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The power-law relationship between creep rate decay and time is one of the intrinsic characteristics of metallic glasses. In the current work, a La30Ce30Ni10Al20Co10 high-entropy metallic glass was selected as the model alloy to test the influences of physical aging and cyclic loading on the power-law creep mechanism, which was probed by the dynamic mechanical analysis in terms of the stochastic activation, and contiguous interplay and permeation of shear transformation zones. It is demonstrated that a notable discrepancy appears between thermal treatment and mechanical treatment on the power-law creep mechanism of this high-entropy metallic glass. On the one hand, physical aging below the glass transition temperature introduces the annihilation of potential shear transformation zones which contribute to creep. On the other hand, cyclic loading can tailor the “forward” jump operations competing with the “backward” ones of shear transformation zones by controlling the interval time (recovery time). The current research offers a new pathway towards understanding the creep mechanism of high-entropy metallic glasses.

International audience; In this work, epoxy networks used for civil engineering constructions are aged at ambient temperature, and the impact of the early stages of aging on mechanical properties is analyzed. It is observed that Young's modulus and the yield stress increase with the aging time whereas the strain at break and the toughness decrease. The structural causes for these variations are investigated by swelling experiments, infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis. By combining experimental results, it is deduced that the aging process occurs in two stages. During the first 7 days of aging, the change in mechanical properties should essentially be attributed to the postcuring, which results in additional crosslinking reactions. When increasing the aging time to 21 days, the crosslinking density reaches a plateau, but a second stage emerges, in which the mechanical properties continue evolving in the same way, with variations of the mechanical parameters being of comparable amplitude. This second stage is attributed to the evolution of the thermodynamic state of the glass, leading to a densification of the epoxy network through minimization of the free volume, and favoring the creation of physical interactions between polymer segments. Therefore, structural relaxation is shown to have a significant impact on the mechanical properties.

The physical aging was investigated of obtained by sol-gel technology nanocomposites based on polyimide (PI) with card substituents and tetraethoxysylane (TEOS). The results for organic-inorganic composites, that contain of 5%. 20% or 50 % of TEOS, demonstrate that at temperatures well below (400K) the glass transition temperature of the polymer can take place changes in the dynamic characteristics of polymer macrochains and its permeability to low molecular probe as well as changes in aggregation of inorganic component. According to the methods of EPR, optical microscopy, etc. changes that occur in the characteristics of sol-gel polyimide based nanocomposites during long-term storage at temperatures much lower than the glass transition temperature of the polymer can be described as follows. The segmental mobility of the organic component is significantly reduced and the dynamic heterogeneity of the polymer increases. The decrease in the relative permeability of aged nanocomposites with increased content of inorganic component as compared with aged pure PI does not correlate with the content of TEOS in contrast to the initial samples of the same composition. PI macrochains chemically bonded to the inorganic phase have limited ability to realize an optimal conformation in the process of thermal relaxation (physical aging) so the increasing the content of the inorganic component has less effect on reducing the permeability of aged composites compared to aged pure PI. This is consistent with changes in the distribution of inorganic aggregates of composites. There are changes in the mean size of aggregates of inorganic particles in the composite and a decrease in their number. Smoothing is observed of the surface of nanocomposite films as well as disappearance of inhomogeneities caused by the surface of support. Due to the chemical bonding of inorganic particles and polyimide matrix, the peculiarities of the physical aging process of such composites are due to the mutual influence of the inorganic and polymer components.

