Kinetics Of Physical Aging Research Articles

Kinetics of physical aging of a silicate glass following temperature up- and down-jumps.

In this article, we investigate the structural relaxation of lithium silicate glass during isothermal physical aging by monitoring the temporal evolution of its refractive index and enthalpy following relatively large (10-40 °C) up- and down-jumps in temperature. The Kohlrausch-Williams-Watts function aptly describes the up- and down-jump data when analyzed separately. For temperature down-jumps, the glass exhibits a typical stretched exponential kinetic behavior with the non-exponentiality parameter β < 1, whereas up-jumps show a compressed exponential behavior (β > 1). We analyzed these datasets using the non-exponential and non-linear Tool-Narayanaswamy-Moynihan (TNM) model, aiming to provide a comprehensive description of the primary or α-relaxation of the glass. This model described both up- and down-jump datasets using a single value of β ≤ 1. However, the standard TNM model exhibited a progressively reduced capacity to describe the data for larger temperature jumps, which is likely a manifestation of the temperature dependence of the non-exponentiality or non-linearity of the relaxation process. We hypothesize that the compressed exponential relaxation kinetics observed for temperature up-jumps stems from a nucleation-growth-percolation-based evolution on the dynamically mobile regions within the structure, leading to a self-acceleration of the dynamics. On the other hand, temperature down-jumps result in self-retardation, as the slow-relaxing denser regions percolate in the structure to give rise to a stretched exponential behavior.

Physical aging in molecular glasses beyond the α relaxation.

The description of kinetics of physical aging, namely the slow evolution of a glass thermodynamic state toward equilibrium, generally relies on the exclusive role of the main α relaxation. Here, we study the kinetics of physical aging over a wide temperature range in five small molecules interacting via van der Waals forces monitoring the time evolution of the glass enthalpic state. To this aim, we employ fast scanning calorimetry, which permits exploring a wide range of aging times. To challenge the role of the α relaxation in the description of physical aging, we employ a model-independent approach, based on the time to reach equilibrium, and a modified version of the single parameter aging model. The latter accounts for the non-linearity of aging making use of the so-called density scaling approach to describe the dependence of the α relaxation time on the glass thermodynamic state. We show that the α relaxation is generally adequate to describe aging at temperatures close to the glass transition and, for lower temperatures, the latest stages of equilibration. In contrast, at low aging temperatures, it fails to catch a wide portion of the time-dependent evolution of the glass thermodynamic state, which is found to be much faster than predicted considering only the α relaxation. Hence, our results and analysis provide compelling arguments that the description of glass equilibration under a wide range of aging conditions is conveyed by different molecular mechanisms, beyond the mere role of the α relaxation.

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.

Dynamics of structural relaxation in bioactive 45S5 glass

Kinetics of physical aging in archetypic 45S5 bioactive silicate glass composition with different types of phase separation are studied in situ below the glass transition temperature (Tg). The qualitative nature of aging is found to be almost independent of the structural differences on the micrometer scale. A well-expressed step-like behavior in the enthalpy recovery kinetics is observed for aging temperatures Ta ∼ 0.90Tg and Ta ∼ 0.85Tg, which, however, disappears when the aging occurs at Ta ∼ 0.95Tg. The overall kinetics are described by a stretched-exponential function with stretching exponent close to 3/7 at Ta ∼ 0.95Tg, and 1/3 when the aging temperature drops to ∼0.90Tg and below. The values correlate well with the predictions of Phillips’ diffusion-to-traps and percolating fractals models. Appearance of step-like behavior at larger departure from Tg is attributed to the hierarchical scheme of approaching equilibrium based on an alignment-shrinkage mechanism of physical aging proposed earlier for chalcogenide glasses.

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.

Description of physical aging kinetics of glassy polymers by interpretation of parameters of the Kohlrausch-Williams-Watts relaxation function via simulation.

The empirical Kohlrausch-Williams-Watts (KWW) relaxation function has been widely used to describe the nonexponential relaxation behavior of glassy polymers during their physical aging. The characteristic relaxation time (τ_{KWW}) and the stretching parameter (β_{KWW}) of the KWW equation are obtained from experimental relaxation data by a curve-fitting procedure. However, knowing the values of KWW parameters is not sufficient to completely characterize the physical aging kinetics of glassy materials in order to compare, for example, the aging rate of different systems. To be able to overcome such problems, at least, an exact interpretation of these parameters is absolutely vital and then it would be quite desirable to convert them into a single relaxation-time constant. This approach, indeed, provides the possibility to directly evaluate and compare the kinetics and aging rate of different systems as well as different methods used for monitoring the physical aging of the same materials. To do these, a method based on the notion of the relaxation spectrum is employed which is modeled via the well-known statistical distribution of log-normal. By detailed numerical simulations, some explicit expressions between KWW parameters with descriptor parameters of the distribution are established. From this point of view, it is tried to interpret τ_{KWW} and β_{KWW} and convert them into a single relaxation-time constant. Finally, the results of suggested simulations are compared with those obtained in the literature.

