| Using a specially designed apparatus, PET and PPTA fibers were heat-treated under a constant tension, and then quenched for study at room temperature. Characterization methods include wide-angle x-ray scattering, small-angle x-ray scattering, polarizing optical microscopy, shrinkage measurements, and mechanical testing. The effects of tension, heat-treatment time and temperature on the structure and property development were determined.;For PET fibers, the imposed tension supresses chain re-coiling and greatly enhances the crystallization rate. Crystallization appears to preceed in two stages: Initial extension of molecular chains and formation of defective crystallites, and subsequent crystallite refinement. These crystallites impose restriction on chains around them and, therefore, reduce the shrinkage of the fibers until it reaches zero at the end of the first stage. During the second stage, the elongation of fibers ceases and there is no detectable shrinkage at 80;During the crystallization, the imposed tension hinders the crystallites and the intervening noncrystalline material from disorienting with respect to the fiber axis. Consequently, the tensile modulus is greatly improved by the increasing number of crystallites and aspect ratio of the crystallites in the first stage. In the second stage, the tensile modulus continues to increase, because of the increasing perfection of the crystallites.;It was found that, for PPTA fibers, threadline tension during heat-treatment has a profound effect on the tensile modulus, while temperature contributes much to the crystallization behavior. When the imposed tension is above a critital level, the tensile modulus is improved via straightening up the pleat structure in PPTA fibers, which leads to a more parallel alignment of chains in the pleats to the fiber axis. On the other hand, the lateral crystallization and axial lattice perfection are found to be uncorrelated with the tensile modulus. |
