Dispersion mechanisms structural relaxation properties

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Thus soluble polymer, interacting in a controlled fashion with colloidal particles, can transform both the equilibrium state and the mechanical properties of dispersions. The possibilities range from equilibrium, low viscosity fluids to nonequilibrium, pseudoplastic pastes with high yield stresses. However, substantial ga( still exist in the ability to, for example, (i) create high viscosity equilibrium fluids with prescribed relaxation spectra, (ii) impart a sol-gel transition at prescribed conditions, or (iii) connect explicitly macromdecular structure with rheological behavior. 

Dispersed systems, i.e. suspensions, emulsions and foams, are ubiquitous in industry and daily life. Their mechanical properties are often tested using oscillatory rheological experiments in the linear regime as a function of temperature and frequency [29]. The complex response function is described in terms of its real part (G ) and imaginary part (G"). Physical properties like relaxation times or phase transitions of the non-perturbated samples can be evaluated. The linear rheology is characterized by the measurement of the viscoelastic moduli G and G" as a function of angular frequency at a small strain amplitude. The basics of linear rheology are described in detail in several textbooks [8, 29] and will not be repeated here. The relations between structure and linear viscoelastic properties of dispersed systems are well known [4,7, 26]. 

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