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Polylactic acid (PLA)/polybutylene adipate terephthalate (PBAT) blend is a fully degradable polymer blend with balanced strength and toughness. However, the performance of PLA/PBAT blends deteriorates significantly after undergoing physical aging. In this work, dicumyl peroxide (DCP) and methyl methacrylate-glycidyl methacrylate copolymer (P(MMA-co-GMA)) were used respectively for reactive compatibilization by dynamic vulcanization. The effects of dynamic vulcanization on the interfacial compatibility and thus on the mechanical properties of PLA/PBAT blends were evaluated using nanomechanical mapping. Results show that the degree of interfacial compatibilization can be determined by the interface length of microscopic Young's moduli distribution between PLA and PBAT. A longer interfacial length indicates an enhanced interfacial interaction between the components in the blend, which leads to improved macroscopic mechanical properties. Moreover, the improvement of interfacial compatibility allows the PLA/PBAT blends to maintain higher mechanical properties after physical aging, which provides an experimental basis for the design of PLA-based blends with both high mechanical properties and resistance to physical aging. [Display omitted] • Visualization of interfacial compatibilization between PLA and PBAT using AFM-based nanomechanical measurements. • Comparing the effects of different interfacial compatibilization methods in PLA/PBAT blends. • PLA/PBAT/P(MMA-co-GMA) blend has excellent mechanical properties and physical aging resistance. • The enhancement mechanism of interfacial compatibilization on the mechanical properties of PLA-based blends was discussed. [ABSTRACT FROM AUTHOR]

The molecular dynamics of the constrained and unconstrained mobile amorphous fractions in semi-crystalline polylactide (PLA) was investigated in the presence of both flow-induced crystalline structures and spherulites by fast scanning calorimetry (FSC) through cooperativity and physical aging concepts. First, the shear rate conditions leading to flow-induced crystallization were characterized by examining the relaxation of shear-induced precursors. At a temperature of 150 °C, the critical relaxation time is so long that cooling down the sheared melt to the crystallization temperature does not affect significantly the precursors. Therefore, highly oriented structures develop. The arrangement of the crystalline fraction, either in flow-induced crystalline structures or spherulites, has no influence on the molecular dynamics of the mobile amorphous fraction, whereas the crystallinity degree was proved to play a significant role. Two kinds of molecular dynamics were distinguished, respectively associated to the α and secondary relaxations. The molecular dynamics related to the α relaxation of the mobile amorphous fraction was investigated in terms of cooperativity: a clear dependence on the coupling and the confinement by the crystals was observed, because a classic decrease of the cooperativity length was found when comparing amorphous to semi-crystalline PLA. On the contrary, the molecular dynamics related to the secondary relaxations of the mobile amorphous fraction was not modified by the presence of the crystals, as observed from close physical aging investigations below the glass transition. The different length-scale of the α and secondary relaxations seems to be the reason why the crystalline environment influences the macromolecular motions rather than the molecular or local motions. [ABSTRACT FROM AUTHOR]

The bulk physical aging behavior of cross-linked polystyrene (PS) is directly compared to that of its linear precursor polystyrene-co-vinylbenzocyclobutene (PS-VBCB) with 8.5 or 12.5 mol% VBCB content. The VBCB units incorporated into a linear PS precursor cross-link with one another upon annealing at 250 °C. Physical aging rates of bulk PS-VBCB films before and after cross-linking are characterized using ellipsometry by monitoring the decrease in thickness during isothermal annealing at a specific aging temperature ( T a ) below the glass transition temperature ( T g ). Physical aging rates show strong dependences on T a values, with neat PS, linear PS-VBCB precursors with 8.5 or 12.5 mol% VBCB, and cross-linked PS with 8.5 mol% VBCB exhibiting maxima at T a - T g = −40 to −30 °C. In contrast, cross-linked PS with 12.5 mol% VBCB exhibits a maximum aging rate at T a - T g = −65 to −45 °C. The distribution of T a -dependent aging rates broadens in cross-linked PS samples compared to linear precursors, which is correlated with increased T g breadth or heterogeneity after cross-linking. Comparisons of aging rates of cross-linked PS with linear precursors demonstrate that cross-linking can lead to an increased, decreased, or unchanged aging rate depending on the values of T a and T a - T g : e.g., when T a - T g > −60 °C, both cross-linked PS samples have a lower aging rate compared to their linear precursors at the same T a - T g whereas the cross-linked PS exhibits a similar or even greater aging rate compared to the linear precursor when T a - T g < −60 °C. These results help to reconcile previous seemingly contradictory observations regarding effects of cross-linking on physical aging rates. To the best of our knowledge, cross-linking induced enhancement of physical aging rates of cross-linked polymers relative to their linear precursors at the same T a - T g value is being reported for the first time here. [ABSTRACT FROM AUTHOR]

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 CO 2 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 CO 2 /CH 4 mixture and an N 2 /O 2 /CO 2 mixture confirm the excellent permselective properties of the PIM-1 membranes even after long aging. [ABSTRACT FROM AUTHOR]

