Effect Of Physical Aging Research Articles

Effect of Physical Aging on the Bending Recovery of PEEK and PI Films

The relationship between the viscoelasticity and bending recovery responses of PI and PEEK films were studied as well as the effect of physical aging treatment on these properties. The residual elastic modulus (Er) and degree of aging of the films were measured from the stress relaxation-recovery experiment using DMA and the enthalpy relaxation endotherms near glass transition temperature using DSC, respectively. The instantaneous bending recovery ratio (BR+) of the films was measured from the change in radius of curvature before and after the bending under the condition of 85 °C/85%RH/24 hr. The DMA test was also performed at 85 °C in order to evaluate the progression of the stress relaxation during bending. The Er and BR+ values showed good correlation, and higher contributions of elastic response or longer delay of the stress relaxation led to improved bending recovery. Films with BR+ values close to zero can be obtained if the conditions of Er <0.05 are satisfied. A physical aging method was proven to be effective for controlling the viscoelastic response of the films; it contributes to the retardation of stress relaxation. The optimum physical aging temperatures were 200 °C for PI and 130 °C for PEEK, respectively. Both films achieved a sufficient aging effect with the treatment time of 24 hours.

The influence of physical ageing on the in-plane shear creep compliance of 5HS C/PPS

Thermoplastic polymer-matrix composites, such as carbon woven fabric reinforced poly(phenylene sulphide) (C/PPS), are increasingly used in the aircraft industry. Primary structural applications, however, are limited due to uncertainty concerning the long-term behaviour. Recent work indicated a progressive creep response over time, which would render these materials unusable for such applications. However, the effect of physical ageing was neglected, which is well known to alleviate the creep behaviour and hence physical ageing is rigorously included in this study on the long-term creep response of C/PPS. Short-term tensile creep tests in the bias direction were performed at temperatures of 50, 60, 65, 70, 75 and 80^{circ}mbox{C} to obtain a master curve using the time–temperature superposition principle. Ordinary horizontal shifting failed to produce a smooth curve and therefore three alternative approaches were used and compared. The physical ageing rate was, however, characterised with horizontal shifting only at 50^{circ}mbox{C} and was implemented by means of the effective time theory (Struik, 1977) to correct the momentary master curves for the influence of physical ageing. The resulting predictions are more realistic and demonstrate that the structural changes in a material reduce the creep rate over time. Hence, the long-term creep compliance tends to increase asymptotically towards a finite value, in contrast to the unbounded momentary response.

Process simulation and optimization of oxygen enriched combustion using thin polymeric membranes: effect of thickness and temperature dependent physical aging

AbstractBACKGROUNDLimited work has been available to model thickness and temperature dependent physical aging encountered in polymeric membranes for gas separation to provide physically intuitive interpretation underlying the process. The Tait equation of states has been integrated within conventional dual mode mathematical model to characterize the physical aging mechanism, whereby the model requires merely temperature dependent parameters. Subsequently, the Tait‐dual mode mechanism model has been incorporated within a succession of states methodology that quantifies transport mechanism in a countercurrent hollow fiber membrane. Finally, the developed mathematical model has been implemented in Aspen HYSYS to constitute an oxygen enrichment plant.RESULTSThe simulation model has been validated with experimental observation with an acceptable error of &lt; 6.5%. The process simulation tool has been used to evaluate the separation performance of oxygen enrichment plant using polymeric membrane with consideration of operating temperature (35–55 °C) and thickness dependent (400–1000 nm) physical aging. It is found that the optimal membrane design that generates highest profitability with physical aging consideration is at film thickness of ∼400 nm and operating temperature of 55 °C. Under such operating condition, the deviation between ideal and non‐ideal simulation model is also the smallest at ∼6%, which implies that physical aging has the least impact to the operability of the plant.CONCLUSIONThe developed simulation model has the potential to be applied for complex membrane system design (multiple membranes or hybrid system), scale up and optimization study that is essentially required for industrial scale application. © 2019 Society of Chemical Industry

Aging and the End Times: Evangelical Eschatology and Experiences of Elderhood in the United States and South Africa

