Calorimetric Glass Transition Temperature Research Articles

Molecular mobility and thermal transitions study in renewable PLA-polyols star-like copolymers

The molecular dynamics mapping in poly(lactic acid) (PLA)-based star-like copolymers is performed herein, employing dielectric spectroscopy techniques supplemented by calorimetry and other structure characterization methods. Next to the neat linear PLA, three star-like copolymers, with 3, 4 and 5 branches were synthesized via ring opening polymerization at the presence of, respectively, glycerol, pentaerythritol and sorbitol (initiators). When increasing the branch number, consequently, the overall molar mass drops, from 55 kg/mol (neat PLA) to 17k (PLA@glycerol), 25k (PLA@pentaerythritol), and 14k (PLA@sorbitol) and, thus, the branch length decreases to ∼6k, ∼6k, and ∼3 k g/mol in the stars. The latter leads to faster segmental mobility, documented by the accelerated α relaxation and the drop in the calorimetric and dielectric glass transition temperatures. Simultaneously, the cooperativity (chains’ fragility) drops. Together these results suggest possibly the increase in free volume, a factor that can potentially favour the physical biodegradability. Next to the segmental dynamics, the local beta relaxation is recorded to have a mild dependence on the type of star (mainly decelerates), while the rare case of Normal Mode relaxation is recorded here only in the stars, providing additional support for the quite short branches. The overall data suggest the co-implementation of two antagonistic effects on mobility, i.e., the chain length changes in parallel to the constraints imposed by the star-like structure. Moreover, the in general low crystallizability of linear PLA further drops in the stars in terms of both suppressed crystalline fraction and nucleation, as well as lower quality of crystals. The materials exhibit single thermal transitions, indicating a homogeneous character, whereas a variety of semicrystalline morphologies can be achieved via both the composition and the thermal treatment. This provides a tool for tuning various properties and, consequently, the manipulation of the macroscopic performance.

AFM dilatometry measurements on ultra‐stable fluoropolymer glasses: Further evidence of extreme fictive temperature reduction

AbstractUltra‐stable amorphous fluoropolymers glasses were created using vacuum pyrolysis deposition (VPD). Glass films with thickness ranging from 90 to 160 nm were grown at a substrate temperature of 0.86 Tg, where Tg is the glass transition temperature of the virgin polymer and is in units of K. Atomic force microscope (AFM) dilatometry measurements were conducted to investigate density behavior of the ultra‐stable glasses. Thickness measurements were made in stepwise fashion over a range of temperatures from ambient to above the Tg. Results show that the intersections of the line for the equilibrium liquid and those for the rejuvenated and stable glasses identifying the fictive temperature Tf result in Tf, rejuvenated = 347.3 K and Tf, stable = 269.5 K, that is, nearly 80 K below the Tg of the rejuvenated material and well below the notional Kauzmann temperature as estimated from the Vogel‐Fultcher‐Tammann (VFT) analysis of the cooling rate dependence of the calorimetric glass transition temperature reported previously. The results corroborate the published calorimetric results on the same ultra‐stable fluoropolymer glasses that witnessed Tf reductions of up to 62.6 K below the Tg of the rejuvenated system. In addition, to demonstrate the versatility of the AFM dilatometry methodology for the thin film response, isothermal de‐aging experiments were carried out to illustrate the devitrification kinetics. We also carried out one of the Kovacs’ signature key experiments, the asymmetry of approach, to further illustrate the method.

Molecular Dynamics and Diffusion in Amorphous Solid Dispersions Containing Imidacloprid.