Assessing suitability of amorphous polymers in durable products requires understanding of long-term effects of physical aging on the material properties. This work shows four polyesters with varying diol composition (poly(ethylene-co-1,4-cyclohexylenedimethylene terephthalate) (PETG1 and PETG2 with ∼30 and ∼60% 1,4-cyclohexylenedimethylene (CHDM), respectively), poly(ethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate) (PETT) with ∼30% 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) and poly(1,4-cyclohexylenedimethylene-co-2,2,4,4-tetramethyl-1,3-cyclobutanediol terephthalate) (PCTT) with ∼80% CHDM and ∼20% TMCD) exposed to thermal treatment at 20, 30 and 40 °C below their respective glass transition temperatures for up to 504 h to accelerate physical aging. The enthalpy relaxation was investigated by differential scanning calorimetry and compared to mechanical changes manifested as tensile yield strength increase. The physical aging rates were found to depend on both chemical structure and composition of CHDM and TMCD segments, where the introduction of TMCD inhibited physical aging. Arrhenius and Vogel-Fulcher-Tamman models were used to fit horizontal shift factors and evaluate the time and temperature dependencies for each polyester. From this study, the two models showed no significant differences in ability to describe the effects of physical aging. The Arrhenius activation energies, E a , were all in the range 118–244 kJ mol−1, were both PETG1 and PETG2 showed no significant difference between E a for enthalpy relaxation and yield strength increase, whereas PETT and PCTT showed ∼19 and ∼107% difference between the two, respectively, suggesting that the relationship between the two phenomena is not independent of chemical structure. The difference between the activation energies suggests that the time scales for physical aging are different when observed as enthalpy relaxation and yield strength. Four PET copolyesters were exposed to thermal treatment at 20, 30, and 40 °C below their respective Tg for up to 504 h to accelerate physical aging, which are rate dependant. Arrhenius and Vogel-Fulcher-Tamman model showed no significant differences in ability to describe the effects of physical aging, with their Arrhenius activation energies ranging 118–244 kJ mol−1, suggesting different time scales for aging in regards to their enthalpy relaxation and yield strength, which are not independent of chemical structure. [Display omitted] • Four glycol modified polyesters are submitted to accelerated physical aging at three different temperatures. • Arrhenius and Vogel-Fulcher-Tamman models were able to describe the results equally well. • Physical aging rate depended on chemical structure, where copolymer TMCD inhibits enthalpy relaxation with yield strength increased. • A de-coupling between thermodynamic and mechanical effects of physical aging was found. [ABSTRACT FROM AUTHOR]

This work is aimed to monitor the physical aging (structural relaxation) of polyester fibers in terms of changes in the number of dye molecules absorbed by aged specimens as a new method and to make some aspects of this complex phenomenon more clear which rarely considered in the literature. Specimens, aged thermally and hygrothermally at the specified temperature and periods, are, thus, conducted to the dye sorption experiments to monitor their physical aging. Both purified and commercial dyes are employed. Independent from dye purities, the number of absorbed dye molecules reduces for both aged thermally and hygrothermally specimens with increasing the aging time because of decreasing of the free volume as well as increasing the crystallinity, revealed by WAXD measurement. However, the rate of aging of hygrothermally aged specimen is faster than that of the thermal one due to the role of water as a plasticizer in the polymeric bulk. The non-exponential character of the relaxation is exhibited by fitting of KWW (Kohlrausch-Williams-Watts) stretched exponential function and two additive exponential models into the relaxation data, constructed from the measuring of the changes in the number of absorbed dye molecules during aging. Using FTIR spectroscopy a correlation between the number of absorbed dye molecules and the amount of trans conformers for the sample upon thermal aging is found which shows the method yields reliable information on physical aging of the polymer. [ABSTRACT FROM AUTHOR]

Two intrinsically microporous polyimides were obtained by high-temperature, one-pot polycondensation reaction of novel triptycene-based dianhydrides containing dimethyl- or diisopropyl-bridgehead groups with a commercially available highly sterically hindered 3,3ʹ-dimethylnaphthidine (DMN) diamine monomer. The dimethyl bridgehead groups in the triptycene building block provided the DMN-based polyimide (TDA1-DMN) with larger surface area (760 m 2 g -1 ) than the diisopropyl-based polyimide (TDA i 3-DMN) (680 m 2 g -1 ), greater fraction of ultramicroporosity, as observed from N 2 and CO 2 NLDFT adsorption analysis, and higher gas permeability and selectivity. Wide-angle X-ray diffraction (WAXD) measurements demonstrated that TDA1-DMN and TDA i 3-DMN exhibited a bimodal pore size distribution, where TDA1-DMN showed smaller d-spacing values and broader intensity peaks. Both TDA-DMN-based polyimides showed very high gas permeabilities with moderate selectivities. For example, fresh TDA1-DMN exhibited an O 2 permeability of 783 Barrer coupled with an O 2 /N 2 selectivity of 4.3 and H 2 permeability of 3050 Barrer with H 2 /N 2 selectivity of 16.7, values that surpassed the 2008 Robeson permeability/selectivity upper bounds. Physical aging of the TDA-DMN polyimide films over a period of 250 days showed relatively small changes in permeability (∼20%) and selectivity (∼5%). [ABSTRACT FROM AUTHOR]