Recent trends in aging studies and popular U.S. discourse reformulate elderhood as a valuable, not necessarily negative, experience, and these new models of aging have extended to a consideration of religious practices that can make old age particularly meaningful. Among evangelical Christians, a shared cosmological (and specifically eschatological) narrative structure provides solace and semiotic coherence in the face of challenges characteristic of the “third” and “fourth age.” What remains less clear is the interplay between transnational religious forces like evangelical ideology and local social contexts in which they are enacted, a process illuminated only through cross-cultural comparison. Drawing on ethnographic fieldwork conducted in Kentucky and in South Africa, I argue that rather than viewing evangelical rhetoric as narrowly determinative, anthropologists ought to broaden common understandings of Christians’ end-times ideology as something that may, contingent on socio-historical context, alternatively help older congregants cope with the physical effects of aging or allow for reconciliation amid rapid societal change. U.S. evangelical churches often address existential concerns faced by a growing population of elders while downplaying the significance of race, yet white South African Christians employ a similar religious cosmology to place their actions during the apartheid era in a symbolically legible narrative. Both settings indicate the malleability of evangelical ideas to foreground certain concerns while erasing others, challenging assumptions about the uniform effects of global evangelicalism.

3D covalent organic framework for morphologically induced high-performance membranes with strong resistance toward physical aging

The physicochemical properties of filler materials are critical considerations influencing the separation performances of mixed-matrix membranes (MMMs). Herein, a three-dimensional covalent organic framework (3D-COF) with a secondary amine-containing backbone was designed to offer large surface area, high porosity and good affinity toward CO2 molecules. Membranes prepared from this 3D-COF filler and a 6FDA-DAM polyimide matrix demonstrated a more significant enhancement in the CO2/CH4 separation performance, which was unattainable by its 2D-COF analogue. Specifically, with 10 and 15 wt% loadings of 3D-COF fillers, the MMMs membranes were able to enhance the CO2 permeability by ~57% and 140%, respectively, at a comparable, if not better, CO2/CH4 selectivity than the unfilled membrane. Furthermore, glassy polymers of high fractional free volume such as 6FDA-DAM tend to suffer from a ubiquitous loss in performance over time due to a physical aging effect. In this regard, the 3D-COF was effective in slowing down the aging process by capitalizing on its high surface area and amine functional groups to immobilize and rigidify the 6FDA-DAM polymer chains, preventing the collapse of the free volume. This allows 97% of the initial membrane performances to be effectively retained after 240 days of aging. Our findings suggest the potential of morphologically-tuned COFs to develop high-performance MMMs with strong practical relevance.

Effect of long-term thermal cycling and moisture on heated Fibre Metal Laminates and glass-fibre epoxy composites

Heated GLARE, a Fibre Metal Laminate with an integrated heater element, has a promising potential as de- or anti-icing system in aircraft structures. To investigate the long-term durability of heated GLARE previously reported thermal cycling tests up to 12,000 cycles are extended up to 144,000 cycles and moisture conditioning is included. Similar testing up to 72,000 cycles is performed on FM906 glass-fibre epoxy laminates with an integrated heater element to study the influence of the aluminium layers in GLARE.In all test results the initial decrease in interlaminar shear values from 0 to about 10,000 thermal cycles is followed by a recovery phase and a resumed decline after roughly 60,000 cycles. The decrease in ILSS is expected to be caused by internal stress relief which is counteracted by the positive effect of physical ageing due to elevated temperature exposure. Thermal cycling of conditioned samples generally resulted in slightly larger decreases in interlaminar shear strength for both heated GLARE and heated FM906 glass-fibre epoxy laminates.

Aging effects of polyamide 12 in selective laser sintering: Molecular weight distribution and thermal properties

The material aging in selective laser sintering SLS of polyamide 12 is one challenge, which has to be overcome for implementation of this technique in serial production. High temperatures and along going processing times lead to chemical and physical aging effects of the supporting partcake material. Resulting aging mechanisms are not understood up to now. The investigations in this study aims at the influence of processing time and temperature on molecular changes and thermal properties of polyamide 12 partcake material in selective laser sintering. The focus of the investigations lays on the global heat exposure of the of the bulk material und thus on global material changes. Gel permeation chromatography analysis was used to determine the molecular weight distribution and changes of polymer structure. The Mark-Houwink plot exhibits a linear chain growth, which is a hint for a resulting solid state polycondensation. With increasing build time and build chamber temperature the average molecular weight is rising, whereby the influence of build time is more significant. The rise of chain length leads to a reduction of crystallization temperature, which was detected by DSC.