The main goal of this study is to develop an experimental toolbox to estimate the self-diffusion coefficient of active ingredients (AI) in single-phase amorphous solid dispersions (ASD) close to the glass transition of the mixture using dielectric spectroscopy (DS) and oscillatory rheology. The proposed methodology is tested for a model system containing the insecticide imidacloprid (IMI) and the copolymer copovidone (PVP/VA) prepared via hot-melt extrusion. For this purpose, reorientational and the viscoelastic structural (α-)relaxation time constants of hot-melt-extruded ASDs were obtained via DS and shear rheology, respectively. These were then utilized to extract the viscosity as well as the fragility index of the dispersions as input parameters to the fractional Stokes-Einstein (F-SE) relation. Furthermore, a modified version of Almond-West (AW) formalism, originally developed to describe charge diffusion in ionic conductors, was exercised on the present model system for the estimation of the AI diffusion coefficients based on shear modulus relaxation times. Our results revealed that, at the calorimetric glass-transition temperature (Tg), the self-diffusion coefficients of the AI in the compositional range from infinite dilution up to 60 wt % IMI content lied in the narrow range of 10-18-10-20 m2 s-1, while the viscosity values of the dispersions at Tg varied between 108 Pa s and 1010 Pa s. In addition, the phase diagram of the IMI-PVP/VA system was determined using the melting point depression method via differential scanning calorimetry (DSC), while mid-infrared (IR) spectroscopy was employed to investigate the intermolecular interactions within the solid dispersions. In this respect, the findings of a modest variation in melting point at different compositions stayed in agreement with the observations of weak hydrogen bonding interactions between the AI and the polymer. Moreover, IR spectroscopy showed the intermolecular IMI-IMI hydrogen bonding to have been considerably suppressed, as a result of the spatial separation of the AI molecules within the ASDs. In summary, this study provides experimental approaches to study diffusivity in ASDs using DS and oscillatory rheology, in addition to contributing to an enhanced understanding of the interactions and phase behavior in these systems.

Infrared Spectral Classification of Natural Bitumens for Their Rheological and Thermophysical Characterization

Natural bitumens consist of many molecules whose chemical composition depends on the oilfield and determines the physicochemical properties of the bitumens as materials. Infrared (IR) spectroscopy is the fastest and least expensive method to assess the chemical structure of organic molecules, which makes it attractive in terms of rapid prediction of the properties of natural bitumens based on their composition evaluated in this way. In this work, IR spectra were measured for ten samples of natural bitumens significantly different in properties and origin. Based on the ratios of certain IR absorption bands, bitumens are proposed to be divided into paraffinic, aromatic, and resinous. In addition, the internal relationship between IR spectral characteristics of bitumens, such as polarity, paraffinicity, branchiness, and aromaticity, is shown. A study of phase transitions in bitumens by differential scanning calorimetry was carried out, and the use of a heat flow differential to find hidden points of bitumens' glass transitions is proposed. Furthermore, the dependences of the total melting enthalpy of crystallizable paraffinic compounds on the aromaticity and branchiness of bitumens are demonstrated. A detailed study of bitumens' rheology in a wide temperature range was carried out, and characteristic features of rheological behavior for different bitumen classes are revealed. Based on the viscous properties of bitumens, their glass transition points were found and compared with the calorimetric glass transition temperatures and nominal solid-liquid transition points obtained from temperature dependences of bitumens' storage and loss moduli. The dependences of viscosity, flow activation energy, and glass transition temperature of bitumens on their IR spectral characteristics are shown, which can be used to predict the rheological properties of bitumens.

Mechanical vs calorimetric glass transition temperature in the oxidation of linoleic acid from condensed κ-carrageenan/glucose syrup systems

Lipid oxidation remains a concern leading to deterioration of the organoleptic quality in processed goods, but it should be minimized by considering changes in the structural properties of foods. The present work investigates the effect of glass transition temperature on the oxidation of linoleic acid at different concentrations of κ-carrageenan/glucose syrup. A combination of 0.5, 1, 2 and 3% (w/w) κ-carrageenan and 82.5, 82, 81, 80% (w/w) glucose syrup were mixed with 1.5% (w/w) linoleic acid and 0.5% (w/w) lecithin to prepare samples with a total solid content of 85% (w/w). Physicochemical properties of these mixtures were recorded using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). This was followed by estimation of the mechanical (Tgm) and calorimetric (Tgc) glass transition temperatures using small deformation dynamic oscillation in-shear and modulated differential scanning calorimetry (MDSC). The linoleic acid oxidation in the condensed polysaccharide/co-solute system was evaluated by monitoring the accumulation of hydroperoxide (ROOH) with UV–vis spectroscopy over a wide temperature range (−25 to 0 °C). The oxidation phase was modelled following a sigmoidal kinetic model indicating initiation and propagation stages of lipid oxidation. ROOH production increased as a function of time and temperature. The structural relaxation of the polymeric matrix influenced the oxidation rate at the initiation stage. At the propagation phase, Tgm appears to control the rate of ROOH formation (kf) and decomposition (kd) in all preparations, as compared to Tgc, an outcome that makes the former an important concept of quality control.