Modification of membranes to improve gas separation properties has been of considerable interest. Crosslinking is one route to modify membranes, but such studies need to be performed on thin membranes to quantify the impact of such modifications at thicknesses relevant to industrial membranes. In this study, the influences of UV irradiation and physical aging on O 2 and N 2 gas permeation properties of thin (∼150 nm) glassy, amorphous poly(arylene ether ketone) (PAEK) copolymer films at 35 °C and 2 atm were investigated. Thin PAEK copolymer films, prepared from tetramethyl bisphenol A (TMBPA) and 4,4′-difluorobenzophenone (DFBP), were UV irradiated on both sides in air or N 2 at 254 nm or 365 nm, which induced crosslinking and, in some cases, photooxidation. Gas permeability decreased and O 2 /N 2 selectivity increased as UV irradiation and aging time increased. At 254 nm, samples irradiated in air have lower permeability coefficients and higher selectivities than samples irradiated in N 2 , which was ascribed to additional decreases in free volume due to photooxidation in samples irradiated in air. Additionally, samples irradiated in air at 254 nm exhibit less physical aging than uncrosslinked and samples irradiated in N 2 at 254 nm, possibly due to interactions among photooxidative polar products that may restrict polymer chain mobility, thereby lowering the aging rate. The influence of water vapor on physical aging of samples irradiated in air was examined. Finally, irradiation at 254 nm leads to more extensive crosslinking and/or photooxidation than irradiation at 365 nm, possibly due to greater UV absorption by the polymer and the higher probability of radical formation at the lower wavelength. [ABSTRACT FROM AUTHOR]

Physical aging in high free volume polymers reduces gas permeability which limits their potential for gas separation. Plasticization is another concern when the separation involves a highly condensable gas such as CO 2 or operates in aggressive conditions. This work reports a straightforward strategy to suppress aging and plasticization of polymers of intrinsic microporosity, specifically PIM-1, with a tailored high CO 2 permeability, by the incorporation of POSS nanoparticles. The nanoparticles enhances gas diffusivity at low particle loadings while induces chain rigidification at high particle loadings. The newly developed membrane materials are promising for CO 2 capture and other gas separation. [ABSTRACT FROM AUTHOR]

Physical aging of both thick and thin films of “high free-volume” glassy perfluoropolymers was studied by monitoring changes in pure gas permeability of O 2 , N 2 and CH 4 . All permeability measurements were done at a fixed temperature of 35 °C for more than 1000 h of aging. Two grades of perfluoropolymers, Teflon AF and Hyflon AD, having different comonomer structures but with similar comonomer ratios were studied to understand the effect of comonomer type and content on the aging behavior. The effect of casting process (solution vs. spin coating) and solvent type (vapor pressure and boiling point) had a significant effect on the absolute permeability of both thick and thin films; however, the aging rates were more affected by thickness and solvent type rather than the casting process for similar thicknesses. After 1000 h of aging, the relative permeability for thin films of Teflon AF 2400 was decreased by 27% compared to only 10% for thick films prepared from Novec 7500 solvent. Teflon AF, which has a higher fractional free volume (FFV) than Hyflon AD, is believed to undergo significant aging well before the initial permeability measurement could be made (after ∼ 1 h of aging) and, therefore, Teflon AF materials showed a lower decrease in relative permeability compared to Hyflon AD for the same aging time. The comonomer type and content has a significant effect on the permeability; the initial absolute oxygen permeability for AF 2400 was an order of magnitude higher compared to AD 60. The physical aging of thin films of the various glassy perfluoropolymers was also tracked by recording changes in the refractive index and thickness with time by ellipsometry. The ellipsometry data also confirmed higher aging rates in Hyflon AD compared to Teflon AF materials. The volumetric aging rate, obtained from the change in the refractive index using the Lorentz–Lorenz equation, and the permeability reduction rate from the ( P 1000 h / P 1 h ) ratio showed an excellent linear correlation. The ( P 1000 h / P 1 h ) ratio also showed a stronger correlation with ( T g −35) °C than with FFV. [ABSTRACT FROM AUTHOR]

Abstract: Most studies using gas permeation to characterize physical aging in thin polymer films have focused on polymers of interest as membrane materials, such as polysulfone (PSF) and Matrimid. Many other physical aging studies, using techniques other than gas permeation, focus on polystyrene (PS). In this work, physical aging in bulk PS films and PDMS-coated thin PS films was studied using well-established gas permeation techniques. The ∼400 nm PS films aged slightly faster than bulk PS. However, the difference between rates of aging in thin and thick films was much less than that reported in PSF and Matrimid films of similar thicknesses. The ∼800 nm films aged in a manner generally similar to bulk PS. Comparison of the normalized oxygen permeability of ∼400 nm films of PS, PSF, and Matrimid revealed that a ∼400 nm PS film experiences a slower decline in relative permeability than a PSF or Matrimid film does. Unlike what has been observed previously in studies of PSF and Matrimid films, PS films do not appear to show aging behavior that is strongly dependent on film thickness or highly accelerated relative to bulk. Because it would be difficult to use the results of PS aging studies to predict the aging behavior of typical gas separation polymers, we suggest that PS is not a good model for the aging behavior of commercially useful gas separation membrane materials. [Copyright &y& Elsevier]

Abstract: Recent studies of physical aging in confined polymer glasses have revealed that aging behavior in confinement often differs from bulk behavior. This study used DSC to characterize physical aging and structural relaxation in bulk polysulfone (PSF) and co-extruded multilayered films of PSF and an olefin block copolymer (OBC) that have average PSF layer thicknesses of 640 nm, 260 nm, and 185 nm. The films were aged isothermally at 170 °C, and the recovered enthalpy upon reheating was measured over time. The films with 640 nm and 260 nm PSF layers had aging rates very similar to that of bulk PSF, while the film with 185 nm PSF layers had an aging rate slightly greater than the bulk value. The cooling rate dependence of the limiting fictive temperature (T f′) in multilayered and bulk PSF samples was also characterized. Values of T f′ were similar for all films at each cooling rate. The results of this work are in general agreement with our previous gas permeation aging study of multilayered PSF films aged at 35 °C, in which the effect of layer thickness on aging behavior was minimal. This stands in contrast to studies with thin, freestanding PSF films, which exhibit accelerated aging relative to bulk and have aging rates that depend strongly on film thickness. [Copyright &y& Elsevier]

