Activation Energy Of Relaxation Research Articles

Flame retardant epoxy vitrimers employing a Meldrum's acid-functionalized and phosphorus-containing bisphenol compound as the cross-linking agent for conventional epoxy

In addition to recycling and reprocessing features, designs of vitrimers possessing critical properties required for conventional cross-linked polymers are of interest. In this work, a newly designed Meldrum's acid (MA)-functionalized and phosphorus-containing bisphenol compound (P-MADV) compound is synthesized. Employing P-MADV as a curing agent, simultaneous formation of permanent and dynamic covalent linkages through one-pot reaction has been achieved in formation of flame retardant epoxy resin vitrimers. The newly synthesized P-MADV compound is an effective cross-linking agent for commercially-available epoxy resins for synthesis of flame-retardant epoxy vitrimers through the conventional cross-linking conditions and processes. The cross-linked epoxy vitrimer passes the flame test under UL 94V-0 specification contributed from the phosphorus-containing groups. The flame retardant vitrimer also exhibits a storage modulus of 1.2 GPa at 50 °C and a Tg (tanδ peak) of 185 °C, which are comparable to the values found with conventional phenolic novolac-cross-linked epoxy resin (storage modulus: 1.6 GPa; Tg: 155 °C). Without addition of catalysts for dynamic bond exchanging reaction, the cross-linked epoxy vitrimer shows stress relaxation time of 397 s at 230 °C and an activation energy of stress relaxation of 97.8 kJ mol−1. The dynamic features warrant the epoxy vitrimer being recyclable under thermally pressing at 230 °C and 16 MPa for 1 h. The recycled samples exhibit thermal and mechanical properties being comparable to the pristine resin. This work has demonstrated an attractive approach for preparation of flame retardant epoxy resin vitrimers in viewpoints of both manufacturing processes and resins properties.

Enthalpy relaxation of sodium aluminosilicate glasses from thermal analysis

AbstractThe sodium aluminosilicate (NAS) glass family is important for many different industrial applications, but glass relaxation has not yet been thoroughly studied in this system. Thermal analysis techniques such as differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC) can provide insight into the enthalpy relaxation of glass by measuring the glass transition temperature (Tg), activation energy, and enthalpy of relaxation. MDSC is mostly used to study nonoxide and low Tg glasses, and there is much debate about whether the nonreversing heat flow analysis method is accurate. To the authors’ knowledge, this is the first paper using MDSC to study these NAS compositions, and one of few papers to report MDSC on high Tg oxide glasses. We report on one set of modulation conditions that obtain a linear response using Lissajous curves, as well as comparing the activation energy calculated from DSC with the enthalpy of relaxation obtained from MDSC. Our results show that the activation energy and enthalpy of relaxation do not give the same compositional minimum in relaxation, and therefore more work is needed to investigate the validity of the nonreversing heat flow approach for high Tg oxide glasses.

Reprocessable Polymer Networks Containing Sulfur-Based, Percolated Dynamic Covalent Cross-Links and Percolated or Non-Percolated, Static Cross-Links.

One method to improve the properties of covalent adaptable networks (CANs) is to reinforce them with a fraction of permanent cross-links without sacrificing their (re)processability. Here, a simple method to synthesize poly(n-hexyl methacrylate) (PHMA) and poly(n-lauryl methacrylate) (PLMA) networks containing static dialkyl disulfide cross-links (utilizing bis(2-methacryloyl)oxyethyl disulfide, or DSDMA, as a permanent cross-linker) and dynamic dialkylamino sulfur-sulfur cross-links (utilizing BiTEMPS methacrylate as a dissociative dynamic covalent cross-linker) is presented. The robustness and (re)processability of the CANs are demonstrated, including the full recovery of cross-link density after recycling. The authors also investigate the effect of static cross-link content on the stress relaxation responses of the CANs with and without percolated, static cross-links. As PHMA and PLMA have very different activation energies of their respective cooperative segmental mobilities, it is shown that the dissociative CANs without percolated, static cross-links have activation energies of stress relaxation that are dominated by the dissociation of BiTEMPS methacrylate cross-links rather than by the cooperative relaxations of backbone segments, i.e., the alpha relaxation. In CANs with percolated, static cross-links, the segmental relaxation of side chains, i.e., the beta relaxation, is critical in allowing for large-scale stress relaxation and governs their activation energies of stress relaxation.

