Enthalpy Relaxation Kinetics Research Articles

Designing Advanced Amorphous/Nanocrystalline Alloys by Controlling the Energy State.

Amorphous alloys, also known as metallic glasses, exhibit many advanced mechanical, physical, and chemical properties. Owing to the nonequilibrium nature, their energy states can vary over a wide range. However, the energy relaxation kinetics are very complex and composed of various types that are coupled with each other. This makes it challenging to control the energy state precisely and to study the energy-properties relationship. This brief review introduces the recent progresses on studying the enthalpy relaxation kinetics during isothermal annealing, for example, the observation of two-step relaxation phenomenon, the detection of relaxation unit (relaxun), the key role of large activation entropy in triggering memory effect, the influence of glass energy state on nanocrystallization. Based on the above knowledge, a new strategy is proposed to design a series of amorphous alloys and their composites consisting of nanocrystals and glass matrix with superior functional properties by precisely controlling the nonequilibrium energy states. As the typical examples, Fe-based amorphous alloys with both advanced soft magnetism and good plasticity, Gd-based amorphous/nanocrystalline composites with large magnetocaloric effect, and Fe-based amorphous alloys with high catalytic performance are specifically described.

Isothermal enthalpy relaxation of amorphous polystyrene studied using temperature-modulated fast scanning calorimetry

The kinetics of enthalpy relaxation of atactic polystyrene after temperature jump-down and jump-up in the glass state is studied using temperature-modulated fast scanning calorimetry (T-M FSC). During the isothermal relaxation, the dynamic heat capacity obtained by T-M FSC underwent a systematic change suggesting a shift of relaxation time. The relaxation time in the nonequilibrium glass state is dependent on the sample's actual temperature T and the nonequilibrium structure that has been indexed by a fictive temperature Tf. The changes in the magnitude and phase angle of the dynamic heat capacity on relaxation prove the influence of the nonequilibrium structure. The influence is indexed by an effective temperature Teff, which is derived from the obtained dynamic heat capacity compared with its temperature dispersion in the liquid state. Teff also corresponds to the mean temperature of the actual and fictive temperatures and can be expressed by Teff≅xT+(1−x)Tf with the nonlinearity parameter x of the Tool–Narayanaswamy–Moynihan model that partitions the activation factor of relaxation into the contributions of T and Tf. Evaluation of Tf and Teff will provide valuable insights into the relaxation time in glass state, which plays an essential role in the glass transition behaviors of amorphous materials.

Fast-scan chip-calorimetry measurement on crystallization and enthalpy relaxation kinetics of isotactic poly(cyclohexene carbonate)

Isotactic poly(cyclohexene carbonate) (iPCHC) is a newly developed degradable semi-crystalline polymer. In this paper, the crystallization kinetics of iPCHC above the glass transition temperature (Tg) and the enthalpy relaxation kinetics below Tg were studied by using a commercial fast-scan chip-calorimeter—Flash DSC 1. Results showed that half crystallization time of iPCHC as a function of crystallization temperature presented the typical bell shape. And iPCHC showed a maximum crystallization rate at 190 °C, with the half crystallization time 153 s. IPCHC enthalpy relaxation time in 85 °C ~ 100 °C temperature range decreases with isothermal temperature. The above results have great significance to the development of processing and application of novel crystallizable polycarbonate materials.

Comparison of three enthalpy relaxation models based on fictive temperature and nonlinear Adam–Gibbs formulation

The phenomenological models based on fictive temperature and nonlinear Adam–Gibbs formulation have been widely used to describe the enthalpy relaxation kinetics. Although the parameters in Adam–Gibbs formulation are physically explicable, the reasonabilities of the parameters obtained from curve-fitting procedures have not been systematically discussed yet. In this work, differential scanning calorimetry was employed to measure the heat capacities of maltitol in both the amorphous and the crystalline states, and the data were used to generate the model parameters. Results showed that the configurational ground state plays an important role in extracting the model parameters. Selecting the crystal rather than the glass as the ground state seems to be effective to improve the reasonabilities of the fitted parameters. It suggested that even if the fit quality seems sound, the physical reasonability of the best-fit parameters still needs to be treated with caution.