Abstract: Generalized fractional Maxwell model is applied to simulate the creep and the relaxation behavior of PEEK and PPS measured at different aging stages and different temperatures. Genetic algorithm and the conjugated gradient method are combined to optimize the model parameters. The results show that the fractional Maxwell model can describe both the creep and relaxation data very well when the model parameters are fitted properly. The momentary creep compliance and the relaxation modulus change with the time at an increasing rate; the fractional orders of the two fractional elements building up the model correspond to the creep or relaxation exponents at the initial glassy stage and the terminal fluid state. The relaxation time τ of the model, which corresponds to the characteristic time of α transition of the sample, shifts towards longer time with increasing the aging time and lowering the aging temperature. In the time domain scaled by the relaxation time τ, all the creep or relaxation curves measured at different aging times and temperatures superpose automatically, which means the time–aging time superposition and the time–temperature superposition present themselves. The shift rates and the temperature shift factors can be then obtained from the model parameters. Good simulation for the creep and relaxation behavior and optimum superposition of the experimental data achieved by the fractional model may offer reliable predictions for the long-term stress-strain response. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental and modeling study of the effects of physical aging on the shape-memory performance of (meth)acrylate-based networks composed of tert-butyl acrylate (tBA) crosslinked by various concentrations of poly(ethylene glycol dimethacrylate) (PEGDMA). The experiments measured the unconstrained recovery response of samples stored at 20 °C (T g  − 36 °C) for zero to 180 days and evaluated the effects of storage on the strain fixity, activation temperature, and initial recovery rate. A thermoviscoelastic model recently developed for amorphous networks near the T g was applied to study the influence of structural and viscoelastic relaxation and the aging time and temperature on the recovery response. Results showed that the activation temperature and the initial recovery rate increased with the aging time, producing a sharper initial recovery response. The thermoviscoelastic model predicted that the magnitude of these effects depended on the aging temperature. There was an optimum aging temperature that maximized the initial recovery rate. These results suggest that physical aging can be manipulated to accelerate the recovery performance of shape-memory polymer devices. [Copyright &y& Elsevier]

Abstract: Industrial gas separation membranes have selective dense layers with thicknesses around 100 nm. It has long been assumed that these thin layers have the same properties as thick (bulk) films. However, recent research has shown that thin films with such thickness experience accelerated physical aging relative to bulk films and, thus, their permeation properties can differ significantly from the bulk. Thin films made from Extem® XH 1015, a new commercial polyetherimide, have been investigated by monitoring their gas permeability. The permeability of the thin films is originally greater than the thick films but eventually decreases well below the permeability of the thick film. The CO2 plasticization of Extem thin films is explored using a series of exposure protocols that indicate CO2 plasticization is a function of film thickness, aging time, exposure time, pressure and prior history. [Copyright &y& Elsevier]

Abstract: The physical aging of polystyrene (PS) confined in a multilayered film arrangement was explored using differential scanning calorimetry (DSC). The multilayered films were produced via multilayer coextrusion and consisted of alternating layers of PS and polycarbonate (PC), with PS layer thicknesses ranging from 50 nm to 500 nm. A 125 μm bulk control film of pure PS was also extruded and studied for comparison. The glass transition temperatures (Tg) of the PS in multilayered films did not appear to be systematically dependent on layer thickness, and Tg values in all PS/PC films were similar to the bulk value of 104 °C. Two approaches were used to investigate the structural relaxation of PS in the layered films. In the first method, PS layers were aged isothermally at 80 °C after annealing above the Tg of PS (135 °C for 15 min) to reset the thermal history and provide a well-defined starting point for aging experiments. Recovered enthalpy data for aged films (calculated from DSC thermograms) showed that the aging rate in the PS layers decreased with decreasing layer thickness. Calculated aging rates were also compared with the fraction of interphase material (which increases significantly with decreasing layer thickness), and the decrease in aging rate for films with thinner layers was found to correlate with an increase in interphase fraction. The elevated Tg of the interphase material (compared to pure PS) was suggested as a possible reason for reduced aging rates in the thin PS layers. In the second method, PS layers were cooled from above their Tg at different rates under confinement by PC layers. After this cooling step was performed, subsequent heating thermograms revealed that the enthalpy recovered upon reheating through the Tg of PS was similar for bulk and nanolayered films. [Copyright &y& Elsevier]

Abstract: The physical aging of polymers in confined environments has been an area of intensive study in recent times. The rate of physical aging in thin films of many polymers used in gas separation membranes is dependent on film thickness and accelerated relative to bulk. In this study, the physical aging of polymer films with alternating glassy polysulfone and rubbery polyolefin layers was monitored by measuring the gas permeability of O2 and N2 as a function of aging time at 35 °C. The alternating layer structures were formed by a melt co-extrusion process. The polysulfone layers have thicknesses ranging from 185 to 400 nm, and the overall thicknesses of the films are on the order of 80–120 μm. The aging of freestanding thin films of polysulfone is rapid and exhibits clear thickness dependence, whereas the aging of multilayered films was observed to be similar to bulk and showed no dependence on layer thickness. At 1000 h of aging time, a 400 nm freestanding PSF film decreased in O2 permeability by 35%, whereas on average the bulk and multilayered films only experienced a decline of 10–15%. A slight increase in O2/N2 selectivity for the multilayered films was observed over the course of aging. [Copyright &y& Elsevier]