Gas Permeation Properties, Physical Aging, and Its Mitigation in High Free Volume Glassy Polymers.

Hundreds of polymers have been evaluated as membrane materials for gas separations, but fewer than 10 have made it into current commercial applications, mainly due to the effects of physical aging and plasticization. Efforts to overcome these two problems are a significant focus in gas separation membrane research, in conjunction with improving membrane separation performance to surpass the Robeson upper bounds of selectivity versus permeability for commercially important gas pairs. While there has been extensive research, ranging from manipulating the chemistry of existing polymers (e.g., thermally rearranged or cross-linked polyimides) to synthesizing new polymers such as polymers of intrinsic microporosity (PIMs), there have been three major oversights that this review addresses: (1) the need to compare the approaches to achieving the best performance in order to identify their effectiveness in improving gas transport properties and in mitigating aging, (2) a common standardized aging protocol that allows rapid determination of the success (or not) of these approaches, and (3) standard techniques that can be used to characterize aging and plasticization across all studies to enable them to be robustly and equally compared. In this review, we also provide our perspectives on a few key aspects of research related to high free volume polymer membranes: (1) the importance of Robeson plots for membrane aging studies, (2) eliminating thermal history, (3) measurement and reporting of gas permeability and aging rate, (4) aging and storing conditions, and (5) promising approaches to mitigate aging.

Effect of Film Thickness and Physical Aging on “Intrinsic” Gas Permeation Properties of Microporous Ethanoanthracene-Based Polyimides

Two ethanoanthracene-based dianhydrides, 9,10-dimethylethanoanthracene-2,3,6,7-tetracarboxylic anhydride (EA-DA) and its more flexible dibenzodioxane-containing derivative (EAD-DA), were synthesized from the same starting material, 9,10-dimethyl-ethanoanthracene-2,3,6,7-tetraol, and used for the preparation of bicyclic intrinsically microporous polyimides (PIM-PIs) by one-pot polycondensation reaction with 3,3′-dimethylnaphthidine (DMN). The resulting organosoluble polyimides, EA-DMN and EAD-DMN, were thermally stable up to 300 °C and had good mechanical properties with tensile strength of 55 and 63 MPa and elongation at break of 15 and 30%, respectively. EA-DMN and EAD-DMN polyimides displayed Brunauer–Emmett–Teller (BET) surface areas of 720 and 800 m2 g–1, respectively. Fresh films showed promising gas separation performance with very high gas permeabilities and moderate gas-pair selectivities, which were both strongly dependent on film thickness. The results obtained in this study shed more light on t...

Physical aging of shape memory polymers based upon epoxy-thiol “click” systems

The physical aging behaviour of two epoxy-thiol shape memory polymers has been studied: diglycidyl ether of bisphenol-A cross-linked with pentaerythritol tetrakis, denoted 4-thiol, with a calorimetric glass transition temperature, Tg, of 51 °C; and the same system modified with tri (2,3-epoxypropyl)isocyanurate in a 30 wt% proportion, denoted 4-thiol-30%iso, with Tg = 63 °C. The aging of both polymers has been measured by torsional creep for aging temperatures between room temperature and the Tg of each polymer. The creep behaviour of each polymer is characterised by a discrete distribution of relaxation times, which shifts with aging time according to a double logarithmic aging rate, μ, and with aging temperature according to an activation energy. The distribution of creep relaxation times for 4-thiol is rather symmetrical, and slightly narrower than that for 4-thiol-30%iso, which is also skewed to longer times. The dependence of μ on temperature displays a peak as the aging temperature reduces below Tg, and is sharper for 4-thiol, in accordance with the narrower distribution, as predicted by a theoretical model based upon structural relaxation kinetics. The other parameters defining the aging behaviour, namely the reduced activation energy (Δh*/R = 92 kK for 4-thiol and 82 kK for 4-thiol-30%iso) and the non-linearity parameter of structural relaxation (x = 0.35 for 4-thiol and 0.25 for 4-thiol-30%iso), have been determined experimentally and are compared with the predictions of the theoretical model. These parameters can be used to predict the effects of physical aging on the shape memory response.

