Molecular Mechanisms of Viscoelastic Behavior

Under applied stress, polymer fibers may deform by either or both of two fundamentally different molecular mechanisms. 

According to the combined first and second laws of thermodynamics, the mechanical work f dx) done on a fiber by applied stress produces an increase in the internal energy dU) or a decrease in the entropy dS).  

Equation 16.1 provides a convenient means of determining whether the fiber s stiffness is energetic or entropic in origin. 

For polymer fibers, the internal energy-cotrtrolled elasticity has an instantaneous nature since it involves the rapid changes of bond lengths and angles. However, the conformational or entropic changes are processes that ate sensitive to the local molecular mobility. This molecular mobility is affected by many factors, such as polymer chain stmcture, temperature, or absorbed moisture or other small molecules, etc. The rates of conformational changes can be described by the Arrhenius equation  

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