Abstract: The effects of physical aging on the thermomechanical properties of polymers were investigated using silicon microcantilever deflection measurements. Polystyrene (PS), polymethylmethacrylate (PMMA), a PS/PMMA blend, or PS-PMMA diblock copolymer were applied to one side of a microcantilever, and the temperature-dependent thermal stress in the polymers was measured. A maximum compressive stress peak was observed for the PS- and PMMA-coated cantilevers during heating but not during cooling, which produced hysteresis. Physical aging of the polymers was found to contribute to the development of the hysteresis properties. The two distinct maximum compressive stress peaks in the PS/PMMA blend coincided with the temperatures at which the pure PS and PMMA peaks occurred, indicating that the glass transitions of each polymer were independent. In contrast, the thermal stress profiles of the PS-PMMA copolymer exhibited a single broad peak at a position intermediate between the peak positions of the pure PS and PMMA, indicating that the PS and PMMA polymers interacted during the glass transition. [Copyright &y& Elsevier]

Abstract: A direct correlation is found between the time evolution of the yield stress in unnotched tensile bars and that of the impact energy measured using notched tensile bars. In both cases a master curve can be constructed with an Arrhenius type of shift function, using the same activation energy. Combining these experimental findings with numerical simulations lead to a maximum hydrostatic stress as a criterion to predict the onset of failure, . Where aging kinetics is not dependent on the polymer’s molecular weight, embrittlement is, and for higher molecular weights a higher is found. Moreover, for high polymers we observe a more stable craze extension and crack propagation after cavitation, causing the computed impact energies to underestimate the experimental ones for all but the lowest molecular weight polymers. Because the maximum in hydrostatic stress under the notch, σ h, increases with the polymer’s age, defined by the value of the state parameter S a, and given the proportionality between S a and the yield stress, σ y, an alternative failure criterion is proposed which is a molecular weight dependent critical value of the evolving yield stress, σ y,c. It predicts a product’s increased sensitivity to damage under the influence of progressive aging as enhanced by temperature. [Copyright &y& Elsevier]

Abstract: The physical aging behavior of thin glassy polysulfone (PSF) films (∼125nm) with different previous histories was tracked using gas permeability measurements. The initial states of these materials were modulated by thermal annealing at fixed temperatures below the glass transition or by exposure to high pressure (800psig (56.2bara)) CO2 for various times. Regardless of the previous history, the nature of the aging response in these samples was consistent with the aging behavior of an untreated film that was freshly quenched from above T g, i.e., permeability decreased and pure gas selectivity increased with aging time. However, the extent of aging-induced changes in transport properties of these materials depended strongly on previous history. The aging behavior was described using Struik’s aging model by allowing the initial conditions to depend on each sample’s previous history. [Copyright &y& Elsevier]

Abstract: The influence of physical aging on the profile of free volume characteristics in thin polysulfone (PSF) films was investigated using variable energy positron annihilation lifetime spectroscopy. The PSF films exhibited decreasing o-Ps lifetime during physical aging, while o-Ps intensity remained constant. The o-Ps lifetime was reduced at lower implantation energies, indicating smaller free volume elements near the film surface (i.e., in the top ∼50nm). These near-surface regions of the films age dramatically faster than bulk PSF. The accelerated aging is consistent with the notion of enhanced mobility near the film surface, which allows polymer near the surface to reach a lower free volume state more quickly than the bulk. No influence of the silicon wafer support on aging behavior was detected. Additionally, the impact of CO2 conditioning on physical aging was briefly examined. The results from these studies were compared to aging behavior of ultrathin PSF films tracked by gas permeability measurements, and favorable agreement was found. [Copyright &y& Elsevier]

Abstract: Membrane-based separations play a key role in energy conservation and reducing greenhouse gas emissions by providing low energy routes for a wide variety of industrially-important separations. For reasons not completely understood, membrane permeability changes with time, due to physical aging, and the rate of permeability change can become orders of magnitude faster in films thinner than one micron. The gas transport properties and physical aging behavior of free-standing glassy polysulfone and Matrimid® films as thin as 18nm are presented. Physical aging persists in glassy films approaching the length scale of individual polymer coils. The films studied ranged from 18–550nm thick. They exhibited reductions in gas permeability, some more than 50%, after ∼1000h of aging at 35°C, and increases in selectivity. The properties of these ultrathin films deviate dramatically from bulk behavior, and the nature of these deviations is consistent with enhanced mobility and reduced T g in ultrathin films. The Struik physical aging model was extended to account for the influence of film thickness on aging rate, and it was shown to adequately describe the aging data. [Copyright &y& Elsevier]