Structural interpretation of the viscous flow and relaxation kinetics in the As-Se and Ge-Se chalcogenide systems

This study investigates the behaviour of As₂₀Se₈₀ and Ge₂₅Se₇₅ chalcogenide glasses near the glass transition temperature using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA), addressing a gap in the literature for these compositions. Precise TMA measurements of viscosity in the range of 10⁶ – 10¹³ Pa·s clarified existing data and improved the quality of fits describing the temperature-dependent viscosity for both compositions. The DSC method enabled a comprehensive analysis of structural relaxation behaviours, which were described using the Tool-Narayanaswamy-Moynihan (TNM) model with the determination of its four parameters. The study discusses the viscosity and structural relaxation behaviours of both glass-formers in relation to their structure, referencing previous results for other binary chalcogenide glasses in the As-Se and Ge-Se systems. The findings indicate significant changes in viscosity and activation energy of structural relaxation with the addition of As or Ge, while the non-linearity and non-exponentiality parameters remain largely unaffected.

Impedance spectroscopy and electrical conductivity of 0.7Bi(1−x)NdxFeO3–0.3BaTiO3 ferroelectric ceramics

0.7Bi[Formula: see text]NdxFeO3–0.3BaTiO3 (BNFO–BTO, [Formula: see text], 0.010, 0.020 and 0.050) ceramics were fabricated using the high-temperature solid-state reaction method. The X-ray diffraction (XRD) patterns revealed that the primary phase in these ceramics was pseudocubic. The Scanning Electron Microscopy (SEM) micrographs exhibited dense microstructures throughout all BNFO–BTO ceramics. Furthermore, the temperature dependence of dielectric behaviors and ferroelectric hysteresis loop shapes suggested the occurrence of a relaxor ferroelectric-type phase transition in BNFO–BTO ceramics. Additionally, the frequency dispersion characteristics and remnant polarization were enhanced with increasing substitution of Bi[Formula: see text] by Nd[Formula: see text]. Conducted impedance analysis on 0.7Bi[Formula: see text]NdxFeO3–0.3BaTiO3 ceramics and two dielectric responses of the grain at high frequency and grain boundary at low frequency were illustrated, respectively. The combination of imaginary impedance ([Formula: see text] and imaginary modulus ([Formula: see text] versus log[Formula: see text]f plots revealed that the charge carriers’ motion was not entirely long-range, short-range migration also exists. Through calculations based on Curie–Weiss Law, the relaxation activation energy ([Formula: see text] and conductance activation energy ([Formula: see text] were investigated, confirming that doping Nd[Formula: see text] effectively mitigates the concentration of oxygen vacancies (OVs) and prevents the formation of oxygen vacancies clusters, ultimately suppressing conductivity in BNFO–BTO ceramics.

Solid state synthesis of lead-free Sm-doped Na0.5Bi4.5Ti4O15 ceramics for enhanced dielectric and electrical properties

Here we report the synthesis of Sm-doped Na0.5Bi4.5Ti4O15 (Na0.5Sm0.5Bi4Ti4O15) lead-free ceramics via a conventional solid-state technique. Investigations of Na0.5Bi4.5Ti4O15 (NBT) and Na0.5Sm0.5Bi4.5Ti4O15 (NSBT) ceramics were demonstrated in detail to understand the composition-based structure-property of Aurivillius compounds and related functional material. Dielectric properties for frequency and temperature in a wide range were analyzed. The conduction activation energy values of NSBT ceramics are obtained to be 1.40 eV, whereas, the NBT ceramics get the value to be 1.31 eV. At higher temperatures, the conduction activation energy value of NSBT ceramics is 1.32 eV for both frequencies of 100 Hz and 1 kHz, whereas, for NBT compounds, the calculated value is 1.27 eV for both frequencies. The simulation performed on the impedance data for capacitive and resistance elements shows well-fitting curves which indicates a single relaxation behavior in the material. Similarly, the AC-conductivity data were analyzed which gives different conduction processes and relaxation activation energies in the NSBT ceramics.

