectroscopy of vibrational energy transfer processes in the N* 2—OCS system

Some experiments on vibrational energy transfer through active nitrogen have been developed for studying microwave spectra in high excited states of N 2O and OCS. The use of a square wave modulation of the excitation of N 2 has allowed the observation of the time variations of the absorption in different vibrational states of OCS. A theoretical model which takes into account the molecular relaxation and propagation phenomena has been derived to fit the experimental results in the (00°1) OCS level. A value of the rate constant K e of the intermolecular V → V energy transfer between active nitrogen and OCS (00°1) has been obtained: K e = (1 ± 0.2) × 10 3 s −1 Torr −1.

Quantum statistical mechanical (QSM) approach to molecular relaxation phenomena: An irreversible thermodynamic theory for radiationless transitions

The quantum statistical mechanical (QSM) approach to molecular relaxation phenomena is extended to allow for nonexponential decay and the incorporation of external disturbances. Microscopic models can be substituted into the final results to study the temporal evolution of multistate systems. The relationship of the present version of QSM theory to Onsager’s theory of linear irreversible processes is discussed. It is shown that kinetics and the principle of detailed balance emerge from QSM theory. The results of QSM theory are related to Fermi’s golden rule, the scattering theoretic formulation of radiationless transitions, and Kubo’s generating function technique.

A Comparison of radiationless transitions involving single-minimum and double minima potential energy surfaces

The quantum-statistical-mechanical (QSM) approach to molecular relaxation phenomena is employed to compare radiationless transitions originating from an electronic state characterized by a single minimum and double minima potential surface for a vibronically active, non-totally symmetric mode. The vibronic level dependence of the decay rates of these two cases has been investigated for both small and large energy gap transitions. It is shown that the behavior of a molecular system is quite different for an initial state possesing a double minima potential surface as compared to the case in which the initial state possesses a single minimum.