Abstract: The dynamic relaxation characteristics of Matrimid® (BTDA–DAPI) polyimide have been investigated using dynamic mechanical and dielectric methods. Matrimid exhibits three motional processes with increasing temperature: two sub-glass relaxations (γ and β transitions), and the glass—rubber (α) transition. The low-temperature γ transition is purely non-cooperative, and displays an identical time–temperature response to both the dynamic mechanical and the dielectric probes with a corresponding activation energy, E A =43kJ/mol. The β sub-glass transition shows a more cooperative character as assessed via the Starkweather method. Comparison of dynamic mechanical and dielectric data for the β process suggests that the dynamic mechanical test (E A =156kJ/mol) is sensitive to a broader, more strongly correlated range of sub-glass motions as compared to the dielectric probe (E A =99kJ/mol). Time–temperature superposition was used to establish mechanical master curves across the glass–rubber (α) relaxation, and these data could be described using the Kohlrausch–Williams–Watts function with an exponent value, β KWW =0.34. The corresponding shift factors were used as the basis of a cooperativity plot for the determination of dynamic fragility. The relation between fragility index (m =115) and β KWW for the Matrimid polyimide was in good agreement with the wide correlation reported in the literature. [Copyright &y& Elsevier]

Abstract: The change in refractive index with time for thin films (∼350nm) formed from glassy 6FDA-based polyimides was monitored by ellipsometry to quantitatively track the physical aging process. The refractive index increased linearly, attributed to the densification of the glassy polyimide, with respect to aging time, on a logarithmic scale; this result is consistent with the decrease in gas permeability during physical aging reported in part I of this series. An excellent correlation was formed between the volumetric aging rate r, computed from the refractive index change by the Lorentz–Lorenz equation, and the permeability reduction rate, ; this relationship depends on the type of gas but appears to be the same for all polymer structures examined. The change in fractional free volume was examined from the refractive index data using parameters determined by group contribution methods. The free volume versus aging time results are well-described by the self-retarding relaxation model of Struik; however, this model does not explain the strong effect of thickness on aging rate. The change in free volume correlates well with the change in gas permeability of these thin films. [Copyright &y& Elsevier]

Abstract: The effect of molecular structure on the kinetics of physical aging of thin films (∼350nm) formed from glassy 6FDA-based polyimides was investigated by tracking the changes in gas permeability (He, O2 and N2) at 35°C for more than 2000h. 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. [Copyright &y& Elsevier]

Abstract: The creep and physical aging behavior of various types of PA6 nanocomposites and unfilled PA6 are described. After annealing far above T g the samples were quenched to room temperature and tested after various ageing times. The creep compliance shows a significant reduction with the addition of exfoliated layered silicate to the matrix polymer. The shape of the creep curves of the nanocomposites is similar to unfilled PA6 and time—ageing time superposition is possible with all materials. The shift rate for superposition is in the same range, but slightly higher in nanocomposites. The creep behavior of nanocomposites conditioned with an equilibrium amount of moisture and dry samples at elevated temperatures shows that the effect of nanofillers is much stronger under these conditions. [Copyright &y& Elsevier]

Abstract: Physical aging of amorphous poly(methyl methacrylate) has been studied by low frequency Raman scattering, broad-band dielectric spectroscopy, low frequency high resolution mechanical spectroscopy and differential scanning calorimetry. The material was subjected to different thermal histories by isothermal aging. A consistent relationship between the changes caused by the physical aging in nanostructure and molecular dynamics has been found. The aging makes the structure more homogeneous at a scale of few nanometers, bringing it to a structural state of lower energy. These structural changes affect mainly the α-relaxation, however, some increase in the relaxation strength as well as an increase in the activation energy of the β-relaxation is also observed. [Copyright &y& Elsevier]

Abstract: The physical aging at 35°C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O2, N2, and CH4, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400nm–62μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer Tg to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150°C below Tg for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14–15% increase in O2/N2 selectivity at an aging time of 1000h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly. [Copyright &y& Elsevier]

Styrene/4-methylstyrene (S/MS) random and gradient copolymers were synthesized by nitroxide-mediated controlled radical polymerization (NM-CRP) and compared to random copolymers made by conventional free radical polymerization (ConvFRP). The gradient copolymers have molecular weight (MW) values approaching 85,000 g/mol, making these some of the higher MW gradient copolymers reported to date. Due to the proximity of the glass transition temperatures (Tg) of polystyrene (PS) and poly(4-methylstyrene) (PMS), there is no significant difference in Tg between the gradient and random copolymers, with both copolymer types yielding single Tgs that typically increase slightly with increasing MS content. While enthalpy relaxation studies demonstrate similarity in random copolymers made by NM-CRP and ConvFRP, they reveal significant differences between random and gradient copolymers. Gradient copolymers exhibit broad enthalpy recovery peaks, whereas random copolymers exhibit narrower enthalpy recovery peaks. The maxima in the enthalpy recovery peaks are at substantially lower temperature, as much as 17 °C, in the gradient copolymers as compared to random copolymers of equal overall composition. While random and gradient copolymers of a given overall composition exhibit similar enthalpy recovery values at a common physical aging time and quench depth relative to Tg, the major differences in the enthalpy recovery peaks indicate that differences in sequence distribution along the chain length can lead to unusual behavior in gradient copolymers relative to random copolymers. [Copyright &y& Elsevier]

