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Macroscopic shrinkage behaviour and microstructural changes in PET as-spun fibres prepared by a normal cooling and by a `Controlled Threadline Dynamics' (LIB) 1 process have been investigated as a function of temperature. For normally cooled spun fibres, the shrinkage and microstructural changes may be respectively divided into two steps with increasing temperature. A significant shrinkage in the low temperature range was attributed mainly to a disorientation process in oriented noncrystalline chains. In the higher temperature region, the still relatively high but gradually decreasing shrinkage is related principally to the diminished disorientation process and a sequential crystallization process. In comparison with the normally cooled spun fibres, a small but monotonically increasing shrinkage was found in the case of the (LIB) samples. Structurally, a monotonic increase in crystallinity and a decrease in fraction of oriented noncrystalline phase were found to accompany the shrinkage behaviour. This indicates that the microstructural change and corresponding shrinkage in the LIB spun fibres is a one-step process. The ultra-high oriented noncrystalline structure which results from the LIB process may promote minimization of distance between molecular chains in the oriented noncrystalline regime, and enhance intermolecular cohesive forces. When the fibres are heat treated, large-scale molecular motion may be restricted because of high intermolecular interaction. Instead, local thermal motion of highly stretched noncrystalline chains may be activated, and the conversion from very taut noncrystalline chains to a crystalline phase may be easier because of smaller intermolecular distances and enhanced cohesive forces. Therefore, the entire process may be characterized mainly as a continuous mass transfer from oriented noncrystalline phase to crystalline phase in situ, eschewing massive molecular recoiling. This may result in restricted disorientation in the noncrystalline phase and consequently accompanied by apparently restricted shrinkage. © Elsevier Science Ltd. |
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