Electro-thermal damage of carbon fiber/epoxy composite laminate

The present work concentrates on the electro-thermal damage mechanism of carbon fiber/epoxy composites subjected to ampere-level Direct Current (DC) currents. A new fully automated electro-thermal tester that enable one to inject currents through the composites and measure the surface temperature in real time has been developed. The DC electrical conductivity and dielectric properties of electrified composites have been investigated, in order to develop a nondestructive testing method for electro-thermal damage. Additionally, Fourier Transform Infrared Spectroscopy (FT-IR), Dynamic Mechanical Analysis (DMA), flexural properties, surface morphology analysis, and Scanning Electron Microscope (SEM) were conducted to exploit in-depth the damage mechanism of electrified composites. Results show that the electrical conductivity is decreased with increasing currents, resulting from the modification of the fiber-to-fiber conduction paths. With higher currents the dielectric properties are apparently enhanced, and this case can be explained by interfacial polarization that arises from the microcracking induced by the residual thermal stress. The dielectric loss tangent can be regard as a characteristic parameter of electro-thermal damage. Furthermore, the deterioration of flexural performances is attributed to the microcracking, which is proved to be the major damage mechanism. High-intensity currents application has an effect of post-curing and physical aging, which significantly increases the glass transition temperature and suppresses the further formation of micro damage.

Effects of aging on poly(1‐trimethylsilyl‐1‐propyne) membranes irradiated with vacuum ultraviolet radiation for gas separation

ABSTRACTIn this study, we investigated the effects of physical aging on the surface and gas‐transport properties of highly gas permeable poly(1‐trimethylsilyl‐1‐propyne) membranes irradiated with vacuum ultraviolet (VUV) radiation. VUV excimer lamp irradiation was performed on one side of the membrane for 6 or 60 min. The gas permeabilities for carbon dioxide (CO2) and nitrogen (N2) were determined through a volumetric measurement method at 23 °C. The gas permeabilities for CO2 and N2 increased temporarily at 7 days after 6 and 60 min of VUV irradiation of the membranes. The change in the gas permeability for N2 was more remarkable than that for CO2. These changes were related to the CO or SiOx ratio. The CO ratio was related to the gas permeability of the membranes VUV‐irradiated for 6 min, whereas the SiOx ratio was related to the gas permeability of the membranes VUV‐irradiated for 60 min. These changes affected the gas selectivities of the membranes. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45973.

Two-scale model for the effect of physical aging in elastomers filled with hard nanoparticles

A two-scale model is developed, and solved numerically, to describe the mechanical behavior of elastomers filled with hard nanoparticles. Of particular interest is the slow recovery of the elastic modulus after large-amplitude oscillatory deformation. To account for this effect, the physical aging of the glassy bridges between the filler particles is captured with two thermal degrees of freedom for the matrix material, namely a kinetic and a configurational one. Formulating the two-scale model enriched with aging in a nonequilibrium thermodynamics context, first results in a constitutive relation for the Cauchy stress tensor. Second, the dynamics of physical aging is described, which eventually results in the slow recovery of the elastic modulus with waiting time. The proposed model is investigated numerically under large amplitude oscillatory shear deformation. Of particular interest in this respect is the coupling of the micro-scale dynamics with the physical aging on the macroscopic scale. This coupling is examined in detail, both in an approximate way using a Gaussian approximation, as well as numerically, under specific conditions. It turns out that the CONNFFESSIT approach (Laso and Öttinger 1993 [46]) can not be employed for the numerical solution of the model under arbitrary loading conditions because of the novel structure of the two-level coupling term. While a procedure for solving the model numerically for the case of strong applied deformation is presented in this paper, other solution methodologies need to be sought for the cases of weak and no applied deformation.

