Reversible recovery from deformation

Influence of Solvents. The stress-strain curves of untreated and ether-extracted corneum in water show marked differences (81). Untreated corneum, extended 5% and relaxed, shows hysteresis similar to that observed for other keratinaceous structures (Figure 35). The deformation mechanism is completely reversible, and hydrogen-bond breakdown and slow reformation may be the major factors determining the stress-strain relationships. With ether-extracted samples, complete recovery is observed from 5% extension but with little or no hysteresis. The more rapid swelling and lack of hysteresis of ether-extracted corneum in water may be related to the breakdown of hydrogen bonds normally shielded from the eflFects of water by the lipid-like materials removed by ether. 

It has been shown (46) that PTT has a very low theoretical crystal modulus, 2.59 GPa (375, 550 psi), compared to 107 GPa (1.55 x 10 psi) of PET (47) because of PTT s highly contracted helical-like conformation, whereas PET chain is fully extended with trans conformation. When a PTT fiber is stretched in situ in a waxd, the fiber period, measured from the Bragg d-spacing, increases immediately and is proportional to the applied strain up to 4% strain before deviating from affine deformation (48). Up to this critical strain the crystal deformation is reversible. The response of microscopic crystalline chains to macroscopic deformation explains why PTT has the best elastic recovery among the three aromatic polyesters. Further, PTT s elastic recovery and permanent set are nearly the same as nylon-6,6 up to 30% strain (25). 

Another type of experiment often done in conjunction with creep is creep recovery, the recoil of strain after the stress is removed, as illustrated in Figure 3.3.2. After the stress has been removed from a viscoelastic material, the deformation reverses itself. We can define a recoverable creep function... 

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