PIM-1/holey graphene oxide (GO) mixed matrix membranes (MMMs) have been prepared and their gas separation performance for CO2/CH4 mixtures assessed. Nanopores have been created in the basal plane of gas-impermeable GO by chemical etching reactions, and the resulting holey flakes have been further chemically functionalized, either with octadecylamine (ODA) or with PIM-1 moieties, to aid their dispersion in PIM-1. It is found that nanopores barely promote gas transport through the graphene-like nanofiller for fresh membranes (tested right after preparation); however, the prepared hybrid PIM-1/holey GO membranes exhibit higher CO2 permeability and CO2/CH4 selectivity than the pure polymer membrane 150 days after preparation and 13 and 15% higher CO2 permeability for filler contents of 0.1% of octadecylamine-functionalized holey GO and 1% of (PIM-1)-functionalized holey GO, respectively. The most significant improvement is observed for the mitigation of physical aging, as MMMs using 10% of (PIM-1)-functionalized holey GO nanofillers are capable of maintaining up to 70% of their initial CO2 permeability after 150 days, whereas only 53% is kept for pure PIM-1 after the same period. The gas permeability of the nanofiller has been rationalized with the aid of the Maxwell–Wagner–Sillars equation., The authors are grateful to EPSRC for funding under grant numbers EP/K016946/1 and EP/M01486X/1. J. M. Luque-Alled is grateful to the Department of Chemical Engineering and Analytical Science─The University of Manchester for funding his Ph.D. studies. P. Gorgojo acknowledges the Spanish Ministry of Economy and Competitiveness and the European Social Fund through the Ramon y Cajal programme (RYC2019-027060-I/AEI/10.13039/501100011033).

This work examined the relationship between changes in molecular orientation and the heat resistance (heat distortion temperature) of polystyrene injection moldings following heat treatment below the glass transition temperature. Molecular orientation around the core layer of the injection moldings was found to be relaxed by the heat treatment. Also, in the untreated specimen, the molecular orientation around the core layer was relaxed from 60°C during the heating process. Since loss tangent (tanδ) also increased from 60°C during the heating process in the untreated specimen, it was considered that the increase in tanδ occurred with the molecular motion due to the relaxation of molecular orientation from 60°C. After the heat treatment, because of the relaxation of molecular orientation around the core layer by the heat treatment, the relaxation of molecular orientation from 60 °C did not occur during the subsequent heating process, and the tanδ of the polymer between 60 and 90 °C was decreased. Because this decrease in the tanδ over this temperature range improved the heat resistance of the material, the enhanced heat resistance by the heat treatment was attributed to the suppression of the relaxation of molecular orientation from 60°C during the heating process. Furthermore, relaxation of molecular orientation and enthalpy relaxation were related to improvement in the heat resistance.

In this work, we investigated the influence of physical aging on polylactic acid (PLA) films using thermal and optical techniques; Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Current (TSDC), and Attenuated Total Reflection Spectroscopy (ATR). The PLA films were aged for different periods: 60, 90, and 120 minutes at a temperature Ta = 43 °C. The result obtained by DSC showed that the effect of physical aging appeared as an endothermic peak, which increased with increasing aging time and evolved towards higher temperatures. TSDC results showed a thermal current peak located between 30 and 80 ° C, which represented the main relaxation mode (α relaxation) of the dielectric manifestation of the glass transition. The intensity of this peak decreased and was shifted to higher temperatures when aging time increased, this result can be explained by a decrease in the molecular mobility of macromolecular chains due to the decrease in the free volume. The effect of physical aging on the PLA by the ATR technique showed a gradual decrease in all absorption bands during the aging period. In particular, the wide absorption band between 3000 and 3700 cm-1 attributed to the hydroxyl group (OH), which disappeared after two hours of aging

Ionic liquids (ILs) have become attractive in membranes for CO₂ separation because of the designable structure, high CO₂ affinity, and good compatibility and stability. Modification of metal–organic frameworks with ILs is an effective strategy to improve the gas separation performance of mixed matrix membranes (MMMs). In this work, task-specific ionic liquids (TSILs) were incorporated to prepare TSIL@ZIF-67/PIM-1 MMMs and improve CO₂ separation performance because of the high CO₂ solubility and great compatibility toward ZIF-67 and PIM-1. The TSILs (TMGHIM and TMGHPhO) selectively and reversibly interact with CO₂ and improve the interfacial compatibility between ZIF-67 and PIM-1, leading to higher CO₂/CH₄ and CO₂/N₂ selectivities than that of the ZIF-67/PIM-1 MMM and the directly blended membrane (TSIL/ZIF-67/PIM-1). The 10 wt % TMGHIM@ZIF-67/PIM-1 MMM shows enhanced CO₂/N₂ and CO₂/CH₄ selectivities of 13.7 and 10.5, respectively; meanwhile, the CO₂ permeability reaches 12,848.5 Barrer and is 2.5 times that of the pure PIM-1 membrane. The TSIL@ZIF-67/PIM-1 MMMs also show good stability against physical aging.

