Tensile force, crystallization induced

If a part or several parts in a body have been strained permanently beyond the limit of plasticity and the external forces and moments are removed, the material in the overstrained region and around it will, in general, be subjected to inherent stresses which are then called residual stresses (Nadai 1963). This description is exact from the macroscopic viewpoint however, from the microscopic viewpoint, the residual stress in each grain may be different, even in a tensile test that induces perfectly uniform deformation from a macroscopic viewpoint as shown in Fig. 1. In such a case, residual stress can be understood as a stress mainly due to the different state of stress existing in the variously oriented crystals before unloading (Johnson and MeUor 1962). 

Crystallization in step-growth polymers such as polyesters and nylons is known to assist their subsequent solid-state polymerization because exclusion of reactive end-groups from crystalline domains enhances their effective concentration in the amorphous domains [14,15]. However, the condensation reaction between the last fraction of end-groups may be hindered by crystallization [16, 17]. The possibility and rate of crystallization can also be enhanced by processes that enhance orientation, such as shearing and fiber drawing [18]. For example, partial replacement of terephthalic units with isophthalic units in PET reduces crystallinity, so that no crystallization in seen in 70 30 random poly(ethylene terephthalate-co-ethylene iso-phthalate) under quiescent conditions. However, heating its amorphous fiber above its Tg under a moderate tensile force results in rapid stress-induced crystallization [19]. The reduction in crystallization by copolymerization has been employed to enhance drawability of melt-spun polyester and polyamide fibers [20]. 

Crystallization Phenomenon Induced by Tensile Force, 191 Photoinduced Formation and Growth of Polymer Crystals, 191 Conclusion, 192... 

On the basis on the carried out structural investigations of amorphous PET fibers simultaneous heat - mechanically modified at isothermal conditions and constant strain stress values it can be make the following conclusions The mechanical strain force applied simultaneously with the linear heating of the studied PET yarns affects significantly the deformation behaviour and samples crystallization kinetics. Moreover in contrast to the results obtained in the first experiment, all of the so treated specimens are partially crystaUine. The role of the tensile stress in the adjustment of the interacting processes of the fluid like deformation and stress-induced crystallization clearly reveals in the ultimate samples deformation. At stress values from 1.56 MPa to 2.16 MPa predominates the fluid like fibers extension, while the further stress increasing leads to the earlier crystallization start and thereby to decrease of the final fibers length. 

When a tensile stress is applied to a polymer sample several mechanisms may contribute to changes in the vibrational spectrum of the polymer under examination. Thus, significant intensity variations can be observed as a consequence of phase transitions, strain-induced crystallization or orientation of the polymer chains. Apart from these effects it has been demonstrated in a series of investigations on the molecular mechanics of stressed polymers, both theoretically and experimentally, that the frequency and shape of absorption bands — predominantly those which contain contributions of skeletal vibrations — are stress sensitive This sensitivity of molecular vibrations to mechanical stress has been interpreted in terms of different mechanisms such as quasielastic deformation (reduction of force constants due to bond weakening under stress), elastic bond stretching or angle bending and conformational varia-... 

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