Semicrystalline polymers linear viscoelasticity

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

The mechanical properties are dependent on both the chemical and physical nature of the polymer and the environment in which it is used. For amorphous polymers, the principles of linear viscoelasticity apply, but these are no longer valid for a semicrystalline polymer. The mechanical response of a polymer is profoundly influenced by the degree of crystallinity in the sample. 

While the term strain hardening is widely used to describe the extensional flow behavior of some polymeric liquids, it lacks any basis in polymer physics. It was introduced by G. I. Taylor in 1934 [ 155] to describe the plastic flow of crystalline metals. It has also been used to describe the behavior of glassy and semicrystalline polymers, where it is said to arise from the effect of strain on solid-state structural features. Its use for melts is based solely on the shape of the curve of 7] (t,e) versus time with strain rate as a parameter. At small values of strain (e =t e) the behavior follows the prediction of linear viscoelasticity, as indicated by Eq. 10.93. Dealy... 

Draw a logB versus temperature plot for a linear, amorphous polymer and indicate the position and name the five regions of viscoelastic behavior. How is the curve changed if (a) the polymer is semicrystalline, (b) the polymer is cross-linked, and (c) the experiment is run faster ... 

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