Papers by Keyword: Physical Aging

Abstract: Two types of carbon fiber-reinforced epoxy composite laminates are chosen for long-term tensile creep tests under different temperatures and load levels. Their time-dependent and non-monotonic deformations indicate clearly both temperature effect and physical aging effect. To characterize these viscoelastic behavior, two phenomenological constitutive models and one physical model are developed. The linear viscoelastic model based on the Boltzmann superposition principle is able to describe reasonably the deformations at relatively lower stress levels and temperatures. The nonlinear viscoelastic model of Schapery’s single-integral form, together with a usage of effective time theory, could describe nicely all the effects of temperature, stress, and physical aging. The physical model based on Ngai’s coupling mechanism concept is further combined with the framework of Schapery’s nonlinear viscoelastic theory, which may provide certain physical understanding about the effect of aging behavior on long-term creep deformation of the laminated composites. Numerical modelling by finite element method are implemented, and comparisons between the experimental and simulation results are demonstrated.

Abstract: 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 T_a = 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

Abstract: The drawback of membrane process that reduces its competitive edge with the conventional separation technologies is ascribed to its decline separative performance over time due to the aging nature of polymeric material. The most widely accepted mechanism that has been thought of governing the volume relaxation process over the course of aging is the dual mode mechanism, whereby it is comprised of two components. The first is the “Lattice contraction” mechanism that describes the uniform collapse of free volume throughout the unrelaxed polymer matrix. The second is the “Diffusion of free volume” mechanism from the interior to the surface of the glassy polymer. Albeit acknowledgement of the dual mode mechanism as the contributing factor, previous aging model renders high implementation challenges to characterize the complicated nature of aging evolution, which requires adaptation of high end computational tools to solve the relatively complex differential equations. In this work, the dual mode mechanism governing the physical aging process has been modelled employing a simple one dimensional finite element numerical solution whereby the film has been divided into many finite slices with equal thickness along the depth of the membrane. The applicability of the mathematical model has been validated with experimental aging data, whereby a small deviation is observed between the two over a wide range of film thicknesses and reasonable intuitive explanation pertaining to the parameters is obtained.

Abstract: The relaxation behaviors of PS below Tg and its variation trend along with aging time were studied by DSC, fluorescence spectrum and XRD. On the basis of DSC results, the kinetics of the volume and enthalpy relaxation process were analyzed, and the influence of the aging time on the relaxation behaviors were discussed. The results of fluorescence spectrum and XRD indicated the presence of the rearrangement of polymer chains in the physical aging of polystyrene.

Abstract: The influence of physical aging on mechanical properties of glassy polymers was investigated in this paper. After annealing above T_g to release the previous thermal and stress history, the polymethyl methacrylate (PMMA) samples were quenched to 27°C, aged for various times (t_a), and were then stretched at the same temperature by two ways: (1) step stresses with four different magnitudes varying from 15MPa to 30MPa; (2) constant rate stretch up to fracture. The physical aging effect was monitored by measuring the initial instantaneous elastic modulus (E) and the fracture strength (σ_f) from the stress-strain curves as a function of t_a up to 1368h. It is shown that both E and σ_f of the material increase with aging time and approach to their asymptotic values, which satisfy the KWW rule, while the isochronous creep compliance decreases with log t_a in a linear manner within the aging time range considered in this paper.

Abstract: In this work, the physical aging and its effect on nonlinear creep behavior of poly(methyl methacrylate) are presented. After annealing above Tg to release the previous thermal and stress history, the samples were quenched to 60oC, aged for various times, and were then tested at three different stress levels (22MPa, 26MPa and 30MPa) at room temperature of 27oC. At each stress level, the creep strain was converted to compliance and measured as a function of test time and aging time. The test results show that higher stress accelerates creep rate of the material while physical aging plays a reverse role. The time-aging time superposition is applicable to build a master creep compliance curve at each stress level, and it is demonstrated that the shift rate deceases with increasing stress. Moreover, based on the time-stress superposition principle, a unified master curve was constructed by further shifting the sub-master curves at 30MPa and 26 MPa to a reference stress level of 22MPa.

Abstract: Different degrees of freedom exist in amorphous matter, and are at the origin of relaxation processes (e.g. structural, mechanical). The effect of physical aging of the glassy network on secondary and primary (and cooperative) relaxation is studied. It is shown that if the temperature of aging is below the liquid glass transition temperature and above that of the secondary relaxation, the strength of this last relaxation changes in a non monotonous complex way while the characteristic time of the primary relaxation increases. This feature is explained taking into account the heterogeneous nature of the glassy network. This concept is in agreement with recent inelastic light scattering observations showing that fluctuations of the elastic constants at the nanometric scale are characteristic of the glassy state.