| When a polymeric material is quenched to below its glass transition temperature, Tg, the material is in a thermodynamic non-equilibrium state for a very long time. During this long period of time, its structure will evolve with time towards the equilibrium state, leading to the aging-time dependent dielectric, thermal and mechanical properties such as dielectric permittivity, thermal conductivity, specific volume, creep compliance and tensile strength etc. This phenomenon is known as physical aging. Polymers and polymer composites are increasingly used in many structural applications, and most of them are used below their Tg, the physical aging behavior of such polymeric materials have been received more and more research attentions in the academic and industrial fields. This thesis focuses on the effects of physical aging via changes in the mechanical response.The mechanical properties of polymers are usually time dependent. Their mechanical response strongly dependents on the mobility of the chains which is affected by temperature, stress and aging time. Creep is the time dependent deformation of materials subjected to a continuous constant stress. Besides the volume recovery and enthalpy recovery measurements, the creep compliance measurement is a very popular and practical method to investigate the physical aging phenomenon. In this thesis, the creep behavior of polymethyl methacrylate (PMMA) at various stresses was measured for various aging times. Moreover, the ageing behavior was monitored by measuring the initial instantaneous elastic modulus (E) and the fracture strength (σf) from the stress-strain curves, which were obtained in uniaxial tensile loadings with a constant strain-rate of 0.00133s-1, as a function of ageing time (te).Prior to the creep tests and constant strain-rate tensile tests, the specimens were heated to 115℃, which was 10℃above the Tg, and hold at that temperature for 30 minutes to remove the previous thermal and mechanical history, and then were allowed to age at various aging temperatures, namely 60℃, 40℃and 27℃for different times of up to over 1000h. All the temperatures were controlled within the rang of±1℃.From the creep tests, it is observed that the creep compliance curves at different stresses (≥14MPa) for a fixed aging time depart from each other, indicating the nonlinear viscoelastic behavior. The time-stress superposition principle was used to analyze this nonlinear effect, and a creep master curve was constructed by shifting the compliance curves at higher stresses to the one at lowest stress (14MPa). It is shown that the master curve covers a wider time range, which is an approximate 2 decades wider than the test duration in the logarithmic time scale, and moreover the stress shift factors depends linearly on the stress in the stress range used in this study. This validates that the time-stress superposition principle provides an accelerated technique for characterization of viscoelastic properties of materials.Some important models for nonlinear viscoelastic creep were briefly reviewed, and two of them, namely Findley model and Struik model, were used to model the nonlinear creep behavior of PMMA, which was aged for 4h at 40℃and tested at 27℃at various stresses. The model parameters were determined via the genetic algorithm (GA). The two models are found to have strong potential to fit the test data. Moreover, the model predictions are compared with the creep master curve obtained by stress shifting technique, and an acceptable agreement is shown within the time range of 105s, though the model predictions are a bit higher than the creep master curve beyond 105s .The aging effect on creep behavior of PMMA was investigated by measuring creep compliance curves at four different stresses, 15-30MPa with an interval of 5MPa, for various aging times (te). The isochronous creep compliance is found to linearly decrease with log te. It is also shown that the creep compliance curves can be superimposed by horizontal shifting the creep response to the one at the reference aging time, indicating the applicability of the time-aging time superposition principle. In this study the reference aging time was chosen as the age of the longest aged test. The master curves for each stress level are created and the corresponding aging shift factor (φa) is found to be proportional to the aging time in a double logarithmic plot. The minus slop of the double logarithmic plot of logφa vs log te is the aging shift rate,μ. It is found thatμdecreases with increasing stress.Moreover, the aging effects on the initial instantaneous elastic modulus (E) and the fracture strength (σf), which obtained from the stress-strain curves, were also discussed. Both E andσf are found to linearly increase with log te in the aging time interval investigated in this study.This study was supported by the National Natural Science Foundation of China (No. 10572123), the Provincial Natural Science Foundation of Hunan (No.05JJ30014) and the Open Project Program of Key Laboratory of Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (No.KF0502).
|
PDF Full Text Request