Impact of ZnO nanoparticle morphology on relaxation and transport properties of PLA nanocomposites

In this work we study the effect of zinc oxide (ZnO) nanoparticle morphology and concentration on the resulting relaxation and transport properties of polylactide (PLA) nanocomposites. Films containing spherical and rod-shaped ZnO nanoparticles were incorporated into an amorphous polylactide (PLA) matrix through a solvent-precipitation and compression moulding method. Morphological analyses carried out by scanning electron microscopy (SEM) together with ultraviolet–visible (UV-Vis) spectroscopy and thermogravimetric analysis (TGA). Results indicate a much better distribution of rod-shaped ZnO within PLA matrix. Relaxation experiments reveal faster physical aging kinetics of PLA in presence ZnO nanoparticles notwithstanding their shape, suggesting the presence of non-interacting surfaces between the amorphous PLA matrix and the ZnO nanoparticles. Interestingly, both helium and oxygen permeability remained stable or increase upon nanoparticle addition, and anisole sorption kinetics showed faster mass transport in the nanocomposites, suggesting that the low interfacial adhesion between PLA and ZnO brings supplementary voids to the material increasing mass transport. Overall, the experimental findings here reported provide a deeper understanding on the influence of metal oxide nanoparticle morphology on the resulting relaxation and gas transport properties of amorphous polymeric nanocomposites.

Physical aging in polycarbonate nanocomposites containing grafted nanosilica particles: A comparison between enthalpy and yield stress evolution

Understanding and controlling physical aging below the glass transition temperature (Tg) is very important for the long-term performance of plastic parts. In this article, the effect of grafted silica nanoparticles on the physical aging of polycarbonate (PC) below the Tg is studied by using the evolution of the enthalpy relaxation and the yield stress. The nanocomposites were found to reach a thermodynamic equilibrium faster than unfilled PC, implying that physical aging is accelerated in presence of grafted nanosilica particles. The Tool-Narayanaswamy-Moynihan model shows that the aging is accelerated by the grafted silica nanoparticles, but the molecular mechanism responsible for physical aging remains unaltered. Furthermore, dynamic mechanical analysis shows that the kinetics of physical aging can be related to a free volume distribution or a local attraction-energy distribution as a result of the change in mobility of the polymer chain. Finally, a qualitative equivalence is observed in the physical aging followed by both the enthalpy relaxation and yield stress. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2069–2081

Crossover between cooperative and fractal relaxation in complex glass-formers

Kinetics of physical aging at different temperatures is studied in situ in arsenic selenide glasses using high-precision differential scanning calorimetry technique. A well-expressed step-like behaviour in the enthalpy recovery kinetics is recorded for low aging temperatures. These fine features disappear when the aging temperature (Ta) approaches the glass transition temperature (Tg). The overall kinetics is described by stretched exponential function with stretching exponent close to 3/5 at Ta > ~0.95 Tg almost independent on glass composition, and 3/7 when the aging temperature drops to ~0.9 Tg. These values are consistent with the prediction of Phillips’ diffusion-to-traps model. Further decrease in aging temperature to ~0.85 Tg leads to the appearance of step-like behaviour and stretching exponent of 1/3 for the overall kinetics, which is the limiting value predicted by random walk on the fractal model. Such behavior is explained as crossover from homogeneous cooperative relaxation of non-percolating structural units to high-dimensional fractal relaxation within hierarchically-arranged two-stage physical aging model.

On the compositional diversity of physical aging kinetics in chalcogenide glasses

Compositional features of enthalpy losses ΔH(t) caused by long-term physical aging at normal conditions are studied at the example of Se-rich chalcogenide glasses As20Se80, As30Se70 and Ge5Se95 obeying “chain-crossing” structural model. The observed relaxation kinetics in general are described by a stretched exponential behavior, but can be also parameterized in terms of multi-step single exponential decays. Microstructural mechanism of natural physical aging in the studied glasses is explained by accepting their structural–topological specificity with a decisive role of preferential chemical environment around Se atoms. The characteristic time constants of aging are shown to increase in more topologically constrained structural fragments capable to relaxation. Independent relaxation stages originated from different chemical environments in As10Se90 and Ge5Se95 glasses allow observation of plateau-like behavior. However, plateaus are found to be smoothed in As20Se80 glass because of overlap of different structural complexes responsible for relaxation.