Impact on activation energy with elevating calcination temperature of barium bismuth ferrite nanocomposite

Barium Bismuth Ferrite nanoparticles having the general formula Ba1-xBixFeO3 were successfully made by the sol-gel system, using nitrates as the fundamental material. Tartaric acid and polyvinyl alcohol were used as chelating agents. The synthesized nanoparticle powder was sintered at 650 ̊C and 750 ̊C for 3 h. Filtering with dilute nitric acid and distilled water is used to remove the impurities. The structural, optical, and dielectric properties were determined at room temperature. Using X-ray diffraction (XRD) analysis, structure, lattice parameter, microstrain, and dislocation density were calculated. Using the Scherrer equation, the peak's crystalline size was ascertained. In FTIR, the vibrational modes of the octahedral and tetrahedral metal complexes in the sample have been studied using the wavenumber in this range. The synthesized nanocomposite contains morphological features, such as interconnecting agglomerates with spherical, irregular, or non-uniform elongated shapes, as demonstrated by SEM images. The activation energy for relaxation and activation energy for conduction was measured from the Cole-Cole plot and the AC conductivity versus frequency plot respectively. The determined activation energies from the two investigations were relatively close to one another. The Barium Bismuth Ferrite nanoparticles samples at room temperature indicate that the samples have implicit candidates for information storage devices for spintronic devices. In the high frequency range, all samples depending upon the temperature demonstrate excellent dielectric behaviour and small dielectric loss, and with stable dielectric constant, these samples make their prospective for the practical application in spintronic devices the main goal of this work.

Low-temperature growth of narrow optical gap highly conducting nc-Ge thin films with superior crystallinity involving dominant orientation

The nc-Ge thin films are grown at a moderately low temperature (∼220 °C) in rf PECVD via optimizing the GeH4 plasma with H2-dilution, within 97.5 ≤ D(H2)(%) ≤ 99.5. At elevated D(H2), while the bonded H-content in the matrix reduces, the film's crystallinity and dc conductivity increase. However, the optical band gap widens for 97.5 ≤ D(H2)(%) ≤ 98.5, via the elevated quantum confinement effect from the increased number of Ge-ncs of dimension below Bohr radius. Conversely, for D(H2) ≥ 98.5%, switching in the quantum effect in larger-sized Ge-ncs induces the band gap narrowing. Significantly reduced ultra-nanocrystalline-Ge (unc-Ge) component occupying the grain boundary zone, minimizes the contribution of grain-boundary defects. However, the persistent amorphous Ge matrix becomes highly defective and less dense, and those induce enhanced O-absorption from the ambient in the dominant Ge-Ox (x < 2) configuration. On increased crystallinity, the activation energies of dipole relaxation (ΔEτ) and resistance (ΔE) in the impedance data reduce dominantly in the a-Ge component than its nc-Ge counterpart, which occurs identically for the dc conductivity when the amorphous component in the matrix sharply disappears at increased D(H2) > 98.5%. A narrow band gap (∼0.90 eV), conducting (σd ∼3.0 ×10−2 S cm−1), and photosensitive nc-Ge network, involving dominant <111>-oriented Ge-ncs (∼35.2 nm) of enriched volume fraction (XC ∼90.2%), obtained at D(H2) ∼99.5%, seems extremely useful for need-based applications as a narrow band gap absorber layer in the bottom sub-cell of flexible high-efficiency cost-effective multijunction thin film solar cells.

Side-Chain-Type Polyimide-Cu Complexes with Suppressed Activation Energy of Relaxation for Advanced High-Temperature Capacitor

Side-Chain-Type Polyimide-Cu Complexes with Suppressed Activation Energy of Relaxation for Advanced High-Temperature Capacitor

A Mechanical Model for Stress Relaxation of Polylactic Acid/Thermoplastic Polyurethane Blends

Polylactic acid (PLA) is considered a promising biodegradable polymer alternative. Due to its high brittleness, composite materials made by melt blending thermoplastic polyurethane (TPU) with PLA can enhance the toughness of PLA. To understand the forced aging caused by stress relaxation in polymer materials, this study explains the stress relaxation experiments of PLA/TPU blends with different mass ratios under applied strain through mechanical model simulations. The Kelvin representation of the standard linear solid model (SLSM) is used to analyze the stress relaxation data of TPU/PLA blends, successfully explaining that the Young’s moduli (E1 and E2) of springs decrease with increasing temperature and TPU content. The viscosity coefficient of the PLA/TPU blends decreases with increasing temperature, and its reciprocal follows the Arrhenius law. For TPU/PLA blends with increased concentration of TPU, the activation energy for stress relaxation shows a linear decrease, confirmed by the glass transition point measured by DMA, indicating that it does not involve chemical reactions.

