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Physical aging is an inherent property of glassy matter, but understanding its microscopic mechanism remains a challenge particularly at the particle level. In this work, we use a confocal microscope to in-situ trace the particle trajectories in a 3D colloidal glass for 73000 s, aiming at resolving the aging dynamics. By calculating the mean square displacement of particle motions, we find that the glass aging with time can be divided into three stages: β relaxation, α relaxation and free diffusion. The system's mean square displacement at each aging state is quantitatively resolved into three contributions of particle dynamics modes: vibration within the nearest-neighbor cages, hopping between cages and cooperative rearrangement. We further calculate the particle's free volume and find that the β-to-α transition is accompanied by the temporary increase of the system-averaged free volume due to pronounced hops of particles. Nevertheless, the temporal autocorrelation of the free volume spatial distribution still obeys a monotonically stretched exponential decay with an exponent of 0.76, which is related to the sub-diffusion dynamics of cooperative rearrangements and hops mixed in α relaxation. According to the resolved vibrational displacements, we calculate the vibrational density of states of this 3D glass, and the characteristic boson peak is reproduced at low frequencies. Our findings shed insight into the particle-level aging dynamics of a real glass under purely thermal activation. [ABSTRACT FROM AUTHOR]

The tensile creep tests for two kinds of epoxy resin matrix composite laminates were performed under 25°C and 50°C. Results indicate that these materials deform in a time‐dependent manner, that is, the tensile strain increases within a short time after the tests began, and then decreases during most of the loading period. Such increase and decrease trends reflect the creep characteristic and effect of physical aging, respectively. A linear viscoelastic model is developed to include both the creep temperature effect and physical aging effect, starting from the Boltzmann superposition principle. In modeling, these two effects both contribute to the transient compliance and result in the increase and decrease of the transient compliance. Time–temperature/aging time superposition principle is adopted to deal with the influences of temperature and physical aging. This theoretical model is further implemented numerically within a finite element framework. To verify the newly proposed model, not only the experimental and numerical results of the tensile deformations of laminates are compared, but also the bending deformations of structural components made from the above laminates are analyzed. Highlights: Tensile creep tests were done for carbon fiber/epoxy laminates.The strain shows creep temperature and aging effect (TE and AE).New constitutive relation was proposed to describe the TE‐ and AE‐caused strains.Structural‐level flexure creep tests and simulation analyses were conducted. [ABSTRACT FROM AUTHOR]

The direct action of salt on asphalt materials has received limited research despite the abundance of studies on salt erosion and its effects on the moisture stability of asphalt concrete. This study aimed to investigate the effects of chloride salt on the rheology of asphalt and erosion mechanisms. Microscopic properties of asphalt mortar were analyzed through scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR), and the erosion degree of salt in asphalt mortar was evaluated based on three major indexes. Rheological properties of asphalt mortar were determined through temperature sweep and multiple stress creep and recovery (MSCR) tests, while the modified water-boiling test was conducted to evaluate the adhesion of asphalt mortar. Results showed that as salinity increases, the agglomerate phenomenon tends to occur in asphalt mortar, leading to physical aging effects on its basic properties. Meanwhile, chloride salt modified asphalt concrete is prone to cracking at low temperatures and has reduced durability at high temperatures, making it unsuitable for standard load or heavy traffic pavement requirements of PG64 modified asphalt. Furthermore, the presence of chloride in asphalt mortar was not conducive to its adhesion. [ABSTRACT FROM AUTHOR]

Shape memory polymers (SMPs) maintain a temporary shape after pre‐straining, wherein the polymer chains are constrained in a non‐equilibrium thermodynamic state. Physical aging lowers the chain conformational energy, which affects the mechanical properties. Herein, we investigate the relationship between physical aging and the shape recovery of SMP sheets, whereas both processes involve motion of polymer chains. We induce conformational changes to polymer chains either by physical aging or via a thermomechanical pre‐straining process. We then quantify structural relaxation via recovered enthalpy measurements using modulated differential scanning calorimetry (MDSC), and the shape recovery performance using dynamic mechanical analysis (DMA). We vary pre‐straining holding time, amount, and rate and observe the relationship between physical aging, recovered enthalpy, and the shape recovery performance. The results indicate that an increase in recovered enthalpy correlates with an increase in characteristic shape recovery time. Further, a maximum decrease in recovery time of 65% is observed at the highest strain rate, and only small amounts of recovered enthalpy occur for aging times longer than 16 h. The results provide insight into the relationship between physical aging and its effects on shape memory, which is important for applications requiring storage for long durations. [ABSTRACT FROM AUTHOR]

Aggregation‐induced emission (AIE) luminogens are attractive dyes to probe polymer properties that depend on changes in chain mobility and free volume. When embedded in polymers the restriction of intramolecular motion (RIM) can lead to their photoluminescence quantum yield (PLQY) strong enhancement if local microviscosity increases (lowering of chain mobility and free volume). Nonetheless, measuring PLQY during stimuli, i.e. heat or mechanical stress, is technically challenging; thus, emission intensity is commonly used instead, assuming its direct correlation with the PLQY. Here, by using fluorescence lifetime as an absolute fluorescence parameter, it is demonstrated that this assumption can be invalid in many commonly encountered conditions. To this aim, different polymers are loaded with tetraphenylenethylene (TPE) and characterized during the application of thermal and mechanical stress and physical aging. Under these conditions, polymer matrix transparency variation is observed, possibly due to local changes in refractive index and to the formation of microfractures. By combining different characterization techniques, it is proved that scattering can affect the apparent emission intensity, while lifetime measurements can be used to ascertain whether the observed phenomenon is due to modifications of the photophysical properties of AIE dyes (RIM effect) or to alterations in the matrix optical properties. [ABSTRACT FROM AUTHOR]

Temperature and physical aging effects on the three-dimensional behaviors, such as shear and bulk properties, of viscoelastic solids are critical for developing polymer composite systems but are rarely addressed. Furthermore, accurate identification of time-temperature-dependent viscoelastic constitutive theories remains a challenging problem. In this work, we propose a robust and automated method to simultaneously identify the viscoelastic, temperature, and physical aging related three-dimensional model parameters based on the Bayesian framework. The proposed method was applied to three-dimensional experimental data from nonisothermal physical aging tests on polycarbonate at different temperatures. The posterior distribution of the model parameters was obtained through Markov chain Monte Carlo (MCMC) simulations, which allowed us to determine the minimum number of material parameters yielding an optimized accuracy. The experimental results are in excellent agreement with the predictions of the identified parameters. Moreover, we have found through the experimental results that the shear properties of polycarbonate are affected by temperature and physical aging, whereas its bulk properties are affected only by temperature and are relatively insignificant in comparison to the shear properties. The proposed method can be applied to polymers or composites to predict their long-term three-dimensional mechanical properties under complex working conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]