Thermodynamic heat-transfer phenomena in nanostructured glassy substances: a comparative study on g-As5Se95 and g-As55Se45

Nanostructurization-driven responses in calorimetric heat-transfer phenomena are compared for glassy arsenoselenides at different levels of their molecular network organization, namely in high-polymerized g-As5Se95 and low-polymerized g-As55Se45, employing multifrequency temperature-modulated DSC-TOPEM® method complemented with Raman scattering microspectroscopy. It is shown that high-polymerized network composed of AsSe3 pyramids interlinked by Se chains with small number of Se8 molecular units decoupled from this network prevails in melt-quenched and nanomilling-derived g-As5Se95. Transition to more polymerized network due to incorporation of destroyed Se8 molecules into glass backbone occurs in this glass under nanostructurization. As a result, nanostructurization-driven calorimetric response in g-As5Se95 dominates by size-induced glass-transition temperature depression. The low-polymerized structure of g-As55Se45 is built of As–Se network enriched with tetra-arsenic selenide molecular units decoupled from this network. Molecular-to-network transition owing to nanomilling-driven destruction of these cage molecules and their incorporation into newly polymerized glass-forming backbone occurs in g-As55Se45 resulting in strong increase in calorimetric glass-transition temperature.

Accelerated aging of 18 years rejuvenated polylactide bottles by fast scanning calorimetry

AbstractThis study focuses on the accelerated physical aging kinetics of rejuvenated polylactide (PLA) that has previously been processed and stored during 18 years under ambient conditions. The finish of a conditioned PLA bottle is subjected to structural relaxation for times varying between 0.001 and 1000.0 min at 45°C, that is, 15°C below its calorimetric glass transition temperature. The recovery of in‐situ aged PLA is monitored by fast scanning calorimetry and it is observed that the thermodynamic equilibrium is reached within the experimental aging time. Moreover, the time evolution of the enthalpy of recovery finds consistency with results obtained for PLA granules which did not undergo additional processing neither prolonged storage nor service life conditions. Thus, it is deduced that both processing and storage do not induce major degradation reactions that could affect the aging kinetics. Therefore, the behavior of a structurally rejuvenated amorphous PLA is depicted through the one of an unaged PLA.

Pressure-annealed high-density amorphous ice made from vitrified water droplets: A systematic calorimetry study on water's second glass transition.

In previous work, water's second glass transition was investigated based on an amorphous sample made from crystalline ice [Amann-Winkel et al., Proc. Natl. Acad. Sci. U. S. A. 110, 17720 (2013)]. In the present work, we investigate water's second glass transition based on the genuine glassy state of high-density water as prepared from micron-sized liquid water droplets, avoiding crystallinity at all stages. All the calorimetric features of water's second glass transition observed in the previous work are also observed here on the genuine glassy samples. This suggests that the glass transition indeed thermodynamically links amorphous ices continuously with deeply supercooled water. We proceed to extend the earlier study by investigating the effect of preparation history on the calorimetric glass transition temperature. The best samples prepared here feature both a lower glass transition temperature Tg,2 and a higher polyamorphic transition temperature Tons, thereby extending the range of thermal stability in which the deeply supercooled liquid can be observed by about 4K. Just before the polyamorphic transition, we observe a spike-like increase of heat capacity that we interpret in terms of nucleation of low-density water. Without this spike, the width of water's second glass transition is 15K, and the Δcp amounts to 3 ± 1JK-1 mol-1, making the case for the high-density liquid being a strong liquid. We suggest that samples annealed at 1.9GPa to 175K and decompressed at 140K to ≥0.10GPa are free from such nuclei and represent the most ideal high-density amorphous glasses.