Dielectric relaxation studies in Sm2O3 doped boro-zinc-vanadate glasses

In this work, the conventional melting procedure was adopted to synthesize glasses with a composition of (ZnO)0.3 – (V2O5)0.3 – (B2O3)0.4-x – (Sm2O3)x, x = 0.002, 0.005, 0.007, 0.01 and 0.02 mol%. The dielectric properties of these glasses were measured. The dielectric constant (εʹ) and dielectric loss (εʹʹ) of glasses are found to vary in the range from 1.9177x102 to 3.9456x103 and from 1.5751 to 6.9095x105 respectively for the temperature range 343 K – 573 K and frequency range 50 Hz to 10 MHz. With increase of frequency, both dielectric constant and loss decreased and increased with increase of temperature. The analysis of dielectric constant and dielectric loss confirmed that the phenomenon of dielectric relaxation is mainly due to the frequency-dependent polarization mechanism. Electric modulus and impedence spectroscopy revealed non-Debye type and a single phase relaxation process. Activation energy for dc conductivity and dielectric relaxation are found to be of same size indicating that the potential barrier encountered by the charge carriers is same in both the processes. The master curves suggested that temperature-independent relaxation is occurring in the present glasses. The high values of measured dielectric parameters suggest that the present glasses are suitable in semiconducting and energy storage applications.

Microwave assisted self-repairable vitrimeric coating for anti-corrosive applications

Vitrimeric coatings are revolutionary and most widely emerging materials for metal corrosion inhibition. As harsh environmental conditions damages polymer-derived protective films and results into micro cracks and corroded metal surfaces. In this work, an active microwave responsive vitrimeric epoxy coating embedded with weathering resistant properties is introduced along with systematic comparative study for the effect of ortho and para position of amine in hardening agent. The developed microwave radiation responsive coating system felicitates self-healing due to disulfide exchange mechanism and corrosion inhibition properties in marine environments. Compared to conventional coatings, prepared graphene oxide (GO) incorporated vitrimeric network showed more promising protection against corrosion in offshore applications with 95.4 % protection efficiency and hydrophobicity with 96-degree contact angle. This study covers the self-healing properties, thermal mechanical study, hydrophobicity measurement, and thorough corrosion examination of synthesized material coated on 304 grade SS substrate. The coating reports excellent self-healing efficiency, reliable mechanical strength (storage modulus-129GPa and loss modulus-83GPa), fast stress relaxation and low activation energy (63KJ/mol) for dynamic disulfide exchange reaction, furthermore low corrosion rate of 1.20 × 10−3mm/y was observed for the coating. Thus, demonstrates its importance for self-healable anti-corrosive application for the marine environment.

Analyzing Temperature – Dependent Structural Changes and Frequency Response of a Ferroelectric Nanocomposite Based on Triglycine Sulfate with Oxidized Multiwalled Carbon Nanotubes

The present work is a continuing study on anomalous properties of the ferroelectric nanocomposite from triglycine sulfate (TGS) and oxidized multiwalled carbon nanotubes (OMWCNT). In this study, the nature of frequency response in a region of 102–107 Hz at low temperatures (63–273 K) will be clarified by analyzing structural changes when reducing temperature. It was found that the interaction between TGS and OMWCNT inhibited the switching of SO4 - groups, leading to a significant increase in relaxation time, activation energies and coercive fields. Consequently, the domain – wall freezing occurred at higher temperatures than those in pure TGS.

Gd3+ substitution effects, electrical and AC impedance characterizations of Gd/Cr co-doped SrBi4Ti4O15 Aurivillius-phase ceramics