The dynamic mechanical behavior of poly(methyl methacrylate) (PMMA) from deep in the glass to the glass transition region has been studied by DMTA and analyzed by using a phenomenological fractional model in which the dynamic stress appears as a non-integer-order derivative of the strain. In order for the model to accurately represent the experimental data, three non-integer values for the derivative order are required. These values are related to two relaxation mechanisms. In the low temperature region (i.e. the β relaxation of PMMA), the derivative order is smaller and near 0.2, which indicates behavior close to the ideal elastic solid (glassy). For higher temperatures (between the β and the α relaxations), the derivative order is higher, indicating more viscoelastic behavior. In this work, modeling of the viscoelastic behavior of polymers using the fractional calculus approach is presented and the extended fractional solid (EFS) model is used to fit the experimental data of PMMA. In addition, the continuous relaxation spectrum H(τ) of PMMA is calculated from the model using the inverse Stieljes integral transformation. Finally, the effect of thermal treatment on the non-integer model parameters and on the distribution of relaxation times is obtained. [Copyright &y& Elsevier]

The physical aging of the epoxy network consisting of a diglycidyl ether of bisphenol A (BADGE n=0) and m-xylylenediamine (m-XDA) were studied by differential scanning calorimetry. The following aging temperatures have been used in this work: 60, 70, 80, 90, 100 and 110 °C. The glass transition temperature and the variation of the specific heat capacities have been calculated using the method based on the intersection of both enthalpy–temperature lines for glassy and liquid states. The endothermic aging peak, relaxation enthalpy and fictive temperature were also calculated for each aging temperature and aging time. [Copyright &y& Elsevier]

The effect of physical aging and side group on the free volume has been studied by positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimeter (DSC) for a series of epoxy resins cured separately with novolac resin and esterified novolac resins, that is, the hydroxyl group of phenol novolac resin was placed by a CH3COO-group, a CH3CH2 CH2COO- and a C6H5 CH2COO-group, which were named EP, EPA, EPB and EPP, respectively. Positron experimental results show that the side group plays an important role in determining free volume properties of epoxy resins, i.e. the stronger the cross-linking of curing agent, the smaller the free volume. This is confirmed by DSC measurement. On the other hand, the free volume relaxation properties were investigated by the Kolrausch–Williams–Watt (KWW), the single exponential and the double exponential functions in aging course for all samples, respectively. We found that ortho-positronium (o-Ps) intensity decreased with increasing aging time, which meant that the free volume diminished resulting from the formation of the local order structure due to the chain segmental rearrangement. The relaxation time constants of both τ and the parameter β have been calculated, which indicated that the order of relaxation rate of free volume characterized by τ−1 measured by PALS was the same as the enthalpy relaxation rate characterized by ∂ΔH/∂(log t) measured by DSC. [Copyright &y& Elsevier]

The effect of physical aging above Tg on ionic aggregation in the zinc salt of sulfonated poly(ethylene-co-propylene-co-ethylidene norbornene) ionomer was investigated by differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and dynamic mechanical thermal analysis (DMTA). The time-dependent development of a high temperature DSC endotherm and a high temperature tan δ relaxation were attributed to the dynamics of microphase separation of ionic aggregates. That conclusion was confirmed by the concurrent growth of a SAXS peak corresponding to the ionic microphase. The aging process at 25 °C occurred over a period of about 1 month, even though the ionomer was ca. 70 °C above Tg, where segmental mobility of the polymer is expected to be high. The kinetics of aging were dependent on temperature; a maximum in the rate of ionic aggregate formation occurred between 10 and 45 °C. [Copyright &y& Elsevier]

The physical aging of a poly(methyl methacrylate) (PMMA) copolymer having a small amount of ethyl methacrylate was investigated below its glass transition temperature (Tg) using dynamic viscoelastic measurements. The results were compared with those from homo-PMMAs to investigate how the comonomer in the copolymer affects the physical aging of the glass. Annealing temperatures were classified into two categories by the glass transition temperature of a poly(ethyl methacrylate) (TgPEMA; ca. 65°C). The relaxation rate of the copolymer was faster than those of the homo-PMMAs when the samples were aged below TgPEMA, but the rates were equal when aged at above TgPEMA. The experimental results are discussed in terms of the segmental motion. [Copyright &y& Elsevier]

Polymers of intrinsic microporosity (PIMs) offer tantalizing combinations of high selectivity and permeability in initial gas permeation measurements. Here, we report characterization of pure- and mixed-gas permeation properties of a thick (∼80 μm) PIM consisting of Tröger's Base (TB) and Ethanoanthracene (EA) films. The effects of feed pressure and temperature on pure-gas permeabilities of CH 4 , N 2 , O 2 , H 2 , and CO 2 were investigated. The physical aging behavior of the thick film was tracked via pure-gas O 2 , N 2 , and CH 4 permeability at 35 °C. Gas permeability decreased noticeably and selectivity increased as aging time increased. Particular attention was given to mixed-gas measurements of CO 2 and CH 4 (50/50) permeabilities at 35 °C and fugacities ranging from 2 to 18 atm to explore whether the rigid, bridged, bicyclic TB and EA units could resist CO 2 -induced plasticization. These results are presented along with pure-gas CO 2 and CH 4 results for membrane samples aged at different times. PIM-EA-TB aged for ∼24 h did not show signs of plasticization across the fugacity range considered. Dual-mode competitive sorption presumably caused the CO 2 /CH 4 mixed-gas selectivity to be slightly higher than its corresponding pure-gas selectivity. However, as aging time increased, aged films underwent progressively more rapid and extensive CO 2 -induced plasticization with increasing fugacity, suggesting a systematic relationship between physical aging and plasticization in PIMs. Consequently, physical aging caused less improvement in mixed-gas CO 2 /CH 4 selectivity than it did in pure-gas selectivity, due mainly to plasticization effects. [Display omitted] • Mixed-gas transport was characterized for CO 2 and CH 4 in PIM-EA-TB films. • A systematic relationship between physical aging and plasticization was observed. • As aging proceeded, PIMs became more susceptible to CO 2 -induced plasticization. [ABSTRACT FROM AUTHOR]