Statistical mechanics of coupled supercooled liquids in finite dimensions

We study the statistical mechanics of supercooled liquids when the system evolves at a temperature TT with a field \epsilonϵ linearly coupled to its overlap with a reference configuration of the same liquid sampled at a temperature T_0T0. We use mean-field theory to fully characterize the influence of the reference temperature T_0T0, and we mainly study the case of a fixed, low-T_0T0 value in computer simulations. We numerically investigate the extended phase diagram in the (\epsilon,T)(ϵ,T) plane of model glass-forming liquids in spatial dimensions d=2d=2 and d=3d=3, relying on umbrella sampling and reweighting techniques. For both 2d2d and 3d3d cases, a similar phenomenology with nontrivial thermodynamic fluctuations of the overlap is observed at low temperatures, but a detailed finite-size analysis reveals qualitatively distinct behaviors. We establish the existence of a first-order transition line for nonzero \epsilonϵ ending in a critical point in the universality class of the random-field Ising model (RFIM) in d=3d=3. In d=2d=2 instead, no phase transition is found in large enough systems at least down to temperatures below the extrapolated calorimetric glass transition temperature T_gTg. Our results confirm that glass-forming liquid samples of limited size display the thermodynamic fluctuations expected for finite systems undergoing a random first-order transition. They also support the relevance of the physics of the RFIM for supercooled liquids, which may then explain the qualitative difference between 2d2d and 3d3d glass-formers.

The Effect of High Protein Powder Structure on Hydration, Glass Transition, Water Sorption, and Thermomechanical Properties.

Poor solubility of high protein milk powders can be an issue during the production of nutritional formulations, as well as for end-users. One possible way to improve powder solubility is through the creation of vacuoles and pores in the particle structure using high pressure gas injection during spray drying. The aim of this study was to determine whether changes in particle morphology effect physical properties, such as hydration, water sorption, structural strength, glass transition temperature, and α-relaxation temperatures. Four milk protein concentrate powders (MPC, 80%, w/w, protein) were produced, i.e., regular (R) and agglomerated (A) without nitrogen injection and regular (RN) and agglomerated (AN) with nitrogen injection. Electron microscopy confirmed that nitrogen injection increased powder particles’ sphericity and created fractured structures with pores in both regular and agglomerated systems. Environmental scanning electron microscopy (ESEM) showed that nitrogen injection enhanced the moisture uptake and solubility properties of RN and AN as compared with non-nitrogen-injected powders (R and A). In particular, at the final swelling at over 100% relative humidity (RH), R, A, AN, and RN powders showed an increase in particle size of 25, 20, 40, and 97% respectively. The injection of nitrogen gas (NI) did not influence calorimetric glass transition temperature (Tg), which could be expected as there was no change to the powder composition, however, the agglomeration of powders did effect Tg. Interestingly, the creation of porous powder particles by NI did alter the α-relaxation temperatures (up to ~16 °C difference between R and AN powders at 44% RH) and the structural strength (up to ~11 °C difference between R and AN powders at 44% RH). The results of this study provide an in-depth understanding of the changes in the morphology and physical-mechanical properties of nitrogen gas-injected MPC powders.

Direct and indirect effects on molecular mobility in renewable polylactide-poly(propylene adipate) block copolymers as studied via dielectric spectroscopy and calorimetry.