Physical aging effects on the dynamic relaxation behavior and mechanical properties of Cu46Zr46Al8 metallic glass

Mechanical relaxation of glassy materials is strongly influenced by their mechanical and physical properties. In the current study, dynamic mechanical relaxation of Cu46Zr46Al8 metallic glass has been investigated by mechanical spectroscopy and molecular dynamics simulations. Our results show that mechanical properties of metallic glasses are highly dependent on their thermal history. α relaxation in the frequency region of the Cu46Zr46Al8 metallic glass was described by Havriliak-Negami (HN) model. In the framework of the quasi-point defects theory, the correlated factor χ could be used to characterize the concentration of “defects” of glassy materials. By molecular dynamics simulations of dynamic mechanical tests at different temperatures and frequencies, it is noted that adjusting of defects is inversely correlated to storage modulus at the atomic level. On the other hand, physical aging below the glass transition temperature Tg reduces the concentration of defects and narrows down the width of shear bands, which corresponds to decreasing of the atomic mobility.

Preliminary techno-economic and environmental assessment of thickness dependent physical aging in oxygen enriched combustion using polymeric membranes

Current study is a pioneering work to demonstrate the methodology and vitality to incorporate physical aging effect in process simulation, economic study and environmental assessment of polymeric membranes under varying thicknesses (25–500 nm) applied in O2 enriched combustion. In this work, the volume relaxation in polymeric membrane during aging has been modelled employing the dual mode mechanism (e.g. Lattice contraction and free volume diffusion) in finite element numerical solution. Later, the dual mode model has been integrated within a succession of states methodology that analytically addresses the separation mechanism of membrane according to the solution-diffusion equation. Finally, the model has been included in Aspen HYSYS process simulator as a membrane unit operation extension to constitute the entire process flow in a combustion plant. Accuracy of the simulation model has been validated with laboratory data (error <5%). It is found that the thinner polymeric membrane is the more economical option, whereby internal rate of return (IRR) can be increased from 2.91% to 82.91% when polymeric membrane thickness is reduced from 500 nm to 25 nm. In addition, incorporation of physical aging mechanism is particularly important in thinner membrane structure, whereby percentage IRR deviation can reach 8.18% for 25 nm thick films.

Effects of aging in dry environment on physical and mechanical properties of a cold-curing structural epoxy adhesive for bridge construction

The effect of physical aging and continuation of curing on the physical and mechanical properties of a commercial cold-curing structural epoxy adhesive used in bridge construction has been investigated during the first year. Since adhesive bridge joints are sealed, only dry conditions were taken into account. Predominant physical aging in the earlier age and predominant curing in the later age affected the physical and mechanical properties. The E-modulus exhibited a maximum in the earlier age due to a maximum mass density caused by physical aging. In the later age, when curing became predominant, mass density and thus the E-modulus decreased while the cross-link density increased. After the first week, the E-modulus development became independent of the curing conditions. Tensile strength and failure strain depended mainly on the cross-link density and their development was thus influenced by the curing conditions but delayed in the earlier age due to physical aging.

Two-subsystem thermodynamics for the mechanics of aging amorphous solids

The effect of physical aging on the mechanics of amorphous solids as well as mechanical rejuvenation is modeled with nonequilibrium thermodynamics, using the concept of two thermal subsystems, namely a kinetic one and a configurational one. Earlier work (Semkiv and Hütter in J Non-Equilib Thermodyn 41(2):79–88, 2016) is extended to account for a fully general coupling of the two thermal subsystems. This coupling gives rise to hypoelastic-type contributions in the expression for the Cauchy stress tensor, that reduces to the more common hyperelastic case for sufficiently long aging. The general model, particularly the reversible and irreversible couplings between the thermal subsystems, is compared in detail with models in the literature (Boyce et al. in Mech Mater 7:15–33, 1988; Buckley et al. in J Mech Phys Solids 52:2355–2377, 2004; Klompen et al. in Macromolecules 38:6997–7008, 2005; Kamrin and Bouchbinder in J Mech Phys Solids 73:269–288 2014; Xiao and Nguyen in J Mech Phys Solids 82:62–81, 2015). It is found that only for the case of Kamrin and Bouchbinder (J Mech Phys Solids 73:269–288, 2014) there is a nontrivial coupling between the thermal subsystems in the reversible dynamics, for which the Jacobi identity is automatically satisfied. Moreover, in their work as well as in Boyce et al. (Mech Mater 7:15–33, 1988), viscoplastic deformation is driven by the deviatoric part of the Cauchy stress tensor, while for Buckley et al. (J Mech Phys Solids 52:2355–2377, 2004) and Xiao and Nguyen (J Mech Phys Solids 82:62–81, 2015) this is not the case.