The production of bio-composites based on vulcanized rubber reinforced by natural fibers has been widely studied. However, the aging phenomenon occurring during the long-term storage generating irreversible damage is limiting the performance of this kind of composites which has not been studied in details. So the objective of this work was to characterize and understand the structure-mechanical property relationships in these bio-composites and to correlate the effective crosslinking density with physical aging as a function of fiber content (0, 20, 30 and 40 wt.%) and fiber surface modification (alkali treatment) for the system polyisoprene (PI) and coir fibers. Furthermore, interfacial adhesion was improved by using maleic anhydride grafted polyisoprene as a coupling agent. From the samples produced, a complete set of morphological (scanning electron microscopy) and mechanical (shear and tension) characterization was performed before and after natural aging. The results showed that the tensile modulus increased with fiber content for the range of conditions studied and the presence of a coupling agent always gave higher tensile strength due to better interfacial stress transfer. However, important elasticity loss (strain at break) was observed due to the low fiber elasticity. Finally, different physical aging mechanisms led to changes in the molecular configuration and a reduction of free volume caused by a post-curing phenomenon of the polyisoprene matrix and low environmental stability of the fibers. These phenomena were shown to increase the Young’s modulus of the neat matrix by eight times compared to the unaged neat polyisoprene, but a smaller variation (five times) was observed for the bio-composites which was related to different crosslinking density and coir fibers state of dispersion in the vulcanized matrix

Polymers exhibit viscoelastic behavior: their mechanical response depends on the loading time, or on the loading frequency. In addition, if a polymer structure has a long service life, the mechanical behavior can also depend on physical aging and chemical degradation. This paper describes a thermodynamically consistent constitutive law taking into account the viscoelastic phenomena and the physical aging. First, an original nonlinear viscoelastic law, depending on the physical aging time, is developed. Then, considering experimental values of dynamic modulus from the literature, the model parameters are identified, using a new method based on the discrete form of the spectrum of relaxation time. The obtained model is numerically implemented and compared to experimental results.

The semicrystalline state of polymers implies the formation of a noncrystalline interphase beside lamellar crystals known as the rigid amorphous fraction (RAF). This devitrifies on heating at temperatures much higher than the one typical of the bulk-amorphous fraction. Its glass transition and physical aging in the glassy state have thus far remained elusive. Here, we study the RAF glassy dynamics in poly(l-lactic acid) (PLLA) applying recently developed aging thermal protocols based on fast scanning calorimetry (FSC). Specifically, semicrystalline samples are aged in different conditions between the glass transition of mobile amorphous fraction and the crystals’ melting temperature (TM). A distinct endothermic peak at temperatures above that of aging develops with time. We provide compelling evidence that in the initial part of aging the origin of this peak is due to the enthalpy recovery of glassy RAF. At longer aging times, the aging peak is at least partly associated with secondary crystallization. Isochronous experiments at different aging temperatures enable to obtain a fair estimate of the RAF glass transition temperature, whose upper limit is about 135 °C. The proposed method holds the promise of gaining new insights into the elusive glassy dynamics of the RAF of semicrystalline polymers., The authors thank Total Corbion (The Netherlands) for kindly providing PLLA as well as Dr. Barbara Immirzi of IPCB-CNR for GPC analyses. D.C. acknowledges funding from Spanish Government “Ministerio de Ciencia, Innovacion y Universidades” (PGC2018-094548-B-I00 (MCIU/AEI/FEDER, UE)) and Basque Government (IT-1175-19).

The acceleration of structural relaxation or physical aging by deformation, known as overaging, has been reported in experiments and simulations of polymer and colloid glasses, and correct accounti...

Upon quenching a polymer melt to a temperature T below its glass transition temperature Tg, structural relaxations, physical aging, enable the material to return to equilibrium. Whereas the physica...

Polymers of intrinsic microporosity (PIMs) have exceptional gas separation performance for a broad range of applications. However, PIMs are highly susceptible to physical aging, which drastically r...

