Glass transition complex behavior

The all-important difference between the friction properties of elastomers and hard solids is its strong dependence on temperature and speed, demonstrating that these materials are not only elastic, but also have a strong viscous component. Both these aspects are important to achieve a high friction capability. The most obvious effect is that temperature and speed are related through the so-called WLF transformation. For simple systems with a well-defined glass transition temperature the transform is obeyed very accurately. Even for complex polymer blends the transform dominates the behavior deviations are quite small. 

This chapter is devoted to a short description of low-strain mechanical properties of polymers in the solid state and in the glass transition region, with an emphasis on the effect of crosslinking on these properties. There are three degrees of complexity in the description of this behavior, depending on the number of variables taken into account in the constitutive equations under consideration. 

D lends of two incompatible polymers with different glass-transition temperatures have properties which differ from the pure components. Their viscoelastic behavior is complex and deviation from the simple time-temperature superposition is generally observed (I). The high... 

Even though diffusion in polymers is generally a complex process, it is possible to find systems that exhibit relatively simple Fickian diffusional behavior. For example, concentration-independent Fickian diffusion has been observed in many polymers when the temperature is far below the polymer s glass transition tem-... 

The curing behavior of PMDA-ODA (Figure 1) has been analyzed previously The overall process is characterized by various physical and chemical steps Decomplexation of the complex formed between NMP and polyamic acid 2 3 6 plasticization of the material by the decomplexed NMP n 12 evaporation of the solvent cycloimidization accompanied by re-formation of anhydride and amine, a side reaction which leads to chain scission 2 vitrification caused by solvent evaporation and imidization and finally molecular ordering of the polyimide near and above the glass transition n 13 14. The particular features of all these processes are heating rate dependent (e.g. compare Figure l.a and l.b) 2 n. 

The conversion of strain mismatch into stress is a function of the stress relaxation modulus exhibited by the polymer. A predictive stress model must incorporate the complex dependencies of the modulus and stress relaxation behavior on temperature, glass transition temperature, degree of cure, crosslink density, solvent-plasticization, and reaction kinetics. 

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