Step-wise kinetics of natural physical ageing in arsenic selenide glasses

The long-term kinetics of physical ageing at ambient temperature is studied in Se-rich As–Se glasses using the conventional differential scanning calorimetry technique. It is analysed through the changes in the structural relaxation parameters occurring during the glass-to-supercooled liquid transition in the heating mode. Along with the time dependences of the glass transition temperature (Tg) and partial area (A) under the endothermic relaxation peak, the enthalpy losses (ΔH) and calculated fictive temperature (TF) are analysed as key parameters, characterizing the kinetics of physical ageing. The latter is shown to have step-wise character, revealing some kinds of subsequent plateaus and steep regions. A phenomenological description of physical ageing in the investigated glasses is proposed on the basis of an alignment–shrinkage mechanism and first-order kinetic equations.

Aging Effects on the Phase Composition and Chain Mobility of Isotactic Poly(propylene)

AbstractChanges in phase composition and chain mobility in injection‐molded isotactic poly(propylene), crystallized from the melt with slow cooling rate and subsequently quenched, associated with aging at temperature well above Tg for 150 and 1 000 h, are studied using time‐domain 1H solid‐state NMR and XRD. All sample exhibit physical aging when exposed to elevated temperatures, and the physical aging kinetics was observed to depend on the morphology of the homopolymer iPP and aging temperatures. The significant increase in the tensile modulus in time was observed for injection‐molded iPP. The observed property changes induced by aging are attributed to microstructural changes within the semi‐rigid and amorphous fractions.magnified image

Increase in effective activation energy during physical aging of a glass

The novel technique of multi-frequency temperature modulated differential scanning calorimetry is employed to measure a decrease in the heat capacity under quasi-isothermal conditions. The effect is used to monitor the kinetics of physical aging in glassy maltitol at ∼5–8 °C below the glass transition temperature. The heat capacity relaxation is analyzed by an isoconversional method that yields effective activation energy for different extents of relaxation. The early stages of aging demonstrate an activation energy ∼145 kJ mol −1 that is significantly smaller than the activation energy of α-relaxation (413 kJ mol −1), which is closely approached at the final stages of aging.

Physical aging of thin 6FDA-based polyimide membranes containing carboxyl acid groups. Part I. Transport properties

The effect of molecular structure on the kinetics of physical aging of thin films (∼350 nm) formed from glassy 6FDA-based polyimides was investigated by tracking the changes in gas permeability (He, O 2 and N 2) at 35 °C for more than 2000 h. The structures studied included homopolymers of 6FDA with 6FpDA and with DAM plus copolymers where a portion of the latter two monomers was replaced with DABA to introduce carboxyl units into the structure for subsequent cross-linking studies. Over this period of aging, the oxygen permeability decreased by a factor of two for the polyimide containing the diamine 6FpDA and by a factor of five for the polyimide containing the diamine DAM. Introduction of DABA units accelerated the aging in the case of 6FpDA and slowed the aging in the case of DAM. Aging rate seems to correlate with the level of free volume of the polymer; the higher the free volume, the faster is the aging. Selectivity for all gas pairs increased upon aging but the rate is not simply explained by free volume alone because the difference in size of the two gas molecules is also reflected in the response to physical aging observed.

Effect of Temperature on Physical Aging of Thin Glassy Polymer Films

The effect of temperature on the kinetics of physical aging of thin films formed from two amorphous glassy polymers, polysulfone based on bisphenol A and poly(2,6-dimethyl-1,4-phenylene oxide), was investigated by monitoring the changes in gas permeability and refractive index. Films with different thicknesses were subjected to isothermal aging at three temperatures, ranging from 35 to 55 °C, for a period of aging of more than 200 days. The rate of permeability loss and the rate of densification determined from the refractive index change by using the Lorentz−Lorenz equation were found to increase with aging temperature. Similar qualitative trends of aging rate were noted by the two measurements. The combination of effects of aging temperature and film thickness on aging behavior were studied and compared with previous research.