Correlation between enthalpy relaxation and mechanical response on physical aging of polycarbonate in relation to the effect of molecular weight on ductile-brittle transition

A series of physical aging experiments at different annealing temperature and times was conducted on three kinds of polycarbonates (PCs) with different molecular weight using a differential scanning calorimetry and a tensile test. The enthalpy relaxation kinetics was well modeled by existing empirical equation using Kohlausch-Williams-Watts (KWW) function and effects of annealing temperature and times for the model parameters were clarified. A good correlation between the kinetics of enthalpy relaxation and that of yield stress was found. A dependence of the molecular weight on the physical aging kinetics could be attributed to its effect on the glass transition temperature.

Thermo‐structural characterization of (As2Se3)100‐x‐(As2Te3)x glasses for infrared optics

AbstractDifferential scanning calorimetry was used to study thermokinetic behavior of (As2Se3)100−x(As2Te3)x infrared glasses (seven compositions along the pseudo‐binary were investigated). Glass‐transition kinetics was described in terms of the Tool‐Narayanaswamy‐Moynihan model and the relaxation motions were interpreted using Raman spectroscopy data. The enthalpy relaxation kinetics was found to be absolutely uninfluenced by changing Se/Te ratio. On the other hand, DSC crystallization behavior changed significantly with increasing As2Te3 content: Te‐rich compositions show marked affinity toward crystallization, whereas in case of the compositions with 0‐34 mol.% As2Te3 crystal growth is significantly suppressed. X‐ray diffraction analysis indicated presence of monoclinic As2Se3, As2Te3, and As2Se(Te)3 phases. The complex crystallization behavior occurring in case of the Te‐rich compositions was described by superposition of two autocatalytic kinetic signals. Based on the information from infrared microscopy the two overlapping crystallization processes can be attributed to the respective formations of spherulitic and needle‐shaped crystallites. Best glass stability was identified in case of the (As2Se3)66(As2Te3)34 composition, which appears to be a potential candidate for optic applications in the far‐infrared region of the spectrum.

Correlation between the structure and relaxation dynamics of (GeS2)y(Sb2S3)1 − y glassy matrices

Differential scanning calorimetry was used to study enthalpy relaxation kinetics of the (GeS2)y(Sb2S3)1−y glasses in the y=0.1–0.9 compositional range. The relaxation behavior was described in terms of the phenomenological Tool-Narayanaswamy-Moynihan (TNM) model. Non-linear optimization procedure was used to determine unique set of relaxation parameters for each studied glass. With increasing GeS2 content the activation energy of enthalpy relaxation was found to steeply decrease whereas the glass transition temperature increased significantly. Both non-linearity and non-exponentiality features were not affected by compositional changes. Raman spectra have shown that the Ge-S structural units, as opposed to the Sb-S units, are significantly interconnected and create a 3-dimensional network capable of solely carrying the relaxation motions, which is in agreement with the interpretation of compositional evolution of relaxation parameters. Moreover, based on the present data the controversy regarding the (GeS2)y(Sb2S3)1−y fragilities found in literature was clarified, and the free-volume concept employing the Avrami growth theory was disputed.

Effect of molar mass on enthalpy relaxation and crystal nucleation of poly (l-lactic acid)

The effect of the chain length/molar mass of poly (l-lactic acid) (PLLA) on the formation of homogeneous crystal nuclei below and above the glass transition temperature (Tg) has been analyzed by fast scanning chip calorimetry, employing Tammann’s nuclei development method. It has been found that the kinetics of crystal nucleation below Tg is independent on the chain length within the investigated range of molar masses from about 60 to 600kDa, similar as the kinetics of enthalpy relaxation. In contrast, at temperatures well above Tg there is detected that crystal nucleation is slower in samples of higher molar mass. This observation is confirmed by non-isothermal nucleation experiments which showed that suppression of nucleation occurs on cooling at rates of 200, 100, and 30K/s in samples with molar masses of around 60, 100, and 600kDa, respectively. The different effect of molar mass on crystal nucleation in the glass and well above Tg is discussed in terms of the length-scale of required chain-mobility and of the critical size of nuclei.