Polymeric materials under nanoconfinements have substantially deviated physical properties with respect to the bulk, especially glass transition temperature, physical aging, and crystallization behavior. Here we highlight the leading methods for creating various confinement systems. Upon these systems, recent advances on hard and soft confinement effect for glass transition, physical aging, mechanical properties and crystallization of polymers are reviewed in details. Furthermore, as nanoconfined systems in extreme conditions are experimentally inaccessible, simulation results describing confinement effect on such systems are also discussed.

This paper provides a viewpoint of the technology of the fast-scanning calorimetry with the relaxation behavior of disordered side chains of poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-C12) around the glass transition temperature of the side chains (Tg,γ). PBTTT is an ideal model of the high-performance copolymer of poly(alkylthiophenes) with side chains. The γ1 relaxation process of the disordered side chains of PBTTT was detected as a small endothermic peak that emerges before the γ2 relaxation process. It shows an increase with increasing temperature as it approaches the glass transition temperature of the disordered side chains of PBTTT. The ductile–brittle transition of PBTTT in low temperatures originating from the thermal relaxation process is probed and illustrated by physical aging experiments. The signature is shown that the relaxation process of the disordered side chain of PBTTT at low temperatures varies from Arrhenius temperature dependence to super Arrhenius temperature dependence at high temperatures. These observations could have significant consequences for the stability of devices based on conjugated polymers, especially those utilized for stretchable or flexible applications, or those demanding mechanical robustness during tensile fabrication or use in a low-temperature environment.

Recent efforts, fostered by a pioneering work by us, have shown the of multiple steps in the recovery of equilibrium of glasses. Jin and McKenna raise concerns regarding the validity of such scenario alleging that the multiple recovery steps would be an artifact arising from poor temperature control in the oven used for isothermal glass equilibration. We critically discuss Jin and McKenna arguments from both the viewpoints of scrutinizing previous literature data and that of the temperature control in the oven. In doing so, we provide compelling arguments that Jin and McKenna conjectures are unjustified and point out the need for efforts to describe glass dynamics significantly below the glass transition temperature, Tg, by accounting for the presence of different relaxation mechanisms active in glass equilibration.

Physical aging deals with slow property changes over time caused by molecular rearrangements. This is relevant for non-crystalline materials like polymers and inorganic glasses, both in production and during subsequent use. The Narayanaswamy theory from 1971 describes physical aging - an inherently nonlinear phenomenon - in terms of a linear convolution integral over the so-called material time $\xi$. The resulting "Tool-Narayanaswamy (TN) formalism" is generally recognized to provide an excellent description of physical aging for small, but still highly nonlinear temperature variations. The simplest version of the TN formalism is single-parameter aging according to which the clock rate $d\xi/dt$ is an exponential function of the property monitored [T. Hecksher et al., J. Chem. Phys. 142, 241103 (2015)]. For temperature jumps starting from thermal equilibrium, this leads to a first-order differential equation for property monitored, involving a system-specific function. The present paper shows analytically that the solution to this equation to first order in the temperature variation has a universal expression in terms of the zeroth-order solution, $R_0(t)$. Numerical data for a binary Lennard-Jones glass former probing the potential energy confirm that, in the weakly nonlinear limit, the theory predicts aging correctly from $R_0(t)$ (which by the fluctuation-dissipation theorem is the normalized equilibrium potential-energy time-autocorrelation function).

We present VITRIFAST, a high throughput optimization procedure to characterize the vitrification kinetics based on calorimetric measurements. By analyzing the temperature dependence of specific heat capacity, the method determines the fictive temperature, Tf , and the enthalpy change during physical aging, ΔH, within only a few seconds. We tested VITRIFAST on the low molecular weight glass-former o - terphenyl (OTP) and on an archetypal glass forming polymer, polystyrene (PS), by analyzing the outcome of two classical sets of experi- ments. By means of fast scanning calorimetry (FSC), we characterized the vitrification kinetics in a wide range of cooling rates and the isothermal physical aging after vitrification at a given rate. In less than 3 minutes, our method could process 18 different calorimetric scans and provided values of Tf and ΔH in excellent agreement with those reported in the literature. VITRIFAST can be employed in the analysis of the temperature dependence of any type of second order derivative of free energy and represents a tremendous advance in the data analysis of calorimetric scans. The method is particularly helpful for fast scanning calorimetry users, considering the extremely large number of heat capacity scans recorded by this technique within a few minutes., S. N and A. A. acknowledge financial support from Action Concert Recherche-ULB under project SADI, and the Fonds de la Recherche Scientifique FNRS under Grant T.0184.20 EXOTICAGE. D.C. acknowledges financial support from the project PGC2018-094548-B-I00 (MICINN-Spain and FEDER-UE) and the project IT-1175-19 (Basque Government).