Gd/Cr co-doped SrBi4Ti4O15 (Sr1-xGdxBi4Ti4O15 + 0.2 wt% Cr2O3, x = 0∼0.08) Aurivillius-phase ceramics were prepared by the traditional solid-state reaction process, the influences of Gd3+ doping concentration (x) on the structures and electrical properties of SrBi4Ti4O15 were investigated, as well as the AC impedance characterizations of the ceramics were analyzed. The incorporation of Gd2O3 and Cr2O3 into SrBi4Ti4O15 induced the lattice distortion of its orthorhombic structure. The donor substitution of Gd3+ for Sr2+ at A-site improved the Curie temperature (TC) and decrease the temperature coefficient of dielectric constant (TKε), as well as change the relaxation activation energy (Earel) of SrBi4Ti4O15 through the lattice distortion caused. Between 425 °C and 700 °C, the DC conduction behaviors of the ceramics was found with a transition from the mixed conduction mechanism involved in both the single-ionized oxygen vacancy and the double-ionized one to the single mode dominated by the latter one. Above TC, the imaginary part of modulus (M″) gradually shifts towards higher frequencies with increasing temperature, indicating the emergence of a new dielectric relaxation mode related to the phase transition process of the ceramics. Owing to the co-doping effects of Gd3+ and Cr3+, the optimized composition with x = 0.03 achieved excellent electrical properties (TC = 550 °C and d33 = 30 pC/N) and good thermal stability (TKε = 4.19 × 10−3/°C between RT and 500 °C, and the lose in d33 is only 3.3% after being annealed at 500 °C for 4 h), which make it promising for stable applications below 500 °C in the high-temperature piezoelectric devices.

Effect of compaction pressure on sinterability, microstructure and electrical behavior of Ba0.5Na0.48Sr0.02TiO3 system

The microstructure of electroceramics is a highly influencing factor to decide its functionality and applicability. Processing conditions used for the conversion of raw powder of electroceramics to the net product are playing a dominant role to modulate microstructure. In the present attempt, various compaction pressures (5 tons/cm2, 7 tons/cm2, and 9 tons/cm2) are used to densify the powder sample to the pellet. The effect of the pressure on sinterability, microstructure, and electrical behavior is systematically revealed. Ba0.5Na0.48Sr0.02TiO3 (BNST) with optimized composition has been prepared by the sol-gel combustion method. The structure is confirmed by X-ray diffraction and accompanied by Rietveld refinements. The compaction pressure effect on sinterability is systematically studied by revealing variation of relative density as a function of temperature at constant holding time as well as a function of holding time at a constant temperature. The final obtained relative density is found to be strongly affected by compaction pressure. Microstructural changes during the sintering process are closely monitored and found porosity, grain size, and grain size distribution are strongly affected by compaction pressure. That indirectly influenced electrical behavior. Relaxation frequency and activation energy of charge relaxation are also found to be varied with the compaction pressure of the powder samples.

Effect of MnO2 addition on structure, electrical and optical properties of Ba(Fe0.5Nb0.5)O3 ceramics

Admixture of Ba(Fe0.5Nb0.5)O3 (BFN) ceramics with different content of MnO2 (0, 0.5, 1.0, 1.5 and 2.0 wt.%) powder were processed through conventional solid-state reaction route. The XRD characterization of all sintered samples confirmed the formation of a single cubic perovskite structure.MnO2 acts as sintering aid, which significantly enhances grain size of the BFN ceramics. The room temperature dielectric constant and conductivity of the MnO2 modified BFN ceramics were suppressed compared to the pure BFN ceramics. However, the dielectric constant increases with the increase in temperature above 250 ?C. The relaxation activation energy (&gt;0.6 eV) and high dielectric constant (?104 at temperature above 250 ?C) of the MnO2 modified BFN ceramics were explained on the basis of ionization of oxygen vacancies, mobile oxygen vacancies and polaron hopping between multiple oxidation states of manganese (Mn2+ and Mn3+). A narrow band gap of 1.45 eV was observed in the 1.5 wt.% MnO2 modified BFN ceramics.

Effects of cooling rate on &lt;i&gt;β&lt;/i&gt; relaxation process and stress relaxation of La-based amorphous alloys