In this work, we study a series of sustainable block copolymers based on polylactide, PLA, and poly(propylene adipate), PPAd, both polymers being prepared from renewable resources. Envisaging a wide range of future applications in the frame of a green and circular economy, e.g., packaging materials replacing conventional petrochemicals, the employment of PPAd aims at lowering the glass transition and melting temperatures of PLA and, finally, facilitation of the enzymatic degradation and compostability. The copolymers have been synthesized via ring opening polymerization of lactides in the presence of propylene adipate oligomers (5, 15 and 25%). The direct effects on the molecular mobility by the structure/composition are assessed in the amorphous state employing broadband dielectric spectroscopy (BDS) and calorimetry. BDS allowed the recording of local PLA and PPAd dynamics in all cases. The effects on local relaxations suggest favoring of interchain interactions, both PLA-PPAd and PPAd-PPAd. Regarding the more important segmental dynamics, the presence of PPAd leads to faster polymer chain diffusion, as monitored by the significant lowering of the dielectric and calorimetric glass transition temperature, Tg. This suggests the plasticizing role of PPAd on PLA (majority) in combination with the lowering of the average molar mass, Mn, in the copolymers from ∼75 to ∼30 kg mol-1, which is the actual scope for the synthesis of these materials. Interestingly, a strong suppression in fragility (chain cooperativity) is additionally recorded. In contrast to calorimetry and due to the high resolving power of BDS, for the higher PPAd fraction, the weak segmental relaxation of PPAd was additionally recorded. Overall, the recordings suggest a strong increase in free volume and two individual dynamic states, one for 0 and 5% PPAd and another for 15 and 25% PPAd. Within the latter, we gained indications for partial phase nano-separation of PPAd. Regarding indirect effects, these were followed via crystallization. Independent of the method of crystallization, namely, melt or cold, the presence of PPAd led to the systematic lowering of crystallization and melting temperatures and enthalpies. The effects reflect the decrease of crystalline nuclei, which is confirmed by optical microscopy as in the copolymers fewer although larger crystals are formed.

Microscopic versus Macroscopic Glass Transition(s) in Blends of Industrial Interest

We investigate by neutron scattering and calorimetry a mixture of styrene-butadiene rubber (SBR) and a commercial resin. The neat materials present a large dynamic contrast, having SBR a much lower glass-transition temperature than the resin. The focus is to exploit neutron scattering selectivity in an isotopically labelled sample where deuterated SBR is the majority component. This direct insight into the resin atomic motions within the mixture allows determining the ‘microscopic’ glass transition of the resin in the presence of the a priori much more mobile SBR. This transition takes place in the vicinity of the initial calorimetric glass transition temperature, mainly dictated by the majority component.

Glass transition effects on the molecular transport of caffeine from condensed k-carrageenan/polydextrose systems

We investigate the effect of added KCl on the glass transition of potassium-κ-carrageenan/polydextrose and molecular transport of caffeine. Samples were prepared with 2% (w/w) κ-carrageenan, 82% (w/w) polydextrose and 1% (w/w) caffeine to a total solids level of 85% (w/w). KCl was added at 0, 50, 100 and 200 mM to induce a dependence of structural properties on the potassium ion concentration. High-resolution spectral data over a wide spectral range for the tertiary system were collected using Fourier transform infrared spectroscopy (FTIR), which were complemented by tangible evidence of its morphology with scanning electron microscopy (SEM). Work then focused on the estimation of the mechanical (Tgm) and calorimetric (Tgc) glass transition temperature using in-shear dynamic oscillation and modulated differential scanning calorimetry (MDSC). Molecular transport of caffeine in the condensed polysaccharide/co-solute system was followed with UV–vis spectroscopy over a wide temperature range (−20 to 30 °C) and modelled with extended diffusion theory. Time-dependent mass transport of the diffusant was correlated to the structural relaxation of the polymeric matrix seen in increasing values of the mechanical glass transition temperature with higher additions of potassium counterions to κ-carrageenan. It was concluded that the mechanical glass transition temperature has a prominent effect on the release kinetics of caffeine in these materials, as compared to the calorimetric glass transition temperature.