The gas separation membrane field has seen exceptional growth since its inception, with extensive research being performed to find novel, high performance polymer membrane materials for application in industrial separations. In this review, we examine recent advances of polymer design strategies to overcome the permeability/selectivity tradeoff relationship and the diminished performance caused by physical aging. We explore fundamental structure–property relationships within current state-of-the-art polymer materials to discern promising routes for continued membrane materials research and development, highlighting promising macromolecular architectures and design strategies providing tailorable polymer free volume for high performance gas separation membranes. Finally, we provide our perspective on encouraging design routes moving forward.

A series of experiments are performed to investigate the effects of physical aging on thermomechanical properties of poly(ethylene terephthalate)-glycol (PETG). The DMA, DSC and XRD tests are used ...

We report the results of an ad hoc evaluation of the thermal transition and physical state of lactobionic acid, carried out by differential scanning calorimetry, which was motivated by the confusion about its physical state in relation to the “melting point.” This work establishes that lactobionic acid is a molecular glass characterized by a glass-liquid transition at around 125°C and 2 minor transitions, respectively, at around 70°C and 40°C. The temperature at which these latter transitions appear and the intensity of the enthalpic peaks, associated with physical aging, are sensitive to the thermal history of the sample and to the presence of small quantities of absorbed water.

Graft block copolymers (BCPs) with poly(4-methyl caprolactone)-block-poly(±-lactide) (P4MCL-PLA) side chains containing 80–100% PLA content were synthesized with the aim of producing tough and sust...

In this paper, the recent development of membrane-based carbon capture technologies is reviewed, including membrane gas separation (MGS) and membrane contactor (MC). The state of the art of both technologies and recent development of membrane materials are discussed. The main challenges such as plasticization and physical aging in MGS, and wetting phenomena in MC are then discussed, followed by attractive attempts to suppress them. In addition, both technologies are compared in terms of their characteristics, carbon capture performances, and techno-economic assessment. Finally, prospects and challenges of membrane-based carbon capture technology, especially at very high CO2 concentration are pointed out.

In the field of calorimetry and in physical aging studies, the departure from equilibrium is commonly measured by the fictive temperature, T fic . For a quantity X , T fic is the temperature at which X ( T fic ) characterizes the nonequilibrium structure of a system, where X ( T ) reflects how the structure depends on T in equilibrium. Here, we demonstrate that the same concept can be applied to variables other than temperature. As an example, we discuss the case of introducing the fictive electric field for quantifying the process of structural recovery in response to removing the sample from its previous equilibrium state by the application of an electric field. The number of fictive fields required and the implications on nonlinear dielectric behavior are also discussed.

cited By 4; International audience; Dynamic relaxation processes are fundamental to understand the mechanical and physical properties of metallic glasses. In the current work, mechanical relaxations in a La 30 Ce 30 Al 15 Co 25 bulk metallic glass were probed by dynamic mechanical analysis. In contrast to many metallic glasses, La 30 Ce 30 Al 15 Co 25 metallic glass shows a pronounced slow β relaxation peak. Physical aging below the glass transition temperature T g leads to an increase of the apparent activation energy and a decrease of the slow β relaxation magnitude. The correlation between the slow β relaxation and the main α relaxation is discussed. © 2019

This letter presents an accelerated physical aging of polystyrene (PS) blocks in polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) diblock copolymers under hard confinement. The three-dimensional hard nanoconfinement was provided by the PMMA component owing to its high elasticity and was formed via self-assembled microphase separation. Aging was observed by measuring enthalpy recovery of the PS blocks in the copolymers for which the degree of polymerization ( N) of PS blocks is fixed, whereas the N of PMMA blocks varies. Our results demonstrate that the aging speed of the PS blocks can increase by a factor of three to that of the neat PS as the N of PMMA blocks increases. Therefore, the hard confinement accelerates physical aging of the PS blocks, i.e., the relatively soft component in the copolymer.