Se-doped GeTe4 glasses for far-infrared optical fibers

Differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) were used to study thermokinetic behavior of Ge21SeyTe79−y infrared glasses (for y up to 8 at.% of Se). Glass transition kinetics was described in terms of the Tool-Narayanaswamy-Moynihan model and the relaxation motions were interpreted using Raman spectroscopy data. Both enthalpy and volume relaxation kinetics were found to be uninfluenced by Se addition. On the other hand, DSC crystallization behavior changed significantly with increasing Se content: initial addition of Se partially delays and decelerates both the initial Te precipitation and the consequent volume-located crystallization of GeTe and Te; additional increase of Se content then further delays and separates the volume-located crystallization processes but the initial surface Te precipitation is no longer affected. From this point of view the fine compositional tuning of basic thermal stability of GeTeSe infrared glasses appears meaningless. On the contrary, glass workability window (determined by means of the TMA crystallization experiments) was found to be significantly affected by Se addition, emphasizing the importance of this type of measurements for the glass technology and material science. The 4 at.% Se content was found optimal with respect to fiber-drawing procedure.

Structural interpretation of the enthalpy relaxation kinetics of (GeTe4)y(GaTe3)1 − y far-infrared glasses

Differential scanning calorimetry was used to study enthalpy relaxation kinetics of the (GeTe4)y(GaTe3)1−y infrared chalcogenide glasses for the compositional range y=0.4−1.0. The relaxation behavior was described in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Direct curve-fitting procedure was used to determine the values of TNM parameters; these results were successfully correlated with data provided by the non-fitting methodology based on the evaluation from constant-ratio (CR) cycles. The addition of GaTe3 into the GeTe4 matrix led to a moderate decrease of activation energy of the relaxation process and to a large increase of the relaxation linearity – the structural relaxation became driven solely by temperature. Interpretation of the compositional evolution of the TNM parameters was used to verify structural information provided by Raman spectroscopy and molecular dynamics simulations: the initial GaTe3 addition causes dilution of the GeTe4 tetrahedral network, which carries the main portion of the relaxation motions; further increase of GaTe3 content then leads to an increase of connectivity outside the GeTe4 sub-networks due to the threefold coordinated Ga atoms bonding with the lone-pair electrons of Te dimers and short chains.

Interplay between the Relaxation of the Glass of Random l/d-Lactide Copolymers and Homogeneous Crystal Nucleation: Evidence for Segregation of Chain Defects.

Random l-isomer rich copolymers of poly(lactic acid) containing up to 4% d-isomer co-units have been cooled from the molten state to obtain glasses free of crystals and homogeneous crystal nuclei. The kinetics of enthalpy relaxation and the formation of homogeneous crystal nuclei have then been analyzed using fast scanning chip calorimetry. It has been found that the relaxation of the glass toward the structure/enthalpy of the supercooled liquid state is independent of the presence of d-isomer co-units in the chain. Formation of homogeneous crystal nuclei in the glassy state requires the completion of the relaxation of the glass. However, nucleation is increasingly delayed in the random copolymers with increasing d-isomer chain defect concentration. The data show that the slower formation of homogeneous crystal nuclei in random l/d-lactide copolymers, compared to the homopolymer, is not caused by different chain-segment mobility in the glassy state but by the segregation of chain defects in this early stage of the crystallization process.

Enthalpy relaxation kinetics of Ge20Te(80-y)Sey far-infrared glasses in the glass transition range

Differential scanning calorimetry was used to study enthalpy relaxation kinetics of the Ge20Te(80-y)Sey infrared chalcogenide glasses for the compositional range y = 0–8. The relaxation behaviour was described in terms of the phenomenological Tool–Narayanaswamy–Moynihan (TNM) model. The direct curve-fitting procedure was used to determine the values of TNM parameters. Compositional evolution of the TNM parameters was interpreted with respect to the involved structural entities and their motions. Based on the joint Raman scattering study, the addition of Se leads to increased amount of edge-shared GeTe4-xSex tetrahedra. While the primary structural basis for the relaxation movements appears not to be affected by addition of Se (constant value of non-linearity), changes of the non-exponentiality parameter indicate increased structural variability occurring within the groups of directly interlinked tetrahedra, which were found to carry the main portion of relaxation movements. Increased activation energy was explained by the presence of significantly stronger Ge–Se bonds and increased amount of edge-shared tetrahedra.

