Viscoelastic behavior above temperature dependence

In compliance with the DMA-experiments there is a distinct difference below and above the glass transition temperature. While below the glass transition temperature a strong dependency of the tangent modulus on strain can be observed, the material shows an almost linear viscoelastic behavior above the glass transition temperature within the considered region. 

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. 

As we have seen above, the transition that separates the glassy state from the viscous state is known as the glass-rubber transition. This transition attains the properties of a second-order transition at very slow rates of heating or cooling. In order to clearly locate the region of this transition and to provide a broader picture of the temperature dependence of polymer properties the principal regions of viscoelastic behavior of polymers will be briefly discussed. 

Polymers can exhibit both viscous and solid mechanical behavior this phenomenon is called viscoelasticity. For a given polymer, the degree of viscous behavior depends on temperature. Below Tg, polymers will behave more or less as elastic solids with very little viscous behavior. Above Tg,... 

The shift factor is the shift in time scale corresponding to the difference between the selected and reference temperature, and the shift factor represents the temperature dependence of the rate of the segmental motion which underlies all viscoelastic behavior the WLF equation demonstrates that all polymers, irrespective of their chemical structure, will exhibit similar viscoelastic behavior at equal temperature intervals (T-Tg) above their respective glass transition temperatures (Tg). Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. Mark JE (ed) (1996) Physical properties of polymers handbook. Springer-Verlag, New York. 

This chapter deals with viscoelastic behavior in the liquid state, particular emphasis being placed upon those aspects associated with the flow properties of polymer melts and concentrated solutions. The time-dependent response of polymers in the glassy state and near the glass transition, one variety of viscoelasticity, was discussed in Chapter 2. The concern in this chapter is the response at long times and for temperatures well above the glass transition. The elastic behavior of polymer networks well above the glass transition was discussed in Chapter 1. The conditions here are similar, and elastic effects may be very important in polymeric liquids, but steady-state flow can now also occur because the chains are not linked together to form a network. All the molecules have finite sizes, and, for flexible-chain polymers, the materials of interest in this chapter, the molecules have random-coil conformations at equilibrium (see Chapters 1 and 7). 

In the above unsteady tests, if one keeps the level of imposed stress and strain low enough, the measured material functions show an independence from these applied stimuli levels, exhibiting only a dependence on time (or frequency). This type of response indicates linear viscoelastic behavior. The primary modes of deformation employed in these tests are either shear or extension. If there is no volume change accompanying the deformation, a single modulus or compliance, whether real or complex, but a function of time (or frequency) and temperature only, suffices to characterize the material behavior. We will define moduli and compliances further below. Let us now start examining these and other key topics in linear viscoelasticity. 

The mechanical behavior of polymer resins exhibits time and temperature dependences, called viscoelastic behavior, not only above, but also below the glass-transition temperature, T. Thus, it can be presumed that the mechanical behavior of polymer composites also depends on time and temperature even below which is within the normal operating-temperature range. Examples in this respect are given by Aboudi et al., Sullivan, Gates, and Miyano et... 

This method can also be apphed to the extrapolation of time- and temperature-dependent creep behavior. Experimental creep curves first need to be obtained at a series of different temperatures over a specific time period, and the values of comphance plotted on a logarithmic time scale. After one creep curve at a chosen temperature is defined as reference, creep curves at other temperatures are then shifted one by one along the log time scale until they superimpose to a single curve in the ideal case. Curves above the reference temperature are shifted to the right, and those below are shifted to the left. This procedure can be apphed to predict longterm creep compliance on the basis of short-term tests at different temperature levels in the range of linear viscoelasticity. 

---