This study presents aging of backsheet materials that were stored for 17 years in the dark at room temperature. The material properties were measured and compared to the results from right after production. After 17 years of storage the strain at break values decreased significantly for all materials. As chemical aging phenomena were excluded, a disentanglement of the polymer chains in the amorphous phase would explain the observed changes. Especially for PVDF the change in the mechanical behavior is considered as very critical and possibly would lead to cracking during operation.

A novel finding made by Cangialosi and coworkers in the physical aging of several polymers way below the glass transition temperature Tg is that equilibrium recovery occurs by reaching a plateau in the enthalpy with partial enthalpy recovery. This observation points to the existence of a much faster mechanism capable of partial equilibrium recovery deep in the glassy state. A similar phenomenon was found in different glassy materials. The generality of the phenomenon indicates that the faster mechanism of equilibrium recovery is universal and fundamental. In this paper the faster mechanism is identified to be the universal JG β-relaxation having dynamic and thermodynamic properties analogous to the α-relaxation, and thus capable of effecting enthalpy and volume recovery far below Tg in several high-Tg polymers. The JG β-relaxation is also the mechanism responsible for the first step of two steps in the approach to equilibrium found in another polymer with much lower Tg. The Coupling Model is used to explain why the first step transpires far below Tg in some polymers but much closer to Tg in another polymer.

A facile evanescent-field imaging approach is developed to probe the aggregation behavior of near-wall colloids/clusters during colloidal gel evolution. Total internal reflection microscope (TIRM) images are directly utilized to access the structural relaxation time via density-fluctuation theory. The behaviors of cluster-cluster aggregation and physical aging of the colloidal gel networks are resolved in both time and space under fractal scaling criteria.

This article belongs to the Special Issue Recent Developments on Calorimetry and Thermal Analysis of Polymers and Nanocomposites., The present work aims to provide insights on recent findings indicating the presence of multiple equilibration mechanisms in physical aging of glasses. To this aim, we have investigated a glass forming polyether, poly(1-4 cyclohexane di-methanol) (PCDM), by following the evolution of the enthalpic state during physical aging by fast scanning calorimetry (FSC). The main results of our study indicate that physical aging persists at temperatures way below the glass transition temperature and, in a narrow temperature range, is characterized by a two steps evolution of the enthalpic state. Altogether, our results indicate that the simple old-standing view of physical aging as triggered by the α relaxation does not hold true when aging is carried out deep in the glassy state., D.C. research was funded by MICINN-Spain and FEDER-UE, grant PGC2018-094548-B-I00; and the Basque Government, grant IT-1175-19. J.M. thanks MICINN /FEDER for grant Ref. PGC2018-094620-A-I00.

This work provides an experimental analysis regarding the fracture behavior of recycled opaque PET (rPET-O) containing titanium dioxide (TiO2) under plane stress conditions. For this purpose, a commercially post-consumer transparent colored/opaque PET flakes mix was processed using a semi-industrial extrusion calendering process. The manufactured rPET-O sheets had a TiO2 content of 1.45 wt.%. The mechanical and fracture properties of unaged and physically aged (1 year) samples were determined through uniaxial tensile experiments and the Essential Work of Fracture (EWF) methodology, respectively, and were compared to those of recycled transparent PET (rPET-T). Under tensile loading, independently of the aging time, rPET-O samples exhibited similar mechanical behavior as rPET-T up to the yield point. The main differences remained in the post-yielding region. The presence of TiO2 particles allowed reducing the strain energy density up to neck formation in aged samples. Regarding the EWF analysis, it is argued that the energy consumed up to the onset of crack propagation (we) for rPET-T was mainly dependent of the molecular mobility. That is, the we value decreased by 26% when rPET-T was physically aged. Interestingly, we values remained independent of the aging time for rPET-O. In fact, it was highlighted that before crack propagation, the EWF response was principally governed by matrix cavitation ahead of the crack tip, which allowed a significant release of the triaxial stress state independently of the molecular mobility. This property enabled rPET-O to exhibit a resistance to crack initiation 17% higher as compared to rPET-T when the material was physically aged. Finally, independently of the aging time, rPET-O exhibited a resistance to crack growth approximately 21% larger than rPET-T due to matrix fibrillation in large scale deformation.

