| With advantages of light weight and high strength, corrosion resistance and designability, polymers are widely applied to various industrious fields, such as aerospace, machinery manufacturing, building energy and bioengineering. Polymers are typical viscoelastic materials, of which mechanical properties are affected by many environmental factors, such as temperature, time, load and so on. Due to their high sensitivity to environmental conditions such as temperature, humidity and time, polymers are easy to aging, which leads to the decrease of the durability of these materials. Nowadays, most engineering design has not taken rheological properties of polymers into consideration, resulting in delayed failure accidents. In practice, polymers are often subjected to dynamic loads. Therefore, the study of dynamic mechanical responses of polymers in their aging process is very necessary. Not only are there theoretical significances, but also wide prospects for engineering applications as well. The main works and creative achievements are listed as follows.1) The dynamic viscoelastic properties of polymers were summarized and related environmental factors were discussed. The time-temperature equivalence principle and the time-aging time equivalence principle were introduced. The differences between viscoelastic constitutive models and their corresponding fractional differential viscoelastic constitutive models were investigated.2) Temperature scan tests of PVC (short for polyvinyl chloride) with different aging time were conducted under different constant levels of frequency by the dynamic mechanical thermal analyzer GABO EPLEXOR500N. The glass transition temperature Tg of PVC with different aging time te under different constant levels of frequency/were determined. And the relationships between glass transition temperature Tg, frequency/and aging time te were obtained. The experimental results indicate that within the aging time te of0d～60d and the frequency/of1Hz～100Hz, the glass transition temperature rgof PVC ranges between67℃～77℃and increases with raising frequency/and lengthening thermal aging time te.3) Frequency scan tests of PVC with different aging time were conducted under the temperature ranging between Tg-60℃～Tg+10℃. And the values of storage modulus E’, loss modulus E" and loss factors tan δ changing along with the frequency f and temperature T were obtained. The experimental results show that the storage modulus E’ of PVC increases with the raise of frequency f, decreases with the raise of temperature T; however, the value of loss factor tan8decreases with the raise of frequency f and increases with the raise of temperature T. The influence of frequency f and temperature T on the loss modulus E" is complicate, when the temperature T ranges between15℃～65℃, the loss modulus E" decreases with the raise of frequency f and increases with the raise of temperature T, while the temperature T ranging between70℃～85℃, it shows a totally opposite variation to the above.4) Based on the time-aging time equivalence principal, the calculation method for dynamical modulus of polymers has been induced creatively from the calculation formula of static creep compliance, and its capability was vertified by experimental data. Aging-time vertical shifting factor and aging-time horizonal shifing factor have been taken into consideration when analyzing the equivalent relationships between dynamic modulus of PVC and aging time, and in the further research the effects of temperatures on aging-time shifting factors have been analyzed, providing a new approach for the research of time-aging time equivalence of dynamical mechanical behaviors of polymers during aging. In addition, based on the time-temperature equivalence principal, taking temperature horizontal shifting factor into consideration, the dynamic viscoelastic master curves of PVC samples are obtained by shifting the frequency sweep curves at different temperatures. These master curves lay the foundation for future research on long-term dynamic mechanical behaviors of polymers.5) The fractional Kelvin model and the fractional Zener model were adopted to analyze the experimental data of PVC samples with different temperatures and related parameters of corresponding models were carried out. These fitting analysis show that within the temperatures of Tg-60℃～Tg and the frequency between0.1～100Hz, the fractional Kelvin model is in good agreement with the experimental data. And the fractional Zener model is capable of describing the relationships between dynamic modulus of PVC and frequency within a broadband of test frequency. |
PDF Full Text Request