Diffusion mechanism for physical aging of polycarbonate far below the glass transition temperature studied by means of dielectric spectroscopy

Dielectric permittivity measurements were used to monitor physical aging of polycarbonate (PC) at about T g – 100 K. The effect of film thickness and cooling rate from the melt was investigated to give new insight into the microscopic nature of the physical aging process. The results show that both the film thickness and the cooling rate play an important role in the kinetics of physical aging. Indications of a diffusive mechanism for physical aging are given, where free volume holes can disappear either at the external surface of the sample or at an internal surface created during the cooling process. Evidence for the existence of the an internal surface is provided.

Physical aging of polycarbonate far below the glass transition temperature: Evidence for the diffusion mechanism

Dielectric permittivity measurements and free volume determination by means of positron annihilation lifetime spectroscopy (PALS) were used to monitor physical aging of polycarbonate (PC) far below the glass transition temperature $(Tg)$. The effect of film thickness and cooling rate from the melt was investigated to give new insight about the microscopic nature of the physical aging process. The results show that both the film thickness and the cooling rate play an important role in the kinetics of physical aging. A strong indication for a fully diffusive mechanism of physical aging is given, at least at the beginning of the aging process, where free volume holes disappear at a boundary which can either be the external surface of the sample or an internal surface defined by the presence of some low-density regions, which are created during the cooling process and have a concentration dependent on the cooling rate. In particular, the amount of internal surface is proportional to the cooling rate from the melt. Evidence for the existence of the low-density regions was provided by the absorption of ethylene glycol (EG), a molecule with a strong tendency to aggregate, in PC samples cooled down at different rates. The amount of absorbed EG was found to be proportional to the cooling rate. In addition, through dielectric spectroscopy in combination with thermogravimetric analysis, it was found that EG tends to locate in regions with large open space rather than in the bulk of PC. The physical origin for the formation of such regions might be related to the evolution of spatial heterogeneities during cooling from the supercooled state and its dependence on the cooling rate.

Physical aging of glassy normal and waxy rice starches: effect of aging time on glass transition and enthalpy relaxation

Physical aging and glass transition characteristics of amorphous normal and waxy rice starches (11 and 15% moisture contents) were investigated under differential scanning calorimetry as function of aging time. Normal rice starch showed higher T g than waxy rice starch. The T g and Δ C p at glass transition gradually increased with aging time, whereas fictive temperature was slightly reduced regardless of moisture content and starch type. The relaxation enthalpy and relaxation peak temperature increased with aging time until structural equilibrium was reached. Enthalpy increase was more significant in the early stage of aging whereas temperature increase was constant during the aging period tested (120 h). Aging kinetic analysis using Cowie and Ferguson model revealed that the amorphous normal and waxy rice starches behaved in different modes for the physical aging. Relaxation distribution parameter ( β) of both starches was in a range of 0.3< β<0.7, but higher at a lower moisture content, and for normal starch than for waxy starch. Maximum relaxation enthalpy for normal starch (1.10 and 2.69 J/g, respectively, at 11 and 15% moistures) was higher than those of waxy starch (0.77 and 2.48 J/g). Based on the characteristic time ( t c), normal starch has slower progression toward an equilibrium than waxy starch. Overall results proved that physical aging kinetics were highly dependent on starch structure and composition.

Influence of Semicrystalline Morphology on the Physical Aging Characteristics of Poly(phenylene sulfide)

We report on the influence of semicrystalline morphology on the physical aging characteristics of poly(phenylene sulfide) or PPS. Specifically, the semicrystalline morphology of PPS was described in terms of a three-phase system comprising a crystalline phase, a mobile-amorphous phase, and a rigid-amorphous phase. The physical aging kinetics were observed to depend on the relative amounts of the mobile-amorphous and rigid-amorphous phases, with accelerated aging rates measured in specimens with higher rigid-amorphous phase fraction. We suggest that the rigid-amorphous phase, which includes chain segments that are more tightly packed relative to the mobile-amorphous phase, is able to accelerate physical aging due to its relative proximity to a state of lower configurational entropy. It is also possible for localized cooperative relaxations within the rigid-amorphous phase to facilitate molecular rearrangements that accelerate the aging process. We also suggest that the configurational entropic state of the...

Physical aging in amorphous polymers: comparison of observations in calorimetric and mechanical tests

Constitutive equations are derived for the kinetics of physical aging in polymeric glasses. An amorphous polymer is treated as an ensemble of cooperatively rearranging regions (CRR) which relax at random times as they are thermally agitated. Structural recovery is modeled as fragmentation and aggregation of CRRs. To verify the governing equations, results of numerical simulation are compared with experimental data in calorimetric and dynamic tensile tests for three commercial grades of poly(ether imide). Fair agreement is demonstrated between observations and predictions of the model.