Effect of physical aging on deformation characteristics of polycarbonate (original) (raw)
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Long time response of aging glassy polymers
Rheologica Acta, 2014
Aging amorphous polymeric materials undergo free volume relaxation, which causes slowing down of the relaxation dynamics as a function of time. The resulting time dependency poses difficulties in predicting their long time physical behavior. In this work, we apply effective time domain approach to the experimental data on aging amorphous polymers and demonstrate that it enables prediction of long time behavior over the extraordinary time scales. We demonstrate that, unlike the conventional methods, the proposed effective time domain approach can account for physical aging that occurs over the duration of the experiments. Furthermore, this procedure successfully describes timetemperature superposition and time -stress superposition. It can also allow incorporation of varying dependences of relaxation time on aging time as well as complicated but known deformation history in the same experiments. This work strongly suggests that the effective time domain approach can act as an important tool to analyze the long time physical behavior of aging amorphous polymeric materials. J J t t
2000
The effect of molecular weight on the viscoelastic performance of an advanced polymer (LaRC TM-SI) was investigated through the use of creep compliance tests. Testing consisted of shortterm isothermal creep and recovery with the creep segments performed under constant load. The tests were conducted at three temperatures below the glass transition temperature of five materials of different molecular weight. Through the use of time-aging-time superposition procedures, the material constants, material master curves and aging-related parameters were evaluated at each temperature for a given molecular weight. The time-temperature superposition technique helped to describe the effect of temperature on the timescale of the viscoelastic response of each molecular weight. It was shown that the low molecular weight materials have higher creep compliance and creep rate, and are more sensitive to temperature than the high molecular weight materials. Furthermore, a critical molecular weight transition was observed to occur at a weight-average molecular weight of M w ∼ 25,000 g/mol below which, the temperature sensitivity of the time-temperature superposition shift factor increases significantly. The short-term creep compliance data were used in association with Struik's effective time theory to predict the long-term creep compliance behavior for the different molecular weights. At long timescales, physical aging serves to significantly decrease the creep compliance and creep rate of all the materials tested. Long-term test data verified the predictive creep behavior. Materials with higher temperature and lower molecular weights had greater creep compliance and higher creep rates.
Modeling of the Postyield Response of Glassy Polymers: Influence of Thermomechanical History
Macromolecules, 2005
The continuous development of constitutive equations for the finite strain deformation of glassy polymers has resulted in a number of sophisticated models that can accurately capture the materials' intrinsic behavior. Numerical simulations using these models revealed that the thermal history plays a crucial role in the macroscopic deformation. Generally, macroscopic behavior is assumed not to change during a test, however, for certain test conditions this does not hold and a relevant change in mechanical properties, known as physical aging, can be observed. To investigate the consequences of this change in material structure, the existing models are modified and enhanced by incorporating an aging term, and its parameters are determined. The result is a validated constitutive relation that is able to describe the deformation behavior of, in our case, polycarbonate over a large range of molecular weights and thermal histories, with one parameter set only.
Physical aging of high-performance thermoplastics: Enthalpy relaxation in PEK-C and PES-C
Journal of Applied Polymer Science, 1995
The developments of physical aging in phenolphthalein poly(ary1-ether-ketone) (PEK-C) and poly(ary1-ether-sulfone) (PES-C) with time at two aging temperatures up to 20 K below their respective glass transition temperatures (T, = 495 and 520 K) have been studied using differential scanning calorimetry (DSC). Substantial relaxation within the aging course of several hours were observed by detecting T, decreasing during physical aging process at the two aging temperatures. The relaxation processes of both polymers are extremely nonlinear and self-retarding. The time dependencies of their enthalpies during the initial stages of annealing were approximately modeled using the Narayanaswamy-Tool model. The structure relaxation parameters obtained from this fitting were used to predict the possibility of physical aging occurring at their respective using temperatures.
Progress in Polymer Science, 1995
The general area of physical aging of polymers is reviewed. Various phenomenological aspects are introduced and discussed in terms of bulk structural changes evidenced by dilatometric and calorimetric studies, and are compared with the wide variety of information available from microstructural investigations involving spectroscopic and scattering techniques. Current models for describing the relaxation kinetics of the non-equilibrium glassy state are compared. Finally, the effects of physical aging on mechanical properties are reviewed, highlighting especially those areas which remain controversial.