Inter-enantiomer conversion dynamics and Johari\u2013Goldstein relaxation of benzophenones

We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted ortho-bromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. Despite the significantly higher mass of the brominated derivative, its dynamic and calorimetric glass transition temperatures are only ten degrees higher than those of benzophenone. The kinetic fragility index of the halogenated molecule is lower than that of the parent compound, and is found to decrease with increasing pressure. By a detailed analysis of the dielectric loss spectra, we provide evidence for the existence of a Johari–Goldstein (JG) relaxation in both compounds, thus settling the controversy concerning the possible lack of a JG process in benzophenone and confirming the universality of this dielectric loss feature in molecular glass-formers. Both compounds also display an intramolecular relaxation, whose characteristic timescale appears to be correlated with that of the cooperative structural relaxation associated with the glass transition. The limited molecular flexibility of ortho-bromobenzophenone allows identifying the intramolecular relaxation as the inter-enantiomeric conversion between two isoenergetic conformers of opposite chirality, which only differ in the sign of the angle between the brominated aryl ring and the coplanar phenyl-ketone subunit. The observation by dielectric spectroscopy of a similar relaxation also in liquid benzophenone indicates that the inter-enantiomer conversion between the two isoenergetic helicoidal ground-state conformers of opposite chirality occurs via a transition state characterized by a coplanar phenyl-ketone moiety.

Decoupling between calorimetric and dynamical glass transitions in high-entropy metallic glasses

Glass transition is one of the unresolved critical issues in solid-state physics and materials science, during which a viscous liquid is frozen into a solid or structurally arrested state. On account of the uniform arrested mechanism, the calorimetric glass transition temperature (Tg) always follows the same trend as the dynamical glass transition (or α-relaxation) temperature (Tα) determined by dynamic mechanical analysis (DMA). Here, we explored the correlations between the calorimetric and dynamical glass transitions of three prototypical high-entropy metallic glasses (HEMGs) systems. We found that the HEMGs present a depressed dynamical glass transition phenomenon, i.e., HEMGs with moderate calorimetric Tg represent the highest Tα and the maximum activation energy of α-relaxation. These decoupled glass transitions from thermal and mechanical measurements reveal the effect of high configurational entropy on the structure and dynamics of supercooled liquids and metallic glasses, which are associated with sluggish diffusion and decreased dynamic and spatial heterogeneities from high mixing entropy. The results have important implications in understanding the entropy effect on the structure and properties of metallic glasses for designing new materials with plenteous physical and mechanical performances.

Manipulation of the Glass Transition Properties of a High-Solid System Made of Acrylic Acid-N,N'-Methylenebisacrylamide Copolymer Grafted on Hydroxypropyl Methyl Cellulose.

Crosslinking of hydroxypropyl methyl cellulose (HPMC) and acrylic acid (AAc) was carried out at various compositions to develop a high-solid matrix with variable glass transition properties. The matrix was synthesized by the copolymerisation of two monomers, AAc and N,N′-methylenebisacrylamide (MBA) and their grafting onto HMPC. Potassium persulfate (K2S2O8) was used to initiate the free radical polymerization reaction and tetramethylethylenediamine (TEMED) to accelerate radical polymerisation. Structural properties of the network were investigated with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), modulated differential scanning calorimetry (MDSC), small-deformation dynamic oscillation in-shear, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The results show the formation of a cohesive macromolecular entity that is highly amorphous. There is a considerable manipulation of the rheological and calorimetric glass transition temperatures as a function of the amount of added acrylic acid, which is followed upon heating by an extensive rubbery plateau. Complementary TGA work demonstrates that the initial composition of all the HPMC-AAc networks is maintained up to 200 °C, an outcome that bodes well for applications of targeted bioactive compound delivery.

PEG-POSS Star Molecules Blended in Polyurethane with Flexible Hard Segments: Morphology and Dynamics.