The dynamic relaxation process and stress relaxation process are two important processes which can reflect the microstructures of materials, for they are closely related to the diffusions, the glass transition phenomena and the microstructural heterogeneities. It is of great significance to explore the relationship among them. In the current research, the &lt;i&gt;β&lt;/i&gt;-relaxation characteristics and stress relaxation behaviors of bulk and ribbon samples obtained by different cooling rates are systematically investigated by taking the typical La-based amorphous alloys as model systems. The experimental results demonstrate that the cooling rate is an important parameter for controlling the energy state of the glass system, which further affects its physical and mechanical properties. Based on the dynamical mechanical spectra, the larger the cooling rate, the greater the low-temperature internal friction is and the smaller the beta relaxation activation energy according to Arrhenius calculations, and the greater the broadening of the beta relaxation behavior in the temperature spectra, suggesting that the higher cooling rate leads to greater atomic mobility and a high degree of heterogeneity in the microstructure. Thermodynamic analysis is conducted to study the slow process of thermal activation and the fast process driven by stress. At low temperature, the activation volume of the strip sample is larger than that of the bulk sample, and the activation volume values of the two samples are almost the same, as the cooling rate only affect the &lt;i&gt;β&lt;/i&gt; relaxation stage, but exert little effect on the &lt;i&gt;α&lt;/i&gt; relaxation, which is consistent with the conclusion that the stress relaxation behavior and &lt;i&gt;β&lt;/i&gt; relaxation behavior are related to the structural non-uniformity of the amorphous alloy. The stress relaxation tests show that the characteristic time of deformation decreases at higher cooling rate, the normalized stress decay is larger, it is easier to deform under an applied force field, and the deformation unit is more likely to activate to accommodate structural deformation. The correlation between stress relaxation and &lt;i&gt;β&lt;/i&gt; relaxation of amorphous alloy is further confirmed, and the proportion of liquid-like region is proportional to the relaxation mode spectrum, which also shows that &lt;i&gt;β&lt;/i&gt; relaxation and stress relaxation are consistent. Finally, by calculating relaxation enthalpy &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$ \Delta {H}_{{\mathrm{r}}{\mathrm{e}}{\mathrm{l}}} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231417_M1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231417_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, the variation of microstructure heterogeneity with cooling rate is experimentally verified. The research sheds new light on further clarifying the relationship among &lt;i&gt;β&lt;/i&gt; relaxation, deformation and microstructural heterogeneity of the amorphous alloy.

Investigation of dysprosium oxide substitution on physical, electrical and dielectric properties in sodium borosilicate glass system

Dysprosium oxide doped sodium borosilicate glasses were prepared in this study using a usual melt-quenching process. Further XRD, FTIR, density measurements, electrical conductivity, and dielectric properties were utilized to explore the prepared glassy samples. We found that all produced samples exhibit amorphous nature through X-ray diffraction. Increase in density and molar volume were found when dysprosium oxide concentration increased, which was ascribed to dysprosium oxide's greater molecular weight. Density data was used to calculate and explain several physical characteristics, including rare earth ion concentration (N), polaron radius (rp), internuclear distance (ri), and field strength (F).

Thermally reprocessable bio-based polyhydroxyurethane vitrimers

Vitrimers present one way to address growing demand for sustainably sourced, recyclable thermoset polyhydroxyurethanes (PHUs). We employ vinylogous urethane formation for dynamic bonding, using amine-terminated bio-sourced PHUs (prepared via terpolymerization of diglycerol dicarbonate (DGC) with Priamine 1074 and 1,10-diaminodecane (DAD)) as crosslinkers with ketone-functional copolymers. The latter are synthesized via atom transfer radical copolymerization (ATRP) of an alkyl methacrylate (C13MA, with average side-chain length of 13 carbons) with (2-acetoacetoxy) ethyl methacrylate (AAEMA) (initial AAEMA mole fraction = 0.1–0.4). By manipulating copolymer functionality, thermoset PHUs (Young's moduli spanning from 31 ± 1.5 kPa to 133 ± 5.7 kPa) are obtained. Furthermore, as ketone group concentration increases, networks became tighter, with slower relaxation rates and increased apparent activation energy of stress relaxation, in agreement with calculated molecular weight between crosslinks obtained rheologically. Finally, thermoset PHUs can be effectively recycled three times through remolding at 100 °C without compromising their mechanical properties.

Dielectric Spectroscopy Studies of Conformational Relaxation Dynamics in Molecular Glass-Forming Liquids.

We review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes of flexible chain or ring moieties, or else the rigid rotation of planar groups, such as conjugated phenyl rings. Comparative analysis of molecules possessing the same (type of) functional group is carried out in order to test the possibility of assigning the dynamic conformational isomerism of given families of organic compounds to the motion of specific molecular subunits. These range from terminal halomethyl and acetyl/acetoxy groups to both rigid and flexible ring structures, such as the planar halobenzene cycles or the buckled saccharide and diazepine rings. A short section on polyesters provides a generalisation of these findings to synthetic macromolecules.