Analysis of Stress Relaxation Processes in Polyimides at High and Low Temperatures
Keywords:heat–resistant polymer, polyimides, stress relaxation, critical state, critical stress, critical strain, limiting parameters, mechanical efficiency.
The article presents an analysis of viscoelastic behavior of thermoplastic heat-resistant polymers (HRP) polyimides under stress relaxation conditions at high (up to 500 °C) and at low (up to –170 °C) temperatures. The main characteristic features of stress relaxation processes are considered on the basis of polyimide PI3 in comparison to other polyimides PI1 and PI2. The polyimides under consideration have different aromatic structures and various hinge-type groups in polymer macromolecules. The difference in structure of macromolecules leads to the different glass transition and relaxation solid-solid transition temperatures. According to the results obtained the relaxation processes in polyimide PI3 are rather well expressed at high as well at low temperatures. On the basis of isochronous-isothermal relationships of relaxing stress the existence of critical state is disclosed and critical parameters expressed by critical stresses and critical strains are determined. Applying limiting parameters (fracture stress, fracture strain) and critical parameters (critical stress, critical strain) the mechanical efficiency of polyimide PI3 is estimated under stress relaxation conditions at high and low temperatures.
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