We herein report the influences of cellulose nanofibril (CNF) and physical aging on the enthalpy relaxation behavior, thermal transition, and dynamic mechanical properties of eco-friendly poly(lactic acid) (PLA) composite films with 0.5–7.0 wt% CNF loadings, which were fabricated by using a facile masterbatch-based melt-compounding process. For the purpose, melt-quenched neat PLA and PLA/CNF composite films were aged physically at 45 °C for different times. The FT-IR spectra revealed that specific hydrogen-bonding interactions exist between the carboxyl groups of PLA matrix and the hydroxyl groups of CNF fillers, and that the interactions are strengthened by the physical aging. Accordingly, the increments of the peak temperature (T p) and area (ΔH relax) of the endothermic enthalpy relaxation at the glass transition temperature region with the aging time were suppressed significantly for the composite films with higher CNF contents, unlike the neat PLA film. In addition, the characteristic relaxation time of PLA/CNF composite films was highly enhanced, indicating the improved enthalpy relaxation and dimensional stability. This result is associated with the fact that the CNF fillers restrict the segmental mobility of PLA chains in the composite films during the physical aging at the glassy state, which was supported by the dynamic mechanical thermal data. Image 1 • PLA/CNF composite films are fabricated by a facile masterbatch melt-compounding. • There exist specific hydrogen-bonding interactions between PLA chains and CNFs. • CNFs retard the enthalpy relaxation of PLA chains by restricting the segmental mobility. • CNFs enchance the dynamic mechanical properties of PLA especially at a glassy state. [ABSTRACT FROM AUTHOR]

Thermomechanical properties during isothermal aging of diethanolamine (DEA) cured diglycidyl ether of bisphenol-A (DGEBA) were tracked using uniaxial compression and differential scanning calorimetry. The epoxy was cured at 70 °C for 24 h, following standard procedure, and then aged at one of four temperatures, T : 55 °C, 65 °C, 76 °C or 105 °C. The initial glass transition of cured DGEBA/DEA was 70 °C and increased proportionally with the logarithm of time indicating that additional crosslinks had formed. Similarly, the yield stress, σ y , increased, by up to 80%, proportionally with the logarithm of aging time until approximately 600 h. After that time period, the increase in σ y slows significantly. Aging also narrowed the yield peak and increased the post-yield flow stress. The changes in σ y due to physical aging were separated from those due to chemical aging by testing samples combining both physical and chemical aging and comparing the results to those from (only) chemically aged samples. The proposed method for separating out the physical contributions can be used to predict the mechanical changes due to physical aging in slowly chemically evolving materials. The aging data are also utilized along with data on σ y as a function of strain rate for unaged material to generate a tentative expression for σ y as a function of strain rate, aging time and temperature. The derivation of this expression requires only the common assumption that the rate of change of σ y / T with respect to log(strain rate) is independent of temperature and of aging time. Image 1057 • Isothermal aging of the engineering relevant DGEBA/DEA material. • Two distinct regions of yield stress behavior when plotted against the log of time. • Discrimination between chemical aging and physical aging effects on yield stress. • Calculation of physical-aging-only effects on the yield stress. • Prediction of yield stress as function of aging time, temperature, and strain rate. [ABSTRACT FROM AUTHOR]

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. Image 1 • Kinetics of enthalpy relaxation of polycarbonate correlates to that of yield stress. • Empirical kinetic equation can be parametrized by annealing conditions and Tg only. • Molecular weight affects aging kinetics through its dependence on Tg. • Molecular weight has more significant effect on brittle stress than on yield stress. • Annealing for a sufficient time saturates the increase in yield stress. [ABSTRACT FROM AUTHOR]

In our previous paper, we reported that the impact value of polycarbonate (PC) increases with increasing molecular weight and decreases upon annealing [Polymer 2012, 53, 895–896]. To clarify these phenomena, the effects of molecular weight and heat treatment on the fracture of PC are investigated in this study. The results show that the stress for craze nucleation increased as the molecular weight increased but was not affected by annealing. The shear modulus and bulk modulus do not exhibit molecular weight dependence, but annealing resulted in a slightly increased shear modulus and a decreased bulk modulus. Furthermore, the amplitude and correlation length of density fluctuations of the Debye–Buche mode in small-angle X-ray scattering profiles were shown to increase upon annealing. On the basis of these results, it is suggested that, as the molecular weight increases, the toughness of PC is enhanced because the increase in molecular weight increases the stress of slippage for entangled molecular chains. In addition, it is proposed that the change in the fraction of fine voids resulting from density fluctuations may affect the toughness, resulting in the reduced toughness observed for annealed PC. Image 1006 • The stress for craze nucleation in the PC increases as the molecular weight increases. • The annealing doesn't affect the stress for craze nucleation in the PC. • The shear modulus and bulk modulus do not exhibit molecular weight dependence. • The bulk modulus clearly decreases by annealing. • Density fluctuations exit in single-component PC and are increased by annealing. [ABSTRACT FROM AUTHOR]