In recent decades, polymers of intrinsic microporosity (PIMs), especially the firstly introduced PIM-1, have been actively explored for various membrane-based separation purposes and widely recognized as the next generation membrane materials of choice for gas separation due to their ultra-permeable characteristics. Unfortunately, the polymers suffer substantially the negative impacts of physical aging, a phenomenon that is primarily noticeable in high free volume polymers. The phenomenon occurs at the molecular level, which leads to changes in the physical properties, and consequently the separation performance and membrane durability. This review discusses the strategies that have been employed to manage the physical aging issue, with a focus on the approach of blending with nanomaterials to give mixed matrix membranes. A detailed discussion is provided on the types of materials used, their inherent properties, the effects on gas separation performance, and their benefits in the suppression of the aging problem. This journal is

ConspectusSince the discovery of polymers of intrinsic microporosity (PIMs) in 2004, the fast size-selective interconnected pore cavities of the polymers have caused the upper bound of membrane performance to be revised, twice. Simultaneously, porous materials have meant that mixed matrix membranes (MMMs) are now a relatively simple method of enhancing transport properties. While there are now reliable routes with mixed matrices to improve the fundamental transport properties of membrane materials, many of the other properties crucial for separation applications remain largely unaddressed. Physical aging severely affects membrane performance over time, especially for those prepared from high fractional free volume polymers. Gradual densification of the glassy polymer chains causes the connected pore channels present in these materials to constrict. Studies now suggest that aging of superglassy polymer materials is a two-step process; a rapid densification occurs within the first few days, followed by a gradual rearrangement of packed chains over longer time frames toward a theoretical equilibrium state. Although advantageous in terms of size selectivity, the considerable drop in permeation over the days and weeks after manufacture greatly impacts material applicability. While often still permeating faster than traditional membrane materials, the continuous gradual collapse of cavities in these polymers are a significant challenge in the application of high free volume polymer membranes. In 2014, we discovered that the porous aromatic framework PAF-1 not only greatly improved the membrane's void space and speed of gas transport but also seemingly froze several glassy polymers in a low-density state, holding the polymer's pore channels open, a process termed as Porosity Induced Side chain Adsorption (PISA).This discovery of PISA fundamentally challenged the conventional wisdom at the time that the aging rate could only be addressed by densification of the polymer. Unlike other high-performance glassy polymers, membranes containing PAF-1 can retain their high permeability for more than a year. Several other examples of antiaging behavior have been subsequently reported by the team, where control of aging rate as a function of gas penetrant, selectivity increases, and stability at higher pressures was reported. These works also demonstrate that these mixed matrix systems had applicability for several other separations, including pervaporation, solvent nanofiltration, and as separators for energy applications. In our subsequent studies, the antiaging mechanism has been elucidated as an effect of the interaction between the polymer's accessible pendant methyl group and the aromatic pore surface of PAF-1 or other antiaging additives. In otherwise identical MMMs, where this hypothesized methyl-π interaction is either absent or interrupted, we find that the antiaging behavior expected by the fixation of the polymer chains to the pore surface and PAF-1 does not occur. As a design approach for mixed matrix membranes, targeted interfacial interactions are a promising pathway for developing other stable membranes, enabling the exciting class of PIM materials to improve industrial separation efficiency.

Poly(N-isopropylacrylamide) (PNIPA), as a smart polymer, can be applied for drug delivery systems. This amorphous polymer can be exposed on a structural recovery process during the storage and transport of medicaments. For the physical aging times up to one year, the structural recovery for PNIPA was studied by advanced thermal analysis. The structural recovery process occurred during the storage of amorphous PNIPA below glass transition and could be monitored by the differential scanning calorimetry (DSC). The enthalpy relaxation (recovery) was observed as overshoot in change heat capacity at the glass transition region in the DSC during heating scan. The physical aging of PNIPA was studied isothermally at 400.15 K and also in the non-isothermal conditions. For the first time, the structural recovery process was analyzed in reference to absolute heat capacity and integral enthalpy in frame of their equilibrium solid and liquid PNIPA.