A star polymer with a polyhedral oligomeric silsesquioxanne (POSS) core and poly(ethylene glycol) (PEG) vertex groups is incorporated in a polyurethane with flexible hard segments in-situ during the polymerization process. The blends are studied in terms of morphology, molecular dynamics, and charge mobility. The methods utilized for this purpose are scanning electron and atomic force microscopies (SEM, AFM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and to a larger extent dielectric relaxation spectroscopy (DRS). It is found that POSS reduces the degree of crystallinity of the hard segments. Contrary to what was observed in a similar system with POSS pendent along the main chain, soft phase calorimetric glass transition temperature drops as a result of plasticization, and homogenization of the soft phase by the star molecules. The dynamic glass transition though, remains practically unaffected, and a hypothesis is formed to resolve the discrepancy, based on the assumption of different thermal and dielectric responses of slow and fast modes of the system. A relaxation α′, slower than the bulky segmental α and common in polyurethanes, appears here too. A detailed analysis of dielectric spectra provides some evidence that this relaxation has cooperative character. An additional relaxation g, which is not commonly observed, accompanies the Maxwell Wagner Sillars interfacial polarization process, and has dynamics similar to it. POSS is found to introduce conductivity and possibly alter its mechanism. The study points out that different architectures of incorporation of POSS in polyurethane affect its physical properties by different mechanisms.

Charge Transport and Dynamics in Imidazolium Chloroaluminate Ionic Liquids

Ion dynamics and charge transport in 1-alkyl-3-methylimidazolium chloroaluminate ionic liquids were investigated over broad temperature and frequency ranges by broadband dielectric spectroscopy and differential scanning calorimetry. The timescale of ion dynamics, as captured by the imaginary component of the complex dielectric modulus as well as the calorimetric glass transition temperature (Tg), is strongly composition-dependent, with the fastest dynamics, lowest Tg, and highest fragility observed for the Lewis acidic 67 mol% AlCl3 + 1-ethyl-3-methylimidazolium chloride composition. The DC ionic conductivity scales directly by the ion dynamics for all compositions indicating the primary effect of ion speciation is an alteration of the underlying ion mobility. The fragility (m) of the 67 mol% AlCl3 + 1-ethyl-3-methylimidazolium chloride ionic liquid, m=148, is among the highest reported fragilities for a glass-forming liquid.

Mechanical versus calorimetric glass transition temperature in the diffusion of nicotinic acid from a condensed gelatin/glucose syrup system

Condensed systems of 25% (w/w) bovine gelatin (Mw = 173 kDa) or 25% (w/w) fish gelatin (Mw = 60 kDa) with 59% (w/w) glucose syrup and 1% (w/w) nicotinic acid to a total solids level of 85% (w/w) were prepared in this study. Analysis was carried out using Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and small deformation dynamic oscillation in-shear. Work allowed elucidation of the micromolecular and macromolecular properties focusing on the estimation of the mechanical (Tgm) and calorimetric (Tgc) glass transition temperature of these systems. A combined protocol of free volume and reaction rate theories was considered to rationalise results in the vicinity of the glass transition region. Following this, a diffusion procedure of nicotinic acid was set up over a wide temperature range (from 5 to −40 °C) using UV–vis spectroscopy and an appropriate chromogenic reaction to detect the presence of the bioactive compound. Time dependence of the mass transfer of nicotinic acid throughout the delivery vehicle of gelatin/co-solute was followed using diffusion theory. The treatise unveiled the correlation between structural relaxation of the vitrified matrix and release kinetics of the diffusant.

Dynamic processes and charge carriers transport in polyvinyl acetate–polyaniline composites

This work studies the effect of composite formation between polyvinyl acetate (PVAc) and conductive polyaniline (PAni) on its conductivity mechanism. To achieve this goal, different concentrations of PAni in mini-emulsion of PVAc are prepared. The dynamic relaxation and electric conductivity are evaluated using broadband dielectric spectroscopy technique. The relaxation rate of the local reorientations (β-process) of ester group follows an Arrhenius relation with activation energy 39 kJ/mole. The coupling of the segmental dynamic (α-process) and terminal fluctuation (β′-process) occurs just above the calorimetric glass transition temperatures, Tg, of pure PVAc. This is reasonable since α-process that originated from a structural rearrangement became very slow below Tg which takes the polymer toward its equilibrium state. There is also decoupling of the charge carriers transport, and the segmental motion, the concept concluded in this manuscript, sounds reasonable if one thinks about the ion motion at T < Tg. This conclusion is straightforwardly deduced from the experimental data, as explained in more detail. The study also confirms that PAni electrical conductivity mechanism is conserved.