Physical properties of maltodextrin DE 10: Water sorption, water plasticization and enthalpy relaxation

Maltodextrins are hydrolysis products of starch. The present research aimed to study physical properties of maltodextrins (water sorption, water plasticization behavior and enthalpy relaxation) and highlighting their functions as food components. The GAB and GT equation were fitted successfully to water sorption and water plasticization data of maltodextrin DE 10. The critical water content and critical water activity values at 25 °C were 14 g water/100 g dry solid and 0.68 aw, respectively. The Kohlrausch–Williams–Watt (KWW model) and extended Adam-Gibbs model fitted well the enthalpy relaxation kinetics data. The non-exponential parameter, β value of maltodextrin DE 10 was within in the range of 0.15–0.93 and its value was dependent on the water activity. At 25 °C, Maltodextrin with aw <0.43 was stable glassy as indicated by very long enthalpy relaxation time. The KWW model gave a more accurate fit than the extended Adam-Gibbs model. Arrhenius model fitted well to enthalpy relaxation data and gave a free activation energy (Ea) of 459.5 kJ/mol.

Enthalpy relaxation kinetics of GeTe4 glass

Differential scanning calorimetry was used to study enthalpy relaxation kinetics of the GeTe4 infrared chalcogenide glass. The relaxation behavior was described in terms of the phenomenological Tool–Narayanaswamy–Moynihan (TNM) and Adam–Gibbs–Scherer (AGS) models. The direct curve-fitting procedure was found to provide the most accurate results under the assumption that high-quality DSC data are obtained and the correct pre-fitting treatment is applied. The recently developed novel evaluation of Δh⁎ from CR cycles and the improved simulation-comparative methodology were then found to provide the best results out of the tested non-fitting methods.

Ductile-brittle transition behaviour of PLA/o-MMT films during the physical aging process

The ductile-brittle transition behaviour of organo modified montmorillonite-based Poly(lactic acid) films (PLA/o-MMT) was analysed using the Essential Work of Fracture (EWF) methodology, Small Punch Tests (SPT) and Enthalpy relaxation analysis. While the EWF methodology could only be applied successfully to de-aged samples, small punch test (SPT) was revealed as more effective for a mechanical characterization during the transient behaviour from duc- tile to brittle. According to differential scanning calorimetry (DSC) results, physical aging at 30°C of PLA/o-MMT samples exhibited slower enthalpy relaxation kinetics as compared to the pristine polymer. Although all samples exhibited an equiv- alent thermodynamic state after being stored one week at 30°C, significant differences were observed in the mechanical performances. These changes could be attributed to the toughening mechanisms promoted by o-MMT.

Enthalpy and high temperature relaxation kinetics of stable vapor-deposited glasses of toluene.

Stable non-crystalline toluene films of micrometer and nanometer thicknesses were grown by vapor deposition at distinct rates and probed by fast scanning calorimetry. Fast scanning calorimetry is shown to be extremely sensitive to the structure of the vapor-deposited phase and was used to characterize simultaneously its kinetic stability and its thermodynamic properties. According to our analysis, transformation of vapor-deposited samples of toluene during heating with rates in excess 10(5) K s(-1) follows the zero-order kinetics. The transformation rate correlates strongly with the initial enthalpy of the sample, which increases with the deposition rate according to sub-linear law. Analysis of the transformation kinetics of vapor-deposited toluene films of various thicknesses reveal a sudden increase in the transformation rate for films thinner than 250 nm. The change in kinetics seems to correlate with the surface roughness scale of the substrate. The implications of these findings for the formation mechanism and structure of vapor-deposited stable glasses are discussed.