Network-Forming Nanoclusters in Binary As\u2013S/Se Glasses: From Ab Initio Quantum Chemical Modeling to Experimental Evidences

Network-forming As2(S/Se)m nanoclusters are employed to recognize expected variations in a vicinity of some remarkable compositions in binary As–Se/S glassy systems accepted as signatures of optimally constrained intermediate topological phases in earlier temperature-modulated differential scanning calorimetry experiments. The ab initio quantum chemical calculations performed using the cation-interlinking network cluster approach show similar oscillating character in tendency to local chemical decomposition but obvious step-like behavior in preference to global phase separation on boundary chemical compounds (pure chalcogen and stoichiometric arsenic chalcogenides). The onsets of stability are defined for chalcogen-rich glasses, these being connected with As2Se5 (Z = 2.29) and As2S6 (Z = 2.25) nanoclusters for As–Se and As–S glasses, respectively. The physical aging effects result preferentially from global phase separation in As–S glass system due to high localization of covalent bonding and local demixing on neighboring As2Sem+1 and As2Sem−1 nanoclusters in As–Se system. These nanoclusters well explain the lower limits of reversibility windows in temperature-modulated differential scanning calorimetry, but they cannot be accepted as signatures of topological phase transitions in respect to the rigidity theory.

Accurate Cure Modeling for Isothermal Processing of Fast Curing Epoxy Resins.

In this work a holistic approach for the characterization and mathematical modeling of the reaction kinetics of a fast epoxy resin is shown. Major composite manufacturing processes like resin transfer molding involve isothermal curing at temperatures far below the ultimate glass transition temperature. Hence, premature vitrification occurs during curing and consequently has to be taken into account by the kinetic model. In order to show the benefit of using a complex kinetic model, the Kamal-Malkin kinetic model is compared to the Grindling kinetic model in terms of prediction quality for isothermal processing. From the selected models, only the Grindling kinetic is capable of taking into account vitrification. Non-isothermal, isothermal and combined differential scanning calorimetry (DSC) measurements are conducted and processed for subsequent use for model parametrization. In order to demonstrate which DSC measurements are vital for proper cure modeling, both models are fitted to varying sets of measurements. Special attention is given to the evaluation of isothermal DSC measurements which are subject to deviations arising from unrecorded cross-linking prior to the beginning of the measurement as well as from physical aging effects. It is found that isothermal measurements are vital for accurate modeling of isothermal cure and cannot be neglected. Accurate cure predictions are achieved using the Grindling kinetic model.

Effect of physical aging on the gas transport and sorption in PIM-1 membranes

Understanding of the properties over long time scales is a key requirement for the successful application of novel polymers as membrane materials. In this light, the physical aging of dense PIM-1 films with different previous histories was monitored for more than 4 years via parallel gas sorption and permeability measurements. The effect of aging on the individual transport parameters, permeability, solubility and diffusivity, was studied on alcohol treated membranes with high excess free volume. Thermal conditioning of these membranes led to accelerated aging and a reduction of the initial gas permeability and diffusivity of the membranes. A long-term CO2 sorption analysis showed aging affected the sorption kinetics much more than the total equilibrium sorption. This was confirmed by permeation studies with six different gases, showing that the reduction of the permeability coefficient of the samples as a function of time is almost entirely due to a reduction of the diffusion coefficient. A renewed alcohol treatment of the aged membrane led to significant rejuvenation of the membrane. To the best of our knowledge, this is the first systematic long term aging study on PIM-1 via simultaneous analysis of sorption and permeation kinetics. Mixed gas permeation measurements with a CO2/CH4 mixture and an N2/O2/CO2 mixture confirm the excellent permselective properties of the PIM-1 membranes even after long aging.