The physical aging of a n epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step a t 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9"C. Samples were aged a t temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only a t an aging temperature of Tg -10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time 7,ff. The rate of relaxation of the system given by 1 / T ,~ decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on 7, three characteristic parameters of the enthalpic relaxation process were obtained (In A = -333, EH = 1020 kJ/ mol, C = 2.1 g/ J ) . Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60"C, where sub-T, peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter p were slightly dependent on temperature, and the characteristic time was found to decrease with temperature.
Modeling of mechanical property degradation by short-term aging at high temperatures
Composites Part B-engineering, 2002
A study on the mechanical property degradation by short-term aging at high temperatures is performed. Samples are manufactured using a thermoplastic polymer (PEEK) and advanced polymer composite-2 (APC-2), and are aged at different temperatures for different times. The flexural properties of the aged samples are measured by 4-point bending tests. An appropriate mathematical equation is proposed to model the degradation data as a function of time and temperature. Numerical techniques are employed to find the optimal values of the parameters using transient temperature profiles. The evaluated results are discussed in comparison with results from previous works to prove the validity of the model. Physical interpretations of the results are provided to support assumptions involved in the model. To illustrate the effectiveness of the model, it was applied to a simulated manufacturing process and a mathematically defined service condition. The model could successfully predict the property loss during manufacturing and service, which could be utilized as a reference in determining the durability and the lifetime of the product. q
Mathematical and Computer Modelling, 2003
Constitutive equations are developed for the linear viscoelastic behavior and volume recovery in glassy polymers. The model is based on the concept of cooperative relaxation that treats a polymer as an ensemble of cooperatively rearranged regions (CR&.). A rearrangement event is thought of as a hop of a CRR trapped in its potential well on the energy landscape to a liquid-like energy level. The energy landscape of a glassy polymer is assumed to be time-independent, whereas the position of the liquid-like state changes with time approaching the equilibrium energy level. Its evolution is described by a fictive temperature that obeys Tool's equation with a characteristic time proportional to the average time for rearrangement. Kinetic equations for volume recovery and stressstrain relations for the viscoelastic response are verified by fitting observations in dilatometric and mechanical (static and dynamic) tests. Fair agreement is demonstrated between the results of numerical simulation and the experimental data for polycarbonate, poly(arylene etherimide), poly(ether ether ketone), and poly(viny1 acetate).
Strain and temperature accelerated relaxation in polycarbonate
Journal of Polymer Science Part B: Polymer Physics, 1988
The nonlinear viscoelastic behavior of glassy polymers and its relationship to ductile yielding is studied by aingleand double-step stress relaxation experiments. In the latter case a small stress relaxation step is superimposed on a specimen at an elevated state of temperature or strain. The reaults show that the changes in the relaxation behaviors in the two cass closely parallel each other. The relaxation behavior at strains near yield closely approximates that at low strain but near Tg. The small strain relaxation response can be described well by a Kohlrad-Williams-Watts (KWW) type function. The interpretation of these data in terms of a coupling model which includes the KWW form is discussed.
Macromolecules, 2003
For the first time, load and depth sensing indentation (DSI) was used in order to monitor physical aging of bisphenol A polycarbonate for 30 months at room temperature and for 1 month at an elevated temperature. The DSI experiments were combined with differential scanning calorimetry, gel permeation chromatography, and infrared spectroscopy. The endothermic peak of polycarbonate shifted toward higher temperatures upon aging at an elevated temperature and did not change its location upon aging at room temperature. The elastic modulus and hardness of polycarbonate increased in a stepwise fashion during aging at room temperature. The molecular weight distribution broadened slightly, and the trans-trans conformational population increased during annealing. No simple correlation between changes in the mechanical properties and the shift of the endothermic peak during annealing was found. These changes seem to be caused by phenomena of different nature; namely, the changes in the mechanical properties appeared to have a reasonable correlation with free volume relaxation of the polymer, whereas the changes in the endothermic peak may be associated with internal energy changes. The similarities and differences between our results and the results of others are discussed.