Direct measurement of the kinetics of volume and enthalpy relaxation of an Au-based bulk metallic glass

Structural relaxation of glasses below their glass transition is a well-studied phenomenon that still poses several open issues. With the advent of bulk metallic glasses with exceptionally low glass transition temperatures, new options are available that are based on the experimental assessment of the time dependence of several different thermodynamic quantities by direct measurements with high accuracy. In this contribution the first direct measurement of the isothermal relaxation of the volume and the enthalpy of an Au-based bulk metallic glassformer are reported and discussed with respect of the characteristics describing the underlying processes.

Kinetics of enthalpy relaxation of milk protein concentrate powder upon ageing and its effect on solubility.

Kinetics of enthalpy relaxation of milk protein concentrate (MPC) powder upon short-term (up to 67 h) storage at 25 °C and aw 0.85, and long-term (up to 48 days) storage at 25 °C and a range of aw values (0-0.85) were studied by differential scanning calorimetry (DSC). The short-term study showed a rapid recovery of enthalpy for the first 48 h, followed by a slower steady increase with time. The non-exponential β parameter was calculated using the Kohlrausch-Williams-Watts function and found to be 0.39. Long-term storage showed that enthalpy relaxation depends on both storage period and water activity. The enthalpy value was much less for lower moisture content (mc) (aw ≤ 0.23, mc ≤ 5.5%) than for higher mc (aw ≥ 0.45, mc ≥ 8%) samples for a particular storage period. The results suggest that the presence of more water molecules, in close proximity to the protein surface facilitates kinetic unfreezing and subsequent motion of molecular segments of protein molecules towards thermodynamic equilibrium. Although de-ageing of stored samples did not reverse storage-induced solubility losses, the timescale of enthalpy relaxation was similar to that of solubility loss. It is suggested that enthalpy relaxation within stored samples allows structural rearrangements that are responsible for subsequent solubility decreases.

Composition dependence and reversibility of photoinduced refractive index changes in chalcogenide glass

Photoinduced refractive index changes are investigated as a function of composition in Ge–As–Se chalcogenide glass by measuring the formation of Bragg reflectors photo-imprinted in polished glass discs. It is shown that the glass network connectivity has a strong effect on the photostructural changes which decrease dramatically when the rigidity percolates through the structure at coordination numbers ⟨r⟩ > 2.4. The Bragg reflector formation is also affected by the refractive index of the glass which varies sharply with ⟨r⟩ and correlates directly with the glass density. The photoinduced refractive index change is shown to reverse upon annealing near the glass transition following a non-exponential kinetic that closely matches the kinetics of structural enthalpy relaxation. The Bragg reflectors can also be reversibly erased and readjusted to a new wavelength through subsequent irradiation with a different photon energy. The composition dependence of photosensitivity is discussed in terms of structural constraints. It is suggested that over-constrained networks remain rigid despite photoexcitation of bond constraints therefore precluding photostructural rearrangements.

Roles of Conformational and Configurational Defects on the Physical Aging of Amorphous Poly(lactic acid)

The roles of poly(lactic acid) chain conformation and configuration on the enthalpy relaxation kinetics of amorphous poly(lactic acid) were examined. Enthalpic relaxation data, which were scaled to the same supercooling from the initial fictive temperature, were taken for three types of the polymer containing various d-lactyl monomers (5.7%, 13.0%, 50%) to assess the effects of configurational defects. The kinetics data were very similar from sample to sample. The effects of configurational defects were assessed using the generalized Kohlrausch−Williams−Watts (KWW) equation solved by the Tool−Narayanaswamy−Moynihan (TNM) equation. The major effect of increasing the d-lactyl contents was to lower the PLA glass transition temperature, thereby accelerating the kinetics of enthalpic relaxation. Configurational defects showed no significant effect on the other KWW/TNM fit parameters (x, Δh, ln A). A slightly larger KWW β stretched exponential parameter is observed for greater (50% d) than for lower (5.7% d) am...