Glass temperature, viscoelastic behavior

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

Summary In this chapter, a discussion of the viscoelastic properties of selected polymeric materials is performed. The basic concepts of viscoelasticity, dealing with the fact that polymers above glass-transition temperature exhibit high entropic elasticity, are described at beginner level. The analysis of stress-strain for some polymeric materials is shortly described. Dielectric and dynamic mechanical behavior of aliphatic, cyclic saturated and aromatic substituted poly(methacrylate)s is well explained. An interesting approach of the relaxational processes is presented under the experience of the authors in these polymeric systems. The viscoelastic behavior of poly(itaconate)s with mono- and disubstitutions and the effect of the substituents and the functional groups is extensively discussed. The behavior of viscoelastic behavior of different poly(thiocarbonate)s is also analyzed. 

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... 

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. 

A common practice is to reduce relaxation or creep data to the temperature Tg thus, the reference temperature is picked as the glass transition temperature measured by some slow technique such as dilatometry. The reason for choosing Tg as the reference temperature is founded on the idea that all amorphous polymers at their glass transition temperature will have similar viscoelastic behavior. This type of corresponding states principal is often expressed in terms of a hopefully universal mathematical relationship between the shift factor aT at a particular temperature and the difference between Tg and this temperature. Perhaps the most well known of these relationships is the WLF equation... 

Rubber is a viscoelastic solid formed by crosslinking a polymer, which is initially a viscoelastic liquid. In spite of this difference there still are some common issues in understanding the physics of the glass temperature and the viscoelastic mechanisms in the softening dispersion (i.e., called the glass-rubber transition zone in Ferry (1980). A case in point can be taken by comparing the viscoelastic behavior of the neat epoxy resin Epon lOOlF (Plazek and... 

It is now well established that the thermomechanical response of glassy polymers and their composites is viscoelastic at temperatures near to, and above the glass transition temperature. Therefore, an accurate long term durability model at elevated temperatures for resins and reinforced plastics (FRP) must necessarily include viscoelastic behavior. This is especially true... 

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