cited By 3; Phase metastability of in-situ formed β-Zr/Ti plays a vital role on mechanical properties of Zr/Ti-based bulk metallic glass composites (β-type BMGCs). In the current study, internal friction behaviors of Ti50.3Zr33.9Cu4.6Ni2.1Be9.1 BMGC containing metastable β-Ti and Ti50.3Zr27.9Cu4.6Ni2.1Be9.1Mo6 BMGC containing stable β-Ti were investigated by mechanical spectroscopy. Prior to the glass transition temperature Tg, Ti50.3Zr33.9Cu4.6Ni2.1Be9.1 BMGC exhibits pronounced internal friction ascribed to the precipitation of ω-Ti in the metastable β-Ti phase. The current investigation not only reveals the fundamental mechanism of the internal friction process of BMGCs, but also provides an effective method to characterize the metastability of β phases in β-type BMGCs. © 2018 Elsevier B.V.

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 (T g). 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 T a ∼ 0.90T g and T a ∼ 0.85T g, which, however, disappears when the aging occurs at T a ∼ 0.95T g. The overall kinetics are described by a stretched-exponential function with stretching exponent close to 3/7 at T a ∼ 0.95T g, and 1/3 when the aging temperature drops to ∼0.90T g 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 T g is attributed to the hierarchical scheme of approaching equilibrium based on an alignment-shrinkage mechanism of physical aging proposed earlier for chalcogenide glasses.

Polymer blending is a promising method to overcome stability obstacles induced by physical aging and swelling of implant scaffolds prepared from amorphous polymers in biomedical application, since it will not bring potential toxicity compared with chemical modification. However, the mechanism of polymer blending still remains unclearly explained in existing studies that fail to provide theoretical references in material R&D processes for stability improvement of the scaffold during ethylene oxide (EtO) sterilization, long-term storage, and clinical application. In this study, amphiphilic poly(ethylene glycol)-co-poly(lactic acid) (PELA) was blended with amorphous poly(lactic-co-glycolic acid) (PLGA) because of its good miscibility so as to adjust the glass transition temperature (Tg) and hydrophilicity of electrospun PLGA membranes. By characterizing the morphological stability and mechanical performance, the chain movement and the glass transition behavior of the polymer during the physical aging and swelling process were studied. This study revealed the modification mechanism of polymer blending at the molecular chain level, which will contribute to stability improvement and performance adjustment of implant scaffolds in biomedical application.

Physical aging of polymer of intrinsic microporosity PIM-1 is one of the major obstacles for its application as a commercial membrane material for gas separation. In this work, physical aging of PIM-1 and matrices of this same polymer containing graphene-like materials were studied. Graphene-like fillers resulted from the functionalization of graphene oxide (GO) with two alkyl chains of different lengths, using octylamine (OA) and octadecylamine (ODA), and further chemical reduction. Extents of membrane aging were evaluated through changes in gas permeability over time; the separation of gas mixtures comprising carbon dioxide and methane, which are of great interest for industrial applications such as the production of biogas or the purification of natural gas, was carried out. 50:50 vol% CO 2 /CH 4 mixtures were used as feed and separation performance analysed for fresh membranes and at intervals of approximately a month up to 155 days. At the end of this testing period, aged PIM-1 membranes showed a CO 2 permeability of (2.0 ± 0.7) × 10 3 Barrer, which corresponds to a CO 2 permeability reduction of 68% from the value obtained right after their fabrication. The addition of alkyl-functionalized GO is shown to be an efficient strategy to retard the physical aging of PIM-1 membranes; filler loadings as low as 0.05 wt% of reduced octyl-functionalized GO showed a CO 2 permeability of (3.5 ± 0.6) × 10 3 Barrer after 5 months, which is almost three quarters higher than that of pure PIM-1 membrane aged for the same time period and represents a reduction of just 39% from its initial value. Moreover, the addition of graphene-like materials to PIM-1 does not affect its mechanical properties.

Changes in the properties of the poly[(trimethylsilyl)prop-1-yne] (PTMSP) film deposited on the walls of a quartz capillary under the action of high temperatures were studied by chromatographic methods. The change in the thermodynamic parameters of sorption of light hydrocarbons related to both physical aging of the PTMSP film and, possibly, its chemical destruction was examined.

© 2018 American Chemical Society. 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.